Spatially resolved photoactivation of dynamic exchange reactions in 3D-printed thiol-ene vitrimers

The covalently cross-linked network structure of vitrimeric polymers is able to be reorganised by a thermoactivated exchange of covalent bonds. Despite numerous options available for steering the exchange rate in dynamic polymer networks, a spatiotemporal onset, macroscopically discernible as a drop in viscosity, is most often difficult to realise. Reported here is the application of a photolatent transesterification catalyst, which releases a strong guanidine base upon irradiation with 405 nm LED light. Incorporated in a visible-light-cured thiol-ene polymer matrix providing ample hydroxy and ester moieties, spatially resolved catalyst activation enables a selective rearrangement of the network topology via dynamic transesterification. Owing to the use of an efficient oligoacylgermane as radical photoinitiator, which absorbs at 450 nm, a wavelength-orthogonality between the light-mediated curing reaction (step growth polymerisation of the thiol-ene resin) and the light-induced cleavage of the photolatent base catalyst is achieved. Moreover, the fast cure rate allows a fabrication of objects by digital light processing 3D printing. Stress relaxation studies confirm excellent creep properties prior to network activation, i.e. in the presence of the unactivated catalyst, and a fast dynamic exchange of covalent bonds, once the catalyst is released by irradiation with 405 nm light. In addition, a spatially controlled activation of transesterification reactions in selected areas of the 3D-printed polymer structure is demonstrated by a reshaping experiment. To reobtain the shape stability of the unactivated polymer network and to preserve the (reorganized) topology, the active catalyst can be removed by a simple heat treatment.