Reversible Addition Fragmentation Chain Transfer-Mediated

Chapter 34 Reversible Addition Fragmentation Chain Transfer -Mediated Copolymerizations: The Challenge of Random Copolymer Blocks 1,2

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Bert Klumperman , James B. McLeary , Eric Τ. Α. van den Dungen , Willem-Jan Soer , and Jelena S. Božović 2

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Downloaded by CORNELL UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch034

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Laboratory of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands UNESCO-Associated Center for Macromolecules and Materials, University of Stellenbosch, Private Bag X1, 7602 Matieland, South Africa 2

The synthesis of random copolymer blocks as part of a block copolymer is potentially one of the unique features of living radical polymerization. However, in this contribution it is shown that subtle variations, such as the choice of a different initiating radical are sufficient to induce variations in copolymer composition at the early stages of the chain growth. The very first few monomer additions in a RAFT-mediated styrene - acrylonitrile copolymerization are studied using in situ H NMR spectroscopy. Initialization behavior is observed in a similar fashion as seen in a RAFT-mediated homopolymerization. In a RAFT-mediated alternating styrene - maleic anhydride copolymerization, the effect of the nature of the leaving group of the RAFT-agent is clearly demonstrated by careful analysis of the resulting (low molar mass) copolymers. l

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

501

502 Introduction

The invention of various living radical polymerization (LRP) techniques induced great excitement about the possibilities of synthesizing polymeric materials with advanced architectures, and precisely controlled chain length ' . In comparison to the well-known living anionic polymerization the choice of monomers is much wider, and in terms of polymerization conditions, LRP also offers great advantages. Among the most versatile LRP techniques is Reversible AdditionFragmentation Chain Transfer (RAFT) mediated polymerization . However, this technique is sometimes hampered with inhibition-like phenomena as well as with retardation phenomena . For RAFT-mediated homopolymerization it was earlier shown that the original RAFT-agent is converted into its single monomer adduct in a highly selective reaction, which is schematically depicted in Scheme l. This selective reaction was termed initialization, and appears to be dependent on monomer type and type of RAFT-agent. This then also raises the question about the effect of initialization on RAFT-mediated copolymerization. 1 2,3

1

2,3


4

5 , 6

A'

AAcD

Scheme 1. A schematic representation of the initialization process in which primary radicals (A) react with styrene (S), and this adduct in turn reacts wit the RAFT-agent (AD) to form the intermediate radical (ASD()A). Upon fragmentation, the single monomer adduct (ASD) is formed and a leaving group-based radical (A) that can again react with monomer to form (AS), etc. A similar process is shown for acrylonitrile (Ac)

In earlier work on Atom Transfer Radical Copolymerization it was already shown that the early stages of the chain growth can deviate from the long chain limit. From modeling studies it was clearly shown that this effect is most likely due to selectivity of the primary radical for addition to the one or the other comonomer in combination with differences in the equilibrium between active and dormant chains for the two different terminal monomer units. 7,8,9


503 Initialization in RAFT-mediated homopolymerization In order to understand copolymerization results, a brief summary of recent discoveries on initialization of RAFT-mediated homopolymerizations will be presented. The investigations into initialization were aimed at unraveling the origin of reported inhibition effects during RAFT-mediated polymerization. Quickly it turned out that this initial period was not a true inhibition period. It is a period in which one monomer unit is inserted in the RAFT-agent between the leaving group and the sulphur atom. In some cases, such as the dithiobenzoate mediated polymerization of methyl acrylate, some chain growth starts to take place before the complete consumption of the RAFT-agent .However, in the dithiobenzoate mediated polymerization of styrene, the decrease in RAFT-agent concentration is pseudozero order in RAFT-agent concentration . From these results it was concluded that the rate-determining step during initialization is the addition of the leaving group radical to monomer. In Figure 5


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Time (min) Figure 1. Relative concentrations of methyl protons of dithiobenzoate species versus time in the in situfreeradical polymerization ofstyrene at 70 °C. Cyanoisopropyl dithiobenzoate was used to mediate the polymerization in the presence ofAIBNasan initiator. 3.56*10~ mol CtDf, 6.75*W molAIBN, 2.40* W mol styrene, 4.84*1Or mol cyanoisopropyl dithiobenzoate. AD is the original RAFT-agent, and ASp is the RAFT agent with i styrene units inserted. (Reproduced from reference 9. Copyright 2004 American Chemical Society.) 3

5

3


4

504 1, a typical example is shown of a cyanoisopropyl dithiobenzoate mediated styrene polymerization. In this polymerization, die initiator-derived primary radicals (cyanoisopropyl radicals, derived from AIBN as initiator) are identical to the leaving group radicals . This makes the interpretation of the NMR spectra somewhat less complicated compared to the system of dissimilar radicals. However, the main conclusions appear to be independent of this variation. 6


Experimental Chemicals Styrene (STY, 99.9%) (Protea Chemicals) was washed with a 0.3 M potassium hydroxide (KOH, 85%) (ACE) solution followed by vacuum distillation to remove die inhibitor and polymer. Acrylonitrile (AN, Aldrich, 99+%) was purified by filtration through a column filled with inhibitor remover for removing hydroquinone and monomethyl ether hydroquinone (Aldrich). Maleic anhydride (MAh, Aldrich, 99%, briquettes) was used as supplied. Solvents: Toluene (Hi-Dry™, anhydrous solvent, Romil Ltd), Methyl Ethyl Ketone (MEK, Merck, 99%), Tetrahydrofuran (THF, Aldrich, AR) were used without further purification. Azo-initiators: Azo bis(isobutyronitrile) (AIBN, Riedel De Haen) was recrystallized from A R grade methanol and found to be -99% pure by *H NMR spectroscopy; l,r-azobis(l-cyclohexanecarbonitrile) (ACHN, Wako, >98%) was used as received. Deuterated solvent (C D 99.6%, 0.1% TMS Sigma-Aldrich) and formic acid (96%, Sigma-Aldrich) were used as received. 6

6

Synthesis of transfer agents The synthesis of bis(thiobenzoyl) disulfide was carried out following the method of Thang et al. with the modifications of de Brouwer et al. The synthesis of cyanoisopropyl dithiobenzoate (CiPDB) was carried out according to the method of Le et α/. and purified by liquid chromatography on a silica column using a 9:9:2 vol. ratio of pentane: heptane: diethyl ether. The product was dried under vacuum to provide the compound with a H NMR purity estimated at - 98%. l

9

2

3

l

Sample preparation for styrene-ce-acrylonitrile reactions A stock solution was prepared by mixing 0.99 g deuterated benzene, 40 mg AIBN (2.41 *10" mol), 0.67 g STY (6.43*10° mol), 0.34 g A N (6.43*10" mol), and 0.29 g CiPDB (1.34*10" mol). The NMR tube was filled to an appropriate level with the stock solution. The tube was flushed with ultra-high purity nitrogen for 10 minutes. At this point a sealed glass insert containing the integration referencestandard (formic acid) was inserted and the tubes were sealed. The use of the reference standard was solely for integration purposes.

4

3

3

Synthesis of poly(styrene-a//-maleic anhydride), SMAh A typical copolymerization of styrene (STY) and maleic anhydride (MAh) was carried out


505 4

as follows: 0.132 g of the RAFT-agent, (CiPDB, 5.97*10" mol) and 14.2 mg of the azo-initiator, (ACHN, 5.82* 10" mol) were accurately weighed, and transferred into a 15-mL, three-necked, round-bottom flask equipped with a magnetic stirrer. 6.02 g of MEK was added as a solvent, followed by the addition of monomers: 0.744 g MAh (7.59* 10" mol) and 0.889 g STY (8.55*10" mol). After the reaction mixture was bubbled with argon for 45 min, an initial sample was removed for gas chromatography (GC) analysis and the flask was immersed in a thermostated oil bath kept at 70 °C, as the reaction temperature. The reaction was carried out under an argon atmosphere. After 12 hrs the reaction was stopped by exposing it to air, and conversion of styrene was 62%, determined by GC analysis. The final reaction mixture was precipitated in an excess of η-heptane (5-fold volumetric excess with respect to the total polymer solution) and then dried under vacuum at 30 °C overnight. 100 mg of dried polymer was dissolved in 10 mL of THF and used for SEC analysis, to determine a molar mass ( M E c ) and a polydispersity index (PDI): M „ E C 1927 g/mol and PDI = 1.12. MALDI analysis was also performed using a similar solution of polymer in THF (lmg/mL). 5

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Analysis

Determination of monomer conversion by G C Monomer, STY conversion was determined by a gas chromatography (GC). Hewlett-Packard (HP-5890) GC, equipped with an AT-Wax capillary column (30 m χ 0.53 mm χ 10 μπι) was used, employing the GC gradient (see Table 1). Toluene was employed as an internal standard. M„,SEC and P D I were measured by size exclusion chromatography (SEC), at ambient temperature using a Waters GPC equipped with Waters model 510 pump and a model 410 differential refractometer. Tetrahydrofuran, THF (Biosolve, stabilised with BHT) with 5 wt% of acetic acid was used as the eluent at a flow rate of 1.0 mL/min. A set of two mixed bed columns (Mixed-C, Polymer Laboratories, 30 cm, 40 °C) was used. Calibration was carried out using narrow molar mass distribution polystyrene (PSTY) standards ranging from 600 to 7*10 g/mol. The masses were calculated using the PSTY calibration curve. Samples were filtered over a 13 mm χ 0.2 μιη PTFE filter, PP housing, Alltech. Data acquisition and processing were performed using WATERS Millennium32 (v3.2 or 4.0) software. 11

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Toluene was added (5 wt% related to MEK) as the internal standard, for the GC analysis.


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Table 1. GC temperature gradient

Temperature ( °C)

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110

220

Time ( min)

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Rate (°C/min)

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Matrix Assisted Laser Desorption/ Ionization - Time Of Flight - Mass Spectrometry (MALDI-ToF-MS) MALDI-ToF-MS analysis was carried out on a Voyager DE-STR from Applied Biosystems. The matrix trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was synthesized according to literature procedures. Potassium trifluoroacetate (PTFA, Aldrich, >99%) was added as cationization agent. The matrix was dissolved in THF at a concentration of 40 mg/mL. The PTFA salt was added to THF at typical concentrations of 1 mg/mL. Polymer was dissolved in THF at approximately 1 mg/mL. In a typical MALDI-ToF-MS analysis the matrix, salt and polymer solution were premixed in a ratio of 10:1:5. The premixed solutions were handspotted on the target well and left to dry. Spectra were recorded in both the linear mode and reflector mode. Additionally, the obtained data were analyzed using in-house developed software. 12

*H Nuclear Magnetic Resonance (Ή NMR) analysis NMR spectra were recorded on a 600 MHz Varian ^'^tnova spectrometer. A 5 mm inverse detection PFG probe was used for the experiments and the probe temperature was calibrated using an ethylene glycol sample in the manner suggested by the manufacturer using the method of Van Geet. H spectra were acquired with a 3 μβ (40°) pulse width and a 4 sec acquisition time. For the U kinetic experiments, samples were inserted into the magnet at 25 °C and the magnet fully shimmed on the sample. A spectrum was collected at 25 °C to serve as a reference. The sample was then removed from the magnet and the cavity of the magnet was raised to the required temperature. Once the magnet cavity had stabilized at the required temperature, the sample was re-inserted (time zero) and allowed to equilibrate for approximately 5 minutes. Additional shimming was then carried out to fully optimize the system and the first spectra were recorded less than 10 min after the sample was inserted into the magnet. Peak assignment was carried out via model compounds and first principles, and automated integration was carried out using ACD Labs 8.0 H processor® using manual peak selection and reintegration to account for overlapping peaks. 4 !

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507 Results a n d Discussion l

The in situ H NMR experiment was conducted in a fashion similar to our previous homopolymerization experiments . However, the resulting N M R spectra are considerably more complex due to the presence of two different monomers. As an example, Figures 2a - 2d show an expansion of the NMR spectra at 5, 30, 60 minutes and 180 minutes. The signals in this part of the NMR spectra are assigned to the ortho protons of the phenyl group of the dithiobenzoate. At time t = 5, the spectrum is largely that of the original cyanoisopropyl dithiobenzoate. Time t = 30 minutes coincides with the end of the initialization period. At this point, the original RAFT-agent is fully converted into a small number of specific monomeric or oligomeric polymer chains. After that, polymerization starts and at t = 60 minutes it is clear that a number of different species are beginning to form, and at time t = 180 minutes the reaction is stopped and the presence of numerous dithiobenzoate species is clear. From the differences among the spectra it is obvious that large differences exist in terms of monomer units neighbouring the thiocarbonyl thio moiety. Detailed investigation, and comparison with the styrene homopolymerization study leads to the observation that addition of the primary radicals to both monomers occurs in the system to produce two mono-adducts, one derived from styrene and one from acrylonitrile. Figure 2a shows that the two forming mono-adducts are formed in unequal amounts. In Figure 3, the decrease in the styrene and acrylonitrile signals are depicted. It is clear that the initial consumption rates of acrylonitrile and styrene monomer are larger than the rates of consumption after initialization is completed, i.e. after 30 minutes. The consumption of monomer prior to initialization is driven by the addition of cyanoisopropyl radicals to the respective monomer units. It is also clear that the predominant mono-adduct that is formed is derived from styrene and this was confirmed by comparison of chemical shifts of species to the homopolymerization studies. One more remarkable point in Figure 3, especially after initialization, is that the styrene concentration decreases linearly with time, whereas that of acrylonitrile seems to fluctuate to some extent. This fluctuation seems to diminish somewhat towards larger reaction times. It is therefore likely that the fluctuation in acrylonitrile consumption rate is caused by chain length dependence of the propagation and chain transfer reactions. In Figure 4, the concentration profiles of the four most prominent adducts are shown. It is apparent that the ratio of the mono-styrene adduct to the mono-acrylonitrile adduct is reasonably constant during the initialization period. This is a fairly logical consequence of the selectivity of the cyanoisopropyl radical towards styrene and acrylonitrile. At approximately 30 minutes both single monomer adducts reach a maximum. The chain extension of the mono-acrylonitrile adduct with a styrene unit commences already from time zero, but there is a significant increase in the rate of this reaction at t = 30 minutes. Conversely, the addition of


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508

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Figure 2a-2d. Expansion of the Ή NMR spectra taken during the STY/AN ymerization after 5, 30, 60 and 180 minutes reaction time. an acrylonitrile unit to the mono-styrene adduct does not take place to anymeasurable extent before the completion of initialization. It is tempting to draw further conclusions with respect to the origin of this difference. One might argue that the large selectivity of the formation of the single monomer adduct is stronger for the styrene adduct than for the acrylonitrile adduct. However, this would be a premature conclusion, since there are two ways to arrive at the dimer adduct. The one way is the reactivation of a single monomer adduct, and the subsequent formation of a dimer adduct. The other way is the addition of two monomers to the initiating cyanoisopropyl radical before



509 addition to the RAFT-agent. Subsequent release of the leaving group from the intermediate radical leads to the formation of the dimer adduct. A similar question applies for example to the dithiobenzoate mediated polymerization of methyl acrylate. Without additional information it is impossible to distinguish between the two scenarios. This will definitely be subject of future investigations. At t = 60 minutes the initial acrylonitrile mono-adduct has been consumed, predominantly by the addition of a styrene monomer unit. At this point the rate of acrylonitrile consumption in the reaction changes, as seen in Figure 3. Although a number of species are not presented in Figure 4 it becomes clear that the nature of the propagating radical and its addition rate coefficients to the respective monomers as well as related intermediate radicals have a significant effect on the rate of consumption of the respective monomers with strong changes in rates of monomer consumption possible early in RAFT mediated copolymerization. The oligomers with a terminal acrylonitrile unit present a shorter lifetime than those with a terminal styrene unit (the role played by the intermediate radical species in affecting the propagating radical concentrations cannot be discounted, meaning that a simplistic comparison of propagation rates could lead to misleading conclusions), this has significant implications to the distribution of the monomer units within the polymeric chains and longer chain polymers would need to be investigated to confirm whether this effect is only observed at short chain lengths. The effect of selectivity of the primary radical is also clearly seen when the copolymerization of STY and maleic anhydride (MAh) is studied. This copolymerization has a large tendency towards alternation. Previous work where pulsed laser polymerization (PLP) was applied to the STY/MAh copolymerization has unambiguously shown that the penultimate unit model is the suitable model to describe this copolymerization . Furthermore, on the basis of the reactivity ratios it can easily be estimated that over the majority of the comonomer feed range, the terminal unit in a growing polymer chain is STY for more than 90%. Recent results from Kajiwara and co-workers at first glance seem to be in contradiction with this observation. They carried out RAFT-mediated copolymerization of STY and MAh, and determined the nature of the intermediate radical by ESR spectroscopy. It was convincingly shown that the intermediate radical carries a MAh terminal unit at both sides. However, this finding is not necessarily in contradiction with the large fraction of STY terminal chain ends reported in conventional copolymerization. The nature of the propagating radical in the reaction may be predominantly styrenic while the intermediate radical is strongly MAh on both sides. This would be the case if addition of a MAh chain end radical to a macroRAFT agent is of larger or comparable rate as its addition to a STY monomer, and simultaneously the fragmentation rate to form a MAh terminal chain end radical is smaller than that to form a STY terminal chain end radical. 13

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40

60

80

100

120

140

160

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Time (min)

Figure 3. Concentration versus time profiles for STY (+) andfor AN (o) durin the copolymerization ofSTY and AN

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Figure 4. Concentration versus time profiles during the copolymerizstion ofS and ANfor single monomer adducts (AAcD and ASD), and dimmer adducts (AAcSD and ASAcD), where A and D are the cyanoisoprpyl and dithiobenzoat moieties, respectively.


511 Figure 5 shows the MALDI-ToF mass spectrum of a STY - MAh copolymer. The copolymer was synthesized via cyanoisopropyl dithiobenzoate mediated copolymerization. With the help of an in-house developed software tool we analyzed the mass spectrum and obtained a copolymerfingerprintas shown in Figure 6. It is easily recognized that the number of STY units in the copolymer is typically one larger than the number of MAh units. If it is taken into account that the polymerization is a virtually ideal alternating copolymerization, the η τγ = n + 1 signature of thisfingerprintcan only originate from the veryfirstone or 15

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Figure 5. MALDI-ToF Mass spectrum of an alternating STY-MAh copolymer, and an expansion thereof The polymer was synthesized in a cyanoisopropyl dithiobenzoate mediated polymerization.



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Figure 6. Copolymerfingerprintof the STY- MAh copolymer shown in Figure 5.

two monomer addition steps. Moreover, if the polymerization is mediated by a cumyl dithiobenzoate it is immediately recognized that the fingerprint show a maximum at η τ γ = n Ah (result not shown). Again, this points at the importance of the first monomer additions. After a few monomer additions, the effect of the cyanoisopropyl or cumyl chain end is no longer a factor in determining the reactivity of the growing chain end. δ

M

Conclusions

Initialization occurs in RAFT-mediated copolymerization just like it does in RAFT-mediated homopolymerizations. This was convincingly shown using in situ H NMR spectroscopy during the RAFT-mediated copolymerization of STY and AN. From the same experiment, indications are obtained that the selectivity of the initial additions of the leaving group radical to both comonomers plays an important role in the early stages of RAFT-mediated polymerization. This is similar to the initiator effect in Atom Transfer Radical Copolymerization as was seen before. Further evidence for this effect of the initiating radical was obtained from the RAFT-mediated copolymerization of STY and MAh. l


513 References

1. 2.

3.


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5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18.

Matyjaszewski, Κ. Controlled Radical Polymerization; ACS Symp. Ser. 685, ACS, Washington, D.C., 1998 Matyjaszewski, K. Controlled/Living Radical Polymerization, Progress in ATRP, NMP, and RAFT; ACS Symp. Ser. 768, ACS, Washington, D.C., 2000 Matyjaszewski, K. Advances in Controlled/Living Radical Polymerization; ACS Symp. Ser. 854, ACS, Washington, D.C., 2003 Chiefari, J.; Chong, Y. Κ. B.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. Α.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559-5562 Monteiro, M . J.; de Brouwer, H. Macromolecules 2000, 34, 349-352. Vana, P.; Davis, T. P.; Barner-Kowollik, C. Macromol. Theory Simul. 2002, 11, 823-835. Perrier, S.; Barner-Kowollik, C.; Quinn, J. F.; Vana, P.; Davis, T. P. Macromolecules 2002, 35, 8300-8306. McLeary, J. B.; McKenzie, J. M . ; Tonge, M . P.; Sanderson, R. D.; Klumperman, B. Chem. Comm. 2004, 1950-1951. McLeary, J. B.; Calitz, F. M . ; McKenzie, J. M . ; Tonge, M . P.; Sanderson, R. D.; Klumperman, B. Macromolecules 2004, 37, 2383-2394 Klumperman, B.; Chambard, G.; Brinkhuis, R.H.G. in Advances in Controlled/Living Radical Polymerization; ACS Symp. Ser. 854, ACS, Washington, D.C., 2003, 180-192. Ziegler, M.J.; Matyjaszewski, K. Macromolecules 2001, 34, 415-424. Matyjaszewski, K. Macromolecules 2002, 35, 6773-6781. McLeary, J. B.; Calitz, F. M . ; McKenzie, J. M . ; Tonge, M . P.; Sanderson, R. D.; Klumperman, B. Macromolecules 2005, 38, 3151-3161 Tacx, J.C.J.F.; Meijerink, N.L.J.; Suen, K. Polymer 1996, 37, 4307-4310. Ulmer, L.; Mattay, J.; Torres-Garcia, H. G.; Luftmann, H. Eur. J. Mass Spectrom. 2000, 6, 49-52. Sanayei, R.A.; O'Driscoll, K.; Klumperman, B. Macromolecules 1994, 27, 5577-5582. Du, F.-S.; Zhu, M.-Q.; Guo, H.-Q.; L i , Z.-C.; L i , F.-M.; Kamachi, M . ; Kajiwara, A. Macromolecules 2002, 35, 6739-6741 Willemse, R. X . E.; Staal, B. B. P.; Donkers, E. H. D.; van Herk, A. M . Macromolecules 2004, 37, 5717-5723.


Reversible Addition Fragmentation Chain Transfer-Mediated

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