Gas bubbles get entrapped continuously during bulk polymerization in stirred batch reactors. Once entrapped, these cannot escape because of the increasing viscosity of the reaction mass. The bubbles also grow with time due to the diffusion of the monomer. This phenomenon is modeled using an Eulerian-Lagrangian population-balance approach along with the equations describing the kinetics of polymerization. It is assumed that there is no nucleation of bubbles. An empirical rate of entrapment, R(entrap,i)(viscosity, bubble diameter) is assumed for the polymerization of methyl methacrylate MMA), an example system. The model parameters are tuned using experimental data on this system under near-isothermal conditions in a 1-L, fully instrumented stainless steel reactor with an anchor agitator. The model predictions are found to be in reasonable accord with experimental data under different conditions of polymerization. The study of this phenomenon has important design implications since the pre-polymerization stage ends as soon as stable clusters of bubbles get formed. Further polymerization is normally carried out in tubular reactors or in molds. POLYM. ENG. SCI., 51:1942-1956, 2011. (C) 2011 Society of Plastics Engineers