Homo- and copolymerization of norbornene with tridentate nickel complexes bearing o-aryloxide-N-heterocyclic carbene ligands

Abstract

A series of new pincer-type tridentate o-aryloxide-N-heterocyclic carbene nickel complexes Ni1–Ni3 were synthesized, and the molecular structure of Ni3 was confirmed by X-ray crystallography. In comparison with the above tridentate complexes, the bidentate bis(aryloxide-NHC) nickel complex Ni4 was also synthesized. On activation with either Et₂AlCl or Me₂AlCl, all nickel complexes showed low activity toward norbornene (NB) homopolymerization. Surprisingly, Ni1–Ni3 catalysts exhibited moderate to high activity toward the homopolymerization (up to 3.21 × 10⁶ g PNB (mol of Ni)⁻¹ h⁻¹) and copolymerization of norbornene with 1-octene (up to 1.39 × 10⁵ g polymer (mol of Ni)⁻¹ h⁻¹) with B(C₆F₅)₃ as the cocatalyst even at 100 °C, showing that tridentate nickel catalysts have high thermal stability. Notably, the tridentate Ni1–Ni3 complexes showed better activities than the corresponding bidentate bis(aryloxide-NHC) nickel complex Ni4. The obtained poly(NB-co-1-octene)s were confirmed to be vinyl-addition copolymers, which exhibited high 1-octene insertion ratio (9%–17%), good thermal stability (T_d > 400 °C), high molecular weight (up to 10⁵), and narrow molecular weight distribution (PDI ≤ 1.91).