In this article, experimental and simulated results are presented for the evolution of the copolymer composition as unsaturations are consumed in the free-radical cross-linking copolymerization of Styrene(St) and BisphenolA glycerolate dimethacrylate (BDMA). Real time FTIR measurements were performed to monitor the depletion of each comonomer double bond during the isothermal curing reaction at 80 degrees C. From the experimental data corresponding to different feed compositions, the initial reactivity ratios and their evolution with conversion were determined via a nonlinear least squares optimization of the integrated form of the copolymerization equation. The reactivity ratio of St increases continuously and exponentially with the overall reaction conversion, while that of BDMA decreases linearly. A modified terminal copolymerization model including the dependence of the reactivity ratios with the overall conversion was proposed. The application of this model provides a consistent fitting for the conversion of each comonomer during all reaction stages, even at high conversion values where large diffusion and topological restrictions for chain movements are present. Simulations show that the concentration of styrene units added to the copolymer increases with the overall reaction conversion, while that for the BDMA double bonds diminishes. Structures rich in homopolymerized styrene are predicted at later stages of the reaction. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 3081-3091, 2010