Photosensitized Formation of Phosphorus-Centered Radicals: Application to the Design of Photoinitiating Systems

The processes involved in the photosensitized decomposition of phosphorus-containing compounds in the presence of a photoinitiator PI were investigated and characterized by laser flash photolysis, electron spin resonance, and molecular modeling (density functional theory calculations). They lie on (i) a photoinitiator/phosphonis-containing compound electron transfer from phosphinites a, b and phosphites c, leading to a phosphoniumyl radical ion R3P center dot+ or a photoinitiator/phosphorus-containing compound hydrogen abstraction reaction from labile P H bonds in phosphine oxides d, phosphonates e, and trialkylphosphine salts f, g and (ii) the subsequent production of phosphorus centered radicals P-center dot in a e and a radical ion P center dot+ in f, g. The interaction rate constants for the key processes (photoinitiator/phosphorus-containing compound, alkoxyl radical/phosphorus-containing compound, and peroxyl radical/phosphorus-containing compound), the associated transient absorption, and electron spin resonance spin trapping spectra were determined. The recorded acrylate polymerization profiles show that the photoinitiator/phosphorus-containing compound (a-g) systems are often efficient. Compared to reference systems, they can lead, in some cases (with benzophenone or a thiopyrylium salt as photoinitiator), to similar or even better polymerization rates. The cationic photopolymerization of epoxides was also feasible with f, g, thereby demonstrating that photoinitiator/f (or g) couples behave as dual radical/cationic photoinitiating systems.