The kinetic Monte Carlo simulation was used to predict the characteristics of the polymer network formation during pre- and postgelation regimes of free-radical cross-linking copolymerization. The simulation naturally considers the presence of multiradicals, primary and secondary cyclization with no preassumptions. The simulation was first validated in the pregel regime by comparing the microstructure with that given by a mean-field model. The Monte Carlo simulation was then used to predict the kinetics and development of the polymer microstructure of the sol and gel fractions up to full conversion, including the complete molecular weight distribution, cross-linking and pendant double bond densities, primary and secondary cyclization and the molecular weight distribution between cross-linking points. The simulation also allows studying the presence and evolution of multiradicals along the polymerization.