Mechanism of high thermal stability of commercial polyesters and polyethers conjugated with bio‐based caffeic acid

In previous report, we discovered that a novel improvement technique to enhance the thermal properties of poly(L‐lactide)s (PLLAs) by terminal conjugation with 3,4‐diacetoxycinnamic acid (DACA). In this study, we clarified the mechanism of the enhancement of thermal stability by using commercial polyesters and polyethers. The effect of thermal improvement by the terminal conjugation of DACA on poly(DL‐lactide), poly(ε‐caprolactone), and poly(ethylene glycol) was almost the same as about 100 °C increase. The amount of residual tin catalyst, which enhances the thermal degradation of polyesters, was reduced at undetected level after the terminal conjugation of DACA probably due to the removal of tin during DACA conjugation process. Furthermore, the π‐π stacking interactions of DACA units and the chemical protection of terminal hydroxyl groups, which enhances intramolecular scission, were also important for the high thermal stability. We clarified that the extreme high thermal stability by DACA conjugation was induced by these above mechanisms.