Tripartite hydrogen-bonding as a driving force for high-concentration cyclization of poly(l-lactide).

A three-step strategy enables efficient cyclization of high-M n PLLA at 0.5 M. Key to success is a persistent tripartite complex driven by hydrogen bonding and ion pairing. Selective intramolecular transesterification at a benzylic ester triggers ring closure, affording cyclic PLLA (M n ≈ 26 000 g mol-1) under synthetically practical conditions. SPM imaging reveals nanorings with diameters consistent with the expected contour length of cyclic PLLA chains.