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On the Nonsingle-Site Character of Bis(2-Dimethylsilyl-indenyl)zirconium(IV) Dichloride/MAO and Bis(2-Trimethylsilyl-indenyl)zirconium(IV) Dichloride/MAO:  Polymerization Characteristics and Mechanistic Implications

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journal contribution
posted on 01.05.2008, 00:00 by Andreas C. Möller, Richard Blom, Ole Swang, Andreas Hannisdal, Erling Rytter, Jon A. Støvneng, Tanja Piel
Ethene polymerization with bis(2-dimethylsilyl-indenyl)zirconium(IV) dichloride (1)/MAO and bis(2-trimethylsilyl-indenyl)zirconium(IV) dichloride (2)/MAO and ethene-co-1-hexene polymerization with 1/MAO are presented. The end group analysis of homopolymers reveals a pronounced dependence of the termination rate on temperature changes. In combination with the high molecular weights obtained, these results are in accord with theoretical predictions. Gel permeation chromatography, Fourier transform infrared, and 13C NMR analyses of copolymerization products from 1/MAO as a function of comonomer concentration at two different temperature series denote its tendency to form inhomogeneous polymer blends. Thermal analysis and fractionation results of one such blend indicate an inhomogeneity in the enchainment process and the existence of multiple active sites of differing geometry. These indications are further supported by AMBER force field and density functional theory studies of the catalyst precursors and the active site of 1/MAO. For this system, δ-agostic interactions for the stabilization of the zirconium cation are favored over β-agostic interactions, which, in contrast to the situation in studies on bis-Cp systems, is a sparsely populated species. The gap in activation enthalphies for β-hydride transfer and elimination is marginalized for these bulky zirconocenes, and conceptually new mechanisms for the isomerization of the vinyl end groups are discussed. Further, unexpected activation of the silicon−hydrogen bond within the ligand framework is observed with an activation enthalpy as low as 14 kcal/mol.