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 ¹³C Hyperpolarized NMR

Abstract

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 C2 symmetric catalyst, [(EBI)ZrMe][B(C6F5)4] (EBI = rac-(C2H4(1-indenyl)2)), and a C2v symmetric catalyst, [(Cp)2ZrMe][B(C6F5)4] (Cp = cyclopentadienyl). Hyperpolarized (13)C 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 (13)C 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 β-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 (13)C NMR signals over the course of several seconds underlines the utility of hyperpolarized NMR for quantifying early events in polymerization reactions.

Quantification of active sites in single-site group 4 metal olefin polymerization catalysis

“…-In the cases where the quantity [C * ] could not be assessed via an independent method, the rate parameters were extracted with an activation efficiency value set arbitrarily (in most cases to 100%) [33,34]. The rate parameters thus determined are then obviously quite questionable.…”

“…Some studies hypothesize that all (pre)catalyst is active; see e.g. [32][33][34]. However, even though metallocene, hemi-metallocene and post-metallocene complexes provide in principle the same global catalytic environment to all propagating centers (hence the "single-site" nomenclature), it is not guaranteed that they all undergo evenly and entirely the transformation into the active species and that the latter species remain active throughout the polymerization process.…”

Applications of dissolution dynamic nuclear polarization in chemistry and biochemistry

“…Chen et al more recently demonstrated that the stereochemistry, mechanisms, and kinetics of olefin polymerization reactions can be investigated by DNP‐enhanced NMR (Figure ) . In this study, two metallocene catalysts, [(EBI)ZrMe]‐[B(C 6 F 5 ) 4 ] (EBI = rac‐(C 2 H 4 (1‐indenyl) 2 )) and [(Cp) 2 ZrMe][B(C 6 F 5 ) 4 ] (Cp = cyclopentadienyl), were used to catalyze the in situ polymerization of hyperpolarized 1‐hexene. Based on signal intensities of isomers, product tacticity was characterized from the hyperpolarized 13 C NMR spectra.…”

“…The application range of reactions that can be monitored by hyperpolarized NMR spectroscopy is readily expanded to organic chemistry, where catalyzed or noncatalyzed rapid reactions have been investigated. Examples include a Grignard reaction, Diels‐Alder reaction, styrene polymerization, and olefin polymerization …”

“…More details of the fit can be found in the original publication. Adapted with permission from Chen et al…”