NOVELTY - A material (A1) comprises substituted 7,7-dimethyl-7,8,9,10-tetrahydro-benzo(b)naphtho(2,3-e)(1,4)dioxine-1,4-dicarbonitrile monomer.    USE - As membrane for separating a gas mixture selected from hydrogen (H2)/carbondioxide (CO2), H2/CO2/carbon monoxide (CO), CO2/methane (CH4), CO2/CH4/hydrogen sulfide (H2S), hydrogen (H2)/nitrogen (N2) and oxygen (O2)/N2 (claimed).    ADVANTAGE - The ultraviolet (UV)-rearranged polymers of polymers of intrinsic microporosity (PIM)-1 membranes can be used for hydrogen (H2)/carbondioxide (CO2) separation with far superior separation performance compared to other polymeric membranes known in the art. The PIM-1 membrane after UV radiation for 4 hours can show H2 permeability of 452 barrer and H2/CO2 selectivity of 7.3. The polymer chains of PIM-1 can undergo a 1,2-migration reaction and be transformed to a close-to-planar like rearranged structure after UV radiation. As a result, the UV-irradiated PIM-1 membrane can exhibit a considerable decrease in both the fractional free volume (FFV) and size of micro-pores. In contrast to the unmodified PIM-1, the UV-irradiated PIM-1 membrane can exhibit exceptionally high gas separation performance that surpasses the upper bounds for gas pairs such as H2/nitrogen (N2), CO2/methane (CH4) and H2/CO2. The UV-irradiated PIM-1 can exhibit better polymer chain packing and a smaller fractional free volume (FFV) compared to the unmodified PIM-1. Compared to pure gas tests, the UV-irradiated PIM-1 membrane can exhibit stable and comparable separation performance in mixed gas tests with and without the presence of carbon monoxide (CO). Thus the UV-rearranged PIM-1 membrane has great potential for the purification and production of industrial hydrogen.    DETAILED DESCRIPTION - A material (A1) comprises substituted 7,7-dimethyl-7,8,9,10-tetrahydro-benzo(b)naphtho(2,3-e)(1,4)dioxine-1,4-dicarbonitrile monomer of formula (I).    n=not defined.    INDEPENDENT CLAIMS are included for the following:    (1) preparing the material (A1) involving: (a) providing an organic polymeric material containing a micro-porous structure and polymer chains; and (b) performing ultraviolet (UV) irradiation on the organic polymeric material to form a UV irradiated organic polymeric material; and    (2) separating a gas mixture, involving: contacting the gas mixture with the material (A1).