A new high-sensitive photo-radical initiator, N-[2-(2-acryloyloxyethoxy)ethyl]-1,8-naphthalimide (NI6), with good thermal stability based on naphthylimide derivative was developed. NI6 was prepared by the condensation of N-[2-(2-hydroxyethoxy)ethyl]-1,8-naphthalimide and acryloyl chloride in the presence of 4-dimethyl-aminopyridine. The film consisting of NI6 and pentaerythritol triacrylate (PETA) showed higher photosensitivity than those containing conventional photo-radical initiators such as acrylic acid 2-(2-{2-[2-(4-benzoyl-phenoxy)-ethoxy]-ethoxy}-ethoxy)ethyl ester, cyclohexylmaleimide, and the resulting film exhibited very high transmittance over 400 nm. The thermal stability of NI6 was very high and no decomposed residues were observed from the film consisting of NI6 after heating at 250 degrees C for 1 h. (c) 2005 Wiley Periodicals, Inc.