NICKEL-ALKALI HETEROBIMETALLIC CATALYSTS FOR ETHYLENE POLYMERIZATION

Polyethylene (PE) is one of the most commonly used plastics in our society due to the low-cost of ethylene and their diverse applications. The physical, chemical and mechanical properties of PE are influenced by its molecular weight, molecular weight distribution and morphology. Production of PE using single-site transition metal based catalysts is advantageous over processes based on free radial chemistry because it allows synthesis of well-defined polymers. However, single site catalysts typically afford one type of polymer under a given set of reaction condition. To obtain different PE products, it is necessary change the reaction conditions or chemically modify the steric or electronic properties of the catalyst. The former could be difficult to do in an industrial plant setting, whereas the latter may consume a tremendous amount of labor, cost, and time. To overcome these drawbacks, our group has been developing creating stimuli-responsive catalysts that are capable of yielding different polyethylene product from a universal catalyst platform. We have created several Ni or Pd catalysts that could switch reactivity by interchanging their pendant secondary cations. In this thesis, we have prepared a new class of nickel phosphine-phenolate complexes bearing a pendant polyethylene glycol (PEG) chain to provide a binding pocket for secondary metals. In the presence of secondary alkali cations such as Li+, Na+, K+, and Cs+, our heterobimetallic complexes displayed significant enhancement in catalytic activity and thermal stability compared to that of their parent monometallic complex and afforded different types of PE depending on the alkali ions used. The nickel-lithium complex showed extraordinary activity at 40 oC and the nickel-cesium displayed a high thermal stability at 90 oC. The binding stoichiometries between nickel and alkali metals were confirmed to be 1:1 both in the solid state by X-ray crystallography and in solution by UV-vis absorption spectroscopy. Interestingly, NMR spectroscopic studies of bimetallic complexes revealed the existence of another isomer, which has not been commonly observed experimentally. DFT calculations showed that chelation of secondary cations has significantly altered the ground state energies of the resulting nickel-alkali complexes, which may also reduce the energy barrier of their ethylene insertion steps. We also took advantage of the tunability of our nickel complex to synthesize bimodal PE in one-pot reactions. Polymerization of ethylene using our nickel complex in the presence of a mixture of Li/Na afforded polyethylene with bimodal molecular weight distributions, which was confirmed by GPC characterization. The ratios of the two molecular weight fractions were found to correlate well with the Li+/Na+ ratio used Lastly, we have prepared a bulky variant of our nickel phosphine-phenolate-PEG catalyst. The introduction of bulkier substituents into the phosphine donor has led to a significant enhancement in catalyst thermal stability and polymer molecular weight. For example, polymerization performed using Ni and Na+ or Cs+ gave ultra-high molecular weight PE at 30 oC. The bulky catalyst also displayed extraordinary productivity in the presence of Cs at 90 oC, yielding PE with high molecular weight, high activity, and narrow polydispersity.