Friedel–Crafts A2 + B4 Polycondensation toward Regioselective Linear Polymer with Rigid Triphenylmethane Backbone and Its Property as Gas Separation Membrane

This report presents an unexpected acid-catalyzed Friedel–Crafts (F–C) hydroxyalkylation polymerization to produce linear polymers containing triphenylmethane repeating units in the backbone. The polymerization of 4-nitrobenzaldehye (NBA, a difunctional A₂ monomer) and 1,4-dimethoxybenzene (DMB, a B₄ monomer with four identical reactive sites) demonstrated very high regioselective reactions at the 2,5 positions of DMB monomer, leaving 3,6 positions unreacted to avoid any branched structure. The produced linear polymers were always terminated with DMB units at both ends, allowing easy synthesis of high-molecular-weight polymers by using excess amount of NBA. The polymerizations used various inexpensive 4-substituted benzaldehydes and para-disubstituted benzenes as the A₂ and B₄ monomers, producing a series of linear polymers with tunable side group compositions. These polymers with high molecular weight and rigid triphenylmethane backbone structure were cast into freestanding membranes and demonstrated excellent gas transport properties with the permeability-selectivity combination outperformed relevant commercial polymer membranes for CO₂/CH₄ separation.