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 Mechanisti
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- 作者：Andreas C. Mö ; ller ; Richard Blom ; Ole Swang ; Andreas Hannisdal ; Erling Rytter ; Jon A. Stvneng ; Tanja Piel
- 刊名：Journal of Physical Chemistry A
- 出版年：2008
- 出版时间：May 1, 2008
- 年：2008
- 卷：112
- 期：17
- 页码：4074 - 4089
- 全文大小：697K
- 年卷期：v.112,no.17(May 1, 2008)
- ISSN：1520-5215
文摘
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/MAOare presented. The end group analysis of homopolymers reveals a pronounced dependence of the terminationrate on temperature changes. In combination with the high molecular weights obtained, these results are inaccord with theoretical predictions. Gel permeation chromatography, Fourier transform infrared, and 13C NMRanalyses of copolymerization products from 1/MAO as a function of comonomer concentration at two differenttemperature series denote its tendency to form inhomogeneous polymer blends. Thermal analysis andfractionation results of one such blend indicate an inhomogeneity in the enchainment process and the existenceof multiple active sites of differing geometry. These indications are further supported by AMBER force fieldand density functional theory studies of the catalyst precursors and the active site of 1/MAO. For this system,
-agostic interactions for the stabilization of the zirconium cation are favored over 
beta2.gif" BORDER=0 ALIGN="middle">-agostic interactions,which, in contrast to the situation in studies on bis-Cp systems, is a sparsely populated species. The gap inactivation enthalphies for beta2.gif" BORDER=0 ALIGN="middle">-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 activationenthalpy as low as 14 kcal/mol.
-agostic interactions for the stabilization of the zirconium cation are favored over beta2.gif" BORDER=0 ALIGN="middle">-agostic interactions,which, in contrast to the situation in studies on bis-Cp systems, is a sparsely populated species. The gap inactivation enthalphies for beta2.gif" BORDER=0 ALIGN="middle">-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 activationenthalpy as low as 14 kcal/mol.