Thioxanthone Derivatives as a New Class of Organic Photocatalysts for Photopolymerisation Processes and the 3D Printing of Photocurable Resins under Visible Light

Emilia Hola,Maciej Pilch,Joanna Ortyl

doi:10.3390/catal10080903

Abstract

In the present paper, novel thioxanthone-based compounds were synthesised and evaluated as a component of photoredox catalysts/photoinitiating systems for the free-radical polymerisation (FRP) of acrylates and the ring-opening cationic polymerisation (CP) of epoxy monomers. The performance of the obtained thioxanthones in two- and three-component photoinitiating systems, in combination with amines, iodonium or sulphonium salt, as well as with alkyl halide, for photopolymerisation processes upon exposure to light emitting diodes (LEDs) with a maximum emission of 405 nm and 420 nm, was investigated. The studied compounds act also as one-component free-radical photoinitiators. Fourier transform real-time infrared spectroscopy was used to monitor the kinetics of disappearance of the functional groups of the monomers during photoinitiated polymerisation. Excellent photoinitiating efficiency and high final conversions of functional groups were observed. Moreover, the influence of thioxanthone skeleton substitution on photoinitiating efficiency was discussed. The photochemical mechanism was also investigated through cyclic voltammetry. It was discovered that thioxanthone derivatives can be used as a metal-free photoredox catalyst active for both oxidative and reductive cycles. Furthermore, a photopolymerizable system based on novel thioxanthone derivatives in a stereolithography three-dimensional (3D) printing technology under visible sources of light was used. The effects of photoinitiator type system and monomer type in photoresins during 3D printing processes were explored. The outcome of this research is the development of high-performance visible photosensitive resins with improved photosensitivity obtained thanks to the development of entirely novel photoinitiating systems specifically adapted for this application.

Highlights

Polymerisation initiated by visible light is considered to be an eco-friendly and sustainable alternative to traditional thermal polymerisation [1,2]
Photoinduced processes possess a lot of attractive features—mild reaction conditions, nearly no release of volatile organic compounds (VOCs), and room temperature operation with the use of convenient light sources [3]
We proposed new thioxanthone-based compounds as sensitizers of iodonium and sulphonium salt for polymerisation processes under visible soft irradiation conditions

Summary

Introduction

Polymerisation initiated by visible light is considered to be an eco-friendly and sustainable alternative to traditional thermal polymerisation [1,2]. There have been many developments in the field of photochemistry—especially in new monomers [4,5]—for monitoring on-line and/or in-situ processes of photopolymerisation [6,7,8], as well as completely new highly efficient photoinitiating systems, which are necessary for photopolymerisation processes [9,10,11]. The search for new, effective components for initiating systems operating in visible light has drawn the attention of scientists from around the world [12]. Light-emitting diodes (LEDs) have attracted increased attention as potential irradiation sources for photopolymerisation processes substituting traditional mercury UV lamps and enabling processes carried out in visible light.

Methods

Results

Conclusion