Moderated copolymerization and its applications. The transfer reaction between styryl radicals and carbon tetrabromide

Abstract

It is proposed to apply the term “moderated” to a copolymerization in which one monomer (B) is present in very low concentration. Moderated copolymerization is a useful technique for studying relatively fast reactions between [graphic omitted] B radicals and active reagents, for example transfer agents. The “moderating” monomer (A) is chosen so that [graphic omitted] A radicals have much lower activity. The incidence of [graphic omitted] A radicals in the propagating chains is thus greatly reduced.

The technique has been applied to measurement of the transfer constant for reaction of styryl radicals with carbon tetrabromide, with methyl methacrylate as moderating monomer. This quantity is too large to permit accurate measurement by studies of conventional type on the styrene + CBr₄ system. A value of 50.9 has been obtained for the transfer constant at 60°C, a figure which is much lower than that published by Thomson and Walters. We discuss possible reasons for the discrepancy, including the influence of non-terminal units in the propagating radicals. The use of moderated copolymerization in studying effects of this kind is described.

Conventional copolymerization kinetics show some inadequacies in moderated copolymerization under conditions of high transfer, and the appropriate expressions for degrees of polymerization are derived.

It is well known that carbon tetrahalides reduce the rate of styrene polymerization. A similar effect with CBr₄ occurs in moderated copolymerization. We suggest that these findings may be attributable to complex formation between styryl radicals and the halide.