OBJECTIVES:Hydroperoxides are key constituents of two-component dental materials. The objective of this study was to evaluate the influence of the hydroperoxide structure on the reactivity and on the mechanical properties of self-cure composites. METHODS:Hydroperoxides HP1-3 were synthesized by selective catalytic oxidation of the corresponding para-substituted cumene precursors and isolated in high purity. They were characterized by 1H NMR and 13C NMR spectroscopy. 16 self-cure composites, based on the redox initiator system hydroperoxide (Cumene hydroperoxide (CHP), HP1-3 or tert.-Amyl hydroperoxide (TAH))/polymerizable thiourea ATU1/copper(II) acetylacetonate, were formulated in Sulzer Mixpac two-component syringes. An equimolar hydroperoxide/ATU1 ratio was selected for each self-cure composite. The reactivity and the final double-bond conversions obtained with these two-component materials was assessed using RT-FTIR spectroscopy. The flexural strength and modulus were measured using a three-point bending setup, after storage of the specimens for 45 min at 37 °C (dry) and for 24 h in water at 37 °C. The working time of each self-cure composite was measured using an oscillating rheometer. RESULTS:CHP derivatives bearing an electron withdrawing group (HP2: ester or HP3: nitrile) in the para position were found to be more reactive than CHP, whereas the compound bearing an electron donating group (tert-butyl, HP1) was less reactive; molecular modelling data were reported for a better understanding of this structure/reactivity/efficiency relationship. All CHP derivatives were more reactive than the aliphatic hydroperoxide TAH. Excellent mechanical properties were obtained with self-cure composites containing either CHP or a para-functionalized CHP derivative. By carefully selecting the amounts of oxidizing/reducing agents and metal catalyst, suitable working times can be obtained with all evaluated hydroperoxides. HP3, thanks to its high reactivity, is nonetheless the most promising compound. SIGNIFICANCE:The curing rate of self-cure composites can be adapted by modifying the structure of the hydroperoxide. In agreement with molecular modelling data, the incorporation of CHP derivatives bearing an electron withdrawing group in the para position is particularly attractive. Indeed, due to a significant reactivity enhancement, the desired properties (working time, flexural strength/modulus) can be obtained by incorporating moderate amounts of hydroperoxide/acylthiourea as well as particularly low contents of metal catalyst to the two-component dental materials.

3D printing of materials which combine fracture toughness, high modulus and high strength is quite challenging. Most commercially available 3D printing resins contain a mixture of multifunctional (meth)acrylates. The resulting 3D printed materials are therefore brittle and not adapted for the preparation of denture bases. For this reason, this article focuses on toughening by incorporation of triblock copolymers in methacrylate-based materials. In a first step, three urethane dimethacrylates with various alkyl spacer length were synthesized in a one-pot two-step synthesis. Each monomer was combined with 2-phenoxyethyl methacrylate as a monofunctional monomer and a polycaprolactone-polydimethylsiloxane-polycaprolactone triblock copolymer was added as toughener. The formation of nanostructures via self-assembly was proven by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The addition of the triblock copolymer resulted in a strong increase in fracture toughness for all mixtures. The nature of the urethane dimethacrylate had a significant impact on fracture toughness and flexural strength and modulus of the cured materials. Most promising systems were also investigated via dynamic fatigue propagation da/dN measurements, confirming that the toughening also works under dynamic load. By carefully selecting the length of the urethane dimethacrylate spacer and the amount of block copolymer, materials with the desired physical properties could be efficiently formulated. Especially the formulation containing the medium alkyl spacer length (DMA2/PEMA) and 5 wt% BCP1 (block copolymer), exhibits excellent mechanical properties and high fracture toughness.

Additive manufacturing is on the verge of replacing established processes in dentistry, as it offers the possibility of manufacturing individual parts simply and cost-effectively. Due to its suitability for a wide variety of materials and, above all, its high precision, the focus is currently on stereolithographic processes. Intrinsic brittleness of the used multifunctional acrylic monomers remains however one of the major challenges. One promising concept is the use of block copolymers (BCPs) guaranteeing minor effects on 3D-printing processing and UV-curing due to initially at least partial solubility, and hence low viscosity impact. A polycaprolactone-polysiloxane (PCL-PDMS-PCL) triblock copolymer is synthesized via ring-opening polymerization of caprolactone and used in radical UV-cured methacrylic resin systems. Small angle X-ray scattering measurements reveal the self-assembly of the BCPs to objects of around 20 nm prior to curing. Subsequently, thermo-mechanical characterization is carried out by dynamic mechanical analysis, flexural testing, and fracture toughness measurements (K-IC). Transmission electron microscopy and scanning electron microscopy micrographs show a homogenous distribution of the BCPs and effective toughening via cavitation and shear yielding. The influence of the crosslink density on the toughness and the high effectiveness of block copolymers for improving fracture toughness is clearly shown.

Objectives: Currently used thiourea-based two-component dental materials may release bitter compounds if they are not properly cured. To address this issue, the objective of this study was to evaluate the potential of acylthiourea oligomers as reducing agents for the development of self-cure composites. Methods: Acylthiourea oligomers ATUO1-3 were synthesized via cotelomerization of the acylthiourea methacrylate ATU1 with butyl methacrylate. They were characterized by 1H NMR spectroscopy and size exclusion chromatography. Self-cure composites based on the redox initiator system cumene hydroperoxide/acylthiourea oligomer/copper(II) acetylacetonate were formulated. The flexural strength and modulus were measured using a three-point bending setup. The double bond conversions were determined using NIR spectroscopy. The working time of each self-cure composite was measured using an oscillating rheometer. Leaching experiments using lightcure composites were performed in DMSO-d6. Results: Acylthiourea oligomers ATUO1-3 were successfully synthesized in good yields. Both the oligomer molecular weight and the amount of thiourea groups were varied. Self-cure composites containing ATUO1 or ATUO2 as reducing agents exhibited excellent mechanical properties and high double-bond conversions. The amounts of reducing agent, cumene hydroperoxide and copper(II) acetylacetonate were shown to have a significant impact on the working time. Moreover, a correlation between flexural modulus and the amount of metal salt was clearly established. Self-cure composites containing the oligomer ATUO1 exhibited a longer working time than materials containing ATU1 or acetylthiourea. Contrary to acetylthiourea, ATUO1 was not able to leach out of light-cured composites. Significance: Acylthiourea oligomers are promising reducing agents for the formulation of two-component dental materials that do not induce a bitter taste in mouth.

Redox initiator systems based on cumene hydroperoxide are currently widely used for the curing of self-cure (SC) and dual-cure (DC) dental materials. Unfortunately, cumene hydroperoxide exhibits a strong odor, which can be unpleasant if high amounts of materials are required for a restoration. In order to reduce smell, innovative cumyl hydroperoxide containing oligomers were prepared and tested as oxidizing agents in current self-curing dental formulations. In a first step, a methacrylate monomer containing a cumyl group was synthesized, namely the (4-isopropylbenzoate) 2-ethyl methacrylate (IBEMA). Then, homopolymer from IBEMA (PIBEMA) and copolymers from IBEMA and methyl methacrylate (MMA) (P(MMA-st-IBEMA)) were successfully synthesized using telomerization in the presence of 2-mercaptoethanol with two different low molecular weights and two monomer ratios for the copolymers. The chain-end sulfide groups of all produced oligomers were quantitatively oxidized to sulfonyl groups. Finally, hydroperoxide groups were obtained on the IBEMA units using the oxidation of isopropyl groups, thus leading to poly(4-(2-hydroxyperoxypropylbenzoate) 2-ethyl methacrylate) (PHPPBEMA) homopolymer and poly(methyl methacrylate-st-(4-(2-hydroxyperoxypropyl)benzoate) 2-ethyl methacrylate) (P(MMA-st-HPPBEMA)) copolymers. Self-cure composites based on the latter were formulated and the working time as well as the mechanical properties (flexural strength and modulus) of the cured materials were assessed. The two-component composites prepared with the oligomers containing the highest amounts of hydroperoxide groups provided high flexural strength and modulus values. To the best of our knowledge, we describe here the first synthesis of hydroperoxide-based oligomeric materials that can be used for well identified application. Indeed, such compounds appeared to be a promising alternative to cumene hydroperoxide for the formulation of odorless SC and DC dental materials.

Polymerization shrinkage represents one of the major drawbacks of dental composites. The incorporation of chain transfer agents into dental formulations is an efficient technology that enables a strong reduction of the shrinkage stress. In this contribution, the synthesis of four new addition-fragmentation chain transfer (AFCT) agents bearing urethane groups is described. These compounds were easily synthesized in three steps. Real-time (RT)-NIR photorheology measurements were performed to evaluate rheological behavior (i.e. time of gelation) and chemical conversion (i.e. double bond conversion at the gel point, final double bond conversion) of dimethacrylate resins containing the synthesized transfer agents. Composites based on these AFCT agents provide good mechanical properties as well as low shrinkage force. The presence of the urethane group results in a significant improvement of the mechanical properties. The addition of urethane AFCT agents to dental composites is an efficient technology that enables the formulation of low shrinkage materials having high flexural strength and modulus.

ID-7 NEW CROSSLINKING DILUENTS FOR DENTAL COMPOSITES Norbert Moszner, Jörg Angermann, Urs Fischer Ivoclar Vivadent AG, Liechtenstein norbert.moszner@ivoclarvivadent.com, joerg.angermann@ivoclarvivadent.com, urs.fischer@ivoclarvivadent.com

New monomers, such as phosphonic acid acrylate 1, bis(methacrylamido) dihydrogen phosphate 2, and cross-linking bisacrylamides 3a-e were successfully synthesized. They show high reactivity and good hydrolytic stability under aqueous acidic conditions. Therefore, they enable the preparation of single-bottle self-etching enamel-dentin adhesives with improved storage stability at room temperature.