The invention relates to a method for producing polyolefins of medium to ultra-high molecular weight by homo- or copolymerization of olefins, specifically ethylene, with other alpha-olefins in the presence of a metallocene-type mixed Ziegler-Natta-catalyst placed onto an organic polymer material for producing shaped bodies, for example by extrusion, injection moulding, film blowing, sinter pressing or ram extrusion. According to the invention the catalyst contains as organic polymer material a partly chloromethylated styrene-divinyl benzene copolymer, a complex compound placed onto same containing Mg, Al and transition metals of the fourth subgroup, as well as reaction products of the latter with other organic compounds and transition metals of the fourth subgroup.