Visible light activated coumarin photocages: an interplay between radical and organobase generation to govern thiol–ene polymerizations

Abstract

Photobase generators (PBGs) are an attractive tool for the latent and spatially governed formation of polymer networks with uniform topologies due to the anionic step-growth mechanistic pathway they unlock. Despite the significant advances made in PBG frameworks, utility in rapid, visible light driven polymer formation and associated structure–reactivity relationships remain scarce. Herein, five coumarinylmethyl PBGs bearing caged tetramethylguanidine (TMG) were synthesized and systematically examined for inducing thiol–ene polymerizations, while benchmarking against the classic ortho-nitrobenzyl (oNB) PBG framework. Quantification of photopolymerization kinetics and bond scission quantum yields with real-time Fourier transform infrared and steady-state UV-vis absorption spectroscopies revealed an increase in the disparity between CC and S–H conversion for derivatives halogenated at the 3-position. Alternatively, incorporation of a π-extended styryl moiety at the same position decreased the conversion gap and enabled uncaging with a blue LED (470 nm). This gap was attributed to the concurrent activation of radical chain-growth and base-catalyzed step-growth mechanisms, which was tempered through the (sub-)stoichiometric addition of tetramethylpiperidinyloxy (TEMPO). These findings paint a detailed picture of PBGs for thiol–ene polymerizations that will inform the selection and optimization of future light-activated catalysts and enable advanced manufacturing of tailored soft materials.