Abstract: The invention relates to a method for preparing polymers of ethylene and propylene, or of copolymers of ethylene and alpha olefins such as propylene 1-butene and 1-hexene or alpha omega dienes such as alpha omega octadiene, respectively, by polymerization within a temperature range from -80.degree. to 150.degree. C. in the presence of a halogen free Ziegler type catalyst system consisting of a cyclopentadienyl compound containing a transition metal and of an aluminum alkyl compound, whereby the polymerization is made in the presence of catalyst system consisting of the following components:(1) a compound containing a transition metal and having the general formula (cyclopentadienyl).sub.n MeY.sub.n, in which n is an integer from 1 to 4, Me is a transition metal, preferably zirconium, and Y being selected from the group of hydrogen, a C.sub.1 -C.sub.5 alkyl group or C.sub.1 -C.sub.5 metal alkyl group or of a compound of the following general formula ##STR1## in which R is a C.sub.1 -C.sub.5 alkyl group or C.