Ethylene-bridged pseudo-C_s symmetric ansa-zirconocene complexes: Synthesis, structures and propylene polymerization behavior

The ethylene-bridged pseudo-C_s symmetric ansa-zirconocene complexes, (1-fluorenyl-2-cyclopentadienylethane)zirconium dichloride (5a) and (1-fluorenyl-2-tetramethylcyclopentadienylethane)zirconium dichloride (5b), were synthesized and their molecular structures have been determined by single crystal X-ray diffraction studies. The complex 5a is very active toward propylene polymerization in the presence of methylaluminoxane (MAO), affording polypropylene with good syndiotacticity. In comparison with the known syndiotactic polypropylene catalytic system iPr(fluorenyl)(cyclopentadienyl)ZrCl₂/MAO, 5a/MAO system is better in thermal stability, higher in activity and lower in syndiotacticity under the identical polymerization conditions. On the other hand, 5b/MAO system shows low activity and produces nearly atactic polypropylene with low molecular weight, which could be attributed to the increased electron density and steric bulkiness around the metal center.