Photoinitiation of free-radical polymerization by aryl disulfide, cyclic anhydride, and phthalimide derivatives
Photoinitiators, crucial to UV-curable coatings, represent a large portion of the total UV-curable coatings market and are thus worthy of investigation. In this dissertation, three classes of compounds, aryl disulfides, maleic anhydrides, and phthalimides, were evaluated for their potential as photoinitiators of free-radical photopolymerization. Several techniques were used to characterize the initiation capacity for each type of compound. Photo-differential scanning calorimetry was used to obtain relative rates of polymerization for each initiation system. Examination of each class of compound proceeded by determination of their initiation efficiency in the presence of monomer only, in the presence of a tertiary amine, and in the presence of a sensitizer and tertiary amine. Furthermore, the photo-differential scanning calorimetry was performed using both the full arc of a medium pressure mercury lamp and at equal absorbance of each sensitizer at a wavelength of 365 nm. The full arc experimentation indicated that the initiator/amine combination initiated polymerization with a rate less than that of a standard Type II photoinitiator. However, addition of a sensitizer to the initiator/amine mixture produced an enhanced rate of polymerization relative to the Type II photoinitiator standard. When compared to a Type I (cleavage) photoinitiator at equal absorbance at 365 nm, the three component initiating system (initiator/sensitizer/amine) yielded approximate polymerization rates of 50% (anhydride), 40% (phthalimide), and 30% (aryl disulfide) than that of the Type II photoinitiator standard. Quenching studies were also performed to determine the degree to which the maleic anhydride and phthalimide derivatives interact with several sensitizers. Maleic anhydride quenches the excited triplet state of each sensitizer examined with a rate constant comparable to the quenching of the sensitizers by a tertiary amine. However, the phthalimides quenched the excited triplet state sensitizers at varying rates depending on the triplet energy of the phthalimide. The triplet energies of the phthalimide derivatives examined in this research, N-phenylphthalimide, N-(3,4-dimethoxyphenyl) phthalimide, and N-(3,4-dicyanophenyl) phthalimide, were determined via phosphorescence to be 67.6 kcal/mol, 61.1 kcal/mol, and 68.6 kcal/mol, respectively. Finally, probable initiation mechanisms for both the maleic anhydride derivatives and the phthalimide derivatives are proposed. The maleic anhydride derivatives are proposed to initiate acrylate polymerization via a dual mechanism involving photosensitization and "chemical sensitization." However, the phthalimides, depending on their respective triplet energies and the concentrations of the phthalimide and amine present, predominately initiate acrylate polymerization via "chemical sensitization."