Enhancing Ethylene Polymerization of NNN-Cobalt(II) Precatalysts Adorned with a Fluoro-substituent

Abstract

Unsymmetrical 2-(1-(2,4-dibenzhydryl-6-fluorophenylimino)ethyl)-6-(1-alkylphenyl-imino)ethyl)pyridine compounds (Ar = 2,6-Me ₂C ₆H ₃ in L1; 2,6-Et ₂C ₆H ₃ in L2; 2,6- ⁱ Pr ₂C ₆H ₃ in L3; 2,4,6-Me ₃C ₆H ₂ in L4; 2,6-Et ₂-4-Me-C ₆H ₂ in L5) were prepared and characterized. The treatment of CoCl ₂ with the compounds L1– L5 afforded the corresponding cobalt complexes Co1– Co5 in excellent yields. The molecular structures of Co3 and Co4 were determined by single-crystal X-ray diffraction, revealing the distorted-square-pyramidal geometry with three nitrogen atoms and two chlorine atoms around the cobalt center. Compared with previous bis(imino)pyridylcobalt analogues, all of the cobalt precatalysts displayed exceptionally higher activities toward ethylene polymerization with 1.32 × 10 ⁷ g (PE) mol ^–1 (Co) h ^–1 at 60 °C in the presence of a co-catalyst MAO or MMAO. These cobalt catalysts produced highly linear polyethylene (PE) waxes with vinyl end groups and low molecular weight ( M _w up to 8.23 kg mol ^–1) along with a relatively lower melting point (all-round T _ms < 128 °C). The narrow dispersity of resultant polyethylenes indicated the single-site active species of the catalytic system.