Fluorinated half-sandwich complexes of titanium, such as CpTiF3, show an increase in activity of up to a factor of 50 compared to chlorinated compounds. In a temperature range of 10-70 degrees C the excess of methylaluminoxane (MAO) can be reduced to an Al:Ti ratio of 300. The highest melting point of 277 degrees C can be obtained using pentamethylcyclopentadienyltitanium trifluoride (CpTiF3)-Ti-*. This fluorinated complex affords polymers with a significantly higher molecular weight than with the chlorinated compound. Alkyl substitution of the cyclopentadienyl ligand can increase the activity.