NOVELTY - Separating at least one gas from mixture of gases, involves: providing polyimide gas separation membrane comprising an aromatic polyimide polymer that comprises several first repeating units of poly(3,3',4,'-diphenylsulfone tetracarboxylic dianhydride-2,4,6-trimethyl-m-phenylenediamine) polyimide (I); contacting the mixture to one side of polyimide gas separation membrane to cause gas to permeate polyimide gas separation membrane; and removing from an opposite side of polyimide gas separation membrane a permeate gas composition comprising a portion of the gas which permeated the membrane.    USE - For separating gas from mixture of gases selected form a mixture of at least one volatile organic compound and atmospheric gases, a mixture of hydrogen and nitrogen, a mixture of at least one gas selected from carbon dioxide and hydrogen sulfide that is mixed with methane, a mixture of carbon dioxide and nitrogen, a mixture of hydrogen and methane, a mixture of olefins and paraffins, and a mixture of iso and normal paraffins (claimed); used in variety of liquid, gas, and vapor separations such as desalination of water by reverse osmosis, non-aqueous liquid separation such as ethanol/water separations, pervaporation dehydration of aqueous/organic mixtures, CO2/CH4, CO2/N2, H2/CH4, O2/N2, H2S/CH4, olefin/paraffin, iso/normal paraffins separations, and other light gas mixture separations; and used in other applications such as for catalysis and fuel cell applications.    ADVANTAGE - The polyimide membrane has high permeance and high selectivity for gas separations; has CO2 permeability of greater than or equal to 50 Barrers and single-gas selectivity for CO2/CH4 of greater than or equal to 15 at 50 degrees C under 791 kPa for CO2/CH4 separation; and has UV cross-linkable functional groups and can be used for the preparation of UV cross-linked polyimide membranes having CO2 permeability of greater than or equal to 20 Barrers and single-gas selectivity for CO2/CH4 of greater than or equal to 35 at 50 degrees C under 791 kPa for CO2/CH4 separation.    DETAILED DESCRIPTION - Separating at least one gas from a mixture of gases, involves: providing a polyimide gas separation membrane comprising an aromatic polyimide polymer that comprises several first repeating units of compound of formula (I); contacting the mixture to one side of the polyimide gas separation membrane to cause the gas to permeate the polyimide gas separation membrane; and removing from an opposite side of the polyimide gas separation membrane a permeate gas composition comprising a portion of the gas which permeated the membrane.    X1=group of formula (i) or (ii);    X2=compound (i) or (ii), or group of formula (iii)-(xiii);    Y'=1,3,5-trimethyl-benzen-2,4-diyl, bis(3,5-dimethylphen-4-diyl)methane, 3,3'-dimethyl-biphen-4,4'-diyl, 2,3,5,6-tetramethyl-benzen-1,4-diyl, 3,5,3',5'-tetramethyl-biphen-4,4'-yl, benzene-1,4-diyl, bis(phen-4-yl)methane, biphen-4,4'-diyl, 1-methyl-benzen-2,4-diyl, oxybis(benzene-4,4'-diyl), 5,5'-oxybis(1,3-dimethylbenzen-2-diyl), or 4,4'-(propane-2,2-diyl)bis((phenoxy-4-diyl)benzene);    n and m=2-500.