Efficient Access to Sequence-Controlled Poly(α-hydroxy acids) with A(BC)D-Type and A(BC)A-Type Triblock Structures via Self-Switchable Ring-Opening Polymerization of Monomer Mixtures.

Synthesizing block-sequence-controlled poly(α-hydroxy acids) of three or four α-hydroxy acids remains challenging in one step. In this study, a strategy was employed using three monomers of -carboxyanhydrides (OCAs) consisting of one α-hydroxy acid (A), asymmetric cyclic diester (B and C, two different α-hydroxy acids of B and C), and symmetric cyclic diester (one α-hydroxy acid of D) with remarkably different activities toward a stereoselective, regioselective, and chemoselective initiator of a zirconium complex. Then, via a self-switchable approach, these monomers can be copolymerized in a well-controlled block sequence of A(BC)D and A(BC)A without an external stimulus. Moreover, upon addition of more monomer mixtures during the copolymerization process, more complicated sequence-controlled poly(α-hydroxy acids) can be achieved with up to 15 blocks.