Similarities in the hydrolysis pseudoequilibrium behavior of methyl- substituted ethoxysilanes

By considering a range of conditions where the condensation time scale significantly exceeds that of hydrolysis, we are able to quantify by ²⁹Si NMR the alkoxide hydrolysis equilibrium constants of silicon sites with varying degrees of ethoxy, siloxy, and methyl group substitution during polymerization of methylethoxysilanes in ethanol/water mixtures. Although there is a strong emphasis in existing literature on differences in hydrolysis behavior with such substitutions, we find a surprising lack of variation in these coefficients: all are near 15±6. This implies that in practice, while hydrolysis rates may vary with substitution, differences in hydrolysis pseudoequilibrium behavior may often be negligible. Surprisingly, we also find that, even under alkaline conditions, sites with exactly one uncondensed group reach hydrolysis pseudoequilibrium, although other sites in the same solution do not.