Hydrolysis and initial polycondensation of phenyltrimethoxysilane and diphenyldimethoxysilane

Abstract

The sol–gel process opens the possibility of producing vitreous solids containing the main structural groups of oxide glasses and polymers by introducing compounds of the type R_nSi(OR′)_4–n into the starting mixture. The Si–C bond is normally stable towards hydrolysis and therefore the ligand R acts as a network modifier in the resulting structure. Two organoalkoxysilanes were studied: phenyltrimethoxysilane (PTMOS) and diphenyldimethoxysilane (DPMOS). The compositional regions in PTMOS and DPMOS systems where the solution is homogeneous and the gelation takes place were investigated. The hydrolysis and initial polycondensation of PTMOS and DPMOS were studied by highresolution ²⁹Si nuclear magnetic resonance. Signals due to the following monomers and oligomers have been identified: PhSi(OMe)₂(OH); PhSi(OMe)(OH)₂; PhSi(OH)₃; Ph₂Si(OMe)(OH); Ph₂Si(OH)₂(where Ph=C₆H₆). Concerning the difunctional alkoxysilane, it was shown that the substitution of the hydroxy group for the methoxy group resulted in an upfield shift of the signal which is opposite to the case for tetrafunctional alkoxides. For the PTMOS, the same substitution induces the conventional downfield shift. ²⁹Si NMR quantitative analysis was performed on both alkoxide solutions during the following of the hydrolysis-condensation reactions and on the final gels. This provided information on the formation of the gels molecular structure.