Flexible Axial Shielding Strategy for the Synthesis of High-Molecular-Weight Polyethylene and Polar Functionalized Polyethylene with Pyridine-Imine Ni(II) and Pd(II) Complexes

Abstract

In this study, several Ni(II) and Pd(II) complexes bearing pyridine-imine ligands with bulky 8-alkyl-2-benzhydryl-1-naphthyl substituents were synthesized and their applications in ethylene (co)polymerization were evaluated. The as-prepared pyridine-imine Ni(II) complexes with 8-alkyl-1-naphthyl substituents (Me and n-Bu) were proven to be highly effective for generating highly branched and high-molecular-weight polyethylene (41.2–82.9 kg/mol) in the ethylene polymerization. Also, the corresponding Pd(II) complexes produced high-molecular-weight hyperbranched polyethylene (36.4–96.2 kg/mol). Polar functionalized E-MA copolymers with very high incorporation ratios (up to 21.7 mol %) could be produced as well by using these Pd(II) complexes. Notably, compared with Ni(II) and Pd(II) complexes with a naphthyl ring unsubstituted in the 8-position, the 8-alkyl-1-naphthyl counterparts generated polyethylene with 1–2 order of magnitude higher molecular weight. Moreover, the 8-alkyl-1-naphthyl Ni(II)/Pd(II) complexes exhibited substantially higher polymerization activities than the rigid 8-aryl-1-naphthyl counterparts and generated polyethylene with comparable molecular weights. In addition, the polyethylenes generated by the Pd(II) and Ni(II) complexes demonstrated very different polyethylene microstructures.