John R. Jones

Microwave-enhanced radiochemistry

The application of microwaves to synthetic organic chemistry is currently experiencing considerable growth. Here we show how the area of radiochemistry, with particular reference to the synthesis of 3H- (or T-), 11C- and 18F-labelled compounds, can benefit. Faster, cleaner, more selective reactions are possible with the formation of much reduced levels of radioactive waste.

Studies of cure schedule and final property relationships of a commercial epoxy resin using modified imidazole curing agents

Abstract A commercial epoxy prepolymer (MY750) was cured with novel modified imidazole curing agents under both isothermal and dynamic scanning conditions. The thermal behaviour of the formulated mixtures was investigated using differential scanning calorimetry and thermogravimetric analysis, and glass transition temperature (Tg) values derived for polymers resulting from both cure schedules. Tg values and the thermal stability of polymers arising from the isothermal cure schedules were generally higher than those for the corresponding dynamic cure. For samples cured by the dynamic curing process, a lower heating rate resulted in higher Tg values and superior thermal stability. The same findings were obtained for samples cured by the isothermal curing process when the relatively low initial cure temperatures were optimal. The results of this study indicate that the initial and post-cure conditions may affect the final properties, and support the view that both the initial and post-cure schedules are of critical importance in governing the nature of the early network formation and the final network structure.

Preparation and characterization of imidazole–metal complexes and evaluation of cured epoxy networks

A series of copper complexes of epoxy-imidazole adducts have been prepared and characterized by 1H nuclear magnetic resonance (NMR) spectroscopy. Differential scanning calorimetry (DSC) was employed to investigate the thermal behaviour of the curing agents and to investigate the medium-term storage stability of a one-pot composition of a commercial epoxy resin when mixed with the complexes. The cure onset temperatures of the mixtures containing copper complexes are ca. 20–50 °C higher than those of the parent epoxy-imidazole adducts and the decrease of cure onset temperatures in the early stages of storage (up to 100 h) is less. The latent nature and improved storage stability of mixtures containing the copper complex were clearly demonstrated and confirmed by the viscosity behaviour of the catalysed mixtures of the commercial epoxy resins MY720 and MY750. 1H NMR and electron paramagnetic resonance (EPR) spectroscopy were employed to monitor the thermal decomposition of the copper(II) complexes, which were found to decompose at 120–130 °C and exist in equilibrium. Glass fibre-reinforced composite samples were prepared using a commercial epoxy resin cured with the complexes and their physico-mechanical properties were evaluated.

Iridium-catalysed labelling of anilines, benzylamines and nitrogen heterocycles using deuterium gas and cycloocta-1,5-dienyliridium(I) 1,1,1,5,5,5-hexafluoropentane-2,4-dionate

A wide range of variously substituted anilines, benzylamines, and nitrogen heterocycles may be conveniently deuterated by exchange with deuterium gas and cycloocta-1,5-dienyliridium(I) 1,1,1,5,5,5-hexafluoropentane-2,4-dionate. The isotopic exchange can be carried out efficiently in dimethylformamide or dimethylacetamide, hence it is directly applicable to the deuteration of polar compounds such as pharmaceuticals. Isotope incorporation is rapid and yields ortho-regiospecificity.

Mechanical properties of tough, high temperature carbon fibre composites from novel functionalized aryl cyanate ester polymers

Abstract Carbon fibre pre-impregnated tape (prepreg) was prepared from a range of prepolymers comprising blends of propenyl-functionalized cyanate ester (CE) oligomers with commercial bis-maleimide (BMI) monomers. Cured neat resin plaques and carbon fibre composites were prepared by autoclave moulding. Various blends of a commercial BMI mixture, a CE and a co-monomer with either allyl or propenyl and cyanate pendant groups were prepared. Differential scanning calorimetry (d.s.c.) data were used to monitor cure of neat resin, while dynamic mechanical thermal analysis (d.m.t.a.) was used to assess cure cycles of composite materials by the measurement of glass transition temperatures (Tg). Composite properties measured were interlaminar shear strength, flexural strength, compressive strength and Mode 1 Fracture toughness (GIC).

Microwave-enhanced aromatic dehalogenation studies: a rapid deuterium-labelling procedure

Rapid (<1 min) and specific deuterium labelling is achieved through the microwave-enhanced dehalogenation of a number of N-4-picolyl-4-halogenobenzamides using deuterated formate as solid deuterium donor and either homogeneous or heterogeneous catalysts; the percentage deuterium incorporation is a function of the kind of solvent used.

Development of a microwave-enhanced isotopic labelling procedure based on the Eschweiler–Clarke methylation reaction

A number of primary and secondary amines have been rapidly methylated under microwave-enhanced conditions using formic acid-formaldehyde mixtures, providing a route to 2H(D)-containing compounds and the potential for 3H(T), 11C, 13C and 14C labelling.

High temperature 1H NMR studies of epoxy cure: A neglected technique

SummaryBy reference to the reaction of a copper complex of phenyl glycidyl ether and 2-ethyl-4-methylimidazole, and Bisphenol A diglycidyl ether, 1H nuclear magnetic resonance spectroscopy at high temperature is shown to be an excellent and under-utilized method for measuring the degree of cure, enabling kinetic parameters to be obtained.

Base-Catalyzed Hydrogen Exchange

Publisher Summary This chapter focuses on the base-catalyzed hydrogen exchange in heterocyclic compounds. Isotopic hydrogen exchange is becoming a widely employed method for preparing labeled compounds, being unsatisfactory only when the specificity of the process is uncertain and when the label can be replaced is high. A better understanding of the various factors involved in the preparation of labeled compounds as well as the stability of the label under widely different conditions is emphasized in the chapter. The conditions necessary to induce exchange vary widely and may be dictated by structural features. Proton-transfer reactions from heterocyclic compounds provide a favorable situation for the operation of a mechanism of exchange. Heterogeneous methods of exchange often involving expensive catalysts were widely used for incorporating tritium and deuterium into heterocyclic compounds. The isotopic exchange procedure is useful as a diagnostic tool for detection of minute traces of labeled compounds that can be seen in the recently developed method for assay of guanine residues in DNA. Many applications have resulted from studies of base-catalyzed isotope exchange reactions of heterocyclic compounds. Primary kinetic hydrogen isotope effects in the base catalyzed exchange of various heterocyclic compounds are tabulated in the chapter.

Studies of temperature and time-dependent network formation in commercial epoxy resins using modified imidazole curing agents

Two commercial epoxy resins, MY750 and MY720, were cured over a range of cure temperatures with a series of novel curing agents based on modified imidazoles. A combination of techniques, including dynamic viscometry, proton nuclear magnetic resonance spectroscopy, differential scanning calorimetry, high performance and gel permeation chromatography, were used to analyse the reaction mixture. It appears that for a given formulation and given resin, the viscosity behaviour near gelation is different for different cure temperatures and the mechanism appears to be dependent not only on the nature of the imidazole curing agent, but also on the cure temperature. Higher temperature cure appears to lead to the formation of a less ‘homogeneous’ reaction mixture at the gel point, containing oligomeric chain lengths of higher polydispersity, whereas in the lower temperature cure, oligomers of a smaller range of chain lengths are present.