Combination and Disproportionation during Copolymerization

It is important to realize that, even if the rate of termination is determined by the rates of chain diffusion, the chain end composition and the ratio of combination to disproportionation are not. Knowledge or prediction of the overall rate of termination offers little insight into the detailed chemistry of the termination processes not involved in the rale-determining step. 

Perhaps because of this complexity, few studies on determining kld/ktt, in cross termination in copolymerization have been reported and most of the available data come from model studies, it is also usually assumed, without specific justification, that penultimate unit effects are unimportant in determining which reactions occur and that values of k klt for the homotermination reactions are similar to those in the corresponding homopolymerizations. 

Three types of model study have been performed. The first approach has been to decompose a mixture of two initiators (/. . one to generate radical A, the other to generate radical B). With this method experimental difficulties arise because the two types of radical may not be generated at the same rate and because homotermination products from cage recombination complicate analysis. 

A second approach has been to use an unsymmetrical initiator which allows the two radicals of interest to be generated simultaneously in equimolar amounts.175 In this case, analysis of the cage recombination products provides information on cross termination uncomplicated by homotermination. Analysis of products of the encounter reaction can also give information on the relative importance of cross and homotermination. However, copolymerization of unsaturated products can cause severe analytical problems. 

A third technique is to examine the products of primary radical termination in polymerizations carried out with high concentrations of initiator.176 177 Values of rtid/Ajc ratios in primary radical termination have been reported for a number of polymerizations carried out with A1BN (model for PM AN ) or AlBMe (model for PMM.V) initiation. 

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