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Incorporating sulfur (S) atoms into polymer main chains endows these materials with many attractive features, including a high refractive index, mechanical properties, electrochemical properties, and adhesive ability to heavy metal ions. The copolymerization involving S-containing monomers constitutes a facile method for effectively constructing S-containing polymers with diverse structures, readily tunable sequences, and topological structures. In this review, we describe the recent advances in the synthesis of S-containing polymers via copolymerization or multicomponent polymerization techniques concerning a variety of S-containing monomers, such as dithiols, carbon disulfide, carbonyl sulfide, cyclic thioanhydrides, episulfides and elemental sulfur (S8). Particularly, significant focus is paid to precise control of the main-chain sequence, stereochemistry, and topological structure for achieving high-value applications. [ABSTRACT FROM AUTHOR]

The desymmetrization copolymerizationof meso-epoxideswith CO2using chiral catalysts or reagents is regardedas a valuable strategy for the synthesis of optically active polycarbonateswith main-chain chirality. The present study demonstrates that thebiphenol-linked dinuclear Co(III) complexes show unprecedented activity,enantioselectivity, and broad substrate scope for coupling CO2with various meso-epoxides to afford thecorresponding isotactic polycarbonates. This investigation also focuseson the mechanistic understanding on the origin of enantioselectivityand highly catalytic activity in the enantiopure dinuclear Co(III)complex mediated copolymerization process, using cyclohexene oxideas a model monomer of meso-epoxides. The kineticstudy by in situinfrared spectroscopy revealed afirst-order dependence on the catalyst concentration in the systemsof biphenol-linked dinuclear Co(III) complex alone or in the presenceof an ionic cocatalyst. An intramolecular bimetallic cooperation mechanismwas proposed to be predominantly responsible for the copolymerizationprocess, wherein alternating chain growth and dissociation take turnsbetween two Co(III) ions from the inside cleft of the catalyst bythe nucleophilic attack of the growing carboxylate species at onemetal center toward the activated epoxide at the other. Density functionaltheory calculations suggested that in the two diastereoisomers ofthe biphenol-linked dinuclear Co(III) complexes the matched configurationwas (S,S,S,S,S)- rather than (S,S,R,S,S)-conformer for CO2/meso-epoxide copolymerizationto give the corresponding polycarbonate with S,S-configuration.The addition of an ionic cocatalyst with bulky cation significantlyimproves both the catalytic activity and enantioselectivity, whilethe presence of a coordination Lewis base caused dramatically a changein the chiral induction orientation. [ABSTRACT FROM AUTHOR]

Trivalent cobalt complexes of salicylaldimine in thepresence ofan inter- or intramolecular nucleophilic cocatalyst have proven tobe excellent catalysts for the copolymerization of CO2andepoxides to selectively afford the corresponding polycarbonates inperfectly alternating nature. Especially, bifunctional cobalt(III)–salencomplexes bearing an appended quaternary ammonium salt are more efficientin catalyzing this copolymerization even at high temperatures andextremely low catalyst loading. The present study focuses on comparativekinetics of two different catalyst systems (binary catalyst systemof salen(III)X 1/nBu4NX and bifunctional catalyst 2bearing an appendedquaternary ammonium salt, X = 2,4-dinitrophenoxide) for coupling CO2and epoxides (propylene oxide or cyclohexene oxide) by meansof in situinfrared spectroscopy. An induction periodwas readily found in the binary catalyst system, and its length significantlydepends on catalyst loading. Contrarily, no induction period was observedin the bifunctional catalyst 2, in which the overallreaction pathway is consistent with the first-order dependence oncatalyst concentration. A reaction order of 1.61 of catalyst concentrationwas obtained from the binary 1/nBu4NX catalyst system, indicating the complexityof the copolymerization. The energies of activation determined forcyclic carbonate and copolymer formation in the coupling reactionof CO2and propylene oxide catalyzed by the binary 1/nBu4NX system are50.1 and 33.8 kJ/mol, respectively, compared to the correspondingvalues in the bifunctional catalyst 2of 77.0 and 29.5kJ/mol. The big difference in the energies of activation for cycliccarbonate versus copolymer formation accounts for the excellent selectivityfor copolymer formation in the bifunctional catalyst systems evenat elevated temperatures. In the coupling system of CO2and 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. [ABSTRACT FROM AUTHOR]

The use of carbon dioxide as a carbon source for the synthesis of organic chemicals can contribute to a more sustainable chemical industry. Because CO2is such a thermodynamically stable molecule, few effective catalysts are available to facilitate this transformation. Currently, the major industrial processes that convert CO2into viable products generate urea and hydroxybenzoic acid. One of the most promising new technologies for the use of this abundant, inexpensive, and nontoxic renewable resource is the alternating copolymerization of CO2and epoxides to provide biodegradable polycarbonates, which are highly valuable polymeric materials. Because this process often generates byproducts, such as polyether or ether linkages randomly dispersed within the polycarbonate chains and/or the more thermodynamically stable cyclic carbonates, the choice of catalyst is critical for selectively obtaining the expected product. [ABSTRACT FROM AUTHOR]

Electrocarboxylation of the C(sp 3 )-O bond in 1,3-oxazolidin-2-ones with CO 2 to achieve β-amino acids is developed. The C-O bond in substrates can be selectively cleaved via the single electron transfer on the surface of a cathode or through a CO 2 •   - intermediate under additive-free conditions. A great diversity of β-amino acids can be obtained in a moderate to excellent yield and readily converted to various biologically active compounds.

Carbon dioxide has attracted broad interest as a renewable C1 feedstock for efficient transformation into value-added organic chemicals; nevertheless, far less attention was paid to its stereochemically controlled catalytic fixation/conversion processes. Here, we report a new strategy for the selective synthesis of chiral carbamates from carbon dioxide via polycarbonate intermediates, which are formed by the desymmetric copolymerization of meso-epoxides using enantiopure dinuclear Co(III) catalyst systems with 99% enantioselectivity. Subsequent degradation reaction of the resultant polycarbonates with various primary or secondary amine nucleophiles can afford optically active carbamates, with the complete configuration retention of the two chiral carbon centers. Our accomplishment reported here opens up a new route to prepare a wide range of CO 2 -based carbamate scaffolds with excellent yields and 99% enantiomeric excess.

Synthesis of stereoregular polyesters with main-chain chirality was achieved for the first time by the asymmetric copolymerization of meso-epoxides and cyclic anhydrides using catalyst systems based on enantiopure bimetallic complexes. The combination of the biphenol-linked dinuclear aluminum complex with tert-butyl groups in the phenolate ortho-positions and a nucleophilic co-catalyst was found to be more efficient in catalyzing this asymmetric copolymerization, affording enantiomerically enriched polyesters (up to 91% ee) with completely alternating structure and narrow molecular weight distribution. It was discovered that the isotactic-enriched poly(cyclopentene phthalate) is a typical semicrystalline material with a melting endothermic peak at 221 °C. This study is expected to provide a promising route to prepare various stereoregular polyesters having a wide variety of physical properties and degradability.

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.

Lewis pair cooperation provides a facile approach for polymerizing dissymmetric divinyl polar monomers such as 4-vinylbenzyl methacrylate in excellent regioselectivity and high reactivity at mild conditions, affording soluble polymers bearing pendant active vinyl groups with high molecular weight (up to 6.4 × 10 5 g/mol) and narrow polydispersity (PDI < 1.5). ESI-TOF MS study demonstrated that the polymerization process only concerned the methacrylic double bond and selectively remained the pendant allylic or styrene C═C bond.

The chiral SalenCo(III)OAc-catalyzed hydrolytic kinetic resolution (HKR) of racemic terminal epoxides to afford both enantioenriched epoxides and diols presents one of the most important achievements in asymmetric synthesis chemistry. Previous studies mainly focused on the development of highly efficient catalysts, while rare reports concerned the mechanistic understanding of metal valence change, associated with the formation of inactive Co(II)-Salen complex. Herein, we report the mechanistic aspects of catalyst deactivation regarding the transformation of Co(III) to Co(II) derivative in the HKR of racemic epoxides catalyzed by SalenCo(III)OAc complexes with an appended 1,5,7-triazabicyclo[4.4.0]dec-5-ene on the ligand framework by means of electrospray ionization mass spectrometry (ESI-MS). Continuous determination of transient cationic species in ESI-MS positive mode in conjunction with UV-vis spectroscopic studies at various time points provides evidence that a certain amount of SalenCo(III)OAc molecules were reduced to the corresponding Co(II) derivatives in the HKR of racemic propylene oxide or styrene oxide. To be accompanied by the reduction of Co(III) to Co(II), the resultant diols were oxidized to α-hydroxy ketones. These analyses along with some control experiments gave a mechanistic understanding of catalyst deactivation of SalenCo(III)OAc-catalyzed HKR of racemic epoxides regarding an unveiled redox reaction between Co(III)-Salen and diol, the hydrolyzed product.

Enantiopure metal-complex catalyzed asymmetric alternating copolymerization of CO(2) and meso-epoxides is a powerful synthetic strategy for preparing optically active polycarbonates with main-chain chirality. The previous studies regarding chiral zinc catalysts provided amorphous polycarbonates with moderate enantioselectivity, and thus, developing highly stereoregular catalysts for this enantioselective polymerization is highly desirable. Herein, we report the synthesis of highly isotactic poly(cyclohexene carbonate)s from meso-cyclohexene oxide using dissymmetrical enantiopure salenCo(III) complexes in conjunction with bis(triphenylphosphine)iminium chloride (PPNCl) as catalyst. The presence of a chiral induction agent such as (S)-propylene oxide or (S)-2-methyltetrahydrofuran significantly improved the enantioselectivity regarding (S,S)-salenCo(III) catalyst systems. Up to 98:2 of RR:SS was observed in the resultant polycarbonates obtained from the catalyst system based on (S,S)-salenCo(III) complex 4d bearing an adamantyl group on the phenolate ortho position, in the presence of (S)-2-methyltetrahydrofuran. Primary ONIOM (DFT:UFF) calculations, which were performed to investigate the effect of the competitive coordination of (S)-induction agent versus cyclohexene oxide to Co(III) center on enantioselectivity, suggest that the (S)-C-O bond in cyclohexene oxide is more favorable for cleavage, due to the interaction between oxygen atom of (S)-induction agent and (S)-C-H of the coordinated cyclohexene oxide. The highly isotactic poly(cyclohexene carbonate) is a typical semicrystalline polymer, possessing a melting point of 216 °C and a decomposition temperature of 310 °C.

Selective transformations of carbon dioxide and epoxides into biodegradable polycarbonates by the alternating copolymerization of the two monomers represent some of the most well-studied and innovative technologies for potential large-scale utilization of carbon dioxide in chemical synthesis. For the most part, previous studies of these processes have focused on the use of aliphatic terminal epoxides or cyclohexene oxide derivatives, with only rare reports concerning the synthesis of CO(2) copolymers from epoxides containing electron-withdrawing groups such as styrene oxide. Herein we report the production of the CO(2) copolymer with more than 99% carbonate linkages from the coupling of CO(2) with epichlorohydrin, employing binary and bifunctional (salen)cobalt(III)-based catalyst systems. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving two electronically different epoxides: epichlorohydrin and propylene oxide. The relative small activation energy difference between copolymer versus cyclic carbonate formation for the epichlorohydrin/CO(2) process (45.4 kJ/mol) accounts in part for the selective synthesis of copolymer to be more difficult in comparison with the propylene oxide/CO(2) case (53.5 kJ/mol). Direct observation of the propagating polymer-chain species from the binary (salen)CoX/MTBD (X = 2,4-dinitrophenoxide and MTBD = 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) catalyst system by means of electrospray ionization mass spectrometry confirmed the perfectly alternating nature of the copolymerization process. This observation in combination with control experiments suggests possible intermediates involving MTBD in the CO(2)/epichlorohydrin copolymerization process.

The mechanism of the copolymerization of CO(2) and epoxides to afford the corresponding polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand framework has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared spectroscopy (FTIR). The single-site, cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation during CO(2)/propylene oxide (PO) copolymerization even at temperatures up to 100 degrees C and high [epoxide]/[catalyst] ratios, and/or low CO(2) pressures. The anchored TBD on the ligand framework plays an important role in maintaining thermal stability and high activity of the catalyst. ESI-MS and FTIR studies, in combination with some control experiments, confirmed the formation of the carboxylate intermediate with regard to the anchored TBD on the catalyst ligand framework. This analysis demonstrated that the formed carboxylate intermediate helped to stabilize the active Co(III) species against decomposition to inactive Co(II) by reversibly intramolecular Co-O bond formation and dissociation. Previous studies of binary catalyst systems based on Co(III)-Salen complexes did not address the role of these nucleophilic cocatalysts in stabilizing active Co(III) species during the copolymerization. The present study provides a new mechanistic understanding of these binary catalyst systems in which alternating chain-growth and dissociation of propagating carboxylate species derived from the nucleophilic axial anion and the nucleophilic cocatalyst take turns at both sides of the Co(III)-Salen center. This significantly increases the reaction rate and also helps to stabilize the active SalenCo(III) against decomposition to inactive SalenCo(II) even at low CO(2) pressures and/or relatively high temperatures.