Urs Karl Fischer

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

Summary 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.

Synthesis and polymerisation of new multifunctional urethane methacrylates

New multifunctional urethane methacrylates were synthesised by addition of methacrylates containing a hydroxy group, such as 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate or glycerol dimethacrylate, to α,α,α′,α′-tetramethyl-m-xylylene diisocyanate. The structure of the new monomers was confirmed by IR, 1H NMR and 13C NMR spectroscopy. The photopolymerisation of the crosslinking monomers in bulk using camphorquinone as photoinitiator results in transparent, crosslinked polymers. Neue multifunktionelle Urethanmethacrylate wurden durch Addition von hydroxygruppenhaltigen Methacrylaten, wie 2-Hydroxyethylmethacrylat, 2-Hydroxypropylmethacrylat oder Glycerindimethacrylat, an α,α,α′,α′-Tetramethyl-m-xylylendiisocyanat hergestellt. Die Struktur der neuen Monomeren konnte durch IR-, 1H NMR- und 13C NMR-Spektroskopie bestatigt werden. Die Photopolymerisation der Vernetzermonomeren in Substanz fuhrte mit Campherchinon als Photoinitiator zu transparenten Polymernetzwerken.

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

Summary 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.

Polymerization of cyclic monomers. VII. Synthesis and radical polymerization of 1,3-bis[(1-alkoxycarbonyl-2-vinylcyclopropane-1-yl)carboxy]benzenes

1,3-Bis[(1-alkoxycarbonyl-2-vinylcyclopropane-1-yl)carboxy]benzenes 1 [RO: CH3O (a), C2H5O (b)] were synthesized by the esterification of the corresponding 1-alkoxycarbonyl-2-vinylcyclopropane-1-carboxylic acids with resorcinol. The structure of the new vinylcyclopropanes was confirmed by elemental analysis and infrared (IR), 1H nuclear magnetic resonance (1H-NMR), and 13C nuclear magnetic resonance (13C-NMR) spectroscopy. The radical polymerization of difunctional 2-vinyl-cyclopropanes in bulk with 2,2′-azoisobutyronitrile (AIBN) results in hard, transparent, crosslinked polymers. During the bulk polymerization of the crystalline bis[(1-methoxycarbonyl-2-vinylcyclopropane-1-yl)carboxy]benzene 1a, an expansion in volume of about 1% took place. The radical solution polymerization of 1a resulted in a soluble polymer with pendant 2-vinylcyclopropane groups.

Monomers for adhesive polymers, 13.1 Synthesis, radical photopolymerization and adhesive properties of polymerizable 2-substituted 1,3-propylidenediphosphonic acids

A family of polymerizable 2-substituted-1,3-propylidenediphosphonic acids was successfully synthesized for an application in dental adhesives. Methacrylates as well as N-alkyl-acrylamides were prepared. Those monomers were synthesized in 4 to 6 steps and were fully characterized by 1H NMR, 13C NMR, 31P NMR spectroscopy and by HRMS. In order to evaluate their reactivity, the copolymerization of each monomer with N,N′-diethyl-1,3-bis-(acrylamido)propane (DEBAAP) in DMF was investigated by photo-DSC. Bis-(4-methoxybenzoyl)diethylgermanium (BMDG) was added as the photoinitiator to each mixture of acidic monomer/DEBAAP/DMF (2/3/5: mol/mol/mol). It was demonstrated that each monomer efficiently copolymerized with DEBAAP. Self-etching enamel-dentin adhesives containing these new diphosphonic acids were formulated and used to mediate a bond between the dental hard tissues (dentin and enamel) and a restorative material. According to both dentin and enamel shear bond strength measurements, 2-methacryloyloxymethyl-1,3-propylidenediphosphonic acid 8, 2-[N-(2-methacryloyloxyethyl)-carbamoyloxymethyl]-1,3-propylidenediphosphonic acid 10a and 2-[N-(10-methacryloyloxydecyl)-carbamoyloxymethyl]-1,3-propylidenediphosphonic acid 10b were the best candidates to enter new formulations. Those diphosphonic acids were significantly more efficient regarding the etching of enamel than the 1,3-bis(methacrylamido)propane-2-yl dihydrogen phosphate 17, a monomer already used in some commercially available dental formulations.

Incorporation of core–shell particles into methacrylate based composites for improvement of the mechanical properties

The fracture toughness of polymeric materials and composites can be enhanced by the incorporation of polymer nanoparticles. The combination of a soft core and a hard shell leads to an improvement of the fracture toughness of the polymeric composites. Thereby, the mechanical resistance of the materials is commonly decreased. In our approach, core–shell nanoparticles consisting of an ethylene glycol dimethacrylate (EGDMA) crosslinked poly(butyl acrylate) (PBA) core and a poly(methyl methacrylate) (PMMA) shell were synthesized. The polymer particles were incorporated into triethylene glycol dimethacrylate (TEGDMA)/urethane dimethacrylate (UDMA) based composites in order to tune the mechanical properties. Different core–shell ratios were applied to study the influence on the fracture toughness and E-modulus. An examination of shell-crosslinking with a TEGDMA content of up to 8% was performed to improve particle stability and dispersibility. The particle sizes and morphologies were characterized by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM) and analytical ultracentrifugation (AUC). Latex particle sizes of 70 to 220 nm were obtained. The mechanical properties (flexural strength, E-modulus and K1c) of polymer composites were investigated in three-point bending tests. Core/shell ratios of 50/50 showed a decreasing effect on flexural strength, E-modulus and K1c. Polymer particles with core/shell ratios of 30/70 led to a significant increase of the mechanical properties with maxima of 1.206 MPa m1/2 (K1c) (increase of 65%), E-modulus of 1.90 GPa (increase of 18%) and flexural strength of 79 MPa (increase of 18%). This study represents the first report of a simultaneous improvement of fracture toughness and E-modulus (at the same time) of additive filled polymer composites. The improvement of mechanical properties makes these materials interesting as tougheners for hard tissue applications like bone cements or dental replacement materials.

Bis(vinylcyclopropane) and bis(methacrylate) monomers with cholesteryl group for dental composites

Abstract Novel bis(vinylcyclopropane)-based monomers with cholesteryl group and high molecular weight were prepared. The phase behaviour of the synthesized monomers was investigated. The obtained macromonomers showed a polymerisation shrinkage that was significantly lower compared to dimethacrylates used in dental composites.

Tetraacylstannanes as Long‐Wavelength Visible‐Light Photoinitiators with Intriguing Low Toxicity

Abstract 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.