title: Synthesis and evaluation of novel MAPO-based photoinitiators
Creators: name: Nikolic, Jelena
Publisher: name: TU Wien
Year: 2024
Subjects: subject: Photopolymer
subject: Photopolymerization

description: For very long, amalgam was the material of choice when it comes to dental restoratives. However, because of its greyish color and health concerns the research focuses on developing of suitable alternatives. Because of their superior aesthetic appearance, photopolymerizable composites are in the focus of the research as the new dental restoratives of choice.In the photopolymerizable formulations, photoinitiators play a crucial role because they directly influence the final properties of the polymerized product. The requirement for the photoinitiators is therefore high reactivity, good compatibility, and solubility in formulations, as well as low toxicity, and no yellowing.Acylphosphine oxides represent promising Type I photoinitiators that can be applied in dentistry. These photoinitiators are highly efficient, show long wavelength absorption and excellent bleaching behavior. Typically, they show an absorption maximum at 350 to 380 nm with tailing to 420 nm. Because of these properties, they are widely used in industrial coatings, especially for the curing of thick and highly pigmented formulations, including white coatings. The commercially available und well-established monoacylphosphine oxide MAPOs are TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide) and TPO-L (ethyl(2,4,6‐trimethylbenzoyl) phenylphosphinate). TPO shows high efficiency and degree of conversion, however it has some drawbacks like higher migration ratio and absorption maximum that is not compatible with the currently used dental lamps. Furthermore, the toxicity of TPO makes it unsuitable for clinical applications. TPO-L on the other hand, shows much lower toxicity, excellent color stability, however, has a lower efficiency than TPO. Therefore, the current research focuses on developing new MAPO derivatives that would have an improved absorption behavior, lower migration ratio and better biocompatibility than TPO and TPO-L.In this work, we focused on synthesis of new MAPO derivatives, that would preferably have an improved efficiency and absorption capabilities than the established MAPO photoinitiators TPO and TPO-L. The current research mainly focuses on the development of new MAPO derivatives through modification of acyl side of the MAPO molecule. In this work, we focused on the synthesis of new MAPO derivatives with modified phosphonyl side of the molecule. Since the phosphonyl radical shows a higher reactivity when compared to benzoyl radical, generated from α-cleavage, the modifications of phosphonyl side of the molecule can influence the overall efficiency of photoinitiators. Furthermore, responsible for the bathochromic shift in MAPO molecules is the nπ*-transition caused by interaction between the empty d-orbital of phosphorus and π*-orbital of carbonyl-carbon atom. The incorporation of heteroatoms that can interact with the d-orbital of phosphorus, can significantly influence the absorption spectra of photoinitiators. To investigate these influences, we focused on the synthesis of new derivatives with heteroatoms that have a higher electronegativity than phosphorus. For this purpose, synthesis of new MAPOs modified with amines, thiols, selenols, phosphines, sterically hindered alcohols, isocyanates and pseudohalides was performed.For the synthesis of new photoinitiators, two approaches were investigated: the nucleophilic and electrophilic.For the nucleophilic reactions, a simple literature known method was used, which enables the synthesis of a wide range of hetero-substituted MAPOs with different electronic structure than carbon-substituted MAPOs.The electrophilic reactions were performed from mesitoylhydrogenphosphane as starting material which was reacted with the corresponding electrophile and oxidated to give the final hetero-substituted MAPO molecule.The obtained MAPO molecules were characterized using UV-Vis spectroscopy and their performance and efficiency examined by photo-DSC.