Copolymer PHA

PHB, along with valuable properties, has certain disadvantages, in particular, high cost and fragility. For overcoming such disadvantages either copolymers PHAs or their blends with other natural polymers, in particular, with chitosan [8] are often used. 

The term UnPHAs includes many different PHAs belonging to the different types indicated earlier (uncommon scl-/mcl-PHAs, semisynthetic/synthetic PHAs and homopolymer/copolymer PHAs) as well as others that have been obtained by physical modifications of the natural ones (i.e. blended polymers). 

In all types of PHAs, P4HB is of the most interest because it was used in the degradable scaffold that resulted in the first successful demonstration of a tissue-engineered tri-leaflet heart valve in a sheep animal model. Its copolymers with PHB and polyhydroxyoctanoate (PHO) are also promising in tissue engineering because of their nontoxic degradation products, stability in tissue culmre media, and the potential to tailor the mechanical and degradation properties to match soft tissue. 

A mutant E. coli strain LS5218 (fadR atoC) was employed for the synthesis of P(3HB-co-3HV) copolymer since this mutant strain constitutively expresses the enzymes involved in the transport and utilization of short chain fatty acids [58, 59]. P(3HB-co-3HV) could be synthesized by a recombinant E. coli strain LS5218 harboring the R. eutropha PHA biosynthesis genes when propionic acid or valeric acid was added as a cosubstrate [58,60]. The P(3HB-co-3HV) copolymer consisting of up to 40 mol% of 3HV could be produced. An alternative method that allowed synthesis of P(3HB-co-3HV) using propionic acid or valeric... 

Holmes PA (1988) Biologically produced PHA polymers and copolymers. In Bassett (ed) Developments in crystalline polymers. Elsevier, London, p 1... 

First, the short chain length PHAs, poly(HASCL), are composed of monomeric units containing up to 5 carbon atoms. The most well-known representatives are poly(3-hydroxybutyrate) (PHB), and its copolymers with hydroxyvalerate. Of all the PHAs, PHB is by far the most commonly encountered in nature [18]. It is the simplest PHA with respect to chemical structure, having a methylene (-CH3) group as the pendent R-unit in Fig. 1. Owing to its enzymatic synthesis, PHB has an exceptional stereochemical regularity. The chains are linear and the chiral centers all are in the R-stereochemical conformation, which implies that this polymer is completely isotactic. 

Second, some organisms are able to incorporate longer pendent chains yielding another class of PHA medium chain length PHA, poly(HAMCL). Poly (HAmcl) is specifically accumulated by fluorescent pseudomonads. When aliphatic hydrocarbons like n-alkane, n-alkanoate, or n-alkanol serve as feedstocks for Pseudomonas oleovorans the resulting PHA is a random copolymer... 

The mechanical properties of PHB and its copolymers have been studied extensively [82, 83]. As shown in Table 3, the material properties of PHAs can be readily controlled by adjusting the polymer composition during the fermentation [84]. 

Hot melt adhesives based on poly(3HB-co-3HV) have also been described [119]. Hot melts are commonly used in bookbinding, bag ending and case and carton sealing and are mostly based on synthetic materials such as polyethylene, polypropylene ethylene-vinyl acetate and styrene block copolymers [119]. Hot melts based on PHAs alleviate the dependence on petroleum based materials and allow the development of biodegradable alternatives based on natural raw materials. 

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