13
4-Hydroxybutyrate
The hydroxy fatty acid 4-hydroxybutyrate (4HB) is a monomer unit in polyhydroxyalkanoates (PHAs) of microbial origin. PHAs are biodegradable, biocompatible, and thermoplastic polymers, which do not undergo abiotic hydrolysis in
aqueous medium. PHA properties—molecular weight, crystallinity, and mechanical
strength—vary according to monomer composition and proportion, which are controllable by specialized culture supplemented medium and by the genetic modification of substrates metabolic transformations in natural and unnatural PHA-producer
strains. In spite of such challenges, 4HB-based biopolymers (Doi et al. 1990)
(Vigneswari et al. 2009) as well as ter- or quarter polymers (Volova et al. 2013,
2016, 2017) have elicited a growing number of studies. Of various copolymers containing 4HB, poly(3- hydroxubutyrate- co-4-hydroxybutyrate) copolymers, P(3HBco-4HB), are of particular interest. P(3HB/4HB)-based devices designed by Tepha
were first-in-class solutions approved by the US Food and Drug Administration for
clinical applications. The degree of crystallinity—the ratio of amorphous to crystalline regions in copolymers—was found to decrease as the fraction of 4HB into
P(3HB/4HB) increased; indeed, biopolymers containing 4HB between 10.4 and
75.0 mol% featured a degree of crystallinity below 55% (Zhila and Shishatskaya
2018). Furthermore, the analysis of ter-polymer P(3HB/3-HV/4HB) demonstrated
the combined effect of 4-hydroxybutyrate and 3-hydroxyvalerate (3-HV) to be more
pronounced than that achievable by either single monomer. The melting temperature of PHAs chaining 4HB with 3HB monomer units was found to decrease as the
fraction of 4HB increased between 10.4 and 75.0 mol%, whereas the temperature of
thermal degradation was unaffected. In copolymers with 4HB content at 10 and
15 mol%, the glass transition temperature was, respectively, 3.4 °C and
−2.6 °C. Copolymers with 4HB content between 29.3 and 75.0 mol% did not show
any glass transition regions. The lower crystallization temperature of P(3HB/4HB)
compared to P(3HB) makes biopolymers able to be readily processed owing to the
maintenance of ductility for longer time periods. The trends observed in melting
and thermal degradation temperature were confirmed in ter-polymers and quarter
polymers, whereas the crystallization temperature was higher than that of P(3HB).
As to the microstructural properties, copolymer-based film surfaces resulted porous
and uneven and more hydrophilic than the P(3HB)-based ones. Finally, copolymers
showed higher elastic properties expressed by elongation at break and lower
mechanical strength expressed by Young’s modulus and tensile strength.
Hydrogen-oxidizing bacteria under autotrophic conditions, which obtain the carbonaceous source from carbon dioxide fixation and the energetic source from
hydrogen oxidation, are important candidates for the commercially suitable production of PHA-based copolymers. However, rather few published studies report the
synthesis of 4HB-containing PHA in autotrophic conditions (Volova et al. 2002).
Volova et al. (2013) managed to produce various PHA copolymers from Cupriavidus
eutrophus B-10646 by handling the supplementation of substrates to the culturing
gaseous medium (CO 2 , CO, and H 2 ) and the duration of cultivation after addition. In
1 Use of Carbon Dioxide in Polymer Synthesis
4-Hydroxybutyrate
The hydroxy fatty acid 4-hydroxybutyrate (4HB) is a monomer unit in polyhydroxyalkanoates (PHAs) of microbial origin. PHAs are biodegradable, biocompatible, and thermoplastic polymers, which do not undergo abiotic hydrolysis in
aqueous medium. PHA properties—molecular weight, crystallinity, and mechanical
strength—vary according to monomer composition and proportion, which are controllable by specialized culture supplemented medium and by the genetic modification of substrates metabolic transformations in natural and unnatural PHA-producer
strains. In spite of such challenges, 4HB-based biopolymers (Doi et al. 1990)
(Vigneswari et al. 2009) as well as ter- or quarter polymers (Volova et al. 2013,
2016, 2017) have elicited a growing number of studies. Of various copolymers containing 4HB, poly(3- hydroxubutyrate- co-4-hydroxybutyrate) copolymers, P(3HBco-4HB), are of particular interest. P(3HB/4HB)-based devices designed by Tepha
were first-in-class solutions approved by the US Food and Drug Administration for
clinical applications. The degree of crystallinity—the ratio of amorphous to crystalline regions in copolymers—was found to decrease as the fraction of 4HB into
P(3HB/4HB) increased; indeed, biopolymers containing 4HB between 10.4 and
75.0 mol% featured a degree of crystallinity below 55% (Zhila and Shishatskaya
2018). Furthermore, the analysis of ter-polymer P(3HB/3-HV/4HB) demonstrated
the combined effect of 4-hydroxybutyrate and 3-hydroxyvalerate (3-HV) to be more
pronounced than that achievable by either single monomer. The melting temperature of PHAs chaining 4HB with 3HB monomer units was found to decrease as the
fraction of 4HB increased between 10.4 and 75.0 mol%, whereas the temperature of
thermal degradation was unaffected. In copolymers with 4HB content at 10 and
15 mol%, the glass transition temperature was, respectively, 3.4 °C and
−2.6 °C. Copolymers with 4HB content between 29.3 and 75.0 mol% did not show
any glass transition regions. The lower crystallization temperature of P(3HB/4HB)
compared to P(3HB) makes biopolymers able to be readily processed owing to the
maintenance of ductility for longer time periods. The trends observed in melting
and thermal degradation temperature were confirmed in ter-polymers and quarter
polymers, whereas the crystallization temperature was higher than that of P(3HB).
As to the microstructural properties, copolymer-based film surfaces resulted porous
and uneven and more hydrophilic than the P(3HB)-based ones. Finally, copolymers
showed higher elastic properties expressed by elongation at break and lower
mechanical strength expressed by Young’s modulus and tensile strength.
Hydrogen-oxidizing bacteria under autotrophic conditions, which obtain the carbonaceous source from carbon dioxide fixation and the energetic source from
hydrogen oxidation, are important candidates for the commercially suitable production of PHA-based copolymers. However, rather few published studies report the
synthesis of 4HB-containing PHA in autotrophic conditions (Volova et al. 2002).
Volova et al. (2013) managed to produce various PHA copolymers from Cupriavidus
eutrophus B-10646 by handling the supplementation of substrates to the culturing
gaseous medium (CO 2 , CO, and H 2 ) and the duration of cultivation after addition. In
1 Use of Carbon Dioxide in Polymer Synthesis