Rc. Chambers et al., INFLUENCE OF STERIC EFFECTS ON THE KINETICS OF ETHYLTRIMETHOXYSILANE HYDROLYSIS IN A FAST SOL-GEL SYSTEM, Chemistry of materials, 5(10), 1993, pp. 1481-1486
The kinetics of acid-catalyzed hydrolysis of ethyltrimethoxysilane (ET
MS) and tetramethoxysilane (TMOS) in dilute methanol have been followe
d at several temperatures by Si-29 NMR. The rates of appearance and di
sappearance of all monomeric silanol intermediates, which are resolved
quantitatively in the NMR spectrum, are fit using integrated rate exp
ressions for competitive-consecutive second-order reactions to obtain
the rate constants for the individual steps of hydrolysis. At all temp
eratures, 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 hydro
lyses demonstrate relatively well-behaved Arrhenius behavior over the
observed temperature range. The trend observed in this study is opposi
te that predicted from earlier reports of general siloxane reactivity.
Previously proposed mechanisms have invoked variations in the inducti
ve effects as the principal rate-controlling parameter. The results re
ported here indicate that instead steric effects control the relative
rates of hydrolysis, which is the first step of the oligomerization pr
oducing 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.