Covalent Adaptable Networks Based on Dynamic Alkoxyamine Bonds

Abstract

Covalent adaptable networks (CANs) introduce a new paradigm to polymer science, by making static network polymers dynamic and thereby recyclable, reprocessable, and self‐healing. The critical feature in CANs is the presence of dynamic covalent linkages within the network structure. A variety of such linkages are introduced into CANs, making the respective networks responsive to various stimuli, such as light, temperature, or pH. Here, CANs based on alkoxyamines as dynamic covalent bonds are reviewed. Alkoxyamines uniquely combine the ability to dynamically form, break, and reform covalent bonds with the possibility to initiate reversible‐deactivation radical polymerization. Polymer networks based on alkoxyamines are therefore both adaptive and quasi‐living, able to remodel the network structure by nitroxide‐exchange reactions (NER) and extend the network structure by nitroxide‐mediated polymerization (NMP). In this review, the concepts behind CANs are first introduced and the properties of nitroxides and derived alkoxyamines are discussed. A special focus is set on the ability to tune the response of alkoxyamines to different stimuli, through alteration of their structure. In addition, possible side reactions during dynamic bond exchange and limitations for polymerization are critically reviewed. Subsequently, examples of alkoxyamine‐based CANs responsive to different stimuli, such as temperature, light, or chemical triggers, are discussed. Properties and applications of CANs based on alkoxyamines are then discussed. Finally, an outlook is provided on challenges that need to be addressed as well as opportunities that lie within these “living” CANs.

Biofunctionalization of Metal–Organic Framework Nanoparticles via Combined Nitroxide‐Mediated Polymerization and Nitroxide Exchange Reaction

“…[22,23] Alkoxyamines that contain a thermo-labile C-ON bond combine the ability to dynamically break, form, and reform covalent bonds with the possibility to initiate reversible-deactivation radical polymerization. [24,25] Alkoxyamines on thermal homolysis generate persistent nitroxide radials that can be used for reversible nitroxide-exchange reactions in the presence of an additional nitroxide radical species or initiate a controlled polymerization of NMP process by monomer addition. Being able to avoid additional mediators or the need for catalyst removal and potential contamination by metals or thiol-based components, favors the use of NMP applying grafting from-method, especially for biological applications, as it is triggered by temperature and needs no further purification.…”

Lignin-Based Covalent Adaptable Network Polymers─When Bio-Based Thermosets Meet Recyclable by Design

“…1 Due to the strong crosslinking between the polymer chains, which is ensured by reactive functional groups on the monomer units, thermosets also display better mechanical properties and thermal stability compared to thermoplastics. 2 They do not melt or dissolve even under harsh conditions 3 and are prone to irreversible damage by high stress. Therefore, thermosets typically cannot be reprocessed or recycled, and after their useful life, disposal accompanied by consequential environmental pollution is usually the only option.…”

“…14 This broad range of bonds makes the so-formed networks more responsive to different types of stimuli, such as light, temperature, chemicals and/or pH. 3 When these are incorporated into resins forming the matrix material, they allow us to tune the properties of new and reprocessable thermosets for target applications.…”

“…Among the wide variety of reversible chemical linkages, the most common are reversible radical interactions, acylhydrazone bonds, boronate ester complexes, DA bonds, diselenide bonds, disulfide bonds, and imine (Schiff base) bonds . This broad range of bonds makes the so-formed networks more responsive to different types of stimuli, such as light, temperature, chemicals and/or pH . When these are incorporated into resins forming the matrix material, they allow us to tune the properties of new and reprocessable thermosets for target applications.…”

Covalent Adaptable Microstructures via Combining Two‐Photon Laser Printing and Alkoxyamine Chemistry: Toward Living 3D Microstructures

“…[43][44][45] The alkoxyamine bond is a particular example of dynamic covalent chemistry, as this bond offers the opportunity for nitroxide exchange reaction (NER) as well as nitroxide mediated polymerization (NMP), [46,47] incorporating dynamic, adaptive, and "living" properties to the polymeric networks. [41,[48][49][50][51][52][53][54][55] However, despite of its versatility, this type of dynamic covalent chemistry has not been explored yet neither in additive manufacturing in general, nor in 2PLP in particular. Creating 3D microstructures consisting of covalent adaptable networks will open the path for direct modification of their molecular structure, allowing modulation of cross-linking density and mesh size postprinting.…”