Influence of the hydroperoxide structure on the reactivity and mechanical properties of self-cure dental composites.

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.

Ethyl-2-(tosylmethyl)acrylate: A promising chain transfer agent for the development of low-shrinkage dental composites.

OBJECTIVE: To evaluate the potential of ethyl-2-(tosylmethyl)acrylate (ASEE) as chain transfer agent for the development of low-shrinkage photopolymerizable dental composites. METHODS: Composites containing 10, 20 and 30 mol% of ASEE in their organic matrix were formulated. Camphorquinone (CQ)/ethyl 4-(dimethylamino)benzoate (EDAB) (0.33 wt%/0.60 wt%), CQ/EDAB/Ivocerin® (0.33 wt%/0.60 wt%/0.10, 0.25 or 0.50 wt%), CQ/EDAB/SpeedCure 938 (SC-938) (0.33 wt%/0.60 wt%/0.30, 0.50 or 1.00 wt%) and Ivocerin® (0.50 wt%) were used as photoinitiator systems. The glass transition temperature (Tg) and the crosslink density were determined by DMTA measurements. The flexural strength/modulus and ambient light working time were assessed according to ISO 4049. The shrinkage force was evaluated using a universal testing machine. The double bond conversion (DBC) was determined by NIR spectroscopy. DBC, flexural strength and modulus were measured after the storage of the specimens in deionized water at 37 °C for 24 h. The DBC, flexural strength and modulus data were analyzed by one-way ANOVA with p = 0.05 as significance level. RESULTS: ASEE-based composites containing the classical initiator system CQ/EDAB exhibited low mechanical properties (flexural strength/modulus) and DBC. The screening of various photoinitiator systems showed that composites based on CQ/EDAB/Ivocerin® (0.33 wt%/0.60 wt%/0.50 wt%), Ivocerin® (0.50 wt%) or CQ/EDAB/SC-938 (0.33 wt%/0.60 wt%/1.00 wt%) were particularly attractive. Indeed, the use of these photoinitiator systems enabled the formulation of composites containing up to 30 mol% ASEE exhibiting excellent mechanical properties, high DBC, good network homogeneity and low shrinkage force values. Interestingly, the addition of SC-938 did not impair the ambient light working time of the uncured composites, whereas the incorporation of 0.50 wt% Ivocerin® resulted in a strong decrease of this value. SIGNIFICANCE: The addition of the allyl sulfone ASEE in combination with the initiator system CQ/EDAB/SC-938 (0.33 wt%/ 0.60 wt%/ 1.00 wt%) is a promising strategy to develop low-shrinkage dental composites which exhibit excellent mechanical properties, low shrinkage force, high DBC and suitable ambient light working time.

Acylthiourea oligomers as promising reducing agents for dimethacrylate-based two-component dental materials.

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 light-cure 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.

Polymerizable thioureas as innovative reducing agents for self-cured and dual-cured 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.

UV-induced photolysis of polyurethanes.

Waste production associated with the use of non-degradable materials in packaging is a growing cause of environmental concern, with the polyurethane (PU) class being notorious for their lack of degradability. Herein, we incorporate photosensitive ortho-Nitrobenzyl units into PUs to achieve controllable photodegradability. We performed their photolysis in solution and thin films which can inform the design of degradable adhesives.

Chain-Length-Dependent Photolysis of ortho-Nitrobenzyl-Centered Polymers.

Herein, we demonstrate that the photochemical cleavage of linear polymers containing a midchain photocleavable moiety strongly depends on the chain length. Based on an ortho-nitrobenzyl (oNB) difunctional reversible addition-fragmentation chain-transfer agent, well-defined poly(methyl acrylate)s (Mn = 1.59-67.6 kg mol-1, D = 1.3-1.4) were synthesized following a core-first approach. Photolysis at λmax = 350 nm of the ortho-nitrobenzyl moiety led to the generation of equally sized polymer segments. The rate of oNB-driven polymer fragmentation, which can be well described by first-order kinetics, strongly increases with increasing molecular weight in a nonlinear fashion, potentially caused by entropic considerations and is compared to the ideal chain model. The current study thus demonstrates that polymer photolysis is dependent on the polymer chain length, with critical implications for photocleavable network design.

High refractive index monofunctional monomers as promising diluents for dental composites.

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 1H 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.

Synthesis and homopolymerization kinetics of 7-(methacroyloxy)-2-oxo-heptylphosphonic acid and its copolymerization with methyl methacrylate.

The synthesis of polymerizable 7-(methacroyloxy)-2-oxo-heptylphosphonic acid M1 destined for self-etch adhesives is described. M1 is characterized by 1H, 13C and 31P-NMR spectroscopy. Its homopolymerization and copolymerization reactivity in the solvents methanol and dioxane between 45 and 70°C in the presence of azobisisobutyronitrile (AIBN) are examined. Polymerization proceeds readily through a thermal free radical initiation. The intensity exponents for the monomer and initiator are only slightly over 1 and approximately 0.5, respectively. This is in accordance with the results typically observed for an ideal free radical polymerization with termination mainly by disproportionation, which is typical for methyl methacrylate (MMA) homopolymerization. The kinetics of copolymerization with MMA are monitored by online 1H-NMR spectroscopy. Two copolymerization reactions for each pair of co-monomers are sufficient to evaluate the copolymerization parameters using the Jaacks method, the Fineman-Ross method and the nonlinear least-squares method. All three methods give similar results for particular monomer M1/MMA couple.

Evaluation of alternative monomers to HEMA for dental applications.

OBJECTIVE: The objective of this work is to find potential alternative monomers to 2-hydroxyethyl methacrylate (HEMA) for dental materials (self-etch adhesives and luting composites). METHODS: Monomers 1-9 were tested as potential HEMA substitutes. Methacrylates 4, 5 and 6 and (N-methyl)acrylamides 7-9 were synthesized and characterized by 1H NMR spectroscopy. The reactivity of each monomer was studied using photo-DSC. Mixtures of monomers 1-9 with the urethane dimethacrylate UDMA (1/1: wt/wt) were formulated and cured. The water sorption and solubility of these materials were determined according to ISO 4049. Luting composites based on monomers 1-9 or on HEMA were formulated. The flexural strength and modulus of elasticity were measured using a three-point bending setup, according to ISO 4049. Self-etch adhesives containing monomers 1-9 or HEMA were prepared and used to mediate a bond between the dental composite Tetric EvoCeram® and both dentin and enamel. The shear bond strength (SBS) was measured using a Zwick universal testing machine. RESULTS: Polymerizable diols 3 and 4 as well as (N-methyl)acrylamides 7-9 were found to be significantly more reactive than HEMA. Resins based on the hydrophilic monomers 3, 7 and 8 exhibited a significantly higher water sorption than the corresponding HEMA-containing material. Luting composites containing monomers 2, 3, 6 and 7 showed similar or even improved mechanical properties compared to the reference material containing HEMA. Self-etch adhesives containing monomers 4 and 9 provided significantly higher dentin SBS than the reference material. SIGNIFICANCE: Some of the evaluated monomers are promising candidates for the development of HEMA-free dental materials.

Monomers for adhesive polymers, 18. Synthesis, photopolymerization and adhesive properties of polymerizable α-phosphonooxy phosphonic acids.

Four polymerizable α-phosphonooxy phosphonic acids 7a, 7b, 9 and 16 were synthesized in seven steps. They were characterized by 1H, 13C and 31P NMR spectroscopy and by high-resolution mass spectroscopy. The copolymerization of acidic monomers 7a, 7b, 9 and 16 with 2-hydroxyethyl methacrylate was studied using a differential scanning calorimeter. Due to the presence of two acidic groups, those monomers are significantly more reactive than 10-methacryloyloxydecylphosphonic acid (MDPA) and 10-methacryloyloxydecyl dihydrogen phosphate (MDP). Self-etch adhesives based on monomers 7a, 7b, 9 and 16 were formulated and used to mediate a bond between a dental composite and the dental hard tissues (dentin and enamel). These adhesives exhibit excellent performances and provide significantly higher dentin and enamel shear bond strength than adhesives based on MDP or MDPA.

Synthesis, radical scavenging activity, protection during storage, and frying by novel antioxidants.

Novel antioxidants, derivatives of trolox, and selected phenolic acids have been prepared in good yields and fully characterized by (1)H NMR, (13)C NMR, and MS. Their antioxidant activities have been assessed by DPPH and ORAC assays, and during frying and accelerated storage tests. Novel phenolic compounds exhibited higher radical scavenging activities than both trolox and α-tocopherol. Trolox hydroxybenzoate showed a significantly higher protection than α-tocopherol under storage conditions. All new antioxidants performed better than α-tocopherol under frying conditions. Moreover, their outstanding thermal stability makes them more valuable than α-tocopherol for frying applications.