The article demonstrates that the combination of N-heterocyclic carbenes, 1,8-diazabicycloundec-7-ene (DBU) and 4-dimethylaminopyridine (DMAP) with simple Lewis acids enables the ring-opening polymerization (ROP) of the macrolactone pentadecalactone in a rapid and efficient manner. Surprisingly, regardless of the nature of the nucleophile, the order of activity is observed to be Mg[X.sub.2] ? Y[Cl.sub.3]? Al[Cl.sub.3] and Mg[I.sub.2] Mg[Br.sub.2] Mg[Cl.sub.2] in every case. Moreover, extension of the study to other cyclic (di)ester monomers makes known the choice of Lewis acid to lead to monomer selective ROP activity and hence control over copolymer composition by choice of Lewis acid.