Structure of methylaluminoxane and the mechanism of active center formation in the zirconocene/methylaluminoxane catalytic system

New data on the structure of polymethylaluminoxane[-Al(CH3)O-](n) (MAO), th e nature of its Lewis acidity, and the mechanism of formation of the active centers of the metallocene catalysts during interaction between MAO and Cp 2ZrMe2 obtained by spectroscopy (Al-27,O-17,H-1, and C-13 NMR and spin prob e ESR) and density functional theory (DFT) calculations are considered. A t hree-dimensional structure of MAO (a cage structure) with four-coordinated aluminum atoms and three-coordinated oxygen atoms is found to he the most p robable. Experimental evidence for the presence of relatively weak Lewis ac id sites in MAO (about 1 site per 50 atoms of Al) is obtained for the first time. The acidity of MAO depends on the size of a MAO molecule n, and it i ncreases with increasing n. A scheme for the formation and structure of Lew is acid sires formed upon interaction between MAO and Al2Me6, which is alwa ys present in the MAO samples is proposed, Cationic intermediates formed in the MAO/Cp2ZrMe2 system at high molar ratios Al/Zr = 200-4000 correspondin g to real conditions of polymerization are characterized for the first time . A scheme of equilibria between them and the most probable precursors of t he active centers in this system are proposed. A new mechanism for the form ation of the [Cp2ZrMe](+)[MAO . Me](-) ionic pair involving the abstraction and occlusion of a methyl ligand in the bulk of a MAO molecule is proposed on the basis of quantum-chemical calculations.