To address the impact of chloride substitution on the performance of a benzhydryl-substituted bis(arylimino)pyridyl–cobalt ethylene polymerization catalyst, this work reports a new family of unsymmetrical N' , N' , N'' -cobalt(II) chloride complexes, Co1 – Co5 , incorporating one fixed N -2-Me-4,6-bis(4,4 ' -dichlorodibenzhydryl)phenyl group while the other N -aryl group is sterically and electronically variable (aryl = 2,6-dimethylphenyl; 2,6-diethylphenyl; 2,6-diisopropylphenyl; 2,4,6-trimethylphenyl; 2,6-diethyl-4-methylphenyl). Full characterization of all five complexes is supported by representative X-ray structures of Co1 and Co2 that display distorted square pyramidal geometries. All cobalt complexes exhibited high activity, under activation with either methylaluminoxane (MAO) or modified MAO (MMAO), with levels reaching up to 14.74 × 10 ⁶ (g of PE)·(mol of Co) ^–1 ·h ^–1 at 60 °C producing polyethylene (PE) wax ( M _w range: 10.3–34.3 kg·mol ^–1 ) with high linearity and narrow dispersity ( M _w / M _n range: 2.1–2.5), highlighting their good control. With both aluminum activators, the most sterically demanding cobalt catalyst, Co3 , afforded the highest molecular weight polyethylene but the lowest catalytic activity.

Binuclear α-diimine nickel catalysts have attracted significant interest in ethylene polymerization due to their distinctive metal–metal cooperativity, which enables catalytic properties not achievable with mononuclear systems. This review summarizes the structural design principles and recent advances of these binuclear catalysts, with an emphasis on how different bridging architectures such as rigid aryl, flexible chain, N-heterocyclic, and specialty bridges regulate intermetallic distance, steric environment, and electronic effects. The impact of these structural factors on catalytic activity, polymer molecular weight, branching density, and mechanical properties is discussed. By establishing structure–performance relationships, this review provides a foundation for the rational design of binuclear nickel catalysts for advanced polyethylene synthesis.

Polyolefin elastomers (POEs) produced by ethylene/α-olefin copolymerization are indispensable for high-end applications such as photovoltaic encapsulation, precision microelectronic protection, advanced foamed footwear, and automotive lightweight components. α-Diimine nickel catalysts that operate through a chain-walking mechanism convert ethylene as the sole feedstock into highly branched polyethylene elastomers, eliminating the need for expensive comonomers. This review systematically analyzes α-diimine nickel complexes developed in recent years via modulation of ligand steric hindrance, electronic effects, and backbone rigidity, and provides a quantitative comparison of their catalytic performance. The compiled data reveal that the catalytic activities span 10⁴–10⁷ g PE (mol Ni)⁻¹ h⁻¹, the molecular weights range from 10⁴ to 10⁶ g mol⁻¹, and the polydispersity indices (PDIs) can be tuned between 1.2 and 29.3, affording polyethylenes with branching densities from 2 to over 186 branches per 1000 carbons. These structural parameters directly govern the mechanical flexibility, elastic recovery, thermal properties, and processability in injection molding, foam extrusion, and film blowing, thus dictating the materials’ suitability for the aforementioned high-value applications. By establishing clear structure–performance relationships, this review offers forward-looking guidance for the industrial scale-up and catalyst design of polyethylene elastomers produced exclusively from ethylene.