Diiodobutadiyne (I–C≡C–C≡C–I) forms 1:1 cocrystals with bis(pyridyl)oxalamide hosts in which the diyne units sit at near-ideal geometry for topochemical 1,4-polymerization, yet the crystals are inert to heat and light. Wilhelm et al. (JACS 2008) showed that hydrostatic pressure of 0.3-10 GPa converts these cocrystals into the ordered conjugated polymer poly(diiododiacetylene) (PIDA), establishing pressure as a distinct activation mode for solid-state polymerization but leaving the underlying chemistry open. Later work settled two of the source paper's open issues: the crystal structure of the pressure-formed polymer phase was solved by a pressure-induced single-crystal-to-single-crystal transformation (CrystEngComm 2013), and the preparation, isolation, and full characterization of PIDA were achieved via companion host systems (JACS 2008; Macromolecules 2011). What remains unresolved is the mechanistic and kinetic core: whether radical or other paramagnetic intermediates form and propagate during pressure-induced chain growth (the source authors proposed electron spin resonance experiments, never reported); and how conversion, chain length, crystallinity, and defect density in the two cocrystals depend on pressure, time, and temperature, including whether sustained lower pressures with longer times or mild heating can reach complete conversion. Direct structural observation during pressurization also remains limited - the 2013 in-situ diffraction reached only unit-cell indexing - so the bond-making steps have not been followed under pressure.

Background

Poly(diacetylenes) (PDAs) are conjugated polymers whose backbone carries alternating ene-yne repeat units. They are made by a topochemical polymerization: monomer diynes (molecules containing two triple bonds) are pre-organized in a crystal lattice at a repeat distance near 5 Angstrom and a tilt near 45 degrees, so that neighboring molecules undergo stereospecific 1,4-addition and the crystal, not a solvent or catalyst, dictates the product's order. A halogen bond is the attractive, highly directional interaction between an electrophilic halogen atom (here iodine, covalently bound to carbon) and a Lewis base such as a pyridine nitrogen or a nitrile; it is a standard tool of crystal engineering, the deliberate design of solids from molecular components. Diiodobutadiyne (1,4-diiodobuta-1,3-diyne, I–C≡C–C≡C–I) is a symmetric diyne whose terminal iodines make it a strong halogen-bond donor. Goroff, Lauher, and co-workers assembled it into host-guest cocrystals - crystals containing both the monomer guest and a hydrogen-bonded oxalamide host - first with bis(nitrile) oxalamide hosts, from which photopolymerization gives the ordered polymer poly(diiododiacetylene) (PIDA), a PDA whose only substituents are iodine atoms (Sun, Lauher, and Goroff, Science 2006). In cocrystals of diiodobutadiyne (compound 1) with the bis(3-pyridyl)oxalamide and bis(4-pyridyl)oxalamide hosts (compounds 3 and 4, giving cocrystals 1·3 and 1·4), the diyne alignment is near the topochemical ideal, but heat and ultraviolet or gamma irradiation fail to induce polymerization. Wilhelm et al. (JACS 2008) discovered that high pressure does: in a diamond-anvil cell (the standard device for generating gigapascal pressures between two diamond faces) and in multianvil presses, cocrystals pressed across 0.3-10 GPa develop the Raman signatures of the PDA backbone (intense bands near 962, 1394, and 2055 cm-1), turn blue and then black, and, by cross-polarization magic-angle-spinning carbon-13 solid-state NMR (a nucleus-specific bulk probe of chemical conversion), reach high polymer content at pressures above roughly 6 GPa. The authors interpreted the acceleration as a kinetic effect - pressure lowering the barrier to 1,4-addition by stabilizing the compact polymeric state - and, for cocrystal 1·4, observed a pressure-induced polymorphic rearrangement of the host lattice. They explicitly left open the microscopic mechanism (proposing electron spin resonance experiments to test for radical or paramagnetic intermediates), the crystal structure of the polymer phase, the dependence of chain length and conversion on pressure, time, and temperature, the isolation of polymer from the pyridyl host, and the relation between the polymorphic rearrangement and polymerization. Two of these issues were later settled. The companion study (Luo et al., JACS 2008) developed a crystallization methodology and a solvent-rinsing isolation protocol for PIDA in bis(nitrile) oxalamide hosts that polymerize spontaneously at room temperature, and fully characterized the polymer; oriented PIDA nanofibers followed (Luo et al., Macromolecules 2011). Most directly, Jin, Plonka, Parise, and Goroff (CrystEngComm 2013) demonstrated a pressure-induced single-crystal-to-single-crystal (SCSC) transformation - the crystal remains a diffraction-quality single crystal while its molecules react - of cocrystal 1·3 at 3.5 GPa and solved the single-crystal X-ray structure of the recovered polymer cocrystal (space group P-1, a = 4.9330(3) Angstrom), quantifying how polymer formation contracts the lattice and degrades the host-guest halogen bonds. In situ Raman showed polymer modes appearing in 1·3 near 1.1 GPa, while 1·4 requires about 5.4 GPa and converts only partially and heterogeneously. The 2013 analysis indicates that pressure-driven repacking of the host and flexibility of the pyridine rings enable the reaction, with the new carbon-carbon sigma bonds forming at the expense of the halogen bonds. What the field has still not delivered for this system is any direct observation of the intermediates of chain growth, an atomically resolved in-situ view of the bond-making steps during pressurization, or a quantitative kinetic map of conversion versus pressure, time, and temperature.

Problem Statement

Establish a quantitative, mechanism-level account of how hydrostatic pressure converts the geometrically ideal but thermally and photochemically inert cocrystals of diiodobutadiyne with bis(pyridyl)oxalamide hosts (1·3 and 1·4) into poly(diiododiacetylene). For these cocrystals the open problems are:

  1. Determine whether radical or other paramagnetic intermediates form in pressure-induced chain initiation and propagation - for example by in-situ high-pressure electron spin resonance measurements on the cocrystals during pressurization, the experiment the source authors proposed and that has never been reported.
  2. Quantify how conversion, polymer chain length, crystallinity, and defect density in 1·3 and 1·4 depend on pressure, time, and temperature - in particular whether complete conversion is achievable at sustained lower pressures with longer reaction times or mild heating within the monomer's stability window, and how the pressure-induced host-lattice rearrangements (exemplified by the polymorphic transition of cocrystal 1·4 and the contrasting onset pressures of 1·3 and 1·4) gate the reaction kinetically or thermodynamically.

Both items are required to resolve the task: settling only the mechanistic determination (item 1) or only the kinetic characterization (item 2) is a partial result, not a complete resolution. Item 2 is complete once conversion, polymer chain length, crystallinity, and defect density have been mapped as functions of pressure, time, and temperature for both cocrystals; the complete-conversion question and the host-rearrangement-gating question are parts of that kinetic map rather than separate deliverables.

References

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