Cyclic Polylactides by Imidazole-Catalyzed Polymerization of L-Lactide

Heating Of L-lactide with imidazole to 98-100 degrees 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 degrees C had little influence on the molecular weight. After 8 h at 150 degrees 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 degrees C. However, the tertiary amine N-methylimidazole also catalyzes the formation of cyclic polylactides together with several byproducts. The reaction mechanisms are discussed.