Poly (D,L-lactic acid) homopolymers: Synthesis and characterization

In this study, poly(D,L-lactic acid or D,L-lactide) (PDLLA) homopolymers were synthesized with different molecular weights. Low molecular weight PDLLAs (MW_n: 500-2300) were synthesized by direct polycondensation of DL-lactic acid without using any catalyst, while high molecular weight PDLLAs ( MW_n: 8×10³-150×10³) were produced by ring-opening polymerization of DL-lactide with stannous chloride as catalyst (initiator). Average molecular weights of PDLLAs were determined by end-group analysis, by viscosimetric measurements and by HPLC. PDLLA homopolymers were characterized by FTIR, NMR and DSC. In the production of PDLLAs by direct polymerization, molecular weights of the PDLLAs increased with the increase in the polymerization time and temperature. However, in the production of PDLLAs by ring-opening polymerization, molecular weights increased with increasing polymerization time and the molar ratio of dimer/initiator, but decreased with increasing temperature. Both low and high molecular weight PDLLAs were amorphous with Tg values around 20°-25°C and 54°-57°C, respectively.