No abstract

The polymerization of simple conjugated dienes has long been of interest: polydienes occur throughout Nature, and polyisoprene and its analogues form the basis of entire industries. In contrast, the polymers of similar small conjugated compounds, diacetylenes, trienes, and triacetylenes, are either unknown or laboratory curiosities. For 40 years, the only viable synthetic method for the 1,4-polymerization of a diacetylene was a topochemical polymerization in a condensed phase. But such an approach is hit or miss: if the diacetylene monomers have a solid-state structure preorganized at distances matching the repeat distance in the final polymer, then thermal or photochemical energy can bring about the polymerization. However, most monomers lack the proper structural parameters and simply do not react. As discussed in this Account, we have developed a supramolecular host-guest strategy that imposes the necessary structural parameters upon a diacetylene monomer that otherwise does not polymerize. We apply this strategy in the synthesis of new types of conjugated polymers made from diacetylenes, triacetylenes, and trienes. To implement the host-guest strategy, we chose a host that would self-assemble into a supramolecular structure with the requisite intermolecular spacing. For diacetylenes, the ideal spacing is 4.9 A, and the oxalamides, which routinely crystallize with a spacing of 5 A, make ideal host molecules. We chose specific oxalamide host substituents that bind to the diacetylene guest molecule, typically through hydrogen bonding. We have focused upon the single-crystal-to-single-crystal polymerizations, allowing us to obtain and characterize the polymers in perfect crystalline form and to define and better understand the reaction trajectories. We have prepared several new classes of polydiacetylenes using this strategy, including the first terminal polydiacetylenes and an aryl-substituted diacetylene. Interestingly, to prepare poly(diiododiacetylene), we used halogen bonds to bind the host and guest. The simplest polydiacetylene known, poly(diiododiacetylene), lacks the side chains that complicate the structures of similar previous polymers. Future studies should provide insights into the role of such side chains in conjugated materials. We further demonstrated the strength of the host-guest strategy by moving from the polydiacetylenes to the polytriacetylenes. Although the structural requirements for a triacetylene polymerization had been stated decades ago, no one had ever found a triacetylene with the requisite spacing of 7.4 A. We designed a series of pyridine-substituted vinylogous amide hosts to achieve this spacing. Cocrystallization of these host molecules with a triacetylene dicarboxylic acid gave us the desired structure. Using thermal annealing, we completed the synthesis of the triacetylene polymer.

Conjugated organic polymers generally must include large substituents for stability, either contained within or appended to the polymer chain. In polydiacetylenes, the substituents fulfill another important role: During topochemical polymerization, they control the spacing between the diyne monomers to produce an ordered polymer. By using a co-crystal scaffolding, we have prepared poly(diiododiacetylene), or PIDA, a nearly unadorned carbon chain substituted with only single-atom iodine side groups. The monomer, diiodobutadiyne, forms co-crystals with bis(nitrile) oxalamides, aligned by hydrogen bonds between oxalamide groups and weak Lewis acid-base interactions between nitriles and iodoalkynes. In co-crystals with one oxalamide host, the diyne undergoes spontaneous topochemical polymerization to form PIDA. The structure of the dark blue crystals, which look copper-colored under reflected light, has been confirmed by single-crystal x-ray diffraction, ultraviolet-visible absorption spectroscopy, and scanning electron microscopy.