Marked Counteranion Effects on Single-Site Olefin Polymerization Processes. Correlations of Ion Pair Structure and Dynamics with Polymerization Activity, Chain Transfer, and Syndioselectivity

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• 作者：Ming-Chou Chen ; John A. S. Roberts ; and Tobin J. Marks
• 刊名：Journal of the American Chemical Society
• 出版年：2004
• 出版时间：April 14, 2004
• 年：2004
• 卷：126
• 期：14
• 页码：4605 - 4625
• 全文大小：589K
• 年卷期：v.126,no.14(April 14, 2004)
• ISSN：1520-5126

文摘

Counteranion effects on the rate and stereochemistry of syndiotactic propylene enchainment bythe archetypal C_s-symmetric precatalyst [Me₂C(Cp)(Flu)]ZrMe₂ (1; Cp = C₅H₄; Flu = C₁₃H₈, fluorenyl) areprobed using the cocatalysts MAO (2), B(C₆F₅)₃ (3)_, B(2-C₆F₅C₆F₄)₃ (4)_, Ph₃C⁺B(C₆F₅)₄^- (5), and Ph₃C⁺FAl(2-C₆F₅C₆F₄)₃^- (6), offering greatly different structural and ion pairing characteristics. Reaction of 1 with3 affords [Me₂C(Cp)(Flu)]ZrMe⁺ MeB(C₆F₅)₃^- (7). In the case of 4, this reaction leads to formation the-methyl dinuclear diastereomers {([Me₂C(Cp)(Flu)]ZrMe)₂(-Me)}⁺ MeB(2-C₆F₅C₆F₄)₃^- (8). A similarreaction with 6 results in diastereomeric [Me₂C(Cp)(Flu)]ZrMe⁺ FAl(2-C₆F₅C₆F₄)₃^- (10) ion pairs. Themolecular structures of 7 and 10 have been determined by single-crystal X-ray diffraction. Reorganizationpathways available to these species have been examined using EXSY and dynamic NMR, revealing thatthe cation-MeB(C₆F₅)₃^- interaction is considerably weaker/more mobile than in the FAl(2-C₆F₅C₆F₄)₃^--derived analogue. Polymerizations mediated by 1 in toluene over the temperature range of -10 to+60 C and at 1.0-5.0 atm propylene pressure (at 60 C) reveal that activity, product syndiotacticity, mand mm stereodefect generation, and chain transfer processes are highly sensitive to the nature of the ionpairing. Thus, the complexes activated with 4 and 5, having the weakest ion pairing, yield the highestestimated propagation rates, while with 6, having the strongest pairing, yields the lowest. The stronglycoordinating, immobile FAl(2-C₆F₅C₆F₄)₃^- anion produces the highest/least temperature-dependent productsyndiotacticity, lowest/least temperature-dependent m stereodefect abundance, and highest productmolecular weight. These polypropylene microstructural parameters, and also M_w, are least sensitive toincreased propylene pressure for FAl(2-C₆F₅C₆F₄)₃^-, but highest with MeB(C₆F₅)₃^-. In general, mmstereodefect production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacialpropylene misinsertion to be the prevailing mm-generating process in all systems at 60 C, being mostdominant with 6, where mm stereodefect abundance is lowest. For 1,3-dichlorobenzene as the polymerizationsolvent, product syndiotacticity, as well as m and mm stereodefects, become indistinguishable for allcocatalysts. These observations are consistent with a scenario in which ion pairing modulates the rates ofstereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity, and also dictates the rate of termination relative to propagation and the preferred terminationpathway. In comparison to 3-6, propylene polymerization mediated by MAO (2) + 1 in toluene reveals anestimated ordering in site epimerization rates as 5 > 4 > 2 > 3 > 6, while product syndiotacticities rankas 6 > 2 > 5 ~ 4 > 3.