TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The membrane may comprise a continuous phase polymer comprising a polyimide having crosslinkable sites; and a molecular sieve material dispersed within the continuous phase polymer, the ratio of crosslinkable sites to imide groups being 3:8-1:16. It may comprise a molecular sieve dispersed within a continuous polymer phase comprising a polyimide polymer material made from monomers A+B+C, where A is a dianhydride of formula (I); B is a diamino cyclic compound without a carboxylic acid functionality; and C is a diamino cyclic compound with a carboxylic acid functionality. The ratio of B to C is 1:4-8:1. The dianhydride is 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA). The monomer C is diaminobenzoic acid (DABA). The monomer B is a diamino aromatic compound. It is a diamino benzene compound having methyl group(s) attached to the benzene ring. The ratio of B to C is 17:3-3:2. The membrane is subjected to esterification conditions in the presence of a diol consisting of ethylene glycol, propylene glycol, 1,3 propanediol, 1,4 butanediol, 1,2 butanediol, benzenedimethanol, or 1,3 butanediol to form a hollow fiber polymer membrane monoester. At least 60% of the carboxylic acid functionality is converted to a monoester. The hollow fiber membrane monoester is subjected to transesterification conditions to form a crosslinked hollow fiber polymer membrane. The monomer B is diamino durene. The molecular sieve is a zeolite, carbon molecular sieve, aluminophosphate zeolite, borosilicate, or silico-aluminophosphate (SAPO).X1 and X2 = halogenated alkyl, phenyl, or halogen;R1-R6 = H, alkyl, or halogen.Preferred Properties: The molecular sieve material dispersed within the continuous phase has an average particle size of less than 1 micron, preferably less than 0.1 micron. EXAMPLE - Monoesterification reaction was carried out by dissolving DABA-copolyimide in tetrahydrofuran (THF) (10 wt.%) under nitrogen atmosphere and thionyl chloride was added. The reaction was heated to reflux and excess thionyl chloride and THF were distilled out of the solution. The residual copolyimide acid chloride (brown crystals) was stored under vacuum at 50degreesC overnight. The acid chloride was dissolved in THF and dropped slowly to excess glycol (70 times excess) dissolved in THF. Starting with a material forming very brittle films, due to low molecular weight, over the acid chloride route, films with high flexibility were obtained. The synthesis of monoester over the acid chloride route could help in the process of hollow fiber spinning.