Study on the formation of a supramolecular conjugated graft copolymer in solution

Article abstract of DOI:10.1002/pola.27060

The formation in solution of a supramolecular graft copolymer bearing conjugated blocks is demonstrated using diffusion ordered NMR spectroscopy (DOSY). A tailor-made poly(3-(2-ethylhexyl)thiophene) (P3EHT) with a phenol end group is synthesized. For this purpose, the chain-growth mechanism of the polymerization of 2-bromo-5-chloromagnesio-3-alkylthiophenes in the presence of a Ni(dppp) catalyst (dppp = 1,3-bis(triphenylphosphino)propane) is exploited, as it enables the use of functionalized initiators to introduce specific end groups. The so-obtained polythiophene was subsequently mixed in solution with poly(4-vinylpyridine) (P4VP) to enable phenol-pyridine hydrogen bonding. The formation of the supramolecular graft copolymer is studied using DOSY-measurements. Based on the results thereof, the amount of P3EHT attached to the P4VP is calculated and the association constant of the hydrogen bond is estimated.

Full text of DOI:10.1002/pola.27060

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Study on the Formation of a Supramolecular Conjugated Graft
Copolymer in Solution
Tine Hardeman,1 Pieter Willot,1 Julien De Winter,2 Thomas Josse,2 Pascal Gerbaux,2
Pavletta Shestakova,3 Erik Nies,4 Guy Koeckelberghs1
1Laboratory for Polymer Synthesis, Division of Polymer Chemistry & Materials, KU Leuven, Celestijnenlaan 200F, 3001
Heverlee (Leuven), Belgium
2Mass Spectrometry Research Group, Interdisciplinary Center for Mass Spectrometry, University of Mons-UMONS,
23 Place du Parc, 7000 Mons, Belgium
3NMR Laboratory, Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences,
Acad G. Bonchev Street, Bl. 9, 1113 Sofia, Bulgaria
4Laboratory for Physical Chemistry of Polymer Materials, Division of Polymer Chemistry & Materials, KU Leuven, Celestijnenlaan
200F, 3001 Heverlee (Leuven), Belgium
Correspondence to: G. Koeckelberghs (E-mail: guy.koeckelberghs@chem.kuleuven.be)
Received 4 November 2013; accepted 5 December 2013; published online 23 December 2013
DOI: 10.1002/pola.27060
ABSTRACT: The formation in solution of a supramolecular graft
copolymer bearing conjugated blocks is demonstrated using dif-
fusion ordered NMR spectroscopy (DOSY). A tailor-made poly(3-
(2-ethylhexyl)thiophene) (P3EHT) with a phenol end group is syn-
thesized. For this purpose, the chain-growth mechanism of the
polymerization of 2-bromo-5-chloromagnesio-3-alkylthiophenes
in the presence of a Ni(dppp) catalyst (dppp 5 1,3-bis(triphenyl-
phosphino)propane) is exploited, as it enables the use of func-
tionalized initiators to introduce specific end groups. The so-
obtained polythiophene was subsequently mixed in solution with
poly(4-vinylpyridine) (P4VP) to enable phenol-pyridine hydrogen
bonding. The formation of the supramolecular graft copolymer is
studied using DOSY-measurements. Based on the results thereof,
the amount of P3EHT attached to the P4VP is calculated and the
C
V
association constant of the hydrogen bond is estimated. 2013
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014,
52, 804–809
KEYWORDS: conjugated polymers; DOSY; graft copolymers;
hydrogen bonding; supramolecular structures
of varying amounts of pentadecylphenol in combination with
poly(4-vinylpyridine) (P4VP), resulting in mesomorphic
structures.6 An all-conjugated covalent graft copolymer with
varying numbers of P3HT side chains has also successfully
been prepared by Wang et al.7 However, no supramolecular
graft copolymers have, to the best of our knowledge, been
made using conjugated polymers, nor has their behavior in
solution been the subject of study. As a prerequisite to favor
the formation of a comb-polymer, H-bonds should be as
strong as possible. Resultantly, phenol and pyridine clearly
emerge as suitable partners, as their H-bond is known to be
among the strongest. Therefore, we combine P4VP polymer
with a tailor-made poly(3-(2-ethylhexyl)thiophene) (P3EHT)
containing a phenol end group (Fig. 1). Since P3EHT can be
used as an electron-donor in organic electronics, while the
pyridine units of P4VP are known to interact with fullerenes
(electron acceptors), supramolecular interactions in such sys-
tems could be exploited for obtaining stabilized bulk hetero-
junction morphologies.
INTRODUCTION Supramolecular interactions can be a useful
tool for obtaining block-copolymeric structures, especially
when more complex species such as, for example, comb-
copolymers are targeted.1,2 First of all, this approach signifi-
cantly reduces synthetic difficulties which are encountered
when preparing the covalent analogs. Moreover, the revers-
ibility of such interactions affords a high degree of versatility
to the system. Hence, the use of supramolecular interactions
allows tuning the properties and composition of these mate-
rials. This is of prime interest in the field of conjugated poly-
mers, as their macromolecular structure has
a large
influence on the resulting properties. Among the wide vari-
ety of supramolecular interactions, hydrogen bonds (H-
bonds) offer the best set of characteristics for this purpose.
This is mainly due to the strength and directionality of H-
bonds, but also to their temperature- and pH-sensitivities.
For these reasons, H-bonds have been widely used for rever-
sibly connecting polymers and other species.3–5 Ruokolainen
et al. have successfully prepared a graft copolymer consisting
Additional Supporting Information may be found in the online version of this article.
C
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2013 Wiley Periodicals, Inc.
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and D is the translational diffusion coefficient (m2 s21). This
equation is applicable for free diffusion and usually a short
gradient pulse approximation is considered.
Size exclusion chromatography (SEC) measurements were
carried out on a Shimadzu 10A GPC system. The column
is a PLgel 5-lm mixed-D-type column and the detection
system consists of a differential refractometer and a UV–
vis spectrophotometer. The GPC system is calibrated
towards polystyrene standards (purchased from Polymer
Laboratories). Before measuring, the polymers are dis-
solved in THF (c ꢀ 1 mg mL21) and filtered over a pore
size of 0.2 lm.
FIGURE 1 Formation of a graft copolymer via hydrogen bond-
ing between P4VP and a P3EHT containing a phenol end group.
Matrix-assisted laser desorption ionization-time-of-flight
(MALDI-ToF) mass spectra were recorded using a Waters
QToF Premier mass spectrometer equipped with a nitrogen
laser of 337 nm with a maximum output of 500 J m22 deliv-
ered to the sample in 4 ns pulses at a 20-Hz repeating rate.
Time-of-flight mass analyses were performed in the reflec-
tion mode at a resolution of about 10,000. The matrix, trans-
2-[3-(4-tert-butyl-phenyl)-2-methyl-2-propenylidene]maloni-
trile, was prepared as a 40 mg mL21 solution in chloro-
form.12 The matrix solution (1 mL) was applied to a stainless
steel target and air-dried. Polymer samples were dissolved in
chloroform to obtain 1 mg mL21 solutions. Then, 1 mL ali-
quots of these solutions were applied onto the target area
(already bearing the matrix crystals) and then air-dried.
EXPERIMENTAL
Reagents and Instrumentation
All reagents were purchased and used without further
purification. (4-Bromo-3-methylphenoxy) (isopropyl) dime-
thylsilane (1), the nickel initiator (2), and 2-bromo-3-(2-eth-
ylhexyl)-5-iodothiophene (3) were synthesized according to
literature procedures.8–10
1H NMR spectra were recorded on a Bruker Avance 300
MHz spectrometer. All DOSY NMR spectra were measured on
Bruker Avance II1 600 NMR spectrometer using a 5-mm
direct detection dual broadband probe, with a gradient coil
delivering maximum gradient strength of 53 G cm21. The
experiments were performed at a temperature of 293 K and
polymer concentrations of 10 mg mL21. The DOSY spectra
were acquired with the double-stimulated echo-pulse
sequence, to eliminate possible convection during the experi-
ments.11 Monopolar sine shaped gradient pulses, gradient
recovery delay (s) of 100 ls, a longitudinal eddy current
delay of 5 ms and three spoiling gradients were used. All
spectra were recorded with 32K time domain data points in
t2 dimension and 32 t1 increments, 16 transients for each t1
increment, and a relaxation delay of 2 s. Diffusion delay (᭝)
of 200 ms and gradient pulse length (d) of 11 ms were used.
The gradient strength G was varied in 32 linear steps from 2
to 95% of the maximum gradient output of the gradient unit
(from 0.68 to 32.65 G cm21) to ensure optimal signal attenu-
ation. The spectra were processed with an exponential win-
dow function (line broadening factor 1), 32K data points in
F2 dimension and 1K data points in the diffusion dimension,
using the fitting routine integrated in the Topspin 2.1 pack-
age. The evaluation of the diffusion coefficients was per-
formed by fitting the diffusion profile (the normalized signal
intensity as a function of the gradient strength G) at the
chemical shift of each signal in the DOSY spectrum with an
exponential function using the variant of Stejskal–Tanner
equation adapted to the particular pulse sequence used:
Synthesis of Poly(3-(2-ethylhexyl)thiophene) with a
Phenol End Group
2-Bromo-3-ethylhexyl-5-iodothiophene (1.05 mmol; 0.422 g)
was brought under an argon atmosphere and dissolved in
dry THF (10 mL). A solution of iPrMgCl.LiCl (1.05 mmol;
0.763 mL) in THF was added to this solution and reacted for
1 h at room temperature. In the meantime, a mixture of the
functionalized initiator (20.0 lmol; 17.7 mg) and dppp (40.0
lmol; 16.5 mg) was purged with argon and dissolved in dry
THF (5 mL). After an initial 15-min reaction period at room
temperature, the monomer solution was added to the reac-
tion mixture and the polymerization was allowed to proceed
for 60 min. The polymerization was quenched by addition of
acidified THF. The latter is obtained by adding a droplet of a
37% HCl solution to 15 mL of THF. The reaction mixture
was then precipitated in methanol and filtered off. Next, the
polymer was purified with a Soxhlet extraction using metha-
nol and chloroform. The chloroform fraction was again pre-
cipitated in methanol, filtered off and dried. The polymer
was obtained as a dark red solid.
Yield: 0.150 g (75%); 1H NMR (300 MHz, CDCl3, d): 6.94 (s,
1H, Ar H in P3HT), 2.74 (d, 2H, Ar-CH2-C in P3HT), 22.43 (s,
3H, Ar-CH3), 1.70–1.29 (m, 8H, CH2), 0.88 (m, 6H, CH3).
ꢂ
ꢀ
ꢁꢃ
I
4
p2
d
2
RESULTS AND DISCUSSION
¼ exp 2DðcdGÞ
D2
(1)
I0
2
Synthesis of P3EHT with Phenol End Group
The synthesis of poly(3-alkylthiophene)s (P3AT) has been
thoroughly investigated, which has led to protocols enabling
here, c is the gyromagnetic ratio of 1H (rad T21 s21), d is
the gradient pulse length (ms), ᭝ is the diffusion delay (ms),
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structure determination, this spectrum can also provide
information on the DP, the efficiency of the initiation and
the living nature of the polymerization. Information on these
parameters can be deducted from the integration values of
the small peaks originating from terminal units of the poly-
mer.26,27 The polymer has two possible chain ends: one aris-
ing from the o-tolyl initiating species (signal e) and one
from the terminal methylene, with either a Br or H in the 2-
position (signal p). Note that given the fact that only one
doublet is found for Hp points to the fact that all chains
bear either a H- or Br-atom at this end. Hence, the DP is
given by:
g
p
DP5
1
(2)
p=21e=3 p=21e=3
in which the first term accounts for the inner and the second
term for the terminal thiophene units. A DPn of 41 was
found. An equivalent way of determining the DP makes use
of the aromatic signals arising from the proton in the 4-
position of the thiophene ring. Once again, the ratio of inte-
gration values of the peak arising from the hydrogen atoms
in the main chain (Hf) and the one from terminal unit (Hx) is
calculated, also giving rise to a DP of 41. This is slightly
lower than the theoretical value of 50, but can be explained
by the fact that the polymerization was not yet fully com-
pleted when the polymerization was terminated. However, it
was chosen to terminate the polymerization at that time, to
assure the living nature prevails over termination. This goal
was indeed achieved, as no evidence of chains end-capped
with bromine (instead of hydrogen) was found with MALDI-
ToF (Fig. 3). In order to verify the efficiency of the initiation,
signals p and e were compared. Taking the number of hydro-
gen atoms that each peak represents into account, this con-
firms that one initiator is built in for one terminal thiophene
unit. Since MALDI-ToF confirms that indeed each chain is
equipped with one initiator unit and one terminal unit (Fig.
3), we can conclude that the initiation was very efficient.
The MALDI-ToF measurement also shows that the number
SCHEME 1 Controlled synthesis of P3EHT bearing a phenol
end group.
a living polymerization and high degree of control for these
materials.13–18 We have opted to use the selective GRIM for
generating the desired monomer, followed by a nickel-
catalyzed polymerization. It is known that the polymerization
of 2-bromo-5-iodo-3-alkylthiophenes proceeds via a controlled
chain-growth mechanism in the presence of a Ni(dppp)-cata-
lyst. The chain-growth mechanism allows the use of external
initiators, which will be built in at the beginning of each
chain.19–24 Hence, the use of a functionalized initiator enables
us to introduce a phenol end group in every P3AT, capable of
engaging in H-bonds. Another benefit of an external initiator
is the faster initiation, which results in a lower polydisper-
sity.25 The initiator is synthesized according to literature
procedures.9 Before polymerization, a ligand exchange is per-
formed with dppp, forming (2). This guarantees an optimal
control over the polymerization (Scheme 1).14,15
By varying the monomer to initiator ratio, the degree of
polymerization (DP) can be controlled. We selected a 50/
1-ratio, which theoretically results in a DP of 50. It was
chosen not to use a higher DP to avoid too much steric hin-
drance for the supramolecular hydrogen bonding induced by
longer chains, as this would reduce the amount of P3EHT
chains attached to P4VP. After termination of the polymeriza-
tion with HCl, the Ni-centers will be replaced by a hydrogen
atom. Hence, while a hydrogen at the end of the polymer
chain is a signature of the living character, a bromine at the
same position implies that the active center, and, conse-
quently, also the living character of the chain, was lost. In a
final step, the phenol group of the polymer is deprotected
using tert-butylammonium fluoride (TBAF) and HCl.
average molar mass of the polymer is equal to 7.7 kg mol21
,
The excellent control over the polymerization is evidenced
by GPC analysis of the polymer, which showed a very low
polydispersity of 1.07. The number average molar mass was
found to be 12 kg mol21, based on SEC measurement using
a polystyrene standard calibration. The polymer is further
analyzed using NMR spectroscopy (Fig. 2). Apart from the
FIGURE 2 1H NMR Spectrum of P3EHT bearing a phenol end
group.
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FIGURE 3 MALDI-ToF spectrum of P3EHT containing a phenol end group, evidencing the perfect initiation en living character of
the polymerization.
corresponding to a DPn of 39, in line with the results of 1H
NMR spectroscopy. The obtained MALDI-Tof spectrum is rep-
resentative, as the low PDI of the polymer enables an effi-
cient ionization of the chains. Finally, no significant
secondary peaks are observed which could be indicative of
side products or uncontrolled polymerization.
the hydrogen-bonded species will differ from their free coun-
terparts, evidence for hydrogen bonding can be found.29,30
This slightly less common technique is chosen because infra-
red spectroscopy (often used to detect hydrogen bonding) is
difficult in solution and dynamic light scattering is compli-
cated for systems containing polythiophenes, as they do not
only scatter but also absorb the light.
Formation of a Supramolecular Graft Copolymer
First, the diffusion coefficients of the pure materials were
measured using DOSY [Fig. 4(a,b)]. These were found to be
5.55 3 10211 m2s21 for P4VP and 14.4 3 10211 m2s21 for
the phenol-initiated P3EHT. The higher value for the former
is expected, as the molar mass of P4VP is more or less five
times higher than the molar mass of the polythiophene. On
the other hand, the conjugation of the polythiophene causes
When the phenol-initiated P3EHT is combined with P4VP, a
supramolecular comb-copolymer should arise as a result of
phenol-pyridine hydrogen bonding. In order to investigate
whether or not hydrogen bonding is taking place, diffusion-
ordered NMR spectroscopy (DOSY) is used.28 This simple
and fast technique is capable of determining diffusion coeffi-
cients of species in solution. As the diffusion coefficient of
FIGURE 4 (a) DOSY spectrum of P4VP in CDCl3 (10 mg mL21). (b) DOSY spectrum of P3EHT with phenol end group in CDCl3 (10
mg mL21).
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viscosity because of the small polymer concentrations. In
order to fully exclude the effect of the viscosity on the mea-
surement, some tetramethyl silane was added as a reference
material.31 It was observed that there was no shift in the
TMS signal going from the pure materials to the mixture.
The diffusion coefficient was found to be 2.23 3 1029
m2s21 in all samples. This proves that the effect of the
increased viscosity is negligible and that the shift toward
lower diffusion coefficients can be ascribed to supramolecu-
lar complex formation.
The results of the DOSY measurements prove the formation
of hydrogen bonds between P4VP and phenol-initiated
P3EHT. However, the strength of this noncovalent interaction
is limited, as only a fraction of the P3EHT chains are coupled
with the P4VP. The measured diffusion coefficients of the
polymers in the blend are represented by the following
equation.
D
measured 5xfree
D
free 1xbound Dbound
(3)
with xfree and xbound the fractions of the polymer in the free
and bound state and Dfree and Dbound their respective diffu-
sion coefficients. From this equation, it is possible to deter-
mine the average fraction of P3EHT chains that is involved
in the supramolecular association. In order to do so, the
assumption has to be made that the diffusion coefficient of
P3EHT in the bound state is equal to the diffusion coefficient
of P4VP in the mixture. This means that we assume that all
P4VP chains have a certain number of binding partners in
the mixture. This is a reasonable assumption, since the molar
mass of the P4VP is sufficiently large and it is likely that the
P3EHT chains are more or less equally distributed over the
P4VP chains for interaction. With the values extracted from
the DOSY spectra, we obtain a value for x(HO-P3EHT)free of
0.54. This means that, on average, slightly more than half of
the HO-P3EHT chains is not interacting with the P4VP via
hydrogen bonding. Hence, 46% is involved in supramolecular
complex formation.
FIGURE 5 DOSY specrum of the 50/50 wt% mixture of P4VP
and the phenol-initiated P3EHT. Two different diffusion coeffi-
cients are clearly observed, although there is a shift towards
lower values compared to the spectra of the pure materials.
the chain to be more rigid than P4VP, which generally leads
to a larger hydrodynamic radius and lower diffusion coeffi-
cient. However, this effect is obviously not large enough to
compensate for the large difference in molar mass.
Subsequently, a 50/50 wt % mixture of both polymers in
solution is measured. This ratio is chosen because it com-
bines the advantages of sufficiently intense signals for both
polymers and an excess of binding partners available for the
phenol-initiated polythiophene. The latter is beneficial, as
upon binding between PV4P and P3EHT the largest shift in
diffusion coefficient is expected for P3EHT. The DOSY spec-
trum of the mixture shows the presence of species with two
different diffusion coefficients (Fig. 5). If complete associa-
tion between both materials occurs, only one diffusion coeffi-
cient of the combined species would be detected. Clearly,
this is not the case. The results from DOSY spectra imply
that not all P3EHT chains are attached to P4VP, despite the
large excess of binding partners. Nevertheless, there is a shift
in the diffusion coefficients toward lower values for both
species in the blend as compared to their individual solu-
tions (Table 1 and Supporting Information Fig. S1). This
decrease of the diffusion coefficients can be explained by the
formation of a supramolecular complex with a smaller diffu-
sion coefficient. This complex is in dynamic equilibrium with
the unbound polymer chains, so that an average diffusion
coefficient of the associated and free polymers is measured.
As the total concentration of pyridine units and P3EHT
chains in solution is known, and also the fraction of P3EHT
chains attached to a pyridine unit, it is possible to calculate
the association constant (see Supporting Information). It was
found to be 8.8. In comparison, the association constant for
hydrogen bonding between free pyridines and phenols in
CCl4 was found to be 44.6.32 The difference between both
values could tentatively be ascribed to a lower “availability”
of both interacting partners in the polymer system.
TABLE 1 The diffusion coefficients of P4VP and P3EHT in
CDCl3 as determined by DOSY
Diffusion Coefficient –
Pure Material
Diffusion Coefficient –
In Mixture
(10211 m2s21
)
Another explanation for the observed changes in diffusion
coefficients could be an increase in viscosity. In the mixture,
the total polymer concentration is higher than for the pure
polymers. Nevertheless, all solutions were still of very low
Polymer
(10211 m2s21
)
P4VP
5.55
14.4
3.90
9.55
P3EHT
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9 F. Monnaie, W. Brullot, T. Verbiest, J. De Winter, P. Gerbaux,
A. Smeets, G. Koeckelberghs, Macromolecules DOI: 10.1021/
ma401809e.
The formation of this supramolecular complex was found
not to have an influence on the absorption spectrum of
P3EHT (Supporting Information Fig. S2). Hence, the aggrega-
tion behavior of P3EHT was not affected by the addition of
P4VP, which can be understood by the relatively low number
of P3EHT chains per P4VP and the only partial complexation.
10 Y. Zhang, K. Tajima, K. Hirota, K. Hashimoto, J. Am. Chem.
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254.
A phenol-initiated P3EHT with a successful initiation, good
control over the molar mass and a low polydispersity was
prepared. This polymer was subsequently mixed in solution
14 A. Yokoyama, R. Miyakoshi, T. Yokozawa, Macromolecules
2004, 37, 1169–1171.
with P4VP in order to form
a supramolecular graft-
15 E. Sheina, J. Liu, M. C. Iovu, D. W. Laird, R. D. McCullough,
Macromolecules 2004, 37, 3526–3528.
copolymer in solution through phenol-pyridine hydrogen
bonding. This blend was studied using DOSY, which showed
that the diffusion coefficients of both components in the
blend decreased with respect to their diffusion coefficients
in pure solution. This effect is ascribed to the formation of a
supramolecular complex, being the first supramolecular
graft-copolymer with a conjugated polymer in solution.
16 R. Miyakoshi, A. Yokoyama, T. Yokozawa, J. Am. Chem.
Soc. 2010, 132, 7803–7810.
17 M. C. Iovu, E. E. Sheina, R. R. Gil, R. D. Mccullough, Macro-
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18 R. S. Loewe, P. C. Ewbank, R. D. McCullough, Macromole-
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19 N. Doubina, S. Paniagua, S. A. V. Soldatova, A. K. Y. Jen, S.
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ACKNOWLEDGMENTS
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. T.H. is a doctoral fellow of
the Fund for Scientific Research (FWO-Vlaanderen). P.W. is a
doctoral fellow of IWT Vlaanderen. The Mons MS Laboratory
acknowledges the ‘Fonds de la Recherche Scientifique (FRS-
FNRS)’ for its contribution to the acquisition of the Waters
QToF Premier Mass Spectrometer. The present work is partially
supported by the FRFC research program (convention No.
2.4508.12).
20 E. Kaul, V. Senkovskyy, R. Tkachov, V. Bocharova, H.
Komber, M. Stamm, A. Kiriy, Macromolecules 2010, 43, 77–81.
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