Gas Separation Properties of Functional Semiladder Polymers Synthesized by Superacid-Catalyzed Polycondensation

Aromatic multifunctional polymers containing xanthene units in the main chain were synthesized via a robust, one-step, metal-free polycondensation of isatin and its derivatives with bisphenols (4,4′-biphenol and bisphenol AF). Electrophilic aromatic substitution occurs at the ortho positions of the bisphenol hydroxyl group, followed by a partial cyclodehydration reaction, yielding copolymers with 9H-xanthene and hydroxyl fragments. The multifunctional polymers were obtained in quantitative yields, exhibiting high chemical and thermal stability. Subsequent postpolymerization reactions using 1-(6-bromohexyl)-1H-indol-2,3-dione with p-terphenyl, N-methylpiperidine, and N-methylpyrrolidine produced polyelectrolytes. Gas permeability tests were conducted on membranes based on 4,4′-bisphenol (2A) and bisphenol AF (2B). Membranes made with bisphenol AF showed improved gas transport properties compared to those with 4,4′-biphenol, particularly membrane 2bB. The inclusion of N-propyl isatin in the fluorinated polymer enhanced gas permeability across all tested gases, with membrane 2bB showing a remarkable selectivity–permeability balance for all gas pairs.