PERSPECTIVE

Comments on “Living Carbocationic Polymerization, IV. Living Polymerization of Isobutylene,” by R. Faust and J. P. Kennedy, 1. Polym. Sci.: Part A: Polym. Chem, 25, 1847 (1987)

MITSUO SAWAMOTO

Kyoto University, Kyoto 606-01, Japan

In the history of polymer chemistry, the 1980s will be remembered as an interesting decade during which a variety of living polymerizations have mushroomed, virtually simultaneously in different research groups with varying backgrounds, to cover almost all possible chain polymerization mechanisms including anionic, cationic, group transfer, metathesis, and so on.’ First reported in 1956 by Szwarc,’ living polymerizations are chain polymerizations that are free from chain transfer, termination, and other chain-breaking re- actions and provide powerful methodologies to syn- thesize macromolecules of precisely controlled archi- tectures and molecular weights, such as monodisperse, end-functionalized, block, and star-shaped polymers. Current journal literature readily attests that exten- sive interest in living polymerizations continues into the late 1990s, where further efforts are being directed to the development of living radical and other new polymerizations.

Among these modern living processes is included living cationic polymerization of vinyl monomers.3 Be- fore the 1980s, carbocationic polymerization has been regarded as one of the least controllable addition po- lymerizations, because its chain carriers, the growing carbocations, are inherently unstable and particularly prone to undergo chain transfer, isomerization, and other side-reactions. For a long time it has therefore been considered fundamentally difficult to achieve liv- ing polymerization therein.

The following paper by Faust and Kennedy4 is the first full article that reported the first example of living cationic polymerization of isobutylene, initiated with a combination of 2-phenyl-2-propyl (cumyl) acetate

,Journal of Polymer Science: Part A Pnlymer Chemist.r);, Vnl. 34,3439-3440 (1996) 0 19% John Wiley & Sons, Inc. CCC 0%97-624X/96/173439-02

and boron trichloride (BCIJ as the initiating system. This was the beginning of the subsequent prolific publications from the Akron group for new initiating systems, living carbocationic polymerizations of other monomers, and related polymer synthesis thereby with precise structural and molecular weight control. The article also constitutes part of the earliest set of papers on well-defined living carbocationic polymerizations that almost simultaneously began to emerge in the mid-1980s for vinyl ethers5 and other cationically polymerizable alkene monomer^.^ The worldwide re- juvenation of the field has thus been ignited in the following years.

The significance and impact of Kennedy’s work in- clude: (i) that it led to the first example of living cationic polymerization of nonpolar hydrocarbon monomers, in contrast to the polar counterparts with hetero-atom containing substituents, such as vinyl ethers and N - vinylcarbazole; (ii) that it involves isobutylene, the monomer of highest industrial importance in carbo- cationic polymerization; and (iii) that it employs a bi- nary initiating system, with cumyl acetate (initiator) and BC1, (coinitiator), contributing to the generaliza- tion of the use of such two-component systems that is now considered essential to most of the modern living polymerizations. ’ z 3

Another point of interest is that, in retrospect, the development of the cumyl acetate/BCl, and subse- quently reported similar initiating systems seems to be a logical consequence, whether intended or not, from deep understanding and knowledge of isobutylene po- lymerization accumulated by Kennedy and his group for over four decades.6 Thus, by a cursory search of his literature one may readily find a few preludes to the discovery, as summarized below in terms of initiating systems:

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3440 SAWAMOTO

(a) Initiation Activity of Alkyl Chlorides (1968)

R-CI + AIEt2CI ------)

(b) “lnlfer” Systems (1980)

Cl-Cl + BC13 - (c) “Quasiliving“ Carbocationic Polymerization (1 982)

W C l + BCIs - (d) Living Carbocationic Polymerization (1 986)

W O g C H 3 + BCl3 - 0

First of all, these systems invariably consist of two components, an initiator (cationogen) and a coinitiator (Lewis acid), and their development demonstrates that proper combination of these ingredients is of prime importance in the design of controlled and living car- bocationic polymerizations. For example, in 1968 Ken- nedy conducted a systematic study of the “activity” of alkyl chloride/Et,AlCl systems in isobutylene poly- meri~at ion.~ In addition to the clear dependence of polymer yield on the structure of the alkyl groups (tert- butyl is the best), it is important in the current context to note that he properly employed Et,AlCl, a relatively weak Lewis acid that cannot initiate polymerization with adventitious water, and thereby simplified his semiquantitative evaluation of initiator activity.

About a decade later Kennedy and Smith’ reported, again in the Journal of Polymer Science, on isobutylene polymerization with the dicumyl chloridefioron tri- chloride (BCls) system, by which they disclosed a con- cept, the “inifer” (initiator-transfer agent), that pro- vided a way to selectively synthesize tert-chloride- capped poly(isobuty1ene). They proposed that the dual function of inifers results in the quantitative attach- ment of the terminal tert-chlorine; the key to this methodology is again judicious choice of inifers and the Lewis acid. This paper also marks the beginning of Kennedy’s extensive use of, and perhaps inclination for, BC1, as coinitiator, which evidently forms the basis of the early phase of his living polymerization research a few years later.

The third systems, coined “quasili~ing,”~ also utilize binary initiating systems to induce polymerizations that are not living in a strict sense but indeed exhibit features (such as a progressive increase in polymer mo- lecular weight against its yield) that are reminiscent of living polymerization. Obviously, as the term “quas- iliving” implies, the 1982/83 publications also suggest that around that time, Kennedy started to devote se- rious efforts to achieve living carbocationic processes. Although the quasiliving polymerizations are charac- terized by a unique technique of continuous and slow

addition of monomer solution, most of the initiating systems employed therein involve BC13 and are in fact akin to those used for the fourth development, the living isobutylene polymerization with cumyl acetate/BCl,.

This brief historical analysis is of course not in- tended to suggest that Kennedy’s discovery* is simply an extension of his preceding work. In sharp contrast, it reminds one the utmost importance of persistent, painstaking, systematic, and creative fundamental re- search focused on a particular field, which often leads to a major breakthrough as witnessed in his particular case. Finally, after its publication just a decade ago, the first full paper on living isobutylene polymerization is still fresh and teaches me much, particularly when I consider the current view that “precision” polymer- ization, not of specially designed exotic monomers but rather well-known commodity monomers like isobu- tylene, will be critically required in the polymer science and technology of the next century.

REFERENCES AND NOTES

1. As recent general reviews for living polymerization, see: (a) 0. W. Webster, Science, 251,887 (1991); (b) T. Aida, Prog. Polym. Sci., 19, 469 (1994).

2. (a) M. Szwarc, Nature, 178,1168 (1956); (b) M. Szwarc, M. Levy, and R. Milkovich, J. Am. Chem. Soc., 78,2656 (1956).

3. As reviews for living cationic polymerization, see: (a) J. P. Kennedy and B. Ivhn, Designed Polymers hy Carho- cationic Macromolecular Engineering: Theory and Prac- tice, Hanser, Munich, pp. 55 ff, 1991; (b) M. Sawamoto, Prog. Polym. Sci., 16, 111 (1991); (c) K. Matyjaszewski and M. Sawamoto, in Cationic Polymerization: Mecha- nisms, Synthesis, and Applications, K. Matyjaszewski, Ed., Marcel Dekker, New York, pp. 288-330, 1996.

4. R. Faust and J. P. Kennedy, J. Polym. Sci.: Part A: Polym. Chem., 25,1847 (1987). A preliminary paper on the same subject: R. Faust and J. P. Kennedy, Polym. Bull., 15, 317 (1986).

5. M. Miyamoto, M. Sawamoto, and T. Higashimura, Mac- romolecules, 17, 265 (1984).

6. (a) J. P. Kennedy, Cationic Polymerization of Olefns: A Critical Inventory, Wiley, New York, 1975; (b) J. P. Ken- nedy and E. Marhchal, Carhocationic Polymerization, Wiley, New York, 1982.

7. (a) J. P. Kennedy, in Polymer Chemistry of Synthetic Elastomers, Part 1, J . P. Kennedy and E. G. M. Tiirngvist, Eds., Wiley-Interscience, New York, pp. 304-308, 1968 (b) J. P. Kennedy, J . Macromol. Sci.-Chem., A3, 885 (1969).

8. J. P. Kennedy and R. A. Smith, J . Polym. Sci., Polym. Chem., Ed., 18, 1523 (1980).

9. R. Faust, A. Fehbrvhri, and d. P. Kennedy, J. Macromol. Sci.-Chem., A18, 1209 (1982-83); see also the papers compiled in the same special issue.