AbstractKinetic and mechanistic features in the early stage of the emulsion copolymerization of styrene and methyl methacrylate were examined using sodium lauryl sulfate and potassium peroxidisulfate as emulsifier and initiator, respectively. Experimental results on copolymer composition, molecular weight distribution, the rate of copolymerization and the number of polymer particles found in the early stage of the reaction showed that: (1) The amount of copolymers produced in the water phase decreases with an increase in the concentration of emulsifier initially charged and can be practically neglected above the critical micelle concentration(CMC), and hence, (2) Polymer particles are regarded to be produced mainly from the emulsifier micelles sufficiently above the CMC. (3) Coagulation of polymer particles can be neglected in the process of particle nucleation.