8th International Electronic Conference on Synthetic Organic Chemistry. ECSOC-8. 1-30 November 2004. http://www.lugo.usc.es/~qoseijas/ECSOC-8/
[F002]
An organometallic selective synthesis of conjugated polymers with improved physical properties.
M. Bouachrine*a, J.-P. Lère-Porte b, J.J.E. Moreaub, F. Serein-Spiraub, T. Lakhlific
*a LRMM, Département de Chimie, Faculté des Sciences et Techniques d'Errachidia, B.P. 509 Boutalamine Errachidia, Maroc (Email : [email protected])
b Laboratoire de Chimie Organométallique, E. N. S. C. Montpellier, France
c LCO, Département de Chimie, Faculté des Sciences de Meknès, Maroc
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Abstract: Conjugated polymers have attracted much interest in recent years due the many possibilities for their applications in opto-electronics and as non-linear optical materials. Synthetic efforts have been made, on the hand, to improve the properties of well-known oligomers and conjugated polymers and on the other, to prepare new polymers, containing original chain units, from a variety of unsaturated monomers. In this paper we present a selective synthesis of conjugated materials based on thiophene by oxidative polymerization of various silyl monomers. The high sensitivity of the silicon atom offers new synthetic route. As shown by GPC, IR, Raman and photoluminescence criteria, Silicon activates the selective coupling of unsaturated monomers, leading to highly conjugated polymers with improved properties for specific applications.
Keywords: Conjugated polymers, Trimethylsilyl, oxidative polymerization
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1. INDRODUCTION
Conductive polymers are currently constituting a growing field of science and engineering for these twenty years since polyacetylene was discovered to be highly conducting upon doping with, for example, iodine [1]. After this discovery a large set of conductive polymers has been synthesized using various techniques [2]. From a practical point of view, the conductive polymers are required to have environmental stability. Particularly, the conductive polymers that have aromatic rings as repeated constituent units show excellent thermal and air stability. Poly(paraphenylene) is counted as typical example among them. Polypyrrole and polythiophene and their derivatives consisting of heterocycles are others examples [3]. However, recently, poly(3-alkylthiophene) was found not only to be soluble in organic solvents [4,5] but even fusible [6]. It should also be mentioned that they exhibit thermochromic [7] and solvatochromic properties [8].
Silicon directed reactions, widely used in organic synthesis to improve the selectivity of carbon-carbon bond formation [9]. The use of this methodology is certainly also interest for selective polymerization reactions. The important role of silicon was early recognized in the group transfer polymerization reaction (GTP) [10,11]. This work describes the use of Silicon on the synthesis of conductive polymers: oxidative coupling of aromatic silylated monomers. The chemical and electrochemical synthesis of poly(3-alkylthiophene) and silyl derivatives are described. The polymers were characterized by different techniques: GPC, FTIR, Raman, RMN, MEB, UV…We wish to report here that Silicon plays an important role in the coupling reaction and yields polymers with high degree of polymerization.
2. EXPERIEMNTAL SECTION
2.1. Preparation of silylated monomers
Trimethylsilyl substituted 3-hexylthiophene were prepared from 3-hexylthiophene. As shown in scheme 1, 2-trimethylsilyl-3-hexylthiophene (HexTSi) and 5-trimethylsilyl-3-hexylthiophene (SiHexT) as well as 2.5-bis(trimethylsilyl)-3-hexylthiophene (SiHexTSi) were synthesized in good yields).
2.2. Polymerization
The polymerization of silylated monomers (HexTSi, SiHexT, SiHexTSi) and that of non silylated monomers (HexT) were performed under similar reactions conditions (scheme 2, scheme 3). For electropolymerization, solutions of 0.1 M of monomers (HexT, HexTSi, SiHexT, SiHexTSi) in Nitrobenzene containing 0.02 M of Bu4NBF4 as supporting electrolyte were oxidized under galvanostatic conditions (I = 3 mA/ cm2, 5°C) [12]. The chemical polymerization was performed at 20°C using FeCl3 in CHCl3. The resulting polymers were washed with methanol until complete eleimination of Iron and purified by precipitation from their solutions in THF. Polymer yields in the range 65-70 % were obtained.
2.3. Characterization
The polymers obtained from monomers (HexT, SiHexT, HexTSi, SiHexTSi) exhibited similar spectroscopic characteristics. lmax = 430 nm, the FT-Raman and FT-IR spectra showed bands in agreement with the values reported for poly(3-nhexylthiophene) [13]. The 1H and 13C NMR spectra also exhibited characteristic resonance consistent with the formation of poly(3-nhexylthiophene) [14]. In particular, no signal which extensive desilylation had occurred during the oxidation coupling. This was further confirmed upon Energy Dispersive X-ray spectroscopic Analysis of the crud film. A very low value was measured for the silicon to sulfur ratio (Si/S < 2. 10-2) in all cases.
Scheme 1 : Preparation of monomers (HexTSi, SiHexT, SiHexTSi)
Scheme 2 : Electrochemical preparation of polymers (PHexT, PSiHexT, PHexTSi, PsiHexTSi)
Scheme 3: Chemical preparation of polymers polymers (PHexT, PsiHexTSi)
3. RESULTS AND DISCUSSION
While polythiophene is insoluble and intractable, poly(3-nhexylthiophene) is soluble in common solvents such as THF or Chloroform and allows easy characterization. The molecular weight can be estimed using GPC chromatography. In table I, are reported the experimental values obtained after calibration of the GPC colon using polystyrene standards.
Table I : GPC data for poly(3-hexylthiophene) obtained by electrochemical polymerization of 3-hexylthiophene (HexT) and silylated 3-hexylthiophene (SiHexT, HexTSi, SiHexTSi).
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Monomers Polymers 10-3 . Mn Mw/Mn 10-2 . DPn
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HexT P(HexT) 21 4,5 1,2
SiHexT P(SiHexT) 57 2,5 3,4
HexTSi P(HexTSi) 55 2,9 3,2
SiHexTSi P(SiHexTSi) 1338 1,3 79,6
Table II : GPC data for poly(3-hexylthiophene) obtained by chemical polymerization of 3-hexylthiophene (HexT) and silylated 3-hexylthiophene (SiHexT, HexTSi, SiHexTSi).
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Monomers Polymers 10-3 . Mn Mw/Mn 10-2 . DPn
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HexT P(HexT) 38.6 2.4 1.96
SiHexTSi P(SiHexTSi) 116.8 1.9 5.96
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As shown in table I, high molecular weight we found after electropolymerization of a bis silylated 3-hexylthiophene (SiHexTSi). In electrogenerated polymers the average molecular weights, Mn is 64 times greater than that measured after elctropolymerization on non silylated monomers. At the same time, the polydipersity Mw/Mn was reduced by a factor of 3.4, interestingly, the polymerization of the monosilylthiophenes (SiHexT and HexTSi) gave a broader distribution of the molecular weights. Moreover the GPC trace was not unimodal and exhibited two peaks and shoulders. Two SiMe3 substituents in 2.5 positions of thiophene ring are necessary. Polymerization of mono silylated 3-alkylthiophene undergoes coupling either with loss of silyl group or loss of the proton. The increased acidity of the reaction medium resulted in competing protodesilylation of the monomer. Thus, the polymerization of mono silylated monomers is complex and gives a broad distribution of molecular weight. The same results were obtained by chemical polymerization (table II). The use of bis silylated monomers also led to polymers with higher average molecular weight and a low polydispersity than those for polymers made from non silylated monomers.
For bis silylated monomers (SiHexTSi) and after formation of the polymer film by electropolymerization, Me3SiF was isolated from the electrolytic mixture and characterized. The role of Me3Si group may be interpreted in terms of an increased stabilization of an intermediate silyl cationic species (scheme 4). The electropolymerization of thiophene proposed to occur via coupling of thiophene radical cation [15]. The coupling of two silylated thiophene radical cation, arising from oxidation of the monomer, would lead to a dimeric dication. The latter is stabilized by a hyperconjugative interaction between the electron deficient p molecular orbital of thiophene ring and a silyl substituent is known [16]. In the next step involving re-aromatisation with cleavage of the Si – F bond is likely to occur . the coupling of C- H bond would liberate a proton. The latter may react with the monomer to give upon protodesilylation, the non silylated was identified in the reaction mixture. Competitive electropolymerizations between silyl and non silyl thiophene units probably occur in the case of monomer HexT and HexTSi.
Scheme 4 : Stabilisation of cationic intermediate and cleavage of the Si-C bond
4. CONCLUSION
The SiMe3 group plays an important role in the oxidative polymerization. The presence of Me3Si groups at the 2 and 5 positions of the thiophene ring seemed necessary. The observed activation, which is proposed to arise from the stabilization of cationic intermediate (scheme 4); allows the preparation of high molecular weight of poly(3-hexylthiophene) with a low polymolecularity. We are currently investigating the use of this methodology for preparation of other conjugated polymers.
REFERENCES
[1] N. Basescu, Z. X. Liu, D. Moses, A. J. Hegger, N. Naarmann, N. Theophilou, Nature, 327, 403, 1987 ; Chemical and Engineering News, June, 22, 1987.
[2] G. Schopf, G. Kossmehl, Adv. Poly. Sci. 1, 34, 1997 and references therein.
[3] A. Pelter, I. Jenking, D. E. Jones, Tetrahedron, 30, Vol 53, 10357, 1997.
[4] B. Xu, S. Holdcroft, Macromolecules, 26, 4457, 1993.
[5] S. Osawa, M. Ito, S. Iwase, M. Yajima, R. Endo, K. Tomaka, Polymer, 33, 914, 1992.
[6] K. Yoshino, S. Nakajima, R. Suimoto, Jpn. J. Appl. Phys. 26, L1038, 1987.
[7] R. Hanna, M. Leclerc, Chem. Mater. 8, 1512, 1996.
[8] M. Leclerc, M. Frechette, J. Y. Bergeron, M. Ranger, I. Levesque, K. Faid, Macromol. Chem. Phys. 197, 2077, 1996.
[9] S. Patai, The chemistry of Organic Silicon Compounds, Wiley, NewYork, 1991.
[10] O. W. Webster, D. Y. Soah, W. Hertler, Macromolecules, 17, 1417,1984.
[11] M. Fontanille, G. Guyot, NATO ASI Series, 1987, 215, 61.
[12] T. A. Skotheim, Hanbook of Conducting Polymers, M. Dekker, NewYork, 1986
[13] M. Bouachrine, T. Lakhilifi, J. Chim. Phys. 95, 95, 987, 1998.
[14] T. A. Chen, X. O. Wu, R. D. Riecke, J. Am. Chem. Soc. , 117, 223, 1995.
[15] J. Roncali, Chem. Rev. 92, 711, 1992 and references therein.
[16] J. Duunogues, Ann. Chim. (Paris), 8, 135, 1983 and references therein.