Phototriggered Base Amplification for Thiol-Michael Addition Reactions in Cross-linked Photopolymerizations with Efficient Dark Cure

In the present work, the photocatalytic activity was investigated toward a thiol-Michael reaction of different combinations of 9-fluorenylmethyl carbamate (Fmoc) derivatives and photocaged amines (PCA) as base amplifiers and the catalytic base, respectively. This phototriggering approach was systemati- cally studied for evaluating its effect on kinetics between thiol and Michael acceptors like acrylate or sulfone wherein, butyl 3-mercaptopropionate (BMP), 1-hexyl acrylate (HA), and ethyl vinyl sulfone (EVS) were used as model reactants. Interestingly, PCAs exhibited low quantum yields by themselves; NPPOC-Hex (2.5 mol %) which when used with BMP and HA, resulted in only 25% thiol conversion; however, when used along with Fmoc-Hex or Fmoc-TMG (2.5 or 5 mol %), it resulted in a higher thiol conversion of 50-60%. Furthermore, use of NPPOC-DEA (5 mol %) with 1 mol % Fmoc-TMG resulted in >70% thiol conversion for the same system. Upon using BMP and EVS nearly complete conversion of functional groups with 5 mol % NPPOC-DEA and 5 mol % Fmoc-DEA was obtained. This enhancement in reaction kinetics and conversion upon addition of an Fmoc derivative to a monofunctional thiol-Michael system was extended to multifunctional derivatives for polymerizing cross-linked polymer networks. Moreover, the kinetic study on model reactants also demonstrated efficient dark curing, resulting in 50-75% thiol conversion with only 30 s irradiation time, leading to validation of the efficacy of Fmoc derivatives and PCAs as photocatalysts for dark cure. Upon precise characterization in cross-linked systems using Raman spectroscopy for TMPTA/PETMP in the presence of 20 mol % NPPOC-DEA and 1 mol % Fmoc-TMG, the extent of dark cure was evaluated for a distance of 16.5 mm, which was observed to undergo maximum conversion and high dark cure propagation upon heating to 70 degrees C. Therefore, Fmoc-PCA catalysis is a practically useful approach for improving the photoinitiated efficiency of the thiol-Michael reaction and enabling photopolymerization in the dark with a marked improvement in photosensitivity.