NIR-sensitized hybrid radical and cationic photopolymerization of several cyanines in combination with diaryliodonium bis(trifluoromethyl)sulfonyl imide

Abstract

A series of cyanines carrying methine chains of different lengths and terminal moieties exhibiting absorption between 750 and 930 nm reacted after excitation in the NIR region with the bis(t-butylphenyl) iodonium cation comprising the [(CF₃SO₂)₂N]⁻ anion (NTf₂)⁻, resulting in the generation of free radicals and conjugate acids. This enabled us to investigate the efficiency of both free radical and cationic polymerization as well as their combination as a hybrid system. Variation of the connecting methine chain length and terminal indolium groups leads to significantly different photopolymerization efficiency by applying NIR-LED exposure. In this study, NIR-LEDs exhibiting an intensity of 1.0 W cm⁻² emitting either at 860 nm or 820 nm were applied for irradiation due to the distinct absorption of the sensitizers. Real-time Fourier-transform infrared spectroscopy (RT-FTIR) showed remarkable initiation efficiency of the initiating system, which indicates higher final conversion and shorter initiating plateau for both free radical and cationic polymerization. The combination of kinetic experiments based on RT-FTIR and DMA investigations of the obtained samples complemented the studies. Surprisingly, some of these systems successfully initiated the cationic polymerization of some oxetane monomers while the initiation of neat epoxides was less efficient. Nevertheless, hybrid photopolymerization of TMPTA or more viscous UDMA following radical polymerization worked in combination with epoxides and oxetanes polymerizing according to cationic polymerization with no phase formation. Here, the release of heat possesses a key function as concluded from UV exposure experiments for comparison.