Intramolecular cyclization of styrene–p-divinylbenzene copolymers

Styrene has been copolymerized at low conversion with minor quantities of p-divinyl-benzene (p-DVB) in (10–15%) solution in toluene and cyclohexane. Under these conditions the molecular weight of the polystyrene formed in the absence of p-DVB was controlled by chain transfer, and the copolymerization coefficients of the styrene and the p-DVB agreed with previous work. Polymer molecular weights were studied as a function of conversion. At very low conversions the number-average (2.2 × 10⁵) and the weight-average (4.4 × 10⁵) molecular weights were unaffected by substituting some of the styrene by p-DVB, but as the reaction continued M?_n increased slowly and M?_w much faster. On the other hand, even at the lowest conversions the intrinsic viscosity was drastically reduced by the introduction of p-DVB, and the radius of gyration, as measured by light scattering, fell. Infrared studies on the polymer show that the concentration of pendent double bonds in low-conversion copolymers is about half of the doubly substituted phenyl groups. It is concluded that the first polymer chains formed are extensively cyclized with the formation of a relatively large number of small rings.     

Visualization and characterization of electroactive defects in the native oxide film on aluminium

Spatial!y localized electrochemical activity at Al/Al2O3 electrodes has been investigated using scanning electrochemical microscopy (SECM) in order to establish the relationship between localized corrosion of Al (and Al alloys) with the defect structure of the native Al2O3 film. Local electron transfer at microscopic defects (2 to 50 microm radius) was visualized in acetonitrile solutions using the nitrobenzene/nitrobenzene radical anion (Eo approximately -1.6 V vs. Ag/Ag+) and tetracyanoquinodimethane/tetracyanoquinodimethane radical anion couples (Eo approximately -0.3 V) as redox mediators for imaging. SECM investigations revealed no significant differences in electrochemical activity at Al/AI203 electrodes in the two mediator solutions, indicating that electrical conduction at the defect sites is weakly dependent on interfacial potential and the electric field across the Al2O3 film. The density of electroactive defects observed by SECM varied by 2 to 3 orders of magnitude between electrodes prepared from the same source of Al (either 99.450% and 99.9995%) suggesting that electrical conduction in the native oxide is very sensitive to surface preparation. Defect densities as low as approximately 3 sites cm(-2) were readily measured by SECM.     

The glass transition time scale and configurational entropy in polymers: an experimental molecular view

We report direct experimental observation of local conformational dynamics in a polymer chain at the calorimetric glass transition temperature Tg. Variable-temperature two-dimensional (2D) solid-state exchange NMR, at natural abundance, reveals segmental dynamics in pure polyisobutylene (PIB) occurring on a time scale of several seconds over the Tg range observed by DSC (203-208 K). To our knowledge, this is the first direct observation of molecular-level conformer interchange (trans-trans/trans-gauche/gauche-gauche) at the caloric glass transition temperature. Our results provide a chronologically accurate and pedagogically advantageous demonstration of molecular processes during a polymer phase transition, relative to traditional bulk mechanical and calorimetric techniques. More importantly, we use a miscible blend to demonstrate a general strategy for quantitative evaluation of configurational entropy changes via combination of temperature-dependent 2D exchange NMR and Adams-Gibbs theory. Our results on the Tg time scale are directly relevant to fundamental understanding of the Tg length scale, i.e., the dimension of cooperatively rearranging regions.     

State-resolved dynamics of oxygen atom recombination on polycrystalline Ag

Rotationally resolved, velocity distributions for desorbed O2 molecules formed by O-atom recombination on the surface of a polycrystalline Ag surface are reported. Surface O atoms are generated by oxygen permeation through a 0.25-mm-thick Ag foil heated to 1020 K. Desorbing O2 molecules are probed by (2 + 1) resonant multiphoton ionization via the C 3Pig (3ssigma), v' = 2 <-- <-- X 3Sigmag-, v" = 0 transition and time-of-flight mass spectrometry. Measured velocity distributions are near Maxwell-Boltzmann and yield average translational energies which are significantly lower than the surface temperature ([Et]/2kB approximately 515 K) and essentially independent of rotational excitation. Comparison of the observed C-X (2,0) resonantly enhanced multiphoton ionization spectrum with spectral simulations suggests that the v" = 0 rotational state distribution is more consistent with the surface temperature, but spectral congestion and apparent intensity perturbations prevent a more quantitative analysis. The calculated, sticking curves show a small barrier energy barrier (approximately 10 meV) beyond which sticking decreases. These observations are consistent with low energy desorption and adsorption pathways involving a weakly bound molecular O2 precursor.     

Synthesis of iron(II), manganese(II) cobalt(II) and ruthenium(II) complexes containing tridentate nitrogen ligands and their application in the catalytic oxidation of alkanes

A series of Fe(II), Mn(II), Co(II) and Ru(II) complexes containing bis(imino)pyridine or bis(amino)pyridine ligands and weakly coordinating triflate (OTf-) or non-coordinating SbF6- anions have been prepared. The complexes have been fully characterized including several solid-state structure analyses. Two unusual mono-chelate six-coordinate bis(imino)pyridine Fe(II) and Mn(II) complexes have been observed. The catalytic properties of the complexes for the oxidation of cyclohexane with H2O2 have been evaluated. Only the Fe(II) complexes have shown catalytic activity, which is mainly due to Fenton-type free radical auto-oxidation.     

表面活性剂模板在空气-水界面ZrO2薄膜中的稳定性

采用模板——十二烷基苯磺酸(DBS-H)在空气-水界面组装ZrO₂薄膜，研究了DBS-H在ZrO₂自组装薄膜中的水溶性、化学稳定性、热稳定性和光化学稳定性。模板的各类稳定性将直接控制ZrO₂薄膜结构，主要表现在层间距变化上。从模板与Na₂SiO₃反应的研究中获得了一种制备ZrO₂ / SiO₂复合氧化物薄膜的新方法，并推测出该复合薄膜的结构。