Poly hydroxyalkanoate s

Poly(hydroxyalkanoate)s have been the focus of attention as a biodegradable and biocompatible substitute for the corresponding conventional non-degradable plastics (3). However, the costs of the large-scale production of these polymers have barred the widespread use. Now, new fermentation strategies have been developed for the efficient production of poly(hydroxyalkanoate)s. 

Poly(hydroxyalkanoate)s are semicrystalline, thermoplastic polyester compounds that can either be produced by synthetic methods or by a variety of microorganisms, such as bacteria and algae. Traditionally known bacterial poly (hydroxyalkanoate)s include poly(3-hydroxybutyrate), poly(hydroxybutyric acid), and poly(3-hydroxy-butyrate-co-valerate) (4). 

Poly(hydroxyalkanoate)s can be modified with long side chains. These t q)es are virtually amorphous owing to the recurring higher alkyl side chains that are regularly spaced along the backbone. 

However, the crystalline fraction has a very low melting point as well as an extremely slow crystallization rate, which seriously limits their potential of use. 

Using poly(hydroxyalkanoate) blends can help to tailor the properties somewhat. Immiscible blends show an improved apparent biodegradability when compared to miscible blends. The bio-degradabUity is mostly controlled by the morphology of the blend. Blends of poly(hydroxyalkanoate)s with other biodegradable poly- 

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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. 

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. 

Nobes, G.A.R., Kazalauskas, R.J., and Marchessault, R.H. (1996) Lipase-catalyzed ring-opening polymerization of lactones a novel route to poly(hydroxyalkanoate) s. Macromolecules, 29 (14), 4829-4833. 

Kowalczuk, M., Adamus, G., Sikorska, W., Htkzer, B., Borcakli, M., Atkin, A.H., 2000, Novel biodegradable packaging materials based upon atactic poly[(R,S)-3-hydroxybutyrate] and natural poly(hydroxyalkanoate)s, Proc. International Food Biopack Conference , Copenhagen, p. 39. 

Sudesh K, Abe H, Doi Y (2000) Synthesis, structure and properties of polyhydroxyalkanoates biological polyesters. Prog Polym Sci 25(10) 1503-1555 Sudesh K, Iwata T (2008) Sustainability of biobased and biodegradable plastics. Clean 36 433-442 Taniguchi I, Kagotani K, Kimura Y (2003) Microbial production of poly(hydroxyalkanoate)s from waste edible oils. Green Chem 5 545-548... 

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