INFLUENCE OF STERIC EFFECTS ON THE KINETICS OF ETHYLTRIMETHOXYSILANE HYDROLYSIS IN A FAST SOL-GEL SYSTEM

The kinetics of acid-catalyzed hydrolysis of ethyltrimethoxysilane (ETMS) and tetramethoxysilane (TMOS) in dilute methanol have been followed at several temperatures by Si-29 NMR. The rates of appearance and disappearance of all monomeric silanol intermediates, which are resolved quantitatively in the NMR spectrum, are fit using integrated rate expressions for competitive-consecutive second-order reactions to obtain the rate constants for the individual steps of hydrolysis. At all temperatures, the rate constants increase with each successive hydrolysis. Specifically, the rate constant for the second step is roughly twice that for the initial hydrolysis. In addition, both ETMS and TMOS hydrolyses demonstrate relatively well-behaved Arrhenius behavior over the observed temperature range. The trend observed in this study is opposite that predicted from earlier reports of general siloxane reactivity. Previously proposed mechanisms have invoked variations in the inductive effects as the principal rate-controlling parameter. The results reported here indicate that instead steric effects control the relative rates of hydrolysis, which is the first step of the oligomerization producing the sol-gel. A comparison of these results with those from the hydrolysis of tetraalkoxysilanes suggests that steric control may be general in the acid-catalyzed hydrolysis of silanes and siloxanes.