Triptycene Polymer with Intrinsic Microporosity
Probably the most important characteristic of the triptycene framework comes from the rigidity of its propeller-like structure and the guarded space between the aromatic faces which has been defined as “Internal Molecular Free Volume” (Long and Swager 2001). In fact, when functionalized triptycenes are used to form polymers, their packing in the solid state becomes very inefficient so that local cavities are created, leading to the formation of a highly microporous material. There is a great variety of examples of the use of this interesting building block for the synthesis of highly microporous materials. For instance, hexa-amino (Kohl et al. 2014) or hydroxo-based triptycenes (Taylor et al. 2014) were used to synthesize discrete molecules with elevated microporosity. Kahveci et al. reported the use of triptycene-based molecules to make very high surface area (up to 3,800 m2 g−1) covalent organic frameworks (COFs), exploiting the trigonal shape of the triptycene to form very well-defined hexagonal channels. They demonstrated the possibility of using these materials for elevated absorption of CH4 and CO2 at 273 K (Kahveci et al. 2013).
When polymerized with tetrafluoroterephthalonitrile via nucleophilic aromatic substitution, these monomers afforded polymers whose BET surface area varied according to the length of the alkyl chain of the bridgehead, proving the versatility of the functionalized triptycenes.
It was found that short alkyl chains (H, Me, Et, Pr) lead to highly porous materials with the highest BET surface area of 1,760 m2g−1 when R = Me, whereas increasing the length of the alkyl chains caused a decrease in surface area due to an elevated fraction of the generated free volume becoming occupied by the flexible side chains.
Because of the high rigidity of the bridged poly-aromatic ring, which often limits the solubility of the obtained material, there are not many examples of triptycene-based polymers that formed robust solution-processable membranes to be used for gas separation.
In the past few years, though, there have been new progresses in this field which led to the synthesis of high-performing triptycene-based materials with excellent performance for gas separation.
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