Michael R. Kember

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.

Mechanistic Investigation and Reaction Kinetics of the Low-Pressure Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by a Dizinc Complex

The reaction kinetics of the copolymerization of carbon dioxide and cyclohexene oxide to produce poly(cyclohexene carbonate), catalyzed by a dizinc acetate complex, is studied by in situ attenuated total reflectance infrared (ATR-IR) and proton nuclear magnetic resonance ((1)H NMR) spectroscopy. A parameter study, including reactant and catalyst concentration and carbon dioxide pressure, reveals zero reaction order in carbon dioxide concentration, for pressures between 1 and 40 bar and temperatures up to 80 °C, and a first-order dependence on catalyst concentration and concentration of cyclohexene oxide. The activation energies for the formation of poly(cyclohexene carbonate) and the cyclic side product cyclohexene carbonate are calculated, by determining the rate coefficients over a temperature range between 65 and 90 °C and using Arrhenius plots, to be 96.8 ± 1.6 kJ mol(-1) (23.1 kcal mol(-1)) and 137.5 ± 6.4 kJ mol(-1) (32.9 kcal mol(-1)), respectively. Gel permeation chromatography (GPC), (1)H NMR spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry are employed to study the poly(cyclohexene carbonate) produced, and reveal bimodal molecular weight distributions, with narrow polydispersity indices (≤1.2). In all cases, two molecular weight distributions are observed, the higher value being approximately double the molecular weight of the lower value; this finding is seemingly independent of copolymerization conversion or reaction parameters. The copolymer characterization data and additional experiments in which chain transfer agents are added to copolymerization experiments indicate that rapid chain transfer reactions occur and allow an explanation for the observed bimodal molecular weight distributions. The spectroscopic and kinetic analyses enable a mechanism to be proposed for both the copolymerization reaction and possible side reactions; a dinuclear copolymerization active site is implicated.

Di-cobalt(II) catalysts for the copolymerisation of CO2 and cyclohexene oxide: support for a dinuclear mechanism?

The synthesis and characterisation of a series of di-cobalt(II) halide complexes, coordinated by a macrocyclic ancillary ligand, is reported. The new complexes show excellent activity as catalysts for the copolymerisation of cyclohexene oxide (CHO) and carbon dioxide, under just 1 atmosphere of pressure of CO2. The complexation of a series of co-ligands has been investigated, including nucleophiles of varying strength, (4-dimethylaminopyridine (DMAP), N-methylimidazole (MeIm) and pyridine), and the anionic donor (Cl) from bulky ammonium salts, ([HNEt3]Cl, [DBU-H]Cl and [MTBD-H]Cl). Structure–activity studies of the complexes, including X-ray crystallography data, in conjunction with mass spectrometry experiments, are used to support a proposed dinuclear mechanism. The initial rate of copolymerisation, determined using in situ attenuated total reflectance infrared (ATR-IR) spectroscopy, shows a first order dependence on both the catalyst concentration and the concentration of cyclohexene oxide. A dinuclear mechanism is proposed in which catalysis occurs on the convex face of the molecule, leading to chain growth from a single site.

Triblock copolymers from lactide and telechelic poly(cyclohexene carbonate)

The preparation of α,ω-hydroxy-telechelic poly(cyclohexene carbonate) from a dizinc catalyst is reported. The telechelic polymer, with an yttrium initiator, can be used to polymerize lactide, yielding new triblock copolymers, substantially derived from renewable resources.