AbstractSummary: Peculiarities of formation kinetics of sequential semi‐interpenetrating polymer networks (semi‐IPNs) based on crosslinked polyurethane (PU) with different cross‐linking density and linear polystyrene (PS), polybutylmethacrylate (PBMA) and polymethacrylate acid (PMAA) have been studied. The experimental data show the dependence of the kinetic parameters of polymerization on Mc, this dependence being different for various monomers. Sharp discrepancy in molecular mass distribution of polymers formed in various matrices has been observed. The differences in dependencies of reaction kinetics and molecular mass distribution are supposed to be connected to various dependence of the chain growth and termination of various monomers on the density of network, i.e. on the confinements imposed by the intranetwork space.