A series of benzyl-substituted, dual-site ansa-metallocenes were synthesized and characterized. Their isolated rac-and meso-diastereomers were studied in propene polymerization after methylaluminoxane or borate activation. Catalysts' polymerization behavior were investigated in various polymerization conditions, and produced polypropenes (PPs) were characterized with NMR, GPC, and DSC. The rac-and meso-diastereomers of these unsymmetric catalysts bearing a SiMe 2 bridge produced PP with similar activity, tacticity, and molar mass. According to quantum chemical calculations, the benzyl group in the catalysts can rotate, having significant energy minima. The reason that the diastereomers produce PP with similar molar mass is linked to these local energy minima and is further discussed.
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, delta-agostic interactions for the stabilization of the zirconium cation are favored over beta-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 beta-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.
Six (Cp)(4‐phenyl‐indenyl)MCl2 (A) and four (Cp)(1‐phenyl‐indenyl)MCl2 (C) (M = Zr/Hf) based complexes were tested in ethene homo‐ and copolymerisation under different conditions to explain how the differences in the complex structure affect the polymerisation process and the formed polymer. Polymerisation experiments reveal that hafnocene catalysts need a higher amount of MAO to reach the maximum activity than zirconocenes. Hafnocenes also incorporate better 1‐hexene. Catalysts with the Cp* or 1,2,4‐Me3Cp ligand show higher activity, and work well with [HNMe2Ph][B(C6F5)4]/TIBA as cocatalyst, but produce polymers with lower $\overline M _{\rm n} $ than the corresponding catalyst with a plain Cp ligand. The Cp* ligand seems to prevent 1‐hexene incorporation. A methyl group in the 2‐position of (Cp)(4‐PhInd)ZrCl2 decreases the activity and $\overline M _{\rm n} $ but favours 1‐hexene insertion. Polymerisation activity is higher with catalysts with a 1‐PhInd‐ligand than with a 4‐PhInd‐ligand.magnified image
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