Unsymmetrical polymeric ladderphanes by sequential polymerization: A new approach for the template synthesis of polymers with well-defined chain length and narrow polydispersity

Article abstract of DOI:10.1002/asia.201000877

Nothing ROMP with that: Unsymmetrical double-stranded ladderphanes are obtained by sequential ring-opening metathesis polymerization and Glaser oxidation of norbornene appended with bisalkyne moieties. Hydrolysis of these ladderphanes gives substituted poly(m-phenylene butadiynylene)s with narrow polymer dispersity index (PDI) and well-controlled degree of polymerization.

Full text of DOI:10.1002/asia.201000877

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DOI: 10.1002/asia.201000877
Unsymmetrical Polymeric Ladderphanes by Sequential Polymerization:
A New Approach for the Template Synthesis of Polymers with Well-Defined
Chain Length and Narrow Polydispersity
Yuan-Zhen Ke,[a, b] Shern-Long Lee,[b] Chun-hsien Chen,*[b] and Tien-Yau Luh*[b]
On the occasion of the 150th anniversary of the Department of Chemistry, The University of Tokyo
There is burgeoning interest in the design and synthesis of
double-stranded (ds) polymers mimicking DNA chemis-
try.[1–9] Most approaches towards unsymmetrical double-
stranded scaffolds are based on the independent syntheses
of two complementary strands, which can then furnish the
double-helical motif through hydrogen bonding.[7,8] Cova-
lently bonded double-stranded polymeric ladderphanes 1[5]
are readily available from the ring-opening metathesis poly-
merization (ROMP)[10] of the corresponding bisnorbornene
derivatives 2 by using the first-generation Grubbs catalyst
3[11] [Equation (1)]. The relatively rigid backbone of poly-
norbornenes, such as those in 5 and 6, offers a supporting
framework for the pending groups coherently aligned to-
wards the same direction.[5,9,12] In addition, the presence of
endo-N-aryl-pyrrolidene-fused moieties in these polymers is
crucial for the stereoselective formation of the comb-like
polymers with isotactic stereochemistry and essentially all
double bonds in the trans configuration when 3 is used as
the catalyst.[12] Replication of a polynorbornene that leads
to a daughter polynorbornene has thus briefly been ex-
plored, and the unsymmetrical double-stranded polybisnor-
bornene 4 is obtained as the intermediate.[9,13] It is envisaged
that an unsymmetrical double-stranded polymer may be
[a] Y.-Z. Ke
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (China)
[b] Y.-Z. Ke, Dr. S.-L. Lee, Prof. C.-h. Chen, Prof. T.-Y. Luh
Department of Chemistry
National Taiwan University
Taipei, Taiwan 106
Fax : (+886)2-2364-4971
E-mail: tyluh@ntu.edu.tw
chhchen@ntu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000877.
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Chem. Asian J. 2011, 6, 1748 – 1751

Table 1. Degree of polymerization of 8a–c, 9a–c, and 10a–d.
formed from sequential polymerizations of a monomer con-
taining two different polymerizable groups, which can un-
dergo polymerization under different reaction conditions.
Sequential polymerizations are well documented for the syn-
thesis of block copolymers.[14] To the best of our knowledge,
the synthesis of unsymmetrical double-stranded polymers
based on this strategy has not been explored. Herein, we
report the unprecedented example on the sequential poly-
merizations of 7 in which the norbornene moiety first under-
goes ROMP to give 8; the two alkynyl groups undergo a
Glaser oxidative coupling reaction[15] following in situ desily-
lation (Scheme 1).
Substrate
Mn
PDI
DP[a]
DP’[a]
F[b]
8a
8b
8c
9a
9b
9c
10a
10b
10c
10d
12
8,600
10,900
14,300
6,70
7,600
10,500
4,600
5,500
6,900
20,000
–
1.15
1.13
1.18
1.18
1.26
1.21
1.17
1.20
1.17
2.66
–
11
14
18
10
12
16
11
14
17
50
–
10
13
17
–
–
–
10
13
17
40
–
0.07
0.06
0.06
0.03
0.04
0.04
0.17
0.18
0.15
–
0.008
[a] Degree of polymerization (DP: by GPC; DP’: by 1H NMR; see:
Ref. [17]. [b] Quantum yields (F) were measured in EtOAc by using cou-
marin I as the reference (F=0.99).
NMR spectra of 10 exhibited characteristic signals due to
1
the terminal groups (d 3.28 ppm in H NMR and d 82.5 and
79.8 ppm in 13C NMR for terminal alkynyl moieties, and d
7.48 ppm in 1H NMR and
d 163.1 and 137.1 ppm in
13C NMR for aryl protons and carbons at end groups, re-
spectively).[17,18] As shown in Table 1, the degrees of poly-
merization (DP’) of 10a–c, based on end-group analyses by
1H NMR spectra, were consistent with the DP obtained by
gel permeation chromatography (GPC), and the polymer
dispersity indices (PDIs) for 10 were comparable to those of
8 and 9. These results suggest that this protocol offers a gen-
eral route for the selective synthesis of poly(arylene butadiy-
nylene) 10 with narrow PDI and controlled chain lengths.
On the other hand, direct polymerization of 11 by the
Glaser reaction gave 10d with DP’=40 and PDI=2.6.
Polymers 9 show characteristic absorptions due to the
aminobenzoate (320 nm) and the substituted diarylbuta-
diyne chromophores (Figure 1a).[17] As shown in Figure 1b,
the emission of 9a in dichloromethane exhibited, in addition
to the intrinsic emission attributed to the substituted
poly(m-arylene-butadiynylene) strand at 391 nm, a broad
charge-transfer band around 478 nm. On the other hand,
polymer 10a exhibited emission at 391 nm, but no peak
around 478 nm was observed. It is known that an aminoben-
zoate moiety such as that in 7 exhibits typical charge-trans-
fer emission at 460 nm.[19] This emission shifted to shorter
wavelength at 434 nm in 8a; this arises from interactions be-
tween the adjacent chromophores, which is typical in related
aminobenzoate-appended polynorbornenes.[12] It is notewor-
thy that the fluorescence profiles of 9a were solvent depen-
dent (Figure 1c). No emission at 478 nm was observed when
cyclohexane was employed and the emission at 340 nm
arose from the aminobenzoate moiety. When the emission
spectrum was recorded in chloroform, neither emission at
340 nm nor at 478 nm was detected. In solvents of higher
polarity such as ethyl acetate, or dichloromethane/acetoni-
trile mixed solvents, the peak at 478 nm emerged as the
prominent emission. These results indicate that the emission
at 478 nm would be different to that of the simple charge-
transfer state of the aminobenzoate moiety. The HOMO
and LUMO energies of the model monomer 12 were esti-
Scheme 1. Synthesis of unsymmetrical ladderphanes 9. a) 3, CH2Cl2,
90%; b) Cu
thylsilyl.
ACHTUNGTRENNUNG(OAc)2, TBAF, pyridine, CH2Cl2, 76% yield. TMS=trime-
Polymerization of 7 in the presence of a catalytic amount
of 3[11] afforded the corresponding diethynyl benzene-ap-
pended polynorbornenes 8a–c (Scheme 1). As ROMP of a
cycloalkene catalyzed by 3 is a living polymerization,[10] 8a–
c of different degrees of polymerization were obtained in
90% yield when different amounts of 3 were used. The re-
sults are summarized in Table 1. Further Glaser oxidation of
8a–c in the presence of tetrabutylammonium fluoride
(TBAF) afforded the corresponding unsymmetrical poly-
meric ladderphanes 9a–c in 76–80% yields (Table 1). It is
noteworthy that the sequence of the addition appeared to
be crucial for this reaction.[16]
Hydrolysis of 9a–c with 10% NaOH in tetrahydrofuran
(THF), dichloromethane, and methanol (2:1:1) in the pres-
ence of Bu4NBr gave 10a–c in 74–77% yields (Table 1). The
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T.-Y. Luh, C.-H. Chen et al.
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Figure 2. a) An STM image of 9a on highly oriented pyrolytic graphite
(HOPG). The sample was dissolved in phenyloctane and deposited on
HOPG. Conditions: Ebias, 0.70 V; itunneling, 40 pA; image size: 22 nmꢁ
22 nm. b) Cartoon expression of linear arrays of the donor–acceptor pairs
of 9.
right side of the stripes. Presumably, the conjugated oligo-
meric diarylbutadiyne moieties in 9 would have better con-
tact with the graphite surface and result in brighter features,
whereas the polynorbornene backbone in 9 may appear
dimmer. As described above, 9 may be considered as a
linear array of the donor–acceptor pairs along the polymeric
backbones. The morphology shown in Figure 2 is consistent
with this property and it seems likely that the donor arrays
and acceptor arrays are arranged in an alternative manner
on the graphite surface.
In summary, we have demonstrated the first synthesis of
unsymmetrical double-stranded polymeric ladderphanes 9
by sequential polymerization of 7. This protocol has also fur-
nished a new template synthesis of polymers such as 10. The
key to the success relies on the well-controlled degree of
polymerization and PDI of 9 by the stereoselective ROMP
of the norbornene derivative 7. Our results can pave the
way to make predictions in other opportunities for new
types of double-stranded ladderphanes. Moreover, the un-
usual photophysical properties of 9 offers an unprecedented
platform for designing novel supramolecular scaffolds,
which constitute well aligned arrays of donor–acceptor pairs.
The scanning tunneling microscopy (STM) results offer ad-
ditional evidence for such assembly. As self-assembled bi-
Figure 1. a) Absorption and b) emission spectra of
7 ((g), lex =
320 nm), 8a ((d), lex =318 nm), 9a ((c), lex =354 nm), and 10a
((a), lex =355 nm) in CH2Cl2. With the exception of 10a (5ꢁ10À6 m),
the concentration for other samples were 4ꢁ10À5 m). The molar extinc-
tion coefficients were calculated on the basis of the molecular weight of
the corresponding monomeric unit. c) Fluorescence spectra of 9a excited
at 320 nm in cyclohexane (·····), CHCl3 (a), EtOAc (g), CH2Cl2
(d), CH2Cl2/CH3CN (1:2 (l)), and CH2Cl2/CH3CN (1:9 (c)), nor-
malized around 390 nm.
mated to be À5.74 and À2.38 eV, whereas the HOMO of 4-
aminobenzoate derivatives was approximately À5.3 eV.[19] It
seems likely that photoinduced electron transfer might take
place between the aminobenzoate moiety and the substitut-
ed poly(m-arylene butadiynylene) in 9.
Like other N-aryl-pyrrolidine-fused single- and double-
stranded polynorbornenes,[5,9,12d]
9 also exhibited well-
aligned ordered images on a graphite surface (Figure 2). It
is interesting to note that the left side of each of the poly-
meric stripes shown in Figure 2 appeared brighter than the
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Chem. Asian J. 2011, 6, 1748 – 1751

Template Synthesis of Polymers
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Acknowledgements
This work is supported by the Natural Science Council of Taiwan, the
National Taiwan University, (in part) the National Science Foundation of
China and Shanghai Institute of Organic Chemistry.
Keywords: electron transfer · glaser oxidation · hydrolysis ·
ladderphanes · ring-opening polymerization
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Received: December 8, 2010
Published online: February 22, 2011
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