Moderately branched ultra‐high molecular weight polyethylene by using N,N′‐nickel catalysts adorned with sterically hindered dibenzocycloheptyl groups

Abstract

Five examples of unsymmetrical 1,2‐bis (arylimino) acenaphthene (L1 – L5), each containing one N‐2,4‐bis (dibenzocycloheptyl)‐6‐methylphenyl group and one sterically and electronically variable N‐aryl group, have been used to prepare the N,N′‐nickel (II) halide complexes, [1‐[2,4‐{(C15H13}2–6‐MeC6H2N]‐2‐(ArN)C2C10H6]NiX2 (X = Br: Ar = 2,6‐Me2C6H3 Ni1, 2,6‐Et2C6H3 Ni2, 2,6‐i‐Pr2C6H3 Ni3, 2,4,6‐Me3C6H2 Ni4, 2,6‐Et2–4‐MeC6H2 Ni5) and (X = Cl: Ar = 2,6‐Me2C6H3 Ni6, 2,6‐Et2C6H3 Ni7, 2,6‐i‐Pr2C6H3 Ni8, 2,4,6‐Me3C6H2 Ni9, 2,6‐Et2–4‐MeC6H2 Ni10), in high yield. The molecular structures Ni3 and Ni7 highlight the extensive steric protection imparted by the ortho‐dibenzocycloheptyl group and the distorted tetrahedral geometry conferred to the nickel center. On activation with either Et2AlCl or MAO, Ni1 – Ni10 exhibited very high activities for ethylene polymerization with the least bulky Ni1 the most active (up to 1.06 × 107 g PE mol−1(Ni) h−1 with MAO). Notably, these sterically bulky catalysts have a propensity towards generating very high molecular weight polyethylene with moderate levels of branching and narrow dispersities with the most hindered Ni3 and Ni8 affording ultra‐high molecular weight material (up to 1.5 × 106 g mol−1). Indeed, both the activity and molecular weights of the resulting polyethylene are among the highest to be reported for this class of unsymmetrical 1,2‐bis (imino)acenaphthene‐nickel catalyst.