Bis-imine Neodymium and Titanium Complexes: Synthesis, Characterization and Behavior in the Polymerization of 1,3-Dienes

Lanthanide and titanium catalysts are well-known in the field of the stereospecific polymerization to be specific for the cis polymerization of 1,3-dienes, butadiene and isoprene in particular. The system TiCl4/AlR3 was the first catalyst used for the polymerization of butadiene, while the ternary system AlEt2Cl/Nd(OCOR)3/Al(iBu)3 is actually used for the commercial production of cis polybutadiene. Over recent years, a novel generation of catalysts, based on the combination of transition metals (e.g., Cr, Fe, Co, V) and lanthanides (e.g., Nd, Pr) complexes with N-, O- and P-donor ligands (e.g., bis-imines, iminopyridines, ketoimines, mono- and bi-dentate phosphines) with methyl-aluminoxanes, was introduced. These new systems resulted to be extremely active in the polymerization of 1,3-dienes, allowing at the same time a better control of the polymerization regio- and stereo-selectivity, and a better regulation of the molecular weight and molecular weight distribution of the resulting polymers. In the wake of these results, we are now reporting on the synthesis and characterization of several bis-imine neodymium and titanium complexes, which, in combination with suitable alkylating agents (e.g., AliBu2H, MAO, TIBAO), have proved extremely active and selective in the polymerization of 1,3-dienes, butadiene and isoprene in particular. Specifically, the catalytic systems obtained by combining bis-imine neodymium complexes with tetraisobutyl-aluminoxane (TIBAO) allowed us to obtain polybutadienes and polyisoprenes with a very high cis content (99.9 and 98%, respectively); this result represents a quite important achievement from the industrial point of view, since even a slight increase in the polymer cis content may lead to a great improvement in the elastic properties of the polymers, and the limited supply of natural rubber is causing an increase in demand for synthetic polyisoprene. Catalysts based on titanium bis-imine complexes gave instead polybutadienes with a lower cis content (85-90%); nevertheless, some dependence of the regioselectivity on the bis-imine ligand structure was observed, which bodes well for the future.