lactide was synthesized by polycondensation and depolymerization reactions from DL-lactic acid and the crude product was recrystallized from dried ethyl acetate. The melting point of the product was 126.3 C. PDLLAs of various molecular weights were synthesized in our laboratory. SnOct2 was purchased from Sigma and used without further purification. ESO was purchased from Kaiqi chemical company and the average number of epoxy groups per molecule was 4. HPLC grade tetrahydrofuran (THF) was purchased from SK Chemicals. Analytical grade chloroform and methanol was purchased from Chongqing Chemical Factory.