POPULATIONS OF OLIGOMERS IN SOL-GEL CONDENSATION

Oligomers of silicon tetramethoxide (TMOS) have been made by condensation of monomers hydrolyzed by substoichiometric water. The concentrations of dimers, trimers and tetramers have been analyzed through Si-29 NMR experiments with polarization transfer. The rates for the formation and consumption of different species vary with the concentration of H+ ions in the reaction bath. At low pH, monomers are consumed faster than oligomers, and cyclic species are formed in large numbers. These rates deviate from statistical models where the reaction rates would be the same for all Si atoms regardless of how many functions they have already reacted. Deviations correspond to a selection of reaction pathways according to the pH of the reaction bath.