Ring‐opening metathesis polymerization of norbornene catalyzed by tantalum and niobium complexes with chelating O‐donor ligands

BACKGROUND: In comparison with group 6 transition metals, such as tungsten and molybdenum, and group 8 metal ruthenium, group 5 metal‐based catalysts for ring‐opening metathesis polymerization (ROMP) have remained much less studied. The few reported ROMP catalysts of group 5 metals require multiple reaction steps to be synthesized, and are highly sensitive to air and moisture. RESULTS: A series of pentavalent tantalum and niobium complexes having catecholato, tropolonato, hinokitiolato, biphenolato and binaphtholato ligands were prepared and their catalytic activities for the ROMP of norbornene (NBE) were studied in the presence of trialkylaluminium as a co‐catalyst. Among these complexes, the tantalum complexes showed high activity upon activation with Buⁱ₃Al. In sharp contrast, the niobium complexes were effectively activated with Me₃Al. The polymers obtained with these complexes had high molecular weights (M_n > 10⁵ g mol⁻¹) and relatively narrow molecular weight distributions (M_w/M_n ≈ 2). CONCLUSION: We found that easily accessible and relatively stable tantalum and niobium complexes with such chelating O‐donor ligands showed high catalytic activity for ROMP of NBE depending on the kind of co‐catalyst. These findings could contribute to future development of ROMP catalysts. Copyright © 2008 Society of Chemical Industry