Materials Science

Copper complexes form effective photo-initiating systems

“Copper complexes G1 or G2 with iodonium salt (and optionally NVK) can work as photo-initiating systems for the radical polymerization of TMPTA in laminate under different visible light irradiations”

Photo-initiation systems make it possible to control exactly when a polymerization reaction occurs. Such systems include a photo-initiator that releases reactive species, such as radicals, cations, or anions, on exposure to light of a particular frequency. The reactive species initiates a chain reaction that results in polymerization. Polymerization does not occur without the light activation and so the exact time and location of polymer creation can be precisely determined. Furthermore, the rate of polymerization can be easily controlled by adjusting the power of the external photo-irradiation source. Due to the high degree of control they provide, photo-initiation systems are widely used across a diverse range of applications, including the creation of biomedical materials and bioconjugation.

The most commonly used photo-initiators are metal complexes since these have intense visible light absorption, long-lived excited states, and suitable redox potentials. Furthermore, they can produce reactive species through either an oxidation cycle or a reduction cycle. Copper complexes can remain in an excited-state for a considerable time giving them the potential to provide good photo-initiators for polymerization reactions. Since copper is more cost-effective and more readily accessible than some of the metal complexes currently employed as photo-initiators, the photo-initiation ability of complexes was evaluated.

Photo-initiating systems activated by soft visible light were developed using three different copper-iodonium salt complexes as the photo-initiator. The light sources used to provide photo-activation were LED, laser diode, and halogen lamp. The release of free radicals was measured at room temperature by electron spin resonance spin trapping (ESR-ST) using a bench-top EPR spectrometer. The captured radicals were identified as aryl radicals and it was found that they can initiate the radical polymerization in TMPTA. The extent of polymerization was assessed by measuring the double-bond content of TMPTA and the epoxy group content of EPOX using real-time Fourier transform infrared spectroscopy.

The G1 and G2 copper complexes were found to be more effective photo-initiators than the well-established camphorquinone (CQ)-based system. Using halogen irradiation, TMPTA conversion was 18% with the CQ/iodonium salt photo-initiator, 48% with the G1 copper/iodonium salt photo-initiator, and 39% with the G2 copper/iodonium salt photo-initiator. In addition, the G1/ iodonium salt/ N-vinylcarbazole photo-initiator system demonstrated good initiation ability for the cationic polymerization of EPOX.

Copper complexes thus can provide efficient catalysts for photopolymerization. Their efficacy as photo-initiators was dependent on the choice of ligand. These findings indicate that copper complexes could provide versatile and cost-effective free radical initiation of polymerization reactions.

Xiao P, et al. Macromolecules 2014, 47, 12, 3837–3844.