Disclosed herein is a method of making a crosslinked membrane such as a crosslinked hollow fiber membrane. The method comprises (a) preparing a polyimide polymer comprising carboxylic acid functional groups from a reaction solution comprising monomers and at least one solvent; (b) treating the polyimide polymer with a diol at esterification conditions to form a monoesterified polyimide polymer; (c) forming a monoesterified polyimide membrane or dense film from the monoesterified polyimide polymer; and (d) subjecting the monoesterified polyimide membrane or dense film to transesterification conditions under a CO2 atmosphere to form a crosslinked polyimide membrane or dense film.