Covalent organic frameworks (COFs) are an emerging class of crystalline porous frameworks,1 which are exclusively built up of organic building units connected by covalent bonds. COFs offer nano-sized pores that are uniform in size, shape and functionality. Their synthesis is based on reticular chemistry, i.e. the framework structure and functionality is determined by the tectonic building blocks one employs. When the monomers react with each other, amorphous networks (the kinetic product) form first. Therefore, reaction conditions allowing for the amorphous network to self-correct to a crystalline framework (the thermodynamic product) - in other words: conditions that render the linkages reversible - have to be found for each COF. To date, the vast majority of COFs is based on boronic acid/boroxine and imine linkages, typically rendered reversible by using appropriate solvent mixtures, modulators and increased temperatures.2 In most cases these conditions are harsh and toxic (e.g. mesitylene/dioxane mixtures). Conventional COFs with boron-based and imine-linkages disintegrate relatively easily under hydrolytic strain. Therefore, recent efforts focus on generating COFs linked by strong heterocyclic linkages such as the imidazole function.3 To date, there are only very few prior reports of imidazole-linked COFs.4 There, a two-step approach is employed: First an imine-linked COF is generated, and second the imine functions are subsequently cyclized - assisted by molecular oxygen - to imidazole functions. Whilst rarely used in COFs, imidazole-linkages are heavily used in polymers, then termed polybenzimidazoles (PBIs). Both linear and network (amorphous) PBIs are in fact industrially established organic materials, especially when they are fully aromatic. As PBIs show outstanding thermal and chemical stability they are e.g. used in fuel cells as membrane materials where they operate efficiently up to 300 ˚C, as well as in flame-retardant textiles e.g. in firefighters' suits.
Conventional PBI synthesis revolves around expensive, toxic and environmentally harmful solvents, and they generally lack crystallinity. We recently developed a novel and green route for synthesizing heterocycle-linked organic materials, using nothing but high-temperature water: hydrothermal polymerization (HTP).5 HTP generates higher crystallinity than conventional syntheses,6 which is highly promising for COFs. With this contribution, we show that PBI-COFs can be generated by HTP, and hence via a clean synthetic route that simultaneously improves the materials properties. We here present HTP of PBI-COFs as well as their characterization by ATR-FT-IR spectroscopy, electron microscopy (SEM and TEM), small and wide-angle X-ray diffraction (SAXS and WAXS), solid-state NMR, and XPS.
References
1. A.P. Cote, A.I. Benin, N.W. Ockwig, M. O'Keeffe, A.J. Matzger, O.M. Yaghi, Science 2005, 310, 1166.
2. X. Feng, X. Ding, D. Jiang, Chem. Soc. Rev. 2012, 41, 6010.
3. C. R. DeBlase, W. R. Dichtel, Macromolecules 2016, 49, 5297.
4. B.C. Patra, S. Khilari, L. Satyanarayana, D. Pradhan, A. Bhaumik, Chem. Comm. 2016, 52, 7592.
5. B. Baumgartner, M. J. Bojdys, M. M. Unterlass, Polym. Chem. 2014, 5, 3771.
6. M. M. Unterlass, Angew.
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