This paper deals with the development of a mathematical model for emulsion copolymerization of styrene and butyl acrylate carried out in the presence of n-dodecyl mercaptan as chain transfer agent (CTA). The model consisted of a system of differential algebraic equations in which the population balances are based on a new approach that reduces significantly the number of equations involved and the corresponding computational time. Most of the unknown kinetic and thermodynamic parameters of the model were estimated from experimental measurements using a stochastic optimization method based on a genetic algorithm. The results showed a fairly good agreement between model predictions and experiments. The model was then successfully validated through additional experiments carried out in batch and fedbatch reactors and clearly showed that the model was able to predict the time-evolution of overall conversion, amounts of each residual monomer, number and weight average molecular weights of the resulting copolymers and average diameters of the corresponding latex particles for different operating conditions, mainly CTA concentration and reaction temperature. The model was finally used to investigate and confirm the effects of CTA concentration, previously observed by several authors, on the kinetics of this polymerization process and on the main properties of the resulting macromolecules and latex particles. (C) 2009 Elsevier Ltd. All rights reserved.