Hafnium Complexes Supported by <i>C</i>,<i>N</i>,<i>N</i>-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–CAryl 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 (Tms reaching up to 130 °C) broaden the potential applications of polyolefin elastomers.