4 - Oil-in-water microemulsion polymerization

Abstract

Microemulsions are formed spontaneously through the mixing of organic and aqueous phases in the presence of a surfactant in the correct proportions. Maintaining the microemulsion structure during the polymerization in direct microemulsions is a significant challenge. Microemulsion polymerization differs from other dispersed-phase polymerization techniques as it involves the polymerization of a thermodynamically stable, rather than a kinetically stable, mixture of monomer, surfactant, and water. The initial concentration of micelles is approximately 1000 times greater than the final concentration of polymer particles. The large number of micelles present in the initial microemulsion gives microemulsion polymerization and its distinct polymerization kinetics. Two consecutive entry events in a single micelle are unlikely and consequently bimolecular termination is virtually absent. The segregation effect on termination enhances the rate of polymerization as the individual segregation of growing chains in different polymer particles reduces the probability of termination, slowing the accumulation of free radicals and giving higher polymerization rates. The confined space effect, on the other hand, results in a decrease in the rate of polymerization as a result of an increased rate of deactivation due to the close proximity of radicals within an individual small reaction volume. Essential features of microemulsion polymerization are as follows: (1) polymerization proceeds under nonstationary state conditions, (2) particle concentration increases throughout the course of polymerization, (3) chain-transfer to monomer/exit of transferred monomeric radical/radical reentry events are operative, and (4) polymers formed are very large with a very broad molecular weight distribution.