The time evolution of the sol-gel process: Si-29 NMR study of hydrolysis and condensation reactions of tetramethoxysilane

The time evolution of the structural intermediate formation obtained during hydrolysis and condensation reactions in tetramethoxysilane (TMOS), methan ol, H2O, HCl (molar ratio 1:5:2: 0.001) sol-gel reaction mixture was invest igated by Si-29 NMR spectroscopy at -75 degrees C. It is obvious that the s ubstitution of the Si methoxy group (OCH3) by a hydroxyl (OH) shifts the re sonance lines downfield. The dimer peaks are located about 9-10 ppm upfield from the corresponding monomers, and the Si located in the middle of the t rimers appeared at the chemical shift of 18-19 ppm upfield from the corresp onding monomers. The peaks of trimers, cyclic tetramers and the other highe r oligomers are relatively broad and weak, and exhibit a multiplet structur e. The time dependence of the concentrations of the individual types of sil icon in the reaction mixture were evaluated from the integral intensity of the corresponding Si atoms from the 29Si NMR spectra. The results obtained represent input data to the various kinetic models of the complex hydrolysi s and condensation reactions.