Two carboxylic acid containing diamines, CADA1 and CADA2, with or without bulky CF3 group were synthesized and polymerized with 6FDA, BTDA and DSDA to form four polyimides, denoted as 6FDACADA1, 6FDA-CADA2, BTDA-CADA1 and DSDA-CADA1. After the thermal induced decarboxylation crosslink reaction, polyimides with 100 times' higher CO2 permeabilities, similar CO2/CH4 selectivities and much better plasticization resistant properties were obtained compared with the un-crosslinked polyimides. The polymer d-spacing increased with the heating temperature and those of the fully crosslinked polyimides, 6FDA-CADA1450 and DSDA-CADA1-450, were 5.81 angstrom and 5.51 angstrom, respectively, which were very close to the estimated distance between the crosslinked polymer chains (5.57 angstrom, calculated by Material Studio). The CO2-induced plasticization phenomenon was not observed at a CO2 pressure up to 30 atm for polyimides after thermally treated at temperatures above 425 degrees C. Moreover, polyimides crosslinked at different temperatures had almost the same selectivities, but their permeabilities gradually increased with the crosslinking temperature. For CO2/ CH4 gas pair, the separation performances of 6FDA-CADA1-425 and BTDA-CADA1-450 membranes surpassed the "2008 Robeson Upper Bound", and other membranes thermally treated at 400 and 425 degrees C were all positioned near the "1991 Robeson Upper Bound".