Recyclable epoxy vitrimers based on imidazolium-catalyzed dynamic ester cross-links

Thermosets, and epoxy resins in particular, constitute an important class of materials on the global polymer market that are valued in plethora of applications requiring superior mechanical properties as well as thermal and chemical stability. As downside, however, they can hardly be recycled and reprocessed due to their permanent cross-linked architecture. In response to this concern, adaptable covalent networks featuring dynamic cross-linking based on associative exchange reactions, also called vitrimers, emerged some years ago. Above a certain temperature, the kinetic of the exchange reactions increases significantly, the topology of the network changes and the material can be reshaped or healed. In the case of epoxy-acid vitrimers, the relaxation of the network relies on transesterification exchanges promoted by catalysts such as zinc carboxylates, organotin compounds, triphenylphosphine, etc. Despite the recognized activity of the latter, the search for alternative non-toxic catalysts with high efficiency, low leaching tendency as well as heat and air sensitivity is still relevant.
As will be discussed in the communication, driven by the attractive properties and catalytic activity of ionic liquids (ILs), we designed epoxy-acid resins in the presence of ILs for the first time and produced unprecedented imidazolium-catalyzed epoxy vitrimers. In addition to outstanding features such as chemical and thermal stability, low vapor pressure, high conductivity, imidazolium salts are known as very efficient homogeneous and heterogeneous catalysts especially in transesterification reactions. In practice, a series of cross-linked epoxy-acid networks were synthesized from diepoxy and diacid compounds cured in the presence of imidazolium acetate catalysts obtained via a single-step and atom economical Radziszewski multicomponent reaction. The impact of the imidazolium on the course of the curing reaction, the structure of networks and the mechanical properties, will be discussed. The vitrimer-like properties of these IL-containing networks, and so the ability of the imidazolium additives to catalyze the transesterification exchange reactions, will also be demonstrated via stress relaxation experiments. The effect of the nature and concentration of the imidazolium derivatives on the network dynamics will also be discussed. Finally, it will be shown that such imidazolium-containing resins can be recycled several times while preserving their excellent mechanical properties.
In addition to demonstrate the capacity of ILs to play the role of catalysts in the field of adaptable networks, this contribution also emphasizes that a low amount of imidazolium within ester networks is likely to induce some relaxation. Moreover, given the great variety of ILs and the large set of reactions they can catalyze, this approach should be applicable to adaptable networks governed by other types of crosslinking bonds and could contribute to the recycling of various polymer resins in the future.