Batch and semibatch emulsion polymerization was investigated with a chain transfer agent (CTA) to optimize key process attributes such as monomer conversion and product properties such as molecular weight and particle size. The test monomer, styrene was polymerized in the presence of the chain transfer agent, n-dodecyl mercaptan, to control the final molecular weight. The process model used for conducting optimization incorporated the nucleation, reaction kinetics, and phase heterogeneity involving the aqueous, monomer droplet and polymer particle phases during the various stages of the reaction. The model developed was able to predict the behavior of emulsion polymerization, while describing the significant effect of CTA on molecular weight distribution (MWD) and polydispersity index (PDI) for both batch and semibatch processes. The optimal operating strategies for the process were developed using reaction temperature and monomer feed rate as process variables to obtain desired process trajectories and product properties.