Thermal degradation behavior of poly[(R)-3-hydroxybutyrate-co-4-hydroxybutyrate]

•The thermal degradation of P(3HB-co-16 mol%−4HB) was investigated by its pyrolyzates and kinetics approaches.•The thermal degradation of P(3HB-co-16 mol%−4HB) proceeded at least in two steps.•Thermal degradation behavior of P(3HB-co-16 mol%−4HB) changed in time and/or temperature•Random degradation in the initial period changes to subsequent 0th-order weight loss meaning unzipping degradation of 3HB units.•The unzipping degradation of 3HB unit from the chain end may stop at the 4HB unit randomly present in the copolymer. The thermal degradation behavior of poly[(R)-3-hydroxybutyrate-co-16 mol%-4-hydroxybutyrate] (P(3HB-co-16 mol%-4HB)) was investigated based on spectroscopic analysis of its pyrolyzates and two kinetic modeling approaches. Pyrolyzate analysis revealed that trans-crotonic acid, cis-crotonic acid, γ-butyrolactone, and their oligomers (dimers and trimers) were produced following thermal degradation of P(3HB-co-16 mol%-4HB). The kinetic analyses confirmed that the thermal degradation behavior of this copolymer changed depending on the time and/or temperature parameters. Specifically, this degradation process proceeded in at least two steps, (i) initial random degradation and (ii) subsequent 0th-order weight loss, which indicated unzipping degradation of the 3HB units. Based on 13C NMR and two-dimensional NMR methods, it was determined that the unzipping degradation of 3HB units starting from the chain end may stop or volatilize an oligomer when the reaction reaches a 4HB unit randomly present in the copolymer.