A catalyst comprising (i) an asymmetric 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—SiR′2—, —R′2Ge—, wherein each R′ is independently a hydrogen atom, C1-C20-alkyl, tri(C1-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; R2 and R2′ are each independently linear C1-10 hydrocarbyl; R5 and R5′ are each independently hydrogen or a C1-20 hydrocarbyl group; R6 and R6′ are each independently hydrogen or a C1-20 hydrocarbyl group; R7 is hydrogen or a C1-20 hydrocarbyl group or is ZR3; Z is O or S, preferably O; R3 is a C1-10 hydrocarbyl group; Ar is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R8; Ar′ is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R8′; and R8 and R8′ are each independently is a C1-20 hydrocarbyl group; with the proviso that at least one of R6 or R7 is not H; and (ii) a cocatalyst comprising a compound of a group 13 metal, e.g. boron.