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 groups about an organic bridging group to control the outcome of the sol-gel polymerization process. The substitution geometry of two triethoxysilyl groups about a carbon-carbon double bond was determined to have a profound effect on sol-gel polymerizations of the E (1) and Z (2) ethenylene-bridged monomers and vinylidene-bridged monomer (3) and on the porosity in the resulting xerogels. ²⁹Si NMR and chemical ionization mass spectrometry were used to elucidate the early sol-gel chemistry in the acid-catalyzed polymerizations 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 conditions. In contrast, cyclization reactions dominated the sol-gel chemistry of 2 beginning with intramolecular cyclization to give the cyclic disilsesquioxane (4) and continued with the formation of cyclic oligomers, including a bicyclic dimer. The cyclization of 2 slowed the rate of gelation compared to 1 and afforded microporous xerogels under acidic conditions and mesoporous gels under basic conditions. The sol-gel chemistry of the vinylidene 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.