Supramolecular graft copolymers in moderately polar media based on hydrogen-bonded aromatic oligoamide units

Article abstract of DOI:10.1039/c2cc35004a

By exploiting hetero-complementary aromatic oligoamide units containing hextuple hydrogen bonds, supramolecular graft copolymers were successfully constructed in moderately polar media.

Full text of DOI:10.1039/c2cc35004a

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Supramolecular graft copolymers in moderately polar media based on
hydrogen-bonded aromatic oligoamide unitsw
Xusong Pan,a Chao Chen,a Jiang Peng,a Yongan Yang,a Yinghan Wang,a Wen Feng,*a
Pengchi Deng*b and Lihua Yuan*a
Received 12th July 2012, Accepted 2nd August 2012
DOI: 10.1039/c2cc35004a
By exploiting hetero-complementary aromatic oligoamide units
containing hextuple hydrogen bonds, supramolecular graft copolymers
were successfully constructed in moderately polar media.
Multiple hydrogen-bonded oligoamides bearing terminal
alkene groups when treated with Grubbs’ catalyst provided
covalently crosslinked zippers in almost quantitative yield, but
failed to afford polymers under the reaction conditions.13
In this report we demonstrate the first construction of supramo-
lecular graft copolymers based on main-chain insertion of hextuple
hydrogen-bonded aromatic oligoamide units that are of high
fidelity and strength in a moderately polar media (Scheme 1).
We initiated our investigation by polymerizing the olefin-
terminated strand 1 (Scheme S4, ESIw) arrayed in the DADDAD
sequence via acyclic diene metathesis (ADMET) polymeriza-
tion14 to obtain the polymer main-chain P1. Monomer 1 was
treated with 1st or 2nd generation Grubbs’ ruthenium catalyst
(10 mol%) in dry toluene at 55 1C for 48 h (Table S1, ESIw).
Under reduced pressure, 1 converted to P1 in only ca. 80%
using 1st generation Grubbs’ ruthenium catalyst. However,
the complete conversion of monomer was observed employing
the 2nd generation catalyst, giving the degree of polymeriza-
tion (DP) of 17. Further enhancement of DP up to 35 was
achieved by using the supramolecular protecting strategy15
(Scheme S5 and Table S1, ESIw) in 2 : 1 molar ratio of 1 and
protecting agent 2a in the ADAADA sequence that is speci-
fically complementary to 1 (B109 MÀ1 in CHCl3). The large
discrepancy in DP with and without 2a is attributed mainly to
completely preventing coordination of oxygens of monomer 1
to ruthenium,6 leading to further chain propagation. In order to
investigate the graft behaviour of P1, the oligoamide-terminated
Multiple hydrogen bonding, along with other non-covalent
forces such as solvophobic,1 metal-coordination2 and ionic
interactions,3 endows supramolecular architectures with high
strength and order due to its directionality, complementarity and
specificity. Supramolecular polymers constructed based on multi-
ple (number of hydrogen bonds >3) hydrogen bonds hold the
high potential for the advanced dynamic, reversible, and self-
healing materials.4,5 So far, few examples6 of supramolecular graft
copolymers based on main-chain insertion of multiple hydrogen
bonding units that are able to pair specifically with complementary
units have been reported. In the meanwhile, the sensitivity in polar
surroundings encoded in hydrogen bonding also constitutes a
hurdle for hydrogen bonding mediated association.7 So far hydro-
gen bonds have been exploited alone,8 or in association with other
non-covalent1,3 and covalent9 interactions to increase the stability
of molecular assemblies in a polar environment. Several examples
were known to involve in forming hydrogen-bonded supramole-
cular polymers in polar solvent.1b–d,8 However, construction of
supramolecular graft copolymers using hydrogen bonding recog-
nition units in polar media still represents a challenging task.
Among various recognition units based on hydrogen bonding
interaction,10 oligoamide strands are a class of association units
with the preorganized backbones containing multiple amide
oxygen and hydrogen atoms that could associate into duplexes
via intermolecular hydrogen bonding.11 Recently, the hierarchical
self-assembly of the molecular duplex was demonstrated by
gelation of solvents underlining the essential function of p–p
stacking interactions in forming an extended network of fibers.12
a College of Chemistry, Key Laboratory for Radiation Physics and
Technology of Ministry of Education, Institute of Nuclear Science
and Technology, State Key Laboratory of Polymer Materials
Engineering, College of Polymer Science and Engineering, Sichuan
University, Chengdu 610064, China. E-mail: lhyuan@scu.edu.cn,
wfeng9510@scu.edu.cn; Fax: +86-28-85418755;
Tel: +86-28-85416996
b Analytical & Testing Center of Sichuan University, Sichuan
University, Chengdu 610064, China. E-mail: pcdeng@yahoo.com
w Electronic supplementary information (ESI) available: Synthesis,
analytical data, calculation of association constant, GPC data, UV
data and 2D-DOSY data. See DOI: 10.1039/c2cc35004a
Scheme 1 Schematic representation of supramolecular graft copolymers
based on hydrogen-bonded aromatic oligoamide units.
c
9510 Chem. Commun., 2012, 48, 9510–9512
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poly(ethylene glycol) 2b and 2c with different lengths of PEG
chains were obtained via click coupling reaction between
alkyne-containing oligoamide and azido-terminated PEG
chains (Scheme S3, ESIw).
The formation of hydrogen-bonded graft copolymers was
1
firstly examined by H NMR technique in moderately polar
Table 1 Association constants (MÀ1) in chloroform–DMF mixturesa
Solventb
Association constants
Compounds
20%
30%
40%
2.5 Â 104
3.1 Â 103
1.5 Â 102
1Á2a
20%
30%
40%
1.5 Â 105
3.2 Â 104
0.9 Â 103
media. In CDCl3–DMF-d7 (3 : 2, v/v), P1 or 2a exhibits
distinct sharp signals over the region of aromatic and amide
protons; however, the signals of P1 turn broad upon gradual
addition of 2a to P1 along with downfield shifts of amide
protons Hd (0.27 ppm) and Hc (0.41 ppm) and upfield shifts of
aromatic protons Ha (0.27 ppm) and Hf (0.17 ppm) (Fig. S2,
ESIw). Two-dimensional NOESY provided direct evidence
for the interaction between P1 and 2a in CDCl3–DMF-d7
(3 : 2, v/v). This is indicated by the presence of cross-strand
NOEs between protons P1-Hd and 2a-Hm, P1-Hd and 2a-Hh,
P1-Hi and 2a-Ha, P1-Hb and 2a-Ha, P1-Hh and 2a-Hc (Fig. S1,
ESIw). These results suggest the occurrence of binding of 2a
with recognition units of P1 along the main chain.
P1Á2a
20%
30%
40%
1.1 Â 105
2.0 Â 104
0.6 Â 103
P1Á2b
20%
30%
40%
0.7 Â 105
1.7 Â 104
2.8 Â 102
P1Á2c
a
Association constants were determined via 1H NMR titration and an
average value from three experiments was obtained at 298 K, with an
b
error of less than 15%. Percentage of DMF-d7 in the mixed solvent
of DMF-d7–CDCl3.
To better demonstrate the formation of hydrogen-bonded
graft copolymers, gel permeation chromatography (GPC)
P1Á2b, P1Á2c and 1Á2a based on the 1H NMR titration experi-
ments (Table 1). For example, the Ka value of P1Á2a was found
to be 1.5 Â 105 MÀ1 in CDCl3–DMF-d7 (4 : 1, v/v), while that
of 1Á2a was determined to be 2.5 Â 104 MÀ1 in the same mixed
solvent, a difference of around one order of magnitude. For
P1Á2b and P1Á2c grafted by oligoamide-terminated PEG
chains, the Ka values are also larger than that of 1Á2a. This
enhancement of binding strength in the graft copolymers over
the monomeric dimer holds true within the range of varying
DMF percentage examined. We ascribe the added stability in
moderately polar media to the solvophobic double-stranded
oligoamide backbone that offers additional aggregation force
and, at the same time, protects hydrogen bonds from polar
solvent molecules that are strongly competitive for hydrogen
bonding (Fig. 1b). The cooperative action of solvophobic
interaction and hydrogen bonding is known to be responsible
for stabilization of intermolecular and/or intramolecular aggre-
gation in polar media.1 Furthermore, the decrease of the binding
strength from P1Á2a to P1Á2c is mainly attributed to an increase
in steric crowding upon increasing the length of the PEG chain16
and competitive hydrogen bonding between recognition units
and PEG chains.17
experiments were performed using
a mixed solvent of
CHCl3–DMF (1 : 1, v/v) as eluent at 30 1C. For the polymer
main chain P1, the number-average molecular weight (Mn) was
determined to be 3.0 Â 104. In contrast, Mn of P1Á2a increased
with increment of the content of 2a, reaching 4.8 Â 104 with two
equivalents of 2a to P1 based on recognition units (Table S2,
ESIw). Also, excess 2a brought extra Mn increment highlighting
the dynamic feature of the non-covalent bond in moderately
polar media. Increasing the temperature from 30 to 50 1C led to
the decline of Mn as shown in temperature-dependent GPC
profiles, indicating decreased stability of the copolymer at higher
temperature (Table S5, ESIw). Furthermore, under the same
conditions, Mn of P1Á2b and P1Á2c exhibited a similar increment
with respect to P1, indicating the successful formation of
H-bonded graft copolymers (Table S3 and S4, ESIw). The above
results contrast sharply with the fact that a mixture of P1 and 1
arrayed in the same sequence fails to exhibit any association in
GPC, indicating the binding specificity of these supramolecular
graft copolymers (Fig. S7, ESIw).
Graft behaviour of main chain P1 was further investigated
1
by two-dimensional diffusion ordered H NMR spectroscopy
(2D-DOSY). As expected for small molecules, 2a, 2b and 2c
display high diffusion constants (6.45 Â 10À10, 5.41 Â 10À10
and 5.04 Â 10À10 m2 sÀ1, respectively), while P1 offers a low
diffusion coefficient of 1.27 Â 10À10 m2 sÀ1, giving an experi-
mental hydrodynamic radius (Rh) of 26.6 A in CDCl3–DMF-d7
(3 : 2, v/v). Graft copolymer P1Á2a has a smaller diffusion
The enhanced stabilizing effect by both hydrogen bonding
and solvophobic aggregation was also revealed by variable-
1
polarity H NMR experiments in CDCl3–DMF-d7 (Fig. 1a).
In the first stage with DMF below 40%, signals of aromatic
and amide protons P1-Hc and 2a-Hb/2a-Hd become considerably
broadened, which is in sharp contrast to the change of the
corresponding proton signals of 1Á2a (Fig. S16, ESIw). This
reflects that the solvophobic aggregation is strengthened with
increasing polarity and hence intermolecular hydrogen bonding
is shielded more efficiently than that in complex 1Á2a (Fig. 1b).
In the second stage with DMF above 40%, signals of protons
P1-Hd and 2a-Ha/2a-Hc shift downfield and grow sharper
along with all other signals, which is in the opposite direction
compared to the change in the first stage. This could be
explained by the predominance of polar solvent molecules in
solution that start to disrupt intermolecular hydrogen bonding and
result in the collapse of the regional large p–p stacking surface of
coefficient of 1.05 Â 10À10 m2
s
À1, corresponding to a larger
Rh of 32.2 A. Furthermore, when 2b or 2c bearing the PEG chain
was mixed with P1, an even smaller diffusion coefficient (0.97 Â
10À10 or 0.87 Â 10À10 m2 sÀ1, respectively) and larger Rh (34.9 A
or 38.9 A, respectively) were obtained. These results indicate
efficient grafting of the polymer main chain with oligoamide-
terminated PEG chains (Table S6, ESIw).
The enhanced binding strength of the side moieties grafted
along the polymer main chain in moderately polar media
(up to 40% DMF in chloroform) was demonstrated by comparing
the association constants (Ka) of recognition units of P1Á2a,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9510–9512 9511

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The authors acknowledge the National Natural Science
Foundation of China (20874062), the Doctoral Program of
the Ministry of Education of China (20090181110047) and
Open Project of State Key Laboratory of Supramolecular
Structure and Materials (SKLSSM201112) for funding
this work, and the Analytical & Testing Center of Sichuan
University for NMR analysis.
Notes and references
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Fig. 1 (a) Stacked partial 1H NMR spectra of the mixture of P1 and
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to loss of shielding capability for hydrogen bonding. Since 1Á2a is
totally disassociated in DMF above 40% (Fig. S16, ESIw), whereas
P1Á2a is subjected to full disassociation only beyond ca. 60%, it is
inferred that the intensified aggregation caused by solvophobic
interaction should be accountable for the added stability of the
hydrogen-bonded graft copolymers in moderately polar media. In
addition, the UV-vis spectra of P1 in CHCl3–DMF (3 : 2, v/v)
(10À5 M) contains a new broad peak around 310 nm at 30 1C with
increasing intensity upon gradual addition of 2a (Fig. S29, ESIw).
This result provides additional evidence for solvophobic aggrega-
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hydrogen bonding which offers a larger p–p stacking surface, and
solvophobic aggregation in a moderately polar environment. As far
as we know, no examples of hydrogen-bonded graft copolymers in
organic polar media have been reported. Further derivatization of
these polymers may provide opportunities for constructing a variety
of polymeric architectures with enhanced stability in polar or
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9512 Chem. Commun., 2012, 48, 9510–9512