Reaction of Trimethylaluminum with [(^tBu)Al(μ₃-O)]₆:  Hybrid <i>tert</i>-Butylmethylalumoxanes as Cocatalysts for Olefin Polymerization

Abstract

The reaction of trimethylaluminum with the hexameric tert-butylalumoxane, [(tBu)Al(μ3-O)]6, has been investigated. Reaction of [(tBu)Al(μ3-O)]6 with 1 equiv of AlMe3 results in the formation of two isomers (A and B) of the hybrid tert-butylmethylalumoxane, [Al7(μ3-O)6(tBu)6Me3]. The structures of compounds A and B, as determined by NMR spectroscopy and mass spectrometry, consist of [Al6(μ3-O)6(tBu)5Me] alumoxane cages, formed via tert-butyl/methyl exchange, in which one of the edges of the Al6O6 cage is complexed to the (tBu)AlMe2 formed during alkyl exchange. The difference between the isomers results from the geometric relationship of the cage Al-Me group and the opened edge. The activity of [Al7(μ3-O)6(tBu)6Me3], for the [Me2C(Cp)(Flu)]ZrBz2-catalyzed polymerization of 1,5-hexadiene, is significantly increased in comparison to [(tBu)Al(μ3-O)]6. The effect of additional equivalents of AlMe3 on the cocatalytic activity of [Al7(μ3-O)6(tBu)6Me3] suggests that a maximum activity is obtained at a [(tBu)Al(μ3-O)]6 to AlMe3 ratio of 1:6. Under conditions of equal Al:Zr ratio the [(tBu)Al(μ3-O)]6(AlMe3)6 system has a higher activity than a representative sample of commercial methylalumoxane (MAO). 1H NMR suggests that the reaction of [(tBu)Al(μ3-O)]6 with 6 equiv of AlMe3 yields (tBu)AlMe2 as the only tert-butyl-containing species and a proposed AlMe3 “free” form of MAO. Whereas the activity of [Al7(μ3-O)6(tBu)6Me3] and MAO shows slight inhibition by the addition of Al(iBu)3, the activity of the [(tBu)Al(μ3-O)]6/(AlMe3)6 system is unaffected. The reaction of Cp2Zr(CD3)2 with [Al7(μ3-O)6(tBu)6Me3] demonstrates that methyl exchange does not occur between a metallocene and the alkyls of the alumoxane cage, but does occur with the complexed (tBu)AlMe2.