Synthesis and Characterization of New Photo Initiators for Self-etching Adhesives

Objective: The purpose was to synthesize and characterize two new types of photo initiators with absorption maxima at 400-460 nm and improved long-term stability in aqueous acidic conditions. Methods: Synthesis of bisacylphosphine-oxide bis-{3-[2-(2-allyloxy)-ethoxymethyl]-2,4,6-trimethylbenzoyl}-phenylphosphine-oxide (BAETBPPO) employed lithiation of P,P-dichlorophenylphosphine and subsequent reaction with 3-(2-allyloxy-ethoxymethyl)-2,4,6-trimethyl-benzoylchloride to form bis-{3-[2-(2-allyloxy)-ethoxymethyl]-2,4,6-trimethylbenzoyl}-phenylphosphine. Oxidation by H₂O₂ gave the desired phenylphosphine-oxide BAETBPPO. The acyl-germanium compound bis-benzoyldiethyl-germanium (BBDG) was prepared by reacting the lithium salt of 2-phenyl-1,3-dithiane with diethyldichloro-germanium to render di-ethyl-bis-(2-phenyl-1,3-dithiane-2-yl)-germanium. Using I₂/CaCO₃ this intermediate was oxidized  to BBDG. The chemical structures of BAETBPPO and BBDG were characterized by ¹H-NMR. Effectivity and storage stability of the photoinitiators in an experimental aqueous self-etching (SE) adhesive based on bisacrylamide and bismethacryamide-phosphate were determined via shear bond strength (SBS) on dentin in comparison to camphorquinone (CQ) / p-dimethylaminoethylbenzoate (DMAEB). SBS was measured according to ISO/TS 11405. Results: After purification by column-chromatography BAETBPPO was isolated as a viscous yellow oil with a yield of 24%, BBDG was isolated as a yellowish powder in a yield of 60%. The spectral data were in agreement with the expected structures. The SBS values of the different initiated SE adhesives are listed below (NN = 5).

	Storage-time: 0days	Storage-time: 14days, 42°C	Storage-time: 28days, 42°C
Initiator	SBS [MPa]	SBS [MPa]	SBS [MPa]
CQ/DMAEB	17.0±4.7	9.7±3.9	-
BAETBPPO	30.3±4.8 a	29.4±3.1 a A	30.3±2.7 a A
BBDG	23.1±3.3	32.7±3.3 a A	29.2±1.6 a A

Means with the same subscript are not significantly different (p<0.05 ANOVA post-hoc Turkey test).

SBS values attained using BAETBPPO and BBDG were unaffected by 4 weeks at 42°C, whereas CQ/DMAEB decreased effectivity already after 2 weeks. Conclusions: The novel photoinitiators BAETBPPO, and BBDG were successfully synthesized. They show high reactivity and good hydrolytical stability in aqueous acid. They appear advantageous as long-term stable initiators for single-bottle SE enamel/dentin adhesives.