Light absorbing products during polymerization of methacrylate monomers photoinitiated with phenyl-1,2-propanedione/amine

The efficiency of a novel photoinitiator, phenyl-1,2-propanedione (PPD), of potential use in dentistry was assessed and the performance of PPD was compared with that of traditional photoinitiators used in dental restorative resins. The photodecomposition of PPD and the polymerization of acrylic monomers photoinitiated by PPD in the presence of dimethylamino ethylmethacrylate, ethyl-4-dimethylaminobenzoate and 4-(N,N-dimethylamino) phenethyl alcohol have been investigated. The photodecomposition of PPD/amine systems in acetonitrile and methacrylate monomers under irradiation at λ = 395 nm resulted in the appearance of light absorbing by-photoproducts. The absorbing species displayed absorption spectra which overlapped that of PPD and were markedly dependent on the amine structure. Irradiation of methacrylate monomers containing PPD in combination with 4-(N,N-dimethylamino) phenethyl alcohol resulted in a continuous increase in absorbance. Since the production of photoproducts increases with irradiation time, the light that reaches the deepest layers decreases. The effect of light screening is a reduced overall photoinitiation rate and consequently a reduced double bond conversion. From the results presented it emerges that the selection of the amine to be used in combination with PPD is a crucial step in order to optimize the efficiency of the PPD/amine photoinitiator system.