Synthesis of tetramethylguanidine-based photobase generators: light-guided dynamics in thioester networks

Abstract

Covalent adaptable networks are crosslinked structures with bonds that undergo reversible exchange reactions when exposed to an external stimulus, enabling a reorganization of their topology. The incorporation of photolatent catalysts allows precise tuning of viscoelastic properties and on-demand activation. Herein, the local control of dynamic bond exchange reactions in thiol–ene resins, bearing thioester linkages and free thiol groups, was investigated, by synthetizing numerous tetramethylguanidine (TMG) based photobase generators with varying UV-Vis absorption profiles to improve the orthogonality between curing reaction and photobase activation. UV-induced activation of the photolatent base yields a strong guanidine base, which is able to efficiently catalyze thiol–thioester exchange reactions. Rheological studies, combined with pH measurements upon UV activation of the bases, identified the most promising photolatent base 1,1,3,3-tetramethylguanidine 2-(naphthalen-2-yl)-2-oxoacetate (TMG-NOA). In addition to light, also temperature can be used to activate TMG-NOA that further increases its applicability. The influence of the developed photobases on the material properties and the curing kinetics were investigated using FTIR- spectroscopy, TGA and DSC measurements. Utilizing the network containing TMG-NOA, the spatially controlled activation of the thiol–thioester exchange mechanism in selected areas was demonstrated at temperatures between 40–70 °C through local imprinting experiments, photochemical drawing and thermally induced healing.