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We report the first sol−gel polymerization of alkoxysilane monomers in supercritical carbon dioxide (SCCD) as the polymerization solvent. Monolithic silica and silsesquioxane gels were formed in SCCD by the nonaqueous polymerization of tetramethoxysilane and 1,4-bis(triethoxysilyl)benzene, respectively, with formic acid (99%). The gels were dried to give high surface area, mesoporous aerogels by simply venting carbon dioxide from the autoclave.

New highly cross-linked polysiloxanes were prepared by sol−gel polymerization of 1,6-bis(diethoxymethylsilyl)hexane (1) and 1,4-bis(diethoxymethylsilyl)benzene (2). Hydrolysis and condensation of 1 and 2 under acidic and basic conditions with 4 equiv of water led to the rapid formation of hexylene- and phenylene-bridged polysiloxane gels. The dry gels (xerogels) were intractable, insoluble materials that were noticeably hydrophobic, exhibiting no swelling in organic solvents or water. Most of the xerogels were high surface area, mesoporous materials. Hexylene-bridged polysiloxanes prepared under acidic conditions were always nonporous regardless of whether they were processed to afford xerogels or supercritically dried as aerogels. Hexylene-bridged polysiloxanes prepared under basic conditions and all of the phenylene-bridged polysiloxanes were mesoporous with surface areas as high as 1025 m2/g.

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