A novel ferrocenium salt as visible light photoinitiator for cationic and radical photopolymerization

A novel hybrid photoinitiator for visible light photopolymerization, (η⁶-3-benzoyl-4-chlorodiphenylamine) (η⁵-cyclopentadienyl) iron hexafluorophosphate (Fc-NBP), was synthesized and studied. Its absorption in the UV and visible light regions showed much stronger activity than those of either the commercialized cationic photoinitiator I-261 or the conventional free radical photoinitiator benzophenone, especially above wavelengths of 350 nm. When exposed to visible light, the photoinitiator under study initiates both cationic polymerization and radical polymerization. The photoinitiator's abilities in the photopolymerization of acrylates and epoxides were evaluated by near infrared (NIR) spectroscopy. The results from NIR clearly indicate that Fc-NBP exhibited high efficiency in photopolymerizing acrylate monomers. In the same lamp, however, benzophenone showed no photoinitiating ability. The photopolymerization rate of the diglycidyl ether of the bisphenol-A epoxy (DGEBA) oligomer was found to be slower than that of acrylates when using Fc-NBP as the photoinitiator. This study shows that the polymerization of epoxide DGEBA can be speeded up by adding a photosensitizer benzoyl peroxide (BPO), but BPO and tertiary amines do not affect the free radical photopolymerization of tripropylene glycol diacrylate (TPGDA). We conclude by providing a possible photoinitiation mechanism.