Chromium complexes supported by NNO-tridentate ligands: an unprecedented activity with the requirement of a small amount of MAO

Abstract

The development of metal catalysts with high activity and thermal stability but with the requirement of a small amount of MAO as a cocatalyst is highly desired for polyolefin industrial application. In this contribution, a series of phenoxy-imine-amine compounds were prepared and used as NNO-tridentate ligands (LH1–LH6: LH = 2,6-(R¹)₂–C₆H₃–NH–C₆H₄–NC-3,5–(R²)₂–C₆H₂–OH; LH1: R¹ = ⁱPr, R² = ^tBu; LH2: R¹ = Me, R² = ^tBu; LH3: R¹ = H, R² = ^tBu; LH4: R¹ = F, R² = ^tBu; LH5: R¹ = Me, R² = H; LH6: R¹ = Me, R² = Cl) to support chromium complexes CrCl₂L(THF) (Cr1(THF)–Cr6(THF)). Upon activation with only 200 equivalents of Al(MAO), these Cr complexes exhibited extremely high activity toward ethylene polymerization and an unprecedented activity as high as 1.18 × 10⁸ g(PE) mol⁻¹(Cr) h⁻¹ was obtained using Cr2(THF)/MAO. Moreover, these Cr catalysts exhibited high thermal stability, with respect to both the activity and the molecular weight. At 100 °C, Cr2(THF)/MAO showed a remarkable activity of 1.01 × 10⁸ g(PE) mol⁻¹(Cr) h⁻¹ and produced polyethylene with a high molecular weight of 24.6 × 10⁴ g mol⁻¹. Therefore, the newly developed NNO-tridentate Cr complexes exhibiting unprecedented activity and excellent thermal stability but only requiring a small amount of MAO (as low as Al/Cr = 100) as a cocatalyst are very promising in the polyolefin industry.