Photopolymerization and initiating mechanism of Michael addition oligomers without photoinitiator

A series of acrylic oligomer which can undergo self-initiation under UV irradiation were synthesized by Michael addition of acetylacetone (acac) with multi-functional acrylates using diazabicycloundecene as catalyst. GC-MS and proton NMR were employed to study the photolysis mechanism of model molecule which was prepared from acac and ethyl acrylate. The β-diketone moiety attached to a fully substituted carbon atom was photoscissile and prone to produce alkyl free radicals by losing acetyl free radical. Polymerization of the acrylated β-diketone-containing oligomers can be thus efficiently triggered regardless of solely irradiated under UV light or blended with pure trimethylolpropane triacrylate. As determined with photo-DSC, the acrylated β-diketone-containing oligomers photopolymerized more slowly than the traditional system of trimethylolpropane triacrylate containing Darocur 1173 at the early stage, but reached a higher final conversion. The rheology measurement showed that the synthesized acrylated β-diketone-containing oligomers exhibited shear thinning property. POLYM. ENG. SCI., 47:1082–1090, 2007. © 2007 Society of Plastics Engineers