Tetraacylstannanes as Long-Wavelength Visible-Light Photoinitiators with Intriguing Low Toxicity.

The first tetraacylstannanes Sn[(CO)R]4 (R=2,4,6-trimethylphenyl (1 a) and 2,6-dimethylphenyl (1 b)), a class of highly efficient Sn-based photoinitiators, were synthesized. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The UV/Vis absorption spectra of 1 a, b reveal a significant redshift of the longest wavelength absorption compared to the corresponding germanium compounds. In contrast to the known toxicity of organotin derivatives, the AMES test and cytotoxicity studies reveal intriguing low toxicity. The excellent performance of 1 as photoinitiators is demonstrated by photobleaching (UV/Vis) and NMR/CIDNP investigations, as well as photo-DSC studies.

Investigations of thiol-modified phenol derivatives for the use in thiol-ene photopolymerizations.

Thiol-ene photopolymerizations gain a growing interest in academic research. Coatings and dental restoratives are interesting applications for thiol-ene photopolymerizations due to their unique features. In most studies the relative flexible and hydrophilic ester derivative, namely pentaerythritoltetra(3-mercaptopropionate) (PETMP), is investigated as the thiol component. Thus, in the present study we are encouraged to investigate the performance of more hydrophobic ester-free thiol-modified bis- and trisphenol derivatives in thiol-ene photopolymerizations. For this, six different thiol-modified bis- and trisphenol derivatives exhibiting four to six thiol groups are synthesized via the radical addition of thioacetic acid to suitable allyl-modified precursors and subsequent hydrolysis. Compared to PETMP better flexural strength and modulus of elasticity are achievable in thiol-ene photopolymerizations employing 1,3,5-triallyl-1,3,5-triazine-2,4,6-trione (TATATO) as the ene derivative. Especially, after storage in water, the flexural strength and modulus of elasticity is twice as high compared to the PETMP reference system.

Synthesis of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide - a tailor-made photoinitiator for dental adhesives.

Because of the poor solubility of the commercially available bisacylphosphine oxides in dental acidic aqueous primer formulations, bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide (WBAPO) was synthesized starting from 3-(chloromethyl)-2,4,6-trimethylbenzoic acid by the dichlorophosphine route. The substituent was introduced by etherification with 2-(allyloxy)ethanol. In the second step, 3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoic acid was chlorinated. The formed acid chloride showed an unexpected low thermal stability. Its thermal rearrangement at 180 ° C resulted in a fast formation of 3-(chloromethyl)-2,4,6-trimethylbenzoic acid 2-(allyloxy)ethyl ester. In the third step, the acid chloride was reacted with phenylphosphine dilithium with the formation of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine, which was oxidized to WBAPO. The structure of WBAPO was confirmed by ¹H NMR, ¹³C NMR, ³¹P NMR, and IR spectroscopy, as well as elemental analysis. WBAPO, a yellow liquid, possesses improved solubility in polar solvents and shows UV-vis absorption, and a high photoreactivity comparable with the commercially available bisacylphosphine oxides. A sufficient storage stability was found in dental acidic aqueous primer formulations.

A partially aromatic urethane dimethacrylate as a new substitute for Bis-GMA in restorative composites.

OBJECTIVES: The objective of this study was to investigate the use of a new, partially aromatic urethane dimethacrylate in visible-light cured resin-based composite restoratives. Selected mechanical properties, such as flexural strength and flexural modulus of elasticity, of model monomer mixtures and composites containing the new urethane dimethacrylate were investigated and compared to the properties of materials that are based on Bis-GMA, at present the most frequently used cross-linker in restorative composites. In addition, the polymerization shrinkage and the water sorption of selected composites were determined. METHODS: The flexural strength, flexural modulus of elasticity, and the water sorption were determined according to ISO 4049:2000. Test specimens (rods: 2 mmx2 mmx25 mm; discs: d=15 mm and h=1 mm) of the investigated composites were prepared in stainless steel molds and light-cured (150 mW/cm2, 2x180 s). The flexural strength and flexural modulus of rods were measured after the samples had been stored under dry conditions or in water for 24 h at 37 degrees C as well as after they had been stored in water for 7 days at 37 degrees C. The water sorption was determined with discs. The polymerization shrinkage was calculated from the densities of the uncured composite pastes and cured composites. RESULTS: Visible light cured mixtures of dimethacrylate diluents with the new urethane dimethacrylate and composites based on these mixtures show a reactivity, flexural strength, flexural modulus of elasticity, polymerization shrinkage and water sorption similar to those of materials that are based on Bis-GMA. The composites did not show any strong deterioration of the mechanical properties after water storage.

Benzoyl germanium derivatives as novel visible light photoinitiators for dental materials.

OBJECTIVES: The objective of this study was to investigate the use of benzoyl germanium derivatives as a novel visible light photoinitiator of resin-based dental composites. Selected mechanical properties, such as flexural strength and flexural modulus, setting time, storage stability, and UV light stability, of the composites based on the novel photoinitiators benzoyltrimethylgermane (BTMGe) or dibenzoyldiethylgermane (DBDEGe) were investigated and compared to the properties of materials that are cured with a mixture of camphorquinone (CQ) and ethyl 4-(N,N-dimethylamino)benzoate (EMBO). METHODS: The flexural strength and flexural modulus of elasticity were determined according to ISO 4049. For this purpose, test specimens (2 mm x 2 mm x 25 mm) of the composites investigated were prepared in stainless steel moulds and light-cured (150 mW/cm2, 2 s x 180 s). The flexural strength and flexural modulus of elasticity were measured after immersing the cured specimens in water for 24h at 37 degrees C and in certain cases, after they had been boiled for 24h in water. In addition, the setting time, curing depth, storage and UV stability of selected composites were determined. RESULTS: The novel photoinitiators BTMGe or DBDEGe can be used to substitute the binary photoinitiator CQ/EMBO in visible light-cured restorative composites. Especially, DBDEGe showed a significantly higher photocuring activity in composites with a filler load of about 60 wt. % in comparison to that of CQ/EMBO. In addition, composites based on BTMGe or DBDEGe showed an improved UV stability and a storage stability comparable to that of CQ/EMBO-based composites.

Sol-gel materials 2. Light-curing dental composites based on ormocers of cross-linking alkoxysilane methacrylates and further nano-components.

OBJECTIVES: The objective of this study was to investigate the use of ormocers, which were synthesized from amine or amide dimethacrylate trialkoxysilanes. Ormocers showed improved biocompatibility in dimethacrylate-diluent-free composite restoratives. Selected mechanical properties, such as flexural strength and flexural modulus of experimental composites containing ormocers were investigated. In addition, the influence of methacrylate-substituted ZrO2 clusters and SiO2 organosols on the mechanical properties of composites was studied. METHODS: The flexural strength and flexural modulus of elasticity were determined according to ISO 4049: 2000. For this purpose, test specimens (2mmx2mmx25mm) of the composites investigated were prepared in stainless steel moulds and light-cured (150mW/cm2, 2x180s). The flexural strength and flexural modulus of elasticity were measured after the samples had been stored in water for 24h at 37 degrees C. RESULTS: While visible light-cured dimethacrylate-diluent-free composite restoratives based on the investigated ormocers showed a similar flexural strength and flexural modulus of elasticity compared to composites that contain only dimethacrylates, their double bond conversion was considerable lower. The simultaneous addition of methacrylate-substituted ZrO2 clusters and SiO2 organosols to the ormocer composite improved the mechanical properties of the composites. SIGNIFICANCE: Ormocers of amine or amide dimethacrylate trialkoxysilanes enabled the preparation of dimethacrylate-diluent-free composite restoratives. Based on the lower cytotoxicity of the ormocers, the prepared restorative composites should show improved biocompatibility. With the addition of nanoparticles, such as methacrylate-substituted ZrO2 clusters or SiO2 organosols, the mechanical properties of composites can be improved.

Bis-(acrylamide)s as new cross-linkers for resin-based composite restoratives.

OBJECTIVES: The objective of this study was to investigate the use of three new bis-(acrylamide)s as cross-linker in resin-based composite restoratives. Selected mechanical properties such as flexural strength and flexural modulus of model composites containing bis-(acrylamide)s were investigated and compared to the properties of composites that are based on only conventional dimethacrylates. In addition, the hydrolytic stability of composites containing an acidic monomer was examined. METHODS: The flexural strength and flexural modulus of elasticity were determined according to ISO 4049:2000. For this purpose, test specimens (2 mm x 2 mm x 25 mm) of the composites investigated were prepared in stainless steel moulds and light-cured (150 mW/cm2, 2 x 180 s). The flexural strength and flexural modulus were measured after the samples had been stored in dry conditions or in water for 24 h at 37 degrees C as well as after they had been stored in water for 7 days at 37 degrees C, and in certain cases, after they had been boiled for 24 h in water. RESULTS: Visible light cured mixtures of dimethacrylates with bis-(acrylamide)s and composites based on these mixtures show a similar reactivity, flexural strength and flexural modulus of elasticity compared to materials that contain only dimethacrylate. The composites did not show any deterioration of the mechanical properties after water storage. Only when strongly acidic monomers were added to the composites containing dimethacrylates or bis-(acrylamide)s did the flexural strength and flexural modulus of the samples decrease after they were stored in water. SIGNIFICANCE: Bis-(acrylamide)s were similarly reactive than dimethacrylates and therefore can be used as diluents to substitute dimethacrylate diluents in composites. Although the bis-(acrylamide)s are entirely soluble in water, non-ionic materials based on bis-(acrylamide)s did not strongly change their mechanical properties during storage in water.