Encyclopedia of Membranes

Living Edition
| Editors: Enrico Drioli, Lidietta Giorno

Triptycene Polymer with Intrinsic Microporosity

  • Mariolino CartaEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-40872-4_1966-1
Triptycene and its derivatives are a class of interesting aromatic compounds with unique, concave, three-dimensional frameworks. Because of the almost perfectly trigonal shape they have been used for several different applications in the field of material chemistry. Typically the synthesis of triptycene-based compounds starts from the formation of the very reactive benzyne intermediate, for example, from the anthranilic acid, which can easily undergo Diels-Alder reaction with anthracene to form the triptycene core as shown in Fig. 1.
Fig. 1

Formation of the triptycene core

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).

A series of triptycene-based network polymers of intrinsic microporosity (PIMs), reported in 2010 (Fig. 2; Ghanem et al. 2010), showed the ability of functionalized triptycenes to be used for selective gas adsorption. These networked PIMs were built from hydroxyl-substituted triptycene units with varying lengths of alkyl chains attached to the bridgehead position (R).
Fig. 2

Synthesis of triptycene network PIMs. R = H, Me, Et, Pr, i Pr, Bu, i Bu, Pent, Oct, Bz

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.

In 2011, Park and Cho realized a polyimide containing a diamino triptycene linked with the commercial hexafluoro dianhydride (6FDI, Fig. 3), which is one of the most used monomers for the formation of high-performance polyimides for gas separation (Cho and Park 2011). The free rotation around the imide linkage provided a low surface area (only 68 m2g−1), which limits the mass transport of the gasses into the membrane and, consequently, the permeability. Despite the low permeability, the excellent selectivities demonstrated by this polymer allowed it to surpass the Robeson upper bound for important gas pairs such as O2/N2 and CO2/CH4.
Fig. 3

Synthesis of 6FDI-DATRI polyimide

In order to increase the free volume, Pinnau et al. used a similar approach synthesizing an extended triptycene dianhydride with bulky isopropyl substituents on the bridgehead, polymerizing it with the commercial tetramethyl phenylenediamine. The combination of the rigidity of the bulky triptycene and the hindrance of the four methyl groups of the phenylenediamine, which restricts the rotation around the imide linkage, afforded a polyimide with very high BET surface area (KAUST-PI-1,750 m2 g−1, Fig. 4). The increase of the free volume led to high permeability which, in combination with decent selectivities, allowed them to obtain a high-performing material for gas separation (Ghanem et al. 2014).
Fig. 4

Synthesis of KAUST-PI-1

A different method was followed by the McKeown group for the synthesis of highly porous and permeable triptycene-based polymers of intrinsic microporosity synthesized via Tröger’s base polymerization (Trip-TB-PIM, Carta et al. 2014). By using a diamino triptycene and reacting it with an excess of dimethoxymethane (DMM), they obtained a polymer which combines the high rigidity of both triptycene and Tröger’s base (TB) cores. The contribution of the two rigid frameworks afforded a highly microporous polymer (Trip-TB-PIM, 900 m2g−1, Fig. 5). Despite the increased stiffness of the backbone, the material resulted soluble in common solvent from which it was casted a robust self-standing thin film membrane. The combination of high surface area, which boosted the permeability, and the polarity of the TB core that enhanced the selectivities for important gas pairs allowed this triptycene-based polymer to go well beyond the Robeson upper bound for H2/N2, O2/N2, and CO2/CH4.
Fig. 5

Triptycene-based Trip-TB-PIM

References

  1. Carta M, Croad M, Malpass-Evans R, Jansen JC, Bernardo P, Clarizia G, Friess K, Lanc M, McKeown NB (2014) Triptycene induced enhancement of membrane gas selectivity for microporous Troeger’s base polymers. Adv Mater (Weinheim, Ger). doi:10.1002/adma.201305783Google Scholar
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© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.School of ChemistryThe University of EdinburghEdinburghUK