A process for preparing a polymer containing derived units of one or more alpha olefins of formula CH2═CHW wherein W is a C3-C10 hydrocarbon radical and optionally from 0 to 81% by mol of derived units of propylene or 1-butene, comprising contacting under polymerization conditions one or more alpha olefins of formula CH2═CHW and optionally propylene or 1-butene in the presence of a catalyst system obtainable by contacting:a) a metallocene compound of formula (I)