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An α-diimine nickel catalyst (Cat-1) with highly conjugated acenaphthylene backbone was synthesized and used in ethylene polymerization. Compared with typical Brookhart catalyst B-Cat, Cat-1 could produce polyethylene with both higher molecular weight (up to 5.1 × 10 5 g mol −1 ) and branching density (up to 135 branches per 1000 carbons) in good polymerization activity. The polyethylene prepared using Cat-1 with higher molecular weight and branching density possessed exceptional mechanical properties (ultimate tensile strength and elongation at break could reach 12.08 MPa and 1148%) and outstanding elastomeric recovery as compared with the polymer prepared using B-Cat. Cat-1 was synthesized using a simple reaction route and the polymerization conditions were suitable for the industrial polymerization process. The novel nickel catalyst Cat-1 is promising for producing highly resilient polyethylene elastomers industrially.KEYWORDS α-diimine nickel catalyst, elastomeric recoverymechanical propertiespolyethylene elastomer

INTRODUCTIONIn recent years, there has been an explosion of research committed to the olefin block copolymers with diverse microstructures. Melting temperatures, glass transition temperatures, and mechanical properties of materials can be altered, when alkyl branches were introduced into carbon hydrogen backbone. 1 This was attributed to the reduction of main chain crystallinity by the alkyl branches. Brookhart et al. demonstrated that some late transition metal catalysts bearing bulky a-diimine ligand exhibited high activities in ethylene polymerization and copolymerization with comonomers, resulting in formation of different branches in the polymer chain. 2-4 Later, more palladium and nickel catalysts were developed and applied to a variety of applications areas. 5-7 These days, some researchers concentrated on new kinds of Ni(II) and Pd(II) complexes, such as dinuclear catalysts, and made astounding advances, 8,9 suggesting a tremendous potential in polymerizing materials with diverse microstructures. With the advantage of the unique chain walking feature of Brookhart-type catalysts, block polymer bearing both plasticity and elasticity can be designed, as the main point is to introduce crystalline or semicrystalline blocks into the amorphous chain segments. Xiao et al. reported a new multiblock polyethylene via chain shuttling polymerization of ethylene between a Brookhart-type catalyst and a bridged zirconocene, where the bridged zirconocene produced linear polyethylene segments. 10 Chen et al. addressed a series of a-diimine Ni(II) and Pd(II) catalyst with different substituents in ethylene (co)polymerization to control the polymer topology. 11-15 Some other researchers concentrated on utilizing the highly x,1-enchainment behavior (chain walking of

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