Process for preparing propylene polymers

A process for polymerizing propylene monomers, optionally together with ethylene comonomers, in the presence of a single-site catalyst comprising

• (i) a complex of formula (I)
• wherein
• M is zirconium or hafnium;
• each X independently is a sigma-donor ligand
• L is a divalent bridge selected from —R′2C—, —R′2C—CR′2—, —R′2Si—, —R′2Si—SiR′2—, —R′2Ge—, wherein each R′ is independently a hydrogen atom or a C1-C20-hydrocarbyl group optionally containing one or more heteroatoms of Group 14-16 of the periodic table or fluorine atoms, or optionally two R′ groups taken together can form a ring;
• R′ and R1′ are each independently hydrogen, C5-C10-aryl or a group —CH(Rx)2 wherein each Rx is independently H or a C1-C10 hydrocarbyl group, and optionally the two Rx taken together can form a ring,
• R2 and R2′ are each independently hydrogen, C5-C10-aryl or a group —C(Ry)3 wherein each Ry is independently H or a C1-10 hydrocarbyl group, or optionally two or three Ry groups taken together can form a ring whereby at least one of R1 or R2 and one of R1′ or R2′ is different from hydrogen and
• whereby R2 together with one of R1, as well as R2′ together with one of R1′ can be part of a further mono- or multicyclic ring condensed to the phenyl ring
• R3 and R3′ are each independently a linear C1 to C6 hydrocarbyl group or a branched or cyclic C4 to C10 hydrocarbyl group, whereby the groups are not branched in α-position
• R4 and R4′ are each independently a tertiary C4 to C10 hydrocarbyl group
• R5 and R5′ are each independently a linear or branched C1 to C10 alkyl group or an C5-C10-aryl group and
• (ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst.