TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The domains of polymeric material are at least 0.5 nm, preferably at least 1 nm in diameter. The host polymer is an aromatic ring-containing polymer, preferably polyamides, polyimides (preferred), polyacetylenes, polyanilines, polysulfones, poly(styrenes), polycarbonates, cellulosic polymers, polycarbonates, polyethers, polyether imides, polyetherketones, poly(arylene ethers), poly(arylene oxides), poly(esteramide-diisocyanate), polyurethanes, polyesters, poly(phenylene oxide)s, poly(pyrrolone)s, polysulfides, poly(ethylene), polymers of intrinsic microporosity, polyvinyl compounds and their copolymers. The polyimide is derived from the reaction of a diamine with a dicarboxylic acid or dianhydride comprising 4,4'-(hexafluoroisopropylidene)-diphthalic anhydride, 3,3',4,4'-bisphenyltetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 1,2,3,5-benzenetetracarboxylic anhydride or 1,4,5,8-naphthalenic tetracarboxylic dianhydride. The polyimide is derived from the reaction of a dicarboxylic acid or a dianhydride with diamine comprising 2,2'-bis(3-amino-4-hydroxylphenyl)hexafluoropropane, 4-(4-aminophenoxy)benzenamine, 3-(4-aminophenoxy)benzenamine, 3-(3-aminophenoxy)benzenamine, 1,4-diamino durene, 2,5-diamino-1,4-benzenedithiol, 5-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane, 6-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane or 3,3'-diaminobenzidine. The permeability of the second polymeric material is at least 50% more permeable to the target gas compared to the host polymer. The second polymeric material is polydisubstituted siloxane, polyalkylene oxide, polyimide, polycarbonate, polyacetylene, polymethacrylate, polyacrylate, polyelectrolyte, poly(ionic liquids), polyvinyl alcohol and/or polyether. The second polymeric material is a non-aromatic ring-containing polymeric material. The second polymeric material contains either a charged end group, or a polar end group such as carbonyl, hydroxyl, amine, ether, or siloxane. The domains are formed by particles of the second polymeric material. The concentration of particles of second polymeric material in (A) is less than 50% w/v, preferably 1% w/v. The molecular weight of the particles of second polymeric material has a number average molecular weight of 50000-10000000. The average particle size of the polymeric particles is at least 5 nm or less than 80 nm. The particles of second polymeric material are core-crosslinked star-polymers. The domains of the second polymeric material comprise segments of the second polymeric material covalently bound to segments of the host polymer. The second polymeric material comprises 0.1-50 wt.% of the polymeric composition. The molecular weight of the host polymer segments is 10000-500000 g/mol. The permeability of the host polymer to the target gas is at least 5 barrer. The selectivity of the host membrane for the target gas species over the second gas species is at least 4. The selectivity of (A) is at least 4. (A) comprises a selective layer thickness of 0.05-100 mm. Preferred Method: The method comprises combining the host polymer with the particles of second polymeric material (which are core crosslinked star-polymers). The host polymer and particles of polymeric composition are combined with a solvent, and the method comprises removing the solvent to produce the polymeric composition comprising a host polymer with the polymeric particles distributed in it. The method for producing gas separation membrane polymeric composition comprises: reacting a host polymeric material or precursor having at least one reactive group of a first type, with domain-forming polymeric material segments having at least one reactive end group of a second type, which is reactive with the reactive group of the first type, to produce a polymeric composition in which multiple segments of the second polymeric material aggregate to form domains within the host polymer, and the domain-forming polymeric material. The host polymeric material precursor being reacted has one or two reactive end groups, and the domain-forming polymeric material segments being reacted have two reactive end groups, and the method involves reacting at least a 2:1 mole ratio of the host polymeric material to domain-forming polymeric material to produce a polymeric composition comprising 3-block units of reacted polymer comprising a segment of domain-forming material between two segments of host polymeric material. The host polymeric material being reacted comprises multiple reactive groups, and the second polymeric material segments each have multiple reactive end groups, so that the reaction yields a cross-linked polymeric composition comprising the host polymeric material and segments of second polymeric material. EXAMPLE - A solution (10 ml) of 4,4'-(hexafluoroisopropylidene)-diphthalic anhydride (250 mg) and polydialkylsiloxane polydimethylsiloxane star polymer (1.3 mg) in dichloromethane, was pored into a level casting ring (diameter 65 mm) and loosely covered. The casting ring was left overnight to allow the dichloromethane to evaporate. Distilled water was added to allow the membrane to be separated from the glass. The membrane was allowed to air dry overnight and then kept for 4 days at 100 degrees C to ensure complete removal of all solvents to give membrane.