Unbridged bicyclic cyclopentadienyl zirconocene complexes: their possible application as fluxional catalysts in propene polymerization

Metallocene complexes are well established highly active single-site catalysts for the homo- and co-polymerization of ?-olefins.1 Their importance lies also in the fact that little changes in their molecular architecture lead to polymers with remarkably different properties. Recently R. Waymouth2 has developed a new catalytic system based on substituted indenyl systems designed to produce elastomeric polypropylene. Relatively slight changes in the catalyst structure are enough to modify the polymerization outcome and we still lack a systematic investigation to understand which structural features are necessary to achieve the desired stereochemical control. In particular, the study of bicyclic metallocenes, where the ring fused to the cyclopentadienyl moiety is saturated, can be very interesting, since tetrahydroindenyl complexes are usually more stable and more resistant to hydrolysis than the corresponding aromatic derivatives. We have recently reported a new synthetic pathway3 that has proved successful for a versatile and "tuned" synthesis of organometallic compounds based on tetrahydroindenyl systems and of related ligands containing seven- and eight-membered saturated rings fused to the cyclopentadienyl moiety. We have studied three series of these novel unbridged bicyclic cyclopentadienyl zirconium complexes bearing different substituents (-H, -CH3, -Ph) in position 2, in combination with methylaluminoxane (MAO) for the polymerization of propene at several temperatures. All the catalysts have been shown to be active; with most catalysts the polymers obtained are fully atactic when the polymerizations are conducted at 30°C, and become partially isotactic at lower temperatures, the content of isotactic pentad [mmmm] ranging from 11% to 21%. The conformation of the saturated ring is likely to play an important role in determining the stereochemical control and/or the rate of interconversion of isospecific and aspecific states. The 13C NMR analysis shows that when a seven-membered ring is associated with a phenyl substituent in position 2, the pattern of the spectrum is similar to that of the sample produced with Waymouth's catalyst; only in this case we can observe a preponderance of the isotactic heptad in the methyl region of 13C NMR spectrum. The appearance of a slight melting peak, similar to that observed in the sample prepared with Waymouth's catalyst, allows us to hypothesize the formation of stereoblocks.