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 Cs-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-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 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 {([Me2C(Cp)(Flu)]ZrMe)2(μ-Me)}+ MeB(2-C6F5C6F4)3- (8). A similar
reaction with 6 results in diastereomeric [Me2C(Cp)(Flu)]ZrMe+ FAl(2-C6F5C6F4)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(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, 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 FAl(2-C6F5C6F4)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 Mw, are least sensitive to
increased propylene pressure for FAl(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.