Photoinitiators from bio-sourced naphthoquinone – the application of naphthoquinone-based vitamins K1 and K3 in free radical photopolymerization

Abstract

Two common bio-sourced naphthoquinone derivatives, vitamin K1 (phylloquinone) and vitamin K3 (menadione), were employed in free radical photopolymerization under LED light exposure at 405 nm. In comparison with juglone, a previously reported natural naphthoquinone derivative, both vitamins exhibited enhanced properties in inducing free radical photopolymerization as sole initiators (monocomponent type II) and in combination with amines, and they can act as typical type II photoinitiators. In a three-component photoinitiating system consisting of an iodonium salt, amine and the vitamin, the polymerization process of TMPTA was significantly accelerated/improved too. These three-component systems resulted in faster reactions than the system that solely contains the iodonium salt or amine as the coinitiator, and they can additionally act as sensitizing dyes. Notably, the more polar vitamin K3 alone can initiate PEGDA polymerization with a high conversion rate, suggesting its potential utility in the development of biopolymer materials. The analysis of steady-state photolysis experiments revealed that K3 rapidly reacted with the electron-rich amine under light but was also more likely to generate by-products. In contrast, the inclusion of the iodonium salt in the three-component system significantly accelerated the photolysis of K3, demonstrating the superior chemical reactivity of the three-component system. Electron spin resonance spin trapping experiments successfully captured the radicals that initiated the photopolymerization reaction. Hence, these results facilitated the further deduction of potential reaction pathways. Finally, K1/iodonium salt and K3/amine, two systems that demonstrated strong performance in free radical polymerization experiments, were successfully applied to direct laser write experiments.