Three-component initiators generally include a light-absorbing photosensitizer, an electron donor that is often an amine, and the third component, which is usually an iodonium salt. To characterize the role of diphenyl iodonium chloride (DPI) in three-component photoinitiator systems containing methylene blue (MB) as the photosensitizer, a systematic series of electron donors was used. The Rhem-Weller equation was used to verify the thermodynamic feasibility for photo-induced electron transfer from the electron donors to the MB. Comparison of the photopolymerization rates of each two-component initiator system (containing the photosensitizer and amine) to those of the corresponding three-component system (with the addition of (DPI) allowed fundamental information regarding the role of the DPI to be obtained. It was concluded that the DPI enhances the photopolymerization kinetics in two ways: (1) it consumes an inactive MB neutral radical and produces an active phenyl radical, thereby regenerating the original methylene blue, and (2) it reduces the recombination reaction of the MB neutral radical and amine radical/cation.

Three‐component photoinitiator systems generally include a light‐absorbing photosensitizer (PS), an electron donor, and an electron acceptor. To investigate the key factors involved with visible‐light activated free radical polymerizations involving three‐component photoinitiators and 2‐hydroxyethyl methacrylate, we used thermodynamic feasibility and kinetic considerations to study photopolymerizations initiated with either rose bengal or fluorescein as the PS. The Rehm–Weller equation was used to verify the thermodynamic feasibility for the photo‐induced electron transfer reaction. It was concluded that key kinetic factors for efficient visible‐light activated initiation process are summarized in two ways: (1) to retard back electron transfer and recombination reaction steps and (2) to use a secondary reaction step for consuming dye‐based radical and regenerating the original PS (dye). Using the thermodynamic feasibility and kinetic data, we suggest three different kinetic mechanisms, which are (i) photo‐reducible series mechanism, (ii) photo‐oxidizable series mechanism, and (iii) parallel‐series mechanism. Because the photo‐oxidizable series mechanisms most efficiently allow the key kinetic factors, this kinetic pathway showed the highest conversion and rate of polymerization. The kinetic data measured by near‐IR and photo‐differential scanning calorimeter verified that the photo‐oxidizable series mechanism provides the most efficient kinetic pathway in the visible‐light activated free radical polymerizations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 887–898, 2009

A series of kinetic experiments were conducted involving visible-light activated free radical polymerizations with three-component photoinitiators and 2hydroxyethyl methacrylate (HEMA). Three-component photoinitiator systems generally include a light-absorbing photosensitizer (PS), an electron donor and an electron acceptor. To compare kinetic efficiency, we used thermodynamic feasibility and measured kinetic data. For this study, 5,10,15,20-tetraphenyl-21H,23H-porphyrin zinc (Zn-tpp) and camphorquinone (CQ) were used as the PSs. The Rehm-Weller equation was used to verify the thermodynamic feasibility for the photo-induced electron transfer reaction. Using the thermodynamic feasibility, we suggest two different kinetic mechanisms, which are (i) photo-reducible series mechanism of CQ and (ii) photo-oxidizable series mechanism of Zn-tpp. Kinetic data were measured by near-IR spectroscopy and photo-differential scanning calorimetry based on an equivalent concentration of excited state PS. We report that the photo-oxidizable series mechanism using Zn-tpp produced dramatically enhanced conversions and rates of polymerizations compared with those associated with the photo-reducible series mechanism using CQ. It was concluded from the kinetic results that the photooxidizable series mechanism efficiently retards back electron transfer and the recombination reaction step. In addition, the photo-oxidizable series mechanism provides an efficient secondary reaction step that involves consumption of the dye-based radical and regeneration of the original PS.Recently, photopolymerization techniques are expanding into a number of growing application areas including optical fibers, video disc coatings, photolithographic resists and aspherical lenses, printing inks, coatings, adhesives, and contact lenses. 1-3 These extensive applications are due to its many advantages including rapid polymerization rates at ambient temperature, lower energy use than thermal polymerization, and solvent-free processes. These advantages have lead to the recognition of photopolymerization as an economical and environmentally friendly process. Additionally, the foremost driving force for the use of

We investigated the formation of ground state donor/acceptor complexes between xanthene dyes (rose Bengal (RB) and fluorescein (FL)) and a diphenyl iodonium salt (DPI) which is dissolved in 2-hydroxyethyl methacrylate (HEMA) monomer. To characterize the association constant of the complex, we have suggested a new analysis model based upon the Benesi-Hildebrand model. Because the assumption of the original Benesi-Hildebrand model is that the absorption bands are due only to the presence of the complex and that the absorption by the free component is negligible; the model cannot be applied to our systems, which is a dye-based initiator system. For each dye, the molar absorptivity of the ground state complex was evaluated as a function of wavelength and this analysis confirmed the validity of the modified Benesi-Hildebrand model. In addition, we observed the RB/DPI photoinitiator system failed to produce a perceptible polymerization rate but the FL/DPI photoinitiator system provided very high rates of polymerization. Based upon the association constant for these complexes, we concluded that the observed kinetic differences arise from the different association constant values of the ground state dye-acceptor complex, resulting in back electron transfer reaction.

In this study, we investigated the effect of electron donor structures on the shelf life of three-component initiator systems which also included methylene blue (MB) as a photosensitizer and diphenyl iodonium salt (DPI) as an electron acceptor. For this research, N-phenylglycine (NPG), N-methyldiethanolamine (MDEA), N,N-diisopropyl-3-pentylamine (DIPA), and 1,4-diazabicyclo[2.2.2]octane (DABCO) were used as electron donors, with different of proton transfer efficiencies and radical/cation persistence. To aid characterization of the shelf-life or dark storage stability of threecomponent initiator systems, the relative polymerization kinetic profile of each freshly prepared initiator system was first obtained using photo-DSC. The standardized photopolymerization reactions were repeated after various dark storage intervals. Thermal stability of each initiator system was compared by applying a ramped temperature program to monomer samples in the DSC in the dark. To analyze the kinetic changes as a function of storage time more quantitatively, we suggested an equation and characterized the shelflifetime constant (k) of three-component initiator systems.From the experiments and analysis, we conclude that the order of shelf-life is consistent with the of radical cation (DHÁ þ ) persistence; DABCO > DIPA > MDEA > NPG, and inversely related to the proton transfer efficiency of the electron donor; NPG > MDEA > DIPA > DABCO. The effects of electron donor structures on thermal stability were consistent with the results of kinetic shelf-life experiments. This investigation provides an effective means to characterize as well as predict shelf lifetimes of initiator systems.