Polybutadiene, living polymer

Brody, H., M. Ladacki, R. Milkovich and M. Szwarc Molecular weight of living polymers. Polybutadiene and polyisoprene. J. Polymer Sci. 25,221 (1957). 

I hope the reader will not be too disappointed that he had to accompany me through some fields of small-molecule or at the most middle-sized molecule chemistry and has not up to here read a word about macromole-cular chemistry, in which according to a quite general opinion, well known to me, I am said to be a special expert. As a matter of fact, except for my former activity in the field of polybutadiene and polystyrene chemistry which touched the problems later on referred to as living polymers, I was never closely connected with macromolecular chemistry. But my own life contained some surprises, and I believe the most astonishing event was my sudden confrontation with macromolecules in consequence of an experiment which led from monomeric ethylene to its lowest possible polymer, ... 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

The first block (polybutadiene or polystyrene) is prepared by anionic polymerization, under high vacuum, in THF dilute solution (less than 5%), at low temperature (—70 °C) with cumylpotassium as initiator. Then, the living polymer is transformated into a hydroxylated polymer (PV—OH) by addition of ethylene oxide under vacuum, or into a carboxylated polymer (PV-COOH) by addition of carbon dioxide under vacuum. 

Hydroxy-terminated liquid polybutadienes are prepared for reactions with diisocyanates to form elastomeric polyurethanes (see Chapter 6). Such materials can be prepared by anionic polymerizations as living polymers and then quenched at the appropriate molecular weight. These polybutadienes can also be formed by a free-radical mechanism. The microstructures of the two products differ, however, and this may affect the properties of the finished products. To form hydroxy terminated polymers by a free-radical mechanism, the polymerization reactions may be initiated by hydroxy radicals from hydrogen peroxide. 

More recent SANS experiments on stretched networks were performed by Hinkley et al. (99) and by Clough et al. (100,101). Hinkley et al. (99) prepared blends of polybutadiene and polybutadiene-de. Both polymers were made by the living polymer technique, end-capped with ethylene oxide, and water-washed to yield the dihydroxy liquid prepolymer. Uniform networks were prepared by reacting the prepolymers with stoichiometric amounts of triphenyl methane triisocyanate. The value of using polybutadiene over polystyrene, of course, is that the networks are elastomeric at ambient temperatures. 

Low-molecular-weight polybutadiene oils result when the polymerization is catalyzed by a mixed system of butyllithium, 1,2-bis(dimethylamino)ethane, and potassium z-butanolate [110-112]. With 1,4-dilithium-1,1-4,4-tetraphenylbutane it is possible to get bifunctional living polymers (seeding technique) [113-118]. 

With the exception of a few commercial polymers such as polyisobutylene, polybutadiene and styrene-butadiene block copolymers, living polymers are prepared in small quantities under stringent conditions. Larger amounts can only be prepared by repeating the synthesis many times, and this is a costly and time-consuming process. In the case of hydrogenated polybutadiene, to prepare samples that resemble polyethylene, the need for a secondary reaction step renders the preparation even more costly. This has so far limited the extent to which it has been possible to use these materials to test models. Cell et al. [ 18] prepared asymmetric stars with structures similar to ethylene-propylene copolymers by hydrogenation of star-branched polyisoprene. The reactions to produce these materials took up to three weeks, and... 

Although there are several mechanisms by which living polymers can be prepared, anionic polymerization to date represents the most successful commercial application. The particular usefulness of lithium alkyls has been with dienes to give cw-polyisoprene and c -polybutadiene, although they can yield isotactic or atactic polymers of styrene and MMA. It is a peculiarity of polymerizations with lithium alkyl that there is no termination step. The rate of polymerization depends on the amount of initiator and monomer present [19]. 

It has been shown recently (10) that such block structures could be tailored precisely by the general method summarized hereabove. It is indeed possible to convert the hydroxyl end-group of a vinyl polymer PA (f.i. polystyrene, or polybutadiene obtained by anionic polymerization terminated with ethylene oxide),into an aluminum alcoholate structure since it is well known that CL polymerizes in a perfectly "living" manner by ring-opening insertion into the Al-0 bond (11), the following reaction sequence provides a direct access to the desired copolymers, with an accurate control of the molecular parameters of the two blocks ... 

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