Ring-opening polymerization of lactones

The synthesis of aliphatic polyesters by the ring-opening polymerization of cyclic monoesters was discovered by Carothers in the 1930s. Since then, a plethora of catalysts and initiators have been discovered to promote this polymerization. Nowadays, steadily increasing attention is paid to organocatalysts and, among them, acids, bases, and H-bond donors and acceptors. Organocatalysts today available for the polymerization of medium size cyclic monoesters such as δ-valerolactone and ε-caprolactone will be reviewed. Special attention will be paid to dual catalysts capable of activating both the initiator and the monomer. The most efficient catalysts promote fast and selective ring-opening polymerization. The mechanism based either on ionic interactions, the establishment of H-bonds or nucleophilic activation will be discussed. The importance of ring size will be highlighted by the organocatalyzed polymerization of β-butyrolactone, γ-butyrolactone and pentadecalactone as a typical macrocyclic monoester.