Hydrolysis and esterification in organically modified alkoxysilanes: A ²⁹Si NMR investigation of methyltrimethoxysilane†

High-resolution ²⁹Si NMR was used to investigate the acid-catalyzed hydrolysis and esterification reactions of methyltrimethoxysilane (MTMS) in methanol. The INEPT experiment, adapted for spin systems with multiple heteronuclear coupling constants, was used to assign the closely 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 based on thermodynamically statistical distributions, irreversible hydrolysis reactions, and reversible hydrolysis reactions were nearly equally successful in accounting for the concentration distributions over a wide range of H₂O/Si ratios (R_w) and temperatures. However, preparation of hydrolyzed MTMS in a nonpseudoequilibrium state unequivocally demonstrated the reversibility of hydrolysis reactions on a short time scale. By measuring the extent of reaction of MTMS systems at high water concentrations, the ratio of the hydrolysis to esterification rate constant was determined to be approximately 100.