Methylene-bridged polysilsesquioxaes: Substitution of a methylene spacer within a silicate matrix

Polysilsesquioxanes with organic bridging groups have been popular as building blocks for organosilicas used for catalyst supports, membranes, high surface area adsorbents, and dielectric films. The organic bridging is an integral part of the network giving rise to the hypothesis that bridged polysilsesquioxanes are more similar to silica in structure than polysilsesquioxanes with pendant organic groups. However, there is only limited evidence to support this organosilica model. To test the hypothesis, we chose to compare the sol-gel polymerizations of the simplest alkylene-bridged polysilsesquioxane monomer, bis(triethoxysilyl)methane (M-0) with two silica precursors: tetraethoxysilane (T-0) and hexaethoxydisiloxane (H-0), a proposed intermediate in T-0's polymerization. M-0 and H-0 differ only on whether the bridging group between the two silicons is a methylene or oxygen, respectively. In spite of faster hydrolysis and condensation rates with M-0, comparisons of the methylene-bridged and silica monomers' sol-gel chemistry and physical properties of the resulting xerogels reveal striking similarities that corroborate the organosilica model. In all three cases, hydrolysis and condensation led to mixtures of acyclic and cyclic intermediates. As one might expect from the structural similarities between the M-0 and H-0 monomers, mass spectrometric analyses of their early hydrolysis and condensation revealed nearly identical reaction pathways. Solid state Si-29 NMR analysis of the xerogels revealed very similar degrees of condensation and surface area analyses of the resulting xerogels (M-1, H-1, and T-1) revealed remarkably similar surface area and pore size distributions, especially between M-1 and H-1.