Novel long-wavelength photoinitiators for dental applications

Abstract

Modern composite tooth fillings consist of inorganic filler and organic methacrylate matrix. This matrix is cured with a photoinitiator system to achieve a solid material. It is known that the state of the art photoinitiator system camphorquinone/ethyl-(dimethylamino) benzoate shows moderate reactivity and low solubility in general but especially in slightly acidic and aqueous primer mixtures. Furthermore higher penetration depth of light is desired, which could be solved by developing photoinitiators that initiate at longer wavelengths. First studies with bisphosphylketones showed the desired red-shift but stability issues concerning water or temperature led to their elimination from the list of possible new photoinitiators. Certain zirconium complexes also had potential but the zirconium central atom coordinated with the monomer double bonds leading to decomposition of the compounds. Another concept is based on combination of diketone systems with heteroatoms with free d-orbitals. Even though the very interesting red-shift concept (-max up to 490 nm) worked out, the reactivity was not in a reasonable range. Benzoyl phenyltelluride was expected to be a red-shifted photoinitiator but instead was found to be a highly reactive TERP-reagent for visible light (400-500 nm) induced controlled radical polymerization. It provides polydispersities (1.2-1.3) among the lowest reported in literature and works best with acrylamides and acrylates. In the field of water-soluble photoinitiators for primer mixtures an improvement could be achieved as well. Monoacylphosphine oxide (MAPO) and bisacylphosphine oxide (BAPO) salts based on sodium or lithium define an important and in the latter case a new class of water-soluble photoinitiators for radical polymerization. These compounds show excellent water-solubility, biocompatibility, storage stability and reactivity. Concerning these properties, the MAPO and BAPO salts are at least in the same range (biocompatibility, stability) or show even better results (reactivity) than state of the art photoinitiators.