Kinetics of microemulsion polymerization

A kinetic model for microemulsion polymerization is proposed considering that the number of micelles is by far larger at any stage of the polymerization than the number of polymer particles or that of radicals in the system. The model assumes that every radical in the aqueous phase enters each micelle separately, propagates therein to form a polymeric radical, and transforms the micelle into a polymer particle. According to the model, propagation of a polymeric radical is stopped only by chain transfer reaction to a monomer or to a chain transfer agent, and the growth of polymer particle is stopped only by desorption of the resultant radical from the particle. The model predicts the rate of polymerization, the molecular weight distribution of produced polymer, the number and the size-distribution of polymer particles produced which agree well with those observed experimentally in the microemulsion polymerization of styrene. The lower rate of polymerization with oil-soluble initiators than with water-soluble initiators even under the same rate of radical production suggests that a pair of radicals generated in a small space such as a micelle or a polymer particle tend to terminate therein bimolecularly with each other.