THEORY OF COUPLED ELECTRONIC-CONFORMATIONAL TRANSITIONS OF CONJUGATED POLYMERS.

Order-disorder transformations of flexible conjugated polymers in dilute solution are characterized by a dramatic modification of polymer conformation over a narrow range of temperature in a pseudo-first-order fashion. A variety of soluble pi -conjugated polydiacetylenes and sigma -conjugated polysilanes have been discovered to undergo rod-to-coil transitions. In addition, true order-disorder phase transitions have been observed in solid films of the polydiacetylenes and polysilanes. The conformational changes in both films and solution are accompanied by large spectroscopic changes indicative of a major modification of the polymer electronic structure at the transition. For the nonpolar conjugated polymers the general features of the phenomena are remarkably insensitive to system-specific details such as solvent quality, substituent structure, and polymer solubility. These observations collectively suggest that there may be a universal physical mechanism responsible for the order-disorder transformations.