Bis(β-ketoimino)nickel(II) Complexes for Random Copolymerization of Norbornene and Methyl 5-Norbornene-2-carboxylate with Controlled Ester Group Incorporation

A series of bis(β-ketoimino)nickel(II) complexes with p-substituted N-phenyl groups, Ni[CH3C(O)CHC(NPhR)CH3]2(Ni1: R = −OCH3; Ni2: R = −CH3; Ni3: R = −CF3), were synthesized, and their general coordination geometry was elucidated by single-crystal X-ray diffraction analysis of Ni3. These complexes were paired with tris(pentafluorophenyl)borane (B(C6F5)3) to catalyze the vinyl addition copolymerization of norbornene (NB) and methyl 5-norbornene-2-carboxylate (NBE). All the catalyst systems exhibited high catalytic activities (>105gpolymermolNi–1h–1) at NBE feed contents of up to 50 mol %, resulting in the production of copolymers with high molecular weights (Mw= 135–355 kg mol–1, Đ= 1.78–2.12). In addition, the content of polar ester groups was precisely controlled by the feed ratio of the monomers. For Ni3, two monomer reactivity ratios were found to be close to unity (Fineman–Ross method: rNB= 0.951, rNBE= 0.903; Kelen–Tüdös method: rNB= 1.15, rNBE= 0.978). Since the copolymerization behaviors were revealed to be independent of the electronegativity of p-substituent, all the catalyst systems of Ni1–Ni3/B(C6F5)3were considered to serve the random copolymerization of NB and NBE. The resulting poly(norbornene-random-methyl 5-norbornene-2-carboxylate)s exhibited the dielectric and surface properties well tunable by compositional modulation.