Poly(ionic liquid)s with Dicationic Pendants as Gas Separation Membranes

Poly(norbornene)s and poly(ionic liquid)s are two different classes of attractive materials, which are known for their structural tunability and thermal stabilities, and have been extensively studied as gas separation membranes. The incorporation of ionic liquids (ILs) into the poly(norbornene) through post-polymerization has resulted in unique materials with synergistic properties. However, direct polymerization of norbornene-containing IL monomers as gas separation membranes are limited. To this end, a series of norbornene-containing imidazolium-based mono- and di-cationic ILs (NBM-mIm and NBM-DILs) with different connectivity and spacer lengths were synthesized and characterized spectroscopically. Subsequently, the poly(NBM-mIm) with bistriflimide [Tf₂N^-] and poly([NBM-DILs][Tf₂N]₂) comprising homo-, random-, and block- (co)polymers were synthesized via ring-opening metathesis polymerization using the air-stable Grubbs second-generation catalyst. Block copolymers (BCPs), specifically, [NBM-mIM][Tf₂N] and [NBM-ImC_nmIm] [Tf₂N]₂ (n = 4 and 6) were synthesized at two different compositions, which generated high molecular weight polymers with decent solubility relative to homo- and random (co)polymers of [NBM-DILs] [Tf₂N]₂. The prepared BCPs were efficiently analyzed by a host of analytical tools, including ¹H-NMR, GPC, and WAXD. The successfully BCPs were cast into thin membranes ranging from 47 to 125 μm and their gas (CO₂, N₂, CH₄, and H₂) permeations were measured at 20 °C using a time-lag apparatus. These membranes displayed modest CO₂ permeability in a non-linear fashion with respect to composition and a reverse trend in CO₂/N₂ permselectivity was observed, as a usual trade-off behavior between permeability and permselectivity.