A simple cocatalyst modification approach for tuning polyolefin branching: halogenated MAO in nickel-catalyzed ethylene polymerization

Abstract

The nickel-diimine catalyst system is known to produce branched polyolefins due to the presence of β-H elimination and reinsertion pathways. Typically, the rate of key reactions is controlled by the design of ligands. On the other hand, in this study, we demonstrated that the degree of branching in nickel-catalyzed ethylene polymerization can be controlled by modifying methylaluminoxane (MAO) with halogenating agents. Specifically, BCl₃-modified MAO led to a reduction in the branching number to fewer than 10 per 1000 main chain carbons when combined with a simple nickel-diimine catalyst, while modification with Me₃SiCl did not. The reactivity of BCl₃ and Me₃SiCl toward aluminoxane and mononuclear alkylaluminum was confirmed by NMR analyses of various nuclei and X-ray total scattering measurements. BCl₃-modified MAO showed a superior effect in terms of reducing the branching number, indicating that the larger size of MAO may contribute to the suppression of chain walking besides the presence of chloride. These straightforward cocatalyst modification strategies offer a novel tuning method for olefin polymerization catalyst systems.