The synthesis of conducting polymers based on heterocyclic compounds

Polymers are best known for their effectiveness as electrical insulators, indeed electrical wiring throughout the world is now sheathed in plastic. However, it was recognized early on that polymers with an appropriate structure ought to be able to conduct electricity. Unfortunately, the same features that might allow this phenomenon also introduce intractability and processing difficulties. As a consequence, it was not until the mid-1970s that the potential of these materials was explored and better-defined materials started to be made. There are now numerous polymers with substantial electrical conductivities and the topic of electrically conducting polymers still continues to excite with many hundreds of new publications printed each year. The backbone structures of some of conjugated polymers are given in Table 6.1. In this chapter we shall deal with electrochemical and chemical syntheses of some relatively simple examples. For electrical conductivity, it is necessary to transfer charge along a conjugated chain, between chains, and also along grain boundaries or between particles. The most energetically difficult process will control the rate of charge transport and this will vary with nature of the polymer, its physical form, and other parameters, but in all cases conjugation along the chain is necessary although it is not sufficient for carbonaceous polymers to simply possess a conjugated chain. To promote conductivity π-overlap along the entire polymer chain length is required to give a half-filled band of delocalized π -electrons. In real systems, distortions of the bonds disrupt the conjugation, and the materials are generally semiconductors. The higher metallic conduction can be achieved by a process known as doping in which electrons are added or more generally removed from the conjugated system (although this is not same as the doping process found in semiconductor technology) The simplest conjugated polymer chain is a polyacetylene chain. Such materials can be prepared by coordination polymerization, or using a sophisticated route involving the degradation of a soluble precursor polymer.