Mechanistic studies of catalytic alkene polymerization and the development of stopped-flow NMR kinetics

A detailed mechanistic understanding of catalytic reactions requires elucidation of reaction kinetics and characterization of catalyst speciation during catalytic processes. A lack of methods capable of providing this information under a wide range of reaction conditions has limited our understanding of many important catalytic processes. An example of a widely used catalytic process for which fundamental mechanistic questions remain is catalytic alkene polymerization. Although homogeneous alkene polymerization catalysts have been known for nearly thirty years, the rates of the fundamental polymerization steps, initiation, propagation, and chain transfer, and the number of active sites are not known for the vast majority of catalysts. This work describes the use of nuclear magnetic resonance (NMR) spectroscopy to study catalytic alkene polymerization. A low temperature method is used to identify and demonstrate the dormancy of metal allyl species following 1-hexene polymerization with [rac-Me2Si(1-indenyl)2Zr(CH2SiMe 3)][B(C6F5)4]. The development of stopped-flow NMR spectroscopy expands the utility of NMR kinetics to faster reactions. Simulated NMR spectra of reactions that proceed on the millisecond timescale (t1/2 < 20 ms) show unusual features. Reaction induced broadening of reactant signals and phase shifted product signals are sensitive to the reaction rate, and the shape of the NMR signals is affected by the kinetic rate law. Modeled spectra show that the appearance of polymerization spectra is sensitive to initiation and propagation rates. Stopped-flow NMR experiments using a modified commercial flow probe determine rac-Me 2Si(1-indenyl)2Zr(Me)(MeB(C6F5) 3)-catalyzed 1-hexene polymerization initiation, propagation, and termination kinetics from a single 1-2 minute experiment. Chain swinging is observed directly, and the influence of solvent polarity on the individual polymerization steps is revealed. A stopped-flow toroid cavity detector is explored as a robust NMR probe for collecting stopped-flow kinetics. Investigations of a suspended central conductor design demonstrate an approach for improving the magnetic field homogeneity in toroid cavities, and initial tests with a stopped-flow toroid cavity show promise for enabling stopped-flow NMR of millisecond timescale reactions. Kinetic modeling of polymer molecular weight distributions demonstrates the effect of fundamental polymerization kinetics on polymer distributions. The importance of modeling band broadening for comparison with molecular weight distributions measured by gel permeation chromatography is shown.