Anionic Polymerization. Kinetics, Mechanisms, and Synthesis 9780841206434, 9780841208421, 0-8412-0643-0

Table of contents :
Title Page......Page 1
Half Title Page......Page 3
Copyright......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Living and Dormant Polymers: A Critical Review......Page 10
Literature Cited......Page 23
Kinetics and Mechanism of Polymerization......Page 25
Block Copolymers......Page 38
ABSTRACT......Page 45
Literature Cited......Page 46
Anionic Two-Phase Block Polymers......Page 49
Graft Polymers......Page 55
Literature Cited......Page 64
I -General......Page 66
3- Cyclic (Ring Shaped) Macromolecules......Page 68
5 - Branched Polymers......Page 69
6 - Model Networks......Page 70
I - Block Copolymers......Page 71
"Grafting from" reactions......Page 74
CONCLUSION......Page 75
LITERATURE CITED......Page 76
5 Relationship of Anion Pair Structure to Stereospecificity of Polymerization......Page 78
Literature Cited......Page 84
6 Some Aspects of Ion Pair-Ligand Interactions......Page 85
Crown ether complexes of fluorenyl carbanion salts......Page 86
Polyamine interactions with carbanion pairs......Page 91
Intramolecular ionic interactions......Page 93
Literature Cited......Page 98
7 The Influence of Aromatic Ethers on the Association of the Polystyryllithium and 1,1-Diphenylmethyllithium Active Centers in Benzene......Page 100
Experimental Section......Page 101
Results and Discussion......Page 102
References:......Page 109
8 The Photoisomerization of Allylic Carbanions......Page 112
Results......Page 113
References......Page 120
9 Solvation of Alkyllithium Compounds Steric Effects on Heats of Interaction of Tetrahydrofurans with Polyisoprenyllithium and Polystyryllithium......Page 121
Results and Discussion......Page 122
Literature Cited......Page 129
ABSTRACT......Page 131
1. Preparation of the strontium salt of one-ended living polystyrene (SrS2)......Page 133
1. Strontium salt of one-ended living polystyrene (SrS2 ) in THF......Page 134
3. Strontium tetraphenylboride in THF......Page 138
Kinetic Measurements......Page 147
Literature Cited......Page 154
11 Dynamics of Ionic Processes in Low Polar Media......Page 157
Electric Field Effects in Ionic Equilibria......Page 159
The Field Modulation Technique......Page 161
Results and Discussion......Page 164
Conclusion......Page 177
Literature Cited......Page 178
12 NMR Characterization of Ion Pair Structure......Page 180
Allylalkalimetal Compounds......Page 181
Model diene polymerization systems......Page 184
Literature Cited......Page 186
13 Polymerization of Methyl Methacrylate Initiated by t-Butyl- and Phenylmagnesium Compounds Factors Influencing the Nature of the Active Centers......Page 188
Nominal Solvent and Solvation-State of the Initiator......Page 189
The Importance of Exact Specification of Initiation Conditions......Page 191
Medium Effect on Molar-Mass Distribution of Polymers Produced by Initiation and Propagation at 250 Κ Using t-Butylmagnesium Compounds......Page 193
Medium Effect on the Steric Dyad and Triad Composition of Polymers Produced by Initiation and Propagation at 250 Κ and 225 K Using t-Butylmagnesium Compounds......Page 194
Kinetics of the Polymerization Initiated by t-Butylmagnesium Bromide in Toluene Solution......Page 196
Effect of Order on Addition of Reagents......Page 198
Conclusions......Page 199
Literature Cited......Page 200
Equilibrium Cyclic Oligomer Formation......Page 201
Kinetics of Anionic Polymerization and Oligomerization......Page 205
Blockcopolymer Synthesis......Page 209
Literature Cited......Page 211
15 Original Anionic Pathway to New PA(PO)2 Star-Shaped Block Polymers Based on Polyvinyl or Polydiene Hydrocarbons and Polyoxirane......Page 212
Metalation of Naphthalene by Potassium in Tetrahydrofuran at Room Temperature......Page 213
Metalation of β-Ethylnaphthalene by Potassium in THF at Room Temperature......Page 215
Synthesis of Naphthalene Terminated Polymers......Page 218
Metalation of Naphthalene Ended Polyvinyl Aromatic Chains by Potassium in THF At RT......Page 219
Star-Shaped Block Copolymerization......Page 220
Conclusion......Page 226
Literature Cited......Page 229
16 Ion Pair Structure and Stereochemistry in Anionic Oligomerization and Polymerization of Some Vinyl Monomers......Page 231
Literature Cited......Page 238
1. Initiation of the Polymerization......Page 239
2. Study of the Nature of the Active Centers......Page 244
3. Study of the Structure of Active Centers......Page 251
Literature Cited......Page 269
18 Kinetics and Mechanism of Anionic Polymerization of Lactones......Page 270
Initiation and Structure of Growing Species......Page 271
Mechanism and Kinetics of Propagation......Page 272
Abstract......Page 280
Literature Cited......Page 281
19 Use of Cryptates in Anionic Polymerization of Heterocyclic Compounds......Page 282
I. Anionic Polymerization of Propylene Sulfide......Page 283
II. Anionic Polymerization of Ethylene Oxide......Page 284
III. Anionic Polymerization of Cyclosiloxanes......Page 292
Acknowledgements......Page 302
Literature Cited......Page 303
20 Anionic Polymerization of Acrolein Kinetic Study and the Possibility of Block Polymer Synthesis: Mechanisms of Initiation and Propagation Reactions......Page 305
Molecular Weights......Page 306
Kinetic Results......Page 307
Block Polymer Synthesis......Page 311
Discussion and Conclusion......Page 321
Literature Cited......Page 324
Experimental......Page 325
Results......Page 327
Discussion......Page 335
Literature Cited......Page 338
Anionic Techniques......Page 340
Cationic Techniques......Page 343
Tertiary Amines......Page 344
Secondary Amines......Page 346
Primary Amines......Page 348
Literature Cited......Page 349
Polymerization of TrMA with BuLi (2, 3)......Page 350
Copolymerization of TrMA and MMA with BuLi (6, 7)......Page 351
Copolymerization of TrMA and MBMA with BuLi (8, 9, 10, 11)......Page 352
Polymerization of TrMA by Optically Active Catalysts (12, 16)......Page 355
Literature Cited......Page 361
24 C-13 NMR Study of the Propagating Live End in the Butyllithium Polymerization of 1,3-Butadiene......Page 363
Results and Discussion......Page 364
Conclusions......Page 379
Literature Cited......Page 382
25 Anionic Polymerization: Some Commercial Applications......Page 384
Polydienes......Page 385
Diene-Styrene Copolymers......Page 389
Literature Cited......Page 400
26 Industrial Applications of Anionic Polymerization: Past, Present, and Future......Page 403
HOMOPOLYMERIZATION......Page 405
BLOCK COPOLYMERS OF HIGH VINYL SOFT SEGMENTS......Page 413
RANDOM COPOLYMERS OF STYRENE/BUTADIENE......Page 416
SUMMARY......Page 418
Literature Cited......Page 419
27 Functionally Terminal Polymers via Anionic Methods......Page 421
Experimental......Page 422
Literature Cited......Page 432
28 Present View of the Anionic Polymerization of Methyl Methacrylate and Related Esters in Polar Solvents......Page 435
Kinetics of the propagation reaction in the polymerization of MMA......Page 437
Tacticities of the polymers......Page 445
Kinetic investigations on the termination reactions......Page 446
Literature Cited......Page 452
29 Anionic Polymerization of Isoprene by the Complexes Oligoisoprenyllithium/Tertiary Polyamines in Cyclohexane. I. Kinetic Study......Page 456
Kinetic study of the propagation reaction by ultraviolet spectroscopy.......Page 458
Variation of νp in function of r = amine/living ends......Page 459
Determination of the orders......Page 462
Discussion......Page 464
Experimental......Page 466
Literature cited......Page 467
30 Preparation of Polymeric Free-Radical Initiators by Anionic Synthesis: Polymeric Azo Derivatives......Page 469
EXPERIMENTAL......Page 470
RESULTS......Page 471
Acknowledgments......Page 473
Literature Cited......Page 474
Reactions of Anionic Sites With Oxygen as a Deactivation Reagent......Page 475
Mechanism of the reaction......Page 476
2 - Oxidation of anionic polymers in solution......Page 478
2.1 - Influence of the oxidation mode......Page 480
2.2 - Influence of the Living End Concentration......Page 481
2.4 -Influence of the Temperature......Page 482
2.5 - Influence of the Anionic Ends Structures......Page 483
3 - Oxidation of Anionic Polymers in the Solid State......Page 484
III- REACTION OF ANIONIC SITES WITH SUIFUR: S8, AS A DEACTIVATING REAGENT......Page 486
Literature Cited......Page 491
32 Cyclic Sulfonium Zwitterions Novel Anionic Initiators for the Control of Cyclooligomerization of Isocyanates......Page 493
Measurement of Kinetics......Page 494
Results and Discussion......Page 495
Literature Cited......Page 504
33 Anionic Polymerization. VII Polymerization and Copolymerization with Lithium-Nitrogen-Bonded Initiator......Page 505
Experimental......Page 506
Results and Discussion......Page 507
Acknowledgement......Page 519
Literature Cited......Page 520
34 Anionic Copolymerization of Butadiene and Isoprene with Organolithium Initiators in Hexane......Page 521
Copolymerization Apparatus and Techniques......Page 523
Kinetic Study......Page 524
Homopolymerization of Butadiene and Isoprene in n-Hexane......Page 526
Kinetic Study on the Copolymerization of Butadiene and Isoprene in Hexane......Page 534
Conclusions......Page 543
LITERATURE CITED......Page 546
35 The Reactivity of Polydiene Anions with Divinylbenzene......Page 548
Experimental......Page 549
Results and Discussion......Page 554
Influence of the DVB/RLi Molar Ratio......Page 556
Conclusion......Page 567
Literature Cited......Page 570
Β......Page 572
C......Page 573
D......Page 574
F......Page 575
I......Page 576
M......Page 577
O......Page 578
Ρ......Page 579
S......Page 581
T......Page 582
Ζ......Page 583

Citation preview

Anionic Polymerization Kinetics, Mechanisms, and Synthesis

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Anionic Polymerization Kinetics, Mechanisms, and Synthesis James E . McGrath,

EDITOR

Virginia Polytechnic Institute and State University

Based on a symposium sponsored by the Division of Polymer Chemistry at the 179th Meeting of the American Chemical Society, March 24-28,

1980.

ACS SYMPOSIUM SERIES 166

AMERICAN

CHEMICAL

SOCIETY

WASHINGTON, D. C. 1981

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Library of Congress CIP Data Anionic polymerization. (ACS symposium series; ISSN 0097-6156; 166) Includes index. 1. Polymers and polymerization—Congresses. I. McGrath, James Ε. II. American Chemical Society. Division of Polymer Chemistry. III. American Chemi­ cal Society. IV. Series. QD380.A54 541.3'93 81-14911 ISBN 0-8412-0643-0 A A C R 2 ACSMC8 166 1-594 1981 Copyright © 1981 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 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, repro­ duce, use, or sell any patented invention or copyrighted work that may in any way be related thereto.

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In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ACS Symposium Series M.

Joan

Comstock,

Series Editor

Advisory Board David L. Allara

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In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide a medium for publishin 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 sub­ mitted by the authors in camera-ready form. Papers are re­ viewed 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.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PREFACE his book is based on an international symposium on anionic polymeriza­ tion. The need for such a symposium grew out of discussions between myself and other members of the Polymer Division Executive Committee, about two years earlier. At that time I pointed out that we had not had a symposium dealing with the subject since 1962! Clearly, much new scien­ tific and important technologica that time. Anionic polymerization dates back at least to the early part of this century. Indeed, sodium-initiated butadiene polymers were investigated as potential synthetic rubbers many years ago. Unfortunately, the derived, high 1,2 microstructure shows a T of about 0 ° C . Electron transfer initiators also were studied by Scott in 1936. The mid 1950's can be pinpointed perhaps as one of the golden eras of anionic polymerization (and, indeed, polymer science). Certainly, the discovery reported by Firestone that lithium-initiated polyisoprene had a structure quite similar to Hevea (natural rubber) should be noted. In addition, of course, one needs to comment on the important discovery by Professor Szwarc, M . Levy, and R. Milkovich that electron-transfer initia­ tion by alkali metal polynuclear aromatic complexes could produce nonterminated living polymers of predictable molecular weights. These novel macromolecular carbanions were further shown to be capable of initiating other monomers to produce well-defined block polymers and to undergo reactions with reagents (e.g. carbon dioxide and ethylene oxide) that could provide functional end groups. In 1957 the Phillips Petroleum Company commercialized perhaps the first styrene-butadiene diblock copolymers. Many groups around the world began to intensively investigate kinetics, mechanisms, and synthesis possibilities of anionic polymerization. Here one should at least mention Professor Morton and his students at Akron (which include myself), Drs. Bywater and Worsfold in Canada, Professor Rempp and his many colleagues in France, and Professor Schulz in Germany. Many other scientists have contributed also to what we currently know about the subject but space prevents us from reviewing their efforts.

T

g

A few books and many reviews on anionic polymerization have appeared in the literature. Indeed, the contributors to the symposium and to this book also have written most of these! Briefly, one must cite the classic book of Professor Szwarc in 1968 and his later edited volumes on xi In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ion pairs. In addition, many of us have benefited from important reviews by Dr. Bywater and Professors Fetters and Rempp, among others. Cur­ rently, I am aware that Professor Morton has a new book that will be published soon. One of the most important discoveries relating to synthesis and physical behavior was made by Dr. Milkovich while at the Shell Develop­ ment Co. He and his colleagues showed that triblock copolymers containing polystyrene-polydiene-polystyrene blocks in appropriate sizes could behave as a physically cross-linked but linear thermoplastic elastomer. Thus Dr. Milkovich was involved with two very crucial discoveries in this field. Interestingly, he received his M . S. degree at Syracuse with Professor Szwarc and his Ph.D. at Akron with Professor Morton. I was pleased that Dr. Milkovich accepted m invitatio t b plenar speake t th symposium, along with Professor The symposium was, and this book is, truly an international contribu­ tion. There are papers from Australia, Belgium, Canada, France, Germany, Japan, Kuwait, Poland, the United Kingdom, and the United States. Invitations also were sent to U.S.S.R. scientists, but, unfortunately, none could attend. Nearly all of the papers presented at the symposium are published in this book. A few manuscripts were not received or were withdrawn for various reasons. The publication time was somewhat longer than initially perceived. I take responsibility for this and thank those authors who submitted manuscripts very promptly for their patience! Those currently actively involved in anionic polymerization will recog­ nize that despite the extensive progress that has been made over the past 25 years, many kinetic, mechanistic, and even synthetic aspects have not been elucidated fully. Thus, it should not be surprising that there are opposing points of view. In the past, controversies have occurred, and although I tried to minimize this factor, I was not completely successful in this regard. However, I was pleased that all of the speakers came to the same room and also contributed to this volume. A few of the papers may be a bit strong; however, I have decided to let the scientific community come to their own decisions on these matters. This book contains very useful new work as well as critical reviews of specific areas. The scope is quite broad and ranges from ion pair structures to various features of the kinetics, mechanisms, and synthesis. Importantly, there are several fine industrial contributions that comple­ ment the academic studies. I hope that this book will be of interest and utility to students and scientists in academia, government, and industry. I would like to thank all of the authors for their contributions to this book. I also would like to gratefully acknowledge the invaluable assistance of the ACS staff. Finally, the secretarial expertise provided by Debbie Farmer and Donna Perdue was indispensable. xii In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

The financial support of the symposium by The Firestone Tire and Rubber Co., Gulf Oil Chemicals Co., the 3M Co., the Shell Development Co., and the Polymer Materials and Interfaces Laboratory of our Univer­ sity was extremely helpful.

JAMES E. MCGRATH Chemistry Department and Polymer Materials and Interfaces Laboratory Virginia Polytechnic Institute and State University Blacksburg, Virginia April

1981

xiii In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1 Living and Dormant Polymers: A Critical Review M I C H A E L SZWARC New York Polymer Center, New York State College of Environmental Science and Forestry, Syracuse, NY 13210

A brief historical polymers and its sideration of various mechanisms of anionic polymerizations. The pertinent papers presented i n this meeting are surveyed. Special attention is devoted to polymerizations involving Li counterions proceeding in hydrocarbon solvents. It is stressed that l i v i n g and dormant polymers participate in such reactions and the consequences of their presence are deduced. Our s t u d i e s o f a n i o n i c p o l y m e r i z a t i o n that l e d us t o the concepts of l i v i n g polymers and e l e c t r o n - t r a n s f e r i n i t i a t i o n r e s u l t e d from a conversation I had with P r o f e s s o r Sam Weissman i n the Spring o f 1955. He t o l d me about h i s e l e c t r o n - t r a n s f e r i n v e s t i g a t i o n s , such as naphthalene-. + phenathrene -> naphthalene + phenanthrene-. As he t a l k e d , i t occurred to me that e l e c t r o n t r a n s f e r t o styrene ought to convert i t s C=C bond i n t o what n a i v e l y could be described as .C-C- . Then, the r a d i c a l end should i n i t i a t e r a d i c a l polymerization while a n i o n i c p o l y m e r i z a t i o n would be simultaneously induced by the c a r b a n i o n i c end. Such a s i t u a t i o n i n t r i g u e d me, and I asked Sam whether he t r i e d to t r a n s f e r e l e c t r o n to styrene. His answer was b r i e f : "No use, i t p o l y merizes." I asked then whether he would approve of our l o o k i n g i n t o t h i s problem and upon g e t t i n g h i s consent, we s t a r t e d our work that summer. With the help of Moshe Levy and Ralph M i l k o v i c h , we developed the a l l - g l a s s , high-vacuum technique and e s t a b l i s h e d that e l e c t r o n t r a n s f e r to s u i t a b l e monomers produces dimeric dianions by coupl i n g the r a d i c a l ends, while the carbanions induce a n i o n i c p o l y m e r i z a t i o n . Moreover, under the s t r i n g e n t p u r i t y c o n d i t i o n s of our experiments, the carbanions r e t a i n , v i r t u a l l y i n d e f i n i t e l y , t h e i r c a p a c i t y of growth. Hence, the p o l y m e r i z a t i o n l e a d i n g to elongation of the p r e v i o u s l y formed chains may continue upon " f e e d i n g " them with a d d i t i o n a l monomers. Our extremely simple but h i g h l y convincing experiments were reported i n 1956(1), and 0097-6156/81/0166-0001$05.00/0 © 1981 American Chemical Society

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

ANIONIC POLYMERIZATION

the term " l i v i n g polymers" was proposed to v i v i d l y convey the idea that these i n t e r e s t i n g macromolecules resume t h e i r growth whenever a s u i t a b l e monomer i s s u p p l i e d . The r a m i f i c a t i o n s of t h i s concept were c l e a r l y o u t l i n e d : the f e a s i b i l i t y of synthes i z i n g polymers of narrow molecular-weight d i s t r i b u t i o n , t a i l o r i n g block-polymers to d e s i r e d p a t t e r n s , the p r e p a r a t i o n of polymers with f u n c t i o n a l end-groups, e t c . These o p p o r t u n i t i e s e q u a l l y apply to l i v i n g polymers whether endowed with one or two growth perpetuating ends. However, the l a t t e r provide a route to synthesis of b i f u n c t i o n a l polymers and e a s i l y y i e l d the ABA t r i b l o c k polymers. Our ideas r a p i d l y caught the a t t e n t i o n of polymer chemists throughout the world and extensive s t u d i e s of l i v i n g polymers began i n many l a b o r a t o r i e s . These i n v e s t i g a t i o n s progressed i n two d i r e c t i o n s : the s y n t h e t i s y n t h e t i c work l e d to r e f i n e polymers, a l l o w i n g f o r more p r e c i s e s t u d i e s o the e f f e c t s o molecular weight d i s t r i b u t i o n on the r h e o l o g i c a l p r o p e r t i e s of polymer s o l u t i o n s and s o l i d polymers. Studies of block-polymers f l o u r i s h e d , as i s evident from the s e v e r a l symposia covering the subjectC2). The s p e c t a c u l a r discovery by Ralph M i l k o v i c h ( 3 ) of the t r i b l o c k polymers y i e l d i n g thermo-plastic rubber and the unexpected d i f f e r e n c e i n the p r o p e r t i e s of t r i b l o c k and d i b l o c k polymers added f u r t h e r impetus to those i n v e s t i g a t i o n s . T h i s subject w i l l be reviewed by Ralph M i l k o v i c h and by T e y s s i e l a t e r during t h i s symposium. Synthesis of f u n c t i o n a l and d i f u n c t i o n a l polymers was g r e a t l y improved by Paul Rempp and h i s a s s o c i a t e s ( 4 ) , and we look forward to S c h u l z d i s c u s s i o n of t h i s subject. Cont r o l l e d p r e p a r a t i o n of v a r i o u s types of macromolecules, such as star-shaped polymers, r e c e n t l y reviewed by Bywater(5), combshaped polymers, e t c . , became f e a s i b l e with the advent of l i v i n g polymers. In f a c t , the s y n t h e s i s of v a r i o u s types of model macromolecules by a n i o n i c p o l y m e r i z a t i o n w i l l be reported here by Rempp, Franta and Hertz. Let me d i g r e s s and ask two questions. Were we the f i r s t to d i s c o v e r l i v i n g polymers? The answer i s no. Review of the l i t erature showed that l i v i n g polymers, although not named i n that way, were d e s c r i b e d by Z i e g l e r ( 6 ) i n 1929 and the t e r m i n a t i o n l e s s p o l y m e r i z a t i o n was considered by Mark and Dostal(7) and by F l o r y ( 8 ) . However, the p o t e n t i a l i t y of those systems was not f u l l y appreciated u n t i l our r e d i s c o v e r y of these i n t e r e s t i n g molecules. A l s o , our work l e d to r e c o g n i t i o n of the necessary c o n d i t i o n s r e q u i r e d f o r such s t u d i e s , namely, the s t r i n g e n t p u r i t y , vacuum techniques, e t c . Is a n i o n i c p o l y m e r i z a t i o n unique i n p r o v i d i n g l i v i n g p o l y mers? I t i s not. The c a t i o n i c a l l y formed l i v i n g polymers are w e l l known today and, i n f a c t , Richards w i l l d e s c r i b e here the techniques of c o n v e r t i n g one kind of l i v i n g polymers to another. I o n i c p o l y m e r i z a t i o n i s much more c o l o r f u l than r a d i c a l p o l y m e r i z a t i o n . While only one species p a r t i c i p a t e s i n r a d i c a l 1

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1. SZWARC

Living

and

Dormant

3

Polymers

homo-polymerization, a v a r i e t y of species may c o - e x i s t and simultaneously c o n t r i b u t e to i o n i c p o l y m e r i z a t i o n . Living polymers f a c i l i t a t e t h e i r c h a r a c t e r i z a t i o n and provide ways of studying t h e i r i n t e r - c o n v e r s i o n , mode of growth, and the degree of t h e i r p a r t i c i p a t i o n i n the o v e r a l l process. Indeed, these s u b j e c t s form the themes of numerous c o n t r i b u t i o n s to t h i s symposium. Free ions and t h e i r r e s p e c t i v e i o n - p a i r s p a r t i c i p a t e i n numerous a n i o n i c and c a t i o n i c p o l y m e r i z a t i o n s ; t h e i r c o n t r i b u t i o n i s a f f e c t e d by the nature of counter-ions, s o l v e n t , and s o l v a t i n g agents whenever added to the r e a c t i n g system. For example, the e f f e c t of cryptâtes on some a n i o n i c polymerizations w i l l be discussed by Mademoiselle B o i l e a u . Moreover, as revealed by the elegant s t u d i e s of P r o f e s s o r Johannes Smid(9), a v a r i e t y of i o n p a i r s may c o - e x i s t i n polymerizin own c h a r a c t e r i s t i c behavior i n t e r e s t i n g f i e l d w i l l be reviewed by P r o f e s s o r Smid, and the dynamics of the interchanges t a k i n g p l a c e between those species w i l l be discussed by Dr. Andre Persoon, who developed the most ingenious techniques a l l o w i n g him to measure the r a t e s of those rapid reactions. T r i p l e ions form r a t h e r unusual s p e c i e s . They c o n t r i b u t e l i t t l e , i f at a l l , to the propagation but t h e i r formation may have a b u f f e r i n g e f f e c t . The i n t r a m o l e c u l a r t r i p l e i o n s ' formation i n the cesium polystyrene system i l l u s t r a t e s t h i s phenomenon(10). The mechanism i s o u t l i n e d i n Scheme 1. The formation of t r i p l e ions increases the c o n c e n t r a t i o n of Cs+ ions and b u f f e r s , theref o r e , the d i s s o c i a t i o n of i o n - p a i r s i n t o f r e e s t y r y l ions which are the main c o n t r i b u t o r s to the propagation.

T r i p l e ion Scheme 1. G e n e r a l l y , two d i s t i n c t processes the formation of t r i p l e i o n s , namely +

2Cat ,An~

could be r e s p o n s i b l e f o r

J

Cat

Î

An" + C a t , A n " , C a t

+

+

+ An~,Cat ,An"~

(a)

or +

2Cat ,An"

+

+

(b)

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4

ANIONIC POLYMERIZATION

Whenever e q u i l i b r i u m (a), which i s concentrâtion-independent i s more important than the conventional d i s s o c i a t i o n of i o n - p a i r s , then i t s e f f e c t , i n c o n j u n c t i o n with the i o n - p a i r s d i s s o c i a t i o n , decreases the a n t i c i p a t e d c o n c e n t r a t i o n of anions. The reverse i s true when e q u i l i b r i u m (b) p r e v a i l s on ( a ) . A somewhat s i m i l a r problem was encountered i n the a n i o n i c propagation i n v o l v i n g the barium s a l t of l i v i n g p o l y - s t y r e n e possessing only one growth-propagating end per m a c r o m o l e c u l e ( l l ) . The p s e u d o - f i r s t order r a t e constant of propagation was found to be independent of the s a l t c o n c e n t r a t i o n . This r e s u l t was accounted f o r by a mechanism o u t l i n e d i n Scheme 2 with the assumption that the f r e e poly-styrene anions are the only s p e c i e s c o n t r i b u t i n g to the growth. 9

2 +

2 B a , (S~'vw\,) Ba

2+

, (S

—

J

'wvx,)

2

Ba

2

, (S 'vw\, ) + s

^

Scheme 2. Denoting and by

by ^

t r

£p^

a n ( e

* ^^

8 3

the r e s p e c t i v e e q u i l i b r i u m constants

the bimolecular r a t e constant of propagation by the f r e e

p o l y - s t y r e n e anions, one f i n d s the observed r a t e constant to be given by

p s e u d o - f i r s t order

1 / 2

k L , /K . , ρ diss triple i . e . , i t i s independent of the s a l t c o n c e n t r a t i o n because w i t h i n the i n v e s t i g a t e d c o n c e n t r a t i o n range the r e l a t i o n triple

K

2 +

d i s s

/[Ba ,(S-ww,) ] 2

was

found to be v a l i d . A s i m i l a r problem concerning the strontium s a l t w i l l be d i s ­ cussed here by P r o f e s s o r Van Beylen, while the problem of the barium s a l t of the p o l y - s t y r e n e endowed with two growth propa­ gating ends w i l l be d e a l t with by Dr. F r a n c o i s . The v a r i o u s kinds of growing species d i f f e r not only i n their propagation but a l s o i n t h e i r stereochemical preferences. Pro­ f e s s o r Hogen-Esch w i l l review t h i s subject i n h i s t a l k on a n i o n i c o l i g o m e r i z a t i o n of some v i n y l monomer, and mechanisms of a n i o n i c , 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 2 - v i n y l p y r i d i n e w i l l be d i s ­ cussed by Dr. F o n t a n i l l e . In t h i s context, the i n t e r e s t i n g paper of Schuerch et a l . ( 1 2 ) deserves a t t e n t i o n . T h e i r work c l e a r l y r e v e a l s the e f f e c t of c a t i o n s o l v a t i o n upon the mode of monomer's approach to the growing c e n t e r s . The a l l c i s p o l y m e r i z a t i o n of isoprene when c a r r i e d out i n hydrocarbon s o l v e n t s with L i counter ions provides the most s p e c t a c u l a r case of s t e r e o s p e c i f i c i t y . I t was reported by Stavely and h i s colleagues(13) i n 1956, and t h e i r d i s c o v e r y g r e a t l y

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

SZWARC

Living

and Dormant

5

Polymers

increased the i n t e r e s t i n a n i o n i c p o l y m e r i z a t i o n . The profound d i f f e r e n c e i n s t e r e o s p e c i f i c i t y of p o l y - i s o p r e n y l l i t h i u m when compared with the s a l t s i n v o l v i n g other a l k a l i c a t i o n s , and the e f f e c t of s o l v a t i o n , f o s t e r e d i n t e r e s t i n the s t r u c t u r e of p o l y d i e n y l s a l t s . The l i v i n g character of these polymerizations allows one to use the NMR as a t o o l f o r such s t u d i e s , and indeed, s i g n i f i c a n t r e s u l t s were reported by v a r i o u s workers. The con­ t r i b u t i o n s of Dr. Worsfold, of Dr. Halasa and h i s c o l l e a g u e s , and of Dr. Bywater cover some of these problems. The understanding of the mechanism of propagation of polymers i n v o l v i n g l i t h i u m c a t i o n s and proceeding i n hydrocarbons stems from the p i o n e e r i n g s t u d i e s o f Worsfold and Bywater(14). Not only was the l i v i n g character of these polymerizations unambiguously e s t a b l i s h e d , but the k i n e t i c s of propagation of l i t h i u m p o l y ­ styrene i n benzene reveale dormant, i n a c t i v e dimeri with a minute f r a c t i o n of growing monomeric l i t h i u m p o l y s t y r e n e s . This c o n c l u s i o n was deduced from the observed square-root depen­ dence of the propagation r a t e on the c o n c e n t r a t i o n of the p o l y ­ mers, i . e . , the r a t e of propagation - k ( K p

where k

d i g g

1/2 1/2 / 2 ) ' [ L i - p o l y S ] ' [S]

i s the propagation constant of the monomeric polymers

and Κ,. i s the d i s s o c i a t i o n contant of the dimers. diss

Since t h i s

r e l a t i o n a p p l i e s even a t polymer concentrations as low as 10 "*M, Dr. Bywater pointed out i n h i s t a l k s that ^ K

n

a

s

t

o

D

e

o

f

t

n

e

i g s

order of 10 ^M or l e s s . The above mechanism served as a prototype accounting f o r other s i m i l a r p o l y m e r i z a t i o n s . However, the k i n e t i c s of propa­ g a t i o n of l i t h i u m polydienes revealed a dependence on the polymer c o n c e n t r a t i o n lower than 1/2, apparently 1/4 or l e s s order. T h i s prompted the assumption that the l i t h i u m p o l y d i e n y l s form higher aggregates than dimers. A u s e f u l q u a s i - q u a n t i t a t i v e technique a l l o w i n g determination of the degree of a s s o c i a t i o n of such polymers was reported by P r o f e s s o r Morton(15). The v i s c o s i t y of concentrated s o l u t i o n s of high molecular weight polymers i s p r o p o r t i o n a l to a power of t h e i r weight average molecular weight, Μ , v i z . , w η - M w where α, although not p r e c i s e l y known, i s w i t h i n the range of 3.3 - 3.5 [see thorough review by P o r t e r and Johnson(16)]. On " k i l l i n g " the aggregated polymers, say by adding a drop of metha­ n o l , the aggregates a r e converted i n t o monomeric dead polymers, and the v i s c o s i t y o f the s o l u t i o n d r a s t i c a l l y decreases. Thus, the r a t i o of v i s c o s i t i e s before and a f t e r methanol a d d i t i o n i s given by N , where Ν denotes the degree of a s s o c i a t i o n of the l i v i n g polymers. a

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6

ANIONIC POLYMERIZATION

In s p i t e of some u n c e r t a i n t y i n the p r e c i s e value of a, t h i s method g i v e s Ν with reasonable accuracy. For example, i f the r a t i o of v i s c o s i t i e s i s 10, then Ν = 2.01, f o r α = 3.3, and Ν = 1.93 f o r α = 3.5. Using t h i s approach, P r o f e s s o r Morton and h i s a s s o c i a t e s confirmed the dimeric nature of l i t h i u m p o l y s t r y l i n benzene, i n agreement with the deduction of Worsfold and Bywater, but they claimed a d i m e r i c nature a l s o f o r l i t h i u m p o l y d i e n y l s i n hydro­ carbon media. The l a t t e r c l a i m c o n f l i c t e d with the k i n e t i c evidence (about 1/4 order) which, a f t e r i n i t i a l d e n i a l s ( 1 7 ) , e v e n t u a l l y was confirmed by the Akron group(18). Controversy arose because the v i s c o s i t y s t u d i e s of other workers(19) l e d to d i f f e r e n t c o n c l u s i o n s . The extension of these i n v e s t i g a t i o n s , i n v o l v i n g l i g h t - s c a t t e r s t u d i e s , again l e d to c o n f l i c t i n g r e s u l t s whe Moreover, Dr. F e t t e r s an f o r the 1/4 order of propagation of the presumably dimeric p o l y dienyls. Strangely enough, they stated(20) that " p l a u s i b l e mechanisms" accounting f o r the dimeric nature of the aggregates and 1/4 order of propagation were presented, without s p e c i f y i n g a s i n g l e one but quoting 12 i r r e l e v a n t r e f e r e n c e s i n support of their claim. Let me suggest a way by which the k i n e t i c and v i s c o s i t y r e ­ s u l t s c o u l d be r e c o n c i l e d , although I have r e s e r v a t i o n s about the v a l i d i t y of the mechanism I am proposing. Let us assume that the polymers with l i t h i u m c a t i o n s are dimeric i n hydrocarbon s o l v e n t s . T h e i r d i s s o c i a t i o n i n t o f r e e l i t h i u m c a t i o n s i s u n l i k e l y i n hydro­ carbon media, although I would not exclude the f e a s i b i l i t y of forming the macro-anions. Hence, I r e j e c t the d i s s o c i a t i o n +

( L i , poly-dienyl")

2

t 2Li

+

+ 2poly-dienyl"

However, an a l t e r n a t i v e mode of d i s s o c i a t i o n of the dimers i s p o s s i b l e , namely ( L i , p o l y - d i e n y l )^ Î +

+

( 2 L i , poly-dienyl

+ (poly-dienyl"),

K

2

T h i s e q u i l i b r i u m i n c o n j u n c t i o n with the e q u i l i b r i a K. 1

leads to the f o l l o w i n g r a t e expression, provided that the f r e e ( p o l y - d i e n y l ~ ) ions a r e the only s p e c i e s c o n t r i b u t i n g to the propagat i o n . Thus, Rate of propagation = k [ ( p o l y - d i e n y l )][M], i . e • 9

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SZWARC

1.

and

Living

Dormant

Rate of propagation/[M] k K p

+

K

l

,K

+

[(Li , P") ] 2

1 / 2

= /(l +

1 / 2 K l

i s short f o r L i , p o l y d i e n y l . +

«j[(Li J p o l y - d i e n y l ) 2]

1/2

= ( l / 2 ) K v +

+

+

/

9

+

+

2

2

a n d

-

2

+

-

2

+

('ww, S , L i ,ww,SD , L i ) = K

u v 1 2

The r a t e of conversion Denoting the i n i t i a l c o n c e n t r a t i o n o f l i t h i u m p o l y s t y r y l by C and by g the f r a c t i o n of l i t h i u m p o l y s t y r y l present i n any form at time t , one f i n d s C = u ^ + 2 K f + K f ) , g = (Κ + K f ) / ( K + 2 K f 2

2

Q

1 2

2

χ

1 2

1

1 2

2 1/2 + K f ) and hence dlng/dt = (k D/C ) (K- + 2 K f z a ο ι iz 0

1 0

+ Κ ί ) /(Κ 2

2 1/2

+ K

λ

f).

1 2

For K

= ( K ^ )

1 2

3

7

2

this

1/2 1/2 expression i s reduced t o dln.g/dt = k D/KC a 1 ο being i n agreement with the experimental f i n d i n g s of p s e u d o - f i r s t order conversion and the p s e u d o - f i r s t order r a t e constant being i n v e r s e l y p r o p o r t i o n a l to the square-root of the i n i t i a l concen-1/2 t r a t i o n of l i t h i u m p o l y s t y r y l , i . e . , C 1/2 ° For k ^ φ ( i 2 ^ becomes curved, but i t s i n i t i a l slope would s t i l l be i n v e r s e l y propor1/2 t i o n a l to [ l i t h i u m p o l y s t y r y l ] The i n t e r e s t i n g f e a t u r e of t h i s kind o f r e a c t i o n i s t h e i r memory. The observed r a t e a t a chosen c o n c e n t r a t i o n of p o l y s t y r y l depends on i t s i n i t i a l c o n c e n t r a t i o n . T h i s i s evident from i n s p e c t i o n of F i g . 2; the e f f e c t a r i s e s from the i n f l u e n c e of products of the r e a c t i o n on i t s r a t e . This i s a general phe­ nomenon. In the system discussed here, the formation of the mixed dimer decreases the c o n c e n t r a t i o n of the r e a c t i v e monomeric l i t h i u m p o l y s t y r y l and thus r e t a r d s the r e a c t i o n . Mixed d i m e r i z a t i o n a l s o a f f e c t s the k i n e t i c s of a n i o n i c co­ polymerization o f styrene and para-methyl s t y r e n e , a system studied by 0 ' D r i s c o l l ( 2 4 ) . The r e a c t i o n was performed i n benzene with l i t h i u m counter-ions and the p l o t s o f In.[styrene] or In(para-methyl styrene] v s . time were both l i n e a r . T h e i r slopes, denoted by and λ , were, however, d i f f e r e n t from each other. K

K

t

h

e

p

l

o

t

o

f

l

n

,

g

v

e

r

s

u

s

t

i

m

e

2

q

2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SZWARC

Living

and Dormant

Polymers

Concentration of living polymers

Figure 2.

Memory effect.

Time

Figure 3.

Copolymerization of styrene and p-methylstyrene initiated by RLi in benzene.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14

ANIONIC POLYMERIZATION

T h i s i s seen i n F i g . 3. These r e s u l t s were accounted f o r by Yamagishi and myself(25) i n the terms of the previous treatment. Denoting again by u and ν the c o n c e n t r a t i o n of the monomeric l i v i n g polymers terminated by s t y r y l or para-methyl s t y r y l u n i t , r e s p e c t i v e l y , and by C the c o n c e n t r a t i o n of a l l the polymers, we find ° 2

K u 1

+ 2K uv + K v 1Z Ζ

n

k

with

l l

10

U

+

and

k

21

V

2

= C

0

=

λ

a

1

Ο n

d

k

12

U

+

k

22

V

=

λ

2

;

being constant and the other symbols having

usual meaning. Since u and ν are v a r i a b l e s , but not each other through the equation k^^vtstyrene

k^^u[para-methy

1 2

=

(

κ )

Κ ι

2

r e l a t e d to

styrene

the f i r s t three equations have to be i d e n t i c a l . the c o n d i t i o n s

κ

their

T h i s leads to

1 / 2

and k

ll

/ k

k

λ

1 - VO ''

1 2 - 21

/ k

U

22

e

"

r

r

l 2 =

1

Moreover

with

and

1

a

n

d

λ

2

C

" ^2 o

1 / 2

as two d i f f e r e n t constants, independent of the

concentrations or composition of the monomers and the concentra­ t i o n of the i n i t i a t o r , i . e . , independent of C . Let i t be s t r e s s e d again that a l l these c o n c l u s i o n s are mathematical consequences of the e m p i r i c a l f i n d i n g s revealed i n F i g . 3, and the assumptions of the e x i s t e n c e of i n e r t homo- and mixed dimers i n e q u i l i b r i u m with the a c t i v e growing monomeric polymers. In c o n c l u s i o n I thank the N a t i o n a l Science Foundation f o r the years of continuous support of our work. Literature Cited 1. a) Szwarc, M.; Levy, M.; M i l k o v i c h , R. J . Amer. Chem. Soc. 1956, 78, 2656. b) Szwarc, M. Nature 1956, 178, 1168. 2. a) Burke, J.J.; Weiss, V., Ed.; "Block and G r a f t Copolymers"; Syracuse U n i v e r s i t y P r e s s : Syracuse, N.Y., 1973. b) Aggarwal, S. L., Ed.; "Block Polymers"; Plenum Press, 1970. c) L a F l a i r , R. T.; " S t r u c t u r e , Morphology and P r o p e r t i e s of Block Polymers"; XXIII, IUPAC, 8, Butterworth, 1971.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

SZWARC

Living

and Dormant

Polymers

15

3. a) M i l k o v i c h , R. ( S h e l l ) , S. A f r i c a n Patent 642,271, 1964. b) M i l k o v i c h , R.; Holden, G.; Bishop, E. T.; Hendricks, W. R. Brit. Patent 1,035,873, 1966; U.S. Patent 3,231,635, 1966. 4. Rempp, P.; Loucheux, M. H. B u l . Soc. Chimique, France, 1958, 1497. 5. Bywater, S. Adv. Polymer Sci. 1979, 30, 89. 6. a) Z i e g l e r , K.; Bahr, K. Chem. Ber. 1928, 61, 253. b) Z i e g l e r , K.; Colonius, H.; Schafer, O. Ann. Chem. 1929, 473, 36; 1930, 479, 150. c) Z i e g l e r , K.; Dersch, F.; Wolltham, H. Ann. Chem. 1934, 511, 13. 7. D o s t a l , H.; Mark, H. Z. Phys. Chem. B. 1935, 29, 299. 8. F l o r y , P. J. " P r i n c i p l e s of Polymer Chemistry"; C o r n e l l Press, 1953. 9. Hogen-Esch, T. E.; 1966, 88, 307. 10. Bhattacharyaa, D. N.; Smid, J.; Szwarc, M. J. Amer. Chem. Soc. 1964, 86, 5024. 11. DeGroof, B.; Van Beylen, M.; Szwarc, M. Macromolec. 1975, 8, 396. 12. Schuerch, C.; e t al. J. Amer. Chem. Soc. 1964, 86, 4481; 1967, 89, 1396. 13. S t a v e l y , F. W.; e t al. Ind. Eng. Chem. 1956, 48, 778. 14. Worsfold, D. J.; Bywater, S. Can. J. Chem. 1960, 38, 1891. 15. Morton, M.; Bostock, E. E.; Livigni, R. Rubber P l a s t i c Age 1961, 42, 397. 16. P o r t e r , R. S.; Johnson, R. Chem. Rev. 1966, 66, 1. 17.(a)Morton, M.; F e t t e r s , L. J.; Bostock, E. E. J. Polymer Sci. 1963, C1, 311. (b)Morton, M.; F e t t e r s , L. J. J. Polymer Sci. 1964, A2, 3111. 18. a) Morton, M.; P e t t , R. Α.; F e t t e r s , J. F.; IUPAC Symposium Tokyo, 1966, 1 69. b) Morton, M.; F e t t e r s , L. J. Rubber Chem. Technol. 1975, 48, 359. 19. e.g., Worsfold, D. J.; Bywater, S. Macromolec. 1972, 5, 393. 20. F e t t e r s , L. J.; Morton, M. Macromolec. 1974, 7, 552. 21. Szwarc, M. Polym. L e t t . 1980, 18, 493. 22. Morton, M.; P e t t , R. Α.; F e t t e r s , L. J. Macromolec. 1970, 3, 333. 23. L a i t a , Z.; Szwarc, M. Macromolec. 1969, 2, 412. 24. O ' D r i s c o l l , K. F.; P a t s i g a , R. J. Polymer Sci. 1965, A3, 1037. 25. Yamagishi, Α.; Szwarc, M. Macromolec. 1978, 11, 1091. 26. Wang, H. C.; Szwarc, M. Macromolec. 1980, 13, 452.

RECEIVED April 7, 1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2 Current Status of Anionic Polymerization MAURICE

MORTON

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

Because of the stron polymerization, especiall been substantial advances, both i n the understanding of the mechanisms involved as well as in the utilization of these systems for the synthesis of new materials. This overview will attempt to highlight the more important advances in both of the above areas, although admittedly with somewhat of a bias toward those particular endeavors pursued i n our own laboratories. Kinetics and Mechanism of Polymerization Homogeneous anionic polymerization involving organoalkali i n i t i a t o r s has been carried out i n two classes of solvents, i.e., polar (ethers, amines), or non-polar (hydrocarbon) media. In the case of the polar solvents, it has been possible to use a wide range of organoalkali compounds, since they are soluble i n these media; but this i s not the case for hydrocarbon solvents which are mainly suitable for organolithium i n i t i a t o r s . Inallsuch studies, these anionic systems have a distinct advantage over other systems, since it i s possible, under proper conditions, to avoid any chain termination reactions, so that the concentration of growing chains can correspond to the concentration of the i n i t i a t o r . Hence the kinetics of the polymerization reaction can thus represent the kinetics of the propagation reaction. Evidence f o r t h e absence o f t e r m i n a t i o n o r t r a n s f e r r e a c t i o n s i n t h e o r g a n o l i t h i u m - i n i t i a t e d p o l y m e r i z a t i o n o f s t y r e n e and i s o p r e n e i s shown i n T a b l e I f o r r e p r e s e n t a t i v e e x a m p l e s o f t h e s e polymers. I t can be seen t h a t these polymers e x h i b i t the expected low v a l u e s , except i n t h e case o f t h e i s o p r e n e p o l y m e r i z e d i n t h e Hi+furan, where a s l o w s i d e r e a c t i o n seems t o o c c u r b e t w e e n t h e s o l v e n t , o n t h e one h a n d , a n d b o t h t h e i n i t i a t o r (MQ v s MS) and t h e g r o w i n g c h a i n s (Ufo v s Mn),on t h e o t h e r hand. 1. K i n e t i c s i n P o l a r M e d i a . These p o l y m e r i z a t i o n s , i n i t i a t e d b y o r g a n o a l k a l i compounds m a i n l y i n e t h e r s o l v e n t s , h a v e 0097-6156/81/0166-0017$06.00/0 ©

1981 American Chemical Society

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18

ANIONIC POLYMERIZATION TABLE I M o l e c u l a r Weight o f A l k y l l i t h i u m - I n i t i a t e d P o l y m e r s i ' l

Monomer

Solvent

M o l e c u l a r Weights ( x l O ~ )

Temp.

3

(°c) Mg

-furan

0 0 -80

Isoprene

n-Hexane

» tf

tt

29

Styrene

Benzene

η

Mw/M

a

142 320 280

136 324 295

53.8

1.06 1.10 1.05

144 355 310

61.2

53.2

n

d

—

Hit-fur an

S t o i c h i o m e t r i c m o l . wt.

Osmotic

Light scattering

values-^ b e e n i n t e n s i v e l y i n v e s t i g a t e d , and t h e r e i s now g e n e r a l a g r e e m e n t t h a t i o n i c d i s s o c i a t i o n a c t u a l l y occurs, e s p e c i a l l y i n the case o f t h e more e l e c t r o p o s i t i v e m e t a l s , i . e . , s o d i u m t h r o u g h c e s i u m . Thus t h e g r o w i n g a n i o n i c c h a i n c a n assume a t l e a s t two i d e n t i t i e s : the f r e e a n i o n and t h e a n i o n - c a t i o n i o n p a i r ( s e v e r a l types o f s o l v a t e d i o n - p a i r s can a l s o be c o n s i d e r e d ) . Furthermore, t h e k i n e t i c s o f t h e s e p r o p a g a t i o n r e a c t i o n s , w h i c h g e n e r a l l y show a f r a c t i o n a l dependency on c h a i n - e n d c o n c e n t r a t i o n r a n g i n g f r o m oneh a l f t o u n i t y , c a n b e s t be e x p l a i n e d b y a s s u m i n g t h a t t h e monomer c a n r e a c t w i t h b o t h t h e f r e e a n i o n a n d t h e i o n - p a i r (4, 5_, 6), b u t a t d i f f e r e n t r a t e s . Thus, f o r example, i n t h e p o l y m e r i z a t i o n o f s t y r e n e b y o r g a n o s o d i u m , t h e r a t e o f p o l y m e r i z a t i o n ( R p ) c a n be expressed as Rp/[S][RS Na] x

1 / 2

= ^

1

/

2

+ kp[RS Na]

V 2

x

(1)

where S r e p r e s e n t s s t y r e n e monomer [RS Na] r e p r e s e n t s t h e t o t a l c o n c e n t r a t i o n o f g r o w i n g c h a i n s of both types, i . e . , the concentration of i n i t i a t o r used. kp = r a t e constant f o r p r o p a g a t i o n by f r e e i o n s + x

kj"

= r a t e constant f o r propagation by i o n p a i r s

K

= ionization

e

constant.

Equation ( l ) y i e l d s d i r e c t l y , from e x p e r i m e n t a l data, t h e +

vc ao ln us et a notf, t khΡe i ocna np a b ie r rc aa tl ec ucloantsetda nit f, k , iwsh known. i l e t h eThe f r e le a at nt ie or n h a r as t e -

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

MORTON

Current

Status of Anionic

19

Polymerization

a c t u a l l y b e e n d e t e r m i n e d e x p e r i m e n t a l l y (4·_, 5_, 6). Hence, i n t h e above s y s t e m , i t h a s b e e n f o u n d (A, % 6) t h a t k p i s s e v e r a l o r d e r s g r e a t e r t h a n k ^ ( c a . \Φ v s . 10 M " ^ " ) a n d t h a t Ke i s o f t h e o r d e r o f 10" , when H ^ f u r a n i s t h e s o l v e n t . The f a c t t h a t much e x p e r i m e n t a l d a t a obey t h e k i n e t i c s o f E q u a t i o n (1) l e n d s c r e d e n c e t o t h e p r o p o s e d mechanism. 2

1

7

2. K i n e t i c s i n Non-Polar Media. P o l y m e r i z a t i o n o f v i n y l monomers i n n o n - p o l a r s o l v e n t s , i . e . , h y d r o c a r b o n m e d i a , h a s b e e n a l m o s t e n t i r e l y r e s t r i c t e d t o t h e o r g a n o l i t h i u m s y s t e m s (7), s i n c e t h e l a t t e r y i e l d homogeneous s o l u t i o n s . I n a d d i t i o n , t h e r e has been a p a r t i c u l a r l y s t r o n g i n t e r e s t i n the p o l y m e r i z a t i o n o f t h e 1 , 3 - d i e n e s , e.g., i s o p r e n e a n d b u t a d i e n e , b e c a u s e t h e s e s y s t e m s l e a d t o h i g h 1,4 c h a i n s t r u c t u r e s , w h i c h y i e l d r u b b e r y polymers. I n the case to a c t u a l l y o b t a i n a polyme c h a i n s t r u c t u r e (7, 9), c l o s e l y r e s e m b l i n g t h e m i c r o s t r u c t u r e of the n a t u r a l rubber molecule. E a r l y studies ( l ) o f the k i n e t i c s o f polymerization o f s t y r e n e , isoprene and butadiene i n hydrocarbon s o l v e n t s i n d i c a t e d a h a l f - o r d e r r a t e dependency o n g r o w i n g c h a i n c o n c e n t r a t i o n , a l t h o u g h t h e r e were c o n f l i c t i n g d a t a a t t h a t t i m e (10_, L L ) w h i c h suggested even lower f r a c t i o n a l o r d e r s f o r the d i e n e s . Since the a p p a r e n t h a l f - o r d e r dependency c o u l d n o t b e r a t i o n a l i z e d , a s i n the case o f t h e p o l a r media, b y an i o n i c d i s s o c i a t i o n mechanism, some o t h e r f o r m o f a s s o c i a t i o n - d i s s o c i a t i o n phenomenon o f f e r e d a p o s s i b l e answer. I n v i e w o f t h e known t e n d e n c y o f o r g a n o l i t h i u m compounds t o u n d e r g o m o l e c u l a r a s s o c i a t i o n i n n o n - p o l a r m e d i a , t h e f o l l o w i n g scheme was p r o p o s e d b y u s ( l _ ) :

(2) (3) Furthermore, the proposed dimeric a s s o c i a t i o n o f the polymer l i t h i u m s p e c i e s was a c t u a l l y c o n f i r m e d (l) b y v i s c o s i t y m e a s u r e ­ ments o n t h e p o l y m e r i z e d s o l u t i o n s , u s i n g t h e w e l l - k n o w n r e l a ­ t i o n (12) w h i c h a p p l i e s t o m e l t s o r c o n c e n t r a t e d s o l u t i o n s 3

1

n = KM * *

U)

where η i s t h e v i s c o s i t y , M i s t h e m o l e c u l a r w e i g h t , and Κ i s a constant which i n c l u d e s t h e c o n c e n t r a t i o n o f the polymer. Because o f the l a r g e exponent i n E q u a t i o n t h i s method was f o u n d t o be a s e n s i t i v e measure o f t h e s t a t e o f a s s o c i a t i o n o f t h e p o l y m e r l i t h i u m c h a i n e n d s , a n d t h i s was f o u n d t o b e v e r y c l o s e t o 2 f o r a l l t h r e e systems s t u d i e d , i . e . , p o l y s t y r e n e , polyisoprene and polybutadiene. T y p i c a l v a l u e s o f N, t h e " a s s o c i a t i o n number", o r w e i g h t a v e r a g e number o f c h a i n s a s s o c i a t e d a t t h e i r e n d s , a r e shown i n T a b l e I I . The v a l u e s o f Ν were o b t a i n e d f r o m t h e r e l a t i o n

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20

ANIONIC POLYMERIZATION Ν = (t /t ) a

^

t

where t = f l o w t i m e o f " a c t i v e " ( n o n - t e r m i n a t e d ) t-fc = " " " terminated chains. a

chains,

The somewhat h i g h e r v a l u e s o f Ν e x h i b i t e d b y t h e p o l y b u t a d i e n y l l i t h i u m were a s c r i b e d t o t h e o c c u r r e n c e o f some c r o s s l i n k i n g d u r i n g t h e p o l y m e r i z a t i o n o f t h i s monomer, a n d t h i s was c o n f i r m e d b y u s e o f a s p e c i a l " c a p p i n g " t e c h n i q u e , a s shown l a t e r . TABLE I I A s s o c i a t i o n i n O r g a n o l i t h i u m Polymer S o l u t i o n s ( l _ , (25°C) Monomer

2)

Solvent 3

(xlO )

Styrene Isoprene If Butadiene ft ft

Benzene n-Hexane HLf-furan n-Hexane n-Hexane H i t - f u r an

1.23 2.08 1.95 4.97 0.25 1.90

Active

Terminated

825 836 3150 1770 1030 2580

81.1 81.1 3130 112 75.5 2570

1.98 1.99 1.00 2.89 2.16 1.00

U n f o r t u n a t e l y , we f o u n d l a t e r ( 7 , 13) t h a t t h e scheme p r o ­ p o s e d i n E q u a t i o n s ( 2 ) a n d ( 3 ) was n o t v a l i d f o r i s o p r e n e a n d b u t a d i e n e , s i n c e t h e i r k i n e t i c o r d e r was c o n s i d e r a b l y b e l o w o n e h a l f , r a n g i n g from o n e - s i x t h t o o n e - f o u r t h , depending on the s o l v e n t used. Rate p l o t s f o r i s o p r e n e i n 3 d i f f e r e n t s o l v e n t s , b u t a d i e n e i n n-hexane a n d b u t a d i e n e i n b e n z e n e , a r e shown i n F i g u r e s 1 t o 4, r e s p e c t i v e l y . T h e r e c a n b e no d o u b t a b o u t t h e l o w k i n e t i c o r d e r e x h i b i t e d b y t h e s e r e a c t i o n s , much l e s s t h a n the one-half o r d e r o r i g i n a l l y claimed. I t i s regrettable t h a t a t t e m p t s were s u b s e q u e n t l y made b y o t h e r s t o r e l a t e t h e s e l o w f r a c t i o n a l o r d e r s f o r i s o p r e n e ( 1 4 ) a n d b u t a d i e n e (15_) t o assumed h i g h e r s t a t e s o f a s s o c i a t i o n t h a n 2, i n o r d e r t o make t h e s e k i n e t i c s f i t t h e mechanism p r o p o s e d i n E q u a t i o n s ( 2 ) and ( 3 ) . T h e r e was e v e n some work ( 1 6 ) w h i c h p u r p o r t e d to prove t h e e x i s t e n c e o f such h i g h e r s t a t e s o f a s s o c i a t i o n f o r t h e s e d i e n e s . However, a l l s u c h c l a i m s were shown t o be i n v a l i d b y overwhelming e x p e r i m e n t a l e v i d e n c e ( 7 , 1 7 ) which d e f i n i t e l y confirmed t h a t not only p o l y s t y r e n e , b u t 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 were a s s o c i a t e d i n p a i r s a t t h e i r o r g a n o l i t h i u m c h a i n ends, i n hydrocarbon media, a t l i t h i u m c o n c e n t r a t i o n s commonly u s e d t o p r e p a r e h i g h p o l y m e r s * -

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

MORTON

Figure 2.

Current

Status of Anionic

Polymerization

21

Dependence of R on organolithium concentration. Butadiene in nhexane at 30°C; [M] = 2.0. p

0

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22

ANIONIC POLYMERIZATION

[RLi]

Figure 3.

or [RLi]

χ 10

Dependence of R on organolithium concentration. Butadiene in ben­ zene at 30°C; [M] = 2.0; [RLi] (A); [RLi] ' (O). p

0

1/4

1 6

-2.0h SLOPE 0.25 ^

ί - -

2.2h Ο

Ο

- SLOPE 0.167

-2.4h -3.0

Figure 4.

-2.8

-2.6 log [RLil

-2.4

-2.2

-2.0

Dependence of R on organolithium concentration. Butadiene in ben­ zene at 30°C; [M] = 2.0. p

0

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

MORTON

Current

Status of Anionic

Polymerization

23

I n v i e w o f t h e e v i d e n c e d e s c r i b e d above, and e l a b o r a t e d b e l o w , t h e r e i s no r e a s o n t o - d a y t o r e l a t e t h e r e a c t i o n o r d e r i n the organolithium p o l y m e r i z a t i o n o f t h e dienes t o t h e s t a t e o f a s s o c i a t i o n , a s e x e m p l i f i e d b y t h e scheme d e s c r i b e d b y E q u a t i o n s (2) and ( 3 ) . T h i s appears a l s o t o be t r u e i n t h e case o f s t y r e n e , as i n d i c a t e d b y some r e c e n t r e s u l t s (IS) w h i c h a l s o a p p e a r e l s e ­ where i n t h i s p u b l i c a t i o n . I n o t h e r w o r d s , t h e r e seems t o b e n o b a s i s f o r assuming t h a t o r g a n o l i t h i u m p o l y m e r i z a t i o n s i n nonp o l a r m e d i a p r o c e e d b y t h i s t y p e o f mechanism, i . e . , where o n l y the unassociated species i s a c t i v e . I n f a c t , i t had been p r o p o s e d (19_) sometime ago t h a t i t w o u l d b e more r e a s o n a b l e t o a s s i g n such a c t i v i t y t o the a s s o c i a t e d complex, a n d t h a t t h e l o w f r a c t i o n a l k i n e t i c o r d e r s h a v e no d i r e c t c o n n e c t i o n w i t h t h e association-dissociation equilibrium. 3. M o l e c u l a r A s s o c i a t i o The d a t a i n T a b l e I I c o n f i r m t h e p r e s e n c e o f a n a s s o c i a t i o n b e t w e e n t h e o r g a n o l i t h i u m c h a i n ends o f p o l y s t y r e n e , p o l y i s o p r e n e and p o l y b u t a d i e n e i n n o n - p o l a r m e d i a ( s u c h a s s o c i a t i o n i s a p p a r e n t l y destroyed i n the presence o f s o l v a t i n g s o l v e n t s , such as H ^ - f u r a n ) . F u r t h e r m o r e , t h e above p o l y m e r c h a i n s seem t o b e a s s o c i a t e d as dimers, although s l i g h t l y higher a s s o c i a t i o n numbers a p p e a r t o b e o b s e r v e d f o r p o l y b u t a d i e n e . S i n c e i t was s u s p e c t e d t h a t some c r o s s l i n k i n g was o c c u r r i n g i n t h e c a s e o f t h e p o l y b u t a d i e n e , a s p e c i a l " c a p p i n g " t e c h n i q u e was d e v i s e d t o overcome t h i s d i f f i c u l t y . S o l u t i o n s o f p o l y s t y r y l l i t h i u m were prepared and t h e i r flow times determined. T h e s e c h a i n s were t h e n "capped" w i t h a few u n i t s o f b u t a d i e n e o r i s o p r e n e , a n d t h i s c o u l d b e done v e r y e f f i c i e n t l y i n v i e w o f t h e f a s t " c r o s s o v e r " r e a c t i o n between b u t a d i e n e o r i s o p r e n e and t h e p o l y s t y r e n e l i t h i u m c h a i n ends (2Ό). Hence t h e a s s o c i a t i o n b e h a v i o r o f t h e new c h a i n ends s h o u l d b e t h e same a s t h a t o f p o l y b u t a d i e n y l l i t h i u m , w i t h o u t t h e r i s k o f a c r o s s l i n k i n g r e a c t i o n i n t e r f e r i n g . Some t y p i c a l d a t a ( Z L ) a r e shown i n T a b l e I I I . I t i s apparent from these data t h a t a l l o f the polymers, i n c l u d i n g butadiene, e x h i b i t an a s s o c i a t i o n as dimers, and t h a t t h e r e i s no reason t o expect any h i g h e r s t a t e s o f a s s o c i a t i o n for polyisoprene o r polybutadiene. This i s confirmed not only by the v i s c o s i t y data on t h e a c t i v e v s . t e r m i n a t e d "capped" polymers, b u t a l s o b y t h e f a c t t h a t t h e r e was no s i g n i f i c a n t i n c r e a s e i n v i s c o s i t y when t h e p o l y s t y r y l l i t h i u m was " c a p p e d " b y b u t a d i e n e o r i s o p r e n e , i . e . , a l l t h r e e t y p e s o f c h a i n ends a r e a s s o c i a t e d i n t h e same way, a s d i m e r s . The change i n f l o w t i m e t h a t s h o u l d o c c u r when a d i m e r i c a s s o c i a t i o n o f c h a i n e n d s m i g h t change t o a h i g h e r s t a t e , e . g . , t e t r a m e r i c , was w e l l i l l u s t r a t e d b y d a t a b a s e d o n l i n k i n g p o l y i s o p r e n y l l i t h i u m b y means o f s i l i c o n c h l o r i d e l i n k i n g a g e n t s ('17) r a n g i n g from d i m e t h y l d i c h l o r o s i l a n e t o t e t r a c h l o r o s i l a n e ( s i l i c o n tetrachloride). S u c h d a t a a r e shown i n T a b l e IV. I ti s obvious

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.40 2.40 0.63 0.60

3

[RLi] (xlO )

3.0 3.2 1.5 1.5

[M]

ft

Butadiene

If

Isoprene

Diene*

1567 1899

Polystyryl Lithium 1591 1974 268.9 398.8

158.4 187.5 26.2 38.4

"Capped" P o l y m e r Active Terminated

F l o w Time ( s e c . )

*The a d d e d d i e n e i n c r e a s e d t h e m o l e c u l a r w e i g h t b y 2000, t h u s c a u s i n g t h e o b s e r v e d increase i n the v i s c o s i t y .

Cyclohexane Benzene

Solvent

A s s o c i a t i o n o f P o l y s t y r y l L i t h i u m "Capped" w i t h B u t a d i e n e a n d I s o p r e n e (30°C) ( 2 1 )

TABLE I I I

slight

1.97 2.00 1.98 1.99

δ

Η

8 2 ο

>

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

S

2

3

2

"capped" w i t h

25°C

2340 4800 825 229

1320 2480 500 235

2280 4770 820 229

Terminated

butadiene.

2

v O.20)

MQ = o s m o t i c m o l . w t ,

Linked

F l o w Time ( s e c . )

Active

= s t o i c h i o m e t r i c mol. wt.

3

^Polyisoprene

M

3

S i OU SiCli** CH SiCl (CH ) SiCl

L i n k i n g Agent

(Cyclohexane,

s

1.2 1.5 1.3 0.81

M

1.2 1.6 1.35 0.85

Primary

3.35 6.0 3.6 1.6

5

(xlO" ) Linked

Number A v e . M o l . Wt.

V i s c o s i t i e s of Associated vs. Linked Polyisoprene (17)

TABLE I V

to

1'

Co

I

2

2

Ν)

26

ANIONIC POLYMERIZATION

a t o n c e t h a t t h e t r i - and t e t r a c h l o r o s i l a n e s c a u s e a s h a r p i n c r e a s e i n t h e f l o w t i m e s of the a c t i v e polymers, presumably by l i n k i n g t h e c h a i n ends t o g e t h e r i n t o t r i - and tetra-branched species. The a c t u a l e x t e n t o f l i n k i n g i s , o f c o u r s e , shown b y a c o m p a r i s o n o f t h e MQ v a l u e s o f t h e p r i m a r y end l i n k e d p o l y m e r chains. These v a l u e s a l s o d e m o n s t r a t e t h e w e l l - k n o w n f a c t ( 1 7 ) t h a t the s i l i c o n t e t r a c h l o r i d e cannot e f f i c i e n t l y l i n k p o l y i s o p r e n y l l i t h i u m i n t o a " t e t r a - s t a r " b u t does so i n t h e c a s e o f polybutadienyl lithium. The f a c t t h a t t h e d i f u n c t i o n a l d i m e t h y l d i c h l o r o s i l a n e h a d no e f f e c t on t h e f l o w t i m e o f t h e p o l y i s o p r e n y l l i t h i u m o f f e r s p e r ­ h a p s t h e m o s t c o n v i n c i n g e v i d e n c e t h a t t h e s e c h a i n ends must h a v e been a s s o c i a t e d i n p a i r s p r i o r to the l i n k i n g r e a c t i o n . The c o r r e s p o n d i n g MQ v a l u e s p r o v i d e t h e n e c e s s a r y c o n f i r m a t i o n t h a t t h e l i n k i n g r e a c t i o n ha a c t u a l l take place should a l s b noted that flow times o be u s e d t o d e t e c t t h e p r e s e n c , , chains. One a d d i t i o n a l i t e m o f e x p e r i m e n t a l e v i d e n c e f o r t h e d i m e r i c a s s o c i a t i o n o f p o l y i s o p r e n y l l i t h i u m was p r o v i d e d b y a l i g h t s c a t t e r i n g s t u d y (21), i n n-hexane a t 25°C., w h e r e i t was f o u n d t h a t t h e m o l e c u l a r w e i g h t o f t h e t e r m i n a t e d p o l y m e r was v e r y c l o s e to one-half t h a t of the a c t i v e polymer. A l l of these data seem t o l e a v e no d o u b t t h a t t h e a c t i v e c h a i n ends i n t h e o r g a n o ­ l i t h i u m p o l y m e r i z a t i o n o f s t y r e n e , i s o p r e n e and b u t a d i e n e , i n n o n - p o l a r s o l v e n t s , are a s s o c i a t e d as p a i r s , a t l e a s t a t c h a i n end c o n c e n t r a t i o n s o f Ι Ο " M or l e s s . T h i s c o n c l u s i o n has a l s o been s u p p o r t e d by data o b t a i n e d i n f o u r o t h e r l a b o r a t o r i e s (22, 23, 24, 2 5 ) , b a s e d on t h r e e d i f f e r e n t e x p e r i m e n t a l techniques, v i z . c r y o s c o p y ( 2 2 ) , v i s c o s i t y ( 2 3 , 2 4 ) , and l o w a n g l e l a s e r l i g h t s c a t t e r i n g T 2 5 ). I t must be c o n c l u d e d , t h e r e f o r e , t h a t t h e k i n e t i c scheme p r o p o s e d i n E q u a t i o n s ( 2 ) and ( 3 ) c a n n o t be v a l i d f o r t h e s e systems. A l t h o u g h the seeming correspondence between the h a l f o r d e r k i n e t i c s and t h e d i m e r i c a s s o c i a t i o n i n t h e c a s e o f s t y r e n e m i g h t v a l i d a t e t h e above k i n e t i c scheme, even t h i s h y p o t h e s i s has b e e n r e c e n t l y c o n t r a d i c t e d ( 1 8 ) . Thus F e t t e r s and Young ( 1 8 ) showed t h a t e v e n t h o u g h t h e o r g a n o l i t h i u m p o l y m e r i z a t i o n o f s t y r e n e i n a r o m a t i c e t h e r s , s u c h a s a n i s o l e and d i p h e n y l e t h e r , had p r e v i o u s l y been found t o obey h a l f - o r d e r k i n e t i c s , t h e s e s o l v e n t s d i d have a marked e f f e c t i n d i s r u p t i n g t h e d i m e r i c a s s o c i a t i o n o f t h e c h a i n e n d s . Hence i t i s d o u b t f u l t h a t E q u a t i o n s ( 2 ) and ( 3 ) can be a p p l i e d e v e n i n t h e c a s e o f s t y r e n e . Another c o m p l i c a t i o n i n t r o d u c e d by the a s s o c i a t i v e p r o p e r t i e s of organolithium s o l u t i o n s i n non-polar solvents i s the f a c t that t h e a l k y l l i t h i u m i n i t i a t o r s a r e t h e m s e l v e s a s s o c i a t e d and c a n b e e x p e c t e d t o " c r o s s a s s o c i a t e " w i t h t h e a c t i v e polymer c h a i n ends. Thus some o f o u r s t u d i e s ( 2 6 ) on t h e e f f e c t o f a d d e d e t h y l l i t h i u m on t h e v i s c o s i t y o f ~ ~ p o l y i s o p r e n y l l i t h i u m s o l u t i o n s i n n-hexane s u p p o r t t h e f o l l o w i n g a s s o c i a t i o n e q u i l i b r i u m 2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

MORTON

Current

(C H Li) 2

5

Status of Anionic

6

+ (PLi)

27

Polymerization

(3)

2(PLi.3C H Li)

2

2

5

where P L i r e p r e s e n t s a p o l y i s o p r e n y l l i t h i u m , a n d t h e e q u i l i b r i u m h i g h l y f a v o r s t h e c r o s s - c o m p l e x ( K e ^ 6 ) . Hence i t i s o b v i o u s that such a cross-complexation would a f f e c t t h e k i n e t i c s o f t h e i n i t i a t i o n r e a c t i o n , r e n d e r i n g any such s t u d i e s o f dubious v a l u e . 4.

S t e r e o s p e c i f i c i t y i n Diene P o l y m e r i z a t i o n

A. Chain M i c r o s t r u c t u r e . I n a d d i t i o n t o t h e i r nont e r m i n a t i n g c h a r a c t e r , t h e homogeneous a n i o n i c p o l y m e r i z a t i o n s d i s c u s s e d h e r e i n h a v e one a d d i t i o n a l f e a t u r e , v i z . , c o n t r o l o f chain s t r u c t u r e o f dienes. A l t h o u g h t h i s does n o t seem t o a p p l y to t h e t a c t i c i t y o f thes the geometrical isomeris s t r i k i n g example o f s u c h a n e f f e c t i s , o f c o u r s e , t h e s y n t h e s i s o f a h i g h c i s - 1 , 4 p o l y i s o p r e n e b y l i t h i u m c a t a l y s i s ( 8 ) . I t has. a l r e a d y b e e n shown b y many i n v e s t i g a t o r s ( 2 7 , 28>, 29,"""30) t h a t both the nature o f t h e a l k a l i metal counter-ion and t h e type o f solvent affect the chain unit structure o f the polydienes, the more e l e c t r o p o s i t i v e m e t a l s a n d / o r t h e more s o l v a t i n g ( p o l a r ) solvents favoring t h e s i d e - v i n y l chain u n i t s t r u c t u r e s (1,2 o r 3,4). The e f f e c t o f s u c h s o l v e n t s i n 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 h a s a l s o been c o r r e l a t e d w i t h t h e a b i l i t y o f t h e d i e n e s , e.g., i s o p r e n e , t o c o p o l y m e r i z e w i t h s t y r e n e ( J L ) , a s i l l u s t r a t e d i n T a b l e V. These d a t a a r e consistent with the previously discussed hypothesis that, i n polar TABLE V ( 3 1 ) E f f e c t o f A l k a l i M e t a l I n t i a t o r s and S o l v e n t s on 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 w i t h I s o p r e n e a t 25°C Initiator n-C^HgLi o r L i Na Na Li Na Li n-C^HgLi L i o r n-Ct^HgLi

Solvent H^-furan H^-furan Dimethyl ether Diethyl ether Hydrocarbons T r i e t h y l amine Di phenyl ether Hydrocarbons

Composition o f i n i t i a l

Wt. %

Styrene

a

% 1,4

80 80 75 68 66 60

30 32 36 51 45 52 82 95

30

16

copolymer a t equimolar charge

ratio.

s o l v e n t s , s u c h a s e t h e r s , t h e p r o p a g a t i n g c h a i n ends a r e i o n p a i r s i n e q u i l i b r i u m with free anions. In t h e case o f the much-studied o r g a n o l i t h i u m p o l y m e r i z a t i o n of butadiene and isoprene, t h e e f f e c t o f ethers and other p o l a r s o l v e n t s i s t o change t h e c h a i n s t r u c t u r e f r o m o n e c o n t a i n i n g

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

28

ANIONIC POLYMERIZATION

>90% 1,4 t o one h a v i n g p r e d o m i n a n t l y 1,2 ( o r 3,4) u n i t s . As e x p e c t e d f o r t h e s e homogeneous s y s t e m s , t h e p l a c e m e n t o f t h e c h a i n u n i t s i s random ( 3 2 ) . However, e v e n i n t h e c a s e o f nonp o l a r m e d i a , t h e c h a i n u n i t s t r u c t u r e may be a f f e c t e d b y s u c h f a c t o r s as t h e c o n c e n t r a t i o n o f t h e l i t h i u m . A recent i n v e s t i g a ­ t i o n ( 3 3 ) , u s i n g h i g h r e s o l u t i o n p r o t o n m a g n e t i c r e s o n a n c e , has shown t h a t t h e c i s - t r a n s r a t i o i n b o t h p o l y i s o p r e n e and p o l y b u t a ­ d i e n e i s a f f e c t e d b o t h b y t h e c o n c e n t r a t i o n o f l i t h i u m and b y t h e presence of d i f f e r e n t hydrocarbon s o l v e n t s , the h i g h e s t c i s con­ t e n t b e i n g o b t a i n e d a t t h e l o w e s t l i t h i u m c o n c e n t r a t i o n and i n t h e a b s e n c e o f any s o l v e n t s . T y p i c a l r e s u l t s f o r p o l y i s o p r e n e a r e shown i n F i g u r e 5. I t was a l s o n o t e d t h a t t h e p r o p o r t i o n o f s i d e v i n y l u n i t s i n b o t h p o l y i s o p r e n e and p o l y b u t a d i e n e was n o t n o t i c a b l y a f f e c t e d by these v a r i a b l e s . B. Structure Studie Organolithium Polymerizatio 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 t h e l i t h i u m p o l y m e r i z e d c o n j u g a t e d d i e n e s made i t o f g r e a t i n t e r e s t t o s t u d y t h e s t r u c t u r e o f t h e p r o p a g a t i n g c h a i n ends i n t h e s e s y s t e m s . T h i s has b e e n done, m a i n l y b y p r o t o n m a g n e t i c r e s o n a n c e s p e c t r o ­ s c o p y ( 2 2 , 34, 35) on b u t a d i e n e ( 2 2 , 3 5 ) , i s o p r e n e ( 3 4 , .35 , 3 6 ) , 2.3- d i m e t h y l b u t a d i e n e (J35, 3 6 ) , s e v e r a l p e n t a d i e n e s T j 7 , 38) and 2.4- h e x a d i e n e ( 3 7 , 38). Typical spectra for polybutadienyl l i t h i u m a r e shown i n F i g u r e 6. As c a n be s e e n , t h e " t r a n s p a r e n t " p e r d e u t e r o b u t a d i e n e was u s e d t o b u i l d c h a i n s o f a b o u t 20 u n i t s . T h i s was done i n o r d e r t o a v o i d any e f f e c t s o f i n i t i a t o r o n t h e f i r s t few c h a i n u n i t s (_25). The e f f e c t o f t h e p o l a r medium ( t e t r a h y d r o f u r a n ) on t h e Ύ h y d r o g e n o f t h e c h a i n - e n d u n i t i s t o move t h e p e a k u p f i e l d , f r o m 4.7 ppm t o 3.3 ppm, and t o i n d i c a t e a more d e l o c a l i z e d c a r b o n - l i t h i u m b o n d . T h i s , o f c o u r s e , t i e s i n w e l l w i t h t h e h i g h s i d e - v i n y l c o n t e n t o f t h e p o l y m e r s made i n such media. On t h e b a s i s o f s u c h s p e c t r a o b t a i n e d i n p o l a r and n o n - p o l a r m e d i a , a mechanism was p r o p o s e d ( 3 5 ) i n which the carbonl i t h i u m bond p a r t i c i p a t e d i n an e q u i l i b r i u m between a l o c a l i z e d and d e l o c a l i z e d s t r u c t u r e , t h e former b e i n g r e s p o n s i b l e f o r the h i g h 1,4 c h a i n s t r u c t u r e w h i l e t h e l a t t e r , h i g h l y f a v o r e d i n p o l a r media, accounted f o r the h i g h s i d e v i n y l c h a i n s t r u c t u r e . A s c h e m a t i c o f t h i s h y p o t h e s i s i s shown i n F i g u r e 7. I t s h o u l d be n o t e d t h a t σ-bonding i s r e s t r i c t e d t o t h e α c a r b o n i n n o n - p o l a r m e d i a , b u t i s p r o p o s e d f o r b o t h t h e α and t h e Ύ c a r b o n i n p o l a r m e d i a . T h i s means t h a t t h e σ-π b o n d e q u i l i b r i u m i l l u s t r a t e d i n F i g u r e 7 s h o u l d l e a d t o a c i s - t r a n s e q u i l i b r i u m i n t h e 1,4 p r o p a g a t i n g c h a i n - e n d u n i t s o n l y i n p o l a r m e d i a . T h i s was a c t u a l l y o b s e r v e d (35_), t o g e t h e r w i t h t h e a b s e n c e o f any s u c h i s o m e r i z a t i o n i n n o n - p o l a r m e d i a ( 3 8 ) , a l t h o u g h t h e r e i s some c o n t r o v e r s y ( 3 4 ) on t h i s p o i n t . IrTfact, a cis-trans isomeriza­ t i o n o f t h e 4,1 p r o p a g a t i n g c h a i n - e n d u n i t s o f p o l y i s o p r e n y l

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Current

MORTON

Figure 5.

Status of Anionic

Polymerization

Effect of organolithium concentration on polyisoprene chain structure: undiluted (O); 0.5M in n-hexane (A); 0.5M in benzene O -

d - BENZENE

AT 2 3 C.

6

ε γ £ a CD C 0 [C D J go [ 2 ~ CH = CH-CH J ,, CH -CH = CH-CHg Li C H

3

2

4

6

4

2

OR

2

^ C H (ref.)

l-CHo-CH

6

CH = C H

7

| 2

2

6

-J

5

I

I

kA l_

I PPm

d - T H F AT - 7 0 ° C . e

C

2 ° 5 [

^ C H - C H =CH-CH -CH -CH = CH-CH Li 2

II N o n - p o l a r (J, m e d i a CH ^CH -CH'^7 >CH „.

2

2

2

2

1t ^ ^ S ^ W 3 H - C H = CH-CH -CH ~CH CH 2

2

2

u

:

2

31

Polymerization

γ

H

^ x ^ > ^CH -CH-CH -CH^>>CH

^

2

2

o r 1,4

2

o

Polar ^media ° 1

· · ·

3

^ RS-[CH -CH-S] 2

a

Thietanes ÇH3

RLi

+ CH ~CH 2

fH3

^ ·.·

>

R-[S-CH~CH -CH ] 2

2

CH -4 2

Figure 10.

Organolithium polymerization of cyclic sulfides.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

X

ANIONIC POLYMERIZATION

Ο ex

, -Ο

•2 r-l •s? o δ l

i " 1-5 H s: •S § l α

fell

in Ο — Ο II CM

to

CsJ

ο11

Ο

Q

+

ïi

•Η en

fI

CO

οII

CM

Ο

4

W ·Η W Ο — C O — Ο

I

ο—ο ! CM S J

I" s:

en

jo I CQ

s: Si

+ 1—1

oc

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

MORTON

Current

Status of Anionic

35

Polymerization

Applied Polymer Symposia

Figure 12.

Τ EM

of an a-methylstyrene-dimethylsiloxane-a-methylstyrene triblock copolymer (46).

CH

3

s-O^HgLi + CH =i-CH=CH 2

Cl SiCH CH SiC.1 3

2

2

3

— > ··· — >

2

+ 6CH =CH3\feBr

s-C^Hg [ C H ^ | L i 3

^

2

6 M g B r C l + ( CH =CH ) S i C H C H S i ( CH=CH ) 2

II + 6SiHCl

3

— >

( I )

x

3

2

2

2

( 11 )

3

(Cl SiCH CH ) SiCH CH Si(CH CH SiCl ) ( I I I ) 3

2

2

3

2

2

2

2

3

3

I + III — > ( s-Ci+Hg [ C H ] ) ( S i C H C H ) S i C H C H S i ( C H C H S i ) ( [ C H ] ^ - C ^ H g ) 3

8

χ

Figure 13.

9

2

2

3

2

2

2

2

3

5

8

Synthesis of an 18-arm, star-branched polyisoprene (49).

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9

36

ANIONIC POLYMERIZATION

q CH -CH=CH-CH=CH-CH 3

C

H

+ Li^2 5 )3

3

N >

L

i [CH-CH=CH-CH] - L i 2

3

CsH 58 n

CHo CHo CH. I _ I L i [ C H - C = C H - C H ] [CH-CH=CH-CH] 2=3 [ C H - C H = C - C H ] L i fH

3

à

2

2

X

2

2

x

(CH ) 0 2

2

where χ = 50-200 CHq

CHoY

CHo

CHc

HO- [CH -C= CH-CH ] [CH-CH= CH-CH] —3 [CH -CH=CH-C-CH ] 0 Η 2

2

J,

χ

Z

ΟΗ(ρ-0 ^Ν0Ο) 6

3

NETWORK Figure 14.

Synthesis of uniform polyisoprene networks.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

χ

2.

MORTON

Current

Star-Branched

Status of Anionic

Polymerization

37

Polymers

In recent years a v a r i e t y o f star-branched p o l y s t y r e n e s , p o l y i s o p r e n e s and s t y r e n e - i s o p r e n e b l o c k copolymers have been p r e p a r e d b y F e t t e r s a n d c o w o r k e r s ( 4 3 ) . The s y n t h e t i c r o u t e involved organolithium polymerization followed by l i n k i n g o f the l i t h i u m c h a i n ends, e i t h e r b y a p o l y f u n c t i o n a l reagent such as a c h l o r o s i l a n e o r b y d i v i n y l benzene. I n t h i s way i t was f o u n d p o s s i b l e t o prepare star-branched polymers o f p r e c i s e s t r u c t u r e , w i t h up t o 15 o r more m o n o d i s p e r s e b r a n c h e s , a n d t o examine t h e i r p r o p e r t i e s . A t y p i c a l scheme i s shown i n F i g u r e 13 f o r t h e s y n t h e s i s o f a n 18-arm s t a r p o l y i s o p r e n e . The e f f i c i e n c y o f t h e l i n k i n g r e a c t i o n i s demonstrated b y t h e narrow, s i n g l e peak o f t h e d i s t r i b u t i o n c u r v e i n t h e g e l p e r m e a t i o n c h r o m a t o g r a m ( 4 9 ). Such p r e c i s e polymers hav behavior, c h a i n dimensions Uniform

Polyisoprene

Networks

I t has l o n g been c o n s i d e r e d h i g h l y d e s i r a b l e t o be a b l e t o c o n s t r u c t an e l a s t i c n e t w o r k h a v i n g a r e l a t i v e l y u n i f o r m s i z e o f network chains. T h i s i s b e c a u s e o f t h e random manner i n w h i c h c r o s s l i n k s are introduced during the conventional process o f r u b b e r v u l c a n i z a t i o n , a n d b e c a u s e t h e r e i s no t h e o r e t i c a l r e l a ­ t i o n which can p r e d i c t t h e e f f e c t o f a d i s t r i b u t i o n o f network c h a i n s i z e o n t h e m e c h a n i c a l p r o p e r t i e s o f t h e n e t w o r k . One way t o achieve such a network would be through 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 a s u i t a b l e monomer, e . g . , i s o p r e n e , b y a d i f u n c t i o n a l i n i t i a t o r , w h i c h c o u l d l e a d t o a r e l a t i v e l y m o n o d i s p e r s e α,ωd i f u n c t i o n a l polyisoprene s u i t a b l e f o r l i n k i n g i n t o a uniform network by a p o l y f u n c t i o n a l l i n k i n g agent. S u c h a n e t w o r k h a s r e c e n t l y b e e n a c c o m p l i s h e d (50_, 51), using t h e scheme shown i n F i g u r e 1 4 . S i n c e t h e p o l y i s o p r e n e d i o l s showed a h i g h d e g r e e o f m o n o d i s p e r s i t y i n c h a i n l e n g t h a s w e l l a s d i f u n c t i o n a l i t y , a n d s i n c e s o l - g e l s t u d i e s showed a h i g h e x t e n t o f r e a c t i o n w i t h t h e t r i - i s o c y a n a t e , i t c o u l d b e assumed t h a t t h e r e s u l t i n g n e t w o r k s were o f a h i g h d e g r e e o f u n i f o r m i t y . M e a s u r e ­ ments o f t h e i r m e c h a n i c a l p r o p e r t i e s i n d i c a t e d t h a t t h e s e u n i f o r m e l a s t i c n e t w o r k s e x h i b i t e d b o t h a more i d e a l e l a s t i c b e h a v i o r a s w e l l a s a h i g h e r s t r e n g t h t h a n c o m p a r a b l e random n e t w o r k s .

ABSTRACT

alkali

The current status of homogeneous a n i o n i c p o l y m e r i z a t i o n by metal d e r i v a t i v e s is reviewed along two lines: a) Mechanism and

kinetics.

b)

S y n t h e s i s of unique polymers.

In the case o f the mechanism, a distinction is made between p o l y m e r i z a t i o n in p o l a r and non-polar media. In the former type

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

38

largely

ANIONIC POLYMERIZATION

of s o l v e n t s ( e t h e r s , amines, etc.) it has been proposed t h a t the propagating c h a i n end c o n s i s t s o f an i o n p a i r in e q u i l i b r i u m w i t h the free i o n i c s p e c i e s , both types of c h a i n end being capable o f chain propagation, although the free carbanion is the more r e a c t i v e , by s e v e r a l orders of magnitude. In the case o f nonp o l a r s o l v e n t s (mainly hydrocarbons), the p o l y m e r i z a t i o n s are restricted to the s o l u b l e o r g a n o l i t h i u m systems, and there is no evidence of the i o n s o l v a t i o n necessary f o r i o n i c dissociation. Instead, there is c o n v i n c i n g evidence t h a t the propagating chain ends, in such cases as s t y r e n e , isoprene and butadiene, are a s s o c i a t e d in pairs, as might be expected f o r such p o l a r species in non-polar media. There is no c l e a r - c u t evidence whether the a c t i v e chains are the a s s o c i a t e d or d i s s o c i a t e d s p e c i e s , or b o t h , s i n c e there is no simple relation between the s t a t e of a s s o c i a t i o n an o n e - h a l f to o n e - s i x t h , A brief review is presented o f the unique s y n t h e t i c capa­ bilities inherent in these homogeneous a n i o n i c systems, i n c l u d i n g the f o l l o w i n g macromolecular s t r u c t u r e s : 1) b l o c k copolymers, 2) star-branched polymers, 3) α,ω-difunctional polymers l i n k e d into networks by p o l y f u n c t i o n a l linking agents.

Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9. 10.

11. 12. 13.

Morton, M.; B o s t i c k , Ε.Ε.; Livigni, R. A. Rubber P l a s t . Age, 1961, 42, 397. Morton, M.; F e t t e r s , L. J.; B o s t i c k , Ε. E. J . Polym. Sci., 1963, C1, 311. Morton, M.; B o s t i c k , Ε. Ε.; C l a r k e , R. G. J . Polym. Sci., 1963, A1, 475. H o s t a l k a , Η.; Figini, R. V.; S c h u l z , G. V. Makromol. Chem., 1964,71, 198. H o s t a l k a , H.; S c h u l z , G. V. Z. P h y s i k . Chem. ( F r a n k f u r t ) , 1965, 45, 286. B a t t a c h a r y y a , D. Ν.; Lee, C. I.; Smid, J.; Szwarc, M. Polymer, 1964, 5, 54; J . Phys. Chem., 1965, 69, 612. Morton, M.; F e t t e r s , L. J. Rubber Chem. T e c h n o l . , 1975, 48, 359. S t a v e l y , F. W. and coworkers. Ind. Eng. Chem., 1956, 48, 778. Schue, F.; W o r s f o l d , D. J.; Bywater, S. Can. J . Chem., 1964, 42, 2884. Lundborg, C.; S i n n , H. Makromol. Chem., 1960, 41, 242; S i n n , H.; Lundborg, C.; ibid., 1961, 47, 86. S p i r i n , Yu. L.; Polyakov, D. K.; Gantmakher, A.R.; Medvedev, S.S. J . Polym. Sci., 1961, 53, 233. Bueche, F. J . Polym. Sci., 1960, 45, 267. Morton, M.; P e t t , R. Α.; Fellers, J. F. P r e p r i n t s IUPAC Macromol. Symp. Tokyo, 1966, 1, 69.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2. MORTON 14. 15. 16. 17. 18. 19. 20. 21. 22.

23. 24. 25.

26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

36.

Current

Status of Anionic

Polymerization

39

W o r s f o l d , D. J.; Bywater, S. Can. J. Chem., 1964, 42, 2884. Johnson, A . F.; W o r s f o l d , D. J. J. Polym. Sci., 1965, A3, 449. W o r s f o l d , D. J.; Bywater, S. Macromolecules, 1972, 5, 393. F e t t e r s , L . J.; M o r t o n , M. M a c r o m o l e c u l e s , 1974, 7, 552. F e t t e r s , L . J.; Young, R. N. Polymer P r e p r i n t s , 1980, 21(1), 34. Brown, Τ. L . J. Organomet. Chem., 1966, 5, 191; Adv. Organomet. Chem., 1965, 3, 365. M o r t o n , M.; Ells, F . R. J. Polym. Sci., 1962, 61, 25. M o r t o n , M.; F e t t e r s J. F . Macromolecules G l a z e , W. Η.; H a n i c a k , J. E.; Moore, M. L.; C h a d h u r i , J. J. Organomet. Chem., 1972, 44, 39. A l J a r r a h , M. M.; Young, R. Ν. Polymer, 1980, 21, 119. Makowski, H . S.; Lynn, M. J. Macromol. Chem., 1966, 1, 443. Hernandez, Α.; Semel, J.; B r o e c k e r , H . C.; Zachmann, H. G.; S i n n , H. Makromol. Chem., R a p i d , Comm., 1980, 1, 75. M o r t o n , M.; P e t t , R. Α.; Fellers, J. F. M a c r o m o l e c u l e s , 1970, 3, 333. F o s t e r , F . C.; B i n d e r , J. L . Adv. Chem. Ser., 1957, 17, 7. M o r i t a , H.; T o b o l s k y , Α. V . J. Am. Chem. Soc., 1957, 79, 5853. T o b o l s k y , Α. V.; Rogers, C. E . J. Polym. Sci., 1959, 40, 73. S t e a r n s , R. S.; Forman, L . E . J. Polym. Sci., 1959, 41, 381. T o b o l s k y , Α . V.; R o g e r s , C. E. J. Polym. Sci., 1959, 38, 205. S a n t e e , E . R.; M a l o t k y , L . O.; M o r t o n , M. Rubber Chem. T e c h n o l . , 1973, 46, 1156. Rupert, J. P h . D . D i s s e r t a t i o n , U n i v e r s i t y of A k r o n , 1975. Schue, F.; W o r s f o l d , D. J.; Bywater, S. J. Polym. Sci., 1969, B7, 821; Macromolecules, 1970, 3, 509. M o r t o n , M . ; Sanderson, R. D . ; S a k a t a , R. J. Polym. Sci., 1971, B9, 61; Macromolecules, 1971, 6, 181. M o r t o n , M.; Sanderson, R. D.; S a k a t a , R.; F a l v o , L. A. Macromolecules, 1973, 6, 186.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

40 37. 38. 39. 40. 41. 42.

43. 44. 45. L. 46. J.

ANIONIC POLYMERIZATION M o r t o n , M.; F a l v o , L . Α.; F e t t e r s , L . J. J. Polym. Sci., 1972, B10, 561. M o r t o n , M.; F a l v o , L . A . M a c r o m o l e c u l e s , 1973, 6, 190. B r o w n s t e i n , S.; Bywater, S.; W o r s f o l d , D. J. M a c r o m o l e c u l e s , 1973, 6, 715. Lachance, P . ; W o r s f o l d , D. J. J. Polym. Sci., Polym. Chem. Ed., 1973, 11, 2295. Holden, G.; M i l k o v i c h , R. U . S. P a t . 3 , 2 3 1 , 6 3 5 ; B e l g . 627,652 (Shell), a p p l . J a n . 1962. M o r t o n , M. E n c y c l o p e d i a of Polymer S c i e n c e and Technology, V o l . 15, John Wiley and Sons, New Y o r k , 1971, p . 508. M o r t o n , M. J. Polym. Sci., Polym. Symp. No. 60, 1977, 1. F e t t e r s , L . J.; 2, 453. M o r t o n , M a u r i c e ; Kammereck, R u d o l f F.; F e t t e r s , J. M a c r o m o l e c u l e s , 1971, 4, 11. M o r t o n , M a u r i c e ; K e s t e n , Y.; Fetters, L. J. Polym. Sci., A p p l . Polym. Symp. No. 26, 113 ( 1 9 7 5 ) .

Received March 5, 1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3 Synthesis of Controlled Polymer Structures R A L P H MILKOVICH ARCO Polymers, Inc., Newtown Square, PA 19073

The development o has led to the emergenc trial importance, the most significant being a family of thermoplastic elastomers. These unique elastomers are presently commercialized by Shell Chemical Company as Kratons and by Phillips Chemical Company as Solprenes. Their uniqueness i s the result of deliberate design of the polymeric structure and composition. The evolution of anionically synthesized block polymers started with an exciting academic discovery and culminated as an industrial economic reality. The recognition of the significance of the electron transfer reaction from sodium to naphthalene in an ether solution, as shown i n Figure 1, led to utilization of the electron transfer to a polymerizable monomer. Figure 2 shows a key step toward the realization that quantitative polymerization reactions could be carried out leading to the synthesis of pure block polymers of controlled structure (1, 2).. The design and synthesis of two-phase block polymers utilizing the " l i v i n g " polymer concept, will be discussed. A more recent development, MACROMER®, a new tool that expands our capability for synthesizing controlled polymer structures, will also be discussed. A n i o n i c Two-Phase Block Polymers

Styrene-butadiene elastomers were developed because of this country's need to have an independent reliable source of rubber during World War II. This rubber was developed as a random copolymer containing about 25% styrene, and i s still prepared by emulsion technology. Today it i s also being prepared by solution anionic chemistry. The random copolymer has no useful properties u n t i l it i s compounded and vulcanized as i n the production of automobile tires, for example. If the basic ingredients of this copolymer (the styrene and butadiene monomeric units) are segregated into segments of homopolymers, several interesting structures can be realized, i . e . : 0097-6156/81/0166-0041$05.00/0 ©

1981 American Chemical Society

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

42

ANIONIC POLYMERIZATION abbabaabbbabaaabb RANDOM COPOLYMER aaaaaaaabbbbbbbbb BLOCK COPOLYMER

T h i s A-B b l o c k polymer can be synthesized a n i o n i c a l l y . The "A" or polystyrene segment i n t h i s case i s prepared by the r e a c t i o n of styrene monomer with b u t y l l i t h i u m i n an appropriate s o l vent. The r a t i o of styrene to i n i t i a t o r used w i l l r e s u l t i n a polystyrene segment of predetermined molecular weight and a very narrow molecular weight d i s t r i b u t i o n . Once a l l the styrene monomer i s q u a n t i t a t i v e l y consumed, a s t y r y l anion w i l l be present at the end of the polyme the presence of any othe monomer u n i t s , or the butadiene, are added to the " l i v i n g poly styrene s o l u t i o n and the polymer chains continue to grow i n s i z e as the polydiene polymer segment i s formed. Again, the r a t i o of monomer added to a c t i v e species present w i l l c o n t r o l the s i z e of the diene polymer segment. A l s o , once the monomer i s consumed, a " l i v i n g " polymer s p e c i e s i s r e t a i n e d - t h i s time i t i s a d i e n y l anion. T h i s anion can be f u r t h e r reacted with a l a r g e v a r i e t y of chemicals as shown i n F i g u r e 3. In t h i s example styrene-isoprene i s being used i n s t e a d of styrene-butadiene. The f i r s t r e a c t i o n i s a d i r e c t termination of the styrene-isoprene terminal group with a protonating reagent to form a d i b l o c k ( 3 ) . In the second r e a c t i o n t h i s d i b l o c k can be f u n c t i o n a l i z e d by r e a c t i n g i t with a v a r i e t y of m a t e r i a l s . Simple f u n c t i o n a l groups such as carboxyl or hydroxyl groups can be introduced. This anion can a l s o be used as the b a s i s f o r o b t a i n ing t y p i c a l v i n y l - t y p e a c t i v i t y . Macromonomers of t h i s type are trademarked MACROMER® by CPC I n t e r n a t i o n a l (_4, 5) . Another r e a c t i o n that can be c a r r i e d out i s a l i n k i n g r e a c tion. I t i s a terminating r e a c t i o n where one uses a d i - or t r i h a l i d e and forms a t r i - or t e t r a - b l o c k polymer (3)· The most obvious r e a c t i o n i s simply to add more styrene and convert the isoprene anion to a s t y r y l anion and grow i t to a d e s i r e d s i z e and form the styrene-isoprene-styrene t r i b l o c k of which we are more f a m i l i a r i n a l l the t h e o r e t i c a l work that has been reported (6, 7) . The l a s t example employs a b i f u n c t i o n a l monomer, divinylbenzene, to form miniblocks of divinylbenzene as i t r e a c t s with a number of d i b l o c k s i n t o what i s c a l l e d a s t a r block c o n f i g u r a t i o n (8). The a c t i v e a n i o n i c s i t e s are now on the divinylbenzene. These m a t e r i a l s are then terminated by protona t i n g agents to o b t a i n the f i n a l product. The number of arms or d i b l o c k s that u n i t e d i n t o a s t a r b l o c k of t h i s type i s c o n t r o l l e d by the r e l a t i v e amounts of the d i b l o c k to the divinylbenzene.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

MILKOVICH

Controlled

Figure 1.

Polymer

Structures

Formation of sodium naphthalene complex.

Na

+

+ *CH=CH

2[*CH-CH2] Na A

+

—Na

+

2

—

—

+ [*CH-CH ]~Na 2

~CHCH C H C H " Na —-POLYMERIZATION +

2

2

l Figure 2.

I ^

'

Φ

Initiation of polymerization of styrene by electron transfer.

S-I®Li®

— — -

S - I - H DIBLOCK

S-I®Li®

Β

-

S-I-R

Z

-

S-I-Z-l-S

-

S-I-S

»

(S-I) (DVB)

S-I®Li®

—

X

S-I®Li® S-I®Li®

X

5

P

V

B

x

MACROMER® TRIBLOCK

TRIBLOCK

Y

STAR BLOCK

R * POLYMERIZABLE FUNCTIONAL GROUP XZX = DIBROMOPROPANE

Figure 3.

Chemical modification of styrene-diene living block polymer.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

+

44

ANIONIC POLYMERIZATION

Professor F e t t e r s a t the U n i v e r s i t y of Akron has i n d i c a t e d that s t a r b l o c k s having over 15 arms can be s y n t h e s i z e d . He has reported s t u d i e s c o n t r a s t i n g d i f f e r e n c e s i n p r o p e r t i e s of the s t a r b l o c k c o n f i g u r a t i o n over other types of polymer c o n f i g u r a t i o n s (9) . The course and some e f f e c t s of these v a r i o u s s y n t h e t i c steps i n the s y n t h e s i s of b l o c k polymers have been i n v e s t i g a t e d v i a GPC a n a l y s i s . The very small hump on the l e f t i n F i g u r e 4 represents the polystyrene r e s i d u e or b l o c k segment. I t i s there because the added diene may have contained i m p u r i t i e s that terminated a small amount of the p o l y s t y r e n e . I t forms a good d e t e c t i n g area to i n d i c a t e the p o s s i b l e s i z e of that segment. The next hump on the shoulder i s the d i b l o c k and the l a r g e r peak i s the t r i b l o c k or l i n k e d m a t e r i a l . I f the t r i b l o c k were prepared by a d d i t i o n of styrene monomer i n t o an b l o c k r a t h e tha linkin t i o n , the hump would no small change i n molecula weight , symmetrical curve that we are more accustomed to seeing i n the l i t e r a t u r e . However, the presence of tljis d i b l o c k peak could be d e s i r a b l e and very b e n e f i c i a l . Some t r i b l o c k s are d i f f i c u l t to thermally process. Perhaps the presence of some of the d i b l o c k m a t e r i a l w i l l be a b e n e f i t i n a i d i n g the p r o c e s s i n g of the mater i a l depending on i t s composition, molecular weight, and intended application. The GPC a n a l y s i s i n F i g u r e 5 shows the r e s u l t of a s y n t h e s i s of a s t a r b l o c k polymer. In t h i s case the v a r i o u s steps and components of a s t a r b l o c k s y n t h e s i s are seen more c l e a r l y . The f i r s t peak on the l e f t i s the p o l y s t y r e n e peak (some of i t was terminated when the isoprene was added to i t ) and the remainder grew to a peak the s i z e of the second one, the d i b l o c k . The presence of the d i b l o c k i s again due to i m p u r i t i e s when the divinylbenzene was added, or the technique used was not h i g h l y r e f i n e d . The f i n a l peak shows that i t i s s u b s t a n t i a l l y higher i n molecular weight than the d i b l o c k , and that i t i s c e r t a i n l y more than double i t s molecular weight. I t has been estimated to have about 10 or 12 arms and i s an example of how one can u t i l i z e chemistry to develop d i f f e r e n t c o n f i g u r a t i o n s of b l o c k polymers. The compositional and two-phase morphological r e l a t i o n s h i p s of "A-B" b l o c k s , the "A-B-A" and s t a r b l o c k s have been studied intensively. I t has been demonstrated that there i s a s u b s t a n t i a l d i f f e r e n c e between random copolymers and b l o c k polymers, and t h i s d i f f e r e n c e i s based s o l e l y on the a r c h i t e c t u r a l arrangement of the monomeric u n i t s . One of the most important d i f f e r e n c e s i s that one Tg i s observed i n the random copolymer, which i s r e l a t e d to the o v e r a l l composition of the polymer. The b l o c k polymer has been shown to have two Tg's - one f o r p o l y s t y r e n e and one f o r the polydiene segment, and that these T g s are not a f f e c t e d by the composition of the b l o c k copolymer. Since we can now s y n t h e s i z e l a r g e q u a n t i t i e s of these pure block polymers, more d e t a i l e d p h y s i c a l s t u d i e s can be c a r r i e d out. The two T g s observed i n f

f

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

MILKOVICH

Controlled

POLYSTYRENE

Polymer

Structures

STYRENE/ ISOPRENE DIBLOCK

STYRENE / ISOPRENE STAR POLYMER

ELUTION VOLUME

Figure 5.

GPC of a starblock: Shell Dutch Patent 646,835, Table III, Example B; Shell Canadian Patent 716,645, Table III, Example B.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

46

ANIONIC POLYMERIZATION

block polymers i n d i c a t e the presence of two d i s t i n c t incompatible e n t i t i e s . We know that when polystyrene and a polydiene are blended together, two Tg's are observed. However, the mixture i s a l s o opaque because the polymers are thermodynamically incompati b l e . The block polymers, on the other hand, are o p t i c a l l y c l e a r , i n d i c a t i n g that i f two d i s t i n c t polymers are present as one molec u l e , then one polymer segment must be d i s p e r s e d very e f f i c i e n t l y i n the other to the extent that i t does not s c a t t e r l i g h t . After much d i s c u s s i o n with my c o l l a b o r a t o r , Dr. Geoffrey Holden, we decided that the morphological concept as represented i n F i g u r e 6 e x p l a i n s the p h y s i c a l phenomena observed. The polystyrene segments are aggregates as spheres t i e d together by the e l a s t i c p o l y diene segments. Dr. Dale Meier, when he f i r s t saw some of the data generated on the b l o c k polymers a t the S h e l l Rubber D i v i s i o n L a b o r a t o r i e s i n Torrence California abl t c a l c u l a t what one would say the the s i z e s of the domains pe has been proven over the years that we were c o r r e c t i n our assumpt i o n s f o r the composition s t u d i e d a t that time. We now know that the composition of b l o c k polymers can g r o s s l y a l t e r t h e i r morphology. Although the "A-B" or styrene-diene d i b l o c k s have c e r t a i n advantages over random copolymers, i t i s a f t e r the second p l a s t i c phase i s added to form at l e a s t an "A-B-A" or a s t a r - t y p e block polymer that the p h y s i c a l c h a r a c t e r i s t i c s of b l o c k polymers as thermoplastic elastomers are r e a l i z e d as i s i n d i c a t e d i n F i g u r e 7. The p o l y s t y r e n e molecular weights are a l l about 26,000 and the polyisoprene molecular weights are about 94,000. These examples were s e l e c t e d from patents (3, 8) to provide some c o n t i n u i t y i n showing these p h y s i c a l e f f e c t s . Although the d i b l o c k has essent i a l l y no t e n s i l e strength (about 100 p s i ) , the t r i b l o c k c o n f i g u r a t i o n (formed by adding styrene to a " l i v i n g " d i b l o c k then t e r minating i t ) gives a polymer s t r u c t u r e showing a dramatic i n c r e a s e i n t e n s i l e s t r e n g t h to about 3000 p s i . The t r i b l o c k prepared by a c o u p l i n g r e a c t i o n using dibromopropane i n t h i s case shows a lower t e n s i l e strength which i s probably due to the presence of d i b l o c k m a t e r i a l . On the other hand, the s t a r b l o c k shows the highest t e n s i l e s t r e n g t h . T h i s could be due to the f a c t that there are a multitude of thermal c r o s s l i n k s f o r the same polymer molecule because of the number of blocks l i n k e d , whereas the t r i b l o c k has only one p o l y s t y r e n e segment a t each end of the molecule. For these p h y s i c a l e f f e c t s to occur i n the most e f f i c i e n t manner, s e v e r a l c r i t e r i a must be s a t i s f i e d : • The polymer segments, f i r s t of a l l , must be thermodynamically incompatible. • The polymer segments must be l i n k e d together chemically. • The Tg's of the two polymer segments must be substantially different.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3.

MILKOVICH

Controlled

Polymer

47

Structures

• The molecular weight d i s t r i b u t i o n of the p l a s t i c or high Tg polymer segment must be very narrow to achieve the maximum phase s e p a r a t i o n . • The molecular weight of the p l a s t i c dispersed phase must be above a c r i t i c a l molecular weight. • To o b t a i n high s t r e n g t h , more than one high Tg polymer segment per molecule must be i n a d i s ­ persed phase. G r a f t Polymers MACROMER® (10) i s a trademark by CPC I n t e r n a t i o n a l of a new f a m i l y of monomers. Because they are synthesized v i a a n i o n i c chemistry, t h e i r molecular weight i s c o n t r o l l e d by the r a t i o of monomer to i n i t i a t o r an weight d i s t r i b u t i o n s . Th that have been i n v e s t i g a t e d are polystyrene, polydiene, and blocks of the two (5, 10). Some of the t y p i c a l MACROMER® f u n c t i o n a l groups that were examined are shown i n Figure 8. These are shown to i n d i c a t e the wide v a r i e t y of f u n c t i o n a l groups that a r e u s e f u l f o r v a r i o u s p o l y m e r i z a t i o n mechanisms ( 4 ) . There are some problems a s s o c i a t e d with the s y n t h e s i s of MACROMERS® when the s t y r y l anion i s reacted with c e r t a i n func­ t i o n a l monomers such as:

M a l e i c Anhydride

H + C= I

/

H

C I

or

O'TVTS)

Vinvl vmyi Chloroacetate

H * I * * / Cl-C-C-0-C = C I H I w H O H f

With maleic anhydride or v i n y l c h l o r o a c e t a t e the v a r i o u s p o i n t s of a n i o n i c a t t a c k are i n d i c a t e d by the arrows - double bonds, α-hydrogen, carbonyls, and of course, the h a l i d e . I t was found that some of these s i d e r e a c t i o n s could be avoided by simply r e ­ a c t i n g the very b a s i c s t y r y l anion with a chemical which would r e s u l t i n a lower base s t r e n g t h anion. T h i s weaker anion w i l l not r e a c t with the carbonyl and α-hydrogens and other p o s s i b l e s i t e s of the f u n c t i o n a l i z i n g r e a c t i o n and l i m i t i t s e l f to simple h a l i d e r e a c t i o n s . T h i s was done by using ethylene or propylene oxide as shown i n Figure 9 f o r the s y n t h e s i s of a polystyrene MACROMER® with a methacrylate f u n c t i o n a l group (8). T h i s intermediate r e a c t i o n step now opens up a whole new opportunity and challenge

American Chemical Society Library 1155Polymerization; 16th St N.McGrath, W. J.; In Anionic ACS Symposium Series; American Chemical Washington. D. C. Society: 20036Washington, DC, 1981.

ANIONIC POLYMERIZATION

Figure 6.

Conceptual morphological

TENSILE STRENGTH AT BREAK (PSI) DIBLOCK

ELONGATION AT BREAK (PSI)

95

1300

TRIBLOCK

(S)

3000

1100

TRIBLOCK

(XZX)

2100

1300

3800

I 100

STAR BLOCK

POLYSTYRENE MOLECULAR WEIGHT ~ 26,000 g/MOLE POLYISOPRENE MOLECULAR WEIGHT~94,000 g/MOLE

Figure 7.

Physical properties of styrene-isoprene block polymers.

O L E F I N — CH = :

—

CH

CH=CH I CH

CH I ESTERS: — 0 — C — 0 = II 0

2

2

ETHER: — 0 — H C = C H

CH

2 2

2

V-HC=CH

2

HYDROXY:

A

EPOXY: — H C — C H

3

— 0 — C — C H = C H — C —OH II II 0 0

3

STYRENE: — ^

3

OH OH I I —CH—CH

2

2

Figure 8.

Typical MA CROMER

functional groups.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3.

MILKOVICH

Controlled

Polymer

Structures

49

to the polymer s c i e n t i s t . I t o f f e r s the means to prepare funct i o n a l i z e d polymers that can be used i n a v a r i e t y of polymerizat i o n mechanisms, i . e . f r e e - r a d i c a l , i o n i c , condensation, or c o o r d i n a t i o n systems to prepare c o n t r o l l e d polymer s t r u c t u r e s , r a t h e r than being l i m i t e d i n the case of the styrene and dienes to a n i o n i c chemistry. Since t h i s work was d i r e c t e d toward p o t e n t i a l commercializat i o n , i t u t i l i z e d the v a r i o u s modes of copolymerization or p o l y m e r i z a t i o n systems - s o l u t i o n , suspension, and l a t e x . Some of the primary comonomers that were i n v e s t i g a t e d i n the MACROMER® s t u d i e s were the a c r y l a t e s , v i n y l c h l o r i d e , styrenes, ethylene, ethylene/propylene, a c r y l o n i t r i l e , and N,N-dimethylacrylamide. A concept has been presented f o r preparing a l a r g e macromonomer - the molecula 10,000 20-30,000 / mole - with a s i n g l e unsaturate chain. How can i t be prove big, i t has f u n c t i o n a l i t y , w i l l r e a c t and how w i l l i t r e a c t with t y p i c a l commercially a v a i l a b l e low molecular weight monomeric species? The best t e s t f o r f u n c t i o n a l i t y would be i n a copolymerizat i o n study. A p o l y s t y r e n e w i t h a methacrylate terminal f u n c t i o n a l group was prepared. A review of r e l a t i v e r e a c t i v i t y r a t i o s i n d i cated that v i n y l c h l o r i d e r e a c t s very r a p i d l y with methacrylates. Therefore, a copolymerization of the p o l y s t y r e n e terminated w i t h a methacrylate f u n c t i o n a l group i n v i n y l c h l o r i d e would be a good t e s t case, and one should observe the disappearance of the MACROMER® i f the r e a c t i o n i s followed by using GPC a n a l y s i s . The GPC t r a c e s that were taken during the course of a cop o l y m e r i z a t i o n r e a c t i o n of v i n y l c h l o r i d e and an 11,000 MW p o l y styrene methacrylate-terminated macromonomer are shown i n F i g u r e 10. The very sharp peak on the l e f t i s the MACROMER® and the curves on the r i g h t are the growing copolymer peaks. One sees that by the time 55% of the v i n y l c h l o r i d e conversion i s completed that the MACROMER® i s v i r t u a l l y completely copolymerized and that i t s peak has almost disappeared. T h i s at l e a s t i n d i c a t e s the existence of a f u n c t i o n a l MACROMER®. A second t e s t was done by using b u t y l a c r y l a t e as the comonomer as shown i n F i g u r e 11. The r e a c t i v i t y r a t i o s i n t h i s case are such that the methacrylate f u n c t i o n a l i t y would r e a c t slower with a c r y l a t e s than with v i n y l c h l o r i d e . As p r e d i c t e d the b u t y l a c r y l a t e i s a t 62% conversion before the MACROMER® peak i s s i g n i f i c a n t l y diminished. These data add v a l i d i t y to the hypothesis that the placement of s i d e chains i n the backbone i s dependent on the t e r m i n a l group of the macromonomer and the r e l a t i v e r e a c t i v i t y of i t s comonomer. An a d d i t i o n a l o b s e r v a t i o n concerning MACROMER® i s that i t w i l l not homopolymerize, and thus d i d not have to be s t a b i l i z e d . I t was demonstrated that MACROMER® w i l l copolymerize with conventional monomers i n a p r e d i c t a b l e manner as determined by the r e l a t i v e r e a c t i v i t y r a t i o s . The copolymer equation:

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

50

ANIONIC POLYMERIZATION Θ sec —Bu •

• CH CH-

sec —Bu -

-CH CH-f—CH CH

2

-CH

® CHo CHo

CH L i

2

Θ Θ 2

CH CH 0Li

2

2

CH

3

2

=

0 II C— C

—CI

0

sec— Bu R

Figure 9.

I 36

I

I

I

MACROMER

I

I

I

I

synthesis.

I

I

I

I

1

1

1

34 32 30 28 26 24 22 ELUTION VOLUME (5 MLS/COUNT) AFTER INJECTION

Figure 10. GPC of SI IMA MACROMER/VCl (—); 9.8% VCl conversion ( ); 55.4%

product: 4.6% VCl conversion VCl conversion (— · —).

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3.

MILKOVICH

Controlled

dMi dM

Polymer

51

Structures

"riMi/M +l 2

2

can be reduced

to the approximation

Γ2Μ2

when Mi i s very low i n molar c o n c e n t r a t i o n . Thus MACROMER® (Mi) c o p o l y m e r i z a t i o n with other monomers ( M 2 ) can be d e s c r i b e d by r values and the monomer feed compositions to g i v e the equation 2

dM dMi/M In a monofunctional MACROMER® having a 15,000 molecular weight, the f u n c t i o n a l i t y can be about one-half of a weight per cent of the t o t a l monomer i t s e l f . T h e r e f o r e , c o n s i d e r i n g a co­ polymer charge on a weight b a s i s of 50/50 wt %, the feed w i l l only c o n t a i n about 3 χ 10" moles of MACROMER®. T h i s i s unique i n i t s e l f ; the o p p o r t u n i t y to c a r r y out c o p o l y m e r i z a t i o n s t u d i e s where one monomer i s of such a low r e l a t i v e c o n c e n t r a t i o n to the other. T h i s presents an o p p o r t u n i t y to study areas of copolymer­ i z a t i o n that may have been questioned i n the past, i . e . : " W i l l a f u n c t i o n a l group attached to a very l a r g e polymer chain r e a c t with the same p r e d i c t a b i l i t y as a t y p i c a l low molecular weight monomer?" and "Are c o n v e n t i o n a l r e l a t i v e r e a c t i v i t y r a t i o s s t i l l v a l i d when one of the monomers i s i n a very low molar concentration? it Some experimental data that were obtained through a s e r i e s of p o l y m e r i z a t i o n s t u d i e s with a methacrylate-terminated MACROMER® with v i n y l c h l o r i d e are shown i n F i g u r e 12. The A l f r e y - G o I d f i n g e r equation was used to c a l c u l a t e the copolymer composition f o r com­ p a r i s o n to the a c t u a l copolymer composition as estimated from GPC a n a l y s i s . A reasonably c l o s e agreement was achieved of the a c t u a l and the t h e o r e t i c a l copolymer compositions, which i n d i c a t e s that the r values are i n the r e g i o n of r i = 10 and r = 0.1. Thus f a r , t h i s phase of t h i s r e s e a r c h area has shown: • That very l a r g e macromonomers having a s e l e c t e d f u n c t i o n a l group can be q u a n t i t a t i v e l y s y n t h e s i z e d . • That these macromonomers do indeed copolymerize with c o n v e n t i o n a l monomers. • That the r e a c t i v i t y of the MACROMER® i s p r e d i c t a b l e . A p o l y s t y r e n e with a f u n c t i o n a l i t y such as a methacrylate group copolymerized w i t h a mixture of e t h y l and b u t y l a c r y l a t e should y i e l d a g r a f t s t r u c t u r e meeting the c r i t e r i a of a thermo­ p l a s t i c elastomer as shown i n F i g u r e 13. The data i n t h i s f i g u r e show that as the MACROMER® content i s i n c r e a s e d , the t e n s i l e 3

2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

52

ANIONIC POLYMERIZATION

I I

36

34

32

30

28

26

24

22

ELUTION VOLUME (5 MLS/COUNT) AFTER INJECTION

Figure 11.

GPC of S11MA MACROMER/BA product: 36.2% (—); 62.3% BA conversion ( ).

COPOLYMER COMP. M,

CONVERSION

M| FEED

M

(%)

(%)

(%)

12.2

35 50

23.7

M,

2

BA conversion

ACTUAL (o)

THEORY (b)

(%)

(%)

85.9

68.7

75.6

8.3

77.9

83.5

84.4

4.6

31.9

87.4

90.9

9.8

83.0

89.4

(o), CALC. FROM GPC ANALYSIS (b), CALC. FROM THEORY FOR

r, =10,

r

2

=0.1

(M,)

+1 ALFREY-GOLDFINGER Eq.

d(M,)

(M,)

(M )

d(M )

(M )

(M| )

2

2

2

(M ) 2

Figure 12.

MACROMER

[Mj]/vinyl chloride [M,] copolymerization.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

+r

2

3.

MILKOVICH

Controlled

Polymer

Structures

53

strength i s i n c r e a s e d . A y i e l d strength i s observed a t 30% MACROMER® content and i n c r e a s e s with i n c r e a s i n g MACROMER® content. Of p a r t i c u l a r i n t e r e s t i s the f a c t that at 30 or 40% MACROMER® contents, t e n s i l e strengths of 1700 and 2200 p s i at a reasonable e l o n g a t i o n percentage are obtained. These are e q u i v a l e n t to tens i l e strengths obtained f o r t y p i c a l v u l c a n i z e d a c r y l i c rubbers. These rubbers were prepared v i a a suspension p o l y m e r i z a t i o n process. We have achieved t h i s thermoplastic e f f e c t here by using the phase s e p a r a t i o n of a p o l y s t y r e n e MACROMER® and the a c r y l i c rubber m o i e t i e s . Again, i t should be s t r e s s e d that these m a t e r i a l s are completely o p t i c a l l y transparent, being c l e a r and c o l o r l e s s . F i g u r e 14 shows the t y p i c a l s t r e s s - s t r a i n behavior and y i e l d p o i n t s of the a c r y l i c based thermoplastic elastomer that one observes i n the styrene-diene-styrene t r i b l o c k polymers. The use of MACROMER® has enabled th s y n t h e s i f i n t e r e s t i n mate r i a l s that can be c a r r i e p o l y m e r i z a t i o n technology graf ture i s synthesized having p a r a l l e l p h y s i c a l e f f e c t s to b l o c k polymer s t r u c t u r e s . a-a-a-a-a-a-a-a-a-a-a b b b b b b b b The s i d e chains are of a predetermined s i z e and composition v i a a n i o n i c chemistry and based on the r e a c t i v i t y placed at the t e r minus of the MACROMER® and the comonomer with which i t w i l l be r e a c t e d . The number of s i d e chains per backbone and a d i s t a n c e apart can be reasonably estimated. T h i s d i s t a n c e between s i d e chains must be s u f f i c i e n t that the backbone can manifest i t s Tg. Considering the e a r l i e r comments of molar c o n c e n t r a t i o n of the MACROMER® i n a t y p i c a l copolymerization r e c i p e , there w i l l not be many MACROMERS® per backbone on a s t a t i s t i c a l b a s i s . Since these m a t e r i a l s are incompatible and are chemically l i n k e d , one would expect to have a domain type s t r u c t u r e as conc e p t u a l i z e d i n F i g u r e 15. P h y s i c a l blends of these polymers were prepared and again, as i n the case of the styrene-dienes, were opaque and cheesy. The copolymer g r a f t s t r u c t u r e s were o p t i c a l l y c l e a r and tough, i n d i c a t i n g that one of the phases i s e f f i c i e n t l y d i s p e r s e d i n the matrix of the other and that the d i s p e r s e d phase i s below the wavelength of l i g h t . The same p h y s i c a l phenomenon a s s o c i a t e d with b l o c k polymers i s shown to occur i n a g r a f t polymer. The use of MACROMER® technology i s another s y n t h e t i c t o o l that the polymer s c i e n t i s t has i n being able to prepare a wide v a r i e t y of new types of polymer s t r u c t u r e s which could be of some p o t e n t i a l commercial i n t e r e s t , such as:

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

54

ANIONIC POLYMERIZATION

MACROMER W T %

YIELD STR (PSI)

TS (PSI) 1300

730

30

270

1700

550

40

1200

2200

400

45

1800

2500

350

50

2700

300 0

240

25

Ε LONG (%)

SI6MA Ml E A B A

Figure 13.

3000

CROMER/acrylic

copolymers.

Ί

-J 100

I I I 200 300 400 % ELONGATION

I 500

I 600

Figure 14. Stress-strain curves for 1:1 ethyl acrylate .butyl acrylate copolymers with S16MA MACROMER at 30%, 40%, and 50% MACROMER contents.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

MILKOVICH

Controlled

Polymer

Structures

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

56

ANIONIC POLYMERIZATION

• Low Tg d i s p e r s e d phase i n high Tg matrix (impact p l a s t i c s ) , • High Tg d i s p e r s e d phase i n low Tg matrix (thermoplastic elastomers), • High Tg d i s p e r s e d phase i n c r y s t a l l i n e polymer matrix, • Low Tg d i s p e r s e d phase i n c r y s t a l l i n e polymer matrix, • High Tg d i s p e r s e d phase i n high Tg matrix, • Hydrophilic-hydrophobic, and • C o n t r o l l e d branch homopolymer. The high Tg d i s p e r s e d phase i n a high Tg matrix i s q u i t e i n t e r ­ e s t i n g and would be appealing to r h e o l o g i s t s . Copolymers with v i n y l c h l o r i d e or a c r y l o n i t r i l e were found to have enhanced a b i l i t y to be pressed i n t t h i sheets I fact th a c r y l o n i t r i l e styrene MACROMER through an I n s t r o n c a p i l l a r hydrophobic m a t e r i a l was developed by r e a c t i n g the p o l y s t y r e n e MACROMER® w i t h Ν,Ν-dimethylacrylamide which developed i n t o a very i n t e r e s t i n g hydrogel (11). The f i n a l example i s perhaps of more of a t h e o r e t i c a l i n t e r e s t . Thus, i t i s now p o s s i b l e with t h i s technique t o prepare c o n t r o l l e d branched homopolymers and compare t h e i r s o l u t i o n , p h y s i c a l , and melt p r o p e r t i e s to s t r a i g h t l i n e a r homopolymers. Conclusion The development of a n i o n i c chemistry has placed a number of powerful t o o l s i n the hands of the polymer chemist. Polymer molecules of predetermined molecular weight, molecular weight d i s t r i b u t i o n , composition, and c o n f i g u r a t i o n can now be synthe­ s i z e d nearing the p u r i t y o f simple organic molecules. Controlled polymeric s t r u c t u r e s have been r e a l i z e d that are h i g h l y d e s i r a b l e as models to advance t h e o r e t i c a l s t u d i e s and indeed have v a s t economic values to i n d u s t r y as p r o f i t a b l e consumer items. Literature Cited 1.

R. Milkovich, M. Szwarc and M. Levy, J.A.C.S. 78, 2656 (1956).

2.

R. Milkovich, Polymerization Initiated by Electron Transfer to Monomer, M.S. Thesis, S.U.N.Y., Syracuse, NY (1957).

3.

R. Milkovich, Process for the Preparation of Elastomeric Block Copolymer, Republic of S. A f r i c a 227,164 (1964).

4.

R. Milkovich, M. T. Chiang, U.S. Patents claiming functional groups and methods of preparation: 3,842,050 (1974); 3,842,057 (1974); 3,842,058 (1974); 3,842,059 (1974); 3,846,393 (1974); 3,862,098 (1975); 3,862,101 (1975); 3,862,102 (1975).

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3.

MILKOVICH

Controlled

Polymer

Structures

57

5.

R. M i l k o v i c h , M. T. Chiang, Polymerizable D i b l o c k , e t c . U.S. Patent 3,842,146 (1974); a l s o 3,862,267 (1975).

6.

G. Holden and R. M i l k o v i c h , U.S. Patent 3,231,635; Process f o r the P o l y m e r i z a t i o n of Block Polymers (1966).

7.

R. M i l k o v i c h , G. Holden, Ε. T. Bishop and W. R. H e i n d r i c k s , Block Copolymers and Compositions Containing Them, Republic of S. Africa 284,464 (1964).

8.

R. M i l k o v i c h , Block Polymers and Process f o r Preparing Them, Canadian Patent 716,645 (1965).

9.

B. J . Bauer and L. J . F e t t e r s , Rubber Chem. and Technol., 51, 406 (1978).

10.

R. M i l k o v i c h , M. T Thermoplastic G r a f

11.

R. M i l k o v i c h , M. T. Chiang, Chemically Joined, Phase Sepa r a t e d , S e l f Cured H y d r o p h i l i c Thermoplastic G r a f t Copolymers and T h e i r P r e p a r a t i o n , U.S. Patents 3,928,255 (1975) and 4,085,168 (1978).

RECEIVED M a r c h 5,

Chiang

Chemically Joined Phase Separated

1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4 Synthesis of Model Macromolecules of Various Types via Anionic Polymerization PAUL REMPP, EMILE FRANTA, and JEAN HERZ Universite Louis Pasteur, Centre de Recherches sur les Macromolecules, 6 rue Boussingault, 67038 Strasbourg Cedex, France

The need for well persity and of known structur est in structure-properties relationship i n dilute solution as well as in the bulk. A great variety of methods have been attempted, to synthesize so-called model macromolecules or tailor made polymers-over the past 20 years. The techniques based on anionic polymerization, when carried out in aprotic solvents, have proved best suited for such synthesis, because of the absence of spontaneous transfer and termination reactions that characterize such systems. The "living" polymers obtained are fitted at chain end with carbanionic sites , which can either initiate further polymerization, or react with various electrophilic compounds, intentionally added to achieve functionalizations. Another advantage of anionic polymerizations i s that difunctional initiators are available, yielding linear polymers fitted at both chain ends with carbanionic sites. In this paper we shall review the various utility of anionic polymerization to the synthesis of tailor made well defined macromolecules of various types. 1

I - HOMOPOLYMERS I -General When a s u i t a b l e monomer i s polymerized by u t i l i z i n g an e f f i c i e n t a n i o n i c i n i t i a t o r where the r a t e constant of i n i t i a t i o n i s higher than the rate constant of propagation, a l i n e a r polymer e x h i b i t i n g a sharp Poisson-type molecular weight d i s t r i b u t i o n i s obtained.-!*! Furthermore, the number average molecular weight of the polymer i s determined by the molar r a t i o of monomer-toi n i t i a t o r used. The range of molecular weights e a s i l y a c c e s s i b l e by a n i o n i c polymerization techniques extends from 1000 to 500,000 approximately. For very low molecular weights the i n i t i a t i o n r e a c t i o n can no more be considered instantaneous and broadening of the molecular weight d i s t r i b u t i o n r e s u l t s . For very high molecul a r weights the amount of i n i t i a t o r i n v o l v e d i s very small, and a p r e c i s e c o n t r o l of the molecular weight of the polymer becomes difficult. T h i s i s true even when d r a s t i c measures are taken to 0097-6156/8l/0166-0059$05.00/0 © 1981 American Chemical Society In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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avoid a l l kinds of " k i l l i n g " i m p u r i t i e s i n the r e a c t o r as w e l l as i n the monomer and solvent used. Some a n i o n i c a l l y polymerizable monomers are l i s t e d below, roughly i n the order of i n c r e a s i n g e l e c t r o a f f i n i t y , i . e . i n the order of i n c r e a s i n g tendency to polymerize a n i o n i c a l l y . Table I A n i o n i c a l l y Polymerizable Monomers L i s t e d i n the Order of Increasing E l e c t r o a f f i n i t y

• • • • • • • • • • • • • • • •

Dimethylaminostyrene Methoxystyrenes Methylstyrenes (ρ, o, m, and α-methyl) Vinylmesitylene Styrene p-chlorostyrene Vinylnapthalenes Isoprene ( p i p e r y l e n e · · .) Butadiene V i n y l p y r i d i n e s (2 or 4) A l k y l m e t h a c r y l a t e ( a l k y l = C i to Ci8 Acrylonitrile Oxiranes Lactones Cyanoacrylates Thiiranes

The choice of the i n i t i a t o r best s u i t e d f o r a given polymer­ i z a t i o n i s of great importance. Fast i n i t i a t i o n r e q u i r e s that the n u c l e o p h i l i c i t y of the i n i t i a t o r be high enough f o r the mono­ mer to be polymerized. However too high a n u c l e o p h i l i c i t y some­ times increases the p r o b a b i l i t y of side r e a c t i o n s . For example, cumylpotassium i s w e l l s u i t e d to i n i t i a t e the polymerization of styrene; i f the same i n i t i a t o r i s used f o r methylmethacrylate polymerization, a s i d e r e a c t i o n on the e s t e r carbonyl i s l i k e l y to occur. Diphenylmethylpotassium, a much weaker n u c l e o p h i l e , i s s t i l l an e f f i c i e n t i n i t i a t o r f o r methylmethacrylate, without s i d e r e a c t i o n , but i t i s unable to i n i t i a t e r a p i d l y and q u a n t i t a t i v e l y the polymerization of styrene. Monomers devoid of p o l a r groups g e n e r a l l y undergo a n i o n i c polymerization i n a p r e d i c t a b l e manner. With p o l a r monomers sometimes side r e a c t i o n s occur during the process; t r a n s f e r r e ­ a c t i o n s i n the case of a c r y l o n i t r i l e , or propylene oxide, and even more so with a l k y l a c r y l a t e s ; d e a c t i v a t i o n s (or " k i l l i n g " ) r e a c t i o n s i n the case of halogen s u b s t i t u t e d styrene or dienes. A n i o n i c " l i v i n g " p o l y m e r i z a t i o n can be c a r r i e d out i n p o l a r as w e l l as i n non-polar s o l v e n t s , provided they are a p r o t i c , and c o n t a i n no s t r o n g l y e l e c t r o p h i l i c f u n c t i o n s . The r a t e of chain growth i s much f a s t e r i n p o l a r than i n non-polar s o l v e n t s .

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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However, f o r the purpose of s y n t h e s i s , they are almost equival e n t . Only f o r diene polymerizations, when high 1,4 u n i t cont e n t s are u s u a l l y d e s i r e d , i t i s of importance to choose nonp o l a r solvents (and L i as a counter ion) as the polymerization media. 2 - F u n c t i o n a l Polymers The terminal carbanionic s i t e s of " l i v i n g " polymers can be reacted w i t h v a r i o u s e l e c t r o p h i l i c compounds of y i e l d (22 For the s y n t h e s i s of s t a r shaped macromolecules, two approached are p o s s i b l e : - One can r e a c t a monofunctional " l i v i n g " precursor polymer s t o i c h i o m e t r i c a l l y with a m u l t i - f u n c t i o n a l d e a c t i v a t o r , whereby the number of branches ρ should be given by the

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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f u n c t i o n a l i t y of the d e a c t i v a t o r . ^ S t a r molecules with up to 12 branches were obtained by t h i s method.12>24 s i m i l a r l y s t a r molecules with A branches l i n k e d to a c e n t r a l lead atom were made by r e a c t i n g the precursor w i t h t e t r a ( d i p h e n y l e t h y l e n e ) lead.^5 The s t a r polymer obtained using these methods are w e l l defined, provided the e f f i c i e n c y of the antagonist f u n c t i o n s i s satisfactory. Pb(C H4-C-C H )4 6

6

5

CH2 -

Star-shaped macromolecules have a l s o been synthesized by u s i n g the raonofunctional a t o r f o r the polymerizatio v i n y l monomer. A small c r o s s l i n k e d nodule i s formed, which i s connected with the ρ chains that have c o n t r i ­ buted to i t s initiation.l§ I t turns out that f l u c t u a ­ t i o n s on the value of ρ w i t h i n a sample remain r a t h e r small, and consequently the s t a r polymers obtained by t h i s method can a l s o be considered as tailor-made p o l y ­ mers. Recently s t a r molecules with deuterium l a b e l e d c e n t r a l nodule have been synthesized according to the same method.27

The s y n t h e s i s of extremely high molecular weight s t a r polymers has been achieved by another method.M F i r s t d i v i n y l ­ benzene i s reacted at very low c o n c e n t r a t i o n with an a n i o n i c i n i ­ t i a t o r (sec. BuLi) to y i e l d a suspension of poly-DVB nodules f i t ­ ted w i t h numerous I n i t i a t i n g s i t e s . Then styrene i s added, and each i n i t i a t i n g s i t e should give y i e l d to a branch. However, the p o l y d i s p e r s i t y of the samples obtained i s very h i g h . 6 - Model Networks Model networks are t r i d i m e n s i o n a l c r o s s l i n k e d polymers whose e l a s t i c a l l y e f f e c t i v e network chains are of known length and of narrow molecular weight d i s t r i b u t i o n . The techniques used to synthesize such networks are d e r i v e d from those developed f o r the s y n t h e s i s of s t a r shaped macromolecules, whereby the i n i t i a ­ tor used must be b i f u n c t i o n a l i n s t e a d of monofunctional.-12»22 The f i r s t step i n v o l v e s p r e p a r a t i o n of a b i f u n c t i o n a l " l i v i n g " precursor, of known molecular weight and low p o l y d i s p e r s i t y . C r o s s l i n k i n g can be achieved e i t h e r by the a d d i t i o n of s t o i c h i o ­ m e t r i c amounts of a m u l t i f u n c t i o n a l e l e c t r o p h i l i c d e a c t i v a t o r , or by adding s m a l l a small amount of a b i f u n c t i o n a l monomer (such as DVB or ethylene g l y c o l d i m e t h a c r y l a t e ) , the p o l y m e r i z a t i o n of which w i l l be i n i t i a t e d by the c a r b a n i o n i c s i t e s of the precur­ sor. In e i t h e r case, the precursor chains become the e l a s t i c a l l y e f f e c t i v e chains of the networks. The experimental c o n d i t i o n s

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can be chosen as to keep the p r o p o r t i o n of d e f e c t s (pendant chains, loops. · . ) very low, and to prevent s y n e r e s i s from occ u r i n g upon c r o s s l i n k i n g . The l a t t e r method was a l s o used to synthesize model networks with l a b e l e d branch points52>2i f o r s t r u c t u r e determination by means of X-ray s c a t t e r i n g or neutron s c a t t e r i n g . A n i o n i c polymerization i s a l s o w e l l s u i t e d f o r the synt h e s i s of networks e x h i b i t i n g a known p r o p o r t i o n of d e f e c t s d i s t r i b u t e d a t random i n the network.22 By using a given p r o p o r t i o n of a monofunctional i n i t i a t o r , together with the b i f u n c t i o n a l one, i t i s p o s s i b l e to introduce a known amount of pendent chains, the average length of which i s h a l f that of the e l a s t i c chains. By c r o s s l i n k i n g a mixture of two ( o r more) " l i v i n g " prec u r s o r s of d i f f e r e n t length, one can introduce a t w i l l a d d i t i o n a l p o l y d i s p e r s i t y among th est f o r a d e t a i l e d i n v e s t i g a t i o l a t i o n s h i p s i n model-networks. I I - BLOCK AND GRAFT COPOLYMERS I - Block Copolymers F a s t growing I n t e r e s t i n block copolymers o r i g i n a t e s from the i n t r a m o l e c u l a r phase separation, which i s r e s p o n s i b l e f o r some unique p r o p e r t i e s i n v o l v i n g many p o t e n t i a l a p p l i c a t i o n s . In t h i s f i e l d again major progress has been accomplished due to the a v a i l a b i l i t y of the " l i v i n g " a n i o n i c polymerization techniques. A r a t h e r obvious method to synthesize block copolymers i s to use a " l i v i n g " precursor as the a n i o n i c i n i t i a t o r f o r the p o l y m e r i z a t i o n of second monomer.22»M However, t h i s method r e q u i r e s that the i n i t i a t i o n r e a c t i o n be f a s t , q u a n t i t a t i v e and f r e e of s i d e r e a c t i o n s . T h i s means that the n u c l e o p h i l i c i t y of the carbanionic s i t e s should be s u f f i c i e n t to attack the second monomer added; i n other words, the monomers have t o be added i n the order of i n c r e a s i n g e l e c t r o a f f i n i t y . On the other hand, i t sometimes happens that the nucleop h i l i c i t y of the end standing precursor carbanion has to be decreased to prevent side r e a c t i o n s . An easy way to achieve that i s an intermediate a d d i t i o n of 1, l-diphenylethylene.-2-5 T h i s procedure i s used e s p e c i a l l y when the second monomer to be added i s a m e t h a c r y l i c e s t e r , t o prevent attack of the e s t e r carbonyl.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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I f the experimental c o n d i t i o n s have been chosen properly the molecular weight and the composition of the sample can be chosen a t w i l l , and the f l u c t u a t i o n s i n composition as w e l l as the p o l y d i s p e r s i t y i n molecular weight remain small. T h i s method a p p l i e s to a great number of systems some of which are l i s t e d below. Table I I Some Block Copolymers Obtained by A n i o n i c P o l y m e r i z a t i o n Techniques •

Two amorphous g l a s s y blocks polystyrene - polydimethylaminostyrene polystyrene - p o l y v i n y l p y r i d i n e (2 or 4) polystyrene poly-2-vinylpyridin

•

One g l a s s y and styrene) polystyrene polystyrene polystyrene polystyrene -

one elastomeric block ( o r poly-a-methyl-

( o r poly-a-methylstyrene) - polyisoprene - polybutadiene -polybutylmethacrylate (or polyvinylmesitylene)polydimethylsiloxanee polystyrene - polypropylene s u l f i d e polymethylmethacrylate - polybutylmethacrylate p o l y v i n y l p y r i d i n e - polypropylene s u l f i d e

•

One g l a s s y and one c r y s t a l l i n e block polystyrene - polyethylene oxide polystyrene - polycaprolactone ( p o l y p r o p i o l a c t o n e ) polymethylmethacrylate - polycaprolactone

•

One c r y s t a l l i n e and one e l a s t o m e t r i c block polyisoprene ( o r polybutadiene) - polyethylene polycaprolactone - polydimethylsiloxane

•

oxide

Two c r y s t a l l i n e blocks polyethylene oxide - polycaprolactone

The s y n t h e s i s of t r i b l o c k copolymers B-A-B can be achieved by means of a b i f u n c t i o n a l i n i t i a t o r : a bifunctional α,ω-dicarbanionic poly-A precursor i s formed, and i s used i n a second step as the i n i t i a t o r f o r the p o l y m e r i z a t i o n of monomer B, w i t h the same c o n d i t i o n s to be observed as above. A number of e f f i c i e n t b i f u n c t i o n a l i n i t i a t o r s are commonly used, i n p o l a r solvent media. Recent work, c a r r i e d out i n s e v e r a l laboratories2é~3fi aimed a t the p r e p a r a t i o n of e f f i c i e n t b i f u n c t i o n a l i n i t i a t o r s s o l u b l e i n non p o l a r s o l v e n t s . Such systems 9

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are e s p e c i a l l y u s e f u l when a polydiene (elastomeric) center block i s wanted, as i n Kraton-type thermoplastic elastomers. Another way of s y n t h e s i z i n g B-A-B t r i b l o c k copolymers i s to use a c o u p l i n g r e a c t i o n . — Monocarbanionic poly-B precursor i s used to i n i t i a t e the polymerization of A. The l i v i n g two block copolymer i s then reacted s t o i c h i o m e t r i c a l l y with an e f ­ f i c i e n t b i f u n c t i o n a l c o u p l i n g agent, such as dibromo-p-xylene or d i m e t h y l d i c h l o r o s i l a n e , or even phosgene. T h i s c o u p l i n g r e a c t i o n y i e l d s the t r i b l o c k copolymers. Another way of s y n t h e s i z i n g block copolyers i t to have two polymers which possess mutually r e a c t i n g chain ends. A p i c ­ turesque example i s the mutual d e a c t i v a t i o n of " l i v i n g " c a t i o n i c p o l y t e t r a h y d r o f u r a n and of " l i v i n g " a n i o n i c polystyrene.2? There are however many other examples of r e a c t i o n s between ω-functional polymers m u l t i b l o c k copolymers, p r e c u r s o r s . F u n c t i o n a l i z a t i o n can be achieved a n i o n i c a l l y , or by polycondensation, or by using t r a n s f e r r e a c t i o n s . However, the a n i o n i c techniques are the only method to achieve an adequate molecular weight c o n t r o l and a low p o l y d i s p e r s i t y . Chemically u n l i k e polymers are incompatible, and i t sometimes happens that the r e a c t i o n medium i s heterogeneous at the beginning. However, once some block copolymer i s formed i t a c t s as a " c o m p a t i b i l i z e r " and the r e a c t i o n medium g r a d u a l l y be­ comes homogeneous. Many examples of such r e a c t i o n s could be quoted. A recent one i s the h y d r o s i l y l a t i o n r e a c t i o n c a r r i e d out between a polystyrene f i t t e d at a chain end with v i n y l s i l a n e groups, and an α,ω-dihydrogenopolydimethylsiloxane. T h i s process i s c a r r i e d out at high c o n c e n t r a t i o n and i t y i e l d s polystyrenep o l y d i m e t h y l s i l o x a n e - p o l y s t y r e n e block copolymers.12 H

I ! Polystyrene -C-Si-CH-CHo

II

I +

I

HSi-OPDMS-OSi-H

I

I

c a t a l y s t , e.g.

Polystyrene

^PtClfc

ι ι CH-Si-CH2-CH2-Si-0-fPDMS|-OSi-CH2-CH2-Si- | PStyrene PI

'

There are a l s o cases where a f u n c t i o n a l polymer i s r e q u i r e d to i n i t i a t e the p o l y m e r i z a t i o n of a second monomer.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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obvious example of t h i s procedure i s given by the p o l y m e r i z a t i o n of a lactam ( p y r r o l i d o n e ) i n the presence of a polymer f i t t e d at a chain end a c y l l a c tarn f unctions.^ >-> Lactams as w e l l as Leuchs anhydrides ( o x a z o l i d i n e diones) polymerize according to a soc a l l e d " a c t i v a t e d monomer" process onto a "promotor" f u n c t i o n . In recent years s e v e r a l attempts have been made to pre­ pare polymers possessing chain end f u n c t i o n s capable of g i v i n g r i s e to f r e e r a d i c a l or to c a t i o n i c s i t e s . A i T h i s research has been mostly aimed at extending the p o s s i b i l i t i e s of synthesis of block copolymers, i n which only one of the blocks i s obtained a n i o n i c a l l y . The s y n t h e s i s of ω-hydroperoxy polymers^ has a l ­ ready been mentioned. Peroxy-or peranhydride f u n c t i o n s have a l s o been introduced i n t o polymer chains.42 Subsequent r a d i c a l poly­ m e r i z a t i o n of a second monomer r e s u l t s i n block copolymers. R e a c t i o n of l i v i n y i e l d s a polymer chain groups. Upon a d d i t i o n of s i l v e r hexafluoroantimonate an end standing oxocarbenium s i t e i s formed, that can be used to i n i t i ­ ate the c a t i o n i c p o l y m e r i z a t i o n of THF.42 2 - G r a f t Copolymers The s y n t h e s i s of g r a f t copolymers can be achieved e i t h e r by " g r a f t i n g from" or by " g r a f t i n g onto" processes.23 The former method seems more v e r s a t i l e , but i t does not allow an adequate s t r u c t u r e c o n t r o l , and i t o f t e n y i e l d s r a t h e r p o l y d i s p e r s e samples. On the contrary " g r a f t i n g onto" methods allow a p r e c i s e c o n t r o l of the s i z e and of the number of g r a f t s , but i t i s only a p p l i c a b l e to a l i m i t e d number of systems. " G r a f t i n g from" r e a c t i o n s T h i s method r e q u i r e s f i t t i n g a backbone chain with metal organic s i t e s , capable of i n i t i a t i n g the polymerization of another monomer, to y i e l d the g r a f t s . M e t a l a t i o n of a polymer-23>A4,45 backbone has been performed by v a r i o u s procedures: d i r e c t m e t a l a t i o n , metal halogen interchange, ad­ d i t i o n of organometallies s u b s t i t u t i o n r e a c t i o n s i n v o l v i n g l i t h i u m - o r g a n i c compounds a c t i v a t e d with TMEDA. Once the back­ bone i s metalated i t o f t e n aggregates and becomes i n s o l u b l e . Therefore when a monomer i s added to form g r a f t s , the i n i t i a t i o n process i s g e n e r a l l y slow and i t does not occur q u a n t i t a t i v e l y . T h i s induces l a r g e f l u c t u a t i o n s on the length of the g r a f t s , and i t prevents any e s t i m a t i o n of the number of g r a f t s a c t u a l l y form­ ed. A s p e c i a l case of " g r a f t i n g from" r e a c t i o n i s the chem­ i c a l m o d i f i c a t i o n that can be induced on a metalated backbone c h a i n by micromolecular reagents. A recent example^ of such r e ­ a c t i o n s i s the o x i d a t i o n of metalated s i t e s on a polystyrene chain, y i e l d i n g v i n y l p h e n o l u n i t s . I t has been shown that i n t h i s case m e t a l a t i o n ( u s i n g BuLi, TMEDA) i s l o c a t e d on the phenyl rings.

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" G r a f t i n g onto" r e a c t i o n s T h i s g r a f t i n g method i s based upon the r e a c t i v i t y of the carbanionic s i t e s of a " l i v i n g " polymer onto e l e c t r o p h i l i c funct i o n s d i s t r i b u t e d along the backbone chain.42 L i v i n g polystyrene or polydienes could thus be g r a f t e d onto backbones bearing e s t e r f u n c t i o n s , or anhydride f u n c t i o n s , or epoxide g r o u p s . — Also b e n z y l h a l i d e functions!!>4? are w e l l s u i t e d f o r such r e a c t i o n s ; they can be obtained by c a t i o n i c chloromethylation of polystyrene under m i l d conditions.12 N i t r i l e and p y r i d i n e groups were a l s o shown to r e a c t under proper c o n d i t i o n s with " l i v i n g " polymer carbanions of high n u c l e o p h i l i c i t y . 2 2 Backbone and g r a f t s can be c h a r a c t e r i z e d independently, and the g r a f t copolymers obtained can be quoted t h e r e f o r e as model macromolecules. I t was shown a l s o that g r a f t i n g occurs at random^, and that f l u c t u a t i o n the molecular weight d i s t r i b u t i o homothetic to that of the backbone chain. Very high molecular weights can be a t t a i n e d e a s i l y by such " g r a f t i n g onto" r e a c t i o n s , even though the extent of g r a f t i n g i s l i m i t e d by s t e r i c f a c t o r s . Recently t h i s g r a f t i n g methods has been used to synthes i z e a m p h i p h i l i c g r a f t copolymers i n which h y d r o p h i l i c g r a f t s are l i n k e d to a hydrophobic backbone. P a r t l y chloromethylated polystyrene i s used to d e a c t i v a t e e i t h e r raonofunctional " l i v i n g " polyethylene o x i d e l i or monofunctional " l i v i n g " p o l y v i n y l p y r i dine. In the l a t t e r case subsequent q u a t e r n i z a t i o n y i e l d s polyelectrolyte grafts.1! As already mentioned, w e l l defined comb-like homopolymers can be made by the same technique!2>U; here a l i v i n g polystyrene i s reacted on a p a r t l y chloromethylated polystyrene backbone, whereby random d i s t r i b u t i o n of the g r a f t s along the chain occurs. I t was a l s o s u c c e s s f u l l y demonstrated that one can synt h e s i z e comb polymers with g r a f t s d i s t r i b u t e d at r e g u l a r i n t e r v a l s along the c h a i n . H e r e the backbone i s a "segmented" p o l y styrene with e l e c t r o p h i l i c f u n c t i o n s remaining on the hinges, obt a i n e d by a chain extension r e a c t i o n ; monofunctional " l i v i n g " polystyrene i s then reacted with the functions remaining on the hinges, to produce the g r a f t s . I t has been e s t a b l i s h e d that the placement of the g r a f t s ( r e g u l a r or at random) does not i n f l u e n c e s i g n i f i c a n t l y the morphology of the comb polymer. CONCLUSION A n i o n i c polymerization techniques have c o n t r i b u t e d to a very l a r g e extent to the development of tailor-made molecules of v a r i o u s types. The long l i f e time of the a c t i v e s i t e s i s a f a c t o r of d e c i s i v e importance f o r such synthesis: i t enables one to choose a t w i l l the molecular weights of the polymers to be made, and i t ensures narrow molecular weight d i s t r i b u t i o n of the samples. I t i n v o l v e s the p o s s i b i l i t y of f u n c t i o n a l i z a t i o n s at one or more chain ends, and of coupling r e a c t i o n s with b i f u n c -

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

REMPP ET AL.

Synthesis of Model

Macromolecules

69

t i o n a l e l e c t r o p h i l i c d e a c t i v a t o r s . I t permits subsequent i n i t i a t i o n of a second monomer to y i e l d block copolymers. The occurence of d i f u n c t i o n a l i n i t i a t o r s g i v e s r i s e to f u r t h e r a p p l i c a t i o n s , e x p e c i a l l y f o r the s y n t h e s i s of t r i b l o c k copolymers and of model networks. The main l i m i t a t i o n of these methods i s the rather small number of monomers that undergo a n i o n i c polymerizations without t r a n s f e r or t e r m i n a t i o n steps. N e v e r t h e l e s s , r e s e a r c h on the v a r i o u s a p p l i c a t i o n s of a n i o n i c p o l y m e r i z a t i o n as an e f f i c i e n t t o o l f o r the s y n t h e s i s of t a i l o r made polymers has been pursued very a c t i v e l y over the past 20 y e a r s , and i t has y i e l d e d a great number o f r e s u l t s of s i g n i f i c a n c e which has a l s o opened a broad f i e l d of p o t e n t i a l a p p l i c a t i o n s .

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

M. S. M. Y.

Szwarc, Advances in Polym. Sci. 12, 127 (1966). Bywater, Progress in P o l y . Sci. 4, 27 (1974). Morton, L. J. F e t t e r s , Macroraol. Rev. 2, 71 (1967). N i t a d o r i , E . F r a n t a , P. Rempp, Makromol. Chem. 179, 927 (1978). M. Schmitt, E . F r a n t a , D. F r o e l i c h , P. Rempp, to be p u b l i s h e d (Makromol. Chem.). P. Chaumont, J. Herz, P. Rempp, Europ. Polym. J. 15, 537 (1979). J. Brossas, G. Clouet, C. R. Acad. S c i . C-280, 1459 (1975). G. B e i n e r t , G. W e i l l , t o be published. M. Winnik, D. H. Richards, to be published. J. M. C a t a l a , G. R e i s s , J. Brossas, Makromol. Chem. 178, 1249 (1977). M. Morton, L. J. F e t t e r s , J. Inomata, D. Rubio, R. Young, Rubber Chem. Techn. 49, 303 (1976). B. V o l l m e r t , J. X. Huang, Makromol. Chemie, 182 (1981). D. G e i s e r , H. Hocker, Polymer B u l l t e i n , 2, 591 (1980). G. H i l d , A. Kohler, P. Rempp, Eur. P o l . J. , 16, 525 (1980). J. S. H i g g i n s , K. Dogson, J. S. Semlyen, Polym. 20, 553 (1979). J. C. G a l i n , M. G a l i n , Makromol. Chem. 160, 321 (1972). C. Strazielle, J. Herz, Europ. Polym. J. 13, 223 (1977). J. C. G a l i n , Martenot, to be published. S. Bywater, Adv. Polym. S c i . 30, 90 (1979). B. J. Bauer, L. J. F e t t e r s , Rubber Chem. Techn. 51. 406 (1978). J. Pannel, Polym. 12, 558 (1971), 13, 2 (1972). F . Candau, E . F r a n t a , Makromol. Chem., 149, 41 (1971). M. Hert, J. Herz, C. Strazielle, Makromol. Chem., 160 (1972).

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23. J. E . L. Roovers, S. Bywater, Macroraol. 5, 384 (1972), 7, 443 (1974). 24. N. Hadjichristidis, A. Guyot, L. J. F e t t e r s , Macromol. 11, 668 (1978). 25. G. B e i n e r t , J. Herz, Makromol. Chem. 181, 59 (1980). 26. D. J. Worsfold, J. G. Zilliox, P. Remmp, Can. J. Chem. 47, 3379 (1969). 27. A. Kohler, J. G. Zilliox, P. Rempp, J. Polacek, I . K o s s l e r , Europ. Polym. J. 8, 627 (1972). 28. H. Eschweg, W. Burchard, Polym. 16, 180 (1975), Makromol. Chem. 173, 235 (1973). 29. J. Herz, P. Rempp, W. Borchard, Adv. Polym. S c i . , 26, 105 (1978). 30. P. L u t z , C. Ipcot, G. H i l d , P. Rempp, Brit. Polym. J., 151 (1977). 31. A. B e l k e b i r - M r a n i Europ. Polym 32. J. B a s t i d e , C. P i c o t , S. Candau, J. Polym. S c i . , A-2, 17, 1441 (1979). 33. P. Rempp, E . F r a n t a , Pure and A p p l . Chem. 30, 229 (1972). 34. L. J. F e t t e r s , J. Polym. Sci., C-26, 1 (1969). 35. D. F r e y s s , P. Rempp, H. Benoit, Polym. L e t t e r s , 2, 217 (1964). 36. R. P. Ross, H. W. Jacobson, W. H. Sharkey, Macromol. 10, 287 (1977). 37. E . A. Mushina, L. Muraviova, T. Samedova, B. A. K r e n t s e l , Europ. P o l y . J., 15, 99 (1979). 38. G. B e i n e r t , P. L u t z , E . F r a n t a , P. Rempp, Makromol. Chem., 179, 551 (1978). 39. Y. Yamishita, e t al. J. Polym. S c i . B-8, 481 (1979). 40. P. Chaumont, B. B e i n e r t , J. Herz, P. Rempp, Polymer, 22, 663 (1981). 41. T. Souel, F. Schue, M. Abadie, D. H. Richards, Polym., 18, 1292 (1978). 42. G. R i e s s , F . P a l a c i n , IUPAC Symp. P r e p r i n t s - H e l s i n k i , 1, 123 (1970). 43. E. F r a n t a , J. Lehmann, L. R e i b e l , S. Penczek, J. Polym. Sci., C-56, 139 (1976). 44. G. Clouet, J. Brossas, Makromol. Chem. 180, 867 (1979). 45. A. J. Chalk, A. S. Hay, J. Polym. S c i . , A-1(7), 691, 1359 (1969). 46. J. M. C a t a l a , J. Brossas, Polym. B u l l . 2, 137 (1980). 47. Y. G a l l o t , P. Rempp, J. Parrod, Polym. L e t t e r s , 1, 329 (1963). 48. P. T a k a k i , R. Asami, M. Pizuno, Macromol., 10, 604 (1977). 49. D. Rahlves, J. Roovers, S. Bywater, Macromol. 10, 604 (1977). 50. F . Candau, P. Rempp, Makromol. Chem., 122, 15 (1969). 51. F . Candau, F. Afshar-Taromi, P. Rempp, Polym., 18, 1253 (1977). 52. J. Selb, Y. G a l l o t , Polym. 20, 1268, 1273 (1979).

RECEIVED June 5,

1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5 Relationship of Anion Pair Structure to Stereospecificity of Polymerization S.

BYWATER

Division of Chemistry, National Research Council of Canada, Ottawa, CanadaK1AO R 9

Spectroscopi active centers i meric model compounds, for example, of butadiene or isoprene show the existence of variable proportions of cis and trans forms at equilibrium dependent on counter-ion and solvent. The new active center formed immediately on monomer addition may not, however, be in its equilibrium configuration. In this case, the rate of isomerization to the stable form becomes important in microstructure determination if it is assumed that the structure is "frozen-in" at the next monomer addition. Charge distribution and counter-ion position also change with reaction conditions, which must have some influence on the proportion of 1,4 and vinyl units in the polymer. Particular examples of correlations between ion-pair properties and polymer microstructure are discussed.

One of the i n t e r e s t i n g features of anionic polymerization i s the v a r i a t i o n in m i c r o s t r u c t u r e of the polymers formed under different conditions. Both v i n y l and diene monomers show these e f f e c t s , most markedly with a c r y l a t e s , butadiene and isoprene and p a r t i c u l a r l y with l i t h i u m as counter-ion. E a r l y models for s t e r e o s p e c i f i c i t y are e s s e n t i a l l y s t a t i c ones, for example with the 1ithium/hydrocarbon/isoprene system, the formation of a s i x membered intermediate with c o o r d i n a t i o n o f the C-Li bond to the incoming monomer i n i t s c i s form ( 1 ) . The c o n f i g u r a t i o n of each terminal unit would then be determined i n the r e a c t i o n leading to i t s formation with no subsequent changes p o s s i b l e . S i m i l a r l y with a c r y l a t e s , complexation of the l i t h i u m with carbonyl groups on incoming monomer and u n i t s of the polymer chain i s the dominant feature of mechanisms for i s o t a c t i c polymer formation (2). Probably the f i r s t departure from these s t a t i c models was 0097-6156/81/0166-0071$05.00/0 Published 1981 A m e r i c a n Chemical Society

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s u g g e s t e d by Bovey e t a l (3_) i n a c r y l a t e p o l y m e r i z a t i o n . They were a b l e t o show u s i n g s p e c i f i c a l l y d e u t e r a t e d monomers t h a t a l t h o u g h i n h y d r o c a r b o n s o l v e n t s monomer a t t a c k d i d o c c u r so as t o m a i n t a i n a l i g n e d e s t e r - g r o u p s i n monomer and p o l y m e r , w i t h t r a c e s o f e t h e r s p r e s e n t , t h e monomer c o u l d a p p r o a c h w i t h e s t e r g r o u p s opposed. T h i s would normally produce a racemic d i a d . Subsequent r o t a t i o n o f the t e r m i n a l g r o u p t o m a x i m i z 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 and hence b r i n g b a c k a meso c o n f i g u r a t i o n was found t o occur. I n i s o p r e n e p o l y m e r i z a t i o n t o o , t h e r e were i n d i c a t i o n s t h a t the s i m p l e p i c t u r e was not a d e q u a t e , f o r t h e c i s - 1 , 4 c o n t e n t was found t o be dependent on r e a c t i o n c o n d i t i o n s , p a r t i c u l a r l y on the i n i t i a t o r c o n c e n t r a t i o n ( 4 ) . The f a c t t h a t b u t a d i e n e u n d e r s i m i l a r c o n d i t i o n s gave a m i x e d c i s / t r a n s p o l y m e r a l s o s u g g e s t e d the mechanism was more c o m p l e x . B o t h t h e s e o b s e r v a t i o n s s u g g e s t t h a t k i n e t i c as w e l l a s t e r e o s t r u c t u r e déterminâ In order to i n c r e a s e the u n d e r s t a n d i n g of these p r o c e s s e s , a v a l u a b l e t o o l i s the s t u d y o f c o n f i g u r a t i o n s and i s o m e r i z a t i o n r a t e s o f low m o l e c u l a r w e i g h t m o d e l s o f the p o l y m e r c h a i n . The a b i l i t y t o s t u d y such compounds i s one o f t h e m a j o r a d v a n t a g e s o f studies in anionic polymerization. The a c t i v e c e n t e r s a r e s t a b l e under many c o n d i t i o n s and can be s t u d i e d a t l e i s u r e by a number o f t e c h n i q u e s , p a r t i c u l a r l y NMR and o p t i c a l s p e c t r o s c o p y . The d i e n e s p r o v i d e a good e x a m p l e . A d d i t i o n o f s e c . o r t - b u t y l 1 i t h i u m t o i s o p r e n e o r b u t a d i e n e u n d e r c o n t r o l l e d c o n d i t i o n s y i e l d s a model one unit, a c t i v e c h a i n u s u a l l y w i t h s m a l l amounts o f two u n i t m a t e r i a l (5^,6). The l a t t e r can be removed i f n e c e s s a r y by c o n v e r t i n g t o t h e m e r c u r y compound w h i c h can be d i s t i l l e d t o remove d i m e r . The m e r c u r y compound can s u b s e q u e n t l y be r e c o n v e r t e d t o t h e l i t h i u m ( o r o t h e r a l k a l i m e t a l ) d e r i v a t i v e by s i m p l e t r e a t m e n t w i t h an a l k a l i m e t a l ( 7 ) . W i t h t h e a c r y l a t e s , due t o e x t e n s i v e a t t a c k o f l i t h i u m a l k y l s on t h e e s t e r g r o u p , t h i s a p p r o a c h c a n n o t be u s e d , but m o d e l s can be made by m e t a l 1 a t i o n o f s u i t a b l e compounds such as m e t h y l i s o b u t y r a t e (8). N.M.R. s t u d i e s on d i e n e models show t h a t t h e p r e f e r r e d c o n f i g u r a t i o n o f the l i t h i u m compound i n h y d r o c a r b o n s i s t r a n s a l t h o u g h some c i s s t r u c t u r e does e x i s t i n e q u i l i b r i u m i . e . t h e a c t i v e c e n t e r i t s e l f does e x i s t i n two forms ( 9 , 1 0 ) .

H

CH

H

H

C—C

Crr.C

\

2

H

(trans) +

M (cis) In p o l a r s o l v e n t s

s u c h as THF,

g e n e r a l l y t h e c i s form i s more

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5.

BYWATER

Anion

Pair

73

Structure

s t a b l e f o r t h e whole s e r i e s o f a l k a l i m e t a l s ( 1 1 ) ( t h e l i t h i u m d e r i v a t i v e o f b u t a d i e n e i n d i e t h y l e t h e r i s an e x c e p t i o n ) . I f the l i t h i u m i s o p r e n e model i s t r a n s f e r r e d from h y d r o c a r b o n s o l v e n t s t o THF a t low t e m p e r a t u r e , i t s c o n f i g u r a t i o n i s f r o z e n i n t h e f o r m stable i n hydrocarbons. On warming h o w e v e r , a t -40° a s l o w i s o m e r i z a t i o n o c c u r s t o t h e form s t a b l e i n THF ( 1 2 ) . This gives an e s t i m a t e o f t h e i s o m e r i z a t i o n r a t e under a s p e c i f i c s e t o f conditions. More i m p o r t a n t a r e t h e r a t e s i n h y d r o c a r b o n s o l v e n t s . These c a n be d e t e r m i n e d i n a d i f f e r e n t way. D i - i s o p r e n y l m e r c u r y as p r e p a r e d by t h e t e c h n i q u e d e s c r i b e d above i s m a i n l y i n i t s c i s form and when r e a c t e d w i t h l i t h i u m s u s p e n s i o n i n h y d r o c a r b o n s o l v e n t s at low temperatures g i v e s c i s - i s o p r e n y l l i t h i u m which slowly isomerizes to the e q u i l i b r i u m t r a n s - r i c h mixture. Rates can be measured between -20° and 0° ( 1 3 ) . E s t i m a t e s c a n a l s o be made o f t h e i s o m e r i z a t i o t u r n out t o be somewha probably c l o s e r t o that o f high polymer. The m i c r o s t r u c t u r e o f p o l y i s o p r e n e p r e p a r e d by l i t h i u m i n i t i a t i o n i n h y d r o c a r b o n s i s 9 5 % 1,4 u n d e r a l l c o n d i t i o n s . The t r a n s 1,4 c o n t e n t however f a l l s f r o m about 20% t o z e r o as t h e m o n o m e r / i n i t i a t o r r a t i o i n c r e a s e s l e a d i n g f i n a l l y t o a 9 5 % c i s 1,4 p o l y m e r . T h i s v a r i a t i o n c a n be e x p l a i n e d w i t h t h e f o l l o w i n g scheme.

J'

:rans*

+ M

» ~

trans,

cis*

where ~ ~ c i s * r e p r e s e n t s a c i s a c t i v e c e n t e r , an i n t e r n a l u n i t appearing unstarred. The newly formed a c t i v e c e n t e r i s e n t i r e l y i n t h e c i s form i . e t h e r e a c t i o n i s s t e r e o s p e c i f i c . T h i s p o i n t can a c t u a l l y be p r o v e d w i t h i n e x p e r i m e n t a l e r r o r . When new monomer a d d s , a c i s c e n t e r i s c o n v e r t e d t o a c i s c h a i n u n i t and o f c o u r s e a t r a n s one t o a t r a n s u n i t . I t i s u n l i k e l y t h a t monomer a d d i t i o n w o u l d change i t s c o n f i g u r a t i o n . Now i f monomer a d d i t i o n i s s l o w , t h e i n i t i a l l y formed c i s a c t i v e c e n t e r may have t i m e t o r e a r r a n g e t o t h e more t h e r m o d y n a m i c a 1 l y s t a b l e t r a n s form and i n the l i m i t o f very slow p o l y m e r i z a t i o n r a t e , t h e p o p u l a t i o n d i s t r i b u t i o n w i l l be t h e e q u i l i b r i u m o n e . I f however t h e r a t e o f monomer a d d i t i o n i s f a s t compared t o i s o m e r i z a t i o n t h e w h o l e a c t i v e c e n t e r p o p u l a t i o n w i l l r e m a i n c i s and w i t h i t t h e p o l y m e r configuration. On t h i s s i m p l e p i c t u r e t h e l o w r a t e p l a t e a u w o u l d g i v e a 66% t r a n s c o n t e n t i n t h e p o l y m e r and t h e h i g h r a t e l i m i t z e r o p e r c e n t i f r a t e s o f a d d i t i o n o f monomer t o t h e two t y p e s o f center are equal. I n f a c t i t c a n be shown t h a t t h e y a r e n o t , t h e c i s a c t i v e c e n t e r s a d d i n g monomer e i g h t t i m e s f a s t e r t h a n t h e trans centers. Knowing t h e r e l a t i v e monomer a d d i t i o n r a t e s , i s o m e r i z a t i o n r a t e s and e q u i l i b r i u m p o p u l a t i o n s i t i s p o s s i b l e t o c a l c u l a t e t h e c i s - t r a n s r a t i o i n t h e p o l y m e r s and compare i t w i t h

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74

ANIONIC POLYMERIZATION

e x p e r i m e n t . Good agreement i s f o u n d ; t h e i m p o r t a n c e o f k i n e t i c and thermodynamic f a c t o r s i n t h i s c a s e i s w e l l e s t a b l i s h e d . A t t e m p t s t o r e p e a t t h e s e measurements on b u t a d i e n e p o l y m e r i z a t i o n l e d t o the r e s u l t t h a t t h e i s o m e r i z a t i o n r a t e s were f a s t e r - t o o f a s t t o measure by t h e t e c h n i q u e s i n use. No d e t a i l e d e x a m i n a t i o n was t h e r e f o r e p o s s i b l e but q u a l i t a t i v e l y i t i s c l e a r t h a t t h i s w i l l r e s u l t i n a m i c r o s t r u c t u r e much c l o s e r t o t h e e q u i l i b r i u m p o p u l a t i o n o f a c t i v e c e n t e r c o n f i g u r a t i o n s i . e . l e s s c i s 1,4 u n i t s in the polymer than w i t h isoprene although t h i s again i s probably a cis-stereospecificreaction. P o l y m e r i z a t i o n o f d i e n e s i n p o l a r s o l v e n t s i s a more c o m p l e x p r o b l e m f o r l a r g e amounts o f v i n y l u n s a t u r a t i o n o c c u r i n t h e polymer. Some 1,4 s t r u c t u r e s do p e r s i s t p a r t i c u l a r l y i n t h e e t h e r s o f l o w e r d i e l e c t r i c c o n s t a n t so i t i s o f i n t e r e s t t o inquire i f cis/trans isomerizatio o f i m p o r t a n c e . T h i s ca p o l y m e r i z a t i o n i n THF. O p t i c a l s p e c t r o s c o p y p r o v i d e s the needed tool. C i s and t r a n s a c t i v e c e n t e r s b o t h show t h e near-UV a b s o r p t i o n s c h a r a c t e r i s t i c o f d e l o c a l i z e d a l l y l i c i o n s and t h e i r pairs. The a b s o r p t i o n maxima a r e however s h i f t e d , t h e t r a n s c e n t e r s a b s o r b i n g at l o n g e r w a v e l e n g t h ( 1 4 , 1 5 ) . I f at -40° a r e l a t i v e l y l a r g e amount o f monomer i s added t o a p o l y b u t a d i e n y l sodium s o l u t i o n at e q u i l i b r i u m , an a l m o s t i n s t a n t a n e o u s s h i f t o f the a b s o r p t i o n maximum o c c u r s t o l o n g e r w a v e l e n g t h , w h i c h s l o w l y r e t u r n s t o i t s o r i g i n a l p o s i t i o n when p o l y m e r i z a t i o n i s o v e r . It appears t h a t i n t h i s s o l v e n t t r a n s c e n t e r s are p r e f e r e n t i a l l y formed on monomer a d d i t i o n w h i c h c a n n o t r e l a x t o t h e more s t a b l e c i s form at t h i s t e m p e r a t u r e . The s i t u a t i o n i s e x a c t l y o p p o s i t e to that observed i n non-polar s o l v e n t s . At h i g h e r t e m p e r a t u r e s however e q u i l i b r i u m i s m a i n t a i n e d . A g a i n we a r e d e a l i n g w i t h a r e a c t i o n w h i c h has a p r e f e r e n t i a l p r o d u c t w h i c h can be d i f f e r e n t from t h e t h e e q u i l i b r i u m f o r m , i n t h i s c a s e i n d i c a t e d by l o w e r i n g t h e t e m p e r a t u r e r a t h e r t h a n i n c r e a s i n g t h e monomer c o n c e n t r a t i o n . P a r a l l e l measurements on p o l y m e r m i c r o s t r u c t u r e show t h a t t h e s e changes a r e r e f l e c t e d i n t h e p r o d u c t . A l t h o u g h the t o t a l 1,4 c o n t e n t i s low, i t i s a l m o s t e n t i r e l y t r a n s at low t e m p e r a t u r e s (16). D i f f e r i n g r a t e s o f monomer a d d i t i o n t o c i s and t r a n s c e n t e r s a r e a l s o i n d i c a t e d by a s h a r p change o f the t e m p e r a t u r e c o e f f i c i e n t o f p o l y m e r i z a t i o n r a t e at -30° where " f r e e z i n g - i n o f s t r u c t u r e f i r s t becomes i m p o r t a n t - t h e t r a n s c e n t e r s add monomer faster in this solvent. In view o f these o b s e r v a t i o n s care should be t a k e n i n i n t e r p r e t a t i o n o f l i t e r a t u r e d a t a on m i c r o s t r u c t u r e , o f t e n d e t e r m i n e d at one p a r t i c u l a r t e m p e r a t u r e and monomer/ initiator ratio. H i e r e s u l t s may not be t y p i c a l o f t h e r e a c t i o n under a l l c o n d i t i o n s . , f

I n p o l a r s o l v e n t s t h e o t h e r i m p o r t a n t v a r i a b l e i s t h e 1,4 t o vinyl ratio. V i n y l u n s a t u r a t i o n i s a l w a y s an i m p o r t a n t p a r t o f t h e p o l y m e r s t r u c t u r e but i t s amount does depend on c o u n t e r - i o n . I t i s t e m p t i n g t o c o r r e l a t e t h i s w i t h c h a r g e d i s t r i b u t i o n a t α and y p o s i t i o n s o f the d e l o c a l i z e d a l l y l i c a c t i v e c e n t e r s . In C NMR 13

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5.

BYWATER

Anion

Pair

75

Structure

s p e c t r a o f t h e 1:1 a d d u c t o f t - b u t y l 1 i t h i u m and b u t a d i e n e f o r example t h e r e s o n a n c e o f t h e γ-carbon moves u p f i e I d and t h a t o f t h e ot-carbon moves down f i e l d on m o v i n g from a h y d r o c a r b o n t o an e t h e r s o l v e n t . I n THF f u r t h e r s h i f t s o f t h e same t y p e o c c u r on c h a n g i n g t h e c o u n t e r - i o n from L i t o K ( 1 1 ) . These chemical s h i f t changes c a n be i n t e r p r e t e d as b e i n g c a u s e d by r e d i s t r i b u t i o n o f c h a r g e from α t o γ p o s i t i o n s i n t h e i o n p a i r s ( T a b l e 1 ) . +

+

Table

I

Charge d i s t r i b u t i o n on b u t a d i e n e one u n i t model and % 1,2 structure i n polybutadiene. % of charge at γ L i (benzene) L i (THF) Na (THF) K (THF) Cs (THF)

3

% 1 ,2

22 37 43 47 50

73 58 44

a) as p e r c e n t a g e o f t o t a l

charge at

87 85 55 41

96 91 83 74

οΗ-γ

I t w i l l be seen t h a t t h e r e i s an a l m o s t e q u a l d i s t r i b u t i o n o f t h e c h a r g e between α and γ p o s i t i o n s i n THF f o r t h e h e a v i e r a l k a l i metal c o u n t e r - i o n s . I f we suppose t h a t i n c r e a s e d c h a r g e p r o d u c e s an i n c r e a s e d r e a c t i v i t y a t a g i v e n p o s i t i o n , t h e n more v i n y l u n s a t u r a t i o n w i l l be p r o d u c e d i n THF t h a n i n h y d r o c a r b o n s o l v e n t s and t h e h i g h e s t v i n y l c o n t e n t w i t h h e a v i e r a l k a l i metal c o u n t e r i o n s . The o r d e r i n THF i s however r e v e r s e d , i . e . t h e h i g h e s t v i n y l s t r u c t u r e s a r e p r o d u c e d by l i t h i u m c a t a l y s i s ( 1 7 ) a l t h o u g h m i c r o s t r u c t u r e determinations i n t h i s solvent normally apply t o r e a c t i o n s w i t h an a p p r e c i a b l e f r e e a n i o n c o n t r i b u t i o n and h e n c e c a n n o t be s i m p l y i n t e r p r e t e d . In d i o x a n e ( 1 8 ) and d i e t h y l e t h e r ( 1 9 ) however t h i s c o m p l i c a t i o n s h o u l d be absent and c h a r g e d i s t r i b u t i o n s u r e l y must f o l l o w t h e same p a t t e r n w i t h c o u n t e r - i o n . A g a i n t h e h i g h e s t 1,2 c o n t e n t i n t h e p o l y m e r o c c u r s w i t h l i t h i u m as c o u n t e r - i o n . The l a r g e c o u n t e r - i o n s p r o d u c e a f a i r l y e v e n l y b a l a n c e d 1,4/1,2 r a t i o as m i g h t be e x p e c t e d from an even a/ y c h a r g e d i s t r i b u t i o n and a l a r g e c o u n t e r i o n t e n d i n g t o b l o c k b o t h positions. I t seems p l a u s i b l e t o s u p p o s e t h a t a h i g h l y s o l v a t e d l i t h i u m c a t i o n and m o d e r a t e l y s o l v a t e d s o d i u m c a t i o n i n e t h e r s o l v e n t s s i t u a t e d c l o s e r t o t h e α-position e f f e c t i v e l y b l o c k t h e terminal p o s i t i o n l e a v i n g p r e f e r e n t i a l attack at the γ p o s i t i o n . Butadiene i s a r e l a t i v e l y simple case being a symmetrical monomer. W i t h i s o p r e n e on t h e o t h e r hand a f u r t h e r c o m p l i c a t i o n a r i s e s b e c a u s e t h e monomer c a n add a t i t s 1 o r 4 p o s i t i o n g i v i n g ζ ÇH 4,1 a c t i v e c e n t e r ( ^H -CH-C-CH'" K ) a ) o r a 1 ,4 c e n t e r 3

+

2

2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

76

ANIONIC POLYMERIZATION +

(-CH2-C-CH-CH X ) b ) . As t h e a d d i t i o n i s i n p r a c t i c e not r e v e r s i b l e t h e mode o f monomer a d d i t i o n d e c i d e s i m m e d i a t e l y i f t h e a c t i v e c e n t r e w i l l p r o d u c e a) 1,4 o r 3,4 b) 4,1 o r 1,2 u n i t s , t h e f i n a l i n t e r n a l c h o i c e i n each case o n l y being d e c i d e d at t h e next monomer a d d i t i o n . A l l e v i d e n c e on configurâtional p r e f e r e n c e so f a r o b t a i n e d has been on t y p e a) c e n t r e s n o r m a l l y formed a l m o s t e n t i r e l y by r e a c t i o n o f b u t y l l i t h i u m w i t h monomer i n h y d r o c a r b o n s o l v e n t s ( 2 0 ) . The p r e s e n c e o f b o t h 1,2 and 3,4 u n i t s i n p o l y m e r s formed i n p o l a r s o l v e n t s shows t h a t h e r e b o t h modes o f a d d i t i o n o c c u r ( T a b l e t l ) . Between 20 and 30% o f t h e v i n y l u n i t s h a v e a 2

Table I I Total

vinyl unsaturatio

Counter-ion Li Na Κ Cs

total

diethyl vinyl

65% 83% 62% 47%

a) P e r c e n t a g e o f t o t a l

ether % l,2 20 26 32 33

a

total

dioxane vinyl % 1,2

93% 91% 62% 53%

18 13 19 23

vinyl

1,2 s t r u c t u r e i n d i e t h y l e t h e r and d i o x a n e . With the free anion the p r o p o r t i o n r i s e s t o ~40% a c c o r d i n g t o r e s u l t s o b t a i n e d i n d i m e t h o x y e t h a n e where i t a p p e a r s t o be r e s p o n s i b l e f o r most polymer f o r m a t i o n . I t s h o u l d be n o t e d t h a t t h e s e f i g u r e s c a n n o t be used t o c a l c u l a t e t h e p r o p o r t i o n o f t h e two t y p e s o f a c t i v e c e n t r e s p r e s e n t b e c a u s e a g a i n t h e i r r e a c t i v i t i e s w i t h monomer may be d i f f e r e n t . T h e i r r e a c t i v i t i e s a t α and γ p o s i t i o n s may be a l s o d i f f e r e n t l e a d i n g t o d i f f e r e n t 1,4 ( 4 , 1 ) t o v i n y l r a t i o s . S t r u c t u r e s o f t y p e b) form have n o t so f a r been p r e p a r e d although t h e r e i s some e v i d e n c e t h a t t h e y p r e f e r t o be i n t h e t r a n s form even i n THF i n c o n t r a s t t o t y p e a) c e n t r e s ( 1 9 ) . Isolation of models o f type b) i s r e q u i r e d t o c o n f i r m t h e i r c o n f i g u r a t i o n a l p r e f e r e n c e and t o measure t h e d i f f e r e n t r a t e s o f monomer a d d i t i o n o f t h e two b e f o r e a good u n d e r s t a n d i n g o f t h e mechanism o f i s o p r e n e p o l y m e r i z a t i o n c a n be o b t a i n e d . W h i l e f o r t h e d i e n e s we have seen much i n f o r m a t i o n c a n be o b t a i n e d from s i m p l e one u n i t a c t i v e p o l y m e r c h a i n m o d e l s , t h i s i s l e s s h e l p f u l i n t h e c a s e o f v i n y l monomers where m i c r o s t r u c t u r e depends on t h e r e l a t i o n s h i p between two o r more monomer u n i t s i n t h e c h a i n and o r i e n t a t i o n o f t h e i n c o m i n g monomer. L i t t l e work has been done i n t h i s f i e l d ( 1 8 ) . A mechanism h a s , h o w e v e r , been proposed t o e x p l a i n the changes i n m i c r o s t r u c t u r e observed i n p o l y α-methylstyrene formed by l i t h i u m c a t a l y s i s i n THF w h i c h does

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5.

BYWATER

Anion

Pair

Structure

77

a t t e m p t t o e x p l a i n t h e s e changes i n terms o f i s o m e r i z a t i o n o f t h e t e r m i n a l u n i t w i t h r e s p e c t t o t h e p e n u l t i m a t e one ( 2 1 ) . At l o w monomer c o n c e n t r a t i o n and h i g h t e m p e r a t u r e , e q u i l i b r i u m between t e r m i n a l meso and r a c e m i c d i a d s w o u l d be m a i n t a i n e d and m i c r o s t r u c t u r e d e t e r m i n e d by t h e thermodynamic s t a b i l i t y o f t h e meso and r a c e m i c u n i t s . The r a c e m i c form i s s u p p o s e d t o be more s t a b l e by 700 c a l / m o l e , i t s c o n c e n t r a t i o n i n c r e a s i n g from 7 2 % a t 0° t o 87% a t -100° a t e q u i l i b r i u m . At h i g h e r monomer c o n c e n t r a t i o n s and l o w e r t e m p e r a t u r e s k i n e t i c f a c t o r s were c l a i m e d t o d o m i n a t e w i t h the meso form b e i n g l e s s e a s i l y formed w i t h a h i g h e r a c t i v a t i o n energy. Once a g a i n t h e i m p o r t a n c e o f b o t h k i n e t i c and t h e r m o dynamic e f f e c t s i s e m p h a s i z e d-,i l l u s t r a t i n g t h e w i d e s p r e a d i m p o r t a n c e o f b o t h o f them i n 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 m o d e l s .

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Stearns, R.S. and Forman, L.E. J . Polym. Sci. (1959) 41, 381. Glusker, D.L., Lysloff, I. and Stiles, E. J . Polym. Sci., (1961) 49, 315. Fowells, W., Schuerch, C., Bovey, F. A. and Hood, F.P. J . Am. Chem. Soc. (1967) 89, 1396. Gebert, W., Hinz, J. and Sinn, H. Makromol. Chem. (1971) 144, 97. Schué, F., Worsfold, D.J. and Bywater, S. J . Polym. S c i . (Pol. Lett. Ed.) (1969) 7, 821. Glaze, W.H. and Jones, P.C. Chem. Commun. (1969) 1434. Bywater, S., Lachance, P. and Worsfold, D.J. J. Phys. Chem. (1975) 72, 2148. Lochmann, L. and Lim, D. J. Organometal . Chem. (1973) 50, 9. Brownstein, S., Bywater, S., and Worsfold, D.J. Macromolecules (1973) 6, 715. Glaze, W.H., Hanicak,J.E.,Moore, M.L. and Chaudhuri, J. J. Organometal Chem. (1977) 44, 39. Bywater, S. and Worsfold, D.J. J. Organometal Chem. (1978) 159, 229. Schué, F., Worsfold, D.J. and Bywater, S. Macromolecules (1970) 3, 509. Worsfold, D.J. and Bywater, S. Macromolecules (1978) 11, 582. Garton, A. and Bywater, S. Macromolecules (1975) 8, 694. Garton, Α., Chaplin, R.P. and Bywater, S. Eur. Polym. J . (1976), 12, 697. Garton, A. and Bywater, S. Macromolecules (1975) 8, 697. Rembaum, Α., E l l s , F.R., Morrow, R.C. and Tobolsky, A.V. J. Polym. Sci. (1962) 61, 166. Salle, R. and Pham, Q.T. J. Polymer Sci. (Chem. Ed.) (1977) 15, 1799. Dyball, D.J., Worsfold, D.J. and Bywater, S. Macromolecules (1979) 12 819. Schué, F. and Bywater, S. Bull. Soc. Chem. Fr. (1970) 271. Wicke, R. and Elgert, K.F. Makromol. Chem. (1977) 178, 3085.

Received February 12, 1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6 Some Aspects of Ion Pair-Ligand Interactions J O H A N N E S SMID Chemistry Department, College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210

Changes in the interand in the structur cation-binding ligands (ethers, crown ethers, polyamines) is briefly reviewed. The complexity o f a n i o n i c p o l y m e r i z a t i o n a r i s e s from the existence o f a v a r i e t y o f species i n the type o f s o l v e n t s o f t e n used i n these r e a c t i o n s : free ions, d i f f e r e n t kinds o f i o n p a i r , triple i o n s , i o n pair dimers, e t c . Cation-binding l i g a n d s , added to c a t a l y z e the p o l y m e r i z a t i o n o r to modify the product s t r u c t u r e , f u r t h e r i n c r e a s e the number o f s p e c i e s : A" + M +

+ A~

+

+ M

A~,M

A~,M

ΑΊ|Μ

+

+

M ,A~,M

+

L

Solvent-or Ligand Separated Ion P a i r s and Free Ions

>< u ι—»

+

The i o n i c equilibria a r e i n f l u e n c e d by many f a c t o r s : con­ c e n t r a t i o n o f ionic s p e c i e s , nature o f c a t i o n and anion (charge derealization, bulky s u b s t i t u e n t s ) , dielectric constant, d o n i c i t y number and specific s t r u c t u r a l features o f the s o l v e n t molecules,

0097-6156/81/0166-0079$05.00/0 © 1981 American Chemical Society

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ANIONIC POLYMERIZATION

80

the presence o f s a l t s o r c a t i o n b i n d i n g l i g a n d s (e.g., glymes, crown e t h e r s , polyamines, cryptands, podands), temperature, pressure, e t c . Any o f these v a r i a b l e s can a l t e r the r e l a t i v e amounts o f the i o n i c s p e c i e s , each o f which can propagate the chain w i t h i t s own c h a r a c t e r i s t i c rate constant and s t e r e o ­ specificity. M o d i f i c a t i o n o f i o n p a i r s t r u c t u r e s by i o n - b i n d i n g l i g a n d s have been s t u d i e d by s e v e r a l techniques, e.g., o p t i c a l , i n f r a r e d and raman spectroscopy, c o n d u c t i v i t y , e l e c t r o n s p i n resonanceand l H , 13C and a l k a l i n u c l e a r magnetic resonance spectroscopy, d i p o l e moment measurements, viscometry and mechanistic s t u d i e s . Much o f the i n f o r m a t i o n p e r t i n e n t t o a n i o n i c p o l y m e r i z a t i o n has been d e r i v e d from non-polymerizable i o n i c systems, e.g., c a r ­ banion s a l t s d e r i v e d from f l u o r e n e , diphenylmethane, t r i p h e n y l methane and 4,5-methylenephenanthren charge d e l o c a l i z e d carbanions phenoxides, c a r b a z y l , e t c . These and other i n v e s t i g a t i o n s have immensely c o n t r i b u t e d to a deeper understanding o f the mechanism o f a n i o n i c p o l y m e r i z a t i o n and have been e x t e n s i v e l y reviewed elsewhere · Some aspects o f l i g a n d i n t e r a c t i o n s w i t h carbanion p a i r s l e a d i n g to d i f f e r e n t types o f i o n p a i r complexes w i l l be b r i e f l y reviewed. The d i s c u s s i o n w i l l focus c h i e f l y on crown ethers and polyamines as l i g a n d s i n t e r a c t i n g w i t h a l k a l i and a l k a l i n e e a r t h c a t i o n s . The polyamines have been used e x t e n s i v e l y i n r e a c t i o n s w i t h organolithium reagents ( 8 ) , while crown e t h e r s , together with the l i n e a r glymes, have been employed i n s e v e r a l s t u d i e s o f s a l t s o f carbanions, οxyanions, s u l f i d e s , c a r b o x y l a t e s , e t c . O p t i c a l spectroscopy has been one o f the important t o o l s i n o b t a i n i n g i n f o r m a t i o n on complex formation constants and on the nature o f the i o n p a i r - s o l v a t i o n and l i g a n d complexes. Crown ether complexes o f f l u o r e n y l carbanion s a l t s Solvent o r l i g a n d i n t e r a c t i o n s w i t h t i g h t i o n p a i r s produce e x t e r n a l l y complexed t i g h t i o n p a i r s and/or l i g a n d separated i o n p a i r s . The s t a b i l i t y o f the complexes depends on s o l v e n t , temperature, type o f crown and the nature o f the c a t i o n . For example, i n e t h e r e a l solvents benzo-15-crown-5 and f l u o r e n y l sodium ( F l ~ , N a ) form the two i s o m e r i c complexes I and I I de­ p i c t e d i n r e a c t i o n 1, but the r a t i o I / I I i s h i g h l y s o l v e n t s e n s i t i v e (9) ( i f the bound s o l v e n t i n I I i s i n c l u d e d i n the s t r u c t u r e o f I I , the two complexes o f course can a c t u a l l y n o t be considered i s o m e r i c ) . +

In these equations, Cr r e f e r s to a crown ether l i g a n d w h i l e S denotes a s o l v e n t molecule. The n o t a t i o n S f o l l o w i n g Fl~,Na+ s i g n i f i e s that i n s o l v e n t s such as dioxane, e t h y l ether, t e t r a hydropyran (THP) o r tetrahydrofuran (THF) the t i g h t i o n p a i r s , e s p e c i a l l y when s m a l l a l k a l i i o n s are i n v o l v e d , contain e x t e r n ­ a l l y complexed s o l v e n t molecules. These must be removed before n

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

SMID

Ion Pair-Ligand

Fl~,Na ,S + Cr 7 η

^

81

Interactions

»

2

Fl~,Na ,Cr + nS

K

Fl",Cr,Na ,S

(1)

^

+ (n-m)S

(ID

a crown l i g a n d can form the e x t e r n a l t i g h t i o n p a i r complex I . The energy o f d e s o l v a t i o n i s expected to i n c r e a s e i n the o r d e r e t h y l ether < THP < THF < DME (1,2 dimethoxyethane) (2). The loose i o n p a i r complex I I i l i k e l t retai tw s o l v e n t molecules s i n c e the c a t i o s i d e o f the crown c a v i t y molecules. Hence, a b e t t e r c a t i o n - s o l v a t i n g s o l v e n t i s l i k e l y to s h i f t e q u i l i b r i u m 2 (2) I

II

i n favor o f the l o o s e i o n p a i r complex I I . T h i s e x p l a i n s the observed values (9) f o r t h e r a t i o s K~ « I I / I w i t h benzo-15-crown5 and F l " , Na+, v i z . , 0 ( e t h y l e t h e r j , 0.52 (THP) and 1.8 (THF). The e f f e c t o f d i e l e c t r i c constant, D, on the formation constant o f a l o o s e i o n p a i r complex i s probably not l a r g e i n s p i t e o f the i n c r e a s e i n the i n t e r l o n i c i o n p a i r d i s t a n c e . The m i c r o s c o p i c r a t h e r than the macroscopic d i e l e c t r i c constant determines to a l a r g e extent the d i f f e r e n c e between the coulomb a t t r a c t i o n energy o f two ions i n a t i g h t and l o o s e i o n p a i r . For example, the formation constant o f the loose i o n p a i r complex I I I between f l u o r e n y l sodium and t r i i s o p r o p a n o l amine borate a t 25C i s n e a r l y i d e n t i c a l i n THF (Κ » 102 W"I, D « 7.4), THP (K - 104 M" , D « 5.6) and dioxane (K - 74 M" , D « 2.4) ( 1 0 ) . 1

1

+

F l " , B-0CH(CH~)CH -N, N a , S η o

OCH(CH )CH 3

2

III

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

82

ANIONIC POLYMERIZATION

For complex I I I , the Na"*" i s probably as a c c e s s i b l e to s o l v a t i o n by solvent molecules as i s the Na i n the t i g h t Fl-,Na+ i o n p a i r . Hence, no e x t e r n a l l y bound s o l v e n t molecules need to be removed. This may be d i f f e r e n t i n o t h e r systems. For example, the formation constant o f a loose i o n p a i r complex between F l " , Na and tetraglyme ( t e t r a e t h y l e n e g l y c o l dimethyl ether) i s n e a r l y four times lower i n dioxane than i n THF (10). T h i s may be caused by s p e c i f i c solvent e f f e c t s r a t h e r than by the d i f f e r ence i n solvent d i e l e c t r i c constant. The f l e x i b l e glyme l i g a n d wraps i t s e l f around the N a i o n , and t h i s may make i t more d i f f i c u l t f o r s o l v e n t molecules to remain bound to N a i n the glyme-separated i o n p a i r . The s p e c i f i c s t r u c t u r e o f the c a t i o n b i n d i n g l i g a n d i s c r i t i c a l i n determining the type of i o n p a i r complex. For example, i n comparison crown-6 and F l ~ , N a for at l e a s t i n THF and THP, although w i t h F l ~ , K t both i o n p a i r complexes are formed (9,11). S u b s t i t u e n t s c l o s e to the crown c a v i t y can a l s o a f f e c t the s t a b i l i t y of the i o n p a i r - l i g a n d complex. *An i n t e r e s t i n g example i s the crown ether IV, an e x c e l l e n t c h e l a t i n g agent f o r L i (12). I t was observed (13) that i n chloroform the t i g h t l i t h i u m p i c r a t e i o n p a i r converts to the crown +

+

+

+

+

IV

separated i o n p a i r complex on a d d i t i o n o f IV. However, i n THP a crown complexed t i g h t i o n p a i r i s formed i n s p i t e o f the b e t t e r c a t i o n - s o l v a t i n g p r o p e r t i e s o f THP which normally favor a l o o s e i o n p a i r complex. Apparently, the bulky methyl groups prevent the L i i o n i n the l o o s e i o n p a i r from complexlng THP molecules. Hence, w h i l e i n chloroform the formation o f the loose i o n p a i r - IV complex requires energy to overcome the coulomb forces on e n l a r g i n g the i n t e r i o n i c i o n p a i r d i s t a n c e , i n THP i t would a l s o r e q u i r e the removal o f THP molecules bound to the t i g h t l i t h i u m p i c r a t e i o n p a i r . Apparently, complexation to IV does not provide enough energy to accomplish both, and i n THP only the crown-complexed t i g h t i o n p a i r i s s t a b l e . Examples o f +

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

SMID

Ion

Pair-Ligand

Interactions

83

t h i s k i n d show the d i f f i c u l t y i n p r e d i c t i n g the nature o f the i o n pair-complex. An important o b s e r v a t i o n i s the c o n c e n t r a t i o n dependency o f the r a t i o I I / I (see equations 1 and 2). In a study o f the com­ p l e x formation between benzo-15-crown-5 and fluorenylsodium i n THF, 2-methyl-THF and THP the r a t i o o f the two complexes I and I I was found to depend not only on s o l v e n t but a l s o on the t o t a l i o n p a i r c o n c e n t r a t i o n (14). The r e s u l t s were r a t i o n a l i z e d by assuming the formation o f dimers o f I I on i n c r e a s i n g the i o n p a i r c o n c e n t r a t i o n as shown i n r e a c t i o n 3. Aggregation o f com­ plex I i s Ka 2 Fr,Cr,Na

+

χ

+

(Fl~,Cr,Na )

(3)

hindered by the e x t e r n a l l aggregation constants, K , depend on the s o l v e n t d i e l e c t r i c constant, t h e i r values at 25°C being 62 M" i n THF, 87 M" in Me-THF (D « 6.24) and 168 K" i n THP. The tendency o f ligand-complexed loose i o n p a i r s to aggregate i s o f t e n r e f l e c t e d i n the s o l u b i l i t y behavior o f these complexes. For example, w h i l e F l ~ , L i has a high s o l u b i l i t y i n THF, i t s l o o s e i o n p a i r complex with dibenzo-14-crown-4 has a s o l u b l i l i t y l e s s than lCT^M (14), D i f l u o r e n y l b a r i u m i s s o l u b l e i n THF up to 0.05 M, but the s o l u b i l i t i e s o f the mixed t i g h t loose i o n p a i r complexes w i t h glymes and crown ethers (e.g., F l " , B a ^ G ^ l - ) are o n l y i n the order o f 0.001-0.01 M (15). S i m i l a r f i n d i n g s have been reported f o r polyamine complexes w i t h organo­ l i t h i u m complexes (see next s e c t i o n ) . Hence, w h i l e e x t e r n a l l i g a n d complexation to a t i g h t i o n p a i r can d r a m a t i c a l l y enhance s a l t s o l u b i l i t y i n a p o l a r media (16), s o l u b i l i t y may decrease when i n t e r a c t i o n with l i g a n d s produces loose i o n p a i r s . a

1

1

1

+

D i f f e r e n c e s i n aggregation constants between t i g h t and loose i o n p a i r s o l v a t i o n o r l i g a n d complexes make i t more d i f f i c u l t to compare r e s u l t s o f i n v e s t i g a t o r s working on the same systems but i n d i f f e r e n t c o n c e n t r a t i o n ranges. For example, U . V - v i s i b l e spectrophotometrie data can e a s i l y by obtained at i o n p a i r c o n c e n t r a t i o n between 10"^-1(Γ^ M, but i n f r a r e d o r nmr measurements on i o n p a i r i n g are f r e q u e n t l y c a r r i e d out at 0.1 M. Conclusions drawn from these s t u d i e s as f a r as i o n p a i r s t r u c t u r e s are concerned may d i f f e r because aggregation at h i g h e r c o n c e n t r a t i o n can l e a d to s h i f t s i n e q u i l i b r i a between d i f f e r e n t ion p a i r species. The s t r u c t u r e o f the i o n p a i r complex i s a l s o s e n s i t i v e to the anion, e s p e c i a l l y the extent o f charge d e r e a l i z a t i o n and the presence o f s u b s t i t u e n t s c l o s e to the a n i o n i c s i t e . Loose i o n p a i r formation r e q u i r e s a c o n s i d e r a b l e i n c r e a s e i n the i n t e r i o n i c i o n p a i r d i s t a n c e , and a charge l o c a l i z e d anion, t h e r e f o r e .

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ANIONIC POLYMERIZATION

84

w i l l favor e x t e r n a l l i g a n d complexetion s i n c e t h i s r e q u i r e s l e s s s t r e t c h i n g o f the i o n i c bond. For complexes o f benzo-15-crown5 w i t h potassium f l u o r e n y l and potassium p i c r a t e , the complex formation constants l e a d i n g to the 1:1 e x t e r n a l l y complexed t i g h t i o n p a i r s are n e a r l y i d e n t i c a l f o r the two s a l t s , i . e . , 8000 f o r potassium f l u o r e n y l (9) and 6000 M~I f o r potassium p i c r a t e (17)· A d d i t i o n o f a second crown produces the l o o s e i o n p a i r A~,Cr,K+,Cr. However, the complexatlon constant f o r adding the second crown i s 1800 M"" f o r the f l u o r e n y l carbanion and o n l y 200 M~l f o r the p i c r a t e s a l t . The lower value f o r p i c r a t e may i n p a r t be due to l e s s charge d e r e a l i z a t i o n , e.g., the f r e e i o n d i s s o c i a t i o n constant f o r potassium f l u o r e n y l i n THF i s 1.6 χ 10~ M (18) as compared to 9.2 χ 10" M f o r potassium p i c r a t e (17). The two NO2 s u b s t i t u e n t s c l o s e to the 0~,ltf*" bond i n p i c r a t e may a l s o h i n d e r the enlargemen t i o n o f a crown e t h e r molecul effects. Crown o r s o l v e n t complexatlon l e a d i n g to loose i o n p a i r s can by i t s e l f cause a d r a s t i c change i n the charge d i s t r i b u t i o n of the anion. A good example i s found i n r e a c t i o n 4 s t u d i e d by Boche e t a l (19). At room temperature the sodium enolate t i g h t ion p a i r with 1

7

8

S

i t s o l e f i n i c nonafulvene s t r u c t u r e i s s t a b l e . At low tempera­ ture, s t r o n g e r s o l v a t i o n f o r c e s cause formation o f a loose i o n p a i r . T h i s i n turn promotes charge d e r e a l i z a t i o n and s t a b i l ­ i z e s the CH3C0-substituted sodium cyclononatetraenide s t r u c t u r e which i s aromatic i n c h a r a c t e r . Loose i o n p a i r s o f such c h a r g e - l o c a l i z e d oxyanion s a l t s as potassium t-butoxide may be d i f f i c u l t to form. T h i s a l k o x i d e i s a t e t r a m e r i c aggregate i n THF (20), and crown a d d i t i o n breaks i t down to the more r e a c t i v e monomeric form. I t i s u n l i k e l y t h a t with benzo-15-crown-5 a 2:1 crown-K* l o o s e i o n p a i r can be formed s i m i l a r to that found w i t h potassium p i c r a t e o r potassium f l u o r e n y l . However, e x t e r n a l complexatlon i t s e l f w i l l s l i g h t l y s t r e t c h the 0"",Κ** bond, and t h i s can have a profound e f f e c t on the anion r e a c t i v i t y (21). While crown complexation to a t i g h t i o n p a i r u s u a l l y i n ­ creases the i n t e r i o n i c i o n p a i r d i s t a n c e , i t may shorten the

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

SMID

Ion

Pair-Ligand

85

Interactions

i o n i c bond when the l i g a n d r e p l a c e s p a r t o f the s o l v a t i o n s h e l l o f a l o o s e i o n p a i r . T h i s i n turn can decrease the f r a c t i o n o f f r e e i o n s formed on i o n p a i r d i s s o c i a t i o n . Hence, i n r e a c t i o n s euch as a n i o n i c v i n y l p o l y m e r i z a t i o n o r proton t r a n s f e r r e a c t i o n s where f r e e anions o f t e n are the r e a c t i v e s p e c i e s , crown e t h e r a d d i t i o n can l e a d to a decrease r a t h e r than an i n c r e a s e i n r e ­ activity. An example i s found i n the i n t e r a c t i o n o f dibenzo-18crown-6 and f l u o r e n y l sodium i n THF (22). The l a t t e r s a l t i s a l o o s e i o n p a i r i n THF w i t h a f r e e i o n d i s s o c i a t i o n constant, equal to 4.0 χ 10"^M. The crown e t h e r converts the s o l v a t e d l o o s e i o n p a i r i n t o a crown-complexed l o o s e i o n p a i r (the U.V. a b s o r p t i o n band remains a t 373 nm, c h a r a c t e r i s t i c f o r a l o o s e i o n p a i r o f F l " " , * ^ (23))» but K drops to 3.3 χ 1(Γ Μ. Replace­ ment o f the bulky THF molecules around the N a by the more p l a n a r crown molecule allow d i s t a n c e , thereby d e c r e a s i n 6

d

+

Polyamine i n t e r a c t i o n s w i t h carbanion

pairs

Polyamine complexes of o r g a n o l i t h i u m compounds have been e x t e n s i v e l y s t u d i e d (8). Recent work by F o n t a n i l l e ejt a l (24) has shown that a l s o i n these systems two i s o m e r i c i o n p a i r complexes can be formed. The spectrophotoroetric " i o n p a i r probe" used was 9 - p r o p y l f l u o r e n y l l i t h i u m ( P F l " , L i ) , which was complexed w i t h tetramethyl ethylene diamine (TMEDA), hexamethyl t r i e t h y l e n e tetramine (HMTT) and tetramethyl t e t r a a z a c y c l o t e t radecane +

Ν

[TMEDA]

Ν

N_

Ν

Ν

Ν

[HMTT] [TMTCT]

(TMTCT) i n cyclohexane o r toluene as s o l v e n t . The amines are powerful L i c h e l a t i n g agents and s t r o n g l y c a t a l y z e the polymer­ i z a t i o n o f p o l y s t y r y l l i t h i u m i n cyclohexane by anion a c t i v a t i o n (8,25). The U.V.-absorption spectrum o f 9 - p r o p y l f l u o r e n y l l i t h i u m i n toluene i s c o n c e n t r a t i o n dependent, showing a t 1(T M i n the 320400 nm r e g i o n only one a b s o r p t i o n band at 368 nm. I t belongs to the ( P F l ~ , L i ) 2 dimer which i s s t a b l e under these c o n d i t i o n s +

3

+

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ANIONIC

86

POLYMERIZATION

but d i s s o c i a t e s on d i l u t i o n i n t o the monomeric i o n p a i r P F l ~ , L i . The a b s o r p t i o n maximum o f the l a t t e r species i s 353 nm and the d i s s o c i a t i o n constant o f the dimer a t 25° equals 2.9 χ 10""% (26). On a d d i t i o n o f minute amounts o f an ether s o l v e n t , E, the 368 nm dimer breaks down o r rearranges i n t o a d i e t h e r a t e , v i z . , PFl~,Li ,THP2, which i s an e x t e r n a l l y s o l v a t e d t i g h t i o n p a i r absorbing a t 359 nm. The same absorption band appears on adding THP to the 353 nm monomeric i o n p a i r . A t higher THP o r THF con­ c e n t r a t i o n two more s o l v e n t molecules are bound forming the loose PF1~ I | L i i o n p a i r absorbing a t 386 nm. The changes a r e shown i n r e a c t i o n 5. +

+

+

(PFf , L i )

(368 nm)

2

[359 nm]

[386 nm]

The dimer ( P F l ~ , L i ) does n o t d i s s o c i a t e on d i l u t i o n when cyclohexane i s the s o l v e n t (24). However, a d d i t i o n o f small q u a n t i t i e s o f TMEDA forms the externally-complexed 1:1 P F l ~ , L i , TMEDA t i g h t i o n p a i r , ^ 3 5 5 nm. Although TMEDA i s an e x c e l l e n t L i * c h e l a t i n g agent, even i n pure TMEDA only a small f r a c t i o n o f loose i o n p a i r s (M).05) can be detected i n the o p t i c a l spectrum. S i m i l a r observations were reported e a r l i e r f o r the u n s u b s t i t u t e d f l u o r e n y l l i t h i u m (2). Models suggest that formation o f a complex o f L i with two TMEDA l i g a n d s i n which a l l four n i t r o g e n atoms can simultaneously i n t e r a c t e f f e c t i v e l y w i t h the c a t i o n i s hindered by the presence o f the e i g h t bulky methyl s u b s t i t u e n t s . Nevertheless, a p r e c i p i t a t e . w h i c h slowly forms i n pure TMEDA was shown to be a P F l " * , L i (TMEDA)2 complex (24). I t d i s s o l v e s i n toluene with r e l e a s e o f one TMEDA l i g a n d and formation o f the 359 nm e x t e r n a l l y complexed P F l ~ , L i , TMEDA t i g h t i o n p a i r . A d d i t i o n o f the polyamine HMTT t o PF1~,Li i n cyclohexane converts the 368 nm ( P F l ~ , L i ) 2 dimer i n t o a species absorbing at 357 nm. T h i s i s the only complex i n s o l u t i o n when the r a t i o r « HMTT/PFl~,Li has reached the value 0.5. T h i s suggests t h a t two P F l ~ , L i i o n p a i r s are complexed to one HMTT-ligand as shown below. Complex V d e r i v e s i t s 2

+

+

+

+

+

+

+

/ — \ l / — \ l .




Li PFL

Li PFL~

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

SMiD

Ion

Pair-Ligand

87

Interactions

s t a b i l i t y i n p a r t from the i o n p a i r - i o n p a i r i n t e r a c t i o n * By adding excess HMTT the 2:1 complex i s converted i n t o a 1:1 loose i o n p a i r complex absorbing at 383 nm (386 nm i n toluene) as shown i n r e a c t i o n 6. +

(PFl"",Li ) 368

2

+ HMTT


ο cr in

cis, trans trans.trans

Figure 3. Energy level diagram showing routes for deactivation of excited trans, trans* conformer.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

112

ANIONIC POLYMERIZATION

no fluorescence corresponding to the transition cis,trans* cis,trans. Apparently, there is no significant barrier to twisting of the excited cis,trans conformer. The related 1,3-diphenyl-2-azaallyl carbanion also exists in solution as an ion pair. Although the absorption maximum in the electronic spectrum is sensitive to the choice of solvent and counterion, surprisingly, no case has been found to date of a solution which simultaneously shows two absorption bands attributable to the coexistence of tight and loose ion pairs. The explanation of this observation is obscure. However, photolysis does effect the transformation of the conformation from trans,trans to cis,trans. No fluorescence has been observed from solutions of this azaallyl ion. A further difference from the diphenylallyl system is that the photostationary state is insensitiv When a photoisomerise carbanion, or of i t s 2-aza analog, is stored in the dark the normal (thermal) conformational equilibrium is reestablished by a process obeying f i r s t order kinetics. The reaction can be conveniently monitored spectrophotometrically. In the case of the diphenylallyl ion, the relaxation rate is much greater for the tight ion pairs than for the loose. It seems probable that the transition state may involve the association of the cation specifically with one of the terminal a l l y l i c carbon atoms. Such a species could be regarded as comprising a styrene moiety together with a benzylic anion, the two being connected by a single carbon-carbon bond about which the barrier to rotation would be expected to be relatively low. The Arrhenius parameters for the cis,trans + trans,trans relaxation are collected in Table 1 for a range of combinations of cation, anion and solvent. In the case of the relaxation of the 1,3-diphenyl-2-aza-allyl carbanion, the absence of separate absorption bands for the two kinds of ion pair prevents the drawing of a general comparative conclusion. It would seem probable that the lithium salt is loosely paired in THF but tightly paired in diethyl ether from a simple consideration of the relative solvating power of these solvents: this view is apparently supported by the pronounced blue shift of the absorption maximum in ether ( x 535 nm) with respect to that in THF (* 562 nm). It is, accordingly surprising, that the Arrhenius parameters for these two systems are found to be so similar. In the poorer solvating solvents diethyl ether and MTHF, the Ε parameter is lower for the sodium salt than for the lithium: that the lithium salt has a higher A factor might be ascribed to the release of some bound solvent on forming the transition state. max

max

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

YOUNG

Photoisomerization

of Allylic

113

Carbanions

TABLE I Arrhenius parameters for thermal relaxation of conformation from cis,trans to trans,trans 1,3 diphenyl-2-aza-allyl anion 3

Κ

Na

Li Ε

log A

Ε

log A

Ε

log A

Ether

79.7

14.8

66.4

11.7

62.9

11.5

MTHF

89.2

15.2

73.8

12.4

THF

82.5

14.8

82.9

13.8

56.8

9.9

DME

78.0

13.7

CH NH

80.4

Solvent

b

C

3

2

The activation energies Ε are expressed in kJ m o l ' .A as s e c " . ^MTHF is 2-methyltetrahydrofuran DME is 1,2-dimethoxyethane

1

and

1

Worsfold and Bywater [13] have demonstrated that the stereochemistry of the propagation of polyisoprenyllithium in hydrocarbon solvents is in part controlled by the rate at which the cis active ends relax to the more stable trans forms (scheme 2). The interesting possibility is accordingly raised that by perturbing the cis t trans equilibrium of the propagating centers of a diene photochemically during polymerisation, i t might be possible to alter the stereochemistry of propagation. Having made extensive studies of the behaviour of models of the propagation center of 1,3-diphenyl butadiene this monomer would be the most appropriate one to investigate. Unfortunately, i t seems that i t cannot be isolated because of extremely f a c i l e dimerisation via the Diels Alder reaction. A cursory examination has, however, been made of the polymerisation of 1-phenylbutadiene by butyllithium in THF, yielding the following results. Tpmnpratnrp

lemperature 0°C -37°

%

3

>

i

n

4

a d

a

d r

d k

i n

t

i

o

e

s

s

n

%

3

>

4

w h e n

addition nominated

28

17

23

13

These results are encouraging and work is in hand to repeat and extend these studies.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

114

ANIONIC POLYMERIZATION

LLP C

Figure 4. Analysis of temperature de­ pendence of fluorescence intensity using Equation 3.

5

6

10 /T 3

Scheme 2.

'-^-cis Li * M — •

ι ^ t r a n s Li +M

^-cis,cisLi -trans,cis Li

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

YOUNG

Photoisomerization

of Allylic

115

Carbanions

References [1] J.W. Burley and R.N. Young, J. Chem. Soc. (B), 1019, (1971). [2] J.W. Burley and R.N. Young, J. Chem. Soc. Perkin 2, 835, (1972). [3]

G.C. Greenacre and R.N. Young, J. Chem. Soc. Perkin 2, 1661, (1975).

[4] J.W. Burley and R.N. Young, J. Chem. Soc. Perkin 2, 1006 (1972). [5]

H.V. Carter, B.J. McClelland and E. Warhurst, Trans. Faraday S o c . , 69

[6]

H.H. Freedman, V.R. Sandel and B.P. Soc., 89, 1762, (1967).

[7]

G.J. Heiszwolf and H. Kloosterziel, Recueil trav. Chim., 86, 1345 (1967).

[8]

R.J. Bushby and G.J. Ferber, Chem. Commun., 407, (1973).

[9]

R.J. Bushby and G.J. Ferber, J. Chem. Soc Perkin 2, 1688, (1976).

Thill,

J.

Amer. Chem

[10] J.W. Burley and R.N. Young, J. Chem. Soc. Perkin 2, 1843, (1972). [11] H.M. Parkes and R.N. Young, J. Chem. Soc. Perkin 2, 249, (1978). [12] R.N.Young, H.M. Parkes and B. Brocklehurst, Makromol. Chem. Rapid Commun 1, 65 (1980). [13] D.J. Worsfold and S. Bywater, Macromolecules, 11, 582, (1978). RECEIVED M a r c h 5,

1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9 Solvation of Alkyllithium Compounds Steric Effects on Heats of Interaction of Tetrahydrofurans with Polyisoprenyllithium and Polystyryllithium R O D E R I C P. Q U I R K Michigan Molecular Institute, Midland, M I 48640

The enthalpies of interaction of tetrahydro furan (THF) an (Me THF) with 0.03 poly (styryl)lithium (PSLi) and poly(isoprenyl)lithium (PILi) have been measured as a function of R ([THF]/[Li]) at 25°C using high dilution solution calorimetry. At low R values (ca. 0.2) the enthalpy of interaction of THF with PILi (-5.8 kcal/mole) i s more exothermic than with PSLi (-4.5 kcal/mole). However, the decrease in enthalpy for Me THF versus THF i s larger for PILi (3.2 kcal/mole) than for PSLi (2.2 kcal/mole) at low R values. The enthalpies decrease rapidly with increasing R values for PILi, but are relatively constant for PSLi. It i s suggested that THF interacts with intact dimer for PILi, but for PSLi this base coordinates to form the unassociated, THFsolvated species. 2

2

Lewis bases effect dramatic changes in microstructure, i n i t i a t i o n rates, propagation rates, and monomer reactivity ratios for alkyllithium-initiated polymerizations of vinyl monomers (1-6). Some insight into the molecular basis for these observations has been provided by a variety of NMR, colligative property, and light-scattering measurements of simple and polymeric organolithium compounds in hydrocarbon and basic solvents (7,8). In general, simple alky1lithiums exist predominantly as either hexamers (for sterically unhindered RLi) or tetramers (for s t e r i c a l l y hindered RLi) in hydrocarbon solvents and as tetramers in basic solvents (9-12). Polymeric organolithium compounds such as poly(styryl)lithium exist as dimers in hydrocarbon solution and are unassociated in basic solvents such as tetrahydrofuran (13-15). The state of association of poly(dienyl)lithiums in hydrocarbon solution is a subject of current 0097-6156/81/0166-0117$05.00/0 ©

1981 American Chemical Society

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

118

ANIONIC POLYMERIZATION

controversy ( L 5 , J j ) ) , a l t h o u g h t h e most r e c e n t r e s u l t s i n d i c a t e d i m e r i c a s s o c i a t i o n f o r t h i s s p e c i e s a l s o (Γ7,_18)· I n s p i t e o f this impressive wealth of data on a s s o c i a t i o n phenomena f o r o r g a n o l i t h i u m compounds, t h e f u n d a m e n t a l n a t u r e o f b a s e - o r g a n o l i t h i u m i n t e r a c t i o n s r e s p o n s i b l e f o r changes i n degree of a s s o c i ­ a t i o n and f o r t h e d r a m a t i c e f f e c t s o f b a s e s on r e a c t i o n s i s n o t understood. We have m e a s u r e d t h e e n t h a l p i e s o f i n t e r a c t i o n o f t e t r a hydrofurans with polymeric organolithiums to c h a r a c t e r i z e the specific nature of base-alky1lithium i n t e r a c t i o n s and these r e s u l t s are r e p o r t e d h e r e i n . Experimental I s o p r e n e (99 + %, A l d r i c h ) and s t y r e n e ( 9 9 % , A l d r i c h ) were p u r i f i e d by i n i t i a l s t i r r i n CaH^ on a h i g h vacuum l i n magnesium ( A l p h a I n o r g a n i c s ) . F i n a l p u r i f i c a t i o n i n v o l v e d d i s t i l ­ l a t i o n f r o m t h i s s o l u t i o n d i r e c t l y i n t o c a l i b r a t e d a m p o u l e s and s e a l i n g w i t h a f l a m e . B e n z e n e ( F i s h e r , C e r t i f i e d ACS) was s t o r e d o v e r c o n e . H^SO^, washed s u c c e s s i v e l y w i t h H^O, d i l . NaHCO^ s o l u t i o n , and H^O, f o l l o w e d by d r y i n g o v e r a n h y d r o u s MgSO^. A f t e r f i l t r a t i o n , f u r t h e r p u r i f i c a t i o n i n v o l v e d s t i r r i n g under r e f l u x o v e r f r e s h l y g r o u n d CaH^, d i s t i l l a t i o n o n t o s o d i u m d i s p e r ­ s i o n , and s t i r r i n g and d e g a s s i n g on t h e vacuum l i n e , f o l l o w e d by d i s t i l l a t i o n and storage over p o l y ( s t y r y 1 ) 1 i t h i u m . Tetrahydrof u r a n ( 9 9 % , A l d r i c h ; and B u r d i c k & J a c k s o n , D i s t i l l e d i n G l a s s ) and 2 , 5 - d i m e t h y l t e t r a h y d r o f u r a n ( A l d r i c h ) were s t o r e d o v e r b e n z o phenone k e t y l and p o t a s s i u m - s o d i u m amalgam and d i s t i l l e d i n t h e d r y box i m m e d i a t e l y b e f o r e u s e . P o l y m e r i z a t i o n s were c a r r i e d out a t 30 C i n a l l g l a s s , s e a l e d r e a c t o r s u s i n g b r e a k s e a l s and s t a n d a r d h i g h vacuum t e c h ­ n i q u e s (3). F o r t h e c a l o r i m e t r i c m e a s u r e m e n t s , a 1 l i t e r sample o f a 0.03M s o l u t i o n o f e a c h p o l y m e r i c l i t h i u m compound w i t h M o f c a . 4,000 was prepared i n benzene s o l u t i o n u s i n g s e c - b u t y l l i t h i u m as i n i t i a t o r and t r a n s f e r r e d t o t h e g l o v e box. Calorimetric m e a s u r e m e n t s were p e r f o r m e d in a recircu­ l a t e d , a r g o n a t m o s p h e r e g l o v e box u s i n g a p p a r a t u s and p r o c e d u r e s w h i c h have b e e n d e s c r i b e d i n d e t a i l p r e v i o u s l y ( 1 2 > 20,2_1 ) . S o l u ­ t i o n s o f o r g a n o l i t h i u m r e a g e n t s were a n a l y z e d u s i n g t h e d o u b l e titration procedure of G i l m a n and Cartledge (2_2) w i t h 1,2dibromoethane. R e s u l t s and

Discussion

Poly(styry1)lithium. The c a l o r i m e t r i c data obtained f o r t h e i n t e r a c t i o n o f t e t r a h y d r o f u r a n (THF) and 2,5-dimethyltetrahydrofuran ( 2 , 5 - M e T H F ) w i t h 0.03M s o l u t i o n s o f p o l y ( s t y r y l ) l i t h i u m i n b e n z e n e as a f u n c t i o n o f R ( [ b a s e ] / [ l i t h i u m a t o m s ] ) a r e shown i n F i g u r e 1. T h e s e d a t a , as w e l l as t h e corresponding data for p o l y ( i s o p r e n y l ) l i t h i u m , are referred to dilute 2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9.

QUIRK

Solvation of Alkyllithium

Compounds

119

Figure 1. Enthalpies of interaction of THF (M) and 2,5-dimethyltetrahydrofuran (%) with 0.03M solutions of polystyryllithium in benzene at 25°C: R = [base]/[Li] where [Li] is the concentration of carbon-bound lithium atoms in the solution.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

120

ANIONIC POLYMERIZATION

s o l u t i o n s o f t h e b a s e i n b e n z e n e a t 25 C as t h e s t a n d a r d s t a t e t o c o r r e c t f o r t h e h e a t s o f s o l u t i o n o f t h e b a s e s . The a b s e n c e o f s i d e r e a c t i o n s ( w h i c h w o u l d consume p o l y ( s t y r y 1 ) 1 i t h i u m and g i v e s p u r i o u s e n t h a l p i e s d u r i n g the c a l o r i m e t r i c measurements) was d e t e r m i n e d by t h e d o u b l e t i t r a t i o n p r o c e d u r e ( 2 2 ) o f t h e b a s e / p o l y ( s t y r y l ) l i t h i u m s o l u t i o n s immediately a f t e r the c a l o r i m e t r i c r u n s . For example, double t i t r a t i o n a n a l y s i s i n d i c a t e d t h a t t h e f i n a l c o n c e n t r a t i o n o f a c t i v e o r g a n o l i t h i u m was 0 . 0 2 6 ^ f o r a 0.027M s o l u t i o n o f p o l y ( s t y r y 1 ) 1 i t h i u m a f t e r a c a l o r i m e t r i c r u n w i t h t e t r a h y d r o f u r a n . The t i m e r e q u i r e d f o r a c a l o r i m e t r i c r u n was g e n e r a l l y l e s s t h a n 30 m i n u t e s . The a c c u r a c y and p r e c i s i o n o f o u r c a l o r i m e t r i c methods were d e t e r m i n e d a t r e g u l a r i n t e r v a l s by c o m p a r i s o n o f o u r d a t a w i t h w e l l - a c c e p t e d values for the h e a t s of s o l u t i o n of standard substances (2_1 ) . The accuracy of our c a l o r i m e t r i bases w i t h polymeric o r g a n o l i t h i u demonstrating that our results are reproducible (± 0.1 k c a l / m o l e ) and i n d e p e n d e n t o f t h e s o u r c e o r method o f p u r i f i c a t i o n o f t h e t e t r a h y d r o f u r a n s . The r e s u l t s a r e s e n s i t i v e t o t h e amount o f n o n - c a r b o n bound l i t h i u m as shown by t h e d a t a in F i g u r e 2. The e f f e c t o f t h e p r e s u m a b l y l i t h i u m a l k o x i d e i m p u r i t i e s i s most a p p a r e n t a t R v a l u e s g r e a t e r t h a n 1.0. I t i s w e l l e s t a b l i s h e d that p o l y ( s t y r y l ) 1 i t h i u m i s predomin a n t l y a s s o c i a t e d i n t o dimers i n hydrocarbon s o l u t i o n s (14,15), while i t i s monomeric i n t e t r a h y d r o f u r a n (L4). Furthermore, c o n c e n t r a t e d s o l u t i o n v i s c o s i t y measurements have shown t h a t t h e e q u i l i b r i u m c o n s t a n t (K ) f o r t h e p r o c e s s shown i n eq 1 [ P S L i = p o l y ( s t y r y l ) l i t h i u m ] has ^ value 8

MPSLi)

2

+ THF

Keq 3 F = £ PSLi»THF

(1)

of a p p r o x i m a t e l y 160 ( L 4 ) . U s i n g t h i s e q u i l i b r i u m c o n s t a n t , i t can be c a l c u l a t e d t h a t upon a d d i t i o n o f t e t r a h y d r o f u r a n (1.2 mmoles) t o 200 ml o f a 0.03M s o l u t i o n o f p o l y ( s t y r y l ) l i t h i u m , more t h a n 9 8 % o f t h e t e t r a h y d r o f u r a n w i l l be c o n v e r t e d t o P S L i * THF. Therefore, i t w o u l d be e x p e c t e d t h a t t h e e n t h a l p i e s o f i n t e r a c t i o n w h i c h we have m e a s u r e d r e f e r t o t h e p r o c e s s shown i n eq 1, i . e . , t h e c o n v e r s i o n by t e t r a h y d r o f u r a n o f d i m e r i c p o l y ( s t y r y l ) l i t h i u m t o s o l v a t e d , monomeric p o l y ( s t y r y l ) l i t h i u m . The d a t a i n F i g u r e 1 show no d r a m a t i c c o n c e n t r a t i o n d e p e n dence f o r t h e i n t e r a c t i o n of e i t h e r t e t r a h y d r o f u r a n or 2,5d i m e t h y l t e t r a h y d r o f u r a n w i t h p o l y ( s t y r y 1 ) l i t h i u m . The a b s e n c e o f d i s t i n c t b r e a k s w i t h i n t h e r a n g e o f R v a l u e s f r o m 0.2 t o 2 c a n be regarded as evidence t h a t the initial base c o o r d i n a t i o n p r o c e s s ( e q 1) i s f o l l o w e d by s u c c e s s i v e c o o r d i n a t i o n w i t h o t h e r t e t r a h y d r o f u r a n m o l e c u l e s as shown i n eq 2. PSLi«THF + nTHF 1Z>.18) 3 f o u r (_15) have b e e n r e p o r t e d b a s e d on l i g h t - s c a t t e r i n g and c o n c e n t r a t e d solu­ t i o n v i s c o s i t y m e a s u r e m e n t s . The most r e c e n t c o n c e n t r a t e d s o l u ­ t i o n v i s c o s i t y s t u d i e s (j^,Γ7), w h i c h i n c l u d e r e s u l t s o f v a r i o u s e n d c a p p i n g and l i n k i n g techniques, provide convincing evidence for predominantly d i m e r i c a s s o c i a t i o n of p o l y ( i s o p r e n y l ) 1 i t h i u m i n h y d r o c a r b o n s o l u t i o n . The e f f e c t o f t e t r a h y d r o f u r a n on the d e g r e e o f a s s o c i a t i o n o f p o l y ( i s o p r e n y l ) l i t h i u m has a l s o b e e n e x a m i n e d by c o n c e n t r a t e d s o l u t i o n v i s c o s i t y measurements ( 13 ) . These r e s u l t s i n d i c a t e t h a t the e q u i l i b r i u m c o n s t a n t f o r the p r o c e s s shown i n eq 3 [ P I L i = p o l y ( i s o p r e n y l ) l i t h i u m ] e x h i b i t s an e q u i l i b r i u m a n
SrSf

β

X

X ^ s 0

0fS

Sr

+

3

"/1.26

X

0 > S r

S

+

β

8

( 1 ·

8

at 20°C)

where

(18)

To o b t a i n the values of Λ and λ a t d i f f e r e n t temperatures we used the f o l l o w i n g equation, r a t h e r than the Walden product which i s known to decrease w i t h temperature ( l b , 1 9 ) . 0

0

β

λ ~(20 Ο ο

β

λ "(τ Ο 0

X

+ o

x

> N a

+(20°C)

+ 0

e

> N a

+(T c)

A Van't Hoff p l o t p r o v i d i n g the corresponding thermodynamic parameters f o r the i o n i c d i s s o c i a t i o n of S r S (eq. 1) i s shown i n 2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

132

ANIONIC POLYMERIZATION

Figure 2.

Plot of equivalent conductance vs. concentration of Sr *, (poly-S') in THF at 20°C (a); Wooster conductance plot in THF at 20°C (b). 2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

10.

DE SMEDT AND VAN BEYLEN

One-Ended

Living

Polystyrene

133

f i g u r e 3. (ΔΗχ° - -1.87 kcal/mol and ASx° » -46.4 e.u.). A curvature of the Van't H o f f p l o t such as observed f o r the t r i p l e i o n formation constant K has been r e p o r t e d i n many other cases and may r e f l e c t the decrease i n exotherraicity of the d i s s o c i a t i o n a t lower temperatures as expected on the b a s i s of the Increase of the d i e l e c t r i c constant with decreasing temperature. Why t h i s i s not the case f o r Κχ i s d i f f i c u l t to say i n cases where s p e c i f i c s o l v a t i o n occurs next to solvent p o l a r i z a t i o n . I f i t were p o s s i b l e to go to lower temperatures a s i m i l a r decrease of the exotherraicity might e v e n t u a l l y a l s o show up f o r Κχ. 2

Table II

Z+

—

S r , ( p o l y - S ) in THF and THP 2

Equilibrium constants Kj and

(THF)

and K- (THP) at differen T, C Kj χ e

(THF)

(THF)

(THP)

20

1.6 (1.6)

3.3 (3.0)

1.2 (1.4)

15

1.9 (1.8)

3.4 (3.0)

1.2 (1.3)

10

2.0 (2.0)

3.5 (3.1)

1.2 Π.4)

5

2.2 (2.1)

3.7 (3.3)

i.2 a . 4 ;

0

2.3 (2.1)

4.1 (3.7)

1.2

a.4)

-5

2.4 (2.3)

4.0 (3.6)

1.2

a.4;

-10

2.6 (2.6)

4.3 (3.9)

1.2 r i . * ;

-15

2.8 (2.7)

4.6 (4.1)

1.1 (1.3)

-20

3.1 (3.0)

4.9 (4.3)

1.1

-25

3.3 (3.2)

5.1 (4.S)

1.1 (1.3)

-30

3.6 (3.7)

5.6 (4.9)

1.1

-35

3.9 (3.9)

5.8 (5.0)

1.1 f i . 3 ;

-40

4.1 (3.7)

5.9 (4.9)

0.92fl.i;

-45

4.4 (4.2)

6.0 (5.1)

-50

4.8 (4.7)

6.1 (5.2)

-55

5.4 (S.3)

6.9 (5.9)

-60

6.2 (S.6)

7.0 (5.8)

-65

6.5 (S.9)

7.1 (5.8)

-70

7.3 (6.6)

7.5 (6.1)

u.s;

a.s;

The S r S - s a l t i s more d i s s o c i a t e d i n THF than the B a S - s a l t (11,15), probably due t o g r e a t e r s o l v a t i o n of the s m a l l e r S r 2

2

2 +

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

134

ANIONIC

POLYMERIZATION

z +

c a t i o n i n the S r S 2 " s a l t than of the B a c a t i o n i n the corresponding s a l t . For a l k a l i ions the s o l v a t i o n energy ( s p e c i f i c and/or through solvent p o l a r i z a t i o n ) exceeds the Coulombic i n t e r a c t i o n energy which binds the ions i n t o i o n p a i r s , and t h e r e f o r e i n THF the d i s s o c i a t i o n of i o n p a i r s i s exothermic ( l b ) . T h i s i s a l s o the case with the SrS2 s a l t , though the s o l v a t i o n of the ( S r S ) c a t i o n i s much smaller than e.g. that of the N a c a t i o n . T h i s i s confirmed by comparing the corresponding AHi°-values (-1.87 and -8.2 kcal/mol r e s p e c t i v e l y ) . In the case of BaS2 i t was already shown that the g a i n i n s o l v a t i o n energy of the ( B a S ) i o n i s smaller than the Coulombic i n t e r a c t i o n energy (ΔΗχ » +0.9 kcal/mol) (11). Furthermore, the entropy change àS\° f o r the d i s s o c i a t i o n of the a l k a l i n e e a r t h p o l y s t y r e n e s a l t s i s s u b s t a n t i a l l y l e s s f l°,SrS ~ * -5 +

+

+

0

AS

s

4 6

e

u

A S

2

d i s s o c i a t i o n of the Na ,S~ contact i o n p a i r (AS = - 60 e.u.) (ytjlb)T h i s smaller decrease i s a t t r i b u t e d to the f a c t that upon d i s s o c i a t i o n of the M , ( S - ) 2 ~ s a l t s , a S" anion remains bound to the M c a t i o n preventing the M * to be reached from a l l s i d e s by the s o l v e n t molecules. The d i f f e r e n c e between A S i , g g ^ and ASi°,B s^also accounts f o r the g r e a t e r s o l v a t i o n of the smaller ( S r S ) c a t i o n formed upon d i s s o c i a t i o n . 2+

2 +

2

e

r

a

+

2.

Strontium

s a l t of one-ended polystyrene In

THP

In THP, which has a lower d i e l e c t r i c constant than THF, and In which conductance measurements were c a r r i e d out at temperatures ranging from -40°C to +20°C, the constant value of the e q u i v a l e n t conductance was maintained throughout the whole c o n c e n t r a t i o n range s t u d i e d (3x10"^ to 8xlO"^M) ( f i g . 4 ) . From these constant h\ v a l u e s , K2 was obtained (see T a b l e I I ) . Since the i n t e r c e p t of the Wooster p l o t ( f i g . 4 ) i s i n d i s t i n g u i s h a b l e from zero, i t was impossible to determine Κχ· The slope of t h i s p l o t gives K2 (values given i n parentheses)· The r e l e v a n t A and λ values are l i s t e d i n T a b l e I I I and were obtained from those i n THF using the Walden r u l e . The v i s c o s i t i e s of THF (19) and THP (20) were taken from the l i t e r a t u r e . Our measurements i n THP thus confirm the mechanism proposed i n the equations (I) and (2) and c l e a r l y i n d i c a t e the lower d i s s o c i a t i o n ( e q . l ) of SrS2 i n THP than i n THF. 0

0

3.

Strontium

t e t r a p h e n y l b o r i d e i n THF

Conductance measurements on strontium t e t r a p h e n y l b o r i d e (SrB2) were performed over a c o n c e n t r a t i o n range of 5 x 1 0 " % to 2xlO~ M and at temperatures extended from -70°C to +20°C. In F i g u r e 5 the r e s u l t s are g r a p h i c a l l y presented as a 1/Λ vs. CA p l o t g i v i n g an accurate i n t e r c e p t , as a r e s u l t of the high degree of d i s s o c i a t i o n of SrB2 i n THF. In t h i s way the values of 6

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

DE SMEDT AND VAN BEYLEN

One-Ended

Living

Polystyrene

135

Figure 4. Plot of equivalent conductance vs. concentration of Sr *, (poly-S~) in THP at 20°C (a); Wooster plot for Sr *, (poly-S) in THP at 20°C (b). 2

2

2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

ANIONIC POLYMERIZATION

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10.

DE SMEDT AND VAN BEYLEN

One-Ended

Living

Polystyrene

137

Table III Limiting equivalent conductances A and \ in THF and THF at different Q

q

temperatures.

Λ

0

X ,triple

λ .triple ο

A ,SrS

(THF)

(THF)

(THP)

(THP)

20

21.0

18.8

12.4

11.1

15

19.6

17.6

11.2

10.1

10

18.4

16.5

10.3

9.2

Τ e

c

V

S r S

5

17.

0

16.

2

Q

Q

2

-5

15.0

13.5

7.6

6.9

-10

14.0

12.6

6.9

6.2

-15

13.0

11.7

6.2

5.6

-20

12.0

10.8

5.5

5.0

-25

11.2

10.0

5.0

4.4

-30

10.2

9.1

4.4

3.9

-35

9.4

8.4

3.7

3.3

-40

8.8

7.9

3.5

3.1

-45

8.0

7.2

-50

7.3

6.6

-55

6.5

5.7

-60

5.8

5.2

-65

5.2

4.7

-70

4.6

4.1

and the corresponding SrBo ^

values f o r the d i s s o c i a t i o n were (SrB) + B~ K (3) SrB +

d

a

2

determined. They are summarized i n T a b l e IV. In a d d i t i o n , Table IV shows the r e l e v a n t thermodynamic parameters. The enthalpy of d i s s o c i a t i o n of the contact i o n p a i r of d i f l u o r e n y l strontium ( S r F l 2 ) was reported to be Δ Η ^ -14.4 kcal/mol a t 20°C, that of the mixed t i g h t - l o o s e Ion p a i r being much l e s s negative (ΔΗ° in THF 2

ΔΗJ - - 0.69 kcal/mol AS! - - 23 e.u.

kcal/mol (15) r e s p e c t i v e l y , we may conclude that these s a l t s are a t l e a s t of the mixed t i g h t - l o o s e type, f o r which there i s but l i t t l e change i n the s o l v a t i o n s t a t e upon d i s s o c i a t o n . Moreover, s i n c e the entropy of d i s s o c i a t i o n f o r the contact i o n p a i r s i s of the order of -50 to -60 e.u., we may again say that the t e t r a p h e n y l b o r i d e s a l t s of Sr and Ba, showing an entropy change of r e s p e c t i v e l y -23 and -22 e.u., are a l s o f o r entropie reasons not of the contact type. In f a c t , the A +-value of ( S r B ) being 12.1 and that of ( B a ^ 26.1 may be taken to i n d i c a t e a double solvent separated i o n p a i r s t r u c t u r e f o r the S r - s a l t and a mixed c o n t a c t - s o l v e n t separated p a i r s t r u c t u r e f o r the BaB2» The double s o l v a t e d +

0

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10.

DE SMEDT AND VAN BEYLEN

One-Ended

Living

139

Polystyrene

2+

c a t i o n of the S r s a l t represented as B ~ 7 / S r / / w i l l upon d i s s o c a t i o n indeed be l e s s mobile than the smaller B"\Ba2+// c a t i o n . The assumption o u t l i n e d above i s a l s o r e f l e c t e d i n the greater K B (=3.05xl0~" M) i n comparison with K (=1.65x10-%) a t 20°C. In c o n c l u s i o n , the f o l l o w i n g e q u i l i b r i a might be w r i t t e n f o r the d i s s o c a t i o n of S r B and BaB , r e s p e c t i v e l y : 5

d

S r

d

B

a

B

2

2

2

2

B-//Sr +//B-—

2

B-

+

2+

//Sr //B-

K

(4)

d

SrB 2+

B-,Ba //B- ς = ± Β " +

2+

//Ba ,B-

K

2

(5)

d

BaB 4.

2

The mixed s a l t SrSB i n THF a t 20°C

When mixed w i t h S r B polystyrene S r S , l i k e forms a mixed s a l t SrS a c c o r d i n g to the e q u i l i b r i u m : 2

SrS

2

+

SrB

2 SrSB

2

K

(6)

a

The formation constant K o f the mixed s a l t SrSB was determined by measuring the v i s c o s i t y of a s o l u t i o n o f such mixture. Indeed the l e n g t h of the S r S molecule i s twice as h i g h as that o f SrSB (and a l s o of dead poly-S) so that we may consider SrSB e q u i v a l e n t to dead poly-S. To a s c e r t a i n a s u f f i c i e n t v i s c o s i t y , a S r S s a l t o f molecular weight of about 58,000 p e r "S"-arm was used (M^/Mn 1.26). Adopting the same method as o u t l i n e d i n r e f . 15, a c a l i b r a t i o n curve was then c o n s t r u c t e d ( f i g . 6 , T a b l e V ) , g i v i n g the s p e c i f i c v i s c o s i t y of mixtures of l i v i n g and r i g o r o u s l y d r i e d dead p o l y s t y r e n e , obtained by t e r m i n a t i o n o f some o f the i n v e s t i g a t e d S r S , as a f u n c t i o n of t h e i r composition r ( r S r S / t o t a l poly-S) f o r a constant t o t a l weight c o n c e n t r a t i o n of p o l y s t y r e n e (83.5 g r / 1 ) . Subsequently the v i s c o s i t y of a mixture of l i v i n g S r S and dead p o l y s t y r e n e of the same t o t a l c o n c e n t r a t i o n was measured without and w i t h a d d i t i o n of S r B . E q u i v a l e n t amounts of l i v i n g S r S and S r B , as were used i n the a

2

2

β

e

2

2

2

2

2

2

case of barium, were not taken i n t h i s case, i n view of the lower s o l u b i l i t y of S r B . I t should a g a i n be s t r e s s e d that the molecular weight d i s t r i b u t i o n of any mixture of l i v i n g and dead polymer i s the same as that of the corresponding mixture of S r S and S r B of the same s p e c i f i c v i s c o s i t y , thus j u s t i f y i n g the method used t o determine the K value f o r any molecular weight d i s t r i b u t i o n . The mixtures of S r S and terminated S used i n determining the c a l i b r a t i o n curve have the same molecular weight d i s t r i b u t i o n as the mixture of S r S and SrSB r e s u l t i n g from the a d d i t i o n of S r B . Indeed samples of the same stock s o l u t i o n were used t o c a r r y out these experiments. Thus, i t was avoided that changes i n molecular weight d i s t r i b u t i o n would a f f e c t the 2

2

2

a

2

2

2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

140

ANIONIC POLYMERIZATION

8.00

5.00

1

« 0.2 8

0

ί0.5

Figure 6. Viscosity of a solution containing SrS and the terminated dead poly-S at various r values (τ = \SrS ]/total poly-S) but at constant weight concentration of total polystyrene (whether as SrS or dead poly-S). The dashed line corresponds to a mixture of living strontium polystyrene ([SrS ] = 4.3 X 10~ M), dead poly­ styrene ([2(terminated S)] = 2.9 Χ /0~ M), and a certain amount of SrB (= 2.4 χ 10 Μ). 2

2

2

2

4

4

2

4

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10.

DE SMEDT AND VAN BEYLEN

Table Viscosity

One-Ended

Living

V

data of the study in

THF

of the mixed β

a t 20

SrSB

t? spec < (b

ο

salt

C

r ο

141

Polystyrene

a )

5.42

>

5. 16

0.09 0.33

5.76

0.50

6.36

0.79

6. 69

( c )

r

5.63

m

(a) (b)

obtained

by

obtained

after

of (c)

termination

the l i v i n g

ratio for with

obtained

of l i v i n g

complete spontaneous SrS

2

from

2

(-

methanol

termination

solution the c a l i b r a t i o n

the m i x t u r e of p o l y s t y r e n e SrB

SrS^ with

curve

(living

(fig.6)

• terminated)

0.28)

r e s u l t s . The r e s u l t s are shown i n f i g . 6 and from the r value determined f o r the above described mixture, K was c a l c u l a t e d to be 100. The mixed s a l t SrSB was assumed to d i s s o c i a t e according to the e q u i l i b r i u m a

SrSB

+

(SrS )

+

B~

K

(7)

b

The conductance of a mixture [SrS2l • 2.19xlO""^M and [SrB2] " 2.08xlO" M was measured and the corresponding [ S r S B ] c o n c e n t r a t i o n and could be c a l c u l a t e d from the f o l l o w i n g set of equations: 4

0

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

142

ANIONIC POLYMERIZATION

[SrSB]

2

K ([SrB ] 2

0

- l/2[SrSB])([SrS ] 2

+

[(SrS) ][B-]/[SrSB]

=

K

~

0

10

2

l/2[SrSB])

b

+

+

[(SrB) ][B-]/([SrB ] -l/2[SrSB]-[(SrB) ]) 2

=

a

» K

Q

» 3.05x10"% SrB

d

2

+

[(SrB) ] + B

+

[(SrS) ] -

t ~J-*o,B~ +

[B~]

+

f(SrB) ]Xo rB

+

> S

I(SrS)+Uo rS

+

+

β

3

L«10 .a

>S

where L i s the measured conductance and a i s the c e l l constant» The f o l l o w i n g λ v a l u e s were used: \q = 38·9 (19)» o,SrB ( P c o n c e n t r a t i o n s were measure K was c a l c u l a t e d r e s u l t i n g i n a mean value of K = 6.72x10"% (see T a b l e V I ) . 0

x

+

β

1 2 β 1

s e e

a r

D

b

Table VI 7+ Conductance of Sr , (poly-S )(B0, ) in THF at 20 C. e

6

* L x 10

2+

[Sr (poly-S~)(B0 ~)]

Λ*

4

(1/Λ.10

2

Λ . ΙΟ

3

LxlO

6

Κ. (calculated from L) D

Ω"

1

1θ\ M

cm mol n" 2

1

-2

1

mol Ω cm

cmV'if

1

1

β"

χ

.0

353

3.54

17.9

5.59

6.33

369

7.55

220

1.80

21.9

4.56

3.95

232

6.68

140

0.905

27.8

3.60

2.51

148

6.86

99.9

0.562

31.9

3.13

1.79

106

6.96

68.9

0.348

35.5

2.81

1.24

73.4

6.76

40.7

0.180

40.6

2.A6

0.73

43.6

7.24

21.9

0.091

43.1

2.32

0.39

23.7

5.83

Cell constant : 1.795 χ 10

5

cm

*L-values corrected for the conductance of the minute amount of remaining Sr(B0.)

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10.

DE SMEDT AND VAN BEYLEN

One-Ended

Living

Polystyrene

143

A l t e r n a t i v e l y A and K f o r the mixed s a l t may be c a l c u l a t e d from a p l o t of 1/Λ vs. [SrSB] A which y i e l d s a s t r a i g h t l i n e (fig.7)· The f o l l o w i n g values may be d e r i v e d from t h i s p l o t : 0

b

0

1/intercept - Λ 38.9 (19) X o

= λ+ + λÔB" · w i l l be equal to 10.5, +

0

S

+

i S r S

r

β

4

9

4

S

a

n

d

s

i

n

c

e

λ

s

ο,B* while the slope -

2

l/K .A * 6.36 y i e l d s K » 6.44x10"%. The measured conductance (L) were c o r r e c t e d f o r a minute f r a c t i o n of remaining SrB2(~5%). b

0

b

I t i s noteworthy a t t h i s p o i n t that only d i s s o c i a t i o n of SrSB a c c o r d i n g t o equation (7) was taken i n t o account and not a c c o r d i n g t o the e q u i l i b r i u m : SrSB ^

(SrB)

K should be much s m a l l e r than K because no propagation was detected i n a q u a s i e q u i v a l e n t mixture of SrS2 and SrB2 (see k i n e t i c measurements) thus the amount of S" i s undetectably s m a l l . C a l c u l a t i o n of the e q u i l i b r i u m constant of the exchange reaction: c

b

(SrB)+

+

S"

(SrS)

K .K 2 a

yields

K

i > e x

+

K

b

= SrB '

K l

« K

K

(9)

i > e x

b

. K d

Β"

+

8.50xl0

6

c

2

j u s t i f y i n g our assumption

that K

b

»

K

Comparing the two d i s s o c i a t i o n s SrB2 ^

c

+

( S r B ) + B" and SrSB

^

(SrS)+ + B " c h a r a c t e r i z e d by the d i s s o c i a t i o n constants K

18.91

-33.9

32.9

8.9

40.9

-53.0

29.8

8.7

37.7

-49.6

18.4

-32.0

26.1

8.6

33.9

-48.6

18.0

-30.4

Physico-chemical parameters of the mixtures D 98K

n/mP -(dD/dT)/K (dD/dP)/atm

3.22

5.45

5.07xl0~

3.55

6.61

6.63x10-

3.87

6.78

8.24x10'

2

v

,-3

-1

.1

1

(dlnn/dT" )/K (dlnn/dP/atra"

,.5 23.7x10"

1.19x10°

7.37x10-

,.5 27.1x10"

1.17x10°

7.17x10'

,-5 30.6x10"

1.15x10°

6.98x10'

4

° The e r r o r s on these small values a r e r e l a t i v e l y l a r g e w h i l e the e r r o r s f o r a l l other experimental values a r e t y p i c a l l y 3-4%. +

f o r an exact agreement the temperature dependence of a shoulg be accounted f o r ; dlna/dT i s [-0.34, -0.04 and"0.16] χ 10" f o r the three mixtures s t u d i e d . dlnad/dP i s always found to be n e g l i g i b l e .

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

166

ANIONIC

Β

+

A

+

s

2AB

Β

+

ABA

+

AB.

s

ABA

POLYMERIZATION

+

BAB

[13]

BAB

+

ABA

+

(A i s the c a t i o n A an omitted here f o r c l a r i t y ) . Such a complex scheme i s however e q u i v a l e n t , from the viewpoint of i o n i z a t i o n , with a simple e q u i l i b r i u m between a non-conducting s t a t e ( i o n - p a i r s and quadrupoles) and a conducting s t a t e ( i o n s and t r i p l e i o n s ) . With the simple assumption o f f a s t e q u l i b r i a w i t h i n each s t a t e (the " h o r i z o n t a l " e q u i l i b r i a i n scheme 13) and noting that u s u a l l y the c o n c e n t r a t i o n o f a l l species i s n e g l i g i b l e compared to the i o n - p a i r c o n c e n t r a t i o n C the r e c i p r o c a l r e l a x a t i o n time a s s o c i a t e d w i t h the slow i o n i z a t i o n mode can be c a l c u l a t e d as: 0

τ"

1

=

K

d

+ + ( k + k )4C /K D

D

0

1 + 9/16.k K (4C /K )

s

F

s

0

2

s

1 + 4C /K 0

K

1/2 1/2 + 1/2 - 1/2 C (1+C /K ) (1+C /K ) d

0

0

3

0

3

k kR kR |k +2| — + — 1 C +3 1

r

l+2Co/K3+2C /K +3Co/K3K3 0

3

T

0

*3

S

2

Co

[14]

K3K3

The d e r i v a t i o n o f t h i s equation from general r e l a x a t i o n theory, w i t h i n the assumptions s t a t e d , i s s t r a i g h t f o r w a r d but tedious and w i l l be published elsewhere. At f i r s t s i g h t equation 14 i s so i n v o l v e d that i t may be everything but u s e f u l f o r any p r a c t i c a l purpose. F o r t u n a t e l y , depending on the ( r e l a t i v e ) s t a b i l i t y o f the d i f f e r e n t aggregated species and on the t o t a l c o n c e n t r a t i o n , d i f f e r e n t l i m i t i n g formes of eq. 14 a r e very i l l u m i n a t i n g . F i r s t , and g r a t i f y i n g , a t low c o n c e n t r a t i o n and/or low s t a b i l i t y o f the aggregated species eq. 14 reduces to:

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

11.

PERSOONS AND EVERAERT

Ionic Processes in Low Polar

r-1

k

d

Media

1/2 1/2 + 2K K C r

d

167

[10]

c

which i s indeed the expression obtained f o r the simple i o n - p a i r d i s s o c i a t i o n . Moreover, as long as the c o n c e n t r a t i o n of quadrupoles i s n e g l i g i b l e compared to the i o n - p a i r s the f i r s t term i n eq. 14, which d e s c r i b e s the d i s s o c i a t i o n , reduces always to k o r depending on the r e l a t i v e magnitude of k , Kjy and kp t o a concentration-dependent erm (which i s always small compared to the recombination term)· Focusing on the recombination term i n eq. 14 we see that t h i s term reduces to: 1

d

d

2k

[14a]

T

K

K

3 3 J at high i o n - p a i r c o n c e n t r a t i o n and/or high s t a b i l i t y of the t r i p l e ions s i n c e then 0 / Κ 3 * < 1 and k « k ± « k assumed to be d i f f u s i o n - c o n t r o l l e d recombination r a t e constants. T h i s i s i n t u i t i v e l y c l e a r s i n c e a t these c o n d i t i o n s the main i o n i c recombination process w i l l be between the a n i o n i c and c a t i o n i c t r i p l e i o n s , the c o n c e n t r a t i o n of which depends on the 3/2 power of the t o t a l i o n - p a i r c o n c e n t r a t i o n . Reasoning along the same l i n e s we note that f o r the case of u n i l a t e r a l t r i p l e i o n formation (e.g. K — » ) a t high i o n - p a i r c o n c e n t r a t i o n , o r f o r the c o n d i t i o n K > C > K "", the recombination term would be g i v e n as: ο

r

+

3

+

3

0

3

Kd

1/2

! [14b]

2kR

κ 3

A l i n e a r c o n c e n t r a t i o n dependence of the r e c i p r o c a l r e l a x a t i o n time upon t o t a l i o n - p a i r c o n c e n t r a t i o n would t h e r e f o r e p o i n t to a main recombination process between a simple i o n and a t r i p l e i o n . C o n s i d e r i n g these d i f f e r e n t l i m i t i n g forms of the recombination term an important t e n t a t i v e c o n c l u s i o n emerges: the c o n c e n t r a t i o n dependence of the r e c i p r o c a l r e l a x a t i o n time i s a d i r e c t measure of the main i o n i c recombination process and y i e l d s t h e r e f o r e i n f o r m a t i o n on the i o n i c species present i n s o l u t i o n . A l i n e a r dependence on t o t a l i o n - p a i r c o n c e n t r a t i o n would t h e r e f o r e i n d i c a t e u n i l a t e r a l t r i p l e i o n formation or, i f both kinds of t r i p l e ions a r e present as i n d i c a t e d by conductance, a s u f f i c i e n t d i f f e r e n c e i n t h e i r s t a b i l i t y . At t h i s p o i n t i t should be noted that the u s u a l method of Fuoss and Draus (19) t o determine s t a b i l i t y constants f o r t r i p l e ions assumes an equal s t a b i l i t y . In many i n v e s t i g a t i o n s t h i s assumption i s o f t e n

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ANIONIC

168

POLYMERIZATION

i m p l i c i t l y accepted even with c o n t r a - i n d i c a t i o n s such as a d i f f e r e n c e i n s i z e of the i o n s . E x p e r i m e n t a l l y a l l the l i m i t i n g cases of the recombination terra are encountered i n a l l systems i n v e s t i g a t e d up to now. In s o l u t i o n s of tetraalkylammonium s a l t s In benzene the r e c i p r o c a l r e l a x a t i o n time i s even dependent on the square of t o t a l c o n c e n t r a t i o n (but here the quadrupoles may be comparable In c o n c e n t r a t i o n w i t h i o n - p a i r s ) which would i n d i c a t e d a preponderance of q u i n t u p l e and t r i p l e ions i n the i o n i c recombination. However a very p u z z l i n g reature of the experimental r e s u l t s , as can be seen d i r e c t l y i n F i g u r e 3, i s the r e l a t i v e l y important i n t e r c e p t which i s always measured f o r systems where the r e c i p r o c a l r e l a x a t i o n time has a higher than square-root dependence on c o n c e n t r a t i o n d i s s o c i a t i o n term of th v a n i s h i n g l y small monomolecular constant f o r systems where the aggregation i n t o quadrupoles i s small. K i s of the order 1-10-% f o r tetrabutylammonium s a l t s i n low p o l a r s o l u t i o n s , the s m a l l e r values a p p l y i n g to benzene-solution (20) and somewhat depending on s i z e and form of the i o n s ; the extent of the d i m e r i z a t i o n of i o n - p a i r s with p i c r a t e i s u s u a l l y small as i n d i c a t e d from conductance. The r e l a x a t i o n behavior i n d i c a t e s a s i t u a t i o n as i f each aggregated form d i s s o c i a t e s i n t o the corresponding ions independently of the other aggrgated s p e c i e s present, indeed i f t h i s would be the case the i n t e r c e p t (which has the dimensions of a r e c i p r o c a l time) c o u l d be e x p l a i n e d as the sum of the d i s s o c i a i t o n r a t e constants of a l l monomolecular d i s s o c i a t i o n processes present. T h i s would be so i f the d i f f e r e n t aggregated s p e c i e s were not i n e q u i l i b r i u m or, which i s the same, would e q u i l i b r a t e more slowly than the i o n i z a t i o n processes. However t h i s assumption i s untenable as confirmed by some measurements on the r e l a x a t i o n of ion-pair-quadrupole e q u i l i b r i a ( 2 1 ) . I t Is t h e r e f o r e not a t r i v i a l problem to d e f i n e which r e l a t i v e l y f a s t monomolecular process present In s o l u t i o n g i v e s r i s e to the i n t e r c e p t . A s a t i s f a c t o r y e x p l a n a t i o n c o u l d only be g i v e n from a d e t a i l e d numerical c a l c u l a t i o n of the extent of p e r t u r b a t i o n of the i o n i z a t i o n mode upon a p p l i c a t i o n of the electric field. The c o n c e n t r a t i o n of i o n i c s p e c i e s , which i s d i r e c t l y p r o p o r t i o n a l to the conductance, i s very small i n these low p o l a r media and s i n c e the r e l a t i v e change i n conductance i s of the order of 1% w i t h f i e l d the a b s o l u t e change i n i o n i c c o n c e n t r a t i o n i s exceedingly s m a l l . As a r e s u l t the change i n c o n c e n t r a t i o n of the aggregated s p e c i e s vanishes almost completely. Otherwise s t a t e d , the aggregation e q u i l i b r i a are not perturbed by the a p p l i c a t i o n of the e l e c t r i c f i e l d which f o r m a l l y , i f we c o n s i d e r the aggregation mode(s), uncouples these equlibria! T h i s means that the d i s s o c i a t i o n term i n eq. 14 should be r e p l a c e d by the sum of a l l monomolecular r a t e constants p e r t a i n i n g to the d i s s o c i a t i o n of a l l aggregates present In s

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PERSOONS AND EVERAERT

Ionic Processes in Low Polar

10 C 6

J

1

o

3 / 2

/M

Media

169

3 / 2

I

2

10*C /M o

Figure 3. Dependence of the reciprocal conductance relaxation time on TBAsalt concentration in media and concentration range where conductance indicates an important fraction of triple ions. TBA-bromide in benzene-nitrobenzene (3, 22 vol %; D — 2, 90) at 298 Κ (Ο). TBA-picrate in benzene-chlorobenzene (16 vol %;D = 2, 78) at 298 Κ (0).

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

170

ANIONIC

POLYMERIZATION

s o l u t i o n , i . e . the i n t e r a c e p t should be g i v e n by k R

Mg /

\

c CH

3

C

0

//\

0 - — CH

3

The three vacancies i n f i l l e d by a covalent bond to e i t h e r an a l k y l group (R) o r h a l i d e (X) and two coordinated l i g a n d s which may be b r i d g i n g X or R, i f the species i s a s s o c i a t e d , or THF, monomer, or the carbonyl groups o f the u l t i m a t e , penultimate, or antipenultimate monomer r e s i d u e s o f the growing c h a i n . I t i s i n the arrangement o f these groups and l i g a n d s around the magnesium atom o f an a c t i v e centre that i t s s t e r e o s p e c i f i c i t y has been and must be sought. The v a r i o u s hypotheses have been discussed and f u l l references given elsewhere ( 2 ) . K i n e t i c a n a l y s i s (1) i n d i c a t e s that the c o n c e n t r a t i o n o f a c t i v e centres i s very low, probably i n the μΜ-mM range. T h i s being so, spectrometric methods are u n l i k e l y to provide d i r e c t evidence of the covalent nature o f the a c t i v e centres o r t h e i r d e t a i l e d s t r u c t u r e s . Such methods a r e v a l i d f o r i n v e s t i g a t i n g the s t r u c t u r e o f the i n i t i a t o r , but the nature of the a c t i v e centre must be deduced from i n d i r e c t evidence; k i n e t i c a n a l y s i s , for i n s t a n c e , has shown that i n the s y n d i o t a c t i c - l i k e or s t e r e o ­ block polymerizations i n i t i a t e d by n-butylmagnesium compounds monomer i s coordinated to the a c t i v e centre ( 8 ) . In the present paper we r e p o r t r e s u l t s obtained with t - b u t y l and phenyl-magnesium compounds which under s u i t a b l e c o n d i t i o n s give h i g h l y i s o t a c t i c polymers. We a r e i n p a r t i c u l a r l o o k i n g to see the extent to which the character o f the p o l y m e r i z a t i o n i s predetermined during the i n i t i a t i o n stage when the s t a b l e and p e r s i s t e n t growth centres a r e formed, and whether monomer i s co­ ordinated to the a c t i v e centre i n these cases. Nominal Solvent and S o l v a t i o n - S t a t e o f the I n i t i a t o r The i n f l u e n c e o f s o l v a t i n g s o l v e n t s , i n our case THF, has been discussed p r e v i o u s l y ( 1 ) . There i s some ambiguity about the s t a t e o f s o l v a t i o n of the i n i t i a t o r i n p o l y m e r i z a t i o n i n nonpolar s o l v e n t s , i n our case toluene. These systems, i n which h i g h l y i s o t a c t i c polymers are formed, a r e sometimes described as " e t h e r - f r e e " or "de-etherated". The b a s i c procedure i s to pump

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

A L L E N ET A L .

Polymerization

of Methyl

Methacrylate

187

o f f the ether i n which the reagent i s prepared and r e d i s s o l v e the reagent i n toluene. The ether removal may be enhanced by r e p e a t ­ ed evaporation and r e d i s s o l v i n g i n toluene o r by h e a t i n g . The most d r a s t i c treatment reported (9) i n v o l v e s h e a t i n g the reagent f o r 8 hours a t 343 Κ under high vacuum ( 1 0 ~ Pa). Not a l l sources s p e c i f y the treatment. The ether component (THF i n our e x p e r i ­ ments) i s o f paramount importance i n determining the k i n e t i c s and s t e r e o s p e c i f i c i t y o f the p o l y m e r i z a t i o n (1,2), I t assumes a v i t a l r o l e i n extant mechanisms proposed to e x p l a i n the s t e r e o s p e c i f i c ­ i t y o f the polymerizations (e.g. 10). We prepared i n i t i a t o r s o l u t i o n s i n toluene a c c o r d i n g to the most d r a s t i c r e c i p e f o r THF removal (9) and used 90 MHz *H NMR to check whether any THF r e ­ mained · S o l u t i o n s o f PhMgBr prepared i n THF were evaporated to dry­ ness under high-vacuum e i g h t hours under high-vacuu toluene s o l u t i o n was f i l t e r e d from some i n s o l u b l e r e s i d u e . THF resonances were observed; t h e i r i n t e g r a t e d i n t e n s i t i e s correspond­ ed to a mole r a t i o of THF:Ph-Mg groups o f 1*2:1. Further e x p e r i ­ ments with s u c c e s s i v e l y l e s s exhuastive procedures showed that only 20 minutes h e a t i n g a t 353 Κ under h i g h vacuum s u f f i c e d to a t t a i n the monoetherate composition. The α and 3 CH2 resonances o f THF i n monoetherate i n i t i a t o r s o l u t i o n s i n toluene were s h i f t e d downfield by 0·12 and 0-62 p.p.m. r e s p e c t i v e l y (11). T h i s confirms t h a t the THF i n these s o l u t i o n s i s , a t l e a s t , predominantly coordinated. On a d d i t i o n o f THF i n excess o f the monoetherate s t o i c h i o m e t r y the resonances s h i f t e d towards t h e i r f i e l d p o s i t i o n s i n THF-toluene s o l u t i o n s . Separate peaks corresponding to f r e e and coordinated THF were not observed. I t was concluded that the e q u i l i b r i u m between these s p e c i e s i s labile. In f u r t h e r experiments the r e s i d u e s were r e d i s s o l v e d i n THF. The NMR s p e c t r a were e s s e n t i a l l y s i m i l a r to those o f s o l u t i o n s o f PhMgBr prepared i n THF. The d e - e t h e r a t i o n procedure does not t h e r e f o r e b r i n g about any i r r e v e r s i b l e changes i n composition o f the reagent. Experiments with t-BuMgBr gave s i m i l a r r e s u l t s . One THF per t-Bu group i s t i g h t l y bound and s u r v i v e s and remains when the r e s i d u e i s d i s s o l v e d i n toluene. The d e - e t h e r a t i o n procedure can be reversed by r e d i s s o l v i n g the r e s i d u e i n THF. I t i s w e l l e s t a b l i s h e d that THF i s i n general d e l e t e r i o u s to the formation o f i s o t a c t i c polymer, e.g. ( 1), and the same i s true w i t h t-BuMgBr and PhMgBr. However, the most e f f i c i e n t i n i t ­ i a t i o n o f i s o t a c t i c growth centres i n toluene s o l u t i o n was not observed w i t h the minimal monoetherate compositions, but when the THF c o n c e n t r a t i o n was s l i g h t l y i n excess (11). Since the r e v e r s ­ i b i l i t y o f THF removal under heat-treatment has been demonstrated t h i s e f f e c t must be due to the presence o f a t r a c e o f excess THF, r a t h e r than decomposition o f i n i t i a t o r d u r i n g the heat treatment necessary to produce the monoetherate. The monoetherate compos­ i t i o n i s c o n s i s t e n t with the s o l u t e being R Mg(THF)2 but i f the 3

2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

188

ANIONIC POLYMERIZATION

s o l u t e i s RMgX, t e t r a h e d r a l c o o r d i n a t i o n can o n l y be maintained with the dimer (RMgX)2(THF) . The proton NMR s p e c t r a of toluene s o l u t i o n s of tBuMgBr and PhMgBr were very s i m i l a r to those o f tBu Mg and Pti2Mg (11) and it must be concluded that the l a t t e r species predominate — a c o n c l u s i o n supported by h a l i d e assay of the toluene s o l u t i o n s . However from the p o i n t o f view of i n i t i a t i o n the s o l u t e s cannot be regarded as d i a l k y l magnesiums; tBu Mg i n toluene does not y i e l d i s o t a c t i c polymer, tBuMgBr and tBuMgCl i n toluene y i e l d polymer i n which syndio- and h e t e r o - t a c t i c t r i a d s cannot be detected by 90 MHz NMR (Figure 1 ) . I t i s suggested that the small excess of THF i s r e q u i r e d to ensure that s u f f i c i e n t h a l i d e , an e s s e n t i a l component of a h i g h l y i s o t a c t i c growth c e n t r e , passes i n t o s o l u t i o n . The optimum amount corresponded to approximately twice the amount corresponding to the monoetherate composition In p o l y m e r i z i n g s o l u t i o XTHF 0-0003. 2

2

2

z

The Importance of Exact S p e c i f i c a t i o n of I n i t i a t i o n C o n d i t i o n s One problem encountered i n the f i e l d i s the apparent i r r e p r o d u c i b i l i t y of the r e s u l t s o f d i f f e r e n t workers, even those i n the same l a b o r a t o r y . T h i s i s p a r t i c u l a r l y the case with molar mass d i s t r i b u t i o n and s t e r i c t r i a d composition. The e x p l a n a t i o n of these apparent i n c o n s i s t e n c i e s comes w i t h the r e a l i z a t i o n that the mechanisms are e n e i d i c and the polymer p r o p e r t i e s are primari l y determined by independent a c t i v e centres of d i f f e r e n t r e a c t i v i t i e s and s t e r e o s p e c i f i c i t i e s whose r e l a t i v e p r o p o r t i o n s are set at the i n i t i a t i o n step, which i s completed i n the f i r s t seconds of the p o l y m e r i z a t i o n . The i r r e p r o d u c i b i l i t i e s a r i s e from i r r e p r o d u c i b i l i t i e s i n the i n i t i a t i o n step which had not been thought r e l e v a n t . Ando, Chûjô and N i s h i o k a (12) noted that these r a p i d exothermic r e a c t i o n s tend to r i s e very s i g n i f i c a n t l y above bath temperature (we have confirmed t h i s e f f e c t ) and a l l o w ed f o r t h i s i n c o n s i d e r i n g the stereochemistry o f the propagation r e a c t i o n . However our r e s u l t s show that the i n f l u e n c e on the i n i t i a t i o n r e a c t i o n s can have a more f a r - r e a c h i n g e f f e c t . Our standard procedure i n p r e p a r a t i v e experiments of i s o t a c t i c polymer i s that used i n e a r l i e r work, where the temperature during i n i t i a t i o n was c a r e f u l l y c o n t r o l l e d . We i n i t i a t e d the p o l y m e r i z a t i o n by mixing i n i t i a t o r s o l u t i o n and monomer s o l u t i o n a l r e a d y thermostatted to a predetermined temperature, u s u a l l y that chosen f o r the p o l y m e r i z a t i o n . At 230 Κ w i t h t-BuMgBr i n THF-toluene mixtures t h i s l e d to a trimodal d i s t r i b ­ u t i o n w i t h the high molar-mass peak i n the 10 range (1,2). More recent work, shown i n Figure 2, has confirmed that trimodal d i s ­ t r i b u t i o n s a r i s e when the mol f r a c t i o n of THF i s below ca 0.3. At one stage experimental convenience l e d us to d i s t i l the mon­ omer onto frozen i n i t i a t o r s o l u t i o n so that i n i t i a t i o n took place at the m e l t i n g temperature of the monomer-toluene-THF mixture. Even though the temperature r o s e w i t h i n a few minutes to 230 Κ 6

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

A L L E N ET AL.

Polymerization

of Methyl

Methacrylate

189

Figure 1. Isotactic, heterotactic, and syndiotactic triad frequencies (i, h, and s) in poly(methyl methacrylate) polymerized and initiated at 225 Κ by t-butylmagnesium bromide (left), t-butylmagnesium chloride (right,), and di-t-butylmagnesium (top) with initial mole fraction of monomer: XMMA = 0.10. Toluene-soluble initiator, X « 0.003 ( · ); toluene-THF solutions with percent THF (100X ) indicated for each point (O). The apex i corresponds to i = 1.00, the base opposite i to i = 0. The i content of any sample is given by the perpendicular distance from the base opposite i. The curves indicate the permitted values of the Bernoullian distribution of i, h, and s. THF

THF

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

190

ANIONIC POLYMERIZATION

and the bulk o f the propagation c a r r i e d out a t t h i s temperature, the polymer was e n t i r e l y d i f f e r e n t , being monomodal and o f mod­ e r a t e molar mass a t a s o l v e n t composition which would have l e d to a t r i m o d a l d i s t r i b u t i o n i f i n i t i a t i o n had occurred a t 230 K. The e f f e c t on the c h a i n c o n f i g u r a t i o n o f the polymers was j u s t as d r a s t i c , t-Butylmagnesium bromide i n toluene s o l u t i o n at 230 Κ produced a polymer o f s t e r i c t r i a d composition i : h : s * 1:1:1 i f i n i t i a t e d a t the m e l t i n g point o f toluene, and i : h : s * 99:0:1 i f i n i t a t o r s o l u t i o n a t 230 Κ was mixed with monomer s o l u t i o n a t 230 K. We have o u r s e l v e s c r i t i c i s e d (1) Petiaud and Quang-Tho Pham (13) f o r assuming i n t h e i r a n a l y s i s o f the s t e r i c t r i a d compos­ i t i o n s o f t h e i r polymers that they had a coherent unimodal molarmass d i s t r i b u t i o n . Our r e s u l t s on a very s i m i l a r system produced polymer o f t r i m o d a l d i s t r i b u t i o wher h k d t b independent and had d i f f e r e n work suggests t h a t it i f a c t produce coherent monomodal samples. Nevertheless incomplete s p e c i f i c a t i o n o f t h e i r i n i t i a t i o n c o n d i t i o n s makes it impossible to be sure. As i n d i c a t e d below, under some circumstances even the order o f a d d i t i o n o f reagents i s an experimental v a r i a b l e . Medium E f f e c t on Molar-Mass D i s t r i b u t i o n o f Polymers Produced by I n i t i a t i o n and Propagation a t 250 Κ Using t-Butylmagnesium Compounds F i g u r e 2 shows g e l permeation chromatograms o f PMMA i n i t i t a t e d and polymerized a t 250 Κ by t-butylmagnesium bromide i n v a r i o u s t o l u e n e - T H F mixtures. The top curve ( X T H F * 0-003) r e f e r s to a toluene s o l u t i o n o f i n i t i a t o r prepared by d i s s o l v i n g the r e s i d u e s remaining a f t e r T H F had been removed by pumping on a high-vacuum l i n e w i t h b r i e f warming. The r e s i d u a l T H F c o r r e s ­ ponded to the optimum amount f o r the production o f i s o t a c t i c growth c e n t r e s . The polymers produced, provided the mol f r a c t i o n of monomer d i d not exceed XMMA 0.20, were i s o t a c t i c ( 1 1 · 0 0 ) w i t h i n the s e n s i t i v i t y o f our assay. As may be seen even though the propagation r e a c t i o n takes a unique stereochemical mode, two independent growth c e n t r e s are o p e r a t i n g s i n c e the d i s t r i b u t i o n i s bimodal. The polymers produced, a f t e r p r e c i p i t a t i o n by methanol, were not f u l l y s o l u b l e i n THF, chloroform, toluene o r any s o l v e n t com­ p a t i b l e with " S t y r a g e l " columns. However a f t e r quick h e a t i n g to a temperature j u s t below T and quick c o o l i n g , samples i n the form of a t h i n f i l m d i s s o l v e d r e a d i l y i n THF and other s o l v e n t s . T h i s , together with the o b s e r v a t i o n that G.P.C. t r a c e s o f the whole samples were i n d i s t i n g u i s h a b l e from those o f the THF-soluble f r a c t i o n o f the sample p r i o r to heat-quench treatment, l e d to the c o n c l u s i o n that the i n s o l u b l e m a t e r i a l i n the untreated polymers was c r y s t a l l i n e . NMR s p e c t r a o f the t r e a t e d samples showed no d e t e c t a b l e syndio- o r h e t e r o - t a c t i c t r i a d peaks, c o n f i r m i n g that no s i g n i f i c a n t degradation had occurred. β

m

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

A L L E N ET AL.

Polymerization

of

Methyl

Methacrylate

191

z

When ΧχΗρ 0.10-0.25 (some t r a c e s have been omitted from Figure 2 f o r c l a r i t y ) trimodal d i s t r i b u t i o n s a r i s e . At high THF (XTHF 0.50) the product i s monomodal of low molar mass. Since the chain s t r u c t u r e of the polymer changes throughout F i g u r e 2, we have not a p p l i e d c a l i b r a t i o n procedures, having n e i t h e r s u i t a b l e standard PMMAs of a p p r o p r i a t e c h a i n s t r u c t u r e nor a sound b a s i s f o r a p p l y i n g a u n i v e r s a l method. The chromatograms are a l i g n e d according to e l u t i o n volume and a mixture of standard polystyrenes i s included to provide a q u a l i t a t i v e r e f e r ­ ence. By way of c o n t r a s t di-t-butylmagnesium gave polymer o f broad monomodal d i s t r i b u t i o n of molar mass - 10 * over the whole range of THF c o n c e n t r a t i o n . 1

Medium E f f e c t on the S t e r i Polymers Produced by I n i t i a t i o 225 R' Using t-Butylmagnesium Compounds The i n f l u e n c e Of THF i s shown i n Figure 1, where the frequen­ c i e s of the i s o - , syndio- and h e t e r o - t a c t i c t r i a d s , i , s and h, of each sample can be read o f f along the a p p r o p r i a t e median (e.g. the apex i corresponding to i = 1·0, the base opposite i c o r r e s ­ ponding to i = 0 ) . The curve represents the B e r n o u l l i a n t r i a d d i s t r i b u t i o n (h = 2 i / s / ) . The b l a c k c i r c l e s r e f e r to sam­ p l e s prepared i n toluene s o l u t i o n c o n t a i n i n g the optimum t r a c e of THF ( X T H F 0.003) f o r i n i t i a t i o n of i s o t a c t i c growth c e n t r e s . I t w i l l be seen that an excess of t h i s , as i n d i c a t e d by mol per­ cent (lOOX-pjjp) on the open c i r c l e s , l e d to syndio t a c t i c - l i k e polymers. Except f o r a few samples having very low i s o t a c t i c content, a l l polymers l i e to the r i g h t of the B e r n o u l l i a n l i n e and have a stereoblock c h a r a c t e r . While toluene s o l u t i o n s of t-butylmagnesium bromide and c h l o r i d e give polymers which were i s o t a c t i c w i t h i n the s e n s i t i v i t y of our assay ( i = 1 - 0 0 ± 0 . 0 1 ) , toluene s o l u t i o n s of di-t-butylmagnesium gave s t e r e o b l o c k polymer with i = 0-33. The a n a l y s i s shown i n Figure 1 i s however incomplete. The medium i s a three-component system c o n s i s t i n g o f two p o l a r com­ ponents (monomer and THF) and one non-polar ( t o l u e n e ) . C o r r e l ­ a t i o n s i n terms of two components o n l y l e a d to incomplete charac­ t e r i z a t i o n and, i f e x t r a p o l a t e d , i n v a l i d c o n c l u s i o n s . In F i g u r e 3 the meso ( i s o t a c t i c ) dyad frequencies of polymers produced by t-butylmagnesium bromide a t 225 Κ are shown as a f u n c t i o n o f the mol f r a c t i o n s of the three components of the medium. I t i s apparent that monomer i t s e l f a c t s as a polar s o l v e n t component decreasing the i s o t a c t i c content i n comparison to toluene. I t should be noted that the polymerizations c a r r i e d out at XMA 1.00 were heterogeneous; monomer was run on to s o l i d monoetherate initiator. In a l l other cases f i l t e r e d s o l u t i o n s of i n i t i a t o r were used. 1

2

1

2

z

=

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

192

ANIONIC POLYMERIZATION

Figure 2.

GPCs of THF

columns in series.

Poly(methyl methacrylate), initiated and polymerized at 250 Κ by t-butylmagnesium bromide in toluene-THF solution (—). Mole fraction of monomer, X = 0.10M. XTHF * indicated in each case. A mixture of standard polystyrene samples of indicated molar mass ( ). All traces are aligned so that the elution volumes correspond. MMA

s

Toluene Figure 3. Meso (isotactic) dyad frequency (m) of poly(methyl methacrylate), prepared at 250 Κ initiated by t-butylmagnesium bromide, as a function of solvent composition. The meso-frequency m is indicated for each point. The solvent composition, given in terms of mole fractions of THF, XTHF, of toluene, Xtoiuene, and of monomer, XMMA, are indicated by the scale on each median, with each apex corresponding to the pure component indicated.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

Polymerization

A L L E N ET AL.

of Methyl

Methacrylate

193

K i n e t i c s o f the P o l y m e r i z a t i o n I n i t i a t e d by t-Butylmagnesium Bromide i n Toluene S o l u t i o n A number o f mechanisms proposed to e x p l a i n the s t e r e o s p e c i f i c i t y i n polymerizations i n v o l v e complexing o f the monomer to a metal p r i o r to a d d i t i o n to the c h a i n ( 2 ) . K i n e t i c evidence has shown that such a mechanism does occur with n-BuMgBr, n-Bu2Mg and s-BuMgBr i n THF-toluene s o l u t i o n . These polymeriza­ t i o n s f o l l o w an i n t e r n a l zero-order r a t e equation. Bateup (1,8) proposed that the mechanism i s

S

k

/ wx/Mg-O \

+

M

c , < * -c k

H

w\,Mg3 \

k / —w\,MMg x

x

3

Kx X

mol/1

1 2 3 4 5 6

0 0.065 0.0107 0.0098 0.0060 0.0034

k

p,x

k

0.45 0.0016 0.010

d,x -1 sec 0 0.00010 0.00011

0.020

0.0007

1/mol»sec

Macromolecules

T a b l e II Blockcopolymerization of ε-Caprolactone L i t h i u m i n THF a t 0°C No b

0> ic) 2 3 4 5 6

Time(hr) 0 0.13 0.83 5 24 120 168

by Poly-a-Methylstyrene iw"

3920 13100 13700 11500 10700 10600 9570

a )

4420 18200 21600 17600 11500 14100 13200

[aMeSt]o=0.41 m o l / 1 [nBuLi] =1.12xl0[€-CL]o=0.58 m o l / 1 a) b y gpc b) p r e p o l y m e r a s a n i n i t i a t o r c ) r e s i d u a l 6 - c a p r o l a c t o n e was o b s e r v e d O

2

Mw/Mn 1.13 1.39 1.58 1.53 1.45 1.33 1.38

mol/1

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

YAMASHITA

ε-Caprolactone

209

Literature Cited 1. Tabuchi, T., Nobutoki, K., and Sumitomo, H., Kogyo Kagaku Zasshi, 71, 1926 (1968), Nobutoki, Κ., Sumitomo, Η., B u l l . Chem. Soc. Japan, 40, 1741 (1967) 2. Perret, R., and Skoulious, Α., Makromol. Chem., 152, 291 (1972) 3. Ito, K., Hashizuka, Κ., and Yamashita, Y., Macromolecules, 10, 821 (1977) 4. Deffieux, Α., and Boileaux, S., Macromolecules, 9, 369 (1976) 5. Jacobson, Η., and Stockmayer, W. H., J. Chem. Phys., 18, 1600 (1950) 6. Flory, P. J., and Semlyen, J. Α., J. Am. Chem. Soc., 88, 3209 (1966) 7. Yamashita, Υ., Polymer Preprints (ACS), 20, 126 (1979) 8. Ito, K., Tomida, Μ. d Yamashita Y. Polym Bull. 1, 569 (1979) 9. Ito, K., and Yamashita, , , , (1978) 10. Nemma, S., and Figueruelo, J. E., Eur. Polym. J., 11, 511 (1975) 11. Yamashita, Y., and Hane, T., J. Polym. Sci., Polym. Chem. Ed., 11, 425 (1973) 12. Yamashita, Υ., Murase, Υ., and Ito, K., J. Polym. Sci., Polym. Chem. Ed., 11, 435 (1973) 13. Yamashita, Y., Iwaya, Y., and Ito, K., Makromol. Chem., 176, 1209 (1975) RECEIVED July 22,

1981.

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15 Original Anionic Pathway to New PA(PO)2 Star-Shaped Block Polymers Based on Polyvinyl or Polydiene Hydrocarbons and Polyoxirane R.

JERÔME

and Ph. TEYSSIE

Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liege, Sart Tilman, 4000 Liege, Belgium G. HUYNH-BA Ε. I. du Pont de Nemours and Company Inc. Polymer Products Department Experimental Station, Wilmington

Naphthalene-terminated p o l y v i n y l aromatics and p o l y i s o p r e n e were obtained s u c c e s s f u l l y . These f u n c t i o n a l polymers were metalated by potassium in THF a t 25°C. The formation o f a s t a b l e di­ negative i o n is observed unless the naphthalene is directly attached t o the end o f the p o l y v i n y l aromatics, in which case a few isoprene u n i t s can be advantageously i n s e r t e d between t h e naphthalene end group and the p o l y v i n y l aromatics. The polymeric and s t a b l e d i n e g a t i v e i o n polymerizes oxirane by both a n i o n i c s i t e s and forms three­ -branched starshaped b l o c k copolymers. Thanks t o t h e i r m u l t i p h a s e c o n s t i t u t i o n , b l o c k copolymers have t h e o r i g i n a l i t y t o a d d a d v a n t a g e o u s l y t h e p r o p e r t i e s o f t h e i r c o n s t i t u t i v e sequences. These v e r y a t t r a c t i v e m a t e r i a l s c a n d i s p l a y n o v e l p r o p e r t i e s f o r new t e c h n o l o g i c a l a p p l i c a t i o n s . In t h i s r e s p e c t , t h e r m o p l a s t i c elastomers a r e demonstrated e x a m p l e s ( l , 2 , 3.) ; t h e y a r e c u r r e n t l y u s e d w i t h o u t a n y m o d i f i ­ c a t i o n a s e l a s t i c b a n d s , s t a i r t r e a d s , s o l i n g s in t h e f o o t w e a r i n d u s t r y , impact r e s i s t a n c e o r f l e x i b i l i t y improvers f o r p o l y ­ s t y r e n e , p o l y p r o p y l e n e and p o l y e t h y l e n e whereas s i g n i f i c a n t d e v e l o p m e n t s a s a d h e s i v e s a n d a d h e r e n d s a r e t o b e n o t e d (h 3). In s o l u t i o n , b l o c k copolymers d i s p l a y i n t e r e s t i n g c o l l o i d a l and i n t e r f a c i a l p r o p e r t i e s . They c a n b e u s e d a s e m u l s i f y i n g a g e n t s in w a t e r - o i l a n d o i l - o i l s y s t e m s (6). I n t h e l a t e r c a s e , the o i l phases a r e s o l i d and t h e y g i v e r i s e t o p o l y m e r i c a l l o y s (j) o r t h e y a r e l i q u i d a n d t h e y a l l o w t h e p r e p a r a t i o n o f l a t e x e s in o r g a n i c medium (0). H o w e v e r , t h e m o l e c u l a r s t r u c t u r e o f b l o c k copolymers based on p o l y b u t a d i e n e PB ( 7 0 $ ) and p o l y s t y r e n e PS b e h a v e a s t h e r m o p l a s t i c e l a s t o m e r s when engaged in m u l t i b l o c k ( P B - P S ) o r t r i b l o c k ( P S - P B - P S ) s t r u c t u r e s b u t n e v e r when i m p l i e d in i n v e r s e t r i b l o c k o r d i b l o c k a r r a n g e m e n t s . Similarly the 9

N

0097-6156/81/0166-0211$05.00/0 © 1981 American Chemical Society

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

212

ANIONIC POLYMERIZATION

s u r f a c e a c t i v i t y o f b l o c k copolymers b a s e d on p o l y o x i r a n e (PO) and p o l y s t y r e n e seems t o d e p e n d on t h e i r m o l e c u l a r p a r a m e t e r s (8). The a c t u a l k n o w l e d g e o f t h e b a s i c m o l e c u l a r s t r u c t u r e p r o p e r t y r e l a t i o n s h i p s r e l i e s m a i n l y on t h e a v a i l a b i l i t y o f w e l l d e f i n e d l i n e a r a r c h i t e c t u r e s : t h e d i , t r i - and m u l t i b l o c k c o p o l y m e r s . New a n d w e l l c o n t r o l l e d m o l e c u l a r s t r u c t u r e s c o u l d u n d o u b t e d l y p r o v i d e a deep u n d e r s t a n d i n g o f t h e b e h a v i o r o f b l o c k c o p o l y m e r s a n d a more e f f i c i e n t m a s t e r i n g o f t h e i r a p p l i c a ­ tions . The p u r p o s e o f t h i s c o n t r i b u t i o n is t o show how t h e d e s i g n o f a new d i a n i o n i c s p e c i e s c a n l e a d t o i n t e r e s t i n g a d v a n c e s in b l o c k c o p o l y m e r ! z a t i o n and e s p e c i a l l y t o o r i g i n a l P A ( P 0 ) stars h a p e d b l o c k c o p o l y m e r s . PA is a h y d r o p h o b i c b l o c k : polystyrene, polytertiarybutylstyren face a c t i v i t y of t h i s nove t e c t u r e is c o n s i d e r e d a n d c o m p a r e d a s f a r as p o s s i b l e w i t h t h e b e h a v i o r o f t h e c o r r e s p o n d i n g ΡΑ-PO d i b l o c k c o p o l y m e r s . 2

M e t a l a t i o n o f N a p h t h a l e n e by P o t a s s i u m Room T e m p e r a t u r e

in

Tetrahydrofuran

at

A p o l y m e r PA t e r m i n a t e d w i t h a s t a b l e d i n e g a t i v e i o n a b l e t o p o l y m e r i z e o x i r a n e is a n i d e a l p a t h w a y t o w a r d s P A ( P 0 ) 2 s t a r shaped b l o c k copolymers. N a p h t h a l e n e is known t o f o r m a s t a b l e l i t h i u m d i a n i o n a t - 8 0 ° C in t e t r a h y d r o f u r a n (THF) a t c o n c e n t r a t i o n s l o w e r t h a n 0 . 5 mol.l""-*- ( 9 - 1 2 ) . U n f o r t u n a t e l y o r g a n o l i t h i u m compounds a r e u n ­ able to polymerize oxirane (13). N a p h t h a l e n e c a n a l s o be m e t a l a t i o n b y s o d i u m a n d p o t a s s i u m in THF b u t no e x p e r i m e n t a l e v i d e n c e f o r a d i a n i o n o f n a p h t h a l e n e s o d i u m o r p o t a s s i u m is t o be f o u n d . A l t h o u g h n a p h t h a l e n e m e t a l a t i o n b y s o d i u m is t h o r o u g h ­ l y d e s c r i b e d ( 9 . , l U , 1 5 . ) , v e r y few r e s u l t s a b o u t p o t a s s i u m a r e p u b l i s h e d (l6, 1777 As t h e r e d u c i n g p o w e r o f t h e a l k a l i m e t a l s decreases from l i t h i u m ( L i / L i = 3 . 0 2 v ) t o potassium (K/K = - 2 . 9 2 v ) a n d f i n a l l y t o s o d i u m ( N a / N a = 2.71v) ( l 8 ) , it is a t t r a c t i v e t o s t u d y in more d e t a i l t h e n a p h t h a l e n e m e t a l a t i o n b y Κ in t h e THF. +

+

+

T h e r e is a g r e a t s i m i l a r i t y in t h e c o u r s e o f t h e n a p h t h a ­ l e n e m e t a l a t i o n b y L i a t - 8 0 ° C o n one h a n d a n d b y Κ a t RT o n t h e o t h e r h a n d . The t i t r a t i o n o f t h e c a r b a n i o n s f o r m e d (l£) p r o v e s t h e p r e s e n c e o f two a n i o n s p e r n a p h t h a l e n e o n l y a t n a p h t h a l e n e c o n c e n t r a t i o n s l o w e r t h a n 0 . 0 3 m o l . l " f o r Κ a t RT ( T a b l e I ) a n d 0 . 5 mol. 1-1 f o r L i at -80°C (9.). In both cases, the d i a n i o n i c 1

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

JERÔME ET A L .

PA(PO)

2

Star-Shaped

Block

213

Polymers

s p e c i e s a r e a b l e t o t r a n s f e r one o f t h e i r e l e c t r o n s t o n e u t r a l naphthalene w i t h f o r m a t i o n o f the well-known naphthalene r a d i c a l a n i o n (NRA) ( 1 0 , 19). T h e n a p h t h a l e n e m e t a l a t i o n b y Κ a t RT p r o c e e d s in t w o s u c c e s s i v e s t e p s : i n i t i a l r e d u c t i o n t o t h e NRA ( F i g u r e 1A) and f i n a l f o r m a t i o n o f t h e d i a n i o n i c s p e c i e s a t n a p h t h a l e n e c o n c e n t r a t i o n l o w e r t h a n 0.03 m o l . l " ( F i g u r e I B ) . The assumed p o t a s s i u m d i a n i o n is s t i l l s t a b l e a t RT a f t e r 100 h r s ; h o w e v e r , in t h e p r e s e n c e o f a n e x c e s s o f n e u t r a l n a p h t h a l e n e , t h e a b s o r p t i o n o f t h e NRA is a g a i n o b s e r v e d ( F i g u r e 1 C ) . The h y d r o l y s i s p r o d u c t s o f t h e p o t a s s i u m n a p h t h a l e n e d i ­ a n i o n h a v e b e e n a n a l y z e d b y p r o t o n NMR s p e c t r o s c o p y . As r e p o r t e d e l s e w h e r e (19.) » t h e y a g r e e w i t h e q [ l ] , w h e r e a s t h e r a t i o b e t w e e n t h e f o r m s I a n d I I is c l o s e t o 3. 1

[I]

[II]

On t h e b a s i s o f a l l t h e s e e x p e r i m e n t a l o b s e r v a t i o n s t h e f o l l o w i n g r e a c t i o n scheme c a n b e p r o p o s e d f o r t h e m e t a l a t i o n o f n a p h t h a ­ l e n e b y Κ in THF a t RT ( e q [ 2 ] ) .

H

K

+

H

K

+

E q [3] t a k e s i n t o a c c o u n t t h e b e h a v i o r o f t h e p o t a s s i u m n a p h t h a ­ l e n e d i a n i o n in t h e p r e s e n c e o f a n e x c e s s o f n a p h t h a l e n e .

H

K

+

H

K

+

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

214

ANIONIC POLYMERIZATION

M e t a l a t i o n o f g - E t h y l n a p h t h a l e n e b y P o t a s s i u m in THF a t Room Temperature Once f i x e d a t t h e e n d o f a p o l y m e r P A , t h e n a p h t h a l e n e is 3 s u b s t i t u t e d ; t o assess t h e i n f l u e n c e o f t h e s u b s t i t u t i o n on t h e m e t a l a t i o n o f n a p h t h a l e n e , β-ethylnaphthalene (EN) was s t u d i e d as a m o d e l compound. I n c o n t r a s t t o t h e p o t a s s i u m n a p h t h a l e n e d i a n i o n , EN g i v e s r i s e t o a d i a n i o n i c s p e c i e s a t c o n c e n t r a t i o n s h i g h e r t h a n 0.03 m o l . l " ( T a b l e I I ) . F u r t h e r m o r e t h e EN d i a n i o n is u n a b l e t o t r a n s f e r one e l e c t r o n t o a n o t h e r n a p h t h a l e n e m o l e c u l e . These sharp d i f f e r e n c e s a r e t o be a t t r i b u t e d t o t h e p a r t i c i p a t i o n o f t h e β-ethyl g r o u p in t h e m e t a l a t i o n p r o c e s s . The EN m e t a l a t i o n b y Κ in THF a t RT h a s b e e n a n a l y z e d b y UV spectrophotometry (Figur d i a n i o n ( F i g u r e 2, Β t l y formed, b u t a d i a n i o n i c species c h a r a c t e r i z e d by spectrum C ( F i g u r e 2) is f i n a l l y f o r m e d w h i c h is s t a b l e in t h e p r e s e n c e o f p u r e n a p h t h a l e n e . The a b s o r p t i o n a t U35nm ( F i g u r e 2C) c o r r e s ­ ponds t o t h e v a l u e r e p o r t e d f o r t h e d i h y d r o n a p h t h a l e n e m o n o a n i o n (DHNA) i+33-35nm (20). DHNA is e a s i l y h y d r o l y z e d i n t o d i h y d r o n a p h t h a l e n e (DHN) w h i c h c a n b e a g a i n m e t a l a t e d t o g i v e DHNA ( e q 1

h).

DHNA

DHN

DHNA

S i m i l a r l y t h e h y d r o l y s i s p r o d u c t o f t h e s t a b l e p o t a s s i u m EN d i a n i o n ( F i g u r e 2C) is m e t a l a t e d b y Κ in THF a t RT a n d t h e c h a r a c t e r i s t i c o f DHNA is a g a i n o b s e r v e d a t U35nm ( F i g u r e 2 D ) . However, t h e s p e c t r a a r e s l i g h t l y d i f f e r e n t . H e n c e , t h e EN d i a n i o n is a c c o r d i n g l y assumed t o b e c o n s t i t u t e d b y a DHNA a n i o n and a l s o b y a n e x t r a c y c l i c c a r b a n i o n r e s u l t i n g f r o m a n i n t r a ­ m o l e c u l a r h y d r o g e n t r a n s f e r ( e q 5, 6). F o r c l a r i f i c a t i o n , t h e 1,2 d i h y d r o s t r u c t u r e is o m i t t e d .

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

JEROME E T A L .

PA(PO)

2

Star-Shaped

Block

Polymers

215

TABLE I Naphthalene M e t a l a t i o n by Potassium in THF a t Room Temperature : Naphthalene

: Carbanions

: Concentration

te"]

(a) : Concentration

[N ] : [N]m.l

: [ C ] m.l 1

1

: U.5 10"

-

3

ί 0.9

ίο"

: h.O

10 "

: 5.9 10 ~

:

2.0

:

2

:

2.0

:

2

:

1.97

:

2

: 9.5 10"

3

: 2.0 10~

2

: 3.0 10"

2

: 7.0 10"

2

: 1.1 ί ο -

1

:

1.57

:

: 1.1 10"

1

: 1.1 ί ο -

1

:

1.00

:

(a) F i n a l constant values

TABLE I I M e t a l a t i o n o f β-ethylnaphthalene (EN) by Κ in THF a t Room Temperature : EN

:

Time o f

:

metalation

:

(hours)

:

(m.l" )

1

:

per EN

: 0.l60

:

120

:

0.32

:

2.0

: 0.125

:

96

:

0.2U

:

1.9

:

: 0.015

:

33

:

0.03

:

2.0

:

:

1

(m.l" )

: [Carbanions] : Nr o f carbanions :

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

:

216

ANIONIC POLYMERIZATION

Figure 1. Visible and UV spectra of (A ) naphthalene radical anion (NRA ) (THF, 25°C); (B) naphthalene dianion (OD values shifted upward by 0.2 unit); (C) naphthalene dianion after neutral naphthalene addition (OD values shifted upward by 1 unit; the NRA concentration is the same in A and B).

Figure 2. The β-ethylnaphthalene metalation by Κ in THF at RT. UV spectra after (A) 10 min, NRA spectrum; (B) 10 h, naphthalene dianion spectrum (OD shifted upward by 0.4 unit); (C) 50 h, isomerized β-substituted naphthalene dianion (OD shifted upward by 0.25 unit); (D) compound corresponding to Spec­ trum C, water deactivated, purified, and again metalated by Κ (OD shifted upward by 1 unit).

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

JEROME E T A L .

PA(PO)

2

Star-Shaped

Block

217

Polymers

[6]

VII Forms I I I a n d I V must b e c o n s i d e r e d t o d e s c r i b e t h e n a p h t h a l e n e t y p e d i a n i o n w h i c h is f i r s t f o r m e d d u r i n g t h e EN m e t a l a t i o n ( F i g u r e 2, B ) . The p r o t o n NMR a n a l y s i s o f t h e h y d r o l y s i s p r o d ­ u c t s (V a n d V I ) o f t h e i s o m e r i z e d EN d i a n i o n ( F i g u r e 2, C) a g r e e s w i t h t h e presence o f t h e e t h y l group on t h e dihydronaphthalene l»ing (19) a n d s u p p o r t A g a i n m e t a l a t e d b y K, ( V I I I ) ; ( e q 7) t h e UV a b s o r p t i o n o f w h i c h ( F i g u r e 2, D) is s l i g h t l y m o d i f i e d in t h e i s o m e r i z e d EN d i a n i o n ( V I I F i g u r e 2, C) b y t h e extra c y c l i c carbanion. C

V

C H

2lQ§)

- * - ,

CH -CH 3

VI

2

l

Qg

[ ] 7

)

VIII

From t h e e x p e r i m e n t a l r e s u l t s , t h e f o l l o w i n g mechanism is p r o ­ p o s e d f o r EN m e t a l a t i o n ( e q 8 )

[8]

H transfer.

Synthesis o f Naphthalene Terminated Polymers The u s e o f l i t h i o n a p h t h a l e n e a n d l i t h i o β-methylnaphthalene as f u n c t i o n a l l y s u b s t i t u t e d a n i o n i c i n i t i a t o r s is v e r y d i s a p ­ p o i n t i n g (21, 22). The d e a c t i v a t i o n o f l i v i n g p o l y a n i o n s o n t o α-bromonaphthalene o r b r o m o - 3 - m e t h y l n a p h t h a l e n e is a n o t h e r u n ­ s a t i s f a c t o r y a p p r o a c h due t o m e t a l h a l o g e n i n t e r c o n v e r s i o n ( e q s 9 , 10) (23). H

*

PS"Li +fulf)l +

( 0 ) + P S - C H2- C - B r 0

Li IX + P S ~ L i

+

,

[9]

(IX)

L i B r + PS-PS

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

[10]

218

ANIONIC POLYMERIZATION

To l i m i t t h i s p r o c e s s , R i c h a r d s h a s p r o p o s e d t o t r a n s f o r m t h e l i v i n g p o l y a n i o n i n t o i t s G r i g n a r d a n a l o g u e w h i c h is much l e s s r e a c t i v e b u t s t i l l a b l e t o i n t e r a c t w i t h t h e u s u a l bromor e a g e n t s (2k). E q u a t i o n s 1 1 and 1 2 summarize t h e G r i g n a r d p o l y ­ mer f o r m a t i o n a n d i t s r e a c t i o n w i t h 3-bromomethylnaphthalene PS~Li

+

PS-MgBr

+

MgBr

+

,

2

PS-MgBr

Br-CH -f

,

2

+

LiBr

[ l l ]

PS-CH -rj

By t h i s p a t h w a y t h e d e g r e e o f p o l y m e r about 5 t o 1 0 $ ( F i g u r e end g r o u p i n d i c a t e s a f u n c t i o n a l i t

2

+ MgBr

2

[12]

c o u p l i n g is r e d u c e d t o

25).

M e t a l a t i o n o f Naphthalene A t RT

E n d e d P o l y d i e n e b y P o t a s s i u m in t h e

The m e t a l a t i o n o f n a p h t h a l e n e 3 - s u b s t i t u t e d b y b o t h a n e t h y l g r o u p o r p o l y i s o p r e n e c h a i n ( P I P ) is c o m p l e t e l y s i m i l a r a s e s ­ t a b l i s h e d b y t h e t i t r a t i o n o f t h e c a r b a n i o n s f o r m e d a n d t h e UV a n a l y s i s o f t h e r e a c t i o n medium ( 2 1 , 2 2 , 2 5 ) . A c c o r d i n g l y t h e naphthalene r a d i c a l a n i o n , naphthalene d i a n i o n and i t s f u r t h e r i s o m e r i z a t i o n by hydrogen t r a n s f e r a r e s u c c e s s i v e l y observed and t h e f i n a l s t a g e o f t h e m e t a l a t i o n c a n be r e p r e s e n t e d b y t h e following structure: PIP

CH-tf~ rl

M e t a l a t i o n o f Naphthalene P o t a s s i u m in THF A t RT

N

RESONANCE FORMS

Ended P o l y v i n y l A r o m a t i c Chains b y

When n a p h t h a l e n e is s u b s t i t u t e d b y a p o l y v i n y l a r o m a t i c c h a i n p o l y s t y r e n e P S , p o l y ( p - t e r t - b u t y l s t y r e n e ) PTBS o r poly(α-methylstyrene) PMS t h e c o u r s e o f t h e m e t a l a t i o n is d e e p l y modified ( 2 1 , 2 2 , 25.). Four c a r b o n i o n s can be formed p e r naphthalene end group ( F i g u r e h) w h e r e a s t h e n a p h t h a l e n e e n d g r o u p is c o m p l e t e l y r e l e a s e d a t t h e end o f t h e m e t a l a t i o n . T h i s phenomenon h a s b e e n t h o r o u g h l y s t u d i e d and w i l l be p u b l i s h e d elsewhere ( 2 2 ) . B r i e f l y when n a p h t h a l e n e is c o n j u g a t e d t o a n a r o m a t i c n u c l e u s t h e p o t a s s i u m d i a n i o n is u n s t a b l e a n d t h e n a p h t h a l e n e e n d g r o u p is r e l e a s e d w h e r e a s t h e p o l y v i n y l a n i o n is f r e e d a n d f i n a l l y i s o m e r ­ i z e d ; eqs 1 3 t o I T summarize t h e s e o b s e r v a t i o n s .

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

JERÔME ET A L . PA{PO)% Star-Shaped

Block

(XII)

R

219

Polymers

(XIII)

S i m i l a r i n s t a b i l i t y o f conjugated a n i o n i c systems has a l r e a d y been o b s e r v e d ; L a g e n d i j k and Szwarc have r e p o r t e d t h a t d i ( a n a p h t h y l ) e t h a n e d i a n i o n ( N N ~ ) in e t h e r s o l v e n t u n d e r g o e s s c i s s i o n o f t h e c e n t r a l C-C b o n d w i t h f o r m a t i o n o f t w o m o n o a n i o n (NN~ —, 2N~) (26). To s u p p r e s s t h e d e l e t e r i o u s e f f e c t o f c o n j u g a t i o n a n d t o o b t a i n a s t a b l e d i a n i o n t e r m i n a t e d PS o r PTBS, a t l e a s t o n e i s o p r e n e u n i t is t o b e i n s e r t e d b e t w e e n t h e p o l y v i n y l aromatic c h a i n and t h e naphthalene end group; t h i s o p p o r t u n i t y has been s u c c e s s f u l l y a p p l i e d . 2

2

Star-Shaped Block

Copolymerization

In the f i r s t step the oxirane polymerization i n i t i a t e d by t h e i s o m e r i z e d EN d i a n i o n h a s b e e n s t u d i e d . The r e s u l t s r e p o r t e d in T a b l e I I I a n d t h e l o w p o l y d i s p e r s i t y o b s e r v e d ( a b o u t 1 . 1 ; F i g u r e 5) a r e c o n c l u s i v e f o r a l i v i n g a n i o n i c system. Moreover t h e i n i t i a t i o n r a t e o f w h i c h is h i g h e r t h a n t h e p r o p a g a t i o n o n e .

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Figure 3. G PC of a naphthalene-terminated polystyrene (M = 3400) obtained by reaction of polystyrylmagnesium bromide with β-bromomethylnaphthalene. n

TABLE I I I . O x i r a n e P o l y m e r i z a t i o n I n i t i a t e d

by t h e (a)

S t a b l e P o t a s s i u m β-ethylnaphthalene D i a n i o n

Sample

[initiator]

Conversion

1

(m.l )

(%)

Mn

Mn

t h e o r . (b)

exper.

PO 1

1 6 . 3 10"

100

2.6 χ 1 0

3

2,700

PO 2

8 . 2 10~

93

5.0 χ 1 0

3

5,200

PO 3

5.U 1 0

95

3 7.7 x 10°

k

-k

7,500

(a) [Oxirane] = 0 . 1 m . l , t i m e : 7 2 h r s , temperature: solvent: THF (Q0%) + Bz ( 2 0 $ ) . - 1

( b ) Mn c a l c u l a t e d o n t h e b a s i s o f t h e monomer o v e r molar r a t i o .

25°C

initiator

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

JERÔME E T A L .

PA(PO)

2

Star-Shaped

Block

221

Polymers

C/N

0

20

40

60

80

100

120

UO ' 160 *

300 500 Time (hrs)

Figure 4. Kinetics of the metalation of naphthalene-terminated PTBS (M = 3400) by K (THF, 25°C, [naphthalene] = 3 X 10' mol/L); C/N = number of carbanions generated per naphthalene end group. n

2

PO 2

Figure 5.

GPC of polyoxirane using potassium β-ethylnaphthalene initiator.

dianion as

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ANIONIC POLYMERIZATION

222

The o x i r a n e p o l y m e r i z a t i o n c a n b e d e s c r i b e d b y eqs 18 a n d 19: H

°3 :-CH

yH-CH -CH -0-

3

2

2

[

1

8

]

XIV

f4H-CH -CH -0-

X I V

2

2

a2 CH -CH -0" 2

[19]

2

f 3

0 3 " "H-CH -CH -0-CH -CH -0 2

2

2

2

From t h e k i n e t i c s o f t h e o x i r a n e p o l y m e r i z a t i o n i n i t i a t e d b y a l c o h o l a t e (l6) a n d b y f l u o r e n y l p o t a s s i u m (27) a n d a s f l u o r e n y l and d i h y d r o n a p h t h a l e n e mono a n i o n (28) h a v e a p p r o x i m a t e l y t h e same b a s i c i t y , t h e k 2 o v e r k p p r a t i o may b e e s t i m a t e d t o 20. T h e r e f o r e , t h e l e n g t h o f t h e t w o g r o w i n g p o l y e t h e r c h a i n s must be l a r g e l y i n d e p e n d e n t o n t h e n a t u r e o f t h e i n i t i a t i n g s i t e . I n t h e s e c o n d s t e p , o x i r a n e h a s b e e n a d d e d a t -30°C t o a n i s o m e r i z e d and s t a b l e naphthalene d i a n i o n f i x e d a t t h e end o f t h e PA c h a i n . O x i r a n e is c o m p l e t e l y p o l y m e r i z e d a t RT, a n d t h e c r u d e p r o d u c t o b t a i n e d is s e p a r a t e d i n t o b l o c k c o p o l y m e r , s t a r t i n g homopolymer PA a n d h o m o p o l y e t h e r . The s e p a r a t i o n scheme is d e s c r i b e d in T a b l e I V a n d t h e r e s u l t s o b t a i n e d f o r a g r e a t l o t o f PA ( P I P , P S , a n d PTBS) b a s e d c o p o l y m e r s a g r e e w i t h a r a t i o o f a b o u t 90$ b l o c k c o p o l y m e r , 10% homo PA a n d o n l y t r a c e s o f homopolyether. The c o m p o s i t i o n o f t h e s t a r - s h a p e d b l o c k c o p o l y m e r is e a s i l y d e t e r m i n e d b y p r o t o n NMR a n a l y s i s ; f r o m t h i s a n d t h e mean number a v e r a g e m o l e c u l a r w e i g h t (Mn) o f t h e s e q u e n c e P A , Mn o f t h e p o l y e t h e r component c a n b e c a l c u l a t e d . The l a t e r is v e r y s i m i l a r t o t h e v a l u e f r o m membrane o s o m e t r y . H y d r o x y l e n d g r o u p o f P A ( P 0 ) s t a r - s h a p e d b l o c k copolymers have been t i t r a t e d a n d t h e i r mean number p e r c o p o l y m e r (1.85) a g r e e s w i t h t h e p r e s e n c e o f t w o polyoxirane branches. On t h e a v e r a g e , t h e p o l y d i s p e r s i t y o f t h e s t a r - s h a p e d b l o c k c o p o l y m e r s v a r i e s b e t w e e n 1.2 a n d 1.3 ( F i g u r e a

r 0

2

6). The m o l e c u l a r p a r a m e t e r s o f a s e r i e s o f w e l l c h a r a c t e r i z e d P A ( P 0 ) b l o c k c o p o l y m e r s c a n b e f o u n d in T a b l e V. 2

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

JEROME E T AL.

PA(PO)

Star-Shaped

2

Block

Polymers

223

TABLE I V S e p a r a t i o n o f PTBS ( P 0 )

2

Copolymers

f r o m C o r r e s p o n d i n g PTBS a n d PO Homopolymers (a) H i g h l y powdery c r u d e p r o d u c t

Ψ

E x t r a c t i o n b y heptane under vigorous s t i r r i n g (b,c) Centrifugation

Soluble Part [S]

Insoluble Part [ i ]

drying

,(a) off

(a,

J-

v

drying o f f

i

E x t r a c t i o n by e i t h e r water o r e t h a n o l a t 30°c( »99

CH3I

-78°

60

CH S0 F

-78°

--e

CH3I

-78°

3

a

b

c

d

g

3

c

2

% Meso

58

50% by volume, b. Triglyme 20% excess over carbanion, 20% excess of crown ether, d. Trace of THF present, Mixture of pentane and butane products, f. About .2M Ethyl carbanion salt prepared in toluene without the presence of THF.

but the stereoselectivity decreases sharply with increasing cation size and -coordination. The results also suggest that the methylation stereochemistry is kinetically determined, and this is confirmed by epimerization (k-tBuO/DMSO) of the mesopentane which yields approximately equal proportions of the meso- and racemic 2,4-di-(2-pyridyl)pentane. The place of sub­ stitution in both the pyridine rings of [2] is a key factor in the stereoselectivity. The methylation of the Li salt of [2b] does yield approximately equal proportions of meso and racemic [6b] under these conditions, and the methylation of the 1lithio-1-(4-pyridyl)-3(2-pyridyl)butane likewise yields a 50/50 mixture of diastereomers. A comparison of the stereochemistry of the 2-vinylpyridine addition to [2a] with that of methylation addition is particu­ l a r l y instructive. Table 2 shows the stereochemistry of forma­ tion of [7a] as a function of counterion size and coordination. Table 2 Stereochemistry of Formation of 2,4,6-Tri(2-pyridyl)-heptane As a Function of Cation and Cation Coordination in THF at -78°C I(m,m) Li, THF Na, THF

d

>95(98.0)

a

>95(92.2)

Na, THF; Na, Ce, THF

b

H(mr) · s. ?

—

00

ο

£ε

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

280

ANIONIC POLYMERIZATION

( F i g . 2 ) suggest t h a t o n l y one t y p e o f i o n - p a i r s is p r e ­ sent in t h e system. E n t h a l p y o f d i s s o c i a t i o n c l o s e t o z e r o and e n t r o p y o f d i s s o c i a t i o n h i g h e r than -25 e.u. i n d i c a t e t h a t a d d i t i o n a l s o l v a t i o n o f i o n s upon d i s s o c i a t i o n is low and e n t h a l p y o f t h i s s o l v a t i o n m e r e l y b a l a n c e s t h e e l e c t r o s t a t i c energy n e c e s s a r y t o s e p a r a t e t h e i o n s . A p p a r e n t l y , t h e o n l y moderate i n c r e a s e o f s o l v a t i o n o f i o n s accompanying t h e d i s s o c i a t i o n is observed because the charges a r e d i f f u s e d in b o t h c a r b o x y l a t e a n i o n and DBCK c a t i o n . D i f f u s i o n o f charge in DBCK c a t i o n means t h a t due t o p o l a r i z a t i o n some p o s i t i v e charge is i n d u ­ ced in t h e e x t e r i o r p a r t o f crown e t h e r m o l e c u l e . Low v a l u e o f t h e i n t e r i o n i c d i s t a n c e ( a 3 A°) i n d i c a t e s t h a t t h e i o n - p a i r s a r e t h e c o n t a c t ones T h e r e f o r e we postulate f o r the structur shape o f crowned c a t i o c a t i o n p e r p e n d i c u l a r l y t o t h e p l a n e o f t h e d i s k , and assume t h a t t h i s is t h e o n l y k i n d o f i o n - p a i r s p a r t i c i ­ p a t i n g in p o l y m e r i z a t i o n . Thus, t h e o t h e r isomer p o s t u ­ l a t e d by Hogen Esch and Smid in t h e p o l y m e r i z a t i o n w i t h c a r b a n i o n s [24,2Ji], where K c a t i o n is w i t h d r a w n from the immediate p r o x i m i t y o f a n i o n , and p h y s i c a l l y sepa­ r a t e d from it by elements o f t h e crown e t h e r does n o t have t o be i n v o l v e d in t h e p o l y m e r i z a t i o n o f $PL. A c t i v a t i o n p a r a m e t e r s f o r p r o p a g a t i o n on macroions were found t o depend s i g n i f i c a n t l y on t h e s t a r t i n g c o n c e n t r a t i o n o f &PL, whereas t h e c o r r e s p o n d i n g a c t i ­ v a t i o n p a r a m e t e r s f o r p r o p a g a t i o n on m a c r o i o n - p a i r s were found t o be v i r t u a l l y independent on t h e composi­ t i o n o f t h e medium. A p p a r e n t l y , s o l v a t i o n o f i o n - p a i r s is n o t v e r y much d i f f e r e n t from s o l v a t i o n o f a c t i v a t e d complex. Dependence o f ΔΗί(-) and AS*(-) on t h e s t a r t i n g c o n c e n t r a t i o n o f 3PL, showing a t y p i c a l compensation phenomenon ( T a b l e 3) c a n o n l y have its o r i g i n in t h e s p e c i f i c s o l v a t i o n o f t h e growing s p e c i e s by m o l e c u l e s o f monomer i t s e l f . I t does n o t mean, however, t h a t m o l e c u l e s o f monomer engaged in t h e s o l v a t i o n s h e l l are p r o p e r l y o r i e n t e d f o r t h e c h e m i c a l change. B e i n g p r o p e r l y o r i e n t e d from t h e s o l v a t i o n view p o i n t t h e s e m o l e c u l e s e n e r g e t i c a l l y d i f f e r from an average m o l e c u l e in b u l k s o l u t i o n and it may be n e c e s s a r y t o remove, r e o r i e n t and/or p r o v i d e a d d i t i o n a l energy t o t h e s e p a r t i c u l a r m o l e c u l e s t o f o r c e them t o p a r t i c i p a t e in the c h e m i c a l change ( p r o p a g a t i o n s t e p ) . At h i g h e r s t a r t i n g c o n c e n t r a t i o n o f 3PL i n c r e a s e s the energy needed t o d e s o l v a t e t h e ground s t a t e in o r d e r t o p e r m i t t h e c h e m i c a l r e a c t i o n t o p r o c e e d . Thus, w i t h i n c r e a s i n g t h e s t a r t i n g c o n c e n t r a t i o n o f monomer: +

+

+

p

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18.

SLOMKOWSKI

AND PENCZEK

281

Lactones 1

1

[&PL]o from 1.0 m o l e - l - t o 3.0 m o l e ' l ' the e n t h a l p y o f a c t i v a t i o n i n c r e a s e s as much as by 10.0 k c a l - m o l e " . Schematic r e p r e s e n t a t i o n o f two modes o f s o l v a ­ t i o n : namely n o n s p e c i f i c f o r the m a c r o i o n - p a i r s and s p e c i f i c f o r m a c r o i o n s is g i v e n below : 1

(where S and M denote s o l v e n t and monomer m o l e c u l e s respectively). Thus, we assume, t h a t in case o f the m a c r o i o n - p a i r s o l v e n t and monomer m o l e c u l e s a r e n o t o r i e n t e d in any s p e c i f i c way and d i s o r d e r l y pack the a v a i l a b l e space around the m a c r o i o n - p a i r is s p e c i f i c a l l y s o l v a t e y due t o the s t r o n g i n t e r a c t i o n between a n e g a t i v e charge and h i g h l y p o l a r monomer and the f u r t h e r passage t o the t r a n s i t i o n s t a t e , b e i n g much l e s s p o l a r t h a n the ground s t a t e o f m a c r o a n i o n , r e q u i r e s h i g h e r energy. T h i s is much more apparent a t h i g h e r monomer c o n c e n t r a t i o n . Comparison o f e n t r o p i e s o f a c t i v a t i o n f o r propaga­ tionβon m a c r o i o n s and on m a c r o i o n - p a i r s l e a d s t o con­ c l u s i o n s b e i n g hand in hand w i t h the a n a l y s i s o f e n t h a l p i e s . The l a r g e n e g a t i v e e n t r o p y o b s e r v e d f o r m a c r o i o n - p a i r s (-52.0 c a l - m o l e " *deg"" ) may r e f l e c t f o r m a t i o n o f t r a n s i t i o n s t a t e w i t h more i o n i c character. Thus, some o f the s o l v e n t and monomer m o l e c u l e s may even become more f i r m l y o r i e n t e d t h a n in the corresponding ground s t a t e . 1

x

Abstract The q u a n t i t a t i v e a s p e c t s o f the elementary re­ actions i n v o l v e d in the p o l y m e r i z a t i o n o f l a c t o n e s a r e d i s c u s s e d . Some earlier papers a r e reviewed and ana­ l y s e d t o g e t h e r w i t h the r e c e n t d a t a o f the p r e s e n t a u t h o r s . I t has been shown, t h a t in the p o l y m e r i z a t i o n o f βPL rate c o n s t a n t o f p r o p a g a t i o n on macroanions (k-p) d e c r e a s e s w i t h i n c r e a s i n g the starting c o n c e n t r a t i o n o f monomer ([βPL] ), whereas the rate c o n s t a n t s o f p r o p a g a t i o n on m a c r o i o n - p a i r s (kΘp) do n o t depend on [βPL] . Thus, the ratio k-p/kΘp does depend on [βPL] . These o b s e r v a t i o n s stem from the h i g h dielectric c o n s t a n t o f βPL and its h i g h s o l v a t i o n ability, a p p a r e n t l y s t r o n g e r f o r i o n s than f o r the i o n - p a i r s . 0

0

0

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

282

ANIONIC

Literature

POLYMERIZATION

Cited

1. St.Slomkowski and St.Penczek, Macromolecules, 9, 367 (1976). 2. St.Slomkowski and St.Penczek, M a c r o m o l e c u l e s , i n press. 3. H . K . H a l l , Jr., Macromolecules, 2, 488 (1969). 4. A.Hamitou, T.Ouhadi, R.Jerome, and P . T e y s s i e , J.Polym.Sci.,Polym.Chem.Ed., 15 865 (1977). 5. K . I t o , Y.Hashizuka, and Y.Yamashita, Macromolecules 10, 821 (1977). 6. E . B i g d e l i and R.W.Lenz, Macromolecules, 11, 493 (1978). 7. K . S . K a z a n s k i i , Chimia i T e c h n o l o g i a Vysokomol. S o e d i n . , Ed. N.S.Enikolopov 5 (1977). 8. P . S i g w a l t and S . B o i l e a u , J.Polym.Sci.: Polymer Symp., 62, 51 (1978). 9. R.D.Lundberg and E.F.Cox in "Ring-Opening Polymerization", Ed. K.C. Frisch and S.L.Reegen, M a r c e l Dekker, New York and London, 1969, p.266. 10. H.Cherdron, H.Ohse, and F . K o r t e , Makromol.Chem., 56, 187 (1962). 11. T . S h i o t a , Y.Goto, and K.Hayashi, J.Appl.Polym.Sci., 11, 753 (1967). 12. T.Tsuda, T . S h i m i z u , and Y.Yamashita, Kogyo Kagaku Z a s s h i , 68, 2473 (1965). 13. Y. E t i e n n e and R.Soulas, J.Polym.Sci., C4, 1061 (1963). 14. V.Jaacks and N.Mathes, Makromol.Chem., 131, 295 (1970). 15. T . B i e l a , L.S.Corley, J.Michalski, St.Slomkowski, St.Penczek, and O.Vogl, Makromol.Chem., in p r e s s . 16. L.S.Corley, St.Slomkowski, St.Penczek, T . B i e l a , nad O.Vogl, Makromol.Chem., in p r e s s . 17. Y.Yamashita, K . I t o , and F . N a k a k i t a , Makromol.Chem., 127, 292 (1969). 18. A.A.Solov'yanov and K . S . K a z a n s k i i , Vysokomol. S o e d i n . , A12, 2114 (1970), ibid A14, 1063 (1972). 19. A . D e f f i e u x and S . B o i l e a u , Polymer,18 1047 (1977). 20. P.Hemery, S . B o i l e a u , and P . S i g w a l t , Europ.Polym.J., 7, 1581 (1971). 21. D.Kotynska, St.Slomkowski, and St.Penczek, in p r e paration. 22. A . D e f f i e u x and S . B o i l e a u , Macromolecules, 9, 369 (1976). 23. T.Ouhadi and J.M.Heuschen, J.Macromol.Sci.,-Chem., A9, 1183 (1975). 24. T.E.Hogen E s c h and J.Smid, J.Phys.Chem., 79, 233, (1975). 25. U.Takaki, T.E.Hogen E s c h , and J.Smid, J.Am.Chem. Soc., 93, 6760 (1971). RECEIVED March 5, 1981. In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

19 Use of Cryptates in Anionic Polymerization of Heterocyclic Compounds SYLVIE BOILEAU Laboratoire de Chimie Macromoléculaire associé au CNRS, Collège de France, 11 Place Marcelin-Berthelot, 75231 Paris Cedex 05, France

The anionic polymerization of heterocyclic monomers i n homogeneous media has been relatively less studied than that of vinyl or dienic monomers. The propagation reaction occurs through active centers that are generally much more stable than carbanions, and the polymerization rates are smaller, thus allowing convenient conductance and kinetic measurements (1,2). However only few monomers, namely propylene sulfide and ethylene oxide, which give true l i v i n g polymers, have been studied i n detail. D i f f i c u l t i e s of different types are encountered for several heterocyclic monomers such as the insolubility of the resulting polymers, chain transfer to the monomer or polymer or associations of ion pairs into higher aggregates. Considerable progress has been made i n this f i e l d i n the last few years by the use of macrobicyclic ligands (I) discovered by Lehn (3).

for instance m = n = p = 2 designated

[222]

These ligands form extremely stable cation inclusion complexes, called cryptates, i n which the cation i s completely surrounded by the ligand and hidden inside the molecular cavity, and this leads to a considerable increase of the interionic distance i n the ion pairs. It has been shown that such ligands have a marked activating effect on anionic polymerizations (4,5,6). Moreover, the aggregates are destroyed and simple kinetic results have been obtained i n the case of propylene sulfide (7,8,9), ethylene oxide (9,10,11) and cyclosiloxanes (12) polymerizations. Though the 0097-6156/81/0166-0283$05.75/0 © 1981 American Chemical Society In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

284

ANIONIC POLYMERIZATION

cryptâtes have been used f o r the a n i o n i c polymerization of other h e t e r o c y c l i c monomers l i k e lactones (4,13,14), c y c l i c carbonates (4) and lactams (15), we w i l l present the r e s u l t s obtained with propylene s u l f i d e , ethylene oxide and c y c l o s i l o x a n e s in t h i s paper. I.

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

Sulfide

Propylene s u l f i d e gives polymers with p e r f e c t l y s t a b l e t h i o l a t e l i v i n g ends in e t h e r e a l s o l v e n t s (16). The propagation r e a c t i o n has been s t u d i e d in THF (7-9JL7-2lT"and in THP (8,9,21,22,23) w i t h N a , C s , B U 4 Î F and s e v e r a l cryptâtes as counterions, by d i l a t o m e t r y . I t has been shown that even f o r non cryptated s p e c i e s , the a s s o c i a t i o n s of i o n p a i r s do not g e n e r a l l y occur in the range of concentration in THF and < 2 10~ m o l e . l " are i o n p a i r s in e q u i l i b r i u m w i t h f r e e ions: +

+

4

S~

M+

S" + M

+

(1)

The f r a c t i o n of f r e e t h i o l a t e anions is given by the r e l a t i o n : [C...]a2 K

(2)

D

(1-a) +

The Kj) has been measured directly f o r N a + [222] as counterion in THF, and c a l c u l a t e d from the i n t e r i o n i c distance ji according to the Fuoss equation f o r other cryptâtes, using the Stokes radius values R obtained from conductimetrie s t u d i e s of the corresponding tetraphenylborides (24). The value of Kj) f o r N a + [222] in THP was deduced from that found in THF assuming that the i n t e r i o n i c distance remains constant in both s o l v e n t s . K i n e t i c measurements were performed at s e v e r a l concent r a t i o n s [C...] of l i v i n g ends f o r a given counterion, at a f i x e d temperature. The values of the apparent propagation rate constant: +

s

+

R

k

p

=

P — [M][C...]

(3)

53

were determined f o r each experiment ( R rate of polymerization, [M] and [C...] * monomer and l i v i n g ends c o n c e n t r a t i o n s ) . According to e q u i l i b r i u m ( 1 ) , kp depends on the f r a c t i o n of f r e e ions: p

k k£ and k

p

-

(1 -

o) kj. + ak

(4)

being the propagation constants of the i o n p a i r s and of

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

19.

BOiLEAU

Cryptâtes

285

the f r e e i o n s , r e s p e c t i v e l y . Examples of p l o t s of k versus α are shown in F i g u r e 1, f o r N a + [222] in THF and in THP, a t -30°C., g i v i n g kj. and k . p

+

The values of kj. and of k found in THF a t -30°C... a r e l i s t e d in T a b l e 1. The values of k may be considered to be the same w i t h i n the experimental e r r o r s . The k values i n c r e a s e r a p i d l y w i t h the s i z e of the c o u n t e r i o n . The most s u r p r i s i n g f a c t is t h a t kj. becomes higher than k with a l l c r y p t a t e d counterions. A p l o t of l o g k-j. versus 1/a g i v e s approximately a s t r a i g h t l i n e ( F i g . 2) and it may be seen that the experimental value f o r k (obtained from k i n e t i c s f o r Na ) is q u i t e f a r from the e x t r a ­ polated value f o r a ». +

β

I t seems t h a t the cannot be r e a d i l y compared p o l a r i z a t i o n of the CH2-S bond in the monomer occurs before the r i n g opening and is induced by the i o n p a i r d i p o l e of the l i v i n g end. Such a m o d i f i c a t i o n should i n c r e a s e with the magnitude of the i o n p a i r d i p o l e and then with the i n t e r i o n i c d i s t a n c e . T h i s leads to an i n c r e a s e of the i n t e r a c t i o n with the monomer, together with the r e a c t i o n rate. I t is understandable that the m o d i f i c a t i o n of the p o l a r i z a t i o n of the CH2-S bond may be q u i t e d i f f e r e n t when the i n t e r a c t i o n occurs with f r e e Ions. From the r e s u l t s shown in F i g u r e 1, it can be seen t h a t , f o r N a + [222], the r e a c t i v i t i e s of c r y p t a t e d i o n p a i r s and of f r e e i o n s are n e a r l y the same in THP whereas k± is three times higher than k in THF. Moreover, k is found to be the same in THF and in THP. The p l o t of l o g k ± versus 1/a f o r the r e s u l t s found in THP g i v e s a s t r a i g h t l i n e p a r a l l e l to the l i n e observed in THF. In a general manner, the Ion p a i r s r e a c t i v i t y is lower in THP than in THF f o r a given counterion because the s e p a r a t i o n of the charges in the t r a n s i t i o n s t a t e is more d i f f i c u l t in a medium of lower d i e l e c t r i c constant. +

II.

A n i o n i c P o l y m e r i z a t i o n of Ethylene Oxide

The propagation r e a c t i o n of ethylene oxide a n i o n i c polymer­ i z a t i o n has been s t u d i e d in THF a t 20°C... with K+, K+ + [222], C s and C s + [TC] as counterions (10,11). [TC] is a s p h e r o i d a l m a c r o t r i c y c l i c tetramine hexaether (25) which forms a very s t a b l e complex with C s . +

+

+

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

286

ANIONIC POLYMERIZATION

Figure 1. Linear dependence of the apparent bimolecular rate constant of living polypropylene sulfide propagation on the fraction of free ions a with Ν a* + [222] as counterion at -30°C...: in THF (); in THP (f); with φ,βΝα + [222] in THP(Vh

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

19.

Cryptâtes

BOILEAU

287

TABLE I Propagation Constants of Free Ions and Ion P a i r s f o r the A n i o n i c P o l y m e r i z a t i o n of Propylene S u l f i d e in THF a t -30°C...

Counterion

5

a

k+

ο A

. l.mole . s e c .

3.7

0.0025

4.3

0.23 ,

1.90

6.5

1.7

3.5

+ [211]

2.53

6.7

4.9

2.8

Na+ + [221]

3.35

7.0

8.9

3.0

Na+ + [222]

4.20

7.2

11.9

5.6

4.55

7.3

8.3

3.3

5.50

7.5

8.7

4.7

Na

10

+

a

0.0054 ,

-

Cs+ .c) Bu4N+ +

Li

Na

+

+

Cs

+

[2 2 2 ] 0

0

s

+ [322]

A

. 1

3.8

-

b

a

)

Obtained from k i n e t i c experiments made with and without