Toward Stereo- and Sequence-Defined Block Copolymers via a Three-Site Organocatalyst

Thedevelopment of sustainable polymers as candidates to replacepetroleum-based plastics is a priority, but still a grand challenge.Here, we report a series of stereo- and sequence-defined copolymerswith isotactic polylactide and alternating/regioregular polythiocarbonateblocks from one-pot, metal-free, and switchable copolymerization of rac-lactide, COS, and epoxides. The highly active organocatalystsincorporating boron and chiral thiourea and tertiary amine groupsare developed based on the site-monomer specific recognition mechanism;in short, the boron site selectively activates epoxides through B-Obonds and the thiourea site selectively activates lactide throughhydrogen bonds. Such block copolymers manifest impressive toughnessand ductility owing to their isotactic polylactide fragments and highmolecular weights, such as P10 with a molecular weightof 92.3 kDa, an ultimate tensile strength of 59 +/- 3 MPa, andan elongation at break of 222 +/- 25%. Our strategy provides distinctiveinsights into designing robust organocatalysts and promising sustainablepolymers.