Catalytic Propene Polymerization: Determination of Propagation, Termination, and Epimerization Kinetics by Direct NMR Observation of the (EBI)Zr(MeB(C6F5)3)propenyl Catalyst Species

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Abstract
Treatment of (EBI)ZrMe2 with B(C6F5)3 and 100 equiv of 1-hexene in toluene-d8 at −40 °C for 1 h, followed by addition of >5 equiv of propene, generates a living polypropene catalyst (3) with a polyhexene “tail”. Upon further addition of propene, speciation of the catalyst and propagation kinetics are measured directly by NMR, yielding a propagation rate law that is first order in [propene] and [living catalyst] with a rate constant of 2.6 M-1 s-1 at −40 °C and the activation parameters ΔH⧧ = 3.4(0.2) kcal/mol and ΔS⧧ = −42(2) cal/(mol K). In the absence of propene, elimination of vinylidene-terminated polymer is observed with a first-order rate constant of 3.3 (0.5) × 10-5 s-1 at −40 °C and ΔH⧧ = 12.9(1.4) kcal/mol and ΔS⧧ = −23(5) cal/(mol K). Use of 1-13C-propene and 1,1-2H-2,3-13C-propene and 1H, 19F, and 13C NMR spectroscopy provide rigorous characterization of the living catalyst and enable the first direct observation of both 1,3-isomerization and chain-end epimerization processes at a metallocene catalyst. Isomerization and epimerization occur with rates that are similar to that of vinylidene termination and are consistent with previous mechanistic hypotheses featuring isomerization via a tertiary alkyl intermediate and switching between enantiofaces of the coordinated alkene.