The Brookhart-type α-diimine nickel and palladium catalysts are one of the most examined systems in the field of olefin polymerization. Even after more than twenty years of extensive research, they continue to attract a lot of attention. In addition to their ease of synthesis and structural versatility, the chain walking capability and great polar group tolerance represent some of their most fascinating features. Followed by some brief descriptions of the chain walking mechanism, this review summarizes some recent advances in α-diimine ligand modifications. Specifically, it is categorized into three aspects: modifications of N-aryl substituents, modifications of ligand backbone structures, and the utilization of dinucleating ligands. Influences of steric and electronic effects on catalytic properties are demonstrated. Some recent developments in copolymerizations of olefins with polar comonomers to produce functionalized polyolefin materials are also discussed.

A series of α-diimine Ni(II) complexes containing bulky phenyl groups, [ArN = C(Naphth)C = NAr]NiBr₂ (Naphth: 1,8-naphthdiyl, Ar = 2,6-Me₂-4-PhC₆H₂ (C1); Ar = 2,4-Me₂-6-PhC₆H₂ (C2); Ar = 2-Me-4,6-Ph₂C₆H₂ (C3); Ar = 4-Me-2,6-Ph₂C₆H₂ (C4); Ar = 4-Me-2-PhC₆H₃ (C5); Ar = 2,4,6-Ph₃C₆H₂ (C6)), were synthesized and characterized. Upon activation with either diethylaluminum chloride (Et₂AlCl) or modified methylaluminoxane (MMAO), all Ni(II) complexes showed high activities in ethylene polymerization and produced highly branched amorphous polyethylene (up to 145 branches/1000 carbons). Interestingly, the sec-butyl branches were observed in polyethylene depending on polymerization temperature. Polymerization of 1-alkene (1-hexene, 1-octene, 1-decene and 1-hexadecene) with C1-MMAO at room temperature resulted in branched polyolefins with narrow M_w/M_n values (ca. 1.2), which suggested a living polymerization. The polymerization results indicated the possibility of precise microstructure control, depending on the polymerization temperature and types of monomers.

Naphthyl-a-diimine nickel complexes with systematically varied ligand sterics, activated by modified methylaluminoxane (MMAO), were tested in the polymerization of higher a-olefin (1-hexene, 1-decene and 1-hexadecene) under suitable conditions. The polymerization results indicated the possibility of precise microstructure control, depending on catalyst structure, polymerization temperature, monomer concentration and types of monomers, which in turn strongly affects the resultant polymer properties. Naphthyl-a-diimine nickel complex bearing chiral bulky sec-phenethyl groups in the o-naphthyl position showed good catalytic activity, and resulted in branched polymers (42-88/1000C) with high molecular weights (M_n: (4.3-15.2) × 10⁴ g·mol^-1) and narrow molecular weight distribution (M_w/M_n = 1.13-1.29, RT), which suggested a living polymerization. The increasing steric hindrance of catalyst leads to enhance insertion for 2,1-insertion of a-olefin and the chain-walking reaction.