Translational diffusion of poly-2,5-(1,3-phenylene)-1,3,4-oxadiazole (PMOD) in solution in 96% sulphuric acid was studied,
and intrinsic viscosity was measured at different stages of thermal degradation. Polymer solution has previously been subjected
to heating at temperature ranging from 75 to 104C and then investigated at 26C. A monotonic decrease in intrinsic viscosity
and the molecular mass, M, of degraded products with increasing degradation temperature was detected. The rate constant of the degradation process
has been obtained from the change in M of the degradation products with time at a fixed solution temperature, and the activation energy of the process was calculated
by using the temperature dependence of the rate constant. The activation energy (E =1028 kJ–1 ) is close to that obtained previously for the hydrolysis of poly-2,5-(1,4-phenylene)-1,3,4-oxadiazole (PPOD) in sulphuric
acid (106 kJ–1 ), the rate constant being approximately twice in the value.