Mechanism of emulsion polymerization

Abstract

Both conventional and inverse emulsion polymerization comprise the emulsification of an immiscible monomer in a continuous medium followed by polymerization with a free radical initiator to give a colloidal sol of polymer particles. Both processes give “emulsion polymerization kinetics,” i.e., a proportionality of both the polymerization rate and polymer molecular weight to the number of particles instead of the inverse relationship between rate and molecular weight observed for mass, solution, and suspension polymerization. The emulsion polymerization process can be divided into particle nucleation and particle growth stages and can be carried out using batch, semicontinuous, or continuous processes. Seeded emulsion polymerization can be used to avoid the particle nucleation stage in all three processes. The many mechanisms proposed for the initiation of emulsion polymerization can be divided into four categories according to the locus of particle initiation: (i) monomer-swollen micelles; (ii) adsorbed emulsifier layer; (iii) aqueous phase; (iv) monomer droplets. These general principles are applied to: (i) the preparation of monodisperse latexes by seeded emulsion polymerization; (ii) the locus of particle initiation for various monomers and initiators; (iii) emulsion copolymerization; (iv) core-shell emulsion copolymerization; (v) polymerization in fine monomer droplets; (vi) inverse emulsion polymerization.