The structure of a synthetic nanotube prepared by the solid-state polymerization of a stacked column of diacetylene-based macrocycles has been determined. A polyether macrocycle monomer with two parallel diacetylene functionalities was prepared. Its crystal structure revealed that the compound crystallizes with structural parameters suitable for topochemical polymerization. Slow annealing of a single crystal for 35 days brought about a single-crystal-to-single-crystal polymerization resulting in the first experimentally determined structure of a tubular addition polymer.