A critical drawback of bis(imino)pyridylcobalt precatalysts is their poor activity with increasing ethylene polymerization temperature. The incorporation of trifluoro-substituents together with benzhydryl steric groups has simultaneously enhanced several parameters (catalytic activity, thermal stability, polymer molecular weights, melting points) of unsymmetrical bis(imino)pyridyl cobalt catalysts in ethylene polymerization. Upon activation with MAO or MMAO, these thermally stable cobalt precatalysts exhibited high activities on the level of 107 g mol− 1 h− 1 at 70 °C, producing linear polyethylene with high molecular weights exceeding 100 kg mol⁻¹ and moderate dispersities (Đ = 2.1–2.8). Notably, the polymerization remained active even at 100 °C with a high activity of 2.22 × 106 g mol− 1 h− 1. Synergistic optimization of ligand substituents and polymerization conditions enabled tuning of activities and polyethylene molecular weights: sterically less hindered precatalysts showed higher activities than their bulkier counterparts, whereas the opposite trend was observed for the molecular weights of the resulting polyethylene. High melting points (131.8–142.4 °C) with sharp endothermic peaks of resulting polyethylene confirmed the formation of strictly linear polyethylene, further evidenced in ¹H and ¹³C NMR spectra. However, the absence of unsaturated chain-end signals in the NMR spectra may suggest that chain-transfer reactions could predominantly occur through interactions with aluminum species under the applied conditions. This strategy of concerted steric and electronic effects is highly attractive for future studies in other catalytic systems.
Ren et al. (2026) studied this question.