.
Angewandte
Communications
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
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 12362 –12366