Regulating the polyethylene microstructure by increasing steric crowding in naphthoxy imine-ligated Ni(ii) complexes

Abstract

Ligands play a prominent role in ethylene polymerization. However, it is a highly challenging task to regulate branching through ligand modifications. Here we report the synthesis of systematically sterically tailored naphthoxy imine-ligated nickel complexes (Ni1, Ni2, and Ni3), their performance in ethylene polymerization, and how the ligand steric controls branching in the resultant PE. Ni1–Ni3 were prepared in one step with an excellent yield (73–93%). The identity of these complexes was unambiguously ascertained using ¹H, ¹³C, 2D NMR spectroscopy, mass analysis, and single-crystal X-ray diffraction. The molecular structure revealed a cis arrangement of alkyl/aryl and donor groups (C–Ni–D), which is necessary for initiating ethylene polymerization. Buried volume contours suggested Ni3 to be sterically the most bulky among the three. When exposed to ethylene, the three nickel complexes Ni1, Ni2, and Ni3 produced polyethylene with excellent activity. As predicted by buried volume calculations, dibenzhydryl-substituted Ni3 outperformed sterically less crowded Ni1 and Ni2. Careful analysis of the resultant PE disclosed that sterically less encumbered Ni1 and Ni2 produce PE with high branching (43–54 branches/1000-C atoms) density. However, the bulkiest Ni3 revealed much lower branching (28 branches/1000-C atoms) and a high TOF of 35 400 mol of PE per mol of Ni per h, along with a high molecular weight of PE (61 000 Da). The steric bulk in Ni3, most likely, reduces chain-walking and thus lowers branching in the resultant PE. As compared to the literature-reported analogous Pd1 catalyst, the Ni3 catalyst discloses high TOF, high molecular weight, and less branched, linear polyethylene.