New multifunctional silorane-based systems were investigated, with respect to their photoreactivity, as potential matrix resins for low-shrinkage/stress dental composites. The objective of this investigation was to synthesize and evaluate the reactivity of a silicon-analogue oxaspirocyclic monomer with silorane-based matrix resin systems during visible-light polymerization. The experimental formulations contained (1) a silicon-containing 1,5,7,11-tetraoxaspiro[5.5]undecane (TOSU-IV), (2) a phenylmethylsilane containing two cyclohexyloxiranyl groups, (3) a cyclotetrasiloxane containing four cyclohexyloxiranyl groups, and (4) a photocationic initiator system. Three main aspects were studied: (1) the photoreactivity of the tetraoxaspiroundecane (TOSU)/silorane reactant mixtures with differential scanning photocalorimetry, (2) oxirane ring-opening reactions of siloranes during binary photopolymerization with Fourier transform infrared (FTIR), and (3) oxaspirocyclic ring-opening reactions of the TOSU reactant during homophotopolymerization and binary photopolymerization with FTIR and NMR. A diallyl ether precursor of TOSU-IV was also included in selected studies. The main findings were as follows: (1) a feasible route for the successful synthesis of a silicon analogue (TOSU-IV) was developed; (2) TOSU-IV was compatible and photoreactive, making possible the reduction of polymerization stress in silorane-based matrix resins; and (3) spectroscopic evidence for both oxirane and oxaspirocyclic ring opening during the visible-light photopolymerization of the test formulations was found. (C) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 336-344, 2007