Polyesters, bacterial, biodegradation

The copolymer poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-co-PHV) produced by A eutrophus has generated more interest than poly-(R)-3-hydroxybutyrate (PHB) homopolymer. Since these bacterial polyesters are biodegradable thermoplastics, their mechanical and physical properties have received much attention. PHB is a relatively stiff and brittle material because of its high crystallinity. However, the physiochemi-cal and mechanical properties of [P(HB-HV)] vary widely and depend on the molar percentage of 3-hydroxyvalerate (HV) in the copolymer (4,5) as shown inTable 1. Propionic acid is converted by a synthetase to propionyl-CoA, and the biosynthetic P-ketothiolase catalyzes the condensation of propionyl-CoA with acetyl-CoA to 3-ketovaleryl-CoA by the acetoacetyl-CoA reductase. The hydroxyvaleryl moiety is finally covalently linked to the polyester by the PHA synthase (6). 

Lenz, R. W. and Marchessault, R. m.. Biomacromole. 2005, 6(1), Bacterial Polyesters Biosynthesis, Biodegradable Plastics and Biotechnol. 

Poly(hydroxyalkanoates) (PHAs) are a very common class of bacterial reserve materials, that have attracted considerable industrial attention (Anderson and Dawes, 1990). These polyesters are biodegradable and biocompatible thermoplastics with physical and mechanical properties dependent on their monomeric composition. The production of PHAs is a typical biotechnological process whose development requires the involvement of several scientific disciplines, i.e. genetics, biochemistry, microbiology, bioprocess engineering, polymer chemistry, and polymer engineering. 

Botana et al. [50] have prepared polymer nanocomposites, based on a bacterial biodegradable thermoplastic polyester, PHB and two commercial montmorillonites [MMT], unmodified and modified by melt-blending technique at 165°C. PHB/Na and PHB/ C30B were characterized by differential scanning calorimetry [DSC], polarized optical microscopy [POM], X-ray diffraction [XRD], transmission electron microscopy [TEM], mechanical properties, and burning behavior. Intercalation/exfoliation observed by TEM and XRD was more pronounced for PHB30B than PHB/Na,... 

R.W. Lenz, R.H. Marchessault, Bacterial polyesters biosynthesis. Biodegradable Plastics Biotechnol. Biomacromol. 6 (2005) 1-8, doi 10.1021/bm049700c. 

Lenz RW, Marchessault RH (2005) Bacterial polyesters biosynthesis, biodegradable plastics and biotechnology. Biomacromolecules 6 1-7... 

Lemes AP, Marcato PD, Ferreira OP, Alves OL, Duran N (2008) Nanotechnology and applications. In Nanocomposites of poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate) reinforced with carbon nanotubes and oxidized carbon nanotubes, pp 615-085 Lenz RW, Marchessault RH (2005) Bacterial polyesters biosynthesis, biodegradable plastics and biotechnology. Biomacromolecules 6 1-8... 

Poly(3-hydroxybutyrate) (1.8) is a bacterial polyester that behaves as an acceptable thermoplastic, yet can be produced from renewable agricultural feedstocks and is biodegradable. It is typically produced not in the pure state. 

When polyester-hydrolyzing activity was isolated using synthetic polyesters such as polycaprolactone, and the enzyme was examined in detail, it was found that it was a cutinase that was responsible for the hydrolysis [113]. Similarly, the polyester domains of suberin were found to be degraded by cutinase. Cutinase is a polyesterase, and similar enzymes may be widely distributed and can degrade a variety of natural and synthetic polyesters. Microbial polyhydroxy-alkanoic acids that are attracting increasing attention as biodegradable polyesters can be hydrolyzed by bacterial polyesterases that share some common features with cutinases [114] and this area is covered in another chapter [115]. 

It is clear that green polymers, as defined by their biodegradability, are almost exclusively biopolymers. The major classes of biopolymer of interest here are proteins and polysaccharides, naturally occurring biopolymers, and these are subdivided into various sub-classes, with different applications, as described above. Other polymers of interest are the bacterial polyesters and polylactides. All of these polymers have the potential to be processed into new materials, but clearly not all of these will have either attractive properties or be economically viable materials. 

In this chapter, solid-state structure and properties relative to the morphologies of several chemically and bacterially synthesized biodegradable polymeric materials are described based mainly on the results obtained for bacterially synthesized polyesters by high resolution solid-state NMR spectroscopy. This chapter briefly discusses polymer blends, which also includes polysaccharides and proteins, since more details are given in other chapters of this book. Several books on biodegradable polymers have been published [1,2], and many review articles on structure and properties of bacterially synthesized polyesters have also been published elsewhere [7-10, 19-22]. 

Poly(3-hydroxybutyrate) (1.8) is a bacterial polyester that behaves as an acceptable thermoplastic, yet can be prodticed from renewable agricultural feedstocks and is biodegradable. It is tyjiically produced not in the pure state, but formed alongside minor amounts of poly(3-hydroxyv alerate). The ratio of these two polymers in a given sample is determined by the ratio of glucose and propionic acid in the medium in which the bacteria live and carry out their metabolic processes. 

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