AB block copoly(3-alkylthiophenes): Synthesis and chiroptical behavior

Article abstract of DOI:10.1021/ma2021503

In a first part of this article, the synthesis of AB type block copoly(3-alkylthiophene)s initiated by Ni(dppp)Cl₂, the most commonly used initiator for these polymers, is investigated. For this study the respective ¹H NMR resonances of all possible end-groups are identified. This result confirms the hypothesis that the Ni(dppp) species can walk back to the beginning of the polymer chain and that propagation can occur at both chain ends. The next part of the article studies the chiroptical behavior of AB-type block copoly(3-alkylthiophene)s with one chiral block and compares the results with those of the corresponding random copolymers. In order to obtain exclusively AB-type block copolymers, the polymers were prepared from a modified Ni initiator. They all have the same degree of polymerization but vary in the length of the respective blocks. The chiroptical behavior was studied by changing the ratio solvent/nonsolvent, meanwhile monitoring the UV-vis and circular dichroism (CD) spectra. Three series were investigated: one in which both blocks aggregate simultaneously, one in which the achiral block stacks before the chiral block, and one in which the chiral block stacks first followed by the achiral block. It was found that when the blocks stack independently, the (chiral/achiral) stacking of the latter is significantly influenced by the former. If both blocks of the polymer chains aggregate simultaneously, Cotton effects which are significantly larger than those of the chiral homopolymer are found.

Full text of DOI:10.1021/ma2021503

ARTICLE
pubs.acs.org/Macromolecules
AB Block Copoly(3-alkylthiophenes): Synthesis and
Chiroptical Behavior
Michiel Verswyvel, Frederic Monnaie, and Guy Koeckelberghs*
Laboratory of Polymer Synthesis, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee (Leuven), Belgium
S Supporting Information
b
ABSTRACT: In a first part of this article, the synthesis of AB type
block copoly(3-alkylthiophene)s initiated by Ni(dppp)Cl2, the most
commonly used initiator for these polymers, is investigated. For this
study the respective 1H NMR resonances of all possible end-groups
are identified. This result confirms the hypothesis that the Ni(dppp)
species can walk back to the beginning of the polymer chain and that
propagation can occur at both chain ends. The next part of the article
studies the chiroptical behavior of AB-type block copoly(3-al-
kylthiophene)s with one chiral block and compares the results with
those of the corresponding random copolymers. In order to obtain exclusively AB-type block copolymers, the polymers were
prepared from a modified Ni initiator. They all have the same degree of polymerization but vary in the length of the respective blocks.
The chiroptical behavior was studied by changing the ratio solvent/nonsolvent, meanwhile monitoring the UVꢀvis and circular
dichroism (CD) spectra. Three series were investigated: one in which both blocks aggregate simultaneously, one in which the achiral
block stacks before the chiral block, and one in which the chiral block stacks first followed by the achiral block. It was found that when
the blocks stack independently, the (chiral/achiral) stacking of the latter is significantly influenced by the former. If both blocks of
the polymer chains aggregate simultaneously, Cotton effects which are significantly larger than those of the chiral homopolymer
are found.
’ INTRODUCTION
tail-to-tail dyad. This has an important consequence on the block
copolymer formation. If two electronically equal polymer blocks
are polymerized using Ni(dppp)Cl2 as the initiator, for instance
P3ATs with different alkyl substituents, the catalyst might travel
along the polymer chain and a batch of AB block copolymers
(originating from unidirectional growth) will be obtained, to-
gether with BAB block copolymers which are formed when
Ni(dppp) walks back to the other end of the polymer after the B
monomer has started to polymerize (bidirectional growth).
Apart from progress on the mechanistic aspects of their
polymerization, the chiroptical properties of (conjugated)
polymers has been a topic of interest for many decades. Green
et al. discovered the sergeants-and-soldiers and majority-rules
behavior in polyisocyanates.8 This implies that the fraction of
one-handed helices in random helical copolymers is higher than
the fraction of chiral monomer or the enantiomeric excess
(sergeants-and-soldiers and majority-rules, respectively) present
in the chain. This results in a chiral response, which increases
nonlinear with the fraction of chiral monomer or the enantiomeric
excess. Meijer and co-workers have revealed that the majority-
rules principle is present in random copoly(3-alkylthiophene)s and
that sergeants-and-soldiers and majority-rules behavior can be
found in stacks of chiral and achiral P3ATs which aggregate at
The field of conjugated polymers emerges at a continuously
increasing speed. For a long time researchers have almost
exclusively been driven by applications, such as transistors,
photovoltaics, light-emitting diodes, etc.1 A major synthetic
breakthrough was realized when the groups of McCullough2
and Yokozawa3 demonstrated that the Ni(dppp)Cl2-initiated
(dppp = 1,3-bis(diphenylphosphino)propane) polymerization
of poly(3-alkylthiophene)s (P3ATs) using a Kumada coupling
reaction proceeds via a controlled chain-growth mechanism.
This allows the polymerization of P3ATs, and later also other
monomers,4 often of predictable molar mass and a controlled
molecular structure. Moreover, with this mechanism, block
copolymers can be prepared by successive monomer additions.5
Also, block copolymers composed of two different alkylthio-
phenes were prepared, and it was found that if the alkyl
substituents differ more than two carbon atoms in length, phase
separation occurs.5b,6
The controlled nature of the Ni(dppp)-mediated polymeri-
zation can be obtained because after reductive elimination (RE),
the Ni(dppp) species remains complexed with the π-conjugated
backbone. Very recently, Kiriy and co-workers postulated the
possible “walking” of the catalyst moiety from one end to the
other during the polymerization.7 When the polymerization is
initiated with Ni(dppp)Cl2 and, consequently, polymers with
two active carbonꢀbromine bonds are formed, this can result in
polymer chains growing from both sides of the initially formed
Received: September 22, 2011
Revised:
November 7, 2011
Published: November 21, 2011
r
2011 American Chemical Society
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Figure 1. Structure of the block and random copolymers examined in this paper. For each polymer different values of m, n, k, and l are synthesized.
the same nonsolvent content.5l We have prepared AB block
copolymers composed of a P3AT and P3AOT block, of which
one or both of them are chiral.5gꢀh We found that the block that
aggregates first imposes restrictions to the aggregation of the
second block, resulting in chiroptical properties which are unusual
for that particular polymer and different from the blend.
in which N is the number of monomer units (degree of
polymerization (DP)) and δ a “stickiness” parameter. Only if δ
equals (or is very close to) 1, the polymer grows (almost)
exclusively on one side. Quantification of δ can predict to which
extent bidirectional growth takes place and whether AB or BAB
block copoly(3-alkylthiophene)s will be obtained.
In this article, the investigation of the chiroptical properties of
block copoly(3-alkylthiophene)s composed of an achiral and a
chiral block is described. While the total length of the block
copolymers is fixed, the fraction of chiral monomers is varied
by changing the length of this block. P3ATs were chosen for
this study, since their polymerization proceeds via a controlled
chain-growth mechanism, allowing to adjust the degree of
polymerization of the blocks at wish and to prepare block
copolymers by successive monomer addition in a one-pot
synthesis. The length of the achiral side chains (butyl vs octyl)
is chosen in such way that the block that aggregates first (the
chiral or achiral) can be tuned. The chiroptical behavior is
compared with random copolymers with the same monomer
composition. Since the exclusive formation of AB-type block
copolymers, which is required for this research, depends on
the fact whether the polymer grows only unidirectionaly or not,
in the first part of this article we describe the investigation
whether unidirectional growth takes place in case of P3ATs. In
the second part of the article, we describe the synthesis of the
actual copolymers and their chiroptical properties are evaluated
(Figure 1).
The first step in the polymerization of P3AT with Ni-
(dppp)Cl2 as a catalyst is a double transmetalation step of two
monomer units on the Ni+II moiety, resulting in a tail-to-tail dyad
(TT-dyad) (see Scheme 1). As long as the polymer chain grows
in only one direction, the TT-dyad remains on one end of the
polymer. Once the catalyst moves to the other end and promotes
a propagation step there, the TT-dyad ends up on the inside of
the polymer. The latter is accompanied by the disappearance of a
carbonꢀbromine bond on the TT-dyad. Consequently, five
different α-methylene protons, and corresponding triplet signals
1
in H NMR spectroscopy, are expected. They are denoted as
Brꢀtail-to-tail (Br-TT), Brꢀhead-to-tail (Br-HT), Hꢀhead-to-
tail (H-HT), tail-to-tailꢀhead-to-tail (TT-HT), and head-to-
tailꢀhead-to-tail (HT-HT) (see Scheme 1). Using these signals,
we can define the fraction of polymer chains grown only in one
direction versus these grown on both directions. The more
polymer chains have grown in both directions, the more TT-
dyads will be moved to the inner polymer chain; i.e., the bromine
on the TT-dyad disappears, and as a consequence the triplet
signal Br-TT weakens in favor of Br-HT. Indeed, the active side
of the polymer chain will bear a hydrogen upon termination, and
the other side retains a bromine.
For this reason, an accurate assignment of the signals is critical
to define the walking behavior of the catalyst during the
polymerization and the “stickiness” (δ). During the past years,
effort was made to assign the triplet signals of the α-methylene
protons of the 1H NMR spectra. Indeed, these signals will differ
significantly depending on their environment. The triplet signals
Br-HT, H-HT, and HT-HT have been already assigned.2a,9
Nevertheless, for the determination of δ, also Br-TT needs to
’ RESULTS AND DISCUSSION
Unidirectional vs Bidirectional Growth. As already men-
tioned, Ni(dppp) remains complexed to the propagating poly-
mer chains in the polymerization of P3AT, and evidence is
present that growth occurs on both sides of the polymer chains.7
For each propagation step, growth at the same end of the
polymer is in any case more likely, since (i) after RE Ni(dppp)
is located more close to the polymer’s growing end and (ii) the
CꢀBr bond polarizes the last thiophene unit, directing the
Ni(dppp) toward this end. Taken this into account, Kiriy showed
that the probability that a polymer chain has grown only on one
side is
1
be identified. When examinating the H NMR spectrum of a
P3AT polymer polymerized with the described protocol, a
deviation of the integration between Br-HT and H-HT is clearly
visible. From the living nature of the polymerization, one expects
that every polymer chain has one Br-end and one H-end.
Nevertheless, a Br-end can also be a Br on a TT-dyad, which is
not covered by the Br-HT signal. Interestingly, another triplet is
PðNÞ ¼ Nδ ꢀ 1
ð1Þ
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Scheme 1. Schematic Representation of the Polymerization of P3AT with Ni(dppp)Cl2 as a Catalysta
a Unidirectional and bidirectional growth and the consequences for the end-groups are shown. RE = reductive elimination; OA = oxidative addition;
TM = transmetalation.
Scheme 2. Synthesis of the Ter(3-alkyl)thiophene 4
Figure 2. 1H NMR spectrum (600 MHz) of the α-methylene protons
in P3HT polymerized with Ni(dppp)Cl2 as a catalyst.
lithium-2,2,6,6-tetramethylpiperidine (LiTMP) and transmeta-
lation with Me3SnCl to 2.
The resonances of the three α-methylenes of 4 were assigned
(Figure 3). This confirms our hypothesis on the assignment of Br-
TT and TT-HT. By using the integration of Br-TT resulting from
unidirectional growth and Br-HT originating from bidirectional
growth, it is possible to calculate the fraction of the polymer
chains with unidirectional growth (P). Doing so for polymer
chains with different degrees of polymerization (N) allows us to
determine δ accurately. Indeed, rearrangement of eq 1 results in
observed more downfield next to Br-HT. By adding its integra-
tion with this of Br-HT, an equal integration is obtained as for H-
HT. This lets us assume that the triplet at 2.54 ppm is Br-TT.
To verify our hypothesis, a ter(3-alkyl)thiophene is prepared
(Scheme 2) with a bromine on the TT-end. This allows us to
assign Br-TT (2.54 ppm) correctly. Moreover, since also the TT-
HT dyad (2.71 ppm) is formed, the corresponding triplet signal
could also be attributed. The synthesis of the terthiophene is
performed in two steps. The bithiophene 3 was coupled with
4-hexyl-2-trimethyltinthiophene (2), which was prepared by
lithiation of 1b on the 5-position with the in situ prepared
logðPÞ
þ 1 ¼ δ
ð2Þ
logðNÞ
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Two series of P3ATs with varying N were prepared. Despite
the use of poly(3-butylthiophene) (P3BT) in the second part of
this article, we opted for poly(3-hexylthiophene) (P3HT) and
poly(3-octylthiophene) (P3OT) because of solubility reasons.
The precursor monomers 6b and 6c are synthesized as described
in the literature.3b The 2-bromo-3-alkylthiophenes were iodi-
nated with iodine and iodobenzene diacetate (Scheme 3). The
precursor monomers 6b and 6c are converted in situ to the
formula which defines the probability to synthesize AB block
copolymers becomes
PðN þ MÞ
PAB
¼
ð3Þ
PðNÞ
with N and M the degree of polymerization of the A and B
block, respectively. The result is visualized in Figure S1. For
instance, for a block copolymer with an A block of 30
monomers and a B block of 10 monomers, the probability
for unidirectional growth is ∼91%. In general, only for a long
first block and a short second block the fraction of BAB block
copolymers becomes negligible.
monomers 7b and 7c with i-PrMgCl LiCl and polymerized with
3
Ni(dppp)Cl2. After polymerization overnight, the reaction mix-
ture is quenched with a 2 M HCl solution, precipitated in
methanol, and dried.
The obtained N and the ratio Br-TT/(Br-HT + Br-TT) (P)
for the synthesized P3HTs and P3OTs are summarized in
Table 1. The degree of polymerization was calculated from
Consequently, since block copolymers with a totally different
composition (AB vs BAB) are required for the chiroptical study
(see further), this Ni(dppp)Cl2 cannot be used as initiator for
these polymers. More in general, this implies that no batch of
exclusively AB block copoly(3-alkylthiophene)s can be obtained
(unless the first block is very large and the second very short),
putting earlier reports on such polymerizations into perspective.
Chiroptical Behavior of Random and Block Copolymers.
In this second part, the study of the chiroptical behavior of AB
block copolymers with one chiral and one achiral block is
described. Since Ni(dppp)Cl2 fails in the exclusive formation
of AB block copolymers, an adjusted initiator must be used. We
opted for the functionalized bromo-(o-tolyl)-nickel(dppp)
which is prepared in situ starting from the air-stable bromo-(o-
tolyl)-bis(triphenylphosphine)nickel 8 and dppp by stirring
these two components for 30 min at room temperature in
THF just before the polymerization. This functionalized initiator
has only one reactive side, unlike the TT-dyad, and can therefore
only grow in one direction. In order to allow comparison, the
total length of all polymers remains the same (i.e., n + m ≈ 40),
1
the H NMR spectra by setting the integration of the outer
α-methylene protons (2.61 to 2.54 ppm) at 2; consequently, the
inner α-methylene protons (2.80 to 2.71 ppm) integrate for N-2.
The observation that Br-HT is always present and that the ratio
of Br-TT/(Br-HT + Br-TT) decreases with increasing degree of
polymerization already points at bidirectional growth,
i.e., δ < 1. Next, the δ for each N was calculated. From these
results, we can conclude that the value of the stickiness for P3HT
and P3OT is 0.68 and 0.73, respectively.
When we apply these values on the one-pot synthesis of
block copolymers with two electronically equivalent blocks,
the ratio of AB and BAB block copolymers formed can be
predicted. We need to realize that during the polymerization of
the first block (A) the catalyst moiety can “walk” from one end
to the other without any influence on the final result of the
block copolymer constitution. To define the probability for a
block copolymer to be built in an AB manner, only the
contributions of P during the polymerization of the second
block needs to be taken into account. For this reason, the
Table 1. Overview of the Number Average Molar Mass (Mn
As Determined by GPC, Polydispersity Index (PDI), the
Obtained Degree of Polymerization (N) As Determined by 1H
NMR Spectroscopy, the Fraction of Unidirectional Growth
(P), and the Stickiness (δ) for the Synthesized Polymers
polymer
Mn (kg molꢀ1
)
PDI
N
P (%)
δ
P3HT13
P3HT18
P3HT20
P3HT24
P3HT27
P3OT12
P3OT18
P3OT22
P3OT29
2.8
3.5
4.9
6.3
7.4
3.8
5.3
7.5
10.0
1.2
1.3
1.2
1.2
1.2
1.3
1.3
1.3
1.4
13
18
20
24
27
12
18
22
29
42
39
38
35
34
48
47
43
41
0.67
0.68
0.68
0.67
0.68
0.71
0.74
0.73
0.74
Figure 3. 1H NMR spectrum of the terthiophene 4 (see full spectrum in
Figure S6).
Scheme 3. Synthesis of the Precursor Monomers 6aꢀ6e and Polymers P3BTn, P3HTn, P3OTn, P3OT*n, and P3BT*
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Scheme 4. Synthesis of the Block Copolymers P3OTn-b-P3OT*m, P3BTm-b-P3OT*n, and P3OTn-b-P3BT*m (A) and the Random
Copolymers P3OTk-co-3OT*l, P3BTl-co-3OT*k, and P3OTk-co-3BTl* (B)
but the lengths of the respective blocks differ. Furthermore, the
length of the achiral substituent was varied in order to allow
separate or simultaneous aggregation of both blocks in the block
copolymer when increasing amounts of nonsolvent (cf. poor
solvent) is added. Moreover, it has been shown that a difference in
length of at least two carbon atoms in the substituents on the two
blocks is sufficient to induce microphase separation.6 As already
mentioned, we synthesized and investigated the random copoly-
mers analogues as well for comparison with the corresponding
block copolymers. The precursor monomers 6a, 6c, 6d, and 6e
are synthesized as depicted in Scheme 3.14,3b,5h All together, we
synthesized poly(3-butylthiophene)-b-poly(3-((S)-3,7-dimethyl-
octyl)thiophene) (P3BT-b-P3OT*), poly((3-butylthiophene)-
co-(3-((S)-3,7-dimethyloctyl)thiophene)) (P3BT-co-3OT*), poly-
(3-octylthiophene)-b-poly(3-((S)-3,7-dimethyloctyl)thiophene)
(P3OT-b-P3OT*), poly((3-octylthiophene)-co-(3-((S)-3,7-di-
methyloctyl)thiophene)) (P3OT-co-3OT*), poly(3-octylthio-
phene)-b-poly(2-(S)-methylbutyl)thiophene) (P3OT-b-P3BT*),
and poly((3-octylthiophene)-co-(2-(S)-methylbutyl)thiophene))
(P3OT-co-3BT*), each time with different lengths of the chiral
blocks (Figure 1).
To start the polymerization of the first block, 7c or 7d is added
to the functionalized initiator with the monomer/initiator ratio
corresponding to the targeted length of the first block. For P3BT-
b-P3OT*, we opted for P3OT* as the first block for solubility
reasons. The initially added monomer is fully consumed after 1 h.
Exploiting the living nature of the polymerization, the second
monomer (7a, 7d, or 7e) is added in such amount that the total
chain length for all the polymers (block A + block B) is the same
(Scheme 4A and Table 2). After stirring overnight, the polymer-
ization is terminated with a 2 M HCl solution, precipitated in
methanol, and subjected to a Soxhlet extraction with methanol,
acetone, hexane, and chloroform. The chloroform fraction is
precipitated in methanol and dried. Termination with acid is
preferred to avoid disproportionation, which would result in BAB
block copolymers.10 The random copolymers were prepared by
adding the two monomers in the appropriate ratio together at the
beginning of the polymerization (Scheme 4B).
Table 2. Ratio of Monomers 7a, 7c, 7d, and 7e to the Initiator
8 Used in the Synthesis of the Corresponding Block and
Random Copolymers
polymer
7c/8
7d/8
P3OT36
40
32
20
8
0
8
P3OT31-b-P3OT*9
P3OT19-b-P3OT*19
P3OT8-b-P3OT*30
P3OT*38
20
32
40
8
0
P3OT31-co-3OT*10
P3OT20-co-3OT*18
P3OT10-co-3OT*25
32
20
8
20
32
polymer
7a/8
7d/8
P3BT41
40
32
20
8
0
8
P3BT32-b-P3OT*9
P3BT11-b-P3OT*24
P3BT10-b-P3OT*30
P3BT29-co-3OT*11
P3BT21-co-3OT*17
P3BT11-co-3OT*30
20
32
8
32
20
8
20
32
polymer
7c/8
7e/8
P3BT*18
0
32
20
8
40
8
P3OT20-b-P3BT*11
P3OT11-b-P3BT*17
P3OT4-b-P3BT*21
P3OT27-co-3BT*9
P3OT17-co-3BT*17
P3OT8-co-3BT*19
20
32
8
32
20
8
20
32
1H NMR spectroscopy is used to determine the length of both
blocks. The total chain length can be obtained by the ratio of the
methyl protons of the initiator (singlet) at 2.49 ppm and the total in-
tegration of all signals of the α-methylene protons (2.83ꢀ2.78 ppm)
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Table 3. Overview of the number average molar mass (Mn),
polydisperisty index (PDI), the degree of polymerization (N),
the fraction achiral monomers and chiral monomers incor-
porated for the synthesized block- and random-copolymers
polymer
Mn (kg molꢀ1
)
PDI
N
7c (%) 7d (%)
P3OT36
10.2
12.7
11.5
11.5
12.2
12.8
11.0
9.8
1.1
1.1
1.2
1.2
1.2
1.1
1.1
1.1
36
40
38
38
38
41
38
35
100
78
49
20
0
0
22
P3OT31-b-P3OT*9
P3OT19-b-P3OT*19
P3OT8-b-P3OT*30
P3OT*38
51
80
100
24
P3OT31-co-3OT*10
P3OT20-co-3OT*18
P3OT10-co-3OT*25
76
52
30
48
70
polymer
Mn (kg molꢀ1
)
PDI
N
7d (%) 7a (%)
P3BT41
8.6
12.1
7.2
1.2
1.2
1.1
1.1
1.1
1.2
1.1
41
41
35
40
40
38
41
0
22
68
75
28
46
74
100
78
32
25
72
54
26
P3BT32-b-P3OT*9
P3BT11-b-P3OT*24
P3BT10-b-P3OT*30
P3BT29-co-3OT*11
P3BT21-co-3OT*17
P3BT11-co-3OT*30
9.8
11.6
11.5
12.3
polymer
Mn (kg molꢀ1
)
PDI
N
7c (%) 7e (%)
Figure 4. (A) Representative CD spectrum of P3OT190-b -P3OT*19 in
a chloroform/methanol 44/56 mixture. (B) Overview of the absolute
value of g values of the negative Cotton effect in CD spectroscopy for the
block and random copolymers in a chloroform/methanol 44/56 mix-
ture. The data points are calculated from the absolute values of the
maximal CD signal near 470 nm.
P3BT*18
3.8
7.9
5.5
4.3
8.1
7.3
6.4
1.2
1.2
1.2
1.2
1.2
1.2
1.2
18
31
28
25
36
34
27
0
64
38
15
75
50
28
100
36
62
85
25
50
72
P3OT20-b-P3BT*11
P3OT11-b-P3BT*17
P3OT4-b-P3BT*21
P3OT27-co-3BT*9
P3OT17-co-3BT*17
P3OT8-co-3BT*19
block copolymers, the two blocks aggregate separately if the
solubility of both blocks is significantly different; in the random
copolymers, suchmicrophase separation is, ofcourse, absent. Ifthe
first polymer of the P3OT-b-P3OT* series is considered
(P3OT31-b-P3OT*9), it is clear that it is composed of a short
chiral block and a long, less soluble achiral block. As a conse-
quence, the achiral block aggregates first. Naturally, as this block is
achiral, the stacking of the this block is achiral; i.e., the polymer
chains are parallel stacked. When the chiral block aggregates (at
higher nonsolvent content), the stacking of this block is governed
by the first block: both blocks adopt more or less the same
orientation, and all polymer chains, alsothe partofthe chiralblock,
are hardly rotated. Therefore, in the final solvent conditions,
P3OT31-b-P3OT*9 shows only a very small chiral response—
much smaller than expected if both blocks would stack indepen-
dently. In the other P3OT-b-P3OT* block copolymers, both
blocks have more or less the same solubility, and they stack
simultaneously. Since the side chains have an equal length,
microphase separation is unlikely and both blocks stack together.
If a sergeant-and-soldier behavior would be present, the chiral
response should be higher than predicted by the mole ratio of
chiral monomer but smaller than that of P3OT* homopolymer.
Interestingly, the chiral response of P3OT19-b-P3OT*19 and
P3OT8-b-P3OT*30 is much larger, pointing to the fact that this
is no sergeant-and-soldiers behavior. This is rather remarkable
since Meijer and co-workers reported sergeant-and-soldier beha-
vior in blends of chiral and achiral P3ATs.5l Nonetheless, it is clear
that in block copolymers of which both blocks stack together the
(see Figures S4 and S5). The ratio of the 1H NMR signal of the
protons of the methyl group(s) of the branched chiral substituent,
and the signal of the α-methylene protons, allows an easy
quantification of the achiral and the chiral monomers incorpo-
rated in the polymer. These two parameters provide us with the
actual length of each block. These lengths, the relative amounts of
each block, Mn, and PDI determined by GPC in THF toward
polystyrene standards are listed in Table 3 for all the synthesized
polymers. Note that the GPC results also confirm that the
copolymers all have a similar molar mass and low PDI.
All the block copolymers were subjected to a solvatochromism
experiment. For this purpose, nonsolvent was added gradually up
to56% methanol (see Supporting Information Figures S2and S3).
No change in the spectra was observed using a higher nonsolvent
percentage. During these additions, UVꢀvis and circular dichro-
ism (CD) spectra were recorded in order to verify whether two
different species which would normally aggregate separately (at
different nonsolvent content), but as a result of too rapid addition
of nonsolvent would be forced to aggregate simultaneously. This
was not the case. The same amount of nonsolvent was added to
the random copolymers. The UVꢀvis and CD spectra are
consistent with those of aggregated P3ATs (see Supporting
Information Figures S2, S3, and S4). The results of the CD signals
(i.e., gabs value, gabs = Δε/ε) under the final solvent conditions for
the block and random copolymers are shown in Figure 4 as a
function of their percentage chiral monomer. In the case of the
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chiral response is higher, meaning that the angle by which the
stacked polymer chains are rotated is higher than in the chiral
homopolymer. An analogous trend is also found in the random
copolymers: the chiral response of P3OT31-co-3OT*10, P3OT20-
co-3OT*18, and P3OT10-co-3OT*25 is higher than that of the
homopolymer (P3OT*38).
Let us consider the P3BT-b-P3OT* series next. In compar-
ison with the achiral block of the previous series (P3OT), P3BT
suffers from a poorer solubility. Note that this poor solubility
made us to use P3OT* as the first block in the synthesis of the
block copolymer. Precipitation of the P3BT block would termi-
nate the polymerization, rendering block copolymer formation
impossible. Therefore, in all P3BT-b-P3OT* block copolymers,
the P3BT block aggregates first (in a achiral way). This hinders
the chiral stacking of the P3OT* block: the chiral block is forced
to adopt more or less the same stacking as the first block. This
results in a smaller than expected chiral response. Interestingly, in
(some of) the random copolymers, the chiral response again
exceeds gabs of P3OT*38.
Finally, the P3BT*-b-P3OT series is studied. In contrast to the
previous block copolymers, here the chiral polymer has the lower
solubility. Hence, in the block copolymers in which the chiral
block has the same or a larger length than the achiral block, the
chiral block aggregate first. Upon aggregation, the polymer
chains of the second, achiral block are forced to stack in the
same way, i.e., rotated toward each other. Therefore, the chiral
response of P3OT*38 and P3OT11-b-P3BT*17 and P3OT4-b-
P3BT*21 do not significantly differ. P3OT20-b-P3BT*21, in
contrast, is composed of a short P3BT* and a long P3OT, of
which the solubility is equal. They therefore aggregate simulta-
neously, and the block copolymer again shows a much higher CD
effect. The random copolymers P3BT*-co-3OT, however, show
in all cases a small chiral response than P3BT*18, which contrasts
to the other series of random copolymers.
In summary, two sets of materials can be distinguished: either
block copolymers of which the blocks aggregate separately or
random copolymers and block copolymers of which the blocks
aggregate simultaneously. In the former polymers, the block
aggregating first imposes its supramolecular structure to the
second and as such determines the stacking behavior of the
second as well. In the latter case, the chiral response can be much
higher than that of the chiral homopolymer, despite the lower
content of chiral moieties. Sergeant-and-soldiers behavior is in all
cases absent, in both block copolymer and random copolymers.
It might seem surprising that the highest chiral response is not
found in polymers with the highest content of chiral monomer.
In this respect, it is instructive to consider the reason why a
(bisignate) Cotton effect can be observed in chiral conjugated
polymers. If the polymer is substituted with achiral (linear) alkyl
groups, the polymer chains stack parallel, maximizing the
π-interactions. If asymmetrically branched (chiral) side chains
are employed, steric hindrance prevents a parallel stacking;
instead the chains are slightly rotated. Note that the eventual
organization is a compromise between π-interactions, which
favor a close, parallel stacking, and steric hindrance, which results
in a larger distance between and a rotation of the polymer chains.
In case of random copolymers or block copolymers of which the
blocks stack simultaneously, the packing of the chains cannot be
as efficient as in the case of a homopolymer. Hence, one might
assume that the distance between the chains becomes larger; i.e.,
the π-interactions are weakened. As a consequence, the branch-
ing of side chain can lead to a larger rotation of the stacked chains.
Since the twists remain moderate (i.e., <45°), this results in
stronger CD effects.11
’ EXPERIMENTAL SECTION
Reagents and Instrumentation. All reagents were purchased
from Sigma-Aldrich, Acros Organics, Merck, and Alfa Aesar. Reagent
grade solvents were dried by a solvent purification system MBRAUN SPS
800 (columns with activated alumina). Bromo-(o-tolyl)-bis(triphenyl-
phosphine)nickel (8),12 the 2-bromo-3-alkylthiophenes 5a,13 5b,14 5c,14
and 5e,14 the precursor monomers 6b3b and 6d,5h and the bithiophene
310 were prepared according to literature procedures. Before each
polymerization, a small aliquot of 2-bromo-5-magnesiochloro-3-
alkylthiophene was quenched with D2O and analyzed by 1H NMR to
verify the quantitative conversion. Gel permeation chromatography
(GPC) measurements were done with a Shimadzu 10A apparatus with
a tunable absorbance detector and a differential refractometer in
tetrahydrofuran as eluent toward polystyrene standards. Mass spectra
were recorded using an Agilent HP5989. 1H nuclear magnetic resonance
(1H NMR) measurements were carried out with a Bruker Avance 300,
400, and 600 MHz. UVꢀvis and CD measurements were performed on
a Perkin-Elmer Lambda 900 UVꢀvis NIR and a JASCO 62 DS
apparatus, respectively.
2-Bromo-3,40,400-trihexylterthiophene (4). A solution of 2,2,6,6-tet-
ramethylpiperidine (0.898 mmol, 0.127 g) in dry THF (7.50 mL) was
cooled to 0 °C and purged with argon. n-BuLi (2.50 M in hexane, 0.860
mmol, 0.340 mL) was added to the solution, and the reaction was
allowed to reach room temperature during 1 h. Subsequently, the
reaction was cooled to 0 °C, and 1b (0.525 mmol, 88.4 mg) in dry
THF (3 mL) was added to the reaction mixture. After 90 min of stirring
at 0 °C, Me3SnCl (0.600 mmol, 0.112 g) in dry THF (2.50 mL) was
added, and the reaction was allowed to reach room temperature over
90 min.
Then, the bithiophene 3 (1.00 mmol, 0.492 g) in dry THF (2.50 mL)
was added to the in situ prepared stannylated compound 2. In a next step,
the reaction mixture was added to Pd(PPh3)4, and the reaction was
stirred overnight at 60 °C. Then H2O was added, and the reaction
mixture was extracted with CH2Cl2. The organic layer was washed with a
saturated NaHCO3 solution, NH4Cl solution, and brine and subse-
quently dried with MgSO4 and concentrated under reduced pressure.
The crude product was purified with column chromatography (SiO2,
petroleum ether), and 5 mg of the crude product was subjected to
recycling GPC. Yield: 26.6 mg (9%). 1H NMR (CDCl3): δ = 6.95 (s,
1H), 6.91 (s, 1H), 6.90 (s, 1H), 6.84 (s, 1H), 2.71 (t, 2H), 2.60 (t, 2H),
2.53 (t, 2H), 1.53 (m, 6H), 1.33 (m, 18H), 0.90 (m, 9H). 13C NMR
(CDCl3): δ = 143.7, 142.9, 140.0, 136.7, 135.4, 134.1, 130.3, 127.2,
126.4, 124.2, 120.1, 107.5, 32.6, 30.4, 29.3, 28.9, 22.6, 14.1. MS (CI):
579/581 (MH+), 501 (MH+ꢀBr).
2-Bromo-5-iodo-3-butylthiophene (6a). 2-Bromo-3-butylthiophene
(5a) (43.0 mmol, 2.49 g) was dissolved in dichloromethane (250 mL),
shielded from light, and kept under argon atmosphere at 0 °C. Iodo-
benzene diacetate (18.0 mmol, 58.0 g) and iodine (16.6 mmol, 4.22 g)
were added to the reaction mixture, and the solution was stirred during 4 h
at room temperature. Afterward, the mixture was extracted with diethyl
ether and washed with a 10% Na2S2O3 solution. The organic layer was
dried with MgSO4, and the solution was concentrated. The crude product
was heatedunder reducedpressure during 3 h to removethe iodobenzene.
Subsequently, the product was dissolved in diethyl ether and activated
carbon was added. Afterward, the product was filtered, concentrated, and
purified byvacuumdistillation (85°C, 0.025mmHg), and apaleyellowoil
was obtained. Yield: 8.11 g (55%). 1H NMR (CDCl3): δ = 6.96 (s, 1H),
2.53 (t, 2H), 1.53 (m, 2H), 1.34 (m, 2H), 0.93 (t, 3H). 13C NMR
(CDCl3): δ = 144.2, 138.0, 111.7, 71.0, 31.8, 28.9, 22.2, 13.8. MS (EI):
343/346 (M•+).
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2-Bromo-5-iodo-3-octylhiophene (6c). The same procedure as
described for 6a was followed, using 2-bromo-3-octylthiophene (5c)
(24.5 mmol, 6.73 g), iodobenzene diacetate (14.7 mmol, 4.73 g), and
iodine (13.6 mmol, 3.44 g). The resulting pure product after vacuum
distillation (152 °C, 0.5 mmHg) was a pale yellow oil. Yield: 5.89 g
(60%). 1H NMR (CDCl3): δ = 6.96 (s, 1H), 2.52 (t, 2H), 1.54 (m, 2H),
1.29 (m, 10H), 0.88 (t, 3H). 13C NMR (CDCl3): δ = 144.3, 138.0,
111.7, 71.0, 31.9, 29.7, 29.3, 29.3, 29.2, 29.2, 29.1, 22.7, 14.1. MS (EI):
400/402 (M•+).
2-Bromo-5-iodo-3-(2-methylbutylthiophene) (6e). The same pro-
cedure as described for 6a was followed, using 2-bromo-3-(2-methyl-
butylthiophene) (5e) (66.4 mmol, 15.5 g), iodobenzene diacetate (35.0
mmol, 11.28 g), and iodine (34.0 mmol, 8.62 g). The resulting pure
product after vacuum distillation (68 °C, 2 mmHg) was a pale yellow oil.
Yield: 18.94 g (79.5%). 1H NMR (CDCl3): δ = 6.92 (s, 1H), 2.56ꢀ2.49
(dd, 1H), 2.36ꢀ2.29 (dd, 1H), 1.64ꢀ1.62 (m, 1H), 1.38ꢀ1.35 (m, 1H),
1.20ꢀ1.15 (m, 1H), 0.93ꢀ0.85 (m, 6H). 13C NMR (CDCl3): δ = 145.5,
140.7, 114.6, 73.2, 38.4, 37.9, 31.4, 21.2, 13.7. MS (EI): 360/358 (MH+).
General Procedure for the Preparation of the Monomers
7aꢀ7e. The precursor monomers 6aꢀ6e (10.0 mmol) were dissolved
in dry THF (66.7 mL) and purged with argon. i-PrMgCl.LiCl (1.22 M in
THF; 10.0 mmol, 8.21 mL) was added to the solution, and the reaction
was stirred during 15 min at 40 °C and another 45 min at room
temperature. The conversion of the GRIM reaction was evaluated by
pouring an aliquot of the reaction mixture in D2O and analyzing it with
1H NMR spectroscopy.
General Procedure for the Polymerization of the Homo-
polymers P3HT13,18,20,24,27,30 and P3OT12,18,22,29. The mono-
mers 7b and 7c were added to a suspension of Ni(dppp)Cl2 in dry THF
(6 mL) under an argon atmosphere. After 1 h, the reaction mixture was
quenched with a 2 M HCl solution, precipitated in methanol, filtered,
and dried under vacuum. The final polymer was a dark red-brown solid.
For the synthesis of all the P3HTx and P3OTx polymers, the general
procedure was performed.
Synthesis of P3HT13. The used reagents were 7b (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (66.7 μmol, 36.3 mg). Yield:
142 mg (86%).
Synthesis of P3HT18. The used reagents were 7b (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (50.0 μmol, 27.1 mg). Yield:
147 mg (89%).
dry in THF (6.60 mL) and stirred for 30 min at room temperature and
under an argon atmosphere. In the next step, the monomer 7a, 7c, 7d, or 7e
was added to the solution, and the mixture was allowed to react overnight.
Afterward, the polymerization was quenched with a 2 M HCl solution and
precipitated in methanol. Next, the polymer was filtered off and fractio-
nated by Soxhlet extraction with methanol, acetone, and chloroform. The
chloroform-soluble fraction was precipitated in methanol, filtered, and
dried in vacuo. The final polymer was a dark red-brown solid. For the
synthesis of all the homopolymers, the general procedure was performed.
Synthesis of P3BT41. The used monomer was 7a (0.11 mmol/mL in
THF, 1.00 mmol, 9.13 mL). Yield: 132 mg (65%).
Synthesis of P3OT36. The used monomer was 7c (0.11 mmol/mL in
THF, 1.00 mmol, 9.13 mL). Yield: 149 mg (76%).
Synthesis of P3OT*38. The used monomer was 7d (0.11 mmol/mL
in THF, 1.00 mmol, 9.13 mL). Yield: 148 mg (66%).
Synthesis of P3BT*18. The used monomer was 7e (0.11 mmol/mL in
THF, 1.00 mmol, 9.13 mL). Yield: 27.3 mg (20%).
General Procedure for the Polymerization of the Block
Copolymers P3OTn-b-P3OT*m, P3BTm-b-P3OT*n, and P3OTn-
b-P3BT*m. First, the initiator 8 (25.0 μmol, 18.9 mg) and dppp (50.0
μmol, 20.6 mg) were dissolved dry in THF (6.60 mL) and stirred for
30 min at room temperature and under an argon atmosphere. In a next
step, the monomer7cor7d wasaddedtothesolution, and the mixturewas
allowed to react for 1 h at room temperature. Subsequently, the second
monomer, 7d, 7a, or 7e, was added, and the reaction mixture was stirred
for 1.5 h at 30 °C and overnight at room temperature. Afterward, the
polymerization was quenched with a 2 M HCl solution and precipitated in
methanol. Next, the polymer was filtered and fractionated by Soxhlet
extraction with methanol, acetone, and chloroform. The chloroform-
soluble fraction was precipitated in methanol, filtered, and dried in vacuo.
The final polymer was a dark red-brown solid. For the synthesis of all the
block copolymers, the general procedure was performed.
Synthesis of P3OT31-b-P3OT*9. The used monomer in the first step
was 7c (0.11 mmol/mL in THF, 0.800 mmol, 7.31 mL) and in the
second step was 7d (0.11 mmol/mL in THF, 0.200 mmol, 1.83 mL).
Yield: 167 mg (83%).
Synthesis of P3OT19-b-P3OT*19. The used monomer in the first
step was 7c (0.11 mmol/mL in THF, 0.500 mmol, 4.56 mL) and in the
second step was 7d (0.11 mmol/mL in THF, 0.500 mmol, 4.56 mL).
Yield: 134 mg (64%).
Synthesis of P3HT20. The used reagents were 7b (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (40.0 μmol, 21.7 mg). Yield:
151 mg (91%).
Synthesis of P3HT24. The used reagents were 7b (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (33.3 μmol, 18.1 mg). Yield:
144 mg (93%).
Synthesis of P3HT27. The used reagents were 7b (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (28.6 μmol, 15.5 mg). Yield:
157 mg (94%).
Synthesis of P3OT12. The used reagents were 7c (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (66.7 μmol, 36.3 mg). Yield:
166 mg (86%).
Synthesis of P3OT8-b-P3OT*30. The used monomer in the first step
was 7c (0.11 mmol/mL in THF, 0.200 mmol, 1.83 mL) and in the
second step was 7d (0.11 mmol/mL in THF, 0.800 mmol, 7.31 mL).
Yield: 152 mg (70%).
Synthesis of P3BT32-b-P3OT*9. The used monomer in the first step
was 7d (0.11 mmol/mL in THF, 0.750 mmol, 6.85 mL) and in the
second step was 7a (0.11 mmol/mL in THF, 0.250 mmol, 2.28 mL).
Yield: 85 mg (61%).
Synthesis of P3BT11-b-P3OT*24. The used monomer in the first
step was 7d (0.11 mmol/mL in THF, 0.500 mmol, 4.56 mL) and in the
second step was 7a (0.11 mmol/mL in THF, 0.500 mmol, 4.56 mL).
Yield: 99 mg (63%).
Synthesis of P3OT18. The used reagents were 7c (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (50.0 μmol, 27.1 mg). Yield:
177 mg (91%).
Synthesis of P3OT22. The used reagents were 7c (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (33.3 μmol, 18.1 mg). Yield:
188 mg (97%).
Synthesis of P3OT29. The used reagents were 7c (0.13 mmol/mL in
THF, 1 mmol, 7.50 mL) and Ni(dppp)Cl2 (28.6 μmol, 15.5 mg). Yield:
162 mg (94%).
General Procedure for the Polymerization of the Homo-
polymers P3BT41, P3OT36, P3OT*38, and P3BT18*. The initiator
8 (25.0 μmol, 18.9 mg) and dppp (50.0 μmol, 20.6 mg) were dissolved
Synthesis of P3BT10-b-P3OT*30. The used monomer in the first
step was 7d (0.11 mmol/mL in THF, 0.250 mmol, 2.28 mL) and in the
second step was 7a (0.11 mmol/mL in THF, 0.750 mmol, 6.85 mL).
Yield: 145 mg (74%).
Synthesis of P3OT20-b-P3BT*11. The used monomer in the first
step was 7c (0.11 mmol/mL in THF, 0.750 mmol, 6.85 mL) and in the
second step was 7e (0.11 mmol/mL in THF, 0.250 mmol, 2.28 mL).
Yield: 47 mg (29%).
Synthesis of P3OT11-b-P3BT*17. The used monomer in the first
step was 7c (0.11 mmol/mL in THF, 0.500 mmol, 4.56 mL) and in the
second step was 7e (0.11 mmol/mL in THF, 0.500 mmol, 4.56 mL).
Yield: 55 mg (35%).
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Synthesis of P3OT4-b-P3BT*21. The used monomer in the first step
was 7c (0.11 mmol/mL in THF, 0.250 mmol, 2.28 mL) and in the
second step was 7e (0.11 mmol/mL in THF, 0.750 mmol, 6.85 mL).
Yield: 95 mg (62%).
the irregular molecular structure of the copolymers, which
complicates a closely stacked structure, resulting in a larger twist
than that of the (regular) chiral homopolymer. Sergeant-and-
soldiers behavior is in all these materials absent.
General Procedure for the Polymerization of the Random
Copolymers P3OTk-co-3OT*l, P3BTl-co-3OT*k, and P3OTk-
co-3BT*l. The initiator 8 (25.0 μmol, 18.9 mg) and dppp (50.0 μmol,
20.6 mg) were dissolved dry in THF (6.60 mL) and stirred for 30 min at
room temperature and under an argon atmosphere. Next, the two
monomers 7c and 7d, 7a and 7d, or 7c and 7e were loaded together to
the initiator and were allowed to react overnight. Afterward, the
polymerization was quenched with a 2 M HCl solution and precipitated
in methanol. Next, the polymer was filtrated and fractionated by Soxhlet
extraction with methanol, acetone, and chloroform. The chloroform-
soluble fraction was precipitated in methanol, filtered off, and dried in
vacuo. The final polymer was a dark red-brown solid. For the synthesis of
all the random copolymers, the general procedure was performed.
Synthesis of P3OT31-co-3OT*10. The used monomers were 7c
(0.11 mmol/mL in THF, 0.750 mmol, 6.85 mL) and 7d (0.11 mmol/
mL in THF, 0.250 mmol, 2.28 mL). Yield: 146 mg (72%).
’ ASSOCIATED CONTENT
S
Supporting Information. Probability of AB-block-co-
b
polymers in function of the length of block A and block B; the
UVꢀvis and CD spectra of the block and random copolymers;
the 1H NMR and 13C NMR characterizations of all new
1
compounds; and all the H NMR spectra of all the homopoly-
mers and copolymers. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: guy.koeckelberghs@chem.kuleuven.be.
Synthesis of P3OT20-co-3OT*18. The used monomers were 7c
(0.11 mmol/mL in THF, 0.500 mmol, 4.56 mL) and 7d (0.11 mmol/
mL in THF, 0.500 mmol, 4.56 mL). Yield: 142 mg (68%).
Synthesis of P3OT10-co-3OT*25. The used monomers were 7c
(0.11 mmol/mL in THF, 0.250 mmol, 2.28 mL) and 7d (0.11 mmol/
mL in THF, 0.750 mmol, 0.750 mL). Yield: 163 mg (76%).
Synthesis of P3BT29-co-3OT*11. The used monomers were 7a (0.11
mmol/mL in THF, 0.750 mmol, 6.85 mL) and 7d (0.11 mmol/mL in
THF, 0.250 mmol, 2.28 mL). Yield: 94 mg (58%).
’ ACKNOWLEDGMENT
We are grateful to the Onderzoeksfonds K.U.Leuven/Re-
search Fund K.U.Leuven and the Fund for Scientific Research
(FWO-Vlaanderen) for financial support. M.V. is a doctoral
fellow of the Fund for Scientific Research (FWO-Vlaanderen).
We are also grateful to Karel Duerinckx for the help and technical
support with NMR spectroscopy.
Synthesis of P3BT21-co-3OT*17. The used monomers were 7a (0.11
mmol/mL in THF, 0.500 mmol, 4.56 mL) and 7d (0.11 mmol/mL in
THF, 0.500 mmol, 4.56 mL). Yield: 112 mg (63%).
Synthesis of P3BT11-co-3OT*30. The used monomers were 7a (0.11
mmol/mL in THF, 0.250 mmol, 2.28 mL) and 7d (0.11 mmol/mL in
THF, 0.750 mmol, 6.85 mL). Yield: 137 mg (68%).
Synthesis of P3OT27-co-3BT*9. The used monomers were 7c (0.11
mmol/mL in THF, 0.750 mmol, 6.85 mL) and 7e (0.11 mmol/mL in
THF, 0.250 mmol, 2.28 mL). Yield: 49 mg (30%).
Synthesis of P3OT17-co-3BT*17. The used monomers were 7c (0.11
mmol/mL in THF, 0.500 mmol, 4.56 mL) and 7e (0.11 mmol/mL in
THF, 0.500 mmol, 4.56 mL). Yield: 11 mg (31%).
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THF, 0.750 mmol, 6.85 mL). Yield: 11 mg (7%).
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’ CONCLUSIONS
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chain if the polymerization is initiated by Ni(dppp)Cl2. Conse-
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addition, both AB- and BAB-type polymers are formed. The
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functional Ni initiator which allows only growth on one side.
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shown that if the two blocks aggregate at different nonsolvent
content, the block aggregating first determines the stacking
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