Norbert Moszner

New developments of polymeric dental composites

The currently used commercial restorative composites contain a mixture of various cross-linking dimethacrylates, glass- and/or silicon dioxide fillers, and a photoinitiator system. They are cured by irradiation with visible light. New developments of polymeric composites for restorative filling materials are mainly focused on the reduction of polymerization shrinkage, and improvement of biocompatibility, wear resistance and processing properties. This can be partially achieved by using new tailor-made monomers and optimized filler particles. This review describes the polymeric chemical aspects of the application of new monomers, e.g. cyclic monomers, liquid-crystalline monomers, ormocers, branched monomers, compomers or Bis-GMA analogues or substitutes for restorative composites. In addition, the contribution of new filler-technologies for the improvement of restorative composites is discussed.

Nanotechnology for dental composites

In dentistry, composites consisting of inorganic fillers such as radiopaque glass, quartz or ceramic particles and an organic resin matrix based on a mixture of dimethacrylates, are used for the restoration of teeth. Important properties of the dental restorative materials can be improved by means of nanotechnology. Inorganic-organic hybrid materials can be used as monomer matrix in dental restoratives to diminish their polymerisation shrinkage and improve their wear resistance and biocompatibility. Inorganic-organic hybrids with tailor-made properties can be created by means of sol-gel processing of hydrolytically condensable, organically modified trialkoxysilanes, which contain radically polymerisable methacrylate groups or cyclic groups capable of ring-opening polymerisation. Furthermore, nanofillers can contribute to increasing the modulus of elasticity or improving the optical properties of the dental composites and are useful as starting compounds for the synthesis of new low temperature processable dental ceramics. Moreover, well designed nano- and microstructured sol-gel components can be used for producing protective and wear resistant coatings for teeth, metal alloys, and glass fillers of special compositions.

Acylgermanes: Photoinitiators and Sources for Ge-Centered Radicals. Insights into their Reactivity

Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane 1 reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radical G• is formed via the triplet state of parent 1. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. Radical G• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 10(8) and 3.2 ± 0.2 × 10(8) M(-1) s(-1), in toluene and acetonitrile, respectively. This is ~1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge-O bonds. LC-NMR analysis of photocured mixtures containing 1 and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales.

Synthesis and polymerization of vinylcyclopropanes

Vinylcyclopropanes are important synthetic intermediates in organic chemistry and are mostly synthesized by the simultaneous introduction of the cyclopropane and the vinyl unit, e. g., by the reaction of trans-1,4-dihalobutenes with β-dicarbonyl compounds or the addition of carbenes to dienes. The polymerization of vinylcyclopropane itself results in vinyl polymers with predominantly 1,2-structural units. The radical polymerization of substituted vinylcyclopropanes results in polymers with mainly 1,5-ring-opened units, whereby radical stabilizing substituents or electron-withdrawing groups increase the radical polymerizability and the ring-opening ability. The vinylcyclopropanes undergo a radical copolymerization with other vinyl monomers, such as methacrylates, and, in comparison to the polymerization of these linear vinyl monomers, the vinylcyclopropanes show a significantly lower volume shrinkage during ring-opening polymerization. Hybrid vinylcyclopropanes are polymerized step-by-step under formation of reactive polymers or polymer networks. Multifunctional cross-linking vinylcyclopropanes can be used as new low-shrinking matrix monomers for photopolymerizable materials. In addition, the sol-gel process of trialkoxysilyl-functionalized vinylcyclopropanes affords low shrinking organic-inorganic nanocomposites.

Sol-Gel Materials, 1. Synthesis and Hydrolytic Condensation of New Cross-Linking Alkoxysilane Methacrylates and Light-Curing Composites Based upon the Condensates

New methacryloyxyalkylaminoalkylalkoxysilanes have been synthesised by Michael addition of the corresponding acryloyloxyalkyl methacrylates with (3-aminopropyl)triethoxysilane (APTES). Low-viscosity polycondensates have been formed by hydrolysis and condensation of these silanes in the presence of ammonium fluoride (NH 4 F). The reaciton of APTES with the addition product of succinic or glutaric anhydride with glycerol dimethacrylate results in the formation of new dimeth-acrylate-functionalised 3-amidopropyltrithoxysilanes. The hydrolic condensation of these silanes was carried out in the presence of 0.5 M HCI. The hydrolysis and condensation of the silanes have been studied by 29 Si NMR spectroscopy. Cross-linked inorganic-organic materials have been obtained by free-radical photopolymerisation of the polycondensates and their mixtures in the presence of camphorquinone and ethyl 4-(dimethylamino)benzoate with visible light (VL). The synthesised polycondensates enable the preparation of diluent-free composites. The mechanical properties of VL-cured polycondensates and composites have been investigated.

Supramolecular Cross-Links in Poly(alkyl methacrylate) Copolymers and Their Impact on the Mechanical and Reversible Adhesive Properties

Hydrogen-bonded, side-chain-functionalized supramolecular poly(alkyl methacrylate)s were investigated as light- and temperature-responsive reversible adhesives that are useful for bonding and debonding on demand applications. Here, 2-hydroxyethyl methacrylate (HEMA) was functionalized with 2-ureido-4[1H]pyrimidinone (UPy) via a hexamethylenediisocyanate (HMDI) linker, to create a monomer (UPy-HMDI-HEMA) that serves to form supramolecular cross-links by way of forming quadruple hydrogen bonded dimers. UPy-HMDI-HEMA was copolymerized with either hexyl methacrylate or butyl methacrylate to create copolymers comprising 2.5, 5, or 10 mol % of the cross-linker. The mechanical properties of all (co)polymers were investigated with stress-strain experiments and dynamic mechanical analysis. Furthermore, the adhesive properties were studied at temperatures between 20 and 60 °C by testing single lap joints formed with stainless steel substrates. It was found that increasing the concentration of the UPy-HMDI-HEMA cross-linker leads to improved mechanical and adhesive properties at elevated temperatures. Concurrently, the reversibility of the bond formation remained unaffected, where rebonded samples displayed the same adhesive strength as regularly bonded samples. Debonding on demand abilities were also tested exemplarily for one copolymer, which for light-induced debonding experiments was blended with a UV-absorber that served as light-heat converter. Single lap joints were subjected to a constant force and heated or irradiated with UV light until debonding occurred. The necessary debonding temperature was comparable for direct heating and UV irradiation and varied between 28 and 82 °C, depending on the applied force. The latter also influenced the debonding time, which under the chosen conditions ranged from 30 s to 12 min.

Exploring the benefits of β-allyl sulfones for more homogeneous dimethacrylate photopolymer networks

β-Allyl sulfones are well known to act as addition fragmentation chain transfer (AFCT) reagents in radical polymerization of monofunctional monomers, thus giving good control over the molecular weight of the formed polymer. Within this work, we investigated the effect of a difunctional β-allyl sulfone (DAS) as a potent AFCT reagent for dimethacrylate photopolymer networks. Similar to thiol-based chain transfer agents, β-allyl sulfones are expected to change the mode of polymerization from a purely radical chain growth mechanism towards a mixed chain growth/step growth-like polymerization process. Resultant materials should have reduced shrinkage stress, increased conversion, and greater toughness. The studied formulations with added chain transfer reagent were analyzed by photocuring and measuring characteristics such as time until gelation, conversion at the gel point, overall conversion and shrinkage stress. The mechanical properties, such as dynamic modulus, hardness and impact resistance were determined from the resulting photopolymer networks. The investigated β-allyl sulfones provide significant advantages over dithiols as chain transfer reagents. Most importantly, β-allyl sulfones do not have a significant odor or do not cause storage stability problems common to thiol–ene formulations. In addition, the β-allyl sulfones more homogeneously regulate a radical methacrylate polymerization. This paves the way to materials with reduced shrinkage stress, sharpened glass transition, and good impact resistance. In addition, hardness and storage or elastic modulus of the materials are significantly higher compared to the dithiol-based materials.

Photoinduced polyaddition of multifunctional azides and alkynes

The photoinduced copper(I)-catalyzed polymerization of multifunctional azides and alkynes is facilitated by the photoreduction of copper(II) acetate generating copper(I) ions without using any additional photoinitiator. The polymerization can only be carried out in solution using at least 15 wt% of methanol. Depending on the catalyst concentration quantitative monomer conversions can be achieved allowing the determination of the mechanical properties. The bifunctional system consisting of a di-azide and di-alkyne exhibited the highest Youngs modulus value of 1600 MPa.

Decisive Reaction Steps at Initial Stages of Photoinitiated Radical Polymerizations

tofollow the very early steps of the radical polymerization.CIDNP is a particularly suitable tool for these investigationssince the subsequent products formed from chemical reac-tions involving radical pairs are observed exclusively.Here, bisacylphosphine oxide 1 serves as an ideal modelcompound because it is a well investigated

Rapid formation of regulated methacrylate networks yielding tough materials for lithography-based 3D printing

Multifunctional methacrylates are highly reactive monomers for radical photopolymerization, but yield brittle materials due to their inhomogeneous and highly crosslinked network architecture. Addition fragmentation chain transfer (AFCT) reagents serve as additives for the regulation of radical network formation and pave the way to photopolymer networks with high toughness. However, AFCT reagents (e.g. β-allyl sulfones) tend to have a negative influence on the reaction speed which limits them for lithography-based 3D fabrication. Vinyl sulfone esters are described as a new class of AFCT reagents for methacrylate-based photopolymerization without the drawback of retardation but good regulation of network architecture. The resulting materials show high network homogeneity, low shrinkage stress, and a significant increase in CC double bond conversion and toughness. This promises great potential for vinyl sulfone esters as AFCT reagents in photopolymer applications. First 3D parts have been successfully fabricated via digital light processing.