Biodegradable polymers bacterial polyesters

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. 

Polyhydroxyalkanoates (PHAs) are a family of biodegradable polymers that have gained a fresh impetus in the recent years. PHAs are polyesters of 7 -hydroxyalkanoic acids produced by a wide variety of bacterial species, under nutrient-limiting conditions (Fig. 8.1). 

Polyfvinyl methyl ether) (PVME) was known to exhibit marginal miscibility with poly(benzyl methacrylate) with LCST behavior. [159] This blend is found to offer distinct similarities to PS/PVME blends. Immiscibility of PVME with a host of other poly(meth) acrylates was observed.[159] PMMA has been shown to be miscible with poly(3-hydroxybutyrate) (PHB) a naturally occurring polyester that can be produced by bacterial fermentation, which has been commercialized as a biodegradable polymer.[160]... 

Amass W, Amass A, Tighe BA (1998) Review of biodegradable polymers uses, current developments in the synthesis and characterization of biodegradable polyesters, blends of biodegradable polymers and recent advances in biodegradation studies. Polym Int 47 89-144 Anderson AJ, Dawes EA (1990) Occurrence, metabolism, metabolic role, and industrial uses of bacterial polyhydroxyalkanoates. Microbiol Rev 54 450-472 Anderson AJ, Haywood GW, Dawes EA (1990) Biosynthesis and composition of bacterial poly(hydroxyalkanoates). Int J Biol Macromol 12 102-105 Bowien B, Kusian B (2002) Genetics and control of CO(2) assimilation in the chemoautotroph Ralstonia eutropha. Arch Microbiol 178 85-93... 

The accessibility of the polymer to water-borne enzyme systems is vitally important because the first step in the biodegradation of plastics usually involves the action of extracellular enzymes which break down the polymer into products small enough to be assimilated. Therefore, the physical state of the plastic and the surface offered for attack, are important factors. Biodegradability is usually also affected by the hydrophilic nature (wettability) and the crystallinity of the polymer. A semicrystalline nature tends to limit the accessibility, effectively confining the degradation to the amorphous regions of the polymer. However, contradictory results have been reported. For example, highly crystalline starch materials and bacterial polyesters are rapidly hydrolysed. 

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