Remote dibenzocycloheptyl substitution on a bis(arylimino)pyridyl-iron ethylene polymerization catalyst; enhanced thermal stability and unexpected effects on polymer properties

Abstract

Four examples of para-dibenzocycloheptyl-substituted 2,6-bis(arylimino)pyridyl-iron(II) chloride complexes, [2,6-{(2-R¹,4-(C₁₅H₁₃),6-R²C₆H₂)NCMe}₂C₅H₃N]FeCl₂ (R¹ = R² = Me Fe1, Et Fe2, iPr Fe3, R¹ = Me, R² = Et Fe4), have been synthesized and characterized by a combination of IR spectroscopy, elemental analysis and single crystal X-ray diffraction. The molecular structures of Fe2 and Fe4 highlight not only their pseudo-square pyramidal geometries but also the variation in configuration of the two remote dibenzo-fused cycloheptyl rings. When activated with MAO or MMAO, Fe1–Fe4 displayed very high activities for ethylene polymerization by operating effectively at 70 °C/10 atm [up to 7.14 × 10⁷ g(PE) mol⁻¹(Fe) h⁻¹] producing high molecular weight linear polyethylene (as high as 98.7 kg mol⁻¹) with a single melting temperature (T_m: >129.7 °C) as well as various levels of vinyl unsaturation. Even at temperatures up to 100 °C, the polymerization activity remained high [up to 1.46 × 10⁷ g(PE) mol⁻¹(Fe) h⁻¹] underlining the good thermal stability of this catalyst class. By contrast at P_C2H4 = 1 atm, all polyethylenes produced using Fe1–Fe4 at run temperatures between 20–60 °C exhibited two melting point values with either MAO (T_m1 93.5–116.2 °C, T_m2 124.9–130.0 °C) or MMAO (T_m1 65.1–85.9 °C, T_m2 116.8–122.8 °C) in line with the bimodality of the lower molecular weight polymers. In particular, when MMAO was employed as a co-catalyst, the polyethylene displayed a wax-like composition and displayed a very narrow dispersity (M_w/M_n range: 1.1–1.6).