Cyclic Polylactides by Imidazole-Catalyzed Polymerization of <scp>l</scp>-Lactide

Abstract

Heating of l-lactide with imidazole to 98−100 °C resulted in complete polymerization within 48 h. Even-numbered cycles resulting from end-to-end cyclization were the only reaction products after 4 h, but the polymerization process was accompanied by intensive racemization. Longer reaction times favored equilibration reactions with formation of odd-numbered cycles. Variation of the monomer−initiator ratio at 120 °C had little influence on the molecular weight. After 8 h at 150 °C, equal quantities of odd- and even-numbered cycles were found, indicating complete equilibration. Other protic heterocycles such as 1,2,4-triazole, benzimidazolyl acetonitrile, uracile, or hypoxanthine were not active as initiators/catalysts at 120 °C. However, the tertiary amine N-methylimidazole also catalyzes the formation of cyclic polylactides together with several byproducts. The reaction mechanisms are discussed.