Synthesis of dinuclear pyridyl-amido hafnium and zirconium complexes toward olefin polymerization with unexpected stereospecificity and ultra-high molecular weight capability

Abstract

A bifunctional pyridyl-imine ligand (syn-L) bridged by the rigid anthracene has been developed to support dinuclear Hf and Zr metal complexes (syn-M₂; M = Hf or Zr). syn-Hf₂ only exists as the racemic diastereomer with a C₂-symmetry, as revealed by both NMR spectroscopy and single-crystal X-ray diffraction, probably because of steric hindrance. The solid-state structure of syn-Hf₂ displays a separation of 6.333 Å between two Hf centers. The mononuclear complex N-anthracenyl-1-(6-naphthyl-2-pyridinyl)HfMe₂ (mono-Hf₁) has also been prepared for control experiments. In the presence of [Ph₃C][B(C₆F₅)₄] as the co-catalyst, dinuclear syn-Hf₂ exhibits significant cooperativity in ethylene homopolymerization and ethylene/1-octene copolymerization as compared to mononuclear mono-Hf₁. In ethylene homopolymerization, syn-Hf₂ shows a high activity of 8025 kg(PE) mol(Hf)⁻¹ h⁻¹ and produces polyethylene with a M_w of 50.5 × 10⁴ g mol⁻¹, which are 3.5× and 3.3× higher than those of mononuclear mono-Hf₁. For ethylene/1-octene copolymerization, syn-Hf₂ displays an exceptional activity of 24 450 kg(polymer) mol(Hf)⁻¹ h⁻¹, nearly 3.0× higher than that of ethylene homopolymerization, revealing a significantly positive comonomer effect. Moreover, the resulting poly(ethylene-co-octene) copolymer from syn-Hf₂ exhibits a M_w of 70.6 × 10⁴ g mol⁻¹ and a 1-octene incorporation of 7.9 mol%, which are 2.5× and 1.4× higher than those from mono-Hf₁ under otherwise identical conditions. In propylene polymerization, syn-Hf₂ exerts an unexpected stereospecificity and affords ultra-high molecular weight atactic polypropylene (UHMWaPP; M_w up to 179 × 10⁴ g mol⁻¹, which is almost 10× higher than that from mono-Hf₁). The UHMWaPP exhibits a stress-at-break value of 3.4 MPa and a strain-at-break value of 1055%, which are 2.6× and 1.6× higher than those from mono-Hf₁. The elastic recovery properties of UHMWaPP are measured through ten repetitive stress–strain cycles at 200% strain, showing a significantly higher strain recovery (SR = 87.6%) than that obtained by mono-Hf₁ (SR = 78.2%).