A catalyst comprising (i) an asymmetric complex of formula wherein M is zirconium or hafnium; each X is a sigma 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, C1-C20-alkyl, tri(Cl-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; R 2 and R 2' are each independently linear C 1-10 hydrocarbyl; R 5 and R 5' are each independently hydrogen or a C1-20 hydrocarbyl group; R 6 and R 6' are each independently hydrogen or a C1-20 hydrocarbyl group; R 7 is hydrogen or a C1-20 hydrocarbyl group or is ZR 3; Z is O or S, preferably O; R 3 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 R 8; Ar' is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R 8'; R 8 and R 8' are each independently is a C1-20 hydrocarbyl group; with the proviso that at least one of R 6 or R 7 is not H; and (ii) a cocatalyst comprising a compound of a group 13 metal, e.g. boron. (i) an asymmetric complex of formula wherein M is zirconium or hafnium; each X is a sigma 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, C1-C20-alkyl, tri(Cl-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; R 2 and R 2' are each independently linear C 1-10 hydrocarbyl; R 5 and R 5' are each independently hydrogen or a C1-20 hydrocarbyl group; R 6 and R 6' are each independently hydrogen or a C1-20 hydrocarbyl group; R 7 is hydrogen or a C1-20 hydrocarbyl group or is ZR 3; Z is O or S, preferably O; R 3 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 R 8; Ar' is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R 8'; R 8 and R 8' are each independently is a C1-20 hydrocarbyl group; with the proviso that at least one of R 6 or R 7 is not H; and (ii) a cocatalyst comprising a compound of a group 13 metal, e.g. boron. M is zirconium or hafnium; each X is a sigma 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, C1-C20-alkyl, tri(Cl-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; R 2 and R 2' are each independently linear C 1-10 hydrocarbyl; R 5 and R 5' are each independently hydrogen or a C1-20 hydrocarbyl group; R 6 and R 6' are each independently hydrogen or a C1-20 hydrocarbyl group; R 7 is hydrogen or a C1-20 hydrocarbyl group or is ZR 3; Z is O or S, preferably O; R 3 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 R 8; Ar' is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R 8'; R 8 and R 8' are each independently is a C1-20 hydrocarbyl group; with the proviso that at least one of R 6 or R 7 is not H; and (ii) a cocatalyst comprising a compound of a group 13 metal, e.g. boron.