Additive-free Covalent Adaptable Networks – AFCAN

Since their inception in 2011, covalent adaptable networks (CANs), which are permanent organic networks with glass-like fluidity, have initiated a paradigm change in the field of polymer science. Indeed these CANs (also called vitrimers) are structurally similar to thermosets, but the linkages between polymer chains are able to exchange, leading to an interesting dynamic crosslinking and polymer reshaping. These CANs may also have self-healing properties and open up new routes for recycling strategies and materials applications such as polymer welding. Hence, these CANs correspond to the expected cornerstone between thermosetting and thermoplastic materials, highly desired by both academic and industrial communities. The most emblematic vitrimer examples are polyester networks, possessing free hydroxyl groups, thermally reshaped via Zn-catalyzed transesterification. However, despite simple exchange reactions, the main issue of these CANs consists in the high quantity of required catalysts for fast transesterification. Indeed, not only the material ageing and the catalyst leaching present environmental impacts, they also entail the loss of properties.
Hence, the AFCAN project proposes two different complementary approaches to design catalyst-free CANs, by targeting two new types of materials: The first one concerns the design of new catalyst-free CANs by either transesterification or transamidation. The second one concerns coordination CANs in which metals are present as intimate constituents of the network and not as free additives. In both cases, the exchange reactions at the heart of the network adaptation properties will be activated or accelerated by fluorine atoms at the a- or ß-positions relative to the carbonyl function.
The AFCAN project relies on the influence of fluorination onto the electrophilicity of a neighbouring carbonyl group. Indeed, the very strong electron-withdrawing ability of a trifluoro-, difluoro- or even fluoromethyl group considerably enhances the reactivity of a vicinal carbonyl function towards the attack of nucleophiles, including weak ones. Indeed, this increased electrophilicity results from a decrease of the electron density on the carbon atom and from a sensible LUMO stabilisation. This reactivity enhancement is also observed for fluorinated esters, which are much more prone to nucleophilic attack than their non-fluorinated counterparts.
Thus, the objectives of the AFCAN project address several challenges, at different levels of scientific maturity and novelty. The first objective consists in the synthesis and study of catalyst-free fluorinated transesterification vitrimers. These materials are expected to display improved transesterification rates and reduced ageing. They will directly have impact on potential industrialization. Secondly, the AFCAN project will interestingly apply these improved catalyst-free vitrimer chemistry to the synthesis of polyhydroxyurethane (PHU) CANs. Indeed, PHU correspond to highly desired isocyanate-free polyurethanes. The literature reports only very limited studies on PHU vitrimers, obtained only via transcarbamoylation. Moreover, since the PHU synthesis suffers from low reactivity, only PHU thermosets have promising applications. Hence, the AFCAN approach is highly innovative and will advantageously valorise catalyst-free transesterification to design novel PHU vitrimers, allowing easy (re)processing of PHU thermosets. Moreover, the AFCAN project will extend this fluorinated approach to transamidation. This part is also highly innovative since the objective is to study catalyst-free transamidation reaction to open route to polyamide vitrimers. This part is a real piece of innovation since only a handful or study report polyamide thermosets. Finally, the AFCAN project aims at the synthesis and study of unprecedented coordination vitrimers containing metals as cross-linkers.