Understanding Hydrolysis and Condensation Kinetics of γ-Glycidoxypropyltrimethoxysilane

Monitoring the kinetics of hydrolysis and condensation of γ-glycidoxypropyltrimethoxy-silane (γ-GPS) was carried out by NMR spectroscopy (²⁹Si-, ¹³C-, and ¹H-). The course of these reactions was followed in 2 wt% aqueous dilution conditions (26% D₂O/74% H₂O), pH 5.4, and temperatures of 26, 50, and 70°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 ²⁹Si- and ¹³C-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 0.026 min^-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°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 68.4 kJ/mol.