Synthesis and Characterization of Biodegradable Poly(lactic acid-co-glycine) via Direct Melt Copolymerization

Instead of the traditional two-step method with cyclic intermediates, a direct reaction of D,L-lactic acid (D,L-LA) and glycine (Gly) as monomers (molar feed ratio LA/Gly: 90/10) and SnO (0.7 wt%) as catalyst is carried out to synthesize a biodegradable poly(lactic acid-co-glycine) [P(LA-co-Gly)] via melt copolycondensation at 170 deg C, absolute pressure of 70 Pa, for 8 h, following a prepolymerization step. The structure of the copolymer was characterized by FTIR and 1H NMR techniques. The GPC results show that the weight average molecular weight (Mw) of the copolymer P(LA-co-Gly) is 3600 g/mol. Under the condition of different molar feed ratios the highest Mw was found to be 5200 with molar feed ratio LA/Gly:98/2. Other characterizations, e.g., intrinsic viscosity, solubility, DSC, and XRD indicated that the copolymer solubility in chloroform and tetrahydrofuran (THF) decreased with the increasing feed charge of Gly and the lowest solubility was found when the molar feed ratio was below LA/Gly:70/30. However, the modified polylactic acid is readily soluble in DMSO. The crystallinity of copolymer gradually disappears as the molar feed ratio of D,L-LA decreases. For LA/Gly > or = 90/10, the XRD pattern has two diffraction peaks, close to 16.8 deg and 19.1 deg, the same position as poly(D,L-lactic acid) (PDLLA). For the soluble P(LA-co-Gly) the intrinsic viscosity, Mw and its dispersity (Mw/Mn) decrease gradually with the increase of feed charge of Gly. These results, especially the molecular weight of the copolymer appear to meet the requirements necessary for a drug delivery applications.