Thermal stability of CO2 adducts of N-heterocyclic carbenes (NHCs) was studied by means of in situ FTIR method with monitoring of the nu(CO2) region of the infrared spectra under various conditions. 1,3-Bis(2,6-diisopropylphenyl)imidazolinium-2-carboxylate (SIPr-CO2) shows higher thermal stability compared with 1,3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate (IPr-CO2). The presence of free CO2 can significantly inhibit the decomposition of NHC-CO2 adducts, while the addition of an epoxide such as propylene oxide has a negative effect on stabilizing these adducts. As zwitterionic compounds, NHC-CO2 adducts were also proved to be effective organic catalysts for the coupling reaction of CO2 and epoxides to afford cyclic carbonates, for which a possible mechanism was proposed. Among these NHC-CO2 adducts, the relatively unstable IPr-CO2 exhibits the highest catalytic activity. The presence of an electrophile such as SalenAlEt could greatly improve the catalytic activity of IPr-CO2 due to intermolecular cooperative catalysis of the binary components.

A series of phosphorus ylide (P-ylide) CO 2 adducts were synthesized and firstly used as organocatalysts for CO 2 transformation. Detailed studies on the cycloaddition reaction of CO 2 with terminal epoxides show that P-ylide CO 2 adducts are efficient metal-free and halogen-free organocatalytsts to mediate this reaction under ambient conditions (25 o C, 1 atm CO 2 ). More importantly, the reactions proceeded with a broad scope, high efficiency, and functional group tolerance and the corresponding cyclic carbonate products were obtained in good to excellent yield (46-99%). Meanwhile, the kinetic study by in-situ FTIR method suggested an intermolecular cooperation effect for effectively accelerating the ring-opening of terminal epoxides. Furthermore, from the investigation on the catalytic diversity of Pylide CO 2 adducts, CO 2 also could be availably converted to functionalized cyclic αalkylidene carbonates, oxazolidinone, N-methylated and N-formylated amines by organocatalytic reactions.

It is of great significance to depolymerize used or waste polymers to recover the starting monomers suitable for repolymerization reactions that reform recycled materials no different from the virgin polymer. Herein, we report a novel recyclable plastic: degradable polycarbonate synthesized by dinuclear chromium-complex-mediated copolymerization of CO with 1-benzyloxycarbonyl-3,4-epoxy pyrrolidine, a meso-epoxide. Notably, the novel polycarbonate with more than 99 % carbonate linkages could be recycled back into the epoxide monomer in quantitative yield under mild reaction conditions. Remarkably, the copolymerization/depolymerization processes can be achieved by the ON/OFF reversible temperature switch, and recycled several times without any change in the epoxide monomer and copolymer. These characteristics accord well with the concept of perfectly sustainable polymers.

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Recycling processes (such as carbon, oxygen, nitrogen cycles etc.) occur each day as an essential character for nature. Managed ecosystems have maintained the natural environment in a state of great harmony for millions of years. In recent decades, various polymers with long-term durability have been developed to be the most predominantly used materials, and have drastically changed the lifestyle of human beings. However, the tremendous growth of plastic debris, with no or very low degradability, over the past decades significantly affects the natural environment. The use of alternative recycling materials is the best option from the environmental point of view. This critical review highlights the most significant progress in chemically recyclable materials, especially several typical systems, which embody perfect recycling between polymers and the starting monomers.

Various CO2, COS and CS2 adducts of N-Heterocyclic olefin (NHO) were synthesized and characterised by single crystal Xray crystallography. The length of C-O bond in NHO-COS adducts is slightly shorter than that in the corresponding NHO-CO2 adducts, while the C-S bond length in NHO-COS adducts is significantly longer than the corresponding C-S distance in the NHO-CS2 adducts, suggesting the negative charge of the NHO-COS adducts is preferentially delocalized on the sulfur atom. The length of the CCS2−CNHO bond is significantly shorter than that of the CCOS−CNHO or CCO2−CNHO bond in the corresponding NHO-COS or NHO-CO2 adducts, implying the relatively high thermal stability of NHO-CS2 adducts. The NHO-CO2, NHO-COS and NHO-CS2 adducts were found to be efficient in catalyzing the cycloaddition reaction of CO2 and epoxides to selectively afford the corresponding cyclic carbonates. Among them, NHO-CO2 adducts were found to be more active, while NHO-CS2 adducts exhibited the lowest activity for this reaction, probably due to their high stability for difficult release of the highly active NHO. NHOs with BBr 3 giving the neutral compound of NHO-BBr 3 , unexpectedly making the ring of THF open, has been shown by Robinson and colleagues 45 . Silyl-functionalized NHOs, obtained by the reaction of NHOs with HSiCl 3 , have been unveiled by the

The asymmetric alternating copolymerization of meso-epoxides with cyclic anhydrides promoted by chiral catalysts or reagents is a powerful strategy for the synthesis of optically active polyesters with main-chain chirality. Herein, we show that, in conjunction with a nucleophilic cocatalyst, enantiopure dinuclear Al­(III) complexes efficiently catalyze this asymmetric copolymerization, exhibiting high activity and achieving enantioselectivities up to 99% ee under mild conditions. Copolymer enantioselectivity and catalytic activity are revealed to be strongly affected by the axial linker, chiral diamine structure, and phenolate ortho-substituents. Density functional theory calculations confirm that the reactions corresponding to ring opening at (R)-C–O and (S)-C–O bonds of the Al-coordinated meso-epoxide during copolymerization exhibit significantly different Gibbs free energies of activation (Δ‡ G). Enantiopure dinuclear Al­(III) complex 3 bearing a hydrogenated binaphthol linker with the matched configuration is demonstrated to be the most active and enantioselective catalyst, featuring a broad substrate scope and allowing one to obtain a wide range of isotactic polyesters with a completely alternating structure and low polydispersity. Notably, most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures between 120 and 240 °C. Additionally, the mixing of selected isotactic (R)- and (S)-polyesters in a 1:1 mass ratio afforded two crystalline stereocomplexes that exhibited enhanced thermal stability as well as new crystallization behavior and therefore significantly differed from the parent enantiopure polymers.

Reversible fixation-release of CO 2 has attracted much attention during the last decades due to the economic and environmental benefits arising from the utilization of renewable resources and the growing concern on the greenhouse effect. 1-3 Numerous systems based on liquid primary or secondary amines have been developed for this process, in which CO 2 is chemically converted into ammonium carbamates or zwitterionic adducts at ambient temperature 4-6 and the fixed CO 2 is released upon heating. 7,8 Likewise, amino-functionalized synthetic polymers or mesoporous materials were also proved to be efficient in CO 2 capture. 9-11 Notably, ethylenimine functionalized mesoporous molecular sieve MCM-41 has higher absorption capacity than either pure polyethylenimine or MCM-41 alone and could be used as highly CO 2 -selective adsorbent for gas mixtures without the preremoval of moisture. 10 Endo and co-workers reported a new type of reversible CO 2 fixation by amidine derivatives and by polymers bearing an amidine moiety both in solution and solid state. 12 The cyclic amidine-functionalized copolymers exhibited better ability to retain CO 2 than corresponding low-molecular weight amidine at ambient temperature and could be applied to reversible fixation-release of CO 2 . The CO 2 fixation by polymers in the solid state may be one of the most simple, economic and effective methods for CO 2 recovery, though the fixing efficiency is relatively low.During the past decade, N-heterocyclic carbenes were studied extensively as versatile ligands 13 and effective organocatalysts. 14 Because of their high basicity, 15 N-heterocyclic carbenes can react rapidly with CO 2 to afford zwitterionic adducts (designated as NHC-CO 2 ), even at very low CO 2 concentrations. 16 In recent studies, we found that both the formation of NHC-CO 2 adducts at 20-50°C in nearly 100% yield and their complete decomposition into free N-heterocyclic carbenes and CO 2 at elevated temperatures were very fast (Figure 1). 17 These observation stimulated us to explore the feasibility of N-heterocyclic carbenes as highly CO 2 -selective adsorbent at low CO 2 concentration. Herein, we report a N-heterocyclic carbene-functionalized synthetic polymer for reversible fixation-release of CO 2 .The synthetic route of zwitterionic NHC-CO 2 adduct-functionalized copolymer (designated as P-NHC-CO 2 ) is shown in Scheme 1. Formation of quaternary ammonium by reaction of 1-(2,6-diisopropylphenyl) imidazole with styrene-4-vinylbenzyl chloride copolymer afforded imidazolium-modified copolymer P-NHC-HCl. Similar to the synthesis of 1-(2,6-diisopropylphenyl)-3-benzyl imidazolium-2-carboxylate (see Supporting Information), P-NHC-CO 2 was readily prepared by deprotonation of P-NHC-HCl using KN(SiMe 3 ) 2 , and followed by reaction with CO 2 . The IR spectrum of the resulted polymer showed a broad absorption peak around 1649 cm -1 , which is assigned to the asymmetric v(CO 2 ) vibrations of NHC-CO 2 adduct moiety. The content of immobilized N-heterocyclic carbene sites could be c...