Copolymerization of cyclohexene oxide and carbon dioxide using (salen)Co(iii) complexes: synthesis and characterization of syndiotactic poly(cyclohexene carbonate)

Peretti K.L., Ajiro H., Cohen C.T., Lobkovsky E.B., Coates G.W.

A highly active cobalt complex ((salph)CoOAc; salph = N,N'-bis(3,5-di-tert-butylsalicylidine-1,2-benzenediamine)) was discovered for the isospecific polymerization of rac-propylene oxide.

Cohen C.T., Chu T., Coates G.W.

Synthetic pathways to (salcy)CoX (salcy = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = halide or carboxylate) complexes are described. Complexes (R,R)-(salcy)CoCl, (R,R)-(salcy)CoBr, (R,R)-(salcy)CoOAc, and (R,R)-(salcy)CoOBzF(5) (OBzF(5) = pentafluorobenzoate) are highly active catalysts for the living, alternating copolymerization of propylene oxide (PO) and CO(2), yielding poly(propylene carbonate) (PPC) with no detectable byproducts. The PPC generated using these catalyst systems is highly regioregular and has up to 99% carbonate linkages with a narrow molecular weight distribution (MWD). Inclusion of the cocatalysts [PPN]Cl or [PPN][OBzF(5)] ([PPN] = bis(triphenylphosphine)iminium) with complex (R,R)-(salcy)CoCl, (R,R)-(salcy)CoBr, or (R,R)-(salcy)CoOBzF(5) results in remarkable activity enhancement of the copolymerization as well as improved stereoselectivity and regioselectivity with maximized reactivity at low CO(2) pressures. In the case of [PPN]Cl with (R,R)-(salcy)CoOBzF(5), an unprecedented catalytic activity of 620 turnovers per hour is achieved for the copolymerization of rac-PO and CO(2), yielding iso-enriched PPC with 94% head-to-tail connectivity. The stereochemistry of the monomer and catalyst used in the copolymerization has dramatic effects on catalytic activity and the PPC microstructure. Using catalyst (R,R)-(salcy)CoBr with (S)-PO/CO(2) generates highly regioregular PPC, whereas using (R)-PO/CO(2) with the same catalyst gives an almost completely regiorandom copolymer. The rac-PO/CO(2) copolymerization with catalyst rac-(salcy)CoBr yields syndio-enriched PPC, an unreported PPC microstructure. In addition, (R,R)-(salcy)CoOBzF(5)/[PPN]Cl copolymerizes (S)-PO and CO(2) with a turnover frequency of 1100 h(-1), an activity surpassing that observed in any previously reported system.

Paddock R.L., Nguyen S.T.

Cherian A.E., Lobkovsky E.B., Coates G.W.

Low molecular weight syndiotactic polypropylene (syndio-PP) with an olefin end group was synthesized using methylaluminoxane-activated bis(phenoxyimine)titanium dichloride ((PHI)2TiCl2) catalysts. Propylene enchainment occurs by a 2,1-insertion mechanism, and termination by β-hydride elimination produces low molecular weight syndio-PP with allyl end groups. Several new (PHI)2TiCl2 complexes with various ligand modifications were found to display a wide range of activities (turnover frequency (TOF) = 3−1200 h-1) and syndiospecificities ([rrrr] = 0.46−0.93) for propylene polymerization. While the TOF increases with increasing reaction temperature and propylene concentration, the molecular weight of the resulting macromonomer shows little variation. This provides strong support for a chain-transfer mechanism involving one molecule of propylene. The allyl-terminated PPs reported herein can be used to synthesize branched polyolefins or other new polyolefin architectures.

Darensbourg D.J., Phelps A.L.

The copolymerization of propylene oxide and CO2 has been investigated employing Cr(salen)N3 complexes as catalysts. Unfortunately the reaction could not be studied in real time via in situ IR spectroscopy, thereby obtaining detailed kinetic data, because of the copolymer limited solubility in most solvents. Investigations employing batch reactor runs concentrating on varying the cocatalyst, the equivalents of cocatalyst, and the steric and electronic structure of the catalyst through modification of the salen ligand were undertaken. It was discovered that the optimal catalyst for copolymer selectivity vs the monomeric propylene carbonate was one that contained a salen ligand with an electron-withdrawing phenylene backbone and electron-donating tert-butyl groups in the phenolate rings. This catalyst was used to investigate the effect of altering the nature of the cocatalyst and its concentration, the three cocatalysts being tricyclohexylphosphine (PCy3), PPN+ N3(-), and PPN+ Cl-, where PPN+ is the large very weakly interacting bis(triphenylphosphoramylidene)ammonium cation. By utilization of more or less than 1 equiv of PCy3 as cocatalyst, the yield of polymer was reduced. On the other hand, the PPN+ salts showed the best activity when 0.5 equiv was employed, and produced only cyclic when using over 1 equiv.

OCHIAI B., ENDO T.

This review describes polymer synthesis utilizing carbon dioxide and carbon disulfide as useful C1 resources. Chain and step copolymerizations are recounted with polymerizations of cyclic carbonates, especially five-membered ones, as monomers derived or potentially obtainable from carbon dioxide. Homopolymerization, copolymerizations, and application as a precursor for reversible chain transfer agents for RAFT polymerization are described for carbon disulfide based polymer synthesis. As polymerizations of monomers from carbon disulfide, ring-opening polymerizations of dithiocarbonates and thiourethanes are described.

Nakano K., Hiyama T., Nozaki K.

Asymmetric amplification in the copolymerization of cyclohexene oxide and carbon dioxide was demonstrated using chiral zinc complexes, prepared from diethylzinc, diphenyl(pyrrolidin-2-yl)methanol, and ethanol.

Darensbourg D.J., Billodeaux D.R.

A series of complexes of the form (salen)AlZ, where H2salen = N,N'-bis(salicylidene)-1,2-phenylenediimine and various other salen derivatives and Z = Et or Cl, have been synthesized. Several of these complexes have been characterized by X-ray crystallography. An investigation of the utilization of these aluminum derivatives along with both ionic and neutral bases as cocatalysts for the copolymerization of carbon dioxide and cyclohexene oxide has been conducted. By studying the reactivity of these complexes for this process as substituents on the diimine backbone and phenolate rings are altered, we have observed that aluminum prefers electron-withdrawing groups on the salen ligands, thereby producing an electrophilic metal center to be most active toward production of polycarbonates from CO2 and cyclohexene oxide. For example, the complex derived from H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine is essentially inactive when compared to the analogous derivative containing nitro substituents in the 3-positions of the phenolate groups. This is to be contrasted with the catalytic activity observed for the (salen)CrX systems, where electron-donating salen ligands greatly enhanced the reactivity of these complexes for the coupling of CO2 and epoxides. While (salen)AlZ complexes are capable of producing poly(cyclohexene oxide) carbonate with low amounts of polyether linkage along with small quantities of cyclic carbonate byproducts, their reactivities, covering a turnover frequency range of 5.2-35.4 mol of epoxide consumed/(mol of Al x h), are greatly reduced when compared to their (salen)CrX analogues under identical reaction conditions.

Coates G.W., Moore D.R.

Given the non-renewable nature of synthetic polymers from petroleum feedstocks, there is increasing interest in developing routes to biodegradable polymeric materials from renewable resources. Polycarbonates made from CO2 and epoxides (see scheme) have the potential to meet these important goals. Reviewed here are well-defined catalysts for epoxide–CO2 copolymerization and related reactions.

Darensbourg D.J., Mackiewicz R.M., Billodeaux D.R.

The rate of the copolymerization reaction of cyclohexene oxide and carbon dioxide in the presence of (salen)CrIIIN3 and various cocatalysts has been determined as a function of CO2 pressure. Carbon dioxide insertion into the (salen)Cr-alkoxide intermediates, afforded following epoxide ring-opening, was shown to be rate-limiting at pressures below 35 bar. Higher pressures of carbon dioxide resulted in catalyst/substrate dilution with a concomitant decrease in the rate of copolymer formation. On the other hand, cyclic carbonate formation was inhibited as the CO2 pressure was increased. The most active (salen)CrN3 catalyst (H2salen = N,N‘-bis(3-tert-butyl-5-methoxysalicylidene)-(1R,2R)-cyclohexenediimine), along with a [PPN][N3] cocatalyst, exhibited a TOF of 1153 mol epoxide consumed/mol chromium·h at 80 °C and a CO2 pressure of 34.5 bar.

Sugimoto H., Inoue S.

An erratum has been published for this article in J Polym Sci Part A: Polym Chem (2005) 43(4) 916. The alternating copolymerization of carbon dioxide and epoxide to produce polycarbonate has attracted the attention of many chemists because it is one of the most promising methodologies for the utilization of carbon dioxide as a safe, clean, and abundant raw material in synthetic chemistry. Recent development of catalysts for alternating copolymerization is based on the rational design of metal complexes, particularly complexes of transition metals with well-defined structures. In this article, the history and recent successful examples of the alternating copolymerization of carbon dioxide and epoxide are described. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5561–5573, 2004

Darensbourg D.J., Mackiewicz R.M., Phelps A.L., Billodeaux D.R.

The design of efficient metal catalysts for the selective coupling of epoxides and carbon dioxide to afford completely alternating copolymers has made significant gains over the past decade. Hence, it is becoming increasingly clear that this "greener" route to polycarbonates has the potential to supplement or supplant current processes for the production of these important thermoplastics, which involve the condensation polymerization of diols and phosgene or organic carbonates. On the basis of the experiences in our laboratory, this Account summarizes our efforts at optimizing (salen)CrIIIX catalysts for the selective formation of polycarbonates from alicyclic and aliphatic epoxides with CO2. An iterative catalyst design process is employed in which the salen ligand, initiator, cocatalyst, and reaction conditions are systematically varied, with the reaction rates and product selectivity being monitored by in situ infrared spectroscopy.

Darensbourg D.J., Mackiewicz R.M., Rodgers J.L., Fang C.C., Billodeaux D.R., Reibenspies J.H.

A detailed mechanistic study into the copolymerization of CO2 and cyclohexene oxide utilizing CrIII(salen)X complexes and N-methylimidazole, where H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine and other salen derivatives and X = Cl or N3, has been conducted. By studying salen ligands with various groups on the diimine backbone, we have observed that bulky groups oriented perpendicular to the salen plane reduce the activity of the catalyst significantly, while such groups oriented parallel to the salen plane do not retard copolymer formation. This is not surprising in that the mechanism for asymmetric ring opening of epoxides was found to occur in a bimetallic fashion, whereas these perpendicularly oriented groups along with the tert-butyl groups on the phenolate rings produce considerable steric requirements for the two metal centers to communicate and thus initiate the copolymerization process. It was also observed that altering the substituents on the phenolate rings of the salen ligand had a 2-fold effect, controlling both catalyst solubility as well as electron density around the metal center, producing significant effects on the rate of copolymer formation. This and other data discussed herein have led us to propose a more detailed mechanistic delineation, wherein the rate of copolymerization is dictated by two separate equilibria. The first equilibrium involves the initial second-order epoxide ring opening and is inhibited by excess amounts of cocatalyst. The second equilibrium involves the propagation step and is enhanced by excess cocatalyst. This gives the [cocatalyst] both a positive and negative effect on the overall rate of copolymerization.

Lu X., Wang Y.

Darensbourg D.J., Mackiewicz R.M., Rodgers J.L., Phelps A.L.

The copolymerization of CO(2) and cyclohexene or propylene oxide has been examined employing (salen)Cr(III)Nu complexes (Nu = Cl or N(3)) as catalysts. The addition of various cocatalysts, including phosphines and PPN+ or Bu4N+ Cl- salts serves to greatly enhance the rate of copolymer production. In these instances, the mechanism of the initiation step appears to be unimolecular in catalyst concentration, unlike the bimolecular process cocatalyzed by N-methylimidazole. The copolymers were produced with >95% carbonate linkages with TOFs in the range 39-494 mol epoxide consumed/mol Cr.h. In the presence of phosphine cocatalysts, no cyclic carbonate was produced as a byproduct.

Yang G., Xie R., Zhang Y., Xu C., Wu G.

Rzhevskiy S.A., Shurupova O.V., Asachenko A.F., Plutalova A.V., Chernikova E.V., Beletskaya I.P.

A comparative study of the copolymerization of racemic propylene oxide (PO) with CO2 catalyzed by racemic (salcy)CoX (salcy = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = perfluorobenzoate (OBzF5) or 2,4-dinitrophenoxy (DNP)) in the presence of a [PPN]Cl ([PPN] = bis(triphenylphosphine)iminium) cocatalyst is performed in bulk at 21 °C and a 2.5 MPa pressure of CO2. The increase in the nucleophilicity of an attacking anion results in the increase in the copolymerization rate. Racemic (salcy)CoX provides a high selectivity of the copolymerization, which can be higher than 99%, and the living polymerization mechanism. Poly(propylene carbonate) (PPC) with bimodal molecular weight distribution (MWD) is formed throughout copolymerization. Both modes are living and are characterized by low dispersity, while their contribution to MWD depends on the nature of the attacking anion. The racemic (salcy)CoDNP/[PPN]DNP system is found to be preferable for the production of PPC with a high yield and selectivity.

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.

Toda T., Nakata T., Fukuoka H., Yoneda H.

Xie X., Huo Z., Jang E., Tong R.

AbstractPrecisely controlling macromolecular stereochemistry and sequences is a powerful strategy for manipulating polymer properties. Controlled synthetic routes to prepare degradable polyester, polycarbonate, and polyether are of recent interest due to the need for sustainable materials as alternatives to petrochemical-based polyolefins. Enantioselective ring-opening polymerization and ring-opening copolymerization of racemic monomers offer access to stereoregular polymers, specifically enantiopure polymers that form stereocomplexes with improved physicochemical and mechanical properties. Here, we highlight the state-of-the-art of this polymerization chemistry that can produce microstructure-defined polymers. In particular, the structures and performances of various homogeneous enantioselective catalysts are presented. Trends and future challenges of such chemistry are discussed.

Pedretti B.J., Zhu C., Watanabe H., Aoshima S., Lynd N.A.

Denk A., Fulajtar E., Troll C., Rieger B.

AbstractThe controlled synthesis of terpolymer structures is often limited by the intrinsic reactivities of the monomers. For the synthesis of a statistical terpolymer from cyclohexene oxide (CHO) and propylene oxide (PO) with CO2, an instrumental solution is demanded. Implementing a setup where one monomer can be added to the reaction mixture over the whole course of the reaction, the random distribution of the epoxides over the whole chain is realized. The successful terpolymerization can be determined with diffusion‐ordered nuclear magnetic resonance spectroscopy and gel permeation chromatography measurements while the statistical microstructure of the generated polymers is indicated in NMR spectroscopy and differential scanning calorimetry measurements. Furthermore, the concept is transferred to the terpolymerization of limonene oxide with PO and CO2 underlining the versatility of the setup.

Brivio M., Veronese L., Biagini P., Tritto I., Po R., Boggioni L., Losio S.

Branched phosphazenium salts are tested as alternative cocatalysts to traditional PPNX salts in the CO2/epoxide copolymerization with different catalysts. Higher molecular weights and comparable or superior conversions and selectivities are achieved.

Wu P., Qi Y., Li Q., Fan R., Qin S., Cai Y., Yuan L., Feng W.

Joshi S.S., Eagan J.M.

The advances in catalytic homogeneous coupling and copolymerization of carbon dioxide with comonomers has enabled sustainable routes to existing polymers and new molecular structures. This chapter reviews selected approaches to coupling CO2 with olefins and epoxides to form monomers and copolymers directly. Both landmark advances and recent directions in the field of homogeneous catalytic conversion of CO2 into macromolecules are covered.

Wang Z., Wang Z., Yin G.

• New chiral salenCr and salanCr complexes with bulky substituents. • Different structures and substituents affect the catalytic performance greatly. • Stereoregular PCHCs and PCHPs with moderate ee for the CHO units. A series of new chiral salenCr and salanCr complexes bearing ( R,R )-1,2-diaminocyclohexane backbone with bulky substituents (cumyl, 1,1-diphenylethyl and trityl) at the ortho-position of the aryloxide moieties were synthesized and investigated as catalysts in the asymmetric copolymerization of cyclohexene oxide (CHO) with CO 2 or phthalic anhydride (PA). The substituents of the Cr complexes affect the catalytic activity as well as stereoselectivity greatly toward the copolymerizations. The different salen/salan backbones of the Cr complexes also play an important role on catalytic performance. Poly(cyclohexene carbonate)s (PCHCs) with number-average molecular weight (M n ) of 4.8-9.1 kg·mol −1 and 19.0-19.4% enantiomeric excess (ee) value of the 1,2-cyclohexanediol unit were prepared by using salenCr with 1,1-diphenylethyl groups and salanCr with cumyl or 1,1-diphenylethyl groups as catalyst, respectively. Poly(cyclohexene phthalate)s (PCHPs) with M n of 1.6 kg·mol −1 and ee value of 7.1-8.7% were obtained by using salenCr with cumyl or 1,1-diphenylethyl groups as catalyst, respectively. While, the salanCr complexes resulted in atactic PCHPs.

Lidston C.A., Severson S.M., Abel B.A., Coates G.W.

Spyridakou M., Gardiner C., Papamokos G., Frey H., Floudas G.

Li R., Dou L., Tong L., Dong W.

Two helical centrosymmetric homotetranuclear Cu (II) complexes, [Cu4(L)2(EtOH)2](ClO4)2·2EtOH·2CHCl3 (1) and [{Cu4(L)2(EtOH)2}{Cu4(L)2(HNO3)2}](NO3)4·3EtOH·3MeOH (2), were synthesized by the reactions of a symmetric bis (salamo)-based ligand H3L with Cu(ClO)2·6H2O and Cu(NO)3·3H2O, respectively, and certified by elemental analyses, UV–Visible absorption spectra, infrared spectra and single-crystal X-ray analysis techniques. X-ray crystal structure analyses reveal that four Cu (II) atoms of complex 1 are attached to two deprotonated ligand (L)3− units with the help of two coordinated ethanol molecules, and then forming a helical centrosymmetric complex by H-bonding and π⋯π interactions. While complex 2 unit cell contains two crystallographically independent but chemically identical homotetranuclear complexes (molecules A and B), eight Cu (II) atoms are coordinated by four deprotonated ligand (L)3− units. The coordination mode of four Cu (II) atoms from molecule A is the same as complex 1; at the same time, oxygen atoms on nitrates are involved in the coordination of another four Cu (II) atoms from molecule B. The short-range interactions in complexes 1 and 2 were calculated by Hirshfeld surfaces analyses. The molecular orbital energy levels, molecular stabilities of complexes 1 and 2 were analyzed by DFT calculations. In addition, antibacterial assays were also investigated in detail.