Formation of thermally rearranged (TR) polybenzoxazoles: Effect of synthesis routes and polymer form

This study demonstrates that thermal treatment of ortho-hydroxy containing polyimides (HPIs) in the solid state certainly leads to the formation of polybenzoxazoles (PBOs). Thermal conversion protocol, including temperature and dwell time, and the form of the sample (film or powder) determine the rearrangement reaction rate of HPI into PBO. Thermal rearrangement kinetics seems to be faster for film samples rather than for powder ones. Also, mild thermal treatment conditions (i.e., low temperatures and short times) seem to result in negligible or low degrees of conversion to PBO. Moreover, synthetic routes of HPI do not alter in any way the thermal conversion pathway. These findings validate the widely reported imide-to-benzoxazole thermal rearrangement mechanism, and contradict the alternative rearrangement pathway, proposed recently, of HPI into poly(biphenylene bisimide) polymers.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Thermal rearrangement reaction from hydroxyl polyimide in the solid state leads to polybenzoxazole. ► Thermal conversion protocol determine the rearrangement reaction rate. ► Reaction kinetics are faster for film rather than powder. ► Mild thermal treatment conditions result in low degree of conversion to PBO.