The calculation results were further corroborated by NMR characterization. By comparing the O NMR spectra of PO mixed with catalysts 3, 5, and 6 (Fig. ), we observed that catalysts we developed an intramolecular site switching method to synthesize copolymers with precise sequences from PO, rac-LA, and COS in one pot. As our expectation, catalyst 4 exhibited a switchable feature toward the mixture of PO, rac-LA, and COS (Fig. ). Only ROCOP of PO/COS occurred from the mixture of PO, LA, and COS, while only ROP of LA occurred in the absence of COS. This phenomenon could be ascribed to the rapid binding of COS to the oxyanion to form a thiocarbonate anion, which can ring open PO but not LA, as revealed by the control experiments (k1 > k3 > k2, Figs. and).