The solution can process single crystal porous organic polymers

Traditional polymer chains are often difficult to crystallize because of poor reversibility of chemical bonds and weak interchain forces. Therefore, the direct preparation of polymer materials in a single crystal state is a challenging issue. Inspired by the existence of multi-level structures and multiple non-covalent interactions in protein crystals, Professor Liu Tianfu, researcher Cao Rong of the Fujian Institute of Physical Construction, Chinese Academy of Sciences, and Professor Omar K. Farha of Northwestern University collaborated to successfully prepare a single crystal organic polymer with porosity, named Polymer-based Hydrogen Bonding Organic Framework (PHOF-1), by rationally selecting polymeric monomers and regulating various non-covalent interactions between polymer chains. The material exhibits solution processing properties by dissolving in organic solvents and maintaining its one-dimensional polymer chain structure. The authors further used the operable properties of this solution to uniformly coat the material on non-woven fabrics and use them for the capture of the toxic gas NH3. This new material type combines the porosity of crystalline materials with the solution processability of polymer materials, which provides a new idea for the development of functional materials.

On May 4, 2023, the results were published in the journal Nature Synthesis under the title “A solution processible single-crystal porous organic polymer”. The first author of the article is Dr. Liu Baitong, who graduated from Liu Tiancai’s research group in 2022 and is currently engaged in postdoctoral research in the research group of Nobel laureate Professor Fraser Stoddart.

Figure 1: One-dimensional polymer chains form a multi-level network structure with open pores through hydrogen bonding and electrostatic interaction. Image source: Nature Synthesis

The monomer of terephthalboronic acid polymerizes under the action of tetramethylammonium to form a tetramer based on reversible B-O bonds and further expands to form a one-dimensional polymer chain. Each molecular chain and the adjacent four molecular chains are crosslinked to each other by hydrogen bonds to form a three-dimensional porous hydrogen-bonded organic framework, and tetramethylammonium is free in the pores as a countering ion. Reversible covalent bonding, multiple hydrogen bonding, and charge synergistic increase the crystallinity of the material, and finally assemble polymer single crystals with open pores.

Figure 2: Reversible change of PHOF-1 between single crystalline, solution, and amorphous states. Image source: Nature Synthesis

PHOF-1 can be dissolved in methanol and regenerated by means of volatile solvents. At the same time, the researchers found that the slow regeneration rate will make the material reappear in a single crystal form, while the rapid regeneration will make the material form an amorphous form. Analysis by cryo-EM and gel permeation chromatography showed that the material can maintain its polymer chain structure in solution. This property gives PHOF-1 ideal solution processability.

Figure 3: Absorption isotherm (a) and penetration experiment (b) of ammonia by PHOF-1. Image source: Nature Synthesis

N2 and CO2 adsorption confirmed that PHOF-1 has microporous properties and high specific surface area. Given that its charged backbone may have a better enrichment effect on gas molecules, the authors studied the adsorption and separation of the toxic gas ammonia. The results of the adsorption isotherm (Figure 3a) and penetration experiment (Figure 3b) of ammonia showed that the material had a good enrichment effect on ammonia. Further combined with the solution processability of the material, PHOF-1 can be uniformly coated on the surface of the non-woven fabric, so as to prepare a composite fiber material for the protection of toxic gases.

This work designed and synthesized a new class of porous hydrogen-bonded organic framework materials, and also provided a new idea for the construction of single-crystal polymer materials. (Source: Science Network)

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