Tuning the electrical conductivity and self-assembly of regioregular polythiophene by block copolymerization: Nanowire morphologies in new di- and triblock copolymers

Article abstract of DOI:10.1002/1521-3773(20020118)41:2<329::AID-ANIE329>3.0.CO;2-M

Simply changing the solvent or evaporation conditions enables the physical properties of polythiophene block copolymers and polyurethane elastomers to be controlled. The copolymers all have high conductivities as well as excellent film-forming and good me

Full text of DOI:10.1002/1521-3773(20020118)41:2<329::AID-ANIE329>3.0.CO;2-M

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devices. Here we present a veryeasysynthetic method to
produce a large number of well-defined block copolymers and
polyurethane elastomers containing regioregular polythio-
phenes. Despite having low percentages of regioregular
polythiophenes, these new copolymers have very high con-
ductivities (of the order of a few ScmÀ1) and form verywell
defined nanowires that reach lengths in the micron range. In
addition, we have found that simplychanging the solvent or
evaporation conditions allows us to control the nanowire
formation and the electrical conductivityof the block
copolymer.
The synthesis of diblock copolymers of head-to-tail-cou-
pled poly(3-alkylthiophenes) (HT-PATs) can be accomplished
byfirst preparing a well-defined PAT (polydispersityindex
(PDI) ˆ 1.2) with 95% of its end groups[9] containing one
proton and one bromine atom (2; Scheme 1). The method[10]
Tuning the Electrical Conductivity and Self-
Assembly of Regioregular Polythiophene by
BlockCopolymerization: Nanowire
Morphologies in New Di- and Triblock
Copolymers**
Jinsong Liu, Elena Sheina, Tomasz Kowalewski, and
Richard D. McCullough*
Over two decades ago the discoveryof electrical conduc-
tivityin conjugated polymers spurred a tremendous amount
of effort aimed at the development of practical conducting
plastics.[1] One of the primarymotivations was the hope of
fabricating inexpensive, lightweight conducting materials.
Those efforts have been coming to fruition in recent years
with the development of polymer-based light-emitting di-
odes,[2] field-effect transistors,[3] el-
ements for active matrix displays,[4]
C6H13
and all-polymer integrated cir-
cuits.[5] The synthesis and study of
regioregular polythiophenes has
produced conjugated polymers
that self-assemble into well-de-
1. butyllithium/ THF
–40 °C
CH2CH2OTHP
Br
S
S
S
2. BrCH2CH2OTHP
–40 °C to 25 °C
1
4
3
1. LDA/THF/ –70 °C
78 %
1. butyllithium
2. ZnCl2
2. ZnCl2
3. [Ni(dppp)Cl2]/ 0 °C
fined superstructures and has
40 %
furthered the use of these materials
in the aforementioned applica-
tions.[6] Formation of ordered su-
permolecular structures in these
regioregular materials correlates
stronglywith their excellent elec-
C6H13
S
C6H13
CH2CH2OTHP
ClZn
S
5
THF / HCl
CH2CH2OTHP
H
H
Br
H
S
S
n
n
[Ni(dppp)Cl ],
55 °C
O
2
40 °C
97 %
2
6
96 %
C6H13
C6H13
S
H
S
n
trical conductivities (thousands or
O
CH2CH2OCCH(CH3)Br
hundreds of ScmÀ1 in comparison
S
BrCCH(CH3)Br
CH2CH2OH
with a few ScmÀ1 for regiorandom
S
N(CH3)3 / THF
25 °C 98 %
n
polymers). Nevertheless, regiore-
gular polythiophenes still have
poor mechanical and processing
properties relative to typical flexi-
7
8
C6H13
methyl acrylate/toluene or
styrene, CuBr, 90 °C
O
Br
S
CH2CH2OC
H
m
PMDTA
ble polymers.
S
n
X
One approach to solving this
problem is to synthesize block co-
9 a: X = -Ph
9 b: X = -COOCH3
polymers that contain conducting
Scheme 1. Synthesis of diblock polymers. LDA ˆ lithium diisopropylamine, dppp ˆ propane-1,3-diylbis-
(diphenylphosphane), THP ˆ tetrahydropyran.
polymer or oligomer units.[7] Such a
block copolymer[8] could self-as-
semble into a number of nanoscale
shown in Scheme 1 is an important modification of methods
morphologies, such as lamellar, spherical, cylindrical, and
vesicular structures, which would lead to the possibilitythat
new electronic/structural copolymers could be designed,
synthesized, and assembled as components in new nano-
[6, 11]
previouslypublished byour group.
The reaction was
optimized and the polymer end groups were characterized by
matrix-assisted laser desorption/ionization mass spectrometry
(MALDI-MS).
Since PATs are chemicallystable polymers, the abilityto
modifythe end groups on one side of the polymer is similar to
polymer- or bead-supported organic synthesis. Excess of
reagents were used to drive the modification reactions to
completion to give >95% yield for all of the steps subsequent
to the synthesis of the HT-PHT. The purification of the
products from all the reaction steps was achieved bysimple
precipitation and filtration, thus each step of the end-group
modification is high yielding (>95%) and is facile to
accomplish. The HT-PHT 2 is functionalized byreaction with
[*] Prof. R. D. McCullough, J. Liu, E. Sheina, T. Kowalewski
Carnegie Mellon University
Department of Chemistry
4400 Fifth Avenue
Pittsburgh, PA 15213-2683 (USA)
Fax : (‡1)412-268-3268
E-mail: rm5g@andrew.cmu.edu
[**] We gratefullyacknowledge support from the NSF (CHE-9807707).
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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329

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thiophene derivative 5 and [Ni(dppp)Cl2] (dppp ˆ propane-
1,3-diylbis(diphenylphosphane)) to yield polymer 6.[12] De-
protection of 6 leads to HT-PHTwith one hydroxyl end group
(7),[12] as verified byNMR and MALDI-MS analysis. The
hydroxy-terminated PHT (7) was further modified byreac-
tion with 2-bromopropionyl bromide to generate an atom
transfer radical polymerization (ATRP)[13] macroinitiator
(8).[14] Diblock copolymers (for example, 9) containing PHT
can be prepared bythe ATRP ™living∫ free-radical polymer-
ization method developed byMatyjaszewski and co-workers
to produce a wide varietyof conventional polymers. As an
example, we have made polyhexylthiophene polystyrene
(PHT-PS; 9a) and polyhexylthiophene polymethylacrylate
(PHT-PMA; 9b) byATRP using 8 as the initiator, styrene or
methyl acrylate as the monomer, and CuBr/N,N,N',N',N''-
pentamethyldiethylenetriamine (PMDTA) as the catalyst[15]
(Scheme 1). The percentage of the PS or PMA block is
completelycontrolled bythe feed ratio of the monomers, as
confirmed byNMR spectroscopyand size-exclusion chroma-
Thin films of all the polymers (PHT-PS (9a), PHT-PA (9b),
polyurethane elastomers (14), PS-PHT-PS (16a), and PA-
PHT-PA (16b)) were generated bythe slow evaporation of
toluene solutions to give magenta to purple films with
excellent mechanical properties. These films were oxidized
byexposure to I 2 vapor to give multifunctional polymers with
high electrical conductivities (Table 1), as determined byfour-
point probe conductivitymeasurements. Surprisinglyhigh
Table 1. Conductivityof block copolymers containing HT-PHT.
PS-PHT diblock copolymers (9a)
wt% of HT-PHT[a]
100%
16800
1.28
37%
30200
1.31
22%
41400
1.32
14%
53400
1.45
[b]
average Mn
[b]
Mw/Mn
conductivity[Scm À1
]
110
4.7
0.08
0.14
PS-PHT-PS triblock copolymers (16a)
wt% of HT-PHT[a]
100%
17900
1.23
52%
25500
1.21
26%
38100
1.25
7.7%
93600
1.51
[b]
average Mn
[b]
Mw/Mn
conductivity[Scm À1
]
96
5.3
0.43
0.05
[16]
PA-PHT-PA triblock copolymers (16b)
tography. High molecular weights and low polydispersities
wt% of HT-PHT[a]
100%
17900
1.23
45%
29700
1.29
18%
50400
1.41
10%
72300
1.66
are produced bythe combination of using the HT-PHT
macroinitiator (8) with a low PDI and the ™living∫ nature of
the ATRP method.
[b]
average Mn
[b]
Mw/Mn
conductivity[Scm À1
]
96
3.3
1.6
0.076
In addition, regioregular HT-PHT can be functionalized on
both the a and w ends, which allows for the synthesis of PS
and PMA triblock copolymers and polyurethane elastomers
containing HT-PHT (Scheme 2). Regioregular PHT 10 with a
mixture of end groups[6, 11] is debrominated bya Grignard
Polyurethane (14)
wt% of HT-PHT
10%
0.13
6.40%
0.048
0.60%
conductivity[Scm À1
]
4.6 Â 10À5
[a] Determined by 1H NMR spectroscopy. [b] Determined by GPC with
polystyrene as standard.
metathesis reaction and quenched with water to yield 11.[9]
A
Vilsmeier reaction on 11 gives polymer 12 as determined by
MALDI-MS. The aldehyde end groups on 12 are reduced with
LiAlH4 to yield a PHT terminated by hydroxymethyl groups
(13). Analysis of the product by MALDI-MS unambiguously
proves the incorporation of the hydroxylmethyl group to both
ends of the polymer chains. We have prepared well-defined
triblock copolymers PS-PHT-PS (16a) and PMA-PHT-PMA
(16b) of high molecular weight with low polydispersities from
the common polymer (15)[15] byusing ATRP with styrene or
methyl acrylate as the monomer.[16] Again the percentages of
electrical conductivities were found in all of the copolymer
samples. While manycopolmyers containing conjugated
polymers and other polymers and blends have been prepared,
no one has reported conductivityvalues near to those
reported here (Table 1). While 100% HT-PHT has a con-
ductivityof 110 Scm À1, a block copolymer of PHT-PS (9a)
containing 37% HT-PHT has a conductivityof about 5 Scm À1
.
À1
The conductivitydrops down to 0.1 Scm
for samples
containing approximately22% of HT-PHT or less. The
conductivityof the block copolymers largelydepends on the
ratio of the conducting and nonconducting blocks and is
related to the structural assembly. The PS-PHT-PS triblock
copolymers (16a) have conductivities as high as 5 ScmÀ1 for a
sample with 52% PHT. Our conducting
PS or PMA corresponded to the feed ratio of the monomer. In
[17]
addition, polymer 13 is easilyconverted
elastomers (14).
into polyurethane
polyurethane copolymers (14) exhibit
C6H13
C6H13
C6H13
conductivities as high as 10À1 ScmÀ1,
POCl3 75 °C
1. RMgBr / THF, reflux
which is much higher than other polyur-
H
n
CHO
n
2. CH3OH
100 %
H
OHC
N-methylformanilide
S
S
S
n
ethane conjugative copolymers or blends
reported in the literature (10À4 ScmÀ1).[18]
Conductivities for blends of conjugative
polymers with polyvinyl chloride and
other conventional polymers have also
been found to exhibit low conductivities
(in the 10À4 ScmÀ1 range).[18] All of our
block copolymers also have excellent
film-forming and good mechanical prop-
erties including elasticityin the polyur-
ethane samples.
97 %
10
12
11
C6H13
C6H13
O
LiAlH4 / THF
O
BrCOCH(CH3)Br
N(CH3)3 / THF
25 °C 92 %
CH2OH
n
CH2OC
HOH2C
25 °C
COCH2
S
13
S
15
n
Br
Br
styrene or methyl
acrylate/toluene
1,4-butanediol
toluene 2,4-diisocyanate
PEG (Mn = 1,500)
CuBr, PMDTA, 90 °C
polyurethane elastomer
14
ABA triblock copolymers
16 a: PS-PHT-PS
16 b: PMA-PHT-PMA
We have found that thin and ultrathin
films these block copolymers containing
Scheme 2. Synthesis of triblock polymers and polyurethane elastomers. PEG ˆ polyethyleneglycol,
PMDTA ˆ N,N,N',N',N''-pentamethyldiethylenetriamine.
330
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polythiophene that were prepared by casting from toluene
followed byfree evaporation of a solvent reproduciblyself-
assembled into verywell-defined ™nanowires∫ spaced later-
allyby30 40 nm (which corresponds to a fullyextended HT-
PHT block) with lengths on the order of micrometers
(Figure 1). Lower percentages of PHT also lead to similar
morphologies as found in drop-cast films, however, a clear
dilution factor of the PHT is observed. As an example, an
atomic force micrograph (AFM) micrograph of a PS-PHT
(9a) sample is shown (Figure 2), where the weight percentage
of PHT is 37% and the film was cast
that thin and ultrathin films prepared byspin-coating from
chloroform had distinctlydifferent, ™uninteresting∫, morphol-
ogywithout anyindications of long-range order (Figure 3).
Suppression of the formation of nanowires under these
conditions maybe pointing to the keyrole of the intrinsic
self-assemblyof polythiophene: this process might have been
kineticallysuppressed here because of the high rate of
solidification associated with spin-coating from a highly
volatile solvent. This kinetic argument was further supported
byexperiments in which thin films were again cast from
from a 0.5 mgmLÀ1 solution in tol-
uene. The samples were then im-
aged using the variable-tapping
force technique (developed in re-
cent years) with the purpose of using
tapping-mode AFM to studyme-
chanical properties of materials at
the
nanoscale.[19]
Interestingly,
™nanowires∫ were clearlydiscerni-
ble onlyunder ™hard tapping∫ con-
ditions (tens to hundreds nanonew-
tons) and were barelyidentifiable
when imaged with forces of the
order of just a few nanonewtons.
Figure 1. ™Nanowire∫ morphology in poly(3-hexylthiophene)-b-poly(styrene) copolymers solvent-cast
from toluene and visualized with tapping-mode AFM. Left: height; right: phase (see text). These and
subsequent images use gray-scale coding, with darker tones corresponding to lower values (for example,
lower heights.)
The formation of
a ™nanowire∫
structure is undoubtedlydictated
by the immiscibility of polystyrene
and poly(3-hexylthiophene). Under
such circumstances nanowires can
be predicted to have a core shell
architecture, with the minoritycom-
ponent (polythiophene) constitut-
ing the core. Such a sheathed struc-
ture is indeed consistent with our
variable-force experiments. Under
™light tapping∫ conditions, the tip
sample force is not high enough to
penetrate through the outer sheath
of the nanowires. Since the polystyr-
ene segments in the sheath can mix
with the chains from adjacent ag-
gregates, the boundaries between
nanowires are not well resolved. In
contrast, under ™hard tapping∫ con-
ditions the probe sample force is
high enough to deform the outer
sheath and ™sense∫ the presence of
a rigid core. These structures have
also been observed in transmission
electron microscopy(TEM) experi-
ments as well.
Figure 2. Under ™light tapping∫ conditions (top) onlythe outer, partiallyoverlapping shells of aggregates
are visualized and the nanowire morphologyis not apparent. In contrast, under ™hard tapping∫ conditions
(bottom), the probe interacts with the outer shells with sufficient force to ™sense∫ the presence of rigid
poly(3-alkylthiophene) cores.
The strong tendencyof regiore-
gular poly(alkylthiophenes) to self-
assemble into stacked aggregates
raises a question about the role of
this intrinsic self-assemblyin the
formation of nanowires. An impor-
tant observation in this regard is
Figure 3. Tapping-mode AFM images of ultrathin films of poly(3-hexylthiophene)-b-poly(styrene) spin-
coated from chloroform. Left: height; right: phase.
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[8] F. S. Bates, Science 1991, 251, 898.
toluene, but under a stream of nitrogen. Under these
accelerated evaporation conditions the formation of nano-
wires was also totallysuppressed.
[9] J. Liu, R. S. Loewe, R. D. McCullough, Macromolecules 1999, 32, 5777.
[10] a) R. D. McCullough, R. D. Lowe, M. Jayaraman, D. L. Anderson, J.
Org. Chem. 1993, 58, 904; b) R. D. McCullough, P. C. Ewbank, R. S.
Loewe, J. Am. Chem. Soc. 1997, 119, 631.
The nanowire morphologies found in block copolymers[20]
of regioregular poly(alkylthiophenes) points to the possibility
of guiding the intrinsic self-assemblyof sufficientlyregular
conjugated polymer chains by coupling them chemically to
incompatible segments. In the simplest case, the obtained
structure is the result of interplaybetween different driving
forces of self-assembly( p stacking versus phase separation).
To rephrase this point, the free-energylandscape of rigid
conjugated molecules has few deep local minima as a result of
strong p interactions which cannot be easilyexplored under
normal conditions. Thus, the molecules are easilytrapped in
the states with a high degree of local stacking, but at the same
time there is a high concentration of defects that adversely
affects the bulk properties. Copolymerization with incompat-
ible flexible segments brings into playcompeting driving
forces of self-assemblythat result in ™more interesting∫ and
easier to explore free-energylandscapes. Identifiyng the
overall features of those energylandscapes maygive us the
abilityto exercise control of the resulting nanostructures, and
effectivelypave the wayto applications of conjugated
polymers as building blocks for future nanoscale and molec-
ular level electronic devices.
[11] Polymer 2 was prepared bythe following method: Distilled diisopro-
pylamine (1.4 mL, 10 mmol) and 2.58m nBuLi (3.7 mL, 9.5 mmol)
were dissolved in dryTHF (50 mL) at À788C. The mixture was then
warmed to room temperature over 5 min and then cooled to À788C.
The monomer 2-bromo-3-hexylthiophene (2.5 g, 10 mmol) was added
to the freshlygenerated lithium diisopropylamine and the reaction
stirred at À708C for 1 h. Anhydrous ZnCl2 (1.43 g,10.5 mmol) was
then added at À708C and the reaction was stirred for 1 h. The reaction
was warmed to 08C and [Ni(dppp)Cl2] (35 mg, 0.065 mmol,
0.6 mol%) was added. The mixture was warmed to room temperature
and then stirred for an additional 30 min. Polymer 2 was precipitated
with methanol. The polymer was washed/fractionated by Soxhlet
extraction with methanol, hexane, dichloromethane, and tetrahydro-
furan. The THF fraction was characterized and used in further
experiments. 1H NMR (CDCl3): d ˆ 6.98 (s, 1H), 2.79 (t, J ˆ 7.7 Hz,
2H), 1.62 (m, 2H), 1.48 (m, 2H), 1.36 (m, 4H), 0.90 (t, J ˆ 6.3 Hz, 3H);
MALDI-MS: Mn ˆ 7634 (PDI ˆ 1.14, end groups: H/Br ca. 95%, H/H
‡
ca. 4%, Br/Br ca. 1%), 7729.02 [M ] (calcd: 7730.12, given a degree of
polymerization of 46 for the PHT); GPC: Mn ˆ 16,800 (PDI ˆ 1.28).
[12] Characterization of 6: 1H NMR 6.98 (s, 49H), 6.89 (d, J ˆ 3.3 Hz, 1H),
6.80 (d, J ˆ 3.3 Hz, 1H), 4.65 (t, J ˆ 3.7 Hz, 1H), 3.82 (m, 2H), 3.65 (m,
2H), 3.11 (t, J ˆ 6.6 Hz, 2H), 2.79 (t, J ˆ 7.63 Hz, 100H), 1.62 (m,
98H), 1.48 1.36 (m, 300H), 0.9 (t, J ˆ 6.3 Hz, 150H); MALDI-MS:
‡
m/z: 7859.36 [M ] (calcd: 7860.88, given a degree of polymerization of
46); GPC: Mn ˆ 17000 (PDI ˆ 1.28). Charaterization of 7: 1H NMR
d ˆ 6.98 (s, 49H), 3.89 (t, J ˆ 6.6 Hz, 2H), 3.07 (t, J ˆ 6.6 Hz, 2H), 2.79
(t, J ˆ 7.6 Hz, 99H), 1.62 (m, 99H), 1.48 1.36 (m, 300H), 0.9 (t, J ˆ
Received: July26, 2001
Revised: September 20, 2001 [Z17609]
‡
6.3 Hz, 148H); MALDI-MS: m/z: 7776.88 [M ](calcd: 7777.35, given
a degree of polymerization of 46); GPC: Mn ˆ 16740 (PDI ˆ 1.28).
[13] a) J.-S.Wang, K. Matyjaszewski, J. Am. Chem. Soc. 1997, 119, 5614;
b) T. E. Patten, J. Xia, T. Abernathy, K. Matyjaszewski,Science 1996,
272, 866; c) K. Matyjaszewski, T. E. Patten, J. Xia, J. Am. Chem. Soc.
1997, 119, 674; d) K. Matyjaszewski, B. Gobelt, H.-J. Paik, C. P.
Horwitz, Macromolecules 2001, 34, 430.
[14] Characterization of 8: 1H NMR 6.98 (s, 49H), 4.38 (m, 1H), 4.40 (m,
2H), 3.19 (t, J ˆ 6.6 Hz, 2H), 2.79 (t, J ˆ 7.6 Hz, 99H), 1.83 (d, J ˆ
6.6 Hz, 3H), 1.62 (m, 99H), 1.48 1.36 (m, 300H), 0.9 (t, J ˆ 6.3 Hz,
148H).
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[16] See Supporting Information.
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