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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, extruded or injection-moulded. 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 macromolecular deformation, enabling reshaping, reprocessing, self-healing, etc. As such, CANs can be seen as an intermediate bridge between thermosets and thermoplastics.

Classification

CANs are currently subdivided in two classes, dissociative CANs and associative CANs,  based on the underlying mechanism of the bond exchange reactions and their resulting temperature dependence.  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), analogous to the S_N1 reaction in organic chemistry.