Controlled Synthesis of an Alternating Donor–Acceptor Conjugated

(5) The incorporation of both monomers was confirmed via 1H NMR and UV–vis spectroscopy as well as chain extension studies. ... (7) The key step in ...
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Controlled Synthesis of an Alternating Donor−Acceptor Conjugated Polymer via Kumada Catalyst-Transfer Polycondensation Alexander D. Todd† and Christopher W. Bielawski*,‡,§ †

Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 689-798, Republic of Korea § Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Kumada catalyst-transfer polycondensation was used to synthesize poly(5,6-difluorobenzotriazole-alt-4-hexylthiophene) (PFBTzHT), an alternating donor−acceptor conjugated polymer, in a controlled manner. Through a series of kinetic studies, a linear relationship between the monomer conversion and the molecular weight of the resulting polymer was observed while maintaining narrow polydispersities (Đs ≤ 1.4). Moreover, the Mn displayed a linear relationship with the initial monomerto-catalyst feed ratio, and polymers with Mns up to 25 kDa were successfully synthesized. All-conjugated block copolymers containing segments of PFBTzHT and poly(3-hexylthiophene) (P3HT) were also obtained in various compositions (30−70% P3HT) via sequential monomer addition in a single vessel.

B

dithienosilole and benzotriazole could be synthesized in a chain-growth fashion using Kumada catalyst-transfer polycondensation (KCTP).5 The incorporation of both monomers was confirmed via 1H NMR and UV−vis spectroscopy as well as chain extension studies. While the work by the Kiriy and Seferos groups have provided fundamental insights and new synthetic routes to various forms of alt-CPs, they do not afford perfectly alternating conjugated systems, which could have implications in the final electronic properties of the polymers. In addition to transition-metal-catalyzed synthesis of D−A CPs, there have been several examples of transition-metal-free approaches using fluoride salts.6 Using such methodology, Sanji et al. showed that treatment of 2-perfluoroaryl-5trimethylsilylthiophenes and 1-pentafluoro-phenyl-4[(trimethylsilyl)ethynyl]benzenes with tetra-n-butylammonium fluoride afforded polymers with tunable molecular weights and narrow Đs.6b,c Despite the success of the method, chain extension studies and block copolymerizations afforded materials with broad polydispersities (Đs ≥ 2). Moreover, the technique currently appears to have a limited monomer scope. The use of a method that meets the necessary requirements of controlled polymerizations, such as KCTP, for the synthesis of alternating CPs would ultimately allow for broad synthetic versatility.

andgap engineering has become an important tool for accessing low band gap π-conjugated polymers (CPs) that exhibit relatively high power conversion efficiencies (PCEs) when used as photoactive layers in polymer-based solar cells.1 One successful strategy for decreasing the bandgap of CPs has been the development of so-called donor−acceptor (D-A) CPs, where the polymer backbone consists of an alternating electron-rich and electron-poor π-conjugated repeat unit. Consequently, bulk heterojunction (BHJ) devices containing D−A CPs display some of the highest reported PCEs to date.1 Conventional synthetic methods for synthesizing D−A CPs have relied on Pd-catalyzed Stille- or Suzuki-type polycondensations and typically afford ill-defined polymers with variable molecular weights and broad polydispersities (Đs). Thus, a method for accessing well-defined D−A CPs with predictable molecular weights and narrow Đs is warranted. Efforts toward the realization of a controlled synthesis of alternating D−A CPs have been recently described. For example, Kiriy and co-workers demonstrated that a D−A monomer bearing a fluorene-linked benzothiadiazole underwent polymerization via Suzuki polycondensation.2 Although a chain-growth mechanism was established, accessing polymers with controlled or high molecular weights proved to be challenging. The same group developed a method for the synthesis of D-A-D CPs that consisted of a thiophene-altnapthalenediimide-alt-thiophene repeat unit.3 The aforementioned system utilized a radical-anion intermediate that exhibited chain-growth kinetics when initiated with a Ni or Pd catalyst. 4 In a recent example, the Seferos group demonstrated that statistical D−A copolymers based on © XXXX American Chemical Society

Received: July 22, 2015 Accepted: October 21, 2015

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DOI: 10.1021/acsmacrolett.5b00505 ACS Macro Lett. 2015, 4, 1254−1258

Letter

ACS Macro Letters

monomer since alkylation of the nitrogen at the 2-position was expected to improve the solubility of the resultant polymer. Considering that LiCl has been shown to accelerate Grignard metathesis (GRIM) reactions13 and to promote KCTPs, i PrMgCl·LiCl was used as a key reagent.7,14 Initial studies focused on optimizing the transmetalation chemistry. As summarized in Scheme 2, the compounds 1a−3a were

We previously reported the controlled KCTP of an AB-type monomer consisting of a thiophene linked to a dialkoxybenzene.7 The key step in obtaining a regioregular alternating CP, poly(thiophene-alt-p-phenylene) (PTPP), was the selective transmetalation at the thiophene with a iPrMgCl·LiCl complex. Regardless, the molecular weight of the PTPPs were successfully controlled up to 39 kDa and displayed narrow Đs. All-conjugated diblock copolymers were also obtained via sequential monomer addition. Building on these findings, we aimed to extend the concept of synthesizing alt-CPs in a controlled manner to a D−A system where a thiophene moiety and an electron-deficient moiety (e.g., benzotriazole) were incorporated into a single monomeric unit. Given that Mghalogen exchange is accelerated with electron-deficient arenes,8 we reasoned that a monomer possessing the general structure of I should undergo transmetalation faster at the electron deficient halogen-arene bond in comparison to the electron rich unit and, thus, afford a regioregular D−A alt-CP. While synthesizing I, a timely report was published by Zhang describing the C−H thienylation of 4-bromo-5,6-difluorobenzothiadiazoles (IIA) and 4-bromo-5,6-difluorobenzotriazoles (IIB) via Pd-catalyzed cross coupling chemistry (Scheme 1).9

Scheme 2. Synthesis of PFBTzT and PFBTzHT

independently treated with one equivalent of iPrMgCl·LiCl and allowed to stir for 30 min at ambient temperature. The resulting mixtures were then quenched with aqueous solutions saturated with NH4Cl and then examined by 1H NMR spectroscopy. Inspection of the NMR spectra indicated that 2a and 3a afforded 2b and 3b, respectively, as a single product. Conversely, treatment of 1a with iPrMgCl·LiCl led to a mixture of monotransmetalated and ditransmetalated products, as well as starting material (see Supporting Information (SI)). With these results in mind, we reasoned that 2a and 3a should afford regioregular D−A CPs with primarily head-to-tail linkages, whereas the corresponding polymer of 1a would be expected to contain head-to-head and tail-to-tail defects. Guided by the aforementioned Grignard metathesis studies, we shifted our attention toward the polymerization of 2b and 3b; key results are summarized in Table 1. When a THF

Scheme 1. (A) Initial Target Monomer Structure; (B) C−H Thienylation of Electron-Deficient Arenes, As Reported by Zhang9

Table 1. Conditions Used to Polymerize 2a or 3aa entry 1 2 3 4

Moreover, when IIA or IIB and 2-bromo- or 2-chlorothiophenes were used as the coupling partners, the aryl-halogen bonds were preserved and a dihalo-substituted conjugated system similar to that of I was obtained. In general, the methods for accessing well-defined fluorine-containing πconjugated polymers represent an emerging field in organic photovoltaics as such materials often show relatively high open circuit voltages,10 electron hole mobilities,11 and PCEs12 when compared to their nonfluorinated analogues. Herein we describe the synthesis of well-defined, alternating D−A CPs via the KTCP of a donor−acceptor monomer, 4thienyl-5,6-difluorobenzotriazole. The method enabled excellent control over the Mn and the Đ of the corresponding polymers and was successfully extended to access well-defined, all-conjugated diblock copolymers. To the best of our knowledge, this is the first reported example of a controlled synthesis of D−A CPs utilizing KCTP. The monomer precursors 1a−3a were prepared by following modifications of literature procedures9 and used to evaluate the steric and electronic parameters necessary to achieve selective transmetalation and subsequent polymerization. Benzotriazole was chosen as the electron-deficient component of the

Grignard reagentb

monomer 2a 3ad 3ad 3ae

i

PrMgCl·LiCl PrMgCl·LiCl i PrMgCl·LiCl i PrMgCl i

Ni catalyst Ni(dppp)Cl2 Ni(dppp)Cl2 Ni(dppe)Cl2 Ni(dppp)Cl2

Mn (kDa)

Đ

c

12.9 5.6

1.22 1.76

Mn and Đ were measured via gel permeation chromatography (GPC) in THF at 23 °C and are reported against polystyrene standards. b Used as a 1.3 M in THF (iPrMgCl·LiCl) or a 2.0 M in THF (iPrMgCl). cThe resulting polymer was insoluble in THF at 23 °C. d Theoretical Mn = 14.6 kDa based on the [3b]0/[Ni]0 = 30 (yield = 75%). eNo precipitate formed upon quenching with 5 M HCl. a

solution of 2b ([2b]0 = 0.02 M) was added to solid Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane, [2b]0/[Ni]0 = 30), a deep purple color formed immediately and was followed by the formation of a precipitate within 15 min. A poorly soluble15 polymeric material was subsequently isolated after vacuum filtration and later identified to be poly(5,6-difluorobenzotriazole-alt-thiophene) (PFBTzT). To enhance the solubility of the aforementioned polymer, focus shifted toward the polymerization of 3b, which features a hexyl side chain on the 4-positon of the thiophene unit. After adding a THF solution of 3b ([3b]0 = 0.02 M) to Ni(dppp)Cl2 ([3b]0/[Ni]0 = 30), the resulting reaction mixture was stirred for 1 h. Upon quenching with 5 M HCl and subsequent workup, a yellow-orange polymer, poly(5,6-diluorobenzotria1255

DOI: 10.1021/acsmacrolett.5b00505 ACS Macro Lett. 2015, 4, 1254−1258

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ACS Macro Letters

polymerization of 3b was dependent on concentration, as solutions containing [3b]0 > 0.02 M typically resulted in uncontrolled polymerizations. Another hallmark of a controlled polymerization process is that the molecular weight of the polymer synthesized is inversely proportional to the initial amount of catalyst added, assuming that the initial monomer concentration remains constant. Thus, a series of polymerizations were conducted with different monomer/catalyst feed ratios ranging from 10− 50 = [3b]0/[Ni(dppp)Cl2)]0; the results are summarized in Figure 1 (top right). The Mn of the PFBTzHT obtained from the aforementioned reaction scaled with respect to the amount of initiator added. In order to obtain polymers with higher molecular weights, the initial monomer concentration must remain lower than or equal to 0.02 M. The upper limit of attainable molecular weights was nearly 25 kDa, corresponding to a polymer with approximately 50 repeat units. Attempts at synthesizing relatively high PFBTzHT (e.g., 32 kDa) led to polymers with an approximate Mn = 20 kDa with narrow Đs (≤1.3). To probe the underlying polymerization mechanism, PFBTzHT (Mn = 6.2 kDa, Đ = 1.30) was subjected to matrix-assisted laser desorption ionization (MALDI) analysis (Figure 1, bottom), and the major species present were found to be Br/H and H/H (recorded as the respective NH4+ adducts) terminated polymers. The former were in accord with an in situ catalyst initiation and catalyst transfer mechanism. As previously noted by Yokozawa, H/H terminated polymers can arise from impure monomers or the presence of excess reagents and did not appear to affect the overall polymerization reaction.16 Collectively, the results of the kinetic experiments as well as the catalyst-feed-ratio studies were consistent with controlled chain-growth processes. It is well-known that CPs synthesized via KCTP have an active Ni(II) species that resides at the terminus of the growing polymer, allowing for chain extension upon exposure to a fresh feed of monomer or access to diblock copolymers.13b,16 Thus, a chain extension experiment was performed in which an aliquot was removed from the reaction mixture after 1 h which afforded a PFBTzHT with an initial Mn = 16.1 kDa and a Đ = 1.29 (87% conversion of monomer as determined by 1H NMR spectroscopy). Afterward, a fresh THF solution of 3b ([3b]0 = 0.02 M) was added to the aforementioned macroinitiator and quenched with 5 M HCl after 1 h. GPC analysis of the resultant polymer after the second addition of 3b showed that a polymer with an Mn = 22.2 kDa and a Đ = 1.47 was produced, in accordance with the predicted Mn. The results from the chain extension study demonstrated that the Ni-terminated PFBTzHT was indeed active for KCTP and indicated that the corresponding polymerization reactions were well controlled. With the success of the chain extension study, subsequent efforts were directed toward finding other suitable Grignard monomers, in particular, 2-bromo-5-chloromagnesio-3-hexylthiophene·LiCl (4, Scheme 3) to synthesize block copolymers. A Ni-terminated PFBTzHT (Mn = 9.9 kDa, Đ = 1.24) macroinitiator (PFBTzHT-1) was added to a THF solution of 4 (0.07 mmol, 0.02 M) and the resulting orange solution was stirred for 45 min until being quenched with 5 M HCl. GPC analysis of the material obtained after the addition of 4 showed a shift to shorter elution times (Mn = 18.1 kDa, Đ = 1.31). Moreover, the 1H NMR spectrum recorded for the product featured signals corresponding to PFBTzHT as well as P3HT. Using similar polymerization conditions, a series of PFBTzHT-

zole-alt-4-hexylthiophene) (PFBTzHT), with a number-average molecular weight (Mn) of 12.9 kDa (theoretical Mn = 14.6 kDa) and a Đ = 1.22 was obtained. PFBTzHT exhibited high solubility in common organic solvents (i.e., THF, CH2Cl2, CHCl3, PhMe, and hexanes) at ambient temperature, and the corresponding solutions appeared yellow-green fluorescent. In order to examine the efficacy of other Ni catalysts, Ni(dppe)Cl2 (dppe = 1,3-bis(diphenylphosphino)ethane) was used; however, only a low molecular weight polymer (Mn = 5.6 kDa) with a broad Đ (1.76) was obtained (entry 3). To examine the effect of additives (i.e., LiCl) on the polymerization of 3b, a reaction analogous to that described above was conducted in the absence of LiCl. After 1 h, an aliquot was removed and terminated with 5 M HCl; however, no precipitate formed. Even after prolonged reaction times (2.5 h), polymerization did not appear to occur. According to the 1 H NMR spectrum of the aforementioned quenched GRIM reaction mixture containing 3a and iPrMgCl, the former underwent transmetalation at the desired 4-position of the benzotriazole with an 87% conversion, which demonstrated that the Grignard monomer has formed. It has been suggested by McNeil that the presence of LiCl can play a critical role in catalyst initiation, especially when the catalyst is generated in situ (self-initiation).13b Indeed, the results from the initial polymerization studies suggested to us that Ni(dppp)Cl2 and LiCl were essential for the successful polymerization of 3b. Intrigued by the narrow Đ and similarity to the predicted theoretical Mn, a series of kinetic studies were undertaken to determine if the polymerization of 3b was controlled. Aliquots were removed at regular intervals (15 min) from the corresponding polymerization mixture, quenched with 5 M HCl, and then examined via 1H NMR spectroscopy (internal standard: 1,3,5-trimethoxybenzene) as well as GPC. As depicted in Figure 1 (top left), a linear relationship between monomer conversion and Mn was observed. Moreover, the Đ for each polymer synthesized was relatively low (≤1.4), consistent with a controlled chain-growth process. The

Figure 1. (top left) Mn and Đ plotted as a function of monomer consumption. Conditions: [3b]0/[Ni]0 = 40, 0.02 M in THF. Aliquots (20 μL) were removed at 15 min intervals, and the polymerization was quenched with 5 M HCl. Conversion of 3a was measured via 1H NMR spectroscopy in CDCl3 using 1,3,5-trimethoxybenzene as the internal standard. (top right) Mn and Đ plotted as a function of catalyst loading. Conditions: 0.02 M in THF, quenched after 1 h with 5 M HCl. The Mns and polydispersities were measured against polystyrene standards via GPC in THF at 23 °C. (bottom) MALDI spectrum of PFBTzHT (Mn = 6.7 kDa, Đ = 1.28). 1256

DOI: 10.1021/acsmacrolett.5b00505 ACS Macro Lett. 2015, 4, 1254−1258

Letter

ACS Macro Letters Scheme 3. Synthesis of PFBTzHT-b-P3HT

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Additional synthesis details, NMR spectra, GPC data, UV−vis spectra, mass spectra, thermogravimetric analysis data, and further characterization details (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

b-P3HT copolymers with varying P3HT compositions (30− 70% P3HT) were synthesized (Table 2). As observed during the synthesis of PFBzHT, relatively dilute conditions (ca. 0.02 M) were required to obtain block copolymers, otherwise mixtures of PFBTzHT and P3HT homopolymers were obtained.17 The absorption properties of PFBTzHT and PFBTzHT-bP3HT were probed via UV−vis spectroscopy in solution and as thin films (see Figures S26 and S27 in the SI). PFBTzHT displayed a λmax = 430 nm in CHCl3 (1.7 × 10−2 mg/mL). Similarly, PFBTzHT-b-P3HT (30−70%, 1.7 × 10−2 mg/mL in CHCl3) showed a λmax = 430 nm with shoulders at 450 nm, arising from the P3HT block. As a film drop casted from CHCl3 (0.15 mg/mL), the λmax of PFBTzHT was unchanged from the solution state. In contrast, thin films of PFBTzHT-bP3HT (drop casted from CHCl3, 0.15 mg/mL) displayed bathochromic shifts of the shoulders at 430 to 600 nm (30% P3HT) and 650 nm (50 and 70% P3HT), typical for P3HT films, when compared to the solution state data. Thus, these data suggested to us that there was minimal electronic communication between the PFBTzHT and P3HT segments of the corresponding block copolymers.

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ACKNOWLEDGMENTS We acknowledge the National Science Foundation (CHE1266323), the Institute for Basic Science (IBS-R019-D1), and the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea for their support. We are grateful to Ian Riddington for conducting the matrix-assisted laser desorption ionization spectrometry experiments.

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(1) (a) Bian, L.; Zhu, E.; Tang, J.; Tang, W.; Zhang, F. Prog. Polym. Sci. 2012, 37, 1292−1331. (b) Jiang, J.-M.; Yuan, M.-C.; Dinakaran, K.; Hariharan, A.; Wei, K.-H. J. Mater. Chem. A 2013, 1, 4415−4422. (2) Elmalem, E.; Kiriy, A.; Huck, W. T. S. Macromolecules 2011, 44, 9057−9061. (3) Senkovskyy, V.; Tkachov, R.; Komber, H.; Sommer, M.; Heuken, M.; Voit, B.; Huck, W. T. S.; Kataev, V.; Petr, A.; Kiriy, A. J. Am. Chem. Soc. 2011, 133, 19966−19970. (4) Tkachov, R.; Karpov, Y.; Senkovskyy, V.; Raguzin, I.; Zessin, J.; Lederer, A.; Stamm, M.; Voit, B.; Beryozkina, T.; Bakulev, V.; Zhao, W.; Facchetti, A.; Kiriy, A. Macromolecules 2014, 47, 3845−3851. (5) Pollit, A. A.; Bridges, C. R.; Seferos, D. S. Macromol. Rapid Commun. 2015, 36, 65−70. (6) (a) Wang, Y.; Watson, M. D. J. Am. Chem. Soc. 2006, 128, 2536− 2537. (b) Sanji, T.; Iyoda, T. J. Am. Chem. Soc. 2014, 136, 10238− 10241. (c) Sanji, T.; Motoshige, A.; Komiyama, H.; Kakinuma, J.; Ushikubo, R.; Watanabe, S.; Iyoda, T. Chem. Sci. 2015, 6, 492−496. (7) Ono, R. J.; Kang, S.; Bielawski, C. W. Macromolecules 2012, 45, 2321−2326. (8) Knochel, P.; Dohle, W.; Gommermann, N.; Kneisel, F. F.; Kopp, F.; Korn, T.; Sapountzis, I.; Vu, V. A. Angew. Chem., Int. Ed. 2003, 42, 4302−4320. (9) He, C.-Y.; Wu, C.-Z.; Zhu, Y.-L.; Zhang, X. Chem. Sci. 2014, 5, 1317−1321. (10) (a) Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D. J. Am. Chem. Soc. 2012, 134, 14932−14944. (b) Wang, X.; Zhang, Z.G.; Luo, H.; Chen, S.; Yu, S.; Wang, H.; Li, X.; Yu, G.; Li, Y. Polym. Chem. 2014, 5, 502−511. (11) (a) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J. Am. Chem. Soc. 2009, 131, 7792−7799. (b) Yum, S.; An, T. K.; Wang, X.; Lee, W.; Uddin, M. A.; Kim, Y. J.; Nguyen, T. L.; Xu, S.; Hwang, S.; Park, C. E.; Woo, H. Y. Chem. Mater. 2014, 26, 2147− 2154. (12) (a) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc. 2011, 133, 4625−4631. (b) Li, Z.; Lu, J.; Tse, S.-C.; Zhou, J.; Du, X.; Tao, Y.; Ding, J. J. Mater. Chem. 2011, 21, 3226−3233. (c) Zhang, Y.; Zou, J.; Cheuh, C.-C.; Yip, H.-L.; Jen, A. K. Y. Macromolecules 2012, 45, 5427−5435. (13) (a) Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333−3336. (b) Lanni, E. L.; McNeil, A. J. J. Am. Chem. Soc. 2009, 131, 16573−16579. (14) (a) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2006, 128, 16012−16013. (b) Wu, S.; Huang, L.; Tian, H.; Geng, Y.; Wang, F. Macromolecules 2011, 44, 7558−7567.

Table 2. Synthesis of Various PFBTzHT-b-P3HT with Predetermined Compositions via Sequential Monomer Additiona PFBTzHT-1 entry 1 2 3

Mn (kDa) b

9.9 8.5b 10.6c

Đ 1.27 1.25 1.28

PFBTzHT-b-P3HT Mn (kDa) d

18.1 12.1e 11.1f

Đ

% P3HTg

yieldh (%)

1.24 1.31 1.62

70 50 30

57 61 57

Mn and Đ were measured via GPC in THF at 23 °C and reported against polystyrene standards. b[3b]0/[Ni]0 = 20. c[3b]0/[Ni]0 = 27. d [4]0/[Ni]0 = 48. e[4]0/[Ni]0 = 26. f[4]0/[Ni]0 = 13. gDetermined via 1 H NMR spectroscopy (CDCl3). hIsolated yield. a

In sum, we report the first controlled synthesis of an alternating D−A CP via Kumada catalyst-transfer polycondensation. Excellent control over the molecular weight of the resultant polymers was achieved along with narrow polydispersities using Ni(dppp)Cl2 as the catalyst. In addition, sequential monomer addition afforded a series of conjugated diblock copolymers. Collectively, the results presented herein demonstrate the feasibility of the controlled synthesis of D−A CPs and ultimately extends the scope of monomers capable of undergoing CTP.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00505. 1257

DOI: 10.1021/acsmacrolett.5b00505 ACS Macro Lett. 2015, 4, 1254−1258

Letter

ACS Macro Letters (15) Recently, Yum et al. (ref 11b) synthesized a similar polymer (Mn = 7.4 kDa) via Pd-catalyzed Suzuki-type polycondensation. (16) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 17542−17547. (17) The GPC chromatogram of the mixture of PFBTzHT and P3HT exhibited a bimodal distribution. As such, the material was subjected to Soxhlet extraction with hexane and CHCl3. The hexane fraction showed 1H NMR resonances corresponding to PFBTzHT whereas analysis of the CHCl3 fraction indicated that only P3HT was present.

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DOI: 10.1021/acsmacrolett.5b00505 ACS Macro Lett. 2015, 4, 1254−1258