Influence of the alkoxide group, solvent, catalyst, and concentration on the gelation and porosity of hexylene-bridged polysilsesquioxanes

1,6-Hexylene-bridged polysilsesquioxanes are hybrid organic-inorganic materials prepared by sol-gel polymerization of 1,6-bis(trialkoxysilyl)hexanes. Due to the trialkoxysilyl groups bonded to each end of the hexylene bridging group, hydrolysis and condensation easily gave network polymers in the form of gels. When the solvent evaporates from the gels, dry gels or xerogels are obtained that may be non-porous or porous depending on whether the network is compliant enough to resist collapse or not. Hexylene-bridged materials appear to lie on the boundary between polysilsesquioxanes with bridging groups that are sufficiently long and flexible to allow porosity to collapse with drying and those with bridging groups that are short and stiff enough to prevent collapse of porosity during drying. Under base-catalyzed conditions, 1,6- bis(triethoxysilyl)hexane polymerizes to yield porous xerogels while under acidic conditions, non-porous xerogels are obtained. In this study, we examine the effects of the alkoxide group on the sol-gel process and the resulting xerogels' porosity and provide a more detailed examination of the relative importance of the sol-gel reaction parameters (concentration, pH, solvent). 1, 6-Bis(trimethoxysilyl)hexane (1), 1,6-bis(triethoxysilyl)hexane (2), and 1, 6-bis(tri-n-propoxysilyl)hexane (3) were polymerized under acidic and basic conditions in methanol, ethanol, or n-propanol, respectively, as well as tetrahydrofuran (THF). The resulting gels were dried to provide xerogels that were characterized by SEM, solid state ¹³C and ²⁹Si Cross Polarization Magic Angle Spinning (CP MAS) NMR spectroscopy, and nitrogen and carbon dioxide sorption porosimetry.