CROSS-LINKING KINETICS IN EMULSION COPOLYMERIZATION

A kinetic model for network structure development during emulsion cross-linking copolymerization of vinyl and divinyl monomers is proposed. By application of the same kinetic parameters as those for bulk polymerizations, it was found that (1) the cross-linking density is much higher in emulsion polymerizations especially at the initial stage of polymerization and (2) polymer networks tend to be highly heterogeneous. Homogeneous networks cannot be formed even under Flory's simplifying assumptions for vinyl/divinyl copolymerization in emulsion polymerizations. Due to the elastic contribution of the free energy change, the monomer concentration in the polymer particles is lower than with linear polymerizations as long as the monomer droplets exist. A lower monomer concentration results in an enhanced tendency toward cross-linking and may decrease the rate of polymerization per polymer particle if the decrease in the bi-molecular termination reaction rate is not significant.