Normally, synthetic organic polymers are insoluble amorphous or polycrystalline products instead of single crystals. As far as polymer science and crystal engineering are concerned, the direct synthesis of single-crystal porous polymers is still a challenge.
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However, several biomacromolecules found in nature have the potential to achieve single-crystal form supported by several supramolecular interactions. This implies that reversible supramolecular interactions could be utilized to modulate the crystallinity of porous polymer materials.
In a study published in the journal Nature Synthesis, the research groups headed by Professor Tianfu Liu and Professor Rong Cao from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences came up with a strategy to obtain single crystals of a porous polymer.
In the hierarchical structures of a few nucleic acids and natural protein crystals, 1D covalent chains produce highly crystalline or even single-crystal products via a synergistic combination of non-covalent supramolecular interactions and then fold into particular spatial conformations as secondary, tertiary, and quaternary structures.
Having been encouraged by this, the scientists visualized that the judicious choice of 1D chains, along with tunable dynamic covalent bonds and supramolecular interactions, may yield direct synthesis of a single-crystal porous polymer with ion channels, preferred cavities, and functionalities.
The researchers developed and synthesized a single-crystal porous Polymer-based Hydrogen-bonded Organic Framework (PHOF-1) via a one-pot route and verified its structure by single-crystal X-Ray diffraction studies.
The scientists found that 1,4-phenylenebisboronic acid monomers polymerize into a tetramer to provide a nine-membered B4O52- cluster (primary structure), which then extends into 1D covalent chains (secondary structure).
These are non-covalently cross-linked by hydrogen bonds and electrostatic interactions (tertiary structure), which eventually progress to afford a hydrogen-bonded organic framework (quaternary structure).
In comparison with insoluble 2D or 3D crosslinked porous polymers, the 1D polymer chains display solution processability and outstanding solubility. The dissolved PHOF-1 retains the sequential 1D chain structure having a very narrow molecular weight distribution and could regenerate in a reversible manner in single-crystal or amorphous states based on the solvent evaporation rate.
Taking benefit of the solution processibility, the scientists constantly coated PHOF-1 onto a non-woven fabric to give a functional composite textile that has the potential of capturing NH3.
This design strategy might pave the way for a new avenue for the exploration of single-crystal porous polymer materials with accurate confined pore spaces, structural information, and direct solution processibility.
Liu, B-T., et al. (2023) A solution processible single-crystal porous organic polymer. Nature Synthesis. https://doi.org/10.1038/s44160-023-00316-4.