Studies on NIR sensitized radical photopolymerization in multifunctional monomers

This thesis describes Near Infrared (NIR) initiated photopolymerization of multifunctional acrylic esters with NIR-LEDs emitting between 750 – 870 nm. These systems comprise a zwitterionic cyanine as sensitizer and an iodonium salt with distinct substitution pattern in both the cation and anion. An optically open window from the UV into the visible part allows embedding of functional materials covering particular this spectral range. Reactivity was studied with a synchronized photo-DSC comprising a self-designed hardware setup for data acquisition. 28 new iodonium salts ([A-I-B]+X-) were studied to show the influence of structural pattern on reactivity. [A-I-B]+X- possesses a different substitution pattern for the cation and the anion, respectively. Electron transfer of the excited state of the polymethine to the iodonium cation bearing different anions such as benzilate, lactate, NO3-, PF6-, SbF6-, p-CH2=CH-Ph-SO3-, p-C12H15-Ph-SO3-, CF3SO3-, C4F9SO3-, B(CN)4-, B(Ph)4-, B(PhF5)4-, N(CN)2- or (SO2-CF3)2N- results in initiating radicals. Although most of these anions do not participate in the redox mechanism, there exists a certain dependence between reactivity and the nature of the anion. Anions with low coordination behavior ((SO2-CF3)2N-, B(Ph)4-,B(PhF5)4-, B(CN)4-) result in acceptable reactivity. This may be explained by the conductivity of the salt in the acrylate monomers such as hexane-1,6-diol diacrylate, tripropylene glycol diacrylate, poly(ethylene glycol) diacrylate and trimethylolpropane triacrylate being different. A high ion mobility leads connects always to a high conductivity. Thus, complete dissociation is necessary to obtain a reactive system. The solubility of the different iodonium salts covers several g/L up to well mixable systems (>2000 g/L). A good solubility is not mandatory to obtain a high reactivity but it is desirable from a practical point of view. Particular the bis(trifluoromethylsulfonyl) imide anion (N(SO2-CF3)2-) results in giant solubilities depending on the iodonium cation. Such photoinitiating systems are interesting because they bleach at the excitation wavelength and open therefore the opportunity to cure coatings with large thickness; that is a length up to several millimeters (14 mm). In addition, NIR-exposure additionally results in formation of light-stable brownish photoproducts. This represents an interesting alternative compared to brownish pigments used in dental materials. These findings require to understand the mechanism of sensitizer decomposition in more detail applying exposure kinetics and a coupling of Ultra Performance Liquid Chromatography (UPLC) with mass spectrometry (MS) to analyze the photoproducts formed. Data show unexpected cleavage of the polymethine chain at typical positions in case of the oxidized species. These were formed as a result of electron transfer between the excited state of the photosensitizer and the iodonium salt. UPLC-MS experiments additionally indicated a certain sensitivity of the system upon adding of acids and radicals generated by thermal decomposition of AIBN. Thus, treatment of the photoinitiator composition (sensitizer and iodonium salt) with either radical or proton forming agents led almost to the same products no matter the system was either exposed with NIR light or not. Futhermore, photo-initiated crosslinking of multifunctional acrylic esters in polymeric binders was investigated based on digital imaging using the Computer-to-Plate (CtP) technology applying laser exposure in the NIR at 830 nm. Iodonium salts derived from several borates and those with the bis(trifluoromethylsulfonyl) imide anion resulted in lithographic materials with high sensitivity being comparable with those used in industrial systems. Finally, iodonium salts comprising the bis(trifluoromethylsulfonyl) imide anion exhibit a lower cytotoxicity compared to those with the tetraphenyl borate anion as determined by the MTT-test using CHO-9-cells