Effect of Cocatalysts on the Catalytic Activities of Tantalum‐ and Niobium‐Based Catalysts for Ring‐Opening Metathesis Polymerization of Norbornene

A series of pentavalent tantalum and niobium complexes with aryloxy ligands was prepared, and their catalytic behavior for the ROMP of norbornene was studied in the presence of an alkylaluminum cocatalyst. Tantalum complexes 1 – 4 showed very high activity for the ROMP of NBE in combination with ⁱBu₃Al to give high‐molecular‐weight polymers. In contrast, the niobium complexes 5 and 6 , as well as NbCl₅, exhibited very high activity upon activation with Me₃Al to give high‐molecular‐weight polymers.