Highly linear polyethylenes tailored with 2,6-bis[1-(p-dibenzo-cycloheptylarylimino)ethyl]pyridylcobalt dichlorides

A novel family of 2,6-bis[1-(p-dibenzocycloheptylarylimino)ethyl]pyridylcobalt dichlorides Co1-Co4 were synthesized and fully characterized by FT-IR, 1H NMR, and elemental analysis as well as X-ray diffraction analysis. The 1H NMR spectra of these paramagnetic cobalt complexes Co1-Co3 showed similar spectra with one set of signals, while Co4 possessed two sets of signals due to the different conformations caused by different substituents on the ortho-position of the N-aryl group. Crystal structures of Co1, Co2 and Co4 revealed similar square-pyrimidal geometry around the cobalt ion, in which two dibenzocycloheptyl groups adapted as cis-conformers, whereas Co3 possessed bistrigonal geometry and the two dibenzocycloheptyl groups were adapted as trans-conformers. On activation with methylaluminoxane (MAO) or modified MAO (MMAO), all these cobalt complexes displayed high catalytic activities towards ethylene polymerization (up to 1.21 × 107 g (PE) mol-1 (Co) h-1) and produced polyethylenes with narrow molecular distributions (PDI ≈ 2.0). Co3 bearing bulkier isopropyl substituents on the ortho-position gave much higher molecular weight of resultant polyethylene than the other cobalt complexes reported herein. The microstructure analysis of the PEs demonstrated that they are highly linear and contain a vinyl end group as the major group and saturated methyl as the minor group, in which the content of the unsaturated polymer chain relied on the ortho-substituent of the N-aryl group.