Here, we report the synthesis of block copolymers from switchable ROP of rac-lactide (LA) and ROCOP of propene oxide (PO)/carbonyl sulfide (COS) using unimolecular trifunctional catalysts (Fig. ). Polylactide (PLA) is among the most attractive bio-based aliphatic polyesters owing to its biodegradability and biocompatibility. PO is one of the most important epoxides with an annual global production of ~ 10 million tons. COS is an emerging monomer for the synthesize of sulfur-containing polymers, produced industrially from abundant CO and elemental sulfur (waste from oil refineries and gas purification). ROCOP of PO/COS, first reported by our group in 2013, has been demonstrated to be a facile route to produce polythiocarbonates with approving optical and mechanical properties. Block copolymers bearing PLA and polythiocarbonate fragments promise to integrate the superior properties of each block. In addition, there are several stereo-, regio-, and chemoselective regulation in ROP of rac-LA and ROCOP of PO/COS. Remarkably, chiral catalyst 4 delivered block copolymers containing isotactic PLA and alternating/regioregular polythiocarbonate blocks.