Separation and Structural Characterization of Cyclic and Open Chain Oligomers Produced in the Partial Pyrolysis of Microbial Poly(hydroxybutyrates)

Abstract

To explore the possibility of using the partial pyrolysis of microbial poly(hydroxybutyrates) (PHBs) for obtaining single 3HB, 4HB, and mixed 3HB/4HB oligomers, the pyrolysis of P(3HB), P(4HB), and P(3HB-co-4HB) was performed and the products obtained were analyzed by HPLC and fast atom bombardment mass spectrometry (FAB-MS). Our results show that the partial pyrolysis of P(3HB) and P(4HB), followed by HPLC separation of the products, affords a suitable route for the synthesis of linear 3HB oligomers and of 4HB oligolides, respectively. The thermal degradation of P(3HB), P(4HB), and P(3HB-co-4HB) copolymers containing 3-34% of 3HB units was investigated by using several techniques: monitoring the time-dependent changes in molecular weight of melt samples in the range 180-200 °C ; direct pyrolysis mass spectrometry ; preparative pyrolysis and subsequent IR, NMR, HPLC, and FAB-MS analysis of the pyrolysate. Contrary to the claim that the rate constant (K d ) of chain scission is not influenced by the composition of the copolyesters, our results show that the rate of chain scission is influenced by the composition of copolyesters and that a mixture of cyclic and open chain oligomers is formed in the pyrolysis process. These findings indicate that P(3HB-co-4HB) copolymers undergo thermal degradation by two competitive random chain scission mechanisms : a β-CH hydrogen transfer at the 3HB units and an ester exchange cyclization at the 4HB units.