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Synthesis and properties of polypyrrole derivatives with liquid crystalline substituents
ELSEVIER
Synthetic
Metals
Synthesis and properties of polypjmole
84 (1997)
177-178
derivatives with liquid crystalline substituents
H. Hasegawa, M. Kijima”
and H. Shirakawa
Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Otsfitute of Materials
Abstract 3- or N-Substituted liquid crystalline pyrroles with a cyclohesylphenyl or a biphenyl as a mesogenic core were synthesized and polymerized giving soluble and fusible polypyrroles. In case of poly(3- or N-substituted pyrrole)s, some mesophases were observed by polarization microscopy. The cast films of poly(3-substituted pyrrole)s showed electrical conductivities of lo’-10” Scm’r upon iodine doping. Keytr~ords:
Poly(S-substituted
pyrrole),
Poly(N-substituted
1. Introduction Recently, electrical conducting polymers with a liquid crystalline substituent have been studied, because they are expected to be an electrical anisotropical material whose main chains are oriented as a result of orientation of the mesogenic substituents by external force in a liquid crystalline phase. Polyacetylene derivatives with liquid crystalline property have been successfully investigated [l], but the conductivity was low because of decrease of coplanarity by the steric hindrance of the large substituent. On the other hand, liquid crystalline polypyrroles [2] or polythiophenes [3] show little success at the present because of difficulties of synthesis, although the conductivities of polypyrroles [4] or polythiophenes [5] were less decreased by introduction of substituents at the 3-position than substituted polyacetylenes [l, 61. In this paper, we investigate synthesis, properties and polymerization method of N- or 3-position substituted pyrrole with a cyclohexylphenyl or a biphenyl as a mesogenic core, and these polymers were characterized in order to develop a liquid crystalline polypyrrole with high conductivities and an electrical anisotropy.
2. Experimental Monomers 3-Substituted pyrroles, 4’-[6-(3-pyrrolyl)hesyloxy]biphenyl4carbonitrile (la) and 4-(4-pentylcyclohexyl)phenyl 6-(3pyrrolyl)hexyl ether (lb) were synthesized according to the reported method [7] (yields : la (34%), lb (87%)). N-Substituted pyrroles, 4’-[6-(l-pyrrolyl)hexyloxyJbiphenyl4-
pyrrole),
Liquid
Crystalline Polymer
carbonitrile (Za) and 4-(4-pentylcyclohexyl)phenyl 6-(lpyrrolyl)hexyl ether (Zb) were synthesized from 6-(lpyrrolyl)hexanol [8] and the phenolic derivative, 4-(4-pentylcyclohexyl)phenol or 4’-hydroxybiphenyl-4-carbonitrile, using Mitsunobu reaction [9] (yields : 2a (94%), 2b (34%)). Similarly, model compounds without pyrrole moiety, (3a, 3b) were synthesized from l-bromohexane and the phenolic derivative by Williamson synthesis.
Polymerization Electrochemical polymerization of monomers (0.05 mol dme3) was carried out using platinum electrode in acetonitrile or CHzCla containing 0.025 mol dmJ tetraethylammonium perchlorate (TEAP) as a supporting electrolyte under an argon atmosphere [lo]. Chemical polymerization of monomers (0.14 mol dme3) was carried out using FeC13 (0.1 mol dms3) and l-naphthalenesulfonic acid (0.04 mol dme3) in chloroform under an argon atmosphere [Z].
Measurements Thermal and optical properties of the monomers and polymers were investigated by differential scanning calorimetry (DSC) or polarizing optical microscopy (POM). Electrical conductivities of the polymers were measured by the four-probe method. Cyclic voltammograms (CV) of monomers ($33 mm01 dm”) were measured in acetonitrile or CH$& containing 0.1 mol dmm3 tetrabutylammonium pcrchlorate (TBAP) with a Beckman Pt disc electrode as the working, a Pt wire as the counter, and a saturated calomel electrode (SCE) as the reference under an argon atmosphere.
(G-I&-O-R 14-s r;” H
Cfi(CHz.)~-0-R 1
R=a: /J---y i;T
Table
3
SCN
2 b : ~GHII
0379-6779/97/$17.00
Q 1997 Elsevier
and electrochemical
properties
Science Sk
All rights rexrved
of monomers.
Epmb 1st
2nd
C*87(*N*70)*Ic C*65(.S*30j*Id
1.36. 1.16;
1.75 1.57
C*85(*N*14)*Id C*35(*N--2 )*ld C-40 l N* 80 *Id C.24 l N* 41 *Id
1.34, 1.79 1.35, 1.67 1.83
bAnodic peak potential a ( ) : Monotropic transition. cDetermined from DSC. d Determined from POM.
author.
PII SO379-6779(96)03892-1
optical,
Phase transition temperaturea
CC) la lb 2a 2b 3a 3b
( CH2),5-0-R
* Corresponding
1. Thermal,
Monomer
-
1.84
values vs. SCE.
H. Hasegawa et al. /Synthetic Metals 84 (1997) 177-l 78
178
Table 2. Synthesis and properties of Pl and P2. Monomer Methoda Yield ~~~ Phase transition temperatureC MwlMnb A mnxd Conductivity e (Scm”) (%) CC) W-4 A 39 8800 1.50 infusible * la 10-7f 11 5000 B 1.47 K* 90(*N* 60 )*I 480 10” 4800 ... ._.....,,.. .... . C... ......... .90 ... ... .. .... .. ........2.18 . ........ ....K.........130 .............I................. . . ......446 ...............................1O4 .................... 50 lb A 8200 1.50 infusible * B 77 2500 1.46 K*130(*S*102)*1 390 1o’3 .. ...... ....................C .... .. ... . . .. ..96 ... .,,......... ..4000 .....,. . .......... 3.05 ......................K*145(*N*135)*1 . ....... . ... . . .. .. . ............ ......430 ....... ........ ........10” ...... ........... 2a B 3 10300 1.27 K* 90.1 * ... ... ..........................C ........ . . .48 .. ......... . 3300 ......... .. .... ... 2.67 ............. . ... ..........K...,,.’....120 ..... .. ’ . I. . . ......................*..............................-.............. .. 2b B 19 6500 1.46 K* 75 I * C 55 3300 3.55 K*170(*S*150)*1 * 1o-s l
l
l
a A: Galvanostatic. B: Potentiostatic. C: Chemical. b Determined by GPC vs. polystyrene absorption. e Cast films on a glass plate upon 12 doping. ‘Free standing cast film.
3. Resultsan& Discussion Thermal and electrical properties of the polymers were generally reflected by properties of monomers. Thus, thermal and electrochemical properties of monomers (1, 2) and the models (3) were investigated and summarized in Table 1. All monomers showed a liquid crystalline phase in the cooling process. CV of monomers (1, 2) showed two oxidation peaks around 1.3 and 1.8 V. In order to assign these peaks, oxidation potential of the hexyl ether with mesogenic core (3) were measured to be about 1.8 V. Thus, the first oxidation peak potentials around 1.3 V are attributed to the oxidation of pyrrole ring by comparing with the data of alkyl substituted pyrroles [6]. Polymerization of pyrrole derivatives are usually carried out by chemical or galvanostatic manners. At first we polymerized 1 by galvanostatic electrochemical The soluble components of the product were method. infusible and the UV/vis spectra showed weak absorptions tailing to 600 nm. In this case electrolytic potential increased to about 3-10 V according to the reaction proceeding. The high electrolytic potential might cause side reactions such as irregular couplings between pyrrole moiety and mesogenic phenyl ether moiety, making the polymer infusible, which can be presumed from results of CV in Table 1. In order to obtain fusible polymers without side reactions, selective oxidative polymerization at the pyrrole ring is necessary. One approach of this is a potentiostatic electrochemical polymerization at a potential lower than the second Epa. Potentiostatic electrochemical polymerizations around first Esa of the pyrrole ring gave mostly soluble and fusible S-Substituted polymers (Pla, Plb) showedan polymers. absorption peak around 450 nm due to a n-n* transition of the conjugated main chain. Oligomeric components (Mn i 5000)of Pla and Plb had a mesophase, Schlieren (in case of Pla) and fan-shaped (in caseof Plb) textures were observed in the cooling process, respectively, although components with the higher molecular weight (Mnk 7000) showed no mesophase. Anothkr approach for the selective oxidation is a chemical polymerization using FeCla and I-naphthalenesulfonic acid [2]. Chemical polymerization of the monomers gave perfectly soluble and fusible polymers but with high polydispersities. S-Substituted nm due to
polymers
a X-X*
showed
transition
an absorption
peak
around
450
of the conjugated main chain.
standard.
’ ( ) : Monotropic
transition.
d * : A tailing
Oligomeric components of Plb had an unidentifiable mesophase, while those of P2b showed a polygonal texture in the cooling processes. More exact characterizations and identification of all mesophases are under way. In conclusion, potentiostatic and chemical polymerizations gave fusible polymers whose oligomeric components had a mesophase, although galvanostatic polymerization gave infusible polymers with low conductivities (Table 2). This suggests that these approaches for the selective oxidation successfully lead to the synthesis of these liquid crystalline polypyrroles. Nevertheless, an improvement of these polymerization conditions is necessary in order to obtain liquid crystalline polypyrroles with a stable enatiotropic mesophase, a high conductivity, a high molecular weight, and a good processability.
Acknowledgment This work was supported by grant-in-aid Society for the Promotion of Science.
from Japan
References
PI
PI [31 141 [51
Fl [71
PI PI [lo]
K. Akagi, H. Goto, Y. Kadokura, H, Shirakawa and K. Araya, Synth. Met., 69 (1995) 13., S. H. Jin, S. J. Choi, W. Ahn, H. N. Cho, and S. K. Choi, Macromolecules, 26 (1993) 1487., J. L. Moigne, A. Hilkerer and F. Kajzar, Makromol. Chem., 192 (1991) 515. F. Vicentini, J. Barrouillet, R. Laversanne, M. Mauzac, F. Bibonne and J. P. Parneix, Liquid Crystals, 2 (1995) 235. N. Koide and H. Iida, Mol. Cryst. Liq. Cry-f., 261 (1995) 427. G. Zotti, G. Schiavon, A. Berlin and G. Pagani, Synth. Met., 40 (1991) 299. M. Sato, S. Tanaka and K Kaeriyama, Synih Met., 28 (1987) 229. M. A. Petit and A. H. Soum, J. Polym. &‘ci.: Part B: Polym, Phys., 25 (1987) 423. P. J. Langley, F. J. Davis and G. R. Mitchell, Mol. Cryst. Liq. Cryst., 236 (1993) 225. A. Bettelheim, B. A. White, S. A. Rayback and R. W. Murray, Inorg. Gem., 26 (1987) 1009. M. S. Manhas, W. H. Hoffman, B. I-al, and A. K. Bose,J. Chem. Sci. Perkin Trans. I, (1975) 461. H. Masuda, S. Tanaka and K. Kaeriyama, J. Polym Sci.: PurtA: Polym. Chem., 28 (1990) 1831.
Synthetic
Metals
Synthesis and properties of polypjmole
84 (1997)
177-178
derivatives with liquid crystalline substituents
H. Hasegawa, M. Kijima”
and H. Shirakawa
Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Otsfitute of Materials
Abstract 3- or N-Substituted liquid crystalline pyrroles with a cyclohesylphenyl or a biphenyl as a mesogenic core were synthesized and polymerized giving soluble and fusible polypyrroles. In case of poly(3- or N-substituted pyrrole)s, some mesophases were observed by polarization microscopy. The cast films of poly(3-substituted pyrrole)s showed electrical conductivities of lo’-10” Scm’r upon iodine doping. Keytr~ords:
Poly(S-substituted
pyrrole),
Poly(N-substituted
1. Introduction Recently, electrical conducting polymers with a liquid crystalline substituent have been studied, because they are expected to be an electrical anisotropical material whose main chains are oriented as a result of orientation of the mesogenic substituents by external force in a liquid crystalline phase. Polyacetylene derivatives with liquid crystalline property have been successfully investigated [l], but the conductivity was low because of decrease of coplanarity by the steric hindrance of the large substituent. On the other hand, liquid crystalline polypyrroles [2] or polythiophenes [3] show little success at the present because of difficulties of synthesis, although the conductivities of polypyrroles [4] or polythiophenes [5] were less decreased by introduction of substituents at the 3-position than substituted polyacetylenes [l, 61. In this paper, we investigate synthesis, properties and polymerization method of N- or 3-position substituted pyrrole with a cyclohexylphenyl or a biphenyl as a mesogenic core, and these polymers were characterized in order to develop a liquid crystalline polypyrrole with high conductivities and an electrical anisotropy.
2. Experimental Monomers 3-Substituted pyrroles, 4’-[6-(3-pyrrolyl)hesyloxy]biphenyl4carbonitrile (la) and 4-(4-pentylcyclohexyl)phenyl 6-(3pyrrolyl)hexyl ether (lb) were synthesized according to the reported method [7] (yields : la (34%), lb (87%)). N-Substituted pyrroles, 4’-[6-(l-pyrrolyl)hexyloxyJbiphenyl4-
pyrrole),
Liquid
Crystalline Polymer
carbonitrile (Za) and 4-(4-pentylcyclohexyl)phenyl 6-(lpyrrolyl)hexyl ether (Zb) were synthesized from 6-(lpyrrolyl)hexanol [8] and the phenolic derivative, 4-(4-pentylcyclohexyl)phenol or 4’-hydroxybiphenyl-4-carbonitrile, using Mitsunobu reaction [9] (yields : 2a (94%), 2b (34%)). Similarly, model compounds without pyrrole moiety, (3a, 3b) were synthesized from l-bromohexane and the phenolic derivative by Williamson synthesis.
Polymerization Electrochemical polymerization of monomers (0.05 mol dme3) was carried out using platinum electrode in acetonitrile or CHzCla containing 0.025 mol dmJ tetraethylammonium perchlorate (TEAP) as a supporting electrolyte under an argon atmosphere [lo]. Chemical polymerization of monomers (0.14 mol dme3) was carried out using FeC13 (0.1 mol dms3) and l-naphthalenesulfonic acid (0.04 mol dme3) in chloroform under an argon atmosphere [Z].
Measurements Thermal and optical properties of the monomers and polymers were investigated by differential scanning calorimetry (DSC) or polarizing optical microscopy (POM). Electrical conductivities of the polymers were measured by the four-probe method. Cyclic voltammograms (CV) of monomers ($33 mm01 dm”) were measured in acetonitrile or CH$& containing 0.1 mol dmm3 tetrabutylammonium pcrchlorate (TBAP) with a Beckman Pt disc electrode as the working, a Pt wire as the counter, and a saturated calomel electrode (SCE) as the reference under an argon atmosphere.
(G-I&-O-R 14-s r;” H
Cfi(CHz.)~-0-R 1
R=a: /J---y i;T
Table
3
SCN
2 b : ~GHII
0379-6779/97/$17.00
Q 1997 Elsevier
and electrochemical
properties
Science Sk
All rights rexrved
of monomers.
Epmb 1st
2nd
C*87(*N*70)*Ic C*65(.S*30j*Id
1.36. 1.16;
1.75 1.57
C*85(*N*14)*Id C*35(*N--2 )*ld C-40 l N* 80 *Id C.24 l N* 41 *Id
1.34, 1.79 1.35, 1.67 1.83
bAnodic peak potential a ( ) : Monotropic transition. cDetermined from DSC. d Determined from POM.
author.
PII SO379-6779(96)03892-1
optical,
Phase transition temperaturea
CC) la lb 2a 2b 3a 3b
( CH2),5-0-R
* Corresponding
1. Thermal,
Monomer
-
1.84
values vs. SCE.
H. Hasegawa et al. /Synthetic Metals 84 (1997) 177-l 78
178
Table 2. Synthesis and properties of Pl and P2. Monomer Methoda Yield ~~~ Phase transition temperatureC MwlMnb A mnxd Conductivity e (Scm”) (%) CC) W-4 A 39 8800 1.50 infusible * la 10-7f 11 5000 B 1.47 K* 90(*N* 60 )*I 480 10” 4800 ... ._.....,,.. .... . C... ......... .90 ... ... .. .... .. ........2.18 . ........ ....K.........130 .............I................. . . ......446 ...............................1O4 .................... 50 lb A 8200 1.50 infusible * B 77 2500 1.46 K*130(*S*102)*1 390 1o’3 .. ...... ....................C .... .. ... . . .. ..96 ... .,,......... ..4000 .....,. . .......... 3.05 ......................K*145(*N*135)*1 . ....... . ... . . .. .. . ............ ......430 ....... ........ ........10” ...... ........... 2a B 3 10300 1.27 K* 90.1 * ... ... ..........................C ........ . . .48 .. ......... . 3300 ......... .. .... ... 2.67 ............. . ... ..........K...,,.’....120 ..... .. ’ . I. . . ......................*..............................-.............. .. 2b B 19 6500 1.46 K* 75 I * C 55 3300 3.55 K*170(*S*150)*1 * 1o-s l
l
l
a A: Galvanostatic. B: Potentiostatic. C: Chemical. b Determined by GPC vs. polystyrene absorption. e Cast films on a glass plate upon 12 doping. ‘Free standing cast film.
3. Resultsan& Discussion Thermal and electrical properties of the polymers were generally reflected by properties of monomers. Thus, thermal and electrochemical properties of monomers (1, 2) and the models (3) were investigated and summarized in Table 1. All monomers showed a liquid crystalline phase in the cooling process. CV of monomers (1, 2) showed two oxidation peaks around 1.3 and 1.8 V. In order to assign these peaks, oxidation potential of the hexyl ether with mesogenic core (3) were measured to be about 1.8 V. Thus, the first oxidation peak potentials around 1.3 V are attributed to the oxidation of pyrrole ring by comparing with the data of alkyl substituted pyrroles [6]. Polymerization of pyrrole derivatives are usually carried out by chemical or galvanostatic manners. At first we polymerized 1 by galvanostatic electrochemical The soluble components of the product were method. infusible and the UV/vis spectra showed weak absorptions tailing to 600 nm. In this case electrolytic potential increased to about 3-10 V according to the reaction proceeding. The high electrolytic potential might cause side reactions such as irregular couplings between pyrrole moiety and mesogenic phenyl ether moiety, making the polymer infusible, which can be presumed from results of CV in Table 1. In order to obtain fusible polymers without side reactions, selective oxidative polymerization at the pyrrole ring is necessary. One approach of this is a potentiostatic electrochemical polymerization at a potential lower than the second Epa. Potentiostatic electrochemical polymerizations around first Esa of the pyrrole ring gave mostly soluble and fusible S-Substituted polymers (Pla, Plb) showedan polymers. absorption peak around 450 nm due to a n-n* transition of the conjugated main chain. Oligomeric components (Mn i 5000)of Pla and Plb had a mesophase, Schlieren (in case of Pla) and fan-shaped (in caseof Plb) textures were observed in the cooling process, respectively, although components with the higher molecular weight (Mnk 7000) showed no mesophase. Anothkr approach for the selective oxidation is a chemical polymerization using FeCla and I-naphthalenesulfonic acid [2]. Chemical polymerization of the monomers gave perfectly soluble and fusible polymers but with high polydispersities. S-Substituted nm due to
polymers
a X-X*
showed
transition
an absorption
peak
around
450
of the conjugated main chain.
standard.
’ ( ) : Monotropic
transition.
d * : A tailing
Oligomeric components of Plb had an unidentifiable mesophase, while those of P2b showed a polygonal texture in the cooling processes. More exact characterizations and identification of all mesophases are under way. In conclusion, potentiostatic and chemical polymerizations gave fusible polymers whose oligomeric components had a mesophase, although galvanostatic polymerization gave infusible polymers with low conductivities (Table 2). This suggests that these approaches for the selective oxidation successfully lead to the synthesis of these liquid crystalline polypyrroles. Nevertheless, an improvement of these polymerization conditions is necessary in order to obtain liquid crystalline polypyrroles with a stable enatiotropic mesophase, a high conductivity, a high molecular weight, and a good processability.
Acknowledgment This work was supported by grant-in-aid Society for the Promotion of Science.
from Japan
References
PI
PI [31 141 [51
Fl [71
PI PI [lo]
K. Akagi, H. Goto, Y. Kadokura, H, Shirakawa and K. Araya, Synth. Met., 69 (1995) 13., S. H. Jin, S. J. Choi, W. Ahn, H. N. Cho, and S. K. Choi, Macromolecules, 26 (1993) 1487., J. L. Moigne, A. Hilkerer and F. Kajzar, Makromol. Chem., 192 (1991) 515. F. Vicentini, J. Barrouillet, R. Laversanne, M. Mauzac, F. Bibonne and J. P. Parneix, Liquid Crystals, 2 (1995) 235. N. Koide and H. Iida, Mol. Cryst. Liq. Cry-f., 261 (1995) 427. G. Zotti, G. Schiavon, A. Berlin and G. Pagani, Synth. Met., 40 (1991) 299. M. Sato, S. Tanaka and K Kaeriyama, Synih Met., 28 (1987) 229. M. A. Petit and A. H. Soum, J. Polym. &‘ci.: Part B: Polym, Phys., 25 (1987) 423. P. J. Langley, F. J. Davis and G. R. Mitchell, Mol. Cryst. Liq. Cryst., 236 (1993) 225. A. Bettelheim, B. A. White, S. A. Rayback and R. W. Murray, Inorg. Gem., 26 (1987) 1009. M. S. Manhas, W. H. Hoffman, B. I-al, and A. K. Bose,J. Chem. Sci. Perkin Trans. I, (1975) 461. H. Masuda, S. Tanaka and K. Kaeriyama, J. Polym Sci.: PurtA: Polym. Chem., 28 (1990) 1831.