A novel trimethyl-substituted carboxyl-containing polyimide was synthesized via a one-pot high-temperature polycondensation reaction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,5-diamino-2,4,6-trimethylbenzoic acid (TrMCA). The polyimide (6FDA-TrMCA) displayed a Brunauer-Emmett-Teller surface area of 260 m2 g-1, demonstrating intrinsic microporosity, in contrast to the related low-free volume COOH-functionalized polyimide 6FDA-DABA. Compared to the nonfunctionalized 6FDA polyimide analogue made from 2,4,6-trimethyl-m-phenylenediamine (TrMPD)-also known as 6FDA-DAM-carboxyl functionalization in 6FDA-TrMCA resulted in reduced surface area, lower fractional free volume, and tighter average chain spacing. Gas permeabilities of 6FDA-TrMCA were typical of functionalized polyimides of intrinsic microporosity (PIM-PIs). For example, at 2 atm and 35 °C, 6FDA-TrMCA showed pure-gas H2 and CO2 permeability of 193 and 144 barrer, coupled with H2/CH4 and CO2/CH4 selectivity of 61 and 45, respectively. Notably, in mixed-gas permeation tests with an equimolar CO2-CH4 mixture at a CO2 partial pressure of 12 atm, 6FDA-TrMCA demonstrated performance located on the 2018 mixed-gas upper bound with a CO2 permeability of â¼98 barrer and CO2/CH4 permselectivity of 38. As the first reported COOH-functionalized PIM-PI homopolymer, 6FDA-TrMCA revealed excellent resistance against CO2-induced plasticization at least up to a CO2 partial pressure of 15 atm, covering the range of typical wellhead CO2 partial pressures (5-10 atm).