Polymerization of bis(triethoxysilyl) ethenes. Impact of substitution geometry on the formation of ethenylene- and vinylidene-bridged polysilsesquioxanes

In this study, we utilized the substitution geometry of triethoxysilyl grou ps about an organic bridging group to control the outcome of the sol-gel po lymerization process. The substitution geometry of two triethoxysilyl group s about a carbon-carbon double bond was determined to have a profound effec t on sol-gel polymerizations of the E (1) and Z (2) ethenylene-bridged mono mers and vinylidene-bridged monomer (3) and on the porosity in the resultin g xerogels. Si-29 NMR and chemical ionization mass spectrometry were used t o elucidate the early sol-gel chemistry in the acid-catalyzed polymerizatio ns of 1-3. Trans substitution about the ethenylene-bridging group in 1 led to acyclic and monocyclic dimers and trimers as condensation products under acidic conditions and only microporous gels under both acidic and basic co nditions. In contrast, cyclization reactions dominated the sol-gel chemistr y of 2 beginning with intramolecular cyclization to give the cyclic disilse s-quioxane (4) and continued with the formation of cyclic oligomers, includ ing a bicyclic dimer. The cyclization of 2 slowed the rate of gelation comp ared to 1 and afforded microporous xerogels under acidic conditions and mes oporous gels under basic conditions. The sol-gel chemistry of the vinyliden e monomer (3) was strongly retarded by the formation of a cyclic dimer (5). Only mesoporous gels were formed under basic conditions after 9 months; no gels were obtained under acidic conditions.