Unsaturated PHAs

Most PHAs are partially crystalline polymers and therefore their thermal and mechanical properties are usually represented in terms of the glass-to-rubber transition temperature (Tg) of the amorphous phase and the melting temperature (Tm) of the crystalline phase of the material [55]. The melting temperature and glass transition temperature of several saturated and unsaturated PHAs have been summarized in Table 2. 

As an example, bulk modification by the organic reaction of unsaturated PHA with sodium permanganate resulted in the incorporation of dihydroxyl or carboxyl functional groups [106]. Due to the steric hindrance of the isotactic pendant chains, complete conversion could not be obtained. However, the solubility of the modified polymers was altered in such a way that they were now completely soluble in acetone/water and water/bicarbonate mixtures, respectively [106]. Solubility can play an important role in certain applications, for instance in hydrogels. Considering the biosynthetic pathways, the dihydroxyl or carboxyl functional groups are very difficult to incorporate by microbial synthesis and therefore organic chemistry actually has an added value to biochemistry. 

The quantitative bulk conversion of unsaturated functional groups in PHAs to epoxides has been achieved by reaction with m-chloroperbenzoic acid as the chemical reagent [107]. No chain scission of the macromolecular chain was observed. Epoxy-modified PHAs are chemically even more reactive than unsaturated PHAs and therefore could be useful in further chemical reactions (e.g. grafting of therapeutic important substances) [108]. 

Another way of modifying unsaturated PHAs in the bulk is by crosslinking of the material. This has been accomplished by either chemical reaction with sulfur or peroxides [109, 110], or by radiation curing [91, 111]. In all cases, crosslinking altered the ultimate material properties drastically, yielding a true rubbery material. The advantages of applying rubbers from crosslinked PHAs over the use of current rubbers will be elaborated in Sect. 4.5. 

UnPHAs Containing Unsaturated or Functionalized Monomers Unsaturated PHAs... 

Kurth et al. 2002 Stigers and Tew 2003). One of them involves the oxidation of unsaturated PHAs i.e. poly[(/ )-3-hydroxyoctanoate-co-(/ )-3-hydroxyundecenoate] with KMnO in the presence of NaHCOj (Lee and Park 2000). Although it allowed the transformation of 50% of the olefins into carboxylic functions, this method involves a decrease in the molecular weight of the polymer. 

Largely unsaturated PHAs have been cross-linked by a very slow chemical procedure (it usually takes days), which implies the conversion of double bonds into epoxy groups and exposure to air (Ashby et al. 20(X)). Other epoxidized bacterial copolyesters, poly[(/ )-3-hydroxyoctanoate-co-(/ )-3-hydroxy-10,ll-epoxyunde-canoate], were cross-linked with succinic anhydride in the presence of 2-ethyM-methylimidazole or with hexamethylene diamine without a catalyst at 90°C (Lee et al. 1999a Lee and Park 1999). 

The dual (C, N) limited growth (D=0.1 h , C/N=16 mol mol ) was used to produce functionalized mcl-PHA in P. putida GPol (Zinn and Hany 2005 Zinn et al. 2004). A given amount of 10-undecenoic acid in the carbon feed resulted in the formation of an identical proportion of olefmic (terminally unsaturated) PHA monomers in the polymer (Table 2). Furthermore, recent experiments revealed... 

The carbojylation of unsaturated PHA has been performed through reactions that involved the conversion of the double bonds to thioethers via the free-radical addition of 11-mercaptoundecanoic acid or (R)-3-mercaptopropionic acid. Transesterification reactions of poly(3-hydro y butyrate) were carried out under reflux of 1,2-dichlorobenzene in the presence of 1,4-butane diol, poly(ethylene glycol) bis(2-aminopropyl ether) with molecular weights of 1000 and 2000, poly(ethylene glycol)methactylate or glycerol at 180 °C. Addition reactions of bromine and the -SH groups of 3-mercaptopropionic acid to the double bond of poly(3-hydro)y-10-undecenoate) were also carried out. The molecular weights of the modified polymers (despite the addition of mercapto acids to the double bonds) remained almost constant. " ... 

Figure 7.2 Oxidative carboxylation of unsaturated PHA using KMn04 as an oxidation agent.

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