Multinuclear NMR investigation of methylaluminoxane

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 A 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 A and 20 < n < 30.