Covalent inclusion of calix[4]arenes in Tröger's base polymers of intrinsic microporosity for physical aging mitigation

Polymeric membrane-based technology offers an energy-efficient solution for post-combustion carbon capture. Owing to their rigid, contorted backbones that endow them with exceptional CO2 permeability, Polymers of Intrinsic Microporosity (PIMs) are among the most promising candidates, but their application is constrained by physical aging. Here, the covalent inclusion of calix[4]arenes as monomeric units in PIMs based on Tröger's Base (PIM-TBs) is presented as a potential strategy for the mitigation of this progressive free-volume collapse, by taking advantage of their intrinsic cavity. For this purpose, upper rim functionalised diamino-tetrapropoxy calix[4]arenes in defined geometrical arrangements (cone, partial cone, and 1,3-alternate) are exploited in the preparation of both homo- (PIM-CXn-TBs) and co-polymers (PIM-Trip-CXn-TBs) with diaminotriptycene units, which are fully characterised in terms of molecular weight distributions, textural parameters and CO2/N2 adsorption properties at room temperature. PIM-Trip-CX3-TB, obtained from the copolymerisation of the 1,3-alternate conformer, and its post-polymerisation dealkylated counterpart (PIM-Trip-CX3(OH)-TB) are solution processed in films and tested in permeation and aging experiments. These studies reveal that, whereas the propylated copolymer mirrors the aging behaviour of pristine PIM-Trip-TB, PIM-Trip-CX3(OH)-TB exhibits a reduced early-stage loss of CO2 permeability at the expenses of membrane flexibility, thus evidencing a concomitant effect of the covalent calix[4]arene incorporation and cavity rigidification promoted by intramolecular hydrogen-bonding interactions.