Preparation of micrometer-sized poly(methyl methacrylate) particles with amphoteric initiator in aqueous media

A previously proposed method based on soap-free emulsion polymerization with an amphoteric initiator for producing micrometer-sized polystyrene particles was extended to application with methyl methacrylate (MMA). The aggregation and dispersion stability of polymer particles, which have ionizable groups arising from initiator radicals, can be controlled by adjusting the pH of the reaction system accompanied with the addition of ionic monomer. Polymerizations were carried out with 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate amphoteric initiator, NH₃/NH₄Cl pH buffer, and sodium p-styrenesulfonate anionic monomer (NaSS) in ranges of MMA concentration (0.58-2.32 kmol/m³ H₂O) and NH₃ concentration (2.5-20 mol/m³ H₂O) at fixed concentrations of 5 mol/m³ H²O initiator, 10 mol/m³ H₂O NH₄Cl, and 1 mol/m³ H₂O NaSS at 65 °C. The addition of NaSS during the polymerization could improve stability in dispersion of particles, which coagulated in the absence of NaSS after the disappearance of monomer drops. An increase in the monomer concentration in the present method could enlarge the particle size without lowering the monodispersity of the particle size distribution. On the other hand, an increase in NH₃ concentration decreased the particle size. The highest monodispersity of particle sizes was obtained at a NH₃ concentration of 5 mol/m³ H₂O, which gave an average size of 1.5 μm and a coefficient of variation of particle size distribution of 2.2% that was much smaller than the standard criterion for monodispersity, 10%.