Mechanistic Study of Regio-Defects in the Copolymerization of Propylene Oxide/Carbonyl Sulfide Catalyzed by (Salen)CrX Complexes

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 ¹H (¹³C) 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 <i>immortal</i> COS/PO copolymerization reaction by eliminating trace amounts of water. We also demonstrated the application of α-OH, ω-OH poly­(propylene monothio­carbonate) for synthesizing a well-defined ABA triblock copolymer, polystyrene-<i>block</i>-poly­(propylene monothiocarbonate)-<i>block</i>-polystyrene (PS-<i>b</i>-PPMTC-<i>b</i>-PS), with a <i>M</i>_n of 10 800 g/mol and a PDI of 1.08 via an atom transfer radical polymerization (ATRP) method.