An analysis of the construction of ion-containing CO2-based polycarbonates via metal-catalyzed immortal copolymerization of propylene oxide (PO) and carbon dioxide in the presence of protic ionic liquids (ILs) as chain transfer agents is described. The reactions were catalyzed by the binary (salen)CoO2CCF3/onium salt system in the absence of added solvent. On the basis of the results of this study, which includes H-1 NMR and MALDI-TOF-MS data, it is clear that the role of the protic IL in this process is dominated by its anion. When the anion of the protic ionic liquid is nucleophilic, such as chloride, the protic ionic liquid acts as a cocatalyst in the reaction system for epoxide ring opening, and the chain transfer reaction initiated by the cation is completely suppressed. However, when the anion of the protic ionic liquid is non-nucleophilic, such as BF4-, the protic ionic liquid acts as an effective chain transfer agent to afford IL-based polycarbonates by rapid and reversible chain transfer processes. The resultant polymer structures were confirmed by H-1 NMR and MALDI-TOF-MS spectroscopies. This work provides a facile method for synthesizing ion-containing polycarbonate as well as an insight into immortal copolymerization processes utilizing protic ionic liquids as chain transfer agents.