Synthesis of bimodal polyethylene by unsymmetric hafnium and zirconium complexes bearing NN-Bidentate amino ligands

Abstract

Bimodal polyethylene is a class of high-performance polyolefin with excellent mechanical property and good processability. However, their synthesis usually requires two distinctive catalysts or two sequential processes that make it being a continuing challenge. In this contribution, a series of hafnium (Hf1–Hf3) and zirconium (Zr1) complexes bearing NN-bidentate amino ligands were developed to synthesize bimodal polyethylene using one catalyst in one reactor, owing to their unique unsymmetric structures. These Hf and Zr complexes were easily synthesized by one-pot reactions of amino ligands with in situ formed MMe₄ (M = Hf or Zr). All metal complexes were characterized by ¹H and ¹³C NMR spectroscopy, and the molecular structure of Hf1 was determined by single-crystal X-ray diffraction. These Hf and Zr complexes exhibited high activity toward ethylene homopolymerization (up to 15600 kg(polymer)·mol⁻¹(M)·h⁻¹) and ethylene/1-octene copolymerization (up to 4320 kg(polymer)·mol⁻¹(M)·h⁻¹). The resulting polymers showed tunable molecular weights with bimodal distributions for most cases. The polymerization results revealed the nature of the ligand strongly influenced the catalytic performance, and those with electron-donating substituents promoted polymerization. Furthermore, the activity of Hf1 was higher than that of Zr1 analogue with the same ligand, underscoring the metal center played a critical role in catalytic properties.