An iterative synthetic route for obtaining symmetric polyynes was developed, consisting of a series of iodination and Stille coupling reactions. The starting materials employed in this pathway are simple and can be prepared easily. Polyynes containing up to seven C≡C bonds were synthesized using this method. This route is particularly effective for accessing polyynes with an odd number of C≡C bonds and has allowed for the synthesis of a new iodine‐capped polyyne, diiododecapentayne.

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 13C 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.

The cover picture shows an iterative synthetic method towards symmetric polyynes. This route encompasses a series of iodination and Stille coupling reactions, in which each cycle symmetrically increases the length of the resulting polyyne by four carbon atoms. The silyl end groups (blue circles) are iodinated by using AgNO3 and NIS (N‐iodosuccinimide), resulting in relatively stable diiodopolyyne (red circles) intermediates. A variety of tin acetylides, depicted by a semicircle, are mixed with the diiodoalkyne in a symmetric double cross‐coupling reaction to produce a polyyne that is increased by two C≡C bonds in length. With this method, tetraynes, pentaynes, hexaynes, and a heptayne have been prepared. Details of the Stille coupling conditions and symmetric polyynes that have been accessed by using this method are discussed in the article by N. S. Goroff et al. on p. 4699 ff. Artwork by Rory Perner is gratefully acknowledged.