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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Abstract

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-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 Mw, 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.

Original languageEnglish
Pages (from-to)4605-4625
Number of pages21
JournalJournal of the American Chemical Society
Volume126
Issue number14
DOIs
Publication statusPublished - Apr 14 2004

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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