Synthesis of bis-(benzocyclohexan-ketoimino) Ni(ii) with different electron groups and their catalytic copolymerization of norbornene and polar norbornene

Abstract

Four N,O-chelating type bidentate ligand nickel(II) complexes, Ni{C₁₀H₈(O)C[ArN]CH₃}₂, two of them containing electron-donating groups (Ni1: Ar = p-PhCH₃, Ni2: Ar = p-PhOCH₃) and another two containing electron-withdrawing groups (Ni3: Ar = p-PhF, Ni4: Ar = p-PhCF₃), were synthesized and the molecular structures of Ni2, Ni3 and Ni4 were determined by X-ray crystallography. Copolymerization of norbornene (NB) and 5-norbornene-2-carboxylic acid methyl ester (NB-COOCH₃) were carried out in toluene with the above complexes as catalyst precursors and B(C₆F₅)₃ as the co-catalyst. All complexes showed high catalytic activity toward the copolymerization (up to 2.80 × 10⁵ g_polymer mol_Ni⁻¹ h⁻¹) and the complexes with electron-withdrawing groups (Ni3 and Ni4) showed higher catalytic activity, and the activity was enhanced with the increased electron-withdrawing ability of the substituent (Ni4 > Ni3). The obtained poly(NB-co-NB-COOCH₃)s were confirmed to be vinyl-addition copolymers and non-crystalline. All copolymers exhibited better solubility in common organic solvents, and showed high molecular weights (up to the grade of 10⁵ g mol⁻¹) and good thermal stability (T_d > 370 °C). Meanwhile, variation of the substituted electron group could slightly affect the above-mentioned copolymers' properties.