Quenching of fluorescence of solutions of polystyrene and copolymers with methyl methacrylate

Quenching of excimer fluorescence from polystyrene and monomer emission from 5 and 10% methyl methacrylate copolymer with styrene has been studied in solution using dibromoethane and iodine as quenching species. From the measured rate constants along with polarization measurement of the emissions, it is concluded that energy migration does not occur in dilute solution of polystyrene. It is proposed that excimer is formed by rotation of a pendant, excited phenyl group with a face-to-face conformation with its nearest neighbour.     

Evidence for selectivity of absorption of volatile organic compounds by a polydimethylsiloxane solid-phase microextraction fibre

Solid-phase microextraction using a 30 microns polydimethylsiloxane fibre has been used to sample the volatile organic compounds from standard mixtures and from mixtures produced by the decomposition of organic compounds. This method of sampling has been compared with the direct injection of an aliquot of headspace gas and shows an enrichment factor of approximately 100 over a 1 ml gas injection for organosulphur gases such as dimethyldisulphide. The performance of the fibre has been evaluated with respect to accuracy and precision at several concentrations in representing the composition of multicomponent mixtures. It was found that the presence of a second component in a gas sample reduced the capacity of the fibre to absorb the primary component. The selectivity of the fibre for various volatile compounds with differing functionality was also studied. It was found that the non-polar polydimethylsiloxane fibre preferentially absorbed the non-polar components of a mixture, e.g. nonane and, correspondingly, under reported the more polar components, e.g. ethanol. Hence, the fibre discriminates in favour of non-polar and against polar components in a mixture in comparison with direct analysis of a headspace sample. Thus, quantitation of a component in a multi-component mixture is liable to error from competitive interference from other components. A major advantage of the technique, however, is that it does not absorb, and therefore introduce, water into the analytical system.     

A novel technique for measuring TH diffusion constant of oxygen in polymer films

A simple technique has been developed facilitating measurement of the diffusion constant for O₂ in polymeric glasses. The method used is such that measurements can be made over a wide range of temperatures. Measured values for the diffusion constant for O₂ in poly(styrene) and poly(methyl methacrylate) are reported.     

Diastereoselective cobalt-catalyzed alkylative aldol cyclizations using trialkylaluminum reagents

Co(acac)2.2H2O serves as an effective precatalyst for alkylative aldol cyclizations of alpha,beta-unsaturated amides with ketones using trialkylaluminum reagents. These reactions provide beta-hydroxylactams containing three contiguous stereocenters with high levels of diastereoselection.