Stable Ultrahighly Branched Polyethylene Synthesis via Externally Robust Chain-Walking Polymerization

Abstract

Recently, late-transition-metal-catalyzed chain-walking polymerization has emerged as a promising method for synthesizing a range of polyethylenes, including linear, branched, and hyperbranched structures, solely from ethylene feedstock. The spatial configuration of the catalyst is a crucial factor in determining the branching patterns observed in the resulting polyethylene. In this study, we designed and employed a series of stereoscopically bulky α-diimine Ni(II) and Pd(II) complexes featuring axial flexible cycloalkyl substituents and an equatorial rigid dibenzobarrelene backbone for ethylene (co)polymerization. These Ni(II) complexes exhibited high catalytic activity, reaching up to 10⁶ g/(mol Ni·h), and yielded very-high-molecular-weight polyethylene with extremely narrow molecular weight distributions (1.00–1.27) and very high branching densities (105–153/1000C) in a living ethylene polymerization behavior under various polymerization temperatures. Most interestingly, the branching density of polyethylene from the cyclohexyl-substituted Ni(II) complex is almost independent of the polymerization conditions and ultrahighly branched polyethylene with ultrahigh molecular weight (up to 1097 kg/mol) can be accessed. On the other hand, the corresponding Pd(II) complexes also demonstrated moderate catalytic activity, achieving up to 10⁵ g/(mol Pd·h), and yielded highly branched polyethylene with high molecular weights ranging from 160.8 to 511.9 kg/mol. Moreover, these Pd(II) complexes were capable of generating high-molecular-weight E-MA copolymers with adjustable incorporation ratios (0.85–2.77 mol %) and high branching densities (106–117/1000C). Through a comparative analysis of single-crystal structures and DFT calculations, we found that the axial flexible cycloalkyl substituents and equatorial rigid dibenzobarrelene backbone of these complexes create an interesting ligand environment that favors chain walking over chain propagation in the nickel-catalyzed ethylene polymerization system. This finding provides valuable insights into the design of more efficient and controllable catalysts for the synthesis of polyethylenes with tailored properties.