Inert covalent bond activation strategy is used to synthesize new materials that can be closed loop recycled and malleable thermoset

Recently, Professor Zhang Wei’s research group of the Department of Chemistry of the University of Colorado Boulder introduced the dynamic aromatic nucleophilic substitution reaction (DySNAr) into the synthesis of a traditional thermoset plastic, and obtained a new class of recyclable alkyl polycyanates .

On September 26, 2022, the study was published in the journal Nature Chemistry under the title “Recyclable and malleable thermosets enabled by activating dormant dynamic linkages.”

Plastics are an indispensable part of daily life, and their excellent properties have made a great contribution to social development and scientific and technological progress. However, with the growing dependence on plastic products, environmental pollution and threats to human health caused by their indegradable properties have become urgent problems to be solved. In order to achieve sustainable ecological and economic development, how to effectively recycle and reuse polymeric materials has become a hot topic. Among them, chemical recycling can effectively degrade polymeric materials into industrial raw materials, which has great application potential. In order to solve the problem that traditional thermosetting plastics are difficult to process and difficult to recycle, dynamic chemical bonds are introduced into the polymeric material skeleton, which reduces the difficulty of material processing and recycling while maintaining the properties of the original material. Although these new materials may become alternatives to traditional materials in the future, the problems of traditional materials remain to be solved.

Reverse synthesis analysis is widely used in the field of synthesis, using the use of cutting chemical bonds at different locations in the target structure to find the optimal synthetic route. In the field of polymer synthesis, different synthesis strategies for the same structure often result in unexpected performance differences. For example, in the synthesis of poly-p-phenylene acetylene (PPE), the single bond of carbon and carbon is cut off, and the polymer with low molecular weight and diyne defect is generally obtained by using the traditional transition metal catalytic coupling reaction; However, if the carbon and carbon triple bonds are cut off, a polymer with higher molecular weight and no diyne defects can be obtained by using dynamic alkyne complex decomposition reactions. More importantly, dynamic reversible reactions can degrade the resulting polymer into small molecules, making closed-loop recovery possible. Therefore, the strategy of reverse synthesis analysis is expected to reveal the potential reversible synthetic route of traditional thermosetting materials.

As a proof of concept, Professor Zhang Wei’s research group at the University of Colorado in the United States used cyanate resin as a model system to illustrate the use of inert covalent bonds that activate traditional thermoset materials to achieve recyclability and machinability. Through the analysis of the retrosynthetic strategy, the authors found that the formation of carbon-oxygen bonds using aromatic nucleophilic substitution reactions may be a potential new method for constructing polycyanates skeletons. It should be noted that only aryl polycyanates can be obtained by using the traditional[2+2+2]trimeric method.

The research team first used 2,4,6-triethoxytriazine (TETA) as a model compound for the reversibility study of the reaction between alkyl substituted cyanate and alcohol, and found that in the presence of a catalytic amount of organic base, the aromatic nucleophilic substitution reaction can be carried out in a reversible manner. The reaction was monitored at different temperatures, and the reaction activation energy was 62.5 kJ/mol calculated by using the Arrennius formula. Subsequently, using TETA and diols of different carbon chain lengths as raw materials, the team synthesized three polycyanates (PCN-A4, PCN-A6, PCN-A12). Alkyl substituted polycyanates have excellent ductility compared to conventional aryl polycyanates . As the alkyl chain grows, the polycyanates become softer and more ductile, while the glass transition temperature can also be reduced to near room temperature. Alkyl polycyanates have extremely high resistance to organic solvents, with a gel fraction of about 99% in a variety of solvents. In addition, alkyl polycyanates exhibit extremely high chemical stability and solvent resistance.

The team then studied the recoverability of the material and found that these polycyanates and potassium carbonate could be efficiently degraded into structural units under the condition of ethanol reflux, and after the ethanol was removed, both the glycol and the TETA could be recovered at high separation rates. Under the same conditions, common plastics such as polypropylene, high-density polyethylene and polystyrene cannot be depolymerized, proving that polycyanates can be selectively and efficiently recovered from plastic mixtures. Polycyanates prepared from recovered TETA and diols and polycyanates prepared from pure raw materials have exactly the same infrared spectrum, similar mechanical properties and glass transition temperature. This recovery method is also applicable to aryl polycyanates and demonstrates the universality of this polycytrite recovery strategy, providing a simple and easy closed-loop recovery method. In addition, the unreacted hydroxyl group in the polycyanates can induce bond exchange based on the aromatic nucleophilic substitution mechanism, so that the polycyanates are plastic.

Figure 1: Study of reversible aromatic nucleophilic substitution reactions

Figure 2: Preparation and characterization of alkyl polycyanarate

Figure 3: Selective recovery of alkyl polycyanarate

This work introduced the dynamic Nucleophilic Aromatic Substitution (DySNAr) into the synthesis of a conventional thermoset plastic and resulted in a new class of recyclable alkyl polycyanates . These polycyanates synthesized from inexpensive, easy-to-read, structurally adjustable structural units offer high chemical stability and closed-loop recyclability, making them valuable in plastics-related industries and significantly improving environmental sustainability. As a routinely effective tool in the field of organic synthesis, retrosynthetic analysis can inspire researchers to design and synthesize existing or entirely new polymers, revealing their unprecedented properties. (Source: Science Network)

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