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  • 80 лет со дня рождения
    академика Кирилла Ильича Замараева
  • XIII Международная конференция по химическим реакторам
    ХИМРЕАКТОР-23
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80 лет со дня рождения
академика Кирилла Ильича ЗАМАРАЕВА

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Strong, adhesive polymers made via stereochemical control

Using a chiral anion catalyst to prepare poly(vinyl ethers) transforms them from niche materials to practical plastics

Taking a cue from chemistry that’s common in the pharmaceutical industry, chemists have used chiral anions to control stereochemistry when making poly(vinyl ethers). The resulting strong, adhesive polymers could find use in lightweight composites for making bicycles, boats, and cars, for example, according to Frank A. Leibfarth, a chemist at the University of North Carolina, Chapel Hill who spoke about the work Tuesday at the American Chemical Society national meeting in Orlando during a session in the Division of Polymer Chemistry.

Until now, poly(vinyl ethers) have been pretty niche products, Leibfarth said, because they’ve been atactic, meaning that the stereochemistry of the ether sidechains coming off the polymer backbone has been random. The resulting polymers are viscous liquids at room temperature and are primarily used as adhesives.

 

 

When the stereochemistry of the sidechains of poly(isobutylvinyl ether) is random, the polymer is a viscous liquid (left). When 91% of the sidechains have the same stereochemistry, the polymer is a solid (right).

Leibfarth and postdoc Aaron J. Teator thought they could improve the properties of poly(vinyl ethers) by controlling the stereochemistry of the ether side chains so that they’re all identical, or, as polymer chemists say, isotactic. Leibfarth said he was inspired by the work of Nobel Prize-winning chemists Karl Ziegler and Giulio Natta, who developed a catalyst that produces isotactic polyolefins, such as polypropylene.

 “This switch from atactic polypropylene to isotactic polypropylene essentially created a multibillion dollar industry because atactic polypropylene is almost useless,” Leibfarth said. But Ziegler-Natta catalysts become poisoned in the presence of Lewis basic heteroatoms like oxygen, which means they can’t be used to make oxygen-rich poly(vinyl ethers).

To control stereochemistry when making poly(vinyl ethers), Leibfarth and Teator turned to chiral anion catalysis, which has been used to control the stereochemistry of small molecules since 2000. The cationic polymerization takes place in a solvent with a low dielectric constant so that the cation at the end of the growing polymer chain tightly pairs with a chiral anion that dictates which side of the chain the next monomer will add to, resulting in an isotactic polymer (shown).

 

 

As the poly(vinyl ether) chain grows, a chiral anion blocks one side, dictating the stereochemistry of the monomer's addition.

The isotactic poly(vinyl ethers) have excellent properties, Leibfarth said. They’re similar to commercial polyolefins but adhere more strongly to polar substrates, such as glass. Leibfarth and Teator also reported the work in Science at the end of March (DOI: 10.1126/science.aaw1703) and have filed a provisional patent application on the process (62/719, 240).

Geoffrey W. Coates, a polymer chemist at Cornell University, called the work “a tour-de-force that combines mechanistic insight, synthesis, stereochemistry, and polymer properties.” He adds, “Given the relative lack of work in stereocontrolled cationic polymerization since it was originally reported by chemist Calvin Schildknecht over 70 years ago, it is great to have such an exciting advance that will revive research in this important area of polymer chemistry.”

 

 

Boron helps to gently couple dinitrogen

Organoboron compounds offer a direct route for synthesizing nitrogen chains that may be used in explosives and pharmaceuticals

 

 

An N4 chain sits sandwiched between two borylene moieties.

Dinitrogen tends to be a loner. Extreme conditions, such as intense radiation in the ionosphere, are needed to coerce two or more N2 molecules to form chains. Various pharmaceuticals and explosives made by humans contain three- and four-atom nitrogen chains. To make these compounds, chemists have to use an indirect route. They first split dinitrogen through a high-temperature and high-pressure industrial process to produce ammonia and amines. Then they stitch together those N1 compounds into the nitrogen chains.

A new study describes a direct and gentle way to make compounds with nitrogen chains using dinitrogen. Marc-André Légaré and Holger Braunschweig of Julius-Maximilians University Würzburg and coworkers report that an organoboron compound can stitch together two N2 molecules under near-ambient conditions to form a complex in which an N4 chain bridges two boron moieties (Science 2019, DOI: 10.1126/science.aav9593).

The study follows work published last year by the Würzburg group in which they succeeded in binding a single dinitrogen molecule between organoboron ligands. This general area of research, known as nitrogen fixation, seeks ways to use highly abundant atmospheric nitrogen in chemical synthesis.

The team made the new nitrogen-chain complex by first synthesizing a dihaloorganoborane precursor via standard methods and then reducing it with a solution of KC8 in the presence of dinitrogen at roughly 4 bar (four times atmospheric pressure) and -30 °C. The group determined the product’s structure using X-ray diffraction and various spectroscopy methods. By teaming up with theoreticians at Goethe University Frankfurt, the chemists studied the molecule’s unusual bonding. They found that the borylene moieties bind dinitrogen similar to how transition metals do—by forming end-on N2-bridging complexes—yet the boron compound’s reactivity is like that of main-group elements.

Légaré says the group is now working to incorporate the nitrogen chains into organic molecules for use in synthesizing pharmaceutical compounds.

“The beauty of this study is the simplicity of the transformation, in which one reduced boron species can lead to spontaneous coupling of N2 under fairly mild conditions,” says Frédéric-Georges Fontaine, a catalysis specialist at Laval University.

Fontaine remarks that chemists might expect such a transformation to be mediated by a transition metal complex or alkaline earth species, but this one is done by a fairly simple boron molecule. He describes the work as an important step in the trend to develop metal-free transformations. This trend has shown that main group elements, used under the right set of conditions, can replace conventional catalysts, which can be costly and challenging to prepare. “The prospect of eventually including N2 as a reagent in metal-free catalysis is really exciting,” he adds.

 

 

Chemists uncover rules of thumb to help with computational screening of MOF catalysts

Relationship between active-site formation energy and bond-breaking energetics can be plugged into algorithms that search for efficient methane-activation catalysts

 

 

A theoretical study has uncovered a key relationship that can be used to search large numbers of MOFs for promising catalysts that can convert methane to methanol. Forming a metal-oxo species to which methane docks is key to that reaction.

 

By studying a large set of metal-organic frameworks (MOFs), researchers have uncovered a simple relationship between the energetics of a reactive site species and the probability of driving methane oxidation in these porous crystalline materials. The discovery provides rules-of thumb that can be incorporated into computational algorithms for screening large numbers of MOFs quickly and cost-effectively in search of catalysts that can activate methane under moderate reaction conditions.

Vast reserves of natural gas, of which roughly 95% is methane, remain untapped because the resource sits in locations that are too remote for the gas to be shipped economically. For years, researchers have worked to develop energy- and cost-effective methods for converting the gas to liquids, which can be transported more easily than gas. Commercial processes for transforming methane to methanol, hydrocarbons, and other liquids already exist. But most of those methods start by converting methane to synthesis gas, a mixture of CO and hydrogen. That process runs at high temperatures (>600 °C) and is economically viable only at very large scales.

Recent studies show that MOFs are promising catalysts for activating methane under mild conditions. But because this class of materials numbers in the tens of thousands, the task of finding or designing the best MOF for the job—for example, converting methane to methanol—is daunting. At the American Chemical Society national meeting in Orlando on Sunday, Andrew S. Rosen reported on an advance that may improve computer-based methods for screening MOF libraries for promising methane activation catalysts.

Rosen, who works with Northwestern University’s Justin M. Notestein and Randall Q. Snurr, highlighted his group’s work in a symposium organized by the Division of Catalysis Science and Technology.

By investigating MOF chemistry via quantum chemical calculations, the researchers found that a MOF’s ability to break the strong C−H bond in methane—the first step in converting the gas to methanol--can be directly related to how easy it is to oxidize the MOF’s surface metal atoms. That oxidation step is critical because it forms the metal-oxo species that functions as the catalyst’s active site (ACS Catal. 2019,  DOI: 10.1021/acscatal.8b05178).

Rosen noted that the study also uncovered a periodic trend: MOFs with later transition metals form less stable but more reactive metal-oxo sites than ones with early transition metals. That finding highlights a key catalysis tradeoff. If a reactive site or intermediate is too reactive, it can dissociate before undergoing the desired catalytic reaction. If it’s too stable, it will be too inert to form the product.

By combining this new correlation with earlier work on methane activation, this group has developed an equation that can describe both the activity toward methane activation and the ability to form the active site, remarked University of Delaware’s Dionisios G. Vlachos, a catalysis specialist. This allows chemists to predict a range of suitable catalyst candidates for methane activation. He added, “Clearly, this study will stimulate interest from computationalists and experimentalists alike.”

 

 

Stir bar contamination may inadvertently catalyze reactions

Traces of metal nanoparticles embedded in used magnetic stirrers can interfere with chemical reactions

 

 

Used stir bars contain traces of metal that can catalyze reactions.

The humble stir bar is a ubiquitous workhorse of the chemistry lab that has swirled its way into researchers’ hearts. But it seems that magnetic stirrers have also been mixing things up in a different way—by smuggling rogue metals into chemists’ reaction flasks (ACS Catal. 2019, DOI: 10.1021/acscatal.9b00294).

Stir bars are typically coated with polytetrafluoroethylene (PTFE), a durable and inert polymer. But they can quickly become discolored, especially when exposed to catalytic metals such as palladium. So Valentine P. Ananikov of the N. D. Zelinsky Institute of Organic Chemistry gathered 60 used stir bars from other labs in his institute and took a closer look at what had caused the stains.

Electron microscopy revealed that the bars’ surfaces were littered with scratches, dents, and cracks that often contained a tangle of polymer filaments. These filaments trapped metal nanoparticles or microparticles, including palladium, gold, platinum, cobalt, or iron, which Ananikov’s team identified by X-ray spectroscopy. “It appears that almost all used stir bars in labs doing intensive catalysis and synthesis experiments are contaminated to varying degrees,” Ananikov says.

The researchers also found that these metals could leach into solution and interfere with reactions. Ananikov’s team tested used stir bars in the palladium-catalyzed Suzuki-Miyaura reaction, which couples aryl halides with boronic acids. First they used a palladium catalyst and a fresh stir bar to carry out a series of these reactions; then they repeated the experiments with no catalyst and a contaminated stir bar. In several cases, the dirty stir bar alone delivered a significant amount of the coupling product, and in one case it even matched the yield from the palladium catalyst. Running the reaction with no catalyst and a brand new stir bar gave no products at all.

Chemists who work with palladium catalysts generally know that dirty stir bars can affect their reactions, but it’s important to understand the extent of the problem, says synthetic chemist Mimi Hii of Imperial College London. “It’s been talked about anecdotally for a while, so it’s really nice to see a systematic study on it.” Because Hii’s group has found palladium to be active at concentrations as low as 5 parts per million, the researchers cleanse their stir bars of palladium with aqua regia, a mixture of nitric acid and hydrochloric acid. Others are not so thorough—indeed, many chemists clean their stir bars with nothing more than a quick scrub and a rinse with acetone and water, Ananikov says.

“I was surprised that there was enough metal there to catalyze reactions,” says Vladimir Gevorgyan, an organic chemist at the University of Illinois at Chicago. He points out that trace amounts of metal can sometimes interact with other metal catalysts to alter their reaction mechanism, so the problem is unlikely to be restricted to palladium chemistry. “I think it’s more general than that,” he says.

Significant stir bar damage can appear within weeks of use, and Ananikov’s theoretical calculations suggest that damaged PTFE binds metal particles much more strongly than pristine PTFE. He advises chemists to use new stir bars when they report reactions that apparently need very low concentrations of metal catalysts, or even none at all. “It’s a cautionary tale for anyone working in catalysis,” Hii says.

Chemical & Engineering News


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