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This compound is more reactive than lactide, because its polymerization is driven by the loss of one equivalent of carbon dioxide per equivalent of lactic acid. Apart from lactic acid and lactide, lactic acid O-carboxyanhydride ("lac-OCA"), a five-membered cyclic compound has been used academically as well. The degree of crystallinity, and hence many important properties, is largely controlled by the ratio of D to L enantiomers used, and to a lesser extent on the type of catalyst used. Use of stereospecific catalysts can lead to heterotactic PLA which has been found to show crystallinity. Polymerization of a racemic mixture of L- and D-lactides usually leads to the synthesis of poly- DL-lactide ( PDLLA), which is amorphous. Progress in biotechnology has resulted in the development of commercial production of the D enantiomer form. Stereoisomers ĭue to the chiral nature of lactic acid, several distinct forms of polylactide exist: poly- L-lactide ( PLLA) is the product resulting from polymerization of L, L-lactide (also known as L-lactide). This condensation reaction is a one-step process, and runs about 100 ☌ lower in temperature. Īnother method devised is by contacting lactic acid with a zeolite. Molecular weights of 128–152 kDa are obtainable thus.
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Carboxylic acid and alcohol end groups are thus concentrated in the amorphous region of the solid polymer, and so they can react.
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Even higher molecular weights can be attained by carefully crystallizing the crude polymer from the melt. Molecular weights of 130 kDa can be obtained this way. Water removal by application of a vacuum or by azeotropic distillation is required to drive the reaction toward polycondensation. The condensation reaction is reversible and subject to equilibrium, so removal of water is required to generate high molecular weight species. This reaction generates one equivalent of water for every condensation ( esterification) step. This process needs to be carried out at less than 200 ☌ above that temperature, the entropically favored lactide monomer is generated. The direct condensation of lactic acid monomers can also be used to produce PLA. The metal-catalyzed reaction tends to cause racemization of the PLA, reducing its stereoregularity compared to the starting material (usually corn starch). The most common route to PLA is the ring-opening polymerization of lactide with various metal catalysts (typically tin octoate) in solution or as a suspension. Two main monomers are used: lactic acid, and the cyclic di-ester, lactide. Several industrial routes afford usable (i.e. The monomer is typically made from fermented plant starch such as from corn, cassava, sugarcane or sugar beet pulp. The name "polylactic acid" is potentially ambiguous or confusing, because PLA is not a polyacid ( polyelectrolyte), but rather a polyester. Without annealing, however, PLA has the lowest heat resistance of the common 3D printing plastics.Īlthough the name "polylactic acid" is widely used, it does not comply with IUPAC standard nomenclature, which is "poly(lactic acid)". Its low melting point, high strength, low thermal expansion, good layer adhesion, and high heat resistance when annealed make it an ideal material for this purpose. PLA is the most widely used plastic filament material in 3D printing. Its widespread application has been hindered by numerous physical and processing shortcomings. In 2010, PLA had the second highest consumption volume of any bioplastic of the world, although it is still not a commodity polymer. PLA has become a popular material due to it being economically produced from renewable resources. It can also be prepared by ring-opening polymerization of lactide [–C(CHĢ, the cyclic dimer of the basic repeating unit. N, formally obtained by condensation of lactic acid C(CHģ)(OH)HCOOH with loss of water (hence its name). Polylactic acid, also known as poly( lactic acid) or polylactide ( PLA), is a thermoplastic polyester with backbone formula (C