Electron delocalization contribution to single crystal thermal expansion of a polydiacetylene

The thermal expansion contribution due to temperature‐dependent π‐electron delocalization is evaluated from spectral measurements on a single crystal polydiacetylene (poly‐2,4‐hexadiyne‐1,6‐diol bisphenylurethane). The observed temperature independence of backbone associated vibrations (less than ±1 cm−1 change in νCC and νCC between 25 and 90°C) implies that thermal conformational fluctuations and equilibrium defect formation (which produce a negative thermal expansion coefficient) do not measurably affect π‐electron delocalization. The separation of equilibrium defects is either much longer than that of nonequilibrium defects or much longer than required to appreciably limit π‐electron delocalization in an effectively defect‐free polymer. Arguments presented indicate that, in the experimental temperature interval, the observed thermal expansion coefficient in the chain direction is over an order of magnitude larger than the delocalization‐associated contribution.