γ‐Irradiation of ultrahigh‐molecular‐weight polyethylene: Electron paramagnetic resonance and nuclear magnetic resonance spectroscopy and imaging studies of the mechanism of subsurface oxidation

The shelf aging of irradiated ultrahigh‐molecular‐weight polyethylene (UHMWPE) causes subsurface oxidation, which leads to failure in UHMWPE orthopedic components, yet the mechanisms causing subsurface oxidation remain unclear. The shelf aging of γ‐irradiated UHMWPE bars has been studied with electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) imaging and with microtoming and Fourier transform infrared microscopy. The bars initially contained only allyl radicals, and upon air exposure, a surface layer of peroxyl radicals formed through the reaction of allyl radicals with oxygen. Importantly, a band of low radical intensity just beneath the peroxyl layer became apparent. NMR imaging showed a zone of altered proton relaxation in this zone. With increasing time, surface peroxyl radicals persisted in comparison with the interior allyl radicals, although oxygen did not appear to penetrate any more deeply into the bar. The area of maximal oxidation and mechanical disruption, measured after 3 years, was at the interface between the zone of exterior peroxyl radicals and the zone of low radical intensity. We present a mechanism involving the intermediacy of sterically strained reactive dialkyl peroxides at this interface to explain subsurface oxidation. We also demonstrate that EPR and NMR imaging provides information that could potentially be used to identify subsurface oxidized UHMWPE components before failure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5929–5941, 2004     

Investigation of transannular cyclisations of verticillanes to the taxane ring system

Treatment of either the verticillenes (3), (6), (13) or (14) with Lewis acids fails to produce the corresponding taxane carbon framework $\text{viz}$ (2), and instead only products, e.g. (8), (9), (16), of rearrangement of the epoxide rings in the substrates are obtained.     

A note concerning the chromatic index of multigraphs

We improve an upper bound for the chromatic index of a multigraph due to Andersen and Gol'dberg. As a corollary we deduce that if no two edges of multiplicity at least two in G are adjacent, then χ′(G) ? Δ(G) + 1. In addition we generalize results concerning the structure of critical graphs due to Vizing and to Chetwynd and Hilton.     

The adsorption of [35S] sulphur on palladium/alumina catalysts

The adsorption of sulphur on two palladium/alumina catalysts, prepared from different precursors, has been investigated using [³⁵S] H₂S.     

Experimental investigation of double ionization to ground and electronically excited states of chlorobenzene, 1,2-, 1,3- and 1,4-dichlorobenzene, 1,2,3-, 1,2,4- and 1,3,5-trichlorobenzene, 1,2,3,4-and 1,2,4,5-tetrachlorobenzene and pentachlorobenzene dications

Double-charge-transfer spectrometry was used to measure double-ionization energies to ground and electronically excited states of various chlorobenzenes. Since OH⁺ was the projectile ion used in these experiments, it is probable that triplet states of the dications were populated because of spin conservation in the double-electron-capture reactions which are the basis of this type of spectrometry. The lowest double-ionization energies for all the molecules studied are within ±0.3 eV of 25.9 eV, except that for 1,3-dichlorobenzene, which is at 26.6 eV. In general, double-ionization energies to three higher lying states (or groups of states) were measured for each molecule. The energies of these states are the same, within experimental uncertainties, for the three trichlorobenzenes, two tetrachlorobenzenes and pentachlorobenzene dications, suggesting that they have the same or very similar distributions of triplet-state energies.