Introduction of constrained cyclic skeleton into β-enaminoketonato vanadium complexes: A strategy for stabilization of active centre of vanadium catalyst for ethylene polymerization

Several novel mono(β-enaminoketonato) vanadium complexes bearing constrained cyclic skeleton, including [(C6H5)C6H3C(O)=C(CH2) n CH=N-Ar]VCl2(THF)2 (V3a: n = 1, Ar = C6H5; V4a: n = 2, Ar = C6H5; V4b: n = 2, Ar = C6F5; V4c: n = 2, Ar = (C3H7)2C6H3; V5a: n = 3, Ar = C6H5), were synthesized and their structure and properties were characterized. The structures of V4c and V5a in solid-state were further confirmed by X-ray crystallographic analysis. Density functional theory (DFT) results indicated that these complexes showed enhanced steric hindrance around the metal center as compared with the acyclic analogues. Upon activation with Et2AlCl and in the presence of ethyl trichloroacetate as a reactivator, all of the complexes exhibited high catalytic activities (107 gPE/(molV·h)) toward ethylene polymerization, and the obtained polymers exhibited unimodal distributions (M w/M n = 2.0–2.3) even produced at elevated temperatures (70–100 °C) and prolonged reaction time. When MAO was employed as a cocatalyst, they only showed moderate catalytic activities (105 gPE/(molV•h)), but the resulting polymers had higher molecular weights (168–241 kg/mol). These vanadium complexes with cyclic skeleton also showed high catalytic activities toward ethylene/norbornene copolymerization. The produced copolymers displayed approximate alternating structure at high in-feed concentration of norbornene. The catalytic capabilities of these complexes could be tuned conveniently by varying ligand structure. Furthermore, the cyclic voltammetry results also proved that these complexes exhibited better redox stabilities than the complexes bearing linear skeleton.