Thermal Stability of High Performance Poly (aryl ether sulfones): Structure/Reactivity Relationships in the Pyrolysis of Oligomeric Model Compounds

The model oligomers 2,2-bis[[4-(phenylsulfonyl)phenoxy]phenyl]propane (ISO) and 4,4′-bis[4- (phenylsulfonyl)phenoxy]-1,1′ -biphenyl (BP) were reacted neat in argon at 425 °C to compare the thermal stability of poly (aryl ether sulfones). Sulfur dioxide was the major gas product, BP generating a slightly higher evolution rate. The weight-average molecular weight increased with reaction time for both oligomers, but after 60 min, the rate of increase was higher for BP. A quantitative molecular level explanation of these observations focuses on the reactive isopropylidene link in ISO. Fission of the weak Ph-SO₂-Ph bond produces phenyl radicals that abstract hydrogen and add to the oligomer backbone. The latter reaction results in increases in M_w and a net weakening of the aryl-SO₂ bond, which leads to increased rates of SO₂ evolution. The value of the relative rate, R_rel, of radical addition to H-abstraction was higher for BP than for ISO due to easily abstractable isopropylidene hydrogens.