Initiation of Polymerization 9780841207653, 9780841210271, 0-8412-0765-8

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Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.fw001

Initiation of Polymerization

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.fw001

Initiation of Polymerization Frederick Ε. Bailey, Jr., EDITOR

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.fw001

Union Carbide Corporation ASSOCIATE EDITORS

E. J. Vandenberg, A. Blumstein, M. J. Bowden, Jett C. Arthur, Joginder Lal, R. M. Ottenbrite Based on a symposium sponsored by the Macromolecular Secretariat, at the 183rd A C S National Meeting, Las Vegas, Nevada, March 28-April 2, 1982

ACS SYMPOSIUM SERIES 212

AMERICAN

CHEMICAL

WASHINGTONG D.C. 1983

SOCIETY

Library of Congress Cataloging in Publication Data

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.fw001

American Chemical Society. National Meeting (183rd: 1982: Las Vegas, Nev.) Initiation of polymerization. (ACS symposium series, ISSN 0097-6156-83/0; 212) Includes bibliographies and index. 1. Polymers and polymerization—Congresses. I. Bailey, Frederick E. (Frederick Eugene), 1927. II. American Chemical Society. Macromolecular Secretariat. III. Title. IV. Series. QD380.A4 1983 ISBN 0-8412-0765-8

668'.9

83-2613

Copyright © 1983 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article 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. 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 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. PRINTED IN THE UNITED STATES OF AMERICA

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.fw001

ACS Symposium Series M. Joan Comstock, Series Editor

Advisory Board David L. Allara

Robert Ory

Robert Baker

Geoffrey D. Parfitt

Donald D. Dollberg

Theodore Provder

Brian M . Harney

Charles N. Satterfield

W. Jeffrey Howe

Dennis Schuetzle

Herbert D. Kaesz

Davis L. Temple, Jr.

Marvin Margoshes

Charles S. Tuesday

Donald E . Moreland

C. Grant Willson

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.fw001

FOREWORD The ACS SYMPOSIUM SERIES 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 ADVANCES IN CHEMISTRY SERIES 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 the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.pr001

PREFACE -ALLTHOUGH THE INITIATION STEP in polymerization is of fundamental importance in polymer synthesis, no in-depth assessment of its status has been made in recent years—years in which great strides were made in understanding and controlling polymer architecture by exploiting knowledge of initiation mechanisms. Such an assessment, therefore, is long overdue. The topic of the initiation step in polymer synthesis fit well into the general format of Macromolecular Secretariat symposia because of its highly interdisciplinary character. The Macromolecular Secretariat is a consortium of five ACS divisions: the Division of Cellulose, Paper, and Textile Chemistry; the Division of Colloid and Surface Chemistry; the Division of Organic Coatings and Plastics Chemistry; the Division of Polymer Chemistry; and the Rubber Division. Each division contributed its particular focus and expertise to the symposium presented here by selecting chairmen who are eminent authorities in their fields, and who also served as associate editors of this volume: Jett C. Arthur, Jr.; Alexandre Blumstein; Murrae J. Bowden; Edwin J. Vandenberg; Joginder Lai; and Ray M . Ottenbrite. The associate editors selected papers for inclusion in the symposium to reflect the most recent advances in control of polymerization processes. Knowledge of the mechanisms and kinetics of initiation extends the range of use and applications of polymers. For their assistance, I am most indebted and grateful. The international character of this symposium, evident from the significant contributions by speakers from outside the United States, was made possible by a Special Educational Opportunity Grant from the Petroleum Research Fund, which provided the registration and travel assistance that enabled many foreign guests to participate. Special thanks are due to the Petroleum Research Fund for this grant, and also to Union Carbide Corporation for sponsoring an informal opening of the symposium prior to the plenary session. FREDERICK E. BAILEY, JR. Union Carbide Corporation South Charleston, WV 25303 October 29, 1982

1 New Syntheses of Functional and Sequential Polymers by Exploiting Knowledge of the Mechanism of Initiation JOSEPH P. KENNEDY

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch001

The University of Akron, Institute of Polymer Science, Akron,OH44325 The f i r s t part of t h i s presentation concerns a b r i e f overview of the mechanism of i n i t i a t i o n of carbocationic polymeri z a t i o n s ; in p a r t i c u l a r i t addresses head group control by protonation, cationation, and the controlled i n i t i a t i o n concept which led to a large new family of block, g r a f t , and bigraft copolymers. Subsequently the f i r s t carbocationic macromer synthesis y i e l d i n g polyisobutenylstyrene is described. The copolymerization of the l a t t e r macromer with acrylates gave r i s e to new graft copolymers. Then head group control with inorganic moieties, i.e., Cl-, NO - and Ø Si-, i s outlined. The first part concludes with a discussion of the s i m i l a r i t y between the mechanisms of i n i t i a t i o n and chain transfer, the appreciation of which led to the i n i f e r concept, which in turn yielded new t e l e c h e l i c s , networks, sequential copolymers, etc. The second part of t h i s presentation focuses on p r a c t i c a l consequences of understanding d e t a i l s of the mechanism of i n i t i a t i o n . The synthesis of a new family of t e l e c h e l i c l i n e a r and t r i - a r m star polyisobutylenes w i l l be described. Among the new prepolymers are t e l e c h e l i c o l e f i n s , epoxides, aldehydes, alcohols, and amines. The preparation of new ionomers and polyisobutylene-based polyurethanes w i l l be outlined and some fundamental properties of these new materials w i l l be discussed. 2

3

0097-6156/83/0212-0003$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4

INITIATION

OF

POLYMERIZATION

The s u r e s t way t o w a r d d e s i r a b l e new p o l y m e r s t r u c ­ t u r e s i s by a s y s t e m a t i c e x p l o i t a t i o n o f t h e d e t a i l e d mechanistic understanding of p o l y m e r i z a t i o n processes. The a i m o f t h i s p r e s e n t a t i o n i s t o e x a m i n e i n some depth the mechanism of c a r b o c a t i o n i c p o l y m e r i z a t i o n s and s u b s e q u e n t l y to apply t h i s i n s i g h t toward the p r e ­ p a r a t i o n o f new p o l y m e r i c m a t e r i a l s p o s s e s s i n g a d v a n ­ tageous combinations of p r o c e s s i n g and/or p h y s i c a l mechanical c h a r a c t e r i s t i c s .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch001

Initiation

i n Carbocationic

Polymerization

E a r l y Developments t i l l the D i s c o v e r y of C o n t r o l ­ led I n i t i a t i o n . U n d e r s u i t a b l e c o n d i t i o n s any e l e c t r o philic s p e c i e s may i n d u c e c a t i o n i c p o l y m e r i z a t i o n s ( 1 ) . As a p r a c t i c a l m a t t e r , t h e m o s t c o n v e n i e n t cationogens are Bronsted a c i d s alone or i n c o n j u n c t i o n with Fried e l - C r a f t s acids ( 1 ) . Systematic r e s e a r c h on t h e i n ­ i t i a t i o n o f c a r b o c a t i o n i c p o l y m e r i z a t i o n became p o s ­ s i b l e by t h e d i s c o v e r y o f c o i n i t i a t i o n by B r i t i s h i n ­ v e s t i g a t o r s (2-4). These workers found that the s t r o n g Lewis a c i d BF alone i s unable to i n i t i a t e isobutylene p o l y m e r i z a t i o n but i n the presence of s u i t a b l e c a t i o n ­ ogens, i . e . , H2O, i m m e d i a t e and v i g o r o u s polymerization ensues. Their formalism: 3

BF e

H BF 0H 3

9

3

+ H 0

— ^

2

+ CH =C(CH ) -» 2

3

2

Η BF 3OH CH3-C(CH3)2BF30Η

Θ

p r o v i d e d v a l u a b l e g u i d a n c e f o r s u b s e q u e n t r e s e a r c h on the mechanism of p r o t i c i n i t i a t i o n and i n t h e c o n t e x t of t h i s p r e s e n t a t i o n i s r e g a r d e d as t h e f i r s t p r o p o s i ­ t i o n of head group c o n t r o l , i . e . , t h e i n c o r p o r a t i o n of a p r o t o n as the "head group" of a m a c r o m o l e c u l e . Not much l a t e r P e p p e r ( 5 ) s u g g e s t e d t h a t c e r t a i n a l k y l h a l i d e s RX i n c o n j u n c t i o n w i t h F r i e d e l - C r a f t s a c i d s MX may a l s o a c t a s i n i t i a t o r s : n

RX + M X „ η

+ C=C

^ ^

9

R^MX n+1

> R-c-ce

A l t h o u g h t h e f o r m a l i s m o f c a t i o n a t i o n by a l k y l h a l i d e s was i n p r i n c i p l e c o r r e c t , t h e s i g n i f i c a n c e o f t h i s m e c h a n i s m f o r h e a d g r o u p c o n t r o l c o u l d n o t be e x p l o i t e d because the c o n v e n t i o n a l F r i e d e l - C r a f t s a c i d s ( B F , A I C I 3 , S n C l i f , ΤiC 1 0 u s e d by t h e e a r l y w o r k e r s w e r e 3

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

Syntheses of Functional and Sequential Polymers

KENNEDY

5

extremely moisture s e n s i t i v e ( i . e . , induced p o l y m e r i z a t i o n by protonation i n the presence of p u t a t i v e t r a c e s of moisture imp u r i t i e s ) and because l a r g e q u a n t i t i e s of C H 3 - head groups a l s o arose by unavoidable chain t r a n s f e r to monomer. In 1965 i t was discovered (6,7) that under conventional "open" l a b o r a t o r y c o n d i t i o n s ( i . e . , i n the presence of m o i s t u r e ) o l e f i n polymerizations can be r e a d i l y i n i t i a t e d by p r o t i c a c i d s or by " a c t i v e organic h a l i d e s (j^-butyl, b e n z y l , a l l y l ) i n conj u n c t i o n with c e r t a i n alkylaluminum compounds (MeaAl, E t 3 A l , E t A l C l , e t c . ) . Since these d i - and t r i a l k y l a l u m i n u m compounds are e f f e c t i v e moisture scavengers and t h e i r r e a c t i o n products with small amounts of H2O a r e p o l y m e r i z a t i o n i n a c t i v e , i n i t i a t i o n can occur only upon the purposeful a d d i t i o n of s u i t a b l e c a t i o n o gens. F o r reasons beyond the scope of t h i s présentation»polymeri z a t i o n s induced by RX/R2AIX combinations a r e chain t r a n s f e r l e s s ( t r a n s f e r l e s s polymerizations a r e discussed i n réf. 1) so that the head groups of polymers formed by these i n i t i a t i n g systems can be r e a d i l y c o n t r o l l e d . For example, (M = monomer): 11

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch001

2

CH

CP3

-Cl/Et AlCl + M CH

»

2

3

0 - ( Μ Λ Μ

Φ

Et AlCl 2

9 2

CH3

3

The head groups of polymers formed by such " c o n t r o l l e d i n i t i a t i o n " systems are determined by the nature of the i n i t i a t o r molecule. A l a r g e number of c o n t r o l l e d i n i t i a t i o n systems g i v i n g r i s e to polymers bearing d e s i r a b l e f u n c t i o n a l head groups e.g., a l l y l , b e n z y l , c y c l o p e n t a d i e n y l , s i l y l , have been d e s c r i b e d ( 1 ) . P o s s i b i l i t i e s o f f e r e d by c o n t r o l l e d i n i t i a t i o n have been ex­ p l o i t e d f o r the p r e p a r a t i o n of new s e q u e n t i a l copolymers and mac­ romers and a r e discussed below. Sequential (Block and G r a f t ) Copolymers by C o n t r o l l e d I n i t i ­ a t i o n . C o n t r o l l e d i n i t i a t i o n by a c t i v e high molecular weight h a l i d e s ( i . e . , macromolecules c o n t a i n i n g t e r t i a r y alkyl or b e n z y l i c h a l i d e s ) i n conjunction with d i - and t r i a l k y l a l u m i n u m coi n i t i a t o r s l e d to the s y n t h e s i s of a l a r g e v a r i e t y of block, g r a f t and b i g r a f t copolymers. Schematically (X and Y = h a l i d e functions): AAAAAAAA-X

X-AAAAAAAA-X

—

+

n

B

>

>

AAAAAAAA-BBBBB diblocks BBBBB-AAAAAAAA-BBBBBB triblocks

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

6

INITIATION

AAAAAAAA

_±_Ξ5—±

I T X X

OF

POLYMERIZATION

AAAAAAAA

I Β

I Ç

? 5

? Β

grafts

Β AAAAAAAA

I

—

I

,

+

N

Η (

? '

, ?

>

:

selective i n i t i a t i o n

AAAAAAAA

r

|

r

Β

î

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch001

bigrafts

C

Β

A l a r g e number of s e q u e n t i a l copolymers comprising many combina­ t i o n s of glassy and elastomeric segments have been prepared (1). The block and g r a f t copolymers prepared by c o n t r o l l e d i n i t i a t i o n are remarkably w e l l defined and many are f r e e of homopolymers. Sequential copolymers obtained by c o n t r o l l e d c a t i o n i c i n i t i a t i o n have been r e c e n t l y comprehensively surveyed (1,8). Macromers by C o n t r o l l e d I n i t i a t i o n . New and unique g r a f t co­ polymers can be prepared by copolymerizing macromers (macromolecu l a r monomers) with conventional monomers. The synthesis of p o l y ( b u t y l acrylate-£-isobutylene), i . e . , the f i r s t g r a f t synthe­ s i s that i n v o l v e s c a r b o c a t i o n i c c o n t r o l l e d i n i t i a t i o n , has recent­ l y been accomplished by the f o l l o w i n g route (9):

CH =ÇH

CH =ÇH

2

2

Me Al/H (^ 3

+ jl-CifHs

2

>

CH -PIB 2

COOBu

R

>

P

o l

y(

b u t v l

acrylate-g-isobutylene

CH -PIB 2

The copolymerization of the p o l y i s o b u t e n y l s t y r e n e macromer with methyl methacrylate and styrene gave f u r t h e r i n t e r e s t i n g new mat e r i a l s (10,11). Inorganic Head Groups by C o n t r o l l e d I n i t i a t i o n . In a d d i t i o n to organic f u n c t i o n a l groups, i n o r g a n i c head groups, e.g., C l - , Br-, 0 N-, can a l s o be introduced i n t o polymers by c o n t r o l l e d i n itiation. For example, the i n c o r p o r a t i o n of CI- head group i n t o p o l y i s o b u t y l e n e has been accomplished by the C l / B C l 3 system (12) 2

2

Q i-C H BCliT + ~ > u

CI

2

+

BC1

3

cr

fy

ft

8

C1-CH -C(CH ) 2

3

2

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Cl-PIB

1.

Syntheses of Functional and Sequential Polymers

KENNEDY

7

S i m i l a r i t y between I n i t i a t i o n and Chain T r a n s f e r : 1. The I n i f e r Concept. A d e t a i l e d a n a l y s i s of the respec­ t i v e mechanisms of i n i t i a t i o n and v a r i o u s chain t r a n s f e r proces­ ses showed that the fundamentals of i n i t i a t i o n by a l k y l h a l i d e s and chain t r a n s f e r to a l k y l h a l i d e s a r e indeed very s i m i l a r . I t i s postulated that an a l k y l h a l i d e that i s able to i n i t i a t e c a r ­ b o c a t i o n i c polymerizations, may a l s o be able to f u n c t i o n as a chain t r a n s f e r agent. Schematically (1): Initiation: 9

+

RX + MX R^MX ^ η ^r— n+1 Chain T r a n s f e r

C = C

>

R-C-C MX

n+1

9

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch001

9

9

{

C=C

^C-C^MX , + RX ~ * w \ £ - C X + R^MX " " > n+1 n+1 Λ

1

^C-CX +

9

R-C-Λχ Ί

n+1

A l k y l h a l i d e s that perform simultaneously as i n i t i a t o r s and chain t r a n s f e r agents a r e i n general termed i n i f e r s . Thus i n the ab­ sence of chain t r a n s f e r to monomer i . e . , when R ^ > R (rate of chain t r a n s f e r to i n i f e r i s l a r g e r than that £o\ionomer; , the t e r m i n i of polymers can be r e a d i l y c o n t r o l l e d by i n i f e r s . Monof u n c t i o n a l i n i f e r s termed m i n i f e r s can be used to c o n t r o l one terminus; b i f u n c t i o n a l i n i f e r s XRX, termed b i n i f e r s , are u s e f u l to c o n t r o l two t e r m i n i ; t r i f u n c t i o n a l i n i f e r s XRX, termed t r i n i f e r s , can c o n t r o l the three t e r m i n i of three-arm s t a r molecules, e t c . The f o l l o w i n g scheme helps to v i s u a l i z e the g i s t of the mechanism l e a d i n g to end group c o n t r o l i . e . , to t e l e c h e l i c p o l ­ ymers, be these l i n e a r or star-shaped, by i n i f e r s (1): CI Cl-l-Cl

CI B C l 3

>

φ J

CI

Cl-R* B C L

Cl-twx*BCU

9

Inifer

e

CI C l - l w ^ C l + C l - R B C Vθ

1

e

2. New Products Prepared by the I n i f e r Technique. A new family of l i n e a r and three-arm s t a r polyisobutylenes have been prepared by the i n i f e r technique. The i n i f e r systems used were cumyl c h l o r i d e (minifer) p-dicumyl c h l o r i d e ( b i n i f e r ) and symt r i c u m y l c h l o r i d e ( t r i n i f e r ) always i n combination with BCI3 coi n i t i a t o r (1) : CH I

3

iCi+H8

-Cj-Cl

CH

3

>·

rp^

CH I

CH I

3

3

W

^^C'^PIB' H:H2-C-C]

CH

3

CH

3

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8

INITIATION

CH

3

CH

3

CH

3

CH

3

CH

3

CH

3

CH

3

OF POLYMERIZATION

3

Cl-Ç-CH ^^PIBw^-(p-^Q^-^^PiBwbCH2-^-Cl 2

CH

CH

CH

3

CH

3

CH

3

3

CI

3

CH _/ Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch001

3

1 CH

\

Z™

3

BC1

3

3

Λ

CH

3

CH 3

CI

/w\,PIBw\,CH -C-Cl ÇH _^C I H3 CH,

ÇH

3

2

3

CI-^-CH2WVPIBWV(J;--^^ CH

3

CH

3

N

\

C

H

/P

A

H 3 P

C CH

3

__ CH

3

VAA,PIBW^CH -Ç-C1 2

CH

3

S i d e - r e a c t i o n s can be avoided without too much d i f f i c u l t y so that v i r t u a l l y t h e o r e t i c a l end group f u n c t i o n a l i t i e s can be obtained, i . e . , F = 1.0, 2.0 or 3.0. Molecular weight c o n t r o l can be r e a d i l y accomplished by a d j u s t i n g reagent concentrations and products ranging from l i q u i d s to rubbery s o l i d s can be prepared. The t e r t i a r y c h l o r i n e t e r m i n i are u s e f u l s t a r t i n g p o i n t s f o r a v a r i e t y of d e r i v a t i z a t i o n s , e.g., d i b l o c k , t r i b l o c k and t r i s t a r block syntheses with styrene, α-methylstyrene (by a l k y l a l uminum chemistry) and/or c y c l i c ethers (by s i l v e r s a l t chemis­ try) . For example, the s y n t h e s i s of the t r i b l o c k PaMeSt-b-PIBb-PaMeSt, a thermoplastic elastomer whose physical-mechanical p r o p e r t i e s a r e determined by the c e n t r a l saturated rubbery ΡΙΒ block sandwiched between two g l a s s y PaMeSt b l o c k s , i s o u t l i n e d by the f o l l o w i n g set of equations: n

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

Syntheses of Functional and Sequential Polymers

KENNEDY

CH

CH

3

3

CH

3

CH

3

CH

3

CH

3

9

Cl-(j:-CH2'w\,PIBw\£-C6^ CH

CH

3

I

3

E t A l C l ; aMeSt | C H C 1 , -50°C 2

2

CH

(pi

3

2

-

P o l y m e r i z a t i o n of

chain

Methyloxirane

I t i s w i d e l y r e c o g n i z e d t h a t E ^ Z n - H ^ O s y s t e m i s one o f t h e most a c t i v e c a t a l y s t s f o r t h e s t e r e o s p e c i f i c p o l y m e r i z a t i o n of oxiranes. A variety of chemical s p e c i e s are formed i n the f o l l o w i n g way: r a p i d f o r m a t i o n o f e t h y l z i n c h y d r o x i d e , i t s a g g r e ­ g a t i o n , and e l i m i n a t i o n o f e t h a n e t o f o r m z i n c o x i d e s t r u c t u r e . The maximum c a t a l y s t a c t i v i t y was a c h i e v e d when the mole r a t i o o f z i n c t o w a t e r was one t o one, where the p r e d o m i n a n t f o r m a t i o n of a s p e c i e s , E t ( Z n O ) H, ( I I I ) , was o b s e r v e d . I f we use less amount o f w a t e r , a n o t h e r s p e c i e s , E t ( Z n O ) Z n E t , ( I V ) , was also produced c o n c u r r e n t l y . C o n t r a r y t o the a n i o n i c n a t u r e o f the former species (III), the latter species (IV) exhibited a c a t i o n i c n a t u r e . F o r i n s t a n c e , more t h a n 95% o f r i n g c l e a v a g e o f m e t h y l o x i r a n e t a k e s p l a c e a t 0-CH bond w i t h s p e c i e s ( I I I ) , w h i l e the c l e a v a g e a t O-CH bond a l s o takes p l a c e c o n c u r r e n t l y w i t h species (IV). 2

a

A r e c e n t GPC study (Γ7) °f poly(methyloxirane) sample p r e p a r e d by E ^ Z n - H ^ O (one t o 0.1 s y s t e m ) c l e a r l y showed the c a t i o n i c n a t u r e o f t h i s c a t a l y s t s y s t e m . More t h a n 50 w e i g h t % o f the p o l y m e r o b t a i n e d was f o u n d t o be o l i g o m e r s i n c l u d i n g pentamer and lower m o l e c u l a r w i g h t compounds i n c o n t r a s t w i t h the h i g h m o l e c u l a r weight p o l y m e r o b t a i n e d w i t h E t Z n - H 0 , one t o one, system. 2

2

30

INITIATION

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POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch003

C-NMR s t u d i e s o f t h e s e p o l y m e r s showed t h a t t h e s p e c t r u m o f t h e o l i g o m e r f r a c t i o n h a d v e r y c o m p l i c a t e d f e a t u r e s owing t o the i r r e g u l a r s t r u c t u r e admixed w i t h h e a d - t o - h e a d a n d t a i l - t o t a i l e n c h a i n m e n t s o f t h e monomeric u n i t , w h i c h a g a i n i s i n s h a r p contrast with the c l e a r - c u t NMR patterns f o r the polymers p r e p a r e d w i t h t h e one t o one c a t a l y s t s y s t e m . The a c t i v e s p e c i e s o f t h e d i e t h y l z i n c - m e t h a n o l s y s t e m was p r e v i o u s l y proven t o be z i n c d i m e t h o x i d e (18). C o n t r a r y t o t h e z i n c - w a t e r s y s t e m , no t r a c e o f c a t i o n i c n a t u r e was o b s e r v e d i n t h e z i n c - m e t h a n o l s y s t e m a t any r a t i o o f t h e two components. When r a c e m i c m e t h y l o x i r a n e i s p o l y m e r i z e d w i t h z i n c d i m e t h ­ o x i d e , D-and L-monomers a r e s e p a r a t e l y i n c o r p o r a t e d i n t o g r o w i n g c h a i n s t o form a n i s o t a c t i c p o l y m e r c o n s i s t i n g o f p o l y ( D - m e t h y l o x i r a n e ) and p o l y ( L - m e t h y l o x i r a n e ) . T h i s s t e r e o s e l e c t i v e polymer­ i z a t i o n c a n be s a t i s f a c t o r i l y e x p l a i n e d i n terms o f t h e e n a n t i o m o r p h i c c a t a l y s t s i t e s model (18). The d * - s i t e s a c c e p t D - m e t h y l oxirane i n preference t o t h e L-monomer, resulting i n the f o r m a t i o n o f -DDDD- i s o t a c t i c s e q u e n c e s . The same s i t u a t i o n i s v a l i d f o r the l * - c a t a l y s t s i t e s . I t was most d e s i r a b l e f o r us t o e l u c i d a t e t h e s t e r e o c o n t r o l mechanism i n terms o f m o l e c u l a r l e v e l c o n s i d e r a t i o n s r a t h e r t h a n a phenomenological a p p r o a c h . No i n f o r m a t i o n , however, was a v a i l ­ able concerning the c h i r a l s t r u c t u r e o f d*- and l * - c a t a l y s t s i t e s , b e c a u s e none o f t h e a c t i v e c a t a l y s t s p o s s e s s e s a w e l l - d e ­ f i n e d s t r u c t u r e . The a c t i v e z i n c m e t h o x i d e , f o r i n s t a n c e , was a disordered powdery substance and i t s c a t a l y s t e f f i c i e n c y was e x t r e m e l y low. S e v e n t e e n y e a r s ago, we i s o l a t e d (1^9) an o r g a n o z i n c complex i n t h e form o f s i n g l e c r y s t a l , w h i c h h a d t h e c o m p o s i t i o n : [ΕtZnOCH ] 3

[Zn(0CH ) ] 3

2

I t was s o l u b l e i n b e n z e n e , a n d t h e b e n z e n e s o l u t i o n e x h i b i t e d a c a t a l y t i c a c t i v i t y f o r t h e o x i r a n e p o l y m e r i z a t i o n a t 80 C, b u t no a c t i v i t y a t room t e m p e r a t u r e . According t o the X-ray a n a l y s i s by K a s a i , t h e o r g a n o z i n c complex c o n s i s t e d o f t w o ^ n a n t i o m o r p h i c d i s t o r t e d c u b e s , d-cube and 1-cube ( 2 0 ) . I n t h e C-NMR s p e c t r u m o f t h e o r g a n o z i n c com­ p l e x , f o u r s i n g l e t s were o b s e r v e d . Two o f them were a s s i g n e d t o t h e i n n e r a n d o u t e r m e t h o x y - c a r b o n s , r e s p e c t i v e l y . We c a r r i e d o u t NMR a n a l y s i s o f a r e a c t i o n s y s t e m i n w h i c h t h e o r g a n o z i n c complex and r a c e m i c m e t h y l o x i r a n e were a l l o w e d t o r e a c t i n b e n z e n e ( 2 1 , 22, 2 3 ) . A s p e c t r u m o f t h e r e a c t i o n s y s t e m a t 30° C was p r o v e n t o be the simple overlapping of the i n d i v i d u a l spectrum of m e t h y l o x i r a n e and t h e z i n c c o m p l e x . No r e a c t i o n t o o k p l a c e a t 3 0 C. I n a s p e c t r u m a t 8 0 C, a number o f new s i g n a l s and shape c h a n g e s i n t h e o r i g i n a l s i g n a l s were o b s e r v e d owing t o t h e o c c u r ­ rence 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 a t 80°C. A l l o f the observed s i g n a l s i n t h e r e a c t i o n s y s t e m c o u l d be e x p l a i n e d r e a s o n a b l y by e

e

3.

TSURUTA

31

Initiation Reactions in Ionic Polymerizations

t h e i n i t i a t i o n mechanism by one o f t h e i n n e r methoxy g r o u p s . T a c t i c i t y d a t a o f p o l y ( m e t h y l o x i r a n e ) p r e p a r e d by the o r g a n o z i n c complex was f o u n d t o a g r e e w e l l w i t h t h e p r e d i c t e d v a l u e s from t h e e n a n t i o m o r p h i c m o d e l . A p o s s i b l e mechanism i s as f o l l o w s : a t 8 0 C, two of the l o n g e s t bonds a r e l o o s e n e d . I f t h e bondl o o s e n i n g t a k e s p l a c e a t d-cube, t h e s t e r i c c o u r s e o f an e n t e r i n g monomer w i l l be i n f l u e n c e d by the c h i r a l s t r u c t u r e a r o u n d the central zinc atom. A molecular model study suggested that L-monomer w i l l be preferentially accepted in this particular example. The i n i t i a t i o n r e a c t i o n t a k e s p l a c e t h r o u g h the n u c l e o p h i l i c a t t a c k by the i n n e r methoxy g r o u p . T h i s may be t h e o r i g i n o f t h e l * - c a t a l y s t s i t e . The p r o b a b i l i t y o f t h e bond l o o s e n i n g i n 1-cube i s e x a c t l y the same as t h a t i n d-cube, so t h a t an e q u a l number of d * - s i t e s and l*-sites will be established i n the reaction system. T h i s e x p l a i n s the e x p e r i m e n t a l r e s u l t s that poly(D-methyloxirane) and p o l y ( L - m e t h y l o x i r a n e ) are formed in e q u i m o l a r amounts i n the r e a c t i o n s y s t e m .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch003

e

Comparison of S t e r e o s p e c i f i c P o l y m e r i z a t i o n of M e t h y l o x i r a n e Ziegler-Natta Polymerization

with

The l a s t t o p i c t h a t i s t o be d i s c u s s e d i s t h e c o m p a r i s o n o f o r g a n o z i n c complex w i t h Z i e g l e r - N a t t a c a t a l y s t s . It i s w i d e l y a c c e p t e d t h a t the f o r m a t i o n of i s o t a c t i c p o l y p r o p y l e n e i s b r o u g h t about by t h e c h i r a l s t r u c t u r e , d* o r 1*, o f t h e t i t a n i u m s p e c i e s i n the c a t a l y s t s y s t e m . The c o n c e p t o f the s t e r i c c o n t r o l i n p r o p y l e n e p o l y m e r i z a t i o n i s t h e same as t h a t i n t h e m e t h y l o x i r a n e p o l y m e r i z a t i o n s w h i c h have been d i s c u s s e d i n the foregoing sections. An active center having d*or 1*c h i r a l i t y i s formed a l o n g t h e c r y s t a l s u r f a c e o f ô' -titanium trichloride. our

v

One of t h e most i m p o r t a n t p o i n t s o f argument about the n a t u r e o f t h e " s u p e r a c t i v e Z i e g l e r c a t a l y s t " i s the r o l e o f e t h y l b e n z o a t e . The w r i t e r w i l l d i s c u s s t h i s i n r e f e r e n c e t o a c a t a l y s t s y s t e m w h i c h was r e p o r t e d (24) r e c e n t l y by K a s h i w a o f M i t s u i P e t r o c h e m i c a l Co. As the f i r s t step f o r the c a t a l y s t p r e p a r a t i o n , the b a l l milled magnesium c h l o r i d e was allowed to stand w i t h t i t a n i u m t e t r a c h l o r i d e a t 80° C f o r two h o u r s . The M g C l ^ T i C l ^ , i . e . , T i C l ^ a d s o r b e d on M g C l ^ was t h e n t r e a t e d w i t h A l E t ^ and ethyl b e n z o a t e (EB) a t 6 0 ° C . The c a t a l y s t ( 2 4 ) ) t h u s o b t a i n e d gave t h e following analytical results:

Ti

CI

Mg

Al

EB

weight percent

0.6

71.0

28.0

0.3

1.2

mole ratio

1.2

1.1

0.8

200

115

32

INITIATION

OF

POLYMERIZATION

I t i s t o be n o t e d t h a t T i , A l and e t h y l b e n z o a t e a r e p r e s e n t i n n e a r l y equal molar r a t i o . R e s u l t s of propylene p o l y m e r i z a t i o n w i t h t h i s type o f c a t a l y s t showed t h a t e t h y l b e n z o a t e e n h a n c e d the r a t e o f f o r m a t i o n o f i s o t a c t i c p o l y p r o p y l e n e , w h i l e i t d e c r e a s e d t h e r a t e f o r a t a c t i c p o l y p r o p y l e n e . E t h y l b e n z o a t e seems t o make t h e c a t a l y t i c s i t e s s t e r i c a l l y more s p e c i f i c .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch003

A c c o r d i n g t o Kashiwa, T i C l , chemisorbs l o o s e l y a t the nons p e c i f i c s i t e so t h a t some o f T i C l ^ m o l e c u l e s may be e x t r a c t e d by ethyl benzoate, which causes the d e a c t i v a t i o n o f the nonspecific site. Furthermore, non-specific sites having the s t r o n g e r a c i d i t y w i l l be p o i s o n e d more e a s i l y by e t h y l b e n z o a t e . R e s u l t s o f s t u d y on t h e m o l e c u l a r w i g h t d i s t r i b u t i o n o f t h e "isotactic fraction" of polypropylene showed that isotactic polymer having higher molecular weight was produced i n the p r e s e n c e o f e t h y l b e n z o a t e . T e r m i n a t i o n r e a c t i o n i s b e l i e v e d t o be a t r a n s f e r o f g r o w i n g c h a i n " from T i - c e n t e r t o A l - c e n t e r and t h e c o o r d i n a t i o n o f e t h y l b e n z o a t e o n t o aluminum s h o u l d be o p p o s i t e to t h e d i r e c t i o n o f e l e c t r o n flow f o r the t r a n s f e r reaction, w h i c h makes t h e t r a n s f e r r e a c t i o n more d i f f i c u l t t o t a k e p l a c e . ?î

S y n d i o t a c t i c propagation of propylene i s know t o be c a t a l y z e d by homogeneous v a n a d i u m c a t a l y s t (1_8). I n t h e p o l y p r o p y l e n e samples p r e p a r e d w i t h t h e homogeneous c a t a l y s t s , the r e l a t i v e p o p u l a t i o n o f i s o - , h e t e r o - and s y n d i o t a c t i c t r i a d s i s i n a c c o r d ance w i t h t h a t p r e d i c t e d from t h e f i r s t o r d e r Markov model ( 2 5 , 2 6 ) . T h e r e i s no c h i r a l s t r u c t u r e a r o u n d t h e homogeneous v a n a d i u m species. The s t e r e o c h e m i s t r y o f t h e e n t e r i n g monomer i s cont r o l l e d by t h e c h i r a l i t y o f t h e g r o w i n g c h a i n end, i n c o n t r a s t w i t h the i s o t a c t i c p r o p a g a t i o n . Inoue e t a l . (21) f o u n d t h a t a p o r p h y r i n - Z n a l k y l c a t a l y s t polymerized methyloxirane t o form a p o l y m e r h a v i n g s y n d i o - r i c h tacticity. The relative p o p u l a t i o n of the t r i a d tacticities suggests that the s t e r e o c h e m i s t r y of the placement of incoming monomer i s c o n t r o l l e d by t h e c h i r a l i t y o f t h e t e r m i n a l and p e n u l t i m a t e u n i t s i n t h e g r o w i n g c h a i n . T h e r e i s no c h i r a l i t y a r o u n d t h e Z n - p o r p h y r i n c o m p l e x . A c h i r a l z i n c complex forms s y n d i o - r i c h p o l y ( m e t h y l o x i r a n e ) , w h i l e c h i r a l z i n c complex, as s t a t e d above, forms i s o t a c t i c - r i c h p o l y ( m e t h y l o x i r a n e ) . The s i t u a t i o n i s j u s t t h e same as t h a t f o r p r o p y l e n e p o l y m e r i z a t i o n s . A c h i r a l v a n a d i u m catalyst produces syndiotactic polypropylene, while chiral t i t a n i u m c a t a l y s t produces i s o t a c t i c polypropylene.

Literature Cited 1. 2. 3.

Imai, N.; Narita, T.; Tsuruta, T. Tetrahedron Lett. 1971, 38, 3517. Narita, T.; Imai, N.; Tsuruta, T. Bull. Chem. Soc. Japan 1973, 46, 1242. Narita, T., Nitadori, Y.; Tsuruta, T. Polymer J . 1977, 9, 191.

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

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch003

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

TSURUTA

Initiation Reactions in Ionic Polymerizations

33

Noren, G. K. J. Org. Chem. 1975, 40, 967. Narita, T.; Yamaguchi, T; Tsuruta, T. Bull. Chem. Soc. Japan 1973, 46, 3825. Tsuruta, T.; Narita, T.; Nitadori, Y.; Irie, T, Makromol. Chem. 1976, 177, 3255. Nitadori, Y.; Tsuruta, T. Makromol. Chem. 1978, 179, 2069. Maeda, M.; Inoue, S. Makromol. Chem., Rapid Commun. 1981, 2, 537. Kimura, M.; Egashira, T.; Nishimura, T.; Maeda, M.; Inoue, S. Makromol. Chem. 1982, 183, in press. Sekiguchi, H. Pure & Appl. Chem. 1981, 53, 1689. Ikeda, M.; Hirano, T.; Nakayama, S.; Tsuruta, T. Makromol. Chem. 1974, 175, 2775. Tsuruta, T.; Tsushima, R. Makromol. Chem. 1976, 177, 337. Aoi, H.; Ishimori, M.; Yoshikawa, S.; Tsuruta, T. J. Organometal. Chem. 1975, 85, 241. Takeichi, T.; Arihara, M.; Ishimori, M.; Tsuruta, T. Tetrahedron 1980, 36, 3391. Aida, T.; Inoue, S. Macromolecules 1981, 14, 1162. Aida, T.; Inoue, S. Macromolecules 1981, 14, 1166. Tsuruta, T. Pure & Appl. Chem. 1981, 53, 1745. Tsuruta, T. J. Polymer Sci. 1972, D6, 179. Ishimori, M.; Tomoshige, T.; Tsuruta, T. Makromol. Chem. 1968, 120, 161. Ishimori, M.; Hagiwara, T.; Tsuruta, T.; Kai, Y., Yasuoka, N.; Kasai, N. Bull. Chem. Soc. Japan 1976, 49, 1165. Tsuruta, T. J. Polymer Sci. Polymer Symposium 1980, 67, 73. Hagiwara, T.; Ishimori, M.; Tsuruta, T. Makromol. Chem. 1981, 182, 501. Tsuruta, T. Makromol. Chem. Suppl. 1981, 5, 230. Kashiwa, N. Paper presented at "International Symposium on Transition Metal Catalyzed Polymerizations: Unsolved Problems" (August, 1981, Michigan Molecular Institute). Doi, Y. Macromolecules 1979, 12, 249; 1012. Doi, Y.; Ueki, S.; Keii, T. Macromolecules 1979, 12, 814. Takeda, N.; Inoue, S. Makromol. Chem. 1978, 179, 1377.

RECEIVED September 10, 1982

4

M u o n i u m as a H y d r o g e n - l i k e P r o b e to S t u d y M o n o m e r Initiation K i n e t i c s 1

2

J. M. STADLBAUER, Β. W. NG, Y. C. JEAN, Y. ITO, and D. C. WALKER

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

University of British Columbia, Department of Chemistry and TRIUMF, Vancouver, B.C. V6T 1Y6 Canada The radioactive muonium atom(Mu) has the same ioni­ zation potential and Bohr radius as the hydrogen atom, but only 1/9 the mass. At the end of its 2.2µsec intrinsic life-time the positive muon (which acts as the Mu nucleus) decays into an ener­ getic positron. This decay can be observed using fast single particle counting methods. Because of this ease of detection and its hydrogen-like pro­ perties, Mu makes an excellent probe to directly study hydrogen atom reactions: for example, hydro­ gen atom initiation of monomer polymerization. We have examined the addition reaction kinetics of Mu with acrylamide, acrylic acid, acrylonitrile, methylmethacrylate and styrene, a l l in aqueous solution. Their second order rate constants were found to be, respectively, 1.9, 1.6, 1.1, 1.0 and 0.11 x 10 M S . As proof that Mu does add across the vinyl bond we have observed the muonium containing free radicals in the pure liquid monomers and obtained their hyperfine coupling constants. 10

-1

-1

First of a l l what is muonium, what is its source, how do we observe i t , and why is it useful? Muonium (Mu) is an atom comprised of a positive muon nucleus, (u ), and a bound elec­ tron. This bound electron can have its spin parallel or antiparallel to the muon nuclear spin resulting in 'triplet' and 'singlet' muonium atoms, respectively. The atom has a mass 1/9 that of the hydrogen atom, H; but because the reduced masses are +

1

Current address: University of Missouri-Kansas City, Department of Physics, Kan­ sas City, MO 64110. 2

Current address: University of Tokyo, Research Center for Nuclear Science and Technology, Tokyo, Japan. 0097-6156/83/0212-0035$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

36

INITIATION

OF

POLYMERIZATION

n e a r l y the same, Mu has e s s e n t i a l l y the same Bohr radius and i o n i z a t i o n p o t e n t i a l as hydrogen. Muonium i s formed when a p o s i t i v e muon thermalizes i n a target and p i c k s up an e l e c t r o n from the stopping medium i n t o a bound s t a t e . Muons, both high energy (28 MeV) and low energy (4.1 MeV), are the product of p o s i t i v e pion decay, i n f l i g h t o r at r e s t , r e s p e c t i v e l y . π+

»μ+ + ν . μ The pion i t s e l f i s g e n e r a l l y produced

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

9

Be

+ lp

> 10

B e

+ ir+.

(1 through the r e a c t i o n (2

Intense high energy proton beams r e q u i r e d f o r these s t u d i e s are c u r r e n t l y a v a i l a b l e a t TRIUMF (Vancouver, Canada), LAMPF. (Los Alamos, U.S.A.), SIN ( V i l l i g e n , S w i t z e r l a n d ) , and KEK (Tsukuba, Japan). Under a transverse magnetic f i e l d the f r e e muon and other diamagnetic muonic species precess with a Larmor frequency of 0.0136 MHz/G, while t r i p l e t muonium precesses a t 1.39 MHz/G. The f a c t o r of 102 between these p r e c e s s i o n frequencies makes i t easy to d i s t i n g u i s h between the species present. The i n t r i n s i c l i f e ­ time of the muon, 2.2 χ 10~^s, allows enough time to study the chemical r e a c t i o n s of muonium i n many chemical environments (Γ-4). When the muon decays i t e j e c t s a p o s i t r o n and two n e u t r i n o s . The p o s i t r o n s , with a maximum energy of 52 MeV, are e a s i l y detectable using nuclear physics s i n g l e - p a r t i c l e f a s t counting systems. The experimental data are tabulated as time histograms that are then computer analyzed. A d e t a i l e d d i s c r i p t i o n of the MSR (Muonium Spin Rotation) method i s a v a i l a b l e elsewhere ( 3 ) . Muonium has been observed i n pure hydrocarbons ( 5 ) , a l c o h o l s (6,7), and water (4). Because Mu r e a c t s slowly with these pure l i q u i d s , g i v i n g observable r e a c t i o n l i f e t i m e s of Mu up to 4us, they can be used as solvents to study various s o l u t e s of i n t e r ­ e s t . As the f r e e t r i p l e t Mu atom r e a c t s with the s o l u t e i t s observed p r e c e s s i o n frequency i s damped and a decay constant, λ , can be obtained. The concentration dependence of the decay con­ stant provides second order chemical r a t e constants f o r Mu a d d i ­ t i o n , a b s t r a c t i o n , s p i n conversion, and o x i d a t i o n - r e d u c t i o n r e a c t i o n s . When analogous hydrogen atom r a t e constants are a v a i l a b l e the k i n e t i c isotope e f f e c t can a l s o be c a l c u l a t e d . Muonium i s u s e f u l as a hydrogen-like probe to study the atomic and r a d i c a l r e a c t i o n s of hydrogen because Mu's r e a c t i o n decay constant i s d i r e c t l y observable, while most hydrogen atom data come from measured r e l a t i v e r a t e constants. The more we know of the r e a c t i o n s of hydrogen, the simplest and most abundant element i n the universe, the sooner we w i l l be b e t t e r able t o understand more complex atoms and t h e i r r e a c t i o n s .

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

STADLBAUER

ET

37

Muonium in Monomer Initiation Kinetics

AL.

Experimental Chemicals. A c r y l o n i t r i l e (AN), methylmethacrylate (MMA), and styrène were s u p p l i e d by A l d r i c h and p u r i f i e d before use. A c r y l i c a c i d (AA) and acrylamide (AM) were purchased from Eastman. The a c r y l i c a c i d contained 200 ppm p-methoxyphenol as i n h i b i t o r . However, a t the s o l u t e concentrations of 5.0 χ 10~ M and 7.4 χ 10~*M the i n h i b i t o r ' s c o n c e n t r a t i o n would be ^ 10"^M and, t h e r e f o r e , could not c o n t r i b u t e s i g n i f i c a n t l y to the observed muonium decay. The water s o l v e n t was t r i p l y d i s t i l l e d , i n i t i a l l y from permanganate s o l u t i o n .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

5

Apparatus. The schematic diagram i n Figure 1 shows the general experimental set-up f o r a low energy 'surface' muon beam. Right and l e f t p o s i t r o n d e t e c t o r s are made of p l a s t i c s c i n t i l l a t ­ or 'paddles' connected to RCA 8575 p h o t o m u l t i p l i e r tubes by lengths of l i g h t pipe; the whole of which are wrapped to exclude outside l i g h t . Detectors can be s e t up f o r double or t r i p l e coincidence counting to help minimize background. Graphite degrader between the d e t e c t o r s helps maximize s i g n a l amplitudes. The Helmholtz c o i l s , which provide the e x t e r n a l magnetic f i e l d , are mounted t r a n s v e r s e l y to the beam. A s i g n a l from the TM ( t h i n s c i n t i l l a t o r - m u o n ) counter s t a r t s the Ins r e s o l u t i o n c l o c k . A s i g n a l from e i t h e r r i g h t or l e f t d e t e c t o r s i n coincidence mode stops the c l o c k i f t h i s s i g n a l appears between a set "gate" of 0 to 4μβ. I f the p o s i t r o n s i g n a l i s not c o i n c i d e n t or appears a f t e r 4ys i t i s not counted and the system s t a r t s over. U s u a l l y about 50,000 good events are counted per minute. MSR: Muonium Spin R o t a t i o n Measurements. For the k i n e t i c p o r t i o n of t h i s study a low energy (4.1 MeV) beam of muons from the M20 channel at TRIUMF was focused on a shallow T e f l o n c e l l with 0.007 cm Mylar as muon windows. These target c e l l s contained approximately 80 ml of sample s o l u t i o n which was bubbled w i t h high p u r i t y He to remove d i s s o l v e d oxygen. In the case of these v o l a t i l e a c r y l i c s o l u t e s the bubbling gas was f i r s t passed through a prebubbler c o n t a i n i n g a s o l u t i o n of the same concentration as the sample. Muonium p r e c e s s i o n s i g n a l s were observed at a f i e l d of 8G provided by Helmholtz c o i l s transverse to the beam and centered on the t a r g e t . Figure 2 shows some time-histograms of both raw and f i t t e d data. The data i s computer f i t t e d using MINUIT, a χ minimization program, to a nine parameter f u n c t i o n . Equation (3) shows that f u n c t i o n with two parameters, background and muon decay, subtracted out. 2

A ( t ) - Αμ cos(o) t + φ ) + A y

M

exp(-xt) cos(u) t - φ ) M

Μ

(3

where A^ and A J J are, r e s p e c t i v e l y , the muon and muonium s i g n a l amplitudes, and ω are t h e i r f r e q u e n c i e s , φ and φ^ t h e i r Μ

μ

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1

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w Si

hd

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>τΙ

>

H Ο Ο

H

g

— ιι

00 Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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C/î Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

40

INITIATION

OF

POLYMERIZATION

i n i t i a l phases, while λ i s the muonium decay constant. Using equation (4), the obseved λ and the known s o l u t e c o n c e n t r a t i o n [S], the bimolecular r a t e constant, k , can be c a l c u l a t e d f o r the r e a c t i o n between Mu and the s o l u t e . M

λ = λ

0

+ k [S]

(4

M

The decay constant f o r the pure water s o l v e n t , λ , has a value o f ( 2 . 4 i 0 . 6 ) x l 0 s " 05-8). Two Xs, r i g h t and l e f t , a r e obtained per experiment and are p l o t t e d against s o l u t e concentra­ t i o n . The slope o f the best l i n e i s taken as k . 0

5

1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

M

MRSR; Muonium R a d i c a l Spin R o t a t i o n . In order t o prove that Mu was indeed adding across the v i n y l double bond o f styrene and the l i q u i d a c r y l i c s , high energy muons (24 MeV) were made to s t r i k e g l a s s , round-bottomed f l a s k s c o n t a i n i n g neat samples. Two methods o f degassing were u t i l i z e d : i ) a freeze-pump-thaw c y c l e followed by vacuum s e a l i n g , and i i ) by bubbling the samples on l i n e using a s p e c i a l probe and ground g l a s s f i t t i n g s . Data were analyzed using a Fast F o u r i e r Transform (FFT) program t o o b t a i n the r a d i c a l frequencies and the hyperfine coupling constant, a , as described elsewhere (9,10). μ

Results Table I l i s t s the decay constants, λ, obtained f o r the d i f ­ f e r e n t concentrations of the monomers s t u d i e d . These λs are the average of the l e f t and r i g h t values obtained f o r each concentra­ t i o n (11). Though s t a t i s t i c a l e r r o r s range from 5% t o 17%, ex­ perimental i r r e p r o d u c i b i l i t i e s i n t a r g e t geometry, f i e l d homoge­ n e i t y , detector t h r e s h o l d s , muon beam asymmetry and background r e s u l t i n a more probable e r r o r of ±25% ( 8 ) . T h i s l e v e l o f r e p r o d u c i b i l i t y i s q u i t e reasonable when compared to r a t e con­ stants obtained by competitive r a t e techniques and d i r e c t p h y s i c a l methods. F i g u r e 3 shows the FFT s p e c t r a o f styrene a t 1500G and 2500G. The l a r g e low frequency peak i s due to the muon while the two higher frequency peaks are due t o the r a d i c a l . A d d i t i o n o f these two r a d i c a l frequencies y i e l d s the muon hyperfine c o u p l i n g constant α . [Peaks a t 23 MHz are due t o the r a d i o frequency (RF) of the c y c l o t r o n . Peaks a t m u l t i p l e s of 125 MHz are due t o the counting system's c l o c k frequency]. These s p e c t r a show that the coupling constant f o r styrene i s P V I I - C 1 . T h i s i n d i c a t e s t h a t the r e a c t i v i t y of the complex i n c r e a s e s as the h y d r o p h o b i c c h a r a c t e r of the complex i n c r e a s e s . T h i s i s i n agreement w i t h the f i n d i n g s o f s e v e r a l r e s e a r c h e r s (Z>§,19_) who have r e p o r t e d t h a t an i n c r e a s e i n the r e a c t i o n r a t e and the b i n d i n g c o n s t a n t can be c o r r e l a t e d w i t h an i n c r e a s e i n h y d r o p h o b i c c h a r a c t e r o f the c a t a l y s t . In the case o f the PNPA, i t was shown t h a t the presence of V I I - C 1 6 had no e f f e c t on the h y d r o l y s i s , but i n the case o f PNPL, a p l o t o f k b v s . [ V I I - C 1 6 ] as shown i n F i g u r e 4 does not y i e l d a s t r a i g h t l i n e and i n d i c a t e s t h a t the V I I - C 1 6 monomer i n h i b i t s the Q

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch007

2

0

s

2

2

2

2

0

s

s

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch007

ISRAEL ET AL.

Micellar Poly(3-alkyl-l-vinylimidazolium)

Salts

3.0 -

^r ^^ T^^J ^^ ^ ^

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-

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15

20

I

I

I

1.40 χ Ι Ο "

5

1.67 Χ Ι Ο "

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25 30 35 4 0

Time (hr) Figure 1. Variation of ln(A oo — At) with time for the hydrolysis of PNPL in the presence of PVII-C16. [PNPL] = 1.66 χ 10~ M, pH= 8.21, 20% EtOH-H 0. 5

2

82

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch007

INITIATION OF POLYMERIZATION

Figure 2. Variation of ln( A oo — At) with time for the hydrolysis of Ν DBS in the presence of PVII-C16. [ Ν DBS ] = 1.15 X 10~ M, pH = 8.21, 20% EtOH-H 0. 4

2

ISRAEL ET AL.

Micellar Poly(3-alkyl-l-vinylimidazolium) Salts

rH

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Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch007

INITIATION OF POLYMERIZATION

Figure 3. Variation of observed rate constants for the hydrolysis of PNPL with varying concentrations of PVII-C1 (Q) and PVII-C16 (φ) at 30 °C. [PNPL] = 1.66 χ 10~ M, pH = 8.21, 20% EtOH-H 0. 5

2

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch007

ISRAEL ET AL.

Micellar

Poly(3-alkyl-l-vinylimidazolium)

•

1

Ο

2

ι

ι

4

6

Salts

- ι -

8

[VII-C ]xi0 (M) l 6

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Figure 4. Variation of k with concentration of VII-C16 for the hydrolysis of PNPL at 30 °C. [PNPL] = 4.49 X 10~ M, pH = 10.4, 20% EtOH-H 0. 0i8

5

2

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch007

INITIATION OF

POLYMERIZATION

h y d r o l y s i s o f the PNPL and the r a t e measurements do not obey the k i n e t i c scheme p r e s e n t e d i n E q u a t i o n ( 4 ) . The mechanism o f t h i s i n h i b i t i o n i s not u n d e r s t o o d at the present time. I n the case o f the a n i o n i c e s t e r s , the a l k a l i n e h y d r o l y s i s o f NDBS and NABS are enhanced by the a d d i t i o n o f c a t i o n i c p o l y e l e c t r o l y t e and p o l y s o a p . The most h y d r o p h o b i c c a t a l y s t ( P V I I - C 1 6 ) gave the l a r g e s t enhancement f o r b o t h s u b s t r a t e s . S a t u r a t i o n behavior was o b s e r v e d f o r b o t h a n i o n i c s u b s t r a t e r e a c t i o n s , i n d i c a t i n g the f o r m a t i o n o f the complex between polymer and s u b s t r a t e ( 2 0 ) . I n the case o f the NABS, the i n c r e a s e i n the h y d r o p h o b i c c h a r a c t e r o f the c a t a l y s t s y i e l d s a v e r y s m a l l i n c r e a s e i n b o t h the b i n d i n g c o n s t a n t (K) and k£ and thus i t can be c o n c l u d e d t h a t the h y d r o p h o b i c i n t e r a c t i o n p l a y s a v e r y minor r o l e . The r a t e enhancement due to the e l e c t r o s t a t i c i n t e r a c t i o n s i n t h i s system i s over 6 f o l d . For the l o n g s i d e - c h a i n e s t e r NDBS we can see the e f f e c t o f both the e l e c t r o s t a t i c and h y d r o p h o b i c i n t e r a c t i o n s . The s h o r t s i d e - c h a i n polymer P V I I - C l produced over a 9 f o l d i n c r e a s e i n the r a t e due p r i m a r i l y t o the e l e c t r o s t a t i c i n t e r a c t i o n . By f a r the most e f f i c i e n t c a t a l y s t s t u d i e d i s the l o n g s i d e - c h a i n p o l y m e r , P V I I - C 1 6 , which combines both e l e c t r o s t a t i c and h y d r o p h o b i c i n t e r a c t i o n s to y i e l d over a 45 f o l d i n c r e a s e i n the r e a c t i o n r a t e . Literature 1.

Cited

Cordes, E . H . and Dunlap, R . B . , Accounts Chem. Res., 1969, 2, 1. 2. Cordes, E . H . and G i t l e r , C., Prog. Bioorg. Phys. Org. Chem., 1970, 8, 271. 3. Fendler, E.J. and Fendler, J.H., Advan. Phys. Org. Chem., 1970, 8, 271. 4. Morawetz, H . , Advan. C a t a l . , 1969, 20, 341. 5. Morawetz, H., Accounts Chem. Res., 1970, 3, 354. 6. Overberger, C . G . and Salamone, J.C., Accounts Chem. Res., 1969, 2, 217. 7. Rudolfo, T., Hamilton, J.A., Cordes, E.H., J. Org. Chem., 1974, 39, 2281. 8. Okubo, T. and Ise, N . , J. Org. Chem., 1973, 38, 3120. 9. Strauss, U.P, Gershfeld, N . L . and Crook, E.V., J. Phys. Chem., 1956, 60, 577. 10. Freedman, H . H . , Mason, J . P . and Medalia, A.I., J. Org. Chem., 1958, 23, 26.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch007

7.

ISRAEL ET AL.

Micellar Poly(3-alkyl-1-vinylimidazolium)Salts87

11. Bender, M . L . and Nakamura, J. Amer. Chem. S o c . , 1962, 84, 2577. 12. Zahn, H. and Schade, F., Chem. B e r . , 1963, 96, 1747. 13. Gnehm, R. and Krecht, O., J. Prakt. Chem., 1906, 73, 521. 14. Bruice, T.C., Katzhendler, J. and Fedor, L . R . , J. Amer. Chem. Soc., 1968, 90, 1333. 15. Salamone, J.C., I s r a e l , S.C., Taylor, P., Snider, B., Polymer, 1973, 14, 639. 16. Salamone, J.C., I s r a e l , S.C., Taylor, P . , Snider, B . , Polym. P r e p r . , 1973, 14(2), 778. 17. Salamone, J.C., I s r a e l , S.C., Taylor, P . , Snider, B . , J. Polym. S c i . , P o l y m . Symp. Ed., 1974, 45, 65. 18. I s r a e l , S.C., Papathomas, K . I . and Salamone, J.C., Polym. P r e p r . , 1981, 22(1), 221. 19. Fendler, J . H . and Fendler, E.J., "Catalysis in M i c e l l a r and Macromolecular Systems", Academic Press, New York, 1975. 20. I s r a e l , S.C., Papathomas, K . I . and Salamone, J.C., Polym. P r e p r . , 22(2), 377. RECEIVED August 24, 1982

8 Epitaxial Polymerization as a Tool for Molecular Engineering JEROME B. LANDO, ERIC BAER, SCOTT E. RICKERT, HEMI ΝΑΕ, and STEPHEN CHING

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch008

Case Western Reserve University, Department of Macromolecular Science, Cleveland, OH 44106 Epitaxial crystallization is the oriented over­ growth of a substance on a crystalline substrate. The interaction between the two is usually highly specific and has profound effects on the morphology and struc­ ture of the crystallizing material. Epitaxial polymeri­ zation is a new phenomenon in the field of macromolecu­ lar science. It combines the epitaxial crystallization of a monomer on a crystalline substrate, followed by solid-state polymerization, which is controlled by the epitaxial crystallization of the monomer. A study of the vapor phase epitaxial polymerization of disulfurni­ tride to polythiazyl, (SN) , resulted in three new crystalline phases of (SN) , and a new appreciation of the catastrophic effect of water on this polymer. The epitaxial polymerization of hexachlorocyclotriphospha­ zene to poly(dichlorophosphazene), (NPCl ) has been investigated. Deposition from both the vapor phase and solution has been studied. The polymer structure and morphology depend upon the monomer epitaxial crystals. The actual monomer morphology and structure have been found to be dependent on the geometry of the substrate. The application of this method to the epitaxial poly­ merization of diacetylenes will also be discussed. x

x

2

x

A topochemical effect in a solid state reaction is any effect on the structure and properties of the product or the kinetics of the reaction that can be directly attributed to the geometric arrangement of the reacting groups or the distance between those groups. The degree of topochemical control in a solid state reaction can vary greatly depending upon the particular system investigated.(1) Reactions to be discussed here, in which there is a crystallographic correlation between the reactant and the resulting product, can occur in solid solution or with the nuclea­ tion and growth of a product phase. Systematic investigation of solid state polymerization reactions began with the discovery that crystalline acrylamide polymerizes when exposed to ionizing radiation. (2,3) Since that 0097-6156/83/0212-0089$06.00/0 © 1983 American Chemical Society

90

INITIATION OF POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch008

time i n v e s t i g a t o r s have been i n t r i g u e d by the p o s s i b i l i t y of pro­ ducing polymers with unusual s t r u c t u r e s , morphologies and p r o p e r t i e s through s o l i d s t a t e p o l y m e r i z a t i o n . Although these goals have been f u l f i l l e d to some extent i n the past twenty-five years, a p e r v a s i v e problem has been a l a c k of c o n t r o l over monomer morphology and s t r u c t u r e . In simple terms we are "stuck" with the s t r u c t u r e s that nature gives us. In the f o l l o w i n g paper, a method w i l l be discussed that allows v a r i a t i o n of monomer s t r u c t u r e and morphology. This i n v o l v e s a technique we have termed e p i t a x i a l polymerization* Monomers are c r y s t a l l i z e d on c r y s t a l l i n e sub­ s t r a t e s . L a t t i c e matching allows the v a r i a t i o n of monomer s t r u c ­ ture and morphology, y i e l d i n g d i f f e r e n t polymer s t r u c t u r e s and morphologies upon p o l y m e r i z a t i o n . Three types of monomers w i l l be discussed i n t h i s paper, c y c l i c s u l f u r - n i t r o g e n compounds, c y c l i c phosphazenes and d i a c e t y l e n e s . Experimental T e t r a s u l f u r Tetramide. T e t r a s u l f u r tetramide (S^N,) was sublimed at 100°C and 10" t o r r . The vapor was passed through Ag S wool at 210°C forming S^^. The hot S , ^ vapor condensed and c r y s t a l l i z e d on a l k a l i h a l i d e s i n g l e c r y s t a l s at -78 0.^±-*Α) Polymerization occurred during heating to room temperature. 5

ο

Polyphosphazene. Hexachlorocyclotriphosphazene (N P^Cl^) was obtained as p u r i f i e d c r y s t a l s from the Army Research L a b o r a t o r i e s . This trimer was deposited as t h i n f i l m s on a l k a l i - h a l i d e substrates from the vapor, melt, and s o l u t i o n . Vapor phases-deposition was accomplished by subliming Ν P^Cl^ at 80°C i n 10 t o r r vacuum. I t was found that d e p o s i t i o n times of around 20 minutes yielded incomplete f i l m s s u i t a b l e f o r e l e c t r o n microscopy. P r i o r to sublimation the a l k a l i h a l i d e c r y s t a l was always annealed at room temperature i n 10"^ t o r r vacuum i n order to ensure removal of adsorbed water from the c r y s t a l s u r f a c e . C r y s t a l l i z a t i o n from s o l u t i o n i n v o l v e d the p r e p a r a t i o n of a 20 wt % s o l u t i o n of the trimer i n decane. The p r e c i p i t a t i o n temperature f o r t h i s s o l u t i o n was 23°C, and a l l e p i t a x i a l d e p o s i ­ t i o n s were done at 29°C, or 6°C above t h i s cloud p o i n t . Deposi­ t i o n times on i n s i t u cleaved s a l t c r y s t a l (100) surfaces were near 10 minutes f o r f i l m thicknesses s u i t a b l e f o r e l e c t i o n micros­ copy. Another method used i n v o l v e d the c a s t i n g of a t h i n f i l m of N^P^Cl^ from decane on a f r e s h l y cleaved a l k a l i h a l i d e surface at room temperature, followed by h e a t i n g to 130°C. The molten trimer f i l m was then slow cooled to 110°C (4°C below the bulk m e l t i n g temperature) and h e l d at that temperature f o r 1 hour. A l l e p i t a x i a l f i l m s were e i t h e r prepared d i r e c t l y f o r micros­ copic examination, or polymerized using a post-polymerization technique f o r the f i r s t time on phosphazene monomer. The trimer f i l m s were i r r a d i a t e d with 2.5 Mrad of γ-radiation from a Co60

8.

LANDO ET AL.

Epitaxial

91

Polymerization

source (24 hours exposure time) at 37°C. The now-activated trimer f i l m s were reacted by annealing i n an i n e r t atmosphere at 140°C f o r two hours. P o l y d i a c e t y l e n e s . Dimethanol-diacetylene (HOCH^CEC-CECCH OH) (DMDA) was used as received from Farehan Chemical Co. Diphenyf urethane-diacetylene ( , the mean-square end-to-end distance of the unperturbed chain. The term " x ( r = o ) i n equation 9 denotes the Gaussian d i s t r i b u t i o n f u n c t i o n expressing the density of end-to-end v e c t o r s , r , i n the v i c i n i t y of r = 0. The existence o f t h i s f u n c t i o n i n the entropy term x

x

x

0

W

Δ β

(1)

=

(6)

k In A s

AS( ) 2

(7)

k In

VP (8)

AS(3) = k In R

χ

s

13.

MCGRATH ET AL.

Polymerization

of

153

Cyclosiloxanes

V = Volume of the system. σ

=

Symmetry number for the chain species ( 2 f o r organosiloxanes).

Α

σ

=

Α

σ

^ = Symmetry number for the r i n g species of "x" repeating u n i t s = 2x f o r organosiloxanes). x

3/2

" s Ρ =

W»(r)

3

"

3 ν

dr = 3/2

s

—

(9) 2πνχ

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

π

Vxb

Z

= X

0

= χ

ν = Number of l i n k s per repeat u n i t , b = Average " e f f e c t i v e l i n k length" 2 = The mean-square end-to-end length averaged over a l l c o n f i g u r a t i o n s of chain s i z e x. X

obviously implies a Gaussian d i s t r i b u t i o n of end-to-end v e c t o r s . This point may, i n f a c t , be a poor assumption for the cases of very short chains (which, i n turn, form small r i n g s ) . The term " v " was defined as the volume element w i t h i n which two termini must meet i n order to form a bond. It appears i n the denominator o f Equation 6 due to the fact that the two atoms which formed the bond broken i n process 1 were constrained to the volume element v p r i o r to the occurrence of process 1. Since o / ( r ) i s the p r o b a b i l i t y of the termini of a^ molecule meeting i n the volume range, v , t h i s f a c t o r a l s o appears i n the entropy term d e s c r i b i n g process 2. The enthalpy terms for the f i r s t two processes presumably cancel each other with the assumption that the r i n g formed i s not small enough to be s t e r i c a l l y s t r a i n e d . T h i s was r a t i o n a l i z e d by noting that the intramolecular bond formed i n process 2 was s i m i l a r i n nature to the one severed i n process 1. The e q u i l i b r i u m constant for process 3 (expressed i n m o l e s / l i t e r ) was derived from equation 8 (equations 10 and 11). The d e f i n i t i o n of "p" i n t h i s context (equation 11) d i f f e r s s

v s

s

w

x

s

3/2

3/2 _3 2πν

Is

_1 N

A

_5/2

3 2πν

_1 (10)

χ 2 2 x

3/2 0

N

A

154

INITIATION OF POLYMERIZATION

< Γ > ο = Vxb χ

[C-M ] [-M - -] [-M -] x

Κ

χ

=

y

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

y

[C-M ]

X

x

=

(11) ρ*

s l i g h t l y from i t s "normal" d e f i n i t i o n ( i . e . f r a c t i o n a l extent o f r e a c t i o n ) . Here, "p" i s defined as the extent o f r e a c t i o n o f the f u n c t i o n a l endgroups i n the chain p o r t i o n o f the system. Moreover, "p" could also be i d e n t i f i e d as the r a t i o o f the concentrations o f a c y c l i c species o f s i z e s χ to x-1 (12). The combination o f equations 10 and 11 p r e d i c t e d s e v e r a l interesting points: 1. The c o n c e n t r a t i o n o f x-mer rings was shown not to be a function of d i l u t i o n . The number o f x-mer rings was p r e d i c t e d to increase l i n e a r l y with d i l u t i o n , thereby making the p r o p o r t i o n o f r i n g s i n the system greater as i t i s diluted. Upon incrementally adding solvent, t h i s process e v e n t u a l l y r e s u l t s i n a " c r i t i c a l d i l u t i o n p o i n t " above which only r i n g s are present. 2. The r i n g s formed are small s p e c i e s . As ρ " ^ l ^ K * [C-M ] and [C-M ] approaches p r o p o r t i o n a l i t y to x"~ / . Jacobson and Stockmayer a l s o reasoned that each subset o f species (e.g. r i n g s vs. chains) w i t h i n the o v e r a l l d i s t r i b u t i o n s must be i n e q u i l i b r i u m with i t s e l f at thermodynamic e q u i l i b r i u m . Therefore, they p r e d i c t e d the normal d i s t r i b u t i o n f o r the chain species whether o r not r i n g species were formed. Figure 1 (reproduced from t h e i r 1950 paper) i l l u s t r a t e s the p r e d i c t e d d i s t r i b u t i o n for r i n g and chain s p e c i e s . One problem o f Jacobson and Stockmayer's i n t e r p r e t a t i o n i s observed i n the cases o f very small but unstrained r i n g s , where much higher concentrations o f r i n g s were formed than were p r e d i c t e d . F l o r y and Semlyen (15) explained t h i s d e v i a t i o n by suggesting that not only d i d two termini have to meet w i t h i n a volume " v " i n order to e s t a b l i s h a bond, they also had to approach each other from a s p e c i f i e d d i r e c t i o n . This d i r e c t i o n was s p e c i f i e d by a s o l i d angle f r a c t i o n 6ω/4π. They explained that t h i s term should appear i n the entropy expression for process 1 i n the inverse form, i . e . 4ττ/δω. In process 2, i f the chains were s u f f i c i e n t l y long, there would be no c o r r e l a t i o n between the p r o b a b i l i t y f o r two termini o f a_ molecule to meet w i t h i n v and to approach w i t h i n the s o l i d angle δω. In t h i s case, the term δω/4ττ would be v a l i d f o r i n c l u s i o n i n t o the entropy term f o r process 2 and, hence, when the entropies for processes 1 and 2 were summed, these terms would cancel (and the equation f o r AS(3) from Jacobson and Stockmayer s theory should be v a l i d ) . However, for short chains, the p r o b a b i l i t y o f approach x

x

s

s

1

x

13.

MCGRATH ET AL.

155

Polymerization of Cyclosiloxanes

100

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

801-

Ιϋ

0

10

20

30

40

50

60

70

D. P. Figure 1. Typical molecular distribution for a ring-chain equilibrium polymer (U).

INITIATION OF POLYMERIZATION

156

of two t e r m i n i o f one chain w i t h i n v would depend on bond angles, s t e r i c f a c t o r s , e t c . , p a r t i c u l a r to the s p e c i f i c system, and would deviate from δω/4π. In that case the p r o b a b i l i t y "P" i n Jacobson and Stockmayer's theory i n the entropy term would be incorrect. There have been s e v e r a l studies conducted i n v e s t i g a t i n g e q u i l i b r i u m d i s t r i b u t i o n s o f polyorganosiloxanes experimentally. T h e o r e t i c a l and e m p i r i c a l weight f r a c t i o n s o f c y c l i c s and t h e i r d i s t r i b u t i o n s with variances i n χ are o f p a r t i c u l a r i n t e r e s t to the s y n t h e t i c polymer chemist. Equation 10 can be rearranged to give the weight f r a c t i o n , w , o f c y c l i c s (equation 12) f o r high extents o f r e a c t i o n (the e m p i r i c a l comparisons were a l l made s

r

3

3 /

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

w

r

Η,

2

.

I

—

= π

2 I N c A

-3/2

(x C )

(12)

x

x=4

c = T o t a l s i l o x a n e concentration i n g/i MQ = Molecular weight o f a repeat u n i t I = The length o f a siloxane bond. J. = C 2x£ x

0

x

at ρ 1 ) . C o n t r i b u t i o n s from the weight f r a c t i o n o f the s t r a i n e d c y c l i c trimer which i s not a p p l i c a b l e to the theory are neglected. Wright and Semlyen compared t h e i r own (ljO values together with Brown and Slusarczuk's (18) c a l c u l a t e d and experimental values o f K f o r polydimethylsiloxane (PDMS). C a l c u l a t e d values were based on c a l c u l a t i o n s o f < r > derived from F l o r y ' s r o t a t i o n a l isomeric state model f o r PDMS (23). E m p i r i c a l values came from measurements o f the concentrations o f x-meric r i n g s according to Jacobson s and Stockmayer s r e l a t i o n s h i p , K = [C-M ]/p . Brown and Slusarczuk (18) e q u i l i b r a t e d PDMS i n toluene at 110 degrees centigrade and a concentration o f 0.22 g/mi o f s i l o x a n e . Values from that e q u i l i b r a t i o n and Wright's and Semlyen's bulk e q u i l i b r a t i o n (17) as a f u n c t i o n o f χ are compared with t h e o r e t i c a l values i n Figure 2. As p r e d i c t e d , the Κ values i n the range χ = 11-40 were experimentally independent o f d i l u t i o n . In d i r e c t c o n t r a s t , the c y c l i z a t i o n constants for χ = 4-10 increase with d i l u t i o n (the increase becoming more pronounced with decreases i n x ) . Siloxane chains f o r χ greater than approximately 15 agreed w e l l with t h e o r e t i c a l values. x

x

1

0

1

x

x

x

χ

These same authors (16) a l s o compared experimental K values as a f u n c t i o n o f χ f o r a s e r i e s o f bulk e q u i l i b r a t e s o f the s t r u c t u r e —fR(CH3)Si-0-}- wherein R e q u a l l e d H, CH3, CH3CH2, CH3CH2CH2, and C F 3 C H 2 C H 2 i n order to assess the e f f e c t o f the s u b s t i t u e n t s i z e on the e q u i l i b r i u m d i s t r i b u t i o n . The K values x

x

x

MCGRATH ET AL.

Polymerization

of

MONOMER

UNITS 10

5

1

1

Cyclosiloxanes

1

1

I

I

I

(x) 15

20

I I I I I I I I IIII II

• -

o

%

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

-1

•

•

Bulk

ο

Solution

3

Calculated

*q "·

-

ο-

-

_

·. *·

\

-3

1

Ι

ι

ι

0.6

0.8

1.0

1.2

1.4

log χ

Figure 2. Molar cyclization equilibrium constants of dimethylsiloxane at 110

an

158

INITIATION OF POLYMERIZATION

for the smallest unstrained rings (x=4 or 5) were found to increase along the s e r i e s R = H Κ > L i (33). At equal molar concentrations, the rates using the hydroxide or s i l o x a n o l a t e of the same metal have been found to be s i m i l a r (30). Tetramethylammonium s i l o x a n o l a t e s e x h i b i t a c t i v i t i e s close to the cesium s i l o x a n o l a t e s (33). Tetramethylammonium hydroxide, s i l a n o l a t e , and s i l o x a n o l a t e r a p i d l y decompose above 130°C y i e l d i n g methanol, methoxy t r i m e t h y l s i l a n e , or methoxysiloxane r e s p e c t i v e l y and trimethylamine (34-35)· i n the cases o f the f i r s t two compounds l i s t e d above, the c a t a l y s t breakdown products are f u g i t i v e at the decomposition temperature. Thus, the usual need for c a t a l y s t n e u t r a l i z a t i o n and removal following polymerization i s eliminated. They are o f t e n termed " t r a n s i e n t " c a t a l y s t s (34). At low c a t a l y s t concentrations and i n the absence of "end-blockers", the degree of polymerization i s approximately

-76

-80

-59

-78

-63

MDM

MDgM

MD M

MDM

MDgM

Source:

MD,M

5

4

307.5

290

270

245

229

194

153

99.5

Reproduced with permission from Ref. 10.

-80

MDM

2

-67

MM

Boiling point, °C

0.9180

0.9099

0.9012

0.8910

0.8755

0.8536

0.8200

0.7636

2 0

Density, d

SOME PHYSICAL CHARACTERISTICS OF LINEAR OLIGOMERS

Melting point, °C

5.

Symbol

TABLE

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

1.3980

1.3970

1.3965

1.3948

1.3925

1.3895

1.3840

1.3774

1

Refractiyi index, ηέ

13.

MCGRATH ET AL.

Polymerization of Cyclosiloxanes

163

i n v e r s e l y r e l a t e d to the c a t a l y s t concentration (36). Russian authors (4) have noted a d e v i a t i o n i n the l i n e a r i t y o f the 1/X vs. c a t a l y s t concentration f u n c t i o n at high c a t a l y s t loadings. It was suggested that t h i s d e v i a t i o n could be a t t r i b u t e d to the presence o f s t a b l e associated s t r u c t u r e s s i m i l a r such as ^4. n

- + 0

Κ

ι/

\i

Si

Si

κ

0

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

4

P r a c t i c a l l y , polymerization temperatures are s e l e c t e d on the b a s i s o f the a c t i v i t i e s o f the c a t a l y s t , c y c l o s i l o x a n e , and endblocker used with the aim o f a r r i v i n g at thermodynamic e q u i l i b r i u m w i t h i n an acceptable time period. In the bulk polymerization o f D4, modest temperature changes reportedly do not a f f e c t the f i n a l e q u i l i b r i u m number average molecular weight o f the polymers (37). Rates o f anionic polymerization are influenced by the number o f siloxane u n i t s present i n the monomer r i n g s . Some c h a r a c t e r i s t i c s are given i n T a b l e s 6 and 7 (10). Due to r i n g s t r a i n i n the three unit r i n g s , a l l o f the c y c l o t r i - siloxanes polymerize f a s t e r than the c y c l o t e t r a s i l o x a n e s . In the dimethylsiloxanes, D3 r e p o r t e d l y polymerizes approximately 50 times f a s t e r than D4 ( 4 ) . Several i n v e s t i g a t i o n s have been performed concerning the r e l a t i v e r e a c t i v i t y o f c y c l i c tetramers s u b s t i t u t e d with v a r y i n g amounts o f phenyl and methyl groups (38-42). Andrianov et a l . (42) studied a n i o n i c copolymerizations o f D4 with varying amounts o f o c t a p h e n y l c y c l o t e t r a s i l o x a n e (10-70 mole percent). They found that the rate o f copolymer formation, the v i s c o s i t y o f the r e s u l t i n g copolymers, and the e q u i l i b r i u m y i e l d o f l i n e a r species a l l decreased r e g u l a r l y as the mole percent o f phenyl tetramer was increased i n the r e a c t i o n mixture. They also noted that i n the e a r l y stages o f conversion, although both monomers had become incorporated i n the l i n e a r p o r t i o n to some extent, the polymers formed were enriched with diphenyl u n i t s . On the basis o f e l e c t r o n i c f a c t o r s , i t was reasoned that i n the diphenyl s u b s t i t u t e d tetramer, the s i l i c o n atoms would be more s u s c e p t i b l e to n u c l e o p h i l i c attack. Conversely, the phenyl s u b s t i t u t e d s i l o x a n o l a t e anion, once formed, would be less r e a c t i v e than the methyl s u b s t i t u t e d analog. These same authors (42) a l s o studied the s t r u c t u r e o f the c y c l i c s as e q u i l i b r a t i o n proceeded. R e d i s t r i b u t i o n type steps had e v i d e n t l y occurred even i n the

?

a

Source:

Dg

6

D

5

D

D

4

3

D

D

Symbol

b

Crystals

290

154

245

210 a

175.8

134

Boiling point, °C

Reproduced with permission from Ref. 10.

At 20mm Hg.

31.5

-32

-3

-44

17.5

64.5

Melting point, °C

1.1770

0.9730

0.9672

0.9593

0.9561

b

Density, d 2 0

INFLUENCE OF RING SIZE ON THE

CHARACTERISTICS OF CYCLIC SILOXANES

TABLE 6.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

1.4060

1.4040

1.4015

1.3982

1.3968

2

0

Refractive index, n ,

13.

MCGRATH ET AL.

Polymerization

165

of Cyclosiloxanes

Ο

cno

ο ο Σ

ο

CM

C7>

η

σι

Ν

σ>

^·

VO

ι—

CM ^JCO ^

Ο

CL

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

>CMQ •ιC

< χ ο CO

-l-> Ο

CM

*

(Ο

Χ

en

τ-

CO

i- ω Η- -ο

CO

ο ο >ο >> •+-> CO LU

Ο

ο

CM

r—

(Tl

Lf)

•ΓCO CD C

in

LO Ν

ί-

Ν

σι

σι

• Ο CM

ι-^

ι—

Ο

Ο

\

ε

η

00

LO

CO

ι— vo σι

Ο

-σ

CL.

Ο

< Ο

ο π: ο.

LO

ι—

>•ιΟ CO

ε ο

en

•ι-

Ο CÛ

ι—

LO

Γ>Η

CO CO

CO CM

CO CO ι—

Ο ι—

rr— ι—

00

stfCO ι—

LO

I— ι—

ο

ε U

α,

ω

i—ι ι—I ο ο

PQ




ο

'ΐCO

Ό Ο U

CO ζπ

ο

^ΐ-

I—I

^ -Γ-

^ ^ Ο ^- Ο Ο ΖΠ ^ ·ρCM Ο Ο CO Ο CO ZLZ II ·Γ— CM •ι_C Ο CO ^ CO ΟCO ' CM ΖΠ CO CM CO LL. ΠΙ COI -c π: ο ο ζπ ο Q_

-—

Ο

α

INITIATION OF POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

166

i n i t i a l stages s i n c e mixed c y c l i c s (diphenyl and dimethyl u n i t s i n the same c y c l i c molecules) were observed soon a f t e r the beginning of the r e a c t i o n . In e q u i l i b r a t i o n r e a c t i o n s , hexaorganyl d i s i l o x a n e or low molecular weight siloxanes terminated with t r i o r g a n y l s i l o x y groups i n the r e a c t i o n mixture can regulate molecular weight by a c t i n g as chain t r a n s f e r agents or "end-blockers". The e f f i c i e n c y of these agents depends on the r e l a t i v e amount of p o s i t i v e charge on t h e i r s i l i c o n atoms. Results of v i s c o s i t y measurements as a f u n c t i o n of time using hexamethyldisiloxane endblocker, D4, and tetramethylammonium hydroxide c a t a l y s t (1J3) are shown i n Figure 4. The i n i t i a l large v i s c o s i t y increase was found to be a r e s u l t of the f a s t e r r e a c t i o n of D4 as compared to hexamethyldisiloxane under these c o n d i t i o n s . Contrasted with the above r e s u l t s was the analogous r e a c t i o n with the exception that a s u l f u r i c a c i d c a t a l y s t instead o f tetramethylammonium hydroxide (see Figure 5) was used (13). The shape of the curve i n Figure 5 represents approximately equal r e a c t i v i t y o f D4 and hexamethyldisiloxane towards an a c i d c a t a l y s t . Many compounds with an e l e c t r o n donor character are reported to have an a c c e l e r a t i n g e f f e c t on the a n i o n i c polymerizations. No doubt, these a l t e r the nature of the ion p a i r . Representative "promoters" reported include tetrahydrofuran (43-45), dimethylformamide (46), s u l f o x i d e s (47,48), and cryptâtes (60). C a t i o n i c polymerization of c y c l o s i l o x a n e s i s w e l l known but used much less f r e q u e n t l y than a n i o n i c r e a c t i o n s . The most frequently used c a t a l y s t s include s u l f u r i c acid and i t s d e r i v a t i v e s (4^49-52)· T r i f l u o r o a c e t i c a c i d has also been used to polymerize D4 i n bulk (53,61). The mechanism of a c i d c a t a l y z e d polymerization i s postulated to be that schematically i l l u s t r a t e d i n equations 16 and 17. However, according to the recent text by Odian (54), there i s no evidence for the rearrangement of the s i l i c o n i u m ion shown i n these equations. A l t e r n a t i v e l y , one could propose that the propagating species i s the t e r t i a r y oxonium i o n . R I

Initiation: /

0

Ί

1 Si R

\

+ HOx

\

/ R-Si-R

R-Si-R

\

SiR2~(0SiR2)3

H A

jr

/

R

0

+

R v

. N

/

Si

I R

I H-(OSiR )3-0-Si 2

I R

+

A"

(16)

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

ON

•—*

Co

ι

ο

fi

1'

•Η.

§'

>β

Ci

>

Η

w

Η

Ο

Ο

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

168

INITIATION OF

POLYMERIZATION

Figure 5. Viscosity vs. time of reaction for equilibration of 1 mol of hexamethyldisiloxane and 1 mol of £> with 4% H SOj, at room temperature (13). 4

2

13.

MCGRATH ET A L .

Polymerization R

Propagation:

0

~0-Si

+

A"

+

169

of Cyclosiloxanes

·

^

^

SiR -(OSiR )3 2

2

R

I +

^OSiR Hj)^A-^f==i

~ ~ ~ ( 0 S i R ) - 0 - S i A"

2

2

SiR -(OSiR )3

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

2

4

(17)

R

2

Our own research (55-58, 62-67) has r e c e n t l y been concerned with the p o l y m e r i z a t i o n o f D4 and o f combinations o f D4 and D"4 ( o c t a p h e n y l c y c l o t e t r a - siloxane) i n the presence o f organofunctional endblockers used as molecular weight r e g u l a t o r s . We have s u c c e s s f u l l y obtained dimethyl amino (62), aminopropyl, g l y c i d o x y p r o p y l , carboxypropyl, hydroxybutyl, and hydroxyphenylpropyl terminated s i l o x a n e oligomers o f c o n t r o l l e d molecular weights by these methods. Transient s i l o x a n o l a t e a n i o n i c c a t a l y s t s prepared by r e a c t i n g four moles o f D-4 with one o f tetramethyl ammonium hydroxide at 80°C are e f f e c t i v e f o r e q u i l i b r a t i n g " n e u t r a l " systems such as the epoxy (59), " b a s i c " dimethyl-amino (64) o r aminopropyl (59,j67) end-blockers and D-4. With " a c i d i c " f u n c t i o n a l i t y on the end-blocker, we have s u c c e s s f u l l y u t i l i z e d t r i f l u o r o a c e t i c a c i d f o r the e q u i l i b r a t i o n s . Further d e t a i l s o f the oligomer synthesis and t h e i r u t i l i z a t i o n i n segmented copolymers w i l l be described i n future p u b l i c a t i o n s .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

R. West and T. J . Barton, J . Chem. Ed., 1980, 57 (3), 165. C. Friedel and J. M. Crafts, Ann., 1965, 136, 203. F. S. Kipping, Proc. Roy. Soc., A, 1937, 159, 139). M. G. Voronkov, V. P. Mileshkevich, and Y. A. Yuzhelevskii, The Siloxane Bond, Plenum Press, N. Y., 1978, 2. A. Ladenburg, Ann. Chem., 1971, 159, 259. C. Friedel and J . M. Crafts, Ann. Chim. Phys., 1970, 19 (5), 334. W. Noll, Chemistry and Technology of Silicones, Academic Press, N.Y., (1968). B. B. Hardman and R. W. Shade, Mat. Technol, 26, Spring, 1980. R. J . H. Voorhoeve, Organohalosilanes: Precursors to Silicones, Elsevier, N.Y., (1967). R. Meals, Encyclopedia of Chemical Technology, 18, 2nd edition, 1969, 221-260.

170 11. 12. 13. 14. 15.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

INITIATION OF POLYMERIZATION C. Eaborn, Organosilicon Compounds, Butterworths Scientific Publications, London, (1960). B. C. Arkles and W. R. Peterson, J r . , ed., Silicon Compounds, Register and Review, Petrarch Systems, Levittown, Pa., (1979). S. W. Kantor, W. T. Grubb, and R. C. Osthoff, J . Amer. Chem. Soc., 1954, 76, 5190. H. Jacobson and W. H. Stockmayer, J . Phys. Chem., 1950, 18, 1600. P. J . Flory and J. A. Semlyen, J . Amer. Chem. Soc., 2966, 88, 3209. P. V. Wright and J. A. Semlyen, Polymer, 11 (9), 1970, 462. P. V. Wright and J. A. Semlyen, Polymer, 10, 1969, 543. J . F. Brown and G. M. Slusarczuk, J . Amer. Chem. Soc., 1965, 87, 931. M. S. Beevers and J . A. Semlyen, Polymer, 1971, 12 (6), 373. T. C. Kendrick, J . Polym. Sci., 1969, A-2, 7, 297. W. C. Davies and D. P. Jones, Polym. Prepr., 1970, 11, 447 J . C. Saam, D. J . Gordon, and S. Lindsey, Macromolecules, 1970, 3, 4. P. J . Flory, V. Crescenzi, and J. E. Mark, J . Amer. Chem. Soc., 1964, 86, 146. J . B. Carmichael and R. Winger, J . Polym. Sci., A, 1965, 971. J . B. Carmichael, Rubber Chem. Technol., 1964, 40, 1084. J . B. Carmichael and D. J. Gordon, J. Phys. Chem., 1967, 71, 2071. Y. A. Yuzhelevskii, E. G. Kagan, and E. B. Dmokhovskaya, Khim. Geterotsikl. Soedin., 1967, 951. J . B. Carmichael and J . Heffel, J . Phys. Chem., 1965, 2218. J . B. Carmichael, J . Macromol. Chem., 1 (2), 1966, 207. W. T. Grubb and R. C. Osthoff, J . Amer. Chem. Soc., 1955, 77, 1405. V. Bazant, V. Chvalovsky', J . Rathousky, Organosilicon Compounds, 1, Academic Press, N.Y., 15 (1965). K. A. Andrianov, Metalorganic Polymers, Interscience, N.Y., (1965). D. T. Hurd and R. C. Osthoff, J . Amer. Chem. Soc., 1954, 76, 249. A. K. Gilbert and S. W. Kantor, J . Polym. Sci., 1959, 40, 35. A. Noshay, M. Matzner, and T. C. Williams, Industrial and Engineering Product Research and Development (I. & E. C. Product Research and Development), 1973, 12, 268. C. L. Lee and O. K. Johanson, J . Polym. Sci., 1966, A-1 (4), 3013. M. Kucera, J . Polym. Sci., 1962, 58, 1263. K. A. Andrianov and S. E. Iakushkina, Polym. Sci, U.S.S.R., 1960, 1, 221-228.

13.

MCGRATH ET AL.

39.

Z. Laita and M. Jelinek, Polym. Sci., U.S.S.R., 1964, 5, 342-353. K. A. Andrianov, S. Ye. Yakushkina, and L. N. Guniava, Vysokomol. soyed., 1966, 8 (12), 2166-2170. K. A. Andrianov et a l . , Vysokomol. soyed., 1970, A14(6), 1268-1276. K. A. Andrianov et a l . , Vysokomol. soyed., 1972, A14(5), 1294-1302. B. Suryanarayanan, B. W. Peace, and K. G. Mayhan, J . Polym. Sci., Polym. Chem. Ed., 1974, 12, 1089. B. Suryanarayanan, B. W. Peace, and K. G. Mayhan, J . Polym. Sci., Polym. Chem. Ed., 1974, 12, 1109. W. A. Fessier and P. C. Juliano, Polym. Prepr., 1971, 12, 151. J . B. Gangi and J . A. Bettelheim, J . Polym. Sci., 1964, A-2, 4011. J . G. Murray, Polym. Prepr., 1965, 6, 163. G. D. Cooper and J . R. Elliott, J . Polym. Sci., A-1 (4). B. Kanner, B. Prokai (Union Carbide Corp.) Ger. Offen. 2,629,138 (C1.C08G 77/78), 13 Jan., 1977, U.S. Appl. 592,129, 30 Jun. 1975. G. Rossmy, R. D. Langenhagen (Goldschmidt), Ger. Offen. 2,714,807, 20, Oct. 1977, Brit. Appl. 76/14,391, 08 Apr. 1976, C. A. 87: 202569z. P. Rosciszeqski, E. Jagielska, K. Bartosiak, Pol. 92,926, 15 Dec. 1977, C. A. 89:75864f. R. E. Moeller (General Electric Co.), Braz. Pedido PI 7,703,261, 20 Feb. 1979, C. A. 90: 188640u. D. T. Hurd, J . Amer. Chem. Soc., 1955, 77, 2998. G. Odian, Principles of Polymerization, 2nd Edition, Wiley, N.Y., 549 (1981). J . S. Riffle, R. G. Freelin, A. K. Banthia, and J . E. McGrath, J . Macromol. Sci. - Chem., 1981, A15(5) 967-998. J . S. Riffle, Ph.D. Thesis, VPI & SU, Blacksburg, Va., Dec. 1980. J . E. McGrath, et a l . , Proceedings of the International Rubber Conference, Kharagpur, India, 1980. J . E. McGrath, J . S. Riffle, I. Yilgor, A. K. Banthia and P. Sormani, Org. Coatings and Plastics Preprints, 1982, 46, 693. J. S. Riffle, I. Yilgör, A. K. Banthia, G. L. Wilkes, and J . E. McGrath, "Epoxy Resins", R. S. Bauer, Editor, ACS Symp. Vol., in press, 1982. S. Boileau, in "Anionic Polymerization: Kinetics, Mechanism and Synthesis," J . E. McGrath, Editor, ACS Symposium Volume 1982, Series No. 166. L. Wilczek and J . Chojnowski, Macromolecules, 1981, 14 (1), 9. T. C. Ward, D. P. Sheehy, J . S. Riffle, and J . E. McGrath, Macromolecules, 1981, 14 (6), 1791.

40. 41. 42. 43. 44.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62.

Polymerization of Cyclosiloxanes

171

172

INITIATION OF POLYMERIZATION

63.

Anionic Polymerization: Kinetics, Mechanism and Synthesis, J. E. McGrath, Editor, ACS Symposium Volume Series No. 166,

64.

J. Ε. McGrath, J . S. Riffle, D. W. Dwight, D. C. Webster and T. F. Davidson, Macromolecules, in preparation, 1982. T. F. Davidson, M. S. Thesis, VPI & SU, Blacksburg, Va.,

(1981).

65.

1980.

66. 67.

S. Tang, E. Meinecke, J . S. Riffle, and J . E. McGrath, Rubber Chem. and Tech., 1980, 54 (5), 1160. J. E. McGrath, J . S. Riffle, A. K. Banthia, I. Yilgor, and G. L. Wilkes, To be published.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch013

RECEIVED October 15, 1982

14 Cationic Photoinitiation Efficiency L. R. GATECHAIR CIBA-GEIGY Corporation, Plastics and Additives Division, Ardsley, ΝY 10502

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch014

S. P. PAPPAS North Dakota State University, Polymers and Coatings Department, Fargo, ND 58105 The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photo­ chemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was in­ vestigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. Sensitized photolysis of both salts resulted in the formation of the same major products as was observed in direct irradiation, iodobenzene, or phenylsulfide and acid. Quantum yields of organic and acidic products were measured using nine sensitizers. Product quantum yields were relatively high (< 1) under conditions where electron transfer sensitiza­ tion was expected to occur. Quantum yields > 2 were obtained in solvents likely to be substrates for hydrogen abstraction, indicating that a chain reaction was involved. Triplet energy sensitiza­ tion resulted in low product quantum yields.

Many recent p u b l i c a t i o n s and patents give evidence of the growing i n t e r e s t i n p h o t o i n i t i a t e d c a t i o n i c polymerization using onium s a l t s . ( 1 - 7 ) The o b j e c t i v e of t h i s study was to b e t t e r un­ derstand the photochemistry of onium s a l t s i n order to improve t h e i r e f f i c i e n c y as photocuring agents. The r e s u l t s of these studies form g u i d e l i n e s for the choice of s e n s i t i z e r s , onium s a l t s and other a d d i t i v e s p o t e n t i a l l y useful i n the c a t i o n i c curing pro­ cess. S e n s i t i z e r s which produce e a s i l y o x i d i z a b l e free r a d i c a l s r e s u l t e d in the highest quantum e f f i c i e n c i e s for Diphenyliodonium hexafluoroarsenate s a l t s (φ> 3 ) . Electron transfer sensitization 0097-6156/83/0212-0173$06.00/0 © 1983 American Chemical Society

INITIATION OF POLYMERIZATION

174

was the most e f f i c i e n t process with t r i p h e n y l s u l f o n i u m hexafluoroarsenate (φ »

+ Epoxy Resin

^

Η

Crossi inked Polymer

Figure 1. Many p h o t o i n i t i a t o r systems have been developed which are capable of producing a c i d i c products. Most of these c a t i o n i c p h o t o i n i t i a t o r s are based on some form of onium s a l t which ( i n the absence of l i g h t ) i s s t a b l e i n the presence of epoxy f u n c t i o n ­ a l r e s i n s . Following i r r a d i a t i o n , an a c i d i c c a t a l y s t i s produced which can i n i t i a t e the formation of polymer. Photochemistry of Aryldiazonium S a l t s . Aryldiazonium s a l t s have long been known to be a c l a s s of photoactive chemicals (see Figure 2 ) . When aryldiazonium s a l t s having n u c l e o p h i l i c anions (such as c h l o r i n e or bromine) are i r r a d i a t e d , the r e s u l t i n g pro­ duct i s n o n a c i d i c . A c i d i c products may, however, be formed during p h o t o l y s i s i n the presence of water.(17) Such chemistry i s o f t e n u s e f u l i n the area of diazo copying, more commonly known as b l u e ­ p r i n t s . _ I f the diazonium s a l t has a nonnucleophilic anion such as P F , BF^, A s F , PC1 , the r e s u l t i n g product ( i . e . , PF ) i s a Lewis acid capable of c a t a l y z i n g the polymerization or epoxy func­ t i o n a l r e s i n s . This chemistry was patented by the American Can Company f o r use i n various kinds of coatings. The c l a s s of diazonium s a l t s , i n general, have the advantage of very strong absorption curves which extend into the v i s i b l e region of the spectrum. Thus, some diazonium s a l t s are s e n s i t i v e to a wide range of wavelengths. They a l s o have disadvantages, i n that many o f the compounds are h i g h l y c o l o r e d , and are, to some degree, thermally unstable, l i m i t i n g the package s t a b i l i t y o f a formulated coating. 6

6

6

Photochemistry of Diaryliodonium S a l t s . The photochemistry of diaryliodonium s a l t s having n u c l e o p h i l i c anions i s i l l u s t r a t e d

14.

GATECHAIR AND PAPPAS

Cationic Photoinitiation Efficiency

175

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch014

in Figure 3.(19) T h i s photochemistry a l s o r e s u l t s i n the forma­ t i o n of nonacidic products. P h o t o l y s i s in a l c o h o l , however, was reported to form acid.(13) C r i v e l l o and Lam, at the General E l e c t r i c Company, found that the p h o t o l y s i s of d i a r y l i o d i u m s a l t s having nonnucleophilic anions r e s u l t s i n the formation of a c i d i c species capable of polymerizing epoxy r e s i n s . ( 1 8 ) T h e i r evidence, based on product a n a l y s i s , sup­ ported the formation of protons as the i n i t i a t i n g species, r a t h e r than the formation of a Lewis acid as i n the photochemistry of diazonium s a l t s (see Figure 4 ) . The p h o t o l y s i s quantum e f f i c i e n c y (φ) ranges from 0.2-0.4 f o r the formation of organic products. (15,18) Photochemistry of T r i a r y l s u l f o n i u m S a l t s . A s i m i l a r mecha­ nism was proposed for the p h o t o l y s i s of t r i a r y l s u l f o n i u m s a l t s having nonnucleophilic anions. The a r y l s u l f o n i u m and to a l e s s e r extent, the aryliodonium s a l t s have improved thermal s t a b i l i t y as compared to the diazonium s a l t s , however, t h e i r absorption maximum occurs at a much shorter wavelength since the aromatic r i n g s are i s o l a t e d by the heteroatom, and therefore not conjugated. Recognizing t h i s d e f i c i e n c y , the workers at General E l e c t r i c , and separately, Smith at 3M (7), discovered that iodonium and sulfonium s a l t s could be s e n s i t i z e d to longer wavelengths by v a r i ­ ous c l a s s e s of aromatic hydrocarbons, aromatic carbonyl compounds, and some c l a s s e s of dyes. S e n s i t i z a t i o n of Onium S a l t s In the curing of coatings, the use of p h o t o s e n s i t i z e r s o f f e r several advantages over d i r e c t e x c i t a t i o n of the onium s a l t s . S e n s i t i z e r s added to onium s a l t s may d r a m a t i c a l l y increase the polymerization rate of r e a c t i v e monomers as compared to unsensit i z e d experiments. (_5) The choice of s e n s i t i z e r w i l l l a r g e l y de­ termine what p o r t i o n of the a v a i l a b l e r a d i a t i o n w i l l be u t i l i z e d i n the curing process. Some s e n s i t i z e r s may r e s u l t i n a dramatic increase i n cure rate simply because they have greater a b s o r p t i v i ­ t y than the onium s a l t at e x c i t a t i o n wavelengths. A l t e r n a t i v e l y , the absorbed l i g h t may be used more e f f i c i e n t l y i n the s e n s i t i z e d system. There are many commercially a v a i l a b l e p h o t o s e n s i t i z e r s . The o v e r a l l cost of coating formulation may be reduced by the use of small amounts of a h i g h l y absorbing p h o t o s e n s i t i z e r i n combination with a reduced concentration of the onium s a l t . In order to choose appropriate s e n s i t i z e r s f o r onium s a l t s , i t i s necessary to understand the p o s s i b l e mechanisms by which s e n s i t i z a t i o n can occur. Energy T r a n s f e r S e n s i t i z a t i o n . Energy t r a n s f e r i s a process where an e x c i t e d state donor molecule i s returned to the ground s t a t e , with simultaneous promotion of an acceptor molecule to i t s

176

INITIATION OF POLYMERIZATION

N CI^*CI->(0>CI Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch014

2

*

^2

(NONACIDIC )

(LEWIS

ACID)

Figure 2. Photolysis of aryldiazonium salts. Photochemistry of these salts is dependent on the nucleophilicity of the anion.

ΐ^0>

C H O H 3

+

+ 2

CH3OH + Η·

(1) (2)

Such processes can lead to increased g r a f t i n g y i e l d s from enhanced H' scavenging by monomer and a b s t r a c t i o n r e a c t i o n s with the back­ bone polymer. Evidence has a l s o been presented (12) to i n d i c a t e that the observed a c i d e f f e c t i s due to an increase i n monomersolvent (MR*) r a d i c a l intermediates i n the g r a f t i n g system. More recent s t u d i e s (21) on the composition of the g r a f t i n g s o l u t i o n s show that oligomer chains a r e of s h o r t e r length i n the presence of

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INITIATION OF

218

POLYMERIZATION

Table I I I . Comparison of T r i m e t h y l o l Propane T r i a c r y l a t e £TMPTA) with A c i d as A d d i t i v e s i n Styrene G r a f t i n g to Polyethylene Graft

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch016

Styrene (% v/v)

Neutral

(%)

H S0 2

4

(0.2M.)

TMPTA (1% v/v)

20

14

19

30

-

37

51

39

40

75

81

73

50

109

134

137

60

89

119

70

89

73

105

80

68

62

59

-

Styrene i n methanol, r a d i a t i o n g r a f t e d to polyethylene f i l m (0.12 mm) at 4.1xlO rad/hr to dose of 2.4xl0 r a d i n a i r . if

5

a c i d , but the numbers of these chains are higher. Because of the smaller s i z e of these chains, the r a d i c a l intermediates a s s o c i a t e d with them can d i f f u s e more r e a d i l y i n t o the swollen backbone polymer to give enhanced y i e l d s due to higher c o n c e n t r a t i o n and higher m o b i l i t i e s a s s o c i a t e d with smaller s i z e . The presence of a c i d can a l s o l e a d to enhanced Trommsdorff peaks i n r a d i a t i o n g r a f t i n g (12), an observation that i s important i n the present work. This r e s u l t can be a t t r i b u t e d to the lower Table IV. Comparison of Divinylbenzene (DVB) with A c i d as A d d i t i v e s i n Styrene G r a f t i n g to Polypropylene Graft Styrene (% v/v)

Neutral

(%)

H S0 2

4

(0.2 M)

DVB

20

37

41

38

30

132

156

92

35

-

136

144

40

75

97

132

50

-

-

111

60

56

51

101

70

41

39

-

80

35

31

55

Styrene i n methanol r a d i a t i o n g r a f t e d to polypropylene f i l m (0.06 mm) a t 4.1x1ο * rad/hr to dose of 2.4xl0 rad i n a i r . 1

5

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch016

16.

ANG ET AL.

Novel Additives

for Enhancing

219

Grafting

M values and l a r g e r numbers of s h o r t e r chains i n the presence of a c i d , s i n c e the lower molecular weights lead to a high s o l u b i l i t y of oligomer r a d i c a l s i n monomer s o l u t i o n , l e a d i n g to an i n c r e a s e i n v i s c o s i t y a t the g e l peak, a r e d u c t i o n i n chain termination and a marked a c c e l e r a t i o n i n g r a f t i n g . With respect to the i n c l u s i o n of p o l y f u n c t i o n a l monomers (e.g. DVB) i n the g r a f t i n g s o l u t i o n , the magnitude of the increment i n copolymerisation y i e l d with these a d d i t i v e s was comparable to the a d d i t i o n of a c i d . P r e l i m i n a r y s t u d i e s show that the g r a f t e d p o l y ­ styrene chains were c r o s s - l i n k e d when DVB was i n the g r a f t i n g s o l u t i o n . In the presence of DVB, branching of the polystyrene can occur. This branching of the growing g r a f t e d chains takes place when one end of the DVB, immobilised during g r a f t i n g , i s bonded to the growing c h a i n . The other end i s unsaturated and f r e e to i n i t i a t e a new chain growth v i a scavenging r e a c t i o n s . The new branched polystyrene chain may e v e n t u a l l y terminate, c r o s s l i n k e d by r e a c t i n g with a neighboring polystyrene chain o r an im­ m o b i l i s e d divinylbenzene r a d i c a l . G r a f t i n g i s thus enhanced mainly through branching of the g r a f t e d chain. The a d d i t i o n of TMPTA, a t r i f u n c t i o n a l monomer, gives s i m i l a r r e s u l t s . The e f f e c t that a p o l y f u n c t i o n a l monomer has on M values of g r a f t e d chains i s c u r r e n t l y being i n v e s t i g a t e d . I t may w e l l be that the r e s u l t i s s i m i l a r to that found with a c i d s and would e x p l a i n the s i m i l a r ­ i t i e s i n degree of g r a f t i n g enhancement observed with both types of a d d i t i v e s . With r a d i a t i o n g r a f t i n g , there i s a l s o an a d d i t i o n a l mecha­ nism f o r enhancement unique to a c i d and not a p p l i c a b l e to the p o l y f u n c t i o n a l monomer a d d i t i v e s . This process i s p a r t i c u l a r l y r e l e v a n t to i r r a d i a t i o n s performed i n a i r and i n v o l v e s the a c i d induced decomposition of peroxy s p e c i e s formed r a d i o l y t i c a l l y i n the backbone polymer, thus generating f u r t h e r s i t e s where copoly­ m e r i s a t i o n may occur (Equation 3). Current evidence (17) indicates that the c o n t r i b u t i o n Ρ - t - 00H + H

+

+ Ρ - t - 0' + H 0 2

(3)

of t h i s pathway to the o v e r a l l a c i d enhancement i n g r a f t i n g i s small compared with the previous processes. S u l f o n i c A c i d Exchange Resins from G r a f t Copolymers S u l f u r i c a c i d i s the most commonly used s u l f o n a t i n g agent f o r preparing i o n exchange r e s i n s from c h e m i c a l l y prepared s t y r e n e divinylbenzene copolymers (22). Reaction a t temperatures up to 100 °C i n d i c a t e s that monosulfonation o n l y i s o c c u r r i n g . Using these r e a c t i o n c o n d i t i o n s , the g r a f t copolymers were s u l f o n a t e d i n an analogous manner, the r e s u l t s (Table V) showing that s u l f o n a t i o n e f f i c i e n c y was very low a t l e s s than 70 C, g r a d u a l l y i n c r e a s i n g with temperature as i n d i c a t e d by the i n c r e a s i n g exchange capacities and reaching a maximum a t 90 °C. Above t h i s temperature the

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

220

INITIATION OF

POLYMERIZATION

Table V. E f f e c t of Temperature on S u l f o n a t i o n of StyrenePolypropylene G r a f t Copolymers as Measured by C a p a c i t i e s c f R e s u l t i n g Ion Exchange Resins . Capacities Copolymer (% Graft)

(m mole/g) with Temp

60

70

80

30

0.4

1.03

1.42

38

0.08

1.08(1.08)°

1.61(1.63)

55

0.10

1.24

1.75(1.77)°

(°c) 103

90

16.1

17.6 b

19.6(19.5)

b

21.1(21.1)°

18.2 18.9

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch016

Copolymers prepared by r a d i a t i o n g r a f t i n g styrene i n methanol to polypropylene powder. S u l f o n a t i o n time, 45 min. C a p a c i t i e s of r e s i n s preswelled with chloroform p r i o r to sulfonation. C a p a c i t i e s of r e s i n s preswelled with 1,2-dichloroethane p r i o r to sulfonation. product begins to char r e s u l t i n g i n a r e d u c t i o n of exchange c a p a c i t y . Time of s u l f o n a t i o n at 90 C i s a l s o important and optimum r e s i n performance was obtained a f t e r 45 minutes (Table V I ) , thus 45 minutes at 90 C was used as standard s u l f o n a t i o n cond i t i o n s f o r a l l subsequent experiments i n c l u d i n g those using DVB copolymers. The data i n Tables V and VI also c l e a r l y show that the c a p a c i t y of the exchange r e s i n i n c r e a s e s with y i e l d of copolymer. The i n c l u s i o n of c h l o r i n a t e d hydrocarbon s o l v e n t s , prev i o u s l y used with chemically prepared r e s i n s (23), to s w e l l the styrene g r a f t copolymer p r i o r to s u l f o n a t i o n d i d not improve the e f f i c i e n c y of the s u l f o n a t i o n . This suggests that l i t t l e g r a f t i n g occurred i n the i n t e r i o r of the polypropylene powder, copolymerisat i o n being predominantly on the surface of the powder.

Table VI. E f f e c t of Time on S u l f o n a t i o n of Styrene-Polyprogylene Copolymers as Measured by C a p a c i t i e s of Ion Exchange Resins C a p a c i t i e s (m mole/g) with Time

(min)

20

30

45

55

65

30

1.17

1.41

17.6

1.79

1.67

38

1.22

1.55

19.6

2.03

1.71

55

1.28

1.64

21.1

1.97

1.78

Copolymer (% Graft)

Copolymers prepared by r a d i a t i o n g r a f t i n g styrene i n methanol to polypropylene powder. S u l f o n a t i o n temperature, 90 C.

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

16.

ANG ET AL.

E f f e c t of DVB

Novel Additives

for Enhancing

221

Grafting

on S t a b i l i t y of Ion Exchange Resins

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch016

Generally the r e s i n s prepared from the g r a f t copolymers of higher y i e l d ( 30%) without DVB s u f f e r e d more degradation during s u l f o n a t i o n than those of lower y i e l d (18%) (Table V I I ) , the l a t t e r r e s i n r e t a i n i n g most of i t s o r i g i n a l exchange c a p a c i t y even a f t e r a l a r g e number of exchanges. P r e l i m i n a r y studies of the e f f e c t of s u l f u r i c a c i d on the polypropylene powder i n d i c a t e d that o x i d a t i o n r e a d i l y occurred as w e l l as noncommitant decomposition of the polystyrene chains. The excessive degradation of the g r a f t copolymers of higher y i e l d during s u l f o n a t i o n appears to be r e l a t e d to the nature of the g r a f t copolymer. As g r a f t i n g i n creased, the copolymer became enriched i n polystyrene not only on Table V I I . E f f e c t of DVB on S t a b i l i t y of Sulfonated Polypropylene G r a f t Copolymer Ion Exchange Resins C a p a c i t i e s (mmole/g) f o r G r a f t Exchange Cycles

(%)

18

30

38

55

61

409

1

1.31

1.76

1.96

2.11

12.28

1.99

3

1.21

1.46

1.62

1.70

1.80

1.90

6

1.20

1.38

1.44

1.50

1.56

1.85

9 11 14 15 17

-

-

1.38

1.43

-

1.34

1.40

1.17

1.24

-

-

Styrene-

-

-

-

1.25

1.29

1.16

1.74

1.24

1.20

1.12

1.72

1.89 1.79

-

Divinylbenzene (1% v/v with respect to styrene) added during synthesis of g r a f t copolymer. the surface but a l s o w i t h i n the bulk of the powder. During s u l f o n a t i o n , polystyrene on the surface of the copolymer was s u l fonated, the p o l a r i t y introduced by the a c i d group enabling the copolymer to s w e l l i n s u l f u r i c a c i d , thus contact of the s u l f o n a t i n g reagent with the aromatic s i t e s deeper w i t h i n the g r a f t copolymer became p o s s i b l e , l e a d i n g to the p o s s i b i l i t y of d i r e c t o x i d a t i o n of the parts of the trunk as s u l f o n a t i o n proceeded. Where the degradation was excessive, fragments of g r a f t copolymer were g r a d u a l l y washed away i n the exchange process, r e s u l t i n g i n a r e d u c t i o n i n exchange c a p a c i t y . With g r a f t copolymer of lower y i e l d (18%), s u l f o n a t i o n was e s s e n t i a l l y confined to the surface of the g r a f t , thus intimate contact of trunk polymer and s u l f o n a t ing reagent was minimised, excessive o x i d a t i o n avoided and higher exchange s t a b i l i t y a t t a i n e d .

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

222

INITIATION OF

POLYMERIZATION

I n c l u s i o n of DVB i n t o the copolymer a l s o minimised the degradation e f f e c t during s u l f o n a t i o n and the r e s i n s produced were more s t a b l e (Table V I I ) . However the m i g r a t i o n of ions i n and out of the g r a f t e d r e s i n s were slower when DVB was included (Table V I I I ) . The r e s u l t s i n d i c a t e that the DVB r e s i n s are s t r u c t u r a l l y Table V I I I . E f f e c t of DVB on Rates of Exchange of Sulfonated Styrene-Polypropylene Graft Copolymer Ion Exchange Resins Graft

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch016

%

DVB 3100 Â) f o r up to 2 hours. The metal oxides acted e i t h e r as e f f e c t i v e photos e n s i t i z e r s , causing increased polymer g r a f t i n g on the f i b e r surface, or as photo-absorbers causing a net decrease i n g r a f t i n g compared to u n s e n s i t i z e d p h o t o g r a f t i n g . Metal oxide-induced g r a f t i n g occurred more r e a d i l y on h y d r o p h i l i c f i b e r s and was accompanied by l e s s homopolymer formation i n comparison to g r a f t ing on more hydrophobic f i b e r s . Antimony and t i n oxides were more e f f e c t i v e on h y d r o p h i l i c f i b e r s , while z i n c oxide was more e f f e c t i v e on hydrophobic f i b e r s . Titanium d i o x i d e was e s s e n t i a l l y i n e f f e c t i v e as a p h o t o s e n s i t i z e r . There was a strong p o s s i b i l i t y that i n i t i a t i o n occurred v i a p h o t o s e n s i t i z e d formation of hydrogen peroxide on the metal oxide surface followed by d e s o r p t i o n and subsequent p h o t o l y s i s o f hydrogen peroxide t o form f r e e r a d i c a l s . R a d i c a l s formed by t h i s means would lead to a b s t r a c t i o n of hydrogen from the f i b e r surface and subsequent g r a f t i n g of poly(methyl a c r y l a t e ) a t these s i t e s . Summary Photo-induced g r a f t i n g of f i b e r s on pre-wetted f i b e r subs t r a t e s occurs by a v a r i e t y of methods. I n t r o d u c t i o n of monomer as a vapor i n the g r a f t i n g process on pre-wetted f i b e r s gives r a p i d p h o t o g r a f t i n g with a minimum o f homopolymer formation. B i a c e t y l vapor g r e a t l y increases the r a t e of p h o t o g r a f t i n g of monomer vapors on pre-wetted f i b e r s and provides a method of f i b e r m o d i f i c a t i o n without l o s s of f i b e r a e s t h e t i c s .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Oster, G. Nature 1954, 173, 300. Oster, G. K.; Oster, G.; Prati, G. J . Am. Chem. Soc. 1957, 79, 595. Oster, G. Phot. Sci. Eng. 1960, 4, 237. Oster, G. U. S. Pat. 2,875,047 Feb. 24, 1959. Oster, G. U. S. Pat. 2,850445 Sept. 8, 1958. Oster, G. Ger. Pat. 1,009,914 Jan. 18, 1956 to June 6, 1957. Nishijima, Y. Kyoto Daigaku Nippon Kogakuseni Kenkyusho Koenshy 1959, 16, 141; Chem. Abstr. 1960, 54, 8616f. Delzenne, G.; Dewinter, W.; Toppet, S.; Smets, G. J. Polym. Sci. A 1964, 2, 1069. Oster, G.; Bellin, J . S.: Holmstrom, B. Experientia 1962, 18, 249.

19.

10. 11. 12. 13. 14. 15. 16. 17. 18.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch019

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

NEEDLES

Photo-Induced Grafting on Prewetted Substrates

257

Chen, C. S. H. J . Polym. Sci. A 1965, 3, 1107, 1127, 1137, 1155. Needles, H. L. J . Polym. Sci. B 1967, 5, 595. Needles, H. L. J . Appl. Polym. Sci. 1967, 11, 719. Needles, H. L . ; Wasley, W. L. Text. Res. J . 1969, 39, 97. Needles, H. L. Polymer Preprints, Am. Chem. Soc. 1969, 10, 302. Needles, H. L. Text. Res. J . 1970, 40, 860. Needles, H. L. Text. Res. J . 1970, 40, 579. Geacintov, N.; Stannett, V.; Abrahamson, E. W.; Hermans, J . J . J . Appl. Polym. Sci 1960, 3, 54. Geacintov, N.; Stannett, V.; Abrahamson, E. A. Makromol. Chem. 1960, 36, 52. Baugh, P. J.; Phillips, G. O.; Worthington, N. W. J . Soc. Dyers and Colourists 1969, 85, 241. Needles, H. L . ; Sarsfield, L. J . Appl. Polym. Symp. 1971, 18, 569. Needles, H. L. J . Appl. Polym. Sci. 1971, 15, 2259. Siddiqui, S. A.; McGee, K.; Lu, W.-C.; Alger, K.; Needles, H. L. Am. Dyestuff Reptr. 1981, 70, No. 7, 20. Ogiwara, Y.; Kubota, H.; Yasunaga, T. J . Appl. Polym. Sci 1975, 19, 887. Howard, G. J.; Kim, S. K.; Peters, R. H. J . Soc. Dyers and Colourists 1969, 85, 468. Seiber, R. P.; Needles, H. L. J . Appl. Polym. Sci, 1975, 19, 2187. Needles, H. L.; Alger, K. W.Schrifenreiche, Special Issue, III 1976. Needles, H. L . ; Alger, K. W.; Tai, A. Textile Res. J . 1978, 48, 123. Needles, H. L . ; Alger, K. W. J . Appl. Polym. Sci. 1978, 22, 3405. Needles, H. L . ; Alger, K. W. J . Appl. Polym. Sci. 1975, 19, 2207.

RECEIVED

August 24, 1982

20 Mechanochemically Initiated Copolymerization Reactions in Cotton Cellulose DAVID N.-S. HON

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

Virginia Polytechnic Institute and State University, Department of Forest Products, Blacksburg, VA 24061 The potentiality of using mechanical stress to i n i tiate graft copolymerization onto cotton cellulose was investigated. A Norton ball mill and a Wiley mill were used. The absorption of mechanical energy by cellulose molecules during milling and its consequence on cellulose properties were taken into consideration. Decreases in degree of polymerization and crystallinity, and increases in accessibility and copper number of milled cellulose were observed. Free radicals formed in the interim were detected by electron spin resonance (ESR) techniques. Three types of mechanoradicals contributing singlet, doublet and triplet ESR signals were identified. ESR studies also revealed that cellulose mechanoradicals were capable of initiating graft polymerization. Methylmethacrylate propagating radicals were identified when the monomer was in contact with cellulose mechanoradicals.. High grafting efficiency was obtained for ball milled and cut fibers, but a higher degree of grafting was obtained from the ball milled fiber.

Cotton i s a major world f i b e r and c e l l u l o s e resource, cont r i b u t i n g to the h e a l t h , s a f e t y , and w e l l being of a l l people. And even more s i g n i f i c a n t , i t i s a renewable organic raw m a t e r i a l by the f i x a t i o n of s o l a r energy by green p l a n t s . Cotton c e l l u l o s e f i b e r (gossypiwn spp.) i s the seed f i b e r o f cotton p l a n t which normally has a higher p u r i t y and a higher molecular weight than other c e l l u l o s e s such as those i s o l a t e d from wood (1). Cotton c e l l u l o s e provides high s t r e n g t h , d u r a b i l i t y and thermal s t a b i l i t y , a b i l i t y to absorb moisture, easy d y e a b i l i t y and wearing comfort. In defiance of these s e r v i c e a b l e p r o p e r t i e s , c o t t o n c e l l u l o s e d i s a l l o w s i t s e l f f o r wider commercial a p p l i c a t i o n s due to poor s o l u b i l i t y i n common inexpensive s o l v e n t s , l a c k of t h e r 0097-6156/83/0212-0259$06.25/0 © 1983 American Chemical Society

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

260

INITIATION OF

POLYMERIZATION

m o p l a s t i c i t y , low dimensional s t a b i l i t y and poor crease r e s i s tance. Many s c i e n t i s t s with r e s t l e s s and probing minds have sought ways to improve cotton c e l l u l o s e p r o p e r t i e s f o r a long time. Many techniques have been developed i n the past decades; of these, g r a f t copolymerization r e a c t i o n s appear to be a p r o p i t i o u s one to synthesize c e l l u l o s e copolymers with unique and usef u l p r o p e r t i e s (_2, 3_). G r a f t copolymers can be synthesized by v a r i o u s i n i t i a t i o n methods such as using u l t r a v i o l e t and i o n i z i n g r a d i a t i o n s , and thermal and chemical r e a c t i o n s (2, 4_) . Despite considerable r e search i n the f i e l d , u n f o r t u n a t e l y , due to the low g r a f t i n g e f f i ciency, high degree of homopolymer formation, c e l l u l o s e copolymers are unable to reach commercial successes (2). In order to e s t a b l i s h a g r a f t i n g technique to achieve c e l l u l o s e copolymers with high g r a f t i n g e f f i c i e n c y , i t has been recognized that mechan i c a l l y generated f r e e r a d i c a l s , i . e . , mechanoradicals, are capable of i n i t i a t i n g g r a f t copolymerization with high g r a f t i n g e f f i c i e n c y (5). Although the s y n t h e s i s of b l o c k - and g r a f t - c o p o l y mers by mechanical f o r c e s has been studied e x t e n s i v e l y f o r synt h e t i c polymers (6, 7), very l i t t l e work has been performed on c e l l u l o s i c m a t e r i a l s . W h i s t l e r and Goatley (8) had i n v e s t i g a t e d the p o s s i b i l i t y of using f r e e r a d i c a l s generated by b a l l - m i l l i n g of corn s t a r c h f o r acrylamide p o l y m e r i z a t i o n . Deter and Huang (9) had attempted to g r a f t a c r y l o n i t r i l e , methyl methacrylate and v i n y l acetate by a v i b r o m i l l . Various degrees of g r a f t i n g were obtained, but v i n y l c h l o r i d e d i d not g r a f t w e l l onto c e l l u l o s e . Hon (_5, _10, 11) had demonstrated that mechanoradicals generated i n wood, high y i e l d pulps, c e l l u l o s e and l i g n i n by a glass-bead m i l l are capable of i n i t i a t i n g g r a f t copolymerization, and a higher degree of g r a f t i n g e f f i c i e n c y was obtained from mechanic a l l y i n i t i a t e d g r a f t i n g systems than from those i n i t i a t e d by ultraviolet light irradiation. O r d i n a r i l y , the disadvantages of using mechanical s t r e s s f o r chemical r e a c t i o n s are a r e l a t i v e l y high energy consumption and equipment complexity. Fortunately, i t has been recognized that g r a f t i n g r e a c t i o n s can be c a r r i e d out d i r e c t l y during polymer p r o c e s s i n g and i n standard equipment, such as i n c u t t e r s and g r i n d e r s , without adding e x t r a energy and production c o s t . D e t a i l s are reported i n t h i s paper. The manufacture of cotton c e l l u l o s e products i n v o l v e s complex conversion methods. In order to convert cotton c e l l u l o s e f i b e r i n t o u s e f u l consumer and i n d u s t r i a l products, mechanical processings, such as g r i n d i n g , crushing, c u t t i n g , e t c . , are i n e v i t a b l y c a r r i e d out at g i n s , t e x t i l e and paper m i l l s , as to render the cotton f i b e r p r o c e s s i b l e and commercially u s e f u l . As a consequence, i t i s opportune i f the e x i s t i n g mechanical o p e r a t i o n energy can be u t i l i z e d to i n i t i a t e g r a f t i n g r e a c t i o n . Accordi n g l y , the p o s s i b i l i t y of u t i l i z a t i o n of t h i s energy f o r i n i t i a t ing g r a f t i n g r e a c t i o n was experimented using a Wiley m i l l and a Norton b a l l m i l l as to simulate a c u t t i n g and a m i l l i n g o p e r a t i o n i n cotton m i l l s . In a d d i t i o n to the e v a l u a t i o n of g r a f t copoly-

20.

HON

Mechanochemically

Initiated Reactions

in Cotton Cellulose

261

mers, the absorption of mechanical energy generated by these m i l l s by c e l l u l o s e molecules and i t s consequence on c e l l u l o s e p r o p e r t i e s were taken i n t o c o n s i d e r a t i o n . Experimental r e s u l t s revealed that a high degree of g r a f t i n g e f f i c i e n c y can be ob­ tained during c u t t i n g or g r i n d i n g process. Only a very short c u t t i n g time i s required f o r the former process to achieve high g r a f t i n g e f f i c i e n c y , whereas a longer m i l l i n g time i s required f o r the l a t t e r process. Experimental

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

M a t e r i a l s . P u r i f i e d acetate-grade cotton f i b e r i n a sheet form was used. Methyl methacrylate was used as the monomer a f t e r p u r i f i c a t i o n by a l k a l i n e e x t r a c t i o n and followed by d i s t i l l a t i o n under reduced pressure. Procedures. Cotton c e l l u l o s e was e i t h e r cut i n a Wiley m i l l or m i l l e d i n a Norton b a l l m i l l (Roalox p o r c e l a i n j a r ) of 1.3 g a l l o n c a p a c i t y . The charge always c o n s i s t s of 3500 grams of high carbon chrome s t e e l b a l l s (3/8" diameter) and 25 grams of cotton f i b e r . The m i l l was r o t a t e d at a constant speed of 60 rpm The change i n degree of p o l y m e r i z a t i o n before and a f t e r mechani­ c a l treatments was determined from l i m i t i n g v i s c o s i t y number ob­ tained by using a c a p i l l a r y viscometer. The measurements were c a r r i e d out i n a thermostat at 298.00 ± 0.05°K i n c u p r i e t h y l e n e diamine s o l u t i o n and converted to degree of p o l y m e r i z a t i o n using the f o l l o w i n g equation (12): DP = 190

[η]

C r y s t a l l i n i t y of cotton c e l l u l o s e was measured using a den­ s i t y method by means of a d e n s i t y gradient column (Techne Inc., Model DC-2). Xylene and carbon t e t r a c h l o r i d e were used to make up the s o l u t i o n . Based on the d e n s i t y data, c r y s t a l l i n i t y of c e l l u l o s e can be c a l c u l a t e d from the f o l l o w i n g equation (13): Crystallinity =

V

Va - V ^ ; a

V q

1 Density = -

where Va, Vc and V are s p e c i f i c volume of amorphous p o r t i o n , c r y ­ s t a l l i n e p o r t i o n and unknown sample, r e s p e c t i v e l y . Due to Kast (14), the values of Va and Vc are 0.680 and 0.628, r e s p e c t i v e l y . A c c e s s i b i l i t y of c e l l u l o s e was determined by an i o d i n e ab­ s o r p t i o n method described by H e s s l e r and Power (15) using the f o l l o w i n g equation: mg of i o d i n e _ (a-b) χ 2.04 g of sample 0.3

χ

2.54

where a i s the volume of 0.02N t h i o s u l f a t e f o r the blank and b i s

262

INITIATION OF POLYMERIZATION

the corresponding volume f o r determination, a r a t i o of the m i l l i ­ grams of i o d i n e absorbed per gram of c e l l u l o s e to 412 (the mg of i o d i n e adsorbed per gram of methocel) gives a value f o r the amor­ phous f r a c t i o n . The percentage of c r y s t a l l i n i t y i s thus equalled to 100 minus percentage of amorphous p o r t i o n . Copper number, a method of determination of reducing end groups i n c e l l u l o s e , was measured using a standard method de­ s c r i b e d by Earland and Raven (16). The average number of chain s c i s s i o n per chain u n i t (S) and the degree of degradation (a) were c a l c u l a t e d based on the f o l ­ lowing equations:

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

ξ

α

=

D =

P

initial _ DP cut

1 DP cut

1 DP i n i t i a l

G r a f t Copolymerization by Norton B a l l M i l l i n g : Cotton f i b e r sheets were t o r n i n t o lengths of 1-2 cm and soaked with methyl methacrylate monomer and water (9:1 r a t i o i n volume) f o r 24 h r s p r i o r to m i l l i n g i n n i t r o g e n atmosphere. G r a f t Copolymerization by Wiley M i l l C u t t i n g : Cotton f i b e r sheets (2 inches i n width) were soaked with methyl methacrylate and water (9:1 r a t i o i n volume) f o r 24 h r s p r i o r to c u t t i n g i n n i t r o g e n atmosphere. The p o l y m e r i z a t i o n was f i n a l l y terminated by the a d d i t i o n of hydroquinone. The copolymerization products were c o l l e c t e d and extracted with benzene to remove the homopolymers. The ungrafted f i b e r under i d e n t i c a l m i l l i n g or c u t t i n g c o n d i t i o n s were a l s o ex­ t r a c t e d with benzene i n c o n s i d e r a t i o n of the p o s s i b l e l o s s of f i ­ ber bundles during e x t r a c t i o n . Degree of g r a f t i n g and g r a f t i n g e f f i c i e n c y were c a l c u l a t e d as f o l l o w s : Degree of g r a f t i n g

Grafting efficiency

A-B (%) = (-=-) x 100 Β (%) = (^f) G—Β

χ 100

where A i s the weight of c e l l u l o s e a f t e r copolymerization and e x t r a c t i o n , Β i s the weight of o r i g i n a l c e l l u l o s e , and C i s the t o t a l weight of products a f t e r copolymerization. E l e c t r o n Spin Resonance (ESR) Studies: ESR s p e c t r a were mea­ sured with an X-band ESR spectrometer (Varian E-12, 100 KHz f i e l d modulation). To avoid d i s t o r t i o n of the s p e c t r a by a power satu­ r a t i o n , the ESR measurements were c a r r i e d out a t a microwave of 3mW. The g-value was measured by comparison with the strong p i t c h provided by V a r i a n A s s o c i a t e s . In a l l cases, ESR s p e c t r a were recorded at 77°K.

20.

HON

Mechanochemically Initiated Reactions in Cotton Cellulose

263

For ESR measurements, the b a l l m i l l e d or cut f i b e r s were t r a n s f e r r e d to a Dewar f l a s k f i l l e d with l i q u i d n i t r o g e n immed i a t e l y a f t e r mechanical treatments i n order to avoid s i g n i f i c a n t decay of unstable f r e e r a d i c a l s . The f i b e r s were then t r a n s f e r r e d slowly to ESR sample tubes together with l i q u i d n i t r o g e n , which was removed by a vacuum pump afterward. Subsequently, the ESR tube was sealed i n vacuum f o r ESR measurements.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

Results and

Discussion

Mechanical E f f e c t on Cotton F i b e r P r o p e r t i e s . I t has been reported e a r l i e r (5, J 7 , 18) that i n the mechanical processing, wood, c e l l u l o s e , and l i g n i n are inescapably absorbing mechanical s t r e s s , i . e . , shear f o r c e s . The consequence of t h i s energy uptake normally leads to the slippage of secondary bonds due to the i n t r a - and i n t e r m o l e c u l a r hydrogen bonds, and the rupture of cov a l e n t bonds, which leads to the shortening of f i b e r length. Mechanoradicals are formed i n the i n t e r i m . The r e d u c t i o n of degree of polymerization of cotton f i b e r was studied a f t e r c u t t i n g and m i l l i n g . Results are shown i n Figures 1 and 2. When cotton sheets were cut through a 6 mm s i e v e of the Wiley m i l l f o r s e v e r a l consecutive c y c l e s , the cotton sheet was disaggregated i n t o f i b r i l l a r bundles as mechanical processing progressed. The f i b e r l o s t i t s DP s i g n i f i c a n t l y at the i n i t i a l s t a t e of c u t t i n g , i . e . , the f i r s t c u t t i n g c y c l e , followed by a slow drop of DP. C a l c u l a t i o n of chain s c i s s i o n showed that 0.19 cleavage takes place per molecular chain at the f i r s t c u t t i n g c y c l e . No s i g n i f i c a n t increment of the reducing end group of c e l l u l o s e , i . e . , aldehyde group, as determined by copper number, was observed (Figure 1). When cotton f i b e r was m i l l e d i n the Norton b a l l m i l l , however, the d r a s t i c change i n DP was observed (•Figure 2). I t was n o t i c e d that a f t e r 50 hrs of m i l l i n g , a blend of two f r a c t i o n s of c e l l u l o s e was formed: one f r a c t i o n of deformed f i b e r r e t a i n e d most of i t s f i b r o u s s t r u c t u r e , and one form of c e l l u l o s e powder has l o s t i t s f i b r o u s s t r u c t u r e completely. For the f i b e r f r a c t i o n , the l o s s of DP was l e s s severe than the powder f r a c t i o n . The l o s s of DP was 17, 31.5, 45.5, 49.5, and 50.5% of t h e i r o r i g i n a l value a f t e r 50, 100, 198, 336 and 400 hours of m i l l i n g , r e s p e c t i v e l y . For the powder f r a c t i o n , the l o s s of DP was s i g n i f i c a n t . Cotton c e l l u l o s e l o s t 50.1, 66.4, 72.3, 77.3 and 77.6% of i t s o r i g i n a l DP value a f t e r the same periods of m i l l i n g , i n d i c a t i n g that mechanochemical chain s c i s s i o n was ext e n s i v e . The r a t e of chain s c i s s i o n and degree of degradation as a f u n c t i o n of m i l l i n g time f o r the f i b e r and powder f r a c t i o n s are shown i n Table I. I t i s c l e a r l y evident that severe degradat i o n of f i b e r took place during m i l l i n g . The increment of reducing end group due to the cleavage of main chains was a l s o observed i n terms of copper number study. Results are a l s o shown i n Figure 2. I t i s obvious that the r a t e of producing reducing end group i n m i l l e d powder was much f a s t e r

264

INITIATION OF POLYMERIZATION

3500

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

10.8

3000h

10

20 Cutting

30 Time

40 (sees)

50

60

Figure 1. Changes in degree of polymerization and copper number of cotton cellulose during cutting with a Wiley mill.

HON

Mechanochemically

Initiated Reactions

in Cotton

Cellulose

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

20.

Figure 2. Changes in degree of polymerization and copper number of cotton cellulose during milling with a Norton ball mill. Key: ψ , · , milled fiber; V , O , milled powder.

265

1.02

1750 1622 1593

198

336

400

Average number of chain s c i s s i o n s

Degree of degradation

a:

0.45

0.20

S:

Degree o f p o l y m e r i z a t i o n .

3. 16

0.98

2209

100

DP:

3.05

0.83

2673

50

0

3211

Control

4

4

6.12 1.96 2.60

1083 892

1.41 2.60

720

729

3.67

1.18

1475

0. 63

10.06 10.77

3.40 3.46

8.10

0

âdo- ) 0

DP

Powder P o r t i o n

3211

0

adO- )

DP

M i l l i n g Time (hrs)

Fiber Portion

Chain S c i s s i o n and Degree of Degradation o f Cotton F i b e r A f t e r M i l l i n g i n a Norton B a l l M i l l

TABLE I

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

δ 2

H

H

Ο

δ ο

H

>

H

2

Ν) ON ON

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

20.

HON

Mechanochemically

Initiated Reactions

in Cotton Cellulose

267

than i n the m i l l e d f i b e r . I t i s i n agreement with the severe r e duction of DP f o r m i l l e d powder. Moreover, the d i r e c t evidence of the rupture of primary bonds was o r i g i n a t e d from ESR s t u d i e s . When cotton f i b e r was cut i n the Wiley m i l l f o r 60 sec a t 298°K i n n i t r o g e n , a s i n g l e t s i g n a l with a l i n e width of 18 gauss, and a g-value of 2.003, due to the alkoxy r a d i c a l s (5), was observed (Figure 3a). This i n d i c a ted that mechanoradicals were generated i n the cut f i b e r due to a chain s c i s s i o n r e a c t i o n . When the cotton f i b e r was m i l l e d i n the Norton b a l l m i l l , an i l l - d e f i n e d f i v e - l i n e s i g n a l was detected (Figure 3b). This spectrum i s very s i m i l a r to those observed from wood c e l l u l o s e m i l l e d a t 77°K (5), which was a s u p e r p o s i t i o n of a s i n g l e t , a doublet and a t r i p l e t s i g n a l (5). Accordingly, three types o f mechanoradicals were produced i n c e l l u l o s e during ball-milling. The assignment of these r a d i c a l s t r u c t u r e s c o n t r i buting to the s i g n a l s has been discussed elsewhere (5). Comparison between the mechanically cut and the m i l l e d f i b e r s c l e a r l y i n d i c a t e d that b a l l - m i l l e d f i b e r e x h i b i t e d more intense ESR s i g n a l than the cut f i b e r , implying that a higher amount o f mechanor a d i c a l s was generated i n b a l l - m i l l e d f i b e r . I t a l s o suggested that the degree of degradation o f the b a l l - m i l l e d f i b e r was much more c r i t i c a l than that o f the cut f i b e r . Much o f the chemical behavior o f c e l l u l o s e f i b e r can be a t t r i b u t e d to c e l l u l o s e s t r u c t u r e . Since c e l l u l o s e i s a h i g h l y c r y s t a l l i n e polymer, i t can absorb mechanical energy e f f i c i e n t l y f o r mechanical s t r e s s r e a c t i o n (5, 19). The mechanically a c t i vated thermal energy, i n a d d i t i o n to rupture o f main chains, may a l t e r morphology o r m i c r o s t r u c t u r e o f cotton c e l l u l o s e . Accordi n g l y , the c r y s t a l l i n i t y and a c c e s s i b i l i t y o f cotton f i b e r may be influenced. When cotton f i b e r was cut through the Wiley m i l l through a 6 mm s i e v e f o r d i f f e r e n t c y c l e s , i . e . , f o r d i f f e r e n t c u t t i n g times, the change o f c r y s t a l l i n i t y was not observed (Figure 4 ) . When cotton f i b e r was m i l l e d f o r 10 h r s , the change of c r y s t a l l i n i t y was h a r d l y recognized, although the increment o f a c c e s s i b i l i t y was observed (see Figure 5). The n o t i c e a b l e change i n c r y s t a l l i n i t y was observed only a f t e r 20 h r s of m i l l i n g . I t should be noted here that the d e s t r u c t i o n o f c r y s t a l l i n i t y h e a v i l y depended upon the m i l l i n g equipment and operation c o n d i t i o n s . Howsmon and Marchessault (19) destroyed the c r y s t a l l i n i t y o f wood c e l l u l o s e i n a r e l a t i v e l y short b a l l - m i l l i n g p e r i o d . F o r z a i t i et a l . (20) had reported that by using a v i b r a t o r y b a l l m i l l , the cotton c e l l u l o s e was converted almost completely to the amorphous form i n 30 mins. As mentioned e a r l i e r , a f i b e r f r a c t i o n and a powder f r a c t i o n of cotton c e l l u l o s e were formed during m i l l i n g ; the changes i n c r y s t a l l i n i t y f o r these two f r a c t i o n s a r e d i f f e r e n t . For the f i b e r f r a c t i o n , the l o s s of c r y s t a l l i n i t y was c r i t i c a l f o r the i n i t i a l 100 h r s of m i l l i n g , as shown i n Figure 4. The r a t e of change i n c r y s t a l l i n i t y was then l e v e l l e d o f f a f t e r 100 h r s o f

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

Figure 3. ESR spectra of cotton cellulose cut in a Wiley mill for 60 s at 298 Κ in nitrogen (a) and cotton cellulose milled in a Norton ball mill for 4 h at 298 Κ in nitrogen (b).

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

Figure 4.

Change in crystallinity of cotton cellulose during cutting and milling. Key: V , cut fiber; ·, milled fiber; O , milled powder.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

270

INITIATION OF

POLYMERIZATION

milling. A f t e r 72 and 200 hrs of m i l l i n g , the l o s s of c r y s t a l l i n i t y was 29.7 and 40.7% of t h e i r o r i g i n a l c r y s t a l l i n i t y v a l u e s , r e s p e c t i v e l y . A d d i t i o n a l s e v e r a l percentage of c r y s t a l l i n i t y may be l o s t i f the m i l l i n g time i s prolonged. But i t i s b e l i e v e d that i f the m i l l i n g time i s prolonged, the powder f r a c t i o n of the c o t ton would probably be increased, which a c t u a l l y has a lower c r y s t a l l i n i t y than the f i b e r f r a c t i o n . For the powder f r a c t i o n , at the i n i t i a l 22 hrs of m i l l i n g , the c r y s t a l l i n i t y was only 34%, which was 46.9% r e d u c t i o n of c r y s t a l l i n i t y of i t s o r i g i n a l value, i . e . , 64.04%. A f t e r 200 hrs of m i l l i n g , about 67% of the c r y s t a l l i n i t y was l o s t . The r e d u c t i o n of c r y s t a l l i n i t y was continuously observed i f the m i l l i n g time was prolonged. Since the consequences of mechanical energy uptake were d i s aggregation of f i b e r bundles, shortening of f i b e r l e n g t h and r e duction of DP, i t i s p l a u s i b l e to consider that new surface areas were a l s o created. A c c o r d i n g l y , the change of a c c e s s i b i l i t y of mechanically t r e a t e d f i b e r s may be observed. The a c c e s s i b i l i t y of mechanically t r e a t e d f i b e r s was evaluated based on the i o d i n e abs o r p t i o n techniques. Results are shown i n F i g u r e 5. Although the c r y s t a l l i n i t y of cotton f i b e r was not i n f l u e n c e d i n the c u t t i n g process, a s l i g h t increase i n a c c e s s i b i l i t y was observed f o r the cut f i b e r . As shown i n Figure 5, the a c c e s s i b i l i t y increased from 10.06% to 17.51% a f t e r 60 sec of c u t t i n g . The i n crease i n a c c e s s i b i l i t y was enhanced when cotton f i b e r was b a l l m i l l e d . During the f i r s t 72 hrs of m i l l i n g , the a c c e s s i b i l i t y of f i b e r f r a c t i o n increased to 32.12% (from 10.06%), whereas f o r the powder f r a c t i o n , i t even increased to 50%. The r a t e of increment of a c c e s s i b i l i t y was slowed down f o r the f i b e r f r a c t i o n even i f the m i l l i n g time was prolonged, but f o r the powder f r a c t i o n , the a c c e s s i b i l i t y was continuously increased. A f t e r 400 hrs of m i l l ing, 72% of a c c e s s i b i l i t y was achieved. Based on the experimental data of c r y s t a l l i n i t y and a c c e s s i b i l i t y , i t i s revealed that the mechanical shear f o r c e s involved i n the c u t t i n g process were able to open up the u n a c c e s s i b l e r e gions i n c e l l u l o s e by c r e a t i n g new surfaces without damaging the c r y s t a l l i n e s t r u c t u r e . I t i s l i k e l y that the surfaces of c r y s t a l l i t e s are made more a c c e s s i b l e during c u t t i n g . The mechanical shear f o r c e s involved i n the m i l l i n g process are able to i n c r e a s e a c c e s s i b i l i t y p e r s p i c u o u s l y i n accompanying with d e s t r u c t i o n of c r y s t a l l i n e regions. The increment of a c c e s s i b i l i t y u s u a l l y set forward the p e n e t r a t i o n of monomer i n t o channels and pores of f i ber to achieve high degree of r e a c t i o n . However, i t should be borne i n mind that the l o s s of c r y s t a l l i n i t y a l s o i s an i n d i c a t i o n of the l o s s of p h y s i c a l p r o p e r t i e s of cotton f i b e r . Hence, the proper c o n t r o l of the mechanical process to achieve high a c c e s s i b i l i t y with l e s s d e s t r u c t i o n of c r y s t a l l i n e s t r u c t u r e s of f i b e r i s b e n e f i c i a l to accomplish the purpose f o r which a chemical m o d i f i c a t i o n i s designed.

20.

HON

Mechanochemically

Milling

Time

100

(hrs)

200

300

400

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

80 0

271

Initiated Reactions in Cotton Cellulose

ο

I

ι

ι

ι

ι

1

1

0

10

20

30

40

50

60

Cutting Figure 5.

Time

1

(sees)

Change in accessibility of cotton cellulose during cutting and milling. Key: O, powder; ·, fiber.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

272

INITIATION OF

POLYMERIZATION

Mechanochemically I n i t i a t e d Graft Copolymerization. According to the ESR study, i t i s c l e a r that mechanoradicals were generated i n c e l l u l o s e e i t h e r by means of mechanical c u t t i n g or b a l l milling. These mechanoradicals may be u t i l i z e d as r e a c t i o n s i t e s f o r the i n i t i a t i o n of v i n y l p o l y m e r i z a t i o n which would r e s u l t i n g r a f t copolymerization of c e l l u l o s e . Based on t h i s p r i n c i p l e , the a b i l i t y of c e l l u l o s e mechanoradicals to i n i t i a t e c o p o l y m e r i z a t i o n was pursued. When cotton f i b e r was b a l l - m i l l e d f o r 4 hrs at 298°K and t r a n s f e r r e d immediately to a sample tube f o r ESR measurement at 77°K, an i l l - d e f i n e d f i v e - l i n e spectrum with a g-value of 2.003 was detected (Figure 6a). When cotton f i b e r was m i l l e d i n the presence of MMA, only a s i n g l e t s i g n a l with a l i n e width of 18 gauss was observed (Figure 6b). This implied that other f r e e r a d i c a l s which generated s i g n a l s other than the s i n g l e t component were i n t e r a c t e d with MMA during m i l l i n g . No MMA-propagating r a d i c a l s were observed. Moreover, immediately f o l l o w i n g the m i l l i n g , MMA was introduced i n t o the mechanically t r e a t e d c e l l u l o s e f o r 2 mins at 298°K and recorded i t s ESR s i g n a l at 77°K, the f i v e - l i n e ESR s i g n a l of c e l l u l o s i c mechanoradicals was converted to an asymmetrical m u l t i p l e t spectrum (Figure 7b), and these s i g n a l s were f u r t h e r i n t e n s i f i e d when the sample was warmed at 298°K f o r 10 mins, the ESR spectrum observed at 77°K was a n i n e - l i n e spectrum, as shown i n F i g u r e 7d. The n i n e - l i n e spectrum o r i g i n a t e d from the c h a r a c t e r i s t i c propagating r a d i c a l s of MMA f o r p o l y m e r i z a t i o n (2_1) . S i m i l a r r e s u l t s were observed when wood c e l l u l o s e was m i l l e d with g l a s s beads at 77°K (5). MMApropagating r a d i c a l s were a l s o detected from cut f i b e r under i d e n t i c a l warm-up treatment, with the exception that the s i g n a l i n t e n s i t y was very weak. This could be due to the low f r e e r a d i c a l c o n c e n t r a t i o n being generated by c u t t i n g . The propagating r a d i c a l s of MMA were not detected when monomer was added to the untreated c e l l u l o s e f i b e r s i n i d e n t i c a l experiments. This c l e a r l y i n d i c a t e d that propagating r a d i c a l s of MMA were created by contact of MMA with c e l l u l o s i c mechanoradicals. Subsequently, i t i s evident that c e l l u l o s i c mechanoradicals are capable of i n i t i a t i n g v i n y l p o l y m e r i z a t i o n . In l i g h t of these f i n d i n g s , g r a f t copolymerization of MMA onto c e l l u l o s e f i b e r was c a r r i e d out at ambient temperature by impregnating monomer i n t o f i b e r p r i o r to c u t t i n g or m i l l i n g . R e s u l t s are shown i n F i g u r e s 8 and 9. For the v i n y l g r a f t copolymerization i n i t i a t e d by the c u t t i n g process, the degree of g r a f t i n g was approached at 50% a f t e r 60 sec of c u t t i n g . The g r a f t i n g e f f i c i e n c y reached i t s maximum of 89% between 20 and 30 sec of c u t t i n g . The i n i t i a t i n g r e a c t i o n f o r the b a l l m i l l system took place s t e a d i l y . A longer i n d u c t i o n p e r i o d seemed to be needed to reach the maximum degree of g r a f t i n g of 196% f o r 5 hrs of m i l l i n g , and the g r a f t i n g e f f i c i e n c y reached i t s maximum a f t e r 2-3 hrs of m i l l i n g . When the g r a f t i n g r e a c t i o n time was prolonged, the degree of g r a f t i n g as w e l l as g r a f t i n g

Mechanochemically

Initiated Reactions in Cotton

Cellulose

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

HON

Figure 6. ESR spectra of cotton cellulose milled in absence (a) and presence (b) of methyl methacrylate for 4 h at 298 K. ESR spectra were recorded at 77 K.

INITIATION OF

POLYMERIZATION

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274

Figure 7. Changes in ESR spectra of cotton cellulose mechanoradicals. Key: a, initial spectrum observed at 77 Κ immediately after milling for 4 h at 298 K; b, cotton cellulose after milling was contacted with methyl methacrylate and warmed at 298 Κ for 2 min; c, 5 min; d, 10 min; recorded at 77 K.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

20. HON Mechanochemically Initiated Reactions in Cotton Cellulose 275

INITIATION OF

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276

Milling

Time

(hrs)

Figure 9. Graft copolymerization of methyl methacrylate onto cotton cellulose induced by mechanical milling with a Norton ball mill from 1 to 4 h.

HON

Mechanochemically

Initiated Reactions in Cotton Cellulose

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

20.

Figure 10. Graft copolymerization of methyl methacrylate onto cotton cellulose induced by mechanical milling with a Norton ball mill from 1 to 5 h.

277

278

INITIATION OF

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

e f f i c i e n c y reduced sharply a f t e r 10 hrs of m i l l i n g (Figure 10). I t i s p o s s i b l e that i f the m i l l i n g was continued, the g r a f t e d PMMA chains might w e l l have a l s o s u f f e r e d cleavage of covalent bonds by mechanical s t r e s s which l e d to low degree of g r a f t i n g . In comparison, a higher degree of g r a f t i n g was achieved f o r the m i l l i n g system than f o r the c u t t i n g system. I t i s p l a u s i b l e that higher mechanical energy was s u p p l i e d f o r the former system, i n which c e l l u l o s e absorbed more energy to produce mechanoradic a l s f o r g r a f t i n g r e a c t i o n . The increment of a c c e s s i b i l i t y and destroying of c r y s t a l l i n e s t r u c t u r e of cotton f i b e r during the m i l l i n g process could a l s o be the f a c t o r s c o n t r i b u t i n g to the high degree of g r a f t i n g . On the other hand, the change i n p h y s i c a l and chemical p r o p e r t i e s of m i l l e d c o t t o n f i b e r were more c r i t i c a l than those of the cut cotton f i b e r . Conclusions On the b a s i s of the experimental f i n d i n g s , the f o l l o w i n g conclusions may be drawn: 1. Cotton c e l l u l o s e i s s u s c e p t i b l e to degradation by mechanical shear f o r c e s supplied by e i t h e r a Wiley m i l l or a Norton b a l l m i l l . Mechanoradicals are produced i n the i n t e r i m . 2. The consequences of the energy absorption by cotton c e l l u l o s e molecules are r e d u c t i o n of degree of p o l y m e r i z a t i o n and i n crement of a c c e s s i b i l i t y and reducing end group. Cotton f i ber s u f f e r e d l e s s degradation and i t s c r y s t a l l i n e s t r u c t u r e was not i n f l u e n c e d when i t was t r e a t e d with a Wiley m i l l . When cotton f i b e r was t r e a t e d with a Norton b a l l m i l l , a deformed f i b e r f r a c t i o n and a powder c e l l u l o s e f r a c t i o n were formed. D r a s t i c changes i n c r y s t a l l i n i t y and a c c e s s i b i l i t y were observed from both f r a c t i o n s . A c c o r d i n g l y , i t i s important to c o n t r o l the mechanical processing p r o p e r l y i n order to avoid l o s i n g p h y s i c a l and chemical p r o p e r t i e s of c o t t o n f i b e r s . 3. Mechanoradicals are capable of i n i t i a t i n g g r a f t copolymerizat i o n . A high degree of g r a f t i n g e f f i c i e n c y and a low degree of homopolymer formation were obtained from the f i b e r s t r e a t e d with the Wiley m i l l and the Norton b a l l m i l l , but a higher degree of g r a f t i n g was obtained from the b a l l - m i l l e d f i b e r . These a u s p i c i o u s f i n d i n g s w i l l c e r t a i n l y shed l i g h t on the development of g r a f t copolymerization techniques w i t h high g r a f t ing e f f i c i e n c y . And the u t i l i z a t i o n of e x i s t i n g mechanical processing energy f o r g r a f t copolymerization r e a c t i o n i s c e r t a i n l y an a t t r a c t i v e mode of o p e r a t i o n f o r commercial a p p l i c a t i o n s .

Literature Cited 1.

Hon, D. N.-S. "Yellowing of Modern Papers" in "Preservation of Paper and Textiles of Historic and Artistic Value II" (Williams, J. C., ed.), Advances in Chemistry Series, 1981, 193, 119.

20.

HON

2.

Stannett, V. "Some Challenges in Grafting to Cellulose and Cellulose Derivatives" in "Graft Copolymerization of Ligno­ cellulosic Fibers" (Hon, D. N.-S., ed.), ACS Symposium Series, in press. Hebeish, A., and Guthrie, J . T. "The Chemistry and Technology of Cellulosic Copolymers", Springer-Verlag, Berlin-Heidelberg­ -New York, 1981, p 351. Hon, D. N.-S. "Graft Copolymerization of Lignocellulosic F i ­ bers", ACS Symposium Series, in press. Hon, D. N.-S. J . Appl. Polym. Sci., 1979, 23, 1487. Casale, A., and Porter, R. S. "Polymer Stress Reactions", Academic Press, New York, Volume 1, 1978, Chapter V. Ceresa, R. J . J . Polym. Sci., 1961, 53, 9. Whistler, R. L., and Goatley, J . L. J . Polym. Sci., 1962, 62, 5123. Deter, W., and Huang, D. C. Faserforsch. Textil. Tech., 1963, 14, 58. Hon, D. N.-S. J . Polym. Sci. Polym. Chem. Ed., 1980, 18, 1957. Hon, D. N.-S. "Modification of Lignocellulosic Fibers." Pa­ per presented at the Second Chemical Congress of the North Americal Continent, San Francisco, CA., Aug., 24-29, 1980, Abstracts of Papers, Part 1, cell-39. Browning, B. L. "Methods of Wood Chenistry", Academic Press, New York, 1967, Vol. 2, p 519-529. Migita, Ν., Yonezawa, Η., and Kondo, T. "Wood Chemistry", Kyoritsu, Tokyo, 1968, Vol. 1, p 102. Kast, Κ. Z. Elektrochemie, 1953, 57, 525. Kessler, L. Ε . , and Power, R. E. Textile Res. J., 1954, 18 (9), p 822-827. Earland, C., and Raven, D. J . "Experiments in Textile and Fiber Chemistry", Butterworth, London, 1971, p 134. Hon, D. N.-S., and Glasser, W. G. TAPPI, 1979, 62, 107. Hon, D. N.-S., and Srinivasan, K. S. V. J . Appl. Polym. Sci., in press. Howsmon, J . Α., and Marchessault, R. H. J . Appl. Polym. Sci., 1959, 1, 313. Forziati, F. Η., Stone, W. Κ., Rowen, J . W., and Appel, W. D. J. Res. of Natl. Bur. of Stand., 1950, 45, 2116. Campbell, D. Macromol. Rev., 1970, 91.

3. 4. 5. 6.

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7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

RECEIVED

Mechanochemically Initiated Reactions in Cotton Cellulose

November 5, 1982

279

21 Factors Affecting the Isomeric Chain Unit Structure in Organolithium Polymerization of Butadiene and Isoprene MAURICE MORTON and J. R. RUPERT

1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch021

The University of Akron, Institute of Polymer Science, Akron, OH 44325

Organolithium initiators are used extensively in the polymerization of butadiene and isoprene, because of their solubility in a variety of solvents. It is well known that non-polar media lead to polymers having a high 1,4 chain unit structure, while polar solvents can exert a dramatic effect in producing a high proportion of 1,2 or 3,4 structures. However, recent work, using the most accurate available NMR spectroscopy, has also shown that, even in nonpolar media, the concentration of initiator and the presence of solvent can have a very significant effect on the cis-l,4/trans-l,4 ratio, without noticeably influencing the proportion of side-vinyl units. Thus, at very low initiator concentrations (~10 M. ), and in the absence of any solvents, i t is possible to attain a cis-1,4 content of 96% for polyisoprene and 86% for polybutadiene. These chain structures have not been found to be noticeably affected by temperature or extent of conversion. -5

The intensive investigations that followed the discovery (l) that lithium and its compounds can lead to the polymerization of isoprene to a very high cis-1,4 configuration, close to that of natural rubber, showed that both the type of alkali metal and the nature of the solvent can have a profound effect on the chain structure of polydienes (2-8). The conclusions reached from these investigations are as follows: 1. In non-polar media, the 1,4 structure is highest for lithium and decreases with increasing electropositivity of the alkali metals. 2. Polar solvents (e.g., ethers, amines, etc.) lead to a higher side-vinyl content (1,2 or 3,4), especially 1

Current address: Monsanto Company, Akron, OH 44313. 0097-6156/83/0212-0283$06.00/0 © 1983 American Chemical Society

284

INITIATION OF

i n t h e case o f l i t h i u m , a p p a r e n t l y b y s o l v a t i n g the m e t a l c a t i o n and thus i n c r e a s i n g t h e i o n i c c h a r a c t e r o f t h e carbon-metal bond. W i t h r e g a r d t o t h e e f f e c t s o f s o l v e n t s on t h e c h a i n m i c r o s t r u c t u r e o f p o l y d i e n e s , t h e most t h o r o u g h l y i n v e s t i g a t e d s y s t e m s h a v e b e e n those i n v o l v i n g o r g a n o l i t h i u m i n i t i a t o r s , s i n c e these operate i n a v a r i e t y o f s o l v e n t s , b o t h p o l a r and n o n - p o l a r , a n d c a n be con­ t r o l l e d t o u n d e r g o l i t t l e o r no s i d e r e a c t i o n s . Furthermore, i n n o n - p o l a r media, such systems y i e l d p o l y b u t a d i e n e s and p o l y i s o p r e n e s h a v i n g a v e r y h i g h (>90%) 1,4 u n i t c o n t e n t , a n d t h e s e a r e important s y n t h e t i c rubbers. M o s t o f t h e d a t a r e f e r r e d t o above w e r e o b t a i n e d i n e a r l i e r work, and were b a s e d o n i n f r a r e d s p e c t r o s c o p y . I n r e c e n t y e a r s , more r e l i a b l e d a t a were o b t a i n e d b y means o f NMR s p e c t r o s c o p y , using both H and C r e s o n a n c e s ( 9 - 1 4 ) . Some o f t h e s e i n v e s t i g a ­ t i o n s suggested t h a t , a s i d e from the dramatic e f f e c t s o f p o l a r s o l v e n t s on t h e c h a i n s t r u c t u r e i n o r g a n o l i t h i u m systems, t h e r e were some s u b t l e e f f e c t s e v e n i n n o n - p o l a r m e d i a , e.g., c a u s e d b y i n i t i a t o r c o n c e n t r a t i o n and type o f non-polar s o l v e n t . S i n n and c o ­ w o r k e r s ( 1 5 ) , f o r e x a m p l e , u s e d i n f r a r e d s p e c t r o s c o p y t o show a n e f f e c t o f b u t y l l i t h i u m c o n c e n t r a t i o n on t h e c h a i n s t r u c t u r e o f p o l y i s o p r e n e a n d p o l y b u t a d i e n e . Hence a n e x t e n s i v e s t u d y was c a r r i e d o u t r e c e n t l y ( 1 3 , 1 4 ) o n t h e i n f l u e n c e o f r e a c t i o n param­ e t e r s on the c h a i n structure" o f p o l y b u t a d i e n e and p o l y i s o p r e n e prepared i n non-polar media. 1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch021

POLYMERIZATION

1 3

Experimental A l l s p e c t r a were o b t a i n e d w i t h t h e V a r i a n HR-300 NMR S p e c t r o m ­ e t e r , u s i n g H i n n o r m a l mode, w i t h o c c a s i o n a l u s e o f F o u r i e r t r a n s f o r m f o r v e r y h i g h m o l e c u l a r weight samples. Hexachlorobutad i e n e was u s e d a s s o l v e n t , w i t h 1% h e x a m e t h y l d i s i l o x a n e a s r e f e r ­ e n c e . The t e m p e r a t u r e s u s e d were 110°C. f o r p o l y i s o p r e n e a n d 125°C. f o r p o l y b u t a d i e n e . The p o l y m e r s a m p l e s were p r e p a r e d w i t h s e c - b u t y l l i t h i u m a s i n i t i a t o r , u s i n g t h e h i g h vacuum t e c h n i q u e s d e s c r i b e d e l s e w h e r e (13). NMR p e a k a s s i g n m e n t s were a s f o l l o w s . For polyisoprene, t h e 3 , 4 - u n i t c o n t e n t was d e t e r m i n e d f r o m t h e o l e f i n i c m e t h y l e n e p r o t o n s a t 4.61 a n d 4.67 ppm, w h i l e t h e t r a n s - 1 , 4 u n i t s w e r e d e t e r m i n e d f r o m t h e m e t h y l p r o t o n r e s o n a n c e a t 1.54 ppm. The c i s - 1 , 4 c o n t e n t was t h e n c a l c u l a t e d b y d i f f e r e n c e . No 1,2 o l e f i n i c m e t h y l e n e p r o ­ t o n s c o u l d be s e e n a t 5,4 ppm, where t h e y w o u l d b e e x p e c t e d . F o r p o l y b u t a d i e n e , t h e 1 , 2 - u n i t c o n t e n t was c a l c u l a t e d f r o m a c o m p a r i ­ son o f t h e 1 , 2 - o l e f i n i c m e t h y l e n e p r o t o n s a t 4.δ ppm w i t h t h e 1,4m e t h i n e p r o t o n s a t 5.4 ppm. The c i s / t r a n s r a t i o was t h e n computed f r o m t h e 1 , 4 - m e t h y l e n e p r o t o n s a t 1.98 a n d 2.03 ppm. These methods made i t p o s s i b l e t o e s t i m a t e t h e c h a i n u n i t s t r u c t u r e s w i t h i n a b o u t 1%. 1

R e s u l t s and D i s c u s s i o n The r e a c t i o n v a r i a b l e s i n v e s t i g a t e d w e r e : a ) i n i t i a t o r c o n c e n ­ t r a t i o n , b ) monomer c o n c e n t r a t i o n , c ) t y p e o f s o l v e n t ( i . e . ,

21.

MORTON AND RUPERT

Isomeric Chain

Unit

285

Structure

b e n z e n e , n-hexane o r c y c l o h e x a n e ) , d ) t e m p e r a t u r e , a n d e ) d e g r e e of conversion. P o l y m e r i z a t i o n s were c a r r i e d o u t g e n e r a l l y a t room t e m p e r a t u r e t o c o m p l e t e ( o r n e a r c o m p l e t e ) c o n v e r s i o n e x c e p t when t h e e f f e c t s o f t h e s e two v a r i a b l e s w e r e b e i n g s t u d i e d .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch021

Polyisoprene E f f e c t o f I n i t i a t o r a n d Monomer C o n c e n t r a t i o n . The e f f e c t o f i n i t i a t o r c o n c e n t r a t i o n ( s - b u t y l l i t h i u m ) and s o l v e n t s on t h e c h a i n s t r u c t u r e o f p o l y i s o p r e n e i s shown i n T a b l e I . The f o l l o w i n g c o n c l u s i o n s c a n b e drawn f r o m t h e s e r e s u l t s : 1. The 3,4 c o n t e n t i s a f f e c t e d v e r y l i t t l e , i f a t a l l , b y t h e i n i t i a t o r o r monomer c o n c e n t r a t i o n , o r b y the type o f s o l v e n t present. 2. The p r e s e n c e o f s o l v e n t d e c r e a s e s t h e c i s - 1 , 4 c o n t e n t , an aromatic s o l v e n t l i k e benzene h a v i n g a g r e a t e r e f f e c t than an a l i p h a t i c solvent l i k e n-hexane. 3. A d e c r e a s e i n i n i t i a t o r c o n c e n t r a t i o n i n c r e a s e s the c i s - 1 , 4 c o n t e n t , i n t h e absence o f s o l v e n t , o r even i n t h e p r e s e n c e o f n-hexane. Table I E f f e c t o f I n i t i a t o r Concentration and Solvents on C h a i n S t r u c t u r e o f L i t h i u m P o l y i s o p r e n e ( P o l y m e r i z a t i o n t e m p e r a t u r e = 20°C) Solvent

M i c r o s t r u c t u r e - m.ol.%

[s-Ci+HgLi]

cis-1,4 Benzene* Benzene* n-Hexane* n-Hexane* None None *

9x10" 4x101x101x103x108x10"

3

5

2

5

3

6

69 70 70 86 77 96

trans-1,4

3,4

25 24 25 11 18 0

6 6 5 3 5 4

[Monomer] = 0.5 M.

Thus i t a p p e a r s t h a t l o w c o n c e n t r a t i o n s o f l i t h i u m , i n g e n e r a l , enhance t h e f o r m a t i o n o f t h e c i s - 1 , 4 s t r u c t u r e , a t t h e e x p e n s e o f the t r a n s - 1 , 4 s t r u c t u r e , and t h a t t h i s e f f e c t i s most n o t i c e a b l e i n t h e p o l y m e r i z a t i o n o f t h e u n d i l u t e d monomer. However, i n t h e p r e s e n c e o f b e n z e n e , no s u c h e f f e c t c a n b e s e e n , p r e s u m a b l y b e c a u s e t h e " s o l v e n t e f f e c t " o f t h i s a r o m a t i c compound c a n c o u n t e r the e f f e c t o f the l i t h i u m concentration. This i s discussed further i n a l a t e r section.

286

INITIATION OF

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E f f e c t o f Conversion. The e f f e c t o f i n c r e a s i n g d e g r e e s o f conversion on the chain s t r u c t u r e i n p o l y m e r i z a t i o n o f u n d i l u t e d i s o p r e n e i s shown c l e a r l y i n T a b l e I I . A s c a n b e s e e n , t h e e x t e n t Table I I E f f e c t o f C o n v e r s i o n on C h a i n S t r u c t u r e of Lithium Polyisoprene (No

s o l v e n t , p o l y m e r i z a t i o n temp. = 20°C. [ s - C ^ H g L i ] = 1 0 "

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Conversion

(%)

M i c r o s t r u c t u r e - mol.

13

29 40 46 48 86

5

M. )

%

Cis-1,4

Trans-1,4

93 92 92 92 95 95

2 3 2 3

6

1 1

4 4

3,4

5 5 5

o f c o n v e r s i o n o f monomer t o p o l y m e r h a s no n o t i c e a b l e e f f e c t o n the chain s t r u c t u r e . Since t h i s i s a non-terminating chain growth r e a c t i o n , t h i s means t h a t e a c h c h a i n m a i n t a i n s a c o n s t a n t c h a i n s t r u c t u r e as i t grows. E f f e c t o f T e m p e r a t u r e . T a b l e I I I shows t h e e f f e c t o f p o l y m e r i z a t i o n temperature on the c h a i n s t r u c t u r e o f l i t h i u m p o l y i s o p r e n e , b o t h i n t h e c a s e o f u n d i l u t e d monomer a n d i n t h e p r e s e n c e o f n-hexane a s s o l v e n t . W i t h i n t h e r a n g e s shown, t h e r e does n o t appear t o be any i n f l u e n c e o f temperature o n the placement o f t h e various isomeric chain unit structures. Table I I I E f f e c t o f P o l y m e r i z a t i o n Temperature on Chain S t r u c t u r e o f L i t h i u m Polyisoprene Temp. ( ° C )

46 20 0 -25 40 -25

*

[s-Ci+HgLi]

1x10" 1x10" 1x10" 1x10" 1x10" 1x10"

5

5

5

5

3

3

[Monomer] = 2.5 M.

Solvent

None None None None n-hexane* n-hexane*

M i c r o s t r u c t u r e - mol. Cis-1,4

Trans-1,4

3,4

95 95 95 93 76 78

1 1 2 3 18 18

4 4 3 4 6 4

%

21.

MORTON AND RUPERT

Isomeric Chain

Unit

287

Structure

Polybutadiene E f f e c t o f I n i t i a t o r C o n c e n t r a t i o n and S o l v e n t s . In t h i s c a s e , t h r e e d i f f e r e n t s o l v e n t s were used t o c a r r y out t h e p o l y m e r i z a t i o n , i n a d d i t i o n t o t h e u s e o f u n d i l u t e d monomer, as shown i n T a b l e IV. I t i s w e l l known t h a t t h e o r g a n o l i t h i u m Table

IV

E f f e c t o f I n i t i a t o r C o n c e n t r a t i o n and S o l v e n t s on C h a i n S t r u c t u r e o f L i t h i u m P o l y b u t a d i e n e ( P o l y m e r i z a t i o n Temp. = 20°C)

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch021

Solvent

Microstructure -

[s-Ci+HgLi]

Cis-1,4 Benzene* Cyclohexane* n-Hexane* n-Hexane* None None

*

[Monomer] = 0.5

8x10" 1x10" 2x103x107x10" 3x10"

6

5

5

2

6

3

52 68 56 30 86 39

Trans-1,4

mol. % 1,2 12 4 7 8 5 9

36 28 37 62 9 52

M.

p o l y m e r i z a t i o n o f b u t a d i e n e does n o t y i e l d as h i g h a c o n t e n t o f c i s - 1 , 4 u n i t s as i n t h e c a s e o f i s o p r e n e . I n f a c t , under p r a c t i c a l p o l y m e r i z a t i o n c o n d i t i o n s i n i n d u s t r y ( [ R L i ] v L O " ) , the t r a n s - 1 , 4 c o n t e n t i s g e n e r a l l y g r e a t e r t h a n 50$. However, t h e d a t a i n T a b l e I V show e x a c t l y how t h e s e r e a c t i o n p a r a m e t e r s a f f e c t t h e c h a i n s t r u c t u r e , as f o l l o w s : 1. A g a i n , as i n t h e c a s e o f i s o p r e n e , t h e s i d e - v i n y l content ( 1 , 2 - u n i t s ) i s not g r e a t l y a f f e c t e d e i t h e r by the i n i t i a t o r c o n c e n t r a t i o n or the presence of solvent. I t does seem t o be i n c r e a s e d s l i g h t l y b y h i g h e r i n i t i a t o r l e v e l s , and a p p a r e n t l y i n c r e a s e s n o t i c e a b l y i n the presence of benzene. 2. Compared t o t h e c a s e o f i s o p r e n e , t h e c i s - 1 , 4 c o n t e n t changes d r a m a t i c a l l y as t h e i n i t i a t o r concentration i s decreased, e s p e c i a l l y i n the absence o f s o l v e n t s , where t h e c i s - 1 , 4 content i s shown t o r i s e f r o m 39$ t o 86$. This also happens i n t h e p r e s e n c e o f s o l v e n t s , a l t h o u g h n o t t o as g r e a t an e x t e n t . 3

I t i s i n t e r e s t i n g t o n o t e t h a t a r e v i e w o f t h e l i t e r a t u r e shows no r e f e r e n c e t o the s u c c e s s f u l s y n t h e s i s o f p o l y b u t a d i e n e , by o r g a n o l i t h i u m i n i t i a t o r s , w i t h a c i s - 1 , 4 c o n t e n t as h i g h as 86$

288

INITIATION OF POLYMERIZATION

a l t h o u g h t h e t r e n d t o w a r d h i g h e r v a l u e s t h a n 50$ h a s b e e n shown. T h i s i s p e r h a p s n o t s u r p r i s i n g , s i n c e t h e a t t a i n m e n t o f 86$ c i s 1,4- was o n l y f o u n d p o s s i b l e i n t h i s w o r k a t e x t r e m e l y l o w i n i t i ­ a t o r c o n c e n t r a t i o n s ( isoprenyl end group > styrenyl end group > terminated. Based on coupling experiments with chlorosilanes, i t is concluded that higher aggregates exist for polydienes, while polystyryllithium molecules are mostly dimeric. The cross-association between active polymer molecules and alkyllithium molecules forming mixed aggregates is demonstrated and their possible roles in affecting propagation rates and cross-propagation rates are suggested. It is fairly well understood that alkyllithiums form rather stable aggregates in which carbon-lithium bond order is maximized by the utilization of all valence orbitals of lithium.(1) Polystyryl lithium molecules are mostly dimeric in solution.(2.»3,4) This has been generally accepted by all the investiaators.(5j However, association numbers of two(D and four(Z>8J has been reported for polydienes. Two methods were used in determining these values, v i z . , light scattering measurements (in vacuo) and viscosity measurements. In the late fifties and early sixties, we, at the Phillips R&D laboratories, had an extensive research program on the copolymerization of dienes and styrene by direct reaction of the initiator with the monomer mixtures as well as by the incremental addition of monomers.(9.) The decrease in solution viscosity was qualitatively apparent in many cases when the active dienyl chain ends were converted to styryl chain ends. Until recently, how0097-6156/83/0212-0291$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

292

INITIATION OF POLYMERIZATION

ever, we did not have quantitative measurements to substantiate this observation. Dynatrol Viscosity Systems are designed for continuous mea­ surement of viscosity in process streams. Dynatrol Viscosity Detectors are i n s t a l l e d d i r e c t l y in process vessels without the need for sampling or analysis. Response i s immediate and contin­ uous. U t i l i z i n g this system, we carried out a series of experi­ ments to study the change in viscosity of polymer solutions when we introduced a change in polymer chain ends.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

Experimental Reactor. The polymerization tests were carried out in a stainless steel reactor with approximately 7 . 6 - l i t e r (2-gallon) capacity. The reactor was b u i l t by Bench Scale Equipment, Dayton, Ohio, s p e c i f i c a l l y for P h i l l i p s R&D to be used in anionic poly­ merization studies. The monomer and solvent tanks are connected d i r e c t l y to the reactor and form a closed system. The weights of monomers and solvent tanks can be read d i r e c t l y and the errors are less than ±1% (±40g.) for the solvent and ±0.1g. for the monomers. For this study, a nitrogen pressure of about 350 k.Pa (50 psig) in the reactor was maintained and the impeller mixing speed was 300 r.p.m.. The temperature was controlled by the automatically regu­ lated steam pressure in the reactor jacket. Materials. P h i l l i p s polymerization grade cyclohexane and butadiene were used. Styrene was a commercial polymerization grade. Solvent was dried over activated Alcoa H151 alumina, and monomers were dried over activated Kaiser 201 alumina before they were transferred to the charge tanks. η - B u t y l l i t h i u m and secbutyl lithium were purchased from Lithium Corporation of America. Chlorosilanes were vacuum d i s t i l l e d before use. Viscosity System. A Dynatrol viscosity system was purchased from Automation Products, Inc., Houston, Texas. The detector (Type CL-10DVT-4) i s inserted in the vessel with the probe f u l l y immersed in the process medium. The drive coil i s excited at a frequency of 120 cps. This produces a pulsating magnetic f i e l d which causes the drive armature to vibrate at a frequency of 120 cps. Mechanical vibration of the drive armature i s transferred along the attached spring rod, through a welded node point to the probe. The probe i s driven into mechanical vibration at the same 120 cps frequency. The amplitude of the vibration of the probe depends upon the viscosity of the process medium. The mechanical vibration of the probe is transferred through a second welded node point along the upper spring rod to the pick­ up end of the detector. The pick-up end consists of an armature and coil arrangement which i s similar to that of the driver end; one exception being that the stator of the pick-up c o i l i s a per-

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

22.

HSIEH AND KITCHEN

Alkyllithium

Initiated

Polymers

293

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

manent magnet. The vibration of the pick-up armature in the f i e l d of this permanent magnet induces a 120 cps A-C voltage in the pick-up coil which i s proportional in magnitude to the amplitude of the pick-up armature vibration. Since the pick-up armature i s being driven by the probe, the magnitude of the voltage generated in the pick-up coil i s a measure of the viscosity of the process media. The detector we used had a range of 10 to 1000 c e n t i poises. The 120 cps output signal from the detector i s fed to the converter, where i t i s converted into a 0-10 MVDC signal compat­ i b l e with 0-10 MVDC recorder. Span and zero controls are located at the convertor. The laboratory set-up i s shown in Figure 1. Procedure. Cyclohexane solvent (3.8 kg.) was introduced to the reactor f i r s t , heated to 50°C and 338 grams of monomer then added. I n i t i a t o r was added at 50°C and the polymerization a l ­ lowed to proceed adiabatically until a peak temperature (65°C to 70°C) had been observed. This generally took about 30 to 40 minutes. A second increment of the monomer (338 grams) was added and allowed to polymerize to quantitative conversion. Peak tem­ perature was reached generally in 10-15 minutes. Cooling was ap­ plied slowly with constant mixing to 78°C. This generally took 20 additional minutes. This temperature was then held constant (±0.2°C) by controlling the steam pressure in the reactor jacket. The response of the viscometer probe immersed in the polymer solu­ tion was recorded continuously. A typical response from the v i s ­ cometer during the polymerization and the termination of the ac­ tive ends i s shown in Figure 2. Extreme care was taken to main­ tain a constant temperature, pressure, and solids concentration while recording the viscometer response. For example, when mono­ mers were used for capping, appropriate amounts of solvent were introduced at the same time to maintain the solids concentration. Because the entire set-up i s a closed system, the control and reproducibility of the polymerization are remarkably good. Since the polymer solution can be drained and rinsed out by the dried solvent from the bottom of the reactor, the closed system rarely needs to be opened and exposed to the atmosphere. For this study, the overall scavenger levels (the difference between levels of added RLi and effective RLi) were in the range of less than 10-4, but more than 10-5 mole/liter,representing 2 to 6% of the total i n i t i a t o r added. Typically, the alkyllithium i n i t i a t o r concentra­ tion i s around 1 χ 10-3 mole/liter. The response of the viscometer represents a relative change. An increase (+) in response (a higher number) indicates that the vibration of the probe has been dampened by a more viscous solu­ t i o n . A decrease (-) in response (a lower number) indicates a less viscous solution. The Dynatrol system has shown i t s e l f to be r e l i a b l e , sensitive, and provides reproducible information. This i s true when the conditions are carefully controlled.

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

294

INITIATION OF POLYMERIZATION

ο 0

SPAN

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

ZERO

CONVERTER 4-20 MA DC

REACTOR

VISCOMETER PROBE

Figure 1.

Figure 2.

ELECTROPNEUMATIC TRANSDUCER 3 - 1 5 PSI

• RECORDING CHART

Dynatrol viscometer installation.

A typical viscosity response curve.

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

22.

HSIEH AND KITCHEN

Alkyllithium Initiated Polymers

295

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

Results and Discussion In the f i r s t series of experiments, polybutadiene samples of about 110,000 molecular weight were prepared in cyclohexane solu­ tion at 15% concentration. The viscosity response of the solu­ tions was continuously measured and changes were recorded when terminated with isopropyl alcohol or when capped with isoprene or styrene (Table I ) . In this table and a l l subsequent tables, B* and S* mean active ("living") polybutadiene and polystyrene mole­ cules, respectively. Terminated polymers are denoted simply as Β and S. BS* and BI* mean polybutadiene molecules with active sty­ rene and isoprene end groups (capped), respectively. SB* and SI* mean polystyrene molecules with active butadiene and isoprene end groups (capped), and BSB* means polybutadiene i n i t i a l l y capped with styrene was capped again with butadiene, etc. To convert B* completely to BS* i s d i f f i c u l t due to an unfavorable cross-propa­ gation rate.(5.) We found that by using 5% of the styrene monomer and adding i t in 8 increments resulted in the maximum observed changes. In the same series of experiments, a polybutadiene sam­ ple of half of the molecular weight was also prepared for compari­ son purposes (Table I ) . Figures 3, 4, and 5 show typical v i s ­ cosity response charts. They correspond to the data in Table I. After the completion of polybutadiene experiments, similar experiments were carried out on polystyrene. The results are shown in Table II and Figures 6 and 7. They are consistent with the findings of similar experiments based on polybutadiene. The viscosity i s very different depending upon the nature of the end group. This i s true whether the polymer i s polybutadiene or polystyrene. Coupling reactions of polybutadienyllithium (B*) with chlorosilanes are well known and well established(10.) and are used for the production of controlled long-chain branched polymers. (JJL) In Table I I I , the peak molecular weight and viscosity response change, before and after termination and coupling reactions, are reported (also see Figures 8-12). We used mono-, d i - , t r i - , and tetrachlorosilanes to produce l i n e a r , t r i chain and tetrachain polybutadiene molecules. Generally, about 2-8% of the precursor, for reasons such as premature terminations, incomplete l i n k i n g , e t c . , remained uncoupled. Therefore, the peak molecular weight from the GPC curves was used here to i l l u s t r a t e the degree of coupling. Figure 13 shows the GPC curves of the precursor and SiCl4-coupled product which includes the minor amount (~6%) of the uncoupled (precursor) material. The presence of such small amounts of the uncoupled material should have very l i t t l e effect on the solution viscosity. It i s significant that the viscosity of the Si d e c o u p l e d polymer did not change within experimental error, in spite of the four-fold increase in actual molecular weight. The R2 SiCl2-coupled polymer with doubling the actual molecular weight of the precursor dropped the viscosity response by 19 units. This decrease in viscosity response i s nearly the

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

296

INITIATION OF POLYMERIZATION

TABLE I VISCOSITY RESPONSE CHANGE OF POLYBUTADIENE 3

5

BY TERMINATION

WITH ISOPROPYL ALCOHOL OR CAPPING WITH ISOPRENE OR STYRENE c

Viscosity Response Change

Action

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

Bj* + ROH

—+· B

-30

l

Βχ* + I

—• Β Ι*

-10

Bi* + S

— • BjS*

-20

ΒχΙ*

-12

χ

'Βχ* + I l ΒχΙ* + Β

+9

—-+ ΒχΙΒ*

ΒχΙΒ* + ROH — + ΒχΙΒ

-30

Βχ* + S

-21

- — • BjS*

Bj^S* + ROH

S* + B

B2* + ROH

-10

-> BiS

Βχ* + S B l

d

B^*

-19

B^B*

+14

---+ B

-30

2

Βχ* versus B *

-21

2

a.

Span 550, zero 420

b.

(Mw/Mn) χ 10" = 111/103 for B\, a typical value 61/57 for B 3

2

c.

About 5% in 2 increments

d.

About 5% in 8 increments

{

Denotes a single experiment

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

22.

HSIEH A N D

KITCHEN

UJ100 CO Ο Û.

CO UJ

oc

Initiated

60

297

Polymers

ALCOHOL TERMINATION

70

H OC
BS* BS).

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

298

INITIATION OF POLYMERIZATION

TABLE II VISCOSITY RESPONSE CHANGE OF POLYSTYRENE BY CAPPING WITH ABOUT 256 BUTADIENE OR ISOPRENE

Experiment

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

A A

Viscosity Response Change

Action

9

S* + B S* + I

— + SI*

Β Β Β Β

ÎS* + B ISB* + ROH [S* + I ^SI* + ROH

— * SB* — + SB —-»· SI* 4- SI

c

fs* + B ISB* + ROH fs* + I ι SI* + ROH

+ * + +

C C C

+30 +17 +39 -68 +22 -46 +17 -35 +9 -25

—-»• SB*

SB* SB SI* SI

Average a.

Experiment

Span

Zero

A B C

550 550 550

490 830 965

Initiation

TWWn χ 10-

3

157/140 116/109 149/129

n-BuLi sec-BuLi sec-BuLi

{

Denotes a single experiment

ALCOHOL TERMINATION

Figure 6. Viscosity response curve of butadiene-capped polystyrene (S* -> SB* -+SB).

10

20

30

MINUTES

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

40

22.

Alkyllithium

HSIEH AND KITCHEN

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

100 90 80 70 60 50 40 30 20 10 0

Figure 7.

Initiated

ISOPRENE ADDED

ALCOHOL TERMINATION

/

20 30 MINUTES

10

299

Polymers

40

50

SI*

Viscosity response curve of isoprene-capped polystyrene (S*

TABLE III VISCOSITY RESPONSE CHANGE OF POLYBUTADIENE BY TERMINATION 3

WITH H 0 OR COUPLING WITH CHLOROSILANES& 2

(Peak Mol.Wt.)C χ 10" Betore After 3

Action

Viscosity Response Change

-

118

-68

B* + S i C U

110

431

-2

B* + MeSiCl3

113

283

-5

134

257

-19

113

117

-64

B* + H 0 2

B* + Me Si Cl 2

B* + Me SiCl 3

2

a.

Span 550, zero 420

b.

Added in 4 increments - 60%, 20%, 20% and 10% of the calculated stoichiometric amount to allow maximum coupling

c.

By GPC

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

300

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

INITIATION OF

Figure 9.

Viscosity response curve of polybutadiene coupled with chlorotrimethylsilane.

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

22.

HSIEH AND KITCHEN

Alkyllithium

100 UJ

(Me) SiCI 2

on

/

CO

Ο

S ce

oc
(sec-BuLi)^ > (n-BuLi)*. It is also known that the rate of propagation is in the order: styrene > isoprene > butadiene. The possible relationship between the rate and degree of aggregation cannot be ignored. The differences in aggregation may also be the key to the mechanism of copolymerization. The so-called "inversion" behavior in copolymer!zations of diene and styrene may well be caused by the differences in degrees of aggregation, which in turn, control the cross-propagation rates. Literature Cited 1. 2. 3. 4.

Brown, T.L. in "Advances in Organometallic Chemistry", Volume 3, F. G. A. Stone; R. West Ed. Academic Press, New York, N.Y., 1965, p. 365. Morton, M.; Bostick, E.E.; Livigni, R. Rubber and Plastics Age 1961, 42, 397. Morton, M.; Fetters, L.J. J. Polym. Sci. 1964, A2, 3311. Johnson, A . F . ; Worsfold, D.J. J. Polym. Sci. 1965, A3, 449.

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

306 5. 6. 7. 8.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

9. 10. 11. 12. 13. 14. 15. 16. 17.

INITIATION OF POLYMERIZATION

Hsieh, H.L.; Glaze, W.H. Rubber Chem. & Technol. 1970, 43, No. 1, 22. Morton, M.; Fetters, L.J.; Bostick, E.E. J. Polym. Sci. 1963, C1, 311. Worsfold, D.J.; Bywater, S. Can. J. Chem. 1964, 42, 2884. Sinn, H.; Lundord, C.; Onsanger, O.T. Macromol. Chem. 1964, 70, 222. Hsieh, H.L. Rubber and Plastics Age 1965, 46, No. 4, 394. Zelinski, R.P.; Wofford, F.F. J. Polym. Sci. 1965, A3, 93. Hsieh, H.L. Rubber Chem. & Technol. 1976, 49, No. 5, 1305. Fetters, L.J.; Morton, M. Macromolecules 1974, 1, 552. Al-Harrah, H.M.F.; Young, R.N. Polymer 1980, 21, 119. Szwarc, M. J. Polym. Sci., Polym. Lett. Ed. 1980, 18, 499. Fetters, L.J.; Morton, M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 199. Hsieh, H.L. J. Polym. Sci. 1965, A3, 153· Selman, C.M.; Hsieh, H.L. Polym. Lett. 1971, 9, 219.

RECEIVED September 28, 1982

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

The Nature of Stereochemical Control in MetalCatalyzed Butadiene Polymerization

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

L. M. STEPHENSON and C. A. K O V A C University of Southern California, Hydrocarbon Research Institute, Los Angeles, CA 90089

Polymerization of c i s , cis-1,4-dideuterio-1, 3-butadiene by several transition metal catalysts has been studied. The existence of non-stereospecific bond forming events is postulated to signal the involvement of a l l y l isomerization in the polymerization mechanism. Trans-1,4-polymers are accompanied by complete scrambling of deute­ rium stereochemistry, contrasting with a more specific process to form cis polymers. Allyl isomerization is thus implicated as a key event in the formation of trans, but not c i s , polymer.

A long sought goal in mechanistic polymer chemistry has been the determination of those factors which lead to c i s , trans or vinyl structures in diene polymers. Various proposals have been made and are summarized in the comprehensive review^ edited by Saltman . The simplest proposal, advanced by Cossee and Arlman^, assigns the dominant role to the nature of the diene coordination. In this mechanism bidentate coordination, e.g. 1, of necessity involving the cisoid conformation of the diene, would lead to cis polymer. Ρ

1

0097-6156/83/0212-0307$06.00/0 © 1983 American Chemical Society

308

INITIATION OF

POLYMERIZATION

Trans polymer backbone i n the Cossee-Arlman scheme was then the r e s u l t of monodentate c o o r d i n a t i o n , e.g. 2, with the more s t a b l e t r a n s o i d conformation adopted.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

2

Kormer, Lobach and o t h e r s ^ were the f i r s t t o suggest an a l t e r n a t i v e process which assigned a dominant r o l e t o a l l y l i s o m e r i z a t i o n . In t h i s mechanism the process begins with b i d e n t a t e c o o r d i n a t i o n , followed by formation of an a l l y l complex. I f t h i s a l l y l complex has a s u f f i c i e n t l i f e t i m e , i t w i l l isomerize from the l e s s s t a b l e , i n i t i a l l y formed, a n t i a l l y l 3 t o the more s t a b l e syn s p e c i e s , As shown below, £ leads t o trans polymer. On the other hand the propagation steps, which i n c l u d e c o o r d i n a t i o n of diene and formation of the next bond i n the chain, could be the f a s t steps. Polymer with a c i s backbone would then be the r e s u l t .

Studies of polymerizing systems and models, p a r t i c u l a r l y using nuclear magnetic resonance techniques, have provided support f o r both p o i n t s of view-*. In t h i s c o n t r i b u t i o n we describe the r e s u l t s of our attempts to devise a chemical probe f o r the involvement of a l l y l i s o m e r i z a t i o n during the p o l y m e r i z a t i o n r e a c t i o n . A c c o r d i n g l y , i t w i l l be appropriate t o b r i e f l y review the key aspects of i s o m e r i z a t i o n i n t r a n s i t i o n metal a l l y l s .

STEPHENSON AND KOVAC

23.

The Nature of A l l y l

Butadiene

Polymerization

309

Isomerization

Work from the group of F a l l e r , i n p a r t i c u l a r , has demon­ s t r a t e d that the most general mechanism f o r a l l y l i s o m e r i z a t i o n proceeds by r o t a t i o n about carbon-carbon bonds i n -allyls (also c a l l e d σ-allyls).

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

antiΤΓ-allyl

T* -

ft* — Ο

Μ

Ρ

Ρ

secondaryCT-allyl

M

primary(T-allyl

A deuterium l a b e l at the primary p o s i t i o n could thus be used as a very s e n s i t i v e probe f o r a l l y l i s o m e r i z a t i o n . Forma­ t i o n of the primary σ-allyl would be expected to scramble any i n i t i a l c o n f i g u r a t i o n of the l a b e l to a random mixture of syn and a n t i deuterium. H

>^^^^ M

^

H

\ Ή '

rotation '

M

310

INITIATION OF

POLYMERIZATION

We^ have p r e v i o u s l y used t h i s method to e s t a b l i s h a l l y l i s o m e r i z a t i o n events i n n i c k e l catalyzed c y c l o d i m e r i z a t i o n of butadiene, and describe here our a p p l i c a t i o n to the polymerizat i o n mechanism. Results

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

The use of c i s , c i s - 1, 4-dideuterio-l,3-butadiene, 8, i n a polymerization r e a c t i o n w i l l provide d i r e c t information about the stereochemistry of the bond forming r e a c t i o n . We have

0

p r e v i o u s l y developed an a n a l y s i s f o r t h i s process, i l l u s t r a t e d above. Oxidative conversion of polymer to s u c c i n i c a c i d or anhyd r i d e , 9, leads to systems which allow the dl/meso r a t i o to be assessed easily**. Our p r e l i m i n a r y a n a l y s i s of t h i s r e a c t i o n a l s o revealed that monomer i s o m e r i z a t i o n to c i s , t r a n s - and t r a n s , trans-diene a l s o occurred. Thus each r e a c t i o n a l s o r e q u i r e d an a n a l y s i s of recovered s t a r t i n g m a t e r i a l ^ . A previous study by P o r r i and A n g l i e t t o ^ , reported while t h i s work was i n progress, employed t h i s same method. Close i n s p e c t i o n of t h i s work r e v e a l s that these authors d i d not take i n t o account the p o s s i b i l i t y of monomer i s o m e r i z a t i o n . Thus, t h e i r i n t e r p r e t a t i o n i s l e s s comp l e t e than that presented below. ^ Two c a t a l y s t systems, R h C l and a l l y l n i c k e l i o d i d e which give high trans polymer were examined and found to give i d e n t i c a l , stereorandom r e s u l t s . Examination of recovered monomer shows no i s o m e r i z a t i o n ; the stereorandomness of the bond forming event must be inherent to the mechanism. 1

1 2

3

23.

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Butadiene

Polymerization

311

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

C a t a l y s t s which lead to c i s polymer show a s i g n i f i c a n t l y higher s t e r e o s e l e c t i v i t y i n the bond forming r e a c t i o n . When (π-allyl n i c k e l i o d i d e ) 2 modified with T i C l ^ ^ ^ i s employed, the 1, 2 deuterium stereochemistry i s 70% d l , and 30% mesο as r e ­ vealed by a n a l y s i s of s u c c i n i c anhydride. In a d d i t i o n , monomer i s o m e r i z a t i o n i s extensive, and could account f o r a l a r g e f r a c ­ t i o n of the mesο s t r u c t u r e s which are formed, j^he use of (π-allyl n i c k e l t r i f l u o r o a c e t a t e ) 2 as c a t a l y s t l e d to a s i m i l a r r e s u l t (32% meso), accompanied by l i t t l e i f any monomer isomer­ ization. Thus, i t appears that i n r e a c t i o n s to form c i s polymer, some, but not always a l l , of the stereochemical information pres­ ent i n the s t a r t i n g diene i s preserved i n the polymer. In con­ t r a s t , none of the i n i t i a l diene stereochemistry can be detected i n the trans polymer. Discussion While s e v e r a l explanations may be given f o r these f i n d i n g s , we focus here on our contention that t h i s r e s u l t supports the im­ portance of a l l y l i s o m e r i z a t i o n i n diene polymerization. We p o s t u l a t e i n i t i a l l y that the l o s s of bond forming s t e r e o s p e c i f i c i t y i n the diene polymerization i s the r e s u l t of a l l y l isomer­ i z a t i o n . T h i s i s p a r t i c u l a r l y w e l l supported by the completely stereorandom bond formation found f o r trans polymer. T h i s i s s a t i s f a c t o r i l y accounted f o r by r a p i d i s o m e r i z a t i o n of the l a b e l , v i a primary a l l y l s , to a random(l:l) mixture of syn and a n t i deuterium. Any other mechanism f o r randomization, f o r example

H

D

Η

one which allows s e v e r a l organometallic conformers to be involved i n the r e a c t i o n , would be u n l i k e l y to r e s u l t i n 1:1 r a t i o s ; i t would be e x c e p t i o n a l l y u n l i k e l y to f i n d the same r a t i o from both trans forming c a t a l y s t s as we do. With t h i s assumption we can then s t a t e that a l l y l i s o m e r i z a t i o n v i a the primary σ-allyl i s slow i n those systems which lead to c i s polymer. With the added knowledge from F a l l e r s work^ that i s o m e r i z a t i o n v i a secondary a l l y l s i s some ten times slower, we would a n t i c i p a t e e s s e n t i a l l y no involvement of such species i n c i s cases. A unique mechanism i s shown i n Figure 1. T h i s scheme s t a r t s with an a l l y l / d i e n e complex w i t h both the growing polymer chain and the deuterium l a b e l i n an a n t i c o n f i g u r a t i o n i n i t i a l l y . Rapid a l l y l i s o m e r i z a t i o n ( l e f t hand branch) leads to trans back­ bone and stereorandom deuterium; r a p i d bond formation ( r i g h t hand 1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

312 INITIATION OF POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

23.

STEPHENSON AND KOVAC

Butadiene Polymerization

313

branch) r e t a i n s the c i s backbone and leads to s t e r e o s p e c i f i c placement of deuterium. Our i n t e r p r e t a t i o n most e a s i l y lends i t s e l f to the idea that the l a c k of complete s t e r e o s p e c i f i c i t y i n the formation of c i s polymer i s due to some i s o m e r i z a t i o n v i a primary a l l y l s . With the r e l a t i v e l y low l e v e l s of i s o m e r i z a t i o n seen, we would expect that backbone i s o m e r i z a t i o n o c c u r r i n g v i a secondary a l l y l s would be e s s e n t i a l l y unobservable. Indeed l i t t l e trans polymer i s seen, i n our two predominantly c i s examples. We cannot exclude the p o s s i b i l i t y that conformers other than those shown i n the r i g h t hand branch of f i g . 1 are i n v o l v e d i n the r e a c t i o n . Some of these could lead to mesο u n i t s i n the polymer; t h i s feature prevents us from being q u a n t i t a t i v e i n some of these arguments. We suggest that i t i s now u s e f u l to attempt to r a t i o n a l i z e the stereochemical course of diene polymerization r e a c t i o n s i n terms of organometallic features which l e a d to f a s t e r or slower a l l y l isomerization rates. Indications pointing i n this d i r e c ­ t i o n are a v a i l a b l e even i n the present study. N i c k e l π-allyl i o d i d e provides trans polymer; however, the simple a d d i t i o n of the Lewis a c i d s p e c i e s , T i C l ^ , converts the p o l y m e r i z a t i o n reac­ t i o n i n t o one y i e l d i n g a high c i s polymer. We propose here that the r o l e of the Lewis a c i d i s one which i o n i z e s the i o d i d e away from the n i c k e l center, p r o v i d i n g a more p o s i t i v e l y charged metal site. T h i s would r e s u l t i n t i g h t e r a l l y l b i n d i n g and slower a l l y l i s o m e r i z a t i o n r a t e s . This a s s o c i a t i o n of c i s polymer with e l e c t r o n e g a t i v e l i g a n d s i n the c a t a l y s t appears to be recognized i n commercial a p p l i c a t i o n as w e l l . ^ Our current work i s o r i e n t e d toward demonstrating q u a n t i t a t i v e r e l a t i o n s h i p s of t h i s type. Experimental General. Proton NMR s p e c t r a were obtained on a V a r i a n XL-200, XL-100, or EM-360 spectrometer, as noted. Chemical s h i f t s are reported i n ppm (6) r e l a t i v e to t e t r a m e t h y l s i l a n e . Experiments f o r which deuterium decoupling was r e q u i r e d were run on the V a r i a n XL-200, using the broadband t r a n s m i t t e r and c o i l as the deuterium de­ coupler, and observing through the normal decoupler c o i l . The instrument was operated unlocked f o r these experiments. Carbon-13 NMR s p e c t r a were obtained using a Varian XL-200 spectrometer operating i n the F o u r i e r transform mode. Polymer samples were run as g e l s or s l u r r i e s i n benzene-d^, unless other­ wise noted. Operating and processing c o n d i t i o n s were standard f o r a l l polymer samples. Chemical s h i f t s are reported r e l a t i v e to t e t r a m e t h y l s i l a n e i n ppm (6). Most deuterated solvents f o r NMR spectra were purchased from Merck, Sharp, and Dohme, and a l l were 99+ atom% D. A n a l y s i s of %-NMR s p e c t r a was performed using modified v e r ­ sions of the computer programs UEA^ and DNMR3.^6

314

INITIATION OF POLYMERIZATION

Raman s p e c t r a were obtained using a Spex spectrometer with a Spectra-Physics argon i o n l a s e r operating at 4880 A. S c a t t e r i n g was measured with a Spex DPC-2 d i g i t a l photometer i n the photoncounting mode. Data are reported i n cm" , r e l a t i v e t o the Rayl e i g h l i n e (20492 cm" ); r e l a t i v e i n t e n s i t i e s are placed i n parentheses f o l l o w i n g the s c a t t e r i n g frequency. Spectra were r e corded at 400 m i l l i w a t t s i n c i d e n t power to give 1 cm"* r e s o l u t i o n ( s l i t s = 100:200:200:100). Samples were prepared by vacuum d i s t i l l a t i o n of butadiene i n t o g l a s s c a p i l l a r y tubes which were then sealed. Gas chromatography was performed using a Hewlett-Packard 5700A Gas Chromatograph equipped with a thermal c o n d u c t i v i t y det e c t o r . Output was recorded and measured with a Hewlett-Packard 3380S I n t e g r a t o r . Synthesis and handling of a i r - and w a t e r - s e n s i t i v e organom e t a l l i c complexes was c a r r i e d out e i t h e r on the bench using standard Schlenk techniques ^ or i n a dry box designed t o maint a i n an atmosphere of l e s s than 1 ppm contaminants. The atmosphere i n the dry box was N2. Solvents f o r use i n polymerizat i o n s and s y n t h e s i s of o r g a n o m e t a l l i c s — d i e t h y l ether, toluene, heptane, benzene, and dioxane—were c a r e f u l l y d r i e d and degassed p r i o r t o use by r e f l u x i n g under n i t r o g e n with sodium benzophenone ^ then f r e s h l y d i s t i l l e d before use. Butadiene was of research p u r i t y ^99.86%) and was purchased from Matheson. When a d d i t i o n a l d r y i n g was r e q u i r e d , anhydrous CaCl2 was used as a drying agent. IR s p e c t r a were obtained using a Perkin-Elmer I n f r a r e d photometer, Model 297. Sampling method i s reported f o r i n d i v i dual cases; absorptions are reported i n c n T , and the i n t e n s i t y i s designated as s ( s t r o n g ) , m (medium), w (weak), or b (broad). M e l t i n g p o i n t s were obtained on an E l e c t r o t h e r m a l c a p i l l a r y melting point apparatus. A l l melting and b o i l i n g p o i n t s are uncorrected. 1

1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

1

1

1

Cis,cis-1,4-dideuterio-l,3-butadiene. The dideuteriobutadiene was synthesized by the method of Stephenson, Gemmer, and Current.^ The dideuteriobutadiene was stored i n a pressure v e s s e l i n the f r e e z e r over anhydrous c a l cium c h l o r i d e u n t i l use. Raman spectrum: 1171 (0) 1216 ( 5 ) , 1226 (98) cm" (93% c i s , c i s - i s o m e r , 7% c i s , t r a n s - i s o m e r , 0% trans,trans-isomer) 9. 1

General Procedure f o r Ozonolysis of Polymers t o S u c c i n i c Anhydride. Polybutadiene (0.5 g) was placed i n a 500 ml 2-necked round bottom f l a s k equipped with a c a p i l l a r y gas bubbler and a magnetic s t i r bar. Trans-polymers were ground t o f i n e powders, while the

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

23.

STEPHENSON AND KOVAC

Butadiene

315

Polymerization

more e l a s t o m e r i c cis-polymers were cut i n t o s m a l l p i e c e s . Anhy­ drous methanol (25 ml) was added to the f l a s k , and the suspension c h i l l e d to -10°C. A Welsbach l a b o r a t o r y ozone generator opera­ t i n g at 90 v o l t s and 8 p s i g oxygen pressure was used t o produce ozone. Oxygen gas was d r i e d by passing through a column of D r i e r i t e b e f o r e e n t e r i n g the o z o n i z e r . A stream of ozone i n oxy­ gen was passed through the r e a c t i o n mixture f o r 3 t o 5 hours, u n t i l l i t t l e or no s o l i d remained i n the f l a s k . The methanol was removed on a r o t a r y evaporator without h e a t i n g , y i e l d i n g a gummy c o l o r l e s s r e s i d u e . To t h i s r e s i d u e was added about 2 ml of f o r ­ mic a c i d ( M a l l i n c k r o d t ) and 1 ml 30% hydrogen peroxide ( M a l l i n c k r o d t ) . The mixture was gently warmed f o r 1 hour, then r e f l u x e d u n t i l no peroxides could be detected by s t a r c h - i o d i d e paper. Evaporation of the solvent l e f t a gummy s o l i d c o n t a i n i n g s u c c i n i c a c i d , which was not f u r t h e r p u r i f i e d before the next s t e p . A c e t i c anhydride (2 ml) was added t o the f l a s k and the mix­ ture was r e f l u x e d f o r s e v e r a l hours, then the solvent was r e ­ moved by r o t a r y evaporation. The l a s t t r a c e s o f solvent were r e ­ moved under vacuum at room temperature, then the t a r - l i k e brown product was repeatedly sublimed t o g i v e 0.2 g (2 mmol) o f white, c r y s t a l l i n e s u c c i n i c anhydride. ^C-NMR (dimethyl s u l f o x i d e - d 6 , 50 MHz, % ) resonances at 6 174.1 and 29.0 ppm Y i e l d = 22% t h e o r e t i c a l . Check f o r Deuterium Exchange during Polymer A n a l y s i s . A sample of a d e u t e r i a t e d polymer produced by r e a c t i o n of cis,cis-1,4-dp-butadiene w i t h π-allyl n i c k e l i o d i d e / T i C l 4 c a t a ­ l y s t was o x i d a t i v e l y cleaved and c y c l i z e d t o d 2 ~ s u c c i n i c anhy­ d r i d e , as d e s c r i b e d elsewhere i n t h i s s e c t i o n . ^C-NMR ( h l o r o form-d, 50 MHz, ^ ) resonance at δ 28.02 ( t , J=21 Hz) ppm. c

A mixture of d ^ - s u c c i n i c anhydride and dQ-succinic anhydride was prepared; t h i s sample e x h i b i t e d C-NMR (chloroform-d,50 MHz, !H) resonances at 28.04 ( t , J=21 Hz) and 28.36 (s) ppm. The absence of the s i n g l e t at 28.36 ppm i n the d£sample insures that < 5% exchange of the deuterium i n the sample could have occurred. 13

A n a l y s i s of Mixtures o f d , l and meso-d9 S u c c i n i c Anhydride. g The method of F i e l d , Kovac and Stephenson was used f o r t h i s analysis. General Procedure f o r the P o l y m e r i z a t i o n o f Butadiene. Unless otherwise noted, the f o l l o w i n g general procedure was used t o e f f e c t p o l y m e r i z a t i o n of butadiene with t r a n s i t i o n metal catalysts.

316

INITIATION OF

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

Polymerizations were c a r r i e d out i n a 3 oz, F i s h e r - P o r t e r pressure b o t t l e which was modified to allow pressure measurements and syringe t r a n s f e r s to be made. T y p i c a l l y , c a t a l y s t s and s o l ­ vents were introduced i n t o the b o t t l e i n the dry box, then the b o t t l e was sealed and removed to the bench. Butadiene was d r i e d by passing i t through anhydrous calcium c h l o r i d e . The amount of butadiene f o r a given polymerization was measured by condensation i n t o a graduated r e c e i v e r , then t h i s volume was bulb-to-bulb t r a n s f e r r e d i n t o the F i s h e r - P o r t e r b o t t l e c o n t a i n i n g c a t a l y s t and s o l v e n t . The r e a c t i o n v e s s e l was s e a l e d , and warmed to the p o l y ­ m e r i z a t i o n temperature by heating i n an o i l b a t h which enclosed the e n t i r e glass p o r t i o n of the b o t t l e . Reported r e a c t i o n tem­ peratures are + 5°C. Reactions were s t i r r e d with a magnetic s t i r bar, and the progress of the r e a c t i o n s was monitored by observing the pressure change. Polymerizations were terminated by c o o l i n g the r e a c t i o n v e s s e l to room temperature and r e l e a s i n g any excess butadiene to the atmosphere. Then the contents of the F i s h e r - P o r t e r b o t t l e were poured i n t o 100 ml of a 1% s o l u t i o n of h y d r o c h l o r i c a c i d i n methanol to p r e c i p i t a t e the polymers. The s o l i d polymers which r e s u l t e d were leached i n t h i s a c i d i c methanol s o l u t i o n to f u l l y remove r e s i d u a l c a t a l y s t (at l e a s t 5 h o u r s ) . I f the polymer s t i l l appeared c o l o r e d , i t was i s o l a t e d , r e d i s s o l v e d i n c h l o r o ­ form, and r e - p r e c i p i t a t e d as described above. F i n a l l y , polymers were washed with anhydrous methanol, i s o l a t e d , and d r i e d i n a vacuum oven at 50°C. P o l y m e r i z a t i o n with π-Allyl N i c k e l Iodide. 20 π-Allyl n i c k e l i o d i d e was prepared by the method of Wilke Polymerizations using t h i s c a t a l y s t were c a r r i e d out with e i t h e r butadiene or c i s , c i s - 1 , 4 - d - b u t a d i e n e as monomer, i n the manner described p r e v i o u s l y i n t h i s s e c t i o n . Representative e x p e r i ­ mental d e t a i l s are given below: ?

Catalyst: Solvent: Monomer: Temperature: Time: Yield:

π-allyl n i c k e l i o d i d e toluene cis-cis-l-4-d -butadiene 50°C 20 hours 0.75 g (51% t h e o r e t i c a l ) 2

-5 mol) 0.04 g (8.8x10 2.0 ml 2.0 ml (2.6x10 mol)

When d e u t e r i a t e d monomer was used, samples were taken f o r Raman spectroscopy before p o l y m e r i z a t i o n and at p e r i o d i c i n t e r ­ v a l s during p o l y m e r i z a t i o n so that the isomeric d i s t r i b u t i o n of the monomer could be monitored. Examination of Raman s p e c t r a f o r the experiment described above showed: (1) (d«-monomer at t=0 minutes) 1216 (10), 1226 (199) cm" . (93% c i s , c i s - i s o m e r ) (2) (d -monomer at t=1200 minutes) 1216 (9), 1226 (206) cm" (94% c i s , c i s - i s o m e r ) . 1

1

2

23.

STEPHENSON AND KOVAC

Butadiene

317

Polymerization

Geometric isomerism was determined on non-deuteriated p o l y ­ mers. Carbon-13 NMR (methylene c h l o r i d e - d 2 , 50 MHz, % ) reso­ nances at δ 130.2 (s) and 32.8 (s) ppm. The absence of a peak at 27.7 ppm i n d i c a t e d that the polymers were 100% trans. A sample of the d e u t e r i a t e d polymer (0.531 g) was ozonized as a suspension i n C ^ C ^ , and the product t r e a t e d with formic acid-hydrogen peroxide as described p r e v i o u s l y i n t h i s s e c t i o n . C y c l i z a t i o n to d 2 - s u c c i n i c anhydrides and d e r i v a t i z a t i o n to form the d2-N-(o-biphenyl) succinimides were a l s o performed by pre­ v i o u s l y described methods. %-NMR (acetone-dfc, 200 MHz, H ) resonances at δ 2.62 ( s ) , 2.62 (d, J=4.4 Hz), 2.84 ( s ) , 2.84 (d, J=4.4Hz), 7.48 (m). Measurement of peak areas by t r i a n g u l a t i o n i n d i c a t e d that the d2-N-(o-biphenyl) succinimide produced i n the above experiment was 55% meso-isomer and 45% d , l isomer.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

2

Polymerization with π-Allyl N i c k e l Iodide/Titanium chloride.

Tetra­

T h i s c a t a l y s t was prepared and used f o r p o l y m e r i z a t i o n i n the f o l l o w i n g manner. A 0.1 M s o l u t i o n of T1CI4 i n toluene was made up and stored i n the dry box. The d e s i r e d amount of π-allyl n i c k e l i o d i d e was weighed out, d i s s o l v e d i n 1.0 ml toluene, and placed i n a F i s h e r - P o r t e r tube. In a l l polymerizations, [Ni] = 3x10" M and the N i / T i r a t i o was « 1. To the π-allyl n i c k e l i o d i d e s o l u t i o n was added an appropriate amount of the stock solu­ t i o n of T i C l ^ , and the mixture was a g i t a t e d at ambient tempera­ ture f o r 20 minutes. Almost immediately upon a d d i t i o n of the T i C l ^ s o l u t i o n , a brown p r e c i p i t a t e formed. A f t e r 20 minutes, the solvent (toluene) was added, and the tube was sealed and r e ­ moved to the bench, where butadiene was t r a n s f e r r e d i n t o the tube to give the d e s i r e d monomer concentration. The tube was placed i n an o i l bath at the polymerization temperature, and the r e s t of the r e a c t i o n and work-up was c a r r i e d out as described i n the gen­ e r a l procedure d e t a i l e d p r e v i o u s l y . T y p i c a l experimental d e t a i l s are given below: 3

Catalyst: Solvent: Monomer: Temperature: Time: Yield:

π-allyl n i c k e l i o d i d e T i C l 4 i n toluene (0.1 M) Toluene 1,3-butadiene 50°C 4.5 hours 0.98 g (61% t h e o r e t i c a l )

3

5

6.5xl0" g (1.4xl0" mol) 0.28 ml (2.8xl0~ mol) 6.0 ml 2.2 ml (0.030 mol) 5

T h i s polymerization was c a r r i e d out a number of times with u n l a b e l l e d butadiene, over a monomer concentration range of

318

INITIATION OF POLYMERIZATION

1J

0.25 M t o 3.0 M. R e l a t i v e areas of the C-NMR peaks at 32.6 and 27.2 ppm i n d i c a t e d that a l l the polymers so produced con­ tained 80 t o 90% c i s - d o u b l e bonds. Cis , cis-1,4-d2-butadiene (_2) was used as monomer i n two cases. Experimental c o n d i t i o n s were the same i n both runs: [Ni] - 3.8x10-3 M, N i / T i - 1.3, [d -butadiene] = 3.0 M, T=55°C. Results of both runs are shown i n Table I . These d e u t e r i a t e d polymers were o x i d a t i v e l y cleaved and the o x i d a t i o n products converted t o the d2~N-(o-biphenyl) s u c c i n i mides f o r d,l/meso a n a l y s i s by ^H-NMR. Run 1 y i e l d e d 30% mesoisomer and 70% d,l/isomer, while Run 2 y i e l d e d 32% meso-isomer and 68% d,l-isomer.

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2

Table I . R e s u l t s of p o l y m e r i z a t i o n of (2) with π-allyl n i c k e l i o d i d e / T i C l 4 .

Polymerization time Yield % c i s i n polymer Monomer Composition ( i n i t i a l ) ^ Monomer Composition ( f i n a l ) 3

b

Run 1

Run 2

330 min 17% 78% 93: 7:0 80:20:0

420 min 21% 83% 93: 7:0 74:26:0

Percentages determined by C-NMR. Reported as r e l a t i v e percentages ( c i s , c i s : c i s , t r a n s :trans, t r a n s ) , as determined from Raman l i n e s at 1226 ( c i s , c i s ) , 1216 ( c i s , t r a n s ) , and 1171 ( t r a n s , t r a n s ) cm"" 1

Polymerization of Butadiene with Rhodium T r i c h l o r i d e . The d e s i r e d amount of RhCl3.3H 0 ( A l f a ) was weighed out and placed i n t o a F i s h e r - P o r t e r tube. In a l l polymerizations, [Rh] = l x l O " M. To t h i s was added a weighed amount of 1,3-cyclohexadiene ( A l d r i c h ) and the d e s i r e d volume o f a 2.5 wt . % s o i n , o f sodium dodecyl s u l f a t e ( A l d r i c h ) i n deionized water. A l l the above operations were c a r r i e d out on the bench, without a p r o t e c ­ t i v e atmosphere. The F i s h e r - P o r t e r tube was then s e a l e d , the monomer was t r a n s f e r r e d i n t o the tube, and p o l y m e r i z a t i o n and work-up were c a r r i e d out as described i n the general procedure elsewhere i n t h i s s e c t i o n . Representative experimental d e t a i l s are given below: 2

2

23.

STEPHENSON AND KOVAC

Catalyst :

Butadiene

319

Polymerization

RI1CI3. 3H 0 1,3-cyclohexadiene Solvent: 2.5 wt.% s o i n , of sodium dodecyl s u l f a t e i n Η 0 Monomer: cis_, cis-1,4-d 2 -butadiene Temperature: 25°C Time: 31 hours Yield: 0.58 g (47% t h e o r e t i c a l ) 2

0.015 0.017 3 ml

5

g ( 5 . 7 x l 0 ~ mol) g ( 2 . 1 x l 0 " mol) 4

2

1.7 ml (0.022 mol)

Geometric isomerism was determine »d on non-deuteriated p o l y H ) resonances at δ 129.3 mers. C-NMR (benzene-d$, 50 MHz, (s) and 32.1 (s) ppm. The absence of a peak at ^27 ppm i n d i c a t e d that the polymers were 100% trans i n s t r u c t u r e . When d e u t e r i a t e d monomer was used, samples were taken f o r Raman spectroscopy before p o l y m e r i z a t i o n and at the end of p o l y ­ m e r i z a t i o n t o a s c e r t a i n t o what extent monomer had isomerized. Examination of Raman s p e c t r a f o r the experiment described above showed: (1) (d -monomer at t=0 minutes) 1216 (10), 1226 (23 (2) d2-monomer at (238) c m . (94% c i s , c i s - i s o m e r ) t=;850 minutes) 1216 (12), 1226 (237) cm" , (93% c i s , c i s isomer). The d e u t e r i a t e d polymer from the above example was o x i d a t i v e l y cleaved and the o x i d a t i o n products were converted t o the d2-N-(o-biphenyl) succinimides f o r d,l/meso a n a l y s i s , •1-H-NMR (acetone-d6, 220 MHz, H ) resonances at δ 7.48 (m), 2.84 ( s ) , 2.84 (d, J=4.4 Hz), 2.62 ( s ) , 2.61 (d, J=4.4 Hz) ppm. Measurement of methylene peak areas by t r i a n g u l a t i o n i n d i c a t e d that the d2~N-(o-biphenyl) succinimide produced i n the above experiment was 48% meso-isomer and 52% d,l-isomer.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

13

2

-1

1

2

Polymerization with π-Allyl N i c k e l T r i f l u o r o a c e t a t e . Polymerizations with t h i s c a t a l y s t were c a r r i e d out by the general procedure described p r e v i o u s l y . Representative e x p e r i ­ mental d e t a i l s are given below: Catalyst : Solvent : Monomer: Temperature: Time: Yield: 13

π-allyl n i c k e l t r i f l u o r o acetate heptane cis,cis-l,4-d2~butadiene 42°C 3.3 hours 0.68 g (47% t h e o r e t i c a l )

5

0.025 g ( 5 . 9 x l 0 ~ mol) 7 ml 2 ml (2.6x10

mol)

C-NMR of the polymer (benzene-d^, 50 MHz, H ) showed resonances a t δ 57.75 ( t , J=20 Hz), 52.38 ( t , J=20 Hz), 128.37 (S). [7-5-81-1] I n t e g r a t i o n of the two t r i p l e t s i n d i c a t e d the polymer was 95% c i s .

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When deuteriated monomer was used, samples were taken for Raman spectroscopy before polymerization and at the end of the polymerization to ascertain the extent to which monomer had isomerized. Raman spectra showed: (1) (d2~monomer at t=0 minutes) 1216 (4.5), 1226 (175.5) cm"1. (96% cis,cis-isomer) (2) (d2-monomer at t=200 minutes) 1216 (15), 1226 (230) cnT1 (91% cis,cis-isomer) A sample of the deuteriated polymer was oxidatively cleaved and derivatized according to previously described procedures to give d2-N-(o-biphenyl) succinimide isomers. -4l-NMR (acetone-d6, 200 MHz, 2 H ) resonances at 6 2.62 (d, J=4.4 Hz), 2.63 (s), 2.85 (d, J=4.5 Hz), 2.85 (s), 7.48 (m). Measurement of methylene proton peak areas by triangulation indicated that the deuteriated derivative produced in the above experiment was 32% meso-isomer and 68% d,l-isomer. Acknowledgment This work has received generous support from the National Science Foundation through Grant 80-12233, and from the Petroleum Research Fund, administered by the American Chemical Society, Grant 11748AC. Literature Cited 1.

2. 3. 4.

5.

6. 7.

See i n p a r t i c u l a r Cooper,W. "Polydienes Coordination C a t a l y s t s " , p. 21-78 and Teyssie, P h . ; Dawans, F. "Theory of Coordination C a t a l y s t s " , p. 79, 138 "The Stereo Rubbers", Saltman, W. E d . , John Wiley and Sons: N.Y. 1977. a) Cossee, P. "Stereochemistry of Macromolecules"; M. Dekker: New York, 1967, V o l . I, p. 145. b) Cossee, P. J . C a t a l . 1964, 3, 80. Arlman, E. J . J . C a t a l . 1966, 5, 178. a) Druz, N . N . ; Zak, A.V.; Lobach, M . I . ; V a s i l i e v , V . A . ; Kormer, V . A . European Polymer J . 1978, 14, 21. b) Kormer, V.A.; Lobach, M . I . Macromolecules 1977, 10, 572. c) Druz, N . N . ; Zak, A.V.; Lobach, M . I . ; Shapkov, P . P . ; Kormer,V.A. European Polym. J . 1977, 13, 875. a) Warin, R.; Julemont, J ; Teyssie, P. J . Organomet. Chem. 1980, 185, 143. b) Dolgoplosk, B.A. Kinetika i Kataliz 1977, 18, 1146. F a l l e r , J . W . ; Thomsen, M . E . ; Mattina, M.V. J. Am. Chem. Soc. 1971, 93, 2842. Graham, C . R . ; Stephenson, L.M. J. Am. Chem. Soc. 1977, 99, 7098.

23.

8. 9.

10.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch023

11.

12. 13. 14. 15. 16. 17. 18 19. 20.

STEPHENSON AND KOVAC

Butadiene Polymerization

321

F i e l d , L . D . ; Kovac, C . A . ; Stephenson, L.M. J . Org. Chem. 1982, 7, 1358. a) Stephenson, L . M . ; Gemmer, R . V . ; Current, S.P. J . Org. Chem. 1977, 42, 212. b) Gemmer, R.V. Ph.D., Thesis, Stanford University, Stanford, C a . , 1975. P o r r i , L . ; A g l i e t t o , M. Makromol. Chem. 1976, 177, 1465. a) Kormer, V . A . ; B a b i t s k i i , B . D . ; Lobach, M . I . ; Chesnokova, N.N. J . Polym. S c i . 1969, C16, 4351. b) Lazutkin, A . M . ; Vashkevich, V . A . ; Medvedev, S.S.; V a s i l i e v a , V . N . Dokl. Akad. Nauk,SSSR 1967. Rinehart, R.W.; Smith, H . P . ; Witt, H . S . ; Romeyn, H. J . Am. Chem. Soc. 1961, 83, 4864; ibid., 1962, 84, 4145. Dolgoplosk, B . A . ; Tinyakova, E . I . Izv. Akad. Nauk. SSSR Ser. Khim. 1970, 2, 344. Durand, J . P . ; Dawans, F . ; Tessie, Ph. J . Polym. S c i . 1970, A1, 979. F e r r e t t i , J.A.; H a r r i s , R . K . ; Johannesen, R.B. J. Mag. Reson. 1970, 3, 84. Keeier, D . A . ; Bensch, G. "DNNMR3-Program 165, Quantum Chemistry Program Exchange", Indiana University, Bloomington, Indiana, 1970. Shriver, D.F. "The Manipulation of A i r - S e n s i t i v e Compounds", McGraw-Hill: New York, NY, 1969. Gordon, A.J.; Ford, R.A. "The Chemist's Companion", John Wiley & Sons: New York, NY, 1972. Rabjohn, N. E d . , Organic Syntheses, C o l l . V. 4, 1963, 484. Wilke, G . ; Bogdanovic, B.; Hardt, P . ; Heimbach, P; Keim, W.; Kroner, M; Oberkirch, W.; Tanaka, K.; Steinrucke, E . ; Walter, D; Zimmerman, H. Angew. Chem. Int. Ed. Eng. 1966, 5, 151.

RECEIVED October 25,

1982

24 Catalytic Control of Architecture and Properties of Butadiene Block Copolymers PH. TEYSSIE, G. BROZE, R. FAYT, J. HEUSCHEN, R. JEROME, and D. PETIT

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch024

University of Liège, Laboratory of Macromolecular Chemistry and Organic Catalysis, Sart-Tilman, 4000 Liège, Belgium It i s shown how a precise c o n t r o l of the initiation and s e l e c t i v i t y of butadiene block polymerization reactions allows the molecular engineering of a v a r i e t y of new heterophase materials. The t y p i c a l examples presented illustrate 3 main p o t e n t i a l i t i e s of these copolymers. First, the stable e m u l s i f i c a t i o n of blends of 2 immiscible homopolymers (PE/PS; PVC/PBD) by the corresponding diblock copolymer. Second, the p o s s i b i l i t y to obtain high performance thermoplastic elastomers from t r i b l o c k copolymers containing endblocks of high cohesive energy density ( i . e . nylon-6, p o l y p i v a l o l a c t o n e ) . T h i r d , the opportunity to t a i l o r "multiblock" systems i n which the "hard" segment i s an i o n i c group (halatotelechelic polymers), imparting to these systems i n t e r e s t i n g r h e o l o g i c a l properties. C a t a l y t i c polymerizations of 1,3-butadiene i n t o e s s e n t i a l l y 1,4 elastomeric polymers are by now pretty well-mastered processes. It i s the purpose of t h i s paper to demonstrate the s t i l l very v i v i d interest i n that time-honored backbone, within the frame of a much broader trend developing now i n the f i e l d of polymeric materials. It i s obvious indeed that industry i s not w i l l i n g anymore to produce many of these necessary materials from new monomers; the key approach here i s thus d i v e r s i f i c a t i o n , but s t a r t i n g from basic feedstocks and materials already available at low prices i n large amounts. On the 0097-6156/83/0212-0323$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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POLYMERIZATION

o t h e r hand, i n a time of e x p l o s i v e development o f "genet i c e n g i n e e r i n g " t e c h n i q u e s i n b i o l o g y , i t i s o n l y too f a i r t o acknowledge a s i m i l a r type o f achievement i n p o l y m e r i c m a t e r i a l s s c i e n c e , i . e . the " m o l e c u l a r e n g i n e e r i n g o f t h e i r b u l k p r o p e r t i e s " . That r e c e n t c a p a b i l i t y t o a c h i e v e , through p r e c i s e ( a l t h o u g h sometimes small) m o d i f i c a t i o n s of molecular s t r u c t u r e s , a " f i n e t u n i n g " o f the f i n a l b u l k p r o p e r t i e s and macroscopic b e h a v i o u r o f these p o l y m e r i c m a t e r i a l s , has a r i s e n from our r a p i d l y i n c r e a s i n g knowledge and m a s t e r i n g of i n i t i a t i o n and p r o p a g a t i o n mechanisms, p a r t i c u l a r l y i n terms o f s e l e c t i v i t y . T h a t a l l o w s us i n t u r n t o c o n t r o l a c c u r a t e l y the m o l e c u l a r a r c h i t e c t u r e of the polymers used, t h a t means a l s o the morphology and f u r t h e r the p h y s i c o - c h e m i c a l as w e l l as p h y s i c o - m e c h a n i c a l b e h a v i o u r of the c o r r e s p o n d i n g f i n a l m a t e r i a l s . A l l o f the examples p r e s e n t e d h e r e , l e a d i n g t o d i f f e r e n t types o f p r o p e r t i e s and a p p l i c a t i o n s , are based on copolymers o f v a r y i n g a r c h i t e c t u r e , but a l l i n v o l v i n g e s s e n t i a l l y 1,4 p o l y b u t a d i e n e b l o c k s . C l e a r l y , t h e i r p r o p e r t i e s w i l l thus r e f l e c t t h r e e i m p o r t a n t pot e n t i a l i t i e s o f heterophase b l o c k copolymer-based mater i a l s : 1) t h e i r " o r g a n i z a t i o n " i n t o mesomorphic phases (1), g i v i n g r i s e t o o r i g i n a l , o f t e n a n i s o t r o p i c , p r o p e r t i e s ; 2) t h e i r a b i l i t y t o s t a b i l i z e l i q u i d emulsions (2,) and more i m p o r t a n t l y f i n e d i s p e r s i o n s o f c o r r e s p o n d i n g homopolymers ( 3 ) , so b r i d g i n g t h e i r " c o m p a t i b i l i t y gap" and a l l o w i n g trie development o f a new " p l a s t u r g y " ; 3) t h e i r use as high-performance e n g i n e e r i n g p r o d u c t s , and p a r t i c u l a r l y as m a t e r i a l s s i m i l a r t o K r a t o n thermoplastic elastomers (4). Experimental A l l of the i m p o r t a n t e x p e r i m e n t a l d e t a i l s , as w e l l as the g e n e r a l p r o c e d u r e s taken from l i t e r a t u r e , were r e p o r t e d i n the r e f e r e n c e s c i t e d i n each s e c t i o n . As a g e n e r a l r u l e , most o f the r e a g e n t s were p u r i f i e d c a r e f u l l y ( p a r t i c u l a r l y f o r m o i s t u r e ) , and a l l o f the r e a c t i o n s c a r r i e d o u t e i t h e r u s i n g vacuum t e c h n i q u e s o r under pure argon atmosphere. Polymer p r o p e r t i e s were i n v e s t i g a t e d by s t a n d a r d methods, u n l e s s o t h e r w i s e ment i o n e d . Polymer b l e n d i n g was u s u a l l y performed f o r 5 minutes a t 190°C on a C.A.M.I.L. t w o - r o l l l a b o r a t o r y mill. Results è-EïËiiîîiDËiY-.EiQ^iëîi}- The u n s a t u r a t e d s t r u c t u r e of the 1,4 p o l y b u t a d i e n e (PBD) backbone i s u n f o r t u n a t e l y

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Butadiene

Block

Copolymers

325

r a t h e r s e n s i t i v e t o a number o f e a s i l y o c c u r r i n g secondar y r e a c t i o n s , and p a r t i c u l a r l y t o d i f f e r e n t t y p e s o f c r o s s - l i n k i n g and o x i d a t i o n p r o c e s s e s ( t h e r m a l , p h o t o c h e m i c a l , and c a t a l y t i c ) . T h i s drawback has been e l e g a n t l y c i r c u m v e n t e d by a f a c i l e c a t a l y t i c hydrogénation o f t h e c h a i n s , u s i n g e f f i c i e n t s o l u b l e Z i e g l e r - t y p e complexes ( 5 ) p r e p a r e d f r o m c o b a l t ( o r n i c k e l ) s a l t s and mixed aluminum a l k y l s i n d i f f e r e n t r a t i o s . T h i s p r o c e s s has a t w o f o l d advantage : 1) i t i s possible t o control i t s s e l e c t i v i t y (varying the r e a c t i o n c o n d i t i o n s and t h e a l u m i n u m - t r a n s i t i o n m e t a l m o l a r r a t i o ) , so t h a t p o l y b u t a d i e n e o n l y c a n be hydrogenated i n the presence of other unsaturated chains l i k e p o l y i s o p r e n e o r p o l y s t y r e n e , w h i c h i n t u r n c a n be h y d r o g e n a t e d l a t e r , i f n e c e s s a r y by c h a n g i n g t h e s e r e a c t i o n parameters; 2 ) t h e hydrogénation i s p r a c t i c a l l y q u a n t i t a t i v e and y i e l d s a product n o t very d i f f e r e n t from t h e correspond i n g p o l y e t h y l e n e s . F o r i n s t a n c e , s t a r t i n g from an a n i o n i c a l l y i n i t i a t e d p o l y b u t a d i e n e ( i n a p o l a r medium) c o n t a i n i n g a c c o r d i n g l y c a . 1 0 - 1 5 % 1 2 u n i t s , one o b t a i n s a p r o d u c t r a t h e r s i m i l a r t o low d e n s i t y l i n e a r p o l y e t h y l e n e LLDPE ( i n f a c t a copolymer o f e t h y l e n e and 1-butène) , d i s p l a y i n g a moderate s e m i - c r y s t a l l i n i t y and a m e l t i n g p o i n t c a 9 0 t o 1 0 0 ° C . On t h e o t h e r hand, t h e u s e o f a l i n e a r pure 1 , 4 PBD ( 9 9 %) o b t a i n e d by Z i e g l e r t y p e c a t a l y s i s ( n i c k e l o r c o b a l t complexes) y i e l d s a high-density p o l y e t h y l e n e - l i k e product, with a higher degree o f c r y s t a l l i n i t y and a m e l t i n g - p o i n t c l o s e t o 130°C.

O b v i o u s l y , t h e r e s i s t a n c e o f t h e s e p r o d u c t s towards l i g h t , oxygen and o t h e r c h e m i c a l s w i l l be much b e t t e r , and c l o s e t o t h a t o f t h e c o r r e s p o n d i n g p o l y o l e f i n s . Moreover, t h e hydrogénation can be stopped a t d i f f e r e n t c o n v e r s i o n s o p e n i n g a much b r o a d e r range o f a p p l i c a t i o n s c o n d i t i o n s . I n d u s t r i a l developments a l r e a d y i n c l u d e s u c c e s s f u l m a t e r i a l s l i k e Kraton G thermoplastic elastomers. sifiers__(PS-b-PBDh)_. Combination o f d i f f e r e n t polymers i n t o h e t e r o p h a s e systems r e p r e s e n t s a v e r y a t t r a c t i v e r o u t e towards new and t a i l o r - m a d e m a t e r i a l s ( 3 J d i s p l a y i n g most o f t h e p r o p e r t i e s o f t h e s t a r t i n g p r o d u c t s . The i n c o m p a t i b i l i t y between p a r t n e r s (a r a t h e r g e n e r a l r u l e ) i s however r e s p o n s i b l e f o r t h e poor p r o p e r t i e s o f many blends, w h i c h d i s p l a y l a r g e domain s i z e w i t h poor i n t e r f a c i a l a d h e s i o n . A c c o r d i n g l y , d i b l o c k copolymers (6) c o n t a i n i n g sequences m i s c i b l e w i t h t h e homopolymers t o be b l e n d e d , have been used t o a l l e v i a t e t h a t s i t u a t i o n as a

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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r e s u l t of t h e i r i n t e r f a c i a l a c t i v i t y ; i n that respect, t h e y have p r o v e n t o be s u p e r i o r (7) t o c o r r e s p o n d i n g t r i b l o c k and g r a f t copolymers r e s p e c t i v e l y ( u n l e s s t h e t r i b l o c k c a n undergo a t r i d i m e n s i o n a l domain o r g a n i z a t i o n p r o v i d i n g an a d d i t i o n a l b o n u s ) . Two d i f f e r e n t t y p e s o f (PS-b-PBDh) d i b l o c k c a n be p r e s e n t l y s y n t h e s i z e d . The f i r s t one by c l a s s i c a l a n i o n i c i n i t i a t i o n ( s - b u t y l - l i t h i u m ) and " l i v i n g " p r o p a g a t i o n o f t h e (PS-b-PBD) copolymer (8) , f o l l o w e d by t h e h y d r o génation p r o c e d u r e d e s c r i b e d "here; as d i s c u s s e d above, t h e r e s u l t i n g p r o d u c t w i l l be c l o s e t o a (PS-bLLDPE) copolymer. The second one came from t h e d i s c o v e r y (9) o f a " l i v i n g " p o l y m e r i z a t i o n o f b u t a d i e n e i n t o a pure (99 %) 1,4 polymer by a b i s η -allylnickel-trif l u o r o a c e t a t e ) c o o r d i n a t i o n c a t a l y s t , f o l l o w e d by s t y ­ rène p o l y m e r i z a t i o n ; u n f o r t u n a t e l y , t h e l e n g t h o f t h e p o l y s t y r e n e b l o c k i s l i m i t e d ( t o a M.W. o f c a . 20,000) by t r a n s f e r r e a c t i o n s . I n a g e n e r a l s t u d y o f t h e u s e f u l n e s s o f t h e f i r s t type c o p o l y m e r s , we have developed ways t o d r a s t i c a l l y improve w i t h them t h e p r o p e r t i e s o f b o t h l o w (LD) and h i g h (HD) d e n s i t y p o l y e t h y l e n e (PE)/PS b l e n d s (2,10). The a d d i t i o n (by h o t - m i l l i n g ) o f moderate amounts o f the s u i t a b l e PS-b-PDBh copolymer g r e a t l y r e d u c e s t h e d i s p e r s e d p a r t i c l e s s i z e a t e v e r y c o m p o s i t i o n ( c a . 2to 5,000 A ) ; moreover, i t s t a b i l i z e s e f f i c i e n t l y t h a t s i t u a t i o n , i . e . even t h r o u g h o u t s t a n d a r d p r o c e s s i n g . A s i g n i f i c a n t enhancement o f b o t h t h e s t r e s s a t b r e a k σβ and t h e e l o n g a t i o n a t break εβ, r e s u l t i n g i n a s t r i ­ k i n g i n c r e a s e o f t h e t o t a l energy a t break (Εβ v a l u e s ) , i s noted ( F i g . 1 ) . The l e v e l o f performance o b t a i n e d depends a s y m p t o t i c a l ­ l y on t h e amount o f copolymer added and, w h i l e a s i g n i ­ f i c a n t improvement i s a l r e a d y noted f o r 0.5 % by w e i g h t o f t h e a d d i t i v e , most o f t h e p e r f o r m a n c e s i n c r e a s e i s r e a c h e d w i t h i n 2-3 %. As e x p e c t e d , t h e l e n g t h o f t h e two b l o c k s has a l s o a g r e a t i n f l u e n c e on t h e f i n a l p r o ­ p e r t i e s : a l t h o u g h s m a l l e r ones a l r e a d y have a g r e a t i m p a c t on t h e i n t e r f a c i a l s i t u a t i o n ( s m a l l e r domains, h i g h e r σ β ) , t h e use o f h i g h e r M.W. d i b l o c k s ( i . e . i n w h i c h each b l o c k i s s i m i l a r i n s i z e t o t h e c o r r e s p o n ­ d i n g homopolymers t o be blended) y i e l d s P S - r i c h b l e n d s d i s p l a y i n g t h e m e c h a n i c a l c h a r a c t e r i s t i c s o f an e x c e l ­ l e n t toughened p l a s t i c , w i t h a h i g h εβ v a l u e (up t o 40 %) a t y p i c a l l y d u c t i l e b e h a v i o u r , and a s t r i k i n g r e s i s t a n c e towards c r y o f r a c t u r e . These f e a t u r e s a r e probably c h a r a c t e r i s t i c o f the importance o f entangle­ ments between c h a i n s o f homo- and copolymer near t h e interface. I n t e r e s t i n g and s i g n i f i c a n t d i f f e r e n c e s a r e a l s o promo-

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Figure 1. Ultimate tensile strength (σ ) and elongation at break (e ) of LDPE (M = 40,000)/PS (M = 10 ) blends. Key: ·, without copolymer; Δ , with 9% of a poly(styrene-b-hydrogenated butadiene), M total = 58,000; O , with 9% of a poly(styrene-b-hydrogenated butadiene), M„ total = 155,000. (Reproduced with permission from Ref. 7. Copyright 1981, John Wiley & Sons, Inc.) Β

B

5

n

n

n

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ted by r a t h e r s u b t l e m o d i f i c a t i o n s i n t h e d i b l o c k s t r u c t u r e . Compared t o "pure" d i b l o c k s p r e p a r e d by c o n s e c u t i ve a n i o n i c p o l y m e r i z a t i o n , " t a p e r e d " b l o c k s o b t a i n e d by a n i o n i c p o l y m e r i z a t i o n o f t h e comonomers m i x t u r e ( 1 1 ) are s t i l l more e f f i c i e n t e m u l s i f i e r s f o r PS/LDPE b l e n d s ; owing t o t h e i r l o w e r m e l t v i s c o s i t y and p a r t i c u l a r i r a s c i b i l i t y c h a r a c t e r i s t i c s , t h e y n o t o n l y a c t as s o l u b i l i z i n g agents o f t h e homopolymers b u t p r o v i d e a t t h e i n t e r f a c e a "graded" modulus r e s p o n s i b l e f o r an improved mechanical response o f the o v e r a l l m a t e r i a l ( 1 2 ) . In o r d e r t o o b t a i n t h i s new t y p e o f macromoïecule, an OH-terminated homopolymer, i . e . PBD o r PS o b t a i n e d by a n i o n i c i n i t i a t i o n combined w i t h o x i r a n e - w a t e r t e r m i n a t i o n , was end-capped (an a l c o h o l d i s p l a c e m e n t r e a c t i o n ) w i t h an a l k o x i d e c a t a l y s t a b l e t o f u r t h e r ensure a p e r f e c t l y " l i v i n g " p o l y m e r i z a t i o n o f CL, t h e r e b y y i e l d i n g the d e s i r e d b l o c k copolymers ( 1 3 ) . I n t h i s case however the PBD hydrogénation ( i f needed) can be conducted p r i o r t o b l o c k c o p o l y m e r i z a t i o n t o a v o i d any i n t e r f e r e n c e from, o r secondary r e a c t i o n w i t h , t h e p o l y e s t e r b l o c k . The f i n a l p r o d u c t s d i s p l a y a number o f a t t r a c t i v e f e a t u r e s (14): t h e y undergo l a m e l l a r mesomorphic o r g a n i z a t i o n ( p e r i o d i c i t y c a . 8 0 Â ) even under t h e form o f h e x a g o n a l s i n g l e c r y s t a l s ( P S - c o p o l y m e r s ) , and e x h i b i t a d u c t i l e b e h a v i o u r as w e l l as a h i g h r e s i s tance t o c r y o f r a c t u r e . Moreover, they are m a c r o s c o p i c a l l y b i o d e g r a d a b l e , a t l e a s t when PCL r e p r e s e n t s t h e c o n t i n o u s phase. The b l e n d i n g approach d e s c r i b e d above has been extended t o t h e s e p r o d u c t s , t a k i n q advantage o f t h e r e m a r k a b l e m i s c i b i l i t y o f PCL w i t h o t h e r p o l y m e r s , i . e . PVC, SAN, p o l y c a r b o n a t e In particular,excellent b l e n d s o f r i g i d PVC w i t h PS and PBD have been p r e p a r e d through h o t - m i l l i n g . A g a i n , they d i s p l a y a very f i n e morphology (domain s i z e c a 5 , 0 0 0 Â ) which i s remarkab l y s t a b l e i n t i m e , and some improved p h y s i c o m e c h a n i c a l p r o p e r t i e s as l o n g as t h e c o r r e s p o n d i n g m o l e c u l a r p a r a meters have been p r o p e r l y o p t i m i z e d ( 1 5 , 1 6 ) . I t s h o u l d be s t r e s s e d here t h a t t h e need f o r s u c E " a n o p t i m i z a t i o n p r o c e s s cannot be o v e r - e s t i m a t e d as r a t h e r minute modif i c a t i o n s c a n l e a d t o improvements o f one o r d e r o f magnitude. Other i n t e r e s t i n g i n d i c a t i o n s have been o b t a i n e d on t h e s e PVC b l e n d s : i n t h e p r e s e n c e o f an e x c e s s rubber y phase, t h e p r e s e n c e o f t h e copolymer ( i . e . PCL-bPBD) promotes i n d e e d good impact r e s i s t a n c e , i n d i c a t i n g a s t r o n g anchorage o f t h a t r u b b e r y phase i n t h e r e s i n matrix.

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I n o t h e r words, t h i s example emphasizes c l e a r l y the v e r y b r o a d a p p l i c a b i l i t y o f the " e m u l s i f i e r s " c o n c e p t d e p i c t e d above, t o p r a c t i c a l problems i n v o l v i n g l a r g e s c a l e polymers.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch024

§1§§ΐ:22!§£§_ίΐΙΙΣ_ΐΐΝ^ The i n t e r e s t of TPË has been l a r g e l y d e m o n s t r a t e d by the numerous and a c t i v e i n v e s t i g a t i o n s l e d i n the f i e l d , and by the c o m m e r c i a l s u c c e s s o f the K r a t o n - t y p e p r o d u c t s , p o l y ( s t y r e n e - c o - d i e n e - c o - s y t r e n e ) . I t i s however a well-known problem t h a t these m a t e r i a l s are u s u a l l y c o n f i n e d t o a p p l i c a t i o n s under r a t h e r m i l d c o n d i t i o n s , due t o the r e l a t i v e l y low Tg of the g l a s s y phase and/or the mediocre t h e r m a l s t a b i l i t y o f the r u b b e r y phase. I n p r i n c i p l e a t l e a s t , answers t o t h a t c h a l l e n g e can be o f f e r e d by the same t y p e o f s y n t h e s i s s t r a t e g y . As d e s c r i b e d p r e v i o u s l y (17), an α,ω-dihydroxyîpolybutadiene can be end-capped w i t h i s o c y a n a t e f u n c t i o n s (through r e a c t i o n w i t h an e x c e s s d i i socyanate), which are f u r t h e r converted i n t o N - a c y l l a c tam g r o u p i n g s (by r e a c t i o n w i t h c a p r o l a c t a m CLM) a c t i v e f o r the p o l y m e r i z a t i o n o f t h a t CLM monomer i n t o N y l o n - 6 b l o c k s a t b o t h ends of the p o l y b u t a d i e n e . Under c l o s e c o n t r o l of the r e a c t i o n c o n d i t i o n s , n e i t h e r the u r e t h a n e nor the u r e a l i n k a g e s formed are b r o k e n d u r i n g the subsequent b l o c k c o p o l y m e r i z a t i o n , and a h i g h y i e l d i n b l o c k copolymer i s o b t a i n e d . That t r i b l o c k copolymer d i s p l a y s i n d e e d a s e t of h i g h p e r f o r m a n c e p r o p e r t i e s : even a t 30 % o f PBD p h a s e , the m a t e r i a l has a n y l o n - 6 c o n t i n u o u s phase, w i t h a c r y s t a l l i n e m e l t i n g p o i n t c a . 225°C and a h i g h t e n s i l e s t r e n g t h ( q u i t e comparable t o t h a t o f p u r e n y l o n - 6 ) . On the o t h e r hand, the r u b b e r y phase i s f i n e l y d i s p e r s e d i n v e r y s m a l l domains ( c a . 250 Â) a l l o v e r the h i g h l y c r y s t a l l i n e n y l o n phase,impart i n g to the m a t e r i a l h i g h e r f l e x i b i l i t y and h y d r o p h o b i c i t y . A l t h o u g h the above p r o d u c t i s not a TPΕ i n the s t r i c t sense o f the t e r m (see ASTM D 1 5 6 6 ) , i t can be p r e p a r e d w i t h a h i g h e r r u b b e r c o n t e n t t o meet t h a t t y p e of b e h a v i o u r . I t has t o be s t r e s s e d however t h a t b l o c k s o f so d i f f e r e n t s o l u b i l i t y p a r a m e t e r s impose a h e t e r o p h a s e s i t u a t i o n w i t h s t r o n g i n t e r m o l e c u l a r i n t e r a c t i o n s (on the p o l a r s i d e ) , even i n the m o l t e n s t a t e w e l l above the c r y s t a l l i n e melting p o i n t : corresponding stained micro­ graphs are v e r y i n f o r m a t i v e i n t h a t r e s p e c t (see r e f . (17), f i g . 4 d ) . Such a s i t u a t i o n r a i s e s o b v i o u s l y s e v e ­ r e r h e o l o g i c a l p r o c e s s i n g p r o b l e m s , w h i c h might be d i f f i c u l t t o s o l v e . Anyhow, i t i m p l i e s a l s o t o work a t a t e m p e r a t u r e w h i c h i s e x c e e d i n g l y d e t r i m e n t a l f o r the

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unsaturated 1,4-polybutadiene b l o c k ; again, p r e l i m i n a r y q u a n t i t a t i v e hydrogénation o f t h e s t a r t i n g p o l y b u t a d i e ne i s t h e answer t o t h a t p r o b l e m , even though t h e d i f u n c t i o n a l m a c r o m o l e c u l e s form a h i g h l y s w o l l e n g e l i n the p r e s e n c e o f an e x c e s s c a t a l y s t .

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e

^it^_§«lY2§2ii2_îPyiii^Ï22lS-§£Elî2£]iE2 h e general i n t e r e s t o f muïtiblock copolymers i s by now w e l l documented p a r t i c u l a r l y i n t h e f i e l d o f TPE. Based on t h e e f f i c i e n t c r o s s - l i n k i n g a c t i o n o f the harder b l o c k s , i n t e r e s t i n g developments have t a k e n p l a c e , l e a d i n g t o some s u c c e s s f u l i n d u s t r i a l m a t e r i a l s such as HYTREL p o l y e t h e r e s t e r t h e r m o p l a s t i c e l a s t o m e r s . The v e r y r a p i d d e v e l o p ments o f i n c r e a s i n g l y s o p h i s t i c a t e d c a t a l y s t s has a l s o promoted new u n e x p e c t e d achievements; a t y p i c a l examp l e (18) i s t h e " c o d i n g " 1,4 p o l y m e r i z a t i o n o f b u t a d i ene by " t a i l o r e d " c a t a l y s t s , namely b i s (η -allylnickel -X) complexes. Under p r e c i s e l y c o n t r o l l e d k i n e t i c c o n d i ­ t i o n s , a m u l t i s t e r e o b l o c k ( p o l y . c i s 1,4-b-poly.trans 1 , 4 ) - b u t a d i e n e c a n be o b t a i n e d , t h a t r e p r e s e n t s t h e f i r s t example o f a t h e r m o p l a s t i c e l a s t o m e r ( s e m i c r y s t a l l i n e m e l t i n g p o i n t c a . 135°C) o b t a i n e d i n one s t e p from one s i n g l e monomer. A c o m p l e t e l y d i f f e r e n t a p p r o a c h has been r e c e n t l y d e v e l o p e d i n t h a t p r o s p e c t , based on t h e assumption t h a t " p r o p e r t i e s s i m i l a r t o those o f m u l t i b l o c k c o p o l y ­ mers c o u l d be r e a c h e d , i n a more v e r s a t i l e manner, by r e p l a c i n g t h e i r h a r d segments by s i n g l e g r o u p i n g s , p r o ­ v i d e d the molecular c h a r a c t e r i s t i c s o f these groupings promote v e r y s t r o n g m u t u a l i n t e r a c t i o n s , a t l e a s t i n the media e n v i s o n e d f o r t h e i r a p p l i c a t i o n s " . That c o n c e p t had l e d t o t h e s y n t h e s i s o f s o - c a l l e d " h a l a t o - t e l e c h e l i c p o l y m e r s " (which means a " s a l t " o r " n e u t r a l i z e d " t e l e c h e l i c polymer, a c i d i c o r b a s i c ) . Although t h a t i s a very g e n e r a l denomination c o v e r i n g a l l t h e c h a i n s formed by any t y p e o f i o n - p a i r c o u p l i n g i n any way, a p a r t i c u l a r l y handy and r e p r e s e n t a t i v e c l a s s o f such s t r u c t u r e s c a n be o b t a i n e d from t h e com­ plete n e u t r a l i z a t i o n of α,ω-dicarboxylato-polymers (PX), by a d i ( o r m u l t i - ) v a l e n t m e t a l d e r i v a t i v e , (19) , according t o the general equation : T

3

n

n

H00C-PX-C00H + — ΜΑ ώ f00C-PX-C00-M l - + 2n HA ν ν ~ I* 2J n ν o

Such a r e a c t i o n has been p e r f o r m e d s u c c e s s f u l l y , s t a r ­ t i n g from a n i o n i c a l l y p r e p a r e d t e l e c h e l i c p o l y m e r s PX, and n e u t r a l i z i n g them q u a n t i t a t i v e l y w i t h v e r y r e a c t i v e

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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m e t a l d e r i v a t i v e s such as m e t a l a l k y I s o r a l k o x i d e s (20); the l a t t e r t e c h n i q u e p r o v e d t o be t h e most v e r s a t i l e one, p r o v i d e d a complete e l i m i n a t i o n o f t h e a l c o h o l evolved i n order t o d i s p l a c e the r e a c t i o n but a l s o t o a v o i d s o l v a t i o n by t h a t a l c o h o l : t h a t meets t h e e s s e n t i a l r e q u i r e m e n t o f any s t e p w i s e p o l y m e r i z a t i o n , i . e . the n e c e s s i t y t o e n s u r e a v e r y h i g h (99 %+) c o n v e r s i o n t o r e a c h t h e h i g h degree o f p o l y m e r i z a t i o n n e c e s s a r y f o r t h e p r o m o t i o n o f t h e most i n t e r e s t i n g p h y s i c a l p r o p e r t i e s , t h a t i n t u r n would be overshadowed i f i o n s o l v a t i o n by t h e a l c o h o l t a k e s p l a c e (see b e l o w ) . I n t h a t way, a b r o a d f a m i l y o f HTP has been s y n t h e s i z e d , wherei n t h e n a t u r e and s i z e o f b o t h t h e polymer and t h e i o n i n v o l v e d c a n be s y s t e m a t i c a l l y m o d i f i e d , as w e l l as t h a t o f t h e s o l v e n t and l i g a n d s . They r e p r e s e n t a c c o r d i n g l y a v e r s a t i l e c l a s s o f m a t e r i a l s w i t h a b r o a d pot e n t i a l range o f " m o l e c u l a r l y e n g i n e e r e d " c h a r a c t e r i s t i c s . The m o s t _ t Y p i c a l _ p r o p e r t Y o f _ h y d r o c a r b o n - s o l u b l e HTP ( i . e . , w h e r e PX i s a polydienê " o r PS, o r p o l y i s o b u tene) w i t h a h i g h enough MW (> 1,000) t o a v o i d e x c e s s i ve charge d e n s i t y ) , i s t h e s t r o n g dependence o f i t s d i l u t e - s o l u t i o n v i s c o s i t y upon c o n c e n t r a t i o n ( 2 0 ) . A l t h o u g h v e r y s i m i l a r t o t h a t o f PX a t v e r y low concent r a t i o n s , i t i n c r e a s e s a b r u p t l y and a s y m p t o t i c a l l y between c a . 1 and 2 %, r e s u l t i n g i n a g e l a t i o n _ p h e n o m e non. T h i s g e l a t i o n c a n u s u a l l y be reduce3 by i n c r e a s i n g the t e m p e r a t u r e , o r upon a d d i t i o n o f s t r o n g l i g a n d s ( F i g . 2 ) . The c r i t i c a l g e l c o n c e n t r a t i o n , C , d e p e n d s e s s e n t i a l l y on t h e n a t u r e o f PX, i t s end-groups, the s o l v e n t , t h e c a t i o n s i z e , and on P X _ m o l e c u l a r w e i g h t f o l l o w i n g t h e r e l a t i o n s h i p Cg = k.M^ (where k " / i s d i r e c t l y p r o p o r t i o n a l to^(r2j|^» , i . e . , depends g

5

1

3

5

on t h e mean end-to-end d i s t a n c e o f t h e f r e e c h a i n ) (21). These a r e o f c o u r s e c l e a r - c u t m a n i f e s t a t i o n s o f t h e e l e c t r o s t a t i c i n t e r a c t i o n s between i o n - p a i r s i n a n o n - p o l a r s o l v e n t , l e a d i n g t o a g g r e g a t e s o f v a r i a b l e s i z e s depend i n g on t h e c o n d i t i o n s ; f o r t h e same r e a s o n s , t h e same type o f i n t e r m o l e c u l a r a s s o c i a t i o n w i l l o b v i o u s l y occur i n t h e n e a t m a t e r i a l . I t i s a l s o n o t e w o r t h y t h a t such a "multiblock" s t r u c t u r e i s a dynamic one, i . e c a r b o x y l i c l i g a n d s e x c h a n g i n g around t h e m e t a l i o n r e s u l t i n g i n a c o n s t a n t s c r a m b l i n g n o t o n l y o f t h e i o n i c aggregat e s b u t a l s o o f t h e c h a i n b l o c k s t h e m s e l v e s . That s i t u a t i o n i s i n f a c t r e s p o n s i b l e f o r t h e dynamic m e c h a n i c a l p r o p e r t i e s of these m a t e r i a l s . W i t h _ c o n c e n t r a t e d s o l u t i o n s {over 50_%)_ a t _ e g u i l i b r i u m as w e l l as w i t h t h e n e a t p r o d u c t s , i t i s p o s s i b l e t o o b s e r v e t y p i c a l SAXS p a t t e r n s , o f t e n e x h i b i t i n g two d i f f r a c t i o n o r d e r s w i t h B r a g g s s p a c i n g i n 1:2 r a t i o s u g 1

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332

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INITIATION OF

Figure 2. Relative viscosity-concentration plots in toluene. Key: A , nonneutralized α,ω-dicarboxylic PBD (M = 4,600); ·, Mg salt of α,ω-dicarboxylic PBD, 25 °C; M, Mg salt of α,ω-dicarboxylic PBD, 80 °C. n

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

24.

TEYSSIE ET AL.

Butadiene

Block

333

Copolymers

g e s t i n g the presence of a l a m e l l a r o r g a n i z a t i o n (22). The c a l c u l a t e d l a m e l l a r t h i c k n e s s seems independent o f t h e n a t u r e o f t h e i o n as w e l l as o f temperature (below a c r i t i c a l d i s s o c i a t i o n v a l u e ) , but again s t r o n g l y r e f l e c t s the mean d i m e n s i o n s o f t h e f r e e c h a i n s . The r h e o l o g i c a l b e h a v i o u r of t h e s e p r o d u c t s has a l s o been i n v e s t i g a t e d "(23) . The s t e a d y - f l o w v i s c o s i t y depends markedly on the s h e a r - r a t e , and f o r v a l u e s h i g h e r t h a n c a . 10 s e c " a s i g n i f i c a n t d i l a t a n t e f f e c t i s obs e r v e d , s i n c e preformed i o n i c c r o s s - l i n k s p r e v e n t t h e c h a i n s t o r e l a x as the d e f o r m a t i o n t i m e - s c a l e d e c r e a s e s . I n _ t e r m s _ o f dynamic m e c h a n i c a l _ b e h a v i o u r , s t o r a g e (G ) and l o s s (G"T m o d u l i have been~dëtërmined (23) o v e r a range of f r e q u e n c i e s f o r d i f f e r e n t p o l y m e r s . A t h i g h e r f r e q u e n c y (>0.5 s e c . " ) , G i s h i g h e r t h a n G", w h i c h i n t u r n p r e s e n t s a maximum c h a r a c t e r i s t i c o f the i o n i c component r e l a x a t i o n mechanism. These e l a s t i c g e l s have r e l a x a t i o n s p e c t r a w h i c h i n d i c a t e a g a i n the i o n s a g g r e g a t i o n : a broad d i s t r i b u t i o n o f r e l a x a t i o n t i m e s app e a r s , the maximum o f w h i c h depends on t h e s t a b i l i t y of the i o n i c network. S i n c e t h e s e m a t e r i a l s d i s p l a y a n i c e t h e r m o r h e o l o g i c a l s i m p l i c i t y ( a t l e a s t when P has a M lower t h a n 20,000, i . e . , n o entanglements i n t e r f e r e n c e ) , master c u r v e s have been e s t a b l i s h e d . The c o r r e s p o n d i n g s h i f t f a c t o r s c o r r e l a t e n i c e l y i n an A r r h e n i u s - t y p e r e l a t i o n s h i p , a l l o w i n g the d e t e r m i n a t i o n o f the a c t i v a t i o n energy o f the secondary r e l a x a t i o n mechanism. F o r d i f f e r e n t i o n s , t h e s e e n e r g i e s a r e d i r e c t l y dependent on t h e i o n e l e c t r o s t a t i c f i e l d . I n _ o t h e r _ w o r d s , t h e s e p r o p e r t i e s can be i n t e r p r e ted i n terms o f a system i n w h i c h the d e f o r m a t i o n p r o c e s s e s are governed by i o n i c m u l t i p l e t s t h e r m a l d i s s o c i a t i o n ; however, w h i l e e l e c t r o s t a t i c a t t r a c t i v e f o r c e s a r e d e t e r m i n a n t i n the a g g r e g a t i o n p r o c e s s , the f r e e c o n f o r m a t i o n o f the macromolecule w i l l c o n t r o l t h e o v e r a l l morphology o f t h e g e l s as w e l l as the mean number of i o n s i n the m u l t i p l e t s . S e v e r a l i n t e r e s t i n g a p p l i c a t i o n s can be e n v i s i o n e d on the b a s i s o f t h a t b e h a v i o u r , b u t a g a i n s a t u r a t e d elastomers w i l l be more s u i t a b l e f o r most o f them, i n terms o f a g e i n g . Here a g a i n , h y d r o g e n a t e d t e l e c h e l i c p o l y b u t a d i e n e i s a p o s s i b l e answer t o t h a t problem, a l t h o u g h t e l e c h e l i c p o l y i s o b u t e n e might a l t e r n a t i v e l y be used i n some c a s e s . 1

1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch024

1

f

x

n

1

General_Conclusion. At t h i s p o i n t , i t i s probably u s e f u l and c e r t a i n l y e n c o u r a g i n g t o s t r e s s a g a i n t h a t a l l o f t h e s e examples c o n f i r m t h e v e r s a t i l i t y o f t h e molecular engineering techniques p r e s e n t l y a v a i l a b l e , as w e l l as t h e i r p o t e n t i a l i t i e s i n b r o a d e n i n g and d i v e r -

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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s i f y i n g the applications of such a time-honored commod i t y polymer as polybutadiene. That is c e r t a i n l y a worthwhile goal i n today's technology and economy. L i t e r a t u r e Cited 1.

2. 3. Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch024

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19.

Sadron, C.; G a l l o t , B. Makromol. Chem. 1973, 164, 301; G a l l o t , B. " L i q u i d C r y s t a l l i n e Order i n Polymers," Blumstein, A, E d . ; Academic: New York, 1978; 191. M a r t i , S.; Nervo, J.; Riess, G. Progr. C o l l o i d . Polym. S c i . 1975 , 58, 114. Paul, D.R. "Polymer Blends," Paul,D.R. and Newman, S.; E d s . ; Academic: New York, 1978; 168. Holden, G.; Bishop, E.T.; Legge, N.R. J . Polymer S c i . 1969, C36, 37. Falk, J.C. J . Polym. S c i . 1971, A - 1 , 9 , 2617; Gillies, G.A. (to Shell O i l Co.), U.S. Patent 3,792,127 (1974). Fayt, R.; J é r ô m e , R. A c t u a l i t e Chimique 1980, 21. Fayt, R.; J é r ô m e , R.; Teyssié, Ph. J. Polym. S c i . Polym. Lett. Ed. 1981, 19, 79. Szwarc, M. "Carbanions, L i v i n g Polymers and E l e c t r on-Transfer Processes," and references t h e r e i n , Wiley-Interscience : New York, 1968. Hadjiandreau, P. P h . D . , Thesis, University of Liege, Liege, 1980. Fayt, R.; J é r ô m e , R.; Teyssié, Ph. J. Polym. S c i . Polym. Phys. Ed. 1981, 19, 1269. Kuntz, I. J. Polym. S c i . 1961, 54, 569. Fayt, R.; J é r ô m e , R.; Teyssié, Ph. J . Polym. S c i . Polym. Phys. Ed. (in p r e s s ) . Heuschen, J.; Jerome, R.; Teyssie, Ph. Macromolecules 1981, 14, 242. Herman, J . J . Ph.D., Thesis, University of Liege, Liege, 1978. Teyssié, Ph.; B i o u l , J.P.; Hamitou, A.; Heuschen,J.; Hocks, L.; Jérôme, R.; Ouhadi, T. ACS Symposium Series No.59, T. Saegusa and E. Goethals Eds., 1977. Heuschen, J.; J é r ô m e , R.; Teyssié, Ph. French Patent 2,383,208 (1977); U.S.Patent 4,281,087(1981). P e t i t . D . ; J é r ô m e , R.; Teyssié, Ph. J . Polym. S c i . Polym. Chem. Ed. 1979, 17, 2903. Teyssié, P h . ; Devaux, A.; Hadjiandreou, Ρ.; J u l é m o n t , M . ; Thomassin, J.M.; Walckiers, E.; Ouhadi, T. "Preparation and Properties of Stereoregular Polymers," Lenz, R.W. and Ciardelli, F.; Eds.; M. R e i d e l : Hingham, Mass., 1980; 144. Moudden, A.; Levelut, A.M.; P i n e r i , M. J . Polym. S c i . Polym. L e t t . Ed. 1977, 15, 1707.

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

24.

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Butadiene Block Copolymers

335

20. Broze, G . ; J é r ô m e , R.; T e y s s i é , Ph. Macromolecules ( i n press) and references t h e r e i n . 21. Broze, G . ; J é r ô m e , R.; T e y s s i é , Ph. Macromolecules ( i n p r e s s ) . 22. G a l l o t , B.; Broze, G . ; J é r ô m e , R. ; Teyssié, Ph. J . Polym. S c i . Polym. L e t t . Ed. 1981, 19, 415. 23. Broze, G . ; J é r ô m e , R.; Teyssié, Ph. Polym. B u l l . 1981, 4, 241; Part V I I , J. Polym. S c i . Polym. Phys. Ed. (to be published).

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch024

RECEIVED December 23, 1982

In Initiation of Polymerization; Bailey, F., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25 Synthesis of Block Sequences by Radical Polymerization W. HEITZ, M . LATTEKAMP, CH. OPPENHEIMER,

1

and P. S. ANAND

2

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

Philipps-Universität, Fachbereich Physikalische Chemie, Polymere, D-3550 Marburg, Federal Republic of Germany

Radical polymerization can be used to synthesize block copolymers either by preparing α,ω - b i f u n c t i o n a l oligomers ( t e l e c h e l i c s ) and using them i n conden­ sation polymerization chemistry or using a precursor containing i n i t i a t o r groups. Problems and requirements i n the synthe­ sis of t e l e c h e l i c s are discussed i n general and s p e c i f i c a l l y for t e l e c h e l i c polybutadiene, polystyrene and poly­ ethylene. Two functional end groups are obtained by use of azo i n i t i a t o r s and percarbonates r e s p e c t i v e l y i f com­ b i n a t i o n i s the only termination r e a c t i o n . The e f f i c i e n c y of the i n i t i a t o r can be >0.9. Dead-end polymerization i s the preferable experimental c o n d i t i o n . Block copolymers can be obtained by r a ­ d i c a l polymerization using polyazoesters as i n i t i a t o r s . F r a c t i o n a l decomposition of the polyazoesters i n the presence of a first monomer resulted i n a precursor containing azo groups. Decomposing the r e s i d u a l azo groups i n presence of a second monomer gave block copolymers along with homopolymers. The f r a c t i o n of block copolymers was 60-80%.

1 2

India.

Current address : Bayer AG, Leverkusen, Federal Republic of Germany Current address: Central Salt & Marine Chemical Research Institute, Bhavnagar,

0097-6156/83/0212-0337$06.00/0 © 1983 American Chemical Society

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INITIATION OF

POLYMERIZATION

Modification of polymers by incorporating block sequences having low glass transition temperatures is a means of changing the mechanical properties and i s e s p e c i a l l y u s e f u l for the formation of thermoplastic elastomers i f the basic polymer i s s e m i - c r y s t a l l i n e . These rubber-like blocks are u s u a l l y formed by i o n i c or t r a n s i t i o n metal catalyzed reactions. Radical polymerization on the other hand i s experimentally simpler and applicable to a wide v a r i e t y of monomers. In the synthesis of Hytrel, poly (butylène terephthalate) i s modified with preformed blocks of poly(tetrahydrofuran)(1) obtained by c a t i o n i c polymerizat i o n . Prepolymers used i n modifying polycondensates must be s t r i c t l y b i f u n c t i o n a l . These products are c a l l e d t e l e c h e l i c s (2). From the analysis of end-groups and number average molecular weight, only average values of f u n c t i o n a l i t y are obtained and these values can be the r e s u l t of a combination of mono-, b i - and t r i - f u n c t i o n a l molecules. The prepolymers can only be considered to be t e l e c h e l i c s i f i t i s proven that b i f u n c t i o n a l i t y i s caused by one polymer homologous s e r i e s . That i s why chromatographic methods p l a y an important r o l e i n t h i s area. The d i r e c t synthesis of block copolymers by r a d i c a l polymerization i s an a l t e r n a t i v e to using preformed blocks with functional endgroups. Polymers with i n i t i a t o r end-groups (3a,3b), with end groups accessible to r a d i c a l a c t i v a t i o n Ti,5J, and p o l y i n i t i a t o r s (6,7) can be used as s t a r t i n g materials. Experimental Section Materials. A l l monomers and solvents were p u r i f i e d by standard procedures. T e l e c h e l i c s . Nitrogen was used as an i n e r t atmosphere. The synthesis i s e i t h e r run i n a flow reactor i f the r e a c t i o n time i s short (e.g. styrene) or a glass pressure v e s s e l i s used as a discontinuous reactor (for butadiene at lower temperatures) or a s t e e l autoclave (with ethylene as the monomer). The flow reactor consisted of a storage b o t t l e for the initiator/monomer solution through which N£ i s bubbled, a p i s t o n pump as used i n l i q u i d chromatography, a c o i l e d c a p i l l a r y ( i . d . 2 mm., length 43 m., volume 135 ml.) immersed i n a thermostat and a back pressure valve at the e x i t .

25.

Η Ε ί τ ζ ET AL.

Synthesis of Block

Sequences

339

Oligobutadiene with two ester-end groups. Eight g. of azobis(methyl isobutyrate) was introduced into a glass pressure v e s s e l ( 'Laborautokav" Buchi Comp., CHUster, Switzerland). Butadiene, 177 g., was condensed into the evacuated v e s s e l . The reaction mixture was heated to 80°C for 24 hrs. Excess of butadiene was d i s t i l l e d o f f , the f i n a l traces at 90°C. i n vacuum. The y i e l d was 60.8 g. of oligomer, corresponding to a conversion o f 30.4% of butadiene. The product had an Fb = 1868.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

f

Oligostyrene with two carbonate end-groups. A s o l u t i o n of 40 g. of dicyclohexyl peroxydicarDonate and 136 g. of styrene i n 300 ml. of toluene was pumped through the flow reactor at 75°C. and a rate o f 1.4 ml./min. The flow reactor was f i l l e d with toluene at the beginning and a forerun was discarded. Oligo­ styrene, 111 g., was obtained from 318 g. o f the reaction mixture a f t e r evaporation of the v o l a t i l e f r a c t i o n at 90°C. i n vacuum, ffix - 1160;Found: C 84.0, H 7.8, 0 8.3; calcd. C 84.2, H 7.6, 0 7.9. Function­ a l i t y by ( ^ - t i t r a t i o n a f t e r h y d r o l y s i s was 1.9. Block copolymer o f styrene and methyl methacrylate. The polyazoester i s made by the r e a c t i o n o f a d i o l and AIBN. The procedure i s described elsewhere (16). Styrene, 300 ml. (2.61 mol), 408 ml. o f methanol and 3.545 g. (13 mmol) o f i n i t i a t o r obtained from d i ­ ethyl ene g l y c o l and AIBN were mixed and heated to 64°C. i n a preheated bath. A f t e r a reaction time of 359 min. (30% decomposition o f the i n i t i a t o r ) the reaction mixture was quenched and the product was removed by f i l t r a t i o n , dissolved i n benzene, p r e c i p i t a t e d with methanol and dried at room temp, i n vacuum. Prepolymer, 52.2 g., was obtained. Three g. o f prepolymer and 6 ml. o f methyl methacrylate (MMA) were dissolved i n 100 ml. benzene and heated f o r 19 hrs. to 75°C. The product was p r e c i p i t ­ ated with methanol, dissolved i n benzene and again pre­ c i p i t a t e d with methanol. The product obtained was 5.77 g., corresponding to 49.3% conversion of MMA. In the p r e c i p i t a t i o n technique, the prepolymer was s t i r r e d for 0.5 hr. i n cyclohexane. MMA was added to t h i s s o l u t i o n or h i g h l y swollen s l u r r y , and immersed i n a preheated bath. The product was i s o l a t e d as given above. Fractionation. One g. of copolymer dissolved i n 25 ml. chloroform was added to 30 g. o f glass beads (50 ym diameter). The solvent was removed i n a rotary evapora­ tor. The polymer-covered glass was dried i n vacuum.

340

INITIATION OF

POLYMERIZATION

A Soxhlet f i t t e d with a cooling jacket was used f o r extraction. Homopolystyrene was extracted with cyclohexane, PMMA with a c e t o n i t r i l e . The solvent was removed, and the polymer was dissolved i n a small amount of benzene and freeze-dried. Results and Discussions

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

Synthesis of T e l e c h e l i c s . The problems of synthe­ s i s of t e l e c h e l i c s by r a d i c a l polymerization can be discussed using the scheme of r a d i c a l polymerization: I ^ 2 RInitiation R.+M RMRM.+n M RM · +TH η

^ '

2RM .

+

n 1"

Propagation

RM H + Τ­ η RM R

Termination

//

RM Η + RM η η

(minus H)

// »

RM R η RH + RM

2n

η

R«+ RM

R M

. η

(minus H)

n

(I - I n i t i a t o r ; R -fragment containing a functional group; R- -primary r a d i c a l ; RM . -macroradical; TH Transfer agent: solvent, monomer, impurity). The i n i t i a t o r i s decomposed into two r a d i c a l s . These r a d i c a l s must add the f i r s t monomer unit with high e f f i c i e n c y . This i s a basic requirement f o r an economic synthesis; otherwise a large f r a c t i o n of the i n i t i a t o r i s wasted. In the propagation reaction, trans­ fer reactions must be s t r i c t l y avoided with the ex­ ception of some s p e c i f i c transfer with the i n i t i a t o r . The main point of concern i n t h i s synthesis i s the termination reaction. Only the combination r e a c t i o n i s allowed. Termination i n polymerization i s mostly the reaction of two macroradicals. In the formation of t e l e c h e l i c s , the r e a c t i o n with primary r a d i c a l s cannot be ignored. The r a t i o of combination to d i s p r o p o r t i o n t i o n reactions may be d i f f e r e n t i n both cases. Even i f primary r a d i c a l s react with each other by combination and macroradicals react by combination, the crossr e a c t i o n can be a c o m p l e t e d i s D r o O o r t i o n a t i o n ( 8 ) . n

Η Ε ί τ ζ ET AL.

25.

341

Synthesis of Block Sequences

A l l r a d i c a l s y n t h e s e s o f t e l e c h e l i c s have two t h i n g s i n common: the p r o b l e m o f the efficiency, f, and the o v e r a l l c o n d i t i o n s .

Ρ

Δίη]

=

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

n

( 1 )

e/2 f [ l ]

with4/M/ - amount o f monomer consumed; e - f u n c ­ t i o n a l i t y ; f - e f f i c i e n c y ; [ I ] - t o t a l amount o f initiator. The i n i t i a t o r i s c o m p l e t e l y consumed u n d e r the c o n ­ d i t i o n s used i n the s y n t h e s i s o f t e l e c h e l i c s . The q u a n t i t i e s o f Eq. ( 1 ) a r e e a s y to d e t e r m i n e i n the o l i g o m e r i c range and can be measured v e r y a c c u r a t e l y . The d e f i n i t i o n o f the e f f i c i e n c y used i n Eq. ( 1 ) i s d i f f e r e n t from the u s u a l one. I n Eq. ( 1 ) , f i s d e f i n e d as

VR-J/M;* f

=

k /R.;^ c

+

% / p

^ j

n

k . / R «

(

k /p .;/R-;

+

tR

2

)

n

w i t h k. , k . , k - rate constants of i n i t i a t i o n , t e r m i n a t i o n oy p r i m a r y r a d i c a l s and between p r i m a r y r a d i c a l s . A t h i g h m o l e c u l a r w e i g h t s the c h a i n t e r m i n a t i o n by p r i m a r y r a d i c a l s can be n e g l e c t e d and Eq. (2) s i m p l i f i e s to the u s u a l d e f i n i t i o n . The e f f i c i e n c y i s h i g h e s t i f b u l k p o l y m e r i z a t i o n can be used. P o l y m e r i z i n g i n bulk r e s u l t s i n e f f i c i e n c i e s >0.9 w i t h s t y r e n e o r b u t a d i e n e as monomers. The c o n d i t i o n s o f s y n t h e s i s s h o u l d be such t h a t the i n i t i a t o r i s c o m p l e t e l y consumed i n the r e a c t i o n . I n o t h e r words the s o - c a l l e d dead-end conditions are used (9j R

v

p

= k

(2fk /k ) - /l7°- /M; 0

p

In

5

5

(3)

t

d

= 2.83

k

5

O

( f / k ^ ) ° - [ l ] °" Q

5

( 1 - e x p ( k t / 2 ) ) (h) d

t

m

= κ [τ] °·

5

Μ

0

(5)

(vp - rate of polymerization: [ M ] , L M . ] a n d [ M J -monomer concentration at the beginning, at time t, and at the end of the reaction) . 0

T

Β

I n t e g r a t i o n o f the r a t e e q u a t i o n o f p o l v m e r i z a t i o n [(Eq.(3)j - u s i n g the f i r s t o r d e r d e c o m p o s i t i o n o f the

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

342

INITIATION OF

POLYMERIZATION

i n i t i a t o r gives an equation r e l a t i n g conversion with time [Eq.(4)]. At long reaction times, i . e . , with respect to i n i t i a t o r decomposition, the conversion of monomer w i l l p r e c i s e l y stop at a p r e d i c t a b l e conversion [Eq.(5)]. The b a s i c point of the dead end polymeri z a t i o n i s that the h a l f - l i f e of the i n i t i a t o r i s so short that the monomer i s not consumed w i t h i n 10 h a l f l i v e s of the i n i t i a t o r . The conversion of the monomer must be l i m i t e d to values lower than 40% i n order to avoid branching. This l i m i t a t i o n makes s o l u t i o n polymerization very uneconomical. But the monomer can serve as i t s own solvent i n bulk polymerization with l i m i t e d convers ion. Eq.(5) by Tobolsky (9) does not take into account the monomer consumption i n the i n i t i a t i o n step and the termination by primary r a d i c a l s . This o v e r s i m p l i f i c a t i o n may explain the deviations from this equation found f o r t e l e c h e l i c s (10). These general statements are discussed with r e f e r ence to three monomers: styrene, ethylene and butadiene. Styrene. Of course the i n i t i a t o r plays an important r o l e i n a successful synthesis of t e l e c h e l i c s . I n i t i a t o r s which have no uniform way of decomposition and i n i t i a t i o n w i l l f a i l to give a product with a c l e a r structure of the end-groups. An example i s dibenzoyl peroxide. The i n i t i a t i o n i s caused e i t h e r by oxybenzoyl r a d i c a l s or phenyl r a d i c a l s . So at l e a s t three polymer homologous series are to be expected. F i g . l a gives a GPC of an oligomer obtained with styrene and high amounts of dibenzoyl peroxide. Due to the d i f ferences i n the end-groups the peaks are not w e l l resolved even at low molecular weight ( high e l u t i o n volume). An aromatic d i a c y l peroxide w i l l never give a product with a c l e a r structure of the end groups. But the s i t u a t i o n i s quite d i f f e r e n t i f azo i n i t i a t o r s are used ( F i g . l b ) . A c l e a r peak series can be seen i n GPC which i s i n agreement with the analysis giving an ester f u n c t i o n a l i t y of two. Nevertheless there are problems with styrene. I t has been known f o r several years that polystyrene r a d i c a l s terminate about 807o by combination and 20% by disproportionation (11,12,13). This i s not i n disagreement with a f u n c t i o n a l i t y 5T two. At low molecular weights termination i s caused mainly by primary r a d i c a l s . The trend found i n the dependence of f u n c t i o n a l i t y on molecular weight may be explained by this fact (14). Using peroxydicarbonates two carbonate end-groups are f i x e d at the polystyrene chain ( F i g . l c ) . Even at a molecular weight of 2000 the f u n c t i o n a l i t y i s é

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

25.

HEITZ ET

AL.

Synthesis of Block

Sequences

343

Figure la. GPC of dibenzoyl peroxide prepared with large amounts of an initiator. Abscissa shows chart distance in centimeters, ordinate shows differential refractive index.

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344

INITIATION OF

POLYMERIZATION

Figure lb. GPC of azobisfmethyl isobutyrate) prepared with large amounts of an initiator. Abscissa shows chart distance in centimeters, ordinate shows differential refractive index.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

Figure le. GPC of dicyclohexyl percarbonate prepared with large amounts of an initiator. Abscissa shows chart distance in centimeters, ordinate shows differential refractive index.

INITIATION OF POLYMERIZATION

346

exactly two. The reason f o r this b i f u n c t i o n a l i t y i s a transfer reaction with the i n i t i a t o r attacking the peroxy bond, i n contrast to the behavior i n presence of ethylene (see below). An appreciable amount of the product i s formed by this reaction as can be concluded from the decomposition rate of the i n i t i a t o r . The e f f i c i e n c y i s >0.9 using azo i n i t i a t o r s and percarbonates i f the monomer concentration i s high. Ethylene. The mechanism of termination i n the polymerization of ethylene i s mainly by combination. This i s known q u a n t i t a t i v e l y f o r a l k y l r a d i c a l s where kd/k =0.12 ( k and k^ are rate constants f o r combin­ ation and disproportionation, respectively). So ethylene should be a good candidate f o r making t e l e c h e l i c s . But a l l e f f o r t s i n t h i s d i r e c t i o n f a i l e d . The reason f o r this f a i l u r e i s that the macroradicals derived from ethylene are highly reactive a l k y l r a d i ­ c a l s . They w i l l abstract hydrogen from any source i n the system. I f t h i s cannot be done from the solvent i t w i l l be done from the i n i t i a t o r (8,15)» The behavior of ethylene i s cTiscussed below with the i n i t i a t i o n by percarbonates as an example. ROCUOCOK ^ 2 ROCO*

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

c

c

II

it

ll

ϋ

ϋ

0

ROCO-

R0.+ C 0

S

o

2

ROCO. + η CH =CH 2

2

^

R0C0CH CH - vs^ /

2

2

II

0

0

hydrolysis HOCHgCHg-^^

Homolytic cleavage of percarbonate w i l l give p r i ­ mary r a d i c a l s which i n contrast to the behavior of d i ­ benzoyl peroxide w i l l not lose carbon dioxide (exception R=t-butyl (8)). These r a d i c a l s are f i x e d as carbonate groups at tKe end of the chain. Hydrolysis w i l l give a hydroxy1 end-group. Chain termination by combination w i l l give d i o l s a f t e r hydrolysis. I f there i s a dispro­ portionation reaction, saturated and unsaturated alcohols with an even number of carbon atoms w i l l r e s u l t . With dimethyl percarbonate as i n i t i a t o r the d i o l s expected a f t e r hydrolysis are present only i n small amounts (Fig.2). The s u r p r i s i n g r e s u l t i s that

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

25.

ΗΕίτζ ET A L .

347

Synthesis of Block Sequences

1

Id

r5

il

Î6

W

t[m] Figure 2. GC of the reaction product of ethylene and dimethyl percarbonate after hydrolysis; numbers correspond to η (S). Abscissa shows time in minutes. Key: O , H0(CHftCHu) H; Q HO(CH,CH,) OH. n

n

American Chemical Society Library 1155 16th St. N. W. Washington, D. C. 20030

348

INITIATION OF

POLYMERIZATION

the saturated alcohol i s the main product and the un­ saturated alcohol i s p r a c t i c a l l y not present. Detailed i n v e s t i g a t i o n showed that the hydrogen i s abstracted from the α - p o s i t i o n of the percarbonate which i s decom­ posed with formation of aldehyde, carbon dioxide and carbonate r a d i c a l s . e

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

V\T -+

KÇHOÇOOÇOR

Λ

il J'

—^ΛΛτΗ +

RCHO

+

C0

o

2

+

. O C O R '

il

In the i n i t i a t i o n r e a c t i o n a carbonate group i s f i x e d at the polyethylene chain-end, and i n the termination reaction a saturated end-group i s formed. Although these experiments f a i l e d to give t e l e c h e l i c s they showed that primary r a d i c a l s from percarbonates w i l l give carbonate end-groups, and t e l e c h e l i c s should be obtainable i f t h i s side reaction can somehow be avoided. Butadiene. For synthesizing t e l e c h e l i c s which u l t i m a t e l y give elastomeric products, the monomer butadiene i s o f considerable i n t e r e s t . Fig.3 shows a GPC of a t e l e c h e l i c polybutadiene prepared from azo b i s ( i s o b u t y r o n i t r i l e ) and butadiene. A c l e a r separation i s observed and the analysis gives a n i t r i l e function­ a l i t y of two. The product i s made by bulk polymeriza­ t i o n . The most important point with respect to a uniform structure of the end-group and the chain i s that the conversion i s l i m i t e d to about 30-40% by proper choice of the dead-end conditions. In f a c t the monomer i s used as a solvent. These conditions cannot be applied with the most commonly used 4,4 -azobis(4-cyanovaleric acid) because of the low s o l u b i l i t y i n butadiene. In Fig.3, the product with the polymerization degree of 1 i s p r a c t i c a l l y not present; t h i s means that cyanoisopropyl r a d i c a l s immediately react with the f i r s t butadiene unit forming a l l y l r a d i c a l s . This i s the true primary r a d i c a l i n the system. The combination of two such r a d i c a l s w i l l give a product with polymerization degree of 2. The e f f i c i e n c y i s >0.9. The n i t r i l e end-groups can subsequently be converted to carboxyl, hydroxyl or amino end-groups. AIBN can be q u a n t i t a t i v e l y converted to azo-esters. Bulk polymerization of butadiene with azo-esters r e s u l t s i n the formation of t e l e c h e l i c polybutadiene with ester end-groups. f

Η Ε ί τ ζ ET AL.

Synthesis of Block

349

Sequences

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

i l

m;

Figure 3. GPC and mass distribution of a telechelic oligobutadiene obtained with high amounts of AIBN (\0). Ρ is degree of polymerization and m represents mass (relative amount). {

350

INITIATION OF P O L Y M E R I Z A T I O N

CH, ι

CH,. ι

3

3

CH,

,

ι

CH, I

3

ROOC-C-N=N-C-COOR — ^ R00C-C4CH -CH=CH-CH f ι

ι

CH,

CH,

3

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

I

3

-N 2

L

2 o

2

Jnι o

CH,

0

3

C-COOR

CH,

3

3

Again the e f f i c i e n c y i s >0.9 and the ester function­ a l i t y i s exactly 2. The ester groups can be converted to carboxylic acids. The whole chain can be hydrogenated to obtain t e l e c h e l i c polyethylene with ester or hydroxyl end-groups. These t e l e c h e l i c s can replace poly(tetrahydrofuran) i n the modification of polyesters. The ester end-groups can be used d i r e c t l y i n a transe s t e r i f i c a t i o n process. Percarbonates are another class of i n i t i a t o r s capable of introducing two functional end-groups into butadiene R0C00C0R 0

R0C0ÎCH CH=CHCHj -0C0R o

0

n

0

0

Primary r a d i c a l s derived from percarbonates w i l l not s p l i t o f f CO2 and they are f i x e d as carbonate endgroups with an e f f i c i e n c y of »0.95. The f u n c t i o n a l i t y i s exactly two. The end-groups can e a s i l y be hydrolyzed, or the chain can be hydrogenated. Block copolymers. Another p o s s i b i l i t y to i n t r o duce s o f t segments "into polymers by r a d i c a l polymerization i s the d i r e c t synthesis of block copolymers using p o l y i n i t i a t o r s . One class of p o l y i n i t i a t o r s i s obtained from AIBN and a d i o l under mild conditions. The r e s u l t i n g polyazoesters show decomposition rates s i m i l a r to low molecular weight azo i n i t i a t o r s (16).

A

I

B

N

H0R0H TT^TTT HCl/benzene 0-5°C

I

3

ι

3

f00C-C-N=N-C-C00R I I

CH, J

η

CH, 3

Fig.4 shows a GPC of a polyazoester prepared with a s l i g h t excess of the d i o l . This excess puts a l i m i t a ­ t i o n on molecular weight and also produces predominant­ l y the polymer homologous series with two OH-end-groups which i s the major peak series of the chromatogram. Small peaks are also present, which means that there are side reactions which w i l l l i m i t the ultimate

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

25.

Δη

ΗΕίτζ ET AL. Synthesis of Block Sequences

351

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352

INITIATION OF

POLYMERIZATION

molecular weight. Using equimolar amounts of d i o l and AIBN average degrees of polymerization ^20 are obtained. These polyazoesters can be used to produce block copolymers i n a two-step procedure. In the f i r s t step a f r a c t i o n of the azo group i s decomposed i n presence of a f i r s t monomer. Thus, an azo group-containing prepolymer i s produced. Decomposition o f the rest of the azo groups i n the presence of a second monomer r e s u l t s i n the formation of block copolymers. Bulk, s o l u t i o n , and p r e c i p i t a t i o n polymerization can be used as polymerization techniques. The e f f i c i e n c y i n the preparat i o n o f the azo group-containing-prepolymer can be as high as 0.8 i f bulk polymerization i s c a r r i e d out i n such a way that short blocks are obtained. In the second step, t y p i c a l values f o r the e f f i c i e n c y are 0.3-0.4. The type of block copolymer produced i n t h i s synthesis i s dependent on the termination reactions of both monomers. I f both monomers terminate by disproportionation diblock copolymers r e s u l t . I f one reacts by combination and one by disproportionation t r i b l o c k copolymers are formed and i f both monomers terminate by combination multiblock copolymers r e s u l t . This process i s unavoidably accompanied by the formation of homopolymers. So the central question of t h i s synthesis i s : How high can the f r a c t i o n of block copolymers be? Styrene/methyl methacrylate (MMA) system was studied as a model (17) . Styrene mainly terminates by combination. Homopolystyrene can be formed from the cleavage of an azo end-group. But the p r o b a b i l i t y i s low that two such macroradicals without an azo group w i l l f i n d each other. The f r a c t i o n of homopolystyrene i n t h i s step i s small. In the f i r s t step about 50% of the azo groups are decomposed; t h i s means a r e a c t i o n time corresponding to one h a l f - l i f e of the i n i t i a t o r . In the second step, a l l the r e s t of the azo groups are decomposed corresponding to a r e a c t i o n time of 10 h a l f - l i v e s o f i n i t i a t o r decomposition. MMA terminates mainly by disproportionation, therefore ABA block copolymers are p r i n c i p a l products. The r e s u l t s o f f r a c t i o n a t i o n of a f i n a l product are given i n Fig.5. The products of Fig.5a are obtained by s o l u t i o n polymerization. The f r a c t i o n of homopolystyrene decreases to about 5%, with increasing r a t i o of MMA/prepolymer used i n the second step. The f r a c t i o n of homopoly(methyl methacrylate) increases to about 35% and the block copolymer f r a c t i o n i s 60%, independent of the MMA/prepolymer r a t i o . The r e s u l t s

25.

ΗΕίτζ ET A L .

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%

Synthesis of Block

Sequences

353

tract

MMA I prepolymer (ml/g) Figure 5a. Fractionation of block copolymers of styrene/MMA obtained by polymerizing MMA (in milliliters) by azogroup-containing-polystyrene (in grams) prepolymer with different MMA/prepolymer ratios (17) using solution polymeriza­ tion. Key: Open symbols, 85 °C; filled symbols, 75 °C. Α, Δ, homopolystyrene; •, homo PMMA; · , O, block copolymer.

354

INITIATION OF

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

%

POLYMERIZATION

fract

ratio MMA / prepolymer (ml/g) Figure 5b. Fractionation of block copolymers of styrene/MMA obtained by polymerizing MM A (in milliliters) by azogroup-containing-polystyrene (in grams) prepolymer with different MM A/prepolymer ratios (11) using precipitation polym­ erization. Key: Open symbols, 85 °C; filled symbols, 75 °C. Α, Δ, homopolystyrene; M, homo PMMA; Φ, O, block copolymer.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch025

25. ΗΕίτζ ET AL.

Synthesis of Block Sequences

355

are somewhat dependent on the technique used i n the preparation. Fig.5b presents r e s u l t s from p r e c i p i t a t i o n poly­ merization. Again homopolystyrene decreases, homo PMMA increases but does not exceed ^20%, and the f r a c t i o n of block copolymer can be nearly 80% of the product. P o l y ( a l k y l acrylates) form soft segments. As acrylate polymerization terminates by combination, multiblock copolymers are formed i f styrene i s the second monomer. These block copolymers show two glass t r a n s i t i o n temperatures ( ^ 3 0 ° C . and 9 0 ° C ) . Phase separation occurs with domain structures depending on the styrene/methyl acrylate r a t i o (18). The advantage of this method oF""synthesis i s that i t covers a vast f i e l d of monomers which are polymerizable by free r a d i c a l s . It i s possible to produce i n the f i r s t step an azo group-containing-polybutadiene which i s not c r o s s - l i n k e d . It i s a soluble, h i g h l y viscous or s o l i d material depending on the molecular weight produced. This method offers a r e a l p o s s i b i l i t y that both blocks can be already any s t a t i s t i c a l / a l t e r ­ nating copolymer (5).

L i t e r a t u r e Cited 1. Allport,D.C.; Mokajer,A.A. in "Block Copolymers"; Allport,D.C.; James,W.H. eds.; Appl. Sci. Publ. London, 1973. 2. Uranek, C . A . ; Hsieh, H . L . ; Buck, O.G. J. Polym. S c i . 1960, 46, 535. 3a. Piirma, I . ; Chou, L . H . J. Appl. Polym. S c i . 1979, 24, 2051. 3b. G u n e s i n , B . Z . ; Piirma, I. J. Appl. Polym. S c i . 1981, 26, 3103. 4. Bamford C . H . Europ. Polym. J. Suppl. 1969, 1. 5. Bamford, C . H . ; Han, X. Polymer 1981, 22, 1299. 6. Smets, G . ; Woodward, A . E . J. Polym. S c i . 1954, 14, 126. 7. Woodward, A.E.; Smets, G. J. Polym. S c i . 1955, 17, 51. 8. Guth, W.; H e i t z , W. Makromol. Chem. 1976, 177, 1835. 9. Tobolsky, A . V . J. Am. Chem. Soc. 1958, 80, 5927. 10. H e i t z , W.; B a l l , P . ; Lattekamp, M. Kautschuk Gummi 1981, 34, 459. 11. O l a j . O . F . ; Ereitenbach, J.W.; Wolf, B. Monatsh. Chem. 1964, 95, 1646.

356 12. 13. 14. 15. 16. 17.

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18.

INITIATION OF POLYMERIZATION

Berger, K . C . ; Meyerhoff, G . Makromol. Chem.1975, 176, 1983. G l e i x n e r , G . ; O l a j , O . F . ; Breitenbach J.W. Makromol. Chem.1979, 180, 2581. Konter, W.; Bömer, B.; K ö h l e r , K.H.; H e i t z , W. Makromol. Chem. 1981, 182, 2619. Guth, W.; H e i t z , W. Makromol. Chem. 1976, 177, 3159. Walz, W.; Bömer, B.; H e i t z , W. Makromol. Chem. 1977, 178, 2527. Oppenheimer, C.; H e i t z , W. Angew. Makromol. Chem.1981, 98, 167. Anand, P . ; S t a h l , H . G . ; H e i t z , W.; Weber, G . ; Bottenbruch, L. Makromol. Chem.1982, 183, 1685.

RECEIVED November 22, 1982

26 Initiator-Accelerator Systems for Dental Resins G. M . BRAUER and H . ARGENTAR

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

National Bureau of Standards, Washington, DC 20234

Initiators and initiator-accelerator systems for curing acrylic resins employed as dental restorative and prosthetic devices are reviewed. These resins are hardened by a free-radical polymerization that is activated by an initiator and heat or light, or by a redox initiator-accelerator system. Dentures are commonly cured by a heating cycle during which benzoyl peroxide (BP) initiator is decomposed to release sufficient radicals to yield a fully hardened device. Most room-temperature, chemically activated systems employ BP-tertiary aromatic amines. Many other potential redox systems are limited by the instability of the uncured components on prolonged storage or the doubtful biocompatibility of the ingredients. Visible or UV energy-cured systems do not require clinical mixing and allow unrestricted working time. Acrylic resins are the materials of choice for almost a l l dental applications wherever synthetic plastics are favored for the restoration of missing teeth or tooth structures. This is not surprising because polymers derived from methacrylate esters f u l f i l l most requisites of a restorative: adequate strength, resilience and abrasion resistance; dimensional stability during processing and subsequent use; translucency or transparency simulating the visual appearance of the oral tissue that i t replaces; satisfactory color stability after fabrication; resistance to oral fluids, food or other substances with which i t may come into contact; satisfactory tissue tolerance; low toxicity, and ease of fabrication into a dental appliance. The largest volume of plastics for dental applications is consumed in the construction and repair of dentures. Other uses include artificial teeth, restoratives—especially for anterior This chapter not subject to U.S. copyright. Published 1983, American Chemical Society

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teeth, p i t - a n d - f i s s u r e s e a l a n t s , r e s i n cements, orthodontic s p l i n t s and bonding r e s i n s , m a x i l l o f a c i a l prostheses, o r a l implants and mouth p r o t e c t o r s . In dental a p p l i c a t i o n s , a f l u i d a c r y l i c r e s i n formulation i s hardened by means of a f r e e - r a d i c a l i n i t i a t e d polymerization that i s e f f e c t e d by one of three means: s u b j e c t i n g a thermally s e n s i t i v e i n i t i a t i n g i n g r e d i e n t to a higher than ambient temperature; s u b j e c t i n g a photoactive i n g r e d i e n t to v i s i b l e or u l t r a v i o l e t r a d i a t i o n ; or b r i n g i n g together i n the r e s i n s o l u t i o n a binary redox i n i t i a t i n g system composed of two chemic a l s , one a reductant (the " a c c e l e r a t o r " or "promotor") and the other an oxidant, i . e . , the i n i t i a t o r . Each of these systems has advantages as w e l l as disadvantages and consequently none i s s t r o n g l y p r e f e r r e d over the others. The i d e a l p o l y m e r i z a t i o n - i n i t i a t i n g system does the f o l l o w i n g : i t produces enough f r e e r a d i c a l s w i t h i n an acceptable time i n t e r v a l so that a mix with a d e s i r a b l e working and curing time f o r the s p e c i f i c end use i s obtained; y i e l d s a polymer with minimum r e s i d u a l monomer and a molecular weight d i s t r i b u t i o n that w i l l r e s u l t i n optimum p h y s i c a l p r o p e r t i e s of the dental r e s t o r a t i v e or device; and, f i n a l l y , produces no undesirable byproducts, such as t o x i c or colored m a t e r i a l s . The i n g r e d i e n t s of the i d e a l system meet the f o l l o w i n g requirements: biocomp a t i b l e and non-toxic; t a s t e l e s s and c o l o r l e s s ; storage s t a b l e f o r extended periods of time under environmental conditions that may be encountered i n t r a n s i t , i n warehouses, i n dental depots or i n the dental o f f i c e ; compatible chemically and p h y s i c a l l y with a l l the other i n g r e d i e n t s of the r e s i n which are encountered i n storage; and r e a d i l y a v a i l a b l e commercially at reasonable p r i c e s . I n i t i a t o r - a c c e l e r a t o r systems f o r a c r y l i c r e s i n s and comp o s i t e s i n dental use have been p r e v i o u s l y reviewed (1). This report updates the information on these systems i n d e n t a l polymers. Thermally I n i t i a t e d Resin Systems Employing Peroxides Heat-cured denture base m a t e r i a l s were introduced i n t o dental use i n 1937. These m a t e r i a l s are prepared from a powderl i q u i d s l u r r y . The l i q u i d i s methyl methacrylate to which are added a p l a s t i c i z e r , c r o s s l i n k i n g agent and i n h i b i t o r . The powder i s poly(methyl methacrylate) containing approximately one percent i n i t i a t o r , u s u a l l y benzoyl peroxide. By s u b j e c t i n g t h i s s l u r r y to elevated temperature (about 75°C to 100°C) f o r one or more hours depending on the temperature employed, s u f f i c i e n t f r e e r a d i c a l s are produced from the i n i t i a t o r to y i e l d a s a t i s f a c t o r y denture. Other i n i t i a t o r s have been proposed. These include the thermally l e s s s t a b l e d i a c y l peroxides, e.g., d i a c e t y l - , b i s ( 2 , 4 - d i c h l o r o b e n z o y l ) - or d i l a u r y l , peroxide. The f i r s t two of these are a v a i l a b l e commercially as a 50% paste

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d i s p e r s i o n i n a phthalate e s t e r , the d i s p e r s i o n being s a f e r i n handling than the neat m a t e r i a l . Binary Redox P o l y m e r i z a t i o n - I n i t i a t i n g Systems

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

The thermal decomposition r a t e of benzoyl peroxide at mouth or ambient temperature i s much too slow to cure a c r y l i c monomers. At such temperatures i n i t i a t o r a c c e l e r a t o r systems are commonly employed. Peroxide-Amine System. Although a great number of redox p o l y m e r i z a t i o n - i n i t i a t i n g systems have been suggested f o r d e n t a l a p p l i c a t i o n s , the d i f f i c u l t y i n meeting the requirements described above has e l i m i n a t e d a l l but a few. By f a r the most popular system i s that c o n s i s t i n g of a t e r t i a r y aromatic amine ( n i t r o g e n and r i n g - s u b s t i t u t e d a n i l i n e ) a c t i n g as the a c c e l e r a t o r and benzoyl peroxide (BP), as the i n i t i a t o r . T h i s system was o r i g i n a l l y suggested i n the e a r l y 1940 s by Schnabel (2, 3). f

Overview of Experimental Data. Most peroxide-amine systems impart c o l o r s to the cured polymers ranging from yellow f o r N,Nd i m e t h y l - p - t o l u i d i n e to b l a c k f o r N,N-dimethyl-p-phenylenediamine (4). Highly e l e c t r o n - d o n a t i n g groups i n the a c c e l e r a t o r molecule u s u a l l y cause the hardened m a t e r i a l to be e s t h e t i c a l l y u n s u i t a b l e . The e f f e c t of i n h i b i t o r , peroxide, i n i t i a t o r and amine a c c e l e r a t o r on the r a t e of p o l y m e r i z a t i o n of poly(methyl metha c r y l a t e ) s l u r r i e s has been s t u d i e d (5_, 6). Time r e q u i r e d to reach the exotherm i n d i c a t e s aromatic peroxides, e s p e c i a l l y j>chlorobenzoyl peroxide, to be the most e f f i c i e n t i n i t i a t o r s . Although p o l y m e r i z a t i o n i n the presence of t h i s compound and N,Ndimethyl-£-toluidine (DMPT) i s more r a p i d i n i t i a l l y , i t i s slower a f t e r the exotherm than i s a system employing BP-DMPT (7). P o l y m e r i z a t i o n using the l a t t e r i n i t i a t o r - a c c e l e r a t o r gives r e s i n s with lower r e s i d u a l monomer. Most chemically a c t i v a t e d denture r e s i n s and f i l l i n g mater i a l s employ the BP-DMPT or BP-N,N-bis(2-hydroxyethyl)-p-toluidine (DHEPT) system. Use of DHEPT i n c r e a s e s the s e t t i n g time somewhat. Methacrylate or dimethacrylate monomers using t h i s a c c e l e r a t o r have improved storage s t a b i l i t y (8) and thus w i l l not g e l prematurely even on exposure to elevated temperatures. Composite r e s t o r a t i v e s c o n t a i n i n g a c r y l i c monomers and i n o r g a n i c r e i n f o r c i n g agents are now being used as f i l l i n g materials. In t h i s a p p l i c a t i o n , l a r g e concentrations of c r o s s l i n k i n g dimethacrylates are incorporated. These l i q u i d s polymerize much more r a p i d l y than l e s s v i s c o u s monomethacrylates because of the Tromsdorff a u t o a c c e l e r a t i o n or g e l e f f e c t a t t r i b u t e d to lessened t r a n s l a t i o n a l m o b i l i t y of growing polymer r a d i c a l s with i n c r e a s i n g v i s c o s i t y of the medium (9^, 10). The aim of a number of recent s t u d i e s has been to develop more r e a c t i v e amines that y i e l d n e a r l y c o l o r l e s s polymers with good c o l o r s t a b i l i t y and improved b i o c o m p a t i b i l i t y .

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Amines with more r i n g s u b s t i t u e n t s , p a r t i c u l a r l y i n the 3 and 5 p o s i t i o n s , e.g. Ν,Ν-dimethyl-sym-xylidine, decrease the curing time and improve the c o l o r s t a b i l i t y of the polymer ( IX). A c c e l e r a t i n g a b i l i t y i s a f u n c t i o n of both r i n g and n i t r o g e n s u b s t i t u t i o n , although r i n g s u b s t i t u t i o n has the greater i n ­ fluence ( 12); c o l o r s t a b i l i t y i s i n f l u e n c e d more by substituents on the r i n g . In general, increased s u b s t i t u t i o n and s t e r i c hindrance of aromatic amines produces l i g h t e r colored m a t e r i a l s . Composites containing aromatic amines having a 3,5-dimethylphenyl group d i s c o l o r l e s s than those with a 4-methylphenyl, and much l e s s than those with an unsubstituted phenyl group. N,N-dimethylp - t e r t - b u t y l a n i l i n e (12, 13) or Ν,N-bis(hydroxyalkyl)-3,5-di-tert b u t y l a n i l i n e s ( 1 4 ) y i e l d hardened r e s i n s that have a very l i g h t shade and e x c e l l e n t c o l o r s t a b i l i t y . T e r t i a r y aromatic amines with l a r g e s u b s t i t u e n t s on the nitrogen atom and molecular weight above 400 can be e f f e c t i v e a c c e l e r a t o r s (15) y i e l d i n g composites with r a p i d curing times. Because of t h e i r low v o l a t i l i t y and reduced d i f f u s i o n r a t e , such a c c e l e r a t o r s should not r e a d i l y penetrate body t i s s u e s . Thus, they would be a n t i c i p a t e d to cause less pulpal i r r i t a t i o n or t o x i c r e a c t i o n s than lower molecular weight amines. V o l a t i l i t y or d i f f u s i o n of the t e r t i a r y amine a c c e l e r a t o r may a l s o be reduced by using polymerizable amines such as those with N-methacryloxyethyl groups that are incorporated i n t o the cured r e s i n (16) or by s u b s t i t u t i n g f o r the low-molecular weight amine a polymeric t e r t i a r y aromatic amine (17). However, s u b s t i ­ t u t i o n of the N-methyl group i n the amine by a b u l k i e r methacryl o y l o x y e t h y l group slows the polymerization process. Formation of i n s o l u b l e polymer i n d i c a t e s that the amines copolymerize, y i e l d ­ ing a c r o s s l i n k e d polymer. Resins cured with an aminoethyl methacrylate a c c e l e r a t o r containing a j>-tolyl or 3,5-xylyl s u b s t i ­ tuent on the n i t r o g e n atom have c o l o r s t a b i l i t y s i m i l a r to those of t h e i r low molecular weight counterparts. The storage s t a b i l i t y of the components of composite formu­ l a t i o n s i s mainly l i m i t e d by the poor s h e l f - l i f e of the BP i n g r e ­ dient (8). Paste formulations, containing BP, but f r e e of a c c e l e ­ r a t o r prematurely harden when stored at 60°C. Formulations using powder-liquid c o n s t i t u e n t s are more s t a b l e at these elevated temperatures. A f t e r extended storage at room temperature f o r two years, composite mixes showed delayed s e t t i n g and decreased mechanical p r o p e r t i e s i n the cured m a t e r i a l . The p u r i t y of the BP and the amine s e l e c t e d g r e a t l y i n f l u e n c e s parameters such as r e a c t i o n r a t e , c o l o r s t a b i l i t y and b i o c o m p a t i b i l i t y (18). With m u l t i f u n c t i o n a l t h i o l s such as p e n t a e r y t h r i t o l t e t r a ( 3 mercaptopropionate), added i n concentrations below 10 percent to t y p i c a l peroxide-amine cured r e s i n s , composites with s i g n i f i c a n t l y reduced s e t t i n g time and e x c e l l e n t c o l o r s t a b i l i t y have been obtained ( J£). Dodecyl mercaptan y i e l d s comparable r e s u l t s ( l l ) . The f r e e r a d i c a l a d d i t i o n of the t h i o l to the double bond of the monomer probably c o n t r o l s the molecular weight of the polymer.

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Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

The polymeric methacrylate m a t e r i a l s , even 24 hours a f t e r hardening, contain a l a r g e number of unreacted groups (20). T h e i r c o n c e n t r a t i o n i n the cured r e s i n i s considerably higher f o r dimethacrylate monomers than f o r those with a s i n g l e methacrylate group i n the molecule. I n f r a r e d r e f l e c t a n c e measurements i n d i c a t e that the r e s i d u a l methacrylate group i n commercial dental com­ p o s i t e s with dimethacrylate i n g r e d i e n t s ranges from 30 to 48 percent. Peroxide-Amine Mechanism and Supporting R e s u l t s . Two chemical mechanisms have been proposed to e x p l a i n how s u b s t i t u ­ ents, s o l v e n t s and e x t e r n a l f a c t o r s a f f e c t the p o l y m e r i z a t i o n i n i t i a t i n g rate. However, the only mechanism that appears to agree with a l l the experimental data so f a r reported i s that i n v o l v i n g an e l e c t o n t r a n s f e r as the key step (21, 22). Accord­ i n g to t h i s mechanism, the amine and peroxide molecules i n t e r a c t to form a c h a r g e - t r a n s f e r complex ( t h i s term i s used to i n d i c a t e a non-polar t r a n s i t i o n s t a t e ) c o n s i s t i n g of an e l e c t r o n d e f i c i e n t amine and a peroxide with an excess e l e c t r o n . The complex sub­ sequently breaks down to y i e l d an aminium c a t i o n , a peroxide f r e e r a d i c a l s u f f i c i e n t l y r e a c t i v e to combine with a monomer molecule to i n i t i a t e the p o l y m e r i z a t i o n and an i n e r t benzoate anion. This mechanism i s supported by the f o l l o w i n g f a c t s , which do not seem to be e x p l a i n a b l e by other mechanisms: The r e a c t i v i t y of the amine as a p o l y m e r i z a t i o n a c c e l e r a t o r depends upon the σ+ value of the meta- or para-substituent of the amine, where σ+ i s the e l e c t r o p h i l i c s u b s t i t u e n t parameter pre­ v i o u s l y described and tabulated (23). When the k i n e t i c r a t e constant (or the r e c i p r o c a l of the p o l y m e r i z a t i o n time with amine and peroxide i n i t i a l concentrations h e l d constant) i s p l o t t e d against the σ+ value on a semi-logarithmic p l o t (Figures 1 and 2 ) , an i n v e r t e d "V" shaped curve r e s u l t s . The k i n e t i c data f o r Figure 1 were taken from a published a r t i c l e d e s c r i b i n g the polymerization of methyl methacrylate i n the presence of BP and v a r i o u s t e r t i a r y aromatic amines as shown i n Figure 2 ( 5 ) . S i m i l a r p l o t s can be obtained from k i n e t i c data of unsaturated p o l y e s t e r r e s i n s (24, 25). T h i s behavior i s independent of the i d e n t i t y of the n i t r o g e n s u b s t i t u e n t s of the amine or the monomer. Previous p u b l i c a t i o n s from t h i s l a b o r a t o r y have pointed out that the σ+ value of the amine r i n g s u b s t i t u e n t y i e l d i n g maximum r e a c t i v i t y , i . e . , minimum p o l y m e r i z a t i o n (or cure) time, occurs at approximately -0.2 i n the cases examined (26-29). The o p t i ­ mum value i s a n t i c i p a t e d to be f a i r l y i n s e n s i t i v e to the type of monomer and experimental c o n d i t i o n s . In Figure 2, p l o t s are shown f o r the p o l y m e r i z a t i o n and corresponding g e l times. The l a t t e r times do not r e v e a l a minimum at the σ+ value where the p o l y m e r i z a t i o n time i s minimum, but r a t h e r show a decided break i n the curve. Since g e l time i n d i c a t e s the c l i n i c a l working time and

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4h

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

-0.8

-0.6 -0.4 -0.2

0

0.2

σ+ Figure 1. Curing times of methyl methacrylate containing fixed initial concentra­ tions (in mol/L) of benzoyl peroxide and aryl-substituted N,N-dimethylaniline versus the σ + value of the aryl substituent of the amine. (Curing times from Ref. 5; σ-f values from Ref. 23.)

Figure 2. The gel (%) and polymeriza­ tion (O) times of unsaturated polyester containing a fixed initial concentration (in mol/L) of benzoyl peroxide and arylsubstituted N,N-bis(3-allyloxy-2-hydroxypropyl)aniline versus the σ + value of the aryl substituent of the amine. The symbols (Â, A) indicate the corresponding time values for the analogous ^-substituted 2-naphthylamine which in the usual sense is not a substituted aniline. (Time values from Refs. 24 and 25; σ + from Ref. 23, except for σ + value for 2-naphthyl ( — 0.61), recalculated from Bier (64).)

-0.8 -0.6

-0.4

-0.2

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polymerization time the p e r i o d i n which a hard polymer i s ob­ t a i n e d , the r a t i o of these two values should be near 1.0 so that "snap hardening" r e s u l t s . I d e a l l y , the r e s p e c t i v e times should be i d e n t i c a l but t h i s i s never r e a l i z e d . The g e l and polymeri­ z a t i o n times approximate each other most c l o s e l y when σ+ i s greater than about -0.2; i . e . , when the amine does not contain an o v e r l y electron-donating r i n g s u b s t i t u e n t . I f the σ+ value of the r i n g s u b s t i t u e n t i s very low, e.g., i n such compounds as t e r t i a r y aromatic amines derived from j>-anisidine or _o-phenylenediamine, g e l a t i o n occurs very r a p i d l y but f i n a l cure to the hard s t a t e takes a considerable time. Previous i n v e s t i g a t i o n s of the spectroscopic behavior of charge-transfer complexes derived from r i n g - s u b s t i t u t e d N,Nd i m e t h y l a n i l i n e have demonstrated the usefulness of the σ+ parameter i n c o r r e l a t i n g the data (30, 31). The s a t i s f a c t o r y f i t t i n g of polymerization r a t e s against the σ+ s u b s t i t u e n t i s i n d i c a t i v e of charge-transfer i n t e r a c t i o n of the aromatic amine. The e f f e c t of solvent upon the r a t e of r e a c t i o n of t e r t i a r y aromatic amines with BP i s not dependent upon the d i e l e c t r i c constant of the solvent or on the monomer formulation (32)· The data of previous i n v e s t i g a t o r s could be c o r r e l a t e d by a simple f u n c t i o n of the solvent r e f r a c t i v e index as the relevant indepen­ dent v a r i a b l e (32a). The c o r r e l a t i o n with the r e f r a c t i v e index a l s o i n d i c a t e s charge-transfer complex formation. This i s i n agreement with the spectroscopic evidence r e v e a l i n g the f a r greater s e n s i t i v i t y of the e l e c t r o n i c energy of charge-transfer complexes between uncharged molecules to the r e f r a c t i v e index of the solvent as compared to the d i e l e c t r i c constant (31). We tested the p r e d i c t i o n that t e r t i a r y aromatic amine a c c e l ­ e r a t o r s c o n t a i n i n g r i n g s u b s t i t u e n t s with σ+ values c l o s e to -0.2 would be the most e f f e c t i v e . A compilation of the σ+ values of r i n g s u b s t i t u e n t (23) l i s t e d the j>-CH2C02C2H5 group as having a σ+ value of about -0.16, suggesting use of a t e r t i a r y aromatic amine w i t h t h i s s u b s t i t u e n t or the corresponding c a r b o x y l i c a c i d or i t s methyl e s t e r as an a c c e l e r a t o r . The o v e r a l l c h a r a c t e r i s t i c s of the composites (hardening time, strength and c o l o r s t a b i l i t y ) c o n t a i n i n g 4-N,N-dimethy1aminopheny l a c e t i c a c i d (DMAPAA) or i t s methyl e s t e r (MDMAPAA) as a c c e l e r a t o r i n g r e d i e n t s compared favorably to r e s t o r a t i v e r e s i n s cured with commonly used t e r t i a r y amines (26). Based on hardening times the approximate order of the a c c e l e r a t i n g a b i l i t y of the r e s p e c t i v e amines was: DMAPAA > Ν, Ν- dime thy 1- sym-xy 1 i d ine > DMPT > MDMAPAA >> DHEPT. This order of r e a c t i v i t y i s dependent on the components ( e s p e c i a l l y monomers) used i n the formulations. Minimal c u r i n g time and maximum t e n s i l e and compressive strength of the cured m a t e r i a l were obtained over a narrow concentration range of a c c e l e r a t o r i n the l i q u i d . This range, which i s depen­ dent on the type of d i l u e n t employed, was approximately the same f o r a l l amines. The b i o c o m p a t i b i l i t y of these amines as expected from s i m i -

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l a r i t y i n s t r u c t u r e s to compounds used m e d i c i n a l l y (33, 3 4 , ) i s good. No mutagenic or c y t o t o x i c e f f e c t s have been observed using the Ames t e s t f o r b a c t e r i a l mutagenicity and the agar overlay t e s t f o r c y t o t o x i c i t y with DMAPAA (35). A second s e r i e s of amines suggested by theory to be r e a c t i v e a c c e l e r a t o r s are the corresponding j)-aminophenethanols. The σ+ value f o r the 2-CH2CH2OH group has not been reported but should be somewhat l e s s than that f o r the 2-CH2CO2C2H5 group and be c l o s e to -0.2. The f o l l o w i n g homologues and d e r i v a t i v e s of DMAPAA(I) and Ν,Ν-dialkylaminophenethanol ( I I ) have been synthesized and tested as a c c e l e r a t o r s (37):

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

(I)

and

f

where R=CH , C H and R =H,CH ,C H . Composites u s i n g a 3 to 1 powder/liquid r a t i o were prepared c o n t a i n i n g a s i l a n i z e d barium g l a s s coated with 1% BP powder and 72.4 percent bis(3-methacryloxy-2-hydroxypropyl) bisphenol A (BIS-GMA), 27.6 percent 1,6-hexamethylene g l y c o l dimethacrylate (1,6-HEDMA), 0.2 percent b u t y l a t e d hydroxytoluene (BHT) and v a r i o u s amines i n the l i q u i d . S e t t i n g times of the formulations v a r i e d from 1.5 to 4.0 min. The N,N-diethylaminophenylacetic a c i d (DEAPAA) was by f a r the most r e a c t i v e a c c e l e r a t o r with a 3 mmolar amine c o n c e n t r a t i o n causing a cure of 4.5 min. Fastest p o l y m e r i z a t i o n f o r the above amines occurred at 17 mmolar concena c c e l e r a t o r . A composite made from powder coated with 1 percent BP and a l i q u i d with 17 mmolar amine has a molar peroxide to amine r a t i o of 6.5 compared to a r a t i o of 1.1 to 1.5 reported as most e f f i c i e n t f o r c u r i n g u n f i l l e d r e s i n s (4, 36). T h i s much l a r g e r excess of peroxide r e q u i r e d to o b t a i n minimum s e t t i n g time should be expected s i n c e only a small p o r t i o n of the peroxide i s access­ i b l e to the amine. P h y s i c a l p r o p e r t i e s ( t e n s i l e strength 36-55 2 MPa, compressive strength 245-303 MPa, water s o r p t i o n 0.5-0.7/cm ) considerably exceeded the minimum requirement of the s p e c i f i c a t i o n f o r d e n t a l composite r e s i n s (37). I f low concentrations are employed i n the formulations e s p e c i a l l y with DEAPAA as a c c e l e r a t o r , the cured composites are n e a r l y c o l o r l e s s . No p e r c e p t i b l e change occurs i n the c o l o r of the specimens c o n t a i n i n g a UV absorber a f t e r 24 hours exposure to a UV l i g h t source. Because of the e x c e l l e n t o v e r a l l p h y s i c a l p r o p e r t i e s , n e a r l y c o l o r l e s s appearance and the p o t e n t i a l l y b e t t e r b i o c o m p a t i b i l i t y , compositions using these a c c e l e r a t o r s should y i e l d improved r e s t o r a t i v e s . 3

2

5

3

2

5

A c r y l i c monomers are polymerized by c a r b o x y l i c acids with t e r t i a r y aromatic amines (38), e.g., 4-N,N-(dimethylamino)pheny1a c e t i c a c i d (8), presumably v i a a charge-transfer complex. Neutral amino a c i d e s t e r a c c e l e r a t o r s should y i e l d monomer formu­ l a t i o n s having b e t t e r s h e l f - l i f e than the corresponding amino a c i d s . This has been e s t a b l i s h e d experimentally (39).

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26.

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ARGENTAR

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367

R e a c t i v i t y of the a c c e l e r a t o r employed i s dependent on the monomer used i n the r e s p e c t i v e formulation. Whereas compositions c o n t a i n i n g DEAPAA cure bis-phenol A dimethacrylate monomer formu­ l a t i o n s f a s t e s t , those containing j>-(dialkylamino)phenethanol r e s u l t i n the more r a p i d polymerization of methyl methacrylate monomer-polymer s l u r r i e s (39). A d d i t i o n of Bronsted a c i d s (40) to s i m i l a r systems increases the p o l y m e r i z a t i o n r a t e but, as has been discussed above, reduces storage s t a b i l i t y . Many peroxyesters, hydroperoxides or peroxides are more storage s t a b l e than BP. However, t h e i r r e a c t i o n with t e r t i a r y amines i s g e n e r a l l y too slow to give a s u f f i c i e n t l y r a p i d cure f o r a c r y l i c r e s i n s . Composite mixes c o n t a i n i n g 2,5-dimethyl-2,5(benzoyl peroxy)hexane, _t-butyl perbenzoate, t - b u t y l hydroper­ oxides or dicumyl p e r o x i d e - t e r t i a r y amines do not harden f o r days. Resins cured with cumene hydroperoxide and DEAPAA harden, but y i e l d h i g h l y c o l o r e d products (39). F a s t e r cures are obtained with _t-butyl peroxymaleic a c i d (TBPM) and primary amines (j>t o l u i d i n e or £-aminophenethanol), but the shades of the r e s u l t i n g composites are unacceptable. Curing i s slower with secondary and t e r t i a r y amines. The l a t t e r compounds y i e l d m a t e r i a l s having higher strength and more d e s i r a b l e shades. Because of the l a r g e concentrations of i n i t i a t o r and t e r t i a r y amine r e q u i r e d f o r s a t i s f a c t o r y cure, the storage s t a b i l i t y of components of com­ p o s i t e r e s i n s containing TBPM and t e r t i a r y amines does not equal those i n i t i a t e d by the l e s s t h e r m a l l y - s t a b l e BP. One unusual and s u r p r i s i n g c h a r a c t e r i s t i c of t e r t i a r y aro­ matic amines i s that i n a d d i t i o n to a c t i n g as a c c e l e r a t o r s , the same compounds i n low concentration i n the presence of oxygen and an i n i t i a t o r may act as i n h i b i t o r s of polymerization (41, 42). This behavior has a l s o been a t t r i b u t e d to the a b i l i t y of the amine to engage i n charge-transfer complex formation. Oxygen a l s o i n h i b i t s r a d i c a l polymerization and r e s u l t s i n uncured f i l m s at the surface of dental sealants (42). Amine-Free Redox Systems Many i n i t i a t o r - a c c e l e r a t o r systems that contain a c c e l e r a t o r s other than amine have been suggested f o r v i n y l polymerization, but only a few have been employed i n d e n t a l r e s i n s . S u b s t i t u t i o n of j>-toluenesulfinic a c i d , a l p h a - s u b s t i t u t e d sulfones and low concentrations of h a l i d e and c u p r i c ions f o r t e r t i a r y amine a c c e l e r a t o r s , y i e l d s c o l o r l e s s products (43-48). Most of these compounds have poor s h e l f - l i f e . They r e a d i l y o x i d i z e i n a i r to s u l f o n i c a c i d s which do not a c t i v a t e polymerization. L a u r o y l peroxide, i n conjunction with a metal mercaptide (such as z i n c hexadecyl mercaptide) and a trace of copper, has been used to cure monomer-polymer s l u r r i e s c o n t a i n i n g methacrylic a c i d (4950). A d d i t i o n of Na s a l t s of saccharine to monomer containing an Ν,Ν-dialkylarylamine speeds up polymerization (51). Methacrylate monomers can be polymerized with _t-butyl-, or

368

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cumyl hydroperoxide and thiourea d e r i v a t i v e s as reducing agent (52, 53, 54). With from 0.5 to 1 percent phenyl-, N - a c e t y l - or N - a l l y l t h i o u r e a composites with e x c e l l e n t c o l o r s t a b i l i t y have been obtained. Cumene- or J:-butyl hydroperoxide or t - b u t y l perbenzoate i n conjunction with a s c o r b i c a c i d or a s c o r b i c palmitate a l s o i n i t i a t e r a p i d polymerization, y i e l d i n g c o l o r l e s s composites with good mechanical p r o p e r t i e s (55). For commercial a p p l i c a t i o n , means must be developed f o r prevention of o x i d a t i o n of a s c o r b i c a c i d or i t s d e r i v a t i v e s on prolonged storage. T r i a l k y l b o r a n e oxides i n i t i a t e polymerization of a c r y l i c monomers at the moist dentin surface (56, 57). Bonding occurs only between the d e n t i n a l c o l l a g e n and any r e t e n t i o n to enamel i s mechanical. P h o t o i n i t i a t e d Polymerization. During the l a s t decade, a number of photochemically-cured m a t e r i a l s have been introduced to the dental p r o f e s s i o n . P r e s e n t l y , such r e s i n s are a v a i l a b l e as r e s t o r a t i v e s , p i t - a n d - f i s s u r e s e a l a n t s , bonding agents and as orthodontic bracket adhesives. The m a t e r i a l s are cured by an appropriate l i g h t source which, by means of s u i t a b l e f i l t e r s , produces r a d i a t i o n i n the near u l t r a v i o l e t region around 360 nm or u t i l i z e s v i s i b l e l i g h t of about 470 nm. For U V - i n i t i a t e d m a t e r i a l , the a c r y l i c r e s i n i s o f t e n composed of a dimethacrylate such as BIS-GMA, a polymerizable d i l u e n t , and a p h o t o i n i t i a t o r , u s u a l l y a benzoin a l k y l ether or diketone. BP (58) or phosphite e s t e r s (59) may be included to a c c e l e r a t e the cure, which should be s u b s t a n t i a l l y completed i n 30 to 60 sec. A d d i t i o n of a photoc r o s s l i n k i n g agent reduces c u r i n g time and lowers s o l u b i l i t y of UV-cured sealants (60). A s i n g l e paste comprising a urethanemethacrylate prepolymer or dimethacrylate, a monomeric d i l u e n t , an alpha diketone (camphorquinone) i n i t i a t o r , dimethylaminoethyl methacrylate (reducing agent) and s i l a n i z e d g l a s s powder y i e l d e d an experimental dental r e s t o r a t i v e with good p h y s i c a l p r o p e r t i e s (61). L i g h t cured m a t e r i a l s do not r e q u i r e mixing by the d e n t i s t and can be manipulated i n d e f i n i t e l y i n the mouth u n t i l t h e i r polymerization i s i n i t i a t e d by exposure to r a d i a t i o n . They s a t i s f y the c o n f l i c t i n g requirements of long working time and short s e t t i n g times (snap hardening) that are d i f f i c u l t to achieve with chemical i n i t i a t o r - a c c e l e r a t o r systems. The r a d i a t i o n e n t e r i n g a composite may be s c a t t e r e d at the r e s i n / p a r t i c l e i n t e r f a c e as w e l l as be absorbed by the p a r t i c l e and r e s i n . The success of r a d i a t i o n - c u r e d m a t e r i a l i s a f u n c t i o n of the l i g h t source, i t s s p e c t r a l d i s t r i b u t i o n and i n t e n s i t y , r a d i a t i o n time, the l i g h t transmission of the r e s t o r a t i v e and l i g h t absorbancy of the surrounding media (62, 63). Degree of cure decreases s l i g h t l y below the r e s t o r a t i o n surface u n t i l a depth i s reached where i t f a l l s o f f r a p i d l y . Generally, a c u r i n g c y c l e of 30 to 60 sec. ensures polymerization up to a

26.

BRAUER AND ARGENTAR

Initiator-Accelerator Systems

depth of at least 3 mm. Alternatively, in deeper cavities the restoration can be build in consecutive layers since the interlayer bond is satisfactory. Furthermore, curing continues after the initiating photochemical reaction is cut off as evidenced by the increase in hardness of specimens "aging" from one hour to 24 hours. Literature Cited 1.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Brauer, G. M. "Biomedical and Dental Applications of Polymers"; Gebelin, C. G.; Koblitz, F. F., Ed.; Plenum, New York, 1981; pp 395-409. Schnabel, E. German Patent 736,481, May 13, 1943. Blumenthal, L. "Recent German Developments in the Field of Dental Materials". FIAT field report No. 1185, Field Information Agency, Technical Office of Military Government for Germany (US), 27 May 1947. Brauer, G. M.; Davenport, R. M.; Hansen., W. C. Mod. Plastics 1956, 34, 153-168, 256. Lal, J.; Green, R. J. Polymer Sci 1955, 17, 403-409. Rose, E. E . ; Lal, J.; Green, R. J. Am. Dent. Assoc. 1958, 56, 375-381. Cornell, J. A.; Powers, C. M. J. Dent. Res. 1959, 38, 606610. Brauer, G. M.; Petrianyk, N.; Termini, D. J. J. Dent. Res. 1979, 58, 1791-1800. Pryor, W. A. "Free Radicals", McGraw-Hill: New York, 1966, p. 317-318. Walling, C. "Free Radicals in Solution" John Wiley: New York, NY, 1957, pp 189-193. Bowen, R. L.; Argentar, H. J. Am. Dent. Assoc. 1967, 75, 918-928. Bowen, R. L.; Argentar, H. J. Dent. Res. 1971, 50, 923-928. Argentar, H.; Tesk. J. A.; Parry, E. E. J. Am. Dent. Assoc. 1981, 102, 664-665. Schmitt, W.; Purrmann, R.; Jochum, P. Ger. Offen. 2, 658, 538, June 30, 1977. Bowen, R. L . ; Argentar, H. J. Dent. Res. 1972, 51, 473-482. Dnebosky, J.; Hynkova, V.; Hrabak, F. J. Dent. Res. 1975, 54 772-776. Hynkova, V . ; Hrabak, F. Makromol. Chem. 1975, 176, 16691678. Koblitz, F. F.; O'Shea, T. M.; Glenn, J. F.; DeVries, K.L. J. Dent. Res. 1977, 56B, No. 791. Antonucci, J. M.; Stansbury, J. W.; Dudderar, D. J. J. Dent. Res. Res. 1982, 61, 270. Ruyter, I. E . ; Svendsen, S. A. Odontol. Scand. 1978, 36, 75-82.

370 21. 22. 23. 24. 25. 26.

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27. 28. 29. 30. 31. 32. 32a. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51.

INITIATION OF POLYMERIZATION Prior, W. A.; Hendrickson, W. H., Jr. J. Am. Chem. Soc. 1975, 97, 1582-1583. Horner, L . ; Brüggemann, H.; Knapp, K. H. Ann. 1959, 626, 119. Brown, H. C.; Okamoto, Y. J Am. Chem. Soc. 1958, 80, 49794987. Mleziva, J. Plasticke Hmoty. Kaucuk, 1964, 1, 225-231. Mleziva, J. Chem. Prumysl. 1965, 15, 80-85. Brauer, G. M.; Dulik, D.; Antonucci, J. M.; Termini, D. J.; Argentar, H. J. Dent. Res. 1979, 58, 1994-2000. Brauer, G. M.; Stansbury, J. W.; Antonucci, J. M. J. Dent. Res. 1981, 1343-1348. Argentar, H. U. S. Patent 4,243,763, Jan. 6, 1981. Argentar, H. U. S. Patent 4,284,551, Aug. 18, 1981. Kravtsov, D. N.; Faingor, D. N. Izo. Akad. Nauk. SSR, Ser. Khim 1968, 289-296; Chem. Abstr. 1968, 69, 66717. Argentar, H. J. Res. Nat. Bur. Stand. (U.S.), 80A, 173-187. Walling, C.; Indictor, N. J. Am. Chem. Soc. 1958, 80, 58145818. Argentar, H. unpublished results. Stecker, P. G., Ed. "The Merck Index," 3rd ed.: Merck: Rahway, NJ 1968, p. 60. Lapkin, V. A. Farmakol. Tokskol (Moscow), 1974, 37, 660662; Chem. Abstr. 1974, 82 68082. Jacobsen, A. H . ; Pettersen, A. H. personal communication. Bowen, R. L.; Argentar, H. J. Appl. Polymer Sci. 1973, 17 2213-2222. American Dental Association Specification No. 27 for Direct Filling Resins; J. Am. Dent. Assoc. 1977, 94, 1191-1194. Hrabak, F.; Hynkova, V. Macromol. Chem. 1981, 182, 15951603. Brauer, G. M.; Stansbury, J. W. unpublished data. Okada, Y.; Kogyo Kagaku Zasshi 1963, 66, 1317-1320; Engl. Summ. 1963, 66, A83. Yates, W. R.; Ihrig, J. L. J. Am. Chem. Soc. 1965, 87 710715. Ruyter, I. E. Acta Odontol. Scand. 1981, 39, 27-32. Hagger, O. Helv. Chim. Acta 1948, 31, 1624-1630. Hagger, O. Helv. Chim. Acta 1951, 34, 1872-1876. Castan, P.; Hagger, O. U. S. Patent 2,567,803, Sept. 11, 1951. Bredereck, H.; Bäder, E. Chem. Ber. 1954, 87, 129-139. Bredereck, H.; Bäder, E. U. S. Patent 2,846,418, Aug. 5, 1958. Brauer, G. M.; Burns, F. R. J. Polymer Sci. 1956, 19, 311321. Stern, H. J.; Shadbolt, L. E . ; Rawitzer, W. L. British Patent 721,641, Jan. 12, 1955. Shadbolt, L. E. Ger. Offen, 1,937,871, Jan. 29, 1970. Lai, J. U. S. Patent 2,833,753, May 6, 1958.

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Initiator-Accelerator Systems

371

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52. Takaaki, S.; Yuji, M. J. Polymer Sci. 1966, A-1, 4, 2735-2746. 53. Sumitomo Chemical Ltd. Brit. Patent 1,177,879, Jan. 14, 1970. 54. Termin, S. C.; Richards, M. C. U. S. Patent 3,991,008, Nov. 9, 1976. 55. Antonucci, J. M.; Grams, C. L.; Termini, D. J. J. Dent. Res. 1979, 58, 1887-1889. 56. Masuhara, E. Deut. Zahnärztl. Z. 1969, 24, 620-628. 57. Nakabayashi, N.; Masuhara, E.; Michida, E.; Ohmori, I. J. Biomed. Mat. Res. 1978, 12. 149-165. 58. Waller, D. E. U. S. Patent 3,709,866, Jan. 9, 1973. 59. Schmitt, W.; Purrmann, R. Ger. Offen. 2,646,416, May 26, 1977. 60. Brauer, G. M. J Dent. Res. 1978, 57, 597-607. 61. Dart, C. E . ; Cantwell, J. B.; Traynor, J. R.; Jaworzyn, J.N. U. S. Patent 4,089,763, May 16, 1978. 62. Cook. W. D. J. Dent. Res. 1980, 59, 800-808. 63. Kilian, R. J. "Biomedical and Dental Applications of Polymers" Gebelin, C. G.; Koblitz, F. F., Ed; Plenum: New York, 1981, pp. 411-417. 64. Bier, A. Rec. Trav. Chim. 1956, 75, 866-870. RECEIVED October 5, 1982

27 Synthesis of Biodegradable Polymers for Biomedical Utilization J. H E L L E R

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

SRI International, Polymer Sciences Department, Menlo Park, CA 94025

Polymer bioerosion is defined as the conversion of an initially water-insoluble material to a water-soluble material and is discussed in terms of three distinct mechanisms denoted as Type I, II, or III. Solubilization by Type I erosion involves hydrolytic bond cleavage occurring in water-soluble polymers that have been insolubilized by covalent crosslinks. Solubilization by Type II erosion involves reactions of pendant groups, and solubilization by Type III erosion involves backbone cleavage. Solubilization by simple dissolution is not considered. Each type of bioerosion is illustrated with specific examples that include gelatin-based surgical aids, enteric-type coatings, erodible sutures, surgical adhesives, oviduct blocking agents, and controlled drug release. Polymer bioerosion can be defined as the conversion of an initially water-insoluble material to a water-soluble material and does not necessarily signify a major chemical degradation. In this review we do not consider a simple dissolution process, and we classify the various bioerosion mechanisms into the the three distinct types shown in Figure 1 . In general terms, Type I erosion encompasses water-soluble polymers that have been insolubilized by hydrolytically unstable crosslinks. Type II erosion includes polymers that are initially water-insoluble and are solubilized by hydrolysis, ionization, or protonation of a pendant group. Type III erosion includes hydrophobic polymers that are converted to small watersoluble molecules by backbone cleavage. Clearly, these three types represent extreme cases, and actual erosion can be a combination of these types. Thus, it is 0097-6156/83/0212-0373$06.00/0 © 1983 American Chemical Society

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p o s s i b l e to develop a combination of Type I and Type I I I e r o s i o n i n which i n i t i a l h y d r o l y s i s i n v o l v e s c r o s s l i n k cleavage with subsequent backbone cleavage o f the high molecular weight waters o l u b l e polymer. S i m i l a r l y , a combination o f Type I I and Type I I I e r o s i o n can be developed i n which i n i t i a l s o l u b i l i z a t i o n i s by i o n i z a t i o n , protonation, or h y d r o l y s i s , followed by backbone cleavage of the s o l u b l e polymer.

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Type I E r o s i o n The most widely used polymer system that bioerodes by Type I erosion or by a combination o f Type I and Type I I I e r o s i o n i s g e l a t i n that has been i n s o l u b i l i z e d by heat treatment, aldehyde treatment, or chromic a c i d treatment. Aside from the photographic i n d u s t r y , i n s o l u b i l i z e d g e l a t i n a l s o f i n d s a p p l i c a t i o n as s u r g i c a l a i d s such as dusting powder for s u r g i c a l gloves or as sponges or f i l m s . Dusting powders are prepared by heating g e l a t i n at 142°C f o r 25 hours, and i n s o l u b i l i z a t i o n i s b e l i e v e d to occur by formation of i n t e r c h a i n amide l i n k s (2). S u r g i c a l sponges or f i l m s are u s e f u l i n a r r e s t i n g s u r g i c a l hemorrhage and are prepared by t r e a t i n g s t e r i l e g e l a t i n with formaldehyde. Although d e t a i l s are p o o r l y understood, i n s o l u b i l i z a t i o n occurs by r e a c t i o n o f g e l a t i n amino groups with formaldehyde followed by r e a c t i o n of the hydroxymethylamino groups Q , j^, Formaldehyde c r o s s l i n k e d g e l a t i n has a l s o been used as a matrix i n c o n t r o l l e d drug r e l e a s e a p p l i c a t i o n s . However, because i n s o l u b i l i z a t i o n of water-soluble polymers by c r o s s l i n k i n g produces hydrogels that are completely permeated by water, they are c l e a r l y unable to immobilize small molecules having a p p r e c i a b l e water s o l u b i l i t y . Consequently, u s e f u l n e s s of these m a t e r i a l s i s l i m i t e d to molecules having extremely low water s o l u b i l i t y or to macromolecules that can be p h y s i c a l l y entangled i n the hydrogel so that they can not d i f f u s e out of the matrix even though they are f r e e l y s o l u b l e i n water. An example of the f i r s t a p p l i c a t i o n i s shown i n F i g u r e 2, which shows r e l e a s e of the h i g h l y w a t e r - i n s o l u b l e hydrocortisone acetate from a g e l a t i n matrix c r o s s l i n k e d with formaldehyde (JO. As i n d i c a t e d by the f i r s t - o r d e r dependence, drug i s released by a simple d i f f u s i o n a l process and the c r o s s l i n k e d g e l a t i n simply provides cohesiveness f o r the drug p a r t i c l e s . Even though r e l e a s e k i n e t i c s are not constant, u s e f u l r e l e a s e over many days i s achieved. The f i r s t - o r d e r drug r e l e a s e a l s o i n d i c a t e s that matrix erosion makes l i t t l e or no c o n t r i b u t i o n to drug r e l e a s e , and, because e r o s i o n of a formaldehyde-crosslinked g e l a t i n i s slow, drug d e p l e t i o n can occur before s i g n i f i c a n t matrix e r o s i o n i s noted. Such a device may be u s e f u l i n a p p l i c a t i o n s where z e r o order k i n e t i c s are not important and removal of the expended device i s not convenient or d e s i r a b l e .

27.

HELLER

Synthesis of Biodegradable

375

Polymers

TYPE I

"X

®

φ

I

TYPE I I —I—Γ-

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Α

—ι—Γ­

Β

Α

C

A

represents a hydrophobic substituent and

Β -> C

represents hydrolysis, ionization or protonation

TYPE I I I

Figure 1. Schematic representation of bioerosion mechanisms. (Reprinted with permission from Réf. 1. Copyright 1980.)

40i

30h

2 20h σ

lOh

-M—b 24

48

72 Time (doys)

96

120

144

Figure 2. Release of hydrocortisone acetate from a cross-linked gelatin matrix. (Reprinted with permission from Réf. 1. Copyright 1980.)

376

INITIATION

OF POLYMERIZATION

Release o f macromolecules from b i o e r o d i b l e hydrogels has received 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 , and most past e f f o r t s have been devoted t o the immobilization o f macromolecules i n hydrogels by p h y s i c a l entanglement (6). However, because o f the growing r e c o g n i t i o n o f the therapeutic p o t e n t i a l o f macromolecules, t h e i r entanglement i n a hydrogel matrix and c o n t r o l l e d r e l e a s e by matrix e r o s i o n i s becoming recognized as an important methodology (7, S).

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

Type I I E r o s i o n Because t h i s type s o l u b i l i z a t i o n i n v o l v e s the r e a c t i o n o f a pendant group, b i o e r o s i o n proceeds without s i g n i f i c a n t changes i n molecular weight. Therefore, unless the polymer backbone a l s o undergoes b i o e r o s i o n , polymers i n t h i s category are only u s e f u l i n t o p i c a l a p p l i c a t i o n s where e l i m i n a t i o n o f high molecular weight, water-soluble macromolecules proceeds with no difficulty. An important c l a s s o f polymers that undergo Type I I e r o s i o n are polymers c o n t a i n i n g c a r b o x y l i c a c i d f u n c t i o n s . These s o l u b i l i z e by i o n i z a t i o n and consequently are i n s o l u b l e a t low pH and s o l u b l e a t high pH. A major a p p l i c a t i o n f o r such polymers i s as e n t e r i c coatings, designed t o p r o t e c t therapeutic agents during passage through the a c i d i c stomach and t o a b r u p t l y d i s s o l v e i n the higher pH environment o f the i n t e s t i n e s . An i n t e r e s t i n g example i s the p a r t i a l e s t e r s o f a methyl v i n y l ether and maleic anhydride copolymer (9., _10, 11 ) . OCH I fCH -CH-CH I COOH Q 3

0

2

CH} | n COOR

These polymers have a c h a r a c t e r i s t i c narrow pH range above which they are s o l u b l e and below which they a r e i n s o l u b l e , and t h i s pH range v a r i e s with the s i z e o f the R-group i n the e s t e r p o r t i o n o f the copolymer. T h i s e f f e c t i s shown i n F i g u r e 3 (J2). T h i s behavior can be r e a d i l y understood by c o n s i d e r i n g the number o f i o n i z e d carboxyls that are necessary t o drag the polymer chain i n t o s o l u t i o n . With r e l a t i v e l y small e s t e r groups, only a low degree o f i o n i z a t i o n i s needed t o s o l u b i l i z e the polymer, and hence the d i s s o l u t i o n pH i s low. As the s i z e of the a l k y l group i n c r e a s e s , so does the hydrophobicity, and p r o g r e s s i v e l y more i o n i z a t i o n i s necessary t o s o l u b i l i z e the polymer r e s u l t i n g i n i n c r e a s i n g l y high d i s s o l u t i o n pH. The same argument holds f o r polymers having the same e s t e r grouping but d i f f e r e n t degrees o f e s t e r i f i c a t i o n . The higher the degree o f e s t e r i f i c a t i o n , the more hydrophobic the polymer and consequently the higher the d i s s o l u t i o n pH.

Figure 3. Relationship between pH of dissolution and size of ester group in half-esters of methyl vinyl ether-maleic anhydride copolymers. (Reprinted with permission from Ref. 12. Copyright 1980, John Wiley & Sons, Inc.)

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

378

INITIATION OF

POLYMERIZATION

Even though p a r t i a l l y e s t e r i f i e d copolymers of methyl v i n y l ether-maleic anhydride copolymers were o r i g n a l l y designed to d i s s o l v e abruptly with an increase i n e x t e r n a l pH, i n a constant pH environment they undergo a c o n t r o l l e d d i s s o l u t i o n process and are t h e r e f o r e u s e f u l m a t e r i a l s f o r the c o n t r o l l e d r e l e a s e of therapeutic agents dispersed w i t h i n them (12). Figure 4 shows polymer d i s s o l u t i o n r a t e and the rate o f hydrocortisone r e l e a s e f o r an η-butyl h a l f - e s t e r of a methyl v i n y l ether-maleic anhydride copolymer f i l m c o n t a i n i n g the dispersed drug. Each p a i r of p o i n t s represents a separate device i n which the amount of drug released by the device i n t o the wash s o l u t i o n was determined by uv measurements and the amount of polymer d i s s o l v e d was c a l c u l a t e d from the t o t a l weight l o s s of the device. The e x c e l l e n t l i n e a r i t y of both polymer e r o s i o n and drug r e l e a s e over the l i f e t i m e of the device provides strong evidence f o r a s u r f a c e - e r o s i o n mechanism and f o r n e g l i g i b l e d i f f u s i o n a l r e l e a s e of the drug. The l a t t e r r e s u l t was independently v e r i f i e d by p l a c i n g a drug-containing f i l m i n water at a pH low enough that no d i s s o l u t i o n of the matrix took place and p e r i o d i c a l l y a n a l y z i n g the aqueous s o l u t i o n f o r hydrocortisone. None was found over s e v e r a l days. Type I I I E r o s i o n Polymers undergoing Type I I I e r o s i o n have found a p p l i c a t i o n s i n (a) absorbable s u r g i c a l sutures, (b) s u r g i c a l adhesives, (c) contraception, and (d) c o n t r o l l e d drug r e l e a s e . Absorbable S u r g i c a l Sutures. The search f o r an optimum absorbable s u r g i c a l suture evolved through various forms o f c o l l a g e n to the modern day catgut (13)· Although catgut i s a strong and e f f e c t i v e suture, i t s u f f e r s from v a r i o u s disadvantages, such as batch-to-batch v a r i a t i o n ; s t i f f n e s s , which r e q u i r e s the use of a c o n d i t i o n i n g storage f l u i d ; and o c c a s i o n a l intense t i s s u e r e a c t i v i t y . For these reasons i t was d e s i r a b l e to develop a s y n t h e t i c m a t e r i a l that could be t a i l o r e d to meet i d e a l suture requirements. The f i r s t s y n t h e t i c absorbable suture that reached commercial production i s Dexon, manufactured by American Cyanamid Co. I t i s p o l y ( g l y c o l i c acid) prepared by the polymerization of g l y c o l i d e (14). Another absorbable 0 II

0

II

iCH -C-0} η 2

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

27.

HELLER

Synthesis of Biodegradable

Polymers

T I M E (hours)

Figure 4. Rate of polymer dissolution and hydrocortisone release from n-butyl half-ester of methyl vinyl ether-maleic anhydride copolymer containing 10 wt% drug. (Reprinted with permission from Ref. 12. Copyright 1980, John Wiley & Sons, Inc.)

379

380

INITIATION OF POLYMERIZATION

suture i s V i c r y l , manufactured by E t h i c o n Company. V i c r y l i s the 10/90 c o p o l y m e r r p o l y ( l a c t i d e - c o - g l y c o l i d e ) and i s prepared by the copolymerization o f 9 parts L(-) l a c t i d e and 1 part g l y c o l i d e (15). Both polymers degrade by a simple h y d r o l y s i s r e a c t i o n , and no c e l l u l a r or enzyme a c t i v i t y i s necessary f o r suture absorption (16, 17)* S u r g i c a l Adhesives. I n many procedures i t i s d i f f i c u l t t o use conventional s u t u r i n g techniques. Consequently, procedures whereby i n c i s i o n s or cut ends o f a r t e r i e s can be joined by means of an adhesive would represent a s i g n i f i c a n t s u r g i c a l advance

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

Q8). Because α-cyanoacrylates contain a double bond s u b s t i t u t e d by two electron-withdrawing s u b s t i t u e n t s , they are h i g h l y s u s c e p t i b l e t o a n i o n i c i n i t i a t i o n , and water i s b a s i c enough t o i n i t i a t e very r a p i d polymerization. Therefore, because moisture can i n i t i a t e polymerization and because the formed polymer i s able t o f i r m l y adhere t o moist surfaces, i t has evoked considerable medical i n t e r e s t as a t i s s u e adhesive 0 9 , 20). However, i t has been observed that methyl α-cyanoacrylate i n these a p p l i c a t i o n s leads t o t i s s u e i n f l a m a t i o n and c e l l n e c r o s i s , and f u r t h e r research has shown that the polymer undergoes a degradation r e a c t i o n that occurs both _in v i v o and i n v i t r o . Because poly(alky1 α-cyanoacrylates) are s t r u c t u r a l l y s i m i l a r t o p o l y ( v i n y l i d e n e cyanide), which has been postulated to degrade by a reverse Knoevenagel r e a c t i o n with e v o l u t i o n o f formaldehyde (21), the f o l l o w i n g degradation mechanism f o r p o l y ( a l k y l cyanoacrylates), which a l s o degrade with e v o l u t i o n o f formaldehyde, has been suggested (22, 23): CN

CN

^CH -C-CH -C^ 0

2

CN CN I l ^H -C-CH 0H + 0 C ^ ι \ C00R C00R

OlP

+

9

C00R

o

1

2 ,

J

C00R

CN

1

CN

+ I C00R

QC^>

H 0 o

1

»

HC^ + 0H^ \ C00R

CN

CN

M]H -C-CH 0H + O l F z

I

z

COOR

o

* 2

OH

-C0 + [CHj I I COOR OH 2

0 - H 0 + H-C-H o

2

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

27.

HELLER

Synthesis of Biodegradable

Polymers

381

The r a t e o f degradation depends on the s i z e o f the e s t e r group, and, as shown i n Figure 5, the r a t e o f degradation as measured by formaldehyde e v o l u t i o n o f the methyl e s t e r a t pH 7.0 i s considerably f a s t e r than the r a t e o f degradation o f the higher e s t e r s . A dependence o f degradation r a t e on polymer molecular weight has a l s o been reported (24). Because poly(methyl α-cyanoacrylate) degrades r e l a t i v e l y r a p i d l y t o methyl α-cyanoacrylate, which i s an i n t e n s e l y n e c r o t i z i n g and pyogenic compound (25, _26), polymerization o f t h i s compound i s c l e a r l y not a v i a b l e s u r g i c a l procedure. However, higher α-cyanoacrylate e s t e r s are considerably l e s s t o x i c , and polymer degradation occurs a t a much lower r a t e so that these m a t e r i a l s may have p o t e n t i a l as s u r g i c a l l y u s e f u l m a t e r i a l s (27, 28, 29). Contraception. The r a p i d polymerization o f methyl a cyanoacrylate and i t s e f f e c t on l i v i n g t i s s u e has been u t i l i z e d i n female s t e r i l i z a t i o n by oviduct blockage (30)* In t h i s procedure methyl α-cyanoacrylate i s i n s t i l l e d i n t o the oviduct where i t r a p i d l y polymerizes i n t o a s o l i d . Subsequent degradation o f the polymer leads t o formation o f scar t i s s u e , which e v e n t u a l l y permanently blocks the oviduct. The o v e r a l l process i s shown i n F i g u r e 6 (31). Major advantage o f t h i s method i s that the methyl cyanoacrylate can be i n s t i l l e d by a s p e c i a l l y - d e s i g n e d t r a n s c e r v i c a l d e l i v e r y device (31) by t r a i n e d paramedical personnel and does not r e q u i r e a s u r g i c a l procedure. Thus, t h i s method may be a t t r a c t i v e t o developing c o u n t r i e s with serious population problems. C o n t r o l l e d Drug Release—Because the degradation products o f Type I I I b i o e r o s i o n are small, water s o l u b l e molecules, the p r i n c i p a l a p p l i c a t i o n o f polymers undergoing such degradation i s f o r the systemic a d m i n i s t r a t i o n o f therapeutic agents from subcutaneous, intramuscular or i n t r a p e r i t o n e a l implantation s i t e s . A p p l i c a t i o n o f Type I I I b i o e r o s i o n to c o n t r o l l e d drug r e l e a s e was f i r s t described i n 1970 (32) and has since then been e x t e n s i v e l y i n v e s t i g a t e d . The v a r i o u s types o f devices c u r r e n t l y under development can be c l a s s i f i e d i n t o (a) d i f f u s i o n a l and (b) monolithic (7.). D i f f u s i o n a l Devices. In these systems a drug-containing core i s surrounded by a b i o e r o d i b l e r a t e - c o n t r o l l i n g membrane. Thus, these devices combine the a t t r i b u t e s o f a r a t e - c o n t r o l l i n g polymer membrane, which provides a constant r a t e o f drug r e l e a s e from a r e s e r v o i r - t y p e device, with e r o d i b i l i t y , which r e s u l t s i n b i o e r o s i o n and makes s u r g i c a l removal o f the drug-depleted device unnecessary. Because constancy o f drug release demands that the b i o e r o d i b l e polymer membrane remain e s s e n t i a l l y unchanged during the d e l i v e r y regime, s i g n i f i c a n t b i o e r o s i o n must not occur u n t i l a f t e r drug d e l i v e r y has been completed. Thus polymer capsules w i l l remain i n the t i s s u e f o r varying lengths o f time a f t e r completion o f therapy.

INITIATION OF

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

382

Figure 5. Homogeneous degradation of α-cyanoacrylate polymers in aqueous acetonitrile. (Reprinted with permission from Ref. 22. Copyright 1966, John Wiley & Sons, Inc.)

27.

HELLER

Synthesis of Biodegradable

383

Polymers

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

ACTION O F M C A W I T H I N OVIDUCT

APPROXIMATE TIME AFTER MCA EJECTION ο few sec

ACTION LIQUID MCA ENTERS OVIDUCT

sec. to minutes

LIOUID MCA—^ SOLID POLY (MCA)

OVIDUCT

/

ν ν

ν

severol days

POLY(MCA) DEGRADES TO SMALL MOLECULES

\

(

•·

INFLAMMATORY RESPONSE COLLAGEN TISSUE DEPOSITED. BLOCKING TUBE

severe I months

Permonent lu bo I blockoge with person's own fibrous tissue; no troces of poly MCA Figure 6. Schematic of the action of methyl cyanoacrylate within an oviduct. (Reprinted from Ref. 31. Copyright 1981, American Chemical Society.)

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

384

INITIATION OF

POLYMERIZATION

Two types of e r o d i b l e d i f f u s i o n a l devices are under development: (1) rod-shaped i n s e r t and (2) microcapsules. Each of these types of devices has c e r t a i n advantages and c e r t a i n disadvantages. Subdermal placement o f rod-shaped devices r e q u i r e s a minor s u r g i c a l procedure, and the device remains v i s i b l e as a small lump. However, because i t can be r e a d i l y removed should t e r m i n a t i o n o f therapy become d e s i r a b l e or necessary, therapy l a s t i n g many months i s p o s s i b l e . On the other hand, microcapsules can be r e a d i l y i n j e c t e d through an 18gauge hypodermic needle, but r e t r i e v a l of microcapsules i s not p o s s i b l e without an extensive s u r g i c a l i n t e r v e n t i o n . For t h i s reason i t has been suggested that the capsules be designed f o r r e l a t i v e l y short-term r e l e a s e , such as three months (33)* Major emphasis f o r the development o f these e r o d i b l e d i f f u s i o n a l systems have been devices that r e l e a s e c o n t r a c e p t i v e s t e r o i d s or n a r c o t i c a n t a g o n i s t s . The polymer systems most e x t e n s i v e l y i n v e s t i g a t e d f o r use as a subdermal capsule f o r the r e l e a s e of l e v o n o r g e s t r e l were v a r i o u s a l i p h a t i c p o l y e s t e r s and, i n p a r t i c u l a r , poly(u)-caprolactone). The r e l e a s e r a t e of l e v o n o r g e s t r e l from such a device, developed by the Research T r i a n g l e I n s t i t u t e and named Capronor, i s shown i n F i g u r e 7 (34). C l e a r l y , e x c e l l e n t constant d a i l y r e l e a s e over many months has been achieved, and those devices are about to undergo Phase I I c l i n i c a l t e s t i n g (35). The polymer most a c t i v e l y i n v e s t i g a t e d f o r use i n a microcapsular d e l i v e r y system i s p o l y ( D L - l a c t i c a c i d ) , and r e l e a s e of norethindrone measured as blood plasma l e v e l from such microcapsules i s shown i n F i g u r e 8 (36). These data show reasonably constant blood l e v e l s and demonstrate that f o r a f i x e d t o t a l weight o f microcapsules, r a t e of drug r e l e a s e and d u r a t i o n o f therapy can be regulated by capsule s i z e . Furthermore, because each capsule f u n c t i o n s as an independent drug d e l i v e r y system, the r a t e of drug d e l i v e r y can a l s o be regulated by v a r i a t i o n i n the t o t a l number of i n j e c t e d microcapsules. M o n o l i t h i c D e v i c e s — I n these systems the drug i s homogeneously dispersed w i t h i n a b i o e r o d i b l e polymer matrix, and r e l e a s e of the drug can be c o n t r o l l e d e i t h e r by d i f f u s i o n or by polymer e r o s i o n . I f e r o s i o n of the matrix i s very much slower than drug d i f f u s i o n , then r e l e a s e k i n e t i c s f o l l o w the Higuchi model (37) and drug r e l e a s e r a t e decreases e x p o n e n t i a l l y with time, f o l l o w i n g t ~ ' ' dependence over a major p o r t i o n of the release rate. I f e r o s i o n i s r e l a t i v e l y f a s t and the drug i s w e l l immobilized i n the s o l i d matrix so that d i f f u s i o n a l r e l e a s e i s minimal, matrix e r o s i o n determines r a t e of drug r e l e a s e . However, i t i s important to d i s t i n g u i s h two types of h y d r o l y t i c e r o s i o n of a s o l i d , hydrophobic polymer. In one, r e f e r r e d to as homogeneous e r o s i o n , the h y d r o l y s i s occurs at a uniform r a t e throughout the matrix. In the other, c a l l e d heterogeneous 2

RELEAS

- I t *

-

Ι

160

ι

170

190

200

210

220

230 DAY

1 . . . I . . . I. . . I . ι . I . i

180

ι ι I t ι ι

"

240

250

260

270

280

ANIMAL SACRIFICED JUNE 23. 1977 (DAY 292)

290

Figure 7. Daily rate of release of norgestrel from poly(u-caprolactone) capsule implanted in rat after 32 days in vitro. (Reprinted with permission from Ref. 4. Copyright 1980, Academic Press, Inc.)

5

10

15

40,

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

300

1

00 LO

ni

to

386

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

INITIATION OF POLYMERIZATION

0

12

24

36

48

60

72

84

96

108

120

132

144

156

168

TIME, days Figure 8. Baboon peripheral serum levels of immune-reactive norethisterone after intramuscular injection of 300 mg of poly(DL-lactic acid) microcapsules containing 75 mg of drug. Size of capsules: A, 10-240 μτη; Β, 65-124 μηι; C, 10-40 μτη. (Reprinted with permission from Ref. 36, p. 75. Copyright 1980.)

180

27.

HELLER

Synthesis of Biodegradable Polymers

387

e r o s i o n , the process i s confined t o the surface o f the device and i s , f o r that reason, commonly r e f e r r e d t o as surface e r o s i o n

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

(38)· Drug r e l e a s e rate f o r matrices undergoing bulk erosion i s nonlinear and d i f f i c u l t t o p r e d i c t because i t i s determined by a combination o f d i f f u s i o n and e r o s i o n . However, drug r e l e a s e from devices undergoing surface e r o s i o n i s p r e d i c t a b l e and can lead t o zero o r d e r - k i n e t i c s provided d i f f u s i o n a l r e l e a s e o f the drug i s minimal and the o v e r a l l surface area o f the device remains e s s e n t i a l l y constant. Many s t u d i e s on drug r e l e a s e from b i o e r o d i b l e monolithic devices have been performed and the majority o f these s t u d i e s used p o l y ( l a c t i c acid) or copolymers o f l a c t i c and g l y c o l i c a c i d s (J_, 7.). Because these polymers undergo a bulk degradation process, drug r e l e a s e i s nonlinear and not amenable t o a d e t a i l e d mechanistic i n t e r p r e t a t i o n . Nevertheless, s e v e r a l systems having p o t e n t i a l f o r the prolonged released o f v a r i o u s therapeutic agents such as c o n t r a c e p t i v e s t e r o i d s , n a r c o t i c antagonists, a n t i c a n c e r agents, and a n t i m a l a r i a l agents have been demonstrated (J29, 40, 41). Because surface e r o s i o n r e s u l t s i n constant and p r e d i c t a b l e r a t e o f drug r e l e a s e , t h i s type o f e r o s i o n i s c l e a r l y p r e f e r r a b l e t o bulk e r o s i o n . However, t o achieve surface e r o d i b i l i t y , a system must be devised i n which the r a t e o f polymer degradation a t the surface o f a device i s very much f a s t e r than the r a t e o f degradation i n the i n t e r i o r . One approach t o surface e r o d i b i l i t y i s t o prepare a polymer that contains linkages that are s t a b l e i n base but are very l a b i l e i n a c i d . Because one such linkage i s an ortho e s t e r , p o l y ( o r t h o e s t e r s ) are c u r r e n t l y under i n t e n s i v e development as monolithic devices f o r zero order drug r e l e a s e (7). Poly(ortho e s t e r s ) were f i r s t d i s c l o s e d i n a s e r i e s o f patents assigned t o the A l z a Corporation (42-45) and were prepared by a t r a n s e s t e r i f i c a t i o n r e a c t i o n as f o l l o w s :

EtO

-fO 0-R} η

OEt * + HO-R-OH

/

0

+ EtOH

Although the A l z a poly(ortho e s t e r ) system has never been s t r u c t u r a l l y i d e n t i f i e d other than by i t s tradename Chronomer, and l a t e r Alzamer, s e v e r a l p u b l i c a t i o n s provide a general d e s c r i p t i o n o f the use o f the polymer f o r the r e l e a s e o f naltrexone (46) and c o n t r a c e p t i v e s t e r o i d s (47, 48, 49). Poly(ortho e s t e r s ) have a l s o been produced by the a d d i t i o n o f d i o l s t o diketene a c e t a l s (50)* P r i n c i p a l l y because o f ease of monomer s y n t h e s i s , polymers were prepared by the a d d i t i o n o f

388

INITIATION OF

POLYMERIZATION

various d i o l s to 3,9-bis(methylene 2,4,8,10-tetraoxaspiro[5,5] undecane), R = H or 3 , 9 - b i s ( e t h y l i d o n e 2,4,8,10-tetraoxaspiro [5,5]undecane), R = CHg. R

0-CH \

2

/

o

C=C

RCH

CH-0

/

/

\

2

CH.R

\

c

C \ O-CH,

/

N

^CH -0 2

HOR'OH

Η

2

0

2

+

CH -0

O-CH,

\ Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

C=C

O-CH^

/ 0

R /

2

C

Η

-

CH -0 \

/

^

/

/ c N

2

!

0-R -

Because poly(ortho esters) are s t a b l e i n a l k a l i n e environments, i n i t i a l attempts to develop a surface-eroding system were based on the i n c o r p o r a t i o n o f sodium carbonate i n t o the bulk m a t e r i a l . Polymer erosion was expected to occur only at the outer surface of a s o l i d device i n which the incorporated b a s i c s a l t i s n e u t r a l i z e d by the e x t e r n a l b u f f e r (51, 52). However, i t was found that as a consequence of an osmotic imbibing o f water caused by the incorporated water-soluble s a l t , a s w e l l i n g f r o n t develops, and drug i s released from the matrix by d i f f u s i o n from the swollen polymer. Because poly(ortho esters) are s t a b l e i n base, no polymer erosion takes p l a c e . Nevertheless, as shown i n F i g u r e 9, constant drug r e l e a s e i s achieved. The use o f o s m o t i c a l l y a c t i v e n e u t r a l s a l t s such as sodium s u l f a t e , a l s o shown i n Figure 9, produces a constant rate o f drug r e l e a s e f o r about 60 days, a f t e r which drug r e l e a s e rate a c c e l e r a t e s up t o drug d e p l e t i o n (j£, 54), The number below the arrow i n d i c a t e s weight l o s s a t 160 days. C l e a r l y , polymer erosion s i g n i f i c a n t l y lags drug r e l e a s e . Furthermore, the r a t e of drug r e l e a s e observed with sodium s u l f a t e i s not that expected from a simple movement o f a s w e l l i n g f r o n t , but instead i n d i c a t e s that the a c t i v e surface area increases with time. T h i s has been v e r i f i e d by observations o f a s u b s t a n t i a l increase i n s i z e and by scanning e l e c t r o n micrograph, which revealed a foam-type i n t e r i o r and heavy surface c r a t e r i n g . Current work i s aimed at producing poly(ortho ester) systems i n which drug r e l e a s e and polymer erosion takes place concomitantly by using incorporated agents that are capable o f lowering the pH at the polymer-water i n t e r f a c e .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

27.

HELLER

Synthesis of Biodegradable

389

Polymers

Time - Days Figure 9. Norethindrone (NE) release from 3,9-bis(methylene 2,4,8,10-tetraoxa­ spiro [5,5] undecane)/1,6-hexane diol polyfortho ester), 63-mm-diameter discs at pH 7.4 and 37 °C. Key: O, 10 wt% NE, 10 wt% Na SO,„ 0.6-mm-thick disc, total drug content 2.4 mg; Δ , 10 wt% NE, 10 wt% Na C0 1.2-mm-thick disc, total drug content 4.0 mg. 2

2

3)

390

INITIATION OF POLYMERIZATION

LITERATURE CITED 1. 2. 3. 4. 5. 6.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Heller, J. Biomaterials 1980, 1, 51-57. Tobolsky, Α. V. Nature 1967, 215, 509-510. Davis, P.; Tabor, B. E . , J. Polymer Sci.: Part A 1963, 1, 799-815. Coopes, I. H. J. Polymer Soi. Part A-1 1970, 8, 1793-1811. Robinson, I. D. J. Appl. Polymer Sci. 1964, 8, 1903-1918. Goldman, R.; Goldstein, L.; Katchalsky, E. "Biochemical Aspects of Reactions on Solid Supports"; Stark, G. R., Ed. Academic Press: New York, 1971, pp 1-78. Heller, J. "Pharmaceutical Applications of Controlled Release Drug Delivery Systems"; Langer, R. S.; Wise, D. L., Eds. CRC Press, in press. Heller, J.; Baker, R. W.; Helwing, R. F.; Tuttle, M. E. J. Biomed. Mater. Res. to be published. Lappas, L. C.; McKeehan, W. J. Pharm. Sci. 1962, 51, 808. Lappas, L. C.; McKeehan, W. J. Pharm. Sci. 1965, 54, 176181. Lappas, L. C.; McKeehan, W. J. Pharm. Sci. 1967, 56, 12571261. Heller, J.; Baker, R. W.; Gale, R. M.; Rodin, J. O. J. Appl. Polymer Sci. 1978, 22, 1991-2009. Goldenberg, I. S. Surgery 1959, 46, 908-912. Frazza, E. J.; Schmitt, E. E. J. Biomed. Mater. Res. Symposium 1971, 1, 43-58. Craig, P. H.; Williams, J. A.; Davis, K. W.; Magoun, A. D.; Levy, A. J.; Bogdansky, S.; Jones, J. P. Jr. Surg. Gynecol. Obstet. 1975, 141, 1-10. Salthouse, T. N.; Matlaga, B. F. Surg. Gynecol. Obstet. 1976, 142, 544-550. Chu, C. C. J. Biomed. Mater. Res., 1981, 15, 19-27. Nathan, H. S.; Nachlas, M. M.; Solomon, R. D.; Halpern, B. D.; Seligman, A. M. Ann. Surg. 1960, 152, 648-659. Leonard, F.; Kulkarni, R. K.; Nelson, J.; Brandes, G. J. Biomed. Mater. Res. 1967, 1, 3-9. Leonard, F.; Hodge, J. W. Jr.; Houston, S; Ousterhout, D. K. J. Biomed. Mater. Res. 1968, 2, 173-178. Gilbert, H; Miller, F. F.; Averill, S. J.; Schmidt, R. F.; Stewart, F. D.; Trumbull, H. L. J. Am. Chem. Soc. 1954, 76, 1074-1076. Leonard, F.; Kulkarni, R. K.; Brandes, G; Nelson, J.; Cameron, J. J. J. Appl. Polymer Sci. 1966, 10, 259-272. Wade, C.W.R.; Leonard, F. J. Biomed. Mater. Res. 1972, 6, 215-220. Venzin, W. R.; Florence, A. J. J. Biomed. Mater. Res. 1980, 14, 93-106.

27. HELLER 25. 26. 27. 28. 29. 30.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch027

31.

32. 33. 34. 35. 36.

37. 38. 39.

40.

41. 42. 43.

Synthesis of Biodegradable Polymers

Cameron, J. L; Woodward, S. C.; Herrmann, J. B. Arch. Surg. 1064, 89, 546. Woodward, S. C.; Herrmann, J. B.; Leonard, F. Fed. Proc. 1964, 23, 495. Woodward, S. C.; Herrmann, J. B.; Cameron, J. L; Brandes, G.; Pulaski, E. J.; Leonard, F. Ann. Surg. 1965, 162, 113122. Lehman, R.A.W.; Hayes, G. J.; Leonard, F. Arch. Surg., 93, 441-446. Leonard, F.; Kulkarni, R. K.; Nelson, J.; Brandes, G. J. Biomed. Mater. Res., 1967, 1, 3-9. Stevenson, T. C.; Taylor, D. S. J. Obstet. Gynaecol. Brit. Comm. 1972, 79, 1028-1039. Hoffman, A. S.; Hale, T. J.; Nightingale, J.A.S.; Halbert, S. Α.; Buckles, R. G. Presented at Second World Congress of Chemical Engineering and World Chemical Exposition, Montreal, Canada, October 4-9, 1981. Yolles, S; Eldridge, J. E . ; Woodland, J.H.R., Polymer News 1970, 1, 9-15. Beck, L. R.; Pope, V. Z.; Cowsar, D. R.; Lewis, D. H.; Tice, T. R., Contracept. Deliv. Syst. 1980, 1, 79-86. Pitt, C. G.; Marks, T.A.; Schindler, A. "Controlled Release of Bioactive Materials"; Baker, R. W., Ed.; Academic Press: New York, 1980, pp. 19-43. Pitt, C. G.; Schindler, A. "Pharmaceutical Applications of Controlled Release Drug Delivery Systems"; Langer, R. S.; Wise, D. L. Eds.; CRC Press, in press. Beck, L. R.; Cowsar, D. R.; Lewis, D. H., "Biodegradables and Delivery Systems for Contraception"; Hafez, E.S.E.; van Os, W.A.A., Eds.; G. K. Hall Medical Publishers: Boston, 1980, pp 63-81. Higuchi, T. J. Pharm. Sci. 1961, 50, 874-875. Heller, J.; Baker, R. W. "Controlled Release of Bioactive Materials"; Baker, R. W., Ed; Academic Press: New York, 1980, pp. 1-17. Wise, D. L.; Schwope, A. D.; Harringan, S. E . ; McCarthy, D. A.; Howes, J. F. "Polymeric Delivery Systems"; Kostelnik, R. J., Ed.; Gordon and Breach Science Publishers: New York, 1978, pp. 75-86. Wise, D. L; Gregory, J. B.; Newberne, P. M.; Bartholow, L. C.; Stanbury, J. B. "Polymeric Delivery Systems"; Kostelnik, R. J., Ed.; Gordon and Breach Science Publishers: New York, 1978, pp. 121-136. Yolles, S; Sartori, M. F. "Drug Delivery Systems", Juliano, R. L., Ed.; Oxford university Press, New York, 1980, pp. 84-111. Choi, N. S.; Heller, J. U.S. Patent 4,093,709, June 6, 1978. Choi, N. S.; Heller, J. U.S. Patent 4,131,648, December 26, 1978.

391

392 44. 45. 46. 47. 48. 49.

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50. 51. 52. 53. 54.

INITIATION OF POLYMERIZATION

Choi, N. S.; Heller, J. U.S Patent 4,138,344, February 6, 1979. Choi, N. S., Heller, J. U.S. Patent 4,180,646, December 25, 1979 Capoza, R. C.; Sendelbeck, L.; Balkenhol, W. J. "Polymeric Delivery System"; Kostelnik, R. J. Ed.; Gordon and Breach Science Publishers, New York, 1978, pp. 59-70. Benagiano, G.; Gabelnick, H. L. J. of Steroid Biochem. 1979, 11, 449-455. Pharriss, B. B.; Place, V. A.; Sendelbeck, L; Schmitt, E. E. J. Reproductive Med. 1976, 1 7 , 91-97. Benagiano, G.; Schmitt, E . ; Wise, D.; Goodman, M. J. Polymer Sci.. Polym. Symp. 1979, 66, 129-148. Heller, J; Penhale, D.W.H.; Helwing, R. F. J. Polymer Sci. Polymer Lett., 1980, 18, 619-624. Heller, J.; Penhale, D.W.H.; Helwing, R. F.; Fritzinger, B. K.; Baker, R. W. Chem. Eng. Progress Symp. Series No. 206, 1981, 77, 28-36. Heller, J; Penhale, D.W.H.; Helwing, R. F.; Fritzinger, B. K. Polymer Eng. Sci. 1981, 21. 727-731. Heller, J.; Penhale, D.W.H.; Helwing, R. F.; Fritzinger, B. K. Controlled Release Delivery Systems"; Roseman, T. J.; Mansdorf, S. Z., Eds.; Marcell Dekker, New York, in press. Heller, J.; Penhale, D.W.H.; Fritzinger, B. K.; Rose, J. E.; Helwing, R. F. Contracept. Deliv. Syst. in press.

RECEIVED October 25, 1982

28 Radiation-Induced Polymerization Reactions for Biomedical Applications G. R. H A T T E R Y and V. D. McGINNISS

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

Battelle Laboratories, Columbus, OH 43201

A partial survey of the growing field of radiation induced polymerization methods used in synthesizing both natural and synthetic polymers w i l l be discussed in this paper. To be included are such subjects as: a) b) c)

Grafting reaction - e.g., e-beam and 60 C o to graft substituents amenable to heparization. Crosslinking reaction - e.g., ultra high molecular weight polyethylene (UHMWPE) for joint replacement. Plasma polymerization - e.g., coating of polymeric surfaces with pinhole free thin layers of material to improve specific mechanical properties.

A review of both past and present work in the f i e l d w i l l be included along with several illustrations of various methods u t i l i z i n g different aspects of radiation chemistry to produce biomaterials. The interaction of electromagnetic radiation with certain types of organic substrates has found widespread interest in biomedi c a l research related applications. Many such studies involve interaction of electromagnetic radiation with organic substrates to develop crosslinked/insoluble network structures. For example, a preformed thermoplastic polymer upon direct interaction with certain types of ionizing radiation can develop crosslinked or network structures having higher melting points, greater tensile strengths and better chemical resistance than the starting thermoplastic polymer material. It is also possible to impregnate certain thermoplastic polymers with 0097-6156/83/0212-0393$06.00/0 © 1983 American Chemical Society

394

INITIATION OF

POLYMERIZATION

low molecular weight compounds (drugs, chemical agents, etc.) followed by c r o s s l i n k i n g v i a r a d i a t i o n processing techniques to produce a composite s t r u c t u r e capable of c o n t r o l l e d release of the encapsulated compounds through the polymer network (Figure 1). S i m i l a r types of r a d i a t i o n induced polymerization reactions can be c a r r i e d out through the use of l i q u i d monomer, oligomer and polymer compositions containing r e a c t i v e v i n y l components (Figure 2).

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

In general, r a d i a t i o n induced polymerization r e a c t i o n s involve c o n s i d e r a t i o n of at l e a s t four major v a r i a b l e s , namely: (1) (2) (3)

type of r a d i a t i o n source type of organic substrate to be i r r a d i a t e d k i n e t i c s of the r a d i a t i o n induced polymerization or c r o s s l i n k i n g reactions network formation and f i n a l chemical, p h y s i c a l and mechanical p r o p e r t i e s of the c r o s s l i n k e d s t r u c t u r e .

(4)

The types of radiaton sources most encountered i n biomedical a p p l i c a t i o n s are o u t l i n e d i n Table I . Mechanisms associated

Table I. 6 0

Co

UV-Vis PLASMA

Radiation Sources X-RAYS 2 0 0 - 7 0 0 nm W A V E L E N G T H S OF LIGHT RADIO FREQUENCIES

with r a d i a t i o n induced polymerization r e a c t i o n s i n v o l v e f r e e r a d i c a l intermediates and can be depicted i n the generalized o u t l i n e s shown i n Figures 3 and 4. More complete d i s c u s s i o n s of r a d i a t i o n processing technologies can be found i n references

(1-5)

.

D i s c u s s i o n of r a d i a t i o n induced polymerization a r t i c l e w i l l focus on the f o l l o w i n g biomedical • • • • •

Bone P r o t h e s i s Polymer - Blood C o m p a t i b i l i t y Immobilization of Reaction Centers Immobilization of Enzymes C o n t r o l l e d Release of Drugs

reactions i n this applications:

28.

HATTERY AND

MCGINNISS

Radiation-Induced

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

Electromagnetic

395

Radiation Source

Electromagnetic Radiation

Thermoplastic Preformed Polymer S t r u c t u r e

Reactions

^ ^

Thermoplastic preformed polymer s t r u c t u r e containing low molecular weight compounds (*)

C r o s s l i n k e d Network Polymers Figure 1. Interaction of electromagnetic radiation with preformed polymer structures.

INITIATION OF

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

Electromagnetic radiation source »J\\\ / / / \ \ \ Electromagnetic radiation / / / \ \ \ =

R vinyl monomers

Multifunctional =: — R vinyl monomers or oligomers Unsaturation J sites

= — R — = — R— z= — Unsaturated polymers photoinitiator is required for light-activated reactions

ν Reactive liquid f coating system

y Solid cross-linked film

Figure 2.

Radiation curing concepts (curing of reactive coatings).

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

HATTERY AND MCGiNNiss

Radiation-Induced

Reactions

Initiating free radicals Α· + π C H = Ç H - ^ A - f C H - C H ^ - * - A - { C H 2 - C H f 2

2

R

n

R

Monomer/unsaturated polymer

Growing polymer (free radical)

R

n

-fCHCH f 2

R

Cured polymer

Figure 3. Ionizing or high-energy electron curing mechanisms.

PI (Photoinitiator)

hv _ R. ^ (Free radicals)

High energy electrons

R- +

Multifunctional unsaturated monomers and polymers Figure 4.

Three-dimensional network formation

Light-induced or high-energy electron curing mechanisms.

398

INITIATION OF

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

RADIATION TREATMENT OF POLYOLEFINS P o l y o l e f i n s , i n p a r t i c u l a r , polyethylenes, have been found to be extremely i n e r t during c l i n i c a l t e s t i n g of implant mate­ r i a l s . (6,7) These m a t e r i a l s are o f t e n wholly accepted by the body t i s s u e s with l i t t l e evidence of foreign-body c e l l r e a c t i o n i n the areas of implantation. For t h i s reason, HDPE (high d e n s i t y polyethylene) has been used i n a multitude of systems f o r biomedical a p p l i c a t i o n s . While the c l i n i c a l p r o p e r t i e s of these m a t e r i a l s are more than acceptable, s e v e r a l problems have a r i s e n with the physical-mechanical p r o p e r t i e s of the i n i t i a l material. Several groups have been involved with analyzing the e f f e c t of wear on these p r o p e r t i e s . (7,8,9) Some have a t ­ tempted to determine whether any increase i n hardness or de­ crease i n c o l d flow could be c o r r e l a t e d to i r r a d i a t i o n . (7) Since s t e r i l i z a t i o n of most of the j o i n t implants i n v o l v i n g p o l y o l e f i n s i s done using r a d i a t i o n , most often Co-60, groups have looked at the properties both p r i o r to and f o l l o w i n g the s t e r i l i z a t i o n dose (10,11) Results i n a l l of these studies have shown s l i g h t but inconsequential increases i n the mechan­ i c a l p r o p e r t i e s . Indeed, studies have shown that s i g n i f i c a n t enhancement of mechanical p r o p e r t i e s does not occur u n t i l the HDPE i s subjected to doses greater than 10 times the s t e r i l i ­ z a t i o n dose of 2.5 MRads. (12,13) At these dose r a t e s , coef i c i e n t of f r i c t i o n drops n o t i c e a b l y f o r HMPE. Very l i m i t e d work has a l s o been done on the d i f f e r e n c e between exposure to low f l u x Co-60 source i r r a d i a t i o n and exposure to Ε-beam r a d i a t i o n of HDPE (10) Results have i n d i c a t e d that the surface of the m a t e r i a l i s a f f e c t e d d i f f e r e n t l y by Ε-beam than by s i m i l a r doses of Co-60. It appears that the Ε-beam i r r a d i ­ ated surface i s more s u s c e p t i b l e to s t r e s s cracking and erab r i t t l e m e n t than that of the Co-60 i r r a d i a t e d surface. R e a l i z i n g that exposure of HDPE to high l e v e l s of i o n i z i n g r a d i a t i o n i n an i n e r t atmosphere has l i t t l e e f f e c t on the p r o p e r t i e s of the m a t e r i a l , some groups (7_, 14,1^5) have begun to study the i n t e r a c t i o n of HDPE and i o n i z i n g i r r a d i a t i o n i n a r e a c t i v e atmosphere. One of the most a c t i v e groups i n t h i s arena has been the U n i v e r s i t y of P r e t o r i a group l e d by Grobbelaar. (7^16) They have studied the p r o p e r t i e s of HDPE i n the presence and absence of gaseous c r o s s l i n k i n g agents (Figure 5). Several s i g n i f i c a n t r e s u l t s can be drawn from t h e i r studies on the e f f e c t of a c e t ­ ylene and a c e t y l e n e / c h l o r o t r i f l u o r o e t h y l e n e on i r r a d i a t e d HDPE. T h e i r studies i n d i c a t e d that d i f f u s i o n of the c r o s s l i n k i n g agents w i t h i n the HDPE matrix reached a depth of only 0.3 mm

28.

HATTERY AND MCGiNNiss

Radiation-Induced

Reactions

(Figure 6). Extension of both time allowed f o r d i f f u s i o n and t o t a l i r r a d i a t i o n did nothing to change t h i s value. Thus, they have found that HDPE composite m a t e r i a l can be produced with a high c r o s s l i n k density at the surface to impart increased abrasion r e s i s t a n c e and l i m i t c o l d flow while the l i m i t e d c r o s s l i n k i n g i n the bulk of the m a t e r i a l leads to continued good shock and embrittlement r e s i s t a n c e (Figure 7). It i s ex­ pected that t h i s work w i l l stimulate i n t e r e s t i n studying im­ provement i n HDPE p r o p e r t i e s through r e a c t i v e gaseous d i f f u ­ sion. Increased wear l i f e of these m a t e r i a l s w i l l be of great a s s i s t a n c e to those who must undergo t o t a l j o i n t replacement at an e a r l y age.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

POLYMER-CERAMIC COMPOSITE MATERIALS The use of ceramic and polymeric c o n s t i t u e n t s i n s p e c i a l a l l o y s and blends has become widespread i n recent years. These "polymeric cements" have been a p p l i e d to a v a r i e t y of uses from pavement r e p a i r and maintenance (17) to a r t i f i c i a l teeth ( 1 8 , ^ , ^ 0 ) and endosseous implantsT (21) Some of these techniques have r e l i e d on i n i t i a t i n g the p o l y ­ m e r i z a t i o n r e a c t i o n by i o n i z i n g r a d i a t i o n . In p a r t i c u l a r , Kamel (21) at Drexel U n i v e r s i t y has been developing a bone r e s t o r a t i v e using an alumina- p o l y ( a c r y l i c a c i d ) composite produced by exposing an aqueous mixture of the blend to γ rad­ i a t i o n of 1 MRad (Figure 8). The p o r o s i t y and c r o s s l i n k density of the system were v a r i e d over wide ranges by varying monomer concentration and a heat treatment step to form anhydrides. Other c o n t r o l parameters included both chemical reactions and p h y s i c a l i n t e r a c t i o n s (Figure 9). V a r i a t i o n of these proper­ t i e s caused a concomitant change i n mechanical p r o p e r t i e s and water absorption. Such c o n t r o l allows the composite to be " t a i l o r e d " to a s p e c i f i c use. This m a t e r i a l can be e a s i l y f a b r i c a t e d and adapted to a number of d i f f e r e n t socket geo­ metries while allowing bone growth i n t o the porous m a t e r i a l . I n i t i a l studies have shown that n e i t h e r the b i o c o m p a t i b i l i t y of the m a t e r i a l nor the r e s i s t a n c e to body f l u i d d i f f u s i o n are a f f e c t e d to a major extent by t h i s c r o s s l i n k i n g method. Long term implantation studies are p r e s e n t l y underway to determine the ultimate e f f e c t of the m a t e r i a l on the implant area. RADIATION INDUCED GRAFTING FOR

INCREASED BLOOD COMPATIBILITY

Many groups have studied the g r a f t i n g of d i f f e r e n t f u n c t i o n a l ­ i t i e s onto the backbone s t r u c t u r e of d i f f e r e n t polymers. Some of these have involved covalent or i o n i c coupling of b i o - a c t i v e compounds to i n e r t substances (22,23) while others have been concerned with exposing the monomer and polymeric substrate to i o n i z i n g r a d i a t i o n . (24,25,26,27)

399

400

INITIATION OF POLYMERIZATION

WEAR AND COLD " FLOW PROBLEMS

HDPE (UNTREATED) -

HDPE

HDPE

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

Figure 5.

RADIATION TREATMENT (y-RAY)

RADIATION TREATMENT (y RAY), HC = CH

Improved HDPE

IMPROVED SURFACE HARDNESS, }· WEAR AND COLD FLOW RESISTANCE PROPERTIES

prostheses through the use of radiation treatment.

High

crosslink

density

at t h e irradiated

surface

(depth o f penetration controlled

by monomer

diffusion)

Lower density

crosslink in t h e bulk

Surface

properties—resistance t o abrasion and

Bulk properties—better

cold shock

flow resistance

Figure 6. Effect of radiation treatment on bulk and surface of HDPE ^CH.

+

CH

HATTERY AND

MCGINNISS

Radiation-Induced

401

Reactions

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

1.5

Dose (kGy) Figure 7. Radiation treatment of HDPE with and without reactive acetylene monomer. (Reprinted from Ref. 7, copyright 1978, and Ref. 16, copyright 1977.)

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402

INITIATION OF

POLYMERIZATION

Figure 8. Radiation-processed polyacrylic acid (FA A) composites for bone restoration. (Reprinted from Ref. 21. Copyright 1977.)

OBJECTIVE

D E V E L O P C O M P O S I T E M A T E R I A L S HAVING M A X I M U M FLEXIBILITY IN VARIATION O F M E C H A N I C A L PROPERTIES WHILE M A I N TAINING BIOCOMPATIBILITY

CONTROL FACTORS • A N H Y D R I D E F O R M A T I O N IS R E L A T E D T O C R O S S L I N K DENSITY • C R O S S L I N K DENSITY C O N T R O L S W A T E R SWELLING OF C O M P O S I T E • S T R O N G P O L Y M E R - F I L L E R INTERACTION Figure 9. Radiation-processed poly acrylic acid (PA A) composites for bone restoration.

28.

HATTERY AND

MCGINNISS

Radiation-Induced Reactions

403

A b r i e f survey of the f i e l d shows that by f a r the most p r e v a l ­ ent polymer substrate i n use i s the s i l i c o n e f a m i l y ,

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

Wilson, at Bishop C o l l e g e , and Eberhart and Elkowitz at U n i v e r s i t y of Texas (27) have i r r a d i a t e d a s i l i c o n e substrate i n the presence of chloromethylstyrene monomer to produce a r e a c t i v e g r a f t polymer that can be quarternized with p y r i d i n e and reacted with sodium heparin to produce a thromboresistant heparinized product that has a higher blood c o m p a t i b i l i t y than the untreated s i l i c o n e . The same group has used e s s e n t i a l l y the same methods to create a heparin g r a f t e d polyethylene surface. The method developed by Wilson, et a l , y i e l d s an i o n i c a l l y bound heparin moiety that a f f e c t s the anticoagulant a c t i v i t y of the heparin molecule to a l e s s e r extent than does the t r a d i ­ t i o n a l method (28) of simple s o l u t i o n coating of m a t e r i a l . Tests have shown that the maximum heparin removal rate i n nor­ mal s a l i n e i s s u f f i c i e n t to prevent c l o t t i n g i n hollow f i b e r a r t i f i c i a l kidneys as predicted by Schmer (29), Future work w i l l develop around a " p u r i f i e d " heparin prepared by Rosenberg and Lam (30) where one t h i r d of the s t a r t i n g mass of heparin contains 85% of the anticoagulant a c t i v i t y Several other groups have also studied the blood c o m p a t i b i l i t y of s i l i c o n e s with various r a d i a t i o n grafted copolymer con­ stituents, Chapiro, et a l (25) have g r a f t e d N - v i n y l p y r r o l i done using Co-60 onto s i l i c o n e i n both "bulk" and " s o l u t i o n " type r e a c t i o n s . It i s i n t e r e s t i n g to note that s i m i l a r work using Ε-beam r a d i a t i o n has not been as s u c c e s s f u l (26), C h a p i r o s studies have shown improvement i n blood c o m p a t i b i l i t y f o r samples with a g r a f t i n g weight increase of greater than 33% (Figure 10). Studies a l s o showed that the g r a f t i n g percent could be c o n t r o l l e d by varying the r a t i o of η-vinyl pyrrolidone i n the solvent. However, i t was found that above approximately 30% g r a f t i n g , the s i l i c o n e becomes b r i t t l e and loses mechanical p r o p e r t i e s . Attempts are presently underway to l i m i t the depth of g r a f t i n g of N - v i n y l pyrrolidone to j u s t the surface of the tubes so that the o r i g i n a l p r o p e r t i e s of the s i l i c o n e can be retained. 1

Other groups have concentrated on the a c r y l a t e and methacrylate hydrogel type m a t e r i a l s . Dincer (22) has shown that heparin can be attached c o v a l e n t l y to c r o s s l i n k e d beads of polymethyl a c r y l a t e to y i e l d increased blood c o m p a t i b i l i t y at very low r a t e s of desorption from the surface. These f i n d i n g s are i n l i n e with Salzman ( 3 2 ) , M e r r i l l (33) and Wong (34) statements that the antithrombogenic e f f e c t does not depend on leaching of heparin from a surface i n t o the blood stream. Further work by Hattery (35) showed that r a d i a t i o n g r a f t i n g of methyl a c r y l a t e

404

INITIATION OF

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

to s i l i c o n e followed by h e p a r i n i z a t i o n by the method of Flake (23) and Dincer (22) could y i e l d a surface that e x h i b i t e d good a n t i c l o t t i n g p r o p e r t i e s . Further t e s t i n g or r e p o r t i n g of these r e s u l t s have been delayed u n t i l a s a t i s f a c t o r y method f o r producing a uniform surface coating has been developed. Another group deeply i n t e r e s t e d i n r a d i a t i o n g r a f t i n g of a c r y l a t e s and methacrylates i s that of Hoffman, et a l at the U n i v e r s i t y of Washington. (26) They have a p p l i e d Co-60 r a d i a t i o n to the production of hydrogel type b i o m a t e r i a l s through g r a f t i n g of s p e c i f i c monomers onto an i n e r t polymer backbone. This work has looked at immobilization of b i o l o g i c a l l y a c t i v e molecules such as enzymes, a l b u m i n s and plasma p r o t e i n s o n t o g r a f t e d hydrogels of hydroxyethylmethacrylate (HEMA) and other f u n c t i o n a l g r a f t i n g agents such as n - v i n y l p y r r o l i d o n e . They have found that c e r t a i n metal ions or polar organic solvents can be used to vary the amount and penetration of the g r a f t monomer as w e l l as changing the r e a c t i o n r a t e . These are by no means the only groups working i n the f i e l d of blood c o m p a t i b i l i t y . However, the ones c i t e d are s u f f i c i e n t to provide i n s i g h t i n t o the progress of r a d i a t i o n synthesis as r e l a t e d to hemocompatibility research. IMMOBILIZATION OF REACTION CENTERS Another area of great i n t e r e s t i n biomaterials research has been that of immobilization of r e a c t i o n centers on an i n e r t substrate to create r e a c t i o n s p e c i f i c c i t e s . One group (36) has been i n t e r e s t e d i n s t a b i l i z a t i o n of c h l o r o p l a s t s f o r use i n s o l a r energy development (Figure 11). Various h y d r o p h i l l i c and hydrophobic monomers were mixed with i s o l a t e d c h l o r o p l a s t s i n a s p e c i f i c b u f f e r s o l u t i o n (Figure 12). The mixture was cooled to below -24C and i r r a d i a t e d with a Co-60 source to 1 MRad. A f t e r i r r a d i a t i o n , r e s i d u a l monomer and c h l o r o p l a s t were washed l e a v i n g the immobilized product stored i n a b u f f e r s o l u t i o n . The authors found that the h y d r o p h i l i c monomer d i d not a f f e c t the e v o l u t i o n of oxygen as much as the hydrophobic monomer. In a d d i t i o n , i t was reported that concentration of s t a r t i n g mater i a l and time a f t e r monomer a d d i t i o n decreased the e f f e c t i v e ness of the c h l o r o p l a s t i n evolving 0£.

The immobilization technique allowed the c h l o r o p l a s t to remain a c t i v e more than 7 times longer than the non-immobilized sample at a c t i v i t y l e v e l s as high as 40% of i n i t i a l values. This i n crease i n s t a b l e l i f e t i m e f o r p h o t o - a c t i v i t y may be tapped f o r f u t u r e use i n conversion of s o l a r energy to chemical and e l e c -

HATTERY AND M c G i N N i s s

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

GRAFT RATIO %

Radiation-Induced #TESTS

CLOTTED TUBES

Reactions

405

UNCLOTTED TUBES

0

16

7

16-22

5

H

9 1

31-39

19

7

12

11-17

9

1

8

Figure 10.

Improvement in short term hemocompatibility of silicone-g-NVP over silicone. Implants for 7 days in lamb carotid arteries.

Figure 11.

Stabilization of chloroplast. (Reprinted with permission from Ref. 36. Copyright 1981, John Wiley and Sons, Inc.)

Figure 12.

Stabilization of chloroplast.

406

INITIATION OF

POLYMERIZATION

t r i c a l energy. However, experiments p r e s e n t l y are only a t the lab stage and much work remains to be performed to determine the f e a s i b i l i t y of the approach. RADIATION INDUCED POLYMERIZATION REACTIONS FOR ENZYME IMMOBILIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

Several studies have been c a r r i e d out on the immobilization of enzymes by r a d i a t i o n ( i o n i z i n g and photochemical) induced polymerization r e a c t i o n s . (37-44) Most of these studies i n ­ volved the use of combinations of h y d r o p h i l i c or hydrophobic monomer/polymer substrates f o r the entrapment of the enzyme catalyst. A l i s t i n g of t y p i c a l hydrophilic/hydrophobic polymer m a t e r i a l s i s contained i n Table I I . The e f f e c t s of hydro­ p h i l i c / h y d r o p h o b i c polymer p r o p e r t i e s i n enzyme a c t i v i t y are

Table II.

Radiation-Induced Immobilization of Enzymes WATER C O N T E N T (%)

POLYMER Ο

II

m N-CH -CH -CH -CH 2

2

2

PNVP

93.7

PAAm

84.8

PHEA

45.9

C H - C (CH )+ 2

3

m

COO ( C H ) OH 2

2

PHEMA

26.0

PHDMMA

13.5 m 2.5

-j-CH -C(CH )j-COOCH3 2

PMMA

3

2.1

28.

HATTERY AND MCGiNNiss

Radiation-Induced

Reactions

407

shown i n Figure 13· In many cases there are very complex r e ­ l a t i o n s h i p s among matrix p o r o s i t y , polymer s t r u c t u r e and net­ work a r c h i t e c t u r e . Enzyme i n i t i a l a c t i v i t y i n a h y d r o p h i l i c polymer s t r u c t u r e decreased r a p i d l y with repeated use (enzyme leakage) but the hydrophobic polymer s t r u c t u r e s retarded l o s s of enzyme a c t i v i t y through repeated usage. Another advantage of using r a d i a t i o n processing techniques to immobilize enzyme m a t e r i a l s on polymer surfaces i s the a b i l i t y to achieve very homogeneous and smooth composite s t r u c t u r e s which are not a t t a i n a b l e by conventional p o l y m e r i z a t i o n techniques. (45)

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

CONTROLLED RELEASE OF MATERIALS FROM RADIATION POLYMERIZED COMPOSITES The g e n e r a l i z e d concept f o r producing composite s t r u c t u r e s capable of c o n t r o l l e d a d d i t i v e r e l e a s e p r o p e r t i e s i n v o l v e s 1) s o l u t i o n or d i s p e r s i o n of a d d i t i v e s i n r e a c t i v e monomer/polymer systems, 2) s u b j e c t i n g the additive/monomer-polymer s o l u t i o n d i s p e r s i o n to r a d i a t i o n , and 3) formation of a c r o s s l i n k e d polymer network which encapsulates the s p e c i f i c agent (Figure 14). T y p i c a l monomer and c r o s s l i n k i n g oligomers u t i l i z e d i n these types of s t u d i e s are shown i n Table I I I . The e f f e c t s of

Table III. Controlled Release of Materials from RadiationPolymerized Composites MONOMERS METHYL A C R Y L A T E (MA) METHYL METHACRYLATE (MMA) 2 - H Y D R O X Y E T H Y L M E T H A C R Y L A T E (HEMA) CROSSLINKING OLIGOMERS POLYETHYLENE G L Y C O L #600 D I A C R Y L A T E (PEGDA) POLYETHYLENE G L Y C O L #400 D I M E T H A C R Y L A T E (PEGDMA) DIETHYLENE G L Y C O L D I M E T H A C R Y L A T E ( D E G D M A ) T R I M E T H Y L O L P R O P A N E T R I A C R Y L A T E (TMPTA) T R I M E T H Y L O L P R O P A N E TRIM ΕΤΗ A C R Y L A T E (TMPTMA)

monomer chemical s t r u c t u r e (hydrophilic/hydrophobic) on r e l e a s e of KC1 from a cured composite system are shown i n Table IV.

408

INITIATION OF

POLYMERS HYDROPHILIC

INCREASED ACTIVITY

% WATER HIGH

POLYMERIZATION

POROSITY DECREASES

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

50%

HYDROPHOBIC

DECREASED ACTIVITY

LOW

INCREASES

Figure 13.

Activity and properties of radiation-induced immobilization of enzymes.

Figure 14.

Controlled release of materials from radiation-polymerized composites.

28.

HATTERY AND MCGiNNiss

Radiation-Induced

409

Reactions

Table IV. Controlled Release of KC1 from Radiation-Polymerized Composites

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

HYDROPHILIC MONOMERS

Ww RELEASE W(%) =— —— RATE Wp + Ww ( / / g / c m min > 2

HEMA PEGDA PEGDMA

32.5 30.7 21.6

4.37 3.87 3.01

HYDROPHOBIC MONOMERS MA MMA DEGDMA TMPTA TMPTMA

7.6 6.8 3.8 3.3 2.4

2.41 2.06 0.69 0.33 0.03

1/ 2

Those polymer composite systems having high W% values (hydrop h i l i c ) (Ww i s the weight of water absorbed to saturate the polymer and Wp i s the weight of the dry polymer) demonstrated higher release rate c a p a b i l i t i e s than those having low W% values. The f u n c t i o n a l i t y of the raonomer/crosslinking oligomer a l s o i n f l u e n c e s the rate of KC1 release i n that monofunctional monomers release f a s t e r than d i f u n c t i o n a l c r o s s l i n k i n g o l i g o mers which i n turn have higher release rates than the t r i f u n c t i o n a l c r o s s l i n k i n g oligomers. This may a l s o be due to the f a c t that t r i f u n c t i o n a l c r o s s l i n k i n g oligomers produce very t i g h t network s t r u c t u r e s r e l a t i v e to higher molecular weight d i f u n c t i o n a l c r o s s l i n k i n g oligomer s t r u c t u r e s . (46) The a d d i t i o n of h y d r o p h i l i c thermoplastic a d d i t i v e s to a ~ T e l a t i v e l y hydrophobic network can a l s o s t r o n g l y i n f l u e n c e the release of KC1 from i t s s t r u c t u r e (Figure 15). (jV7) An example of another type of drug c o n t r o l l e d release composite system i s shown i n Table V and Figure 16. (48)

410

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

INITIATION OF

0

1

2

3

4

Release Rate ( m g / c m ^ m i n Figure 15.

1 / 2

)

Controlled release of KCl from radiation-polymerized polyethylene 600 glycol (PEG 600).

DEGDMA/

HATTERY AND MCGiNNiss

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

28.

Radiation-Induced

Reactions

-Radiation Cured Composite Control Factors Effecting Drug Release Rates • Relative concentrations of each drug • Molecular weight of the drug • Content and composition of drugs • Polarity of the composite • Network of the polymer Figure 16.

Controlled release of multicomponent cytotoxic agents from radiationpolymerized composites.

411

412

INITIATION OF POLYMERIZATION

Table V. Controlled Release of Multicomponent Cytotoxic Agents from Radiation-Polymerized Composites. CYTOTOXIC A G E N T S

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

1 ( 2 - T E T R A H Y D R O F U R Y L ) - 5 FLUOROURALIL (FT 207) M I T O M Y C I N C (MMC) A D R Y A M Y C I N (ADM) MONOMERS

POLYMERS

DEGDMA TMPTMA

PMMA PEG 1 0 0 0 PMAC

Source: Reprinted from Ref. 48. Copyright 1980.

Plasma polymerization r e a c t i o n s have a l s o been u t i l i z e d to modify hydrogel polymer surfaces f o r enhanced c o n t r o l l e d r e lease c a p a b i l i t i e s . In one study an ummodified polymer hydrog e l e x h i b i t e d very rapid release rates f o r an entrapped drug i n aqueous medium (Figure 17 and 18). M o d i f i c a t i o n o f the hydrog e l surface with an Argon i o n plasma ( c r o s s l i n k i n g or casing of the surface) or plasma polymerization of the surface i n the presence of t e t r a f l u o r o e t h y l e n e monomer (Figure 19) produced an improved s t r u c t u r e f o r drug c o n t r o l l e d release c a p a b i l i t i e s (Table VI),(49,50)

Table VI. Rates of Drug Release from Unmodified and Modified Hydrogel Structures HYDROGEL STRUCTURE

DRUG

RELEASE

UNMODIFIED

41.8/yg/hr-cm'

CASED

40

TFE

3-9

COATED

//g/hr-cm

2

/yg/hr-cm

2

RATE

28.

HATTERY AND MCGiNNiss

Radiation-Induced

Reactions

413

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

Objective—to modify hydrogel surface so as to control the rate of drug diffusion for release at a specified rate

Unmodified hydrogel Figure 17.

Hydrogel modification by plasma treatment.

Drug release

D r u g (D) in s w o l l e n hydrogel D r u g = pilocarpine, progesterone Figure 18. Control of drug release rate through hydrogels by plasma treatment. (Reprinted from Ref. 49. Copyright 1977.)

414

POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

INITIATION OF

Figure 19.

Hydrogel modification by plasma treatment.

28. HATTERY AND MCGINNISS

Radiation-Induced Reactions

415

LITERATURE CITED

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

1.

Chapiro, A., "Radiation Chemistry of Polymeric Systems", Interscience Publisher, a division of John Wiley and Sons, Inc., New York, 1962. 2 Wilson, J. E., "Radiation Chemistry of Monomers, Polymers, and Plastics", Marcel Dekker, Inc., New York, 1974. 3. McGinniss, V. D., Nowacki, L. J. and Nablo, S. V., ACS Symposium, No. 107, page 51-70, 1979. 4. McGinniss, V. D., National Symposium on Polymers in the Service of Man, ACS, 15th State-of-the-Art Symposium, pages 175-180, 1980. 5. Hollahan, J. R., and Bell, A. T., "Techniques and Applications of Plasma Chemistry", John Wiley and Sons, 1974. 6. Charnley, J., Clin Orth Rel Res., 95 pg. 9 (1978). 7. Grobbelaar, C. J. et a l , J. Bone Jt Sug, 60-B, pg. 370 (1978). 8. Dumbleton, J. H., Shen, C., Wear, 37, pg 279 (1976). 9. Nusbaum, H. J., et a l . , J. Appl Poly. Sc., 23, pg 777 (1979). 10. Hattery, G. R., unpublished report, Battelle Columbus Laboratories, 1980. 11. DuPlessis, T.A., Paper presented at Cong South Africa Assoc. Physicists Med Biology, Bellville, (1977). 12. Dumbleton, J. H., Shem, C., J. Appl. Poly. Sci., 18, pg 3493 (1974). 13. Dumbleton, J. H. et a l , Wear, 29, pg 163 (1974). 14. Mitsui, H. et a l , Polym. Jour., 13, pg 108 (1972). 15. Hagiwara, M. et a l , Poly. Sci, B, Polym. Let., 11, pg 613 (1973). 16. Grobbelaar, C. J. Radiat. Phys. Chem, 9, pg 647 (1977). 17. Nandi, U. S., Personal Communication, 1981. 18. Hodash, M. et a l , Oral Surg, 24, pg 831 (1967). 19. Young. F. A., J. Biomed Mater Res. Symp., 2, pg 281 (1972). 20. Reed, O. M. et a l , J. Biomed Mater Res. Symp., 2, pg 296 (1972). 21. Kamel, I. L., Radiat. Phys. Chem. 9, pg 711 (1977). 22. Dincer, A. K., Sc. D. Thesis, "Covalent Coupling of Heparin to Synthetic Polymer Surfaces", MIT, 1977. 23. Flake, J., S. M. Thesis, "Aminolysis and Heparinzation of Polymethylacrylate for Biomedical Application", MIT, 1976. 24. Hattery G. R., S. M. Thesis, "Radiation Induced Crosslinking in Polymethyl acrylate", MIT, 1978. 25. Chapiro, A. et a l , Radiat. Phys. Chem., 15, pg 423 (1980). 26. Hoffman, A. S. et a l , Trans. Amer. Soc. Artif. Internal Organ, 18, pg 10 (1972).

416 27. 28. 29. 30. 31.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch028

32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

INITIATION OF POLYMERIZATION

Wilson, J. E., et a l , J. Macromol Sci. Chem. A16, pg 769 (1981). Grode, G. A., et a l , J. Biomed Mater. Res. Symp., 3, pg 77 (1972). Schmer, G., Trans. Am. Soc. Aritif. Intern. Organ, 19, pg 188 (1973). Rosenberg, R. D., Lam. L. H., Ann. N.Y. Acad. Sci., 283, pg 404 (1977). Yosada, H., Reforjo, M. F., J. Polym. Sci, Part A-2, pg 5093 (1964). Salzman, E. W., Blood, 38, pg 509 (1971). Merrill, E. W., et a l , J. Appl. Physiol., 28, pg 723 (1970). Wong, P. S. L., et a l , Fed, Proc., p. 441 (1969). Hattery, G. R., Unpublished report, MIT, (1977). Yoshii, F., et a l , Biotech Bioeng, 23, pg 833 (1981). Kumakura, M. et a l . Journal of solid-phase Biochemistry, 2, No 3, 279 (1977). Kaetsu, I., Kumakura, M., and Yoshida, M., Biotech. Bioeng., 21, 867 (1979). Ibid, 863 (1979). Ibid, Polymer, 20, 3 (1979). Ibid, 9 (1979). Ichimura, K., and Watanabe, Poly. Sci. Poly. Chem., 18, 891 (1980). Yoshida, M., Kumakura, M., and Kaetsu, I., J. Macromol. Sci-Chem., A14, No. 4, 541 (1980). Ibid, 555 (1980). Kaetsu, I., et a l , Biomed. Mat. Res., 14, 199 (1980). Yoshida, M., Kumakura, M. and Kaetsu, I., Polymer, 19, 1375 (1978). Ibid, 1379 (1978). Kaetsu, I., et a l , Biomaterials, 1, 17 (1980). Colter, K. D., Shen, M., and Bell, A. T., Biomat., Med. Dev. Art. Org., 5, No 1, 13 (1977). Ibid, 1 (1977).

RECEIVED November 9, 1982

29 Mechanisms of Electron-Transfer Initiation of Polymerization: A Quantitative Study MICHAEL SZWARC

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

University of California, San Diego, Department of Chemistry, La Jolla, CA 92093

Initiation of v i n y l , vinylidene or diene polymerization is commonly visualized as an addition of some moiety, X, to a monomer, M, resulting in the formation of a reactive end-group capable of sustaining propagation of polymerization. Depending upon whether X is a radical, a cation, or an anion, the ensuing polymerization is propagated through a radical, cationic, or anionic mechanism, as shown schematically below:

In each case one end of a growing polymer is inert while the other is active. Still another mechanism of i n i t i a t i o n was proposed in the 1950fs (1)9 namely, i n i t i a t i o n by electron-transfer to monomer. In such an i n i t i a t i o n , the electron is transferred from a suitable donor, A or AT, to a monomer, M, converting i t into a monomeric radical-anion

The r e s u l t i n g monomeric r a d i c a l - a n i o n s e i t h e r dimerize dimeric d i a n i o n s .

into

e.g.,

or react with monomer, y i e l d i n g then dimeric r a d i c a l - a n i o n s ,

0097-6156/83/0212-0419$06.00/0 © 1983 American Chemical Society

420

INITIATION OF

M7

+ Μ

+

~Μ.Μ·

k

POLYMERIZATION

a

e.g., Ph.CHiCH^ + Ph.CH:CH + Ph.CH. CH . CH . CH. Ph 2

2

2

The dimeric dianions i n i t i a t e anionic polymerization from both ends of the macromolecules, M +

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

"M.M"

"M.M

propagated

M.M"

whereas the dimeric r a d i c a l - a n i o n s could i n i t i a t e anionic polymerization from one of t h e i r ends and simultaneously a r a d i c a l polymerization from the other end. However, t h i s i s an u n l i k e l y event. Most probably, they d i s p r o p o r t i o n a t e , dimerize, or are reduced to dianions, v i z . 2"Μ.Μ·

+

"M.M"

+

2M

2~Μ.Μ·

+

"Μ.Μ.Μ.Μ"

or ~Μ.Μ·

+ A (or Α") «* "Ή.Μ~+ Α

+

(or Α)

In t h i s paper I w i l l discuss q u a n t i t a t i v e aspects of the e l e c t r o n - t r a n s f e r e q u i l i b r i a , both homogeneous and heterogeneous, the fate of the monomeric r a d i c a l - a n i o n s , and the methods leading to the d e s i r e d thermodynamic and k i n e t i c data. Since f l a s h - p h o t o l y s i s was e x t e n s i v e l y used i n our work, a few words about i t s b a s i c feature are i n place. A d e t a i l e d d e s c r i p t i o n of i t s usage i n our systems i s given elsewhere (_2). P r i n c i p l e s of

Flash-Photolysis

Our experimental set-up i s shown schematically i n F i g . 1. The i n v e s t i g a t e d s o l u t i o n , u s u a l l y about 10"^ M i n the a c t i v e i n g r e d i e n t , i s introduced i n t o a c y l i n d r i c a l , 10 cm. long quartz c e l l with o p t i c a l l y f l a t end windows. The c e l l i s placed between two p a r a l l e l f l a s h lamps, separated from them by cuvets containing a l i g h t f i l t e r i n g s o l u t i o n that absorbs UV l i g h t . A properly c o l l i m a t e d beam of monitoring l i g h t passes through the c e l l and i s focused on the s l i t of monochromator. By choosing the d e s i r e d wavelength, one allows the monochromatic l i g h t to reach a p h o t o m u l t i p l i e r , and i t s output i s a m p l i f i e d and fed i n t o an o s c i l l o s c o p e . On t r i g g e r i n g the scope, but not the f l a s h lamps, one gets on the screen of the o s c i l l o s c o p e a h o r i z o n t a l z e r o - l i n e , showing how much l i g h t of a d e s i r e d wavelength passes through the unphotolyzed s o l u t i o n . Thereafter the f l a s h lamps are

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

29.

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421

] Flash Lamp Reaction Cell

Scope Figure 1. Experimental setup for flash photolysis. Scope registers changes in absorbance resulting from destruction of reagents and formation of products. For systems regenerating reagents, curve displayed on scope returns to its original baseline.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

422

INITIATION OF

POLYMERIZATION

t r i g g e r e d , a f t e r the scope has been a c t i v a t e d ^ 100 lisec. earlier. The ensuing p h o t o l y s i s converts some of the o r i g i n a l reagents i n t o t r a n s i e n t species. The r e a c t i o n of the l a t t e r , t a k i n g place i n the dark period f o l l o w i n g a f l a s h , y i e l d s the f i n a l products. The progress of the dark r e a c t i o n i s revealed by changes i n the absorbance of the monitoring l i g h t , d i s p l a y e d on the screen of the o s c i l l o s c o p e as a curve g i v i n g the i n t e n s i t y of the transmitted l i g h t as a f u n c t i o n of time. The systems discussed here are p e r f e c t l y r e v e r s i b l e — f o r any wavelength the enhanced absorbance or bleaching decays to a z e r o - l i n e as shown i n F i g . 1. Therefore, f l a s h i n g could be repeated over and over again, l e a v i n g unaltered the ultimate composition of the photolyzed s o l u t i o n . By v a r y i n g the s e t t i n g of the monochromator, a family of curves i s obtained, each d e p i c t i n g the r e t u r n of the photolyzed s o l u t i o n to i t s o r i g i n a l s t a t e but monitored at a d i f f e r e n t wavelength. From such curves one c a l c u l a t e s the o p t i c a l density f o r each wavelength of the photolyzed s o l u t i o n at a chosen time, say 100 ysec. a f t e r each f l a s h . This allows one to construct a d i f f e r e n c e spectrum — the d i f f e r e n c e i n the absorbance of the t r a n s i e n t s and the o r i g i n a l reagents at that time. Q u a n t i t a t i v e Treatment of Homogeneous E l e c t r o n - T r a n s f e r Initiation A d i f f e r e n c e spectrum obtained by f l a s h i n g a THF s o l u t i o n of dimeric dianions of a Ot-methyl styrene, K ,C(CH3)(Ph).CH .CH2.C(CH )(Ph),K = Κ+,-α.ΟΓ,Κ*, i s shown i n F i g . 2 ( 3 ) . It fades with time; however, i t s shape remains unchanged i n d i c a t i n g that the t r a n s i e n t s formed by f l a s h d i r e c t l y regenerate the o r i g i n a l dimeric dianions, no other intermediates or products being formed. The d i f f e r e n c e spectrum r e s u l t s from the absorbance of the intermediates and bleaching of the dimers. Hence, the spectrum o f the intermediates i s constructed by adding the known spectrum of the photolyzed dimers to the observed d i f f e r e n c e spectrum. Such a procedure i s i l l u s t r a t e d i n F i g . 3. The r e s u l t i n g spectrum of the intermediates c l o s e l y resembles that of Ot-methyl styrene r a d i c a l - a n i o n s reported by three independent groups (4), who used pulse r a d i o l y s i s i n t h e i r s t u d i e s . I t follows that the p h o t o l y s i s leads to d i r e c t or i n d i r e c t p h o t o - d i s s o c i a t i o n of the dimeric-dianions i n t o r a d i c a l - a n i o n s of Ot-methyl styrene, ot , i . e . , +

+

2

+

K, and

-αα-,Κ

+

3

+

• 2Κ ,α*

t h e i r d i m e r i z a t i o n takes place i n the subsequent dark p e r i o d . Analogous studies of f l a s h p h o t o l y s i s of THF s o l u t i o n s of

SZWARC

Mechanisms of Electron-Transfer Initiation

423

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

29.

Figure 3. The calculated absorption spectrum in THF of a—, K* deduced from the difference spectrum and the known spectrum of K ,~aa,K\ Key: O, a~; ·, observed difference spectrum (~5 ms afterflash);X, ~aa. +

424

INITIATION OF

dimeric dianions of 1,1-diphenyl ethylene, C a t , C(Ph) -CH -CH -C(Ph) , Cat*, "D-D", C a t +

2

POLYMERIZATION

+

2

led to the spectrum o f the respective transient shown in F i g . 4 (_5). Its s i m i l a r i t y with the spectrum of 1,1-diphenyl ethylene r a d i c a l - a n i o n s , D~, reported by Hammill (6) who r a d i o l y z e d frozen 2-MeTHF s o l u t i o n of that hydrocarbon, implies that f l a s h - p h o t o l y s i s of those dimeric dianions again leads to t h e i r p h o t o - d i s s o c i a t i o n i n t o the monomeric r a d i c a l anions, v i z . +

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

Cat ,~D.D~,Cat

+

2DT,Cat

+

For both systems r e c i p r o c a l s of Δ o p t i c a l density are l i n e a r with time. For the α-methyl styrene system, t h i s i s shown i n F i g . 5 where 1/Δ (opt. density) at 340 nm, 400 nm, and 600 nm i s p l o t t e d vs. time, and for the 1,1-diphenyl ethylene system i n F i g . 6 g i v i n g p l o t s of 1/Δ (opt. d e n s i t y ) at 390 nm, 470 nm, and 750 nm vs. time. L i n e a r i t y of those p l o t s proves the bimolecular character of the processes through which the t r a n s i e n t s regenerate the dimeric dianions. Slopes of those p l o t s provide, t h e r e f o r e , the values of the r e s p e c t i v e d i m e r i z a t i o n constants, k^, d i v i d e d by ε where ε i s the r e s p e c t i v e e f f e c t i v e molar absorbance and & i s the length of the c e l l . The e f f e c t i v e molar absorbances were determined by various methods O , 5) and t h e r e a f t e r the d i m e r i z a t i o n constants of D~,Cat and a~,Cat were c a l c u l a t e d . T h e i r values, l i s t e d i n Table 1, show t h e i r dependence on the nature of c a t i o n — i n c r e a s i n g with i t s r a d i u s . S i g n i f i c a n t l y , the d i m e r i z a t i o n of r a d i c a l - a n i o n s i s slower than of small, n e u t r a l r a d i c a l s the l a t t e r combination being d i f f u s i o n c o n t r o l l e d . Apparently, the r e p u l s i o n of the n e g a t i v e l y charged p a r t i c l e s ( f o r free ions) or of the unfavorably oriented d i p o l e s ( f o r i o n - p a i r s ) leads to the i n e r t i a . S u r p r i s i n g l y , 0t~,Cat dimerize much slower than D~,Cat , an unexplained f i n d i n g . +

+

+

+

The d i m e r i z a t i o n i s therefore rate determining i n the i n i t i a t i o n of polymerization by aromatic r a d i c a l - a n i o n s . i t s rate i s

-dtr

'>

because due to the extremely short r e l a x a t i o n time of the e l e c t r o n - t r a n s f e r e q u i l i b r i u m ( t h i s r e l a x a t i o n time i s given by T=l/|kf[M] + k^tA]}, kf and k denoting the forward and backward r a t e constants of the e l e c t r o n t r a n s f e r . Thus, Τ i s at the most 1 Usee, and the e q u i l i b r i u m concentration o f the monomeric r a d i c a l - a n i o n s i s unperturbed by t h e i r d i m e r i z a t i o n . A numerical example i s i l l u m i n a t i n g . b

Mechanisms of Electron-Transfer Initiation

SZWARC

425

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

AOD

t(ms) •

Ο

I

20

40

•

!

60

•

Ι­

80

Figure 5. Plots of 1/Δ (OD) vs. time for the Κ*~αά~,Κ* system. Recorded at λ 340, 400, and 600 nm.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

426

INITIATION OF

0

1

POLYMERIZATION

2

Figure 6. Flashphotolysis of ~DD~ with D (excess in THF. Typical plots of 1/Δ (OD) vs. time obtained by monitoring the dark reaction proceeding in the presence of a large excess of D at 470, 390, and 750 nm. Key: 750 nm (slope = 1.13 Χ 10 ); %, 470 nm (slope = 1.98 χ 10 ); A, 390 nm (slope = 1.02 χ 10 ). 4

3

4

TABLE I D i m e r i z a t i o n Constants of D-,Cat Cat

+

8

lO" ^

and a-,Cat +

Msec(D. ,Cat )

i n THF at 25°C

7

+

10 K, Msec(a",Cat ) —α

Li

+

1.2

-

Na

+

3.5

0.2

10.0

1.0 - 1.2

30.0

_

K

+

Cs

+

29.

Mechanisms

SZWARC

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421

Consider the i n i t i a l c o n d i t i o n s of a 1 M THF s o l u t i o n of a monomer with 10~^M of an i n i t i a t o r A~,Cat . For as low K as 10""5, the e l e c t r o n - t r a n s f e r e q u i l i b r i u m , e s t a b l i s h e d w i t h i n a few psec, converts 27% of A ~ , C a t i n t o monomerT,Cat . Even f o r as low as 10^ M ~ l s e c ~ l , i t takes less than 0.2 sec to produce 95% of a l l the dimeric dianions expected i n the q u a n t i t a t i v e conversion. In that time less than 50 monomer molecules are added t o each of the formed growing centers, provided the propagation constant i s not l a r g e r than 250 M ~ l s e c ~ l - a rather high value, while at the completion of p o l y m e r i z a t i o n 10^ molecules are added. The d i m e r i z a t i o n competes with monomer a d d i t i o n t o monomeric radical-anions, +

t r

+

+

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

M~ + Μ

+

·Μ.Μ~

k

a

Most l i k e l y kg i s smaller than the propagation constant, i . e . , k < 250 M""lsec""l. However, since the c o n c e n t r a t i o n of the monomer i s at l e a s t 10^ times l a r g e r than that of the monomeric r a d i c a l - a n i o n s , the a d d i t i o n competes e f f i c i e n t l y with the d i m e r i z a t i o n . Nevertheless, the r e s u l t i n g dimeric r a d i c a l - a n i o n s , ·Μ.Μ~, play no r o l e i n the p o l y m e r i z a t i o n because t h e i r d i f f u s i o n c o n t r o l l e d d i s p r o p o r t i o n a t i o n (rate constant ^ M ~ l s e c ~ l ) destroys them as soon as formed. Hence, r a d i c a l propagation i s imperceptible i n such systems. The dimeric dianions are s t a b l e . The r a t e of d i s s o c i a t i o n of Κ ,~α.θΓ,Κ was determined by the f o l l o w i n g procedure (_7). α-methyl styrene perdeuterated i n phenyl groups, a5j), was prepared and converted i n t o dimeric dianions, K , ~ a 5 « a 5 ~,K . T h e i r THF s o l u t i o n was mixed with a s o l u t i o n of ordinary p r o t i c dimers, K , α.α"",Κ . The r e v e r s i b l e d i s s o c i a t i o n - a s s o c i a t i o n a

+

+

+

+

D

+

D

+

+

+

Κ ,-α.α-,Κ

m = 243,

K ,~a.a5 ,K

m = 248,

. ^diss Κ+,-αξρ.α^ρ,Κ-'- ^ ! 2aT ,K

+

;

1 S S

m = 238,

^ 2«τ,κ+

+

D

+

5D

+

+

forms mixed dimers, K ,~a.a5j),K * For a 50:50 mixture the rate of mixed dimers formation i s equal to 1/2 that of d i s s o c i a t i o n , k ^ g g . The mixture was kept at a constant temperature, and at d e s i r e d time i n t e r v a l s (12h, 24h, e t c . ) a l i q u o t s were removed and protonated by methanol. The r e s u l t i n g hydrocarbons were i s o l a t e d and analyzed by mass-spectrometer. The a n a l y s i s gives the f r a c t i o n of the homo-dimers, masses 238 and 248, converted i n t o mixed dimers, mass 243, i n a predetermined time i n t e r v a l , and t h i s allows the

428

INITIATION OF POLYMERIZATION

=

c a l c u l a t i o n of the f i r s t order d i s s o c i a t i o n constant, k ^ i s s 6.10"8 s e c at 25°C. In c o n j u n c t i o n with the determined a s s o c i a t i o n constant, i t gives the e q u i l i b r i u m constant of the dissociation, K^iss 10" · Assuming a p l a u s i b l e value of 15 e.u. f o r AS of d i s s o c i a t i o n , one f i n d s the heat of d i s s o c i a t i o n ΔΗ ^ 24 kcal/mole. Another approach l e d to the d i s s o c i a t i o n constant of Na ,~DD",Na (jB). Since 1,1-diphenyl ethylene does not add to i t s dimeric d i a n i o n s , a s o l u t i o n of Na ,~DD~,Na was mixed with r a d i o a c t i v e 1,1-diphenyl ethylene and the k i n e t i c s of exchange was i n v e s t i g a t e d . The r e s u l t s l e d to the upper l i m i t of the r e s p e c t i v e d i s s o c i a t i o n constant, namely - 1

%

Μ

+

+

+

iSdiss < " eec"*. F l a s h - p h o t o l y s i s l e d a l s o to the determination of e l e c t r o n - t r a n s f e r e q u i l i b r i u m constants, e.g., Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

1 0

7

biphenylideT (B7),Na

+

+ 1,1-diphenyl ethylene (D) +

biphenyl (B) + 1,1-diphenyl ethylene r a d i c a l - a n i o n T (D7),Na ; K +

t r

This i s achieved by flash-photοlyzing a THF s o l u t i o n of Na ,~DD~,Na i n the presence of a mixture of biphenyl and 1,1-diphenyl ethylene of known composition. N e i t h e r hydrocarbon reacts with the d i a n i o n s , although both being at much higher c o n c e n t r a t i o n than Na ,~D.D~,Na . F l a s h p h o t o l y s i s p h o t o - d i s s o c i a t e s some of the dimers i n t o D7,Na and then the e q u i l i b r i u m +

+

+

+

+

D7,Na

+

+ Β

+ B7,Na

+

K

l/ tr

i s r a p i d l y e s t a b l i s h e d due to r e l a t i v e l y high concentrations of Β and D 10~ M). In the dark period f o l l o w i n g a f l a s h , the photolyzed dimers are regenerated by the r e a c t i o n 3

+

+

2D7,Na -> Na ,"DD",Na

+

k

d

+

Since [D7,Na ] = X/( 1+[B]/ [ D ] K ) , where 1/2 X i s the concentration of the d i s s o c i a t e d dimers, tr

-d(l/X)/dt = k /(l+[B]/[D]K )2 d

tr

i . e . , p l o t s of the r e c i p r o c a l of the o p t i c a l d e n s i t y at any chosen wavelength are again l i n e a r with time. This i s shown by F i g . 7 and t h e i r slopes give { k / ( l + [ B ] / [ D ] K ) } f c ε. In the absence of the added biphenyl, an analogous p l o t has a slope k /& ε(Fig. 8), and hence the r a t i o of both slopes gives ( l + [ B ] / [ D ] K ) . Since [B]/[D] i s known, K i s derived from that r a t i o . 2

tr

d

d

2

t r

t r

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Mechanisms of Electron-Transfer Initiation

Figure 7. Plots of 7/Δ (OD 470) vs. time for different [Trph]/[D] ratios. Slopes must be corrected when used for plot shown in Figure 8 because of change in effective extinction coefficient at 470 nm due to absorbence of Trph*,Na\ Key to [Trph]/[D] ratios: O , 204; Δ , 434; •, 593; and Q, 847.

0

0.2

0.4

0.6

0.8

1.0

8

10

1.2

[B]/[D] 2

4

6

Figure 8. Square root reciprocals of corrected slopes of lines 1/Δ (OD 470) vs. time (see caption to Figure 7) plotted as functions [Trph]/[D] (A) [B]/[D] (O).

430

INITIATION OF POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

E l e c t r o n - t r a n s f e r t o monomer i s n o t t h e o n l y mode o f i n i t i a t i o n by r a d i c a l - a n i o n s , a l t h o u g h i t i s unique f o r n o n - p o l a r monomers. W i t h some p o l a r monomers, e s p e c i a l l y c y c l i c ones, t h e i n i t i a t i o n resembles p r o t o n a t i o n . F o r example, t h e r e a c t i o n o f sodium n a p h t h a l e n i d e w i t h e t h y l e n e o x i d e f o l l o w s t h e r o u t e (9)

and t h e r e d u c t i o n o f t h e a d d u c t b y n a p h t h a l e n i d e f o l l o w e d b y t h e a d d i t i o n o f another molecule o f epoxide y i e l d s the para or ortho diadduct

The b i m o l e c u l a r r a t e c o n s t a n t o f t h e f i r s t a d d i t i o n i s ^ 1 M ~ l s e e " * ( 1 0 ) . The s u b s e q u e n t p r o p a g a t i o n i s due t o alkoxide ions. A s i m i l a r p r o c e s s was r e p o r t e d f o r t h e i n i t i a t i o n o f c y c l i c - t e t r a - d i m e t h y l s i l o x a n e by s a l t s o f n a p h t h a l e n i d e (11). The e v i d e n c e f o r t h i s mechanism i s t w o - f o l d : the presence o f a r o m a t i c m o i e t y i n t h e r e s u l t i n g p o l y - g l y c o l and t h e quantitative analysis of the solution l e f t a f t e r p r e c i p i t a t i o n of the polymer, demonstrating o n l y o n e - h a l f o f the u t i l i z e d naphthalenide being converted into naphthalene. ( I t has been c l a i m e d t h a t a t v e r y low c o n c e n t r a t i o n s o f t h e n a p h t h a l e n i d e , p o l y m e r s w i t h o n l y one g r o w i n g g r o u p were formed ( 1 0 ) . H y d r o g e n a b s t r a c t i o n f r o m s o l v e n t was p r o p o s e d a s a n e x p l a n a t i o n . However, t e r m i n a t i n g i m p u r i t i e s become s i g n i f i c a n t a t v e r y low c o n c e n t r a t i o n s o f i n i t i a t o r s and t h e i r a c t i o n may a c c o u n t f o r the observations.) A n o t h e r v a r i a n t o f i n i t i a t i o n by n a p h t h a l e n i d e was p r o p o s e d by S i g w a l t (12), who s t u d i e d t h e a n i o n i c p o l y m e r i z a t i o n o f propylene sulphide. Formation o f propylene i n the course o f t h i s r e a c t i o n l e d him t o a f o l l o w i n g mechanism: sodium n a p h t h a l e n i d e + p r o p y l e n e s u l p h i d e -»· n a p h t h a l e n e + NaS* + p r o p y l e n e

29.

SZWARC

Mechanisms

of Electron-Transfer

431

Initiation

NaS* d i m e r i z e and t h e f o r m e d s o d i u m d i s u l p h i d e , NaSSNa, i n i t i a t e s the p r o p a g a t i o n . I t w o u l d be a d v a n t a g e o u s t o d e m o n s t r a t e t h e p r e s e n c e o f S-S bond i n t h e r e s u l t i n g p o l y m e r . Heterogeneous E l e c t r o n - T r a n s f e r

Initiation

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

R e a c t i o n s o f s o l i d a l k a l i m e t a l p a r t i c l e s w i t h monomer o r i t s s o l u t i o n s lead to heterogeneous e l e c t r o n - t r a n s f e r initiation. The t r a n s f e r t a k e s p l a c e on t h e s u r f a c e o f t h e m e t a l t o t h e a d s o r b e d m o l e c u l e s o f t h e monomer and y i e l d s a d s o r b e d monomeric r a d i c a l - a n i o n s w i t h t h e p o s i t i v e l y c h a r g e d p a r t i c l e s a c t i n g as c o u n t e r - i o n s . Detachment r e q u i r e s n o t o n l y d e s o r p t i o n o f the adsorbed r a d i c a l - i o n s but a l s o removal o f m e t a l c a t i o n s from the m e t a l l a t t i c e . The h i n d r a n c e o f d e s o r p t i o n d o e s n o t a f f e c t t h e m o b i l i t y o f r a d i c a l - a n i o n s on t h e m e t a l s u r f a c e . Hence, t h e i r d i m e r i z a t i o n w i t h formation of s t i l l adsorbed d i m e r i c d i a n i o n s i s very l i k e l y , and t h e s e may grow and f o r m l i v i n g o l i g o m e r s . Degree o f p o l y m e r i z a t i o n o f t h e a t t a c h e d o l i g o m e r s depends on t h e i r l i f e t i m e on t h e s u r f a c e , and t h e l i f e t i m e i s s h o r t e n e d by a c a t i o n s o l v a t i n g s o l v e n t that f a c i l i t a t e s removal of the c a t i o n f r o m t h e m e t a l l a t t i c e and t h e r e f o r e t h e d e s o r p t i o n . This i s d e m o n s t r a t e d by O v e r b e r g e r ( 1 3 ) , who s t u d i e d t h e c o - p o l y m e r i z a t i o n o f s t y r e n e and m e t h y l m e t h a c r y l a t e initiated by a f i n e s u s p e n s i o n o f p a r t i c l e s o f m e t a l l i c l i t h i u m . S t y r e n e and m e t h y l m e t h a c r y l a t e compete f o r t h e s i t e s on t h e m e t a l s u r f a c e , t h e f o r m e r b e i n g f a v o r e d by t h e h i g h p o l a r i z a b i l i t y o f i t s ïï e l e c t r o n s . Hence, o n l y s t y r e n e p o l y m e r i z e s on t h e s u r f a c e , y i e l d i n g a b l o c k o f l i v i n g polystyrene. The e v e n t u a l l y d e s o r b e d l i v i n g p o l y s t y r e n e i n i t i a t e s i n s o l u t i o n polymerization of methyl methacrylate b e c a u s e t h i s monomer i s g r e a t l y p r e f e r r e d t o s t y r e n e i n a n i o n i c co-polymerization. The g r e a t e r h i n d r a n c e o f d e s o r p t i o n , t h e h i g h e r the p e r c e n t a g e o f s t y r e n e i n the r e s u l t i n g block-polymer. I n d e e d , as d e m o n s t r a t e d by t h e O v e r b e r g e r s t u d y , the s i z e of p o l y s t y r e n e b l o c k i n c r e a s e s with d e c r e a s i n g s o l v a t i o n power o f t h e medium and t h e r e a c t i o n s p e r f o r m e d i n the a b s e n c e o f e t h e r s y i e l d s c o - p o l y m e r w i t h 28% o f s t y r e n e a t 1% conversion. On t h e o t h e r h a n d , t h e f a c i l e r e m o v a l o f Na c a t i o n s from the sodium l a t t i c e might e x p l a i n the f o r m a t i o n o f homo-poly-methyl-methacrylate i n s i m i l a r experiments i n v o l v i n g sodium d i s p e r s i o n i n s t e a d o f l i t h i u m d i s p e r s i o n . F u r t h e r e v i d e n c e o f monomer a d s o r p t i o n on a m e t a l s u r f a c e i s p r o v i d e d b y t h e e x t e n s i v e s t u d i e s o f R i c h a r d s and h i s c o - w o r k e r s (14). As i s w e l l known, a l k y l b r o m i d e s i n t e t r a h y d r o f u r a n v i g o r o u s l y r e a c t w i t h a l k a l i m e t a l s , say l i t h i u m , y i e l d i n g the Wurtz c o u p l i n g p r o d u c t s . The v i o l e n t r e a c t i o n s l o w s down on a d d i t i o n o f a r o m a t i c monomers l i k e s t y r e n e , and t h e n a t u r e o f +

432

INITIATION OF

the products i s d r a s t i c a l l y changed (15). For example, when an equimolar mixture of ethylbromide and styrene r e a c t s i n tetrahydrofuran with m e t a l l i c l i t h i u m , the a l k y l capped dimer, C H5.CH(Ph).CH2.CH2.CH(Ph).C H forms 90% o f the products, about 5% appear as C2H5.CH2.CH(Ph).C2H5, the remainder being a mixture of an a l k y l capped trimer and butane. The t a i l - t o - t a i l s t r u c t u r e of the capped dimer was demonstrated by the n.m.r. technique. It seems that styrene and ethylbromide compete f o r the s i t e s on the l i t h i u m surface. The adsorption of styrene possessing e a s i l y p o l a r i z a b l e TT e l e c t r o n s i s more favorable than that of ethylbromide. Hence, Wurtz coupling i s hindered, while e l e c t r o n - t r a n s f e r to the adsorbed styrene y i e l d s r a d i c a l - a n i o n s and t h e i r m o b i l i t y on the surface allows f o r t h e i r d i m e r i z a t i o n . E v e n t u a l l y , the dimeric dianions are desorbed; and s i n c e t h e i r r e a c t i o n with ethylbromide i s f a s t e r than propagation, the e t h y l capped dimers are the main products. Further support f o r the proposed mechanism i s provided by the r e s u l t s of experiments i n v o l v i n g phenylbromide instead o f ethylbromide (16). The p o l a r i z a b l e IT e l e c t r o n s of t h i s a r y l compound allow i t to e f f e c t i v e l y compete with styrene f o r the s i t e s on the l i t h i u m surface and thus the Wurtz coupling r e a c t i o n becomes dominant. S i m i l a r r e s u l t s were obtained with e t h y l t o s y l a t e . Although the r e a c t i o n of t o s y l a t e with l i v i n g polystyrene i s r a p i d and q u a n t i t a t i v e , y i e l d i n g e t h y l capped polymers, i t s r e a c t i o n with the monomer and m e t a l l i c l i t h i u m produces only 10% of the e t h y l capped polymers, the remainder being evolved as butane. Again, the aromatic nature of t o s y l a t e allows i t to compete with styrene f o r the l i t h i u m s i t e s . An i n t e r e s t i n g extension of t h i s p i c t u r e i s provided by the behavior of p-xylylene dibromide (17). With butadiene as monomer and tetrahydrofuran as solvent, t h e i r r e a c t i o n on m e t a l l i c l i t h i u m leads to an unusual co-polymer, 2

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

POLYMERIZATION

2

5

-(BD) -CH2.C H4.CH2.CH2.C H4.CH2-(BD)ni(BD-butadiene moiety) B

6

6

I t s composition i s determined by the i n i t i a l r a t i o of the reagents i n the feed. N.m.r. a n a l y s i s confirmed the above s t r u c t u r e and showed that the p-xylylene moieties i n the polymeric chains e x i s t e x c l u s i v e l y as dimers. Unexpectedly, the v i c i n y l d i h a l i d e s react d i f f e r e n t l y (18). For example, an equimolar mixture of styrene and 1,2-dibromoethane r e a c t s on l i t h i u m metal y i e l d i n g a head-to-head, t a i l - t o - t a i l polystyrene, ethylene and l i t h i u m bromide. Apparently, the adsorbed styrene i s reduced and dimerized to the d i a n i o n s , ~CH(Ph).CH .CH .CH(Ph)" 2

2

29.

433

Mechanisms of Electron-Transfer Initiation

SZWARC

and the l a t t e r r a p i d l y react with the dibromide y i e l d i n g the unconventional head-to-head, t a i l - t o - t a i l polymer with e l i m i n a t i o n of ethylene nj~CH(Ph) .CH .CH .CH(Ph)"+"CH(Ph) -CH -CH^CHCPh)"} 2

2

-[CH(Ph) .CH -CH .CH(Ph)—CH(Ph) -CH -CH . (Ph)] 2

2

B r C H

2

2

2

2

C H

2

B r >

nC H +2nLiBr 2

4

Other l i n k i n g agents were i n v e s t i g a t e d , e.g., dibromo-dimethyl-silane and dichlorophenylphosphine (19). I n t e r e s t i n g products were obtained with diepoxides, namely "M.M.CH -CH.R.CH.CHo.M.M." 2

etc.,

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch029

I I OLi O L i y i e l d i n g the r e s p e c t i v e p o l y - o l s on h y d r o l y s i s . E l e c t r o n - T r a n s f e r Step-Wise Reaction An example of such a r e a c t i o n i s d i s c u s s e d . Reaction o f b i s ( 1 , 1 - d i p h e n y l ethylene) l i n k e d by a chain of a l i p h a t i c hydrocarbon y i e l d s on r e d u c t i o n with a l k a l i metals o r s u i t a b l e e l e c t r o n donors, e.g., naphthalenide, a product o f p o l y - d i m e r i z a t i o n . For example:

Ph

Ph

The r e s u l t i n g poly-carbanions are protonated and y i e l d the r e s p e c t i v e hydrocarbon (20). Other examples are provided by the work of Hocker and h i s co-workers, and those discussed i n the preceding s e c t i o n .

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434

INITIATION OF POLYMERIZATION

Literature Cited 1. (a) Szwarc, M.; Nature 178, 1168 (1956). (b) Szwarc, M.; Levy, M. and Milkovich, R.; J. Amer. Chem. Soc. 78, 2656 (1956). 2. Rämme, G.; Fisher, M.; Claesson, S. and Szwarc, M.; Proc. Roy. Soc. A327, 467 (1972). 3. Wang, H. C.; Levin, G. and Szwarc, M. ; J. Phys. Chem. 83, 785 (1979). 4. (a) Katayama, M.; Hatada, M.; Hirota, K.; Yamazaki, H. and Ozawa, Y.; Bull. Chem. Soc. Japan 38, 851, 2208 (1965). (b) Schneider, C. and Swallow, A. J.; Makromolek. Chem. 114, 155, 172 (1968). (c) Metz, D. J.; Potter, R. C. and Thomas, J. K.; J. Polymer Sci. A5, 877 (1967). 5. (a) Wang, H. C.; L i l l i e , E. D.; Slomkowski, S.; Levin, G. and Szwarc, M.; J. Amer. Chem. Soc. 99, 4612 (1977). (b) L i l l i e , E. D.; Slomkowski, S.; Levin, G. and Szwarc, M.; ibid., 99 4608 (1977). 6. Hamill, W. H.; "Radical Ions", p. 321, Kaiser, E. T. and Kevan, L., Eds., Interscience (1968). 7. Asami, R. and Szwarc, M.; J. Amer. Chem. Soc. 84, 2269(1962). 8. Spach, G.; Monteiro, M.; Levy, M. and Szwarc M.; Trans. Faraday Soc. 58, 1809 (1962). 9. Richards, D. H. and Szwarc, M.; Trans. Faraday Soc. 55, 1654 (1959). 10. Kazanskii, S.; Solovyanov, A. A. and Entelis, S. G.; Europ. Polymer J. 7, 1421 (1971). 11. Morton, M.; Rembaum, A. and Bostick, E. E . ; J. Polymer Sci. 32, 530 (1958). 12. (a) Boileau, S.; Champetier, G. and Sigwalt, P.; J. Polymer Sci. C16, 3021 (1967). (b) Favier, J. P.; Boileau, S. and Sigwalt, P.; Europ. Polymer J. 4, 3 (1968). 13. Overberger, C. G. and Yamamoto, N.; J. Polymer Sci. B3, 569 (1965); A4, 3101 (1964). 14. Richards, D. H.; Polymer 19, 109 (1978). 15. Davis, A.; Richards, D. H. and Scilly, N. F. ; Makromolek. Chem. 152, 121, 133 (1972). 16. Cunliffe, A. V.; Paul, N. C.; Richards, D. N. and Thompson, D.; Polymer 19, 329 (1978). 17. Richards, D. H. and Scilly, N. F.; Brit. Polymer J. 2, 277 (1970); 3, 101 (1971). 18. Richards, D. H.; Scilly, N. F.; Chem. Comm., p. 1515 (1968). 19. (a) Richards, D. H. et a l . ; Europ. Polymer J. 6, 1469 (1970). (b) Cunliffe, A. V.; Hubbert, W. J. and Richards, D. H.; Makromolek. Chem. 157, 23, 39 (1972). (c) Richards, D. H. et al.; Polymer 16, 654, 659, 665 (1975). 20. Hocker, H. and Lattermann, G.; J. Polymer Sci. Symposium 54, 361 (1976). RECEIVED January 10,1983

30 Initiation of Polymerization with High-Energy Radiation VIVIAN STANNETT North Carolina State University, Chemical Engineering Department, Raleigh, NC 27650

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

JOSEPH SILVERMAN University of Maryland, Institute for Physical Science and Technology, Laboratory for Radiation and Polymer Science, College Park, MD 20742

High energy photons and electrons interact with organic liquids to produce excited species, positive molecule ions and electrons. Most of the electrons rapidly recombine with their geminate cation radicals to form additional excited molecules. Some of the latter lose their excitation energy by c o l l i s i o n with ether molecules; the remainder break into free radicals. For this reason most of the polymerization reactions of vinyl monomers in bulk, emulsion and in solution are initiated by free radicals. A small proportion of the ejected electrons have sufficient energy to escape the coulombic forces of their corresponding positive molecule ions. These free electrons lose their energy to the surroundings, become thermalized, and can generate anionic species either by simple capture or by dissociative electron capture. Both the positive and negative ionic species can, under suitable conditions, i n i t i a t e cationic and anionic polymerizations, respect i v e l y . An important point of fundamental interest is that the early positive and negative species are paramagnetic, and that the mechanisms by which they are converted to propagating carbonium ions and carbanions are as yet not well understood. The subsequent chain growth of the radical, cationic and anionic i n i t i a t i n g species has led to new fundamental information of considerable value to both radiation and polymer chemists. Apart from the fundamental interest in radiation induced polymerization, there is considerable practical interest including industrial exploitation. The i n i t i a t i o n process has essentially zero activation energy in markec contrast to chemical i n i t i a t i o n . This has interesting kinetic consequences as w i l l be discussed later. In addition, i t leads to an overall lower activation energy for the polymerizaton i t s e l f . This, coupled with the ease of control of the rate of initiation including, i f necessary, the rapid removal of the source, lessens the chance of run-away 0097-6156/83/0212-0435$06.00/0 © 1983 American Chemical Society

436

INITIATION OF POLYMERIZATION

p o l y m e r i z a t i o n s . T h e r e a r e a number o f o t h e r p r a c t i c a l a d v a n t a g e s o f r a d i a t i o n : f o r example, the l a c k o f any r e s i d u a l c a t a l y s t o r catalyst f r a g m e n t s and the. v i r t u a l l y u n l i m i t e d range o f c h a i n initiation r a t e s w h i c h c a n be r e a d i l y o b t a i n e d . T h e r e a r e some possible disadvantages, in particular, changes i n the polymer itself brought about by the concurrent radiolysis. I n any p r a c t i c a l r a d i 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 e r e q u i r e d dose i s so low, however, t h a t t h e s e e f f e c t s w o u l d n o r m a l l y be n e g l i g i b l e .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

Radiation

Initiated

Free

Radical

Initiation

A l l o r g a n i c l i q u i d s , i n c l u d i n g v i n y l monomers, p r o d u c e f r e e r a d i c a l s on i r r a d i a t i o n . The G v a l u e ( r a d i c a l s p r o d u c e d p e r 100 eV of absorbed radiation energy), which i s the r a d i a t i o n yield, depends on t h e i r s t r u c t u r e and c a n v a r y from 0.7, f o r pure styrene, f o r example, t o an a p p a r e n t value o f 19 f o r c a r b o n t e t r a c h l o r i d e i n styrene s o l u t i o n . I n t h e c a s e o f b u l k monomers, i t i s e a s y t o c a l c u l a t e t h e rate of i n i t i a t i o n from t h e G v a l u e s , t h e dqse r a t e , and t h e p h y s i c a l c o n s t a n t s o f t h e monomer. I f t h e k /k^ v a l u e s a r e known f o r t h e monomer i n q u e s t i o n a t the t e m p e r a t G r e u s e d , t h e r a t e s o f i n i t i a t i o n c a n be c a l c u l a t e d from t h e o b s e r v e d s t e a d y s t a t e r a t e s of p o l y m e r i z a t i o n . Thus f o r t h e s i m p l e s t c a s e , 2

2

R k R. = 1

— 2

k (M) Ρ

(

1

)

2

R. c o m b i n e d w i t h t h e measured dose r a t e c a n be t h e n u s e d t o c a l c u l a t e t h e G v a l u e f o r r a d i c a l s ; t h i s method was d e v e l o p e d by Chapiro (jO. S t y r e n e i s a p a r t i c u l a r l y w e l l b e h a v e d monomer and the k /k v a l u e s a r e known o v e r a w i d e t e m p e r a t u r e r a n g e . I t c a n be u therefore, t o measure the r a t e s of initiation in solution. Other methods, f o r example, the use of radical s c a v e n g e r s such as DPPH, c a n a l s o be u s e d t o o b t a i n a d i r e c t v a l u e for the r a t e of r a d i c a l p r o d u c t i o n . W i t h b u l k monomers t h e R. v a l u e s a r e a f u n c t i o n o n l y o f t h e dose r a t e and t h e k i n e t i c s a r e quite comparable t o those o f the benzoyl peroxide initiated system, f o r example. 2

Radiation i n i t i a t i o n f o r solution vinyl polymerization i s more c o m p l e x . A s p e c i a l f e a t u r e o f r a d i a t i o n i s t h a t i t a t t a c k s all components o f the system including the s o l v e n t . With a m o n o m e r - s o l v e n t m i x t u r e , t h e r e f o r e , t h e r a t e o f i n i t i a t i o n c a n be r e p r e s e n t e d by a l i n e a r e q u a t i o n as f o l l o w s : R. = D [A(M)G ( i ) + B ( S ) G ( i ) ] (2) ι m s Where D i s t h e dose r a t e and G ( i ) and G ( i ) a r e t h e G v a l u e s f o r i n i t i a t i o n f o r t h e monomer a n d s o l v e n t , r e s p e c t i v e l y . (M) and ( S ) a r e t h e monomer and s o l v e n t c o n c e n t r a t i o n s and A and Β c o n s t a n t s t o c o n v e r t the G v a l u e s i n t o a p p r o p r i a t e u n i t s , i . e . , moles, l i t e r s , seconds.

30.

STANNETT AND SILVERMAN

437

High-Energy Radiation Initiation 2

The method o f C h a p i r o (1_) u s i n g known k / k v a l u e s f o r the p u r e monomer can t h e n be u s e d t o c a l c u l a t e t i e o v e r a l l R. i f the r a d i c a l y i e l d s a r e p u r e l y a d d i t i v e and i d e a l k i n e t i c s a p p l y . The latter i m p l i e s t h a t a l l the s o l v e n t r a d i c a l s r e a c t immediately w i t h monomer and the t e r m i n a t i o n s t e p i s by the r e c o m b i n a t i o n of the growing chains. If the purely additive effect, sometimes c a l l e d the s i m p l e d i l u t i o n e f f e c t , a p p l i e s , then a p l o t of the overall rates of initiation versus m, the mole fraction of monomer, g i v e s a s t r a i g h t l i n e . I f the p l o t c u r v e s , i t i n d i c a t e s t h a t e n e r g y t r a n s f e r i s t a k i n g p l a c e as w i l l be d i s c u s s e d later. Curvature above the l i n e shows s e n s i t i z a t i o n and b e l o w i n d i c a t e s d e a c t i v a t i o n by the s o l v e n t . A more u s e f u l a n a l y s i s has been made by C h a p i r o (1) who showed t h a t :

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

t

1

Γ

R u

K

s

3 / 2

Γ

(l-m)l

1 / 2

m

po Where R and R are the rates of polymerization in s o l u t i o n and piire monomer*, r e s p e c t i v e l y . Φ ^ = Φ /Φ l * Φ * φ are the m o l a r y i e l d c o n s t a n t s f o r f r e e r a d i c a l p r o d u c t i o n per u n i t e x p o s u r e d o s e and u n i t volume from the s o l v e n t and monomer, r e s p e c t i v e l y ; ( t h e s e a r e a l m o s t p r o p o r t i o n a l t o the G v a l u e s ) . V w

ΓΘ

πι

e

r

a n a

e

5

and V a r e the r e s p e c t i v e m o l a r v o l u m e s . P l o t s o f R /R versus m g i v e a f a m i l y o f c u r v e s a c c o r d i n g t o the v a l u e o f ^φ ^ ° as shown i n F i g u r e 1. When Φ^^=1.0, as o b t a i n e d w i t h s t y r e n e i n b e n z e n e or methyl methacrylate i n e t h y l a c e t a t e , f o r example, the k i n e t i c s are s i m i l a r to those o b t a i n e d w i t h c a t a l y s t - i n i t i a t e d polymeriza­ t i o n s y s t e m s f o r w h i c h t h e r e i s no e f f e c t o f the s o l v e n t on the initiation rate. m

The c u r v e s can u s u a l l y be f i t t e d w i t h a c o n s t a n t v a l u e of ^rel' ^ °~ examples may be f o u n d i n r e f e r e n c e 1. T h i s i s not a l w a y s t h e c a s e , however. V a l u e s of $ γ be c a l c u l a t e d from e q u a t i o n 3 a t v a r i o u s monomer c o n c e n t r a t i o n s . I f t h e s e a r e not c o n s t a n t i t i s u s u a l l y assumed t h a t e n e r g y t r a n s f e r p r o c e s s e s a r e o p e r a t i v e . The t e r m e n e r g y t r a n s f e r p r o c e s s i n t h i s work i s u s e d i n i t s most g e n e r a l s e n s e . The u s u a l m e a n i n g i m p l i e s r a p i d t r a n s f e r of e x c i t a t i o n from an a b s o r b i n g s i t e ( s u c h as a s o l v e n t m o l e c u l e ) over s e v e r a l molecular l e n g t h s t o an e n e r g y s i n k w h i c h becomes r e a c t i v e ( s u c h as a s o l u t e w h i c h becomes an effective p r o d u c e r of r a d i c a l s ) . We use the t e r m to a l s o i n c l u d e f a s t , but ordinary, chemical reactions which effectively transfer the reactive site. Because these two mechanisms a r e e q u i v a l e n t , we shall describe the solvent effect i n terms o f n e u t r a l e x c i t e d species a r i s i n g from the d i r e c t a b s o r p t i o n o f r a d i a t i o n by the monomer and s o l v e n t and the t r a n s f e r of e x c i t a t i o n by e x c h a n g e processes. Regardless of the details of the mechanism, the processes are equivalent and are described below in the conventional sense of radiation-produced excited species that a

n u r n

e r

m

τ

β

a

v

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438

INITIATION OF

POLYMERIZATION

Figure 1. Relative rates of radiation-induced polymerization in bulk and in solution (Eq. 3) at various 0 values. (Reprinted with permission from Réf. 1. Copyright 1962, John Wiley & Sons.)

30.

S T A N N E T T

A N D

i n t e r a c t w i t h monomer and s o l v e n t o r a n exchange o f e n e r g y , thus M* - S

+

S* + M

439

High-Energy Radiation Initiation

S I L V E R M A N

thereby

producing

free

radicals

S* + M

->

S» + M

(4)

M" + S

+

M. + S

(5)

An e x t r e m e example o f e n e r g y t r a n s f e r b e h a v i o r was r e p o r t e d by M i l l e r and S t a n n e t t (2) w i t h s t y r e n e i n n - d i b u t y l d i s u l f i d e . The results are presented i n Table I and show a p r o g r e s s i v e decrease in Φ^ with increasing amounts of d i s u l f i d e . This i m p l i e s t h a t e n e r g y t r a n s f e r p r o c e s s e s , p r e d o m i n a t e l y from s t y r e n e t o t h e d i s u l f i d e , a r e t a k i n g p l a c e . The o v e r a l l k i n e t i c s showed that classic k i n e t i c s were o t h e r w i s e n o r m a l . A number o f o t h e r e x a m p l e s a r e p r e s e n t e d i n r e f e r e n c e (_1) and by S t a n n e t t e t a l . ( A ) .

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

ΓΘ

The kinetics of radiation induced polymerization i n s o l u t i o n i n w h i c h e n e r g y t r a n s f e r i s o p e r a t i v e have been d e v e l o p e d by N i k i t i n a and B a g d a s s a r i a n ( 3 ) . A term was i n t r o d u c e d , Ρ r e p r e s e n t i n g t h e r e l a t i v e p r o b a b i l i t y o f e n e r g y t r a n s f e r from S to S v e r s u s t r a n s f e r from S t o M. T h i s l e a d s t o a m o d i f i e d e x p r e s s i o n f o r F /R : r po l

5

e

Γ

Vpo F

K

L

. m + (1-m)

1/2

13/2 Γΐ + φ . Ρ . (1-m)vi^sn ' V /V ι , ρ (1-m) J s m 1+ P ~*

(6)

rel m

The d e v e l o p m e n t and l i m i t a t i o n s o f t h i s e q u a t i o n have been discussed by C h a p i r o ( 1). The d a t a with styrene and d i b u t y l d i s u l f i d e were a n a l y z e d l e a d i n g t o s i n g l e v a l u e s o f Φ -=8.5 and P ^ ^ = 3 . 1 . T i e d e g r e e o f f i t o f t h e d a t a i s shown i n F i g u r e 2. Using the value obtained f o r Φ^ι after correcting f o r energy transfer, G ( r a d i c a l ) value (strictly speaking, the G value f o r chain i n i t i a t i o n f o r d i b u t y l d i s u l f i d e was e s t i m a t e d t o be 3.1. The b e h a v i o r and v a l u e s were f o u n d t o be i n good agreement w i t h other data p u b l i s h e d f o r v a r i o u s d i s u l f i d e s . When t h e φ ^ ^ v a l u e s a r e l e s s t h a n u n i t y and p l o t s o f R_. v e r s u s m c u r v e befow t h e φ=1.0 l i n e , t h e r e i s d e a c t i v a t i o n by t h e solvent whereas curvature above the l i n e signifies Φ^ 1 · 0 , i n d i c a t i n g s e n s i t i z a t i o n o f t h e p o l y m e r i z a t i o n by t h e s o l v e n t . The e f f e c t s o f s o l v e n t s on the r a t e s o f i n i t i a t i o n can have c o n s i d e r a b l e i m p l i c a t i o n s f o r r a d i a t i o n g r a f t i n g . T h i s was c l e a r l y shown f o r t h e g r a f t i n g o f s t y r e n e t o c e l l u l o s e a c e t a t e i n s o l u t i o n ( 4 ) . The u s e o f p y r i d i n e w i t h a Φ 0 compared w i t h c i n e t h y l formamide w i t h found t o g i v e s i m i l a r y i e l d s o f g r a f t Γθ c o p o l y m e r . However, t h e a c c o m p a n y i n g hemepolymer was o n l y 8% i n t h e c a s e o f p y r i d i n e compared w i t h 3 2 % w i t h d i m e t h y l formamide as the s o l v e n t u n d e r c o m p a r a b l e c o n d i t i o n s . >

Γβ

%

Φ |^^·^

w

a

s

W h i l e t h e e n e r g y t r a n s f e r model i s r e m a r k a b l y s u c c e s s f u l i n summarizing the p o l y m e r i z a t i o n k i n e t i c s o f v i n y l monomer solu­ tions, t h e r e may be o t h e r e x p l a n a t i o n s w i t h c e r t a i n monomer-

440

INITIATION OF POLYMERIZATION

Table

I:

The V a r i a t i o n Radiation Styrene

r

e

l

DBS

Free

0.005 M r a d p e r h o u r

44.0

10

20

23.8

f o r the

Radical Polymerization of

i n n-Dibutyl Disulfide

5

Solvent

16.6

at 25°C. ( r e f . 2) 40

17.3

60

80

13.6

6.9

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Φ

Percent

Φ . with Percent rel

Initiated

Dose R a t e Vol

of

0.2

0.4

0.6

Mole F r a c t i o n

0.8

1.0

o f Monomer

Figure 2. Relative rates for radiation-induced polymerization of styrene in dibutyl disulfide at 25 °C. Key: , equation 6 with values; 0 i — 8.5; P i — 3.1. (Reprinted with permission from Ref. 2. Copyright 1969, John Wiley & Sons.) re

re

30.

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Radiation

Initiation

441

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

solvent systems (_5) · The principal observations l e a d i n g t o an a l t e r n a t i v e e x p l a n a t i o n are t h e s e : (1) the G v a l u e f o r i o n p a i r formation i n gaseous hydrocarbon monomers i s ^4; (2) G (free ions) i n neat hydrocarbon l i q u i d monomers i s ^ 0 . 1 ; (3) the G v a l u e f o r p r o p a g a t i n g r a d i c a l c h a i n s i n neat h y d r o c a r b o n liquid monomers i s CH-CH + CH 2

3

2

2

3

ι NO. Ζ.

J

2

.3

|

2

r.o„ Δ

occur

0

2

( l c 0

radical)

=

C-

J

(11) NO. /.

The i s o b u t y l e n e mechanism a p p e a r s p l a u s i b l e and i s s t r o n g l y supported by mass spectrometer s t u d i e s ( 9 ) . The n i t r o e t h y l e n e m e c h a n i s m r e m a i n s however h i g h l y s p e c u l a t i v e . C o n s i d e r a b l e r e l e ­ vant i n f o r m a t i o n o f e l e c t r o n t r a n s f e r p o l y m e r i z a t i o n s i s however a v a i l a b l e from the work o f Szwarc and o t h e r s ( 1 0 ) . I n the c a s e o f p u r e s t y r e n e , t h e mechanism f o r f o r m a t i o n o f the p r o p a g a t i n g c a r b o n i u m i o n i s n o t c l e a r . R e c e n t r e s u l t s w i t h

30.

Radiation

Initiation

443

p i c o s e c o n d p u l s e r a d i o l y s i s h a s , however, c l a r i f i e d t h e s i t u a t i o n . The p r i n c i p a l e a r l y e f f e c t a p p e a r s t o be i o n i z a t i o n l e a d i n g t o t h e c a t i o n r a d i c a l and t h e a n i o n r a d i c a l ( b y e l e c t r o n c a p t u r e ) . W i t h i n 10 p s , t h e c a t i o n r a d i c a l adds a monomer t o become t h e d i m e r i z e d . c a t i o n i c s p e c i e s , w i t h an e s t i m a t e d f i r s t o r d e r r a t e c o n s t a n t o f 8 χ 1 0 M" sec" (_5 ). The d i m e r c a t i o n d i s a p p e a r s by a f i r s t order process with a l i f e t i m e o f 20 ns (_5). T h i s i s p r e s u m a b l y t h e t r i m e r i z a t i o n and thus the f i r s t step o f the c a t i o n i c p o l y m e r i z a ­ t i o n , t h e c a l c u l a t e d r a t e c o n s t a n t i s 4 χ 10 M*sec . T h i s i s i n e x c e l l e n t a g r e e m e n t w i t h t h e v a l u e s e s t i m a t e d from a c o m b i n a t i o n of c o n d u c t i v i t y and p o l y m e r i z a t i o n r a t e s t u d i e s w i t h radiation initiation (JLl'i?.) · Chemically initiated polymerizations are somewhat l o w e r b u t t h e s e a r e n o t s t r i c t l y r e l e v a n t s i n c e t h e y were c o n d u c t e d i n c h l o r i n a t e d s o l v e n t s . I t has been w e l l e s t a b l i s h e d t h a t s o l v a t i o n and o t h e r e f f e c t s r e d u c e t h e r a t e c o n s t a n t s w i t h free cationic polymerizations. 9

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High-Energy

STANNETT AND SILVERMAN

1

1

These r e s u l t s

are consistent with

the f o l l o w i n g mechanisms:

Ionization Φ CH=CH -^W* φ CH-CH + e" 2

(12)

2

Dimerization of cation r a d i c a l φ CH-CHt + $CH=CH

0

2

^

8

x

2

l

^

n

M

• HCH C-CH CH

s

0

lOps

(13)

0

J 2

2J

Φ

Φ

Tirimerization by c a t i o n i c a d d i t i o n + 4xl0 M~ s~ · HCH C-CH CH + φ0Η=Οί — • HCH C-CH CH-CH CH 2 I I 20 ns I2 | 2| φ φ φ φ φ 6

0

0

1

1

+

0

ο

0

0

2

(14)

Formation of anion r a d i c a l φ0Η=0Η

2

+ e

-* (|CH=CH-

(15)

The close proximity o f t h e d i m e r c a t i o n and t h e a n i o n radical i n t h e s p u r c o u l d l e a d t o some of the high yield of d i m e r s and t r i m e r s f i r s t r e p o r t e d by M a c h i , S i l v e r m a n and M e t z (13) . P u l s e r a d i o l y s i s r e s u l t s were a l s o o b t a i n e d ( 5 ) s u g g e s t i n g t h a t the r o l e o f methanol i s to serve as a r a p i d p r o t o n donor t o the a n i o n r a d i c a l c o n v e r t i n g the l a t t e r t o a n e u t r a l p r o p a g a t i n g f r e e r a d i c a l . The r e s u l t i n g m e t h o x i d e a n i o n c o u l d n e u t r a l i z e t h e c a t i o n r a d i c a l c o n v e r t i n g i t to an a d d i t i o n a l n e u t r a l p r o p a g a t i n g f r e e r a d i c a l . M e t h a n o l h a s , i n d e e d , been o b s e r v e d t o s c a v e n g e b o t h t h e c a t i o n i c a n d a n i o n i c s p e c i e s (5) a n d t o i n c r e a s e t h e r a t e o f 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 (11). The G ( i ) values f o r the y i e l d of p o s i t i v e or negative initiating s p e c i e s , which lead to estimates o f the r a t e s o f i n i t i a t i o n , a r e d i f f i c u l t t o d e t e r m i n e . The methods a v a i l a b l e a r e t h e s c a v e n g i n g o f i o n s w i t h v a r i o u s compounds a n d t h e e l e c t r i c a l

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

444

INITIATION OF

POLYMERIZATION

c o n d u c t i v i t y method u s i n g s t e a d y s t a t e and p u l s e d c h a r g e c o l l e c t i o n t e c h n i q u e s . They do n o t a l w a y s show good a g r e e m e n t ; however the r a n g e o f v a l u e s i s n o t t o o l a r g e i n most c a s e s . S i n c e t h e values enter into the r a t e e x p r e s s i o n as a s q u a r e r o o t , the errors become even less significant i n e v a l u a t i n g the other k i n e t i c p a r a m e t e r s . The f r e e i o n y i e l d s o f o n l y a few monomers have b e e n m e a s u r e d d i r e c t l y ( 1 1 , 1 4 - 1 5 ) . However t h o s e f o r a l a r g e number o f o t h e r o r g a n i c l i q u i d s have been d e t e r m i n e d and have been c o l l e c t e d t o g e t h e r i n an i m p o r t a n t and u s e f u l p u b l i c a t i o n ( 1 5 ) . A p a r t f r o m the v a l u e s t h e m s e l v e s , i n t e r e s t i n g t r e n d s a r e p r e s e n t e d i n r e f e r e n c e 16. The y i e l d s i n c r e a s e w i t h t e m p e r a t u r e t o a l i m i t e d e x t e n t , f o r e x a m p l e , by a b o u t s i x - f o l d from -78 C t o +110 C f e r a number of dienes. The free ion yields also increase with increasing dielectric constant, but w i t h some i m p o r t a n t except i o n s , n o t a b l y the a l k a n e s ; f o r the l a t t e r , f r e e i o n y i e l d s a r e greater f o r branched alkanes than f o r normal alkanes although t h e r e a r e no s i g n i f i c a n t d i f f e r e n c e s i n the d i e l e c t r i c constant. W i t h p o l a r o r g a n i c o f compounds the c o r r e l a t i o n i s o f t e n g o o d . [See F i g u r e 3 (1^6).] T h i s e n a b l e s one t o make an e s t i m a t e f o r , s a y , m e t h y l e n e c h l o r i d e where no v a l u e s have b e e n d e t e r m i n e d . It c o u l d be a s k e d w h e t h e r t h e s e f r e e i o n y i e l d s a r e r e a l l y e q u a l t o the y i e l d o f c h a i n i n i t i a t i o n . O n l y one d i r e c t c o m p a r i s o n has been made, w i t h e t h y l v i n y l e t h e r i n n - p e n t a n e and n e o p e n t a n e s o l u t i o n s where G ( f r e e i o n ) v a l u e s r a n g e 0.16 t o 1.0 (18). The r a t e s o f p o l y m e r i z a t i o n o f s t y r e n e were p r o p o r t i o n a l t o the s q u a r e r o o t o f the computed o v e r a l l G ( i ) v a l u e s . In t h i s c a s e , at least, i t appears that a l l the free i o n s do indeed participate i n the i n i t i a t i o n process. I t i s b e l i e v e d t h a t the f r e e p o s i t i v e and n e g a t i v e s p e c i e s a n n i h i l a t e e a c h o t h e r i m m e d i a t e l y on c o n t a c t . T h i s i s b o r n e out by the strict square r o o t r e l a t i o n s h i p w h i c h i s f o u n d between the r a t e s o f p o l y m e r i z a t i o n and the dose r a t e o f the r a d i a t i o n . The g r o w i n g c h a i n ends a r e t h e r e f o r e f r e e i n n a t u r e , i . e . , with no i o n - p a i r component. T h i s makes r a d i a t i o n i n i t i a t e d i o n i c p o l y m e r i z a t i o n an e x c e l l e n t method f o r s t u d y i n g f r e e i o n p o l y m e r i z a t i o n : e x a m p l e s o f t h e power o f t h i s method have been p r e s e n t e d for p-methoxy s t y r e n e (1_9) and the v i n y l e t h e r s ( 16, 20,21 ) . The work o f H a y a s h i e t a l . (22), however, has indicated t h a t an i o n p a i r s component can be i n t r o d u c e d by a d d i n g strong e l e c t r o n s a c c e p t o r s such as p y r o m e l l i t i c anhydride. Finally, it is clear that certain monomers, notably styrene, can polymerize simultaneously by free radical, free anion, and free c a t i o n mechanisms. A f t e r the extremely rapid reactions l e a d i n g to the production of the chain initiators, interactions between these different mechanisms a p p e a r to be m i n i m a l but a r e unknown. However i t i s l o g i c a l t o b e l i e v e t h a t the a n i o n and c a t i o n c h a i n s w i l l t e r m i n a t e e a c h o t h e r . There are s u f f i c i e n t q u a n t i t a t i v e data f o r neat s t y r e n e at room t e m p e r a t u r e to estimate the r e l a t i v e r a t e s at which each process w i l l proceed.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

30.

STANNETT AND SILVERMAN

High-Energy

Radiation

Initiation

445

Figure 3. Correlation between the G (free ions) and dielectric constant, D, values for a series of halogenated compounds used to estimate methylene chloride values. (Reprinted with permission from Ref. 17. Copyright 1982, Butterworth & Co. (Publishersj Ltd.)

446

INITIATION OF POLYMERIZATION

As indicated earlier, G=0.7 for free radical i n i t i a t i o n and about 0.1 for the free ions. The termination rate constants are 3 χ 107 M' 1 sec'1 for free radicals and 2 χ 1011 M" 1 sec"1 for the free ions. The propagation rate constants have been determined to be 30, 4 χ 106 and about 105 M" 1 sec" 1 for free radical, cationic and anionic polymerization, respectively. The steady state rates of polymerization, R , are given by R = k (M) (R./k )% 1 Ρ Ρ t

(16)

using the correct rate constants for each case.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch030

R

= 1.04 χ 10" 9 G(i) Ρ D

(17)

where R. is the i n i t i a t i o n rate in M "''sec Ρ is the specific gravity, and D is the dose rate in rads per second. For a typical case of gamma polymerization in which D=100, R. is 6.6 χ 10' 0 and 0.943 χ 1 θ " 8 M" 1 sec' 1 for radicals and ions, respectively. The rates of polymerization (M^sec" 1 ) calculated by means of equation 17 are as follows. radical

1.23 χ 1 θ " 5 -3 cation 7.6 χ 10 -4 anion 1.9 χ 10 The ratio is 1:618:15.4 for the free radical, cationic and anionic respectively. The overwhelming mechanism is clearly cationic under super dry conditions with about 2.5% anionic and a negligible radical contribution. Literature Cited 1. Chapiro, A., "The Radiation Chemistry of Polymeric Systems", Interscience Publishers, New York, NY 1962. 2. M i l l e r , L. A. and Stannett, V . , J. Poly. S c i . , A1, 7, 3159 (1969). 3. Nikitina, T. and Eagdassarian, Sbornik Rabot po Radiatsionnoi Khimii, Moscow Academy of Sciences, U.S.S.R., p. 183 (1955) see also Ref. 1 pages 266-269. 4. Stannett, V . , Wellons, J. D., and Yasuda, H . , J. Poly. S c i . , C4, 551 (1964). 5. Tagawa, S., Silverman, J . , Kobayashi, H., Katasumura, Y., Washio, M., and Tabata, Y., Radiation Phys. & Chem., in press (1982). 6. Huang, R. Y. M. and Chandramouli, P., J. Poly. S c i . , P, Polymer 7 , 245 (1969).

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30.

STANNETT A N D SILVERMAN

High-Energy Radiation Initiation

447

7. Davison, W. H. T . , Pinner, S. H . , and Worrall, R., Chem. and Ind., 1274 (1957). 8. Yamaoka, H., Williams, F . , and Hayashi, K., Trans. Farad. Soc., 63, 376 (1967). 9. Lampe, F. W., J. Phys. Chem., 63, 1986 (1959). 10. Szwarc, M., "Carbanions, Living Polymers and Electron Transfer Processes", Wiley, N.Y. (1968). 11. Williams, F . , Hayashi, Ka., Ueno, K., Hayashi, K., and Okamura, S., Trans. Farad. Soc. 63, 1501 (1967). 12. Hayashi, Ka., Hayashi, K., and Okamura, S., Polym. J. (Japan) 4, 426 (1973). 13. Machi, S., Silverman, J . , and Metz, D. J . , J. Phys. Chem, 76, 930 (1972). 14. Hayashi, Ka., Yamazawa, Y . , Takagaki, T . , Williams, F . , Hayashi, K., and Okamura, S., Trans. Farad. Soc. 63, 1489 (1967). 15. Hayashi, Ka., Hayashi, K., and Okamura, S., J. Poly. S c i . , A1, 9, 2305 (1971). 16. Allen, A. O., "Yields of Free Ions Formed in Liquids by Radiation", N.B.S. Reference Data System NSRD-NBS 57 (1976). 17. Deffieux, A., Hsieh, W. C . , Squire, D. R. and Stannett, V . , Polymer, 23, 65 (1982). 18. Kubota, H., Kabanov, Y a . , Squire, D. R. and Stannett, V . , J. Macromol. S c i . - Chem. A12, 1299 (1978). 19. Deffieux, A., Squire, D. R. and Stannett, V . , Polymer B u l l . , 2, 469 (1981). 20. Hsieh, W. C . , Kubota, H., Squire, D. R., and Stannett, V . , J. Poly. S c i . - Chem. 18, 2773 (1980). 21. Deffieux, A., Hsieh, W. C., Squire, D. R. and Stannett, V . , Polymer 22, 1575 (1981). 22. Hayashi, K., I r i e , M . , and Yanamoto, Y . , J. Poly. S c i . , Sym. Series 56, 173 (1977). R E C E I V E D January 10,1983

American Chemical S o c i e t y Library U 5 5 16îh St. N . W. Washington, 0. C. 20038

31 Temperature and Dilution Effects in Polymerization Induced by Peroxides Undergoing Rapid Decomposition (t ^ΧΑ)""* R - m + _____ Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch031

tDMF

The s o l u t i o n polymerization of ΜΑΗ i s s u b j e c t t o a d i l u t i o n e f f e c t , i . e . the y i e l d of poly-ΜΑΗ i s d r a m a t i c a l l y reduced when the s o l v e n t c o n c e n t r a t i o n i s a t l e v e l s which are completely s a t i s ­ f a c t o r y f o r the polymerization of r e a c t i v e monomers. T h i s may be a t t r i b u t e d t o the s h o r t h a l f l i f e of the ΜΑΗ excimer and/or com­ p l e x o r the i n t e r a c t i o n of the e x c i t e d species with the s o l v e n t . Table I . Homopolymerization o f Maleic A n h y d r i d e

Temp, °C 60 80 100 80

a

t

tBPP ,min

1 / 2

345 30 2.5 30

Solvent

-acetone benzophenone benzene xylene

ml

_ _

1 1 5 5 50

a

PMAH Yield,% 30 40 62 57 62 5 16 8 + 45

b

i m m o l e t - b u t y l peroxypivalate (tBPP) added i n 4 p o r t i o n s over 20 min t o 4.9g (50 mmoles) ΜΑΗ; t o t a l r e a c t i o n time k 60 min Waxy s o l i d ; i n f r a r e d spectra i n d i c a t e presence of aromatic structures

31.

GAYLORD

Temperature and Dilution Effects

453

Polymerization o f Norbornene

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch031

Analogous t o the s i t u a t i o n with ΜΑΗ, the polymerization of norbornene shows a strong dependence on the c a t a l y s t h a l f l i f e , i . e . bulk polymerization does not occur unless the c a t a l y s t h a l f l i f e i s l e s s than 2 h r s a t the r e a c t i o n temperature. F u r t h e r , even when the c a t a l y s t h a l f l i f e i s 30 min, the higher the temper­ ature the higher the y i e l d o f polynorbornene, probably r e f l e c t i n g a strong dependence of the c o n c e n t r a t i o n o f polymerizable species and/or the propagation r a t e on the temperature (_5). NMR s p e c t r a of the polymer i n d i c a t e t h a t a rearrangement o f the s t r u c t u r e occurs during propagation.

The s o l u t i o n polymerization o f norbornene occurs only when low concentrations o f s o l v e n t s are present (6). Table I I . Homopolymerization o f Norbornene b Catalyst DsBPDC tBPP tBPA DsBPDC AIBN

V 2 min 30 30 30 90 60

Temp,°C 70 80 130 50 70

Solvent _

_

acetone

0.3

benzene dioxane

tBPA

75

120

ml

benzene

2.0 0.05 1.85 10.0 0.045

Polymer Yield,% 12 12 58 2 0 11 3 0 20

1 mmole c a t a l y s t added i n 4 p o r t i o n s over 20 min t o 1.84g (20 mmoles) norbornene; t o t a l r e a c t i o n time 60 min DsBPDC = d i - s e c - b u t y l peroxydicarbonate; tBPA = t - b u t y l peroxyacetate; AIBN = a z o b i s i s o b u t y r o n i t r i l e Polymerization o f S u b s t i t u t e d Norbornenes The D i e l s - A l d e r adducts o f eye1opentadiene (CPD) with ΜΑΗ and with N-phenylmaleimide (NPMI) c o n t a i n the norbornene moiety and undergo polymerization under the same c o n d i t i o n s as ΜΑΗ and nor­ bornene, i . e . i n the presence o f r a d i c a l c a t a l y s t s undergoing r a p i d decomposition a t the r e a c t i o n temperature and s u b j e c t t o a s i g n i f i c a n t d i l u t i o n e f f e c t (7_, 8) .

454

INITIATION OF

Table I I I . Homopolymerization

Adduct endo

b Catalyst BPO° tBPP tBPA tBHP-70

exo

tBPA

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch031

tBHP-70

o Temp, C

POLYMERIZATION

o f CPD-MAH Adduct

Solvent^

ml

80 80 120 147 170 240

dioxane dioxane CB xylene

5 20 2 0.3

-

-

120 147 240

CB xylene

-

Polymer Yield,% 0 0 50 50 47 80 72 65 35

2 0.3

-

" 1 . 2 5 mmoles c a t a l y s t added i n 4 p o r t i o n s over 20 min t o 2g k (12 mmoles) adduct; t o t a l r e a c t i o n time 60 min BPO = benzoyl peroxide; tBHP-70 = 70% t - b u t y l hydroperoxide with 20% d i - t - b u t y l peroxide ^ Reaction time 5 h r s CB = chlorobenzene a Table IV. Homopolymerization

Adduct endo

b Catalyst C

BP0 tBPP tBPA tBPB tBHP-70

exo

tBPP tBPA tBHP-90

ο Temp, C

of CPD-NPMI Adduct

Solvent^

ml

80 80 85 120 150 150 210

CB dioxane DCE CB CB

20 20 10 3 2

-

-

85 120 155 260

DCE CB CB

16 3 3 _

-

Polymer Yield,% 0 0 9 60 53 0 20 0 0 60 30

1.25 mmoles c a t a l y s t added i n 4 p o r t i o n s over 20 min t o 2g (8.4 mmoles) adduct; t o t a l r e a c t i o n time 60 min tBPB = t - b u t y l perbenzoate; tBHP-90 - 90% t - b u t y l hydroperoxide Reaction time 5 h r s DCE = 1,2-dichloroethane The d i l u t i o n e f f e c t i s very s i g n i f i c a n t i n some conjugated diene-MAH copolymerizations which a l s o r e q u i r e the use of c a t a ­ l y s t s a t temperatures where they have a s h o r t h a l f l i f e . Thus,

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch031

31.

GAYLORD

Temperature and Dilution

Effects

455

whereas the copolymerization of butadiene and ΜΑΗ occurs r e a d i l y a t 80 C on the a d d i t i o n of tBPP t o a s o l u t i o n c o n t a i n i n g as much as 90% dioxane, the copolymerization o f CPD and ΜΑΗ i n the pres­ ence o f tBPP a t 80 C r e q u i r e s t h a t the dioxane c o n c e n t r a t i o n be l e s s than 25% {T). The copolymerization of CPD and NPMI a l s o r e ­ q u i r e s the use of c a t a l y s t s which have s h o r t h a l f l i v e s a t the r e a c t i o n temperature and takes place i n bulk o r i n the presence of low concentrations of s o l v e n t s (8). The copolymerization of butadiene with ΜΑΗ y i e l d s an unsat­ urated 1:1 a l t e r n a t i n g copolymer while the copolymerizations of CPD with ΜΑΗ and with NPMI y i e l d s a t u r a t e d 1:2 copolymers. The copolymerization of the CPD-MAH adduct with ΜΑΗ and the copoly­ m e r i z a t i o n of the CPD-NPMI adduct with NPMI y i e l d the same c o ­ polymers as are obtained from CPD and ΜΑΗ and CPD and NPMI, r e s ­ p e c t i v e l y . The CPD copolymerizations may a c t u a l l y proceed through the r a p i d formation of the adducts, followed by t h e i r copolymer­ i z a t i o n with ΜΑΗ and NPMI, r e s p e c t i v e l y . The homopolymerizations of the CPD-MAH and CPD-NPMI adducts, as w e l l as the copolymerizations of the adducts with ΜΑΗ and NPMI, r e s p e c t i v e l y , a t temperatures above the exo-endo i s o m e r i z a t i o n temperature of the adducts, y i e l d products c o n t a i n i n g both endo and exo s t r u c t u r e s , i n d i c a t i n g t h a t retrograde d i s s o c i a t i o n of the adducts has regenerated the charge t r a n s f e r complex which under the i n f l u e n c e of the r a p i d l y decomposing c a t a l y s t undergoes e x c i t a t i o n and p o l y m e r i z a t i o n . The d i l u t i o n e f f e c t s i n these polymerizations a t temperatures below and above the i s o m e r i z a t i o n temperature, may be a t t r i b u t e d t o the s h o r t h a l f l i f e of the ex­ c i t e d adducts and the e x c i t e d complexes, r e s p e c t i v e l y .

X = N-0

or 0

456

INITIATION OF POLYMERIZATION

Mechanisms involving rearrangements in the norbornene moiety to yield products with 2,7 linkages, analogous to the rearrangement noted in the homopolymerization of norbornene, have been proposed previously {]_, 8) . However, there is as yet no direct evidence of rearrangement in the products of the adduct homopolymerizations and copolymerizations. A significant temperature effect is noted in the bulk polymerizations of the liquid Diels-Alder adducts of CPD with acrylonitrile (AN) and methyl acrylate (MA) (9). Table V. Homopolymerization of CPD-AN and CPD-MA Adducts

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch031

Catalyst

*l/2 mole-% min

Temp Time ~C hr

Q Mp, C

MW (vpo)

10 13 16 27 54

138-150 193-198

425 892

0 0 3 11 32

110-113 139-144

1640 2015

Yield %

CPD-AN Adduct tBPP BPO tBPA tBHP-90

6 5 10 2 10

30 30 3.5 2 2

80 100 150 220 220

1 39 1 2 2

CPD-MA Adduct tBPP tBPO tBPA tBHP-90

10 5 10 2 10

30 30 3.5 2 2

80 100 150 220 220

1 39 1 2 2

Catalyst added over 20 min to 25 mmoles adduct; total reaction time as indicated The yield of homopolymer increases with temperature. The increase in the molecular weight of the polymer with increasing catalyst concentration at the same temperature may be attributed to an increased concentration of polymerizable, i.e. excited species, as the catalyst concentration increases and rapid decomposition occurs. Literature Cited 1. 2. 3. 4.

Dannenberg, E. M.; Jordan, M. E . ; Cole, H. M. J. Polym. Sci. 1958, 11, 253. Gaylord, N. G. Applied Polymer Symposium 1975, 26, 197. Gaylord, N. G.; Maiti, S. J. Polym. Sci., Polym. Lett. Ed. 1973, 11, 253. Gaylord, N. G.; Koo, J. Y. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 107.

GAYLORD

5.

Gaylord, N. G.; Mandal, B. M.; Martan, M. J. Polym. Sci., Polym. Lett. Ed. 1976, 14, 555. Gaylord, N. G.; Deshpande, A. B.; Mandal, B. M.; Martan, M. J. Macromol. Sci.-Chem. 1977, A11, 1053. Gaylord, N. G. Polymer Preprints 1976, 17, 666. Gaylord, N. G.; Martan, M. Polymer Preprints 1981, 22, 11. Gaylord, N. G.; Schildknecht, E. A.; unpublished results.

6. 7. 8. 9.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch031

RECEIVED January

Temperature and Dilution Effects

457

31.

10,1983

32 Homogeneous Lanthanide Complexes as Polymerization and Oligomerization Catalysts: Mechanistic Studies P. L . WATSON and T. HERSKOVITZ

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

Central Research and Development Department, E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, DE 19898

The trivalent lanthanide complexes M(η5-C5Me5) 2 CH 3 •L (M = Yb, Lu; L = diethyl ether, tetrahydrofuran or trimethylaluminum) and divalent M(η5-C5Me5)2•L complexes (M = Yb, Eu, Sm; L = diethyl ether or tetrahydrofuran) are catalysts for polymerization of ethylene (hexane, 30-100°C, 60 psig; Mv product ~105) and oligomerization of propene. The initial insertion reactions of propene with Lu(η 5 -C 5 Me 5 ) 2 CH 3 •L, studied by NMR, give well characterized Lu(η 5 -c 5 Me 5 ) 2 -isobutyl and -2,4-dimethylpentyl complexes. Prior dissociation of ligand L is necessary. Constants for the dissociative pre-equilibria are determined from the rates of the propene insertion reactions and ligand exchange rates. Rates of ethylene polymerization in the presence of various ligands L are modified qualitatively as for the propene insertion reactions viz. L = no ligand > diethyl ether > THF > Al (CH3)3 (slowest rate). Ethylene propagation is faster than propene propagation by ~104. Much e f f o r t has been devoted during the l a s t 30 years toward understanding the mechanisms o p e r a t i v e i n the c o o r d i n a t i o n c a t a l y s i s o f ethylene and α-olefin p o l y m e r i z a t i o n u s i n g Z i e g l e r Natta systems (metal h a l i d e and aluminum a l k y l , sometimes with Lewis base m o d i f i e r s ) . Aspects o f the complex heterogeneous r e a c t i o n s have been e l u c i d a t e d (1-5) but the intimate mechanistic d e t a i l - f o r example the r o l e o f i n h i b i t o r s and promoters, k i n e t i c s and thermodynamics o f chain growth, modes o f chain t r a n s f e r and t e r m i n a t i o n - comes p r i m a r i l y from s t u d i e s o f homogeneous c a t a l y s t s (_5~Z) Ethylene p o l y m e r i z a t i o n c a t a l y z e d by the w e l l - c h a r a c t e r i z e d homogeneous lanthanide complexes [ M ( C H R ) R ' ] studied,

·

h

5

4

2

2

0097-6156/83/0212-0459$06.25/0 © 1983 American Chemical Society

a

s

b

e

e

n

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

460

INITIATION OF

POLYMERIZATION

i n a d d i t i o n to some model r e a c t i o n s with s u b s t i t u t e d o l e f i n s . ( 8 / £ ) Mechanistic d e t a i l s of the a c t u a l o l e f i n i n s e r t i o n r e a c t i o n s were obscured i n these systems by the s t a b i l i t y of the a l k y l - b r i d g e d dimeric s t r u c t u r e s and by the slow r a t e of α-olefin i n s e r t i o n r e l a t i v e t o f u r t h e r i n s e r t i o n o f a second o l e f i n and/or β-hydrogen elimination. S t e r e o s p e c i f i c p o l y m e r i z a t i o n o f 1 , 3 - d i e n e s ( 1 0 - 1 8 ) (to buta­ diene) and isoprene homo- and copolymers), d i m e r i z a t i o n of propene ( 1 9 ) and r e c e n t l y s t e r e o s p e c i f i c p o l y m e r i z a t i o n of acetylene ( 2 0 ) to high c i s - c o n t e n t p o l y a c e t y l e n e have a l l been reported u s i n g lanthanide c a t a l y s t s . Sen ( 2 1 ) has reported the p r e p a r a t i o n o f c a t i o n i c europium systems (which perhaps f u n c t i o n as c a t i o n i c i n i t i a t o r s ) f o r p o l y m e r i z a t i o n of norbornadiene and 1 , 3 - c y c l o h e x a diene. Described i n t h i s paper i s a model system - one i n which w e l l c h a r a c t e r i z e d lanthanide complexes e x h i b i t high c a t a l y s t a c t i v i t i e s f o r ethylene p o l y m e r i z a t i o n but where the corresponding oligomer­ i z a t i o n of propene i s s u f f i c i e n t l y slowed so t h a t stepwise i n s e r ­ t i o n of the o l e f i n can be s t u d i e d q u a n t i t a t i v e l y and a l l important intermediates observed or i s o l a t e d . Emphasized i n t h i s paper i s the e f f e c t of added Lewis a c i d s and bases on the r a t e of o l e f i n i n s e r t i o n s , and comparison between ethylene and propene r e a c t i o n s . The c a t a l y s t s , of general s t r u c t u r e M(n -Cp*)2^3·L (M = Yb, Lu; Cp* = C ( C H ) ; L = C H L i , ether, THF or A l ( C H ) ) , polymerize ethylene a t r a t e s comparable t o Z r ( b e n z y l ) / A l 0 . Thus they are l e s s a c t i v e by two orders of magnitude than the zirconium-aluminoxane systems reported r e c e n t l y by Kaminsky and Sinn, ( 2 2 ) but much ι more a c t i v e than most other reported homogeneous and heterogeneous ethylene p o l y m e r i z a t i o n c a t a l y s t s . (23_, 2 4 ) 5

5

3

5

3

3

4

3

2

3

Within the context of lanthanide chemistry i t i s i n t e r e s t i n g to note r e p o r t s by Chinese ( 1 0 , 1 5 ) and Russian ( 1 2 ) s c i e n t i s t s that a c t i v i t i e s o f lanthanide-based Z i e g l e r - N a t t a c a t a l y s t s f o r p o l y m e r i z a t i o n of 1 , 3 - d i e n e s show much higher a c t i v i t y f o r e a r l y metals than f o r the end-row metals Er-Lu. The p o t e n t i a l f o r higher a c t i v i t y analogs of the complexes d e s c r i b e d here i s thus evident . Experimental A l l s y n t h e t i c manipulations were c a r r i e d out i n a Vacuum Atmospheres HE-63 Drybox under a very slow continuous purge of dry N using p r e d r i e d glassware. Solvents (THF, ether, toluene, pentane) were d r i e d by d i s t i l l a t i o n from Na/benzophenone under N2. Ethylene and propene were purchased from Matheson (Research grade, 9 9 . 9 8 ° / m i n and 9 9 . 7 % min r e s p e c t i v e l y ) . NMR measurements were made on a N i c o l e t 3 6 0 MHz s p e c t r o ­ meter. Both 1 H and 1 3 c chemical s h i f t s were referenced to TMS by c o r r e c t i n g the s h i f t s r e l a t i v e to r e s i d u a l deuterated solvent peaks. Paramagnetic m a t e r i a l s were r e f e r e n c e d (in Hz) to the h i g h e s t f i e l d s o l v e n t peak (+ = down f i e l d ) . Samples f o r NMR 2

32.

WATSON AND HERSKOViTZ

Homogeneous

Lanthanide

461

Complexes

were approximately 0.05 M i n lanthanide complex. O l e f i n was condensed i n t o the sample on the vacuum l i n e and then the sample was sealed under vacuum. Elemental analyses were done by Franz Pascher, M i c r o a n a l y t i s c h e s Lab., Bonn, West Germany. Ethylene p o l y m e r i z a t i o n s were c a r r i e d out i n a g l a s s 500cc batch r e a c t o r with high speed (1000 rpm) s t i r r i n g and a thermocouple f o r i n t e r n a l temperature sensing. The s t a i n l e s s s t e e l head a l s o had an i n l e t f o r gases and septum p o r t f o r c a t a l y s t i n j e c t i o n and sampling. Absence of a l l poisons was ensured i n each p o l y m e r i z a t i o n by e i t h e r : a) t i t r a t i o n with s a c r i f i c i a l c a t a l y s t under 10 p s i g C H u n t i l polyethylene formation ensued; or b) a d d i t i o n o f 10~ moles of tetraneophylzirconium. T y p i c a l runs i n v o l v e d p r e e q u i l i b r a t i o n of s o l v e n t (cyclohexane, 150 mL) with C H (60 p s i g ) i n the r e a c t o r , followed by i n j e c t i o n o f c a t a l y s t (10~3 t o 10~ mol). Reaction times were u s u a l l y 60-120 sec and were terminated with e t h a n o l / a c e t i c a c i d quench. Average p o l y m e r i z a t i o n r a t e s were c a l c u l a t e d from the t o t a l polymer y i e l d obtained during the r e a c t i o n . Ethylene uptake p r o f i l e s were always monitored using a mass flowmeter (Brooks 5810) on the ethylene feed l i n e , and recorded f o r a n a l y s i s . I t should be noted t h a t the a b b r e v i a t i o n Cp* = n 5 - c ( C H ) i s used i n the experimental s e c t i o n below. 2

4

4

2

4

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

5

5

3

5

Li[MCp* (CH^) ] (THF) ^ 3 , Zb (M = Lu) , 36 (M = Yb) . (25) To a 0°C s o l u t i o n of L i [ M C p * C l ] (THF) 3 (26) was added 2 eq. CH3ÏX. A f t e r 30 min the s o l u t i o n was evaporated to dryness. Ether e x t r a c t s of the s o l i d residues were f i l t e r e d and the f i l t r a t e cooled, y i e l d i n g m i c r o c r y s t a l l i n e white (2b) or yellow (£g) product. For 2A: -^H NMR (glyme-d ) δ-1.77 (s, 2CH , LU-CH3) , 1.86 (m, 12H, THF), 2.28 (s, 30H, r i n g CH3, overlapping THF) and 3.68 (m, 12H, THF). Anal. C a l c d . f o r [ C H L i L u O , i . e . 1 THF): C, 56.11; H, 8.33; L i , 1.25; Lu, 31.44. Found: C, 56.59; H, 8.40; L i , 1.17; Lu, 31.20. Less than 0.15% C l found. For 2B: Anal. C a l c d . f o r [ C 3 H L i O Y b ] : C, 58.6; H, 8.60; L i , 0.90; Yb, 24.83. Found: C, 58.53; H, 8.83; L i , 1.00; Yb, 24.82. [N.B. Samples f o r elemental a n a l y s i s were s t o r e d under vacuum f o r s e v e r a l weeks r e s u l t i n g i n some slow l o s s o f THF]. 9

9

2

2

10

3

2 6

4

60

4 6

3

Li[MCp* (CH^) 3, 3A (M = Lu), 3B (M = Yb). Solid L i [ M C p * ( C H ) ] ( T H F ) 3 was heated under vacuum a t 75° f o r 1-2 ! H NMR i n glyme-d^^o showed no THF or ether. 2

2

3

9

2

days.

MCp* Al(CH^) , £b (M = Lu), (M = Yb). To a s t i r r e d s l u r r y of Li[MCp* (CH3) ] ( s o l v e n t - f r e e ) i n toluene was added 2 eq. A l ( 0 ^ 3 ) 3 . A f t e r most of the s o l i d s were taken i n t o s o l u t i o n the mixture was f i l t e r e d . The product was c r y s t a l l i z e d (-40°C) from the concentrated f i l t r a t e (M = Lu, white; M = Yb, p u r p l e ) . For 4A: Ε NMR (toluene-dg) 62.06 (s, 30H, Cp*), -0.11 (s, 6H, b r i d g i n g CH3) and -0.21 (s, 6H, t e r m i n a l CH3)· A n a l . C a l c d . f o r [C H AlLu]: C, 54.13; H, 7.95; A l , 5.07; Lu, 32.86. 9

4

2

λ

2 4

4 2

2

462

INITIATION OF POLYMERIZATION

Found: C, 54.16; H, 7.94; A l , 4.86; Lu, 32.70. For 4B: H NMR (toluene-dg) overlapping peaks a t +92 (Wi ^30 Hz) and +62 Hz (est. Wi ^40 Hz), i n r a t i o 10:4. Anal. Calcd. f o r [ C 4 H A l Y b ] : C, 54.32; H, 7.98; A l , 5.09; Yb, 32.61. Found: C, 53.90; H, 7.95; A l , 4.88; Yb, 32.50. 2

42

MCp* CH 'ether, 5& (M = L u ) , 5B, (M = Yb).(27) M C p * A l ( C H ) was d i s s o l v e d i n d i e t h y l ether. On c o o l i n g the s o l u t i o n t o -30°C c r y s t a l s o f the product formed (5A white; 5B orange). For 5&: ! H NMR (toluene-d ) 6-0.05 (s, 3Îi7 Lu-CH^), 1.09 ( t , 6H, e t h e r ) , 2.19 (s, 30H, Cp*), 3.49 (q, 4H, e t h e r ) ; ( c y c l o h e x a n e - d ) : -0.95 (Lu-CH ) , 1.20 ( e t h e r ) , 1.95 (Cp*), 3.56 ( e t h e r ) . 3-H NMR (-90° 0.04 M L u C p * C H - e t h e r and 0.4 M ether i n t o l u e n e - d ) : 6-0.55 (s, Lu-Me), 0.41 ( t , CH coord, e t h e r ) , 0.60 ( t , CH coord, ether), I. 17 ( t , CH f r e e e t h e r ) , 2.04 (s, Cp*), 2.79 (q, CH coord, e t h e r ) , 2.86 (q, coord, e t h e r ) , and 3.16 (q, CH f r e e e t h e r ) . Anal. C a l c d . f o r [ C H O L u ] : C, 56.17; H, 8.11, 0, 2.99; Lu, 32.73. Found: C, 56.04; H, 8.11, 0, 2.90; Lu, 33.20. For 5β: ! H NMR (toluene-d ) J = +9 Hz, Wi = 90 Hz. A n a l . C a l c d . f o r [ C H O Y b ] : C, 56.37; H, 8.14; 0, 3.00; Yb, 32.49. Found: C, 56.05; H, 8.13; O, 2.90; Yb, 32.35. 9

3

2

3

4

8

12

3

3

8

3

3

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

3

2

2

2 5

4 3

8

25

43

[MCp* CH ] , 2h (M = L u ) . To a s t i r r e d s o l u t i o n o f MCp* CH *ether i n toluene was added 1-3 eq. N E t . Solvent was removed under vacuum. The r e s u l t i n g r e s i d u e s were washed with n-pentane, c o l l e c t e d by f i l t r a t i o n and d r i e d . 1H NMR ( c y c l o h e x a n e - d , 0.05 M, 25°C): 6-1.10 (s, 3H, Lu-CH ) and 2.01 (s, 30H, Cp*); ( t o l u e n e - d , 0.05 M, 25°C): 6-0.5 (s, 3H, Lu-CH ) and 2.13 (s, 30H, Cp*). Anal. Calcd. f o r [ C H L u ] : C, 54.77; H, 7.22; Lu, 38.00. Found: C, 54.44; H, 7.19; Lu, 37.90. 2

2

3

9

3

3

12

3

8

3

2 1

3 3

MCp* CH -THF, (M = L u ) , 6B (M = Yb). MCp* CH .THF (M = Yb, Lu) was prepared by d i s s o l v i n g MCp* CH *ether i n THF. Evaporation of s o l v e n t followed by r e c r y s t a l l i z a t i o n from n-pentane (-40°) gave the white (Lu) or orange (Yb) product. For 6A: H NMR (glyme-d ) 6-1.18 (s, 1CH , Lu-CH ), 1.80 (m, 4H,~THF overlapping Cp*), 1.89 (s, 10CH , Cp*) and 3.64 (m, 4H, THF) ( t o l u e n e - d ) : 6-0.79 (s, 1CH , Lu-CH ), 1.21 (m, 4H, THF), 1.93 (s, 10CH , Cp*) and 3.28 (m, 4H, THF). ^ NMR (-90°, 0.04 M LuCp* CH «THF and 0.02 M THF i n t o l u e n e - d ) : 6-0.32 (s, Lu-CH ), 1.13 (unres., 3 and 3 ' CH o f coord. THF), 1.57 (m, f r e e THF), 2.27 (s, Cp*), 3.04 (unres., α CH of coord. THF), 3.38 (unres., o/ CH o f coord. THF), 3.81 (m, f r e e THF). Anal. C a l c d . f o r [ C H L u O ] : C, 56.38; H, 7.76; Lu, 32,86. Found: C, 56.51; H, 7.81; Lu, 32.75. Anal. Calcd. f o r [ C H O Y b ] : C, 56.58; H, 7.79; Yb, 32.61. Found: C, 56.61; H, 7.83; Yb, 32.75. 2

3

2

2

3

3

1

l0

3

3

3

3

8

3

3

2

8

3

3

2

2

2

2 5

2 5

4 1

4 1

Results and D i s c u s s i o n Synthesis and C h a r a c t e r i z a t i o n of Lutetium- and YtterbiumMethyl Complexes. The s y n t h e t i c s t r a t e g y o u t l i n e d i n Scheme 1

32.

WATSON AND HERSKOViTZ

Homogeneous

Lanthanide

Complexes

463

allows the p r e p a r a t i o n of a f a m i l y of methyl complexes having the general s t r u c t u r e MCp* CH 'L (M = Lu, Yb; Cp* = n - C ( C H ) ; L = Lewis bases: ether, THF, CH3L1; and Lewis a c i d s : AlMe , MCp* CH ). (25, 26, 27) S t a t i c s t r u c t u r e s of s e v e r a l complexes - [YbCp* (CH ) ] L i ( T H F ) ( e t h e r ) , YbCp* CH ·THF and YbCp* CH -ether - have been confirmed by X-ray c r y s t a l l o g r a p h y . Ligand arrangements are represented adequately by the drawings i n Scheme 1. A l l these s t r u c t u r e s c o n t a i n a b a s i c sandwich arrangement of metal atom between r| -C5(CH )5 r i n g s (mutually staggered) s i m i l a r t o s t r u c t u r e s reported f o r r e l a t e d halogen complexes.(26) The methyl group and l i g a n d L are coordinated i n the plane between the n 5 - C ( C H ) r i n g s . An ORTEP drawing of YbCp* CH*ether i s provided i n the abstract,(28) and of YbCp* CH «THF i n F i g u r e 1. V a r i a b l e temperature 1H NMR provides information about the l a b i l i t y of l i g a n d s L i n LuCp* CH *L complexes (Eq. 1). For a l l l i g a n d s L considered here Eq. 1 l i e s s t r o n g l y t o the left. D i s s o c i a t i v e rather than a s s o c i a t i v e exchange i s confirmed 5

2

3

5

3

5

3

2

3

2

2

2

3

2

3

2

3

5

3

5

3

5

2

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

2

2

LuCp* CH^-L Ζ J

^

0

3

3

•

LuCpSCH-, Z

+

L

(1)

3

by the independence of 1H NMR coalescence temperatures on concentration of excess L. Exchange of coordinated ether or THF (L) i n 5A o r 6A with uncoordinated L i s extremely r a p i d a t 25°C i n toluene or cyclohexane. At -90°C, l i m i t i n g s p e c t r a show sharp separate resonances f o r coordinated L and added f r e e L. Intramolecular exchange - presumably r o t a t i o n a l - of the chemically e q u i v a l e n t a, s i t e s and 3, 3^ s i t e s o f coordinated L i s f a s t on the NMR time s c a l e above -90°C f o r ether and above -65°C f o r THF. Coalescence measurements show t h a t i n t e r m o l e c u l a r l i g a n d exchanges then become r a p i d a t higher temperatures. Dis­ s o c i a t i v e l i g a n d exchange r a t e s at 15°C (Eq. 1 forward, extrapo­ l a t e d from data i n Figure 2) are 4.5xl0 s " f o r ether and 1.2xl0 s " f o r THF i n toluene-dg. Data shown i n Figure 2 i n d i c a t e t h a t these l i g a n d d i s s o c i a t i o n processes are 2-3 times f a s t e r i n cyclohexane than i n toluene. The s t r u c t u r e of M C p * A l ( C H ) (4, see Scheme 1) i s i n f e r r e d from the NMR spectrum, and by analogy with the s t r u c t u r e s of YbCp* AlCl (26) and Y ( C H ) A 1 ( C H ) . ( 2 9 ) Exchange w i t h A l ( C D ) occurs w i t h i n minutes a t I5°C. B r i d g e - t e r m i n a l methyl group s i t e exchange (extrapolated from coalescence temperatures i n F i g u r e 2) a t 15°C i s ^0.3 s " . I f t h i s exchange r e s u l t s from d i s s o c i a t i o n of A l ( C H ) , then 0.3 s " i s the r a t e of Eq. 1 forward. However, i f the exchange i s i n t e r m o l e c u l a r then 0.3 s " i s an upper l i m i t f o r the d i s s o c i a t i v e process. Since the resonance f o r f r e e , added A l ( C H ) (at δ-0.3, superimposed on the l o w e r - f i e l d h a l f of the coalescenced b r i d g e - t e r m i n a l resonance) i s a l s o broadened i t i s l i k e l y that i n t e r m o l e c u l a r exchange i s 5

3

2

2

1

1

4

5

3

5

4

2

3

4

3

3

1

1

3

3

1

3

3

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

464 INITIATION OF POLYMERIZATION

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

WATSON AND HERSKOViTZ Homogeneous Lanthanide Complexes

MCH-d -

tol-d

MCH-d

ether

THF

THF

THF

A1(CH ) 3 t o l - d g

Al(CH )

5A

6A

6A

6A

4A

4A

8

8

14

3 MCH-d..14

tol-d

14

b

àv

38.3

32.9^

42.4

e

52.75^

e

X

170.1

146.2

188.4

234.4

120.3

c

-1 8 0MHz k (s ) 27.08

1

1

94.5

-10

-5.5

-5 9

-65

172.3

46.4

19.1

148.0

190.8

765.5

206.3

84.8

657.5

847.7

448.0

1054.8

237.4^ 100.1

545.6

e

122.8

Τ (°C) Δν 360MHz k ( s " )

Coalescence measurements' for ligand exchange.

120

107

-10

7

11

-63

-48

-55

S 0.3

~1.2xl0

~4.5xl0

1

1 4

3

5

Τ (°C) k ^ C s " )

a. Measurements were made on s o l u t i o n s 0.5M i n complex and 0.5M i n added l i g a n d L. Changes o f t h e c o o r d i n a t e d - l i g a n d l i n e shapes w i t h t e m p e r a t u r e were i n d e p e n d e n t o f c o n c e n t r a t i o n o f added L. b. Δν = ν(free) - ν(coord). c. C a l c u l a t e d f o r two s i t e model, k = /2πΔν a t c o a l e s c e n c e . d. By e x t r a p o l a t i o n . e . P r o t o n s α t o oxygen. ^. P r o t o n s β t o oxygen, g. Toluene-dg and m e t h y l c y c l o h e x a n e - d .

3

3

tol-dg

ether

5A

tol-dgS

ether

5A

Solvent

Ligand

Complex

Figure 2.

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

32.

WATSON

AND

Homogeneous Lanthanide Complexes

HERSKOViTZ

467

o c c u r r i n g . Again the exchange r a t e i s independent of the amount of added l i g a n d , A l (0113)3. The dimer [MCp* CH3]2* §r can a l s o be viewed as a Lewis a c i d adduct. Monomer and dimer are i n r a p i d e q u i l i b r i u m down t o -60°C, g i v i n g r i s e to a s i n g l e LU-CH3 resonance i n the H NMR spectrum of The l i m i t i n g spectrum o f §A at -90°C shows a 1:1:2 p a t t e r n f o r the Cp* peaks and a 1:1 s e t f o r the LU-CH3 groups. The s t r u c t u r e shown i n Scheme 1 i s proposed f o r t h i s dimer. Presumably s t e r i c bulk c o n s t r a i n s the two LuCp* fragments t o be mutually orthogonal and prevents b r i d g i n g o f both methyl groups. The methyl complexes are thus w e l l - c h a r a c t e r i z e d with r e s p e c t to s t r u c t u r e and l a b i l i t y o f l i g a n d L. Comparatively, l a b i l i t y o f L decreases i n the s e r i e s MCp* CH3 ^ether>THF>AlMe . As w i l l be seen, t h i s i s a l s o approximately the o r d e r i n g o f e q u i l i b r i u m constants Κ f o r Eq. 1. 2

1

2

r

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

2

3

Reaction o f Methyl Complexes with Propene. Products. Reaction o f [MCp* CH3] / §, with propene i n i t i a l l y y i e l d s i s o b u t y l complexes MCp* CH CH(CH ) , 9, as shown i n Scheme 2. Secondary, slower r e a c t i o n o f propene with 9 g i v e s the 2,4-dimethylpentyl species J.Q, and subsequently higher oligomers. Confirmation o f the s t r u c t u r e s comes from s e v e r a l sources. GC-MS of hydrolyzed samples o f 9 and 10 show isobutane and 2,4-dimethylpentane ( r e s p e c t i v e l y ) as the only C^ or C7 isomers formed, confirming the r e g i o s p e c i f i c i t y o f the i n s e r t i o n s . -^H NMR spectra of 9A and 10A (and o f H s p e c i f i c a l l y - l a b e l l e d samples o f 9A and 10A) are f u l l y assigned (30) and c o n s i s t e n t with proposed s t r u c t u r e s . A d d i t i o n a l l y , 9A i s a l s o formed from the r e a c t i o n of LuCp* H,(31) 11A,.with isobutene ( r e a c t i o n 5, Scheme 2 ) , and 10A i s formed by a d d i t i o n o f e i t h e r 2,4-dimethylpentene t o LuCp* H or by a d d i t i o n o f 4-methylpentene t o LuCp* CH3. Polypropene i s not formed i n these systems. A n a l y s i s (by h y d r o l y s i s then GC) of mixtures o f propene (60 p s i ) and 8A (0.1 M i n cyclohexane) r e v e a l s that oligomers a t l e a s t up to C 4 are formed. However the product d i s t r i b u t i o n above C^Q becomes p r o g r e s s i v e l y more complex, i n d i c a t i n g chain t r a n s f e r and chain t e r m i n a t i o n processes which compete w i t h propagation. Both § and XQ are t h e r m a l l y unstable with h a l f - l i v e s at 25°C o f ^3 h r . The thermal decomposition of 9A i n cyclohexane i s under study as a model f o r c h a i n t e r m i n a t i o n f o r these systems. Both 3-hydrogen e l i m i n a t i o n ( g i v i n g 11A and isobutene) and β-alkyl e l i m i n a t i o n (giving 8A and propene) are important k i n e t i c a l l y a c c e s s i b l e processes, both f o r 8A and f o r the longer chain LuCp* (CH CHCH ) CH complexes. (31) K i n e t i c s . K i n e t i c a n a l y s i s of the r e a c t i o n o f the dimer 8A with propene confirms t h a t d i s s o c i a t i o n t o the a c t i v e monomer 7A i s necessary and t h a t the rate-determining step i s i n s e r t i o n of «propene i n t o the Lu-CH bond o f 7A (reactions 1 and 2 i n Scheme 2). At 15°C Κ = k i / k . ! = 4.1x10-3 M'^and k = 1.22x10"! M S " 2

2

2

2

3

2

2

2

2

2

2

2

2

3

n

3

3

_1

2

1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

468 INITIATION OF

Scheme 2. POLYMERIZATION

32.

WATSON AND HERSKOViTZ

469

Homogeneous Lanthanide Complexes

i n cyclohexane. Previous k i n e t i c s t u d i e s (27) o f the r e a c t i o n of LuCp*2CH3-ether, 5, with propene a l s o showed the presence o f a d i s s o c i a t i v e p r e e q u i l i b r i u m g i v i n g 7A and ether followed by a slower o l e f i n i n s e r t i o n . The r a t e expression d e r i v e d (Eq. 2) i s c o n s i s t e n t with Scheme 2. Knowing k e x a c t l y , values f o r 2

-diZ&'L] dt

=

kik [7A-L][propene] k^lL]

the p r e e q u i l i b r i u m constants Κ = k / k _ can be d e r i v e d from the observed r a t e s o f r e a c t i o n o f the complexes with propene under p s e u d o - f i r s t order c o n d i t i o n s . These are shown i n F i g u r e 3 f o r v a r i o u s l i g a n d s L. I n h i b i t i o n o f propene i n s e r t i o n by ether o r monomer 7A i s m i l d but k i n e t i c a l l y important a t the c o n c e n t r a t i o n s used f o r NMR experiments (^0.05 Μ ) , slowing the r a t e s by a f a c t o r o f 20-30. I n h i b i t i o n i s very pronounced w i t h the stronger l i g a n d s Al(CH )3 - T h i s k i n e t i c i n h i b i t i o n i s due t o the thermo­ dynamic s t a b i l i t y o f t h e adducts (see Figure 4 ) . In f a c t , complexation o f 7L by both A l f C H ^ ^ and THF slows the propene i n s e r t i o n r e a c t i o n s u f f i c i e n t l y t h a t only very low steady-state concentrations o f the i n i t i a l L u - i s o b u t y l product are observed. Observed r a t e s f o r r e a c t i o n o f propene with LuCp* CH *L (L = LuCp*2CH , ether, THF, AlMe ) are a l l dependent on propene c o n c e n t r a t i o n . The h i g h e s t energy t r a n s i t i o n s t a t e i s t h e r e f o r e always t h a t f o r the o l e f i n i n s e r t i o n , not f o r l i g a n d d i s s o c i a t i o n . I n h i b i t i o n o f the r e a c t i o n o f i s o b u t y l complex 9A with propene does not occur with added ether, i n d i c a t i n g t h a t formation o f 9A-ether i s not s i g n i f i c a n t . However, THF does i n h i b i t t h i s r e a c t i o n and a value o f Κ = k4/k_4 (Scheme 2) = 'VLxlO M " " i s obtained. The r e l a t i v e a b i l i t y o f 7A and 9A t o coordinate THF i s probably l a r g e l y s t e r i c i n o r i g i n ? F i n a l l y , the r a t e o f propene i n s e r t i o n i n t o the Lu-C bond o f i s o b u t y l complex 9A, V L . l x l O " ^ M~1 s"^ i s a good estimate o f the r a t e o f propagation d u r i n g f u r t h e r o l i g o m e r i z a t i o n o f propene. 1

Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

(2)

?

a

n

d T

H

1

F

3

2

3

3

3

-2

1

Polymerization o f Ethylene by Methyl-Lutetium and -Ytterbium Complexes. Products. Ethylene (60 p s i , 30-100°C) i s polymerized r a p i d l y by 10"" t o Ι Ο " M cyclohexane s o l u t i o n s o f the methyl complexes 3, 4, 5, 6, and 8 (Table). Low Mw oligomers were not formed ( a n a l y s i s by GC) i n experiments l a o r 3a, Table. A t 160°C both the Lu and Yb e t h e r a t e s produced very l i t t l e polyethylene CVL0~ o f the Table y i e l d s a t 40-100°) r e f l e c t i n g thermal decomposition o f the a c t i v e s p e c i e s . Inherent v i s c o s i t i e s of the polymer (Table) show t h a t M ^ 1 0 and t h a t p o l y m e r i z a t i o n a t higher temperatures reduces the molecular weight (Figure 5 ) . I n f r a r e d s p e c t r a l a n a l y s i s o f the polyethylenes obtained i n the Table showed no H(R)C=CR groups t h a t would r e s u l t from α-olefin formation then i n c o r p o r a t i o n i n t o growing polymer then chain t e r m i n a t i o n v i a 4

6

3

5

v

2

e l

3

3

3

k

3

2

1.2xl0~

-

3 . U X 1 0 "

1.7xl0~

2.3xl(T

observed

5

6

5

4

( s

1 )

2

2

b

2

l.OxlO"

4.1xl0~

2.4xl0"

1.4xl0~

2

3

5

4

3

(M)

1.9xl0"

K

1

1

1

-