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′
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(C1-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′
; and
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.