The semicrystalline poly(monothiocarbonate)s were prepared by the copolymerization of carbonyl sulfide (COS) and ethylene oxide, an achiral epoxide, using a bifunctional chromium(III) complex as catalyst. The resultant copolymer, possessing perfectly alternating structure, high molecular weight, and narrow polydispersity, has a melting temperature of 128.2 °C, with a melting enthalpy up to 75.44 J/g. Moreover, an ABA triblock copolymer containing the “hard” semicrystalline poly(ethylene monothiocarbonate) (A) and the “soft” amorphous poly(propylene monothiocarbonate) (B) is synthesized by stepwise addition of epoxides. The tensile testing demonstrates the triblock copolymer may have the potential as a thermoplastic elastomer.

Carbonyl sulfide (COS) as a carbon source for copolymerization with epoxides has recently received some attention. The introduction of sulfur atom can provide enhancement of important polymer properties compared to the corresponding copolymer from CO2. However, the synthesized copolymers are all amorphous, therefore hindering them to be used as structural materials. Herein, we report the synthesis and characterization of semicrystalline poly(thiocarbonate)s derived from enantiopure epichlorohydrin and COS employing the single-site bifunctional catalyst. The catalyst shows excellent regioselectivity for epichlorohydrin ring-opening at methylene carbon. The copolymerization mechanism has been studied by means of NMR and ESI-MS methods. It is found the reaction temperature plays an important role in the crystallization behavior of the resultant copolymers. That is, at ambient temperature the propagating monothiocarboxylate species favors the nucleophilic attack at the chloromethylene of epichlorohydrin to form an epoxy ring end group, along with the release of chloride ion as a new initiator. This chain termination results in low molecular weight and board distributed copolymers, in accordance with the amorphousness. Alternatively, at reduced temperature such as −25 °C, the monothiocarboxylate species prefers consecutive alternating enchainment of COS and epichlorohydrin to give copolymers with enhanced molecular weights. Of importance, the formed polymer is a typical semicrystalline thermoplastic, possessing a Tg of 15.6 °C and a Tm of 96.7 °C.

Unprecedented activity (TOF > 270 000 h–1), high polymer selectivity (>99%), and excellent durability (TON > 600 000) were observed in the copolymerization of carbonyl sulfide (COS) and epoxides mediated by the single-site bifunctional chromium catalyst containing a Lewis acidic metal center and a sterically hindered organic base in a molecule, selectively producing the corresponding poly(thiocarbonate)s with completely alternating structure, high molecular weight, and narrow monodispersity. No oxygen–sulfur exchange reaction occurred even at an elevated temperature of 80 °C. Nevertheless, this catalyst was not efficient for CO2/epoxides copolymerization. The presence of CO2 completely inhibits the reactivity of COS. Contrarily, the corresponding Co(III) complex with the same ligand showed very low activity for COS/epoxides copolymerization but excellent activity (TOF > 13 000 h–1) for CO2/epoxides copolymerization at both ambient and high temperatures, affording the polycarbonates with >99% carbonate linkages and high molecular weight up to 467.0 kg/mol.

Regioselective ring opening of three-membered heterocyclic compounds (epoxides or N-substituted aziridines) at various temperatures was observed in coupling reactions with CO2 by the use of an aluminum–salen catalyst in conjunction with intramolecular quaternary ammonium salts as cocatalysts, affording the corresponding five-membered cyclic products with complete configuration retention at the methine carbon. Notably, this bifunctional aluminum-based catalyst exhibited nearly 100% regioselectivity for the ring opening at the methylene C–O bond for various terminal epoxides. This was true for those bearing an electron-withdrawing group, such as styrene oxide or epichlorohydrin, thereby affording the synthesis of various enantiopure cyclic carbonates that have previously been obtained only rarely by other methods. An intramolecular cooperative catalysis is suggested to contribute to the high activity and excellent stereochemistry control observed. Surprisingly, the highly selective ring opening at the methine carbon of N-substituted aziridines was found in the coupling with CO2, predominantly giving 5-substituted oxazolininones with retention of configuration as a result of double inversion at the methine carbon.

Trivalent cobalt complexes of salicylaldimine in the presence of an inter- or intramolecular nucleophilic cocatalyst have proven to be excellent catalysts for the copolymerization of CO2 and epoxides to selectively afford the corresponding polycarbonates in perfectly alternating nature. Especially, bifunctional cobalt(III)–salen complexes bearing an appended quaternary ammonium salt are more efficient in catalyzing this copolymerization even at high temperatures and extremely low catalyst loading. The present study focuses on comparative kinetics of two different catalyst systems (binary catalyst system of salen(III)X 1/nBu4NX and bifunctional catalyst 2 bearing an appended quaternary ammonium salt, X = 2,4-dinitrophenoxide) for coupling CO2 and epoxides (propylene oxide or cyclohexene oxide) by means of in situ infrared spectroscopy. An induction period was readily found in the binary catalyst system, and its length significantly depends on catalyst loading. Contrarily, no induction period was observed in the bifunctional catalyst 2, in which the overall reaction pathway is consistent with the first-order dependence on catalyst concentration. A reaction order of 1.61 of catalyst concentration was obtained from the binary 1/nBu4NX catalyst system, indicating the complexity of the copolymerization. The energies of activation determined for cyclic carbonate and copolymer formation in the coupling reaction of CO2 and propylene oxide catalyzed by the binary 1/nBu4NX system are 50.1 and 33.8 kJ/mol, respectively, compared to the corresponding values in the bifunctional catalyst 2 of 77.0 and 29.5 kJ/mol. The big difference in the energies of activation for cyclic carbonate versus copolymer formation accounts for the excellent selectivity for copolymer formation in the bifunctional catalyst systems even at elevated temperatures. In the coupling system of CO2 and cyclohexene oxide, the energy of activation for copolymer (Ea) formation is 47.9 kJ/mol for the binary 1/nBu4NX catalyst system, higher than 31.7 kJ/mol determined in the bifunctional catalyst 2.