A process for the preparation of a random propylene copolymer comprising polymerizing propylene and at least one C2-10 alpha olefin (especially ethylene) in the presence of a catalyst; wherein said catalyst comprises: (i) a complex of formula (I): wherein M is zirconium or hafnium; each X is a sigma ligand; L is a divalent bridge selected from —R′2C—, —R′2C—CR′2—, —R′2Si—, —R′2Si—SiR2—, —R′2Ge—, wherein each R′ is independently a hydrogen atom, C1-C20-hydrocarbyl, tri(C1-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; each R1 is a C4-C20 hydrocarbyl radical branched at the β-atom to the cyclopentadienyl ring, optionally containing one or more heteroatoms belonging to groups 14-16, or is a C3-C20 hydrocarbyl radical branched at the β-atom to the cyclopentadienyl ring where the β-atom is an Si-atom; n is 0-3; each R18 is the same or different and may be a C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms belonging to groups 14-16; each R4 is a hydrogen atom or a C1-6-hydrocarbyl radical; each W is a 5 or 6 membered aryl or heteroaryl ring wherein each atom of said ring is optionally substituted with an R5 group; each R5 is the same or different and is a C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms belonging to groups 14-16; and optionally two adjacent R5 groups taken together can form a further mono or multicyclic ring condensed to W optionally substituted by one or two groups R5; and (ii) a cocatalyst comprising an organometallic compound of a Group 13 metal.