12842
J. Phys. Chem. 1993,97, 12842-12847
Kinetics and Activation Parameters of Electrochemical Polymerization of 3-Alkylthiophenes in the Presence of Various Aromatic Additives Yen Wei,’ Jing Tian, David Clahn, Bin Wang, and Deryn Cbu Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 191 04 Received: June 1 , 1993; In Final Form: September 14, 19938 Electrochemical polymerization of 3-alkylthiophenes was performed in the presence of a small amount of 2,2’-bipyrrole, indole, 5-methoxyindoleI 2,2’-bithiophene, or 2,2’:5’,2’’-terthiophene. The presence of these additives effects a significant increase in the rate of polymerization. The applied potentials required for the polymerization to proceed can be reduced to values lower than the oxidation potentials of the monomers. The reaction rate was found to be first order in the monomer concentration and 0.5 order in theadditive concentration. The first-order dependence of the rate in monomer concentration was also observed in the polymerization with 2,2’-bithiophene or 2,2’:5’,2’’-terthiophene as the monomer. The activation energies for the polymerization of 3-methylthiophene were measured both in the absence and in the presence of 2,2’-bithiophene or 2,2’:5’,2’’terthiophene. The presence of the additives results in a decrease in the activation energy and an increase in the preexponential factor. The activation enthalpies, activation entropies, and Gibbs free energies of activation were also evaluated. All the results are interpreted based on the mechanism that the polymer chain growth is accomplished by electrophilic aromatic substitution reaction of radical cation intermediates with neutral monomers.
SCHEME I
Introduction The electrochemical oxidative polymerization of aromatic compounds has been widely utilized in the preparation of electrically conducting polymers.’ Among those polymers, polythiophene and its derivatives have drawn considerable attention for their high conductivity,Z environmental ~tability,~ and processibility.4 A great deal of research effort has been devoted to the preparations and the investigation of structureproperty relationshipof these polymers.6 There have been a few reports on the kinetics and mechanism of the electrochemical polymerization.7-12 The mechanism of polymer growth on the electrode remains as a subject of debate. The electrochemical polymerization of thiophene waS thought to involve coupling of cation radicals generated from oxidation of the monomers and oligomers at the electrode surface via a typical step-growth mechanism.7-10 Recently, we have developed a new methodology for both electrochemical and chemical syntheses of conducting polymers, such as polyanilines,13 polypyrrole~,~~ and polythiophenes.lsJ6 In this method for preparation of polythiophenes,lsJ6 a small amount of oligomer of thiophene such as 2,2’-bithiophene and 2,2‘:5’,2‘’-terthiophene was introduced to the 3-alkylthiophene polymerization system. The presence of the bithiophene or terthiophene was found to significantlyincrease the rate of polymerization,1~J6the conjugation length of the polymer chains,17and the uniformity of the polymer films.Is On the basis of the experimental results, we have proposed a mechanism for the oxidative polymerization of thiophene monomers.l6 According to this mechanism, the polymer growth is accomplished by electrophilic attack of the radical cation at the growing chain end on the neutral monomer followed by oxidation and deprotonation as illustrated in Scheme I. The process is neither a classical step-growth nor a classicalchain polymerization but something in between. In an effort to extend the synthetic method and to further elucidate the polymerization mechanism, we have investigated the effects of other types of aromatic compounds, such as 2,2’bipyrrole, indole, 5-methoxyindole,5,5’-dmethyl-2,2’-bithiophene, and p-hydroquinone, on the electrochemical polymerization of 3-alkylthiophenes. The activation energy for the electrochemical *Abstract published in Advance ACS Absrracts, November 1, 1993.
0022-3654f 93f 2O97-12842SO4.OO/O
.R
.
.R
Polymer
.‘I
-
2H+
CkfoR
-- - /
\ / \ / \
polymerization of 3-methylthiophenewas measured for the first time both in the absence and in the presence of 2,2’-bithiophene or 2,2’”’,2’’-terthiophene in order to get further insights into the reasons for the observed change in the rate of polymerization. Other thermodynamic parameters of activation were also evaluated.
Experimental Section Materials and Instrumentation. 3-Methylthiophene (99+%, Aldrich) was doubly distilled under reduced pressure. 2,2‘Bithiophene (97%, Aldrich), 2,2’:5’,2’’-terthiophene (99%, Aldrich), indole (99+%, Aldrich), 5-methoxyindole(99%. Aldrich), and lithium perchlorate anhydrous (99.596, Alfa) were used as received. Acetonitrile(99.996, Fisher) were dried and stored over molecular sieves. 2,2’-Bipyrrole was prepared according to Rapoport’s procedurel8 and was purified by recrystallization followed by sublimation. 5,5’-DimethyL2,2’-bithiopheneand 3-npentylthiophene were prepared using Tamao’s method.*g ElecQ 1993 American Chemical Society
Electrochemical Polymerization of 3-Alkylthiophenes trochemical polymerization was carried out on an EG&G PAR Model 273 potentiostat/galvanotat. In general, a three-electrode single-compartment cell was employed for the polymerization with platinum plates as both working and counter electrodes and a saturated calomel electrode (SCE) as reference. The control of polymerizationtemperature (jz0.3 “C)was achieved by means of a jacketed cell through which water was circulated from a Polytemp thermostat. All the electrochemical reactions were carried out at 22 OC unless otherwise specified in the text. The areas of the working and counter electrode were 3.0 and 6.0 cm2, respectively. The oxidation potentials were measured by linear sweeping voltammetry as the anodic peak values. The conductivities of the polymer films were measured by the standard fourprobe method. The infrared spectra of the polymer-KBr pellets were recorded on a Perkin-Elmer Model 1600 FTIR spectrophotometer. Electrochemical Polymerization. The general procedures for the polymerization and data treatment are similar to those previously reported. l6 3-Methylthiophenewas polymerized using the potential cycling technique with and without addition of the thiophene oligomers or other aromatic heterocyclic compounds. The electrolyte consisted of 0.20 M monomer in deareated 0.10 M LiC104 acetonitrile solution containing 0-2.0 mM additive as specified in the text. In general, the potential was cycled between -0.2 and 1.5 V (or other up-switch potentials as specified in the text) at a sweeping rate of 100 mV/s. The poly(3-n-pentylthiophene) films were prepared potentiostatically at an applied potential of 1.5 V vs SCE on indium-tin oxide (ITO) working electrodes in 0.10 M LiC104 acetonitrile solution containing 0.20 M monomer. The polymerization of 2,2’-bithiophene alone was carried out by cycling the potential between -0.2 and 1.15 V vs SCE at a sweeping rate of 50 mV/s. The bithiophene concentration was varied from 0.010 to 0.040 M in 0.10 M LiC104 acetonitrile solution. Similarly,poly(2,2’:5’,2’’-terthiophene) was also synthesized by cycling the potential between -0.2 and 1.2 V vs SCE at a sweeping rate of 50 mV/s. The concentration of 2,2’:5’,2’’-terthiophene was varied from 0.12 to 1.0 mM in 0.10 M LiC104acetonitrile solution. The activation parameters were determined from the polymerization rates measured in a temperature range 0-32 OC. Data treatments were performed using TK! Solver Linear Regression Model software. Each kinetic data point was taken from an average of at least four parallel experimental measurements. The rate constants were reproducible typically to f20%.
Results and Discussion The polymerization of 3-methylthiophene with the potential cycling technique was monitored continuously by the change in the anodic and cathodic charges during the reaction process. In each anodic scan, the total anodic charge (Qa) is attributed to the consumption of polymerization and oxidation of electroactive polymer (i.e., doping of the polymer) already deposited on the electrode. In each cathodic scan, the charge (ec) is corresponding to the reduction of the amount of electroactive polymer already deposited on the electrode.’6.20 Therefore, the total cathodic charge during the polymerization is proportional to the amount of electroactive polymer formed in the polymerization. The rate of polymer formation can be expressed as the change in the cathodic charge with respect to the number of cycles scanned, which is proportional to the reaction time (t). Hence, the rate of polymerization (dQ,/dr) can be determined from the slope of the plot of cathodic charge (ec) against the cyclic number. Figure 1 shows some typical Qc-r plots for the electrochemical polymerization of 3-methylthiophene (a) alone and in the presence of a small amount of (b) 2,2’-pyrrole, (c) indole, and (d) 5-methoxyindole. As indicated by the steepness of the slopes in the Qc-r plots, the rate of polymerization is increased significantly in the presence of these additives.
The Journal of Physical Chemisrry, Vol. 97, No. 49, 1993 12843 1
40 1
1
O-t 0
c
1
I
I
I
I
5
10
15
20
25
Cycle Number
Figure 1. Plots of total cathodic charge against the number of cycles for the electrochemical polymerization of 3-methylthiophene(0.2 M) (a) in the absence and in the presence of (b) 2,2’-bipyrrole (0.2 mM), (c) indole (0.5 mM), and (d) 5-methoxyindole (0.5 mM). The potential was cycled between -0.2 and 1.5 V vs SCE at a scan rate of 100 mV/s.
The effect of these additives on the rate is similar to that of 2,2‘-bithiopheneand 2,2’:5‘,2’’-terthiophene and can be interpreted by the mechanism we have proposed.16 Thus, in the absence of the additives,the polymerization of 3-methylthiophenestarts with the oxidation of the monomer forming a monomer radical cation, which undergoesan electrophilicaromatic substitutionona neutral monomer followed by oxidation and deprotonation to afford a dimer (e.g., 2,2’-bithiophene). The dimer is then oxidized and the process repeats, leading eventually to the formationof polymer. The oxidation potential (Eox)of the thiophene monomer is known to be higher than the dimer, other oligomer, and the polymer.9 Moreover, the oxidation potential decreasesrapidly as the number of the thiophene unit is increased from one (monomer) to five (pentamer) and then slows down and levels off as the number is further increa~ed.~ Therefore, the oxidation of monomer to form the dimer should be the slowest step, Le., the rate-determining step, during the polymerization in the absence of the additives. On the other hand, the polymerization was found to be facilitated significantly by introducing a small amount of 2,2’-bithiophene or 2,2’:5’,2’’-terthiophene to the monomer solution.’5J6 Under this circumstance, the oligomers will be oxidized first due to their lower oxidation potentials, and the polymerization could bypass the slowest step, resulting in the increase in the rate of polymerization. The bithiophene and terthiophene function effectively as the initiators in the polymerization. According to the mechanism we proposed, any aromatic compound that has a lower oxidation potential than the monomer and whose oxidation intermediate can undergo electrophilicsubstitution reactions with the neutral monomers should facilitate the oxidative polymerization. Indeed, this generalization is in good agreement with the observed rate enhancement effect of 2,2’-bipyrrole, indole, and 5-methoxyindoleon the polymerizationof 3-methylthiophene. As shown in Table I, all these additives have lower oxidation potentials than the monomer and have an unhindered a-position upon which the polymer chain couldgrow. Becausetheseadditives function as the initiators, poly(3-methylthiophene) synthesized in the presence of the additives has essentially the same chain structural features as that obtained in the absence of the additives, as evidenced by their nearly identical cyclic voltammograms and infrared spectra (Figure 2). A similar additiveeffect was observed in the polymerization of 3-n-pentylthiophene. The rate enhancement effect was also observed in the polymerization system with 2,2’-bithiopheneas the monomer (Eox = 1.35 V vs SCE) and 2,2’-bipyrrole as the additive ( P x= 0.50 V). As demonstrated in Figure 3, the rate of polymerization of 0.04 M 2,2’-bithiophene a? the up-switch potential of 1.05 V in the presence of 0.1 mol % 2,2’-bipyrrole (b) is higher than that in the absence of the additives (a). The cyclic voltammogram and IR spectrum of the polymer synthesized in the presence of
Wei et al.
12844 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
TABLE I: Onset and Peak Oxidation Potentials of the Monomers and Additives’ 1.70
oxidation potential @x (V vs SCE) 1.82
2,2’-bithiophene
1.12
1.35
2,2’:5’,2”-terthiophene
0.95
1.02
1.00
1-10
0.37
0.50
0.98
1.07
0.92
1.os
0.70
1.04
comwunds 3-methvlthio~hene
structure
onset of oxidation (V vs SCE)
~~
Q-Q
2,2’-bipyrrole
H
H
m
indole
H
5-methoxyindole H
phydroquinone
#The potentials of the compounds (2.0 mM) were measured by linear sweeping voltammetry in 0.1 M LiCIOd-acetonitrilesolution at a sweeping rate of 100 mV/s from -0.2 to 2.0 V vs SCE. The anodic peak potential was used to represent the oxidation potential (Eox). 3.0
0 b
0
o O
I
0
m
.
“ . “ I
2
I
4
.
I
6
,
1
.
8
I
.
I
1 0 1 2
Cycle Number
Figure 3. Plots of the cathodic charge against the number of cycles for the polymerization of 2,2’-bithiophene (0.04 M)(a) in the absence and (b) in the presence of 2,2’-bipyrrole(0.04 mM). The potentialwas cycled between -0.2 and 1.05 V at a scan rate of SO mV/s.
I
I
I
eDQQ
LWO
LOO0
cm+
i
wo
Figure 2. Infrared spectra of poly(3-methylthiophene)prepared (a) in the absence and in the presence of (b) 0.25 mol % indole and (c) 0.25 mol 46 5-methoxyindole. 2,2’-bipyrrole are essentially the same as those of the poly(2,2’bithiophene) synthesized in the absence of the additive. It should be noted that the rate-enhancing additives must meet both the oxidation potential and the reactivity requirements. p-Hydroquinone as an additive did not facilitate the polymerization of 3-methylthiophenethough its oxidation potential (1.04 V vs SCE) is much lower than the monomer (1.82 V). This is because the oxidized hydroquinone intermediate cannot undergo the electrophilic aromatic substitution reactions. Furthermore, we have also introduced 5,5’-dimethyL2,2’-bithiophene(Eox= 1.10 V) as the additive to the polymerization of 3-methylthiophene. Again, no significant rate enhancement was observed because all the
a-positions in the compound are not available for the polymer chain growth. In the presence of the rate-enhancing additives, the applied potential in the polymerization could be reduced to the values below the oxidation potential of the monomer while maintaining reasonably high reaction rates, because the additives can be oxidized at lower anodic potentials. For example, at an upswitch potential of 1.4 V, 3-methylthiophene polymerizes at a low rate (Figure 4a) but at much higher rates when indole and 2,2’bipyrrole are present (Figure 4, b and c, respectively). At 1.3 V, the polymerization of 3-methylthiophene in the presence of 2,2’bipyrrole proceeds (Figure 4d), but no noticeable reaction was observed in the absence of the additive. The decrease in the required applied potential for the polymerization offers many advantages. It is well documented that the a-position in the monomer is more reactive than the @-position,and therefore, the a,a-linkages are predominant in the polymer chains.4..z1~zz More a,a-linkages results favorably in longer conjugation chain length. The high applied potentials would lead to polymer chains with
Electrochemical Polymerization of 3-Alkylthiophenes 2o
G E a!
v
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12845
7
7
40
15-
F
6m
10-
u
a0
5m
5-
0
’ 0
10
20
30
0
40
Figure 4. Relationshipbetween the cathodic charge and the number of cycles in the polymerization of 3-methylthiophene (0.2 M) with the potential cycling range of -0.2 to 1.4 V at a scan rate of 100 mV/s (a) in the absence and in the presence of (b) indole (0.2 mM) and (c) 2,2’bipyrrole (0.2 mM) and (d) with the potential cycling range of -0.2 to 1.3 V in the presence of 2,2’-bipyrrole (0.2 mM).
-
rate = dQ,/dt = P p p [ M I ][A]’.’
(1) where [MI]is the concentration of 3-methylthiophene, [A] is the concentration of 2,2’-bipyrrole, and krP is the apparent rate
15
20
25
Cycle Number
Figure 5. Plots of the cathodic charge (Qo)against the number of cycles (t) in the polymerization of 3-methylthiophenein the presence of 0.20 mM 2,2’-bipyrrole at the monomer concentrations of (a) 0.10, (b) 0.20, and (c) 0.25 M. The potential was cycled between -0.2 and 1.5 V at a scan rate of 100 mV/s. 0.5
more unfavorable structures, Le., a,@-or @,@-linkages, and hence shorten the conjugation Moreover, the polymer chains degrade at high potentials because of over~xidation.~~ Therefore, by lowering the applied potential in the polymerization, both the formation of mislinkages and the chain degradation could be minimized to afford the polymer chains with better structural regularity and longer conjugation length. The presence of the additives was also found to improve the uniformity of the polymer film deposited on the electrode. This is attributed to the increase in the number of growing polymer chains in the system because the additives function as initiators, hence the nuclei for the film growth.I6 In general, the polymers prepared in the presence of the additives exhibit higher conductivities. For example,thepoly(3-n-pentylthiophene)film prepared in the presence of 2 mol 9% 2,2’-bithiophene on an I T 0 electrode with thickness of 10 pm exhibited a conductivity of 2 S/cm, which is about 10 times higher than the polymer prepared without the additive (0.2 S/cm) under otherwise identical conditions. The increase in conductivity might have resulted from the longer conjugation length” and more uniform films. The former would facilitate the intrachain electron transfer, while the latter would assist the interchain electrons-transfer process. We have studied the kinetics of electrochemicalpolymerization of 3-methylthiopheneinthe presence of the additives. As a typical example, the electrochemical polymerizations of 3-methylthiophene (MI) were carried out using potential cycling technique with the monomer concentration [MI]varied from 0.010 to 0.40 M in thepresenceof0.20 mM 2,2’-bipyrrole. Thecathodiccharge is plotted against the number of cycles (i.e., reaction time t), as shown in Figure 5. The rate of polymerization (dQc/dt, the slope of Qc-t plot) remains constant as the polymerization proceeds and increases linearly with an increase in the monomer concentration. The plot of log(dQc/dt) versus log[Ml] gives a straight line with a slope of 1.06 (Figure 6), indicating that the rate is first order in monomer. The effect of the bipyrrole additive concentration ([A]) on the rate of polymerization was measured by varying the bipyrrole concentration from 0.10 to 2.0 mM at a fixed 3-methylthiophene concentration of 0.20 M. The rate was also found to increaseas the bipyrrole concentration was increased. The plot of log(dQc/dr) vs log[Al gives a slope of 0.42 (Figure 7). Thus, the rate is approximately one-half order on the additive concentration. The empirical rate expression could be therefore written as
10
5
0
Cycle Number
-
y = 1,0364 + 1.0624~RA2= 0.990
-1.1
-1.0
-0.9
-0.8 -0.7
-0.6 -0.5
log[Methylthlophene]
Figure 6. Logarithmic plot of the polymerization rate (dQc/dt)against
theconcentration of 3-methylthiophene. ThedQ,/dtvalueain mCcyclc1 were taken from the slopes of the Qc-t plots in Figure 5. 0.4 y = 0.39746 t 0.41861X R A 2 = 0.995
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
log[Bipyrrole]
Figure 7. Logarithmic plot of dQc/dtagainst the concentration of 2,2’bipyrrole for the polymerization of 3-methylthiophene (0.20 M) in the presenceof 2,2’-bipyrroleof various concentrations(0.10-2.0 mM) with the potential cycling range of -0.2 to 1.5 at a scan rate of 100 mV/s.
constant in mC M-I” cycle-’. This is in exact agreement with the results we previously reported on the electrochemical polymerization of 3-methylthiophene in the presence of 2,2’bithiophene or 2,2‘:5‘,2’’-terthiophene and with the electrophilic aromatic substitution mechanism proposed.16 The first-order dependence of the rate on the monomer concentration was also observed in the electrochemical polymerization of 3-methylthiophene (M1),16 2,2’-bithiophene (Mz), or 2,2’:5‘,2‘‘-terthiophene (M3) alone. 2,2’-Bithiophene at various concentrations (0.014.04 M) in 0.1 M LiC104 acetonitrile solution was polymerized by cycling the potential between 4.2
12846 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
Wei et al.
A
y = 1.7793+ 1.0416X R”2 = 0.989
2.2
-
-. .
-2.1
-1.9
-1.7
-1.5
-1.3 3.3
log[Bithiophenel
Figure 8. Logarithmic plot of dQ,/dt against the concentrationof 2,2‘bithiophene for the polymerization of 2,2’-bithiophene at various concentrations (0.01-0.04 M) with the potential cycling range of -0.2 to 1.15 V at a scan rate of 50 mV/s.
and 1.15 V at a sweeping rate of 50 mV/s. The rate of polymerization (dQc/dt) was found to be directly proportional to the concentration of the monomer (Le., 2,2’-bithiophene in this case). Plottinglog(dQc/dt)vslog[M~](Figure 8) yieldsa straight line with a slope of 1.04, indicating a first-order dependence. A similar result was obtained for the polymerization of 2,2’:5’,2”terthiophene in a concentration range of 0.12-1.0 mM, in which the slope of log(dQc/dt)-log[M3] plot was 0.99. These results, again, can be readily interpreted according to the proposed mechanism. Thus, in the absence of a rate-enhancing additive, the slowest step in the polymerization is the oxidation of the monomers to form corresponding dimeric species because the monomers have the highest oxidation potential in comparison with the subsequently formed oligomers and polymers. The rate of polymerization without the additives can be expressed as rate = dQc/dt = kapp’[M,]
(2) where n = 1,2, or 3 and kapp’is the apparent rate constant in mC M-I cycle-’. To develop a better understanding of the additive effects in the oxidative polymerization of the thiophene monomers, the activation parameters were evaluated for the first time for the polymerization of 3-methylthiophene both in the absence and in the presence of 2,2’-bithiophene or 2,2’:5’,2”-terthiophene.3-Methylthiophene was polymerized in the temperature range 0-32 OC. The rate (dQc/dt) and the apparent rate constant (kapp or k a p f ) were determined from the plot of thecathodic chargeversus cycle number. In general, the rate of polymerization increases as the temperature is increased. The Arrhenius plot exhibits a linear relationship between the logarithmic rate constant of polymerization (In kapp’) and inverse absolute temperature (1 / T),as shown in Figure 9. According to the Arrhenius equation
In kapp’= In A - E,/RT
(3)
the apparent activation energy (E,) and preexponential factor ( A ) can be determined from the slope and intercept of the plot, re~pectively.2~ At the reaction temperatures higher than 32 OC, the In kawdeviates slightlyfrom the straightline (i.e., the recorded dQcfdt values were lower than the expected), owing probably to excessive diffusion of the radical cations away from the vicinity of theelectrode. Such a diffusion is evidenced by the color change in theelectrolyte at the higher temperatures. 3-Methylthiophene was also polymerized in the presence of 2,2’-bithiophene or 2,2’: 5’,2’‘-terthiophene a t various temperatures. The plot of In kapp vs 1 / T yields a straight line (Figure lo), indicating that the polymerization also obeys the Arrhenius equation. As listed in Table 11, the activation energy in the presence of the bithiophene or terthiophene is reduced by about 3 kJ/mol and the preexponential factor A is increased by about 1-2 orders of magnitude in comparison with the polymerization system in the absence of
3.4
3.5
3.6
1 n (x1000)
Figure 9. Plot of Ink*@against 1/ Tfor the electrochemicalpolymerization
of 3-methylthiophene.
:k Y
5
7.1
V.,
I
3.3
.
1
3.4
.
*
3.5
3.6
1fr (x1000)
Figure 10. Plot of In kap@ against 1/T for the electrochemical polymerization of 3-methylthiophene in the presence of (a) 2,2’bithiophene and (b) 2,2’:5’2’’-terthiophene.
TABLE 1I: Representative Apparent Rate Constants and Activation Parameters for the Polymerization of 3-Methylthiophene (3MT) in the Absence and in the Presence of 2,2’-Bithiophene and 2,2‘:5’,2“-Terthiophene and for the Polymerization of the Bithiophene and Terthiophene Alone 3MT + 3MT + bithio- terthiomonomer 3MT‘ bithiopheneO terthiopheneO pheneb pheneb kPPPat 22 “C 12c 119od 174od SOW 421Oe kPPPat15 OC 1Oe 106od 158od 46Oe 402Oe E , (kJ mol-I) 12 15 9.2 8.5 4.7 A 5.7 X 1.6 X 5.6 X 1.6 X 2.9 X I 03 IO’ 10s lo“ lo“ AH*kJ mol-I 13 10 6.8 6.2 2.4 AS* J K-l mol-L -180 -150 -160 -170 -170 AG*kJ mol-I 67 55 49 58 52 The polymerization of 3-methylthiophene(0.20 M) was carried out in 0.1 M LiC104 acetonitrile solution in the absence or in the presence of 0.1 mol % 2,2’-bithiophene or 0.25 mol W 2,2”5’2”-terthiophene. The applied potential was cycled between -0.2 and 1.5 V vs SCE at scan rate of 100 mV/s. The polymerization of 2,2’-bithiophene (5.0 mM) and 2,2’:5’2”-terthiophene (0.5 mM) was performed in 0.1 M LE104 acetonitrile solution with potential cyclingranges of 4 . 2 to 1.15 and 4 . 2 to 1.2 V, respectively, at a scan rate of 50 mV/s. In mC M-l cycle-I. In mC M-l.5 cycle-I. the additives. These results suggest that both theactivation energy and the preexponential factor contribute to the observed enhancement of the polymerization rate by the additives. According to simple collision the factor A consists of collision frequency and steric components. The former is related to the population of reactive species in the system. In the absence of the additives, only a small fraction of 3-methylthiophene monomers should be oxidized because the up-switch potential applied in the polymerization (ie., 1.5 V) is much lower than the oxidation potential of the monomer (1.82 V). In the presence of
Electrochemical Polymerization of 3-Alkylthiophenes
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12847
the bithiophene or terthiophene under the same reaction conditions, many more reactive radical cations are produced because the applied potential is higher than the oxidation potential of either additive. Hence, the number of growing polymer chains is increased. As a consequence, the collision frequency should be greater in the system with the additives and so should the preexponential factor and the polymerization rate. This is consistent with the experimental observations. The steric effect may not contribute to the A factor significantly because the growing polymer chain ends have the same stereochemical structure after the initiation, regardless of whether the additives are present or absent. We have also measured the activation energies of the electrochemical polymerizations with 2,2’-bithiophene and 2,2’:5’,2’’terthiophene as monomers at the up-switch potentials of 1.15 and 1.20 V, respectively, and the results are listed in Table 11. As one would anticipate, the activation energies are lower than those with 3-methylthiophene as monomer. Because lower up-switch potentials were employed, the A values are smaller than those for the polymerization of 3-methylthiophene in the presence of the additives. Other activation parameters, Le., activation enthalpy (AH’) and activation entropy (AS*), were determined from slope and intercept of the plot of ln(kaPP/T) against 1/T, respectively, according to the following eq~ation:~~,*5
+
+
ln(kaPp/T) = -AH*/T M * / R ln(k/h) (4) where R is the gas constant, k is Boltzmann’s constant, and h is Planck’s constant. All the data are listed in Table I1 along with the Gibbs free energy of activation (AG*)calculated at 25 OC. The change in AH*is similar to that in E,, and the values are of the same order of magnitude (2-1 2 kJ mol-’) as those reported for the electrochemical electron exchange involving metallocene redox couples.26 The negative pS* values in all the systems suggest the formation of ordered activated complexes. There are several possible reasons for the negative AS*which are consistent with the proposed mechanism. Firsr, the radical cations generated during the polymerization are much more solvated in the polar LiC104-acetonitrile media in comparison with the corresponding neutral reactants. Second, the polymer chain growth involves bimolecular reactions between the radicals cations and the incoming monomers. Both factors may have contributed to the observed entropy decrease in the activated complexes.
Conclusions We have found that the electrochemical polymerization of 3-alkylthiophenes is significantly facilitated by the presence of a small amount of 2,2’-bipyrrole, indole, 5-methoxyindole, 2,2’bithiophene, or 2,2’:5’,2’’-terthiophene in the polymerization system. All these rate-enhancing additives have oxidation potentials lower than the monomers and can be incorporated into the polymer backbones. In the presence of these additives, the polymerization proceeds a t the applied potentials lower than the oxidation potentials of the monomers to afford the electroactive polymers of improved quality. The polymerization of 3-methylthiophene in the presence of the additive is first order in the monomer concentration and one-half order in the additive concentration. For comparison, the electrochemical polymerization was also carried out using 2,2’-bithiophene or 2,2’:5‘,2”terthiophene as monomer. The rate was found to be first order in the bithiophene or terthiophene concentration. The activation energies for the polymerization of 3-methylthiophene were measured both in the absence and in the presence of 2,2’bithiophene or 2,2’:5’,2”-terthiophene. The enhancement of the polymerization rate in the presence of the additives is attributed to the decrease in the activation energy and the increase in the preexponential factor. Other thermodynamic parameters such as activation enthalpy, activation entropy, and Gibbs free energy of activation are also evaluated. When 5,5‘-dimethyl-2,2‘-
bithiophene or p-hydroquinone was used as additive in the polymerization of 3-alkylthiophenes, no significant change in the reaction rate was observed. All the results can be interpreted by, and are consistent with, the proposed polymerization mechanism.
Acknowledgment. This work was supported in part by E. I. Du Pont de Nemours & Co. through a Young Faculty Award to Y. Wei. We thank Professor A. W. Addison for helpful discussions on kinetics of electrochemical reactions and Dr. W. Wang for his assistance in the preparation of 5,5’-dimethyl-2,2’-bithiopheneV References and Notes (1) Diaz, A. F.; Bargon, J. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol 1, pp 81-115. (b) Tourillon, G.; Gamier, F . J . Electroanal. Chem. Interfacial Electrochem. 1982, 135, 173. (c) Roncali, J.; Gamier, F . Synrh. Met. 1986, 15, 323. (d) Shi, L.-H.; Roncali, J.; Gamier, F . J . Electroanal. Chem. 1989,263,155. (e) Diaz, A. F.; Kanazawa, K. K.; Gardini, G. P. J. Chem. SOC.,Chem. Commun. 1979,635. (2) (a) Sato, M.; Tanaka, S.; Kaeriyama, K. J . Chem. Soc., Chem. Commun.1985, 713. (b) Kaneto, K.; Yoshino, K.;Inuishi, Y. Solid State Commun.1983, 46, 389. (3) (a) Wang, Y.; Rubner, M. F. Synrh. Met. 1990, 39, 153. (b) Gustafsson, G.; Inganss, 0.;Nilsson, J. 0.;Leidberg, B. Synth. Met. 1988, 26, 297. (4) (a) Souta Maior, R. M.; Hinkelmann. K.;Eckert, H.; Wudl, F. Macromolecules 1990, 23, 1268. (b) Leclerc, M.; Diaz, F. M.; Wegner, G. Makromol. Chem. 1989, 190, 3105. (c) Guay, J.; Kasai, P.; Diaz, A,; Wu, R.; Tour, J. M.; Dao, L. H. Chem. Mater. 1992, 4 , 1097. (d) Hill, M. G.; Penneau, J.-F.; Zinger, B.; Mann, K. R.; Miller, L. L. Chem. Mater. 1992, 4, 1106. (5) (a) Chen, T.-A.; Rieke, R. D. J . Am. Chem. SOC.1992,114,10087. (b) Back, R.; Lennox, R. B. Lungmuir 1992, 8, 959. (c) Masuda, H.;
Kaeriyama, K.; Suezawa, H.; Hirota, M. J . Polym. Sci.: Part A: Polym. Chem. 1992,30,945. (d) Elsenbaumer, R. L.; Jen, K. J.; Oboodi, R. Synth. Met. 1986, 15, 169. (6) (a) Chung, T.-C.; Kaufman, J. H.; Heeger, A. J.; Wudl, F . Phys. Rev. B 1984,30,702 and references therein. (b) Zerbi, G.; Chierichetti, B.; Inganiis, 0. J. Chem. Phys. 1991, 94, 4646. (c) Glenis, S.; Tourillin, G.; Gamier, F. ThinSolid Films 1984,122,9. (d) Tanguy, J.; Pron, A.; Zagorska, M.; Kulszewicz-Bajer, I. Synth. Met. 1991,45,81. (e) Roncali, J.; Garreau, R.; Yassar, A.; Marque, P.; Gamier, F.; Lemaire, M. J . Phys. Chem. 1987, 91,6706. (f) Chaos, S.;Wrighton, M. S. J . Am. Chem. Soc. 1987,109,6627. (7) Genies, E. M.; Bidan, G.; Diaz, A. F. J . Electroanal. Chem. 1983, 149, 101. (8) Bargon, J.; Mohmand, S.;Waltman, R. J. IBMJ. Res. Dev. 1983, 27, 330. (9) Diaz, A. F.; Crowley, J.; Bargon, J.; Gardini, G. P.; Torrance, J. B. J . Electroanal. Chem. 1981, 121, 335. (10) (a) Waltman, R. J.; Bargon, J.; Diaz, A. F. J . Phys. Chem. 1983,87, 1459. (b) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G. Chem. Mater. 1993, 5 , 430. (1 1) Satoh, M.; Imanishi, K.; Yoshino, K. J. Electroanal. Chem. 1991, 317, 139. (12) John, R.; Wallace, G. G. J . Elecrroanal. Chem. 1991, 306, 157. (13) (a) Wei, Y.; Jang, G.-W.; Chan, C.-C.; Hsueh, K. F.; Hariharan, R.; Patel, S. A.; Whitecar, C. K. J . Phys. Chem. 1990, 94, 7716. (b) Wei, Y.; Sun, Y.; Tang, X . J . Phys. Chem. 1989, 93, 4878 and references therein. (14) Wei, Y.; Tain, J.; Yang, D.-C. Makromol. Chem., Rapid Commun. 1991, 12, 617. (15) Wei, Y.; Jang, G.-W.; Chan, C.-C. J. Polym. Sci., Part C Polym. Lett. 1990, 28, 219. (16) Wei, Y.; Chan, C.-C.; Tain, J.; Jang, G.-W.; Huseh, K.F. Chem. Mater. 1991, 3, 888. (17) (a) Wei, Y.;Tian, J. Polymer 1992,33,4872. (b) Wei, Y.; Tian, J. Macromolecules 1993, 26, 457. (18) (a) Rapport, H.; Castagnoli, N., Jr. J. Am. Chem. SOC.1962, 84, 2178. (b) Rapport, H.; Bordner, J. J. Org.Chem. 1964, 29, 2727. (19) Tamao, K.; Kcdama, S.; Nakajima, I.; Kumada, M. Tetrahedron 1982,38, 3347. (20) Zotti, G.; Cattarin, S.;Comisso, N. J. Electroanal. Chem. 1987,235, 259. (21). (a) Waltman, R. J.; Bargon, J. Tetrahedron 1984, 40, 3963. (b) Roncali, J.;Lemaire, M.;Garreau,R.;Garnier, F.Synth. Met. 1987,18,139. (22) Niziurski-Mann, R. E.; Scordilis-kelly, C.; Liu, T.-L.; Cava, M. P.; Carlin, R. T. J. Am. Chem. SOC.1993,115, 887. (23) (a) Krische, B.; Zagorska, M. Synth. Met. 1989, 28. C263. (b) Marque, P.; Roncali, J.; Gamier, F. J . Electroanal. Chem. 1987, 218. 107. (c) Odin, C.; Nechtschein, M. Synrh. Met. 1991, 44, 177. (24) Connor, K. A. ChemicalKinetics;VCH Publishers: New York, 1990; pp 188-190. (25) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, Part A , Structure and Mechanisms; Plenum Press: New York, 1990; Chapter 4. (26) Nielson, R. M.; McManis, G. E.; Golovin, M. N.; Weaver, M. J. J. Phys. Chem. 1988, 92, 3441. (b) Gennett, T.; Milner, D. F.; Weaver, M. J. J . Phys. Chem. 1985,89, 2787.
