Polyhydroxybutyrate copolymers

Polyhydroxybutyrate. Copolymers of hydroxybutyrate and hydroxyvaler-ate possess many mechanical properties in common with synthetic polyolefins, and so have attracted much attention as replacements for these materials in environments where biodegradability is an important parameter (see Poly(3-HYDROXYALKANOATES)). Carswell-Pomerantz and co-workers (217,218) have reported a detailed study of the radicals formed on irradiation of such materials. They found that the yield of radicals at 77 K was G(R) = 1.7 0.2, independent of copolymer composition, but that on irradiation at 300 K, the yield of radicals was reduced for the copol5mers because of their lower glass-transition temperatures compared with the homopolymer. At low temperatures a significant contribution to the ESR spectra from radical anions was noted. These radicals were observed to decay on warming to produce scission radicals. At still higher temperatures, radicals produced by abstraction of a methylene proton adjacent to the carbonyl group were detected. 

After many years of research initiated in the 1970s, ICI introduced polyhydroxybutyrate-valerate copolymers in 1990. Because of their particularly interesting manufaeturing technology, these materials are dealt with in depth separately in the next section. 

Table 31.3 Some selected properties of polyhydroxybutyrate-valerate copolymers (Biopol—Zeneca)...

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

If the homopolymer decomposes at the fabrication temperature another approach is to make a copolymer that can be melt processed at a lower temperature. For example, polyhydroxybutyrate decomposes at the processing temperature (190°C), whereas the copolymer with valeric acid can be processed at 160°C without decomposition. These aliphatic polyesters are biodegradable and most importantly, the decomposition products are not toxic, hence their use in medical applications (e.g., sutures). 

Metabolix s PHBV (polyhydroxybutyrate valerate) was initially developed by ICI. PHBV and related copolymers are made in a pilot plant using different bacteria to create compositions with up to 70% crystallinity. Elongation can be manipulated from 5% to 100%, and melting points range between 135 and 185 °C (275-365 °F). 

Polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV) are also being researched for use in medical devices. The PHB homopolymer is crystalline and brittle, whereas the copolymers of PHB with PHV are less crystalline, more flexible, and easier to process. These polymers typically require... 

Polyhydroxybutyrate-valerate copolymer Biodegradable polyester used in degradable plastic... 

Though the interest for synthetic biodegradable polymers is still increasing, few of them are currently available. The best known products are polyhydroxybutyrate, poly(e-caprolactone), poly(glycolic acid), poly(lactic acid) and their copolymer. 

The two most common PHAs are polyhydroxybutyrate (PHB) and a copolymer of polyhy-droxybutyrate and polyhydroxy valerate (PHBV). PHAs can be used as films, fibers, and coatings for hot beverage cups made of paper. 

Another important family of biobased polyesters is the polyhydroxyalkanoates (PHAs) the most common members of this family are polyhydroxybutyrate (PHB) and its copolymer, polyhydroxybutyrate-valerate (PHBV). 

The nomenclature of PHAs and their classification are still in the process of evolving, because new structures are continuing to be discovered. The main biopolymer in the PHA family is a homopolymer polyhydroxybutyrate (PHB). The most conunon copolymer is poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV), whose chemical structure is presented in Figure 9.4. However, there are others, such as poly(hydroxybulyrate-co-hydroxyhexanoate) (PHBHx), poly(hydroxybutyrate-co-hydroxyoctanoate) (PHBO) and poly(ltydroxybulyrate-co-hydroxyoctadecanoate) (PHBOd), for example. 

Ahmed, T., Margal, H., Lawless, M., Wanandy, N. S., Chiu, A., Foster, L. J. R., Polyhydroxybutyrate and its copolymer with polyhydroxy-valerate as biomaterials Influence on progression of stem cell cycle. Binmocromoip.cuip.s. 10, 2707-2715 (2010), DOl 10.1021/ bml007579. 

Besides the common polyhydroxybutyrate (PHB), other polymers of this class are produced by a variety of organisms such as poly-4-hydroxybutyrate, PHV, and polyhy-droxyhexanoate. PHV is a naturally occurring bacterial polyester, which was first isolated by Wallen and coworkers [172,173]. Also, PHV/PHB copolymers have been studied to make a wide range of thermally processable polyesters, which exhibit the necessary long-term degradation profile required for a degradable fracture fixation device [174]. 

In the presence of limited nutrients, bacteria can be induced to make polyhydroxybutyrates and valerates, which are processed into a copolymer known as BiopoP. BiopoP has properties similar to polypropylene, but it is biodegradable and obtained from nonpetroleum sources. 

Polyhydroxybutyrate (PHB) is a polyhdroxyalkmioate polymer with pl tic-like properties that is synthesized by some soil bacteria (e.g. Alcaligenes eutrophus). The genes for the pathway have been cloned and inserted into plants with successful production of PHB. Additional research is required to optimize the properties the butyrate-valerate copolymer has superior functionality but it remains more difficult to express this material in plants compared to the pure polymer (75). Improved expression levels and localization would also assist in separations and in the economic cost to produce this renewable biodegradable plastic. 

Noda I, Green PR, Satkowski MM, Schechtman LA (2005) Preparation and properties of a novel class of polyhydroxyalkanoate copolymers. Biomacromolecules 6 580-586 Omidvar V, Akmar ASN, Marziah M, Maheran AA (2008) A transient assay to evaluate the expression of polyhydroxybutyrate genes regulated by oil palm mesocarp-spedfic promoter. Plant Cell Rep 27 1451-1459... 

Barham PJ, Keller A (1986) The relationship between microstructure and mode of fracture in polyhydroxybutyrate. J Polym Sd Polym Phys Ed 24 69-77 Bond EB (2003) Fiber spinning behavior of a 3-hydorxybutyrate/3-hydroxyhexanoate copolymer. Macromol Symp 197 19-31... 

Pouton CW, Majid MIA, Notarianni LJ (1988) Degradation of polyhydroxybutyrate and related copolymers. Proc Int Symp Control Release Bioact Mater 15 182-183 Ramsay BA, Saracovan I, Ramsay JA, Marchessault RH (1994) A method for the isolation of microorganism producing extracellular long-side-chain poly(P-hydroxyalkanoate) depolymerase). J Environ Polym Degrad 2 1-7... 

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