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

Counteranion effects on the rate and stereochemistry of syndiotactic propylene enchainment by the archetypal C_s-symmetric precatalyst [Me₂C(Cp)(Flu)]ZrMe₂ (1; Cp = C₅H₄; Flu = C₁₃H₈, fluorenyl) are probed using the cocatalysts MAO (2), B(C₆F₅)₃ (3), B(2-C₆F ₅C₆F₄)₃ (4), Ph₃C ⁺B(C₆F₅)₄^- (5), and Ph₃C⁺-FAI(2-C₆F₅C₆F ₄)₃^- (6), offering greatly different structural and ion pairing characteristics. Reaction of 1 with 3 affords [Me₂C(Cp)(Flu)]ZrMe⁺ MeB(C₆F₅) ₃^- (7). In the case of 4, this reaction leads to formation the μ-methyl dinuclear diastereomers {([Me ₂C(Cp)(Flu)]ZrMe)₂(μ-Me)}⁺ MeB(2-C ₆F₅C₆F₄)₃^- (8). A similar reaction with 6 results in diastereomeric [Me ₂C(Cp)(Flu)]ZrMe⁺ FAI(2-C₆F₅C ₆F₄)₃^- (10) ion pairs. The molecular structures of 7 and 10 have been determined by single-crystal X-ray diffraction. Reorganization pathways available to these species have been examined using EXSY and dynamic NMR, revealing that the cation-MeB(C ₆F₅)₃^- interaction is considerably weaker/more mobile than in the FAI(2-C₆F₅C ₆F₄)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, m and mm stereodefect generation, and chain transfer processes are highly sensitive to the nature of the ion pairing. Thus, the complexes activated with 4 and 5, having the weakest ion pairing, yield the highest estimated propagation rates, while with 6, having the strongest pairing, yields the lowest. The strongly coordinating, immobile FAI(2-C ₆F₅C₆F₄)₃^- anion produces the highest/least temperature-dependent product syndiotacticity, lowest/least temperature-dependent m stereodefect abundance, and highest product molecular weight. These polypropylene microstructural parameters, and also M_w, are least sensitive to increased propylene pressure for FAI(2-C₆F₅C₆F₄)₃ ^-, but highest with MeB(C₆F₅)₃ ^-. In general, mm stereodefect production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacial propylene misinsertion to be the prevailing mm-generating process in all systems at 60 °C, being most dominant with 6, where mm stereodefect abundance is lowest. For 1,3-dichlorobenzene as the polymerization solvent, product syndiotacticity, as well as m and mm stereodefects, become indistinguishable for all cocatalysts. These observations are consistent with a scenario in which ion pairing modulates the rates of stereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity, and also dictates the rate of termination relative to propagation and the preferred termination pathway. In comparison to 3-6, propylene polymerization mediated by MAO (2) + 1 in toluene reveals an estimated ordering in site epimerization rates as 5 > 4 > 2 > 3 > 6, while product syndiotacticities rank as 6 > 2 > 5 ∼ 4 > 3.