In Situ Determination Of Tacticity, Deactivation, And Kinetics In [Rac-(C2h4(1-Indenyl)(2))Zrme][B(C6f5)(4)] And [Cp2zrme][B(C6f5)(4)]-Catalyzed Polymerization Of 1-Hexene Using C-13 Hyperpolarized Nmr

The stereochemistry, kinetics, and mechanism of olefin polymerization catalyzed by a set of zirconium-based metallocenes was studied by NMR using dissolution dynamic nuclear polarization (DNP). Hyperpolarized 1-hexene was polymerized in situ with a C-2 symmetric catalyst, [(EBI)ZrMe][B(C6F5)(4)] (EBI = rac-(C2H4(1-indenyl)2)), and a C-2v symmetric catalyst, [(Cp)(2)ZrMe][B(C6F5)(4)] (Cp = cyclopentadienyl). Hyperpolarized C-13 NMR spectra were used to characterize product tacticity following initiation of the reaction. At the same time, a signal gain of 3 orders of magnitude from C-13 hyperpolarization enabled the real time observation of catalyst-polymeryl species and deactivation products, such as vinylidene and a Zr-allyl complex. The compounds appearing in the reaction provide evidence for the existence of beta-hydride elimination and formation of a dormant site via a methane-generating mechanism. The presence of a deactivating mechanism was incorporated in a model used to determine kinetic parameters of the reaction. On this basis, rate constants were measured between 0.8 and 6.7 mol % of catalyst. The concentration dependence of the rate constants obtained indicates a second-order process for polymerization concomitant with a first-order process for deactivation. The simultaneous observation of both processes in the time evolution of C-13 NMR signals over the course of several seconds underlines the utility of hyperpolarized NMR for quantifying early events in polymerization reactions.