Block copolymers containing organic semiconductor segments by RAFT polymerization

Article abstract of DOI:10.1039/c1ob05276d

Approaches to the synthesis of block copolymers containing organic semiconductor segments (polythiophene, perylene diimide) by RAFT polymerization have been explored. A method involving transformation of a vinyl derivative to a macro-RAFT agent provides f

Full text of DOI:10.1039/c1ob05276d

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PAPER
Block copolymers containing organic semiconductor segments by RAFT
polymerization†‡
Ming Chen,* Matthias Ha¨ussler,* Graeme Moad* and Ezio Rizzardo
Received 21st February 2011, Accepted 9th June 2011
DOI: 10.1039/c1ob05276d
Approaches to the synthesis of block copolymers containing organic semiconductor segments
(polythiophene, perylene diimide) by RAFT polymerization have been explored. A method involving
transformation of a vinyl derivative to a macro-RAFT agent provides for the synthesis of block
copolymers which are joined by a short non-hydrolysable linkage.
Block copolymers attract interest because of their ability to self-
Introduction
assemble to give nanophase separation into periodic domains.
Control of radical polymerization with the addition of thio-
The dimensions of these domains can be in the range of 5–
carbonylthio compounds that serve as reversible addition frag-
50 nm which encompasses that required for many semiconductor
applications.18–22 Block copolymers may also be added as a minor
component and control the morphology of a blend by acting as
a compatibiliser or structure director.22–24 Recent reviews on the
use of block copolymers in organic electronics include those by
Segalman et al.,25 Kim et al.,19 Scherf et al.26 and Darling.22 The
block copolymers containing conjugated segments are a sub-class
of rod-coil polymers. Several relevant reviews have appeared on
the use15,25 and self-assembly27 of such block copolymers.
There are three main processes whereby a RAFT synthesized
polymer can be combined with fully conjugated polymers to
form block copolymers. The post polymerization “grafting to”
strategy involves coupling of a RAFT-synthesized polymer to
a semiconductor segment. The “grafting through” strategy in-
volves RAFT polymerization of a (macro)monomer with pendant
semiconductor functionality. These strategies are included in our
recent review.15 This paper focuses on the “grafting from” strategy
wherein macro-RAFT agents based on organic semiconductor
or analogous oligomeric species are prepared by end-group
modification of an organic semiconductor.
mentation chain transfer (RAFT) agents was first reported in
1998.1,2 Since that time much research carried out in these
laboratories and elsewhere3–11 has demonstrated that polymer-
ization with reversible addition-fragmentation chain transfer is
a reversible deactivation radical polymerization (RDRP);12 an
extremely versatile process that satisfies most of the established
criteria for a living polymerization13,14 It can be applied to form
polymers with a narrow molecular weight distribution. These
may be homopolymers or copolymers and can be formed from
most monomers amenable to radical polymerization. A variety of
architectures including stars, blocks, microgel and hyperbranched
structures, and supramolecular assemblies are accessible.
While fully conjugated polymers, such as those that see use
in organic semiconductors, cannot be directly made by RDRP
methods (ATRP, NMP, RAFT), these methods can be used to
form materials which comprise segments of these polymers either
as blocks or as pendant units.15–17
Significant benefits of RAFT polymerization are the ability to
form polymers with narrow molecular weight distributions and
uniform compositions with the possibility of eliminating the low
molecular weight “impurities” which can act as hole or electron
traps in organic semiconductors while, at the same time, targeting
the modest molecular weights that offer advantages in solubility,
processing and film forming characteristics.
CSIRO Materials Science and Engineering, Bag 10, Clayton South, Victoria,
Australia. E-mail: graeme.moad@csiro.au; Fax: +613 9545 2445; Tel: +613
9545 2509
† This paper is dedicated to the memory of Professor Athelstan L. J.
Beckwith whose innovation and enthusiasm helped to inspire several
generations of free radical chemists.
‡ Electronic supplementary information (ESI) available: 1H NMR spectra.
Synthesis of compounds 12 and 32. Results of radical polymerizations
performed in the presence of compounds 9–12. Further details of RAFT
polymerizations with 30.
RAFT synthesized block copolymers based on poly(3-
hexylthiophene) (P3HT) have been reported by Iovu et al.28 Yang
et al.29 and Palaniappan et al.30 These studies made use of the
macro-RAFT agents 1a or 1b to form ‘Z’-connected blocks where
the thiocarbonylthio functionality in the product is positioned at
the block linkage. This means that the block will be cleaved on
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thiocarbonylthio group removal/transformation, for example, by
radical induced reduction (refer Scheme 1).11,31
in preparing polymers for light harvesting applications.33,35,43
McLeary, Klumpermanandcolleagues44–50 observed that complete
conversion of the initial RAFT agent to a species incorporating a
single monomer unit is common to many well-behaved RAFT
polymerizations including those of St44,47 and methyl acrylate
(MA)46,49 with dithioester RAFT agents, and N-vinylpyrrolidone48
and vinyl acetate48 with xanthate RAFT agents. They termed this
behaviour selective initialization. Moad et al.51 proposed that the
chain length dependence of propagation in radical polymerization
(kp(1)ꢀkp(2)) is such that, as long as the transfer constant of the
RAFT agent is high (Ctr>10), there will be substantial conversion
to the single monomer “chain” before oligomerization to provide
a two unit or longer chain. A further important consideration in
minimizing byproducts is to select the ‘R’ group of the RAFT
agent to be the same as the initiator derived radical. Thus, in
the present work we use a cyanoisopropyl RAFT agent and use
azobis(isobutyronitrile) (AIBN) as initiator.
Scheme 1 Overall process for RAFT polymerization and thiocar-
bonylthio end-group removal.
This is of significance since the presence of the thiocarbonylthio
group has been shown to be detrimental to some applications.
For example, certain RAFT agents and macro-RAFT agents
effectively quench the fluorescence of coumarone derivatives
and acenaphthalene units.32–34 No quenching is observed for the
RAFT-synthesized polymers from which the thiocarbonylthio
end-group had been removed, for example, by aminolysis32 or
radical-induced reduction.33
For ‘R’-connected macro-RAFT agents, the thiocarbonylthio
group remains at the chain end and the block linkage formed is a
carbon-carbon bond so the structure should remain intact during
processing.35 Grande et al.36 reported the terthiophene RAFT
agent 2 which was used to form a polythiophene macro-RAFT
agent by an electrodeposition process using cyclic voltammetry.
The block juncture formed with use of this form of macro-
RAFT agent is a carbon–carbon bond. In this paper we explore
the use of this technology for the synthesis of block copolymers
containing organic semiconductor segments.
Results and discussion
Synthesis and use of ester-linked macro-RAFT Agents
In initial studies, we prepared a macro-RAFT agent based on the
tetrathiophene 552 which was prepared by a palladium-catalyzed
cross-coupling process’. The Vilsmeier–Haack reaction using one
equivalent of POCl3/DMF provided the monoaldehyde 6 in 52%
isolated yield. This was transformed to a macro-RAFT agent 8 by
the sequence of reactions shown in Scheme 2.
Rajaram et al.24 made use of the ‘R’-connected macro-RAFT
agent 3 to form a P3HT block copolymer. The RAFT agents
2 and 3 possess a relative long connecting chain between the
polythiophene chain and the thiocarbonylthio group and include
a potentially hydrolysable ester or amide linkage. This linkage is
retained at the block juncture in the RAFT synthesized polymer.
A method of synthesizing macro-RAFT agents suitable for
forming ‘R’-connected block copolymers involves the insertion
of a single monomer unit into a RAFT agent structure to form
a new macro-RAFT agent. Zard and coworkers37–42 were the
first to exploit this method when they applied xanthate transfer
chemistry to selectively insert a single unit of a less-activated
monomer such as a vinyl ester or a vinyl amide. The xanthate
transfer process failed (by providing an oligomeric product) when
applied to more activated monomers such as styrene (St) or acrylic
monomers. We have previously used single unit monomer insertion
with dithioester RAFT agents and appropriate St derivatives
Scheme 2 Macro-RAFT agent synthesis from tetrathiophene.
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Table 1 Polymerizations with tetrathiophene macro-RAFT agent 8a
Reactant mole ratio
AIBN Time/h Conv.b (%) Mn (calc) Mn
Ðf
c
d
Monomer
8
St
1577 8.2
1640 8.2
1907 8.2
2278 8.2
1
1
1
1
16
16
16
6
41
91
76
26
8900
12,000 1.10
11,000 1.20
23,900 1.24
11,400e 1.11
MMA
MA
AA
18900
15900
5900
a Bulk polymerization at 70 ◦C. b Conversions estimated from H NMR
spectrum. c Theoretical molecular weights calculated using the expression
Mn(calc) = ([monomer]/[RAFT agent]) ¥ fractional conversion ¥ MW
of monomer + MW of RAFT agent. d Molecular weights from GPC
in polystyrene equivalents. e GPC after esterification of PAA segment
with diazomethane.4 Estimated molecular weight of free acid is 9500.
f Molecular weight dispersity = Mw/Mn.
1
Bulk polymerizations of St, methyl methacrylate (MMA), MA,
and acrylic acid (AA) were carried out at 70 ◦C. The macro-
RAFT agent 8 provided good control over the molecular weight
dispersity (Ð)53 as shown in Table 1. However, the correspondence
between found (GPC) and calculated molecular weights is poor.
Higher than calculated molecular weights can often be attributed
to either incomplete usage of the RAFT agent (i.e., a low transfer
constant, in this case Ð should also be high) or the RAFT agent
concentration being lower than expected (i.e., an impure RAFT
agent). Lower than expected molecular weights suggest there is a
source of chain ends other than the RAFT agent. The polystyrene
equivalent molecular weights for poly(MA) and poly(AA) are
expected to be significantly lower than actual molecular weights.54
The oligothiophene block may also affect elution behaviour.
The monomer conversions shown in Table 1 were also lower
than expected for the reaction conditions. This suggested some
interference of the oligothiophene on the course of polymerization.
To test this hypothesis a series of control polymerizations of St,
MA and MMA were conducted in the presence of thiophene
and thiophene-containing compounds (9–12). The addition of
compounds 9–11 provided a lowering of molecular weight vs.
the control and significant retardation manifest as a reduced
conversion of monomer to polymer. Moreover, analysis by GPC
with diode array detection showed that some of the thiophene
derivative was incorporated into the polymer. The effect was
most profound with the acrylate (MA) and was small with
MMA. This order correlates with the intrinsic activity of the
corresponding propagating radicals.55,56 Compound 12 in which
the 2- and 5-positions of the thiophene are substituted appeared
to be substantially less reactive (some lowering of molecular weight
was observed but no retardation). Details of these experiments are
provided in the ESI.‡
Scheme 3 Macro-RAFT agent synthesis from poly(3-hexylthiophene)
derivative.
Solution polymerizations of St, MMA, MA, and AA were
carried out at 70 ◦C in the presence of the P3HT bis-macro-
RAFT agent 17. Polymerization conditions and outcome are
shown in Table 2. Good control (low Ð) was observed for
MMA polymerization. Only low conversion was obtained for
polymerization of St. Polymerization of MA gave a poly(MA)
with a high dispersity and a lower than expected molecular
weight. Attempted polymerization of AA provided an insoluble
product which was not characterized. The results with MA and
AA suggest interference of the P3HT block on the course of their
polymerization.
Synthesis and use of carbon-linked macro-RAFT Agents
While 8 and 17 are effective RAFT agents, they (like 2 and 3) have
the disadvantage that the polythiophene segment is connected
by a long, potentially hydrolysable, linkage. We therefore chose
to evaluate single unit monomer insertion. The experimental
protocol was first tested on 5-methyl-2-vinylthiophene (20).
There are a number of reports of radical polymerization of
vinylthiophene derivatives.58–63 This work demonstrated that the
double bonds of vinylthiophene derivatives are highly reactive58
and also suggested that it was important to block the 5-
position to avoid side reactions such as transfer to monomer and
crosslinking.63 RAFT polymerization of 2- and 3-vinylthiophene
and of 2,5-dibromo-3-vinylthiophene was reported by Mori et
al.64 Polymerization of the 2- or 3-vinylthiophene with or without
The procedure used by McCullough and coworkers57 for the
preparation of an ATRP macro-initiator was adapted as shown
in Scheme 3 to provide a bis-macro-RAFT agent based on P3HT.
Thus, regioregular P3HT (Mn, = 4094, Xn = 18), synthesized by
the Grim process with a mixture of end-groups (13, X = H or Br),
was debrominated by Grignard metathesis and quenched with
water to yield 14. The Vilsmeier–Haack reaction provided the
telechelic polymer with a,w-aldehyde groups which were reduced
to hydroxymethyl end-groups with LiAlH4. The P3HT 16 was
converted to the bis-macro-RAFT agent 17 by esterification. The
transformation to the bis-RAFT agent was followed by NMR.
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Table 2 Polymerizations with P3HT bis-macro-RAFT agent 17
(Mn,4094)a
Reactant mole ratio
d
e
Monomer
17
AIBN Conv.b (%) Mn (calc) Mn
Ðf
St
24400 45
12.2
8.6
76.2
8900
45300
44700
—
15100 1.16
54500 1.21
73000 1.45
MMA
MA
AA
7300 13.5 3.66
7300 13.5 3.66
6900 11.5 3.47
87.3
c
—
—
a Solution polymerization at 70 ◦C for 16 h. Chlorobenzene (CBz) was
used as solvent for St (687 mg with 300 mg CBz), MMA (660 mg with 1
g CBz) and MA (568 mg with 1 g CBz). A mixture of 1 : 1 (w/w) of CBz
and acetonitrile was used for AA (475 mg with 1 g solvent). b Conversions
estimated from 1H NMR. c Not determined (polymer precipitated during
polymerization). d Theoretical molecular weights calculated were using the
expression Mn(calc) = ([monomer]/[RAFT agent]) ¥ fractional conversion
¥ MW of monomer + MW of RAFT agent. e Molecular weights from
GPC in polystyrene equivalents. f Molecular weight dispersity = Mw/Mn.
Scheme 5 Macro-RAFT agent synthesis from poly(3-hexylthiophene)
macromonomer and use in RAFT polymerization to form
poly(3-hexylthiophene)-block-polystyrene.
RAFT agent gave only low molecular weight polymers. RAFT
polymerization of 2,5-dibromo-3-vinylthiophene was successful
providing a low dispersity polymer. The result again indicates the
importance of blocking the 2 and 5 positions of the thiophene unit
to radical attack.
The RAFT agent (21) was synthesized from the vinylthiophene
20 and butyl cyanoisopropyl trithiocarbonate 18 as shown in
Scheme 4 to provide a 68% isolated yield.
Fig. 1 Regions 2.4–3.4 and 4.9–5.9 ppm of 1H NMR spectra of
vinyl-P3HT (22) (lower trace) and (impure) P3HT macro-RAFT agent (23
(upper trace). Signal assignments for 22 d 5.51 ( CHH), 5.25 ( CHH),
2.60 (t, vinyl-Tp-CH2), 2.55 (t, Br–Tp–CH2). Signal assignments for 23
d 5.88 (m, Tp–CH–S), 3.38 (m, S–CH2C3H7), 2.8 (t, Tp–CH2), 2.55 (t,
Br–Tp–CH2) (Tp = thiophene).
PSt with molecular weight similar to the P3HT macro-RAFT
agent (23) (Fig. 2b). The amounts of these impurities might be
minimized by optimization of the reaction conditions. A balance
needs to be achieved between the need for high conversion of 22
and minimization of by-products from termination reactions. Up
to two moles of the termination products is expected for each mole
of AIBN decomposed.
Scheme 4 RAFT agent synthesis from vinylthiophene.
A P3HT macro-RAFT agent 23 was synthesized by an
analogous procedure as shown in Scheme 5. Vinyl terminated
P3HT 22 (Mn (GPC) 2400 polystyrene equivalents, Ð 1.39,
Mn (NMR) ~5000) was synthesized by Grignard metathesis
polymerization.65,66 The single unit monomer insertion was con-
ducted in chlorobenzene solvent with a two-fold excess of 18. This
1
reaction gave ~90% conversion of 22 (based on the H NMR
spectra, Fig. 1) and provided at 36% isolated yield of the P3HT
macro-RAFT agent (the low isolated yield is partly a consequence
of the small scale of the reaction and losses during precipitation).
Very high conversion for the single unit monomer insertion step is
important since 22 and 23 are not readily separated by chromatog-
raphy or other means. The experimental conditions also provided
a component twice the molecular weight of the precursor P3HT
(23) (Fig. 2a). This component appeared inert in the subsequent
RAFT polymerization and is therefore thought to be an impurity
(25) formed by termination by combination. Residual 22 and other
impurities formed by termination by disproportionation (26, 27)
or cross-combination with cyanoisopropyl radicals (28) most likely
account for a peak in the GPC chromatogram of the P3HT-block-
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The RAFT agents 21 and 23 were evaluated in St polymerization
(Table 3 and Table 4 respectively). Marked retardation was seen
for high concentration of 21. Conversions obtained with lower
concentrations 21 were consistent with those obtained in styrene
polymerization with other RAFT agents under the conditions
indicated.54,67
The impure macro-RAFT agent 23 (contaminated with 25–28)
was used in RAFT polymerization. The GPC chromatograms of
both the macro-RAFT agent 23 and P3HT-block-PSt 24 are shown
in Fig. 2. The assignment of the peak attributed to the P3HT block
copolymer was confirmed by UV-visible spectrophotometry and
GPC with photodiode array detection (Fig. 2b) which showed
that the high molecular weight peak contained both P3HT and
polystyrene while the two low molecular weight peaks contained
only P3HT (see discussion above). Pure P3HT-block-PSt 24 was
isolated by recycle GPC.
The perylene diimide macro-RAFT agent 30 was prepared
from the styrene derivative 29 by single unit monomer insertion
into RAFT agent 18 (Scheme 6). The reaction was performed
with 1 : 1 : 0.05 ratio of 29 : 18 : AIBN. The single unit monomer
insertion appeared to be quantitative based on 29 and provided a
92% isolated yield after chromatography. This crystalline product
30 was not contaminated by dimers as might be formed by radical-
radical termination.
Fig. 2 Gel permeation chromatogram (GPC) obtained with (a) refractive
index detection and (b) UV detection with wavelength 530 nm of
macro-RAFT agent 23 (—) and P3HT-block-polystyrene 24 (- - - -) (see
Experimental and Table 4).
Table 3 Bulk styrene polymerizations with RAFT agent 21a
Reactant mole
ratio
21 AIBN Temp./◦C Time/h Conv. (%) Mn (calc) Mn
Ðd
b
c
St
500 10
5000e 10
5000e 10
5000f 10
1
1
1
0
70
70
70
110
16
16
16
16
0
—
—
—
22
17
41
11800
9210
21700
11800 1.10
12600 1.11
24000 1.11
a Bulk polymerization at 70 ◦C. b Theoretical molecular weights calculated
using the expression Mn(calc) = ([monomer]/[RAFT agent]) ¥ fractional
conversion ¥ MW of monomer + MW of RAFT agent. c Molecular weights
from GPC in polystyrene equivalents. d Molecular weight dispersity =
Mw/Mn. e Duplicate experiments. f Thermal initiation.
Scheme 6 RAFT agent synthesis from perylene diimide derivative.
Table 4 Solution styrene polymerization with macro-RAFT agent 23
(Mn,~5000)a
The macro-RAFT agent 30 was evaluated in the polymerization
of the triarylamine monomer (31) and the benzobisthiadiazole
derivative (32) We have previously reported on the design of 31
and the RAFT homopolymerization of this monomer.68 There
also are several reports on RAFT polymerization of related
triarylamine-based monomers in the literature.69–73 The position
of the diphenylamine group meta to the styrene double bond
and blocking the para positions of the phenyl substituents are
important in minimizing side reactions during polymerization.68
The GPC showed that the macro RAFT agent was completely
consumed during polymerization, confirming the purity of the
Reactant mole ratio
b
St
23
AIBN Temp./◦C Time/h Conv. (%) Mn
Ðc
2000
4d
1
70
20
57
86600 1.12
a Solution polymerization (687 mg St with 300 mg chlorobenzene) at
70 ◦C for 16h. b Molecular weights from GPC in polystyrene equivalents.
c Molecular weight dispersity = Mw/Mn of P3HT-block-polystyrene com-
ponent (refer Fig. 1a). d impure RAFT agent (see text).
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Table 5 Polymerizations with perylene diimide macro-RAFT agent 30a
Organic semiconductors by their nature may be reactive towards
radicals. The present work also provides another demonstration
of the high reactivity of thiophene units in radical polymerization
and the importance of substituting both the 2- and 5-positions of
thiophene units in derived (macro)RAFT agents and monomers
to minimize side reactions during radical polymerization.
Reactant mole ratio
c
d
Monomer
30 AIBN
Conv.b (%) Mn (calc) Mn
Ðe
31
32
50
20
1
1
0.1
0.1
40
40
6900
4500
8400 1.3
3200 1.2
a Solution polymerization at 70 ◦C for 16 h (500 mg 26 with 1.17 g
chlorobenzene; 1.0 g 27 with 2.0 g chlorobenzene). b Conversions estimated
from 1H NMR. c Theoretical molecular weights calculated were using the
expression Mn(calc) = ([monomer]/[RAFT agent]) ¥ fractional conversion
¥ MW of monomer + MW of RAFT agent. d Molecular weights from
GPC in polystyrene equivalents. e Molecular weight dispersity = Mw/Mn.
Experimental
Materials and instruments
AIBN (TCI) was recrystallized from methanol. 2-(3-Methylthien-
2-yl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane,74
5,5¢-dibromo-
2,2¢-bithiophene,75 were prepared according to published literature
methods. The RAFT agents, 4-(butylthiocarbonothioylthio)-
4-cyanopentanoic acid (18) and butyl 2-cyanopropan-2-yl-
carbonotrithioate (4), were synthesized by the method of Thang
and coworkers.76,77 Monomers St, MMA and MA were filtered
through basic alumina and flash distilled immediately before use.
Catalysts tris(dibenzylideneacetone)dipalladium (Pd2dba3) and
HP(t-Bu)3]BF4 were obtained from Strem. The N,N-di-p-tolyl-
3-vinylaniline (31) was prepared as previously described.68 All
other starting materials were obtained from Aldrich and used as
received.
RAFT agent, and a high transfer constant for 30 in the polymer-
ization is indicated by the relatively low dispersity obtained (Ð 1.3,
Table 5).
For polymerization of 32 while the dispersity of the block
copolymer was also low (Ð 1.2), the molecular weight distribu-
tion obtained was distinctly bimodal. Moreover, a contaminant
containing the perylene diimide chromophore and with molecular
weight similar to 30 is observed by GPC. These observations can
be seen as indicative of side reactions involving 32 and again
emphasizes the importance of protecting the 2- and 5 positions
of thiophene units. The possibility of such side reactions was
also indicated in the model studies with compounds 11 and 12
(vide infra). The successful RAFT homopolymerization of suitably
substituted monomers containing the benzobisthiadiazole unit
(e.g., 33) has been reported elsewhere.68
1H and 13C NMR spectra were recorded on a Bruker Av400
spectrometer (1H 400.13 MHz; 13C 100.63 MHz) at 25 ◦C in
deuterated solvents as stated. Chemical shifts are expressed as
parts per million downfield from (external) tetramethylsilane. High
resolution Positive ion Electron Impact mass spectra (HRMS-
EI) were obtained with a ThermoQuest MAT95XL mass spec-
trometer using an ionization energy of 70 eV. Accurate mass
measurements were obtained with a resolution of 5000-10000
using perfluorokerosene as the reference. High resolution positive
ion electrospray mass spectra (HRMS-ESI) were acquired with a
Micromass Q-TOF II mass spectrometer using a cone voltage of
50V and a capillary voltage of 3.0kV. The sample was introduced
by direct infusion at a rate of 5 ml min-1 using NaI as an internal
calibrant. Molecular weights were determined by gel permeation
chromatography (GPC) performed with tetrahydrofuran (THF,
1.0 mL min-1) as eluent at 30 ◦C using a Waters GPC instrument
equipped with a Waters 2414 refractive index detector, and a
Waters 2996 diode array detector, a series of four Polymer
Laboratories PLGel columns (3 ¥ Mixed-C and 1 ¥ Mixed-E each
30 cm ¥ 7.5 mm), and Empower Software. The GPC was calibrated
with low dispersity polystyrene standards (Polymer Laboratories
EasiCal, Mp from 264 to 256000) and molecular weights are
reported as polystyrene equivalents. Recycling preparative GPC
was performed with a JAI LC-9201 Separations Module equipped
with a RI-50s Refractive Index Detector, a UV-3740 Single
Wavelength Detector and 2 JAI preparative columns (JAIGEL-
2H (20 ¥ 600 mm) and JAIGEL-2.5H (20 ¥ 600 mm)) connected
Conclusions
Transformation of a vinyl compound to a macro-RAFT agent
provides a route to ‘R’-connected block copolymers where the
block linkage is short and does not contain hydrolysable ester or
amide groups. The method is applicable to vinylthiophene and to
St derivatives and has been demonstrated with the preparation of
block copolymers containing organic semiconductor segments.§
◦
in series at room temperature (~22 C) with chloroform (3.5 mL
min-1) as eluent. Thin layer chromatography (TLC) was performed
with silica gel 60 F254 sheets (Merck) and eluting solvent as
indicated. Melting points were obtained with a Buchi B-545
melting point apparatus. Ultraviolet-visible absorption spectra
(UV) were obtained for solutions in 1 cm path-length quartz
cuvettes using a Cary 5E spectrophotometer.
§ The synthesis of the RAFT agents 8, 17, 20, 23 and 30 was previously
described in a patent application35 and are mentioned in a recent review.15
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(3,3¢¢¢-Dimethyl-[2,2¢:5¢,2¢¢:5¢¢,2¢¢¢-quaterthiophen]-5-yl)methyl
4-(((butylthio)carbonothioyl)thio)-4-cyanopentanoate (8)
was dissolved in dichloromethane (10 mL). After stirring for 15
min, a solution of 7 (0.64 g, 1.65 mmol) and DMAP (8 mg,
0.07 mmol) in dichloromethane (10 mL) was added dropwise.
A precipitate was formed immediately. After 4 h, the precipitate
was filtered and washed with dichloromethane (5 mL). The filtrate
was concentrated by rotary evaporation. The red crude product
was further purified by silica gel column chromatography with
ethyl acetate : n-hexane = 1 : 4 (v/v) as the eluent to afford the
title compound (8) as a yellow viscous oil (1.05 g, 96.3% yield).
TLC (CHCl3) Rf 0.66. 1H NMR (CDCl3): d 7.15 (d, J = 5.2
Hz, 1H, 5¢¢¢-H), 7.13 (dd, J = 3.8 Hz, 2H, 3¢,4¢¢-H), 7.05 (dd,
2H, J = 3.8 Hz, 4¢,3¢¢-H), 6.89 (d, J = 5.2 Hz, 1H, 4¢¢¢-H),
6.88 (s, 1H, 3-H), 5.22 (s, 2H, -OCH2), 3.33 (t, 2H, J = 7.5 Hz,
-SCH2), 2.67 (m, 2H, C(O)CH2), 2.56 (m, 1H, CH2CMeCN), 2.43
(s, 3H, 3¢¢¢-CH3), 2.41 (m, 1H, CH2CMeCN), 2.38 (s, 3H, 4-CH3),
1.87 (s, 3H, C(CN)CH3), 1.68 (m, 2H, SCH2CH2), 1.43 (m, 2H,
3,3¢¢¢-Dimethyl(2,2¢:5¢,2¢¢:5¢¢,2¢¢¢-quaterthiophene)
(5). A
solution of 5,5¢-dibromo-2,2¢-bithiophene (2.7 g, 8.3 mmol)
and 4,4,5,5-tetramethyl-2-(3-methylthiophen-2-yl)-1,3,2-dioxa-
borolane (4.1 g, 18.3 mmol) in toluene (100 mL) and N,N-
dimethylformamide (DMF) (80 mL) was carefully degassed by
purging with nitrogen. A solution of tripotassium phosphate
monohydrate (12.6 g, 55 mmol) in 70 mL water was then added.
The resulting emulsion was degassed, and the catalyst system
(Pd2dba3 [186.6 mg, 0.18 mmol], [HP(t-Bu)3]BF4 [105 mg, 0.36
mmol]) was added. Afterwards, the mixture was stirred at room
temperature overnight. After removal of the toluene solvent under
reduced pressure, the mixture was poured into water (300 mL) and
was extracted with chloroform (3 ¥ 40 mL). The organic phase
was washed with water (30 mL), dried over magnesium sulfate,
and the solvent evaporated. Purification by flash chromatography
on silica gel [hexane : ethyl acetate = 4 : 1 (v/v)] followed by
recrystallization from hexane/dichloromethane yielded yellow
SCH2CH2CH2), 0.93 (t, 3H, J = 7.5 Hz, SCH2CH2CH2CH3). 13
C
NMR (CDCl3): d 13.6, 15.5, 22.1, 24.8, 29.7, 29.8, 33.8, 36.7,
46.3, 61.0, 77.2, 119.0, 123.4, 123.8, 124.0, 126.1, 126.3, 130.8,
131.5, 132.6, 132.9, 133.8, 134.2, 134.9, 135.0, 135.8, 136.4, 137.0,
171.2, 216.9 (C S). HRMS (EI) Found 661.0400 Calculated for
C30H31NO2S7 661.0394.
1
crystals of 5 (2.6 g, 87.5%). H NMR (CDCl3): d 7.15 (d, J =
5.2 Hz, 2H, 5,5¢¢¢-H), 7.13 (d, J = 3.8 Hz, 2H, 3¢,4¢¢-H), 7.05 (d,
2H, J = 3.8 Hz, 4¢,3¢¢-H), 6.89 (d, J = 5.2 Hz, 2 H, 4,4¢¢¢-H), 2.43
1
(s, 6H, -CH3). The H NMR spectrum was consistent with that
a,x-Dihydroxymethyl-poly(3-hexylthiophene) (16)
reported.52
a,w-Dihydroxymethyl-poly(3-hexylthiophene) (Xn = 18 by 1H-
NMR) was prepared according to procedure of Liu et al.57 the
polymer was purified by Soxhlet extraction with methanol (~16 h)
and chloroform (~16 h).
3,3¢¢¢-Dimethyl-[2,2¢:5¢,2¢¢:5¢¢,2¢¢¢-quaterthiophene]-5-carbal-
dehyde (6). Phosphorus oxychloride (1.04 g, 6.8 mmol) was
slowly added into a solution of 5 (2.41 g, 6.73 mmol) and DMF
(0.51 mL, 6.8 mmol) dissolved in 50 mL dichloromethane. The
mixture was stirred for 15 min at room temperature and then
heated at 70 ◦C for 2 h. The dark red solution was then cooled
and poured into ice water and neutralized (pH = 7) through the
addition of potassium hydroxide. The organic layer was separated
and the water phase three times extracted with dichloromethane
(30 mL). The combined organic phases were consecutively washed
with sodium bicarbonate and brine solution and dried over
MgSO4. Purification by silica gel column chromatography with
n-hexane : ethyl acetate in a ratio 4 : 1 (v/v) gave red crystals of 6
a,x-Bis(((4-(((butylthio)carbonothioyl)thio)-4-
cyanopentanoyl)oxy)methyl)-poly(3-hexylthiophene) (17)
A mixture of RAFT agent 4 (0.291 g, 1.0 mmol) and dicyclohexyl
carbodiimide (0.25 g, 1.2 mmol) was dissolved in dichloromethane
(120 mL). After stirring for 15 min, a solution of 16 (0.3 g,
0.1 mmol) and 4-dimethaminopyridine (4 mg, 0.04 mmol) in
dichloromethane (10 mL) was added dropwise. This solution
was stirred at room temperature overnight and afterwards con-
centrated by rotary evaporation. The concentrated solution was
precipitated into methanol (100 mL) and a brown solid was
obtained, which was filtered and washed with additional methanol
(3 ¥ 20 mL) and dried in a vacuum oven until constant weight
(0.31 g, 93.9% yield). Due to the additional purification steps and
the removal of oligomeric P3HT, Xn as estimated from 1H NMR
increased to 21, which was used to calculate the molecular weight
of the macro-RAFT agent used in the polymerization (Mn, =
4094). 1H NMR (CDCl3): d 6.89 (s, Tp-H), 5.25 (s, -OCH2),
3.33 (t, -SCH2), 2.79 (TpCH2C5H11), 2.67 (m, C(O)CH2), 2.56 (m,
CH2CMeCN), 2.41 (m, CH2CMeCN), 1.87 (s, C(CN)CH3), 1.70
(m, TpCH2CH2C4H9 and SCH2CH2),1.43 (m, TpC2H4C3H6CH3
and SC2H4CH2), 0.93 (m, Tp-C5H11-CH3 and SCH2CH2CH2CH3)
(Tp = thiophene).
1
(1.35 g, 52%). H NMR (CDCl3): d 9.81 (s, 1H, -CHO), 7.54 (s,
1H, 3-H), 7.24 (d, J = 3.8 Hz, 1H, 3¢-H), 7.17 (m, 3H, 4¢,4¢¢,5¢¢¢-H),
7.06 (d, J = 3.9 Hz, 1H, 3¢¢-H), 6.90 (d, 1H, J = 5.1 Hz, 4¢¢¢-H),
2.48 (s, 3H, 4-CH3), 2.43 (s, 3H, 3¢¢¢-CH3).
(3,3¢¢¢-Dimethyl-[2,2¢:5¢,2¢¢:5¢¢,2¢¢¢-quaterthiophen]-5-yl)metha-
nol (7). To a suspension of 6 (1.35 g, 3.5 mmol) in 30 mL
dry ethanol was added 265 mg (7 mmol) sodium borohydride
in small portions. The reaction mixture quickly changed colour
from orange to yellow and the stirring was continued for 30 min.
Removal of the solvent under reduced pressure followed by
purification with flash silica gel column chromatography using
chloroform as eluent yielded yellow crystals of 7 (1.32 g, 97.2%).
1H NMR (CDCl3): d 7.15 (d, J = 5.2 Hz, 1H, 5¢¢¢-H), 7.13 (dd,
J = 3.8 Hz, 2H, 3¢,4¢¢-H), 7.05 (d, 1H, J = 3.8 Hz, 4¢-H), 7.02 (d,
1H, J = 3.8 Hz, 3¢¢-H), 6.89 (d, J = 5.2 Hz, 1H, 4¢¢¢-H), 6.81 (s,
1H, 3-H), 4.77 (s, 2H, -OCH2), 2.43 (s, 3H, 3¢¢¢-CH3), 2.38 (s, 3H,
4-CH3).
2-Methyl-5-vinylthiophene (20)
13.9 mL butyl lithium (1.6 M solution in hexane) was added to
a solution of methyltriphenylphosphonium bromide (8.5 g, 23.8
mmol) in THF (40 mL). The resulting solution was stirred for 3 h
when of 5-methylthiophene-2-carbaldehyde (19) (2.0 g, 15.9 mmol)
(3,3¢¢¢-Dimethyl-[2,2¢:5¢,2¢¢:5¢¢,2¢¢¢-quaterthiophen]-5-yl)methyl
4-(((butylthio)carbonothioyl)thio)-4-cyanopentanoate (8). A mix-
ture of 4 (0.53 g, 1.81 mmol) and DCC (0.41 g, 1.98 mmol)
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in THF (10 mL) was added. The reaction mixture was stirred
overnight then poured into 500 mL ice water. The product was
extracted with diethyl ether and the extracts washed with brine and
dried over magnesium sulphate. Flash column chromatography on
silica gel eluting with hexane/chloroform (5 : 1) gave 2-methyl-5-
vinylthiophene (20) (0.38 g, 19.1%). 1H NMR (CDCl3): d 6.74 (d,
1H, J = 3.5 Hz, 3 Tp-H), 6.72 (dd, 1H, J = 17.1 and 10.8 Hz,
Tp-CH =), 6.60 (d, 1H, J = 2.5 Hz, 2 Tp-H), 5.43 (d, 1H, J = 17.3
Hz, = CHH), 5.04 (d, 1H, J = 10.8 Hz, = CHH), 2.45 (s, 3H, CH3).
Ph-H), 7.47–7.53 (m, 3H, Ph-H), 7.36 (d, 2H, J = 8.2, Ph-H), 7.33
(d, 1H, J = 2.3 Hz, Ph–H), 6.73 (dd, 1H, J = 17.6 and 10.9 Hz,
CHH), 5.76 (d, 1H, J = 17.6 Hz, CHH), 5.31 (s, 2H, St-CH2-
N-), 5.25 (d, 1H, J = 10.9 Hz, CH CH2), 1.36 (s, 9H, -C(CH3)3),
1.29 (s, 9H, -C(CH3)3). 13C NMR (CDCl3) d 31.3, 31.8, 34.4,
35.5, 43.5, 114.0, 122.8, 122.9, 123.7, 125.7, 126.1, 126.3, 126.4,
128.0, 128.7, 129.5, 129.6, 131.0, 131.5, 132.6, 134.1, 134.2, 136.4,
136.6, 137.1, 143.7, 150.3, 162.9, 164.2. UV (dichloromethane):
lmax 528, 489, 457 nm. HRMS (EI) Found 694.2804 Calculated
for C47H38N2O4 694.2826.
Butyl (3-cyano-3-methyl-1-(5-methylthiophen-2-yl)butyl)
carbonotrithioate (21)
Perylene diimide based macro-RAFT agent (30)
2-Methyl-5-vinylthiophene (20) (375 mg, 3.02 mmol), butyl 2-
cyanopropan-2-yl-carbonotrithioate 18 (705 mg, 3.02 mmol) and
AIBN (5 mg, 0.03 mmol) were placed in an ampoule and the
solution degassed through three freeze-pump-thaw cycles. The
ampoule was sealed under vacuum and heated in a constant-
temperature oil bath at 70 ◦C for 20 h. The crude product
was purified by silica gel flash column chromatography with
hexane/chloroform (5 : 1 v/v) as the eluent to provide 21 (730 mg,
67.6%). TLC (hexane/CHCl3 3 : 1 v/v) Rf 0.16. 1H NMR (CDCl3):
d 6.91 (d, 1H, J = 3.4 Hz, 4 Tp-H), 6.60 (d, 1H, 3.4 Hz, 3 T-H),
5.64 (dd, 1H, J = 10.4 and 4.3 Hz, Tp-CHCH3), 3.35 (t, 2H, J =
7.35, S-CH2), 2.45 (s, 3H, Tp-CH3), 2.33 (m, 2H, Tp-CHCH2),
1.67 (m, 2H, S-CH2CH2), 1.42 (s, 6H, C(CN)(CH3)2), 1.30 (s, 2H,
CH2CH3), 0.93 (t, 3H, J = 7.3 Hz, CH2CH3). 13C NMR (CDCl3): d
13.6, 15.5, 22.0, 26.7, 27.7, 29.9, 31.8, 36.6, 46.3, 47.6, 123.7, 125.1,
127.1, 138.5, 140.7, 221.9 (C S). HRMS (ESI) Found 380.0592
Calculated for C16H23NS4Na+ 380.0611.
A solution of above monomer (29) (0.50 g, 0.7 mmol) 18 (0.16 g, 0.7
mmol) and AIBN (6 mg, 0.035 mmol, ~5 mol%) in chlorobenzene
(3 mL) was placed in an ampoule which was degassed through
three freeze-thaw evacuate cycles then heated 16 h at 70 C. The
product was purified by flash column chromatography on silica
◦
gel with dichloromethane as◦the eluent to yield red crystals of 30
1
(0.60 g, 92%) m.p. 217–219 C. TLC (CHCl3) Rf 0.07. H NMR
(CDC13): d 8.67 (b, 2H, perylene-H), 8.46 (b, 4H, perylene-H),
8.33(b, 2H, perylene-H), 7.20 (b, 1H, Ph-H), 5.42 (m, 1H, -S-
CH(Ph)-CH2-), 5.35 (s, 2H, Ph-CH2-N-), 3.26 (t, 2H, J = 7.45
Hz, -CH2-CH2-S-), 2.34 (m, 2H, -CH(Ph)-CH2-C-), 1.36 (s, 9H,
-C(CH3)3), 1.34 (s, 6H,–C(CN)(CH3)2), 1.29 (s, 9H, -C(CH3)3),
0.86 (t. 3H, J = 7.3 Hz, -CH2CH3). 13C NMR (CDCl3) d 13.5,
22.0, 27.1, 27.6, 29.9, 31.2, 31.7, 31.9, 34.3, 35.5, 36.6, 43.3, 45.2,
51.6, 123.0, 123.1, 123.7, 123.9, 126.1, 126.4, 127.9, 128.5, 128.8,
129.1, 129.7, 130.0, 131.3, 131.7, 132.6, 134.4, 134.5, 137.3, 138.2,
143.7, 150.2, 163.1, 164.3, 221.7. UV (dichloromethane): lmax 528,
482, 462, 313 nm. HRMS (ESI) Found 950.3038 Calculated for
C24H18N2OS3Na+ 950.3096.
Synthesis of P3HT-Macro-RAFT agent (23)
A polymerization tube was charged with vinyl-end-functionalized
P3HT (22)65,66 (Mn (GPC) 2400, Ð 1.39, 120 mg, 2.44 ¥ 10-5 mol) of
butyl 2-cyanopropan-2-yl-carbonotrithioate (11.4 mg, 4.9 ¥ 10-5
mol), AIBN (0.082 mg, 5 ¥ 10-7 mol), and chlorobenzene (1.35
mL). The mixture was degassed through three freeze-pump-thaw
cycles, sealed under vacuum and heated in a constant-temperature
oil bath at 70 ◦C for 20 h. The macro-RAFT agent (23) was
precipitated three times from a large excess of methanol, filtered,
washed with methanol, and dried under vacuum to constant mass
(45 mg, 36%). 1H NMR (CDCl3): d 6.98 (s, Tp-H), 5.88 (m,
Tp-CH), 3.38 (m, S-CH2), 2.8 (t, Tp-CH2), 2.55 (t, Br-Tp-CH2),
1.71 (m, Tp-CH2CH2), 1.44 and 1.35 (m, CH2 of hexyl and butyl
groups), 0.94 (m, CH3 of hexyl and butyl groups). Expansions of
the NMR spectrum are shown in Fig. 1. The GPC chromatogram
is shown in Fig. 2.
RAFT Polymerization
Prior to polymerization the monomers were flash distilled. The
monomer, AIBN, RAFT agent and solvent (concentrations as
indicated in Table 1–Table 5) were combined in an ampoule which
was degassed through three freeze-pump-thaw cycles, sealed under
vacuum and heated in a thermostatted oil bath for the stated time.
Polymerization was terminated by rapidly cooling the ampoule.
A small sample of the reaction mixture was retained to determine
the monomer conversion by 1H NMR spectroscopy. The polymers
were precipitated three times by addition into a rapidly stirred large
excess of non-solvent (methanol for St and n-hexane for MA and
MMA), filtered, washed with non-solvent, and dried in vacuum to
constant mass. The conditions, conversions and molecular weights
of the polymers obtained are shown in Table 1–Table 5.
N¢-(Di-2,5-tert-butylphenyl)-N-(4-vinylbenzyl)perylene diimide
(29)
Notes and references
A mixture of N-(di-2,5-tert-butylphenyl)perylene diimide78 (1.0 g,
1.7 mmol), 4-vinylbenzyl chloride (0.40 g, 2.6 mmol) in t K2CO3
(2.3 g, 17.5 mmol) in DMF (20 mL) was heated for 16 h at
70 ◦C. Purification by flash silica gel column chromatography,
using dichloromethane as the eluent, yielded red crystals of 29
(0.95 g, 80%) m.p. 233–235 ◦C. 1H NMR (CDC13): d 8.61 (d, 2H,
J = 8.0 Hz, perylene-H), 8.30 (dd, 4H, J = 8.0, 5.3 Hz, perylene
-H), 8.12 (d, 2H, J = 8.2 Hz, perylene -H), 7.60 (d, 1H, J = 8.7 Hz,
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