Synthesis of poly(3-alkylthiophene)-block-poly(arylisocyanide): Two sequential, mechanistically distinct polymerizations using a single catalyst

Article abstract of DOI:10.1021/ja106999q

Block copolymers of poly(3-hexylthiophene) and a poly(arylisocyanide) were synthesized in a single pot via the addition of 2-bromo-3-hexyl-5- chloromagnesiothiophene followed by n-decyl 4-isocyanobenzoate to a solution of Ni(1,3-bis(diphenylphosphino)prop

Full text of DOI:10.1021/ja106999q

Published on Web 09/21/2010
Synthesis of Poly(3-alkylthiophene)-block-poly(arylisocyanide): Two
Sequential, Mechanistically Distinct Polymerizations Using a Single Catalyst
Zong-Quan Wu,†,‡ Robert J. Ono,† Zheng Chen,† and Christopher W. Bielawski*,†
Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin, Austin, Texas 78712, United States,
and School of Chemical Engineering, Hefei UniVersity of Technology, Hefei, Anhui 230009, P.R. China
Received August 4, 2010; E-mail: bielawski@cm.utexas.edu
nide)s have been prepared from a large pool of readily available
Abstract: Block copolymers of poly(3-hexylthiophene) and a
poly(arylisocyanide) were synthesized in a single pot via the
addition of 2-bromo-3-hexyl-5-chloromagnesiothiophene followed
by n-decyl 4-isocyanobenzoate to a solution of Ni(1,3-bis(diphe-
nylphosphino)propane)Cl2. The respective mechanistically distinct
polymerizations proceeded in a controlled fashion and afforded
well-defined block copolymers with tunable molecular weights and
monomers. They may adopt helical architectures,7b,8 and recent
reports have prompted their utility in OPV devices.9 Hence, block
copolymers containing P3HT and poly(isocyanide)s are attractive
targets for study in various electronic applications.
Scheme 1. Synthesis of Poly(3-hexylthiophene)-block-poly(n-decyl
4-isocyanobenzoate) via Sequential Monomer Addition
compositions. The block copolymers exhibited microphase sepa-
ration characteristics in the solid state.
The self-assembly of semiconducting block copolymers has
recently emerged as a promising approach to achieving the hier-
archical and morphological control needed for understanding and
optimizing the charge separation and shuttling processes inherent
to organic photovoltaic (OPV) devices.1 Block copolymers contain-
As summarized in Scheme 1, initial efforts focused on exploring
ing regioregular poly(3-hexylthiophene) (P3HT) are widely studied
for such applications due to P3HT’s excellent electronic properties
and synthetic accessibility2 as well as the numerous methods
available for its modification.3 The preparation of these block
copolymers typically involves either elaborating an end-function-
alized polythiophene into an appropriate macroinitiator for the chain
extension of a second block via a polymerization process that is
mechanistically distinct from that of P3HT4 or coupling preformed
homopolymers with complementary end-functionalities.5 Such
methodologies, however, can be complex and inefficient, frequently
resulting in materials that contain inseparable homopolymer impuri-
ties. A convenient method that employs a single set of reaction
conditions and/or a single catalyst system to grow two mechanisti-
cally distinct homopolymers in a sequential fashion6 would
overcome these issues and facilitate access to a broad range of
P3HT-containing block copolymers.
the ability to grow a poly(phenylisocyanide) (PPI) from P3HT.
Using standard methods,2 2-bromo-3-hexyl-5-chloromagnesio-
thiophene (generated in situ from 2,5-dibromo-3-hexylthiophene
and iPrMgCl) was polymerized in THF using Ni(dppp)Cl2 ([mono-
mer]0/[Ni]0 ) 30) (dppp )1,3-bis(diphenylphosphino)propane) to
generate 1. When no further molecular weight increase was
observed by size exclusion chromatography (SEC), an aliquot was
removed for further analysis (see below), and n-decyl 4-isocy-
anobenzoate (2) ([2]0/[Ni]0 ) 30) was added to the reaction vessel.
After 1 h, the mixture was poured into excess methanol, and the
precipitated solids were collected in 82% isolated yield (over the
two polymerization steps) via filtration.
As shown in Figure 1A, SEC analysis revealed that the isolated
material was of higher molecular weight and lower polydispersity
(Mn ) 7.28 kDa; polydispersity index (PDI) ) 1.15) than that
contained within the aliquot removed after completion of the
thiophene polymerization (Mn ) 4.19 kDa; PDI ) 1.35).4h These
results were consistent with the chain extension of 1 and, combined
with 1H NMR spectroscopy, which revealed signals attributable to
both P3HT and PPI, indicated that 3 had formed.
The narrow molecular weight distributions exhibited by the
copolymer prepared in our preliminary experiment suggested to us
that the polymerization of 2 may have occurred via a controlled
chain-growth process. To investigate, a series of chain extension
polymerizations were performed by equally dividing a THF solution
of macroinitiator 1 (Mn ) 2.70 kDa; PDI ) 1.31) and adding
different quantities of 2 to each fraction. As shown in Figure 1B,
a linear correlation between the Mn of the crude reaction polymer
products and the feed ratio of 2/1 was observed, and each copolymer
analyzed exhibited a narrow polydispersity (PDI < 1.35). These
results support the established2 quasi-living nature of GRIM
polymerizations and that the polymerization of 2, as initiated by 1,
also proceeded in a similarly controlled manner.10
Regioregular P3HT is commonly prepared via a Ni-catalyzed
Grignard metathesis (GRIM) polymerization of 2,5-dibromo-3-
hexylthiophene.2 In a generally accepted mechanism, the polym-
erization proceeds via the oxidative addition of a Ni(0) catalyst
into a Br-terminated P3HT, followed by transmetalation with a
magnesiothiophene monomer and reductive elimination. A reactive
Ni(II) species resides at the P3HT terminus in the resting state of
this catalytic cycle (1, Scheme 1). While treatment of this species
with various organomagnesium reagents effectively quenches the
polymerization reaction, often with concomitant installation of a
functional group,3 we reasoned that the resulting Ni(II) complex
may possess sufficient reactivity to catalyze the polymerization of
other monomers, such as isocyanides, which have been reported
to polymerize under similar conditions.7 Moreover, poly(isocya-
† The University of Texas at Austin.
‡ Hefei University of Technology.
9
14000 J. AM. CHEM. SOC. 2010, 132, 14000–14001
10.1021/ja106999q  2010 American Chemical Society

C O M M U N I C A T I O N S
assembling into higher order structures in the solid state, a film of
3b drop-casted from CHCl3 was investigated by tapping-mode
atomic force microscopy (AFM). As shown in Figure 2A, the film
exhibited a nanofibrillar morphology, consistent with other films
of P3HT-containing block copolymers.4,6 The nanofibrils were
unidirectionally aligned and exhibited long-range order with
persistent lengths of up to 1 µm (cf. Figure S4, Supporting
Information). Differential scanning calorimetry (DSC) of 3c
provided further evidence that these block copolymers were capable
of undergoing phase separation, as glass transition (Tg) and melting
(Tm) temperatures assignable to both P3HT and PPI phases were
observed (Figure 2B).
In conclusion, we have prepared a block copolymer containing
regioregular P3HT and a poly(arylisocyanide) in one pot via the
sequential addition of monomers that may be further modified. A
single Ni complex was used to effect two mechanistically distinct
polymerizations, each of which proceeded in a controlled fashion
to give well-defined copolymers with low polydispersities and Mn’s
proportional to the monomer-to-catalyst feed ratios. The copolymers
were found to aggregate in solution and exhibited microphase
separation in the solid state, features which warrant their study in
OPV and other electronic devices.
Figure 1. (A) Representative size exclusion chromatograms of homopoly-
mer 1 (black) and its respective block copolymer 3b (red); see Table 1 for
Mn and PDI data. (B) Plot of Mn and Mw/Mn values of 3 measured as a
function of the feed ratio of 2 to 1 (Mn ) 2.70 kDa; PDI ) 1.31). Mn and
Mw/Mn were determined by SEC (eluent ) THF, 25 °C).
As summarized in Table 1, a variety of copolymers with different
molecular weights and compositions were synthesized using the
aforementioned method by simply varying the initial feed ratio of
monomers. In addition, all of the copolymers synthesized were
isolated in high yields (81-90%) and exhibited narrow, monomodal
distributions by SEC (Figure S2, Supporting Information). Col-
lectively, these results support the successful union of two
mechanistically distinct polymerization reactions within a single
reaction vessel to obtain well-defined block copolymers containing
P3HT.
Acknowledgment. We thank the U.S. DOE Office of Basic
Energy Sciences (EFRC Award DE-SC0001091) for support.
Table 1. Selected Molecular Weight and Polydispersity Dataa
Supporting Information Available: Synthetic procedures, spec-
troscopic and AFM data, and size exclusion chromatograms. This
material is available free of charge via the Internet at http://pubs.acs.org.
3
[2]0/[1]0b
Mnc (kDa)
Mw/Mnc
P3HTd (wt %)
yielde (%)
3a
3b
3c
3d
3e
34/1 (2.70)
16/1 (4.19)
25/1 (7.58)
12/1 (5.47)
10/1 (11.6)
9.29
7.28
11.6
7.72
13.7
1.30
1.15
1.13
1.17
1.17
29
57
65
71
85
90
82
85
85
81
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Figure 2. (A) AFM phase image of 3b drop-casted from CHCl3 ([3b]0 )
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During the course of our synthesis and characterization studies,
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(10) No polymerization of 2 was observed in the presence of isolated P3HT
that was free of Ni.
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