Photolabile ROMP gels using ortho-nitrobenzyl functionalized crosslinkers

This paper describes a series of four ortho-nitrobenzyl substituted bis(norbornene) crosslinkers that are suitable for the preparation of photoresponsive organogels and hydrogels through ring-opening metathesis polymerization. As measured by several techniques - visual inspection, rheology, and release of network-bound polymers - organogels preapred using these four crosslinkers varied in their sensitivity to UV irradiation by about two orders of magnitude. The reactivity of the gels show qualitative correlation with the stability of the intermediate benzylic radicals. Gels with larger crosslink densities required longer UV irradiation time to dissolve than gels with smaller crosslink densities. Together, these results demonstrate how rational changes to the structures of crosslinkers and gels can tune photosensitivity. Finally, we show a proof-of-concept photochemical release experiment of a physically entrapped fluorescent polymer from a photolabile ROMP hydrogel.