Controlled Isospecific Polymerization of α‑Olefins by Hafnium Complex Incorporating with a trans-Cyclooctanediyl-Bridged [OSSO]-Type Bis(phenolate) Ligand

Treatment of trans-cyclooctanediyl-bridged [OSSO]-type ligand 1 with Hf­(CH2Ph)4 in toluene afforded the corresponding hafnium­(IV) dibenzyl complex 5 as pale yellow crystals. X-ray crystallographic analysis of 5 revealed that the six-coordinated hafnium center incorporated in the [OSSO]-type ligand 1 adopted cis-α configuration, and two benzyl ligands were coordinated to hafnium center by η1-fashion. In the polymerization of 1-hexene, the combination of hafnium complex 5 and B­(C6F5)3, (Ph3C)­[B­(C6F5)4], or dMAO (dried methylaluminoxane) as an activator provided poly­(1-hexene)­s with perfect isotacticity ([mmmm] > 95%) and high molecular weights (Mw = 74 800–421 000 g mol–1). The combination of 5/dMAO catalyst system, in particular, exhibited fairly high activity of 2100 g mmol (5)−1 h–1, which is quite higher than those of previously reported [OSSO]-type hafnium complexes. In addition, the polymerization of 4-methyl-1-pentene (4-MP) catalyzed by the 5/(Ph3C)­[B­(C6F5)4] system at room temperature produced completely isotactic poly­(4-MP)­s with good activity (54–101 g mmol (5)−1 h–1) and relatively narrow PDI (Mw/Mn = 1.3). Furthermore, the propylene polymerization catalyzed by the 5/dMAO system at different temperatures (0–70 °C) for 1 h afforded highly isotactic polypropylenes, where the catalyst activity and isotacticity depend on the polymerization temperature. Thus, the activity greatly increased from 500 to 17 000 g mmol (5)−1 h–1 when the polymerization temperature was increased. In contrast, isotacticity and melting transition temperature (Tm) were elevated by decreasing polymerization temperature (at 0 °C: [mmmm] = 93.7%, Tm = 156.2 °C; at 70 °C: [mmmm] = 86.8%, Tm = 140.9 °C).