Switchable polymerization provides the opportunity to regulate polymer sequence and structure in a one‐pot process from mixtures of monomers. Herein we report the use of O2 as an external stimulus to switch the polymerization mechanism from the radical polymerization of vinyl monomers mediated by (Salen)CoIII?R [Salen=N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine; R=alkyl] to the ring‐opening copolymerization (ROCOP) of CO2/epoxides. Critical to this process is unprecedented monooxygen insertion into the Co?C bond, as rationalized by DFT calculations, leading to the formation of (Salen)CoIII?O?R as an active species to initiate ROCOP. Diblock poly(vinyl acetate)‐b‐polycarbonate could be obtained by ROCOP of CO2/epoxides with preactivation of (Salen)Co end‐capped poly(vinyl acetate). Furthermore, a poly(vinyl acetate)‐b‐poly(methyl acrylate)‐b‐polycarbonate triblock copolymer was successfully synthesized by a (Salen)cobalt‐mediated sequential polymerization with an O2‐triggered switch in a one‐pot process. 相似文献
The synthesis of solid catalysts for the co‐catalyst‐free cycloaddition of CO2 has attracted much attention. Herein, we report a hierarchical porous organic polymer, Py‐Zn@MA, that is able to catalyze the cycloaddition reaction of epoxides and CO2 without using any additives or co‐catalyst to afford turnover frequency (TOF) values as high as 250 and 97 h−1 at 130 °C by using pure and diluted CO2 (simulating flue gas), respectively. These results are superior to those obtained from previously reported heterogeneous co‐catalyst‐free systems. The high activity of Py‐Zn@MA is mainly attributed to its bifunctional nature with ZnBr2 and pyridine activating the epoxide in a cooperative way. Notably, Py‐Zn@MA can be easily prepared on a large scale without using any catalyst and the chemicals are cost effective. Moreover, Py‐Zn@MA shows good substrate universality for the cycloaddition reactions of epoxides. Our designed porous organic polymer Py‐Zn@MA material has the potential to serve as an efficient catalyst for the direct conversion of flue gas with epoxides into value‐added cyclic carbonates. 相似文献
Due to the axial group initiation in traditional (salen)CoX/quaternary ammonium catalyst system, it is difficult to construct single active center propagating polycarbonates for copolymerization of CO2/epoxides. Here a redox‐responsive poly(vinyl cyclohexene carbonate) (PVCHC) with detachable disulfide‐bond backbone is synthesized in a controllable manner using (salen)CoTFA/[bis(triphenylphosphine)iminium, [PPN]TFA binary catalyst, where the axial group initiation is depressed by judiciously choosing 3,3′‐dithiodipropionic acid as starter. While for those comonomers failing to obtain polycarbonate with unimodal gel permeation chromatography (GPC) curve, a versatile method is developed by combination of immortal copolymerization and prereaction approach, and functional aliphatic polycarbonates having well‐defined architecture and narrow polydispersity can be prepared. The resulting PVCHC can be further functionalized with alkenes by versatile cross‐metathesis reaction to tune the physicochemical properties. The combination of immortal polymerization and prereaction approach creates a powerful platform for controllable synthesis of functional CO2‐based polycarbonates.
In today's world, a major scientific challenge is preserving the delicate balance between industrial growth and a pollutant free terrestrial environment. Thus, “greener” syntheses of commodity materials, capture and utilization of gaseous industrial by‐products have become research areas of increasing significance. The pioneering work of Inoue showed a potential utilization of CO2, a major petrochemical by‐product, and opened a new field of research. Inoue discovered the (porphyrin)Al(III)X catalyst systems, (X=Cl? or alkoxide) which copolymerize CO2 with epoxides to produce polycarbonates. This catalyst can also copolymerize epoxides and cyclic anhydrides to form polyesters. The current record describes our research aimed towards mechanistic understanding and further developments of (porphyrin)M(III)X catalyst systems. This detailed account shows the influence of the porphyrin ligands (tetraphenylporphyrin (TPP), octaethylporphyrin (OEP), tetrakis‐pentafluorophenylporphyrin (TFPP)), metal centers (Al, Cr, and Co) and Lewis base co‐catalysts on the individual reaction steps and equilibria involved in the copolymerization processes. 相似文献
Direct coordinative copolymerization of ethylene with functionalized co‐monomers is a long‐sought‐after approach to introducing polyolefin functionality. However, functional‐group Lewis basicity typically depresses catalytic activity and co‐monomer incorporation. Finding alternatives to intensively studied group 4 d0 and late‐transition‐metal catalysts is crucial to addressing this long‐standing challenge. Shown herein is that mono‐ and binuclear organoscandium complexes with a borate cocatalyst are active for ethylene + amino olefin [AO; H2C=CH(CH2)nNR2] copolymerizations in the absence of a Lewis‐acidic masking reagent. Both activity (up to 4.2×102 kg mol−1⋅h−1> atm−1>) and AO incorporation (up to 12 % at 0.2 m [AO]) are appreciable. Linker‐length‐dependent (n) AO incorporation and mechanistic probes support an unusual functional‐group‐assisted enchainment mechanism. Furthermore, the binuclear catalysts exhibit enhanced AO tolerance and enhanced long chain AO incorporation. 相似文献
Five niobium cluster compounds of the AI2[Nb6Cl18] type (AI = organic cation: [nPr4N]+, [nBu4N]+, [BMIm]+, [Ph4P]+, and [PPN]+) are obtained through treatment of [Nb6Cl14(H2O)4] · 4H2O with excess of thionyl chloride in the presence of an organic chloride, AICl. Single‐crystal structure studies show that the compounds consist of discrete cations and cluster [Nb6Cl18]2– anions. The cluster unit of the hydrated cluster starting material is oxidized by two electrons. Powder diffraction studies and NMR spectroscopic measurements show all compounds to crystallize without co‐crystallized solvent molecules. They are air and water stable. The solubility in organic solvents changes to a great extent on changing the type of cation. The ESI‐MS spectra of [nPr4N]2[Nb6Cl18] and [Ph4P]2[Nb6Cl18] show the pseudomolecular peak of the anionic cluster as well as additional signals, which involve simultaneously chloride mass loss and reduction processes. 相似文献
The regioselective ring‐opening reactions of some epoxides with ammonium thiocyanate in the presence of a series of new 9H‐thioxanthen‐9‐one‐fused azacrown ethers, i.e., 7 – 11 (Scheme 1), and also of dibenzo[18]crown‐6 ( 12 ), Kryptofix®22 ( 13 ), and benzo[15]crown‐5 ( 14 ) were studied (Tables 1 and 2). The epoxides were subjected to cleavage by NH4SCN in the presence of these catalysts under mild conditions in various aprotic solvents. Reagents and conditions were identified for the synthesis of individual β‐hydroxy thiocyanates in high yield and with more than 90% regioselectivity. The results can be discussed in terms of a four‐step mechanism (Scheme 2): 1) formation of a complex between catalyst and NH4SCN, 2) release of SCN? from the complex, 3) reaction of the released SCN? at the sterically less hindered site of the epoxide, and 4) regeneration of the catalyst. The major advantages of this method are the high regioselectivity, the simple regeneration of the catalyst, the reuse of it through several cycles without a decrease of activity, and the ease of workup of the reaction mixtures. 相似文献
A series of water‐insoluble iron(III) and manganese(III) porphyrins, FeT(2‐CH3)PPCl, FeT(4‐OCH3)PPCl, FeT(2‐Cl)PPCl, FeTPPCl, MnT(2‐CH3)PPOAc, MnT(4‐OCH3)PPOAc, MnT(2‐Cl)PPOAc and MnTPPOAc, in the presence of imidazole (ImH), F?, Cl?, Br? and acetate were used as catalysts for the aqueous‐phase heterogeneous oxidation of styrenes to the corresponding epoxides and aldehydes with sodium periodate. Also, the effect of various reaction parameters such as reaction time, molar ratio of catalyst to axial base, type of axial base, molar ratio of olefin to oxidant and nature of metal centre on the activity and oxidative stability of the catalysts and the product selectivity was investigated. Higher catalytic activities were found for the iron complexes. Interestingly, the selectivity towards the formation of epoxide and aldehyde (or acetophenone) was significantly influenced by the type of axial base. Furthermore, Br? and ImH were found to be the most efficient co‐catalysts for the oxidation of olefins performed in the presence of the manganese and iron porphyrins, respectively. The optimized molar ratio of catalyst to axial base was different for various axial bases. Also, the order of co‐catalyst activity of the axial bases obtained in aqueous medium was different from that reported for organic solvents. The use of a convenient axial base under optimum reaction catalyst to co‐catalyst molar ratio in the presence of the manganese porphyrin gave the oxidative products with a conversion of ca 100% in a reaction time of less than 3 h. However, the catalytic activity of the iron porphyrins could not be effectively improved by increasing the catalyst to co‐catalyst molar ratio. 相似文献