Chain cleavage mechanism of palm kernel oil derived medium-chain-length poly(3-hydroxyalkanoates) during high temperature decomposition

Medium-chain-length poly(3-hydoxyalkanoates) (mcl-PHAs) are natural polyesters having the molecular formula of [∼OCHR-CH2-COO∼]n, composed of monomers each having 6 to 14 carbon atoms, with relatively bulky alkyl pendant group. They are primarily synthesized by fluorescent pseudomonads under unbalanced nutrient availability as carbon and energy storage compounds. From thermogravimetic analysis, the mcl-PHAs produced from Pseudomonas putida grown on palm kernel oil showed a drastic decomposition at temperature above 195 °C. An increase of HO-CHR∼∼ terminals was observed when the mcl-PHAs were thermally decomposed at 160-180 °C, which was presumably due to the hydrolytic chain cleavage initiated at -COO-CHR∼∼, the main skeletal bonds of the biopolyester. At 190 °C, molecules with RCH≡CH∼∼ were also observed as minor decomposition products from the FTIR and NMR measurements and this could be probably due to the dehydration of HO-CHR∼ groups of hydroxyl acids, producing terminally unsaturated oligomers. These observations were consistent with the detection of three new unsaturated monomers: C4:1, C6:1 and C8:1 in the methanolyzed mcl-PHAs in gas chromatography analysis.