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Covalent adaptable networks (CANs) are a type of polymer material that closely resemble thermosetting polymers (thermosets). However, they are distinguished from thermosets by the incorporation of dynamic covalent chemistry into the polymer network. When a stimulus (for example heat, light, pH, ...) is applied to the material, these dynamic bonds become active and can be broken or exchanged with other pending functional groups, allowing the polymer network to change its topology. This introduces reshaping, (re)processing and recycling into thermoset-like materials.

Background

Historically, polymer materials have always been subdivided in two categories based on their thermomechanical behaviour. Thermoplastic polymer materials melt upon heating and become viscous liquids, whereas thermosetting polymer materials remain solid as a result of cross-linking.

Thermoplastics consist of long polymer chains that are stiff at service temperatures but become softer with increasing temperature. At low temperatures, the molecular motion of the polymer chains is limited due to chain-entanglements, resulting in a hard and glassy material. Increasing the temperature will lead to a transition from a hard to a soft material at the glass transition temperature (T_g) yielding a visco-elastic liquid. In the case of (semi-)crystalline polymer materials, viscous flow is achieved when the melting point (T_m) is reached and the intermolecular forces in the ordered crystalline domain are overcome. Thermoplastics regain their solid properties upon cooling and can thus be reshaped by polymer processing methods such as extrusion and injection moulding and they can also be recycled. Examples of thermoplastic polymers are polystyrene, polycarbonate, polyethylene, nylon, Acrylonitrile butadiene styrene (ABS), etc.

Thermosets, on the other hand, are three-dimensional networks that are formed through permanent chemical cross-linking of multifunctional compounds. This is an irreversible process that results in infusible and insoluble polymer networks with superior properties compared to thermoplastics. When a thermoset is exposed to heat, it maintains its dimensional stability and thus cannot be reshaped. These polymer materials are generally used for demanding applications (e.g. wind turbines, aerospace, etc.) that require chemical resistance, dimensional stability and good mechanical properties. Typical thermosetting materials include epoxy resins, polyester resins, polyurethanes, etc.

In the framework of sustainability, the combination of the mechanical properties of thermosets with the reprocessability of thermoplastics through the introduction of dynamic bonds has been the topic of numerous research studies. The use of non-covalent interactions such as hydrogen bonding, pi-stacking or crystallization that lead to physical cross-links between polymer chains is one way of introducing dynamic cross-linking. The thermoreversible nature of the physical cross-links results in polymer materials with enhanced mechanical properties without compromising reprocessability. The properties of these physical networks are highly dependent on the used backbone and type of non-covalent interactions, but typically they are brittle at low temperature and become elastic or rubbery above T_g. Upon further heating the physical cross-links disappear and the material behaves as a visco-elastic liquid, allowing them to be reprocessed. These materials are also known as thermoplastic elastomers.

Covalent adaptable networks (CANs) instead use dynamic covalent bonds that are able to undergo exchange reactions upon application of an external stimulus, typically heat or light. In absence of a stimulus these materials behave as covalently cross-linked materials, showing high chemical resistance and dimensional stability, but when the stimulus is applied the dynamic bonds become activated which enables the network to rearrange its topology on a molecular level. As a result, these materials are able to undergo permanent deformations, enabling reshaping, reprocessing, self-healing, etc. As such, CANs can be seen as an intermediate bridge between thermosets and thermoplastics.

In 2011, the research group of French researcher Ludwik Leibler developed a specific class of CANs based on an associative exchange mechanism (see subsection Classification). By adding a suitable catalyst to epoxy/acid polyester based networks, they were able to prepare a permanent epoxy network that showed a gradual viscosity decrease upon heating. This type of behaviour is typical for vitreous silica and had before never been seen in organic polymer materials. Therefore, the authors introduced the name Vitrimers for these kind of materials.

Classification

CANs are currently subdivided in two groups, dissociative CANs and associative CANs, based on the underlying mechanism of the bond exchange reactions (i.e. the order in which the bond forming and breaking occurs) and their resulting temperature dependence.  

Dissociative CANs

The exchange mechanism of dissociative CANs requires a bond-breaking event prior to the formation of a new bond (i.e., an elimination/addition pathway). Upon application of a stimulus the equilibrium shifts to the dissociated state, resulting in a temporarily decreased cross-link density in the network. When a sufficient amount of dynamic bonds dissociate due to the equilibrium being shifted below the gel point, the material will suffer a loss of dimensional stability and show a sudden and drastic viscosity decrease.  After removal of the stimulus the bonds reform and, in the ideal case, the original cross-link density is restored. This temporary decrease in cross-link density enables very fast topology rearrangements in dissociative CANs, such as viscous flow and stress relaxation, which allows the reprocessing of covalently cross-linked polymer networks. Additionally, dissociative CANs can be solubilized in good solvents.

Associative CANs

In contrast to dissociative CANs, networks in associative CANs do not depolymerize upon application of a stimulus and maintain a near constant cross-link density. Here, the exchange mechanism relies on the formation of a new bond before fragmentation of another bond (i.e. an addition/elimination pathway). This means that bond exchange occurs via a temporarily more cross-linked intermediate state. However, in practice, this small increase will often be negligible, resulting in a practically constant cross-link density. As a result, associative CANs remain insoluble in inert solvents, even at elevated temperatures.

In the case of Vitrimers, associative exchange is triggered by heat and the viscosity of these materials is controlled by chemical exchange reactions, leading to a linear dependence of viscosity with inverse temperature according to the Arrhenius law. The decreased viscosity caused by rapid dynamic bond exchanges enables stress relaxation and network topology rearrangements in these materials.

Dynamic chemistries used in CANs

Dissociative exchange

Diels-Alder

One of the most well-known dissociative chemistries often implemented in CANs is the Diels-Alder (DA) reaction, which is reversible at elevated temperatures. The [4+2] cycloaddition between furan (i.e. a diene) and maleimide (i.e. a dienophile) occurs through a concerted mechanism and is promoted at 40 °C, while the equilibrium of this reversible reaction gradually shifts towards the dissociated products at temperatures above 90 °C. However, prolonged exposure to high temperatures (> 100 °C) can result in undesirable oxidation of furan moieties and homopolymerization of maleimide moieties, i.e. irreversible side reactions.

Already in 1966, the first patent concerning polymer materials containing reversible furan-maleimide bonds was filed by the American chemical company DuPont. But more recently academic researchers have integrated reversible DA bonds in CANs and other polymeric materials for various applications, such as shape memory, self-healing, reversible linear polymerization, protection strategies, surface modification, ...

Associative exchange

Transamination of vinylogous urethanes

Vinylogous urethanes (VU) have proven to be interesting chemical moieties for vitrimer applications since VU are known to undergo associative transamination reactions in the presence of pendant primary amines. In 2015, it was shown that materials based on VU linkages are able to be reprocessed at higher temperatures (150 °C) without addition of a catalyst. These materials can be easily prepared via the polycondensation reaction of multifunctional acetoacetate and primary amine monomers. Additionally, VU bonds have also already been incorporated in perfluoropolyether and polydimethylsiloxane matrices.

General scheme of the reversible transamination reaction of vinylogous urethanes.

Although the exchange reactions are already active without the presence of a catalyst, it has been shown that the introduction of certain additives can enhance or inhibit the exchange rate. While the addition of an acids like p-Toluenesulfonic acid (pTsOH) and sulfuric acid enhanced the exchange rate, the opposite effect was seen when a strong base like triazabicyclodecene (TBD) was added. This observation was linked to a difference in promoted exchange mechanism.