Reactivities of conjugated dienes towards the 1-cyano-1-methylethyl and 1-phenylethyl radicals

Abstract

Study of the reactions of monomers with the 1-cyano-1-methylethyl and 1-phenylethyl radicals, derived from azobisisobutyronitrile and 1,1′-azobis(1-phenylethane) respectively, has been extended to the cases of isoprene, 2,3-dimethyl-1,3-butadiene and 2,3-diphenyl-1,3-butadiene. The procedure involves examination by ^(13)C-NMR of end-groups in binary copolymers of the dienes with methyl methacrylate, made using ^(13)C-enriched samples of the azo compounds as initiators. Selectivity in the reactions of the initiating radicals with monomers is compared with that in the reactions of the monomers with polymethacrylonitrile and polystyrene radicals, as deduced from consideration of appropriate copolymerizations. The 1-phenylethyl radical models the polystyrene radical rather more satisfactorily than the 1-cyano-1-methylethyl radical models the polymethacrylonitrile radical. There is further discussion of the effects of penultimate monomeric units on selectivity in the reactions of growing polymer radicals with monomers.

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