A new procedure for the synthesis of π-conjugated polymers via ligand-free iron(III)-catalyzed oxidative homocoupling reaction of grignard reagents

Abstract

The FeCl₃-catalyzed oxidative homocoupling reaction of Grignard reagents was used for the synthesis of π-conjugated polymers in this study. In the presence of an oxidant (1,2-dichloroethane), three polymers were successfully prepared, namely, poly(9,9-dioctylfluorene-2,7-diyl), poly(N-butylcarbazol-3,6-diyl) and poly(2,5-dihexyloxyphenylene-1,4-diyl). In comparison with the polymers obtained by Suzuki coupling, the polymers in this work showed no obvious difference in chemical structures or optical properties. On the basis of the relatively low cost of iron(III)-catalysts and Grignard reagents, this route would be an attractive alternative for the synthesis of π-conjugated polymers.

---

1. Introduction

Since the conductivity of doped polyacetylenes was revealed,^1–3 considerable interest has been aroused to prepare π-conjugated polymers and to explore their chemical and physical properties. After four decades of intensive research, a large number of conjugated polymers, such as polyacetylenes,^1–7 polyanilines,⁸ polythiophenes,⁹ polyphenylene-ethylenes,¹⁰ polyphenylenes,¹¹ and polyfluorenes,¹² have been obtained and are widely used as conducting materials,^1–6,9b,9c polymer light emitting diodes,^9d–9f,12 solar cells,¹³ field effect transistors,¹⁴ biochemical sensors,¹⁵ nonlinear optics,¹⁶ electrochromic devices¹⁷etc.

However, in comparison with the investigations that focused on the properties of π-conjugated polymers, research regarding the synthetic methodology of the polymers has been a little insufficient. Summarily, there have been two main ways to prepare π-conjugated polymers, namely, via a reduction or oxidation process,¹⁸ or via a metal-catalyzed coupling method (including Kumada coupling,¹⁹ Stille coupling,²⁰ and Suzuki coupling²¹). Although these well-established coupling routes have played a very important role in the synthesis of many π-conjugated polymers, there still remain some inevitable problems. For instance, during the synthesis of alkyl polyfluorenes via the FeCl₃ oxidative route, irregular structures can be generated,^18c thus resulting in poor optoelectronic properties. As for the reductive methods, Yamamoto and Ullmann coupling polymerizations are somewhat unsatisfactory because highly toxic and moisture-sensitive Ni(COD)₂^18e is used in the former and a large amount of zinc powder is wasted in the latter.^18d Palladium-catalyzed Suzuki coupling polymerization is, undoubtedly, an efficient method for the synthesis of various polyarylenes due to the availability of the monomers and the convenient polymerization route, specifically the compatibility of the functional groups and the designability of the chemical structures used as monomers.²¹ However, the use of expensive metal catalysts and ligands can cause an increase in the cost of the target polymers.^19–21 Accordingly, exploring and developing a cheaper and more convenient procedure for the synthesis of conjugated polymers are urgently required. It is noted that aryl Grignard reagents can be easily converted into the corresponding biaryl compounds with high yields in the presence of FeCl₃ and 1,2-dihaloethane.²² Nevertheless, there is little information about the synthesis of conjugated polymers via the oxidative coupling of bis-Grignard reagents.²³ Previously, we have reported the synthesis of polyfluorene using a Pd(II)-catalyzed base-free oxidative homocoupling reaction of diboronic esters.²⁴ Our interest is to find cheap and practical methods for the synthesis of π-conjugated polymers. Thus, as part of an extended investigation, in this study we report the synthesis of π-conjugated polymers using the ligand-free iron(III)-catalyzed oxidative homocoupling of Grignard reagents.

2. Experimental

2.1 Materials

Fluorene and carbazole were industrial products, and were recrystallized from ethanol before use. 2,7-Dibromo-9H-fluorene (1),²⁵ 2,7-dibromo-9,9-dioctyl-fluorene (2),²⁵ 1,4-dibromo-2,5-dihexyloxybenzene (4),²⁶ and 3,6-dibromo-N-butylcarbazole (6)²⁷ were prepared as previously reported. Diisobutylaluminium hydride (DiBALH, 1 M in hexane) and ferric chloride (FeCl₃, 0.2 M in 2-methyltetrahydrofuran) were purchased from Aldrich without further purification. Both 1,2-dichloroethane and 1,2-dibromoethane were commercially obtained from Aldrich and distilled over P₂O₅ before use.)

2.2 Measurement

Proton nuclear magnetic resonance (¹H NMR) spectra were recorded with a Varian Mercury 300 spectrometer and carbon nuclear magnetic resonance (¹³C NMR) spectra were recorded with a Bruker DRX 400 spectrometer. UV-Visible absorption and photoluminescence (PL) spectra were measured with a Hitachi UV2800 spectrometer and a Hitachi F-4500 spectrometer, respectively. Traces of bromine in the polymers were determined on an American Dionex 500 ion chromatograph. A Waters Breeze GPC system was employed to deduce the number-average molecular weight (M_n) and molecular weight distribution (M_w/M_n) based on polystyrene standards with THF as an eluent.

2.3 Synthesis

Synthesis of 9,9-dioctyl-2,7-dibromomagnesiofluorene (3). Under an argon atmosphere, a mixture of magnesium turnings (5.833 g, 240 mmol) and 120 mL of dry THF in an oven-dried flask was stirred and 0.5 mL of DiBALH (1 M in THF) was added. After being kept at room temperature for 0.5 h, the mixture was heated to 50 °C during a period of 10 min, then a solution of 9,9-dioctyl-2,7-dibromofluorene (2) (13.15 g, 24 mmol) in 60 mL of dry THF was added slowly through a syringe. The mixture was stirred at room temperature for 6 h to give a transparent grayish solution, which was kept carefully and directly used for polymerization.

Synthesis of 1,4-dibromomagnesio-2,5-dihexyloxybenzene (5). Under an argon atmosphere, a mixture of magnesium turnings (0.972 g, 40 mmol) and 15 mL of dry THF in an oven-dried flask was stirred and 0.08 mL of DiBALH (1 M) was added. The mixture was heated to 50 °C over a period of 10 min, and a solution of 1,4-dibromo-2,5-dihexyloxybenzene (4) (1.744 g, 4 mmol) in 10 mL of dry THF was added slowly through a syringe. The solution was allowed to stir at room temperature for 6 h and the obtained grayish transparent solution was kept carefully and directly used for polymerization.

Synthesis of 3,6-dibromomagnesio-N-butylcarbazole (7). Under an argon atmosphere, a mixture of magnesium turnings (0.972 g, 40 mmol) and 25 mL of dry THF in an oven-dried flask was stirred and 0.04 mL of DiBALH (1 M in THF) was added. The mixture was heated to 50 °C over a period of 10 min, and a solution of 3,6-dibromo-N-butylcarbazole (6) (1.524 g, 4 mmol) in 10 mL of dry THF was added slowly through a syringe. The solution was allowed to stir for 6 h and the generated grayish mixture was kept carefully and directly used for polymerization.

General polymerization route

Under an argon atmosphere, a mixture of FeCl₃ (1 mL, 0.2 M in 2-methyltetrahydrofuran), 1,2-dichloroethane (0.396 g, 4 mmol), and 18 mL of THF in a 50 mL oven-dried flask was stirred for 5 min at room temperature. Consequently, the prepared Grignard reagents (2 mmol) were added carefully through a syringe during a period of about 10 min. The mixture was allowed to react for a set time, and then dilute aq. HCl was added to quench the reaction. After removal of THF under vacuum, the residue was extracted with CHCl₃ and washed with 10% EDTA twice, and 15 mL of saturated aq. NaCl solution three times. The organic layer was concentrated in vacuo and precipitated in 500 mL of stirring methanol. The obtained solid was redissolved in CHCl₃ and reprecipitated in a mixture of methanol and THF (1 : 2, v/v) for poly(9,9-dioctyl-2,7-fluorene), or acetone for poly(N-butylcarbazole) or methanol for poly(2,5-dihexyloxy-1,4-phenylene). Characterization: Poly(9,9-dioctyl-2,7-fluorene) (PFO) ¹H NMR (300 MHz, CDCl₃δ in ppm): 7.88–7.79 (m, 2H), 7.75–7.60 (m, 4H), 2.22–1.97 (m, 4H), 1.30–1.02 (m, 20H), 0.91–0.75 (m, 10H). ¹³C NMR (125 MHz, CDCl₃, δ in ppm): 151.91, 140.63, 140.12, 126.25, 121.59, 120.03, 55.43, 40.46, 31.86, 30.11, 29.27, 23.90, 22.65, 14.10. Poly(N-butylcarbazol-3,6-diyl) (PBC), ¹H NMR (300 MHz, CDCl₃δ in ppm): 8.60–8.35 (br, 2H), 7.97–7.74 (br, 2H), 7.57–7.30 (br, 2H), 4.43–4.12 (br, 2H), 1.99–1.72 (br, 2H), 1.04–0.74 (br, 3H). ¹³C NMR (125 MHz, CDCl₃δ in ppm): 140.09, 133.39, 125.58, 123.78, 118.99, 109.00, 43.06, 31.32, 20.66, 13.97. Poly(2,5-dihexyloxy-1,4-phenylene) (PHP), ¹H NMR (300 MHz, CDCl₃, δ in ppm): 7.10 (br, 2H), 4.01–3.75 (br, 4H), 1.83–1.49 (br, 4H), 1.40–1.12 (m, 12H), 0.98–0.67 (br, 6H). ¹³C NMR (125 MHz, CDCl₃, δ in ppm): 150.90, 127.60, 117.43, 69.64, 31.67, 29.51, 25.78, 22.62, 14.02.

3. Results and discussion

On the basis of the wide applications of fluorene-based polymers,¹² our initial efforts were occupied with the synthesis of poly(9,9-dioctylfluorene) via the homopolymerization of 9,9-dioctyl-2,7-dibromomagnesiofluorene (3, Fig. 1). To realize the polymerization, high yields of bis-Grignard reagents were required. Usually, there were two ways for the preparation of Grignard reagents: one was the metal-halogen exchange between halogenated aryl compounds and isopropyl magnesium chloride;²⁸ and the other was direct formation via the reaction of halogenated aryl compounds with magnesium.²⁹ Considering that magnesium was cheaper and readily available, we chose the directly synthetic procedure, e.g., 3 was prepared from the reaction of magnesium with dibromoalkylfluorene (2). It was noted that the directly synthetic procedure needed an “activated magnesium”,³⁰ which was obtained in many cases by pretreating magnesium with I₂ or dibromoethane.²⁹ We found that diisobutylaluminum hydride (DiBALH) was more effective for the improvement of the reactivity of magnesium. When a catalytic amount of DiBALH was used, 2 quickly disappeared during a period of 1 h, and the HPLC results showed that 3 was formed in a high yield after the reaction was carried out at room temperature for 6 h.

Fig. 1 The procedure for the synthesis of polyfluorene.

Subsequently, we investigated the oxidative homopolymerization of the obtained 9,9-dioctyl-2,7-dibromomagnesiofluorene (3). There had been a lot of oxidants used for the Fe(III) catalyzed homocoupling of Grignard reagents.^22,31 Initially, we chose dibromoethane as an oxidant, and poly(9,9-dioctylfluorene-2,7-diyl) (PFO, Fig. 1) was obtained with a number average molecular weight (M_n) of 5700 after being extracted with acetone for 24 h in a Soxhlet extractor. However, further optimization of the polymerization conditions, including changing the reaction temperature from −25 °C to refluxing and adding a ten-fold amount of 1,2-dibromoethane, did not enhance M_n. Such results may be attributed to the fact that 1,2-dibromoethane was apt to be decomposed and the generated HBr may inevitably quench the Grignard reagent, resulting in a suppression of chain growth. Interestingly, when 1,2-dichloroethane (Fig. 1) was used as an oxidant, PFO was obtained with a M_n of more than 7300 after being extracted with acetone, and the yield was more satisfactory (Table 1). It was noted that the reaction gave the polymer with a wide polydispersity, as can be seen in Table 1. Removing the fractions with low molecular weight by precipitation of the obtained polymer in a mixture solvent of THF and methanol (2 : 1, v/v) gave a polymer fraction with a M_n of 11 960 and a PDI of 1.34. This result is superior to that previously reported by our group via the Pd(II)-catalyzed oxidative homopolymerization of diboronic esters,²⁴ suggesting that Fe(III) catalyzed oxidative homocoupling can give a high molecular weight of PFO which can meet the requirements of optoelectronic devices.

Table 1 Polymer fractions obtained in different precipitating solvents

Processing method	M n ^a	Polydispersity	Yield (%)
a GPC curves of the samples were shown in supplementary information. b Precipitated in methanol. c Extracted with acetone for 24 h in a Soxhlet extractor. d Reprecipitated in a mixture of methanol and THF (1 : 2, v/v).
A^b	4870	2.19	98
B^c	7300	1.59	75
C^d	11 960	1.34	35

---

To investigate the reason why Fe(III) catalyzed oxidative homocoupling gave such a wide PDI, we then looked into the relationship between the molecular weight and reaction time, and the results are summarized in Table 2. Surprisingly, the largest polymer fraction had come up to 10 300 during a very short period (one minute). Although the largest polymer fraction was near to 12 000 when the reaction was run for only 5 min, a lot of oligomers with a molecular weight of ∼3000 were observed. Further prolongation of the reaction time did not increase the molecular weight of PFO. Assuming that the impurities in the Grignard reagent (3) may influence chain growth, we then put 3 into dilute aq. HCl, and the product (9,9-dioctylfluorene) in the organic layer displayed a HPLC purity of 98.7%. This result implied that the wide PDI of PFO can not be attributed to the impurities in monomer 3, and may be due to the special mechanism of the iron-catalyzed homocoupling of bis-Grignard reagents,²² as shown in Fig. 2. Initially, the oxidation of a bis-Grignard reagent by FeCl₃ generated an oligomer P1, accompanying the formation of a low valent iron A. The oxidative addition of A to the dihaloethane and the subsequent β-halogen elimination transforms A to the iron species B. Consequently, the transmetalation of magnesium to iron afforded a species C. Finally, the polymer intermediate P2 was obtained via the reductive elimination of C, accompanying the regeneration of the active species A. In the catalytic cycle, 1,2-dihaloethane (1,2-dichloroethane or 1,2-dibromoethane) played a role to oxidize the iron species A (Feⁿ) to the iron species B (Feⁿ⁺²), undergoing a reductive elimination step.²² Because the iron species B and C displayed complicated oxidation states,²² including Fe¹⁺, Fe²⁺ and Fe³⁺, the progress of B to C to P2 may produce the polymer with a wide PDI.

Fig. 2 A proposed mechanism for Fe(III)-catalyzed homopolymerization.²²

Table 2 The correlation between the largest fraction of PFO and the reaction time using 1,2-dichloroethane as oxidant. All reactions were carried out at room temperature^a

Entry	Time (t)	M n	Polydispersity
a GPC curves of the samples are shown in the ESI†.
1	1 min	10 300	1.27
2	2 min	10 990	1.30
3	3 min	11 070	1.29
4	5 min	11 810	1.29

---

The chemical structure of the obtained PFO was confirmed by ¹H NMR and ¹³C NMR (See ESI†). In comparison with that of precursor 2, the ¹H NMR spectrum of PFO (Fig. 3) showed no extra signal in the area of 1.6–2.2 ppm, suggesting that no cross-coupling product between 3 and the oxidant (1,2-dichloroethane) was produced.

Fig. 3 A comparison of the ¹H NMR spectrum of precursor 2 (A) with that of the obtained PFO (B).

To further confirm the chemical structure of PFO, the UV-vis and PL spectra of the PFO prepared from traditional Suzuki coupling were compared with those of the PFO obtained in this work (Fig. 4). As can be seen from Fig. 4, the two samples showed no obvious difference, indicating that Fe(III) catalyzed oxidative coupling gave a polymer with the same chemical structure as that of the polymer prepared by Suzuki coupling.

Fig. 4 UV-vis (top) and PL (bottom) spectra of PFO obtained using different coupling routes. The concentration of all samples in THF was 4 × 10⁻⁶ M for UV-vis and 4 × 10⁻⁸ M for PL, respectively.

Considering that poly(alkyloxyphenylene)s³² and alkyl substituted polycarbazoles³³ had emerged as a very attractive class of excellent photoelectric materials, we then tried to synthesize poly(2,5-dihexyloxyphenylene-1,4-diyl) (PHP) and poly(N-butyl-carbazol-3,6-diyl) (PBC) by Fe(III) catalyzed oxidative coupling, and the synthetic route is shown in Fig. 5. The data of the obtained polymers are summarized in Table 3. The chemical structures of PHP and PBC were verified by ¹H NMR and ¹³C NMR (see experimental section and ESI†).

Fig. 5 The synthetic route of poly(alkyloxyphenylene) and poly(N-alkylcarbazole).

Table 3 Synthesis of PHP and PBC^a

Polymer	M n	Polydispersity
a In the presence of 2 eq. 1,2-dichloroethane and 10% FeCl3. All the reactions were carried out at room temperature for 18 h.
PHP	2800	1.27
PBC	3700	1.24

---

Compared to the UV-vis spectra of the corresponding precursors, those of PHP and PBC showed red-shifts, suggesting that the polymers had the expanded π-conjugated system (Fig. 6). It is noteworthy that PHP and PBC each displayed a lower molecular weight in comparison with PFO. The ion chromatograph (Table 4) also indicated that the Br content in PHP and PBC was much higher than that in PFO, suggesting that the formation of monomers 5 and 7 (Fig. 5) was more difficult than that of monomer 3 (Fig. 1). PHP and PBC seem to be considered as the oligomers, however, casting a toluene solution of PHP or PBC on glass plates gave a smooth film. The lower M_n may also be attributed to the special process of the iron-catalyzed homocoupling of bis-Grignard reagents (Fig. 2),²² as well as being potentially due to steric hindrance resulting from the long side chains in PHP³⁴ and the twisted and zigzag-shape structure in PBC.³⁵ Further, more in-depth details will soon be investigated.

Fig. 6 UV-Vis spectra of PHP and PBC in THF. For comparison, the spectra of the corresponding precursors 4 and 6 are also given.

Table 4 The chemical analysis of the polymers

Polymers	Terminal Br content (%)^a
a Measured by ion chromatography.
PHP	0.436
PBC	0.404
PFO	0.013

---

4. Conclusions

Three π-conjugated polymers, poly(9,9-dioctylfluorene-2,7-diyl) (PFO), poly(N-butylcarbazol-3,6-diyl) (PBC) and poly(2,5-dihexyloxyphenylene-1,4-diyl) (PHP) have been successfully synthesized via Fe(III)-catalyzed oxidative homocoupling of Grignard reagents. The advantages of this route include a short reaction period and low cost, making it an attractive alternative for the synthesis of π-conjugated polymers.

Acknowledgements

Financial support from Ministry of Science and Technology of China (2010BAK67B12 and 2011ZX02703) are gratefully acknowledged.

References

1. T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed., 1974, 12, 11.
2. H. Shirakawa, Rev. Mod. Phys., 2001, 73, 713.
3. H. Shirakawa and E. J. Louis, J. Chem. Soc., Chem. Commun., 1977, 578.
4. G. Natta, I. Pasquon and A. Zambelli, J. Polym. Sci., 1961, 156, 387.
5. C. K. Chiang, M. A. Druy and S. C. Gau, J. Am. Chem. Soc., 1978, 100, 1013.
6. B. M. Quillan, G. B. Street and T. C. Clarke, J. Electron. Mater., 1982, 11, 471.
7. H. Shirakawa, T. Otaka and G. Piao, Synth. Met., 2001, 117, 1.
8. (a) C. F. Liu, Polym. J., 1993, 25, 363; (b) Y. K. Kang, Synth. Met., 1992, 52, 319.
9. (a) T. Yamamoto, K. Sanechika and A. Yamamoto, J. Polym. Sci., Polym. Lett. Ed., 1980, 18, 9; (b) K. T. Jen, G. G. Miller and R. L. Elsenbaumer, J. Chem. Soc., Chem. Commun., 1986, 1346; (c) S. Jin and G. Xue, Macromolecules, 1997, 30, 5753; (d) K. Yoshino, Y. Manda and M. Onoda, Solid State Commun., 1989, 70, 111; (e) X. J. Wang, M. R. Andersson and M. E. Thompson, Thin Solid Films, 2004, 468, 226; (f) M. R. Andersson, M. Berggren and G. Gustafsson, Synth. Met., 1995, 71, 2183; (g) M. Leclerc and G. J. Daoust, J. Chem. Soc., Chem. Commun., 1990, 273; (h) T. A. Chen and R. D. Rieke, J. Am. Chem. Soc., 1992, 114, 10087; (i) S. Hotta, S. Rughooputh, A. J. Heeger and F. Wudl, Macromolecules, 1987, 30, 212; (j) R. D. McCullough, Adv. Mater., 1998, 10, 93.
10. (a) J. H. Burroughes, D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature, 1990, 347, 539; (b) D. Braun and A. Heerger, Appl. Phys. Lett., 1991, 58, 1982.
11. (a) G. Grem, G. Leditzky, B. Ullrich and G. Leising, Adv. Mater., 1992, 4, 36; (b) V. Scherf and K. Muellen, Macromolecules, 1992, 25, 3546; (c) J. Huber, K. Mullen, J. Salbeck, H. Schenk, U. Scherf, T. Stehlin and R. Stern, Acta Polym., 1994, 45, 244.
12. (a) O. Yutaka, U. Masao and M. Keiro, Jpn. J. Appl. Phys., 1991, 30, 1941; (b) G. Klaerner and R. D. Miller, Macromolecules, 1998, 31, 2007; (c) W. L. Yu, J. Pei and W. Huang, Chem. Commun., 1999, 1837; (d) S. Setagesh, A. C. Grimsdale and T. Weil, J. Am. Chem. Soc., 2001, 123, 946; (e) S. L. Mcfarlane, D. G. Piercey and L. S. Coumont, Macromolecules, 2009, 42, 591; (f) J. Pei, W. L. Yu and W. Huang, Chem. Commun., 2000, 1631; (g) W. Y. Wong, L. Liu and D. M. Cui, Macromolecules, 2005, 38, 4970; (h) C. S. Wu and Y. Chen, Macromolecules, 2009, 42, 3729.
13. (a) J. Hou, Z. Tan, Y. Yan, Y. He, C. Yang and Y. Li, J. Am. Chem. Soc., 2006, 128, 4911; (b) J. Hou, L. Huo, C. He, C. Yang and Y. Li, Macromolecules, 2006, 39, 594; (c) J. Hou, C. Yang, C. He and Y. Li, Chem. Commun., 2006, 871; (d) E. Zhou, Z. Tan, C. Yang and Y. Li, Macromol. Rapid Commun., 2006, 27, 793.
14. X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen and S. Marder, J. Am. Chem. Soc., 2007, 129, 7246.
15. (a) F. He, Y. Tang, S. Wang, Y. Li and D. Zhu, J. Am. Chem. Soc., 2005, 127, 12343; (b) F. He, Y. Tang, M. Yu, F. Feng, L. An, H. Sun, S. Wang, Y. Li, D. Zhu and G. Bazan, J. Am. Chem. Soc., 2006, 128, 6764; (c) Y. Tang, F. Feng, F. He, S. Wang, Y. Li and D. Zhu, J. Am. Chem. Soc., 2006, 128, 14972; (d) X. Feng, L. Liu, S. Wang and D. Zhu, Chem. Soc. Rev., 2010, 39, 2411; (e) X. Duan, L. Liu, X. Feng and S. Wang, Adv. Mater., 2010, 12, 1602.
16. S. Ellinger, K. R. Graham, P. Shi, R. T. Farley, T. Steckler, R. Brookins, P. Taranekar, J. Mei, L. Padilha, T. Ensley, H. Hu, S. Webster, D. Hagan, E. V. Stryland, K. Schanze and J. R. Reynolds, Chem. Mater., 2011, 23, 3805.
17. (a) F. B. Koyuncu, E. Sefer, S. Koyuncu and E. Ozdemir, Polymer, 2011, 52, 5772; (b) B. Kim, J. Kim and E. Kim, Macromolecules, 2011, 44, 8791.
18. (a) A. A. Sergei and M. K. Valerii, Macromol. Chem. Phys., 2000, 201, 809; (b) P. Kovacic and J. J. Oziomek, J. Org. Chem., 1964, 29, 100; (c) Y. Ito, T. Shimada, J. Ha, M. Vacha and H. Sato, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 4338; (d) Q. Pei and Y. Yang, J. Am. Chem. Soc., 1996, 118, 7416; (e) T. Yamamoto, A. Morita, Y. Miyazaki, T. Maruyama, H. Wakayama, Z. H. Zhou, Y. Nakamura, T. Kanbara, A. Sasaki and K. Kubota, Macromolecules, 1992, 25, 1214.
19. (a) R. D. McCullough and R. D. Lowe, J. Chem. Soc., Chem. Commun., 1992, 1, 70; (b) R. D. McCullough, Synth. Met., 1995, 69, 279; (c) A. Yokoyama, R. Miyakoshi and T. Yokozawa, Macromolecules, 2004, 37, 1169; (d) R. Miyakoshi, A. Yokoyama and T. Yokozawa, J. Am. Chem. Soc., 2005, 127, 17542; (e) R. S. Loewe, S. M. Khersonsky and R. D. McCullough, Adv. Mater., 1999, 11, 250; (f) E. E. Sheina, J. Liu, M. C. Iovu, D. W. Laird and R. D. McCullough, Macromolecules, 2004, 37, 3526.
20. (a) A. Fazio, B. Gabriele, G. Salerno and S. Destri, Tetrahedron, 1999, 55, 485; (b) R. D. McCullough, P. C. Ewbank and R. S. Loewe, J. Am. Chem. Soc., 1997, 119, 633.
21. (a) S. Guillerez and G. Bidan, Synth. Met., 1998, 93, 123; (b) C. Kowitz and G. Wegner, Tetrahedron, 1997, 53, 15553; (c) T. I. Wallow and B. M. Novak, J. Am. Chem. Soc., 1991, 113, 7411; (d) W. Yang, Q. Hou, C. Liu, Y. Niu, J. Huang, R. Yang and Y. Cao, J. Mater. Chem., 2003, 13, 1351; (e) M. Ranger, D. Rondeau and M. Leclerc, Macromolecules, 1997, 30, 7686; (f) M. Ranger and M. Leclerc, Chem. Commun., 1997, 1597; (g) C. F. Shu, R. Dodda and F. Wu, Macromolecules, 2003, 36, 6698; (h) Q. Peng, Z. Y. Lu, Y. Huang, M. G. Xie, S. H. Han, J. B. Peng and Y. Cao, Macromolecules, 2004, 37, 260; (i) Q. Hou, Q. Zhou, Y. Zhang, W. Yang, R. Yang and Y. Cao, Macromolecules, 2004, 37, 6299; (j) C. H. Chou, S. L. Hsu, K. Dinakaran, M. Y. Chiu and K. H. Wei, Macromolecules, 2005, 38, 745.
22. (a) T. Nagano and T. Hayashi, Org. Lett., 2005, 7, 491; (b) G. Cahiez, C. Chaboche, F. M. Betzer and M. Ahr, Org. Lett., 2005, 7, 1943.
23. M. S. Maji, T. Pfeifer and A. Studer, Chem.–Eur. J., 2010, 16, 5872.
24. C. Yuan, J. Hong, Y. Liu, H. Lai and Q. Fang, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 4098.
25. (a) M. Ghaemy and M. Barghamadi, J. Appl. Polym. Sci., 2009, 114, 3464; (b) D. Zeng, J. Chen, Z. Chen, W. Zhu, J. He, F. Yu, H. Huang, H. Wu, C. Liu, S. Ren, J. Du, J. Sun, E. Xu, A. Cao and Q. Fang, Macromol. Rapid Commun., 2007, 28, 772.
26. Q. Fang and T. Yamamoto, Polymer, 2003, 44, 2947.
27. (a) K. Smith, D. M. James, A. G. Mistry, M. Bye and D. J. Fauikner, Tetrahedron, 1992, 48, 7479; (b) T. Zhao, Z. Liu, Y. Song, W. Xu, D. Zhang and D. Zhu, J. Org. Chem., 2006, 71, 7422.
28. (a) P. Knochel, W. Dohle, N. Gommermann, F. F. Kneisel, F. Kopp, T. Korn, I. Sapountzis and V. A. Vu, Angew. Chem., Int. Ed., 2003, 42, 4302; (b) A. Krasovskiy and P. Knochel, Angew. Chem., Int. Ed., 2004, 43, 3333; (c) F. Kopp, A. Krasovskiy and P. Knochel, Chem. Commun., 2004, 2288; (d) H. Ren, A. Krasovskiy and P. Knochel, Org. Lett., 2004, 6, 4215; (e) H. Ren, A. Krasovskiy and P. Knochel, Chem. Commun., 2005, 543; (f) A. Krasovskiy, B. F. Straub and P. Knochel, Angew. Chem., Int. Ed., 2006, 45, 159.
29. (a) R. D. Rieke and H. Xiong, J. Org. Chem., 1991, 56, 3109; (b) A. M. Caporusso, L. A. Aronica, R. Geri and M. Gori, J. Organomet. Chem., 2002, 648, 109.
30. F. M. Piller, P. Appukkuttan, A. Gavryushin, M. Helm and P. Knochel, Angew. Chem., Int. Ed., 2008, 47, 6802.
31. (a) G. Cahiez, A. Moyeux, J. Buendia and C. Duplais, J. Am. Chem. Soc., 2007, 129, 13788; (b) M. S. Maji and A. Studer, Synthesis, 2009, 14, 2467.
32. (a) R. Miyakoshi, K. Shimono, A. Yokoyama and T. Yokozawa, J. Am. Chem. Soc., 2006, 128, 16012; (b) E. L. Lanni and A. J. McNeil, J. Am. Chem. Soc., 2009, 131, 16573.
33. M. C. Stefan, A. E. Javier, I. Osaka and R. D. McCullough, Macromolecules, 2009, 42, 30.
34. K. Ono, A. Adachi, K. Okita, M. Goto and Y. Yamashita, Chem. Lett., 1998, 545.
35. H. Lai, J. Hong, P. Liu, C. Yuan, Y. Li and Q. Fang, RSC Adv., 2012, 2, 2427.

---

Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c2ra21127k

---