Xiaochen
Wang
and
Mingfeng
Wang
*
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore. E-mail: mfwang@ntu.edu.sg; Fax: +65 6794 7553; Tel: +65 6316 8746
First published on 17th June 2014
This article describes the synthesis of donor–acceptor (D–A) type copolymers based on benzo[1,2-b:4,5-b′]dithiophene and 2,1,3-benzothiadiazole via direct-arylation cross-coupling polycondensation. To achieve high performance polymerization, we have systematically investigated the reaction factors including catalysts, solvents, ligands, bases, additives, concentration of reactants and phase transfer agents. In particular, 1,2-dimethylbenzene (ODMB), as a nonpolar high boiling point solvent, is a superior medium to perform this direct-arylation polymerization. In this nonpolar aromatic solvent, Pd2dba3/(o-MeOPh)3P, accompanied with a base potassium carbonate and an additive pivalic acid, serves as an efficient catalyst system to obtain high-quality polymers. Our optimized condition gave the polymer with a weight-average molecular weight (Mw) as high as 60 kg mol−1 in nearly quantitative yield and excellent C–H selectivity.
The protocols of C–H direct arylation towards synthesis of conjugated polymers have usually been borrowed from what has been learned from the synthesis of small organic molecules for pharmaceutics.13,14 For example, Fagnou et al. have explored an effective synthetic protocol, which involves palladium acetate (Pd(OAc)2) as a catalyst, N,N-dimethylacetamide (DMAc) as a solvent, potassium carbonate (K2CO3) as a base and pivalic acid (PivOH) as an additive, for direct arylation of aromatic compounds.15,16 This synthetic protocol has been successfully applied to the synthesis of many conjugated polymers.17–29 The catalyst Pd(OAc)2 is particularly efficient in highly polar solvents such as DMAc, in which most low polar or nonpolar conjugated polymers show limited solubility. Therefore, highly polar solvents are not ideal reaction media for synthesis of conjugated polymers, particularly for those decorated with hydrophobic alkyl side chains.
Very recently, there has been some progress in exploring low polar or nonpolar solvents as the reaction media for synthesis of conjugated polymers. For instance, Ozawa et al. have used tetrahydrofuran (THF) instead of highly polar solvents to synthesize poly(3-hexylthiophene-2,5-diyl) (P3HT), to ensure the solubility of the resulting polymers during polymerization.30 Herrmann's catalyst (trans-Di-μ-acetatobis[2-[bis(2-methylphenyl)phosphine]benzyl]dipalladium), in the presence of an appropriate ligand, was proven to be an effective catalyst in this reaction system to afford high-molecular-weight P3HT with high regioregularity (98%), whereas the reaction catalyzed with Pd(OAc)2 in the same solvent was not reproducible and frequently provided low molecular weight products. Later, Leclerc et al. have utilized this reaction condition with modifications to synthesize a series of D–A type conjugated polymers.31–36 The reaction was typically performed with heating at 120 °C. The overheated solvent (THF) and the necessity of using a sealed and pressurized reaction container, however, may compromise the reproducibility of the polymerization and raise the cost as well as safety concerns for performing and scaling up the synthesis. In addition, some monomers cannot be polymerized in THF with Herrmann's catalyst.36 More recently, Ozawa et al. have reported another efficient catalytic system based on tris(dibenzylideneacetone)dipalladium(0)-chloroform adducts (Pd2(dba)3·CHCl3) for polycondensation of 2,7-dibromo-9,9-dioctylfluorene and 1,2,4,5-tetrafluorobenzene.37 This catalytic system was sufficiently reactive in both THF and in toluene to afford the polyphenylene derivative with high molecular weight and high yield.
Despite these recent advances, little study has been carried out in exploring high boiling point nonpolar solvents for efficient synthesis of conjugated polymers via direct arylation polycondensation. Even less has been understood with reaction factors that affect the direct arylation polycondensation in nonpolar solvents.
In this article, we report such a direct arylation polycondensation system. Our target polymer, denoted as PBDTBT, consists of alternating benzo[1,2-b:4,5-b′]dithiophene (BDT) as an electron donor (D) and 2,1,3-benzothiadiazole (BT) as the electron acceptor (A). Both BDT and BT have been among the most popular building blocks in a variety of D–A conjugated polymer semiconductors.1–6,38–47 Two long branched 2-hexyldecyloxy groups were introduced to the BDT segment to afford good solubility of PBDTBT in a variety of solvents.
To optimize the polymerization of BDT and BT under the scheme of direct arylation, we have systematically examined a broad range of factors, including a series of low polar or nonpolar solvents, catalysts, ligands, bases, additives, reactant concentrations and phase transfer agents. Our optimized condition for direct arylation gives high molecular weight PBDTBT in a nearly quantitative yield with good regioregularity.
Entry | Catalyst | Ligand | Solvent | Base | Yield (%) | M n (kg mol−1) | M w (kg mol−1) | PDI | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a HDBDT (0.2 mmol) and BrBT (0.2 mmol) were polymerized in solvent (1 mL), in the presence of catalyst (5 mol%), ligand (10 mol%), base (0.6 mmol) and PivOH (0.06 mmol), at 100 °C for 24 h. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | Pd(OAc)2 | PCy3·HBF4 | DMAc | K2CO3 | 98 | 11.0 | 32.2 | 2.9 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | Herrmann's catalyst | (o-MeOPh)3P | THF | Cs2CO3 | 88 | 4.1 | 9.8 | 2.4 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | Pd2dba3 | (o-MeOPh)3P | THF | Cs2CO3 | 99 | 16.0 | 32.8 | 2.0 |
To further compare the polymers prepared using these different catalytic systems, absorption spectra of the polymers were collected (Fig. 1). As expected, PBDTBT synthesized using Herrmann's catalyst/THF, due to its much lower molecular weight, shows an absorption peak at a shorter wavelength (545 nm) than the other two polymers synthesized using Pd(OAc)2/DMAc (568 nm) and Pd2dba3/THF (638 nm), respectively. Surprisingly, the polymers synthesized using Pd(OAc)2/DMAc and Pd2dba3/THF, respectively, despite their similar Mw, show much different optical absorption properties. The latter shows an absorption peak and the onset at longer wavelengths, corresponding to a longer average conjugation length. The average conjugation length of CPs is determined by maximum effective conjugation length, degree of polymerization (molecular weight) and structural defects.4 Here, the difference of the conjugation length between PBDTBTs prepared with Pd(OAc)2/DMAc and Pd2dba3/THF, respectively, might be mainly attributed to the structural defects, which is further discussed in the final section (Optical properties). The possible structural defects in PBDTBTs prepared by direct arylation polymerizations are shown in Scheme 2.
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Fig. 1 Absorption spectra of chloroform solutions of PBDTBTs synthesized with different catalysts: line 1, 2, and 3 correspond to the polymers synthesized under the condition shown in entry 1, 2, and 3, respectively, in Table 1. |
To obtain more information about the molecular structures of PBDTBTs synthesized using Pd(OAc)2/DMAc and Pd2dba3/THF, 1H-NMR spectra of these polymers were collected. As highlighted with green rectangles in Fig. 2, marked regio-irregular sequence peaks at around 7.9 and 8.8 ppm are observed in the spectrum (line a) of PBDTBT synthesized using Pd(OAc)2/DMAc. These two peaks may be assigned to the protons of 6-unsubstituted benzo[1,2-b:4,5-b′]dithiophene and protons at 5 and 6 positions of benzothiodiazole which linked on the 3 and/or 7 positions of BDT, respectively, as shown in Scheme 2. In contrast, only negligible peaks exist in those regions for the polymer synthesized using Pd2dba3/THF (line b).
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Fig. 2 1H-NMR spectra of PBDTBTs synthesized under different conditions: line a and b correspond to the polymers synthesized under the condition shown in entry 1 and 3 in Table 1; line c corresponds to the polymer synthesized under the optimized condition in ODMB (entry 1, Table 5). |
All the results described above indicate that Pd2dba3 is a superior catalyst in THF, compared to Herrmann's catalyst in the same solvent and Pd(OAc)2 in DMAc for direct arylation polymerization of HDBDT and BrBT. Therefore, in the following sections, we focus on Pd2dba3 catalyst and discuss how other factors influence the direct arylation polymerization.
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Scheme 3 Chemical structures and abbreviations of solvents used in the direct arylation polymerization. |
Entry | Solvent | Boiling point (°C) | Dielectric constant | Dipole moment (10−10 C m) | Yield (%) | M n (kg mol−1) | M w (kg mol−1) | PDI | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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a The physical properties of the solvents were from ref. 50. b HDBDT (0.2 mmol) and BrBT (0.2 mmol) were polymerized in a solvent (1 mL), in the presence of Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), Cs2CO3 (0.6 mmol) and PivOH (0.06 mmol), at 100 °C for 24 h. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | THF | 66 | 7.58 | 5.67 | 99 | 16.0 | 32.8 | 2.0 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | DIO | 101.3 | 2.209 | 1.50 | 97 | 14.1 | 31.0 | 2.2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | MOB | 153.7 | 4.33 | 4.00 | 96 | 13.2 | 23.7 | 1.8 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
4 | MB | 110.6 | 2.24 | 1.23 | 99 | 13.6 | 24.3 | 1.8 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
5 | DMB | 137–140 | — | — | 96 | 11.3 | 22.5 | 2.0 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
6 | TMB | 164.7 | 2.279 | 0.23 | 95 | 6.8 | 11.9 | 1.7 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
7 | ODMB | 144.4 | 2.266 | 1.47 | 98 | 14.2 | 31.1 | 2.2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
8 | THN | 207.6 | 2.733 | 1.33 | 95 | 5.2 | 8.2 | 1.6 |
It is worthwhile to note that, in ODMB, the polymerization gives a comparable result to that in THF, indicating that ODMB is a promising reaction medium for Pd2dba3-catalyzed coupling polymerization. As mentioned in Introduction, most of direct arylation polymerizations for the synthesis of conjugated polymers have been carried out in high-boiling-point, highly polar solvents such as DMAc, in which high-molecular-weight polymer products often show limited solubility. In contrast, ODMB described above is a nonpolar aromatic solvent with a boiling point of 144.4 °C. Based on the “like dissolves like” principle, ODMB should possess good solubility for aromatic compounds, particularly for nonpolar and low polar molecules. Therefore, ODMB should be favorable for synthesis of a broad range of conjugated polymers. In addition, the significantly higher boiling point of ODMB than that of THF allows polymerization to be readily performed under ambient pressure.
The results of copolymerization of HDBDT and BrBT in the presence of different bases are summarized in Table 3. Carbonates were examined firstly, due to their wide use in previous direct arylation polymerizations. Among the tested carbonates, potassium carbonate (K2CO3) gave optimal results; cesium carbonate (Cs2CO3) showed an acceptable polymerization result; sodium carbonate (Na2CO3) was ineffective, only giving trace oligomer/polymer; calcium carbonate (CaCO3) and barium carbonate (BaCO3) did not give any oligomer/polymer at all after a brief purification procedure.
Entry | Base | Yield (%) | M n (kg mol−1) | M w (kg mol−1) | PDI | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a HDBDT (0.2 mmol) and BrBT (0.2 mmol) were polymerized in ODMB (1 mL), in the presence of Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), base and PivOH (0.06 mmol), at 100 °C for 24 h. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | Cs2CO3 (3 eq.) | 98 | 14.2 | 31.1 | 2.2 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | K2CO3 (3 eq.) | 98 | 19.5 | 46.5 | 2.4 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | Na2CO3 (3 eq.) | Trace | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
4 | CaCO3 (3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
5 | BaCO3 (3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
6 | K3PO4 (3 eq.) | 96 | 18.0 | 39.7 | 2.2 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
7 | KOAc (3 eq.) | 85 | 5.4 | 8.5 | 1.6 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
8 | t-BuOK (3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
9 | TEA (3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
10 | DIPEA (3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
11 | DBU (3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
12 | TEDA (3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
13 | K2CO3 (2 eq.) | 97 | 18.6 | 42.8 | 2.3 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
14 | K2CO3 (5 eq.) | 98 | 22.0 | 54.4 | 2.5 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
15 | K2CO3 (10 eq.) | 97 | 20.2 | 47.3 | 2.3 |
The performance of carbonates should be related to their basicity and solubility. Solubility of the carbonates, both in typical solvents51,52 and water, follows an order of Cs2CO3 > K2CO3 > Na2CO3 > CaCO3 ≈ BaCO3. In a Pd-catalyzed cross-coupling reaction, it is important to maintain a reasonable concentration of basic anions in the reaction system.53 We speculate that the concentration of carbonate from K2CO3 in ODMB at the polymerization temperature (i.e. 100 °C) should be at a favorable level, compared to other carbonates, for the direct arylation polymerization of HDBDT and BrBT.
As K2CO3 showed better performance than other metal carbonates in the direct arylation polymerization, we further tested potassium bases with other anions. The polymerization results are shown in Table 3. Potassium phosphate (K3PO4) is also an effective base, outranked only by K2CO3. Compared to K3PO4, potassium acetate (KOAc) gives polymer products with a lower reaction yield and a lower Mn, which may be caused by its weak alkaline and less effective coordination with reactive catalytic centers. When potassium tert-butoxide (t-BuOK) was added to the polymerization system, the reaction mixture turned to dark brown while being heated in an oil bath. This phenomenon suggested that some reagents were decomposed or some side reactions occurred in the presence of such a strong base.
Finally, organic bases such as triethylamine (TEA), diisopropylethylamine (DIPEA), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and triethylenediamine (TEDA) were tested in the copolymerization of HDBDT and BrBT. But none of them resulted in the formation of polymer products. The presence of organic bases may lead to debromination of dibromobenzothiadiazole as the main reaction54 and consequently interferes with the polymerization.
As K2CO3 showed the best performance among all the bases tested above, we further optimized the equivalence of K2CO3 (entries 2, 13, 14 and 15, Table 3) in the reaction mixtures. With the increase of K2CO3 from 2 to 5 equivalents, the molecular weight of the resulting polymers increased gradually. Further increase of the amount of K2CO3 to 10 equivalents, the molecular weight of the resulting polymers cannot be further improved.
The effect of additives on the Pd2dba3-catalyzed copolymerization of HDBDT and BrBT in ODMB is summarized in Table 4. Surprisingly, the Pd2dba3-catalyzed polymerization was quenched upon addition of DIPEA as an additive. The presence of DIPEA may result in debromination of bromide monomers and thus quenches the coupling reaction.54 In contrast to DIPEA, the addition of PivOH to the reaction mixture promoted the coupling copolymerization. For example, the addition of 0.5 equivalent (vs. the monomer) PivOH resulted in a more than 6-fold increase of molecular weight of the formed polymer, compared to that of the polymer synthesized without PivOH. In addition, this increase of molecular weight was accompanied by an increase of the reaction yield from 68% to 98%. A further increase of the amount of PivOH up to 1 equivalent (vs. the monomer) did not lead to significant improvement of the polymerization (entry 5, Table 4).
Entry | Additive | Yield (%) | M n (kg mol−1) | M w (kg mol−1) | PDI | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a HDBDT (0.2 mmol) and BrBT (0.2 mmol) were polymerized in ODMB (1 mL), in the presence of Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), K2CO3 (0.6 mmol) and additive, at 100 °C for 24 h. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | None | 68 | 3.4 | 7.7 | 2.3 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | PivOH (0.3 eq.) | 98 | 19.5 | 46.5 | 2.4 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | DIPEA (0.3 eq.) | 0 | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
4 | PivOH (0.5 eq.) | 98 | 21.9 | 53.6 | 2.4 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
5 | PivOH (1 eq.) | 97 | 21.1 | 51.0 | 2.4 |
Entry | Concentration | Yield (%) | M n (kg mol−1) | M w (kg mol−1) | PDI | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a HDBDT (0.2 mmol) and BrBT (0.2 mmol) were polymerized in ODMB (1, 2, or 4 mL), in the presence of Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), K2CO3 (1 mmol) and PivOH (0.1 mmol), at 100 °C for 24 h. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 0.2 M | 98 | 24.5 | 60.1 | 2.4 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | 0.1 M | 99 | 19.5 | 45.0 | 2.3 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | 0.05 M | 97 | 13.8 | 26.6 | 1.9 |
Entry | PTA | Yield (%) | M n (kg mol−1) | M w (kg mol−1) | PDI | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a HDBDT (0.2 mmol) and BrBT (0.2 mmol) were polymerized in ODMB (2 mL), in the presence of Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), K2CO3 (1 mmol), PivOH (0.1 mmol) and PTA (0.06 mmol), at 100 °C for 24 h. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | None | 99 | 19.5 | 45.0 | 2.3 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | 18-Crown-6 | 96 | 10.2 | 16.7 | 1.6 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | Aliquat336 | Trace | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
4 | TBAB | Trace | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
5 | TBAF | Trace | — | — | — | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
6 | TBAPF6 | 99 | 16.8 | 35.9 | 2.1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
7 | Water (0.4 mL) | 99 | 9.2 | 15.7 | 1.7 |
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Fig. 3 Absorption spectra of chloroform solutions of PBDTBTs synthesized under different conditions: lines 1–5 correspond to the polymers entry 1–5 listed in Table 7 with the same sequence. |
Entry | M n/Mw (kg mol−1) | Polymerization condition | λ abs in CHCl3 (nm) | λ onset in CHCl3 (nm) | FWHM (nm) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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a Details of the polymerizations: DMAc (1 mL), Pd(OAc)2 (5 mol%), PCy3·HBF4 (10 mol%), K2CO3 (0.6 mmol), and PivOH (0.06 mmol). b ODMB (1 mL), Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), KOAc (0.6 mmol) and PivOH (0.06 mmol). c ODMB (2 mL), Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), K2CO3 (1 mmol), PivOH (0.1 mmol) and water (0.4 mL). d ODMB (2 mL), Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), K2CO3 (1 mmol), PivOH (0.1 mmol) and TBAPF6 (0.06 mmol). e ODMB (1 mL), Pd2dba3 (5 mol%), (o-MeOPh)3P (10 mol%), K2CO3 (1 mmol) and PivOH (0.1 mmol). All these polymerizations were carried out with HDBDT (0.2 mmol) and BrBT (0.2 mmol) at 100 °C for 24 h. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 11.0/32.2 | Pd(OAc)2/DMAca | 329, 365, 567 | 678 | 210 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | 5.4/8.5 | Pd2(dba)3/ODMBb | 333, 366, 572 | 685 | 185 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | 9.2/15.7 | Pd2(dba)3/ODMBc | 334, 370, 590 | 691 | 163 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
4 | 16.8/35.9 | Pd2(dba)3/ODMBd | 336, 385, 643 | 699 | 139 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
5 | 24.5/60.1 | Pd2(dba)3/ODMBe | 337, 387, 648 | 702 | 128 |
As expected, for the polymers synthesized in ODMB, gradual bathochromic shifts of both the absorption peak and the onset of absorption were observed with the increase of the molecular weight, accompanied with a decrease of the FWHM. The PBDTBT prepared using Pd(OAc)2/DMAc (line 1) showed a slight hypsochromic shift in its absorption spectrum, compared with the PBDTBT synthesized using Pd2dba3/ODMB and with a much lower molecular weight (line 2). In other words, the conjugation length of PBDTBT prepared using Pd(OAc)2/DMAc with an Mn/Mw of 11.0/32.2 kg mol−1, was even inferior to that of PBDTBT polymerized in ODMB with an Mn/Mw of 5.4/8.5 kg mol−1. These results imply that some structural defects, presumably caused by the relatively poor regioselectivity in the polymerization, may exist in the polymer prepared using Pd(OAc)2/DMAc.9,11,22,23,27
To further probe the structure of the polymers synthesized in ODMB, 1H-NMR spectrum of a representative PBDTBT prepared under the optimized reaction condition (entry 1 in Table 5) was collected and the result is presented in Fig. 2 for comparison. A close inspection of the NMR spectrum reveals only slight distortion without any distinguishable peaks in the regions around 8.8 and 7.9 ppm. This result further suggests that the relatively poor C–H regioselectivity observed in Pd(OAc)2-catalyzed polymerization in DMAc was suppressed in the Pd2dba3-catalyzed polymerization in ODMB.
In addition, the PBDTBTs that we synthesized here via direct arylation polymerization in ODMB (line 3 and 4) shows similar or even red-shifted absorption compared with the same type of polymer with a similar Mn but synthesized via Stille coupling by other research groups.40,41 Again, these results suggest the good regioregularity of the polymers synthesized via Pd2dba3-catalyzed direct arylation polymerization in ODMB.
When purifying the reaction mixtures from different batches of polymerizations, we tried to remove unreacted starting materials and low molecular weight organic and inorganic impurities and collect all the resulting polymers and oligomers present in final products in order to completely compare the polymerizations under different reaction conditions.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4py00565a |
This journal is © The Royal Society of Chemistry 2014 |