Structure-reactivity relationships of novel monomeric photoinitiators

• Two different groups of methacrylates based on Irgacure 2959, Irgacure 184, benzophenone and acetophenone were synthesized.
• Photopolymerization efficiency of the initiators was determined using photo-DSC.
• The effect of polymerizable group structure on photoinitiation efficiency was found to be minimal for fast polymerizing systems.
• Irgacure 2959, Irgacure 184 and BP-based MPIs showed similar or increased activity compared to their commercial analogs.

Two different groups of methacrylates containing Type I or Type II photoinitiating moieties were synthesized and evaluated for use in UV curing applications. The first, a novel group of monomeric photoinitiators (MPIs) were synthesized by reactions of tert-butyl α-bromomethacrylate (TBBr) with 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (Irgacure 2959), 1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 184) and 4-hydroxyacetophenone to give monomers MPI1, MPI3 and MPI4 respectively; and conversion of MPI1 to MPI2 by cleavage of tert-butyl ester groups with trifluoroacetic acid. The second group of photoinitiators were synthesized by reaction of 2-isocyanatoethyl methacrylate (IEM) with Irgacure 2959 (MPI5) and 4-hydroxybenzophenone (MPI6). All these MPIs’ absorption range in the UV region was found to be similar to their nonmonomeric analogs. Their photoinitiating abilities in the polymerizations of hexane-1,6-diol diacrylate (HDDA), 2-hydroxyethyl methacrylate (HEMA) and trimethylolpropane triacrylate (TMPTA) were studied using photodifferential scanning calorimeter and the kinetic parameters were correlated with the structures of the photoinitiating systems. Photoinitiating activities of investigated Type I photoinitiators, including small molecule commercial analogs, during polymerizations of HDDA and TMPTA are very slightly influenced by MPIs structure. However, the MPI structure is found to be important for the curing of HEMA, and some of the synthesized MPIs have better efficiency than commercial ones tested. The MPIs based on Irgacure 2959, Irgacure 184 and benzophenone (BP), with similar or increased photoinitiating activity compared to their commercial precursors, appear to be promising photoinitiators. An extra advantage of MPIs is their incorporation into the final formulations, reducing undesired migration of small molecules.