Dinuclear nickel (II) complexes bearing two pyrazolylimine ligands: Synthesis characterization, and catalytic properties for vinyl-type polymerization of norbornene

A series of dinickel (II) complexes of bis-2-(C₃HN₂(R₁)₂-3,5)(C(R₂)N(C₆H₃(CH₃)₂-2,6)Ni₂Br₄ (complex 1: R₁ = CH₃, R₂ = Ph; complex 2: R₁ = CH₃, R₂ = 2,4,6-trimethylphenyl; complex 3: R₁ = R₂ = Ph; complex 4: R₁ = Ph, R₂  = 2,4,6-trimethylphenyl) were synthesized and characterized. The solid-state structures of complexes 1, 2 and 3 have been confirmed by X-ray single-crystal analyses to be in the form of a dinuclear and bromine-bridged structure. However, there is an equilibrium that shifts between the monomer and dimmer in toluene based on the characterization of UV-vis spectrophotometry. Activated by methylaluminoxane (MAO), these complexes are capable of catalyzing the polymerization of norbornene with moderate activity up to 6.64 × 10⁵ gPNBE/(molNi·h). The influences of polymerization parameters such as reaction temperature and Al/Ni molar ratio on catalytic activity and molecular weight of the polynorbornene were investigated in detail. The influence of the bulkiness of the substituents on polymerization activity was also studied. The obtained polynorbornenes were characterized by means of ¹H NMR, FTIR and TG techniques. The analyses results of polymers’ structures indicated that the norbornene polymerization is vinyl-type polymerization rather than ROMP.