Hydrolysis and polymerization of dimethyldiethoxysilane, methyltrimethoxysilane and tetramethoxysilane in presence of aluminum acetylacetonate. A complex catalyst for the formation of siloxanes

Hydrolysis and polymerization of dimethyldiethoxysilane (DMDE), methyltrimethoxysilane (MTMS) and tetramethoxysilane (TMOS) in the presence of aluminum acetylacetonate (Al(acac)(3)) have been investigated by infrared and NMR spectroscopy. In the absence of acidic catalyst, Al(acac)(3) catalyzes the hydrolysis of all the silanes. The catalytic activity of Al(acac)(3) is less than that of HNO3, but larger than that of NH3. The hydrolysis rate increases with increasing concentration of Al(acac)(3) in DMDE. The hydrolysis of TMOS occurs rapidly after an inductive period, which becomes longer with addition of Al(acac)(3). The results are explained by assuming an Al(acac)(3) catalyzed hydrolysis and a silanol catalyzed hydrolysis. The addition of Al(acac)(3) causes changes in polymerization of the resultant silanols. In DMDE and MTMS, it stabilizes the silanols at the early stage, and then enhances their polymerization. The polymerization in TMOS leads to the formation of precipitates that have a high degree of polymerization. The polymerization appears to proceed via a deprotonation mechanism including transfer of protons from silanols to Al(acac)(3). The present results strongly suggest that, besides acids and bases, metal complexes can be used as catalysts for the formation of siloxanes under ambient conditions.