Nickel(II) Complexes with Three-Dimensional Geometry α-Diimine Ligands: Synthesis and Catalytic Activity toward Copolymerization of Norbornene

A series of three-dimensional geometry 9,10-dihydro-9,10-ethanoanthracene-11,12-diimines (L1–L4) and their nickel(II) dibromide complexes (C1–C4) were synthesized and characterized. The nickel complexes C1–C4, with three-dimensional geometry, exhibited very high activities for norbornene (NB) homopolymerization with only B(C 6 F 5 ) 3 as cocatalyst,: for C2 even up to 5.53 × 10 7 g of polymer/((mol of Ni) h). To investigate the activation of polar monomer, complexes C2 and C3 were selected for copolymerization of NB and 5-norbornene-2-yl acetate (NB-OCOMe) in relatively high activities (1.6–5.8 × 10 5 g of polymer/((mol of Ni) h)) and high molecular weights ((0.2–2.8) × 10 5 g/mol) as well as narrow molecular weight distributions (MWD < 2 for all polymers) depending on the variation of feed ratios. The reactivity ratios of the NB and NB-OCOMe monomers for C2/B(C 6 F 5 ) 3 system by the Kelen–Tüdös method were determined to be r NB-OCOMe = 0.05 and r NB = 6.72, respectively. Moreover, the mechanism of polymerization catalyzed by the novel three-dimensional geometry nickel(II) complexes was presented and supported by an end group analysis of the polymer and density functional theory (DFT) calculations of the reaction. The substituent effect of the catalysts and the interaction between Ni 2+ and NB were discussed, and the results showed that α-diimine nickel complexes with greater steric hindrance and smaller HOMO–LUMO gaps could achieve higher reactivity.