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

Catalytic systems based on unbridged substituted indenyl systems are becoming of interest in the production of elastomeric polypropylene. (1) If properly substituted, the interconversion between chiral and achiral torsional isomers of these metallocenes can be slowed down to the time scale of the polymerisation reaction: in this case the polymer obtained will be characterised by alternating sequences of isotactic and atactic stereoblocks. 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. In the framework of this area of research the study of bicyclic metallocenes, where the ring fused to the cyclopentadienyl moiety is saturated, can be very interesting. (2, 3) These kinds of complexes, besides their higher stability and resistance to hydrolysis, introduce a conformational flexibility parameter that can be finely tuned. Although several zirconium and hafnium complexes have been already studied and compared, very little is known about the behaviour of the corresponding titanium derivatives. Since the titanium metal allows a closer interaction between the two bent metallocene moieties, in principle, the effect of the substituents should be stronger in its complexes than with the other Group 4 metallocenes. We report here our recent findings regarding the study of the reactivity of two new titanium complexes (1a and 1b) as fluxional catalysts in propene polymerisation.