Copolymer poly 3-hydroxybutyrate

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). 

Poly(3-hydroxybutyrate) (PHB) and its copolymer poly(3-hydroxybutyrate-co-3-bydroxyvalerate) (PHBV) are ratber resistant towards moisture and tbeir cbemical hydrolysis at neutral pH and ambient temperature proceeds slowly through random chain scission [78,79]. The rate of chemical hydrolysis decreases with increasing crystallinity [80]. There is no agreed explanation to how the copolymer composition affects the hydrolysis rate. It has been suggested that it is the crystalhnity, rather than the composition that affects the hydrolysis rate. On the other hand, PHBVs of the same crystalhnity, but different compositions (45-71 mol% HV), showed decreasing hydrolysis rates with increasing HV-content [81]. 

Another terpolymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-4HB-co-3HHx)] was found to have better thermal stability due to the introduction of 4HB and 3HHx monomers into P(3HB) (Xie and Chen 2008). It has lower crystalUnity and better flexibility compared to P(3HB) homopolymer and copolymers. Poly(3-hydroxybutyrate-C( -3-hydroxyvalerate-co-4-hydroxybutyrate) [P(3HB-co-3HV-co-4HB)] also showed superior properties over those of 3HB and its copolymer (Madden et al. 2000 Chanprateep and Kulpreecha 2006). Table 2.3 shows the comparison of different types of PHA polymer. 

Many bacteria can use glycerol as a carbon source. As it is a by-product of the production of bio-diesel it can be achieved chet ly. Therefore it seems to be a good alternative to glucose for die production of Polyhydroxyalkanoates. Sodiumvalerate is often used as a precursor in order to obtain the copolymer Poly-3-hydroxybutyrate-co-3-hydroxyvalerate. As Biorelated Potymers Sustainable Polymer Science and Technology Edited by Chiellini et al., Kluwer Academic/Plenum Publishers, 2001 147... 

Tiipathi, L., Wu, L.P., Chen, J. and Chen, G.Q. (2012) Synthesis of Diblock copolymer poly-3-hydroxybutyrate -block-poly-3-hydroxyhexanoate [PHB-b-PHHx] by a -oxidation weakened Pseudomonas putida KT2442. Microbial Cell Factories, 44,1-11. 

The major commercial microial polyesters are polyhydroxyalkanoates (PHAs) (see Poly(3-HYDROXYALKANOATES)) Poly(3-hydroxybutyrate) (PHB) (8) is brittle, but as the number of carbon atoms in the side chains of PHAs increases, eg, poly(3-hydrox5Tvalerate) (PHV) (9), the properties tend from polypropylene-like to elastomeric. Copolymers, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), have a range of properties depending on composition (Table 1). 

In air, the thermooxidative degradation and stability of commercial and laboratory made polyhydroxyalkanoates such as the homopolymer poly(3-hydroxybutyrate) and the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) was investigated by Carraso et al. [20]. It was indicated that the presence of hydroxyvalerate within the copolymer led to a thermally more stable material (with an increase of 14°C). 

As mentioned earlier, the family of bacterial-synthesized polyhydroxyesters so-called polyhyroxyalkanoates (PHAs), poly(3-hydroxybutyrate) or PHB is the most important one and represents an interesting alternative to synthetic polymers. Another interesting variant is the copolymer poly(3-hydroxybutyrate-co-3-hy-droxyvalerate) or PHB/HV. Due to its high hydrophobicity, the preparation of... 

The copolymer poly(3-hydroxybutyrate-co -hydroxybutyrate) (P(3HB-co-4HB)) has also been produced by R. eutropha and has potential medical applications. As with other copolymers, thermal properties of P(3HB-co-4HB) improve with increasing amounts of 4HB monomer (Ishida et al. 2001 Kasuya et aL 1996). There are many instances in the literature where P(3HB-co-4HB) is produced (Cavalheiro et al. 2012 Ishida et al. 2001 Kim et al. 2005 Volova et al. 2011), and in most cases, a 4HB precursor molecule (y-butyrolactone, 4-hydroxybutyrate, etc.) is typically added to the culture. Depending on the amount of precursor feeding, the 4HB fraction of PHA produced by R. eutropha can vary from 0 to 100 mol% (Cavalheiro et al. 2012 Ishida et al. 2001 Kim et al. 2005 Volova et al. 2011). 

The copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) or poly(3HB-co-3HV) was produced in transgenic Arabidopsis thaliana and in the seeds of transgenic Brassica napus (oilseed rape) by diverting metabolic pools of acetyl CoA and threonine into PHA production through four introduced genes that are targeted for expression in the plastids (Slater et al. 1999). The four genes involved are ... 

Two types of polyhydroxyalkanoate (PHA) biosynthesis gene loci (phb and pha) of Pseudomonas sp. strain 61-3, which produces a blend of poly-3-hydroxybutyrate [PHB] homopolymer and a random copolymer poly(3-hydroxybutyrate-co-3-hydroxyalkanoate) [P(3HB-co-3HA] consisting of 3HA units of 4 to 12 carbon atoms, were cloned and analyzed at the molecular level [274]. In the phb locus, three open reading frames encoding polyhydroxybutyrate (PHB) synthase (PhbCPs), 6-ketothiolase (PhbAPs), and NADPH-dependent acetoacetyl coenzyme A reductase (PhbBPs) were... 

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