The mechanistic details of the coupling reaction of propylene oxide with carbon dioxide catalyzed by copper(I) cyanomethyl to yield cyclic carbonate were elucidated by density functional theory (DFT) calculations at the B3LYP/6-311G** level. Our results reveal that the overall reaction is stepwise and considered to include two processes. In process 1, CO2 insertion into the Cu(I)-C bond of copper(I) cyanomethyl affords activated carbon dioxide carriers. In process 2, O-coordination of propylene oxide molecule to the electrophilic copper center of carriers occurs. Herein, three possible pathways were investigated, and the calculated reaction free energy profiles were compared. It was found that carrier 8 reacting with propylene oxide is more favored than the other two carriers (6 and 7) both kinetically and thermodynamically. Several factors, such as the composition of catalyst, the coordinate environment of copper, and the symmetry of frontier molecular orbitals, affected the reaction mechanisms, and the outcomes were identified. The overall reaction is exothermic. In addition, natural bond orbital (NBO) analysis has been performed to study the effects of charge transfer and understand the nature of different interactions between atoms and groups. The present theoretical study explains satisfactorily the early reported experimental observations well and provides a clear profile for the cycloaddition of carbon dioxide with propylene oxide promoted by NCCH2Cu.