Synthesis of novel biodegradable material poly(lactic acid-trimesic acid) via direct melt copolycondensation and its characterization

Directly starting from lactic acid (LA) and trimesic acid (TMA), novel biodegradable material poly(lactic acid-trimesic acid) (PLT), a modified polylactic acid (PLA) with terminal carboxyl, was synthesized via melt copolycondensation. The optimal synthetic conditions, including catalyst kinds and dosage, prepolymerization time, copolymerization temperature and time, were discussed. When L-lactic acid (L-LA) and TMA as molar feed ratio n(L-LA)/n(TMA) 120/1 was prepolymerized for 8 h at 140 °C, the copolycondensation catalyzed by 0.9 wt % SnCl₂ at 190 °C for 8 h gave PLT with the biggest intrinsic viscosity ([η]) 1.91 dL∙g⁻¹, and the corresponding weight-average molecular weight (M_w) was 14,100 Da. Serial L-PLTs at different molar feed ratios were synthesized and characterized with FTIR, ¹H NMR, GPC, DSC, and XRD. Increasing n(L-LA), M_w increased first, and the biggest M_w was 17500 Da at n(L-LA)/ n(TMA) 240/1, then decreased. Using D,L-lactic acid (D,L-LA) instead of L-LA, the influences of LA stereochemical configuration were investigated. The peak phenomenon of M_w was similar, but the biggest M_w was 23,100 Da at n(D,L-LA)/n(TMA) 320/1. The serial L-PLTs had a certain crystallinity (10.2%∼23.0%), while all D,L-PLTs were amorphous. These differences may be in touch with the reaction mechanism of direct melt copolycondensation. The method was simple and practical for the synthesis of PLA biomedical materials applied in drug delivery carrier, and vessel substitution material.