Copolymers bearing DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) and DBN (1,5-diazabicyclo[4.3.0]-non-5-ene) moieties fixed carbon dioxide under atmospheric pressure. The copolymers bearing DBN moieties fixed carbon dioxide faster than those bearing DBU moieties owing to the lower steric hindrance around the imine structure. These copolymers held trapped CO2 under a N-2 flow at 25 degrees C, whereas the corresponding low-molecular weight amidines release trapped CO2 under the same conditions. The trapped CO2 in the copolymers was quantitatively released by a N-2 flow at 120 degrees C. The CO2 trapping efficiency of a copolymer bearing DBN moieties is competitive with that of an amidine-containing polymer, whereas the preparative method for the copolymer bearing DBN is easier. The higher CO2 trapping abilities of the DBN derivatives were supported by computational calculation.