Thermal degradation mechanism of poly(hexamethylene carbonate)

Thermal degradation behaviors of poly(hexamethylene carbonate) (PHC) were investigated by means of thermogravimetric analysis (TGA), proton nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Six types of PHC samples with different end groups and molecular weights were synthesized to systematically investigate the thermal degradation mechanism. Thermal degradation behaviors of the PHC samples were examined under both isothermal and non-isothermal conditions. The PHC samples showed distinct thermal degradation behaviors from other aliphatic polycarbonates (poly(propylene carbonate), poly(trimethylene carbonate) and poly(butylene carbonate)). The results indicated that the chain-end structure makes a slight effect on the thermal stability of PHC regardless of the molecular weight. During the non-isothermal degradation of PHC, four main reactions were illustrated: unzipping, intramolecular transesterification, β-H transfer and decarboxylation reactions. Intramolecular transesterification reaction dominantly occurs below 300 °C accompanying with unzipping reaction which can only be induced by hydroxyl end group, and releasing cyclic hexamethylene carbonate monomer and dimer. Above 300 °C, the four degradation reactions take place simultaneously.