Investigation of Barton esters as radical photoinitiators

Different photoinitiators based on Barton thiohydroxamic esters,O‐acyl‐N‐hydroxy‐pyridine‐2(1H)‐thione andO‐acyl‐N‐hydroxy‐thiazole‐2(3H)‐thione derivatives, were tested in photopolymerization reactions through time‐resolved FTIR spectroscopy. Good rates of polymerization and final monomer conversion were obtained for some compounds. The excited state processes, investigated by time resolved absorption spectroscopy, lie on a fast singlet state cleavage leading to thiyl and alkyl radicals. Both the pyridine‐2(1H)‐thiyl and alkyl radicals are able to initiate a polymerization, in contrast with the thiazole‐2(3H)‐thiyl radical. A triplet state is observed for some derivatives. Computational studies help to describe the excited state properties and show a strong difference in the spin localization in the formed thiyl initiating radicals.