Generation of hyperpolarized spin states prior to an NMR experiment results in signals that are enhanced by several orders of magnitude. NMR spectra can then be acquired in rapid succession, permitting the characterization of time dependence on the sub-second to second time scale. Here, we utilize hyperpolarization by dissolution dynamic nuclear polarization (D-DNP) to study kinetics and mechanisms at early times in catalyzed polymerization reactions, revealing the relationship between catalyst and monomer structure with mechanisms of initiation, elongation and deactivation. For this purpose, monomers are hyperpolarized on 13C or 1H spins prior to injection into a catalyst solution for in-situ NMR detection. The resulting signals indicate reaction products that are formed, including the polymer product and its stereostructure, products of side reactions, as well as catalyst bound species. Secondly, model equations can be used for the determination reaction kinetics under concomitant influence of spin relaxation. In a first example, we illustrate the application of this technique for the characterization of single site olefin polymerization, using metallocene catalysts to react hyperpolarized hexene. The time evolution of signals from monomer and polymer species under catalysis by two zirconium based catalysts leads to a kinetic model comprising reaction and deactivation processes. A deactivation product is observed by correlation of chemical shifts to those known from the monomer. In a second example, we determine kinetic rates for initiation, and for the formation of polymer units with E and Z stereostructure for a ring opening metathesis polymerization using a ruthenium catalyst. Isotope enrichment of monomer or polymer species is not required in either case, simplifying synthetic requirements.

Early time kinetics and mechanisms of polymerization reactions by hyperpolarized NMR

253rd National Meeting (2017)

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