Heterobimetallic Nickel-Alkali Catalysts for Tunable Ethylene Polymerization

Polyethylene (PE) is one of the most common plastics used today because of its low cost and numerous applications. The physical and chemical characteristics of PE are dependent on its molecular weight, molecular weight distribution, and morphology. PE is prepared using a variety of methods, such as free radical polymerization that yields broadly dispersed polymers. In contrast, coordination polymerization using a single-site, transition-metal catalyst affords narrowly dispersed PE. Such well-defined materials possess many desirable properties suitable for commercial applications. Typically, to obtain different PE using single-site catalysts, the reaction conditions or the structure of the catalyst would need to be modified. The disadvantage of this approach is that it can be extremely costly and might not be feasible on an industrial scale. To enable access to diverse PE materials, we aimed to create catalysts that are responsive to metal ion stimuli and are capable of providing polymers with different morphologies. We have developed a nickel olefin polymerization catalyst that is thermally stable and can be turned on in the presence of external alkali ions. To further improve our design, we also synthesized a ligand variant containing 2,6-dimethoxyphenyl groups to increase the catalyst steric bulk. We found that the polymer molecular weight was significantly enhanced using this second-generation catalyst. When we varied the solvent incrementally from 2% ether/98% toluene to 100% hexane, the polymer molecular weight distribution went from monomodal to multimodal. We also observed that the polymer weights varied significantly when using 1:0 to 1:1 catalyst to cesium ratios.