Alumoxo-heteropoly compounds as weakly coordinating anions for metallocenes in the oligomerization of alkenes

The in situ synthesis of methylalumoxane (MAO) on the crystalline structure of heteropoly acids generates a highly active and weakly coordinating comp ound for the metallocene catalyst. The MAO-phase formed by the controlled h ydrolysis of trimethylaluminum covers the Keggin unit of the heteropoly com pound, as is verified with several spectroscopic techniques. The primary Ke ggin structure of the heteropoly compound is preserved and MAO-formation in duces an intensive charge transfer in the VIS-region. Al-27-NMR of alumoxo- phosphotungstate illustrates the reversible interaction of the Lewis acid A l of the anchored alumoxane with the bridging oxygen atoms of the phosphotu ngstate Keggin structure. The interaction of the alumoxo-heteropoly compound with an ansa-metallocene produces a highly active catalyst for the co-oligomerization of ethene and propene. The softness of the heteropoly compound combined with the pseudo- liquid phase formation of the alumoxo-heteropoly compounds exceeds the acti vity of the homogeneous system and the stability of the heterogeneous MAO-a nchored materials. The composition of the Keggin structure determines the c atalytic potential of the metallocene which is transformed in a temperature dependent oligomerization and a fluctuating alternation of monomer and com onomer. The molecular weight and the methyl branching of the oligomer molec ules are designing tools for the physicochemical features of a synthetic lu bricant. A comparable charge delocalization over each heteropoly compound r esults in a single-sited catalytic system inducing a small dispersity of ol igomer products. (C)) 1999 Elsevier Science B.V. All rights reserved.