From zirconium to titanium: the effect of the metal in propylene polymerisation using fluxional unbridged bicyclic catalysts

Catalytic systems based on unbridged substituted indenyl systems are becoming of interest in the production of elastomeric polypropylene. A full understanding of the structural features necessary to control this kind of behaviour has not yet been achieved, since relatively slight changes in the molecular architecture can lead to polymers with remarkably different properties. We report here our recent findings regarding the study of bicyclic zirconium and titanium complexes as fluxional catalysts in propylene polymerisation. Most of them have been synthesised according to a synthetic procedure that allowed us to prepare a series of complexes in which the ring fused to the cyclopentadienyl moiety is saturated and of different sizes, thus introducing a flexibility parameter that can be finely tuned. The results obtained show that the stereoselectivity induced by this class of catalysts strongly depends both on the structure of the ligand and on the nature of metal atom (Zr vs. Ti). The titanium-based catalysts yield polypropylenes with new and interesting microstructures, in particular when an higher stability is achieved through a careful choice of the substitution pattern of the ligands.