A 6FDA (4,4'-hexafluoro isopropylidene diphthalic anhydride) based copolyimide has been synthesized containing the diamines DABA (3,5-diamino benzoic acid) and a 15-crown-5-diamine to separate propylene/propane mixtures. With the carboxylic acid group of DABA crosslinking the polymer chains is possible leading to a suppression of plasticization of the polymer. Thereby the loss of separation performance at higher pressures of membranes made from such a polymer,can be avoided. The crown ether group allows to incorporate silver ions by complexation in the membrane polymer. As a consequence facilitated transport of olefins is possible. Pure gas permeation and sorption experiments at 35degreesC and feed pressures up to 8 bar have been carried out with propylene and propane using non-crosslinked and EG-crosslinked copolyimide membranes, the latter ones with and without silver ions.