ISSN:
1022-1352
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
By means of 27Al, 1H, 17O and 13C NMR spectroscopy the structure of the methylaluminoxane (MAO) cocatalyst of Kaminsky-Sinn catalysts was investigated. We have found that the 27Al NMR resonance line of MAO is extremely broad at room temperature (δ ≈ 60, Δω1/2 50 ± 10 kHz). At elevated temperatures (40-120°C) MAO exhibits an 27Al resonance at δ 110 ± 10, Δω1/2 15-10 kHz, that is, within the range reported for the aluminoxane clusters [(t-Bu)Al(μ3-O)]6 and [(t-Bu)Al(μ3-O)]9 with cage structures. The 27Al resonance at δ 149-153, usually attributed to MAO, belongs to AlMe3 present in MAO samples. The 17O NMR resonance of MAO at 50°C (δ 67, Δω1/2 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 27Al and 17O 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 MeAlO3 environment. Upon increasing the temperature these oligomers reversibly break into smaller MAO units. Based on the 27Al 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.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/macp.1997.021981206
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