AbstractThe thermal expansion coefficient of poly(oxy‐2,6‐dimethoxy‐1,4‐phenylene), PPOO, was measured from 150 to 500 K, using a thermomechanical analyzer connected to a computer for the data handling. Poly(oxy‐2,6‐dimethyl‐1,4‐phenylene), PPO, and some brominated poly(oxy‐2,6‐disubstituted‐1,4‐phenylene)s were studied as reference materials. The apparent glass transition temperatures of the high molar mass polymers were 456 K for PPOO and 487 K for PPO, and the changes in cubic thermal expansion coefficients atTgwere 6,01 · 10−4K−1for PPOO and 4,23 · 10−4K−1for PPO. The activation of the β‐relaxation was found for PPOO at 260 K and attributed to the hindered torsional oscillations of the backbone phenylene groups. Another transition was found at 160 − 170 K and interpreted as the γ‐relaxation due to the oscillatory motion of the methoxy groups. Reasons for the differences in the thermal expansio