A population balance approach is described to follow the time evolution of molecular properties in free-radical copolymerization. The model formulation is based on the two dimensional orthogonal collocation on finite difference (2-OCFD) and method of generating functions (GF) which was properly adopted to calculate time-conversion behavior of styrene-methyl methacrylate copolymerization. A comprehensive model which uses the free volume theory to account for diffusion controlled termination and propagation reactions as well as the variation of the initiator efficiency with respect to the monomer conversion was developed. The model is capable of predicting the conversion, composition and molecular weight development up to limiting conversions. The plasticizing effect of a blowing agent on the rate of copolymerization has been investigated. The phenomenological aspect of rate data is analyzed for the copolymerization of styrene with methyl methacrylate over a full range of conversion and the gel effect index is used to systematize the observations. (C) 2011 Elsevier Ltd. All rights reserved.