Customizing Polyolefin Morphology by Selective Pairing of Alkali Ions with Nickel Phenoxyimine-Polyethylene Glycol Catalysts

In the present work, we have prepared nickel phenoxyimine-polyethylene glycol (PEG) catalysts with sterically bulky <i>N</i>-aryl substituents and investigated their ethylene homo- and copolymerization behavior. We have found that different nickel catalyst and alkali ion (Na⁺ or K⁺) combinations yielded polyethylene with different branching microstructures and molecular weights. Our heterobimetallic catalysts can copolymerize ethylene and nonpolar α-olefins with high activity but are strongly inhibited in the presence of polar vinyl olefins. We demonstrate that our heterobimetallic catalysts are significantly more stable in ethylene homopolymerization in comparison to conventional nickel phenoxyimine systems on the basis of time-dependent activity studies. This work showcases the versatility of Lewis acid tunable catalyst constructs to prepare customized polyolefins and suggests that similar design strategies could be applied to other catalyst systems

Fine-Tuning Nickel Phenoxyimine Olefin Polymerization Catalysts: Performance Boosting by Alkali Cations

To gain a better understanding of the influence of cationic additives on coordination–insertion polymerization and to leverage this knowledge in the construction of enhanced olefin polymerization catalysts, we have synthesized a new family of nickel phenoxyimine–polyethylene glycol complexes (<b>NiL0</b>, <b>NiL2</b>–<b>NiL4</b>) that form discrete molecular species with alkali metal ions (M⁺ = Li⁺, Na⁺, K⁺). Metal binding titration studies and structural characterization by X-ray crystallography provide evidence for the self-assembly of both 1:1 and 2:1 <b>NiL</b>:M⁺ species in solution, except for <b>NiL4</b>/Na⁺ which form only the 1:1 complex. It was found that upon treatment with a phosphine scavenger, these <b>NiL</b> complexes are active catalysts for ethylene polymerization. We demonstrate that the addition of M⁺ to <b>NiL</b> can result in up to a 20-fold increase in catalytic efficiency as well as enhancement in polymer molecular weight and branching frequency compared to the use of <b>NiL</b> without coadditives. To the best of our knowledge, this work provides the first systematic study of the effect of secondary metal ions on metal-catalyzed polymerization processes and offers a new general design strategy for developing the next generation of high performance olefin polymerization catalysts