The synthesis of sequence-controlled block copolymers from a mixture of cyclic ester monomers represents a significant challenge. In this study, we demonstrate the selective and self-switchable ring-opening polymerization (ROP) of three different cyclic ester monomers (lactide (LA), rac-β-butyrolactone (rac-β-BL) and ε-caprolactone (ε-CL)) to block copolymers of the form (AB) x (BC) y using a highly selective yttrium catalyst. Each block contains units from two monomers and completely excludes one monomer. The switch from formation of one block to the next is strictly controlled by the presence or absence of LA in the reaction mixture rather than relative concentrations of each monomer. Individual block length and composition are determined by the initial monomer concentrations.