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

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.