Siloxy-substituted alumoxanes: synthesis from polydialkylsiloxanes and trimethylaluminium, and application as aluminosilicate precursors

Abstract

The interaction of AlMe₃ with the polydialkylsiloxanes (OSiRMe)_x and (OSiPh₂)₃ for long periods at elevated temperatures leads to the rupture of the silicon–oxygen framework and yields dimeric dimethylaluminium siloxides [Me₂Al(OSiMe₂R)]₂, R = Me (1), n-C₆H₁₃(2), n-C₈H₁₇(3), n-C₁₄H₂₉(4), n-C₁₈H₃₇(5), -CH₂CH₂CF₃(6), Ph (7), and [Me₂Al(OSiMePh₂)]₂(8), respectively. Hydrolysis of the dimethylaluminium siloxides results in the formation of oligomeric siloxy-substituted alumoxanes having the composition [Al(O)(OH)_x(OSiMe₂R)_1–x]_n, R = Me (9), n-C₆H₁₃(10), n-C₈H₁₇(11), n-C₁₄H₂₉(12), n-C₁₈H₃₇(₁₃), –CH₂CH₂CF₃(14), Ph (15), and [Al(O)(OH)_x(OSiMePh₂)_1–x]_n(16). The extent of hydrolysis, physical properties, and the ceramic yield of aluminosilicate upon their pyrolysis are highly dependent on the nature of the alkyl substituent, R, on silicon. The aluminium siloxides and siloxy-alumoxanes have been characterised by IR, ¹H, ¹³C, ¹⁷O, ²⁷Al and ²⁹Si NMR spectroscopy, mass spectrometry, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray (EDX) analysis. The aluminosilicate has been characterised by scanning electron microscopy (SEM), EDX, XPS and X-ray diffraction (XRD).