Simultaneous interpenetrating polymer networks (IPNs) based on polyether polyurethane (PUR) and poly(methyl methacrylate-co-trimethylolpropane trimethacrylate) (PMMA) were prepared in bulk at GO degrees C. The gelation time of the individual PMMA network was found to be faster when using azobis(isobutyronitrile) (AIBN) in the presence of stannous octoate (SnOc(2)) as activating system, compared to pure AIBN. Kinetic measurements have shown that the synergistic SnOc(2) role concerns the initiation of the free-radical polymerization. The results were explained by assuming the formation of a cyclic complex between the nitrile groups of AIBN and the tin(II) atom of SnOc(2).