Azeotropic dehydrative polycondensation

Keywords synthesis, PLA, polycondensation, azeotropic dehydrative polycondensation, ring-opening polymerization, enzyme-catalyzed polymerization, molecular weight, kinetics, mechanism... 

Figure 3.3 Flow diagram of the azeotropic dehydrative polycondensation of LA...

Ajioka et al. [26] have synthesized PLA with a higher than 300 000 by azeotropic dehydrative polycondensation of LA in the presence of a catalyst and an organic solvent. These polymers have good mechanical properties and can be processed into products such as cups, film, and fiber, which can be used as compostable materials. 

Azeotropic dehydration and condensation polymerization (route 2 in Figure 8.2) yields directly high molar mass polymers. The procedure, patented by Mitsui Toatsu Chemicals [13, 14], consists of the removal of condensation water via a reduced pressure distillation of lactic acid for 2-3h at 130°C. The catalyst (in high amounts) and diphenyl ester are added and the mixture is heated up to reflux for 30-40 h at 103°C. Polycondensated PLA is purified to reduce residual catalyst content to the ppm range [5,10,15]. 

The fourth step in the manufacturing process is the polymerization of lactic acid to polylactide, or PLA. PLA can be polymerized via direct polycondensation reaction through azeotropic dehydration. High molecular weight polymers are difficult to synthesize due to the equilibrium between the free acids, water, and the polymer. Dean-Stark trap can be used to remove excess water during the reaction. m-Xylene can be added to lactic acid that added together in a flask at 138 C for 30 hours. After water is removed, the Dean-Stark trap can be replaced with a molecular sieve to recycle the azeotropic mixture. The resultant mixture can be polymerized to PLA at 138°C (Kim and Woo 2002). 

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