Covalent Adaptable Networks Based on Dynamic Alkoxyamine Bonds

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.