From the mixture of PO and LA, under a nitrogen atmosphere, at 40 °C for 0.8 h, PLA was firstly synthesized with an Mn of 3.9 kDa and LA conversion of 30% (Fig. ). When COS ([COS]0:[PO]0 = 1.2:1.0) was charged into the system, ROCOP of PO/COS was triggered whereas ROP of LA was completely turned off, affording a triblock copolymer with an Mn of 33.4 kDa at the sixth hour (Fig. ). After draining COS and charging N2, ROP of LA restarted, delivering a pentablock copolymer with an Mn of 42.1 kDa at the eighth hour, as revealed by unimodal GPC peaks (Fig. ) and diffusion-ordered NMR (Fig. ). The PLA block was highly isotactic with a Pm of 0.82 according to C NMR (Fig. ). The polythiocarbonate block had alternating and regioregular structures, as determined by NMR analysis (Fig. ).