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February 2019:
Congratulations to Jihyun Kim for winning Best Poster Award at the Minerva-Gentner Symposium on MR Spectroscopy & Molecular Imaging

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Catalysis & Polymerization

Kinetics and Mechanisms in Polymerization Catalysis
Owing to the widespread application of polymer materials, an accurate understanding of the relationship between synthesis and the properties of the final product is of high relevance. Using hyperpolarization of nuclear spins, we are able to mesure the initial time period in a polymerization reaction immediately following catalyst activation. A sufficiently large signal to observe the time evolution of 13C NMR signals, revealing the kinetics and mechanisms of the catalyzed reaction governed by the interaction of the monomer and catalyst, and determining the structure of the final product. Hyperpolarized real-time NMR therefore represents a unique way of accessing the activity, deactivation pathways, stereo structure formation, and other properties of modern polymerization catalysts.

Case Study: Metallocene Catalyzed Reactions
Real-time 13C NMR of metallocene catalysisHyperpolarized 1-hexene is used as the monomer in a metallocene catalyzed polymerization reaction. Initially strong monomer signals are observed in 13C NMR spectra (red labels). These signals decay at later time both because the monomer is consumed, and because of spin relaxation. At the same time, hyperpolarization is carried to reaction product, as monomers are integrated into the polymer (blue signals). From these signals, it is possible to determine the tacticity of the product appearing shortly after initiation of the reaction. The kinetics of the reaction can be determined from the time evolution of the monomer signal intensities, as well as from those of polymer. In the figure with logarithmic vertical scale, signal depletion due to spin relaxation follows a linear line, whereas with catalyst additon, an initially larger slope followed by a curvature is observed. The latter is due to the polymerization reaction occurring in conjunction with catalyst deactivation. Rate constants for these processes can be determined from the fit to a kinetic model.

Chemical shift correlations in metallocene catalyzed reactionThe high dynamic range observed in 13C NMR spectra allows the observation of minor species arising from side reactions. Signals from these species can be correlated through the reaction by manipulating the spin populations of the initially hyperpolarized spin. The origin of the corresponding signals can then be identified even in transiently appearing species. Here, these chemical shift correlations are shown for vinylidene (1') and Zr-allyl species (2') species, providing evidence for β-H elimination.