Highlights
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Hexaarylbiimidazoles were examined as thiol-ene photopolymerization initiators.
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Affixing hydroxyhexyl groups to the HABI core significantly improved solubility.
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HABIs show excellent sensitivity as visible photoinitiators for thiol-enes.
Abstract
Objectives
The aim of this study is to determine if hexaarylbiimidazoles (HABIs) are efficient, visible light-active photoinitiators for thiol–ene systems. We hypothesize that, owing to the reactivity of lophyl radicals with thiols and the necessarily high concentration of thiol in thiol–ene formulations, HABIs will effectively initiate thiol–ene polymerization upon visible light irradiation.
Methods
UV–vis absorption spectra of photoinitiator solutions were obtained using UV–vis spectroscopy, while EPR spectroscopy was used to confirm radical species generation upon HABI photolysis. Functional group conversions during photopolymerization were monitored using FTIR spectroscopy, and thermomechanical properties were determined using dynamic mechanical analysis.
Results
The HABI derivatives investigated exhibit less absorptivity than camphorquinone at 469 nm; however, they afford increased sensitivity at this wavelength when compared with bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. Photolysis of the investigated HABIs affords lophyl radicals. Affixing hydroxyhexyl functional groups to the HABI core significantly improved solubility. Thiol–ene resins formulated with HABI photoinitiators polymerized rapidly upon irradiation with 469 nm. The glass transition temperatures of the thiol–ene resin formulated with a bis(hydroxyhexyl)-functionalized HABI and photopolymerized at room and body temperature were 49.5 ± 0.5 °C and 52.2 ± 0.1 °C, respectively.
Significance
Although thiol–enes show promise as continuous phases for composite dental restorative materials, they show poor reactivity with the conventional camphorquinone/tertiary amine photoinitiation system. Conversely, despite their relatively low visible light absorptivity, HABI photoinitiators afford rapid thiol–ene photopolymerization rates. Moreover, minor structural modifications suggest pathways for improved HABI solubility and visible light absorption.
1
Introduction
Resin formulations primarily composed of dimethacrylate-based monomers, including bisphenol A diglycidyl ether dimethacrylate (bisGMA), triethyleneglycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA), have been commonly utilized as the continuous polymeric phase in composite dental restorative materials for several decades . The long-standing success of these materials is attributable to several factors, including their high modulus and glass transition temperature upon polymerization, and adequate hydrophobicity and concomitant low water sorption. Unfortunately, the radical-mediated polymerization of these dimethacrylates is strongly inhibited by oxygen , yielding tacky surfaces owing to incomplete surface polymerization. Additionally, as the molecular weight evolution of these polymerizations proceeds via chain-growth , large amounts of unreacted, extractable monomer often remain after polymerization has ceased, potentially leading to acute toxicity or sensitization . Moreover, as this chain-growth mechanism yields a low gel point conversion, frustration of the shrinkage accompanying polymerization occurs early in the reaction, resulting in the development of high shrinkage stress levels in the polymerized material . Attempts to mitigate high shrinkage stress, the clinical consequences of which include marginal discoloration and debonding, enamel and dentin crack formation, and secondary caries , by limiting the conversion via the utilization of resin formulations that vitrify early during their polymerization in turn affords materials with undesirably high residual methacrylate monomer concentrations .
In recent years, several polymerization mechanisms have emerged as potential alternatives to the prevalent dimethacrylate polymerization for composite dental restorative materials, including the cation-mediated ring-opening of oxiranes , copper(I)-catalyzed azide–alkyne cycloadditions , and radical-mediated thiol–ene additions . The utilization of thiol–ene polymerizations for dental restorative materials is particularly promising owing to their potential as toxicologically safer alternatives to acrylics and their unique and desirable combination of characteristics relative to other radical polymerization systems . In contrast to the chain-growth mechanism typically associated with (meth)acrylate systems, thiol–ene polymerizations proceed via a radical-mediated step-growth mechanism between multifunctional thiol and non-homopolymerizable vinyl monomers , where functional groups can coreact on any size monomer, oligomer, or polymer, such that the molecular weight grows geometrically . Here, a thiyl radical adds to a vinyl, which subsequently abstracts a hydrogen from a thiol, generating a thioether moiety and regenerating a thiyl radical (see Fig. 1 a ). These thiol–ene systems demonstrate nearly all of the advantages of typical radical-mediated polymerizations in that they polymerize rapidly, do not require solvents for processing, are optically clear, and provide an excellent range of mechanical properties . Additionally, owing to their step-growth polymerization mechanism, thiol–ene polymerizations display delayed gelation , form homogeneous polymer networks with very narrow glass transitions , and exhibit less shrinkage per mole of double bonds reacted (12–15 cm 3 mol −1 for thiol–enes vs. 22.5 cm 3 mol −1 for methacrylates) , leading to reduced polymerization shrinkage stress . Furthermore, thiol–ene polymerizations demonstrate extraordinary resistance to oxygen inhibition , attributable to facile hydrogen abstraction by peroxy radicals from the ubiquitous thiol.
As composite dental restorative materials are typically cured in situ via photopolymerization, visible light irradiation is used in preference to ultraviolet light for clinical acceptability, necessitating the incorporation of a visible light-absorbing photoinitiator in the resin formulation. The most commonly employed visible light-active photoinitiator for dimethacrylate-based formulations is camphorquinone (CQ) which, in order to afford sufficient radical-generation activity, is used in combination with a tertiary amine . Thus, CQ is a ‘Type II’ photoinitiator ( i.e. , H-abstraction) where, upon photoexcitation, it abstracts a hydrogen from the tertiary amine H-donor to yield a reactive, initiating radical centered on the amine and a relatively unreactive camphorquinone-centered radical . As hydrogen abstraction only proceeds while the Type II photoinitiator remains in its excited state, the lifetime of which is often short , the radical generation quantum yield for these compounds can be low. In contrast, ‘Type I’ photointiators undergo direct homolysis upon irradiation to yield two polymerization-initiating radicals at typically high radical generation efficiencies . Moreover, the necessity for Type II photoinitiators to be formulated with a co-initiator can lead to potentially deleterious consequences such as co-initiator discoloration and toxicity .
Although the CQ/tertiary amine photoinitiation system is employed extensively in dimethacrylate dental formulations, it displays poor reactivity in thiol–ene-based resins . Conversely, direct-cleavage-type photoinitiators have been shown to efficiently initiate thiol–ene formulations upon irradiation . Notably, whereas the absorbance peak of CQ is centered at approximately 470 nm , only a very few single component photoinitiators, such as benzoyl phosphine oxides , titanocenes , and benzoyl germanes , absorb well into the visible spectral region. Hexaarylbiimidazoles (HABIs), a class of photoinitiators first synthesized by Hayashi and Maeda , are presented here as promising direct cleavage, visible light-active photoinitiators specifically for thiol–ene systems. Upon irradiation, HABIs undergo homolytic cleavage to yield two lophyl radicals ( Fig. 1 b) that are unreactive with oxygen and show slow recombination rates , attributable to steric hindrance and electron delocalization ; indeed, HABI photoinitiators show no initiation activity in (meth)acrylate formulations without the presence of a hydrogen-donating coinitiator . Thiols are commonly used as coinitiators in conjunction with HABIs , where hydrogen abstraction by the HABI-derived lophyl radicals yields initiating thiyl radicals ( Fig. 1 b), suggesting a particular suitability for HABI photoinitiators in thiol–ene formulations. Unfortunately, commercial HABIs often exhibit poor absorption in the visible spectum, sometimes requiring a photosensitizer, and low solubility in common resins and organic solvents . Here, we examine the hypothesis that HABIs will effectively initiate thiol–ene polymerization upon visible light irradiation by describing the synthesis of a novel, dihydroxy-functionalized HABI with enhanced solubility and visible light absorptivity, and investigating its utilization as a photoinitiator in thiol–ene formulations, focusing on its influence on the polymerization kinetics and thermomechanical properties of the resultant polymers.
2
Materials and methods
2.1
Materials
The monomers bisphenol A bis(2-hydroxy-3-methacryloxypropyl) ether (bisGMA, Esstech Inc., Essington, PA, USA) and triethylene glycol dimethacrylate (TEGDMA, Esstech) were formulated as a mixture consisting of 70 wt% bisGMA and 30 wt% TEGDMA and used as a model dimethacrylate-based resin. The monomers pentaerythritol tetra-3-mercaptopropionate (PETMP, Evans Chemetics, Teaneck, NJ, USA) and 1,3,5,-triallyl-1,3,5-triazine-2,4,6(1 H ,3 H ,5 H )-trione (TATATO, Sigma-Aldrich, St. Louis, MO, USA) were similarly formulated as a mixture, such that the thiol ( i.e. , mercapto) and ene ( i.e. , allyl) functional groups were present at a 1:1 stoichiometric ratio, and used as a model thiol–ene resin. Prior to thiol and ene component mixing, 0.01 wt% tris( N -nitroso- N -phenylhydroxylaminato)aluminum (Q1301, Wako Chemicals, Richmond, VA, USA) was added to the PETMP as a radical polymerization inhibitor to preclude premature thiol–ene polymerization. Camphorquinone (CQ, Esstech), ethyl 4-dimethylaminobenzoate (EDAB, Esstech), bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819, BASF, Florham Park, NJ, USA), 2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,2′-biimidazole ( o -Cl-HABI, TCI America, Portland, OR, USA), and 2,2′-bis(2-chloro-4-hexan-1-ol-phenoxy)-4,4′,5,5′-tetraphenyl-1,2′-biimidazole ( p -HOH-HABI) were used as visible light-active photoinitiating systems. CQ was used in conjunction with EDAB as a co-initiator (1 wt% CQ:0.5 wt% EDAB). Irgacure 819, o -Cl-HABI, and p -HOH-HABI were used at the concentrations as indicated in the text. All commercial monomers and photoinitiators were used without further purification, the synthesis of p -HOH-HABI is described in reference , and the structures of all materials used are shown in Fig. 2 .
The solubility of the HABI photoinitiators was determined by addition of a photoinitiator to a liquid monomer or organic solvent (by weight ratio) and heating the combined materials in an oven (50–60 °C) while mixing. Samples were then kept at room temperature for 30 min and, if no recrystallization occurred, the photoinitiator loading was recorded as soluble. Photoinitiator concentration increments of 5 wt% were employed in this study. Thus, for example, the solubility of o -Cl-HABI in dimethylformamide (DMF) is <20 wt%, which means that its solubility in DMF is above 15 wt% but below 20 wt%.
2.2
Methods
2.2.1
Light sources and intensity measurement
Blue light was provided by an LED-based dental lamp (G-Light, GC America) equipped with a longpass filter (435 nm cut-on wavelength) yielding a single emittance curve peak centered at 469 nm (full width at half maximum (FWHM) 29 nm), while violet light was provided by a collimated, LED-based illumination source (Thorlabs M405L2-C) with an emittance centered at 405 nm (FWHM 13 nm), used in combination with a current-adjustable LED driver (Thorlabs LEDD1B) for intensity control. UV light was provided by a high pressure mercury vapor lamp (EXFO Acticure 4000) equipped with a filter to isolate the 365 nm spectral line (FWHM 10 nm). Irradiation intensities were measured with an International Light IL1400A radiometer equipped with a broadband silicon detector (model SEL033), a 10× attenuation neutral density filter (model QNDS1), and a quartz diffuser (model W).
2.2.2
UV–vis spectrophotometry
UV–vis spectrophotometry was performed on 1 wt% samples of photoinitiator in toluene using an Agilent Technologies Cary 60 UV–vis spectrophotometer. Spectra were collected from 200 to 800 nm on solutions using a 1 mm pathlength quartz cuvette both in the dark and under irradiation once the radical concentration reached equilibrium. HABI photodissociation and subsequent recombination was examined by monitoring 554 nm and 598 nm for o -Cl-HABI and p -HOH-HABI, respectively, the wavelengths where the visible light absorbance by the generated lophyl radicals was greatest ( i.e. , λ max ), while the sample solutions in the cuvette were irradiated with 405 nm for 9.5 min to ensure radical concentration equilibration, then for a further 10 min after the light was turned off.
2.2.3
EPR spectroscopy
Electron paramagnetic resonance (EPR) spectroscopy was performed on 1 wt% samples of HABI photoinitiators in toluene using a Bruker EMX spectrometer. A TE 102 cavity (ER4102ST, Bruker), 100 kHz modulation frequency, and 1 G pp modulation amplitude were used for all experiments. Optical access within the cavity was afforded by a 10 mm × 23 mm grid providing 50% light transmittance to the sample. All sample solutions were held in a 3.2 mm inner diameter quartz EPR sample tube which was inserted into the spectrometer cavity for analysis. The sample solutions were irradiated in situ with 405 nm light at 10 mW cm −2 and spectra were collected once the radical concentration reached steady state. All experiments were performed at room temperature.
2.2.4
FTIR spectroscopy
Resin formulations were introduced between glass microscope slides separated by spacers (250 μm thick for bisGMA/TEGDMA and 50 μm thick for PETMP/TATATO) to maintain constant sample thickness during polymerization. Each sample was placed in a Thermo Scientific Nicolet 6700 FTIR spectrometer equipped with a horizontal transmission accessory, as described elsewhere , and spectra were collected from 2000 to 7000 cm −1 at a rate of 3 every 2 s. The functional group conversion upon irradiation was determined by monitoring the disappearance of the peak area centred at 2571 cm −1 corresponding to the thiol group stretch, 3083 cm −1 corresponding to the allylic vinyl group stretch, and 6164 cm −1 corresponding to the methacrylate group stretch. The respective sample thicknesses for methacrylate and thiol–ene formulations were selected to ensure that the functional group peaks remained within the linear regime of the instrument detector while affording good signal to noise and maintaining optically thin and isothermal polymerization conditions. All experiments were performed in triplicate, and the photoinitiator concentrations and irradiation intensities were as indicated in the figure captions.
2.2.5
Dynamic mechanical analysis
Cross-linked polymer films were prepared from bisGMA/TEGDMA formulations containing 1 wt% CQ/0.5 wt% EDAB and from PETMP/TATATO formulations containing 1 wt% p -HOH-HABI which were polymerized between glass microscope slides separated by 250 μm thick spacers for 20 min at either 23 °C (room temperature) or 37 °C (body temperature) under 469 nm irradiation at 10 mW cm −2 . Samples of approximately 15 mm × 5 mm × 0.25 mm were cut from the cured films and mounted in a TA Instruments Q800 dynamic mechanical analyzer equipped with a film tension clamp. Experiments were performed at a strain and frequency of 0.1% and 1 Hz, respectively, scanning the temperature from −20 °C to 200 °C twice at 1 °C min −1 , and the elastic moduli ( E ′) and tan δ curves were recorded; the repeated temperature scan was used to determine the influence of dark polymerization at temperatures greater than the glass transition temperature ( T g ). The T g was assigned as the temperature at the tan δ curve peak.
2
Materials and methods
2.1
Materials
The monomers bisphenol A bis(2-hydroxy-3-methacryloxypropyl) ether (bisGMA, Esstech Inc., Essington, PA, USA) and triethylene glycol dimethacrylate (TEGDMA, Esstech) were formulated as a mixture consisting of 70 wt% bisGMA and 30 wt% TEGDMA and used as a model dimethacrylate-based resin. The monomers pentaerythritol tetra-3-mercaptopropionate (PETMP, Evans Chemetics, Teaneck, NJ, USA) and 1,3,5,-triallyl-1,3,5-triazine-2,4,6(1 H ,3 H ,5 H )-trione (TATATO, Sigma-Aldrich, St. Louis, MO, USA) were similarly formulated as a mixture, such that the thiol ( i.e. , mercapto) and ene ( i.e. , allyl) functional groups were present at a 1:1 stoichiometric ratio, and used as a model thiol–ene resin. Prior to thiol and ene component mixing, 0.01 wt% tris( N -nitroso- N -phenylhydroxylaminato)aluminum (Q1301, Wako Chemicals, Richmond, VA, USA) was added to the PETMP as a radical polymerization inhibitor to preclude premature thiol–ene polymerization. Camphorquinone (CQ, Esstech), ethyl 4-dimethylaminobenzoate (EDAB, Esstech), bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819, BASF, Florham Park, NJ, USA), 2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,2′-biimidazole ( o -Cl-HABI, TCI America, Portland, OR, USA), and 2,2′-bis(2-chloro-4-hexan-1-ol-phenoxy)-4,4′,5,5′-tetraphenyl-1,2′-biimidazole ( p -HOH-HABI) were used as visible light-active photoinitiating systems. CQ was used in conjunction with EDAB as a co-initiator (1 wt% CQ:0.5 wt% EDAB). Irgacure 819, o -Cl-HABI, and p -HOH-HABI were used at the concentrations as indicated in the text. All commercial monomers and photoinitiators were used without further purification, the synthesis of p -HOH-HABI is described in reference , and the structures of all materials used are shown in Fig. 2 .
The solubility of the HABI photoinitiators was determined by addition of a photoinitiator to a liquid monomer or organic solvent (by weight ratio) and heating the combined materials in an oven (50–60 °C) while mixing. Samples were then kept at room temperature for 30 min and, if no recrystallization occurred, the photoinitiator loading was recorded as soluble. Photoinitiator concentration increments of 5 wt% were employed in this study. Thus, for example, the solubility of o -Cl-HABI in dimethylformamide (DMF) is <20 wt%, which means that its solubility in DMF is above 15 wt% but below 20 wt%.
2.2
Methods
2.2.1
Light sources and intensity measurement
Blue light was provided by an LED-based dental lamp (G-Light, GC America) equipped with a longpass filter (435 nm cut-on wavelength) yielding a single emittance curve peak centered at 469 nm (full width at half maximum (FWHM) 29 nm), while violet light was provided by a collimated, LED-based illumination source (Thorlabs M405L2-C) with an emittance centered at 405 nm (FWHM 13 nm), used in combination with a current-adjustable LED driver (Thorlabs LEDD1B) for intensity control. UV light was provided by a high pressure mercury vapor lamp (EXFO Acticure 4000) equipped with a filter to isolate the 365 nm spectral line (FWHM 10 nm). Irradiation intensities were measured with an International Light IL1400A radiometer equipped with a broadband silicon detector (model SEL033), a 10× attenuation neutral density filter (model QNDS1), and a quartz diffuser (model W).
2.2.2
UV–vis spectrophotometry
UV–vis spectrophotometry was performed on 1 wt% samples of photoinitiator in toluene using an Agilent Technologies Cary 60 UV–vis spectrophotometer. Spectra were collected from 200 to 800 nm on solutions using a 1 mm pathlength quartz cuvette both in the dark and under irradiation once the radical concentration reached equilibrium. HABI photodissociation and subsequent recombination was examined by monitoring 554 nm and 598 nm for o -Cl-HABI and p -HOH-HABI, respectively, the wavelengths where the visible light absorbance by the generated lophyl radicals was greatest ( i.e. , λ max ), while the sample solutions in the cuvette were irradiated with 405 nm for 9.5 min to ensure radical concentration equilibration, then for a further 10 min after the light was turned off.
2.2.3
EPR spectroscopy
Electron paramagnetic resonance (EPR) spectroscopy was performed on 1 wt% samples of HABI photoinitiators in toluene using a Bruker EMX spectrometer. A TE 102 cavity (ER4102ST, Bruker), 100 kHz modulation frequency, and 1 G pp modulation amplitude were used for all experiments. Optical access within the cavity was afforded by a 10 mm × 23 mm grid providing 50% light transmittance to the sample. All sample solutions were held in a 3.2 mm inner diameter quartz EPR sample tube which was inserted into the spectrometer cavity for analysis. The sample solutions were irradiated in situ with 405 nm light at 10 mW cm −2 and spectra were collected once the radical concentration reached steady state. All experiments were performed at room temperature.
2.2.4
FTIR spectroscopy
Resin formulations were introduced between glass microscope slides separated by spacers (250 μm thick for bisGMA/TEGDMA and 50 μm thick for PETMP/TATATO) to maintain constant sample thickness during polymerization. Each sample was placed in a Thermo Scientific Nicolet 6700 FTIR spectrometer equipped with a horizontal transmission accessory, as described elsewhere , and spectra were collected from 2000 to 7000 cm −1 at a rate of 3 every 2 s. The functional group conversion upon irradiation was determined by monitoring the disappearance of the peak area centred at 2571 cm −1 corresponding to the thiol group stretch, 3083 cm −1 corresponding to the allylic vinyl group stretch, and 6164 cm −1 corresponding to the methacrylate group stretch. The respective sample thicknesses for methacrylate and thiol–ene formulations were selected to ensure that the functional group peaks remained within the linear regime of the instrument detector while affording good signal to noise and maintaining optically thin and isothermal polymerization conditions. All experiments were performed in triplicate, and the photoinitiator concentrations and irradiation intensities were as indicated in the figure captions.
2.2.5
Dynamic mechanical analysis
Cross-linked polymer films were prepared from bisGMA/TEGDMA formulations containing 1 wt% CQ/0.5 wt% EDAB and from PETMP/TATATO formulations containing 1 wt% p -HOH-HABI which were polymerized between glass microscope slides separated by 250 μm thick spacers for 20 min at either 23 °C (room temperature) or 37 °C (body temperature) under 469 nm irradiation at 10 mW cm −2 . Samples of approximately 15 mm × 5 mm × 0.25 mm were cut from the cured films and mounted in a TA Instruments Q800 dynamic mechanical analyzer equipped with a film tension clamp. Experiments were performed at a strain and frequency of 0.1% and 1 Hz, respectively, scanning the temperature from −20 °C to 200 °C twice at 1 °C min −1 , and the elastic moduli ( E ′) and tan δ curves were recorded; the repeated temperature scan was used to determine the influence of dark polymerization at temperatures greater than the glass transition temperature ( T g ). The T g was assigned as the temperature at the tan δ curve peak.
3
Results
3.1
HABI absorbance and photolysis
The UV–vis absorption spectra presented in Fig. 3 show the absorption by solutions of (a) o -Cl-HABI and (b) p -HOH-HABI both in the dark and under 405 nm irradiation, and the absorbance and molar absorptivity of the four photoinitiating systems examined in this study at three wavelengths tabulated in Table 1 . Both HABIs display extended absorption tails well into the visible spectral region; however, whereas o -Cl-HABI displays higher absorbance at shorter wavelengths, the absorbance tail of p -HOH-HABI extends further into the visible and hence affords higher absorbance in the blue region of the spectrum ( i.e. , 469 nm). In contrast, CQ exhibits good absorptivity at 469 nm but absorbs poorly at 405 nm and 365 nm. Finally, Irgacure 819 demonstrates negligible absorbance at 469 nm but relatively strong absorbance at 405 nm and 365 nm.
| Photoinitiator | λ = 469 nm | λ = 405 nm | λ = 365 nm | |||
|---|---|---|---|---|---|---|
| Absorbance | Molar absorptivity (M −1 cm −1 ) | Absorbance | Molar absorptivity (M −1 cm −1 ) | Absorbance | Molar absorptivity (M −1 cm −1 ) | |
| CQ/Amine | 0.268 | 51.4 | 0.035 | 6.80 | 0.011 | 2.05 |
| Irgacure 819 | 0.003 | 1.39 | 1.44 | 696 | 2.07 | 1002 |
| o -Cl-HABI | 0.007 | 5.69 | 0.287 | 219 | 0.494 | 376 |
| p -HOH-HABI | 0.014 | 14.7 | 0.096 | 99.0 | 0.390 | 401 |
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