Microemulsion polymerization of styrene in the presence of a cationic emulsifier

The principal subject discussed in the current paper is the radical polymer ization of styrene in the three- and four component microemulsions stabiliz ed by a cationic emulsifier. Polymerization in the o/w microemulsion is a n ew polymerization technique which allows to prepare the polymer latexes wit h the very high particle interface area and narrow particle size distributi on. Polymers formed are very large with a very broad molecular weight distr ibution. In emulsion and microemulsion polymerizations, the reaction takes place in a large number of isolated loci dispersed in the continuous aqueou s phase. However, in spite of the similarities between emulsion and microem ulsion polymerization, there are large differences caused by the much large r amount of emulsifier in the latter process. In the emulsion polymerizatio n there are three rate intervals. In the microemulsion polymerization only two reaction rate intervals are commonly detected: first, the polymerizatio n rate increases rapidly with the reaction time and then decreases steadily . Essential features of microemulsion polymerization are as follows: (1) po lymerization proceeds under non-stationary state conditions; (2) size and p article concentration increases throughout the course of polymerization; (3 ) chain-transfer to monomer/exit of transferred monomeric radical/radical r e-entry events are operative; and (4) molecular weight is independent of co nversion and distribution of resulting polymer is very broad. The number of microdroplets or monomer-starved micelles at higher conversion is high and they persist throughout the reaction. The high emulsifier/water ratio ensu res that the emulsifier is undissociated and can penetrate into the microdr oplets. The presence of a large amount of emulsifier strongly influences th e reaction kinetics and the particle nucleation. The mixed mode particle nu cleation is assumed to govern the polymerization process. At low emulsifier concentration the micellar nucleation is dominant while at a high emulsifi er concentration the interaction-like homogeneous nucleation is operative. Furthermore, the paper is focused on the initiation and nucleation mechanis ms, location of initiation locus, and growth and deactivation of latex part icles. Furthermore, the relationship between kinetic and molecular weight p arameters of the microemulsion polymerization process and colloidal (water/ particle interface) parameters is discussed. In particular, we follow the e ffect of initiator and emulsifier type and concentration on the polymerizat ion process. Besides, the effects of monomer concentration and additives ar e also evaluated. (C) 2001 Elsevier Science B.V. All rights reserved.