Direct Evidence for the Sulfonium-Mediated Photopolymerization of 1,2-Dithiolanes

Light-induced, initiatorless homopolymerization of 1,2-dithiolanes is widely attributed to radical propagation. However, several experimental observations, including copolymerization behavior and EPR measurements, are inconsistent with this assignment. Here, this reaction is examined using in situ photoNMR, revealing first-order monomer consumption that is incompatible with a propagating radical mechanism. PhotoNMR further enables direct observation of a sulfonium intermediate attributed to a charge-transfer complex, corroborated through steady-state and transient absorption spectroscopy. Together, these data establish an activated-monomer-type polymerization, proceeding through a cationic rather than radical pathway. This same intermediate also enables controlled chain growth and is leveraged as a photoinitiator for cationic vinyl ether polymerization, extending the scope of dithiolane photochemistry beyond homopolymer formation and opening new opportunities for dithiolane-based photopolymerization strategies.