Thermal degradation behavior of bacterial poly(3-hydroxybutyrate-co-3-mercaptopropionate)

Poly(3-hydroxybutyrate-co-3-mercaptopropionate) [P(3HB-co-3MP)] is a bacterial polyhydroxyalkanoate (PHA) with thio/oxo ester linkages. In this study, the thermal degradation behavior of P(3HB-co-3MP) was investigated to understand the characteristic features of thermostable thioester linkage. To this end, P(3HB-co-3MP) copolymers were biosynthesized using Ralstonia eutropha H16 supplemented with 3,3′-dithiodipropionic acid as the 3 MP precursor. First, the comonomer composition, thermal properties, and crystalline component of the prepared copolymers were characterized by nuclear magnetic resonance, differential scanning calorimetry, and Fourier transform infrared spectroscopy, respectively. The change in molecular weight during repeated melting of the copolymers was also studied. A 6 mol% 3 MP copolymer showed molecular weight reduction via random polymer chain scission to an extent similar as that of P(3HB), whereas a 22 mol% 3 MP copolymer showed relatively suppressed molecular weight reduction. This observation indicated that thioester linkages conferred thermal stability to the polymer chain depending on its content. Furthermore, the volatile decomposition products of P(3HB-co-3MP) pyrolysis were also investigated. Because volatile decomposition products containing sulfur atoms were barely detected, P(3HB-co-3MP) may be suitable for chemical recycling via pyrolysis in the same way as P(3HB).