2.1

Materials

Ethyl 2-​(hydroxymethyl)​acrylate 1 and Ivocerin ^® were purchased from Synthon Chemicals GmbH (Germany). 2,3-Dihydroxypropyl methacrylate 3 was purchased from ABCR (Germany). 4-​(2,​2-​Dimethyl-​1,​3-​dioxolan-​4-​yl)​butan-​1-​ol 10 as well as ethyl 2-​(4-hydroxy-2-oxabutyl)​acrylate 2 and N -​methyl-​ N -​(2-​hydroxyethyl)​acrylamide 7 were synthesized according to procedures reported in the literature . Aerosil OX50 and R709, 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)-phenyl]propane (Bis-GMA), HEMA and glycidyl methacrylate were purchased from Evonik (Germany). Boron trifluoride diethyl etherate was purchased from Alfa Aesar (Germany). MDP was purchased from Orgentis (Germany). Camphorquinone (CQ) was purchased from Rahn (Switzerland). Bis-[(2-methacryloyloxyethoxy-carbonyl)-amino]-2,2,4(2,4,4)-trimethylhexane (UDMA) and N , N ′-diethyl-1,3-bis-(acrylamido)propane (DEBAAP) were produced by Ivoclar Vivadent AG (Liechtenstein) and MinAscent Technologies GmbH (Germany), respectively. Ytterbium fluoride was purchased by Sukgyung AT (Korea). All other reagents were purchased from Sigma-Aldrich (Switzerland). Column chromatographies were performed on Macherey-Nagel silica gel 60 (40–63 μm).

2.2

Measurements

¹ H NMR spectra were recorded on a DPX-400 spectrometer using TMS as internal reference for spectroscopic chemical shifts. Data are given in the following order: chemical shift in ppm, multiplicity (s, singlet; br s, broad singlet; d, doublet; t, triplet; m, multiplet), coupling constant in Hertz, assignment. The viscosity of the monomers was measured at 23 °C with an Anton-Paar Rheometer MCR 302 using a CC17-SN3382 measuring system.

2.3

Syntheses

2.3.1

Synthesis of monomer 4

2.3.1.1

4-(2,2-Dimethyl-1,3-dioxolan-4-yl)butyl methacrylate 11 ( Fig. 2 )

Methacryloyl chloride (42.8 g, 409.5 mmol) was added dropwise, at 0 °C, to a solution of alcohol 10 (67.95 g, 390 mmol) and triethylamine (43.6 g, 430 mmol) in dichloromethane (DCM) (500 mL) ( Fig. 2 ). The reaction mixture was stirred for 10 min at 0 °C and for 3 h at room temperature (RT). Deionized water (250 mL) was added and the solution was stirred for 30 min. The phases were separated and the aqueous phase was extracted with DCM (1 × 250 mL). The combined organic layers were dried over sodium sulfate and the solvent was removed under reduced pressure. Hexane (150 mL) was added and the mixture was filtered through a pad of aluminum oxide (150 g). The solution was concentrated under reduced pressure. 62.2 g (256.7 mmol) of the desired product were isolated.

Synthesis of 5,6-dihydroxyhexyl methacrylate 4.

Yield: 66%. Aspect: colorless oil. ¹ H NMR (400 MHz, CDCl ₃ ): δ = 1.35 (s, 3H, CH ₃ ); 1.41 (s, 3H, CH ₃ ); 1.39–1.76 (m, 6H, CH ₂ ); 1.94 (s, 3H, CH ₃ ); 3.51 (t, ³ J _HH = 6.5 Hz, 1H, CH ₂ O); 4.01–4.13 (m, 2H, CH ₂ O and CHO); 4.15 (t, ³ J _HH = 6.5 Hz, 2H, CH ₂ OCO); 5.54–5.57 (m, 1H, C CH ₂ ); 6.10 (s, 1H, C CH ₂ ).

2.3.1.2

5,6-Dihydroxyhexyl methacrylate 4

Pyridinium p -toluenesulfonate (PPTS, 1.13 g, 4.5 mmol) was added to a solution of methacrylate 11 (9.53 g, 39.3 mmol) in methanol (MeOH) (250 mL). The reaction mixture was stirred for 16 h. The solution was concentrated under reduced pressure and further MeOH (250 mL) was added. The solution was stirred for 5 additional hours at RT and was concentrated under reduced pressure. The crude product was purified by column chromatography (eluent = hexane/ethyl acetate: 1/1). 5.95 g (29.4 mmol) of the desired product were isolated.

Yield: 75%. Aspect: slightly yellow oil. ¹ H NMR (400 MHz, CDCl ₃ ): δ = 1.35–1.60 (m, 4H, CH ₂ ); 1.64–1.76 (m, 2H, CH ₂ ); 1.94 (s, 3H, CH ₃ ); 2.13 (br s, 1H, OH); 2.31 (br s, 1H, OH); 3.44 (dd, ² J _HH = 10.9 Hz, ³ J _HH = 7.5 Hz, 1H, CHC H ₂ O); 3.67 (dd, ² J _HH = 10.9 Hz, ³ J _HH = 3.0 Hz, 1H, CHC H ₂ O); 3.69–3.77 (m, 1H, CHO); 4.17 (t, ³ J _HH = 6.5 Hz, 2H, CH ₂ OCO); 5.54–5.57 (m, 1H, C CH ₂ ), 6.10 (s, 1H, C CH ₂ ).

2.3.2

Synthesis of monomers 5 and 6

2.3.2.1

2-Hydroxy-3-methoxypropyl methacrylate 5

A solution of glycidyl methacrylate (43.90 g, 309 mmol) in MeOH (200 mL) was added dropwise, at 0 °C, to a solution of boron trifluoride diethyl etherate (1.80 g, 12.7 mmol) in MeOH (450 mL). The mixture was stirred for 1 h at RT. The solvent was removed under reduced pressure. The crude product was dissolved in ethyl acetate (100 mL) and hexane (200 mL) was added. The cloudy solution was filtered through a pad of silica gel and the solvent was removed under reduced pressure. The material was purified by distillation (bp: 57 °C (0.02 mbar)). 41.26 g (237 mmol) of the desired monomer were isolated.

Yield: 77%. Aspect: colorless liquid. The NMR spectroscopic data of the major isomer is provided below. ¹ H NMR (400 MHz, CDCl ₃ ): δ = 1.95 (s, 3H, CH ₃ ); 2.56 (br s, 1H, OH); 3.40 (s, 3H, CH ₃ ); 3.43 (dd, ² J _HH = 9.6 Hz, ³ J _HH = 6.1 Hz, 1H, C H ₂ OCH ₃ ); 3.49 (dd, ² J _HH = 9.6 Hz, ³ J _HH = 4.3 Hz, 1H, C H ₂ OCH ₃ ); 4.01–4.09 (m, 1H, CH OH); 4.22 (t, ³ J _HH = 5.0 Hz, 2H, CH ₂ OCO); 5.58–5.61 (m, 1H, C CH ₂ ); 6.13 (s, 1H, C CH ₂ ).

2.3.2.2

2-Hydroxy-3-ethoxypropyl methacrylate 6

Monomer 6 was synthesized according to the procedure described for the synthesis of monomer 5 . Glycidyl methacrylate (28.80 g, 203 mmol) and dry ethanol (400 mL) were used for the synthesis. The monomer was purified by distillation under vacuum (bp: 67 °C (0.03 mbar)). 24.25 g (129 mmol) of the desired monomer were isolated.

Yield: 64%. Aspect: colorless liquid. The NMR spectroscopic data of the major isomer is provided below. ¹ H NMR (400 MHz, CDCl ₃ ): δ = 1.21 (t, ³ J _HH = 7.0 Hz, 3H, CH ₂ CH ₃ ); 1.96 (s, 3H, CH ₃ ); 2.60 (br s, 1H, OH); 3.43–3.59 (m, 4H, CH ₂ O); 4.01–4.10 (m, 1H, CH OH); 4.18–4.28 (m, 2H, CH ₂ OCO); 5.58–5.62 (m, 1H, C CH ₂ ); 6.14 (s, 1H, C CH ₂ ).

2.3.3

Synthesis of monomers 8 and 9

2.3.3.1

N -​Methyl-​ N -​(3-​hydroxypropyl)​acrylamide 8

Monomer 8 was prepared from 3-(methylamino)propan-1-ol (54.05 g, 606 mmol) according to the procedure described for the synthesis of monomer 7 . 43.25 g (302 mmol) of the desired monomer were isolated.

Yield: 50%. Aspect: colorless liquid. ¹ H NMR (CDCl ₃ ): δ = 1.63–1.80 (m, 2H, CH ₂ ); 2.93 and 3.01 (2s, 3H, CH ₃ N); 3.40–3.62 (m, 4H, CH ₂ OH and CH ₂ N); 4.10 (br s, 1H, OH); 5.59 and 5.67 (2dd, ² J _HH = 2.0 Hz, ³ J _HH = 10.5 Hz, 1H, CH C H ₂ ); 6.24 and 6.27 (2dd, ² J _HH = 2.0 Hz, ³ J _HH = 16.8 Hz, 1H, CH C H ₂ ); 6.53 and 6.62 (2dd, ³ J _HH = 10.5 Hz, ³ J _HH = 16.8 Hz, 1H, C H CH ₂ ).

2.3.3.2

N -Methyl- N -(6-hydroxyhexyl)acrylamide 9

Magnesium oxide (6.00 g, 148.9 mmol) was dispersed for 15 min in a mixture of 6-(methylamino)hexan-1-ol (4.05 g, 30.9 mmol), tetrahydrofuran (THF) (30 mL) and deionized water (12 mL). The solution was then cooled down to −10 °C. A solution of acryloyl chloride (2.72 g, 30.0 mmol) in THF (10 mL) was added dropwise to the reaction mixture. After stirring for 2 h at 0 °C, the mixture was filtered through Celatom ^® . The solvent was removed under reduced pressure and the crude product was purified via column chromatography (first ethyl acetate/acetonitrile (3/1) than ethyl acetate/methanol (9/1)). 3.17 g (17.1 mmol) of the desired product were isolated.

Yield: 57%. Aspect: slightly yellow oil. ¹ H NMR (CDCl ₃ ): δ = 1.27–1.47 (m, 4H, CH ₂ ); 1.52–1.66 (m, 4H, CH ₂ ); 1.76 (br s, 1H, OH); 2.99 and 3.05 (2s, 3H, CH ₃ N); 3.35 and 3.45 (2t, ³ J _HH = 7.5 Hz, 2H, CH ₂ N); 3.62 and 3.65 (2t, ³ J _HH = 6.4 Hz, 2H, CH ₂ O); 5.67 and 5.68 (2dd, ² J _HH = 2.0 Hz, ³ J _HH = 10.4 Hz, 1H, CH C H ₂ ); 6.32 and 6.33 (2dd, ² J _HH = 2.0 Hz, ³ J _HH = 16.8 Hz, 1H, CH C H ₂ ); 6.56 and 6.58 (2dd, ³ J _HH = 10.5 Hz, ³ J _HH = 16.8 Hz, 1H, C H CH ₂ ).

2.4

Photopolymerization procedure

Photopolymerizations were carried out with a Perkin Elmer differential scanning calorimeter (DSC, Pyris Diamond). 0.5 mol% Ivocerin ^® was added to each monomer as a photoinitiator. The experiments were performed under a stream of nitrogen. A sample (ca. 1.0 mg) of each mixture was placed in an uncovered aluminum DSC pan. The DSC chamber was purged with nitrogen for 4 min. The acquisition was then started. After 1 min of acquisition, the samples were irradiated for 2 min at 37 °C with a LED curing light (Bluephase, monowave LED with a maximum intensity at 470 nm, Ivoclar Vivadent AG). The incident light intensity was about 20 mW cm ⁻² . Each experiment was repeated three times. The heat flux was monitored as a function of time using the DSC under isothermal conditions. Double-bond conversion (DBC) was calculated as the quotient of the overall heat of polymerization evolved [Δ H _p (J g ⁻¹ )] and the theoretical heat of polymerization obtained for 100% conversion of the mixtures [Δ H _{0 p} (J g ⁻¹ )] (Eq. (1) ).

Δ H _{0 p} was calculated according to the following formula (Eq. (2) ):

where Δ H _{0 i} is the theoretical heat of polymerization of monomer i (i = methacrylates or monomer 1 or monomer 2 , Δ H _{0 i} = 54.8 kJ mol ⁻¹ ; i = N -(methyl)acrylamides, Δ H _{0 i} = 74.7 kJ mol ⁻¹ ) and M _i its molar mass.

The rate of polymerization ( R _p ) was calculated according to the following formula:

where Q is the heat flow per s during the reaction and m the mass of the mixture in the sample.

2.5

Water sorption and solubility

UDMA/monomers 1–9 : 1/1 (wt/wt) mixtures were prepared. Ivocerin ^® (0.2 wt%) was added as photoinitiator. A similar mixture based on HEMA was also formulated as a reference. For each mixture, specimen discs (diameter = 15.0 mm, depth = 1.0 mm) were prepared and cured with a visible light source (Spectramat SP2 polymerization unit, Ivoclar Vivadent AG) for 2 × 3 min (3 min for each side of the disc). Water sorption and solubility were measured according to ISO 4049 .

2.6

Luting composite formulation

Luting composites C1-C10 based on monomers 1–9 as well as on HEMA were formulated. Those formulations contain 8.83 wt% of monofunctional monomer (HEMA or alternative monomer), 35.67 wt% of UDMA, 0.27 wt% of CQ, 0.26 wt% of ethyl 4-(dimethylamino)benzoate (EDAB), 37.83 wt% of fumed silica OX-50 and 17.14 wt% of ytterbium fluoride (YbF ₃ ). The fumed silica OX-50 was modified with the silane coupling agent 3-methacryloyloxy-propyltrimethoxysilane prior to use. The composites were prepared as follows: CQ and EDAB were added to the monomer mixture and the solution was stirred until complete dissolution of the photoinitiator system. Both fillers were subsequently added. The composites were then mixed in a three roll mill (Exakt Apparatebau, Norderstedt, Germany). Mechanical properties were measured after storage in distilled water at 37 °C for 24 h, according to ISO 4049 . The flexural strength (FS) and flexural modulus of elasticity (E) were determined using a Z 010 universal testing machine (Zwick, Germany).

2.7

Adhesive formulation and shear bond strength measurement

One-step self-etch adhesives based on monomers 1–9 as well as on HEMA were formulated. Those formulations contain 10.00 wt% of monofunctional monomer (HEMA or alternative monomer), 35.40 wt% of DEBAAP, 10.00 wt% of MDP, 20.00 wt% of Bis-GMA, 15.00 wt% of water, 5.00 wt% of isopropanol, 0.90 wt% of CQ, 0.42 wt% of EDAB, 1.25 wt% of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (Irgacure 819), 0.03 wt% of butylated hydroxytoluene (BHT) and 2.00 wt% of fumed silica Aerosil R709. For SBS measurements, freshly extracted bovine mandibular incisors were embedded in unsaturated polyester resin (ViscoVoss). Flat dentinal and enamel surfaces were prepared with 120-grit and 400-grit wet silicon carbide paper on the labial side of the embedded teeth. The adhesive was first rubbed on the prepared surface (dentin or enamel) with a microbrush for 20 s. The adhesive layer was strongly air dried and light cured for 10 s with a LED curing light (Bluephase G2, polywave LED with a broadband spectrum from 385 to 515 nm and 2 maxima at 410 and 470 nm, Ivoclar Vivadent AG). A polyethylene mold with a central 2.38 mm diameter circular hole was fixed on the surface. A composite (Tetric EvoCeram ^® , Ivoclar Vivadent AG) was inserted in the mold and light-cured (Bluephase G2) for 20 s. The samples were finally stored in water at 37 °C for 24 h before being tested. The SBS was measured using a Z010 universal testing machine (Zwick, Germany) at a crosshead speed of 0.8 mm min ⁻¹ . 10 samples were tested for each adhesive.

2.8

Statistical analysis

The experimental results were analyzed in a one-way analysis of variance (ANOVA) based on n-number of specimens: flexural modulus and strength testing (n = 8), shear bond strength (n = 10). For statistical significance the P -value was set below 0.05.