Structure of Methylaluminoxane and the Mechanism of Active Center Formation in the Zirconocene/Methylaluminoxane Catalytic System
Kinetics and Catalysis
, E-ISSN: 1608-3210
Boreskov Institute of Catalysis SB RAS
New data on the structure of polymethylaluminoxane[-Al(CH3)O-](n) (MAO), the nature of its Lewis acidity, and the mechanism of formation of the active centers of the metallocene catalysts during interaction between MAO and Cp2ZrMe2 obtained by spectroscopy (Al-27,O-17,H-1, and C-13 NMR and spin probe ESR) and density functional theory (DFT) calculations are considered. A three-dimensional structure of MAO (a cage structure) with four-coordinated aluminum atoms and three-coordinated oxygen atoms is found to he the most probable. Experimental evidence for the presence of relatively weak Lewis acid 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 increases with increasing n. A scheme for the formation and structure of Lewis acid sires formed upon interaction between MAO and Al2Me6, which is always present in the MAO samples is proposed, Cationic intermediates formed in the MAO/Cp2ZrMe2 system at high molar ratios Al/Zr = 200-4000 corresponding to real conditions of polymerization are characterized for the first time. A scheme of equilibria between them and the most probable precursors of the active centers in this system are proposed. A new mechanism for the formation 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.