AbstractThe activation enthalpies for the structural relaxation of poly(oxy‐2,6‐dimethoxy‐1,4‐phenylene) and poly(oxy‐2,6‐dimethyl‐1,4‐phenylene) were determined by studying the dependence of the limiting fictive temperature on the cooling rate, using a differential scanning calorimeter (DSC) interfaced to a computer. The molar mass dependence of the activation enthalpies complied with the empirical equation, ΔH˙= ΔH ∞˙−A/M va, where ΔH ∞˙is the activation enthalpy of an infinite chain length polymer, Mvis the viscosity‐average molar mass,a= 0,64, andAis a constant that depends on the free volume of the chain ends of the polymers. The values of ΔH ∞˙are 433 kJ/mol for poly(oxy‐2,6‐dimethoxy‐1,4‐phenylene) and 44