Ethylene/α-olefin homo- and copolymerization using a dinuclear catalyst of nickel

Considering the essence of high electron density of ligands, frameworks with high electron donating groups are important in the transition-metal catalytic systems. As benzhydrol groups have been investigated for their beneficial electronic effects, a novel homo-dinuclear Ni (II) catalyst based on benzhydrol-substituted ligand was synthesized, characterized, and used for ethylene polymerization. The catalyst was further examined and compared with its mononuclear analogue. The maximum activity for the polymerization was then obtained at molar ratio of Al/Ni: 1500/1, polymerization temperature of 15 ℃ and the monomer pressure of 1.5 bar within 5 min of the polymerization which was 2.64 × 10⁶ g mol⁻¹Ni h⁻¹. Effects of cocatalyst type (e. g., modified methylalumoxane (MMAO), triisobutylaluminum (TiBA), and triethylaluminum (TEA)) in the polymerization were also investigated wherein MMAO outperformed in terms of the greatest activity. As the polymerization temperature elevated, the polyethylene (PE) microstructure changed from crystalline into amorphous form, while both of the activity of the catalyst and the Viscosity Average Molecular Weight (\(\overline{M }\) _v) of the obtained polymer were diminished. With regard to increasing of ethylene pressure in the reactor (up to 4 bars), the \(\overline{M }\) _v increased and reached to the maximum value of 1.44 × 10⁶ g mol⁻¹. The catalyst was also active in the presence of the long-chain α-olefin monomers, such as 1-hexene and 1-octene, the comonomers. The co-monomer addition decreased crystallinity of the polymer (from 25 to 15%) and led to higher branching density of the obtained copolymers.