Mechanism, kinetics and modelling of the inverse-microsuspension homopolymerization of acrylamide

Abstract

The polymerization of water-soluble monomers is industrially often carried out in inverse-microsuspension or inverse emulsion. Although the kinetics of these processes have been investigated extensively over the past decade, there has been no mechanism proposed which can predict rates and molecular weights. In this paper a general mechanism is developed for inverse-microsuspension polymerization in paraffinic media with oil-soluble initiators. It is compared with experimental data for acrylamide polymerizations and is found to predict conversion, molecular weight and particle characteristics quite well. The mechanism consists of the initiation, propagation, transfer and termination reactions that are common to all free-radical polymerizations. It also includes three newly proposed steps: the reaction between a macroradical and an interfacial emulsifier, which has been found to dominate over the conventional bimolecular reaction; a long-chain branching reaction with terminal unsaturated carbons; and the mass transfer of primary radicals and oligoradicals between organic and aqueous phases. These have a profound effect on the kinetics, where the rate is found to depend on the initiator level to a power greater than one-half and to be inversely proportional to the surface emulsifier concentration. Owing to the importance of unimolecular termination and long-chain branching in inverse microsuspension, certain classes of emulsifier have been identified as being most suitable for the production of ultra-high-molecular-weight polymers.