Inorganic and Organometallic Polymers. Macromolecules Containing Silicon, Phosphorus, and Other Inorganic Elements 9780841214422, 9780841212077, 0-8412-1442-5

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Inorgani Organometallic Polymers

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

ACS

SYMPOSIUM

SERIES

360

Inorganic and Organometallic Polymers Macromolecules Containing Silicon, Phosphorus, and Other Inorganic Martel Zeldin, EDITOR Indiana University-Purdue University at Indianapolis

Kenneth J. Wynne, EDITOR Office of Naval Research

Harry R. Allcock, EDITOR The Pennsylvania State University

Developed from a symposium sponsored by the Divisions of Inorganic Chemistry, of Polymer Chemistry, Inc., and of Polymeric Materials: Science and Engineering at the 193rd Meeting of the American Chemical Society, Denver, Colorado, April 5-10, 1987

American Chemical Society, Washington, DC 1988

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Library of Congress Cataloging-in-Publication Data Inorganic and organometallic polymers/Martel Zeldin, editor, Kenneth J. Wynne, editor, Harry R. Allcock. p.

cm.—(ACS symposium series; 360)

"Developed from a symposium sponsored by the Divisions of Inorganic Chemistry, of Polymer Chemistry, Inc., and of Polymeric Materials: Science and Engineering at the 193rd Meeting of the American Chemical Society, Denver, Colorado, April 5-10, 1987." Includes bibliographies and indexes. ISBN 0-8412-1442-5 1. Inorganic polymers—Congresses 2. Organometallic polymers—Congresses I. Zeldin, Martel, 1937II. Wynne, Kenneth J., 1940. III. Allcock, H. R. IV. American Chemical Society. Division of Inorganic Chemistry. V. American Chemical Society. Division of Polymer Chemistry. VI. American Chemical Society. Division of Polymeric Materials: Science and Engineering. VII. Series. QD196.I535 546-dc19

1988 87-30630 CIP

Copyright © 1988 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates tne copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

ACS Symposium Series M. Joan Comstock, Series Editor 1988 ACS Books Advisory Board Harvey W. Blanch

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In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Foreword SYMPOSIUM SERIES

The ACS was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing except that, in order to save time, the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of th Advisory Board an integrity symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

IN CHEMISTRY SERIES

ADVANCES

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Preface THE FIELD OF INORGANICO - RGANIC MACROMOLECULES

is entering a phase of rapid development and change. For the past 20 years this area has grown steadily, mainly through fundamental studies by a small number of academic, government, and industrial scientists. Today, the burgeoning interest in this field is driven by the search for new high-performance materials and by the recognitio of organic polymers with a wide range of hitherto intractable engineering problems. Thus, great interest is directed toward the electrical, photochemical, mechanical, and biomedical properties of the new polymers as well as their unusual behavior at low and high temperatures. Polysiloxanes (silicones) began as a scientific curiosity in the 1930s, but their current widespread use in industrial and consumer applications is well-known. The recent emergence of polyphosphazenes, polysilanes, and organoelement-oxo polymers derived from the sol-gel process appears to be following a similar pattern—led first by long-range, fundamental, academic research, and then developed into an expanding technology by work in industrial and government laboratories. It is a stimulating experience to be a part of, or to follow, the current growth and expansion of this diverse field. It is a direct consequence of the needs, opportunities, challenges, and broad interdisciplinary nature of the subject that prompted the symposium on which this book is based to survey the state of the art and current perspectives in inorganic and organometallic polymers. The contributions in this book fall into two categories: topical reviews and specialist reports by symposium participants and invited contributors. The topical reviews provide a thorough survey of a particular subject area and may also contain recent results from the authors' laboratories. The specialist contributions are shorter chapters that describe particularly exciting recent research progress. Together, the topical reviews and the specialist contributions provide an in-depth look into past accomplishments and currently stimulating new efforts. We gratefully acknowledge financial support for the symposium from the following organizations: Celanese Corporation, Dow Corning Corporation, ACS Division of Inorganic Chemistry, ACS Division of Polymer Chemistry, Inc., ACS Division of Polymeric Materials: Science and

xi

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Engineering, Eastman Kodak Laboratories, and Petroleum Research Fund of the American Chemical Society. We also acknowledge the generous financial support for the symposium and the preparation of the volume from the Office of Naval Research.

MARTEL ZELDIN Indiana University-Purdue University at Indianapolis Indianapolis, IN 46223

KENNETH J. WYNNE Office of Naval Research Arlington, VA 22217

HARRY R. ALLCOCK The Pennsylvania State University University Park, PA 16802 September 11, 1987

xii In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 1

An Introduction to Inorganic and Organometallic Polymers Kenneth J. Wynne Chemistry Division, Offic

"Inorganic and organometallic" in the context of t h i s Symposium is meant to describe macromolecules which usually contain inorganic elements in the chain and organic moieties as pendant groups. The s u b t i t l e delineates elements of primary focus: "Macromolecules Containing S i l i c o n , Phosphorus and Other Inorganic Elements." The term "macromolecules" implies that the subject matter includes chain molecules that may be b u i l t up of repeat u n i t s , as well as more complex r i n g , branched, or crosslinked species (for example, see Organo-Oxo-Element Macromolecules Related to Sol-Gel Processes, and contributions by Murray (p. 408) or Seyferth (p. 143)). P r i o r reviews concerning inorganic and organometallic macromolecules are contained in texts by Stone and Graham (1), Andrianov (2), Borisov (3), Allcock (4,5), and Voronkov (6), and volumes based on previous ACS symposia edited by Rheingold (7), and Carraher, Sheats and Pittman (8, 9). The present e f f o r t is t o p i c a l in nature, and the order of presentation approximates that of the Symposium presentations. The impetus for t h i s symposium volume is the considerable progress which has been made in the l a s t few years in inorganic and organometallic macromolecules. T o t a l l y new macromolecules have been brought into existence by the development of new synthetic methods or improvement of known synthetic routes. Thus, Neilson (p. 283) describes a new polymerization reaction that gives high molecular weight poly(diorganophosphazenes), -(R PN)n -, in which organic pendant groups are bonded through P-C bonds. These polyphosphazene analogs of polysiloxanes were not previously accessible, and the development of structure-property relationships in this new subclass of macromolecules w i l l no doubt y i e l d important information and perhaps s i g n i f i c a n t applications. 2

This chapter is not subject to U.S. copyright. Published 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

2

A n o t h e r example o f i n t e r e s t i n g new i n o r g a n i c polymers is f o u n d in the work o f L a s o c k i (p. 166), who r e p o r t s the s y n t h e s i s of p o l y s i l a z o x a n e s , - [ ( R SiOj^CR^SiNR) ] -, and f i n d s s u r p r i s i n g l y b e t t e r t h e r m a l s t a b i l i t y compared wïtfi t h e i r p o l y s i l o x a n e a n a l o g s . The d e s i g n o f f u n c t i o n a l i z e d polymers w i t h a s p e c i f i c u t i l i z a t i o n is seen in new p o l y s i l o x a n e s used by Z e l d i n (p. 199) as phase t r a n s f e r c a t a l y s t s . N o v e l f u n c t i o n a l p o l y p h o s p h a z e n e s have been r e p o r t e d as w e l l by A l l c o c k (p. 250). The i n t r o d u c t i o n o f t r a n s i t i o n m e t a l c y c l o p e n t a d i e n y l , m e t a l c a r b o n y l and c a r b o r a n e m o i e t i e s i n t o p o l y p h o s p h a z e n e macromolecules is r e p r e s e n t a t i v e o f t r u l y n o v e l c h e m i s t r y a c h i e v e d a f t e r c a r e f u l model s t u d i e s w i t h corresponding molecular systems. West (p. 6 ) , M i l l e r (p. 4 3 ) , Z e i g l e r ( 1 0 ) , and Sawan (p. 112) o u t l i n e the s y n t h e s i s of a wide v a r i e t y o f s o l u b l e , p r o c e s s a b l e p o l y d i o r g a n o s i l a n e s , a c l a s s o f polymers which not l o n g ago was t h o u g h t t o be i n t r a c t a b l e . M a t y j a s z e w s k i (p. 78) has found s i g n i f i c a n t improvements in the s y n t h e t i c method f o r polydiorganosilane synthesi unusual s u b s t i t u t e d polydiorganosilanes r e p o r t s s y n t h e t i c r o u t e s t o a number o f new p o l y c a r b o s i l a n e s and p o l y s i l a z a n e s w h i c h may be used as p r e c u r s o r s t o c e r a m i c m a t e r i a l s . New c a t a l y t i c p o l y m e r i z a t i o n r o u t e s t o p o l y s i l a n e s ( H a r r o d , (p. 89)) and p o l y s i l a z a n e s ( L a i n e , (p. 124)) have been d i s c o v e r e d . S i n g l e r ( p . 268) d e s c r i b e s t h e use o f B C l ^ in t h e more e f f i c i e n t s y n t h e s i s of - ( P N C 1 ) -, the s t a r t i n g h i g h polymer f o r most polyphosphazene p o l y m e r s c u r r e n t l y under i n v e s t i g a t i o n . These r e s u l t s in the a r e a o f p o l y m e r i z a t i o n c a t a l y s t s a r e i m p o r t a n t , as the s y s t e m a t i c development o f e f f i c i e n t c a t a l y t i c r o u t e s f o r i n o r g a n i c and o r g a n o m e t a l l i c macromolecules w i l l make such m a t e r i a l s more g e n e r a l l y a c c e s s i b l e and u t i l i z a b l e . T h i s r e s e a r c h is a l s o c l o s e l y r e l a t e d t o u n d e r s t a n d i n g o f mechanisms o f c h a i n growth. E f f o r t s aimed a t the e l u c i d a t i o n o f p o l y m e r i z a t i o n mechanisms i n c l u d e t h o s e o f S i n g l e r (p. 268) in p o l y p h o s p h a z e n e s , L i p o w i t z (p. 156) in p o l y s i l a z a n e s , and Z e i g l e r and W o r s f o l d in p o l y s i l a n e s . In c o n t r a s t w i t h c a r b o n c h e m i s t r y , the mechanisms o f p o l y m e r i z a t i o n r e a c t i o n s l e a d i n g t o i n o r g a n i c and o r g a n o m e t a l l i c m a c r o m o l e c u l e s a r e o f t e n not w e l l u n d e r s t o o d . Such s t u d i e s are c r i t i c a l in e l u c i d a t i n g pathways o f c h a i n growth, t e r m i n a t i o n , and b r a n c h i n g so t h a t t h e s e f e a t u r e s may be c o n t r o l l e d . O f t e n t i m e s m e c h a n i s t i c s t u d i e s l e a d t o more e f f i c i e n t s y n t h e t i c methods, f o r example b y i m p r o v i n g y i e l d s o r s h o r t e n i n g r e a c t i o n times. In the s e c t i o n on b o r o n - c o n t a i n i n g p o l y m e r s , P a i n e (p. 378), N e i l s o n ( p . 385), and P a c i o r e k (p. 392) p r e s e n t p i o n e e r i n g work aimed a t the p r e p a r a t i o n o f l i n e a r c h a i n m a c r o m o l e c u l e s w i t h B-N backbones. T h i s work is s i g n i f i c a n t because c o n d e n s a t i o n r e a c t i o n s o f B-N compounds tend to p r o d u c e compounds w i t h r i n g s t r u c t u r e s r a t h e r t h a n c h a i n s . One o b v i o u s p o t e n t i a l u t i l i z a t i o n o f such n o v e l macromolecules is as p r e c e r a m i c m a t e r i a l s , much in the same way as p o l y a c r y l o n i t r i l e is used as a p r e c u r s o r f o r c a r b o n . O r i e n t a t i o n o f t h e B-N polymer may be t r a n s f e r r e d in p a r t t o the c e r a m i c s o l i d s t a t e a g a i n in manner s i m i l a r to c a r b o n c h e m i s t r y . However, the p r o p e r t i e s o f BN p o l y m e r s and c e r a m i c m a t e r i a l s d i f f e r g r e a t l y from t h e i r c a r b o n a n a l o g s due t o l o c a l i z e d e l e c t r o n i c s t a t e s in t h e BN bond. The consequences o f t h i s c o n t r a s t i n g e l e c t r o n i c s t r u c t u r e on m a t e r i a l s p r o p e r t i e s w i l l be i n t e r e s t i n g t o see as t h i s new r e s e a r c h a r e a develops. 9

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

1. WYNNE

Introduction

3

A p l u r a l i t y o f p a p e r s in t h i s volume c o n c e r n l i n e a r c h a i n m a c r o m o l e c u l e s . Fundamental t o u n d e r s t a n d i n g t h e p h y s i c a l and m e c h a n i c a l b e h a v i o r and c h e m i c a l and p h y s i c a l s t a b i l i t y o f t h e s e m a c r o m o l e c u l e s is a f a m i l i a r i t y w i t h phase t r a n s i t i o n b e h a v i o r , an a r e a w e l l known in o r g a n i c polymer c h e m i s t r y ( 1 1 ) . As with organic p o l y m e r s , amorphous and s e m i c r y s t a l l i n e i n o r g a n i c and o r g a n o m e t a l l i c macromolecules a r e known. C r y s t a l l i n i t y a r i s e s f r o m m a i n c h a i n o r d e r o r s i d e c h a i n c r y s t a l l i z a t i o n as d i s c u s s e d by S i n g l e r (p. 268) f o r polyphosphazenes and West (p. 6) and M i l l e r ( p . 43) f o r p o l y s i l a n e s . The l a t t e r work d e m o n s t r a t e s that c r y s t a l l i z a t i o n b e h a v i o r p l a y s a c r i t i c a l r o l e in c o n t r o l l i n g main c h a i n c o n f o r m a t i o n and o p t i c a l t r a n s i t i o n s in p o l y ( d i o r g a n o s i l a n e s ) . Thus, i m p o r t a n t s t r u c t u r e / p r o p e r t y r e l a t i o n s h i p s a r e emerging t h a t a r e r e l e v a n t t o e l e c t r o n i c and o p t i c a l m a t e r i a l s a p p l i c a t i o n s f o r these m a t e r i a l s . In a d i f f e r e n t v e i n , s i d e chain c r y s t a l l i z a t i o n has r e s u l t e d in t h e f i r s t l i q u i d c r y s t a l l i n e i n o r g a n i c and o r g a n o m e t a l l i c macromolecules v i z . unusual p o l y ( d i a l k o x y phosphazenes) d e s c r i b e d b In t h i s c a s e , t h e f l e x i b l s t r u c t u r a l r e q u i r e m e n t s on t h e n a t u r e o f t h e pendant group. Unique c o m b i n a t i o n s o f p r o p e r t i e s c o n t i n u e t o be d i s c o v e r e d in i n o r g a n i c and o r g a n o m e t a l l i c macromolecules and s e r v e t o c o n t i n u e a h i g h l e v e l o f i n t e r e s t w i t h r e g a r d t o p o t e n t i a l a p p l i c a t i o n s . Thus, A l l c o c k d e s c r i b e s h i s c o l l a b o r a t i v e work w i t h S h r i v e r (p. 250) t h a t l e d t o i o n i c a l l y c o n d u c t i n g p o l y p h o s p h a z e n e / s a l t complexes w i t h t h e h i g h e s t ambient temperature i o n i c c o n d u c t i v i t i e s known f o r p o l y m e r / s a l t e l e c t r o l y t e s . E l e c t r o n i c c o n d u c t i v i t y is found v i a t h e p a r t i a l o x i d a t i o n o f u n u s u a l p h t h a l o c y a n i n e s i l o x a n e s (Marks, p. 224) which c o n t a i n s i x - c o o r d i n a t e r a t h e r t h a n t h e u s u a l four-coordinate Si. P a r t o f t h i s symposium was d i r e c t e d t o t h e s y n t h e s i s , p r o p e r t i e s and a p p l i c a t i o n s o f i n o r g a n i c and o r g a n o m e t a l l i c m a c r o m o l e c u l e s w i t h network s t r u c t u r e s . The s e c t i o n on organo-oxo macromolecules r e l e v a n t t o s o l - g e l p r o c e s s i n g a d d r e s s e s t h e i n t e r e s t i n g s y n t h e s i s and c h a l l e n g i n g c h a r a c t e r i z a t i o n e f f o r t s in t h i s a r e a . B r i n k e r (p. 314) o u t l i n e s t h e complex c h e m i c a l and p h y s i c a l f a c t o r s w h i c h a f f e c t network f o r m a t i o n and s t r u c t u r e r e s u l t i n g from t h e h y d r o l y s i s o f a t e t r a a l k o x y s i l a n e . The i n t e r e s t i n g p r o p e r t i e s o f h y b r i d o r g a n i c / i n o r g a n i c network s t r u c t u r e s a r e d e s c r i b e d in the work o f Schmidt ( p . 333) and W i l k e s ( p . 354). In c o n c l u s i o n , some t r e n d s c a n be g l e a n e d from an e x a m i n a t i o n o f t h e c o n t e n t o f t h e symposium as a whole. The growth in r e s e a r c h e f f o r t s a d d r e s s i n g t h e s y n t h e s i s and p r o p e r t i e s o f p o l y ( d i o r g a n o s i l a n e s ) w i l l l i k e l y c o n t i n u e . The u n i q u e p h o t o p h y s i c a l p r o p e r t i e s o f t h i s newly d e v e l o p e d c l a s s o f i n o r g a n i c m a c r o m o l e c u l e s (12) t o g e t h e r w i t h ready s y n t h e t i c r o u t e s w i l l be c o n t r i b u t i n g f o r c e s h e r e , and no doubt new v e c t o r s w i l l a r i s e . A n o t h e r a r e a o f i n c r e a s e d a t t e n t i o n w i l l be organo-oxo macromolecules d e r i v e d from s o l - g e l p r o c e s s i n g methods, e i t h e r a s c o p o l y m e r s o r b l e n d s . Complex d e p e n d e n c i e s o f organo-oxo m a c r o m o l e c u l a r c o m p o s i t i o n and s t r u c t u r e on s t a r t i n g m a t e r i a l s a n d p r o c e s s i n g c o n d i t i o n s ( i n c l u d i n g k i n e t i c e f f e c t s ) w i l l l e a d to c h a l l e n g i n g and i n t e r e s t i n g s c i e n c e . Important m e c h a n i c a l , o p t i c a l and s t r u c t u r a l a p p l i c a t i o n s c o u p l e d a g a i n w i t h emerging s y n t h e t i c approaches w i l l be among t h e d r i v e r s f o r c o n t i n u e d h i g h a c t i v i t y in t h i s area.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

4

INORGANIC AND ORGANOMETALLIC POLYMERS

F i n a l l y , one a d d i t i o n a l comment c o n c e r n i n g t h e n a t u r e o f p r o g r e s s from i n t e r d i s c i p l i n a r y r e s e a r c h is e v i d e n t from t h e r e s u l t s r e p o r t e d in t h i s Symposium Volume. Once a g a i n i t is s e e n t h a t most r a p i d p r o g r e s s is made when s y n t h e t i c chemists c o l l a b o r a t e w i t h t h e i r c o l l e a g u e s in m a t e r i a l s s c i e n c e o r p h y s i c s t o d e t e r m i n e p r o p e r t i e s o f new i n o r g a n i c and o r g a n o m e t a l l i c po l y m e r s . Acknowledgment The a u t h o r thanks t h e O f f i c e o f N a v a l R e s e a r c h contribution.

Literature

f o r support of

this

Cited

1. Stone, F. G. Α., Graham, W. A. G. Inorganic Polymers, Academic Press, New York 1962 2. Andrianov, Κ. A. Metalorganic New York 1965. 3. Borisov, S. N., Voronkov, M. G., Lukevits, E. Ya. Organosilicon Heteropolymers and Heterocompounds, Plenum Press, New York 1970. 4. Allcock, H. R. Phosphorus-Nitrogen Compounds, Academic Press, New York 1972. 5. Allcock, H. R. Heteroatom Ring Systems and Polymers, Academic Press, New York 1967. 6. Voronkov, M. G., Mileshkevich, V. P., Yuzhelevskii, Yu. A. The Siloxane Bond, Consultants Bureau, New York 1978. 7. Rheingold, A. L. Homoatomic Rings, Chains and Macromolecules of Main-Group Elements, Elsevier Scientific Publishing Co., New York 1977. 8. Carraher, C. E., Sheats, J. E., Pittman, C. U. Organometallic Polymers, Academic Press, New York 1978. 9. Carraher, C. E., Sheats, J. E., Pittman, C. U. Advances in Organometallic and Inorganic Polymers, Marcel Dekker, Inc. New York 1982 10. Zeigler, J. M., Harrah, L. Α., Johnson, A. W., Polymer Preprints, 28 (1), 424 (1987). 11. Wunderlich, B. Macromolecular Physics/Crystal Melting, Academic Press, New York 1980. 12. Abkowitz, M., Knier, F. E., Stolka, M., Wigley, R. J., Yuh, H.-J., Solid State Commun., 62, 547 (1987). RECEIVED October 27,

1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 2

Polysilane High Polymers: An Overview Robert West and Jim Maxka Department of Chemistry, University of Wisconsin, Madison, WI 53706 The history and described. The polysilanes (polysilylenes) are linear polymers based on chains of s i l i c o n atoms, which show unique properties resulting from easy delocalization of sigma electrons in the s i l i c o n - s i l i c o n bonds. Polysilanes may be useful as precursors to s i l i c o n carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors.

The p o l y s i l a n e s are compounds c o n t a i n i n g c h a i n s , r i n g s , o r t h r e e d i m e n s i o n a l s t r u c t u r e s of s i l i c o n atoms j o i n e d by c o v a l e n t bonds. R e c e n t l y , p o l y s i l a n e h i g h polymers have become the s u b j e c t of i n t e n s e r e s e a r c h in numerous l a b o r a t o r i e s . These polymers show many unusual p r o p e r t i e s , r e f l e c t i n g the easy d e r e a l i z a t i o n of sigma e l e c t r o n s in the s i l i c o n - s i l i c o n bonds. I n f a c t , the p o l y s i l a n e s e x h i b i t b e h a v i o r u n l i k e t h a t f o r any o t h e r known c l a s s of m a t e r i a l s . In t h i s c h a p t e r , an i n t r o d u c t i o n and overview of p o l y s i l a n e c h e m i s t r y w i l l be p r e s e n t e d , c o n c e n t r a t i n g on the l i n e a r h i g h polymers ( p o l y s i l a n e s ) and t h e i r t e c h n o l o g i c a l a p p l i c a t i o n s . P o l y s i l a n e polymers were reviewed in 1986,(1) and a more g e n e r a l review of p o l y s i l a n e c h e m i s t r y appeared in 1982.(2) Historical P o l y ( d i p h e n y l s i l y l e n e ) may have been prepared as e a r l y as the 1920's by F. S. K i p p i n g , the g r a n d f a t h e r of o r g a n o s i l i c o n c h e m i s t r y ; but the p o l y m e r i c or o l i g o m e r i c p r o d u c t s were not c h a r a c t e r i z e d . The f i r s t c e r t a i n p r e p a r a t i o n of a l i n e a r p o l y s i l a n e came in 1949, when C h a r l e s Burkhard of the G e n e r a l E l e c t r i c Company Research Laborat o r i e s , p u b l i s h e d a c l a s s i c paper d e s c r i b i n g the s y n t h e s i s of p o l y ( d i m e t h y l s i l y l e n e ) , ( M e 2 S i ) . ( 3 ) The polymer was o b t a i n e d by condensing d i m e t h y l d i c h l o r o s i l a n e w i t h sodium m e t a l , in essence the same p r o c e s s used today f o r the s y n t h e s i s of p o l y s i l a n e s . Burkhard d e s c r i b e d ( M e 2 S i ) q u i t e c l e a r l y and a c c u r a t e l y , as an i n s o l u b l e , i n f u s i b l e , and g e n e r a l l y q u i t e i n t r a c t a b l e m a t e r i a l . I tisnow c l e a r t h a t p o l y ( d i m e t h y l s i l y l e n e ) is a t y p i c a l among p o l y s i l a n e s , but t h i s n

u

0097-6156/88/0360-0006$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2. WEST AND MAXKA

7

Polysilant High Polymers

was net r e a l i z e d at the t i m e . The d i s c o u r a g i n g p r o p e r t i e s of p o l y t d i m e t h y l s i l y l e n e ) perhaps c o n t r i b u t e d to the n e g l e c t of t h i s f i e l d over the f o l l o w i n g 25 y e a r s . ( 4 ) In any event, between 1951 and 1975, no papers appeared on p o l y s i l a n e h i g h polymers. However, l i n e a r p e r m e t h y l p o l y s i l a n e s o f the type Me(SiMe2)nMe were prepared and s t u d i e d , e s p e c i a l l y by Kumada and h i s s t u d e n t s , ( 5 ) and c y c l i c p o l y s i l a n e s were b e i n g i n v e s t i g a t e d in s e v e r a l laboratories.(£ι_7) S t u d i e s of the p e r m e t h y l c y c l o s i l a n e s , (Me2Si)« where η = 4 to 7, showed t h a t these compounds e x h i b i t remarkable d e r e a l i z a t i o n of the r i n g sigma e l e c t r o n s , and so have e l e c t r o n i c p r o p e r t i e s somewhat l i k e those of a r o m a t i c hydrocarbons.(6) I n t e r e s t in p o l y s i l a n e polymers was f i n a l l y reawakened by the work of Yajima and H a y a s h i , who found t h a t p o l y ( d i m e t h y l s i l y l e n e ) c o u l d be used as a p r e c u r s o r to s i l i c o n c a r b i d e . ( 9 ) The discovery, or r e d i s c o v e r y , o f s o l u b l e p o l y s i l a n e s at W i s c o n s i n was q u i t e accidental.(10) In one attempt to p r e p a r e c y c l o s i l a n e s c o n t a i n i n g both phenyl and methyl groups co-condensed w i t h a l k a l the d e s i r e d r i n g compound, and to our s u r p r i s e i t proved to be somewhat s o l u b l e and m e l t a b l e . The i n t r o d u c t i o n of phenyl groups a l o n g the c h a i n breaks up the c r y s t a l l i n i t y of (Me2$i)n polymer. T h i s a d v e n t i t i o u s f i n d i n g l e d to s y n t h e s i s of the " p o l y s i l a s t y r e n e " f a m i l y of Me2Si-PhMeSi copolymers.(11) At almost the same t i m e , s o l u b l e p o l y s i l a n e s were r e p o r t e d by T r u j i l l o { 1 2 ) at Sandia L a b o r a t o r i e s and by Wesson and W i l l i a m s ( 1 3 ) at Union C a r b i d e Co. Research in p o l y s i l a n e polymers grew s l o w l y at f i r s t a f t e r t h i s reawakening. But w i t h i n the past few y e a r s , both the unusual s c i e n t i f i c i n t e r e s t and the t e c h n o l o g i c a l p o s s i b i l i t i e s " of the p o l y s i l a n e s have been r e c o g n i z e d , and a c t i v i t y in t h i s f i e l d has increased sharply. Commercial manufacture of both p o l y ( d i m e t h y l ­ s i l y l e n e ) and " p o l y s i l a s t y r e n e " is now b e i n g c a r r i e d out in Japan, so t h a t these two polymers are r e a d i l y a v a i l a b l e in q u a n t i t y . S y n t h e s i s of P o l y s i l a n e s P o l y ( s i l y l e n e ) polymers are u s u a l l y made by the r e a c t i o n of d i o r g a n o d i c h l c r o s i l a n e s w i t h sodium m e t a l , in an i n e r t d i l u e n t a t temperatures above 100°C. (11) Rapid s t i r r i n g is o r d i n a r i l y used so t h a t the sodium is f i n e l y d i s p e r s e d , speeding the r a t e of r e a c t i o n . E i t h e r homopolymers o r copolymers can be s y n t h e s i z e d : R RR

S1CI2

Na.

solvent ^ 0 7 ; » >100°C

~ - S i — OR I

miJr

+

H

2

+

I RO — S i ~ ^ I

Strong o x i d i z i n g agents such as m - c h l o r o p e r b e n z o i c (MCPBA) a c i d r e a c t w i t h p o l y s i l a n e s , t o i n s e r t oxygen atoms between the silicons:(17)

™

— S i —

Si

ι

ι

~

M

C

P

B

A

> ~w4i-

also

v

O—Si' ~ I

P o l y s i l a n e s c o n t a i n i n g Si-Η groups a r e r e a c t i v e in v a r i o u s ways, f o r i n s t a n c e in a d d i t i o n t o o l e f i n s c a t a l y z e d by p l a t i n u m . R e a c t i v i t y of o r g a n i c s u b s t i t u e n t groups has a l s o been o b s e r v e d ; an example is hydrogen h a l i d e a d d i t i o n t o the C=C double bond in p o l y s i l a n e s c o n t a i n i n g c y c l o h e x e n y l e t h y l groups:(18)

P h o t o c h e m i s t r y and C r o s s l i n k i n g When i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t , a l k y l p o l y s i l a n e s undergo s c i s s i o n almost e x c l u s i v e l y , but c r o s s l i n k i n g as w e l l as s c i s s i o n is observed f o r a r y l - s u b s t i t u t e d polysilanes.(19,20) A l t h o u g h the mechanism of p h o t o l y s i s has not been e l u c i d a t e d in d e t a i l , e x h a u s t i v e p h o t o l y s i s of p o l y s i l a n e s (RR'Si)n in the presence of a s i l y l e n e t r a p p i n g agent, t r i e t h y l s i l a n e , l e d t o the s i l y l e n e product E t a S i - S i R R ' - H as w e l l as d i s i l a n e , HSiRR'-SiRR'H.(21) These f i n d i n g s suggest t h a t t h e r e a r e a t l e a s t two primary s t e p s , s i m p l e s c i s s i o n t o s i l y l r a d i c a l s (A) and e l i m i n a t i o n of s i l y l e n e s , R2S1 (B). These two p r o c e s s e s c o u l d a l s o occur simultaneously(C).

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

10 Reaction

A R

2

2

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Reaction

1

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R R R I I I Si — S i — S i ~ 2

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1

2

I

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Reaction

r

Rs

2

2

J. RI Scheme 1.

2

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R e a c t i o n s f o r the P h o t o d e g r a d a t i o n P o l y s i l a n e Polymers.

of

The s i l y l r a d i c a l s formed in the i n i t i a l s c i s s i o n appear to undergo f u r t h e r r e a c t i o n s , which may be complex. A possible s e c o n d a r y r e a c t i o n is hydrogen t r a n s f e r from an a l p h a carbon atom g i v e Si-Η and a s i l i c o n carbon double bond:(21)

I -Si-

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3

P h o t o c h e m i c a l c r o s s l i n k i n g of a l k e n y l p o l y s i l a n e s takes p l a c e r e a d i l y , presumably v i a r a d i c a l a d d i t i o n t o the u n s a t u r a t e d C=C l i n k a g e s on a d j a c e n t c h a i n s . C r o s s l i n k i n g of any p o l y s i l a n e can be c a r r i e d out p h o t o c h e m i c a l l y i f the polymer is mixed w i t h a multiply-unsaturated compound such as p h e n y l t r i v i n y l s i l a n e ; a d d i t i o n of s i l y l groups to the c a r b o n - c a r b o n d o u b l e bonds of the a d d i t i v e then p r o v i d e s the c r o s s l i n k i n g . ( 2 0 ) Thermal c r o s s l i n k i n g of p o l y s i l a n e s c o n t a i n i n g p o l y u n s a t u r a t e d c r o s s l i n k i n g a d d i t i v e s can a l s o be c a r r i e d ^ u t , w i t h a f r e e - r a d i c a l i n i t i a t o r such as ALBN. Among the o t h e r c r o s s l i n k i n g systems which have been d e v i s e d , an i n t e r e s t i n g example is the c r o s s l i n k i n g of a l i q u i d m i x t u r e of an o l i g o m e r i c p o l y s i l a n e c o n t a i n i n g Si-Η bonds t o g e t h e r w i t h a p o l y u n s a t u r a t e d compound, brought about by the a d d i t i o n of foPtCU as a h y d r o s i l y l a t i o n c a t a l y s t ; t h i s p r o c e d u r e p r o v i d e s "room temperature v u l c a n i z a t i o n " of polysilanes.(22)

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2.

WEST AND MAXKA Electronic

Polysilane High Polymers

11

Spectra

The p o l y s i l a n e polymers a l l show s t r o n g e l e c t r o n i c a b s o r p t i o n bands in the u l t r a v i o l e t r e g i o n , f a l l i n g between 300 and 400 nm.(19) A b s o r p t i o n s p e c t r a f o r s o l u t i o n s of a few polymers are shown in F i g u r e 2. The e l e c t r o n i c t r a n s i t i o n s are of o-c* type, r e f l e c t i n g e x t e n s i v e d e r e a l i z a t i o n of σ-electrons in the c a t e n a t e d s i l i c o n atoms. The a b s o r p t i o n maxima depend on the nature of the o r g a n i c substituents. Simple, unhindered d i a l k y l p o l y s i l a n e s absorb near 300 nm, but the i n t r o d u c t i o n of s t e r i c a l l y - h i n d e r i n g groups s h i f t s the maximum t o l o n g e r wavelength. A r y l groups d i r e c t l y a t t a c h e d t o s i l i c o n a l s o produce bathochromic s h i f t s ; as an example, (PhMeSi)n has i t s a b s o r p t i o n maximum at 340 nm. The l o n g e s t wavelength maxima are found f o r the s o l u b l e d i a r y l p o l y s i l a n e s r e c e n t l y r e p o r t e d by M i l l e r et a l . , ( 2 4 ) which have A x 395 nm. Thus both e l e c t r o n i c and s t e r i c e f f e c t s i n f l u e n c e the a b s o r p t i o n s p e c t r a of p o l y ­ s i l a n e s . (25) A r y l s u b s t i t u e n t s can i n f l u e n c e the e l e c t r o n i c spectrum by a l l o w i n g m i x i n g of sigm paper from the NTT r e s e a r c Many p o l y s i l a n e s a l s o show s t r i k i n g changes in t h e i r uv s p e c t r a w i t h temperature.(27) An example is shown in F i g u r e 3 f o r ( n - p e n t y l 2 S i ) in hexane s o l u t i o n . As the temperature is lowered the o r i g i n a l a b s o r p t i o n band at 313 nm d e c r e a s e s and a new band at 356 nm grows in. These changes are r e v e r s i b l e , a l t h o u g h m i c r o c r y s t a l s a p p a r e n t l y form i f the s o l u t i o n is kept f o r a time at low temperatures, making the r e v e r s a l q u i t e slow. The changes w i t h s t e r i c h i n d r a n c e and temperature must be due to c o n f o r m a t i o n a l e f f e c t s a l o n g the polymer c h a i n s . I t is now g e n e r a l l y agreed t h a t the thermochromism r e s u l t s from an i n c r e a s e in the p r o p o r t i o n of t r a n s - t o gauche c o n f o r m a t i o n s in the polymer c h a i n as the temperature is d e c r e a s e d . S i m i l a r l y , the i n t r o d u c t i o n of s t e r i c a l l y h i n d e r i n g s u b s t i t u e n t s c o u l d i n c r e a s e the amount of t r a n s junctions. E v i d e n c e in f a v o r of t h i s model is p r e s e n t e d in s e v e r a l r e c e n t papers,(28) as w e l l as the chapter by M i c h l in t h i s volume. Pure o l i g o m e r i c p o l y s i l a n e s of moderate c h a i n l e n g t h would be v e r y u s e f u l f o r d e t e r m i n i n g c o n f o r m a t i o n a l p r e f e r e n c e s and s t u d y i n g c o n f o r m a t i o n a l changes, but none are y e t a v a i l a b l e . The c l o s e s t a p p r o x i m a t i o n is the c y c l i c oligomer, ( M e 2 S i ) i e . The c r y s t a l s t r u c t u r e f o r t h i s compound, i l l u s t r a t e d in F i g u r e 4, shows t h a t i t has a c o n f o r m a t i o n q u i t e u n l i k e t h a t f o r o r g a n i c 16-membered r i n g s . T y p i c a l 1 6 - r i n g c a r b o c y c l e s e x h i b i t a "square" or d i a m o n d - l a t t i c e type s t r u c t u r e w i t h e i g h t t r a n s and e i g h t gauche t o r s i o n a l a n g l e s . An example is 1 , 1 , 8 , 8 - t e t r a m e t h y l c y c l o h e x a d e c a n e , which has e i g h t t r a n s j u n c t i o n s between 175.6 and 180°, and e i g h t gauche t o r s i o n s between 50.3 and 59.3°, a l l normal v a l u e s . (30) In c o n t r a s t , the c y c l o s i l a n e (Me2Si)i& shows no t o r s i o n a l angles in e i t h e r the normal t r a n s or normal gauche range.(30) I n s t e a d i t has e i g h t "gauche- e c l i p s e d " j u n c t i o n s between 83.9 and 93.4°, and e i g h t " t r a n s - e c l i p s e d " angles between 158.0 and 169.5°. These r e s u l t s suggest t h a t the view of p o l y s i l a n e polymers as c o n s i s t i n g of t r a n s and gauche l i n k a g e s may be o v e r s i m p l i f i e d . In the p o l y s i l a n e s , i n t e r m e d i a t e t o r s i o n a l a n g l e s may be much more important than they are in carbon polymers. m a

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

F i g u r e 1. G e l permeation chromatograph f o r ( P h M e S i ) n ( M e 2 S i ) copolymer, m=n, showing bimodal m o l e c u l a r weight d i s t r i b u t i o n . m

250

300 Wavelength, nm

350

400

F i g u r e 2. UV s p e c t r a of s o l u t i o n s of h i g h m o l e c u l a r weight p o l y s i l a n e s in THF a t 23°C; ( — ) , (n-DodecylMeSi)n ; (···) (n-Hexyl Si) ; ( ), ( C y c l o h e x y l M e S i ) n [ i n c y c l o h e x a n e ] ; ί ), (PhMeSi)n. 2

Q

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

WEST AND MAXKA

Polysilane High Polymers

F i g u r e 4. ORTEP diagram from x - r a y c r y s t a l s t r u c t u r e of (Me2Si)ie. Hydrogens have been o m i t t e d f o r c l a r i t y .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

14

INORGANIC AND ORGANOMETALLIC POLYMERS

Even more c o m p l i c a t e d changes take p l a c e in s o l i d f i l m s of p o l y s i l a n e polymers as the temperature is changed. These are d i s c u s s e d in the c h a p t e r by M i l l e r et a l . in t h i s volume.(31) Nmr

S p e c t r a and C o n f i g u r a t i o n 1 3

2 9

P r o t o n , C and S i NMR s p e c t r a f o r p o l y s i l a n e s have been recorded.(32) The p r o t o n NMR p r o v i d e l i t t l e s t r u c t u r a l i n f o r m a t i o n , but i n t e g r a t i o n of areas under the p r o t o n resonances is q u i t e u s e f u l f o r d e t e r m i n i n g the c o m p o s i t i o n of copolymers. The S i NMR s p e c t r a are of p a r t i c u l a r i n t e r e s t because they r e f l e c t the c o n f i g u r a t i o n of the polymer c h a i n . ( 3 2 ) Some Si s p e c t r a of a l k y l p o l y s i l a n e s a r e shown in F i g u r e 5. Symmetricallys u b s t i t u t e d polymers such as (n-hexyl2Si)η have no c h i r a l i t y s i n c e t h e r e can be a p l a n e of symmetry through each s i l i c o n atom. The S i resonance is t h e r e f o r e a s i n g l e narrow l i n e . However f o r d i a l k y l p o l y s i l a n e s w i t h two d i f f e r e n t a l k y l groups on each s i l i c o n (RR'Si)n, each s i l i c o n ato for a particular s i l i c o c h e m i s t r y of o t h e r nearby s i l i c o n atoms. For such polymers, a r a t h e r symmetrical c l u s t e r of peaks is observed ( F i g u r e 5 ) . These r e s u l t s are c o n s i s t e n t w i t h a t a c t i c s t r u c t u r e s , h a v i n g a s t a t i s t i c a l ( B e r n o u l l i a n ) d i s t r i b u t i o n of r e l a t i v e c o n f i g u r a t i o n s . ( 3 2 , 3 3 ) For a r y l p o l y s i l a n e s the r e s u l t s are q u i t e d i f f e r e n t . ( 3 4 ) The S i NMR f o r (PhSiMe)n is shown in F i g u r e 6; i t c o n s i s t s of t h r e e broad l i n e s w i t h r e l a t i v e i n t e n s i t y 3:3:4, each l i n e e v i d e n t l y c o n t a i n i n g a c l u s t e r of r e s o n a n c e s . The p a t t e r n s f o r o t h e r a r y l a l k y l p o l y s i l a n e s d i f f e r , but in g e n e r a l two or t h r e e broad resonances are found; none of the a r y l compounds s t u d i e d so f a r has g i v e n a symmetrical p a t t e r n l i k e those observed f o r the a l k y l p o l y ­ s i l a n e s of F i g u r e 5. The r e s u l t s f o r a r y l s i l a n e s are not f u l l y understood, but the s p e c t r a may r e f l e c t p a r t i a l t a c t i c i t y in these polymers. Further work is needed; s t u d i e s of model compounds w i t h known r e l a t i v e c o n f i g u r a t i o n would be p a r t i c u l a r l y h e l p f u l . S i l i c o n - 2 9 NMR of p o l y s i l a n e copolymers a l s o shows g r e a t promise, e s p e c i a l l y f o r d i s t i n g u i s h i n g b l o c k - l i k e from f u l l y random copolymers. 2 9

2 9

2 9

2 9

T e c h n o l o g i c a l A p p l i c a t i o n s of

Polysilanes

P o s s i b l e ways in which p o l y s i l a n e s may be u s e f u l i n c l u d e , 1. As p r e c u r s o r s to s i l i c o n c a r b i d e c e r a m i c s ; 2. As p h o t o i n i t i a t o r s in r a d i c a l r e a c t i o n s ; 3. As p h o t o c o n d u c t i v e m a t e r i a l s , and 4. As p h o t o r e s i s t s in m i c r o e l e c t r o n i c s . The l a s t of these uses w i l l be t r e a t e d in the c h a p t e r by M i l l e r , ( 3 1 ) and so w i l l not be covered here. P o l y s i l a n e s as P r e c u r s o r s to S i l i c o n C a r b i d e . The o r i g i n a l p r o c e s s f o r thermal g e n e r a t i o n of s i l i c o n c a r b i d e ceramic from p o l y m e r i c p r e c u r s o r s was p i o n e e r e d by Yajima and Hayashi.(9) The s t a r t i n g m a t e r i a l s are e i t h e r p o l y ( d i m e t h y l s i l y l e n e ) or the c y c l o s i l a n e (Me2Si)&. T h e r m o l y s i s of these m a t e r i a l s at 400-450°C l e a d s to a complex s e r i e s of changes in which i n s e r t i o n of CH2 groups takes p l a c e i n t o many of the S i - S i bonds, l e a v i n g hydrogen bound to silicon.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2.

WEST AND MAXKA

(nHex S0 ζ η o

Polysilane High Polymers

(nHexSiMe)

(nBuSIMe)

π

π

15

(nPrSIMe) ·»

1 ppnn

F i g u r e 5. solution.

2

9

S i NMR s p e c t r a f o r d i a l k y l p o l y s i l a n e s in benzene

8

F i g u r e 6.

2 9

29

Si

Sl.ppm

NMR f o r (PhSiMe)n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

16

(Me Si) 2

450°C Ar

n

\

H 2 l / 2 ^CH -sr 2

(CH ) 3

2

+

Cl[Si-CH ] Cl 0

v

(linear) Scheme I

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. £

Z

3

e

[ HDïS ( HD)]^-

3

Z

Z

E

£

Z

Z

TD6w-

£

10 UD ( HD)TS [ HDJS ( HD)]TDU KD ( H D ) T S ^ H D T S ( H D ) TD Z

Z

\D HOJS (^HD)]ID

2

3

+

£

TD HDTS ( HD)]TD

TD6W HDTS ( HD)TD + Z

3

3

Z

£

£

Z

Z

£

TD HD ( HD)TS HDÎS ( HD)TD

2


U +

Z

3

£

£

TD HDTS ( HD)TD

siUOitsoquOonjoj

HXHSJAaS

INORGANIC AND ORGANOMETALLIC POLYMERS

26

T

According to the available reports discussed above, the R R S i ( C H 2 C l ) C l / m e t a l reactions (M = Na, Mg) do not appear to give high molecular weight polysilmethylenes and at this time, this reaction has not found useful application in preceramic polymer synthesis. However, the introduction of crosslinking processes into the basic metal-effected dehalogenative polymerization of (CH3)2Si(CH2Cl)Cl was shown by Schilling and his coworkers [15] to result in formation of polycarbosilanes whose pyrolysis gave higher ceramic yields. Thus, cocondensation of (CH3)2Si(CH2Cl)Cl with CH3S1CI3 by reaction with potassium in T H F produced a branched polycarbosilane, [((CH3)2SiCH2) (CH3Si)y] , which on pyrolysis to 1 2 0 0 ° C gave a 31% yield of β -SiC. (This yield still is too low, and in further work, Schilling developed useful SiC polymer pre­ cursors which, however, are polysilanes, not polycarbosilanes [15]. Another procedure for the synthesis of 1,3-disilacyclobutanes is the pyrolysis of monosilacyclobutanes (eq. 5), [9, 14, 16], but this method has difficulties and disadvantages [14]. One of these is that polysilmethylene formation is a side-reaction when it is carried out in the gas-phase x

\ /

C

H

Si

2

_ _

N

h

CH

2

i

9

h

>

2

temp.

\ /

C

\i R

F

H

2 s /

R

+ 2 CE

Si

X:H

n

0

R'

2

Such polymerization apparently is the sole process when the butane thermolysis is carried out in the liquid phase (eq. 6).

RR'Si=CH

2

+ C H 2

(5)

A

4

— ^

silacyclo-

f

[RR SiCH ] 2

x

(6)

A more useful thermolytic polymerization which produces linear poly­ silmethylenes is that of 1,3-disilacyclobutanes carried out in the liquid phase. Such polymerization of l,l,3,3,-tetramethyl-l,3-disilacyclobutane was reported first by Knoth [17] (eq. 7). This process was studied in some detail by Russian workers [18]. l,l,3,3-Tetramethyl-l,3-disilacyclobutane is more thermally stable than 1,1-dimethyl-l-silacyclobutane. / (CH ) Si 3

C H

300°C

2>

2

^i(CH ) 3

2

^

[(CH ) SiCH ] 3

2

2

n

(7)

^CHJT A r y l and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH2]2 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19].

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

3.

SEYFERTH

27

Polycarbosilanes

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate poly­ merization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. CH /

2

\

R'Li

η R Si

CH

2

N

V

CH

f

> R [R SiCH CH CH ] Li

2

2

2

2

2

(8)

n

2

The first intermediate, resulting from c y c l o - [ ( C H 3 ) S i C H ] ring opening by R L i , is R ( C H ) S i C H S i ( C H ) C H L i , a bulky "neopentyF-type reagent. Its attack at another c y c l o - [ ( C H 3 ) S i C H ] molecule will not be very favorable and so the polymerizatio doe t I fact it is possible to polymerize 3 using an organolithium initiato 2

3

2

2

3

2

2

2

2

2

2

2

(CH ) Si 3

2

H

2

ι-

CH

^CH ^

3

2

3 Transition metal catalysts are especially effective in initiating the ring-opening polymerization of 1,3-disilacyclobutanes to give polysil­ methylenes, as reported by various workers [23-30]. Products ranging from low molecular weight telomers to high molecular weight polymers ( M » 1 0 ) could be prepared, depending on experimental conditions. A large variety of transition metal catalysts was applicable: Pt/C, H P t C l - 6 H 0 , P t C l , I r C l " , R u C l - , A11CI4-, P d C l , RUI3, [olefin P t C l ] , ( E t S ) P t C l , ( B u P ) P t C l , (Ph P) Pt(CH ) , (Pr As) PtCl , P d B r , AUCI3, C u C l , CuCl, if-crotylnickel and -chromium compounds, and others. The telomeric products of lower molecular weight were obtained when the catalytic 1,3-disilacyclobutane ring opening was carried out in the presence of small amounts of trialkylsilanes, R3S1H [23, 24, 26], or carbon tetrabromide [26]. The high molecular weight [ ( C H 3 ) S i C H ] polymers are very thermally stable (2.0% weight loss after heating at 4 5 0 ° C in vacuo for 30 min)[26]. However, pyrolysis under argon at 1 0 0 0 ° C leaves little or no ceramic residue, so they are not useful preceramic polymers. Their thermal stability is poor in the presence of oxygen, polydimethylsiloxanes and formaldehyde and oxides of carbon being obtained as oxidation products. They are inert toward concentrated mineral acids and alkalis at room temeprature, but chlorinolysis to lower molecular weight products occurs on photoinduced chlorination at 2 0 ° C [26]. Attempted conversion 5

n

2

2

6

2

2

2

2

2

2

2

2

6

2

3

4

2

2

2

4

3

2

3

2

3

2

2

2

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2

2

28

INORGANIC AND ORGANOMETALLIC POLYMERS

of [ ( C H 3 ) 2 S i C H ] to [ C H 3 ( C l ) S i C H ] by treatment with the (CH )3S1CI/AICI3 silicon-methyl cleavage reagent did result in the desired methyl group cleavage, but the polymer of initial molecular weight 250,000 underwent chain scission, i.e., S i - C H cleavage as well, to give [ C H 3 ( C l ) S i C H ] products of average molecular weight 2300 [31]. Most of the polymerization studies were carried out with [(CH3) _ SiCH ] . The platinum-catalyzed polymerization of cyclo-[CH3(EtO)S i C H ] and cyclo-[CH3(Cl)SiCH ] gave only low yields of viscous liquid polymers [25]. Each of these monomers was copolymerized with c y c l o [(CH3) SiCH ] . Since the S i - Η bond also is activated by transition metal catalysts, such catalyzed ring-opening polymerization of cyclo[ C H 3 ( H ) S i C H ] very likely would present complications. Fairly low molecular weight [CH3(H)SiCH ] oligomers were, however, accessible by another route: the L1AIH4 reduction of the [CH3(Cl)SiCH ] oligomers mentioned above [31]. In spite of their potentially useful reactive S i - H functionality, pyrolysis of these oligomers gave a ceramic yield of only 5%. Thus, a facile thermal cross-linking process did not occur. Polydimethylsilylene Mark and his coworkers polymer was prepared by HoPtCl6 6H 0-catalyzed ring opening polymeri­ zation (at 2 5 ° C for 3 days) of l,l,3,3-tetramethyl-l,3-disilacyclobutane. The polymeric product was fractionated by precipitation from benzene by addition of methanol and the fractions obtained were characterized using viscosity, osmotic pressure and dielectric constant measurements. Samples of the polymer which has been crosslinked by X-radiation [32a] or a peroxide cure [32d] also were investigated (stress-strain isotherms and thermoelastic properties) in order to obtain useful information about the statistical properties of these chain molecules. The experimental and theoretical studies indicated that the most appropriate model for the [ ( C H 3 ) S i C H ] chain is one in which there is no strong preference for any conformation: most are of identical energy [32b], Also studied was the stress-optical behavior of polydimethylsilylene [32c,d]. The polysilmethylenes, however, apparently are not useful polymers, either as such or as precursors for silicon carbide. A rubbery material could be prepared by treatment of [ ( C H 3 ) S i C H ] with dicumyl peroxide in the presence of fused silica and ground quartz, but no utility was claimed for this product [26]. The probable reason that polysilmethylenes are not useful preceramic polymers is that on pyrolysis they undergo chain scission. The reactive chain end thus generated, most likely a free radical center, then undergoes "back-biting," i.e., SH2 attack further along the chain with resulting extrusion of a small, volatile, cyclic species ( c y c l o - [ ( C H ) S i C H ] (n = 2,3,4...) in the case of polydimethylsilmethylene, as shown in Scheme 3. The result is that no residue is left behind as the polymer pyrolysis proceeds; all or most of the polymer is converted to volatiles. An apparent exception to this is the case of c y c l o - [ H S i C H ] which was reported to have been polymerized by catalytic activation with H P t C l 6 « 6 H 0 at 7 5 ° C in vacuo to give a "viscous liquid or a clear cake" which was claimed to "have a linear polycarbosilane structure" [33]. Pyrolysis of this material to 9 0 0 ° C , it was claimed, gave an 85% yield of silicon carbide. It seems doubtful that a linear polymer has been formed. Polysilmethylene chains also are formed in the thermal, coppercatalyzed reactions of methylene chloride and chloroform with elemental 2

n

2

n

3

2

2

n

2

2

n

2

2

2

2

2

2

2

2

2

2

n

2

n

2

2

2

n

2

3

2

2

2

2

2

n

n

2

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

3. SEYFERTH

29

Polycarbosilanes

R R R R R R I I I I I I [-Si-CH,-Si-CH,-Si-CH -Si-CH -Si-CH -Si-CH -] I I I I 2 | 2 | 2 R R R R R R 0

2

2

0

0

2

o

J

ι R [-ii-CH^-li-CH^-Si-CH^-Si-CH.-ii-CH.. I ι I I | R R R R R 2

2

[-Si-CH,-Si-CHR

2

2

+

2

CH,

«h I

«L-CH--] 2 R

CH,

R R Si

SiR

2

CH

2

2

(volatile)

f u r t h e r S2 a t t a c k down H

+

Rι -Si-CH--] I R 2

the

chain Scheme HI

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

J

30

INORGANIC AND ORGANOMETALLIC POLYMERS

silicon. Reported first by Patnode and Schiessler in 1945, the CH2CI2 - S i / C u reaction produced C l 3 S i C H 2 S i C l 3 , C l 3 S i C H 2 S i C l 2 H and cyclo[ C l 2 S i C H 2 l 3 as isolable products [34]. Also formed in this reaction was a viscous liquid product of approximate composition [Cl2SiCH2]nHowever, it was the work of Fritz and Worsening [35] which led to the utilization of the CH2CI2-S1/CU reaction in the formation of silmethylenes of higher molecular weight. The production of such species was o p t i ­ mized when the reaction was carried out in a fluidized bed reactor. For ease of analysis, all Si-Cl bonds in the product mixture were reduced to S i - Η ; this permitted the application of HPLC in the separation of indivi­ dual compounds in the product mixture. Those polysilmethylenes thus isolated and identified which contain four or more silicon atoms are shown in Figure 1. The CHCI3-S1/CU reaction was carried out under the same conditions and also was followed by S i - C l to S i - Η reduction and H P L C . The branched polycarbosilane products thus isolated that contain four or more silicon atoms are depicted in Figure 2. These mixtures (or the higher molecular weight portions of these mixtures) present interest­ ing possibilities as precerami The polycarbosilane fibers are not strictly polysilmethylenes. However, their main repeat units are [CH3(H)SiCH2] and [(CH3)2SiCH2], so they will be discussed here. The preparation of these polymeric precursors to silicon carbide as effected by S. Yajima and his coworkers was an important development [36]. The chemistry involved is fairly complex [36-39]. It is based on the thermal rearrangement of polydimethylsilylene (derived from sodium condensation of (CH3)2SiCl2), initially, very likely to a polysilmethylenetype polymer as a result of a Kumada-type free radical rearrangement (which, in its simplest example converts, (CH3)3SiSi(CH3)3 to (CH3)3SiCH2Si(CH3)2H [40]). In the case of polydimethylsilylene, such a rearrangement would give as product [CH3(H)SiCH2]n- However, it is clear that the thermal conversion effected at 4 5 0 - 4 7 0 ° C does not stop there. IR and NMR spectroscopic studies have shown that the Yajima polycarbosilane has a more complicated structure than [CH3(H)SiCH2]n> and crosslinking and cyclization processes [such as that shown below (no mechanism implied)] have been suggested [41]. LHI.

\ χ A CH

"

3

Si

v

\

A Η CHJ

\ / A x

CH,

CHJ

C

H

\y

\

A

Sr

Η CH,

\ SΛ

/ \

Η CH,

^CH?

/

\

.Si

CHJ

\ ./ e

\ C

H

SI

A \ CH, Η

CHJ

AN

' \ A " \

/< X

Η CH,

CH, CH,

CH, CH,

\ /

c

\ l / Si

H

CH,

\ l

A Si,\ CH, Η

CH,

a\Ia^ / ° κ Si Si ^ S / i \ H

\ / A CHj

X CH,

H c

\

SI

A \

CH, CH,

/

CHJ.

\ /

C

H

CH,

H

\ A \ A " \ A ' / \ / < A \

Η CH,

S

Η CH,

CH, CH,

X

/

C H

X

CH, CH,

CH,

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

3.

SEYFERTH

31

Polycarbosilanes

H Si(CH -SiH )2CH -SiHj H Si(CH -SiH ) Me 3

2

3

2

2

U

H Si

SiH

2

2

3

Si

H Si 2

2

2

SH

1

I

3

CH ) S«H 2

%

Si

Si H

5

3

2

H Si(CH -S»H ) Mc 3

H H Si(CH -SiH ) CH -SiH H Si(CH - S i H ) C H - S i H

2

2

8

2

3

2

3

2

2

3

2

2

4

H Si S i ^ ^ S^ i H i«

3

2

2

Si(CH -SiH ) CH -SiH 2

U

2

2

3

2

2

2

I

3

J

l ((Sir SiH -CH ) SiH

Si H

2

2

6

2

H Si(CH,-SiH,) CH,-SiH

Si H Si(CH,-SiH,) Mc S i ( C H ; - SiH J C H , - S i H Si(CH;-SiH;) CH]-SiH Si(CH^-SiH ) CH,-SiH

3

3

8

3

2

H H H H

3

5

3

5

3

3

6

2

SiH

H Si 2

H Si

3

7

/

\

H

A

^ S i

2

/

> i

\

SiH

3

3

2

"Si

J ( S i H - S«
Λ

c73 .2Ρ

*δ ο ο >

DO

.S -4-·

c

8o c Κ rt

ο •P S -g.

υ& .& * 1

s I 3

ο

00 «•«

UN

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Ο

3.

SEYFERTH

Polycarbosilanes

39

These ring opening reactions proceed via S i - C (or in the latter case, Si-Si) bond scission. Anothe C=C bond scission (eq. 12

The hydrosilylation reaction has served in the polymerization of vinylsilicon hydrides (eq. 13) [53], For the most part,ρ -addition pre­ dominated.

Polysilaacetylides have been prepared by the reaction of BrMgCECMgBr [54], ( M g C E C ) and L i C = C L i [55] with dichlorosilanes (eq. 14). In general, high molecular weight products were not obtained. n

Silaarylene and silaarylene-siloxane polymers have been reviewed [56]. Recent work of interest on such systems is that of Koide and Lenz on poly(silaarylene-siloxanes) [57] and of Ishikawa et al. [58] on poly(p-(disilanylene)phenylenes), synthesized as shown in eq. 15.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

40

INORGANIC AND ORGANOMETALLIC POLYMERS

(15)

(R -

E t and Ph)

Ishikawa's polymers readily underwent photochemical degradation at specific frequencies, an observation which suggested their application as positive UV resists.

Acknowledgment I acknowledge with thanks the generous support of our research in the polycarbosilane area by the Air Force Office of Scientific Research.

Literature Cited For

a

review of preceramic polymers, "Ceramics via Polymer Pyrolysis," see: K. J. Wynne and R. W. Rice, Ann. Rev. Mater. Sci., 14 (1984) 297-334. 2. G. Fritz and E. Matern, "Carbosilanes. Syntheses and Reactions," Springer-Verlag, Berlin, 1986. 3. E. G. Rochow, "An Introduction to the Chemistry of the Silicones," Wiley, New York; 1st edition, 1949, Chapter 3. 4. (a) L. H. Sommer, G. M. Goldberg, J. Gold and F. C. Whitmore, J. Am. Chem. Soc., 69 (1947) 980; (b) B. Bluestein, J. Am. Chem. Soc., 70 (1948) 3068. 5. J. Τ. Goodwin, Jr., W. E. Baldwin and R. R. McGregor, J. Am. Chem. Soc., 69 (1947) 2247. 6. (a) J. T. Goodwin, Jr., U.S. patent 2,507,512 (1950); Chem. Abstr., 45 (1951) 3410; (b) H. A. Clark, U.S. patent 2,507,517 (1950); Chem.Abstr., 45 (1951) 2259; (c) H. A. Clark, U.S. patent 2,507,521 (1950); Chem. Abstr., 45 (1951) 2265; (d) J. T. Goodwin, Jr., U.S. patent 2,544,079 (1951); Chem. Abstr., 45 (1951) 6654; (e) J. T. Goodwin, Jr., Brit. patent 684,102 (1952). 7. L. H. Sommer, F. A. Mitch and G. M. Goldberg, J. Am. Chem. Soc., 71 (1949) 2746. 8. (a) J. T. Goodwin, Jr., U.S. patent 2,483,972 (Oct. 4, 1949); Chem. Abstr., 44 (1950) 2011e; Brit, patent 624,550 (June 10, 1949); Chem. Abstr., 44 (1950) 6632; (b) J. T. Goodwin, Jr., U.S. patent 2,607,791 (1952); Chem. Abstr., 48 (1954) 13732. 9. For a review, which includes the synthesis, properties and reac­ tions of 1,3-disilacyclobutanes see: R. Damrauer, Organometal. Chem. Rev., A, 8 (1972) 67-133. 10. W. H. Knoth and R. V. Lindsey, Jr., J. Org. Chem., 23 (1958) 1392. 11. R. Müller, R. Köhne and H. Beyer, Chem. Ber., 95 (1962) 3030. 12. N. S. Nametkin, V. M. Vdovin, V. I. Zav'yalov and P. L. Grinberg, Izv. Akad. Nauk SSSR, Ser. Khim. (1965) 929. 13. (a) W. A. Kriner, J. Org. Chem., 29 (1964) 1601. (b) W. A. Kriner, U.S. patent U.S. 3,178,392 (1965). 14. N. Auner and J. Grobe, J. Organomet. Chem., 188 (1980) 151. 15. (a) C. L. Schilling, Jr., J. P. Wesson and T. C. Williams, Amer. Ceram. Soc. Bull., 62 (1983) 912. (b) C. L. Schilling, Jr., J. P. Wesson and T. C. Williams, J. Polymer Sci., PolymerSymposia, No.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

3. SEYFERTH

16.

17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

32.

33. 34.

Polycarbosilanes

41

70 (1983) 121. (c) C. L. Schilling, Jr., Brit. Polymer J., 18 (1986) 355. (a) N. S. Nametkin, V. M. Vdovin, L. E. Gusel'nikov and V. I. Zav'yalov, Izv. Akad. Nauk SSSR, Ser. Khim., (1966) 584 [563]. (b) N.S. Nametkin, L. E. Gusel'nikov, V. M. Vdovin, P. L. Grinberg, V. I. Zav'yalov and V. D. Oppengeim, Dokl. Akad. Nauk S.S.S.R., 171 (1966) 630[1116]. (c) N. S. Nametkin, R. L. Ushakova, L. E. Gusel'nikov, E. D. Babich and V. M. Vdovin, Izv. Akad. Nauk S.S.S.R., Ser. Khim., (1970) 1676 [1589]. (d) L. E. Gusel'nikov and M. C. Flowers, Chem. Commun., (1967) 864. (e) M. C. Flowers and L. E. Gusel'nikov, J. Chem. Soc. B, (1968) 419. (f) C. M. Golino, R. D. Bush and L. H. Sommer, J. Am. Chem. Soc., 97 (1975) 7371. (g) N. S. Nametkin, V. M. Vdovin and V. I. Zav'yalov, Izv. Akad. Nauk S.S.S.R., Ser. Khim., (1965) 1448. (h) N. Auner and J. Grobe, J. Organomet. Chem., 197 (1980) 13. (i) N. Auner and J. Grobe, J. Organomet. Chem., 197 (1980) 147. (j) N. Auner and J. Grobe, Z. anorg. allg. Chem., 485 (1982) 53. W. H. Knoth, U.S. 4166a. Review of ring opening reactions of silacyclobutanes: N. S. Nametkin and V. M. Vdovin, Izv. Akad. Nauk SSSR, Ser. Khim. (1974) 1153. N. S. Nametkin, V. M. Vdovin and V. I. Zav'yalov, Dokl. Akad. Nauk SSSR, 162 (1965) 824. N. S. Nametkin, V. M. Vdovin and V. I. Zav'yalov, Izv. Akad. Nauk SSSR Ser. Khim. (1964) 203. N. S. Nametkin, V. M. Vdovin, V. A. Poletaev and V. I. Zav'yalov, Dokl. Akad. Nauk. SSSR, 175 (1967) 1068. D. Seyferth and J. Mercer, unpublished work, 1976. N. S. Nametkin, V. M. Vdovin and P. L. Grinberg, Izv. Akad. Nauk SSSR, Ser. Khim. (1964) 1123. D. R. Weyenberg and L. E. Nelson, J. Org. Chem. 30 (1965) 2618. W. A. Kriner, J. Polymer Sci., Part A-1, 4 (1966) 444. W. R. Bamford, J. C. Lovie and J. A. C. Watt, J. Chem. Soc. C (1966) 1137. G. Levin and J. B. Carmichael, J. Polymer Sci., Part A-1, 6 (1968) 1. Ν. S. Nametkin, Ν. V. Ushakov and V. M. Vdovin, Vysokomolekul. Soedin., 1 (1971) 29. V. S. Poletaev, V. M. Vdovin and N. S. Nametkin, Dokl. Akad. Nauk SSSR, 203 (1972) 1324. V. A. Poletaev, V. M. Vdovin and N. S. Nametkin, Dokl. Akad. Nauk SSSR, 208 (1973) 1112. E. Bacque', J.-P. Pillot, M. Birot and J. Dunogues, Paper presented at the NATO Advanced Workshop on "The Design, Activation and Transformation of Organometallics into Common and Exotic Materials," Cap d'Agde, France, Sept. 1-5, (1986). (a) J. H. Ko and J. E. Mark, Macromolecules, 8 (1975) 869. (b) J. E. Mark and J. H. Ko, Macromolecules, 8 (1975) 874. (c) M. A. Llorente, J. E. Mark and E. Saiz, J. Polymer Sci., Polymer Phys. Ed., 21 (1983) 1173. (d) V. Galiatsatos, and J. E. Mark, Polymer Preprints, 28 (1987) 258. T. L. Smith, Jr., U.S. patent 4,631,179 (Dec. 23, 1986). (a) W. I. Patnode and R. W. Schiessler, U.S. 2,381,000 and 2,381,002 (1945); Chem. Abstr., (1945) 4888. (b) See also: Α. V.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

42

35. 36. 37. 38.

39. 40.

41. 42. 43. 44. 45. 46. 47. 48. 49.

50.

51. 52. 53. 54. 55.

56. 57. 58.

INORGANIC AND ORGANOMETALLIC POLYMERS Topchiev, N. S. Nametkin and V. I. Setkin, Chem. Techn., 4 (1952) 508; Dokl. Akad. Nauk SSSR, 82 (1952) 927. G. Fritz and A. Worsching, Z. anorg. allg. Chem., 512 (1984) 103. S. Yajima, Am. Ceram. Soc. Bull., 62 (1983) 893. S. Yajima, M. Omori, J. Hayashi, K. Okamura, T. Matsuzawa and C. Liaw, Chem. Lett. (1978) 551. For early Yajima papers see: (b) S. Yajima, J. Hayashi and M. Omori, Chem. Lett. (1975) 931. (c) S. Yajima, K. Okamura and J. Hayashi, Chem. Lett. (1975) 1209. (d) S. Yajima, M. Omori, J. Hayashi, K. Okamura, T. Matsazawa and C. Liaw, Chem. Lett. (1976) 551. (e) S. Yajima, J. Hayashi, M. Omori and K. Okamura, Nature, 260 (1976) 683. (f) S. Yajima, K. Okamura, J. Hayashi and M. Omori, J. Am. Ceram. Soc., 59 (1976) 324. S. Yajima, Y. Hasegawa, J. Hayashi and M. Iimura, J. Mater. Sci., 13 (1978) 2569. (a) Κ. Shiina and M. Kumada, J. Org. Chem., 23 (1958) 139. (b) H. Sakurai, R. Koh, A. Hosomi and M. Kumada, Bull. Chem. Soc. Japan, 39 (1966) 2050. Chem. Commun. (1986) Y. Hasegawa and K. Okamura, J. Mater. Sci., 21 (1986) 321. Y. Hasegawa and K. Okamura, J. Mater. Sci., 18 (1983) 3633. Nippon Carbon Co. in Japan, via Dow Corning Corp. in the United States. S. Yajima, T. Iwai, T. Yamamura, K. Okamura and Y. Hasegawa, J. Mater. Sci., 16 (1981) 1349. W. Verbeek and G. Winter, Ger. Offenlegungsschrift 2,236,078 (1974). G. Fritz and K.-P. Worns, Z. anorg. allg. Chem., 512 (1984) 103. G. Fritz, J. Grobe and D. Kummer, Advan. Inorg. Chem. Radiochem., 7 (1965) 349. N. S. Nametkin, V. M. Vdovin and V. I. Zav'yalov, Izv. Akad. Nauk SSSR Ser. Khim. (1965) 1448. (a) V. M. Vdovin, K. S. Pushchevaya, N. A. Belikova, R. Sultanov, A. F. Plate and A. D. Petrov, Dokl. Akad. Nauk SSSR, 136 (1961) 96. (b) N. S. Nametkin, V. M. Vdovin, K. S. Pushchevaya and V. I. Zav'yalov, Izv. Akad. Nauk SSSR, Ser. Khim. (1965) 1453. (a) N. S. Nametkin, V. M. Vdovin, E. Sh. Finkel'shtein, M. S. Yatsenko and Ν. V. Ushakov, Vysokomolekul. Soedin.B11(1969) 207. (b) J. C. Salamone and W. L. Fitch, J. Polymer Sci., Part A 9 (1971) 1741. K. Shiina, J. Organomet. Chem., 310 (1986) C57. H. Lammens, G. Sartori, J. Siffert and N. Sprecher, J. Polymer Sci. B, Polymer Lett., 9 (1971) 341. (a) J. W. Curry, J. Am. Chem. Soc., 78 (1956) 1686. (b) J. W. Curry, J. Org. Chem., 26 (1961) 1308. L. K. Luneva, A. M. Sladkov and V. V. Korshak, Vysokomolekul. Soedin. A9, No. 4 (1967) 910. D. Seyferth, Third International Conference on Ultrastructure Processing of Ceramics, Glasses and Composites, San Diego, Feb. 23-27, 1987. W. R. Dunnavant, Inorg. Macromol. Rev. 1 (1971) 165. N. Koide and R. W. Lenz, J. Polymer Sci., Polymer Symposia, No. 70 (1983) 91. (a) M. Ishikawa, N. Hongzhi, K. Matsusaki, K. Nate, T. Inoue and H. Yokono, J. Polymer Sci., Polymer Lett. Ed., 22 (1984) 669. (b) M. Ishikawa, Polymer Preprints, 28 (1987) 426.

RECEIVED October 23, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 4

Soluble Polysilane Derivatives: Chemistry and Spectroscopy 1

1

1

1

R. D. Miller , J. F. Rabolt , R. Sooriyakumaran , W. Fleming , G. N. Fickes , B. L. Farmer, and H. Kuzmany 2

3

4

1

IBM Almaden Research Center, San Jose, CA 95120-6099 Department of Chemistry, University of Nevada, Reno, NV 89557 Mechanical and Material 2

3

4

Institut of Festkorperphysik, University of Vienna, Vienna, Austria Polysilanes represent an interesting class of radiation sensitive polymers for which new applications have been discovered. Even though the polymer backbone is composed only of silicon-silicon single bonds, all high molecular weight polysilanes absorb strongly in the UV. The position of this absorption depends not only on the nature of the substituents, but also on the conformation of the backbone. In this regard, materials where the polymer backbone is locked into a planar zigzag conformation absorb at much longer wavelengths than comparable materials where the backbone is either disordered or nonplanar (e.g., helical). A planar zigzag backbone is often observed for symmetrically substituted derivatives in the solid state when there are also significant intermolecular interactions such as side chain crystallization. These materials are strongly thermochromic. We will discuss the thermochromism exhibited by a number of polysilanes in the solid state. Polysilane derivatives are also quite sensitive to light and ionizing radiation. The predominant process is chain scission leading to the production of lower molecular weight fragments. Certain applications which are dependent on this radiation sensitivity are described.

Polysilane derivatives, i.e., polymers containing only silicon in the backbone, are attracting considerable attention as a new class of radiation sensitive polymers with interesting spectroscopic properties (1). The first diaryl substituted polymers were probably first synthesized over 60 years ago by Kipping (2). In 1949, Burkhard (3) reported the preparation of the simplest substituted derivative poly(dimethylsilane). These early materials attracted relatively little scientific interest because of their intractable nature until the pioneering work of Yajima, who showed that poly(dimethylsilane) could J>e converted through a series of pyrolytic steps into 0-SiC fibers with very high tensile strengths (4-7). Around the same time, a number of workers reported the preparation of a soluble homopolymer (8) and some soluble copolymers (9-10). The preparation of soluble derivatives restimulated interest in substituted silane polymers and, as a result, a number of new applications for high molecular weight polysilane derivatives have recently appeared. In this regard, they have been investigated as oxygen insensitive initiators for vinyl polymerization (11) 0097-6156/88/0360-0043$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

44

and as a new class of polymeric photoconductors exhibiting high hole mobilities (12-14). The discovery that polysilane derivatives were radiation sensitive, coupled with their stability in oxygen plasmas, has also led to a number of microlithographic applications (15-19). Synthesis High molecular weight, linear, substituted silane polymers are usually prepared by a Wurtz type coupling of the corresponding dichlorosilanes initiated by sodium dispersed in an aromatic or hydrocarbon solvent (1,15,23). Recently, lower molecular weight materials have been prepared from monosubstituted silanes using a variety of titanium containing catalysts (20,21). We have prepared a large number of soluble polysilanes by the modified Wurtz coupling (23). The mechanism of polymer formation is not known with certainty, but it appears to be surface initiated, since the use of soluble aromatic radical anions does not lead to the production of high polymer. Recently, some evidence has been presented to support the involvement of radicals in the polymerization process (22).

R ^ S i C l o + Na

-

- S i l R

+ 2NaCl

2

n

Our initial studies (23) were performed in toluene, and Table I shows the results from the polymerization of a number of representative monomers. The data reported in Table I are for "direct" addition of the monomer to the sodium dispersion. Inverse addition often leads to higher molecular weights, although the overall polymer yields are usually lower (15,23). The results in Table I show that, under these reaction conditions, a bimodal molecular molecular weight distribution is normally obtained. Furthermore, it is obvious that the crude polymer yields drop precipitously as the steric hindrance in the monomer increases. In an effort to improve the yield of high polymer, a solvent study was conducted using a typical sterically hindered monomer dichloro-di-n-hexylsilane, and the results are shown in Table II. The use of as little as 5% by volume of diglyme in toluene led to a 6-7 fold increase in the yield of polymer. As the proportion of diglyme cosolvent decreased, the fraction of high molecular weight material in the isolated polymer increased. This polymerization can be also conducted around room temperature by using ultrasound activation both during the generation of the dispersion (24) and during the monomer addition. Interestingly, the characteristic blue color of the reaction mixture was not observed during the addition of monomer to the dispersion in pure xylene, but appeared soon after some diglyme cosolvent was added, with an attending large increase in the viscosity of the reaction mixture. The use of diglyme to improve the yield of high polymer is not limited to dichloro-di-n-hexylsilane and we have achieved significant improvements with a number of other dialkyl substituted monomers. Our preliminary studies indicate that there is usually little advantage in the use of polyether cosolvents for the polymerization of aryl substituted monomers unless the aryl ring contains a polar group such as an alkoxy substituent. Initially, it was felt that the role of the polyether solvent was cation complexation, thus promoting anionic propagation (15). However, the use of varying amounts of the specific sodium ion complexing agent, 15-crown-5, in the polymerization of dichloro-di-n-hexylsilane failed to significantly improve the polymerization yield and led to low molecular weight materials (entries 6 and 7). Furthermore, the addition of 16% by volume of the nonpolar solvent heptane to the toluene also resulted in a good yield of high molecular weight material. These results imply that specific cation complexation probably does not play a significant role in the polymerization process and suggest instead that there is a significant bulk solvent effect for this particular example.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

4.

MILLER ET AL.

Soluble Polysilane Derivatives

45

Table I. Some Representative Polysilanes Produced in Toluene Using Sodium in a Direct Addition Mode Yield

Polymer

(PhMeSi)n

n

(cyc-Hexyl M e S i )

n

[(C

1 0

Si£

1 3

H

2 1

)

2

6

n

Si=f

M /M

3

w

193

1.81

9.9

1.69

83.2

240

2.9

2.5

3.1

1.2

297

644

2.17

7.4

13.3

1.79

281

524

1.86

14.6

20.5

1.40

n

R

0.72

4.6

0.27

3.2

4.5

1.40

1120

1982

1.77

1.10

1.2

1.13

521

1693

3.2

1.18

1.42

1.21

3

n

χ ΙΟ"

w

5.6

12

n

M

3

107

32

(n-Dodecyl M e S i )

6

24

n

n

(n-Hexyl M e S i )

[(C H )2

χ ΙΟ"

n

60

(/3-Phenethyl M e S i )

(n-PrMe)Si

M

(%)

2.4

3.12

5.3

Table II. Solvent Effects on the Polymerization of Dichloro-di-w-Hexylsilane Entry

Additive in T o l u e n e

Y i e l d (%)

1

Toluene only

6

1982 1.2

1120 1.1

1.77 1.13

3.12

2

30%

Diglyme

37

1073 31.7

561 13.5

1.9 2.3

1.42

3

10%

Diglyme

36

1358 26.6

712 13.0

1.9 2.1

2.61

4

5%

34

1008 22.3

539 12.6

1.87 1.79

3.41

22

37.6

14.0

2.7

-

5

Diglyme

25% Diglyme 75% Xylene (Ultrasound)

M

n

χ 10-3

M

w

χ 10-3

Mw/Mn

_ / H i g h Molecular w t \ V L o w Molecular wt /

6

15-Crown-5 0.5 m m o l / m m o l N a

18

6.4

4.5

1.42

—

7

15-Crown-5 1.1 m m o l / m m o l

13

6.4

4.6

1.38

-

27

1386 1.12

679 1.09

2.04 1.03

8

16%

Heptane

Na 9.61

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

46

INORGANIC AND ORGANOMETALLIC POLYMERS

Spectral Properties and Polymer Morphology One of the most intriguing properties of catenated silane derivatives is their unusual electronic spectra (25). The observation that low molecular weight silicon catenates absorb strongly in the U V was first reported by Gilman (26,27) and has been extensively studied (25). In this regard, the high molecular weight, substituted silane polymers all absorb strongly in the UV-visible region. This is an unusual feature for a polymer containing only sigma bonds in the backbone framework and differs markedly from comparable carbon backbone analogs. Earlier theoretical studies successfully treated this phenomenon using a linear combination of bond orbitals model (LCBO) (25). These predictions have been confirmed more recently using more sophisticated theoretical approaches (28-32). Our initial spectroscopic studies on unsymmetrically substituted, atatic polysilanes derivatives resulted in a number of preliminary conclusions (33). We found that both the Α and the ε/Si-Si bond depended somewhat on the molecular weight, increasing rapidly at first with increased catenation and finally approaching limiting values for high molecular weight materials (33). These limiting values seem to be reached around a degre studies at ambient temperature 305-325 nm with those polymers containing sterically demanding substituents being red shifted to longer wavelengths. The effect of the steric bulk of substituents on the absorption spectra of alkyl substituted polymers has also been reported by other workers (19). Those polymers containing aryl substituents directly bonded to the silicon backbone are usually red shifted to longer wavelengths (^max ~ 335-350 nm). This shift, which has been previously documented for short silicon catenates (34), was suggested to result from interactions between the σ and σ* levels of the silicon backbone and the π and π * states of the aromatic moiety (25). It has recently been predicted from theoretical studies, that the extent of this perturbation should also be dependent on the geometric orientation of the aromatic substituent relative to the silicon backbone (31). Recently, variable temperature studies on certain polysilane derivatives both in solution and in the solid state indicate that the nature of the substituent effects is more complex than first indicated. This is shown dramatically in Figure 1. The absorption spectrum of poly(di-n-hexylsilane)(PDHS) in solution is not unusual and shows a broad featureless long wavelength maximum around 316 nm. The spectral properties of films of PDHS were, however, quite unexpected (35). Figure 1 shows that a spectrum of PDHS run immediately after baking at 100°C to remove solvent resembles that taken in solution except for the anticipated broadening. However, upon standing at room temperature, the peak at 316 nm is gradually replaced by a sharper band around 372 nm. This process is completely thermally reversible upon heating above 50°C and recooling. DSC analysis of the sample revealed a strong endotherm (ΔΗ = 4.0 Kcal/mol) around 42°C. The corresponding cooling exotherm was observed around 20°C, the exact position being somewhat dependent on the cooling rate. This thermal behavior is reminiscent of that observed for a number of isotactic polyethylene derivatives substituted with long chain alkyl substituents (36-39). For the polyolefins, the thermal transition has been attributed to the crystallization of the hydrocarbon side chains into a regular matrix. However, the significant changes in the electronic spectrum of PDHS at the transition temperature suggest that the nature of the backbone chromophore is also being altered during the process. IR and Raman spectroscopic studies on films and powders of PDHS indicate that the hexyl side chains are crystallizing into a hydrocarbon type matrix (40). This is indicated by the presence of a number of sharp characteristic alkane bands which become dramatically broadened above the transition temperature. Similar changes are observed for n-hexane below and above the melting point. CPMAS S i N M R studies on PDHS also show that the rotational freedom of the side chains increases markedly above the transition temperature (41,42). All of the spectral evidence φ

Μ

2 9

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

4.

MILLER ET AL.

Soluble Polysilane Derivatives

47

supports the suggestion that the side chains of PDHS are crystallizing below 42°C and that side chain melting is to a large extent associated with the observed thermal endotherm. While it was somewhat unexpected that conformational changes in a sigma bonded backbone would produce such a dramatic effect on the absorption spectrum, this is not totally without precedent in silicon catenates. In this regard, Bock et al. (43) have suggested that conformational equilibration in short chain silicon catenates is responsible for the complexity of their photoelectron spectra. Recent theoretical studies (32) on larger silicon catenates have not only confirmed the dependence of the electronic properties on the backbone conformation, but have also suggested that the all trans, planar zigzag conformation should absorb at significantly longer wavelengths than the gauche conformations. This data strongly suggests that the red shift observed for PDHS upon cooling below 42°C is associated with a backbone conformational change initiated by side chain crystallization and, furthermore, that the backbone below the transition temperature is trans or planar zigzag. This hypothesis was confirmed by wide-angle X-ray diffraction and Raman studies on stretch-aligned films of PDHS (44,45). At room temperature, W A X D analysis yielded over 40 distinct reflections from 5-45° 20 of which were indexed (44). Th strong meridional spot on th zigzag nature of the polymer backbone. At 60°C, a single sharp reflection remains on the equator (13.5A) corresponding to the silicon-silicon interchain distance. The maintenance of this strong reflection for long periods of time above the transition temperature suggests that all order is not lost, even though the side chain melting transition is associated with a conformational disordering of the silicon backbone resulting in a 60 nm blue shift in the absorption spectrum (44). This thermochromic behavior is not limited to PDHS itself but extends to other symmetrical derivatives with longer n-alkyl chains (38,42). The thermochromism of the higher homologs is, however, frequently more complex than that observed for PDHS. Although all of the higher homologs examined (up to C ) show side chain crystallization as evidenced by their IR and Raman spectra and the observation of a reversible endotherm (ΔΗ = 4.5-8.0 Kcal/mol) by DSC ( T m ~ 43-55° C), the nature of the spectral shifts displayed by these materials is variable. For example, although the heptyl and octyl derivatives behave almost identically to PDHS, the decyl derivative is more complex (see Table III). In the latter case, there exist at least two phases, each observable by DSC analysis. The low temperature phase, characterized by a thermal transition around 31°C, has a UV absorption maximum around 350 nm. Another phase, with a transition near 45°C, absorbs at much longer wavelengths, around 380 nm. The previous analysis would suggest that the phase which absorbs at the longer wavelength is more likely to contain the silicon backbone in a planar zigzag conformation. The tetradecyl derivative apparently forms a single phase ( T m ~ 54° C) and absorbs at 347 nm. The nature of the backbone conformation(s) for those materials absorbing around 350 nm has not yet been determined but it is most probably not planar zigzag. Apparently, the longer alkyl chains can still crystallize without enforcing a planar zigzag conformation of the backbone. A less likely alternative than a nonplanar backbone for those materials which absorb around 350 nm would be a planar zigzag backbone conformation where the silicon-silicon valence angles are significantly distorted from those values observed for PDHS by the size of the substituents and/or the mode in which they crystallize. Due to the nature of the orbital interactions in sigma bonded systems, the H O M O - L U M O excitation energy will be quite sensitive to changes in these valence angles even more so than to changes in the silicon-silicon bond lengths (31). On the basis of the above information, we submit that significant intermolecular interactions in the solid state (e.g., side chain crystallization) are a necessary but not always sufficient condition to force a planar zigzag conformation of the silicon backbone and in their absence the backbone either disorders or adopts ^ j e j g a ^ ^ ^ % " $ β $ $ * · 1 4

Library 1155 16th St., H.W. Washington. O.C. 20036

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

48

INORGANIC AND ORGANOMETALLIC POLYMERS

Table III. Transition Temperatures and Absorption Maxima of Some Di-w-alkyl Polysilane Films Polymer

λ shift (nm)

T (°C) m

p o l y ( d i - n - h e x y l silane)

317

374

42

p o l y ( d i - n - h e p t y l silane)

317 —

374

45*

p o l y ( d i - n - o c t y l silane)

317 —

374

43*

p o l y ( d i - n - d e c y l silane)

320 —

383

45

350)

(28)

347

54

(320 — poly(di-n-tetradecyl silane)

322

* measured by IR

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Soluble Polysilane Derivatives

If the alkyl substituent in a dialkyl substituted polysilane is either too short or is branched, it cannot pack properly to allow side chain crystallization, and hence, one would expect very different, properties from PDHS and its higher homologs. In order to study this possibility, we synthesized and characterized the di-n-butyl (PDBS), di-n-pentyl (PDPS) and di-5-methylhexyl polymers (PDMHS). A film of the branched chain 5-methylhexyl polymer absorbed at 315 nm and the position of this maximum was relatively insensitive to temperature. Likewise, no thermal transition was observed in the region where the side chain melting transition was seen for PDHS and its higher homologs. The thermal and optical characteristics of PDBS and PDPS are very similar and the latter will be used for illustrative purposes. The A of PDNPS both in solution and as a solid film occurs around 314 nm. The absorption maximum of the film is unusually narrow relative to other polysilane samples with a full width at half height of 22 nm (see Figure 2). Cooling to - 60°C has little effect except for the appearance of a small, broad shoulder at 340 nm. Heating, however, results in a dramatic change which is shown in Figure 2. Above 75° C, the absorption broadens markedly (w^ = 58 nm) but the X ^ remains essentially unchanged. DSC analysis reveals the presence of a wea 74°C. It is significant that th observed in PDHS (42° C, Δ H = 5 Kcal/mol). In the latter case, the transition has been attributed to a combination of side chain melting and the subsequent disordering of the planar zigzag backbone. The other spectroscopic data for PDPS (46) is also very different from that described for PDHS. Figure 3 shows a comparison of the Raman spectra for the two materials. Particularly obvious is the absence of the strong characteristic band at 689 cm-l for PDHS which had previously been assigned as a silicon-carbon vibration in the rigidly locked planar zigzag conformation. The CPMAS 29si N M R spectra of PDHS and PDPS are also markedly different (Figure 4). In particular, it should be noted that the silicon signal for PDPS, which is relatively narrow, is significantly upfield from that of PDHS below the transition temperature. It is interesting that at 87°C, which is above the respective phase transition temperatures of each polymer, the silicon signal of each sample becomes a narrow, time averaged resonance around - 23.5 ppm relative to TMS. All of the comparative spectral data suggests that PDPS exists in a regular structure in the solid state below 74° C which is, however, different from the planar zigzag form of PDHS. We have successfully stretch-aligned samples of PDPS, and the film diffraction pattern is compared with that of PDHS in Figure 5. From an analysis of this data, it is obvious that the backbone of PDPS is not planar zigzag but is, in fact, helical (44). The diffraction data suggests that PDPS exists as a 7/3 helix in the solid state where each silicon-silicon bond is advanced by approximately 30° from the planar zigzag conformation (Figure 6). A stable helical conformation for polysilane derivatives containing bulky substituents in the absence of intermolecular interactions is also consistent with recent calculations (47). The data shows for the first time that significant deviation from the trans backbone conformation, even in a regular structure, results in a large blue shift (~60 nm) in the U V absorption spectrum. On the basis of the results described, it now seems that the absorption spectra normally observed for alkyl polysilane derivatives ( A ~ 305-325 nm) may arise either from conformational disorder in the backbone or from systematic deviations from the planar zigzag conformation even in regular structures and do not represent the intrinsic limiting absorption for an alkyl substituted polysilane chain. In a continuing study of substituent effects on the spectra of polysilane derivatives, we have succeeded in the preparation of the first soluble poly(diarylsilane) homopolymers. Materials of this type have traditionally proved to be insoluble and intractable. Very recently, West and co-workers have reported the preparation of some soluble copolymers which contain diphenylsilylene units (48,49). m a x

2

ma

m a x

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

50

INORGANIC AND ORGANOMETALLIC POLYMERS 0.8 p

400

Figure 2. Variable temperature U V spectra of a poly(di-n-pentylsilane) (PDPS) film.

I

1600

1

1400

ι 1200

ι 1000

ι

ι

I

800

600

400

Frequency (cm

1

L_ 200

)

Figure 3. Raman spectra of PDHS (upper) and PDPS (lower) at 21°C.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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MILLER ET AL.

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Soluble Polysilane Derivatives

-23.5

-23.5

87°C

-29.2

23.0

I

-33°C

~~i—'—ι—'—ι—•—ι 50 (ppm)

ι— —ι—•—ι—'—ι—'—ι—'—ι 0 50 (ppm) 1

Poly(di-n-hexylsilane)

Poly(di-n-pentylsilane) 29

Figure 4. Variable temperature CPMAS Si NMR spectra of PDHS (left) and PDPS (right).

Figure 5. Room-temperature, wide-angle X-ray diffraction (WAXD) pictures of stretch-oriented samples of PDHS (left) and PDPS (right).

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

52

The soluble substituted diaryl homopoiymers were prepared by the condensation of the respective substituted silyl dichlorides with sodium (50). The yields of these polymerizations were predictably low (~10%) as expected based on previous studies on sterically hindered monomers. The most extraordinary feature of the soluble poly(diarylsilanes) is their remarkable absorption spectra. In this regard, they absorb around 400 nm, making them by far the most red shifted of any polysilane derivatives studied thus far. Some representative spectral data is shown in Table IV. Included in the table for comparison are data for poly(methylphenylsilane) and a random copolymer containing diphenylsilylene units. The long wavelength absorption band of the diaryl polysilanes in solution is exceptionally narrow (11-16 nm) for a polymeric material containing a backbone chromophore. Surprisingly, there is very little difference in the spectra recorded either in dilute solution or as films except for some broadening in the solid sample. The origin of the large spectral red shifts observed for these materials is under active investigation. At this point, however, they seem much too large to be attributed to typical electronic substituent effects alone. In this regard, poly(phenylmethylsilane) i re ~ 25-30 nm. This shift has bee substituent π and π* orbital addition of a second aromatic group contributed similarly to the spectrum, which is highly unlikely based on the study of model compounds (34,51,52), the materials would be expected to absorb around 360-365 nm. In this regard, the random 1:1 copolymer prepared from the condensation of diphenyl silyl dichloride with hexylmethylsilyl dichloride absorbs around 350 nm. It therefore seems likely that the explanation of the unexpectedly large red shifts observed for the soluble diaryl polysilane homopoiymers requires something other than a simple electronic substituent effect. One intriguing, albeit tentative, explanation is that these spectral shifts also are conformational in origin. It is interesting that the magnitude of the observed shifts (50-60 nm) relative to poly(methylphenylsilane) is very similar to that observed for the poly(di-n-alkylsilanes) when they undergo a change from a disordered backbone to a regular planar zigzag arrangement (35,40). Although the close correspondence between the magnitude of the spectral shifts observed in each system may be fortuitous, it does raise the possibility that the diaryl derivatives adopt an extended planar zigzag conformation even in solution without the intervention of significant intermolecular interactions. This argument does not necessarily require that these polymers must exist exclusively as rigid rods, but the position of the long wavelength transition would argue that all-trans runs of significant length be present in solution. While the suggestion that the origin of the red shifts observed for the soluble diaryl polysilanes is conformationalisintriguing, the final answer must await further structural and spectroscopic studies both in solution and in the solid state. Photochemistry Since both the X and the e depend on the molecular weight up to a degree of polymerization of 40-50 (33), any process which induces significant chain scission should result in bleaching of the initial absorbance. Conversely, crosslinking processes which maintain or increase the molecular weight of the polymer should lead to very little bleaching. All of the polysilane derivatives which we have examined thus far undergo significant bleaching upon irradiation (15) both in solution and as films (see Figure 7 for a representative example) albeit the rate varies somewhat with structure. In general, the alkyl substituted materials bleach more rapidly than the aryl derivatives in the solid state and among the former those with bulky substituents usually bleach most rapidly. GPC examination of the polymer solutions after irradiation show that the molecular weight is continuously max

SiSi

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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MILLER ET AL.

53

Soluble Polysilane Derivatives

Figure 6. Projection of the 7/3 helical conformation of PDPS onto a plane normal to the helical axis. Silicon atom represented b th large filled circles carbo atoms by the smaller. Table IV. UV-Visible Spectroscopic Data for Some Soluble Diaryl Polysilanes in Hexane Polymer

(PhMeSi)

-KPhfeSiJ^-co—[Me

A

n

Hexyl S i - ^

m

a

x

ε χ

10-*/SiSi

340

90

350

5.5

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

54

reduced. Alkyl derivatives such as poly(methyldodecylsilane) cleanly scission and the molecular weight decreases regularly upon irradiation. The GPC traces of such materials show no high molecular weight tail. On the other hand, aromatic polysilane derivatives such as poly(phenylmethylsilane) are not cleanly converted to lower molecular weight materials upon irradiation and the GPC analysis of these irradiated materials clearly show the presence of a higher molecular tail suggesting that concurrent crosslinking is occurring (33). Materials such as PDHS which absorb at ~370 nm after standing for some time to allow side chain crystallization to orient the backbone into a planar zigzag configuration can also be bleached upon irradiation at 365 nm with the formation of lower molecular weight fragments. The soluble poly(diarylsilanes) are bleached very readily upon irradiation at 404 nm in solution, but much more slowly in the solid state. One possible explanation for this latter observation is that the absence of readily available α-hydrogens for disproportionation of the silyl chain radicals produced by photoscission promotes rapid recombination resulting in chain repair (vide infra). The quantum yields for both chain scission (Φ ) and crosslinking ( Φ ) can be estimated from plots of 1/Mn and 1/M versus the absorbed dose in photochemical and other radiation induced quantum yields for scission ar and vary from ~0.5-1.0. Th alky y crosslink during irradiation ( Φ = 0-0.08). On the other hand, aromatic materials such as poly(methylphenylsilane) and related derivatives are more prone to crosslinking ( Φ ~ 0.12-0.18), although chain scission is still the predominate process ( Φ / Φ ~ 5-7). One particularly clear trend is that the efficiency for both scission and crosslinking is significantly reduced in the solid state relative to solution by as much as a factor of 50-100. The presence of air is not necessary for photobleaching, although in vacuum, the efficiency depends somewhat on the nature of the substituents. This suggested that the polysilanes might have some potential as positive functioning e-beam resists (vide infra). To conveniently assess their sensitivity toward ionizing radiation, samples of poly(p-t-butylphenyl methylsilane) poly(di-n-butylsilane) and poly(di-n-pentylsilane) were placed in Pyrex tubes, sealed under vacuum and subjected to y-radiolysis from a C o source. After irradiation, the samples were opened and analyzed by GPC. In each case, polymer scission was the predominate pathway and the G(s) values varied from ~0.14 to 0.4. Under the same conditions, the G(s) value of P M M A was calculated to be 1.2. The alkyl polysilane derivatives were ~3 times more sensitive than the aromatic polymer. The observed crosslinking was relatively minor in each case and the ratios of G(s)/G(x) were all greater than 20. Previous photochemical studies on acyclic silane oligomers have shown that both silyl radicals and substituted silylenes are intermediates in the irradiation (55,56). It has recently been suggested that monomelic silylene species are also major gas phase intermediates in the laser induced photoablation of a number of alkyl substituted copolymers (19). Accordingly, we have exhaustively irradiated a number of high molecular weight polysilane derivatives at 254 nm in the presence of trapping reagents such as triethylsilane and various alcohols (57). With the former reagent, the major products were the triethylsilane adducts of the substituted silylene fragment extruded from the polymer chain. Also produced in reasonable yield were disilanes carrying the substitution pattern of the polymer chain. The disilane fragments apparently accumulate in the photolysis mixture, because they are only weakly absorbing at 254 nm. These results suggest that silylenes and silyl radicals are both generated upon exhaustive photolysis of the linear substituted silane polymers. The silanes produced by abstraction of a hydrogen atom from the trapping reagent are subjected to further chain degradation until they reach the stage where they no longer efficiently absorb the light. The results from the photolysis in alcoholic solvents were also consistent with silylenes and silyl radicals as intermediates (57). These studies lead us to postulate the mechanism shown 8

W

χ

χ

δ

χ

6 0

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

χ

4.

MILLER ET AL.

55

Soluble Polysilane Derivatives

below for the photodegration of polysilane high polymers in solution. It is not known whether the silylenes are produced concurrently with or subsequently to the formation of the silyl chain radicals.

72 '.

"2

1

h

Si — Si — Si I I I

w w w

Rj

/

Si:

R

"2

1

Rj

n v

w w w

Rj

Si:

+

2 w w w

Si I

2

I + (Et) SiH — • (Et) Si — Si — H 3

3

w w w

3

R*>

Si — H I

+

(Et) Si . 3

R-,

R|

w w w

». /

Si . + (Et) SiH — • I

1

"1 1

\

R| h

h

Si — H — • I (Et) SiH 3

R9

1

Rj 1

R| 1

H — Si — Si — H + (EtkSi — Si — H I I I R

~>"propagation 1"

iNa

(1) 2

2

[CISiRiR ]- + Na+ -> SiRÏR : + NaCl

I "propagation 2"

i "propagation 3"

This can result in polymodality of the molecular weight distribution when the exchange between the chain carriers is slow and they lead to the independent growth. Polymodality of the MWD is observed indeed. This phenomenon has been explained by differences in diffusion of the short and long polymer chains, but multiplicity of the chain carriers seems to us to be a more probable explanation. The origin of the reaction which limits the chain growth is not known. Probability of cyclization, one of the side reactions, should decrease strongly with the polymerization degree in agreement with the

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

6.

MATYJASZEWSKI ET AL.

New Synthetic Routes to Polysilanes

81

Jacobson-Stockmayer theory (13 ). Usually transfer and termination reactions in polymerization have energies of activation higher than propagation. Thus, we attempted coupling reactions at temperatures lower than usually applied. In order to assure the access of dichlorosilane to the surface of the sodium, the dispersion was continuously regenerated by ultrasonication. We have observed a dependence of the yield, polymerization degree, and polydispersity of polysilanes on temperature and also on the power of ultrasonication. In the ultrasonication bath the simplest test of the efficiency of cavitation is the stability of the formed dispersion. It must be remembered that the ultrasonic energy received in the reaction flask placed in the bath depends on the position of the flask in the bath (it is not the same in each bath), on the level of liquid in the bath, on temperature, on the amount of solvent, etc. When an immersion probe is used the cavitation depends on the level of the meniscus in the flask as well. The power is usually adjusted close to 50% of the output level but it varies with the reactio conditions. The immersion-typ temperatures. GPC traces of poly(phenylmethylsilylenes) prepared in the ultrasonication bath are shown in Fig. 1. In contrast to thermal condensation, monomodal high molecular weight polymer is formed. Oligomeric cycles (mostly cyclic pentamer), formed usually in high yield (cf. Table 1), can be very easily separated from the reaction mixture by precipitation with isopropanol. The molecular weight of polysilanes decreases and polydispersity increases with temperature. The coupling reaction is usually rapid and completed in a short time after the addition of dichlorosilane. Ultrasonication accelerates the recovery of the sodium surface and enables large momentary excess of sodium to chlorosilanes. We have, however, observed a change in the molecular weight and polydispersity at longer reaction times. This is illustrated in Fig. 2. Because the coupling reaction is usually carried out in excess sodium ([Na]o/[Si-Cl]o=1.2), degradation of the polymer is possible. We had prepared polysilanes and tested this possibility by their reaction with and without sodium in the presence of ultrasonication. Monomodal and bimodal polysilane was used. Reaction without sodium led to random degradation whereas ultrasonication with sodium favored degradation of the low molecular weight polymer. Thus, the monomodality of polysilanes obtained by ultrasonication can have its origin in the selective and more rapid degradation of the low molecular weight polymer, which might have a different microstructure than the high polymer, and may also favor the formation of the high polymer by reaction conditions different than those in a typical thermal process. If two independent mechanisms of chain growth are assumed, a lower temperature will favor the process proceeding with the lower activation energy (high polymer). If polymodality has its origin in diffusion phenomena, cavitation can eliminate the formation of low oligomers. Eventually, a large excess of sodium may also influence the kinetics of coupling and favor the formation of high polymer. The degradation of polysilanes in the presence of sodium indicates that the polymer is a kinetic product and is easily degraded to

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Fig. 1. GPC traces of the polymerization mixtures of phenylmethyldichlorosilane ([M]o= 0.32 mol/L) and sodium ([Na]o= 0.7 mol/L) in toluene after 6 hours and quenching with dry ethanol.

C/5

Μ

ο

r

>

Η

Μ

3

ο ο ο > ο

>

ο

>

Ο *

to

00

6. MATYJASZEWSKI ET AL.

New Synthetic Routes to Polysilanes

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

83

84

INORGANIC AND ORGANOMETALLIC POLYMERS

thermodynamically more stable cyclic oligomers. Thus, some earlier experiments performed with soluble initiators (biphenyl radical anion) ( 1 Π in which only oligomers were formed may indicate very rapid polymerization as well as very rapid degradation. This means that isolation of the high polymer could have been possible but at shorter reaction times. We believe that the reproducibility of our results will enable more systematic kinetic and mechanistic studies of the reductive coupling of dichlorosilanes leading to the formation of high polymers. Modification

of

Polysilanes

The presence of alkali metal during reductive coupling practically prevents the synthesis of polysilanes with groups which can be easily cleaved under basic conditions, such as pendant alkoxy or amino substituents. Polysilanes with pendant functional groups are not known. We have recently discoverd a very efficient method of functionalization of polysilanes. Model reactions sho rapidly and quantitatively R-Si(CH ) -Si(CH3)2-R + 2 H O S 0 C F -> CF S020-Si(CH )2-Si(CH3)2-OS02CF3 + 2RH 3

2

2

3

3

3

where R = C H , C H , CI 6

5

(2)

3

Bis l,2-(trifluoromethanesulfonlyloxy)tetramethyldisilane is a stable and reactive compound towards different nucleophiles (14). It reacts rapidly with pyridine forming mono and (at a 1:2 ratio) disalts with pyridine. With secondary amines this compound forms the corresponding disilyldiamine. Dialkoxydisilanes were prepared in good yields in the reaction with different alcohols: B: CF S0 0-Si(CH )2-Si(CH )2-OS0 CF + 2ROH -> RO-Si(CH ) -Si(CH ) -OR -BH (3) 3

2

3

3

2

3

3

2

3

2

where RO-: C H 0 - , C F C H 0 - , CH =CHCH 0-, (CH ) CO-, etc 3

3

2

2

2

3

3

We used a similar reaction for the modification of polysilanes. Poly(phenylmethylsilylene) has been chosen as the model polysilane because cleavage of the aryl groups is much faster than that of the alkyl groups. The displacement reaction proceeds very rapidly at room temperature and immediately after addition of the acid the sharp signal of the benzene is found. ÏH-NMR spectra of the initial polymer indicate the presence of different iso-, syndio-, and atactic structures. The chemical shifts of the methyl groups after displacement of the phenyl groups by triflate groups move downfield approximately 1.2 ppm (cf. Fig. 3). A similar chemical shift of methyl groups is observed for mono and disilanes with triflate groups. Polysilanes bearing triflate groups are very reactive and form an insoluble gel with a trace of moisture. The triflate group is hydrolized to silanol which rapidly condenses with the remaining triflate groups to a siloxane unit, linking intra or intermolecularly silicon atoms:

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

6. MATYJASZEWSKI ET AL.

New Synthetic Routes to Polysilanes

a

Fig. 3. iH-NMR spectra of poly(phenylmethylsilylene) ([(SiPhMe)n]o=0.42 mol/L) (a), after reaction with triflic acid ([CF3SC>3H]o= 0.34 mol/L (b), and poly(methylmethoxysilylene) ([(SiOMeMe) lo= 0 · mol/L) (c) in CD Cl 3 using CH2CI2 as internai standard. n

4 2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

85

86

INORGANIC AND ORGANOMETALLIC POLYMERS

CF3SO2O

Ph ...-(Si)n-... CH

+ 11HOSO2CF3

...-(Si)n-... + H 0 2

CH

3

O-Si-...

3

...-(Si)n-... CH3

(4)

Polymers with triflate groups react with alcohols to form alkoxysubstituted polysilanes. This reaction occurs readily in the presence of bases. The best results were obtained using triethylamine and hindered pyridine. In Fig. 3c the NMR spectrum of the reaction mixture containing the excess of triethylamine is shown, the methyl groups from the polymer chains absorb in the range typical for alkoxysilanes. Reaction in the presence of unsubstituted pyridine leads to the formation of insoluble polymer probably by attack at the p-C atom in the silylated pyridine. Polysilanes with alkox conventional polysilanes. They degrade rapidly in the presence of light in agreement with the facile formation of silylene from dialkoxydisilanes. Properties of these polymers are currently being investigated. Ring-Opening

Polymerization

The formation of the linear polymer from the cyclic monomer requires a decrease of the free energy. Because usually entropy is lost during polymerization, the main driving force for the ring-opening process is the release of the angular strain upon conversion of the cycles to linear macromolecules. Thus, a majority of three- and four-membered rings can be readily and quantitatively converted into polymers. Nonpolymerizability may have its origin either in thermodynamics or in kinetics. Some monomers, in spite of the strain, cannot be homopolymerized but readily copolymerize. l-Benzyl-2,2dimethylaziridine or tetramethyloxirane can serve as the examples (15). On the other hand unsuccessful polymerization can be ascribed to the lack of the initiator which could lead to the formation of active species capable of a significant decrease of the energy barrier for propagation. Therefore oxetanes, thietanes, azetidines, and aziridines cannot be polymerized by an anionic but only by a cationic mechanismQJL). Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly (ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers; only strained (and more electrophilic) monomers can react with the anion. We have attempted the polymerization of octaphenylcyclotetrasilane, which is the only commercially available strained cyclopolysilane.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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MATYJASZEWSKI ET AL.

87

New Synthetic Routes to Polysilanes

We have used different anionic, cationic, and metathesis initiators but after the reaction of the initiator with one monomer molecule (initiation, ki) no subsequent propagation was observed:

Ph Ph PhPh ki I I I I BuLi + PhgSi4 -> Bu-Si-Si-Si-Si-Li+ I I I I PhPh Ph Ph

(+Ph Si4) k 8

p

k

d

PhPhPhPhPhPhPhPh I I I I I I I I Bu-Si-Si-Si-Si-Si-Si-Si-Si"Li+ I I I I I I I I PhPhPhPhPhPhPhPh

(5) This may suggest that either propagation is very slow due to the low reactivity of the derived species or that the equilibrium constant K=k /kd is very low which means that the cyclic tetramer is more stable than the polymer chain. This is apparently in agreement with the high yield (up to 40%) of cyclic tetramer Kipping experiment of th very low because in addition to cyclic tetramer, large quantities of cyclic pentamer and hexamer were formed. Only very recently was existance of polysilanes with two aryl substituents proved by Miller who prepared soluble diaryl polysilanes with long p-alkyl substituents ( 1 2 ) · Yields and stabilities of these polymers were not reported. The interactions between bulky phenyl substituents in the polymer chain can give more steric hindrance than the deformation of the valency angles in the four membered ring. Similar interactions prevent the polymerization of 1,1-diphenylethylene and 2,2diphenyloxirane (16). Thus, octaphenylcyclotetrasilane can be thermodynamically more stable than linear perphenylpolysilane and no initiator exists capable of converting this cycle to the linear polymer. On the other hand, anions formed by the ring cleavage of octaphenylcyclotetrasilane may be very unreactive due to steric hindrances as well as to the formation of the tight silyl ion pair with alkali metals. We have attempted to decrease these interactions by using cryptands in order to better solvate the alkali metals and form the loose, separated ion pair. However, in addition to the rapid cleavage of one ring no propagation was observed. Using a 10 fold excess of monomer to butyl lithium in the presence of cryptand [2.1.1], only 10% of the monomer was converted to the linear chain; the rest did not react even after a longer time or at higher temperatures. The polymerizabilty of cycles should increase for rings with smaller substituents. It has been reported that substitution of the chlorine atoms by alkyl groups in octachlorotetracyclosilane using methyl lithium led to the ring cleavage, indicating the possibility of polymerization. The sensitivity towards cationic or anionic systems should strongly depend on the structure of the substituents. Alkoxy groups may strongly stabilize positive charge and in a way similar to vinyl ethers can facilitate cationic polymerization. On the other hand, cyano groups should favor the anionic process. The facile displacement of phenyl groups by triflic acid was used as the first step in the synthesis of the four-membered cycles with alkoxy and cyano groups. During this substitution different isomers were formed and at the ratio 3:1 ( [ H O S 0 2 C F 3 ] o : [ P h g S i 4 ] o ) seventeen signals of methoxy groups were p

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

88

INORGANIC AND ORGANOMETALLIC POLYMERS

observed by NMR. The presence of different isomers in the puckered four-membered ring indicates small differences in reactivities of different aryl groups. The excess of the acid was observed by NMR at a ratio higher than 4:1 which showed difficulty in the formation of the geminal ditriflate. In the preliminary experiments, the ring-opening polymerization of octamethoxycyclotetrasilane led to a polymer with molecular weight M = 1 0 and high polydispersity. The conditions leading to better defined polymers are at present being investigated. 4

n

Acknowledgments. The financial support for this work by the Office of Naval Research is gratefully acknowledged. Literature Cited 1. West, R. J. Organomet. Chem. 1986, 300, 327 2. Hiraoka, H., et al., U. S. Patent 4, 464,460 (1984) 3. Hofer, D. C., Miller, R 469,16 4. Hofer, D. C. , Miller, R. D., Willson, C. G., Neureuther,A. ibid, 1984, 469, 108 5. Yajima, S., Hasegawa, Y., Hayashi, J., Ijmura, M. J. Mater. Sci. 1978, 13, 2569 6. Mazdyasni, K., West, R., David, L . D. J. Amer. Cer. Soc. 1978, 61, 504 7. West, R., Wolff, A. R., Petersen, D. J., J. Radiation Curing, 1986, 13, 35 8. West, R., David, L . D., Djurovich, P. I., Stearley, K. L . , Srinivasan, K. S. V., Yu, H. J . Amer. Chem. Soc. 1981, 103, 7352 9. Kepler, R. G., Zeigler, J. M . , Harrah, L . Α., Kurtz, S. R. Bull. Am. Phys. Soc. 1983, 28, 362 10. Kajzar, F. , Messier, J., Rosilio,C. J. Appl. Phys., in press 11. Zeigler, J. M . Polvmer Preprints 1986 ,27, 109 12. Zeigler, J. Μ. , Harrah, L. A. , Johnson, A. W. Polymer Preprints 1987, 28, 424 13. Jacobson, W. , Stockmayer, W. H. J. Chem. Phvs. 1950, 18, 1600 14. Chen, Y. L . , Matyjaszewski, K. J. Organomet. Chem., submitted 15. Penczek, S. , Kubisa, P., Matyjaszewski, K. Adv. Pol. Sci. 1980, 37,1 16. Penczek, S. , Kubisa, P., Matyjaszewski, K. Adv. Pol. Sci. 1986, 68/69, 1 17. Miller, R., Rabolt, J. R., Sooriyakumaran, R., Fickes, G.N., Farmer, B.L., Kuzmany, Η. , Polymer Preprints 1987, 28, 422 RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 7

Polymerization of Group 14 Hydrides by Dehydrogenative Coupling John F. Harrod Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canad The dehydrocoupling organosilane germane under the c a t a l y t i c influence of titanocene and zirconocene derivatives is reviewed. Primary organosilanes generally give RSiH -terminated o l i g o organosilylenes containing 10 to 20 s i l i c o n atoms, depending on the catalyst. Polymer s t r u c t u r a l assignments are based on a combination of ir, nmr and ms spectroscopies and on molecular weight studies using gpc and vpo. Primary germanes give three-dimensional gels with the titanium-based catalysts but secondary germanes give short chain linear oligomers. It is proposed that the polymerization proceeds by a r e p e t i t i v e insertion of s i l y l e n e moieties i n t o a m e t a l - s i l i c o n bond. 2

The p r o d u c t i o n o f e x t r e m e l y l o n g c h a i n s o f c a r b o n atoms is g r e a t l y facilitated by t h e common e x i s t e n c e o f compounds with metastable multiple carbon-carbon bonds. I n a d d i t i o n t o the h i g h l y f a v o r a b l e thermodynamics of polymerization o f C=C and CEC b o n d s , the initiation modes, which t y p i c a l l y produce a p r o p a g a t i n g species with one a c t i v e e n d , g e n e r a l l y p r e c l u d e t h e f o r m a t i o n o f r i n g s by intramolecular c o u p l i n g o f the two c h a i n e n d s . The thermodynamic stability o f polymer r e l a t i v e t o o l e f i n is f a v o r a b l e f o r those cases where one o f the c a r b o n s c a r r i e s o n l y hydrogen atoms. If both c a r b o n atoms c a r r y s u b s t i t u e n t s , the c e i l i n g temperature, above which the polymer is u n s t a b l e relative t o monomer, is generally t o o low f o r t h e polymer t o be u s e f u l . The near absence of s i l i c o n a n a l o g s o f the s u b s t i t u t e d o l e f i n s and a c e t y l e n e s has precluded a p a r a l l e l evolution of p o l y s i l y l e n e chemistry. Indeed, the s u c c e s s f u l s t r a t e g y f o r p r o d u c i n g d o u b l e bonds between elements o f the t h i r d and lower p e r i o d s has been t o so encumber t h e atoms on either side o f the d o u b l e bond w i t h b u l k y s u b s t i t u e n t s that the m o l e c u l e has no i n c l i n a t i o n t o p o l y m e r i z e The l a r g e r s i z e o f silicon compared t o c a r b o n makes a much h i g h e r s t e r i c encumbrance n e c e s s a r y t o s t a b i l i z e t h e monomer. The same c o n s i d e r a t i o n s apply even more so t o the h e a v i e r congeners o f group 14.

0097-6156/88/0360-0089$06.00/0 © 1988 American Chemical Society

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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P o l y ( d i m e t h y l s i l y l e n e ) was f i r s t r e p o r t e d by Burkhard in 1949 (2^, b u t the l a c k o f s o l u b i l i t y and g e n e r a l i n t r a c t a b i l i t y o f t h i s material discouraged further studies at that time. The d e m o n s t r a t i o n by Yajima and H a y a s h i t h a t p o l y ( d i m e t h y l s i l y l e n e ) can be p y r o l y z e d to s i l i c o n c a r b i d e , and the subsequent development o f s i l i c o n carbide fibres v i a this route, lead to a dramatic r i s e in i n t e r e s t in p o l y ( o r g a n o s i l y l e n e s ) and t h e i r chemistry (3^). More recently t h e y have a t t r a c t e d i n c r e a s i n g a t t e n t i o n s i n c e potential uses in the a r e a s o f m i c r o l i t h o g r a p h y (4) and r e p r o g r a p h y (5^) have been identified. Certain p o l y ( o r g a n o s i l y l e n e s ) have also been shown to have unusual thermochromic behavior (6,7 ) and t e m p e r a t u r e - d e p e n d e n t t r a n s i t i o n s in c h a i n c o n f o r m a t i o n ( £ ) . These developments have been made p o s s i b l e by the a p p l i c a t i o n o f m o d i f i e d Wurtz-Fittig-type coupling to the production of linear p o l y s i l y l e n e s , as shown in E q u a t i o n 1 · R» η RR'SiCl

2

+ 2

(M is a group 1 m e t a l o r

alloy)

The contributions o f West and c o - w o r k e r s in t h i s area have been noteworthy (9_). The W i s c o n s i n group showed that the solubility of p o l y ( d i m e t h y l s i l y l e n e ) can be g r e a t l y enhanced by inclusion of a second s u b s t i t u e n t t h r o u g h c o p o l y m e r i z a t i o n and t h a t E q u a t i o n 1 has c o n s i d e r a b l e g e n e r a l i t y (J_0 ) . I t is now clear that p o l y ( o r g a n o s i l y l e n e s ) with very h i g h molecular weights (ca. 10 ) can be s y n t h e s i s e d by t h i s type o f r e a c t i o n . The p o l y m e r s thus o b t a i n e d have s u f f i c e d t o a l l o w the development o f some new technologies and to p o i n t the way t o o t h e r s . A number o f a s p e c t s of the W u r t z - F i t t i g - t y p e c o u p l i n g d e t r a c t from i t s attractiveness as a c o m m e r c i a l l y v i a b l e r o u t e to p o l y s i l y l e n e s . Among the most serious d i f f i c u l t i e s a r e the p o o r c o n t r o l o f m o l e c u l a r w e i g h t and polydispersity, p r o d u c t i o n o f low m o l e c u l a r weight eyelies, the hazards associated with handling hot, molten a l k a l i metals, the limited tolerance of functional groups on the silicon to the r e a c t i o n c o n d i t i o n s and r e l a t i v e l y h i g h c o s t . We have r e c e n t l y r e p o r t e d an a l t e r n a t i v e r o u t e t o polysilyl­ enes, involving the c a t a l y t i c e l i m i n a t i o n o f H between two S i - H moieties (11,12,13). The r e a c t i o n is homogeneously c a t a l y s e d by t i t a n o c e n e and z i r c o n o c e n e d e r i v a t i v e s and in p r i n c i p l e s h o u l d be easier to u n d e r s t a n d and c o n t r o l than the heterogeneous WurtzFittig reaction. I t is thus c l e a r t h a t t h e s e polymers p r o v i d e at least an i m p o r t a n t complement to those made by the Wurtz-Fittigt y p e c o u p l i n g and t h a t improvements in the performance o f c a t a l y s t s could lead to a v i a b l e commercial route for the large scale production of polysilanes. In t h i s paper a d e s c r i p t i o n o f the progress we have made t o date in u n d e r s t a n d i n g the reaction mechanism and c h a r a c t e r i z i n g the polymers w i l l be d e s c r i b e d . The scope o f the c a t a l y s i s and some o f i t s present l i m i t a t i o n s will also be d i s c u s s e d . Finally, some o f the major q u e s t i o n s that remain to be answered b e f o r e t h i s c h e m i s t r y c a n be successfully applied to the general synthesis of polysilylenes will be addressed. 6

2

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Dehydrogenative Coupling The f o r m a t i o n o f an X - X bond by t h e e x t r u s i o n o f H from two X - H containing m o l e c u l e s is a r e a c t i o n of considerable generality. Until recently, such reactions have n o t been systematically exploited f o r the formation o f polymers, although c l o s e l y r e l a t e d r e a c t i o n s in which the thermodynamics o f the p r o c e s s a r e enhanced by s c a v e n g i n g t h e hydrogen w i t h oxygen ( o x i d a t i v e coupling) have been e x p l o i t e d to c o m m e r c i a l advantage (14). Some o f the reasons for the l a c k o f p r o g r e s s in t h i s a r e a a r e t h e r e l a t i v e l y low r e a c t i v i t y o f many o f the more i n t e r e s t i n g t y p e s o f monomer, a l a c k o f thermodynamic d a t a which a l l o w p r e d i c t i o n o f whether a reaction is f e a s i b l e o r n o t , and a l a c k o f s p e c i f i c i t y in c a s e s when there are s e v e r a l X - H bonds in the same m o l e c u l e . In the c o u r s e o f s t u d y i n g the r e a c t i o n s o f S i - Η compounds w i t h dialkyltitanocenes, with a view t o the s y n t h e s i s of new h y d r i d o s i l y l t i t a n o c e n e complexes we a d v e n t i t i o u s l y d i s c o v e r e d t h a t phenylsilane undergoe coupling to a l i n e a influence of dimethyltitanocene. The ease w i t h which t h i s r e a c t i o n proceeds i n i t i a l l y i n d u c e d us t o u n d e r e s t i m a t e the s i g n i f i c a n c e o f the o b s e r v a t i o n . Further s t u d i e s q u i c k l y r e v e a l e d t h a t the r a p i d dehydrogen­ a t i v e c o u p l i n g o f p r i m a r y o r g a n o s i l a n e s t o o l i g o m e r s and t h e s l o w e r coupling o f secondary s i l a n e s t o d i m e r s c a n be e f f e c t e d under ambient c o n d i t i o n s w i t h compounds o f t h e type C p M R (M = T i , R = alkyl; M = Z r , R = a l k y l or H)(11,12,13). None o f the other m e t a l l o c e n e s , m e t a l l o c e n e a l k y l s , o r m e t a l l o c e n e h y d r i d e s o f groups 4, 5 o r 6 have shown any measurable a c t i v i t y f o r p o l y m e r i z a t i o n under ambient c o n d i t i o n s , a l t h o u g h vanadocene c a t a l y z e s the slow, s t e p w i s e o l i g o m e r i z a t i o n o f p h e n y l s i l a n e in r e f l u x i n g t o l u e n e ( 1 5 ) · Excessive m e t h y l a t i o n o f the Cp l i g a n d s d e a c t i v a t e s t h e c a t a l y s t s . For example, t h e mixed c y c l o p e n t a d i e n y l - p e n t a m e t h y l c y c l o p e n t a d i e n y l (CpCp ) complexes o f T i and Z r a r e a c t i v e , b u t t h e b i s Cp complexes are not (16,17). However, t h e C p M R complexes, where M = Th o r U a r e c a t a l y t i c a l l y active; the former for the dimerization of primary silanes and the l a t t e r for their o l i g o m e r i z a t i o n (V7 ) . A p r o b l e m w i t h t h e s e compounds is t h a t they are t o o r e a c t i v e and have a tendency t o r e a c t w i t h the C - H bonds o f substituents on the s i l i c o n (JM3). Bis (indenyl) dimethyl titanium does n o t c a t a l y z e the p o l y m e r i z a t i o n r e a c t i o n , b u t does e f f e c t a slow s t e p w i s e o l i g o m e r i z a t i o n o f p h e n y l s i l a n e t o dimer and t r i m e r a t room t e m p e r a t u r e . The b i s ( i n d e n y l ) z i r c o n i u m a n a l o g is a c t i v e as a polymerization catalyst, g i v i n g e s s e n t i a l l y the same p r o d u c t as the b i s ( c y c l o - p e n t a d i e n y l ) complex. 2

2

2

2

2

Replacement o f one o f the Cp groups in the t i t a n o c e n e zirconocene-based catalysts by an alkyl group d e s t r o y s catalytic activity. It is thus c l e a r t h a t the c o n s t r a i n t s c a t a l y t i c a c t i v i t y are extremely severe. A r e m a r k a b l e f e a t u r e o f the p o l y m e r i z a t i o n r e a c t i o n s is absence o f any e v i d e n c e in nmr s p e c t r a o f r e a c t i o n m i x t u r e s intermediate low m o l e c u l a r weight o l i g o m e r s . This behavior quite distinct from those systems mentioned above, which dimers and t r i m e r s and i t is b e l i e v e d t h a t t h e s e represent mechanistically d i s t i n c t processes.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

or the on the for is give two

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Normally, no s m a l l c y c l o p o l y s i l a n e s a r e o b s e r v e d in these reactions* Two e x c e p t i o n s we have noted a r e the v e r y slow r e a c t i o n of p h e n y l s i l a n e under the i n f l u e n c e o f C p T i M e and the reaction of b e n z y l s i l a n e under the i n f l u e n c e o f d i m e t h y l t i t a n o c e n e a t very long reaction times. From b o t h o f t h e s e r e a c t i o n s we i s o l a t e a s i n g l e isomer o f the c y c l o h e x a s i l a n e , in c a . 10 p e r c e n t y i e l d in the c a s e o f the p h e n y l s i l a n e and c a . 60 p e r c e n t y i e l d in the case of the benzylsilane. These i s o m e r s a r e b e l i e v e d t o be the alltrans isomers. The p h e n y l d e r i v a t i v e is identical to that p r e v i o u s l y r e p o r t e d by Hengge (J_9), the b e n z y l d e r i v a t i v e is a new compound. In the case of the benzylsilane reaction, the cyclohexasilane is p r o d u c e d from p o l y m e r , following essentially complete c o n v e r s i o n o f the monomer t o l i n e a r p o l y s i l a n e o f a b o u t 10 silicon u n i t s average l e n g t h . T h i s o b s e r v a t i o n is p r o b a b l y v e r y i m p o r t a n t in u n d e r s t a n d i n g the f a c t o r s t h a t l i m i t c h a i n l e n g t h . 2

2

The Polymers The p o l y m e r i z a t i o n o f p r i m a r E q u a t i o n 2. The r a t e o f p o l y m e r i z a t i o n is s t r o n g l y dependent on the

η RSiH

3

R Ψ H-f-Si-^H

+

(n-1 ) H

(2)

2

Η

steric demands o f R. The r e l a t i v e r a t e s f o r a number o f d i f f e r e n t silane reactions, c a t a l y z e d by d i m e t h y l t i t a n o c e n e and measured under c o - h y d r o g e n a t i o n c o n d i t i o n s w i t h c y c l o h e x e n e as described below, is (20); P h e n y l (13) > p - T o l y l (10) B e n z y l (1) > n - h e x y l (1)

> Phenylmethyl (4.6) > Cyclohexyl (0.5).

> PhSiD

3

(3.6)

>

C y c l o h e x y l - and p h e n y l m e t h y l s i l a n e s do n o t p o l y m e r i z e , but give d i m e r . W i t h t i t a n i u m - b a s e d c a t a l y s t s the v a l u e o f η is about 10 and does n o t v a r y v e r y much w i t h R o r e x p e r i m e n t a l conditions; with z i r c o n i u m based catalysts, η c a n be as h i g h as 20. P o l y ( p h e n y l - and p - t o l y s i l y l e n e s ) p r o d u c e d w i t h t h e s e c a t a l y s t s a r e brittle g l a s s e s and p o l y ( b e n z y l - and n - h e x y l s i l y l e n e s ) a r e v i s c o u s oils. A l l o f the polymers a r e a t a c t i c and h i g h l y s o l u b l e in most organic solvents. They have been shown by a variety of spectroscopic methods t o be l i n e a r and S i H R t e r m i n a t e d (21_). The SiH R t e r m i n i can be d e t e c t e d by the use o f S i - n m r u s i n g a DEPT pulse sequence. They a r e a l s o e v i d e n t from the p r e s e n c e of a strong S i H i n f r a r e d b e n d i n g mode a t about 910 cm · The l i n e a r nature of the polymers and t h e i r d e g r e e s o f p o l y m e r i z a t i o n have been d e t e r m i n e d u s i n g mass s p e c t r o s c o p y and p a r t i c u l a r l y by comparing the mass spectrum o f a linear poly(phenylsilylene), produced by d e h y d r o g e n a t i v e c o u p l i n g w i t h t h a t r e p o r t e d by Hengge et a l . (J_9) f o r h e x a p h e n y l - c y c l o h e x a s i l a n e . The d a t a a r e shown in T a b l e I and a t t e n t i o n is drawn t o the f a c t t h a t the c y c l i c p o l y m e r , which does not s e p a r a t e i n t o fragments w i t h a s i n g l e Si-Si bond cleavage, gives heavy i o n s in h i g h abundance w h i l e the linear polymer, which is fragmented by a s i n g l e S i - S i bond cleavage, gives heavy fragments in only very low abundance. The 2

2 9

2

2

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hexabenzylcyclohexasilane d e s c r i b e d above behaves in v e r y similar fashion. The v e r y s h a r p , c o n v e n t i o n a l nmr s p e c t r a o f t h e s e c y c l i c compounds a l s o c o n f i r m the absence o f s m a l l r i n g compounds in the normal polymer p r o d u c t s , where they would be e a s i l y d e t e c t a b l e . The degrees o f p o l y m e r i z a t i o n d e t e r m i n e d by mass s p e c t r o s c o p y are c o r r o b o r a t e d by vapor p r e s s u r e osmometry and g e l permeation chromatography s t u d i e s (21 ) . The t e r t i a r y hydrogens of the polymer backbone show no detectable f u r t h e r r e a c t i v i t y under p o l y m e r i z a t i o n c o n d i t i o n s but t h e y can be h y d r o s i l a t e d u s i n g the c l a s s i c a l S p e i e r - t y p e (22,23), or our new z i r c o n i u m - b a s e d c a t a l y s t s (see below) ( V7), to give polymers with fully, or p a r t i a l l y substituted backbones. The presence of S i - Η at every silicon atom and the attendant opportunity for further f u n c t i o n a l i z a t i o n is one of the most interesting features of t h e s e new p o l y m e r s . The p o t e n t i a l for u t i l i z a t i o n o f S i - Η groups on p o l y ( s i l y l e n e s ) f o r c r o s s - l i n k i n g and other modes o f f u n c t i o n a l i z a t i o n has a l r e a d y been r e c o g n i z e d by West (8). Synthesis o Fittig-type c o u p l i n g ha dialkyldichlorosilanes with methyldichlorosilane. G r e a t c a r e must be e x e r c i s e d t o m a i n t a i n n e u t r a l c o n d i t i o n s d u r i n g the work-up in o r d e r t o a v o i d r e a c t i o n o f the S i - Η f u n c t i o n s ( £ ) . Cohydrogenation of

Olefins

Thermodynamics o n l y s l i g h t l y favor E q u a t i o n 1 and the copious evolution o f H can have u n d e s i r a b l e e f f e c t s on the c o u r s e o f the reaction. The i n c l u s i o n of an i n t e r n a l o l e f i n in the reaction mixture completely suppresses the evolution of hydrogen and increases the rate of polymerization. With titanocene-based catalysts the olefin undergoes h y d r o g é n a t i o n but the polymerization, except f o r a rate i n c r e a s e , p r o c e e d s in the same way as in the absence o f o l e f i n (20_). C o h y d r o g e n a t i o n is u s e f u l f o r the s y n t h e s i s o f polymers from gaseous s i l a n e s , in p a r t i c u l a r S i H and MeSiH , since it allows the progress of the reaction to be monitored by gas uptake. Another i m p o r t a n t advantage of cohydrogenation is t h a t i t can make E q u a t i o n 1 much more thermodynamically viable because o f the h i g h h e a t o f hydrogénation of olefins. In this respect the reaction resembles olefin h y d r o g e n a t i o n - d r i v e n C - H bond a c t i v a t i o n s s t u d i e d by C r a b t r e e (24)· T i t a n o c e n e c a t a l y s t s do n o t c a t a l y z e the h y d r o s i l a t i o n o f most internal olefins, a l t h o u g h t h e y can a t t a c h a c t i v e o l e f i n s s u c h as styrene, or norbornene to the growing polymer c h a i n ends. The zirconocene-based catalysts, on the o t h e r hand, c a n be powerful hydrosilation catalysts and the r e m a r k a b l e copolymer synthesis shown in E q u a t i o n 3 c a n be e a s i l y a c h i e v e d under m i l d conditions 2

4

3

(17). With cyclohexene, polymerization o c c u r s more r a p i d l y than hydrosilation. After p o l y m e r i z a t i o n has proceeded t o completion, t h e r e is a slow h y d r o s i l a t i o n t o i n t r o d u c e c y c l o h e x y l groups onto the polymer c h a i n , to a maximum e x t e n t o f about 50 p e r c e n t o f t h e S i - Η groups. W i t h more r e a c t i v e o l e f i n s , s u c h as s t y r e n e , hydro­ silation occurs more r a p i d l y than p o l y m e r i z a t i o n and the polymerization r e a c t i o n is s u p p r e s s e d . As in the polymerization reaction, the r e a c t i v i t y o f p r i m a r y s i l a n e s is much g r e a t e r than

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Cp' ZrMe 2

Ph Ph H4Si-Si4t:H H £ )

2

2n PhSiH-

+

(2n-1) H

9

(3)

that of secondary s i l a n e s and the p r o d u c t o f h y d r o s i l a t i o n of styrene by p h e n y l s i l a n e is almost entirely phenyl(1-phenylethyDsilane. This s u g g e s t s t h a t the i n c o r p o r a t i o n o f cyclohexyl groups i n t o the polymer does n o t o c c u r by s i m p l e h y d r o s i l a t i o n . We currently favor a mechanism which i n v o l v e s cleavage of the preformed polymer c h a i n by the c a t a l y s t t o g i v e an intermediate which is capable of y i e l d i n g a l k y l s i l a n e , b u t does not require direct b r e a k i n g o f the t e r t i a r y S i - Η bond o f the polymer c h a i n . C o u p l i n g o f Germanes D i m e t h y l t i t a n o c e n e is e x t r e m e l (15). Even s e c o n d a r y germane but t h e r e seems to be a s e v e r e c o n s t r a i n t on the c h a i n length, as in the case of primary s i l a n e p o l y m e r i z a t i o n . Polymerization of diphenylgermane can be c a r r i e d out under two d i f f e r e n t regimes using dimethyltitanocene as catalyst. Addition of freshly recrystallized d i m e t h y l t i t a n o c e n e to an e x c e s s o f diphenylgermane r e s u l t s in s t e a d y e v o l u t i o n o f hydrogen a t c a . 60°C., b u t the c o l o u r of the solution remains yellow until a l l of the germane is consumed. During this period, the formation of tetraphenyldigermane and s m a l l amounts o f h i g h e r o l i g o m e r s can be o b s e r v e d by nmr s p e c t r o s c o p y . When almost a l l o f the monomer is consumed t h e r e is a d r a m a t i c change in c o l o u r t o d a r k p u r p l e accompanied by a surge of gas. The r e s u l t i n g p u r p l e s o l u t i o n is much more active for the f u r t h e r dehydrogenative c o u p l i n g of diphenylgermane and produces p r i m a r i l y octaphenyltetragermane. The d a r k p u r p l e p r o d u c t can be p r o d u c e d d i r e c t l y by r e a c t i o n o f dimethyltitanocene and diphenylgermane in a molar r a t i o o f about 1:2. The s t r u c t u r e of this d a r k c o l o u r e d compound w i l l be d i s c u s s e d f u r t h e r below. P r i m a r y germanes undergo c o u p l i n g to i n s o l u b l e g e l s under the influence of dimethyltitanocene, presumably because the backbone hydrogens show s u f f i c i e n t activity to lead to crosslinking. Perhaps the most i n t e r e s t i n g a s p e c t o f t h i s r e a c t i o n is t h a t it points t o the p o s s i b i l i t y t h a t s i m i l a r r e a c t i o n s o f s i l a n e s may be a c h i e v a b l e w i t h more a c t i v e c a t a l y s t s . G e l s produced from c o u p l i n g of primary s i l a n e s c o u l d have i n t e r e s t i n g electronic properties s i n c e t h e y a r e the homologs o f s i l i c o n monohydride, a m a t e r i a l o f c o n s i d e r a b l e c u r r e n t i n t e r e s t t o the e l e c t r o n i c s i n d u s t r y (25^). In the p r e s e n c e o f vanadocene, phenylgermane undergoes f a c i l e s t e p w i s e c o n v e r s i o n to o l i g o m e r s a t about 6 0 ° C . Results obtained w i t h germanes a l s o p r o v i d e models for the kinds of r e a c t i o n s t h a t may be o c c u r r i n g in the silane polymer­ ization reaction as well. F o r example, we have succeeded in c a r r y i n g out the r e a c t i o n shown in E q u a t i o n 4 (26 ) . The analogous reactions with t r i p h e n y l s i l a n e , or t r i p h e n y l s t a n n a n e , were not Cp Ti(CH ) 2

3

2

+ Ph GeH 3

Cp Ti(CH )(GePh ) 2

3

3

+ CH

4

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

(4)

7.

HARROD

95

Polymerization of Group 14 Hydrides

successfully demonstrated, b u t the f a c t t h a t i t o c c u r s w i t h the germane adds credence t o the h y p o t h e s i s t h a t an analogous r e a c t i o n is the first step in the reaction of silanes with dime t h y 1 t i t a n o c e n e · The Mechanism o f P o l y m e r i z a t i o n The compounds J,, 2 and £ have a l l been i s o l a t e d from r e a c t i o n s of phenylsilane with either dimethyltitanocene (12) or dimethylzirconocene (J_3). A l l o f the e v i d e n c e p o i n t s t o the fact that these compounds a r e p r o b a b l y r e s t i n g s p e c i e s and a r e not involved in the c a t a l y t i c c y c l e . They do n e v e r t h e l e s s g i v e some i n d i c a t i o n of the complex s e r i e s o f r e a c t i o n s t h a t t r a n s f o r m the d i m e t h y l m e t a l l o c e n e to a c t i v e c a t a l y s t . Using the titanocene-catalyzed co-hydrogenation of cyclohexene, we have s t u d i e d the k i n e t i c s o f the p o l y m e r i z a t i o n o f a number o f p r i m a r y s i l a n e s (2CJ) The r a t e law was found to be: Rate

=

k[catalyst]

We a t t r i b u t e the form o f t h i s r a t e law t o be due to the pseudoe q u i l i b r i u m 6. We r e f e r t o 6 as a p s e u d o - e q u i l i b r i u m , because i t is in fact a steady state r a t h e r than a t r u e e q u i l i b r i u m . If R

o

Cp Ti 2

^TiCp

N

2

+

RSlH ^=a 3

2 Cp Ti(H)(SiH R) 2

2

(6)

H 1

4

compound 4 is a p a r t i c i p a n t in the c a t a l y t i c c y c l e , a n y t h i n g which changes the throughput r a t e of the cycle will alter the c o n c e n t r a t i o n o f 4. Compounds 1 c a n be o b s e r v e d in s o l u t i o n by nmr and the R = Ph compound has been s t r u c t u r a l l y c h a r a c t e r i z e d by X ray a n a l y s i s (12). A p s e u d o - e q u i l i b r i u m s u c h as E q u a t i o n 6 leads n a t u r a l l y t o the r a t e law o f E q u a t i o n 5 i f i t is assumed t h a t the e q u i l i b r i u m l i e s to the l e f t and the r a t e o f r e a c t i o n is c o n t r o l l e d by the unimolecular transformation of the titanium(IV)silylhydride, 4. The most p l a u s i b l e first step for the decomposition of 4 is an α - h y d r i d e e l i m i n a t i o n from the SiH R group, f o l l o w e d , o r accompanied, by l o s s o f H from the complex t o give a C p T i = S i H R complex. I t is then assumed that propagation o c c u r s by some k i n d o f r e p e t i t i v e i n s e r t i o n o f the s i l y l e n e i n t o a Ti-Si bond. A p o s s i b l e mechanism o f t h i s k i n d is shown in Scheme 1· Two e q u a l l y p l a u s i b l e r o u t e s f o r the p r o p a g a t i o n s t e p a r e shown on the lower c e n t e r and lower r i g h t o f Scheme 1· This type of mechanism is a t t r a c t i v e s i n c e i t e x p l a i n s why s e c o n d a r y silanes w i l l only dimerize. A number o f f e a t u r e s o f the s i l a n e and germane p o l y m e r i z a t i o n reactions show unequivocally that there are a t least two indépendant mechanisms o p e r a t i n g . T h i s dichotomy is most e v i d e n t in s i t u a t i o n s where the p r o d u c t s o f r e a c t i o n c a n be compared f o r the i n d u c t i o n p e r i o d t h a t p r e c e d e s the f o r m a t i o n o f compound 1, and the p r o d u c t s t h a t are p r o d u c e d a f t e r the f o r m a t i o n o f 1 ( 1 2 , 1 5 , 2 0 ) . 2

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

96

INORGANIC AND ORGANOMETALLIC POLYMERS

Table I .

Mass Spectra of a Linear Oligo(phenylsilylene) and of Hexaphenylcyclohexasilane

Oligophenylsilylene Ion

Mass

Hexaphenylcylcohexasilane

(abundance)

Ion

Mass

(abundance)

636

(30)

530

(20)

Ph Si H

7

532

(0.45)

P h

Ph Si H

3

499

(0.35)

Ph Si H

Ph Si H

4

452

(1.13)

Ph.Si.H,

452

(50)

376

(16)

5

5

5

4

4

5

S i

6

H

6 6

5

4

5

5

5 4

Ph Si H

2

422

Ph Si H

3

346

(7.18)

Ph Si H

3

316

(12.4)

Ph Si,H

344

(75)

2

287

(27.0)

Ph Si

315

(20)

259

(24.0)

Ph Si

259

(60)

240

(25.0)

Ph Si 2

3

238

(12)

Ph Si H

211

(42)

Ph si

2

210

(16)

Ph SiH

183

(47)

Ph SiH

183

(80)

PhSi

105

(100)

PhSi

105

(100)

4

4

3

4

Ph Si H 3

Ph si 3

Ph Si 3

Ph Si H 2

2

2

1.

3

2

2

3

Q

3 3

5

4

3

3

2

2

5

From réf. 1 .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

7. HARROD

97

Polymerization of Group H Hydrides

Cp TiMe 2

+

2

RSiH

3

.Me Cp Ti 2

+

CH.

+

MeRSiH

SiH R 2

CP Ti 2

RSiH

0

0

Cp Ti^ 2

SiH R 2

Cp,Ti=Si RSiH

RSiH,

Cp Ti 2

0

RS1H ^SiH R Cp Ti SiH R

0

2

Cp Ti 2

^TiCp

V*

/

2

2

Cp Ti 2

s

SiHRSiH R 2

2

Si-H R

^SiHRSiH R 2

Cp Ti=Si

Cp Ti

λ

2

2

SiH R 2

-RSiH

+ RSiH

n

0

J-HH,

R ii-H

ft \

Cp Ti=SiRSiH R 2

2

Cp

Tl

2 ^SxRSiH R RH Si 2

k

2

C p

2 ^S 1HRS iHRS i H R T

£

-HJ Cp Ti=Si 2

SiHRSiH R 2

Scheme 1.

Mechanism of t i t a n o c e n e c a t a l y z e d s i l a n e p o l y m e r i z a t i o n .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

In the two regimes the p r o d u c t s a r e q u i t e d i f f e r e n t . For example, in the titanium catalyzed oligomerization of diphenylgermane described above, one c a t a l y t i c regime seems t o i n v o l v e no gross reduction of the titanium(IV) and g i v e s rise mainly to tetraphenyldigermane, the other i n v o l v e s the reduction of the titanium to give the d i p h e n y l g e r m y l analogue o f 1,, which is a p o w e r f u l c a t a l y s t f o r the t e t r a m e r i z a t i o n o f d i p h e n y l g e r m a n e . The c h a r a c t e r i z a t i o n o f the p u r p l e t i t a n i u m i n t e r m e d i a t e as an analogue o f 1 is based on the o b s e r v a t i o n o f an e s r spectrum o f the triplet state and on the s i m i l a r i t y o f i t s H-nmr spectrum to t h a t of 1 (11). A similar dichotomy was o b s e r v e d in the titanium catalyzed polymerization o f p r i m a r y s i l a n e s c o u p l e d t o the h y d r o g é n a t i o n of norbornene (2(0). A t low c a t a l y s t concentration ( c a . 0.004M), essentially complete conversion o f norbornene to an e q u i m o l a r mixture of norbornane and b i s - p h e n y l s i l y l - ( a n d / o r 1,2-diphenyldisilyl)norbornane was observed. Under t h e s e conditions no evidence for reductio catalyst concentration dimethyltitanocene to J, and 2 o c c u r s and the catalytic reaction produces mainly p o l y s i l a n e (DP c a . 10) and norbornane in c a . 80 per cent y i e l d s , and s i l y l a t e d norbornanes in about 20 p e r cent yield. Our present preferred hypothesis is that the reactions o c c u r r i n g in the regime where g r o s s r e d u c t i o n o f the t i t a n i u m does not occur are metal catalyzed radical chain reactions. Oligomerization under t h e s e c o n d i t i o n s p r o c e e d s by the stepwise coupling of s i l y l or germyl r a d i c a l s . Following reduction of the titanium, we b e l i e v e t h a t p o l y m e r i z a t i o n o c c u r s by some s o r t of rapid a d d i t i o n mechanism in which the intermediates are not observable because they are s h o r t l i v e d , o r because they are spectroscopically s i l e n t ( e . g . , c a n n o t be seen in the nmr because they are paramagnetic). A l i k e l y c a n d i d a t e f o r the mechanism in this c a s e is the r a p i d r e p e t i t i v e i n s e r t i o n o f s i l y l e n e moieties, produced by α - h y d r i d e e l i m i n a t i o n from 4, i n t o a m e t a l - s i l y l b o n d , as shown in Scheme 1 · At the present time we do n o t know what the termination reaction is. G i v e n the f a c t t h a t the polymers i s o l a t e d from co­ hydrogenation reactions do n o t have d i f f e r e n t molecular weight properties from t h o s e p r o d u c e d by s i m p l e polymerization, hydrogenolysis can be e x c l u d e d as a l i k e l y t e r m i n a t i o n r e a c t i o n . This leaves spontaneous homolysis of a metal-Si bond, followed by hydrogen a b s t r a c t i o n to n e u t r a l i z e the r e s u l t i n g s i l y l r a d i c a l , or reductive elimination of a s i l y l with a hydride, o r o f two silyl ligands. There is n o t h i n g in the p r e s e n t l y a v a i l a b l e information that a l l o w s us t o d i s c r i m i n a t e between t h e s e a l t e r n a t i v e s and t h e y must be c o n s i d e r e d as e q u a l l y p l a u s i b l e . I t is c l e a r that any r e a c t i o n t h a t l e a d s t o polymer c h a i n t e r m i n a t i o n w i t h - S i H R groups will stop f u r t h e r c h a i n growth since neither titanium, nor z i r c o n i u m based c a t a l y s t s a r e a c t i v e f o r s e c o n d a r y s i l a n e s . The f o r m a t i o n o f c y c l o h e x a s i l a n e s and the z i r c o n i u m catalyzed hydrosilation of poly(phenylsilylene), r e f e r r e d to above, both suggest that s l o w c l e a v a g e o f polymer c h a i n s may o c c u r in the presence of the catalysts. Such c l e a v a g e may a l s o play an n

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

7. HARROD

Polymerization of Group 14 Hydrides

99

important r o l e in l i m i t i n g c h a i n l e n g t h s and i t w i l l be the focus of f u r t h e r study. L i k e many homogeneously c a t a l y z e d r e a c t i o n s , the o v e r a l l c y c l e ( o r c y c l e s ) in t h e s e p o l y m e r i z a t i o n r e a c t i o n s p r o b a b l y c o n t a i n s too many s t e p s to be e a s i l y a n a l y z e d by any s i n g l e approach. Both kinetics and model compound s t u d i e s have thrown l i g h t on some o f the steps. However, as indicated above, many o f the model compounds isolated from the r e a c t i o n s of primary silanes with metallocene a l k y l s and h y d r i d e s a r e too u n r e a c t i v e t o e x p l a i n the polymerization results. Conclusions The catalyzed dehydrogenative coupling of s i l i c o n and germanium hydrides has been achieved with high r e a c t i o n rates and high conversions. T h i s r e p r e s e n t s a major new r o u t e to the s y n t h e s i s o f S i - S i and o f Ge-Ge b o n d s The low d e g r e e s o f p o l y m e r i z a t i o n o f the products of primary organosilan limitation on t h e i r us is a pre-requisite. F u r t h e r s t u d i e s on the mechanism(s) of the reaction are being pursued, p a r t i c u l a r l y w i t h a view to under­ standing the n a t u r e o f the c h a i n t e r m i n a t i o n p r o c e s s ( e s ) . It is possible that such knowledge w i l l a l l o w some c o n t r o l over the factors l i m i t i n g chain length. The selective reactions of t e r m i n a l S i - Η groups with appropriate c o u p l e r s is a p r o m i s i n g method f o r the conversion of our oligomers to h i g h m o l e c u l a r weight m a t e r i a l s and we are presently studying such reactions. I t is l i k e l y t h a t runs of twenty silicon atoms already exhibit many o f the desirable features manifest by h i g h e r molecular weight m a t e r i a l s and the coupling together o f t h e s e c h a i n s can g i v e them the dimensional stability and mechanical properties necessary for certain applications· The p r i n c i p l e of forming novel polymeric materials by dehydrogenative coupling is of considerable generality. The catalytic reactions o f S i - Η and OH a r e w e l l know (27)· S i m i l a r reactions with the heavier chalcogens might lead to some interesting new m a t e r i a l s . Of even g r e a t e r interest is the c a t a l y t i c f o r m a t i o n o f h i g h m o l e c u l a r w e i g h t p o l y ( s i l a z a n e s ) by the elimination of H between silanes and p r i m a r y a m i n e s . This r e a c t i o n has a l r e a d y been s u c c e s s f u l l y c a r r i e d o u t w i t h the a i d of t r a n s i t i o n m e t a l complex c a t a l y s t s ( 2 8 ) . 2

Acknowledgments The N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l of Canada, the Fonds FCAR du Q u é b e c , the Dow C o r n i n g C o r p o r a t i o n , and Esso Canada a r e a l l thanked f o r t h e i r f i n a n c i a l s u p p o r t o f this work. My c o l l a b o r a t o r s , without whose e x p e r i m e n t a l s k i l l s none o f the work d e s c r i b e d above would have been done, a r e thanked and their c o n t r i b u t i o n s a r e r e c o g n i z e d by c i t a t i o n in the r e f e r e n c e s .

Literature Cited 1.

Fink, J. M.; Michalczyk, M. J.; Haller, K.; West, R. J. Chem. Soc., Chem. Communs. 1983, 1010.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

100 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

Burkhard, C. A. J. Am. Chem. Soc. 1949, 71, 963. Yajima, S; Shishodo, T.; Kayano, H. Nature 1976, 264, 237. Miller, R. D.; Hofer, D.; McKean, D. R.; Wilson, C. G.; West, R.; Trefonas, P. T. III ACS Symposium Series No. 266; American Chemical Society, Washington, DC, 1984, pp. 293-310. Stolka, M.; Yuh, H.-J.; McGrane, K.; Pai, D. M. J. Polymer Sci., Part A, Polymer Chemistry 1986, 25, 823. Trefonas, P.; Damewood, J. R.; West, R. Organometallics 1985, 4, 1318. Harrah, L. Α.; Ziegler, J. M. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 209. Gobbi, G. C.; Fleming, W. W.; Sooriyakamuran, R.; Miller, R. D. J. Am. Chem. Soc. 1986, 108, 5624. For an excellent review of the current status of organopolysilane chemistry, see West, R., J. Organometal. Chem. 1986, 300, 327. Helmer, B; West, R. J. Organometal. Chem. 1982, 236, 21. Trefonas, P.; Djurovich, Miller, R. D.; Hofer, 1983, 21, 819. Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Organomet. Chem. 1985, 279, C11. Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Am. Chem. Soc. 1986, 108, 4059. Aitken, C. T.; Harrod, J. F.; Samuel, E, Can. J. Chem. 1986, 64, 1677. Finkbeiner, H. L.; Hay, A. S.; White, D. M. Polymerization by oxidative coupling, in Polymerization Processes, Wiley-Interscience, New York, 1977, p. 537. Malek, A.; Harrod, J. F., unpublished results. Aitken, C.; Harrod, J. F., unpublished results. Barry, J.-P. ; Harrod, J. F., unpublished results. The reactivity of organoactinides with C-H bonds has been extensively studied by Marks et al.; see e.g., Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A. J.; Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40. Hennge, E.; Lunzer, F. Monatheft. Chem. 1976, 107, 371. Harrod, J. F. ; Yun, S. S. Organometallics, in press. Aitken, C. T.; Harrod, J. F.; Gill, U. Can. J. Chem., in press. Speier, J. L. Adv. Organomet. Chem. 1979, 17, 407. Harrod, J. F.; Chalk, A. J. In Organic Synthesis via Metal Carbonyls, Wender, I., Pino, P. Eds.; Wiley: New York, 1977; Vol. 2, p. 673. Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1982, 104, 107. Stein, H. J.; Peercy, P. S. Appl. Phys. Lett. 1979, 34(9), 604. DeMeo, Ε. Α.; Taylor, R. W. Science 1984, 224, 245. Harrod, J. F.; Malek, Α.; Rochon, F.; Melanson, R. Organo­ metallics, in press. See e.g., Sommer, L. H.; Lyons, J. E. J. Am. Chem. Soc. 1967, 89, 1521. Blum, Y; Laine, R. M. Organometallics 1986, 5, 2081.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 8

Polysilylene Preparations D.J.Worsfold Division of Chemistry, National Research Council of Canada, Ottawa, Ontario K1A 0R9, Canada The reductive couplin refluxing toluene has been studied with a view to determining the mechanism of the reaction. The reaction behaves as a chain reaction, rather than a simple polycondensation, in that high molecular weight material is formed near the start of the reaction. Also the formation of high molecular weight material is not dependant on equivalent proportions of reactants. The complex mixture of products of widely separated molecular weight can be interpreted by the simultaneous operation of two routes to polymer. One of these seems to be promoted by the presence of organometallic species, and could be v i a an anionic step chain growth. The reaction kinetics suggest that the growing chains have a comparatively long l i f e t i m e . A slow build up of rate suggest an initial formation of the growing chain ends at a rate dependant on the sodium surface area, but the subsequent chain propagation reaction is independant of surface area. Copolymerization experiments show a large difference in r e a c t i v i t y of aryl and a l k y l substituted dichloromethylsilanes.

The d i s c o v e r y t h a t s o l u b l e h i g h m o l e c u l a r weight p o l y s i l a n e s may be prepared by t h e r e d u c t i v e coupling o f d i c h l o r o d i a l k y l s i l a n e s by a l k a l i metals (1,2) has l e d t o c o n s i d e r a b l e work on t h e p r o p e r t i e s o f t h i s i n t e r e s t i n g c l a s s o f polymers ( 3 , 4 , 5 ) . The p r e p a r a t i o n o f t h e polymers l e a v e s much t o be d e s i r e d a s f r e q u e n t l y t h e h i g h polymer is o n l y a minor p r o d u c t . Mechanistic s t u d i e s of t h e r e a c t i o n w i t h a v i e w t o i m p r o v i n g t h e r e l e v a n t y i e l d s have been few (6). The major ones by Z e i g l e r (7,8,9) showed t h a t a s i l y l e n e d i r a d i c a l was n o t i n v o l v e d in t h e r e a c t i o n , and s t r e s s e d t h e importance o f polvmer solvent interactions. T h e r e appear t o be t h r e e types o f p r o d u c t , a low m o l e c u l a r w e i g h t product a p p e a r i n g t o be s m a l l r i n g compounds ( P I ) , an i n t e r m e d i a t e p r o d u c t ( P I I ) , and t h e u s u a l l y d e s i r e d h i g h m o l e c u l a r weight polymer ( P H I ) . Previous s t u d i e s have c o n c e n t r a t e d o n l y on P H I ,

0097-6156/88/0360-0101 $06.00/0 Published 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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a l t h o u g h P I I c o u l d be u s e f u l f o r some purposes. However, t o u n d e r s t a n d the r e a c t i o n f u l l y i t is n e c e s s a r y to c o n s i d e r a l l t h r e e p r o d u c t s , and a l s o the p o s s i b i l i t y t h a t more than one mechanism is operating. Three such p r o d u c t s are formed in the a n i o n i c p o l y m e r i z a t i o n of c y c l i c s i l o x a n e s t o p o l y s i l o x a n e s by a rearrangement mechan i s m , but i n t e r c h a n g e s between the t h r e e p r o d u c t s d i d not appear as p o s s i b l e w i t h the p o l y s i l a n e s . Experimental The p r e p a r a t i o n s were c a r r i e d out as u s u a l in a t h r e e necked f l a s k f i t t e d w i t h a s t a i n l e s s s t e e l s t i r r e r , condenser, and n i t r o g e n i n l e t (10,11). The s o l v e n t s and d i c h l o r i d e s were added by s y r i n g e . The sodium was added as a f r e s h l y c u t b l o c k and, p r i o r t o the r e a c t i o n , m e l t e d in the r e f l u x i n g s o l v e n t and s t i r r e d t o form sand. The o v e r a l l s u r f a c e a r e a of the sodium c o u l d be c o n t r o l l e d by the s t i r r e r speed. When n e c e s s a r y the s t i r r e r speed was measured by a t a c h o meter. Toluene, the majo s e v e r a l p o r t i o n s of s u l p h u r i c a c i d , washed, d r i e d , and s t o r e d over c a l c i u m h y d r i d e on a vacuum r a c k . The t o l u e n e was d i s t i l l e d out immediately p r i o r t o the r e a c t i o n . The d i c h l o r i d e s were vacuum d i s t i l l e d a t the time of the r e a c t i o n i n t o t h r e e f r a c t i o n s , and the m i d d l e f r a c t i o n , about 60% of the t o t a l , u s e d . The d i c h l o r i d e s were o b t a i n e d from P e t r a c h , s t o r e d in a n i t r o g e n g l o v e bag and handled by syringe. The p r i n c i p a l d i c h l o r i d e s used in t h i s study were h e x y l m e t h y l dichlorosilane (HMDS) and p h e n y l m e t h y l d i c h l o r o s i l a n e (PMDS), some c o p o l y m e r i z a t i o n s of the l a t t e r and d i m e t h y l d i c h l o r o s i l a n e (DMDS) were a l s o made. The u s u a l p r a c t i c e was t o add the d i c h l o r i d e (20 v o l . % ) in one p o r t i o n t o the r e f l u x i n g r e a c t i o n m i x t u r e a t the s t a r t o f the r e a c t i o n . T h i s u s u a l l y e n t a i l e d c o o l i n g the r e a c t i o n m i x t u r e when PMDS was used t o p r e v e n t a t o o v i g o r o u s r e f l u x a t the b e g i n n i n g o f the r e a c t i o n . In a l l cases the r e a c t i o n t u r n e d dark p u r p l e and ultimately viscous. When samples were removed d u r i n g the c o u r s e of the r e a c t i o n , the r e a c t i o n was t e r m i n a t e d by adding t o water, o r t o an a l k y l l i t h i u m , o r in some cases t o t r i m e t h y l c h l o r o s i l a n e . The u n r e a c t e d d i c h l o r i d e s c o u l d be determined by d i r e c t i n j e c t i o n i n t o the gas chromatograph (GC), o r by the r e a c t i o n p r o d u c t w i t h an a l k y l l i t h i u m . The r e a c t i o n mixture a t the end was treated carefully with water, and washed. The r e a c t i o n m i x t u r e was a n a l y s e d a t t h i s p o i n t by g e l p e r m e a t i o n chromatography (GPC). The h i g h polymer c o u l d be isolated by p r e c i p i t a t i o n in i s o p r o p y l a l c o h o l . The intermediate f r a c t i o n P I I was p a r t l y p r e c i p i t a t e d w i t h the P H I o r remained in s o l u t i o n a c c o r d i n g t o the c o n d i t i o n s of the reaction. Further f r a c tions of P I I c o u l d be isolated by p r e c i p i t a t i o n in e t h a n o l or methanol. PI was s o l u b l e in e t h a n o l . The m o l e c u l a r weight d i s t r i b u t i o n s were determined o n l y in terms o f the p o l y s t y r e n e e q u i v a l e n t from the p o l y s t y r e n e c a l i b r a t i o n of the GPC columns. T h i s can o n l y be regarded as s e m i q u a n t i t a t i v e , as the method determines o n l y the r e l a t i v e hydrodynamic volumes. I f the polysilane c h a i n was significantly stiffer than p o l y s t y r e n e , the m o l e c u l a r weights would be e s t i m a t e d too h i g h . I f the c h a i n s are

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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branched t h e m o l e c u l a r weights found would be t o o low. The e r r o r s c a n be upto a f a c t o r of 2, b u t t h i s is s m a l l compared t o t h e range covered. The complete r e a c t i o n m i x t u r e was a n a l y s e d t o g i v e t h e p r o p o r t i o n s o f t h e t h r e e d i f f e r e n t m o l e c u l a r weight p r o d u c t s ( 7 ) . RESULTS Products If samples were taken f o r t h e GPC and GC i t was found t h a t a f t e r a l l t h e d i c h l o r i d e was consumed t h a t t h e p r o d u c t was d i s t r i b u t e d i n t o t h e t h r e e peaks on t h e GPC P I , P I I , P H I ( F i g u r e 1 ) . A l t h o u g h h i g h m o l e c u l a r weight m a t e r i a l was p r e s e n t as soon as t h e d i c h l o r i d e was consumed, c o n t i n u e d r e f l u x w i t h sodium caused changes in t h e mole­ c u l a r weight, p a r t i c u l a r l y o f P I I ( T a b l e I ) . T h i s was v e r y T a b l e I . Chang Continue Time (mins) 60 Mw

χ 10"

3

1.3

in MW P I I

f PMDS

100

200

300

400

2.5

3.9

4.8

5.0

marked in t h e case of PMDS. T h i s meant t h a t when t h e r e a c t i o n m i x t u r e was added t o i s o p r o p y l a l c o h o l t o r e c o v e r t h e polymer, t h e p r o p o r t i o n o f P I I i n c l u d e d was dependent on t h e time t h e r e a c t i o n was l e f t to reflux. Hence a t s h o r t r e f l u x times t h e polymer c o u l d appear monomodal as P H I , w h i l e a t l o n g e r r e f l u x times t h e p r e c i p i t a t e d polymer appeared mostly t o be P I I whereas t h e a c t u a l y i e l d o f P H I was t h e same. Hence t h e marked v a r i a t i o n s t h a t appear in t h e l i t e r a ­ t u r e o f the p r o p o r t i o n s o f t h e two peaks in t h e GPC o f t h e p r e c i p i ­ t a t e d p r o d u c t ( 7 , 8 ) , c o u l d , in p a r t , be an a r t i f a c t o f t h e time o f reflux, and t h e u n f o r t u n a t e c h o i c e of i s o p r o p y l a l c o h o l as t h e precipitant. Isopropyl alcohol has a l i m i t i n g solubility for p o l y s i l a n e s j u s t in t h e range o f m o l e c u l a r weights c o v e r e d by P I I . Low M o l e c u l a r Weight

Product

The lowest m o l e c u l a r weight product c o u l d be i s o l a t e d by i t s s o l u ­ b i l i t y in e t h a n o l o r methanol. I t was found t o have as i t s major p r o d u c t a c y c l i c pentamer. T h i s was c o n f i r m e d in t h e HMDS c a s e from i t s H and C s p e c t r a which were c o n s i s t a n t w i t h i t s m o l e c u l a r f o r m u l a , but c o n t a i n i n g many i s o m e r s . The mass spectrum gave a p a r e n t peak a t 640.8 w i t h an a p p r o p r i a t e mix o f i s o t o p a m e r s . The UV spectrum had a maximum a t 261mu in agreement w i t h t h e assignment o f Watanabe e t a l . (12) f o r c y c l i c pentamers. The PMDS low MW p r o d u c t a l s o gave H and C s p e c t r a in a c c o r d w i t h a c y c l i c p r o d u c t , and t h e m o l e c u l a r weight determined by f r e e z i n g p o i n t d e p r e s s i o n was 560 (600 f o r a pentamer). In many p o l y m e r i z a t i o n s c y c l i c m a t e r i a l is produced by a c o n c u r r e n t b a c k b i t i n g r e a c t i o n as l i n e a r polymer is formed. For example d i o x a n is formed in t h e c a t i o n i c p o l y m e r i z a t i o n o f e t h y l e n e o x i d e t o p o l y o x y e t h y l e n e , and p o l y o x y e t h y l e n e c a n be degraded t o 1

1 3

1

1 3

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dioxan with a c i d c a t a l y s t s . Thus i t is p o s s i b l e t h a t t h e f o r m a t i o n o f P I t h e low MW product in t h e s e p o l y s i l a n e s is an i n t e g r a l p a r t o f t h e p o l y m e r i z a t i o n p r o c e s s , and n o t a s e p a r a t e s i d e r e a c t i o n . F o r example, Na/K a l l o y was r e a c t e d w i t h HMDS in t e t r a h y d r o f u r a n ( T H F ) . The f i r s t product a t s h o r t r e a c t i o n time (1-2 min) was a polymer w i t h a m o l e c u l a r weight up t o 1 0 , which r a p i d l y degraded a t l o n g e r times t o t h e pentamer. A l s o h i g h MW polymer from t h e r e a c t i o n o f HMDS in t o l u e n e w i t h Na, on r e f l u x i n g w i t h Κ in THF degraded t o t h e low M.W. c y c l i c material. 5

C h a i n Nature o f R e a c t i o n A l t h o u g h t h e c o u p l i n g r e a c t i o n o f sodium w i t h a d i c h l o r i d e t o form a polymer would appear t o be a form o f a c o n d e n s a t i o n reaction, the r e a c t i o n appears more l i k e a c h a i n r e a c t i o n . Normally a c o n d e n s a t i o n polymerization will produce f i r s t dimers and t r i m e r s which then c o u p l e in t u r n t o produce a r a p i d i n c r e a s e in m o l e c u l a r weight when a l l t h e monomer has r e a c t e d should form o n l y whe present. In t h i s r e a c t i o n i t is p o s s i b l e t o i s o l a t e h i g h m o l e c u l a r w e i g h t m a t e r i a l from near t h e s t a r t of t h e r e a c t i o n w i t h molecular weight above 1 0 , F i g u r e 2. Moreover c o n s i d e r a b l e h i g h MW polymer is formed when e i t h e r sodium o r t h e d i c h l o r i d e is in e x c e s s , F i g u r e 2. T h i s s u g g e s t s t h a t i f t h e i n i t i a t i o n r e a c t i o n is a r e a c t i o n between sodium and t h e d i c h l o r i d e , t h e subsequent c h a i n growth r e a c ­ t i o n must be v e r y much f a s t e r in o r d e r t h a t l o n g c h a i n s s h o u l d form. T h e r e is some e v i d e n c e t h a t m o l e c u l e s c o n t a i n i n g sequences o f s i l i c o n atoms r e a c t f a s t e r t h a n those w i t h s i n g l e atoms ( 1 3 ) . N e v e r t h e l e s s , t h e c h a i n growth in g e n e r a l cannot be t o o r a p i d because t h e m o l e c u l a r w e i g h t o f t h e h i g h MW m a t e r i a l i n c r e a s e s d u r i n g t h e c o u r s e o f t h e r e a c t i o n , T a b l e I I , a s d i c h l o r i d e is consumed i n d i c a t i n g t h e c h a i n c a r r i e r s have a l i f e t i m e comparable t o t h e r e a c t i o n t i m e . 5

TABLE I I .

PHI

Time (mins) HMDS used % Yield P H I % Μ χ 10" w 5

ττ

Formation with 10 33 14 2.0

20 82 26 3.0

Time f o r HMDS 33 97 37 4.5

180 100 45 4.5

A d d i t i o n of Organometallics Z e i g l e r has shown by t r a p p i n g experiments t h a t t h e r e a c t i o n i n t e r ­ m e d i a t e was n o t a s i l y l e n e d i r a d i c a l formed by t h e d e h a l o g e n a t i o n o f t h e d i c h l o r i d e . T h i s was c o n f i r m e d h e r e by u s i n g d i m e t h o x y d i m e t h y l s i l a n e as a t r a p p i n g reagent. Other p o s s i b i l i t i e s f o r t h e r e a c t i o n i n t e r m e d i a t e a r e c h a i n end r a d i c a l s o r a l k a l i m e t a l s i l a n e c h a i n e n d s . A l s o p o s s i b l e a r e r a d i c a l a n i o n s as in t h e s i n g l e e l e c t r o n t r a n s f e r (SET) p o l y m e r i z a t i o n scheme d e s c r i b e d by H e i t z ( 1 4 ) . The l a t t e r scheme demonstrates s e v e r a l s i m i l a r i t i e s t o the present reaction p a r t i c u l a r l y in t h e f o r m a t i o n o f h i g h e r m o l e c u l a r weight m a t e r i a l n e a r t h e s t a r t o f p o l y m e r i z a t i o n s which would appear t o be s i m p l e polycondensations·

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F i g u r e 1. GPC t r a c e s o f complete p r o d u c t s of t h e r e d u c t i v e c o u p l i n g o f d i c h l o r o s i l a n e s w i t h Na in r e f l u x i n g t o l u e n e . ···· H e x y l m e t h y l dichlorosilane, phenylmethyldichlorosilane.

F i g u r e 2. GPC t r a c e s o f p r o d u c t s o f t h e r e d u c t i v e c o u p l i n g o f h e x y l m e t h y l d i c h l o r o s i l a n e w i t h Na in r e f l u x i n g t o l u e n e . A, on a d e f i c i t o f Na; B, w i t h normal Na a f t e r 20% consumption o f d i c h l o r i d e .

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The r o l e of i o n i c s p e c i e s in t h e r e a c t i o n was s t u d i e d by adding o r g a n o m e t a l l i c compounds a t t h e s t a r t of the r e a c t i o n . Either dibutylmercury o r diphenylmercury were added t o t h e sodium sand in r e f l u x i n g t o l u e n e b e f o r e t h e d i c h l o r i d e was added. A yellow c o l o r was produced i n d i c a t i n g t h e r e a c t i o n o f the mercury compound w i t h t h e sodium t o g i v e an organosodium compound. This color instantly disappeared on a d d i t i o n o f t h e d i c h l o r i d e . With HMDS t h e f i n a l p r o d u c t had a lower MW f o r P H I , and u s u a l l y a more complex GPC. W i t h PMDS a very g r e a t i n c r e a s e in r a t e o c c u r r e d , and most of t h e p r o d u c t was now P I I , Figure 3· T h i s p r o d u c t behaved in an i d e n t i c a l f a s h i o n t o t h e P I I in t h e absence of added o r g a n o m e t a l l i c in t h a t on c o n t i n u e d r e f l u x w i t h sodium i t i n c r e a s e d in MW w h i l s t r e m a i n i n g a c o n s t a n t f r a c t i o n o f t h e r e a c t i o n p r o d u c t , Figure 3· A f t e r short r e a c t i o n times t h e r e a c t i o n s o l u t i o n c o u l d be s e p a r a t e d from t h e sodium, washed w i t h water, d r i e d , and a g a i n r e f l u x e d w i t h sodium. The p r o d u c t P I I a g a i n would show an i n c r e a s e in m o l e c u l a r weight, a l t h o u g h any o f t h e o r i g i n a l a c t i v e c h a i n ends c o u l d be e x p e c t e d t o be d e s t r o y e d by the washing The i n f e r e n c e may w i t h HMDS, a r e a f f e c t e d by t h e presence o f o r g a n o m e t a l l i e s , p r e s u mably in t h e r o l e of p r o d u c i n g more a c t i v e c h a i n s . In t h e case of PMDS t h i s then becomes t h e major r e a c t i o n as seen by t h e i n c r e a s e in r a t e and t h e absence o f P H I . The l a t t e r h a v i n g perhaps a s e p a r a t e mechanism o f f o r m a t i o n . F o r HMDS t h e e f f e c t is s m a l l e r b u t perhaps t h e i n c r e a s e in c h a i n s is i n d i c a t e d by t h e d e c r e a s e in MW o f P H I . A l t h o u g h f a r from c o n c l u s i v e t h i s is e v i d e n c e f o r some i n t e r m e d i a c y o f a l k a l i m e t a l s i l a n e s p e c i e s in t h e p r o d u c t i o n of these products, w h i c h in o u r hands were o f t e n the major p o l y m e r i c s p e c i e s . Attempts t o promote radical reactions by adding azobisi s o b u t y r o n i t r i l e t o HMDS p o l y m e r i z a t i o n s gave no marked e f f e c t . E f f e c t o f Sodium S u r f a c e

Area

Because of t h e heterogeneous n a t u r e o f t h e r e a c t i o n , t h e p a r t i c l e s i z e , and hence t h e s u r f a c e a r e a of t h e sodium, might have some e f f e c t on t h e r e a c t i o n . In t h e s e r e a c t i o n s t h e sodium was added as a b l o c k , melted, and s t i r r e d t o g i v e t h e d i s p e r s i o n . A c c o r d i n g t o t h e speed o f t h e s t i r r e r t h e p a r t i c l e s i z e v a r i e d from about 1mm t o .1mm in d i a m e t e r . T h i s change caused a change in t h e r a t i o of t h e p r o d u c t s of t h e r e a c t i o n , p a r t i c u l a r l y in t h e case o f HMDS., Figure 4. In t h i s case t h e l a r g e y i e l d o f t h e P H I compared t o t h e low MW c y c l i c s P I a t low Na s u r f a c e a r e a , suggests t h e f o r m a t i o n of t h e l a t t e r is promoted by t h e l a r g e s u r f a c e a r e a o f t h e sodium. In t h e c a s e o f PMDS a r e l a t i v e l y s m a l l e f f e c t was found. Reaction

Kinetics

The r a t e o f consumption o f PMDS was t o o f a s t t o f o l l o w , however, t h i s was not t h e case f o r HMDS, and t h e d i s a p p e a r a n c e o f d i c h l o r i d e was f o l l o w e d w i t h time. The dependence of t h e r e a c t i o n on t h e s u r f a c e a r e a of sodium is shown in F i g u r e 5. There is an i n i t i a l p e r i o d o f i n c r e a s i n g r a t e , and t h i s is v e r y dependent on t h e sodium s u r f a c e area. T h i s i n d u c t i o n p e r i o d c a n be a s s o c i a t e d w i t h an i n c r e a s e in t h e number of a c t i v e l y growing c h a i n s w i t h c o m p a r a t i v e l y l o n g l i f e -

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

8.

WORSFOLD

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Préparations

1

I

10

3

10

107

ι 4

10

5

MW

F i g u r e 3. GPC t r a c e s o f p r o d u c t s o f t h e r e d u c t i v e c o u p l i n g o f p h e n y l m e t h y l d i c h l o r o s i l a n e w i t h Na in r e f l u x i n g t o l u e n e in t h e p r e s e n c e o f 3.4 χ 10" M HgBu . After 5 mins, 120 min, 240 mins. 2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

108

INORGANIC AND ORGANOMETALLIC POLYMERS

1.0

r

Consumption of Dichloride at Different Na Surface Areas 0.8

Η

0.6f-

0.4

0.2

0

10

20 TIME, min

30

40

F i g u r e 5. F r a c t i o n of h e x y l m e t h y l d i c h l o r o s i l a n e r e m a i n i n g , p l o t t e d v e r s u s time, a t v a r i o u s s u r f a c e a r e a s of sodium per mole d i c h l o r i d e Δ, 0.20; • , 0.67; Ο , 4.64 m e t e r . 2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

8. WORSFOLD times

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however,

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Polysilylene Preparations this

period.

relatively

The l a t e r

independent

of

stages

of t h e r e a c t i o n a r e ,

the surface

area

of

sodium

(Table I I I ) . Table

III.

Dependence o f Maximum Rate o n Sodium S u r f a c e A r e a

S u r f a c e Area / Mole D i c h l o r i d e Meter

Max. Rate M/sec χ 1 0

3

2

0.72 0.85 1.16

0.20 0.67 4.64

The r a t e d e c r e a s e s towards t h e end o f t h e r e a c t i o n which s u g g e s t s t h e r a t e is dependent on t h confirmed by d e c r e a s i n t h r e e , w h i l s t k e e p i n g t h e sodium c o n s t a n t , when i t was found t h a t u n d e r comparable s t i r r i n g c o n d i t i o n s t h e f r a c t i o n r e a c t i o n c u r v e s nearly coincided. A s l i g h t l y l o n g e r i n d u c t i o n p e r i o d w i t h t h e lower monomer b e i n g c o n s i s t a n t w i t h a s l o w e r b u i l d up of t h e r e a c t i o n centers at this concentration. The apparent l a c k o f dependence o f t h e p r o p a g a t i o n r e a c t i o n on t h e s u r f a c e a r e a o f t h e sodium s u g g e s t s t h a t t h e r e a c t i o n of a c h l o r i n e ended c h a i n w i t h sodium is p r o b a b l y f a s t and n o t t h e r a t e determining step. The r a t e d e t e r m i n i n g p r o c e s s is p r o b a b l y t h e r e a c t i o n o f t h e sodium ended c h a i n w i t h t h e d i c h l o r i d e . T h i s l a t t e r r e a c t i o n is presumably n o t on t h e sodium s u r f a c e because o f t h e l a c k o f dependence on t h e s u r f a c e a r e a . T h i s is s u p p o r t e d by t h e o b s e r v a ­ t i o n t h a t i f t h e sodium is allowed t o s e t t l e p a r t way through t h e r e a c t i o n most o f t h e polymer appears t o be in t h e s o l u t i o n and n o t a b s o r b e d on t h e sodium s u r f a c e v i a t h e l o n g l i v e d a c t i v e c h a i n ends. Visual observation, however, suggests t h a t some polymer is a t t a c h e d t o t h e s o l i d phase. T h i s is p a r t i c u l a r l y n o t i c e a b l e i f w a t e r is added t o t h e r e a c t i o n m i x t u r e when t h e p u r p l e c o l o r is o n l y d i f f i c u l t l y discharged. The source o f t h e c o l o r is n o t c l e a r . It is p o s s i b l e t h a t i t is Na e n t r a p p e d in t h e p r e c i p i t a t i n g NaCl t o g i v e c o l o r c e n t e r s as in t h e p r e p a r a t i o n o f sodium a l k y l s . Another p o s s i ­ b i l i t y is t h a t i t is due t o a charge t r a n s f e r r e a c t i o n between t h e sodium and polymer o r c y c l i c m a t e r i a l . However, a f t e r i s o l a t i o n o f t h e polymer i t is n o t p o s s i b l e t o r e g e n e r a t e t h e c o l o r in t o l u e n e s o l u t i o n w i t h Na. The r e a c t i v e c h a i n end a l s o might be c o l o r e d as a r e t h e c e n t e r s in l i v i n g a n i o n i c p o l y m e r i z a t i o n . But i f so t h i s would i n d i c a t e t h e attachment o f t h e m a j o r i t y of t h e a c t i v e c e n t e r s to t h e s o l i d s u r f a c e because t h e s u p e r n a t a n t l i q u i d is o n l y very s l i g h t l y colored. E f f e c t o f Reaction

Termination

Reagent

I f t h e r e a c t i o n has l o n g l i v e d i n t e r m e d i a t e s , i t is p o s s i b l e t h a t t h e c h a i n end c o u l d a l t e r n a t e between a c h l o r i n e ended and a sodium ended c h a i n as a d i c h l o r i d e was added t o g i v e t h e c h l o r i n e end and then

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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r e a c t e d f u r t h e r w i t h sodium t o r e g e n e r a t e t h e sodium end. On remov i n g an a l i q u o t t h e sodium is no l o n g e r in c o n t a c t w i t h t h e r e a c t i o n m i x t u r e and t h e c h a i n c o u l d be e x p e c t e d t o a t t a i n t h e c h l o r i n e ended form. Then i f t h e r e a c t i o n was t e r m i n a t e d w i t h m e t h y l l i t h i u m in d i e t h y l e t h e r t h i s would r a p i d l y cap t h e end. I f , however, water was added c o u p l i n g o f t h e c h a i n s c o u l d be expected l e a d i n g t o a h i g h e r m o l e c u l a r weight. T h i s was indeed found. With HMDS, a f t e r 57% r e a c t i o n on t e r m i n a t i o n w i t h C H L i o r H 0 , P H I had MWs o f 1.3 and 2 . 5 x l 0 , a f t e r 84% r e a c t i o n 2.4 and 3.1x10 , r e s p e c t i v e l y . This c o n f i r m s t h e l o n g l i f e o f t h e a c t i v e c h a i n s and t h e p r o b a b i l i t y t h a t i t is t h e c h l o r i n e end a t t h i s time. I t was found a l s o t h a t t h e polymer r e s i d e d mostly in t h e s o l u t i o n , and n o t l a r g e l y absorbed v i a i t s a c t i v e end t o t h e sodium s u r f a c e . 3

2

5

s

Copolymerization A few c o p o l y m e r i z a t i o n s o f PMDS and DMDS were c a r r i e d o u t By f o l l o wing t h e c o n c e n t r a t i o n o t h a t PMDS r e a c t s about f o u is r e f l e c t e d in t h e p o l y m e r i c p r o d u c t s o f t h e r e a c t i o n where a t e a r l y s t a g e s o f t h e r e a c t i o n t h e polymer c o m p o s i t i o n is f a r h i g h e r in PMDS than t h e f e e d . A f e e d r a t i o o f PhMe/Me of 0.75 gave a p r o d u c t r a t i o PhMe/Me of 1.59. I t is a l s o apparent t h a t in t h e l a t e r s t a g e s o f t h e r e a c t i o n o n l y DMDS is p r e s e n t . This should lead t o i n s o l u b l e polymer i f i n i t i a t i o n and p r o p a g a t i o n both o c c u r a t t h i s s t a g e . In fact comparatively little insoluble polymer forms, and t h i s is f u r t h e r e v i d e n c e f o r t h e l o n g l i f e o f t h e a c t i v e polymer c h a i n s . In t h e l a t e r s t a g e s of t h e r e a c t i o n much DMDS adds as a d i m e t h y l s i l y l r i c h b l o c k onto t h e e x i s t i n g p h e n y l m e t h y l s i l y l b l o c k s and remains in solution. The r e s u l t i n g polymers would have a graded composition a l o n g t h e c h a i n as t h e c o m p o s i t i o n o f t h e f e e d changes. 2

2

Conclusions Much is l e f t n o t undestood in t h i s r e a c t i o n . I t would appear t h a t more than one r o u t e l e a d s t o t h e h i g h e r MW m a t e r i a l , most l i k e l y two t o account f o r t h e appearance o f t h e two h i g h e r MW peaks in t h e GPC. The low m o l e c u l a r weight c y c l i c s a r e perhaps t h e p r o d u c t s o f a s i m u l taneous growth and b a c k b i t i n g mechanism in one o f t h e s e r o u t e s . One of t h e mechanisms shows some e v i d e n c e o f i o n i c o r a l k a l i m e t a l s i l y l i n t e r m e d i a t e s , and t h i s maybe a l s o t h e r o u t e t o t h e c y c l i c s because t h i s is promoted in THF s o l u t i o n . T h i s r e a c t i o n would then form most of t h e p r o d u c t s and be t h e major f a c t o r d e c i d i n g t h e k i n e t i c s o f t h e reaction. The r e a c t i o n k i n e t i c s in t h e case o f HMDS s u g g e s t s a two s t a g e mechanism f o r t h e major d i c h l o r i d e consuming r e a c t i o n . F i r s t a slow Na s u r f a c e dependent b u i l d up o f l o n g l i v e d a c t i v e c e n t e r s then a p r o p a g a t i o n s t e p in which t h e Na s u r f a c e is n o t r a t e d e t e r m i n i n g . The r e a c t i o n which o c c u r s on t h e s u r f a c e must be f a s t . The o t h e r mechanism, i f in f a c t t h e r e is one, is perhaps v i a a r a d i c a l a n i o n as p e r t h e SET mechanism suggested by H e i t z (14,15) f o r other polymerizations. The a b i l i t y of t h e l o n g s i l i c o n c h a i n t o delocalize e l e c t r o n s would a s s i s t in s t a b i l i z i n g either radical a n i o n s o r s t r a i g h t a n i o n i c s p e c i e s . A s i m p l e r a d i c a l is p r e f e r r e d by Z e i g l e r (9) f o r t h e f o r m a t i o n o f the h i g h e s t MW f r a c t i o n in t h e p o l y m e r i z a t i o n o f PMDS, a p r o d u c t f o r whose r o u t e no e v i d e n c e has a c c r u e d in t h i s work.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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,

F i g u r e 6. Fraction o f p h e n y l r a e t h y l d i c h l o r o s i l a n e ( • ) and d i m e t h y l d i c h l o r o s i l a n e (O) r e m a i n i n g as a f u n c t i o n o f time in a copolymerization. Literature

Cited

1. Wesson, J.P.; Williams, T.C. J. Polym. Sci., Polym. Chem. Ed., 18, 959 (1980). 2. Trujillo, R.E. J. Organometal. Chem., 198, C27 (1980). 3. West, R.; David, L.D.; Djurovich, P.I.; Stearley, K.L.; Srinivasan, K.S.V.; Yu, H. J. Amer. Chem. Soc., 103, 7352 (1981). 4. Harrah, L.A.; Zeigler, J.M. J. Polym. Sci., Polym. Let. Ed., 23, 209 (1985). 5. Hofer, D.C.; Miller, R.D.; Miller, C.G. SPIE Vol. 469, 16, Advances in Resist Technology (1984). 6. West, R. J. Organometal. Chem., 300, 327 (1986). 7. Miller, R.D.; Hofer, D.; McKean, D.R.; Willson, C.G.; West, R.; Trefonas, P.T. Materials for Microlithography, ACS Symp. Series 226, 293 (1984). 8. Zeigler, J.M. Polym Preprints, 27(1), 109 (1986). 9. Zeigler, J.M. Polym Preprints, 28(1), 424 (1987). 10. Wesson, J.P.; Williams, T.C. J. Polym. Sci., Polym. Chem. Ed., 17, 2833 (1979). 11. Xing-Hua Zhang, R.; West, J. Polym. Sci., Polym. Chem. Ed., 22, 159 (1984). 12. Watanabe, H.; Muraoka, T.; Kageyama, M.; Yoskizumi, K.; Nagai, Y. Organometallics, 141, 3 (1984). 13. Weyenberg, D.R.; Atwell, W.H. Pure Appl. Chem., 19, 343 (1969). 14. Heitz, W. Makromol. Chem., Macromol. Symp., 4, 35 (1986). 15. Koch, W.; Risse, W.; Heitz, W. Macromol. Chem., Suppl., 12, 105 (1985). RECEIVED September 23, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 9

Characterization of Copolydiorganosilanes with Varying Compositions Samuel P. Sawan, Yi-Guan Tsai, and Horng-Yih Huang Polymer Science Program, Department of Chemistry, University of Lowell,

Soluble polydiorganosilane homo and copolymers have recently shown great potential in such areas as precursors for the preparation of s i l i c o n carbide fibers (1), as p h o t o i n i t i a t o r s in alkene polymerization (2), as photoconductors (3), and as positive or negative self-developing photoresists for photolithographic a p p l i c a tions (4). A number of copolydiorganosilane copolymers have been reported recently (5) in which the copolymer contained equal amounts of both monomers in the feed. In this work seven copolymers, including; dimethyl-co-cyclohexylmethyl, cyclohexylmethyl-co-n-propylmethyl, c y c l o h e x y l m e t h y l - c o - n - p r o p y l m e t h y l , phenylmethyl-co-isopropylmethyl, phenylmethyl-co-n-propylmethyl, p h e n y l m e t h y l - c o - c y c l o h e x y l m e t h y l and n-propylmethyl-co-isopropylmethyl silane with varying percentages of the r e s p e c t i v e comonomers in the feed have been prepared and characterized.

The p r e v i o u s d i s c o v e r y o f p o l y d i m e t h y l s i l a n e by Burkhard in 1949 (6) i n d i c a t e d t h a t t h i s m a t e r i a l was i n s o l u b l e in common o r g a n i c s o l v e n t s , h i g h l y c r y s t a l l i n e and r e a d i l y decomposed w i t h o u t m e l t i n g when h e a t e d above 250°C. By i n t r o d u c i n g l a r g e r o r g a n o - s u b s t i t u e n t s o n t o t h e s i l i c o n b a c k b o n e , p o l y s i l a n e p o l y m e r s w e r e shown t o r e a d i l y d i s s o l v e in o r g a n i c s o l v e n t s and show m e l t i n g b e h a v i o r common t o many a m o r p h o u s c a r b o n b a c k b o n e d p o l y m e r s . With these p r o p e r t i e s , p o l y s i l a n e s c a n be m o l d e d , c a s t e d a n d p o t t e d i n t o v a r i o u s shapes o r even drawn i n t o f i b e r s . P o l y s i l a n e polymers p o s s e s s s e v e r a l r a t h e r remarkable properties. Some o f t h e u n i q u e p r o p e r t i e s o f t h e s e i n o r g a n i c polymers r e s u l t from t h e delocalization o f sigma e l e c t r o n s in t h e i r u n i n t e r r u p t e d s i l i c o n backbone w i t h a c o n c u r r e n t p h o t o a b l a t i v e sens i t i v i t y t o UV r a d i a t i o n . S e v e r a l p o l y s i l a n e s have been c h a r a c t e r i z e d and a l l show a s t r o n g UV a b s o r p t i o n . The maximum a b s o r p t i o n w a v e l e n g t h in t h e i r UV s p e c t r a is s t r o n g l y dependent on t h e organo-substituents ( 5 ) , m o l e c u l a r weight and t h e c o m p o s i t i o n o f 0097-6156/88/0360-0112$06.00/0 © 1988 American Chemical Society

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

9. SAW AN ET AL.

113

Characterization of Copolydiorganosilanes

copolymers* A l s o , t h e a b s o r p t i o n maxima may be temperature depen­ dent f o r some p o l y s i l a n e s such as p o l y ( d i - n - h e x y l s i l a n e ) ( 7 ) . The photochemical degradation of high molecular weight p o l y s i l a n e s a r e q u i t e u n u s u a l and is t h e key t o p o t e n t i a l uses o f t h e s e polymer* A p h o t o l y t i c cascade mechanism has been proposed in w h i c h b o t h s i l y l e n e a n d s i l y l r a d i c a l g e n e r a t i o n o c c u r in t h e p h o t o d e g r a d a t i o n ( 8 ) . P h o t o s c i s s i o n and p h o t o c r o s s l i n k i n g b o t h may o c c u r when t h e polymer is exposed t o UV l i g h t and is d i c t a t e d by the c o m p o s i t i o n o f t h e p o l y s i l a n e * The r a t e o f p h o t o d e g r a d a t i o n is a l s o s t r o n g l y dependent on t h e s u b s t i t u e n t s and c o p o l y m e r composition* A l t h o u g h c o n s i d e r a b l e i n t e r e s t e x i s t s in t h e p o l y s i l a n e s t h e number o f systems t h a t have been i n v e s t i g a t e d is l i m i t e d * Thus, a need t o p r e p a r e and c h a r a c t e r i z e a d d i t i o n a l p o l y s i l a n e copolymers is r e a d i l y apparent. I t is e x p e c t e d that systematic i n v e s t i g a t i o n s , as p r e s e n t e d in t h i s r e p o r t , w i l l y i e l d a d d i t i o n a l and v a l u a b l e i n s i g h t i n t o t h e u n i q u e b e h a v i o r o f t h i s c l a s s o f polymers. Experimental P o l y d i o r g a n o s i l a n e copolymers were s y n t h e s i z e d by a d d i n g 2.2 moles o f a sodium d i s p e r s i o n in a l i g h t o i l ( A l d r i c h ) a t a c o n s t a n t r a t e (320 meq/min) i n t o a t o l u e n e s o l u t i o n which c o n t a i n e d a t o t a l o f 1 mole o f d i o r g a n o d i c h l o r s i l a n e monomers.

Rj^ I η Cl-Si-Cl I

R2

R3 +

I 2.2(n+m) Na m Cl-Si-Cl I Toluene,110°C

R4

>

Rj^ R3 I I -(Si) -(Si) I I n

R2

m

R4

A l l monomers ( P e t r a r c h ) used in t h i s study were c a r e f u l l y frac­ t i o n a l l y d i s t i l l e d p r i o r t o u s e . The c o n d e n s a t i o n p o l y m e r i z a t i o n was c a r r i e d o u t a t a p p r o x i m a t e l y 110°C and a l l o w e d t o r e f l u x f o r 4 h o u r s a f t e r t h e a d d i t i o n o f t h e sodium m e t a l was completed. The h i g h m o l e c u l a r weight copolymers were o b t a i n e d a f t e r two p r e c i p i t a ­ t i o n s from t o l u e n e i n t o e t h y l a c e t a t e . M o l e c u l a r w e i g h t s o f t h e c o p o l y m e r s were determined by g e l p e r m e a t i o n c h r o m a t o g r a p h y (GPC) w i t h f o u r μ - s t y r a g e l ( W a t e r s ) columns c a l i b r a t e d u s i n g p o l y s t y r e n e standards. C h l o r o f o r m was u s e d a s t h e e l u a t e a t a f l o w r a t e 1.5 m l / m i n . An LKB-2140 U l t r a v i o l e t P h o t o d i o d e A r r a y d e t e c t o r was u s e d t o d e t e c t t h e polymer w i t h a scan range from 190 t o 370 nm. P r o t o n and carbon-13 n u c l e a r m a g n e t i c resonance (NMR) s p e c t r a were r e c o r d e d on a IBM Instruments 270 MHz NMR Spectrometer on 6-8 weight p e r c e n t s o l u t i o n s in d e u t e r a t e d c h l o r o f o r m . Ultraviolet s p e c t r a were r e c o r d e d on an IBM U l t r a v i o l e t Spectrophotometer Model 9420 u s i n g c h l o r o f o r m s o l u t i o n s c o n t a i n i n g 2 χ 1 0 " g/ml o f t h e copolymers. 5

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

114

INORGANIC AND ORGANOMETALLIC POLYMERS

R e s u l t s and

Discussion

The p r e p a r a t i o n o f h i g h m o l e c u l a r weight polymers r e q u i r e s c a r e f u l a t t e n t i o n t o t h e p u r i t y o f the r e a g e n t s employed and the r e a c t i o n conditions used. A t y p i c a l y i e l d f o r high molecular weight p o l y d i o r g a n o s i l a n e u s i n g the Wurtz r e a c t i o n ranged from 10 to 35%. A l l p o l y m e r s p r e p a r e d in t h i s s t u d y w e r e f o u n d t o be s o l u b l e in c h l o r o f o r m , t o l u e n e and t e t r a h y d r o f u r a n and c o u l d be drawn i n t o f i b e r s o r c a s t i n t o f i l m s . G e l p e r m e a t i o n c h r o m a t o g r a p h i c (GPC) a n a l y s e s showed a l l samples t o be bimodal in t h e i r m o l e c u l a r weight d i s t r i b u t i o n e x c e p t f o r c-hex/n-pro (50/50) and c - h e x / i - p r o (25/75), copolymers 5 and 9, r e s p e c t i v e l y . M o l e c u l a r w e i g h t s were c a l c u l a t e d from a c a l i b r a ­ tion curve prepared using polystyrene standards without hydrodynamic c o r r e c t i o n f o r t h e s i l a n e p o l y m e r s . The c a l c u l a t e d m o l e c u l a r w e i g h t s r a n g e d f r o m s e v e r a l t e n s o f t h o u s a n d s up t o several m i l l i o n s . T a b l e I shows the m o l e c u l a r w e i g h t s o b s e r v e d f o r each mode in the bimoda been made t o determine p o l y m e r s , a p r e v i o u s r e p o r t (9) i n d i c a t e s t h a t t h e a c t u a l weight average m o l e c u l a r w e i g h t s f o r p o l y s i l a n e s are h i g h e r than c a l c u ­ l a t e d by a f a c t o r o f 2 t o 3. The lowest m o l e c u l a r weight o b s e r v e d f o r a l l polymers p r e p a r e d in t h i s study was f o r copolymer 16, a copolymer p r e p a r e d u s i n g 75 mole % c y c l o h e x y l m e t h y l and 25 mole % p h e n y l m e t h y l s i l a n e . The low m o l e c u l a r weight o b s e r v e d f o r t h i s copolymer may be due t o the two b u l k y s i d e g r o u p s o f t h e monomers ( p h e n y l and c y c l o h e x y l ) s t e r i c a l l y i n t e r f e r i n g w i t h p r o p a g a t i o n d u r i n g the p o l y m e r i z a t i o n . A l l copolymers show s t r o n g UV a b s o r p t i o n maxima r a n g i n g from 304 t o 340 nm as shown in T a b l e I and F i g u r e 1. The absorbance maxima were found t o be dependent on the comonomers employed and the c o m p o s i t i o n o f the c o p o l y m e r s . Copolymers which c o n t a i n the p h e n y l group on t h e polymer c h a i n show a g r e a t e r r e d s h i f t in t h e i r UV spectrum such t h a t t h e X appears near 340 nm. This red s h i f t i n g due t o a r y l s u b s t i t u t i o n has been r e p o r t e d by o t h e r s (10) and is due t o σ-ττ e l e c t r o n r e s o n a n c e o f t h e p h e n y l g r o u p and s i l i c o n backbone. I t is i n t e r e s t i n g t o n o t e t h a t as the amount o f p h e n y l monomer c o n t e n t is r e d u c e d in the copolymer t h e X is ob­ served to decrease. T h i s is p r o b a b l y due t o l e s s c o n j u g a t i v e in­ t e r a c t i o n o f the p h e n y l s u b s t i t u e n t w i t h the s i l i c o n backbone. As shown in T a b l e I , c y c l o h e x y l and i s o p r o p y l s u b s t i t u e n t s a l s o r a i s e d the a b s o r p t i o n maxima t o l o n g e r wavelengths as compared with linear a l k y l substituted polysilanes. The r e d s h i f t i n g be­ h a v i o r o f c y c l o h e x y l s u b s t i t u t e d p o l y s i l a n e s has been a t t r i b u t e d t o an e x t e n d e d t r a n s c o n f o r m a t i o n o f t h e p o l y m e r c h a i n due t o the b u l k y c y c l o h e x y l s i d e groups ( 1 1 ) . The i s o p r o p y l group a l s o ap­ p e a r s t o s t e r i c a l l y a f f e c t c h a i n c o n f o r m a t i o n such t h a t polymers c o n t a i n i n g t h i s r e s i d u e show r e d s h i f t i n g in t h e i r UV s p e c t r a . T h i s e f f e c t is e v i d e n c e d in an e x a m i n a t i o n o f t h e UV s p e c t r a o f copolymers p r e p a r e d from η-propyl m e t h y l and i - p r o p y l m e t h y l s i l a n e monomers. As the c o n t e n t o f the i s o p r o p y l r e s i d u e is i n c r e a s e d from 25 t o 75 mole %, the X is o b s e r v e d t o i n c r e a s e from 307.4 t o 312.4 nm, r e s p e c t i v e l y ( F i g u r e 2 ) . The c h a n g e in t h e maximum a b s o r p t i o n w a v e l e n g t h o f t h e m

a

x

m

m

a

a

x

x

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

9.

SA WAN ET AL.

Table I. Sample Number

115

Characterization of Copolydiorganosilanes

Properties o f Copolydiorganosilanes

Monomer Feed C o m p o s i t i o n A Β

Mole % A in A +Β

1

c-Hexyl

Me

Me

Me

75

2

c-Hexyl

Me

Me

Me

50

3

c-Hexyl

Me

Me

Me

25

4

c-Hexyl

Me

n-Propyl

Me

75

5

c-Hexyl

Me

n-Propyl

Me

50

6

c-Hexyl

Me

7

c-Hexyl

Me

iso-Propy

8

c-Hexyl

Me

i e o - P r o p y l Me

50

9

c-Hexyl

Me

i s o - P r o p y l Me

25

10

Phenyl

Me

n-Propyl

Me

75

11

Phenyl

Me

n-Propyl

Me

50

12

Phenyl

Me

n-Propyl

Me

25

13

Phenyl

Me

i s o - P r o p y l Me

75

14

Phenyl

Me

i s o - P r o p y l Me

50

15

Phenyl

Me

i s o - P r o p y l Me

25

16

Phenyl

Me

c-Hexyl

Me

75

17

Phenyl

Me

c-Hexyl

Me

50

18

Phenyl

Me

c-Hexyl

Me

25

19

n-propyl

Me

i s o - P r o p y l Me

75

20

n-Propyl

Me

i s o - P r o p y l Me

50

21

n-Propyl

Me

i s o - P r o p y l Me

25

22

n-Propyl

Me

100

Mw χ 10~

3

1100 8 1300 14 1100 7 840 7 600

5 1000 5 1200 1300 1 910 3 850 7 1300 8 980 4 740 4 54 8 150 8 180 4 1400 34 1700 14 1200 10 1300 23

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

A.max tnm; 307.6 307.8 304.4 317.2 313.4

323 320.8 340 336 309.6 340 339.8 333.6 340 340.6 326.2 307.4 308.2 312.4 307

116

INORGANIC AND ORGANOMETALLIC POLYMERS

1: 2: 3: 4: 5: 6: 7:

c-Hex/Me (50/50). c-Hex/n-Pro (50/50). c-Hex/ieo-Pro (50/50). Ph/n-Pro (50/50). Ph/ieo-Pro (50/50). Ph/c-Hex (50/50). n-Pro/iio-Pro (50/50).

Wavelength, F i g u r e 1. UV

g Ί

190

(nm)

Spectra f o r Representative Polysilane

r—τ—j

220

.

.

1

250

1

•

1

.

r—j

280 Wavelength,

310

.

.

Copolymers.

1

340

r—,

370

(nm)

F i g u r e 2. The UV S p e c t r a o f P o l y ( n - p r o p y l - m e t h y l - c o - i s o - p r o p y l m e t h y l s i l a n e ) Copolymers w i t h S i m i l a r M o l e c u l a r Weights H a v i n g the F o l l o w i n g C o m p o s i t i o n s : (1) 75/25, (2) 50/50 and (3) 25/75.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

9.

SA WAN ET AL.

Characterization of Copolydiorganosilanes

117

copolymers a l s o depends upon t h e m o l e c u l a r weight o f t h e copolymer. The UV a b s o r p t i o n s p e c t r a f o r copolymer 5, c-hex/n-pro ( 5 0 / 5 0 ) , w i t h d i f f e r e n t m o l e c u l a r w e i g h t s a r e shown in F i g u r e 3. These s p e c t r a r e s u l t from t h e UV a n a l y s i s o f t h e v a r i o u s f r a c t i o n s t h a t pass through t h e p h o t o d i o d e a r r a y detector during the chromatographic a n a l y s i s . The λ f o r polymer f r a c t i o n s w i t h m o l e c u l a r w e i g h t s o f 1040, 52, 11 and 5 χ 1 0 were found a t 314, 313.5, 308.2 and 304 nm, r e s p e c t i v e l y . A similar observation f o r the wavelength dependent s h i f t in X as a f u n c t i o n o f m o l e c u l a r w e i g h t has been r e p o r t e d f o r l i n e a r ^ s i l a n e o l i g o m e r s (12,13) in which t h e s i l i c o n c h a i n a c t s as a σ - σ o r σ - π chromophore. The p r o t o n d e c o u p l e d c a r b o n 13 NMR s p e c t r a f o r t h r e e p o l y ( c y c l o h e x y l m e t h y l - c o - i s o p r o p y l m e t h y l ) copolymers a r e shown in F i g u r e 4. The backbone m e t h y l group is o b s e r v e d as o c c u r r i n g be­ tween -4 and -1 ppm and c o n s i s t s o f m u l t i p l e r e s o n a n c e s which a r e due t o polymer m i c r o s t r u c t u r e . M u l t i p l e r e s o n a n c e s a r e a l s o ob­ s e r v e d f o r t h e m e t h y l and t e r t i a r y c a r b o n o f t h e i s o p r o p y l group and f o r t h e methine carbo t u r a l assignments f o r a l s o been found t h a t i n c r e a s i n g t h e b u l k y c h a r a c t e r o f t h e sub­ s t i t u e n t y i e l d e d b r o a d e r r e s o n a n c e p e a k s in t h e c a r b o n - 1 3 NMR spectra. Μ

χ

3

m

a

x

One. a d d i t i o n a l peak o c c u r r i n g a t 29.5 ppm was always a p p a r e n t in the C NMR s p e c t r a o f a l l copolymers p r e p a r e d in t h i s s t u d y . Proton coupled C NMR s p e c t r a r e v e a l a t r i p l e t f o r t h i s r e s o n a n c e w i t h a c o u p l i n g c o n s t a n t o f 127 Hz ( F i g u r e 5 ) . Such NMR s p e c t r a l r e s u l t s a r e c o n s i s t e n t w i t h an a d d i t i o n a l methylene r e s o n a n c e in the copolymers. E v i d e n c e in t h e l i t e r a t u r e s u g g e s t s t h a t t h e con­ d e n s a t i o n r e a c t i o n may p r o c e e d v i a a r a d i a l pathway ( 1 4 ) . Thus, a methylene group may be r e a d i l y g e n e r a t e d d u r i n g t h e c o n d e n s a t i o n p o l y m e r i z a t i o n due t o s i l y l r a d i c a l a b s t r a c t i o n o f a m e t h y l group hydrogen atom. F o u r i e r transform i n f r a r e d spectroscopic examination of these polymers a l s o s u p p o r t s t h i s c o n c l u s i o n . As can be o b s e r v e d in F i g u r e 6, a p e a k o c c u r r i n g a t 2086 cm is s e e n in a l l t h e copolysilanes. T h i s absorbance has been a s s i g n e d t o a S i - H stretching absorption (15). Such a m o i e t y is e x p e c t e d i f s i l y l r a d i c a l a b s t r a c t i o n o f a hydrogen o c c u r s . E x a m i n a t i o n o f t h e in­ f r a r e d s p e c t r a f o r a S i - C H o - S i v i b r a t i o n a l peak which s h o u l d be l o ­ c a t e d b e t w e e n 1000 a n d 1100 cm is i n c o n c l u s i v e due t o t h e p r e s e n c e o f a m u l t i t u d e o f absorbances in t h i s r e g i o n . P o l y d i o r g a n o s i l a n e s undergo p h o t o d e g r a d a t i o n r e a d i l y when ex­ p o s e d t o m i d o r d e e p UV r a d i a t i o n . In t h i s study, the p h o t o d e g r a d a t i o n was c a r r i e d o u t by e x p o s i n g c o p o l y s i l a n e s o l u t i o n s (5 χ 10 g/ml in c h l o r o f o r m ) t o a low p r e s s u r e mercury UV l i g h t s o u r c e and r e c o r d i n g t h e UV s p e c t r a o f such s o l u t i o n s as a f u n c t i o n of t h e exposure t i m e . As shown in F i g u r e 7 f o r copolymer 20, i . e . n-pro/i-pro (50/50), the X is o b s e r v e d t o s h i f t t o s h o r t e r wavelengths w i t h i n c r e a s i n g exposure t i m e . The s h i f t in t h e maxi­ mum absorbance w a v e l e n g t h is accompanied by a change in t h e e x t i n c ­ t i o n c o e f f i c i e n t f o r t h e p h o t o p r o d u c t formed d u r i n g irradiation. Such b l e a c h i n g s p e c t r a a r e v e r y s i m i l a r t o s p e c t r a r e p o r t e d f o r c y c l o s i l a n e s w i t h d i f f e r e n t r i n g s i z e s ( 1 6 ) . The c y c l o s i l a n e s show an i n c r e a s e in t h e i r e x t i n c t i o n c o e f f i c i e n t s and a s h i f t t o l o n g e r m

a

x

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

118

INORGANIC AND ORGANOMETALLIC POLYMERS

M o l e c u l a r Weight CI) 1040 χ 10 (2) 52 χ 10 3

3

Wavelength, (nm) F i g u r e 3. UV S p e c t r a f o r P o l y ( c y c l o h e x y l - m e t h y l - c o - i s o - p r o p y l m e t h y l s i l a n e ) (50/50) w i t h D i f f e r e n t M o l e c u l a r W e i g h t s .

(a) c - H e x / i s o - P r o (b) c - H e x / i s o - P r o (c) c - H e x / i s o - P r o

(25/75) (50/50) (75/50)

F i g u r e 4. 67 MHz C a r b o n - 1 3 NMR S p e c t r a f o r P o l y ( c y c l o h e x y l methy1-co-n-propy1-methy1s i l a n e ) Copolymers.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

9.

SA WAN ET AL.

Characterization of Copolydiorganosilanes

30.0

'2θ!θ

I

I

ι

ι

,0

.°

ι

ι

I I 1 ι ι

".° ppm

F i g u r e 5. 67 MHz C a r b o n - 1 3 NMR S p e c t r a f o r P o l y ( n - p r o p y l m e t h y l s i l a n e ) . ( a ) P r o t o n Decoupled (b) P r o t o n C o u p l e d .

4000 WAVENUMBERS C M " F i g u r e 6. FT-IR Spectrum f o r P o l y ( n - p r o p y l - m e t h y l s i l a n e ) .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

119

120

INORGANIC AND ORGANOMETALLIC POLYMERS

F i g u r e 7. UV S p e c t r a f o r P o l y ( n - p r o p y 1 - m e t h y l - c o - i s o - p r o p y 1m e t h y l s i l a n e ) (50/50) as a F u n c t i o n o f UV Exposure Time*

w a v e l e n g t h s in t h e i r X as t h e r i n g s i z e is i n c r e a s e d . These UV s p e c t r a l r e s u l t s i n d i c a t e that the p o l y d i o r g a n o s i l a n e s are cleaved i n t o s m a l l f r a g m e n t s , perhaps c y c l i c , d u r i n g i r r a d i a t i o n . The r a t e o f p h o t o d e g r a d a t i o n of the high molecular weight s p e c i e s was examined by p l o t t i n g t h e i n t e n s i t y a t t h e o r i g i n a l maximum a b s o r p t i o n w a v e l e n g t h as a f u n c t i o n o f t h e exposure t i m e . As shown in F i g u r e s 8 and 9, such p l o t s y i e l d a s t r a i g h t l i n e w i t h v a r y i n g s l o p e s d e p e n d i n g upon t h e copolymer composition. Copolymers c o n t a i n i n g t h e p h e n y l group showed t h e h i g h e s t s o l u t i o n d e g r a d a t i o n r a t e s o f a l l copolymers s t u d i e d . The p h o t o d e g r a d a t i o n r a t e was f o u n d t o i n c r e a s e d i r e c t l y w i t h t h e c o n t e n t o f p h e n y l r e s i d u e s in t h e c o p o l y m e r s . Copolymers w h i c h c o n t a i n a h i g h r a t i o of b u l k y g r o u p s , such as c y c l o h e x y l and i s o p r o p y l , show a h i g h e r d e g r a d a t i o n r a t e ' as compared t o l i n e a r a l k y l s u b s t i t u t e d polysilanes. m

a

x

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

9.

SAWANETAL.

121

Characterization of Copolydiorganosilanes

1.2-1

Exposure Time (sec) F i g u r e 9. Change in UV Absorbance f o r P o l y ( p h e n y l - m e t h y l - c o - n p r o p y l - m e t h y l s i l a n e ) Copolymers a t T h e i r R e s p e c t i v e λ as a F u n c t i o n o f UV Exposure Time. Μ

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Χ

INORGANIC AND ORGANOMETALLIC POLYMERS

122 Acknowledgment s

The a u t h o r s w i s h t o thank D i g i t a l Equipment f i n a n c i a l s u p p o r t f o r t h i s work and Dr. C o l l i n s technical assistance.

Literature 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16.

Corporation f o r Conley f o r h i s

Cited

Yajima, S. Am. Ceram. Soc. Bull. 1983, 62, 893. West, R.; Maxka, J.; Sinclair, R.; Cotts, P. Presented at the 193rd ACS National Metg., Sym. on Organomet. Polym., Denver, 1987. Kepler, R.G.; Zeigler, J.M.; Harrah, L.A.; Kurtz, S.R. Bull. Am. Phys. Soc. 1993, 28, 362. Zeigler, J.M.; Harrah L.A.; Johnson, A.W. SPIE Adv. Resist. Tech. 1985, 539, 166. Zhang, X.H.; West, 23, 479; J. Polym. Burkhard, C.A. J. Am. Chem. Soc. 1949, 71, 963. Miller, R.D.; Hofer, D.; Rabolt, J.; Fickes, G.N. J. Am. Chem. Soc. 1985, 107, 2172. Trefonas III, P.; West, R.; Miller, R.D. J. Am. Chem. Soc. 1985, 107, 2737. West, R. J. Organomet. Chem. 1986, 300, 327. Pitt, C.G.; Carey, R.N.; Toren, E.C. Jr. J. Am. Chem. Soc. 1972, 94, 3806. Trefonas III, P.; Djurovich, P.I.; Zhang, X.H.; West, R. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 819; J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 823. MacKay, K.M.; Watt, R. Organometal. Chem. Rev. 1969, 4, 137. Gilman, H.; Atwell, W.H.; Schwebke, G.L. J. Organometal. Chem. 1964, 2, 369. Zeigler, J.M. A.C.S. Polym. Prep. 1986, 27(1), 109. Yajima, S.; Hasegawa, Y.; Hayashi, J.; Iimura, M. J. Mate. Sci. 1978, 13, 2569. West, R. Pure Applied Chem. 1982, 54, 5, 1041.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 10

Synthetic Routes to Oligosilazanes and Polysilazanes Polysilazane Precursors to Silicon Nitride 1

Richard M. Laine , Yigal D. Blum, Doris Tse, and Robert Glaser

2

Inorganic and Organometalli

Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, com­ pounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the gene­ ral synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. The latter part of the chapter focusses on catalyzed dehydrocoupling reactions as a route to polysilazanes. Non-oxide c e r a m i c s such as s i l i c o n c a r b i d e ( S i C ) , s i l i c o n n i t r i d e (S13N4), and boron n i t r i d e (BN) o f f e r a wide v a r i e t y o f u n i q u e p h y s i c a l p r o p e r t i e s such as h i g h hardness and h i g h s t r u c t u r a l s t a b i l i t y under e n v i r o n m e n t a l extremes, as w e l l as v a r i e d e l e c t r o n i c and o p t i c a l p r o p e r t i e s . These advantageous p r o p e r t i e s p r o v i d e the d r i v i n g f o r c e f o r i n t e n s e r e s e a r c h e f f o r t s d i r e c t e d toward d e v e l o p i n g new p r a c t i c a l a p p l i c a t i o n s f o r these m a t e r i a l s . These e f f o r t s o c c u r d e s p i t e the c o n s i d e r a b l e expense o f t e n a s s o c i a t e d w i t h t h e i r i n i t i a l p r e p a r a t i o n and subsequent t r a n s f o r m a t i o n i n t o f i n i s h e d p r o d u c t s . The expense in p r e p a r a t i o n d e r i v e s from the need f o r h i g h p u r i t y products. The c o s t in f a b r i c a t i n g t h r e e - d i m e n s i o n a l shapes o f S i C , βΙβΝ^ or BN a r i s e s from the need t o s i n t e r powders o f these m a t e r i a l s at h i g h temperatures (>1500°C) and o f t e n under h i g h p r e s s u r e . Coat­ i n g s o f these m a t e r i a l s a r e a t p r e s e n t produced by c h e m i c a l o r

Current address: Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195 On leave from the Department of Chemistry, Ben Gurion University, Beer Sheva, Israel

2

0097-6156/88/0360-0124$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

10. LAINE ET AL.

Synthetic Routes to Polysilazanes

125

p h y s i c a l vapor d e p o s i t i o n (CVD o r PVD). I n each i n s t a n c e , t h e p r o c e s s is equipment and energy i n t e n s i v e and, has numerous l i m i t a tions. I f i n e x p e n s i v e methods f o r the s y n t h e s i s and f a b r i c a t i o n o f non-oxide c e r a m i c s can be d e v e l o p e d , then the number o f u s e f u l a p p l i c a t i o n s c o u l d be enormous. One p o t e n t i a l s o l u t i o n t o these problems, suggested some 20 y e a r s ago by C h a n t r e l l and Popper ( 1 ) , i n v o l v e s the use o f i n o r g a n i c o r organom e t a l l i c polymers as p r e c u r s o r s to the d e s i r e d c e r a m i c m a t e r i a l . The concept ( 2 ) c e n t e r s on t h e use of a t r a c t a b l e ( s o l u b l e , m e l t a b l e o r m a l l e a b l e ) i n o r g a n i c p r e c u r s o r polymer t h a t c a n be shaped a t low temperature ( a s one shapes o r g a n i c polymers) i n t o a c o a t i n g , a f i b e r or as a m a t r i x ( b i n d e r ) f o r a ceramic powder. Once t h e f i n a l shape is o b t a i n e d , t h e p r e c u r s o r polymer can be p y r o l y t i c a l l y t r a n s f o r m e d i n t o the d e s i r e d c e r a m i c m a t e r i a l . With c a r e f u l c o n t r o l o f the p y r o l y s i s c o n d i t i o n s , the f i n a l p i e c e w i l l have t h e a p p r o p r i a t e p h y s i c a l and/or e l e c t r o n i c p r o p e r t i e s The c o m m e r c i a l i z a t i o n o c e r a m i c f i b e r f a b r i c a t e d by p y r o l y s i s o f f i b e r s o f p o l y c a r b o s i l a n e ( 3 ) , has sparked i n t e n s i v e e f f o r t s t o s y n t h e s i z e polymer p r e c u r s o r s of use in t h e f a b r i c a t i o n o f Si^N^ and BN f i b e r s , c o a t i n g s and binders. I n t h i s r e g a r d , o l i g o - and p o l y s i l a z a n e s have r e c e n t l y been shown t o o f f e r c o n s i d e r a b l e p o t e n t i a l as p r e c u r s o r s t o S13N4. The o b j e c t i v e o f t h i s c h a p t e r is t o p r o v i d e an o v e r v i e w o f t h e g e n e r a l methods f o r s y n t h e s i z i n g o l i g o - and p o l y s i l a z a n e s and t o show how some o f these r o u t e s have been used t o s y n t h e s i z e o l i g o - and p o l y s i l a z a n e p r e c e r a m i c polymers. I n t h e l a t t e r p a r t o f t h e c h a p t e r , we w i l l emphasize our own work on t r a n s i t i o n m e t a l c a t a l y z e d dehydrocoupling reactions. General Synthetic

Approaches

H i s t o r i c a l l y , o l i g o - and p o l y s i l a z a n e s y n t h e t i c c h e m i s t r y has passed through t h r e e s t a g e s of development w i t h q u i t e d i f f e r i n g g o a l s . I n i t i a l e f f o r t s , b e g i n n i n g w i t h the work o f Stock and S o m i e s k i in 1921, were d i r e c t e d simply towards the p r e p a r a t i o n and c l a s s i f i c a t i o n of the g e n e r a l p r o p e r t i e s of p o l y s i l a z a n e s ( 4 , 5 ) . The commercial s u c c e s s o f p o l y s i l o x a n e s o r s i l i c o n e s in the f i f t i e s and s i x t i e s prompted s t u d i e s on the s y n t h e s i s o f p o l y s i l a z a n e s as p o t e n t i a l a n a l o g s (without much s u c c e s s ) . As mentioned above, c u r r e n t i n t e r e s t d e r i v e s from t h e i r use as s i l i c o n n i t r i d e p r e c e r a m i c p o l y m e r s . R e a c t i o n s ( l ) - ( 5 ) i l l u s t r a t e known methods f o r f o r m i n g s i l i c o n n i t r o g e n bonds o f p o t e n t i a l use in t h e f o r m a t i o n o f o l i g o - and p o l y silazanes. The most common method of f o r m i n g s i l a z a n e s is v i a ammon o l y s i s o r a m i n o l y s i s as shown in r e a c t i o n (1) ( 4 , 5 ) :

f

R S i C l + 2R NH 3

2

> R SiNR 3

f

,

2

+ R NH 2

+ C 1 2

""

Si-N bonds c a n a l s o be formed by a d e h y d r o c o u p l i n g r e a c t i o n by a l k a l i (6-11) o r t r a n s i t i o n m e t a l s ( 1 2 ) :

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

X

( )

catalyzed

126

INORGANIC AND ORGANOMETALLIC POLYMERS

f

R SiH + R NH 3

catalyst

2

In r a r e nitrogen despite the S i - N

>

R

3

S

i

N

R

»

2

^

+

( 2

)

i n s t a n c e s , one can a l s o o b t a i n d i s p l a c e m e n t o f oxygen by as i l l u s t r a t e d by r e a c t i o n ( 3 ) ( 1 3 ) . R e a c t i o n (3) proceeds t h e f a c t t h a t t h e S i - 0 bond is 25-30 k c a l / m o l e s t r o n g e r than bond ( 1 4 ) .

R S i - 0 H + R' NH 3

> R SiNR

2

f

3

2

+ H 0

(3)

2

Verbeek and W i n t e r have used a d e a m i n a t i o n / c o n d e n s a t i o n r e a c t i o n t o form new S i - N bonds as e x e m p l i f i e d by r e a c t i o n ( 4 ) ( 1 5 , 1 6 ) .

f

2R Si(NHR ) 2

>

2

I t has been s u g g e s t e d t h a t r e a c t i o n (4) p a s s e s t h r o u g h an R Si=NR intermediate, although no s u b s t a n t i a t i o n e x i s t s ( 1 7 , 1 8 ) .

f

2

Van Wazer (19) has d e s c r i b e d t h e use o f a r e d i s t r i b u t i o n i l l u s t r a t e d in ( 5 ) , t o form l i n e a r o l i g o m e r s from c y c l i c

2R SiCl 2

i

2

+ -[R SiNH] -l 2

3

reaction, species:

> ClR SiNHSiR Cl + 2

2

Cl-[R SiNH] -SiR Cl 2

2

2

(5)

W i t h the e x c e p t i o n o f r e a c t i o n ( 3 ) , t h e u t i l i t y o f a l l o f t h e above r e a c t i o n s f o r t h e s y n t h e s i s o f o l i g o - and p o l y s i l a z a n e s has been examined in v a r y i n g d e t a i l . I n the f o l l o w i n g p a r a g r a p h s , we attempt t o examine t h e p e r t i n e n t s t u d i e s in each a r e a e s p e c i a l l y as i t r e l a t e s to the s y n t h e s i s of preceramic polymers. Polymerization

by Ammonolysis and A m i n o l y s i s

The s i m p l i c i t y o f t h e a m m o n o l y s i s / a m i n o l y s i s o f d i h a l o h a l o s i l a n e s , r e a c t i o n s ( 6 ) and ( 7 ) , made them the o r i g i n a l method o f c h o i c e f o r the s y n t h e s i s o f o l i g o - and p o l y s i l a z a n e s , e s p e c i a l l y because o f the analogy to the h y d r o l y t i c synthesis of p o l y s i l o x a n e s . The ammonol y s i s o f d i h a l o s i l a n e s , r e a c t i o n ( 6 ) , has been found t o be e x t r e m e l y s e n s i t i v e to s t e r i c f a c t o r s (6).

1

R RSiCl

2

4 - 3xNH

1

+

> -[R RSiNH] ~ + 2xNH Cl"

3

x

4

1

(6)

F o r example, w i t h R - R - H, r e a c t i o n ( 6 ) g i v e s o n l y o l i g o s i l a z a n e s ; w i t h R = C H and R = H, both c y c l i c and o l i g o s i l a z a n e s a r e formed; and w i t h R > C H and R = H then one o b t a i n s m o s t l y c y c l o t r i - and c y c l o t e t r a s i l a z a n e s as the p r o d u c t s . 1

3

1

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

10.

LAINE ET AL.

127

Synthetic Routes to Polysilazanes

A m i n o l y s i s o f d i h a l o s i l a n e s , r e a c t i o n ( 7 ) , is a l s o e x t r e m e l y s e n s i t i v e t o t h e s t e r i c b u l k of the amine ( 2 0 ) . F o r R = CH3, 1

1

xR RSiCl

2

+ 3xR'NH

1

t

,

+

> -[R RSiNR ] - + 2 x R N H C l ~

2

χ

(7)

3

f

R » CH3 and R > E t , t h e p r o d u c t s a r e p r e d o m i n a n t l y t h e c y c l o t r i - and c y c l o t e t r a s i l a z a n e s and/or simple d i s i l a z a n e s , e.g., R ^ R S i ( N H R ) . F o r r e a c t i o n s where R = H o r CH3, R = H and R = CH3, t h e p r o d u c t s can be m o s t l y l i n e a r o l i g o s i l a z a n e s depending on c o n d i t i o n s ( s e e below) (21-23). The p r e f e r e n c e f o r c y c l o t r i - and c y c l o t e t r a s i l a ­ zanes, in these r e a c t i o n s , is not s u r p r i s i n g g i v e n t h a t t h e hydro­ l y s i s of d i h a l o s i l a n e s a l s o l e a d s t o c y c l o t r i - and c y c l o t e t r a siloxanes. ?

2

1

1

The s e a r c h f o r s i l i c o n n i t r i d e p r e c u r s o r s has renewed i n t e r e s t in ammonolysis as a r o u t e t f o l l o w e d up on e a r l y s t u d i e p r e p a r a t i o n o f - [ H S i N M e ] - from MeNH and H S i C l . We f i n d t h a t t h e m o l e c u l a r weight of t h i s o l i g o m e r is g r e a t l y a f f e c t e d by t h e r e a c t i o n conditions. O r i g i n a l l y , S e y f e r t h and Wiseman were a b l e t o produce m i x t u r e s of o l i g o m e r s o f - [ H S i N M e ] - and t h e c y c l o t e t r a m e r ( 2 2 ) . Removal o f the c y c l o t e t r a m e r by d i s t i l l a t i o n gave a polymer w i t h Mn - 600 D a l t o n s (x = 10). We have now succeeded in p r e p a r i n g quan­ t i t i e s of - [ H S i N M e ] - w i t h Mn « 1000-1200 (x = 18-20) w i t h l e s s than 10% c y c l o m e r s ( 2 3 ) . The r e s u l t i n g o l i g o s i l a z a n e c a n be used d i r e c t l y as a p r e c u r s o r f o r t r a n s i t i o n metal c a t a l y z e d d e h y d r o c o u p l i n g p o l y ­ m e r i z a t i o n (see b e l o w ) . 2

x

2

2

2

2

2

x

x

A r a i e t a l . ( 2 4 ) have d e v i s e d a n o v e l approach t o ammonolysis wherein the s i l y l h a l i d e is m o d i f i e d by c o m p l e x a t i o n t o p y r i d i n e ( p y ) p r i o r to ammonolysis:

RHSiCl

2

+ 2py

> RHSiCl » 2py

R H S i C l « 2 p y + NH3 2

(8)

2

> NH C1 ( o r py.HCl) + - [ R H S i N H ] ~ 4

x

(9)

R = Me o r H

The o l i g o s i l a z a n e s formed v i a t h i s approach g i v e h i g h e r m o l e c u l a r weights than o b t a i n a b l e by d i r e c t ammonolysis. When R = H, t h e p r o d u c t is a p o l y d i s p e r s e d polymer w i t h a weight average m o l e c u l a r weight (M^j) o f « 1300 D. The polymer is n o t s t a b l e and c r o s s l i n k s t o form an i n t r a c t a b l e p r o d u c t in a s h o r t time in t h e absence o f solvent. P y r o l y s i s o f t h i s m a t e r i a l g i v e s a n 80% c e r a m i c y i e l d o f s i l i c o n n i t r i d e mixed w i t h s i l i c o n . S i l i c o n is found even when p y r o l y s i s is conducted under an ammonia atmosphere. The t e n t a t i v e l y proposed o l i g o m e r s t r u c t u r e is ( 2 5 ) :

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

128

INORGANIC AND ORGANOMETALLIC POLYMERS

/

H sr

N

SiH

3

SiH

2

\

-SIH

2

J.

S 2

1

H

^

N

cm iH . 2 IH

2

A

„

S I H

2

H

Of i n t e r e s t is t h e f a c t t h a t IR and NMR e v i d e n c e p o i n t t o t h e p r e s e n c e o f SiH3 groups in the o l i g o s i l a z a n e . The SiH3 groups must a r i s e as a consequence o f a r e d i s t r i b u t i o n p r o c e s s , a l t h o u g h further study remains t o v e r i f y thes polymer, - [ M e H S i N H ] - , ca l y s i s of t h i s m a t e r i a l give hig yields compositio the c e r a m i c p r o d u c t was not r e p o r t e d . x

R i n g Opening P o l y m e r i z a t i o n Base and a c i d c a t a l y z e d r i n g opening p o l y m e r i z a t i o n o f c y c l o t r i s i l o x a n e , - [ M e 2 S i O ] 3 - [e.g. r e a c t i o n ( 1 0 ) ] , is a well-known method o f g e n e r a t i n g h i g h m o l e c u l a r weight p o l y s i l o x a n e s .

L

R0- + χ [ M e S i O ] - l 2

>

3

R0-

RO-[Me SiO] 2

3 χ

(10)

= a l k o x i d e or hydroxide

A number of groups have attempted t o d e v e l o p t h e analogous r e a c t i o n f o r c y c l o t r i - and c y c l o t e t r a s i l a z a n e s . Ring opening p o l y m e r i z a t i o n r e p r e s e n t s an a l t e r n a t i v e t o d i r e c t ammonolysis even though t h e c y c l o m e r p r e c u r s o r s a r e n o r m a l l y made by ammonolysis o r a m i n o l y s i s . Thus, A n d r i a n o v e t a l . (26) attempted t o c a t a l y z e p o l y m e r i z a t i o n of a number o f a l k y l and a l k y l / a r y l c y c l o s i l a z a n e s u s i n g c a t a l y t i c amounts of KOH o r o t h e r s t r o n g bases a t temperatures o f up t o 300°C. I n g e n e r a l , the r e a c t i o n s proceed w i t h e v o l u t i o n o f NH3, hydrocarbons and t h e f o r m a t i o n o f i n t r a c t a b l e , c r o s s l i n k e d , b r i t t l e p r o d u c t s even at low t e m p e r a t u r e s . C o n t r a r y t o what is o b s e r v e d w i t h c y c l o t r i s i l o x a n e s , no e v i d e n c e was found f o r the f o r m a t i o n o f l i n e a r p o l y ­ silazanes. C o p o l y m e r i z a t i o n o f m i x t u r e s o f c y c l o s i l a z a n e s and c y c l o s i l o x a n e s gave somewhat more t r a c t a b l e polymers w i t h l e s s e v o l u t i o n o f h y d r o c a r b o n s o r ammonia, however v e r y l i t t l e was done to c h a r a c t e r i z e t h e r e s u l t i n g m a t e r i a l s . An e x p l a n a t i o n f o r the l a c k o f f o r m a t i o n o f l i n e a r p o l y s i l a z a n e s in base c a t a l y z e d r i n g opening is suggested by one o f F i n k ' s p u b l i c a t i o n s ( 7 ) . F i n k o b s e r v e s t h a t o l i g o s i l a z a n e amides w i l l undergo f r a g m e n t a t i o n t o s i m p l e amides under t h e c o n d i t i o n s employed by A n d r i a n o v :

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

129

Synthetic Routes to Polysilazanes

10. LAINE ET AL.

-> L i N ( S i R )

R3SiN(Li)SiR N(SiR )2 2

3

3

+ 0.5(R SiNSiR )

2

3

3

(Π)

2

Presumably, any type o f s t r o n g base w i l l r e a c t w i t h c y c l o s i l a z a n e s t o form amido s p e c i e s . I f amido s p e c i e s a r e e x t r e m e l y s u s c e p t i b l e t o f r a g m e n t a t i o n then r e a c t i o n s such as r e a c t i o n (11) would promote depolymerization r a t h e r than p o l y m e r i z a t i o n . Consequently, the only way p o l y m e r i c s p e c i e s c a n form would be t h r o u g h a c o n d e n s a t i o n p r o c e s s that l e a d s t o t r i s u b s t i t u t e d n i t r o g e n s and f u s e d r i n g s . Rochow e t a l . (27-28) have examined a c i d c a t a l y z e d r i n g o p e n i n g p o l y m e r i z a t i o n as an approach t o t h e f o r m a t i o n o f p o l y s i l a z a n e s . They f i n d t h a t h e a t i n g c y c l o t r i - and c y c l o t e t r a s i l a z a n e s w i t h ammo­ nium h a l i d e c a t a l y s t s a t temperatures o f 160°C f o r 6-8 h r e s u l t s in a c o n d e n s a t i o n p o l y m e r i z a t i o n p r o c e s s t h a t e v o l v e s ammonia and l e a d s t o the f o r m a t i o n o f waxy p o l y s i l a z a n e s . A n a l y t i c a l r e s u l t s s u g g e s t t h a t t h e s e p o l y s i l a z a n e s c o n s i s t o f r i n g s l i n k e d by s i l y l b r i d g e s as i l l u s t r a t e d by t h e f o l l o w i n

SiR

R Si 2

2

Polymers of t h i s type were found t o have t h e h i g h e s t m o l e c u l a r w e i g h t , ^ * 10K D, of any p o l y s i l a z a n e s produced t o d a t e . U n f o r t u n a t e l y , no one has examined t h e u t i l i t y o f t h e s e compounds as preceramic polymers. Rochow e t a l . (29) a l s o r e p o r t t h a t h e a t i n g t r i - and c y c l o t e t r a ­ s i l a z a n e s under ammonia l e a d s t o the f o r m a t i o n o f l i n e a r o l i g o d i m e t h y l s i l a z a n e s , r e a c t i o n (12); however, they were never a b l e

NH

3

+ ^[Me SiNH] -J 2

> NH -[Me SiNH] -H

3

2

2

(12)

x

t o o b t a i n c o n v e r s i o n s much g r e a t e r than 10% and m o l e c u l a r were always on t h e o r d e r o f ^ * 1200 D.

weights

R i n g opening p o l y m e r i z a t i o n c a n a l s o be c a t a l y z e d by t r a n s i t i o n m e t a l s as shown in r e a c t i o n (13) ( 1 2 , 3 0 ) . The p r o c e s s appears t o i n v o l v e h y d r o g e n o l y s i s o f t h e Si-N bond, g i v e n t h a t r e a c t i o n (13) Ru (C0) /135°C/lh/H 3

1 2

2

(Me Si) NH + ^[Me SiNH] 3

2

2

4

Me Si-[Me SiNH] ~SiMe 3

2

x

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

3

(13)

INORGANIC AND ORGANOMETALLIC

130

POLYMERS

r e q u i r e s o n l y l h t o e q u i l i b r a t e in t h e p r e s e n c e o f as l i t t l e as 1 atm of and more t h a n 30h w i t h o u t a s o u r c e o f hydrogen. The v a l u e o f χ in M e S i - [ M e 2 S i N H ] - S i M e v a r i e s depending on t h e r e l a t i v e p r o p o r t i o n s of the capping agent, ( M e S i ) N H , t o the t e t r a c y c l o m e r . Because the c a t a l y s t i n d i s c r i m i n a n t l y c l e a v e s S i - N bonds in both the t e t r a c y c l o m e r and t h e p r o d u c t o l i g o m e r s in r e a c t i o n ( 1 3 ) , e q u i l i b r a ­ t i o n p r e v e n t s t h e growth o f h i g h m o l e c u l a r weight s p e c i e s . 3

x

3

3

2

I f r e a c t i o n (13) is c a r r i e d out under H2 in the absence o f c a p p i n g agent then o l i g o m e r s w i t h M « 2000 D c a n be i s o l a t e d f o l l o w i n g d i s t i l l a t i o n o f t h e v o l a t i l e c y c l o m e r s . We b e l i e v e t h a t in t h i s case the p r o d u c t polymers a r e both condensed r i n g systems and hydrogen capped l i n e a r o l i g o m e r s . n

None o f the above d e s c r i b e d r i n g opening p o l y m e r i z a t i o n methods h a s , as y e t , proved u s e f u l f o r the f o r m a t i o n o f p o l y s i l a z a n e p r e c e r a m i c polymers. However, S i - N bond c l e a v a g e and r e f o r m a t i o n as i t o c c u r s in r e a c t i o n ( 1 3 ) , is p r o b a b l thermoset s t e p in t r a n s i t i o m e r i z a t i o n o f h y d r i d o s i l a z a n e s ( 3 1 ) , as d e s c r i b e d below. Deamination/Condensation

Polymerization Reactions

R e a c t i o n (4) was the f i r s t r e a c t i o n s u c c e s s f u l l y used t o s y n t h e s i z e preceramic p o l y s i l a z a n e s . Verbeek e t a l . found t h a t f u s i b l e p o l y ­ s i l a z a n e r e s i n s c o u l d be produced by p y r o l y s i s o f b i s - o r t r i s a l k y l a r a i n o s i l a z a n e s ( o r m i x t u r e s ) (15,16):

R Si(NHMe) 2

RSi(NHMe)

Δ

2

0

0

8

0

0

2

Δ

5

2

3

R

s

0

C

/

3

h

c

—>

M e N I l 2

> MeNH

2

+ L(R siNMe) 2

3

+ polymer

+ polymer

(14)

(15)

Me o r P h e n y l

The c y c l o t r i s i l a z a n e (R = Me) produced in r e a c t i o n (14) is r e c y c l e d a t 650°C [by r e a c t i o n w i t h MeNH the r e v e r s e o f r e a c t i o n (14)] t o i n c r e a s e the y i e l d o f p r o c e s s i b l e polymer. Physicochemical char­ a c t e r i z a t i o n o f t h i s m a t e r i a l shows i t t o have a s o f t e n i n g p o i n t a t 190°C and a C : S i r a t i o of 1:1.18. F i l a m e n t s 5-18 μιη in d i a m e t e r can be spun a t 315°C. The p r e c u r s o r f i b e r is then r e n d e r e d i n f u s i b l e by exposure t o a i r and t r a n s f o r m e d i n t o a c e r a m i c f i b e r by h e a t i n g t o 1200°C under Ν . The ceramic y i e l d is on the o r d e r o f 54%; a l t h o u g h , the c o m p o s i t i o n o f the r e s u l t i n g amorphous p r o d u c t is n o t r e p o r t e d . The approach used by Verbeek is q u i t e s i m i l a r to t h a t employed by Y a j i m a e t a l . (13) in the p y r o l y t i c p r e p a r a t i o n o f p o l y c a r b o s i l a n e and i t s t r a n s f o r m a t i o n i n t o S i C f i b e r s . 2>

2

R e a c t i o n (15) has r e c e n t l y been s t u d i e d in g r e a t e r d e t a i l by a group at M a r s h a l Space F l i g h t C e n t e r (31-33). Wynne and R i c e propose ( 2 ) the f o l l o w i n g s t r u c t u r e f o r t h e p o l y s i l a z a n e produced when R = Me:

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

10. LAINE ET AL.

Synthetic Routes to Polysilazanes

131

CHo

s

£ y

, < H CHo

NH

I

Jx

CHo

The M a r s h a l l group has o p t i m i z e d r e a c t i o n (14) t o o b t a i n a p o l y ­ s i l a z a n e w i t h Μ^^ » 4000 D a l tons which can be hand drawn t o g i v e 1020 μιη p r e c e r a m i c f i b e r s exposure to humid a i r an c e r a m i c y i e l d s , 5 5 % , as found by Verbeek e t a l . The c e r a m i c p r o ­ d u c t s a r e m a i n l y amorphous S i C and S i ^ N ^ w i t h some Si02 ( a con­ sequence of the h u m i d i t y t r e a t m e n t s ) . +

U n f o r t u n a t e l y , the c h e m i s t r y of the d e a m i n a t i o n / c o n d e n s a t i o n process is p o o r l y u n d e r s t o o d ; t h u s , l i t t l e can be s a i d about the mechanisms and k i n e t i c s of the amine e l i m i n a t i o n s t e p , the n a t u r e of the " s i l a i r a i n e " (7,8) i n t e r m e d i a t e or the c o n d e n s a t i o n s t e p . I t seems r e a s o n a b l e t o p r e d i c t t h a t i f one c o u l d l e a r n to c o n t r o l the r e l a t i v e r a t e s f o r these two s t e p s , more c o n t r o l c o u l d be e x e r t e d over the c o n d e n s a t i o n p r o c e s s and the p r o p e r t i e s of the p r e c u r s o r polymer. Si-Cl/Si-N R e d i s t r i b u t i o n Polymerization Reactions In the p a s t f i v e y e a r s , r e s e a r c h e r s a t Dow C o r n i n g have made a c o n c e n t r a t e d e f f o r t to e x p l o r e and d e v e l o p the use of S i - C l / S i - N r e d i s t r i b u t i o n r e a c t i o n s as a means of p r e p a r i n g t r a c t a b l e , p o l y s i l a ­ zane p r e c u r s o r s t o S13N4 ( 3 4 - 4 2 ) . I n i t i a l work f o c u s s e d on the r e a c t i o n of c h l o r o s i l a n e s and c h l o r o d i s i l a n e s w i t h h e x a m e t h y l disilazane:

MeSiCl

3

+

(Me Si) NH

MeCl SiSiMeCl 2

3

2

> Me SiCl

2

+

3

(Me Si) NH 3

+

Polymer

> Me SiCl

2

3

+

(16)

Polymer

(17)

S u r p r i s i n g l y , p y r o l y s i s of these p o l y s i l a z a n e polymers gave m o s t l y S i C r a t h e r than the e x p e c t e d S i N . I t was l a t e r found t h a t the use of H S i C l o in p l a c e of MeSiClo p r o v i d e s u s e f u l p r e c u r s o r s t o S Î O N A (42): 3

4

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

132

INORGANIC AND ORGANOMETALLIC POLYMERS

HN(SiMe ) 3

2

+ HS1CI3

> Me SiCl + 3

-[HSi(NH) 5] [HSiNH(NHSiMe )] χβ

^

χ

3

(18)

« 3,500 D a l t o n s « 15,000 D a l t o n s

The sequence o f r e d i s t r i b u t i o n r e a c t i o n s l e a d i n g t o t h e polymer in r e a c t i o n (18) is i l l u s t r a t e d by t h e f o l l o w i n g :

shown

HSiCl

(19)

3

+ (Me Si) NH 3

Me SiNHSiHCl 3

2

3

3

+ (Me Si) NH

(Me SiNH) SiHCl 3

> Me SiCl + Me SiNHSiHCl

2

2

3

2

> Me SiCl + (Me SiNH) SiHCl

2

3

3

(20)

2

+ (Me Si) 3

2(Me SiNH)SiHCl

> ( M e S i ) N H + Me SiNHSiHClNHClHSiNHSiMe

3

3

2

(22)

3

The polymer produced in r e a c t i o n (18) c a n be spun t o g i v e 15-20 μ m fibers. These f i b e r s must then be made i n f u s i b l e p r i o r t o b e i n g p y r o l y z e d in o r d e r t o o b t a i n h i g h q u a l i t y c e r a m i c f i b e r s . The c u r i n g p r o c e s s i n v o l v e s vapor phase r e a c t i o n o f t h e spun p r e c u r s o r w i t h HSiCl . T h i s f i n a l r e d i s t r i b u t i o n r e a c t i o n removes r e s i d u a l M e S i groups, reduces c a r b o n c o n t e n t and c r o s s l i n k s t h e p r e c u r s o r p o l y ­ silazane. A f t e r a p p r o p r i a t e p r o c e s s i n g t h e r e s u l t i n g amorphous ceramic f i b e r c o n s i s t s o f 96% S i N , 2% C and 2% 0 and the o v e r a l l c e r a m i c y i e l d is a p p r o x i m a t e l y 45-55%. 3

3

3

4

C a t a l y t i c Dehydrocoupling—Dehydrocyclization Reactions C a t a l y t i c d e h y d r o c o u p l i n g , as shown in r e a c t i o n ( 3 ) , was p i o n e e r e d by F i n k (6-10) and l a t e r s t u d i e d by A n d r i a n o v e t a l ( 4 3 ) . A l t h o u g h F i n k d e s c r i b e d the s y n t h e s i s o f the f i r s t p o l y s i l a z a n e s , (9) r e a c t i o n ( 2 3 ) , t h i s r o u t e t o o l i g o - and p o l y s i l a z a n e s remained u n e x p l o r e d

H NRNH 2

2

+ 2R SiH 2

é 2

°

b

> H

2

+ -[RN^-R Si) N] 2

2

x

u n t i l the work o f S e y f e r t h and Wiseman, (11,20-22,44,45) twenty later.

(23) years

S e y f e r t h and Wiseman f i n d t h a t cyclomers and o l i g o m e r s o f t h e type - [ M e S i H N H ] - , produced by ammonolysis o f M e S i H C l , w i l l r e a c t w i t h a s t r o n g base, eg KH, t o undergo " d e h y d r o c y c l i z a t i o n " , r e a c t i o n ( 2 4 ) , a k i n t o r e a c t i o n ( 2 3 ) . The r e s u l t i n g p r o d u c t s a r e s o l u b l e , t r a c t a b l e , s h e e t l i k e polymers t h a t can be spun i n t o f i b e r s and g i v e e x t r e m e l y h i g h c e r a m i c y i e l d s upon p y r o l y s i s . x

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

10.

LAINE ET AL.

133

Synthetic Routes to Polysilazanes

1%KH

•[MeSiHNH] x

x » 4.9

Me

Me

H

„

"\/~- R S i ^ - N R ) S i R (26) 2

2

1

2

M o d e l i n g s t u d i e s where R - R = Me p r o v i d e e v i d e n c e in f a v o r o f a s i l a i m i n e i n t e r m e d i a t e ; however, e f f o r t s t o t r a p t h e s i l a i m i n e s p e c i e s were u n s u c c e s s f u l . Thus, t h e s u p p o r t f o r a s p e c i f i c r i n g c l o s u r e mechanism remains i n c o n c l u s i v e .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

134

INORGANIC AND ORGANOMETALLIC POLYMERS

T r a n s i t i o n Metal Catalyzed

Dehydrocoupling Polymerization

Reactions

T r a n s i t i o n m e t a l c a t a l y z e d d e h y d r o c o u p l i n g , r e a c t i o n ( 2 7 ) , o f f e r s an a l t e r n a t i v e to the a l k a l i metal d e h y d r o c y c l i z a t i o n process

R SiH 2

f

+ R NH

2

catalyst

2

>

^

+

[ R 2

s

i N R

»

(27)

] χ

and p r o v i d e s r o u t e s t o d i f f e r e n t types o f o l i g o - and p o l y s i l a z a n e s (12,46-50). We have examined, in a p r e l i m i n a r y f a s h i o n , t h e k i n e t i c s and mechanisms i n v o l v e d in r e a c t i o n (27) u s i n g r e a c t i o n (28) ( 5 0 ) : Ru (C0) /70°C/THF 3

2

T

Et SiH

+ R NH

3

> H

2

2

+ Et SiHNR'

(28)

3

We f i n d t h a t the r a t e s o (28) a r e governed by an example, when R* = n-Pr, η-Bu o r s-Bu, t h e r a t e o f r e a c t i o n e x h i b i t f i r s t o r d e r dependence on [ E t S i H ] a t c o n s t a n t amine c o n c e n t r a t i o n . However, the r a t e o f r e a c t i o n e x h i b i t s i n v e r s e n o n - l i n e a r dependence on [n-PrNH ] and [n-BuNH ], but p o s i t i v e n o n - l i n e a r dependence on [ s BuNH ] a t c o n s t a n t [ E t S i H ] . F u r t h e r m o r e , i f R* = t-Bu, then t h e r a t e of r e a c t i o n is almost independent of both [t-BuNH ] and [ E t S i H ] . S t u d i e s o f the r a t e dependence on c a t a l y s t c o n c e n t r a t i o n f o r r e a c t i o n (28) where R'NH is n-BuNH r e v e a l r e l a t i v e c a t a l y s t a c t i v i t i e s t h a t a r e i n v e r s e l y dependent on [ R u ( C 0 ) ] . Similar studies with R NH = t-BuNH r e v e a l t h a t t h e r a t e o f r e a c t i o n is l i n e a r l y dependent on [Ru (C0)^ ]. P i p e r i d i n e is u n r e a c t i v e under the r e a c t i o n c o n d i t i o n s studied. 3

2

2

2

3

2

2

3

2

f

3

1 2

2

2

3

2

Based on our p r e v i o u s work w i t h R u ( C O ) ^ / a m i n e c a t a l y t i c systems, we can propose mechanisms t h a t account f o r these o b s e r v a t i o n s ; a l t h o u g h , a d d i t i o n a l work w i l l have t o performed t o v a l i d a t e some o f our assumptions. The r a t e vs r e a c t a n t c o n c e n t r a t i o n s t u d i e s suggest t h a t t h e r e a r e t h r e e d i f f e r e n t r a t e d e t e r m i n i n g s t e p s in t h e c a t a l y t i c c y c l e f o r r e a c t i o n ( 2 8 ) , depending on the s t e r i c demands o f t h e amine. I n t h e c a s e s where t h e r a t e o f r e a c t i o n is i n v e r s e l y depen­ dent on [RNH ], one c a n assume t h a t t h e amine is n o t p a r t i c i p a t i n g in the r a t e d e t e r m i n i n g step o r t h e r e a r e competing r e a c t i o n s where t h e dominant r e a c t i o n is i n h i b i t i o n . The c o n c l u s i o n then is t h a t t h e r a t e d e t e r m i n i n g s t e p is p r o b a b l y o x i d a t i v e a d d i t i o n o f E t S i H t o t h e a c t i v e c a t a l y s t . The i n v e r s e dependence c a n be i n t e r p r e t e d in l i g h t of Kasez e t a l * s work (51) on the low temperature r e a c t i o n s o f p r i m a r y amines w i t h R u ( C 0 ) ^ , and our work on a m i n e / R u ( C O ) ^ i n t e r a c t i o n s (52,53). 3

2

2

3

3

2

3

2

Kaesz e t a l . have shown t h a t s i m p l e , p r i m a r y amines ( e . g . MeNH ) w i l l r e a c t w i t h R u ( C O ) ^ t o form μ-acetamido l i g a n d s a t t e m p e r a t u r e s as low as -15°C. We f i n d t h a t s i m p l e p r i m a r y , s e c o n d a r y and t e r t i a r y a l k y l amines w i l l r e a c t w i t h R u ( C O ) a t t e m p e r a t u r e s o f 70-150°C t o undergo c a t a l y t i c d e u t e r i u m f o r hydrogen exchange r e a c t i o n s on t h e h y d r o c a r b o n groups and t r a n s a l k y l a t i o n ( 5 2 ) . We have found t h a t a 2

3

2

3

1 2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

10.

135

Synthetic Routes to Polysilazanes

LAINE ET AL.

v a r i e t y o f a l i p h a t i c and a r o m a t i c amines w i l l r e a c t w i t h M 3 ( C 0 ) (M = Ru o r Os) by b i n d i n g through t h e n i t r o g e n c o i n c i d e n t w i t h o x i d a ­ t i v e a d d i t i o n o f an a l p h a C-H bond t o form ^ - i m i n i u m ) H M 3 ( C O ) ^ and ^ - i m i n i u m ) 2 H M 3 ( C O ) 9 complexes ( 5 3 ) . We suggest t h a t w i t h n-PrNH and n-SuNH t h e r e is s u c c e s s f u l c o m p e t i t i o n between amine and s i l a n e f o r s i t e s o f c o o r d i n a t i v e u n s a t u r a t i o n on the a c t i v e c a t a l y s t species. 1 2

2

2

2

W i t h r e g a r d t o t h e s-BuNH r e s u l t s , i t is l i k e l y t h a t s t e r i c b u l k , e s p e c i a l l y a t t h e t e r t i a r y C-H a l p h a t o the NH - group, l i m i t s i t s a b i l i t y t o b i n d t o t h e a c t i v e c a t a l y s t s i t e and t h e r e f o r e i t cannot compete w i t h E t 3 S i H , a l t h o u g h i t can s t i l l f u n c t i o n as a r e a c t a n t . I f t h i s l o g i c is c o r r e c t , then we c a n a l s o suggest t h a t S i - N bond f o r m a t i o n p r o b a b l y o c c u r s by n u c l e o p h i l i c a t t a c k d i r e c t l y on t h e s i l i c o n moiety bound t o m e t a l r a t h e r than t h r o u g h i n i t i a l l i g a t i o n a t the m e t a l f o l l o w e d by r e a c t i o n . Because t h e r a t e o f s i l a z a n e forma­ t i o n is dependent on both [Et3SiH] and [s-BuNH ] we s u g g e s t t h a t t h e r a t e l i m i t i n g s t e p in t h i bond. On c h a n g i n g amin s i g n i f i c a n t r e d u c t i o n in the r e a c t i o n r a t e . As d i s c u s s e d above, t h e f a c t t h a t t h i s r a t e is almost c o m p l e t e l y independent o f changes in b o t h [ E t 3 S i H ] and [t-BuNH ] suggests t h a t c a t a l y s t a c t i v a t i o n must become t h e r a t e d e t e r m i n i n g s t e p . 2

2

2

An a l t e r n a t e r a t i o n a l e t o t h e above c y c l e c a n be proposed i f t h e amine s e r v e s as both a s p e c t a t o r l i g a n d and r e a c t a n t in t h e c a t a l y t i c cycle. I n the case o f n-PrNH and n-BuNH , t h e complex c o n t a i n i n g the s p e c t a t o r l i g a n d can add a second amine ( c a u s i n g i n h i b i t i o n ) o r i t can r e a c t w i t h s i l a n e . I n h i b i t i o n may i n c l u d e s t a b i l i z a t i o n o f the c l u s t e r towards f r a g m e n t a t i o n g i v e n t h e s t a b i l i t y o f t h e (μ^iminium)HM3(C0) Q and ^ - i m i n i u m ) H M 3 ( C 0 ) 9 complexes ( 5 3 ) . In this r e g a r d , we f i n d t h a t p i p e r i d i n e w i l l r e a c t w i t h R u 3 ( C 0 ) t o form v e r y s t a b l e b i s ( p i p e r i d i n o ) c l u s t e r complexes ( 5 3 ) . I f we attempt t o c a r r y out r e a c t i o n (28) u s i n g p i p e r i d i n e , a s t e r i c a l l y undemanding secondary amine, we observe no r e a c t i o n . I f p i p e r i d i n e is a c t i n g a s a l i g a n d t o t o t a l l y d e a c t i v a t e by i n h i b i t i o n , then a d d i t i o n o f p i p e r i d i n e t o r e a c t i o n (28) r u n w i t h n-BuNH s h o u l d i n h i b i t o r t o t a l l y poison the r e a c t i o n . I n f a c t , t h e a d d i t i o n o f more than one equivalent of p i p e r i d i n e per equivalent of c a t a l y s t served only t o s l i g h t l y a c c e l e r a t e t h e r e a c t i o n r a t h e r than i n h i b i t i t . Thus, we must c o n c l u d e t h a t the s p e c t a t o r l i g a n d concept does n o t appear t o be valid. These r e s u l t s s u p p o r t t h e f o l l o w i n g v e r y g e n e r a l mechanism f o r d e h y r o c o u p l i n g as c a t a l y z e d by M = R u 3 ( C 0 ) : 2

2

2

1

2

2

1 2

2

1 2

M + RNH

2

M + Et SiH 3

E t S i M H + RNH 3

;

^

^

M(RNH ) 2

> Et SiMH 3

-> E t S i N H R + MH 3

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

136

There appear to be c o n s i d e r a b l e s t e r i c demands i n v o l v e d in f o r m a t i o n of the S i - N bond in r e a c t i o n ( 2 8 ) . Support f o r the importance of s t e r i c c o n s t r a i n t s in d e h y d r o c o u p l i n g comes from s t u d i e s on the s y n t h e s i s of o l i g o s i l a z a n e s from PhSiU^ and NH3. When r e a c t i o n (29) is run at 60°C., NMR and e l e m e n t a l a n a l y s i s Ru (C0) /60°C/THF 3

PhSiH

3

1 2

+ NH3

> H

2

+ H-[PhSiHNH] -Η

(29)

χ

viscous

oil,M

«

n

1000

c o n f i r m t h a t the r e s u l t i n g o l i g o m e r is e s s e n t i a l l y l i n e a r ( 4 9 ) . In o r d e r f o r t h i s to o c c u r , c h a i n growth must o c c u r by s t e p w i s e a d d i t i o n at the PhSiH2 end caps w i t h o u t c o m p e t i t i o n from r e a c t i o n a t the i n t e r i o r Si-Η bonds. T h i s i m p l i e s t h a t s t e r i c s e l e c t i v i t y a f f e c t s the f a c i l i t y of the o x i d a t i v e a d d i t i o n s t e p . I f the o l i g o m e r produced in r e a c t i o n (29 90°C., h i g h e r m o l e c u l a r weigh e l e m e n t a l a n a l y s i s now i n d i c a t pendan group and imino c r o s s l i n k s as shown in r e a c t i o n ( 3 0 ) . 2

Ru (CO) /90°C/16h — > H 3

H-[PhSiHNH] -H + NH x

3

1 2

2

+

N H

0.5

-[PhSiHNH] [PhâiNH] -

(30)

x y

solid, M « ' η

1400

The s t r u c t u r a l changes t h a t o c c u r a t 90°C are a g a i n i n d i c a t i v e o f steric constraints. We f i n d no e v i d e n c e t h a t the N-Il bonds in the - [ P h S i H N H ] - backbone p a r t i c i p a t e in the c r o s s l i n k i n g p r o c e s s o b s e r v e d in (30) w h i c h is in k e e p i n g w i t h our o b s e r v a t i o n concerning p i p e r i d i n e * s l a c k of r e a c t i v i t y . We a l s o o b s e r v e s i m i l a r b e h a v i o r f o r r e a c t i o n s of n - h e x y l s i l a n e w i t h NH (23). χ

3

The f o l l o w i n g r e a c t i o n s were performed to demonstrate the u t i l i t y o f t r a n s i t i o n m e t a l d e h y d r o c o u p l i n g f o r the s y n t h e s i s of d i f f e r e n t types of p o l y s i l a z a n e s and a n o v e l p o l y s i l o x a z a n e : Ru (C)

/60°C/THF

3

HMe SiNHSiMe H + NH3 2

> H

2

2

+ -[Me SiHN] 2

Mn

(31)

x

« 2000

(volatiles

distilled

off)

Ru (C0) /60°/THF 3

xHMe SiNHSiMe H + NH3 2

2

+ yPhSiH

1 2

>

3

H

2

+

[HMe SiNHSiMe H] [PhSiHNH] 2

2

x

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

y

(32)

10. LAINE ET AL.

137

Synthetic Routes to Polysilazanes

Ru (C0) /60°/THF 3

1 2

H M e S i 0 S i M e H + NH< 2

-> H

2

2

+ - [ M e S i H N M e S i O ] - (33) 2

2

x

Mn « 5000-7000

P r e c e r a m i c Polymers

by D e h y d r o c o u p l i n g

We have a l s o e x p l o r e d the use o f the d e h y d r o c o u p l i n g r e a c t i o n f o r the s y n t h e s i s o f p r e c e r a m i c p o l y s i l a z a n e s s t a r t i n g from t h e p r e c u r s o r MeNH-[H SiNMe] -H. As d e s c r i b e d above, M e N H - [ H S i N M e ] - Η is produced by a m i n o l y s i s o f H S i C l w i t h MeNHo under c o n d i t i o n s where χ » 18-20 (Mn « 1150). 2

x

2

2

χ

2

S e y f e r t h and Wiseman r e p o r t e d t h a t o l i g o m e r s o f M e N H - [ H S i N M e ] - Η where χ » 10 gave a 38% ceramic y i e l d when p y r o l y z e d t o 900°C (21,22). The c e r a m i c p r o d u c Because MeNH-[H S i N M e ] bonds, t h e p o s s i b i l i t y o f f o r m i n g c h a i n extended and/or branched polymers u s i n g t h e d e h y d r o c o u p l i n g r e a c t i o n e x i s t s . We f i n d t h a t treatment o f t h i s p r e c u r s o r , as in r e a c t i o n ( 3 4 ) , does l e a d t o species with higher 2

2

χ

χ

Ru (C0) /90°C/THF 3

1 2

MeNH-[H SiNMe] -H 2

x

molecular weights. Depending on the r e a c t i o n c o n d i t i o n s and time, i t is p o s s i b l e t o produce t r a c t a b l e , p r o c e s s a b l e polymers w i t h v i s c o e l a s t i c p r o p e r t i e s u s e f u l f o r making ceramic c o a t i n g s , f i b e r s and t h r e e d i m e n s i o n a l o b j e c t s (when used as a b i n d e r f o r s i l i c o n n i t r i d e powder). With i n c r e a s e d r e a c t i o n times o r h i g h e r t e m p e r a t u r e s more c r o s s l i n k i n g o c c u r s in r e a c t i o n ( 3 4 ) , and t h e r e s u l t i n g p r o d u c t f i r s t t u r n s i n t o a g e l and then i n t o an i n t r a c t a b l e r e s i n . Figure 1 shown below i l l u s t r a t e s t h e changes in m o l e c u l a r weight and d i s p e r s i o n as r e a c t i o n (34) proceeds ( 4 9 ) . The b i m o d a l d i s t r i ­ b u t i o n s u g g e s t s t h a t t h e r e is more than one mechanism f o r polymer growth. At l e a s t one mechanism, i f not more, l e a d s t o g e l a t i o n a s e v i d e n c e d by t h e d i s p a r i t y between M » 2300 and M » 25K a t 65 h . T h i s d i s p a r i t y is t y p i c a l o f a g e l a t i o n v i a b r a n c h i n g p r o c e s s ( 5 4 ) . n

w

Table I l i s t s the molecular weights and v i s c o e l a s t i c p r o p e r t i e s f o r t h e p r e c u r s o r s and s e l e c t e d polymers produced in r e a c t i o n ( 3 4 ) . I t a l s o c o n t a i n s the c e r a m i c y i e l d s o b t a i n e d on p y r o l y s i s t o 900°C and the c o m p o s i t i o n o f t h e c e r a m i c p r o d u c t . The s a l i e n t f e a t u r e s o f the r e s u l t s summarized in Table I are : (1) The m o l e c u l a r weight and t h e s t r u c t u r e ( e x t e n t o f b r a n c h i n g and/or

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

138

Figure 1. Size exclusion chromatography of Ru3(C0)i2 catalyzed polymerization of MeNH-[H SiNMe] -H as a function of time. Polystyrene standards used f o r c a l i b r a t i o n . 2

x

Table I. Pyrolysis Studies on MeNH- [^SiNMe]-H Oligomers and Polymers X

Oligomer

-[H SiNMe] 2

x

ϊδϊ

Viscosity

Ceramic Y i e l d

SÎ3N4

(GPC)

(poise)

(% a t 900°C)

(Percent)

600-700

1

40

80-85

1150

5

45-50

80-85

1560

18

60-65

80-85

1620

100

65-70

80-85

χ = 10 -[H SiNMe] 2

x

χ = 19 -[H SiNMe] 2

x

R u ( C 0 ) / 9 0 C/THF 3

1 2

or 30h -[H SiNMe] 2

x

Ru (C0) /90°C/THF 3

1 2

f o r 65h

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

10.

LAINE ET AL.

139

Synthetic Routes to Polysilazanes

c r o s s l i n k i n g ) of the o l i g o - o r p o l y s i l a z a n e p l a y a r o l e in the t o t a l c e r a m i c y i e l d ; (2) C a t a l y t i c d e h y d r o c o u p l i n g can be used to i n c r e a s e the o v e r a l l c e r a m i c y i e l d and modify the v i s c o e l a s t i c p r o p e r t i e s of the p r e c u r s o r polymer; and, (3) The c o m p o s i t i o n of the c e r a m i c p r o d u c t (83% S i N ^ and 17%C f o r a l l p r e c u r s o r s ) appears to be d e f i n e d by the monomer u n i t and appears to be independent of m o l e c u l a r weight or v i s c o e l a s t i c p r o p e r t i e s . T h i s l a t t e r p o i n t is e x t r e m e l y i m p o r t a n t to s y n t h e t i c c h e m i s t s in t h a t i t s t r o n g l y s u g g e s t s t h a t one can use custom-designed " m o l e c u l a r b u i l d i n g b l o c k s " to s y n t h e s i z e precursors to a wide range of known m a t e r i a l s and p o s s i b l y to some t h a t a r e unknown. 3

C o n c l u d i n g Remarks I t is i n t r i g u i n g to note t h a t good s y n t h e t i c r o u t e s to t r a c t a b l e , h i g h m o l e c u l a r weight (>20K) p o l y s i l a z a n e s a r e s t i l l not a v a i l a b l e d e s p i t e the e x t e n s i v e e f f o r t s and p r o g r e s s made in the l a s t f i v e y e a r s and the f a c i l i t y w i t prepared. P a r t of the proble f o r the above d i s c u s s e d approaches a r e s t i l l p o o r l y u n d e r s t o o d . We would l i k e to suggest here some a d d i t i o n a l r e a s o n s f o r t h i s l a c k of success. C o n s i d e r the e f f e c t s of the N-R group on the s t r u c t u r e and r e a c t i v i t y of p o l y s i l a z a n e s as s u s c e p t i b l e to hydrogen bonding e f f e c t s . These e f f e c t s a l o n e s h o u l d f a v o r r i n g c l o s u r e over the r e s p e c t i v e s i l o x a n e a n a l o g s . Moreover, f o r R - H, a new type of depolymerization r e a c t i o n , analogous to r e a c t i o n ( 4 ) , is a v a i l a b l e as i l l u s t r a t e d in reaction (35). Thus, r e a c t i o n (35) c o u l d a l s o c o n t r i b u t e to the

H-[R SiNH] 2

x y

[R SiNH] 2

y

-[R SiNH] 2

x y

-H

+

[R Si«N][R SiNH] ^ -H 2

2

y

1

(35)

i n s t a b i l i t y of o l i g o s i l a z a n e s . In f a c t , the p o l y s i l o x a z a n e produced in r e a c t i o n (33) cannot r e a r r a n g e v i a r e a c t i o n (35) w h i c h may explain why we are a b l e to produce t h i s polymer w i t h such h i g h (5000-7000 D). F o r the case where R > Me, i t has a l r e a d y been shown t h a t s t e r i c e f f e c t s work in f a v o r of r i n g c l o s u r e ( 2 1 - 2 3 ) . A n o t h e r l i k e l y but h e r e t o f o r e u n r e c o g n i z e d problem w i t h the common a m m o n l y s i s / a m i n o l y s i s method of p o l y s i l a z a n e s y n t h e s i s is t h a t the b y p r o d u c t w i l l always c o n t a i n C I " s a l t s . Walsh f i n d s t h a t the S i - C l and S i - N bond s t r e n g t h s are e q u i v a l e n t (about 100 k c a l / m o l e ) ( 1 4 ) . G i v e n the w e l l known a b i l i t y of F~ t o promote c l e a v a g e and rearrangement of bonds a t s i l i c o n , i t may be t h a t C I " e x h i b i t s s i m i l a r p r o p e r t i e s w i t h the r e l a t i v e l y weak Si-N bonds (compared t o S i - 0 bonds); a l b e i t , to a l e s s e r degree than F". Support f o r t h i s i d e a comes from the i d e n t i f i c a t i o n of S i H o l i g o m e r caps formed d u r i n g the ammonol y s i s of H S i C l « 2 p y , see r e a c t i o n (9) above ( 2 4 ) . The r e d i s t r i b u t i o n r e a c t i o n n e c e s s a r y to o b t a i n the S i H groups c o u l d r e a d i l y be promoted by C I " and r e p r e s e n t s another mechanism whereby growth of h i g h m o l e c u l a r weight p o l y s i l a z a n e s is p r e v e n t e d . If redistribution 3

2

2

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

140

r e a c t i o n s to form c h a i n c a p p i n g s p e c i e s a r e promoted by C I " , then p r o c e s s e s which a v o i d ammonolysis may p r o v i d e the key t o making h i g h m o l e c u l a r weight p o l y s i l a z a n e s . The r e s u l t s shown in the F i g u r e suggest t h a t c a t a l y t i c d e h y d r o c o u p l i n g may o f f e r the o p p o r t u n i t y t o surmount t h i s problem i f a p p r o p r i a t e l y s e l e c t i v e c a t a l y s t s can be developed. I n the a r e a of p r e c e r a m i c p o l y s i l a z a n e s , s u f f i c i e n t p r o g r e s s has been made t o produce p r e c u r s o r s f o r s i l i c o n n i t r i d e f i b e r s , c o a t i n g s and as b i n d e r s f o r s i l i c o n n i t r i d e powder. However, p a r t i c u l a r problems s t i l l remain t o be s o l v e d p a r t i c u l a r l y w i t h r e g a r d t o r e d u c i n g i m p u r i t y l e v e l s and improving d e n s i f i c a t i o n d u r i n g p y r o l y s i s . Acknowledgment s We would l i k e t o thank Dr. Andrea Chow and Mr. R i c h a r d H a m l i n f o r t h e i r c o n t r i b u t i o n s in the c h a r a c t e r i z a t i o n of the polymer s p e c i e s and f o r d i s c u s s i o n s c o n c e r n i n g r a t e f u l l y acknowledge suppor D e f e n s e S c i e n c e s O f f i c e through O f f i c e of N a v a l R e s e a r c h C o n t r a c t s N00014-84-C-0392 and N00014-85-C-0668.

Literature 1. 2.

3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Cited

Chantrell, P. G. and Popper, E. P.; "Special Ceramics", E. P. Popper Ed.; New York; Academic (1964) pp. 87-102. For recent reviews see a. Wynne, K. J. and Rice, R. W.; Ann. Rev. Mater. Sci. (1984) 14, 297. b. Wills, R. R.; Mark, R. Α.; Mukherjee, S. Α.; Am. Ceram. Soc. Bull. (1983) 62, 904-915. c. Rice, R. R.; Cer. Bull. (1983) 62, 889-892. Yajima, S.; Shishido, T.; Kayano, H.; Nature (London) (1976), 264, 237. Stock, A. and Somieski, K.; Ber. Dtsch. Chem. Ges.(1921) 54, 740 see also ref 5. a. Brewer, S. D. and Haber, C. P.; J. Am. Chem. Soc. (1948) 70, 361. b. Osthoff, R. C. and Kantor, S. W.; Inorg. Synth. (1957) 5, 61. Fink, W.; Angew. Chem. Inter. Nat. Ed. (1966) 5, 760-776. Fink, W.; Chem. Ber. (1963) 96, 1071-1079. Fink, W.; Helv.Chim. Acta. (1964) 47, 498-508. Fink, W.; Helv. Chim. Acta. (1964) 49, 1408-1415. Fink, W.; Helv. Chim. Acta. (1964) 51, 954. Wiseman, G. H.; Wheeler, D. R.; and Seyferth, D.; Organomet. (1986) 5, 146-152. Blum, Y. D. and Laine, R. M.; Organomet. (1986) 5, 2081. Gilman, H.; Hofferth, B.; Melvin, H. W. and Dunn, G. E.; J. Am. Chem. Soc.(1950) 72, 5767-5768. Walsh, R.; Acc. Chem. Res. (1981) 14, 246-252. Verbeek, W.; U. S. Patent No. 3,853,567 Dec 1974. Winter, G. and Verbeek, W.; and Mansmann, M.; U. S. Patent No. 3,892,583 Jul 1975. a. Parker, D. R. and Sommer, L. H.; J. Am. Chem. Soc. (1976) 98, 618-620. b. Parker, D. R. and Sommer, L. H.; J. Organomet. Chem. (1976) 110, C1-C4. For other studies on the Si=N bond see the following ref. and ref 7.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

10. LAINE ET AL. 18. 19.

20.

21. 22.

23. 24. 25. 26.

27. 28. 29. 30. 31. 32.

33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

43.

44. 45.

Synthetic Routes to Polysilazanes

141

Elseikh; M. and Sommer, L. H.; J. Organomet. Chem. (1980) 186, 301-308. a. Van Wazer, J. R.; Moedritzer, L.; Moedritzer, K. and Groves, W.; U. S. Patent No. 3,393,218 July, 1968. b. see also Wannagat, U.; Pure and Appl. Chem. (1966)13,263-279. Seyferth, D. and Wiseman, G. H.; "Ultrastructure Processing of Ceramics, Glasses and Composites", L. L. Hench and D. R. Ulrich, Eds. (1984) pp. 265-275. Wiseman, G. H.; Ph.D. Dissertation, M. I. T., Aug. 1984. a. Seyferth, D.; Wiseman, G. H. and C. Prud'homme, J. Am. Ceram. Soc.(1983) 66, C13-C14. b. Seyferth, D. and Wiseman, G. H.; Am. Chem. Soc. Poly. Div. Prprt. (1984) 25, 10-12. Laine, R. M.; Blum, Y. D.; Dodge, A. unpublished results. Arai, M. and Isoda, T.; Japan Kokai Tokkyo Koho JP. 61 89,230. Arai, M.; Sakurada, S.; Isoda, T. and Tomizawa, T.; Am. Chem. Soc. Polymer Div. Polym. Prpts. (1987) 27, 407. a. Zhdanov, Α. Α.; Kotrelev, G. V.; Kazakova, V. V. and Redkozubova, Ye. P.; Polym. Andrianov, Κ. Α.; V.; J. Organomet. Chem. (1965) 3, 129-137. c. Andrianov, K. Α.; Kotrelev, G. V.; Kamaritski, Β. Α.; Unitski, I. H. and Sidorova, Ν. I.; J. Organomet. Chem. (1969) 16, 51-62. Rochow, E. G.; Mon. Chem. (1964)95,750-765. Kruger, C. R. and Rochow, E.; J. Poly. Sci. A, (1964) 2, 31793189. Redl, G. and Rochow, E. G; Angew. Chem. (1964) 76, 650. Zoeckler, M. T. and Laine, R. M.; J. Org. Chem. (1983)48,2539. Chow, A. W.; Hamlin, R. D.; Blum, Y. and Laine, R. M.; submitted to J. Pol. Sci. a. Penn, B. G.; Daniels, J. G.; Ledbetter, III, F. E.; and Clemons, J. M.; Poly. Eng. and Sci. (1986) 26, 1191-1194. b. Penn, B. G.; Ledbetter III, F. E.; Clemons, J. M. and Daniels, J. G.; J. Appl. Polym. Sci.(1982) 27, 3751-3761. Penn, B. G.; Ledbetter III, F. E. and Clemons, J. M.; Ind. Eng. Chem. Process. Des. Dev. (1984) 23, 217-220. Gaul Jr, J. H.; U. S. Patent No. 4,340,619 Jul 1982. Baney, R. H.; Gaul, J. H.; U. S. Patent 4,310,651 1982. Gaul Jr, J. H.; U. S. Patent No. 4,395,460 Jul 1983. Gaul Jr, J. H.; U. S. Patent No. 4,312,970 Jan 1985. Cannady, J. P.; U. S. Patent No. 4,535,007 Aug 1985. Cannady, J. P.; U. S. Patent No. 4,543,344 Sep 1985. Bujalski, D. R.; Eur. Patent Appl. EP 175,382 Mar 1986. Baney, R. H.; Bujalski, D. R.; Eur. Patent Appl. EP 175,383 Mar 1986. Legrow, G. E.; Lira, T. F.; Lipowitz, J. and Reaoch, R. S.; "Better Ceramics Through Chemistry II", Mat. Res. Symp. Proc. Vol. 73, Brinker, C. J.; Clark, D. E.; and Ulrich, D. R.; Eds. (1986) pp 553-558. Andrianov, Κ. Α.; Shkol'nik, M. I.; Syrtsova, Zh. S.; Petrov, K. I.; Kopylov, V. M.; Zaitseva, M. G. and Koroleva, Ε. Z.; Dokl. Akad. Nauk. SSSR (1975) 223, 347-350. Seyferth, D.; Prud'Homme, C.C.;Wiseman, G. H.; U. S. Patent 4,397,828 Aug 1984. Seyferth, D. and Wiseman, G. H.; U. S. Patent 4,482,669 Nov. 1984.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

142 46.

47.

48. 49.

50. 51. 52. 53. 54.

Blum, Y. D.; Laine, R. M.; Schwartz, Κ. B.; Rowcliffe, D. J.; Bening, R. C.; and Cotts, D. B. "Better Ceramics Through Chemistry II" Mat. Res. Soc. Symp. Proc., Brinker, C. J.; Clark, D. E. and Ulrich, D. R. Eds. (1986) 73, pp 389-393. Schwartz, K. B.; Rowecliffe, D. J.; Blum, Y. D. and Laine, R. M.; "Better Ceramics Through Chemistry II" Mat. Res. Soc. Symp. Proc., Brinker, C. J.; Clark, D. E. and Ulrich, D. R. Eds. (1986) 73 pp 407-412. Laine, R. M. and Blum, Y. D.;U. S. Patent 4,612,383 Sept 1986. Laine, R. M.; Blum, Y. D.; Hamlin, R. D.; Chow, Α.; in "Ultrastructure Processing of Glass, Ceramics and Composites II" D. D. Mackenzie and D. R. Ulrich, Eds., J. Wiley and Sons, (1987) in press. Biran, C.; Blum, Y. D., Laine, R. M.; Glaser, R. and Tse, D. S.; manuscript submitted for publication. Lin, Y. C.; Knobler, C. B. and Kaesz, H. D.; J. Am. Chem. Soc. (1981) 103, 1216-1218 and references therein. Wilson, Jr., W. B. 107, 361-368. Eisenstadt, Α.; Giandomenico, C.; Fredericks, M. F.; and Laine, R. M.; Organomet. (1985) 4, 2033. Billmeyer, Jr., F. W.; "Textbook of Polymer Science" 2nd Ed. John Wiley and Sons, Ν. Υ., Ν. Y., 1971 pp. 272-278.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 11

Organosilicon Polymers as Precursors for Silicon-Containing Ceramics Recent Developments Dietmar Seyferth, Gary H. Wiseman, Joanne M. Schwark, Yuan-Fu Yu, and Charles A. Poutasse Department of Chemistry Massachusett Institut

f Technology

Following general comments about the preceramic polymer approach to the preparation of ceramic materials, we describe the preparation of a novel polysilazane by dehydrocyclodimerization of the ammonolysis product of CH SiHCl , or coammonolysis products of CH SiHCl with other chlorosilanes, the conversion of such polymers to ceramic products and their use in upgrading Si-Η con­ taining organosilicon polymers, e.g., methylhydrogenpolysiloxane, to more useful ceramic precursors. 3

2

3

2

Silicon-containing ceramics include the oxide materials, silica and the silicates; the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride; silicon oxynitride and the sialons; main group and transition metal suicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the prepara­ tion of a l l of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. Silicon carbide, SiC [ 1 ] and silicon nitride, S13N4 [ 2 ] , have been known for some time. Their properties, especially high thermal and chemical stability, hardness, high strength, and a variety of other pro­ perties have led to useful applications for both of these materials. The "conventional" methods for the preparation of SiC and S13N4, the high temperature reaction of fine grade sand and coke (with additions of sawdust and NaCl) in an electric furnace (the Acheson process) for the former and usually the direct nitridation of elemental silicon or the reaction of silicon tetrachloride with ammonia (in the gas phase or in solution) for the latter, do not involve soluble or fusible intermediates. For many applications of these materials this is not necessarily a dis­ advantage (e.g., for the application of SiC as an abrasive), but for some of the more recent desired applications soluble or fusible (i.e., processable) intermediates are required. The need for soluble or fusible precursors whose pyrolysis will give the desired ceramic material has led to a new area of macromolecular science, that of preceramic polymers [3]. Such polymers are needed for a number of different applications. Ceramic powders by themselves are 0097-6156/88/0360-0143$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

difficult to form into bulk bodies of complex shape. Although ceramists have addressed this problem using the more conventional ceramics techniques with some success, preceramic polymers could, in principle, serve in such applications, either as the sole material from which the shaped body is fabricated or as a binder for the ceramic powder from which the shaped body is to be made. In either case, pyrolysis of the green body would then convert the polymer to a ceramic material, hopefully of the desired composition. In the latter alternative, shrinkage during pyrolysis should not be great. Ceramic fibers of diverse chemical compositions are sought for application in the production of metal-, ceramic-, glass- and polymermatrix composites [3c]. The presence of such ceramic fibers in a matrix, provided they have the right length-to-diameter ratio and are distributed uniformly throughout the matrix, can result in very consider­ able increases in the strength (i.e., fracture toughness) of the resulting material. T o prepare such ceramic fibers, a suitable polymeric precursor is needed, one which can be spun by melt-spinning dry-spinning or wet-spinning techniques [4 or without a prior cure step) Some materials with otherwise very useful properties such as high thermal stability and great strength and toughness are unstable with respect to oxidation at high temperatures. An example of such a class of materials is that of the carbon-carbon composites. If these materials could be protected against oxidation by infiltration of their pores and the effective coating of their surface by a polymer whose pyrolysis gives an oxidation-resistant ceramic material, then one would have available new dimensions of applicability of such carbon-carbon composite materials. In order to have a useful preceramic polymer, considerations of structure and reactivity are of paramount importance. Not every inorganic or organometallic polymer will be a useful preceramic polymer. Some more general considerations merit discussion. Although preceramic polymers are potentially "high value" products jf the desired properties result from their use, the more generally useful and practical systems will be those based on commercially available, relatively cheap starting monomers. Preferably, the polymer synthesis should involve simple, easily effected chemistry which proceeds in high yield. The preceramic polymer itself should be liquid or, if a solid, it should be fusible and/or soluble in at least some organic solvents, i.e., it should be processable. Its pyroly­ sis should provide a high yield of ceramic residue and the pyrolysis volatiles preferably should be non-hazardous and non-toxic. In the requirement of high ceramic yield, economic considerations are only secondary. If the weight loss on pyrolysis is low, shrinkage will be minimized as will be the destructive effects of the gases evolved during the pyrolysis. There are important considerations as far as the chemistry is concerned. First, the design of the preceramic polymer is of crucial importance. Many linear organometallic and inorganic polymers, even if they are of high molecular weight, decompose thermally by formation and evolution of small cyclic molecules, and thus the ceramic yield is low. In such thermolyses, chain scission is followed by "back-biting" of the reactive terminus thus generated at a bond further along the chain. Thus high molecular weight, linear poly(dimethylsiloxanes) decompose thermally principally by extruding small cyclic oligomers, (Me2SiO) , η = 3,4,5 . . . When a polymer is characterized by this type of thermal decomposition, the ceramic yield will be low and it will be necessary to convert the n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Ceramics

145

linear polymer structure to a highly cross-linked one by suitable chemical reactions prior to its pyrolysis. In terms of the high ceramic yield requirement, the ideal preceramic polymer is one which has functional substituent groups which will give an efficient thermal cross-linking process so that on pyrolysis non-volatile, three-dimensional networks (which lead to maximum weight retention) are formed. Thus, preceramic polymer design requires the introduction of reactive or potentially reactive functionality. In the design of preceramic polymers, achievement of the desired elemental composition in the ceramic obtained from them (SiC and S13N4 in the present cases) is a major problem. For instance, in the case of polymers aimed at the production of SiC on pyrolysis, it is more usual than not to obtain solid residues after pyrolysis which, in addition to S i C , contain an excess either of free carbon or free silicon. In order to get close to the desired elemental composition, two approaches have been found useful in our research: (1) The use of two comonomers in the appropriate ratio in preparation of the polymer and (2) the use of chemical or physical combination appropriate ratio. Preceramic polymers intended for melt-spinning require a compromise. If the thermal cross-linking process is too effective at relatively low temperatures ( 1 0 0 - 2 0 0 ° C ) , then melt-spinning will not be possible since heating will induce cross-linking and will produce an infusible material prior to the spinning. A less effective cross-linking process is required so that the polymer forms a stable melt which can be extruded through the holes of the spinneret. The resulting polymer fiber, however, must then be "cured", i.e., cross-linked, chemically or by irradiation, to render it infusible so that the fiber form is retained on pyrolysis. Finally, there still are chemical options in the pyrolysis step. Certainly, the rate of pyrolysis, i.e., the time/temperature profile of the pyrolysis, is extremely important. However, the gas stream used in the pyrolysis also is of great importance. One may carry out "inert" or "reactive" gas pyrolyses. An example of how one may in this way change the nature of the ceramic product is provided by one of our preceramic polymers which will be discussed in more detail later in this paper. This polymer, of composition [ ( C H 3 S i H N H ) ( C H 3 S i N ) b ] > gives a black solid, a mixture of S i C , S13N4, and some free carbon, on pyrolysis to 1 0 0 0 ° C in an inert gas stream (nitrogen or argon). However, when the pyrolysis is carried out in a stream of ammonia, a white solid remains which usually contains less than 0.5% total carbon and is essentially pure silicon nitride. At higher temperatures (>400°C), the N H 3 molecules effect nucleophilic cleavage of the Si-C bonds present in the polymer and the methyl groups are lost as C H 4 . Such chemistry at higher temperatures can be an important and sometimes useful part of the pyrolysis process. The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C / l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 S1O2. (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). The Yajima polycarbosilane, while it was one of the first, is not the only polymeric precursor to silicon carbide which has been developed. a

m

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

Another useful system which merits mention is the polycarbosilane which resulted from research carried out by C . L . Schilling and his coworkers in the Union Carbide Laboratories in Tarrytown, New York [6]. More recently, a useful polymeric precursor for silicon nitride has been developed by workers at Dow Corning Corporation [7].

New Silicon-Based Preceramic Polymer Systems: Recent Research at

MXT:

In earlier work [ 8 ] , we have developed a process for the preparation of useful preceramic polymers using commercially available CH3S1HCI2 as the starting material. In the initial step, this chlorosilane was treated with ammonia to give oligomeric, mostly cyclic [CH3S1HNH] (n~5). This product, itself not a useful preceramic material, was subjected to the base-catalyzed dehydrocyclodimerization (DHCD) reaction (eq. 1) in which the species present in the [ C H 3 S i H N H ] oligomer mixture are linked together via cyclodisilazane units n

n

I 2 - Si—N — I I

H

base

H

2

+ >;

(1)

(e.g., KH)

H

I

The repeating unit in the [ C H 3 S i H N H ]

n

cyclics is 1.

CH, \

H-N

CH 3

I • Si —- Ν I

I

Η

Η

Η

CH,

/

Η Η I Ν

/

Si

\

CH,

/

Si^ Η

\

N H

« H

N

CH,

For example, the cyclic tetramer is the 8-membered ring compound 2. On the basis of equation 1, the adjacent NH and SiH groups provide the functionality which permits the molecular weight of the [ C H 3 S i H N H ] cyclics to be increased. Thus, [ C H 3 S 1 H N H ] cyclics will be linked together via four-membered rings as shown in 3. n

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

11.

SEYFERTH ET AL.

Silicon-Containing

H /

R

\

\

Si

R

- N

Si-

I \ Ν

I

H R

/

\ l

-Si R

/ H

H /

R

/

Si

/

147

Ceramics

Si

H

I -N

R

\ l Si-

-N

I

I

N-

-Si

I N -

I \ N-

R

\

HI

R

H

/ -Si / \

- CH,

3

We believe that a sheet-like network polymer (which, however, is not flat) having the general functional unit composition [(CH3SiHNH) (CH3SiN)t>( C H 3 S i H N K ) ] results. When hydrogen evolution ceases, a "living" polymer with reactive silylamide functions is present. On reaction of the latter with an electrophile ( C H 3 I or a chlorosilane), the neutral poly­ silazane is formed and can be isolated in the form of an organic-soluble white solid when the reaction is carried out in T H F solution. Depending on solvent and reaction conditions, the molecular weight of this product is between 800 and 2500 g/mol. The molecular weight can be approxi­ mately doubled by using a difunctional chlorosilane (such as ClMe2SiCH2CH2SiMe2Cl) to quench the "living" polymer. Such polysilazanes, when pyrolyzed in a stream of argon, leave behind a black ceramic residue equivalent to 80-85% of the weight of the originally charged polymer. Analysis gave % Si, C and Ν values which could be rationalized in terms of a composition of (by weight) 67% S13N4, 28% SiC and 5% C . The volatiles evolved during the pyrolysis consisted of H2 and C H 4 and a trace of N H 3 . Initial evaluation of the polysilazane shows it to have promise in three of the main potential applications of preceramic polymers: in the preparation of ceramic fibers and of ceramic coatings and as a binder for ceramic powders. This polysilazane undergoes thermal crosslinking too readily to permit melt-spinning, but it can be dry-spun. It was clear that if melt-spinning was to be successful we would have to prepare a less reactive polysilazane with fewer reactive Si(H)-N(H) groups. To achieve this, we have studied polysilazanes derived by the denydrocyclodimerization of the products of coammonolysis of CH3SiHCl2 and CH3(Un)SiCl2, where Un is an unsaturated substituent such as vinyl, CH2=CH, or allyl, C H 2 C H = C H 2 . These groups were used because a melt-spun fiber requires a subsequent cure step to render it infusible. If this is not done, the fiber would simply melt on pyrolysis and no ceramic fiber would be obtained. The vinyl and allyl groups provide C=C functionality which can undergo UV-catalyzed Si-Η additions, reactions which after melt-spinning in principle should lead to extensive further crosslinking without heating. To provide the required monomers, we ammonolyzed mixtures of CH3S1HCI2 and CH (CH =CH)SiCl2 and CH3S1HCI2 and C H ( C H = C H C H ) S i C l , respectively. The r ( C H 3 S i H N H ) ( Ç R 3 ( U j i ) S i N H ) ] oligomers were a

c

n

3

2

2

3

x

American Chemical Society Library

v

2

n

1155 16th St., N.W. In Inorganic and OrganometallicD.C. Polymers; Zeldin, M., et al.; Washington, 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2

148

INORGANIC AND

ORGANOMETALLIC POLYMERS

prepared using CH3SiHCl2/CH3(Un)SiCl2 ratios of 3, 4 and 6, although other ratios can be used. This gives materials which still contain many (CH3)Si(H)-N(H)- units, so polymerization by means of the process of eq. 1 still will be possible. However, dilution of the ring systems of the cyclic oligomers with CH3(Un)Si units will decrease the cross-linking which will occur on treatment with the basic catalyst. The coammonolysis of CH3S1HCI2 and CH3(Un)SiCl2 in the indicated ratios was carried out by the procedure as described for the ammonolysis of C H 3 S 1 H C I 2 [8]. A mixture of c y c l i c oligomeric silazanes is to be expected in these reactions, [(CH3(H)SiNH) (CH3(Un)SiNH) ] , with more than one ring size present. In each preparation, the soluble, liquid products were isolated and used in the base-catalyzed polymerizations. When a ratio CH3SiHCl2/CH3(Un)SiCl2 of x : l (x>l) was used in the ammonolysis reaction, the CH3(H)Si/CH3(Un)Si ratio in the soluble ammonolysis product usually was somewhat less than x:l. The addition of the various ammonolysis products to suspensions of catalytic amounts of K H in dry tetrahydrofuran resulted in hydrogen gas evolution with formatio followed by treatment wit potassium silylamide present in solution. The resulting silazane generally was obtained in high (>90%) yield and these products, always white powders, were soluble in organic solvents such as hexane, benzene, toluene and THF. Their average molecular weights were in the 800-1200 range. The presence of unchanged vinyl and allyl groups was proven by their IR and * H NMR spectra. A typical elemental analysis led to the empirical formula S i C i 5NH4 4 which could be translated into a composition x

y

n

[(CH SiHNH)(CH3[CH =CH]SiNH)2.3]x. These polysilazanes, upon pyrolysis to 1 0 0 0 ° C under argon or nitrogen, gave black ceramic materials in good yield (73-86%, by weight). Analysis of the ceramic produced by such pyrolysis of one of these polysilazanes gave a composition 67.5 wt.% S13N4, 22.5 wt.% SiC, and 10.0 wt.% unbound carbon. Another such ceramic sample, obtained by pyrolysis of another polysilazane, had the composition 68.8 wt.% S13N4, 21.7 wt.% SiC, and 9.9 wt.% C . The possibility of "curing" fibers pulled from polysilazanes of the type prepared here was demonstrated in the following experiment: Fibers were pulled from a concentrated syrup of a polysilazane derived from a 4:1 molar ratio CH3SiHCl2/CH3(CH2=CH)SiCl2 coammonolysis product by DHCD in T H F . Some fibers were pyrolyzed without any further treatment. These melted in large part, leaving very little in the way of ceramic fibers. Other fibers were subjected to UV irradiation for 2 hours. These, on pyrolysis under argon, did not melt and ceramic fibers were obtained. This polysilazane then is a good candidate for meltspinning. In order to obtain a SÎC/SÎ3N4 mixture rich in S13N4 by the preceramic polymer route, one requires a polymer which is richer in nitrogen than the C H 3 S 1 H C I 2 ammonolysis product, (CH3SiHNH) . In designing such a preceramic polymer, one would like to retain the facile chemical and thermal cross-linking system which the Si(H)-N(H)-unit provides. We have found that the coammonolysis of C H 3 S 1 H C I 2 and H S 1 C I 3 serves our purposes well. For H S 1 C I 3 , ammonolysis introduces three Si-N bonds per silicon atom, so the ammonolysis product of C H 3 S i H C l 2 / H S i C l 3 mixtures will contain more nitrogen than the ammonolysis product of CH3S1HCI2 alone. 3

2

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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SEYFERTH ET AL.

Silicon-Containing Ceramics

149

In order to define the optimum system, we have investigated the ammonolysis of C H 3 S i H C l 2 / H S i C l 3 mixtures in various ratios in two solvents, diethyl ether, E t 0 , and tetrahydrofuran, T H F . C H 3 S i H C l / H S 1 C I 3 mol ratios of 6, 3 and 1 were examined. In both solvents, the 6:1 and 3:1 ratios produced polysilazane oils with molecular weights in the range 390-401 g/mol and 480 g/mol, respectively. When a 1:1 reactant ratio was used, waxes of somewhat higher (764-778 g/mol) molecular weights were obtained in both solvents. In the 1:1 reaction carried out in Et20 the yield of soluble product was only 40%, but in T H F it was nearly quantitative. The oils produced in the 6:1 and 3:1 reactions in Et20 appeared to be stable on long-term storage at room temperature in the absence of moisture (e.g., in the inert atmosphere box). However, the waxy product of 1:1 (Et20) reactions and all the coammonolysis products prepared in T H F formed gels (i.e., became insoluble) after 3-4 weeks at room temperature, even when stored in a nitrogen-filled dry box. The pyrolysis of the coammonolysis products was studied The 6 C H 3 S Î H C 1 2 / 1 H S 1 C I 3 ammonolysi since it contains the leas expected, low ceramic yields were obtained on pyrolysis of these products. Pyrolysis of the 3:1 products gives increased ceramic yields, while pyrolysis of the most highly cross-linked 1:1 ammonolysis products gives quite good ceramic yields, 72% for the product prepared in Et20$ 78% for that prepared in T H F . A l l of the ammonolysis products were submitted to the KH-catalyzed dehydrocyclodimerization reaction in order to obtain more highly crosslinked products that would give higher ceramic yields on pyrolysis. In all cases, the standard procedure [8] was used (1% K H in T H F , followed by quenching with C H 3 I (or a chlorosilane)). In every reaction, the product was a white solid which was produced in virtually quantitative yield. The proton NMR spectra of these products, as expected, showed an increase in the S i C H 3 / S i H + NH proton ratio, while the relative SiH/NH ratio was unchanged. In all cases, the molecular weights of the solid products were at least double that of the starting ammonolysis product, so the desired polymerization had occurred. Although the increase in molecular weight in these D H C D reactions is not great, any increase is useful for further processing. The reactions bring the advantage that the oils are converted to more easily handleable solids. Pyrolysis of the white solids obtained in these KH-catalyzec dehydrocyclodimerization reactions (under argon from 50-950°C) produced black ceramic residues, with the exception of the 1:1 T H F ammonolysisderived solid which left a brown residue. The ceramic yields were excellent (all greater than or equal to 82%, with the highest being 88%). Analysis of bulk samples of the ceramic materials produced in the pyrolysis of the various KH-catalyzed dehydrocyclodimerization products showed that our goal of a higher S i 3 N 4 / S i C ratio has been achieved: for the 1:1 ammonolysis product-derived polymers, 86% S 1 3 N 4 , 8% SiC and 5% C (THF ammonolysis) and 83% S 1 3 N 4 , 11% SiC and 6% C (Et20 ammonolysis); for the 3:1 and 6:1 ammonolysis product-derived polymers: 77% S 1 3 N 4 , 18-19% SiC and 4-5% C (Et20 ammonolysis) and 74% S 1 3 N 4 , 20% SiC and 5-6% C (THF ammonolysis). These polymers may be used in the preparation of quite pure silicon nitride if the pyrolysis is carried out in a stream of ammonia (a reactive gas) rather than under nitrogen or argon. The ammonia reacts with the 2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2

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INORGANIC AND ORGANOMETALLIC POLYMERS

polymer at higher temperatures to cleave methyl groups from silicon (eq. 2), so that essentially all carbon is lost. Thus pyrolysis of the dehydro­ cyclodimerization product of the 1:1 (THF) ammonolysis

+

CH

4

(2)

product to 1000°C in a stream of ammonia gave a white ceramic residue in high yield which contained only 0.29% C., the remainder being silicon nitride. Other commercially available RS1CI3 compounds are CH3S1CI3 (a cheap by-product of the Direct Process) and CH2=CHSiCl3 (also an inexpensive starting material) and both were included in this study. In both cases, 6:1, 3:1 and 1:1 CH3SiHCl2/RSiCl3 ammonolysis products were prepared and submitted t ceramic yields obtained o In the C H 3 S i H C l 2 / C H 3 S i C l 3 experiments: 78-86%. In all cases, a black ceramic residue resulted when the pyrolysis to 1000°C was carried out in a stream of argon. As expected, the carbon content (in the form of SiC and free C) was higher than that of the C H 3 S i H C l 2 / H S i C l 3 - d e r i v e d ceramics: 12-18% SiC, up to 9.5% C . Nonetheless, higher S13N4 contents than those obtained when C H 3 S 1 H C I 2 is used alone (-67%)) were obtained: - 76-80% S13N4. To produce a ceramic material containing only S13N4, the white solid polysilazane derived from DHCD of the oil obtained by ammonolysis of 6:1 C H 3 S i H C l 2 / C H 3 S i C l 3 was pyrolyzed in a stream of ammonia (to 1000°C). A white ceramic residue containing only 0.36% by weight C resulted. With 6:1, 3:1 and 1:1 CH3SiHCl2/CH2=CHSiCl3 ammonolysis products DHCD gave white solids whose pyrolysis resulted in increased carbon content and decreased S 1 3 N 4 (vs. the CH3SiHCl2/CH3SiCl3 examples): -69-73% S 1 3 N 4 , 9-13% SiC, 12-18% C (by weight). Our research also has been directed at SiC precursors. It began with an examination of a potential starting material in which the C:Si ratio was 1, the ratio desired in the derived ceramic product. Available methylsilicon compounds with a 1 C / l Si stoichiometry are CH3S1CI3 and CH3S1HCI2. The latter, in principle, could give [ C H 3 S i H ] cyclic oligomers and linear polymers on reaction with an alkali metal. In practice, the Si-Η linkages also are reactive toward alkali metals. Thus, mixed organochlorosilane systems containing some CH3S1HCI2 have been treated with metallic potassium by Schilling and Williams [9]. It was reported that the CH3SiHCl2~based contribution to the final product was (CH3SiH)n.2(CH3Si)o.8> i.e., about 80% of the available Si-Η bonds had reacted. Such Si-Η reactions lead to cross-linking in the product, or to formation of polycyclic species if c y c l i c products are preferred. Nevertheless, we have used this known reaction for CH3S1HCI2 with an alkali metal as an entry to new preceramic polymers. When the reaction of CH3S1HCI2 with sodium pieces was carried out in tetrahydrofuran medium, a white solid was isolated in 48% yield. This solid was poorly soluble in hexane, somewhat soluble in benzene, and quite soluble in T H F . Its ! H NMR spectrum (in C D C I 3 ) indicated that extensive reaction of S i - Η bonds had occurred. The 6 (SiH)/6(SiCH3) n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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integration led to a constitution [ ( C H 3 S i H ) o . 4 ( C H 3 S i ) o . 6 l n C H 3 S 1 H units are ring and chain members which are not branching sites; the CH3S1 units are ring and chain members which are branching sites. In our reactions it is expected that mixtures of polycyclic and linear (possibly cross-linked) polysilanes will be formed. (Attempts to distill out pure compounds from our preparations were not successful. Less than 10% of the product was volatile at higher temperatures at 1Q~4 torr.). The ceramic yield obtained when the [(CH3SiH)n.4(CH3Si)n.6]n polymer was pyrolyzed (TGA to 1000°C) was 60%; a gray-black solid "was obtained whose analysis indicated a composition 1.0 SiC + 0.49 Si. The reaction of methyldichlorosilane with sodium in a solvent system composed of six parts of hexane and one of THF gave a higher yield of product which was soluble in organic solvents. Such reactions give a colorless oil in 75 to over 80% yield which is soluble in many organic solvents. In various experiments the molecular weight (cryoscopic in benzene) averaged 520-740 and the constitution (by * H NMR) [ ( C H S i H ) n . 7 6 ( C H 3 S i ) o . 2 4 Î n to [ ( C H S i H ) . 9 ( C H S i ) o . i ] This less cross-linked material (compare gave much lower yields yields ranging from 12-27% in various runs). Again, the product was (by analysis) a mixture of SiC and elemental silicon, 1.0 SiC + 0.42 Si being a typical composition. These results are not especially promising, and it was obvious that further chemical modification of the [ ( C H 3 S i H ) ( C H 3 S i ) ] products obtained in the CH3SiHCl2/Na reactions was required [10]. A number of approaches which we tried did not lead to success, but during the course of our studies we found that treatment of the [ ( C H 3 S i H ) ( C H 3 S i ) y ] products with alkali metal amides (catalytic quantities) serves to convert them to materials of higher molecular weight whose pyrolysis gives significantly higher ceramic yields. Thus, in one example, to 0.05 mol of liquid [(CH3SiH)n.85(CH3Si)n.i5] in T H F was added, under nitrogen, a solution of about 1.25 mmol (2.5 mol%) of [(CH3)3Si]2NK in T H F . The resulting red solution was treated with methyl iodide. Subsequent nonhydrolytic workup gave a soluble white powder in 68% yield, molecular weight 1000, whose pyrolysis to 1 0 0 0 ° C gave a ceramic yield of 63%. The proton NMR spectra of this product showed only broad resonances in the S i - Η and S 1 - C H 3 regions. In the starting [ ( C H 3 S i H ) ( C H 3 S i ) y ] material, the observed proton NMR integration ratios, S1CH3/ SiH, ranged from 3.27-3.74. This ratio was quite different in the case of the product of the silylamide-catalyzed process, ranging from 8.8 to 14. Both Si-Η and Si-Si bonds are reactive toward nucleophilic reagents. In the case of the alkali metal silylamides, extensive structural reorgani­ zation, involving both S i - Η and Si-Si bonds of the [(CH3SiH) (CH3Si) ] polysilane, that results in further cross-linking, must have taken place. While these silylamide-catalyzed reactions provided a good way to solve the problem of the low ceramic yield in the pyrolysis of [(CH3SiH) ( C H 3 S i ) y ] , the problem of the elemental composition of the ceramic product remained (i.e., the problem of Si/C ratios greater than one) since only catalytic quantities of the silylamide were used. As noted above, K H - c a t a l y z e d polymerization of the CH3S1HCI2 ammonolysis product gives a polymeric silylamide of type [(CH3SiHNH) (CH3SiN) (CH3SiHNK) ]. In a typical example, a = 0.39, b = 0.57, c = 0.04, so there is only a low concentration of silylamide functions in the polymer. This polymeric silylamide reacts with electrophiles other than H

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methyl iodide, e.g., with diverse chlorosilanes, and it has been isolated and analyzed. Since it is a silylamide, we expected that it also would react with [ ( C H 3 S i H ) ( C H 3 S i ) y ] polysilane-type materials. Not only would it be expected to convert the latter into material of higher molecular weight, but it also would be expected to improve the Si/C ratio (i.e., bring it closer to 1). As noted above, pyrolysis of [ ( C H 3 S i H N H ) ( C H 3 S i N ) b ( C H 3 S i H N K ) ] gives a ceramic product in 80-85% yield containing S13N4, SiC and excess carbon. Thus, combination of the two species in the appropriate stoichiometry, i.e., of [ ( C H 3 S i H ) ( C H 3 S i ) y ] and [ ( C H 3 S i H N H ) ( C H 3 S i N ) b ( C H 3 S i H N K ) ] n > and pyrolysis of the product (which we will call a "graft" polymer) after C H 3 I quench could, in principle, lead to a ceramic product in which the excess Si obtained in pyrolysis of the former and the excess C obtained in the pyrolysis of the latter combine to give SiC. Accordingly, experiments were carried out in which the two polymer systems, [ ( C H 3 S i H ) ( C H 3 S i ) ] and the "living" polyme ilylamide, [(CH SiHNH) (CH SiN) (CH SiHNK) ] were mixed in THF solution in allowed to react at room experiments were carried out with the [(CH3SiH)x(CH3Si) ] materials prepared in hexane/THF as well as with those prepared in alone.) After quenching with methyl iodide, nonhydrolytic workup gave a new polymer in nearly quantitative yield (based on weight of material charged). The molecular weight of these products was in the 1800-2500 range. Their pyrolysis under nitrogen gave ceramic products in 74-83% yield. Thus, the reaction of the two polymer systems gives a new polymer in close to quantitative yield, which seems to be an excellent new preceramic polymer in terms of ceramic yield. In an alternative method of synthesis of [ ( C H 3 S i H ) ( C H 3 S i ) y ] [ ( C H 3 S i H N H ) ( C H 3 S i N ) ] "combined" polymers, the polysilyl amide was generated in situ in the presence of [ ( C H 3 S i H ) ( C H 3 S i ) y ] . This, however, gave materials that were somewhat different. In one such experiment, a mixture of ( C H 3 S i H N H ) cyclics (as obtained in the ammonolysis of C H 3 S 1 H C I 2 in T H F ) and the [ ( C H 3 S i H ) ( C H 3 S i ) ] material (x = 0.76; y = 0.26) in THF was treated with a catalytic amount of K H . After the reaction mixture had been treated with methyl iodide, the usual workup gave an 89% yield of hexane-soluble white powder, molecular weight - 2750. On pyrolysis, this material gave a 73% yield of a black ceramic. The "combined" polymer prepared in this way ("in situ polymer") was in some ways different from the "combined" polymer prepared by the first method ("graft" polymer). Principal differences were observed in their proton NMR spectra and in the form of their T G A curves. This suggests that the two differently prepared polymers have different structures. It is likely that in the "in situ" preparation intermediates formed by the action of K H on the ( C H 3 S i H N H ) cyclics are intercepted by the reaction with the [ ( C H 3 S i H ) ( C H 3 S i ) ] also present before the [(CH SiHNH) (CH3SiH)b(CH3SiHNK) ] polymer (which is the starting reactant used in the "graft" procedure) has a chance to be formed to the extent of its usual molecular weight. Thus, less of the original C H 3 S 1 H N H protons are lost and/or more of those of the [ ( C H 3 S i H ) ( C H 3 S f r y ] system are reacted. The T G A curves of the "graft" polymer and the "in situ" polymer are different as well. Noteworthy in the former is a small weight loss between 1 0 0 ° C and 200°C., which begins at around 1 0 0 ° C . This initial x

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small weight loss occurs only at higher temperature (beginning a t ~ 1 7 5 ° C ) in the case of the "in situ" polymer. This difference in initial thermal stability could well have chemical consequences of importance with respect to ceramics and both kinds of polymers may be useful as preceramic materials. Further experiments showed that the "combined" polymers may be converted to black ceramic fibers. Pyrolysis of pressed bars of the "combined" polymer to 1 0 0 0 ° C gave a black product of irregular shape (74-76% ceramic yield). In other experiments, SiC powder was dispersed in toluene containing 20% by weight of the "combined" polymer. The solution was evaporated and the residue, a fine powder of SiC with the "combined" polymer binder, was pressed into bars and pyrolyzed at 1000°C. A ceramic bar (6% weight loss, slightly shrunk in size) was obtained. The ceramic products obtained in the pyrolysis of the "combined" polymers have not been studied in detail, but some of them have been analyzed for C., N , and Si The compositions of the ceramic materials obtained cover the rang Thus, as expected, they ar which is obtained in the pyrolysis of the [(CH3SiH) (CH3Si)y] materials alone is not present, so that objective has been achieved. By proper adjustment of starting material ratios, we find that the excess carbon content can be minimized [11]. The "living polymer" intermediate in our polysilazane synthesis is useful in "upgrading" other Si-Η containing polymers for application as precursors for ceramic materials. Another example of this application is provided by such "upgrading" of methylhydrogenpolysiloxane, [CH3S1( H ) 0 ] . Such a linear polymer, average molecular weight 2000-5000 (vendor data), on pyrolysis under argon to 1000°C., left a black ceramic residue of only 13%. In this study, experiments were carried out with a [CH3Si(H)0] prepared using conditions under which the yield of the cyclic oligomers (m = 4, 5, 6, . . . .) is maximized [12], as well as with the commercial [ C H 3 S i ( H ) 0 ] polymer of higher molecular weight, with presumably high linear content. In one approach, the polymeric silylamide was prepared as described above (using the product of CH3SÎHC12 ammonolysis in THF) and to this "living" polymer solution were added slowly the [ C H 3 S i ( H ) 0 ] oligomers (high cyclic content). An immediate reaction with some gas evolution occurred. The resulting clear solution was treated with CH3I to react with any remaining silylamide or silanolate units. Silylamide/siloxane weight ratios of 1:1 and 1:5 were used. In both cases the polymeric product was an organic-soluble white solid of moderate (1700 and 2400, respectively) average molecular weight. In both cases pyrolysis under nitrogen or argon gave high char yields (78% and 76%, respectively, by TGA). Pyrolysis to 1 0 0 0 ° C of a bulk sample of the 1:1 by weight polymer gave a black solid in 80% yield. Elemental analysis indicated a "composition": 1 SiC + 0.84 S13N4 + 2.17 S1O2 + 2.0 C . (This is not meant to reflect the composition in terms of chemical species present. No doubt, rather than separate S13N4 and S1O2, silicon oxynitrides are present). On the other hand, pyrolysis under gaseous ammonia gave a white ceramic solid in 78.5% yield. It is suggested that here also high temperature nucleophilic cleavage of S1-CH3 bonds by N H 3 occurred and that the ceramic product is a silicon oxynitride. This was confirmed by analysis. The white solid contained less than 0.5% carbon. x

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In an alternate approach to this "graft" procedure, the "in situ" procedure was used. A mixture (-1:1 by weight) of the C H 3 S 1 H C I 2 ammonolysis (in T H F ) product, [ C H 3 S 1 H N H ] , and the C H 3 S 1 H C I 2 hydrolysis product, [ C H 3 S i ( H ) 0 ] , in T H F , was added to a suspension of a catalytic amount of K H in T H F . Hydrogen evolution was observed and a clear solution resulted. After quenching with C H 3 I , further work-up gave the new polymer, a soluble white powder, average molecular weight 1670. Pyrolysis to 1 0 0 0 ° C gave a black ceramic solid in 84% yield (by TGA). Pyrolysis of bulk sample under argon yielded a black ceramic (73%). Analysis indicated the "composition": 1 SiC + 1.03 S13N4 + 1.8 S1O2 + 2.63 C . In place of the (mostly) cyclic [ C H 3 S i H O ] oligomers, a commercial methylhydrogenpolysiloxane (Petrarch PS-122) of higher molecular weight, presumably mostly linear species, may serve as the siloxane component. When a 1:1 by weight ratio of the preformed polymeric silylamide and the [ C H 3 S i ( H ) 0 ] polymer was used and the reaction mixture was quenched with C H 3 I , the usual work-up produced a soluble white solid molecular weight 1540. Pyrolysis material (77% yield by TGA) solid (73% yield) whose analysis indicated a "composition": 1 SiC + 1.5 S13N4 + 3.15 S1O2 + 3.6 C . Application of the "in situ" procedure [1:1 by weight ratio of ( C H 3 S i H N H ) and [ C H 3 S i ( H ) 0 ] gave a soluble white powder, average molecular weight 1740, pyrolysis yield (TGA) 88% (black solid). The fact that the pyrolysis of these polymers under a stream of ammonia gives white solids, silicon oxynitrides which contain little, if any, carbon, in high yield is of interest. The ceramics applications of these silicon oxynitride precursor systems are receiving further study by Yu and Ma of Universal Energy Systems [13]. The chemistry which is involved in the "graft" and "in situ" procedures and the structures of the hybrid polymers which are formed remain to be elucidated. However, there is no doubt that these procedures are useful ones. We have used them also to form new and useful hybrid preceramic polymers from the Yajima polycarbosilane which contains a plurality of [CH3Si(H)CH2] units [14]. m

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Conclusions We have described new routes to useful preceramic organosilicon polymers and have demonstrated that their design is an exercise in functional group chemistry. Furthermore, we have shown that an organosilicon polymer which seemed quite unpromising as far as application is concerned could, through further chemistry, be incorporated into new polymers whose properties in terms of ceramic yield and elemental composition were quite acceptable for use as precursors for ceramic materials. It is obvious that the chemist can make a significant impact on this area of ceramics. However, it should be stressed that the useful applications of this chemistry can only be developed by close collaboration between the chemist and the ceramist.

Acknowledgments The work reported in this paper was carried out with generous support of the Office of Naval research and the A i r Force Office of Scientific Research. The results presented here derive from the Ph.D. dissertations of Gary H . Wiseman and Joanne M. Schwark and the postdoctoral research of Dr. Yuan-Fu Yu and Dr. Charles A. Poutasse. I thank these coworkers for their skillful and dedicated efforts.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Literature Cited 1.

2.

3.

4. 5. 6. 7.

8.

9.

10.

11. 12. 13.

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Gmelin Handbook of Inorganic Chemistry, 8th Edition, SpringerVerlag: Berlin, Silicon, Supplement Volumes B2, 1984, and B3, 1986. Messier, D. R.; Croft, W. J. in "Preparation and Properties of Solid-State Materials," Vol. 7, Wilcox, W. R., ed.; Dekker: New York, 1982, Chapter 2. a. Wynne, K. J.; Rice, R. W. Ann. Rev. Mater. Sci. (1984) 14, 297. b. Rice, R. W. Am. Ceram. Soc. Bull. (1983) 62, 889. c. Rice, R. W. Chem. Tech. (1983) 230. Billmeyer, F. W. Jr., Textbook of Polymer Chemistry, 2nd edition, Wiley: New York, 1984, Chapter 18. Yajima, S. Am. Ceram. Soc. Bull. (1983) 62, 893. Schilling, C. L., Soc. Bull. (1983) 62, LeGrow, G. E.; Lim, T. F.; Lipowitz, J.; Reaoch, R. S. in "Better Ceramics Through Chemistry II," edited by C. J. Brinker, D. E. Clark and D. R. Ulrich, Materials Research Society, Pittsburgh, 1986, pp. 553-558. a. Seyferth, D.; Wiseman, G. H. J. Am. Ceram. Soc. (1984) 67, C-132. b. U. S. Patent 4,482,669 (Nov. 13, 1984). c. Seyferth, D.; Wiseman, G. H. in "Ultrastructure Processing of Ceramics, Glasses and Composites," 2, edited by L. L. Hench and D. R. Ulrich, Wiley: New York, 1986, Chapter 38. a. Schilling, C. L., Jr.; Williams, T. C., Report 1983, TR-83-1, Order No. AD-A141546; Chem. Abstr. 101 196820p. b. U. S. Patent 4,472,591 (Sept. 18, 1984). Very much the same study, with the same results, was carried out by Sinclair and Brown-Wensley at 3M prior to our investigation. This work came to our attention when the U.S. patent issued: Brown-Wensley, Κ. Α.; Sinclair, R. A. U. S. patent 4,537,942 (Aug. 27, 1985). This "combined polymer approach" has been patented: Seyferth, D.; Wood, T. G.; Yu, Y.-F; U. S. patent 4,645,807 (Feb. 24, 1987). Seyferth, D.; Prud'homme, C.; Wiseman, G. H. Inorg. Chem. (1983) 22, 2163. Yu, Y.-F.; Ma, T.-I in "Better Ceramics Through Chemistry II," edited by C. J. Brinker, D. E. Clark and D. R. Ulrich, Materials Research Society, Pittsburgh, 1986, pp. 559-564. Seyferth, D. and Yu, Y.-F., U. S. patent 4,650,837 (Mar. 17, 1987).

RECEIVED October 23, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 12

NMR Characterization of a Polymethyldisilylazane A Precursor to Si—C—N—O Ceramics Jonathan Lipowitz, James A. Rabe, and Thomas M. Carr

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Si and C NMR spectroscopy was used to characterize a polymethyldisilylazane polymer, a precursor to Si-C-N-O ceramics. Polymer is prepared by reaction of mixed chloromethyldisilanes with hexamethyldisilazane. The polymer is a low MW (M ~2000), glassy oligomer with a polycyclic, cage-like structure and a broad MW distribution. Si and C NMR spectra give only broad signals. A simplified polymerization reaction using sym-tetrachlorodimethyldisilane was followed by Si NMR to give insight into development of polymer structure. The broad NMR signals were shown to result from a multitude of chemical environments about the Si atoms which develop early in the reaction, not from restricted motion of Si atoms in the polymer molecules or from Ν quadrupole broadening. n

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Ceramics c a n be p r e p a r e d by p y r o l y s i s o f v a r i o u s o r g a n o s i l i c o n polymers. The advantages o f c e r a m i c f o r m a t i o n from polymers i n c l u d e t h e a b i l i t y t o p r e p a r e shapes d i f f i c u l t t o a c h i e v e by c o n v e n t i o n a l powder p r o c e s s i n g methods, such as f i l m s and f i b e r s ; the u s e o f lower p r o c e s s i n g temperatures than in c o n v e n t i o n a l methods; t h e a b i l i t y t o a c h i e v e v e r y h i g h p u r i t y because r e a g e n t s can be p u r i f i e d by w e l l - e s t a b l i s h e d methods such as d i s t i l l a t i o n and r e c r y s t a l l i z a t i o n ; t h e a b i l i t y t o v a r y ceramic c o m p o s i t i o n by v a r i a t i o n o f polymer c o m p o s i t i o n ; and t h e a b i l i t y t o c r e a t e unique m e t a s t a b l e c e r a m i c c o m p o s i t i o n s which cannot be a c h i e v e d by c o n v e n t i o n a l p r o c e s s i n g (3). T h i s t e c h n o l o g y has been r e v i e w e d r e c e n t l y by s e v e r a l a u t h o r s ( 4 - 1 1 ) .

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Current address: Technical Center, Owens Corning Fiberglas, Granville, OH 43023

0097-6156/88/0360-0156$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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One o f t h e main advantages o f t h e polymer r o u t e t o c e r a m i c s is the p r e p a r a t i o n o f ceramic f i b e r s , a shape d i f f i c u l t t o a c h i e v e by o t h e r methods. Ceramic f i b e r - b a s e d composites a r e becoming an i n c r e a s i n g l y i m p o r t a n t group o f s t r u c t u r a l m a t e r i a l s (12, 13). A c e r a m i c f i b e r w i t h Si-C-N-0 c o m p o s i t i o n c a n be p r e p a r e d by m e l t - s p i n n i n g , c u r e and p y r o l y s i s o f a p o l y m e t h y l d i s i l y l a z a n e polymer p r e c u r s o r (14, 15), which is t h e r e a c t i o n p r o d u c t o f a m i x t u r e o f 50 mol % 1,1,2,2- t e t r a c h l o r o - 1 , 2 - d i m e t h y l d i s i l a n e ( l a ) , 40 mol % 1 , 1 , 2 - t r i c h l o r o - 1 , 2 , 2 - t r i m e t h y l d i s i l a n e ( l b ) and 10 m o l % 1,2-dichloro-l,1,2,2- tetramethyldisilane +1,1-dichloro-l,2,2,2tetramethyldisilane (Ic). T h i s m i x t u r e is r e a c t e d w i t h excess h e x a m e t h y l d i s i l a z a n e (HMDZ) to g e n e r a t e o l i g o m e r i c s p e c i e s ( p o l y m e t h y l d i s i l y l a z a n e "polymer") w i t h t h e approximate c o m p o s i t i o n I I , as determined b y e l e m e n t a l a n a l y s i s and M from GPC a n a l y s i s , [Me

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],

(II)

a l o n g w i t h M e ^ S i C l and M e ^ S i C l and HMDZ a r e removed by g r a d u a l l y i n c r e a s i n g temperature t o about 250° C d u r i n g r e a c t i o n . The polymer is a g l a s s y m a t e r i a l w i t h a h i g h l y c r o s s l i n k e d presumably c a g e - l i k e p o l y c y c l i c s t r u c t u r e and is r e a d i l y s o l u b l e in a r o m a t i c and a l i p h a t i c h y d r o c a r b o n s . Number average m o l e c u l a r w e i g h t s (R ) o f c a . 2000 a r e found b y GPC. Polymers a r e r e a c t i v e w i t h b o t h oxygen and m o i s t u r e and must be h a n d l e d in a d r y and i n e r t atmosphere. A b r o a d MW d i s t r i b u t i o n is found b y GPC, s u g g e s t i n g t h a t a v a r i e t y o f s t r u c t u r e s a r e p r e s e n t . The m i x t u r e o f c h l o r o d i s i l a n e monomers and t h e i r h i g h average f u n c t i o n a l i t y (f=3.4) suggest a complex h i g h l y c r o s s l i n k e d s t r u c t u r e . The i n t e n t o f t h e e f f o r t d e s c r i b e d in t h i s paper was t o o b t a i n i n s i g h t i n t o t h e s t r u c t u r e o f t h i s complex polymer and t o c h a r a c t e r i z e t h e s t r u c t u r e s p r e s e n t u s i n g NMR. Experimental A p p r o x i m a t e l y 1 g polymer and 0*06 M C r ( a ç a c ) ^ were d i s s o l v e d in CDC1 to prepare s o l u t i o n s f o r S i and C NMR s p e c t r o s c o p y . NMR s p e c t r a were r u n on a V a r i a n XL-200 FT-NMR i n s t r u m e n t . To a i d in o b t a i n i n g q u a n t i t a t i v e d a t a , t h e s o l u t i o n was doped w i t h 0.06 M chromium a c e t y l a c e t o n a t e [ C r ( a c a c ) ) ] t o remove p o s s i b l e s i g n a l a r t i f a c t s r e s u l t i n g from l o n g s p i n - l a t t i c e r e l a x a t i o n times ( T ' s ) and t^ç n u c l e a r ^ O v e r h a u s e r e f f e c t , well-known f e a t u r e s a s s o c i a t e d with S i and C NMR s p e c t r o s c o p y . T h i s p e r m i t s q u a n t i t a t i v e signal acquisition. From t h e l i t e r a t u r e (16) and a d d i t i o n a l work done in t h i s l a b o r a t o r y , i t was e x p e c t e d t h a t C r ( a c a c ) ^ would be an i n e r t s p e c i e s . A s o l u t i o n o f HMDZ (2.04 g, 12.67 mmole), l , l , 2 , 2 - t e t r a c h l o r o - l , 2 - d i m e t h y l d i s i l a n e , l a (0.99 g, 4.34 mmole), 9

g

0

,

3

m

m

o

9

in

C D C 1

t

0

5

m

l

v

u

m

e

w

a

s

Cr ( a c a c ) ^ ( ° 2 § » 3 .° o l p r e p a r e d and S i NMR d a t a were sampled a t 10 minute i n t e r v a l s . 1 , 1 , 2 , 2 - t e t r a c h l o r o - l , 2 - d i m e t h y l d i s i l a n e was p r e p a r e d by t h e method o f S a k u r a i e t a l . ( 1 7 ) . H e x a m e t h y l d i s i l a z a n e e n r i c h e d t o 99% Ν was p r e p a r e d by a N i - c a t a l y z e d r e a c t i o n o f t r i m e t h y l s i l a n e w i t h 99% e n r i c h e d NH . R e a c t i o n was c a r r i e d o u t in a 5 L f l a s k on a 0

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

158

INORGANIC AND ORGANOMETALLIC POLYMERS

vacuum l i n e u s i n g 0.67 atm (0.14 mole) Me^SiH (PCR Research C h e m i c a l s , G a i n e s v i l l e , F L ) and 0.33 atm 10.07 mole) NH (Stohler I s o t o p e C h e m i c a l s , Waltham, MA) and 25 g N i powder, 99.9%, 500-600 urn, s p h e r i c a l (Johnson Matthey, I n c . , Seabrook, NH). N i powder was washed w i t h r e a g e n t grade THF and vacuum degassed b e f o r e use. R e a c t i o n c o n d i t i o n s a r e s i m i l a r t o t h a t d e s c r i b e d in ( 1 8 ) . The bottom s e c t i o n o f t h e f l a s k c o n t a i n i n g N i powder was h e a t e d 96 h r s at 170° C. D i s t i l l a t i o n gave 7.1 g (0.044 mole) (Me S i ) NH o f 98% p u r i t y by gc a n a l y s i s . The o n l y i m p u r i t y found was h e x a m e t h y l d i s i l o x a n e . A nmr experiment t o f o l l o w f o r m a t i o n o f p r o d u c t s and l o s s o f r e a c t a n t s was performed j u s t as d e s c r i b e d f o r u n l a b e l e d HMDZ. F o r m a t i o n o f p r o d u c t s and l o s s o f r e a c t a n t s was v i r t u a l l y i d e n t i c a l t o t h a t shown in F i g u r e 5. 2

Results 13 To g a i n i n s i g h t i n t o th performed. From e x a m i n a t i o be seen t h a t t h i s e x p e r i m e n t a approac yield There is l i t t l e a d d i t i o n a l i n f o r m a t i o n o b t a i n e d from e x a m i n a t i o n o f the b r o a d s i g n a l s in a S i NMR spectrum ( F i g u r e 2) o f t h i s polymer, except f o r the o b s e r v a t i o n o f r e s i d u a l Me^SiÇl b y - p r o d u c t a t c a . + 32 ppm and Me^SiNH groups a t c a . +2 ppm. H NMR^hows o n l y Si-CH^ groups. I t was hoped t h a t the b r o a d , f e a t u r e l e s s S i NMR spectrum c o u l d be s i m p l i f i e d by the u s e o f a pure d i s i l a n e s t a r t i n g m a t e r i a l . S i n c e t h e polymer i t s e l f is h i g h l y c r o s s - l i n k e d and the p u r e , s y m m e t r i c a l 1 , 1 , 2 , 2 - t e t r a c h l o r o - l , 2 - d i m e t h y l d i s i l a n e is l i k e l y t o produce a s i m p l e r polymer o f s i m i l a r s t r u c t u r e , i t was chosen f o r d e t a i l e d s t u d y . Polymer p r e p a r e d from the s y m - t e t r a c h l o r o d i s i l a n e e x h i b i t s s i m i l a r p r o p e r t i e s t o t h a t from the m i x t u r e ( I ) o f chlorodisilanes. Indeed, the s y m - t e t r a c h l o r o d i s i l a n e is t h e predominant m o l e c u l a r s p e c i e s in the m i x t u r e o f d i s i l a n e s ( I ) i s e d . The S i NMR spectrum o f the polymer t h a t r e s u l t s from the r e a c t i o n of HMDZ w i t h s y m - t e t r a c h l o r o d i m e t h y l d i s i l a n e ( C ^ M e S i - S i M e C i p , l o o k s v e r y much l i k e t h a t o f polymer p r e p a r e d from the m i x t u r e o f d i c h l o r o s i l a n e s ( F i g u r e 2 ) . There is no o b s e r v a b l e f i n e s t r u c t u r e t h a t would l e a d t o an i n c r e a s e d u n d e r s t a n d i n g o f t h e c h e m i c a l s t r u c t u r e o f t h i s simpler oligomer. 29 The b r o a d , f e a t u r e l e s s n a t u r e o f t h e S i NMR s i g n a l is l i k e l y due t o one o f t h r e e phenomena t h a t a r e well-known t o cause such e f f e c t s in NMR s p e c t r o s c o p y : a m u l t i p l i c i t y o f c h e m i c a l and p h y s i c a l environments about the s i l i c o n atoms; r e s t r i c t e d segmental motj^n in p o l y c y c l i c o r c a g e - l i k e m o l e c u l e s ; o r j-J e b r o a d e n i n g due to S i magnetic i n t e r a c t i o n s w i t h t h e numerous Ν quadrupoles. To o b t a i n i n f o r m a t i o n on the r e a s o n f o r NMR s i g n a l b r o a d e n i n g and i n s i g h t i n t o the c h e m i c a l n a t u r e o f t h e s e p r e - c e r a m i c polymers, attempts were made t o f o l l o w the r e a c t i o n o f t h e s y m - t e t r a c h l o r o d i s i l a n e and HMDZ by S i NMR s p e c t r o s c o p y . From i n s p e c t i o n o f F i g u r e 3, which is t h e S i NMR spectrum o f the r e a c t i o n m i x t u r e a f t e r 10 m i n u t e s , i t is seen t h a t r e a c t i o n is q u i t e r a p i d and t h a t a s i g n i f i c a n t amount o f the t r i m e t h y l s i l a z a n e monoadduct o f s y m - t e t r a c h l o r o d i s i l a n e has formed, a l o n g w i t h M e ^ S i C l b y - p r o d u c t and a s m a l l amount o f the sym-1,2- d i a d d u c t . As r e a c t i o n p r o c e e d s , r a p i d consumption o f s t a r t i n g m a t e r i a l s w i t h r e s u l t a n t n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

12.

LIPOWITZ ET AL.

U

Characterization of a Polymethyldisilylazane

CDCI

159

3

Intensity

Π

— ι

Γ

90 p p m

1 40

1

Τ 0

Γ

r -10

C h e m i c a l Shift, δ (ppm)

13 F i g u r e 1. CDC1 .

C NMR Spectrum o f P o l y m e t h y l d i s i l y l a z a n e Polymer in

3

Intensity

30

—ι 10

20

1 c

-10

-20

C h e m i c a l Shift, δ (PPm) 29 F i g u r e 2. S i NMR Spectrum o f P o l y m e t h y l d i s i l y l a z a n e Polymer in CDC1 ; 0.06 M in C r ( a c a c ) 3

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

160

INORGANIC AND ORGANOMETALLIC POLYMERS

f o r m a t i o n o f monoadduct and d i a d d u c t s p e c i e s is seen. About 40 m i n u t e s a f t e r i n i t i a l m i x i n g , f o r m a t i o n o f the t r i a d d u c t s p e c i e s is o b s e r v e d , which remains as a minor component throughout the c o u r s e o f the r e a c t i o n . A c o m p i l a t i o n o f t h e S i NMR c h e m i c a l s h i f t s o f t h e s e low m o l e c u l a r weight s p e c i e s is shown in T a b l e I . A f t e r 80 minutes i t was o b s e r v e d t h a t the o n l y s i g n i f i c a n t changes were the n e a r - t o t a l consumption o f t h e [ M e C ^ S i ] ^ starting m a t e r i a l and the monoadduct s p e c i e s , a l o n g w i t h i n c r e a s e d d i a d d u c t formation. I n a d d i t i o n , t r i a d d u c t c o n t e n t is a t i t s maximum c o n c e n t r a t i o n , w i t h o n l y s m a l l amounts o f t h i s s t r a i n e d s p e c i e s w i t h [Me«Si] -SiMe- l i n k a g e s formed. From i n s p e c t i o n o f F i g u r e 4, o b t a i n e d a f t e r 10 h o u r s , i t can be seen t h a t , a l o n g w i t h M e ^ S i C l and HMDZ s p e c i e s , a p p r e c i a b l e amounts o f h i g h e r o r d e r o l i g o m e r s w i t h [ = S i N H ] S i M e - t y p e l i n k a g e s a r e formed. These o b s e r v a t i o n s l e a d t o t h e f o l l o w i n g c o n c l u s i o n s r e g a r d i n g the r e a c t i o n scheme o f [ M e C ^ S i L and HMDZ. The i n i t i a l s t e p i n v o l v e s t h e f a s t l i g a n d exchange r e a c t i o n o f HMDZ w i t h [ M e C ^ S i K t by-product. A s i m i l a r f a s c r e a t e the s y m m e t r i c a l 1,2-diadduct and a d d i t i o n a l M e ^ S i C l . Steric h i n d r a n c e causes f o r m a t i o n o f two t r i m e t h y l s i l y l a z a n e groups on a s i l i c o n atom t o y i e l d [Me^SiNH] ~SiMe-type s p e c i e s t o be slow. This would account f o r the p r e s e n c e o f o n l y minor amounts o f t r i a d d u c t and t h e f a c t t h a t t h e 1,1-diadduct is n o t o b s e r v e d . The f o r m a t i o n o f h i g h e r o r d e r s p e c i e s than t r i a d d u c t i n v o l v e s s e l f - r e a c t i o n o f monoadduct and d i a d d u c t s p e c i e s as w e l l as r e a c t i o n between t h e s e s p e c i e s t o c r e a t e a m u l t i p l i c i t y o f l i n e a r and c y c l i c s p e c i e s w i t h S i - S i - N H - S i - S i - t y p e l i n k a g e s . No s i n g l e h i g h e r o r d e r s p e c i e s t h a n t r i a d d u c t is formed in s u f f i c i e n t c o n c e n t r a t i o n t o s t a n d out from the m u l t i p l i c i t y o f s p e c i e s p r o d u c e d . The v a l i d i t y o f t h e c o n c l u s i o n r e g a r d i n g the r e a c t i o n scheme can be v e r i f i e d from F i g u r e 5, which p l o t s r e l a t i v e moles o f each s p e c i e s vs time. There is a r a p i d r i s e in monoadduct and d i a d d u c t content i n i t i a l l y . S h o r t l y a f t e r i n i t i a l mixing, a r a p i d decrease in monoadduct w i t h a s i g n i f i c a n t l y s l o w e r d e c l i n e in d i a d d u c t c o n t e n t is seen. T r i a d d u c t is slow t o form and slow t o r e a c t . A r a p i d d e c r e a s e in HMDZ c o n t e n t f o l l o w e d by a g r a d u a l i n c r e a s e in HMDZ is found due t o t h e f o l l o w i n g c o n d e n s a t i o n r e a c t i o n s : 2

2

=Si-NH-SiMe

3

+ Me SiNH-Si= - 2 ^ > =Si-NH-Si= + [ M e ^ i ^ N H 3

(III)

I t is o b s e r v e d t h a t t h e l o n g - t e r m i n c r e a s e in HMDZ is s i g n i f i c a n t l y l e s s t h a n the s h o r t - t e r m d e c r e a s e in t h e i n i t i a l r e a c t i o n p e r i o d . T h i s is s i g n i f i c a n t b e c a u s e , in terms o f the proposed r e a c t i o n scheme ( F i g u r e 6 ) , i t would be d i f f i c u l t t o e n v i s i o n f o r m a t i o n o f p o l y c y c l i c o r c a g e - l i k e s t r u c t u r e s w i t h o u t r e g e n e r a t i o n o f HMDZ t h a t is n e a r l y e q u a l in magnitude t o t h a t o f t h e i n i t i a l consumption o f HMDZ, l o n g b e f o r e l a r g e r m o l e c u l e s o f r e s t r i c t e d segmental m o t i o n a r e p r o d u c e d . However, b r o a d s i g n a l s a r e f o r m i n g as shown by t h e r e d u c t i o n in t o t a l i n t e g r a t e d i n t e n s i t y o f sharp s i g n a l s . No s i l i c o n r e s o n a n c e from a m o l e c u l e w i t h more than 5 S i atoms ( t r i a d d u c t ) can be d i s t i n g u i s h e d from the m u l t i t u d e o f S i environments p r o d u c e d . W i t h t h i s e v i d e n c e , i t would seem t h a t b r o a d

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

12.

LIPOWITZ ET AL.

Characterization of a Polymethyldisilylazane

161

(Me Si) NH 3

2

Me

CI Me|H]SiNHSiMe3 2

CI

Me Me CI MeSiSiNH[|j]Me CI

Intensity

2

CI M*Si[|i]NHSiMe3 CI 2

3

Me SiCI 3

Me dUNHSiMeaJa CI —Γ 20

-T30

Γ™ 40

10

"I -20

-10

0

29 F i g u r e 3. S i NMR Spectrum o f 10-Minute R e a c t i o n P r o d u c t o f ( M e S i ) NH, 2.5 M, and ( M e C l S i ) 0.87 M, in CDC1 ; 0.06 M in Cr(acac; . 3

2

2 >

3

3

on TABLE I . of

S i NMR C h e m i c a l S h i f t s in t h e R e a c t i o n ( M e S i ) N H and ( M e C l S i ) 3

2

6 (

Starting Materials (Me S i ) NH (MeOl Si) 2

2

2

By-Product Me SiCl

2 9

2

S i ) in ppm R e l a t i v e t o Me^Si 2.24 17.45

30.08

3

Monoadduct 7.48 1.91 21.61

Me„Si NH-ClMeSi,-Si Cl.Me 3 a b c l

(a) (b) (c)

1,2-Diadduct [Me Si -NHClMeSi l 3

a

b

6.60 (a) 2.20 ( b ) *

2

Triadduct [Me„Si NH] -MeSL - S i MeCl-NHSi,Me 3 a 2 b e α 5 0

0

3.42 17.65 3.97 5.65

(a) (b) (c) (d)

* D o u b l e t o b s e r v e d ( c a . 0.1 ppm s e p a r a t i o n ) due t o t h e p r e s e n c e o f e r y t h r o and t h r e o d i a s t e r e o m e r s .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

162

x10 Intensity

Γ"

20

30

40

—Γ" -10

10

-30

-20

29„. F i g u r e 4. S i NMR Spectrum o f 10-Hour R e a c t i o n P r o d u c t o f ( M e S i ) N H and ( M e C l S i ) in CDC1 ; 0.06 M in C r ( a c a c ) . 3

2

2

2

3

3

4η

Me SiCI

3H

3

Total Signal Due to Sharp Lines (x 1/3) Moles

2Η (Me Si) NH 3

(CI MeSi) 2

2

^ - - ^

Monoadduct

1 VI

Ί 2 10

F i g u r e 5.

3

2

4

Ν

tria-Adduct

M II I 5 6 789 100 T i m e (min)

sym-Diadduct = 5

1 2

Γ Γ 3

I M l 11 4

R e a c t i o n o f ( M e S i ) N H and ( M e C l S i ) 3

2

2

2

5 6 7 89 1000

in CDC1 .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

3

12.

LIPOWITZ ET AL.

Characterization of a Polymethyldisilylazane

Me Me CISI-SiCI CI CI

163

Me Me RNHSi-SiCI + RCI CI CI

R NH 2

fast

mono-adduct + RCI

fast J R NH 2

Me Me R NH RNHSi-SiNHR slow NH CI R tris-adduct R = Me Si 2

Me Me RNHSi-SiNHR + RCI CI CI 1,2-di-adduct

3

Further reactions include primarily mono- anddi-adduct reactions:

(

linear SiSiNHSiSi cyclic dimers Si—Si—NH

Self-condensations and cross-condensations

dimers

I

/Si-Si NH NH \ Si-Si' N

F i g u r e 6.

R e a c t i o n Scheme.

s i g n a l s a r e n o t due t o r e s t r i c t e d segmental motiojj in c a g e - l i k e moieties. I n s t e a d , i t would appear t h a t e i t h e r Ν quadrupolar i n t e r a c t i o n s o r p r o d u c t i o n o f numerous S i - N speçies would cause t h i s lack of spectral d e t a i l . S u b s t i t u t i o n o f 99% N - e n r i c h e d HMDZ, w h i c h has no q u a d r u p o l a r n u c l e i , in t h e r e a c t i o n m i x t u r e r e s u l t s in the same s i g n a l b r e a d t h . T h i s i n d i c a t e s t h a t t h e r e a c t i o n o f HMDZ and [MeCl Si]« r e s u l t s in a continuum o f s p e c i e s t h a t ^ g a n n o t be r e s o l v e d By The Ν chemical 5y NMR tteecc hh n i q u e s a t t h e p r e s e n t t i m e . s h i f t s a r e shown in T a b l e I I .

15 N-NMR C h e m i c a l

TABLE I I .

15 Ν Chemical

Compound (Me Si) 3

1 5 2

-351.4

1 5

3

2

-350.3

1 5

(Me S i N H S i C l M e ) (1,2-diadduct)

J

1 5

2

1 5

N H S i C l M e S i M e ( N HSiMe ) (triadduct) a

* ppm r e l a t i v e

Shift*

-358.4

NH

Me Si NHSiClMeSiCl Me (monoadduct)

Me S i

Shifts

b

5

t o CH N0 3

1

-349.2 (a) -352.2 (b) d o u b l e t

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

164 Conclusions

As a r e s u l t o f the above work, the f o l l o w i n g has been a c h i e v e d . A proposed r e a c t i o n scheme f o r f o r m a t i o n o f p o l y m e t h y l d i s y l a z a n e o l i g o m e r , a c e r a m i c p r e c u r s o r , has been d e v e l o p e d , g i v i n g some i n s i g h t i n t o s t r u c t u r a l f e a t u r e s o f t h i s type of r e s i n - l i k e molecule. I n a d d i t i o n , the cause f o r b r o a d f e a t u r e l e s s s i g n a l s in the NMR s p e c t r o s c o p y o f t h e s e polymers has been d e t e r m i n e d t o be the m u l t i p l i c i t y o f environments about the S i atoms w h i c h d e v e l o p e a r l y in t h e r e a c t i o n b e f o r e a p p r e c i a b l e p o l y m e r i z a t i o n o c c u r s . Acknowledgments T h i s work was made p o s s i b l e by s u p p o r t from the D e f e n s e Advanced R e s e a r c h P r o j e c t s Agency (DARPA) and the A i r F o r c e Wright A e r o n a u t i c a l L a b o r a t o r i e s (AFWAL) under C o n t r a c t F33615-83-C-5006 t o Dow C o r n i n g C o r p o r a t i o n encouragement o f t h e C o n t r a c (AFWAL).

Literature Cited 3. Lipowitz, J., Freeman, Η. Α., Chen, R. T., and Prack, E. R. Adv. Ceram. Mater. 1987, 121. 4. Rice, R. W. Am. Ceram. Soc. Bull. 1983, 62, 889. 5. Yajima, S. ibid., 893. 6. West, R., David, L. D., Durovich, P. I., Yu, Η., and Sinclair, R. ibid., 899. 7. Wills, R., Markle, R. Α., and Mukherjee, S. P. ibid., 904. 8. Schilling, Jr., C. L., Wesson, J. P., and Williams, T. C. ibid., 912. 9. Walker, Jr., Β. E., Rice, R. W., Becher, P. F., Bender, Β. Α., and Coblenz, W. S. ibid., 916. 10. Baney, R. H. Division of Industrial and Engineering Chemistry, I. Structural, Electronic and Refractory Ceramics, ACS Symposium on Materials in Emerging Technologies, 1985. 11. K. J. Wynne and R. W. Rice. An. Rev. Mater. Sci. 1984, 14, 297. 12. Mah, T., Mendiratta, M. G., Katz, A. P. and Mazdiyasni, K. S. Am. Ceram. Soc. Bull. 1987, 66, 304. 13. Marshall, D. B. and Ritter, J. E. ibid. 1987, 66, 309. 14. Gaul, Jr., J. H. U.S. Patent 4 340 619, 1982. 15. Baney, R. H. In Ultrastructure Processing of Ceramics, Glasses, and Composites. Ed. Hench, L. L. and Ulrich, D. R. Wiley, J. New York, 1984, Chapter 20.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

12. 16.

17. 18.

LIPOWITZ ET AL.

Characterization of a Polymethyldisilylazane

165

Harris, R. K. Kennedy, J. D., McFarlane, W. In NMR and the Periodic Table. Ed. Harris, R. K. and Mann, Β. E., Academic Press, London, 1978, 309-340. Sakurai, H., Watanabe, T. and Kumada, M. J. Organometal. Chem. 1967, 7,15. Kotzsch, H. J., Draese, R., and Vahlensieck, H. J. U.S. Patent 4 115 427, 1978.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 13

Silicon-Nitrogen-Containing Rings and Polymers Z. Lasocki, B. Dejak, J. Kulpinski, E. Lesniak, S. Piechucki, and M. Witekowa Institute of Polymers, Technical University, Lodz, Poland It has been of synthesizing useful polymers from the following types of silazane (Si-N) containing ring compounds: cyclic bis (silyl) imidates, N-acylcyclosilazoxanes, N-arylcyclosilazoxanes and Ν , Ν ' -diarylcyclodisilazanes. The imidates and acylcyclosilazoxanes are readily hydrolyzed and decomposed on heating. The interesting internal mobility of these molecules will be discussed. Arylcyclosilazoxanes undergo anionic, ring-opening polymerization and copolymerization with cyclosiloxanes to yield linear polysilazoxanes of remarkable thermal stability. Another method of improving the thermal stability of polysiloxanes is based on the polycondensation of functional cyclodisilazane oligomers with α, ωdihydroxypolysiloxanes. High temperature reactions of diarylcyclodisilazanes with phenylenediamines yield a variety of cyclolinear polysilazane oligomers and polymers with aromatic spacing groups. Their structure and properties will be discussed. In a research program to explore the possibilities of synthesizing useful polymeric materials from silazane (Si-N) containing ring compounds, reactions of the following ring systems were studied: (I) cyclic silylamides of various ring sizes, (II) cyclosilazoxanes; six- and eight-membered, (III) cyclodisilazanes Formation of polymer chains or networks from these compounds can be conceived either as a ring-opening polymerization process or a polyreaction involving exocyclic functional groups.

Cyclic Bisfsilylamidesï: Structure and Rearrangements In an attempt to prepare polymers of type Ic or Id from acid amides and 1,3dichlorodisiloxanes or 1,5-dichlorotrisiloxanes, it was found that bimolecular cyclization prevailed over polycondensation and cyclic bis(silylamides) in the amido (la) or imidato (lb) isomeric forms were exclusively formed in high yields

0097-6156/88/0360-0166S06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Rings and Polymers

167

Ο r C

M

Me

, I I +

I Me

I η R - C - N H + η CISi 4 O S i * CI I I 1,2

Π .

Me

R 1

1

M

e

"I

ι

!

I Me

I L Me

(Id)

2

Me

Me

C-ΝΓ

/ R-C \\

^Y

Si' Me^

(Ic)

I 1.2 Me

Me

Me

R

χ

4 0 - C = N - S i 4 0Si-)-H

-2n HC1

Me

-i

-2n HC1

2

Me

1

N — S H OSi->

Me

Me

Ο

e

N

Me

\ Y / N—Si / \ Me Me

Y = O, O-Si-0 I Me

(lb)

(la) Scheme 1

(1). This reaction bears a close similarity to the one discovered by J.F. Klebe who found that acid amides with dialkyldichlorosilanes yield disilaoxadiazines (2). The interesting intramolecular mobility of these compounds was studied by *H NMR spectroscopy and the mechanism of internal silyl migrations within their molecules arose some discussion in the literature (3,4). The present cyclic bis(silylamides) are a family of very reactive compounds, sensitive to moisture and heat, readily hydrolyzed to form amides and cyclosiloxanes and readily decomposed at moderate temperatures with elimination of nitrile. However, many attempts to produce from them linear polymers by ringopening polymerization failed. In a few cases compounds la and lb were found to be interconvertible, e.g. for R = Me and Υ = Ο the two isomers in benzonitrile, pyridine or carbon tetrachloride are found to be in a tautomeric equilibrium, which was quantitatively studied by H NMR (5). The tautomer in the amido form , N acetyltetramethylcyclodisilazoxane, is the main component in the tautomeric mixture at -30°C., but its signals have almost completely disappeared from the spectrum at 90°C., where the imidato isomeric form, 2,4,6-pentamethyl-2,4-disila-1,3,5dioxazine, predominates. The amido tautomer is the first example of a stable fourmembered silazoxane ring structure; it represents an intermediate case between the long known disilazane ring, reviewed in 1968 by W. Fink (6) and cyclodisiloxane, recently synthesized by R. West et al. (7). A n isomeric ring structure to the cyclodisilazoxane, the disiloxazetidine, has been quite recently reported (8). Alkyl substituents R in I and particularly R = H tend to stabilize the amido form of the molecule, whereas electronegative groups (R = CCI3, P-XC6H4; X = l

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

168

INORGANIC AND ORGANOMETALLIC POLYMERS

H, N O 2 , Cl, O C H 3 ) favor the imidato isomer, e.g. the reaction product of trichloroacetamide with 1,5-dichlorohexamethyltrisiloxane exists in the eightmemberedringimidato form (lb, R = C C I 3 , Y = O S i M e 2 0 ) . Theringinthat form, however, does not lose its internal mobility and two out of the three methylsilyl singletsinthe H NMR and C NMR spectra undergo coalescence at 46°C (AG* = 17.1 kcal/mole) and 90°C (AG* = 18.5 kcal/mol), respectively. This behavior is best explained as the result of an intramolecular silyl migration resulting in a degenerate rearrangement and involving a transannular attack of nitrogen on silicon, formation of an unstable intermediate in the amido six-membered ring form to afford finally an identical structure with the initial molecule except for the respective positions of silicons (a) and (b), which have been interchanged (9). l

Ms^

(a)/

1 3

Me

Si-0

N'

N

Me Si' • I Me

Me I

Ο

Me

< b )

Me

II

Si'

0

/

Me

v ( a ) /

Me

Si-0

I s

Me-Si^° Si-Me

N

II CI3C-C

Me

Me

-

o' \

v

Me

Me N

N-Si Me

Si'

*

/ ( b ) N

Me

The dynamic NMR behavior of the chloroacetamide derivative of hexamethyltrisiloxane (I, R = C I C H 2 , Y = 0 S i M e 2 0 ) offers a good example of the structural versatility of cyclic bis(silylamides) (10). The C NMR spectrum of this compound at -50°C is consistent with the amido form of the molecule and contains singlets at (δ ppm) 174.21 (C = O), 45.11 (CH C1), 2.47 and 0.79 ( S 1 C H 3 ) . At -10°C new signals, which can be assigned to the imidato isomer come into sight at (δ ppm) 157.24 (C = N) and 46.41 ( C H 2 C I ) . Three additional weak singletsinthe methylsilyl region of the spectrum are also observed. At 10°C the carbonyl signal has disappeared and coalescence phenomena are apparentinboth chloromethyl and methylsilyl regions. Finally, at 68°C the spectrum is consistent with the sole existence of the imidato form of the molecule, which undergoes a degenerate rearrangement (i.e. sharp singlets at 45.63,1.23 and 0.97 ppm, 0) (10). S i NMR spectroscopy is a reliable tool for the structure determination of siloxane derivatives, since the chemical shifts can be assigned to definite ring sizes and hence to the isomeric forms of the compounds being examined. For instance, in the spectrum of the hexamethyltrisiloxane derivative of p-chlorobenzamide (I, R = P - C I Q 5 H 4 , Y = O S i M e 2 0 ) at -32°C there are three singlets at (δ ppm) -12.82 (a), -15.01 (b) and -21.03 (c); signals (a) and (b) undergo coalescence at 23°C yielding one peak at -14.67 ppm 0 , while the chemical shift of silicon (c) is almost unaffected. In the spectra of model six-memberedrings,the chemical shifts are at (δ ppm) -4.45 ( 2 S i , N-Si-O) and -8.66 (ISi, O-Si-O) for N-phenylhexamethylcyclotrisilazadioxane and -5.78 ( 2 S i , N-Si-O) and -9.81 (ISi, O-Si-O) for Nacetylhexamethylcyclotrisilazadioxane. On the other hand, it is known that on passing from hexamethylcyclotrisiloxane to octamethylcyclotetrasiloxane thereisa diamagnetic shift from (0 ppm) -9.2 to - 2 0 . 0 ( 1 1 ) . Therefore, the examined compound can be safely assigned the eight-membered imidatoringstructure lb (9). To summarize the effects of the nature of groups R and Y on the structure of bis(silylamides) (I) it can be concluded that relative increase of the π characterinthe carbon-nitrogen bond is probably responsible for the relative stabilization of the 1 3

?

29

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

13.

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Si-N-Containing

Rings and Polymers

169

amido form of the molecules by hydrogen and methyl substituents R. This effect is pronounced in the trisiloxane derivatives of the amides due to the absence of considerable strain in the six-membered rings and the conformation lability of the eight-membered structures. Silazoxane and Silazane Linear and Cyclo-linear Polymers Introductory Remarks. In contrast with the popularity and usefulness of the polysiloxane chains, which constitute the structural backbone of silicones, the knowledge of polymers based on the silazane unit is still limited. In the sixties there was still some hope of the possibility of producing long chain polysilazane molecules and a number of laboratories were active in seeking convenient methods for their synthesis (e.g. see review by Aylett (12)). Although the energy of the silicon-nitrogen bond is ca. 30 kcal/mole lower than that of the siloxane linkage, the extensive (p-d) π bonding tends to increase the SiN bond strength. Indeed, some low molecular weight silazanes were found to be stable to high temperatures; for example the four-membered cyclodisilazanes which are among the most decompose over 500°C in spit (11). This raised hope that polysilazanes would surpass silicones in thermo­ stability. However, the polarity of the silicon-nitrogen bond and, particularly the basicity of nitrogen, make these compounds sensitive to hydrolysis in acidic media, thus limiting their general utility. It has become fairly evident that for most silazane systems the equilibrium between rings and chains lies well to the side of the ring compounds and the concentration of linear oligomers is very low. VanWazer and Moedritzer have shown that equilibration of N-trimethylhexamethylcyclotrisilazane with dichlorodimethylsilane yields mainly the cyclic trimer. The concentration of linear oligomers decreases rapidly with increasing chain lengths and the dimer, 1,3dichlorotetramethyldisilazane, is the main linear component at equilibrium (14). In a similar equilibration study of octamethylcyclotetrasiloxane with dichloro­ dimethylsilane it was found that the amount of rings at equilibrium is negligibly low (15). The reluctance of silazanes to build long chain molecules arises, therefore, from thermodynamic grounds. Varying the substituents on silicon and nitrogen does little to change the situation. Hydrogen on silicon, however, does appear to promote silazane chains formation (12,16). It has been recently claimed that unsubstituted cyclosilazanes can be polymerized by heat to yield soluble, high molecular weight polysilazanes, which were further pyrolyzed to P-Si3N4 and p-SiC ceramics (11). However, the linear chain structure of the polymers was not established and some decomposition of the material during polymerization was observed. The reasons for the high thermodynamic stability of the silazane rings with respect to chains are not clear. Quantum mechanical calculations have shown that ρπ - ρ π interactions between non-bonded nitrogen atoms are stronger in silazane rings relative to similar interactions between oxygens in cyclosiloxanes, which results in relatively high N-N bond orders (IS). It has been suggested that there are larger angular strains, in cyclosiloxanes. Moreover, the Si-0 bond energy is higher in linear structures and increases with chain length, the reverse being true for the SiN energy in cyclic and linear polysilazanes (19). Whatever the reason, it is an experimental fact that cyclosilazanes of various ring sizes (four-, six- and eight) are reluctant to polymerize by a ring-opening process although they readily undergo rearrangements with ring expansions and contractions.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

170

INORGANIC AND ORGANOMETALLIC POLYMERS

In the present research program, which is directed to synthesizing useful polymers from silazane containing ring compounds, three general pathways were followed: the first consists of using preformed silazoxane rings to produce silazoxane chains byring-openingpolymerization, the second is based on a combination of disilazane rings with polysiloxane chains to obtain silazane-siloxane block copolymers, the third takes advantage of the susceptibility of silazanes to nucleophilic attack by amines and to transamination, which with afunctional reagents, may result in polyreaction and formation of chains containing alternating silazane and organic units. Silazoxane Linear Polymers and Copolymers Several sets of six- and eight-membered N-arylcyclosilazoxanes (Ila-e) have been prepared from aniline and dichlorosiloxanes:

ο. MeoSi^ SiMe, I I Ο Ν Si' Μβ2

(lia)

Ο

(lib)

s

N

0^ SiMe

2

X = p-Me; p-OMe; m-Cl; m,p-Br

/

\

0

X

2

Ο / SiMe

2

Me^Si' SiMe2

' \ Me2Si

Me Si 2

\ ©-


I Ph

etc.

Propagation (B) N

I 1

s , p

/

, ° -

s

^ h

.'

/

S

ΐ

!

O - S *i / \

IN/ c1 :

ι c: Ph

S'/ \ / \/ c:

CÎ I Ph

0"K+

a:

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

172

INORGANIC AND ORGANOMETALLIC POLYMERS

Chain Transfer J

V

N

V

wvSi. Si | N " I Ph

Si

X

/

\ '

Si S Ν ."O'wv/ i t Ph

\ / Si Ν I Ph

0"K

4

O-K *\/ΝΛ» Si ^

\ / *w^SL

\ / Si N

0 * w ^

\ / Si X

\ / Si N " O"K x

+

vwv

.* + 'WN.Si-O.

SiO" K+

+

\ / Si

.Si I Ph-N.

\ / Si

Si ^ I N-Ph Si' / \ s

(lib)

Propagation (A) will result in the expected ordered sequence of siloxane and silazane units in the polymer chain. However, nucleophilic attack by the silanolate anion (probably ion pair) on the ring silicon adjacent to nitrogen (propagation (B)) will yield a chain containing some silazane units separated by a single siloxane unit. Chain transfer at the site shown in the scheme will produce an unstable amide anion, which, by a fast back-biting reaction, will yield the cyclic diazasiloxane, lib. The anionic mechanism of polymerization of cyclosilazoxanes is also supported by the fact that compounds Ha and lie are able to copolymerize with cyclosiloxanes in similar conditions to their polymerization. Copolymerization of Ha with hexamethylcyclotrisiloxane (D3) at 130°C in vacuo yielded polymers of reasonably high molecular weight. However, on increasing the comonomer ratio Ha:D3 from 0.8:1 to 2:1, the molecular weight was found to decrease from 80,000 to 16,000, the highest ratio of comonomer units found in the chain being 1.25:1 (22). This effect is also reflected by the fall of the intrinsic viscosity of polymer solutions in toluene upon increasing the comonomer ratio. The linear structure of the polymers containing both siloxane and silazane units in their chains was well established by spectroscopic methods, e.g. the ! H NMR spectrum contains only the peaks attributable to the phenyl groups and the two different positions of the methyl-silyl protons, the latter giving approximately the expected integration (22). Copolymerizations of ejght-membered He with octamethylcyclotetrasiloxane yielded polymers whose Mn never exceeded 7,200 (21). From these results it is evident that there is a definite limiting number of silazane units that can be introduced into a siloxane chain by copolymerization of silazoxane and siloxane rings. The six-membered and eight-membered

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

13.

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Si-N- Containing Rings and Polymers

173

cyclosilazoxanes containing two silazane bonds (lib, d and e) do not copolymerize with cyclosiloxanes by the anionic process, although l i b is able to polymerize alone. The ultimate member of the six-membered ring series, N-triphenylhexamethylcyclotrisilazane (23) neither homopolymerizes nor copolymerizes with cyclosiloxanes. The silazoxane copolymers are viscous liquids similar in all respects to silicone oils. Surprisingly, their thermal stability was found to be much higher. Polydimethylsiloxane chains depolymerize completely at ca. 3 5 0 ° C leaving practically no residue. In the case of N-phenylpolysilazoxanes the highest rate of weight loss is at 550°C., as revealed by a sharp peak on the D T G line for a copolymer of Ha with D3 (1:1 molar ratio, M = 70,000). At 800°C there remains up to 40% (w/w) of a solid residue which does not change on further heating and is probably composed of silica, silicon carbide and silicon nitride (22). Therefore, the mechanism of chain splitting must be different from that of polysiloxane. However, it follows from isothermal kinetic studies of degradation of these type of polymers, that random scission of the polymer chain followed by complete depolymerization of the kinetically active fragments a mechanism established by M Zeldin et al. for trimethylsily here to quite a large extent with 5-10% of the initial copolymer weight remaining as a solid glassy residue (25). Thus there is some controversy between the conclusions concerning the close similarity of thermal degradation mechanisms on the one hand and the experimental facts which suggest fundamental differences in the thermal behavior of polysiloxane and polysilazoxane chains, on the other. The reasons for the obviously higher thermostability of N-phenylsilazoxane linear polymers are not clear at the moment and the problem requires further studies. Substituting phenyl groups on nitrogen in all cyclosilazoxanes (Ila-e) was intended to increase the resistance to hydrolysis of the monomers and polymers by decreasing the basicity of their molecules. A kinetic study of methanolysis of cyclosilazoxanes Ha and b has shown that the reaction is inhibited by bases. Methanolysis in slightly acidic acetate buffer solution proceeds fairly fast (26,22). Pseudo-first-order rate constants were measured using the U V technique and partial catalytic rate constants for all the acidic species present in the solution (methoxonium ion, undissociated acetic acid and methanol molecules) were determined. It was found that general acid catalysis operates in this reaction. This fact, together with other kinetic tests of mechanism such as substituent effects on the benzene ring, activation parameters and solvent kinetic isotope effects suggest a mechanism involving concerted proton transfer to the substrate and nucleophilic attack on silicon by a solvent molecule (26.27). Since proton transfer to electronegative elements such as oxygen or nitrogen is usually fast and diffusion controlled, it follows that in the present case the nitrogen basicity must be very low as a result of extensive (p-d) π conjugation with both adjoining silicon atoms coupled with the electron withdrawing effect of the phenyl group. n

Siloxane-cyclodisilazane Block Copolymers (29) Polycondensation of difunctional oligomeric cyclodisilazanes with a,codihydroxypolysiloxanes proved to be another successful attempt to synthesize silazane modified polysiloxanes of improved thermostability. The cyclodisilazane oligomers Ilia (see scheme below) are readily available from common and cheap materials, dichlorodimethylsilane and ammonia (6). The chlorine atoms in these compounds can be easily exchanged to other functional groups, such as amino, hydroxyl, etc.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

W. Fink (13) and K . A . Andrianov (28) and others reported a series of attempts to combine the cyclodisilazane monomers and oligomers into a polymeric system with siloxanes. In most of these syntheses some ring cleavage and crosslinking of the linear block copolymer occurred due to the susceptibility of the disilazane ring to nucleophilic attack by the functional groups of the second component of the reaction mixture. The only fully successful attempt seems to have been the polycondensation of the lithium salts of hydroxycyclosilazanes or hydroxy siloxanes with chlorofunctional comonomers (13). In the present work it has been found that the side reactions resulting in cleavage of the disilazane ring can be largely avoided if comparatively long polysiloxanediol chain molecules are made to react with the oligomeric dichlorocyclodisilazanes where the hydrogen chloride formed is carefully and continuously neutralized by an amine. The idealized polycondensation of cyclodisilazane oligomer Ilia, composed of an average number of 8.36 rings, with commercial α,ω-dihydroxypolydimethylsiloxanes of 100, 150 and 200 siloxane units in the average chain is represented below, as well as the structure of the cyclodisilazane-siloxane linear block copolymer which is formed. The reaction was carried out in benzene at 80° and heating the residue at material completely soluble in benzene, toluene and some other solvents. The intrinsic viscosity of toluene solutions is increased on passing from the initial polysiloxanes to the copolymers; i.e., from [η] = 0.113 - 0.159 to [η] = 0.225 0.325. The IR spectrum of the copolymer contains all the bands attributable to the silazane ring. It can be concluded that the polycyclodisilazane blocks have been preserved and the copolymer has a linear structure. On the other hand, crosslinking and gel formation occurred when equivalent amounts of oligomeric dimethylsiloxane-a,co-diols were taken as components; e.g. on polycondensation of Ilia with 1,7-dihydroxyoctamethyltetrasiloxane rubber-like, insoluble material was formed.

Me I Cl-Si

ι

.MB

Me

Me

Me Me i—Me—ι I I I CI + HO-Si-O H ~ -r^Si-OH I I I Me J n Me Me

X

4-N'" N

S

—Si

v

Si^ Me' M e

Me

v

Me

I

Me Me

I

(-Et NHCl)

3

6

I

Me

0

x = 8.36 n = 100,150,200

Et N C H

(Ilia)

i

3

6

.Me

HCkSi-O'wvSi-O-Si +-N I I ! 'Si' Me Me Me M e ' Me s

ΜΓΊ

I Me

^

4-0-vw.Si-0-Si 4- ΝΓ -Si Si^ » Me Me' Me ^ s i

I

K

N

Me Me

I . O'w^Si I Me

The copolymers have a remarkably improved thermostability with respect to the polysiloxanediols. While the polysiloxane homopolymers undergo 15% weight loss at 350-360°C., as determined by the thermogravimetric experiments, the same percent weight loss is observed at 4 3 0 - 5 1 0 ° C for various samples of

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

13.

LASOCKI ET AL.

Si-N-Containing

175

Rings and Polymers

cyclosilazoxane-siloxane block copolymers obtainedfromequivalent proportions of the reagents. However, increased stability by ca. 50°C (using the same test) was also evident for a sample prepared with 3% (w/w) of cyclosilazane oligomer, which corresponds to 25% of the equivalent amount. Thus, copolymerization of polysiloxane diols with dichlorocyclodisilazanes is an efficient method of modifying polysiloxanes to achieve considerably higher thermostability of the material. Polysilazanes with Aromatic Spacing Groups (30)

The susceptibility of cyclodisilazanes to nucleophilic attack by aromatic amines has also been used to prepare silazane containing polymers. Polysilazane cyclo-linear chains with aromatic spacing groups, synthesized by polycondensations of difunctional cyclodisilazanes with bis-phenols and NjN'-diorganosilane diamines, have been reported (13). In the present work a series of N,tf-diphenyltetramethylcyclodisilazanes with substituents on the benzeneringwere prepared from bis(phenylamino)silanes (23) (for spectroscopic data (31)) and th t d HIb treated with aniline and phenylene-diamine HIb with aniline yield bis(phenylamino)tetramethyldisilazan (se scheme below)ina reversible reaction whose equilibrium is shifted to therightwith decreasing temperature. With m-phenylenediamine, the unsymmetrical linear disilazane IVb' is formed which undergoes recyclization to the unsymmetrical cyclodisilazane HIb' The following set of equilibria is thus established.

(IVb)

M e

2

(nib')

On prolonged heating, further cleavage reactions of the silazaneringsoccur and a fraction, consisting of linear oligomers of the idealized structure shown below, was collected as a resinous residue after evaporating the lower boiling material. The molecular weight of the oligomeric fraction is ca. 1000 and the structure is established by IR and NMR spectroscopy and elemental analysis.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

176

INORGANIC AND ORGANOMETALLIC POLYMERS

NH-Si(Me2)-N-Si(Me2)-NH

Si

-φ

•Ν

Si Me

m

2

Similar results were obtained when o-phenylenediamine was reacted with HIb. A range of low boiling compounds were separated from the reaction mixture after 10-12 hr heating at 220°C. These have been identified as diazadimethylsilaindane Vb (m.p. 76-78°C.; lit. (32) m.p. 75-83°C) and bis(phenylamino)dimethylsilane VIb (m.p. 46-48°C.; lit. (31) m.p, 45 ± 2°C). They are probably formed by thermal decomposition of the intermediate unsymmetrical disilazane IVo". At 190-225°C/0.3 torr, a crystalline fraction with

^^NH-S^Me^-NH^^ 2

vb — • (ΟΓ

jfCH

(Vllb)

NH-Si(Me2)-NH Me2

.Si

Vllb + VIb

•Cg^

Me

2

(VHIb)

NH NH ^Si^ Me 2

m.p. 209-211°C was collected. The analytical data for this compound suggest structure VHIb, (for C18H28N4S13: Mcaic 384.7, m/e 384.7; cale %:C.,56.19; H, 7.34; N, 14.56; Si, 21.90; found %:C.,56.22; H, 7.56; N, 15.19; Si, 21.79; IR (υ, cm-l): 3381 s (NH), 970, 890 s (Si-N-Si); H NMR (δ, ppm, SiMe int. standard) 0.51 d (18 H) S1CH3, 3.82 s (2 H) NH, 6.38-6.75 m (8 H) HATl

4

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

13.

LASOCKI ET AL.

Si-N-Containing

Rings and Polymers

177

V I H b is formed by dimerization of the diazasilaindane, V b , and subsequent reaction of the cyclic dimer, V l l b , with bis(phenylamino)dimethylsilane, VIb, as it was shown in separate experiments starting from V b and V I b . It was subsequently found that V H I b is formed in fairly good yield whenever o-phenylenediamine is brought into reaction with difunctional silanes, such as dichlorodimethylsilane, bis(diethylamino)dimethylsilane and hexamethylcyclotrisilazane. Some negligible amounts of linear oligomers, obtained as a non-volatile, solid brown residue, were also formed on reaction of HIb with o-phenylene­ diamine; these were almost completely soluble in toluene. No high molecular fraction was detected among the reaction products. When HIb is heated with p-phenylenediamine at 2 0 0 - 3 0 0 ° C under conditions similar to the other phenylenediamines, mainly insoluble, probably crosslinked, oligomers (rubber-like solid) are formed. On the other hand, pphenylenediamine with bis(diethylamino)dimethylsilane at 180°C yielded a toluenesoluble solid whose molecular weight and elemental analysis agree well with the probable cyclophane structure I X ; for C32H48N8S14: M i c 657.16 found Mcryoscopic 648, m/e 657.16 found %: C., 58.40; H , 7.42 is being further investigated. H

H

(IX) ΗΝ

NH

Thus, the tendency of silazanes to form ring structures, including macrorings, is extraordinary. Conclusion Cyclosilazanes are found to be reluctant to polymerize by the ring-opening process, probably for thermodynamic reasons. On the other hand, six- and eight-membered silazoxane rings are able to undergo anionic polymerization under similar conditions to those which have been widely used for cyclosiloxane polymerization provided there is no more than two silazane units in the cyclic monomer. They can also copolymerize with cyclosiloxanes; however, the chain length of the linear polymer formed is substantially decreased with increasing proportion of silazane units. The thermostability of siloxane-silazane copolymers of both random and block structure is found to be much higher (i.e. 100-200°C) with respect to polysiloxanes. This effect is brought about by introducing only a few silazane entities into the polymer chain. The reasons for the effect are not clear and the mechanism of thermal degradation of polysilazoxanes will require further experimental studies.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

178 Acknowledgments

This work was supported in part by the Polish Academy of Sciences, Programs CPBP 03-13 and 03-14. I am sincerely grateful to Ms. Sharon Fricke (IUPUI) for typing and producing the manuscript.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

Dejak, B.; Lasocki, Z. J. Organometal. Chem. 1972, 44, C39. Klebe, J. F. J. Amer. Chem. Soc. 1968, 90, 5246. Boer, F. P.; vanRemoertere, F. P. J. Amer. Chem. Soc. 1970, 92, 801. Klebe, J. F. Acc. Chem. Res. 1970, 3, 299. Dejak, B.; Lasocki, Z. J. Organometal. Chem. 1983, 246, 151. Fink, W. Helv. Chim. Acta 1968, 51, 1011. West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343. Gilette, G. and West, R. presented at the 19th Organosilicon Symposium, Lousiana State University, Baton Rouge, LA, April 26-27, 1985. Dejak, B.; Lasocki, Z. 275. Dejak, B.; Lasocki, Z. unpublished results. Engelhardt, G.; Magi, G.; Lipmaa, E. J. Organometal. Chem. 1973, 54, 115. Aylett, B. J. Organomet. Chem. Rev. 1968, 3, 151. Fink, W. J. Paint. Technol. 1970, 42, 220. VanWazer, J. R.; Moedritzer, K. J. Chem. Phys. 1964, 41, 3122. Moedritzer, K. Organomet. Chem. Rev. 1966, 1, 179. Redl, G.; Rochow. E.G. Angew Chem. 1964, 76, 650. Matsumoto, M.; Niwata, K.; Tanaka, S. presented at the 7th International Symposium on Organosilicon Chemistry, Gunma Symposium, September, 1984. Kirichenko, Ε. Α.; Ermakov, A. J.; Samsonova, I. N. Zh. Fiz. Khim. U.S.S.R. 1977, 51, 2506. May, L. Α.; Rumba, G. Ya. Latv. P.S.R. Zinat. Akad. Vest., Khim. Ser., 1964, 1, 23. Lasocki, Z.; Witehowa, M., Syn. React. Inorg. Metal-Org. Chem. 1974, 4, 231. Lasocki, Z.; Witekowa, M. unpublished results. Lasocki, Z.; Witekowa, M. J. Macromol. Sci. Chem. 1977, A11, 457. Kulpinski, J.; Lasocki, Z.; Piechucki, S. Bull. Pol. Acad. Sci. Chem. (in press). Zeldin, M.; Quian, B.; Choi, S. J. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 1361. Kang, D. W.; Rajendran, D. P.; Zeldin, M. J. Polym. Sci., Polym. Chem. Ed. 1986, 24, 1085. Lasocki, Z.; Witekowa, M. J. Organometal. Chem. 1984, 264, 49. Lasocki, Z.; Witekowa, M. J. Organometal. Chem. 1986, 311, 17. Andrianov, Κ. Α.; Emelianov, U. N. Usp. Khim. 1977, 66, 2066. Kulpinski, J.; Lasocki, Z.; Lesniak, E. unpublished results. Kulpinski, J.; Lasocki, Z. unpublished results. Albanov, A. I.; Voronkov, M. G.; Dorokhova, V. V.; Kulpinski, J.; Larin, M. F.; Lasocki, Z.; Piechucki, S.; Brodskaia, E. I.; Pestunovich, V. A. Izv. Akad. Nauk U.S.S.R. 1982, 1781. Wieber, M.; Schmidt M. Z. f. Naturforschung 1963, 18b, 849. Anderson, H. H. J. Amer. Chem. Soc. 1951, 73, 5802.

RECEIVED October 13, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 14

Recent Advances in Organosiloxane Copolymers 1

2

J. D. Summers, C. S. Elsbernd, P. M. Sormani , P. J. A. Brandt , C. A. Arnold, I. Yilgor , J. S. Riffle, S. Kilic, and J. E. McGrath 3

4

Department of Chemistry and Polymer Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg VA 24061 Organosiloxane copolymers have been of great interest over the past several years due to the somewhat unique c h a r a c t e r i s t i c s imparted by the siloxane blocks. The excellent UV and oxidative resistance of these structures together with t h e i r outstanding thermal s t a b i l i t y and wide temperature use range make them candidates as modifiers for a multitude of applications. Organosiloxane materials have now been prepared which demonstrate a variety of interesting properties such as atomic oxygen resistance, biocompatibility, high gas permeabilities, and hydrophobic surfaces. In t h i s paper, the synthesis and properties of functional polysiloxane oligomers which are the precursors for the copolymers are discussed. A d d i t i o n a l l y , a summary and review of the preparation and c h a r a c t e r i s t i c s of the organosiloxane copolymers is given. The general theme of our research focuses on multiphase copolymer systems obtained v i a l i v i n g polymers, engineering polymers (both thermoplastics and thermosets) and polysiloxane systems. Organosiloxane copolymers have been of i n t e r e s t f o r a number of years due to the unusual c h a r a c t e r i s t i c s of polysiloxanes, such as their thermal and UV s t a b i l i t y , low glass t r a n s i t i o n temperature, very high gas permeability and, e s p e c i a l l y , their low surface energy character­ i s t i c s ( 1 - 2 ) . In the polydimethylsiloxane chain, 1, many workers have demonstrated that the methyl groups are oriented toward the a i r or vacuum surface. This provides a hydrophobic, low surface energy c h a r a c t e r i s t i c , which has a number of secondary but important applications including enhanced biocompatibility and more recently, resistance to atomic oxygen and oxygen plasmas. Our current work on 1

Current address: Experimental Station, Ε. I. du Pont de Nemours and Company, Wilmington, DE 19898 Current address: Specialty Chemical Division, 3M Center, St. Paul, MN 55144 Current address: Mercor, Inc., Berkeley, CA 94710 Correspondence should be addressed to this author. 0097-6156/88/0360-0180$06.00/0 © 1988 American Chemical Society

2

3

4

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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181

Organosiloxane Copolymers

siloxane chemistry includes the preparation of functional oligomers both by e q u i l i b r a t i o n processes employing the commonly available c y c l i c tetramer, Ό^, and to some extent, l i v i n g polymerizations whic u t i l i z e the lithium siloxanolate i n i t i a t e d polymerization of the c y c l i c trimer, D (1-4). The novel oligomers are then incorporated into high performance block, segmented and g r a f t copolymers. 3

~Si-0'

I CH„ Functional Polysiloxane Oligomers For e q u i l i b r a t i o n processes, one must synthesize both oligomers and what are termed dimers or disiloxanes Our primary i n t e r e s t is in the u t i l i z a t i o n of thes both l i n e a r block or segmente oughened networks such as the epoxy and imide systems (3-27). The generalized structure of the oligomers of i n t e r e s t is shown in Scheme 1.

CH.

Y

I I

CH,

I

X-R-Si-0-(-Si-0-)-Si-R-X CH

Y

3

CH

Variables:

X, R, Υ, η

3

Scheme 1. General Structure of an α,ω-Difunctional Polysiloxane.

The o v e r a l l synthesis of f u n c t i o n a l l y terminated oligomers involves e q u i l i b r a t i o n of the c y c l i c tetramer in the presence of a functional disiloxane as i l l u s t r a t e d in Scheme 2. In this case, one u t i l i z e s a c a t a l y s t which can be e i t h e r an a c i d i c or basic moiety. A basic c a t a l y s t (for example) attacks the s i l i c o n of the tetramer and i n i t i a t e s the ring-opening polymerization. However, in the presence of the disiloxane, the polymerization undergoes what is e f f e c t i v e l y a very useful chain transfer reaction. This occurs because, l i k e the monomer and growing chain, the silicon-oxygen-silicon bond of the disiloxane is s u f f i c i e n t l y polar that i t is attacked by the active ionic species and, thus, takes part in the e q u i l i b r a t i o n process. On the other hand, one takes advantage of the fact that the s i l i c o n a l k y l (or aryl) bond is more covalent in nature and, thus, stable during the polymerization. The e q u i l i b r a t i o n process is generalized in Scheme 3 wherein D " refers to the c y c l i c , tetrameric s t a r t i n g material and "MM" denotes a disiloxane with a terminal group associated with each "M". This shorthand nomenclature system is one t r a d i t i o n a l l y u t i l i z e d by siloxane chemists. As one can note from Scheme 3, the dimer w i l l be incorporated into the chain and the chain ends w i l l be i d e n t i c a l with the terminal units of "MM" or, as we have c a l l e d i t , the disiloxane. This entire concept is well-known and, in n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

182

INORGANIC AND ORGANOMETALLIC POLYMERS

I 4-Si-O-}I Y +

"Cyclics" χ = 3 or 4 Y = CH , C H , CH = CH , or CH CH CF

X

CH.

!

CH

I

3

3

6

5

2

2

2

3

0

3

X-R-Si-O-Si-R-X II CH CH

"End-Blocker" R = a l k y l , a r y l , aralkyl or a chemical bond X = -NH , -N(CH ) r "OH, " C l , 2

Catalyst, heat

3

2

Λ -CH-CH

|

2

CH.

Y

I I

I

X-R-Si-O-f-Si-O-4—Si-R-X

I C H

I Y

3

Ί C H

3

+

Scheme 2.

Cyclics General Synthesis of "End-Blocked" Polysiloxanes v i a E q u i l i b r a t i o n Processes.

x D

(x + 4)

4

+ MM

• MD M χ

χ cat. MD M + MD M x y Scheme 3.

• MD.

, .M + MD. .M (x+w) (y-w)

E q u i l i b r a t i o n Polymerization Processes (Shown u t i l i z i n g base catalysis)*

f a c t , methyl terminated polydimethylsiloxane f l u i d s and o i l s have been prepared in this way f o r some time. However, the additional feature which we have focused on is to design the endgroups to be organofunctional. Many oligomers with various f u n c t i o n a l i t i e s have been prepared in our laboratory ( 4 ) . However, the amino terminated species has been studied most extensively due to i t s wide u t i l i t y as a component of a large number of segmented copolymers (e.g. imides, amides, ureas, e t c . ) . In order to prepare functional oligomers of t h i s type, one must f i r s t prepare the disiloxane. One route to t h i s was pointed out some years ago by Saam and Speier (28). They showed that i t was possible to react allylamine with a protecting reagent such as hexa-

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Organosiloxane Copolymers

methyldisilazane to produce the protected amine material, plus ammonia which could be removed by d i s t i l l a t i o n . The protected allyamine group undergoes h y d r o s i l y l a t i o n with dimethylchlorosilane in the presence of various platinum c a t a l y s t s (such as c h l o r o p l a t i n i c acid) to produce mostly the chlorosilane intermediate. Purification of t h i s material by vacuum d i s t i l l a t i o n is desirable. In order to prepare the disiloxane, one may hydrolyze the chlorosilane to produce f i r s t a s i l a n o l , which r e a d i l y undergoes dehydration to produce a stable siloxane bond. At the same time, water is able to e a s i l y hydrolyze the l a b i l e s i l i c o n - n i t r o g e n bond to regenerate the desired primary amine group. Many other methods have been reported for the preparation of analogous disiloxanes. For example, unsaturated n i t r i l e s can be hydrosilylated and subsequently reduced to produce an amine group. A l t e r n a t i v e l y , allylamine can also be protected v i a imine formation. The general area is of great interest now since i t has been demonstrated that the available l i n e a r siloxane segmented copolymers have a variety of interesting properties A number of other reactive endgroup aromatic amines, silylamines and silanes (4). In addition to the dimethylsiloxane repeat unit in the backbone, i t is possible to produce a number of other important structures, such as methyl-vinyl, diphenyl, trifluoropropyl-methyl, and methyl-silane. A l l of these have been demonstrated to be useful for various reasons in our laboratory and elsewhere. The synthetic process for oligomeric species is to react the aminopropyl-functional disiloxane with the c y c l i c tetramer as i l l u s t r a t e d in Scheme 4. (CH )

I CH

II

CH 3

1

CH

2

1

CH

s

2

Si

/

H-NH-CH.-^Si-O-Si-fCH-^r-NH- + (CH ) - S i 2 23. ι 2 3 2 0

3

\

0 \«.' Si

Si"(CH )

I { C E

(DSX)

CH.

I J

3

+ J catalyst heat CH.

I J

0

CH Θ Θ Θ Θ

3

( D

4

)

3

H N-fCH ) '•( "Si-0-^-Si-fCH -hr-NH 2 2 3 η Ζ ό Ζ 0

]

3 2

CH

o

0

+

Cyclics

3

C y c l i c s are removed v i a vacuum stripping Catalyst must be neutralized for maximum s t a b i l i t y Mn is controlled at equilibrium v i a i n i t i a l [D 1/[DSX] r a t i o Other endgroups possible with appropriate DSX and catalyst 4

Scheme 4.

Synthesis of Difunctional Bis-(Aminopropyl) Polydimethylsiloxane Via An E q u i l i b r a t i o n Polymerization Process,

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INORGANIC AND ORGANOMETALLIC POLYMERS

Here one may choose a suitable catalyst, t y p i c a l l y a potassium or tetramethylammonium siloxanolate. The process then goes through a series of e q u i l i b r a t i o n steps (as discussed e a r l i e r ) to produce the ring-chain equilibrium between the linear oligomer and the c y c l i c species. C y c l i c species are predominantly the tetrameric c y c l i c , which can be removed by vacuum d i s t i l l a t i o n . In the case of the preferred quaternary ammonium catalysts, one may decompose the catalyst by simply heating the reaction mixture b r i e f l y to about 150°C. This w i l l deactivate the catalyst, as indeed was pointed out a number of years ago by G i l b e r t and Kantor (29). E f f e c t i v e l y , i t is possible to obtain a stable, l i n e a r , functionalized oligomer free of c y c l i c s . This can be v e r i f i e d v i a chromatography (e.g. GPC and/or HPLC). In the case of a potassium siloxanolate catalyst, one must neutralize the equilibrated product (for example, with ion exchange systems) to achieve maximum s t a b i l i t y . We have already demonstrated that i t is possible to synthesize a variety of molecular weights. The number average molecular weight at equilibrium is governed by the i n i t i a l molar r a t i o of th general, the DSX incorporate ammonium catalyst. Some c h a r a c t e r i s t i c s of aminopropyl terminated polydimethylsiloxane oligomers are i l l u s t r a t e d in Scheme 5.

Mn (q/mole) 600 1000 1800 2400 3800 CH. i 3 H

2 N

Tq°C

6 11 22 30 50

-115 -118 -121 -123 -123

CHι 3

-eCH t3-f-Si-0-^Si-eCH +3-NH 2

2

CH

Scheme 5.

η

3

CH

2

3

Characteristics of Aminopropyl Terminated Polydimethylsiloxane Oligomers Synthesized by a Base Catalyzed E q u i l i b r a t i o n Process,

The number average molecular weights of the oligomers can be obtained yia t i t r a t i o n of the amine endgroups. They can also be estimated by H-NMR up to reasonably high molecular weights, by comparing the integrated r a t i o of the methylene protons at the end of the chain to the s i l i c o n methyls in the backbone. The t i t r a t i o n values for the endgroups and the number average molecular weight determined by proton NMR agree quite nicely in the case of the polydimethylsiloxane system described bearing α,ω-aminopropyl groups. Hydroxyalkyl terminated oligomers are not so simple. Problems can occur i f the hydroxyl group interacts with the growing species to produce a silicon-oxygen-carbon bond. In this case, one may f i n d e f f e c t i v e l y

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Organosiloxane Copolymers

lower -OH concentrations than anticipated. In f a c t , i t may be necessary to also block the hydroxyl group during the e q u i l i b r a t i o n . Organosiloxane

Copolymers

During the past few years (4, 15-27, 30-36), we have prepared block or segmented copolymers from a variety of hard segments. In p a r t i c u l a r , we have focused on polyureas, polyurethanes, polyamides and polyimides. In addition, we have u t i l i z e d these oligomers to modify epoxy networks. Thus, the e q u i l i b r a t i o n procedure works extremely well from a synthetic point of view. Although r e l a t i v e l y l i t t l e (1,7,37) has been reported with respect to the detailed k i n e t i c s and mechanisms involved in these o v e r a l l processes, we have begun to explore this important aspect and some of our results have been recently reported (38,39). In this paper, a review of the u t i l i z a t i o n of polysiloxane oligomers for the preparation of soluble, high performance segmented polyimide-siloxane copolymers and various elastomeric copolymers w i l Much of the early e f f o r in reference 9. In the synthesis of organosiloxane copolymers i t should be noted that one has the choice of introducing either a silicon-oxygen-carbon link or a silicon-carbon l i n k between the two d i s s i m i l a r segments. In small molecules, quite d i s t i n c t l y d i f f e r e n t hydrolytic s t a b i l i t i e s are observed. For example, one often uses silicon-oxygen-carbon bonds as protecting groups which are stable under anhydrous neutral or basic conditions, but which quickly revert when treated with aqueous a c i d i c environments. The benefit of the silicon-carbon bond is that i t is s i g n i f i c a n t l y more h y d r o l y t i c a l l y stable than the silicon-oxygen-carbon bond. However, Noshay, Matzner and coworkers (9) showed that the silicon-oxygen-carbon bond in hydrophobic high molecular weight copolymers was much more stable than would have been predicted from small molecule considerations. Apparently the hydrophobic environment protects the p o t e n t i a l l y hydrolyzable group. The chemistry for synthesizing functional polysiloxane oligomers appropriate for producing block copolymers with silicon-carbon links between the blocks has been reviewed e a r l i e r in this monograph. Representative structures have been depicted in Scheme 1. One generally prepares the Si-C linked f u n c t i o n a l i t i e s by conducting h y d r o s i l y l a t i o n reactions between Si-Η groups and the corresponding unsaturated, functional endgroup. In some cases, as previously described, the endgroup must either f i r s t be protected or subsequently further reacted after h y d r o s i l y l a t i o n , in order to f i n a l l y produce the desired functionality. Polysiloxanes, 2, appropriate for producing Si-O-C units between the blocks in copolymers are shown below. The copolymerization reaction involves 0 II

X = N(CH ) , OCH CH , CI, 0"C-CH 3

2

2

3

3

2 Y = CHl

CH

3

Y

CH, 3

3'

CH=CH2' CH CH CF 2

2

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

attack of a nucleophile on the terminal s i l i c o n atom of the siloxane with either concerted or subsequent elimination of the functional group "X". Noshay and coworkers developed an interesting route for the preparation of p e r f e c t l y alternating siloxane poly(arylene ether sulfone) copolymers v i a reaction of hydroxyl groups with silylamines (X - N(CH ) in 2) which is reviewed in reference 9. A variety of d i f f e r e n t hard segments were incorporated into the resulting copolymers. These workers observed that, in fact, the p r o c e s s i b i l i t y of these copolymers was very dependent upon the d i f f e r e n t i a l s o l u b i l i t y parameter between the hard segment and the non-polar polydimethylsiloxane block (14). In some cases, the microphase separation was apparently so well developed that the materials developed an extremely high v i s c o s i t y in the melt, rendering them e s s e n t i a l l y non-processible. This phenomenon occurred even though the materials were l i n e a r , and indeed, solvent-castable from a variety of appropriate organic l i q u i d s Although this v i s c o s i t y e f f e c t is general with microphas p a r t i c u l a r l y pronounced Despite some of the melt fabrication problems, the organosiloxane systems produced by the silylamine-hydroxyl reaction (9) produced interesting, perfectly alternating copolymers. 3

2

Polycarbonate (15-18) and aromatic polyester (19,20) copolymers produced using this same general route have been investigated in our laboratories. The chemistry is i l l u s t r a t e d in Scheme 6. The perfectly alternating copolymers developed very uniform morphology (17) reminiscent of the t r i b l o c k styrene-diene materials. Typical s t r e s s - s t r a i n and dynamic mechanical behavior is shown in Figures 1 and 2. By contrast, i f the copolymers were prepared by either a randomly coupled route or v i a an in-situ generation of the hard block (16,17), the morphology was not regular and, indeed, the physical properties of both elastomeric and r i g i d compositions were quite d i f f e r e n t from those of the perfectly alternating systems. The aromatic polyester/siloxane copolymers synthesized in our laboratories included a series of siloxanes which were themselves comprised of "blocky" structures. Both diphenylsiloxane sequences as well as trifluoropropyl-methylsiloxane units were combined with dimethylsiloxane units. These copolymers are of considerable i n t e r e s t as multiphase damping materials (19) due to the fact that two, three or even more relaxations can be designed into these copolymers as a function of block length and composition. Indeed, s i g n i f i c a n t loss peaks ranging from -120 to well over 200°C have been achieved (19,20). Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end f u n c t i o n a l i t y present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A c h a r a c t e r i s t i c s t r e s s - s t r a i n curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. I t was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best s i l i c a reinforced, chemically crosslinked, s i l i c o n e rubber

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Organosiloxane Copolymers

187

m a t e r i a l s d e s c r i b e d in t h e l i t e r a t u r e . Various other fundamentally o r i e n t e d s t u d i e s which d e s c r i b e t h e k i n e t i c s and mechanisms of t h e s y n t h e s i s o f t h e s e o l i g o m e r s a r e p r o v i d e d in t h e r e f e r e n c e s h e r e i n . A number of o t h e r h a r d segments such as t h e i m i d e s and amides have a l s o been i n v e s t i g a t e d . One was a b l e t o show a good c o r r e l a t i o n

STRAIN.%

Figure 1. Stress-strain curves of polycarbonate-polydimethylsiloxane block copolymers (Crosshead Speed: 5 cm/min). (Reproduced from Refs. 1 5 , 18. Copyright 1980, American Chemical Society.)

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

Figure 2. Storage moduli and loss tangents versus temperature f o r three t y p i c a l "block copolymers; Frequency: 11 Hz; IYL of blocks expressed in g/mole. (Reproduced from Refs. 15» 18. Copyright 1980, 198^ American Chemical Society.)

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

14. SUMMERS ET AL.

A B C D Ε

Organosiloxane Copolymers

PSX-1150-HMDI-81 PSX-2740-HMDI-91 PSX-3740-HMDI-96 Filled high structure silica Filled low structure silica

Modulus (ΜΡα) 110.4 56 41

Figure 3 . Stress versus % elongation behavior f o r siloxane-urea segmented copolymers from H-MDI as a function of molecular weight of oligomer used and the hard segment content a t 25 °C. (Reproduced with permission from Ref. 2 5 . Copyright 1984 IPC Business Press, Ltd.)

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

189

190

INORGANIC AND ORGANOMETALLIC POLYMERS

Figure 4. Dynamic m e c h a n i c a l b e h a v i o r o f s i l o x a n e - u r e a copolymers p r e p a r e d from H-MDI. Curve A: PSX-1 50-HMDI-81; c u r v e Β : PSX1770-HMDI-87; c u r v e C: PSX-770-HMDI-91; c u r v e D: PSX-3680-HMDI94. (Reproduced w i t h p e r m i s s i o n from R e f . 2 5 . C o p y r i g h t 1984 IPC B u s i n e s s P r e s s , L t d . )

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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191

Organosiloxane Copolymers

I

CH, 3

0 „

CH, I 3

H-f-O-C^H—C-C^H—0-C-^-0-CH—C-C^HT-OH 6 4 J 6 4 a 6 4 | 6 4 L

J

CH

CH

3

(A)

CH.

CH, 3 (CH, ) ,Ν-hsi-cHrSi-N ( CH )

I

I

j

J Z

D

CH

3

J

ο

CH (B)

3

Solvent

—f—A-Β—]— n 1

Scheme 6.

Δ

(e.g.

Chlorobenzene)

+ HN(CH,) 3 2

J

Perfectly Alternating Polycarbonate-Polydimethylsiloxane Block Copolymers.

I

j

CH, 3

CH, 3

H N- ( CH ) J-hsi-0-}^Si-(CH ) Ν Η 2

2

2

CH

Γ

2

CH,

0

OCN-R-NCO

(aromatic or cycloaliphatic)

+

H N-R -NH

(chain extender, optional)

J H

? 3

-f-rUREA HARD^j-t—Si-0 ) J ^ SEGMENT j fe

J

n

a

Scheme 7.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

between the i n t e n s i t y of the hydrogen bond formation and the ultimate mechanical properties at room temperature. In t h i s regard, the urea linked systems were preferred. On the other hand, thermal s t a b i l i t y was best for the polyimide systems. Brief discussion of the novel surface properties in these elastomers is appropriate. The bulk microphase separation characteristics of thermoplastic elastomers has been studied in great d e t a i l and is very important. However, less information is known about the surface structure in these two-phase materials. A number of investigators have been interested in these areas. Some of the f i r s t work, described in reference 26, focused on the (at that time) somewhat unique siloxane enhancement at the air/vacuum surface in pure copolymers and even in homopolymer-block copolymer blends. E s s e n t i a l l y , the polydimethylsiloxane displays very low surface energy which provides a thermodynamic d r i v i n g force for migration to the a i r or vacuum i n t e r f a c e . In contrast to the homopolymer, the siloxane segment is chemically linked and cannot macroscopically phase separate. X-ray photoelectro ESCA) conducted demonstrate predominantly siloxane, even when the bulk siloxane compositions were r e l a t i v e l y low. An important c r i t e r i o n , though, was the development of microphase structure in the bulk, which apparently frees the siloxane microphase to more e a s i l y migrate to the a i r or vacuum interface. Many other studies of this phenomenon have appeared in the l i t e r a t u r e since that time. In the cases of the siloxane modified polyimides, discussed b r i e f l y below, i t is of great interest in both the area of oxygen plasma processing and atomic oxygen resistance in outer space applications. In the presence of atomic oxygen, apparently the siloxane structure on the surface is converted to an organosilicate-type, ceramic-like material which provides protection during the etching process. Further discussion of t h i s phenomenon is provided in references 32 and 40-43. Although most of the multi-block or segmented thermoplastic elastomer studies have u t i l i z e d the readily available c y c l i c tetramer (D^) structure as a starting monomer (4), there has also been i n t e r e s t in materials derived from the c y c l i c trimer ί ^). c y c l i c trimer has the advantage that one can produce predictable molecular weights with narrow d i s t r i b u t i o n s , due to the a b i l i t y for the c y c l i c trimer to produce l i v i n g lithium siloxanolate chain ends. This has been described by a number of authors (9). Recently, polymers based upon t-butylstyrene and (44) have been studied. These materials, again, r e l y somewhat upon the lower s o l u b i l i t y parameter associated with the t-butylstyrene group r e l a t i v e to styrène. As a r e s u l t , the subsequent block polymers that were prepared process e a s i l y and may lead to a t t r a c t i v e specialty elastomeric and r i g i d copolymers. As mentioned e a r l i e r , siloxanes impart a number of b e n e f i c i a l properties to polymeric systems into which they are incorporated, including enhanced s o l u b i l i t y , resistance to degradation in aggressive oxygen environments, impact resistance and modified surface properties. These p a r t i c u l a r advantages render polysiloxanemodified polyimides a t t r a c t i v e for aerospace, microelectronic and other high performance applications (40-43). 0

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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193

The thermal-mechanical properties of siloxane-modified polyimides are a function of the weight f r a c t i o n of incorporated siloxane, molecular weight of the siloxane blocks and chemical architecture of both the siloxane and imide segments. Thus, copolymers with high concentrations of incorporated siloxane (>50%), where the siloxane is the continuous phase, behave as thermoplastic elastomers, whereas lower siloxane concentrations r e s u l t in more r i g i d materials which behave e s s e n t i a l l y as modified polyimides. At high polyimide compositions, the upper glass t r a n s i t i o n value and most mechanical properties approach those of the unmodified controls. However, the surface structure can s t i l l be strongly dominated by the low surface energy polysiloxane microphase. The great difference in s o l u b i l i t y parameters of the siloxane and imide segments is a d r i v i n g force for microphase separation, p a r t i c u l a r l y when higher molecular weight siloxane oligomers are incorporated. A d d i t i o n a l l y , because the siloxane component possesses a r e l a t i v e l y low surface energy i t w i l l migrate to the a i r or vacuum i n t e r f a c e , y i e l d i n g a surfac This e f f e c t is observed eve incorporation. In aggressive oxygen environments, the surface siloxane segments convert to a ceramic-like s i l i c a t e (SiO ) which provides a protective outer coat to the bulk material. The conversion of polysiloxane to s i l i c o n dioxide in oxygen plasma has been documented within the l i t e r a t u r e of the electronics industry as well as in our laboratory. The copolymers investigated were largely based upon 3 , 3 ' , 4 , 4 ' benzophenone tetracarboxylic dianhydride (BTDA), the meta-substituted diamine 3,3'-diaminodiphenyl sulfone (DDS) and aminopropyl-terminated polydimethylsiloxane oligomers of various molecular weights in the range of 950 to 10,000 grams per mole. The incorporated siloxane oligomer has been varied from 5 to 70 weight percent. Conversion of the poly(amic acid) to the f u l l y imidized polyimide was accomplished by two d i f f e r e n t techniques. The f i r s t , and most common, route was via bulk thermal imidization with the loss of water. In t h i s case, temperatures near the glass t r a n s i t i o n temperature of the f u l l y imidized product must be employed in order to achieve complete imidization. In the second method, imidization was accomplished at lower temperatures in the range of 150 to 170°C by means of a solution imidization procedure ( 4 2 , 4 3 ) . Although in both cases, tough, transparent, f l e x i b l e and soluble films were obtained. The choice of imidization method a f f e c t s material properties. The solution imidized materials are markedly more soluble than the bulk imidized systems, f o r example, but their thermal and mechanical properties do not vary s i g n i f i c a n t l y . The polysiloxane-amic acid intermediates were prepared as previously described (42,43) then converted to the f u l l y imidized segmented copolymers by either the bulk or solution imidization route. The bulk-thermal imidization technique involved casting the amic acid solutions onto a substrate, then removing solvent in a vacuum oven at 100°C f o r several hours. The films were then thermally cycled at τ 0 0 , 200 and 300°C for an hour at each temperature in a forced a i r convection oven to complete the c y c l i z a t i o n . The solution imidization procedure has been further described by Summers, et. a l . ( 4 2 , 4 3 ) . A representative structure for the poly(imide siloxane) copolymers is depicted in Figure 5.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

M

MM

§

r

S3

Μ

ζ ο

>

Figure 5.

Structural Representation of Poly(Imide-Siloxane) Segmented Copolymers (PSX = Polydimethylsiloxane),

Ζ

s

ο ο > ζ > ζ ο

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Organosiloxane Copolymers

One of the major goals of t h i s endeavor was to s o l u b i l i z e the normally intractable polyimides by the incorporation of siloxane segments, and, optionally, by solution imidization. S o l u b i l i t i e s of a series of siloxane-modified polyimide copolymers were evaluated in a variety of solvents as indicated in Table I. Copolymer s o l u b i l i t y was found to be a function of the siloxane oligomer concentration

Table I.

S o l u b i l i t i e s of Bulk (B) and Solution (S) Imidized Poly(Imide Siloxane) Segmented Copolymers

Solvent

Control

20% PSX

40% PSX

60% PSX

B

B

B

B

S

S

S

S

N-methylprrolidinone (NMP)

S

S

M S

M S

M S

S

S

Dimethylacetamide (DMAc)

I

S

M S

Tetrahydrofuran (THF)

I

I

I

I

I

S

S

S

Methylene Chloride (CH C1 )

I

I

I

I

I

S

S

S

2

Key:

2

S = Soluble;

M » Marginally soluble;

I = Insoluble

as well as the method of imidization. The solution imidized copolymers and also the solution imidized control (no siloxane) were a l l soluble in dipolar, aprotic solvents such as Nmethylpyrollidinone (NMP) and dimethylacetamide (DMAc). The s o l u b i l i t y of the solution imidized copolymers and control in NMP or DMAc ranged from «»10 weight percent for the control t o «20 weight percent f o r the copolymers. At 40 weight percent siloxane, the solution imidized copolymers were soluble in a variety of alternative solvents, including tetrahydrofuran, methylene chloride and diglyme. The bulk imidized materials were less soluble than those which were solution imidized, requiring a t least 10 weight percent siloxane at s o l i d s concentrations of less than 5 percent to achieve s o l u b i l i t y in NMP and DMAc. The bulk imidized control was not soluble in d i p o l a r , aprotic solvents. I n t r i n s i c v i s c o s i t i e s of the copolymers ranged from 0.50 to 0.85 d l / g , i n d i c a t i n g that high molecular weight copolymers were obtained by both imidization techniques over the entire composition and segment molecular weight range. Values of the upper glass t r a n s i t i o n temperatures of the siloxane modified polyimides were found to be a function of both the l e v e l of incorporated siloxane as well as the siloxane molecular weight (Table I I ) . The upper t r a n s i t i o n temperature of the solution

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

196

Table I I . Upper Glass Transitions of Poly(Imide Siloxane) Segmented Copolymers

WT. % PSX

PSX Mn

CONTROL CONTROL

IMIDIZATION METHOD

[nl

(NMP

25°C)

BULK SOLN

1.36

Τ (°C) g 272 265

f

10 10

900 900

BULK SOLN

0.62 0.63

256 251

10 10

2100 2100

BULK SOLN

0.78 0.73

261 260

10

5000

BULK

0.71

264

10

1000

20 20

900 900

BULK SOLN

0.78 0.67

246 240

20 20

2100 2100

BULK SOLN

0.60 0.57

258 252

20

5000

BULK

0.51

262

40 40

900 900

BULK SOLN

0.55 0.58

225 218

imidized materials was found to be only s l i g h t l y depressed from i t s bulk imidized analogue. Generally, the upper t r a n s i t i o n temperature increased with greater s i l o x i n e oligomer molecular weight and with decreasing siloxane incorporation. In many cases, the copolymers' upper t r a n s i t i o n temperature was depressed only s l i g h t l y r e l a t i v e to that of the control, i n d i r e c t l y indicating that good microphase separation was achieved. The lower temperature siloxane t r a n s i t i o n is d i f f i c u l t to detect by DSC f o r low levels of siloxane incorporation, i . e . , 80%) synthesis of symmetric and unsymmetric organic anhydrides in well-stirred organic solvent/water suspensions from an acid chloride in the organic phase and a carboxylate ion in the aqueous phase in as little as 10 min at ambient temperature. The phasetransfer process is depicted in Scheme 1 in which the catalyst is acylated in CH2CI2 and the intermediate is carried into the water where reaction with a carboxylate ion generates the acid anhydride. Since the rate of anhydride diffusion into the organic phase is considerably faster than hydrolysis, cessation of agitation isolates phases and desired product. It has also been determined that there is selective transport of more hydrophobic carboxylate ions (e.g., p-toluate vs. isobutyrate) at the mixed solvent interface which suggests an interfacial component in the process. In order to examine more closely the question o activity and transport facility polymeric catalyst (viz. polysiloxanes) which can effect transacylation at the mixed solvent interface without the catalyst partitioning into the aqueous medium.

Scheme 1 ,

The feasibility of bonding pyridinyl groups to silicon which contains a hydrolytically sensitive functional group has recently been demonstrated (5-7). 2Fluoro-3-(dimethylchlorosilyl)pyridine and 3-fluoro-4-(dimethylchlorosilyl)pyridine as well as 2-, 3-, and 4-(dimethylchlorosilyl)pyridine were prepared by the reaction of the corresponding lithiopyridines with excess Me2SiCl2- Hydrolysis of the pyridinyl substituted chlorosilanes gave disiloxanes which were insoluble in water. In the present report we will describe extension of this work to include pyridinyl dichlorosilanes which can be hydrolyzed to polysiloxanes. These polymers can be N oxidized and the resultant derivatives have been shown to be effective hydrophobic transacylation catalysts.

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Catalysts for Transacylation Reactions

Results and Discussion Synthesis and Spectroscopic Characterization.

3-(Methyldichlorosilyl)-

pyridine (1) was synthesized by the reaction of 3-lithiopyridine, preparedinsim from 3-bromopyridine and n-butyllithium, with a large excess of methyltrichlorosilane at 76° (Scheme 2). The product is a colorless distillable liquid which is soluble in aromatic, aliphatic and chlorinated hydrocarbons and extremely sensitive to moisture and protonic solvents. The infrared spectrum of 1 exhibits a pair of intense absorptions at 505 cm and 550 cnr characteristic of symmetric and asymmetric stretching vibrations, respectively, of the S1CI2 group (8). Bands at 710(s)/760(vs) cm" and 455(m) cm" are consistent with r- and t-ring vibrations, respectively, associated with Si-C ring modes observed in phenylsilanes (9). The *H-NMR spectrum of 1 contains a singlet at 0.60 ppm assigned to Me3Si protons and a complex pattern of peaks in the aromatic region corresponding to the pyridinyl ring protons (viz. δ 8.57, d/d, H ; 8.33, d/d, H ; 7.60 d/t, H and 6.88, d/d/d H ) The integrated areas of the multiplets and comparison of the pattern wit with the assignments (10). The mass spectrum of 1 exhibits polyisotopic ion clusters with relative abundances diagnostic of a dichloro derivative for the molecular ion M * (m/e: 195, 193, 191) and fragment ions (M-CH )+ (m/e: 180, 178, 176) and (M-Py)+ (m/e: 117, 115, 113). The appearance of Py (m/e: 78) indicates a competition in fragmentation -1

1

1

1

2

6

4

5

+

3

+

H O - f SiMeO^-H χ

Scheme 2. in which ion current can be carried either by the resonance stabilized aromaticringor the electropositive metalloid. Conspicuous by its absence, but common to all silylsubstituted pyridines and their oxide derivatives, is the (M-HCN) * which is charac­ teristic of ring fragmentation elimination evident in MS of 3-substituted pyridines (11). Hydrolysis of 1 is carried out at room temperature by the slow addition of aqueous NH3 at 0°C (Scheme 2). The resulting hydroxyl end-functional polysiloxane (2) is dispersed as fine droplets in the aqueous medium and can be separated by centrifugation. When neat, 2 is a colorless fluid with molecular weight ranging from +

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

202 3

4

1 χ 10 to 1 χ 10 depending primarily on the length of time the polymerisheat-dried (e.g. < 100°C) under vacuum. Upon standing at room temperature for several days or heating for shorter periods of time above 100°C., the fluid becomes a rubbery semi­ solid characteristic of a high molecular weight polymer resulting from chain-end condensation. The IR spectrum of 2 displays a strong, broad band in the 1050-1100 cm region characteristic of the Si-O-Si stretching vibrationinsiloxanes (12). Additionally a strong broad band centered at 3,200 cm due to retained water is superimposed on terminal OH groups. Attempts to reduce or completely remove the water with conventional drying agents or by azeotropic distillation with organic solvents were unsuccessful. The *H-NMR spectrum of 2 in CDCI3 (Figure 1) exhibits broad unresolved resonances in the aromatic region similar to those found in the monomer. Broad signals with lack of resolution are consistent with magnetic non-equivalence of the methyl group protons resulting from a mixture of triad tacticities. Species 2 was end-blocked to give 3 and simultaneously dehydrated by refluxing with a mixture of (Me3Si)2N Me3SiCl clear colorless viscous fluid under vacuum or in air and is soluble in organic solvents but immiscible with water. The IR spectrum of 3 differs from 2 by the appearance of new bands at 845 cm-V870 cm" which are characteristic of the end-block Si(CH3)3 group (13). The intensity of the bands decrease with increase in polymer molecular weight thus supporting the assignment. There is also a significant decrease in the OH absorption confirming an anhydrous polymer. The !H-NMR spectrum of 3 is given in Figure 1. The principal difference between the spectrum of 2 and 3 is the appearance of a singlet at -0.07 ppm in 3 which has been assigned to the terminal trimethylsiloxyl protons. The relative area of the peak decreases with increase in molecular weight. Thus, if the molecular weightisnot too large (i.e. < 10,000), the ^ - N M R spectrum is a convenient method for determining the average degree of polymerization (viz. A / A = 1.4; D = 8.3 M = 1,300). The NMR data agree well with other methods of molecular weight determination. When 3 is treated with m-chloroperoxybenzoic acid oxidation at the ring nitrogen occurs (Scheme 2). Product 4 is a pale yellow viscous fluid which is soluble in organic solvents but immiscible with water having a partition ratio of 24:1 in a CH2CI2/H2O medium. The IR spectrum of 4 gives direct evidence for the presence of + the Ν - Ο bond with a characteristic stretching vibration at 1250 cm' (14). The band is intense and broad thus masking the much sharper CH rocking modes in the same region. It is noteworthy that a new medium intensity absorption occurs in 4 at 920 cm , which is a window in 3. This band is analogous to the one assigned to + the Ν - Ο deformation mode at 800-900 cm reported by Kireev et al. (14) and Shindo (15). The *H-NMR spectrum of a low molecular weight oligomer of 4 is given in Figure 1. A salient feature of the spectrum is the appearance of two rather than three unresolved signals in the aromatic region. Similar spectral changes (i.e. the merging of H » protons and the upfield shift of H » protons by ~ 0.5 ppm) upon N-oxidation have been observed with other 3-substituted pyridines (10). The position of the highfield methyl proton signalin4 is comparable to that foundin3. -1

-1

1

M e

M e

3

p

n

1

-1

-1

3

4

2

6

Thermal Analysis of 2. 3 and 4. The TGA of 2 and 3 (x = 10) under argon is given in Figure 2. Both polymers have approximately the same temperature at which

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Figure 1. 60 MHz *H NMR Spectra of Hydroxy Terminated-(2), Trimethylsiloxyl End-Capped-(3) and Trimethylsiloxyl End-Capped/N-oxidized-(4) Poly(methyl 3-pyridinylsiloxane).

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

JCC77

•Ρ 3»

100.0

300.0

400.0

Temperature

500.0 (*C)

600.0

700.0

Figure 2. T G A of Hydroxy Terminated-(2), Trimethylsiloxyl End-Capped-(3) and Trimethylsiloxyl End-Capped/N-oxidized-(4) Poly(methyl 3-pyridinylsiloxane); Heating Rate: 40°C/min, Under Ar.

200.0

800.0

w

r

ο

•τ

η

>

3 w Η

ο

ο >

ο w

> ζ

η

> g

Ο »

s

15.

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Catalysts for Transacylation Reactions

205

the onset of weight loss occurs, presumablyinpart by depolymerization (16,12). No attempt, thus far, has been made to identify the volatile products. If consistent with other polysiloxanes (e.g. Me2, Me/Ph), cyclic oligomers are formed. The larger residue fraction for the hydroxy terminated species (28% vs. 9%) may be the result of crosslinking by pyridinyl decomposition perhaps involving end-group (nucleophilic) attack at the 2/6 positions of the ring. The TGA of 4, however, indicates a lower onset of weight loss accompanied by a higher (40%) amount of residue. This suggests that the 1-oxide plays a significant role not only in chain scission but also in the crosslinking process. The order of stability from DSC data is 3 > 2 « 4 as judged from the exotherm maximum. In order to determine the nature of the degradation process, DSC experiments were carried out on 3 in which heating was terminated at different points in the heating program; i.e. prior to, at and post exothermic maximum. With the exception of the samples heated above 500°C., the residues from 3 were clear viscous yellow fluids solubleinCH2CI2. The IR spectrum of the soluble residues were almost identical to the unheated material, the only change being the appearance of a new weak band at 940 cnr which suggests some N-oxidation b ambient air Indeed the band is not present when precaution and the analyzer. The combinatio viscosity and solubility in organic solvents of the residues suggests that heating to 450°C promotes principally redistribution polymerization with little, if any, decomposition and crosslinking. In contrast, if 3 is heated above 500°C., the residue is a black insoluble solid with a relatively featureless IR spectrum containing broad bandsinthe SiOSi and SiC regions. The absence ofringmodes and the appearance of a sharp band at 2350 cm-1, which corresponds to the SiH vibration, implies that degradation involves crosslinking by pyridineringdecomposition and H migration. The latter is a well known pathway in mass spectrometry of organosilanes (IS). Clearly, 3 displays remarkable thermal stability for its molecular weight. Further studies of the thermal properties and degradation process of 2-4 are underway and will be reported elsewhere. 1

Catalysis in Transacylation Reactions. The principal objective of the study was to evaluate 4 as an effective organic soluble lipophilic catalyst for transacylation reactions of carboxylic and phosphoric acid derivatives in aqueous and two-phase aqueous-organic solvent media. Indeed 4 catalyzes the conversion of benzoyl chloride to benzoic anhydride in well-stirred suspensions of CH2CI2 and 1.0 M aqueous NaHC03 (Equations 1-3). The results are summarized in Table 1 where yields of isolated acid, anhydride and recovered acid chloride are reported. The reaction is believed to involve formation of the poly(benzoyloxypyridinium) ion intermediate (5) in the organic phase (Equation 1) and 5 then quickly reacts with bicarbonate ion and/or hydroxide ion at the interphase to form benzoate ion (Equation 2 and 3). Apparently most of the benzoate ion is trapped by additional 5 in the organic layer or at the interphase to produce benzoic anhydride (Equation 4), an example of normal phaseMe II

-6 S i - O *

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

Ο

Ο

Ο

transfer catalysis (19). The same result is observed for catalysis by 3-(trimethylsilyl)pyridine 1-oxide (6), l,3-dimethyl-l,3-bis-3-(l-oxypyridinyl) disiloxane (7) and poly(4-vinyl pyridine 1-oxide) (8). Interestingly, catalysis by pyridine 1-oxide (9) furnishes nearly equal amounts of anhydride and hydrolysis product. This has been taken as evidence for a high proportion of inverse phase-transfer catalysis (la), i.e., reaction between 1-benzoyloxypyridinium ion and bicarbonate ion in the aqueous phase. It is particularly significant that water-soluble (8) and organic-soluble (4, 6 and 7) catalysts behave quite similarly in two-phase media. This argues for a symmetry­ like behavior in catalysis at the interphase of the water-organic solvent suspension. Hydrolysis of diphenyl phosphorochloridate (DPPC) in 2.0 M aqueous sodium carbonate is also believed to be a two-phase process. DPPC is quite insoluble in water and forms an insoluble second phase at the concentration employed (i.e. 0.10 M). It seems highly significant that the hydrophobic silicon-substituted pyridine 1-oxides (4, 6,7) are much more effective catalysts than hydrophilic 8 and 9. In fact, 4 is clearly the most effective catalyst we have examined for this reaction (ti/2 < 10 min). Since derivatives of phosphoric acids are known to undergo substitution reactions via nucleophilic addition-elimination sequences (20) (Equation 5), we believe that the initial step in hydrolysis of DPPC occurs in the organic phase. Moreover, the

o

(RO) P-X + :Nu 2

o

-

0

(RO) P-X — (RO) P-Nu + :X 2

+

2

Nu

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

(5)

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207

3

Table 1. Catalysis in Transacylation Reactions

Benzoyl Chloride in CH2CI2 — 1.0 M NaHC03 Catalysts (0.10 equiv)

(9) (8) (6) (7) (4)

0

Acid (%)

0.4 42.0 8.3 4.9 8.1 5.0

Anhydride (%) Recovered Acid Chloride (%

0.0 49.4 81.0 91.6 79.1 89.3

>90.0 1.6 0.7 1.5 2.0 3.0

Diphenyl Phosphorochloridate in 2.0 M NaHC03 Catalysts (0.10 equiv)

(9) (8) (6) (7) (4)

D

Acid (%)

14.2 22.4 39.2 72.3 66.0 85.9

Recovered Acid Chloride (%)

72.6 71.9 47.3 22.9 23.5 10.7

a

Reaction mixtures contained either 4 mMole of benzoyl chloride or 1 mMole of diphenyl phosphorochloridate. Reactions were run for 30 minutes at 22-23°C. b

9, Pyridine 1-oxide; 8, Poly(4-vinylpyridine 1-oxide); 6, 3-(Trimethylsilyl)pyridine 1-oxide; 7, l,3-Dimethyl-l,3-bis-3-(l-oxypyridinyl)disiloxane; 4, Me3SiO endblocked poly(methyl 3-(l-oxypyridinyl)siloxane).

relatively high catalytic activity of 4 provides strong evidence for the importance of association or binding between the hydrophobic catalyst and DPPC prior to the first catalytic step. It is noteworthy that polysiloxanes, long recognized and utilized because of their inertness and hydrophobicity, are now shown to be important platforms for chemical catalysis even in water containing media provided they are appropriately functionalized. Moreover, to our knowledge this is the first exampleinthe published literature of a polysiloxane serving as a catalystinorganic synthesis (21). The importance of hydrophobic binding interactionsinfacilitating catalysis in enzyme reactions is well known. The impact of this phenomenon in the action of synthetic polymer catalysts for reactions such as described above is significant. A full investigation of a variety of monomelic and polymeric catalysts with nucleophilic sites is currently underway. They are being used to study the effect of polymer structure and morphology on catalytic activityintransacylation and other reactions.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

208

INORGANIC AND ORGANOMETALLIC POLYMERS

Acknowledgment

Acknowledgement is made to Reilly Tar and Chemical Corporation and the Dow Corning Corporation for partial support of this research and to the National Science Foundation for funds to purchase the thermal analyzer (grant CHE 84-10776). J.-m. X. and C.-x. T. express their sincerest gratitude to the Synthetic Research Institute of Tianjin (PRC) for leaves-of-absence to carry out the research.

Liturature Cited 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

(a) Hirl, M.A.; Gamson, E.P.; Klotz, I.M. J. Am. Chem. Soc. 1979, 101, 6020. (b) Delaney, E.J.; Ward, L.E.; Klotz, I.M. J. Am. Chem. Soc. 1982, 104, 799. Lehn, J.-m. Acc. Chem. Res. 1978, 11, 49. Bender, M.; Kamiyama, M. Cyclodextrin Chemistry. Reactivity and Structure Concepts in Organic Chemistry; Springer-Verlag: Berlin, 1978; Vol. 16. (a) Fife, W.K.; Xin W.K.; Zhang, Z.-d. J Z.-d. Tetrahedron Lett. 1986, 27, 4933. (d) Fife, W.K.; Zhang, Z.-d. ibid. 1986, 27, 4937. Zeldin, M.; Xu, J.-m. J. Organomet. Chem. 1987, 320, 267. Zeldin, M.; Xu, J.-m.; Tian, C.-x. ibid. 1987, 326, 341. Zeldin, M.; Xu, J.-m.; Tian, C.-x.; Polymer Preprints 1987, 28(1), 417. Smith, A.L. Spectrochim. Acta. 1960, 16, 87. Maslowsky, E., Jr. Vibrational Spectra of Organometallic Compounds: WileyInterscience: New York, 1977, p. 65-66, 410-411. Pouchert, C.J.; Campbell, J.R. The Aldrich Library of NMR Spectra: Aldrich Chemical Co. Inc., Milwaukee, 1974; Vol IX. Budzikiewicz, H.; Djerassi, C.; Williams, D.H. Mass Spectrometry of Organic Compounds: Holden-Day Inc.: San Francisco, 1967, Chapt. 20. See ref. 9, p. 111-116. Smith, A.L. Analysis of Silicones: J. Wiley and Sons: New York, 1974. Kireev, G.V.; Leont'ev, V.B.; Kurbatov, Y.V.; Otroshchenko, O.S.; Sadykov, A.S. Izv. Akad. Nauk SSSR. Ser. Khim. 1980, 5, 1034. Shindo, H. Chem. and Pharm. Bull. (Tokyo) 1958, 6, 117. Grassie, N.; Francey, K.F.; MacFarlane, I.G. Polym. Degrad. and Stability, 1980, 2, 67; (b) Grassie, N.; Francey, K.F.; MacFarlane, I.G. ibid, 1980, 2, 53. Kang, D.W.; Rajendran, G.P.; Zeldin, M. J. Polym. Sci.: Part A: Polym. Chem. 1986, 24, 1085. Spalding, T.R. in "Mass Spectrometry of Inorganic and Organometallic Compounds," Litzow, M.R.; Spalding, T.R. Ed., Elsevier Scientific Publishing Co., 1973; Chapter 7. Dehmlow, E.V.; Dehmlow S.S. Phase Transfer Catalysis: 2nd ed.; Verlag Chemie; Weinheim, 1983. Jencks, W.P. Catalysis In Chemistry And Enzymology: McGraw-Hill, New York, 1969. A patent (FR 2474891; see CA 96:124298c) assigned to B.A. Ashby, GE Company, claims a platinum-siloxane complex which catalyzes hydrosilation of vinylsiloxanes or SiOH groups in the preparation of silicone resins and rubbers.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 16

Photochemical Behavior of Organosilicon Polymers Bearing Phenyldisilanyl Units Mitsuo Ishikawa and Kazuo Nate Department of Applied University, Saijo-cho, Higashi-Hiroshim Engineering Research Laboratory, Hitachi Ltd., Totsuka, Yokohama 244, Japan The synthesis and photochemical behavior of p o l y s i l o ­ xanes containing p h e n y l ( t r i m e t h y l s i l y l ) s i l o x y units and p-(disilanylene)phnylene polymers have been reported. I r r a d i a t i o n of thin l i q u i d films of polysiloxanes con­ taining phenyldisilanyl u n i t s , and both phenyldisilanyl units and alkenyl groups ( v i n y l , allyl, and butenyl) with a Xe-Hg lamp afforded transparent films which are insoluble in common organic solvents. I r r a d i a t i o n of poly[p-(1,2-diethyldimethyldisilanylene)phenylene] and poly[p-(1,2-dimethyldiphenyldisilanylene)phenylene] with a low-pressure mercury lamp in solutions resulted in homolytic s c i s s i o n of s i l i c o n - s i l i c o n bonds, f o l ­ lowed by the formation of silicon-carbon unsaturated compounds and hydrosilanes. I r r a d i a t i o n of thin s o l i d films of p-(disilanylene)phenylene polymers in a i r led to the formation of photodegradation products involving Si-OH and S i - O - S i groups. Lithographic applications of a double-layer system of the l a t t e r polymers are also described. The π - e l e c t r o n s y s t e m - s u b s t i t u t e d o r g a n o d i s i l a n e s such as a r y l - , a l k e n y l - , and a l k y n y l d i s i l a n e s a r e p h o t o a c t i v e under u l t r a v i o l e t i r r a d i a t i o n , and t h e i r p h o t o c h e m i c a l b e h a v i o r has been e x t e n s i v e l y s t u d i e d (1) . However, much l e s s i n t e r e s t has been shown in the p h o t o c h e m i s t r y o f polymers b e a r i n g π - e l e c t r o n s u b s t i t u t e d d i s i l a n y l units (2-4). I n t h i s p a p e r , we r e p o r t the s y n t h e s i s and p h o t o ­ chemical behavior of polysiloxanes i n v o l v i n g p h e n y l ( t r i m e t h y l s i l y l ) s i l o x y u n i t s and s i l i c o n p o l y m e r s in which the a l t e r n a t e a r r a n g e ­ ment o f a d i s i l a n y l e n e u n i t and a phenylene group is found r e g u l a r l y in the polymer backbone. We a l s o d e s c r i b e l i t h o g r a p h i c a p p l i c a t i o n s o f a d o u b l e - l a y e r system o f the l a t t e r p o l y m e r s .

0097-6156/88/0360-0209$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

210

INORGANIC AND ORGANOMETALLIC POLYMERS

Polysiloxanes

Containing Phenyldisilanyl Units

In 1975, we found t h a t i r r a d i a t i o n o f p e n t a m e t h y l p h e n y l d i s i l a n e w i t h a l o w - p r e s s u r e mercury lamp l e a d s t o the t r a n s i e n t f o r m a t i o n o f a silene. I n t h e p r e s e n c e o f a t r a p p i n g agent such as a l c o h o l , the s i l e n e t h u s formed r e a c t s w i t h a l c o h o l t o g i v e a d d i t i o n p r o d u c t s , w h i l e in the absence o f the t r a p p i n g a g e n t , i t undergoes p o l y m e r i ­ z a t i o n to give n o n v o l a t i l e substances (5).

We i n i t i a l l y t h o u g h t t h a t i f p o l y s i l o x a n e s which a r e i n c o r p o ­ r a t e d w i t h p h e n y l d i s i l a n y l u n i t s and hydroxy groups in the polymer c h a i n c o u l d be p r e p a r e d , they would undergo p h o t o c r o s s l i n k i n g on exposure t o u l t r a v i o l e t r a d i a t i o n . I n o r d e r t o l e a r n whether such p o l y s i l o x a n e s undergo p h o t o c r o s s l i n k i n g , we s y n t h e s i z e d s i l o x a n e copolymers i n v o l v i n g p h n e y l ( t r i m e t h y l s i l y l ) s i l o x y u n i t s in the polymer b a c k b o n e , and s t u d i e d t h e i r p h o t o c h e m i c a l b e h a v i o r (€0 . P o l y s i l o x a n e s ( l ) - ( 4 ) b e a r i n g p h e n y l d i s i l a n y l u n i t s c o u l d be r e a d i l y p r e p a r e d by c o p o l y m e r i z a t i o n o f 1 , 3 , 5 - t r i p h e n y l [ t r i s ( t r i ­ m e t h y l s i l y l ) ] c y c l o t r i s i l o x a n e and c y c l o p o l y s i l o x a n e s in the p r e s e n c e o f a c a t a l y t i c amount o f an i n t e r c a l a t i o n compound p r e p a r e d from g r a p h i t e and p o t a s s i u m m e t a l in a r a t i o o f 8 : 1 , in h i g h y i e l d s . The p o l y m e r s 1 - 4 thus o b t a i n e d a r e c o l o r l e s s g r e a s e l i k e , and a r e s o l u b l e in common o r g a n i c s o l v e n t s .

LMe SiSi(Ph)0] 3

+

3

(Me SiO) 2

+

4

[R(Me)Si)]

3

o

r

4

1, ζ = ο 2,

R = CH =CH-

3,

R = CH =CHCH -

4,

R = CH =CHCH CH -

2

2

2

2

2

2

The UV spectrum o f the polymer 1 (Mn, 28,000) e x h i b i t s a c h a r a c t e r i s t i c a b s o r p t i o n a t 235 nm, i n d i c a t i n g the p r e s e n c e o f p h e n y l d i s i l a n y l u n i t s in t h e m o l e c u l e . I t is w e l l known t h a t p e n t a ­ m e t h y l p h e n y l d i s i l a n e shows a c h a r a c t e r i s t i c a b s o r p t i o n a t 231 nm ( 7 - 9 ) , b u t t r i m e t h y l p h e n y l s i l a n e e x h i b i t s an a b s o r p t i o n a t 211 nm. The p o l y m e r s 2 - 4 a l s o show s t r o n g a b s o r p t i o n in t h e s i m i l a r r e g i o n , ana p h o t o a c t i v e under U V - i r r a d i a t i o n .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

16.

ISHIKAWA AND NATE

Polymers Bearing Phenyldisilanyl Units

211

I r r a d i a t i o n o f a hexane s o l u t i o n o f the polymer 1 w i t h a 15-W l o w - p r e s s u r e mercury lamp under a n i t r o g e n atmosphere, p r o d u c e d the i n s o l u b l e c r o s s l i n k e d polymer whose IR spectrum shows a weak a b s o r p ­ t i o n a t 2150 cm"* a t t r i b u t e d t o the s t r e t c h i n g v i b r a t i o n o f an S i - H bond. In o r d e r t o l e a r n more about the p h o t o c r o s s l i n k i n g p r o c e s s , we synthesized 1,1-bis(trimethylsiloxy)-1-phenyl(trimethyl)disilane (5) as a model compound and examined i t s p h o t o c h e m i c a l b e h a v i o r in s o l u ­ tions. Compound 5 c o u l d r e a d i l y be p r e p a r e d by c o h y d r o l y s i s o f 1 , 1 d i c h l o r o - 1 - p h e n y l ( t r i m e t h y l ) d i s i l a n e w i t h a l a r g e excess of c h l o r o t r i m e t h y l s i l a n e in h i g h y i e l d . 1

Ph Cl (Ph)SiSiMe 2

+

3

2Me SiCl 3

— •

Me Si-0-Si-0-SiMe 3

3

Me Si 3

U n l i k e p e n t a m e t h y l p h e n y l d i s i l a n e , the p h o t o l y s i s o f compound 5 in the p r e s e n c e o f methanol in hexane gave no p r o d u c t s a r i s i n g from the r e a c t i o n o f a s i l e n e w i t h m e t h a n o l , b u t p o l y m e r i c s u b s t a n c e s were o b t a i n e d as main p r o d u c t s , in a d d i t i o n t o s m a l l amounts o f b i s ( t r i m e t h y l s i l o x y ) p h e n y l s i l a n e (6) (4%) and b i s ( t r i m e t h y l s i l o x y ) p h e n y l m e t h o x y s i l a n e (7) (5%).

hv MeOH

Ph ι M e S i - O - S i - O - S i Me 3

3

H

+

Polymeric

+

Ph I M e S i - O - S i - O - S i Me 3

3

OMe

substances

I r r a d i a t i o n o f the compound 5 in i s o p r o p y l b e n z e n e under s i m i l a r c o n s i t i o n s a f f o r d e d m- and p - [ b i s ( t r i m e t h y l s i l y l ) p h e n y l s i l y l ] i s o ­ p r o p y l b e n z e n e (8) (m:p=1.4:l) and m- and p - ( t r i m e t h y l s i l y l ) i s o ­ p r o p y l b e n z e n e (9) (m:p=3:l) in 45 and 24% y i e l d s , respectively, t o g e t h e r w i t h 9% y i e l d o f h y d r o s i l a n e 6. The f o r m a t i o n o f p r o d u c t s 8 and 9 may be b e s t u n d e r s t o o d in terms o f h o m o l y t i c a r o m a t i c s u b s t i t u t i o n o f s i l y l r a d i c a l s p r o d u c e d by h o m o l y t i c s c i s s i o n o f an S i - S i bond in compound 5, w h i l e h y d r o s i l a n e 6 c a n be e x p l a i n e d by hydrogen a b s t r a c t i o n o f t h e b i s ( t r i m e t h y l s i l o x y ) p h e n y l s i l y l r a d i c a l (Scheme 1 ) . On the b a s i s o f the p h o t o c h e m i c a l r e a c t i o n o f the model com­ pound 5, i t seems l i k e l y t h a t the p h o t o c h e m i c a l l y g e n e r a t e d s i l y l r a d i c a l s p l a y an i m p o r t a n t r o l e in the p h o t o c r o s s l i n k i n g o f the polymer 1. F i r s t , p h o t o c h e m i c a l bond s c i s s i o n o f t h e S i - S i bonds in the polymer c h a i n s t a k e s p l a c e t o g i v e s i l y l r a d i c a l s . The r e s u l t ­ i n g r a d i c a l s undergo h o m o l y t i c a r o m a t i c s u b s t i t u t i o n t o g i v e p h e n y l e n e l i n k a g e s , o r a b s t r a c t hydrogen from m e t h y l s i l y l groups t o form s i l y l m e t h y l e n e r a d i c a l s which would r e s u l t in the f o r m a t i o n o f s i l y l m e t h y l e n e l i n k a g e s (Scheme 2 ) .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

212

INORGANIC AND ORGANOMETALLIC POLYMERS

Ph 5

Me S i - 0 - S i - O - S i M e .

JH^

+

Me S i -

\ ^ i s o p r o p y l benzene

-\y^ 6

HÇ(CH ) 3

N

V i s o p r o p y l benzene ^

2

PhSi(0SiMe ) 3

Ph

H

SiMe

2

Ç(

C H

3

)

2

3

Ph

-^Si-O-SiMe - ( W I

~~Si-0-SiMe - 0 ~ -

2

SiMe

.

+

-SiMe,

2

3

3

Ph

Ph

^Si-O-SiMe

-0 H.

Z

^•Si-O-SiMe -0 ' w ,

2

^Si-0-SiMe- 0 ' w

2

SiMe

/s/vSi-O-SiMe - 0 '

I

SiMe

3

2

3

Ph I

^Si-0-SiMe -0^w 2

+

-H

^Si-0-SiMe -0 2

SiMe

3

Me Me.Si . J

+

Me

^O-Si-0-SiMe - 0 ~ I

CH

•

~ 0 - S i - 0 - S i M e -0~>

2

ι

2

CH

3

+ Scheme

2

Me SiH 3

2.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

16.

ISHIKAWA AND NATE

Polymers Bearing Phenyldisilanyl Units

Me I

Me ι

'wO-Si-O-SiMe

/^O-Si-O-SiMe

-0^

λ

-

H

-

? 2

j CH

I

-OA/N.

ΛΛO-Si-O-SiMe

ι Ph

-ΟΛΛ/ z

0

2

>\M)-Si-0-SiMe

2

213

,

-ΟΛ/Ν, 2

Ph

When a t h i n l i q u i d f i l m w i t h a t h i c k n e s s o f a p p r o x i m a t e l y 2 ym p r e p a r e d by s p i n c o a t i n g o f a 15% benzene s o l u t i o n o f polymer 1 was i r r a d i a t e d w i t h a 500-W Xe-Hg lamp f o r 300 s in a i r , a t r a n s p a r e n t s o l i d f i l m was o b t a i n e d . The UV spectrum o f t h i s s o l i d f i l m shows t h a t an a b s o r p t i o n a t 235 nm due t o p h e n y l d i s i l a n y l u n i t s v a n i s h e s a f t e r U V - i r r a d i a t i o n (Figure 1). This c l e a r l y indicates that p h o t o l y t i c c l e a v a g e o f s i l i c o n - s i l i c o n bonds l e a d i n g t o the c r o s s linking occurred. Simila photolysi f th thi l i q u i d film under a n i t r o g e n atmospher whose UV s p e c t r a show n units. We have i n v e s t i g a t e d the p h o t o c h e m i c a l b e h a v i o r o f polymer 1 under v a r i o u s c o n d i t i o n s in a i r and found t h a t UV i r r a d i a t i o n o f the t h i n l i q u i d f i l m s w i t h a t h i c k n e s s o f l e s s t h a n 10 μπι, i n d e e d produced t r a n s p a r e n t s o l i d f i l m s . However, when the f i l m s w i t h a t h i c k n e s s o f 100 ym were i r r a d i a t e d w i t h a mercury lamp, c r o s s l i n k i n g l e a d i n g t o t h e s o l i d f i l m s o c c u r r e d o n l y on t h e s u r f a c e o f the f i l m s , b u t i n s i d e remained as l i q u i d a f t e r p r o l o n g e d i r r a d i a t i o n . In t h s e s c a s e s , t h a s u r f a c e o f the f i l m s was found t o be s l i g h t l y opaque. T h e r e f o r e , most o f the l i g h t would n o t be t r a n s m i t t e d t o the i n s i d e o f the f i l m s . S i n c e c r o s s l i n k i n g o f the polymer o c c u r s as the r e s u l t o f the i n i t i a l f o r m a t i o n o f s i l y l r a d i c a l s , the s i l o x a n e polymer c o n t a i n i n g b o t h p h e n y l d i s i l a n y l u n i t s and f u n c t i o n a l groups which undergo r a d i c a l p o l y m e r i z a t i o n s h o u l d produce s o l i d m a t e r i a l whatever the t h i c k n e s s o f the f i l m s . To a s c e r t a i n t h i s , we have examined the p h o t o c h e m i c a l b e h a v i o r o f the polymers 2 - 4 . I r r a d i a t i o n o f t h i n f i l m s p r e p a r e d by s p i n c o a t i n g o f a 15% benzene s o l u t i o n o f p o l y m e r s 2 and 3 on a q u a r t z p l a t e w i t h a mercury lamp in a i r gave s o l i d f i l m s . The a b s o r p t i o n a t 235 nm o b s e r v e d in the UV s p e c t r a o f the s t a r t i n g f i l m s a g a i n d i s a p p e a r e d a f t e r UV i r r a d i a t i o n , as o b s e r v e d in the s i m i l a r p h o t o l y s i s o f 1 , i n d i c a t i n g t h a t the r a d i c a l s c i s s i o n o f s i l i c o n - s i l i c o n bonds occurred. IR s p e c t r a o f the r e s u l t i n g f i l m s show t h a t i n t e n s i t y o f the bands a t 1595 c m f o r polymer 2 and a t 1640 c m " f o r polymer 3 , due t o the C=C s t r e t c h i n g f r e q u e n c i e s d e c r e a s e d and a b r o a d band a t 1725 c m " a t t r i b u t e d t o the C=0 s t r e t c h i n g f r e q u e n c i e s was o b s e r v e d . S i m i l a r i r r a d i a t i o n o f the t h i n f i l m s o f 2 and 3 under a n i t r o g e n atmosphere a l s o a f f o r d e d t r a n s p a r e n t s o l i d f i l m s whose IR s p e c t r a , however, show no C=0 s t r e t c h i n g f r e q u e n c i e s . I n t e n s i t y o f the bands due t o the C=C s t r e t c h i n g f r e q u e n c i e s in the r e s u l t i n g f i l m s decresed to a c e r t a i n extent. A l l o f the p h o t o c r o s s l i n k i n g d e s c r i b e d here b e g i n w i t h s u r f a c e o f the f i l m s . The p h o t o c r o s s l i n k i n g l e a d i n g t o the s o l i d f i l m s under a n i t r o g e n atmosphere would p r o c e e d by the mechanism analogous - 1

1

1

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

214

INORGANIC AND ORGANOMETALLIC POLYMERS

0

250

300

350

Wave Length (nm)

Figure 1.

UV spectra of a thin l i q u i d film of 1: A, before i r r a d i a t i o n ; B, after irradiation.(Reproduced with permission from Ref. 6. Copyright 1986 John Wiley & Sons, Inc.)

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

16.

ISHIKAWA AND NATE

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Units

215

t o t h a t o b s e r v e d in s o l u t i o n (see Scheme 2 ) . I n the a i r , however, the s i l y l r a d i c a l s p r o d u c e d on the s u r f a c e o f the f i l m s r e a c t w i t h oxygen t o g i v e s i l y l p e r o x y r a d i c a l s which would r e s u l t in f o r m a t i o n of s o l i d f i l m s . I r r a d i a t i o n o f t h i c k f i l m s o f 2 and 3 a l s o l e d t o c r o s s l i n k i n g , however, the i n s i d e o f the f i l m s c o u l d n o t be completely solidified. The p h o t o c h e m i c a l b e h a v i o r o f l i q u i d f i l m s o f 2 and 3 is n o t v e r y much d i f f e r e n t from t h a t o f 1, because v i n y l and a l l y l - s u b s t i t u t e d s i l i c o n compounds a r e r a t h e r i n a c t i v e toward radical polymerization. I n c o n t r a s t t o the p h o t o c h e m i c a l b e h a v i o r o f 2 and 3 , polymer 4 gave t h i c k s o l i d f i l m s . I n t h i s c a s e , the r a d i c a l s formed in the f i l m s in which c r o s s l i n k i n g o c c u r r e d on the s u r f a c e would i n d u c e the r a d i c a l r e a c t i o n o f b u t e n y l groups i n s i d e o f the f i l m s . C o n s e q u e n t l y , p h o t o c r o s s l i n k i n g o c c u r r e d no m a t t e r what the t h i c k n e s s o f the f i l m s t o g i v e i n s o l u b l e s o l i d f i l m s . p-Disilanylenephenylene

Polymers

N e x t , we have s y n t h e s i z e examined t h e i r p h o t o c h e m i c a f i l m s (3) (K. N a t e , M . I s h i k a w a , H . N i , H . Watanabe, Y . S a h e k i , O r g a n o m e t a l l i c s , in p r e s s ) . To o u r knowledge, s i l i c o n p o l y m e r s , in which the a l t e r n a t e arrangement o f a d i s i l a n y l e n e u n i t and a p h e n y l ene group is found r e g u r a r y in t h e p o l y m e r backbone have n o t been r e p o r t e d so f a r (10). F i r s t , we attempted t o p r e p a r e p o l y [ p - ( t e t r a methyldisilanylene)phenylene] by the c o n d e n s a t i o n o f 1,4-bis(chlorod i m e t h y l s i l y l ) b e n z e n e w i t h sodium m e t a l in t o l u e n e . However, the p o l y m e r s t h u s o b t a i n e d were m a i n l y i n s o l u b l e and i n t r a c t a b l e w h i t e solid materials. T h i s may be a s c r i b e d t o the h i g h c r y s t a l l i n i t y o f the p o l y m e r s . I n o r d e r t o r e d u c e the c r y s t a l l i n i t y , we used 1 , 4 bis (chloroethylmethylsilyl)benzene and 1 , 4 - b i s ( c h l o r o m e t h y l p h e n y l s i l y l ) b e n z e n e as the s t a r t i n g monomers. The r e a c t i o n o f t h e s e monomers w i t h sodium d i s p e r s i o n a t 709 0 ° C under a n i t r o g e n atmosphere a f f o r d e d p o l y [ p - d i e t h y l d i m e t h y l d i s i l a n y l e n e ) p h e n y l e n e ] (10) and p o l y [ p - ( 1 , 2 - d i m e t h y l d i p h e n y l d i s i l a n y l e n e ) p h e n y l e n e ] (11), r e s p e c t i v e l y , in good y i e l d s .

+

10,

2NaCl

R = Et

11 , R = Ph Both polymers 10 and 11 a r e s o l u b l e in common o r g a n i c s o l v e n t s , m e l t w i t h o u t d e c o m p o s i t i o n , and can be drawn i n t o t h e f i b e r s . M o l e c u l a r w e i g h t s o f the polymers 10 and 11, d e t e r m i n e d by g e l p e r m e a t i o n chromatography w i t h t e t r a h y d r o f u r a n as the e l u a n t a f t e r p u r i f i c a t i o n by r e p r e c i p i t a t i o n from b e n z e n e - e t h a n o l , showed a b r o a d monomodal m o l e c u l a r w e i g h t d i s t r i b u t i o n . The degree o f p o l y m e r i z a t i o n depends on p a r t i c l e s i z e o f sodium m e t a l . Polymers w i t h m o l e c u l a r w e i g h t s o f 2 3 , 0 0 0 - 3 4 , 0 0 0 a r e always o b t a i n e d , i f f i n e sodium p a r t i c l e s a r e u s e d .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

216

INORGANIC AND ORGANOMETALLIC POLYMERS

The NMR c h e m i c a l s h i f t s o f the polymers 10 and 11 a r e o n l y s l i g h t l y dependent on the m o l e c u l a r w e i g h t . The NMR spectrum o f the polymer 10 shows t h r e e s h r a p resonances a t δ 0 . 3 4 , 0 . 9 4 , and 7.28 ppm, due t o M e S i , E t S i , and phenylene r i n g p r o t o n s , w h i l e the polymer 11 r e v e a l s s h r a p resonances a t 6 0 . 6 2 , 7 . 2 8 , and 7.35 ppm, a t t r i b u t e d t o MeSi and p h e n y l r i n g p r o t o n s . The polymers 10 and 11 show c h a r a c t e r i s t i c s t r o n g a b s o r p t i o n bands a t 262 and 254 nm, r e s ­ p e c t i v e l y , s i g n i f i c a n t l y r e d - s h i f t e d r e l a t i v e to pentamethylphenyld i s i l a n e which e x h i b i t s an a b s o r p t i o n band a t 231 nm. When a benzene s o l u t i o n o f the polymer 10 was p h o t o l y z e d upon i r r a d i a t i o n w i t h a 6-W l o w - p r e s s u r e mercury lamp f o r 2 h , s o l u b l e p h o t o d e g r a d a t i o n p r o d u c t s were p r o d u c e d . I n o r d e r t o l e a r n the i n f l u e n c e o f i r r a d i a t i o n time in t h e m o l e c u l a r w e i g h t o f p h o t o ­ d e g r a d a t i o n p r o d u c t s , benzene s o l u t i o n s c o n t a i n i n g 10 w i t h m o l e c u l a r w e i g h t o f 23,000 were p h o t o l y z e d f o r a d e f i n i t e time under the same c o n d i t i o n s , and the m o l e c u l a r w e i g h t s o f the r e s u l t i n g p h o t o p r o d u c t s were d e t e r m i n e d by GPC The m o l e c u l a r w e i g h t o f the p h o t o p r o d u c t s d e c r e a s e d in the e a r l y s t a g e p a s s e d t h r o u g h a minimu c r e a s e d g r a d u a l l y w i t h i n c r e a s i n g r e a c t i o n time (29,000 a f t e r 6 h , and 37,000 a f t e r 12 h ) , i n d i c a t i n g t h a t some c r o s s l i n k i n g r e a c t i o n s o c c u r r e d d u r i n g the p h o t o l y s i s ( F i g u r e 2 ) . ! H NMR s p e c t r a o f the r e s u l t i n g p h o t o p r o d u c t s show very b r o a d resonances f o r M e S i , E t S i , and p h e n y l r i n g p r o t o n s . I n a d d i t i o n , a m u l t i p l e resonances w i t h low i n t e n s i t y due t o an S i - Η p r o t o n is o b s e r v e d a t δ 4.2 ppm in a l l the photoproducts. UV s p e c t r a o f the p r o d u c t s show d i s a p p e a r a n c e o f t h e o r i g i n a l s t r o n g band a t 262 nm, and IR s p e c t r a r e v e a l a band w i t h medium i n t e n s i t y a t 2120 cm~l a t t r i b u t e d t o the s t r e t c h i n g f r e q u e n c i e s an S i - Η b o n d . We c a r r i e d o u t the p h o t o l y s i s o f 10 in the p r e s e n c e o f an e x c e s s o f methanol in benzene. I n c o n t r a s t t o the p h o t o l y s e s in the absence o f m e t a n o l , the m o l e c u l a r w e i g h t o f t h e p h o t o p r o d u c t s d e c r e a s e d c o n t i n u o s l y w i t h i n c r e a s i n g r e a c t i o n t i m e , and f i n a l l y r e a c h e d a c o n s t a n t v a l u e (Mw = 1 , 8 0 0 ) , a f t e r 4 h - i r r a d i a t i o n . The % NMR s p e c t r u m o f the p h o t o p r o d u c t s shows s i g n a l s a t δ 3.4 and 4.2 ppm, due t o C H 3 - 0 and S i - Η p r o t o n s , in a d d i t i o n t o the b r o a d s i g n a l s c o r r e s p o n d i n g t o M e S i , E t S i , and p h e n y l r i n g p r o t o n s . The r e s o ­ nances w i t h low i n t e n s i t i e s a t δ 2.2 and 6.2 ppm, a s s i g n e d t e n t a ­ t i v e l y as c y c l o h e x a d i e n y l r i n g p r o t o n s a r e a l s o o b s e r v e d . Assuming t h a t o u r assignment f o r the c y c l o h e x a d i e n y l r i n g p r o t o n s is c o r r e c t , t h e r e s u l t i n d i c a t e s t h a t the r e a r r a n g e d s i l i c o n - c a r b o n u n s a t u r a t e d compounds a r e formed in the p h o t o d e g r a dative processes. The p h o t o r e a r r a n g e d s i l e n e i n t e r m e d i a t e s , however, a r e i n v o l v e d o n l y as minor p r o d u c t s f o r the p r e s e n t p o l y m e r i c systems. In f a c t , the r a t i o o f the c y c l o h e x a d i e n y l group t o the d i s i l a n y l e n e p h e n y l e n e u n i t in t h e p h o t o p r o d u c t s which show a maximum v a l u e o f the c y c l o h e x a d i e n y l r i n g p r o t o n s , is c a l c u l a t e d t o be a p p r o x i m a t e l y 1:19 by ! H NMR s p e c t r o s c o p i c a n a l y s i s . When a benzene s o l u t i o n o f 10 in the p r e s e n c e o f m e t h a n o l - d i was p h o t o l y z e d under t h e same c o n d i t i o n s , the p r o d u c t s c o n t a i n i n g b o t h C H 3 - 0 and S i - Η groups were a g a i n o b t a i n e d . The IR spectrum o f the p r o d u c t shows no a b s o r p t i o n due t o an S i - D s t r e t c h i n g f r e q u e n ­ c i e s , i n d i c a t i n g t h a t no d i r e c t r e a c t i o n o f the p h o t o - e x c i t e d 10 w i t h methanol is i n v o l v e d . The r e l a t i v e r a t i o o f an S i - Η t o C H 3 - 0

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

ISHIKAWA AND NATE

3x10

4

Polymers Bearing Phenyldisilanyl Units

217

L

I

I

I

I

1

1

L

Ο

1

2

3

4

5

6

Time (h)

Figure

2.

Plot

of

time

for

the

molecular weights the

presence

photolysis of

methanol

of

of in

products

10:

A,

in

vs.

irradiation

benzene;

B,

benzene.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

in

INORGANIC AND ORGANOMETALLIC POLYMERS

218

t o [ M e ( E t ) S i S i ( M e ) ( E t j C f c H ^ n in the p h o t o p r o d u c t o b t a i n e d from 4 h i r r a d i a t i o n o f 10 is d e t e r m i n e d t o be 1 : 1 . 5 - 2 . 0 : 3 . 2 - 3 . 8 ( n ) . These r e s u l t s c l e a r l y i n d i c a t e the f o r m a t i o n o f a d i f f e r e n t t y p e o f s i l i c o n - c a r b o n d o u b l e bonded i n t e r m e d i a t e s . T h i s t y p e o f s i l e n e may be p r o d u c e d from h o m o l y t i c s c i s s i o n o f s i l i c o n - s i l i c o n b o n d s , f o l l o w e d by hydrogen a b s t r a c t i o n o f the r e s u l t i n g r a d i c a l from e i t h e r a m e t h y l group o r methylene o f an e t h y l group on the o t h e r s (Scheme 3 ) .

Et E t

Et Et

Et

^Si-Si-^Vsi-Si-w Me Me ^

=

/

Me

Et

* ^ S i -

Et Et

- S i - ^ y S i - S i ^

Me

Et

+

v

^ Me Me

Et

^Si-H

h

Me

Et Et

•

Me Me Et Et

CH =Si-^Vsi-Si Λ Λ

+

2

CH CH=Si^ySi-Si ™>

^ Scheme

3.

On the b a s i s o f the methoxy c o n t e n t s in the p h o t o p r o d u c t s , t h e s e s i l è n e s seem t o be formed as main p r o d u c t s in the p o l y m e r i c s y s t e m s , a l t h o u g h t h i s t y p e o f s i l e n e is p r o d u c e d o n l y as a minor p r o d u c t in the p h o t o l y s i s o f a r y l d i s i l a n e s . The p h o t o l y s i s o f p o l y m e r Π in the absence o f methanol p r o ­ ceeded w i t h c o n s i d e r a b l y d i f f e r e n t f a s h i o n from t h a t o f 10. Irradi­ a t i o n o f a benzene s o l u t i o n o f Π r e s u l t e d in the d e c r e a s e o f the m o l e c u l a r w e i g h t o f the p h o t o p r o d u c t s ( F i g u r e 3 ) . No i n c r e a s e o f the m o l e c u l a r w e i g h t o f t h e p r o d u c t s w i t h i n c r e a s i n g r e a c t i o n time was o b s e r v e d . M o r e o v e r , the p h o t o d e g r a d a t i o n o f 11 p r o c e e d e d f a s t e r t h a n t h a t o f 10. T h u s , the p h o t o l y s i s o f a benzene s o l u t i o n o f Π f o r 1 h w i t h a l o w - p r e s s u r e mercury lamp l e d t o the f o r m a t i o n o f the p h o t o p r o d u c t whose m o l e c u l a r w e i g h t was d e t e r m i n e d t o be 5 , 9 0 0 , w h i c h remained unchanged a f t e r 4 h - i r r a d i a t i o n . NMR s p e c t r a o f the p r o d u c t s d i s p l a y b r o a d s i g n a l s a t δ 0 . 6 , 4 . 9 , and 6 . 8 - 7 . 8 , a t t r i b u t e d t o M e S i , H S i , and p h e n y l r i n g p r o t o n s , respectively. I r r a d i a t i o n o f 11 in the p r e s e n c e o f methanol f o r 1 h a f f o r d e d t h e p r o d u c t w i t h m o l e c u l a r w e i g h t o f 2,000 which remained c o n s t a n t during further i r r a d i a t i o n . IR and ^-H NMR s p e c t r a show t h a t the r e s u l t i n g p r o d u c t s i n v o l v e b o t h C H 3 - 0 and S i - Η g r o u p s . The r e l a t i v e r a t i o o f an S i - Η and C H 3 - 0 group t o [ P h ( M e ) S i S i ( M e ) ( P h ) C 6 H 4 ] is c a l c u l a t e d as b e i n g 1 : 1 . 0 - 1 . 7 : 2 . 5 - 3 . 0 ( n ) . When s i m i l a r p h o t o l y s i s o f 11 in t h e p r e s e n c e o f MeOD was c a r r i e d o u t , a g a i n the p r o d u c t whose H NMR r e v e a l s the r e s o n a n c e due t o t h e S i - Η p r o t o n was o b s e r v e d . The r e l a t i v e r a t i o o f the S i - Η and C H 3 - 0 p r o t o n s was i d e n t i c a l w i t h t h o s e o f the p r o d u c t s o b t a i n e d in the p r e s e n c e o f n o n - d e u t e r a t e d m e t h a n o l . The f o r m a t i o n o f the m e t h o x y s i l y l group can be u n d e r s t o o d by the a d d i t i o n o f methanol a c r o s s the s i l i c o n - c a r b o n d o u b l e b o n d s . NMR s p e c t r a o f a l l p h o t o p r o d u c t s o b t a i n e d from the p h o t o l y s e s o f 11 in the p r e s e n c e o f methanol r e v e a l no r e s o n a n c e s a t t r i b u t e d t o the c y c l o h e x a d i e n y l ring protons. T h i s i n d i c a t e s t h a t the p h o t o c h e m i c a l d e g r a d a t i o n o f the p o l y m e r 11 g i v e s no r e a r r a n g e d s i l e n e i n t e r m e d i a t e s , b u t p r o d u c e s n

1

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

ISHIKAWA AND NATE

Polymers Bearing Phenyldisilanyl Units

2

3

219

4

Time (h)

Figure

3.

Plot

of

time

for

the

molecular weights the

presence

photolysis of

methanol

of

of in

products

11:

A,

in

vs.

irradiation

benzene;

B,

benzene.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

in

INORGANIC AND ORGANOMETALLIC POLYMERS

220

e x c l u s i v e l y s i l a e t h e n e i n t e r m e d i a t e s and h y d r o s i l a n e s (Scheme 4 ) . I t seems l i k e l y t h a t the i n c r e a s e o f the m o l e c u l a r w e i g h t o b s e r v e d in t h e p h o t o l y s i s o f 10 in the absence o f methanol may be a s c r i b e d t o t h e p o l y m e r i z a t i o n o f the p h o t o r e a r r a n g e d s i l e n e i n t e r m e d i a t e s .

Scheme

4.

The a b s o r p t i o n s p e c t r films e x h i b i t a broad s t r u c t u r e l e s pectively. The a b s o r p t i o n maxima o f t h s e s bands a r e n o t d i f f e r e n t from t h o s e in a THF s o l u t i o n . We have examined the p h o t o c h e m i s t r y o f the t h i n f i l m s o f the p o l y m e r s 10 and 11, in o r d e r t o l e a r n whether the polymers can be used as p o s i t i v e deep UV r e s i s t s . Thus, i r r a d i a t i o n of t h i n f i l m s w i t h a t h i c k n e s s o f a p p r o x i m a t e l y 0.1 ym p r e p a r e d by s p i n c o a t i n g o f a 5% t o l u e n e s o l u t i o n o f t h e polymer 10 on a q u a r t z p l a t e in a i r w i t h a 500-W Xe-Hg lamp f o r 30 s gave p h o t o d e g r a d a t i o n p r o d u c t s , which a r e s o l u b l e in 2 - e t h o x y e t h a n o l . The a b s o r p t i o n a t 262 nm o b s e r v e d in UV s p e c t r a o f the s t a r t i n g f i l m s v a n i s h e d a f t e r U V - i r r a d i a t i o n , showing t h a t the s c i s s i o n o f t h e s i l i c o n - s i l i c o n bonds occurred. IR s p e c t r a o f the r e s u l t i n g f i l m s showed t h a t i n t e n s i t y o f the band a t 500 c m " a s s i g n e d as the S i - S i s t r e t c h i n g f r e q u e n c i e s d e c r e a s e s r e m a r k a b l y , and s t r o n g a b s o r p t i o n bands a t 3350 and 1060 cm"" , and a b r o a d band a t 1720 c m " appeared a f t e r U V - i r r a d i â t i o n . The f a c t t h a t o n l y v e r y weak a b s o r p t i o n band a t 2140 c m " due t o t h e S i - Η s t r e t c h i n g f r e q u e n c i e s was o b s e r v e d , i n d i c a t e s t h a t t h e s i l y l r a d i c a l g e n e r a t e d p h o t o c h e m i c a l l y from the s c i s s i o n o f t h e s i l i c o n - s i l i c o n bonds were scavenged by oxygen in a i r . I n d e e d , IR s p e c t r a o f t h e p r o d u c t s o b t a i n e d from t h e s i m i l a r p h o t o l y s i s under r e d u c e d p r e s s u r e o b v i o u s l y show the a b s o r p t i o n due t o the S i - H s t r e t c h i n g f r e q u e n c i e s , b u t t h e absence o f the band a t 3350 and 1720 c m " . 1

1

1

1

1

I r r a d i a t i o n o f a s o l i d f i l m p r e p a r e d from the polymer 11 in a i r a f f o r d e d p h o t o d e g r a d a t i o n p r o d u c t s which a r e s o l u b l e in 2 - e t h o x y ­ ethanol. IR s p e c t r a o f the p r o d u c t s show s t r o n g a b s o r p t i o n bands a t t r i b u t e d t o S i - O H and S i - O - S i s t r e t c h i n g f r e q u e n c i e s . In c o n t r a s t t o the p r o d u c t s from 10, t h e s e p r o d u c t s show a b s o r p t i o n due t o t h e S i - Η bond. T h i s r e s u l t i n d i c a t e s t h a t some o f t h e s i l è n e s would be formed in t h i s s y s t e m . The i n t e n s e a b s o r p t i o n a t 254 nm in t h e UV spectrum a g a i n d i s a p p e a r e d a f t e r U V - i r r a d i a t i o n . The main r o u t e f o r t h e p h o t o d e g r a d a t i o n o f the polymers 10 and 11 in t h e s o l i d f i l m s in a i r c o n s i s t s o f r a d i c a l s c i s s i o n o f the s i l i c o n - s i l i c o n b o n d s , analogous t o t h a t o b s e r v e d in s o l u t i o n , b u t in t h e f i l m s , t h e r e s u l t i n g s i l y l r a d i c a l s r e a c t w i t h oxygen t o g i v e

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Polymers Bearing Phenyldisilanyl

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221

s i l o x a n e s and s i l a n o l s as the f i n a l p r o d u c t s . F o r the polymer 11, t h e f o r m a t i o n o f s i l è n e s which would be t r a p p e d by oxygen o r m o i s t u r e in t h e a i r a r e a l s o i n v o l v e d as a minor d e g r a d a t i v e pathway. Lithographic Applications I t has been r e p o r t e d t h a t o r g a n o s i l i c o n polymers show h i g h e t c h i n g r e s i s t a n c e a g a i n s t an oxygen p l a s m a , and can be used as the t o p i m a g i n g l a y e r in the d o u b l e - l a y e r r e s i s t system. The t y p i c a l o r g a n o s i l i c o n polymers r e p o r t e d t o d a t e f o r the l i t h o g r a p h i c a p p l i c a t i o n s a r e the s i l o x a n e polymers h a v i n g v i n y l groups (11) o r c h l o r o m e t h y l groups (12) , p o l y s t y r e n e s h a v i n g t r i m e t h y l s i l y l groups (13) , and s i l y l s u b s t i t u t e d novolacs (14,15). P o l y s i l a n e s have a l s o been found t o s e r v e as e x c e l l e n t 0 reactive ion etching b a r r i e r s for d o u b l e - l a y e r r e s i s t a p p l i c a t i o n s (16). 2

The 0 r e a c t i v e i o n e t c h i n g r a t e s o f poly(disilanylenephenylene) 10 and 11, and p o l y i m i d e (PIQ) were measured As can be seen in F i g u r e 4, the polymers 1 a g a i n s t the oxygen p l a s m a The l i t h o g r a p h i c a p p l i c a t i o n s o f a d o u b l e - l a y e r r e s i s t system in w h i c h the p o l y [ p - ( d i m e t h y l d i p h e n y l d i s i l a n y l e n e ) p h e n y l e n e ] film (0.2 ym t h i c k ) was used as the t o p i m a g i n g l a y e r have been examined (K. N a t e , T . Inoue, H . Sugiyama and M . I s h i k a w a , J . A p p l . P o l y m . S c i . in p r e s s ) . I n t h e s e s t u d i e s , the PIQ (2.0 ym t h i c k ) was used as an u n d e r l a y e r . T h u s , t h e f i l m c o n s i s t i n g o f the polymer 11 and PIQ p r e p a r e d on a s i l i c o n wafer was exposed t o deep U V - l i g h t w i t h the use o f Canon c o n t a c t a l i g n e r PLA-521 t h r o u g h a photomask f o r 5 t o 6 s (UV i n t e n s i t y : 72 mV/cm a t 254 nm). The r e s u l t i n g f i l m was t h e n d e v e l o p e d w i t h a 1:5 m i x t u r e o f t o l u e n e and i s o p r o p y l a l c o h o l f o r 15 s and r i n s e d w i t h i s o p r o p y l a l c o h o l f o r 15 s. A positive r e s i s t p a t t e r n was o b t a i n e d a f t e r t r e a t m e n t o f the f i l m p a t t e r n w i t h 0 RIE under the c o n d i t i o n o f 0.64 W/cm (RF power:7 MHz, 0 pressure:3 mtorr). 2

2

2

2

2

The SEM p h o t o g r a p h o f the d o u b l e - l a y e r r e s i s t p a t t e r n t h u s o b t a i n e d is shown in F i g u r e 5. I n t h i s way, submicron l e v e l l i n e p a t t e r n s where the minimum l i n e w i d t h is 0.5 ym and the a s p e c t r a t i o is above 3.0 c o u l d r e a d i l y be o b t a i n e d . Summary I r r a d i a t i o n o f t h i n l i q u i d f i l m s o f p o l y s i l o x a n e s 1-4, containing p h e n y l d i s i l a n y l u n i t s in a i r l e d t o h o m o l y t i c s c i s s i o n o f s i l i c o n s i l i c o n bonds which underwent c r o s s l i n k i n g t o g i v e s o l i d m a t e r i a l s . I r r a d i a t i o n o f t h i c k f i l m s o f 1-3 a l s o l e d t o c r o s s l i n k i n g , however, the i n s i d e o f t h e f i l m s c o u l d n o t be s o l i d i f i e d , w h i l e polymer 4 afforded thick s o l i d f i l m s . Similar i r r a d i a t i o n of s o l i d films p r e p a r e d from p - ( d i s i l a n y l e n e ) p h e n y l e n e polymers 10 and 11 gave photodegradation products. The main r o u t e f o r p h o t o d e g r a d a t i o n c o n s i s t s o f r a d i c a l s c i s s i o n o f s i l i c o n - s i l i c o n bonds in the polymer backbone. The r e s u l t i n g s i l y l r a d i c a l s r e a c t w i t h oxygen t o g i v e s i l o x a n e s and s i l a n o l s as the f i n a l p r o d u c t s . I t has been found t h a t polymers 10 and 11 show v e r y h i g h e t c h i n g r e s i s t a n c e a g a i n s t the oxygen p l a s m a , and can be u s e d as the t o p imaging l a y e r in the d o u b l e - l a y e r r e s i s t system.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

222

2h

Etching Time (min)

Figure

Figure

5.

4.

Etching rate

Double-layer the

top

of

resist

imaging

10 a n d

pattern

11,

and

polyimide.

p r e p a r e d by

using

layer.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

11

as

16. ISHIKAWA AND NATE

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223

Acknowledgments The c o s t o f t h i s work was d e f r a y e d in p a r t from G r a n t - i n - A i d f o r S p e c i a l P r o j e c t R e s e a r c h , O r g a n i c C h e m i c a l R e s o u r c e s , 61111002, t o w h i c h t h e a u t h o r s ' thanks a r e d u e . We a l s o e x p r e s s o u r a p p r e c i a t i o n t o S h i n - e t s u C h e m i c a l C o . , L t d . , and T o s h i b a S i l i c o n e C o . , L t d . , f o r a g i f t of organochlorosilanes.

Literature 1. 2. 3. 4.

5.

6. 7. 8. 9. 10.

11.

12.

13.

14. 15. 16.

Cited

(a) Ishikawa, M. Pure Appl. Chem. 1978, 50, 11. (b) Ishikawa, M.; Kumada, M. Adv. Organomet. Chem. 1981, 19, 51. Ishiakwa, M.; Imamura, N.; Miyoshi, N.; Kumada, M. J. Polym. Sci. Polym. Lett. Ed. 1983, 21, 657. Ishikawa, M.; Ni, H.; Matsuzaki, Κ.; Nate, K.; Inoue, T.; Yokono, H. J. Polym. Sci. Polym. Lett. Ed. 1984, 22, 669. (a) West, R.; David, L. D.; Djurovich, P. I.; Steraley, K. L.; Srinivasan, K. S. V.; (b) Zhang, X. H.; West, 1984, 22, 159. (c) Trefonas, P.; West, R.; Miller, R. D. J. Am. Chem. Soc. 1985, 107, 2737. (d) Trefonas, P.; Damewood, Jr.; West, R.; Miller, R. D. Organometallics 1985, 4, 1318. (a) Ishikawa, M.; Fuchikami, T.; Kumada, M. J. Organomet. Chem. 1976, 118, 155. (b) Ishikawa, M.; Oda, M.; Nishimura, K.; Kumada, M. Bull. Chem. Soc. Japan 1983, 56, 2795. Nate, K.; Ishikawa, M.; Imamura, N.; Murakami, Y. J. Polym. Sci. Polym. Chem. Ed. 1986, 24, 1551. Gilman, H.; Atwell, W. H.; Schwebke, G. L. J. Organomet. Chem. 1964, 2, 369. Sakurai, H.; Kumada, M. Bull. Chem. Soc., Japan 1964, 37, 1894. Hague, D. N.; Prince, R. H. J. Chem. Soc. 1965, 4690. Nate, K.; Sugiyama, H.; Inoue, T.; Ishikawa, M. Abstr. Meeting Electrochem. Soc. Oct. 7-12, 1984, New Orleans, L. A. Abstract No. 530, p 778, 1984. Shaw, J. M.; Hatzakis, M; Paraszczak, J.; Liutkus, J.; Babich, E. Regional Technical Conference, "Photopolymers PrinciplesProcess and Materials" p 285, Ν. Y. November 1982. Morita, M.; Tanaka, Κ.; Imamura, S.; Tamamura, T.; Kogure, I. The 44th Conference of Japanese Applied Phisics, No. 28a-T-1, Abstract p 243, September 1983. Suzuki, N.; Saigo, K.; Gokan, H.; Ohnishi, Y. The 44th Confer­ ence of Japanese Applied Physics, No. 26a-U-7, Abstract p 258, September 1983. Saotome, Y.; Gokan, H.; Suzuki, M; Ohnishi, Y. J. Electrochem. Soc. 1985, 132, 909. Wilkins Jr., C. W.; Reichmanise, E.; Wolf, T. M.; Smith, Β. C. J. Vac. Sci. Technol. 1985, Β 3(1), 306. Hofer, D. C.; Miller, R. D.; Willson, G. C. Proceedings of SPIE, Advances in Resist Technology, 469, 16, Santa Clara, California, March 1984.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 17

Routes to Molecular Metals with Widely Variable Counterions ana Band-Filling Electrochemistry of a Conductive Organic Polymer with an Inorganic Backbone 1

Tobin J. Marks John G Gaudiello Glen E. Kellogg Department of Chemistry and Materials Research Center, Northwestern University, Evanston, IL 60201 This contribution reviews recent results on [Si(Pc)O] (Pc = phthalocyaninato) solid state electrochemistry and the structural interconversions that accompany electrochemical doping/undoping processes. In acetonitrile/(n-Bu) N BF-4, it is found that a significant overpotential accompanies initial oxidation of aspolymerized [Si(Pc)O] . This can be associated with an orthorhombic-->tetragonal structural transformation. Once in the tetragonal crystal structure, cycling between tetragonal doped {[Si(Pc)O](BF ) } and tetra­ gonal undoped [Si(Pc)O] is relatively facile. Evidence is presented for continuous tuning of the band-filling between y = 0.00 and 0.50. In comparison, electrochemical oxidation of monoclinic β-Ni(Pc) under the same conditions is also accompanied by a signif­ icant overpotential in forming tetragonal Ni(Pc)(BF )0.48. However, electrochemical undoping produces the monoclinic γ-Ni(Pc) phase with far less band structure tunability than in the silicon polymer. Experiments with tosylate as the anion indicate that tetragonal {[Si(Pc)O](tosylate) } can be tuned contin­ uously between y = 0.00 and 0.67. For the anions PF-6, SbF-6, CF (CF ) SO-3 (n = 0,3,7), it appears that the maximum accessible oxidation levels are largely dictated by packing limitations (anion size). Evidence is also presented for reversible reductive doping of [Si(Pc)O] in THF/(n-Bu) N BF-4. n

+

4

n

4

y

n

n

4

y

3

2

n

n

+

n

4

A major b a r r i e r t o u n d e r s t a n d i n g fundamental r e l a t i o n s h i p s between m o l e c u l a r a r c h i t e c t u r e , e l e c t r o n i c s t r u c t u r e , and c h a r g e t r a n s p o r t in m o l e c u l a r m e t a l s d e r i v e s from o u r i n a b i l i t y t o i n t r o d u c e p o t e n 1

Current address: Department of Chemistry, Michigan State University, East Lansing, MI 48824 0097-6156/88/0360-0224$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

17.

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225

t i a l l y i n f o r m a t i v e p e r t u r b a t i o n s w i t h o u t major, d e l e t e r i o u s r e o r g a n i z a t i o n s in c r y s t a l s t r u c t u r e . Over the p a s t s e v e r a l y e a r s we have shown t h a t r o b u s t , h i g h l y c r y s t a l l i n e , s t r u c t u r a l l y w e l l d e f i n e d polymers o f t h e t y p e [ M ( P c ) 0 ] , M - S i , G e ; Pc - p h t h a l o c y a n i n a t o (1,11) (1-5) o f f e r a u n i q u e o p p o r t u n i t y t o e x p e r i m e n t w i t h the p r o p e r t i e s o f a ( p h t h a l o c y a n i n e ) m o l e c u l a r m e t a l ( I I I ) (6n

Φ" c b

••

Φ. Φ I

11) under c o n d i t i o n s in Pc^ s u b u n i t s a r e r i g i d l y h e l d in a c o f a c i a l o r i e n t a t i o n . I n e a r l i e r c o n t r i b u t i o n s , we d i s c u s s e d in d e t a i l t h e e f f e c t s on e l e c t r i c a l , o p t i c a l , a n d magnetic p r o p e r t i e s , hence band s t r u c t u r e , o f s e q u e n t i a l l y v a r y i n g Pc-Pc i n t e r p l a n a r s p a c i n g (hence bandwidth) (1-2) and c h e m i c a l l y i n t r o d u c e d o f f - a x i s , c h a r g e - c o m p e n s a t i n g c o u n t e r i o n s (X in { [ M ( P c ) 0 ] X } ) (1-5.12-14). However, t h e scope o f c h e m i s t r y p o s s i b l e i n c r e a s e s d r a m a t i c a l l y when e l e c t r o c h e m i c a l approaches (15-18) are u s e d f o r d o p i n g . Thus, we r e c e n t l y commun­ i c a t e d (7,19,20) t h a t a f a r g r e a t e r range o f c o u n t e r i o n s c a n be i n t r o d u c e d , s u b s t a n t i a l t u n i n g o f the b a n d - f i l l i n g c a n be e f f e c t e d , phase boundary/ f r e e energy i n f o r m a t i o n c a n be o b t a i n e d , and r e d u c ­ t i v e (η-type) d o p i n g can be c a r r i e d out. I n a d d i t i o n , the [ M ( P c ) 0 ] system has p r o v i d e d v a l u a b l e i n s i g h t i n t o s e v e r a l p o o r l y u n d e r s t o o d , g e n e r a l a s p e c t s o f c o n d u c t i v e polymer e l e c t r o c h e m i s t r y . I n v i e w o f t h e r a p i d l y e x p a n d i n g n a t u r e o f t h i s a r e a , we f e l t i t t i m e l y t o r e v i e w a number o f r e c e n t developments. Thus we summar­ i z e h e r e r e c e n t r e s u l t s on [ S i ( P c ) 0 ] s o l i d s t a t e e l e c t r o c h e m i s t r y and c l o s e l y r e l a t e d s t r u c t u r a l i n t e r c o n v e r s i o n s . +

y

n

n

n

Electrochemical

Methodology

The f i l m - l i k e n a t u r e o f most c o n d u c t i v e polymers is r e a d i l y adapt­ able t o e l e c t r o c h e m i c a l experimentation by simply a t t a c h i n g p r e s s u r e c o n t a c t s and immersing t h e specimen in a n e l e c t r o l y t e o r by s t u d y i n g f i l m s which have been e l e c t r o p o l y m e r i z e d d i r e c t l y on the w o r k i n g e l e c t r o d e (15-18). However, the b a s i c m o r p h o l o g i c a l u n i t o f [ S i ( P c ) 0 ] c o n s i s t s o f s m a l l c r y s t a l l i t e s (1-5.21-23). We thus u t i l i z e d two d i f f e r e n t approaches f o r e l e c t r o c h e m i c a l exper­ iments. Gram-scale r e a c t i o n s can be c a r r i e d out b y d o p i n g t h e polymer as a f i n e l y - g r o u n d s l u r r y in a s t a n d a r d three-compartment c e l l (Figure 1). T h i s a p p r o a c h is p a r t i c u l a r l y u s e f u l f o r p r e ­ p a r i n g m a c r o s c o p i c amounts o f doped polymer s u f f i c i e n t f o r p h y s i c a l c h a r a c t e r i z a t i o n by a b a t t e r y o f t e c h n i q u e s . Alternatively, s m a l l e r s c a l e e x p e r i m e n t s can be c a r r i e d out b y p r e s s i n g a s m a l l amount o f t h e f i n e l y ground polymer a g a i n s t a p l a t i n u m d i s k e l e c ­ trode (Figure 2). T h i s method has t h e advantage o f more r a p i d n

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INORGANIC AND ORGANOMETALLIC POLYMERS

SSCE Reference Ag Reference

0.3 M TBA(BF ) in CH CN 4

3

0.1 - 0.6 g polymer In CH CN 3

Pt mesh Working

Pt mesh Counter 10 mm frit Ε porosity

micro stir bar

F i g u r e 1. A p p a r a t u s f o r s l u r r y - s c a l e e l e c t r o c h e m i c a l exper­ iments w i t h { [ S i ( P c ) 0 ] X y } m a t e r i a l s . I n t h e c a s e shown, t h e equipment is c o n f i g u r e d f o r s t u d i e s in a c e t o n i t r i l e / ( n — B u ) 4 ~ N+BF4. n

•

PAR 273 Digital Potentlostat

3 Electrode Vacuum E-Chem Cell , 0 . 3 M TBA(BF ) v

V

4

in

CHtCN

\

Ag Reference^ U Teflon filter

Pt Counter

) Pt disk Working 1 - 2 mg polymer

F i g u r e 2. A p p a r a t u s f o r m i c r o s c a l e e l e c t r o c h e m i c a l e x p e r i m e n t s with {[Si(Pc)0]Xy} materials. I n t h e case shown, t h e equipment is c o n f i g u r e d f o r s t u d i e s in a c e t o n i t r i l e / ( n - B u ) 4 N B F 4 . n

+

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k i n e t i c s and is analogous t o t e c h n i q u e s employed f o r c h a r a c t e r i z i n g i n o r g a n i c e l e c t r o d e m a t e r i a l s (24-27). I t has n o t been s u i t a b l e , in t h i s c o n f i g u r a t i o n , f o r l a r g e - s c a l e s y n t h e s e s . Both types o f e x p e r i m e n t s a r e c o n t r o l l e d b y a PAR Model 273 p o t e n t i o s t a t / g a l v a n o s t a t i n t e r f a c e d t o a Z e n i t h microcomputer u s i n g l o c a l l y d e v e l o p e d software. E x p e r i m e n t s were c a r r i e d o u t under r i g o r o u s l y anhydrous and a n a e r o b i c c o n d i t i o n s u s i n g p u r i f i e d e l e c t r o l y t e s and s o l v e n t s . Both types o f experiments u t i l i z e d s i l v e r wire q u a s i - r e f e r e n c e e l e c t r o d e s t h a t were r e f e r e n c e d d i r e c t l y t o a sodium s a t u r a t e d calomel e l e c t r o d e a t the completion o f the experiment. In both c a s e s , t h e k e y e l e c t r o c h e m i c a l e x p e r i m e n t is one in w h i c h t h e p o t e n t i a l is i n c r e m e n t a l l y s t e p p e d and t h e r e s u l t i n g c u r r e n t f l o w m o n i t o r e d a f t e r each s t e p . When t h e c u r r e n t decays t o a p r e d e t e r mined b a c k g r o u n d l e v e l , t h e n e x t p o t e n t i a l s t e p is a u t o m a t i c a l l y executed. T h i s e x p e r i m e n t h a s been v a r i o u s l y c a l l e d s e q u e n t i a l c o n t r o l l e d p o t e n t i a l c o u l o m e t r y (CPC), e l e c t r o c h e m i c a l p o t e n t i a l s p e c t r o s c o p y (ECPS), o r e l e c t r o c h e m i c a l v o l t a g e s p e c t r o s c o p y (EVS) (18,25,28,29), and y i e l d l e v e l as a f u n c t i o n o i t has b e e n shown t h a t c o u l o m e t r i c a l l y - d e t e r m i n e d d o p i n g s t o i c h i o m e t r i e s a r e in good agreement w i t h t h o s e d e t e r m i n e d b y e l e m e n t a l a n a l y s i s (19,20,30). rSi(Pc)01

n

Oxidative

Doping

in

+

Acetonitrile/(n-Bu)/,N BF/,

S l u r r y - s c a l e e l e c t r o c h e m i c a l d o p i n g o f [ S i ( P c ) 0 ] in a c e t o n i t r i l e / (n-Bu)4N BF4 is i l l u s t r a t e d in F i g u r e 3. I n t e r e s t i n g l y , t h e i n i t i a l o x i d a t i o n e x h i b i t s a s u b s t a n t i a l o v e r p o t e n t i a l w h i c h is n o t o b s e r v e d on subsequent r e o x i d a t i o n f o l l o w i n g e l e c t r o c h e m i c a l undoping. X - r a y d i f f r a c t i o n d a t a argue t h a t t h e i n i t i a l o v e r p o t e n t i a l (commonly r e f e r r e d t o as t h e " b r e a k - i n " p r o c e s s (15—18. 32,32) in o t h e r c o n d u c t i v e polymer systems b u t p o o r l y u n d e r s t o o d ) is l a r g e l y s t r u c t u r a l in o r i g i n . Thus, a s - p o l y m e r i z e d [ S i ( P c ) 0 ] has b e e n a s s i g n e d a c l o s e l y - p a c k e d o r t h o r h o m b i c c r y s t a l s t r u c t u r e (3), whereas { [ S i ( P c ) 0 ] ( B F 4 ) } has a t e t r a g o n a l c r y s t a l s t r u c t u r e ( F i g u r e 4) (12-14). The o r t h o r h o m b i c - ^ t e t r a g o n a l reorganization n e c e s s a r i l y i n v o l v e s 6-8 À d i s p l a c e m e n t s o f t h e t i g h t l y - p a c k e d [ S i ( P c ) 0 ] c h a i n s t r a n s v e r s e t o t h e backbone d i r e c t i o n . Subsequent e l e c t r o c h e m i c a l undoping o f t e t r a g o n a l { [ S i ( P c ) 0 ] ( B F 4 ) } yields a more open [ S i ( P c ) 0 ] phase w h i c h c a n be i n d e x e d as a n undoped t e t r a g o n a l c r y s t a l s t r u c t u r e ( F i g u r e 4) ( 3 0 ) . T h i s polymer is s p e c t r o s c o p i c a l l y , d i f f r a c t o m e t r i c a l l y , and e l e c t r o c h e m i c a l l y i d e n t i c a l t o m a t e r i a l which c a n be p r e p a r e d b y t h e r m a l l y u n d o p i n g t e t r a g o n a l { [ S i ( P c ) 0 ] ( 1 3 ) 0 3 7 ) n in vacuo. n

+

n

V

n

n

v

n

n

From a s t r u c t u r a l s t a n d p o i n t , i t is n o t s u r p r i s i n g t h a t t e t r a g o n a l [ S i ( P c ) 0 ] r e a d i l y undergoes o x i d a t i v e d o p i n g w i t h a s m a l l e r o v e r p o t e n t i a l than the orthorhombic polytype n o r t h a t c y c l i n g between t e t r a g o n a l doped and undoped s t r u c t u r e s is r e l a t i v e l y facile. F u r t h e r m o r e , t h e d i f f r a c t i o n d a t a e v i d e n c e a smooth, monot o n i c i n c r e a s e in { [ S i ( P c ) 0 ] ( B F 4 ) } l a t t i c e p a r a m e t e r s (-0.03(2) Â in c and +0.27(6) Â in a) as y = 0-»Ό.50. T h i s is in a c c o r d w i t h a s i m p l e V e i g a r d ' s law/homogeneous d o p i n g p i c t u r e . Further support f o r a c o n t i n u o u s t u n i n g o f t h e b a n d - f i l l i n g is d e r i v e d from t h e smooth c h a r a c t e r o f t h e EVS c u r v e s ( F i g u r e 3 ) . T r a n s i t i o n s between phases o f g r e a t l y d i f f e r i n g s t r u c t u r e s and f r e e e n e r g i e s would be n

y

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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228

i n d i c a t e d by s t e p s and p l a t e a u s (15-18.24-27). Infrared spectro­ scopy, X - r a y d i f f r a c t i o n , and c o u l o m e t r y i n d i c a t e t h a t r e p e a t e d e l e c t r o c h e m i c a l c y c l i n g between doped and undoped s t a t e s o c c u r s w i t h o n l y minor i r r e v e r s i b l e d e c o m p o s i t i o n o f the polymer. The m i c r o d o p i n g e x p e r i m e n t s a r e in good agreement w i t h the above e l e c t r o c h e m i s t r y ( F i g u r e 5 ) , the p r i n c i p a l d i f f e r e n c e b e i n g a d i m i n u t i o n in a l l h y s t e r e s e s which a r e a p p a r e n t l y e l e c t r o d e c o n t a c t related. A l t h o u g h l e s s pronounced t h a n in the s l u r r y e x p e r i m e n t s , the " b r e a k - i n " o v e r p o t e n t i a l is c l e a r l y e v i d e n t , w h i l e the h y s t e r ­ e s i s in d o p i n g and u n d o p i n g the t e t r a g o n a l phase is now v e r y s m a l l . P h y s i c a l d a t a on the { [ S i ( P c ) 0 ] ( B F 4 > y } system p r o v i d e an i n t r i g u i n g p i c t u r e o f how the c o l l e c t i v e p r o p e r t i e s o f a m o l e c u l a r m e t a l v a r y as b a n d - f i l l i n g is tuned o v e r a w i d e r range t h a n has e v e r b e f o r e b e e n p o s s i b l e (19,20,30,31). As d i s c u s s e d in d e t a i l e l s e w h e r e b o t h t h e r m o e l e c t r i c power and o p t i c a l r e f l e c t i v i t y d a t a e v i d e n c e an e v o l u t i o n from an i n s u l a t o r t o a m o l e c u l a r m e t a l . I n t e r e s t i n g l y , b o t h measurements show an insulator->metal transition a t y«0.15-0.20, f o l l o w e state with progressivel adherence o f the b e h a v i o r (e.g., the plasma f r e q u e n c y , the thermo­ e l e c t r i c power) t o t h a t p r e d i c t e d by s i m p l e t i g h t - b i n d i n g band t h e o r y is g r a t i f y i n g (19,20,31). M a g n e t i c s u s c e p t i b i l i t y measure­ ments e v i d e n c e a l a r g e C u r i e - l i k e component f o r y £ 0.20, indi­ c a t i n g , as a l s o i m p l i e d by the t h e r m o e l e c t r i c power and o p t i c a l r e s u l t s , t h a t t h e r e is s u b s t a n t i a l l o c a l i z a t i o n o f the e l e c t r o n i c s t r u c t u r e a t low d o p i n g l e v e l s . T h i s presumably a r i s e s from disorder. A t h i g h e r d o p i n g l e v e l s , the s u s c e p t i b i l i t y is s u b s t a n ­ t i a l l y more P a u l i - l i k e as is the c a s e f o r most p h t h a l o c y a n i n e m o l e c u l a r m e t a l s (1.2.7-14). I n t e r e s t i n g l y , f o u r - p r o b e conduc­ t i v i t y measurements show a sharp r i s e in c o n d u c t i v i t y between y=*0 and y«0.20, f o l l o w e d by a l e v e l i n g - o f f beyond y « 0 . 2 0 . n

What I f The Backbone?

P h t h a l o c y a n i n e R i n g s Are

Not

Connected By A

Polymer

To p r o b e the importance o f the [ S i ( P c ) 0 ] e n f o r c e d - s t a c k i n g super­ s t r u c t u r e in the e l e c t r o c h e m i c a l d o p i n g / u n d o p i n g p r o c e s s , exper­ iments were a l s o c a r r i e d out w i t h s u b l i m e d N i ( P c ) , w h i c h has a m o n o c l i n i c , s l i p p e d - s t a c k c r y s t a l s t r u c t u r e (/?-Ni(Pc), F i g u r e 6) (34). I t is f o u n d t h a t the o n s e t p o t e n t i a l f o r o x i d a t i v e d o p i n g in a c e t o n i t r i l e / ( n - B u ) 4 N B F 4 is r o u g h l y comparable t o t h a t o f o r t h o ­ rhombic [ S i ( P c ) 0 ] ( F i g u r e 7 ) . The a b r u p t r i s e in y v s . Ε i n d i c a t e s t h a t few i n t e r m e d i a t e s t o i c h i o m e t r i e s a r e a c c e s s e d b e f o r e N i ( P c ) ( B F 4 ) y , y«0.48, is r e a c h e d ; i . e . , t h e r e is a major change in c r y s t a l s t r u c t u r e . X - r a y d i f f r a c t i o n d a t a i n d i c a t e t h a t the doped m a t e r i a l has a t e t r a g o n a l c r y s t a l s t r u c t u r e s i m i l a r t o N i ( P c ) ( 4 > 0 . 3 3 (Î) Ni(Pc)(Cl04) .42 (£) ( F i g u r e 6 ) . In c o n t r a s t to { [ S i ( P c ) 0 ] ( B F 4 ) o . 5 o ) n » e l e c t r o c h e m i c a l u n d o p i n g o f Ni(Pc)(BF4)Q.48 does n o t take p l a c e o v e r a p a r t i c u l a r l y b r o a d p o t e n t i a l range. R a t h e r , the EVS c u r v e is r a t h e r s t e e p ( F i g u r e 7) s u g g e s t i n g a major s t r u c t u r a l r e o r g a n i z a t i o n upon undoping. Upon complete undoping, powder X - r a y d i f f r a c t i o n r e v e a l s t h a t the e l e c t r o c h e m i c a l l y undoped N i ( P c ) is n o t s i m i l a r in s t r u c t u r e t o t e t r a g o n a l [ S i ( P c ) 0 ] nor does i t e x h i b i t the s t a r t i n g 0 - N i ( P c ) s t r u c t u r e . R a t h e r , the t e t r a g o n a l c r y s t a l s t r u c t u r e has c o l l a p s e d t o a n o t h e r known, monon

+

n

BF

a

n

d

0

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

17.

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229

0.6 -i

D

0.5

"x o

0.4

y

"o

Έ σ ο.

0.3

0.2 DOPING [Si(Pc)0], • -Orthorhomlc • -Tetragonal

0.1 D) Φ "D

0.0

2.00

ΐ.έο' ' ' ί.όο' ' ' ό.έο' ' ' ό.όο' Eoppi .

Volts

F i g u r e 3. S l u r r y e l e c t r o c h e m i c a l v o l t a g e s p e c t r o s c o p y exper­ iments w i t h { [ S i ( P c ) 0 ] ( B F 4 ) } ( 0.00-0.50) m a t e r i a l in acetonitrile/(n-Bu)4N

Tetragonal

Orthorhombic

F i g u r e 4. S t r u c t u r a l t r a n s f o r m a t i o n s accompanying e l e c t r o ­ c h e m i c a l d o p i n g and u n d o p i n g o f { [ S i ( P c ) 0 ] ( B F 4 ) } m a t e r i a l s , y = 0.00-0.50 y

n

0.6

ο

0.5

[si(Pc)o]

n

/

BF;

"Ό 0.4

t σ ο. «.ο ο Φ k. ο> φ "Ο

0.3

5

Second CycU (Tetragonal)

0.2 0.1

-1

0.0

ζ.υΰ

-τ—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—I 1.50 1.00 0.50 0.00 -0.50

Eoppi .

Volts

F i g u r e 5. M i c r o s c a l e e l e c t r o c h e m i c a l v o l t a g e s p e c t r o s c o p y e x p e r i m e n t s w i t h { [ S i ( P c ) 0 ] ( B F ) } m a t e r i a l s in a c e t o n i t r i l e / (n-Bu)4N BF4. Data p o i n t s were a c q u i r e d a t 50 mV i n t e r v a l s . 4

y

n

+

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

230

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^(virgin)

Tetragonal (doped)

7(undoped)

F i g u r e 6. Structura N i ( P c ) ( B F 4 ) m a t e r i a l s , y = 0.00-0.50. y

0.6

BF^ 0.5 -[Si(Pc)0]

0.4

Τ Ό

a

-Ni(Pc)

D 0.3

CL 0.2

-I

Ο 0.1 0.0

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I ι

2.00 1.50

1.00

Eappl .

0.50

-0.50

0.00

VoltS

F i g u r e 7. Comparison o f s l u r r y e l e c t r o c h e m i c a l v o l t a g e s p e c t r o ­ scopy e x p e r i m e n t s w i t h N i ( P c ) and t e t r a g o n a l [ S i ( P c ) 0 ] . n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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c l i n i c s l i p p e d - s t a c k 7 - N i ( P c ) phase ( F i g u r e 6) u s u a l l y p r o d u c e d by p r e c i p i t a t i o n from a c i d (35,36). This structure d i f f e r s p r i n c i ­ p a l l y from t h e i n i t i a l β phase in h a v i n g a s m a l l e r t i l t i n g a n g l e o f the p e r p e n d i c u l a r t o t h e N i ( P c ) p l a n e w i t h r e s p e c t t o t h e c r y s t a l lographic b (stacking) axis. Hence, i t is s t r u c t u r a l l y somewhat more c l o s e l y r e l a t e d t o t h e t e t r a g o n a l p a c k i n g arrangement t h a n is the β phase. These r e s u l t s c l e a r l y i n d i c a t e t h a t t h e e n f o r c e d M(Pc) s t a c k i n g is a major s t r u c t u r a l f a c t o r f a c i l i t a t i n g t h e e l e c ­ t r o c h e m i c a l t u n i n g o f t h e [ S i ( P c ) 0 ] band s t r u c t u r e . n

rSi(Pc)01

n

Electrochemistry

With Other Counterions

The e l e c t r o c h e m i c a l t e c h n i q u e s d e s c r i b e d above c a n be employed w i t h a wide v a r i e t y o f e l e c t r o l y t e s t o probe t h e n a t u r e o f t h e d o p i n g p r o c e s s , f a c t o r s c o n t r o l l i n g t h e u l t i m a t e degree o f b a n d - f i l l i n g a c h i e v a b l e , and band s t r u c t u r e - c o u n t e r i o n i n t e r a c t i o n s . I n F i g u r e 8 is shown a d o p i n g / u n d o p i n g e x p e r i m e n t w i t h t e t r a g o n a l [ S i ( P c ) 0 ] in a c e t o n i t r i l e / E t 4 N t o s y l a t e seen t h a t the b a n d - f i l l i n range: { [ S i ( P c ) 0 ] ( t o s ) } , y=0 -+ « 0 . 6 7 . The shape o f the EVS c u r v e i m p l i e s a homogeneous d o p i n g p r o c e s s . X-ray d i f f r a c t i o n data s u p p o r t t h i s p i c t u r e and r e v e a l a monotonie e x p a n s i o n in t e t r a g o n a l l a t t i c e p a r a m e t e r s (+0.70(5) Â in a and -0.03(2) Â in ç ) (20,30). B r o a d e r l i n e w i d t h s in t h e { [ S i ( P c ) 0 ] ( t o s ) y } d i f f r a c t i o n p a t t e r n s i m p l y a l e s s o r d e r e d c r y s t a l s t r u c t u r e t h a n in { [ S i ( P c ) 0 ] ( B F 4 ) y } . +

y

n

n

n

The charge t r a n s p o r t and o p t i c a l p r o p e r t i e s o f t h e { [ S i ( P c ) 0 ] ( t o s ) y } m a t e r i a l s as y=0 -*> 0.67 a r e r e m i n i s c e n t o f t h e { [ S i ( P c ) 0 ] ( B F 4 ) y } system, b u t w i t h some noteworthy d i f f e r e n c e s . Again there is an i n s u l a t o r - t o - m e t a l t r a n s i t i o n in t h e t h e r m o e l e c t r i c power n e a r y«0.15-0.20. Beyond t h i s d o p i n g s t o i c h i o m e t r y , t h e t o s y l a t e s a l s o show a c o n t i n u o u s e v o l u t i o n t h r o u g h a m e t a l l i c phase w i t h d e c r e a s i n g b a n d - f i l l i n g . However, t h e t r a n s i t i o n seems somewhat smoother t h a n in t h e BF4 system f o r y^0.40, p o s s i b l y a consequence o f a more d i s o r d e r e d t o s y l a t e c r y s t a l s t r u c t u r e . Both { [ S i ( P c ) 0 ] ( t o s ) y } o p t i c a l r e f l e c t a n c e s p e c t r a and f o u r - p r o b e c o n d u c t i v i t i e s a r e a l s o c o n s i s t e n t w i t h a t r a n s i t i o n t o a m e t a l a t y«0.15-0.20. Repeated e l e c t r o c h e m i c a l c y c l i n g l e a d s t o c o n s i d e r a b l y more decomp o s i t i o n t h a n in t h e t e t r a f l u o r o b o r a t e system. n

n

n

E l e c t r o c h e m i c a l e x p e r i m e n t s w i t h t e t r a g o n a l { S i ( P c ) 0 ] and acetonitrile/(n-Bu)4N PF6 o r acetonitrile/(n-Bu)4N SbF£ r e v e a l d o p i n g b e h a v i o r s i m i l a r t o the t e t r a f l u o r o b o r a t e system w i t h one exception. L i m i t i n g stoichiometries obtained are {[Si(Pc)0](PF6)o.47) ([Si(Pc)0](SbF6)o.4i) . The lower s t o i c h i o m e t r i e s a c h i e v e d a r e e x p l i c a b l e in terms o f t h e g r e a t e r s i z e s o f t h e l a t t e r two a n i o n s and t h e space a v a i l a b l e in t h e c o u n t e r i o n " t u n n e l s " o f the { [ S i ( P c ) 0 ] X y } c r y s t a l structures. These r e l a t i o n s h i p s c a n be p u t on a more q u a n t i t a t i v e f o o t i n g by c o n s i d e r i n g a n i o n v a n d e r Waals d i a m e t e r s as shown in F i g u r e 9. F u r t h e r v e r i f i c a t i o n t h a t p a c k i n g c o n s i d e r a t i o n s a r e i m p o r t a n t in d i c t a t i n g a c h i e v a b l e d o p i n g s t o i c h i o m e t r i e s is p r o v i d e d by e l e c t r o c h e m i c a l e x p e r i m e n t s w i t h a s e r i e s o f p e r f l u o r o a l k y l s u l f o n a t e s as t h e tetrabutylammonium s a l t s in a c e t o n i t r i l e . As c a n be seen in F i g u r e 10, EVS c u r v e s a r e s i m i l a r t o o t h e r a n i o n s , however t h e " l e v e l i n g - o f f " p o i n t o f l i m i t i n g Si(Pc) oxidation state f a l l s with increasing anion bulk. In a l l cases, the X-ray d i f f r a c t i o n p a t t e r n s are c o n s i s t e n t w i t h n

+

a

n

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d

n

n

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I I 2.00

I I I I

I I I I I I I I I I I I I I I I I I I I I

F i g u r e 8. S l u r r y e l e c t r o c h e m i c a l v o l t a g e s p e c t r o s c o p y e x p e r i ments w i t h { [ S i ( P c ) 0 ] ( t o s y l a t e ) y } m a t e r i a l s , y = 0.00-0.67 in acetonitrile/Et4N (tosylate"). n

+

Van der Waals diameter (Â)

Counterion

B-4—F

F — Ρ

)

F

F—Sb F

F

Maximum y y &ic yobs c

5.9

0.56

0.50

6.4

0.52

0.47

6.9

0.48

0.41

F

F i g u r e 9. S p h e r i c a l a n i o n s i z e e f f e c t s on maximum p o s s i b l e { [ S i ( P c ) 0 ] X y } d o p i n g l e v e l s as seen from a s i m p l e p a c k i n g model. n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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the now u b i q u i t o u s t e t r a g o n a l { [ S i ( P c ) 0 ] X y } c r y s t a l s t r u c t u r e . A l t h o u g h v a n d e r Waals d i a m e t e r s a r e more d i f f i c u l t t o e s t i m a t e f o r t h e s e more f l e x i b l e c o u n t e r i o n s , assuming e l o n g a t e d C F 3 ( C F 2 ) S 0 3 g e o m e t r i e s y i e l d s l i m i t i n g s t o i c h i o m e t r i e s in r e a s o n a b l e agreement w i t h experiment ( F i g u r e 1 1 ) . I n o t h e r work (19,20), we have a l s o shown t h a t s u l f a t e c a n be e l e c t r o c h e m i c a l l y i n t r o d u c e d as a c o u n t e r i o n u s i n g [ ( n - B u ) 4 N ] 2 S O 4 in a c e t o n i t r i l e . I n t h i s case, the f i n a l product s t o i c h i o m e t r y , {[Si(Pc)0](S04)o.09)n> l i m i t e d by t h e o x i d a t i v e s t a b i l i t y o f t h e s u l f a t e anion. T h e r m o e l e c t r i c power, o p t i c a l r e f l e c t i v i t y , m a g n e t i c s u s c e p t i b i l i t y , and f o u r - p r o b e e l e c t r i c a l c o n d u c t i v i t y measurements e v i d e n c e b e h a v i o r t y p i c a l o f an [ S i ( P c ^ ) 0 ] compound where p«0.20. Tha t is, t h e r e is no e v i d e n c e t h a t t h e more concen­ t r a t e d c o u n t e r i o n charge h a s i n d u c e d s i g n i f i c a n t l o c a l i z a t i o n o f the band s t r u c t u r e . n

n

+

is

+

n

Reductive

f S i ( P c ) 0 1 Doping n

In p r i n c i p a l , i t shoul b a s e d m e t a l by i n t r o d u c i n g e l e c t r o n s i n t o t h e S i ( P c ) l o w e s t unoccu­ p i e d m o l e c u l a r o r b i t a l (LUMO) r a t h e r t h a n removing e l e c t r o n s from the h i g h e s t o c c u p i e d m o l e c u l a r o r b i t a l (HOMO). M o l e c u l a r o r b i t a l c a l c u l a t i o n s a t t h e DVM-Χα l e v e l (37-39) i n d i c a t e t h a t t h e LUMO is o f a p p r o p r i a t e S i ( P c ) π s p a t i a l c h a r a c t e r t o form a c o n d u c t i o n band. I n c o n t r a s t t o t h e HOMO, however, i t a l s o c o n t a i n s an a p p r e c i a b l e amount o f S i - 0 c h a r a c t e r , so t h a t i n t e r e s t i n g m e t a l e f f e c t s may be o b s e r v e d in t h e p h y s i c a l p r o p e r t i e s . I n i t i a l exper­ iments i n d i c a t e t h a t t e t r a g o n a l [ S i ( P c ) 0 ] c a n i n d e e d be reduct i v e l y doped t o form a n i o n i c , e x t r e m e l y a i r - s e n s i t i v e [ S i ( P c ^ " ) 0 ] products. An example o f a m i c r o s c a l e d o p i n g experiment, w h i c h i l l u s t r a t e s c y c l i n g between p- and η-doping, is shown in F i g u r e 12. F u r t h e r c h a r a c t e r i z a t i o n o f t h i s i n t e r e s t i n g system is in p r o g r e s s . n

n

Conclusions These r e s u l t s i l l u s t r a t e t h a t e l e c t r o c h e m i c a l t e c h n i q u e s c a n be employed t o s y n t h e s i z e a v a s t range o f [ S i ( P c ) 0 ] - b a s e d m o l e c u l a r m e t a l s / c o n d u c t i v e polymers w i t h wide t u n a b i l i t y in o p t i c a l , m a g n e t i c , and e l e c t r i c a l p r o p e r t i e s . Moreover, t h e s t r u c t u r a l l y w e l l - d e f i n e d and w e l l - o r d e r e d c h a r a c t e r o f t h e polymer c r y s t a l s t r u c t u r e o f f e r s the opportunity to explore s t r u c t u r e / e l e c t r o ­ c h e m i c a l / c o l l e c t i v e p r o p e r t i e s and r e l a t i o n s h i p s t o a d e p t h n o t p o s s i b l e f o r most o t h e r c o n d u c t i v e polymer systems. On a p r a c t i c a l note, the p r e s e n t study h e l p s t o d e f i n e those parameters c r u c i a l t o the f a b r i c a t i o n , from cheap, r o b u s t p h t h a l o c y a n i n e s , o f e f f i c i e n t energy storage d e v i c e s . n

Acknowledgments T h i s r e s e a r c h was s u p p o r t e d b y t h e NSF-MFL program t h r o u g h t h e M a t e r i a l s R e s e a r c h C e n t e r o f N o r t h w e s t e r n U n i v e r s i t y ( G r a n t DMR8520280) and b y t h e O f f i c e o f N a v a l R e s e a r c h . We thank Dr. M. G. Kanatzidis for helpful discussions.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

234

INORGANIC AND ORGANOMETALLIC POLYMERS

0.6

σ "Ό

Ί

CF (CF ) S0j 3

0.5

2

n

0.4

σ Έ σ

0.3

CL 0.2 0.1 {

Ο "Ό

0.0

:

2.0θ'

' ' îio'

' ' Î.Ôo'

' ' 0 . 5 θ ' ' ' Ô.ÔO" '

'-0.50

F i g u r e 10. S l u r r y e l e c t r o c h e m i c a l v o l t a g e s p e c t r o s c o p y experiments w i t h t e t r a g o n a l [ S i ( P c ) 0 ] and v a r i o u s p e r f l u o r o a l k y l s u l f o n a t e s (as t h e tetrabutylammonium s a l t s in a c e t o n i t r i l e ) . n

Counterion

!>

Van der Waals length (A)

Maximum y y ic yobs ca

6.4

0.52

0.55

9.4

0.35

0.38

15.7

0.21

0.26

F i g u r e 11. P e r f l u o r o a l k y l s u l f o n a t e a n i o n s i z e e f f e c t s on maximum a c h i e v a b l e ( [ S i ( P c ) 0 ] X y J d o p i n g l e v e l s as seen from a simple p a c k i n g model.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

17.

MARKS ET AL.

Routes to Molecular Metals

F i g u r e 12. M i c r o - s c a l e e l e c t r o c h e m i c a l v o l t a g e s p e c t r o s c o p y showing o x i d a t i v e as w e l l as r e d u c t i v e d o p i n g o f t e t r a g o n a l { S i ( P c ) 0 ] in T H F / ( n - B u ) N B F 4 . +

n

4

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

235

INORGANIC AND ORGANOMETALLIC POLYMERS

236 Literature

Cited

1. Marks, T. J. Science 1985, 227, 881-889. 2. Diel, B. N.; Inabe, T.; Lyding, J. W.; Schoch, K. F., Jr.; Kannewurf, C. R.; Marks, T. J. J. Am. Chem. Soc. 1983, 105, 1551-1567. 3. Dirk, C. W.; Inabe, T.; Schoch, K. F., Jr.; Marks, T. J. J. Am. Chem. Soc. 1983, 105, 1539-1550. 4. Dirk, C. W.; Inabe, T.; Lyding, J. W.; Schoch, K. F., Jr.; Kannewurf, C. R.; Marks, Τ. J. J. Polym. Sci., Polym. Symp. 1983, 70, 1-29. 5. Dirk, C. W.; Mintz, Ε. Α.; Schoch, K. F., Jr.; Marks, T. J. J. Macromol. Sci.-Chem. 1981, A16, 275-298. 6. Almeida, M.; Kanatzidis, M. G.; Tonge, L. M.; Marks, T. J.; Marcy, H. O.; McCarthy, W. J.; Kannewurf, C. R. Solid State Commun., in press. 7. Gaudiello, J. G.; Marcy, H. O.; McCarthy, W. J.; Moguel, M. K.; Kannewurf, C. R.; 8. Inabe, T.; Liang, W.-B.; J. W.; McCarthy, W. J.; Carr, S. H.; Kannewurf, C. R.; Marks, T. J. Synth. Met. 1986, 13, 219-229. 9. Inabe, T.; Nakamura, S.; Liang, W.-B.; Marks, T. J.; Burton, R. L.; Kannewurf, C. R.; Imaeda, K.-I. J. Am. Chem. Soc. 1985, 107, 7224-7226. 10. Inabe, T.; Marks, T. J.; Burton, R. L.; Lyding, J. W.; McCarthy, W. J.; Kannewurf, C. R.; Reisner, G. M.; Herbstein, F. H. Solid State Commun. 1985, 54, 501-503. 11. Schramm, C. J.; Scaringe, R. P.; Stojakovic, D. R.; Hoffman, Β. M.; Ibers, J. Α.; Marks, T. J. J. Am. Chem. Soc. 1980, 102. 6702-6713. 12. Inabe, T.; Gaudiello, J. G.; Moguel, M. K.; Lyding, J. W.; Burton, R. L.; McCarthy, W. J.; Kannewurf, C. R.; Marks, T. J. J. Am. Chem. Soc. 1986, 108, 7595-7608. 13. Inabe, T.; Moguel, M. K.; Marks, T. J.; Burton, R.; Lyding, J. W.; Kannewurf, C. R. Mol. Cryst. Liq. Cryst. 1985, 118, 349352. 14. Inabe, T.; Kannewurf, C. R.; Lyding, J. W.; Moguel, M. K.; Marks, T. J. Mol. Cryst. Liq. Cryst. 1983, 93, 355-367. 15. Diaz, A. F.; Bargon, J. In Handbook of Conducting Polymers; Skotheim, T. Α., Ed.; Marcel Dekker: New York, 1986; Vol. 1, pp 81-115. 16. Street, G. Β., ibid. pp. 265-291. 17. Burgmayer, P.; Murray, R. W., ibid., pp. 507-523. 18. Shacklette, L. W.; Toth, J. E.; Murthy, N. S.; Baughman, R. H. J. Electrochem. Soc. 1985, 132, 1529-1535. 19. Gaudiello, J. G.; Almeida, M.; Marks, T. J.; McCarthy, W. J.; Butler, J. C.; Kannewurf, C. R. J. Phys. Chem. 1986, 90, 49174920. 20. Almeida, M.; Gaudiello, J. G.; Butler, J. C.; Marcy, H. O.; Kannewurf, C. R.; Marks, T. J. Synth. Met., in press. 21. Zhou, X.; Inabe, T.; Marks, T. J.; Carr, S. Η., submitted for publication. 22. Zhou, X.; Marks, T. J.; Carr, S. H. J. Polym. Sci., Phys. Ed. 1985, 23, 305-313.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

17.

MARKS ET AL.

Routes to Molecular Metals

237

23. Zhou, X.; Marks, T. J.; Carr, S. H. Polymeric Mats. Sci. Eng. 1984, 51, 651-654. 24. Rouxel, J.; Brec, R. Ann. Rev. Mat. Sci. 1986, 16, 137-162. 25. Thompson, A. H. Rev. Sci. Instrum. 1983, 54, 229-237. 26. Whittingham, M. S. Ann. Chim. Fr. 1982, 7, 204-214, and references therein. 27. Thompson, A. H. J. Electrochem. Soc. 1979, 126, 608-616. 28. Kaufman, J. H.; Mele, J. E.; Heeger, A. J.; Kaner, R.; MacDiarmid, A. G. J. Electrochem. Soc. 1983, 130, 571-574. 29. Kaufman, J. H.; Kaufer, J. W.; Heeger, A. J.; Kaner, R.; MacDiarmid, A. G. Phys. Rev. B. 1982, 26. 2327-2330. 30. Gaudiello, J. G.; Kellogg, G. E.; Tetrick, S. M.; Marks, T. J., submitted for publication. 31. Gaudiello, J. G.; Kellogg, G. E.; Tetrick, S. M.; Almeida, M.; Marks, T. J.; Marcy, H. O.; McCarthy, W. J.; Butler, J. C.; Kannewurf, C. R., submitted for publication. 32. Pickup, P. G.; Osteryoung, R. A. J. Am. Chem. Soc. 1984, 106 2294-2299, and references 33. Kaufman, F. B.; Schroeder, Chambers, J. Q. J. Am. Chem. Soc. 1980, 102, 483-488. 34. Brown, C. J. J. Chem. Soc. A 1968, 2488-2493. 35. Brown, C. J. J. Chem. Soc. A 1968, 2494-2498, and references therein. 36. Assour, J. M. J. Phys. Chem. 1965, 69, 2295-2299. 37. Pietro, W. J.; Marks, T. J.; Ratner, M. A. J. Am. Chem. Soc. 1985, 107, 5387-5391. 38. Pietro, W. J.; Ellis, D. E.; Marks, T. J.; Ratner, M. A. Mol. Cryst. Liq. Cryst. 1984, 105, 273-287. 39. Ciliberto, E.; Doris, Κ. Α.; Pietro, W. J.; Reisner, G. M.; Ellis, D. E.; Fragalà, I.; Herbstein, F. H.; Ratner, M. Α.; Marks, T. J. J. Am. Chem. Soc. 1984, 106, 7748-7761. RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 18

A New Approach to the Synthesis of Alkyl Silicates and Organosiloxanes 1

George B. Goodwin and Malcolm E. Kenney Department of Chemistry, Case Western Reserve University, Cleveland,

A new route to alkyl silicates and organosiloxanes is described. This route has three steps. These are (1) the protonation of a silicate (obtained by collection, mining, or laboratory or commercial synthesis), (2) the esterification of the silicic acid formed by the protonation, and (3) the organodealkoxylation of the alkyl silicate resulting from the esterification. An important feature of this route is that it does not depend on elemental silicon. The route is illustrated with the synthesis of (EtO) Si from γ-Ca SiO , Ca SiO O, and portland cement, (EtO) SiOSi(OEt) from Ca ZnSi O , (n-PrO) SiO(n-PrO) SiOSi(On-Pr) from 4

3

2

2

3

7

3

the

9

(EtO) Si O 10

4

3

Ca Si O , 3

2

4

6

7

[5.5.1]

from

2

and [5.3.3]

Cu Si O .6H O 6

6

18

2

3

3

isomers of

and from

Na Ca Si O , 4

4

6

18

and the [5.5.1] and [5.3.3] isomers of Me Si O from the [5.5.1] and [5.3.3] isomers of (EtO) Si O . 10

6

10

6

7

7

As is w e l l known, a number o f d i f f e r e n t k i n d s o f s i l i c a t e i o n s a r e f o u n d in s i l i c a t e s . Thus, among gem s i l i c a t e s b e n i t o i t e , BaTiSi3C>9, c o n t a i n s t h e c y c l o t r i s i l i c a t e i o n and aquamarine ( b e r y l ) , Be3Al2SigOi8» c o n t a i n s t h e c y c l o h e x a s i l i c a t e i o n ( 1 , 2 ) . S i m i l a r l y , among common s i l i c a t e m i n e r a l s hemimorphite, Zn4Si207(OH)2.Η2θ, c o n t a i n s t h e d i s i l i c a t e i o n and e n s t a t i t e , MgSi03, c o n t a i n s t h e i n f i n i t e c h a i n s i l i c a t e i o n ( 1 ) . I n t h e c a s e o f t h e common s y n t h e t i c s i l i c a t e s , p o r t l a n d cement c o n t a i n s phases w h i c h c a n be a p p r o x i m a t e d as Ca2Si04 and Ca3SiC>40 and which c o n t a i n t h e o r t h o s i l i c a t e i o n ( 3 , 4 ) , F i g u r e 1. Some o f t h e many s i l i c a t e s t h a t a r e r e a d i l y a v a i l a b l e have s i l i c a t e i o n s w i t h frameworks t h a t a r e s i m i l a r t o o r t h e same as t h o s e p r e s e n t in common a l k y l s i l i c a t e s and common o r g a n o s i l o x a n e s (5,7). I n l i g h t o f t h i s , a s y n t h e t i c approach t o a l k y l s i l i c a t e s and o r g a n o s i l o x a n e s b a s e d on s u b s t i t u t i o n r e a c t i o n s becomes conceivable. 1

Current address: Glass R&D Center, PPG Industries, Inc., Pittsburgh, PA 15238 0097-6156/88/0360-0238$06.00/0 © 1988 American Chemical Society

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

18.

GOODWIN AND

KENNEY

F i g u r e 1. S t r u c t u r e s (c) c y c l o t r i s i l i c a t e ,

Alkyl Silicates and Organosiloxanes

o f (a) o r t h o s i l i c a t e , (b) and (d) c y c l o h e x a s i l i c a t e

disilicate, ions.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

239

240

INORGANIC AND ORGANOMETALLIC POLYMERS

The complete p r o c e s s f o r s y n t h e s i z i n g such s p e c i e s u s i n g t h i s a p p r o a c h would e n t a i l the a c q u i s i t i o n o f an a p p r o p r i a t e n a t u r a l s i l i c a t e o r the p r e p a r a t i o n o f an a p p r o p r i a t e s y n t h e t i c s i l i c a t e and t h e n the c o n v e r s i o n o f t h i s s i l i c a t e i n t o the a l k y l s i l i c a t e or o r g a n o s i l o x a n e by s u i t a b l e s u b s t i t u t i o n r e a c t i o n s . I n terms o f bond c l e a v a g e , t h i s p r o c e s s c o u l d e n t a i l no d e s t r u c t i o n and r e f o r m a t i o n o f framework s i l i c o n - o x y g e n bonds, and, in terms o f o x i d a t i o n number, i t would e n t a i l no r e d u c t i o n and r e o x i d a t i o n o f the s i l i c o n . T h i s p r o c e s s c o n t r a s t s w i t h the e l e m e n t a l - s i l i c o n p r o c e s s e s sometimes u s e d f o r a l k y l s i l i c a t e s (8) and the e l e m e n t a l - s i l i c o n p r o c e s s e s g e n e r a l l y u s e d f o r o l i g o m e r i c and p o l y m e r i c o r g a n o s i l o x a n e s (6 ,7) . S i n c e the s i l i c o n in t h e s e p r o c e s s e s is o b t a i n e d from q u a r t z , t h e s e p r o c e s s e s e n t a i l , in terms o f bond c l e a v a g e , the d e s t r u c t i o n o f f o u r s i l i c o n - o x y g e n bonds p e r s i l i c o n and the subsequent r e f o r m a t i o n o f the r e q u i r e d number o f such bonds. In terms o f o x i d a t i o n number, t h e y e n t a i l the r e d u c t i o n o f the s i l i c o n from f o u r t o z e r F i g u r e s 2 and 3. Because t h e s e p r o c e s s e s r e q u i r e r e d u c t i o n and r e o x i d a t i o n o f the s i l i c o n , t h e y r e q u i r e l a r g e amounts o f energy p e r u n i t o f product. T h i s makes them i n h e r e n t l y u n a t t r a c t i v e and makes a s e a r c h f o r r e p l a c e m e n t s f o r them w o r t h w h i l e . This n a t u r a l l y leads to a c o n s i d e r a t i o n o f the s i l i c a t e - b a s e d s u b s t i t u t i o n a p p r o a c h t o t h e s e compounds. A number o f p i e c e s o f work i n d i c a t i n g t h a t t h i s a p p r o a c h c a n be d e v e l o p e d have been r e p o r t e d . I n one, a commercial 3.25:1 Si02:Na20 sodium s i l i c a t e was s u c c e s s i v e l y p r o t o n a t e d and b u t o x y l a t e d to a mixture of polymeric b u t y l s i l i c a t e s (9). In a second, c h r y s o t i l e a s b e s t o s , Mg3Si205(0H)4, was s u c c e s s i v e l y p r o t o n a t e d and a l l y l o x y l a t e d t o a p o l y m e r i c a l l y l s i l i c a t e , a p p a r e n t l y w i t h s i l o x a n e framework p r e s e r v a t i o n ( 1 0 ) . A l s o , in a v e r y minor b y p r o d u c t r e a c t i o n p s e u d o w o l l a s t o n i t e , Ca3Si309, was p r o t o n a t e d and p r o p o x y l a t e d t o a p r o p y l s i l i c a t e w i t h framework p r e s e r v a t i o n (11). I n o t h e r e f f o r t s , ( E t 0 ) 3 S i 0 S i ( 0 E t ) 3 was a l k y l d e a l k o x y l a t e d t o o r g a n o d i s i l o x a n e s ( 1 2 ) , and o l i g o m e r i c and p o l y m e r i c o r g a n o a l k o x y s i l o x a n e s were a l k y l d e a l k o x y l a t e d t o o r g a n o s i l o x a n e s w i t h o r a p p a r e n t l y w i t h framework p r e s e r v a t i o n (13,14). A l s o , monomeric and o l i g o m e r i c m e t a l s i l i c a t e s were p r o t o n a t e d and s i l y l a t e d t o o r g a n o s i l y l s i l o x a n e s w i t h f u l l o r s u b s t a n t i a l framework p r e s e r v a t i o n ( 1 5 ) . In the p r e s e n t paper, a r o u t e t o a l k y l s i l i c a t e s and o r g a n o s i l o x a n e s w h i c h has elements in common w i t h t h e s e p i e c e s o f work and w h i c h is b a s e d on the s u b s t i t u t i o n a p p r o a c h is i l l u s t r a t e d and d i s c u s s e d . THE

ROUTE

In the r o u t e u s e d in t h i s work, a monomeric o r o l i g o m e r i c s i l i c a t e is p r o t o n a t e d , the r e s u l t i n g s i l i c i c a c i d is e s t e r i f i e d , and the a l k y l s i l i c a t e is o r g a n o d e a l k o x y l a t e d . I n e a c h s t e p , the s i l o x a n e framework is f u l l y o r s u b s t a n t i a l l y p r e s e r v e d . F o r the case s t a r t i n g w i t h a m e t a l d i s i l i c a t e the r e a c t i o n s c a n be r e p r e s e n t e d as :

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

18.

GOODWIN AND KENNEY

Alkyl Silicates and Organosiloxanes

241

F i g u r e 2. V a r i a t i o n o f o x i d a t i o n number o f s i l i c o n w i t h s t e p and v a r i a t i o n o f number o f oxygens bonded t o s i l i c o n w i t h s t e p in d i r e c t or e l e m e n t a l - s i l i c o n process f o r (EtO)4Si.

F i g u r e 3. V a r i a t i o n o f o x i d a t i o n number o f s i l i c o n w i t h s t e p and v a r i a t i o n o f number o f oxygens bonded t o s i l i c o n w i t h s t e p in u s u a l p r o c e s s f o r (Me2SiO)4.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

242 0 SiOSi0 3

6 3

-

+ H

+

+ ROH

>

( H O ) S i O S i ( O H ) - x R O H + ROH 3

3

3

3

> (R0) Si0Si(0R)

3

(R0) Si0Si(0R)

(HO) SiOSi(OH) -xROH 3

+ R'MgX

> R Si0SiR 3

3

(1)

3

(2) (3)

3

I t is p o s s i b l e t h a t some e s t e r i f i c a t i o n o f the s i l i c a t e i o n o c c u r s in t h i s r o u t e b e f o r e the i o n is f u l l y p r o t o n a t e d and thus t h a t the f i r s t two s t e p s o f the r o u t e o v e r l a p . A n o v e l f e a t u r e o f the r o u t e is t h a t i t l e a d s t o monomeric and o l i g o m e r i c a l k y l s i l i c a t e s from m e t a l s i l i c a t e s in good y i e l d and w i t h f u l l o r s u b s t a n t i a l s i l o x a n e framework p r e s e r v a t i o n . Further, i t leads to oligomeric organosiloxanes of intermediate m o l e c u l a r w e i g h t from a l k y l s i l i c a t e s in good y i e l d and w i t h s i l o x a n e framework p r e s e r v a t i o n . EXAMPLES ( E t Q ) & S i from 7-Ca?Si04 s l o w l y t o a a c o o l e d s u s p e n s i o n o f 7-Ca2Si04 (Cerac I n c . , Milwaukee, WI) in e t h a n o l . The r e s u l t i n g s u s p e n s i o n was f i l t e r e d and t h e s o l i d was washed. The f i l t r a t e and the washings were combined and s l o w l y added t o an e t h a n o l - t o l u e n e s o l u t i o n t h a t was b e i n g d i s t i l l e d a t a moderate r a t e . A f t e r the r e s u l t a n t had been d i s t i l l e d u n t i l a s u b s t a n t i a l amount o f d i s t i l l a t e had been c o l l e c t e d , the s u s p e n s i o n o b t a i n e d was f i l t e r e d and the s o l i d was washed. The f i l t r a t e and washings were combined, c o n c e n t r a t e d , b u l b - t o - b u l b d i s t i l l e d , and f r a c t i o n a l l y d i s t i l l e d . W i t h the a i d o f gas chromatography, gas chromatography-mass s p e c t r o m e t r y , and i n f r a r e d s p e c t r o s c o p y , the p r o d u c t was compared t o a u t h e n t i c (Et0)4Si. The r e s u l t s showed t h a t i t was q u i t e p u r e (99.8%) ( E t 0 ) 4 S i ( c o n t a i n e d y i e l d 33%). The ( E t 0 ) 4 S i c o u l d have been c o n v e r t e d t o Me4Si w i t h a m e t h y l G r i g n a r d reagent i f d e s i r e d (16). ( E t O U S i from MONOCLINIC Ca^SiO^O. M o n o c l i n i c C a S i 0 4 0 ( C o n s t r u c t i o n T e c h n o l o g y L a b o r a t o r i e s , I n c . , S k o k i e , I L ) was t r e a t e d in a manner s i m i l a r t o t h a t u s e d w i t h 7-Ca2Si04. The p r o d u c t was r e l a t i v e l y pure (95%) ( E t 0 ) 4 S i ( c o n t a i n e d y i e l d 4 2 % ) . 3

( E t 0 ) 4 S i from PORTLAND CEMENT. P o r t l a n d cement (Maryneal z e r o C A Type I I I , Lone S t a r I n d u s t r i e s , I n c . , Houston, TX) was a l s o t r e a t e d in a s i m i l a r f a s h i o n . The p r o d u c t a g a i n was r e l a t i v e l y p u r e (94%) ( E t 0 ) 4 S i ( c o n t a i n e d y i e l d 4 1 % ) . 3

( E t O ) ^ S i O S i ( O E t ) ^ from C a 9 Z n S i 9 0 (HARDYSTONITE). An H C l - e t h a n o l s o l u t i o n was added s l o w l y t o a s u s p e n s i o n o f Ca2ZnSi207 ( p r e p a r e d by s i n t e r i n g ZnO and C a S i 0 ( w o l l a s t o n i t e ) ) in e t h a n o l . The s u s p e n s i o n formed was s t i r r e d a t ambient temperature and f i l t e r e d . The f i l t r a t e was c o o l e d and added s l o w l y t o an e t h a n o l - t o l u e n e s o l u t i o n t h a t was b e i n g d i s t i l l e d a t a moderate r a t e . A f t e r the r e s u l t a n t had been d i s t i l l e d u n t i l a s u b s t a n t i a l amount o f d i s t i l l a t e had been c o l l e c t e d , the m i x t u r e p r o d u c e d was decanted. The d e c a n t a t e was c o n c e n t r a t e d and f r a c t i o n a l l y d i s t i l l e d . The p r o d u c t was compared t o a u t h e n t i c ( E t O ) S i O S i ( O E t ) w i t h the a i d 7

3

3

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

18. GOODWIN AND KENNEY

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243

o f gas chromatography, gas chromatography-mass s p e c t r o m e t r y , and i n f r a r e d spectroscopy. The r e s u l t s showed t h a t i t was r e l a t i v e l y p u r e (97%) ( E t O ) S i O S i ( O E t ) ( c o n t a i n e d y i e l d 2 9 % ) . The ( E t O ) S i O S i ( O E t ) c o u l d have been c o n v e r t e d t o E t S i O S i E t w i t h an e t h y l G r i g n a r d reagent i f d e s i r e d (12). 3

3

3

3

3

3

( n - P r 0 ) ^ S i 0 ( n - P r 0 ) 9 S i 0 S i ( 0 n - P r H from C a S i 0 9 (PSEUDOWOLLASTONITE) . To a s u s p e n s i o n o f Ca Si C>9 ( p r e p a r e d b y h e a t i n g C a S i 0 ( w o l l a s t o n i t e ) ) in n - p r o p a n o l was added an HCl-np r o p a n o l s o l u t i o n (and t h e a d d i t i o n f u n n e l w a s h i n g s ) . The r e s u l t i n g m i x t u r e was d i s t i l l e d u n t i l a s u b s t a n t i a l amount o f d i s t i l l a t e h a d been c o l l e c t e d and t h e s u s p e n s i o n p r o d u c e d was filtered. The s o l i d was washed, t h e f i l t r a t e and washings were combined, and t h e r e s u l t a n t was c o n c e n t r a t e d and mixed w i t h an HC1-n-propanol s o l u t i o n . A f t e r t h e s o l u t i o n formed h a d been d i s t i l l e d u n t i l a s u b s t a n t i a l amount o f d i s t i l l a t e h a d been c o l l e c t e d , t h e remainder was t w i c e d i l u t e d w i t h pentane filtered and c o n c e n t r a t e d . Th d i s t i l l e d and f r a c t i o n a l l t o g r a p h y , gas chromatography-mass s p e c t r o m e t r y , and i n f r a r e d s p e c t r o s c o p y , t h e p r o d u c t was shown t o be r e l a t i v e l y pure (94%) (n-PrO) SiO(n-PrO)2SiOSi(On-Pr) (contained y i e l d 18%): IR ( n e a t ) 2964 ( s ) , 2938 ( s ) , 2880 ( s ) , 1465 (m) 1088 ( v s ) cm' ; GCMS m/z [ r e l i n t e n s i t y ] 529 [ ( M - 0 P r ) , 7 ] , 235 [ ( ( H 0 ) S i 0 ) + , 1 0 0 ] . 3

3

3

3

3

3

3

1

+

7

3

2

THE [ 5 . 5 . Π and [5.3.3] ISOMERS o f ( E t 0 ) m S i ^ 0 7 from CufiSifi0ia-6H90 (DIOPTASE). An H C l - e t h a n o l s o l u t i o n was added s l o w l y t o a s u s p e n s i o n o f C u g S i g O i g * 6 H 0 (Ward's N a t u r a l S c i e n c e E s t a b l i s h m e n t , I n c . , R o c h e s t e r , NY) in an e t h a n o l - t o l u e n e solution. The r e s u l t i n g m i x t u r e was d i s t i l l e d u n t i l a s u b s t a n t i a l amount o f d i s t i l l a t e h a d been c o l l e c t e d a n d the s u s p e n s i o n formed was c o o l e d and f i l t e r e d . The s o l i d was washed and t h e f i l t r a t e and washings were combined. A f t e r t h e s o l u t i o n p r o d u c e d h a d been c o n c e n t r a t e d , i t was mixed w i t h an H C l - e t h a n o l s o l u t i o n , e t h a n o l , and t o l u e n e . The m i x t u r e formed was d i s t i l l e d u n t i l a s u b s t a n t i a l amount o f d i s t i l l a t e h a d been c o l l e c t e d , and t h e r e m a i n d e r was d i s t i l l e d t w i c e . By gas chromatography, gas chromatography-mass s p e c t r o m e t r y , i n f r a r e d s p e c t r o s c o p y , and ^Si n u c l e a r magnetic r e s o n a n c e s p e c t r o s c o p y , t h e p r o d u c t was shown t o c o n t a i n a s u b s t a n t i a l amount (43%) o f t h e [5.5.1] isomer o f ( E t O ) i o S i 6 0 ( c o n t a i n e d y i e l d 20%): GCMS [ r e l i n t e n s i t y ] 685 [ ( M - 0 E t ) , 6 ] , 415 [ ( ( H 0 ) S i 0 ) , 100]; S i NMR (39.7 MHz, CDC1 ) δ -95.56 (middle type S i ) , -101.55 ( b r a n c h type S i ) . I t was a l s o shown t o c o n t a i n a s i g n i f i c a n t amount (29%) o f t h e [5.3.3] isomer o f ( E t O ) i o i 6 ° 7 ( c o n t a i n e d y i e l d 14%): GCMS m/z [ r e l i n t e n s i t y ] 685 [(M-0Et)+, 1 2 ] , 415 [ ( ( H O ) S i 0 ) , 100]; S i NMR (39.7 MHz, CDC1 ) δ -94.69 ( m i d d l e type S i ) , -96.65 ( m i d d l e type S i ) , -101.27 ( b r a n c h type Si). 2

7

+

+

7

6

2 9

8

3

s

+

7

6

2 9

8

3

THE [5.5.1] and [5.3.3] ISOMERS o f (EtO)ιnSifiOy from N a 4 Ç a 4 S i 6 0 i . Na4Ca4SigOi8 ( p r e p a r e d b y s i n t e r i n g Na2C0 , C a C 0 , a n d S i 0 ) was t r e a t e d in a manner s i m i l a r t o t h a t u s e d w i t h C u g S i g O i g * 6 H 0 . A s u b s t a n t i a l p a r t (46%) o f t h e p r o d u c t was t h e [5.5.1] isomer o f ( E t O ) i o S i 6 0 ( c o n t a i n e d y i e l d 31%): GCIR (Ar m a t r i x ) 2984 (m), 2936 (m), 2905 (m), 1161 ( s ) , 1105 ( v s ) cm" . A s i g n i f i c a n t p a r t 8

3

3

2

2

7

1

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

244

(28%) o f t h e p r o d u c t was t h e [5.3.3] isomer o f (EtO^QSifcOy ( c o n t a i n e d y i e l d 20%): GCIR (Ar m a t r i x ) 2983 (m), 2935 (m), 2904 (m), 1162 ( s ) , 1106 ( v s ) cm" . 1

THE [5.5.1] and [5.3.3] ISOMERS o f MeioaioOy from t h e [5.5.1] and [5.3.3] ISOMERS o f ( E t O ) m S J 6 0 7 . A s o l u t i o n o f CH MgCl in t e t r a h y d r o f u r a n was s l o w l y added t o a c o o l e d s o l u t i o n o f t h e [5.5.1] and [5.3.3] isomers o f ( E t O ) i o i 6 ° 7 ( p r e p a r e d as above) in tetrahydrofuran. The r e s u l t i n g m i x t u r e was s t i r r e d f o r a c o n s i d e r a b l e p e r i o d o f time w h i l e b e i n g k e p t c o o l and t h e n was concentrated. The c o n c e n t r a t e was s t i r r e d w i t h a m i x t u r e o f d i l u t e HC1 and pentane, and t h e r e s u l t i n g o r g a n i c phase was s e p a r a t e d and washed w i t h an aqueous N a C l s o l u t i o n . I t was t h e n d i s t i l l e d u n t i l a s u b s t a n t i a l amount o f d i s t i l l a t e h a d been collected. The o i l formed was f l a s h chromatographed and t h e e l u a t e was t w i c e f r a c t i o n a l l y d i s t i l l e d . The p r o d u c t was shown b y gas chromatography, gas chromatography-mass s p e c t r o m e t r y , gas chromatography-infrared spectroscopy chromatography, and S to c o n t a i n a s u b s t a n t i a l amount (60%) o f t h e [5.5.1] isomer o f M e i o l 6 ° 7 ( c o n t a i n e d y i e l d on the b a s i s o f c o n t a i n e d [5.5.1] isomer o f ( E t O ) i o S i 0 38%): IR (neat) 2966 (m), 1260 ( s ) , 1079 ( v s ) cm" ; GCMS m/z [ r e l i n t e n s i t y ] 415 [(M-Me)+, 68]; S i NMR (39.7 MHz, CDC1 ) δ -19.07 (D type S i ) ; -63.55 (T t y p e S i ) . I n a d d i t i o n , i t was shown t o c o n t a i n a s i g n i f i c a n t amount (29%) o f t h e [5.3.3] isomer o f Μ β χ ο ΐ 6 ° 7 ( c o n t a i n e d y i e l d on t h e b a s i s o f c o n t a i n e d [5.3.3] isomer o f ( E t O ) i o S i 6 0 7 2 9 % ) : IR ( n e a t ) 2966 (m), 1263 ( s ) , 1083 (vs) cm" ; GCMS m/z [ r e l i n t e n s i t y ] 415 [(MMe) , 83]; S i NMR (39.7 MHz, C D C I 3 ) δ -17.98 (D t y p e S i ) , -20.93 (D t y p e S i ) , -62.73 (T type S i ) . The [5.5.1] and [5.3.3] isomers of MeioSi607 known compounds (17,18). 3

s

2 9

s

6

7

1

2 9

3

δ

1

+

2 9

a

r

e

DISCUSSION REACTANTS and CHEMISTRY, STEP 1. The s i l i c a t e s u s e d in t h e examples g i v e n in t h e f i r s t o r p r o t o n a t i o n s t e p o f t h e r o u t e have s i l i c a t e i o n s w i t h v a r i o u s s t r u c t u r e s . Thus, 7-Ca2Si04, mono­ c l i n i c Ca3Si040, and t h e p r i n c i p a l s i l i c a t e phases in p o r t l a n d cement, a p p r o x i m a t e l y Ca3SiÛ40 and Ca2Si04 c o n t a i n t h e o r t h o s i l i c a t e i o n ( 3 , 4 ) , Ca2ZnSi20y c o n t a i n s t h e d i s i l i c a t e i o n (19), Ca3Si3U9 c o n t a i n s t h e c y c l o t r i s i l i c a t e i o n ( 1 ) , and CugSigO^g.6H2O and Na4Ca4SigO;Lg c o n t a i n t h e c y c l o h e x a s i l i c a t e i o n ( 1 , 2 0 ) . The s t r u c t u r a l and c h e m i c a l d i v e r s i t y o f t h e s e s i l i c a t e s show t h a t a wide v a r i e t y o f s i l i c a t e s c a n be employed in t h i s s t e p . V a r i o u s methods c a n be u s e d t o o b t a i n t h e s i l i c a t e s n e c e s s a r y f o r the step i n c l u d i n g small s c a l e c o l l e c t i o n (e.g., d i o p t a s e ) , m i n i n g , s y n t h e s i s from s i l i c a ( e . g . , N a 4 C a 4 S i g 0 i g ) , and s y n t h e s i s from o t h e r s i l i c a t e s ( e . g . , Ca3Si3U9 and C a 2 Z n S i 2 0 y ) . The r e s u l t s o b t a i n e d so f a r i n d i c a t e t h a t t h e s i l i c a t e s b e s t s u i t e d f o r u s e are o f t e n calcium s i l i c a t e s . No a c i d s o t h e r t h a n HC1 have been t r i e d in t h e s t e p . However, i t is l i k e l y t h a t i t c a n be c a r r i e d o u t w i t h o t h e r s t r o n g acids. From t h e r e s u l t s o f s e v e r a l e x p e r i m e n t s i t h a s b e e n l e a r n e d t h a t i s o p r o p a n o l and n - b u t a n o l can be u s e d t o make a l k y l s i l i c a t e s

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

18.

GOODWIN AND KENNEY

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by t h e r o u t e and thus t h a t t h e y c a n be u s e d in t h e f i r s t s t e p . T h i s s u g g e s t s t h a t a v a r i e t y o f a l c o h o l s c a n be u s e d in i t (however, as is o b v i o u s , t h e c h o i c e o f t h e a l c o h o l is governed b y the n a t u r e o f t h e a l k o x y groups needed in t h e second s t e p ). The f a c t t h a t t h e a l k y l s i l i c a t e s p r o d u c e d b y t h e r o u t e r e t a i n t h e s i l o x a n e framework o f t h e p a r e n t s i l i c a t e s f u l l y o r s u b s t a n t i a l l y , F i g u r e 4, shows t h a t t h e framework is a t l e a s t l a r g e l y r e t a i n e d in t h e f i r s t s t e p . A number o f e x p e r i m e n t s i n d i c a t e one f a c t o r a i d i n g s t r u c t u r e r e t e n t i o n in t h i s s t e p is t h e use o f an amount o f HC1 t h a t is j u s t s l i g h t l y above t h a t w h i c h is s t o i c h i o m e t r i c a l l y r e q u i r e d (105-110%). Other experiments i n d i c a t e t h a t in some i n s t a n c e s t h e u s e o f a low t e m p e r a t u r e (~-10 ° C ) l i k e w i s e h e l p s w i t h s t r u c t u r e r e t e n t i o n . A l s o p e r t i n e n t t o s t r u c t u r e r e t e n t i o n in t h i s s t e p a r e r e s u l t s from e x p e r i m e n t s showing t h a t t h e r o u t e y i e l d s a l k y l s i l i c a t e s h a v i n g poor s t r u c t u r e r e t e n t i o n when t h e i n t e r m e d i a t e s i l i c i c a c i d c o n c e n t r a t i o n is h i g h and good r e t e n t i o n when i t is low. These r e s u l t s s u g g e s (-0.04M) a i d s s t r u c t u r is e a s i l y u n d e r s t a n d a b l e in terms o f t h e ease w i t h w h i c h s i l i c i c a c i d s r e a c t w i t h themselves. S i n c e i t is w e l l known t h a t a l c o h o l s form s t r o n g h y d r o g e n bonds, i t is v e r y p r o b a b l e t h a t t h e a l c o h o l in t h i s s t e p h y d r o g e n bonds t o t h e s i l i c i c a c i d ( o r a c i d s ) p r o d u c e d in i t , and as a r e s u l t c r e a t e s a p r o t e c t i v e s h e a t h around each s i l i c i c a c i d molecule. T h i s s h e a t h is c l e a r l y v e r y i m p o r t a n t in framework r e t e n t i o n in v i e w o f t h e r e a c t i v i t y o f s i l i c i c a c i d s . STEP 2. The e x p e r i m e n t s w i t h e t h a n o l , n - p r o p a n o l , i s o p r o p a n o l , and n - b u t a n o l a l r e a d y d e s c r i b e d o r mentioned i n d i c a t e t h a t a v a r i e t y o f a l c o h o l s c a n be u s e d in t h e second s t e p . I t appears t h a t u n h i n d e r e d p r i m a r y and s e c o n d a r y a l c o h o l s g e n e r a l l y w i l l be suitable. The a c i d in t h i s s t e p c l e a r l y f u n c t i o n s as a c a t a l y s t ( a c i d s a r e known t o c a t a l y z e t h e e s t e r i f i c a t i o n o f s i l a n o l s ) ( 2 1 ) . The t o l u e n e , when employed, s e r v e s t o d r i v e t h e e s t e r i f i c a t i o n t o c o m p l e t i o n b y f o r m i n g a w a t e r - t o l u e n e - e t h a n o l a z e o t r o p e (12% water) ( 2 2 ) . I t a l s o renders the r e a c t i o n s o l u t i o n a poor s o l v e n t f o r t h e b y p r o d u c t s a l t s and thus f a c i l i t a t e s t h e s e p a r a t i o n o f t h e s e s a l t s ( t h e pentane, when used, s e r v e s t h i s same f u n c t i o n ) . The f a c t t h a t t h e a l k y l s i l i c a t e s p r o d u c e d b y t h e r o u t e f u l l y o r s u b s t a n t i a l l y r e t a i n t h e o r i g i n a l s i l o x a n e framework a l s o shows t h a t t h e framework is a t l e a s t l a r g e l y r e t a i n e d in t h e second step. The r e s u l t s p e r t a i n i n g t o s i l i c i c a c i d c o n c e n t r a t i o n a l r e a d y m e n t i o n e d l e a d t o t h e c o n c l u s i o n t h a t a low s i l i c i c a c i d concentration aids this structure retention. A l s o a i d i n g i t , no doubt, is t h e a b i l i t y o f t h e a l c o h o l t o s h e a t h and p r o t e c t t h e s i l i c i c acid. (The r i n g o p e n i n g o c c u r i n g in t h e c o n v e r s i o n o f t h e S13O96" i o n t o ( n - P r 0 ) 3 S i 0 ( n - P r 0 ) 2 S i 0 S i ( 0 n - P r ) 3 c o u l d o c c u r in e i t h e r t h e f i r s t o r second step o f the s y n t h e s i s . I t is a t t r i b u t a b l e t o a p r o t o n - a s s i s t e d c l e a v a g e o f t h e s i l o x a n e framework enhanced b y t h e s t r a i n i n h e r e n t in t h e r i n g . The rearrangement w h i c h o c c u r s in the c o n v e r s i o n o f t h e S i ^ O ^ g ^ - l ° ^ [5.3.3] isomer o f (EtO)ioSi6°7 a l s o c o u l d o c c u r in e i t h e r t h e f i r s t o r second s t e p n t

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In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

18.

GOODWIN AND KENNEY

of the s y n t h e s i s . framework c l e a v a g e

Alkyl Silicates and Organosiloxanes

247

I t c a n be a s c r i b e d t o a p r o t o n - a s s i s t e d f o l l o w e d by r i n g - c o n t r a c t i n g r i n g c l o s u r e s . )

STEP 3. The l a s t o r o r g a n o d e a l k o x y l a t i o n s t e p c a n no doubt be c a r r i e d o u t w i t h a v a r i e t y o f G r i g n a r d r e a g e n t s , and thus i t is c l e a r l y f l e x i b l e too. From t h e r e s u l t s o f v a r i o u s e x p e r i m e n t s , i t is a p p a r e n t t h a t framework r e t e n t i o n in t h i s s t e p is a i d e d b y a low temperature (~0 ° C ) and a moderate G r i g n a r d r e a g e n t t o a l k o x y group r a t i o (-2:1). UTILITY. T h i s r o u t e appears t o o f f e r a p a t h t o a l k y l s i l i c a t e s t h a t is s i m p l e and has t h e p o t e n t i a l o f h a v i n g low energy requirements. I t does n o t o f f e r a c o r r e s p o n d i n g low-energy p a t h t o o r g a n o s i l o x a n e s because a G r i g n a r d r e a g e n t is u s e d in t h e o r g a n o d e a l k o x y l a t i o n s t e p and thus e l e m e n t a l magnesium is u l t i m a t e l y r e q u i r e d f o r t h i s s t e p . However, i t s h o u l d be p o i n t e d o u t t h a t e l e m e n t a l magnesium is needed o n l y f o r t h e f o r m a t i o n o f the S i - C bonds in t h i s r e q u i r e d f o r the formatio t h i s way t h e new r o u t e is advantageous. Further, i t s existence reemphasizes the f a c t t h a t p o s s i b l e paths t o o r g a n o s i l o x a n e s t h a t a r e n o t b a s e d o n e l e m e n t a l s i l i c o n cannot be summarily d i s m i s s e d . The r o u t e a l s o p r o v i d e s a s a t i s f a c t o r y p a t h t o some a l k y l s i l i c a t e s f o r w h i c h no o t h e r s a t i s f a c t o r y r o u t e s a r e a v a i l a b l e . Some o f t h e s e a l k y l s i l i c a t e s may be o f i n t e r e s t in t h e s y n t h e s i s of ceramics by the s o l - g e l technique. ACKNOWLEDGMENT. We thank R a l p h E. Temple, D a l e R. P u l v e r , a n d Gordon F e a r o n f o r h e l p f u l d i s c u s s i o n s . We a l s o thank Diamond Shamrock and Dow C o r n i n g f o r f i n a n c i a l s u p p o r t . LITERATURE CITED 1. 2. 3.

4. 5.

6.

7.

8. 9.

Liebau, F. Structural Chemistry of Silicates; SpringerVerlag: Berlin, 1985. Fleischer, M. Glossary of Mineral Species 1980; Mineralogical Record: Tucson, AZ, 1980. Helmuth, R. Α.; Miller, F. M.; O'Connor, T. R.; Greening, N. R. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.; John Wiley: New York, 1979; Vol. 5, p 163. Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford: Oxford, 1984; p 1017. Arkles, B. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, Μ., Ed.; John Wiley: New York, 1982; Vol. 20, p 912. Stark, F. O.; Fallender, J. R.; Wright, A. P. In Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, 1982; Vol. 2, p 305. Hardman, Β. B.; Torkelson, A. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, Μ., Ed.; John Wiley: New York, 1982; Vol. 20, p 922. Ayen, R. J.; Burk, J. H. Mater. Res. Soc. Symp. Proc. 1986, 73 (Better Ceram. Chem. 2), 801. Iler, R. K.; Pinkney, P. S. Ind. Eng. Chem. 1947, 39, 1379.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

248 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20. C: 21. 22.

Bleiman, C.; Mercier, J. P. Inorg. Chem. 1975, 14, 2853. Calhoun, H. P.; Masson, C. R. J. Chem. Soc., Dalton Trans. 1980, 1282. Smith, B. Ph.D. Thesis, Chalmers Technical High School, Gothenburg, Sweden, 1951 as quoted in Eaborn, C. Organosilicon Compounds; Academic Press: New York, 1960; p 14. Wacker-Chemie G.m.b.H. Br. Patent 732 533, 1955. Compton, R. Α.; Petraitis, D. J. U. S. Patent 4 309 557, 1982. Lentz, C. W. Inorg. Chem. 1964, 3, 574. George, P. D.; Sommer, L. H.; Whitmore, F. C. J. Am. Chem. Soc. 1955, 77, 6647. Jancke, H.; Engelhardt, G.; Magi, M.; Lippman, Ε. Ζ. Chem. 1973, 13, 392. Menczel, G. Acta Chim. Acad. Sci. Hung. 1977, 92, 9. Deer, W. Α.; Howie, R. Α.; Zussman, J. Rock-Forming Minerals; John Wiley: New York, Ohsato, H.; Takéuchi, Cryst. Struct. Commun. 1986, 42, 934. Eaborn, C. Organosilicon Compounds; Academic Press: New York, 1960; p 295. Horsley, L. H. Azeotropic Data II; Advances in Chemistry 35; American Chemical Society: Washington, DC, 1962; p 61.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 19

Current Status of Polyphosphazene Chemistry Harry R. Allcock Department of Chemistry, Pennsylvania State University, University Park, PA 16802 Inorganic polymer chemistry is an area of research that links the classical fields of ceramics, metals, and organic polymers, and provides opportunities for the synthesis of new substances that combine the properties of all three. Polyphosphazenes are inorganic macromolecules that illustrate the possibilities available for a wide range of other inorganic systems. In this article, it will be demonstrated that, depending on the synthesis method and molecular structure, it is possible to bias the properties toward those of ceramics, metals, or organic high polymers, and also to develop polymers of biomedical interest.

Polyphosphazenes comprise some of the most i n t e n s i v e l y s t u d i e d i n o r g a n i c macromolecules. They i n c l u d e one o f the o l d e s t known s y n t h e t i c polymers and many of the newest. In m o l e c u l a r s t r u c t u r a l v e r s a t i l i t y , they s u r p a s s a l l o t h e r i n o r g a n i c polymer systems ( w i t h over 300 d i f f e r e n t s p e c i e s now known), and t h e i r uses and d e v e l o p i n g a p p l i c a t i o n s a r e as broad as in many a r e a s o f o r g a n i c polymer chemistry. Moreover, the m o l e c u l a r s t r u c t u r a l , s y n t h e t i c , and p r o p e r t y nuances o f t h e s e polymers i l l u s t r a t e many of the a t t r i b u t e s , problems, and p e c u l i a r i t i e s o f o t h e r i n o r g a n i c m a c r o m o l e c u l a r systems. Thus, t h e y p r o v i d e a "case s t u d y " f o r an u n d e r s t a n d i n g o f what may l i e ahead f o r o t h e r systems now b e i n g probed at the exploratory l e v e l . I n s h o r t , an u n d e r s t a n d i n g of polyphosphazene c h e m i s t r y forms the b a s i s f o r an a p p r e c i a t i o n of a wide v a r i e t y o f r e l a t e d , i n o r g a n i c - b a s e d macromolecular systems and o f the r e l a t i o n s h i p between i n o r g a n i c polymer c h e m i s t r y and the r e l a t e d f i e l d s o f o r g a n i c polymers, c e r a m i c s c i e n c e , and m e t a l s . T h i s r e l a t i o n s h i p is i l l u s t r a t e d in F i g u r e 1. The s c i e n c e o f s o l i d s is the s c i e n c e o f s u p r a m o l e c u l a r systems in which the t h r e e d i m e n s i o n a l s o l i d s t r u c t u r e is h e l d t o g e t h e r by c o v a l e n t bonds

0097-6156/88/0360-0250$06.00/0 © 1988 American Chemical Society

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Thermally stable, but brittle, heavy and difficult to fabricate

CERAMICS

y

for Synthesis of New Materials

METALS Ductile, strong and good electrical conductors, but heavy and prone to oxidation and corrosion

Tough, lightweight, easy to fabricate and corrosion-resistant, but unstable to heat and oxidation

F i g u r e 1. The r e l a t i o n s h i p between t h e t h r e e c l a s s i c a l m a t e r i a l s a r e a s o f c e r a m i c s , p o l y m e r s , and m e t a l s .

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( c e r a m i c s ) , m e t a l l i c bonds ( m e t a l s ) , i o n i c a t t r a c t i o n s ( s a l t s ) , or a c o m b i n a t i o n of c o v a l e n t and van der Waals f o r c e s ( l i n e a r or branched polymers). In p r a c t i c e , i t is c o n v e n i e n t to c o n s i d e r s o l i d systems in terms of the t h r e e - c o r n e r e d arrangement shown in F i g u r e 1, in » which the h i t h e r t o s e p a r a t e and i n s u l a t e d f i e l d s of o r g a n i c polymers, c e r a m i c s , and m e t a l s form the c o r n e r s t o n e s of t r a d i t i o n a l knowledge and p r a c t i c e f o r m a t e r i a l s r e s e a r c h . A c r u c i a l p o i n t to be emphasized is t h a t f u t u r e developments in the m a t e r i a l s f i e l d w i l l almost c e r t a i n l y occur on the c o n n e c t i n g l i n e s t h a t j o i n these d i s c i p l i n e s or in the c e n t r a l a r e a t h a t l i e s between a l l t h r e e . T h i s is one of the main purposes of i n o r g a n i c polymer r e s e a r c h — t h e s e a r c h f o r new and u s e f u l compounds and m a t e r i a l s t h a t combine the p r o p e r t i e s of polymers w i t h those of c e r a m i c s and/or m e t a l s . Such h y b r i d m o l e c u l e s and s u p r a m o l e c u l a r s o l i d s o f f e r the promise of systems w i t h the f l e x i b i l i t y , s t r e n g t h , toughness, and ease of f a b r i c a t i o n of p o l y m e r s , w i t h the h i g h temperature o x i d a t i v e s t a b i l i t y of c e r a m i c s , and the e l e c t r i c a l or c a t a l y t i c p r o p e r t i e s of metals. Polyphosphazen is p o s s i b l e in one r e p r e s e n t a t i v Macromolecules d i f f e r from s m a l l m o l e c u l e s in a number of c r i t i c a l p r o p e r t i e s . F i r s t , the l i n e a r c h a i n s t r u c t u r e c o n f e r s e l a s t i c i t y , toughness, and s t r e n g t h on the s o l i d s t a t e system. This is a consequence of the r e o r i e n t a t i o n a l freedom of the s k e l e t a l bonds and of t h e i r a b i l i t y to absorb impact or undergo e l a s t i c d e f o r m a t i o n by means of c o n f o r m a t i o n a l changes r a t h e r than bond c l e a v a g e . However, the n a t u r e of the s k e l e t a l bonds and the elements i n v o l v e d can have a p o w e r f u l i n f l u e n c e on the t o r s i o n a l b a r r i e r f o r i n d i v i d u a l s k e l e t a l bonds. Second, l i n e a r c h a i n polymers are t h e r m o d y n a m i c a l l y u n s t a b l e at elevated temperatures. E n t r o p i e i n f l u e n c e s f a v o r a breakdown to s m a l l m o l e c u l e s e i t h e r by random f r a g m e n t a t i o n or by depolymerization. The l a t t e r p r o c e s s i n v o l v e s a r e v e r s i o n of the polymer to monomer or s m a l l m o l e c u l e r i n g s . Depolymerization to s m a l l r i n g s is a f e a t u r e common to many i n o r g a n i c polymers a t temperatures above 200-250°C. T h i r d , the i n t r o d u c t i o n of c r o s s l i n k s between c h a i n s c o n f e r s i n s o l u b i l i t y and i n c r e a s e d s o l i d s t a t e r i g i d i t y , o f t e n accompanied by improved t h e r m a l s t a b i l i t y . H i g h degrees of c r o s s l i n k i n g c o n f e r c e r a m i c - t y p e p r o p e r t i e s on the s o l i d , whether the backbone atoms are carbon atoms or i n o r g a n i c s p e c i e s . And f i n a l l y , i r r e s p e c t i v e of the types of elements in the backbone, the p r o p e r t i e s of a l i n e a r polymer w i l l depend on the s i d e groups a t t a c h e d to t h a t backbone. T h i s p r i n c i p l e u n d e r l i e s a l l p o l y o l e f i n and p o l y v i n y l m a c r o m o l e c u l a r s c i e n c e and t e c h n o l o g y . It a p p l i e s e q u a l l y w e l l to i n o r g a n i c polymer systems. An i m p o r t a n t d e v e l o p i n g a r e a t h a t l i e s in the r e g i o n between polymer c h e m i s t r y , c e r a m i c s c i e n c e , and m e t a l s , i n v o l v e s the s e a r c h f o r new e l e c t r i c a l l y - c o n d u c t i n g s o l i d s . L i n e a r polymers may conduct e l e c t r i c i t y by e l e c t r o n i c or i o n i c mechanisms. As w i l l be d i s c u s s e d , polyphosphazenes have been s y n t h e s i z e d t h a t , depending on the s i d e group s t r u c t u r e , conduct by e i t h e r of t h e s e two processes. The h i s t o r i c a l development of polyphosphazene c h e m i s t r y is compared in F i g u r e 2 w i t h those of o t h e r i n o r g a n i c polymer systems. I t s o r i g i n s can be t r a c e d to the l a t e 1 8 0 0 s , (I) a l t h o u g h the f i r s t f

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253

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1900

1910

1920

1930

1940

1950

1960

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1990

F i g u r e 2. Approximate sequence o f development o f d i f f e r e n t a r e a s o f i n o r g a n i c polymer c h e m i s t r y from 1900 t o t h e p r e s e n t . The broken l i n e s r e p r e s e n t a c o n t i n u i n g b u t low l e v e l o f activity. V e r t i c a l b a r s i n d i c a t e a b r u p t b e g i n n i n g s and ends o f transient research e f f o r t s .

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s t a b l e p o l y ( o r g a n o p h o s p h a z e n e s ) were not mid-1960 s.

synthesized u n t i l

the

f

Synthesis

of Polyphosphazenes

Three approaches have been developed f o r the s y n t h e s i s of p o l y phosphazenes. These a r e : (1) The macromolecular s u b s t i t u t i o n r o u t e ; (2) The c y c l i c t r i m e r or t e t r a m e r s u b s t i t u t i o n / p o l y m e r i z a t i o n r o u t e , and (3) D i r e c t s y n t h e s i s from o r g a n o s i l y l p h o s p h a z e n e monomers. T h i s l a s t method is d e s c r i b e d in d e t a i l in another Chapter and w i l l not be c o n s i d e r e d f u r t h e r in t h i s review. M a c r o m o l e c u l a r S u b s t i t u t i o n Route. The c u r r e n t surge in p o l y phosphazene r e s e a r c h is m a i n l y a r e s u l t of the development in the mid I 9 6 0 s (2-4) of a s u b s t i t u t i v e r o u t e to the s y n t h e s i s of organo phosphazene h i g h polymers. B e f o r e t h a t time, o n l y a s p o r a d i c i n t e r e s t in the s u b j e c t e x i s t e d because the known polymers cross linked poly(dihalophosphazenes) h y d r o l y t i c a l l y unstable The s u b s t i t u t i v e method of s y n t h e s i s is i l l u s t r a t e d in Scheme I . Thermal p o l y m e r i z a t i o n of h e x a c h l o r o c y c l o t r i p h o s p h a z e n e ( I ) y i e l d s an u n c r o s s l i n k e d , o r g a n i c s o l v e n t - s o l u b l e h i g h polymer ( I I ) ( 2 - 4 ) . This r e a c t i o n must be c o n t r o l l e d c a r e f u l l y to a v o i d the f o r m a t i o n of the c r o s s l i n k e d , i n s o l u b l e polymer known as " i n o r g a n i c r u b b e r " ( 1 ) . The c y c l i c t r i m e r ( I ) , o b t a i n e d from phosphorus p e n t a c h l o r i d e and ammonium c h l o r i d e , is a v a i l a b l e c o m m e r c i a l l y and is now used on a l a r g e s c a l e in the p o l y m e r i z a t i o n r e a c t i o n . U n c r o s s l i n k e d poly(dichlorophosphazene) ( I I ) is a h i g h l y r e a c t i v e m a c r o m o l e c u l a r intermediate. The c h l o r i n e atoms can be r e p l a c e d by a wide v a r i e t y of o r g a n i c or o r g a n o m e t a l l i c s i d e g r o u p s . Replacement of the c h l o r i n e atoms c o n v e r t s the h y d r o l y t i c a l l y - s e n s i t i v e i n t e r m e d i a t e t o one of many w a t e r - s t a b l e p o l y m e r i c d e r i v a t i v e s , u s u a l l y w i t h o u t c l e a v a g e of the backbone bonds. T h i s r e a c t i o n is the b a s i s of n e a r l y a l l the polyphosphazene c h e m i s t r y and t e c h n o l o g y developed d u r i n g the l a s t 20 y e a r s . The i m p o r t a n t consequence of t h i s r e a c t i o n pathway is t h a t m o l e c u l a r d i v e r s i t y can be a c h i e v e d by m a c r o m o l e c u l a r s u b s t i t u t i o n r a t h e r than by the more c o n v e n t i o n a l method of p o l y m e r i z a t i o n or c o p o l y m e r i z a t i o n of d i f f e r e n t monomers. I t a l l o w s polymers w i t h d i f f e r e n t s i d e groups or c o m b i n a t i o n s of s i d e groups to be p r e p a r e d w i t h an i d e n t i c a l backbone and m o l e c u l a r weight d i s t r i b u t i o n . Thus, d i r e c t comparisons can be made of the e f f e c t s of d i f f e r e n t s i d e groups on p h y s i c a l properties. I t a l s o a l l o w s g r o s s or s u b t l e p r o p e r t y changes to be d e s i g n e d i n t o a macromolecule, a f e a t u r e t h a t a s s i s t s both l a b o r a t o r y s t u d i e s and t e c h n o l o g i c a l i n n o v a t i o n s . Because of these advantages, the s u b s t i t u t i o n r o u t e has been a major reason f o r both the r e c e n t e x p a n s i o n of r e s e a r c h in polyphosphazenes and the use of these polymers in advanced t e c h n o l o g y . Four examples of the e f f e c t s of changes in s i d e group s t r u c t u r e a r e i l l u s t r a t e d in the polymers d e p i c t e d as I I and V I - V I I I . This method of s y n t h e s i s can be used as a p r e l u d e to the g e n e r a t i o n of a w i d e r s t r u c t u r a l v a r i e t y i f o r g a n i c r e a c t i o n c h e m i s t r y is c a r r i e d out on the o r g a n i c s i d e g r o u p s . F o r example, the glucosylphosphazene polymer shown as X has been p r e p a r e d by the c h e m i s t r y shown in Scheme II. 1

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

19. ALLCOCK

Polyphosphazene Chemistry

Scheme 1.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

255

256

INORGANIC AND ORGANOMETALLIC POLYMERS

0CH CFH

CI

o

I

f

P -

Ν = Ρ -

I

I

OCH CF

Cl

2

3

VI

II

(Microcrystalline, filmand fiber-forming polymer: stable to water, hydro­ phobic)

(Elastomer: sensitive to water)

NHCH3'

OCH CF 2

3

I Ν = Ρ -

Ν = Ρ -

I

I

0CH (CF2)CF H 2

NHCH3

2

VIII

VII

(Water-soluble and waterstable)

(Elastomer: stable to water, hydrophobic)

RONa

CH 0H t

1

CI

ONo

°-T"

CF,C00H

-N-P—

-

II

NoCI

4îs -H-I OR

IX

Scheme 2.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

19.

ALLCOCK

Polyphosphazene Chemistry

257

D e p r o t e c t i o n of X, and subsequent o x i d a t i o n , r e d u c t i o n , and a c e t y l a t i o n r e a c t i o n s can, w i t h c a r e , be c a r r i e d out w i t h o u t d e c o m p o s i t i o n o f the i n o r g a n i c backbone. R e a c t i o n s o f t h i s type a r e of p a r t i c u l a r i n t e r e s t f o r t h e s y n t h e s i s o f b i o a c t i v e o r biocompatible polyphosphazenes. I t s h o u l d be noted t h a t most of t h e s u b s t i t u t i o n - b a s e d s y n t h e s i s work w i t h p o l y ( o r g a n o p h o s p h a z e n e s ) has been preceded by e x p l o r a t o r y s t u d i e s a t the s m a l l m o l e c u l e , model compound l e v e l , o f t e n w i t h t h e use of c y c l i c t r i m e r I as a model f o r polymer I I ( 6 ) . C y c l i c T r i m e r S u b s t i t u t i o n / P o l y m e r i z a t i o n Route. I t is o f t e n e a s i e r t o r e p l a c e c h l o r i n e atoms in phosphazenes by o r g a n i c s i d e groups a t the c y c l i c t r i m e r o r t e t r a m e r l e v e l than a t the h i g h polymer l e v e l . T h i s is p a r t i c u l a r l y t r u e when o r g a n o m e t a l l i c r e a g e n t s a r e employed as h a l o g e n replacement n u c l e o p h i l e s . Thus, an a l t e r n a t i v e r o u t e t o p o l y ( o r g a n o p h o s p h a z e n e s ) i n v o l v e s the s y n t h e s i s o f o r g a n o - s u b s t i t u t e d c y c l i c t r i m e r i c o r t e t r a m e r i c phosphazenes f o l l o w e d by p o l y m e r i z a t i o n o f thes So f a r , i t has prove t h a t bear o r g a n i c s i d e groups o n l y , but some s p e c i e s t h a t bear b o t h halogeno and o r g a n i c s i d e groups can be induced t o p o l y m e r i z e . S e v e r a l c y c l i c t r i m e r s t h a t f a l l i n t o t h i s c a t e g o r y a r e shown in e q u a t i o n s 1-3. The r i n g s t r a i n i n h e r e n t in s p e c i e s XV i n c r e a s e s i t s propensity for polymerization. A s u b s t a n t i a l number o f organoh a l o g e n o c y c l o p h o s p h a z e n e s have now been p o l y m e r i z e d ( 7 - 1 7 ) . After p o l y m e r i z a t i o n , t h e h a l o g e n atoms can be r e p l a c e d by treatment w i t h a l k o x i d e s , a r y l o x i d e s , p r i m a r y o r s e c o n d a r y amines, o r o r g a n o m e t a l l i c reagents. Different

Classes

of P o l y ( o r g a n o p h o s p h a z e n e s )

W i t h t h i s s y n t h e t i c and m o l e c u l a r s t r u c t u r a l d i v e r s i t y , polyphosphazene c h e m i s t r y has d e v e l o p e d i n t o a f i e l d t h a t r i v a l s many a r e a s o f o r g a n i c polymer c h e m i s t r y w i t h r e s p e c t t o t h e t a i l o r e d s y n t h e s i s o f polymers f o r s p e c i f i c e x p e r i m e n t a l o r t e c h n o l o g i c a l uses. Indeed, h y b r i d systems a r e a l s o a v a i l a b l e in which o r g a n i c polymers bear phosphazene u n i t s as s i d e g r o u p s . T h i s is d i s c u s s e d in another Chapter. An u n d e r s t a n d i n g o f modern polyphosphazene c h e m i s t r y can be o b t a i n e d by c o n s i d e r i n g t h e f o l l o w i n g a s p e c t s o f the f i e l d . ( 1 ) Phosphazene polymers w i t h s i d e group systems t h a t a l l o w the h i g h i n h e r e n t f l e x i b i l i t y of t h e backbone t o become m a n i f e s t . Such polymers a r e r u b b e r y e l a s t o m e r s (18-20) s e v e r a l o f which have been d e v e l o p e d in government and i n d u s t r y as h i g h t e c h n o l o g y m a t e r i a l s . Other examples a r e o f i n t e r e s t as s o l i d i o n i c c o n d u c t i o n media ( 2 1 ) . (2) Polyphosphazenes w i t h b i o c o m p a t i b l e o r b i o l o g i c a l l y a c t i v e g r o u p s — i n c l u d i n g s e v e r a l examples t h a t a r e w a t e r - s o l u b l e , b i o d e g r a d a b l e , o r a r e b i o a c t i v e as s o l i d s . (3) Membrane m a t e r i a l s . ( 4 ) Polyphosphazenes w i t h t r a n s i t i o n m e t a l s in the s i d e g r o u p s , s e v e r a l of which have i n t e r e s t i n g c a t a l y t i c o r e l e c t r o a c t i v e p r o p e r t i e s . ( 5 ) Polymers w i t h r i g i d , s t a c k a b l e s i d e groups t h a t can impart e i t h e r s e m i c o n d u c t i v i t y or l i q u i d c r y s t a l l i n e c h a r a c t e r . The e l a s t o m e r c h e m i s t r y and t e c h n o l o g y a r e d i s c u s s e d in a n o t h e r C h a p t e r . A s p e c t s 2-5 a r e i l l u s t r a t e d by the examples g i v e n in t h e following sections.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Scheme 3.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

19.

ALLCOCK

Biocompatible

Polyphosphazene Chemistry

259

o r B i o a c t i v e Polyphosphazenes

T h i s s u b j e c t can be c o n s i d e r e d in terms o f f i v e d i f f e r e n t types o f molecules or m a t e r i a l s : (a) b i o l o g i c a l l y i n e r t , w a t e r - i n s o l u b l e polymers; (b) w a t e r - i n s o l u b l e polymers t h a t bear b i o l o g i c a l l y a c t i v e s u r f a c e groups; ( c ) w a t e r - s w e l l a b l e p o l y m e r i c g e l s , o r a m p h i p h i l i c polymers t h a t f u n c t i o n as membranes; (d) w a t e r - i n s o l u b l e but b i o e r o d a b l e polymers t h a t erode in aqueous media w i t h c o n c u r r e n t r e l e a s e o f a l i n k e d o r e n t r a p p e d b i o a c t i v e m o l e c u l e ; and ( e ) w a t e r - s o l u b l e polymers t h a t bear b i o a c t i v e agents as s i d e g r o u p s . (a) B i o l o g i c a l l y - I n e r t , W a t e r - I n s o l u b l e Polymers. Polymers o f t h i s type a r e o f b i o m e d i c a l i n t e r e s t as m a t e r i a l s f o r the c o n s t r u c t i o n o f a r t i f i c i a l h e a r t v a l v e s , b l o o d v e s s e l s , s o f t t i s s u e p r o s t h e s e s , or as c o a t i n g s f o r d e v i c e s such as pacemakers. A number of d i f f e r e n t polyphosphazenes a r e b e i n g c o n s i d e r e d f o r such u s e s , but s p e c i a l a t t e n t i o n has been f o c u s e d on two c l a s s e s — t h o s e t h a t bear f l u o r o a l k o x y s i d e groups Both types a r e h y d r o p h o b i group arrangements, can e x i s t as e l a s t o m e r s o r as m i c r o c r y s t a l l i n e f i b e r - o r f i l m - f o r m i n g m a t e r i a l s . P r e l i m i n a r y s t u d i e s have suggested t h a t these two c l a s s e s of polyphosphazenes a r e i n e r t and b i o c o m p a t i b l e in subcutaneous t i s s u e i m p l a n t a t i o n e x p e r i m e n t s . (b) W a t e r - I n s o l u b l e Polymers t h a t Bear B i o l o g i c a l l y - A c t i v e S u r f a c e Groups. Three examples w i l l be g i v e n o f polyphosphazenes t h a t have these c h a r a c t e r i s t i c s . F i r s t , t h e a n t i c o a g u l e n t h e p a r i n has been attached to the surface of a poly(aryloxyphosphazene) v i a a q u a t e r n i z e d a r y l o x y s i d e group system ( 2 3 ) . Second, dopamine has been l i n k e d c o v a l e n t l y t o a p o l y ( a r y l o x y phosphazene) v i a a d i a z o - c o u p l i n g t e c h n i q u e ( 2 4 ) . Experiments showed t h a t r a t p i t u i t a r y c e l l s in c u l t u r e responded t o t h e s u r f a c e - b o u n d dopamine in a s i m i l a r manner t o t h a t found when t h e dopamine was f r e e in s o l u t i o n . T h i r d , a poly[bis(phenoxy)phosphazene] has been c o a t e d on porous alumina p a r t i c l e s , s u r f a c e n i t r a t e d , reduced t o t h e a m i n o - d e r i v a t i v e , and then c o u p l e d t o the enzyme g l u c o s e - 6 - p h o s p h a t e dehydrogenase o r t r y p s i n by means o f g l u t a r i c d i a l d e h y d e . The i m m o b i l i z e d enzymes were more s t a b l e than t h e i r c o u n t e r p a r t s in s o l u t i o n , and they c o u l d be used in c o n t i n u o u s f l o w enzyme r e a c t o r equipment ( 2 5 ) . ( c ) W a t e r - S w e l l a b l e P o l y m e r i c G e l s o r A m p h i p h i l i c Polymers t h a t F u n c t i o n as Membranes. Two polyphosphazene systems have been s t u d i e d in d e t a i l ( 2 6 ) — m i x e d s u b s t i t u e n t polymers t h a t bear both methylamino and t r i f l u o r o e t h o x y groups ( X V I ) , w i t h the polymer c h a i n s l i g h t l y c r o s s l i n k e d v i a t h e methylamino u n i t s by g a m m a - i r r a d i a t i o n , and p o l y [ b i s ( m e t h o x y e t h o x y e t h o x y ) p h o s p h a z e n e (XVII), again l i g h t l y c r o s s l i n k e d by gamma r a y s . The l a t t e r polymer is w a t e r - s o l u b l e in the u n c r o s s l i n k e d s t a t e ( 2 7 ) . I t a l s o f u n c t i o n s as a s o l i d e l e c t r o l y t e medium f o r s a l t s such as l i t h i u m t r i f l a t e ( 2 1 ) . (d) W a t e r - I n s o l u b l e but B i o e r o d a b l e Polymers t h a t Erode in Aqueous Media w i t h C o n c u r r e n t R e l e a s e o f a L i n k e d o r Entrapped B i o a c t i v e Molecule. The f i r s t examples o f polyphosphazenes t h a t f u n c t i o n in t h i s way were s p e c i e s t h a t c o n t a i n e d e t h y l g l y c i n a t e o r o t h e r amino

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

260

INORGANIC AND ORGANOMETALLIC POLYMERS

Cl

C H5

s

6

Ρ

S

\

Ν

C6H

5

Ν

II ci

cij

Ρ Cl'

^

(D

p' "α

/ Ν

XI

CH

XII

CH Si(CH )

X

2

3

3

p '

^

\

Ν

Ν

hea

N

Civ Cl'

\

/

CI

(2) CH

Cl

3

Ν

XIII

XIV

heat , •N = P - N = P - N = P P F

'

P

% /

(3)

N

F

Ν XV

XVI

NHCH

OCH CH OCH CH OCH

3

Ν - Ρ I OCH CF 2

XVI

2

2

2

2

3

Ν - Ρ -

I

3

OCH CH OCH CH OCH 2

2

2

2

3

XVII

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

19.

ALLCOCK

Polyphosphazene Chemistry

261

a c i d e s t e r s i d e groups ( 2 8 ) . H y d r o l y s i s r e s u l t s in a breakdown o f the polymer t o amino a c i d , e t h a n o l , and phosphate, a l l o f which can be m e t a b o l i z e d , and ammonia which can be e x c r e t e d . Subsequent a p p l i c a t i o n o f t h i s c h e m i s t r y has c o n f i r m e d the b i o c o m p a t i b i l i t y of the system and has l e d t o t h e development of a c o n t r o l l e d drug r e l e a s e system ( 2 9 ) . A d i f f e r e n t approach r e p o r t e d r e c e n t l y (30) i n v o l v e s t h e s y n t h e s i s and e v a l u a t i o n o f polyphosphazenes t h a t c o n t a i n b o t h a r y l o x y and i m i d a z o l y l s i d e g r o u p s . H y d r o l y t i c removal o f t h e i m i d a z o l y l u n i t s takes p l a c e w i t h a c o n c u r r e n t e r o s i o n o f the s o l i d polymer in a manner t h a t is a p p r o p r i a t e f o r the c o n t r o l l e d r e l e a s e o f e n t r a p p e d drug m o l e c u l e s . ( e ) W a t e r - S o l u b l e Polymers t h a t Bear B i o a c t i v e S i d e Groups. P o l y phosphazenes t h a t bear methylamino ( 3 1 ) , g l u c o s y l , o r a l k y l e t h e r (27) s i d e groups a r e s o l u b l e in water. P o l y [ b i s ( m e t h y l a m i n o ) phosphazene] was used in e a r l y work as a c o o r d i n a t i o n c a r r i e r f o r p l a t i n u m a n t i t u m o r drug l i n k a g e o f a wide range a n e s t h e t i c s ( 3 3 ) , s t e r o i d s ( 3 4 ) , a n t i b a c t e r i a l agents ( 3 5 ) , o r p r o t e i n s t o polyphosphazenes which, in c o n j u n c t i o n w i t h the w a t e r - s o l u b i l i z i n g groups, o f f e r s a promise f o r a v a r i e t y o f pharmacological a p p l i c a t i o n s . Polyphosphazenes

w i t h T r a n s i t i o n M e t a l s in the S i d e Groups

T h i s a s p e c t o f polyphosphazene c h e m i s t r y has r e c e n t l y been reviewed elsewhere (36) and o n l y a b r i e f account w i l l be g i v e n h e r e . M e t a l l o p h o s p h a z e n e s a r e a new type o f macromolecule d e s i g n e d t o b r i d g e t h e gap between polymers and m e t a l s . A l t h o u g h s t i l l a t an e x p l o r a t o r y s t a g e of l a b o r a t o r y development, they may p r o v i d e a c c e s s t o e l e c t r o n i c a l l y - c o n d u c t i n g polymers, m a g n e t i c a l l y - a c t i v e polymers, m a c r o m o l e c u l a r c a t a l y s t s , e l e c t r o d e m e d i a t o r systems, o r polymers c r o s s l i n k e d by m e t a l atoms. T r a n s i t i o n m e t a l s have been l i n k e d t o c y c l o - and p o l y phosphazenes by f o u r d i f f e r e n t methods. F i r s t , and most o b v i o u s , t h e l i n k a g e makes use o f the c o o r d i n a t i n g power of t h e backbone n i t r o g e n atoms. The p l a t i n u m d i c h l o r i d e adduct ( r e f e r r e d t o e a r l i e r ) f a l l s into this category. Second, as a l o g i c a l development o f t h e f i r s t approach, p o l y phosphazenes have been s y n t h e s i z e d t h a t bear phosphine u n i t s connected t o a r y l o x y s i d e groups ( 3 7 ) . The phosphine u n i t s b i n d o r g a n o m e t a l l i c compounds, such as those o f i r o n , c o b a l t , osmium, o r ruthenium (38). In s e v e r a l cases, the c a t a l y t i c a c t i v i t y of the m e t a l is r e t a i n e d in the m a c r o m o l e c u l a r system ( 3 9 ) . A s i m i l a r b i n d i n g o f t r a n s i t i o n m e t a l s has been a c c o m p l i s h e d t h r o u g h n i d o c a r b o r a n y l u n i t s l i n k e d t o a polyphosphazene c h a i n ( 4 0 ) . T h i r d , m e t a l l o c e n e u n i t s , such as f e r r o c e n e o r r u t h e n o c e n e , have been l i n k e d t o phosphazene c y c l i c t r i m e r s o r t e t r a m e r s and t h e s e were p o l y m e r i z e d and s u b s t i t u t e d t o g i v e polymers o f t h e type mentioned p r e v i o u s l y ( 4 1 ) . Polyphosphazenes w i t h f e r r o c e n y l groups c a n be doped w i t h i o d i n e t o form weak s e m i c o n d u c t o r s . Polymer c h a i n s t h a t bear both r u t h e n o c e n y l and f e r r o c e n y l s i d e groups a r e p r o s p e c t i v e e l e c t r o d e m e d i a t o r systems. F i n a l l y , a t t h e c y c l i c t r i m e r and t e t r a m e r l e v e l s , compounds o f

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types XVIII and XIX have been s y n t h e s i z e d , and m e t a l - s u b s t i t u t e d high polymers have a l s o been p r e p a r e d . The development of o r g a n o m e t a l l i c r e a c t i o n c h e m i s t r y a t the s m a l l m o l e c u l e l e v e l , as e x e m p l i f i e d by compounds X V I I I and XIX is an i l l u s t r a t i o n of the key r o l e p l a y e d by s m a l l m o l e c u l e model compound work in the development of h i g h p o l y m e r i c phosphazene c h e m i s t r y . Polyphosphazenes w i t h R i g i d , S t a c k a b l e

Side

Groups

The c o n n e c t i o n between polymer c h e m i s t r y and c e r a m i c s c i e n c e is found in the ways in which l i n e a r macromolecules can be c o n v e r t e d i n t o g i a n t " u l t r a s t r u c t u r e " systems, in which t h e whole s o l i d m a t e r i a l comprises one g i a n t m o l e c u l e . T h i s t r a n s f o r m a t i o n c a n be a c c o m p l i s h e d in two w a y s — f i r s t by the f o r m a t i o n of c o v a l e n t , i o n i c , or c o o r d i n a t e c r o s s l i n k s between polymer c h a i n s , and second, by t h e i n t r o d u c t i o n of c r y s t a l l i n e o r d e r . I n t h e second approach, s t r o n g van d e r Waals f o r c e s w i t h i n t h e c r y s t a l l i n e domains c o n f e r r i g i d i t y and s t r e n g t h n o t u n l i k e present. A d i f f e r e n c e between m i c r o c r y s t a l l i t e - b a s e d u l t r a s t r u c t u r e and c o v a l e n t l y - c r o s s l i n k e d systems is t h a t m i c r o c r y s t a l l i t e s melt at s p e c i f i c t e m p e r a t u r e s , a l l o w i n g the polymer t o be f a b r i c a t e d by h e a t i n g a t modest t e m p e r a t u r e s . Subsequent c o o l i n g of the system below the c r y s t a l l i z a t i o n temperature a l l o w s the p h y s i c a l p r o p e r t y advantages of the s o l i d s t a t e t o become m a n i f e s t . Liquid c r y s t a l l i n i t y is a l s o p o s s i b l e i f some o r d e r is r e t a i n e d in t h e molten s t a t e . C r y s t a l l i n e o r d e r n o t o n l y adds m e c h a n i c a l s t r e n g t h , i t a l s o p r o v i d e s o p p o r t u n i t i e s f o r t h e appearance of o t h e r p r o p e r t i e s t h a t depend on s o l i d s t a t e o r d e r — s u c h as e l e c t r o n i c c o n d u c t i v i t y . F o r these r e a s o n s , s e v e r a l e x p l o r a t o r y s t u d i e s have been made o f t h e f e a s i b i l i t y of attachment of c r y s t a l l i z a b l e s i d e groups t o p o l y phosphazenes. Three examples a r e g i v e n below. TCNQ-Polyphosphazene Systems. T e t r a c y a n o q u i n o d i m e t h a n e (XX) s a l t s c r y s t a l l i z e in t h e form of s t a c k e d a r r a y s t h a t a l l o w e l e c t r i c a l semiconductivity (42). A l t h o u g h t h i s phenomenon has been s t u d i e d in many l a b o r a t o r i e s , i t has not been p o s s i b l e t o f a b r i c a t e c o n d u c t i v e f i l m s o r w i r e s from these s u b s t a n c e s because of the b r i t t l e n e s s t h a t is c h a r a c t e r i s t i c o f o r g a n i c s i n g l e c r y s t a l s . However, i t seemed p o s s i b l e t h a t , i f the f l e x i b i l i t y and ease of f a b r i c a t i o n o f many polyphosphazenes c o u l d be combined w i t h t h e e l e c t r i c a l p r o p e r t i e s o f TCNQ, c o n d u c t i n g polymers might be a c c e s s i b l e . In r e c e n t s t u d i e s , i t has been found t h a t TCNQ u n i t s can be a t t a c h e d t o q u a t e r n i z e d polyphosphazene m o l e c u l e s , ( 4 3 ) . The q u a t e r n i z a t i o n s i t e s a r e e i t h e r backbone n i t r o g e n atoms or amino o r phosphino u n i t s on t h e s i d e c h a i n s . A l t h o u g h t h e l o a d i n g o f TCNQ was not h i g h , t h e p o l y m e r i c system p o s s e s s e d s u f f i c i e n t e l e c t r i c a l s e m i c o n d u c t i v i t y t o suggest t h a t the TCNQ u n i t s were f o r m i n g s t a c k e d arrays w i t h i n the s o l i d matrix. This suggested that other s i d e groups might behave in a s i m i l a r manner. Polyphosphazene-Phthalocyanine S t r u c t u r e s . Thus, a r e l a t e d s t u d y was c a r r i e d o u t w i t h copper p h t h a l o c y a n i n e u n i t s l i n k e d c o v a l e n t l y t o a poly(aryloxyphosphazene) (44). Non-polymeric copper p h t h a l o c y a n i n e forms o r d e r e d s t a c k e d s t r u c t u r e s in t h e c r y s t a l l i n e s t a t e . When

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

ALLCOCK

Polyphosphazene Chemistry

ο

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In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

264

INORGANIC AND ORGANOMETALLIC POLYMERS

O(CH CH 0) .^^-N

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XXI

•FIRST ATTEMPTS TO SUBSTITUTE CROSSLINKED ( N P C l ) 2

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-ISOLATIO CONVERSIO

•MIXED SUBSTITUENT ELASTOMERS

-AMINO ACID DERIVATIVES, [ N P ( N H C H ) ] -MIXED SUBSTITUENT ARYLAMINO AND ALKOXY DERIVATIVES -POLYMERIZATION OF ORGANO-HALO TRIMERS -POLYMER-BOUND DYES 3

2

n

-DIRECT SYNTHESIS FROM N-SILYL-PHOSPHORAMINES -CARBORANYL DERIVATIVES -BIOACTIVE AND BIOERODABLE POLYMERS (STEROIDS, DOPAMINE, PROCAINE, ETC.)

. PHOSPHINE-BOUND TRANSITION METAL CATALYSTS .VINYL POLYMERS WITH PHOSPHAZENE SIDE GROUPS

-K-

-METALLOCENE DERIVATIVES -RADIATION CROSSLINKING . TCNQ DERIVATIVES .ENZYME IMMOBILIZATION

• PHTHALOCYANINE DERIVATIVES •METAL CARBONYL (Fe, Ru) DERIVATIVES -SOLID ELECTROLYTES • ORGANOSILICON DERIVATIVES -LIQUID CRYSTALLINE POLYMERS

1

1990—

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3.

synthesis the

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approximate

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o f developments

o f p o l y ( o r g a n o p h o s p h a z e n e s ) from t h e e a r l y

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In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

1

s to

19.

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Polyphosphazene Chemistry

265

t r e a t e d w i t h i o d i n e o r o t h e r dopants, t h e s e a r e t r a n s f o r m e d i n t o e l e c t r i c a l semiconductors (45-46). I n t h e phosphazene p o l y m e r i c system, some p h t h a l o c y a n i n e group s t a c k i n g a l s o appears t o o c c u r , even though t h e p h t h a l o c y a n i n e c o n c e n t r a t i o n is low. Doping w i t h i o d i n e g e n e r a t e d weak e l e c t r i c a l s e m i c o n d u c t i v i t y . L i q u i d C r y s t a l l i n e Polymers. The t h i r d example is t h e r e c e n t s y n t h e s i s in our l a b o r a t o r y o f a l i q u i d c r y s t a l l i n e polyphosphazene (47). The s t r u c t u r e o f t h i s macromolecule is shown in XXI. T h i s p a l e y e l l o w polymer shows t y p i c a l m i c r o c r y s t a l l i n e p r o p e r t i e s a t temperatures below 118°C. H e a t i n g above 118°C l e a d s t o t h e f o r m a t i o n of a mesophase, in which some m o l e c u l a r o r d e r is r e t a i n e d . The mesophase m e l t s above 127°C t o g i v e an i s o t r o p i c m e l t . T h i s is a s t r i k i n g i l l u s t r a t i o n o f the way in which a h i g h l y f l e x i b l e polymer c h a i n , c o u p l e d t o r i g i d u n i t s v i a f l e x i b l e s p a c e r groups, a l l o w s l i q u i d c r y s t a l l i n e s t a c k i n g of the aromatic azide u n i t s to occur. I t appears from r e c e n t p a r a l l e l work t h a t t h e same phenomenon e x i s t s even when a non-mesogeni H i s t o r i c a l Perspective The development o f s y n t h e t i c r o u t e s t o new polyphosphazene s t r u c t u r e s began in the mid I960's ( 2 - 4 ) . The i n i t i a l e x p l o r a t o r y development of t h i s f i e l d has now been f o l l o w e d by a r a p i d e x p a n s i o n o f s y n t h e s i s r e s e a r c h , c h a r a c t e r i z a t i o n , and a p p l i c a t i o n s - o r i e n t e d work. The i n f o r m a t i o n shown in F i g u r e 3 i l l u s t r a t e s the sequence o f development of s y n t h e t i c pathways t o polyphosphazenes. I t seems c l e a r t h a t t h i s f i e l d has grown i n t o a major a r e a o f polymer c h e m i s t r y and t h a t polyphosphazenes, as w e l l as o t h e r i n o r g a n i c macromolecules, w i l l be used i n c r e a s i n g l y in p r a c t i c a l a p p l i c a t i o n s where t h e i r unique p r o p e r t i e s a l l o w t h e s o l u t i o n o f d i f f i c u l t e n g i n e e r i n g and b i o m e d i c a l problems. Acknowledgment s Our work has been s u p p o r t e d m a i n l y by t h e Army R e s e a r c h O f f i c e , A i r F o r c e O f f i c e o f S c i e n t i f i c Research, O f f i c e o f N a v a l R e s e a r c h , and the P u b l i c H e a l t h S e r v i c e t h r o u g h t h e N a t i o n a l H e a r t , Lung, and Blood Institute. Literature 1. 2. 3. 4. 5. 6. 7. 8.

Cited

S t o k e s , H. N. Am. Chem. J . 1879, 19> 782. A l l c o c k , H. R.; K u g e l , R. L. I n o r g . Chem. 1966, 5^» 1016. A l l c o c k , H. R.; K u g e l , R. L.; V a l a n , K. J . I n o r g . Chem. 1966, 5, 1709. A l l c o c k , H. R.; K u g e l , R. L. I n o r g . Chem. 1966, 1716. S e e l , F.; Langer, J . Angew Chem. 1956, 6 £ , 461; Z. Anorg. A l l g . Chem. 1958, 295, 316. A l l c o c k , H. R. A c c t s . Chem. Res. 1979, 12, 351. A l l c o c k , H. R.; Moore, G. Y. Macromolecules 1975, 377. A l l c o c k , H. R.; P a t t e r s o n , D. B. I n o r g . Chem. 1977, JJ6, 197.

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266 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.

INORGANIC AND ORGANOMETALLIC POLYMERS Allcock, H. R.; Schmutz, J. L.; Kosydar, Κ. M. Macromolecules 1978, 11, 179. Ritchie, R. L.; Harris, P. J.; Allcock, H. R. Macromolecules 1979, 12, 1014. Scopelianos, A. G.; O'Brien, J. P.; Allcock, H. R. J. Chem. Soc., 1980, 198. Allcock, H. R. Polymer 1980, 21, 673. Allcock, H. R.; Ritchie, R. J.; Harris, P. J. Macromolecules 1980, 13, 1332. Allcock, H. R.; Connolly, M. S. Macromolecules 1985, 18, 1330. Allcock, H. R.; Lavin, K. D.; Riding, G. H. Macromolecules 1985, 18, 1340. Whittle, R. R.; Desorcie, J. L.; Allcock, H. R. Acta. Cryst. 1985, C41, 546. Allcock, H. R.; Brennan, D. J.; Graaskamp J. M. Macromolecules (submitted). Rose, S. H. J. Polym. Sci (B) 1968, 6, 837. Singler, R. E.; Hagnauer, 1982, 11. Tate, D. P. J. Polym. Sci., Polym. Symp. 1974, 48, 33. Blonsky, P. M.; Shriver, D. F.; Austin, P. E.; Allcock, H. R. Polym. Mater. Sci. Eng. 1985, 53, 118. Wade, C. W. R.; Gourlay, S.; Rice, R.; Hegyli, Α.; Singler, R. E.; White, J. in Organometallic Polymers, Carraher, C. E.; Sheats, J. E.; Pittman, C. U. Eds.; Academic: New York, 1978; 283-286. Neenan, T. X.; Allcock, H. R. Biomaterials 1982, 3, 2, 78. Allcock, H. R.; Hymer, W. C.; Austin, P. E. Macromolecules 1983, 16, 1401. Allcock, H. R.; Kwon, S. Macromolecules 1986, 19, 1502. Allcock, H. R.; Gebura, M.; Kwon, S. (unpublished work). Allcock, H. R.; Austin, P. E.; Neenan, T. X.; Sisko, J. T.; Blonsky, P. M.; Shriver, D. F. Macromolecules 1986, 19, 1508. Allcock, H. R.; Fuller, T. J.; Mack, D. P.; Matsumura, K.; Smeltz, Κ. M. Macromolecules 1977, 10, 824. Grolleman, C. W. J.; deVisser, A. C.; Wolke, J. G. C.; van der Goot, H.; Timmerman, H. J. Controlled Release 1986, 3, 143. Laurencin, C.; Koh, H. J.; Neenan, T. X.; Allcock, H. R.; Langer, R. S. J. Biomed. Mater. Res, (in press). Allcock, H. R.; Cook, W. J.; Mack. D. P. Inorg. Chem. 1972, 11, 2584. Allcock, H. R.; Allen, R. W.; O'Brien, J. P. J. Am. Chem. Soc. 1977, 99, 3984. Allcock, H. R.; Austin, P. E.; Neenan, T. X. Macromolecules 1982, 15, 689. Allcock, H. R.; Fuller, T. J. Macromolecules 1980, 13, 1338. Allcock, H. R.; Austin, P. E. Macromolecules 1981, 14, 1616. Allcock, H. R.; Desorcie, J. L.; Riding, G. H. Polyhedron, 1987, 6, 119. Allcock, H. R.; Fuller, T. J.; Evans, T. L. Macromolecules 1980, 13, 1325. Allcock, H. R.; Lavin, K. D.; Tollefson, Ν. M.; Evan, T. L. Organometallics 1983, 2, 267. Dubois, R. Α.; Garrou, P. E.; Lavin, K. D.; Allcock, H. R. Organometallics 1986, 5, 460.

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Allcock, H. R.; Scopelianos, A. G.; Whittle, R. R.; Tollefson, Ν. M. J. Am. Chem. Soc. 1983, 105, 1316. Allcock, H. R.; Riding, G. H.; Lavin, K. D. Macromolecules 1987, 20, 6. Melby, L. R.; Harder, P. J.; Hertler, W. R.; Mahler, W.; Benson, R.; Mochel, W. E. J. Am. Chem. Soc. 1962, 84, 3374. Allcock, H. R.; Levin, M. L.; Austin, P. E. Inorg. Chem. 1986, 25, 2281. Allcock, H. R.; Neenan, T. X. Macromolecules 1986, 19, 1495. Nohr, R. S.; Kuznesof, P. M.; Wynne, K. J.; Kenney, M. E.; Srebenmann, P. G. J. Am. Chem. Soc. 1981, 103, 4371. Marks, T. J. Science (Washington, D.C.) 1985, 227, 881. Kim, C.; Allcock, H. R. Macromolecules (in press); Polym. Prepr. 1987, 28(1), 446. Singler, R. E.; Willingham, R. Α.; Lenz, R. W.; Furukawa, A.; Finkelmann, H. Macromolecules (in press); Polym. Prepr. 1987, 28(1), 488.

RECEIVED September 30, 198

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 20

Phosphazene Polymers: Synthesis, Structure, and Properties Robert E. Singler, Michael S. Sennett, and Reginald A. Willingham Army Materials Technology Laboratory, Watertown, MA 02172-0001 An overview of th unique class of polymers with a phosphorus-nitrogen backbone i s presented, with a focus on poly(dichlorophosphazene) as a common intermediate for a wide variety of poly(organophosphazenes). Melt and solution polymerization techniques are i l l u s t r a t e d , including the role of c a t a l y s t s . The elucidation of chain structure and molecular weight by various d i l u t e solution techniques i s considered. Factors which determine the properties of polymers derived from poly(dichlorophosphazene) are discussed, with an emphasis on the role that the organic substituent can play i n determining the f i n a l properties.

The s t u d y of open-chain polyphosphazenes has a t t r a c t e d i n c r e a s i n g a t t e n t i o n i n r e c e n t y e a r s , b o t h from the s t a n d p o i n t o f fundamental r e s e a r c h and t e c h n o l o g i c a l development. The polyphosphazenes a r e l o n g c h a i n s o f a l t e r n a t i n g p h o s p h o r u s - n i t r o g e n atoms w i t h two s u b s t i t u e n t s a t t a c h e d t o phosphorus. These polymers have been the s u b j e c t o f s e v e r a l r e c e n t r e v i e w s ( 1 - 3 ) . I n t e r e s t has stemmed from the c o n t i n u i n g s e a r c h f o r polymers w i t h improved p r o p e r t i e s f o r e x i s t i n g a p p l i c a t i o n s as w e l l as f o r new polymers w i t h n o v e l properties. F i g u r e 1 p r o v i d e s an o v e r v i e w of the two s t e p s y n t h e s i s p r o c e s s , p i o n e e r e d by A l l c o c k (4) and i n use today by a number o f workers and l a b o r a t o r i e s : f o r m a t i o n of a s o l u b l e r e a c t i v e polymer i n t e r m e d i a t e ( I I ) from which i s d e r i v e d a l a r g e number o f polymers v i a substitution reactions. S i n c e the i n i t i a l d i s c l o s u r e by A l l c o c k , workers have sought t o answer v a r i o u s q u e s t i o n s : 1) What i s the n a t u r e of the p o l y m e r i z a t i o n p r o c e s s (mechanism)? 2) What i s the s t r u c t u r e of p o l y ( d i c h l o r o p h o s p h a z e n e ) t h a t d i s t i n g u i s h e s i t from the i n s o l u b l e " i n o r g a n i c r u b b e r " ( I I I ) ? 3) The s u b s t i t u t i o n p r o c e s s g i v e s a s e e m i n g l y e n d l e s s v a r i e t y o f p r o d u c t s . What a r e the l i m i t a t i o n s o r

This chapter is not subject to U.S. copyright. Published 1988, American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

20.

Phosphazent Polymers

SINGLERETAL.

269

CROSSLINKED MATRIX

HNRR'-Et3N OR / r

/

I ι

{N=P}

loAr Γ

X

'I ι

{N=P}

\ ?

X

NRR' I

η

±N=P}

X

OR

OAr

NRR'

IV

V

VI

F i g u r e 1. S y n t h e s i s o f p o l y ( d i c h l o r o p h o s p h a z e n e ) and poly(organophosphazenes).

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

270

INORGANIC AND ORGANOMETALLIC POLYMERS

c o n t r o l l i n g f a c t o r s i n the s u b s t i t u t i o n process? 4) How do the above f a c t o r s c o n t r o l the p r o p e r t i e s of the poly(organophosphazenes) ( e g . IV, V, V I ) ? 5) Are any o f these polymers t e c h n o l o g i c a l l y u s e f u l o r of commercial i n t e r e s t ? T h i s paper w i l l p r o v i d e an o v e r v i e w of the p o l y m e r i z a t i o n p r o c e s s e s and the p r o p e r t i e s of p o l y ( d i c h l o r o p h o s p h a z e n e ) . This paper w i l l a l s o d i s c u s s the v a r i o u s f a c t o r s which i n f l u e n c e the p r o p e r t i e s o f the poly(organophosphazenes) and show how these f a c t o r s have r e s u l t e d i n a c l a s s o f polymers w i t h a wide range o f p r o p e r t i e s , i n c l u d i n g s e v e r a l examples o f c u r r e n t commercial i m p o r t a n c e . Poly(dichlorophosphazene) The p o l y m e r i z a t i o n of h e x a c h l o r o c y c l o t r i p h o s p h a z e n e ( I ) has been the s u b j e c t o f numerous i n v e s t i g a t i o n s ( 5 ) . The r e a c t i o n ( I > I I , III) i s markedly i n f l u e n c e d by the presence of t r a c e i m p u r i t i e s . The c o n v e n t i o n a l r o u t e t o I I i s a m e l t p o l y m e r i z a t i o n a t 250 °C of h i g h l y purified trimer (NPC^)^ Proper s e l e c t i o n of r e a c t i o o b t a i n I I and a v o i d the f o r m a t i o n of I I I . For l a r g e s c a l e i n d u s t r i a l p r o c e s s e s , v a r i o u s a c i d s and o r g a n o m e t a l l i c compounds can be u t i l i z e d as c a t a l y s t s t o prepare s o l u b l e polymer, b o t h i n b u l k and i n s o l u t i o n ( 2 ) . The advantages of c a t a l y z e d p o l y m e r i z a t i o n s i n c l u d e lower r e a c t i o n t e m p e r a t u r e s , h i g h e r y i e l d s , and the use of c o n v e n t i o n a l l a r g e s c a l e equipment. S i z e e x c l u s i o n chromatography (GPC) and o t h e r d i l u t e s o l u t i o n t e c h n i q u e s have been a p p l i e d t o the c h a r a c t e r i z a t i o n of I I ( 6 , 7 ) . Polymers o b t a i n e d from the b u l k p o l y m e r i z a t i o n t y p i c a l l y have h i g h m o l e c u l a r w e i g h t s and broad m o l e c u l a r weight d i s t r i b u t i o n s (MWD's). C a t a l y z e d p r o c e s s e s g e n e r a l l y g i v e narrower MWD's but lower m o l e c u l a r weight polymer. A l t h o u g h q u e s t i o n s s t i l l remain as t o the n a t u r e of the p o l y m e r i z a t i o n mechanism ( 7 ) , i t i s g e n e r a l l y thought t o be a c a t i o n i c , c h a i n growth, r i n g opening p o l y m e r i z a t i o n p r o c e s s ( F i g u r e 2 ) . E v i d e n c e f o r t h i s i n c l u d e s the e f f e c t i v e n e s s of Lewis a c i d c a t a l y s t s , e s p e c i a l l y B C l ^ , f o r m a t i o n of h i g h m o l e c u l a r weight polymer e a r l y i n the p o l y m e r i z a t i o n , and d i l u t e s o l u t i o n parameters o b t a i n e d on I I which p o i n t t o randomly c o i l e d polymer c h a i n s r e l a t i v e l y f r e e of l o n g - c h a i n b r a n c h i n g f o r low t o moderate c o n v e r s i o n s t o h i g h polymer. One way t o overcome the m o l e c u l a r weight l i m i t a t i o n s i n a s o l u t i o n c a t a l y z e d process i s by t a k i n g advantage o f the " l i v i n g " n a t u r e of the p o l y m e r i z a t i o n ( 7 ) . For the B C l ^ c a t a l y z e d p o l y m e r i z a t i o n , one can add monomer ( t r i m e r ) t o the e x i s t i n g polymer t o i n c r e a s e the m o l e c u l a r weight i n a s t e p w i s e f a s h i o n ( F i g u r e 3 ) . T r i m e r i s p o l y m e r i z e d i n the presence of BC1~ i n a t r i c h l o r o b e n z e n e s o l u t i o n i n a s e a l e d ampoule a t 210 °C f o r 48 h o u r s . For the second and t h i r d s t a g e s , t r i m e r i s added i n s o l u t i o n e q u a l t o the amount i n stage 1. The B C l ^ c o n c e n t r a t i o n i s h e l d c o n s t a n t . Each stage i s c a r r i e d t o g r e a t e r than 95 % c o n v e r s i o n . L i g h t s c a t t e r i n g measurements on the polymer o b t a i n e d from s t a g e 3 show MW > 10 , thus c o n f i r m i n g t h a t h i g h m o l e c u l a r weight I I can be o b t a i n e d i n h i g h conversion i n a c a t a l y z e d s o l u t i o n process ( 8 ) .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Phosphazene Polymers

20. SINGLERETAL.

[NPCI l3 — 2

[NPCI J 2

n

BULK - UNCATALYZED HIGH PURITY TRIMER NECESSARY - OTCERWISE GEL FORMATION HIGH POLYMER (MW - 10 ) 6

AT LOW CONVERSION «30%), GEL FREE, 250°C, 40-100 hr BULK - CATALYZED TRIMER PURITY LESS CRITICAL LOWER TEMPERATURES (170°C - 220°C) WITH HIGHER CONVERSIONS 050%) OF GEL-FREE POLYMER AT SHORTCR TIMES LOWER MW POLYMER MO*) SOLUTION - CATALYZED SAME COMMENT INERT SOLVENT GENERAL MECHANISM CATIONIC - CHAIN GROWTH - RING OPENING

F i g u r e 2. G e n e r a l comments on t h e p o l y m e r i z a t i o n

BCI [NPCI l3

process.

3

2

" [NPCI ] 2

TCB, 210°C SEALED TUBE

n

STEPWISE PROCESS FIRST STAGE: 15 wt% TRIMER IN C6H3CI3 (3g/16g). BCI3-0.66g. 48 hr. 210°C. 95% CONVERSION. SOLUBLE POLYMER. SECOND STAGE: NEW TRIMER SOLUTION ADDED TO POLYMER. IBCI3J ~ CONSTANT. SAME t, T, % CONVERSION. THIRD STAGE: REPEAT STAGE

M «

1 2 3

13,000 100,000 322,000

n

M » w

37,000 118,000 536,000 ( M ~ 6 χ 10 )

*GPC MW DE7IRMI NATION. *LIGHT SCATTERING.

w

6

f

POLYSTYRENE STANDARDS.

SENNE1T (1986)

F i g u r e 3. S o l u t i o n p o l y m e r i z a t i o n w i t h BC1~.

Stepwise

process.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

272

INORGANIC AND ORGANOMETALLIC POLYMERS

Poly(organophosphazenes) The s y n t h e s i s of poly(organophosphazenes) r e p r e s e n t s p r o b a b l y the b e s t example of a c e n t r a l theme of i n o r g a n i c macromolecules: P r e p a r a t i o n of a r e a c t i v e p o l y m e r i c i n t e r m e d i a t e , p o l y ( d i c h l o r o p h o s phazene), and subsequent use i n a wide v a r i e t y of s i d e group replacement r e a c t i o n s ( F i g u r e 1 ) . T h i s concept has been demonstrated i n a number of l a b o r a t o r i e s (3) and has p r o v i d e d a wide v a r i e t y o f polymers w i t h d i f f e r e n t p r o p e r t i e s . T a b l e I s e r v e s t o i l l u s t r a t e how the n a t u r e and s i z e of the s u b s t i t u e n t a t t a c h e d t o the P-N backbone can i n f l u e n c e the p r o p e r t i e s of the p o l y ( o r g a n o p h o s p h a z e n e s ) . The g l a s s t r a n s i t i o n temperatures range from -84 °C f o r ( N P ( O C H C H > ) t o around 100 °C f o r the p o l y ( a n i l i n o p h o s p h a z e n e s ) . Polymers range from e l a s t o m e r s t o f l e x i b l e f i l m f o r m i n g t h e r m o p l a s t i c s or g l a s s e s a t room t e m p e r a t u r e . I n the case o f p o l y ( a l k o x y p h o s p h a z e n e s ) ( I V ) o r p o l y ( a r y l o x y p h o s phazenes) (V) a d r a m a t i c change i n p r o p e r t i e s can a r i s e by employing combinations of s u b s t i t u e n t s ( N P ( O C H ) ) are s e m i c r y s t a l l i n i n t r o d u c t i o n of two or more s u b s t i t u e n t s o f s u f f i c i e n t l y d i f f e r e n t s i z e , e l a s t o m e r s are o b t a i n e d ( F i g u r e 4 ) . Another requirement f o r e l a s t o m e r i c b e h a v i o r i s t h a t the s u b s t i t u e n t s be randomly d i s t r i b u t e d a l o n g the P-N backbone. T h i s p r i n c i p l e was f i r s t demonstrated by Rose ( 9 ) , and subsequent work i n s e v e r a l i n d u s t r i a l l a b o r a t o r i e s has l e d t o the development of phosphazene e l a s t o m e r s of commercial i n t e r e s t . A phosphazene f l u o r o e l a s t o m e r and a phosphazene elastomer w i t h mixed a r y l o x y s i d e c h a i n s a r e showing promise f o r m i l i t a r y and commercial a p p l i c a t i o n s . These e l a s t o m e r s a r e the s u b j e c t of a n o t h e r paper i n t h i s symposium ( 1 0 ) . S t u d i e s have shown t h a t not a l l phosphazene copolymers a r e n e c e s s a r i l y elastomers (11,12). Figure 5 contrasts s e m i c r y s t a l l i n e homopolymers w i t h an e l a s t o m e r i c copolymer, and t h e n w i t h some s e m i c r y s t a l l i n e a r y l o x y c o p o l y m e r s . Note i n F i g u r e 5 t h a t t h e r e i s a d e c r e a s i n g o r d e r of c r y s t a l l i n i t y from top t o bottom. The i n t e r m e d i a t e c a s e s r e p r e s e n t two c l a s s e s o f c r y s t a l l i n e copolymers which a r e d i s t i n g u i s h a b l e by t h e i r t h e r m a l t r a n s i t i o n b e h a v i o r and X-ray c r y s t a l s t r u c t u r e p a t t e r n s . I n c r e a s i n g the d i f f e r e n c e s i n the s i z e and n a t u r e of the s u b s t i t u e n t s on the phenoxy r i n g w i l l produce amorphous c o p o l y m e r s , but the polyphosphazene u n i t c e l l appears t o be u n u s u a l l y t o l e r a n t of p e r t u r b a t i o n s on a more l i m i t e d s c a l e ( 1 2 ) . As evidenced by the s t r u c t u r e s i n F i g u r e 5, some c a r e must be t a k e n i n s e l e c t i n g substituents to achieve desired p r o p e r t i e s , e s p e c i a l l y i f the g o a l i s t o p r e p a r e amorphous polymers. The s i d e c h a i n s u b s t i t u e n t s can a f f e c t the p r o p e r t i e s of the polyphosphazenes i n y e t a n o t h e r way. Whereas ( N P ( 0 C H C H ^ ) ) i s amorphous, i n c r e a s i n g the s i d e c h a i n l e n g t h by u s i n g l o n g c h a i n a l c o h o l s can r e s u l t i n polymers w h i c h a r e s e m i c r y s t a l l i n e ( 1 3 ) . Presumably these polymers assume more of the c h a r a c t e r of p o l y ( e t h y l e n e o x i d e ) , as the s i d e c h a i n l e n g t h i n c r e a s e s . The morphology of the s e m i c r y s t a l l i n e polyphosphazenes i s complex. T a b l e I p r o v i d e s examples o f phosphazenes w i t h two f i r s t o r d e r t r a n s i t i o n s denoted by T ( l ) and Tm. The T ( l ) i s an i n t e r m e d i a t e t r a n s i t i o n t o a p a r t i a l l y ordered s t a t e . Between T ( l ) 2

6

5

2

3

2

n

n

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

n

SINGLERETAL.

Phosphazene Polymers

Table I. Summary of T r a n s i t i o n and Decomposition Temperatures (°C) f o r Various Polyphosphazenes

V

POLYMER

T(U* 33·

-66

[Cl2PNln [(CH3CH20)2PNI

-84

[(CF3CH 0) PNJ

-66

90

240*

360

6

160

390

380

-24

66

370

380

4

167

365

410

n

2

2

[(C H 0) PNl 6

5

2

n

n

i(3-CIC H 0) PNI 6

4

2

[(4-CIC H 0) PNl 6

4

2

I([CH ] N) PN] 3

2

2

i(C H NH) PN] 6

5

2

n

n

-4

n

105

n

i(4-CH OC H NH) PN] 3

6

4

2

i(CF CH 0)(C3F CH 0)PN] 3

2

7

2

-77

n

[(CF CH 0)(HCF C F CH )PN] 3

2

2

3

6

2

[(C H 0)(4-C H C H 0)PN] 6

5

2

5

6

4

[(C H )(4-CIC H 0)PNJ 6

5

6

4

n

n

n

-68 -27 5

77,94

•BY DIFFERENTIAL THERMAL ANALYSIS OR DIFFERENTIAL SCANNING CALORIMETRY. BY THERMAL MECHANICAL ANALYSIS EXCEPT WHERE NOTED. ^THERMAL DECOMPOSITION TEMPERATURES BY THERMAL GRAVIMETRIC ANALYSIS. MOLECULAR WEIGHT CHANGES HAVE BEEN REPORTED BELOW 200°C. f

F i g u r e 4. C o n t r a s t i n g s y n t h e s i s o f homopolymers and c o p o l y m e r s showing p o s s i b l e copolymer s t r u c t u r e s w h i c h a r e randomly d i s t r i b u t e d a l o n g t h e polymer backbone.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

274

INORGANIC AND ORGANOMETALLIC POLYMERS

DECREASING

CRYSTALLINITY

0O- 3 CH

{fN}n

{P-Nln OQ-CH

{p-ίθί

1

3

iP-N}n

0CI

CH3

XH3

ELASTOMER

F i g u r e 5. E f f e c t o f s i d e c h a i n s u b s t i t u e n t s on polymer c r y s t a l l i n i t y . Polymers w i t h two p a r a s u b s t i t u e n t s (second row) are more c r y s t a l l i n e than polymers w i t h mixed para and meta s u b s t i t u e n t s .

and Tm i s a mesomorphic s t a t e . However, d e t a i l e d a n a l y s i s (14-16) shows t h a t the polymers w i t h a T ( l ) t r a n s i t i o n a r e not nematic o r s m e c t i c i n n a t u r e , but r a t h e r have a pseudohexagonal phase e x h i b i t i n g dynamic d i s o r d e r when c h a r a c t e r i z e d by X-ray d i f f r a c t i o n e x p e r i m e n t s . Polyphosphazenes such as (NPCOCH^CF^W^ have been termed " c o n d i s " o r c o n f o r m a t i o n a l ^ disordered c r y s t a l s by Wunderlich (17). To show a n o t h e r example o f the e f f e c t o f s i d e c h a i n s t r u c t u r e on polymer p r o p e r t i e s , i t has been r e c e n t l y demonstrated t h a t l i q u i d c r y s t a l l i n e s i d e c h a i n phosphazenes can be p r e p a r e d by a t t a c h i n g a mesogenic group through a f l e x i b l e s p a c e r t o the phosphazene polymer c h a i n ( 1 8 ) . Copolymer V I I ( F i g u r e 6) e x h i b i t s a s t r o n g r e v e r s i b l e l i q u i d c r y s t a l l i n e phase between 123 and 175 °C. M i c r o s c o p i c a n a l y s i s i n the l i q u i d c r y s t a l l i n e r e g i o n i s shown i n F i g u r e 7. A s i m i l a r polyphosphazene w i t h a d i f f e r e n t s u b s t i t u t e d phenylazo mesogen s i d e c h a i n has a l s o been prepared which shows l i q u i d c r y s t a l l i n e o r d e r ( 1 9 ) . F u r t h e r work i s underway t o e l u c i d a t e the exact n a t u r e o f t h i s s t a t e and t o prepare a d d i t i o n a l l i q u i d c r y s t a l l i n e s i d e c h a i n polyphosphazenes. n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

20.

Phosphazene Polymers

SINGLERETAL.

275

Polyphosphazene

-TN-P L- i-ln OCH2CF3

F i g u r e 6. G e n e r a l s t r u c t u r e f o r phosphazenes w i t h mesogenic s i d e groups. Example i s a mixed s u b s t i t u e n t polymer ( V I I ) where R r e p r e s e n t s the t r i f l u o r o e t h o x y group and the mesogen w i t h f l e x i b l e spacer i s r e p r e s e n t e d by the c u r l i c u e and r e c t a n g u l a r box.

F i g u r e 7. O p t i c a l m i c r o g r a p h o f V I I showing t e x t u r e o f t h e mesophase a t 182 °C. M a g n i f i c a t i o n 320 X.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

276

INORGANIC AND ORGANOMETALLIC POLYMERS

Conclusion The polyphosphazenes a r e h i g h m o l e c u l a r weight polymers w i t h a wide range o f n o v e l and p o t e n t i a l l y u s e f u l p r o p e r t i e s . The l a r g e number of d i f f e r e n t pendant groups w i t h w i d e l y v a r i e d f u n c t i o n a l i t y which can be a t t a c h e d t o t h e P-N backbone demonstrate t h e u n u s u a l m o l e c u l a r d e s i g n p o t e n t i a l o f t h i s c l a s s o f polymers. Undoubtedly, some o f these w i l l h o l d promise f o r f u t u r e r e s e a r c h and development. Literature Cited 1. 2.

3. 4. 5. 6.

7. 8. 9. 10.

11. 12. 13. 14.

15. 16. 17. 18. 19.

Tate, D.P. and Antowiak, T.A. Kirk-Othmer Encycl. Chem. Technol. 3rd. Ed. 1980, 10, 939. Singler, R.E.; Hagnauer, G.L.; Sicka, R.W. In Polymers for Fibers and Elastomers; Arthur, J.C., Ed.; ACS Symposium Series, No. 260. American Chemical Society, Washington, D.C., 1984, p 143. Allcock, H.R. Chem. Allcock, H.R. Phosphorus-Nitrogen York, 1972. Hagnauer, G.L. J. Macromol. Sci.- Chem. 1981, A16, 385. Hagnauer, G.L.; Koulouris, T.N. In Liquid Chromatography of Polymers and Related Materials-III; Jack Cazes, Ed.; Marcel Dekker, Inc.; New York, 1981; p 99. Sennett, M.S.; Hagnauer, G.L.; Singler,R.E.; Davies,G. Macromolecules 1986, 19, 959. Sennett, M.S. unpublished results. Rose, S.H. J. Polym. Sci. Β 1968, 6, 837. Penton, H.R. In Inorganic and Organometallic Polymers; Zeldin, M.; Allcock, H. and Wynne, K. Eds., ACS Symposium Series, No. xx, American Chemical Society, Washington, D.C., 1987. Dieck, R.L. and Goldfarb, L. J. Polym. Sci. Poly Chem. Ed. 1977, 15, 361. Beres, J.J.; Schneider, N.S.; Desper, C.R.; Singler, R.E. Macromolecules 1979, 12, 566. Allcock, H.R.; Austin, P.E.; Neenan, T.X.; Sisko, J.T.; Blonsky, P.M.; Shriver, D.F. Macromolecules 1986, 19, 1508. Schneider, N.S.; Desper, C.R.; Beres, J.J. In Liquid Crystalline Order in Polymers; Blumstein, Α., Ed., Academic Press, N.Y., 1978, p 299. Kojima, M.; Magill, J.H. Makromol. Chem. 1985, 186, 649. Yeung, A.S.; Frank, C.W.; Singler, R.E. Polym. Prepr. ACS Div. Polym. Chem. 1986, 27(2), 214. Wunderlich, B. and Grebowicz, J. In Polymeric Liquid Crystals; Blumstein, Α., Ed.; Plenum Press, New York, 1985, 28, 145. Singler, R.E.; Willingham, R.A.; Lenz, R.W.; Furukawa, Α.; Finkelmann, H. Macromolecules 1987, in press. Allcock, H.R. and Kim, C. Macromolecules 1987, in press.

RECEIVED

September

1,

1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 21

Polyphosphazenes: Performance Polymers for Specialty Applications H. R. Penton Ethyl Technical Center, Ethyl Corporation, Baton Rouge, LA 70898 Although polyphosphazenes were f i r s t synthesized the laboratory nearly one hundred years ago, t h e i r technological importance i s only now being r e a l i z e d . Today commercial applications f o r polyphosphazene specialty elastomers exist i n aerospace, marine, oil exploration and i n d u s t r i a l fields. This paper describes the properties and applications f o r this unique class of elastomers. N e a r l y one hundred y e a r s ago, Stokes r e p o r t e d t h e f i r s t s y n t h e s i s o f a polyphosphazene, which he d e s c r i b e d a s " i n o r g a n i c r u b b e r " ( 1 ) . Stokes had s y n t h e s i z e d p o l y ( d i c h l o r o p h o s p h a z e n e ) by t h e t h e r m a l polymerization o f hexachlorocyclotriphosphazene. Although t h e p o l y ( d i c h l o r o p h o s p h a z e n e ) , o r c h l o r o p o l y m e r as i t i s o f t e n c a l l e d , had a l l t h e c h a r a c t e r i s t i c s o f a s t r o n g e l a s t o m e r , t h e polymer r a p i d l y h y d r o l y z e d upon exposure t o a t m o s p h e r i c m o i s t u r e ; l o s i n g i t s rubber q u a l i t i e s due t o e x c e s s i v e c r o s s l i n k i n g and u l t i m a t e breakdown t o form p h o s p h o r i c a c i d , ammonia and h y d r o c h l o r i c a c i d . In a d d i t i o n t o t h e h y d r o l y t i c i n s t a b i l i t y o f t h e c h l o r o p o l y m e r , thermal p o l y m e r i z a t i o n o f hexachlorocyclotriphosphazene g e n e r a l l y y i e l d e d o n l y g e l l e d polymer due t o e x c e s s i v e b r a n c h i n g and c r o s s l i n k i n g as t h e c o n v e r s i o n t o polymer exceeded f i f t y p e r c e n t ( 2 ) . The problems a s s o c i a t e d w i t h t h e s y n t h e s i s and h a n d l i n g o f c h l o r o p o l y m e r were a major b a r r i e r t o t h e development o f t h e s e polymers u n t i l A l l c o c k and Kugel found t h a t c h l o r o p o l y m e r c o u l d be o b t a i n e d a s a s o l u b l e , g e l - f r e e polymer i f c o n v e r s i o n s were l i m i t e d t o l e s s than f i f t y p e r c e n t ( 3 ) . Subsequent replacement o f t h e c h l o r i n e s w i t h metal a l k o x i d e s o r a r y l o x i d e s y i e l d e d organo-substit u t e d polyphosphazenes which were b o t h t h e r m a l l y and h y d r o l y t i c a l l y stable (4). A l l c o c k * s d i s c o v e r y t h a t s t a b l e polyphosphazenes c o u l d be p r e pared under c o n t r o l l e d c o n d i t i o n s opened t h e door f o r t h e commerc i a l development o f t h i s c l a s s o f polymers, and i n 1970, t h e f i r s t polyphosphazene e l a s t o m e r s were s y n t h e s i z e d and t h e t e c h n o l o g y subs e q u e n t l y d e v e l o p e d by F i r e s t o n e T i r e and Rubber Company ( 5 ) . Today, polyphosphazenes a r e c o m m e r c i a l l y a v a i l a b l e , and r e p r e s e n t 0097-6156/88/0360-0277S06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

278

INORGANIC AND ORGANOMETALLIC POLYMERS

t h e f i r s t new c l a s s o f s e m i - i n o r g a n i c e l a s t o m e r s t o be c o m m e r c i a l l y d e v e l o p e d s i n c e s i l i c o n e rubber ( 6 ) . Polyphosphazenes a r e unique i n t h e w o r l d o f polymers i n t h a t t h e v a r i o u s end p r o d u c t s can be d e r i v e d from a s i n g l e p r e c u r s o r polymer, p o l y d i c h l o r o p h o s p h a z e n e ( I ) . D i f f e r e n t polymers a r e p r e pared by m o d i f y i n g t h e o r g a n i c groups used t o d i s p l a c e t h e c h l o r i n e s o f I . F o r i n s t a n c e , i f I i s r e a c t e d w i t h t h e sodium s a l t s o f t r i f l u o r o e t h a n o l and a mixed f l u o r o t e l o m e r a l c o h o l , a p o l y ( f l u o r o alkoxyphosphazene) e l a s t o m e r ( I I ) i s o b t a i n e d h a v i n g a unique s e t of p r o p e r t i e s i n c l u d i n g a wide s e r v i c e temperature range, f u e l and oil resistance, low temperature flexability, high temperature s t a b i l i t y , and e x c e l l e n t f l e x f a t i g u e and damping c h a r a c t e r i s t i c s ( 7 ) . Replacement o f t h e c h l o r i n e s w i t h phenoxy and p-ethylphenoxy s u b s t i t u e n t s y i e l d s a p o l y ( a r y l o x y p h o s p h a z e n e ) ( I I I ) ; a non-halogenated, flame r e s i s t a n t , low smoke p r o d u c i n g e l a s t o m e r ( 7 ) . Ç1 [P=N] CI

I

OCH n

[f

[

OCH (CF ) CF H 2

2

x

2

x = 1, 3, 5, 7 . . . II

f

n

OC H -p-C H

III

Chemistry The c o n v e n t i o n a l r o u t e t o p r e p a r e I g e n e r a l l y i n v o l v e s a h i g h temp e r a t u r e melt p o l y m e r i z a t i o n o f h e x a c h l o r o c y c l o t r i p h o s p h a z e n e , o r t r i m e r ( I V ) . Recent s t u d i e s have demonstrated t h e e f f e c t i v e n e s s o f v a r i o u s a c i d s and o r g a n o m e t a l l i c s as c a t a l y s t s f o r t h e p o l y m e r i z a t i o n o f IV ( 8 ) . A l t e r n a t e r o u t e s f o r t h e p r e p a r a t i o n o f c h l o r o polymer which do n o t i n v o l v e t h e r i n g opening p o l y m e r i z a t i o n o f t r i m e r have been r e p o r t e d i n t h e p a t e n t l i t e r a t u r e ( 9 , 10). These r o u t e s i n v o l v e a c o n d e n s a t i o n p o l y m e r i z a t i o n p r o c e s s and may prove t o be o f t e c h n o l o g i c a l importance f o r t h e p r e p a r a t i o n o f low t o moderate m o l e c u l a r weight polyphosphazenes. The t r u e v a l u e o f t h e c h l o r o p o l y m e r ( I ) l i e s i n i t s u s e as an i n t e r m e d i a t e f o r t h e s y n t h e s i s o f a wide v a r i e t y o f p o l y ( o r g a n o phosphazenes) as shown i n F i g u r e 1. The n a t u r e and s i z e o f t h e s u b s t i t u e n t a t t a c h e d t o t h e phosphorus p l a y s a dominant r o l l i n d e t e r m i n i n g t h e p r o p e r t i e s o f t h e polyphosphazene. Homopolymers p r e p a r e d from I , i n which t h e R groups a r e t h e same o r , i f d i f f e r ent, similar i n molecular s i z e , t e n d t o be s e m i - c r y s t a l l i n e t h e r m o p l a s t i c s . I f two o r more d i f f e r e n t s u b s t i t u e n t s a r e i n t r o duced, t h e r e s u l t i n g polymers a r e g e n e r a l l y amorphous e l a s t o m e r s . (See F i g u r e 1.) A p p l i c a t i o n s o f Polyphosphazenes Poly(Fluoroalkoxyphosphazene) Elastomers. When I i s s u b s t i t u t e d w i t h a m i x t u r e o f t r i f l u o r o e t h o x i d e and t e l o m e r f l u o r o a l k o x i d e s , an elastomer I I i s obtained having a f l u o r i n e content o f approximately 55 p e r c e n t . A s m a l l amount o f an u n s a t u r a t e d c u r e s i t e may a l s o be i n c o r p o r a t e d i n t o t h e polymer t o promote v u l c a n i z a t i o n .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

21.

PENTON

279

Polyphosphazenes: Performance Polymers OR

-

κ

CN -

p

[n

n

OR

F i g u r e 1.

Synthesis of

poly(organophosphazenes).

The f l u o r i n e c o n t e n t o f I I g i v e s i t e x c e l l e n t r e s i s t a n c e t o f u e l s , o i l s , most h y d r a u l i c f l u i d s and c h e m i c a l s . S i n c e t h e r e a r e no C-C and C-H bonds a l o n g t h e polymer backbone, I I d i s p l a y s e x c e l l e n t r e s i s t a n c e t o d e g r a d a t i o n by a t m o s p h e r i c oxygen and ozone. In a d d i t i o n , t h e i n h e r e n t l y f l e x i b l e n a t u r e o f t h e Ρ41 backbone a l l o w s t h i s e l a s t o m e r t o be used a t temperatures down t o -65°C, and g i v e s th e polymer e x c e l l e n t f l e x f a t i g u e r e s i s t a n c e o v e r a broad temperature range (-65 t o 175°C). The phosphazene f l u o r o e l a s t o m e r s can be compounded and processed u s i n g c o n v e n t i o n a l rubber equipment. T y p i c a l filler systems c o n s i s t o f s i l i c a s , c l a y s o r carbon b l a c k s e i t h e r a l o n e o r i n c o m b i n a t i o n . The r e s u l t i n g compound can be c u r e d w i t h p e r o x i d e s , s u l f u r and r a d i a t i o n . Depending on th e t y p e and amount o f f i l l e r used, vulcanizates having an excellent balance of physical properties are obtained (Table I ) . Elastomers of II offer s i g n i f i c a n t p r o p e r t y advantages o v e r f l u o r o p o l y m e r e l a s t o m e r s and fluorosilicones, p a r t i c u l a r l y where a combination of low temperature f l e x i b i l i t y as w e l l as h i g h temperature s t a b i l i t y , good f l e x - f a t i g u e r e s i s t a n c e and f u e l , o i l o r c h e m i c a l r e s i s t a n c e a r e r e q u i r e d . (See T a b l e I.) Because o f i t s e x c e l l e n t range o f p r o p e r t i e s and r e l i a b i l i t y , poly(fluoroalkoxyphosphazene) elastomers are used as seals, gaskets, and shock mounts i n demanding m i l i t a r y , a e r o s p a c e , p e t r o l e u m and i n d u s t r i a l a p p l i c a t i o n s . I n a d d i t i o n , a p p l i c a t i o n s under development f o r t h i s e l a s t o m e r i n c l u d e f u e l hoses f o r a r t i c use, c o a t e d f a b r i c s f o r p r o t e c t i v e c l o t h i n g , s e a l a n t s , c o a t i n g s and medical devices. Poly(Aryloxyphosphazene) Elastomers. Poly(aryloxyphosphazene) e l a s t o m e r s , I I I , p r e p a r e d by th e r e a c t i o n o f c h l o r o p o l y m e r w i t h mixed phenoxides, o f f e r e x c e l l e n t f i r e r e s i s t a n c e without the i n c o r p o r a t i o n o f halogens i n t h e polymer o r as an a d d i t i v e . I n a d d i t i o n , i n a f i r e s i t u a t i o n smoke e v o l u t i o n from t h e s e polymers

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

280

i s m i n i m a l and th e r e s u l t i n g non-acidic (11).

o f f - g a s e s have low

toxicity

and

are

Table I. P r o p e r t i e s o f P o l y ( f l u o r o a l k o x y phosphazene) Elastomers PHYSICAL PROPERTY

UNITS

VALUE

Density Tensile Strength 100% Modulus Elongation Compression Set (70 h r β 150°C) Hardness, Durometer TR-10 B r i t t l e Point Temperature Rang Tear R e s i s t a n c e

g/ml MPa ( p s i ) MPa(psi) % %

1.75-1.85 6.9-13.8 (1000-2000) 2.8-13.8 ( 400-2000) 75-250 15-25

Shore A °C °C

35-90 -56 -68

E x c e l l e n t Bonding t o M e t a l s and F a b r i c s E x c e l l e n t V i b r a t i o n Damping C h a r a c t e r i s t i c s E x c e l l e n t F l e x L i f e i n Dynamic A p p l i c a t i o n s Excellent Weatherability Fungus R e s i s t a n t Flame R e s i s t a n t R e s i s t a n t t o Broad Range o f F l u i d s L i q u i d Oxygen Compatible Poly(aryloxyphosphazene) elastomers can be cured with p e r o x i d e s , s u l f u r and r a d i a t i o n . The r e s u l t i n g v u l c a n i z a t e s a r e r e s i s t a n t t o a t t a c k by m o i s t u r e and o i l s and have been found t o have desirable characteristics for electrical insulation a p p l i c a t i o n s where f i r e s a f e t y i s a c o n c e r n ( T a b l e I I ) ( 1 2 ) . r e s i s t a n t , low smoke, c l o s e d c e l l foams w i t h e x c e l l e n t p r o p e r t i e s ( T a b l e I I I ) have a l s o been developed from p o l y ( a r y l o x y p h o s p h a z e n e ) elastomers (13). Applications f o r t h e s e foams, which can be produced as e i t h e r s l a b s t o c k o r tube s t o c k , a r e b e i n g d e v e l o p e d f o r m i l i t a r y , aerospace and commercial u s e s . (See T a b l e I I and I I I . ) F

i

r

e

Summary A l t h o u g h polyphosphazene c h e m i s t r y i s n e a r l y 100 y e a r s o l d , t h e t e c h n o l o g i c a l p o t e n t i a l o f t h i s c l a s s o f polymers i s o n l y now b e i n g realized. The poly(fluoroalkoxyphosphazene) elastomers o f f e r a unique c o m b i n a t i o n o f p r o p e r t i e s i n c l u d i n g a wide o p e r a t i n g temperature range, e x c e l l e n t f u e l and o i l r e s i s t a n c e and low temperature prop­ e r t i e s s u p e r i o r t o t h o s e o f t h e f l u o r o s i l i c o n e s and f l u o r o p o l y m e r elastomers. Poly(aryloxyphosphazene) elastomers offer outstanding fire r e s i s t a n c e w i t h o u t the presence o f h a l o g e n . In a f i r e these e l a s t o m e r s produce low l e v e l s o f smoke as w e l l as low t o x i c i t y and non-corrosive off-gases.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

21.

PENTON

Polyphosphazenes: Performance Polymers

281

Table I I . Properties o f Poly(aryloxyphosphazene) Wire and C a b l e I n s u l a t i o n / J a c k e t i n g PHYSICAL PROPERTY

UNITS

VALUE

Tensile Strength Elongation Tear R e s i s t a n c e , Die Β Halogen Water A b s o r p t i o n Hardness, Durometer Heat A g i n g (168 h r 0 158°C) Tensile Retention Elongation Retention LOI NBS Smoke D e n s i t Flaming Non-Flaming D i e l e c t r i c Const., 10 Hz Volume R e s i s t i v i t y

MPa ( p s i )

9.0-11.0 (1300-: 200-400

Table

%

kN/m ( p p i ) 13 (75) 0.05 3.25 mg/cm 83 Shore A

%

% % %

110 45 44

DM

65-75

ohm-cm

3.9 3.2-1014

I I I . P r o p e r t i e s o f P o l y ( a r y l o x y p h o s p h a z e n e ) Foams

PROPERTY

VALUE

UNITS 3

Density lb/ft Compressive R e s i s t a n c e p s i Tensile Strength psi Compression S e t % Thermal C o n d u c t i v i t y BTU»in/hr»ft C Water Vapor perm*in Permeability LOI NBS Smoke D e n s i t y Non-Flaming Flaming Flame Speed Index A c i d Gas G e n e r a t i o n mg HCl/g 2o

4.5-5.5 2-8 25-35 25 0.26 0.19 44 40 70 11 0

References 1. H. N. Stokes, Amer. Chem. J . , 19, 782 (1897). 2. H. R. Allcock, Phosphorus-Nitrogen Compounds, Academic Press, New York (1972). 3. H. R. Allcock and R. L. Kugel, J . Amer. Chem. Soc., 87, 4216 (1965). 4. H. R. Allcock and R. L. Kugel, Inorg. Chem., 5, 1709 (1966). 5. D. F. Lohr and J . A. Beckman, Rubber and P l a s t i c s News, 16 (1982). 6. H. R. Penton, International Rubber Conference Summaries, Stuttgart, Fed. Reb. of Germany, 55 (1985).

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

282 7. 8. 9. 10.

H. R. Penton, Kautschuk Gummi Kunst., 39, 301 (1986). G. L. Hagnauer, J . Macromol. S c i . , Chem. Ed., A16, 385 (1981). Ε. D. Hornbaker and Η. M. Li, U.S. Patent: 4,198,381 (1980). R. De Jaeger, M. Helioui and E. Puskaric, U.S. Patent: 4,377,558 (1983) 11. P. J . Lieu, J . H. Magill and Y. C. A l a r i e , J . F i r e and Flammability, 11, 167 (1980). 12. J . T. Books, D. M. Indyke, W. O. Muenchinger, International Wire and Cable Symposium Proceedings, Cherry Hill, New Jersey (USA), 1 (1984). 13. W. B. Mueller, J . C e l l u l a r P l a s t i c s , 22, 53 (1986). RECEIVED

September

1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 22

Poly(alkyl/arylphosphazenes) Robert H . Neilson , R. Hani , G. M . Scheide , U. G. Wettermark , P. Wisian-Neilson , R. R. Ford , and A. K. Roy 1

1

1

2

1

2

2

Department of Chemistry, Texas Christian University, Fort Worth, TX 76129 Department of Chemistry Southern Methodist University Dallas

1

2

A representative series of the title polymers and copolymers were prepared by the thermally induced condensation polymerization of the corresponding N-silylphosphoranimines. The polymers were characterized by standard dilute solution techniques, thermal analysis (DSC and TGA), multinuclear NMR spectroscopy, and X-ray diffraction measurements. The polymers have molecular weights (M ) ranging from 25,000 to 200,000 with symmetrical molecular weight distributions (M /M = 2) and, in general, exhibit solution behavior typical of random coil polymers in "good" solvents (e.g., THF or CHCl ). Deprotonation/substitution reactions of the N-silylphosphoranimine "monomers" were used to prepare a variety of functionalized phosphazene precursors containing, for example, silyl, vinyl, and phosphinyl substituents. A chain growth process is suggested as the polymerization mechanism on the basis of studies of polymer molecular weight vs. extent of reaction. w

w

n

3

In recent years, many poly(phosphazenes), [ R o P N ] , with a variety of substituents at phosphorus have been prepared and they often exhibit useful properties including low temperature flexibility, resistance to chemical attack, flame retardancy, stability t o UV radiation, and reasonably high thermal stability. ( 1 , 2 ) Compounds containing biologically, catalytically, or electrically active side groups are also being investigated. ( 3 , 4 ) The most commonly used synthetic route t o poly(phosphazenes) is the ringopening/substitution method developed by Allcock and coworkers. (1 ) This procedure involves the initial preparation of poly(dichlorophosphazene), [ C I P N ] , by the ring-opening polymerization of the cyclic trimer and subsequent nucleophilic displacement of the chlorine atoms along the chain. In each case, the substituents at phosphorus must be introduced after polymerization since the fully substituted cyclic phosphazenes d o not polymerize. ( 5 , 6 ) A c o m m o n feature of poly(phosphazenes) prepared in this manner is that the organic substituents are bonded t o phosphorus through oxygen or nitrogen links, thereby providing pathways for decomposition or depolymerization on heating above about 200°C. It has been postulated that directly P-C bonded alkyl or aryl side groups might enhance the thermal or chemical stability of the polymers and give rise t o interesting physical properties. ( 6 - 8 ) Furthermore, the alkyl substituted poly (phosphazenes), [ R o P N ] , are isoelectronic with the well-known siloxane polymers, [ R S i O ] , and the extent of tnis analogy would be interesting to pursue. The published attempts to prepare the directly P-C bonded polymers by t h e s u b s t i t u t i o n m e t h o d have g e n e r a l l y n o t b e e n s u c c e s s f u l . T r e a t m e n t of poly(dihalophosphazenes) with organometallic reagents (e.g., RMgX or RLi) results in either incomplete substitution under mild conditions or undesired reactions such as chain cleavage a n d / o r crosslinking under more vigorous conditions. (7 - 1 0 ) n

2

n

n

2

n

0097-6156/88/0360-0283$06.00/0 © 1988 A m e r i c a n Chemical Society

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

284

D u r i n g t h e last f e w y e a r s , a n e w g e n e r a l m e t h o d f o r t h e s y n t h e s i s of poly(phosphazenes) has been under investigation in our laboratory. The approach is based o n the premise that suitably constructed N-silylphosphoranimines can eliminate substituted silanes t o form cyclic a n d / o r linear phosphazenes (eq 1). This type of condensation polymerization reaction has several potential advantages, the most important of which is the ability t o incorporate the desired phosphorus substituents directly into the starting Si-N-P compound. This procedure has resulted in the successful preparation of the first fully P-C bonded poly(phosphazene), [ M e P N ] (1), as described in preliminary reports. (11 - 1 3 ) 2

n

R

R

Me SiN=P-X

>

3

Me SiX 3

+

(1)

[-N=P-]

n

R

R

We report here the results of our recent studies of poly(alkyl/arylphosphazenes) with particular emphasis on the following areas: (1) the overall scope of, and recent improve­ ments in, the condensation polymerization method; (2) the characterization of a repre­ sentative series of these polymer mometry, light scattering, and DSC), NMR spectroscopy, and X-ray diffraction; (3) the preparation and preliminary ther­ molysis reactions of new, functionalized phosphoranimine monomers; and (4) the mechanism of the polymerization reaction.

Results and Discussion Synthesis. The N-silyl-P-trifluoroethoxyphosphoranimine reagents, used as "monomers" in this polymer synthesis, are readily prepared from either PCU or P h P C I in a straightforward, 3-step reaction sequence as described elsewhere, (i 4 - 1 6 ) These compounds are obtained as colorless, distillable, air-sensitive liquids in yields of 50-75% based o n starting PCU or PhPCI . On heating at ca. 160-200°C for 2-12 days, these phosphoranimines readily eliminate M e S i O C H C F (eq 2) t o form the polymeric phosphazenes, e.g., 1-4. Copolymers 2

2

3

2

3

R

- Me SiOCH CF 3

ι

*

Me SiN=P-0CH CF 3

2

2

3

["Ν^Λ

2

()

R R

R

I-N+ln

[(-N-f-) (-N.f-)y]„ Ph R x

R

1: R = R

R

= Me

5: R = Me

2: R = R' = Et

6: R = Et

3: R = Me, R' = Ph

χ= y

4: R = Et, R' = Ph such as 5 and 6 are prepared by heating equimolar mixtures of the dialkyl- and phenyl/alkyl substituted phosphoranimines.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

22. NEILSON ET AL.

Polyfalkyl/arylphosphazenes)

285

These thermolysis reactions normally produce polymeric products, free of the cyclic analogs, in essentially quantitative yield and in sufficient purity t o give satisfactory elemental analysis upon removal of the silyl ether byproduct under vacuum. Final purification is generally achieved by precipitation of the polymer into a non-solvent such as hexane. With the exception of poly(diethylphosphazene) (2), which is insoluble in all c o m m o n solvents (see below), the new polymers are readily soluble in C h L C U and CHCU. In addition, the phenyl substituted compounds (3-6) are soluble in THF ancfvarious aromatic solvents. None of the polymers are water-soluble; however, [ M e P N ] (1) is soluble in a 50:50 water/THF mixture. All of the soluble polymers (1 and 3-6) give high resolution NMR spectra ( H, C , and P ) that are completely consistent with their proposed structures. As observed for other types of poly (phosphazenes), the Ρ chemical shifts of these alkyl/aryl substituted polymers are consistently ca. 15-30 p p m upfield from those of the analogous cyclic trimers and tetramers. Some important structural information is provided by C NMR spectroscopy, par­ ticularly for the phenyl/alkyl derivatives 3 and 4 . These polymers are rare examples of phos­ phazenes that contain two different substituents at each phosphorus atom in the chain. Thus, they have the possibility of being stereoregular The fact that the structures are completely atactic, however, is confirmed b C NMR spectrum (ca. 22 ppm More recently, w e have extended the use of the condensation polymerization method t o include many other combinations of alkyl and aryl groups in both homopolymers as well as copolymers. Future systematic studies of these materials should provide valuable information on the structure/property relationships which are applicable to this class of polymers. The possibility of incorporating unsaturated groups into the polymers, as potential sites for crosslinking, is also being explored. For example, the vinyl (7, 8), ally! (9), and butenyl (10) substituted phosphoranimines have been prepared and subjected t o the usual thermolysis and cothermolysis conditions (eq 3). In these cases, all of which have a terminal 2

n

1 3

3 1

3 1

1 3

3

R

R'

x Me SiN=/>OCH CF 3

2

(CH ) 2

\ H

3

n

2

+

y Me SiN=P0CH CF 3

2

3

— η

Ae R' = Ph, Me

/ /

(3)

7: R = Me, η = 0 8: R = Ph, η = 0 9: R = Ph, η = 1

Swell able, crosslinked polymers (x:y = 1:2, 1:5, 1:10, 1:20)

10: R = Me, η = 2 vinyl group, crosslinking occurs during the thermal polymerization process, leading t o the formation of tough, insoluble rubber-like materials. Only when very high proportions of c o monomers (e.g., x:y ^ 1:50) are employed, d o w e obtain soluble, characterizable polymers. In contrast, when substituted vinyl groups (see below) are involved, the thermal crosslinking is suppressed so that soluble phosphazenes bearing intact -CH=CHR substituents are o b ­ tained. Characterization. The series of poly (alkyl/aryiphosphazenes) (1-6) was studied by a variety of standard dilute solution techniques including viscosity measurements, membrane os-

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

286

mometry, size exclusion chromatography, and light scattering. Taken together, these studies demonstrate (17) that the poly(alkyl/arylphosphazenes) exist as extended, flexible

chains in good solvents such as THF or CHCI3, with average chain lengths of several hundred to a thousand repeat units and symmetrical molecular weight distributions (MJM

-2). The intrinsic viscosities (ca. O.15-O.50 d L / g ) were determined at 3 0 ° C in CHCI3 for the dimethyl polymer 1 and in THF for the phenyl substituted polymers 3 - 6 . In all cases, the plots of reduced viscosity vs. concentration were quite linear at low concentration (ca. O.11.0%) with Huggins constants in the range (ca. O.30-O.45) characteristic of good polymer/solvent interactions. Absolute number average molecular weights ( M ) , determined by membrane osmometry, fall in the general range of 20,000-100,000, with those of the dimethyl (1) and phenyl/methyl (3) polymers and the corresponding copolymer (5) typically being greater than 50,000. Initially, the routine characterization of these poly(phosphazenes) was hindered by their anomalous behavior in size exclusion chromatography ( S E C ) experiments. The analyses were attempted o n commercial columns, with either /zStyragel or glass bead pack­ ings, at a variety of temperatures and concentrations In addition several different solvents including mixed solvent system reproducible chromatograms wer between the polymer and the column materials. Recently, however, w e found that these SEC problems are completely circumvented by the addition of a small amount (ca. O.1 weight percent) of an ionic species such as (n-Buj^NBr to the THF mobile phase. Under these condi­ tions, classic S E C behavior is observed and consistent, non-tailing chromatograms are o b ­ tained. Moreover, the molecular weights (M ) measured by S E C , relative t o narrow molecular weight polystyrene standards, agree very well (within ca. 20-30%) with the values deter­ mined by membrane osmometry. The absolute weight average molecular weights ( M ) of several samples of [PhiMeJPN]^ from different preparations were determined by light scattering measurements in dilute THF solutions. The M values, which range from 73,000 t o 202,000, are consistently (ca. 30%) higher than those obtained by S E C determinations. These results, when combined with the membrane osmometry data, confirm that the S E C experiments provide valid repre­ sentations of the molecular weight distributions of these poly(alkyl/arylphosphazenes). The radius of gyration < S > ' for the highest molecular weight sample was found t o be 2 4 9 A which is consistent with the pojymer having a randomly coiled configuration in solu­ tion. Indeed, the calculated ratio < S * > / / M (ca.O.31k m o l / g ) is essentially identical t o the value obtained for high molecular weight poly(dichlorophosphazene). (18) Also, a good correlation between M (light scattering) and intrinsic viscosity is ob­ served for [ P h ( M e ) P N ] over the molecular weight range of the samples studied. Thus, the Mark-Houwink relationship, [η] = K{M f, yields values of Κ = 1.44 χ 1 0 " (with [η] in d L / g ) and a = O.66. These data again indicate a well solvated, extended-chain structure of the polymer in THF. In D S C experiments, the symmetrically substituted poly(dialkylphosphazenes) 1 and 2 show sharp endothermic melt transitions [4.0 and 7.7 kJ/(mole of repeat unit), respectively], indicating a fairly high degree of crystallinity. On the other hand, none of the phenyl substituted polymers (3 and 4) and copolymers (5 and 6) show a melt transition; therefore, the side group asymmetry disrupts the crystalline order in the solid state. Poly(dimethylphosphazene) (1) shows a glass transition of - 4 6 ° C which increases t o - 3 ° C and 3 7 ° C for the 2 5 % (5) and 50% (3) phenyl substituted analogues. A similar trend is noted for the phenyl/ethyl derivatives (4 and 6), although poly(diethylphosphazene) itself does not exhibit a discernible glass transition. n

n

w

w

2

Z

1

2

Z

1

2

W

w

n

4

Thermogravimetric analysis (TGA) of these poly(phosphazenes) shows their d e c o m ­ position onset temperatures in an inert atmosphere t o be ca. 3 5 0 t o 4 0 0 ° C , depending o n the side group. These temperatures are ca. 2 5 - 7 5 ° C higher than that reported for commer­ cial materials based o n the fluoroalkoxy substituted polymer, [ ( C F Q C H 0 ) ^ P N ] . (19 ) Inter­ estingly, methyl rather than phenyl side groups yield the more stable materials, as shown by 2

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

22. NEILSON ET AL.

287

Polyfalkyl/arylphosphazenes)

the results for polymers 1, 3, and 5. We have also recently reported that sllylmethyl groups (i.e., MeoSiChL) have a significant stabilizing influence (20). Samples of the poly(dialkylphosphazenes) 1 and 2 displayed X-ray powder diffraction patterns characteristic of crystalline regions in the materials. The peaks in the diffraction pattern of 1 were of lower amplitude and greater angular breadth than those of 2. These data indicate that poly(diethylphosphazene) (2) is highly crystalline while poly(dimethylphosphazene) (1 ) is more amorphous with smaller crystalline zones. This high degree of crystallinity is probably responsible for the insolubility of 2 as noted above. All of the phenyl substituted polymers 3-6 were found t o be quite amorphous in the X-ray diffraction studies, a result that is further evidence for an atactic structure of the poly(alkylphenylphosphazenes) 3 and 4 and for a random substitution pattern in the copolymers 5 and 6. Derivatization reactions. In addition t o changing the substituents in the primary precursor phosphines, ( M e S i ) N P R R \ t w o other approaches have been used t o prepare polymers with more complex substituents attached t o the backbone by direct P-C linkages. These are (a) a l t e r a t i o n of R a n d R' in t h e i m m e d i a t e N - s i l y l p h o s p h o r a n i m i n e p r e c u r s o r s , M e S i N = P(OCHpCF2)RR\ and (b) alteration of R and R' in the preformed poly(alkyl/aryl phosphazenes), p R ' P = N ] . Som in other papers ( 2 1 - 2 3 ) , whil poly(alkyl/arylphosphazenes) has been recently reported. (20) The basic reaction which is employed in the derivatization of the phosphoranimine monomers is the deprotonation of 11 with n-BuLi followed by quenching with various electrophiles (eq 4). 3

2

3

n

^

(1) n-BuLi

Me SiN=ÇOCH CF 3

2

Me

R >

3

Me SiN=POCH CF 3

(2) EX

2

(4)

3

CH E 2

11: R = Me or Ph

12: Ε = a l k y l , a l l y l , /

SiMe R , PPh 2

2

This process has also been extended t o include the Peterson olefination reaction (eq 5) of 12 (where Ε = Me Si) to give a series of substituted vinyl derivatives 13. Both 12 and 13 have typically been obtained in high yield and have been fully characterized by NMR spectroscopy and elemental analysis. 3

Me

(1) n-BuLi

Me SiN=i>OCH CF 2

3

£H SiMe 2

>

3

3

Me

(2) R C=0 2

Me SiN=POCH CF 2

3

(5)

3

HC N

(3) Me SiC1 3

CR

2

13 R C(0) = Me C(0), PhC(0)Me, PhC(0)H, PhC(0)CF , 2

2

3

PhC(0)CH=CH , e t c . 2

A number of copolymers 14 have been prepared by cothermolysis of the new deriva­ tives, 12 and 13, with the simplest phosphoranimine precursors, M e S i N = P(OCH CF )(Me)R (11). The copolymers 14 derived from the Peterson olefination products 13 are soluble, non-crosslinked materials with molecular weights in the 30,000 -100,000 range. This implies 3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2

3

INORGANIC AND ORGANOMETALLIC POLYMERS

288 R

R R = Me or Ph Ε' = CH PPh , 2

2

CH=CHR'

(R' = Me, Ph)

14 that these systems might allow for subsequent crosslinking via a "curing" process. Finally, the series of copolymers bearing pendant diphenylphosphine ligands have reasonably highmolecular weights when the ratio x:y is ca. 5:1 and the phosphines readily coordinate t o F e ( C O ) upon treatment with F e ( C O ) . In a d d i t i o n t o p r o v i d i n g m a n y n e w p r e c u r s o r s t o f u n c t i o n a l i z e d p o l y ( a l k y l / a r y l p h o s p h a z e n e s ) , t h e d e p r o t o n a t i o n / s u b s t i t u t i o n r e a c t i o n s of t h e s e Nsilylphosphoranimines serve as useful models for similar chemistry that can be carried out on the preformed polymers. New reactions and experimentation with reaction conditions can first be tried with monomers before being applied t o the more difficult t o prepare polymeric substrates. A considerable amount of preliminary work [e.g. with the silylated monomers (2 2 ) and polymers ( approach. 4

2

g

The polymerization process. The successful synthesis of poly(alkyi/arylphosphazenes) by the condensation polymerization process leads t o a number of questions concerning the mechanism of this reaction (eq 1). It was initially assumed that a step growth mechanism, as is usually the case for condensation reactions, was operative since the polydispersity ( M / M ) values of the polymers are close t o the theoretical limit of 2.0 expected for a step growth process. The possibility of a step growth mechanism is precluded, however, by the results of experiments in which the polymerization is stopped prior t o completion. Even when the reaction is ca. 20% complete, the contents of the polymerization ampoule are found t o be fairly high molecular weight ( M ^ 62,000) polymer and unreacted monomer. No indication of the presence of linear or cyclic oligomers is found in the NMR spectroscopic and SEC studies of these reaction mixtures. Such data clearly indicate that some type of a chain growth mechanism must be occurring. While our studies of the nature of the polymerization process have not yet fully elucidated the mechanism, they have produced a very significant practical improvement. In a study of the effect of varying the leaving group on phosphorus, a series of alkoxy and aryloxy substituted phosphoranimines were prepared in high yield by treatment of the Pbromophosphoranimine, M e S i N = PR(R')Br, with the appropriate alcohol or phenol in the presence of EtgN. The alkoxy derivatives are considerably more stable than the trifluoroethoxy analogs and d o not yield poly(phosphazenes) upon thermolysis. On the other w

n

w

3

hand, the thermal decomposition of the phenoxy substituted monomers is an efficient, high yield, and inexpensive new preparative route to the poly(alkyl/arylphosphazenes).

Conclusion The condensation polymerization of N-silyl-P-trifluoroethoxyphosphoranimines is a useful synthetic route t o a new class of polymers, i.e., the poly(alkyl/arylphosphazenes). In our ef­ forts t o expand the scope and potential applications of this methodology, studies are con­ tinuing in a variety of areas including: (1) synthesis of new monomers containing reactive functional groups [e.g., ( C H ) C H = C H , C H P P h , ChLBr, and S i M e R ] ; (2) thermolysis reac­ tions of many of these functionalized monomers; (3) cierivatization reactions of preformed polymers by (a) deprotonation/substitution of P-Me groups, (b) electrophilic substitution on the P-Ph groups, (c) acid coordination t o the backbone nitrogens; and (4) studies of the ef­ fects of leaving group variation in the alkoxy and aryloxy substituted phosphoranimines. 2

n

2

2

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

22. NEILSON ET AL.

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289

Acknowledgments The authors thank the U.S. Army Research Office for financial support of this research at TCU and SMU. The light scattering and the thermal analysis experiments were conducted, respectively, by Dr. G.L. Hagnauer (Army Materials Technology Laboratory, Watertown, MA) and Dr. J.J. Meister (SMU).

Literature Cited 1. (a) Allcock, H.R. Chem. Eng. News 1985, 63(11), 22. (b) Allcock, H.R. Angew. Chem., Int. Ed. Engl. 1977, 16, 147. 2. Singler, R.E.; Hagnauer, G.L.; Sicka, R.W. ACS Symposium Series 1984, 260, 143. 3. Allcock, H.R. ACS Symposium Series 1983, 232, 49. 4. Blonsky, P.M.; Shriver, D.F.; Austin, P.; Allcock, H.R. J. Am. Chem. Soc. 1984, 106, 6854. 5. Allcock, H.R. Acc. Chem. Res. 1979, 12, 351. 6. Allcock, H.R.; Patterson, D.B 7. Allcock, H.R.; Evans, T.L.; Patterson 8. Allcock, H.R.; Desorcie, J.L.; Harris, P.J. J. Am. Chem. Soc. 1983, 105, 2814. 9. Allcock, H.R.; Chu, C.T.-U. Macromolecules 1979, 12, 551. 10. Evans, T.L.; Patterson, D.B.; Suszko, P.R.; Allcock, H.R. Macromolecules 1981, 14, 218. 11. Wisian-Neilson, P.; Neilson, R.H. J. Am. Chem. Soc. 1980, 102, 2848. 12. Neilson, R.H.; Wisian-Neilson, P. J. Macromol. Sci.-Chem. 1981, A16, 425. 13. Wisian-Neilson, P.; Roy, A.K.; Xie, Z.-M.; Neilson, R.H. ACS Symposium Series 1983, 232, 167. 14. Neilson, R.H.; Wisian-Neilson, P. Inorg. Chem. 1982, 21, 3568. 15. Wisian-Neilson, P.; Neilson, R.H. Inorg. Chem. 1980, 19, 1875. 16. Wisian-Neilson, P.; Neilson, R.H. Inorg. Synth., in press. 17. Neilson, R.H.; Hani, R.; Wisian-Neilson, P.; Meister, J.J.; Roy, A.K.; Hagnauer, G.L. Macromolecules 1987, 20, 910. 18. Hagnauer, G.L. ACS Symposium Series 1980, 138, 239. 19. Critchley, J.P.; Knight, G.J.; Wright, W.W. "Heat Resistant Polymers"; Plenum Press: New York, 1983; Chapter 8, and references cited therein. 20. Wisian-Neilson, P.; Ford, R.R.; Neilson, R.H.; Roy, A.K. Macromolecules 1986, 19, 2089. 2 1 . Roy, A.K.; Hani, R.; Neilson, R.H.; Wisian-Neilson, P. Organometallics 1987, 6, 378. 22. Wettermark, U.G.; Wisian-Neilson, P.; Scheide, G.M.; Neilson, R.H. Organometallics 1987, 6, 959. 23. Roy, A.K.; Wettermark, U.G.; Wisian-Neilson, P.; Neilson, R.H. Phosphorus and Sulfur, in press. RECEIVED

September

1,

1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 23

Hybrid Inorganic-Organic Polymers Derived from Organofunctional Phosphazenes Christopher W. Allen Department of Chemistry, University of Vermont, Burlington, V T 05405 Carbon chain polymer systems can be prepared by homo- or copolymerization reactions of cyclophosphazenes with o l e f i n i c exocyclic groups. The resulting polymers and copo­ lymers exhibit flame retardancy and have a large number of reactive s i t e s for further synthetic transformations. In alkenylpentafluorocyclotriphosphazenes the r e a c t i v i t y of the o l e f i n i c center may be mediated by placement of an electron donating group on the o l e f i n or by placing an insu­ l a t i n g function between the o l e f i n and the phosphazene. A broad range of homo- and copolymers have been prepared from vinyloxyphosphazenes. These materials undergo f a c i l e thermal decomposition. The v o l a t i l e products of these processes have been i d e n t i f i e d and some insight has been gained into the k i n e t i c s and mechanism of the polymer ther­ molysis reactions.

The primary approach t o the development o f main group inorganic polymer chemistry has been i n the preparation, c h a r a c t e r i z a t i o n and u t i l i z a t i o n o f polymers with an inorganic backbone which may, or may not, be protected by organic substituents ( l a ) . Important members of t h i s c l a s s of materials which are discussed i n t h i s volume and elsewhere includes R CAB),η

la.

(CHCH ) 2

n

lb.

R

R A B t R S i R S i ; SS; R ^ i O ; R P N ; SN 2

2

2

2

χ = 2-4

0097-6156/88/0360-0290$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

23.

ALLEN

Hybrid Inorganic-Organic Polymers

291

poly(siloxanes), poly(phosphazenes) and more recently poly( s i l a n e s ) , poly(carbosilanes) and p o l y ( s i l a z a n e s ) . Our approach has been to invert the r o l e s of the organic and inorganic e n t i t i e s thereby producing polymers with carbon chain backbones and inorganic substituents ( l b ) . V i n y l polymers with t r a n s i t i o n metal substituents have been studied extensively.with most i n t e r e s t being shown i n metallocene d e r i v a t i v e s . Inorganic main group e n t i t i e s as substituents on carbon chain polymers have received sporatic study with the most i n t e r e s t being shown i n a c y c l i c substituents involving organosilanes. V i n y l carboranes can also be polymerized and copolymerized to polymeric species having the boron c l u s t e r as a substituent. Our primary focus has been on the use of organofunçtional cyclophosphazenes as precursors to the desired polymers. We have conducted a systematic study of the e f f e c t of v a r i a t i o n of both the phosphazene and o l e f i n i c components within the general structure of an organofunçt i o n a l phosphazene (2) These studies have allowed for both an understanding of the polymerizatio of monomers and the incorporatio p e r t i e s , such as flame retardency, of the phosphazenes i n t o t r a d i t i o n a l organic polymers. 9

R X

E-C=CH

X=C1,F R=Me,0Et,0Me Y=N,NPX2N

9

\ / .P v

/

X

V

P

2 \^

^>

E =

P X

C

H

°' 6 4

2

Y ALKENYL PHOSPHAZENES We i n t i a l l y demonstrated that our basic approach was f e a s i b l e by preparing copolymers (3$ R=Me) of 2-(propenyl)pentafluorocyclotriphosphazene (4) and styrene or related monomers. ' The copolymers, which have good s o l u b i l i t y and thermal s t a b i l i t y , do not burn or produce smoke when exposed to a flame. Nucleop h i l i c s u b s t i t u t i o n reactions of the types which are well known i n phosphazene chemistry can be c a r r i e d out on the phosphazene moiety i n these copolymers thus allowing for the preparation of a broad spectrum of related materials. However, the strong electron withdrawing nature of the N P F c unit transforms the propenyl group into a highly polar o l e f i n which undergoes comp e t i t i v e anionic addition i n the course of i t s preparation and serves as a termination s i t e i n the copolymerization r e a c t i o n . 6

3

3

6

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

292

Ph

3

The electron d e f i c i e n two ways either by placement of an electron donating substituent on the o l e f i n or by i n s u l a t i n g the o l e f i n from the phosphazene ring by other groups. The introduction of an alkoxy function as the electron donating group on the o l e f i n leads to 2-(l-ethoxy vinyl)pentafluorocvclotriphosphazene, N3P3F5C(0Et)=CH2 ( 5 ) . A comparison of the nmr chemical s h i f t s of the o l e f i n i c βcarbon i n 4 and 5 gives evidence for a s i g n i f i c a n t reduction i n the o l e f i n p o l a r i t y by addition of the electron donating ethoxy function. The change i n o l e f i n p o l a r i t y i s also demonstrasted by the absence of anionic addition to the o l e f i n i c center i n 5. F a c i l e copolymerization of 5 with styrene or methylmethacrylate occurs to y i e l d materials (e.g. 3; R=0Et) with higher degrees of phosphazene incorporation but properties s i m i l a r to the copoly­ mers derived from 4. The use of an a r y l group as an i n s u l a t i n g f u n c t i o n ^ , 14 i exemplified i n α-methylstyrylphosphazenes such as the meta and para isomers of N3P3F5C£H4C(CH3)=CH2 (6) which also have been induced to undergo copolymerization with styrene and methylmethacrylate. These l a t e r compounds have the highest degree of phosphazene incorporation into copolymers which we have yet observed. Reactivity r a t i o and Q,e c a l c u l a t i o n s show t h a t , i n general, ( i . e . , i n 4,5,6) the phosphazene moiety e x h i b i t s a very strong electron withdrawing e f f e c t without any s i g n i f i c a n t mesomeric i n t e r a c t i o n . Thus, the model of the e l e c t r o n i c s t r u c ­ ture of the o l e f i n i c center i n alkenylphosphazenes which a r i s e s from quantitative studies of r e a c t i v i t y i n copolymerization reac­ t i o n s i s very s i m i l a r to that deduced from spectroscopic studies of arylphosphazenes. Further examination of these r e a c t i v i t y and Q,e data allow for a c l e a r understanding of the polymerization behavior and how i t i s modified by the phosphazene. Although most systems are best described by the terminal model, the r e a c t i v i t y patterns exhibited i n the copolymerization of ot-methylstyryl pentafluorocyclophosphazene (6) with methylmethacrylate can only by q u a n t i t a t i v e l y f i t by a pennultimate model.1* 1 1

s

9

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

23. ALLEN

293

Hybrid Inorganic-Organic Polymers

In order to obtain homopolymers from the organofunctional phosphazenes, monosubstituted rather than 1,1-disubstituted o l e ­ f i n s of the type described above, are required. The styrene d e r i v a t i v e , N ^ F ^ H J ^ C h L , was prepared from the phenylmethyl ether d e r i v a t i v e , N P C H.CrOMe)=CH , v i a hydrostannation and subsequent B-elimination of the t r i a l k y l t i n methoxide. Polymerization of the s t y r y l phosphazene lead to a high molecular weight polymer with pentafluorocyclotriphosphazene units at the para p o s i t i o n of each phenyl group (7; E=C H^, X=F). The TGA of t h i s material shows a s i g n i f i c a n t retention of i n v o l a t i l e material to over 1000°, suggesting the i n t r i g u i n g p o s s i b i l i t y 3

3

6

2

6

(CHCH 3 2

Ε

n

Χ /

\ Ν

Ν

ι

II ΧοΡ

^ ΡΧ

9

Ν of using the p y r o l y s i s of polymers of type l b as a new route to ceramic s o l i d s . VINYLOXYPHOSPHAZENES In an attempt to generate convenient routes to organofunctional phosphazenes which could undergo homopolymerization reactions, we explored the reactions of l i t h i u m enolates, p a r t i c u l a r l y that of acetaldehyde, with halophosphazenes which leads to the series of organofunctional monomers_N3P3Cl6-ni0CH=ÇH2)n i (n=l-6), N P F „ (0CH=CH2Jn(n=l-5) and N ^ C l s . n T o C H ^ Î n (n=l,2). In these systems, an oxygen atom acts as the insul a t i n g function between the phosphazene and the o l e f i n . Other, related monomers, can be prepared by nucleophilic s u b s t i t u t i o n reactions with, for example alkoxide ions, of the chlorine atoms i n N P C1 0CH=CH (8). The appropriate monosubstituted derivat i v e (n=lj i n many cases has been polymerized by r a d i c a l i n i t i a t i o n to y i e l d the l i n e a r high polymer with a cyclophospbazene moiety as part of each repeating u n i t , e.g. 7$ E=0, X=C1. However, not a l l of the vinyloxyphosphazene monomers w i l l undergo r a d i c a l polymerization; those with amino substituents are unreact i v e . The C nmr data indicate that these species e l e c t r o n i c a l l y resemble v i n y l ethers (which do not undergo r a d i c a l polymerizat i o n ) whereas the reactive derivatives resemble v i n y l acetate. These data demonstrate an excellent example of e l e c t r o n i c effect transmission i n cyclophosphazene systems. The large number of reactive s i t e s per monomer unit i n these highly functionalized polymers has allowed for further synthetic 15,

ï7

3

3

5

n

18

3

3

5

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

6

294

INORGANIC AND ORGANOMETALLIC POLYMERS

transformations based on the chemistry of the phosphazene unit thereby allowing preparation of the amino substituted polymers by reaction of 7 (E=0, X=C1) with the appropriate amine. The broad range of vinyloxyphosphazene monomers which are a v a i l a b l e allow for the formation of copolymers of two d i f f e r e n t organofunctional phosphazene monomers. Consequently, we can apply polymerization r e a c t i v i t y studies to d i r e c t l y explore differences i n chemical behavior based on changes i n substituent, ring s i z e , etc. of the inorganic monomer. We have prepared a series of copolymers of 8 and i t s tetrameric analog, N4P4Cl70CH=CH2 and shown that 8 has the greater r e a c t i v i t y i n t h i s system. In a j o i n t study with Dr. van de Grampel of the University of Groningen, we have explored the copolymerization of 8 with a vinyloxycyclophospha(thia)zene. As opposed to the alkenylphosphazene polymers, the vinyloxychlorophosphazene polymers are thermally l a b i l e undergoing an exothermic decomposition v i a HC1 e l i m i n a t i o n as low as 100° followed by a more comple peratures. The f i r s t ste reaction with low weight l o s s . The s o l i d state k i n e t i c s of t h i s f i r s t step have been measured. The a c t i v a t i o n energy for the f i r s t step i s greater for the tetramer than for the trimer d e r i vative. Since t h i s i s the reverse of the behavior observed f o r displacement reactions of the (NPCl2)3 and (NPCl2)4 , i t implies that the b a r r i e r i s p r i m a r i l y associated with the chain reorganization energy needed to bring the phosphorus-chlorine bond i n proximity with a carbon-hydrogen group. After the exothermic c r o s s - l i n k i n g step, there i s a second process which appears to r e s u l t i n the elimination of oxobridged phosphazene dimers, e.g. (N3P3Cl5)2Û and, by i m p l i c a t i o n , to b u i l d an oxo c r o s s - l i n k between the polymer chains. Copolymerization with organic monomers suggest a s i m i l a r i t y of behavior between the vinyloxyphosphazene and v i n y l a c e t a t e . The thermal l a b i l i t y of the polymers cont a i n i n g the vinyloxyphosphazene i s even more pronounced i n the copolymers. In order to overcome some of these problems a larger i n s u l a t i n g function was sought. The reactions of 2-hydroxyethyl methylmethacrylate with N^P^Clr leads to N3P3CI5OCH2CH2OC(0)C(CH3) = CH2 which undergoes r a d i c a l or thermal homo and copolymerization which i f not c a r e f u l l y c o n t r o l l e d i s accompanied by extensive c r o s s - l i n k i n g . 20

8

EXTENSIONS TO OTHER SYSTEMS Our studies have convinced us that t h i s approach to hybrid organic-inorganic polymers i s a valuable one leading to a wide variety of new, i n t e r e s t i n g , materials. In addition to new phosphazenes, we are now expanding our studies to the preparation of pentamethylvinylcyclotriborazene polymers and we are i n v e s t i gating various s i l i c o n containing systems.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

23.

ALLEN

Hybrid Inorganic-Organic Polymers

295

ACKNOWLEDGMENTS This work was supported i n part by the Office of Naval Research; c o l l a b o r a t i v e e f f o r t s with Dr, van de Grampel are supported, i n part, by a NATO t r a v e l grant, LITERATURE CITED 1. Pittmann, J r . , C.U.; Rausch, M.D., Pure Appl. Chem., Pure Appl. Chem., 1986, 58, 617. 2. Dzhardimalieva, G.I.; Zhorin, V.A.; Ivkva, I.N. Pomogailo, A.D.; Enikopolyan, N.S., Dokl. Akad. Nauk SSSR, 1986, 287, 654. 3. Billingham, N.C.; Jenkins, A.D.; K r o n f l i , E.B.; Walton, D.R.M. J . Polym. S c i . , Polym. Chem. Ed., 1977, 15, 683; Lakshmana Rao, V.; Babu, G.N., J . Macromol. S c i . , Chem., 1983, A20, 527; Ledwith Α.; Chiellini, E.; Solaro R. Macromolecules, 1979 Vassi, I.W., Eur. Polym Semenov, O.B.; Yanovsky, Yu. G., Inter. J . Polym. Mater., 1980, 8, 225. 4. Wright, J.R.; Kingen, T.J., J . Inorg. Nucl. Chem. 1974, 36, 3667; Klingen, T.J., i b i d , 1980, 42, 1109. 5. Allen, C.W., J . Polym. S c i . , Polym. Symp., 1983, 70, 79. 6. Dupont, J.G., Allen, C.W., Macromolecules., 1979, 12, 169. 7. Allen, C.W.; Dupont, J.G., Ind. Eng. Chem. Prod. Res. Dev., 1979, No.18,80. 8. Allcock, H.R., Phosphorus-Nitrogen Compounds; Academic Press: New York, 1972; Allen, C.W., "Cyclophosphazenes and Related Compounds" i n The Chemistry of Inorganic Rings, Ed. I. Haiduc and D.B. Sowerby, Academic Press, i n press. 9. Allen. C.W.; White, A.J., Inorg. Chem., 1974, 13, 1220; Allen, C.W., J . Organometal. Chem., 1977, 125, 215; Allen, C.W.; Green, J.C., Inorg. Chem., 1980, 19, 1719. 10. Allen, C.W.; Bright, R.P.; Ramachandran, K., ACS Symp. Ser., Phosphorus Chemistry, 1981, 171, 321. 11. Allen, C.W.; Bright, R.P., Inorg. Chem. 1983, 22, 1291. 12. Allen. C.W.; Birght, R.P., Macromolecules, 1986, 19, 571. 13. Shaw, J.C.; Allen, C.W., Inorg. Chem., 1986, 25, 4632. 14. Allen, C.W.; Shaw, J.C., Phosphorus and Sulfur, 1987, 30, 97. 15. Allen, C.W.; Ramachandran, K.; Bright, R.P.; Shaw, J.C., Inorg. Chim. Acta, 1982 64, L109. 16. Ramachandran, K.; A l l e n , C.W., Inorg. Chem. 1983, 22, 1445. 17. Allen, C.W.; Bright, R.P., Inorg. Chim. Acta 1985, 99, 107. 18. Brown, D.E.; Allen, C.W., Inorg. Chem., 1987, 26, 934. 19. Allen, C.W.; Ramachandran, K.; Brown, D.E., Inorg. Syn., i n press. 20. van de Grampel, J.C.; Jekel, A.P.; Dhathathreyan, K.; Allen, C.W.; Brown, D.E., Abstract 319, 193rd American Chemical Society Meeting, Denver, A p r i l , 1987. RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 24

Polybis(pyrrolyl)phosphazene R. C. Haddon , S. V. Chichester , and T. N. Bowmer 1

1

2

AT&T Bell Laboratories, Murray Hill, NJ 07974-2070 Bell Communications Research, Inc., Red Bank, NJ 07701-7020 1

2

Current progress in the synthesis and properties of pyrrolylphosphazenes is summarized. The differences in reactivity of the cyclic trimer (NPCl ) , and high polymer (NPCl ) , toward the pyrrolide nucleophile are discussed. Efforts to induce electronic conductivity in the polyphosphazenes are reviewed with particular emphasis on polybis(pyrrolyl)phosphazene. 2 3

2 x

It is clear from the contents of this volume that polyphosphazene chemistry is currently receiving a great deal of attention. Commercialization of these materials is still in its infancy but appears to hold significant potential. Much of the current level of interest in this field stems from the synthetic versatility of the phosphazene polymers, as developed by Allcock and coworkers. " As a result of this, the preparative chemistry of the polyphosphazenes is the most highly developed of all the inorganic polymer systems. This has lead to the synthesis of a wide variety of substituted polyphosphazenes, " many of which were specifically designed for a given chemical, mechanical or biological property. Noticeably absent from the properties exhibited by the polyphosphazenes has been electronic conductivity and in general the materials are insulators. It should be noted, however, that suitably functionalized polyphosphazenes have recently been shown to serve as electrolytes for ionic conductivity. 1

2 4

4

2 5

4

6

The question of electronic conductivity in the polyphosphazenes inevitably raises questions regarding the electronic structure of the phosphazene linkage. " This matter has been the subject of controversy in the literature, but experimentally the situation is now well known. In spite of the fact that the phosphazene backbone is fully conjugated, bond equalized and possesses bond lengths which are indicative of partial double bond character, the evidence suggests that these are localized systems. 7

12

4,13

0097-6156/88/0360-0296S06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

24. HADDON ET AL.

Poly bis (pyrrolyl)phosphazene

297

The possibility of inducing electronic conductivity within the P-N linkage is being actively pursued within a number of research groups, ' but the phosphazenes themselves present a daunting challenge (in spite of their proximity to polysulfurnitride in the Periodic Table). 4,11 14,15

An alternative approach to electronic conductivity in the polyphosphazenes, utilizes the 'outrigger approach' in which the appropriate functionality for electronic mobility is provided by the substituents. A successful implementation of this methology which made use of polyphosphazene bound copper phthalocyanine units was recently reported by Allcock and Neenan. Iodine doping of these materials led to conductivities in the semiconductor range. It is also possible to prepare semiconducting tetracyanoquinodimethane (TCNQ) salts, in which quaternized polyphosphazenes serve as counterions. In a variant of the outrigger approach we have synthesized polybis (pyrrolyl) phosphazene (PBPP); this new polymer undergoes electrochemical oxidation t 4

16

16

17

18

Apart from the ability of PBPP to undergo electroxidation, this polymer is significantly different from previously reported polyphosphazenes, both in preparation and in properties. Although a complete picture of the chemistry of this polymer has yet to emerge, our current progress will be reviewed in the present article. Preparation of Pyrrolylphosphazenes The first pyrrolylphosphazenes were apparently prepared by McBee and coworkers in 1960, and although the work was never documented in the chemical literature, hexakis- (pyrrolyl) cyclotriphosphazene (1) and octakis(pyrrolyl)cyclotetraphosphazene were described in a Technical Report of the Defense Technical Information Center. Compound 1 was reported to be produced in 26% yield from the interaction of hexachlorocyclotriphosphazene [(NPC^ ) ] with excess potassium pyrrolide in refluxing benzene over a 24 hour period. Lithium pyrrolide and pyrrolyl magnesium bromide were found to be unsatisfactory reagents for the preparation of 1. 19

2

3

The yield of the potassium pyrrolide reaction is considerably improved by the use of a phase transfer catalyst. However, in exploratory work on this substitution reaction we have found that sodium pyrrolide reacts with (NPC^ ) in tetrahydrofuran at room temperature to produce 1 in quantitative yield in less than 3 hours. The structure of the compound is reproduced in Figure 1. 20

2

3

20

In general, the small molecule substitution chemistry provides an excellent model for the analogous high-polymer synthesis. However, in attempting to extrapolate from the reaction of pyrrolyl salts with (NPC£ ) to the corresponding reaction with the high polymer (NPC^ ) , we discovered significant discrepancies in the two reaction pathways. High yields of 1 may be isolated from the reaction of (NPC/ ) with all of the alkali metal pyrrolide salts which were tested, except lithium. In the case of the polymer (NPC^ ) , 4,13

2

2

3

X

21

2

3

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

298

however, we found that some of the alkali metal pyrrolides brought about substantial chain cleavage and occasionally produced the cyclic trimer (1) in high yield. Furthermore there is apparently a facile cross-linking reaction available to PBPP, as a substantial fraction of the isolated polymer is often insoluble (although chemical analyses indicate complete replacement of chloride). There is little mention in the literature of the use of amide salts in substitution reactions on chlorophosphazene precursors. The anilide anion was shown to be a powerful nucleophile in substitution reactions on various trimer derivatives, but investigations of such reactions with the high polymer have not been reported. Where strong nucleophiles (such as amide salts) with low steric requirements are employed, the usual pentacoordinate transition state (Scheme 1), may be a viable reaction intermediate which can undergo alternative modes of decomposition, perhaps involving chain cleavage and/or cross-linking. 22

I

: pI

+

2X

ο ο

ο

ι

+

2x K C «

M)

sl = P —

ό (PBPP)

Nevertheless, through scrupulous purification of the reaction components and rigorous control of the reaction conditions it is possible to isolate the polymer in a state of good purity, by the reaction of potassium pyrrolide with (NPC/ ) in tetrahydrofuran at room temperature (Equation 1). Addition of water to the reaction mixture precipitates the polymer as a white rubbery solid which hardens on drying. A P N M R spectrum of a typical reaction product is given in Figure 2. 2

X

31

Properties of Poly bis (pyrroly I)phosphazene (PBPP) The solubility properties of the polymer are very sensitive to the mode of isolation. PBPP is partially soluble in tetrahydrofuran immediately after precipitation from the reaction mixture, but when the polymer has been dried the solubility is less predictable. 21

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

24. HADDON ET AL.

Figure 1.

Poly bis (pyrrolyl)phosphazene

Structure of crystalline [ Ν Ρ Ο Κ ^ Η ^ ^ (1)

299

20

Scheme 1.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

300

31

NP Ν

P

NP Ν

-30 8 (PPM, EXT H P 0 ) 3

Figure 2.

31

4

P NMR spectrum of reaction to produce PBPP.

Brittle colorless films of PBPP may be cast from tetrahydrofuran solution. The insoluble portion of PBPP is swelled by the tetrahydrofuran and gives rise to free-standing films on solvent evaporation. Differential scanning calorimetry experiments on PBPP show a glass transition temperature at 40 °C, and some indication of a melting transition at 170°C. PBPP undergoes electrochemical oxidation to produce black films with conductivities in the semiconductor range. More highly conducting materials are produced by heating PBPP in an inert atmosphere (without doping). As may be seen from the thermogravimetric analysis in Fig. 3, PBPP exhibits some resistance toward the thermal depolymerization process which is characteristic of many polyphosphazenes. The conductivity and thermal involatility probably arise from the tendency of PBPP to undergo cross-linking as exemplified in Scheme 2. The structure found for the trimer (Fig. l ) , suggests that these should be viable processes in the polymer. 17

21

13

20

20

Future Prospects Although the picture is far from complete, the available evidence suggests that PBPP is rather different from most polyphosphazenes. The polymer may be induced to be an electronic conductor, but perhaps as a result of this tendency to cross-link, the material is more sensitive and difficult to handle than most polyphosphazenes and the thermal depolymerization reaction is inhibited. Nevertheless the synthetic versatility offered by the polyphosphazenes

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

24. HADDON ET AL.

Polybis(pyrrolyl)phosphazene

Scheme 2.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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302

suggests that PBPP and the phthalocyanine derivatives represent the forerunners of a new class of polymers with interesting electronic solid state properties. 18

LITERATURE

CITED

1. 2. 3. 4. 5.

Penton, H. R. Polym. Prepr. 1987, 28, 439. Allcock, H. R.; Kugel, R. L.; Valan, K. J. Inorg. Chem. 1966, 5, 1709. Allcock, H. R.; Kugel, R. L. Inorg. Chem. 1966, 5, 1716. Allcock, H. R. Chem. Eng. News 1985, 63 (11), 22. Neilson, R. H.; Ford, R. R.; Hani, R.; Roy, A. K.; Scheide, G. M.; Wettermark, U. G.; Wisian-Neilson, P. Polym. Prepr. 1987, 28, 442. 6. Blonsky, P. M.; Shriver, D. F.; Austin, P.; Allcock, H. R. J. Am. Chem. Soc. 1984, 106, 6854. 7. Craig, D. P.; Paddock, J. P., Ed.; Academic Press, New York, 1971, Vol. 2. 8. Dewar, M. J. S.; Lucken, E. A.C.;Whitehead, M. A. J. Chem. Soc. 1960, 2423. 9. Salem, L. "Molecular Orbital Theory of Conjugated Systems"; Benjamin: New York, 1966; p. 158. 10. Haddon, R. C. Chem. Phys. Lett. 1985, 120, 310. 11. Haddon, R.C.;Mayo, S. L.; Chichester, S. V.; Marshall, J. H. J. Am. Chem. Soc. 1985, 107, 7585. 12. Ferris, K. F.; Friedman, P.; Friedrich, D. M., ACS Meeting, Denver, 1987. 13. Allcock, H. R. "Phosphorus-Nitrogen Compounds", Academic Press, New York, 1972. 14. Wisian-Neilson, P.; Roy, A. K.; Xie, Z.-M.; Neilson, R. H. ACS Symposium Series 1983, 232, 167. 15. Barendt, J. M.; Haltiwanger, R.C.;Norman, A. D. Inorg. Chem. 1986, 25, 4323: Thompson, M. L.; Tarassoli, Α.; Haltiwanger, R.C.;Norman, A. D. Inorg. Chem. 1987, 26, 684. 16. Allcock, H. R.; Neenan, T. X. Macromol. 1986, 19, 1495. 17. Allcock, H. R.; Levin, M. L.; Austin, P. E. Inorg. Chem. 1986, 25, 2281. 18. Haddon, R.C.;Stein, S. M.; Chichester-Hicks, S. V.; Marshall, J. H.; Kaplan, M. L.; Hellman, M. Y. Mater. Res. Bull. 1987, 22, 117. 19. McBee, E. T.; Johncock, P.; French, S. E.; Braendlin, H. P. U.S. Govt. Res. Rep. AD 1960, 254985. 20. Craig, S. L.; Cordes, A. W.; Stein, S. M.; Chichester-Hicks, S. V.; Haddon, R. C. Acta Crystallogr., in press. 21. Haddon, R.C.;Chichester-Hicks, S. V.; Bowmer, T. N. to be published. 22. Allcock, H. R.; Smeltz, L. A. J. Am. Chem. Soc. 1976, 98, 4143. RECEIVED

September

2 2 , 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 25

Skeletal Stabilization as the Basis for Synthesis of Novel Phosph(III)azane Oligomers and Polymers Elizabeth G. Bent, Joseph M . Barendt, R. Curtis Haltiwanger, and Arlan D. Norman Department of Chemistry and Biochemistry University of Colorado

Reactions of PhPCl or P(NR ) (R = alkyl) with orthoaminated benzenes yield new phosph(III)azanes which contain extended P(III)-N skeletons stabilized by bridging ο-C Η groups. The novel [(PhP)(C H N PPh)] is formed from the PhPCl - 1,2-(NH ) C Η reaction; the dimer (n = 2) is fully characterized. From 1,2-(NH ) C Η or 1,2,3-(NH ) C H reactions with P(NR ) (R = Me, Et), phosphazanes are obtained which contain P N or P N skeletal units stabilized around the arene rings. The intermediates C H (NH) (PNEt ) and C H (NH)[N(PNEt ) ](PNEt ) in the 1,2-(NH ) C Η - P(NEt ) reaction have been characterized. The two-phosphorus compound displays 1,4- A-B condensation monomer reactivity. Preliminary studies of its oligomer/polymer chemistry are reported. 2

6

2

3

4

6

2

2

2

6

4

2

n

4

2

6

4

2

3

6

3

2

3

4

2

3

2

3

6

2

2

4

2

2

2

6

2

2

6

4

4

2

3

I n o r g a n i c polymers based on a l t e r n a t i n g main group e l e m e n t - n i t r o g e n s k e l e t o n s ( e . g . I - IV) are of i n t e r e s t f o r t h e i r p o t e n t i a l as elastomers, high-temperature o i l s , e l e c t r i c a l conductors, b i o l o g i c a l m o l e c u l e c a r r i e r s , and p r e c u r s o r s t o ceramic m a t e r i a l s (1_ - 6). \/ I - f - P=rN-4-η I

I I -f-P—Ν 4 η II

I I - ( - Β — Ν - ) -η III

, \ / I -f-Si—Ν-^-η N

IV

S i g n i f i c a n t r e c e n t advances have occured w i t h phosphazenes (J_) and t o a l e s s e r e x t e n t s i l i z a n e s ( 6 ) ; i n c o n t r a s t , polymers based on p h o s p h o r u s ( I I I ) and n i t r o g e n are v i r t u a l l y unknown. Because such systems o f f e r o p p o r t u n i t i e s f o r metal c o o r d i n a t i o n , and i n t h e i r o x i d i z e d forms c o u l d be v a l u a b l e polymer p r e c u r s o r s t o PON c e r a m i c s ( 2 ) , we have begun s y s t e m a t i c s t u d i e s of the s t r u c t u r a l and r e a c t i v i t y f a c t o r s necessary f o r t h e i r formation. Reactions that i n p r i n c i p l e could y i e l d l i n e a r phosph(III)azanes g e n e r a l l y do not 8 ) ; i n s t e a d they produce low m o l e c u l a r weight o l i g o m e r s . Among t h e s e , t h e four-membered r i n g 0097-6156/88/0360-0303$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

1 , 3 , 2 , 4 - d i a z a d i p h o s p h e t i d i n e s (V) are most common. Only w i t h s m a l l s u b s t i t u e n t s on phosphorus and n i t r o g e n (R and R = Me or E t ) have h i g h e r o l i g o m e r s ( e . g . V I ) been o b t a i n e d ( 9 ) . ?

μ

R R .P.

«< > P R V

%P

N

P

>" ~~ v RN PR

h

I

RP

N

H

P h

P h

Κ >

X

\

I

>N

.Ν

NR

V

Ν—Ρ

N

H

P

h

< > Ν

/

Ν Ph

Ph

VI

P h

VII

R e a c t i o n s of PX3 w i t h RNH2 c o u l d form c y c l i c - l i n e a r polymers based on connected P2N2 r i n g s , e.g. [ P 2 ( N R ) 3 l ; however, so f a r o n l y the d i n u c l e a r o l i g o m e r V I I has been o b t a i n e d ( 8 ) Apparently a c o m b i n a t i o n of s m a l l bon i n t e r g r o u p r e p u l s i o n s betwee 0_, 7.» 8.» 10) s t r o n g l y f a v o r s m a l l r i n g o l i g o m e r s and d i s f a v o r polymers. I n no case has P ( I I I ) - N oligomer-polymer i n t e r c o n v e r s i o n , analogous t o what o c c u r s w i t h phosphazenes and s i l a z a n e s (1^, 6 ) , been seen. Recause low o l i g o m e r s of l i n e a r p h o s p h ( I I I ) a z a n e s appear t h e r m o d y n a m i c a l l y p r e f e r r e d at a l l r e a s o n a b l e t e m p e r a t u r e s , we have undertaken a s y n t h e t i c s t r a t e g y i n v o l v i n g s k e l e t a l s t a b i l i z a t i o n as a t e c h n i q u e t o o b t a i n these polymers. We seek t o c o n s t r u c t p h o s p h o r u s - n i t r o g e n s k e l e t o n s which c o n t a i n s t a b i l i z i n g u n i t s between a d i a c e n t n i t r o g e n atoms, f o r example as i n V I I I , c r e a t i n g r i g i d extended u n i t s which w i l l not n

a l l o w s m a l l r i n g f o r m a t i o n but w i l l f a v o r m o l e c u l a r e x t e n s i o n based on r i g i d 1,4- r e a c t i v e u n i t s . Thus we have sought t o s y n t h e s i z e r e a c t i v e A-B type i n t e r m e d i a t e monomers ( I X ) or o l i g o m e r s / p o l y m e r s ( V I I I ) d i r e c t l y based on such systems. Our s t u d i e s so f a r are p r o t o t y p i c a l and have c o n c e n t r a t e d m a i n l y on r e a c t i o n s t r a t e g i e s ; however, they i n d i c a t e the approach has c o n s i d e r a b l e p o t e n t i a l . Amine-Phosphorus(III)

H a l i d e Condensation

Reactions

Since amine-phosphorus(III) h a l i d e condensation r e a c t i o n s are w e l l - e s t a b l i s h e d low-energy r o u t e s to p h o s p h o r u s - n i t r o g e n bonds, they p r o v i d e a l o g i c a l s t a r t i n g p o i n t f o r e x a m i n a t i o n of s k e l e t a l l y s t a b i l i z e d molecule s y n t h e s i s . F o r our i n i t i a l s t u d i e s , r e a c t a n t s t h a t are phenyl s u b s t i t u t e d as opposed t o a l k y l s u b s t i t u t e d

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

25.

BENT ET AL.

Novel Phosph(III)azane Oligomers and Polymers

on phosphorus were chosen, so as to maximize the p o s s i b i l i t y of obtaining c r y s t a l l i n e products. R e a c t i o n of P h P C l w i t h 1,2-(NH ) 2 6 i+ t o l u e n e i n the presence of E t N to remove hydrogen c h l o r i d e proceeds smoothly a t r e f l u x as: c

2

H

i

n

2

3

2n P h P C l

+ η 1,2-(NH ) C ^ J^ ^* 3

2

2

2

6

1/n

[(PhP)(C H N PPh)] 6

4

2

n

(1)

X The c h a r a c t e r of p r o d u c t X depends on r e a c t i o n d i l u t i o n . Oligomers (n _> 2)/polymers form; dimer f o r m a t i o n i s minimized at h i g h r e a c t a n t concentrations. The s t r u c t u r e of dimer XI (X, where η = 2 ) , o b t a i n e d by x - r a y s i n g l e c r y s t a l a n a l y s i s ( F i g u r e 1), shows t h a t the d e s i r e d s k e l e t a l s t a b i l i z a t i o n based upon 1,4- c o n d e n s a t i o n u n i t s has been a c h i e v e d . XI i s a c y c l o p h a n e type r i n g of a l t e r n a t i n g phosphoru atoms are b r i d g e d by o-CgH C symmetry of the m o l e c u l e i s m a i n t a i n e d i n s o l u t i o n a l s o , s i n c e t h e P(31) NMR spectrum e x h i b i t s two temperature independent t r i p l e t s ( J = 17.2 Hz) at 85.0 pom and 112.0 ppm. 2 v

In a d d i t i o n to b e i n g a s k e l e t a l l y s t a b i l i z e d m o l e c u l e , XI c o n t a i n s an u n u s u a l r i n g s t r u c t u r e . The r i n g has f o u r n i t r o g e n and two phosphorus atoms i n a p l a n e w i t h the two r e m a i n i n g phosphorus atoms above the r i n g , which a l o n g w i t h the o r i e n t e d phenyl groups forms a w e l l - d e f i n e d c a v i t y i n a boat shaped e i g h t membered r i n g . T h i s geometry i s undoubtedly imposed by the s k e l e t a l s t a b i l i z i n g unit. Other e i g h t membered p h o s p h o r u s ( I I I ) - n i t r o g e n r i n g s , (MePNMe)i* (9) and (PrNCH CH NP)i (11) are crown shaped c o n t a i n i n g a l l c h e m i c a l l y e q u i v a l e n t phosphorus atoms. I n X I , because the phosphorus atoms are of two s e t s and one s e t i s i n a h i g h l y p r o t e c t e d environment, the p o s s i b i l i t y of s e l e c t i v e , " c a v i t a n d , " c o o r d i n a t i o n at phosphorus s i t e s e x i s t s . D e t a i l e d c h a r a c t e r i z a t i o n of the o l i g o m e r / p o l y m e r p r o d u c t s (X) 2

2

X

+

XIII

from E q u a t i o n 1 are i n p r o g r e s s . The p r o d u c t s are v i s c o u s o i l s , s o l u b l e i n h y d r o c a r b o n s o l v e n t s , c o n s i s t e n t w i t h t h a t expected o f l i n e a r oligomer/polymers· They e x h i b i t broad a p p r o x i m a t e l y e q u a l a r e a P(31) resonances at 95 - 102 ppm and 114 - 120 ppm, c o n s i s t e n t w i t h phosphorus environments of b r i d g i n g and d i a z a p h o s p h o l e t y p e s . G i v e n the p h y s i c a l p r o p e r t i e s and the c o r r e l a t i o n of P(31) and H ( l ) NMR s p e c t r a l parameters between dimer XI and the h i g h e r o r d e r p r o d u c t s , we i n f e r X has the c y c l i c - l i n e a r s t r u c t u r a l p r o p e r t i e s c h a r a c t e r i s t i c of X I . S t u d i e s of the o l i g o m e r / p o l y m e r p r o d u c t s to

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

305

306

INORGANIC AND ORGANOMETALLIC POLYMERS

M

X

PTa0C2000>50-O.04-RTl 70

I

PCS) 3 8 d o g s

I

BGSJ 3 2 0 d o y a |

Ι

60 —

50

CO

g

Z.

40 30

ιο :

ti

O.1

O.2

O.3 O.4 s (1/nm)

O.5

O.6

Figure 11. Aging effect on the SAXS profile of a TEOS-PTMO material (PTMO MW=2000) region, the scattering intensity should increase as the electron density of the cluster increases with aging. On the other hand, the peak position and the tail region of the SAXS curve should remain relatively unaltered since the aging process as described would not change the correlation distance between the TEOS rich clusters (or conversely the PTMO rich regions). The aging data presented represent only the initial efforts to probe the time dependence observed in the TEOS-PTMO hybrid materials. Comprehensive studies are currently underway which will, it is hoped, provide the structural as well as mechanical data necessary to understand this phenomenon. Conclusions Hybrid materials with TEOS and triethoxysilane encapped PTMO have been successfully prepared by the sol-gel process. High transparency is observed for most samples, and many of the mechanical properties are considerably improved relative to the earlier reported TEOS-PDMS systems. From the dynamic mechanical spectroscopy, an increase of PTMO molecular weight from 650 to 2000 results in a decrease in both the modulus and the glass transition temperature of the final product. The SAXS results indicate that a correlation distance exists in the samples, and this distance increases as PTMO molecular weight increases. A cluster model is thus suggested to account for the experimental results. The TEOS content is changed from 50 to 70 wt% to study its effect on the final structure of the materials. Besides an increase of the stiffness and glass transition temperature, a speculated change in structure is observed at 70 wt% TEOS. This change is in line with that anticipated from simple theoretical consideration of composition. Titaniumisopropoxide, which is known to have a fast rate of condensation, has been successfully added to the TEOS-PTMO systems by employing a different reaction procedure. The resulting materials are yellow or orange in color but usually transparent. The modulus increases significantly with the addition of titanium, and the ultimate strength is improved with respect to the pure TEOS-PTMO hybrid systems. This is tentatively attributed to a catalytic effect on condensation caused by the titanium component, which results in a tighter network structure. Glass transition temperatures observed in dynamic mechanical spectra indicate that the phase separation is also enhanced for Ti containing systems.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

376

Aging is significant in both the pure and Ti containing TEOS-PTMO systems and, similar to other reported pure sol-gel glasses, it is due to further curing of the TEOS based species. This results in an increase of the modulus and a decrease of the elongation at break. However, no change is observed in the correlation length shown by SAXS. Therefore, a structural alteration via a change in the dispersion of the PTMO oligomers is not the cause of these observations. Further study on this subject is currently being undertaken. Acknowledgments The authors wish to thank Dr. James G. Carlson for providing the endcapped PTMO oligomers.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20. 21. 22. 23. 24. 25.

Dislich, H. Angew. Chem. Int. Ed. Engl. 1971, 10(6), 363. Yoldas, B. E. J. Mat. Sci. Mukherjee, S. P. J. Non-Cryst. Dislich, H. J. Non-Cryst. Solids 1983, 57, 371. Mackenzie, J. D. J. Non-Cryst. Solids 1982, 48, 1. Yamane, M.; Aso, S.; Okano, S.; Sakaino, T. J. Mat. Sci. 1979, 14, 607. Sakka. S; Kamiya, K. J. Non-Cryst. Solids 1980, 42, 403. Yoldas, B. E. J. Mat. Sci. 1979, 14, 1843. Yamane, M.; Inoue, S.; Yasumori, A. J. Non-Cryst. Solids 1984, 63, 13. Zerda, T. W.; Artaki, I.; Jonas, J. J. Non-cryst. Solids 1986, 81, 365. Strawbridge, I.; Craievich, A. F.; James, P. F. J. Non-cryst. Solids 1985, 72, 139. Schmidt, H. Mat. Res. Soc. Symp. Proc. 1984, 32, 327. Wilkes, G. L.; Orler, B.; Huang, H. Polym. Prep. 1985, 26(2), 300. Huang, H.; Orler, B.; Wilkes, G. L. Polym. Bull. 1985, 14(6), 557. Huang, H.; Glaser, R. H.; Wilkes, G. L. Polym. Prep. 1987, 28(1), 434. Huang, H.; Orler, B.; Wilkes, G. L. Macromolecules 1987, 20(6), Brinker, C. J.; Scherer, G. W.J.Non-Cryst. Solids 1985, 70, 301. Schaefer, D. W.; Keefer, K. D. Mat. Res. Soc. Symp. Proc. 1986, 73, 211. Parkhurst, C. S.; Doyle, L. Α.; Silverman, L. Α.; Singh, S.; Anderson, M. P.; McClurg, D.; Wnek, G. E.; Uhlmann, D. R. Mat. Res. Soc. Symp. Proc. 1986, 73, 769. Critchfield, F. E.; Koleske, J. V.; Magnus, G.; Dodd, J. L. J. Elastoplastics 1972, 4, 22. Hashimoto, T.; Shibayama, M.; Fujimura, M.; Kawai, H. Memoirs of the Faculty of Engineering, Kyoto University, 1981, 43. Wallace, S.; Hench, L. L. Mat. Res. Soc. Symp. Proc. 1984, 32, 47. Philipp, G.; Schmidt, H. J. Non-Cryst. Solids 1984, 63, 283. Enns, J. B.; Gillham, J. K. J. Appl. Polym. Sci. 1983, 28, 2567. Flory, P. J. Principles of Polymer Chemistry Cornell University Press, 1953.

RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 30

Precursors to Nonoxide Macromolecules and Ceramics C. K. Narula, R. T. Paine, and R. Schaeffer Department of Chemistry, University of New Mexico, Albuquerque,

There is great interest in developing molecular precursors for boron-nitrogen polymers and boron nitride solid state materials, and one general procedure is described in this report. Combinations of B-trichloroborazene and hexamethyldisilazane lead to formation of a gel which, upon thermolysis, gives hexagonal boron nitride. The BN has been characterized by infrared spectroscopy, x-ray powder diffraction and transmission electron microscopy. Non-oxide ceramic m a t e r i a l s are t y p i c a l l y prepared from simple s t a r t i n g reagents by using c l a s s i c a l e n e r g e t i c r e a c t i o n c o n d i t i o n s ( h i g h t e m p e r a t u r e , high p r e s s u r e , CVD, e t c . ) . Although these r o u t e s p r o v i d e u s e f u l m a t e r i a l s f o r many a p p l i c a t i o n s , t h e r e i s i n c r e a s i n g need f o r a l t e r n a t e low energy procedures which p e r m i t more f l e x i b l e p r o c e s s i n g of t h e ceramic e n d - p r o d u c t . One s y n t h e t i c r o u t e which has a t t r a c t e d r e c e n t a t t e n t i o n i n v o l v e s t h e combination of simple reagents i n t o an adduct c o n t a i n i n g a l l t h e elements of i n t e r e s t f o r the f i n a l s o l i d s t a t e product. F o l l o w i n g thermal o r photochemical a c t i v a t e d i n t r a m o l e c u l a r e l i m i n a t i o n - c o n d e n s a t i o n c h e m i s t r y , an o l i g o m e r i c or p o l y m e r i c ceramic p r e c u r s o r i s o b t a i n e d which can be e a s i l y handled and converted by p y r o l y s i s t o t h e d e s i r e d ceramic e n d - p r o d u c t . This concept has been s u c c e s s f u l l y a p p l i e d i n t h e p r o d u c t i o n of s i l i c o n c a r b i d e and s i l i c o n n i t r i d e (1). R e l a t i v e l y l i t t l e a t t e n t i o n has been g i v e n , on t h e o t h e r hand, t o t h e development of m o l e c u l a r p o l y m e r i c p r e c u r s o r s f o r Group 13-15 b i n a r y c e r a m i c s . The p o t e n t i a l importance of p o l y m e r i c p r e c u r s o r s i n t h i s area can be i l l u s t r a t e d by a b r i e f overview of s e l e c t e d c h e m i s t r y known t o p r o v i d e t e c h n o l o g i c a l l y i m p o r t a n t boron nitride. Boron n i t r i d e may be o b t a i n e d i n t h r e e p r i m a r y c r y s t a l l i n e modifications ( 2 ) : α , β, and χ . The most commonly encountered α form has a g r a p h i t i c s t r u c t u r e (hexagonal c e l l , a = 2.504 Â, ç = 6.661 Â ) . For many y e a r s , t h i s m o d i f i c a t i o n has been prepared from combinations of cheap boron and n i t r o g e n c o n t a i n i n g r e a g e n t s , e . g . B ( 0 H ) and (NH )C0, B ( 0 H ) , C and N o r KBH and NH C1 ( 3 - 5 ) . More 3

2

3

2

4

4

0097-6156/88/0360-0378$06.00/0

© 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

30. N A R U L A E T A L .

Precursors to Nonoxide Macromolecules

379

r e c e n t l y , chemical vapor d e p o s i t i o n (CVD) t e c h n i q u e s u t i l i z i n g m i x t u r e s o f BCI3 and NH3, BF3 and NH3 o r B2H6 and NH3 have been employed f o r t h e s y n t h e s i s o f h-BN ( 3 , 6 - 1 0 ) . P e r t i n e n t t o o u r own s t u d i e s , s e v e r a l i n v e s t i g a t o r s have a l s o used s u b s t i t u t e d borazenes as p r e c u r s o r s i n c l a s s i c a l t h e r m o l y s i s o r CVD p r e p a r a t i o n s o f h-BN. For example, Constant and Feurer (H) p y r o l y z e d h e x a c h l o r o borazene, (ClBNC1)3 a t 900-950°C on s i l i c o n s u b s t r a t e s and observed t h e d e p o s i t i o n o f amorphous BN which was subsequently converted t o h-BN by e l e c t r o n bombardment. I n thorough, systematic t h e r m o l y s i s s t u d i e s r e p o r t e d by Rice and coworkers (12) and Paciorek and coworkers ( 1 3 ) , i t was found t h a t a v a r i e t y o f s u b s t i t u t e d borazenes (XBNYJ3 produced boron r i c h BN s o l i d s t a t e materials. At t h e i n i t i a t i o n o f o u r s t u d i e s s e v e r a l years ago, t h e r e had been r e l a t i v e l y few w e l l documented r e p o r t s on t h e f o r m a t i o n o f p o l y m e r i c B-N compounds. An e x p l a n a t i o n f o r t h i s s i t u a t i o n may be found i n t h e h i s t o r i c a l development o f m o l e c u l a r b o r o n - n i t r o g e n chemistry (14-16). In a t t e n t i o n was g i v e n t o aminoboranes, iminoboranes and borazenes and progress made was impressive. For a v a r i e t y o f reasons, advances i n B-N polymer c h e m i s t r y were f r u s t r a t e d . A f t e r t h a t p e r i o d i n t h e USA, f u r t h e r e f f o r t s t o e x p l o r e p o l y m e r i c m a t e r i a l s were few and t h i s area attracted l i t t l e attention. Nonetheless, s e v e r a l r e p o r t s i n t h e p a t e n t and open l i t e r a t u r e suggested t h a t borazene compounds c o u l d be employed as monomers f o r modest m o l e c u l a r w e i g h t oligomers (13,11-24). F u r t h e r m o r e , a t a n t a l i z i n g p a t e n t r e p o r t i n 1976 by Taniguchi and coworkers ( 2 5 ) suggested t h a t s p i n n a b l e f i b e r s c o u l d be o b t a i n e d from a borazene (H NBHPh)3 based polymer which were, i n t u r n , converted t o boron n i t r i d e . More r e c e n t l y , Paciorek and coworkers have r e p o r t e d on t h e f o r m a t i o n o f boron n i t r i d e m a t e r i a l s from s e v e r a l condensed s u b s t i t u t e d borazenes ( 2 3 , 2 4 ) . Recognizing t h e need f o r fundamental s y n t h e t i c c h e m i s t r y l e a d i n g t o b o r o n - n i t r o g e n preceramic p o l y m e r s , we embarked on a program t o develop improved condensation r o u t e s f o r aminoboranes, iminoboranes and borazenes. I n p a r t i c u l a r , borrowing from t h e p r i n c i p l e s o f o r g a n i c polymer s y n t h e s e s , c h e m i s t r y was sought which would r e s u l t i n t h e c r o s s l i n k i n g o f borazene monomer b u i l d i n g blocks. I t was r a t i o n a l i z e d t h a t c r o s s l i n k i n g t h r o u g h amide b r i d g e s would produce preceramic o l i g o m e r s w i t h "excess" n i t r o g e n ( e . g . N:B = 1 . 5 ) w h i c h , upon p y r o l y s i s , might r e t a i n more n i t r o g e n than found i n p y r o l y s i s p r o d u c t s from monomeric borazenes. I n a d d i t i o n , i t was a n t i c i p a t e d t h a t t h e c r o s s l i n k i n g might r e s u l t i n boron n i t r i d e m a t e r i a l s w i t h a porous m a c r o s t r u c t u r e . 2

Results I t i s w e l l known t h a t some s i l y l a m i n e s , i n combination w i t h h a l o b o r a n e s , p r o v i d e a f a c i l e r o u t e t o aminoboranes ( E q u a t i o n 1) (26). M e l l e r and F u l l g r a b e (19) have p r e v i o u s l y r e p o r t e d t h a t a R BC1 + R NSiMe 2

2

3

-> R BNR 2

v a r i a t i o n of t h i s reaction w i l l

(1)

2

p r o v i d e coupled

substituted

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

380

borazenes. Indeed, t h e f o l l o w i n g model r e a c t i o n s were examined i n our work and b i s - b o r a z i n y l amines 1 and 2 were i s o l a t e d i n high y i e l d (Equations 2 and 3). Compounds"! and~2 were f u l l y c h a r a c t e r i z e d by elemental a n a l y s i s and s p e c t r o s c o p i c techniques (27). (Cl)(Me) B N Me 2

3

3

+ ( M e ^ i ^ N H -> [ M e ^ N g M e ^ N H

3

(2)

ι C 1 ( M e )

B

N

M e

2 3 3 3

+

(

M e

S i ) 3

2

N M e

"* [

M e

B 2

N

M e

]

3 3 3 2

N M e

( 3 )

2 The Β NMR s p e c t r a f o r these compounds show resonances a t 37.4 and 28.0 (1) and 37.2 and 27.6 (2), r e s p e c t i v e l y . Extending t h e model c h e m i s t r y t o t h e f o r m a t i o n of preceramic p o l y m e r s , we found t h a t o l i g o m e r i c g e l s are o b t a i n e d by r e l a t e d reactions involving B-trichloroborazene 4-6. I n each case, a t r a n s p a r e n of i t s c o n t a i n e r . Some s p e c i f i c e x p e r i m e n t a l d e t a i l s f o r t h e f o r m a t i o n of 3 are p r o v i d e d , and a summary of t h e p r o c e s s i n g of t h e o l i g o m e r i s g i v e n i n F i g u r e 1. The g e l s may be formed i n a v a r i e t y 1 1

CH CI C1

B

N

H

3 3 3 3

+

(

M e

s i ) 3

2

N H

1 [ ( H N )

—

3 3 3 3 n B

N

]

H

+

M e

s i c l 3

( ) 4

3 CH CI C1

B

N

H

3 3 3 3 * (

M e

s i 3

)

N M e 2

—

2

2

> [(

N M e

B

N

H

)3 3 3 3l

+

M e

n

3

S i C 1

(5

>

4 CH CI C 1

B

N

M e

3 3 3 3

+

(

M e

s i 3

)

N H

2

2

2

N H

B

N

M e

»[( ) 3 3 3^ 3

+

M e

S i C 1 3

6

5 of o t h e r s o l v e n t s , and t h e p r o c e s s i n g of these m a t e r i a l s i s described separately ( 2 7 ) . T r i c h l o r o b o r a z e n e , CI3B3N3H3 (50 mmol) and (Me3Si)2NH (75 mmol) were combined a t -78°C i n CH2C12 and t h e m i x t u r e s t i r r e d f o r 30 min a t -78°C and then s l o w l y warmed t o 25°C over two h o u r s . A c o l o r l e s s gel formed which was i s o l a t e d by vacuum e v a p o r a t i o n of the v o l a t i l e s . The r e s u l t i n g c o l o r l e s s g l a s s y s o l i d was p y r o l y z e d i n vacuo a t 900°C f o r 24 hours i n a q u a r t z tube and t h e evolved v o l a t i l e s i d e n t i f i e d as NH and NH4CI. The remaining s o l i d was b r i e f l y (2 hours) heated i n a i r a t 1200°C i n o r d e r t o remove minor carbon i m p u r i t i e s and t o improve c r y s t a l l i n i t y . This s o l i d was then t r e a t e d a t room temperature w i t h 40% aqueous HF t o remove b o r i c a c i d and s i l i c a formed i n small q u a n t i t i e s . The s o l i d o b t a i n e d a t 900°C was i d e n t i f i e d as boron n i t r i d e ; however, t h e m a j o r i t y of t h e m a t e r i a l was amorphous. A f t e r t r e a t m e n t a t 1200°C, w h i t e c r y s t a l l i n e b o r o n - n i t r i d e was o b t a i n e d i n about 55% y i e l d . 3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

30. N A R U L A E T A L .

Precursors to Nonoxidt Macromolecules

900 C VAC. -EtjO

1200 C AJR -NH

-NH

-NKQ

e

Figure 1 .

381

e

3

WHITE S0LI0

Summary o f borazene gel p r o c e s s i n g .

The boron n i t r i d e o b t a i n e d i n t h i s study was c h a r a c t e r i z e d by i n f r a r e d s p e c t r o s c o p y , powder x - r a y d i f f r a c t o m e t r y and t r a n s m i s s i o n e l e c t r o n microscopy. Trace elemental analyses were a l s o performed by energy d i s p e r s i v e x - r a y a n a l y s i s and carbon a r c emission spectroscopy. R e p r e s e n t a t i v e s p e c t r a a r e d i s p l a y e d i n Figures 2 - 4 . The i n f r a r e d spectrum o f 3 ( F i g u r e 2) c o n t a i n e d i n a KBr p e l l e t shows a broad a b s o r p t i o n i n t h e r e g i o n 1425-1065 c n H , a sharp a b s o r p t i o n a t 710 c n H , and t h e spectrum c l o s e l y resembles s p e c t r a r e p o r t e d i n t h e l i t e r a t u r e ( 6 , 8 , 2 8 ) . The broad band c e n t e r e d a t 3300 cm-1 suggests t h e presence o f r e t a i n e d N-H f u n c t i o n a l i t i e s ; however, t h i s f e a t u r e may a l s o be seen i n some commercial samples o f h-BN. The x - r a y powder p a t t e r n ( F i g u r e 3) i s a l s o s i m i l a r t o p a t t e r n s r e p o r t e d i n t h e l i t e r a t u r e (29.). The l i n e a t 2Θ ~26°corresponds t o t h e 002 r e f l e c t i o n , a l i n e a t 2Θ - 4 2 - 4 4 ° corresponds t o o v e r l a p o f t h e 100 and 101 r e f l e c t i o n s , and a l i n e a t 2Θ - 5 4 ° i s t h e 004 r e f l e c t i o n . The l i n e s a r e broader than t y p i c a l l y observed from h i g h l y c r y s t a l l i n e commercial h-BN samples, and t h i s l i n e broadening may r e s u l t from small c r y s t a l l i t e s i z e o r from t h e presence o f t h e p r e v i o u s l y observed t u r b o s t r a t i c structure modification (30)· The measured spacing d(002) = 3 . 3 5 - 3 . 3 6 A agrees very w e l l w i t h t h e r e p o r t e d d(002) = 3.330 A f o r h-BN prepared by c o n v e n t i o n a l methods. Most t u r b o s t r a t i c m o d i f i c a t i o n s show a g r e a t e r d e v i a t i o n i n d(002) = 3 . 5 - 3 . 6 A ; t h e r e f o r e , i t i s assumed t h a t any t u r b o s t r a t i c d i s o r d e r i s s m a l l . The m a c r o s t r u c t u r e o f t h e boron n i t r i d e o b t a i n e d here i s porous w i t h pores ~2 urn i n d i a m e t e r . There i s no evidence f o r m i c r o p o r o s i t y and t h e BET s u r f a c e area i s ~35 m2 g - 1 . Transmission e l e c t r o n micrographs ( F i g u r e 4) show r e g i o n s o f w e l l developed crystallinity. The c r y s t a l l i n g g r a i n s a r e - 5 - 1 0 nm on a s i d e and 30-40 nm l o n g . The BN (002) l a t t i c e f r i n g e s a r e c l e a r l y v i s i b l e .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

382

7oT

4000

3590

3180

2770

2360

1950

1540

1130

720

310

cm"

Figure 2 .

Figure 3.

I n f r a r e d spectrum o f BN.

X - r a y powder d i f f r a c t i o n a n a l y s i s o f BN.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

30. NARULA ET AL.

Figure 4 .

Precursors to Nonoxide Macromolecules

Transmission e l e c t r o n micrograph o f

383

BN.

Conclusion The r e s u l t s of t h i s study c l e a r l y r e v e a l t h a t c r o s s l i n k i n g of borazene r i n g s may be achieved w i t h s i l y l a m i n e s and t h e r e s u l t i n g o l i g o m e r i c g e l s may be e f f i c i e n t l y processed t o g i v e hexagonal boron n i t r i d e i n good y i e l d s and w i t h good c r y s t a l u n i t y . The c h a r a c t e r i s t i c s of t h e g e l s formed by t h i s s y n t h e t i c r o u t e lend themselves t o a v a r i e t y of p r o c e s s i n g modes, e . g . s o l - g e l and aerogel p r o c e s s i n g , not a v a i l a b l e i n t h e s y n t h e t i c procedures n o r m a l l y a p p l i e d i n boron n i t r i d e p r e p a r a t i o n s . Additional studies are i n progress which are designed t o t a k e advantage of these features. Acknowledgment i s made t o Sandia N a t i o n a l L a b o r a t o r y f o r of t h i s r e s e a r c h . Literature 1. 2. 3.

support

Cited

Wynne, K . J . ; R i c e , R.W. Ann. Rev. Mater. S c i . 1984, 14, 297. Hoard, J.L.; Hughes, R.E. I n The Chemistry of Boron and Its Compounds; M u e t t e r i e s , E.L. Ed. J . Wiley and Sons, NY 1967. M e l l e r , A. Gmelin Handbuck der Anorganische Chemie. Boron Compounds Second Supplement 1983, 1, 304.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

384 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Kalyoncu, C.S. Ceram. Eng. Sci. Proc. 1985, 1356. Archer, N.J. Chem. Soc. (London) Spec. Pub. No. 30 1977, 167. Rand, M.J.; Roberts, J.F. J. Electro. Chem. Soc. 1968, 115, 423. Pierson, H.O. J. Composit. Mat. 1975, 9, 228. Takahashi, T.; Itoh, H.; Takeuchi, A. J. Cryst. Growth 1979. 47, 245. Sano, M.; Aoki, M. Thin Solid Films 1981, 83, 247. Chopra, K.L.; Agarwal, V.; Vankar, V.D.; Deshpandey, C.V.; Bunshah, R.F. Thin Solid Films 1985, 126, 307. Constant, G.; Feurer, R. J. Less Common Metals 1981, 82, 113. Bender, B.A.; Rice, R.W.; Spann, J.R. Ceram. Eng. Sci. Proc. 1985, 6, 1171. Paciorek, K.J.L.; Harris, D.H.; Kratzer, R.H. J. Polym. Sci. Polym. Chem. 1986, 24, 173. Lappert, M.F. In Developments in Inorganic Polymer Chemistry; Lappert, M.F. Ed., Elsevier Publ. Co., New York, 1962. Gaines, D.F.; Borlin, Muetterties, E.L. Ed., Steinberg, H.; Brotherton, R.J. In Organoboron Chemistry; Vol. 2, J. Wiley, NY 1966. Wagner, R.I. U.S. Pat. 3,288,726 Nov. 29, 1966; Chem. Abstr. 1977, 66 38349W. Wagner, R.I.; Bradford, J.L. Inorg. Chem. 1962, 1, 99. Meller, Α.; Füllgrabe, H.J. Z. Naturforsch. 1978, 33b, 156. Gerrand, W.; Mooney, E.F.: Pratt, D.E. J. Appl. Chem. 1963, 13, 127. Aubrey, D.W.; Lappert, M.F. J. Chem. Soc. 1959, 2927. Gerrand, W.; Hudson, H.R.; Mooney, E.F. J. Chem. Soc. 1962, 113. Paciorek, K.J.L.; Kratzer, R.H.; Harris, D.H.; Smythe, M.E.; Kimble, P.F.U.S. Pat 4,581,468 Apr. 8, 1986; Chem Abstr. 1986, 105, 80546g. Paciorek, K.J.L.; Kratzer, R.H.; Harris, D.H.; Smythe, M.E. Polym. Preps. 1984, 25, 15. Taniguchi, I.; Koichi, H.; Takayoshi, M. Jpn. Kokai 76 53,000. Chem Abstr. 1976, 85, 96588. Nöth, H. Z. Naturforschg. 1961, 16b, 618. Narula, C.K.; Paine, R.T.; Schaeffer, R. manuscript in preparation. Brame, E.G. Margrave, J.L.; Meloche, V.W. J. Inorg. Nucl. Chem. 1957, 5, 48. Pease, R.S. Acta Cryst. 1952, 5, 356. Biscoe, J.; Warren, B.E. J. Appl. Phys. 1942, 13, 364; Thomas, J.; Weston, N.E.; O'Connor, T.E. J. Am. Chem. Soc. 1963, 84. 4619; Economy, J.; Anderson, R. Inorg. Chem. 1966, 5, 989; Matsuda, T.; Uno, N.; Nakae, H.; Hirai, T. J. Mat. Sci. 1986, 21, 649.

RECEIVED

September

1,

1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 31

Boron-Nitrogen Polymer Precursors S. Yvette Shaw, Donn A. DuBois, and Robert H . Neilson

1

Department of Chemistry, Texas Christian University, Fort Worth, TX 76129 The synthesis of tractable, by the ready formation of (RBNR) . In an attempt to circumvent this problem, we have prepared a series 3

of new mono- and diborylamines, EN(CH ) N(E')B(Ph) [Ε = H, E' = SiMe (2), B(NMe ) (3), B(Ph)NMe , (4); Ε = SiMe , E' = SiMe (5), B(NMe ) (6), B(Ph)NMe (7); 8: Ε = E' = B(NMe ) ; 9: Ε = Ε' = SiMe H], by lithiation of the 1,3,2-diazaboracyclohexane ring system (1: Ε = Ε' = Η) followed by quenching with electrophiles. The crystal and molecular structure of 8 has been deter­ mined by single crystal X-ray diffraction. Further chemistry of these com­ pounds has been explored by reactions involving their pendent groups. For example, the bis(trimethylsilyl) compound 5 reacts with PhBCl via Si-N bond cleavage to afford 11 [E = SiMe , Ε' = B(Ph)Cl]. Also, the bis(dimethylamino)boryl derivative 8 undergoes transamination reactions with 1,3-diaminopropane to give compounds containing two (12) or three (13) BN C rings linked together. Preliminary thermolysis studies show that some of the diborylamines (e.g., 3, 4, and 11) are potentially useful precursors to boron-nitrogen oligomers/polymers. 2 3

2 2

3

2

2

3

3

2 2

2 2

2

2

3

2

3

There has been considerable interest in recent years in the preparation of ceramic materials via pyrolysis chemistry of preceramic polymers. In particular, many important advances, most of which are described elsewhere in this Symposium volume, have been made in the syn­ thesis of silicon carbide (SiC) and silicon nitride ( S i N ) from appropriate polymeric materials. As pointed out by Wynne and Rice [1], however, polymeric routes t o boron nitride (BN), another important ceramic material, have been largely unexplored. This stems mainly from a lack of well characterized polymers containing boron and nitrogen. In the 1950s and 1960s, the syntheses of numerous amine- and aminoboranes were reported along with many un­ successful attempts t o polymerize such reagents. [2] The high thermal stability of the cyclic trimers [i.e., borazenes, (RBNR)J is generally cited as the reason for the failure of the B-N "monomers" t o polymerize. In fact, t o our knowledge, all of the more recent attempts t o prepare BN polymer precursors involve linking these borazene rings together t o form cyclomatrix or at least cyclolinear polymers. [3,4] Within this context, our main objective is to prepare and characterize tractable, high molecular weight, linear polymers containing the boron-nitrogen backbone. Aside from their potential as preceramic materials, the linear B-N polymers are expected to exhibit interesting and potentially useful properties themselves. For example, since they are the exact isoelectronic analogues of polyacetylene, these polymers should have similar electrical conductivity and possibly better physical/mechanical properties. 3

1

4

Correspondence should be addressed to this author. 0097-6156/88/0360-0385506.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

386

In order to overcome the problem of borazene ring formation, two different ap­ proaches, both involving diborylamines as condensation monomers, are being studied in our laboratory. The incorporation of a linear N-B-N-B unit along with other structural features should prevent these systems from condensing to the 6-membered borazene rings upon thermolysis. In the first method, acyclic diborylamines containing both the Si-N and the B-X groups (I) are the starting compounds. A few such species have recently been prepared and characterized. [5] In this case, the bulky t-Bu group on boron serves both to stabilize the primary aminoborane precursors to I and to help inhibit [6] formation of the borazene ther­ molysis products.

Me Si

l

3

Bu R

Ν—B

R

W

Me SiX

i«

w

ν

3

X

l

I : R, R", = Me, E t , B u , P h , e t c ,

X = Cl , NMe , 0CH CF 2

2

3

The second approach involves linking the nitrogen atoms of the N-B-N-B backbone through bridging alkylene units by use of the 1,3,2-diazaboracycloalkane ring systems (I I) (eq 2). The bridges are intended to provide structural rigidity in order to prevent the boronnitrogen backbone from condensing to the cyclic trimer. The alkylene moieties should also enhance the solubility of the polymeric products in organic solvents, thereby facilitating their characterization.

R

R'

/

EX Δ

or

*;

1/n

base

8

II:

R, R ,

X = as i n eq 1

Ε = H,

SiMe

x

3

m = 2, 3, 4

We report here some of our recent results in the second of these areas. Specifically, the synthesis, structure, reactivity, and preliminary thermolysis studies of several new monoand diborylamines based on the 2-phenyl-1,3,2-diazaboracyclohexane ring system (1) are described.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

31. SHAW ET AL.

Boron-Nitrogen Polymer Precursors

387

Results and Discussion S y n t h e s i s . The 2-phenyl-1,3,2-diazaboracyclohexane ring system (1) was selected as the starting material in this study because: (1) it is prepared easily and in high yield by a three step synthesis from B C I ; (2) the N-H bonds are potential sites for deprotonation and substitu­ tion reactions; and (3) the trimethylene bridge enhances the rigidity of the N-B-N backbone which should prevent cyclization upon thermolysis of an appropriate precursor. The compounds described herein were prepared by three methods. The first route in­ volves deprotonation/substitution at the N-H sites of 1, the second consists of a cleavage reac­ tion of an Si-N derivative of 1 with PhBCI , and the third route is a transamination reaction be­ tween a bis(dimethylamino)boryl derivative of 1 and an aliphatic diamine. In the first approach, compound 1 is deprotonated by treatment with one equivalent of n-BuLi. Quenching of the resulting anion with various electrophiles produces the monosubstituted products 2-4 (eq 3). 3

2

Ph

Ph (1 (2)

ECl

2 : Ε = SiMe 3:

3

Ε = B 2

2

4 : Ε = BCPrONMe,

These new derivatives were isolated in good yields (60-94%) as high boiling liquids and were fully characterized by NMR spectroscopy ( H , C , and B ) and elemental analysis. The proton NMR of the starting material 1 shows a well-resolved multiplet and quintet for the trimethylene bridge. Upon monosubstitution, however, three complex multiplets are ob­ served, indicative of the unsymmetrical structures of these derivatives. Also, the nonequivalence of the N-C carbon atoms is clearly apparent in the C NMR spectra of 2-4. Further substitution of the diazaboracyclohexane ring system may be accomplished by the same reaction sequence, starting with compounds 2-4, t o afford the disubstituted products 5-8 (eq 4). The yields of these reactions ranged from 70 t o 82%. The symmetrically 1

1 3

1 1

1 3

Ph

^A^

H

Ph

J^J^U (2)

2-4

NAN-

(4)

E /

E C1 /

5 : Ε = Ε' = SiMe 6:

Ε = SiMe , 3

7 : Ε = SiMe , 3

8:

3

Ε' = B(Nr1e > 2

E

/

2

= B(Ph)NMe

Ε = E' = BH)

1^·-

Η Φ

Η

Η I Φ

Η I Β I /Ν

Η Η

Ν Β — Ν I I I /Β^ Ν Φ I Ν Β /Ν^ \ I Φ Η Η Η

/Φ < Η

Scheme Î.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

397

INORGANIC AND ORGANOMETALLIC POLYMERS

398

SiMe I 3

NH

Q

Ε I

H N-

-Β I Me Si-f-N 2

2

N- •SlMe I NH

J I

3

3

0

*B" I NH

I

SiMe„

0

The c o n d e n s a t i o n p r o c e s s a p p a r e n t l y i n v o l v e s e l i m i n a t i o n of trimethylsilylamine, (CH ) SiNH^ which, being unstable, d i s p r o p o r t i o n a t e s i n t o h e x a m e t h y l d i s n a z a n e and ammonia. In the condensable v o l a t i l e s c o l l e c t e d , these m a t e r i a l s were p r e s e n t i n a 1:1 r a t i o s u p p o r t i n g the p o s t u l a t e d p r o c e s s . F u r t h e r t h e r m o l y s i s a t 200°C (52 h ) , f o l l o w e d by 4 h a t 250-260°C, r e s u l t e d i n a m i x t u r e of d o u b l y - b r i d g e d t e t r a m e r s and octamers, x=2 and 4. At t h i s s t a g e , 53* of th From the tetramer/octame be melt drawn as shown Figure i n ammonia, from 65-950°C, r e s u l t e d i n w h i t e , pure boron n i t r i d e fibers. The f i b e r s e x h i b i t e d no weight l o s s under t h e r m o g r a v i m e t r i c c o n d i t i o n s , both i n a i r and i n n i t r o g e n up t o 1000°C ( 1 2 ) . 3

f

P o t e n t i a l N o n - C y c l i c P r e c u r s o r s of P r e c e r a m i c Polymers. Boranes such as b i s ( t r i m e t h y l s i l y l ) a m i n o t r i m e t h y l s i l y l a m i n o c h l o r o b o r a n e s can be viewed as monomers f o r p r e c e r a m i c polymer and, u l t i m a t e l y , boron nitride production. I n t e r m o l e c u l a r d e h y d r o h a l o g e n a t i o n of t h i s borane would be thus expected t o y i e l d e i t h e r the dimer o r the p o l y m e r i c system. CI I (Me Si) NBNHSiMe 3

2

3

(Me.Si) NB — 0

Me S IN 3

NSiKe. BN (S i M e ) 3

2

and/or [(Me Si) NB-NSiMe ] 3

2

3

x

Bis ( t r i m e t h y l s i l y l ) aminotr imethylsilylaminochloroborane was o b t a i n e d i n a 70* y i e l d f o l l o w i n g the procedure of W e l l s and C o l l i n s ( 10). No t r i e t h y l a m i n e h y d r o c h l o r i d e was formed and the s t a r t i n g m a t e r i a l was r e c o v e r e d unchanged a f t e r p r o l o n g e d r e f l u x i n g w i t h a f i v e - f o l d excess of s p e c i a l l y d r i e d t r i e t h y l a m i n e . However, when the t r i e t h y l a m i n e employed was not so r i g o r o u s l y d r i e d and 10- t o 2 0 - f o l d excess was used, an oxygen-bridged compound,

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

32. PACIOREK ET AL.

F i g u r e 1.

F i g u r e 2.

Boron Nitride and Its Precursors

M e l t spun BN p r e c u r s o r

M e l t spun BN p r e c u r s o r

399

f i b e r s , m a g n i f i c a t i o n 500x.

fibers, magnification

5,000x.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

400

( M e S i ) NBNHSlMe 3

2

3

0

1 (Me Si) NBNHSlMe 3

2

3

II was o b t a i n e d i n a 35% y i e l d . C o n d u c t i n g the r e a c t i o n i n benzene u s i n g the r e q u i r e d q u a n t i t y of water i n the presence of t r i e t h y l ­ amine gave 55% y i e l d of t r i e t h y l a m i n e h y d r o c h l o r i d e , but o n l y a 7% y i e l d of the oxygen-bridged product. The s u r p r i s i n g f i n d i n g was the r e l a t i v e l y high y i e l d ( M8%) of m a t e r i a l which i s e i t h e r (Me SiNH) BN(SiMe ) or [(Me S i ) Ν] BNH (the d i f f e r e n t i a t i o n c o u l d not Be made from the mass S p e c t r a l f r a g ­ m e n t a t i o n p a t t e r n ) , t o g e t h e r w i t h h e x a m e t h y l d i s i l a z a n e and aminob i s ( t r i m e t h y l s i l y l ) am NB(NH )NHSiMe , or i t were a l s o observed, but i n low c o n c e n t r a t i o n s , i n the r e a c t i o n where the h i g h y i e l d of the oxygen-bridged compound was r e a l i z e d . The major d i f f e r e n c e between these r e a c t i o n s was the r e l a t i v e r a t i o of t r i e t h y l a m i n e and the presence of benzene. Based on these r e s u l t s , i t i s c l e a r t h a t the process i n v o l v i n g hydrogen c h l o r i d e e l i m i n a t i o n and oxygen s u b s t i t u t i o n , (Me .Si).NBNHS iMe . I NEt. I 2 (Me Si) NBNHSiMe + H 0 ^ 0 + 2 Et N-HCl 3

2

3

P 1

3

3

2

3

2

3

( M e S i ) NBNHSiMe 3

2

3

i s not t h e o n l y one o c c u r r i n g . Apparently, a more e x t e n s i v e h y d r o l y s i s of the c h l o r o b o r a n e takes p l a c e c o n c u r r e n t l y , i . e . ,

(Me.Si) NBNHSiMe. j l à 0

= • ( M e . S i ) NH NEt J Ζ 0

3

+ B ( 0 H ) . + [H.NSiMe.] + Et.N-HCl j ζ J J

Inasmuch as t r i m e t h y l s i l y l a m i n e i s u n s t a b l e , i t w i l l decompose i n t o ammonia and h e x a m e t h y l d i s i l a z a n e . The ammonia thus formed c o u l d be visualized to form the amino compound (Me^si) NB(NH.) NHSiMe . Since (Me S i ) NB(NHSiMe ) . was prepared by Wells and C o l l i n s (j_3) v i a i n t e r a c t i o n of tne c h l o r o b o r a n e w i t h hexa­ m e t h y l d i s i l a z a n e , i t s p r o d u c t i o n here c o u l d f o l l o w the same p a t h . The ni trogen-bridged analogue, [(Me Si ) NBNHSiMe^NH, Compound I , was formed i n a 57% y i e l d from b i s t r i m e t h y l s i l y l ) aminotrimethylsilylaminochloroborane and aminobis ( t r i m e t h y l s i l y l )aminotrimethylsilylaminoborane i n the presence of t r i e t h y l a m i n e by h e a t i n g a t 100°C over an 18 h p e r i o d , i . e . 2

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

32. PACIOREK ET AL.

Boron Nitride and Its Precursors

NH

CI

0

( M e S i ) NBNHSiMe 3

401

+

2

( M e S i ) NBNHSiMe 3

2

3

JEt N 3

( M e S i ) NBNHSiMe 3

2

3

NH

+

(Me Si) NBNHSlMe 3

2

Et N-HCl Q

3

1 3

I The n i t r o g e n - b r i d g e d compound e x h i b i t e d p h y s i c a l and s p e c t r a l c h a r a c t e r i s t i c s v e r y s i m i l a r t o those of the oxygen-bridged com­ pound. T h i s i s i l l u s t r a t e d by the c l o s e m e l t i n g p o i n t s (169-171°c and 158-160°C, r e s p e c t i v e l y (the spectrum of y - i m i d o - b i s [ b i s ( t r i m e t h y l s i l y l ) a m i n o t r i m e t h y l s i l y l amino]borane i s g i v e n i n F i g u r e 3 ) . Thermal b e h a v i o r of I and I I i s a l s o s i m i l a r as shown by DSC t r a c e s , where i n the oxygen-bridged compound two s t r o n g endotherms were observed a t 70 and 75°C and i n the n i t r o g e n - b r i d g e d m a t e r i a l a t 85 and 105°C. No phase t r a n s i ­ t i o n r e s p o n s i b l e f o r these endotherms c o u l d be v i s u a l l y observed on heating. Both m a t e r i a l s , on r e m e l t i n g i n a i r , e x h i b i t e d the same m e l t i n g p o i n t s as those o b t a i n e d under i n e r t - c o n d i t i o n s p o i n t i n g to t h e i r t h e r m a l , o x i d a t i v e , and h y d r o l y t i c s t a b i l i t i e s . Investigation of the c o n d e n s a t i o n r e a c t i o n s of Compound I are c u r r e n t l y i n progress. The mass s p e c t r a l breakdown p a t t e r n s f o r the two bridged compounds were v e r y s i m i l a r , w i t h the e x c e p t i o n t h a t the peaks were one mass u n i t h i g h e r i n the case of the oxygen-bridged m a t e r i a l . B o t h compounds produced s t r o n g m o l e c u l a r i o n s a t 533 ( I ) and 534 (II). The p r o c e s s e s r e s p o n s i b l e f o r s e v e r a l of the major fragments c o u l d be i d e n t i f i e d from the m e t a s t a b l e s . In the case of μ -imido-bis[bis(trimethylsilyl)aminotrimethylsilylamino]borane, these a r e : 533 518

+

+

• δίδ* +

• 429

429^ 429

(Μ)

+

• 341 •316

+

+

+

15 [Me]

m*

503.4

+

89

[H^NSiMe^]

m*

355.3

+

88

[HNSiMe^]

m*

271.0

+

113

m*

232.8

[BNHNSiMe^]

In view of the absence of the a d d i t i o n a l p r o t o n , the e q u i v a l e n t of the 341 i o n , namely the 3 4 2 i o n , would not be expected t o be formed to any s i g n i f i c a n t e x t e n t i n the case of the oxygen-bridged compound. The f r a g m e n t a t i o n p a t t e r n c o n f i r m s i t and no m e t a s t a b l e for t h i s process was found. In the oxygen-bridged compound, m e t a s t a b l e s were observed a t m/e 504.5, 357.5, 213, and 82.5. The processes r e s p o n s i b l e have been i d e n t i f i e d as: +

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

F i g u r e 3. I n f r a r e d spectrum o f y-imido-bis[bis(trimethylsilyl)aminotrimethylsilylamino]borane ( I ) .

w

C/5

w

κ;

r

Ο

> r η

w Η

ο

ο >

ο ο

> ζ

ο > ο

Ο

32. PACIOREK ET AL.

Boron Nitride and Its Precursors

1

534" (Μ)

•δίθ" "

+

15 [Me]

403

m* 504.4

519

+

• 430

+

+

89 [H^NSiMe^]

m*

356.3

519

+

• 332

+

+

187 [B(NHSiMe^)g]

m*

212.4

+

113

m*

82.3

259"

•146

+

The 332* i o n i s s p e c i f i c t o the o x y g e n - b r i d g e d p r o d u c t . It is s p e c u l a t e d t h a t i t i s formed by the f o l l o w i n g path from the 519 ion: (Me Si) N Q

0

(Me.Si) N

v

9

; Β — 0 — B —NHSiKe Me^SiHN \^ | Me—Si—Ν—SiMe 3

J

2

Z

Β | | M e - S i +N

—

Me~SiHN

5

^

0

HC — H

SiMe

CH

2

519

NHSiMe^

+

332

+

In o r d e r t o o b t a i n i n s i g h t i n t o t h e n a t u r e o f b o n d i n g i n c a t e n a t e d BN s p e c i e s , t h e c r y s t a l and m o l e c u l a r s t r u c t u r e s o f Compounds I and I I were o b t a i n e d . Both compounds c r y s t a l l i z e i s o m o r p h o u s l y i n the m o n o c l i n i c space group P2 /n. For I . a = 12.876(2) Â , b = 14.828(3) \ c = 18.628(4) Â; β - 97.07(2) ; V = 3529(1) Â ; Ζ = 4, agd t h e m o l e c u l a r w e i g h t based on a c a l c u l a t e d d e n s i t y o f 1.005 g/cm was 533.85, i n e x c e l l e n t agreement w i t h t h e m o l e c u l a r i o n observed i n the mass spectrum (533 m/e). Refinement led to R = 4.1% and R = 4.2% f o r 5093 independent r e f l e c ­ tions. The s t r u c t u r e o f I , g i v e n i n F i g u r e 4, c o n f i r m s t h e p r e s e n c e of a c e n t r a l Ν BNBN framework t h a t i s e s s e n t i a l l y p l a n a r w i t h a B ( 1 ) - N ( 1 ) - B ( 2 J a n g l e o f 135.9(2)°. The r e m a i n i n g B-N-B a n g l e s a r e c l o s e t o 120 . The B-N bond d i s t a n c e s r e f l e c t a c o m p e t i t i o n f o r TT-electron d e n s i t y . The s h o r t e s t B-N bonds, (B(2)-N(5) 1.405(3) % and B ( ( l ) - N ( 3 ) 1.423(3) Â, a r e t o n i t r o g e n s bonded t o one s i l i c o n , w h i l e the l o n g e s t B-N bonds B(2)-N(4) 1.485(3) λ and B ( l ) - N ( 2 ) 1.487(3) Â a r e t o n i t r o g e n s bonded t o two s i l i c o n s . For I I , a = 12.826(3) Â, b = 14.822(4) Â, and c - 18.557(4) λ; β - 95.98(2)°; V = 3509(1) Â ; Ζ = 4, F.W. = 534.8 amu, and t h e c a l c u l a t e d d e n s i t y i s 1.012 g/cm . Refinement l e d t o R = 4.3% and R * 4.0% f o r 4458 independent r e f l e c t i o n s . Structure I I , given i n F i g u r e 5, d i f f e r s from I o n l y i n the e x i s t e n c e o f a c e n t r a l B-O-B u n i t w h e r e i n the boron oxygen d i s t a n c e s [ B ( l ) - 0 ( 1 ) 1.380(3) and B ( 2 ) - 0 ( l ) 1.389(3)] a r e somewhat s h o r t e r than t h e c o r r e s p o n d i n g B-N d i s t a n c e s f o r I [ B ( l ) - N ( l ) 1.428(3) and B ( 2 ) - N ( l ) 1 . 4 4 6 ( 3 ) ] . t

3

f

1

w f

f

w f

Acknowledgment Support o f t h i s r e s e a r c h from t h e S t r a t e g i c Defense S c i e n c e s O f f i c e through C o n t r a c t N00014-85-C-0659 from the O f f i c e o f N a v a l R e s e a r c h and t h e c r y s t a l s t r u c t u r e a n a l y s i s by C. S. Day o f C r y s t a l y t i c s Co. are g r a t e f u l l y acknowledged.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

404

INORGANIC AND ORGANOMETALLIC POLYMERS

Figure 4. A p e r s p e c t i v e drawing of C H ^ N g S i Β with n o n h y d r o g e n atoms r e p r e s e n t e d by thermal v i b r a t i o n ellipsoids drawn t o encompass 50* o f t h e i r e l e c t r o n d e n s i t y ; hydrogen atoms a r e r e p r e s e n t e d by a r b i t r a r i l y s m a l l spheres which a r e i n no way r e p r e s e n t a t i v e o f t h e i r t r u e thermal motion.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

32.

PACIOREK ET AL.

Boron Nitride and Its Precursors

405

F i g u r e 5. A p e r s p e c t i v e drawing of i a 5 6 4 6 , 2 , w i t h n o n h y d r o g e n atoms r e p r e s e n t e d by thermal v i o r a t i o n ellipsoids drawn t o encompass 50* o f t h e i r e l e c t r o n d e n s i t y ; hydrogen atoms a r e r e p r e s e n t e d by a r b i t r a r i l y s m a l l spheres f o r purposes o f clarity. c

H

N

0 S i

B

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

406

INORGANIC AND ORGANOMETALLIC POLYMERS

Literature Cited 1. Wynne, K. J., Rice, R. W. Ann. Rev. Mater. Sci. 1984, 14, 297. 2. Steinberg, H., Brotherton, R. J. Organoboron Chemistry, Vol. 2, pp. 175-434; J. Wiley, New York, 1966. 3. Taniguchi, I., Harada, Κ., Maeda, T. Jpn. Kokai 76 53,000, 11 May 1976. 4. Paciorek, K. J. L., Harris, D. H., Kratzer, R. H. J. Polym. Sci. 1986, 24, 173. 5. Paciorek, K. J. L., Kratzer, R. Η., Harris, D. H., Smythe, M. E. Polymer Preprints 1984, 25, 15. 6. Groszos, S. J., Stafiej, S. F. J. Am. Chem. Soc. 1958, 80, 1357. 7. Toeniskoetter, R. H., Hall F. R. Inorg. Chem. 1963, 2, 29. 8. Ohashi, O., Kurita, Y., Totani, T., Watanabe, H., Nakagawa, T., Kubo, M. Bull. Chem. Soc. Japan 1962, 35, 1317. 9. Gerrard, W., Lappert, M. F., Pearce, C. A. J. Chem. Soc. 1957, 381. 10. Wells, R. L., Collins, A. L. Inorg. Chem. 1966, 5, 1327. 11. Geymayer, P., Rochow, 12. Without additional the fibers are amorphous based on transmission electron microscopy (TEM) with islands of microfibrils or microcrystallites. Selected area electron diffraction (SAED) of the microcrystallites indicates that they could be BN, B O , or graphite. Auger electron spec­ troscopy showed the fibers to be essentially free of carbon which is supported by their colorless appearance. X-ray diffraction produces a weak signal corresponding to hexagonal BN. The fact that the fibers do not change in shape when heated at 1000°C rules out the presence of significant amounts of B O , which melts at 460°C. Further characterization of the fibers in regard to composition, structure, susceptibility to "graphitization", as well as strength and density is in progress. 13. Wells, R. L., Collins, A. L. Inorg. Chem. 1968, 7, 419. 2

3

2

RECEIVED

3

September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 33

Electron Transport in and Electrocatalysis with Polymeric Films of Metallotetraphenylporphyrins B. A. White , S. A. Raybuck , A. Bettelheim , K. Pressprich, and Royce W. Murray 1

2

3

4

Kenan Laboratorie

Chapel Hill, NC 27514

Amino-, hydroxy-, and pyrrole-substituted tetraphenylporphyrins undergo electro-oxidative polymerizations analogous to those of aniline, phenol, and pyrrole, to form electroactive polyporphyrin films on electrode surfaces. The voltammetry of differently metallated forms of these poly-porphyrins has been studied as has the rate of electron-hopping diffusion within the polymers. The films are demonstrably permeated by axial ligands and are remarkably pinhole free. The rate constants for electron hopping in the polymers have values parallel to those of heterogeneous electron transfer rate constants for corresponding metalloporphyrin monomers at solid electrode surfaces, consistent with lack of dominance of ionic or polymer chain motions on the barrier to electron transfers within the poly-porphyrin films. The cobalt- m e t a l l a t e d polymer is an electrocatalyst for reduction of dioxygen. T h i s r e s e a r c h r e s t s upon o x i d a t i v e e l e c t r o c h e m i c a l p o l y m e r i z a t i o n (ECP) of s o l u t i o n s of the f u n c t i o n a l i z e d m e t a l l o t e t r a p h e n y l p o r p h y r i n monomers shown i n F i g . I . These o x i d a t i o n s l e a d to f o r m a t i o n of t h i n , c r o s s - l i n k e d p o l y m e r i c f i l m s of the m e t a l l o t e t r a p h e n y l p o r p h y r i n s on the e l e c t r o d e s u r f a c e . The f i l m s c o n t a i n from c a . 4 t o 500 m o n o l a y e r - e q u i v a l e n t s of p o r p h y r i n s i t e s , which a r e i n h i g h c o n c e n t r a t i o n ( c a . IM) s i n c e the polymer backbone c o n s i s t s s o l e l y of the p o r p h y r i n s themselves as the backbone u n i t s . The p o l y m e r i c f i l m s adhere to the e l e c t r o d e and 1

Current address: The Upjohn Company, Kalamazoo, MI 49001 Current address: Department of Chemistry, Harvard University, Cambridge, M A 02138 Current address: Nuclear Research Center—Negev, Atomic Energy Commission, POB 9001 Beer Sheva, Israel 4 Correspondence should be addressed to this author. 2

3

0097-6156/88/0360-0408$06.00/0 © 1988 American Chemical Society

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

33. WHITE ET AL.

Polymeric Films of Mctallotetraphenylporphyrins

X

jT

Η

V.

(PN

Ν

X

XJT

X

y

Y*

H

χ

abbreviation

H

NHp H (m-NH )TPP 2

"0

N H

2

-0-NMe

2

2

H (p-NH )TPP 2

2

H (p-NMe )TPP 2

2

H (p-OH)TPP 2

-o-O

H (p-pyr)TPP 2

F i g . 1. S t r u c t u r e s o f o x i d a t i v e l y e l e c t r o p o l y m e r i z a b l e tetraphenylporphyrins. The p o r p h y r i n s can be p o l y m e r i z e d as m e t a l l a t e d f o r m s , or t h e m e t a l can be i n s e r t e d i n t o t h e polymer i n some c a s e s .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

409

410

INORGANIC AND ORGANOMETALLIC POLYMERS are s o l u b l e i n most s o l v e n t s , which a l l o w s i n v e s t i g a t i o n of t h e i r e l e c t r o c h e m i c a l r e a c t i v i t y as s u r f a c e f i l m s ( 1 , 2 ) , the e l e c t r o c a t a l y t i c r e a c t i v i t y of the c o b a l t p o r p h y r i n as a d i o x y g e n r e d u c t i o n c a t a l y s t ( 2 , 4 ) , and the p e r m e a b i l i t y of the f i l m s as membranes ( 5 ) . A d d i t i o n a l l y , s i n c e the p o r p h y r i n s i t e s are i n c l o s e c o n t a c t e l e c t r o n hopping o c c u r s , o f t e n f r e e l y , between s i t e s which are a d j a c e n t members of a redox c o u p l e p a i r . The c u r r e n t s due to these e l e c t r o n hopping ( o r s e l f exchange) r e a c t i o n s have been i n v e s t i g a t e d f o r a s e r i e s of p o r p h y r i n redox c o u p l e s (6) i n o r d e r to probe the m o l e c u l a r f a c t o r s c o n t r o l l i n g t h e i r r a t e . E l e c t r o c h e m i c a l P o l y m e r i z a t i o n of

Metallotetraphenylporphyrins

Our s t r a t e g y f o r e l e c t r o c h e m i c a l p o l y m e r i z a t i o n (ECP) of m e t a l complexes was based on an analogy to the ECP of s m a l l f u n c t i o n a l i z e d monomers (7-12) l i k e v i n y l - s u b s t i t u t e d a r o m a t i c s , p y r r o l e , a n i l i n e , and p h e n o l The i d e a i s to a t t a c h such a c t i v a t i n g groups ont ( o r o r g a n i c m o i e t y ) , an t h i s f u n c t i o n a l i z e d m e t a l complex to form i t i n t o a polymer. Forming a f i l m on the e l e c t r o d e from t h i s polymer depends on the e l e c t r o c h e m i c a l l y o x i d i z e d or reduced f u n c t i o n a l i z e d m e t a l complex e i t h e r g r a f t i n g d i r e c t l y onto some f u n c t i o n a l group present on the e l e c t r o d e s u r f a c e , or undergoing r a p i d c o u p l i n g , o l i g o m e r i z a t i o n , n u c l e a t i o n , and p r e c i p a t i o n as a f i l m b e f o r e mass t r a n s p o r t c a r r i e s the a c t i v a t e d monomer ( o r o l i g o m e r ) away i n t o the s o l u t i o n . We have had c o n s i d e r a b l e p r e v i o u s s u c c e s s w i t h t h i s s t r a t e g y f o r m e t a l p o l y p y r i d i n e complexes (13-15), f o r a l a r g e number of f u n c t i o n a l i z e d m e t a l complexes, and i t works f o r the compounds shown i n F i g . 1, as f r e e bases and as v a r i o u s l y m e t a l l a t e d forms ( 1 , 2 ) . Tetra(o-aminophenyl)porphyrin, H ( o - N H ) T P P , can f o r the purpose of e l e c t r o c h e m i c a l p o l y m e r i z a t i o n be s i m p l i s t i c a l l y viewed as f o u r a n i l i n e m o l e c u l e s w i t h a common p o r p h y r i n s u b s t i t u e n t , and one e x p e c t s that t h e i r o x i d a t i o n should form a p o l y ( a n i l i n e ) m a t r i x w i t h embedded p o r p h y r i n s i t e s . The p a t t e r n of c y c l i c v o l t a m m e t r i c o x i d a t i v e ECP (1) of t h i s f u n c t i o n a l i z e d m e t a l complex i s shown i n F i g . 2A. The growing c u r r e n t - p o t e n t i a l envelope r e p r e s e n t s a c c u m u l a t i o n of a polymer f i l m t h a t i s e l e c t r o a c t i v e and conducts e l e c t r o n s at the p o t e n t i a l s needed t o c o n t i n u o u s l y o x i d i z e f r e s h monomer t h a t d i f f u s e s i n from the b u l k solution. I f the f i l m were not f u l l y e l e c t r o a c t i v e at t h i s p o t e n t i a l , s i n c e the f i l m i s a dense membrane b a r r i e r t h a t prevents monomer from r e a c h i n g the e l e c t r o d e , f i l m growth would soon cease and the e l e c t r o d e would become p a s s i f i e d . T h i s was the case f o r the p h e n o l i c a l l y s u b s t i t u t e d p o r p h y r i n i n F i g . 1. 2

2

f l

C h a r a c t e r i z a t i o n of the E l e c t r o p o l y m e r i z e d

M

Films

Each m e t a l l o t e t r a p h e n y l p o r p h y r i n has i t s own p a r t i c u l a r c u r r e n t p o t e n t i a l growth p a t t e r n ; those f o r o t h e r complexes b e s i d e s t h a t i n F i g . 2A are d i s p l a y e d elsewhere ( 1 6 ) . We have not attempted to u n r a v e l the d e t a i l s of the o x i d a t i v e redox s t a t e s i n v o l v e d i n the f i l m growth. That of F i g . 2A o b v i o u s l y i s not s i m p l y

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

33. WHITE ET AL.

Polymeric Films of Metallotetraphenylporphyrins

•1.0 O.5 O.0 ι—I—ι—ι—ι—ι—I—ι—ι—ι—ι—I

Fig. 2. Curve A: Electropolymerization of lmH H (o-NH )TPP in O.1M Et^NClO^/CH^CN by sweeping potential at 200mV/s on Pt electrode. Numbers represent scan number. Curve B; Cyclic voltammogram of an electropolymerized film of poly-[H (oNH )TPP] on a Pt electrode, inO.1MEt^NClO^/CHoCN at 200 mV/s. Integration of the charge under the wave shows that coverage is 3.5X1°"° mol/cm^ of the porphyrin sites. Curve C: Rotated disk electrode voltammetry of the Os(lII,Il) reaction forO.2mM [0s(bpy) Cl ] inO.1M EfyNClO^/CI^CN at a naked ( ) and at a polymer coated ( ) Pt disk electrode. The polymer is poly[Co(o-NH )TPP] at 2.8X10-9 mol/cm . 2

2

2

2

2

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

411

412

INORGANIC AND ORGANOMETALLIC POLYMERS

p o l y ( a n i l i n e ) - l i k e n o r i n f a c t would we expect i t t o be g i v e n the o v e r l a p of t h e a n i l i n e and p o r p h y r i n r i n g o x i d a t i o n p o t e n t i a l s and the i n t e r r u p t i o n of growth of long l i n e a r p o l y ( a n i l i n e ) c h a i n s e x p e c t e d g i v e n from the s t e r i c b u l k i n e s s of the p o r p h y r i n s u b s t i t u e n t . However, we do b e l i e v e the c o u p l i n g c h e m i s t r y of the o x i d i z e d p o r p h y r i n i n v o l v e s mainly (perhaps not e x c l u s i v e l y ) s h o r t ( d i m e r , t r i m e r ? ) h e a d - t o - t a i l coupled a n i l i n e u n i t s . Such c o u p l i n g has been supported by Raman s t u d i e s of the aminop o r p h y r i n f i l m s by S p i r o and a s s o c i a t e s (who i n d e p e n d e n t l y d i s c o v e r e d t h e i r ECP as p a r t of t h e i r own i n t e r e s t i n t h i s t o p i c ) ( 1 7 ) . Many u n c e r t a i n t i e s on the other hand, remain about t h e d e t a i l s of the r a d i c a l c a t i o n c o u p l i n g r e a c t i o n s , and t h e i r p r o d u c t s , t h a t a c t to b i n d the c e n t r a l m e t a l l o t e t r a p h e n y l p o r p h y r i n u n i t s together. The problem i s c o m p l i c a t e d i n an a n a l y t i c a l s e n s e , s i n c e t h e e x t r e m e l y s m a l l q u a n t i t i e s of polymer t h a t a r e formed by ECP i n a f i l m l i m i t the number of techniques u s e f u l l y brought t o bear on the i n t e r c o n n e c t i n g polymer s t r u c t u r e The e l e c t r o c h e m i c a metallotetraphenylporphyri s t r a i g h t f o r w a r d l y a n a l y z e d ( 1 , 2 ) . With few e x c e p t i o n s , when transferred to a fresh supporting e l e c t r o l y t e s o l u t i o n , f i l m s formed from ECP r e a c t i o n s l i k e F i g . 2A e x h i b i t e l e c t r o c h e m i c a l r e d u c t i o n waves a t or very near the p o t e n t i a l s observed f o r r e d u c t i o n s of the c o r r e s p o n d i n g monomers d i s s o l v e d i n s o l u t i o n s . For example, a f i l m formed o x i d a t i v e l y as i n F i g . 2A g i v e s i n f r e s h e l e c t r o l y t e the r e d u c t i v e c y c l i c voltammogram of F i g . 2B. The i n d i c a t e d f o r m a l p o t e n t i a l E i of the c o r r e s p o n d i n g monomer (-1.17V) i n s o l u t i o n i s very near t h a t of the s u r f a c e f i l m (-1.13V v s . SSCE). That f o r m a l p o t e n t i a l s of s u r f a c e f i l m s on c h e m i c a l l y m o d i f i e d e l e c t r o d e s a r e near those of t h e i r c o r r e s p o n d i n g d i s s o l v e d monomers (13,18) i s a c t u a l l y a common, and quite u s e f u l , observation. I n the present c a s e , i t demonstrates t h a t the e l e c t r o n i c s t r u c t u r e s of the p o r p h y r i n r i n g s embedded i n the polymer f i l m a r e not s e r i o u s l y p e r t u r b e d from t h a t of the monomer. The r e d u c t i o n e l e c t r o c h e m i s t r y of ECP p o r p h y r i n f i l m s f u r t h e r m o r e responds t o added a x i a l l i g a n d s i n t h e expected ways. We have t e s t e d t h i s ( 2 , 6 ) f o r the ECP form of the i r o n complex of tetra(o-amino)phenyl)porphyrin by adding c h l o r i d e and v a r i o u s n i t r o g e n e o u s bases t o the c o n t a c t i n g s o l u t i o n s , o b s e r v i n g the F e ( I I l / l I ) wave s h i f t t o expected p o t e n t i a l s based on t h e monomer b e h a v i o r i n s o l u t i o n . T h i s i s a d d i t i o n a l e v i d e n c e t h a t the e s s e n t i a l p o r p h y r i n s t r u c t u r e i s preserved d u r i n g the o x i d a t i o n of the monomer and i t s i n c o r p o r a t i o n i n t o a p o l y m e r i c f i l m . Another p o i n t of importance about the f i l m s t r u c t u r e i s t h e degree t o which i t can be permeated by v a r i o u s i o n s and m o l e c u l e s . I t i s of course e s s e n t i a l t h a t s u p p o r t i n g e l e c t r o l y t e i o n s be a b l e to p e n e t r a t e the f i l m , e l s e the e l e c t r i c a l double l a y e r a t the e l e c t r o d e / p o l y m e r i n t e r f a c e c o u l d not be charged t o p o t e n t i a l s t h a t d r i v e e l e c t r o n t r a n s f e r s between t h e polymer and t h e e l e c t r o d e . The e l e c t r o n e u t r a l i t y requirements of p o r p h y r i n s i t e s as t h e i r e l e c t r i c a l charges a r e changed by o x i d a t i o n o r r e d u c t i o n a l s o c o u l d not be s a t i s f i e d w i t h o u t e l e c t r o l y t e p e r m e a t i o n . With the p o s s i b l e e x c e p t i o n of the p h e n o l i c s t r u c t u r e i n F i g . 1, t h i s l e v e l of p e r m e a b i l i t y seems to be met by the ECP p o r p h y r i n s . g

o

n

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

33.

WHITE ET AL.

Polymeric Films of Metallotetraphenylporphyrins

Another l e v e l of p e r m e a b i l i t y i s t h a t of a x i a l b a s e s , such as p y r i d i n e . As noted above, we have seen the p o r p h y r i n e l e c t r o c h e m i s t r y to respond to a d d i t i o n of a x i a l bases to the c o n t a c t i n g s o l u t i o n s ; t h i s i m p l i c i t l y demonstrates t h a t bases such as p y r i d i n e are a b l e to p e n e t r a t e the f i l m s t o the a x i a l p o r p h y r i n sites in i t s interior. I f the permeating m o l e c u l e i s s u f f i c i e n t l y b u l k y , of c o u r s e , the degree to which i t can p a r t i t i o n i n t o the polymer ( d i s s o l v e i n , w i t h p a r t i t i o n c o e f f i c i e n t Ρ = C / C ) , and i t s m o b i l i t y w i t h i n the polymer ( d i f f u s i o n c o e f f i c i e n t w i t h i n , D ^ ) , may become s m a l l . T h i s i s i n f a c t o b s e r v e d , as i l l u s t r Kid by the r o t a t e d d i s k e l e c t r o d e voltammogram of a s o l u t i o n of the b u l k y permeant [ O s ( b p y ) C 1 ] at a ECP f i l m of c o b a l t t e t r a ( o a m i n o p h e n y l ) p o r p h y r i n i n F i g . 2C. In order t o be o x i d i z e d , t h i s complex must permeate through the f i l m to the electrode/polymer i n t e r f a c e . The d i f f e r e n c e between the l i m i t i n g c u r r e n t s observed f o r the o s m i u m ( I I / I I I ) o x i d a t i o n at the naked ( ) as opposed t o the polymer-coated (- impedance of the polyme complex. A n a l y s i s of how the l i m i t i n g c u r r e n t s depend on the e l e c t r o d e r o t a t i o n r a t e (5) g i v e s a p e r m e a b i l i t y PD 4X10~ cm / s . T h i s i s over 1,000 X s m a l l e r than t h e 3 i f f u s i o n c o e f f i c i e n t f o r t h i s osmium complex d i f f u s i n g f r e e l y i n the a c e t o n i t r i l e s o l v e n t ( o b t a i n e d from the l i m i t i n g c u r r e n t at the naked Pt e l e c t r o d e ) , and the observed PD ^ c o r r e s p o n d s to a v e r y low p e r m e a b i l i t y of the polymer f i l m t o i u c h b u l k y permeants. Given our a b i l i t y t o prepare sandwich e l e c t r o d e m i c r o s t r u c t u r e s from these p o l y m e r i c p o r p h y r i n s i n the f a c e of the known (19) s e n s i t i v i t y of these m i c r o s t r u c t u r e s to f a i l u r e v i a p i n h o l e d e f e c t s i n the f i l m , we b e l i e v e that the above f i l m permeation ( F i g . 2C) occurs m a i n l y by a membrane-dissolving mechanism r a t h e r than by t r a n s p o r t of the osmium complex through an assortment of h o l e s and d e f e c t s i n the polymer f i l m . That i s , we b e l i e v e the " h o l e s " i n the f i l m s t h a t permeants pass through i n o r d e r to reach the e l e c t r o d e / p o l y m e r i n t e r f a c e , are a p p r o x i m a t e l y m o l e c u l a r s i z e d and mainly r e p r e s e n t the i n t e r s t i c e s between randomly coupled p o r p h y r i n u n i t s , r a t h e r than h o l e s t h a t are m o l e c u l a r l y l a r g e spaces as might be caused by i n t e r v e n t i o n by p a r t i c u l a t e m a t t e r or by m e c h a n i c a l or t h e r m a l l y - i n d u c e d s t r e s s t e a r s i n the f i l m . B e t t e r evidence f o r t h i s a s s e r t i o n c o u l d come from a comparison of PD among permeants of a graded m o l e c u l a r s i z e , as done p r e v i o u s l ^ r o r a d i f f e r e n t ECP polymer f i l m ( 2 0 ) . The r e s u l t s of c u r r e n t experiments (5) are p r o m i s i n g i n t h i s regard. Q

2

t

i n

2

1

P

Q

Ί

E l e c t r o n Transport

i n ECP

Metallotetraphenylporphyrin

Films

The a b i l i t y , i n a v o l t a m m e t r i c experiment l i k e F i g . 2B, t o e l e c t r o c h e m i c a l l y charge the e q u i v a l e n t of many monolayers of p o r p h y r i n s i t e s even though the i n d i v i d u a l s i t e s are r e l a t i v e l y immobile w i t h i n the polymer, i m p l i e s the e x i s t e n c e of an e f f i c i e n t

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

413

414

INORGANIC AND ORGANOMETALLIC POLYMERS

s i t e - s i t e e l e c t r o n t r a n s p o r t mechanism. The e l e c t r o n t r a n s p o r t mechanism f o r f i l m s l i k e the p o l y m e r i c p o r p h y r i n s was i n f a c t i d e n t i f i e d some time ago, as a hopping of e l e c t r o n s (21) between the l o c a l i z e d e l e c t r o n i c s i t e s that the redox monomer s i t e s i n the polymer r e p r e s e n t . The p r o c e s s can be r e p r e s e n t e d , f o r a polymeric i r o n tetra(o-aminophenyl)porphyrin c h l o r i d e , of f i r s t a p o l y - [ F e ( I I I ) T P P ( X ) ] s i t e next to the e l e c t r o d e / p o l y m e r i n t e r f a c e becoming reduced by an e l e c t r o n t r a n s f e r from the e l e c t r o d e : Electrode(e") + poly-[Fe(III)TPP(X)] —> Electrode + poly[ F e ( I I ) T P P ] + X" The reduced p o l y - [ F e ( I I ) T P P ] p o r p h y r i n s i t e now f i n d s i t s e l f next to a f r e s h p o l y - [ F e ( I I I ) T P P ( X ) ] s i t e one polymer l a t t i c e u n i t f u r t h e r i n t o the polymer. An e l e c t r o n hopping - or s e l f exchange - r e a c t i o n can then ensue, r e p e a t e d l y , i n s u c c e s s i v e l a y e r s and s i t e s : poly-[Fe(II)TPP]

+ poly-[Fe(III)TPP(Cl) +

poly-[Fe(II)TPP]

The r e a c t i o n amounts to a v e c t o r i c a l l y d i r e c t e d c u r r e n t i n the sense of o c c u r r i n g down a c o n c e n t r a t i o n g r a d i e n t of reduced p o l y [ F e ( I I ) T P P ] s i t e s emanating from the r e d u c i n g electrode/polymer i n t e r f a c e . The magnitude of the c u r r e n t c l e a r l y conveys i n f o r m a t i o n about the r a t e of the p o l y - [ F e ( I I I ) T P P ( X ) ] - p o l y [ F e ( I I ) T P P ] s e l f exchange r e a c t i o n . The above m e c h a n i s t i c aspect of e l e c t r o n t r a n s p o r t i n e l e c t r o a c t i v e polymer f i l m s has been an a c t i v e and c h e m i c a l l y r i c h r e s e a r c h t o p i c (13-18) i n polymer coated e l e c t r o d e s . We have c a l l e d (19) the p r o c e s s "redox c o n d u c t i o n " , s i n c e i t i s a nonohmic form of e l e c t r i c a l c o n d u c t i v i t y t h a t i s i n t r i n s i c a l l y d i f f e r e n t from t h a t i n metals or s e m i c o n d u c t o r s . Some of the s p e c i a l c h a r a c t e r i s t i c s of redox c o n d u c t i v i t y are n o n - l i n e a r c u r r e n t - v o l t a g e r e l a t i o n s and a narrow band of c o n d u c t i v i t y c e n t e r e d around e l e c t r o d e p o t e n t i a l s t h a t y i e l d the n e c e s s a r y m i x t u r e of o x i d i z e d and reduced s t a t e s of the redox s i t e s i n the polymer (mixed v a l e n t form). E l e c t r o n hopping i n redox c o n d u c t i v i t y i s o b v i o u s l y a l s o p e c u l i a r t o polymers whose s i t e s comprise s p a t i a l l y l o c a l i z e d e l e c t r o n i c s t a t e s . I n the present c a s e , the e l e c t r o n hopping c h e m i s t r y i n the p o l y m e r i c p o r p h y r i n s i s an e s p e c i a l l y r i c h t o p i c because we can m a n i p u l a t e the a x i a l c o o r d i n a t i o n of the p o r p h y r i n , t o l e a r n how e l e c t r o n s e l f exchange r a t e s respond to a x i a l c o o r d i n a t i o n , and because we can compare the s e l f exchange r a t e s of the d i f f e r e n t redox couples of a g i v e n m e t a l l o t e t r a p h e n y l p o r p h y r i n polymer. To measure these c h e m i c a l e f f e c t s , and a v o i d p o t e n t i a l l y competing k i n e t i c phenomena a s s o c i a t e d w i t h m o b i l i t i e s of the e l e c t r o n e u t r a l i t y - r e q u i r e d c o u n t e r i o n s i n the p o l y m e r s , we chose a s t e a d y s t a t e measurement technique based on the sandwich e l e c t r o d e microstructure (19). F i g u r e 3 i l l u s t r a t e s the c o n s t r u c t i o n of a sandwich e l e c t r o d e , which i s a d e l i c a t e l y assembled yet p o w e r f u l e x p e r i m e n t a l t o o l . The p o r o s i t y of the Au e l e c t r o d e evaporated

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

33. WHITE ET AL.

Polymeric Films of Metallotetraphenylporphyrins

onto the t h i n polymer f i l m ( t h i c k n e s s d) a l l o w s i n s p e c t i o n of the f i l m ' s voltammetry, and permits the i o n and s o l v e n t access f o r adjustment of i t s average o x i d a t i o n s t a t e to a mixed v a l e n t form. F i g u r e 4 shows the a p p l i c a t i o n (6) of p o t e n t i a l s to the Pt and Au e l e c t r o d e s of the sandwich ( v s . a r e f e r e n c e e l e c t r o d e elsewhere i n the c o n t a c t i n g e l e c t r o l y t e s o l u t i o n ) so t h a t they span the E ° of the p o l y - [ C o ( I l / l ) T P P ] couple ( F i g . 4B). There i s a consequent r e d i s t r i b u t i o n of the c o n c e n t r a t i o n s of the s i t e s i n the two o x i d a t i o n s t a t e s to a c h i e v e the s t e a d y s t a t e l i n e a r g r a d i e n t s shown i n the i n s e t . F i g u r e 4C r e p r e s e n t s s u r f a c e p r o f i l o m e t r y of a d i f f e r e n t f i l m sample i n order t o determine the f i l m t h i c k n e s s from t h a t the a c t u a l p o r p h y r i n s i t e c o n c e n t r a t i o n (O.85M). The f l o w of s e l f exchange-supported c u r r e n t i s e x p e r i m e n t a l l y p a r a m e t e r i z e d by a p p l y i n g F i c k ' s f i r s t law to the c o n c e n t r a t i o n - d i s t a n c e diagram i n F i g . 4B: f

i,. = nFAD C lim e

là

where C /d r e p r e s e n t s p o r p h y r i n s i t e s and the m o b i l i t y of the e l e c t r o n i s g i v e n as the " e l e c t r o n d i f f u s i o n c o e f f i c i e n t " D (13,18). D i s proportional t o the apparent s e l f exchange r a t e a s f o r m u l a t e S by Saveant (22) e

D

3

e

a p p

= 10 k ex

C_

Co

(Λ)

(2)

2

3 where C i s the t o t a l c o n c e n t r a t i o n (mol/cm ) of p o r p h y r i n s i t e s s e p a r a t e d by the average d i s t a n c e Λ . A p p l i c a t i o n of these two e q u a t i o n s to l i m i t i n g c u r r e n t s l i k e t h a t of F i g . 4B, f o r f i l m s p o l y m e r i c p o r p h y r i n s c o n t a i n i n g a v a r i e t y of m e t a l s , mixed v a l e n t s t a t e s , and added l i g a n d s g i v e s r e s u l t s p l o t t e d i n the important F i g . 5. We can say s e v e r a l t h i n g s about the apparent s e l f exchange rate constants. F i r s t , they vary by n e a r l y 10 X, even though the e l e c t r o l y t e i o n s and the mode of ECP c o u p l i n g are the same throughout. (Tetra(o-aminophenyl)porphyrin, in differently m e t a l l a t e d forms, was used i n each c a s e . ) T h i s i s commanding e v i d e n c e t h a t the p r i m a r y response of k i s t o the p a r t i c u l a r c h e m i s t r y of the p o r p h y r i n tfiat p e r t a i n s to the p o r p h y r i n ' s m e t a l , i t s o x i d a t i o n s t a t e , and a x i a l l i g a n d . The a l t e r n a t i v e r e s u l t would have been a k that v a r i e d l i t t l e ex w i t h p o r p h y r i n m e t a l because the e l e c t r o n hopping was e n e r g e t i c a l l y c o n t r o l l e d by the r a t e of movements of e l e c t r o l y t e i o n or p o l y m e r i c l i n k a g e s w i t h i n the polymers. S e c o n d l y , F i g . 5 shows t h a t the p o l y m e r i c r a t e c o n s t a n t s p a r a l l e l v a l u e s of heterogeneous r a t e c o n s t a n t s that have been observed f o r the e l e c t r o c h e m i c a l r e a c t i o n s of s o l u t i o n s of the c o r r e s p o n d i n g d i s s o l v e d p o r p h y r i n monomers. (The s l o p e of the l i n e i s O.5). T h i s re-emphasizes what was s a i d above, t h a t measurements of e l e c t r o n hopping i n polymers can g i v e r a t e c o n s t a n t s t h a t are m e a n i n g f u l i n the c o n t e x t of the metalloporphyrin s i n t r i n s i c e l e c t r o n t r a n s f e r chemistry. T h i r d l y , one experiment to measure D was c a r r i e d out w i t h the sandwich of e l e c t r o d e s and p o l y m e r i c p o r p h y r i n f i l m exposed t o a gas phase r a t h e r than a f l u i d l i q u i d b a t h i n g environment. T h i s a p p

p p

1

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

415

416

INORGANIC AND ORGANOMETALLIC POLYMERS

*

POROUS Au

REDOX POLYMER FILM

SOLID Pt

R,NX; 4 ' SOLVENT OR

CASE

F i g . 3·

GAS

\ ^

Diagram (cross-section) of a sandwich electrode.

E , V vs. SSCE -O.5 -1.0 -1.5 P t

O.0

7

S

E

'L

Au

I

. . 1,

C

U

J .

4 0 um 300nm[~^

,

κ.

u

d=90nm JL.

F i g . 4. Voltammograms i n O.1M Et^NClO^/CHoCN; Γ = 1.2X1Ο" mol/cm . Curve A; Cyclic voltammetry of Pt/poly-Co(o-NH )TPP at 20 rnV/s; S = 200uA/cm^. Curve Β ; Four-electrode voltammetry of Pt/poly-Co(o-NH4)TPP/Au sandwich electrode with E^u = O.0 V; E ^ scanned negatively at 5 mV/s; S = 400uA/cm . Curve C.; Surface profilometry of a poly-Co(o-NH )TPP f i l m on Sn0 /glass; Γτ = 7.6X10"9 mol/cm . (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.) 8

τ

2

2

2

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

33. WHITE ET AL.

Polymeric Films of Metallotetraphenylporphyrins

-2.50

-2.00

-1.50

F i g . 5· P l o t o f a p p a r e n t e l e c t r o n s e l f exchange r a t e c o n s t a n t s k|^P, d e r i v e d from polymer D values f o r f i l m s c o n t a i n i n g the i n d i c a t e d m e t a l s , mixed v a l e n t s t a t e s , and l i g a n d s , a l l i n a c e t o n i t r i l e , u s i n g E q u a t i o n 2, v s . literature h e t e r o g e n e o u s e l e c t r o n t r a n s f e r r a t e c o n s t a n t s k° f o r the c o r r e s p o n d i n g monomers i n n i t r i l e s o l v e n t s . See Ref. 6 f o r details. (Reproduced from Ref. 6. C o p y r i g h t 1987 American Chemical S o c i e t y . ) e

n o v e l r e s u l t was o b t a i n e d by e l e c t r o l y t i c a l l y c o n v e r t i n g a p o r p h y r i n f i l m t o the 1:1 F e ( I I I / I I ) mixed v a l e n t form, then removing the sandwich from the e l e c t r o l y t e s o l u t i o n , d r y i n g i t , e x p o s i n g i t t o a p y r i d i n e / a c e t o n i t r i l e vapor b a t h , and scanning the v o l t a g e a p p l i e d to the sandwich ( 7 ) . The r a t e c o n s t a n t f o r e l e c t r o n s e l f exchange, as measured by D , i s by t h i s experiment shown t o be s e n s i t i v e to a change of polymer s o l v a t i o n s t a t e , d e c r e a s i n g i n the b a t h i n g gas by 3X as compared to immersing the f i l m i n p y r i d i n e / a c e t o n i t r i l e l i q u i d . Hopefully, continuing experiments w i l l g i v e us i n s i g h t s i n t o the m o l e c u l a r e f f e c t s that a r e important to s o l v e n t - d e p r i v e d e l e c t r o n s e l f - e x c h a n g e s . E l e c t r o c a t a l y s i s of Dioxygen R e d u c t i o n by C o b a l t

Porphyrins.

Dioxygen r e d u c t i o n e l e c t r o c a t a l y s i s by m e t a l m a c r o c y c l e s adsorbed on or bound to e l e c t r o d e s has been an important a r e a of i n v e s t i g a t i o n (23) and has a c h i e v e d a s u b s t a n t i a l m o l e c u l a r s o p h i s t i c a t i o n i n terms of s t r u c t u r e d d e s i g n of the m a c r o c y c l i c c a t a l y s t s ( 2 4 ) . S i n c e t h e r e have been few o t h e r e l e c t r o c h e m i c a l s t u d i e s of p o l y m e r i c p o r p h y r i n f i l m s , we e l e c t e d to i n s p e c t the d i o x y g e n e l e c t r o c a t a l y t i c e f f i c a c y of f i l m s of e l e c t r o p o l y m e r i z e d cobalt tetraphenylporphyrins. A l l the f i l m s e x h i b i t e d some a c t i v i t y , t o d i f f e r i n g e x t e n t s , w i t h f i l m s of the c o b a l t t e t r a ( o a m i n o p h e n y l p o r p h y r i n ) b e i n g the most a c t i v e ( 2 - 4 ) . C u r i o u s l y , t h i s compound, both as a monomer i n s o l u t i o n and as an e l e c t r o p o l y m e r i z e d f i l m , a l s o e x h i b i t e d two e l e c t r o c h e m i c a l waves

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i n t h e range of p o t e n t i a l s expected f o r C o ( I I l / I I ) r e a c t i v i t y . The s i t u a t i o n i s complex because the r e l a t i v e prominence of these two f e a t u r e s depends on the e l e c t r o l y t e and i n t h e case of t h e polymer, the t h i c k n e s s of the polymer f i l m ( 3 ) . Both waves show c a t a l y t i c a c t i v i t y , and r o t a t e d r i n g d i s k measurements r e v e a l t h a t e l e c t r o l y s i s a t the p o t e n t i a l of the f i r s t (more p o s i t i v e ) C o ( I I I / I I ) wave produces a l a r g e r p o r t i o n o f the two e l e c t r o n t ^ C ^ r e d u c t i o n product than does e l e c t r o l y s i s a t the second wave p o t e n t i a l . The p r o p o r t i o n s furthermore,depend on t h e polymer f i l m t h i c k n e s s ; b e i n g 26 a n d 1 0 % f o r 3.4X10~ (about f o u r p o r p h y r i n m o n o l a y e r s ) and 1.6X10" mol/cm (about 16 l a y e r s ) , r e s p e c t i v e l y , a t t h e p o t e n t i a l o f the f i r s t wave. G e n e r a l l y s i m i l a r r e s u l t s were o b t a i n e d i n 1M NaOH, except t h a t t h i c k e r polymer f i l m s i n t h a t case r a t h e r s t a b l y e f f e c t r e d u c t i o n of d i o x y g e n w i t h no d e t e c t a b l e ^ 0 2 p r o d u c t , i . e . , a f o u r e l e c t r o n pathway. I n t e r p r e t i n g these r e s u l t s on a d e t a i l e d m o l e c u l a r b a s i s i s d i f f i c u l t because we have a t present no d i r e c t s t r u c t u r a l data p r o v i n g the n a t u r e o f th seems c r i t i c a l t o the the d i s s o l v e d monomeric p o r p h y r i n , i n C H ^ C ^ s o l v e n t , r e v e a l a s t r o n g tendency f o r a s s o c i a t i o n , e s p e c i a l l y f o r t h e t e t r a ( o aminophenyl)porphyrin. From t h i s o b s e r v a t i o n , we have s p e c u l a t e d ( 3 ) that the s p l i t C o ( I I l / l I ) wave may r e p r e s e n t r e a c t i v i t y of n o n - a s s o c i a t e d ( d i m e r ? ) and a s s o c i a t e d forms of the c o b a l t t e t r a ( o - a m i n o p h e n y l ) p o r p h y r i n s , and t h a t these s t a t e s p l a y d i f f e r e n t r o l e s i n the dioxygen reduction chemistry. That d i m e r i c c o b a l t p o r p h y r i n s i n p a r t i c u l a r can y i e l d more e f f i c i e n t f o u r e l e c t r o n d i o x y g e n r e d u c t i o n pathways i s w e l l known ( 2 4 ) . Our r e s u l t s suggest that e f f o r t s t o i n c o r p o r a t e more s t r u c t u r a l l y w e l l d e f i n e d d i m e r i c p o r p h y r i n s i n t o polymer f i l m s may be a w o r t h w h i l e l i n e of f u t u r e r e s e a r c h . 9

Acknowledgments T h i s r e s e a r c h was supported by g r a n t s from the N a t i o n a l S c i e n c e F o u n d a t i o n and t h e O f f i c e of N a v a l R e s e a r c h . H e l p f u l c o n v e r s a t i o n s w i t h P r o f e s s o r J . P. Collman of S t a n f o r d U n i v e r s i t y a r e a l s o acknowledged. Literature

Cited

1. White, Β. Α.; Murray, R. W. J. Electroanal. Chem. 1985, 189, 345. 2. Bettelheim, Α.; White, Β. Α.; Raybuck, S. Α.; Murray, R. W. Inorg. Chem. 1987, 26, 1009. 3. Bettelheim, Α.; White, Β. Α.; Murray, R. W. J. Electroanal. Chem. 1987, 217, 271. 4. Bettelheim, Α.; Reed, R. Α.; Hendricks, Ν. H.; Collman, J. P.; Murray, R. W. J. Electroanal. Chem., in press. 5. Pressprich, K., University of North Carolina, unpublished results, 1987. 6. White, B. A.; Murray R. W. J. Am. Chem. Soc. 1987, 109, 2576. 7. Faulkner, L. R.; Shaw, B. R.; Haight, G. P., Jr. J. Electroanal. Chem. 1982, 140, 147.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

33. WHITE ET AL. Polymeric Films of Metallotetraphenylporphyrins 419 8. Kanazawa, Κ. Κ.; Diaz, Α. F.; Geiss, R. H.; Gill, W. D., Kwak, J. F.; Logan, J. Α.; Rabolt, J. F., Street, G. B. J. Chem.Soc.Chem. Commun. 1979, 854. 9. Frommer, J. E.; Chance, R. R. Encyclop. Science and Technol., 2nd Ed., Vol. 5, John Wiley and Sons, 1986, p. 462. 10. Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980, 111, 111. 11. Bazier, M.; Lund, H., Eds., Organic Electrochemistry, M. Dekker, Ν. Y., 1983. 12. Adams, R. Ν.; Electrochemistry at Solid Electrodes, M. Dekker, Inc., Ν. Υ., 1969, p. 329, 363. 13. Murray, R. W.; Ann. Rev. Mats. Sci. 1984, 14, 145. 14. Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, J. S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151. 15. Leidner, C. R.; Sullivan, B. P.; Reed, R. Α.; White, Β. Α.; Crimmins, M. T.; Murray, R. W.; Meyer, T. J. Inorg. Chem. 1987, 26, 882. 16. White, Β. Α.; Ph.D. Chapel Hill, NC, 1986. 17. Su, Y. O.; Macor, Κ. Α.; Miller, L. Α., Spiro, T. G. 191 ACS National Meeting, N. Y. City, April 1986, Div. Inorg. Chem. Abstr. 50. 18. Murray, R. W. Electroanalytical Chemistry, Vol. 13, A. J. Bard, Ed., M. Dekker, 1984. 19. Pickup, P.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 1991. 20. Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J. Am. Chem. Soc. 1982, 104, 2683. 21. Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Kramer, S. R.; Chambers, J. Q. J. Am. Chem. Soc. 1980, 102, 483. 22. Andrieux, C. P.; Saveant, J. M. J. Electroanal. Chem. 1981, 123, 171. 23. Yeager, E. J. Molec. Cat. 1986, 38, 5. 24. Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J. Am. Chem. Soc. 1980, 103, 6027. RECEIVED

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1,

1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 34

Electronically Conducting Films of Poly(trisbipyridine)-Metal Complexes C. Michael Elliott, J . G. Redepenning, S. J . Schmittle, and E. M . Balk Department of Chemistry, Colorado State University, Fort Collins,

Polymers films formed from trisbipyridineruthenium complexes and coated on electrode surfaces have been found to have interesting e1ectrochromic and conductiνity properties.

Many e x a m p l e s o f e l e c t r o d e s m o d i f i e d w i t h p o l y m e r s c o n t a i n i n g m e t a l t r i s b i p y r i d i ne c o m p l e x e s h a v e been r e p o r t e d i n t h e l a s t decade. Most o f these m a t e r i a l s f a l 1 i n t o a s u b - c a t e g o r y o f i o n i c c o n d u c t o r s o f t e n r e f e r r e d t o as " r e d o x " c o n d u c t o r s ( 1 - 2 ) . The d r i v i n g f o r c e r e s p o n s i b l e f o r charge t r a n s p o r t across a redox c o n d u c t i n g f i l m coated on an e l e c t r o d e and h a v i n g s p a t i a l l y f i x e d e l e c t r o a c t i v e sites is the establishment o f a concentration gradient within the f i l m . T h i s g r a d i e n t i s g e n e r a t e d by t h e o x i d a t i o n o r r e d u c t i o n o f e l e c t r o c h e m i c a l l y a c t i v e s i t e s which a r e i n d i r e c t c o n t a c t w i t h t h e e l e c t r o d e . The c h a r g e i s t h e n propagated t h r o u g h t h e f i l m by s i t e - s i t e e l e c t r o n exchange o r s o ca I 1 ed " e l e c t r o n h o p p i n g . " In contrast, electronically c o n d u c t i n g p o l y m e r s , such as polvDvrrole, a r e ohmic i n n a t u r e and c h a r g e t r a n s p o r t i s i n r e s p o n s e t o an e l e c t r o s t a t i c p o t e n t i a l gradient r a t h e r than a concentration gradient (3). We r e p o r t h e r e s t u d i e s on a p o l y m e r f i l m w h i c h i s f o r m e d b y t h e t h e r m a l p o l y m e r i z a t i o n o f a monomeric c o m p l e x t r i s ( 5 , 5 ' b i s [ ( 3 - a c r y 1 y 1 - 1 - p r o p o x y ) c a r b o n y 1 1 - 2 , 2 ' - b i p y r i d i n e ) r u t h e n i urn(11) as i t s t o s y l a t e sa I t , I (4). Polymer f i l m s formed from I ( p o l y - 1 ) a r e i n s o l u b l e i n a l l s o l v e n t s t e s t e d and possess e x t r e m e l y good chemical and e l e c t r o c h e m i c a l s t a b i l i t y . Depending on t h e f o r m a l o x i d a t i o n s t a t e o f t h e r u t h e n i u m s i t e s i n p o l y - I t h e m a t e r i a l can e i t h e r a c t as a r e d o x c o n d u c t o r o r as an e l e c t r o n i c ( o h m i c ) conductor h a v i n g a s p e c i f i c c o n d u c t i v i t y which i s s e m i c o n d u c t o r 1 i k e i n magnitude.

0097-6156/88/0360-0420506.00/0 © 1988 American Chemical Society

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Re su 1 t s and Pi scuss ion A c y c l i c v o 1 tammogram o f a p o l y - I f i l m c o a t e d on a t i n o x i d e e l e c t r o d e i s shown i n F i g u r e 1. One o f t h e more i n t e r e s t i n g p r o p e r t i e s o f I and polymers formed from i t , as e v i d e n c e d by t h e voltammogram i n F i g u r e 1, i s t h e e x t e n s i v e a r r a y o f s t a b l e o x i d a t i o n s t a t e s t h a t a r e a c c e s s i b l e by e l e c t r o c h e m i c a l r e d u c t i o n . As t h e e l e c t r o d e p o t e n t i a l i s v a r i e d b e t w e e n -O.8 and - 2 . 2 V v s . A g / A g (-O.303 V v s . f e r r o c e n e / f e r r i c e n i u r n ) t h e r u t h e n i u m s i t e s a r e c o n v e r t e d , by s e g u e n t i a l one e l e c t r o n s t e p s , f r o m t h e 2+ o x i d a t i o n s t a t e a l 1 t h e way t o t h e f o r m a l 4 - s t a t e . I n t h e course o f these r e d u c t i o n s t h e r u t h e n i u m a l m o s t c e r t a i n l y remains as a d r u t h e n i um( I I ) t h r o u g h o u t a n d t h e r e d u c i n g e q u i v a l e n t s e n t e r t h e l o w - l y i n g π o r b i t a l s of the very e l e c t r o n d e f i c i e n t , esters u b s t i t u t e d b i p y r i d i n e 1 igands ( 5 ) . I t was n o t e d e a r l y i n o u r s t u d i e s o f monomer i c a n a l o g s o f Ï t h a t s i g n i f i c a n t changes o c c u r r e d i n t h e v i s i b l e s p e c t r u m upon c o n v e r s i o n between o x i d a t i o v i v i d , visual difference a r e a l s o v e r y apparent i n p o l y - I and th u s make i t o f i n t e r e s t as an e 1 e c t r o c h r o m i c m a t e r i a l ( 6 ) . (n a d d i t i o n t o i t s u n u s u a l o p t i c a l p r o p e r t i e s , p o l y - I is r a t h e r unique i n s e v e r a l other respects. In the formal 2 + o x i d a t i o n s t a t e t h e f i x e d s i t e s i n p o l y - I a r e p o s i t i v e l y charged. At - 2 . 2 V, h o w e v e r , i n t h e 4 - f o r m a l o x i d a t i o n s t a t e , t h e f i x e d s i t e s a r e n e g a t i v e l y charged. Thus by s i m p l y changing t h e a p p l i e d p o t e n t i a l t h e polymer may be c o n v e r t e d from an anion exchanger, t o a n e u t r a l polymer and f i n a l 1 y t o a c a t i o n exchanger. I t is also possible t o sterical ly block oxidation states of a p a r t i c u l a r c h a r g e - t y p e f r o m f o r m i n g i n t h e p o l y m e r by u s i n g 3 s o l u t i o n e l e c t r o l y t e h a v i n g one c h a r g e - t y p e i o n t h a t i s t o o l a r g e t o e n t e r t h e po 1 ymer ( 7 - 8 ) . Po 1 y ( 1 , 1 - d i m e t h y 1 - 3 , 5 - d i m e t h y 1 e n e p i p e r d i n i urn hexaf 1 u o r o p h o s p h a t e ) , 1 1 , i s j u s t such an e l e c t r o l y t e . F i g u r e 2 i s a eye l i e vo1tammogram o f a p 1 a t i num e 1 e c t r o d e coated with a f i l m o f p o l y - I i n an a c e t o n i t r i i e s o l u t i o n c o n t a i n i n g a normal e l e c t r o l y t e , TBA PF^~ ( t e t r a - N - b u t y 1 ammonium h e x a f 1 u o r o p h o s p h a t e ) and t h e same e l e c t r o d e i n an a c e t o n i t r i l e s o l u t i o n o f I I. As can be seen t h e f i r s t t w o r e d u c t i o n s , 2 + / 1 + and 1 + / 0 , ( w h i c h o v e r l a p one a n o t h e r ) a r e p r e s e n t i n b o t h vo 1 tammograms a t about, t h e same p o t e n t i a l s . In both o f these cases charge b a l a n c e w i t h i n t h e polymer i s m a i n t a i n e d by t r a n s p o r t o f P F ~ i o n s i n a n d o u t o f t h e f i 1 m. The t h i r d a n d s u b s e q u e n t r e d u c t i o n s , however, r e q u i r e t h e t r a n s p o r t o f c a t i o n s i n t o t h e f i l m i n order t o m a i n t a i n charge balance. I n t h e case o f t h e TBA PFç~ e l e c t r o l y t e t h e c a t i o n s can e n t e r t h e f i l m a n d t h e r e d u c t i o n s t a k e p l a c e , b u t f o r t h e p o l y m e r i c c a t i o n case t h e r e d u c t i o n s a r e absent because t h e c a t i o n s a r e t o l a r g e t o penetrate t h e p o l y - I f i l m (8). F i g u r e 3 shows t h e eye l i e vo1tammogram o f an a c e t o n i t r i 1 e s o l u t i o n c o n t a i n i n g 9 - f l u o r e n o n e on a bare Pt e l e c t r o d e and on a P o l y - I coated Pt e l e c t r o d e u s i n g I I as t h e e l e c t r o l y t e . The o n l y s i g n i f i c a n t d i f f e r e n c e between t h e vo1tammograms i s t h e presence o f t h e p o l y - I r e d u c t i o n a n d o x i d a t i o n waves f o r t h e c o a t e d electrode. We h a v e p r e v i o u s l y d e t e r m i n e d by o t h e r means t h a t p o l y - I f i l m s s h o u l d n o t be permeable t o m o l e c u l e s as l a r g e as 9 +

#

+

6

+

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F i g u r e 1. C y c l i c vo1tammogram on a t i n o x i d e e l e c t r o d e m o d i f i e d w i t h a t h i n f ï 1 m o f po 1 v - I . A sweep r a t e o f 50mV/s was e m p l o y e d i n CH CN c o n t a i n i n g O.1 M TBAPF . Ε v s . A g ( O . 1 M AgN0 in DMS0)/Ag. +

3

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σον

F i g u r e 2. C y c l i c v o I t a m m o g r a m s on a Pt e l e c t r o d e c o a t e d w i t h poly-I: ( ) , CH CN. O.1 M TBAPF ; ( ) , CH CN, 50 mM p o l y c a t i o n i c e l e c t r o l y t e , II. Scan r a t e = 50 mV/s. Ε v s . A g / A g . (Reproduced f r o m R e f . 7. C o p y r i g h t I985 American Chemical S o c i e t y . ) 3

6

3

+

F i g u r e 3. C y c l i c vo1tammograms s h o w i n g 9 - f l u o r e n o n e r e d o x a c t i v i t y a t a b a r e Pt e l e c t r o d e ( ) and a t a Pt e l e c t r o d e coated w i t h p o l y - 1 ( ). Both vo 1 tammograms were run i n t h e same CH CN, 50 mM I I e l e c t r o l y t e s o l u t i o n and r e f e r e n c e d t o At / A g . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m Ref. 8. C o p y r i g h t 1986 Ε Isevier.) 3

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f l u o r e n o n e ; t h e r e f o r e , F i g u r e 3 suggests t h a t t h e f l u o r e n o n e must be b e i n g reduced a t t h e p o l y m e r / s o l u t i o n i n t e r f a c e . In o r d e r t o produce i d e n t i c a l vo1tammograms f o r 9-f1ourenone on t h e bare and coated e l e c t r o d e s under these c o n d i t i o n s r e q u i r e s t h a t t h e f i l m i n t h e f o r m a l z e r o - v a l e n t o x i d a t i o n s t a t e have s i g n i f i c a n t e l e c t r o n i c c o n d u c t i v i t y (7). This i n t e r p r e t a t i o n i s borne o u t by t h e r e s u l t s shown i n F i g u r e 4 below. Murray, e t . a l . have d e s c r i b e d t h e c o n s t r u c t i o n o f s o - c a l l e d "sandwich e l e c t r o d e s " in which a polymer f i l m is sandwiched between two m e t a l l i c e l e c t r o d e s (9). I f one o f t h e e l e c t r o d e s i s p o r o u s , i o n s f r o m so 1 u t i on w î I 1 be ab 1 e t o e n t e r and 1 ea ve t h e p o l y m e r , thus t h e o x i d a t i o n s t a t e o f t h e polymer can be changed . A l s o , t h e two e l e c t r o d e s a r e e l e c t r i c a l l y separated by the polymer f i l m and thus each can be c o n t r o l l e d a t a d i f f e r e n t p o t e n t i a l . A s a n d w i c h e l e c t r o d e o f p o l y - I was c o n s t r u c t e d and e x a m i n e d as f o l lows: I η a so 1 u t i o n o f I I , u s i n g a b i p o t e n t i o s t a t , t h e two e l e c t r o d e s were c o n t r o l l e d such t h a t a smal 1 p o t e n t i a l d i f f e r e n c e ( c a . 20 mV) was m a i n t a i n e e l e c t r o d e s was scanne b e y o n d , t h e po 1 y - 1 2 + / I + and 1 + / 0 coup 1 e wh i I e ma i n t a i η i ng t h e s m a l l , c o n s t a n t p o t e n t i a l d i f f e r e n c e between them. F i g u r e 4 shows t h e c u r r e n t f l o w i n g b e t w e e n t h e t w o e l e c t r o d e s as a f u n c t i o n o f potential. At p o t e n t i a l s p o s i t i v e o f t h e 2 + / 1 + c o u p l e no measurable c u r r e n t f l o w s between t h e e l e c t r o d e s as a r e s u l t o f t h e l a r g e r e s i s t a n c e o f t h e f i l m (>1 X 1 0 *- Ω-cm) when i n t h e 2 + oxidation state. However, a t p o t e n t i a l s s u f f i c i e n t l y n e g a t i v e t o f u l l y reduce t h e p o l y - I f i l m t o t h e f o r m a l z e r o - v a l e n t o x i d a t i o n s t a t e , t h e f i l m passes c o n s i d e r a b l e c u r r e n t . Based on e s t i m a t e s of the f i l m thickness the c a l c u l a t e d s p e c i f i c resistance of the r e d u c e d f i l m i s a b o u t 1 X 1 0 Ω-cm wh i ch i s a t 1 e a s t 1 X 1 0 more c o n d u c t i v e t h a n i t i s i n t h e 2+ f o r m . P e r f o r m i n g t h e same experiment as above u s i n g a c o n v e n t i o n a l e l e c t r o l y t e a l l o w s one t o access t h e n e g a t i v e formal o x i d a t i o n s t a t e s o f p o l y - I . These experiments c o n f i r m t h a t t h e zero o x i d a t i o n s t a t e o f p o l y - I i s by f a r t h e most c o n d u c t i v e f o r m . Once a p o l y - I f i l m i n a sandwich e l e c t r o d e c o n f i g u r a t i o n i s r e d u c e d t o t h e z e r o - v a l e n t f o r m i t can be r e m o v e d f r o m t h e e l e c t r o c h e m i c a l e e l 1 and e x a m i n e d d r y , e . g . a f t e r t h e a c e t o n i t e t h a t swel I s t h e f i l m e v a p o r a t e s , ( t h e f i l m i n t h i s o x i d a t i o n s t a t e must, however, be p r o t e c t e d from 0 ) . In t h e d r y form z e r o - v a l e n t p o l y - I i s about a f a c t o r o f two more c o n d u c t i v e t h a n when swol 1 en w i t h a c e t o n i t r i 1 e. G e n e r a l l y , t h e c o n d u c t i v i t y o f redox c o n d u c t i n g p o l y m e r s d e c r e a s e s when t h e s o l v e n t i s r e m o v e d , o s t e n s i b l y b e c a u s e o f more r e s t r i c t e d l o c a l m o t i o n o f t h e c o u n t e r i o n s . The z e r o - v a l e n t p o l y - I f i l m c o n t a i n s no c o u n t e r i o n s . When immersed i n t o l u e n e t h e f i l m g i v e s t h e same r e s i s t a n c e as when i n t h e d r y f o r m . F i g u r e 5 i s a p l o t o f t h e l o g a r i t h m o f t h e f i l m r e s i s t a n c e (measured w i t h t h e e I e c t r o d e i n a t o i uene b a t h ) v s . T~ . The c u r v e i s f a i r 1 y 1 i n e a r o v e r t h e e n t i r e r a n g e f r o m room t e m p e r a t u r e down t o 77K, s u g g e s t i n g t h a t t h e c o n d u c t i o n mechanism i s a t h e r m a l l y a c t i v a t e d p r o c e s s . However, b e f o r e c o n s i d e r i n g t h i s d r y f i l m c o n d u c t i v i t y d a t a f u r t h e r , a model must be d e v e l o p e d t o e x p l a i n why z e r o v a l e n t p o l y - I f i l m s s h o u l d conduct a t a l l . Z e r o - v a l e n t p o l y - I d i f f e r s i n a number o f ways f r o m o t h e r 1

3

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-O.7

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F i g u r e 4. P l o t o f p o l y - I c o n d u c t i v i t y as a f u n c t i o n o f p o t e n t i a l . A s e r i e s o f p o t e n t i a l s t e p o f 20mV were e m p l o y e d on a s a n d w i c h electrode. Each p o t e n t i a l was h e l d u n t i l F a r a d a i c c u r r e n t ceased, where upon a DC c o n d u c t i v i t y measurement, ΔΕ = 60MV, was t a k e n , b e f o r e p r o c e d i n g w i t h t h e next p o t e n t i a l . The r e s u l t s a r e f o r O.05 M II e l e c t r o l y t e i n a c e t o n i t r i l e v s . A g / A g . +

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17-1

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F i g u r e 5. Conductivity o f a po l y - 1 film vs. 1/T(K). Experimental procedure: A p o l y - I s a n d w i c h e l e c t r o d e was p o t e n t i o s t a t e d i n i t s Ru° s t a t e u s i n g p o l y c a t i o n i c e l e c t r o l y t e , I I , t h e n removed from t h e e l e c t r o c h e m i c a l c e l l and p l a c e d i n a toluene bath. The t o l u e n e was q u i c k l y f r o z e n t o 77K, where r e s i s t a n c e o f t h e Ru p o l y - 1 was d e t e r m i n e d u s i n g a DC b i a s o f 20mV. The t e m p e r a t u r e was s l o w l y r e t u r n e d t o 298K, w i t h p e r i o d i c r e s i s t a n c e measurements. The break f r o m l i n e a r i t y occurs a t t h e toluene melting p o i n t .

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e l e c t r o n i c a l l y c o n d u c t i n g polymers. F i r s t l y , i t i s amorphous and p r o b a b l y h i g h 1 y cross 1 inked. S e c o n d l y , i t c o n t a i n s no e x t e n d e d conjugation. L a s t l y , a t l e a s t in a f o r m a l sense, i t i s not mixed valent. A c l o s e examination o f t h e e l e c t r o c h e m i s t r y o f both t h e p o l y - I f i l m and t h e monomer i n s o l u t i o n , suggests t h a t t h i s l a t t e r p o i n t , w h i l e t r u e i n t h e macroscopic sence, p r o b a b l y i s not t r u e on t h e m i c r o s c o p i c l e v e l . The redox p o t e n t i a l s f o r t h e 1+/0 and 0 / 1 - c o u p l e s i n s o l u t i o n a r e r e s p e c t i v e l y -O.850 and -1.006 V v s . SCE. The e x a c t p o t e n t i a l s f o r t h e a n a l o g o u s c o u p l e s i n t h e polymer a r e more d i f f i c u l t t o measure b u t , whatever t h e i r a b s o l u t e v a l u e s a r e , t h e i r s e p a r a t i o n i s l i k e l y t o be smal 1. C o n s i d e r a t i o n o f t h e Nernst e g u a t i o n shows t h a t i f t h e same 156 mV s e p a r a t i o n i n p o t e n t i a l s e x i s t s b e t w e e n t h e 1+/0 and 0 / 1 - c o u p l e s i n t h e polymer as e x i s t s i n s o l u t i o n , then t h e zero v a l e n t s t a t e s h o u l d be d i s p r o p o r t i o n a t e d t o about 5% i n t o t h e 1+ and 1 - forms a t room temperature. I n r e a l i t y t h e n , z e r o - v a l e n t p o l y - I i_s a m i x e d valent material. Two unique f e a t u r e s o f t h i s m a t e r i a l s h o u l d be noted a t t h i s p o i n t . F i r s t polymers, there are n t h a n t h e c o u n t e r i o n p a i r g e n e r a t e d by t h e d i s p r o p o r t i o n a t i o n . This means t h a t both t h e c a t i o n s and anions can move by e l e c t r o n exchange under f o r c e o f a v o l t a g e g r a d i e n t . Second, u n l i k e o t h e r mixed v a l e n c e polymers t h e degree o f mixed v a l e n c e c h a r a c t e r i s n o t d e t e r m i n e d by t h e d o p a n t c o n c e n t r a t i o n . I n a sense t h e population of 1 + s t a t e i s t h e r m a l l y i n d u c e d , and t h u s t h e c o n c e n t r a t i o n o f charge c a r r i e r s i s t e m p e r a t u r e dependent i n a Boltzmann f a s h i o n . To an e x t e n t t h e n , r e d u c e d p o l y - I has some s i m i l a r i t i e s t o an i n t r i n s i c semiconductor: t h e number o f p o s i t i v e and n e g a t i v e c h a r g e c a r r i e r s i s e q u a l and t h e i r absolute c o n c e n t r a t i o n i s e x p o n e n t i a l l y d e p e n d e n t on t h e a b s o l u t e temperature. E x t e n d i n g t h i s a n a l o g y one s t e p f u r t h e r , the s e p a r a t i o n between t h e 1+/0 and 0 / 1 - p o t e n t i a l s i s analogous t o a band gap e n e r g y . T h e r e i s , h o w e v e r , no e v i d e n c e , d i r e c t o r o t h e r w i s e , t o s u g g e s t t h a t r e d u c e d p o l y - I has any l o n g r a n g e e l e c t r o n de 1 oca I i z a t i o n analogous t o a band s t r u c t u r e . In f a c t , t h e v i s i b l e spectrum o f t h e reduced f i l m i s q u a l i t a t i v e l y s i m i l a r t o t h e s o l u t i o n spectrum o f s o l u b l e a n a l o g s in t h e same o x i d a t i o n s t a t e ; a f a c t which would argue t h a t t h e redox s t a t e s a r e p r o b a b l y f a i r l y wel 1 l o c a l ized w i t h i n t h e polymer. The d a t a i n F i g u r e 5 can now be c o n s i d e r e d i n 1 i g h t o f t h e c o n d u c t i o n model d e v e l o p e d a b o v e . As s t a t e d p r e v i o u s l y , c o n d u c t i o n i n reduced p o l y - I behaves l i k e an a c t i v a t e d process. There a r e two sources t h a t p o t e n t i a l l y c o u l d be r e s p o n s i b l e f o r t h i s behavior. The f i r s t i s t h e B o l t z m a n n t y p e c o n c e n t r a t i o n dependence o f t h e 1+ and 1 - s t a t e s d i s c u s s e d above. The number o f charge c a r r i e r s is expected t o decrease approximately e x p o n e n t i a l l y w i t h T~^. The second i s t h e a c t i v a t i o n b a r r i e r t o se 1f-exchange between 1+ and 0 s i t e s and 0 and 1 - s i t e s . For low c o n c e n t r a t i o n o f charge c a r r i e r s both processes are expected t o c o n t r i b u t e t o t h e measured r e s i s t a n c e . A n o t h e r f a c t o r w h i c h a p p e a r s t o be o f i m p o r t a n c e i n t h e c o n d u c t i v i t y b e h a v i o r o f z e r o - v a l e n t p o l y - I i s t h e absence o f n o n e l e c t r o a c t i v e i o n s i n t h e p o l y m e r . The d i f f e r e n c e i n p o t e n t i a l between t h e 2+/1+ and 1+/0 c o u p l e s i n s o l u t i o n i s s i g n i f i c a n t l y smal 1er t h a n t h e d i f f e r e n c e i n p o t e n t i a l between t h e 1+/0 and 0 / 1 -

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couples. The degree o f d i s p r o p o r t i o n a t i o n and t h u s t h e numbers o f p o t e n t i a l charge c a r r i e r s , s h o u l d t h u s be s i g n i f i c a n t l y l a r g e r i n the 1+ form. N o n e t h e l e s s , t h e zero v a l e n t form i s a t l e a s t f i v e times more c o n d u c t i v e . One i s drawn t o t h e c o n c l u s i o n t h a t e i t h e r t h e s e l f - e x c h a n g e r a t e s between t h e o x i d a t i o n s t a t e s i n v o l v e d ( i e . 2 + , 1+ and 0) a r e i n n a t e l y l o w e r f o r t h e 1+ s t a t e o r t h a t t h e presence of ions in the polymer a d v e r s e l y a f f e c t s charge transport. Based on t h e eye 1 i c v o l t a m m e t r y o f t h e monomer i n s o l u t i o n t h e heterogeneous charge t r a n s f e r f o r t h e 2+/1 + c o u p l e i s a t l e a s t as f a s t as t h e charge t r a n s f e r f o r t h e 1+/0 c o u p l e . This w o u l d argue t h a t t h e d i f f e r e n c e s i n c o n d u c t i v i t y between t h e 1 + and 0 f o r m s o f p o l y - I l i e s i n t h e p r e s e n c e and a b s e n c e , r e s p e c t i v e l y , o f n o n - e 1 e c t r o a c t i v e ions w i t h i n t h e polymer. One p o s s i b l e way t h a t t h e presence o f n o n - e 1 e c t r o a c t i v e ions in the polymer c o u l d a d v e r s e l y a f f e c t t h e c o n d u c t i v i t y o f t h e f i l m s i s by deepening t h e p o t e n t i a l w e l l f o r charge l o c a l i z a t i o n . Consider, f o r example, PF ~ ions r e s i d i n g i n t h e polymer m a t r i x . The c a t i o n i c s i t e s w i t h i l o c a l i z e around t h e PF^ t o move e i t h e r t h e anion must a l s o p h y s i c a l l y move or c o n s i d e r a b l e e n e r g y must be expended t o o v e r c o m e t h e e l e c t r o s t a t i c f o r c e s h o l d i n g the ion p a i r s t o g e t h e r . In the z e r o - v a l e n t polymer t h e c a t i o n s and a n i o n s a r e b o t h c a p a b l e o f " m o v i n g " by s i t e - s i t e exchange. A d d i t i o n a l 1 y, t o t h e e x t e n t t h a t t h e charges o f b o t h t h e 1+ and 1- ion s i t e s are d e l o c a l i z e d , t h e p o t e n t i a l w e l l s f o r each w i l l n e c e s s a r i l y be s h a l l o w e r than f o r a t o t a l l y localized i o n case ( i e . PFg~"). I t is thus p o s s i b l e t o r a t i o n a l i z e in several ways why, despite the smaller degree of d i s p r o p o r t i o n a t i o n , t h e n e u t r a l , " i o n - f r e e " zero v a l e n t form i s a much b e t t e r conductor than t h e 1+ form. Summary Polymer f i l m s o f I have a wide range o f r e v e r s i b l e r e d u c t i o n electrochemistry. The f i x e d s i t e s are s t a b l e i n o x i d a t i o n s t a t e s t h a t r a n g e f o r m t h e f o r m a l 2+ t o 4 - f o r m s . O v e r t h i s r a n g e o f o x i d a t i o n s t a t e s t h e polymer changes t h r o u g h an a r r a y o f c o l o r s . A d d i t i o n a l l y , because t h e f i x e d s i t e s change f o r m a l charge, t h e f i lm can be e i t h e r a c a t i o n e x c h a n g e r , a n e u t r a l p o l y m e r o r an a n i o n exchanger depending upon t h e a p p l i e d v o l t a g e . By e m p l o y i n g a s o l u b l e c a t i o n i c p o l y m e r as t h e s o l u t i o n e l e c t r o l y t e polymer f i l m s can be s t e r i c a l l y b l o c k e d from r e d u c i n g beyond t h e f o r m a l zero v a l e n t form. In t h e z e r o - v a l e n t form t h e polymer i s an ohmic conductor both i n s o l u t i o n and d r y . A model has been proposed which d e s c r i b e s t h e c o n d u c t i v i t y o f t h e polymer and i n p a r t accounts f o r i t s ohmic n a t u r e and semiconductor-1 i k e t e m p e r a t u r e dependence.

Acknowledgment S u p p o r t o f t h i s work u n d e r a g r a n t f r o m t h e U. S. D e p a r t m e n t E n e r g y , O f f i c e o f B a s i c E n e r g y S c i e n c e s , DE-FG02-87ER13666 g r a t e f u 1 1 y acknowledged.

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References 1.

R. W. Murray, "Chemically Modified Electrodes", in Electroanalytical Chemistry, Vol. 13, (A. J. Bard, ed.), M. Dekker, NY, 191 (1984). 2. a) H. D. Abruna, P. Denisevich, M. Umana, T. J. Meyer, and R. W. Murray, J. Am. Chem. Soc., 103, 1 (1981); b) P. Denisevich, K. W. Willman, and R. W. Murray, J. Am. Chem. Soc., 103, 4727 (1981); c) P. Denisevich, H. D. Abruna, C. R. Leidner, T. J. Meyer, and R. W. Murray, Inorg. Chem., 21, 2153 (1981); d) P. K. Ghosh and T. G. Spiro, J. Εlectrochem. Soc., 128, 1281 (1981); e) J. M. Calvert, B. P. Su11ivan, and T. J. Meyer, Adv. Chem. Ser., 192, 159 (1982); f) J. S. Facci, R. H. Schmehl, and R. W. Murray, J. Am. Chem. Soc., 104, 4959 (1982); g) J. M. Calvert, R. H. Schmehl, B. P. Su11ivan, J. S. Facci, T. J. Meyer, and R. W. Murray, Inorg. Chem., 22, 1983, 2151. 3. J. L. Bredas and G. (1985). 4. C. M. Elliott and J. G. Redepenning, J. Εlectroanal. Chem., 197, 219 (1986). 5. C. M. Εlliott and E. J. Hershenhart, J. Am. Chem. Soc., 104, 7519 (1982). 6. Chem. and Εng. News, 61, 24, 21 (1983). 7. C. Μ. E l i o t t , J. G. Redepenning, and E. M. Balk, J. Am. Chem. Soc., 107, 8302 (1985). 8. C. M. Ε11iott, J. G. Redepenning, and Ε. M. Balk, J. Electroanal. Chem., 213, 203 (1986). 9. P. Pickup, W. Kutner, C. R. Leidner, and R. W. Murray, J. Am. Chem. Soc., 106, 1991 (1984). RECEIVED September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 35

Copper Chloride Complexes with Poly(2-vinylpyridine) A. M . Lyons , E. M . Pearce , M . J. Vasile , A. M . Mujsce , and J. V. Waszczak 1

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AT&T Bell Laboratories, Murray Hill, NJ 07974 Polytechnic 2

Metal containing polymers were prepared by complexing copper(II) chloride to poly(2-vinylpyridine)(P2VPy) in aqueous methanol. Through control of the steric environment of the ligand and selection of the solvent system, soluble, linear complexes were formed, where one pyridine moiety is coordinated to each copper atom. The thermal decomposition of the complexes was studied by thermogravimetric analysis, pyrolysis mass spectroscopy, optical, infrared and x-ray photoelectron spectroscopy, and magnetic susceptibility measurements. The complexes decompose in several steps including loss of water, reduction of Cu(II), evolution of halogen, and fragmentation and condensation of the polymer structure. Thermal decomposition of the complex results in a composite of metallic copper and carbon. Increasing the copper chloride concentration in the complex significantly increases the polymer char yield.

The synthesis of inorganic compounds via the thermal decomposition of metal containing polymer precursors offers greater flexibility than conventional processing methods. The object of this study is to extend the processing advantages inherent to polymeric materials, such as film forming, fiber drawing, and molding of complex shapes, to inorganic compounds. In order to exploit these processing techniques, high molecular weight linear systems are required since crosslinked networks are not easily processable. The study of inorganic compound formation from polymer precursors has been limited to refractory materials. Ceramics such as silicon carbide , silicon nitride , and silicon carbide-titanium carbide composites have been prepared by pyrolyzing silicon based polymers. Electrically conductive films, fibers, and molded articles of carbon, prepared from polymeric precursors such as poly(acrylonitrile) and phenol-formaldehyde, are well known . Recently, conductive materials prepared from poly(phosphazenes) have been reported . The conductivity of these materials is limited to Ω-cm without subsequent doping. 1

2

3

4

5

In this study, we extend the range of inorganic materials produced from polymeric precursors to include copper composites. Soluble complexes between poly(2-vinylpyridine) (P2VPy) and cupric chloride were prepared in a mixed solvent of 95% methanol 5% water. Pyrolysis of the isolated complexes results in the formation of carbonaceous composites of copper. The decomposition mechanism of the complexes was studied by optical, infrared, x-ray photoelectron and pyrolysis mass spectroscopy as well as thermogravimetric analysis and magnetic susceptibility measurements.

0097-6156/88/0360-0430506.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

35. LYONS ET AL.

Copper Chloride Complexes

431

Experimental Poly(2-vinylpyridine), with M =36,000, was purchased from Aldrich Chemical Co. and purified by two precipitations from ethanol into deionized water. CuCl -2H 0 (ACS purity) and poly(4-vinylpyridine) from Mallinckrodt and Polysciences Inc. respectively, were used as received. Complexes with different ratios of Cu to pyridine moiety were formed in solution (95% methanol, 5%H 0). Due to decreasing solubility of the complex with increasing copper concentration, the concentration of reagents was varied as shown in Table I. v

2

2

2

Table I. Reactant specifications for the preparation of CuCl :P2VPy complexes 2

Complex (Cu:Py Ratio)

CuCl -2H 0 (moles)

P2VPy (moles)

1:4 1:2 1:1

1.25 χ ΙΟ"

5.00 χ 10"

2

2

3

Solvent (mis) 3

22

The complexes were isolated by freezing the solutions in liquid nitrogen and removing the solvent under a vacuum of œ ΙΟΟμπι of Hg as the solution thawed. Total solution concentration of 10~ M was employed for the continuous variation analysis. A series of C u C l solutions was used to generate a curve from which the background correction factors were determined, as C u C l does not follow Beer's Law in this region. Infrared spectra were recorded on a Perkin Elmer model 680 spectrophotometer as mulls in nujol or fluorolube. The magnetic susceptibility of the copper complexes was measured from 4.2 to 300 Κ by the Faraday method . X-ray photoelectron spectroscopy (XPS) was performed with a Perkin Elmer hemispherical spectrometer. T G A analysis was performed on a Perkin Elmer TGS-2 thermobalance in purified N at a flow rate of 30cc/min, heating rate of 10°C/min, and sample weight of 30mg. The materials were dried by heating in N for 2 hours at 30°C before beginning each experiment. The material formed in the T G A was analyzed by reheating to 600°C in 0 . The carbonaceous char volatilized, leaving a residue of CuO (as confirmed by x-ray diffraction). Pyrolysis mass spectroscopy was conducted with a Hewlett-Packard model 5985B gas chromatograph/quadrupole mass spectrometer, operated at « Î O Torr and 70eV electron-impact ionization energy. Samples were introduced into the mass spectrometer via a glass lined direct insertion probe (DIP). The samples were decomposed in the DIP to a nominal temperature of 300°C at a heating rate of 30°C/min. 2

2

2

6

2

2

2

- 6

Results and Discussion

Complex Preparation Three complexes were isolated with nominal copper to pyridine ratios of 1:4, 1:2, and 1:1. The green solutions yielded either green (1:4 and 1:2) or yellow brown (1:1) hygroscopic powders. The complexes could be redissolved after isolation. Films prepared by evaporating the solutions were transparent, and of the same color as the powders. Upon complexation, shifts in the UV-visible spectra of cupric chloride are manifested as a shoulder at approximately 370 nm, and a shift in the visible absorption from 865 to 850 nm. The method of continuous variation (Job's Method) was employed using the new, 370 nm, absorption. The results indicate one monomer residue (pyridine 7

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

432

moiety) is coordinated to each copper cation. When a sterically unhindered amine, such as poly(4-vinylpyridine), is used as a ligand, two monomer residues complex to one copper atom. This results in the formation of insoluble, blue, crosslinked gels. Coordination of copper to pyridine is also observed in the IR spectrum of the complexes. Shifts relative to the uncomplexed material are observed which are in agreement with model compounds and dilute complexes of cupric chloride and P2VPy . In addition, new bands were observed at 1622 and 1540 c m which could be attributed to charge transfer from the pyridine ring to C u ^ . Partial reduction of C u ^ to C u ^ is suggested by the magnetic susceptibility results. Curie-Weiss behavior was observed from 20-300°C for the three P2VPy-CuCl complexes, and moments of 1.5 - 1.6 B . M . were calculated. These values are low compared to the spin-only value of 1.73 B.M., or the experimentally observed moments of 1.8-2.0 B . M . for Cu(II) complexes . These results indicate that 22% of the copper is in the C u ^ state. XPS results also show that copper is predominately in the C u ^ state with some C u ^ also present. 8

9

- 1

2

10

D é c o m p o s i t i o n Process Thermograms for P2VPy and the 1:4, 1:2, and 1:1 complexes are shown in Figure 1. The first step in the decomposition sequence occurs at 170°C with the evolution of water and reductio T G A from 35 to 220°C is in copper atom. Loss of water was confirmed by MS results (Figure 2). The reduction of Cu(II) to Cu(I) was demonstrated by the X P S results. The bands for C u ^ disappear with the concomitant growth of the C u ^ bands. In addition, the magnetic moments decrease from 1.5 to O.2 B . M . , and significant shifts are observed in the visible absorption spectrum. A l l complexed pyridine rings acquire a net positive charge during this transition, accounting for the perseverance of the 1622 and 1540 c m vibrations. The second major event observed in the T G A results occurs with an extrapolated onset temperature of 320°C and is due to the decomposition of the polymer. This value is 70°C lower than that for uncomplexed P2VPy. Polymer decomposition was studied by MS and a stepwise evolution of gaseous products was observed, as shown for selected ions evolved from the 1:4 complex in Figure 2. This decomposition pattern is similar for all complexes, regardless of copper concentration. Initially, HC1 is generated and two maxima are observed in the mass spectra at nominal temperatures of 153°C and at 170°C. The second maximum of HC1 is associated with the decomposition of the polymer backbone. The polymeric ligand forms a mixture of compounds upon decomposition. Initially, a conjugated dimer (207 a.m.u.) and trimer (311 a.m.u.) with relative ratios of 100:24 are evolved (the concentration of an ion is not directly proportional to the number of counts, however changes in ion counts are related to changes in concentration). The structure of the ions is postulated in Figure 3. Both species exhibit maxima at œl70°C, but the dimer continues to be the most abundant species throughout the polymer decomposition. Lower molecular weight molecules begin to appear at 165°C, but increase continuously with higher temperatures. These include a conjugated dimer (194 a.m.u., Figure 3), monomers (106, 93 and 79 a.m.u.), and benzene (78 a.m.u.). The relative ratios of these species are compared to the 207 ion at nominal temperatures of 170, 220, and 270°C in Table II. - 1

Examination of the mass spectrum of P 2 V P Y taken during the maximum decomposition rate reveals the major decomposition products as methylpyridine (93 a.m.u.), protonated vinyl pyridine (106 a.m.u.), and protonated dimer (211 a.m.u.) with ion ratios 74:100:59 respectively. Trimeric and tetrameric protonated species (316 and 421 a.m.u.) are also observed but in relatively small amounts. Protonated ions, rather than the simple monomers and dimers observed for the decomposition of poly(styrene) by M S , may be created by a mechanism similar to that reported for the decomposition of 2-(4-heptyl)pyridine in the mass spectrometer. The unsaturated nature of the volatiles generated during decomposition of the complexes, compared to the monomeric species released by the virgin polymer, probably 11

12

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

35.

LYONS ET AL.

Copper Chloride Complexes

1

O.001 0

433

I

80

16

TEMPERATUR (°C)

Figure 1. Thermograms of P2VPy and complexes of C u C l with P2VPy with ratios of 1:4, 1:2, and 1:1. 2

18

20000-

ι nu

0-

5000-

H 0 2

1

1

1000-

r

1

3 6 MID.

7 9 im.ii.

r

PYRIDINE

04000-

1 2 0 7 tm.D.

01000-

DIMER ^ y * - v ^ _ ^ ~ ~ 1

311

imn.

0-

TRIMER I

1 9 4 Linn

DIMER

i

I

I

1

A .

Λ/*Λ~*^

1000-

o100

200 APPR0X. TEMPERATURE TO

300

Figure 2. Evolution of selected ions from pyrolysis of the 1:4 complex in the mass spectrometer as a function of temperature.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

434

INORGANIC A N DORGANOMETALLIC POLYMERS

ι

ι

·

c=c—c=c+ 207 a.m.u. H

H

H H

c=c—c=c—c=c+

194 imu. Figure 3. Proposed structures of selected ions evolved during the pyrolysis of CuCl :P2VPy complexes. 2

Wt. % Cu IN COMPLEX Figure 4. Total char yield (solid line) and polymer char yield (dashed line) of complexes pyrolyzed to 800°C vs weight % copper in the complex.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

35. LYONS ET AL.

Copper Chloride Complexes

435

Table II. Relative mass intensities evolved from the 1:4 chloride complex at three temperatures during thermal decomposition in the mass spectrometer. Nominal temperature values are given. Nominal Temperature

a.m.u.

170°C

220°C

270°C

207

100

100

100

311 194 193 130 106 93 79 78 36

24 7 7 7 8 8 11 22 144

11 47 53 18 22 19 19 28 67

10 35 35 25 35 35 22 29 20

results from dehydrogenation of the polymer during HC1 evolution. Double bonds may be created along the chain backbone which would react to form crosslinks or cyclized structures. Both mechanisms are known to increase char yields in organic polymer systems . The increases in both total char yield ( — - ° f —\ j polymeric char wt. of precursor . , wt. of carbonaceous products \ , . „. . ~ m . , , . yield ( ; 1 are shown m figure 4. Copper* ' is reduced to wt. of polymer in precursor copper metal during the decomposition of the polymer. 13

w t

c

c n a r

a n c

1

Conclusions P2VPy and C u C l react in solution to form a soluble complex in which one pyridine moiety is coordinated to each copper atom. Approximately 22% of the copper atoms are in the Cu(I) state. The decomposition of the polymer complexes occurs via a stepwise process. A copper composite results where the where the polymer char yield increases with increased copper complexation. P2VPy was initially thought to be an ideal ligand for the pyrolytic formation of pure copper as it thermally decomposes to form no char. The presence of C u C l significantly changes the decomposition mechanism of the polymeric ligand and results in high polymer char yields upon pyrolysis. Dehydrogenation of the backbone may occur when HC1 is generated. This would result in the formation of a conjugated polymer intermediate, which would subsequently decompose to form high char yields. 2

2

The authors would like to thank Pat Gallagher, Frank DiSalvo, William Reents Jr., and Costas Tzinis for helpful discussions. Literature

Cited

[1] Yajima, S.; Hayashi J.; Omori, M.; Okamura, K. Nature 1976, 261, 685. [2] Penn, B. G.; Ledbetter, F. Ε. III; Clemons, J. M.; Daniels, J. G. J. Appl. P o l y m . Sci. 1982,

27,

3751.

[3] Yajima, S. et.al.,J.Materials Sci. 1981, 16, 1349.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

436 [4] Lyons, A. M. Chemistry

J. Non-Crystalling

and Applications

Solids

1985, 70, 99. Knop, Α.; Scheib, W.

of Phenolic Resisns; Springer: Berlin, 1979.

[5] Allcock, H. R. International Conference on Ultrastructure in Organic and Inorganic Polymers, [6] DiSalvo, F. J.; Safran, S. Α.; Haddon, R. C.; Waszczak, J. V. Phys. Rev. B 1979, 20, 4883. DiSalvo, F. J.; Waszczak, J. V. Phys. Rev. B 1981, 23, 457. [7] Skoog, D. Α.; West, D. M. Fundamentals of Analytical Chemistry; Saunders College Pub.: New York, 1982; 4th ed., p 552. [8] Goldstein, M.; Mooney, E. F.; Anderson, Α.; Gebbie, H. A. Spectrochimica Acta 1965, 21, 105. [9] Inagaki, N.; Suganuma, R.; Katsuura, K. Europ. Polym. J. 1978, 14, 151. [10] Hathaway, B. J.; Billing, D. E. Coordin. Chem. Rev. 1970, 5, 143. [11] Luderwald, I.; Vogl, O. Makromol. Chem. 1979, 180, 2295. [12] Biemann, K. Mass Spectrometry; McGraw-Hill: New York, 1962; p 130. [13]

Quinn, C. B. J. Polym.

RECEIVED

Sci., Polym. Chem. Ed. 1977, 15, 2587.

September 1, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 36

Cationic and Condensation Polymerization of Organometallic Monomers Kenneth E. Gonsalves and Marvin D. Rausch 1

2

Department of Chemistry and Chemical Engineering, Stevens Institute o f Technology Hoboken NJ 07030 Department of Chemistry MA 01003

1

2

1,1'-Bis(β-aminoethyl)ferrocene was synthesized via a 6-step process starting with ferrocene. This monomer was then copolymerized with various aromatic and ali­ phatic diacid chlorides as well as with diisocyanates, leading to ferrocene-containing polyamides and polyureas having moderately high to low viscosities. The above monomer and 1,1'-bis(β-hydroxyethyl)ferrocene were also utilized as chain extenders. Three types of isopropenylmetallocene monomers were synthesized and subjected to polymerization and copolymerization by cationic initiators: (1) isopropenylferrocene, (2) (η -isopropenylcyclopentadienyl)dicarbonylnitrosylmolybdenum; and (3) 1 , 1 ' - d i i s o p r o p e n y l c y c l o p e n t a dienylstannocene, and related derivatives of each. 5

There i s c u r r e n t l y c o n s i d e r a b l e i n t e r e s t i n o r g a n o m e t a l l i c polymers, s i n c e polymers c o n t a i n i n g m e t a l s might be e x p e c t e d t o p o s s e s s p r o ­ p e r t i e s d i f f e r e n t from those o f c o n v e n t i o n a l o r g a n i c p o l y m e r s . 1""^ Two major approaches t o t h e f o r m a t i o n o f m a t e r i a l s o f t h i s type have i n v o l v e d t h e d e r i v a t i z a t i o n o f preformed o r g a n i c polymers w i t h o r g a n o m e t a l l i c f u n c t i o n s ^ and t h e s y n t h e s i s and p o l y m e r i z a t i o n o f o r g a n o m e t a l l i c monomers t h a t c o n t a i n v i n y l s u b s t i t u e n t s > ^ F o r the t r a n s i t i o n m e t a l s , c o n d e n s a t i o n p o l y m e r i z a t i o n s have a l s o been investigated. However, t h e r e a c t i o n s have g e n e r a l l y been c o n d u c t e d a t e l e v a t e d t e m p e r a t u r e s , and t h e r e s u l t i n g p r o d u c t s have o f t e n n o t been w e l l c h a r a c t e r i z e d . ^ * ^ Ferrocene-Containing

P o l y a m i d e s and P o l y u r e a s

We now r e p o r t a c o n v e n i e n t method f o r t h e i n t e r f a c i a l p o l y c o n d e n s a t i o n o f 1,Ι -bis(3-aminoethyl)ferrocene (1) w i t h a v a r i e t y o f d i a c i d c h l o r i d e s and d i i s o c y a n a t e s , l e a d i n g t o f e r r o c e n e - c o n t a i n i n g p o l y amides and p o l y u r e a s . 9 I n some i n s t a n c e s , we have been a b l e t o ob­ serve f i l m formation a t the i n t e r f a c e . Moreover, t h e p o l y m e r i z a t i o n r e a c t i o n s c a n be c o n v e n i e n t l y conducted a t ambient t e m p e r a t u r e s i n contrast to e a r l i e r high-temperature organometallic condensation 1

0097-6156/88/0360-0437$07.25/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

438

INORGANIC AND ORGANOMETALLIC POLYMERS

p o l y m e r i z a t i o n s , w h i c h f r e q u e n t l y l e d to u n d e s i r a b l e s i d e r e a c t i o n s . We a l s o f i n d t h a t the r e l a t e d monomer, 1,1 - b i s ( 3 - h y d r o x y e t h y l ) f e r r o ­ cene (2) r e a c t s w i t h d i a c i d c h l o r i d e s and d i i s o c y a n a t e s t o form f e r r o c e n e - c o n t a i n i n g p o l y e s t e r s and p o l y u r e t h a n e s , r e s p e c t i v e l y . Monomers (_1) and (2) a r e shown i n Scheme I . Monomers 1 and 2^ have been s y n t h e s i z e d s t a r t i n g from f e r r o c e n e , u t i l i z i n g m o d i f i c a t i o n s o f procedures o u t l i n e d p r e v i o u s l y by Sonoda and M o r i t a n i H and by R a t a j c z a k e t a l . 1 2 The i n t e r m e d i a t e d i a c i d , 1,1 f e r r o c e n e d i c a r b o x y l i c a c i d , was s y n t h e s i z e d a c c o r d i n g t o the more c o n v e n i e n t procedure o f Knobloch and RauscherA® Monomer 1 was vacuum d i s t i l l e d p r i o r t o use (bp 120°C, 1 mm Hg). D e t a i l s of the s y n t h e t i c r o u t e s a r e g i v e n i n Scheme 1. I t s h o u l d be emphasized t h a t i n c o n t r a s t t o p r e v i o u s f e r r o c e n e - c o n t a i n i n g monomers,13 ± and 2 p o s i t i o n the r e a c t i v e amino and h y d r o x y l groups two methylene u n i t s removed from t h e f e r r o c e n e n u c l e u s . T h i s f e a t u r e m i n i m i z e s s t e r i c e f f e c t s and a l s o enables 1 and 2_ t o undergo the Schotten-Baumann reaction r e a d i l y withou e f f e c t p r o v i d i n g any c o n s t r a i n t s . 1 4 , 1 v i g o r o u s , e x o t h e r m i c , and i n s t a n t a n e o u s . I n t e r f a c i a l or s o l u t i o n polycondensation, w i t h or without s t i r ­ r i n g , was the g e n e r a l procedure u t i l i z e d f o r the p r e p a r a t i o n of the polyamides and polyureas,l° D e t a i l s a r e g i v e n i n Table I . An i m p o r t a n t p o i n t t o be noted i s t h a t , i n the u n s t i r r e d i n t e r f a c i a l condensation p o l y m e r i z a t i o n of 1 w i t h sebacoyl c h l o r i d e or t e r e p h t h a l o y l c h l o r i d e i n the o r g a n i c phase and t r i e t h y l a m i n e as t h e p r o t o n a c c e p t o r , immediate f i l m f o r m a t i o n took p l a c e a t the i n t e r ­ f a c e . The polyamide f i l m s were removed a f t e r 1 h, d r i e d , and u t i l i z e d f o r t a k i n g e l e c t r o n micrographs. a

Attempts t o o b t a i n m o l e c u l a r w e i g h t s o f these new i r o n - c o n t a i n ­ i n g polyamides i n m - c r e s o l s o l u t i o n have not- been s u c c e s s f u l , due to the v e r y l i m i t e d s o l u b i l i t i e s of the m a t e r i a l s i n o r g a n i c s o l v e n t s . S i m i l a r d i f f i c u l t i e s have p r e v i o u s l y been encountered i n the mole- ^ c u l a r weight détermination o f n y l o n 66 (polyhexamethyleneadipamide). However, t h e i n t r i n s i c v i s c o s i t y v a l u e s g r e a t e r t h a n 1.0 f o r the polyamides o b t a i n e d from 1_ and t e r e p h t h a l o y l c h l o r i d e o r s e b a c o y l c h l o r i d e are comparable t o i n t r i n s i c v i s c o s i t i e s o f n y l o n s h a v i n g number average m o l e c u l a r w e i g h t s between 10,000 and 18,000.16° -jhe low [η] v a l u e s o b t a i n e d f o r the p o l y u r e t h a n e s can be a t t r i b u t e d to premature p r e c i p i t a t i o n from s o l u t i o n and, i n t h e case o f polymers o b t a i n e d from _1 and TD1, t o decreased r e a c t i v i t y imposed by s t e r i c effects.18 19 The polyamides and p o l y u r e a s e x h i b i t e d b r o a d , i n t e n s e N-H s t r e t c h e s around 3300 cm~l. A v e r y s t r o n g c a r b o n y l s t r e t c h i n g v i b r a t i o n was p r e s e n t a t 1630 cm~l. The amide I I band was e v i d e n t near 1540 cm"!. I n a d d i t i o n , sp C-H s t r e t c h e s o c c u r r e d around 3100 cm"! and asymmetric and symmetric s p ^ c-H s t r e t c h e s a t 2950 and 2860 cm~l, r e s p e c t i v e l y . The p o l y u r e t h a n e showed the c a r b o n y l ab­ s o r p t i o n near 1700 cm"l and C-0 s t r e t c h e s i n the v i c i n i t y of

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Organometallic Monomers

36. GONSALVES AND RAUSCH

439

ο CH C0C1, A l C l ? 3

CH C1 , 2

2

3 h

F

v

|

N

50°C, 5 h

0

e f

a

5% NaOCl

I *

^

(58%)

CH OH, H SO 3

2

k

1

^

LiAlHt,,

^ ^ ^ C 0

C H

2

Et 0 2

^ ^ 4 Ï ^ C H

(90%)

25°C,

2 h (N )

KCN, H 0

I

2

Th

2

0 H

^^33^CH CH NH

2

THF

3

2

(98%)

«^^^^-CH CN PC1 ,

*

e

(75%)

'

3

,

F

*

2

L i A l H , A1C1 ^ H

yC^rW

Ec

1

3

2» 3 h

>-=j=^

(85%)

I

(73%)

'NaOH EtOH, Δ H CH OH 2

^

L i A l H i + , THF ^

I ^

2

| H CH OH

H C00H

2

2

2

2 (70%)

(98%)

Scheme I .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2

2

INORGANIC AND ORGANOMETALLIC POLYMERS

440

f

Table I.

Polycondensâtion Reactions between 1,1 -Bis(3-aminoethyl)ferrocene (1) and (3-hydroxyethyl)ferrocene (2) with Diacid Chlorides and Diisocyanates

monomer (M^ i

a

i

a

i

a

i

a

i

a

process (base used)

(M ) 2

% yield dL/g

e

UI (Et N)

72

1.50

sebacoyl chloride (CCl^)

UI (Et N)

85

O.37

sebacoyl chloride (CCl^)

UI (NaOH)

39

O.59

terephthaloyl

chloride ( C I ^ C l ^

b

3

3

sebacoyl chloride (CCl^)

I

(Et N)

51

1.09

adipoyl chloride (CCl^)

UI (Et N)

47

O.53

45

O.80

51

O.16

S

46

O.20

TDI (CHC1 )

UI

58

O.16

TDI (CHC1 )

S

53

O.10

S

67

f

1

terephthaloyl

2

terephthaloyl

b

3

3

chloride(CH C1 ) 2

2

S

b

S

chlorid

(Et N) 3

(pyridine)

(m-xylene, reflux 2 i 1 1

TDI° (Me S0, 115 2

a

°C)

3

3

MDI

d

Monomer i n aqueous phase b

UI, unstirred i n t e r f a c i a l S, solution I, S t i r r e d i n t e r f a c i a l C

TDI: tolyene 2,4-diisocyanate (80%) +2,6,

isomer

(20%)

d

MDI : methylenebis(4-phenylisocyanate)

6

I n t r i n s i c v i s c o s i t y determined

i n m-cresol at 3?°C

^insoluble i n m-cresol 1220 and 1280 cm ^. Similar absorptions were present i n the polyester. The polyamides and polyureas are thus assessed to have structures outlined i n Scheme I I . Model Compounds. Further elucidation of these polymer structures was done by synthesizing model analogs. 20 It has been demonstrated by Hauser and coworkers that 3-aminoethylferrocene (_3) undergoes reactions t y p i c a l of the amino funct i o n a l group.

Fe

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

36.

GONSALVES AND RAUSCH

Organometallic Monomers

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

441

INORGANIC AND ORGANOMETALLIC POLYMERS

442

They showed t h a t _3 a f f o r d e d a p i c r a t e on b e i n g t r e a t e d w i t h s a t u r ­ a t e d a l c h o h o l i c p i c r i c a c i d and formed N,N,N-trimethyl-3-ferrocenylethylammonium i o d i d e on b e i n g t r e a t e d w i t h m e t h y l i o d i d e . 3-Aminoe t h y l f e r r o c e n e (3) was s y n t h e s i z e d by a m o d i f i c a t i o n o f t h e l i t e r a ­ t u r e method^O and was c h a r a c t e r i z e d by e l e m e n t a l a n a l y s i s , IR and -4i NMR s p e c t r o s c o p y . Following the procedure of Pittman e t a l . , ^ 1 the m e t h i o d i d i d e o f Ν,Ν-dimethylaminomethylferrocene was c o n v e r t e d t o f e r r o c e n y l a c e t o n i t r i l e by r e f l u x i n g w i t h sodium c y a n i d e i n deoxygenated w a t e r . The l a t t e r compound was then reduced t o t h e amine _3 w i t h L 1 A I H 4 . I n o r d e r t o i s o l a t e pure _3, i n s t e a d o f p a s s i n g hydrogen c h l o r i d e gas i n t o an e t h e r s o l u t i o n o f 3,^° 6N H2SO4 was used. The p r e c i p i t a t e d ferrocenylammonium s u l f a t e was f i l t e r e d under n i t r o g e n and t r e a t e d w i t h aqueous sodium h y d r o x i d e t o o b t a i n t h e f r e e amine _3 i n the o r g a n i c l a y e r . Pure 3 - a m i n o e t h y l f e r r o c e n e (3) was ob­ t a i n e d i n 70% y i e l d by d i s t i l l a t i o n under*vacuum (b.p. 120°C/1 t o r r ) . A s u s p e n s i o n o f 3 - a m i n o e t h y l f e r r o c e n e (_3) i n deoxygenated w a t e r c o n t a i n i n g an excess o p r e c i p i t a t e immediatel l e n t o f pure t e r e p h t h a l o y l c h l o r i d e i n d r y benzene. E l e m e n t a l a n a l y ­ s i s and an IR spectrum i n d i c a t e d t h e y e l l o w p r e c i p i t a t e t o have t h e s t r u c t u r e h_ ( y i e l d 100%) .

I n _3, t h e amino f u n c t i o n a l group i s two methylene u n i t s removed from the f e r r o c e n e n u c l e u s . I t appears from t h e i n s t a n t a n e o u s and q u a n t i ­ t a t i v e f o r m a t i o n o f 4_ from _3 t h a t t h i s f e a t u r e m i n i m i z e s s t e r i c e f f e c t s and a l s o e n a b l e s _3 t o undergo t h e Schotten-Baumann r e a c t i o n r e a d i l y w i t h o u t t h e c l a s s i c a l α-metallocenylcarbenium i o n e f f e c t s p r o v i d i n g any c o n s t r a i n t s . ^ The IR spectrum o f 4^ showed the c h a r a c t e r i s t i c N-H s t r e t c h a t 3320 c n T M s ) , t h e amide 1 ( c a r b o n y l ) s t r e t c h a t 1625 c m " ( s ) , t h e amide I I (N-H) s t r e t c h a t 1540 c m " l ( s ) , and t h e amide I I I band a t 1310 cm~ (m). I n a d d i t i o n , c h a r a c t e r i s t i c a b s o r p t i o n s o f t h e f e r r o c e n y l group were e v i d e n t a t 1100 and 1000 cm"l ( i n d i c a t i n g an u n s u b s t i t u t e d c y c l o p e n t a d i e n y l r i n g ) and a t 800 cm~l. 1

1

I n a d d i t i o n t o t h e above r e a c t i o n , 1 , l ' - b i s ( 3 - a m i n o e t h y l ) f e r r o c e n e (3) was r e a c t e d w i t h two e q u i v a l e n t s o f b e n z o y l c h l o r i d e

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

36.

GONSALVES AND RAUSCH

Organometallic Monomers

i n the presence o f t r i e t h y l a m i n e . A p r o d u c t (5) whose spectrum c l o s e l y resembled t h a t o f 4_ was o b t a i n e d .

443

infra-red

As b e f o r e , an N-H s t r e t c a t 1650 c m ( s ) , amide I cm"l(m) . _ 1

I t i s w e l l e s t a b l i s h e d t h a t p r i m a r y a m i n o - f u n c t i o n a l groups, p a r t i c u l a r l y a l i p h a t i c ones, r e a c t i n s t a n t a n e o u s l y w i t h i s o c y a n a t e s t o form ureas a t ambient t e m p e r a t u r e . Indeed t h i s was o b s e r v e d when β-aminoethy1ferrocene (3) and f r e s h l y d i s t i l l e d p h e n y l i s o c y a n a t e were shaken v i g o r o u s l y . A y e l l o w p r e c i p i t a t e s e p a r a t e d out immedi­ a t e l y . E l e m e n t a l a n a l y s i s and an IR spectrum o f the p r o d u c t i n d i c a t e t h i s compound t o have the s t r u c t u r e 6^.

Fe

The c h a r a c t e r i s t i c a b s o r p t i o n s o f t h e u r e a group were e v i d e n t i n t h e IR spectrum: -NH s t r e t c h 3320 c m ~ ( s ) ; amide I 1625 c n T ^ m ) ; amide I I 1560 c m " ( s ) ; amide I I I 1240 c n T ^ m ) . The y i e l d o f 6^ was a g a i n q u a n t i t a t i v e . When t h e diamine 1 was s i m i l a r l y r e a c t e d w i t h p h e n y l ­ i s o c y a n a t e , a y e l l o w p r e c i p i t a t e was observed i m m e d i a t e l y . The IR spectrum o f t h i s p r o d u c t (7) was s i m i l a r t o t h a t o f 6. The -NH s t r e t c h o c c u r r e d a t 3300 cm"~l(s) and t h e c a r b o n y l a b s o r p t i o n a t 1630 cm"" (s). The amide I I band o c c u r r e d a t 1550 cm~"l(s,br) and t h e amide I I I a t 1260 αιΓ^ιη). 1

1

1

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

444

INORGANIC AND ORGANOMETALLIC POLYMERS

Segmented P o l y ( e t h e r u r e t h a n e ) F i l m s C o n t a i n i n g F e r r o c e n e the Hard Segments

Units i n

Segmented p o l y ( e t h e r u r e t h a n e s ) were s y n t h e s i z e d from p o l y p r o p y l e n e g l y c o l (PPG) and 4,4 M e t h y l e n e - b i s ( p h e n y l - i s o c y a n a t e ) (MDI) using l,l -bis($-aminoethyl)ferrocen f e r r o c e n e (2) as c h a i n e x t e n d e r s f

Synthesis o f Polyurethanes. I n t h e "prepolymer method" employed i n t h i s s t u d y , MDI (2 e q u i v a l e n t s ) and PPG (1 e q u i v a l e n t ) were r e a c t e d a t 60°C i n t h e p r e s e n c e o f 1% d i b u t y l t i n d i l a u r e a t e as c a t a l y s t , i n the m e l t . The c o u r s e o f t h e p o l y m e r i z a t i o n s was f o l l o w e d s p e c t r o s c o p i c a l l y by o b s e r v i n g t h e i n t e n s i t y o f t h e ^NCO peak i n t h e IR (Scheme I I I ) . MDI i t s e l f e x h i b i t s a s t r o n g NCO peak a t 2260 cm ^. A f t e r t h e above r e a c t i o n had p r o c e e d e d f o r 30 min, a s m a l l a l i q u o t was w i t h ­ drawn from t h e r e a c t i o n v e s s e l and i t s IR spectrum r e c o r d e d . I t showed a s t r o n g a b s o r p t i o n a t 2260 cnT^ c h a r a c t e r i s t i c o f t h e — N C O group. S i m i l a r l y , a f t e r 60 min t h e a b s o r p t i o n f o r —NCO was s t i l l strong. The p r e p o l y m e r thus o b t a i n e d had r e a c t i v e — NCO groups. When t h i s prepolymer was c u r e d i n a vacuum oven f o r 12 h a t 60 C, the r e s u l t i n g m a t e r i a l a l s o e x h i b i t e d a s t r o n g broad a b s o r p t i o n a t 2245 cm~l. L i k e w i s e , a s i m i l a r prepolymer allowed t o cure a t c a . 20°C i n a i r showed t h e above a b s o r p t i o n . Thus i n a l l c a s e s , t h e p r e ­ polymer p o s s e s s e d r e a c t i v e i s o c y a n a t e end groups. A f t e r t h e MDI and PPG had r e a c t e d t o form the p r e p o l y m e r , 1 (1 e q u i v a l e n t ) i n d r y DMF was added as t h e c h a i n e x t e n d e r . The r e a c t i o n was then c o n t i n u e d a t ambient t e m p e r a t u r e , c a . 20°C, f o r 3 h followed by c u r i n g a t 20°C f o r 24 h i n vacuum. No i s o c y a n a t e group a b s o r p t i o n was observed i n t h e IR spectrum o f t h e b l o c k p o l y ( u r e a u r e t h a n e ) polymer ( B P U l a ) . A l t e r n a t i v e l y , t h e c u r i n g was a l s o c a r r i e d o u t a t 60°C i n a vacuum oven f o r 24 h . No i s o c y a n a t e group a b s o r p t i o n was o b s e r v e d i n t h e IR spectrum o f t h i s polymer (BPU1). However, t h e d i f f e r e n c e i n c u r i n g p r o c e d u r e s p r o d u c e d dark brown f i l m s h a v i n g d i f f e r e n t s o l u b i l i t i e s . The former was s o l u b l e and t h e l a t t e r i n s o l u b l e i n DMF. The complete d i s a p p e a r a n c e o f t h e —NCO peak i n t h e IR s p e c t r a of these dark-brown t r a n s l u c e n t f i l m s i s a good i n d i c a t i o n t h a t com­ p l e t e c h a i n e x t e n s i o n o r c u r e had o c c u r r e d . I n t h e l a t t e r case,

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

36.

GONSALVESANDRAUSCH

Organometallic Monomers

445

(MDI) + CHrr-H>-CH--CH -^ OH 2

r

(PPG) Dibutyl t i n dilaureate

-IL-* NC0:

30 mi η

V

2260 (s)

30 min PREPOLYMER

-^

V

NC0

cm M2260 ( s ) 1730 ( s )

20*C/3 h

e

DMF,60C/3 h

cured a t 20 C f o r 24 h e

viscous residue

No NCO peak i n IR (BPUla)

cured a t 60 C f o r 24 h e

No NCO peak i n IR (BPUR2)

cured a t 60°C f o r 24 h No NCO peak i n IR (BPU1)

Scheme I I I .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

446

however, the i n s o l u b i l i t y o f the f i l m i n DMF a l s o i n d i c a t e s t h a t b i u r e t b r a n c h i n g and c r o s s l i n k i n g c o u l d have o c c u r r e d on c u r i n g a t 60°C f o r 24 h, as might be a n t i c i p a t e d . The above prepolymer on treatment w i t h 2_ as the c h a i n e x t e n d e r i n dry DMF d i d not proceed a t ambient temperature. The m i x t u r e had t o be h e a t e d t o 60°C f o r 3 h b e f o r e the r e a c t i o n was complete. A f t e r c u r i n g a t 60°C f o r 24 h, the y e l l o w , t r a n s l u c e n t b l o c k p o l y u r e t h a n e f i l m (BPUR2) a g a i n showed the absence o f the —NCO peak i n the IR spectrum i n d i c a t i n g t h a t c u r i n g had been complete. The f a c t t h a t a h i g h e r temperature had to be used i n the case of 2_ as the c h a i n ex­ t e n d e r compared t o 1. i s i n k e e p i n g w i t h the lower o r d e r o f r e a c ­ t i v i t y of d i o l s w i t h d i i s o c y a n a t e s as compared to diamines w i t h diisocyanates. On the b a s i s o f t h e i r i n f r a r e d s p e c t r a the polymers BPUla and BPUR2 were a s s e s s e d t o have the s t r u c t u r e s 8 and 9_ r e s p e c t i v e l y as g i v e n i n F i g u r e 1. 1

Polymer BPUla showed the c h a r a c t e r i s t i c NH s t r e t c h a t 3350 cm a l o n g w i t h the c o r r e s p o n d i n g amide I and I I a b s o r p t i o n s a t 1 7 3 5 ( s ) _ ^ and 1530 (br) cm" r e s p e c t i v e l y . A c a r b o n y l a b s o r p t i o n a t 1650 cm was a s c r i b e d to the urea group i n t h i s polymer. A s t r o n g C-O-C (br) peak a t 1100 cm" and CH s t r e t c h e s at 3050, 2940, and 2860 cm" were a l s o observed. The polymer BPUR2 a l s o showed the c h a r a c t e r i s t i c — NH s t r e t c h a t 3300 cm" and the amide I and I I a b s o r p t i o n s a t 1720 (s) and 1530 (s) cm" , r e s p e c t i v e l y . A g a i n the C-O-C s t r e t c h was observed a t 1100 cm" and the CH s t r e t c h e s a t 3050, 2940, and 2860 cm" . These c o r r e l a t i o n s compare w e l l w i t h t h e IR s p e c t r a o f s i m i l a r segmented p o l y u r e t h a n e s . " 1

1

1

1

1

1

1

The i n c l u s i o n of _1 as an i n t e g r a l p a r t o f the p o l y u r e t h a n e system was c o n f i r m e d i n the s y n t h e s i s of B P U l a . A f t e r the r e a c t i o n had been completed, the m i x t u r e was poured i n t o e x c e s s d i e t h y l e t h e r , r e s u l t i n g i n a y e l l o w g e l a t i n o u s p r e c i p i t a t e . The y e l l o w v i s c o u s m a t e r i a l was s e p a r a t e d and a l l o w e d to cure a t ambient temperature f o r 24 h. The r e s u l t i n g t r a n s l u c e n t y e l l o w f i l m was s o l u b l e i n DMF. The amount of i r o n determined by e l e m e n t a l a n a l y s i s i n t h i s sample was 1.1% (4% f e r r o c e n e ) . Thus the IR s p e c t r a , e l e m e n t a l a n a l y s e s , as w e l l as p r e c i p i t a t i o n procedure f o r BPUla a l l p o i n t towards the i n c l u s i o n of 1 i n t h e p o l y u r e t h a n e s v i a c h e m i c a l l i n k a g e , and not by j u s t mere p h y s i c a l compounding. The p e r c e n t a g e of i r o n i n the o t h e r p o l y u r e t h a n e s , BPU1 and BPUR2, were determined to be a p p r o x i m a t e l y 1.9 and 1.2%, r e s p e c t i v e l y , c o r r e s ­ ponding t o 6.7% and 5% f e r r o c e n e u n i t s i n t h e copolymers. The m o l e c u l a r weight d i s t r i b u t i o n (MWD) o f the l i n e a r p o l y u r e ­ thanes were determined by GPC. The s o l v e n t used was THF and the i n s t r u m e n t c a l i b r a t e d _ b y narrow MWD p o l y s t y r e n e s . Polymer BPUla had an M of 56,000 (M :12,500); and BPUR2 M o f 97,000 (M :9,100). w η w η Mass S p e c t r o m e t r y . Mass s p e c t r o m e t r i c (MS) a n a l y s i s has been u t i l i z e d f o r polymer and copolymer s t r u c t u r a l i d e n t i f i c a t i o n ^ . Recently Dussel et a l . u t i l i z e d pyrolysis-MS to c h a r a c t e r i z e 2 5

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

S

Ï

i

ο" >enen s> ta

2

2

0 ^ ^ ^ < ^ C M 2

! c

2

2

C H

NH-C—HH-€H

- { O ) ^ H ^ - ^ C H

\vJ/

* ^ F S V /R\ ^ ^ " ^ 0 X ^ 2

2

c

-

Fe i.

^

2

2

β—CH CH NH~

F i g u r e 1. Average s t r u c t u r e o f segmented p o l y u r e t h a n e c o n t a i n i n g ferrocene units. (Reproduced w i t h p e r m i s s i o n from R e f . 53· C o p y r i g h t 1986 John W i l e y . )

-ΚΚ-€Η -0-^ —

BPUR2

-fCH-CH —0-^

3

f"

BPUla

INORGANIC AND ORGANOMETALLIC POLYMERS

448

segmented c o p o l y ( e t h e r u r e t h a n e - u r e a ) (PEUU), by i d e n t i f y i n g k e y fragments and r e l a t i n g them t o the b u i l d i n g b l o c k s o f PEUU — p o l y e t h e r s , d i i s o c y n a t e s , and the diamine. R i c h a r d s e t a l . 2 6 have a l s o u t i l i z e d t h i s t e c h n i q u e f o r the s t r u c t u r a l a n a l y s i s o f a p o l y ( e t h e r urethane-urea) (Biomer). G e n e r a l l y , the d e c o m p o s i t i o n o f the macrom o l e c u l a r c h a i n s i s i n i t i a t e d by s c i s s i o n o f the urea bonds f o l l o w e d by s p l i t t i n g o f the urethane bonds T h i s i s f o l l o w e d by the e v a p o r a ­ t i o n o f the p o l y e t h e r b u i l d i n g b l o c k s . 2 5 Based on the above s t u d i e s , the i d e n t i t y o f the d i i s o c y a n a t e component i n BPUla was deduced by the presence o f fragments a t m/z 250 [ m e t h y l e n e - b i s ( 4 ) p h e n y l - i s o c y a n a t e ] (MDI), 224 (4-amino-4 - i s o c y a n a t o - d i p h e n y l m e t h a n e ) , and 198(diamino-diphenylmethane).27 The presence o f the d i a m i n e , as the c h a i n e x t e n d e r was e v i d e n t from the presence o f the fragments m/z 324, a s s i g n a b l e t o l , l - b i s ( $ - i s o - ^ c y a n a t o e t h y l ) f e r r o c e n e , and 298, a s s i g n a b l e t o the s t r u c t u r e 10. f

2

10

The i n t e n s e s i g n a l s b e l o n g i n g t o the i o n s e r i e s m/z 59, 117, 175, 133, 291, 349, e t c . a r e 58 mass u n i t s a p a r t , c o r r e s p o n d i n g t o t h e mass o f the repeat u n i t s o f PPG.26 Thus t h e c o m p o s i t i o n o f BPUla can be regarded as PPG, end-capped w i t h MDI and c h a i n extended by 1_. The MS o f BPUR2 was s i m i l a r . 13

13 C-NMR. The C-NMR spectrum o f BPUR2 i n a c e t o n e - d ^ a l s o p r o v i d e d i n f o r m a t i o n r e g a r d i n g the i n c o r p o r a t i o n o f BHF i n t o the polymer v i a c h a i n e x t e n s i o n . The δ s h i f t s a t 68.8, 69.7, and 85.4 are w i t h i n the range e x p e c t e d f o r c y c l o p e n t a d i e n y l carbons.28 The methylene carbons a d j a c e n t t o the c y c l o p e n t a d i e n y l r i n g s e x h i b i t the r e s o n ­ ances a t δ 72.3 (C-J and 65.7 ( C ) ppm r e s p e c t i v e l y . The a b s o r b ances a t 75.9 and 73.8 ppm can be a s s i g n e d t o the methine and methylene carbons and the δ s h i f t a t 17.8 ppm t o the m e t h y l c a r ­ bons i n the PPG segments. The a r o m a t i c carbons were a s s i g n e d t h e δ s h i f t s 119.2, 129.7, 136.4 and 138.2 ppm and the methylene c a r b o n , b r i d g i n g the benzene r i n g s , 41 ppm.29 The absorbance a t δ 154.3 ppm b e l o n g s t o the c a r b o n y l c a r b o n o f the urethane group. The i n ­ tense s i g n a l s a t 30.2 and 205.9 a r e o f the s o l v e n t a c e t o n e . The 1 3 spectrum o f BPUla was s i m i l a r but o f l e s s e r r e s o l u t i o n owing t o i t s l i m i t e d s o l u b i l i t y i n a c e t o n e . The above s p e c t r a o f BPUR2 i s g i v e n i n F i g u r e 2. a

C

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

?

n

2

p

p

m

2

?

—0CH CH -

2

0—C—NH- - ^ Q ^ C H - ^ ^ - N H - C - 0 C H C H ,

F i g u r e 2. NMR spectrum o f BPUR2 i n a c e t o n e - d ^ , 2000 s c a n s . (Reproduced w i t h p e r m i s s i o n from R e f . 53. C o p y r i g h t 1986 John W i l e y ) .

—[Ο—CH—CH -}r

CH,

450

INORGANIC AND ORGANOMETALLIC POLYMERS

C a t i o n i c P o l y m e r i z a t i o n and C o p o l y m e r i z a t i o n cene Monomers

of Isopropenylmetallo-

Only a few examples o f t h e c a t i o n i c p o l y m e r i z a t i o n r e a c t i o n s o f o r g a n o m e t a l l i c monomers have been r e p o r t e d i n t h e l i t e r a t u r e . K u n i t a k e and c o w o r k e r s ^ f i r s t r e p o r t e d t h e c a t i o n i c p o l y m e r i z a t i o n of v i n y l f e r r o c e n e , and Korshak and c o - w o r k e r s ^ p o l y m e r i z e d 1 , 1 diisopropenylferrocene with cationic i n i t i a t o r s . Recently, J a b l o n s k i and C h i s t i f u r t h e r i n v e s t i g a t e d t h e c a t i o n i c p o l y m e r i z a ­ t i o n of 1,1'-diisopropenylferrocene.^ 1

T

1

The Q-e v a l u e s o f v i n y l f e r r o c e n e determined i n t h e f r e e r a d i c a l c o p o l y m e r i z a t i o n s t u d i e s - * have shown t h a t t h e m e t a l l o c e n e group i s s t r o n g l y e l e c t r o n c o n t r i b u t i n g , and i t i s a l s o known t h a t t h e i s o p r o p e n y l group i s s t r o n g l y e l e c t r o n c o n t r i b u t i n g . F u r t h e r m o r e , t h e i s o p r o p e n y l group has s u b s t a n t i a l advantage o v e r t h e v i n y l group i n c a t i o n i c p o l y m e r i z a t i o n r e a c t i o n s as seen i n a comparison o f t h e relative polymerizabilitie these r e a s o n s , and s i n c range o f η^-cyclopentadienyl-metal monomers w h i c h c o n t a i n i s o p r o p e n y l units,33 f e l t t h a t these new o r g a n o m e t a l l i c monomers s h o u l d be p o t e n t i a l l y a t t r a c t i v e c a n d i d a t e s f o r polymers prepared by c a t i o n i c i n i t i a t i o n c o n d i t i o n s . As an i n i t i a l s t e p i n our i n v e s t i g a t i o n s i n t h i s u n c h a r t e d a r e a , we s t u d i e d i n some d e t a i l t h e c a t i o n i c p o l y ­ m e r i z a t i o n r e a c t i o n s o f i s o p r o p e n y l f e r r o c e n e ( I F ) i t s e l f . We were p a r t i c u l a r l y i n t e r e s t e d i n d e t e r m i n i n g the e f f e c t o f t h e i n t e r a c t i o n of t h e p r o p a g a t i n g carbenium i o n w i t h t h e f e r r o c e n y l group on t h e r e a c t i v i t y and p o l y m e r i z a b i l i t y o f t h i s monomer and o f t h e r e l a t e d i s o p r o p e n y l m e t a l l o c e n e monomers, whose s t r u c t u r e s are shown below, i n c l u d i n g α-trifluorovinylferrocene (TVF), (η^-isopropenylcyclopentadienyl)dicarbonylnitrosylmolybdenum (IDM), and i t s t r i c a r b o n y l m e t h y l a n a l o g (ITMM) 2

w

e

f 3 -C=CH H

CH ^g^C^CH I 2

H

? 3

3

^ ^ F ^ C=CH I Fe

0

9

2

2

Fe

ψ

J NO

CO

CO (IF)

(TVF) CH

(IDM)

H

^ 3

^r-C=CH

3

g

C=CH

2

Sn CO

I

CO CO

^ UM

Y" 2 CH 3

(ITMM) (DIS)

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

36.

Organometallic Monomers

GONSALVES AND RAUSCH

451

Ferrocene Monomers. A l t h o u g h the i s o p r o p e n y l f e r r o c e n e monomer has been r e p o r t e d p r e v i o u s l y by s e v e r a l groups o f i n v e s t i g a t o r s , 3 4 - 3 7 we were a b l e t o develop a more f a c i l e , dependable, h i g h y i e l d synthes i s f o r t h i s compound, u t i l i z i n g the t h r e e - s t e p p r o c e d u r e shown below:

» ^^C-CH

CHoCOCl,AlCl

±

*

1 ^ ?

^ 7

CH C1 ,O.5h 2

2

(11)

f 3 H

3

CH Mgl/THF^^C-OMgl > I CH 0-10°C, 16h ^ ?

(12, 53%)

H

A

^

^

(13)

H

?3 F i g u r e 4, was p r e p a r e d from t h e r e a c t i o n o f nBuSnCl3 i t h s i l v e r c y c l o h e x a n o a t e ( 5 ) . The l a t t e r r e a c t i o n was employed by Anderson (6) i n s y n t h e s i z i n g o r g a n o t i n t r i c a r b o x y l a t e s . Both t h e drum _1 and l a d d e r _3 undergo s t r u c t u r a l changes i n s o l u t i o n i n i n t e r c o n v e r s i o n p r o c e s s e s as i n d i c a t e d by S n NMR A mechanism associated w i t h t h i s proces T

T

2

2

2

2

2

6

2

6

3

6

2

2

2

2

2

2

6

1

2

2

w

1 1 9

Discussion S y n t h e s i s . Both t h e c y c l o p e n t a n o a t e drum c o m p o s i t i o n , _1, and the c y c l o h e x a n o a t e drum, [ n - B u S n ( 0 ) 0 C C H i 1 3 6 » A» p r e p a r e d by a c o n d e n s a t i o n r e a c t i o n of η-butyl s t a n n o i c a c i d w i t h t h e c o r r e s p o n d i n g c a r b o x y l i c a c i d , E q u a t i o n 1. w

2

e

r

e

6

6 n-BuSn(0)OH + 6 RC0 H

• [n-BuSn(0)0 CR]

2

2

6

+ 6 H 0

(1)

2

I n a subsequent r e a c t i o n , n-BuSnCl3 was r e a c t e d w i t h t h e s i l v e r s a l t of c y c l o h e x a n e c a r b o x y l i c a c i d i n t h e presence o f wet s o l v e n t . T h i s r e a c t i o n gave the l a d d e r f o r m u l a t i o n , _3, i d e n t i f i e d above, E q u a t i o n 2. 6 n-BuSnCl [(n-BuSn(0)0 CC H ) 2

6

n

2

+

3

+ 10 A g C H C 0 ~ 6

n

2

+ 4 H0

n-BuSnCO^CeHx ) 3 ] x

2

2

•

+ 10 A g C l + 8 HC1

(2)

Other than t h e c h l o r o d e r i v a t i v e , 2^, the drum and l a d d e r com­ pounds a r e p r e p a r e d i n h i g h y i e l d , > 70%. A l l a r e s o l u b l e i n o r g a n i c s o l v e n t s and show c h a r a c t e r i s t i c i n f r a r e d s p e c t r a . The drum com­ pounds 1^ and 4^ e x h i b i t a s y m m e t r i c a l d o u b l e t f o r t h e c a r b o x y l a t e s t r e t c h i n g f r e q u e n c y , V ^ Q Q , c e n t e r e d near 1550 cm and a s i n g l e Sn-0 s t r e t c h , V g , near 600 cm" . I n c o n t r a s t , t h e open-drum s t r u c t u r e s , 2^ and _3, snow an u n s y m m e t r i c a l V^-QQ d o u b l e t i n t h e same r e g i o n as t h a t f o r t h e drums and t h e presence of two Sn-0 s t r e t c h e s near 600 cm" . The drum compounds a r e t h e r m a l l y q u i t e s t a b l e . F o r example, the c y c l o p e n t a n e c a r b o x y l i c a c i d drum, ^L, was heated a t 300°C f o r 3 h i n vacuum. The m a t e r i a l o b t a i n e d was s o l u b l e i n CDCI3, and S n NMR showed a s i n g l e l i n e a t -491.4 ppm, compared t o t h e s t a r t i n g m a t e r i a l -485.8. The s h i f t i s presumably due t o l o s s of s o l v e n t m o l e c u l e s present i n the c r y s t a l i n the s t a r t i n g m a t e r i a l . 1

1

nQ

1

1 1 9

1 1 9

S n NMR Data. H y d r o l y t i c a l l y , t h e drum f o r m u l a t i o n s a r e more s t a b l e than t h e open-drum forms. The h y d r o l y s i s r e a c t i o n i n E q u a t i o n 3 illustrates this.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

472

INORGANIC AND ORGANOMETALLIC POLYMERS

F i g u r e 2. ORTEP p l o t of [n-BuSn(0)0 CC H ] ·ϋ Η , 1. The v i e w i s down the pseudo Sg a x i s . The t e r m i n a l carbon atoms of the n-Bu groups a r e o m i t t e d f o r purposes of c l a r i t y . (Reproduced from R e f . 5. C o p y r i g h t 1987 American Chemical S o c i e t y . ) 2

5

9

6

6

6

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. 2

2

2

2

2

F i g u r e 3. 0RTEP p l o t of [ ( n - B u S n ( 0 ) 0 C P h ) n - B u S n ( C l ) ( 0 C P h ) ] , 2S i x of the e i g h t p h e n y l groups and a l l hydrogen atoms have been o m i t t e d f o r purposes of c l a r i t y . (Reproduced from Ref. 5. C o p y r i g h t 1987 American Chemical S o c i e t y . )

e*

*> c*

1

ο

ι

5*

ο

-ι

Ο

Λ"

f

Ο

H >

So w

Ο r S

Χ

00

INORGANIC AND ORGANOMETALLIC POLYMERS

>^

4-1

CM ι—ι CO /-s

.

^ W

H

«

0) «H Ο Ο

O H

H

Cu

U

c co

ω Pu

3

s

3 c ι , ο c μ ϋ OÛ •Η μ ω

PQ

< ο

-Η




οο

U Η CM Ο

ο

Η

^ ti

/—s Cj »û Ο &p CO --

4J

Τ ο υ 4J

LO

ο ^ · Η

Η

^

Ο f*5 Η

Ο

& Ο

ro/ ο v

ο

1

ΙΟ

ο

Ο

μ ω

^

Β ο τ) • 4-ι α) >ί π) υ eu β Ό Ο Ο ι ,ο μ Ί μ α •Η cd eu Pu ο pti

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

38.

HOLMES ET AL.

Oligomeric Organotin Compounds

l

l

f

[(R Sn(0)02CR)2R Sn(0 CR)3]2 + 2 H 0 2

475

• [R Sn(0)0 CR]

2

2

6

+ 4 RC0 H

(3)

2

1 1 9

Our i n v e s t i g a t i o n u s i n g S n NMR e s t a b l i s h e s the r e t e n t i o n of the drum and l a d d e r s t r u c t u r e s i n s o l u t i o n and shows t h e i r i n t e r c o n v e r s i o n a c c o r d i n g t o E q u a t i o n 3. F i g u r e 5 d i s p l a y s S n NMR s p e c t r a of a sample of the l a d d e r compound, _3, i n s l i g h t l y m o i s t CDCI3. H y d r o l y s i s i s e v i d e n t as the drum peak a t -486 ppm grows i n i n t e n s i t y r e l a t i v e to s i g n a l s a s s i g n e d t o the l a d d e r . As F i g u r e 5 emphasizes, the S n peak a t -532 ppm f o r _3 appears as a major i n t e r m e d i a t e . S i g n a l s i n t h i s r e g i o n a r e p r e s e n t i n the s p e c t r a of a l l samples except t h a t f o r the c h l o r o d e r i v a t i v e , _2, w h i c h a l s o l a c k s a h i g h f i e l d s i g n a l i n the 600-630 ppm region. The a c t i o n of excess c y c l o p e n t a n e c a r b o x y l i c a c i d on a CDCI3 s o l u t i o n of the c y c l o p e n t a n e drum _1 causes i t s s i g n a l a t -485.8 ppm t o d i s a p p e a r and g i v e t o f o r m a t i o n of the l a d d e ments have shown t h a t the h y d r o l y s i s p r o c e s s g i v e n i n E q u a t i o n 3 i s r e v e r s i b l e , i . e . , a drum forms from a l a d d e r c o m p o s i t i o n and, i n the presence of excess a c i d , the drum can be opened up to y i e l d the ladder formulation. 1 1 9

1 1 9

S t r u c t u r a l D e t a i l s . F o r JL, the hexameric "drum" has i d e a l i z e d S m o l e c u l a r symmetry. The geometry of the stannoxane framework of the m o l e c u l e c o n s i s t s of six-membered r i n g s i n a c h a i r c o n f o r m a t i o n . Each Sn atom i s bonded t o t h r e e framework oxygen atoms, where the Sn0 bonds are a l l of comparable s t r e n g t h and have l e n g t h s r a n g i n g from 2.075(7)A t o 2.093(7)A. The oxygen atoms of the framework a r e t r i c o o r d i n a t e . The sum of the t h r e e Sn-O-Sn a n g l e s about these oxygen atoms range from 331.8° t o 333.9°. The Sn atoms, w h i c h are a l l c h e m i c a l l y e q u i v a l e n t , are h e x a c o o r d i n a t e d , w i t h the c o o r d i n a t i o n sphere b e i n g completed by an η-butyl group and two oxygen atoms from d i f f e r e n t c a r b o x y l a t e groups. Each of the four-membered r i n g s of the core i s spanned by a c a r b o x y l a t e group t h a t forms a s y m m e t r i c a l b r i d g e between two Sn atoms. The Sn-0 bonds t o the b r i d g i n g carboxy­ l a t e atoms a r e l o n g e r than the c o r e bonds and range from 2.155(8)Â t o 2.173(8)A. The "unfolded-drum" or " l a d d e r " compound _2 has c r y s t a l l o g r a p h i c C± symmetry. T h i s corresponds t o the i d e a l i z e d m o l e c u l a r symmetry and, t h e r e f o r e , t h e r e a r e t h r e e c h e m i c a l l y i n e q u i v a l e n t types of Sn atoms i n the m o l e c u l e , a l t h o u g h a l l a r e h e x a c o o r d i n a t e d . The oxygen atoms i n the open form can be s u b d i v i d e d i n t o two t y p e s , as i n the case of the drum m o l e c u l e : t r i c o o r d i n a t e framework oxygen atoms and the d i c o o r d i n a t e oxygen atoms of the b r i d g i n g c a r b o x y l a t e l i g a n d s . The geometry about the oxygen atoms of the framework of the open form tends toward p l a n a r i t y . I n t h i s case the sums of the a n g l e s about the t r i c o o r d i n a t e oxygen atoms i s 357.4° f o r 01 and 355.3° f o r 02. The geometry about the Sn atoms i s b e s t d e s c r i b e d as d i s t o r t e d octahedral. The g e n e r a l s t r u c t u r a l f e a t u r e s of the " l a d d e r " , [ ( n - B u S n ( 0 ) 0 C C H ) n - B u S n ( 0 C C H ) 3] , _3, shown i n F i g u r e 4, a r e v e r y s i m i l a r t o t h a t found f o r 2^, the p r i n c i p a l d i f f e r e n c e b e i n g a 6

2

6

n

2

2

6

1x

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

476

u| υ

υ

1 1 9

F i g u r e 5. T i n NMR s p e c t r a of [ ( n - B u S n ( 0 ) 0 C C H ! ) nB u S n ( 0 2 C C H ) ] , 2» CDC1 i n d i c a t i n g h y d r o l y s i s t o a "drum" form. The upper spectrum i s f o r a sample c o n t a i n i n g Ρι+Οχο» added t o reduce h y d r o l y s i s r e l a t i v e t o t h e m i d d l e spectrum w h i c h has no added d r y i n g agent. The lower spectrum i s r e c o r d e d on t h e l a t t e r sample one week l a t e r . D i d e n t i f i e s t h e l i n e due t o drum f o r m u l a t i o n and U, t h e l i n e s due t o the u n f o l d e d drum. (Reproduced from R e f . 5. C o p y r i g h t 1987 American C h e m i c a l S o c i e t y . ) 2

6

1

2

i n

6

n

3

2

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

38. HOLMES ET AL.

Oligomeric Organotin Compounds

477

replacement of the C l atom i n 1_ by a pendant c y c l o h e x a n o a t e group. L i k e _2, the m o l e c u l e , _3, has c r y s t a l l o g r a p h i c C-^ symmetry. The Sn-0 bond l e n g t h s of the b r i d g i n g c a r b o x y l a t e s a r e i n g e n e r a l l o n g e r ( a v e r a g i n g 2.15(3)A) than the Sn-0 framework bonds ( a v e r a g i n g 2.06(2)A). I t i s i n t e r e s t i n g to note t h a t the bond a n g l e s a t the " p y r a m i d a l " framework oxygen atoms i n the drum, _1, average 100.4(3)° i n the distannoxane r i n g and a r e a t 133.2(3)° i n the t r i s t a n n o x a n e r i n g . T h i s compares w i t h an average bond angle a t oxygen w i t h i n the distannoxane r i n g of 102.9(1)° i n the u n f o l d e d drum, _2, which has n e a r l y p l a n a r oxygen atoms and s l i g h t l y s h o r t e r Sn-0 framework bonds, a v e r a g i n g 2.063(3)A f o r _2 compared to 2.086(7)A f o r 1. A l t h o u g h the number of bonds b r o k e n and formed i s conserved i n the h y d r o l y s i s of an u n f o l d e d drum of the type 3 t o a drum s t r u c t u r e , E q u a t i o n 3, the entropy change i s f a v o r a b l e . Ladder t o Drum C o n v e r s i o n I t i s i n t e r e s t i n g t o s p e c u l a t e how the drum s t r u c t u r e i s forme the energy d i f f e r e n c e betwee s m a l l . By way of i l l u s t r a t i o n , we a p p l y the h y d r o l y s i s p r o c e s s t o the l a d d e r f o r m u l a t i o n , J3. M e c h a n i s t i c a l l y , f o r t h i s example, a c o n s e r v a t i o n i s expected i n the number of bonds t o be b r o k e n and formed i n e x e c u t i n g the h y d r o l y s i s of E q u a t i o n 3. A minimum of n i n e S n - c a r b o x y l a t e oxygen bonds are r e q u i r e d t o be c l e a v e d . D u r i n g t h i s cleavage the S n - c a r b o x y l a t e oxygen bonds must assume c i s p o s i t i o n s at the t i n atoms. These c h e l a t i n g groups are i n t r a n s p o s i t i o n s i n the open-drum s t r u c t u r e . The bonds formed i n c o m p l e t i n g the convers i o n to the drum s t r u c t u r e w i t h t h i s c o n s t r a i n t c o n s i s t of t h r e e Snc a r b o x y l a t e oxygen bonds and s i x Sn-0 bonds. F i g u r e 6 shows a p o s s i b l e i n i t i a l i n t e r m e d i a t e and F i g u r e 7 i l l u s t r a t e s a proposed form j u s t p r i o r to c l o s u r e to y i e l d the drum. P h o s p h i n i c A c i d R e a c t i o n s . R e a c t i o n of n - b u t y l s t a n n o i c a c i d w i t h diphenylphosphate i n s t e a d of a c a r b o x y l i c a c i d a l s o r e s u l t s i n the f o r m a t i o n of a drum c o m p o s i t i o n [n-BuSn(O) 0 P ( O P h ) ] 6 (Chandrasekhar, V.; Holmes, J . M.; Day, R. O.; Holmes, R. R., u n p u b l i s h e d work). However, when d i p h e n y l p h o s p h i n i c a c i d i s r e a c t e d w i t h n - b u t y l s t a n n o i c a c i d under r e f l u x i n t o l u e n e , a new s t r u c t u r a l form of t i n i s o b t a i n e d (_7) . The r e a c t i o n proceeds a c c o r d i n g t o E q u a t i o n 4 g i v i n g the s t a b l e o x i d e c o m p o s i t i o n i n 90% y i e l d , mp 198-208°C dec. 2

[ (n-BuSn(0H)0 PPh ) 0][Ph P0 ]

3 n-BuSn(0)0H + 4 P h P 0 H 2

2

2

2

+ 2

2

3

H0 2

2

2

(4)

X-ray a n a l y s i s ( F i g u r e 8) shows t i n ( I V ) p r e s e n t i n an oxygencapped c l u s t e r m o l e c u l e . The b a s i c framework c o n s i s t s of a t r i stannoxane r i n g i n a cyclohexane c h a i r arrangement. H y d r o x y l groups comprise the oxygen components of the r i n g system. A t r i c o o r d i n a t e d oxygen atom caps one s i d e of t h i s framework w h i l e t h r e e a d d i t i o n a l d i p h e n y l p h o s p h i n a t e groups b r i d g e a d j a c e n t h e x a c o o r d i n a t e d t i n atoms. I t i s noted t h a t t h r e e S n 0 r i n g u n i t s form as a consequence of the presence of the unique capping oxygen atom. These t h r e e four-membered r i n g s c o n t a i n the l a t t e r atom and form a p o r t i o n of a cube. 2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

478

INORGANIC AND ORGANOMETALLIC POLYMERS

F i g u r e 6. A s u g g e s t e d i n i t i a l i n t e r m e d i a t e r e s u l t i n g i n the h y d r o l y s i s of a l a d d e r s t r u c t u r e as r e p r e s e n t e d by E q u a t i o n 3. (Reproduced f r o m R e f . 5 . C o p y r i g h t 1987 American C h e m i c a l S o c i e t y . )

Figure 7· Suggested s t r u c t u r a l r e p r e s e n t a t i o n j u s t p r i o r t o c l o s u r e t o y i e l d t h e drum. (Reproduced from R e f . 5 . C o p y r i g h t I987 American C h e m i c a l S o c i e t y . )

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

38. HOLMES ET AL.

Oligomeric Organotin Compounds

479

The oxygen-capped c l u s t e r can be viewed as a h y d r o l y s i s product of the drum j u s t as the drum i s viewed as a h y d r o l y s i s product of the l a d d e r , c f . E q u a t i o n 5 (R = P h P ) . 2

!

[R Sn(0)0 R] 2

f

+ 2 R0 H 4- 2 H 0

6

2

• 2[(R Sn(OH)0 R) 0][R0 ]

2

2

drum

3

(5)

2

cluster

The schematic ( F i g u r e 9) f o r a drum i n d i c a t e s how i t i s r e l a t e d t o two c l u s t e r m o l e c u l e s . F o r m a l l y , two b r i d g i n g phosphinates r e a r r a n g e , two oxygen atoms a r e added, f o u r Sn-0 bonds are c l e a v e d , the s i x Sn-O-Sn l i n k a g e s become Sn(OH)Sn u n i t s , and two phosphinates a r e added t o hydrogen bond t o the h y d r o x y l s . The S n NMR spectrum f o r the oxygen-capped c l u s t e r e x h i b i t s a s i n g l e resonance w i t h t r i p l e t c h a r a c t e r c e n t e r e d a t -498.5 ppm ( | J ( S n - 0 - P ) I = 132.0 H z ) . T h i s o b s e r v a t i o n i s c o n s i s t e n t w i t h the presence of t h r e e e q u i v a l e n t t i n atoms p r o v i d e d by a c l u s t e r u n i t which has the hydrogen-bonde exchange among the t h r e A c u b i c arrangement, [n-BuSn(0)0 P (CgHi ι) ] i+, i s prepared (Kumara Swamy, K. C.; Day, R. O.; Holmes, R. R., accepted by JACS f o r p u b l i c a t i o n ) from a c o n d e n s a t i o n of n - b u t y l s t a n n o i c a c i d w i t h d i c y c l o h e x y l p h o s p h i n i c a c i d i n t o l u e n e s o l u t i o n ; mp 263-265°C, Sn NMR, -462.8 ppm; J ( S n - 0 - P ) = 116 Hz. As shown i n F i g u r e 10, the c o r e of the m o l e c u l e i s d e f i n e d by t i n atoms and t r i v a l e n t oxygen atoms which occupy the c o r n e r s of a d i s t o r t e d cube, each f a c e of w h i c h i s d e f i n e d by a four-membered S n 0 r i n g . The top and bottom f a c e s of the cube are open, w h i l e each of i t s f o u r s i d e s i s spanned d i a g o n a l l y by a phosphinate b r i d g e between two t i n atoms. The phos­ p h i n a t e b r i d g e s a r e r e q u i r e d by symmetry t o be s y m m e t r i c a l . I n a d d i t i o n , a b u t t e r f l y s t r u c t u r e ( F i g u r e 11) w h i c h c o n t a i n s two t i n atoms i n a d i m e r i c r e p r e s e n t a t i o n [n-BuSn(OH) ( 0 P ( C g H n ) ) ] a r i s e s i n the same r e a c t i o n as the cube (Kumara Swamy, K. C.; Day, R. O.; Holmes, R. R., u n p u b l i s h e d w o r k ) . The cube may be viewed as a c o n d e n s a t i o n product of the d i m e r i c c l u s t e r a c c o r d i n g t o the e x p r e s ­ s i o n i n E q u a t i o n 6. Hence, the d i m e r i c c l u s t e r may s e r v e as a p r e ­ c u r s o r on the way to the c u b i c form. 1 1 9

2

1 1 9

3 1

2

2

1 1 9

2

2

2

2

2[n-BuSn(0H)(0 P(C H ) ) ] 2

6

1 1

2

2

2

• [n-BuSn(0)0 P(C H χ) ]

2

2

6

1

2

2

4

+ 4(C H ) P0 H 6

n

2

2

(6)

2

The common s t r u c t u r a l u n i t i n a l l of the o l i g o m e r i c forms d i s ­ covered so f a r i s the four-membered d i m e r i c distannoxane r i n g , S n 0 . The number of t i n atoms found i n these s t r u c t u r e s ranges from two t o s i x , e x c l u d i n g f i v e . F u r t h e r , c o n d e n s a t i o n and c o u p l i n g r e a c t i o n s a r e expected t o produce a d d i t i o n a l o l i g o m e r i c forms, r e v e a l i n g o t h e r i n t e r e s t i n g f e a t u r e s of t h i s new s t r u c t u r a l c l a s s of o r g a n o t i n compounds. 2

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2

480

INORGANIC AND ORGANOMETALLIC POLYMERS

F i g u r e 8. ORTEP p l o t of [ ( n - B u S n ( O H ) 0 P P h ) 0 ] [ P h P 0 ] . Pendant atoms of the t h r e e η-Bu groups and of the e i g h t Ph groups a r e o m i t t e d f o r purposes of c l a r i t y . Hydrogen-bonding i n t e r a c t i o n s a r e shown as dashed l i n e s . (Reproduced from R e f . 7 · C o p y r i g h t I987 American Chemical S o c i e t y . ) 2

2

3

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

38. HOLMES ET AL.

F i g u r e 11.

Oligomeric Organotin Compounds

481

ORTEP p l o t o f [n-BuSn(OH) ( 0 P ( C H ) ) ] . 2

6

n

2

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

482 Acknowledgments

Support o f t h i s r e s e a r c h by t h e donors o f t h e P e t r o l e u m Research Fund, a d m i n i s t e r e d by the American Chemical S o c i e t y and by t h e N a t i o n a l S c i e n c e F o u n d a t i o n (CHE 8504737) i s g r a t e f u l l y acknowledged. We thank the U n i v e r s i t y o f M a s s a c h u s e t t s Computing Center f o r generous a l l o c a t i o n o f computing time. Literature

Cited

1. Lambourne, H. J. Chem. Soc. 1922, 121(2), 2533. 2. Lambourne, H. J. Chem. Soc. 1924, 125, 2013. 3. Chandrasekhar, V.; Day, R. O.; Holmes, R. R. Inorg. Chem. 1985, 24, 1970. 4. Holmes, R. R.; Schmid, C. G.; Chandrasekhar, V.; Day, R. O.; Holmes, J. M. J. Am. Chem. Soc. 1987, 109, 1408. 5. Chandrasekhar, V.; Schmid, C. G.; Burton, S. D.; Holmes, J. M.; Day, R. O.; Holmes, 6. Anderson, H. H. Inorg. 7. Day, R. O.; Holmes, J. M.; Chandrasekhar, V.; Holmes, R. R. J. Am. Chem. Soc. 1987, 109, 940. RECEIVED

September

1,

1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Chapter 39 N M R Characterization of the Compositional and Configurational Sequencing of Tri-n-butyltin Polymers Jon M . Bellama and William F. Manders Department of Chemistry and Biochemistry, University of Maryland,

119

The chemical shift and structure of the Sn NMR spectrum of the copolymer poly(tri-n-butyltin methacrylate/methyl methacrylate) exhibit strong solvent functionality. The values of J( Sn, C) are also solvent dependent, increasing as the donor ability of the solvent increases. The Sn chemical shift and structure are linearly related to J( Sn, C) in oxygen- and nitrogen-containing solvents. The tri-n-butyltin groups were removed from the tin-containing polymers to give poly(MAA/MMA). The tin-stripped polymers were better resolved and free from the intense butyl-carbon signals. The com­ positional and configurational sequencing of the original polymer was investigated. The α-methyl and carbonyl regions of poly(MAA/MMA) contained well­ -resolved configurational triads. The carbonyl region, however, is also split by compositional effects. The α-methyl region is insensitive to com­ positional sequencing; integration of α-methyl triad gave quantitative information concerning tacticity. Compositional sequences were assigned in the carbonyl region of the C spectra of poly(MAA/MMA) and in the Sn spectra of poly (TBTM/MMA). Both syndiotactic and heterotactic splittings were observed in the carbonyl spectrum. In the Sn spectrum only the syndiotactic compositional triad was observed. n

119

13

119

3

119

13

13

119

119

T r i b u t y l t i n and t r i p h e n y l t i n compounds a r e used as wood p r e s e r ­ v a t i v e f u n g i c i d e s , a g r o c h e m i c a l f u n g i c i d e s , and as m i t i c i d e s and b i o c i d e s i n marine p a i n t s . A copolymer of t r i - n - b u t y l t i n m e t h a c r y l a t e (TBTM) and methyl m e t h a c r y l a t e (MMA), w h i c h i s the a c t i v e agent i n a n t i f o u l i n g 0097-6156/88/0360-0483$06.00/0 © 1988 American Chemical Society

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

p a i n t s , can be s y n t h e s i z e d e i t h e r by the f r e e r a d i c a l c o p o l y m e r i z a t i o n of the r e s p e c t i v e v i n y l monomers w i t h a p e r o x i d e i n i t i a t o r or by e s t e r i f i c a t i o n o f f r e e a c i d groups i n a copolymer o f m e t h a c r y l i c a c i d (MAA) and methyl m e t h a c r y l a t e by b i s ( t r i - n - b u t y l t i n ) o x i d e (TBTO) ( 3 ) . The o b j e c t i v e of t h i s study i s t o c h a r a c t e r i z e t i n - c o n t a i n i n g polymers on a m o l e c u l a r l e v e l by means of h i g h f i e l d , h i g h r e s o l u ­ t i o n , m u l t i n u c l e a r F o u r i e r Transform N u c l e a r Magnetic Resonance (FT-NMR) ( 4 ) . T h i s study i s g e n e r a l l y an a p p l i e d approach d e a l i n g w i t h c o m p o s i t i o n and c o n f i g u r a t i o n o f s p e c i f i c f o r m u l a t i o n s o f the copolymer. P u b l i c a t i o n s c o n c e r n i n g pendant t r i o r g a n o t i n polymers a r e l i m i t e d t o s y n t h e t i c methods and a p p l i c a t i o n s ( f o r a recent r e v i e w , see r e f e r e n c e 5^). There a r e a l s o s e v e r a l p e r t i n e n t r e p o r t s on the c h e m i s t r y of monomeric t r i o r g a n o t i n c a r b o x y l a t e s , a l k o x i d e s , and ha 1 i d e s (6.-8). EXPERIMENTAL S y n t h e s e s . I s o t a c t i c p o l y ( m e t h y l m e t h a c r y l a t e ) was s y n t h e s i z e d by the method of T s u r u t a e t a l . (9). Under a n i t r o g e n atmosphere, a q u a n t i t y of 6 mL (0.056 mole) of methyl m e t h a c r y l a t e (MMA) d r i e d over 4A m o l e c u l a r s i e v e was d i s s o l v e d i n 24 mL of s i m i l a r l y d r i e d t o l u e n e . To the g l a s s v i a l c o n t a i n i n g the r e a c t i o n was added 0.65 mL of 1.6 M n - b u t y l l i t h i u r a , and the r e a c t i o n was kept a t -78°C i n a d r y i c e / i s o p r o p a n o l b a t h . The p o l y m e r i z a t i o n was h a l t e d 24 h r l a t e r w i t h the a d d i t i o n of h y d r o c h l o r i c a c i d and methanol (methanol/water - 4.1 by volume). The polymer was d r i e d i n vacuo a t 50°C, r e d i s s o l v e d i n methylene c h l o r i d e , p r e c i p i t a t e d by b e i n g poured i n t o w a t e r - c o n t a i n i n g methanol, and d r i e d i n vacuo a t 50°C. T a c t i c i t y and c o m p o s i t i o n were v e r i f i e d w i t h U NMR. Y i e l d : 47%. I s o t a c t i c p o l y ( m e t h y l m e t h a c r y l a t e / m e t h a c r y l i c a c i d ) , a copo­ lymer of methyl m e t h a c r y l a t e and m e t h a c r y l i c a c i d , was s y n t h e s i z e d by the p a r t i a l h y d r o l y s i s of i s o t a c t i c poly(MMA) a c c o r d i n g t o the method of K l e s p e r e t a l . (10-13). A h y d r o l y z i n g m i x t u r e of 8 mL d i o x a n e and 4 mL m e t h a n o l i c KOH ( 1 0 % by weight Κ0Η) was mixed w i t h 250 mg of polymer i n c l o s e d v i a l s a t 85°C f o r 48 h r . S a p o n i f i e d polymer s e p a r a t e d from the s o l u t i o n and adhered t o the w a l l s of t h e v i a l . The p r e c i p i t a t e d polymer was d i s s o l v e d i n water and then p r e c i p i t a t e d a g a i n w i t h a few drops o f HC1. The s o l u t i o n was warmed and the c o a g u l a t e d polymer removed, washed w i t h w a t e r , and d r i e d i n vacuo a t 50°C. The ·*0 NMR spectrum i n d i c a t e d a p p r o x i ­ m a t e l y 25% h y d r o l y s i s . Y i e l d : 73%. A t a c t i c poly(methyl methacrylate/methacrylic a c i d ) , the copolymer of methyl m e t h a c r y l a t e (MMA) and m e t h a c r y l i c a c i d (MAA), was s y n t h e s i z e d " d i r e c t l y " as a prepolymer to be e s t e r i f i e d w i t h b i s ( t r i - n - b u t y l t i n ) oxide (TBTO). Two f o r m u l a t i o n s of p o l y (MMA/MAA) were s y n t h e s i z e d , a 1:1 and a 2:1 MMA and MAA copolymer whose syntheses d i f f e r o n l y i n the p r o p o r t i o n of monomer r e a c t e d . The p r e p a r a t i o n was c a r r i e d out i n a 3 L 4-neck round-bottom f l a s k f i t t e d w i t h thermometer, s t i r r e r , a d d i t i o n f u n n e l s , and r e f l u x condenser. The i n i t i a l components were mixed and heated t o a moderate r e f l u x (66-67°C) f o r 10 min. The added charges were then f e d s i m u l t a n e o u s l y and e v e n l y over a 3 h r p e r i o d w h i l e mainl

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

39.

Tri-n-butyltin Polymers

BELLAMA AND MANDERS

485

t a i n i n g moderate r e f l u x . The f i n a l charges were then f e d e v e n l y o v e r a one hr and a 5 min p e r i o d w h i l e m a i n t a i n i n g the same tem­ p e r a t u r e and a g i t a t i n g f o r at l e a s t one hr to i n s u r e t h a t a l l the polymer i s d i s s o l v e d from the s i d e s of the v e s s e l . The s o l v e n t i s removed i n vacuo at 60°C. I n a second method, a 1 g sample of p o l y ( t r i - n - b u t y l t i n m e t h a c r y l a t e / m e t h y l m e t h a c r y l a t e ) was d i s s o l v e d i n 4 mL of c h l o r o ­ form. One mL of c o n c e n t r a t e d HC1 was added dropwise w i t h s h a k i n g u n t i l no more p r e c i p i t a t e appeared. The p r e c i p i t a t e was removed and then shaken a g a i n w i t h a c l e a n b a t c h of c h l o r o f o r m and HC1. The p r e c i p i t a t e was d r i e d i n vacuo at 60°C. The y i e l d of poly(MMA/MAA) was 70%. T r i - n - b u t y l t i n m e t h a c r y l a t e (TBTM) was s y n t h e s i z e d by chemists a t the David T a y l o r N a v a l Ship Research and Development Center (DTNSRDC), A n n a p o l i s , M a r y l a n d . The methods of Dyckman et a l . (Ο and Monterraoso et a l . (2_) were used, i n which 30 g m e t h a c r y l i c a c i d was s l o w l y added to 103.8 g b i s ( t r i - n - b u t y l t i n ) o x i d e i n 300 mL benzene i n a f l a s k equippe C o o l i n g kept the temperatur added, the s o l u t i o n was heated g r a d u a l l y and m a i n t a i n e d at 30°C w h i l e the water of r e a c t i o n was removed i n vacuo and benzene was added to r e p l a c e the benzene l o s t d u r i n g t h i s p e r i o d . When the s o l u t i o n became c l e a r , the benzene was removed and the r e s u l t i n g p a l e y e l l o w v i s c o u s l i q u i d was d i l u t e d w i t h 100 mL e t h e r and c o o l e d t o -20°C. The product s e p a r a t e d as l o n g , t h i c k , t r a n s p a r e n t needles. The y i e l d was c l o s e to t h e o r e t i c a l and the ra.p. was 18°C. A f r e e - r a d i c a l s y n t h e s i s of p o l y ( t r i - n - b u t y l t i n m e t h a c r y l a t e / m e t h y l m e t h a c r y l a t e ) was a l s o c a r r i e d out by the DTNSRDC c h e m i s t s . Two d i f f e r e n t f o r m u l a t i o n s of the t i n - c o n t a i n i n g copolymer were prepared. The f i r s t c o n t a i n e d a 1:1 r a t i o of TBTM to MMA u n i t s and the second c o n t a i n e d a 1:2 r a t i o of TBTM t o MMA u n i t s . The s y n t h e s i s f o r each d i f f e r s o n l y i n the r e l a t i v e p r o p o r t i o n of monomer loaded i n t o the r e a c t i o n v e s s e l . Commercial b e n z o y l p e r o x i d e (1 percent by weight based on the sum of TBTM and MMA) was employed as the i n i t i a t o r . 0.8 mol of TBTM f r e s h l y r e c r y s t a l l i z e d from e t h e r and 0.8 mol MMA were added to 860 g of benzene i n a f l a s k equipped w i t h a thermometer and r e f l u x condenser. To t h i s m i x t u r e was added 4.3 g (0.018 mol) of b e n z o y l p e r o x i d e . The f l a s k and contents were warmed at the r e f l u x temperature of benzene (80.1°C) w i t h s t i r r i n g . A f t e r 24 hours r e a c t i o n time the benzene was removed i n vacuo at 50 °C. N u c l e a r Magnetic Resonance. A l l s p e c t r a used i n t h i s study were c o l l e c t e d on e i t h e r a Bruker Instruments I n c . CXP-200 or WM-400 s u p e r c o n d u c t i n g spectrometer o p e r a t i n g at f i e l d s t r e n g t h s of 4.7 Τ and 9.4 Τ r e s p e c t i v e l y . F o r a f i e l d s t r e n g t h of 4.7 Τ the NMR f r e ­ q u e n c i e s f o r *H, C , and S n n u c l e i are 200.00, 50.31, and 74.60 MHz, r e s p e c t i v e l y ; at 9.4 T: 400.00, 100.62, and 149.20 MHz, respectively. The S n s p e c t r a were c o l l e c t e d at 4.7 and 9.4 T, w h i l e most ^H and C s p e c t r a were c o l l e c t e d at 9.4 T. Conditions f o r s p e c t r a l a c q u i s i t i o n always i n c l u d e d quadrature phase d e t e c t i o n and broad-band p r o t o n d e c o u p l i n g f o r C and S n spectra. D e c o u p l i n g f o r * S n s p e c t r a was gated on d u r i n g a c q u i s i t i o n to 13

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INORGANIC AND ORGANOMETALLIC POLYMERS 1J

m i n i m i z e a n e g a t i v e NOE, w h i l e d e c o u p l i n g f o r C s p e c t r a employed a t w o - l e v e l scheme w i t h low power on d u r i n g the r e l a x a t i o n d e l a y t o induce a f a v o r a b l e NOE. Normal l e v e l s were used d u r i n g a c q u i s i ­ t i o n . N o r m a l l y a π/2 p u l s e was used w i t h a r e l a x a t i o n d e l a y of 3 ( T i > . The v a l u e of T^ f o r S n n u c l e i i n t i n - c o n t a i n i n g polymers was measured as a p p r o x i m a t e l y 1 s. For spectra a relaxation d e l a y of 3 s was used. P r i m a r y c h e m i c a l s h i f t r e f e r e n c e s used were i n t e r n a l TMS f o r *H s g e c t r a , e x t e r n a l TMS f o r and i n t e r n a l tetramethyltin for ^ S n . Most s p e c t r a were i n d i r e c t l y r e f e r e n c e d to the above standards u s i n g the i n t e r n a l deuterium l o c k frequency as a secondary r e f e r e n c e . A l l s p e c t r a were a c q u i r e d w i t h a d e u t e r i u m l o c k . Where the a p p r o r i a t e d e u t e r a t e d s o l v e n t was not a v a i l a b l e , a c o n c e n t r i c tube c o n t a i n i n g a d e u t e r a t e d s o l v e n t was employed. 1 1 9

9

RESULTS AND

DISCUSSION 1 1 9

S o l v e n t E f f e c t s i n the poly(MMA/TBTM) s y n t h e s i z e the a p p r o p r i a t e monomers were s o l u t i o n s i n benzene w i t h a p p r o x i ­ m a t e l y 33% s o l i d s (weight to volume). The p a r t i c u l a r f o r m u l a t i o n chosen as r e p r e s e n t a t i v e of the c l a s s c o n t a i n e d a 1:1 r a t i o of pen­ dant methyl to t r i - n - b u t y l t i n groups. I n p r e p a r i n g the dry polymer, the benzene was removed i n vacuo w i t h n o m i n a l l y 5% by weight r e s i d u a l s o l v e n t . A phenoraenological study was performed to determine the e f f e c t of s o l v e n t on ^ S n NMR s p e c t r a of these o r g a n o r a e t a l l i c polymers. Samples were d i s s o l v e d i n c h l o r o f o r m , benzene, n-hexane, a c e t o n e , t e t r a h y d r o f u r a n , methanol, and p y r i d i n e . The S n NMR s p e c t r a i n these s o l v e n t s are g i v e n i n F i g u r e 1. The appearance and l o c a t i o n of the S n resonance changes d r a s t i c a l l y over the range of s e l e c t e d s o l v e n t s . The c h e m i c a l s h i f t moves u p f i e l d i n the o r d e r c h l o r o f o r m , benzene, n-hexane, acetone, t e t r a h y d r o f u r a n , p y r i d i n e , and methanol. The amount of s t r u c t u r a l i n f o r m a t i o n and, converse­ l y , the broadening of the resonance i n c r e a s e s i n the same o r d e r w i t h methanol and p y r i d i n e r e v e r s e d . S e v e r a l phenomena may c o n t r i b u t e to the appearance and l o c a ­ t i o n of the ^ S n resonance. The s i t u a t i o n i s f u r t h e r c o m p l i c a t e d by the f a c t t h a t the t r i - n - b u t y l t i n group i s a t t a c h e d to a h i g h m o l e c u l a r weight polymer whose s o l v e n t i n t e r a c t i o n s may a l s o bear on the l * S n resonance. I t i s w e l l known t h a t o r g a n o t i n compounds w i t h at l e a s t one e l e c t r o n e g a t i v e s u b s t i t u e n t can r e a d i l y expand t h e i r c o o r d i n a t i o n number to 5 or 6, and t h a t donor s o l v e n t s may promote such expan­ s i o n . I n the absence of c o o r d i n a t i n g s o l v e n t s , i t appears l i k e l y t h a t t i n w i l l expand i t s c o o r d i n a t i o n number by s e l f - a s s o c i a t i o n w i t h e s t e r groups i n o t h e r p a r t s of the polymer. The a d d i t i o n of one s o l v e n t m o l e c u l e w i l l r e h y b r i d i z e t i n from sp^ t o sp-*d w i t h p l a n a r R groups. I t i s g e n e r a l l y observed t h a t a d d i t i o n of e l e c t r o n d e n s i t y at the t i n c e n t e r i n R3Sn0R monomers w i l l move the S n resonance u p f i e l d . I t has been shown t h a t ^ ( 8 η , C ) i s s e n s i t i v e to changes i n bond o r i e n t a t i o n i n Me3SnCl brought about by c o o r d i n a t i n g s o l v e n t s . C o u p l i n g between a c t i v e n u c l e i i s more e f f i c i e n t through bonds w i t h a h i g h e r p e r c e n t s c h a r a c t e r , and the 9

1 1 9

1 1 9

9

9

1 1 9

1 1 9

1 3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

39.

BELLAMA AND MANDERS

120

100

Tri-n-butyltin Polymers

80

60

40

20

0

487

PPM

1 1 9

F i g u r e 1. S n NMR s p e c t r a of 10% w/v poly(TBTM/MMA) i n s e v e r a l s o l v e n t s . From the t o p : c h l o r o f o r m , benzene, n-hexane, a c e t o n e , t e t r a h y d r o f u r a n , methanol, and p y r i d i n e .

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INORGANIC AND ORGANOMETALLIC POLYMERS

i n c r e a s e d c o u p l i n g i s m a n i f e s t e d by an i n c r e a s e i n the c o u p l i n g constant. On r e h y b r i d i z a t i o n o f t i n from s p t o s p d the Sn-C bonds i n the e q u a t o r i a l p o s i t i o n s r e h y b r i d i z e from s p t o sp^ (25% s t o 33% s c h a r a c t e r ) . The S n - C c o u p l i n g c o n s t a n t s observed i n C s p e c t r a i n v a r i o u s s o l v e n t s are g i v e n i n Table I . A r e p r e s e n t a t i v e spectrum 3

3

3

1 1 9

1 3

1 3

Table I.

l i y

1 : J

Observed S n - C Coupling Constants i n Poly(Tri-n-butyltin Methacrylate/Methyl M e t h a c r y l a t e ) i n Various Solvents a

lj(H9 13 (Hz) 360 350 360

Solvent

S n j

2j(H9

b

c )

S n >

13

c )

b

3j(H9sn,13 )b C

(Hz) 63.0 64.3 65.2

(Hz)

Benzene Hezane 18.0 Chloroform i-Propanol Dioxane 37 Acetone 370 67.9 19.1 Tetrahydrofuran 380 68.3 21.3 n-Butanol 69.6 21.5 Ethanol 70.7 23.2 Pyridine 410 72.4 23.6 Methanol 74.4 420 24.9 Observed i n ° 0 s p e c t r a . Where no v a l u e s are g i v e n the c o u p l i n g c o n s t a n t was e i t h e r too s m a l l t o be r e s o l v e d (^J) o r s i m p l y not recorded (*J). The v a l u e s l i s t e d f o r J ( H S n , * C ) a c t u a l l y an average o f J( S n , C ) and J ( S n , C ) , w h i c h were not r e s o l v e d . n

9

3

a

n

l l 9

1 3

n

1 1 7

r

e

1 3

e l e c t r o n d e n s i t y a t the t i n c e n t e r i n I^SnOR monomers w i l l move the S n resonance u p f i e l d . I t has been shown t h a t J ( S n , C ) i s s e n s i t i v e t o changes i n bond o r i e n t a t i o n i n Me3SnCl brought about by c o o r d i n a t i n g s o l v e n t s . C o u p l i n g between a c t i v e n u c l e i i s more e f f i c i e n t through bonds w i t h a h i g h e r percent s c h a r a c t e r , and the i n c r e a s e d c o u p l i n g i s m a n i f e s t e d by an i n c r e a s e i n the c o u p l i n g constant. On r e h y b r i d i z a t i o n o f t i n from s p t o s p d the Sn-C bonds i n the e q u a t o r i a l p o s i t i o n s r e h y b r i d i z e from s p t o sp^ (25% s t o 33% s c h a r a c t e r ) . The H S n - * C c o u p l i n g c o n s t a n t s observed i n * C s p e c t r a i n v a r i o u s s o l v e n t s are g i v e n i n Table I . A r e p r e s e n t a t i v e spectrum i s g i v e n i n F i g u r e 2. The c o u p l i n g i s not observed i n ** Sn s p e c t r a due t o the l a c k o f r e s o l u t i o n . The v a l u e s o f * J , and J a r e l a r g e r f o r r e c o g n i z e d donor s o l v e n t s such as methanol and p y r i ­ d i n e , and s m a l l e r f o r n o n - c o o r d i n a t i n g s o l v e n t s such as benzene, hexane, and c h l o r o f o r m . The s t e r i c demands of the c o o r d i n a t i n g phenomenon are e v i d e n t i n the s e r i e s MeOH, EtOH, i-PrOH, and n-Bu0H. Methanol ( w i t h J = 74.4 Hz) i s most e f f i c i e n t i n d o n a t i n g t o a t i n f l a n k e d by t h r e e b u l k y η-butyl groups. The c o u p l i n g c o n s t a n t s f o r EtOH and n-BuOH are 70.7 and 69.6 Hz, r e s p e c t i v e l y , w h i l e t h a t f o r the secondary a l c o h o l , i-PrOH, i s 66.0 Hz, com­ parable t o chloroform. I t appears t h a t s u b s t i t u t i o n a t the ctcarbon g r e a t l y h i n d e r s d o n a t i o n , w h i l e the s i z e o f the unbranched 1 1 9

1

3

1 1 9

1 3

3

3

9

3

3

9

3

3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

BELLAMA AND MANDERS

Tri-n-butyltin Polymers

489

β

PPM 1 3

F i g u r e 2. C NMR spectrum of η-butyl r e g i o n of 25% w/v poly(TBTM/MMA) i n methanol a t 300 Κ w i t h J ( S n , C ) i n d i c a t e d . n

x 1 9

1 3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

primary a l c o h o l i s l e s s c r i t i c a l . The same r e a s o n i n g may a l s o e x p l a i n the order MeOH > p y r i d i n e f o r the c o u p l i n g c o n s t a n t s . P y r i d i n e i s regarded as the b e t t e r c o o r d i n a t i n g s o l v e n t , but i t s b u l k may h i n d e r i t s a b i l i t y t o donate. The l ^ S n s p e c t r a of the polymer i n methanol, n - b u t a n o l , and i - p r o p a n o l shows broadening of the S n s i g n a l , and the J( S n , C ) v a l u e s both i n c r e a s e i n the order i - p r o p a n o l < nb u t a n o l < methanol. Both the appearance and the p o s i t i o n of the s i g n a l i n d o n a t i n g s o l v e n t s can be a t t r i b u t e d to the a b i l i t y of the s o l v e n t to c o o r d i n a t e w i t h t i n . The move u p f i e l d of the ^ S n NMR s i g n a l as the d o n a t i o n t o t i n i n c r e a s e s can be i n t e r p r e t e d as an i n c r e a s e i n s h i e l d i n g of the t i n c e n t e r . The broadening of the s i g n a l , however, cannot be e x p l a i n e d as e a s i l y . A v e r a g i n g of the c h e m i c a l s h i f t between a c o o r d i n a t e d and a n o n - c o o r d i n a t e d t i n would broaden the s i g n a l , a l t h o u g h monoraeric t r i o r g a n o t i n c a r b o x y l a t e s do not show a marked b r o a d e n i n g . The r a t e of s o l v e n t exchange on the t i n a t t a c h e d t o the polymer may be slowe l i n e - b r o a d e n i n g may no p e r a t u r e s t u d i e s were performed on the s o l u t i o n i n p y r i d i n e w i t h any a p p r e c i a b l e change i n the appearance of the spectrum. An i n c r e a s e i n temperature would be expected to i n c r e a s e the r a t e of s o l v e n t exchange and narrow the s i g n a l . The broadening of the s i g n a l may a l s o r e s u l t from the many p r o c e s s e s i n f l u e n c i n g the t r a n s v e r s e r e l a x a t i o n t i m e , T 2 , w h i c h determines the l i n e w i d t h when a v e r a g i n g of c h e m i c a l environments i s not s i g n i f i c a n t . I n d o n a t i n g s o l v e n t s the s u b t l e e f f e c t s d e t e r m i n i n g the chemic a l s h i f t i n c h l o r o f o r m , benzene, and hexane are a p p a r e n t l y masked. I n hexane, which i s c o n s i d e r e d a poor s o l v e n t , s e l f - a s s o c i a t i o n i s p o s s i b l e and would e x p l a i n the appearance of the ** Sn spectrum. C h l o r o f o r m and benzene are e x c e l l e n t s o l v e n t s f o r o r g a n o m e t a l l i c p o l y m e r s , and the s t r u c t u r e and d o w n f i e l d p o s i t i o n support a w e l l s o l v a t e d , u n a s s o c i a t e d environment. 1 1 9

3

1 1 9

1 3

9

9

D e t e r m i n a t i o n of C o n f i g u r a t i o n a l Sequences. D u r i n g the p o l y m e r i z a t i o n of methyl m e t h a c r y l a t e , c o n f i g u r a t i o n a l sequences are g e n e r a t e d as a r e s u l t of the i n e q u i v a l e n c e of the methyl and methyl formate s u b s t i t u e n t s on the carbon backbone ( 1 4 ) . T h i s isomerism, termed t a c t i c i t y , r e s u l t s i n t h r e e d i f f e r e n t arrangments: i s o t a c t i c , w i t h a l l methyl groups on one s i d e of the extended c h a i n ; s y n d i o t a t i c , w i t h methyl groups a l t e r n a t i n g r e g u l a r l y from one s i d e t o the o t h e r of the backbone; and a t a c t i c , w i t h the methyl groups d i s t r i b u t e d randomly. C o n f i g u r a t i o n a l sequences can be f u r t h e r c h a r a c t e r i z e d as c a t e n a t e d monomer dyads of e i t h e r meso (m) w i t h b o t h methyl groups on the same s i d e of the backbone, or raceraic ( r ) w i t h methyl groups on opposing s i d e s of the backbone. At the t r i a d l e v e l t h r e e d i f f e r e n t sequences are p o s s i b l e : i s o t a c t i c , ram; s y n d i o t a c t i c , r r ; and h e t e r o t a c t i c , mr. I t i s w e l l known t h a t the m e c h a n i c a l and p h y s i c a l p r o p e r t i e s of v i n y l polymers are dependent upon t h e i r s t e r e o c h e m i c a l conf i g u r a t i o n . I t i s c r i t i c a l , t h e r e f o r e , t h a t the s t e r e o r e g u l a r i t y of poly(TBTM/MMA) be determined a c c u r a t e l y and c o n v e n i e n t l y i f the f i e l d performance of the m a t e r i a l i s t o be p r e d i c t e d w i t h any c e r t a i n t y . The e f f e c t i v e n e s s of o r g a n o m e t a l l i c polymers as an a n t i -

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

39.

BELLAMA AND MANDERS

Tri-n-butyltin Polymers

491

f o u l i n g agent i s a r e s u l t of the g r a d u a l h y d r o l y s i s of t h e o r g a n o t i n moiety and i t s subsequent s o l u b i l i z a t i o n (1_). I t has been shown t h a t the r a t e of h y d r o l y s i s of p o l y ( m e t h y l methacrylate) i s s t r o n g l y dependent on the t a c t i c i t y of the polymer. The r a t e o f h y d r o l y s i s of t h e i s o t a c t i c polymer i s a t l e a s t an o r d e r o f magni­ tude g r e a t e r than e i t h e r the h e t e r o t a c t i c o r s y n d i o t a c t i c p l a c e ­ ment. The r e l e v a n c e of t a c t i c i t y t o f i e l d performance i s o b v i o u s . N u c l e a r magnetic resonance has proved t o be a v a l u a b l e t o o l i n d e t e r m i n a t i o n of c o n f i g u r a t i o n a l sequences i n poly(MMA) ( 1 4 ) . I n F i g u r e 3 i s shown the H NMR of poly(MMA) s y n t h e s i z e d w i t h an a n i o n i c p o l y m e r i z a t i o n c a t a l y s t known t o produce predominantly i s o ­ t a c t i c sequences. I n these polymers, t h e *H NMR spectrum of t h e methylene u n i t s i n the polymer backbone g i v e s an u n e q u i v o c a l d e t e r ­ m i n a t i o n of t a c t i c i t y . The methylene s i g n a l , o c c u r r i n g about 1.8 ppm, i s d i a g n o s t i c f o r dyad sequences ( r o r m). I n the s y n d i o t a c ­ t i c groups ( r ) , the protons a r e m a g n e t i c a l l y e q u i v a l e n t and appear as a s i n g l e peak a t the c h e m i c a l s h i f t p o s i t i o n In isotactic g r o u p i n g s (m), however and hence appear as a d o u b l e about 0.35 ppm) on e i t h e r s i d e of t h e s y n d i o t a c t i c peak. Because of t h e i n e q u i v a l e n c e , each l i n e of t h e d o u b l e t i s s p l i t i n t o two s i g n a l s by the geminal ^H-^H c o u p l i n g of about 15 Hz so t h a t a t o t a l of 4 l i n e s a r e seen. I n t e g r a l s of t h e s i n g l e t and the quar­ t e t a l l o w the t a c t i c i t y a t the dyad l e v e l t o be determined quan­ titatively. I t i s , of c o u r s e , more d e s i r a b l e t o i n c r e a s e the l e n g t h of t h e sequence a n a l y z e d . The number of d i s t i n g u i s h a b l e η i s g e n e r a l l y l i m i t e d by t h e r e s o l u t i o n of the s p e c t r o m e t e r . The t a c t i c i t y of the polymer i n F i g u r e 4 i s predominantly i s o t a c t i c a t the dyad l e v e l . The t h r e e α-methyl resonances a t 0.8, 1.0, and 1.2 ppm comprise the α-methyl t r i a d (mm, mr, r r , r e s p e c t i v e l y ) . T h i s i d e n t i f i c a t i o n can o n l y be made w i t h t h e knowledge p r o v i d e d by t h e methylene dyad a n a l y s i s . I n t e g r a t i o n of t h e α-methyl resonance shows t h e polymer t o be 86.4% i s o t a c t i c , 8.4% h e t e r o t a c t i c , and 3.1% s y n d i o t a c t i c . The *H s p e c t r a of copolymers o f MMA and MAA a r e p o o r l y r e s o l v e d , and a l t h o u g h t h e β-methylene and α-methyl s i g n a l s show s t r u c t u r e , very l i t t l e q u a n t i t a t i v e i n f o r m a t i o n i s a v a i l a b l e . The b u t y l resonances a t 0.8, 1.2, 1.3, and 1.6 ppm overwhelm the u s e f u l p r o t o n s i g n a l s a t the β-methylene and α-methyl p s o i t i o n s , i . e . , t h e r e s o l u t i o n i s poor because of c o m p o s i t i o n a l s e q u e n c i n g . The spectrum of s e v e r a l samples of poly(MMA/MAA) a r e g i v e n i n F i g u r e s 4-5. I n F i g u r e 4 t h e s i x unique carbons a r e matched w i t h t h e i r r e s p e c t i v e s i g n a l s . The d o w n f i e l d c a r b o n y l resonance i s the f r e e a c i d . The r e s o l u t i o n has improved r e l a t i v e t o the *H s p e c t r a , and q u a n t i t a t i o n should be p o s s i b l e . I n F i g u r e 5 t h e spectrum of a p a r t i a l l y s a p o n i f i e d sample of i s o t a c t i c poly(MMA) i s g i v e n . I t i s l i k e l y the poly(MMA/MAA) produced i n the h y d r o l y s i s w i l l a l s o be i s o t a c t i c . Based on t h i s assumption t h e i s o t a c t i c c a r b o n resonances can be a s s i g n e d : α-methyl, 21.0 ppm; βm e t h y l e n e , 51.0 ppm;

-CO2H,

180.0 ppm;

and - C O 2 C H 3 , 177.6 ppm.

The

c a r b o n y l resonances a r e g i v e n as the c e n t e r frequency o f t h e com­ p o s i t i o n a l t r i a d t o be d i s c u s s e d l a t e r . The q u a t e r n a r y and methoxy carbons do not c o n t a i n c o n f i g u r a t i o n a l i n f o r m a t i o n . These polymers were s y n t h e s i z e d w i t h f r e e - r a d i c a l p o l y m e r i z a t i o n c a t a l y s t s and a r e

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

492

INORGANIC AND ORGANOMETALLIC POLYMERS

I

A_AJ 0

PPM

F i g u r e 3. *H NMR o f i s o t a c t i c poly(MMA) i n CDCI3 a t 300 Κ w i t h 1% TMS. Upper t r a c e i s x8 v e r t i c a l e x p a n s i o n .

I - O C H q

40

30

20

PPM

1 3

F i g u r e 4. C NMR spectrum of 25% w/v poly(MMA/MAA) prepolymer i n p r y i d i n e a t 360 K.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

39.

BELLAMA AND MANDERS

ÇH,

ÇOOH

ÇH,

Tri-n-butyltin Polymers

COOR

CH,

ÇOOR

ÇH,

493

COOR

-CHj-C-CHj-Ç-CHj-Ç-CHj-Ç-CHj-Ç-CHj-Ç-CHj-Ç-CHj-iCOOH

CH,

COOH

CH,

COOH

CH,

COOR

CH,

1 3

F i g u r e 5. C NMR o f c a r b o n y l r e g i o n o f 25% w/v poly(MMA/MAA) prepolymer i n p y r i d i n e a t 360 K. The s y n d i o t a c t i c peaks a r e l a b e l l e d a c c o r d i n g t o c o m p o s i t i o n . The weak h i g h e r f i e l d peaks a r e caused by h e t e r o t a c t i c polymer; t h e r e i s t o o l i t t l e i s o t a c t i c m a t e r i a l t o be seen.

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

494

INORGANIC AND ORGANOMETALLIC POLYMERS

expected t o be p r i m a r i l y s y n d i o t a c t i c and h e t e r o t a c t i c . The a methyl r e g i o n s of the spectrum of t h e t i n c o n t a i n i n g polymer are e s s e n t i a l l y i d e n t i c a l , which i s i n d i c a t i v e of a c o n f i g u r a t i o n a l sequence u n a f f e c t e d by the change i n c o m p o s i t i o n . The i s o t a c t i c a m e t h y l peak occurs a t 21 ppm, w h i l e the l a r g e r peaks a t 19 and 18 ppm a r e a s s i g n e d as h e t e r o t a c t i c and s y n d i o t a c t i c , r e s p e c t i v e l y , as shown i n F i g u r e 6. The 8-methylene r e g i o n p r o v i d e s an a n a l y s i s a t the dyad l e v e l ; the peak a t 53 ppm i s raceraic, w h i l e the peak a t 51 ppm i s raeso. The c a r b o n y l r e g i o n i n c l u d e s 4 s e t s of t r i a d s ; t h e d o w n f i e l d p a i r i s a t t r i b u t e d t o the f r e e a c i d , w h i l e the u p f i e l d p a i r i s the methyl e s t e r . The i d e n t i t y was determined by comparing the polymer c o n t a i n i n g an e q u a l number of f r e e a c i d and methyl e s t e r groups w i t h the polymer c o n t a i n i n g t w i c e as many methyl e s t e r u n i t s as f r e e a c i d u n i t s . The t r i a d c e n t e r e d a t 180.5 ppm i s t h e s y n d i o t a c t i c sequence. The spectrum of t h e α-methyl r e g i o n i s w e l l r e s o l v e d w i t h r e s p e c t t o t a c t i c i t y , b u t u n l i k e the c a r b o n y l r e g i o n i s i n s e n s i t i v e to compositiona the α-methyl r e g i o n a r Table I I .

C o m p o s i t i o n and T a c t i c i t y R a t i o s Taken from 1 3 NMR S p e c t r a of Poly(MMA/MAA)

C

a

Copolymer Designation

COOCH3

Isotactic (%)

COOH

A*> B*> 1.0 C* 2.0 D 0.9 E 2.4 T a c t i c i t y p e r c e n t s Are ±0.5% T i n - s t r i p p e d polymers Prepolymers c

c

a

b

c

3.0 4.0 5.0 2.0 2.0

Heterotactic (%)

Syndiotactic (%)

32 37 37 35 40

65 59 58 63 58

D e t e r m i n a t i o n of C o m p o s i t i o n a l Sequences. I n the c o p o l y m e r i z a t i o n of MMA and MAA o r TBTM, c o m p o s i t i o n a l dyads and t r i a d s a r e generated. These sequences a r e determined by the r e l a t i v e con­ c e n t r a t i o n o f monomers as w e l l as by t h e i r r e l a t i v e r e a c t i v i t y . These c o m p o s i t i o n a l sequences c h a r a c t e r i z e the m a t e r i a l and a l l o w p r e d i c t i o n s of a c t i v i t y based on s t r u c t u r e by comparison w i t h f i e l d t e s t e d polymers. The r e l a t i v e r e a c t i v i t i e s ( i n f r e e - r a d i c a l c o p o l y m e r i z a t i o n s ) of TBTM and MMA a r e 0.79 and 1.00 r e s p e c t i v e l y ( 1 5 ) . W i t h e q u a l c o n c e n t r a t i o n s of monomer, an excess of MMA i n t h e polymer would be expected. I n the f o l l o w i n g d i s c u s s i o n A w i l l r e p r e s e n t the MAA o r TBTM u n i t and Β w i l l r e p r e s e n t the MMA u n i t . F o r Α-centered t r i a d s f o u r d i f f e r e n t arrangements a r e p o s s i b l e : AAA, AAB, BAA, and BAB. Analogous sequences a p p l y t o the B-centered t r i a d s . F o r a random compositon, B e r n o u l l i a n s t a t i s t i c s should apply ( 1 4 ) . W i t h (the p r o p o r t i o n of TBTM) equal t o 0.5, t h e p r o b a b i l i t i e s o f each o f t h e Α-centered t r i a d s i s P^ o r 0.25. The AAB and BAA t r i a d s a r e i n d i s t i n g u i s h a b l e and appear as a s i n g l e resonance. 2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

39.

BELLAMA AND MANDERS

Tri-n-butyltin Polymers

I I I I I I I I I I l l t I I I I I I I 1 I I I I I 1 1 1 1 1 1 1 1 1 l i 1111 n 111 »

20

15

PPM

\ \\ CH

CH

3

J CH

3

Ί

ÇOOR

3

ÇH

coo

3

-CH -Ç-CH -C-CH -C-CH -C^CH -CCOOR COOR COOR C H COOR 2

2

2

2

2

2

3

1 3

F i g u r e 6. C NMR spectrum of α-methyl r e g i o n of 25% w/v poly(MMA/MAA) prepolymer i n p y r i d i n e a t 360 K. L a b e l l e d according to t a c t i c i t y .

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

495

INORGANIC AND ORGANOMETALLIC POLYMERS

496

As w i t h the c o n f i g u r a t i o n a l a n a l y s i s , the t r i - n - b u t y l t i n groups a r e s t r i p p e d from the polymers f o r c o m p o s i t i o n a l a n a l y s i s . The ™C NMR s p e c t r a of the r e s u l t i n g poly(MMA/MAA) and prepolymers show t h a t the c a r b o n y l s p e c t r a of the prepolymer and the t i n s t r i p p e d polymer a r e n e a r l y i d e n t i c a l . The s y n d i o t a c t i c comp o s i t o n a l t r i a d s are assigned i n Figure 5 w i t h A representing the f r e e a c i d c e n t e r e d t r i a d and Β r e p r e s e n t i n g the methyl e s t e r cen­ t e r e d t r i a d . The r a t i o of the i n t e n s i t i e s of the Α-centered t r i a d a r e a p p r o x i m a t e l y c o r r e c t f o r a random c o m p o s i t i o n . The s t r u c t u r e s of the ^ S n resonance o f s i g n a l s of the s e v e r a l polymers a r e s i m i l a r i f the i m p u r i t i e s a r e i g n o r e d ( t h e sharp peak on the d o w n f i e l d s h o u l d e r i s TBTM). The observed t r i a d i s comparable t o the c o m p o s i t i o n a l t r i a d s i d e n t i f i e d i n the C c a r b o n y l spectrum. The * S n t r i a d s i g n a l s appear as s h o u l d e r s r a t h e r than as w e l l - r e s o l v e d peaks (as i n the s p e c t r u m ) . The s p e c t r a c o n t a i n no i n f o r m a t i o n r e g a r d i n g t a c t i c i t y , a l t h o u g h i t i s assumed the observed t r i a d i s s y n d i o t a c t i c . By comparing the v a r i o u s polymers, c o m p o s i t i o n a B e g i n n i n g w i t h the d o w n f i e l and AAA where A i s the TBTM u n i t and Β i s the MMA u n i t . 9

1 3

1 9

Literature 1.

2. 3. 4. 5.

6.

7. 8. 9. 10. 11. 12. 13. 14. 15.

Cited

Dyckman, E. J.; Montemarano, J. A. In Antifouling Organometallic Polymers: Environmentally Compatible Materials, Report No. 4186, Naval Ship R & D Center, Bethesda, MD, 1974. Montermoso, J. C.; Andrews T. M.; Marinelli, L. P. J. Polym. Sci., 1958, 32, 523. Okawara, R.; O'Hara, M. In Organotin Compounds; Sawyer, Α. Κ., Ed.; Marcel Dekker, Inc., New York, 1971; Vol. 2, Chapter 5. Farrar, T. C.; Becker, E. D. Pulse and Fourier Transform NMR, Academic: New York, 1971. Davies, A. G.; Smith, P. J. Comprehensive Organometallic Chemistry, Wilkinson, G., Ed., Pergammon Press: New York, 1982, Chapter 11. Somasekharan, K. N.; Subramanian, R. V. ACS Symposium Series No. 121, American Chemical Society; Washington, D.C., 1980; pp 165-181. Neumann, W. P. The Organic Chemistry of Tin, Interscience Publishers: New York, 1970; Chapter 2. Neumann, W. P. ibid., Chapter 7. Tsuruta, T.; Makisomoto, T.; Kanai, H. J. Macromol. Sci., 1966, 1, 31. Klesper, E.; Johnsen, Α.; Gronski, W.; Wehrli, F. Makromol. Chem., 1975, 176, 1071. Klesper, E. J. Polym. Sci., 1968, Β 6, 313. Klesper, E. J. Polym. Sci., 1968, Β 6, 633. Klesper, E.; Gronski, W. J. Polym. Sci., 1969, Β 7, 727. Bovey, F. A. High Resolution NMR of Macromolecules, Academic: New York, 1972, Chapter 3. Ghanem, Ν. Α.; Messiha, Ν. N.; Ikaldesus, Ν. E.; Shaaban, A. F. Eur. Poly. J., 1979, 15, 823.

R E C E I V E D October 23, 1987

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Author Index Allcock, Harry R., 250 Allen, Christopher W., 290 Archer, Ronald D., 463 Arnold, C. Α., 180 Balk, Ε. M , 420 Barendt, Joseph M , 303 Bellama, Jon M 483 Bent, Elizabeth Bettelheim, Α. Blum, Yigal D., 124 Bowmer, T. N., 296 Brandt, P. J. Α., 180 Brinker, C. J., 314 Bunker, B. C , 314 Carr, Thomas M , 156 Chandrasekhar, V., 469 Chen, C. L., 78 Chichester, S. V., 296 Day, R. O., 469 Dejak, B., 166 Downing, J. W., 61 DuBois, Donn Α., 385 Elliott, C. Michael, 420 Elsbernd, C. S., 180 Farmer, B. L., 43 Fickes, G. N., 43 Fife, Wilmer K., 199 Fleming, W., 43 Ford, R. R., 283 Gaudiello, John G., 224 Glaser, Raymond H., 354 Glaser, Robert, 124 Gonsalves, Kenneth E., 437 Goodwin, George B., 238 Haddon, R. C , 296 Haltiwanger, R. Curtis, 303 Hani, R., 283 Harris, D. H., 392 Harrod, John F., 89 Holms, Joan M , 469 Holms, Robert R., 469 Huang, Hao-Hsin, 354 Huang, Horng-Yih, 112 Ishikawa, Mitsuo, 209 Kamiya, K., 345 Karatsu, T., 61 Kellogg, Glen E., 224 Kenney, Malcolm E., 238 Kilic, S., 180

Kim, H. K., 78 Kirkpatrick, R. J., 314 Klingensmith, 61 Kratzer, R. H., 392 Krone-Schmidt, W., 392 Kulpinski, J., 166 Kuzmany H. 43

Lasocki, Z., 166 Lee, Annabel Y., 463 Lipowitz, Jonathan, 156 Lyons, A. M , 430 Manders, William F., 483 Marks, Tobin J., 224 Matyjaszewski, K . , 78 Maxka, Jim, 6 McGrath, J. E., 180 Michl, J., 61 Miller, R. D., 43,61 Mujsce, A. M , 430 Murray, Royce W., 408 Narula, C. K., 378 Nate, Kazuo, 209 Neilson, Robert H., 283,385 Norman, Arlan D., 303 Ochaya, Ven O. 463 Paciorek, K . J. L., 392 Paine, P. T., 378 Pearce, Ε. M , 430 Penton, H. R., 277 Piechucki, S., 166 Poutasse, Charles Α., 143 Pressprich, K . , 408 Rabe, James Α., 156 Rabolt, J. F., 43 Rausch, Marvin D., 437 Raybuck, S. Α., 408 Redepenning, J. G., 420 Rifile, J. S., 180 Roy, A. K., 283 Sakka, S., 345 Sawan, Samuel P., 112 Schaefler, R., 378 Scheide, G. M , 283 Schmid, Charles G., 469 Schmidt, Η. K., 333 Schmittle, S. J., 420 Schwark, Joanne M , 143

498 In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

499

INDEX Sennet, Michael S., 268 Seyferth, Dietmar, 21,143 Shaw, S. Yvette, 385 Singler, Robert E., 268 Sooriyakumaran, R., 43 Sormani, P. M , 180 Summers, J. D., 180 Swamy, K . C. Kumara, 469 Tallant, D. R.,314 Tetrick, Stephen M , 224 Tian, Cheng-xiang, 199 Tramontano, Valentino J., 463 Tsai, Yi-Guan, 112 Tse, Doris, 124 Vasile, M J., 430 Wallraff, G . M , 61 Wang, Bing, 463

Ward, K . J., 314 Waszczak, J. V., 430 West, Robert, 6 Wettermark, U . G., 283 White, Β. Α., 408 Wilkes, Garth L., 354 Willingham, Reginald Α., 268 Wiseman, Gary H., 143 Wisian-Neilson, P., 283 Witekowa, M , 166 Worsfold, D. J., 101 Wynne, Kenneth J., 1,392 Xu, Jian-min, 199 Yilgor, I., 180 Yoko, Y., 345 Yu, Yuan-Fu, 143 Zeldin, Martel, 199

Affiliation Index Army Materials Technology Laboratory, 268 A T & T Bell Laboratories, 296,430 Bell Communications Research, Inc., 296 Carnegie Mellon University, 78 Case Western Reserve University, 238 Colorado State University, 420 Dow Corning Corporation, 156 Ethyl Corporation, 277 Fraunhofer-Institut fur Silicatforschung, 333 Hiroshima University, 209 Hitachi Ltd., 209 IBM Almaden Research Center, 43 Indiana University-Purdue University at Indianapolis, 199 Kyoto University, 345 Massachusetts Institute of Technology, 21,143 McGill University, 89 Mie University, 345 National Research Council of Canada, 101 Northwestern University, 224 Office of Naval Research, 1,392

The Pennsylvania State University, 250 Polytechnic University, 430 SRI International, 124 Sandia National Laboratories, 314 Southern Methodist University, 283 Stevens Institute of Technology, 437 Technical University, Lodz, Poland, 166 Texas Christian University, 283,385 Ultrasystems Defense and Space, Inc., 392 University of Colorado, 303 University of Illinois, 314 University of Lowell, 112 University of Maryland, 483 University of Massachusetts, 437,463,469 University of Nevada, 43 University of New Mexico, 378 University of North Carolina, 408 University of Texas at Austin, 61 University of Vermont, 290 University of Vienna, 43 University of Wisconsin, 6 Virginia Polytechnic Institute and State University, 180,354 Washington State University, 43

Subject Index A

a form of boron nitride preparation, 378-379 structure, 378

Absorption spectra, poly(di-n-alkylsilanes), 64,65/ W-Acetyltetramethylcyclodisilazoxane, formation, 167

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

499

INDEX Sennet, Michael S., 268 Seyferth, Dietmar, 21,143 Shaw, S. Yvette, 385 Singler, Robert E., 268 Sooriyakumaran, R., 43 Sormani, P. M , 180 Summers, J. D., 180 Swamy, K . C. Kumara, 469 Tallant, D. R.,314 Tetrick, Stephen M , 224 Tian, Cheng-xiang, 199 Tramontano, Valentino J., 463 Tsai, Yi-Guan, 112 Tse, Doris, 124 Vasile, M J., 430 Wallraff, G . M , 61 Wang, Bing, 463

Ward, K . J., 314 Waszczak, J. V., 430 West, Robert, 6 Wettermark, U . G., 283 White, Β. Α., 408 Wilkes, Garth L., 354 Willingham, Reginald Α., 268 Wiseman, Gary H., 143 Wisian-Neilson, P., 283 Witekowa, M , 166 Worsfold, D. J., 101 Wynne, Kenneth J., 1,392 Xu, Jian-min, 199 Yilgor, I., 180 Yoko, Y., 345 Yu, Yuan-Fu, 143 Zeldin, Martel, 199

Affiliation Index Army Materials Technology Laboratory, 268 A T & T Bell Laboratories, 296,430 Bell Communications Research, Inc., 296 Carnegie Mellon University, 78 Case Western Reserve University, 238 Colorado State University, 420 Dow Corning Corporation, 156 Ethyl Corporation, 277 Fraunhofer-Institut fur Silicatforschung, 333 Hiroshima University, 209 Hitachi Ltd., 209 IBM Almaden Research Center, 43 Indiana University-Purdue University at Indianapolis, 199 Kyoto University, 345 Massachusetts Institute of Technology, 21,143 McGill University, 89 Mie University, 345 National Research Council of Canada, 101 Northwestern University, 224 Office of Naval Research, 1,392

The Pennsylvania State University, 250 Polytechnic University, 430 SRI International, 124 Sandia National Laboratories, 314 Southern Methodist University, 283 Stevens Institute of Technology, 437 Technical University, Lodz, Poland, 166 Texas Christian University, 283,385 Ultrasystems Defense and Space, Inc., 392 University of Colorado, 303 University of Illinois, 314 University of Lowell, 112 University of Maryland, 483 University of Massachusetts, 437,463,469 University of Nevada, 43 University of New Mexico, 378 University of North Carolina, 408 University of Texas at Austin, 61 University of Vermont, 290 University of Vienna, 43 University of Wisconsin, 6 Virginia Polytechnic Institute and State University, 180,354 Washington State University, 43

Subject Index A

a form of boron nitride preparation, 378-379 structure, 378

Absorption spectra, poly(di-n-alkylsilanes), 64,65/ W-Acetyltetramethylcyclodisilazoxane, formation, 167

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

500

INORGANIC A N D ORGANOMETALLIC POLYMERS

Acyclic diborylamines, prevention of borazene ring formation, 386 Acyclic silane oligomers, intermediates in irradiation, 54 Adhesive, synthesis via organic network formers, 337,338/,339 Aerogels, description, 317 Aging effects of tetraethoxysilanepoly(tetramethylene oxide) systems occurrence, 372-373 on mechanical properties, 373t S A X profile, 374,375/ temperature, 373,374/ Alkenyl polysilanes, photochemical cross-linking, 10 Alkenylphosphazenes electronic structure of olefinic center, 292-293 properties, 291-292 structure, 292 synthesis of ceramic solids, 29 Alkoxy-substituted polysilanes, formation, 86 Alkyl silicates effect of alcohols on synthesis, 244-245 structures, 245,246/ synthesis by elemental-silicon process, 240,241/ by silicate-based substitution approach, 240-247 synthesis step 1, 244-245 synthesis step 2, 245,247 synthesis step 3, 247 Alkylated polysilanes, thermochromic behavior, 64 Alkylpolysilanes photodegradation, 9-10 * Si-NMR spectra, 14,15/ Amine-phosphorus(III) halide condensation products, 305-307 reaction, 305 Amino-terminated polysiloxane oligomers, preparation, 182-183 Aminoboranes, formation via silylamines and haloboranes, 379 (£-Aminoethyl)ferrocene, reactions, 440,442 Aminolysis of dihalosilanes, reaction, 127 Aminopropyl-terminated poly(dimethylsiloxane) oligomers, characteristics, 184 Ammonolysis analysis of products, 149 preparation of silicon-containing ceramics, 149 products, 148 Ammonolysis of dihalosilanes modified reaction, 127 reaction, 126 Amorphous silicon carbide, formation from polysilanes, 16 9

Angular scattering variable, definition, 361 Anionic polymerization, 1,3-disilacyclobutanes, 27 Aromatic polyester copolymers dynamic mechanical behavior, 186,188/ preparation, 186 stress-strain behavior, 186,187/ Aryl-substituted polysilanes, photodegradation, 9-10 N-Arylcyclosilazoxanes polymerization, 170-172 preparation, 170 resistance to hydrolysis, 173 Arylpolysilanes, S i - N M R spectra, 14,15/ Atactic, definition, 490 Atactic poly(methyl methacrylate/methacrylic acid), synthesis, 484-485 29

Bimodal molecular-weight distributions, polysilanes, 8-9,12/ Biocompatible-bioactive polyphosphazenes biologically inert water-insoluble polymers, 259 water-insoluble but bioerodible polymers, 259,261 water-insoluble polymers that bear biologically active surface groups, 259 water-soluble polymers that bear bioactive side groups, 261 water-swellable polymeric gels-amphiphilic polymers that function as membranes, 259-260 Bis(aminopropyl)poly(dimethylsiloxane), synthesis via an equilibration polymerization process, 183-184 1,1 '-Bis(£-aminoethyl)ferrocene polycondensation, 437-438,440/ reactions with benzoyl chloride, 442-443 with phenylisocyanate, 443 structure, 438-439 synthesis, 438 1,1 '-BisO0-hydroxyethyl)ferrocene polycondensation, 438,440/ structure, 438-439 Bis(borazinyl)amines, characterization and preparation, 380 Bis(trimethylsiloxy)phenylsilyl radical, role in photo-cross-linking, 211-213 1,1 -Bis(trimethylsiloxy)-1 -phenyl(trimethyl)disilane irradiation, 211-212 photolysis, 211 synthesis, 211

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

501

INDEX Bis(trimethylsilyl)aminotrimethylsilylaminochloroborane formation of nitrogen-bridged analogue, 400-401 intermolecular dehydrohalogenation, 398,400 mass spectral breakdown, 398 preparation, 398 l,2-Bis-(trifluoromethanesulfonyloxy)tetramethyldisilane, reactions, 84 Boranes, intermolecular dehydro­ halogenation, 398 Borate systems FTIR spectra, 322,323/ stability of B-O-B bonds toward hydrolysis, 322,324/,325 Borazene gels, processing, 380,381/ Borazene ring formation, prevention, 386 Borazine pyrolysis studies, 394-399 synthesis, 393-394 Boron nitride crystalline modifications, 378 formation, 394 infrared spectrum, 381/,383 transmission electron micrograph, 382/383 X-ray powder diffraction analysis, 382/,383 Boron nitride precursors crystal, 398 mass spectra, 401,403 melt spin, 398,399/ Boron-nitrogen polymer precursors preliminary thermolysis studies, 390 preparation, 385-390 problems in cyclic formation, 385 Branched poly(carbosilanes), structures, 29,31/ Bulk-thermal imidization description, 193 solubilities, 195/ [n-BuSn(OH)(0 P ( C H ) ) ] , ORTEP plot, 479,481/ [(n-BuSn(OH)O PPh ) O l P h Ρ Ο Λ ORTEP plot, 477,480/ [(w-BuSn(O)O CC H ) -w-BuSn( O X C J O 1 , O K f t P plot, 471,474/ [H-BuSn(0)0 ar.H ] -C H ORTEP plot, 471,472/ [(w-BuSn(O)O CPh) -/i-BuSn(Cl)(O CPh) ] , ORTEP plot, 471,473/ [*-BuSn(O)0.P(C H ) ] ORTEP plot, 479,4*1/ e

2

n

2

2

e

6

Capacitance device sensor, scheme, 341,342/

2

Catalysis in transacylation reactions conversion of benzoyl chloride to benzoic anhydride, 205 hydrolysis of diphenyl phosphorochloridate, 206,207/ importance of hydrophobic binding interactions, 207 Catalytic dehydrocoupling, reaction, 131-132 Catalytic elimination of H , production of polysilylenes, 90 Catalytically active polymers development, 199 examples, 199-200 Cationic polymerization, isopropenylferrocene, 453-460 Ceramic fiber with Si-C-N-O composition, preparation, 157 2

, advantage preparatio polymers, 156-157 Chemical vapor deposition techniques, preparation of α form of boron nitride, 379 C H (NH) [P(NEt )], molecular structure, 309,311/ 6 4 2 ( 2^1r > 307,308/ C H N [P(S)NMe ] [P(S)(NMe ) ] , structure, 307,308/ Chromophoric chain segments of polysilanes electronic effects of substituents, 68,69/ lowest energy excitation, 68,69/,70 Si-Si molecular orbitals, 70,71/,72 C-NMR spectra of poly(TBTM/MMA), 488,489/,490 segmented poly(ether urethanes), 448,449/ Coammonolysis, pyrolysis of products, 149 Cobalt(III) polymer, synthesis via organic reactions, 465 Cohydrogenation of olefins advantages, 93 effect on rate of polymerization, 93-94 Compositional sequences of tri-/i-butyltin polymers, C-NMR spectra, 496 relative reactivities, 493 1,2-Condensation dimer of C H (NH)[NP(NEt ) ](PNEt ), molecular structure, 3Γ10,311/ Condensation polymerization of polyphosphazenes advantages, 284 characterization, 285-287 derivatization reaction, 287-288 reaction, 284 synthesis, 284-285 Conduction model, for poIy(trisbipyridine)-metal complexes, 420-428 6

C

4

2

H

6

N

3

P

N E T

structure

3

2

13

1S

e

4

2

2

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

2

2

502

INORGANIC AND ORGANOMETALLIC POLYMERS

Configurational sequences of tri-w-butyltin polymers C - N M R spectra, 491-495 composition and tacticity ratios, 493/ *H-NMR spectra, 491,492 Contact lenses improvement of mechanical strength of materials, 339,340/ structure of polymer, 339,340/ synthesis of materials, 339 Contrast enhancement lithography, description, 57 Coordination polymers, synthesis, 463-468 Copoly(diorganosilanes) change in U V absorbance vs. U V exposure time, 120,121/ effect of molecular weight on U V spectra, 114,117,118/ of substituents on U V spectra Fourier transform-IR spectrum properties, 114,115/ proton-coupled C - N M R spectra, 117,118-119/ U V spectra, 114,116/ U V spectra vs. U V exposure time, 117,120/ Copolymerization, reaction rate of phenylmethyldichlorosilane vs. that of dimethyldichlorosilane, 110,111/ Copper chloride complexes with poly(2-vinylpyridine) complex preparation, 431-432 infrared spectra, 431 isolation, 431 loss of water, 432,433/ pyrolysis mass spectroscopy, 431 reactant specifications for preparation, 431/ relative mass intensities, 432,435/ structure of evolved ions, 432,434/ thermograms, 432,433/ thermogravimetric analysis, 431 total char yield and polymer char yield, 434/,435 Coupling of germanes model for polymerization reaction, 94-95 to insoluble gels, 94 Covalently cross-linked systems, comparison with microcrystallite-based ultrastructure, 262 Cyclic bis(silylamides) effects of substituents on stability, 167-169 properties, 167 structure, 167 1 3

1 3

29

structure determination by Si-NMR spectroscopy, 168 synthesis from silazane-containing ring compounds, 166-167

Cyclic trimer, preparation of block polymers, 192 Cyclic trimer substitution-polymerization route for polyphosphazene synthesis cyclic trimers with halogens and organic side groups, 257,260 illustration, 257-258 Cyclohexanoate drum compound S n - N M R data, 471,475 synthesis, 471 Cyclohexasilicate ion, structure, 238,239/ Cvclopentanoate drum compound S n - N M R data, 471,475 synthesis, 471 Cyclopentanoate ladder compound, S n - N M R data, 475,476/ Cyclosilazoxanes, anionic mechanism of polymerization, 170-172 I19

I19

119

Deamination-condensation polymerization reactions, reaction, 130-132 Decoupling pyrolysis studies, 137,138/, 139 size exclusion chromatography, 137,138/ synthesis of preceramic polysilazanes, 137-139 Dehydrocyclization reaction, mechanism, 133 Dehydrogenative coupling absence of low-molecular-weight oligomers, 91-92 constraints on catalytic activity, 91 description, 91 Deprotonation-substitution of 2-phenyl1,3,2-diazaboracyclohexane reactions, 387 yields, 387-388 Derivatization reactions for polyphosphazenes cothermolysis, 287-288 deprotonation, 287 Peterson olefination reaction, 287 Dialkoxydisilanes, preparation, 84 1,3,2-Diazaboracycloalkane ring systems, prevention of borazene ring formation, 386 1,3,2,4-Diazadiphosphetidines, structure, 304 Dichlorodialkylsilanes, products of reductive coupling, 101 1,1 '-Diisopropenylferrocene cationic polymerization, 450 unreactivity in polymerization, 455 Dimethyltitanocene, coupling of germanes, 94 Dioxygen reduction electrocatalysis by metal macrocycles, efficacy, 417-418

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

503

INDEX Diphenylgermane, polymerization, 94 1,3-Disilacyclobutanes anionic polymerization, 27 formation of high-molecular-weight product, 23-26 preparation from (chloromethyl)chlorosilane, 23 synthesis, 22-29 p-Disilanylenephenylene polymers, synthesis, 215 Disilicate ion, structure, 238,239/ 1,1 -Disubstituted silacyclopentanes, ring-opening polymerization, 33,38 Drawability of S i ( O C H ) solution, 2

g

4

influencing factors, 348 Drum compounds formation from phosphinic acid, 477,480/ molecular symmetry, 475 schematic, 479,480/

Ε Electrocatalysis of dioxygen reduction by cobalt porphyrins, efficacy, 417-418 Electrochemical polymerization of metallotetraphenylporphyrins, film formation, 409 Electron hopping, description, 420 Electron transport in metallotetraphenylporphyrin films apparent self-exchange rate constants, 415,417/ electron hopping, 414 mechanism, 413-414 sandwich electrode, 414,416/ Electropolymerized films characterization, 409,412-413 electron transport, 413-417 permeation, 412-413 Elemental-silicon process disadvantage, 240 variation in oxidation number of silicon and number of oxygens bonded, 240,241/ Emission spectra, poly(di-w-alkylsilanes), 62,63/ (EtO) Si, synthesis, 242 (EtO) Si Ο isomers, synthesis from Na Ca^i Ο 243-244 (EtO) SiQSi(OEt) , synthesis from C a Z n S i O 242-243 4

4

2

2

r

F Ferrocene monomers, formation of isopropenylferrocene monomer, 451

Field effect transistor sensor, scheme, 341,342/ Fluorescence polarization, polysilanes, 64,66,67/ Functional polysiloxane oligomers equilibration polymerization processes, 181-182 general structure, 181 overall synthesis, 181-182

Gel formation, availability of monomer for silicate systems, 318-319 Gel structure during consolidation difference between evolution during consolidation and that in solution, 325

MASS and CPMASS N M R spectra, 325,327/,328 Raman spectra of silicates, 325,326/ stability of siloxane bonds, 328-329 Gelation, description, 317 Gels applications for dried gels, 317 densification, 317-318 description, 318 drying, 317 factors influencing structure, 318 S i - N M R spectra, 322 Germanes, coupling, 94 Glasslike inorganic network, modification by organic groupings, 334,336/ Group 14 hydrides, polymerization, 89-99 29

H Haloboranes, formation of aminoboranes, 379 (Halomethyl)halosilanes, synthesis of high-molecular-weight poly(silmethylenes), 22 Hexachlorocyclotriphosphazene, polymerization, 270 Hexakis(pyrrolyl)cyclotriphosphazene structure, 297,299/ synthesis, 297-298 High-molecular-weight polymers, molecular-weight distribution, 114,115/ High-molecular-weight polysilanes, ultraviolet spectra, 11,12/ High-molecular-weight poly(silmethylenes), synthesis by (halomethyl)halosilanes, 22 Highest occupied molecular orbital, for polysilanes, 70,71/72

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

504

INORGANIC A N D ORGANOMETALLIC POLYMERS

Hybrid inorganic-organic polymers synthesis via alkenylphosphazenes, 291-293 via (vinyloxy)phosphazenes, 293-294 inorganic substituents, 291-294 organic substituents, 290-291 Hydrosilylation, reaction, 38 Hydroxy-terminated poly(dimethylsiloxane) disadvantages of use in glass networks, 355-356 incorporation into tetraethoxysilane glass network, 355 Hydroxy-terminated poly(methyl-3-pyridinylsiloxane) *H-NMR spectrum, 202,203/ IR spectrum, 202 synthesis, 201-202 thermal analysis, 202,204/,205

Isopropenylferrocene carbenium ion, stability, 458 Isopropenylferrocene copolymerization with α-methylstyrene, 455,456/,457 Isopropenylferrocene homopolymers H - N M R spectrum, 453,454/ IR spectra, 453 structure, 453 Isopropenylferrocene monomer homopolymerization reactions, 451,452/ synthesis via ferrocene monomers, 451 Isopropenylferrocenyl carbenium ion, stability, 455 Isopropenylmetallocene monomers cationic polymerization, 450 structures, 450 Isotactic, definition, 490 Isotactic poly(methyl methacrylate),

Imidization, techniques, 193 M-Imidobis[bis(trimethylsilyl)aminotrimethylsilylaminojborane infrared spectrum, 401,402/ mass spectral breakdown, 401,403,404-405/ INDO/S method, nature of chromophoric chain segments, 66,68-71 Infrared spectrum, of boron nitride, 381/,383 Inorganic and organometallic macromolecules applications, 3 with network structures, 3 Inorganic compound formation from polymer precursors, examples, 430 Inorganic networks addition of organic groupings by polymerization, 334,336/ addition of organic groupings by substitution, 334,336/ characterization, 334 typical structure, 334,336/ Inorganic polymer chemistry, relationship to organic polymers, ceramic science, and metals, 250,251/,252 Inorganic polymer research, main purposes, 252 Inorganic polymers, definition, 1 Inorganic polymers based on alternating main group element-nitrogen skeletons, structures, 303 Intractability, problem for linear transition metal coordination polymers, 463 Inverse addition, definition, 23 Isopropenylferrocene, cationic polymerization, 453,455

L

1

Ladder compounds conversion to drum form, 477,478/ general structural features, 475,477 molecular symmetry, 475 Linear boron-nitrogen polymers preparation, 385-390 properties, 385 Linear chain macromolecules behavior, 3 preparation, 2 Linear phosph(III)azanes, formation, 303-304 Linear polysilane, preparation, 6 Linear poly(silmethylenes), preparative procedures, 22 Linear polysilylenes, production, 90 Linear transition metal coordination polymers problems with intractability, 463 reaction kinetics, 464 Low-molecular-weight product of reductive coupling, isolation, 103-104 Low-molecular-weight silicon catenates, electronic spectra, 46 Low-temperature coupling in the presence of ultrasound chain-growth mechanism, 80 degradation of polysilanes, 81,84 dependence of yield, polymerization degree, and polydispersity, 81 effect on molecular weight, 81,83/ gel permeation chromatographic traces, 81,82/ limitation of chain growth, 80 monomodality of polysilanes, 80

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

505

INDEX Low-temperature reductive coupling in presence of ultrasound, preparation of polysilanes, 79 Lowest unoccupied molecular orbital, for polysilanes, 70,71/,72

M Macromolecular substitution route for polyphosphazene synthesis advantages, 254 effects of changes in side group structure, 254,256-257 illustration, 254-255 preparation of glucosylphosphazene polymer, 254,256-257 Macromolecules critical properties, 252 definition, 1 development, 1 Mass spectrometry, segmented poly(ether urethanes), 446,448 (Me Si) , X-ray crystal structure, 11,13/ Me Si Ο isomers, synthesis from ( E t o \ S i O 244 Metallocene monomers, synthesis, 458-459 Metallophosphazenes, description, 261 Metallotetraphenylporphyrins electrochemical polymerization, 409 structure, 408, 410/ Metals, relationship to inorganic polymers, 250,251/,252 Methyl-terminated poly(dimethylsiloxane) oligomers, preparation, 18 J -182 3-(Methyldichlorosilyl)pyridines hydrolysis, 201-202 infrared spectrum, 201 mass spectrum, 201 synthesis, 201 Methylpoly(silylmethylenes), preparation, 22 Microcrystallite-based ultrastructure, comparison with covalently cross-linked systems, 262 Microlithography problems, 57 single and multilayer process, 55,56/ Microscale electrochemical experiments, apparatus, 225,226/,227 Modification of polysilanes H - N M R spectra, 84,85/ model reactions, 84 Molecular metals, structure, 225 Molecular-weight distributions, determination, 102-103 2

0

6

r

1

Ν Network formers, definition, 334 Nicalon fibers, formation, 32

Nonoxide ceramic materials classical preparation, 378 preparation via polymeric ceramic precursor, 378 Nonoxide ceramics expense in preparation, 124-125 properties, 124 Nonpolymerizability, causes, 86 Nuclear magnetic resonance, collection of spectra, 485-486 Number-average molecular weights, determination, 184-185

Ο Octaphenylcyclotetrasilane, polymerization, 86-88

, 477,478 phosphinic acid reactions, 477,479,480/ structural symmetry, 475,477 synthesis, 471-475 Oligosilazanes, structure from formation via modified ammonolysis, 127-128 Organic network formers applications, 337-342 basic principles, 335,337 effect of reaction conditions on reaction sequence, 337,338/ synthesis of adhesive material, 337,338/339 Organic polymers, relationship to inorganic polymers, 250,251/,252 Organofunctional cyclophosphazenes, precursors of inorganic-organic polymers, 291 Organometallic polymers definition, 1 formation, 437 Organometallics, effect on reductive coupling of dichlorosilanes, 104,106,107/ Organosilicon polymers, lithographic applications, 221,222/ Organosilicon polymers bearing phenyldisilanyl units, photochemical behavior, 209 Organosiloxane copolymers, synthesis, 185 Organosiloxanes, synthesis by elemental-silicon process, 240,241/ Ormocer, definition, 334 Ormosils, definition, 334 Orthosilicate ion, structure, 238,239/ Oxidatively electropolymerizable tetraphenylporphyrins, structures, 409,410 Oxygen-capped cluster, schematic, 479,480/

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

506

INORGANIC AND ORGANOMETALLIC POLYMERS

Pentamethylphenyldisilane, products of irradiation, 210 Perfectly alternating siloxane poly(arylene ether sulfone) copolymers, preparation, 186,191 2-Phenyl-1,3,2-diazaboracyclohexane ring system advantages to study, 387 cleavage reaction, 387-389 deprotonation-substitution, 387-388 transamination reaction, 387,389 2-Phenyl-1,3,2-di[bis(dimethylamino)boryljazaboracyclohexane, molecular structure, 389-390 Phenylaminoborazines, condensation mechanism, 395-396 N-Phenylsilazoxane linear polymers thermostability, 173 Phosph(III)azane oligomers/polymer properties, 305,307 structure, 305 Phosphinic acid reactions, formation of a drum composition, 477 Phosphorus-nitrogen skeletons with stabilizing units, structure, 304 Photochemistry of polysilanes primary process, 73 secondary processes, 73-75 Photoconductors, polysilanes, 18 Photoelectrochemical properties of titanium dioxide films apparatus, 350/ current-bias potential curves, 350/351 photocurrent vs. heating time, 351/ porosity of film vs. heating temperature, 351,352/ preparation of gel films, 348,350 scanning electron micrograph, 351,352/ Photoinitiation by polysilanes efficiency, 17 photopolymerization, 1 7/ [(PhP)(C Al >TPPh)] , structure, 305,306/ [PhSn(0)O CC? H 1 , schematic of drum structure, 469,470/ Phthalocyanine molecular metals apparatus for electrochemical experiments, 225,226/227 effect of counterions on electrochemistry, 231,232/,233,234/ electrochemical structural relationships, 228,230/ microscale electrochemical voltage spectroscopy experiments, 227-228,229/ reductive doping, 233,235/ slurry electrochemical voltage spectroscopic experiments, 228,230/,231,232/ slurry-scale electrochemical doping, 227,229/ 2

2

Phthalocyanine molecular metals— Continued structural transformations accompanying electrochemical doping, 227,229/ Platinum-catalyzed polymerization, poly(dimethylsilylene), 28 P(III)-N oligomer-polymer interconversion, occurrence, 304 Poly(amic acid), conversion to fully imidized polyimide, 193 Polyamides, structures, 440-441 Poly(aryloxyphosphazene) elastomers applications, 279-280 properties, 280-281/ Polybis(pyrrolyl)phosphazene cross-linking, 300-301 electroxidation, 297 future prospects, 300,302

properties, 298,300 thermogravimetric analysis, 300,301/ Polycarbonate copolymers dynamic mechanical behavior, 186,188/ preparation, 186 stress-strain behavior, 186,187/ Poly(carbosilane) definition, 21 preparation, 26,29,32/, 145-146 structure, 21 variations, 32 Polycyclic carbosilanes, examples, 33,34-37/ Poly(cyclohexylmethyl-c0isopropylmethylsilane), 117,118/ Poly(cyclohexylmethyl-c0-wpropylmethylsilane) copolymers, C - N M R spectra, 117,118/ Poly(diarylsilane) homopolymer, preparation, 49,52 Poly(dichlorophosphazene) characteristics, 277 effect of organic groups on end product, 278 polymerization process, 270,271/ preparation routes, 278,279/ solution polymerization with BC1 270,271/ Poly[/?-(diethyldimethyldisilanylene)phenylene] formation of C=Si intermediates, 216,218 H - N M R spectrum, 216 molecular weights of products vs. irradiation time, 216,217/ photochemistry of thin films, 220-221 preparation, 215 properties, 215 Poly(di-H-hexylsilane) absorption maximum, 49,50/ l s

1

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

507

INDEX Poly(di-/i-hexylsilane)—Continued absorption spectra, 46,48/,64,65/ C P / M A S S i - N M R spectra, 46,49,51/ emission spectra, 62,63/ experimental procedures for photochemistry, 62 film diffraction pattern, 49,51/ IR and Raman spectroscopic studies, 46 phases, 47,48/ Raman spectra, 49,50/ thermochromic behavior, 47 Poly[/K 1,2-dimethyldiphenyldisilanylene)phenylene] formation of silaethene intermediates and hydrosilanes, 220 H - N M R spectrum, 216 lithographic applications of a double-layer resist system, 221,222/ molecular weights of product irradiation time, 218,219/ photochemistry of thin films, 220-221 preparation, 215 properties, 215 Poly(dimethylsilane), properties, 112 Poly(dimethylsiloxane) bulk microphase separation characteristics, 192 structure, 21 Poly(dimethylsiloxane) chain, structure, 180-181 Poly(dimethylsilylene) applications, 90 platinum-catalyzed polymerization, 28 properties, 90 synthesis, 6-7 Poly(dimethylsilylmethylene), structure, 21 Poly(diorganophosphazenes), synthesis, 1 Poly(diorganosilane) copolymers molecular-weight determination, 113 synthesis, 113 Polydiorganosilanes, synthesis, 2 Poly(di-n-pentylsilane) absorption maximum, 49,50/ C P / M A S S i - N M R spectra, 49,51/ film diffraction pattern, 49,51/ helical conformation, 49,53/ Raman spectra, 49,50/ Poly(rt-dipentylsilane), ultraviolet spectrum vs. temperature, 11,13/ Poly(diphenylsilylene), preparation, 6 Polyf/?-(disilanylene)phenylenes] lithographic applications, 221,222/ synthesis, 39 Poly(fluoroalkoxyphosphazene) elastomers applications, 279 preparation, 278 properties, 279,280/ Poly(imide-siloxane) copolymers, structure, 193,194/ 29

1

29

Polymer-silylamide applications, 153 properties, 152-153 pyrolysis products, 153 reactions, 153 synthesis, 152 Polymeric boron-nitrogen compounds, formation, 379-383 Polymeric films of metallotetraphenylporphyrins characterization, 409,412-413 electrocatalysis of dioxygen reduction, 417-418 electron transport, 413-417 structure, 408 Polymerization by aminolysis, reaction, 127 Polymerization by ammonolysis modified reaction, 127

Polymerization mechanisms, elucidation, 2 Polymerization of W-arylcyclosilazoxanes mechanism, 171-172 reaction, 170-171 Polymerization of primary organosilanes degrees, 93 mechanism, 92 properties of polymers, 92 rates, 92 Polymerization of primary silanes mechanisms, 95,98 products, 95,96/ pseudoequilibrium, 95 rate law, 95 routes for the propagation step, 95,97 termination reaction, 98-99 Polymerization of silanes, cation complexation, 44 Polymers, ceramic formation, 156-157 Polymers with triflate groups, properties, 86 Polv(methyl methacrylate) (MMA), H-NMR spectra, 491,492/ Poly(methyl methacrylate-methyl methacrylate) ( M M A - M M A ) C - N M R spectra, 491,492/,493,494-495/ composition and tacticity ratios, 493/ Poly(methyldisilylazane) C - N M R spectrum, 158,159/ experimental procedures, 157-158 N - N M R chemical shifts, 163/, 164 reaction scheme, 160,162-163/ S i - N M R shifts of low-molecular-weight species, 160,161/ S i - N M R spectrum, 158,159/ Poly(organophosphazenes) classes, 257-265 effect of substituents, 272,273/,273/,274/ morphology, 272,273/,274 synthesis, 278,279/ 1 3

1 3

1 5

29

29

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

508

INORGANIC AND O R G A N O M E T A L L I C POLYMERS

Poly(organosilylenes), thermochromic behavior, 90 Poly(phenylmethyl-c0-«-propylmethylsilane) copolymers, change in U V absorbance, 120,121/ Polyphosphazene chemistry, historical development, 252,253/,254 Polyphosphazenes applications, 278-281 biocompatible-bioactive compounds, 259-261 characteristics, 250 characterization, 285-287 commercial development, 277-278 definition, 268 derivatization reactions, 287-288 diffraction patterns, 287 electronic structure, 296 historical perspective, 264/,26 intrinsic viscosities, 285 melt transition, 285 N M R spectra, 285 polymerization process, 270,271/,288 synthesis, 254 thermogravimetric analysis, 286-287 two-step synthesis process, 268,269/ various end products from single precursor, 278 weight-average molecular weights, 285 Poly(phosphazenes) condensation polymerization, 284 synthesis by ring-opening substitution method, 283 Polyphosphazenes with rigid, stackable side groups formation, 262 liquid-crystalline polymers, 264-265 polyphosphazene-phthalocyanine structures, 262,265 tetracyanoquinodimethane-polyphosphazene systems, 262-264 Polyphosphazenes with transition metals in side groups, methods for linking metals, 261-263 Poly(w-propylmethyl-co-isopropylmethylsilane) copolymers, 114,116/, 117,120/ Poly(w-propylmethylsilane) C - N M R spectra, 117,119/ F T - I R spectrum, 117,119/ Poly(silaacetylides), preparation, 38 Poly(silaarylene-siloxanes), synthesis, 38 Polysilane high polymers, photodegradation mechanism, 54-55 Polysilanes applications, 78 bimodal molecular-weight distribution, 8-9,12/ bleaching upon irradiation, 52,54,56/ catalytic polymerization routes, 2 1 3

Polysilanes—COM/MM?*/ cross-linking, 10 definition, 6,43 discovery, 43-44 electronic spectra, 11,12-13/, 14 high-resolution patterns, 57,58/ microlithography, 55,56/,57 modification, 84 nature of chromophoric chain segments, 66,68-71 photochemical quantum yields, 54 photoconductors, 18 photodegradation reactions, 9-10 photoinitiators, 17/ polarization of the emission, 64,66,67/ precursors to silicon carbide, 14,16-17 preparation, 79-88 primary photochemical process, 73

sensitivity toward ionizing radiation, 54 S i - N M R spectra, 14,15/ submicron images, 57 substituent effects, 46,48/ synthesis, 7-8,44,45/ technological applications, 14-18 unique properties, 112-113 variable-temperature studies, 46 29

Wurtz condensation reaction, 78-79 Polysilanes with alkoxy groups, properties, 86 Polysilastyrene formation of silicon carbide, 17 synthesis, 7 Polysilazanes acid-catalyzed ring-opening polymerization, 129 catalytic polymerization routes, 2 curing of fibers, 148 general synthetic approaches, 125-126 melt spinning, 147-148 properties, 147,148 structure, 129 use in applications of preceramic polymers, 147 Polysilazanes with aromatic spacing groups structure, 175-176 synthesis by polycondensation, 175-177 Polysilazoxanes, synthesis, 2 Poly(silmethylenes) formation, 26,28-29 properties, 28-29 structures, 29,30/ Polysiloxanes copolymerization reaction, 185-186 structures, 185 use as hydrophobic catalysts, 200

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INDEX

509

Polysiloxanes containing phenyldisilanyl units irradiation, 211,213,215 photochemical behavior, 213,215 preparation, 210 U V spectra of thin liquid film, 213,214/ U V spectrum, 210 Poly[/?-(tetramethyldisilanylene)phenylene], preparation, 215 Poly(tetramethylene oxide)-modified sol-gel glasses effects of aging, 372-375 of molecular weight, 359-364 of tetraethoxysilane content, 365-368 of titanium addition, 368-372 example procedures, 357-358 materials, 356 reaction scheme, 356-357 sample nomenclature, 358/ Poly(tri-w-butyltin methacrylate-methy methacrylate) ( T B T M - M M A ) C - N M R spectrum, 488,489/,490 determination of compositional sequences, 493,496 of configurational sequences, 490-495 H - N M R spectra, 491,492/ S n - C coupling constants, 488/ solvent effects in the S n - N M R spectra, 486,487/,490 Poly(trisbipyridine)-metal complexes absence of nonelectroactive ions, 427-428 conduction as activated process, 427 conductivity vs. potential, 421,424,425/ cyclic voltammogram, 421,422-423/ film resistance vs. reciprocal temperature, 424,426/ optical properties, 421 properties vs. those of other electronically conducting polymers, 424,427 sandwich electrode configuration, 424 Polyureas, structures, 440-441 Polyurethanes average structure, 446,447/ molecular-weight distribution, 446 synthesis, 444-445 Poly(4-vinylpyridine 1 -oxide), use as nucleophilic catalysts, 200 Preceramic organosilicon polymers, synthesis, 143-154 Preceramic polymers applications, 143-144 design, 144-145 effect of pyrolysis, 145 factors determining usefulness, 144 preparation of silicon nitride mixtures, 148 preparation using chlorosilane, 146 1 3

Preceramic polymers—Continued problems with composition of ceramic product, 145 products, 146-147 synthesis by decoupling, 137-139 Precursors to silicon carbide, polysilanes, 14,16-17 Preparation of silica glass fibers composition and properties of tetraethoxysilane solutions, 346/ viscosity vs. time, 346,347/ Primary organosilanes mass spectra, 92-93,96/ polymerization, 92 Primary silanes, polymerization mechanism, 95-99 Properties, polysilanes, 8/ («-PrO) SiO(/i-PrO) SiOSi(0«-Pr) , 3

2

3

catalysts, 200 Pyrolysis of monosilacyclobutanes, synthesis of monosilacyclobutanes, 26 Pyrrolylphosphazenes, preparation, 297

1

1 1 9

1 3

119

R Radical disproportionation reaction, documentation, 74-75 Reaction kinetics of reductive coupling of dichlorosilanes, effect of sodium surface area, 106,108/, 109/ Red shifts, origin for soluble poly(diarylsilanes), 52 Redistribution reactions, sequence, 132 Redox conduction, description, 414 Redox conductors, charge transport, 420 Reductive coupling of dichlorosilanes with sodium addition of organometallics, 101 chain nature of reaction, 104/, 105/ change in molecular weight of intermediate product, 103/ experimental procedures, 102-103 gel permeation chromatographic traces of complete products, 103,105/ low-molecular-weight product, 103-104 reaction kinetics, 106,108/, 109/ reaction termination reagent, effect, 109-110 sodium surface area, effect, 106,108/ Refractories, applications, 392-393 Restructuring polymers effect after gelation, 320 effects at short-length scales, 320,322 process, 319-320

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

510

INORGANIC AND ORGANOMETALLIC POLYMERS

Ring-opening polymerization discussion, 86 1,1-disubstituted silacyclopentanes, 33,38 preparation of polysilanes, 79 Ring-opening polymerization of silazanes catalysis by transition metals, 129-130 reaction, 128-129

Sandwich electrode application of potentials, 415,416/ description, 414-415,416/,424 Science of solids, definition, 250,252 Scratch-resistant coating, properties, 339,341/ Segmented poly(ether urethanes) average structure, 446,447/ C - N M R spectrometry, 448,449 mass spectrometry, 446,448 molecular-weight distribution, 446 synthesis, 444-446 Silazane systems effect of substituents, 169 equilibrium, 169 thermodynamic stability, 169 Silazane-containing ring compounds, synthesis of useful polymers, 170-177 Silazanes catalytic dehydrocoupling, 131-132 deamination-condensation polymerization reactions, 130-132 formation via aminolysis, 125-128 via deamination-condensation reaction, 126 via dehydrocoupling reaction, 125-126 via redistribution reaction, 126 problems in synthesis, 139-140 ring-opening polymerization, 128-129 transition metal catalyzed dehydrocoupling, 134-137 Silazoxane linear polymers and copolymers formation, 170-173 properties, 173 Silica glass fibers, preparation, 345-348 Silicate gels, Raman spectra, 320,321/ Silicate systems availability of monomer, 318-319 hydrolysis, 318 restructuring, 318 Silicate-based substitution process examples, 242-244 reactions, 240,242 utility, 247 Silicon carbide, conventional method of preparation, 143 Silicon carbide fiber precursor, preparation, 145

Silicon carbide fibers, process, 32-33 Silicon carbide precursor preparation silylamide-catalyzed reactions, 151-152 starting materials, 150 yield, 150-151 Silicon catenates, dependence of electronic properties on backbone conformation, 47 Silicon nitride conventional method of preparation, 143 preparation, 149-150 Silicon-containing ceramics, description, 143 Silicon-nitrogen bond, polarity, 169 Siloxane-modified polyimide copolymers solubilities, 195/ upper glass transitions, 195,196/ Siloxane-modified polyimides microphase separation, 193

1 3

Siloxane-cyclodisilazane block copolymers polycondensation reaction, 174 synthesis, 173-174 thermostability, 174-175 Silyl cleavage reaction of 2-phenyl1,3,2-diazaboracyclohexane derivatives reaction, 388 yields, 388-389 N-Silyl-P-(trifluoroethoxy)phosphoranimine reagents formation of polyphosphazenes, 284-285 preparation, 284 Silylamines, formation of aminoboranes, 379 S i 0 glasses applications, 335 introduction of organic network modifiers, 335,338/ Si(OC H ) solution factors influencing drawability, 348 spinnability, 346,347/,348 Slurry-scale electrochemical experiments, apparatus, 225,226Λ227 S n - N M R spectra, S n - C coupling constants, 488/ solvent effect of poly(TBTM-MMA), 486,487/,490 Sodium surface area, effect on reductive coupling of dichlorosilanes, 106,108/'. 109/ Sol-gel process advantages, 333 formation of pure inorganic materials, 333-342 limitation, 355 silica glass fiber preparation, 346-348 Sol-gel process for making glasses and ceramics schematic, 314,315/ shrinkage vs. temperature, 314,316/ uses, 314,317 2

2

119

g

l l 9

1 3

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

511

INDEX Sol-gel reactions production of low-temperature glasses, 354 two-step network-forming polymerization process, 355 Sol-gel techniques inorganic networks, 334-336 interpenetrating networks, 341 organic network formers, 335-341 Sol-gel-derived inorganic polymers, structural evolution, 318-329 Soluble high-molecular-weight polysilanes, preparation, 101 Soluble poly(diarylsilanes) absorption spectra, 52,53/ origin of large spectral red shifts, 52 Soluble polysilanes discovery, 7 polymerization process, 44,45/ yield of high polymer, 44,45/ Solution imidization description, 193 solubilities, 195/ structural representation, 193,194/ Spinnability of Si(OC H ) solution, log M versus log [η] plots, 346,347/,348 Stober process, description, 319-320 Styrylphosphazene, polymerization, 293 Syndiotactic, definition, 490 2

4

R

Τ Tacticity, definition, 490 Tensile strength of silica glass fibers, vs. cross-sectional area, 348,349/ Tetra(0-aminophenyl)porpnyrin, electrochemical polymerization, 409,411/ Tetraethoxysilane-poly(dimethylsilane) systems calculated electron densities of components, 364/ dynamic mechanical spectra, 359,361,362/ effect of tetraethoxysilane content, 365-368 model, 361,363 Tetraethoxysilane-poly(tetramethylene oxide) systems aging effects, 372-375 calculated electron densities of components, 364/ characterization methods, 358 dynamical mechanical spectra, 359,360/,361 example procedures, 357-358 materials, 356 mechanical properties, 359/ mechanical properties of titanium-containing systems, 368,369/ reaction scheme, 356,357 sample nomenclature, 358/

Tetraethoxysilane-poly(tetramethylene oxide) systems—Continued S A X profiles, 361,362/,363 schematic model for structure, 364,366/ stress-strain behavior, 359,360/ tetraethoxysilane contents, effect, 365 titanium addition, effect, 368-372 1,1,3,3-Tetramethyl-1,3-disilacyclobutane, preparation, 23 Thermolytic polymerization of 1,3-disilacyclobutanes, reaction, 26 Thermoplastic elastomers, bulk microphase separation characteristics, 192 Titanium dioxide coating films color, 348,349/ photoelectrochemical properties, 348,350-352 preparation, 348-352

g tetraethoxysilane poly(tetramethylene oxide) systems dynamic mechanical behavior, 370,371/ mechanical properties, 368-369/ S A X profiles, 370,371/ Transacylation catalysis, 205-206,207/ phase-transfer process, 200 Transamination condensations intermediates, 307,309 monomer-dimer equilibrium, 310 products, 309 reactions, 307 Transamination reaction of 1,3-diaminopropane, reaction, 389 Transition metal catalysts, initiation of ring-opening polymerization of 1,3-disilacyclobutanes, 27 Transition metal catalyzed dehydrocoupling polymerization reactions mechanism, 134-136 reaction, 134 steric constraints, 136 Transmission electron micrograph, of boron nitride, 382/,383 jfl-Triamino-N-triphenylborazine, pyrolysis, 395 £-Triamino-N-tris(trimethylsilyl)borazine composition of monomer and dimer, 396-398 ring-opening mechanism, 396 0-Trianilinoborazine, pyrolysis, 395 Tributyltin compounds, applications, 483 Tri-w-butyltin methacrylate, synthesis, 485 Tri-H-butyltin polymers, synthesis, 483-484 £-Trichloro-N-triphenylborazine, synthesis, 393 £-Trichloro-N-tris(trimethylsilyl)borazine pyrolysis, 394-395 synthesis, 393-394

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

512

INORGANIC AND O R G A N O M E T A L L I C POLYMERS

Trimethylsiloxyl end-capped N-oxidized poly(methyl-3-pyridinylsiloxane) catalyst for transacylation reactions, 205-206,207/ H - N M R spectrum, 202,203/ IR spectrum, 202 synthesis, 202 thermal analysis, 202,204/,205 Trimethylsiloxyl end-capped poly(methyl-3-pyridinylsiloxane) H - N M R spectrum, 202, 203/ IR spectrum, 202 synthesis, 202 thermal analysis, 202,204/,205 Trimethylsilyl radicals, disproportionation, 74 Triphenyltin compounds, applications, 483 Tris{5,5'-bis[(3-acrylyl-1 -propoxy)carbonyl]2,2'-bipyridine}ruthenium(II), properties of polymer films, 42 1

1

U Unsymmetrically substituted, atactic polysilanes, spectroscopic studies, 46 Uranyl dicarboxylate polymers, polymerization, 464 Uranyl polymers coordination of ligands, 467 Ύ-ray sensitivity, 465,466/ infrared and thermal studies, 467-468

Urea-linked copolymers chemistry, 186,191 dynamic mechanical behavior, 186,190/ properties, 186,191-192 stress-strain behavior, 186,189/ V (Vinyloxy)phosphazenes, synthesis of inorganic-organic polymers, 293-294 Vinylferrocene, destabilization, 457-458 W Wurtz condensation reaction, synthesis of polysilanes, 78-79 X

Xerogels description, 317 formation of dense glasses and ceramics, 317

Ζ Zirconium polymer, synthesis, 464-465 Zirconium Schiff base polymer, number-average molecular weights, 467

In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.