Red light activatable photobase generator for rapid thiol-nucleophilic polymerization

Photobase generators (PBGs) are molecules capable of releasing reactive bases when exposed to a photonic field. These compounds have been increasingly used as photolatent catalysts for light-induced polymerization processes such as ionic polymerization and ring opening polymerization. However, most PBGs require UV light trigger, with inherent limitations of low penetration depth, harmful irradiance, and high equipment cost. Herein, we introduce a long-wavelength visible-light responsive PBG based on a tetramethylguanidine (TMG) conjugated with a photoremovable methylene blue moiety. Under irradiation of red light (lambda max = 660 nm), TMG is rapidly released, enabling the initiation of thiol nucleophilic addition to a range of electrophilic substrates including (meth)acrylate, propiolate, and glycidyl groups. The photopolymerization was demonstrated by red lightinduced crosslinking of resins, prepared by mixing pentaerythritol tetrakis(3-mercaptopropionate) with poly (ethylene glycol) functionalized with electrophilic substrates. We further highlight the advantage of long wavelength visible light initiation by photopolymerization behind a tissue spacer, and fabrication of thick (up to 5 cm) plastic samples.