Method: The photoinitiator system consists of methylene blue (MB+; dye absorbing at 650nm), diisopropylethylamine (DIPEA; reductant), and diphenyliodonium salt (DPI+; oxidant). 2-Hydroxyethyl methacrylate (HEMA) was used as monomer with camphorquinone (CQ) and ethyl 4-N,N-dimethylaminobenzoate (EDMAB) as the initiator control. Monomer conversion kinetics were followed by FTIR, initiator consumption was determined by UV/Vis and photopolymerizations were triggered with appropriate red or blue light for the experimental and control systems, respectively.
Result: When MB+ is exposed to 650 nm light in the presence of DIPEA, rapid photo-bleaching produces neutral, colorless leuco-methylene blue (LMB) through an unusual two-electron, one-proton transfer without radical formation. On time-scales that extend for hours, LMB is reoxidized to MB+ by the DPI+ with production of phenyl radicals that initiate latent polymerization. In comparing post-cure potential, photopolymerizations of HEMA were interrupted at 5% conversion and then stored in the dark with final conversion measured at 85±1% with MB+ versus 10±1% with CQ. A photo-masked exposure of HEMA that extended only 170±130μm into the shadowed region with CQ initiation provided 3.7±0.7mm shadow cure with MB+ due to diffusion of LMB into the shadow region. At equal absorbed photon doses, a 1.2cm thick HEMA sample was uniformly polymerized with MB+ photo-activation while the CQ analog remained liquid monomer.
Conclusion: Compromised light access and significant attenuation within materials are issues that complicate dental material photocuring. While there are obvious practical concerns with MB+ as a photoinitiator, the unique reaction mechanism and photocuring behavior demonstrated for this photo-activated redox process further expands the versatility of photopolymerization.