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Simple One-Pot Syntheses of Water-Soluble Bis(acyl)phosphane Oxide Photoinitiators and Their Application in Surfactant-Free Emulsion Polymerization In memoriam Pascal Le Floch

Georgina Müller, Michal Zalibera, Georg Gescheidt, Amos Rosenthal, Gustavo Santiso-Quinones, Kurt Dietliker, Hansjörg Grützmacher* The sodium salt of the new bis(mesitoyl)phosphinic acid (BAPO-OH) can be prepared in a very efficient one-pot synthesis. It is well soluble in water and hydrolytically stable for at least several weeks. Remarkably, it acts as an initiating agent for the surfactant-free emulsion polymerization (SFEP) of styrene to yield monodisperse, spherical nanoparticles. Time-resolved electron paramagnetic resonance (TR-EPR) and chemically induced electron polarisation (CIDEP) indicate preliminary mechanistic insights.

1. Introduction Bis(acyl)phosphane oxides (R1PO(COR2)2; BAPOs) are wellestablished and highly reactive photoinitiators (PIs) that have found widespread use in industrial applications of radical polymerizations, leading to pigmented and clear coatings, adhesives, inks, photoresists, printing plates, and dental restoring materials.[1,2] There is increasing

Dr. G. Müller, Dr. A. Rosenthal, Dr. G. Santiso-Quinones, Dr. K. Dietliker, Prof. H. Grützmacher Department of Chemistry and Applied Biosciences, ETH Zurich CH-8093 Zurich, Switzerland E-mail: [email protected] Dr. M. Zalibera, Prof. G. Gescheidt Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austria

interest toward photopolymerization in an aqueous medium, specifically for biomedical applications such as tissue engineering or the preparation of hydrogel-type materials.[3] Although several concepts have been developed to convert known photoinitiators into water-compatible forms,[4–7] these approaches are mostly based on a tedious synthetic procedures. Water-soluble and hydrolytically stable PIs, which can be activated under mild conditions with LED light remain a formidable challenge. Here, we report a simple one-pot procedure for the synthesis of the new BAPO-type PI bis(mesitoyl)phosphinic acid (BAPO–OH) and its sodium salt. Moreover, we report preliminary results concerning the use of this initiator in surfactant-free emulsion polymerizations (SFEPs) which may represent a valuable contribution to the development of sustainable methods for the preparation of stable dispersions of polymer particles in water.

Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400743

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Depending on the reaction conditions, 2 forms distinct crystals as determined by X-ray diffraction with single crystals.[11] The dimer (BAPO–OH)2 (2)2 (Figure 1a) arises from a concentrated toluene solution, (BAPO–OH)2×EtOH (2)2×EtOH (Figure 1b) from an ethanol solution, and (BAPO–OH)×H2O (2)×H2O from H2O (for details see Supporting Information). Furthermore, the structure of the imidazolium salt (C3N2H5)+(BAPO–O)− 3d as an example for 3 was obtained (Figure 1c). Note that in all structures the quality of the collected data allowed the refinement of the positions of the hydrogen atoms. The structure of BAPO–OH consists of Scheme 1. One-pot procedure for the synthesis of BAPO–OH 2 from red phosphorus and dimers held together by short hydrogen sodium metal. Deprotonation leads to ammonium salts, (NR3H)+(BAPO–O)− 3. bonds between the P(=O)OH moieties of two neighboring molecules (O1– H4ai 1.579(3) Å, O1LO4′ 2.499(2) Å; see 2. Results and Discussion Figure 1a). Two different P–O bond lengths are observed, P1–O1 1.4983(1) Å and P1–O4 1.5365(2) Å, which can be Bis(mesitoyl)phosphinic acid, (MesCO)2P(O)OH = BAPO–OH, attributed to the localization of the acidic proton H4a is easily accessible from red phosphorus and sodium metal at oxygen atom O4. The structure of the adduct with in a one-pot procedure according to Scheme 1. In dimethoxethanol, (2)2×EtOH, is best described as aggregate with yethane (DME) as the solvent and with naphthalene, C10H8, as redox promoter, sodium and phosphorus are converted three components: (i) a BAPO–OH unit with two difto sodium phosphide, Na3P. After in situ protonation to give ferent P–O bond lengths [P2–O24 1.482(2) Å, P2–O25 1.5302(1) Å], (iii) the anion (BAPO–O)− with two equally NaPH2(NaOtBu)x (step 2),[8,9] reaction with mesitoylchloride, MesCOCl (step 3) and treatment with acetic acid, HOAc long P–O bonds [P1–O19 1.502(1) Å, P1–O20 1.501(2) Å], (step 4), the phosphaenol 1[10] is cleanly obtained. Finally, and (iii) an ethoxonium ion (EtOH2)+ which are closely aqueous H2O2 is added (step 5) to afford the desired product linked via hydrogen bridges. In the imidazolium salt 3d, the P–O bond lengths are likewise almost equal [P1–O1 BAPO–OH (2) at high yield (87% based on P). 1.497(1) Å, P1–O2 1.500(1) Å]. The imidazolium cation Crystals of BAPO–OH 2 melt at T = 134.7 °C and are takes a bridging position between two (BAPO–O)− anions thermally stable up to 250 °C before decomposition begins (see Supporting Information for details). The pKa and a polymer strand is formed via O1–H1 and O2–H2 hydrogen bonding (N1-O1 2.67 Å, N2-O2 2.67 Å). In the value of 2 is 2.48 and was determined by titration with structure of BAPO–OH×H2O (not shown here), water 0.1 M NaOH in water (c = 1.295 mg mL−1; T = 298 K). Consequently, various nitrogen bases fully deprotonate BAPO– molecules (O5) form short hydrogen bonds between two OH to give the corresponding salts 3a–d (Scheme 1). molecules of BAPO–OH and layers are formed (O3-O5 disThe sodium salt Na(BAPO–O) 4 is easily accessible by in tance: 2.57 Å). Both P–O bonds have the same length of situ deprotonation of BAPO–OH with NaHCO3 and exhibits 1.507 Å and are slightly elongated compared to a typical P=O double bond (1.47 Å) as in the commercially availgood water solubility of up to 4.5 mg mL−1. Remarkably, this able BAPO derivative IRGACURE 819.[11–13] A charactercompound shows a very good stability under basic conditions, and an aqueous solution of 4 (31P NMR δ = 1.64 ppm) istic structural feature of all BAPO compounds are the long bonds between the phosphorus center and the can be kept at pH = 11 for at least one month under exclucarbon centers of the acyl groups[14] which are about sion of light without decomposition. Diffusion NMR experiments (diffusion ordered spectroscopy, DOSY) were 1.87 Å. These distances are significantly longer than the carried out to determine the degree of aggregation of 2 in typical distances between a tetracoordinated λ5,σ4-P(V) D2O. The evaluation of the data reveal that 2 likely forms center and carbon atoms in a sp2 valence configuration dimeric aggregates in solution as also determined in the (≈1.78 Å). In all compounds, the mesityl rings are rotated solid state (see Figure 1b). On the contrary, the sodium out of conjugation with the carbonyl groups. The torsalt 4 very likely forms solvated monomeric species in sion angle O1–C1–C11–O2 between the carbonyl groups aqueous solution (see Supporting Information for details). varies between 70 and 95°.

2

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Simple One-Pot Syntheses of Water-Soluble Bis(acyl)phosphane Oxide Photoinitiators . . .

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Figure 1. Thermal ellipsoids at 50% probability, all hydrogen atoms apart from those involved in hydrogen bonding are omitted for clarity. Selected bond lengths [Å] are listed (for further details see Supporting Information). a) ORTEP plot of (BAPO–OH)2 (2)2 (thermal ellipsoids at 50% probability, all hydrogen atoms apart from H4a/ H4ai are omitted for clarity): P1–C1 1.871(2), P1–C11 1.868(2), P1–O1 1.4983(1), P1–O4 1.5365(2), C1–O3 1.215(2), C11–O2 1.211(2), O4–H4a 0.92(3). b) ORTEP plot of (BAPO–OH)2×EtOH [(2)2×EtOH]: P1–C1 1.870(2), P1–C11 1.878(2), P1–O19 1.502(1), P1–O20 1.501(2), P2–C21 1.870(2), P2–C31 1.875(2), P2–O24 1.482(2), P2–O25 1.532(2), C1–O18 1.207(2), C11–O21 1.212(2), C21–O22 1.207(3), C31–O23 1.209(2), O24– H1 1.44(4), O26–H1 1.05(3), O20–H2 1.23(5), O26–H2 1.22(5), O19–H3 1.47(4), O25–H3 0.97(4). c) ORTEP plot of (C3N2H5)+(BAPO–O)−3d: P1–C1 1.878(1), P1–C11 1.878(1), P1–O1 1.497(1), P1–O2 1.500(1), C1–O3 1.225(1), C11–O4 1.225(1), N1–O1 2.6699(14), O1–H1 1.844(19), N1–H1 0.853(19), C1–P1–C11 95.07(5), C1–P1–O1 113.11(5), C1–P1–O2 107.57(5), C11–P1–O1 108.07(5), C11–P1–O2 112.22(5), O3–C1–P1 113.38(8), O4– C11–P1 113.89(8), O3–C1–C11–O4 95.91(9), O3–C1–P1–O1 164.52(9), O3–C1–P1–O2 62.83(10), O4–C11–P1–O1 60.76(9), O4–C11–P1–O2 166.88(8), O3–C1–C11–O4 95.91(9).

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Figure 2a shows a comparison of the UV/vis spectra of BAPO–OH (2), its sodium salt Na(BAPO–O) (4), and the imidazolium (Im) salt (C3N2H5)(BAPO–O) 3d in aqueous solution. All compounds show the typical long-wave absorption at about 380–400 nm for the n–π* excitation of the C=O group.[1] The photoactivity of 2 was investigated by timeresolved electron paramagnetic resonance (TR-EPR) upon laser-flash photolysis at λ = 355 nm in acetonitrile. Figure 2b shows the TR-EPR spectrum, obtained between 50 and 200 ns after the laser pulse. It consists of two signals: The central signal at g = 2.0005(1) and two lines (emission/absorption (E/A)) spaced by 34.9(1) mT centered at g = 2.0040(2). In analogy to well-described BAPOs,[15–19] the central signal can be assigned to the mesitoyl whereas the E/A doublet, with the characteristically large isotropic hyperfine coupling constant of 34.9 mT clearly shows the presence of a phosphanoyl radical. The E/A character of the doublet indicates that the chemically induced electron polarization (CIDEP) effect observed in the spectrum is dominated by the radical pair mechanism.[15–17] The hyperfine coupling AP of 34.9 mT is the largest value reported so far for an acyl-phosphonyl radical.[15,18,20] It indicates a high degree of s-character and spin localization at the phosphorus. Thus, EPR reveals the α-cleavage of 2 upon photoexcitation in the n–π* region of its absorption spectrum leading to the formation of two reactive radicals. Generally, the preparation of polymer lattices under surfactant-free conditions requires polar end-groups on the growing polymer chains, which stabilize the colloidal particles.[21–24] Typically, water-soluble thermal initiators such as potassium persulphate (KPS), 2,2-azobis-(2-amidinopropane)dihydrochloride (V-50), 2,2′-azobis isobutyramidine (AIBA), or azobis(cyanovaleric acid) (V501) are employed.[25–27] To provide amphiphilic reaction conditions, we have utilized the sodium salt Na(BAPO–O) 4 for the surfactant-free emulsion polymerization (SFEP) of styrene (S) Accordingly, we have tested varying amounts of 4 as the initiator (Table 1).[28] Irradiation of the reaction solution with a blue LED (λ = 465 nm) or UV light at room temperature led to characteristic polymer architectures. The diameter of the polymer particles obtained by UV irradiation ranges from d = 209 to 228 nm. The size of the particles decreases with increasing amount of PI. The smallest particles were obtained with 2.04 mol% of Na(BAPO–O) (4) with respect to monomer and a styrene to water ratio of 0.5 mL/10 mL under blue LED irradiation. From dynamic light scattering (DLS) measurements for two experiments under the same conditions, values for the average particle size of Zave = 97 nm and Zave = 123 nm were obtained. The average size of the particles obtained from a scanning electron

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Figure 2. a) Comparison of UV/vis spectra in water of BAPO–OH (2) and its sodium salt Na(BAPO–O) (4) and its imidazolium salt (C3N2H5) (BAPO–O) (3d) at c = 0.1 × 10−3 M in the range of λ = 300–480 nm. b) TR-EPR spectrum of BAPO–OH 2 in CH3CN, integrated signal intensity observed 50–200 ns after the laser pulse. The signal of the acyl radical is in the center at g = 2.0005(1), the signals of the HO–PO(COMes)l radical [g = 2.0040(2); AP = 34.9 mT] is indicated by asterisks.

microscopy (SEM) image is d = 79 nm. While the average molecular weight is rather small and shows a broad distribution (Mw = 1500–3500 g mol−1 and Mw/Mn = 1.4–2.1 for different experiments; see Supporting Information for details), the polydispersity index (PDI) for the size distribution is remarkably narrow and below PDI < 0.1 in all experiments indicating a monodisperse size distribution. In Figure 3, two SEM images of these spherical polymer particles at different magnifications are shown. The ζ-potential found for the lattices described herein, ranges from −56 to −61 mV. These values are an indication for polymer dispersions with a very good stability (see Table S2, Supporting Information).

3. Conclusion In summary, a facile one-pot procedure for the preparation of the new acidic photoinitiator BAPO–OH 2 and its Table 1. Photoinitiated SFEP of styrene (S) with 4a) in 10 mL water and an irradiation time of 30 min under blue LED or UV light.

Exp. no. S: water ratio mol% 4b) [mL]

Blue LED d [nm]

UV d [nm]

0.5:10

380c)

228c)

1

0.64

2

2.04

123 79d)

231c)

3

3.21

138c)

225c)

0.32

240c)

218d)

5

1.02

c)

220

209d)

6

1.60

222c)

210d)

4

1.0:10

c)/97c),

Figure 3. SEM images of monodisperse polymer beads obtained in the photoinitiated SFEP (blue LED irradiation) of styrene with Na(BAPO–O) (4) at a magnification 200 K× (left) and 30 K× (right).

easily accessible water-soluble salts has been developed. The photoinitiated system with the use of Na(BAPO–O) (4) thus operates under principles usually valid for thermally initiated surfactant-free emulsion polymerization. Conveniently, this process can be carried out at ambient temperature to afford stable lattices. The spherical polystyrene nanoparticles exhibit a very narrow size distribution.[29] Water-soluble photoinitiators with ambiphilic character like 4 open up new perspectives for a wide range of applications taking advantage of their activity at interfaces. We are presently exploring the scope of these initiators for the production of hydrogels and the generation of 3D structures. Moreover, initial investigations by time-resolved optical methods and CIDNP spectroscopy indicate that the photoinduced reactivity of 4 offers new reaction pathways.

Supporting Information

a)

4 was prepared in situ from 1 eq. BAPO–OH/1.2 eq. NaHCO3; b) With respect to styrene; c)Diameter d determined by DLS; d) Diameter d determined by SEM.

4

Supporting Information is available from the Wiley Online Library or from the author.

Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400743 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Simple One-Pot Syntheses of Water-Soluble Bis(acyl)phosphane Oxide Photoinitiators . . .

Macromolecular Rapid Communications www.mrc-journal.de

Acknowledgements: This work was supported by the ETH Zurich, TU Graz, and the BASF Company. M.Z. thanks the Pro Bono foundation for financial support. Received: December 30, 2014; Revised: January 7, 2015; Published online: ; DOI: 10.1002/marc.201400743

Keywords: electron paramagnetic resonance; nanoparticles; phosphorus; photoinitiators; polymerization; radicals

[1] K. Dietliker, A Compilation of Photoinitiator Commercially Available for UV Today, SITA Technology Limited, Edinburgh 2002. [2] K. Dietliker, J. V. Crivello, Photoinitiators for Free Radical Cationic & Anionic PhotopolymerisationWiley, Vol. III, Chichester 1998. [3] a) N. Moszner, I. Lamparth, J. Angermann, U. K. Fischer, F. Zeuner, T. Bock, R. Liska, V. Rheinberger, Beilstein J. Org. Chem. 2010, 6, 26; b) B. Ferse, K.-F. Arndt, N. Oelsner, Z. Phys. Chem. 2014, 228, 209; c) H. Lin, D. Zhang, P. G. Alexander, G. Yang, J. Tan, A. W.-M. Cheng, R. S. Tuan, Biomaterials 2013, 34, 331. [4] Cyclodextrines as solubility promoter for hydrophobic PIs: a) I. C. Alupei, V. Alupei, H. Ritter, Macromol. Rapid Commun. 2002, 23, 55; b) S. Li, F. Wu, M. Li, E. Wang, Polymer 2005, 46, 11934. [5] Examples of known water-soluble PIs: Acylphosphinates: a) T. Majima, W. Schnabel, W. Weber, Makromol.Chem. 1991, 192, 2307; b) B. D. Fairbanks, M. P. Schwartz, C. N. Bowman, K. S. Anseth, Biomaterials 2009, 30, 6702; c) B. D. Fairbanks, S. P. Singh, C. N. Bowman, K. S. Anseth, Macromolecules 2011, 44, 2444; Specifically functionalized hydrophilic α-hydroxyketones: d) S. Knaus, H. F. Gruber, J. Macromol. Sci. A 1996, 33, 869; e) R. Liska, J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1504; Benzophenones: 6e), Thioxanthones: 6e); f) J. Qiu, J. Wei, J. Appl. Polym. Sci. 2014, 131, 40659. g) F. Karasu, D. Balta, R. Liska, N. Arsu, J. Inclusion Phenom. Macrocyclic Chem. 2010, 68, 147; h) T. Hayakawa, K. Horie, Dent. Mater. 1992, 8, 351; For a review see: i) W. A. Green, Eur. Coat. J. 1994, 5, 274. [6] For a phenacylpyridinium oxalate in the photopolymerization of hydrophilic monomers in water, see: M. A. Tasdelen, B. Karagoz, N. Bicak, Y. Yagci, Polym. Bull. 2008, 59, 759. [7] An aqueous dispersion of a BAPO photoinitiator is commercially available (Irgacure 819-DW, 50% PI). This photoinitiator allows initiation with visible light, but shelf life stability of the dispersion may be limited in certain formulations. [8] M. Podewitz, J. D. van Beek, M. Wörle, T. Ott, D. Stein, H. Rüegger, B. H. Meier, M. Reiher, H. Grützmacher, Angew. Chem. 2010, 122, 7627. [9] M. Podewitz, J. D. van Beek, M. Wörle, T. Ott, D. Stein, H. Rüegger, B. H. Meier, M. Reiher, H. Grützmacher, Angew. Chem. Int. Ed. 2010, 49, 7465. [10] G. Becker, W. Becker, M. Schmidt, W. Schwarz, M. Westerhausen, Z. Anorg. Allg. Chem. 1991, 605, 7.

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[11] CCDC 1021460 (BAPO–OH)2 (2)2, CCDC 1021457 (BAPO– OH)2(EtOH) (2)2 × EtOH, and CCDC 1020197 (C3N2H5)+(BAPO– O)− (3d) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif. [12] R. H. Sommerlade, S. Boulmaaz, J.-P. Wolf, J. Geier, H. Grützmacher, M. Scherer, H. Schönberg, D. Stein, P. Murer, S. Burkhardt (Ciba Speciality Chemicals Holding Inc., Switzerland), WO2005014605A1, 2005. [13] H. Grützmacher, J. Geier, D. Stein, T. Ott, H. Schönberg, R. H. Sommerlade, S. Boulmaaz, J.-P. Wolf, P. Murer, T. Ulrich, CHIMIA Int. J. Chem. 2008, 62, 18. [14] T. Ott, PhD Thesis, ETH Zurich (Switzerland) 2008. [15] I. Gatlik, P. Rzadek, G. Gescheidt, G. Rist, B. Hellrung, J. Wirz, K. Dietliker, G. Hug, M. Kunz, J.-P. Wolf, J. Am. Chem. Soc. 1999, 121, 8332. [16] D. Hristova, I. Gatlik, G. Rist, K. Dietliker, J.-P. Wolf, J.-L. Birbaum, A. Savitsky, K. Möbius, G. Gescheidt, Macromolecules 2005, 38, 7714. [17] U. Kolczak, G. Rist, K. Dietliker, J. Wirz, J. Am. Chem. Soc. 1996, 118, 6477. [18] S. Jockusch, N. J. Turro, J. Am. Chem. Soc. 1998, 120, 11773. [19] G. W. Sluggett, P. F. McGarry, I. V. Koptyug, N. J. Turro, J. Am. Chem. Soc. 1996, 118, 7367. [20] I. Gatlik, PhD Thesis, University of Basel (Switzerland) 2001. [21] A. Kotera, K. Furusawa, Y. Takeda, Kolloid Z. Z. Polym. 1970, 239, 677. −, Kolloid Z. Z. Polym. 1970, [22] A. Kotera, K. Furusawa, K. Kudo 240, 837. [23] J. W. Goodwin, J. Hearn, C. C. Ho, R. H. Ottewill, Colloid Polym. Sci. 1974, 252, 464. [24] J. W. Goodwin, R. H. Ottewill, R. Pelton, G. Vianello, D. E. Yates, Br. Polym. J. 1978, 10, 173. [25] K. Tauer, R. Deckwer, I. Kühn, C. Schellenberg, Colloid Polym. Sci. 1999, 277, 607. [26] Examples of nitroxide mediated controlled free-radical emulsion polymerization employing water-soluble poly(sodium acrylate) alkoxyamine macroinitiators without surfactant have also been reported: a) G. Delaittre, J. Nicolas, C. Lefay, M. Save, B. Charleux, Chem. Commun. 2005, 5, 614; b) C. Dire, S. P. Magnet, L. Couvreur, B. Charleux, Macromolecules 2008, 42, 95. [27] A. M. Telford, B. T. T. Pham, C. Neto, B. S. Hawkett, J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3997. [28] Recently, the photoinitiated radical emulsion polymerization of various monomers employing BAPOs under irradiation with UV or blue LED light has been reported: a) P. Laurino, H. F. Hernandez, J. Bräuer, K. Krüger, H. Grützmacher, K. Tauer, P. H. Seeberger, Macromol. Rapid Commun. 2012, 33, 1770; b) P. Seeberger, H. Grützmacher, K. Tauer, J. Braeuer, P. Laurino (Max-Planck-Gesellschaft, Germany, ETH Zürich, Switzerland), WO2012052147A1, 2012; c) P. Seeberger, H. Grützmacher, K. Tauer, J. Braeuer, P. Laurino (Max-Planck-Gesellschaft, Germany, ETH Zürich, Switzerland), WO2012052148A1, 2012. [29] In contrast, a very broad size distribution was observed in SFEP with other photoinitiated systems: M. Ratanajanchai, D. Tanwilai, P. Sunintaboon, J. Colloid Interface Sci. 2013, 409, 25.

Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400743 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Simple one-pot syntheses of water-soluble bis(acyl)phosphane oxide photoinitiators and their application in surfactant-free emulsion polymerization.

The sodium salt of the new bis(mesitoyl)phosphinic acid (BAPO-OH) can be prepared in a very efficient one-pot synthesis. It is well soluble in water a...
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