Evaluation of Aromatic Thiols as Photoinitiators
Dillon M. Love, Benjamin D. Fairbanks, Christopher N. Bowman
Abstract
The unique photodynamics of aromatic thiols (relative to alkyl thiols) allowed their employment as effective ultraviolet and visible light photoinitiators (PIs) for acrylate photopolymerizations, radical-mediated thiol–ene coupling (TEC) network polymerizations, solventless and initiatorless small-molecule TEC reactions, and hydrogel network polymerizations. Twenty-six thiol structures were evaluated as radical generating PIs in the polymerization of n-hexyl acrylate. Initiator effectiveness follows the trend nonheterocyclic aromatic thiols > heterocyclic aromatic thiols ≈ thioacids > alkyl thiols. Substituted versions of thiophenol exhibited the highest photoinitiation efficiency with electron-withdrawing substituents increasing effectiveness. Ortho- and para-substituted mercaptobenzoic acids and (trifluoromethyl)thiophenols were generally the most effective, leading to 100% acrylate conversion at a loading of 3 mM (0.1 mol % with respect to acrylate) when irradiated with 320–390 nm light and at loadings of 30 mM when irradiated with 405 nm light. Results demonstrate the potential of aromatic thiols as oxygen insensitive photoinitiators for bulk polymerizations and for specialty polymer synthesis via aromatic thiol-functionalized macroinitiators.