Seven- and eight-membered saturated fused ring zirconium and titanium metallocenes: a route for the synthesis of elastomeric polypropylene

The discovery by Waymouth and Coates[1] that a single catalytic species (i.e. (IndPh)2ZrCl2) can produce elastomeric polypropylene aroused an increasing interest on unbridged substituted indenyl systems. The basis of a "fluxional" behaviour is the free rotation around the ligand-metal bond and the consequent possibility of interconversion between chiral and achiral rotational isomers on the time scale of the polymerisation reactions. By a careful choice of the reaction conditions and on the structure of the catalyst, it is then possible to obtain a polymer constituted by isotactic/atactic stereoblocks, that impart interesting elastomeric properties. Despite the number of studies carried out in the last years, a clear structure/performance correlation is still lacking and there are several open questions. Our contribution in this field is the development of a new family of unbridged bicyclic metallocenes, where the ring fused to the cyclopentadienyl moiety is a saturated one. They are interesting complexes, since, besides their higher stability and resistance to hydrolysis, they introduce a conformational flexibility parameter that can be finely tuned.