Comparative study on the hydrolysis kinetics of substituted ethoxysilanes by liquid-state 29Si NMR

Liquid-state 29Si NMR was used to investigate the ammonia-catalyzed initial hydrolysis of different substituted ethoxysilanes dissolved in methanol. The following ethoxysilanes were used: tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DBS). Their reaction expresses were obtained by assuming first-order in the reactant ethoxysilanes and varying other reactant ammonia and water concentrations. Individual kinetic rate constants for the initial hydrolysis of each ethoxysilanes, the rate-limiting step for total reactions, were calculated, which were independent of any reactant concentration. The results indicated that the steric factors have more important effect on the reactive reactivity than inductive factors, so that the initial hydrolysis rate constants for the studied ethoxysilanes decreased in an unusual order DDS > TEOS > MTES. For these three ethoxysilanes, a common conclusion was drawn that the substitution of the hydroxyl group for the ethoxyl group resulted in a downfield shift. In the case of DDS, however, the trend was opposite to the previous studies in which the hydrolyzed resultants had upfield shift compared with DDS monomer.