Biphenyl(isatin-co-trifluoroacetophenone)-Based Copolymers Synthesized Using the Friedel–Crafts Reaction as Mechanically Robust Membranes for Efficient CO₂ Separation

In this study, we report a series of ether bond-free (i.e., full carbon backbone) copolymers, prepared for the first time by a simple one-step Friedel–Crafts (F–C) polycondensation reaction using biphenyl-isatin (BP-Isa), a torsion-resistant rigid segment with high CO2 selectivity, and biphenyl-trifluoroacetophenone (BP-TFAPh), a highly permeable ladder-type segment. The copolymer exhibits a high molecular weight, moderate BET surface area, and excellent thermophysical properties. (BP-Isa)x-(BP-TFAPh)y copolymers containing over 50% TFAPh loading show a perfect combination of mechanical properties and gas separation properties, outperforming most commercial gas separation polymers, most F-C polymers, and high-performance PIM-PI-1 polymer membranes. (BP-Isa)x-(BP-TFAPh)y copolymer membranes with loadings over 50% also exhibited a good CO2 permeability of 394–526 Barrer and good CO2/N2 and CO2/CH4 selectivities of 24.4–29.0 and 19–25, respectively. Also, the 50:50 composition of the copolymer films exhibited excellent antiaging properties for up to one month, with good resistance to plasticization at pressures of up to 15 atm.