Activation of Cp₂ZrX₂ (X = Me, Cl) by Methylaluminoxane As Studied by Electrospray Ionization Mass Spectrometry: Relationship to Polymerization Catalysis

Activation of Cp 2 ZrCl 2 and Cp 2 ZrMe 2 by methylaluminoxane (MAO) in toluene is largely complete at Al:Zr ratios of 100:1 to 200:1 as revealed by electrospray ionization mass spectrometry (ESI MS). The anions present undergo chlorination in the case of Cp 2 ZrCl 2 . DFT calculations reveal that chlorination of MAO is favorable and involves dissociation of Me 3 Al, followed by association of Me 2 AlCl. Ethylene polymerizations were conducted using these catalyst precursors in toluene. The activity vs [Zr] data are essentially identical, while higher MW polyethylene is formed with a narrower MWD at lower Al:Zr ratios in the case of Cp 2 ZrMe 2 . The activity vs [Zr] data could be fit to a model which invokes bimolecular deactivation of growing chains. ESI MS reveals that a dinuclear Zr 2 cation with m / z 557 is formed on exposure of [Cp 2 Zr(μ-Me) 2 AlMe 2 ] + to ethylene, in addition to other cations that are dinuclear with respect to Zr. Labeling experiments using ethylene- d 4 indicate that these dinuclear cations are derived from ethylene, either through direct incorporation in the case of m / z 557, or indirectly through incorporation of deuterium following e.g. β-H elimination. These experiments shed light on the need for high Al:Zr ratios for ethylene polymerization using soluble metallocene catalysts. The active catalyst [Cp 2 ZrR][MAO(Me)] (R = H, Et or a higher homologue) suffers a second order deactivation, and thus activity improves upon dilution of the catalyst precursor at constant [Al].