Synthesis and Solution Aggregation of Polystyrene-Oligo(p-phenyleneethynylene)-Polystyrene Triblock Copolymer

Article abstract of DOI:10.1021/ma035503e

The solution aggregation and synthesis of polystyrene-oligo(p-phenyleneethynylene) (OPE)-polystyrene triblock copolymer are studied. Variation of physicochemical interactions between the relative lengths of the blocks and the constituent blocks allow a tailoring of morphologies. The use of the solvent composition to influence chain conformations is also demonstrated. It is found that the triblock copolymer exhibits unique solvatochromatic behaviors through planarization of the conjugated backbone and aggregation-induced π-π stacking.

Full text of DOI:10.1021/ma035503e

1172
Macromolecules 2004, 37, 1172-1174
rigid unsubstituted OPEs and flexible polystyrene is
expected to induce the formation of novel supramolecu-
lar assemblies through the microphase separation in the
solid state and selective solvation in solution.
Syn th esis a n d Solu tion Aggr ega tion of
P olystyr en e-Oligo(p-p h en ylen eeth yn ylen e)-
P olystyr en e Tr iblock Cop olym er
As shown in Scheme 1, a bifunctional phenylene-
ethynylene trimer, 1, with an amino end group and an
iodo terminal group was synthesized and then coupled
with a carboxyl acid-terminated polystyrene (PS-COOH,
2) to yield a polystyryl-OPE, 3.17 Carboxyl terminal
polystyrene was prepared by anionic polymerization
initiated with sec-BuLi and subsequently adding ultra-
pure carbon dioxide into the polymerization solution.18
The formation of the triblock copolymer, 4, was accom-
plished under Pd/Cu cross-coupling condition, using 1,4-
diethynylbenzene to connect two polystyryl-OPE to-
gether. The incorporation of rigid block into polymeric
chain was proven by spectroscopic studies. After the
Ku n Li a n d Qin g Wa n g*
Department of Materials Science and Engineering,
The Pennsylvania State University,
University Park, Pennsylvania 16802
Received October 6, 2003
Revised Manuscript Received December 16, 2003
The supramolecular organization of π-conjugated
polymers is a topic of great importance because it has
a profound effect on the macroscopic properties of the
materials such as luminescence quantum efficiency and
charge-carrier mobility.1,2 One of the most promising
strategies to manipulate supramolecular structures is
the incorporation of conjugated polymer chains into
rod-coil block copolymer architectures as the rodlike
component.3 Aggregation and microphase separation of
rod-coil block copolymers containing π-conjugated poly-
mers and oligomers have yielded a number of nanoscale
morphologies, such as lamellar, spherical, cylindrical,
and vesicular structures with tunable optical and
electronic proprieties.4-9 Variation of physicochemical
interactions between the constituent blocks and the
relative lengths of the blocks will allow a tailoring of
morphologies and formation of well-defined conductive
domains in nanoscale dimensions. Compared to the
effort directed at understanding the self-assembly of
rod-coil diblock copolymers, triblock copolymers con-
taining conjugated moieties are much less studied.10,11
In this Communication, we describe the synthesis of a
coil-rod-coil triblock copolymer consisting of oligo(p-
phenyleneethynylene) (OPE) as the rod segment and
polystyrene as the coil segment. It is found that this
triblock copolymer exhibits unique solvatochromatic
behaviors through aggregation-induced π-π stacking
and planarization of the conjugated backbone.
The phenyleneethynylene polymers (PPEs) and oli-
gomers are a class of materials with demonstrated
applications in displays, sensors, and molecular elec-
tronics.12 Its unique optoelectronic properties associated
with strong aggregation behavior spark the ideas of the
incorporation of OPEs into block copolymer architec-
tures.13-15 Most approaches to OPE containing block
copolymers include the introduction of alkyl or alkyoxy
side chains, generally at R-positions of the phenyl rings
in OPEs, to increase solubility and processability.
Recently, Coudret and co-workers reported the synthesis
of unsubstituted OPE designed for self-assembly mono-
layers.16 Scheme 1 outlines our synthetic route used to
prepare the triblock copolymer containing unsubstituted
OPEs. This convergent approach circumvents the in-
solubility problem of OPEs without introducing the
R-position substituents and allows extension of the
chemistry to longer OPEs and other architectures. A
large ø value (the Flory-Huggins segment-segment
interaction parameter) due to significant differences in
structures and solubility parameters between the highly
1
coupling reaction of 1 and 2, the H NMR spectrum of
3 shows no signal for residual amine groups at 3.92 ppm
and affords a doublet at 7.75 ppm and a singlet at 7.54
ppm, which are characteristic signals of benzylic protons
from OPE. Four signals at 90-94 ppm in the 13C NMR
spectrum of 3 are assigned to ethine carbons from OPE.
1
In the H NMR spectrum of 4, the absence of a doublet
around 7.75 ppm corresponding to aromatic protons
adjacent to the iodo group, and the increased intensity
of a singlet at 7.54 ppm indicates the presence of
heptamer OPE in the triblock copolymer. The Raman
spectrum of 4 reveals an intense signal at 2205 cm-1
attributed to the CtC bonds. The proposed structures
were proved by gel permeation chromatography (GPC)
studies as well. For 2, the number-average molecular
weight (Mn) was calculated to be 2791 with a polydis-
persity (PDI) of 1.10 using THF as the eluent and
polystyrenes as standards. For 3, a Mn of 3141 (PDI )
1.14) was determined. Dimerization of polystyryl-OPE
results in the triblock copolymer with an approximately
double Mn of 6128 (PDI ) 1.17). The transformation of
1 into 2 was accompanied by a blue shift of the
absorption maximum from 345 to 335 nm in the UV/
vis spectra. This 10 nm shift is attributed to the
electron-withdrawing effect of amido bonds, which
further confirmed the formation of polystyryl-OPE,
rather than the mixture of two starting materials. Since
there was no special steric or push-pull effect present
in the triblock copolymer, as expected, the π-π* absorp-
tion of 4 is red-shifted to 368 nm due to the increase in
conjugation length. The fluorescence spectrum of 4 in
toluene gives an emission maximum at 406 nm with two
small shoulders at about 424 and 467 nm derived from
low-energy vibronic sidebands and a quantum yield of
0.78.
The intermolecular aggregation of OPE derivatives
has been generally observed in both solution and solid
states with a substantial influence on optical prop-
erties.19-21 We investigated the solvatochromism of 4
to study its aggregation behavior in solution. The
copolymer 4 is found to dissolve readily in solvents such
as THF, dichloromethane, chloroform, toluene, and
1,1,2,2-tetrachloroethane (TCE) at room temperature to
give a clear, colorless solution. The absorption spectra
are independent of concentration. Even in relatively
concentrated solutions (1.5 g/L, measured in a 1 mm
cell), only one broad band is detected. However, the
* To whom correspondence should be addressed. E-mail:
wang@matse.psu.edu.
10.1021/ma035503e CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/17/2004

Macromolecules, Vol. 37, No. 4, 2004
Communications to the Editor 1173
Sch em e 1. Syn th esis of Tr iblock Cop olym er 4
F igu r e 2. UV/vis absorption spectra of 4 in TCE/MeOH
mixtures. The solvent composition is marked by the volume
percentage of TCE.
F igu r e 1. UV/vis absorption spectra of PS-OPE-PS, 4, in
toluene/MeOH mixtures and thin film at room temperature.
The “aggregate band” is observed at 410 nm. The solvent
composition is marked by the volume percentage of toluene.
It is noteworthy that the triblock copolymer 4 exhibits
different solvatochromatic behaviors in different solvent
systems, which reveals the fact that 4 can readily adopt
diverse assemblies in response to the external stimuli.
Addition of methanol to a solution of 4 in TCE leads to
the reverse solvatochromatic shift (Figure 2). When
more than 50 vol % methanol is added, the PS-OPE-
PS TCE solution begins to show the hypsochromic shift
behavior. At a 1:2 TCE/methanol ratio, the λmax is blue-
shifted to 332 nm with the formation of an aggregation
peak at 410 nm. The observed hypsochromic shift (36
nm) is considerable and can be interpreted by the
molecular exciton model, assuming H-type aggregates
formed by a parallel orientation of OPE segments.28,29
The observed blue shift is a consequence of interchain
π-π interaction between parallel OPE segments raised
from aggregates, which accompanied by strong fluores-
cence quenching. Fluorescence spectroscopy of 4 (Figure
3) illustrated that upon increasing the methanol content
absorption spectrum changes dramatically if a nonsol-
vent such as methanol is added. As shown in Figure 1,
addition of methanol to a solution of 4 in toluene leads
to the main transition band (λmax) red-shifted from 368
to 376 nm and, concomitantly, with the development of
an additional aggregate-induced band at 410 nm. At
about 65 vol % methanol, the aggregate band is com-
pletely developed, and the solution spectrum of 4 is
almost superimposable to that of thin films. Similar
effects have been observed in dialkyl- and dialkoxy-
substituted poly(phenyleneethynylene)s as well as other
conjugated polymers such as poly(alkylthiophene)s.22-24
The general consensus about the bathochromic shift in
λmax is caused by the increased effective conjugated
length as a result of the conjugated chain backbone
becoming more planar due to the formation of ag-
gregates in solution or solid states.18,25-27

1174 Communications to the Editor
Macromolecules, Vol. 37, No. 4, 2004
Ack n ow led gm en t. This work was supported by the
Pennsylvania State University, the Center for Optical
Technologies, and the Pennsylvania Department of
Community and Economic. Assistance with SANS stud-
ies by Drs. L. Guo and P. Thiyagarajan at Argonne
National Laboratory is appreciated.
Su p p or tin g In for m a tion Ava ila ble: Synthetic proce-
dures and characterization data for trimer 1, PS-COOH 2,
polystyryl-OPE 3, and triblock copolymer 4; fluorescence
spectra of 4 in TCE, TCE/MeOH, and toluene/MeOH; the
quantum yield of 4 in TCE/MeOH mixtures as a function of
volume fraction of methanol; SANS studies of 4 in TCE and
toluene. This material is available free of charge via the
Internet at http://pubs.acs.org.
F igu r e 3. Fluorescence spectra of 4 in TCE/MeOH mixtures
at room temperature. The solvent composition is marked by
the volume percentage of TCE. Excitation was performed at
332 nm.
Refer en ces a n d Notes
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of the TCE/methanol mixture from 0 to 65 vol % the
fluorescence intensity decreases and the quantum yield
declines from 0.78 to 0.17. Although reduced photolu-
minescence quantum yields have also been observed in
toluene/methanol mixtures, the TCE/methanol system
exhibits larger decrease of fluorescent efficiency. This
is due to the presence of densely packed aggregates,
further confirming the H-type aggregate formation in
TCE/MeOH. Small-angle neutron scattering (SANS)
experiments were performed to rationalize the observed
solvatochromatic properties. SANS in toluene evidenced
the presence of nanoscopic aggregates of 4, and the
calculated average aggregation number is about 5 (see
Supporting Information for details). As for the case of
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aggregation. Without the steric repulsion from R-posi-
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formation of parallel aligned OPE assemblies as well.
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In conclusion, we described the synthesis, character-
ization, and solution aggregation of a coil-rod-coil
triblock copolymer 4. It has been demonstrated that the
use of the solvent composition to influence chain con-
formations and thus to manipulate the packing of the
conjugated polymer blocks is important for achieving
control in the assembly of conducting polymers and
associated optical characteristics. Our ongoing work
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