A computationally efficient Monte Carlo method was used to simulate the reaction kinetics and molecular structure development during free-radical copolymerizations with divinyl monomers. A single parameter was used to describe the reduced reactivity of the pendent vinyl groups incorporated within the polymer backbone. The simulation results were compared with published experimental data for the bulk copolymerization of methyl methacrylate with different levels of ethylene glycol dimethacrylate. The model was able to effectively predict the reaction kinetics, the gel point, and sol–gel fractions in both the pre- and postgel regimes, including the swelling index of the gel. In the postgel regime the cross-linked molecule becomes the primary locus of reactions, and all chains eventually become part of this massive cross-linked polymer network. The Monte Carlo method allows the determination of the complete molecular structure as it evolves with time, including properties like cross-linking density, number of free chain ends, primary cycles and loops, and the fraction of unreacted pendent vinyl groups.