Syntheses of amphiphilic diblock copolymers containing a conjugated block and their self-assembling properties

Article abstract of DOI:10.1021/ja0010812

An efficient approach to the syntheses of amphiphilic rod - coil diblock and coil - rod - coil triblock copolymers was developed. Each diblock copolymer consists of a perfectly monodispersed oligo(phenylene vinylene) covalently bonded to a poly(ethylene g

Full text of DOI:10.1021/ja0010812

J. Am. Chem. Soc. 2000, 122, 6855-6861
6855
Syntheses of Amphiphilic Diblock Copolymers Containing a
Conjugated Block and Their Self-Assembling Properties
Hengbin Wang,† H. Hau Wang,‡ Volker. S. Urban,§, Kenneth C. Littrell,§
P. Thiyagarajan,§ and Luping Yu*,†
Contribution from the Department of Chemistry and The James Franck Institute, The UniVersity of
Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637, Chemistry and Materials Science DiVisions
and Intense Pulsed Neutron Source, Argonne National Laboratory, 9700 South Cass AVe.,
Argonne, Illinois 60439, and European Synchrotron Radiation Facility, Grenoble, France
ReceiVed March 27, 2000
Abstract: An efficient approach to the syntheses of amphiphilic rod-coil diblock and coil-rod-coil triblock
copolymers was developed. Each diblock copolymer consists of a perfectly monodispersed oligo(phenylene
vinylene) covalently bonded to a poly(ethylene glycol) block with a very low polydispersity (<1.05). The
structure and basic physical properties of these copolymers were characterized by various spectroscopic
techniques such as NMR, MALDI-TOF, GPC, DSC, UV/vis, and the fluorescence study. These diblock
copolymers were shown to possess remarkable self-assembling abilities, and long cylindrical micelles (>1
µm) were formed. TEM, SANS, and AFM studies showed that the core of the micelles has a diameter of
∼8-10 nm and was composed of an OPV block. TEM and SANS studies revealed that these OPV-PEG
micelles have a cylindrical OPV core surrounded by a PEG corona. Cryo-TEM and SANS studies indicate
that fibers were formed even in very dilute THF/H2O solutions. Since the conjugated OPV blocks exhibit
liquid crystallinity and electric and optical properties, these micelles are interesting for studying the electroactive
effect in a nanometer scale.
During the past few decades, scientists have been fascinated
by the microphase separation properties of block copolymers,
especially diblock copolymers.1-12 Because the two covalently
connected blocks are physically incompatible, the system will
undergo a phase separation to form a highly symmetric
morphology of repeated structures, such as spheres, cylinders,
or lamellae, formed on nanometer scales. Diblock copolymers
that have flexible chains in both blocks have been extensively
studied.1-7 Our current understanding of the phase behavior of
diblock copolymers is largely based on these systems.2,8
However, in recent years, there has been a growing interest in
diblock copolymers with rod-coil structures, based on which
new morphologies and phase separation behaviors have been
observed.9-12 Our group is interested in this area because the
self-assembling ability of the diblock copolymers offers a chance
to organize functional species into nanometer domains. This
research area provides an opportunity to synthesize functional
polymers with well-defined supramolecular architectures. New
materials with promising physical, chemical, electronic, and
optical properties can be generated. One kind of functional
species in which we are interested is conjugated polymers
because of their unique electrical and optical properties, such
as high electrical conductivity upon doping, high third-order
optical nonlinearity, and electroluminescent properties.13-16
Incorporation of a conjugated polymer chain into heterogeneous
diblock copolymers creates a new class of self-assembling
electroactive materials in which the classical lamella, cylindrical,
and spherical morphologies of diblock copolymers allow for
one-, two-, and three-dimensional confinement of the electro-
active regions. All of these morphologies may be useful for the
fabrication of novel dielectric and electrically conductive
polymer devices.
† The University of Chicago.
‡ Chemistry and Materials Science Divisions, Argonne National Labo-
rator.
§ Intense Pulsed Neutron Source, Argonne National Laborator.
Synchrotron Radiation Facility.
(1) Aggarwal, S., Ed. Block Copolymers; Plenum Press: New York, 1970.
(2) Bates, F. S.; Fredrickson, G. H. Annu. ReV. 1990, 41, 525.
(3) Szwarc, M.; Levy, M.; Milkovich, R. J. Am. Chem. Soc. 1956, 78,
2656.
(4) Cornelissen, J. J. L. M.; Fischer, M.; Sommerdijk, N. A. J. M.; Nolte,
R. J. M. Science 1998, 280, 1427.
(5) Ruokolainen, J.; Makinen, R.; Torkkeli, M.; MaKela, T.; Serimaa,
R.; Brinke, G. T.; Ikkala, O. Science 1998, 280, 557.
(6) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728.
(7) Massey, J.; Power, N. K.; Manners, I.; Winnik, M. A. J. Am. Chem.
Soc. 1998, 120, 9533.
Recently, we have succeeded in synthesizing OPV-polyiso-
prene diblock copolymers via a two-stage approach.17 Both
blocks in this copolymer system are hydrophobic and are found
to form lamella structures in a wide range of composition. An
interesting question is whether this trend applies to the am-
(13) Skotheim, T. A., Ed. Handbook of Conducting Polymers; Marcel
Dekker, Inc.: New York, 1986; Vols. 1 and 2.
(8) Bates, F. S.; Fridrickson, G. H., Phys. Today 1999, 52, 32.
(9) Radzilowski, L. H.; Wu, J. H.; Stupp, S. I. Macromoleculars 1993,
26, 879.
(10) Li, W.; Maddux, T.; Yu, L. Macromolecules 1996, 29, 7329.
(11) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903.
(12) (a) Marsitzky, D.; Brand, T.; Geerts, Y.; Klapper, M.; Mu¨llen, K.
Macromol. Rapid Commun. 1998, 19, 385. (b) Francke, V.; Ra¨der, H. J.;
Geerts, Y.; Mu¨llen, K. Macromol. Rapid Commun. 1998, 19, 275.
(14) Skotheim, T. A., Ed. ElectroresponsiVe Molecular and Polymeric
Systems; Marcel Dekker, Inc.: New York, 1991: Vol. 2.
(15) Kiess, H. G., Ed. Conjugated Conducting Polymers; Springer Series
in Solid State Science; Springer-Verlag: Berlin, 1992; p 102.
(16) Goodson, T., III; Li, W. J.; Gharavi, A.; Yu, L. P. AdV. Mater. 1996,
9, 639 .
(17) Li, W. J.; Wang, H. B.; Yu, L. P.; Morkved, T. L.; Jaeger, H. M.
Macromolecules 1999, 32, 3034-3044.
10.1021/ja0010812 CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/06/2000

6856 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Yu et al.
Scheme 1. Synthesis of Building Blocks for Diblock Copolymers
spectrophotometer. Emission spectra were collected by using a Shi-
madzu RF-5301PC spectrofluorophotometer.
phiphilic type of rod-coil diblock copolymer. In this paper,
we describe a new approach for the synthesis of a diblock
copolymer system consisting of an oligo(phenylene vinylene)
and poly(ethylene glycol), an amphiphilic type of rod-coil
diblock copolymer. It was found that these diblock copolymers
exhibit remarkable self-assembling capabilities, particularly the
capability to form wormlike cylindrical micelles even at very
low concentrations.
Transmission Electron Microscopy. Films were prepared by
placing a drop of the copolymer solution onto carbon-coated or holey
TEM grids. The grids were dried in the air or with the atmosphere of
a solvent. The specimen may be exposed to osmium tetraoxide overnight
or washed with a drop of phosphotungstic acid to get a stain effect.
The specimen was then examined in a Philips CM120 electron
microscope operated at 120 KV.
Small-Angle Neutron Scattering. Scattering experiments were
performed by using the time-of-flight instrument SAND at the Intense
Pulsed Neutron Source at Argonne National Laboratory. By using
neutrons with wavelengths in the range of 0.5-14.0 Å by time-of-
flight and a 40 × 40 cm2 position sensitive proportional counter at a
fixed sample-to-detector distance of 2 m, SAND produces data in the
scattering vector q (q ) 4π sin(θ/λ), where θ is half the scattering
angle and λ is the wavelength of the incident neutrons) range of 0.004-
0.8 Å-1 in a single measurement. To obtain best contrast for the SANS
signals, deuterated THF was used as a solvent. In addition D2O was
used for the investigation of the effect of water on the self-assembly
of the diblock copolymer in deuterated THF. The solutions were
measured in Suprasil (quartz) cylindrical cells with 2 mm path length.
The data for each sample were corrected for the backgrounds from the
instrument, the Suprasil cell, solvent, and sample transmission and
placed on an absolute scale by using the routine procedures at IPNS.18
Synthesis. A general synthetic procedure is outlined in Scheme 1.
Compounds A and B were synthesized by similar procedures described
previously.17
General Procedure for the Heck Reaction. A (0.83 g, 2.06 mmol),
tri-o-tolylphosphine (0.125 g, 0.412 mmol), NBu3 (0.74 mL, 4.12
mmol), Pd(OAc)2 (0.023 g, 0.103 mmol), and 5 (1.00 g, 2.06 mmol)
were dissolved in anhydrous DMF (10 mL). The mixture was heated
Experimental Section
Materials. 1,4-Dichlorobezene, 1-bromohexane, methyl 4-methyl-
benzoate, triethyl phosphite, poly(ethylene glycol) methyl ether (average
Mn ca. 1900, 5000) and the other conventional reagents were used as
received. Divinylbenzene was purified according to a literature
procedure.10 Tetrahydrofuran was dried by distillation from sodium
metal. Ethylene glycol dimethyl ether and N,N-dimethylformamide were
purified by distillation from calcium hydride.
1H NMR spectra were recorded on a Bruker AM 400 spectrometer.
A differential scanning calorimeter (TA instruments DSC 10) was used
to determine the thermal transitions in a heating rate of 10 °C min-1
.
The results were reported as the maxima and minima of their
endothermic or exothermic peaks. A polarized optical microscopy
(Nikon Optiphoto-2, magnification ×400) equipped with a high-
temperature stage (Creative devices 50-600) was used to observe the
thermal transitions and to analyze the anisotropic texture. Molecular
weight distributions were determined by using gel permeation chro-
matography (GPC) with a Waters Associates liquid chromatograph
equipped with a Waters 510 HPLC pump, Waters 410 differential
refractometer, and Waters 486 tunable absorbance detector. THF was
used as the eluent and polystyrene as the standard. Elemental analyses
were performed by Atlantic Microlab, INC. MALDI-TOF spectra were
performed by Washington University Resource for Biomedical and Bio-
organic Mass Spectrometry lab with dithranol as the matrix. UV-vis
spectra were collected by using a Shimadzu UV-2401PC recording
(18) Thiyagarajan, P.; Epperson, E. J.; Crawford, K. R.; Carpenter, J.
M.; Klippert T. E.; Wozniak D. J. Appl. Crystallogr. 1997, 30, 280-293.

Syntheses of Amphiphilic Diblock Copolymers
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 6857
at 90 °C for 12 h and poured into 100 mL of methanol after being
cooled to room temperature. The precipitate was collected by suction
filtration and washed with methanol. The product was purified by
column chromatography using a mixture of chloroform and hexane as
the eluent, yielding pure 6 as a yellowish orange solid.
General Procedure for the Wittig Reaction. Compound B (0.95
g, 1.65 mmol) was dissolved in anhydrous DME (10 mL), then dropped
slowly into the mixture of compound 6 (1.00 g, 1.24 mmol), NaH (0.041
g, 1.70 mmol), and DME (10 mL). The resulting solution was stirred
under reflux for 5 h and poured into 100 mL of methanol after being
cooled to 0 °C. The precipitate was collected by suction filtration and
washed with methanol. The product was purified by column chroma-
tography by using a mixture of chloroform and hexane as the eluent to
give pure 7 as a yellowish orange solid.
Compound A. 1H NMR (CDCl3, ppm): δ10.25 (s, 1 H), 7.64 (s, 1
H), 7.51 (d, J ) 8.3 Hz, 2 H), 7.48, (s, 1 H), 7.44 (d, J ) 8.3 Hz, 2 H),
7.34 (d, J ) 16.1 Hz, 1 H), 7.12 (d, J ) 16.1 Hz, 1 H), 6.74 (dd, J )
10.9 Hz, 1 H), 5.80 (d, J ) 17.6 Hz, 1 H), 5.29 (d, J ) 10.9 Hz, 1 H),
3.01 (t, J ) 7.8 Hz, 2 H), 2.77 (t, J ) 7.8 Hz, 2 H), 1.62 (m, 4 H),
1.40 (m, 4 H), 1.33 (m, 8 H), 0.89 (m, 6 H). Anal. Calcd for C28H38O:
C, 86.52; H, 9.51. Found: C, 86.37; H, 9.48.
Compound B. 1H NMR (CDCl3, ppm): 7.46 (d, J ) 8.1 Hz, 2 H),
7.41 (s, 1 H), 7.32 (s, 1 H), 7.31 (dd, J1 ) 8.1 Hz, J2 ) 2.3 Hz, 2 H),
7.24 (d, J ) 16.1 Hz, 1 H), 6.95 (d, J ) 16.1 Hz, 1 H), 4.03 (m, 4 H),
3.17 (d, J ) 21.8 Hz, 2 H), 2.68 (m, 4 H), 1.61 (m, 4 H), 1.33 (m, 4
H), 1.26 (m, 8 H), (0.89 (m, 6 H). Anal. Calcd for C31H46BrO3P: C,
64.46; H, 8.03. Found: C, 64.46; H, 8.04.
1 H), 7.05 (d, J ) 16 Hz, 1 H), 7.00 (d, J ) 16 Hz, 1 H), 3.93 (s, 3
H), 2.78 (m, 8 H), 2.70 (m, 4 H), 1.64 (m, 12 H), 1.42 (m, 12 H), 1.35
(m, 24 H), 0.90 (m, 18 H). Anal. Calcd for C84H109BrO2: C, 81.98; H,
8.93; Br, 6.49. Found: C 81.62; H, 8.85.
Compound 9. 1H NMR (CDCl3, ppm): 7.54 (s, 4 H), 7.50 (d, J )
8.3 Hz, 4 H), 7.46 (s, 2 H), 7.45 (s, 2 H), 7.43 (d, J ) 8.3 Hz, 4 H),
7.39 (d, J ) 16.1 Hz, 2 H), 7.36 (d, J ) 16.1 Hz, 2 H), 7.05 (d, J )
16.1 Hz, 2 H), 7.03 (d, J ) 16.1 Hz, 2 H), 6.74 (dd, J1 ) 11.0 Hz, J2
) 17.5 Hz, 2 H), 5.78 (d, J ) 17.5 Hz, 2 H), 5.26 (d, J ) 11.0 Hz, 2
H), 2.77 (m, 8 H), 1.64 (m, 8 H), 1.35 (m, 8 H), 1.25 (m, 16 H), 0.90
(m, 12 H). Anal. Calcd for C66H82: C, 90.56; H, 9.44. Found: C, 90.29;
H, 9.43.
Preparation of Diblock Copolymer Precursors. Compound 7
(0.8355 g, 0.679 mmol) and KOH (0.183 g, 3.4 mmol) were dissolved
in THF (15 mL) and 0.6 mL of H2O was added. The resulting solution
was heated to reflux for 12 h, and then poured into 100 mL of methanol
at room temperature. The precipitate was collected by suction filtration
and washed with methanol. The product was further purified by flash
chromatography using a mixture of chloroform and methanol as the
eluent to give a yellow solid (OPV6-acid).
The mixture of poly(ethylene glycol) monomethyl ether (MW )
1900) (0.63 g 0.33 mmol), OPV6-acid (0.368 g, 0.3 mmol), and PPh3
(87 mg, 0.33 mmol) in 15 mL of THF was heated to 80 °C. When all
the starting material dissolved, the solution was cooled to 35 °C and
DEAD (63 mg, 0.36 mmol) was added. After the resulting solution
was stirred at 35 °C for 24 h, the THF was distilled out by vacuum
distillation. The product was further purified by flash chromatography
using a mixture of chloroform and methanol as the eluent to give
compound 8 as a yellow solid.
Compound 1. 1H NMR (CDCl3, ppm): 7.41 (s, 1 H), 7.40 (d, J )
7.9 Hz, 2 H), 7.32 (s, 1 H), 7.20 (d, J ) 15.9 Hz, 1 H), 7.17 (d, J )
7.8 Hz, 2 H), 6.95 (d, J ) 15.9 Hz, 1 H), 2.68 (m, 4 H), 2.36 (s, 3 H),
1.59 (m, 4 H), 1.33 (m, 8 H), 0.90 (m, 6 H). Anal. Calcd for C27H37Br:
C, 73.45; H, 8.46; Br, 18.10. Found: C, 73.39; H, 8.46; Br, 17.98.
1
Compound 8 (n ) 110). H NMR (CDCl3, ppm): 8.06 (d, J ) 8
Hz, 2 H), 7.58 (d, J ) 8 Hz, 2 H), 7.54-7.52 (m, 8 H), 7.49-7.46 (m,
5 H), 7.44 (s, 1 H), 7.39 (d, J ) 16 Hz, 2 H), 7.39 (d, J ) 16 Hz, 1
H), 7.28 (d, J ) 16 Hz, 1 H), 7.06 (d, J ) 16 Hz, 2 H), 7.056 (d, J )
16 Hz, 1 H), 7.05 (d, J ) 16 Hz, 1 H), 7.00 (d, J ) 16 Hz, 1 H), 4.49
(t, J ) 5.1 Hz, 2 H), 3.85 (t, J ) 5.1 Hz, 2 H), 3.82 (t, J ) 5.1 Hz, 4
H), 3.64 (m, 432 H), 3.46 (t, J ) 5.1 Hz, 4 H), 3.38 (s, 3 H), 2.78 (m,
8 H), 2.68 (m, 4 H), 1.64 (m, 12 H), 1.42 (m, 12 H), 1.35 (m, 24 H),
0.91 (m, 18 H).
1
Compound 2. H NMR (CDCl3, ppm): 10.25 (s, 1 H), 7.65 (s, 1
H), 7.55 (s, 4 H), 7.50 (s, 1 H), 7.45-7.42 (m, 4 H), 7.40 (d, 1 H),
7.37 (d, J ) 16.0 Hz, 1 H), 7.31 (d, J ) 16.1 Hz, 1 H), 7.19 (d, J )
8.0 Hz, 2 H), 7.15 (d, J ) 16.1 Hz, 1 H), 7.05 (d, J ) 16.1 Hz, 1 H),
7.02 (d, J ) 16.0 Hz, 1 H), 3.03 (t, 2 H), 2.77 (m, 6 H), 2.38 (s, 3 H),
1.64 (m, 8 H), 1.42 (m, 8 H), 1.34 (m, 16 H), 0.90 (m, 12 H). Anal.
Calcd for C56H74O: C, 86.32; H, 9.57. Found: C, 87.44; H, 9.75.
Compound 3. 1H NMR (CDCl3, ppm): 7.54-7.51 (m, 8 H), 7.46-
7.42 (m, 7 H), 7.39 (d, J ) 16 Hz, 1 H), 7.38 (d, J ) 16 Hz, 2 H), 7.34
(s, 1 H), 7.31 (d, J ) 16 Hz, 1 H), 7.28 (d, J ) 16 Hz, 1 H), 7.20 (d,
J ) 8 Hz, 2 H), 7.054 (d, J ) 16 Hz, 1 H), 7.048 (d, J ) 16 Hz, 1 H),
7.042 (d, J ) 16 Hz, 7.01 (d, J ) 16 Hz, 1 H), 6.99 (d, J ) 16 Hz, 1
H), 2.77 (m, 8 H), 2.73 (m, 4 H), 2.38 (s 3 H), 1.65 (m, 12 H), 1.43
(m, 12 H), 1.34 (m, 24 H), 0.90 (m, 18 H). Anal. Calcd for C84H109Br:
C, 84.01; H, 9.26. Found: C, 83.76; H, 9.35.
Compound 4. 1H NMR (CDCl3, ppm): 7.54 (b, 8 H), 7.49 (d, J )
8 Hz, 2 H), 7.46-7.34 (m, 15 H), 7.31 (d, J ) 16 Hz, 1 H), 7.18 (d,
J ) 8 Hz, 2 H), 7.06-6.99 (m, 6 H), 6.71 (dd, J1 ) 18 Hz, J2 ) 10
Hz, 1 H) ), 5.77 (d, J ) 18 Hz, 1 H), 5.25 (d, Jj ) 10 Hz, 1 H), 2.76
(m, 12 H), 2.37 (s, 3 H), 1.66 (m, 12 H), 1.35 (m, 24 H), 0.90 (m, 18
H). Anal. Calcd for C93H118: C, 90.38; H, 9.62. Found: C, 90.16, H,
9.67.
Result and Discussion
Synthesis. In our two-stage approach, we had to synthesize
long conjugated molecules such as oligothiophene, oligo-
(phenylene vinylene) with mono- or difunctionalities.10,16,17
These oligomers were then coupled with living polymeric
species. It became apparent that this approach has several
limitations. The most critical one is the limited solubility of
OPVs, which causes difficulty in purification and lowers the
efficiency in any further coupling reaction. To overcome this
problem, we developed a convergent approach to the diblock
copolymers. Scheme 1 shows the synthesis of asymmetric
building blocks. Scheme 2 shows the coupling of these blocks
at the final step to the block copolymers. The advantage of this
approach is that the solubility of the smaller pieces of the
conjugated block is sufficient for coupling and the resulting
block copolymers are soluble in the reaction medium. Therefore,
the coupling reaction became more efficient. The product and
the starting materials were found to have enough contrast in
solubility and polarity that the purification became much easier
than with the previous approach. This approach also permits
easy synthesis of triblock copolymers and other architectures
(Scheme 2).
Compound 5. 1H NMR (CDCl3, ppm): 8.04 (d, J ) 8.3 Hz, 2 H),
7.56 (d, J ) 8.3 Hz, 2 H), 7.43 (s, 1 H), 7.37 (d, J ) 15.2 Hz, 1 H),
7.35 (s, 1 H), 7.01 (d, J ) 16.1 Hz, 1 H), 3.93 (s, 3 H), 2.69 (m, 4 H),
1.57 (m, 4 H), 1.43 (m, 4 H), 1.32 (m, 6 H), 0.90 (m, 6 H). Anal.
Calcd for C28H37BrO2: C, 69.27; H, 7.68; Br, 16.46. Found: C, 69.54;
H, 7.79; Br, 16.33.
Compound 6. 1H NMR (CDCl3, ppm): 10.25 (s, 1 H), 8.04 (d, J )
8.3 Hz, 2 H), 7.66 (s, 1 H), 7.60 (d, 2 H), 7.57 (s, 4 H), 7.51 (s, 1 H),
7.48 (d, 1 H), 7.47 (s, 1 H), 7.46 (s, 1 H), 7.41 (d, J ) 16.1 Hz, 1 H),
7.38 (d, J ) 16.1 Hz, 1 H), 7.15 (d, J ) 16.1 Hz, 1 H), 7.07 (d, J )
16.1 Hz, 2 H), 3.94 (s, 3 H), 3.03 (t, J ) 7.8 Hz, 2 H), 2.78 (m, 6 H),
1.64 (m, 8 H), 1.42 (m, 8 H), 1.34 (m, 16 H), 0.91 (m, 12 H). Anal.
Calcd for C57H74O3: C, 84.82; H, 9.24. Found: C, 84.57; H, 9.31.
As outlined in Scheme 1, a monofunctionalized OPV block
with a vinyl end functional group (Compound 4) and a
difunctionalized OPV block with bromide and methyl ester end
functional groups (7) were synthesized stepwise. The R group
(a hexyl hydrocarbon chain) located at the para positions of
phenyl rings in the OPV block is introduced to increase the
solubility and render the block hydrophobic. These OPVs show
1
Compound 7. H NMR (CDCl3, ppm): 8.04 (d, J ) 8 Hz, 2 H),
7.58 (d, J ) 8 Hz, 2 H), 7.54-7.52 (m, 8 H), 7.49-7.46 (m, 5 H),
7.44 (s, 1 H), 7.39 (d, J ) 16 Hz, 2 H), 7.39 (d, J ) 16 Hz, 1 H), 7.28
(d, J ) 16 Hz, 1 H), 7.06 (d, J ) 16 Hz, 2 H), 7.056 (d, J ) 16 Hz,

6858 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Yu et al.
Scheme 2. Convergent Approach for the Syntheses of Diblock Copolymers
Table 1. Thermal Transition Temperatures of Diblock Copolymers
8 (n)45) diblock A diblock B diblock C diblock D
the typical physical properties observed in corresponding
polyphenylene vinylenes, such as strong photoluminescence and
electroluminescent properties.16 Hydrolysis of compound 4
yielded an OPV containing a carboxylic acid group, which was
later reacted with commercially available monodispersed poly-
(ethylene glycol) monomethyl ether to yield compounds 8. Two
PEGs with Mn of 1900 and 5000 were used. The PEG chain is
flexible and strongly hydrophilic. PEG also exhibits some special
properties, such as water solubility and complexation capacity
with alkali metal cations, which can induce various liquid
crystalline supramolecular structures.19-21 Further coupling
between compounds 8 and 4 yielded the final diblock copoly-
mers. Five diblock copolymers (copolymers A-D, 8) with three
types of OPV blocks were synthesized. When a divinyl
compound was used to couple with compound 8, a triblock
copolymer was obtained. The resulting copolymers were purified
by column chromatography (silica gel) using a chloroform/
methanol mixture as the eluent. Preliminary results show that
the triblock copolymer behaves similarly with diblock copoly-
mers (its detailed studies will be published elsewhere).
Tm1 (°C) 43
Tm2 (°C) NA
Tm3 (°C) 126
39
NA
175
39
150
234
46
NA
NA
55
150
235
All of the resulting block copolymers showed a narrow
molecular weight distribution with polydispersity of less than
1.05, as determined from GPC (inset in Figure 1). MS
spectroscopic studies using MALDI-TOF showed a close match
between calculated and experimental molecular weights (Figure
1). Fine peaks in the mass spectrum vary by the mass of one
ethylene oxide unit (44.01), indicating that the polydispersity
of the copolymer mirrors that of the PEG used in the synthesis.
The chemical structures of these copolymers were confirmed
by using 1H NMR. All of the chemical shifts corresponding to
the OPV and PEG blocks appear in the 1H NMR spectrum. The
ratio of ethylene oxide protons to the aromatic protons is
consistent with the ratio calculated according to the number of
repeating units in poly(ethylene glycol) and the OPV blocks.
Thermotropic Phase Behavior. Because the all-trans-OPVs
are rigid molecules,22,23 it was observed that all of the
copolymers manifest a reversible thermotropic liquid crystalline
phase. Table 1 presents the thermal transition temperatures of
Figure 1. (a) MALDI-TOF spectrum of diblock copolymer B. (b) GPC
traces of three copolymers.
the diblock copolymers, deduced from DSC traces. The first
transition, Tm1, corresponds to the melting point of the PEG
block and the third thermal transition, Tm3, corresponds to the
LC-isotropic transition. A visible crystalline melting peak
corresponding to the OPV block can be observed only for an
OPV with 13 benzene rings.24 Optical polarized microscopic
studies of these copolymers revealed a Schlieren texture and
also confirmed that the Tm3 is the LC-isotropic phase transition
temperature.
Electronic Properties. UV-vis and Emission spectra of the
copolymer B solution in THF and THF/water mixture solvents
(0.02 mg/ml) are shown in Figure 2. In pure THF, the copolymer
exhibited an intense transition with absorption maximum at 418
nm resulting from the OPV block (inset to Figure 2). Small-
angle neutron scattering studies show that the diblock copolymer
molecules self-assemble into cylindrical micelles at this con-
centration. Addition of water as a nonsolvent for the OPV block
results in a significant solvatochromic effect. At a THF/H2O
ratio (v/v) of 1:1, the absorption maximum was blue-shifted to
393 nm, accompanied by a slight decrease in the absorption
(19) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509.
(20) Lee, M.; Cho, B. K.; Kim, H.; Yoon, J. Y.; Zin, W. C. J. Am. Chem.
Soc. 1998, 120, 9168.
(21) Schroter, J. A.; Tschierske, C.; Wittenberg, M.; Wendorff, J. H. J.
Am. Chem. Soc. 1998, 120, 10669.
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1993, 26, 5281. (b) Yu, L. P.; Bao, Z. N. AdV. Mater. 1994, 6, 156.
(24) Collings, P. J. Liquid crystals; Princeton University Press: Princeton,
New Jersey, 1990.

Syntheses of Amphiphilic Diblock Copolymers
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 6859
Figure 4. AFM image of diblock copolymer B film (TopMatrix
Discoverer, on Mica substrate, Contact mode).
when a Holey grid was used as the substrate for the casting
film. It can be seen that the whole film is composed of
interwoven fibers several thousands nanometers in length.
Because the micelles are entangled with each other, they can
support themselves over holes as wide as a micrometer in the
holey grid. In various solvent conditions, cylindrical micelles
with different lengths were observed and no staining was needed.
The diameters of the fibers are all identical (8-10 nm) and
long fibers (several micrometers) can be obtained when the
samples are air-dried. All copolymers show identical behavior
except copolymer A & C which forms fibers with smaller
diameters because of its short OPV block. AFM studies also
indicated the formation of fibers, as shown in Figure 4.
However, the fibers are obviously aggregated to form bundles.
These fibers can be observed in films as cast from a diluted
solution. This seems to indicate that the fiber may have been
formed in the diluted solution state. Cryo-TEM (CTEM) images
of the copolymers in vitrified solutions (THF/water 1:10)
confirmed this hypothesis (Figure 3b). Cylindrical micelles were
observed in these samples with the same diameter. It is
interesting to note that after the copolymer solution was
solidified by freezing, the micelles were not straight, rather they
were often curved, looped, and entangled with each other. This
deformation of fibers may be due to the effect of the matrix or
may reflect a snapshot of fiber motion in solution. Although
the TEM micrographs are two-dimensional projections of
microstructures in three-dimensional specimens, it is hard to
tell where a micelle begins and ends. Continuous strands with
lengths of thousands of nanometers were identified.
Figure 2. (A) Fluorescence spectra of diblock copolymer B in THF/
H2O mixtures (excited at 410 nm). (B) UV-vis absorption spectra of
copolymer B in THF/H2O mixtures.
intensity caused by the formation of aggregates in solution. The
formation of aggregates also dramatically reduces the fluores-
cence intensity with a concurrent reversal of the relative
intensities of the three emissions. At a 1:1 THF/H2O (v/v) ratio,
the fluorescence maximum red-shifted to 542 nm. Photoemission
of the copolymer in a THF solution, excited at 410 nm, produces
a typical spectrum for poly(phenylene vinylene) in solution: a
strong fluorescence at 480 nm with two small shoulders at about
508 and 555 nm derived from low-energy vibronic sidebands.25-27
Phase Separation Behavior. Diluted solutions of these
diblock copolymers in mixed THF/water solvents at various
volume ratios were used for the self-assembling studies. A THF/
water mixture solvent was used because the copolymers do not
have good solubility in pure water. Both THF and H2O are good
solvents for the PEG, but the OPV is insoluble in H2O and only
has limited solubility in THF.
Films cast from solution (0.5 mg/mL) were studied by
transmission electron microscopy (TEM) under various condi-
tions. Figure 3a shows the TEM image of copolymer B obtained
Figure 3. (a) TEM images of copolymer B films prepared by placing a drop of the copolymer solution onto holey TEM grids. The grids were dried
in the air or atmosphere of a solvent. The specimen was then examined in a Philips CM120 electron microscope operated at 120 kV. (b) Cryo-TEM
Images. Cryo-TEM samples were prepared by placing a drop of the copolymer solution on a copper grid and vitrified by rapidly plunging into
liquid ethane (90 K). It was subsequently transferred under liquid nitrogen to a grid box and mounted in a Gatan cryo specimen holder (model 626,
Gatan Inc., Warrendale, PA) and examined at low temperature (-172 °C) in a Philips CM120 electron microscope operated at 100 kV. (c) TEM
images of the AgTf complex of copolymer B.

6860 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Yu et al.
Figure 6. Scattering intensity versus q of copolymer B in d8-THF/
D2O mixture showing the correlation peak shifts to a smaller value
with increasing water concentration.
Figure 5. Neutron scattering intensity versus scattering vector (q, Å-1
)
of copolymer B in d8-THF showing three different conditions: no water,
8.3% D2O, and 10.7% D2O. The sharp peak in the presence of 10%
D2O indicates the spatial correlation between the rods.
The formation of cylindrical micelles in solution is further
confirmed by SANS studies. SANS is a nondestructive technique
that can provide information on size, shape, and interactions
among particles from the size of 1 to 100 nm at relevant solution
conditions. It was found that solutions of copolymer B showed
strong neutron scattering intensities (Figure 5, curve without
the addition of D2O). The data show a form factor typical of
infinitely long, rodlike objects. From a Modified Guinier analysis
for rodlike objects of the SANS data we obtained a rod radius
of 7.71 ( 0.07 nm for the diblock copolymer B.28 This result
indicates that copolymer molecules in solution self-assemble
into cylindrical micelles at concentrations of 0.05-2.5 wt % in
d8-THF. More detailed analysis of the SANS data revealed that
the cross-section of an elliptical shell characterized with 5.0
and 10.0 nm semiaxis and 2.0 nm shell thickness.29
Moreover, interesting results were observed when D2O was
added into the d8-THF solution (0.05 mg/mL) of diblock
copolymer B. A sudden and dramatic change in the scattering
pattern was observed upon addition of 10.7 wt % D2O (Figure
5): the forward neutron scattering intensity increased by an order
of magnitude in the low q region, indicating that the cylindrical
micelles formed by copolymer B become less entangled and
orient parallel to each other. Furthermore, a correlation peak
appears at a q value (length scale of 19.6 nm) that corresponds
to the center-to-center correlation between the cross-sections
of the rodlike particles. Thus, the originally distinct and
randomly oriented rodlike particles condense into a phase of
more ordered packed rods. Further addition of water continu-
ously shifts the peak position to smaller scattering vectors
(Figure 6). This indicates that the addition of water swells the
system such that the distance between the center of the rods
monotonic increases from 19 nm (at 10% water) to 23 nm (at
27% water) (Figure 7). It is interesting to note the linear
relationship between water content and the distance between
Figure 7. Plot of the rod-rod distance (2π/q) against D2O weight
percent of copolymer B in THF/ D2O mixture showing a linear
relationship.
the centers of the rods with a slope of 0.74 nm/g. The
aggregation is also accompanied by the change in their electronic
absorption and emission spectra.
These fibers are much like the giant wormlike micelles
observed in the coil-coil diblock system.30 An important
question about these fibers is the packing structure of the diblock
molecules, especially that of the OPV block in the core of the
fiber. Traditional micelle organization is not consistent with the
TEM and SANS results and thermodynamically not favored
since the length of the conjugated block is about 8.5 nm
corresponding to the diameter of the fiber obtained from TEM
studies. A layered structure is a possible packing mode, as shown
in other “hairy-rod” molecules.31 However, identification of
detailed molecular packing requires more extensive scattering
work and will be the topic of our next publication.
Complexation with Metal Ions. One of the special properties
of PEG is its complexion capability with metal cations.19,32 It
is thus interesting to dope metal cations into the copolymer
solution. Complex copolymers were prepared by mixing THF
solutions of OPV-PEG114 (0.05 wt %) with an appropriate ratio
(25) Gettinger, C. J.; Heeger, A. J. J. Chem. Phys. 1994, 101 (2), 1673.
(26) Halger, T. W.; Pakbaz, K.; Voss, K. F.; Heeger, A. J. Phys. ReV. B
1991, 44, 8652.
(27) Hempenius, M. A.; Langeveld-Voss, B. M. W.; van Haare, J. A. E.
H.; Janssen, R. A. J.; Sheiko, S. S.; Spatz, J. P.; Mo¨ller, M.; Meijer, E. W.
J. Am. Chem. Soc. 1998, 120, 2798.
(28) Porod, G. Small Angle X-ray Scattering; Glatter, O., Kratky, O.,
Ed.; Academic Press: New York, 1982; Chapter 2, pp 17-51.
(29) Urban, V. S.; Wang, H. H.; Thiyagarajan, P.; Littrell, K. C.; Wang,
H. B.; Yu, L. P. J. Appl. Crystalgraph. 2000, in press.
(30) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960.
(31) Gonzalez-Ronda, L.; Martin, D. C.; Nanos, J. I.; Politis, J. K.; Curtis,
M. D. Macromolecules 1999, 32, 4558,

Syntheses of Amphiphilic Diblock Copolymers
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 6861
of silver triflate salt (R ) 0.33, R is the ratio of the salt to the
EO repeating unit). It was found that silver triflate induced a
strong aggregation of the cylindrical micelles, packing them into
parallel fiber bundles (Figure 3c). The silver ions selectively
chelate with the poly(ethylene glycol) block, causing a sharp
contrast in the TEM micrograph (Figure 3c). The PEG block
became much darker than the OPV block (the central block).
The diameter of the whole fiber is about 15-17 nm and the
size of the core (∼8-10 nm) is about the same size as the
diameter of micelles observed without salt. When the sample
was stained with osmium tetraoxide, a similar internal structure
such as Figure 3c was observed because osmium tetraoxide will
not stain the OPV block but will selectively stain the PEG
block.33 The result is interesting because it indicates the
possibility of preparing an ionic conductive wire with an
electronic conductive core.
and poly(ethylene oxide) as the coil block were synthesized.
These copolymers organize into cylindrical micelles in solution
and in cast films on a nanometer scale, as observed by using
TEM, AFM, and SANS. These OPV-PEG micelles have a
cylindrical OPV core surrounded by a PEG corona. The detailed
fiber structure is still unknown and needs further study. These
nanometer-sized micelles are interesting because the OPV blocks
exhibit liquid crystallinity, electric, and optical properties. These
built-in physical properties distinguish these diblock copolymers
from others. It is possible to further manipulate these nanofibers.
For example, a magnetic field may be used to align these fibers
because these diblock copolymers are liquid crystalline materi-
als. Studies on electric and optical properties are in progress.
Acknowledgment. We gratefully acknowledge the financial
support of the National Science Foundation, Air Force Office
of Scientific Research, and Office of Naval Research. This work
also benefited from the support of the NSF MERSEC program
at the University of Chicago and from the use of the Intense
Pulsed Neutron Source funded by the U.S. Department of
Energy, BES-Materials Science, under contract W-31-109-ENG-
38 to the University of Chicago.
Conclusion
A novel series of amphiphilic rod-coil copolymers with
different lengths of oligo(phenylene vinylene) as the rod block
(32) Chandrasekhar, V. AdVances in Polymer Science; Springer-Verlag:
Berlin, Heidelberg, 1998.
(33) Lipic, P. M.; Bates, F. S.; Hillmyer, M. A. J. Am. Chem. Soc. 1998,
120, 8963.
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