Disulfide-containing monomers for dynamic UV-cured networks

Photopolymerization is a well-established technology which has gained importance over the past years for its environmental-friendly and sustainable features. Thanks to the low-energy need and fast-curing processes, UV-curable materials are nowadays one of the main choices in applications such as inks and coatings production. Despite excellent surface and bulk properties, the non-recyclability of the crosslinked coatings has been recognised as an issue by several organizations, due to the increasing attention to environmental regulations. Dynamic covalent networks (DCN), combining the benefits of thermosets and the re-processability of thermoplastics, have acquired relevance during the last few years as a possible solution to overcome the recyclability issue. Recently, dynamically crosslinked linear disulfide networks have attracted a lot of interest, in sectors like self-healing and targeting applications , . Indeed, disulfide bonds are known to be responsive to plenty of external stimuli, like pH, light, temperature, and redox conditions , and cleavage of disulfide with the formation of thiyl radicals could be easily achieved. Based on these distinctive features, the introduction of disulfide-containing crosslinkers into UV-curable networks was studied as a first step for developing a reversible photopolymerizable coating. Due to the wide field of application of photopolymers, both cationic and radical photocrosslinking mechanisms were tested. In the former case, 2-hydroxyethyl disulfide (HEDS) was added in an epoxy formulation, exploiting the addition mechanism between the diol and the epoxy ring for the creation of a polymeric network. Then in the latter, following the work of Zhang et al. (2020), a disulfide-containing diacrylate monomer was synthesized starting from HEDS and crosslinked through a radical photopolymerization reaction.