Polypyrrole: An Electrochemical Approach to Conducting Polymers

Until recent times, almost all of the known organic polymers were essentially electrically insulating, with room-temperature conductivities of 10−10 ohm−1 cm−1 or less. The desirability of having low-density, flexible, processible conductors provided the impetus for finding ways of enhancing the electronic conductivity of polymers. The electrical and optical properties of these materials depend on the electronic structure and basically on the chemical nature of the repeating unit. The general requirements of the electronic structure in these polymers were recognized and described many years ago.1 The electronic conductivity is proportional to both the density and the drift mobility of the carriers. The carrier drift mobility is defined as the ratio of the drift velocity to the electric field and reflects the ease with which carriers are propagated. Enhancing the electrical conductivity of polymers then requires an increase in the carrier mobility and the density of charge carriers. The particular importance of the delocalized π-electrons to form energy bands of high-mobility carriers was stressed early on2 and a large number of polymers were considered as having this characteristic.3 These same considerations have been discussed from a somewhat broader perspective that include polymeric charge-transfer complexes and organometallics.4 The dramatic conductivity enhancements reported recently5 in polyacetylene when treated with strong oxidants have spurred a resurgence of interest in these systems including several articles reviewing the electronic structure of these polymers.6–8