Comparative Kinetic Studies of the Copolymerization of Cyclohexene Oxide and Propylene Oxide with Carbon Dioxide in the Presence of Chromium Salen Derivatives. In Situ FTIR Measurements of Copolymer vs Cyclic Carbonate Production

Eberhardt R., Allmendinger M., Rieger B.

Highly efficient formation of poly(propylene carbonate) can be achieved in the coupling of CO 2 and propylene oxide assisted by 4-(N,N-dimethylamino)pyridine (DMAP) and catalyzed with salen chromium(III) chloride by using DMAP/Cr ratios of less than 2. Under these conditions a possible backbiting mechanism is suppressed, leading to only minor amounts of cyclic carbonate as a side product.

Eberhardt R., Allmendinger M., Luinstra G.A., Rieger B.

The quantitative synthesis of a series of new zinc(II) sulfinate complexes (4a−e) by insertion of SO2 into zinc−ethyl bonds of β-diiminate complexes ((ArNC(CH3)CH(CH3)CNAr)Zn(O(SO)Et); Ar = 2,6-diisopropylphenyl (4a), 2,6-diethylphenyl (4b), 2-ethyl-6-isopropylphenyl (4c), 2,6-diphenylphenyl (4d), 2,6-bis(4-tert-butylphenyl)phenyl (4e)) is described. X-ray structure analysis reveals a dinuclear, μ-ethylsulfinate-bridged structure for 4a in the solid state, which in solution exists in an equilibrium with the mononuclear species. The easily accessible complexes 4a−c are highly active catalysts for the alternating copolymerization of CO2 and cyclohexene oxide, leading to products with narrow molecular weight distributions.

Allen S.D., Moore D.R., Lobkovsky E.B., Coates G.W.

Despite recent advances regarding catalysts for CO2/epoxide copolymerization, the development of high-activity catalysts for alternating polymerization of CO2 and commodity epoxides, such as propylene oxide, remains a challenge. A new class of unsymmetrically substituted beta-diiminate zinc complexes is reported that exhibits unprecedented activity for CO2/propylene oxide copolymerization. The polymers formed are of high molecular weight (Mn approximately 35 kg/mol) and have narrow polydispersities (PDI approximately 1.1), consistent with a living polymerization.

Darensbourg D.J., Wildeson J.R., Yarbrough J.C.

Zinc complexes derived from benzoic acids containing electron-withdrawing substituents have been synthesized from ZnII(bis-trimethylsilyl amide)2 and the corresponding carboxylic acid (2,6-X2C6H3COOH, where X = F, Cl, or OMe) in THF and structurally characterized via X-ray crystallography. The 2,6-difluorobenzoate complex crystallizes from THF or CH3CN as a seven membered zinc aggregate, where the metal atoms are interconnected by a combination of 10 μ-benzoates and μ4-oxo ligands, that is, [(2,6-difluorobenzoate)10O2Zn7](solvent)2, solvent = THF (1) and CH3CN (1a). On the other hand, the 2,6-dichlorobenzoate zinc derivative crystallizes from THF as a dimer, [(2,6-dichlorobenzoate)4Zn2](THF)3 (2), where the two zinc centers are bridged by three benzoate ligand. One of the zinc centers possesses a tetrahedral ligand environment where the fourth ligand is a unidentate benzoate, and the other zinc center has an octahedral arrangement of ligands which is accomplished by the additional binding of three THF molecules. Upon dissolution of complex 1 or 2 in the strongly binding pyridine solvent, disruption of these zinc carboxylates occurs with concomitant formation of mononuclear zinc bis-benzoates with three pyridine ligands in the metal coordination sphere. Complexes 1 and 2 were found to be effective catalysts for the copolymerization of cyclohexene oxide and carbon dioxide to afford polycarbonates devoid of polyether linkages, that is, completely alternating copolymers. Although these catalysts or catalyst precursors in the presence of CO2/propylene oxide afforded mostly propylene carbonate, they did serve as efficient catalysts for the terpolymerization of carbon dioxide/cyclohexene oxide/propylene oxide. The reactivities of these zinc carboxylates were very similar to those previously reported analogous complexes which have not been structurally characterized. Hence, it is suggested here that all of these zinc carboxylates provide similar catalytic sites for CO2/epoxide coupling processes.

Paddock R.L., Nguyen S.T.

Darensbourg D.J., Wildeson J.R., Yarbrough J.C., Reibenspies J.H.

Dimeric phenoxide derivatives of zinc and cadmium have been synthesized from the reaction of the corresponding metal bistrimethylsilylamide and two equivalents of 2,6-F2C6H3OH in tetrahydrofuran. The zinc analogue, [Zn(O-2,6-F2C6H3)2·THF]2 (1), has been characterized in the solid state via X-ray crystallography, where the zinc centers are shown to possess distorted tetrahedral geometry containing two bridging phenoxides and a terminal phenoxide and THF ligand. The distance between the metal centers (Zn···Zn) was found to be 3.059 A, and the THF ligands lie on opposite sides of the plane formed by the two zinc atoms and two bridging phenoxide ligands' oxygen atoms. There are several Zn···F nonbonding distances involving the bridging phenoxide ligands that are less than the van der Waals internuclear distance. In addition, both the zinc and cadmium dimeric derivatives have been prepared such that the labile THF ligands are replaced by the sterically encumbering basic phosphine, PCy3. The solid-state structu...

Jacobsen E.N.

The discovery of the metal salen-catalyzed asymmetric ring-opening (ARO) of epoxides is chronicled. A screening approach was adopted for the identification of catalysts for the addition of TMSN3 to meso-epoxides, and the chiral (salen)CrN3 complex was identified as optimal. Kinetic and structural studies served to elucidate the mechanism of catalysis, which involves cooperative activation of both epoxide and azide by two different metal centers. Covalently linked bimetallic complexes were constructed on the basis of this insight, and shown to catalyze the ARO with identical enantioselectivity but 1−2 orders of magnitude greater reactivity than the monomeric analogues. Extraordinarily high selectivity is observed in the kinetic resolution of terminal epoxides using the (salen)CrN3/TMSN3 system. A search for a practical method for the kinetic resolution reaction led to the discovery of highly enantiomer-selective hydrolytic ring-opening using the corresponding (salen)CoIII catalyst. This system displays extraordinary substrate generality, and allows practical access to enantiopure terminal epoxides on both laboratory and industrial scales.

Mang S., Cooper A.I., Colclough M.E., Chauhan N., Holmes A.B.

The copolymerization of 1,2-cyclohexene oxide (CHO) with carbon dioxide was performed, using supercritical carbon dioxide (scCO2) as both solvent and reactant, to produce an aliphatic polycarbonate...

Nozaki K., Nakano K., Hiyama T.

Beckman E.J.

Darensbourg D.J., Holtcamp M.W., Struck G.E., Zimmer M.S., Niezgoda S.A., Rainey P., Robertson J.B., Draper J.D., Reibenspies J.H.

A series of zinc phenoxides of the general formula (2,6-R2C6H3O)2Zn(base)2 [R = Ph, tBu, iPr, base = Et2O, THF, or propylene carbonate] and (2,4,6-Me3C6H2O)2Zn(pyridine)2 have been synthesized and characterized in the solid state by X-ray crystallography. All complexes crystallized as four-coordinate monomers with highly distorted tetrahedral geometry about the zinc center. The angles between the two sterically encumbering phenoxide ligands were found to be significantly more obtuse than the corresponding angles between the two smaller neutral base ligands, having average values of 140° and 95°, respectively. In a noninteracting solvent such as benzene or methylene chloride at ambient temperature, the ancillary base ligands are extensively dissociated from the zinc center, with the degree of dissociation being dependent on the base as well as the substituents on the phenolate ligands. That is, stronger ligand binding was found in zinc centers containing electron-donating tert-butyl substituents as opposed to electron-withdrawing phenyl substituents. In all instances, the order of ligand binding was pyridine > THF > epoxides. These bis(phenoxide) derivatives of zinc were shown to be very effective catalysts for the copolymerization of cyclohexene oxide and CO2 in the absence of strongly coordinating solvents, to afford high-molecular-weight polycarbonate (Mw ranging from 45 × 103 to 173 × 103 Da) with low levels of polyether linkages. However, under similar conditions, these zinc complexes only coupled propylene oxide and CO2 to produce cyclic propylene carbonate. Nevertheless, these bis (phenoxide) derivatives of zinc were competent at terpolymerization of cyclohexene oxide/propylene oxide/CO2 with little cyclic propylene carbonate formation at low propylene oxide loadings. While CO2 showed no reactivity with the sterically encumbered zinc bis(phenoxides), e.g., (2,6-di-tert-butylphenoxide)2Zn(pyridine)2, it rapidly inserted into one of the Zn−O bonds of the less crowded (2,4,6-trimethylphenoxide)2Zn(pyridine)2 to provide the corresponding aryl carbonate zinc derivative. At the same time, both sterically hindered and sterically nonhindered phenoxide derivatives of zinc served to ring-open epoxide, i.e., were effective catalysts for the homopolymerization of epoxide to polyethers. The relevance of these reactivity patterns to the initiation step of the copolymerization process involving these monomeric zinc complexes is discussed.

Super M., Berluche E., Costello C., Beckman E.

The copolymerization of 1,2-epoxycyclohexane (cyclohexene oxide, CHO) and carbon dioxide was performed using no other solvent than carbon dioxide itself. A CO2-soluble, ZnO-based catalyst was synthesized and used to catalyze the polymerization. Polymerizations were conducted at different temperatures, pressures, and mole fractions of CHO giving rise to polymer yields as high as 69% with catalyst activities as high as 400 g of polymer/g of Zn. The best results were obtained at temperatures of 100−110 °C and at mole fractions of CHO above XCHO = 0.15. This polymer was >90% polycarbonate with weight average molecular weights (Mw) ranging from 50 000 to 180 000.

Leitner W.

Hansen K.B., Leighton J.L., Jacobsen E.N.

McGuire T.M., Ning D., Williams C.K.

Liu G., Su Y., Chuang W., Ko B.

Ge Q., Dong B., Chen Y., Wang X., Wang X., Fan C., Chen C., Lin S., Pan Q., Ng F.T.

This study achieved catalytic degradation of polycarbonates and transformation of CO2 into propylene carbonate under ambient conditions.

Eisenhardt K.H., Fiorentini F., Lindeboom W., Williams C.K.

Hassan M., Bhat G.A., Darensbourg D.J.

A wide variety of aliphatic polycarbonates are readily synthesized via the copolymerization of epoxides and carbon dioxide. Nevertheless, these polymeric materials have limited utility, in part because of their lack...

Chernikova Elena V., Beletskaya Irina P.

Carbon dioxide (CO2) plays a vital role in organic and polymer chemistry as a source of cheap and available raw material for the synthesis of many valuable products, including polymer materials with a specified set of properties, and as a solvent for chemical reactions. This review is devoted to the synthesis, properties and applications of polycarbonates obtained by copolymerization of CO2 with epoxides, a hot topic that has aroused great interest among the scientific community and industry representatives. The existing data on the catalytic systems used for the synthesis of polycarbonates are analyzed and summarized, depolymerization of polycarbonates, which is a key aspect in the polymer recycling, is discussed, information on the properties and applications of polycarbonates is systematized, and prospects for the development of this area of chemistry are considered.Bibliography — 438 references.

Stahl S., Luinstra G.A.

The action of three different Co/Zn double-metal-cyanide (DMC) catalysts in the propoxylation of polyols was made comparable in a kinetic study, using pulse-wise feeding of propylene oxide (PO). Key insights...

Yang L., Liu S., Zhou Z., Zhang R., Zhou H., Zhuo C., Wang X.

Xie W., Wang S., Lv J., You Z., Yu W., Liu X., Wang M., Ma X.

Cycloaddition reaction of CO2 to epoxides is an important procedure for both CO2 conversion and cyclic carbonate synthesis. Homogeneous alkali halide and quaternary ammonium salt are the general catalysts, while they have the problems of halogen corrosion and residue, intricate separation, and catalyst recycling. Here, a series of heterogenous and halogen-free bifunctional yttrium-coordinated g-C3N4 were synthesized and employed to catalyze the cycloaddition of CO2 to butylene oxide. Excellent butylene oxide conversion (94.5%) and butene carbonate selectivity (95.0%) were achieved over the typical Y/g-C3N4-0.5 catalyst without any halogen addition. The characterizations demonstrate that the coordinated Y species function well as the preferred Lewis acid sites for epoxide ring opening and butene carbonate desorption, and the uncondensed amino groups are supplied as Lewis base sites to activate CO2. It is the synergistic effect based on acid-base duality that enhances the catalytic performance of Y/g-C3N4 to cycloaddition of CO2 with epoxide.

Bhat G.A., Darensbourg D.J.

The use of carbon dioxide as an alternative feedstock for the production of polymeric materials represents a promising strategy for the valorization of CO2. In this regard, there remains much current attention focusing on the synthesis of polycarbonates from CO2 and epoxides. The use of metal coordination complexes as catalysts for the alternating copolymerization process under mild reaction conditions has significantly advanced our synthetic capability and mechanistic understanding of these coupling reactions. This review’s focal point is the advancements made in the area of CO2/epoxide polymerization processes involving a variety of main group and transition metal complexes as catalysts for the preparation of these thermoplastics. A brief discussion of the development of organocatalysts, whose design is based on their metal counterparts, is also presented.

Zhuo C., Yu X., Cheng Y., Liu S., Zhu M., Wang X.

Lishchenko Y.L., Sotnik S.O., Volochnyuk D.M., Ryabukhin S.V., Gavrilenko K.S., Kolotilov S.V.

Catalytic reactions of 3-membered heterocycles, such as epoxides and aziridines, with CO2 in the presence of metal–salen complexes are considered in this review (salens is a class of ligands, derived from diamines and salicylaldehyde or its substituted analogs). These reactions lead to copolymers or cyclic carbonates/oxazolidinones, which are important products of modern fine and industrial organic synthesis. The influence of metal ion, substituents in salen and the nature of cocatalyst on the catalytic performance of the systems based on metal–salen complexes, reported during two recent decades, is analyzed. It was shown that formation of bi- or polynuclear complexes, as well as incorporation of the organic cocatalyst (such as tetraalkylammonium cation) in the molecule of the metal-containing catalyst improved the catalytic performance of metal–salen species due to cooperative effect on several active sites in activation of epoxide and its reaction with CO2. The ways for creation of heterogeneous metal–salen catalysts for reactions of 3-membered heterocycles with CO2 are also summarized.

Wang Y., Liu Z., Guo W., Zhang C., Zhang X.

Castro-Ruiz A., Grefe L., Mejía E., Suman S.G.

A bioinspired Co(iii) catalyst efficiently transforms epoxides and CO2 into cyclic carbonates. The mechanism was found to combine the cooperative effects of the metal center and amino acid residues in the outer coordination sphere.

Gao Z., Gao B., Meng S., Yang Z., Liang Z., Sun Z., Zhou Y., Pang X.