Effect of temperature and concentration of SnCl₂ on depolymerization process of L-lactide synthesis from L-lactic acid via short polycondensation

Lactide is a group of cyclic ester, which is used to synthesize polylactic acid (PLA) by ring opening polymerization method. The properties of PLA depend strongly on the quality of the lactide monomer from which it is synthesized. In this research, lactide was synthesized from L-lactic acid through short polycondensation process without catalyst and depolymerization process with SnCl₂ catalyst. The influences of temperature and catalyst concentration on the depolymerization were investigated to achieve high yield, high concentration and exact stereoform of the lactide produced. Polycondensation process of L-lactic acid was carried out gradually at 120 to 180°C for 5 h. Polycondensation produced oligomer with molecular weight (Mn) 2272-2390, (Mw) 2699-2911 and PDI 1.03 to 1.24. Yields of oligomer were between 79.63-83.64%. Depolymerization process was carried out at 76 torr for 3 h with variation of temperature of 190-220°C and concentration of SnCl₂ 0.05-0.2 % (w/w). The crude lactide yield increased with the raising of SnCl₂ concentration and temperature up to 210°C. At temperature of 220°C, the reaction rate of oligomerization for heavy residue was much faster than the depolymerization for lactide produce, which resulted in a lower yield of lactide and more residu obtained. The highest yield of crude lactide was 78.8% with mass of SnCl2 catalyst used 0.1 (w/w) and temperature 210°C. Lactide was characterized by using fourier-transform infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance (¹H NMR). From ¹H NMR analysis, optical purity of lactide produced in depolymerization process confirmed to be L-lactide and no stereoform of racemic formed. Temperature and catalyst concentration did not influence to stereoform of lactide but it affected to L-lactide concentration in crude lactide produced. The highest L-lactide concentration obtained in this study was 81.03%.