A multistep process comprising the following steps: step a) polymerizing propylene and optionally one or more monomers selected from ethylene or alpha olefins of formula CH.sub.2.dbd.CHT.sup.1 wherein T.sup.1 is a C.sub.2-C.sub.10 alkyl radical in the presence of a catalyst system supported on an inert carrier, comprising: ii) one or more metallocene compounds of formula (I): ##STR00001## ii) an alumoxane or a compound capable of forming an alkyl metallocene cation; and optionally iii) an organo aluminum compound; step b) contacting, under polymerization conditions, in a gas phase, propylene or ethylene with one or more alpha olefins of formula CH.sub.2.dbd.CHT, wherein T is hydrogen or a C.sub.1-C.sub.10 alkyl radical, and optionally a non-conjugated diene, in the presence of the polymer obtained in step a) and optionally in the presence of an additional organo aluminum compound; provided that an homopolymer is not produced; wherein: the compound of formula (I) is described in the application.