QUANTUM-CHEMICAL PM-3 STUDY OF THE THERMAL-STABILITY OF HETEROCYCLIC FRAGMENTS OF HETEROPOLYMERS .3. THERMAL STABILITIES OF DIFFERENT COMPONENTS OF HETEROPOLYMER REPEATING UNITS

Dissociation energies (E(D)) of different components of repeating unit s of heteropolymers (HPs) have been calculated by the semiempirical PM -3 method. Comparison of E(D) values of heterocycles (HCs) and of sing le bonds contained in phenyl-substituted benzoheterocycles have shown that for polybenzazoles and HPs with carbonyl-containing HCs the initi al degradation in the absence of weak links (uncyclized units, structu ral defects, etc.) preferably starts with HC homolytic decomposition. For these two groups of HCs the relationship between the E(D) of HCs a nd the initial degradation temperature of corresponding HPs is, on the whole, similar to the one found upon calculation of benzoheterocycles . In HPs based on quinoline and quinoxaline the single bond between HC and phenyl (Ph) group is less stable than the HC, and the breaking of that single bond can accompany the initial HP degradation. An analysi s of relative stabilities of X-Ph bonds of Ph-X-Ph fragments of HPs wi th X = O, S, CH2, CO and of the relationship between their stabilities and those of other fragments of the HP repeating unit has been perfor med by using both calculated E(D) values and those obtained from therm ochemical data. E(D) values for Ph-X-Ph decompositions turned out to b e close to those for some HCs.