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 wherein M is zirconium or hafnium; each X independently is a sigma -donor ligand L is a divalent bridge selected from -R' 2 C-, -R' 2 C-CR' 2 -, -R' 2 Si-, -R' 2 Si-SiR' 2 -, -R' 2 Ge-, wherein each R' is independently a hydrogen atom or a C 1 -C 20 -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 1 and R 1' are each independently hydrogen, C 5 -C 10 -aryl or a group-CH(R x) 2 wherein each R x is independently H or a C 1 -C 10 hydrocarbyl group, and optionally the two R x taken together can form a ring, R 2 and R 2' are each independently hydrogen, C 5 -C 10 -aryl or a group-C(R y) 3 wherein each R y is independently H or a C 1-10 hydrocarbyl group, or optionally two or three R y groups taken together can form a ring whereby at least one of R 1 or R 2 and one of R 1 or R 2' is different from hydrogen and whereby R 2 together with one of R 1, as well as R 2' together with one of R 1' can be part of a further mono- or multicyclic ring condensed to the phenyl ring R 3 and R 3' are each independently a linear C 1 to C 6 hydrocarbyl group or a branched or cyclic C 4 to C 10 hydrocarbyl group, whereby the groups are not branched in α-position R 4 and R 4' are each independently a tertiary C 4 to C 10 hydrocarbyl group R 5 and R 5' are each independently a linear or branched C 1 to C 10 alkyl group or an C 5 -C 10 -aryl group and (ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst. (i) a complex of formula wherein M is zirconium or hafnium; each X independently is a sigma -donor ligand L is a divalent bridge selected from -R' 2 C-, -R' 2 C-CR' 2 -, -R' 2 Si-, -R' 2 Si-SiR' 2 -, -R' 2 Ge-, wherein each R' is independently a hydrogen atom or a C 1 -C 20 -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 1 and R 1' are each independently hydrogen, C 5 -C 10 -aryl or a group-CH(R x) 2 wherein each R x is independently H or a C 1 -C 10 hydrocarbyl group, and optionally the two R x taken together can form a ring, R 2 and R 2' are each independently hydrogen, C 5 -C 10 -aryl or a group-C(R y) 3 wherein each R y is independently H or a C 1-10 hydrocarbyl group, or optionally two or three R y groups taken together can form a ring whereby at least one of R 1 or R 2 and one of R 1 or R 2' is different from hydrogen and whereby R 2 together with one of R 1, as well as R 2' together with one of R 1' can be part of a further mono- or multicyclic ring condensed to the phenyl ring R 3 and R 3' are each independently a linear C 1 to C 6 hydrocarbyl group or a branched or cyclic C 4 to C 10 hydrocarbyl group, whereby the groups are not branched in α-position R 4 and R 4' are each independently a tertiary C 4 to C 10 hydrocarbyl group R 5 and R 5' are each independently a linear or branched C 1 to C 10 alkyl group or an C 5 -C 10 -aryl group and M is zirconium or hafnium; each X independently is a sigma -donor ligand L is a divalent bridge selected from -R' 2 C-, -R' 2 C-CR' 2 -, -R' 2 Si-, -R' 2 Si-SiR' 2 -, -R' 2 Ge-, wherein each R' is independently a hydrogen atom or a C 1 -C 20 -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 1 and R 1' are each independently hydrogen, C 5 -C 10 -aryl or a group-CH(R x) 2 wherein each R x is independently H or a C 1 -C 10 hydrocarbyl group, and optionally the two R x taken together can form a ring, R 2 and R 2' are each independently hydrogen, C 5 -C 10 -aryl or a group-C(R y) 3 wherein each R y is independently H or a C 1-10 hydrocarbyl group, or optionally two or three R y groups taken together can form a ring whereby at least one of R 1 or R 2 and one of R 1 or R 2' is different from hydrogen and whereby R 2 together with one of R 1, as well as R 2' together with one of R 1' can be part of a further mono- or multicyclic ring condensed to the phenyl ring R 3 and R 3' are each independently a linear C 1 to C 6 hydrocarbyl group or a branched or cyclic C 4 to C 10 hydrocarbyl group, whereby the groups are not branched in α-position R 4 and R 4' are each independently a tertiary C 4 to C 10 hydrocarbyl group R 5 and R 5' are each independently a linear or branched C 1 to C 10 alkyl group or an C 5 -C 10 -aryl group and (ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst.