Disclosed is a process for preparing a polyester or copolymer containing ester functionalities. The process can comprise: providing an optionally substituted lactone having a ring size of from 6 to 40 carbon atoms; and subjecting said lactone to metal mediated ring-opening polymerization using as catalyst a compound according to general formula (I): wherein M can be Al, Cr, Mn and Co; X and X′ are independently a heteroatom; Y and Y′ can be, independently, selected from O, N, S, P, C, Si, and B; Z can be selected from hydrogen, borohydrides, aluminum hydrides, carbyls, silyls, hydroxide, alkoxides, aryloxides, carboxylates, carbonates, carbamates, amidos, thiolates, phosphides, and halides; L1 and L2 can be independently an organic ligand linking X and Y together and linking X′ and Y′ together, respectively; and L3 is an optional organic ligand linking Y and Y′ together.