Soapless emulsion polymerization of methyl methacrylate in water in the presence of calcium sulfite

A kinetic study performed on the polymerization of the MMA-K₂S₂O₈-CaSO₃-H₂O system shows that two different shapes of the time-conversion curve were obtained according to the presence or absence of calcium sulfite powder in the reaction mixture. In the absence of the powder, the polymerization behavior is similar to that of the MMA-K₂S₂O₈-H₂O system reported earlier. An attempt was made to apply the kinetic model of the rate of polymerization and the self-nucleation model of polymer particle formation in soapless emulsion polymerization to the present system. One is able to express the experimental time-conversion curves reasonably well. The other can account for the number of polymer particles formed in polymerizations initiated with potassium persulfate alone or calcium sulfite alone but not that in polymerization initiated by both. In the latter case the rate of radical generation and the ionic strength were appreciably enhanced compared with the former. These enhancements might affect the mechanism of polymer particle formation; certain nucleations, such as an aggregative nucleation, might occur simultaneously in addition to the self-nucleation. In the presence of the solid phase the time-conversion curves were similar to that of emulsion polymerization rather than soapless emulsion polymerization, and the molecular weight of polymer formed was smaller than in the absence of the solid phase. It was shown experimentally that the rate of polymerization was remarkably enhanced by an increase in the solid content, whereas the molecular weight of polymer was only slightly affected. As a result, it is considered that calcium sulfite solid provides an important polymerization locus and probably also plays a significant role in radical generation.