Sustainable Synthesis of a Fluorinated Arylene Conjugated Polymer

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Letter Cite This: ACS Macro Lett. 2018, 7, 1232−1236

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Sustainable Synthesis of a Fluorinated Arylene Conjugated Polymer via Cu-Catalyzed Direct Arylation Polymerization (DArP) Robert M. Pankow, Liwei Ye, and Barry C. Thompson* Department of Chemistry and Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089-1661, United States

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S Supporting Information *

ABSTRACT: Direct arylation polymerization (DArP) has enabled the more sustainable synthesis of conjugated polymers through the reduction of waste, elimination of toxic and hazardous byproducts, and through the reduction of overall synthetic steps. However, DArP methodologies have almost exclusively relied on the use of noble metal catalysts, such as those with Pd, counter to the efforts toward sustainability. Herein, we report the optimized synthesis of a flourinated arylene conjugated copolymer via DArP using a low-cost, sustainable copper catalyst. Through optimization of the polymerization conditions, we are able to lower the loading of the catalyst from 50 to 5 mol %. As an example, we are able to obtain molecular weights of 16.4 kDa, accompanied by a yield of 54%, with a loading of only 5 mol % for the Cu catalyst. The synthesized polymers were characterized using 1H and 19F NMR spectroscopy, showing agreement with those previously prepared with a minimization or exclusion of defects. This work also demonstrates that Cu-catalyzed DArP methodologies can be compatible with substrates that do not possess directing groups, which can help to facilitate C−H functionalization, allowing for a broadening of substrate scope.

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Daugulis et al. have reported the synthesis of biaryls through the cross-coupling of a fluoroarenes and an aryl iodide.11,12 This method requires only a mild base, such as K3PO4, and is suitable for catalytic amounts of the copper catalyst (10 mol %). Given these parameters, we were interested in extending this methodology toward the synthesis of conjugated polymers in an effort to show that copper catalysts can potentially replace Pd catalysts in DArP. Transcribing these conditions for conjugated polymers is not direct, however, since Daugulis originally used high concentrations (1 M) and a stoichiometric imbalance between the cross-coupling partners. Conditions such as these are not favorable for conjugated polymer synthesis, since conjugated polymers typically possess low solubility and require an accurate stoichiometric balance to achieve high molecular weights (Mn). Shown in Scheme 1, our efforts have focused on optimizing the synthesis of poly[(9,9-dioctylfluorene-2,7-diyl)(2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-diphenylene)] (PDOF-

onjugated polymers have become promising for a wide variety of applications, ranging from optoelectronic devices to biomedical applications. Conventionally, their synthesis has relied on cross-coupling methods that often invoke toxic, hazardous reagents and extended synthetic routes. Through direct arylation polymerization (DArP), their synthesis has become simplified, and many of the hazards associated with a conjugated polymer synthesis can be circumvented through the use of C−H activation.1−5 DArP, however, still relies on unsustainable metal catalysts, such as Pd, which hinder the effort toward greater sustainability that DArP seeks to address. Other efforts to improve the sustainability of DArP have remained limited.6,7 As an effort to employ more sustainable transition metal catalysts, such as copper, oxidative coupling has become a successful method for conjugated polymer synthesis, but these methods are restricted to the preparation of homopolymers and require a stoichiometric amount of oxidant.8,9 Methods for copper-catalyzed aryl−aryl cross couplings using C−H activation are known, but their application toward a perfectly alternating conjugated copolymer synthesis has only just come under investigation. As the first report, we have previously disclosed conditions for the Cu-catalyzed DArP of various thieno[3,4-c]pyrrole-4,6-dione (TPD) copolymers, and we became interested in expanding the scope of this methodology to other substrates.10 We had postulated that the carbonyls on TPD can function as a directing group, helping to facilitate C− H functionalization, and thus, we were determined to know whether or not Cu-DArP is compatible with substrates that do not possess this type of functionality. © XXXX American Chemical Society

Scheme 1. Synthesis of PDOF-OD Using the Conditions Outlined in Table 1

Received: August 15, 2018 Accepted: September 17, 2018

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ACS Macro Letters Table 1. Optimization of Cu-DArP Conditions for PDOF-OD entry

cat. mola (%)

base (eq.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

50 50 50 50 25 15 5 15 5 15 5 5 5 5

K2CO3 (4) K2CO3 (40) K3PO4 (4) K3PO4 (40) K3PO4 (4) K3PO4 (4) K3PO4 (4) K3PO4 (40) K3PO4 (40) K3PO4 (4) K3PO4 (4) K3PO4 (4) K3PO4 (4) K3PO4 (4)

solventb (M) DMA DMA DMA DMA DMA DMA DMA DMA DMA DMA DMA DMA DMA DMA

(0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.5) (0.5) (0.25) (0.5) (0.25)

temp (°C)

time (h)

Mnc (kDa), Đc

yieldc (%)

140 140 140 140 140 140 140 140 140 140 140 140 120 140

72 72 72 72 72 72 72 72 72 16 16 16 16 36

10.1, 2.16

64 0 78 64 72 52 48 0 0 71 51 54 36 62

24.5, 2.18 insoluble 4.3, 2.62 2.7, 2.56 1.46, 1.91

20.4, 3.18 6.79d 16.4, 2.23 4.67, 5.20 insoluble

a

Loading based on equivalents to each monomer. 99.999%-Puratrem Cu(I) iodide was used as the copper source with a 1:1 ratio to phenanthroline. bN,N-Dimethylacetamide (DMA). cDetermined for polymer products after purification. dThe polymer product was only soluble in hot 1,2-dichlorobenzene (DCB) causing much of the higher-Mn portions of the sample to be filtered off before measurement using GPC.

decrease in Mn was observed (4.3 kDa) for 25 mol % (entry 5), but the yield remained similar to that of entry 3 (72%). A similar trend was observed for lower loadings (15 and 5 mol %), where the value for Mn significantly diminished, while the yield remained moderate (entries 6 and 7). Interestingly, no polymer product was obtained when higher equivalents of base (40 equiv) was employed with a lower catalyst loading (entries 8 and 9). Taking into account the results from entries 5−9, we presumed that the concentration of the monomers is too low (0.1 M), which is generally unfavorable for step-growth polymerizations such as the one under study. Since the solubility of the PDOF-OD polymer is reasonable in DMA, as evidenced by the value for Mn with entry 3 (24.5 kDa), we envisioned that increasing the concentration of the reaction mixture from 0.1 to 0.5 M would help to facilitate the polymerization at lower catalyst loadings. As shown with entry 10, an increase in the concentration from 0.1 M (entry 6) to 0.5 M with a 15 mol % catalyst loading led to a significant increase in the value for Mn (20.4 vs 2.7 kDa) and yield. The reaction time was also decreased from 72 (entry 6) to 16 h (entry 10) due to visible gelation of the reaction mixture. This result significantly improved upon our previously disclosed conditions for Cu-catalyzed DArP, which required higher catalyst loadings (50 mol %) and prolonged reaction times (72 h).10 Aside from the increase in concentration, we believe the low pKa of 2 and good solubility of PDOF-OD also contribute to the improvement in Mn and yield. When the catalyst loading is further lowered from 15 mol % (entry 10) to 5 mol % (entry 11), the solubility of the isolated polymer product became significantly lower, presumably due to a higher Mn being achieved. The portion of polymer soluble in DCB from this sample, allowing for measurement by GPC, gave a Mn of 6.79 kDa and a yield of 51%. Support for the claim that the insoluble materials obtained were consequential of a high Mn being achieved, as opposed to branching, cross-linking, or other structural defects, is provided in the discussion regarding the 1H and 19F NMR analysis below. In regards to the polymerization reaction, in order for branching or cross-linking to occur, it would require the activation of a C−H bond on fluorene, which is relatively not acidic. A previous study of PDOF-OD synthesis using Pd-

OD), which has been previously prepared using Pd-catalyzed DArP.13,14 The focus of optimization was in regards to catalyst loading, where the equivalents of base, catalyst loading, ligand, and solvent concentration were varied, as detailed in Table 1. The Supporting Information (SI) includes complete synthetic details in regards to the synthesis for monomers and polymers. Briefly, polymerizations were performed in a high-pressure vessel under a N2 atmosphere for the allotted reaction time and temperature. The reaction mixture was then cooled, and solids were dissolved in hot 1,2-dichlorobenzene and then precipitated into a cold 10% (v/v) ammonium hydroxide/ methanol solution. The polymer was then filtered off and washed sequentially with water, methanol, acetone, and hexanes. The polymer was then collected and dried overnight under high vacuum. Based on our previous study with TPD Cu-catalyzed DArP, for the synthesis of PDOF-OD (Scheme 1), we initially employed K2CO3 as a base (4 equiv), CuI as the copper source (50 mol %), phenanthroline (phen) as the ligand (50 mol %), and N,N-dimethylacetamide (DMA) as the solvent, at 140 °C for 72 h (entry 1).10 We found this provides a good value for Mn (10.1 kDa) and yield (64%). Adding an excess of base (40 equiv, entry 2) was detrimental to the reaction and no polymer product was afforded, despite being a successful strategy for promoting C−H functionalization in our previous study and a report on Pd-catalyzed DArP by Leclerc et al.10,15 Since Daugulis et al. had success employing K3PO4 as a base for certain substrates, we figured this might be an improvement over K2CO3. Indeed, as shown in entry 3, K3PO4 as a base provided a significant increase in Mn and yield (24.5 and 78%, respectively) relative to entry 1. This demonstrates the capacity for these conditions to yield perfectly alternating conjugated copolymers, without the need for a directing group to help facilitate C−H functionalization. Increasing the equivalents of K3PO4 to 40 equiv only afforded insoluble polymer product with a slightly diminished yield (64%, entry 4). Given the high catalyst loading (50 mol %) in addition to the high equivalents of base for entry 4, we presume the insolubility of the polymer product is likely due to a Mn that is too high to allow for solubility in 1,2-dichlorobenzene (DCB). With optimal conditions for 50 mol % catalyst loading, we sought to probe lower loadings (entries 5−7). A significant 1233

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ACS Macro Letters

section 5 of SI and Figure S1).13,20−23 The proposed location for the octafluorobenzene end-group is slightly upfield (δ7.38) from that of the tetrafluorobenzene model compound (δ7.08), but this is likely due to further deshielding brought upon by the additional tetrafluorobenzene group attached and is close to the range of chemical shift observed for these compounds (δ 7.35−7.27).20,24 19F NMR spectroscopy was also performed for structural analysis (see SI for spectra), and we observed two singlet resonances at δ-135.0 and δ-138.9, which are in good agreement with the literature values of δ-135.0 and δ139.0.13,14 Minor resonances were not observed in the 19F NMR spectra at −134.1 and −134.5 ppm, which are believed to be associated with a terminal octafluorobiphenyl unit. Also, resonances in the range of 24−25 ppm were not observed, which would indicate the oxidative coupling of the octafluorobiphenyl units yielding perfluorinated oligo(pphenylenes).13,24 Based on a previous study, the presence of branching or cross-linking in PDOF-OD prepared via DArP can be determined by comparing the number of repeat units calculated using NMR spectroscopy with that from GPC.13 Because end-groups were not observed in the 19F spectrum, the end-groups present in 1H NMR were used for the analysis. Using the 1H NMR spectrum from entry 10 as an example (Figure 2), integral ratios between the end group c−c′ and the

catalyzed DArP suggests that branching may occur where 1 and 2 oxidatively couple to form branched structures.13 The conditions employed for the aforementioned study used PtBu2Me·HBF4 as the phosphine ligand, DMA as the solvent, and Pd(OAc)2 as the palladium source. It should be noted that the ligand (PtBu2Me·HBF4) and solvent (DMA) may be the cause of the observed defects, since in a subsequent study with PCy3 as the ligand and toluene as the solvent, branching defects were not reported.14 For reference, in the case of Cucatalyzed oxidative couplings, Cu(OAc)2 is generally employed for substrates with greater reactivity than fluorene, such as benzothiazole or other azaheterocycles, and those using Cu(I) catalysts require more reactive bases.8,9,12,16−18 For example, a study regarding copper-catalyzed dehydrogenative, or oxidative, couplings between benzothiazole and pentafluorobenzene required excess lithium alkoxide as a base, likely ruling out branching or cross-linking in our case.19 Further structural analysis using 1H NMR spectroscopy, discussed below, provides additional evidence for the absence or minimization of branching with the reaction conditions reported here. Operating under the hypothesis that the conditions in entry 11 resulted in an insoluble, high Mn polymer, the concentration was lowered from 0.5 M (entry 11) to 0.25 M (entry 12). This afforded a more soluble polymer product with a Mn of 16.4 kDa and a yield of 54%. We also attempted to achieve more soluble polymer product by lowering the reaction temperature from 140 to 120 °C (entry 13). However, this provided significantly lower Mn (4.67 kDa) and yield (36%) for the given reaction time (16 h). It is possible that extending the reaction time may allow for improved Mn and yield at lower temperatures. Characterization of the PDOF-OD polymers was performed using 1H NMR spectroscopy in order to confirm the proposed structure. 1H NMR data for Table 1 is provided in the SI. As shown in Figure 1, the structure of PDOF-OD matches identically with that previously reported with resonances centered at δ7.94 and δ7.59 (ppm).13,14 The assignment of end groups was performed by comparing the obtained spectrum for PDOF-OD to that of model compounds with similar structure, for which a detailed discussion is provided in the SI (see

Figure 2. Integral ratio between c−c′ and C−C′ protons of PDOFOD (entry 10 of Table 1): collected in CDCl3 at 25 °C and 500 MHz.

polymeric protons C−C′ (annotated in Figures 1 and 2) show a ratio of 1:25, which is in close agreement with the estimate for Mn provided by the GPC (1:30). This suggests that the potential branching defects observed using Pd(OAc)2, (PtBu2Me·HBF4), and DMA based conditions are suppressed with the Cu-catalyzed methodology presented here, where PDOF-OD with branching-defects are reported to have a much greater disparity between the NMR and GPC data (1:25 vs 1:127, respectively).13 Given that the c−c′ proton is adjacent to the b−b′ proton, which is the proposed location for branching, any instance of branching would likely have a direct effect on the integration of this resonance (c−c′). We

Figure 1. 1H NMR of PDOF-OD synthesized using the conditions outlined in Table 1 (entry 3): collected in CDCl3 at 25 °C and 500 MHz. 1234

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(6) Pankow, R. M.; Ye, L.; Gobalasingham, N. S.; Salami, N.; Samal, S.; Thompson, B. C. Investigation of Green and Sustainable Solvents for Direct Arylation Polymerization (DArP). Polym. Chem. 2018, 9 (28), 3885−3892. (7) Matsidik, R.; Luzio, A.; Hameury, S.; Komber, H.; McNeill, C. R.; Caironi, M.; Sommer, M. Effects of PNDIT2 End Groups on Aggregation, Thin Film Structure, Alignment and Electron Transport in Field-Effect Transistors. J. Mater. Chem. C 2016, 4, 10371−10380. (8) Faradhiyani, A.; Zhang, Q.; Maruyama, K.; Kuwabara, J.; Yasuda, T.; Kanbara, T. Synthesis of Bithiazole-Based Semiconducting Polymers via Cu-Catalysed Aerobic Oxidative Coupling. Mater. Chem. Front. 2018, 2, 1306. (9) Huang, Q.; Qin, X.; Li, B.; Lan, J.; Guo, Q.; You, J. Cu-Catalysed Oxidative C−H/C−H Coupling Polymerisation of Benzodiimidazoles: An Efficient Approach to Regioregular Polybenzodiimidazoles for Blue-Emitting Materials. Chem. Commun. 2014, 50 (89), 13739− 13741. (10) Pankow, R. M.; Ye, L.; Thompson, B. C. Copper Catalyzed Synthesis of Conjugated Copolymers Using Direct Arylation Polymerization. Polym. Chem. 2018, 9 (30), 4120−4124. (11) Do, H.-Q.; Khan, R. M. K.; Daugulis, O. A General Method for Copper-Catalyzed Arylation of Arene C−H Bonds. J. Am. Chem. Soc. 2008, 130 (45), 15185−15192. (12) Do, H.-Q.; Daugulis, O. Copper-Catalyzed Arylation and Alkenylation of Polyfluoroarene C−H Bonds. J. Am. Chem. Soc. 2008, 130 (4), 1128−1129. (13) Lu, W.; Kuwabara, J.; Iijima, T.; Higashimura, H.; Hayashi, H.; Kanbara, T. Synthesis of π-Conjugated Polymers Containing Fluorinated Arylene Units via Direct Arylation: Efficient Synthetic Method of Materials for OLEDs. Macromolecules 2012, 45 (10), 4128−4133. (14) Saito, H.; Kuwabara, J.; Kanbara, T. Facile Synthesis of Fluorene-Based π-Conjugated Polymers via Sequential Bromination/ Direct Arylation Polycondensation. J. Polym. Sci., Part A: Polym. Chem. 2015, 53 (19), 2198−2201. (15) Grenier, F.; Goudreau, K.; Leclerc, M. Robust Direct (Hetero)Arylation Polymerization in Biphasic Conditions. J. Am. Chem. Soc. 2017, 139, 2816−2824. (16) Liu, C.; Yuan, J.; Gao, M.; Tang, S.; Li, W.; Shi, R.; Lei, A. Oxidative Coupling between Two Hydrocarbons: An Update of Recent C−H Functionalizations. Chem. Rev. 2015, 115 (22), 12138− 12204. (17) Qin, X.; Feng, B.; Dong, J.; Li, X.; Xue, Y.; Lan, J.; You, J. Copper(II)-Catalyzed Dehydrogenative Cross-Coupling between Two Azoles. J. Org. Chem. 2012, 77 (17), 7677−7683. (18) Do, H.-Q.; Daugulis, O. Copper-Catalyzed Arylation of Heterocycle C−H Bonds. J. Am. Chem. Soc. 2007, 129 (41), 12404−12405. (19) Fan, S.; Chen, Z.; Zhang, X. Copper-Catalyzed Dehydrogenative Cross-Coupling of Benzothiazoles with Thiazoles and Polyfluoroarene. Org. Lett. 2012, 14 (18), 4950−4953. (20) Wakioka, M.; Kitano, Y.; Ozawa, F. A Highly Efficient Catalytic System for Polycondensation of 2,7-Dibromo-9,9-Dioctylfluorene and 1,2,4,5-Tetrafluorobenzene via Direct Arylation. Macromolecules 2013, 46 (2), 370−374. (21) Hernández, M. C. G.; Zolotukhin, M. G.; Maldonado, J. L.; Rehmann, N.; Meerholz, K.; King, S.; Monkman, A. P.; Fröhlich, N.; Kudla, C. J.; Scherf, U. A High Molecular Weight Aromatic PhOLED Matrix Polymer Obtained by Metal-Free, Superacid-Catalyzed Polyhydroxyalkylation. Macromolecules 2009, 42 (23), 9225−9230. (22) Lu, W.; Kuwabara, J.; Kanbara, T. Polycondensation of Dibromofluorene Analogues with Tetrafluorobenzene via Direct Arylation. Macromolecules 2011, 44 (6), 1252−1255. (23) Luo, Z.-J.; Zhao, H.-Y.; Zhang, X. Highly Selective PdCatalyzed Direct C−F Bond Arylation of Polyfluoroarenes. Org. Lett. 2018, 20 (9), 2543−2546. (24) Heidenhain, S. B.; Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Mori, T.; Tokito, S.; Taga, Y. Perfluorinated Oligo(p-

hypothesize that this suppression of branching-defects (βdefects) is likely due to the sensitivity of the Cu-catalyst to steric hindrance and the general lower reactivity of the Cucatalyst toward C−H functionalization.10,1,25 In summary, we have presented the optimization of CuDArP conditions that allow for a substantial decrease in the loading of the catalyst, from 50 to 5 mol %. This was achieved through optimization of the base employed and increasing the concentration of the monomers. Lowering the catalyst loading is without great sacrifice to the values for Mn and yield for PDOF-OD, which are 16.4 kDa and 54% for 5 mol% Cu catalyst, respectively. Structural analysis of the synthesized polymers using 1H and 19F NMR spectroscopy shows agreement with previously reported values and shows a minimization or exclusion of defects, specifically branching caused by oxidative or dehydrogenative couplings. Employing Cu-DArP for substrates without directing groups successfully demonstrates the capacity for this methodology to be applied to a broader scope. Future work will seek to improve upon the polymerization conditions and expand the substrate scope.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00618.

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Experimental procedures including the synthesis and characterization for the monomers and polymers (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Barry C. Thompson: 0000-0002-3127-0412 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (MSN under Award Number CHE-1608891) and the Dornsife/Graduate School Fellowship (to R.M.P.). We thank Thomas Saal for assistance with the 19F NMR experiments.

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REFERENCES

(1) Pouliot, J.-R.; Grenier, F.; Blaskovits, J. T.; Beaupré, S.; Leclerc, M. Direct (Hetero)Arylation Polymerization: Simplicity for Conjugated Polymer Synthesis. Chem. Rev. 2016, 116 (22), 14225−14274. (2) Gobalasingham, N. S.; Thompson, B. C. Direct Arylation Polymerization: A Guide to Optimal Conditions for Effective Conjugated Polymers. Prog. Polym. Sci. 2018, 83, 135−201. (3) Bohra, H.; Wang, M. Direct C−H Arylation: A “Greener” Approach towards Facile Synthesis of Organic Semiconducting Molecules and Polymers. J. Mater. Chem. A 2017, 5, 11550−11571. (4) Lombeck, F.; Marx, F.; Strassel, K.; Kunz, S.; Lienert, C.; Komber, H.; Friend, R.; Sommer, M. To Branch or Not to Branch: C−H Selectivity of Thiophene-Based Donor−Acceptor−Donor Monomers in Direct Arylation Polycondensation Exemplified by PCDTBT. Polym. Chem. 2017, 8 (32), 4738−4745. (5) Wang, Q.; Takita, R.; Kikuzaki, Y.; Ozawa, F. PalladiumCatalyzed Dehydrohalogenative Polycondensation of 2-Bromo-3Hexylthiophene: An Efficient Approach to Head-to-Tail Poly(3Hexylthiophene). J. Am. Chem. Soc. 2010, 132, 11420−11421. 1235

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ACS Macro Letters Phenylene)s: Efficient n-Type Semiconductors for Organic LightEmitting Diodes. J. Am. Chem. Soc. 2000, 122 (41), 10240−10241. (25) Daugulis, O.; Do, H.-Q.; Shabashov, D. Palladium- and Copper-Catalyzed Arylation of Carbon−Hydrogen Bonds. Acc. Chem. Res. 2009, 42 (8), 1074−1086.

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