TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The cross-linking agent comprises at least two functional groups in its chemical structure chosen from primary amines, secondary amines, alcohols or thiols, (preferably primary and/or secondary di-, tri- or polyamine; or di-, tri- or polyol or di-, tri- or polythiol). The cross-linking agent is an aliphatic alcohol, aliphatic aromatic alcohol or aromatic alcohol, aliphatic thiol, aliphatic aromatic thiol or aromatic thiol; ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, pentanediol, hexanediol, heptanediol, octanediol, nonanediol, or decanediol; hydrazine, aliphatic amine, aliphatic aromatic amine or aromatic amine; or at least one compound selected from: 1,2-diaminoethane, 1,3-diaminopropane, diaminobutane, diaminopentane, diaminohexane, diaminoheptane, diaminooctane, diaminononane, diaminodecane, ethylene diamine, N,N'-dimethylethylene diamine, N, N'- diethylethylenediamine, diethylenetriamine, triethylenetetraamine, tetraethylene pentaamine, pentaethylenehexamine, tris(2-aminoethyl)amine, m-xylylendiamine, p-xylylenediamine, multifunctional aniline derivatives, phenylene diamines, methylene dianiline, or oxydianiline. The organic solvents (s2) are chosen from N-methylpyrrolidone, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, 1,4-dioxane, gamma-butyrolactone, water, alcohols, ketones or formamide.    TECHNOLOGY FOCUS - POLYMERS - Preferred Method: The step (iii) involves: solidifying the cast film by immersing the film cast on the substrate, possibly after an evaporation, into a phase inversion coagulation medium, where the medium comprising at least one multifunctional, nucleophile cross-linking agent. The polyimide polymer is present in 6-35 wt.% of the casting solution. The coagulation bath used in the phase inversion membrane solidification comprises any organic solvent or its mixture, selected from water, and the organic solvents (s2), where the resulting membrane is stable to organic solvents (s2). Preferred Components: The polyimide casting solution (s1) optionally comprises at least one additive comprising at least one multifunctional nucleophile cross-linking agent. The polyimide is at least one copolymer derived from (a) the co-condensation of benzophenone 3,3',4,4'-tetracarboxylic acid dianhydride and mixture of di(4-aminophenyl) methane and toluene diamine, or mixture of 4,4'-methylenebis (phenyl isocyanate) and toluene diisocyanate; or (b) the condensation of 1H,3H-Benzo(1,2-c:4,5-c')difuran-1,3,5,7-tetrone with 5,5'-cabonylbis(1,3-isobenzofuranidione), 1,3-diisocyanato-2-methylbenzene and 2,4-diisocyanato-1-methylbenzene. The polyimide polymer has a compound of formulae (I)-(III). The polyimide polymer is a polyimide copolymer comprising 10-90 (preferably 20)% of compound of formula (IV) and 90-10% of compound of formula (V). The cross-linking agent is at least one compound selected from: polyethyleneimine, polyallylamine, polyvinylamine, polyether diamines based predominantly on a polyethylene oxide backbone with a molecular weight of 50-20000, trimethoxysilylpropyl-substituted polyethyleneamine having a molecular weight of 200-200000, or polyethyleneamine having a molecular weight of 1000-200000. The polymer casting solution comprises mixture of organic solvents (s2). EXAMPLE - No suitable example given.