in solution, in suspension or in the gas phase, in the presence of a catalyst which is formed from a metallocene as the transition metal compound and a cocatalyst, which comprises using as the metallocene a compound of the formula I ##STR13## in which M1 is a metal of group IVb of the periodic table,R1 and R2 are identical or different and are a hydrogen atom, a C1 -C10 -alkyl group, a C1 -C10 -a