Lactide synthesis optimization: investigation of the temperature, catalyst and pressure effects

The most applicable method to produce polylactic acid (PLA) is via ring opening polymerization of lactide, which involves three steps: polycondensation of low-molecular-weight PLA (LMW PLA), depolymerization or dimerization of a prepolymer into a cyclic dimer of lactic acid (lactide) and ring opening polymerization of lactide to achieve high-molecular-weight PLA. This work aimed to optimize the reaction conditions of lactide synthesis. Reactions at different temperatures and pressures were carried out in the presence of tin (II) oxide, tin (II) chloride, tin (II) octate, antimony trioxide, sulfuric acid and no catalyst in a semi-batch reactor. Different analytical techniques such as gas chromatography, gel permeation chromatography, differential scanning calorimetry, nuclear magnetic resonance and Fourier transform infrared spectroscopy have been used to evaluate the LMW PLA properties, rate of lactide synthesis and racemization. Results showed that the highest rate of conversion occurred in the presence of tin (II) chloride and octate. Synthesis rate increased with the increase in reaction temperature and the decrease in pressure. Increasing temperature also resulted in higher amounts of optical and chemical impurities in the crude lactide.