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The higher the stereochemical purity of the lactide monomer, the higher the stereochemical purity of the obtained PLA, which controls material properties such as melting point, crystallinity and crystallization rate, and mechanical strength [8, 9, 88].

The strong dependence on D‐isomer content presents an opportunity to control polymer properties. NatureWorks Ingeo PLA is easily processable and suitable as amorphous biopackaging material as a result of its relatively high meso‐lactide content. The downside is the poor resistance to elevated temperatures (low heat distortion temperature, HDT) during transportation, storage, and use of articles produced from this bioplastic. meso‐Lactide—which contains an L‐ and a D‐isomer—is an unavoidable side product of lactide production and must be separated from L‐ and D‐lactides of high stereochemical purity.

Kolstad [9] investigated the crystallization behavior of copolymers of L‐lactide and meso‐lactide. He found that every 1% of meso‐lactide comonomer—or D‐isomer—causes a 3°C reduction in the melting point of the PLA copolymer. With 3% meso‐lactide in PLA, crystallization is more than two times slower than PLLA under the same conditions. With 6% meso‐lactide incorporation, the difference can be up to 10 times!

This underlines the need for a low meso‐lactide content in the monomer mixture for semicrystalline PLA, because meso‐lactide formation by racemization cannot be avoided during melt polymerization of lactides. According to Gruber and coworkers, racemization, which lowers the stereochemical purity of the PLA, is believed to be driven by factors such as temperature, pressure, time at a given temperature or pressure, the presence of catalysts or impurities, and relative concentrations of the two enantiomers at any given time during the polymerization process [88].

PLA grades for more demanding applications that require better heat resistance are achievable by stereocomplexation with PDLA [89]. This is only effective with PLA grades of high stereochemical purity. To prepare high‐quality PLA, it is necessary to start with lactide monomers with the highest possible stereochemical purity, that is, the lowest meso‐lactide content that is technically and economically achievable by purification.

D‐Lactide can be obtained if one has the appropriate biochemistry to produce the D‐enantiomer of lactic acid by fermentation of carbohydrates. Copolymerization of controlled mixtures of L‐ and D‐lactides subsequently offers the advantage of precise control over PLA properties. Moreover, D‐lactide is the monomer for the production of poly(D‐lactide), which is able to form high‐melting stereocomplex PLA via 1 : 1 racemic cocrystallization with P(L)LA, as will be discussed in Chapter 5 [89].