Charge Transfer Complexes as Dual Thermal and Photochemical Polymerization Initiators for 3D Printing and Composites Synthesis

The development of visible light 3D printing technology and the preparation of high-tech composites under mild conditions remain a huge challenge. This article is devoted to the development of dual thermal/photochemical initiators both for visible light 3D printing and composite preparation under mild conditions. Dual thermal and photochemical polymerization initiators are presented, a dual ability scarcely encountered in the literature. In detail, here, a series of charge transfer complexes (CTCs) that are demonstrated to be formed between phosphines (electron donors) and iodonium salt as electron acceptor are investigated as dual thermal and photochemical free radical polymerization (FRP) initiators. The CTCs exhibit excellent photo and/or thermal initiating properties for the benchmarked methacrylate resin and can be efficiently used for the curing of glass fibers or carbon fiber composites. To the best of our knowledge, this is the first access to high-tech carbon fiber composites using dual photo/thermal initiators. The formations of the CTCs were confirmed by several techniques, such as P-31 NMR, UV-vis, and molecular orbital (MO) calculations. Laser write experiments at 405 nm show excellent spatial resolution performances. These systems are commercially available, stable, and environmentally friendly. Additionally, safer storage of the thermal initiators is expected (as formed in situ) compared to traditional thermal initiators (e.g., this is highly worthwhile for the replacement of peroxides). Dual approaches (photo/thermal curing) can also be used, as the same initiating system can initiate both polymerization processes, e.g., fast curing by light of the surface associated with a dark thermal curing in depth.