The influence of monomer ionization and hydrolysis on the radical polymerization kinetics of 2-(dimethylamino)ethyl methacrylate in aqueous solution

The aqueous solution radical polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) is investigated using in situ nuclear magnetic resonance spectroscopy. While fully ionized DMAEMA is stable over the time scale of polymerization, the non-ionized monomer hydrolyzes at pH > 6.0 to form ionized methacrylic acid (MAA) and 2-(dimethylamino)ethanol, with the hydrolysis rate measured as a function of temperature and pH. Fully ionized DMAEMA polymerizes in water 5 times faster than non-ionized DMAEMA in dimethyl sulfoxide due to the reduced rate coefficient for radical-radical termination. When polymerizations are conducted at pH in the range of 8.0 to 10.1, the reaction is complicated by DMAEMA hydrolysis, with the MAA product copolymerizing to form poly(DMAEMA-co-MAA). Thus, the system requires consideration as a terpolymerization of ionized and non-ionized DMAEMA species with ionized MAA, with the relative rates of polymerization and hydrolysis dependent on reaction temperature, pH, and initiator concentration.