Chemically recyclable crosslinked thiol-ene photopolymers via thiol-disulfide exchange reactions

Conventional crosslinked photopolymers possess permanent network structures, which cannot be remolded/recycled and generally discarded as waste after use. Research has begun to incorporate dynamic bonds into crosslinked photopolymers to make them reprocessable/recyclable, however, previous studies typically require tedious pre-syntheses to obtain photoreactive building blocks that contain desired dynamic linkages/functionalities. In this study, we report a simple, one-step, and scalable synthesis of chemically recyclable crosslinked thiol-ene photopolymers that contain dynamic disulfide bonds from commercially available building blocks. Specifically, liquid polysulfides containing both reactive thiol end groups and internal disulfide bonds, together with multifunctional alkenes, are selected as the building blocks to simultaneously incorporate disulfide bonds during network formation via thiol-ene photopolymerization. The incorporated dynamic disulfide bonds allow these thiol-ene networks to be chemically recycled into photoreactive thiol oligomers through base-catalyzed thiol-disulfide exchange reactions. The resulting thiol oligomers can be effectively reused, together with the original multifunctional alkenes, for the next-generation syntheses of crosslinked thiol-ene photopolymers with chemical compositions and material properties that are nearly identical to those of the originally crosslinked sample. This chemical recycling process can be repeated many (infinite in theory) times to produce recycled thiol-ene photopolymer networks with full property retention. Overall, the unique chemistry demonstrated in this study could potentially provide a route towards a circular economy of crosslinked photopolymers.