A dimeric aluminum(salen) catalyst of formula I: wherein: (a) at least one R group is selected from L-A, and/or (b) at least one of X group is a divalent C3-7 heterocyclene group, containing a ring atom which is a quaternary nitrogen atom paired with a counterion selected from Cl, Br and I; and/or (c) at least one X group is a C2-5 alkylene chain or a C1-3 bisoxyalkylene chain, substituted by a group -Q-L-A; and/or (ii) (a) one R group is L-A′, or (b) one X group is a divalent C3-7 heterocyclene group, containing a ring atom which is a quaternary nitrogen forming part of an ammonium linking group bound to a solid support and paired with a counterion selected from Cl, Br and I; or (c) one X group is a C2-5 alkylene chain or a C1-3 bisoxyalkylene chain, substituted by a group -Q-L-A′.