Polymerization of lactones and lactides initiated by cyclodextrins

Abstract

Cyclodextrins (CDs) were found to initiate ring-opening polymerizations of lactones and lactides selectively to give polyesters in high yields. The order of the polymer yield of β-butyrolactone with CDs is α-CD ≅ β-CD> γ-CD> no CD, while the order of the polymer yield of δ-valerolactone (δ-VL) is β-CD> α-CD>γ-CD> no CD. The yields of the polyesters depend on the cavity size of CDs and on the structures of lactones, indicating that the reaction took place via inclusion of lactones in the CD cavity. The β-CD - adamantane inclusion complex did not show any polymerization activity for δ-VL under the same conditions because adamantane strongly inhibits the formation of the inclusion complex between β-CD and δ-VL. The included monomer in the CD cavity is activated by the formation of hydrogen bonds between the hydroxyl group of CDs and the carbonyl oxygen of monomers in the initiation step, which was observed by FT-IR spectroscopy. The products were found to be a polymer chain attached to the C2-hydroxyl group of a single glucopyranose unit of CD via an ester bond. The lactones are activated by the other remaining secondary hydroxyl groups to give the propagation step by way of insertions of monomers between CD and the polymer chain. The initiation and the propagation steps of the polymerization of lactones by CDs were observed by solid state 13C NMR techniques.