QUANTUM-CHEMICAL PM-3 STUDY OF THE THERMAL-STABILITY OF HETEROCYCLIC FRAGMENTS OF HETEROPOLYMERS .2. 6-MEMBERED HETEROCYCLES

Dissociation energies (ED) of various heterocyclic fragments of repeat ing units of thermally stable heteropolymers have been calculated by t he semiempirical PM-3 method. Fragments with heterocycles containing c arbonyl and/or amine groups and/or oxygen (or sulfur) atoms (benzazole s, phthalimides, benzoxazinones, etc.) have fairly close ED values. Th e correlation between ED values of fragments and the initial degradati on temperature (TD) of heteropolymers comprising these fragments revea ls that the TD values depend both on ED of corresponding heterocycles and on the manner of heterocycle decomposition: whether it leads to an immediate weight loss (polyimides, polybenzoxazinone, etc.) or just t o a chain scission (polybenzazoles). Quinoline and quinoxaline are cha racterized by higher ED values, but it seems that decomposition of the se compounds during the initial degradation of polyquinoline and polyq uinoxaline proceeds via mechanisms different from the straightforward homolytic decomposition characteristic of other considered heterocycle decompositions.