Naturally occurring polyesters such as polyhydroxybutyrate (PHB), which belongs to a class called polyhydroxylalkanoates (PHA), can be produced commercially through fermentation by microorganisms. They are placed in a 'bioreactor' and fed with a carbon source, which forces them to produce the desired product. This strategy essentially exploits a natural process that certain microorganisms undergo when excess carbon is available, where the polymers act as a storage for energy and carbon.[1] Biosynthesis of PHB involves a series of enzymatically catalyzed reactions in bacteria. For example, in Alcaligenes eutrophus, PHB is primarily derived from acetyl-coenzyme A. Various acyl transferases and reductases are involved in the biosynthesis of monomer-coenzyme A complex. PHB synthase completes the pathway by performing polymerization of these monomers. Due to the high cost of bacterial PHB production, transgenic plants have been explored as an alternative to lower the cost of PHB production. Transgenic Arabidopsis thaliana plants containing PHB biosynthesis genes from A. eutrophus were created and tested for their ability to produce and accumulate PHB.[2] As is the case with many biodegradable polymers, PHB can be made into a biodegradable thermoplastic by the addition of a suitable plasticizer. Also, copolymers comprising two different monomers can also be produced. One example of such is poly(hydroxybutyrate-co-valerate), which can be mixed with citrate ester plasticizers to produce thermoplastic.[3]


external image 200px-Poly-%28R%29-3-hydroxybutyrat.svg.png


Figure 1. Core structure of poly-(R)-3-hydroxybutyrate


Synthetic aliphatic polyesters have also been developed. Polylactic acid (PLA) is an example of such polymers. In this case, the monomer used, lactic acid, is a natural product, which could be obtained from microorganisms through fermentation.[1] Many species of bacteria, such as those belonging to the genus Lactobacilli, are used to mass-produce lactic acid. Organisms such as L. amylophilus produce predominantly the L isomer of lactic acid, while others, including L. jensenii, produce mainly the D isomer or a mixture. Synthesis of polylactic acid usually proceeds through cyclic dimers of lactic acid, also known as lactides.Two polymerization methods are possible. Condensation polymerization involves removal of water as esters are formed. This route, however, cannot be used to produce high molecular weight polymers, unless additives, such as chain extenders, are added. The additives must later be removed, adding to the cost of production. As a result, condensation is more expensive to perform. To obtain higher chain lengths without the use of additives, Mitsui Chemicals in Japan has developed azeotropic condensation methods. As the name implies, the water is removed azetropically, increasing the efficiency of polymerizatioin. Unfortunately, this method requires a high quantity of toxic catalysts, which may remain in the polymer. Another method of PLA production involves ring-opening polymerization (ROP) of lactides. High molecular weight polymers can be produced using this method. The mechanism may be either ionic or coordination-insertion, where the latter involves a catalyst. Ionic ROP may suffer from high levels of racemization, transesterification and impurities. Various copolymers can be produced to modify the characteristics of the final product. Examples of monomers that can be used to produce copolymers include glycolide and valerolactone. Due to its intrinsic brittle nature, plasticizers are necessary to produce PLA thermoplastic. Plasticizers such as oligomeric lactic acid and lactide monomers can be used.[4]
external image Polylactic-acid-2D-skeletal.png

Figure 2. Core structure of poly(lactic acid)



lactides.png


Figure 3. Structures of L-lactide, meso-lactide and D-lactide

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  1. ^ Stevens ES. 2002. Green Plastics: An introduction to the new science of biodegradable plastics. Princeton (NJ): Princeton University Press.
  2. ^ Poirier Y, Clague DE, Klomparens K, Somerville C. 1992. Polyhydroxybutyrate, a biodegradable thermoplastic, produced in transgenic plants. Science. 256(5056):520-23.
  3. ^ Kotnis MA, O’Brien GS, Willett JL. 1995. Processing and mechanical properties of biodegradable poly(hydroxybutyrate-co-valerate)-starch compositions. Journal of Environmental Polymer Degradation. 3(2):97-105.
  4. ^ Belgacem MN, Gandini A. 2008. Monomers, polymers and composites from renewable resources. 1st ed. Kidlington (UK): Elsevier.