Diffusion‐controlled polymerization of some alkyl methacrylates

Abstract

A study has been made of the nonstationary‐state free‐radical polymerization of methyl, n‐butyl, isobutyl, and 3,5,5‐trimethylhexyl methacrylate in mixed solvents of varying viscosity. It has been found that the termination reaction is diffusion controlled over the complete viscosity range in each case. Values of the ratio k_t/k_p have been obtained for each monomer in ethyl acetate solvent. The relative decreases in this ratio as the size of the alkyl group increases have been compared with decreases calculated (a) on the basis of translational diffusion of macroradicals controlling termination, (b) on the basis that segmental diffusion of the radical chain end in the coiled polymeric free radical is the rate‐controlling process in termination. Although it has been necessary to consider an oversimplified model of segmental diffusion, the calculated dependence of k_t on the size of the alkyl group is in accord with that observed experimentally, whereas decreases calculated on the basis of translational diffusion do not agree with those observed. The results are interpreted as signifying that segmental diffusion of the radical chain end is the rate‐determining step in the termination of the free‐radical polymerization of the alkyl methacrylates, and as providing a simple method of calculating the relative values of the diffusion‐controlled termination rate constant for the polymerization of different monomers.