HYDROLYSIS AND ESTERIFICATION IN ORGANICALLY MODIFIED ALKOXYSILANES -A SI-29 NMR INVESTIGATION OF METHYLTRIMETHOXYSILANE

High-resolution Si-29 NR IR was used to investigate the acid-catalyzed hydrolysis and esterification reactions of methyltrimethoxysilane (MT MS) in methanol, The INEPT experiment, adapted for spin systems with m ultiple heteronuclear coupling constants, was used to assign the close ly spaced resonances of the MTMS hydrolysis products. Due to the rapid reaction rates, only the pseudoequilibrium concentration distribution for the resulting hydrolysis products could be determined, Models bas ed on thermodynamically statistical distributions, irreversible hydrol ysis reactions, and reversible hydrolysis reactions were nearly equall y successful in accounting for the concentration distributions over a wide range of H2O/Si ratios (R(w)) and temperatures, However, preparat ion of hydrolyzed MTMS in a nonpseudoequilibrium state unequivocally d emonstrated the reversibility of hydrolysis reactions on a short time scale, By measuring the extent of reaction of MTMS systems at high wat er concentrations, the ratio of the hydrolysis to esterification rate constant was determined to be approximately 100.