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Chemoselective terpolymerization can produce polymer materials with diverse compositions and sequential structures, and thus have attracted considerable attention in the field of polymer synthesis. However, the intrinsic complexity of three‐component system also brings great chanllenge, in regard to the reactivity and selectivity of different monomers. Herein, we report the terpolymerization of CO2/epoxide/anhydride by a binary organocatalytic C3N3‐Py‐P3/TEB (triethylborane) system. Both the activity and chemoselectivity were highly dependent upon the molar ratio of C3N3‐Py‐P3 to TEB, and sequence‐controlled poly(ester‐carbonate) copolymers were readily synthesized through one‐pot/one‐step methodology by tuning the stoichiometric ratio of phosphazene/TEB. In particular, C3N3‐Py‐P3/TEB with a molar ratio of 1/0.5 exhibited an unprecedentedly high chemoselectivity for ring‐opening alternating copolymerization (ROAC) of cyclohexene oxide (CHO) and phthalic anhydride (PA) first and then ROAC of CO2/CHO. Thus, well‐defined triblock polycarbonate‐b‐polyester‐b‐polycarbonate copolymers can be produced from the mixture of CO2, CHO and PA using a bifunctional initiator. With C3N3‐Py‐P3/TEB=1/1, tapered copolymers were obtained, while random copolymers with high content of polycarbonate (PC) were synthesized with further increasing the amount of TEB. The mechanism of the unexpected chemoselectivity was further investigated by DFT calculations. [ABSTRACT FROM AUTHOR]