A mechanistic investigation of a three-component radical photoinitiator system comprising methylene blue, N-methyldiethanolamine, and diphenyliodonium chloride

Three-component systems, which contain a light-absorbing species (typically a dye), an electron donor (typically an amine), and a third component (usu ally an iodonium salt), have emerged as efficient, visible-light-sensitive photoinitiators. Although three-component systems have been consistently fo und to be faster and more efficient than their two-component counterparts, these systems are not well understood and a number of distinct mechanisms h ave been reported in the literature. In this contribution, photodifferentia l scanning calorimetry and in situ, time-resolved, laser-induced, steady-st ate fluorescence spectroscopy were used to study the initiation mechanism o f the three-component system methylene blue, N-methyldiethanolamine and dip henyliodonium chloride. Kinetic studies based upon photodifferential scanni ng calorimetry reveal a significant increase in polymerization rate with in creasing concentration of either the amine or the iodonium salt. However, t he laser-induced fluorescence experiments show that while increasing the am ine concentration dramatically increases the rate of dye fluorescence decay , increasing the DPI concentration actually slows consumption of the dye. W e concluded that the primary photochemical reaction involves electron trans fer from the amine to the dye. Pie suggest that the iodonium salt reacts wi th the resulting dye-based radical (which is active only for termination) t o regenerate the original dye and simultaneously produce a phenyl radical ( active in initiation) derived from the diphenyliodonium salt. O 2000 John W iley & Sons, Inc.