Acetal, crossed polars

Cross-polarization/magic-angle spinning NMR studies of polymorphism cortisone acetate. Spectrochim. Acta, Part A 1990, 46A (6), 927-935. 

Fig. 21. Top Experimental ( H)- C cross-polarization spectrum of freshly prepared poly-e-caprolactone nanospheres in an aqueous dispersion (wc—100 MHz, full proton decoupling). Five polymer signals ( P ) are detected within the given spectral range, other resonances can be assigned to the surfactant polyvinyl alcohol and small amounts of polyvinyl acetate. Bottom Corresponding ( H)- C cross-polarization spectrum of a solid bulk sample of poly-f-caprolactone under static conditions.

Fig. 23. Experimental ( H)- C cross-polarization spectra of poly-f-caprolactone nanospheres in aqueous dispersion after different freezing histories (wc 100MHz, full proton decoupling) (a) dispersion after 4 weeks storage at room temperature, (b) after freezing for 1 h and thawing, (c) after freezing for 22 h and thawing, (d) after two freezing/thawing cycles. The signals at 23 and 45 ppm correspond to the surfactant polyvinyl alcohol and residual polyvinyl acetate, respectively. ...

Polyethylene has to a large extent dominated morphological studies in polymer systems however, it is a good model for other systems. Optical microscopy of acetal shows the same characteristic patterns found in polyethylene when a spherulite is viewed under cross-polar imaging (Figure 5.14). 

Silyi enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in DMSO or certain other polar aprotic solvents." The reaction has been performed with R , R = hydrogen or alkyl, though best yields are obtained when r = r = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyi enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-Bp3-Et20," ceric ammonium nitrate," and lead tetraacetate." If R =0R (in which case the substrate is a ketene silyi acetal), dimerization with TiCU leads to a dialkyl succinate (34, r =0R)." ... 

However, solvent viscosity, rather than polarity, has been a useful tool for mechanistic purposes. Although the quantum yield of the ortho-rearranged product of 4-methylphenyl acetate (20) does not change with viscosity of the medium, the formation of 4-methylphenol (22) is highly sensitive to this factor. Thus, its quantum yield is 0.45 in ethanol (1.00 cP) but only 0.02 in Carbowax 600 (109 cP) (Scheme 8 Table 3) [13], This clearly supports the mechanism involving caged radical pairs. A related aspect is the intramolecular nature of the process confirmed by the lack of cross-coupling products in crossover experiments with mixtures of different esters [10]. 

SPE methods with different cartridge packings have been employed for the pre-concentration and clean up of sulfonated azo dyes from waters and soil extracts [110,111], The extraction of solid samples has been carried ont by sonication or Soxhlet extraction and the extracts treated like the water samples. C18 cartridges and columns [111] were used followed by the elution with aqueous organic solvents in the presence of TEA with recovery yields always greater than 65% [93,111], Higher recoveries have been obtained by using C18 columns, pre-conditioned with an ammonium acetate buffer and elnted with methanol [111], The use of styrene-divinylbenzene [93,112], as well as of cross-linked polymeric sorbents with sulfonate functions, was shown to be suitable in the SPE of the more polar componnds [111],... 

Fig. 38. Substrate dependence of the PS/PMMA domain structure spin-cast from THF a-c on SiOx d-f on octadecyl mercaptane (ODM) [352]. The SFM images have lateral dimensions of 14x14 pm2. a,d as spin cast b after immersion in cyclohexane to remove PS. e after immersion in acetic acid to remove the PMMA-rich phase. The cross-sections (c,f), which were recorded along the lines in (a,b,c,d), reveal the vertical distribution of the PS (dark grey) and PMMA (light grey) phases. The error bar in (c) indicates the accuracy of the superposition procedure. PMMA preferentially adsorbs on the more polar SiOx surface to form a homogeneous layer next to the substrate. On the ODM a PS/PMMA bilayer is observed. Courtesy of U. Steiner...

E. Lau and A. Bacic, Capillary gas chromatography of partially methylated alditol acetates on a high-polarity, cross-linked, fused-silica BPX 70 column,./. Chromatogr., 637 (1993) 100-103. 

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