Dialkoxy-substituted, C₁-symmetric metallocenes: Synthesis and catalytic behavior in the propylene polymerization reaction

The synthesis of a series of C₁-symmetric metallocene complexes rac-[1-(5,6-dialkoxy-2-methyl-1-η⁵-indenyl)-2-(9-η ⁵-fluorenyl)ethane]zirconium dichlorides (alkyl: n-butyl, n-hexyl, n-octyl, n-decyl) is described. These complexes are versatile catalysts in the polymerization of propylene after in situ activation with triisobutylaluminum (TIBA) and Ph₃C[B(C₆F₅)₄] in toluene and heptane solution. All catalysts show higher solubility and improved polymerization properties in industrially used hydrocarbon solvents (e.g. heptane). However, the molecular weights and isotacticity values of the resulting polypropylene materials are decreased compared to the ethoxy-bridged analogue rac-[1-(5,6-ethylenedioxy-2-methyl-η⁵-indenyl)-2-(9- η⁵-fluorenyl)ethane]zirconium dichloride. A possible explanation is based on enhanced interaction of the active catalyst centers with Al(III) scavenger molecules even at low Al:Zr ratios, leading to reversible chain transfer.