Charlotte K. Williams

An overview of CO2 capture technologies

In this paper, three of the leading options for large scale CO2 capture are reviewed from a technical perspective. We consider solvent-based chemisorption techniques, carbonate looping technology, and the so-called oxyfuel process. For each technology option, we give an overview of the technology, listing advantages and disadvantages. Subsequently, a discussion of the level of technological maturity is presented, and we conclude by identifying current gaps in knowledge and suggest areas with significant scope for future work. We then discuss the suitability of using ionic liquids as novel, environmentally benign solvents with which to capture CO2. In addition, we consider alternatives to simply sequestering CO2—we present a discussion on the possibility of recycling captured CO2 and exploiting it as a C1 building block for the sustainable manufacture of polymers, fine chemicals, and liquid fuels. Finally, we present a discussion of relevant systems engineering methodologies in carbon capture system design.

Polymers from Renewable Resources: A Perspective for a Special Issue of Polymer Reviews

The field of polymers derived from non‐petrochemical feedstocks is gaining a great deal of momentum from both a commercial and academic sense. Using annually renewable feedstocks, such as biomass, for the production of new plastics can have both economic and environmental benefits. Fundamental research in the production, modification, property enhancement, and new applications of these materials is an important undertaking. The new materials, concepts, and utilizations that result from these efforts will shape the future of polymers from renewable resources. This issue of Polymer Reviews focuses on the production and properties of renewable resource polymers and highlights current trends and research directions.

Biocompatible Initiators for Lactide Polymerization

The review summarizes recent developments in the preparation and use of new initiators for the ring opening polymerization of lactide. The review compares different classes of initiator including metal complexes, classed according to their group in the periodic table, and carbon‐based initiators/organocatalysts. Emphasis is placed on the polymerization kinetics and the control exhibited by the different types of initiators. Where useful properties, such as high rates or stereocontrol, have been observed a more detailed examination of the initiator is provided. A further focus of the review is initiators displaying low toxicity and biocompatibility.

Synthesis of functionalized biodegradable polyesters

This tutorial review summarizes recent developments in the syntheses of functionalized aliphatic polyesters. These polymers are attracting attention as sustainable alternatives to petrochemicals and for applications in medicine. Two main syntheses are described: step polymerization using mild chemo/enzymatic catalysis and ring opening polymerization, which is usually initiated by metal complexes. The methods are compared and their utility illustrated with reference to interesting new materials.

A bimetallic iron(iii) catalyst for CO2/epoxide coupling

A novel di-iron(III) catalyst for the copolymerisation of cyclohexene oxide and CO(2) to yield poly(cyclohexene carbonate), under mild conditions, is reported. The catalyst selectivity was completely changed on addition of an ammonium co-catalyst to yield only the cis-isomer of the cyclic carbonate, also under mild conditions. Additionally, the catalyst was active for propylene carbonate and styrene carbonate production at 1 atm pressure.

Highly Active Dizinc Catalyst for the Copolymerization of Carbon Dioxide and Cyclohexene Oxide at One Atmosphere Pressure

A novel dizinc complex having a macrocyclic ancillary ligand shows remarkable activity at only one atmosphere of CO2 for the copolymerization of CO2 and cyclohexene oxide. Carbon dioxide is an attractive reagent for synthetic chemistry as it is abundant, inexpensive, of low toxicity, and is the waste product of many chemical processes. The copolymerization of carbon dioxide and epoxides, known for several decades, is a particularly promising route to activate and use CO2 as a renewable C-1 source. [1–5] Furthermore, if cyclohexene oxide is used, the resulting copolymer has a high glass transition temperature and tensile strength, but it is also degradable. The first report of this type of copolymerization came from Inoue et al. in 1969, and they used diethyl zinc and alcohols to produce poly(propylene carbonate), albeit with very low turnover numbers (TONs). Subsequently several research groups developed more active and controlled catalysts, and notable for their activity are the zinc phenoxide, zinc b-diiminate, and chromium(III)– or cobalt(III)–salen complexes. The zinc b-diiminate complexes show very high turnover frequencies (TOFs), as well as excellent control for the copolymerization of CO2 and cyclohexene oxide. Recent mechanistic studies by Coates and co-workers suggest that the most effective b-diiminate complexes are loosely associated dimers under the polymerization conditions. This proposal has led to the deliberate preparation of various dimetallic zinc catalysts, and among these dizinc catalysts, the anilido aniline complexes show particularly high TONs and TOFs because they can operate at low catalyst loadings. 24] However, all of the known high activity catalysts require substantial (> 7 atm) pressures of carbon dioxide, which significantly increases the overall energy requirement of the process. Although catalysts which operate at only one atmosphere of CO2 are known, 23, 26,27] so far the best reported TON was 20 and the highest TOF was 3.3 h . We report the preparation of a dimetallic zinc complex (Scheme 1) having a macrocyclic ancillary ligand, which shows very high activity for the copolymerization of cyclohexene oxide and carbon dioxide under mild pressures. The macrocyclic ligand H2L 1 was prepared in two steps with 84% overall yield from commercial reagents (see the Supporting Information) by using an adaptation of a synthetic route described previously. The dimetallic zinc complex, [LZn2(OAc)2] was synthesized by the deprotonation of H2L 1 using potassium hydride, and subsequent reaction with zinc acetate. The complex was isolated as a white solid in 70 % yield (Scheme 1). The stoichiometry of the complex was confirmed by elemental analysis, which was in agreement with the calculated values, and the identification of a fragment peak in the FAB mass spectrum for the molecular ion less an acetate group. The H NMR spectrum at 25 8C shows broadened resonances, consistent with several diastereoisomers being present, which are fluxional on the NMR timescale. When the sample was heated to 110 8C coalescence was observed (see Figure S1 in the Supporting Information). A single resonance was observed for the aromatic protons and the signal for the NH groups was a broadened resonance at d = 4.78 ppm. The methylene groups are diastereotopic, therefore four broadened resonances were observed from d = 3.32–2.46 ppm, each with an integral corresponding to 4H. The signals for the tertbutyl groups and the methyl group of the acetate resonate as singlets with integrals corresponding to 18H and 6H, respectively. The methyl groups on the ligand backbone are also diastereotopic and are observed as two singlets, each with a relative integral corresponding to 6H. The complex was tested at low pressures for the copolymerization of carbon dioxide and cyclohexene oxide (Table 1). Thus, at only one atmosphere of CO2, 80–100 8C, and a 0.1 mol% catalyst loading, poly(cyclohexene carbonate) was produced with a TON in the range of 430–530 and a TOF in the range of 18–25 h 1 (Table 1, entries 1–3). There are very few catalysts that are effective at such a low pressure, 23, 26,27] the most active of which is a dizinc Scheme 1. The synthesis of the dizinc complex [LZn2(OAc)2]. Reagents and conditions: a) KH, THF, 78 8C!RT, 1 h; b) Zn(OAc)2, THF, RT, 16 h.

Sustainable polymers from renewable resources

Renewable resources are used increasingly in the production of polymers. In particular, monomers such as carbon dioxide, terpenes, vegetable oils and carbohydrates can be used as feedstocks for the manufacture of a variety of sustainable materials and products, including elastomers, plastics, hydrogels, flexible electronics, resins, engineering polymers and composites. Efficient catalysis is required to produce monomers, to facilitate selective polymerizations and to enable recycling or upcycling of waste materials. There are opportunities to use such sustainable polymers in both high-value areas and in basic applications such as packaging. Life-cycle assessment can be used to quantify the environmental benefits of sustainable polymers.

Charge Recombination in Organic Photovoltaic Devices with High Open-Circuit Voltages

A detailed charge recombination mechanism is presented for organic photovoltaic devices with a high open-circuit voltage. In a binary blend comprised of polyfluorene copolymers, the performance-limiting process is found to be the efficient recombination of tightly bound charge pairs into neutral triplet excitons. We arrive at this conclusion using optical transient absorption (TA) spectroscopy with visible and IR probes and over seven decades of time resolution. By resolving the polarization of the TA signal, we track the movement of polaronic states generated at the heterojunction not only in time but also in space. It is found that the photogenerated charge pairs are remarkably immobile at the heterojunction during their lifetime. The charge pairs are shown to be subject to efficient intersystem crossing and terminally recombine into F8BT triplet excitons within approximately 40 ns. Long-range charge separation competes rather unfavorably with intersystem crossing--75% of all charge pairs decay into triplet excitons. Triplet exciton states are thermodynamically accessible in polymer solar cells with high open circuit voltage, and we therefore suggest this loss mechanism to be general. We discuss guidelines for the design of the next generation of organic photovoltaic materials where separating the metastable interfacial charge pairs within approximately 40 ns is paramount.

Comparison of structurally analogous Zn2, Co2, and Mg2 catalysts for the polymerization of cyclic esters

Three dimetallic monoethoxide complexes supported by a binucleating phenoxide ligand, LM2Cl2OEt (M = Zn, Co, or Mg), were prepared and shown by X-ray crystallography to be structurally analogous. Comparative studies of their cyclic ester polymerization reactivity revealed different trends for reactions with epsilon-caprolactone and lactide, however, implicating complicated effects of metal ion variation in these polymerizations.

Metal‐Size Influence in Iso‐Selective Lactide Polymerization

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group 13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a Pi value of 0.81–0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (Ps=0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu.