Noteworthy Chemistry

July 21, 2014

Make self-healing materials from bioderived polylactide. The widespread commercial use of polylactide (PLA), a thermoplastic polymer derived from renewable sources such as corn, has sparked a drive to enhance PLAs’ mechanical durability. One way to achieve this goal is to incorporate a mechanism for healing or repair when the material becomes worn.

J. P. Brutman, P. A. Delgado, and M. A. Hillmyer* at the University of Minnesota (Minneapolis) are exploring the development of PLA vitrimers. They describe vitrimers as cross-linked polymers with flow dynamics that rely on the transient nature of their network structure. They use bond-exchange reactions to topologically rearrange the network structures, which improves processibility and mechanical, chemical, and thermal robustness.

The authors use a tin-catalyzed transesterification reaction to generate PLA vitrimers from hydroxyl-functionalized, four-arm poly-(±)-lactide and methylenediphenyl diisocyanate (MDI). They predicted that this commercially relevant strategy, including the choice of nontoxic materials, should promote rapid chain relaxation (i.e., short repair times).

To optimize material properties, the authors independently varied the isocyanate (NCS)/hydroxyl (OH) ratio and the catalyst/OH ratio. As the MDI content was increased at a constant tin-catalyst content, higher cross-link densities were achieved, but stress relaxation decreased before leveling off near the stoichiometric ratio (0.9:1 NCS/OH). Varying the catalyst loading produced only minimal changes in the plateau moduli for 0.75:1 NCS/OH, but transesterification relaxation kinetics became faster as the amount of catalyst or the temperature was increased.

Healing mechanical damage in these PLA vitrimers was directly related to the NCS content, the amount of catalyst used, and the dynamic nature of the network structure. The authors state that the healing characteristics of the PLA vitrimers rival those of other polyester vitrimers and demonstrate the versatility of this strategy for generating a range of material performance. (ACS Macro Lett. DOI: 10.1021/mz500269w; LaShanda Korley)

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Telescope the synthesis of anti-amino alcohols. β-Amino alcohols are useful building blocks for organic synthesis; some of the synthesized products have biological activity. These compounds are usually prepared in a sequence that consists of protecting the oxygen and nitrogen atoms of an amino acid with benzyl groups, reducing the carboxyl group to aldehyde with DIBAL-H (i-Bu2AlH), and adding a Grignard reagent. J. M. Padrón and coauthors at the University of La Laguna (Spain) and San Luis National University (Argentina) telescoped the second and third reactions into a one-pot synthesis that starts with a protected amino acid (see figure).

One-pot synthesis of anti-β-amino alcohols from benzylated amino acids

The authors first established optimal conditions for the one-pot sequence, such as the ratios of reagents and the best solvent (which turned out to be ethyl ether). The products have the anti configuration, which corresponds to the nonchelating Felkin–Anh pathway. No racemization occurs with alanine or phenylalanine derivatives as substrates or several Grignard reagents.

The authors used their method to prepare anti-2-amino-1,3-diols and anti-3-amino-1,4-diols. They also synthesized naturally occurring, biologically active spisulosine and sphinganine. This simple, straightforward procedure, which avoids the isolation of the usually unstable aldehyde, is an overall improvement for preparing β-amino alcohols. (J. Org. Chem. DOI: 10.1021/jo500481j; José C. Barros).

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A blood test detects “mad cow” disease in asymptomatic humans. “Mad cow” disease—more scientifically, variant Creutzfeldt–Jakob disease (vCJD)—is a transmissible, degenerative brain disorder. Whereas it famously can be caused by eating the meat of an animal infected with bovine spongiform encephalopathy, secondary cases have been identified in people who received blood transfusions from donors who were thought to be healthy but later developed vCJD. Before now, it was a challenge to detect vCJD cases because there were no diagnostic assays.

O. Andréoletti at the National Veterinary School of Toulouse (France) and colleagues in France, the United Kingdom, and Germany developed a blood test that can detect vCJD prions, or misfolded proteins, in animals and humans during early stages of infection. They optimized a technique called protein misfolding cyclic amplification that propagates misfolded prions. This procedure allows efficient, specific in vitro amplification of any vCJD agent in the blood. The assay accurately and consistently identified infected animals in the asymptomatic phase of infection and three out of four patients with vCJD without any false positives among the healthy controls.

Because vCJD can incubate asymptomatically for years before the onset of clinical disease, a test like this one, which can diagnose vCJD in humans and animals shortly after initial infection, may help prevent future outbreaks. In light of new concerns about the blood-borne transmission of vCJD, this assay can help to reliably screen and identify at-risk blood donors. (PLoS Pathog. DOI: 10.1371/journal.ppat.1004202; Abigail Druck Shudofsky)

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Produce hydrogen with quantum dots and visible light. Hydrogen, a clean, renewable energy carrier, can be obtained in one step by splitting water, the most abundant natural resource on Earth. This process, unfortunately, is only easy on paper; and ideally, it should be carried out using energy from sunlight. A high-performance photocatalyst is the key to water-derived hydrogen.

L.-Z. Wu and colleagues of the Chinese Academy of Sciences (Beijing) developed a hybrid catalyst system from a nickel salt and cadmium sulfide (CdS) quantum dots (QDs). This system exhibits an impressive ability to generate hydrogen from glycerol and water.

The researchers prepared the catalyst by adding NiCl2 to 3-mercaptopropionic acid (MPA)–stabilized CdS QDs. By using a variety of advanced spectroscopic techniques, they confirmed that Ni2+ bonds to the “hanging” S2- ion on the surface of MPA-CdS QDs to form catalytically active sites. Under irradiation by visible light (410 nm wavelength), the hybrid catalyst generated hydrogen at the rate of 74.6 μmol/(h•mg) with a quantum yield of 12.2% from a water–glycerol mixture. The authors believe that this is the greatest rate of irradiation-driven hydrogen production from water and glycerol to date.

Mechanistic studies revealed that •OH radicals, along with hydrogen and carbon radicals, are involved in the catalytic cycle. Given the low cost of starting materials for the catalyst and the hydrogen sources, this technique may lead to a practical solution to affordable hydrogen production. (ChemSusChem DOI: 10.1002/cssc201400028; Xin Su)

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Make hydrogels with strong, tunable multicolor emissions. Functional hydrogels with strong light emission are difficult to prepare because forming aggregates of conventional luminophores in aqueous media normally quenches luminescence processes. Energy transfers usually lead to the formation of hydrogels that emit single colors, even when multiple luminophores with different colors are used to make them. C. He, Y. Q. Dong, H. Wang, and co-workers at Beijing Normal University report a simple process for preparing multicolor light-emitting hydrogels from a single luminogen.

The luminogen used by the researchers was 9-[bis(4-propoxyphenyl)methylene]-9H-fluorene (1; see figure). Its aggregates in solvent mixtures that contain various amounts of water emit strong light of different colors because of variations in the aggregates’ structures. 

The morphologies, and hence the emission colors of the aggregates of 1, can be preserved by forming hydrogels in the aqueous mixtures. The emission colors can be reversibly tuned by modulating the morphologies from the crystalline state to the amorphous state. This is accomplished by heating and then cooling the hydrogels or by fuming them with organic solvent vapors.

This work provides a simple process for preparing advanced hydrogels that have multicolor emissions and respond to stimuli. The hydrogels have several potential applications, especially in the fields of bioimaging and bioengineering. (J. Mater. Chem. C DOI: 10.1039/C4TC00741G; Ben Zhong Tang)

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How stable is carbonyldiimidazole to atmospheric water? N,N-Carbonyldiimidazole, commonly known as CDI, often is used in chemical synthesis to convert a carboxylic acid functional group to a reactive acylating agent. But CDI is vulnerable to degradation by atmospheric moisture. K. M. Engstrom and co-workers at AbbVie (North Chicago, IL) describe their studies on the stability of CDI to moisture, particularly as it might affect the stability or purity of the material when it is used in a manufacturing plant.

The authors’ key findings were that

  • water is involved in the rate-determining step of CDI degradation; and
  • the key attributes that affect the stability of a given lot are particle size (as measured by particle size distribution) and chloride content.

The degradation rate is higher in lots of CDI that contain higher chloride content and smaller particles. It also increases at higher air flow rates, humidity, and exposed surface area. (Org. Process Res. Dev. DOI: 10.1021/op400281h; Will Watson)

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Luminogen that forms hydrogels with multicolored emissions