Polydiacetylenes (PDA's) were among the first side chain substituted conducting polymers studied and are deserving of special mention. In many ways the structural properties of these PDA's are very similar to the P3AT and other more recent systems with one remarkable exception; many PDA's can be prepared in essentially single crystal form[,] through a solid-state polymerization. The actual process is rather straightforward. Singles crystals of the monomer are prepared by slow precipitation of the monomer in solution. Subsequent exposure to intense an ultra-violet, x-ray or -ray sources initiates the reaction process which directly couples the conjugated diacetylenic linkages to form a single polymer chain. The resulting polymers are both soluble and fusible.
The thermal behavior of the materials is also quite interesting[,]. At temperatures somewhat below the isotropic melt temperature, Tm, the main chains and side chains can undergo a reversible conformational transition. However, if heated above Tm the single crystal structure can no longer be recovered and another solid-state structure is obtained. A similar result is obtained if the resultant polymers are cast from solution.