Poly Hydroxyalkanoates

4 Poly(Hydroxyalkanoates) Unlike bio-based PE, PET, and PEA, the poly(hydroxyalkanoates) (PHA) are bioplastics synthesized by bacteria. It was the first bacterial polymer to be harvested commercially. PHAs are deposited within the bacterial cells of many species as a lipoic material (Bnrdon, 1946). It is also unusual in that PHAs though hydrophobic still rapidly biodegrade in the environment. All bacterial polymers are not necessarily biodegradable (Steinbuchel, 2005) PHAs biodegradability is attributed to its saturated polyester chemical structure. 

PHAs are manufactured by Telles (United States), Biomer (Germany), Mitsubishi Gas and Kaneka (Japan), PHB Industrial S/A (Brazil), and Metabolix (United States). 

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, poly(3-hydroxybutyrate-co-4-hydroxybutyrate) poly(3-hydroxyoctanoate-co-hydroxyhexanoate) poly(3-hydroxyoctanoate-co-hydroxyhexanoate). 

FIGURE 4.17 Schematic diagram of poly(lactic acid) manufacture from L-lactic acid. Reproduced with permission from (Gupta et al. 2007). 

A copolymer of P3HB with valeric acid, P(3HB-co-3HV), with the structures shown above, cau also be bacterially produced and available commercially. In the United States, PHB is manufactured by Telles (ADM/Metabolix) and Meridian Inc. PHBV is manufactured by Metabolix Inc. 

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Poly(hydroxyalkanoic acid)s BiopoF Monsanto Environmental Biosynthesis by bacteria or transgenic plants... 

Finally, the synthesis of biodegradable poly(hydroxyalkanoic acids) (PHA) by bacteria or genetically modified plants should be mentioned. The microbiology, biochemistry, and genetics of PHA biosynthesis have been reviewed by several authors52,105,405 407 and are beyond the scope of this chapter. 

PHA degradation, 43 PHAs. See Poly(hydroxyalkanoic acid)s (PHAs)... 

Poly(hydroxyalkanoic acid)s (PHAs), 27, 31, 41-43 biodegradable, 90 Poly(4-hydroxybenzoic acid) (PHBA), 49-50... 

Poly(hydroxyalkanoates) (PHAs), of which poly(hydroxybutyrate) (PHB) is the most common, can be accumulated by a large number of bacteria as energy and carbon reserve. Due to their bio degradability and bio compatibility these optically active biopolyesters may find industrial applications. A general overview of the physical and material properties of PHAs, alongside with accomplished applications and new developments in this field is presented in this chapter. 

Keywords. Poly(hydroxyalkanoates), Biopolyesters, Properties, Modification, Crosslinking, Bio-... 

Since poly(hydroxybutyrate), a naturally abundant poly(hydroxyalkanoate) (PHA, Fig. 1), was first isolated and characterized by Lemoigne in 1925, PHAs have been studied extensively by biochemists who referred to them as lipids [1, 2]. Further research on PHAs however, made it clear that these materials are in fact water-insoluble polyesters, which are synthesized and accumulated intra-cellularly as storage compounds by many different bacteria. Using various types of substrates, an almost infinite variety of PHAs can be synthesized [3-9]. 

Since the chemical structure and monomer composition of a specific polymer are the most important factors in determining the polymer s physical and material properties, a short recapitulation of typical representatives of microbially synthesized poly(hydroxyalkanoates) is presented in this section. A more detailed overview on this issue is available from References [19-21], but is not within our scope here. The monomer composition of PHAs depends on the nature of the carbon source and the microorganisms used. This way, numerous monomers have been introduced into PH A chains [3-9]. PHAs have been divided roughly into two classes [19]. 

Fig. i. Chemical structure of poly(hydroxyalkanoate) repeating unit... 

Poly(3-hydroxybutyrate) is a biopolymer produced by numerous bacteria in nature as an intercellular carbon and energy reserve and belongs to the class of poly (hydroxyalkanoate)s (PHAs). In 1925, the French microbiologist Maurice Lemoigne discovered and characterized PHB extracted from Bacillus megaterium. However, it is produced by a various number of microorganisms such as Cupriavidus necator or Ralstonia eutroph. PHAs are biodegradable polyesters with a structure as shown in Fig. 1. 

Lu J, Tappel RC, Nomura CT (2009) Mini-review biosynthesis of poly(hydroxyalkanoates). J Macromol Sci, Part C Polym Rev 49 226-248... 

Poly-(3-hydroxybutanoic acid) (PHB), belongs to the large family of poly-(hydroxyalkanoates) (PHAs), high molecular weight natural polymers produced by various microorganisms and stored in cell cytoplasm (200). Low molecular weight PHB, also present in bacteria and are primarily involved in transport of ions and DNA across inner bacterial membrane (201). PHB could be developed as a valuable biocompatible material with possible applications in gene delivery after cytotoxic, safety, and efficacy evaluations. 

Liu, S.J. Steinbuchel, A. Exploitation of butyrate kinase and phosphotransbutyrylase from Clostridium acetobutylicum for the in vitro biosynthesis of poly(hydroxyalkanoic acid). Appl. Microbiol. Biotechnol., 53, 545-552 (2000)... 

All these polyesters are produced by bacteria in some stressed conditions in which they are deprived of some essential component for their normal metabolic processes. Under normal conditions of balanced growth the bacteria utilizes any substrate for energy and growth, whereas under stressed conditions bacteria utilize any suitable substrate to produce polyesters as reserve material. When the bacteria can no longer subsist on the oiganic substrate as a result of depletion, they consume the reserve for energy and food for survival or upon removal of the stress, the reserve is consumed and normal activities resumed. This cycle is utilized to produce the polymers which are harvested at maximum cell yield. This process has been treated in more detail in a paper (71) on the mechanism of biosynthesis of poly(hydroxyalkanoate)s. 

General Trend Polysaccharide-graft-poly(hydroxyalkanoate)s... 

Scheme 1 a Ring-opening polymerization of cyclic esters in the presence of SnOct2 and hydroxo-initiator, and b its application to the synthesis of polysaccharide-gra/t-aliphatic polyesters (or poly(hydroxyalkanoate)s)... 

It is further attractive that polymer blending may offer opportunities not only to improve the processability and modify the physical properties of CEs, but also to alter the thermal instability and/or mechanical brittleness of the second component polymers, e.g., many aliphatic polyesters including bacterial poly(hydroxyalkanoate)s. Recent developments in the area of cellulose ester/polymer blends are reviewed below. 

Keywords. Controlled drug delivery, Drug release, Microspheres, Degradation, Erosion, Polylactide, Poly(glycolide-co-lactide), Poly(e-caprolactone), Poly(hydroxyalkanoates) Polyanhydrides, Polycarbonates, Poly(orthoesters), Poly( l,5-dioxepan-2-one)... 

Ashby, R.D., Solaiman, D.K.Y. andFoglia, T.A. 2004. Bacterial Poly(hydroxyalkanoate) Polymer Production from the Biodiesel Co-Product Stream../. Polym. Environ., 12, 105-112. 

Figure 7.4. Transport of fatty acids (FA) and metabolic reactions in P-oxidation for the formation of acetyl CoA and (R)-3-hydroxyacyl CoA, key intermediates for poly-hydroxyalkanoate biosynthesis. Note The gene products PhbA, PhbB, PhbC, and PhaC correspond to the enzymes 3-ketothiolase, acetoacetyl CoA reductase, PHB synthase, and PFIA synthase, respectively.

Ashby, R. D., and Foglia, T. A. 1998. Poly(hydroxyalkanoate) biosynthesis from triglyceride substrates. Appl. Microbiol. Biotechnol., 49,431-437. 

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