Studies in heterocyclic polymers. Part VI. A weight-loss study of the formation of a polybenzoxazole

The cyclisation of an aromatic poly(o-hydroxyamide) to form polybenzoxazole has been studied by combining isothermal weight-loss and thermogravimetric analyses. The thermogravimetric data were analysed by the method of Gyulai and Greenhow [4] to derive values for the apparent activation energy of the cyclisation process which was found to double at about 50% conversion (74–155 kJ mole^?1. A similar result was observed when the process was studied in the presence of low concentrations of p-toluene sulphonic acid, which appears to have a moderate catalytic effect upon the cyclisation process. The apparent order of reaction of the process has been found to be close to 2, contrary to previous studies which have assumed it to be unimolecular.