Article pubs.acs.org/Macromolecules
Mixed-Ligand Approach to Palladium-Catalyzed Direct Arylation Polymerization: Highly Selective Synthesis of π‑Conjugated Polymers with Diketopyrrolopyrrole Units Masayuki Wakioka,* Rina Takahashi, Nobuko Ichihara, and Fumiyuki Ozawa* International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan S Supporting Information *
ABSTRACT: The combined use of P(2-MeOC6H4)3 (L1) and TMEDA as ligands effectively prevents defect formation in palladium-catalyzed direct arylation polymerization (DArP) to give donor−acceptor type alternating copolymers (DA polymers) with diketopyrrolopyrrole (DPP) units. The reaction of 3,6-bis(5-bromo-2-thienyl)diketopyrrolopyrrole (1-Br) and 3,4-dicyanothiophene (2-H) in toluene at 100 °C for 6 h in the presence of only L1 afforded a notable amount of insoluble materials via branching and cross-linking; in addition, the soluble part (Mn = 3100) included a large quantity of homocoupling defects (12.5%). In contrast, in the presence of L1 and TMEDA, the formation of insoluble materials was completely suppressed, and homocoupling defects decreased to 1.6%. Furthermore, the molecular weight of poly(1-alt-2) remarkably increased (Mn = 24 500, 97% yield). The mixed ligand catalyst using L1 and TMEDA was also effective against DArP of 1-Br with thienopyrroledione (3-H) or 3,4-propylenedioxythiophene (4-H) to afford DA polymers with DPP units in high yields (poly(1-alt-3): Mn = 36 800, 100% yield; poly(1-alt-4): Mn = 19 000, 80% yield). Thermal and optical properties of the resulting polymers are reported.
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INTRODUCTION The palladium-catalyzed direct arylation polymerization (DArP) has attracted much attention as a viable alternative to the existing synthetic means of π-conjugated polymers based on cross-coupling reactions.1−3 In particular, DArP has a distinct advantage over cross-coupling polymerization in the synthesis of alternating copolymers consisting of donor−acceptor combinations of repeating units (DA polymers), which have proven to be a valuable component of bulk heterojunction polymer solar cells.4−7 DA polymers have so far been prepared by Migita−Stille cross-coupling polymerization.8 However, this method needs toxic tin reagents for monomer preparations, and the polymerization reaction forms a highly toxic side product such as Me3SnBr. It is expected that the DArP process, which proceeds via C−H bond activation, provides a simple yet fundamental solution for these problems.1−3 In 2010, we reported the first successful example of DArP to afford highly head-to-tail regioregular poly(3-hexylthiophene).9,10 We also reported the synthesis of several kinds of DA polymers with well-defined structures via DArP.11−13 In these studies, we pointed out that the use of good solvents for π-conjugated polymers such as toluene and THF could be an effective approach to highly selective DArP systems. The importance of solvent choice in DArP has also been indicated by other groups.14,15 We therefore developed a novel palladium catalyst that exhibits high catalytic activity in those solvents, using P(2-MeOC6H4)3 (L1) as the ligand.9,11−13,16 Many © XXXX American Chemical Society
research groups have applied this catalyst to the synthesis of DA polymers.17−30 More recently, we reported that the combined use of L1 and TMEDA as ligands effectively prevents defect formation in DArP of 2,6-diiododithienosilole (DTS-I 2) and thienopyrroledione (TPD-H2) to give poly(DTS-alt-TPD).31,32 DArP has two side reactions to afford structural defects. One is homocoupling that causes deterioration in physical properties of polymers,33−36 and several ways of preventing this side reaction have been presented.37,38 On the other hand, the other side reaction to afford branching defects is still an unsolved problem inherent in DArP.39−42 The branching defects are generated by direct arylation at undesirable C−H positions and eventually converted to cross-linking defects leading to insoluble materials. We found that the mixed ligand catalyst using L1 and TMEDA entirely suppressed this side reaction. Moreover, this catalyst effectively reduced homocoupling defects as well. In this study, we tested the capability of the mixed ligand catalyst in DArP of diketopyrrolopyrrole (DPP) derivatives (Scheme 1). DPP-based DA polymers prepared by Migita− Stille cross-coupling polymerization have proven to be a competent component of optoelectronic devices.43−50 In Received: December 11, 2016 Revised: January 6, 2017
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DOI: 10.1021/acs.macromol.6b02679 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
insoluble materials (>50%) was formed and remained in the thimble upon Soxhlet extraction with PhCl (run 1). Insoluble materials were already observed after 6 h (run 2) but negligible in 10 min (run 3). The soluble products in all runs were oligomeric compounds with low molecular weight. In this connection, Leclerc et al. reported that the reaction of 1a-Br and 2-H successfully proceeded in toluene at 120 °C for 18 h in the presence of Herrmann−Beller catalyst (2 mol %), L1 (8 mol %), PivOH (0.3 equiv), and Cs2CO3 (2.3 equiv) to afford poly(1a-alt-2) with Mn = 14 000 in 92% yield.23 Thus, we tested the same reaction conditions; however, most of the product was insoluble in hot PhCl, and this situation did not improve even in a short reaction time (run 4).20 In contrast, in the presence of TMEDA, poly(1a-alt-2) with good solubility was obtained in almost quantitative yield, where no trace of insoluble materials remained in the extraction thimble (runs 5−7 in Table 1). The molecular weight was unchanged by the amount of TMEDA, whereas homocoupling defects decreased with increasing amount of TMEDA. Thus, the amount of homocoupling defects (1a-1a + 2-2) decreased from 12.5% (run 2) to 1.6% (runs 6 and 7) with the aid of TMEDA. Figure 1 shows aromatic regions of the 1H NMR spectra of soluble products of runs 3, 2, and 6 in Table 1. These spectra were recorded in C2D2Cl4 at 130 °C on a 800 MHz spectrometer. The signal assignments were performed with model compounds of substructures and structural defects (see the Supporting Information). Figure 1a shows the spectrum of the product after 10 min of the reaction in the absence of TMEDA (run 3). Besides the main signals due to cross-coupling units (A1 and A2), signals arising from homocoupling units (B1, B2, C1, and C2) and terminal groups (a1−a4, b1−b4, c1−c4, and d1−d4) are observed. The average number of cross-coupling and homocoupling bonds in each molecule was estimated to be 17.6 (1a-2), 0.65 (2-2), and 0.06 (1a-1a) by peak integration. Moreover, the average number of four kinds of terminal groups was estimated to be 0.89 (1a-Br), 0.63 (1a-H), 0.43 (2-H), and 0.05 (1a-Br adjacent to the DCT−DCT unit). Thus, almost the same number of 2-2 bonds (0.65) and 1a-H groups (0.63) was
Scheme 1. DArP of Diketopyrrolopyrrole Derivatives
particular, DPP derivatives with 2-thienyl groups at the 3,6positions adopt highly planar structures reinforced with internal hydrogen bonding, thereby exhibiting high carrier mobility.44,51 On the other hand, such monomers having plural C−H bonds have a marked tendency to form branching and cross-linking defects upon DArP.29,30,41 Herein, we describe that the mixed ligand catalyst is highly effective against such DArP systems to afford DPP-based DA polymers with well-controlled structures.
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RESULTS AND DISCUSSION Effects of TMEDA on DArP. First of all, DArP of 1a-Br and 3,4-dicyanothiophene (DCT-H2, 2-H) was examined in the absence of TMEDA (runs 1−3 in Table 1). The reaction was conducted in toluene at 100 °C using Pd2(dba)3·CHCl3 (1 mol %), L1 (4 mol %), and PivOH (1 equiv) as catalyst precursors and Cs2CO3 (3 equiv) as a base. After 24 h, a large quantity of Table 1. DArP of 1-Br and C−H Monomers (M-H)a ligand (mol %)
homocoupling (%)f
soluble product
run
monomers
L1
TMEDA
reaction time
insoluble materials
1 2 3 4b 5 6 7 8 9 10 11
1a-Br + 2-H 1a-Br + 2-H 1a-Br + 2-H 1a-Br + 2-H 1a-Br + 2-H 1a-Br + 2-H 1a-Br + 2-H 1a-Br + 3-H 1a-Br + 3-H 1b-Br + 4-H 1b-Br + 4-H
4 4 4 8 4 4 4 4 4 4 4
0 0 0 0 10 30 50 0 30 0 50
24 h 6h 10 min 4h 6h 6h 6h 6h 6h 48 h 24 h
yes yes no yes no no no no no no no
c
yield (%) 41 85 92 25 97 97 97 96 100 93 80
d
Mn (Mw/Mn) 3700 (1.3) 3100 (1.2) 4300 (1.7) 3700 (2.3) 24700 (3.1) 24500 (2.7) 24600 (2.9) 24700 (2.8) 36800 (1.5) 13000f 19000f
e
1-1
M-M
4.7 4.8 0.3 5.5 0.6 0.2 0.6 g g 2.7 1.0
7.2 7.7 3.5 8.6 2.0 1.4 1.0 g g 3.4
