Recent Advances in α-Diimine Nickel Complexes for the Synthesis of Polyethylene Elastomers via Chain Walking

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.