The equimolar polyaddition cross-linking reaction of poly(methyl methacrylate-co-2-methacryloyloxyethyl isocyanate) with tri(oxytetramethylene) glycol leading to polyurethane (PU) networks was carried out in methyl benzoate at a 0.1 mol/L concentration of functional groups at 80 degreesC. Simultaneously, the free-radical cross-linking copolymerization of methyl methacrylate with tri(oxytetramethylene) dimethacrylate leading to polymethacrylate (PM) networks was progressed at a dilution of 1/10 in the presence of CBr4 as a chain transfer agent. The simultaneous interpenetrating networks (SINs) were formed via the topological cross-links between PU and PM network structures introduced by the respective intramolecular cross-linking reactions. Under the same polymerization conditions, no gelation was observed for respective cross-linking homopolymerizations leading to PU and PM homo-networks because respective intramolecular cross-linking reactions would be enhanced at a diluted solution. The topological cross-link formation between PU and PM network polymer precursors was pursued by SEC monitored with a multichannel UV-vis spectrometer device. The enhanced occurrence of intermolecular cross-linking via the topological cross-link formation was reflected on the variations of both the UV-vis monitored SEC curves and the molecular weights of the resulting PU and PM network polymer precursors with conversion. In addition, the respective actual gel points were compared with the corresponding theories for PU and PM networks formation or gelation. The swelling ratio of the resulting gel was quite high, suggesting a rather low cross-link density of PU/PM SIN produced through the topological cross-link formation between PU and PM network polymer precursors.