Conjugated block copolymers via functionalized initiators and click chemistry

Article abstract of DOI:10.1002/pola.26984

Conjugated block copolymers are potentially useful for organic electronic applications and the study of interfacial charge and energy transfer processes; yet few synthetic methods are available to prepare polymers with well-defined conjugated blocks. Here

Full text of DOI:10.1002/pola.26984

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Conjugated Block Copolymers via Functionalized Initiators
and Click Chemistry
Kendall A. Smith,1 Deanna L. Pickel,2 Kevin Yager,3 Kim Kisslinger,3 Rafael Verduzco1
1Rice University, Department of Chemical and Biomolecular Engineering, MS-362, 6100 Main Street, Houston, Texas 77005-1892
2Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
3Center for Functional Nanomaterials, Brookhaven, National Laboratory, Upton, New York 11973
Correspondence to: R. Verduzco (E-mail: rafaelv@rice.edu)
Received 31 July 2013; accepted 14 October 2013; published online 6 November 2013
DOI: 10.1002/pola.26984
ABSTRACT: Conjugated block copolymers are potentially useful for
organic electronic applications and the study of interfacial charge
and energy transfer processes; yet few synthetic methods are avail-
able to prepare polymers with well-defined conjugated blocks.
Here, we report the synthesis and thin film morphology of a series
of conjugated poly(3-hexylthiophene)-block-poly(9,9-dioctylfluor-
ene) (P3HT-b-PF) and poly(3-dodecylthiophene)-block-poly(9,9-
dioctylfluorene) (P3DDT-b-PF) block copolymers prepared by
functional external initiators and click chemistry. Functional group
control is quantified by proton nuclear magnetic resonance spec-
troscopy, size-exclusion chromatography, and matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry. The thin
film morphology of the resulting all-conjugated block copolymers
is analyzed by a combination of grazing-incidence X-ray scattering,
atomic force microscopy, and transmission electron microscopy.
Crystallization of the P3HT or P3DDT blocks is present in thin films
for all materials studied, and P3DDT-b-PF films exhibit significant
PF/P3DDT co-crystallization. Processing conditions are found to
impact thin film crystallinity and orientation of the p–p stacking
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direction of polymer crystallites.
2013 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 154–163
KEYWORDS: block copolymers; click chemistry; conducting poly-
mers; conjugated block copolymers; crystallization; morphol-
ogy; organic photovoltaics; P3DDT; P3HT; PFO; self-assembly;
self-organization; X-ray
persity. All-conjugated BCPs are typically synthesized using
one or more condensation polymerization steps, making it
difficult to synthesize well-defined all-conjugated BCPs. An
alternative is to use a controlled polymerization method
along with sequential monomer addition, but this is applica-
ble to a limited set of monomers.12–14 Click chemistry can
provide an efficient method to couple conjugated macrore-
agents and can give control over both block molecular
weights independently. The application of click chemistry to
synthesize conjugated BCPs has been limited due to the diffi-
culty in obtaining functionalized materials from polyconden-
sation reactions and removing homopolymer impurities.
However, recent synthetic innovations have demonstrated
improved control over the functionality of conjugated poly-
mers through the use of functional external initiators.13–16
This method enables the preparation of well-defined macro-
reagents for use in click coupling reactions to prepare conju-
gated BCPs.
INTRODUCTION Conjugated block copolymers (BCPs) are
comprised of two or more p-conjugated polymer blocks and
are promising for use in organic electronic applications.1–4
Recent work has demonstrated the potential of conjugated
BCPs for use in organic photovoltaics (OPVs), and they may
also be useful for organic light-emitting diodes (OLEDs), thin
film transistors, or microelectronics.5–7 Furthermore, under-
standing the mechanism of charge separation at the interface
between donor and acceptor organic semiconductors
remains a challenge for the development of OPVs, and conju-
gated BCPs provide a way to control this interface through
the covalent linking of two conjugated polymers.8 However,
further progress requires addressing both synthetic and
processing challenges. Only a handful of examples of conju-
gated BCPs with both p-type and n-type polymer blocks
have been reported, and the relationship between different
processing steps and film microstructure—including polymer
crystallinity and microphase segregation—remains unclear.7,9–11
Here, we demonstrate an efficient click-coupling route for
the synthesis of conjugated BCPs using functionalized initia-
tors for both blocks, and the resulting materials are used to
Conjugated polymers are typically made by step-growth or
condensation polymerization reactions, which provide little
control over the final polymer molecular weight and polydis-
Additional Supporting Information may be found in the online version of this article.
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study the effects of processing conditions on film microstruc-
ture. A series of poly(3-hexylthiophene)-block-poly(9,9-dio-
ctylfluorene) (P3HT-b-PF) and poly(3-dodecylthiophene)-
block-poly(9,9-dioctylfluorene) (P3DDT-b-PF) BCPs are pre-
pared, and functionality of the precursor materials is quanti-
fied by proton nuclear magnetic resonance spectroscopy (1H
NMR), size-exclusion chromatography (SEC), and matrix-
assisted laser desorption/ionization time-of-flight mass spec-
trometry (MALDI-TOF MS). 1H NMR was used to monitor
conversion of functional groups during each reaction step.
The microstructure of P3DDT-b-PF and P3HT-b-PF films is
analyzed by grazing-incidence X-ray scattering (GIXS) and
transmission electron microscopy (TEM), and we find signifi-
cant co-crystallization of both blocks for P3DDT-b-PF but
predominantly P3HT crystallization for P3HT-b-PF BCPs. This
may be due to a mismatch of crystal melting temperatures
or of crystal lattice spacings for P3HT and PF polymer
blocks.
applied to the target via the dried droplet method.18 Mass
spectra were collected in reflection mode, and the instrument
was externally calibrated with polystyrene standards.
Differential Scanning Calorimetry
Thermograms were recorded on a TA Instruments differen-
tial scanning calorimetry (DSC) 2920 equipped with a refri-
gerated cooling system against an empty sealed pan as
ꢀ
reference. In a typical run, the sample was heated to 250 C
at 5 ꢀC min21, cooled to 40 ꢀC at 5 ꢀC min21, then equili-
brated for 10 min before heating to 250 ꢀC at 5 ꢀC min21
.
Second heating cycles are reported. Plots are baseline sub-
tracted for easier viewing.
Grazing Incidence Small-/Wide-Angle X-ray Scattering
Grazing incidence wide-angle X-ray scattering (GIWAXS) and
grazing incidence small angle X-ray scattering (GISAXS)
measurements were carried out at the X9 undulator beam-
line at the National Synchrotron Light Source, Brookhaven
National Laboratory. An X-ray energy of 14.0 keV was
selected using a silicon monochromator, and images were
collected using two area detectors: a Pilatus 300k (Dectris)
for small-angle measurements (3091 mm distance) and a
Photonic Sciences fiber-coupled CCD for wide-angle data
(370 mm distance). Sample measurement was carried out
under vacuum which is in the range of 2–3 3 1026 bar.
GIWAXS images presented herein are median filtered and
processed using the GIXSGUI package for Matlab (Math-
works), data are corrected for X-ray polarization, detector
sensitivity and geometrical solid-angle.19 Plots are normal-
ized and plotted on a logarithmic scale.
EXPERIMENTAL
Instrumentation
Nuclear Magnetic Resonance Spectroscopy
1
Solution H nuclear magnetic resonance (NMR) spectroscopy
was performed on a 500-MHz Varian Inova NMR. Chloro-
form-d (CDCl3, Cambridge Isotope Laboratories) was used as
the solvent with TMS (0.05%) as an internal standard. Data
were processed using SpinWorks 3.1.8.1.17
Size-Exclusion Chromatography
Polymer molecular weights and polydispersities (PDIs) were
obtained by SEC using an Agilent 1200 module equipped
with three PSS SDV columns in series (100, 1000, and
10,000 Å pore sizes), an Agilent variable wavelength UV–visi-
ble (UV–vis) detector, a Wyatt Technology HELEOS II multi-
angle laser light scattering (MALLS) detector (k 5 658 nm),
and a Wyatt Technology Optilab reX RI detector. This system
enables SEC with simultaneous refractive index (SEC-RI),
UV–vis (SEC-UV–vis), and MALLS (SEC-MALLS) detection.
THF was useꢀd as the mobile phase at a flow rate of 1 mL
min21 at 40 C. Weight average molecular weights (Mw) are
determined by light scattering with dn/dc values calculated
assuming 100% mass recovery of the injected sample. Poly-
dispersity (PDI) was determined using SEC-RI calibrated
with a set of monodisperse polystyrene standards (Astra
Software Version 5.3.4).
Transmission Electron Microscopy
TEM images were acquired on a JEOL JEM-1400LaB6 TEM
operating at an accelerating voltage of 120 keV. All the TEM
images, which were collected using a Gatan 2K CCD camera.
Samples were prepared by casting solutions of 3 mg ml21
polymer solution at 1500 rpm for 30 s onto poly(3,4-ethyle-
nedioxythiophene)
poly(styrenesulfonate)
(PEDOT:PSS)
coated silicone substrates. Substrates were then placed in
water to allow the PEDOT:PSS layer to dissolve and the poly-
mer film was collected onto 300 mesh copper TEM grids
with lacey carbon. Samples were then annealed at about 165
ꢀC for about 15 min before measurements.
Synthesis
Materials
Anhydrous tetrahydrofuran (THF), anhydrous toluene, dichloro-
methane, chloroform, tetrahydrofuran, hexanes, acetone, metha-
nol, 1,3-bis(diphenylphosphino)propane]dichloronickel(II) (Ni
[dppp]Cl2), isopropylmagnesium chloride (iPrMgCl, 2M in THF),
9,9-dioctyl-2,7-dibromofluorene, 3-hexylthiophene, 3-bromothio-
phene, magnesium, 1-bromododecane, copper(I) bromide
(CuBr), 18-crown-6, cesium fluoride (CsF), N,N,N0,N00,N00-pentam-
ethyldiethylene-triamine (PMDETA), poly(3,4-ethylenedioxythio-
phene) poly(styrenesulfonate) (PEDOT:PSS), 4-bromophenyl
ethanol, tosyl chloride, bis(tri-tert-butylphosphine) palladium(0)
(Pd[t-Bu3P]2), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide
(EDC), 4(dimethylamino), pyridine (DMAP), 5-hexynoic acid,
Matrix-Assisted Laser Desorption/Ionization–Time of
Flight Mass Spectrometry
Matrix-assisted laser desorption/ionization–time of flight
(MALDI-TOF) spectra were collected using a Bruker Dalton-
ics Autoflex II mass spectrometer, which is equipped with an
N2 laser (k 5 337 nm), operating at a frequency of 25 Hz
and an accelerating voltage of 20 kV. trans22-[3-(4-tert-
butylphenyl)22-methyl-2-propenylidene]malononitrile (DCTB)
(>98%, TCI) was used as the matrix. Solutions of DCTB (20
mg mL21) and the analyte (10 mg mL21) were prepared in
THF and then mixed in a 10:2 ratio. A volume of 1 lL was
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tetrabutylammonium fluoride solution in THF (TBAF, 1M in
THF), tosyl chloride, and azidotrimethylsilane were obtained
from commercial sources and used as received. 2,5dibromo-3-
hexylthiophene, 2,5dibromo-3-dodecylthiophene, and 70-bromo-
90,90-dioctyl-fluoren-20-yl-4,4,5,5,-tetramethyl-[1,3,2]dioxaborolane
(2) were synthesized as previously described.20–24
Azide-Functionalized Poly(9,9-dioctylfluorene)
Azide-functionalized poly(9,9-dioctylfluorene) (PF-azide) was
prepared by dissolving PFAtosylate (0.32 g, 0.64 mmol) in
5 mL of anhydrous THF followed by addition of azidotrime-
thylsilane (0.2 mL, 1.5 mmol) and 1M tert-butyl ammonium
fluoride solution (1.5 mL, 1.5 mmol). The reaction was
allowed to proceed overnight and the product was isolated by
precipitation into methanol, followed by washed with acetone.
2-(4-Bromophenyl)ethyl 4-Methylbenzenesulfonate (1)
4-Bromophenyl ethanol (1.96 g 9.75 mmol) was dissolved in
ꢀ
dichloromethane (8 mL) at 0 C. Tosyl chloride (2.34 g, 12.2
PF1
mmol) and pyridine (1.4 mL) were added, and the reaction
was allowed to warm to room temperature. The reaction
was stirred at room temperature for at least 48 h and the
reaction mixture was washed with water, extracted with
dichloromethane, and dried over sodium sulfate. Solvent was
removed by rotary evaporation, and the product was col-
lected as a white solid.
1H NMR (500 MHz, CDCl3, d): 8.0–7.5 (m, 80H, ArAH), 2.31–
1.91 (m, 53H, ArACACH2A), 1.39–0.97 (m, 283H, ACH2A),
0.97–0.50 (m, 136H, ACH2A), 3.63–3.55 (t, 2H,
ArACH2ACH2AN3), 3.01–2.91 (t, 2H, ArACH2ACH2AN3)
PF2
1H NMR (500 MHz, CDCl3, d): 8.0–7.5 (m, 185H, ArAH),
2.31–1.91 (m, 131H, ArACACH2A), 1.39–0.97 (m, 698H,
ACH2A), 0.97–0.50 (m, 330H, ACH2A), 3.63–3.55 (t, 2H,
ArACH2ACH2AN3), 3.01–2.91 (t, 2H, ArACH2ACH2AN3)
1H NMR (500 MHz, CDCl3, d): 7.68–7.62 (d, 2H, ArAH),
7.38–7.32 (d, 2H, ArAH), 7.31–7.25 (d, 2H, ArAH), 7.0–6.94
(d, 2H, ArAH), 4.23–4.17 (t, 2H, CH2), 2.95–2.88 (t, 2H, CH2),
2.47–2.43 (s, 3H, ArACH3),
Alkyne-Functionalized Poly(3-alkylthiophene)
Alkyne-functionalized poly(3-alkylthiophene) (P3AT-alkyne)
2-(4-Bromophenyl)ethyl 4-Methylbenzenesulfonate
Bis(tri-tert-butylphosphine)palladium(0)
was prepared using
elsewhere.25
a
general procedure described
Similar to previously reported synthesis, (1) (0.25 g 0.87
mmol) and bis(tri-tert-butylphosphine)palladium(0) (82 mg,
0.16 mmol) were dissolved in 3 mL of anhydrous toluene in
an inert atmosphere and heated to 70 C for 3 h. Material
was used without further purification.
P3HT1
1H NMR (500 MHz, CDCl3, d): 7.13–6.64 (s, 88H, ArAH),
3.09–2.37 (t, 176H, ArACH2A), 2.08–1.09 (m, 845H,
ACH2A), 1.09–0.60 (m, 265H, ACH3), 4.52–4.41 (t, 2H, ArA
16
ꢀ
CH2ACH2AOACOA), 4.27–4.18 (t,
AOACOA)
2
H, ArACH2ACH2
Tosyl-Functionalized Poly(9,9-dioctylfluorene)
Similar to previously reported synthesis, 70-bromo-90,90-dioctyl-
fluoren-2-yl-4,4,5,5,-tetramethyl-[1,3,2]dioxaborolane (2) (1.23
g, 2.07 mmol) was dissolved in 110 mL THF along with
18-crown-6 (6.61 g, 25.3 mmol), CsF (1.6 g, 12.6 mmol), and
5 mL water.16 The monomer solution was purged with N2
for 30 min before adding the crude product solution from the
preparation of 2-(4-bromophenyl) ethyl 4-methylbenzenesulfonate
bis(tri-tert-butylphosphine) palladium(0). The solution was
stirred overnight at room temperature before quenching
with 1 mL 5M HCl. The polymer was recovered by precipita-
tion in methanol and washed with copious amounts of
acetone to collect a light yellow/green solid. Yield: 394 mg,
49%:
P3HT2
1H NMR (500 MHz, CDCl3, d): 7.13–6.64 (s, 41H, ArAH),
3.09–2.37 (t, 84H, ArACH2A), 2.08–1.09 (m, 442H, ACH2A),
1.09–0.60 (m, 124H, ACH3), 4.52–4.41 (t, 2H, ArACH2A
CH2AOACOA), 4.27–4.18 (t, 2H, ArACH2ACH2AOACOA)
P3DDT1
1H NMR (500 MHz, CDCl3, d): 7.13–6.64 (s, 109H, ArAH),
3.09–2.37 (t, 230H, ArACH2A), 2.08–1.09 (m, 2332H,
ACH2A), 1.09–0.60 (m, 352H, ACH3), 4.52–4.41 (t, 2H, ArA
CH2ACH2AOACOA), 4.27–4.18 (t, 2H, ArACH2ACH2A
OACOA)
P3DDT2
PF1
1H NMR (500 MHz, CDCl3, d): 7.13–6.64 (s, 43H, ArAH),
3.09–2.37 (t, 85H, ArACH2A), 2.08–1.09 (m, 881H, ACH2A),
1.09–0.60 (m, 130H, ACH3), 4.52–4.41 (t, 2H, ArACH2AC
H2AOACOA), 4.27–4.18 (t, 2H, ArACH2ACH2AOACOA)
1H NMR (500 MHz, CDCl3, d): 8.0–7.5 (m, 81H, ArAH), 2.31–
1.91 (m, 52H, ArACACH2A), 1.39–0.97 (m, 276H, ACH2A),
0.97–0.50 (m, 135H, ACH2A), 4.34–4.21 (t, 2H,
ArACH2ACH2AOTs), 3.11–2.96 (t, 2H, ArACH2ACH2AOTs),
3.11–2.96 (t, 3H, ASO2APhACH3)
Poly(3-alkylthiophene)-b-poly(9,9-dioctylfluorene)
Poly(3-alkylthiophene)-b-poly(9,9-dioctylfluorene) (P3AT-b-
PF) was synthesized using a procedure described else-
where.16 In a typical reaction, PF-azide (4) (106 mg, 9.6
mmol), P3AT-alkyne (5) (128 mg, 8.53 mmol), and copper(I)
bromide (8.8 mg, 61.5 mmol) were added to a dry 50-mL
round bottom flask and purged with nitrogen. Anhydrous
THF (9 mL) was added by syringe and the reaction was
PF2
1H NMR (500 MHz, CDCl3, d): 8.0–7.5 (m, 188H, ArAH),
2.31–1.91 (m, 128H, ArACACH2A), 1.39–0.97 (m, 661H,
ACH2A), 0.97–0.50 (m, 317H, ACH2A), 4.34–4.21 (t, 2H,
ArACH2ACH2AOTs), 3.11–2.96 (t, 2H, ArACH2ACH2AOTs),
3.11–2.96 (t, 3H, ASO2APhACH3)
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purged via needle while N,N,N0,N00,N00-pentamethyldiethylene-
1.01–0.50 (m, 251H, ACH3), 4.49–4.39 (t, 2.00H, ArA
CH2ACH2AOACOA), 4.24–4.12 (t, 2.11H, ArACH2A
CH2AOACOA), 4.65–4.55 (t, 1.61H, ArACH2ACH2AN3C2HA),
3.34–3.17 (t, 1.75H, ArACH2ACH2A N3C2HA)
triamine (0.1 mL, 0.48 mmol) was added. The reaction was
ꢀ
allowed to proceed overnight at 40 C. The reaction mixture
was passed through a short basic alumina column followed
by copious amounts of THF. Product was concentrated in a
rotary evaporator, and then precipitated into hexanes, col-
1
lected by filtration, and washed with boiling hexanes. Full H
RESULTS AND DISCUSSION
NMR spectra for BCPs synthesized are included in the Sup-
porting Information Figures S10–S15.
The use of functionalized initiators has been shown to be an
effective route for the synthesis of functional polythiophenes
and polyfluorenes. Grignard metathesis polymerization
(GRIM) initiated by a functionalized nickel catalyst results in
well-defined polythiophenes with good control over the
functionality.15,25 Functionalized polyfluorenes can be syn-
thesized through catalyst-transfer Suzuki-Miyaura polymer-
ization initiated by a functionalized Pd catalyst.13,16 Together,
these chemistries provide routes to separately synthesize
alkyne and azide functionalized polythiophenes and poly-
fluorenes which can subsequently be coupled using click
chemistry. Click chemistry provides an efficient means to
couple polymeric materials together, but requires that the
starting materials be well functionalized in order for the
final BCPs to be well defined. Thus, we extensively character-
ized the precursor materials described herein. Synthesis of
the alkyne functionalized P3AT was recently described so
this work will focus more on the synthesis of the azide func-
tionalized PF. Characterization of both materials is more
extensive than the previous report.25
P3HT1-b-PF1
1H NMR (500 MHz, CDCl3, d): 8.10–7.41 (m, 67H, ArAH),
7.15–6.88 (s, 92H, ArAH), 3.01–2.35 (t, 199H, ArACH2A),
2.35–1.88 (m, 46H, ACH2A), 1.88–1.01 (m, 1032H, ACH2A),
1.01–0.50 (m, 397H, ACH3), 4.49–4.39 (t, 2.00H,
ArACH2ACH2AOACOA), 4.24–4.12 (t, 1.98H, ArACH2ACH2
AOACOA), 4.65–4.55 (t, 2.05H, ArACH2ACH2AN3C2HA),
3.34–3.17 (t, 2.51H, ArACH2ACH2A N3C2HA)
P3HT1-b-PF2
1H NMR (500 MHz, CDCl3, d): 8.10–7.41 (m, 63H, ArAH),
7.15–6.88 (s, 86H, ArAH), 3.01–2.35 (t, 187H, ArACH2A),
2.35–1.88 (m, 41H, ACH2A), 1.88–1.01 (m, 981H, ACH2A),
1.01–0.50 (m, 373H, ACH3), 4.49–4.39 (t, 2.00H,
ArACH2ACH2AOACOA), 4.24–4.12 (t, 1.92H, ArACH2A
CH2AOACOA), 4.65–4.55 (t, 1.86H, ArACH2ACH2AN3C2HA),
3.34–3.17 (t, 2.72H, ArACH2ACH2A N3C2HA)
P3HT2-b-PF2
Our overall synthetic approach to prepare conjugated BCPs
is shown in Scheme 1. Alkyne-functionalized poly(3-alkylth-
iophene) (P3AT) is prepared through the use of a functional-
ized nickel catalyst with protected hydroxyl functionality.
This details of this synthesis were recently described.25
P3AT with a hydroxyl functionality is obtained after poly-
merization and deprotection, and the functional group is
subsequently converted to an alkyne through a Steglich
esterification with 5-hexynoic acid. The resulting alkyne-
functionalized polythiophenes were found to have good sta-
bility, solubility, and reactivity in click reactions with azide-
functionalized poly(ethylene glycol).25 This procedure was
applied to the synthesis of both P3HT-alkyne and P3DDT-
alkyne (Table 1). 1H NMR analysis reveals the presence of a
hydroxyl group after polymerization and deprotection, and a
clean shift in the peaks is observed after conversion to the
alkyne [Fig. 1(A)].
1H NMR (500 MHz, CDCl3, d): 8.10–7.41 (m, 97H, ArAH),
7.15–6.88 (s, 42H, ArAH), 3.01–2.35 (t, 102H, ArACH2A),
2.35–1.88 (m, 79H, ACH2A), 1.88–1.01 (m, 797H, ACH2A),
1.01–0.50 (m, 290H, ACH3), 4.49–4.39 (t, 2.00H, ArA
CH2ACH2AOACOA), 4.24–4.12 (t, 1.90H, ArACH2ACH2A
OACOA), 4.65–4.55 (t, 1.93H, ArACH2ACH2AN3C2HA),
3.34–3.17 (t, 1.83H, ArACH2ACH2A N3C2HA)
P3DDT1-b-PF1
1H NMR (500 MHz, CDCl3, d): 8.10–7.41 (m, 49H, ArAH),
7.15–6.88 (s, 69H, ArAH), 3.01–2.35 (t, 149H, ArACH2A),
2.35–1.88 (m, 37H, ACH2A), 1.88–1.01 (m, 1642H, ACH2A),
1.01–0.50 (m, 296H, ACH3), 4.49–4.39 (t, 2.00H, ArA
CH2ACH2AOACOA), 4.24–4.12 (t, 2.08H, ArACH2ACH2A
OACOA), 4.65–4.55 (t, 1.86H, ArACH2ACH2AN3C2HA),
3.34–3.17 (t, 1.48H, ArACH2ACH2A N3C2HA)
P3DDT1-b-PF2
1H NMR (500 MHz, CDCl3, d): 8.10–7.41 (m, 89H, ArAH),
7.15–6.88 (s, 108H, ArAH), 3.01–2.35 (t, 228H, ArACH2A),
2.35–1.88 (m, 58H, ACH2A), 1.88–1.01 (m, 2600H, ACH2A),
1.01–0.50 (m, 476H, ACH3), 4.49–4.39 (t, 2.00H,
ArACH2ACH2AOACOA), 4.24–4.12 (t, 2.03H, ArACH2A
CH2AOACOA), 4.65–4.55 (t, 1.30H, ArACH2ACH2AN3C2HA),
3.34–3.17 (t, 1.94H, ArACH2ACH2A N3C2HA)
Azide-functionalized poly(9,9-dioctylfluorene) (PF-azide) is
prepared through the use of an externally added, functional-
ized palladium catalyst. The palladium catalyst Pd(t-Bu3P)2
is reacted with (1) using
a method similar to those
described by Yokozawa et al.14 The resulting Pd complex ini-
tiates the polymerization of (2) resulting in tosylate-
functionalized PF (PF-tosylate). The tosylate functional group
is converted to an azide in a one-step reaction with tert-
butyl ammonium fluoride (TABF) and azidotrimethylsilane in
chloroform, yielding PF-azide after precipitation, filtration,
and washing with methanol and acetone. This procedure was
applied to synthesize two PF-azide polymers, and 1H NMR
P3DDT2-b-PF2
1H NMR (500 MHz, CDCl3, d): 8.10–7.41 (m, 83H, ArAH),
7.15–6.88 (s, 39H, ArAH), 3.01–2.35 (t, 90H, ArACH2A),
2.35–1.88 (m, 61H, ACH2A), 1.88–1.01 (m, 1359H, ACH2A),
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SCHEME 1 Synthetic scheme for the preparation of P3AT-alkyne, PF-azide, and P3AT-b-PF block copolymers.
analysis indicates quantitative transformation of PF-tosylate
to PF-azide [Fig. 1(B)].
functionality can be estimated. The molecular weight esti-
mates by 1H NMR and SEC are in good agreement for all
macroreagents except for P3DDT1 and PF2 (Table 1). For
these samples, the higher DP estimates from 1H NMR analy-
sis reflects incomplete functionalization, roughly 62 and 52%
on a molar basis for P3DDT1 and PF2, respectively. Table 1
also shows that higher molecular weight polymers (P3HT1,
P3DDT1, and PF2) have a lower degree of functionality as
compared to lower molecular weight polymers P3HT2,
P3DDT2, and PF1.
The degree of functionality can be analyzed using a combina-
tion of 1H NMR, SEC-MALLS, and MALDI-TOF. As shown in
Figure 1, NMR peaks corresponding to the expected func-
tional groups are present for hydroxyl and alkyne functional-
ized polythiophenes and tosylate and azide-functionalized
PFs. 1H NMR provides an estimate of polymer molecular
weight through comparison of the integrated intensities of
peaks corresponding to protons on the functional group and
main-chain backbone. By comparing 1H NMR molecular
weight estimates to SEC-MALLS estimates, the degree of
MALDI-TOF mass spectrometry (MALDI-TOF MS) provides an
independent measurement of the degree of functionality of
P3AT and PF polymers. MALDI-TOF MS indicates the desired
functionalized product is the major product for all P3AT poly-
mers, and the degree of functionality ranges from 60 to 90%.
The percent functionality is calculated based on the monoiso-
topic peak of each major distribution for a particular mer. As
detail in Supporting Information (Tables S1 and S2), at least
three mer distributions are chosen and an average and stand-
ard deviation is calculated. The result of this calculation are
generally lower than that predicted by 1H NMR and SEC-
MALLS, but comparable to previous studies that have found
significant variability in the degree of functionality for P3HT
depending on the structure of the nickel initiator.26 The
MALDI-TOF technique samples from the lower molecular
weight spectrum of the polymers; therefore, it is possible that
MALDI-TOF reflects poor functionality only in the lower-
molecular weight fraction of the polymer samples. Consistent
with the measurements by 1H NMR and SEC-MALLS, MALDI-
TOF measurements show a higher degree of functionality for
P3DDT2 and P3HT2 compared with P3DDT1 and P3HT1.
TABLE 1 Characteristics of Conjugated Polymer Macroreagents
Mn MALLS
Sample (kg mol21
PDI,
DP,
DP, 1H
Functionality
MALDI (%)c
)
SEC-RI SECa NMRb
P3HT1
P3HT2
11.9
7.3
1.23
1.20
1.25
1.20
1.49
1.75
72
44
68
37
13
16
88
41
109
43
13
31
6266
7162
8065
9163
9661
4260
P3DDT1 17.1
P3DDT2 9.2
PF1
PF2
4.9
6.2
a
DP calculated from MALLS Mn divided by repeat unit molecular
weight.
b
DP calculated from the relative integrated intensities of aromatic
hydrogens to polymer functional group hydrogens.
c
Reported percent functionality is relative abundance of all polymer
species containing desired functional groups averaged over at least
three mer distributions with deviation from these values.
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FIGURE 1 1H NMR of (A) PF-tosylate and PF-azide, (B) P3AT-OH and P3AT-alkyne, and (C) final P3AT-b-PF product.
As can be seen in Table 1 and Figure 2, MALDI-TOF shows a
high degree of functionality for PF1. For this polymer, the
estimated DP calculated from the MALLS molecular weight is
resulting products are summarized in Table 2. Although PF1
and PF2 have similar molecular weights, they differ with
respect to functionality and were both investigated in the
synthesis of BCPs. The click coupling reaction is carried out
in the presence of copper(I)bromide and PMDETA in a THF
solution under a nitrogen atmosphere at 40 ꢀC. A modest
excess of the PF reagent is utilized as this reagent is easily
removed in subsequent processing by washing the BCP prod-
uct with hexanes. Successful coupling is revealed through
both 1H NMR (Fig. 1 and Supporting Information Figs. S9–
S14) and SEC analysis (Fig. 3 and Supporting Information
Fig. S1). As shown in Figure 3 and Supporting Information
Figure S1, SEC traces of the BCP show significant shifts to
shorter elution times as compared to the macroreagents by
both RI detection and by UV absorbance at wavelengths
selective for each block. Due to the similarities in molecular
weight between PF1 and PF2, relatively small differences are
observed when comparing BCPs that only differ in the PF
macroreagent used. A significant difference in the final BCP
molecular weight and composition is observed for BCPs that
differ in the P3HT or P3DDT macroreagent used in the
1
in good agreement with the DP measured by H NMR and in
agreement with the expected DP based on the monomer to
catalyst ratio. For PF2, we targeted a polymer molecular
weight twice that of PF1 by adjusting the catalyst to mono-
mer ratio, but SEC-MALLS indicates that PF2 and PF1 have
comparable molecular weights. 1H NMR and SEC-MALLS
indicate poor functionality, and MALDI-TOF indicates a signif-
icant amount of OH functionalized product as an impurity
(see Tables S1 and S2 in Supporting Information). Our previ-
ous work has shown poor functionality for PF with molecu-
lar weighs greater than 7 kg mol21 16
Thus, for the reaction
.
conditions detailed herein, we obtain PF with a high degree
of functionality and molecular weight controlled by catalyst
to monomer ratio only for polymer chains with an average
molecular weight of 5 kg mol21
.
P3AT-b-PF BCPs are synthesized using a copper-catalyzed
click reaction between PF-azide and P3AT-alkyne, and the
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FIGURE 2 MALDI-TOF analysis of PF1-tosylate.
synthesis. For example, the molecular weight difference
between P3HT1-b-PF2 and P3HT2-b-PF2 corresponds to the
molecular weight difference between P3HT1 and P3HT2,
roughly 4.6 kg mol21, and the P3HT content of P3HT1-b-PF2
is higher by 26 wt %. The BCPs generally have lower molec-
ular weights than the sum of the corresponding homopoly-
mer, which may be due to the presence of homopolymer
impurities in the final products. We expect that most of the
homopolymer impurities are P3HT or P3DDT homopolymer
since PF is removed by washing with hexanes.
FIGURE 3 Size exclusion chromatography data for P3DDT2-b-
PF2 (A) differential refractive index (SEC-RI), (B) UV–vis absorb-
ance signal for P3DDT2 block (measured at 500 nm), and (C)
UV–vis absorbance signal for PF2 block (measured at 300 nm
and corrected for the contribution of P3DDT2).
A comparison of the film morphologies of the synthesized
BCPs gives some insight into the role of polymer block ratio
and the crystallization behavior of the constituent polymer
blocks. Previous work has found that P3HT crystallization
typically dominates thin film morphology for BCPs with one
P3HT block.27–29 In P3HT-b-PF all-conjugated BCPs, we had
determined that P3HT tends to dominate the crystallization
at larger weight fractions of P3HT and likewise PF at low
P3HT weight fraction, with co-crystallization being seen
between about 30 and 50 wt % P3HT.16,30 With the materi-
als presented herein, we can compare P3HT and P3DDT all-
conjugated BCPs. Since P3DDT has a crystallization tempera-
ture similar to that of PF, we expect that BCPs of these mate-
rials may allow for enhanced competition between the
crystallization of the P3AT and PF blocks. Additionally, the
larger size of the P3DDT unit cell may be more compatible
with the crystal structure of PF.
ꢀ
Polymer film samples were heated above 230 C then slowly
cooled to room temperature. As shown by GIWAXS images
shown in Figure 4 and line cuts of GIWAXS data are also
shown in Supporting Information Figure S2. Crystallization
of each block is determined by observing the characteristic
peaks at qz 5 0.38, 0.77, and 1.14 for P3HT, qz 5 0.22, 0.46,
0.68, and 1.14 for P3DDT, and qz 5 0.5 for PF. While P3HT
and PF have readily distinguished peaks, peaks for P3DDT
and PF overlap but can be readily distinguished in the line
cuts (see Supporting Information Fig. S2). Crystallinity of the
PF block is suppressed for copolymers where the P3HT is
the majority block, and crystallization of both blocks is
observed for a ratio near 50 wt % P3HT, similar to our pre-
vious report. However, for P3DDT2-b-PF2, which is 62%
P3DDT, we see both crystal forms which suggest that the
molecular weight ratio may not be the deciding factor for
whether both crystals may be observed. A very weak PF
peak can also be discerned in the line cut for P3DDT1-b-PF2
(82% P3DDT). The lower crystallization temperature of
P3DDT or the larger size of the monomer unit may be
responsible for the observed co-crystallization of P3DDT and
PF polymer blocks. Analysis of the polymers by DSC is
TABLE 2 Characteristics of Conjugated Block Copolymers
Mn MALLS
Sample
(kg mol21
)
PDI, SEC-RI
P3AT (wt %)a
P3HT1-b-PF1
P3HT1-b-PF2
P3HT2-b-PF2
P3DDT1-b-PF1
P3DDT1-b-PF2
P3DDT2-b-PF2
15.8
13.9
8.3
1.19
1.24
1.33
1.27
1.36
1.31
78
78
52
84
82
65
25.1
22.9
11.9
a
P3AT wt % determined by integrating aromatic regions of BCP to
obtain the relative degree of polymerization and the respective molecu-
lar weight of the P3AT and PF repeat units.
160
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FIGURE 4 GIWAXS analysis of BCP films after thermal annealing at 230 ꢀC.
detailed in Supporting Information (Fig. S3). As an example,
the DSC for P3HT2-b-PF2 and its constituent polymers is
shown in Figure 5. The transitions associated with the P3HT
block (200–220 ꢀC) are very evident and are at tempera-
tures similar to those of the constituent homopolymer; how-
ever, there is only a broad and weak transition (see inset)
implications for applications in organic electronics. For
example, the P3HT component has anisotropic hole mobility:
for transistor applications one would want to emphasize the
out-of-plane lamellae orientation so as to enhance in-plane
conduction; whereas for photovoltaic applications one would
instead want to emphasize the in-plane lamellae orientation
so as to enhance out-of-plane mobility.31
ꢀ
spread from 105 to 155 C that may be attributable to PF. In
general, BCPs show thermal transitions at temperature simi-
lar to their P3AT constituents. For copolymers containing
P3DDT, the location of the thermal transition for the P3DDT
block (150–175 ꢀC for P3DDT1 and 120–150 ꢀC for
P3DDT2) overlaps significantly with that of the PF (110–145
ꢀC) making evaluation of PF crystallization in the BCP diffi-
cult by DSC.
Assessment of GISAXS data in Supporting Information (Fig.
S6) shows a weak peak for the P3HT-b-PF, but none for
P3DDT-b-PF. This may be due to the reduced contrast
between the P3DDT block and PF relative to that of P3HT.
As detailed in Figure 7, a comparison of TEM and AFM
images of P3HT1-b-PF1 reveals that this peak may be due to
A variety of annealing conditions were investigated including
solvent annealing in the presence of vapor chloroform for 5
days, as well as 24 h of annealing at 165 ꢀC. Additionally,
casting conditions were varied to cast from room tempera-
ture chloroform and refluxing chloroform. Results for all
these conditions may be found in Supporting Information
Figures S4 and S5. As detailed in Figure 6, casting from
refluxing chloroform resulted in some randomization of the
orientation of crystals evidenced in GIWAXS, particularly evi-
dent in the secondary reflections for the as cast samples.
Heating to 230 ꢀC effectively erases the initial morphology
and the usual out-of-plane orientation of the spacing
between crystallites through alkyl-side chains is observed. As
expected, this case of high-temperature annealing followed
by slow cooling gives rise to very good order with the p–p
stacking peak clearly visible. Lower temperature annealing
conditions give rise to orientation distributions of the P3HT
component that deviate markedly from that observed for
pure P3HT thin films. This tunability of the ordering has
FIGURE 5 Differential scanning calorimetry curves from second
heating cycle of PF2, P3HT2, and P3HT2-b-PF2.
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FIGURE 6 GIWAXS analysis for P3DDT2-b-PF2 under different processing conditions. First row shows samples cast from room
temperature chloroform while second row shows samples cast from refluxing chloroform. Columns denote as cast films, 165 and
230 ꢀC thermally annealed films, and films annealed in presence of chloroform vapor for five days at room temperature.
scattering of regularly sized P3HT crystallites embedded in
an amorphous matrix. As detailed in Supporting Information
(Fig. S7), no such crystallites are seen by TEM in P3DDT-b-
PF although topologies similar to those of P3HT-b-PF are
seen in AFM. Uneven patches are observed in all P3DDT-b-
PF samples, however these do not appear to be macroscopic
phase separation as elemental analysis shows uniform
changes in both sulfur and carbon signals suggesting that
the patterns are due to variations in film thickness and/or
density (Supporting Information Fig. S8). As shown in Sup-
porting Information Figure S9, an AFM survey of other sam-
ples show similar results showing few if any difference in
topology based on the block size or processing conditions
and no evidence of ordered nanostructures. This suggests
that a periodic nanostructure characteristic of microphase
segregation is not observed for these materials under any of
the processing conditions tested, and crystallization of one
or both blocks is the predominant characteristic of thin film
morphology.
CONCLUSIONS
Herein, we demonstrated a synthetic route for preparing
P3AT-b-PF block copolymers using click chemistry. P3AT and
PF macroreagents are prepared through the use of function-
alized, external initiators, and subsequent modification pro-
vides PF-azide and P3AT-alkyne macroreagents. This method
enabled control over the molecular weight of the P3AT block,
FIGURE 7 Comparison of (A) AFM and (B) TEM images for P3HT1-b-PF1.
162
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