r from 2 to 50, which comprises carrying out thepolymerization in the presence of a catalyst whose transition metal component is a compound of the formula I ##STR4## in which M.sup.1 is titanium, zirconium, hafnium, vanadium, niobium or tantalum,R.sup.1 and R.sup.2 are identical or different and are a hydrogen atom, a halogen atom, a C.sub.1 -C.sub.10 -alkyl group, a C.sub.1 -C.sub.10 -alkoxy grou