Described is a metallocene polymerization process for producing ethylene polymers. The process exhibits very high productivity and is particularly useful for the production of ethylene homopolymers or high density polyethylene (HDPE). The process is conducted in a liquid reaction medium between about 25 °C and 250 °C and comprises feeding continuously or semi-continuously into the liquid reaction medium a catalyst solution comprising an unsupported zirconium or hafnium biscyclopentadienyl metallocene having one or more isobutyl groups on each cyclopentadienyl ring thereof.