Niobium complexes as catalytic precursors for the polymerization of olefins

Until now niobium compounds have been very rarely applied for the polymerization of olefins, notwithstanding their moderate cost, limited toxicity and the large availability of stable oxidation states, which makes them versatile precursors. The data reported to date on ethylene and norbornene polymerization catalysts are discussed, with particular reference 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 polymerization 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, whilst for ethylene polymerization the process of optimization of the niobium-based catalytic systems still needs hard work.