QUANTUM-CHEMICAL PM-3 STUDY OF THE THERMAL-STABILITY OF HETEROCYCLIC FRAGMENTS OF HETEROPOLYMERS .1. 5-MEMBERED HETEROCYCLES - AZOLES AND IMIDES

Semiempirical PM-3 calculations of the mechanisms of the decomposition of oxazole and maleimide have been carried out. It has been shown tha t thermal decomposition of the considered heterocycles proceeds via a consecutive breaking of two ring bonds, the breaking of the second bon d being the rate-determining step. Both an activation barrier of heter ocycle decomposition and its dissociation energy (ED) can be used as a measure of heterocycle thermal stability. The correlation between ED values of azole and imide containing model compounds and the initial d egradation temperatures of corresponding heteropolymers makes it possi ble to clarify some essential features of the thermal degradation of p olyazoles, polybenzazoles and polyimides.