Nonshrinking, photopolymerizable polycarbosiloxanes through ring-opening polymerization of disilaoxacyclopentane monomers

(Chemical Equation Presented) Shrinkage during polymerization and processing of silica and hybrid organic-inorganic xerogels prohibits net-shape fabrication and introduces stresses that often result in cracking. In this study, we have used a new class of hybrid organic-inorganic monomers based on the ring-opening polymerization of two strained disilaoxacyclopentyl groups bridged by alkylene or arylene groups to control the physical and mechanical properties of the resulting monolithic gels. Shrinkage is virtually eliminated by the ring-opening polymerization chemistry which, in contrast to that of alkoxysilane sol-gels, does not require water or solvent, or produce any condensation byproducts. The bridging group can be varied in length and flexibility and the bridged monomers can be copolymerized with the monomer 2,2,5,5-tetramethyl-2,5-disila-l-oxacyclopentane, to permit molecular engineering of the glass transition temperature (T_g) and coefficient of thermal expansion in the resulting hybrid materials. Most importantly, we have demonstrated for the first time that the ring-opening polymerizations of disilaoxacyclopentane monomers can be performed using Brønsted acid catalysts. This has also enabled the use of photoacid generators to fabricate thin films and bulk samples of these hybrid materials.