A porous polymer featuring a flexible backbone and rigid, fluorinated side-chains is prepared via Ring-Opening Metathesis Polymerization (ROMP). This polymer exhibits ultrahigh CO2 permeability (> 21000 Barrer) and exceptional plasticization resistance (CO2-induced plasticization pressure > 51 bar). By generating ultramicroporosity through pre-designed side chains instead of the polymer backbone, a promising new design strategy is presented for gas-separation membranes.