Radical/anionic SRN1-type polymerization for preparation of oligoarenes

Article abstract of DOI:10.1002/anie.201206096

Radicals and anions: The TEMPO-mediated oxidation of magnesiated iodoarenes A provides highly regioregular poly(m-phenylenes) by strictly alternating anionic/radical cross-over chain-growth polymerization. Poly(m-phenylenes) with mean molecular weight (M<

Full text of DOI:10.1002/anie.201206096

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DOI: 10.1002/anie.201206096
Polymer Synthesis
Radical/Anionic SRN1-Type Polymerization for Preparation of
Oligoarenes**
Sandip Murarka and Armido Studer*
Synthetic polymers have strongly influenced and changed
modern life. Their worldwide annual production rose to 230
million tons in the year 2009.[1] Polymers have been prepared
either by chain-growth or by step-growth polymerization.[2]
These two classes can be divided into subclasses by further
specifying the chemical nature of the polymerization process
(Figure 1).
Figure 1. Classification of polymerization processes.
Chain-growth polymerization[3] has been achieved by
radical, coordination, or ionic chemistry. Herein, we intro-
duce a novel chain-growth polymerization technique that
occurs by an unprecedented strongly alternating radical/
anionic cross-over process. The new method does not use any
Scheme 1. Radical/anionic cross-over polymerization processes (I=ini-
tiator).
transition metal and allows preparation of oligoarenes, which
have found applications in materials science.[4,5] As metal
impurities alter physical properties of a synthetic polymer,
materials prepared by transition-metal-based catalysis have
to be rigorously purified. Most transition metals used in such
polymerizations are expensive, and the necessity of using
sophisticated purification procedures leads to a further
increase of costs.
It is known that a radical polymerization can be trans-
formed into an anionic polymerization by a cross-over process
(Scheme 1). Such a transformation is best achieved by
termination of the radical process by a reagent that installs
a functional group that is able to initiate an anionic
polymerization.[6] The reverse process is also well-established.
However, alternating in situ radical/anionic cross-over poly-
merizations are unknown. Our design principle is based on
the knowledge that an arene radical anion bearing an anionic
leaving group X (for example, Br, I) readily fragments XÀ,
generating an aryl radical (ArC).[7] Such radicals react with
various nucleophiles (NuÀ) to radical anions (Ar-NuCÀ), which
upon oxidation (electron transfer to the starting aryl halide)
À
provide the arylation products Ar Nu in a chain process.
These established transformations are classified as SRN1
reactions[8,9] (nucleophilic radical substitutions), and aryl
À
Grignard reagents (Ar MgX) can act as nucleophiles in
such processes.[10] We assumed that aryl Grignard reagents
bearing an anionic leaving group at the arene moiety are
suitable monomers in unprecedented radical/anionic SRN1-
type polymerizations for preparation of oligoarenes. Reaction
of an initiator radical with such a monomer should generate
a radical anion, which will fragment XÀ to give an aryl
radical.[11] This reactive radical will then add to the anionic
[*] S. Murarka, Prof. Dr. A. Studer
Organisch-Chemisches Institut
À
monomer to give the corresponding radical anion by C C
bond formation. Subsequent XÀ elimination generating the
chain-extended aryl radical completes the chain propagation
step. The mechanism of the novel polymerization process is
depicted in Scheme 1, exemplified for the preparation of
poly(m-phenylene).[4,5]
Westfꢀlische Wilhelms-Universitꢀt
Corrensstrasse 40, 48149, Mꢁnster (Germany)
E-mail: studer@uni-muenster.de
[**] We thank the Westfꢀlische Wilhelms-University for supporting our
work. Dr. Heinrich Luftmann and Dr. Matthias Letzel (WWU
Mꢁnster) are acknowledged for conducting the MS studies.
Aryl Grignard (Ar-MgX) reagents undergo highly effi-
cient homocoupling to biaryls (Ar-Ar) by oxidation with the
2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO).[12,13]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206096.
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This homocoupling, which most likely proceeds via a biaryl
radical anion intermediate, was applied as initiation step. As
monomer for initial studies, we chose aryl magnesium
compound 1a, which is readily generated from the sym-
metrical bisiodide by I–Mg exchange, with commercially
available iPrMgCl (Table 1).[14] Oxidative homocoupling of
1a should lead to a biaryl radical anion bearing two iodide
substituents. Elimination of an iodide will generate the
corresponding biaryl radical, which in turn should be able to
initiate polymerization (Scheme 2).
Polymerization for 1 h provided 2a with a Mn of
3400 gmolÀ1 and a PDI of 2.04 in 40% yield (Table 1,
entry 1). Yield and Mn were increased upon increasing
polymerization time to 24 h (entries 2–6). Polymerization
did not occur at 08C (entry 7), and reactions were best
conducted at a concentration of 0.5m (entries 8 and 9).
Interestingly, the highest Mn (19600 gmolÀ1) and yield (81%)
resulted in the absence of TEMPO (entry 10). We assume that
traces of dioxygen acted as oxidant to initiate the polymer-
ization in this case, which further reduces the cost of the
process.
However, the TEMPO initiation method has also benefits,
as varying the initiator amount allowed for a rough adjust-
ment of the molecular weight of 2a. The largest polymers
were obtained with 10 mol% of TEMPO, and at higher
initiator loading, Mn decreased (entries 11–16). The non-
linearity of the amount of added initiator with respect to Mn
indicates that the SRN1 polymerization is not a living process.
Most likely, the reaction of the polymeric aryl radical with the
anionic monomer 1a is faster than initiation. The anionic
leaving group influences reaction outcome, as the Br deriv-
ative 1b as monomer delivered a moderate result (entry 17),
and also the countercation influences polymerization. The Li
(Li-1a) and Zn derivatives (Zn-1a) afforded worse results
(see the Supporting Information). In contrast to anionic
polymerizations, where moisture has to be carefully excluded
from the reaction mixture, the radical/anionic SRN1 polymer-
ization is not sensitive in that regard, as hydrolysis of the
monomer leads to an inactive aryliodide that does not
influence the reaction outcome.
Table 1: Polymerization of Mg compound 1a under different conditions.
Entry
TEMPO
[mol%]
Conc
t
[h]
Yield
[%]
Mn
[gmolÀ1 [a]
PDI
[molLÀ1
]
]
1
2
3
4
5
6
5
5
5
5
5
5
5
5
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.0
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
2
3
40
46
42
66
66
66
–
68
50
81
60
70
59
54
50
26
59
3400
5700
6600
2.04
2.20
1.87
2.16
1.83
1.95
–
1.89
2.04
1.74
1.97
1.80
1.68
1.61
1.60
1.60
2.23
5
8800
15
24
24
24
24
24
24
24
24
24
24
24
24
11700
12000
–
5500
9300
19600
11300
13700
13300
10400
10300
6500
7[b]
8
9
10
11
12
12
14
15
16
17[c]
–
The high regioregularity of the poly(m-phenylene) syn-
thesis was confirmed by 13C NMR analysis (Figure 2), which
clearly shows the meta connectivity of the arene moieties
(only four resonances for the arene C atoms), and therefore
a mechanism where polymerization occurs by an anionic
process through aryne intermediates[15] can be unambiguously
excluded.
7.5
10
20
30
40
50
10
9400
[a] Polystyrene standard was used for GPC analysis. [b] Reaction at 08C.
[c] Conducted with 1b (X=Br) to give 2b (X=Br, R=H, Br, iPr).
Scheme 2. Initiation by oxidative homocoupling of 1a.
Polymerizations were conducted in tetrahydrofuran
(THF), and TEMPO was used as oxidant in most experiments
and was added at À408C. We noted that initiation occurred
upon warming the reaction mixture to room temperature.
Initial polymerizations were performed with 5 mol%
TEMPO (0.5m), and polymer 2a was isolated by repeated
precipitation. Analysis was conducted by NMR spectroscopy,
mass spectrometry, and gel permeation chromatography
(GPC).
Figure 2. 13C NMR spectrum of 2a (75 MHz, CDCl3, RT;
Mn =19600 gmolÀ1, PDI=1.74).
Absorption and emission spectra of 2a showing the typical
curves for this substance class can be found in the Supporting
Information. As analyzed by mass spectrometry (MALDI,
matrix-assisted laser desorption), synthetic 2a contains three
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series of poly(m-phenylenes) that differ in the termination
moiety. iPr (2a-iPr), H (2a-H), and I (2a-I) were identified as
terminating substituents, and all polymers contain at least one
iodine atom (see the structure of 2a in Table 1). Assuming
that the three different polymer types are ionized to the same
extent, integration of the peaks of the individual series in the
MALDI spectra allows for estimation of the relative ratio of
the three series (see the Supporting Information, MALDI
analysis was conducted for Mn < 10000 gmolÀ1). We found
that for the lower molecular weight polymers (region 1500 to
3000 gmolÀ1), the ratio of 2a-iPr to 2a-H to 2a-I-termination
is 21:27:52. For medium-sized poly(m-phenylene) (5000 to
6800 gmolÀ1) we noted a change in the relative abundance
with a lowering of the “iPr-fraction” (2a-iPr/2a-H/2a-I =
11:29:60) and the largest series (7500 to 10000 gmolÀ1
)
showed an even lower fraction of the iPr-terminated poly-
mers, while the relative amount of H-terminated macro-
molecules remained nearly constant throughout all mass
regions (2a-iPr/2a-H/2a-I = 7:27:66). The source of H-termi-
nated polymers is not quite clear to us at this moment, as
mass-spectrometric analysis shows no deuterium incorpora-
tion either by performing the polymerization process in
deuterated THF or/and by quenching the reaction with D2O.
Other solvents, such as Et2O and C6H5CF3, provided worse
results. The I-terminated series were either formed by
oxidation of the polymeric radical anion, by dimerization of
two polymeric aryl radicals, or by I-abstraction of the
polymeric aryl radical from isopropyl iodide (isopropyl
iodide was formed as byproduct in the initial I–Mg exchange
reaction). iPr-terminated poly(m-phenylenes) most likely
derived from reaction of the polymeric aryl radical with
unreacted iPrMgX.
As all of the polymers contain at least one iodine
substituent, the molecular weight of 2a (Mn = 13400 gmolÀ1,
PDI = 1.70) could be further increased by transforming the
polymer to the corresponding magnesiated 2a-MgCl followed
by oxidative homocoupling with TEMPO[12,18] to give H-
terminated poly(m-phenylene) in a quantitative yield with
Mn = 21300 gmolÀ1 and PDI = 1.50 (see the Supporting
Information). Moreover, end-group modification can be
achieved by transforming the polymer 2a (Mn =
5700 gmolÀ1, PDI = 2.20) into the corresponding magnesiated
2a-MgCl, followed by oxidative cross-coupling[19] with 3 in the
presence of TEMPO to give 4 (Mn = 6500 gmolÀ1, PDI =
1.90).[20] Alkoxyamine 4 was then successfully used as
a macroinitiator for the nitroxide-mediated polymerization
(NMP)[21,22] of styrene to give poly(m-phenylene)-block-poly-
(styrene) copolymer 5, which was isolated as a white solid
(30%, 80700 gmolÀ1, PDI = 1.23, Scheme 3).[23]
Scheme 3. Synthesis of block copolymer 5 by end-group modification
of 2a and subsequent NMP of styrene.
Table 2: End-capping during polymerization.
Entry
3
Yield
[%]
Mn
[gmolÀ1 [a]
PDI
Ratio
2a/4-H/4-iPr/4-I/6
[mol%]
]
1
2
3
4
5
6
7
8
5
10
15
20
25
30
40
50
72
67
72
65
73
70
72
71
16000
11700
11700
8400
7700
8700
1.62
1.56
1.40
1.35
1.28
1.29
1.26
1.24
73:03:05:12:08[b]
52:08:07:18:14[b]
21:13:09:16:41[b]
22:00:00:25:53[b]
09:00:00:19:72[c]
09:00:00:20:71[c]
00:00:00:13:87
00:00:00:11:89
6100
4400
[a] Polystyrene standard was used for GPC analysis. [b] 2a contains 2a-
H, 2a-iPr, and 2a-I (ratios are given in the Supporting Information).
[c] For the 2a series, only 2a-I was detected by MALDI-MS.
Supporting Information).[24] With 5 mol% 3, a slight reduc-
tion of Mn was noted, and around 25% of the polymers
contained the alkoxyamine moiety derived from 3 as the end
group (Table 2, entry 1). By increasing the amount of the end-
capping reagent 3, a decrease of Mn was achieved (entries 2–
8). Thus, this approach offers an alternative for rough
adjustment of the molecular weight. Importantly, upon
increasing the concentration of 3, a gradual decrease of
formation of polymers of type 2a was observed and at higher
loadings of 3, 2a was not identified. The only polymers
formed under these conditions were 4-I and 6. Overall yield
remained nearly constant in these polymerizations, and the
PDI gradually decreased upon increasing the loading of 3.
To study the scope of the novel polyarene synthesis, other
monomers were then tested. Under optimized conditions, the
more electron-rich poly(m-phenylene) 7 was successfully
Along with the possibility to chemically postmodify the
iodinated oligoarene, we found that end-group tailoring can
also be achieved by polymerizing the anionic monomer in the
presence of an additional aryl Grignard derivative lacking the
anionic leaving group. This was shown by polymerizing
monomer 1a in the presence of varying amounts (5 to
50 mol%) of alkoxyamine 3 using the air initiation procedure
(Table 2). End-group analysis was performed with MALDI-
MS as discussed above. We looked at a Mn range between
2500 and 4500 gmolÀ1 for these MS investigations (see the
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prepared from the corresponding iodoaryl magnesium deriv-
ative (Scheme 4). The method also allowed the synthesis of
the chiral poly(m-phenylene) 8,[16,17,25] and random copoly-
heating the reaction mixture to 708C and poly(p-phenylene)
10 was obtained in 41% yield with Mn of 4100 gmolÀ1.
Initiation with TEMPO was less efficient in that case.[26] We
were pleased to find that highly fluorescent poly(naphtha-
lene)[27] 11 was successfully prepared by this novel method.
In conclusion, we have introduced a novel method for
preparation of polyarenes. The process, which occurs by an
alternating radical/anionic chain growth polymerization, does
not require any transition metal. Initiation can be achieved
either by traces of air or upon adding the commercially
available TEMPO radical. Polymerizations are experimen-
tally easy to conduct. In contrast to known anionic chain-
growth polymerizations, strict exclusion of moisture is not
necessary to run these reactions. The process has been
successfully applied to the preparation of poly(m-phenyl-
enes), poly(p-phenylenes), and a polynaphthalene, which are
interesting compounds in materials science, documenting the
potential of the novel approach. The polyarenes thus obtained
contain an iododaryl moiety at one terminus of the polymer
chain. This functional chain end is readily chemically further
transformed, as documented by successful metalation of the
polyarene with subsequent oxidative homo and cross-cou-
pling. Furthermore, end-group tailoring can be achieved upon
polymerizing the anionic monomer in the presence of an aryl
Grignard reagent, which lacks the ionic leaving group
(capping reagent). Mn and PDI of the resulting polyarenes
decrease upon increasing the concentration of the capping
reagent. At higher concentration of the capping reagent,
complete capping was achieved. Future studies will be
devoted to identify an improved initiation method and to
further extending the anionic/radical SRN1-type polymeri-
zation process towards the use of other anionic haloarenes as
monomers.
Received: July 30, 2012
Revised: September 19, 2012
Published online: October 29, 2012
Scheme 4. Poly(arenes) 7–11 prepared by SRN1-type polymerization.
Keywords: chain-growth polymerization · Grignard reagents ·
.
oligoarenes · radical anions · TEMPO
mers consisting of m-phenylene and p-phenylene moieties
were successfully obtained (9). Interestingly, NMR analysis of
polymer 9 (see the Supporting Information) revealed that the
monomer composition (3:1) used is well-reflected in the
polymer composition. Therefore, rate constants of the reac-
tion of the two different growing aryl radicals with the two
different aryl Grignard reagents are similar. For preparation
of 7 and 8, the TEMPO method provided better results as
compared to the air-initiated processes.
Poly(p-phenylene) synthesis turned out to be more
challenging. To solubilize the polymer, we had to install an
alkyl group, and to ensure regioselective metalation we chose
an alkylated iodobromoarene as substrate. Iodine–magne-
sium exchange on 4-bromo-1-iodo-2-octylbenzene with
iPrMgCl provided the corresponding aryl magnesium deriv-
ative, which compared to 1a was far less reactive for steric
reasons. Another problem is the generally less efficient halide
elimination in bromoaryl radical anions as compared to
iodide elimination in the corresponding iodoarylradical
anions. Nevertheless, polymerization was achieved upon
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