Main-chain degradable amphiphilic diblock copolymers composed of a hydrophilic-functionalized polyester and PLA were facilely prepared by one-pot ring-opening polymerization (ROP) via actively manipulating the catalytic states of an acid–base catalytic system. The resultant block copolymers showed low critical micelle concentration (CMC) in water and were capable of forming stable micelles with optimal hydrodynamic particle size (average diameter 83 nm) and narrow particle distribution.