Finely-Tuned Bis(imino)pyridylcobalt Complexes Enhance Ethylene Polymerization: The Role of Bulky and Halogen Substituents

The bis(imino)pyridylcobalt complexes have been finely tuned through using the aniline derivative bearing a <i>meta</i>-chloro substituent, besides its <i>ortho</i>- and <i>para</i>-di(4-fluorophenyl)methyl and <i>ortho</i>-methyl substituents for the series of 2-[1-(3-chloro-4,6-bis((di(4-fluorophenyl)methyl)-2-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridylcobalt(II) chlorides (2,6-Me₂Ph, <b>Co1</b>; 2,6-Et₂Ph, <b>Co2</b>; 2,6-<i>i</i>Pr₂Ph, <b>Co3</b>; 2,4,6-Me₃Ph, <b>Co4</b>; and 2,6-Et₂-4-MePh, <b>Co5</b>). The compounds were characterized using elemental analysis, ¹H/¹³C NMR, FT-IR spectroscopy, and the single-crystal X-ray diffraction used in confirming the molecular structures of <b>Co1, Co2, Co4,</b> and <b>Co5</b>. The newly synthesized precatalysts, maintaining steric influences with the addition of an electron-withdrawing <i>meta</i>-chloro group, achieved higher activities along with better thermal stability, and controlled molecular weights of polyethylenes obtained. Upon activation with either MAO or MMAO, all catalysts exhibited remarkable activity for ethylene polymerization, for example, 9.2 × 10⁶ g mol^-1 h^-1 by <b>Co1</b> at 70 °C with 30 min and 18.0 × 10⁶ g mol^-1 h^-1 by <b>Co4</b> with the first 5 min. <b>Co4</b> demonstrated exceptionally thermal stability with the peak activity of 8.9 × 10⁶ g mol^-1 h^-1 at 70 °C and slightly decreased to 7.2 × 10⁶ g mol^-1 h^-1 at 80 °C, and even maintained an activity of 1.6 × 10⁶ g mol^-1 h^-1 at 100 °C. More importantly, all resultant polyethylenes were characterized as having vinyl-terminal and high-linear feature along with narrow dispersity; the molecular weights could be adapted in the ranges from 6.4 to 50.0 kg mol^-1. In comparison with previous cobalt analogs, the current system performed better thermal stability and polymerization efficiency. Therefore, such robust complex catalysts are potentially considered for the polyethylene industry.