Polymerization of l‐Lactide Catalyzed by Metal Acetylacetonate Complexes

ABSTRACT

Ring‐opening polymerizations (ROPs) of

‐lactide (LA) were performed at 120°C, 140°C, 150°C, and 180°C in bulk catalyzed by acetylacetonate (acac) complexes of manganese(II), nickel(II), copper(II), zirconium(IV), dibutyltin(IV), and vanadyl(V). These experiments revealed that only the acac complexes of Zr and Bu₂Sn were reactive enough to enable quantitative polymerization within a few hours. Further ROPs with Bu₂Sn(acac)₂ and Zr(acac)₄ showed that Bu₂Sn(acac)₂ had a slightly lower reactivity than the Zr complex. The ROPs catalyzed with Zr(acac)₄ yielded number average molecular weights (M_n) up to 70,000 g mol⁻¹. The matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectra indicated an exclusive formation of cycles in the mass range of up to m/z 13,000 for Zr(acac)₄ and up to m/z 18,000 for Bu₂Sn(acac)₂. Unusually high melting temperatures (187°C–192°C) were also achieved along with high crystallinities. Furthermore, four lactide/glycolide copolymerizations catalyzed by Zr(acac)₄ were performed, and the topology and the extent of transesterification were characterized by MALDI‐TOF mass spectrometry and ¹³C NMR spectroscopy.