The kinetics and mechanism of the photoinitiated polymerization of 1,6-hexanediol dimethacrylate (HDDMA) in a poly(methyl methacrylate) (PMMA) matrix were studied. The maximum double-bond conversion, the maximum polymerization rate, the intrinsic reactivity, and the kinetic constants for propagation and termination were calculated. For this system, a reaction-diffusion termination mechanism occurred from the start of the polymerization, and it was predominantly maintained until high monomer concentrations, probably because of the relatively high intermolecular attraction force between the PMMA matrix and HDDMA monomer. In addition, a comparative study of the photoinitiated polymerization of methacrylic monomers in four different polymeric matrices [styrene-butadiene-styrene (SBS), polystyrene (PS), polybutadiene (PB), and PMMA] was carried out. The aggregation state, vitreous or rubbery, of the monomer-matrix system and the intermolecular strength of attraction in the monomer-matrix system and growing macroradical and matrix systems were the principal factors influencing the kinetic and mechanistic behavior of these systems. When PB and SBS were used as matrices, crosslinked polymerized products were obtained as a result of the participation of double bonds of the matrix in the polymerization process (copolymerization). PS sequences in the SBS and PS matrices also took part in the polymerization process through the coupling of the benzylic radical to the growing macroradical.