Controlled chain-growth Kumada catalyst transfer polycondensation of a conjugated alternating copolymer

Article abstract of DOI:10.1021/ma300013e

The Ni-catalyzed Kumada catalyst transfer polycondensation of a novel biaryl monomer, 2-(4-bromo-2,5-bis(2-ethylhexyloxy)phenyl)-5- chloromagnesiothiophene (2), afforded the respective π-conjugated alternating copolymer poly(thiophene-alt-p-phenylene) (PTPP). Under optimized conditions, the polystyrene-equivalent number-average molecular weight (M_n) of the copolymers prepared using this method were varied between 6.4 and 39 kDa by adjusting the initial monomer-to-catalyst ratios and, in all cases, the resulting materials exhibited narrow polydispersity indices (PDIs ≥ 1.33). Moreover, the M_n of the copolymers produced were found to increase linearly with monomer conversion. The ability of PTPP to be utilized as a macroinitiator for further copolymerization was confirmed through a series of chain extension experiments as well as block copolymerizations involving 2-bromo-5-chloromagnesio-3-hexylthiophene. MALDI-MS analysis showed that the major population of the PTPP prepared using the aforementioned method contained H/Br end groups, as would be expected for efficient catalyst transfer during the polymerization with minimal occurrence of chain termination. Collectively, these results were consistent with a controlled polymerization reaction and constituted the first such example in which a conjugated polymer with an alternating repeat unit was produced via a chain-growth process.

Full text of DOI:10.1021/ma300013e

Article
pubs.acs.org/Macromolecules
Controlled Chain-Growth Kumada Catalyst Transfer
Polycondensation of a Conjugated Alternating Copolymer
Robert J. Ono, Songsu Kang, and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, Texas 78712,
United States
S
* Supporting Information
ABSTRACT: The Ni-catalyzed Kumada catalyst transfer
polycondensation of a novel biaryl monomer, 2-(4-bromo-
2,5-bis(2-ethylhexyloxy)phenyl)-5-chloromagnesiothiophene
(2), afforded the respective π-conjugated alternating copoly-
mer poly(thiophene-alt-p-phenylene) (PTPP). Under opti-
mized conditions, the polystyrene-equivalent number-average
molecular weight (Mn) of the copolymers prepared using this
method were varied between 6.4 and 39 kDa by adjusting the
initial monomer-to-catalyst ratios and, in all cases, the resulting materials exhibited narrow polydispersity indices (PDIs ≤ 1.33).
Moreover, the Mn of the copolymers produced were found to increase linearly with monomer conversion. The ability of PTPP to
be utilized as a macroinitiator for further copolymerization was confirmed through a series of chain extension experiments as well
as block copolymerizations involving 2-bromo-5-chloromagnesio-3-hexylthiophene. MALDI-MS analysis showed that the major
population of the PTPP prepared using the aforementioned method contained H/Br end groups, as would be expected for
efficient catalyst transfer during the polymerization with minimal occurrence of chain termination. Collectively, these results were
consistent with a controlled polymerization reaction and constituted the first such example in which a conjugated polymer with
an alternating repeat unit was produced via a chain-growth process.
aforementioned polymerization metrics but could also enable
the preparation of more advanced macromolecular structures
such as block copolymers and surface-grafted polymers.
Moreover, the successful preparation of such well-defined
structures could potentially facilitate greater control over the
polymer morphology, which is another important parameter
that governs the performance of organic electronic devices.6,7
We reasoned that such controlled polymerizations may be
achieved by applying the Ni-catalyzed Kumada catalyst transfer
polycondensation (CTP) to monomers that feature “built in”
alternating repeat units. Recently, during the course of our
investigations, Huck and co-workers reported a synthesis of the
alternating copolymer poly(9,9′-dioctylfluorene-alt-benzothia-
diazole) (PF8BT) using Pd-catalyzed Suzuki CTP.8 While the
chain-growth nature of the aforementioned polymerization was
elegantly demonstrated, only polymers having moderate
molecular weights (up to ca. 10 kDa or 19 repeat units) were
isolated and efforts toward controlling the polymerization (e.g.,
establishing a linear relationship between polymer molecular
weight and monomer conversion) appeared to be underway.
We chose to continue our pursuit of Ni-catalyzed Kumada
CTPs because they offer, as a complementary methodology,
several potential advantages over analogous Suzuki-type
polycondensations. For instance, Kumada CTPs do not require
INTRODUCTION
■
McCullough’s1 and Yokozawa’s2 seminal reports on the chain-
growth polymerization of 3-hexylthiophene have enabled the
corresponding polymer, P3HT, to become one of the most
prevalent conjugated polymers (CPs) in the literature. Despite
the synthetic versatility and impressive electronic properties of
P3HT, significant research efforts are now being directed
toward the synthesis of new CPs that feature properties tailored
toward specific applications. For example, in the field of organic
photovoltaics, a growing number of CPs have been shown to
outperform P3HT when incorporated into solar cells,3,4
generally owing to the former’s broad absorption overlap
with the solar spectrum and better energy matching with
commonly used electron acceptors (e.g., phenyl-C61-butyric
acid methyl ester; PCBM). Many of these new, high
performance CPs are effectively “donor−acceptor” (D−A)
polymers, designed to feature reduced optical bandgaps (and
hence broader absorptions that extend into the red) by virtue of
incorporating electron-donating and -withdrawing components
into the polymer main chain.5 Structurally, nearly all of these
types of CPs take the form of an alternating copolymer
comprised of donor and acceptor groups (e.g., -[D-A]-n) and
are generally synthesized by metal-catalyzed step-growth
polycondensations. As such, these methods do not permit
rigorous control over polymer molecular weight or polydisper-
sity (Mw/Mn) and oftentimes afford ill-defined materials. A
controlled, chain-growth synthesis of donor−acceptor CPs
would not only allow for precise control over the
Received: January 3, 2012
Revised: February 4, 2012
Published: February 23, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of PTPP
a
R = t-butyl or i-propyl. R′ = 2-ethylhexyl.
a
the use of specially synthesized catalysts or ex-situ initiation.
Furthermore, despite the evidence that a chain-growth
mechanism is operative in the Kumada CTP of monomers
having relatively large molecular lengths (e.g., fluorenes,
carbazoles, oligothiophenes, etc.), control over such polymer-
izations largely remains an unsolved problem, presumably due
to inefficient catalyst transfer across the length of the
monomer.9−15 Here, we report the first controlled, chain-
growth synthesis of a π-conjugated alternating copolymer,
poly(thiophene-alt-p-phenylene) (PTPP), via the Kumada
catalyst transfer polycondensation of a novel bicyclic monomer.
Table 1. Polycondensation of 2 with Various Ni Catalysts
equiv of
LiCl
Grignard
reagent
b
b
entry
catalyst
Mn
Mw/Mn
1
2
3
Ni(dppe)Cl2
Ni(dppp)Cl2
Ni(dppe)Cl2
Ni(dppp)Cl2
Ni(dppe)Cl2
Ni(dppp)Cl2
0
iPrMgCl
iPrMgCl
iPrMgCl
iPrMgCl
tBuMgCl
tBuMgCl
12 000
3 700
2.03
1.32
1.74
1.33
2.03
1.53
0
1.0
1.0
1.0
1.0
12 000
14 500
15 200
10 500
c
4
5
6
a
Polymerizations were carried out by treating 1 with 1.0 equiv of
Grignard reagent in THF ([1]0 = 0.1 M) for 20 min at 0 °C, followed
by warming to 23 °C and addition of the Ni catalyst ([1]0/[Ni]0 =
33). In all cases, the expected Mn was based on a degree of
polymerization of 33 and calculated to be 13 700 Da. Determined by
GPC based on polystyrene standards (eluent: THF). Conversion of 2
RESULTS AND DISCUSSION
■
b
We elected to incorporate thiophene and dialkoxybenzene into
a single monomer unit because the Kumada catalyst transfer
polymerizations of both thiophenes1,2 and dialkoxybenzenes16
have already been independently established.17 Premonomer 1
was prepared in two steps by the Suzuki coupling of 2-
thiopheneboronic acid and 1,4-dibromo-2,5-di(2-
ethylhexyloxy)benzene followed by iodination with N-iodosuc-
cinimide in 57% overall yield (see the Supporting Information
for additional details). As summarized in Scheme 1, treatment
c
to polymer: 87%; isolated yield: 71%.
was much narrower, but only materials with relatively low Mns
were obtained (entry 2).
It has been shown that the addition of LiCl to Kumada CTP
reactions can have beneficial effects on the chain-growth
polymerization characteristics of aryl-Grignard monomers. For
example, Yokozawa and co-workers reported that the addition
of LiCl conferred controlled, chain-growth characteristics to the
polymerization of 1-bromo-4-chloromagnesio-2,5-dihexyloxy-
benzene to afford poly(p-phenylene)s with high molecular
weights and narrow PDIs (e.g., 1.18).16 We thus performed the
polymerization of 2 with the aforementioned Ni catalysts in the
presence of 1.0 equiv of LiCl. As shown in Table 1, the addition
of LiCl had little effect on the polymerization when
Ni(dppe)Cl2 was used as the catalyst, as the molecular weight
of PTPP remained unchanged from that which was obtained
from the corresponding reaction without LiCl, although a slight
decrease in polydispersity was observed (entry 3). When
Ni(dppp)Cl2 was used, however, the Mn of the resulting
polymer increased significantly compared to that obtained from
the analogous polymerization without LiCl, while the polymer’s
PDI remained relatively low (entry 4). After quenching the
polymerization with HCl(aq), the pure homopolymer, poly-
(thiophene-alt-p-phenylene) (PTPP), was isolated in 71% yield
by means of precipitation from methanol, followed by
collection with the aid of vacuum filtration. When the
polymerization was repeated in the presence of an internal
standard (1,4-dihexyloxybenzene) and monitored over time by
1H NMR spectroscopy, we found that the polymerization
proceeded rapidly at 23 °C, consuming 87% of 2 within 15 min
(Figure S1). Substituting tBuMgCl for iPrMgCl to facilitate the
Grignard metathesis reaction did not lead to any significant
molecular weight increase or narrowing of the polydispersity of
the respective polymers (entries 5 and 6).
i
of 1 with 1.0 equiv of PrMgCl at 0 °C in THF resulted in the
1
fast, quantitative conversion to 2, as deduced by H NMR
spectroscopic analysis of the reaction mixture after quenching
with HCl(aq). The reaction was complete within 20 min, and
metalation occurred selectively at the α-position of the
thiophene, leaving the aryl bromide intact, also evidenced by
1H NMR spectroscopy of the corresponding quenched product,
which revealed signals that were identical to that of 2-(4-
bromo-2,5-bis(2-ethylhexyloxy)phenyl)thiophene (i.e., the pre-
cursor to 1).
Polymerization was carried out at 23 °C by the addition of a
Ni catalyst (3 mol %) to the THF solution of 2 and monitored
by gel permeation chromatography (GPC) over time until the
respective polymer molecular weight ceased to increase. Two
Ni catalysts, Ni(dppp)Cl2 and Ni(dppe)Cl2 (dppp = 1,3-
bis(diphenylphosphino)propane; dppe = 1,2-bis-
(diphenylphosphino)ethane), were investigated for the poly-
merization of 2, as they have been shown to be suitable
catalysts for the homopolymerization of 2,5-dibromo-3-
alkylthiophenes as well as 2,5-dibromo-1,4-dialkoxyben-
zenes.2,16,18 As summarized in Table 1, both of the
aforementioned catalysts afforded polymeric products. When
Ni(dppe)Cl2 was used as the catalyst, a relatively high number-
average molecular weight (Mn) polymer was obtained;
however, the resulting polymer also exhibited a broad
polydispersity index (PDI) (Table 1, entry 1). In contrast,
when Ni(dppp)Cl2 was used, the PDI of the resultant polymer
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Figure 1. (A) Mn and Mw/Mn of PTPP plotted as a function of monomer conversion ([1]0/[Ni(dppp)Cl2]0 = 56); see text for further details. (B)
Mn and Mw/Mn of PTPP plotted as a function of Ni(dppp)Cl2 loading (conversion of 2 to polymer: 78−85%). Solid line indicates the theoretically
expected Mn at a given Ni(dppp)Cl2 loading. (C) MALDI mass spectrum of PTPP (Mn = 10 800 Da, Mw/Mn = 1.25, as determined by GPC). The
peak at m/z ∼ 7257 reflects electronic noise. For all polymerizations, [1]0 = 0.10 M.
Scheme 2. Proposed Mechanistic Pathways Leading to the Formation of H/Br end-Capped PTPP
Elegant mechanistic studies by McNeil and co-workers
revealed that the role of LiCl in the controlled formation of
poly(p-phenylene) and P3HT depends on the catalyst used. In
Ni(dppe)Cl2-catalyzed polymerizations, LiCl was found to have
no effect on the polymerization rate, whereas in Ni(dppp)Cl2-
catalyzed polymerizations a rate dependence on LiCl was
observed.18,19 Our results were consistent with these
observations: the addition of LiCl had a significant effect on
the polymerization of 2 when Ni(dppp)Cl2 was used as the
catalyst, affording PTPP with a high molecular weight and low
polydispersity, and little effect when Ni(dppe)Cl2 was used. We
therefore presume that the polymerization of 2, as catalyzed by
Ni(dppp)Cl2, exhibits a rate dependence on [LiCl], which may
serve to explain the rapid kinetics of polymerization observed in
the presence of LiCl.
The Mn of the PTPP obtained using Ni(dppp)Cl2 as the
catalyst in the presence of 1.0 equiv LiCl (Mn = 14 500 Da, Mw/
Mn = 1.33) was close to the theoretically expected value of
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13 700 Da, assuming both quantitative initiation and
conversion of monomer to polymer. These results suggested
to us that the polymerization was proceeding via a chain-growth
mechanism. To probe whether the polymerization was
controlled, the molecular weight of the polymer formed in
the reaction was monitored as a function of monomer
conversion. Briefly, a polymerization reaction was set up
under the aforementioned conditions (i.e., catalyst = Ni(dppp)-
Cl2, 1.0 equiv LiCl, 23 °C in THF) at an initial monomer-to-
catalyst ratio of 56 in the presence of an internal standard (1,4-
dihexyloxybenzene); aliquots were then removed at regular
intervals, quenched with aqueous HCl, and poured into excess
methanol. After collection of the precipitated solids via
filtration, the isolated polymers and filtrates were analyzed by
1
GPC and H NMR spectroscopy, respectively. As shown in
Figure 1A, a linear dependence of the Mn versus the percent
conversion of monomer over the course of the polymerization
was observed. Moreover, the PDIs of the isolated polymers
remained at or below 1.3 over the course of the polymerization
reaction. When the polymerization of 2 was performed using
varying initial catalyst loadings, the Mn increased proportionally
with the initial monomer-to-catalyst ratio ([1]0/[Ni(dppp)-
Cl2]0) while PDIs remained relatively low (Figure 1B),
indicating that the molecular weight of PTPP was successfully
controlled over a wide Mn range (i.e., 6.4−39 kDa).
Furthermore, the experimentally determined Mns were in
good agreement with the theoretically expected values at all of
the initial catalyst loadings explored. Taken together, these
results suggested to us that the polymerization followed a
controlled chain-growth mechanism. It is also worth noting that
PTPP, even at high molecular weights, maintained excellent
solubility in THF (ca. 80 mg mL−1), presumably due to the
presence of the branched alkoxy side chains in every repeat
unit.
End-group analysis using matrix-assisted laser desorption
ionization (MALDI) mass spectrometry has been shown to
provide important insight into polymerization mechanisms.2,8,20
We therefore obtained MALDI mass spectra of a sample of
PTPP (isolated after quenching the polymerization with HCl).
As shown in Figure 1C, we observed a major population of
signals that corresponded to H2O adducts of H/Br end-capped
PTPP, as calculated by the formula 414.3n (mass of n repeat
units) + 1 (H) + 79.9 (Br) + 18 (H2O).21 The presence of H/
Br end-capped polymers, along with a negligible population of
polymers bearing Br/Br or H/H end groups, is a hallmark of
efficient catalyst transfer in self-initiated catalyst transfer
polycondensation reactions.2,20 Collectively, these results
suggested to us that nearly every polymer grew via catalyst
transfer to the end of the polymer chain, as opposed to the
catalyst freely diffusing through the reaction mixture to initiate
a new polymerization or transferring to another chain.
Therefore, we concluded that the polymerization proceeded
through a chain-growth mechanism with minimal chain
termination (Scheme 2).
To further demonstrate the controlled, chain-growth nature
of the aforementioned Kumada CTPs, a series of chain
extension and block copolymerization experiments were
performed. First, a monomer addition experiment was
conducted, wherein a fresh feed of monomer 2 was added to
an unquenched, stirring solution of PTPP (PTPP-1) (initial
conditions: [1]0/[Ni(dppp)Cl2]0 = 20; conversion of 2 to
PTPP-1: 80%; PTPP-1 Mn = 8700 Da, Mw/Mn = 1.25). As
shown in Figure 2A, the GPC trace of the polymer formed at
Figure 2. GPC traces of polymers obtained from chain extension
polymerization experiments. (A) A monomer addition experiment:
PTPP-1 (dashed line) ([1]0/[Ni(dppp)Cl2]0 = 20, conversion of 2 to
polymer: 80%; PTPP-1: Mn = 8700 Da, Mw/Mn = 1.25); PTPP-1′
(solid line) ([remaining and added 1]0/[Ni(dppp)Cl2]0 = 31;
conversion of 2 to polymer: 72%; PTPP-1′: Mn = 17 000 Da, Mw/
Mn = 1.29). (B) A block copolymerization experiment (polymerization
order: PTPP followed by P3HT). PTPP (dashed line): [1]0/
[Ni(dppp)Cl2]0 = 18, conversion of 2 to polymer: 86%; polymer:
Mn = 11 600 Da, Mw/Mn = 1.29. PTPP-b-P3HT (solid line): [3]0/
[Ni(dppp)Cl2]0 = 51, conversion of 3 to polymer: 96%; polymer: Mn
= 21 900 Da, Mw/Mn = 1.33. (C) A block copolymerization
experiment (polymerization order: P3HT followed by PTPP).
P3HT (dashed line): [3]0/[Ni(dppp)Cl2]0 = 60; conversion of 3 to
polymer: 84%; polymer: Mn = 7500 Da, Mw/Mn = 1.22. P3HT-b-
PTPP (solid line): [1]0/[Ni(dppp)Cl2]0 = 24, conversion of 2 to
polymer: 67%; polymer: Mn = 9200 Da, Mw/Mn = 1.70.
the conclusion of this reaction (PTPP-1′) was clearly shifted
toward higher molecular weight ([remaining and added 1]0/
[Ni(dppp)Cl2]0 = 31; PTPP-1′: conversion of 2 = 72%, Mn =
17 000 Da, Mw/Mn = 1.29) with respect to PTPP-1. No
discernible signal attributable to the presence of unreacted
PTPP-1 was observed, suggesting to us that the added feed of 2
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a
Scheme 3. Synthesis of Diblock Copolymers PTPP-b-P3HT and P3HT-b-PTPP
a
R′ = 2-ethylhexyl.
was successfully polymerized via chain extension from the
terminus of PTPP-1.
appearing at 500 nm for the thin film. The emission spectra of
PTPP in solution and as a thin film exhibited maxima at 523
and 629 nm, respectively. These results were consistent with
those obtained for similar copolymers (e.g., poly-
(alkoxyphenylene−thienylene)s) previously reported in the
literature.15,25−28 The absorption spectra of the diblock
copolymer PTPP-b-P3HT were also recorded in dilute
CHCl3 solution as well as in the solid state as a thermally
annealed thin film (Figure S4). In this case, a large red shift of
absorption was observed upon concentration (λmax = 464 →
505 nm), likely a consequence of the π−π stacking and
subsequent planarization of the P3HT chains.29,30 Moreover,
vibronic structures reminiscent of P3HT homopolymer thin
films were clearly visible at 560 and 605 nm. Compared to the
solid state absorption of a P3HT homopolymer, however, the
corresponding spectrum of PTPP-b-P3HT exhibited an
absorption maximum at higher energies (λmax = 56031 and
505 nm for P3HT homopolymer and PTPP-b-P3HT,
respectively). We believe that the solid state spectrum of
PTPP-b-P3HT reflects the summed profiles of PTPP and
P3HT homopolymers; thus, the blue-shifted λmax of PTPP-b-
P3HT compared to that of P3HT is a consequence of the
overwhelming contribution of PTPP to the copolymer’s
absorption profile, rather than a morphological and/or energy
transfer effect. Regardless, the absorption characteristics of
PTPP-b-P3HT should be amenable to further fine-tuning by
controlling the relative block lengths of the respective
polymers, an otherwise laborious synthetic task that is
effectively streamlined by the methodologies presented here,
thus highlighting the potential utility of Kumada CTPs for
producing conjugated alternating copolymers with tunable
optical properties.
We subsequently shifted our attention to the formation of
diblock copolymers, choosing to incorporate P3HT as the
second block because of its well-behaved polymerization
behavior via Kumada CTP (Scheme 3).9 As shown in Figure
2B, addition of 2-bromo-5-chloromagnesio-3-hexylthiophene20
(3) to an unquenched solution of PTPP (Mn = 11 600 Da, Mw/
Mn = 1.29) afforded a polymer with a monomodal GPC profile
that was of higher molecular weight (Mn = 21 900 Da) than
that obtained for its parent PTPP homopolymer. Furthermore,
the PDI of the product remained low (Mw/Mn = 1.33),
indicating that the unquenched PTPP in solution was active
and effectively initiated the polymerization of 3, which also
proceeded in a chain-growth manner. The formation of a
diblock copolymer, consistent with the expected structure (i.e.,
1
PTPP-b-P3HT), was further confirmed by H NMR spectros-
copy, which revealed signals attributable to both PTPP and
P3HT (Figure S10).
Considering recent reports that have shown that the order of
polymerization can affect the outcome of block copolymeriza-
tions involving Kumada CTPs,13,22,23 we next attempted to
synthesize the aforementioned diblock copolymer P3HT-b-
PTPP by reversing the order of monomer addition. Thus, a
THF solution of 2 ([1]0/[Ni(dppp)Cl2]0 = 24) was added to a
stirring solution of unquenched P3HT (Mn = 7500 Da, Mw/Mn
= 1.22). In this case, the resulting copolymer (Mn = 9200 Da,
Mw/Mn = 1.70) displayed a broader molecular weight
distribution than the parent P3HT homopolymer, and the
conversion of 2 remained relatively low (conversion of 2 to
polymer: 67%), even after prolonged reaction periods (Figure
2C). This result suggested to us that P3HT was not an efficient
macroinitiator for the polymerization of PTPP, possibly due to
the strong association of the Ni catalyst with P3HT.22,24
Nevertheless, successful chain extension polymerizations of
PTPP were achieved, which provided additional support that
chain-growth polymerization of 2 proceeded in a controlled
fashion.
In conclusion, we have demonstrated that Kumada catalyst
transfer polycondensation is a viable method for synthesizing
well-defined conjugated alt-copolymers. We found that
controlled chain-growth characteristics were conferred to the
polymerization when Ni(dppp)Cl2 was used as the catalyst in
the presence of LiCl, a result that was consistent with efficient
intramolecular catalyst transfer within the growing polymer
backbone, despite the relatively large length of the repeat unit.
As a benefit of this chain-growth mechanism, excellent control
over copolymer molecular weight, polydispersity, and end-
group composition was obtained, all of which facilitated the
successful preparation of a new, well-defined block copolymer,
The absorption and fluorescence spectra of the PTPP
synthesized using the aforementioned procedures were
acquired in dilute CHCl3 solution and in the solid state (as
thin films) (Figure S3). A slight bathochromic shift was
observed in the respective absorption spectra upon concen-
tration (λmax = 467 nm → 477 nm), with a small shoulder
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(15) Wang, F.; Wilson, M. S.; Rauh, R. D.; Schottland, P.;
Thompson, B. C.; Reynolds, J. R. Macromolecules 2000, 33, 2083−
2091.
(16) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am.
Chem. Soc. 2006, 128, 16012−16013.
PTPP-b-P3HT. These results unambiguously show, for the first
time, that conjugated polymers with alternating repeating units
can be synthesized in a controlled, chain-growth manner. Many
low-bandgap and D−A conjugated polymers exhibit complex,
alternating structures but are currently synthesized by means
that do not permit the aforementioned degree of control over
the polymerization. It is expected that the method presented
herein will be amenable to attaining control over the synthesis
of such D−A and other conjugated polymers and enable access
to more complex macromolecular structures thereof.
(17) Our decision to use a monomer that metalates at the thiophene
as opposed to the phenyl moiety was based on the work of Yokozawa
and co-workers, who demonstrated that the order of polymerization
was critical to the outcome of forming block copolymers containing
P3HT and poly(p-phenylene) (PPP) segments: successive polymer-
ization of the phenylene monomer followed by the thiophene
monomer yielded a well-defined block copolymer with narrow
molecular weight distribution, while reversing the order of monomer
addition resulted in polymers with broad molecular weight
distributions.22 In other words, we anticipated that if metalation
occurred at the thiophene unit, the propagating end of the resulting
polymer (i.e., polymer−NiLBr) would feature a terminus where Ni is
connected to a phenyl group, reminiscent of a Ni-terminated PPP, and
ultimately result in a controlled polymerization process. In contrast,
metalation at the phenyl unit would lead to polymers with termini that
are reminiscent of Ni-terminated P3HT and afford ill-defined
polymers.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and additional character-
ization details. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bielawski@cm.utexas.edu.
■
(18) Lanni, E. L.; McNeil, A. J. Macromolecules 2010, 43, 8039−8044.
(19) Lanni, E. L.; McNeil, A. J. J. Am. Chem. Soc. 2009, 131, 16573−
16579.
Notes
The authors declare no competing financial interest.
(20) Lohwasser, R. H.; Thelakkat, M. Macromolecules 2011, 44,
3388−3397.
ACKNOWLEDGMENTS
■
(21) The signals observed at m/z 7746 and 8575 were assigned to an
H/I terminated polymer as adducts of Li and dihydroxybenzoic acid
(DHB, the matrix material) according to the following formula 414.3n
(mass of n repeat units) + 1 (H) + 127 (I) + 7 (Li) + 154 (DHB). An
H/I terminated polymer can arise from the generation of a small
concentration of (2,5-bis(2-ethylhexyloxy)-4-(5-iodothiophen-2-yl)
phenyl)magnesium chloride formed during the Grignard metathesis
reaction, where the metalation of 1 occurs at the phenyl instead of at
the thiophene moiety. This monomer can then initiate a polymer-
ization reaction, thereby establishing an iodo group on one end of a
polymer chain and, after catalyst transfer and quenching with HCl, a
hydrogen group on the other.
This material is based upon work supported by the National
Science Foundation through the Centers for Chemical
Innovation (CCI) under Grant CHE-0943957 as well as the
Robert A. Welch Foundation (F-1621). We thank Dr. Karin
Keller for conducting the MALDI mass spectrometry and Dr.
Johanna Brazard for assistance with the fluorescence spectros-
copy experiments as well as for helpful discussions.
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