Preparation of High Purity Lactide Using a High-Boiling-Point Alcohol Immobilization Method

Abstract

During the lactide producing process, volatile impurities, such as lactic acid and its dimer, are simultaneously distilled out with lactide. To remove these impurities, many post-treatments were needed. Herein, we present a novel approach to obtain high purity lactide by adding high-boiling-point alcohols to immobilize these volatile impurities. A series of high-boiling-point alcohols were screened and pentaerythritol was found to be the best immobilization reagent. When the molar ratio of lactic acid’s carboxylic group and pentaerythritol’s hydroxyl group was 15:1, lactide with 99% GC purity was directly distilled out and no traces of lactic acid and its dimer were detected. The results of HPLC-MS revealed that the distillation residues were esters of pentaerythritol with lactic acid or its oligomer. This indicated that volatile impurities were immobilized in distillation residues. The distillation residues were reused for 4 runs, and the purities of lactide were all over 98%.

Efficient Synthesis of Lactide with Low Racemization Catalyzed by Sodium Bicarbonate and Zinc Lactate

“…5,11−14 However, the yields of lactide when using such catalysts are often below 80% because the catalysts can also catalyze side reactions. 15,16 For instance, according to Jiang et al, 15 the yield of lactide was 77.5% when using SnCl 2 at a catalyst loading of 0.1 mol % relative to OPLA. In addition, Cao et al 16 used SnO as a catalyst for lactide production and only achieved a 73% yield with a catalyst loading of 0.4 wt %.…”

“…15,16 For instance, according to Jiang et al, 15 the yield of lactide was 77.5% when using SnCl 2 at a catalyst loading of 0.1 mol % relative to OPLA. In addition, Cao et al 16 used SnO as a catalyst for lactide production and only achieved a 73% yield with a catalyst loading of 0.4 wt %. Some other metal compounds have also been used as catalysts; however, their catalytic performances are also poor due to the same problems as mentioned above.…”

“…In general, the mechanism of the depolymerization to form lactide is the backbiting reaction of the hydroxyl group on the PLA chain under reduced pressure (<20 kPa) and high temperature (180–240 °C) with a catalyst, which can be a salt or an oxide of Sn or Zn. ,− However, the yields of lactide when using such catalysts are often below 80% because the catalysts can also catalyze side reactions. , For instance, according to Jiang et al, the yield of lactide was 77.5% when using SnCl 2 at a catalyst loading of 0.1 mol % relative to OPLA. In addition, Cao et al used SnO as a catalyst for lactide production and only achieved a 73% yield with a catalyst loading of 0.4 wt %. Some other metal compounds have also been used as catalysts; however, their catalytic performances are also poor due to the same problems as mentioned above. , Hong et al showed that only 68% lactide was produced when using zinc oxide as the catalyst at a depolymerization temperature of 230–250 °C.…”

Syntheses and chemical transformations of glycolide and lactide as monomers for biodegradable polymers

“…A large number of different catalytic systems have been proposed since, which makes it possible to obtain lactide in high yield, selectivity, and optical purity [103,104,155,156]. Additionally, there appeared new versions of its synthesis using microwave radiation [157][158][159], various solvents [160], oligomer modifiers (e.g., polyhydric alcohols) [161][162][163], etc. Some approaches are similar to methods for glycolide obtainment.…”

“…Further studies have shown that polyhydric high-boiling alcohols affect the depolymerization of oligomers, and an increase in the number of hydroxyl end groups is not the sole reason for that. Wang et al found that when oligomers are modified with polyhydric alcohols with further depolymerization, the acid residue and its lowmolecular weight oligomers are bound (immobilized) as esters that have a higher boiling point; therefore, they are not distilled with lactide during oligomers depolymerization and do not cause contamination (Table 8, entry 22) [163]. The depolymerization of oligomers modified with pentaerythritol shows a crude lactide yield of 93% and purity of L-lactide after purification approximating 99%.…”