Understanding hydrolysis and condensation kinetics of gamma-glycidoxypropyltrimethoxysilane

Monitoring the kinetics of hydrolysis and condensation of -glycidoxypropyltrimethoxy-silane (-GPS) was carried out by NMR spectroscopy (Si-29-, C-13-, and H-1-). The course of these reactions was followed in 2wt% aqueous dilution conditions (26% D2O/74% H2O), pH 5.4, and temperatures of 26, 50, and 70 degrees C. At ambient temperature, hydrolysis and condensation proceed at very different time scales: a few hours for the hydrolysis versus several weeks for the condensation. Distortionless Enhancement by Polarization Transfer (DEPT) sequences by Si-29- and C-13-NMR spectroscopy were optimized for determining the complete spectral assignment for each hydrolysis step, i.e., RSi(OMe)(3-n)(OH)(n) (with R=(CH2OCH)CH2OCH2CH2CH2-;andn=1, 2, 3). A pseudo-first order rate constant for the first hydrolysis step, T0((OMe)3)+H2O -> T0((OMe)2OH)+MeOH, was calculated to be 0.026min(-1). Simultaneously to the condensation reactions, we have observed epoxy ring opening of the glycidyl- group. All three processes (hydrolysis, condensation, and epoxy ring opening) are dramatically accelerated with temperature increases from 26 to 70 degrees C. The activation energy of the epoxy ring opening leading to the formation of a diol structure at the extremity of the glycidoxypropyl- chain was estimated to be 68.4kJ/mol.