Innovative Approach to Chiral Polyurethanes: Asymmetric Copolymerization with Isocyanates.

The introduction of nitrogen-containing functional groups to chiral polymer backbones enables the tailoring of physical properties and offers opportunities for further post-polymerization modification. However, the substrate scope of such polymers is extremely limited because monomers having nitrogen-containing groups can change coordination state with respect to the metal centers, thus decreasing the activity and enantioselectivity and even poisoning the catalyst completely. In this paper, we report our attempts to carry out the asymmetric copolymerization of meso-epoxide with highly reactive isocyanates. In particular, we found that biphenol-linked bimetallic Co(III) complexes with multiple chiral centers are very efficient in catalyzing this asymmetric copolymerization reaction, affording optically active polyurethanes with a completely alternating nature and a high enantioselectivity of up to 94% ee. Crucially, we identified that the steric hindrance at the phenolate ortho position of the ligand strongly influences the catalytic activity and product enantioselectivity. In addition, density functional theory calculations revealed that the highly sterically bulky substituents change the mechanism from bimetallic to monometallic, and result in the unexpected inversion of the chiral induction direction. Moreover, the high stereoregularity of the produced polyurethanes enhances their thermal stability. This study offers a versatile methodology for the synthesis of chiral polymers containing nitrogen functionalities.

Closed-loop recycling of sulfur-rich polymers with tunable properties spanning thermoplastics, elastomers, and vitrimers.

The development of closed-loop recycling polymers that exhibit excellent performance is of great significance. Sulfur-rich polymers possessing excellent optical, thermal, and mechanical properties are promising candidates for chemical recycling but lack efficient synthetic strategies for achieving diverse structures. Herein, we report a universal synthetic strategy for producing polytrithiocarbonates, a class of sulfur-rich polymers, via the polycondensation of dithiols and dimethyl trithiocarbonate. This strategy has excellent compatibility with a wide range of monomers, including aliphatic, heteroatomic, and aromatic dithiols enabling the synthesis of polytrithiocarbonates with diverse structures. The present synthesis strategy offers a versatile platform for the construction of thermoplastics, elastomers, and vitrimers. Notably, these polytrithiocarbonates can be easily depolymerized via solvolysis into the corresponding monomers, which can be repolymerized to virgin polymers without changing the material properties.

Synthesis of Helical-Shaped Axially Chiral Bisoxime Ethers via Chiral Phosphoric-Acid-Catalyzed Sequential Enantioselective Condensations.

A successful synthesis of helical-shaped axially chiral bisoxime ethers is reported. This approach utilized symmetric L-shaped diketone scaffolds as carbonyl components for the enantioselective condensation with hydroxylamines, delivering dual axially chiral oxime ethers with up to 99% ee. Additionally, the axially chiral mono-oxime ethers of azabicyclic ketones with high ee's were also successfully produced. Various chiral bicyclic lactams can be readily synthesized via Beckmann rearrangement, demonstrating a potential application in organic synthetic chemistry.

A Powerful Strategy for Synthesizing Block Copolymers via Bimetallic Synergistic Catalysis.

Block copolymers, comprising polyether and polyolefin segments, are an important and promising category of functional materials. However, the lack of efficient strategies for the construction of polyether-b-polyolefin block copolymers have hindered the development of these materials. Herein, we propose a simple and efficient method to obtain various block copolymers through the copolymerization of epoxides and acrylates via bimetallic synergistic catalysis. The copolymerization of epoxides and acrylates proceeds in a sequence-controlled manner, where the epoxides-involved homo- or copolymerization occurs first, followed by the homopolymerization of acrylates initiated by the alkoxide species from the propagating polymer chain, thus yielding copolymers with a block structure. Notably, the high monomer compatibility of this powerful strategy provides a platform for synthesizing various polyacrylate-based block copolymers comprising polyether, polycarbonate, polythiocarbonate, polyester, and polyurethane segments, respectively.

Electroreductive Ring-Opening Carboxylation of 1,3-Oxazolidin-2-ones with CO2 for Accessing ß-Amino Acids.

Electrocarboxylation of the C(sp3)-O bond in 1,3-oxazolidin-2-ones with CO2 to achieve ß-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 CO2• - intermediate under additive-free conditions. A great diversity of ß-amino acids can be obtained in a moderate to excellent yield and readily converted to various biologically active compounds.

Sequence Control in Asymmetric Terpolymerizations of meso-Epoxides, CO2 , and Phthalic Anhydride: Unprecedented Statistical Ester-Carbonate Distributions.

The statistical terpolymerization of epoxides, CO2 and cyclic anhydrides remains challenging, mainly because epoxide/CO2 and epoxide/anhydride copolymerizations typically proceed at considerably different rates. Herein, we report the syntheses of novel chiral terpolymers with unprecedented statistical distributions of carbonate and ester units (up to 50 % junction units) via the one-pot reaction of cyclohexene oxide, phthalic anhydride, and CO2 under mild conditions using enantiopure bimetallic aluminum-complex-based catalyst systems. Notably, all resulting terpolymers exhibited excellent enantioselectivities (≥96 % ee) that were independent of the carbonate-ester distribution. The statistical compositions of the carbonate and ester units in the resulting terpolymers were determined via 1 H and 13 C NMR spectroscopies. Furthermore, thermal properties were tuned by altering the ester content of the chiral terpolymer without influencing the enantioselective ring-opening step involving the meso-epoxide. This asymmetric terpolymerization methodology is also compatible with a variety of meso-epoxides to afford the corresponding terpolymers with 17 %-25 % junction units and excellent enantioselectivities (94 %-99 % ee). The present study is expected to provide new guidelines for preparing a broad range of biodegradable polymers with excellent enantioselectivities and adjustable properties.

Direct and Selective Electrocarboxylation of Styrene Oxides with CO2 for Accessing ß-Hydroxy Acids.

Highly selective and direct electroreductive ring-opening carboxylation of epoxides with CO2 in an undivided cell is reported. This reaction shows broad substrate scopes within styrene oxides under mild conditions, providing practical and scalable access to important synthetic intermediate ß-hydroxy acids. Mechanistic studies show that CO2 functions not only as a carboxylative reagent in this reaction but also as a promoter to enable efficient and chemoselective transformation of epoxides under additive-free electrochemical conditions. Cathodically generated α-radical and α-carbanion intermediates lead to the regioselective formation of α-carboxylation products.

Cationic P,O-Coordinated Nickel(II) Catalysts for Carbonylative Polymerization of Ethylene: Unexpected Productivity via Subtle Electronic Variation.

Transition-metal-catalyzed copolymerization of ethylene with carbon monoxide affords polyketones materials with excellent mechanical strength, photodegradability, surface and barrier properties. Unlike the widely used and rather expensive Pd catalysts, Ni-catalyzed carbonylative polymerization is very difficult since the strong binding affinity of CO to Ni deactivates the highly electrophilic metal center easily. In this study, various cationic P,O-coordinated Ni complexes were synthesized using the electronic modulation strategy, and the catalyst with strong electron-donating substituents exhibits an excellent productivity of 104  g polymer (g Ni)-1 , which represents a rare discovery that a Ni complex could operate with such exceptional efficiency in comparison with Pd catalysts. Notably, those Ni catalysts were also efficient for terpolymerization of ethylene, propylene with CO for producing commercial polyketone materials with low melting temperatures and easy processibility.

Controlled Disassembly of Elemental Sulfur: An Approach to the Precise Synthesis of Polydisulfides.

The usage of elemental sulfur (S8 ) for constructing sulfur-containing polymers is of great significance in terms of sulfur resource utilization or fabrication of high-performance polymers. Currently, the random disassembly of S8 hinders its direct use in the precise synthesis of sulfur-containing polymers. Herein, we provide an effective strategy for controlling the dismantlement of S8 to synthesize polydisulfides, a promising category of dynamic bonds containing polymers. In this strategy, the completely alternating copolymerization of one sulfur atom, which is orderly derived from S8 , with episulfides is achieved with MTBD (7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) as catalyst and [PPN]SbF6 ([PPN]+ is bis(triphenylphosphine)iminium) as cocatalyst. Delightedly, the living- polymerization feature, and the good monomer compatibility allows for the access to diverse polydisulfides. Furthermore, the density functional theory (DFT) was employed to elaborate the copolymerization process.

COS-triggered oxygen/sulfur exchange of isatins: chemoselective synthesis of functionalized isoindigos and spirothiopyrans via self-condensation and the thio-Diels-Alder reaction.

Herein, we present the first organocatalytic oxygen/sulfur atom exchange reaction (O/S ER) of isatins by employing carbonyl sulfide (COS) as a novel sulfuring reagent under mild reaction conditions. 8-Diazabicyclo[5.4.0]undec-7-ene (DBU) exhibited excellent activity in this approach. Remarkably, the chemical transformations of in situ generated 3-thioisatins can be tuned via the judicious choice of reaction solvents in a one pot process, enabling the selective formation of either functionalized isoindigos in CH3CN via a self-condensation process or spirothiopyrans in DMSO in the presence of conjugated dienes via the thio-Diels-Alder reaction. Mechanistic studies with experimental and density functional theory approaches revealed that the O/S ER between isatins and COS results in the formation of 3-thioisatins as the key intermediates, which further undergo solvent-controlled transformations to generate isoindigos or spirothiopyrans, respectively. The easily-accessible substrates and operational simplicity make the process suitable for further exploration. The practicality of this transformation was demonstrated by the gram-scale synthesis of isoindigo-based drug molecules and donor-acceptor conjugated polymers.

Enantioselective, Stereoconvergent Resolution Copolymerization of Racemic cis-Internal Epoxides and Anhydrides.

Unprecedented enantioselective resolution copolymerization of racemic cis-internal epoxides and anhydrides was mediated by dinuclear aluminum complexes with multiple chirality, affording optically active polyesters with two contiguous stereogenic centers, and the unreacted substrates in good enantioselectivity. Unexpected stereoconvergence is observed in this resolution copolymerization, where the selectivity factor for the enantioselective formation of copolymer significantly exceeds the kinetic resolution coefficient based on the unreacted epoxide at various conversions. Catalytic activity and copolymer enantioselectivity are strongly influenced by the phenolate ortho-substituents of the ligand set, as well as the axial linker and its chirality. An enantiopure binaphthol-linked bimetallic AlIII complex allows stereoconvergent access to the stereoregular semi-crystalline polyesters and a concomitant kinetic resolution of the epoxide substrates.

Randomly Distributed Sulfur Atoms in the Main Chains of CO2 -Based Polycarbonates: Enhanced Optical Properties.

Polymeric materials possessing both high refractive indices and high Abbe numbers are much in demand for the development of advanced optical devices. However, the synthesis of such functional materials is a challenge because of the trade-off between these two properties. Herein, a synthetic strategy is presented for enhancing the optical properties of CO2 -based polycarbonates by modifying the polymer's topological structure. Terpolymers with thiocarbonate and carbonate units randomly distributed in the polymers' main chain were synthesized via the terpolymerization of cyclohexene oxide with a mixture of CO2 and COS in the presence of metal catalysts, most notably a dinuclear aluminum complex. DFT calculations were employed to explain why different structural sequence were obtained with distinct bimetallic catalysts. Varying the CO2 pressure made it possible to obtain terpolymers with tunable carbonate linkages in the polymer chain. More importantly, optical property studies revealed that terpolymers with comparable thiocarbonate and carbonate units exhibited a refractive index of 1.501 with an enhanced Abbe number as high as 48.6, much higher than the corresponding polycarbonates or polythiocarbonates. Additionally, all terpolymers containing varying thiocarbonate content displayed good thermal properties with Tg >109 °C and Td >260 °C, suggesting little loss in the thermal stability compared to the polycarbonate. Hence, modification of the topological structure of the polycarbonate is an efficient method of obtaining polymeric materials with enhanced optical properties without compromising thermal performance.

Development of High-Capacity and Water-Lean CO2 Absorbents by a Concise Molecular Design Strategy through Viscosity Control.

The exponentially increasing viscosity of water-lean CO2 absorbents during carbon capture processes is a critical problem for practical application, owing to its strong correlation with systems' mass transfer properties, as well as convenience of transportation. In this work, a concise strategy based on structure-viscosity relationships is proposed and applied to construct a series of functionalized ethylenediamines as single-component absorbents for post-combustion CO2 capture. These nonaqueous absorbents have outstanding viscosities (50-200 cP, 25 °C) at their maximal CO2 capacities (up to 22 wt % or 4.92 mol kg-1 , 1 bar), and are readily regenerated at low temperatures (50-80 °C) under ambient pressure. Additional capture of CO2 through physisorption could also be achieved by operating at high pressures. The CO2 capture and release process is systematically investigated by means of 13 C NMR spectroscopy, differential scanning calorimetry (DSC), in situ FTIR analysis, and density functional theory (DFT) calculations, which could provide sufficient spectroscopic details to reveal the ease of reversibility and enable rational interpretation of the absorption mechanism.

Enantioselective Resolution Copolymerization of Racemic Epoxides and Anhydrides: Efficient Approach for Stereoregular Polyesters and Chiral Epoxides.

Herein we report an efficient strategy for preparing isotactic polyesters and chiral epoxides via enantioselective resolution copolymerization of racemic terminal epoxides with anhydrides, mediated by enantiopure bimetallic complexes in conjunction with a nucleophilic cocatalyst. The chirality of both the axial linker and the diamine backbones of the ligand are responsible for the chiral induction of this kinetic resolution copolymerization process. The catalyst systems exhibit exceptional levels of enantioselectivity with a kinetic resolution coefficient exceeding 300 for various racemic epoxides, affording highly isotactic copolymers (selectivity factors of more than 300) with a completely alternating structure and low polydispersity index. Most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures in the range from 77 to 160 °C.