se, in the presence of a catalyst which is formed from the components comprising a metallocene as the transition metal compound and a cocatalyst, wherein the metallocene is a compound of the formula I or Ia ##STR9## in which M1 is a metal of group IVb, Vb or VIb of the periodic table,R1 and R2 are identical or different and are a hydrogen atom, a C1 -C10 -alkyl group, a C1 -C10 -alkoxy group, a C6