The two-component Camphorquinone (CQ)/aromatic amine system is well-established and clearly corresponds to the reference system used in all photopolymerizable dental adhesives and composites. However, this CQ/amine system still suffers from the presence of aromatic amines that can be referenced as toxic. Therefore, the aim of this work is to develop amine-free photoinitiating systems (PISs) for the polymerization of a representative dental methacrylate resins upon blue light irradiation. The proposed strategy is based on the in-silico design (by molecular modelling) of new hydrogen donors (amine-free) bearing a copolymerizable moiety (methacrylate functionality) to ensure their low migration/leaching properties from the synthesized polymer. The new proposed PISs are compared to the well-established CQ/amine system for the polymerization of different methacrylate blends upon exposure to a commercial blue dental LED centered at 477 nm.
Molecular orbitals calculations are used to design new hydrogen donors exhibiting low C–H bond dissociation energies. Based on this in-silico design, the syntheses of new co-initiators are reported here for the first time. Real-time FTIR experiments are used to monitor the photopolymerization profiles. Color indexes measurements were also carried out to investigate the bleaching properties of the new proposed systems.
Three new co-initiators are proposed as alternatives to aromatic amines in dental materials in combination with camphorquinone. The performances of the new proposed amine-free PISs for the photopolymerization of thick (1.4 mm) samples of methacrylate upon exposure to a blue dental LED under air are excellent. Similar or better polymerization performances are obtained with the new proposed amine-free systems compared to those reached with the CQ/amine reference. Excellent bleaching properties are also found. The involved chemical mechanisms are investigated through molecular orbitals calculations.
Currently in dental materials, the development of safer chemistries in adhesives, polymers and composites is a huge challenge. [ ] For example, bisphenol-A free-materials were proposed. The use of chemicals less sensitive to leaching is also urgently needed. In this context, the development of safer initiating systems is now required. The two-component Camphorquinone(CQ)/aromatic amine system is well-established and clearly corresponds to the reference system used in almost all photopolymerizable dental adhesives and composites. The chemical mechanisms of this latter Type II initiating system corresponds to a hydrogen abstraction reaction between the camphorquinone triplet state and an amine acting as hydrogen donor (r1 in Scheme 1 ). [ ] More in detail, this latter process corresponds to an electron followed by proton transfer reaction leading to the formation of initiating amino-alkyl radicals that are known as powerful species to initiate polymerization processes i.e. their rate constants of addition onto (meth)acrylate double bond being very high (10 5 -10 7 M −1 s −1 depending of the aminoalkyl radical structures). [ ]
However, this CQ/amine system still suffers from the presence of aromatic amines that can be sometimes toxic. In the Table 1 , the pictograms and information from the European Chemicals Agency (ECHA) on two well-established amines used in dental materials (i.e. ethyldimethylaminobenzoate (EDB) and dimethylaminobenzonitrile (DMABN)) are reported [ ].
Therefore, the search for amine-free systems is an important challenge. Previous works have already proposed sulfonates and sulfinates [ ] as new classes of coinitiators combined with CQ for the photopolymerization upon blue dental LEDs. New hydrogen donors bearing C–H labile hydrogen were also proposed e.g. based on a piperonyl structure [ ] or a germanium hydride [ ]. The proposed strategy here is to continue the development of new co-initiators by the proposition of new hydrogen donors bearing a copolymerizable moiety (methacrylate functionality) to ensure its low migration/leaching properties from the synthesized polymer. Despite recent elegant works, there is still an urgent need for efficient blue light photosensitive systems with high bleaching ability. [ ]
The proposed strategy is based on the in-silico design of new hydrogen donors by molecular modelling of the C–H Bond Dissociation Energy (BDE). We proposed here three new co-initiators ( HDa1 , HDb1 , HDc1 – Scheme 2 ). These products have already been synthesized but using other reaction conditions [ ] and for other applications. Indeed, to our knowledge they have never been tested as co-initiators. Therefore, in this work, their initiating ability will be compared to EDB used as benchmark aromatic amine for the polymerization of methacrylates under blue light irradiation (Smartlite focus from Dentsply Sirona). We will also show that combining these new hydrogen donors with an iodonium salt (Iod) increases the reactivity of the system leading to similar performance than CQ/EDB. The aim of this study is also to obtain better bleaching properties than current amine-based systems. For the new hydrogen-donors, a comparison of their initiating ability with those of the non-methacrylated derivatives ( HDa2 , HDb2 , HDc2 – Scheme 1 ) will be also provided.
Sesamol ( HDa2 ) and triethylamine were obtained from Sigma Aldrich. 6-hydroxy-3-coumaranone ( HDb2 ), 5-hydroxy-2(3 H )-benzofuranone ( HDc2 ) and methacrylic anhydride were obtained from TCI Chemicals. HDa1 , HDb1 and HDc1 were efficiently prepared in one step by acylation of commercially available sesamol, 6-hydroxy-3-coumaranone or 5-hydroxy-2(3 H) benzofuranone using methacrylic anhydride in the presence of triethylamine in CHCl 3 . The synthetic procedure used for the preparation of these new hydrogen-donors is described below. Camphorquinone (CQ) was obtained from Sigma Aldrich and used as a representative Type II photoinitiating system. Ethyldimethylaminobenzoate (EDB) was used as additive in multicomponent systems and obtained from Sigma Aldrich. Speedcure 938 (Iod) was obtained from Lambson Ltd (UK – Wetherby). Spectrum® TPH®3 resin (a benchmark methacrylate formulation in dental materials) from Dentsply Sirona was used for the preparation of the dental compositions. The commercial TPH3 resin is used unfilled and provided without initiator by Dentsply Sirona. The added initiating systems are given weight percent.
Syntheses of HDa1, HDb1 and HDc1
1 H-NMR spectra were recorded at room temperature on a Varian 300 MHz spectrometer. 1 H-NMR chemical shifts (δ) are reported in ppm relative to the residual CHCl 3 in CDCl 3 signal (δ =7.26 ppm). Data for 1 H-NMR spectra are reported as follow: chemical shift (multiplicity, coupling constants, integrations). Abbreviations are as follows: s (singulet), d (doublet), dd (doublet of doublet), m (multiplet).
The reaction products HDa1 , HDb1 and HDc1 were purified by flash chromatography on silica gel (70–230 mesh).
General procedure for the syntheses of HDa1 , HDb1 and HDc1 : the starting material Hda2 , HDb2 or HDc2 (1 eq) was introduced in a two-neck flask and then dissolved in CHCl 3 . After cooling to 0 °C methacrylic anhydride (1.6 eq) and then triethylamine (1.6 eq) were added successively dropwise via a dropping funnel. The reaction mixture was stirred at 0 °C until dissolution of the starting material (∼15 min) and then warmed to room temperature and stirred for another 24 h. At the end of the reaction the organic phase was successively washed with distilled water (30 mL), 1 N HCl (20 mL) and 1 N NaHCO 3 (20 mL). The organic phase was dried over anhydrous MgSO 4 , filtered and evaporated under reduce pressure. The crude reaction mixture was purified by flash- chromatography on silica gel using as eluent a 1:3 mixture of AcOEt/hexane.
1,3-Benzodioxol-5-yl-2-methylprop-2-enoate HDa1 was prepared according to the general procedure starting from sesamol (1.00 g, 7.24 mmol), methacrylic anhydride (1.72 mL, 11.58 mmol) and triethylamine (1.61 mL, 11.58 mmol) in CHCl 3 . After purification of the crude reaction mixture the product was obtained as a colorless oil (1.38 g, 93% yield). 1 H-NMR (300 MHz, CDCl 3 ) δ 6.80 (d, J =8.4 Hz, 1 H), 6.65 (d, J =2.2 Hz, 1 H), 6.56 (dd, J = 8.4, 2.2 Hz, 1 H), 6.33 (s, 1 H), 5.99 (s, 2 H), 5.74 (s, 1 H), 2.05 (s, 3 H).
(3-Oxo-1-benzofuran-6-yl) 2-methylprop-2-enoate HDb1 was prepared according to the general procedure starting from 6-hydroxy-3-coumaranone (0.50 g, 3.33 mmol), methacrylic anhydride (0.79 mL, 5.32 mmol) and triethylamine (0.74 mL, 5.32 mmol) in CHCl 3 . After purification of the crude reaction mixture the product was obtained as an orange solid (0.61 g, 84% yield). 1 H- NMR (300 MHz, CDCl 3 ) δ 7.68 (d, J =8.1 Hz, 1 H), 6.97 (d, J =1.9 Hz, 1 H), 6.86 (dd, J = 8.1, 1.9 Hz, 1 H), 6.37 (s, 1 H), 5.82 (s, 1 H), 4.66 (s, 2 H), 2.06 (s, 3 H).
(2-Oxo-3H-1-benzofuran-5-yl) 2-methylprop-2-enoate HDc1 was prepared according to the general procedure starting from 5-hyrdoxy-2(3 H)-benzofuranone (0.50 g, 3.33 mmol), methacrylic anhydride (0.79 mL, 5.32 mmol) and triethylamine (0.74 mL, 5.32 mmol) in CHCl 3 . After purification of the crude reaction mixture the product was obtained as a yellow oil (0.48 g, 66% yield). 1 H-NMR (300 MHz, CDCl 3 ) δ 7.11−7.02 (m, 3 H), 6.34 (s, 1 H), 5.77 (s, 1 H), 3.75 (s, 2 H), 2.05 (s, 3 H).
The 1H-NMR spectra of compounds HDa1 , HDb1 and HDc1 were in accordance with reported data [ ].
A blue LED@477 nm representative for dental materials usage (Smartlite Focus from Denstply Sirona ∼300 mWcm −2 in the selected conditions) was used for the irradiation of the photocurable samples.
The photosensitive formulations were deposited on a polypropylene film and the sample thickness was controlled using a mold (1.4 mm – thick samples). The samples were polymerized under air using the blue LED. The evolution of the double bond content of the methacrylate function was continuously followed by real-time Fourier Transform Infrared (FTIR) spectroscopy (JASCO FTIR 6600) at about 6165 cm −1 for thick samples.
The l*,a*,b* colorimetric values of the synthesized polymers were determined using a spectrophotometer from thorlabs in transmission experiments. [ ]
The C–H bond dissociation energy for the different hydrogen donors were calculated at the UB3LYP/6−31G* level using the density functional theory approach. The procedure has been presented in [ ].
Results and discussions
Syntheses of the new Co-initiators
Based on our previous work, we have efficiently and easily synthesized in one step three new hydrogen donors HDa1 , HDb1 and HDc1 ( Scheme 3 ) using an acylation reaction of commercially available starting materials with methacrylic anhydride (for more details see the Experimental Section and the 1 H-NMR spectra in the Supporting Information). A high purity is validated for the prepared compounds by NMR spectra after flash chromatography purification. To the best of our knowledge, these new compounds have never been used in radical photopolymerization and more particularly as co-initiators in Type II photoinitiating systems. Thanks to the presence of a labile hydrogen born by the methylene group of the heterocylic moiety and the introduction of a copolymerizable part, these compounds act both as coinitiators and copolymerizable monomers and are therefore much less subject to migration.