Precision synthesis of copolymers with defined sequence structures from mixed monomers remains a challenge due to the requirements for stringent control of monomers enchainment order. In the last decade, significant progresses have been made in one-step sequence-selective block copolymerization from three-component monomer mixtures comprising cyclic anhydride, epoxide, and a third monomer such as CO₂, cyclic ester, N‑sulfonyl aziridine (Az), etc. On the other hand, it is still rarely reported for one-step sequence-selective synthesis of A_nB_m type block copolymers from classic binary monomer mixtures. We have found that ethylene oxide (EO) and Az compose a valid pair provided with a two-component metal-free catalyst. Optimal Lewis acid/base ratio allows the two monomers to strictly block-copolymerize in a reverse order (EO-first) as compared with the conventional anionic approach (Az-first). Livingness of the copolymerization facilitates one-pot synthesis of multiblock copolymers by addition of mixed monomers in batches. Calculation results reveal that the Lewis acid accelerates the enchainment of EO and decelerates that of Az because of its much stronger coordination with the oxyanion than the sulfonamide anion. This unique Janus catalytic effect is key to enlarge the activity difference and fulfill strictly reversed monomers enchainment order. The same principle also applies well to a binary monomer mixture comprising cyclic anhydride (A) and epoxide (B). The catalyst allows the excess epoxide to act as “the third monomer” and actualizes one-step tailored synthesis of (AB)_nB_m type block copolymers. The Janus catalytic effect enables us to regulate the relative activity of the monomers and control the microstructure of the copolymer, i.e. from gradient to block with the same molar mass and comonomer composition, simply by changing the Lewis acid/base ratio. The influence of microstructure on the chromatographic behavior of gradient copolymers with different gradient strengths is revealed by liquid chromatography at critical conditions and gradient liquid adsorption chromatography. In both cases, the elution volume of gradient copolymer increases with the gradient strength.

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