"Although this achievement is limited to laboratory-scale testing at the gram level, I am optimistic about the kilogram-level experiments in the factory," said Liu Peng, an alumnus of Shanghai University of Applied Sciences and a Ph.D. graduate from the University of Fribourg in Switzerland.

Recently, he and his collaborators successfully achieved controllable degradation of polystyrene. In the study, he locked the degradability by inserting mechano-responsive groups into the polymer chains.

This degradation process does not require a large amount of solvents or significant energy consumption, and it is expected to help the field of degradable polymers transition from basic research to practical application.

Liu Peng hopes that this method can be integrated with existing industrial production lines to produce new types of polymers that are mechanically controllable in degradation, and to see some applications in real life.

Taking the commonly used polystyrene-based plastic foam in life as an example, after its use, it can be recycled, crushed, and hydrolyzed into other valuable small molecule compounds.Alternatively, after being discarded into the natural environment, it can be degraded into small molecular compounds under the action of natural forces such as wind, friction, and tidal forces. This is achieved by activating the mechano-responsive groups in the polymer chains, in the mildly alkaline seawater, and through the activities of animals and microorganisms, thereby reducing the negative impact on the environment.

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It is also reported that the results of this experiment confirm: the molecular weight of the final degradation products is generally below 1000Da, so there is no need to worry about the problem of microplastic pollution.

Secure a "lock" for polymer degradation

At present, plastic pollution is increasingly affecting human life. Developing new methods or materials to solve and alleviate plastic pollution has become increasingly urgent.

(Note: The term "1000Da" refers to 1000 Daltons, a unit of measurement for molecular mass.)The reason why plastic accumulates in nature and causes pollution is largely due to the fact that most of the plastic products we use in our daily lives are polymers with all-carbon chains.

Due to the stability of carbon-carbon bonds, polymers with all-carbon chains require hundreds to thousands of years to naturally degrade in the natural environment.

Although in recent years, more and more degradable and recyclable polymers have been reported, most of them achieve degradation by inserting unstable chemical groups into the all-carbon chain polymers.

There is a drawback to these polymer materials: the inserted unstable chemical groups will continuously trigger the degradation of the polymer, which greatly shortens the service life of plastic products and limits the usage environment of these materials.

It is precisely against this background that Liu Peng conceived the idea of whether a new type of controllable degradable polymer can be developed: it will only trigger degradation when we need it to degrade.Even when we do not need to degrade, it can still possess performance that is the same as or similar to traditional materials.

This is like adding a lock to the polymer chain, where the degradable groups in the polymer are locked during the product's daily use.

When the product is used up and discarded, the lock is opened with a key to activate the degradable groups, thus achieving the degradation of the polymer and reducing plastic pollution.

At the same time, such controllable degradable polymers need to be easy to synthesize, and it is best to be compatible with the current industrial production line of traditional plastics, in order to achieve the maximum transformation of research and development applications.Achieving Copolymerization of Different Types of Monomers and Cyclobutene Monomers

At the beginning of the research, the first issue Liu Peng considered was the design and synthesis of monomers.

He and his colleagues designed several types of stimuli-responsive monomers at that time. After considering the stability of the monomers, the convenience of synthesis, and the economy and convenience of mechanical triggering, they ultimately chose cyclobutene monomers.

These types of monomers can be easily obtained through a one-step [2+2] cyclization addition of commercial reagents.

In order to maximize the simplification of this method, they chose the simplest, which is also widely used in the industry, free radical polymerization.Under this polymerization method, the team achieved copolymerization of different types of monomers and cyclobutene monomers, thereby successfully introducing mechano-responsive groups into traditional polymer chains.

By measuring the reactivity of the polymer monomers and characterizing the copolymers, they determined that the copolymers were random copolymers, ensuring that the cyclobutene monomers could be evenly distributed along the polymer backbone.

Subsequent material property characterization data showed that copolymers containing mechano-responsive groups, such as polystyrene-co-cyclobutene, have very similar thermodynamic and mechanical properties compared to traditional styrene.

After confirming that the cyclobutene monomer would not have too much impact on the properties of the copolymer, they mechanically activated the copolymer.

Specifically, the team first subjected the copolymer to ultrasonic stimulation under solution conditions.Under the stimulation of ultrasonic waves, the research group clearly observed the process of cyclobutane ring-opening rearrangement to form olefin double bonds on the nuclear magnetic resonance (NMR) spectrum, thereby confirming the feasibility of mechanical triggering of cyclobutane and the release of degradable groups.

Through further hydrolysis experiments, they obtained a large number of small molecular components, enabling the controllable degradation of copolymers.

In recent years, although many papers have reported polymer activation induced by ultrasonic waves, these methods need to be carried out under extremely diluted solution conditions, which greatly limits their application range.

Therefore, they tested more practical methods such as ball milling and cryogenic grinding, both of which showed better mechanical activation and degradation, and neither of these methods required the involvement of solvents.

Finally, the related paper was published in Nature Chemistry titled "Mechanically triggered on-demand degradation of polymers synthesized by radical polymerizations" [1].Liu Peng is the first author and co-corresponding author, with Professor Nico Bruns from the Adolphe Merkle Institute in Switzerland serving as the co-corresponding author.

Cross-national and interdisciplinary collaboration

Although the paper was published in a high-impact journal of Nature, Liu Peng did not have an easy journey. He encountered difficulties such as the impact of the pandemic on experiments, starting from scratch with new knowledge, and the transition of research when changing jobs.

Fortunately, Professor Nico Bruns, Professor Christoph Weder, and Professor Michael Mayer from the Adolphe Merkle Institute in Switzerland provided him with a considerable degree of autonomy in terms of experimental time and funding.

"The experiments during the revision of the paper were taken over by a postdoctoral fellow from the Swiss team (Sètuhn Jimaja), who completed this part of the work excellently. He is also the second author of this paper, and I am very grateful for his help," said Liu Peng.At the same time, Liu Peng and others have also cooperated with experts in the field of computational chemistry simulation, further confirming the design and degradation mechanism through computational simulation.

In terms of the analysis of degradation products, they have cooperated with the German Waters GmbH company, using UPLC-QToF analysis to determine the molecular weight and structural formula of the final products.

It is also reported that this method has shown good degradation for polystyrene, polymethyl methacrylate, and polyacrylate.

However, it is not very suitable for polyolefin materials such as polyethylene and polypropylene, which currently account for the largest market share. Therefore, Liu Peng and others plan to apply this method to polyolefin materials.

Compared with vinyl and acrylic polymers, the synthesis of polyolefin polymers is much more complex, because the monomers of polyolefin materials are generally gases, and the synthesis conditions are more demanding. Therefore, the requirements for degradable groups that can be copolymerized are also relatively high.Currently, Liu Peng has designed and synthesized a series of different types of new monomers.

Preliminary experimental results show that these monomers and olefin monomers have excellent copolymerization, and the final polymer also shows excellent degradability, but the specific experiments are still in progress.