Substituent effects on the sol-gel chemistry of organotrialkoxysilanes

Silsesquioxanes are a family of siloxane network polymers that have become important as vehicles for introducing organic functionalities into sol-gel materials. However, there has not been a systematic study of the capacity o f organotrialkoxysilanes to form gels through the sol-gel process. In this study, we examined the sol-gel chemistry of:organotrialkoxysilanes (RSi(OR ')(3)) with different organic groups (R = H, Me, Et, Pr, i-Pr, n-Bu, i-Bu, t-Bu, hexyl, octyl, decyl, dodecyl, hexadecyl, octadecyl, cyclohexyl, vinyl , phenyl, benzyl, phenethyl, chloromethyl, chloromethylphenyl, and tridecaf luoro-1,1,2,2-tetrahydrooctyl) with methoxide or ethoxide substituents on s ilicon, at varying monomer concentrations, and under acidic, neutral, and b asic conditions. Gels were obtained from the sol-gel polymerization of the monomers with R '= Me and R = H, Me, vinyl, chloromethyl, chloromethylpheny l, hexadecyl, and octadecyl and R ' = Et and R = H, Me, Et, chloromethyl, v inyl, dodecyl, hexadeeyl, and octadecyl. Formation of gels, even with these monomers, was often circumvented by phase separation phenomena, giving ris e to crystalline oligomers, resinous materials, and precipitates. Gels obta ined from these polymerizations were processed as xerogels and characterize d by solid-state,NMR, microscopy, and nitrogen sorption porosimetry.