Plastics are polymers, molecules formed of many identical units bound to each other like pearls in a necklace. For a polymer to be electrically conductive it must “imitate” a metal – the electrons in the bonds must be freely mobile and not bound fast to the atoms. One condition for this is that the polymer consists of alternate single and double bonds, termed conjugated double bonds. Polyacetylene is the simplest possible conjugated polymer. It is obtained by polymerisation of acetylene, shown to the left.