An Unsymmetrical Iron(II) Bis(imino)pyridyl Catalyst for Ethylene Polymerization: Effect of a Bulky Ortho Substituent on the Thermostability and Molecular Weight of Polyethylene

Three new iron(II) 2,6-bis(imino)pyridyl complexes bearing bulky and unsymmetrical substituted aniline groups, [2-(CH(CH3)(C6H5))-4-R1-6-R2-C6H2NC(CH3)]2C5H3NFeCl2 (1, R1 = H, R2 = methyl; 2, R1 = methyl, R2 = methyl; 3, R1 = H, R2 = methoxy), were synthesized. The unsymmetrical bis(imino)pyridyl ligand L2 contains three isomers that can be detected by 1H NMR and 13C NMR spectroscopy. However, the corresponding complex 2 exhibits only one isomer in solution. X-ray diffraction of 2 confirms that the iron complex adopts a Cs configuration. These complexes activated by methylaluminoxane (MAO) have high catalytic activities for ethylene polymerization and produce linear polyethylenes with bimodal or broad molecular weight distribution. The steric and electronic effects of the ortho substituents on the aniline moiety distinctly affect the molecular weight of the obtained polyethylene. In comparison with 1 and 3, having a single ortho substituent on the aryl rings, and the typical complex 4, containing o-diisopropyl substitution, complex 2 has better thermal stability and produces much higher molecular weight polyethylene. Even at 70 °C, the 2/MAO system still keeps high activity and relatively stable kinetics.