Direct synthesis of poly(p-phenyleneethynylene)s from calcium carbide

Abstract

An efficient method for the preparation of poly(p-phenyleneethynylene)s (PPEs) from direct coupling reactions between aryl diiodides and the inexpensive chemical feedstock calcium carbide is developed. A variety of PPEs can be prepared in high yields (71–93%) with the degree of polymerization between 36 and 128, offering fluorescence quantum efficiencies in the range of 0.34–0.71.

---

The Sonogashira reaction (Scheme 1) is among the most powerful tools for the construction of a C–C triple bond via Pd/Cu catalysed sp²–sp coupling reaction between aryl or alkenyl halides and terminal alkynes.¹ Such reactions are essential for preparing acetylenic compounds that are important building blocks of natural products, pharmaceuticals and molecular materials.² Due to environmental and economic pressures, over the past decade, development of this reaction has focused on new catalysts³ and new techniques such as microwave,⁴ ultrasonic irradiation⁵ or micro flow reactions.⁶ However, little investigation has been done on a more sustainable source of sp-carbon. Traditional synthetic methodology previously relied on the use of terminal alkynes such as substituted acetylenes, including expensive (trimethylsilyl)acetylene and acetylene gas,⁷ as sp-carbon providers. The former is a more favourable source due to its convenient handling but it is expensive and requires an extra deprotection step in some cases. The latter is appreciably more economical but its hazardous nature i.e. high flammability and explosiveness is prohibitive to broader adoption in routine laboratory experiments and industrial production. Therefore a better, safer and more economical source of acetylene is highly desirable for the Sonogashira reaction.

Scheme 1 Typical Sonogashira coupling reaction.

Acetylene gas is produced by treating calcium carbide with water. The commercial process for making the calcium carbide is electric arch furnace of coke and lime. Since the first launch of a calcium carbide plant in 1894, it has become an important primary chemical feedstock for a broad range of commodity and specialty chemicals. With a constant low price and significant safety advantages over acetylene gas, if acetylenic derivatives can be synthesized directly from calcium carbide, such a process would offer significant cost and environmental advantages over existing reactions. The first direct use of calcium carbide in the Sonogashira reaction was reported in 2006 by Zhang,⁸ whereby symmetrical diarylethynes were synthesized in the presence of non-commercially available amino phosphine ligands. Subsequently, we reported the use of calcium carbide as a provider of sp-C in the synthesis of diarylethynes in the presence of a commercial Pd/Cu catalyst system.⁹ Recently, propagylamine derivatives have been prepared directly from calcium carbide via a Sonogashira type reaction.¹⁰ These examples demonstrated that calcium carbide could effectively serve as a C₂ synthon in the synthesis of small molecular acetylenic derivatives,^8–10 the transfer of this simple and economical source of acetylene into efficient synthesis of acetylenic containing polymer remains a challenge.

Poly(p-phenyleneethynylene)s (PPEs) are an attractive family of conjugated polymers in which aryl groups are fully π-conjugated by acetylene linkers. Such structural alignment extends electronic delocalization across the polymer backbone, providing properties such as charge mobility, electrical conductivity and photoluminescence.¹¹ PPEs have found a large number of applications especially in the area of sensor,¹² solar cell¹³ and display technologies.¹⁴ Typically, PPEs are prepared by polycondensation between aryl diiodide 1 and various acetylene sources through the Sonogashira coupling reaction as depicted in Scheme 2.

Scheme 2 Synthetic method for PPEs via Sonogashira coupling.

Diethynylarenes¹⁵ were first introduced as coupling partners and later on TMS-acetylene was used.^2d,16 Later, the Bunz and Li groups reported the synthesis of PPEs directly from acetylene gas.¹⁷ This methodology gave high molecular weight homo-PPEs, considered to be economical due to the low price of acetylene gas. However, the success of this method relies heavily on close monitoring and regulating of stoichiometric acetylene gas, which require difficult equipment set up. We therefore investigated a direct synthesis of PPEs from calcium carbide which is less hazardous and provides a more economical carbon source than the acetylene gas. Herein, we would like to report that calcium carbide may be conveniently used in the condensation polymerization of diiodoarene to synthesize high molecular weight PPEs bearing various side chains (Scheme 2).

In our first model study, the polymerization was the coupling reaction between 1,4-dibutoxy-2,5-diiodobezene 1a and calcium carbide at room temperature in the presence of a Pd(OAc)₂, CuI and PPh₃ catalyst system. The resulting polymer was precipitated in excess methanol. The degree of polymerization was determined from the polymer solution in THF by gel permeation chromatography using standard polystyrene for universal calibration (Table 1). In the first attempt (entry 1), 1% of Pd(OAc)₂ was used in a similar manner to the synthesis of the small molecules⁹ but the reaction gave only a low molecular weight oligomer, which was precipitated in methanol, along with the recovery of the starting diiodo compound. The polymerization was significantly improved by the increase of palladium amount to 5 mol%, which yielded 84% of a orange solid (entry 2) corresponding to the desired PPE (M_w = 11.7 kDa) without residual starting material 1a. Next, a set of experiments were carried out to determine the best solvent and base system (entries 2–7). The mixture of THF–DBU in a ratio 2 : 1 was found to be the best condition to produce the highest degree of polymerization (DP_n = 36) and narrow polydispersity index (PDI) (M_w/M_n = 2.3) (entry 5). The slightly lower yield was mainly due to the loss of the high molecular weight portion, which was poorly soluble in CH₂Cl₂ solvent needed for the second precipitation.

Table 1 Synthesis of dibutoxy-PPEs 2a from calcium carbide via the Sonogashira coupling reaction^a

Entry	Pd(OAc)2 (mol%)	Base/solvent	Yield^b (%)	M w (× 10^3)	DPn^c	M w/Mn^c
a Unless noted, CaC2 (6 equiv.), Pd(OAc)2, PPh3 (10 mol%), CuI (10 mol%) for 20 h. b Precipitation in CH3OH from CH2Cl2. c Determined with GPC using universal calibration with standard polystyrene. d TMS-acetylene were used instead of CaC2.
1	1	TEA/THF	N.A.	—	—	—
2	5	TEA/THF	84	11.7	20	2.3
3	5	TEA/MeCN	90	6.8	13	2.1
4	5	TEA/DMF	N.A.	—	—	—
5	5	DBU/THF	71	20.2	36	2.3
6	5	K2CO3/THF	N.A.	—	—	—
7	5	Cs2CO3/THF	N.A.	—	—	—
8^d	5	DBU/MeCN/THF	100	9.6	17	2.3

---

The ¹H and ¹³C NMR spectra of the orange solid product showed clean signals corresponding to PPE without end groups and diyne signals, suggesting a high molecular weight and defect free polymer (Fig. 1a and S13†). For comparison, the polymerization reaction between TMS-acetylene (entry 8) and 1a was tested using literature-reported conditions.^16c The PPE obtained from these conditions also showed no diyne signal in ¹³C NMR (Fig. S19†) but displayed small peaks in the aromatic region suggesting the end group (Fig. 1b). The results agreed well with the GPC data showing a lower degree of polymerization (DP_n = 17) comparing with the polymer obtained from our optimized calcium carbide method (entry 5). Both CaC₂ and TMS-acetylene polymerization (entries 5 and 8) gave higher molecular weight and lower PDI than those from the conventional Pd-catalyzed coupling polymerization using the terminal alkynes.^14g The high molecular weight and narrow PDI of PPEs have been attributed to the slow formation of the alkyne terminated monomers and oligomers that reduces the chance of their homocoupling.^17b The homo-coupling side reaction causes the stoichiometric imbalance of the iodo and terminal alkyne groups that lead to low molecular weight and high PDI. The formation of the alkyne terminated monomers and oligomers in the CaC₂ polymerization is governed by either the slow release of acetylene gas or calcium acetylide from CaC₂. The release of these active species is self-controlled by HI, a by-product generated from the alkyne/iodocondensation,⁹ which is quite different from the direct use of acetylene gas that required very slow stirring.^17b With the optimized conditions in hand, we then tested compatibility of the reaction with diiodobenzene containing different side chains. A series of diiodobenzenes (1b–f), prepared according to published procedures, were subjected to the Sonogashira coupling reaction with CaC₂ under the optimized reaction conditions (Table 1, entry 5) and the results are summarized in Table 2. The dialkoxy-PPEs containing simple alkyl chains, such as 2b, 2c and 2d, were synthesized efficiently in good to excellent yields (Table 2, entries 2–4). Polymerization of 1c, bearing double ethylhexyloxy groups, gave the PPE with highest DP_n = 128 and PDI = 2.0 when the reaction was carried out at room temperature for 20 h. This high degree of polymerization is attributed to the excellent solubility of the corresponding polymer in the reaction medium. This result suggested that the lower degree of polymerization of the other monomers was thus limited by the polymer solubility rather than the reaction efficiency. Interestingly, the polymerization condition is also compatible with the monomers containing multiple oxygenous substituents such as 1e and 1f (Table 2, entries 5 and 6). PPE 2e and 2f were prepared in 87 and 93% yields, respectively, as shown by the absence of end groups in the aromatic region in ¹H NMR spectra (Fig. S17 and S18†), suggesting efficient polymerization of the monomers. The GPC measurement showed the degree of polymerization of 2eca. 51 while that of 2f could not be determined due to its poor solubility in THF. All PPEs were obtained as deep orange solids which typically gave blue to green emission in chloroform solution, except 2f which was soluble only in DMSO (Table S1†). The photophysical properties of the polymers including maximum absorption wavelength (λ_abs), maximum emission wavelength (λ_em), molar absorptivity (ε) and fluorescence quantum efficiency (Φ_f) were determined as presented in Table 2. Typically, each PPE exhibited a broad absorption band with the absorption maximum around 445 nm and showed the emission maximum around 475 nm, representing a rather small Stoke’s shift (Fig. S20†). These PPEs also gave relatively high quantum efficiencies (0.34–0.71). These photophysical parameters are consistent with the defect free π-conjugated PPEs reported with other polymerization methods,^2d especially the high quantum efficiency.

Fig. 1 ¹H NMR spectrum of PPE 2a derived from: (a) CaC₂; (b) TMS-acetylene.

Table 2 Synthesis and photophysical properties of PPEs

Entry	Substituents	Yield^a (%)	M w (× 10^3)	DPn^b	M w/Mn^b	λ abs/nm	λ em/nm	Log ε	Φ f
a Isolated yield by reprecipitation from CH3OH/CH2Cl2. b Determined by GPC using PS as reference. c Dissolved in CH2Cl2 and used quinine sulfate as reference standard. d Performed in DMSO.
1	X: R1 = R2 = O(CH2)3CH3: 1a	71	20.2	36	2.3	447	476	4.32	0.65
2	X: R1 = R2 = O(CH2)7CH3: 1b	78	40.9	48	2.4	448	476	4.30	0.66
3	X: R1 = R2 = OCH2CH(CH2CH3)((CH2)3CH3): 1c	83	91.3	12	2.0	48	479	4.41	0.71
4	X: R1 = OMe, R2 = OCH2CH(CH2CH3)((CH2)3CH3): 1d	93	26.9	47	2.2	450	478	4.02	0.67
5	X: R1 = R2 = O(CH2)2O(CH2)2OCH3: 1e	87	26.9	51	1.5	430	470	4.02	0.67
6	X: R1 = R2 = O(CH2)3OH: 1f	93	N.A.	N.A.	N.A.	445	481	4.11	0.34^d

---

In conclusion, this work demonstrates a novel and efficient polymerization of diiodobenzene derivatives to the corresponding defect free PPEs, directly from a low cost feedstock, calcium carbide, at ambient temperature. The polymerization method is compatible with various alkyl, ethylene glycol and aliphatic alcohol substituents. The extension of this polymerization method for preparation of PPEs with more functional pendant groups is in progress.

The authors wish to thank Prof. Pierre Dixneuf for discussions. This study is financially supported by the Thailand Research Fund (TRF-RSA5480004) and Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. This work is part of the Project for Establishment of Comprehensive Center for Innovative Food, Health Products and Agriculture supported by the Thai Government Stimulus Package 2 (TKK2555, SP2), the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission (AM1006A-56) and the Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University (RES560530126-AM).

Notes and references

1. (a) K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975, 16, 4467–4470; for a review see: (b) R. Chinchilla and C. Nájera, Chem. Rev., 2007, 107, 874–922; (c) H. Doucet and J. C. Hierso, Angew. Chem., Int. Ed., 2007, 46, 834–871; (d) R. Chinchilla and C. Nájera, Chem. Soc. Rev., 2011, 40, 5084–5121.
2. (a) M. Yamashita, H. Horiguchi and K. Hirano, J. Org. Chem., 2009, 74, 7481–7488; (b) C. J. Li, W. T. Salaven, IV, V. T. John and S. Banergee, Chem. Commun., 1997, 1569–1570; (c) M. Iyoda, A. Vorasingha and Y. Kuwatani, Tetrahedron Lett., 1998, 39, 4701–4704; (d) U. H. F. Bunz, Macromol. Rapid Commun., 2009, 30, 772–805.
3. (a) M. B. Thathagar, J. Beckers and G. Rothenberg, Green Chem., 2004, 6, 215–218; (b) C. G. Frost and L. Mutton, Green Chem., 2010, 12, 1687–1703; (c) R. Thorwirth, A. Stolle and B. Ondruschka, Green Chem., 2010, 12, 985–991; (d) H. Doucet and J. C. Hierso, Angew. Chem., Int. Ed., 2007, 46, 834–871; (e) S. Liu and J. Xiao, J. Mol. Catal. A: Chem., 2007, 270, 1–43; (f) C. A. Fleckenstein and H. Plenio, Chem.–Eur. J., 2007, 13, 2701–2716; (g) L. Yang, L. Zhao and C. J. Li, Chem. Commun., 2010, 46, 4184–4186; (h) R. Luque and D. J. MacQuarrie, Org. Biomol. Chem., 2009, 7, 1627–1632.
4. (a) P. Appukkuttan, W. Dehaen and E. V. D. Eycken, Eur. J. Org. Chem., 2003, 4713–4716; (b) H. Huang, H. Liu, H. Jiang and K. Chen, J. Org. Chem., 2008, 73, 6037–6040; (c) K. M. Dawood, W. Solodenko and A. Kirschning, ARKIVOC, 2007, 5, 104–124.
5. (a) A. R. Gholap, K. Venkatesan, R. Pasricha, T. Daniel, R. J. Lahoti and K. V. Srinivasan, J. Org. Chem., 2005, 70, 4869–4872; (b) S. S. Palimkar, P. H. Kumar, R. J. Lahoti and K. V. Srinivasan, Tetrahedron, 2006, 62, 5109–5115.
6. (a) L. M. Tan, Z. Y. Sem, W. Y. Chong, X. Liu, Hendra, W. L. Kwan and C. L. Lee, Org. Lett., 2013, 15, 65–67; (b) R. Javaid, H. Kawanami, M. Chatterjee, T. Ishizaka, A. Suzuki and T. M. Suzuki, Chem. Eng. J., 2011, 167, 431–435; (c) B. K. Singh, N. Kaval, S. Tomar, E. V. D. Eycken and V. S. Parmar, Org. Process Res. Dev., 2008, 12, 468–474; (d) C. G. Frost and L. Mutton, Green Chem., 2010, 12, 1687–1703.
7. (a) C. J. Li, D. Li and C. Costello, Org. Process Res. Dev., 1997, 1, 325–327; (b) M. Pal and N. G. Kundu, J. Chem. Soc., Perkin Trans. 1, 1996, 449–451; (c) J. Moon, M. Jeong, H. Nam, J. Ju, J. H. Moon, H. M. Jung and S. Lee, Org. Lett., 2008, 10, 945–948.
8. W. Zhang, H. Wu, Z. Liu, P. Zhong, L. Zhang, X. Huang and J. Cheng, Chem. Commun., 2006, 4826–4828.
9. P. Chuentragool, K. Vongnam, P. Rashatasakhon, M. Sukwattanasinitt and S. Wacharasindhu, Tetrahedron, 2011, 67, 8177–8182.
10. Z. Lin, D. Yu, Y. N. Sum and Y. Zhang, ChemSusChem, 2012, 5, 625–628.
11. (a) U. H. F. Bunz, Chem. Rev., 2000, 100, 1605–1644; (b) U. H. F. Bunz, Adv. Polym. Sci., 2005, 177, 1–52; (c) J. Zheng and T. M. Swager, Adv. Polym. Sci., 2005, 177, 151–179; (d) G. Voskerician and C. Weder, Adv. Polym. Sci., 2005, 177, 209.
12. (a) T. S. Corbitt, L. Ding, E. Ji, L. K. Ista, K. Ogawa, G. P. Lopez, K. S. Schanze and D. G. Whitten, Photochem. Photobiol. Sci., 2009, 8, 998–1005; (b) E. L. Dane, S. B. King and T. M. Swager, J. Am. Chem. Soc., 2010, 132, 7758–7768; (c) G. He, N. Yan, J. Yang, H. Wang, L. Ding, S. Yin and Y. Fang, Macromolecules, 2011, 44, 4759–4766; (d) I. B. Kim, B. Erdogan, J. N. Wilson and U. H. F. Bunz, Chem.–Eur. J., 2004, 10, 6247–6254; (e) R. L. Phillips, O. R. Miranda, D. E. Mortenson, C. Subramani, V. M. Rotello and U. H. F. Bunz, Soft Matter, 2009, 5, 607–612; (f) R. L. Phillips, O. R. Miranda, C. C. You, V. M. Rotello and U. H. F. Bunz, Angew. Chem., Int. Ed., 2008, 47, 2590–2594; (g) S. W. Thomas, III, G. D. Joly and T. M. Swager, Chem. Rev., 2007, 107, 1339–1386; (h) J. H. Wosnick, C. M. Mello and T. M. Swager, J. Am. Chem. Soc., 2005, 127, 3400–3405; (i) X. Zhao, Y. Liu and K. S. Schanze, Chem. Commun., 2007, 2914–2916.
13. (a) H. Hoppe, D. A. M. Egbe, D. Mühlbacher and N. S. Sariciftci, J. Mater. Chem., 2004, 14, 3462–3467; (b) J. K. Mwaura, X. Zhao, H. Jiang, K. S. Schanze and J. R. Reynolds, Chem. Mater., 2006, 18, 6109–6111; (c) F. Silvestri and A. Marrocchi, Int. J. Mol. Sci., 2010, 11, 1471–1508.
14. (a) C. A. Breen, S. Rifai, V. Bulović and T. M. Swager, Nano Lett., 2005, 5, 1597–1601; (b) A. Montali, P. Smith and C. Weder, Synth. Met., 1998, 97, 123–126; (c) N. G. Pschirer, T. Miteva, U. Evans, R. S. Roberts, A. R. Marshall, D. Neher, M. L. Myrick and U. H. F. Bunz, Chem. Mater., 2001, 13, 2691–2696; (d) C. Schmitz, P. Pösch, M. Thelakkat, H. W. Schmidt, A. Montali, K. Feldman, P. Smith and C. Weder, Adv. Funct. Mater., 2001, 11, 41–46; (e) A. F. Thünemann and D. Ruppelt, Langmuir, 2001, 17, 5098–5102; (f) Y. Xu, P. R. Berger, J. N. Wilson and U. H. F. Bunz, Appl. Phys. Lett., 2004, 85, 4219–4221; (g) J. Shinar, L. S. Swanson, F. Lu and Y. Ding, US Pat., 5 334 539, 1994.
15. R. Giesa and R. C. Schulz, Makromol. Chem., 1990, 191, 857–867.
16. (a) M. Banno, T. Yamaguchi, K. Nagai, C. Kaiser, S. Hecht and E. Yashima, J. Am. Chem. Soc., 2012, 134, 8718–8728; (b) A. Khan and S. Hecht, Chem. Commun., 2004, 300–301; (c) A. Khan, S. Müller and S. Hecht, Chem. Commun., 2005, 584–586.
17. (a) C. J. Li, W. T. Slaven, IV, Y. P. Chen, V. T. John and S. H. Rachakonda, Chem. Commun., 1998, 1351–1352; (b) J. N. Wilson, S. M. Waybright, K. McAlpine and U. H. F. Bunz, Macromolecules, 2002, 35, 3799–3800.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and spectroscopic data of 1a–f and PPE 2a–f. See DOI: 10.1039/c3py01068f

---