Synthesis and transfer of chirality in supramolecular hydrogen bonded conjugated diblock copolymers

Article abstract of DOI:10.1021/ma502357a

The synthesis of a block copoly(3-alkylthiophene) consisting of two different P3AT blocks equipped with an H-donor and -acceptor functionality is presented. The P3ATs were synthesized using a functionalized Ni-initiator. By a series of postpolymerization reactions, including click chemistry, an H-donor and -acceptor entity was attached to the end of the polymer chains. Evidence for a quantitative functionalization of the polymers was provided by ¹H NMR and MALDI-ToF analyses. Chiral side chains were implemented on one of both blocks, allowing the study of the influence of the H-bond formation on the chiral self-assembly using UV-vis and circular dichroism spectroscopy.

Full text of DOI:10.1021/ma502357a

Article
pubs.acs.org/Macromolecules
Synthesis and Transfer of Chirality in Supramolecular Hydrogen
Bonded Conjugated Diblock Copolymers
Frederic Monnaie,† Ward Ceunen,† Julien De Winter,‡ Pascal Gerbaux,‡ Valentina Cocchi,§
Elisabetta Salatelli,§ and Guy Koeckelberghs*,†
†Laboratory for Polymer Synthesis, KU Leuven, Celestijnenlaan 200F, Box 2404, B-3001 Heverlee, Belgium
‡Organic Synthesis and Mass Spectrometry Laboratory, Research Institute for Science and Engineering of Materials, University of
Mons-UMONS, 20 Place de Parc, B-7000 Mons, Belgium
§Dipartimento di Chimica Industriale “Toso Montanari”, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
S
* Supporting Information
ABSTRACT: The synthesis of a block copoly(3-alkylthiophene) consisting of two
different P3AT blocks equipped with an H-donor and -acceptor functionality is
presented. The P3ATs were synthesized using a functionalized Ni-initiator. By a
series of postpolymerization reactions, including click chemistry, an H-donor and
-acceptor entity was attached to the end of the polymer chains. Evidence for a
1
quantitative functionalization of the polymers was provided by H NMR and
MALDI-ToF analyses. Chiral side chains were implemented on one of both blocks,
allowing the study of the influence of the H-bond formation on the chiral self-
assembly using UV−vis and circular dichroism spectroscopy.
predictable molar masses, and perfect control over the
molecular structure. Hence, it also provides the possibility to
prepare all-conjugated block copolymers by successive
monomer addition24,30−50 and even control over the end-
groups. In the specific case when Ni(dppp) is used as a catalyst,
H/Br-terminated CPs are obtained.14,15 As far as the end-
functionalizing of the conjugated polymers is concerned,
different approaches can be used. In the case of a controlled
chain-growth polymerization, a Grignard reagent equipped with
a functional group can be added at the end of the
polymerization.51−54 The main drawback of this approach is
that it requires a controlled polymerization, which is only the
case for a selected number of CPs. The approach also suffers
from the fact that both mono- and dicapping can occur,51−54
leading to a sample that is contaminated with polymers
containing two functional groups. A second approach makes
use of postpolymerization reactions: the Br atom can be
converted into carboxylic acid by making use of a GRIM
reaction and the addition of CO2,55 or an aldehyde function can
be introduced at the H-terminated thiophene by a Vilsmeier
reaction.56 A third method makes use of an initiator which is
equipped with a functional group. This requires the polymer-
ization to proceed via a chain-growth mechanism, but the
polymerization does no longer need to be controlled. Applied
to P3ATs, this concept results in polymers end-capped at one
end with the functional group of the initiator and at the other
end with a H atom. Different functionalized polymers have
INTRODUCTION
■
Different methodologies have been developed in the past to
obtain advanced macromolecular structures. Most of those
procedures rely on the creation of covalently bonded
macromolecules. Nevertheless, in the past decades, noncovalent
interactions, with a special emphasis on hydrogen bonds, have
triggered more and more interest from different research
groups and can now be regarded as an extra toolbox.1
Multiple hydrogen bonds (MHBs) have shown to be
versatile, and by making use of the directional character of
the interaction, different kinds of highly structured entities can
nowadays be obtained. One way of exploiting H-bonding in
polymer science is the formation of so-called pseudo block
copolymers, i.e. supramolecular block copolymers designed
using various building blocks, such as poly(benzyl methacry-
late) and poly(n-butyl acrylate),2 self-complementary poly(alkyl
acrylates),3 and telechelic poly(isobutylene)s and poly(ether
ketone)s.4 The H-bonding is not only limited to two blocks but
also led to the development of polymers of a supramolecular
nature.5−7 These interactions were also implemented in
conjugated entities such as perylenes.8 Meijer et al. extended
the work on conjugated assembly by focusing on chiral oligo(p-
phenylenevinylene)perylene bisimide.9 This led to the further
development of conjugated polymer networks for applications
such as organic photovoltaics.10,11
Contrarily to their nonconjugated counterparts, conjugated
polymers (CP) equipped with H-bonding entities have not yet
been so extensively employed. The ability to obtain poly(3-
alkylthiophene)s (P3AT)s12−15 and some other conjugated
polymers16−29 via a controlled chain-growth polymerization has
opened new platforms to synthesize CPs with low dispersities,
Received: November 21, 2014
Revised: December 4, 2014
© XXXX American Chemical Society
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already been obtained by this method.57−62 The functional
groups make it possible to form a diblock conjugated
copolymer that cannot be synthesized by successive monomer
addition.58,60,63 Another asset is that they can be used to
decorate nanoparticles (NPs).53 We recently prepared a variety
of such hybrid materials using different functional groups and
nanoparticles.64
Hardeman et al. showed that these functionalized initiators
are not restricted to form covalent bonds but can also be used
as partners for noncovalent interactions such as single H-bonds
in the formation of graft copolymers.65 Besides these single
hydrogen bond P3ATs, Enders et al. reported oligo- and
polythiophenes with MHB moieties.66 In the present work, we
report the formation of a block copolymer (Figure 1)
matric crystals) and then air-dried. FT-IR spectra were recorded using
a Bruker Alpha-p apparatus in ATR mode. UV−vis and circular
dichroism (CD) measurements were performed on a PerkinElmer
Lambda 900 UV−vis NIR and a JASCO 62 DS apparatus, respectively.
Synthesis of N-(6-Acetamidopyridin-2-yl)-4-ethynylbenza-
mide (7). 5 (500 μmol, 75.6 mg), 4-(dimethylamino)pyridine
(DMAP) (10.0 μmol, 1.22 mmol), and triethylamine (TEA) (550
μmol, 76.2 μL) were dissolved in dry THF (2.5 mL) under a N2
atmosphere. This mixture was added dropwise to a solution of 6 (0.50
mmol, 82.3 mg) in dry THF (1 mL). The reaction mixture was
refluxed for 30 h and cooled to room temperature. Subsequently, the
mixture was extracted with ethyl acetate and washed with brine. The
organic layers were collected and dried over MgSO4, and the solvents
were removed under reduced pressure. The product was purified using
column chromatography on silica gel, eluting with ethyl acetate. The
product was isolated as a white solid. Yield: 19.6 mg (14%). 1H NMR
(CDCl3): δ = 8.23 (s, 1H), 8.06 (d, J = 8.2 Hz, 1H), 7.94 (d, J = 8.2
Hz, 1H), 7.86 (d, J = 8.6 Hz, 2H), 7.77 (t, J = 8.2 Hz, 1H), 7.63 (d, J =
8.6 Hz, 2H), 7.60 (s, 1H), 3.25 (s, 1H), 2.22 (s, 3H). 13C NMR
(CDCl3): δ = 168.48, 164.61, 149.52, 149.36, 141.10, 132.58, 127.11,
126.30, 109.84, 109.71, 82.51, 80.21, 24.81. MS: m/z = 279 (M+), 237
(M+ − CH3CO). Melting point: 191.3−193.6 °C.
Synthesis of 1-(Prop-2-ynyl)pyrimidine-2,4-dione (8). Hex-
amethyldisilazane (HMDS) (15.5 mmol, 2.50 g) and trimethylsilyl
chloride (TMSCl) (1.60 mmol, 174 mg) were dissolved in dry
acetronitrile (20 mL) under an argon atmosphere, and the mixture was
added dropwise to a solution of uracil (4.00 mmol, 448 mg) in dry
acetonitrile under an argon atmosphere. The mixture was refluxed for
4 h and was then cooled to room temperature, after which propargyl
bromide in toluene (4.80 mmol, 80 wt %, 714 mg) and KI (10.0 μmol,
1.67 mg) were added to the solution. The mixture was then refluxed
for 16 h, extracted with ethyl acetate, and washed with NaHCO3 and
brine. The product was dried over MgSO4, and the solvents were
removed under reduced pressure. The product was purified using
column chromatography on silica gel, eluting with ethyl acetate/
methanol (97/3). The product was obtained as white crystals. Yield:
186 mg (31%). 1H NMR (CDCl3): δ (ppm) = 8.66 (s, 1H), 7.46 (d, J
= 8.2 Hz, 1H), 5,79 (dd, J = 8.2 Hz, J = 1.6 Hz, 1H), 4.56 (d, J = 2.6
Hz, 2H), 2.50 (t, J = 2.5 Hz, 1H). Melting point: 167.8−170.4 °C.
Synthesis of P1a. The precursor initiator 1 (50.0 μmol, 44.9 mg)
and 1,3-bis(diphenylphosphino)propane (dppp) (100 μmol, 41.2 mg)
were dissolved in dry THF (4 mL), purged with argon, and stirred for
15 min. Subsequently, monomer 4a in dry THF (8.67 mL) was purged
with argon and added to the initiator solution. For the synthesis of 4a,
the precursor monomer 3a (1.00 mmol, 359 mg) was dissolved in dry
THF (8 mL) and purged with argon, and i-PrMgCl·LiCl (1.28 M in
THF, 1.00 mmol, 0.87 mL) was added to the solution. The reaction
was stirred for 60 min at room temperature. To verify the conversion,
a small aliquot (0.2 mL) was quenched with D2O after 30 min and
analyzed by 1H NMR. After polymerizing for 1 h, the reaction mixture
was terminated with a 2 M HCl solution. The mixture was
concentrated, and the polymer was precipitated in MeOH. Next, the
polymer was filtered and fractionated by Soxhlet extraction with
methanol and chloroform. The chloroform fraction was concentrated,
and the polymer was precipitated in methanol, filtered, and dried in
vacuo. The final polymer was recovered as a dark red-brown solid.
Yield: 90.0 mg (59%).
Figure 1. Block copolymer formed by MHB.
composed of two conjugated P3AT blocks, by noncovalent
interactions, more in particular MHBs. MHBs rather than
single H-bonds will be used to increase the interaction strength.
By introducing chiral alkyl side chains on one block, the
stacking behavior of the different blocks can be monitored, and
the influence of the block copolymer formation on the self-
assembly can be studied. Previous studies have indicated that
the block that aggregates first imposes an influence on the
stacking behavior of the other block.46,63 Here it will be tested
whether this principle also holds when the block copolymer
formation is realized by supramolecular H-bond formation
(Figure 1).
EXPERIMENTAL SECTION
■
All reagents were purchased from TCI, Sigma-Aldrich, Acros Organics,
and ABCR. Reagent grade solvents were dried by a solvent purification
system MBRAUN SPS 800 (columns with activated alumina). The
precursor initiator 1,63 2-acetamido-6-aminopyridine (5),67 4-ethynyl-
benzoyl chloride (6),68 and the precursor monomers 3a46 and 3b46
were synthesized according to literature procedures.
The gel permeation chromatography (GPC) measurements were
performed using a Shimadzu 10A apparatus with a tunable absorbance
detector and a differential refractometer in THF as eluent calibrated
toward polystyrene standards. 1H and 13C nuclear magnetic resonance
(1H NMR) measurements were carried out with a Bruker Avance 300,
400, and 600 MHz. Mass spectra were recorded using an Agilent
HP5989. 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/m2 delivered to the sample in 4 ns
pulses at 20 Hz repeating rate. Time-of-flight mass analyses were
performed in the reflection mode at a resolution of about 10 000. The
matrix, trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-
malonitrile (DCTB), was prepared as a 40 mg/mL solution in
chloroform.69 The matrix solution (1 μL) was applied to a stainless
steel target and air-dried. Polymer samples were dissolved in
chloroform to obtain 1 mg/mL solutions. Then, 1 μL aliquots of
these solutions were applied onto the target area (already bearing the
Synthesis of P1b. The same procedure as described for P1a was
followed, using 3b (1.00 mmol, 401 mg). Yield: 128 mg (62%).
Synthesis of P2a. P1a (283 μmol, 43.1 mg) was dissolved in THF
(50 mL), purged with argon, and shielded from light. Tetrabuty-
lammonium fluoride trihydrate (TBAF·3H2O) (340 μmol, 107 mg)
was added, and the reaction was stirred at room temperature for 16 h.
Water was added and extracted with CHCl3, dried over MgSO4, and
concentrated. The polymer was precipitated in methanol, filtered, and
dried in vacuo. P2a was obtained as a dark red-brown solid. Yield: 39.0
mg (90%).
Synthesis of P2b. The same procedure as described for P2a was
followed, using P1b (518 μmol, 101 mg) and TBAF·3H2O (622 μmol,
196 mg). Yield: 93.0 mg (92%).
B
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Scheme 1. Overview of the Polymer Synthesis Including Polymerization of 4a and 4b with the Use of Initiator 2, Followed by
the Different Postpolymerization Reactions Yielding P4a and P4b, and Synthesis of the Hydrogen Donor 7 and Acceptor 8
Entities
against disproportionation.57 Since the polymerization requires
the use of Grignard monomers, protection of the alcohol
function is needed.
Synthesis of P3a. P2a (158 μmol, 24.0 mg) was dissolved
together with diphenylphosphoryl azide (DPPA) (3.15 mmol, 867
mg) in dry THF (60 mL) and cooled to 0 °C, and then 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) (3.15 mmol, 480 mg) was
Polymers P1a and P1b were synthesized with the Ni(dppp)-
added. The mixture was stirred for 12 h at room temperature,
mediated polymerization. Therefore, the precursor monomers
concentrated, and precipitated in methanol. Subsequently, the polymer
2-bromo-5-iodo-3-alkylthiophene (3a and 3b) were converted
was washed thoroughly with methanol and dried under vacuo. P3a was
to the actual monomers 5-magnesiochloro-2-bromo-3-alkylth-
iophene (4a and 4b) using i-PrMgCl·LiCl. Prior to the
initiation, a ligand exchange using 2 equiv of dppp was
performed (Scheme 1). The dppp ligand, in contrast to PPh3
present in 1, results in a controlled polymerization.57 Hereby,
we were able to tune the polymerization degree (DP) by
varying [M]0/[In]. This ratio was set to 20 for both
polymerizations. The polymerization was terminated after 1 h
by treating the mixture with a 2 M HCl solution in THF.
Molar masses and dispersities of P1a and P1b (Table 1)
were determined by GPC toward polystyrene standards. It is
recovered as a dark red-brown solid. Yield: 21.3 mg (89%).
Synthesis of P3b. The same procedure as described for P3a. P2b
(400 μmol, 79 mg), DPPA (8.00 mmol, 2.20 g), and DBU (8.00
mmol, 1.22 g) were used. Yield: 67.0 mg (86%).
Synthesis of P4a. An argon-purged solution of N,N,N′,N″,N″-
pentamethyldiethylenetriamine (PMDTA) (9.03 μmol, 1.56 mg) in
dry THF (20 mL) was added to a suspension of CuBr (6.78 μmol, 970
μg) in dry THF (20 mL). To this mixture, a solution of P3a (113
μmol, 17.2 mg) and 7 (612 μmol, 170 mg) in dry THF (200 mL) was
added. The mixture was stirred for 24 h under an argon atmosphere.
Then, an aqueous solution of NH3 was added. The mixture was
subsequently extracted with CHCl3, dried over Na2SO4, concentrated,
precipitated in methanol, and filtered. P4a was obtained as a dark red-
brown solid. Yield: 13.4 mg (79%).
Table 1. M , Đ, and DP for P1a and P1b
̅
n
Synthesis of P4b. The same procedure as described for P4a was
followed, using P3b (214 μmol, 45 mg), 8 (1.16 mmol, 174 mg)
PMDTA (18.5 μmol, 3.21 mg), and CuBr (13.9 μmol, 1.99 mg). Yield:
43.0 mg (96%).
a
n
a
b
polymer
M
̅
(kg/mol)
Đ
DP
P1a
P1b
3.6
5.3
1.1
1.1
15
16
a
Determined by GPC in THF toward polystyrene standard.
Determined by H NMR
b
1
RESULTS AND DISCUSSION
■
Polymer Synthesis. For the synthesis of P3ATs equipped
with H-donor and -acceptor entities, it is important to realize
that Grignard monomers (4a and 4b, Scheme 1) are employed
during the polymerization. Since Grignard reagents are
incompatible with acidic entities (H-donors), different
postpolymerization reactions, including click chemistry proce-
dures, are employed to introduce the functionality. Therefore, a
protected alcohol Ni-initiator63 is synthesized and used in the
polymerization.
important to note that GPC tends to overestimate the molar
mass of P3ATs.70 For both polymers low dispersities were
obtained, which is in line with the controlled nature of the
polymerization.
1
A part of the H NMR spectra of P1a and P1b is presented
in Figure 2. The spectrum of P1a shows the o-tolyl function of
the initiator at 2.49 ppm (a), the doublet of multiplets at 2.70
ppm originating from the internal α-methylene protons (b),
and more upfield a part of a multiplet of a doublet originating
from the terminal α-methylene (c). Note that in the figure only
the upfield part of the doublet of c is integrated due to
The functional Ni-initiator was prepared by a oxidative
insertion of Ni(PPh3)4 in an appropriately functionalized o-tolyl
bromide. The o-tolyl group was used to enhance the stability
C
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the chains were H-terminated, indicating the high degree of
1
control over the polymerization, which is in line with the H
NMR analysis.
Postpolymerization Reactions. After preparation of the
protected P3ATs, a series of postpolymerization reactions were
performed on P1a and P1b (Scheme 1) in order to obtain the
hydrogen donor and acceptor functionalized P3ATs. It is
important to note that all postpolymerization reactions must be
1
quantitative. Their conversion was monitored by H NMR
spectroscopy (Figure 4). In particular, the signal corresponding
to the CH2 next to the functional group was monitored. In a
first step, the silyl protecting group was removed by the use of
TBAF·3H2O. Upon deprotection, a small shift in the 1H NMR
from 4.74 (P1a and P1b) to 4.71 ppm (P2a and P2b) can be
observed. Next, the alcohol function was converted to an azide
with DBU and DPPA, resulting in a shift from 4.71 (P2a and
P2b) to 4.34 ppm (P3a and P3b).
Finally, in a last postpolymerization step, the donor (7) and
Figure 2. Zoom of the α-methylene and o-tolyl region of P1a (left)
and P1b (right).
acceptor (8) units were coupled to the azide groups using a
1
Cu(I)AAC click reaction. H NMR indicated a shift to 5.60
ppm for the CH2 in P4a and 5.49 ppm for P4b was observed.
Note also the presence of the singlet at 4.9 ppm, originating
from the H-acceptor moiety, where it shifts from 4.6 ppm
before the coupling to 4.9 ppm after the coupling. Besides H
NMR spectroscopy, FT-IR measurements also supported the
quantitative conversion (Figure S20).
overlapping signals. A DP of 15 (Table 1) was derived from the
integration values using eq 1 with x = 1 (only half of the signal
is integrated).
1
b + c
DP =
For the synthesis of 7 (Scheme 1), acetyl chloride was
reacted with 2,6-diaminopyridine to yield 5. After purification,
4-ethynylbenzoyl chloride (6)68 was added in order to produce
the donor unit 7. The acceptor unit 8 (Scheme 1) was
synthesized starting from uracyl, following a two-step
procedure that involves the addition of hexamethyldisilizane
and trimethylsilyl chloride, followed by the addition of
propargyl bromide and KI.
MALDI-ToF analysis of P4a and P4b (Figure 5) shows that
after a series of three postpolymerization reactions the polymer
chains were still all H-terminated and equipped with the correct
acceptor or donor entity. MALDI-ToF spectra of all polymers
are presented in the Supporting Information (Figures S4−S11).
1H NMR Analysis as Evidence for MHB Formation. For
the study of the self-assembly of the supramolecular block
copolymer, solvents are required to allow the formation of the
a
3
c
x
+
(1)
The same pattern is also observed in the spectrum of P1b: the
o-tolyl function of the initiator at 2.49 ppm (a), a broad triplet
at 2.80 ppm originating from the internal α-methylene protons
(b), and more upfield a triplet originating from the terminal α-
methylene (c) which corresponds to a H-terminated unit. Here,
it is clear that the α-methylene protons of Br-terminated
thiophene, resonating at 2.57 ppm,46,71 are not observed in the
1H NMR spectrum. This indicates that the polymerization
proceeded in a controlled manner. A DP of 16 (Table 1) was
calculated using the integration values (x = 2) from Figure 2 in
combination with eq 1.
MALDI-ToF analysis (Figure 3) of P1a and P1b shows that
all chains were indeed initiated by the external initiator and that
Figure 3. MALDI-ToF spectra of P1a (left) and P1b (right).
D
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1
Figure 4. H NMR spectrum of postpolymerization of P1a−P4a (left) and P1b−P4b (right).
Figure 5. MALDI-ToF spectra of P4a (left) and P4b (right).
H-bonds. The 1H NMR spectra of the donor (7), the acceptor
(8), and an equimolar mixture of 7 and 8 in different solvents
and solvent mixtures are presented in Figure 6. Upon H-bond
formation, the NMR spectra of 7, 8, and the 7 + 8 mixture are
expected to be different, highlighting then the formation of the
noncovalent complexes. For the self-assembly studies, two
different solvents are required. The first solvent must dissolve
both P4a and P4b, while remaining compatible with the
subsequent formation of H-bonds. In a second step, we will add
a nonsolvent that result in the self-assembly but leaves the H-
bonds intact.
a′) as well as for the other protons (b, c, d, f, g, and b′),
concluding that CHCl3 can be used as solvent for these
aggregation experiments (Figure 6). By using a solvent mixture
of CDCl3/CD3CN (50/50), obvious shifts were observed as
well. This provides the evidence that upon addition of CH3CN
the polymer not only self-assembles but also that the H-bonds
are not disrupted.
Next, it is investigated whether the H-bonds are also formed
between the functionalized polymers. Since the polymers start
to precipitate in CD3CN, the 1H NMR spectrum is only
1
recorded in CDCl3. Again, H NMR was recorded for P1a,
THF and CHCl3 are able to solubilize poly(3-
alkylthiophene)s and are candidates as good solvent.
Acetonitrile is able to precipitate the polymers and might also
keep the H-bonded entities interacting with each other. Note
that methanol, being a protic solvent, was obviously not
considered. For all experiments described below, 0.5 mM
deuterated solutions of 7, 8, and mixture of 7 and 8 were
prepared. When THF-d8 was used as a solvent, no difference
P1b, and an equimolar mixture of both polymers (Figure 7),
based on the DP values determined from the 1H NMR spectra.
It is clear that the amide protons a and a′ are strongly shifted in
the mixture spectrum, indicating that H-bonds are indeed
formed. Also, small shifts can be observed for several other
protons such as b, c, d, b′, and c′.
Self-Assembly Study. Polymers P4a and P4b and an
equimolar mixture of P4a and P4b were subjected to a
solvatochromism experiment. By using a mixture of the H-
donor and -acceptor polymers and comparing the results which
the spectra obtained from the separate polymers (P4a and
P4b), the effect of H-bond formation, i.e. block copolymer, on
the stacking behavior can be monitored by CD spectroscopy. In
the solvatochromism experiment, CH3CN is gradually added as
a nonsolvent to CHCl3 solutions of P4a, P4b, and a 1:1 mixture
1
(Figure S21) between the H NMR spectra for the donor or
acceptor and the mixture of both was observed, demonstrating
that the H-bonds are disrupted in THF. Therefore, THF is not
the appropriate solvent for these aggregation studies. CDCl3, in
1
contrast, did induce significant differences between the H
NMR data for the mixture and the individual compounds.
Indeed, a clear shift is observed for the amide protons (a, e, and
E
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1
Figure 6. H NMR of 7, 8, and 7 + 8 in CDCl3 (top three spectra) and in CDCl3/CD3CN 50/50 (bottom three spectra).
1
Figure 7. H NMR of P4a, P4b, and P4a and P4b mixed in equimolar amounts in CDCl3.
of P4a and P4b. In order to ensure reproducibility, the
nonsolvent was added slowly by a syringe pump (0.5 mL/min).
After the addition of the different nonsolvent amounts, CD and
UV−vis spectra were measured. Upon analyzing the CD results
(Figure 8) of the chiral P4a and the equimolar mixture of P4a
and P4b, it is obvious that the chiral expression of the mixture
is lower than the response of pure P4a. Important to note here
is that the results are presented in g/L P4a used in the
experiment. This allows a direct comparison of the chiral
expression of the mixture and P4a. This shows that the H-bond
formation reduces the chiral stacking to about 35% of the
original chiral response by the presence of the achiral P4b in
the mixture. When considering the UV−vis spectra of both P4a
and P4b (Figure 8 and Figure S22, respectively), the polymer
chains of P4b start to stack at lower amounts of CH3CN than
in the case of P4a (42% versus 58%, respectively). Previous
research has shown that, for a block copolymer composed of
blocks which differ more than 2C’s in the length of the side
chain, the blocks tend to microphase separate.38 Taking a
difference in side chain length (8C’s compared to 4C’s) of both
monomer units in the mixture of P4a and P4b into account, a
microphase separation can be expected. It is also been shown
that the block that aggregates first dictates or influences the
stacking behavior (chiral or achiral) of the block that aggregates
later on (first come, first served principle46). Since the achiral
block self-assembles first, these experiments suggest that this
principle is also present in this supramolecular block
copolymer.
F
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Figure 8. UV−vis (top) and CD (bottom) spectra in CHCl3 and added amount (%) of CH3CN of P4a (cstart = 0.025 g/L) (left) and the equimolar
mixture P4a (cstart = 0.011 g/L) and P4b (cstart = 0.014 g/L) (right), Δε is expressed in L g−1 cm−1; g L−1 is the mass concentration of P4b.
Figure 9. UV−vis (top) and CD (bottom) spectra in CHCl3 and added amount (%) of CH3CN of P2a (cstart = 0.025 g/L) (left) and the equimolar
mixture P2a (cstart = 0.011 g/L) + P2b (cstart = 0.014 g/L) (right). Δε is expressed in L g−1 cm−1; g L−1 is the mass concentration of P2a.
In order to prove that the reduction of Δε is indeed
originating from the H-bond formation and the effect of the
achiral block (P4b) on the chiral block (P4a) and not from the
formation of some sort of mixed aggregates of both blocks, the
experiment is repeated using P2a, P2b, and an equimolar
mixture of both. These polymers are equipped with an alcohol
function and are not able to from the H-bonds. The UV−vis
experiments confirm that P2b (Figure S22) aggregates at lower
amounts of CH3CN compared to P2a (Figure 9, as was also the
case for P4a and P4b). If the formation of mixed aggregates is
G
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amounts for CH3CN are depicted in Figure 9. The CD spectra
of the mixture and P2a are clearly similar, and no reduction of
chiral expression is observed. Therefore, it can be concluded
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this H-bond formation results in a quenching effect of the chiral
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ASSOCIATED CONTENT
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(21) Huang, L.; Wu, S.; Qu, Y.; Geng, Y.; Wang, F. Macromolecules
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* Supporting Information
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1
compounds; GPC data, H NMR, FT-IR, and MALDI-ToF
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spectra of the polymers. This material is available free of charge
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́
(24) Boon, F.; Hergue, N.; Deshayes, G.; Moerman, D.; Desbief, S.;
De Winter, J.; Gerbaux, P.; Geerts, Y. H.; Lazzaroni, R.; Dubois, P.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: guy.koeckelberghs@chem.kuleuven.be (G.K.).
■
(25) Erdmann, T.; Back, J.; Tkachov, R.; Ruff, A.; Voit, B.; Ludwigs,
S.; Kiriy, A. Polym. Chem. 2014, 5, 5383.
Notes
(26) Pammer, F.; Jager, J.; Rudolf, B.; Sun, Y. Macromolecules 2014,
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful to the Onderzoeksfonds KU Leuven/Research
Fund KU Leuven and the Fund for Scientific Research (FWO-
Vlaanderen). F.M. and W.C. are grateful to IWT for a doctoral
fellowship. The Mons laboratory thanks the “Fonds National
pour la recherche Scientifique” (FRS-FNRS) for financial
support in the acquisition of the Waters QToF Premier and for
continuing support. The present project is partially supported
by the FRFC grant 2.4508.12. V.C. is grateful to the University
of Bologna (Marco Polo Funds 2013).
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