Activity and Thermal Stability of Cobalt(II)-Based Olefin Polymerization Catalysts Adorned with Sterically Hindered Dibenzocycloheptyl Groups

Abstract

Five examples of unsymmetrical 2-(2,4-bis(dibenzocycloheptyl)-6-methylphenyl- imino)ethyl)-6-(1-(arylyimino)ethyl)pyridine derivatives (aryl = 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 and 2,6-Et ₂-4-MeC ₆H ₂ in L5) were prepared and characterized. Treatment with CoCl ₂ offered the corresponding cobalt precatalysts Co1– Co5, which were characterized by FT-IR and NMR spectroscopy as well as elemental analysis. The molecular structures of Co3 and Co4 determined by single crystal X-ray diffraction revealed distorted square pyramidal geometries with τ ₅ values of 0.052–0.215. Activated with either MAO or MMAO, the precatalysts displayed high activities in ethylene polymerization, where Co1 with the least bulky substituents exhibited a peak activity of 1.00 × 10 ⁷ g PE mol ⁻¹ (Co) h ⁻¹ at 60 °C. With MAO as a cocatalyst, the activity was reduced only by one order of magnitude at 90 °C, which implies thermally stable active sites. The polymerization product was highly linear polyethylene with vinyl end groups. Co3 with the most sterically hindered active sites was capable of generating polyethylene of high molecular weight, reaching 6.46 × 10 ⁵ g mol ⁻¹. Furthermore, high melting point and unimodal molecular weight distribution were observed in the resulting polyethylene. It must be stressed that the thermal stability of the catalyst and the molecular weight of the obtained polyethylene attain the highest values reported for the unsymmetrical 2,6-bis(imino)pyridylcobalt (II) chloride precatalysts.