Vat photopolymerization 3D printing with light-responsive thiol-norbornene photopolymers

The present study highlights the synthesis of a photosensitive norbornene monomer containing an o-nitrobenzyl ester (o-NBE) group, which undergoes controlled bond cleavage upon UV exposure. As the o-NBE chromophores are transparent in the visible light region, photopolymers are readily formed by visible light-induced thiol-ene chemistry using a long-wavelength absorbing photoinitiator. Selective photolysis of the o-NBE moieties is then obtained by irradiation with light in the UV-A spectral region. The sequence-dependent orthogonality of the curing and cleavage reaction are confirmed by FT-IR spectroscopy. Owing to their constrained ring system, norbornene moieties are highly reactive in radical-induced thiol-ene reactions, which enables the vat photopolymerization 3D printing of functional structures with reasonable build speed. Once printed, the wettability of the 3D objects' surface was conveniently tuned by UV-induced cleavage of the o-NBE groups (release of free carboxylic acids) and related photo-oxidation of the non-reacted thiol moieties (formation of sulfonic acids). With this approach, we were able to introduce wettability gradients onto 3D-printed objects, which is interesting for microfluidic devices and lab-on-chip technologies.