Microemulsion polymerization of styrene in the presence of anionic emulsifier

Polymerization in o/w microemulsions is a new polymerization technique which allows to prepare the ultrafine latex particles with low particle size distribution, high molecular weight polymer products and special dispersions with high solids-content. This article presents a review of the current literature from the field of radical polymerization of styrene in o/w microemulsions. We focus on the initiation locus and its relation with the kinetic and molecular weight parameters. In particular, we follow the effect of initiator and emulsifier concentration on the polymerization process. In addition, the effects of some additives and temperature on the reaction kinetics are discussed in more detail. The essential features of styrene microemulsion polymerization are as follows: (i) Microemulsion polymerization proceeds under non-stationary state conditions. (ii) Microemulsion polymerization is slower than emulsion polymerization. (iii) Chain-transfer to monomer/exit of transferred monomeric radical/re-entry events govern the polymerization process. (iv) Average number of radicals per particles is very low (much less than 0.5) due to the strong desorption of monomeric radicals from particles. (v) Size and the number of particles increase throughout the course of polymerization. (vi) MWD of resulting linear polymer chains in microemulsion is very broad. (vii) At higher temperatures, the additional radicals comes from the thermally-induced initiation and accumuration of PSt oligomers in microdroplets is supposed to reduce the mass transfer from monomer droplets to the reaction loci. (C) 1999 Elsevier Science B.V. All rights reserved.