Effect of metal compounds on thermal degradation behavior of aliphatic poly(hydroxyalkanoic acid)s

Effect of metal compounds on the thermal degradation behaviors of poly(3-hydroxybutyric acid) (P(3HB)), poly(4-hydroxybutyric acid) (P(4HB)), and poly(ɛ-caprolactone) (PCL) was investigated by means of thermogravimetric and pyrolysis-gas chromatograph mass spectrometric analyses. Na and Ca compounds accelerated a random chain scission of P(3HB) molecules resulting in a decrease of thermal degradation temperature, whereas the contribution of Zn, Sn, Al compounds to the thermal degradation of P(3HB) was very small. In contrast to P(3HB), Zn, Sn and Al compounds induced the thermal degradation of PCL at lower temperature range by catalyzing the selective unzipping depolymerization from ω-hydroxyl chain end. Transesterification reaction of PCL molecules could be facilitated by the presence of Ca compound, while the gravimetric change was detected at almost identical temperature region regardless of the content of Ca compound. According to the lactonizing characteristic of monomer unit, the thermal degradation of P(4HB) progressed by the cyclic rupture via unzipping reaction from the ω-hydroxyl chain end or/and random intramolecular transesterification at the main chain with a release of γ-butyrolactone as volatile product. Each of metal compounds used in this study was effective to catalyze the cyclic rupture of P(4HB) molecules, and the degradation rate was accelerated by the presence of metal compounds.