Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 39

polar solvents including an 80 : 20 acetone-water mixture, methanol and
DMSO, indicating a considerably enhanced hydrophilicity of the modified
PHA. 89 . Despite the fact that the KMnO 4 treatment allowed the hydroxylation
of about half of the pendant vinyl groups present in the PHA, the reaction
performed with 9-borobicyclononane (a hydroboration-oxidation reaction)
hydroxylated most of the vinyl groups (almost 100%) present in poly[(R)-3-
undecenoate]. Molecular weight and polydispersity of hydroxylated PHU
were found to be 10 000 and 1.23, respectively, while those of the originals
were 32 000 and 2.42. The decomposition temperatures of the original and
hydroxylated PHU were 280 and 200 1C, respectively. 112
The carboxylation 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 113 or (R)-3-
mercaptopropionic acid. Transesterification reactions of poly(3-hydroxy
butyrate) were carried out under reflux of 1,2-dichlorobenzene in the pres-
ence of 1,4-butane diol, poly(ethylene glycol) bis(2-aminopropyl ether)
with molecular weights of 1000 and 2000, poly(ethylene glycol)methacrylate
or glycerol at 180 1C. Addition reactions of bromine and the -SH groups
of 3-mercaptopropionic acid to the double bond of poly(3-hydroxy-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. 114
Copolymers of PHAs containing repeating units with unsaturated or
brominated pendant side chains have been obtained from cultures of
Pseudomonas oleovorans grown on mixtures of octanoic acid and undecenoic
acid or 11-bromoundecanoic acid as carbon sources. These polymers,
bearing reactive functionalities, have been used to graft acetylated maltosyl
units either by anti-Markovnikov addition to the double bond or SN2 sub-
stitution of the halogen. De-O-acetylation of the sugar moieties yielded PHAs
with new properties. 115
All the chemical modifications will significantly change the properties of
PHAs. Table 2.5 shows some typical changes in properties resulting from the
modification of PHA MCL.

d n 2 r 4 n g | 8

.

2.5 Outlook
Different metabolic pathways guide the production of different types of
PHAs. Thus the choice of carbon source for the production of these PHAs has
unequal importance for PHA SCL and PHA MCL . For PHA MCL , the choice of the
carbon source is crucial for the final structure of the biopolymer. On the
contrary, for PHA SCL , almost any carbon source can be used. Thus, more in
depth studies in this field were performed for PHA SCL . It was thus shown
that the Cupriviadus genus has great potential for this type of production.
Recent work on mixed microbial cultures has also shown an interesting
perspective, especially knowing that these systems are usually cheaper to
establish. Also, for a long time, the assumed hypothesis of the necessity of