Kinetics and Mechanism of Emulsion Polymerization

The course of emulsion polymerization may be considered as involving three intervals: Interval I, where particle formation takes place. The end of this interval is not dependent upon the degree of conversion, but on the total amount of polymer formed. With usual recipes, it ends at about 1-5% conversion. Interval II lasts from the end of Interval I until monomer disappears as a separate phase. In this interval, the particle number is usually found to be constant, the particle volume increases proportional to conversion, the monomer concentration in the particles is approximately constant, and therefore the termination is also constant within the particles. Interval III starts when the monomer disappears as a separate phase. The transition from Intervals II to III is determined by the degree of conversion and differs for different monomers. In cases where the solubility of monomer in the water phase is low, the monomer present in the aqueous phase may usually be neglected compared to the monomer present in the polymer particles. This will usually hold with most monomers at ordinary conditions, where the amount of polymer per unit of water is relatively high, and when the concentration of monomer in the particles is much higher than in the aqueous phase. In this case, the particle volume during Interval III will decrease slightly due to the contraction by polymerization. The concentration of monomer in the particles generally decreases during Interval III, which leads to an increased viscosity within the particles and thereby also to a decrease in the value of the termination constant. The present paper deals chiefly with the kinetics of emulsion polymerization in the three intervals and the application of kinetics to the study of the mechanism. Several review articles on emulsion polymerization have recently appeared in the literature. The reason for presenting a new review article is that a vast number of papers on emulsion polymerization have recently been published, which have broadened the theoretical and experimental knowledge of the process. Several experimental and theoretical studies on the mechanisms of particle formation have appeared. New theories for particle formation outside the micelles have appeared. Also the relative effectiveness of micelles and particles in capturing radicals from the aqueous phase has been discussed in several papers.