Ester-Activated Vinyl Ethers as Chain Transfer Agents in Radical

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Ester-Activated Vinyl Ethers as Chain Transfer Agents in Radical Photopolymerization of Methacrylates Gernot Peer,†,‡ Anna Eibel,§ Christian Gorsche,† Yohann Catel,‡,∥ Georg Gescheidt,§ Norbert Moszner,‡,∥ and Robert Liska*,†,‡ †

Institute of Applied Synthetic Chemistry, TU Wien, Getreidemarkt 9/163 MC, 1060 Vienna, Austria Christian-Doppler-Laboratory for Photopolymers in Digital and Restorative Dentistry, Getreidemarkt 9, 1060 Vienna, Austria § Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria ∥ Ivoclar Vivadent AG, 9494 Schaan, Liechtenstein

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‡

S Supporting Information *

ABSTRACT: The regulation of multifunctional methacrylates to yield tough photopolymers is a widely researched topic, whereby addition−fragmentation chain transfer (AFCT) agents represent one viable class of additives. Vinyl ethers have been described as potent AFCT agents in radical polymerization but are unexamined in network formation via photopolymerization. In this article, we present a sterically hindered vinyl ether as AFCT agent for methacrylate networks, which shows enhanced acid stability opposed to vinyl ethers described in the literature. After synthesis and confirmation of the efficient regulation in a monofunctional system, the reactivity and mechanical properties in a difunctional methacrylate were evaluated. An increase in double bond conversion, significantly lower shrinkage stress, and high toughness were assessed and substantiated the great potential of this compound in photopolymerizable resins.

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INTRODUCTION Photopolymerization of multifunctional (meth)acrylates is traditionally applied in thin film applications such as coatings1 and printing inks,2 but recent advantages, which helped to overcome the limitation of light penetration, led to an extended scope with 3D applications. Although acrylates are more reactive than methacrylates, the latter are much less toxic and thus favored in these recent fields such as dental restoratives,3 tissue engineering,4 or additive manufacturing5 (or 3D printing) for biomedical applications. However, the problem of polymerization-induced shrinkage stress and resulting brittle materials seem to be adamant. Numerous strategies exist nowadays to overcome these limitations to yield tougher materials.6 The use of methacrylate monomers with low resulting shrinkage such as bisphenol A glycidyl methacrylate or high molecular weight methacrylates is one way to obtain tough polymers, but the high viscosity requires specific processing. The use of ring-opening monomers such as vinylcyclopropanes is another viable option to reduce shrinkage stress and is investigated especially in dental restoratives.7 Additives such as inorganic nanoparticles, organic nanogels, or rubbers do not alter the network formation itself but increase the toughness.8,9 Another class of additives are chain transfer agents (CTAs), which modify the network architecture and furthermore alter the polymerization kinetics. In a monofunctional system, the addition of CTAs typically lowers the resulting molecular weight, which confers to a lower © XXXX American Chemical Society

cross-link density in networks. This leads to a more homogeneous network with defined thermomechanical properties and reduced shrinkage stress due to the shift of the gel point (transition from liquid to solid state) to higher conversion. Thiol−ene chemistry portrays a widely used and well-evaluated CTA technology and demonstrates remarkable reactivity together with increased toughness in methacrylate systems,10,11 whereas current research tries to overcome difficulties such as restricted storage stability,12 characteristic odor,13 or softening of the material.14 Addition−fragmentation chain transfer (AFCT) agents represent an adequate alternative to thiol−ene chemistry and have been known since the 1980s.15,16 Depending on the structure, these agents can be categorized into reversible (RAFT) and irreversible agents.17 Especially agents, which undergo an irreversible mechanism, are expedient in radical photopolymerization as they maintain the high reactivity of the methacrylic polymerization opposed to RAFT agents. Common structural features of AFCT agents are a carbon−carbon double bond with an adjacent activating group (A, e.g., ester moiety), a particular central atom (Y, e.g., carbon atom), and a leaving group (L, e.g., tosyl moiety). The propagating radical attacks the reactive double bond and forms an intermediate radical which Received: January 14, 2019 Revised: February 22, 2019

A

DOI: 10.1021/acs.macromol.9b00085 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Chain transfer mechanism of AFCT agents. A = activating group, Y = center atom, and L = leaving group.

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undergoes β-scission to form a leaving group radical (Figure 1). There are numerous substance classes thoroughly investigated as AFCT agents such as allyl sulfides,18,19 allyl sulfones,20,21 halides,20 or phosphonates,20 and an extensive review on addition−fragmentation chemistry was published by Moad et al.17 It is noteworthy that a vast majority of leaving groups are based on heteroatoms, while carbon-based leaving groups are solely found in activated vinyl ethers as AFCT agents.22 These vinyl ethers (VE) contain an oxygen atom as center atom and thus show an irreversible character through the formation of a ketone. Benzyl vinyl ethers are known to effectively regulate the polymerization of styrene and methyl methacrylate in solution but are unexplored in methacrylic network formation via photopolymerization.23 A challenging synthesis and significant acid sensitivity are attributed to vinyl ethers, wherefore they might have not been investigated any further.17 Vinyl ethers with a tertiary leaving group have not yet been used as AFCT agents in polymerization but showed utility in organic synthesis.24 Additionally methyl methacrylate oligomers and derivatives with tertiary leaving groups have been examined to have chain transfer attributes. It has to be mentioned that they show highly reversible (RAFT agent) character due to the carbon as central atom.25−27 We recently showed that the substitution of carbon to oxygen as center atom in an AFCT reagent has a huge beneficial impact on the reactivity when we compared β-allyl sulfones with vinyl sulfonates.28 Calculations revealed that the free energy of the formed α-ketoester is significantly lower than the vinyl sulfonate, and therefore an irreversible nature can be assumed.28 Furthermore, different activating groups were investigated in allyl sulfones, where an ester functionality showed the overall best performance.29 These insights should be used to alter a methyl methacrylate dimer (MMA-D) to a vinyl ether with a tertiary leaving group (Figure 2). Herein we

EXPERIMENTAL SECTION

Materials and General Methods. Ethyl 2-hydroxyisobutyrate (Fluka), ethyl bromoacetate (Fluka), sodium hydride (NaH, TCI), nbutyllithium (n-BuLi, Sigma-Aldrich), diisopropylamine (DIPA, Sigma-Aldrich), 1H-benzotriazol-1-methanol (BzMeOH, TCI), methanesulfonyl chloride (MsCl, Fluka), triethylamine (TEA, SigmaAldrich), diazabicycloundecene (DBU, TCI), and benzyl methacrylate (BMA, Sigma-Aldrich) were purchased from respective companies and used as received unless otherwise noted. The photoinitiator bis(4-methoxybenzoyl)diethylgermane (BMDG), the chain transfer agent methyl methacrylate dimer (MMA-D), the primer (2-((2-(ethoxycarbonyl)allyl)oxy)ethyl)phosphonic acid (ECAP), and the monomers 1,10-decanediol dimethacrylate (D3MA) and urethane dimethacrylate (UDMA, isomeric mixture; CAS: 72869-864) were kindly provided by Ivoclar Vivadent AG and used as received. Commercial grade methylene chloride (CH2Cl2, Donau Chemie) and tetrahydrofuran (THF, Donau Chemie) were dried using a PureSolv system (Inert, Amesbury, MA). Diethyl ether (Et2O) (Donau Chemie) petroleum ether (PE, Donau Chemie) and ethyl acetate (EE, Donau Chemie) were used without further purification. All monomers were used without removal of inhibitor. NMR spectra were recorded on a Bruker Avance at 400 MHz for 1 H (100 MHz for 13C); chemical shifts are given in ppm and were referenced to the solvent residual peak (CDCl3). Multiplicities are termed s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). Coupling constants (J values) are given in hertz. Silica gel chromatography was performed with a Büchi MPLC system equipped with the control unit C-620, fraction collector C-660, and UV photometer C-635. HR-MS analysis was performed from methanol solutions (concentration: 10 μM) by using a HTC PAL system autosampler (CTC Analytics AG, Zwingen, Switzerland), an Agilent 1100/1200 HPLC with binary pumps, degasser, and column thermostat (Agilent Technologies, Waldbronn, Germany), and an Agilent 6230 AJS ESI-TOF mass spectrometer (Agilent Technologies, Palo Alto, CA). For the investigation of the cytotoxicity of selected synthesized monomers, the in vitro XTT test30 was performed using a L929 mouse cell line. To evaluate the potential of the monomers to induce gene mutations, an Ames test31 was performed. Synthesis of Ethyl 2-(2-Ethoxy-2-oxoethoxy)-2-methylpropanoate (IM1). For the synthesis of IM1, a suspension of NaH in mineral oil (60%, 9.16 g, 229.1 mmol) in a 1000 mL three-necked flask (purged with argon) was mixed with 500 mL of dry THF and cooled with ice water to 0−5 °C. Ethyl 2-hydroxyisobutyrate (30.3 g, 229.1 mmol) was added dropwise, and the suspension was stirred for 1 h at this temperature. Ethyl bromoacetate (38.3 g, 229.1 mmol) was added dropwise, and the reaction solution was stirred overnight at ambient temperatures. The reaction was quenched with 150 mL of saturated NH4Cl solution, the aqueous phase was extracted with ethyl acetate (3 × 150 mL), and the combined organic phases were dried with Na2SO4 and filtered. The solvent was evaporated, and the crude product was purified via distillation (bp: 70 °C at 0.07 mbar) to obtain a colorless liquid (35.8 g, 72%). 1H NMR (400 MHz, CDCl3) δ (ppm): 4.20 (4H, m, −CH2−, 3J = 7.1 Hz), 4.10 (2H, s, −CH2− O−), 1.47 (6H, s, −C−CH3), 1.27 (6H, t, −CH3, 3J = 7.1 Hz). 13C NMR (100 MHz, CDCl3) δ (ppm): 174.0 (CO), 170.4 (CO), 78.5 (C4), 63.3 (C2), 61.3 (C2), 61.0 (C2), 24.6 (C1), 14.3 (C1). HR-MS (ACN, ESI+, m/z): calcd: 241.1046 [M + Na]+; found: 241.1061 [M + Na]+. Synthesis of Ethyl 2-((1-Ethoxy-2-methyl-1-oxopropan-2yl)oxy)-3-hydroxypropanoate (IM2). For the synthesis of IM2 anhydrous THF (600 mL) in a 1000 mL three-necked flask purged with argon was cooled to −10 °C, and n-butyllithium (2.5 M solution in hexane, 83 mL; 206.2 mmol) and freshly distilled diisopropylamine (34 mL, 240.6 mmol) were added. The solution was allowed to stir at

Figure 2. Modification of MMA-D to vinyl ether EOE to enhance reactivity.

present an ester-activated vinyl ether with a sterically hindered leaving group (EOE) to show the dramatic increase in reactivity when employing an irreversible system. The paper explores the AFCT properties of the vinyl ether in a monofunctional monomer. Subsequently, different VE concentrations in a dimethacrylate network and the impact on photoreactivity and (thermo)mechanical properties were evaluated. The methyl methacrylate dimer was used as reference to observe the drastic change in AFCT efficiency upon introduction of oxygen as central atom. B

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Macromolecules 0 °C for 30 min and then cooled to −70 °C. IM1 (15 g, 68.7 mmol) in anhydrous THF (60 mL) was added dropwise, and the solution was stirred for 1 h. 1H-Benzotriazole−methanol (20.5 g, 137.5 mmol) was added as a solid in portions over a period of 30 min. The reaction solution was stirred for another 3 h at −70 °C and then quenched with 200 mL of saturated NH4Cl. The aqueous phase was extracted with Et2O (3 × 200 mL) and washed with 1 N NaOH (1 × 300 mL) and brine (1 × 200 mL). The organic layer was dried over Na2SO4 and filtered, and the solvent was evaporated. The crude product (10.6 g, 42.7 mmol, 90% purity) was used without further purification. 1 H NMR (CDCl3, 400 MHz): δ = 4.20 (5H, m, −CH2−CH3, O− CH−), 3.84 (2H, m, −CH2−OH), 1.51 (3H, s, −C−CH3), 1.44 (3H, s, −C−CH3), 1.27 (6H, t, −CH3, 3J = 7.1 Hz). 13C NMR (CDCl3, 100 MHz): δ = 175.4 (CO), 171.4 (CO), 79.0 (C4), 76.2 (C3), 64.3 (C2), 61.8 (C2), 61.3 (C2), 27.3 (C1), 22.7 (C1), 14.3 (C1), 14.3 (C1). HR-MS (ACN, ESI+, m/z): calcd: 271.1152 [M + Na]+; found: 271.1165 [M + Na]+. Synthesis of Ethyl 2-((1-Ethoxy-2-methyl-1-oxopropan-2yl)oxy)acrylate (EOE). For the synthesis of EOE crude ethyl 2-((1ethoxy-2-methyl-1-oxopropan-2-yl)oxy)-3-hydroxypropanoate (10.61 g, 42.7 mmol) was diluted with 130 mL of anhydrous CH2Cl2 under an argon atmosphere and cooled to 0 °C. Triethylamine (5.62 g, 55.6 mmol) and methanesulfonyl chloride (5.87 g, 51.3 mmol) were added dropwise, and the solution was allowed to stir for 30 min at ambient temperature. The reaction was quenched with 100 mL of saturated NaHCO3 solution, and the aqueous phase was extracted with CH2Cl2 (3 × 70 mL). The combined organic phases were dried over Na2SO4 and filtered, and the solvent was evaporated. Without further purification the crude product was diluted with 90 mL of anhydrous THF, and the solution was cooled to 0 °C. DBU was added (19.52 g, 128.2 mmol), and the solution was stirred overnight at ambient temperatures before quenching with 90 mL of water. The aqueous phase was extracted with CH2Cl2 (4 × 70 mL), organic phases were dried over Na2SO4 and filtered, and the solvent was evaporated. The crude product was purified via silica gel column chromatography (petrol ether/ethyl acetate 10/1) and isolated in a yield of 4.83 g as a colorless liquid (31% with respect to step 2). 1H NMR (CDCl3, 400 MHz): δ = 5.53 (1H, d, C = CH2, 2J = 2.7 Hz), 4.55 (1H, d, C = CH2, 2J = 2.7 Hz), 4.22 (4H, qq, −CH2, 3J = 7.1 Hz), 1.59 (6H, s, −C−CH3), 1.29 (6H, 2 t, −CH3, 3J = 7.1 Hz). 13C NMR (CDCl3, 100,6 MHz): δ = 14.3 (C1), 14.3 (C1), 24.6 (C1), 61.6 (C2), 79.7 (C4), 100.3 (C2), 147.9 (C4), 163.7 (CO), 173.4 (CO). HR-MS (ACN, ESI+, m/z): calcd: 253.1046 [M + Na]+; found: 253.1058 [M + Na]+. Laser Flash Photolysis (LFP). LFP experiments were performed on a LKS80 spectrometer (Applied Photophysics, UK). Samples were excited with the frequency-tripled light from a Spitlight Compact 100 (InnoLas, Germany) solid state Nd:YAG laser at 355 nm (∼15 mJ/ pulse, 8 ns). Rate constants for the addition of the BMDG-derived germyl radical to the double bonds of BMA, MMA-D, and EOE were determined in pseudo-first-order experiments. Solutions of BMDG in acetonitrile containing monomer or AFCT at concentrations in the range 0.01−0.075 M and providing absorbance of ∼0.3 at 355 nm were prepared (A355 ∼ 0.3). Static solutions were saturated with argon before the measurements. The decay of the germyl radical was recorded at its absorption maximum (480 nm) determined from its transient absorption spectrum. Preparation of Resin Formulations and Photopolymer Specimens. For photo-DSC studies, benzyl methacrylate (BMA) was used as monomer. Formulations with 0 and 20 mol % chain transfer agent (EOE and MMA-D) were prepared, and 1 mol % BMDG as photoinitiator was added. BMA and other monomers were used without removal of inhibitor as no classical kinetic study with low conversion is performed. For RT-NIR-photorheology measurements, a 1:1 molar mixture of UDMA and D3MA termed 2M was applied as difunctional monomer resin. Formulations with 0, 5, 10, and 20 db% EOE and 20 db% MMA-D were prepared (db%···AFCT double bond represents X% of all double bonds in the respective formulation). 1 wt % BMDG as photoinitiator was added to all formulations. The formulations were

mixed via vortex mixer and homogenized via an ultrasonic bath for 10 min at ambient temperature. The formulations prepared for RT-NIR-photorheology were also used to cast specimens in clear silicon molds for dynamic mechanical thermal analysis (DMTA), swelling tests, Dynstat impact resistance, and tensile tests. For DMTA measurements and Dynstat impact resistance tests rectangular specimens (5 × 2 × 40 mm3 for DMTA, 5 × 1 × 8 mm3 for swelling test, and 10 × 4 × 15 mm3 for Dynstat) and for tensile testing specimens according to ISO 527-2, type 5b (dumbbell-shaped test pieces with a total length of 35 mm and a parallel region dimension of 2 × 2 × 12 mm3), were fabricated. The formulations were placed into a Lumamat 100 light chamber (provided by Ivoclar Vivadent AG) with six Osram Dulux L Blue 18 W lamps. The emitted wavelength spectrum was 400−580 nm at a measured total intensity of ∼20 mW cm−2 determined with an Ocean Optics USB 2000+ spectrometer at the position of the samples. Exposure was performed for 10 min on the top and 10 min on the backside of the samples. The samples were polished with sandpaper to ensure comparable sample geometries of 99.5%) and analyzed via 1H NMR spectroscopy. The solvent signal was used as internal reference. The conversion was calculated from the integral ratio between the double bond peaks before and after curing, while the −CH2− peak from BMA served as a reference signal. Gel Permeation Chromatography (GPC). GPC was performed with a Waters GPC using three columns connected in series (Styragel HR 0.5, Styragel HR 3, and a Styragel HR 4) and a Waters 2410 RI detector. The columns were maintained at 40 °C, and a flow rate of 1.0 mL min−1 was applied. Polystyrene standards were used for molecular weight calibration, and THF served as solvent. The samples from the conversion evaluation via 1H NMR spectroscopy were used for this purpose. The CDCl3 was evaporated, and the residue polymers were dissolved in THF without precipitation prior to GPC characterization (∼5 mg mL−1). The dissolved samples were filtered via a syringe filter and injected into the measurement vials. RT-NIR-Photorheology. An Anton Paar MCR 302 WESP rheometer with a P-PTD 200/GL Peltier glass plate and a PP25 measuring system was used to conduct the RT-NIR-photorheology experiments. In addition to the rheometer, a Bruker Vertex 80 FTIR spectrometer was used to analyze the conversion over time of the sample. Details for the setup and the measurement procedure are described in the literature, and every formulation was measured in triplicate.32 For each measurement, an exact amount of monomer formulation (150 μL) was placed at the center of the glass plate, and the measurements were conducted at 25 °C with a gap of 200 μm. The formulations were sheared with a strain of 1% and a frequency of 1 Hz. UV light was used to initiate the reaction, which was emitted via an Exfo OmniCureTM 2000 device with a broadband Hg lamp (300 s, 400−500 nm, 1 W cm−2 at the tip of the light guide, ∼10 mW cm−2 on the surface of the sample, measured with an Ocean Optics USB 2000+ spectrometer). The methacrylate double bond conversion (DBC) was determined by recording a set of single spectra (time interval ∼0.26 s) with an OPUS 7.0 software and then integrating the respective double bond bands at ∼6160 cm−1. The ratio of the double bond peak area at the start and the end of the measurement gave the DBC. Dynamic Mechanical Thermal Analysis (DMTA). An Anton Paar MCR 301 device with a CTD 450 oven and an SRF 12 C

DOI: 10.1021/acs.macromol.9b00085 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. Synthetic pathway to a sterically hindered activated vinyl ether (EOE).

Figure 4. (a) Photocleavage of BMDG to yield a germyl radical G· and a benzoyl-type radical B·. (b) Reaction of G· with the double bond of EOE yields the addition product G-EOE·. The addition rate constant kadd is determined and compared with the results for MMA-D and BMA. measuring system was used to perform the DMTA measurements. The prepared DMTA samples (one specimen per formulation) were tested in torsion mode with a frequency of 1 Hz and a strain of 0.1%. The temperature was increased from −100 to 200 °C with a heating rate of 2 °C min−1. The glass transition temperature was defined as the temperature at the maximum dissipation factor (tan δ). Swelling Tests. The respective polymer specimens (four specimen per formulation, rectangular shaped) were weighed (mstart) and then submerged in ethanol. The samples were stored at ambient conditions for 7 days, and the ethanol was replaced once after 3 days. The polymer specimens were dried using a paper towel and then weighed (mswollen). Afterward, the discs were placed in a 60 °C vacuum oven and dried until a constant weight was reached (mdry). The degree of swelling S (eq 1) and the gel fraction GF (eq 2) of the polymer networks were determined as follows: S=

accordance with Bischoff et al., IM1 was deprotonated with LDA (in situ prepared from n-BuLi and diisopropylamine) and treated with benzotriazole-1-methanol (BzMeOH) as an organic source of formaldehyde.35 The work-up involves treatment with 1 N NaOH, which affected the final yield. However, it is necessary to effectively remove benzotriazole from the crude product (IM2). The last step was performed without further purification after 1H NMR spectroscopy revealed a conversion of >90% and no significant byproducts. Subsequent mesylation and elimination with DBU afforded the crude product while no significant saponification was observed upon application of the strong base. The crude product was purified via column chromatography to yield 23% pure product after three steps. The aforementioned acid sensitivity of vinyl ethers17 has not been seen with EOE, as weeks of storage under acidic conditions (1 N acetic acid in acetonitrile/water mixture) only led to minor degradation (Figure S1). For the investigation of the cytotoxicity of the synthesized compound EOE an in vitro XTT test30 was carried out. The XTT test is based on the cleavage of the yellow tetrazolium salt XTT [= (sodium-3′-(1-phenylaminocarbonyl)-3,4tetrazolium)bis(4-methoxy-6-nitro)-benzenesulfonic acid hydrate)] to form an orange, water-soluble formazan dye by dehydrogenase activity in active mitochondria. The higher the XTT50 value of a compound, the lower is its cytotoxicity. The XTT50 value determined for EOE was >819 μg/mL,36 which corresponds to a relatively low cytotoxicity. Finally, it should be mentioned that the bacterial reverse mutation test of EOE, the so-called Ames test,31 proved that the EOE did not induce gene mutations.37 Laser Flash Photolysis (LFP). LFP is ideally suited for obtaining quantitative kinetic data describing the reactivity of the primary Ge-centered radicals toward their addition to double bonds.21,28 As a model reaction, we have investigated the photocleavage of the initiator BMDG and the subsequent addition of the primary germyl radical G· to EOE (see Figure 4). The goal was to compare the reactivity of EOE with the reference compound MMA-D and the monomer BMA. Laser-flash photolysis (355 nm excitation) of BMDG gives rise to a transient absorption spectrum revealing a distinct

mswollen − mdry

GF =

mdry

(1)

mdry mstart

(2)

Tensile Tests. A Zwick Z050 equipped with a 1 kN load cell was used for tensile tests. Five specimens per sample were measured. The specimens were fixed between two clamps and strained with a traverse speed of 5 mm min−1. A stress−strain plot was recorded simultaneously. Dynstat Impact Tests. Dynstat impact tests have been performed with the manufactured polymer specimens (10 × 2 × 20 mm3) according to DIN 53 435. For every sample, four different specimens were measured. The impact resistance is determined by the ratio of work required to break the specimen to the cross section of the sample at the fracture site. For the measurements, a 5 kpcm (0.5 J) hammer was used, and the impact resistance value was normalized to the width and thickness of the tested specimen.

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RESULTS AND DISCUSSION Synthesis of EOE. The synthesis was conducted in accordance with Hiersemann et al., who prepared allyl vinyl ethers in a three-step synthesis (Figure 3).33 The first step involves the Williamson etherification of ethyl 2-hydroxyisobutyrate together with ethyl bromoacetate, which yields 72% pure product (literature known IM134) after facile purification via distillation. In the second step, which was performed in D

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Table 1. Second-Order Rate Constants kadd for the Addition of G· to EOE, MMA-D, and BMAa

absorption band at 480 nm (Figure 5) attributed to the germyl radical G·.38,39 The benzoyl-type radical B· does not display

compound

kadd/108 M−1 s−1

EOE MMA-D BMA

1.31 ± 0.10 3.06 ± 0.23 5.50 ± 0.27

a

Errors are reported as twice the standard deviation from least squares analysis of the data.

The rate constant of (1.31 ± 0.10) × 108 M−1 s−1for EOE toward the G· is lower than that of MMA-D. The monomer BMA displays the highest addition rate constant among the studied compounds (Table 1). These data provide the basis for providing reaction mixtures with an appropriate monomer/ AFTC ratio to attain the desired polymer composition in terms of the dominating Ge-based polymer end groups. The fact that the rate constant of the AFCT agent EOE is of the same order of magnitude as for the monomer shows that EOE is included into the polymer chain from the start of the photopolymerization. Another decisive factor for the efficiency of an AFCT agent is the rate of β-scission to form a leaving group radical. In previous studies with β-allyl sulfone-based AFCT agents it has been possible to determine the corresponding rate constants via LFP experiments (kfrag ∼ 104 s−1).21,28,29 Unfortunately, we have not been able to obtain the β-scission rate constants for EOE due to unfavorable absorption properties, since the adduct radical G-EOE· does not reveal transient absorbance above 300 nm. However, we anticipate that EOE displays comparable fragmentation kinetics as previously reported for β-allyl sulfones. Assessing Photoreactivity via Photo-DSC. To get a first insight into the potential of EOE as a transfer agent, a mixture of pure BMA and mixtures of BMA together with 20 mol % EOE and methyl methacrylate dimer (MMA-D) were prepared, and 1 mol % of BMDG was added as photoinitiator. The photoreactivity of the prepared mixtures was assessed via photo-DSC43 at 25 °C with a UV/vis broadband light source together with a filter (400−500 nm). BMA as a monofunctional monomer does not gel but does solidify; therefore, only a moderate autoacceleration should be detectable. The resulting polymer chain is readily soluble in both chloroform and THF, thus a suitable monomer for first tests. The photoDSC measurements reveal kinetic data such as tmax and t95, which are the times when the maximum of heat is evolving and where 95% of the total heat has evolved, respectively (Table 2). At first glance, both AFCT agents seem to perform similarly, and no significant difference is visible from the kinetic point of view. EOE and MMA-D do increase t95 marginally from ∼100 to 125 s, while tmax does not increase but even is decreased from 19 to ∼17 s. However, when looking at the released heat, a drastic difference between the activated vinyl ether and MMA-D is visible. The mixture with EOE releases twice the heat than the mixture containing MMA-D, and assuming an energy neutral transfer reaction,21 this also implies a conversion twice as high. This assumption can be confirmed via 1H NMR spectroscopy. EOE decreases the double-bond conversion only marginally from 58% for pure BMA to 50% for the mixture, whereas MMA-D leads to a significantly lower conversion of 24% of the methacrylate. A high conversion of EOE itself additionally demonstrates the advantage of the activated vinyl ether and its irreversible nature

Figure 5. Transient absorption spectrum recorded 200−300 ns after laser excitation (355 nm) of BMDG in argon-saturated acetonitrile solution (A355-0.3). The inset shows the absorbance spectrum of the parent photoinitiator (BMDG) before laser irradiation.

absorptions in the wavelength range attainable to the LFP method and can only be observed by time-resolved IR and EPR spectroscopy.40,41 Accordingly, monitoring the decay of the G· absorption band in the presence of AFCT agents or monomers allows determining the second order addition rate constants kadd. Exponential fitting of the time decay traces obtained at various AFCT agent or monomer concentrations yielded pseudo-firstorder rate constants kexp.42 The second order addition rate constants (kadd) are obtained from the slopes of their linear dependence on the AFCT agent or monomer concentration, according to eq 3 (k0 represents the estimated rate constant for the decay of the radicals in the absence of a quencher).42 kexp = k 0 + kadd·c BMA/MMA − D/EOE

(3)

Figure 6 shows the corresponding linear dependence of the pseudo-first-order rate constants on the AFCT agent or monomer concentration. Table 1 summarizes the addition rate constants kadd.

Figure 6. Pseudo-first-order decay rate constant (kexp) of radical G· versus AFCT agent and monomer concentrations (excitation wavelength: 355 nm; monitoring wavelength: 480 nm). Secondorder addition rate constants kmonomer are obtained from the slopes. E

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Macromolecules Table 2. Analysis of BMA Polymerized Neat and with AFCT Agentsa photo-DSC

NMR

GPC

CTA

tmax/s

t95/s

ΔH/J g−1

DBCBMA/%

DBCAFCT/%

Mn/kDa

Đ

EOE MMA-D

18.9 ± 0.8 17.1 ± 0.5 16.9 ± 1.0

98 ± 3 127 ± 3 125 ± 0

191 ± 4 133 ± 1 63 ± 2

58 ± 1 50 ± 2 24 ± 2

30 ± 2 12 ± 2

3.6 1.2 1.4

3.4 1.9 2.3

tmax = time to maximum of polymerization rate; t95% = time to 95% of heat evolution; ΔH = measured heat of polymerization; DBC = double bond conversion calculated via 1H NMR spectroscopy; Mn = number-average molecular weight derived from GPC; Đ = polydispersity derived from GPC. a

with 0, 5, 10, and 20 db% (db%···AFCT double bond represents X% of all double bonds in the respective formulation) of EOE in 2M (2M_EOE 5, 2M_EOE 10, and 2M_EOE 20) were prepared to see the change of the polymer network properties with increasing amount of CTA. A mixture of 2M together with 20 db% MMA-D (2M_MMA-D 20) was used as a reference, and for all formulations 1 wt % of BMDG was used as a photoinitiator. The in situ measurement of the double-bond conversion in NIR spectroscopy as well as rheological data during the photopolymerization is realized in RT-NIR-photorheology, which serves as a powerful tool to track different important parameters while curing.32 The time until gelation (tgel), conversion at gel point (DBCgel), the final conversion (DBC), the final shrinkage stress (normal force, FN,final), the time when 95% of the final storage modulus is reached (t95G′), and the final storage modulus (G′) provide a comprehensive picture of the photopolymerization reaction and help to visualize the effect of the AFCT agents. Gelation (G′/G″ = 1; Figure S4) of difunctional methacrylates occurs after a short period of time (100 °C → 31 °C) was observed. Dynstat impact tests and tensile tests further elucidated the capability of EOE to regulate the network formation as both impact resistance and strain at break could be increased significantly. The addition of different amounts of EOE to a dimethacrylate represents a viable tool to tailor the (thermo)mechanical properties and reduce the shrinkage stress, while especially the increase in toughness is highly demanded for photopolymers. Therefore, sterically hindered activated vinyl ethers can be added to the toolbox of efficient AFCT agents for methacrylates, which could gain increased importance as the variety of applications for photopolymers with specific properties (e.g., medicinal devices, prosthetics, tissue engineering) increases steadily.

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ACKNOWLEDGMENTS

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REFERENCES

The authors acknowledge funding by the Christian Doppler Research Association and the company Ivoclar Vivadent AG within the framework of the Christian Doppler Laboratory for “Photopolymers in Digital and Restorative Dentistry”. The financial support by the Austrian Federal Ministry for Digital and Economic Affairs and the National Foundation for Research, Technology and Development is gratefully acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00085. Acid stability tests of EOE; GPC plots, photoreactor plots of EOE in BMA; G′/G″ plot and statistical data of RT-NIR-photorheology measurements in 2M as test matrix; storage modulus, DBC plots, and NMR spectre of storage stability tests in 2M as test matrix; tan δ plot and numerical data of DMTA measurements in 2M as test matrix; numerical data of swellability tests in 2M as test matrix numerical data of tensile tests in 2M as test matrix; numerical data of Dynstat impact tests in 2M as test matrix (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anna Eibel: 0000-0002-0986-2999 Christian Gorsche: 0000-0002-6374-3595 Georg Gescheidt: 0000-0002-6827-4337 Robert Liska: 0000-0001-7865-1936 Notes

The authors declare no competing financial interest. I

DOI: 10.1021/acs.macromol.9b00085 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.9b00085 Macromolecules XXXX, XXX, XXX−XXX