Facile synthesis and gas transport properties of Hünlich's base-derived intrinsically microporous polyimides

Tröger's base (TB) has been utilized as an important building block in designing ladder polymers of intrinsic microporosity (PIMs) and microporous polyimides (PIM-PIs) for membrane-based gas separations due to its unique V-shaped bicyclic structure and versatile molecular chemistry. Nearly a century after its discovery, Hünlich's base (HB) was recently reintroduced as a valuable diamine derivative of TB made by a single-step reaction of 2,4-diaminotoluene and formaldehyde, spurring use in molecular devices such as molecular tweezers and photo-switches. Unlike TB, HB has not been explored as a building block of PIMs and PIM-PIs for membrane-based gas separations. In this study, we synthesized two soluble PIM-PIs for the first time by reaction of HB as diamine and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) or 9,10-diisopropyltriptycene tetracarboxylic dianhydride (TDAi3), respectively. 6FDA-HB exhibited high Brunauer-Teller-Emmett (BET) surface area of 415 m2 g−1 and fractional free volume (FFV) of 0.26. The gas separation performance of the 6FDA-HB polymer was similar to other 6FDA-based dimethyl-substituted Tröger's base polyimide analogues, exhibiting O2, CO2 and H2 permeability of 62, 286, and 391 Barrer with O2/N2, CO2/CH4 and H2/CH4 selectivity of 4.4, 26 and 36, respectively. Compared to 6FDA-HB, the triptycene-containing Hünlich's base polyimide (TDAi3-HB) displayed a higher BET surface area (501 m2 g−1) owing to the presence of the rigid bridged tricyclic triptycene contortion site, resulting in about two-fold increase in O2 permeability to 188 Barrer coupled with slightly compromised O2/N2 selectivity of 4.1. Beside the merit of facile synthesis, the Hünlich's base-derived polyimides experienced relatively modest effects of physical aging on gas permeation properties.