Zhang, Y., Qi, Y., Yin, Y., Sun, P., Li, A., Zhang, Q., & Jiang, W. (2020). ACS Sustainable Chemistry & Engineering, 8(7), 2865-2873.
Lactide is an important monomer for the industrial production of polylactic acid (PLA), which is one of the most promising materials to replace petroleum-based polymers. Addressing issues such as low yield and purity of lactide when synthesized using metal catalysts and the racemization observed in recently reported catalytic systems, a new catalytic approach-using a composite system of Zn(La)2 and NaHCO3-has been developed. This system offers high yield and purity, with lactide yield reaching over 95.63% and achieving a purity of 97.86% in just 3.25 hours.
Catalytic Synthesis of Lactide
Low oligomeric PLA (OPLA), obtained through the polycondensation of lactic acid, serves as the raw material for lactide synthesis. Under conditions of 195 °C and 3 Torr, Zn(La)2 catalyzes the depolymerization of OPLA to produce lactide. As the reaction progresses, the residual oligomer's molecular weight increases, and the lactide yield gradually decreases. At this point, NaHCO3 is added as a co-catalyst. Under these reaction conditions, the yield of lactide gradually increases, and the reaction is terminated when almost all the substrate has been converted into lactide.
Zhang, Chao, Liqiong Liao, and Shaoqin Gong. Journal of applied polymer science 110.2 (2008): 1236-1241.
The polymerization was conducted in a single-mode microwave reactor equipped with an internal infrared thermometer for temperature measurement. Typically, 1.02 grams of trimethylene carbonate (TMC) and varying amounts of zinc lactate were added to a 10 mL vial. The vial was then sealed with a cap and purged with a stream of high-purity (>99.999%) nitrogen gas for 30 minutes to remove moisture and oxygen. The vial was subsequently subjected to microwave irradiation at 120 °C for different time intervals. After the irradiation was completed, the polymerization product was dissolved in chloroform and filtered to remove the catalyst. A portion of the solution was taken and dried to determine the TMC conversion using 1H NMR. Another portion of the solution was concentrated using a rotary evaporator and precipitated in excess methanol. The precipitate was collected by filtration and vacuum-dried at room temperature for gel permeation chromatography (GPC) analysis.
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