Upconversion-Nanoparticle-Assisted Radical Polymerization at λ = 974 nm and the Generation of Acidic Cations

Upconversion nanoparticles (UCNPs) sensitised photolytic cleavage of the blue/UV photoinitiator bis(4-methoxybenzoyl) diethyl germanium (1). 1 initiated radical photopolymerisation in combination with NaYF4:TmYb@NaYF4 UCNPs. A lambda=974 nm near-infrared (NIR) laser served as the excitation source to generate blue and UV light. In addition, a metal-free photo-ATRP system (ATRP=atom transfer radical polymerisation), composed of iso-propyl thioxanthone (ITX), N,N,N',N '',N ''-pentamethyldiethylenetriamine (PMDETA) and -bromo(iso-butyl) ethylester (alpha-BrBuEt) was used instead of 1. This resulted in a significant smaller polydispersity compared to the UCNP/1 system. Control of polymerization was successfully demonstrated by means of a chain extension experiment showing control of chain termination. For the first time, we demonstrated photolytic formation of acidic cations in such NIR UCNP systems composed of an ITX/iodonium photoinitiator for the sensitized generation of acidic cations. In addition, 1 also resulted in crosslinking of 1,6-hexanediol diacrylate (HDDA) in combination with UCNPs and NIRlaser excitation. UVfilter materials such as TiO2 do not significantly interfere with crosslinking.