Aluminum Silanolates Acid Strength and Polymerization Catalyst Activity

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Abstract
Two aluminum silanolates, diethyl(trimethylsiloxy)aluminum (I) and tris(trimethylsiloxy)-aluminum (II), were compared with the corresponding aluminum alcoholates in terms of their acid strength and in their catalyst behavior in the polymerizations of cyclic ethers and vinyl ether. The reference aluminum alcoholates were diethylaluminum t-butoxide (III) and aluminum t-butoxide (IV). The Lewis acid strength was examined by means of two methods, the Hammett indicator method and the shift of the IR carbonyl band (ΔνC=O) of n-butyraldehyde. Four aluminum compounds, I–IV, were weaker Lewis acids than triethylaluminum; i.e., the replacement of the ethyl group of triethylaluminum with either a siloxy group or an alkoxy group brought about a decrease in the acid strength. The acid strength of I and II were demonstrated to be stronger than that of the reference alcoholates, III and IV. A difference in polymerization catalyst activity was also observed between aluminum silanolates and the reference alcoholates; it was reasonably explained by the difference in acid strength.

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