David C. Sherrington

Utilisation of homogeneous and supported chiral metal(salen) complexes in asymmetric catalysis

The first salen ligand and Cu complex were discovered in 1889, and gradually the potential catalytic activity of subsequent salen species has been recognised, particularly in the case of achiral salen complexes in oxidation reactions. The development of chiral salen metal complexes and catalysts in the last decade has however stimulated a very rapid growth in the chemistry and application of these species. The variety of asymmetric reactions in which particular chiral metal salen complexes are proving useful grows steadily, and there is no evidence of this growth slowing. This review summarises the key work and references on soluble chiral metal salen complex catalysts categorised according to the metal centre. It also describes the work to date on producing supported heterogeneous chiral analogues of some of these.

Self-assembly in synthetic macromolecular systems via multiple hydrogen bonding interactions

Self-assembly yielding supramolecular systems is a relatively new and fascinating area in polymer science. By combining a knowledge of organic and bio-organic chemistry with synthetic polymer chemistry many self-assembling structures can be developed in synthetic polymer systems via exploitation of inter- and/or intramolecular interactions. This review describes some double, triple and quadruple hydrogen bonding systems from the recent literature which have been used in this context.

High internal phase emulsions (HIPEs) — Structure, properties and use in polymer preparation

High internal phase emulsions (HIPEs) are concentrated systems possessing a large volume of internal, or dispersed phase. The volume fraction is above 0.74, resulting in deformation of the dispersed phase droplets into polyhedra, which are separated by thin films of continuous phase. Their structure, which is analogous to a conventional gas-liquid foam of low liquid content, gives rise to a number of peculiar and fascinating properties including high viscosities and viscoelastic rheological behaviour. Like dilute emulsions, HIPEs are both kinetically and thermodynamically unstable; nevertheless, it is possible to prepare metastable systems which show no change in properties or appearance over long periods of time.

Facile, versatile and cost effective route to branched vinyl polymers

Abstract Free radical one-step polymerisation of methyl methacrylate in the presence of a crosslinking comonomer and balancing levels of a chain transfer agent allow facile high conversion synthesis of polymers with a branched architecture. Broad GPC curves have been obtained for these materials and branching has been demonstrated using GPC coupled with a triple detection system allowing evaluation of g ′, the Zimm branching factor. In addition the bridging residues in branched polymers arising from use of an unsaturated crosslinking comonomer have been chemically cleaved to yield the linear oligomers of much narrower molecular weight distribution that comprise the branched structure. The methodology promises to be generic, utilises only readily available starting materials and holds out good prospects for providing branched vinyl polymers on a technically useful scale at low cost.

Polymer-supported metal complex alkene epoxidation catalysts

Abstract Following a brief background description of homogeneous alkene epoxidation catalysts and early attempts to immobilise these, the most recent developments in the supporting of these species are reviewed. The area is divided into non-chiral and chiral catalysts. Within the former the key combinations that are described are the Mo(VI) and V(V)/alkyl hydroperoxide systems, upon which a great deal of structural evaluation has also been carried out, and W(VI)/H2O2. A brief summary of results using immobilised Cu, Mn, Fe, Ru and Ti with alkyl hydroperoxide is also given. In the case of chiral catalysts the main systems described are those mimicking the Sharpless tartrate ester/Ti(IV)/hydroperoxide system for epoxidation of allylic and related alcohols, and those related to Jacobsen’s chiral Mn(III) (salen) system for epoxidation of internal non-functionalised cis-alkenes.

Evaluation of methods aimed at complete removal of template from molecularly imprinted polymers

Polymers imprinted with clenbuterol were used to study the influence of various post-polymerization treatments [e.g., thermal annealing, microwave assisted extraction (MAE), Soxhlet extraction and supercritical fluid template desorption] on the bleeding of residual template. The aim of the study was to reduce the bleeding to levels that would allow the use of the materials as affinity phases for extraction of clenbuterol from bovine urine at concentrations below 1 ng ml-1. After treatment, the clenbuterol imprinted polymers were packed into solid-phase extraction columns and the bleeding was estimated by quantifying the amount of template released in 10 ml of methanol-acetic acid (9 + 1 v/v). This was followed by an assessment of selectivity and recovery in comparison with non-treated material. The lowest bleeding level was found after MAE using 100% trifluoroacetic acid for 3 x 20 min at 100 degrees C. The collected eluate contained in this case 3 ng ml-1 of clenbuterol. The same material was subsequently used for the extraction of clenbuterol from spiked bovine urine. The resulting selectivity and recovery were lower compared with those obtained using the untreated material. A milder but still efficient method to reduce the bleeding level was found to be MAE with formic acid. In this case a bleeding level of 14 ng ml-1 was found after only a 1 h extraction time. In a second model system, using a polymer imprinted with L-phenylalanine anilide, the bleeding was reduced to a similar level by extensive on-line washing in good swelling solvents containing acid or base additives and after thermal annealing of the polymers in the dry state.

Study of the formation of the open-cellular morphology of poly(styrene/divinylbenzene) polyHiPE materials by cryo-SEM

The formation of the interconnected morphology of open-cell styrene/divinylbenzene (DVB) PolyHIPE copolymers has been studied by scanning electron microscopy on frozen HIPE samples at different stages of polymerisation, a technique known as cryo-SEM. The transition from discrete emulsion droplets to interconnected cells was observed to occur around the gelpoint of the polymerising system. This would suggest that the formation of holes between adjacent cells is due to the contraction of the thin monomeric films on conversion of monomer to polymer, as a result of the higher density of the latter.

On-line solid-phase extraction with molecularly imprinted polymers to selectively extract substituted 4-chlorophenols and 4-nitrophenol from water.

Three polymers have been synthesised using 4-chlorophenol (4-CP) as the template, following different protocols (non-covalent and semi-covalent) and using different functional co-monomers, 4-vinylpyridine (4-VP) and methacrylic acid (MAA). The polymers were evaluated to check their selectivity as molecularly imprinted polymers (MIPs) in solid-phase extraction (SPE) coupled on-line to liquid chromatography. The solid-phase extraction procedure using MIPs (MISPE), including the clean-up step to remove any interferences, was optimised. The 4-VP non-covalent polymer was the only one which showed a clear imprint effect. This MIP also showed cross-reactivity for the 4-chloro-substituted phenols and for 4-nitrophenol (4-NP) from a mixture containing the 11 priority EPA (Environmental Protection Agency) phenolic compounds and 4-chlorophenol. The MIP was applied to selectively extract the 4-chloro-substituted compounds and 4-NP from river water samples.

Preparation, structure and morphology of polymer supports

The use of polymer resin beads as an aid to synthesis is becoming an increasingly common feature in both academic and industrial synthesis laboratories. The large majority of users employ materials sourced commercially and adopt or adapt procedures already described in the literature without thinking too deeply about the physico-chemical aspects of the support. Success can be immediate, but more often a learning curve needs to be traversed. The present article seeks to describe the chemistry of synthesising supports and to present a user-friendly description of their key physico-chemical properties. A qualitative and pictorial view of how specific morphologies can be generated, and the relevance of these, is also presented. It is hoped that this insight will be of advantage to users in planning and pursuing their chemistry using polymer supports.

Non-covalent and semi-covalent molecularly imprinted polymers for selective on-line solid-phase extraction of 4-nitrophenol from water samples

Two molecularly imprinted polymers (MIPs) have been synthesised for the selective extraction of 4-nitrophenol (4-NP) from water samples. One polymer was synthesised via a non-covalent approach and the other via a semi-covalent approach. The selectivity of the polymers for 4-NP was evaluated when these polymers were applied in on-line solid-phase extraction (MISPE) coupled to reversed-phase HPLC. The MISPE conditions for both MIPs were optimised and a clean-up step was included to eliminate non-specific interactions. Differences between the two MIPs were observed with the non-covalent MIP being the more selective of the two, whereas the recoveries were slightly higher for the semi-covalent MIP. The performance of the imprinted polymers in the MISPE of real water samples was also evaluated.