Studies in polymerization IX . The occlusion of free radicals by polymers: physical factors determining the concentration and behaviour of trapped radicals

Abstract

A technique has been developed for estimating the concentration of trapped free radicals in polyacrylonitrile which utilizes the reaction between the radicals and aa-diphenyl- B -picryl hydrazyl. It has been shown that during the photopolymerization of acrylonitrile at 25° C the probability of a given radical being trapped per second increases with the rate of reaction, but that the fraction of the total number of radicals produced which becomes trapped is approximately constant, and about 1 %. One implication of this is that chain termination by burial of radicals at 60° C is not likely to be important kinetically. The activity of the photopolymer in initiating polymerization of the monomer has been studied as a function of the temperature and the degree of swelling of the polymer. The latter has been modified by adding to the monomer before photopolymerization a solvent, or a second monomer. The results indicate the manner in which the behaviour of radicals is affected by their mean degree of occlusion. As the latter increases from zero the fast reaction at first increases from a very small value, passes through a maximum and then decreases to zero. The low value of the termination coefficient of occluded radicals has permitted the observation of a thermal after-effect for the first time.