For mild and ecofriendly polymerization conditions, Redox Initiating Systems (RISs) are more and more attractive. Indeed, the redox polymerization can be carried out at room temperature without any external energetic stimulus. Moreover, these latter processes can be very efficient under air and without purification of the monomers/resins. In this work, new redox initiating systems based on dihydropyridines are reported. An excellent reactivity is found in presence of copper complexes as catalyst, acid and peroxide. A full control of the gel time is possible by the selection of the appropriated reactant concentrations. These systems were characterized by optical pyrometry experiments as well as FTIR spectroscopy. Tack-free surfaces as well as potential photoactivation for on-demand polymerization can be obtained.

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.

Photocuring 3D printing of materials exhibiting high fracture toughness and excellent mechanical properties (flexural strength/modulus) is challenging. Nowadays, most of the photocurable 3D printing resins are based on a mixture of multifunctional (meth)acrylates and provide therefore brittle materials. This article describes further developments of a toughening strategy based on the incorporation of block copolymers in low crosslink density methacrylate-based materials. Six dimethacrylates bearing a bisphenol A core and urethane groups are successfully synthesized. Various spacers between the bisphenol A core and the methacrylate groups are selected. Each monomer is combined with (octahydro-4,7-methano-1H-indenyl)methyl acrylate as a monofunctional monomer and a poly(epsilon-caprolactone)-polydimethylsiloxane-poly(epsilon-caprolactone) triblock copolymer is added as toughener. It is shown that the addition of the triblock copolymer results for all mixtures in a strong increase of the fracture toughness. Moreover, the higher the amount of monofunctional monomer, the stronger the increase. The nature of the urethane dimethacrylate is found to have a significant influence on the fracture toughness, flexural strength, and flexural modulus of cured materials. Two of the synthesized dimethacrylates are identified as promising candidates for the development of fracture-tough photocuring 3D printing materials. Innovative photocurable resins based on novel urethane macromonomers, a monofunctional acrylate, and a triblock copolymer, are evaluated for the preparation of fracture tough denture base materials. This article highlights the high efficiency of triblock copolymers as toughening agents in low crosslink density dimethacrylate networks. image

3D printing of fracture tough dental materials that additionally exhibit excellent mechanical properties is challenging. Nowadays, most of the 3D printing dental materials contain a mixture of highly reactive dimethacrylates. The corresponding printed materials exhibit a high crosslink density and are particularly brittle. They are therefore not suitable for additive manufacturing of fracture tough denture bases. Recently, an efficient technology based on the combination of a urethane dimethacrylate macromonomer with a monofunctional monomer and a poly(epsilon-caprolactone)-polydimethylsiloxane-poly(epsilon-caprolactone) triblock copolymer was developed. Materials exhibiting a low crosslink density, excellent mechanical properties, and high fracture toughness were obtained. In this article, further developments of this highly efficient technology are described. A wide range of monofunctional monomers (both methacrylates and acrylates) was evaluated in this system. It was shown that the structure of the selected monofunctional monomer has a strong influence on the mechanical properties (flexural strength and modulus) as well as on the fracture toughness of the light cured materials. Thanks to the formation of nanostructures, a strong increase of fracture toughness was obtained upon addition of the toughening agent (poly(epsilon-caprolactone)-polydimethylsiloxane-poly(epsilon-caprolactone) triblock copolymer). The most promising materials were the ones based on the following monofunctional monomers: 2-phenoxyethyl methacrylate, (octahydro-4,7-methano-1H-indenyl)methyl acrylate, isobornyl acrylate, tetrahydrofurfuryl methacrylate and 4-tert-butylcyclohexyl acrylate. Indeed, these materials exhibited excellent mechanical properties (90.0 +/- 3.8 MPa < flexural strength < 102.6 +/- 4.7 MPa; 2402 +/- 90 MPa < flexural modulus < 2714 +/- 68 MPa) combined with high fracture toughness (1.89 +/- 0.06 MPa m(1/2) <= maximum stress intensity factor (K-max) <= 2.18 +/- 0.08 MPa m(1/2); 418 +/- 14 J m(-2) <= work of fracture (W-f) <= 591 +/- 25 J m(-2)). The measured K-max and W-f were even significantly higher than the values reported for Probase Hot (K-max = 1.44 +/- 0.18 MPa m(1/2); W-f = 270 +/- 30 J m(-2)), a clinically proven and well-established PMMA based denture base material from Ivoclar. The successful DLP 3D printing of a monoblock denture using the most promising formulation confirmed the great potential of this technology for the development of 3D printing fracture tough denture bases.

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.

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.

OBJECTIVE:To evaluate polymerizable acylthioureas as reducing agents in two-component dental materials. METHODS:Acylthioureas 1 and 2 were synthesized and characterized by 1H and 13C NMR spectroscopy. Self-cured composites based on the redox initiator system cumene hydroperoxide/acylthiourea 1 or 2/copper(II) acetylacetonate were formulated. Various amounts of cumene hydroperoxide, acylthiourea and copper(II) acetylacetonate were used. An equimolar cumene hydroperoxide/acylthiourea ratio was selected for each self-cured 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 composite was determined using an oscillating rheometer. RESULTS:Acylthioureas 1 and 2 were synthesized in three to four steps. In combination with cumene hydroperoxide and copper(II) acetylacetonate, both prepared compounds were found to be effective reducing agents. The higher the amount of cumene hydroperoxide and acylthiourea in the self-cured composite, the higher the flexural modulus and the faster the polymerization (lower working times). Similarly, it was shown that increased copper(II) acetylacetonate amounts result in an acceleration of the curing as well as in an improvement of the mechanical properties. The self-cured composite containing 1.25 wt% of cumene hydroperoxide in the monomer mixture of the first paste and 2.00 wt% of acylthiourea 1 in the monomer mixture of the second one provided excellent mechanical properties as well as an optimal working time. SIGNIFICANCE:Polymerizable acylthioureas can be used as reducing agents in two-component dental materials. Due to the presence of the methacrylate group, such structures should be efficiently incorporated into the network during polymerization and should not leach out of the composite after curing. As a result, such dental materials are not expected to exhibit bitterness properties.

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.

Objectives. To evaluate high refractive index methacrylates as diluents for the formulation of radiopaque esthetic bulk-fill composites. Methods. 2-(4-Cumylphenoxy)ethyl methacrylate 1, 2-(2-phenylphenoxy)ethyl methacrylate 2 and 2-[2-(2-phenylphenoxy)ethoxy]ethyl methacrylate 3 were synthesized and characterized by H-1 NMR spectroscopy. The reactivity of each monomer was studied using photo-DSC. Bulk-fill composites based on monomers 1-3 were formulated. Translucency (before and after light cure) was measured using a spectrophotometer. The depth of cure and the water sorption of these materials were determined according to ISO 4049. The flexural strength and modulus of elasticity were measured using a three-point bending setup, according to ISO 4049. The shrinkage force was assessed based on a method described by Watts et al. using a universal testing machine. Results. Monomers 1-3 were easily synthesized in two steps. They exhibit a low viscosity and a high refractive index (1.553-1.573). Monofunctional methacrylates 1-3 were found to be more reactive than triethylene glycol dimethacrylate (TEGDMA). Bulk-fill composites based on these monomers were successfully prepared. They exhibit a high depth of cure and excellent esthetic properties (low transparency). These composites provide higher flexural modulus as well as lower water sorption than a corresponding material based on TEGDMA. Methacrylates 1 and 3 are particularly interesting as they led to composites showing lower shrinkage force. Significance. Methacrylates 1-3 are promising diluents for the formulation of highly esthetic radiopaque bulk-fill composites. (c) 2020 Published by Elsevier Inc. on behalf of The Academy of Dental Materials.

ABSTRACT Tetrabenzoylgermane 1 and various substituted tetrabenzoylgermanes 2 – 7 were investigated as visible light (VL) photoinitiators (PIs) for dental dimethacrylate resins and dimethacrylate‐based composites. The tetrabenzoylgermanes 1 – 7 show a very strong VL absorption between 400 and 450 nm. Substituents on the benzoyl chromophore strongly influence their properties such as melting point, solubility, absorption behavior, or PI reactivity. A good photobleaching behavior and a very high reactivity as VL PI was found in photo‐differential scanning calorimeter experiments for selected tetrabenzoylgermanes. Composite pastes containing only ∼0.1 wt % of Ge‐PI exhibited a sufficient photocuring due to the high PI‐reactivity of the tetraacylgermanes. Among the investigated germane PIs, tetrakis(2‐methylbenzoyl)germane 2 shows the best performance as VL PI for restorative composites and enables the composites to be photocured using an LED with an emission maximum of 500 nm. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135 , 46115.

The multistep synthesis of four new acidic vinylcyclopropanes is described. These monomers were fully characterized using H-1 NMR, C-13 NMR and P-31 NMR spectroscopy and high-resolution mass spectrometry. The homopolymerization of the corresponding polymerizable di-tert-butyl phosphates as well as the copolymerization of each acidic monomer with 1,1-diethoxycarbonyl-2-vinylcyclopropane were investigated using photo-differential scanning calorimetry with bis(4-methoxybenzoyl)diethylgermane as photoinitiator. Self-etch adhesives based on the synthesized acidic vinylcyclopropanes are able to achieve a strong bond between a dental composite and both dentin and enamel. Flowable composites containing these acidic vinylcyclopropanes exhibit good mechanical properties. The replacement of methacrylates by vinylcyclopropanes leads to a significant reduction of the shrinkage stress of flowable composites. (c) 2017 Society of Chemical Industry

Polymerization shrinkage of dental composites remains a major concern. Free-radically polymerizable cyclic monomers can be a conceivable alternative to methacrylates for the development of low-shrinkage composites. In this study, the one-step synthesis of the novel low viscosity difunctional vinylcyclopropanes 1 – 4 is described. Photopolymerization kinetics of these monomers are investigated by photo-differential scanning calorimeter, using bis(4-methoxybenzoyl)diethylgermane as photoinitiator. Real-time near-infrared photorheology measurements are performed to evaluate rheological behavior (i.e., time of gelation, polymerization-induced shrinkage force) and chemical conversion (i.e., double bond conversion at the gel point, final double bond conversion) of the vinylcyclopropanes in situ. The potential of these monomers as reactive diluents in dental restorative materials is evaluated. Composites based on vinycyclopropanes 1 – 4 show good mechanical properties and exhibit significantly lower volumetric shrinkage and shrinkage stress than corresponding dimethacrylate-based materials. The results indicate that such monomers are promising candidates for the replacement of commonly used low viscosity dimethacrylates such as triethylene glycol dimethacrylate in dental composites.

Polymerization shrinkage of methacrylate-based dental composites remains a major concern in restorative dentistry. Cyclic monomers, such as 1,1-disubstituted 2-vinylcyclopropanes, might represent an interesting alternative to dimethacrylates in order to reduce shrinkage. In this contribution, the synthesis of seven crosslinking vinylcyclopropanes bearing urethane groups is described. These monomers were synthesized by esterification of 1-ethoxycarbonyl-2-vinylcyclopropane-1-carboxylic acid with either ethylene glycol or diethylene glycol, followed by a reaction with selected diisocyanates. In order to evaluate the reactivity of the synthesized vinylcyclopropanes, their photopolymerization behavior was investigated by photo-differential scanning calorimetry. Real-time (RT)-NIR-photorheology measurements were performed to evaluate rheological behavior (i.e. time of gelation, polymerization induced shrinkage force) and chemical conversion (i.e. double bond conversion at the gel point, final double bond conversion) of the vinylcyclopropanes in situ. Composites based on these monomers show good to excellent mechanical properties and exhibit low shrinkage. The presence of urethane groups provides a significant improvement of the mechanical properties. The replacement of methacrylates by urethane vinylcyclopropanes appears to be a promising approach to develop low-shrinkage dental composites without sacrificing the mechanical properties.

ABSTRACT Polymerization shrinkage of methacrylate‐based dental composites remains a major concern in restorative dentistry. Cyclic monomers that undergo ring‐opening polymerization are known to exhibit reduced polymerization shrinkage compared to methacrylates. In this article, the synthesis of four crosslinking 1,1‐disubstituted 2‐vinylcyclopropanes bearing rigid spacers is described. These monomers were synthesized by esterification of 1‐ethoxycarbonyl‐2‐vinylcyclopropane‐1‐carboxylic acid with the corresponding diols. The photopolymerization kinetics of these monomers was investigated by photo‐differential scanning calorimeter using bis(4‐methoxybenzoyl)diethylgermane as the photoinitiator. The synthesized vinylcyclopropanes (VCPs) were shown to be more reactive than the frequently used reactive diluent triethylene glycol dimethacrylate. Composites based on these VCPs showed good mechanical properties and exhibited a significantly reduced volumetric shrinkage and shrinkage stress compared to a corresponding dimethacrylate‐based restorative material. This work highlights the excellent potential of VCPs as alternatives to methacrylates in the development of low‐shrinkage dental composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45577.

Bis(4-methoxybenzoyl) diethylgermane (BMDG) is used for the photopolymerization of six 1,1-disubstituted 2-vinylcyclopropanes (VCPs). A significantly higher photo-polymerization reactivity compared to the conventional camphorquinone (CQ)/amine or CQ/amine/iodonium salt photoinitiator systems is observed. Furthermore, the influence of the electron-withdrawing substituents on the monomer reactivity is investigated. 1-Ethoxycarbonyl-1-ethylcarbamoyl2-vinylcyclopropane 5 and 1-ethoxycarbonyl-2-vinylcyclopropanecarboxylic acid 2 are found to be the most reactive monomers. 1,1-Disubstituted VCP-based dental composites containing BMDG are formulated. Those materials exhibit good mechanical properties as well as a low polymerization shrinkage.