Enantioselective Epoxide Polymerization Using a Bimetallic Cobalt Catalyst

Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using a Bimetallic Chromium Catalyst: A New Route to Semicrystalline Polyols

Hydroxy-telechelic poly(propylene oxide) (PPO) is widely used industrially as a midsegment in polyurethane synthesis. These atactic polymers are produced from racemic propylene oxide using chain shuttling agents and double-metal cyanide catalysts. Unlike atactic PPO, isotactic PPO is semicrystalline with a melting temperature of approximately 67 °C. Currently there is no practical route to hydroxy-telechelic isotactic PPO using racemic propylene oxide as the monomer. In this paper, hydroxy-telechelic isotactic PPO is synthesized from racemic propylene oxide with control of molecular weight using enantioselective and isoselective bimetallic catalysts in conjunction with chain shuttling agents. The discovery of an easily accessible bimetallic chromium catalyst is reported for this transformation. Diol, triol, and polymeric chain shuttling agents are used to give hydroxy-telechelic isotactic PPO of varying functionality and structure. Detailed quantum chemical studies are used to reveal the polymerization mechanism and origin of stereoselectivity.

Bimetallic Schiff base complexes for stereoselective polymerisation of racemic-lactide and copolymerisation of racemic-lactide with ε-caprolactone

Bimetallic Schiff-base complexes with different bridging parts and silyl-substituents were applied for stereoselective ROP ofrac-LA, and copolymerisation ofrac-LA/ε-CL.

Group 4 alkoxide complexes containing [NNO]-type scaffold: synthesis, structural characterization and polymerization studies

A series of new dinuclear group 4 complexes containing [NNO]-type ligands are utilized as catalysts for polymerization reactions.

Crystalline CO2-based polycarbonates prepared from racemic catalyst through intramolecularly interlocked assembly

The crystalline stereocomplexed polycarbonates can be prepared by mixing enantiopure polymers with opposite configuration, which derived from the asymmetric copolymerization with CO₂ using enantiopure catalyst or/and chiral epoxides. Herein, we develop a powerful strategy for producing crystalline intramolecular stereocomplexed polycarbonates from racemic catalysts, which possess similar thermal stability and crystalline behaviour in comparison with the stereocomplexes by mixing opposite enantiopure polymers. Living polymer chains shuttle between catalyst molecules with different configurations to produce diastereomeric active species which is suggested to be responsible for the formation of isotactic multiblock polycarbonates in racemic bimetallic cobalt catalyst-mediated stereoselective copolymerization of CO₂ and meso-epoxides. Solid-state NMR spectroscopy study suggests that the interaction in the carbonyl and methine regions is responsible for the strong crystallization capacity and compact package structure in the crystalline polycarbonates.

Crystalline polycarbonates can be formed by mixing enantiopure polymers with opposite configurations. Here, the authors produce crystalline intramolecular stereocomplexed polycarbonates using a racemic catalyst, and show that these display similar properties to those formed by mixing the two enantiomers.

A well-defined, versatile photoinitiator (salen)Co-CO2CH3 for visible light-initiated living/controlled radical polymerization

The control of the polymerization of a wide range of monomers under mild conditions by a single catalyst remains a major challenge in polymer science. We report a versatile, well-defined organocobalt salen complex to control living radical polymerization of different categories of monomers, including acrylates, acrylamides and vinyl acetate, under visible light irradiation at ambient temperature. Both household light and sunlight were effectively applied in the synthesis of polymers with controlled molecular weights and narrow polydispersities. Narrowly dispersed block copolymers (Mw/Mn < 1.2) were obtained under various conditions. The structures of the polymers were analyzed by 1H NMR, 2D NMR, 13C NMR, GPC, MALDI-TOF-MS and isotopic labeling experiments, which showed that the ω and α ends of the polymer chains were capped with (salen)Co and -CO2CH3 segments, respectively, from the photoinitiator (salen)Co-CO2CH3. The ω end was easily functionalized through oxygen insertion followed by hydrolysis from 18O2 to -18OH. This robust system can proceed without any additives, and offers a versatile and green way to produce well-defined homo and block copolymers.

Spectroscopic properties of a series of Co(II) coordination polymers and the influence of Co(II) coordination environment on photoelectric property

Four Co(II) coordination polymers, [Co(suc)]n 1, [Co(pdc)]n 2, {[Co7(suc)4(OH)6(H2O)3]·8H2O}n 3, {[Co(bdc)(phen)(H2O)]·H2O}n 4 (H2suc=succinic acid, H2pdc=pyridine-3,4-dicarboxylic acid, H2bdc=1,2-benzenedicarboxylic acid, phen=1,10-phenanthroline) were hydrothermally synthesized and characterized by X-ray single-crystal diffraction, surface photovoltage spectroscopy (SPS), electrical conductivity, thermogravimetric analysis (TG), ultraviolet visible and near-infrared absorption spectrum (UV-Vis-NIR), infrared spectrum (IR), and elemental analysis. The structural analyses indicate that the coordination numbers of the Co(II) ions are 4, 5, 6 and 6 for the polymers 1-4, respectively. And polymers 1 and 2 exhibit 3D structure formed by suc(2-) and pdc(2-) anions bridging Co(II) ions, respectively. Polymer 3 exhibits a 2D structure with suc(2-) anions bridging seven-nuclear [Co7(OH)6(H2O)3](3-) unit and polymer 4 is a 1D structure bridged by bdc(2-) anions. The surface photoelectric properties of the cobalt polymers were mainly studied by SPS. The results of SPS reveal that all polymers possess certain photoelectric conversion property in the range of 300-800 nm. The influences of the structure, coordination micro-environment of central metal ion and structural dimensionality on response bands of SPS were discussed.

Preparation of biocompatible, biodegradable and sustainable polylactides catalyzed by aluminum complexes bearing unsymmetrical dinaphthalene-imine derivatives via ring-opening polymerization of lactides

Aluminum complexes bearing unsymmetrical dinaphthalene-imine ligands were employed for lactide polymerization.

Modulation of electronic and redox properties in phenolate-rich cobalt(iii) complexes and their implications for catalytic proton reduction

The redox, spectroscopic and catalytic activity of a series of cobalt complexes with phenolate-rich environments was investigated. The complex [Co^III(L^Cl)MeOH] shows considerable proton reduction in MeCN:HOAc with TON = 10.8.

Magnesium complexes bearing N,N-bidentate phenanthrene derivatives for the stereoselective ring-opening polymerization of rac-lactides

Magnesium complexes based on N,N-bidentate phenanthrene derivatives derived from Schiff bases were synthesized and investigated as catalysts for rac-lactide polymerization.

Zr(iv) complexes containing salan-type ligands: synthesis, structural characterization and role as catalysts towards the polymerization of ε-caprolactone, rac-lactide, ethylene, homopolymerization and copolymerization of epoxides with CO2

Three new Zr(iv) complexes bearing salan-type diamine bis(phenolato) ligands were synthesized and their activities towards the ROP of ε-CL, rac-LA and homopolymerization, copolymerization and coupling of epoxides with CO₂ were investigated.

Ring-opening copolymerization (ROCOP): synthesis and properties of polyesters and polycarbonates

This feature article highlights the opportunities presented by ring-opening copolymerization (ROCOP) as a controlled route to prepare polyesters and polycarbonates.

Tuning ligand electronics and peripheral substitution on cobalt salen complexes: structure and polymerisation activity

A series of cobalt salen complexes, where salen represents an N2O2 bis-Schiff-base bis-phenolate framework, are prepared, characterised and investigated for reversible-termination organometallic mediated radical polymerisation (RT-OMRP). The salen ligands contain a cyclohexane diimine bridge and systematically altered para-substituted phenoxide moieties as a method to examine the electronic impact of the ligand on complex structure and reactivity. The complexes are characterised by single crystal X-ray diffraction, cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and computational methods. Structural studies all support a tailorable metal centre reactivity altered by the electron-donating ability of the salen ligand. RT-OMRP of styrene, methyl methacrylate and vinyl acetate is reported and suggests that cobalt-carbon bond strength varies with the ligand substitution. Competing β-hydrogen abstraction affords long-chain olefin-terminated polymer chains and well controlled vinyl acetate polymerisations, contrasting with the lower temperature associative exchange mechanism of degenerative transfer OMRP.

A broadly applicable and practical oligomeric (salen) Co catalyst for enantioselective epoxide ring-opening reactions

The (salen) Co catalyst (4a) can be prepared as a mixture of cyclic oligomers in a short, chromatography-free synthesis from inexpensive, commercially available precursors. This catalyst displays remarkable enhancements in reactivity and enantioselectivity relative to monomeric and other multimeric (salen) Co catalysts in a wide variety of enantioselective epoxide ring-opening reactions. The application of catalyst 4a is illustrated in the kinetic resolution of terminal epoxides by nucleophilic ring-opening with water, phenols, and primary alcohols; the desymmetrization of meso epoxides by addition of water and carbamates; and the desymmetrization of oxetanes by intramolecular ring opening with alcohols and phenols. The favorable solubility properties of complex 4a under the catalytic conditions facilitated mechanistic studies, allowing elucidation of the basis for the beneficial effect of oligomerization. Finally, a catalyst selection guide is provided to delineate the specific advantages of oligomeric catalyst 4a relative to (salen) Co monomer 1 for each reaction class.

Partial Kinetic Resolution of Oxanorbornenes by Ring-Opening Metathesis Polymerization with a Chiral Ruthenium Initiator

We report the first kinetic resolution by ring-opening metathesis polymerization (KR-ROMP). The polymerization profile showed a solvent-dependent variation of selectivity (S) over the course of the reaction. In tetrahydrofuran and dichloromethane, the resolution selectivity increased over the course of the reaction, while in toluene the selectivity was much higher in the beginning of the reaction and decreased throughout. Evidence suggests that the change in selectivity might be attributed to the chiral secondary structure of the growing polymer chain.

A concise review of computational studies of the carbon dioxide–epoxide copolymerization reactions

The production of polycarbonates from carbon dioxide and epoxides is an important route by which waste CO₂ is converted into useful products. This review surveys the use of computational chemistry toward understanding this reaction.

Bimetallic catalysis in the highly enantioselective ring–opening reactions of aziridines

Structural and kinetic evidence for bimetallic mechanism.

A trinuclear Zn3(OAc)4-3,3′-bis(aminoimino)binaphthoxide complex for highly efficient catalytic asymmetric iodolactonization

Using a harmony of the tri-zinc atoms, 1 mol% Zn₃(OAc)₄-3,3′-bis(aminoimino)binaphthoxide catalyzed asymmetric iodolactonization in up to 99.9% ee.

Enantioselective polymerization of epoxides using biaryl-linked bimetallic cobalt catalysts: a mechanistic study

The enantioselective polymerization of propylene oxide (PO) using biaryl-linked bimetallic salen Co catalysts was investigated experimentally and theoretically. Five key aspects of this catalytic system were examined: (1) the structural features of the catalyst, (2) the regio- and stereoselectivity of the chain-growth step, (3) the probable oxidation and electronic state of Co during the polymerization, (4) the role of the cocatalyst, and (5) the mechanism of monomer enchainment. Several important insights were revealed. First, density functional theory (DFT) calculations provided detailed structural information regarding the regio- and stereoselective chain-growth step. Specifically, the absolute stereochemistry of the binaphthol linker determines the enantiomer preference in the polymerization, and the interaction between the salen ligand and the growing polymer chain is a fundamental aspect of enantioselectivity. Second, a new bimetallic catalyst with a conformationally flexible biphenol linker was synthesized and found to enantioselectively polymerize PO, though with lower enantioselectivity than the binaphthol linked catalysts. Third, DFT calculations revealed that the active form of the catalyst has two active exo anionic ligands (chloride or carboxylate) and an endo polymer alkoxide which can ring-open an adjacent cobalt-coordinated epoxide. Fourth, calculations showed that initiation is favored by an endo chloride ligand, while propagation is favored by the presence of two exo carboxylate ligands.

Mechanistic basis for high stereoselectivity and broad substrate scope in the (salen)Co(III)-catalyzed hydrolytic kinetic resolution

In the (salen)Co(III)-catalyzed hydrolytic kinetic resolution (HKR) of terminal epoxides, the rate- and stereoselectivity-determining epoxide ring-opening step occurs by a cooperative bimetallic mechanism with one Co(III) complex acting as a Lewis acid and another serving to deliver the hydroxide nucleophile. In this paper, we analyze the basis for the extraordinarily high stereoselectivity and broad substrate scope observed in the HKR. We demonstrate that the stereochemistry of each of the two (salen)Co(III) complexes in the rate-determining transition structure is important for productive catalysis: a measurable rate of hydrolysis occurs only if the absolute stereochemistry of each of these (salen)Co(III) complexes is the same. Experimental and computational studies provide strong evidence that stereochemical communication in the HKR is mediated by the stepped conformation of the salen ligand, and not the shape of the chiral diamine backbone of the ligand. A detailed computational analysis reveals that the epoxide binds the Lewis acidic Co(III) complex in a well-defined geometry imposed by stereoelectronic rather than steric effects. This insight serves as the basis of a complete stereochemical and transition structure model that sheds light on the reasons for the broad substrate generality of the HKR.

Asymmetric copolymerization of CO2 with meso-epoxides mediated by dinuclear cobalt(III) complexes: unprecedented enantioselectivity and activity

Unprecedented enantioselectivity and catalytic activity was observed in the asymmetric copolymerization of CO2 with meso-epoxides (including the less reactive cyclopentene oxide) mediated by the dinuclear Co(III) complex (S,S,S,S)-1 under mild conditions. The resultant copolymers possess more than 99 % carbonate linkages and a perfectly isotactic structure.