Five para-substituted monoacyltrimethylgermane derivatives, i.e., p-fluorobenzoyltrimethylgermane (pFBG, λmax = 405 nm), p-methoxybenzoyltrimethylgermane (pMBG, λmax = 397 nm), ...benzoyltrimethylgermane (pHBG, λmax = 409 nm), p-cyanobenzoyltrimethylgermane (pCBG, λmax = 425 nm), and p-nitrobenzoyltrimethylgermane (pNBG, λmax = 429 nm) are investigated via a combination of pulsed laser polymerization with subsequent electrospray ionization and mass spectrometry (PLP-ESI-MS) as well as femtosecond transient absorption spectroscopy. The relative initiation efficiencies of the initiating benzoyl radical fragments of pFBG, pMBG, and pHBG are determined using PLP-ESI-MS. The para-substituted derivatives with the electron-donating groups, pFBG and pMBG, display a factor 1.5 and 1.3, respectively, superior overall initiation efficiency compared to the unsubstituted pHBG. In contrast, the derivatives pCBG and pNBG carrying electron-withdrawing groups display only weak initiation behavior at a factor 4 higher total energy of ∼112 J (∼28 J for typical PLP experiments with pMBG, pFBG, and pHBG at ∼320 J and 90 000 pulses). The differences in the initiation efficiencies are representative for two classes of monoacyltrimethylgermane initiators, i.e., efficient initiators and weak initiators, each distinct in their specific radical cleavage mechanism. The efficient initiators pMBG, pFBG, and pHBG show an ultrafast intersystem crossing within 2–4 ps after pulse irradiation and subsequent formation of benzoyl and trimethylgermyl radical fragments. In contrast, the weak initiators pCBG and pNBG relax to the ground state after photoexcitation via a dominating ultrafast internal conversion (IC) within 13 and 2 ps, respectively, disallowing effective initiation under typical PLP conditions (∼320 J/pulse with 90 000 pulses resulting in ∼28 J total energy per sample). pCBG features weak initiation behavior additionally forming methyl and p-cyanobenzoyldimethylgermyl radicals at a factor 4 higher total energy of ∼112 J. Consistent with a considerably faster IC relaxation, pNBG features a factor 10 weaker monomer conversion than pCBG.
Supramolecular polymer networks based on polyacrylates with hydrogen bonding 2-ureido-41H-pyrimidinone (UPy) side chains are of considerable interest due to the stimuli-responsive nature imparted by ...the reversible cross-links formed by dimerized UPy groups. Previously reported UPy-containing acrylic monomers are solid and show limited miscibility with comonomers, and this has stifled their (co)polymerization in bulk. We here report the synthesis of a liquid 2-ureido-41H-pyrimidinone methacrylamide (UPy-OPG-MAA), which was made by connecting the UPy motif and methacrylamide (MAA) via an amine-terminated oligo(propylene glycol) (OPG) linker. The new monomer was miscible with conventional methacrylates. This permitted the photoinitiated free-radical bulk copolymerization with hexyl methacrylate (HMA) to afford a series of copolymers (poly(UPy-OPG-MAA-co-HMA)) in which the UPy-OPG-MAA content was varied between 0 and 20 mol %. The investigation of the mechanical properties of these copolymers by dynamic mechanical analysis and adhesion tests revealed that the introduction of the UPy groups leads to an increase of the stiffness in the glassy state, the formation of a rubbery plateau above the glass transition temperature, and a significant increase of the adhesive strength. Joints bonded with poly(UPy-OPG-MAA-co-HMA) could be debonded on demand using light or heat.
Display omitted
•Synthesis of five novel polymerizable addition-fragmentation chain transfer agents.•Development of dental composites exhibiting low shrinkage stress.•Low chain transfer agent ...leaching ability.
One of the major drawbacks for restorative dentistry is the polymerization shrinkage and the consequential shrinkage stress in methacrylate-based dental composites. An efficient way to reduce shrinkage stress is the incorporation of addition fragmentation chain transfer agents (AFCT agents). In this contribution, five novel polymerizable allyl sulfones were synthesized in four to five steps. A photo-DSC study was carried out in order to evaluate the influence of the addition of each AFCT agent on the polymerization rate of a dimethacrylate-based monomer mixture. The glass transition temperature (Tg) of the resulting networks was measured using DMTA. Dental composites based on these new chain transfer agents (CTAs) were prepared. A combination of camphorquinone, ethyl 4-(dimethylamino)benzoate and bis-(4-t-butylphenyl)-iodonium hexafluorophosphate was selected as photoinitiator system. Low shrinkage force values, suitable ambient light working time and excellent mechanical properties were obtained for all CTA based materials. Composites based on three of the synthesized CTAs even exhibited higher flexural modulus than the corresponding CTA-free material. Furthermore, a significant reduction of unreacted CTA leaching was found if a polymerizable moiety was introduced in the CTA structure. Hence, the incorporation of polymerizable allyl sulfones in dental composites is a promising strategy to obtain low shrinkage materials exhibiting improved biocompatibility.
Three series of regioisomeric bisadducts of C60, namely, C62(anisyl)4 and the mixed systems C62(anisyl)2‐(COOEt)2, and C61(COOEt)2(NCOOEt), were synthesized starting from the 1,2‐monoadducts ...C61(COOEt)2 (1), C61‐(anisyl)2 (2), and C60(NCOOEt) (4) by using the Bingel and Bamford‐Stevens reactions, and nitrene additions. In the case of C61(COOEt)2(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis‐1 isomers of C62(anisyl)4 and C62(anisyl)2(COOEt)2 were not formed. The transannular 6,6 bonds in the cis‐1 isomer 42 of C61(COOEt)2(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C62‐(COOEt)4 and C60(NCOOEt)2, which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans‐3 for sterically demanding addends (e.g., combinations of C(anisyl)2 and C(COOEt)2) and cis‐1, e, and trans‐3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)2). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend‐independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.
A compression of the fullerene cage along the a axis (see Figure) occurs when 1,2‐monoadducts are formed. This leads to increased reactivities of the cis‐1, e, and trans‐3 double bonds. An addend‐independent distortion of the 60fullerene framework thus determines the regioselectivity of subsequent additions to 6,6 double bonds.
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.
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.
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.
Acylthiourea oligomers are promising reducing agents for the formulation of two-component dental materials that do not induce a bitter taste in mouth.
•Three acylthiourea oligomers were synthesized by cotelomerization of an acylthiourea monomer with butyl methacrylate.•Self-cure composites based on acylthiourea oligomers were formulated and evaluated.•Materials exhibiting excellent mechanical properties and suitable working times were successfully obtained.
Because of the poor solubility of the commercially available bisacylphosphine oxides in dental acidic aqueous primer formulations, ...bis(3-{2-(allyloxy)ethoxymethyl}-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)ethoxymethyl}-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)ethoxymethyl}-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.
Display omitted
•For mild polymerization conditions, Redox Initiating Systems (RISs) are more and more attractive.•New redox initiating systems based on dihydropyridines are reported.•An excellent ...reactivity is found in combination with copper catalysts, acid and peroxide.•A full control of the gel time is possible by the selection of the appropriated reactants.•Chemical mechanisms are proposed for the new redox systems.
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.
To evaluate polymerizable acylthioureas as reducing agents in two-component dental materials.
Acylthioureas 1 and 2 were synthesized and characterized by
H and
C 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.
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.
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.
Display omitted
•Synthesis of four innovative addition-fragmentation chain transfer agents bearing urethane groups.•Evaluation of the rheological behavior using real ...time-NIR-photorheology.•Formulation and characterization of innovative low-shrinkage dental composites exhibiting excellent mechanical properties.•Influence of the urethane group on the mechanical properties of the tested 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.
PDF