The catalyst precursor 9-fluorenylidene-1 -cyclopentadienylidene-2-hex-5-enylidene zirconium dichloride proved to be highly active in the heterogeneously catalyzed polymerization of ethylene using silica gel/partially hydrolyzed trimethylaluminum (PHT) as cocatalyst. The substitution of position 4 of the fluorenylidene fragment and position 3 of the cyclopentadienylidene ring improves the catalyst activity. The introduction of a phenyl group into the bridge increases the catalyst activity and the molecular weight of the polymer. The prepolymerization of this catalyst system leads to a major change in catalyst and polymer properties. A significant increase in catalyst activity and a lower molecular weight of the produced polyethylene is observed. The presence of hydrogen during prepolymerization or polymerization of ethylene produces a broader molecular weight distribution indicating a higher number of different active centers.