Niobium complexes as catalytic precursors for the polymerization of olefins

A review. Until now niobium compds. have been very rarely applied for the polymn. of olefins, notwithstanding their moderate cost, limited toxicity and the large availability of stable oxidn. states, which makes them versatile precursors. The data reported to date on ethylene and norbornene polymn. catalysts are discussed, with particular ref. to the evolution of the approach to the design of these systems, starting from isoelectronic, isolobal or isostructural relationships with Group 4 metallocene catalysts. The catalytic performances in ethylene and norbornene polymn. of the niobium precursors with cyclopentadienyl, diene, amido, amidinate, pyrazolylborate ligands have been compared. The most recent results have been discussed, comparing the reported activities for complexes with phenolate, bis(imino)pyridine and N,N-dialkylcarbamate ligands. Significant improvements in norbornene ROMP have been recently achieved, while for ethylene polymn. the process of optimization of the niobium-based catalytic systems still needs hard work.