'Switch' catalysis: from monomer mixtures to sequence-controlled block copolymers

A 'Switch' catalysis method is reviewed whereby a single catalyst is switched between ring-opening polymerization and ring-opening copolymerization cycles. It allows the efficient synthesis of block copolymers from mixtures of lactones, epoxides, anhydrides and carbon dioxide. In order to use and further develop such 'Switch' catalysis, it is important to understand how to monitor the catalysis and characterize the product block copolymers. Here, a step-by-step guide to both the catalysis and the identification of block copolymers is presented.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Dinuclear iron(iii) complexes bearing phenylene-bridged bis(amino triphenolate) ligands as catalysts for the copolymerization of cyclohexene oxide with carbon dioxide or phthalic anhydride

Dinuclear iron(iii) complexes bearing rigid ortho-, meta-, and para-phenylene bridges as effective catalysts for CHO/CO₂ and CHO/PA copolymerizations.

Bi-functional heterogeneous catalysts for carbon dioxide conversion: enhanced performances at low temperature

Novel bi-functional catalysts allowing to decrease the reaction temperature for the synthesis of cyclic carbonates below 150 °C were successfully synthesized.

One-pot copolymerization of epoxides/carbon dioxide and lactide using a ternary catalyst system

A PLA/PPC multiblock copolymer was prepared using a novel ternary catalytic system that proceeds via an intermolecular chain transfer mechanism.

Cycloaddition of carbon dioxide to epoxides catalysed by supported ionic liquids

Simple hybrid silica-imidazolium based ILs as highly active and selective catalysts for the carbonation of epoxides in batch and continuous flow systems under mild reaction conditions.

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

CO₂ conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO₂ utilization as a measure for global CO₂ mitigation. Whereas the latter focuses on reducing the end-of-pipe problem "CO₂ emissions" from todays' industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO₂-based chemistry does not depend primarily on the absolute amount of CO₂ emissions that can be remediated by a single technology. Rather, CO₂-based chemistry is stimulated by the significance of the relative improvement in carbon balance and other critical factors defining the environmental impact of chemical production in all relevant sectors in accord with the principles of green chemistry.

Highly Active and Readily Accessible Proline-Based Dizinc Catalyst for CO2 /Epoxide Copolymerization

In the pursuit of CO₂ -based materials, the development of efficient catalysts for the alternating copolymerization of CO₂ and epoxides to give polycarbonates is receiving particular attention. Desirable attributes for such catalysts are high copolymerization activity at low CO₂ pressure, as well as chemo- and stereocontrol over the formed polymer. Here, we report a novel chiral zinc catalyst that can be isolated in 97 % yield from commercial sources, and that produces polycarbonates selectively from neat cyclohexene oxide under 1 bar of CO₂ pressure at temperatures above 50 °C. At 80 °C reaction temperature, TONs of 1684 and initial TOFs up to 149 h^-1 were measured, producing an isotactic-enriched polycarbonate with a probability P_m of 65 % for the formation of a meso diad. Insight into the dinuclear nature of the active species and the copolymerization progress has been gained from structural and spectroscopic studies.

Di-Zinc-Aryl Complexes: CO2 Insertions and Applications in Polymerisation Catalysis

Two new di-zinc-aryl complexes, [LZn2 Ph2 ] and [LZn2 (C6 F5 )2 ], coordinated by a diphenol tetraamine macrocyclic ligand have been prepared and fully characterised, including by single-crystal X-ray diffraction experiments. The complexes' reactivities with monomers including carbon dioxide, cyclohexene oxide, phthalic anhydride, isopropanol and phenol were investigated using both experimental studies and density functional theory calculations. In particular, [LZn2 Ph2 ] readily inserts carbon dioxide to form a carboxylate, at 1 bar pressure, whereas [LZn2 (C6 F5 )2 ] does not react. Under these conditions [LZn2 Ph2 ] shows moderate activity in the ring-opening copolymerisation of cyclohexene oxide/carbon dioxide (TOF=20 h-1 ), cyclohexene oxide/phthalic anhydride (TOF=33 h-1 ) and the ring-opening polymerisations of rac-lactide (TOF=99 h-1 ) and ϵ-caprolactone (TOF=5280 h-1 ).

Selective Synthesis of Cyclic Carbonate by Salalen-Aluminum Complexes and Mechanistic Studies

Salalen-aluminum complexes were synthesized and used as catalysts in the reactions of CO₂ with different epoxides. The reaction of cyclohexene oxide and CO₂ was thoroughly investigated. In particular, the effect of the reaction conditions (nature and equivalents of the co-catalyst, CO₂ pressure, and temperature) and of the ligands (substituents on the ancillary ligand, nature of the labile ligand, and nature of the nitrogen-donor atoms) on the results of this reaction was studied. The cycloaddition reaction of CO₂ with terminal epoxides bearing different functional groups was realized. Moreover, NMR mechanistic studies provided information on the catalytic cycle. Interestingly, the characterization of an intermediate species in the mechanism of the reaction of cyclohexene oxide with CO₂ , catalyzed by one of the salalen-aluminum complexes, was accomplished.

Mechanistic Studies of Cyclohexene Oxide/CO2 Copolymerization by a Chromium(III) Pyridylamine-Bis(Phenolate) Complex

Chromium(III) chlorido amine-bis(phenolate) complexes paired with nucleophilic co-catalysts are a promising family of catalysts for the copolymerization of CO₂ and epoxides to selectively produce polycarbonates with a very high degree of carbonate linkages. Single-component catalyst systems can be prepared, where the neutral nucleophile, 4-dimethylaminopyridine (DMAP), is coordinated to the metal site to provide a stable octahedral Cr^III complex. These complexes possess the potential for both anionic (from the chlorido ligand) or neutral (DMAP) nucleophilic epoxide ring-opening during the proposed rate-determining initiation step. Concentration effect studies support a first-order dependence of the polymerization rate on the concentration of single-component catalyst. End-group analysis of polycarbonates by MALDI-TOF MS indicate the presence of predominantly DMAP-initiated chains as well as the occurrence of chain-transfer events resulting in ether linkages, likely from the presence of cyclohexene diol formed by the reaction of cyclohexene oxide and adventitious water.

Mechanistic Insights into the Carbon Dioxide/Cyclohexene Oxide Copolymerization Reaction: Is One Metal Center Enough?

A detailed study on the mechanism for the alternating copolymerization of cyclohexene oxide (CHO) and CO₂ mediated by an [Al{amino-tri(phenolate)}]/NBu₄ I binary catalyst system was performed by using DFT-based methods. Four potential mechanisms (one monometallic and three bimetallic) were considered for the first propagation cycle of the CHO/CO₂ copolymerization. The obtained Gibbs free energies provided a rationale for the relative high activity of a non-covalent dimeric structure formed in situ and thus for the feasibility of a bimetallic mechanism to obtain polycarbonates quantitatively. Gibbs free energies also indicated that the alternating copolymerization was favored over the cyclic carbonate formation.

A Bimetallic Aluminium(Salphen) Complex for the Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide

A bimetallic aluminium(salphen) complex is reported as a sustainable, efficient and inexpensive catalyst for the synthesis of cyclic carbonates from epoxides and carbon dioxide. In the presence of this complex and tetrabutylammonium bromide, terminal and internal epoxides reacted at 50 °C and 10 bar carbon dioxide pressure to afford their corresponding cyclic carbonates in yields of 50-94 % and 30-71 % for terminal and internal cyclic carbonates, respectively. Mechanistic studies using deuterated epoxides and an analogous monometallic aluminium(salphen) chloride complex support a mechanism for catalysis by the bimetallic complex, which involves intramolecular cooperative catalysis between the two aluminium centres.

Polymers from sugars: cyclic monomer synthesis, ring-opening polymerisation, material properties and applications

This feature article gives an overview of sugar-based polymers that can be made by ring-opening-polymerisation and their applications.

Switchable catalytic processes involving the copolymerization of epoxides and carbon dioxide for the preparation of block polymers

The synthesis of block polymersviaa one-pot process with segments prepared by different or the same mechanistic pathways.

An organic polymer-grafted ionic liquid as a catalyst for the cycloaddition of CO2 to epoxides

Chemical fixation of CO₂ to a cyclic carbonate catalyzed by benzyl chloride polymer-supported imidazolium ionic liquids.

Oxygenation of RZn(N,O)-type complexes as an efficient route to zinc alkoxides not accessible via the classical alcoholysis path

The oxygenation of alkylzincs incorporating a 2-ester substituted pyrrolate ligand (L) leads to zinc alkoxides with an uncommon structural motif in the solid state: a trimer [(L)Zn(μ-O^tBu)]₃ with the central [Zn₃(μ-OR)₃] ring or a heterocubane [(L)Zn(μ₃-OEt)]₄.

Highly recyclable polymer supported ionic liquids as efficient heterogeneous catalysts for batch and flow conversion of CO2 to cyclic carbonates

A series of cross-linked polymer supported ionic liquids based on quaternary ammonium salts have been prepared using a facile method of radical polymerization.

Kinetic and mechanistic investigation for the copolymerization of CO2 and cyclohexene oxide catalyzed by trizinc complexes

The mechanism of CO₂/epoxide copolymerization catalyzed by Schiff base trizinc complexes.

Synthesis, characterization and catalytic activity of rare-earth metal amides incorporating cyclohexyl bridged bis(β-diketiminato) ligands

The bridged bis(β-diketiminato) ligands supported rare-earth amides exhibited high catalytic activity towards the hydrophosphination of β-nitroalkene and α,β-unsaturated carbonyl derivatives with an excellent regioselectivity.

CO2-Driven stereochemical inversion of sugars to create thymidine-based polycarbonates by ring-opening polymerisation

A strategy using CO₂ to invert the stereochemistry and cyclise natural sugars is presented and applied towards thymidine-based polymers by ring-opening polymerisation.

β-Diketiminate complexes of the first row transition metals: applications in catalysis

Although β-diketiminate complexes have been widely explored in stoichiometric studies, their use as catalysts is largely underdeveloped.

Bio-derived polymers for coating applications: comparing poly(limonene carbonate) and poly(cyclohexadiene carbonate)

Two fully bio-based polycarbonates, poly(limonene carbonate) and poly(cylcohexadiene carbonate), were post-functionalized via thiol–ene reactions and tested as future coating materials.

The ability of a zinc pyrrolidine complex to catalyze the synthesis of cyclic carbonates from carbon dioxide and epoxides: a mechanistic theoretical investigation

The reaction mechanism for the synthesis of cyclic carbonates from carbon dioxide and epoxides catalyzed by a zinc pyrrolidine complex has been elucidated using the density functional level of theory.