Influence of Backbone Substituents on the Ethylene (Co)polymerization Properties of α‑diimine Pd(II) and Ni(II) Catalysts

A series of α-diimine ligands with different substituents on the acenaphthyl backbone were synthesized and characterized. The corresponding Ni­(II) and Pd­(II) complexes were prepared and used in ethylene polymerization and copolymerization with methyl acrylate. In ethylene polymerization, these Ni­(II) complexes showed activities of up to 1.6 × 10⁷ g/((mol of Ni) h), generating polyethylene with a molecular weight (M_n) of up to 4.2 × 10⁵. Interestingly, these Ni­(II) complexes behave very similarly in ethylene polymerization except for the complex with two methoxy substituents on the ortho position of the acenaphthyl backbone, in which case about 3 times higher polyethylene molecular weight and much lower branching density were observed. The ligand substituent effect is much more dramatic for the Pd­(II) complexes. In ethylene polymerization, activities of up to 1.7 × 10⁵ g/((mol of Pd) h) and a polyethylene molecular weight (M_n) of up to 4.7 × 10⁴ could be obtained. The Pd­(II) complex with two methoxy substituents on the ortho position of the acenaphthyl backbone demonstrated much higher activity and generated polyethylene with about 3 times higher molecular weight than that for the classic Pd­(II) complex. A similar trend was maintained in ethylene–methyl acrylate copolymerization.