Previous studies demonstrate that decarboxylation cross-linking improves both the plasticization resistance and gas separation properties of a polymer precursor. However, when the decarboxylation reactions were carried out at low temperatures (350-400 degrees C), gas permeability was low because of the intrinsically low gas permeability of the polymer precursor. In this work, we synthesized a series of highly permeable polymer precursors having a triptycene-based Troger's base (TB) backbone structure and different amounts of carboxylic acid side groups. The TB polymer precursors were heated in a temperature range of 275-400 degrees C to induce cross-linking. After cross-linking, a more open polymer structure was obtained, resulting in an increase in the free volume fraction and gas permeability. The lowest cross-linking temperature was 350 degrees C and the separation performances of the cross-linked polymers surpassed the "2008 Robeson Upper Bound" for the CO2/CH4 and O-2/N-2 gas pairs. Moreover, the cross-linked polymers were not plasticized by CO2 at a pressure up to 30 bar.