This article reports synthesis and structure property studies of block copolyimides synthesized using diamino room temperature ionic liquids (RTIL) as diamine monomers. Specifically, polyimide oligomers of different lengths were synthesized using 2,2-bis (3,4-carboxylphenyl) hexafluoropropane dianhydride (6FDA) and diamino RTIL (1,3-di(3-aminopropyl) imidazolium bis[(trifluoromethyl) sulfonyl] imide). These oligomers were copolymerized with 6FDA and m-phenylenediamine (MDA) using in situ polymerization to form (6FDA-RTIL)-(6FDA-MDA) block copolyimides. The impact of the length and relative concentration of 6FDA-RTIL oligomer in the copolymer on the resulting thermal, physical, and gas transport properties was monitored. As the concentration of the 6FDA-RTIL segments increased, the backbone of the block copolyimides became more flexible resulting in a decrease in the glass transition temperature (T-g) and an increase in the density. The permeabilities of the RTIL containing copolyimides were consistently lower than those of the base polyimide, 6FDA-MDA, with some increase in selectivities. Interestingly, the permeabilities of films produced with the low molecular weight oligomers were very different than those produced with same composition of the high molecular weight oligomers. This may be indicative of very different morphologies within these copolyimides. (C) 2015 Wiley Periodicals, Inc.