Small quantities of regio-defects in a regio-/stereoregular polymer weaken its tacticity and properties. This work clarified the origin of the regio-defect in the process of synthesizing poly(monothiocarbonate) through the copolymerization of propylene oxide (PO) and carbonyl sulfide (COS) catalyzed by a (salen)CrCl complex accompanied by bis(triphenylphosphoranylidene)ammonium chloride ([PPN]Cl). Quantitative characterization results from the MALDI-TOF-MS and H-1 (C-13) NMR spectroscopy suggested that the chain transfer reaction resulted in the regio-defect in the final copolymer, i.e., tail-to-tail (T-T) diad and dithiocarbonate (DTC) unit. The chain transferring to water in the reaction system led to the production of a (salen)Cr-OH intermediate, which initiated the copolymerization via either attacking PO first to result in formation of a T-T diad or first activating COS to produce mercapto (-SH) end-capped dormant chains via decarboxylation, thus generating a DTC unit in the final product through another chain transfer reaction and regrowth of the chain. The content of regio-defect in the final copolymer was directly related to the water content in the system. It is essential to reduce the regio-defect for an immortal COS/PO copolymerization reaction by eliminating trace amounts of water. We also demonstrated the application of alpha-OH, omega-OH poly(propylene monothiocarbonate) for synthesizing a well-defined ABA triblock copolymer, polystyrene-block-poly(propylene monothiocarbonate)-block-polystyrene (PS-b-PPMTC-b-PS), with a M-n of 10 800 g/mol and a PDI of 1.08 via an atom transfer radical polymerization (ATRP) method.