The present invention relates to the field of polymerisation catalysts, and systems comprising said catalysts for polymerising carbon dioxide and an epoxide, a lactide and/or lactone, and/or an epoxide and an anhydride. The catalyst is of formula (I): Wherein M 1 and M 2 are independently selected from Zn(ll), Cr(ll), Co(ll), Cu(ll), Mn(ll), Ni(ll), Mg(ll), Fe(ll), Ti(ll), V(ll), Cr(lll)-X, Co(lll)-X, Ni(lll)-X, Mn(lll)-X, Fe(lll)-X, Ca(ll), Ge(ll), Al(lll)-X, Ti(lll)-X, V(lll)-X, Ge(IV)-(X) 2 or Ti(IV)-(X) 2 . R 3A is different from R 3B ; and/or at least one occurrence of E 3 , E 4 , E 5 and E 6 is different to a remaining occurrence of E 3 , E 4 , E 5 and E 6 . A ligand, a process of asymmetric N-substitution of a symmetrical ligand and a process for the reaction of:(i) carbon dioxide with an epoxide; (ii) an epoxide and an anhydride; and/or (iii) a lactide and/or a lactone, in the presence of a catalyst is also described.