It is particularly important to be able to synthesize well-defined block copolymers from one-pot monomer mixtures in a metal-free manner. Herein, switchable copolymerization between the ring-opening alternating copolymerization (ROAC) of anhydrides with epoxides and the ring-opening polymerization (ROP) of lactones using imidazolium ionic liquids has been investigated in detail. It is found that a simple ionic liquid, 1-ethyl-3-methylimidazole chlorine (EMIMCl), was efficient to bridge ROAC and ROP, producing well-defined block polyesters from anhydrides, epoxides and lactones in a one-step strategy. The DFT calculation and experimental results indicated that the sequential transformation of mixed monomers mainly depends on the synergistic effect of anions and cations of ionic liquids, where the generating alkoxide anions are the key to regulate ROAC and ROP, and imidazolium cations (EMIM⁺) contribute to the monomer activation and inhibition of potential side reactions. EMIMCl exhibits high catalytic activity and selectivity in a multi-component system, which enables the switchable copolymerization of mixed monomers from ROAC to ROP. This study provides a new route for the facile synthesis of block polyesters using a metal-free catalytic system.