The formation of ordered polydiacetylenes (PDAs) from conjugated triynes and longer polyynes has proven challenging due to the low stability of the starting materials and the possibility of varying regiochemistry for the polymerization. We report here the preparation of host-guest cocrystals of diiodohexatriyne and diiodooctatetrayne, each with a bis(nitrile)oxalamide host. Single-crystal X-ray diffraction studies show that the halogen-bonding interactions between the host and guest align the diiodopolyyne monomers with the proper parameters for 1,4-topochemical polymerization. Using Raman spectroscopy, solid-state C-13 MAS NMR, and single-crystal X-ray diffraction, we have demonstrated the formation of a single highly ordered PDA, poly(iodoethynyliododiacetylene) (PIEDA), from the 1,4-polymerization of diiodohexatriyne. Diiodooctatetrayne also forms ordered cocrystals, but attempts to form polymer from these crystals led to disordered species. This work represents the first reported single-crystal-to-single-crystal 1,4-polymerization of a conjugated triyne.