Saran S Kumar and Saritha A.
Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, 690525, Kerala, India

Since Hermann Staudinger’s groundbreaking work, which first posited the covalent structure of polymers, new topics and concepts with applications in everyindustry have been created. Synthetic organic polymers are ubiquitous in modern life due to their affordability, light weight, and variety of useful qualities. Chemical engineers have created a variety of methods over the past century that provide precise control over the size and dispersity of synthetic polymers as well as their composition, functionality, and structure, which in turn determine their chemical, physicochemical, and mechanical properties.

Based on how they react to heat, which is directly related to the structure of the polymer chains, polymers are frequently categorised. The first category consists of thermoplastics, which, when heated above the glass transition temperature, Tg, flow macroscopically like viscoelastic liquids. They are made up of polymer chains that are not chemically linked to one another yet have enough molecular mass to become entangled. They may be repeatedly melted and moulded due to their molten state’s molecular mobility. However, applications requiring rubbery solids, dimensional stability at high temperatures, or exceptional environmental stress cracking resistance are not suitable for thermoplastics.[1]

The second class of polymeric materials, thermosets, can be created by covalently linking polymer chains to overcome these drawbacks. The three-dimensional network prevents polymer chain dispersion caused by heat or solvents, rendering thermosets insoluble by nature. Because they cannot flow and have an elastic modulus that depends on the crosslinking density, thermosets display a theoretically limitless rubbery plateau above the Tg. Due to these properties, thermosets are advantageous for uses needing high mechanical strength, solvent resistance, or creep resistance, such as structural applications, coatings, adhesives, electronics, and composites. However, after thermosets are cured, the topology of the network and the macroscopic morphology are “set,” making further processing impossible.

The decision of whether to use a thermoset or a thermoplastic for a particular application is thus fraught with difficult compromises with regard to processability, durability, and recyclability. Scientists have worked to combine the malleability of thermoplastics with the performance benefits of thermosets because of the intellectual challenge and social desire for recyclable high performance materials. The current approach is to build a network of polymer chains in which the crosslinks can be broken and repaired using a triggered stimulus within a particular temperature window. The first examples include nanostructured thermoplastic elastomers and polymers with polar or hydrogen-bonding groups in the main chain that use phase separation to physically crosslink polymer chains.[2]
This methodology has been widely applied to supramolecular networks with non-covalent transient crosslinks that function via hydrogen bonds, – interactions, and metal complexation. The thermodynamic equilibrium constant K, which governs the equilibrium between the associated and the dissociated forms and, consequently, the connectedness of the system, describes these interactions. Rate constants for the bond generation and rupture of transitory linkages, which relate to the system’s dynamicity, are another characteristic of such systems. The viscoelastic characteristics of reversible networks are significantly influenced by the temperature dependence of the thermodynamic and kinetic constants. Supramolecularly crosslinked materials frequently exhibit high rates of exchange and K values that significantly drop with temperature, which makes them unsuitable for applications requiring creep resistance.

Though theoretically comparable to non-covalently crosslinked networks, polymer networks that are crosslinked by reversible covalent bonds are distinguished by the strength and longevity of the covalent association. A variety of stimuli can be employed to shift the equilibrium between the associated and dissociated states depending on the properties of the reversible covalent bond. As an illustration, reversible covalent networks have been created using both chemically and thermally activated groups, such as boronic esters, disulfides, acylhydrazones, imines, and acetals. Other thermally activated groups include Diels-Alder adducts, alkoxyamines, urethanes, and urea. The equilibrium constant of the reversible covalent crosslinking group, which controls the degree of crosslinking, is the main cause of the observed modulation of macroscopic characteristics. When temperatures are high, the dissociation process accelerates more quickly than the association reaction. In the dissociated condition, these materials consequently suffer from a loss of network integrity, which is bad for applications that call for simple shaping and welding. Recently, a fundamentally novel approach to polymer networks that combines the malleability and reprocessability of thermoplastics with the permanent, temperature-independent structure of thermosets was developed. Leibler and colleagues introduced these brand-new polymer compounds, known as vitrimers, in 2011.[3]

Vitrimers
Vitrimers are a class of polymeric materials that possess dynamic covalent bonds, allowing them to undergo reversible chemical reactions while maintaining their structural integrity. The term “vitrimers” is derived from the Latin word “vitrum,” which means glass, as these materials exhibit properties similar to glass, such as high transparency and rigidity.

Vitrimers are a type of polymers that can be reshaped, reprocessed, and repaired multiple times without losing their properties or performance. This is because the dynamic covalent bonds in vitrimers can break and reform in response to external stimuli, such as heat, light, or chemical triggers. This unique property makes vitrimers highly versatile and desirable for a wide range of applications in various fields, including automotive, aerospace, electronics, packaging, and coatings.One of the key advantages of vitrimers is their ability to be recycled and reprocessed, reducing waste and promoting sustainability. Additionally, vitrimers can be tailored to exhibit specific properties, such as mechanical strength, thermal stability, and chemical resistance, by adjusting their chemical composition and processing conditions.Vitrimers have been the subject of extensive research and development, and they hold great promise for future technological advancements. They offer numerous opportunities for innovation in areas such as advanced composites, self-healing materials, shape memory materials, and recyclable polymers, among others. However, there are still challenges to overcome in terms of scaling up production, optimizing properties, and understanding the long-term performance of vitrimers in various applications.[4]

In summary, vitrimers are a class of polymeric materials that possess dynamic covalent bonds, allowing them to undergo reversible chemical reactions and exhibit unique properties such as recyclability, reprocessability, and versatility. They have the potential to revolutionize various industries and contribute to the development of more sustainable and advanced materials in the future

Vitrimer Chemistry
The chemistry of vitrimers is centered around their dynamic covalent bonds, which are responsible for their unique properties and behavior. Vitrimers are typically formed by polymerizing monomers that contain functional groups capable of undergoing reversible reactions, such as transesterification, disulfide exchange, imine exchange, or other similar reactions. These dynamic covalent bonds can break and reform under specific conditions, allowing the vitrimer to undergo reshaping, reprocessing, and self-healing.[5]

The chemistry of vitrimers can vary depending on the specific type of dynamic covalent bond used and the desired properties of the material. Some common examples of dynamic covalent bonds used in vitrimers include:

• Transesterification: This is a type of reaction that involves the exchange of ester groups between two polymer chains. Transesterification reactions are typically catalyzed by a base or an acid, and they can occur at relatively low temperatures. This type of dynamic covalent bond is commonly used in vitrimers based on polyesters or polycarbonates.

  

• Disulfide exchange: This reaction involves the exchange of disulfide bonds (-S-S-) between two polymer chains, leading to the formation of new disulfide bonds. Disulfide exchange reactions are typically redox reactions and require a catalyst, such as a thiol or a disulfide compound, to occur. Disulfide exchange is commonly used in vitrimers based on polymers containing thiol or disulfide functional groups.
• Imine exchange: This reaction involves the exchange of imine bonds (-C=N-) between two polymer chains, leading to the formation of new imine bonds. Imine exchange reactions are typically catalyzed by a base or an acid, and they can occur at moderate temperatures. Imine exchange is commonly used in vitrimers based on polymers containing imine functional groups, such as polyimines.

These are just a few examples of the chemistry involved in vitrimers, and there are other types of dynamic covalent bonds that can be used as well. The choice of dynamic covalent bond and its specific chemistry depends on the desired properties of the vitrimer material, such as its mechanical strength, thermal stability, and reprocessability.

Overall, the chemistry of vitrimers is focused on the reversible nature of dynamic covalent bonds, which allows for their unique properties, such as reshaping, reprocessing, and self-healing capabilities. The careful design and control of the dynamic covalent bond chemistry is crucial in tailoring the properties and performance of vitrimers for various applications.

Types of Vitrimers
Vitrimers can be classified based on several criteria, including their chemical composition, processing methods, and properties. Here are some common ways in which vitrimers can be classified:

1. Chemical composition: Vitrimers can be classified based on their chemical composition, such as the type of polymer backbone or the specific dynamic covalent bonds used.[6][7] For example:
   • Epoxy-based vitrimers: These vitrimers are based on epoxy resins and typically undergo dynamic exchange reactions at the epoxy groups, allowing for reshaping and reprocessing.
   • Polyurethane-based vitrimers: These vitrimers are based on polyurethane polymers and undergo dynamic exchange reactions at the urethane linkages, which can enable reshaping and reprocessing.
   • Polyester-based vitrimers: These vitrimers are based on polyesters and typically undergo transesterification reactions, allowing for reshaping and reprocessing through exchange of ester bonds.
2. Processing methods: Vitrimers can also be classified based on the processing methods used to form and shape them.[8] For example:
   • Melt-processable vitrimers: These vitrimers are typically processed as a molten material, either by melting the monomers or polymers, and then cooled to solidify. They can be shaped using traditional processing techniques such as injection molding or extrusion.
   • Solvent-processable vitrimers: These vitrimers are processed using solvents, where the monomers or polymers are dissolved in a solvent, shaped into the desired form, and then the solvent is evaporated to solidify the vitrimer.
3. Properties: Vitrimers can also be classified based on their specific properties, such as mechanical properties, thermal properties, or self-healing capabilities. For example:
   • High-temperature vitrimers: These vitrimers are designed to withstand high temperatures without significant degradation of their properties. They may find applications in aerospace or automotive industries where high-temperature performance is crucial.
   • Self-healing vitrimers: These vitrimers have the ability to autonomously repair damage or cracks, either through thermally or chemically triggered dynamic covalent bond reactions, allowing for self-repairing of the material.
   • Shape memory vitrimers: These vitrimers can “remember” a specific shape and can recover that shape when subjected to external stimuli, such as heat or light, due to the dynamic covalent bonds allowing for reversible shape changes.

These are just some examples of how vitrimers can be classified based on their chemical composition, processing methods, and properties. Vitrimers are a rapidly evolving field of research, and new classification schemes may emerge as the understanding and applications of these materials continue to advance.

Applications of Vitrimers
Vitrimers have a wide range of potential applications due to their unique properties and versatility. Here are some examples of applications where vitrimers have shown promise:

1. Automotive and aerospace industries: Vitrimers can be used in the manufacturing of lightweight, high-strength components for automotive and aerospace applications. Their ability to undergo reshaping and reprocessing allows for complex shapes to be fabricated, and their self-healing properties can provide improved durability and damage tolerance in harsh environments.
2. Adhesives and coatings: Vitrimers can be used as adhesives and coatings due to their excellent adhesion properties and the ability to self-heal. They can be used in applications where bonding and sealing of various substrates are required, such as in construction, electronics, and packaging.
3. 3D printing: Vitrimers can be used in 3D printing to fabricate complex and functional objects with dynamic properties. The ability to undergo reshaping and reprocessing allows for post-processing modifications, and the self-healing properties can help repair any printing defects or damage during use.
4. Biomedical applications: Vitrimers have shown potential in biomedical applications, such as drug delivery, tissue engineering, and medical devices. Their dynamic properties and biocompatibility make them suitable for applications where controlled release of drugs, shape-changing implants, or self-healing materials are desired.
5. Packaging materials: Vitrimers can be used in packaging materials to provide improved barrier properties, such as gas and moisture resistance, and better mechanical strength. Their reprocessability and self-healing properties can also extend the lifespan of packaging materials and reduce environmental waste.
6. Coatings for corrosion protection: Vitrimers can be used as coatings for corrosion protection in various industries, such as oil and gas, marine, and infrastructure. Their ability to self-heal can provide enhanced corrosion resistance and durability, extending the lifespan of coated structures and reducing maintenance costs.
7. Soft robotics: Vitrimers can be used in soft robotics, where materials with dynamic properties and shape-changing capabilities are desired. Vitrimers can enable the fabrication of soft robotic devices with programmable shape changes, allowing for complex and adaptable movements.

These are just some examples of the wide range of potential applications of vitrimers. As research and development in this field continue to progress, we can expect to see even more innovative applications of vitrimers in various industries and technologies.[9]

Sustainability of Vitrimers
Vitrimers, as a class of dynamic materials, have the potential to offer sustainability benefits in various applications due to their unique properties and characteristics. Here are some aspects related to the sustainability of vitrimers:

1. Recycling and Reprocessing: Vitrimers are capable of undergoing reshaping and reprocessing due to their dynamic covalent bonds, which can be reversed under appropriate conditions. This property allows for efficient recycling and repurposing of vitrimer materials, reducing waste and promoting circular economy principles. Vitrimers can be recycled by applying heat, pressure, or other stimuli to trigger the exchange reactions at their dynamic covalent bonds, resulting in reshaping and reprocessing of the material without significant loss of performance or quality. This can help reduce the environmental impact of waste generated from traditional plastic materials that are difficult to recycle or dispose of.
2. Self-healing: Vitrimers can exhibit self-healing properties, where their dynamic covalent bonds can undergo exchange reactions to repair damage or cracks in the material. This property can extend the lifespan of vitrimer materials, reducing the need for frequent replacement and reducing waste. Self-healing vitrimer materials can potentially reduce the environmental impact associated with material damage or degradation, and contribute to the development of more durable and long-lasting products.
3. Reduced Energy Consumption: The reprocessing and reshaping of vitrimer materials typically occurs at lower temperatures compared to traditional thermosetting or thermoplastic materials, which often require high temperatures for processing. This can lead to reduced energy consumption during material processing, resulting in lower greenhouse gas emissions and reduced environmental impact.
4. Potential for Bio-based Materials: Vitrimers can be synthesized from renewable, bio-based feedstocks, such as plant-derived monomers or renewable polymers. This can reduce dependence on fossil-based resources and contribute to the development of more sustainable materials. Additionally, the ability of vitrimers to be reshaped and reprocessed without significant loss of performance or quality can extend the lifespan of bio-based materials, potentially reducing the need for frequent replacement and waste generation.
5. Tailored Material Properties: Vitrimers can be designed and tailored to have specific material properties, such as mechanical strength, toughness, and thermal stability, through the choice of monomers, dynamic covalent bonds, and processing conditions. This allows for the development of materials with optimized properties for specific applications, potentially reducing material waste and improving resource efficiency.[10][11]

It’s important to note that the sustainability of vitrimers also depends on factors such as the specific application, processing methods, end-of-life management, and environmental impact assessment of the overall life cycle of the material. However, the unique properties of vitrimers, such as their recyclability, self-healing, potential for bio-based materials, and reduced energy consumption during processing, suggest that they have the potential to contribute to more sustainable material solutions in various applications. Further research and development in this area may lead to more sustainable vitrimer materials and applications in the future.

REFERENCES

1. M. Biron, Thermosets and composites, Elsevier, 2003
2. R.M. Ibarra, Recycling of thermosets and their composites, in: Thermosets, Elsevier, 2018: pp. 639–666.
3. M. Capelot, M.M. Unterlass, F. Tournilhac, L. Leibler, Catalytic control of the vitrimer glass transition, ACS Macro Lett. 1 (2012) 789–792.
4. J. Zheng, Z.M. Png, S.H. Ng, G.X. Tham, E. Ye, S.S. Goh, X.J. Loh, Z. Li, Vitrimers: Current research trends and their emerging applications, Mater. Today. 51 (2021) 586–625.
5. M. Capelot, D. Montarnal, F. Tournilhac, L. Leibler, Metal-catalyzed transesterification for healing and assembling of thermosets, J. Am. Chem. Soc. 134 (2012) 7664–7667.
6. F. Gamardella, F. Guerrero, S. De la Flor, X. Ramis, A. Serra, A new class of vitrimers based on aliphatic poly (thiourethane) networks with shape memory and permanent shape reconfiguration, Eur. Polym. J. 122 (2020) 109361.
7. B. Hendriks, J. Waelkens, J.M. Winne, F.E. Du Prez, Poly (thioether) vitrimers via transalkylation of trialkylsulfonium salts, ACS Macro Lett. 6 (2017) 930–934.
8. C. Taplan, M. Guerre, J.M. Winne, F.E. Du Prez, Fast processing of highly crosslinked, low-viscosity vitrimers, Mater. Horizons. 7 (2020) 104–110.
9. M. Guerre, C. Taplan, J.M. Winne, F.E. Du Prez, Vitrimers: directing chemical reactivity to control material properties, Chem. Sci. 11 (2020) 4855–4870.
10. W. Alabiso, S. Schlögl, The impact of vitrimers on the industry of the future: Chemistry, properties and sustainable forward-looking applications, Polymers (Basel). 12 (2020) 1660.
11. X.-L. Zhao, Y.-Y. Liu, Y. Weng, Y.-D. Li, J.-B. Zeng, Sustainable epoxy vitrimers from epoxidized soybean oil and vanillin, ACS Sustain. Chem. Eng. 8 (2020) 15020–15029.