Evaluation of N-aromatic maleimides as free radical photoinitiators: A photophysical and photopolymerization characterization

Photopolymerizable compositions were prepared using acrylate monomers in co mbination with various N-aromatic maleimides. N-aromatic maleimides were se gregated into two groups: those that could adopt a planar conformation and those that could not adopt a planar conformation. The maleimides were chara cterized using single-crystal X-ray diffraction spectroscopy, laser flash p hotolysis spectroscopy, UV-vis absorption spectroscopy, and photodifferenti al scanning calorimetry. Planar N-aromatic maleimides were found to have a low relative excited-state triplet yield, showing significant shift of the primary maleimide UV absorption band with changes in solvent polarity, and did not initiate free radial polymerization upon direct UV excitation. Twis ted N-aromatic maleimides have a higher relative triplet yield, show neglig ible shift of the primary maleimide UV absorption band, with solvent polari ty, and initiate free radical polymerization upon direct excitation. Additi on of benzophenone was found to dramatically increase the initiation effici ency of both planar and twisted N-aromatic maleimides to levels approaching that of conventional cleavage photoinitiators.