End group-functionalization and synthesis of block-copolythiophenes by modified nickel initiators

Article abstract of DOI:10.1021/ma200846v

Polythiophenes with alcohol, tosylate, azide, ethynylene, carboxylic acid, and amine end groups were prepared by a combination of functionalized nickel initiators and postpolymerization reactions. The azide and ethynylene polymers were subsequently used in a click reaction to produce a conjugated block copolymer. Finally, a conjugated triblock-copolymer was synthesized by means of a chain growth polymerization initiated by a binuclear nickel initiator.

Full text of DOI:10.1021/ma200846v

ARTICLE
pubs.acs.org/Macromolecules
End Group-Functionalization and Synthesis of
Block-Copolythiophenes by Modified Nickel Initiators
Alfons Smeets,† Pieter Willot,† Julien De Winter,‡ Pascal Gerbaux,‡ Thierry Verbiest,† and
Guy Koeckelberghs†,*
†Laboratory of Molecular Electronics and Photonics, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium
‡Mass Spectrometry Research Group, Interdisciplinary Center for Mass Spectrometry, University of Mons-UMONS, 20 Place du Parc,
B-7000 Mons, Belgium
S Supporting Information
b
ABSTRACT: Polythiophenes with alcohol, tosylate, azide,
ethynylene, carboxylic acid, and amine end groups were pre-
pared by a combination of functionalized nickel initiators and
postpolymerization reactions. The azide and ethynylene poly-
mers were subsequently used in a click reaction to produce a
conjugated block copolymer. Finally, a conjugated triblock-
copolymer was synthesized by means of a chain growth
polymerization initiated by a binuclear nickel initiator.
’ INTRODUCTION
moiety performs a nucleophilic attack on Ni(dppp)Cl2, forming
the chloro organonickel(dppp) initiator, which then reacts with
more monomer, starting the propagation cycle. As a consequence,
a functional group can also conveniently be introduced in the
beginning of each polymer chain by initiating the polymerization
from such organonickel species equipped with the desired func-
tional group. Several methodologies have been reported for the
synthesis of such molecules and their use as initiators for the
P3HT polymerization. Luscombe and co-workers prepared arylꢀ
Ni complexes from (in situ prepared) Ni(PPh3)4 or Ni(cod)2
and the corresponding aryl bromides or chlorides.24ꢀ26 Kiriy re-
acted aryl bromides with either Ni(PPh3)4 or diethyl(bipyridyl)-
nickel, followed by a ligand exchange with dppp.27,28 A Ni-initiator
capable of initiating a NMP was also prepared through this
methodology.29 We prepared Ni-initiators bearing a (protected)
alcohol or acetylenic group by reacting Ni(PPh3)4 with the ap-
propriate aryl bromide and used them to polymerize P3HT,
P3OATs, poly(3-alkylsulfanylthiophene)s and poly(phenylene)s.30
In this manuscript, we first expand the latter protocol by syn-
thesizing a broad variety of functional Ni initiators, offering the
possibility to prepare conjugated polymers equipped with alco-
hol, tosylate, azide, amine, ethynyl and carboxylic acid functional
groups. Next, the click reaction was used to couple two different
conjugated polymers (in casu P3AT and P3AOT) resulting in an
all-conjugated block copolymer with a nonconjugated spacer.
Finally, we prepared a bifunctional Ni-initiator and used it to
initiate a P3HT polymerization. Moreover, the living nature of
Conjugated polymers (CPs) remain intensively studied ma-
terials because they hold promise as active layers in applications
in the field of molecular electronics.1 A major break-through was
achieved by the groups of McCullough2,3 and Yokozawa,4,5 who
demonstrated that the Ni(dppp)Cl2-initiated (dppp =1,3-bisdi-
phenylphosphinopropane) polymerization of poly(3-hexylthio-
phene) (P3HT) proceeds via a controlled chain-growth mechan-
ism. This allows the synthesis of well-defined polythiophenes
with predicted molar mass and narrow polydispersities. More-
over, the controlled nature of the polymerization can also be
exploited to end-cap the polymer chains, introducing functional
groups, which can, after some transformation, act as initiators for
a controlled radical polymerization of a second block. In this way,
block copolymers composed of a P3HT block and a nonconju-
gated block (e.g., PMMA, PS, ...) can be prepared.6ꢀ8 Alterna-
tively, the living nature can be used to prepare all-conjugated
block copolymers by successive addition of the respective mono-
mers. In this way, the synthesis of block copolymers composed of
two different P3AT blocks,9ꢀ14 a P3AT and a poly(3-alkoxythio-
phene) (P3AOT) block,15,16 a poly(selenophene) block,17 or a
poly(fluorene) block18 and a poly(pyrrole)-block-poly(phenylene)19
have been achieved.
End-capping of polymer chains requires a living nature of the
polymerization. This is indeed the case for P3ATs and it has also
been found for some other conjugated polymers (to a certain
extent) using either Ni- or Pd-catalysts.20ꢀ23 Nevertheless, some
other CPs polymerize via an albeit chain-growth, but not-living,
mechanism. However, if the mechanism of the Ni(dppp)Cl2-
initiated polymerization is examined into more detail, it becomes
clear that a chloro organonickel(dppp) species can be considered
as the actual initiator. More specifically, an arylmagnesium
Received: April 12, 2011
Revised:
July 4, 2011
Published: July 13, 2011
r
2011 American Chemical Society
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this polymerization could be exploited for the synthesis of a
P3AOT-block-P3HT-block-P3AOT triblock copolymer.
supernatant was concentrated in vacuo and the product precipitated
in MeOH, yielding a yellow-brown powder.
Yield: 0.125 g (4%).
1H NMR (CDCl3): δ = 7.61 (m, 24 H), 7.25 (m, 36 H), 5.62 (s, 2 H),
1.47 (s, 6 H).
’ EXPERIMENTAL SECTION
Synthesis of Block Copolymer Using the Click Reaction.
Synthesis of P3HT-2-P3AOT. An argon purged solution of PMDTA
(5.60 μmol, 0.97 mg) in dry THF (5 mL) was added to a suspension of
CuBr (4.51 μmol, 0.650 mg) and dry THF (5 mL). A solution of P3HT-
2-N3 (68.7 μmol, 11.4 mg) and P3AOT-E (126 μmol, 30.0 mg) in dry
THF (50 mL) was added to this solution and the reaction mixture was
stirred overnight under an argon atmosphere. Then, chloroform and an
aqueous solution of NH3 were added and the mixture was extracted. The
organic layer was dried over Na2SO4 and concentrated to a volume of
approximately 15 mL, precipitated in methanol and filtered off. The
polymer was further purified by means of Soxhlet extraction with
pentane, hexane, and heptane and finally with chloroform. The chloro-
form solution was precipitated in MeOH, filtered off, and dried in vacuo.
Yield = 18.9 mg (48%).
Synthesis of Triblock Copolymer. An argon-purged mixture of
18 (0.0486 mmol, 0.0740 g) and dppp (0.107 mmol, 0.0440 g) in dry
THF (6.5 mL) was stirred for 30 min at room temperature. In the next
step, 2-magnesiochloro-4-hexyl-5-bromothiophene (0.800 mmol) in
dry THF (5.5 mL) was added to the initiator solution and stirred for
1 h at room temperature. Next, 3.8 mL of the mixture was quenched with
HCl, precipitated in MeOH, filtered off, and dried in vacuo. Washing
with acetone yielded P3HT-bth-P3HT as a dark purple solid. To the
remaining solution (8.2 mL), 2-bromomagnesio-5-bromo-3-(3,7-(S)-
dimethyloctyloxy)thiophene (1.65 mmol) (prepared as for P3AOT-
TMS) in dry THF (12 mL) was added and the mixture was allowed to
react overnight. The polymer was then precipitated in MeOH, further
purified by means of Soxhlet extraction with pentane, hexane, heptane
and finally with THF. The THF solution was precipitated in MeOH,
filtered off and dried in vacuo, yielding P3AOT-P3HT-P3AOT as a
black solid.
Reagents and Instrumentation. All reagents were purchased
from Sigma-Aldrich, Acros Organics, Merck, Wako, ABCR, and Alfa
Aesar. Reagent grade solvents were dried by distillation over a suitable
drying agent. Bromo-[2-methyl-4-(trimethylsilylethynylene)phenyl]bis-
(triphenylphosphine)nickel30 (1) and 2-magnesiochloro-4-hexyl-5-bro-
mothiophene4 was prepared according to literature procedures. Before
each polymerization, a small aliquot of 2-magnesiochloro-4-hexyl-5-
bromothiophene was quenched with D2O and analyzed by 1H NMR to
verify its conversion. Gel Permeation chromatography (GPC) measure-
ments 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 MHz, 400 MHz
and 600 MHz. 31P NMR measurements were carried out with a Bruker
Avance 400 MHz. MALDIꢀToF mass spectra were recorded using a
Waters QToF Premier mass spectrometer equipped with a nitrogen
laser, operating at 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
10000. The matrix, trans-2-[3-(4-tert-butyl-phenyl)-2-methyl-2-prope-
nylidene]malononitrile (DCTB), was prepared as 20 mg/mL solution in
chloroform. 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 matrix crystals, and then
air-dried. IR spectra were recorded using a Bruker Alpha-p apparatus in
ATR mode.
Synthesis of the Bifunctional Initiator. Synthesis of 2-Bromo-
3-methylthiophene (16). N-Bromosuccinimide (50.0 mmol, 8.88 g)
was added portion wise to a solution of 3-methylthiophene (15)
(50.0 mmol, 4.91 g) in CHCl3 (20 mL) and AcOH (20 mL), which
had been cooled to 0 °C, shielded from light and was equipped with a
CaCl2 drying tube. The reaction was then allowed to reach room
temperature overnight, after which aqueous NaHSO3 and NaHCO3
were added. The mixture was then extracted with hexane, the organic
layer was dried over MgSO4 and the solvent removed in vacuo. The
crude product was then vacuum distilled (66 °C, 16 mmHg), and the
pure product was isolated as a light yellow oil.
Yield (P3HT-bth-P3HT) = 26.5 mg (64%).
Yield (P3AOT-P3HT-P3AOT) = 185 mg (32% based on P3AOT).
’ RESULTS AND DISCUSSION
Initiator Design. In a previous communication,30 we estab-
lished a protocol for the synthesis of arylnickel complexes and
their subsequent application as functional initiators for the
synthesis of conjugated polymers. This allowed us to insert
end groups into CPs even if their polymerization is not con-
trolled. We decided to expand upon this protocol by inserting a
wider variety of functional groups, which could then be employed
in postpolymerization reactions. The end groups in question
should allow straightforward coupling of the CP to the desirable
substrates, i.e., other polymers or nanoparticles. Two candidates
which immediately come to mind are azide and ethynylene
functionalities, as they can be employed in so-called CuAAC
click chemistry, which is robust and can be achieved using mild
conditions,31,32 fulfilling the requirement stated earlier. Two
other functional groups we deemed worth pursuing are amine
and carboxylic functionalities, because they could be useful for
coupling CPs with nanoparticles33 or surfaces.34 As for the choice
of the CPs, we decided to use poly(3-hexylthiophene) (P3HT)
as main CP, because the polymerization of P3HT with function-
alized nickel initiators yields polymers with a high degree of
functionalization.30 As second polymer we decided to use poly-
(3-(3,7-(S)-dimethyloctyloxy)thiophene (P3AOT). Although
the polymerization of P3AOT is less well-behaved and its
Yield =7.40 g (83%).
1H NMR (CDCl3): 7.16 (d, 1 H, 5.5 Hz), 6.77 (d, 1 H, 5.5 Hz), 2.20
(s, 3 H).
Synthesis of 5,50-Dibromo-4,40-dimethyl-2,20-bithiophene (17). A
solution of 16 (20.0 mmol, 3.54 g) in dry DMSO (40 mL) was purged
with argon and added to a solution of PdCl2(PhCN)2 (0.600 mmol,
0.230 g), AgNO3 (40.0 mmol, 6.79 g) and KF (40.0 mmol, 2.32 g) in dry
DMSO (40 mL). The mixture was allowed to react for 3 h at 60 °C, after
which hexane was added. The resulting precipitate was then removed by
filtration and subsequently washed with hexane and diethyl ether. The
organic layer was then washed with a NaCl solution, dried over MgSO4
and the solvent was removed in vacuo. Finally, the crude product was
purified by column chromatography (SiO2, eluent: hexane) and isolated
as a white solid.
Yield: 2.02 g (57%).
1H NMR (CDCl3): δ = 6.76 (s, 2 H), 2.17 (s, 6 H).
Synthesis of 18. A solution of 17 (2.00 mmol, 0.704 g) in dry toluene
(50 mL) was added under argon atmosphere to Ni(PPh3)4 (5.00 mmol,
5.54 g). The reaction was allowed to proceed overnight, after which
toluene (50 mL) was added and the solution was filtered. The
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Scheme 1. Synthesis of the Ni Initiatorsa
organomagnesium or organozinc compounds) are available. The
azide functionality, however, has to be formed through post-
polymerization reactions of a precursor end group, as no direct
method of protection is available. As precursor end group, we
selected an alcohol functionality, because of its easy protection,
deprotection and synthetic versatility.
The synthesis of the protected acetylene-functionalized
initiator, bromo-[2-methyl-4-[(trimethylsilyl)ethynyl]phenyl]bis-
(triphenylphosphine)nickel(II) (1) has been described previously.30
For the (protected) alcohol-functionalized initiator (5), 4-bromo-3-
methylphenol (2) was alkylated with ethylene carbonate in DMF
using K2CO3 as base, forming 3. Subsequently, 3was protected using
TBDMSCl and imidazole in DMF, yielding 1-bromo-2-methyl-
4-[2-(tert-butyldimethylsilyloxy)ethyloxy]benzene (4). Reaction of
4 with Ni(PPh3)4 resulted in the formation of bromo[2-methyl-
4-[2-(tert-butyldimethylsilyloxy)ethyloxy]phenyl]bis(triphenylphos-
phine)nickel(II) (5). In order to introduce carboxyl groups in the
polymers, we first selected an oxazoline moiety as a protecting
group compatible with the organomagnesium monomer. The
corresponding nickel initiator could be prepared by converting
4-bromo-3-methylbenzoic acid (6) to its acid chloride (7) by
means of oxalyl chloride, which was reacted with 2-amino-2-
methyl-1-propanol and SOCl2, yielding 2-(4-bromo-3-methyl-
phenyl)-4,5-dihydro-4,4-dimethyloxazole (8). Analogous to the
synthesis of previous initiators, 8 was then reacted with Ni-
(PPh3)4, forming the nickel initiator 9. However, after polymer-
ization, it turned out that the carboxylic acid could not be de-
protected without degradation of the polymer. Indeed, treatment
with KOH in THF did not result in any reaction, while treatment
with aqueous HCl (either in dioxane or chloroform) or MeI and
subsequently NaOH resulted in degradation reactions. Because
of our inability to convert the oxazoline end group back to the
carboxylic acid, we were forced to consider a different protecting
group. We decided to protect the carboxylic acid by esterificating
it with tert-butanol.
In order to synthesize the COOtBu functionalized initiator, 7
was reacted with KOtBu, yielding tert-butyl(4-bromo-3-methyl)-
benzoate (10). Reaction of 10 with Ni(PPh3)4 yielded the COOtBu-
functionalized initiator (11).
a Key: (i) ethylene carbonate, K2CO3; (ii) TBDMSCl, imidazole;
(iii) Ni(PPh3)4; (iv) oxalyl chloride, DMF; (v) 2-amino-2-methyl-1-
propanol, SOCl2; (vi) KOtBu; (vii) (1) DIAD, phthalimide, (2) PPh3,
N2H2; (viii) Boc2O, DMAP.
The introduction of an amine group into the polymers
required the synthesis of a nickel initiator with an amine func-
tionality, which also needs to be protected against the organo-
magnesium monomer. At first we decided to employ STABASE
protection (STABASE = 1,1,4,4-tetramethyldisilyl azacyclopentane)
for the amine. However, we were unable to isolate the necessary
STABASE-protected precursor for the nickel initiator as it spon-
taneously decomposed back to the free amine during purifica-
tion. We therefore had to select an alternative and switched to
Boc protection instead. First, we synthesized 2-(4-bromo-3-
methylphenyloxy)ethylamine (12) by performing a Mitsunobu
reaction of 3 with phthalimide, subsequent hydrazinolysis of the
phthalimide intermediate yielded the primary amine 12 after
work-up. The amine 12 was then protected using Boc2O after
functionalization usually incomplete,30,35 it is still a worthwhile
choice because of its substantially different physicochemical
properties compared to P3HT, which should allow for easy dis-
crimination of both CPs using standard spectroscopic techni-
ques. For the reasons mentioned above, we decided to synthesize
P3HT with azide, amine, and carboxylic end groups, while a
P3AOT would be synthesized with an ethynylene end group.
In order to synthesize both CPs with the desired end groups,
the corresponding nickel initiators (Scheme 1) were synthesized
from Ni(PPh3)4 and the appropriate aryl bromide. Although
all four functional end groups should not pose any problems
in this respect, they are unfortunately incompatible with the
polymerization conditions of both P3HT and P3AOT, because
both polymerizations proceed through the nickel-catalyzed
coupling of an organomagnesium (or organozinc) monomer.
The end groups in question thus had to be either protected or
formed after polymerization from a precursor moiety. To ethy-
nylene, amine and carboxylic functional groups, the former
applies, as well established methods for protection (against either
1
which H NMR and MS analysis indicated that the amine had
been bis-protected to form 13. Finally, reaction of 13 with
Ni(PPh3)4 yielded the corresponding nickel initiator 14.
Synthesis of the Polymers. The polymers were prepared by
a Ni(dppp)-mediated polymerization of metalated thiophenes,
as depicted in Scheme 2. In the case of P3AOT, the actual
monomer, 2-bromomagnesio-5-bromo-3-(3,7-(S)-dimethyloc-
tyloxy)thiophene, was prepared in situ from MgBr2 and 2-lithio-
5-bromo-3-(3,7-(S)-dimethyloctyloxy)thiophene, which was in
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Scheme 2. Synthesis of the Polymers
Figure 1. 31P NMR spectrum of 5 (6.70 μmol) after 30 min of ligand
exchange with dppp (13.3 μmol) in THF (0.5 mL).
revealed that the main end groups are Br/TMS, with some
minor H/Br content. This is the expected result, as the polym-
erization of poly(3-alkoxythiophenes) is not controlled,35 yield-
ing polymer chains in which chain end degradation of the
polymer-Ni complex occurred. Signals corresponding to the loss
of a C10H21• radical are also detected in the mass spectrum and
were previously ascribed to an aromatic β-cleavage from the
ionized polymer chain.38
In order to prepare P3HT with an alcohol functionality, 5 was
first reacted with dppp, after which a solution of 2-magnesio-
chloro-4-hexyl-5-bromothiophene was added, resulting in the
formation of P3HT-2-OTBDMS. The presence of the ethyl-
1
OTBDMS moiety was confirmed by H NMR, with the ethyl
protons giving rise to two triplets, respectively at 4.19 and 3.61 ppm.
Also, MALDIꢀToF analysis indicates that the majority of the
P3HT is OTBDMS/H terminated with some remainder H/H
terminated P3HT (see Figure S38, Supporting Information).
H/H-terminated polymers require Ni(dppp)X2 (X = Cl or Br) as
initiator. Since the 31P NMR spectra indicate the absence of such
species at the moment the monomer is introduced, such Ni-
complex must be formed when the monomer is added. The pre-
sence of nonfunctionalized P3HT is not necessarily detrimental,
as it will simply not react during postpolymerization reactions
(coupling reactions, block copolymer formation, ...) and can
thereafter be removed by fractionation.
For the polymerization of the carboxyl-functionalized P3HT,
2-magnesiochloro-4-hexyl-5-bromothiophene was first transme-
talated with ZnBr2 to 2-bromozincio-4-hexyl-5-bromothiophene.
Then, 11 was allowed to undergo ligand exchange with dppp
and used to initiate the polymerization of the 2-bromozincio-
4-hexyl-5-bromothiophene, resulting in the formation of P3HT-
COOtBu. The MALDIꢀToF analysis (see Figure S39, Support-
ing Information) confirmed that the polymerization reaction is
quantitatively initiated by 11. Decomposition reactions, i.e. losses
of isobutene (56 u) from the ionized oligomers, were also de-
tected and are characteristic of the COOtBu moiety. Surprisingly,
the dominant ion series did not correspond to the tBu/H end
groups, but to tBu/C4H7O end groups. This composition was
unambiguously demonstrated by an accurate mass measurement
performed on ionized C4H7Oꢀ(3HT)15ꢀCOOtBu (measured
mass, 2753.3833; u/calculated mass, 2753.3812 u/0.8 ppm error).
This suggests that a reaction with THF occurs. McNeil and co-
workers also observed such reaction in the PPP polymerization.39
In order to find out whether this reaction occurs at the stage of
the monomer formation or during the actual polymerization, a
turn prepared from 2-bromo-3-(3,7-(S)-dimethyloctyloxy)thio-
phene and LDA.35 The monomer in the P3HT polymerization is
2-magnesiochloro-4-hexyl-5-bromothiophene which was prepared
from 2-bromo-5-iodo-3-hexylthiophene and iPrMgCl. The
COOtBu and Boc2N moieties are not compatible with organomag-
nesium reagents and thus polymerization conditions which use
organomagnesium monomers cannot be employed. Fortunately,
the synthesis of P3HT can also be accomplished by an organo-
zinc monomer3,36 using the same catalyst and conditions, only
transmetalation of the organomagnesium monomer with ZnBr2
is required. Because the Ni-catalyzed polymerization of P3HT is
controlled when dppp is used as ligand,25,37 ligand exchange was
first performed in situ before the monomers were added, thus
forming the actual initiator. In order to verify whether the ligand
exchange indeed takes place (quantitatively), we monitored this
reaction in a separate experiment by 31P NMR spectroscopy for
all initiators. After 30 min, the sole species detected in all cases
except for 1 were liberated PPh3, excess dppp (2 equiv were
used) and the end-product (Figure 1). The apparent broadness
of the peaks can be attributed to the fact that it is actually a (less
resolved) doublet. No starting product, nor degradation com-
pounds (e.g., Ni(dppp)2) were found (see Supporting Informa-
tion, Figures S23ꢀS26). For all P3HTs, [M0]/[In] = 15.
Thus, the synthesis of the ethynylene-functionalized P3AOT
was accomplished by first performing ligand exchange on 1 with
dppp, after which this mixture was used to initiate the polymer-
ization of 2-bromomagnesio-5-bromo-3-(3,7-(S)-dimethylocty-
loxy)thiophene, yielding P3AOT-TMS. MALDIꢀToF analysis
(see Figure S37, Supporting Information) of P3AOT-TMS
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Table 1. Mn and PDI for the Homopolymers Determined by
GPC Calibrated toward Polystyrene Standards in THF
Scheme 3. Post-Polymerization Deprotection of P3HT-2-
OTBDMS and conversion to P3HT-2-N3
polymer
Mn (kg/mol)
PDI
P3HT-2-OTBDMS
P3AOT-TMS
1.9
8.1
7.1
7.3
1.3
1.6
1.2
1.2
P3HT-COOtBu
P3HT-NBoc2
1H NMR spectrum was recorded of the zincated compound,
quenched with D2O (see Supporting Information, Figure S27).
No signs of reaction with THF were observed, demonstrating
that this reaction takes place during the polymerization, termi-
nating the polymerization. The 1H NMR spectrum of the poly-
mer, however, does not contain signals matching the THF end
group, while those corresponding to the initiator moiety are
clearly present.
Finally, the amine-functionalized P3HT was also synthesized
using2-bromozincio-4-hexyl-5-bromothiophene, whichwas added
to a solution of 14, after the latter underwent ligand exchange
1
with dppp, forming P3HT-NBoc2. The H NMR spectrum of
P3HT-NBoc2, contains two methyl signals from the o-methyl
moiety, indicating partial deprotection during polymerization or
work-up. This could also be confirmed by recording the MALDI
mass spectrum of the P3HT-NBoc2 sample (see Figure S40,
Supporting Information). Indeed, this spectrum features signals
corresponding to both the bis-protected polymer chains and
the monoprotected molecules, respectively P3HT-NBoc2 and
P3HT-NHBoc. Nevertheless, since isobutene losses are de-
tected in the MALDI spectrum, we cannot rule out that the ions
corresponding to P3HT-NHBoc can also be generated by de-
compositions upon MALDI analysis of P3HT-NBoc2 molecules
into P3HT-NHBoc molecules. In addition, as already observed
when analyzing the P3HT-COOtBu sample, polymer ions
bearing a C4H7O residue as end group dominate the MALDI
mass spectrum.
The molar masses and polydispersities (Table 1) were deter-
mined by GPC calibrated toward polystyrene standards. The DP
of the P3HTs was also determined by 1H NMR integration. For
P3HT-2-OTBDMS, it was found to be equal to 10, which
correlates well with the GPC result. For P3HT-COOtBu (26)
and P3HT-NBoc2 (27) significantly higher than expected molar
masses were found, which suggests an inefficient initiation by the
respective Ni-initiators. The polydispersity, however, remains
low. GPC, on the other hand, gives higher values. It should be
noted that GPC tends to overestimate the molecular weight of
P3HTs and that this error increases with higher molar mass.40
This effect is also observed for P3AOTs which can be explained
by the more rigid conformation of P3AOTs in THF, which
explains the relatively high value for P3AOT-TMS. Its higher
polydispersity can be attributed to the fact that the polymeriza-
tion is not controlled.
possible to achieve direct conversion of the alcohol to the azide,
in order to limit the number of postpolymerization reactions, as
the removal of remaining reactants from the resulting polymer is
not always straightforward and can be detrimental for the final
yield. Subsequent literature study revealed that diphenylpho-
sphorylazide (DPPA) is able to achieve quantitative conversion
of alcohols to azides in small molecule synthesis, using mild
conditions.41 We found that, when reacting P3HT-2-OH with
DPPA in the presence of DBU, a substantial shift in ppm value for
the ethyl protons is observed, as expected. However, the IR
spectrum of the resulting P3HT did not contain the expected
azide peak around 2100 cmꢀ1. Careful examination of the aro-
matic region of the 1H NMR spectrum showed that the reaction
stopped at the stage of the diphenyl phosphonate. We were there-
fore forced to shift our approach to a two-step conversion, first
converting P3HT-2-OH with p-toluenesulfonyl chloride and
NaOH to the tosylate P3HT-O-Ts, which was isolated and
thereafter converted to the azide functionalized polymer (P3HT-
2-N3) by reaction with tetrabutylammonium azide. Although the
pathway through the tosylate requires an additional step, there is
also a benefit related with it, because the tosylate moiety in itself
is an interesting functionality as it can serve as a springboard for
many other functional groups besides the azide.
Postpolymerization Transformations. The next step was
the transformation of the prepared polymers to those with the
initially desired functional group, either through deprotection or
conversion to another functionality (Scheme 3). For P3HT-2-
OTBDMS, in addition to the deprotection reaction, conversion
to the azide was required. Deprotection was achieved by reaction
with TBAF, which furnished the alcohol-functionalized polymer
P3HT-2-OH. Next, the alcohol functionality in P3HT-2-OH
had to be converted into an azide. We wondered whether it was
Analysis of P3HT-2-N3 by IR spectroscopy (absorption at
2103 cmꢀ1, see Figure S50, Supporting Information) and
MALDIꢀToF confirmed the successful insertion of the azide. In
the MALDI mass spectrum, presented in Figure S41, Supporting
Information, signals corresponding to ionized P3HT end-capped
with a hydrogen atom and the azide-containing group were
clearly identified. In addition, a characteristic nitrogen loss was
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The deprotection of P3HT-NBoc2 proved to be unexpectedly
difficult, as treatment with an excess of trifluoroacetic acid only
achieved partial deprotection to the monoprotected amine.
Substituting TFA for p-toluenesulfonic acid and performing the
reaction under reflux conditions for 48 h, finally, yielded P3HT-
NH2, as indicated by the shift of the ethyl protons in the 1H NMR
spectrum (Figure 3). This was also confirmed by the MALDIꢀ
ToF analysis (see Figure S43, Supporting Information).
Finally, P3HT-COOtBu was deprotected to P3HT-COOH.
Initial attempts employing KOH were not successful. Fortu-
nately, the use of NBu4OH readily resulted in a clean and
quantitative deprotection (Scheme 4), as shown by the MALDIꢀ
ToF spectra (see Figure S44, Supporting Information) and IR
spectroscopy, which show a shift from 1713 cmꢀ1 (ester carbonyl)
to 1684 cmꢀ1 (carboxylic acid carbonyl) (see Figures S47ꢀS50,
Supporting Information).
Block Copolymer Formation. The successful synthesis of
P3AOT-E and P3HT-2-N3 provided us with two polymeric
substrates suitable for application in CuAAC click-chemistry
(Scheme 5). As both polymers are only soluble in either chloro-
form or THF, the standard CuSO4-ascorbic acid protocol could
not be used. Instead we used a combination of CuBr and N,N,N0,
N00,N00-pentamethyldiethylenetriamine (PMDTA) in THF,42
with a slight excess of P3HT-2-N3 to perform the click reaction,
resulting in the formation of P3HT-2-P3AOT.
Figure 2. Chemical shift of the ethyl protons in function of the end
group in the postpolymerization reactions of P3HT-2-OTBDMS.
In order to remove unreacted homopolymers, the crude
P3HT-2-P3AOT was successively fractionated with pentane,
hexane, heptane and chloroform, with the P3HT-2-P3AOT
being dissolved in the latter fraction. GPC analysis of this fraction
(Figure 4) showed a shift to higher molar mass compared to both
P3AOT-E and P3HT-2-N3, confirming the formation of block
copolymer while excluding the possibility of a blend of the two
homopolymers. 1H NMR analysis clearly shows the R-methylene
signals for the P3AOT (4.21 ppm) and the P3HT (2.78 ppm)
block. From the integration of these signals it can also be deduced
that the ratio P3AOT:P3HT is 4:1.
Bifunctional Ni-Initiator. Recent research43 suggests that
during the polymerization of P3HT using a Ni-catalyst, ring
walking of the Ni-center allows the polymerization to proceed
from both chain ends. However, when two significantly different
monomers are polymerized, propagation can be expected to
occur only at one side, since the complexation of the Ni-moiety is
more favored for one system than to the other one and the Ni-
catalysts cannot cross this barrier. We therefore decided to
investigate whether it would be possible to grow P3HT simulta-
neously from both chain ends, using two nickel atoms propagat-
ing at the same time in opposite directions. The experimental
verification of this hypothesis could be achieved by initiating the
polymerization of P3HT with an initiator carrying two nickel
atoms. In order to mimic the properties of a P3HT chain, we
decided to use a bithiophene moiety, with methyl substituents
ortho to the nickel moiety for stability reasons. Therefore, 5,
50-dibromo-4,40-dimethyl-2,20-bithiophene (17) was prepared
through palladium coupling of 2-bromo-3-methylthiophene44
(16), which was in turn prepared by bromination of 3-methyl-
thiophene (15). Subsequently, 17 was allowed to react with 2
equiv of Ni(PPh3)4, resulting in the formation of the bifunctional
nickel initiator 18 (Scheme 6). Although 18 was success-
fully isolated, it showed extremely rapid decomposition in
solution, which severely complicated its analysis and ex-
plains its low yield. However, when combined with dppp,
the stability was sufficient to allow initiation of the polymerization.
Figure 3. Chemical shift of the ethyl protons in the deprotection of
P3HT-NBoc2.
observed and definitively confirmed the presence of the azide
moiety. The presence of P3HT polymer chains with H/H as the
end groups was also highlighted by the MALDI measurement,
which is due to the fact that they were already present for P3HT-
2-OTBDMS and are not affected by the postpolymerization
reactions. As a final confirmation, the different conversion steps
of P3HT-2-OTBDMS to P3HT-2-N3 can all be monitored by
observing the chemical shifts of the ethyl protons, as illustrated in
Figure 2.
In the case of P3AOT-TMS, the TMS moiety was removed
using TBAF, yielding the ethynylene functionalized polymer
P3AOT-E. Again deprotection was confirmed both by 1H NMR
and MALDIꢀToF (see Figure S42, Supporting Information). As
already observed when analyzing the protected molecules
(P3AOT-TMS), the MALDI process also leads to the loss of a
C10H21• radical from the ionized P3AOT-E polymers.
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Scheme 4. Post-Polymerization Deprotection of the Polymers
Scheme 5. Block Copolymer Formation
chains are present (Supporting Information, Figure S45). GPC-
analysis of P3HT-bth-P3HT results in an Mn of 3.8 kg/mol and a
PDI of 1.6.
If one takes into consideration that (a) the polymerization
does indeed proceed on both sides and (b) the polymerization of
P3HT is controlled when employing dppp-ligated nickel cata-
lysts, it should be possible to prepare a diblock-copolymer by
extending the P3HT block on P3HT-bth-P3HT by addition of a
second monomer. Therefore, after the polymerization of P3HT-
bth-P3HT, one batch was quenched with acid, while 2-bromo-
magnesio-5-bromo-3-(3,7-(S)-dimethyloctyloxy)thiophene was
added to the other batch and the polymerization was allowed to
proceed further. If the conditions mentioned above are met, the
latter should result in the formation of a diblock-copolymer of
P3HT and P3AOT on both sides of the bithiophene moiety, in
the form of P3AOT-P3HT-bth-P3HT-P3AOT. Essentially, this
can be regarded as a P3AOT-P3HT-P3AOT triblock-copoly-
mer, and to the best of our knowledge, it is the first time that such
a structure, comprised of entirely conjugated blocks, has been
Figure 4. GPC elution profile for P3HT-2-P3AOT compared with its
constituting homopolymers.
When 2-magnesiochloro-4-hexyl-5-bromothiophene was added
to a solution of 17 which had first undergone ligand exchange
1
with dppp, polymerization did occur, as expected. H NMR of
1
the H+-terminated P3HT-bth-P3HT shows that polymerization
did indeed proceed on both sides, as only one singlet at 2.42 ppm
is visible for both 4,40-methyls and there is a clear shift from 17
(Figure 5). Also, the presence of only one triplet at 2.62 ppm,
arising from the R-methylenes from hydrogen terminated P3HT
chains,27 indicates that the polymerization is controlled. The fact
that there is no distinct triplet for the R-methylenes of P3HT
next to the bithiophene moiety is probably due to the high
similarity of the bithiophene to the P3HT backbone. MAL-
DIꢀToF spectroscopy reveals that indeed only H/H terminated
prepared by a chain-growth polycondensation. In the H
NMR spectrum, the R-methylene protons of the P3AOT-
block (4.22 ppm) and P3HT-block (2.80 ppm) have a ratio of
3.5:1. By comparing the integration of these with the singlet of
the bithiophene methyls at 2.42 ppm, it can be concluded that
the P3HT-block contains 7 units and the P3AOT-block 24 on
each side. The disappearance of the triplet at 2.62 ppm upon
addition of the second monomer, clearly demonstrates a
successful initiation of the P3AOT-blocks on both sides of
the P3HT-bth-P3HT.
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Scheme 6. Synthesis of the Bifunctional Nickel Initiator and Subsequent Polymerizationa
a (i) NBS; (ii) PdCl2(PhCN)2, AgNO3, KF; (iii) Ni(PPh3)4; (iv) (1) dppp, (2) 2-magnesiochloro-4-hexyl-5-bromothiophene; (v) 2-bromomagnesio-5-
bromo-3-(3,7-(S)-dimethyloctyloxy)thiophene or H3O+.
’ CONCLUSION
In conclusion, we have succeeded in preparing several poly-
thiophenes with ethynylene, alcohol, tosylate, azide, amine, and
carboxylic acid functionalities. We also demonstrated that these
end groups can be transformed in a straightforward manner and
can be used forthe synthesis of a conjugated block copolymer with
two electronically different blocks. For the first time, a bifunc-
tional nickel initiator was synthesized and used to prepare a
triblock-copolymer consisting of fully conjugated blocks.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
1H NMR, 13C NMR, 31P NMR, IR, and MALDIꢀToF spectra of
Figure 5. 1H NMR comparison of 17 and block copolymers.
selected compounds and/or polymers. 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.
’ ACKNOWLEDGMENT
We are grateful to the Katholieke Universiteit Leuven (GOA)
and the Fund for Scientific Research (FWO-Vlaanderen) for
financial support. A.F. and P.W. are doctoral fellows of IWT-
Vlaanderen. P.G. and JDW (FRS-FNRS research associate and
research fellow) are grateful to the FRS-FNRS for financial sup-
port in the acquisition of the Waters QToF Premier mass spec-
trometer and for continuing support.
Figure 6. GPC elution profile of P3HT-bth-P3HT compared with
P3AOT-b-P3HT-b-P3AOT.
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