Multinuclear NMR investigation of methylaluminoxane

✍ Scribed by Dmitrii E. Babushkin; Nina V. Semikolenova; Valentina N. Panchenko; Anatolii P. Sobolev; Vladimir A. Zakharov; Evgenii P. Talsi

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 506 KB
• Volume: 198
• Category: Article
• ISSN: 1022-1352
• DOI: 10.1002/macp.1997.021981206

✦ Synopsis

Abstract

By means of ^27^Al, ^1^H, ^17^O and ^13^C NMR spectroscopy the structure of the methylaluminoxane (MAO) cocatalyst of Kaminsky‐Sinn catalysts was investigated. We have found that the ^27^Al NMR resonance line of MAO is extremely broad at room temperature (δ ≈ 60, Δω~1/2~ 50 ± 10 kHz). At elevated temperatures (40–120°C) MAO exhibits an ^27^Al resonance at δ 110 ± 10, Δω~1/2~ 15–10 kHz, that is, within the range reported for the aluminoxane clusters [(t‐Bu)Al(μ~3~‐O)]~6~ and [(t‐Bu)Al(μ~3~‐O)]~9~ with cage structures. The ^27^Al resonance at δ 149–153, usually attributed to MAO, belongs to AlMe~3~ present in MAO samples. The ^17^O NMR resonance of MAO at 50°C (δ 67, Δω~1/2~ 1.7 kHz) is within the range typical for three‐coordinate oxo ligands, but it can be attributed only to a minor part of MAO oligomers. It was shown that the intensity of ^27^Al and ^17^O NMR resonances of MAO increases with increasing temperature, whereas the width of both resonances is almost constant. The results obtained lead to the conclusion that at ambient conditions MAO forms oligomers (MeAlO)~n~ with cage structure and MeAlO~3~ environment. Upon increasing the temperature these oligomers reversibly break into smaller MAO units. Based on the ^27^Al NMR data the average radius (R) of MAO oligomers is approximately 5.1 ± 0.3 Å at 120°C. This radius corresponds to (MeAlO)~n~ species with 9 < n < 14. At ambient conditions the predominant part of MAO forms oligomers with R = 7 ± 0.5 Å and 20 < n < 30.