Noteworthy Chemistry

October 7, 2013

 

This soy-based alkyd coating resists corrosion and bacteria. The quest continues for low–volatile organic compound (VOC), high-performance surface coatings that do not rely on petroleum feedstocks. Antibacterial coatings are of particular interest to the food and medical-device industries.

Alkyd resins—oil-modified polyesters—are used as coatings because of their high gloss, solvent resistance, and low cost. Conventional methods for producing these resins involve organic solvents that release unacceptable levels of VOCs into the atmosphere. Waterborne alkyds release almost no VOCs, but they dry more slowly than their oil-based counterparts, and they are not as resistant to water, acids, and bases.

S. Pathan and S. Ahmad* at the National Islamic University (New Delhi, India) synthesized soy-based alkyd coatings modified with butylated melamine formaldehyde (BMF). Their flexible adhesive coatings are highly scratch-resistant and withstand impacts of >150 lb/in. (>26.8 kg/cm). The coatings are antibacterial and corrosion-resistant. They are safe to use at temperatures as high as 200 °C.

Pathan and Ahmad treated soy oil (1) and glycerol (2) with NaOH under nitrogen to produce soy monoglyceride (3), which was then treated with phthalic anhydride (4) to produce the soy alkyd (5). The alkyd was neutralized with Et3N to make it “waterborne” (6). [It is not clear in the article whether “waterborne” means water-soluble.—Ed.]

Synthesis of waterborne soy-based alkyd

Finally, the authors dissolved 6 and BMF (7) in various ratios in aq MeOH. With p-toluenesulfonic acid (TsOH) as the catalyst, they produced 3-D thermoset polymers (8).

Incorporating BMF into the soy alkyd increases the hydrophobicity of the coatings, which promotes corrosion resistance. The coatings protected the substrates against acid, alkaline, and tap-water corrosion. Adding BMF also increases the antibacterial activity of the soy alkyd, particularly against Staphylococcus aureus (and to a lesser extent, Escherichia coli), a useful feature for food-packaging materials. (ACS Sustainable Chem. Eng. 2013, 1, Article ASAP; Nancy McGuire)

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Metal-free polymersomes are biocompatible. M. J. Isaacman, E. M. Corigliano, and L. S. Theogarajan* at the University of California, Santa Barbara, designed polymeric vesicles that are made by strain-promoted azide–alkyne cycloaddition (SPAAC) coupling. They used their metal-free coupling strategy to join a hydrophobic, difunctional diazide-poly(dimethylsiloxane) (≈5.3 kDA) block with a hydrophilic, bicyclo[6.1.0]nonyne functionalized poly(ethylene glycol) (≈2 kDA) or a poly(methyloxazoline) (≈1.5 kDa) block.

The authors assembled polymersomes from these amphiphilic triblocks under aqueous conditions and compared them with their copper-catalyzed azide–alkyne cycloaddition (CuAAC) counterparts. One noticeable difference was that the strain-assisted polymer vesicles were smaller than those made using CuAAC, perhaps because of chelation effects. Comparative biocompatibility studies showed that the metal-free polymer vesicles were not cytotoxic, suggesting that SPAAc is a desirable strategy for “designer” drug-delivery vehicles. (Biomacromolecules 2013, 14, 2996–3000; LaShanda Korley)

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Make white-light–emitting micelles with FRET. One strategy for synthesizing white-light emitters, fluorescence resonance energy transfer (FRET), can be used to obtain the base colors that constitute white light and to finely modulate the contribution from each one, usually in the form of supramolecular assemblies. FRET has been used to make thin-film–, gel-, and vesicle-based organic white-light emitters. X. Zhang, D. Görl, and F. Würthner* at the University of Würzburg (Germany) have added micelles to the family of white-light emitters.

The authors chose hydrophobic biscarbazoles that had two-, three-, and six-carbon alkyl spacers as energy donors to be loaded into micelles. In a low-polarity micelle interior environment, the one with the shortest spacer fluoresced in the UV region (320–400 nm) only in its extended form. In contrast, the biscarbazoles with longer spacers exhibited red-shifted emission in the cyan-to-blue range (380–550 nm) as a result of intramolecular π–π stacking of the two carbazole units.

The biscarbazoles were loaded into micelles that were formed by an orange-light–emitting amphiphilic perylene bisimide with an average diam of 6–9 nm. When the system was excited by UV light, FRET between the energy source and the micelle produced light that spanned the entire visible spectrum. The system emitted white light with a FRET efficiency of 0.46. The light had CIE chromaticity coordinates of (0.34, 0.30), comparable with those for pure white light (0.33, 0.33). (CIE is the Commission Internationale de l’Eclairage.) (Chem. Commun. 2013, 49, 8178–8180; Xin Su)

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Enhance the power output of a solar cell by harvesting more light. Solar cells based on CdTe are promising photovoltaic devices because they are inexpensive and easy to make. Their spectral responses to short-wavelength light, however, is poor (cutoff wavelength <500 nm). This limits the average module efficiency to ≈12.6%.

To harvest light in the short-wavelength region, T. Ren, W.-J. Dong, and coauthors at Shanghai Jiao Tong University, Washington State University (Pullman), and the University of Toledo (OH) synthesized two luminescent down-shifting (LDS) molecules (1 and 2 in the figure). These fluorogens absorb <500-nm photons and re-emit them in the solid state at >550 nm with high efficiency to make the light absorbable by CdTe cells.

Luminescent down-shifting molecules

The LDS molecules are tetraphenylethylene–malononitrile adducts linked by a furan (1) or thiophene (2) bridge. Because they display strong intramolecular charge transfer and aggregation-induced emission, they have large Stokes shifts and high fluorescence quantum yields in the solid state (see table).

Compound Stokes
shift, nm
Quantum
yield
1 129 0.93
2 143 0.84

The desirable photonic attributes allowed the authors to improve the spectral responses of CdTe solar cells when they are doped in polymer films as LDS materials. The short-circuit current density of the CdTe cells increases by 9%–10% when the LDS-containing polymer film is applied to the cell’s surface. (Energy Environ. Sci. 2013, 6, 2907–2911; Ben Zhong Tang)

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A modified synthesis makes downstream chemistry flexible. In the original route to XL228, an IGF-1R, Src, and Bcr-Abl inhibitor, N. R. Guz* and E. Goldman at Exelixis (South San Francisco, CA) and H. Leuser at CARBOGEN-AMCIS (Aarau, Switzerland) used 2,4,6-trichloropyrimidine as the starting material. This compound was subjected to successive SNAr displacements with 3-amino-5-cyclopropylpyrazole, 3-isopropyl-5-(aminomethyl)isoxazole, and N-methylpiperazine. This method, however, required many purification steps, making it unsuitable for scale-up.

The authors’ alternative route began with 2-amino-4,6-dichloropyrimidine and formed the isoxazole ring via a 1,3-dipolar cycloaddition reaction that is similar to the method for making the isoxazole used in the original sequence. Reactivity–selectivity principles were then used to determine the optimum order of carrying out the remaining two SNAr displacement reactions on the symmetrical intermediate. Adding the pyrazole, followed by reaction with the methylpiperazine, was the preferred order; it gave XL228 in 16% overall yield on an 11-kg scale. (Org. Process Res. Dev. 2013, 17, 1066–1073; Will Watson)

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Image crystals in 3-D by using digital holography. Controlling crystal shape and size distributions is crucial in a variety of manufacturing processes, especially in the pharmaceutical industry. Conventional methods for monitoring the shape and size of crystals, such as focused beam reflectance measurement and process video microscopy, provide 2-D planar images and do not give depth information.

To monitor the crystal growth process more accurately, A. Rajendran and coauthors at Nanyang Technological University (Singapore), the University of Alberta (Edmonton), and ABB Global Industries & Services (Bangalore, India) developed a digital holography method for imaging crystal samples on-line and in 3-D. Their method is based on observing the scattering of light when it interacts with crystals.

In the authors’ setup, a laser beam is focused by a microscope objective lens to produce a diverging spherical wavefront, which passes through a pinhole to illuminate suspended crystals. Light that encounters the crystals is scattered and creates an interference pattern with unscattered light. The pattern is numerically reconstructed to provide detailed information about the 3-D shapes and sizes of the samples.

The authors used their method to measure the crystal size distribution of transparent oxalic acid crystals. The results were consistent with those from traditional optical microscopy. They also integrated the monitoring system with a jacketed crystallizer and a flow-through cell, allowing real-time tracking of the size distribution of potash alum [KAl(SO4)2·12H2O] as it crystallizes. (Cryst. Growth Des. 2013, 13, 3969–3975; Xin Su)

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