Ring-opening polymerizations(ROP)of epoxides and lactones share the common feature of proceeding through alkoxide/hydroxyl end groups,but their one-pot sequential polymerization has been a major challenge as the active species that work for one can either be inactive or lead to uncontrolled polymerization for the other,especially in the case of conventional metal-based initiating/catalytic systems.The development of organocatalytic polymerization techniques has offered new opportunities for the fulfillment of such synthetic tasks.Recently,we have found that controlled sequential ROP of epoxides and lactones can be performed leading to well-defined block copolymers provided the organic catalysts are adequately chosen and finely tuned to the monomers to polymerize.For example,the use of a mild phosphazene base enables the sequential ROP of ethylene oxide,ε-caprolactone and L-lactide to generate the corresponding triblock terpolymer,and a “catalyst switch” strategy(the successive use of a strong phosphazene base and an acid catalyst)enables the one-pot synthesis of well-defined block copolymers based on various polyethers,polyesters and polycarbonates with two,three or four block types.