Polymer chemists at A*STAR have synthesized materials with intrinsic, microscale porosity that are well suited for reusable gas and liquid separations because of their robust, ladder-like frameworks.
Polymers tend to pack tightly together in the solid state, easily filling any voids with their flexible chain structure. Recently, however, polymers of intrinsic microporosity (PIMs) have emerged that use rigid, fused organic ring structures to open up tiny voids in the plastic material. Researchers are considering PIMs as replacements for the activated carbons and zeolites used during industrial gas adsorption processes because they can be chemically tuned to specific contaminants, and do not require thermal regeneration.
Chemists can synthesize PIMs through a process, known as nucleophilic substitution, which enables covalent bonds to simultaneously form between two aromatic ring compounds. This type of polymerization tends to generate 'ladder' structures: two long parallel chains intermittently bridged by chemical 'rungs'. While ladder polymers have an abundance of micropores for gas adsorption, the extra stiffness from the rigid ring components makes them notoriously difficult to dissolve for solution processing.
Ranganathan Krishnan and Anbanandam Parthiban from the A* STAR Institute of Chemical and Engineering Sciences tackled this problem by turning to a non aromatic ring compound called octafluorocyclopentene (OFCP) that contains numerous fluorine atoms. Fluorine units can impart special characteristics to ladder polymers such as Teflon-like chemical resistance, improved solubility, and high thermal stability, notes Krishnan. As well, the team anticipated that the more flexible OFCP ring could lend sufficient 'kinks' to the polymer backbone to produce an organosoluble PIM.
Krishnan and Parthiban reacted OFCP with rigid benzene-type rings to synthesize fluorinated ladder polymers. Previous studies have suggested that nucleophilic substitution could only add two aromatic precursors to the OFCP ring. However, the scientists found that with their reaction conditions, nucleophilic substitution occurred twice, in distinct steps, at four positions on the OFCP ring.
The ability to assemble PIMs piece-by-piece gave the researchers broad control over their structure: they produced several kinds of linear and ladder polymers by controlling the degree of nucleophilic substitution. In addition, the fluorinated chains had good solubility in the common organic solvent dimethylformamide.
Having a high surface area is key for PIM purification technology, and the A*STAR team found they could reliably tune this property by choosing different aromatic partners for the OFCP ring. Membrane prototypes based on ladder polymers are the next focus of Krishnan and Parthiban as they strive to improve real world gas separations.
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Krishnan Ranganathan et al. Soluble, microporous ladder polymers formed by stepwise nucleophilic substitution of octafluorocyclopentene, Polym. Chem. (2015). DOI: 10.1039/c5py00359h