Hafnium Complexes Supported by C,N,N-Tridentate, Pincer-Type Ligands Containing a Six- or Seven-Membered Metallacycle for the Synthesis of High-Performance Olefin Block Copolymers

This study presents a series of hafnium (Hf1 and Hf2) and zirconium (Zr1 and Zr2) complexes, supported by C,N,N-tridentate, pincer-type ligands, that feature a six-membered metallacycle ortho-fused to a five-membered N,N-chelate. These complexes display remarkable catalytic properties in ethylene homopolymerization, with activities as high as 19,500 kg(PE)·mol^–1(cat)·h^–1, producing polyethylenes with impressive molecular weights of up to 1342 kg·mol^–1. Interestingly, these complexes exhibit an unexpected high selectivity for ethylene in ethylene/1-octene copolymerization, with incorporation levels lower than 0.3 mol %. For comparison, Hf4 with an expanded seven-membered metallacycle has also been synthesized and showed even lower activity. The activation of these metal complexes has been investigated using NMR and MS spectroscopy, revealing that for Hf2 with a six-membered metallacycle, the initial olefin insertion preferentially occurs into the Hf–C_Aryl bond. In contrast, the ligand in Hf4 is not modified in the presence of the monomer, indicating a different activation pathway associated with the seven-membered metallacyclic structure. Given its high activity and selectivity for ethylene during copolymerization and significant efficiency in chain transfer reactions, Hf2 emerges as a promising candidate for synthesizing hard segments in the production of olefin block copolymers (OBCs) in combination with the conventional C,N,N–Hf complex (py-Hf), whose high melting points (T_ms reaching up to 130 °C) broaden the potential applications of polyolefin elastomers.