Emulsion Polymerization in a Hybrid Carbon Dioxide/Aqueous Medium

The development of a new reaction medium is reported, based on a biphasic mixture of carbon dioxide and water for emulsion polymerizations: a surfactant-free aqueous emulsion polymerization of methyl methacrylate using potassium persulfate under a varying head pressure of CO 2 (0−350 bar) at 75 °C. The resulting polymer is a stable latex with particles of submicron size. The effect of CO 2 on polymerization is relatively small, until there is a significant change at high CO 2 pressure. This is seen by examining the molecular weight distribution in the form of the log(number distribution), P ( M ) (readily obtained by GPC). At pressures of 140 bar and below, the P ( M ) show the form expected for chain-stopping events dominated by transfer and by diffusion-controlled termination. At 280 bar, ln P ( M ) is significantly steeper than its lower-pressure counterpart at relatively low conversion (45%). This is attributed to swelling by supercritical CO 2 reducing the viscosity of the particles, allowing more rapid termination.