Synthesis of a water-soluble polydiacetylene has been achieved by topochemical polymerization in the solid state of the bis(N-methylimidazolium)diacetylene monomer. Structural characterization for the monomer by X-ray diffraction and NMR spectroscopy supports a photopolymerization initiated at the surface. Characterization of the polymer (NMR, UV and Raman spectroscopy, and dynamic light scattering) is given along with a molecular modelling interpretation of the polymerization in the solid state.