Effect of the Electron Donor Structure on the Shelf-Lifetime of Visible-Light Activated Three-Component Initiator Systems

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 Biphenyl 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 three-component 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 shelf-lifetime 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 (DH center dot(+)) 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. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114:1535-1542, 2009