Recently, Dr. Hu Yixin from Duke University in the United States and his team have synthesized some functional groups.

Compared to ordinary carbon-carbon bonds, the chemical bonds contained in these functional groups are somewhat weaker, which allows for the organization of some bonds to break without destroying the ordinary carbon-carbon bonds, thereby expanding the potential applications of mechanochemical chemistry.

The greatest application prospect of this achievement lies in the ability to safely and stably store hydrogen fluoride in high molecular weight solid materials, and to efficiently release it under the stimulation of mechanical force.

At the same time, when exposed to air, the high molecular weight solid materials will begin to degrade and gradually liquefy with the participation of moisture.

In addition, the high molecular weight solid materials can also produce hydrogen fluoride in situ, which will make the distribution of hydrogen fluoride more uniform and more convenient for subsequent reactions such as the reaction of functional groups on the high molecular chain.According to the introduction, polymer mechanochemistry—primarily involves the transmission of mechanical forces through polymer chains, enabling specific chemical bonds or functional groups on the chains to undergo chemical reactions under the influence of external forces.

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Sometimes, these chemical reactions involving mechanical forces can produce unique chemical products, and these products cannot be generated by heat or light activation.

In daily life, mechanochemistry is ubiquitous. Simply tearing open a packaging bag involves some principles of mechanochemistry.

Previously, the academic community has created many types of force-responsive functional groups. After being subjected to external forces, some will change color, some can emit fluorescence, and some can release small molecules.

Within this field, one of the research focuses is to design mechanochemical groups and embed them into polymer chains, allowing polymer materials to release small molecules under the stimulation of mechanical forces.Benefiting from the properties of the released small molecules, the aforementioned strategy can theoretically enable force-responsive polymeric materials to achieve various new functionalities.

Previously, the team led by Hu Yixin and another team have reported several mechanochemical groups capable of releasing hydrogen chloride (HCl) molecules.

The aqueous solution of hydrogen chloride is what people commonly refer to as hydrochloric acid. Leveraging its acidity, hydrochloric acid can catalyze various hydrolysis and polymerization reactions and has a wide range of industrial applications.

The chloride ion is an excellent leaving group. From a chemical perspective, hydrogen chloride (HCl) can be easily released through mechanical force.

Thus, the research group shifted their focus to hydrogen fluoride (HF), as it is very similar to hydrogen chloride (HCl) but also very unique.At the same time, they set their research goal as: Can a mechanochemical group that releases hydrogen fluoride in response to force be designed and synthesized?

Unlike chloride ions, fluoride ions are not an excellent leaving group. Therefore, the design concept of force-responsive generation of hydrogen chloride cannot be simply transplanted to the force-responsive generation of hydrogen fluoride.

Currently, there is no mechanoresponsive functional group that can release hydrogen fluoride. Because hydrofluoric acid is highly corrosive, there are risks in both storage and use.

If a polymer can release hydrogen fluoride under simple and direct external stimulation, and the polymer itself has the characteristics of thermal stability, then the safety and convenience of the use and storage of hydrogen fluoride will be greatly improved.

At the same time, the application of fluoride ions in hydrogen fluoride can also expand the application of protons. For example, fluoride ions can react with silyl functional groups.Previously, the research group had conducted studies on the release of hydrogen chloride, and this study is based on the previous one. However, they still do not have full confidence in being able to make the fluoride ions depart.

So they made two structures: one is gem-difluorocyclopropane (gDFC) with a methyl substitution (M1), and the other is gDFC with an alkoxy substitution (M2).

As a result, they found that the ring-opening structure included the migration of fluoride ions, which was due to the substitution by oxygen atoms, leading to changes in the ring-opening structure. In the gDFC without oxygen atom substitution previously, there was no migration of fluoride ions observed.

For this reason, the collaborators of the team where Hu Yixin is located, conducted molecular dynamics simulations to try to explain the principle of fluoride ion migration.

Through further research, they found that although the two have similarities in structure, the different number of carbon substituents led to significant differences in the results of the chemical reactions.According to the report, the open-loop structure of M1 becomes stable after ultrasonication; while the open-loop structure of M2 becomes unstable after ultrasonication, and hydrolysis will occur in the presence of water, causing the polymer chains to break and subsequently releasing hydrogen fluoride.

They also found that the hydrolysis process after the ring-opening of M2 is an acid-catalyzed process. Therefore, after the production of hydrogen fluoride, more hydrogen fluoride will be catalyzed, so this is a self-catalyzed amplification release process.

It is understood that this is also the first case of force-responsive groups releasing hydrogen fluoride, which is quite different from the thermodynamic release of hydrogen fluoride in the past.

At the same time, they also found that the intermediate can react with water to produce hydrogen fluoride. This also means that intermediates similar to A2 can release hydrogen fluoride.

For the production of hydrogen fluoride, the team also conducted multiple verifications. In addition to observing the production of hydrogen fluoride in nuclear magnetic resonance spectroscopy, they also confirmed the existence of fluoride ions through a fluorescent probe of organic small molecules for fluoride ions.At the same time, they also confirmed through experiments that the fluoride ions did indeed detach from the polymer chain and entered the solution.

After confirming the production of hydrogen fluoride, they began to consider whether the produced hydrogen fluoride could be used for the degradation of polymers.

It is understood that a collaborator of the team had previously synthesized a degradable polymer material [1]. Due to the siloxane functional groups on the main chain, this polymer material can be rapidly degraded under acidic conditions or in the presence of fluoride ions.

Therefore, for the mechanochemical groups synthesized in this study, they used ultrasonic stimulation to generate hydrofluoric acid, and used the produced hydrofluoric acid to perform in situ degradation of polymers containing siloxane functional groups.

This involves the competition of two reactions: one is the autocatalytic rate of hydrofluoric acid produced by P2 (the polymer chain composed of M2), and the other is the reaction rate of hydrofluoric acid with siloxane.The results show that the autocatalytic rate of hydrofluoric acid is far higher than the reaction rate of hydrofluoric acid and siloxane, which allows P3 to be degraded more completely, thus laying an experimental foundation for the realization of force-responsive degradation of macromolecular materials.

In fact, the team originally intended to follow the design concept of force-responsive release of hydrogen chloride.

The student who originally participated in this project also initially tried to transplant the design concept of force-responsive release of hydrogen chloride to the force-responsive release of hydrogen fluoride, and obtained the M1 structure.

Unfortunately, this approach did not work at all. Later, Hu Yixin found that M1 could not release hydrogen fluoride, which is likely due to the difficulty for fluoride ions to leave.

At the same time, after the force-opening ring of M1, the product linked with the fluorine atom and the oxygen atom is a quaternary carbon, which makes it difficult for the fluorine atom or the oxygen atom to be "attacked" by other reagents.When the research hit a bottleneck, Hu Yixin suddenly thought of another system she was studying at the same time [2], which might provide some new ideas.

After several attempts at synthesis, she successfully designed the M2 structure. After the M2 force open loop, the product is linked to a tertiary carbon with a fluorine atom and an oxygen atom, which allows the fluorine atom to leave better.

Later, Hu Yixin found that the difference from M1 was that after the polymer chain of M2 was subjected to ultrasonication, concentrated and added deuterated chloroform, the peaks of the proton nuclear magnetic spectrum were very clear.

However, if M2, like M1, has no reaction of fluorine ion leaving, the splitting of the proton nuclear magnetic spectrum on the polymer chain should not be so clear.

Moreover, after analysis by size exclusion chromatography, the quality of the polymer chain also decreased a lot.Thus, Hu Yixin sought to identify the cause of the discrepancy in M2 by exploring various variables.

At that time, she was unaware that the integration moments of the fluorine peak of hydrogen fluoride and hydrofluoric acid were changing, and the displacement of hydrofluoric acid was also not fixed.

"So there was a period of stagnation in the middle, feeling that there were too many variables and not sure what the mechanism was. Just then, by chance, my friend came to visit me from the UK, and I went to New York with her for two days. The day after I came back, I found that just by adding water, it could be degraded and release hydrogen fluoride," she said.

This further made her realize that sometimes people need to take a break to change their way of thinking and also need to spend time organizing their thoughts.

To verify the correctness of the product structure, she also performed characterizations such as mass spectrometry and two-dimensional nuclear magnetic resonance spectroscopy, and compared the chemical shift when directly placing hydrofluoric acid in the same deuterated tetrahydrofuran.In fact, in the research, Hu Yixin was not clear whether hydrofluoric acid was actually produced, so she first assumed that this molecule could produce a high concentration of hydrofluoric acid, and could even react with glass.

Therefore, she was fully equipped to detect the changes during the ultrasonic process with a camera, and also very exaggeratedly posted several "Do not approach, hydrofluoric acid is produced" warning signs around the ultrasonic device to remind colleagues to pay attention to safety.

As a result, with the repetition of the experiment, it was found that the concentration of hydrofluoric acid produced was not very high, and it was not as dangerous as imagined.

Finally, the related paper was published in JACS with the title "Self-Amplified HF Release and Polymer Deconstruction Cascades Triggered by Mechanical Force" [3].

Hu Yixin is the first author, Professor Stephen L. Craig from Duke University in the United States, Professor Heather J. Kulik and Professor Jeremiah Johnson from the Massachusetts Institute of Technology in the United States, and Professor Lin Yangju, a graduate of Professor Craig's group, served as co-corresponding authors.For the system discovered in this instance, there are many potential directions worth exploring. Utilizing the in situ force response to generate hydrofluoric acid, as well as the characteristic of force-responsive self-degradation, to achieve changes in the material's properties at the macroscopic level, is the new direction that the research group is currently most interested in.