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The diffusion coefficients of small paramagnetic tracers (nitroxide spin probes) and spinlabeled poly(ethylene oxide) in hydrogels were measured at 300 K, using two-dimensional spatial-spectral ESR imaging (ESRI). The gels were prepared by copolymerization of 2-hydroxyethyl methacrylate (HEMA) and 2-(2-hydroxyethoxy)ethyl methacrylate (DEGMA), in the presence of a fixed molar concentration of glycol dimethacrylate as the cross-linker and 4,4'-azobis(4-cyanopentanoic acid) as the initiator. Variation of the amount of water in equilibrium with the gel was achieved by polymerizing different molar ratios of the monomers.Experimental concentration profiles of the diffusant were measured as a function of time, and each profile was obtained by image reconstruction, from a complete set of projections. The diffusion coefficients were determined by simulating all experimental profiles, using the Fick model of diffusion for an initially confined tracer diffusing into a finite system. The diffusion coefficients depend on the amount of water in the gels. The variation of the diffusion coefficients D with the weight fraction of the copolymer in the gel is consistent with the free volume model. The variation of D with the molecular mass M of the diffusant scales as M~°'s in gels containing >67 wt % water and as M~1A for the gel containing 55 wt % water. The advantages and present limitations of 2D spatial-spectral ESRI for measuring transport properties in polymers are discussed.

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The translational diffusion coefficients of a small paramagnetic molecule (the deuteriated nitroxide PDTEMPONE) as tracer were measured in solutions of polystyrene (PS) in toluene and dimethylformamide (DMF) and in cross-linked polystyrene (cPS) networks swollen by the same solvents, using two-dimensional spatial-spectral electron spin resonance imaging (2D ESRI). PS solutions containing 30, 40, and 50 w/w % polymer were examined in toluene at 300, 305, 310, and 315 K and in DMF at 300 K. PS networks prepared with 0.5, 1.0, 2.5, and 5.0 mol % divinylbenzene as cross-linker were swollen in toluene or DMF and studied at 300 K. Two-dimensional images describing the distribution of the diffusant in the sample were reconstructed from a complete set of projections obtained as a function of the magnetic field gradient applied along the symmetry axis of the sample. The concentration profile of the diffusant at a given time was obtained by integrating absorption ESR spectra in slices perpendicular to the spatial axis, which was divided into 256 points. These profiles were obtained as a function of time, and the diffusion coefficients were determined by simulating the experimental diffusion profiles using Fick's model of diffusion. The diffusion coefficients were found to depend on the solvent, temperature, and PS concentration in the solutions and are significantly reduced by cross-linking in the swollen networks. The temperature dependence of the diffusion coefficients shows an Arrhenius behavior, and their dependence on the polymer concentration in the solutions is consistent with the free volume theory. Results from this study were compared with those obtained for solvent selfdiffusion and diffusion of other tracers in PS solutions and swollen networks.

A novel I2-catalyzed cross-dehydrogenative aromatization of cyclohexanones and anilines to synthesize N,N'-diaryl-o-phenylenediamines has been unprecedentedly developed with dimethyl sulfoxide and oxygen employed as mild terminal oxidants. To prove the rationality of the two separate dehydration steps of the proposed mechanism, a resulting I2-catalyzed cross-dehydrogenative aromatization of cyclohexenones and anilines to synthesize diarylamines has also been reported.