Frontal Polymerization of Covalent Adaptable Networks

Frontal polymerization is a curing method that is known for its high conversion, short reaction times and low energy consumption. However, the resulting materials are typical thermosets, allowing no reprocessing, reshaping nor recycling. Herein, a new approach is pursued, which combines the energy efficiency of frontal polymerization with the unique post-processability of covalent adaptable networks. Thus, selected thiol-acrylate resins, bearing a sufficiently high number of ester linkages and free hydroxyl groups, were investigated, using phosphate esters as transesterification catalysts. The amount of phosphate ester and thiol was varied and its influence on material properties and frontal polymerization kinetics were analyzed. The reaction kinetics were studied with FTIR and photo-DSC measurements, showing a trend towards lower reactivity and higher conversions with an increased thiol content. The obtained networks exhibited tunable bond exchange rates by varying either the amount of thiol or of the catalyst. DMA measurements reveal a higher network homogeneity with increasing thiol content. Moreover, reprocessing, recycling as well as reshaping of the material was successfully demonstrated. Concludingly, these findings could significantly lower energy consumption and increase circularity in future thermoset production.