Influence of the Grafting Density on the Self-Assembly in Poly(phenyleneethynylene)-g-poly(3-hexylthiophene) Graft Copolymers

Article abstract of DOI:10.1021/acs.macromol.5b01830

Conjugated graft copolymers consisting of a chiral poly(phenyleneethynylene) (PPE) backbone and poly(3-hexylthiophene) side chains (P3HT) with different grafting degrees were synthesized. While PPE was prepared by classical Sonogashira couplings, the end-

Full text of DOI:10.1021/acs.macromol.5b01830

Article
pubs.acs.org/Macromolecules
Influence of the Grafting Density on the Self-Assembly in
Poly(phenyleneethynylene)‑g‑poly(3-hexylthiophene) Graft
Copolymers
Joost Steverlynck,† Julien De Winter,‡ Pascal Gerbaux,‡ Roberto Lazzaroni,§ Philippe Leclere,§
̀
and Guy Koeckelberghs*,†
†Laboratory for Polymer Synthesis, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium
§
‡Organic Synthesis and Mass Spectrometry Laboratory, Interdisciplinary Center for Mass Spectrometry, and Laboratory for
Chemistry of Novel Materials, Center for Innovation and Research in Materials and Polymers (CIRMAP), University of
Mons-UMONS, 7000 Mons, Belgium
S
* Supporting Information
ABSTRACT: Conjugated graft copolymers consisting of a
chiral poly(phenyleneethynylene) (PPE) backbone and poly-
(3-hexylthiophene) side chains (P3HT) with different grafting
degrees were synthesized. While PPE was prepared by classical
Sonogashira couplings, the end-functionalized P3HT was
prepared by a controlled Kumada catalyst transfer poly-
condensation (KCTP) allowing the installation of an acetylene
end group. After some postpolymerization reactions on the
PPE to introduce azide groups, the P3HT was clicked to the
PPE through the CuAAC coupling reaction. Subsequently, the
(chiral) self-assembly of these materials was studied by means of UV−vis and CD spectroscopy, AFM, and DSC. Finally,
fluorescence spectroscopy is used to study the quenching of the PPE fluorescence by P3HT.
synthesis of linear structures. These include homopolymers and
different types of (block) copolymers. In recent years some
progress has been made in the synthesis of conjugated
polymers with a controllable degree of branching.7 The field
of graft copolymers is even less explored, mainly due to
synthetic challenges. Nonetheless, these materials offer a whole
new range of possibilities, like production of solar cells with
broadband absorbance and the study of energy transfer
processes.8,9 In comparison to block copolymers the ratio
between the polymers can be more effectively tuned. The
synthesis of conjugated graft copolymers has only been
reported for two systems and this by the “grafting from” and
“graft through” methods.10,11 However, chirality has never been
introduced, although it offers a lot of opportunities for both
characterization of the material and its properties. For instance,
circular dichroism (CD) can be observed in chiral conjugated
polymers and used to explore the supramolecular structure of
these materials.12,13 Also, the influence of the grafting density in
CPs has never been investigated, although it can be assumed
that it severely affects the self-assembly and, hence, the
properties. In this report, we describe the synthesis, (chiral)
self-assembly, and emission characteristics of a series of grafted
conjugated polymers consisting of a PPE backbone with P3HT
INTRODUCTION
■
For nonconjugated polymers, extensive research has already
been performed in the field of graft copolymers. Progress in the
domain of controlled polymerizations allowed the production
of well-defined molecular brushes.1 These structures show
some unusual properties that are not observed for their linear
counterparts, making new applications possible. These features
include their wormlike behavior and compact molecular
dimension.2 A lot of parameters like the length of the backbone
and side chains as well as the grafting density can be optimized
to obtain the desired properties. By variation of these
parameters, different precisely defined nanostructures can be
obtained which can act as template for inorganic nanostructures
or as very stable micelles that can be used as drug carriers.2 This
type of polymers can also lead to super soft elastomers and
photonic materials.2
Conjugated polymers (CPs) have been investigated for
decades, as they show some unique properties and applications
compared with their nonconjugated counterparts. Especially
their optoelectronic properties together with their solution
processability make them very useful for implementation as
active material into low-cost electronics like organic photo-
voltaics (OPVs), organic field effect transistors (OFETs), and
organic light-emitting diodes (OLEDs).3−6
Despite the fact that graft copolymers can show a unique
behavior compared to the linear analogues, up until now, in the
field of conjugated polymers the focus mainly has been on the
Received: August 19, 2015
Revised: November 25, 2015
© XXXX American Chemical Society
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Scheme 1. Structure and Synthesis of the Monomers
side chains and investigate the influence of the different grafting
density on these properties.
RESULTS AND DISCUSSION
■
Monomer Synthesis. While the monomer 1 for the
synthesis of P3HT is commercial, the chiral monomer 4 was
synthesized according to literature procedures.7 Monomer 3
was obtained by alkylation of hydroquinone with 6-chlorohex-
anol, followed by iodination with ICl (Scheme 1).14−18
Methods using NIS and KIO3 with I2 in acidic medium lead
to formation of side products. A chiral side chain was used,
since it allows the study of the self-assembly with chiral
techniques, e.g., CD.
1
Figure 1. H NMR signal of α-CH2 of P3HT (P1).
Polymer Synthesis. Acetylene end-capped P3HT (P1) was
obtained by polymerization of 2-chloromagnesio-5-bromo-
hexylthiophene, in situ prepared by a modified GRIM reaction,
using the KCTP (Scheme 2). After 15 min of polymerization
the end-capper was added in excess.19−22 After another 15 min
the reaction was terminated by adding acid.
GPC calibrated against polystyrene standards results in an
M value of 4.4 kg/mol and a dispersity of 1.2, which is in line
̅
n
with the expected living KCTP. The M value corresponds to
̅
n
26 units. This is higher than expected but can be explained by
the fact that this technique overestimates the molar mass of
P3HT.26,27 When using the correction factor of 1.3, determined
by Seferos et al., we obtain a degree of polymerization of 20.27
Scheme 2. Synthesis of Polymer P1
This is close to the targeted M value of 18.
̅
n
The PPE polymer with functionalized side chains was
obtained by polymerizing monomers 3 and 4 via the
Sonogashira reaction. The polymerization was carried out in
THF with Pd(PPh3)4 as catalyst, Et3N as base, and CuI as
cocatalyst (Scheme 3). Iodobenzene was used as a chain
stopper to limit the molar mass in order to obtain soluble
material and a fixed DP of 30 units.28−30 GPC analysis results in
an M value of 11.1 kg/mol and a dispersity of 1.4. The higher
̅
n
dispersity compared to P3HT can be explained to the step-
growth polymerization mechanism.
To obtain the polymer with terminal azide groups in the side
chains, two postpolymerization reactions were performed. First,
the alcohol groups were converted to better leaving groups, i.e.,
tosyl groups, which were subsequently converted to azide
groups by reaction with sodium azide in the presence of 18-
crown-6 (Scheme 4).31−35
Analysis of the MALDI-ToF spectra indicates a mixture of
mainly Br/acetylene-terminated polymers and another series of
polymers, attributed to dialkyne P3HT, as minor product.23,24
Based on the MALDI analyses, this second distribution should
represent less than 25% of contamination (see Supporting
Information, S9).
1H NMR spectroscopy was used to monitor these
postpolymerization transformations (Figure 2). This reveals
that the reactions occurred quantitatively.
1
In the H NMR spectrum, next to the signal for the internal
α-CH2, there is a clear triplet signal corresponding to a α-CH2
at a terminal unit with a bromine end (Figure 1).25 Since also
some dicapped polymers are present (MALDI-ToF), an exact
determination of the DP by 1H NMR spectroscopy is
impossible.
For grafting P3HT to the PPE backbone the CuAAC click
reaction was used with CuBr/PMDTA as catalyst system
(Scheme 5). The high yields of this reaction should allow a high
control over grafting density and enable us to obtain high
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Scheme 3. Synthesis of Polymer P2
Scheme 4. Synthesis of Polymer P4 via Two Postpolymerization Reactions
is a clear increase in molar mass upon increasing the ratio of P1
to P4. The dispersities of the graft copolymers have the same
value due to purification by preparative GPC. Consequently, a
study of the influence of the grafting degrees on the properties
can be considered valid.
Since the correction factor for the real M and M measured
̅
̅
n
n
by GPC for both P3HT and PPE are known and the amount of
P3HT and PPE in the graft copolymers is known as well (from
1H NMR), the real M of the graft copolymers can be
̅
n
calculated. This reveals that GPC overestimates M by a factor
̅
n
of 2.
1
Figure 2. H NMR signals of −OCH2 and −CH2 groups next to the
functional end group of the side chains of P2, P3, and P4.
In order to check if the aimed degrees of functionalization
correspond to what was aimed for, a H NMR analysis was
1
performed. Already at first glance, it is obvious that the peaks
corresponding to P3HT increase dramatically (Figure 4).
To have a precise determination of the grafting percentages,
the integration values of the −OCH2 peaks (a) and the N3−
CH2 protons (b) of PPE were determined (Figure 5).
The grafting percentage is then calculated by the following
formula:
degrees of functionalization. This reaction is often used to
synthesize graft copolymers by the “grafting to” method.36−39
By adjusting the ratio between P1 and P4, percentages of
functionalization of 10 (P5), 25 (P6), 50 (P7), and 100 (P8)
were aimed for (see Supporting Information). Note that gel
formation could have been anticipated (especially for P8) as
some dicapped P3HT is present. However, this is not observed.
GPC and H NMR of the Graft Copolymers. To remove
residual homopolymers, the graft copolymers were purified by
preparative GPC. The purified graft copolymers were analyzed
1
no. of nonreacted azide functions
grafting % = 1 −
no. of initial azide functions
b/2
a/4
by GPC; chromatograms and corresponding M and dispersity
̅
× 100% = 1 −
× 100%
n
values are displayed in Figure 3 and Table 1, respectively. There
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Scheme 5. Synthesis of the Four Graft-Copolymers P5−P8
peak.40 Upon increasing the methanol content the absorption
band is red-shifted for P3HT due to aggregation. This occurs
starting from 30% MeOH. For PPE, this red-shift is not
observed; aggregation is only visible by the appearance of an
extra peak around 480 nm.41 Aggregation occurs starting from
60% MeOH (see Supporting Information). From the spectra of
the graft copolymers in 90% MeOH a clear contribution of PPE
is still visible for P5 (Figure 6). Also, for P6 the characteristic
band at 480 nm of PPE can still be observed. However, for P7
and P8 the UV−vis spectra are nearly identical to the P3HT
homopolymer P1, although the fine structure is somewhat less
defined. This is in line with the fact that the P3HT content
increases dramatically from P5 to P8.
Also, spectra in 90% MeOH were simulated using the spectra
of the corresponding homopolymers and their respective mass
fraction in order to investigate the influence of the backbone
and side chains on their aggregation (Figure 7). For this
simulation, the behavior of the graft copolymers was modeled
as a linear combination of both homopolymers. The fraction of
the two polymers contributing to the spectrum of the graft
copolymers was calculated using the mass fraction determined
Figure 3. Chromatograms of homopolymers P1 and P4 and of the
graft copolymers P5−P8.
While aimed for 10, 25, 50, and 100%, values of 10, 24, 38, and
50% were achieved (Table 1). It is clear that the deviation
increases with increasing degrees of functionalization. This can
be explained by the increasing steric hindrance, which is in
general the limiting factor for the “grafting to” method. Note
that this nicely correlates with [d/(20·2)]/(a/4) (Table 1).
This formula corresponds with the calculations of grafting %
using the α-CH2 protons of the P3HT side chains and
assuming a degree of polymerization of 20. This result suggests
that the degree of polymerization is indeed close to 20.
UV−Vis Spectroscopy. UV−vis experiments were per-
formed on both homopolymers (P1 and P4) and the four graft
copolymers (P5−P8). For all of them ten solutions were made
with an increasing amount of methanol in comparison to
chloroform (Supporting Information, S11−16). In pure
chloroform both P3HT and PPE absorb around 440 nm,
although the P3HT absorption is characterized by a broader
1
from H NMR. If the two polymers would not influence each
other, the simulated and experimental spectra would coincide.
However, the peak around 600 nm, originating from aggregated
P3HT, is less pronounced in the experimental spectra
compared to the simulated spectra for P6−P8. This less
pronounced fine structure demonstrates that the stacking of the
P3HT in the graft copolymers is complicated due to their
covalent bond to the PPE backbone. Note that UV−vis cannot
provide much information on the PPE aggregation, as the UV−
vis spectrum is rather insensitive for self-assembly and the
smaller fraction of PPE present. In summary, the UV−vis study
indicates that the PPE compromises the self-assembly of the
P3HT.
Table 1. M and Đ Values of Homopolymers P1 and P4 and of the Graft Copolymers P5−P8 and Aimed vs Calculated Grafting
̅
n
Percentages
grafting % (NMR)
polymer
M (kg/mol)
Đ
aimed grafting (%)
1 − [(b/2)/(a/4)] × 100%
[d/(20·2)]/(a/4) × 100%
̅
n
P1
P4
P5
P6
P7
P8
4.4
11.1
15.8
27.4
45.9
50.3
1.2
1.4
1.2
1.2
1.2
1.2
10
25
10
24
38
50
11
24
37
50
50
100
D
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1
Figure 4. H NMR spectra of graft copolymers P5−P8.
contains chiral side chains, allowing chiral stacking, resulting in
a CD signal. Such signal was also observed for the
homopolymer. Naturally, since the P3HT is achiral, no CD is
observed.
For the graft copolymers, only a Cotton effect is observed for
P5, the graft copolymer with the lowest grafting density. The
region of the Cotton effect is the same as in P4, showing that
the Cotton effect originates from the PPE. The other polymers
do not show any CD, pointing at the absence of chiral self-
assembly. Interestingly, the CD effect of P5 changes sign
compared to homopolymer P4. This rules out that the effect
originates from some PPE that does not contain any P3HT, as
this would result in a smaller, positive Cotton effect. This also
shows that the P3HT influences the (chiral) self-assembly of
the PPE. Combined with the UV−vis results, these data show
that the PPE and P3HT influence each other’s self-assembly
and that the chiroptical behavior of these conjugated graft
copolymers is no simple superposition of the two polymers.
Atomic Force Microscopy (AFM). Figures 9a,b illustrate
the typical topographic Tapping Mode AFM images of thin
deposits of P7 and P8 (prepared from 0.01−0.1 mg/mL
solutions). Compared to what is generally observed for linear
conjugated polymers,42−45 here the polymer chains are not at
all organized as fibrillary assemblies.
1
Figure 5. H NMR spectrum of P6 in the region 2.5−4.5 ppm.
As suspected from the spectroscopic data, it appears that
upon solvent evaporation the polymer chains do not form
organized assemblies; instead, they tend to collapse into round-
shaped objects ranging from about 20 to 100 nm in diameter,
whatever the solvent used (THF or chloroform). The smallest
objects may contain only few polymer chains while the larger
ones are made of more chains. The aggregate size mainly
depends on the solvent evaporation kinetics and the initial
polymer concentration (larger aggregates are observed when
using more concentrated polymer solutions). This lack of
organization is most probably due to the fact that the PPE
segments are far from each other because of the presence of the
P3HT side chains, which prevents any π−π stacking and long-
range assembly of the conjugated backbone. For similar
reasons, the P3HT branches cannot interact to form fibrils.
For the P8 graft polymer, and whatever the solvent, the
morphology is also showing nonorganized round-shaped
objects despite the higher grafting density.
Figure 6. UV−vis spectra of homopolymers P1 and P4 and graft
copolymers P5−P8 in MeOH/CHCl3 (9/1). cP1 = 0.026 g/L; cP4
0.026 g/L; cP5 = 0.018 g/L; cP6 = 0.019 g/L; cP7 = 0.020 g/L; cP8
0.032 g/L.
=
=
CD Spectroscopy. Also, CD spectra for 90% MeOH
solutions of homopolymers and graft copolymers were obtained
(Figure 8). From the self-assembly of the homopolymers, it is
clear that in these conditions both the P3HT and PPE self-
assembleif this would be possible. The PPE backbone
Differential Scanning Calorimetry (DSC). DSC measure-
ments were performed on the polymer powders (Supporting
Information, S16). P1 shows a clear melting peak at 178 °C.
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Figure 7. Experimental vs simulated UV−vis of graft copolymers P5−P8 in MeOH/CHCl3 (9/1).
emit at different wavelengths due to a difference in the Stokes
shift. A first look at the fluorescence spectra shows a dramatic
decrease of the fluorescence of the PPE backbone for increasing
grafting densities. The amount of quenching was calculated
(see Supporting Information). As the fluorescence and the
extinction coefficient of the homopolymers and the mass
fraction of PPE and P3HT are known, energy transfer from the
PPE backbone to the P3HT side chains can be calculated. An
enormous degree of quenching is found for the fluorescence of
the PPE backbone (Table 2). The smaller quenching for P5 in
comparison to the other graft copolymers may be due to the
presence of a very small amount of homopolymer P4. Indeed,
while very small amounts of homopolymer (a few percent) are
not visible by GPC or CD, this amount can already result in a
significant fluorescence.
Figure 8. CD spectra of graft copolymers P5−P8 in MeOH/CHCl3
(9/1). cP1 = 0.026 g/L; cP4 = 0.026 g/L; cP5 = 0.018 g/L; cP6 = 0.019
g/L; cP7 = 0.020 g/L; cP8 = 0.032 g/L.
CONCLUSION
■
We report the first use of the “grafting to” method for the
synthesis of conjugated graft copolymers with a backbone and
side chains of different chemical nature (PPE and P3HT,
respectively). To achieve this goal, the CuAAC click reaction
was used to click acetylene end-functionalized P3HT to azide-
functionalized PPE. GPC demonstrates the synthesis of graft
copolymers with an increasing grafting density. Using 1H NMR
analysis, grafting percentages of 10, 24, 38, and 50% were
calculated for P5−P8. This is close to what was aimed for
except for P8. The (chiral) self-assembly was studied by means
of UV−vis and CD spectroscopy, AFM, and DSC. These
techniques showed that the aggregation behavior is not a linear
combination of backbone and side chains independent of each
other. The aggregation of the backbone as well as the side
Also, for P8 an onset of a melting peak at 138 °C is observed.
Besides the lower melting temperature compared to P1, also
the melting enthalpy dropped from 5 J/g to 3 mJ/g. For all
other graft copolymers no melting peaks were observed. This
result, in accordance with the UV−vis and AFM observations,
indicates that the presence of a small amount of PPE is
sufficient to disrupt the crystallization of poly(3-hexyl-
thiophene).
Fluorescence Spectroscopy. Fluorescence was measured
for solutions of the polymers in pure chloroform at an
excitation wavelength of 440 nm. The λmax,em for the PPE
backbone is around 473 nm, while that of the P3HT side chains
is around 574 nm (Figure 10). Although P3HT and PPE
absorb at the same wavelength region around 440 nm, they
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Figure 9. Tapping mode AFM height images (5.0 μm × 5.0 μm) of (a) P7 and (b) P8. The Z-range is 50 nm. The solvent was THF for P7 and
chloroform for P8.
comparison between the experimental and simulated
spectra of the graft copolymers (P5−P8) in 90% MeOH;
DSC thermograms of P1 and graft copolymers P5−P8
(PDF)
AUTHOR INFORMATION
Corresponding Author
*(G.K.) E-mail: guy.koeckelberghs@chem.kuleuven.be.
■
Notes
The authors declare no competing financial interest.
Ph.L. is F.R.S.-FNRS Senior Research Associate.
ACKNOWLEDGMENTS
Figure 10. Fluorescence spectra of graft copolymers P5−P8 in CHCl3.
cP1 = 0.0026 g/L; cP4 = 0.0026 g/L; cP5 = 0.0018 g/L; cP6 = 0.0019 g/
L; cP7 = 0.0020 g/L; cP8 = 0.0032 g/L.
■
We are grateful to the Onderzoeksfonds K.U. Leuven/Research
Fund K.U. Leuven and the Fund for Scientific Research (FWO-
Vlaanderen) for financial support. J.S. is grateful to IWT for a
doctoral fellowship. Research in Mons is supported by the
Science Policy Office of the Belgian Federal Government (PAI
Table 2. Calculated Percentages of Energy Transfer
graft copolymer
quenching PPE
́
7/5), the European Commission/Region Wallonne FEDER
P5
P6
P7
P8
−6
−96
−99
−99
program, and the “Fonds National pour la Recherche
Scientifique” (FRS-FNRS). The MONS laboratory is grateful
to the “Fonds National pour la Recherche Scientifique” (FRS-
FNRS) for financial support for the acquisition of the Waters
QToF Premier mass spectrometer and for continuing support.
chains is compromised by the covalent bonding to each other.
AFM data indicate that there is no self-assembly of the polymer
chains in the solid state. This is indicated by a disappearance of
the melting peak in DSC, the disappearance of fine structure in
the UV−vis spectra, and the disappearance of a chiral response
of the backbone in the CD spectra. Quenching of the
fluorescence of PPE was observed even when the ratio of
PPE/P3HT is small.
ABBREVIATIONS
■
PPE, poly(phenyleneethynylene); P3HT, poly(3-hexyl-
thiophene); PMDTA, N,N,N′,N″,N″-pentamethyldiethylene-
triamine; CD, chiral dichroism; DSC, density scanning
calorimetry; GPC, gel permeation chromatography; MALDI-
ToF, matrix assisted laser desorption ionization−time of flight;
GRIM, Grignard methatesis; KTCP, Kumada catalyst transfer
polymerization; CuAAC, copper assisted azide click chemistry;
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01830.
■
M , number-average molecular weight; DP, degree of polymer-
ization; AFM, atomic force microscopy.
̅
n
S
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DOI: 10.1021/acs.macromol.5b01830
Macromolecules XXXX, XXX, XXX−XXX