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 units of heteropolymers (HPs) have been calculated by the semiempirical PM-3 method. Comparison of E_D values of heterocycles (HCs) and of single bonds contained in phenyl-substituted benzoheterocycles have shown that for polybenzazoles and HPs with carbonyl-containing HCs the initial degradation in the absence of weak links (uncyclized units, structural defects, etc.) preferably starts with HC homolytic decomposition. For these two groups of HCs the relationship between the E_D of HCs and 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 analysis of relative stabilities of X-Ph bonds of Ph-X-Ph fragments of HPs with X = O, S, CH₂, CO and of the relationship between their stabilities and those of other fragments of the HP repeating unit has been performed by using both calculated E_D values and those obtained from thermochemical data. E_D values for Ph-X-Ph decompositions turned out to be close to those for some HCs.