Heterobimetallic Catalysts for Ethylene Homo- and Co-Polymerization
Polyethylene (PE) is one of the most important plastics in the world. The incorporation of polar functional groups into PE, even in low concentrations, can dramatically improve its material properties. The discovery that late transition metal Ni and Pd complexes can promote the coordination-insertion copolymerization of ethylene and polar olefins was a major scientific breakthrough. Unfortunately, their catalytic activities are significantly reduced in the presence of polar monomers compared to in the presence of non-polar monomers. It is believed that the formation of metal-monomer chelated complexes prevents efficient copolymerization. As a new strategy to avoid coordination inhibition by polar monomers, we first designed a new class of mononuclear nickel complexes that have pedant polyethylene glycol (PEG) chains that can bind alkali metal ions. Solution titration studies by UV-vis absorption spectroscopy demonstrated that optimal metal binding is achieved by selecting the appropriate PEG chain length to match the size of the alkali ions. Upon the addition of the alkali salts, the resulting heterobimetallic nickel-alkali metal catalysts were found to be more active in ethylene polymerization, compared to their parent nickel catalysts. They also produced polymers with higher molecular weight and higher branching compared to in the absence of the alkali ions. To test our strategy on other molecular systems, we also designed cationic palladium phosphine-phosphonate complexes with secondary metal-binding PEG chains. Solution studies by 1H NMR spectroscopy showed that a 1:1 binding stoichiometry between Pd and alkali ions can be achieved. The structure of the palladium-sodium species was confirmed by X-ray crystallography. Our heterobimetallic complexes exhibit significant enhancement in catalytic activity and thermal stability compared to their parent monometallic catalysts. For the copolymerization of ethylene and acrylates, our heterobimetallic complexes also showed higher activity and produced copolymer with higher molecular weight. Mechanistic studies conducted by low-temperature NMR spectroscopy revealed that the addition of alkali ions can accelerate ethylene insertion by the palladium acrylate intermediate.