Polyalkanoates
Many bacteria are a potential source of naturally occurring polyesters, mainly poly (b-hydroxyalkanoate) s with a general structure
when the polymer is poly(b-hydroxybutyrate) R ¼ CH3 [69]. The material is found in the form of hard crystalline granules in many bacterial cells. The most common one, poly(b-hydroxybutyrate), was discovered back in the 1920s and identified as a polymer of D-( )-b-hydroxybutyric acid. In the native state this polymer may reach molecular weights of 1,000,000 or higher. It forms a compact right-handed helix with a twofold screw axis and a repeat unit of 5.96 A ˚ . Because poly (b-hydroxybutyrate) is stereoregular, it is highly crystalline. The substitution in the b-position makes it thermally unstable. This limits its use in plastics. It was found, however, that oxygen starvation of bacterial cultures results in formation of a copolymer of b-hydroxybutyric acid with b-hydroxyvaleric acid instead of a homopolymer. Further investigations showed that as many as 11 different b-hydroxy acid constituents are present in different naturally occurring polyalkanoates, depending upon the bacterial source and conditions of growth. Today, a family of products, marketed under the trade name of Biopol, is available commercially with a range of properties, such as melting point, toughness, flexibility, and others. The melting points range from 80 to 180C. It was reported that Pseudomonas oleovorans microorganism can be forced to produce a thermo plastic elastomer by growing it on a substrate containing sodium octanoate [71]. The product is poly (b-hydroxy octanoate). It contains crystalline regions that act as physical cross-links. Also, addition of poly (ethylene glycol) of molecular weight 200 to cultivation media of Alcaligines eutrophus during polymer formation, where the carbon source used is 4-hydroxybutyrate, affects the type of ester that it produces [73]. Addition of this glycol in amounts from 0 to 2% by weight results in increased amounts of 4-hydroxybutyrate incorporation from 66 to 86 mol.% into the produce, poly(3-dydroxyalkanoate). When, however, the amount of the glycol is increased to 4%, the amount of incorporation of the 4-hydroxybutyrate decreases to 64%. The product then also contains a diblock copolymer of poly(3-hydroxyalkanoate) and poly (ethylene glycol) [73].
