Synthesis and Manufacture of PLA

Abstract PLA belongs to the family of aliphatic polyesters derived from a-hydroxy acids. The basic building block of PLA, lactic acid, (2-hydroxy propionic acid), can exist in optically active D- or L-enantiomers, which can be produced by carbohydrate fermentation or chemical syntihesis. Depending on the proportion of the enantiomers, PLA of with variable material properties can be derived. This allows the production of a wide spectrum of PLA polymers to match performance requirements. 

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

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Another distinction should be made regarding the extent of functionalization. Whereas endcapping and copolymerization are performed during polymer synthesis and therefore involve all the polymer chains, the postpolymerization modifications do not necessarily affect the complete bulk of the material. Depending on the experimental procedure applied, it is also possible that only the PLA chains on the surface (of a manufactured device or likewise) are modified (surface functionalization). 

Figure 8.4 Thermal history of PLA polymer during one- and two-stage PLA bottle manufacturing. (Reprinted from Polydactic acid) - Synthesis, Structures, Properties, Processing, and Applications-, Chapter 14, L.-T. Urn., Processing of Polydactic Acid), Copyright (2010), with permission from John Wiley Sons, Inc. [94J.)...

Some metal cations such as sodium and potassium in the feed increase racemization risk, while other metals (Al, Fe) are catalytically active in transesterification, resulting in competitive polylactide formation [68,69]. Through corrosion, metals may be released in the residue and will build up there [6, 75]. Some patents discuss the presence of acid impurities in the process [6, 7, 67, 78], Mono- and dicarboxylic fermentation acids are responsible for stoichiometric imbalance in the lactic acid polycondensation reaction. Consequently, the composition of the obtained lactic acid oligomer chains can differ from pure PLA, resulting in impeded and incomplete catalytic depolymerization of the oligomers into lactide. In PLA manufacture, degradation reactions play a role, mainly via intramolecular chain scission, and this may also affect lactide synthesis. 

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