- Email: [email protected]
The routes towards processible and stable conducting poly(thiophene)s
Synthetic Metals, 55-57 (1993) 1221-1226
122 1
T H E R O U T E S T O W A R D S P R O C E S S I B L E AND S T A B L E CONDUCTING POLY(THIOPHENE)S
O. P EI , 0. INGANJi,S, G. GUSTAFSSON$ AND M. GRANSTROM Laboratory of Applied Physics, Dept. of Physics, IFM, LinkSping University, S-581 83, Link6ping, Sweden SUNIAX Corp., 5357 Overpass Road, Santa Barbara, CA 93111, USA M. ANDERSSON, T. HJERTBERG£ AND O. WENNERSTROM Dept. of Organic Chem. and £Polymer Techn., Chalmers University of Technology S-412 96, GSteborg, Sweden J.E. OSTERHOLM, J. LAAKSO AND H. J,i~RVINEN Neste Oy Research Center, Kulloo, SF-06850, Finland
ABSTRACT The underlying nature of the thermal instability of doped poly(3alkylthiophene)s, a fatal problem for their application, is discussed. The so-called "thermal undoping" is attributed to interactions due to steric hindrance of the long flexible side chains, which twist the conjugated main chain and kick out the dopants. Thus the routes to suppress thermal undoping are to avoid or alleviate the side chain interactions by separating the side chains from each other, or from the main chains, and also leaving space around the main chains to accommodate dopants. Accordingly, poly(3-(4-octylphenyl)thiophene), random copolymers of 3methylthiophene and 3-octylthiophene, and regular copolymers of thiophene and 3octylthiophene are prepared. Thermal undoping is significantly suppressed in these polymers. Some of these polymers are soluble and fusible in the neutral state. After doping they become highly conductive, yet remain stable even at elevated temperatures. Thermochromism, solvatochromism and thermal undoping are thus all related to side chain mobility.
INTRODUCTION Conducting polymers are usually intractable due to the rigid conjugated main chains. Many efforts have been made in the search for processible conducting polymers[l]. One major advance in recent years is the solubilization of 0379-6779/93/$6.00
© 1993 - Elsevier Sequoia. All rights reserved
1222
conjugated polymers by attaching long flexible side chains onto the conjugated main chains. Poly(3-alkylthiophene)s or PATs with the alkyl side chains being nbutyl or longer are a notable example[2]. The addition of long alkyl chains not only renders the polymer soluble and in some cases fusible, but also results in such associated p h e n o m e n a as thermochromism and solvatochromism[3-5]. The driving force is believed to be the torsion along the conjugated main chains, caused by steric interaction between the bulky side chains after thermal excitation or in good solvents. Unfortunately, all PATs doped either chemically or electrochemically are unstable, especially at elevated temperatures[6,7]. The i n s t a b i l i t y of P3AT increases with increasing length of the side chains. The m e c h a n i s m of this "thermal undoping" has been suggested to be due to the side chain mobility[8,9]. Torsion of the conjugated main chain and the kicking out of the dopant anions by the mobile side groups causes the thermal undoping, a mechanism similar to that driving the t h e r m o c h r o m i s m in n e u t r a l PATs. An a l t e r n a t i v e i n t e r p r e t a t i o n attributes thermal undoping to the high oxidation potentials of PAT[6]. The doped PAT is such a strong oxidant t h a t it readily retrieves electrons from somewhere, especially when the main chains have been distorted by the side chains and the oxidation potentials increase further. Possible electron sources are the dopants and methylene groups on the alkyl chains adjacent to the thiophene rings. However, whatever happens during the thermal undoping, the instability of conducting PAT is an intrinsic property of the polymers, due to the introduction of long flexible side chains. Some other processible conducting polymers have also been prepared which usually contain long side groups to achieve processibility. To our knowledge at least some of them are inevitably unstable[10,11], probably due to the attached long side groups as well. There is thus an conflict between obtaining processible conducting polymers by attaching long side chains to the conjugated main chain, and the instability the attached side chains bring about. Based on the above analysis, the routes to avoid thermal undoping should be to avoid or alleviate the side chain interaction by separating the side chains from each other or from the main chains and leaving space around the main chains to accommodate dopants[12,13]. To achieve this goal, poly(3-(4-octylphenyl)thiophene) (POPT), random copolymers of 3-methylthiophene and 3-octylthiophene (POTMT), and regular copolymers from thiophene and 3-octylthiophene with well-distributed octyl side groups (PTOT and PDTOT) were prepared. All of these polymers are soluble in chloroform, and POTMT, PTOT, and PDTOT are also fusible. In POPT, the side chains are separated from the main chain by a stiff benzene bridge. The rigidity of the main chain is expected to be enhanced and the effect of side chain interaction on the main chain alleviated. In the other polymers the side chains are
1223
separated from each other in order to avoid side chain interaction and to leave space for the dopants. 1. PQPT A POPT chain is more rigid than a poly(3-octylthiophene) (POT) chain as indicated by its peculiar solvatochromism in chloroform/methanol solutions and its suppressed t h e r m o c h r o m i s m at elevated temperatures[13]. The optical absorption spectrum of POPT in a pure chloroform solution shows a sharp peak at 2.7eV due to bandgap absorption, with a few minor peaks at the lower energy side. When adding methanol to the solution, the absorption intensity of the sharp peak decreases without energy shift, while the absorption in the region 2-2.5eV increases. We suggest that there are at least two phases present in the solutions. One is the planar, u-electron extended, and one is the twisted POPT chains. There occurs twisted-to-planar main chain transition as more methanol is added. POPT is not as rigid as poly(3-phenylthiophene), resulting from the competing effects of enhanced n-electron extension due to the phenyl bridge and steric hindrance due to the long octyl group. POPT is infusible. Electrochemical doping and undoping of POPT in an acetonitrile solution shows polaron formation at low doping levels with four absorption peaks at 2.5, 1.5, 1.2, and <0.5 eV. At deep doping levels, bipolarons are dominating. When heating FeC13-doped POPT at 110 °C, we saw a similar bipolaron-to-polaron transition as the material was thermally undoped. Our interpretation of this phenomenon is main chain torsion due to the thermally enhanced side chain interactions, bipolaron dissociation and polaron stabilization. The conductivity stability of doped POPT has been improved, compared to POT (Fig. 1), but is not as good as desired. In the following will show that separating the flexible side chains is a more efficient way to stabilize doped poly(thiophene)s. 2. PQTMT POTMT r a n d o m copolymers were p r e p a r e d by e l e c t r o c h e m i c a l l y copolymerizing 3-methylthiophene and 3-octylthiophene from acetonitrile solutions containing these monomers and LiC104 as the supporting electrolyte, with subsequent electrochemical reduction. Ratios of the monomer units in the resulting copolymers, MT/OT, are close to the feed ratios of the preparation solutions. S o l v a t o c h r o m i s m in c h l o r o f o r m / m e t h a n o l s o l u t i o n s and thermochromism in solution-cast thin films are significantly alleviated in those POTMTs with high MT/OT ratios, indicative of a very rigid polythiophene main chain. Steric interactions between the side chains have been efficiently suppressed. The stability of doped POTMTs with various MT/OT ratios is displayed by the evolution of their optical absorption spectrum at 110 °C in vacuum, as shown in
1224
~ 0.8 o~
-I
P O T M T (o~
÷
÷
0
D
~-2
MT
=00.6
0
÷
÷ 0
"t't+ +
0
-3
;.~0.4
POPT
0 0 0
OO
-4
o POT . 5 , 1 , 1 , 1 , 1 , 1
0
20
40
.
I
,
60
T i m e (rain)
I
.
I
80
.
I
,
0
0
40
80
120
T i m e (min)
Fig. 1. Conductivity decay of some substituted poly(thiophene)s at 110°C in lab air. The samples were solution-cast thin films of around ll~m thickness, doped with FeCI3,eH20 in acetonitrile. Fig. 2. Intensity evolution at 110 °C in vacuum of the NIR peak at 0.8 eV on the optical absorption spectra of POTMT(CIO4") films doped in acetonitrile at 1.2 V vs Ag/AgCI. MT/OT ratios shown in the fig. are the feed ratios for copolymerization.
Fig. 2. The stability is improved to a greater extent with higher MT/OT ratio. FeCI3doped POTMT (2MT/1OT) is very stable. Its conductivity decreases only a little after two hours' heating at 110 °C in lab air(Fig. 1). Cyclic voltammetry indicates a high redox potential in this copolymer, close to t h a t in PATs. T h u s the t h e r m a l undoping in PATs is not due to their high oxidation potentials. It is r a t h e r due to side chain mobility and the kicking out of the dopants by the mobile side chains. To confirm this argument, we r a n the optical absorption spectrum evolution at 110 °C in v a c u u m of POTMT (2MT/IOT) films doped with dopants of different sizes, e.g., perchlorate(C104-), tosylate(TsO'), a n d dodecylbenzene sulfonate(DBS'). Fig.3 clearly shows the effect of dopant size on the stability of the copolymer. Bulky dopants can easily be kicked out, while small dopant can not.
POTMT are random copolymers, a n d there m u s t be some OT-to-OT linkages which will cause some side chain interactions. The better material is t h a t with a regular structure and absent OT-to-OT linkages. Thus PTOT with octyl groups attached to every second thiophene unit on a polythiophene chain and PDTOT with octyl groups attached to every third thiophene unit on a polythiophene chain were
1225
synthesized. Solvatochromism and t h e r m o c h r o m i s m in these regular copolymers were investigated as above. They indicate t h a t the rigidity of a PTOT chain is lower t h a n t h a n of a PDTOT chain, with the rigidity of a POTMT (2MT/1OT) random copolymer chain lying between. This r a n d o m copolymer has a density of long side chains a little bit higher than PDTOT. Thus both side chain density and OT-to-OT linkages are crucial to the rigidity of the conjugated polymer m a i n chains. The methyl groups on POTMT may also exert some effects. The conductivity stability of the two regular copolymers, doped with FeC13, are shown in Fig. 1. PTOT is as stable as the random copolymer, while PDTOT is even more stable. The evolution of the optical absorption spectrum of the doped copolymers at l l 0 ° C in vacuum indicates a stability sequence of PTOT
1
0.9
o 0.8 ,.Q
~
~
i
. . . .
I
. . . .
I
. . . .
I
,
,
,
,
P O T M T (2MT/IOT)
0.8 o.6
.8
< 0.7
<
~
. . . .
\
o.4
0.6
~ 0.2
DBS-
Z
0
. . . ,
0
. . . .
10
,
. . . .
20
t
. . . .
30
,
. . . .
4O
0.5 i
. . . .
50
Time (min)
i . . .
60
0.4
7O
0
................... 5 10
'
15
20
T i m e (h)
Fig. 3. Intensity evolution at 110 °C in vacuum of the NIR peak at 0.8 eV on the optical absorption spectra of POTMT(2MT/1OT) films electrochemically doped in acetonitrile. The dopants are shown in the figure. Fig. 4. Intensity evolution at 110 °C in vacuum of the NIR peak at 0.8 eV on the optical absorption spectra of three substituted poly(thiophene) films doped with FeCI3o6H20 from acetonitrile.
.... 25
1226
CONCLUSIONS Solvatochromism and thermochromism in POT are both driven by the mobile side chains. Decreasing the side chain density and number of OT-to-OT links will alleviate these chromisms. Thermal undoping in doped POT is also due to the steric effects of the side chains. It may be suppressed by separating the OT units with 3-methylthiophene or thiophene units. The insolubility and infusibility of most conjugated polymers are due to both the high rigidity of the main chains and interchain %-electron extension. Without the interchain ~-electron extension, most polymers would be soluble and fusible, provided they were free of crosslink and their molecular weights were not very high. Dilutely attached long flexible side chains may separate conjugated main chains from each other in a good solvent or at an elevated temperature. Then slight distortion along the main chain will render the polymer soluble and fusible. In this specifically tailored conjugated polymer, free space is available along the main chains to accommodate dopants so that the doped polymer is stable. REFERENCES [1] J.R. Reynolds and M. Pomerantz, in "Electroresponsive Molecular and Polymeric Systems", ed. T. A. Skotheim, Dekker, New York, 1991, Vol.2, p. 187. [2] G. Gustafsson, O. Ingan~s, W. R. Salaneck, J. Laakso, M. Loponen, T. Taka, J. E. 0sterholm, H. Stubb, and T. Hjertberg, in "Conjugated Polymers", Eds. J. L. Bredas and R. Silbey, Kluwer Academic, Dordrecht, 1991, p. 315. [3] S. D. D. Rughoopugh, A. J. Heeger, and F. Wudl, J. Polym. Sci., Polym. Phys. Ed., 25(1987)101. [4] O. Ingan~s, W. R. Salaneck, J.-E. Osterholm, and H. Laakso, Synth. Met., 22 (1988)395. [5] W. R. Salaneck, O. Ingan~s, J. E. Osterholm, B. Themans, and J. L. Bredas, Synth. Met., 28(1989)451. [6] G. Gustafsson, O. Ingan~s, J. O. Nilsson, and B. Liedberg, Synth. Met., 26 (1988)297. [7] Y. Wang and M. F. Rubner, Synth. Met., 39(1990)153. [8] J. T. Lopez-Navarrete and G. Zerbi, Chem. Phys. Lett., 175(1990)125. [9] M. GranstrOm and O. Ingan~s, Synth. Met., 48(1992) 21. [10] M. Rehahn, A. D. Schluter, G. Wegner, and W. J. Feast, Polymer, 30(1989) 1054. [11] H. Masuda, S. Tanaka, and K. Kaeriyama, J. Chem. Soc. Chem. Commun., (1989)725. [12] Q. Pei and O. Ingan~s, Synth. Met., 46(1992)353. [13] Q. Pei, H. J~irvinen, J. E. 0sterholm, O. Ingan~is, and J. Laakso, Macromolecules, (1992), in the press.
122 1
T H E R O U T E S T O W A R D S P R O C E S S I B L E AND S T A B L E CONDUCTING POLY(THIOPHENE)S
O. P EI , 0. INGANJi,S, G. GUSTAFSSON$ AND M. GRANSTROM Laboratory of Applied Physics, Dept. of Physics, IFM, LinkSping University, S-581 83, Link6ping, Sweden SUNIAX Corp., 5357 Overpass Road, Santa Barbara, CA 93111, USA M. ANDERSSON, T. HJERTBERG£ AND O. WENNERSTROM Dept. of Organic Chem. and £Polymer Techn., Chalmers University of Technology S-412 96, GSteborg, Sweden J.E. OSTERHOLM, J. LAAKSO AND H. J,i~RVINEN Neste Oy Research Center, Kulloo, SF-06850, Finland
ABSTRACT The underlying nature of the thermal instability of doped poly(3alkylthiophene)s, a fatal problem for their application, is discussed. The so-called "thermal undoping" is attributed to interactions due to steric hindrance of the long flexible side chains, which twist the conjugated main chain and kick out the dopants. Thus the routes to suppress thermal undoping are to avoid or alleviate the side chain interactions by separating the side chains from each other, or from the main chains, and also leaving space around the main chains to accommodate dopants. Accordingly, poly(3-(4-octylphenyl)thiophene), random copolymers of 3methylthiophene and 3-octylthiophene, and regular copolymers of thiophene and 3octylthiophene are prepared. Thermal undoping is significantly suppressed in these polymers. Some of these polymers are soluble and fusible in the neutral state. After doping they become highly conductive, yet remain stable even at elevated temperatures. Thermochromism, solvatochromism and thermal undoping are thus all related to side chain mobility.
INTRODUCTION Conducting polymers are usually intractable due to the rigid conjugated main chains. Many efforts have been made in the search for processible conducting polymers[l]. One major advance in recent years is the solubilization of 0379-6779/93/$6.00
© 1993 - Elsevier Sequoia. All rights reserved
1222
conjugated polymers by attaching long flexible side chains onto the conjugated main chains. Poly(3-alkylthiophene)s or PATs with the alkyl side chains being nbutyl or longer are a notable example[2]. The addition of long alkyl chains not only renders the polymer soluble and in some cases fusible, but also results in such associated p h e n o m e n a as thermochromism and solvatochromism[3-5]. The driving force is believed to be the torsion along the conjugated main chains, caused by steric interaction between the bulky side chains after thermal excitation or in good solvents. Unfortunately, all PATs doped either chemically or electrochemically are unstable, especially at elevated temperatures[6,7]. The i n s t a b i l i t y of P3AT increases with increasing length of the side chains. The m e c h a n i s m of this "thermal undoping" has been suggested to be due to the side chain mobility[8,9]. Torsion of the conjugated main chain and the kicking out of the dopant anions by the mobile side groups causes the thermal undoping, a mechanism similar to that driving the t h e r m o c h r o m i s m in n e u t r a l PATs. An a l t e r n a t i v e i n t e r p r e t a t i o n attributes thermal undoping to the high oxidation potentials of PAT[6]. The doped PAT is such a strong oxidant t h a t it readily retrieves electrons from somewhere, especially when the main chains have been distorted by the side chains and the oxidation potentials increase further. Possible electron sources are the dopants and methylene groups on the alkyl chains adjacent to the thiophene rings. However, whatever happens during the thermal undoping, the instability of conducting PAT is an intrinsic property of the polymers, due to the introduction of long flexible side chains. Some other processible conducting polymers have also been prepared which usually contain long side groups to achieve processibility. To our knowledge at least some of them are inevitably unstable[10,11], probably due to the attached long side groups as well. There is thus an conflict between obtaining processible conducting polymers by attaching long side chains to the conjugated main chain, and the instability the attached side chains bring about. Based on the above analysis, the routes to avoid thermal undoping should be to avoid or alleviate the side chain interaction by separating the side chains from each other or from the main chains and leaving space around the main chains to accommodate dopants[12,13]. To achieve this goal, poly(3-(4-octylphenyl)thiophene) (POPT), random copolymers of 3-methylthiophene and 3-octylthiophene (POTMT), and regular copolymers from thiophene and 3-octylthiophene with well-distributed octyl side groups (PTOT and PDTOT) were prepared. All of these polymers are soluble in chloroform, and POTMT, PTOT, and PDTOT are also fusible. In POPT, the side chains are separated from the main chain by a stiff benzene bridge. The rigidity of the main chain is expected to be enhanced and the effect of side chain interaction on the main chain alleviated. In the other polymers the side chains are
1223
separated from each other in order to avoid side chain interaction and to leave space for the dopants. 1. PQPT A POPT chain is more rigid than a poly(3-octylthiophene) (POT) chain as indicated by its peculiar solvatochromism in chloroform/methanol solutions and its suppressed t h e r m o c h r o m i s m at elevated temperatures[13]. The optical absorption spectrum of POPT in a pure chloroform solution shows a sharp peak at 2.7eV due to bandgap absorption, with a few minor peaks at the lower energy side. When adding methanol to the solution, the absorption intensity of the sharp peak decreases without energy shift, while the absorption in the region 2-2.5eV increases. We suggest that there are at least two phases present in the solutions. One is the planar, u-electron extended, and one is the twisted POPT chains. There occurs twisted-to-planar main chain transition as more methanol is added. POPT is not as rigid as poly(3-phenylthiophene), resulting from the competing effects of enhanced n-electron extension due to the phenyl bridge and steric hindrance due to the long octyl group. POPT is infusible. Electrochemical doping and undoping of POPT in an acetonitrile solution shows polaron formation at low doping levels with four absorption peaks at 2.5, 1.5, 1.2, and <0.5 eV. At deep doping levels, bipolarons are dominating. When heating FeC13-doped POPT at 110 °C, we saw a similar bipolaron-to-polaron transition as the material was thermally undoped. Our interpretation of this phenomenon is main chain torsion due to the thermally enhanced side chain interactions, bipolaron dissociation and polaron stabilization. The conductivity stability of doped POPT has been improved, compared to POT (Fig. 1), but is not as good as desired. In the following will show that separating the flexible side chains is a more efficient way to stabilize doped poly(thiophene)s. 2. PQTMT POTMT r a n d o m copolymers were p r e p a r e d by e l e c t r o c h e m i c a l l y copolymerizing 3-methylthiophene and 3-octylthiophene from acetonitrile solutions containing these monomers and LiC104 as the supporting electrolyte, with subsequent electrochemical reduction. Ratios of the monomer units in the resulting copolymers, MT/OT, are close to the feed ratios of the preparation solutions. S o l v a t o c h r o m i s m in c h l o r o f o r m / m e t h a n o l s o l u t i o n s and thermochromism in solution-cast thin films are significantly alleviated in those POTMTs with high MT/OT ratios, indicative of a very rigid polythiophene main chain. Steric interactions between the side chains have been efficiently suppressed. The stability of doped POTMTs with various MT/OT ratios is displayed by the evolution of their optical absorption spectrum at 110 °C in vacuum, as shown in
1224
~ 0.8 o~
-I
P O T M T (o~
÷
÷
0
D
~-2
MT
=00.6
0
÷
÷ 0
"t't+ +
0
-3
;.~0.4
POPT
0 0 0
OO
-4
o POT . 5 , 1 , 1 , 1 , 1 , 1
0
20
40
.
I
,
60
T i m e (rain)
I
.
I
80
.
I
,
0
0
40
80
120
T i m e (min)
Fig. 1. Conductivity decay of some substituted poly(thiophene)s at 110°C in lab air. The samples were solution-cast thin films of around ll~m thickness, doped with FeCI3,eH20 in acetonitrile. Fig. 2. Intensity evolution at 110 °C in vacuum of the NIR peak at 0.8 eV on the optical absorption spectra of POTMT(CIO4") films doped in acetonitrile at 1.2 V vs Ag/AgCI. MT/OT ratios shown in the fig. are the feed ratios for copolymerization.
Fig. 2. The stability is improved to a greater extent with higher MT/OT ratio. FeCI3doped POTMT (2MT/1OT) is very stable. Its conductivity decreases only a little after two hours' heating at 110 °C in lab air(Fig. 1). Cyclic voltammetry indicates a high redox potential in this copolymer, close to t h a t in PATs. T h u s the t h e r m a l undoping in PATs is not due to their high oxidation potentials. It is r a t h e r due to side chain mobility and the kicking out of the dopants by the mobile side chains. To confirm this argument, we r a n the optical absorption spectrum evolution at 110 °C in v a c u u m of POTMT (2MT/IOT) films doped with dopants of different sizes, e.g., perchlorate(C104-), tosylate(TsO'), a n d dodecylbenzene sulfonate(DBS'). Fig.3 clearly shows the effect of dopant size on the stability of the copolymer. Bulky dopants can easily be kicked out, while small dopant can not.
POTMT are random copolymers, a n d there m u s t be some OT-to-OT linkages which will cause some side chain interactions. The better material is t h a t with a regular structure and absent OT-to-OT linkages. Thus PTOT with octyl groups attached to every second thiophene unit on a polythiophene chain and PDTOT with octyl groups attached to every third thiophene unit on a polythiophene chain were
1225
synthesized. Solvatochromism and t h e r m o c h r o m i s m in these regular copolymers were investigated as above. They indicate t h a t the rigidity of a PTOT chain is lower t h a n t h a n of a PDTOT chain, with the rigidity of a POTMT (2MT/1OT) random copolymer chain lying between. This r a n d o m copolymer has a density of long side chains a little bit higher than PDTOT. Thus both side chain density and OT-to-OT linkages are crucial to the rigidity of the conjugated polymer m a i n chains. The methyl groups on POTMT may also exert some effects. The conductivity stability of the two regular copolymers, doped with FeC13, are shown in Fig. 1. PTOT is as stable as the random copolymer, while PDTOT is even more stable. The evolution of the optical absorption spectrum of the doped copolymers at l l 0 ° C in vacuum indicates a stability sequence of PTOT
1
0.9
o 0.8 ,.Q
~
~
i
. . . .
I
. . . .
I
. . . .
I
,
,
,
,
P O T M T (2MT/IOT)
0.8 o.6
.8
< 0.7
<
~
. . . .
\
o.4
0.6
~ 0.2
DBS-
Z
0
. . . ,
0
. . . .
10
,
. . . .
20
t
. . . .
30
,
. . . .
4O
0.5 i
. . . .
50
Time (min)
i . . .
60
0.4
7O
0
................... 5 10
'
15
20
T i m e (h)
Fig. 3. Intensity evolution at 110 °C in vacuum of the NIR peak at 0.8 eV on the optical absorption spectra of POTMT(2MT/1OT) films electrochemically doped in acetonitrile. The dopants are shown in the figure. Fig. 4. Intensity evolution at 110 °C in vacuum of the NIR peak at 0.8 eV on the optical absorption spectra of three substituted poly(thiophene) films doped with FeCI3o6H20 from acetonitrile.
.... 25
1226
CONCLUSIONS Solvatochromism and thermochromism in POT are both driven by the mobile side chains. Decreasing the side chain density and number of OT-to-OT links will alleviate these chromisms. Thermal undoping in doped POT is also due to the steric effects of the side chains. It may be suppressed by separating the OT units with 3-methylthiophene or thiophene units. The insolubility and infusibility of most conjugated polymers are due to both the high rigidity of the main chains and interchain %-electron extension. Without the interchain ~-electron extension, most polymers would be soluble and fusible, provided they were free of crosslink and their molecular weights were not very high. Dilutely attached long flexible side chains may separate conjugated main chains from each other in a good solvent or at an elevated temperature. Then slight distortion along the main chain will render the polymer soluble and fusible. In this specifically tailored conjugated polymer, free space is available along the main chains to accommodate dopants so that the doped polymer is stable. REFERENCES [1] J.R. Reynolds and M. Pomerantz, in "Electroresponsive Molecular and Polymeric Systems", ed. T. A. Skotheim, Dekker, New York, 1991, Vol.2, p. 187. [2] G. Gustafsson, O. Ingan~s, W. R. Salaneck, J. Laakso, M. Loponen, T. Taka, J. E. 0sterholm, H. Stubb, and T. Hjertberg, in "Conjugated Polymers", Eds. J. L. Bredas and R. Silbey, Kluwer Academic, Dordrecht, 1991, p. 315. [3] S. D. D. Rughoopugh, A. J. Heeger, and F. Wudl, J. Polym. Sci., Polym. Phys. Ed., 25(1987)101. [4] O. Ingan~s, W. R. Salaneck, J.-E. Osterholm, and H. Laakso, Synth. Met., 22 (1988)395. [5] W. R. Salaneck, O. Ingan~s, J. E. Osterholm, B. Themans, and J. L. Bredas, Synth. Met., 28(1989)451. [6] G. Gustafsson, O. Ingan~s, J. O. Nilsson, and B. Liedberg, Synth. Met., 26 (1988)297. [7] Y. Wang and M. F. Rubner, Synth. Met., 39(1990)153. [8] J. T. Lopez-Navarrete and G. Zerbi, Chem. Phys. Lett., 175(1990)125. [9] M. GranstrOm and O. Ingan~s, Synth. Met., 48(1992) 21. [10] M. Rehahn, A. D. Schluter, G. Wegner, and W. J. Feast, Polymer, 30(1989) 1054. [11] H. Masuda, S. Tanaka, and K. Kaeriyama, J. Chem. Soc. Chem. Commun., (1989)725. [12] Q. Pei and O. Ingan~s, Synth. Met., 46(1992)353. [13] Q. Pei, H. J~irvinen, J. E. 0sterholm, O. Ingan~is, and J. Laakso, Macromolecules, (1992), in the press.