HYDROXYVALERIC ACID COPOLYMER

Hydroxybutyric acid-hydroxyvaleric acid copolymer membranes were treated with ozone and grafted with acrylic acid. The membranes were further grafted with chitosan or chitooligosaccharide via esterification and immobilised with hyalmonic acid. Antibacterial activities were investigated. Surface properties were characterised using a water contact angle goniometer. 26 refs. 

Another example where metabolic pathway engineering has made a dramatic impact is in the biodegradable polymer field. The polymer of this family most widely studied is poly-P-hydroxybutyrate (PHB) (46). Another member of the PHA family commercialized by Imperial Chemical Industries (ICI), which later became Zeneca under the trade name Biopol, is a copolymer consisting of p-hydroxybutyric acid and P-hydroxyvaleric acid. This biodegradable polymer was first used in plastic shampoo bottles by the Wella Corporation [198]. In the early part of 1996, the Biopol product line, was purchased from Zeneca by the Monsanto Company. 

The bacterially derived polymers of 3-hydroxyalkanoic acids, PHA, of which a copolymer of 3-hydroxybutyric and 3-hydroxyvaleric acids (BiopoP ) developed originally by ICI is a commercial example, are relatively crystalline materials whose crystallinity, melting temperature, etc. can be varied by increasing the length of the side-chain (R) ... 

Moreover, P3HB shows a crystalline structure because of its stereoregularity similarity to isotactic polypropylene. In order to improve mechanical properties and processability, and thus to obtain a more flexible material, P3HB copolymers are nowadays used. The first attempt to synthesize a random copolymer is a blend of 3-hydroxybutanoic acid and 3-hydroxyvaleric acid, obtaining poly(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV) copolymer. Other blends are obtained using, for example, hydroxyhexanoic acid, 4-hydroxybutanoic acid, and hydroxyoctanoic acid. Physical and mechanical properties of these copolymers have been reported (Lee and Park, 2005, Marchessault and Yu, 2005, Williams and Martin, 2005) as well... 

Thus, owing to the thermoplastic properties of PHAs and their biodegradability, they have attracted industrial interest, and bacteriologically produced PHB and its copolymers with 3-hydroxyvaleric acid are commercially available. Although PHAs are water-insoluble, hydrophobic and partially crystalline polymers, they can be degraded by a large variety of microorganisms. 

The immediate functional requirement is that of appropriate strength and rigidity to hold the tissue in place. However, healing processes take place in a relatively short space of time, in a matter of days for soft tissue and weeks for bones. Once it is complete, the suture or plate becomes redundant and indeed can impose undesirable constraints on the healed tissues. It may be preferable, therefore, for the material to be removed from the site, either physically or by dissolution or degradation. This probably represents the simplest and most obvious indication for intentionally degradable polymers. A variety of aliphatic polyesters, including homopolymers and copolymers of glycolic acid, lactic acid, hydroxybutyric acid and hydroxyvaleric acid are possibilities here. Several of these materials are now in clinical use for absorbable sutures. ... 

An example of a commercial biodegradable polyester is Biopol-a copolymer of hydroxybutyric and hydroxyvaleric acids, achieved during fermentation of sugars from sugar beet in the presence of bacteria that transform glucose into polymer. Biopol is one of the polymers with an ideal biodegradability profile, decomposing into carbon dioxide and water. Because of its stiff nature it is useful for bottles and canisters. 

Aliphatic polyesters such as PHAs, and more specifically homopolymers and copolymers of hydroxybutyric acid and hydroxyvaleric acid, have been confirmed to be readily biodegradable. Such polymers are actually synthesized by microbes, with... 

It is well known that certain kinds of bacteria, e.g., Azotobacter, Bacillus, Alcaligenes, and Zoogloea, accumulate high amounts of poly[(/ )-3-hydroxybutyrate] when they are cultured under limited nitrogen conditions. (/ )-p-Hydroxybutyric acid ester of high optical purity (100% e.e.) was obtained by alcoholysis of this biopolymer [9]. When propionic acid and glucose were used as the carbon sources, Alcaligenes eutrophus produced a copolymer of (/ )-p-hydroxybutyric acid and (/ )-P-hydroxyvaleric acid. (/ )-p-Hydroxy-valeric acid ester (100% e.e.) was obtained from this copolymer by a procedure similar to that described above [16] (Scheme 6). 

This is a copolymer consisting of hydroxybutyrate and hydroxyvalerate units incorporated randomly along the chain. The hydroxyvalerate content may be varied by adding controlled amounts of a simple organic acid. 

The following examples, found in R. eutropha, illustrate the formation of copolymers (cf. [37]). With propionic acid as an additional carbon source, the 3-ketothiolase catalyzes the condensation of the propionyl-CoA unit with acetyl-CoA to form 3-ketovaleryl-CoA, which is reduced to 3-hydroxyvalerate moieties and polymerized by the synthase [27]. 

Unfortunately, only a few short-chain-length poly(HA)s [poly(HASCL)], namely poly(3HB), its copolymers with 3-hydroxyvalerate, and poly(3-hy-droxyvaleric acid) [poly(3HV)], can be prepared as a milky suspension of de-... 

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). 

Alicagenes eutropha produces a copolymer of hydroxyvalerate and hydroxybutyrate when deprived of key nutrients, such as amino acids and minerals. The product, biopol, represents up to 90 percent of the dry weight of the bacterium. It is comparable to polypropene in physical properties, has better flexibility at low temperatures, and is biodegradable to CO2 and water within months. However, the polymer (trade name Biopol) is not currently cost-competitive with synthetic polymers because of the high costs of the fermentation substrates and the fermentation plants. 

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