Formation of a miscible supramolecular polymer blend through self-assembly mediated by a quadruply hydrogen-bonded heterocomplex

Article abstract of DOI:10.1021/ja0631854

A supramolecular network polymer consisting of a pair of immiscible polymers, poly(butyl)-methacrylate (PBMA) and polystyrene (PS), is described. A urea of guanosine (1, UG) and 2,7-diamido-1,8-naphthyridine (2, DAN), which form an exceptionally strong qu

Full text of DOI:10.1021/ja0631854

Published on Web 08/11/2006
Formation of a Miscible Supramolecular Polymer Blend
through Self-Assembly Mediated by a Quadruply
Hydrogen-Bonded Heterocomplex
Taiho Park and Steven C. Zimmerman*
Contribution from the Department of Chemistry, Roger Adams Laboratory,
600 S. Mathews AVenue, UniVersity of Illinois, Urbana, Illinois 61801
Received May 6, 2006; E-mail: sczimmer@uiuc.edu
Abstract: A supramolecular network polymer consisting of a pair of immiscible polymers, poly(butyl)-
methacrylate (PBMA) and polystyrene (PS), is described. A urea of guanosine (1, UG) and 2,7-diamido-
1,8-naphthyridine (2, DAN), which form an exceptionally strong quadruply hydrogen-bonding complex, are
displayed at 1-10 mol % along the main backbone of PBMA and PS, respectively. 1H NMR studies show
heterocomplexation between UG and DAN exclusively. This high-fidelity, high-affinity supramolecular
connection of two different polymer coils at the molecular level produces a polymer blend. Blends containing
different weight ratios of the polymers and mole percent of the recognition units were characterized by
AFM and DSC experiments with no isolated domains observed and a single glass-transition temperature
(Tg). The Tg is tunable by varying the weight ratio of the polymers in the blend. In addition, viscosity
measurements, size-exclusion chromatography (SEC), and dynamic light-scattering (DLS) studies
demonstrate the formation of a supramolecular network structure.
Self-assembly mediated by hydrogen bonding is a well-
documented approach to creating structures with nanometer
dimensions.1 Beyond the efficiency that such a building block
approach affords in the production of oligomeric or polymeric
structures, there are significant practical consequences as a result
of the suprastructures being formed reversibly. The ability to
form and break bonds in response to temperature or stress can
confer properties that are not achievable in covalently bonded
systems. Specific examples of unique macroscopic properties
arising in such reversible supramolecular polymers have been
reported recently. In particular, the self-associating unit 2-ureido-
4[1H]-pyrimidinone2 has been displayed along polymer back-
bones3 or on their ends4 to produce more complex polymeric
structures.
of the polymers, including the use of compatibilizers such as
block or grafted copolymers.6 Another approach has been the
introduction of reactive groups to covalently connect individual
polymers within the blend.7 The use of hydrogen bonding to
improve interfacial properties and favor blending is particularly
appealing because of the potential for relatively low process
temperature (reversibility), and because it may lead to mixing
at the molecular level.8 To achieve desired blends, monomers
possessing weak hydrogen-bonding moieties such as hydroxyl,
carboxyl, pyridyl, and amino groups have been employed. This
method has been limited to only a few polymer pairs; the weak
intermolecular contacts means that a high mole percent of
(4) (a) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.;
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New materials with tunable properties have also been
prepared through the formulation of two or more immiscible
polymers. Indeed, polymer blends have been a central topic in
material science.5 Numerous efforts have been made to minimize
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J. AM. CHEM. SOC. 2006, 128, 11582-11590
10.1021/ja0631854 CCC: $33.50 © 2006 American Chemical Society

Miscible Supramolecular Polymer Blend
A R T I C L E S
provides a significant challenge for molecular recognition-driven
blend formation. Beyond determining if the highly stable UG‚
DAN heterocomplex could drive the formation of a miscible
and reversible supramolecular network structure, it was of
interest to see if the properties of such a blend might be tuned
by varying the mole percent of the recognition units and the
relative amounts of the two polymers within the mixture.
Specifically, we describe the synthesis of UG-incorporated
PBMA (UG-PBMA) 3 and DAN-incorporated PS (DAN-PS) 4
and detailed studies of the resulting suprastructure, its revers-
ibility, the miscibility of the normally immiscible polymer pair,
and the ability to tune the properties of the polymer blend.
Figure 1. Formation of a strong complex from UG and DAN.
comonomers is required. As a result, the properties of polymers
can become quite different from those lacking the hydrogen-
bonding groups. Lower mole percentage incorporation of
recognition units has successfully reduced interfacial energies
between two immiscible phases without blend formation.9
Heterocomplexes that form with high stability and fidelity
might overcome the disadvantages of these simpler modules.10
For example, even at a low mole percent incorporation of such
modules along the polymer backbones, the assembly process
would be thermodynamically driven, leading to efficient revers-
ible, intermolecular cross-linking between immiscible polymeric
chains and thus mixing at the molecular level.11 Recently, we
reported that guanosine urea 1′ (UG) weakly self-associates (Ka
≈ 200 M-1) but very strongly complexes with DAN 2, (1‚2,
Ka ≈ 5 × 107 M-1), thus representing a heterocomplex with
unparalleled affinity and pairing fidelity (see Figure 1).12 Indeed,
the free energy of the quadruple hydrogen bonding is ca. 10%
that of a C-C bond. Herein, we report polymer blends of poly-
(butyl)methacrylate (PBMA) and polystyrene (PS) driven by
appended UG and DAN recognition units. These two polymers
were chosen because of their documented immiscibility.13 Thus,
the strong repulsive interactions between PS and PBMA
Results and Discussion
Monomer Synthesis and Copolymerization. To obtain a
random copolymer, we chose UG-BMA 7 and BMA 8, because
their radical propagation sites are similar; this means that their
copolymerization reactivities (r7 ) k77/k78 and r8 ) k88/k87)
would also be similar.14 UG-PBMA 3 was prepared by radical
copolymerization4g,h of 7 and 8 in DMSO at 60 °C with AIBN,
as outlined in Scheme 1. The esterification reaction of the
carboxylated UG 512a and 6 using EDCI and DMAP activation
produced UG-BMA 7 in 84% yield. Monomer 7 was very
soluble in methanol; thus, the remaining 7 was easily removed
by precipitating the copolymer in methanol. Alternatively, UG-
PBMA 3 could be prepared from prepolymer 9 with hydroxyl
groups where the carboxylated UG 5 could be attached.
Likewise, DAN-PS 4 was obtained by the radical polymeri-
zation of styrene (St) and DAN-St 12 in DMSO at 90 °C, with
AIBN used as the initiator. DAN-St 12 was synthesized by
treating carboxylated styrene (St-CO2H) 11 with amino naph-
thyridine 1312a and EDCI. Styrene 11, in turn, was synthesized
by the substitution reaction of 4-vinylbenzyl chloride with 2
equiv of sebacic acid in the presence of K2CO3 and 18-crown-6
in DMF.
The properties of a representative group of copolymers
prepared in this study are summarized in Table 1. Mole ratios
of recognition units in a copolymer were varied by varying the
monomer feed. The molecular weights of the copolymers were
kept relatively low to minimize physical entanglement of
polymer coils in semidilute solutions. The molecular weights
and the polydispersity indices (PDI) were determined by gel
permeation chromatography (GPC) in THF with standard
polystyrene calibration. The PDI values are typical for these
types of polymerization reactions. The mole percentage of
recognition unit in 4 was estimated from the 1H NMR integration
ratio of aromatic H (6.62 ppm) and benzyl CH2 (5.05 ppm) in
chloroform-d. Vinylic hydrogens in 12, observed at 5.25, 5.75,
and 6.70 ppm, were not detected in 4 (Figure 2), indicating that
the starting monomers were completely removed. In case of 3,
DMSO-d6 was used as a result of the peak broadness observed
in chloroform-d. Two peaks at 5.97 and 5.63 ppm, assigned as
the vinylic hydrogen atoms in 7, disappeared in copolymer 3,
for which the mole percentage of recognition unit was estimated
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A R T I C L E S
Scheme 1
Park and Zimmerman
Table 1. Characterization Results of Copolymers Containing
Recognition Units UG or DAN
1
from H NMR integration ratio of aromatic H (8.00 ppm) and
OCH2 (4.22-3.82 ppm). The copolymer composition deter-
mined by NMR was in good agreement with the monomer feed
ratios. Thus, functional monomers were randomly incorporated
into the resulting copolymers, as expected from the structural
similarity of radical propagation sites in BMA 8 and UG-BMA
7, and St and DAN-St 12. The average number of functional
group (F) per chain was calculated from the experimentally
determined monomer mole ratios and the number averaged
molecular weights (Mn) of the polymers.
1H NMR Evidence for Intermolecular Heterocomplexation
of UG and DAN Units Attached to Polymers. Intermolecular
heterocomplexation of UG and DAN units was readily observed
by 1H NMR in chloroform-d (Figure 3). Unlike the very clean
spectrum of UG 1′ that weakly self-associated, that of 3a, in
which UG units were attached to PBMA, exhibited multiple
recognition
unit (F)
mol %
unit (F)b
avg no. of
F per chaind
c
c
polymera
Mn
Mw
3a
3b
4a
4b
4c
PBMA
PS
UG
UG
DAN
DAN
DAN
4
10
1
4
7
17
11
15
13
16
22
10
28
16
29
23
33
29
21
4.2
5.7
1.4
4.2
8.2
a Polymerization was carried out in DMSO at 60 °C (24 h) for 3 or at
90 °C (48 h) for 4. b Estimated from the 1H NMR spectrum. c Relative
molecular weights against polystyrene standards calculated from GPC in
THF. d Calculated from the number average molecular weight (Mn) and
mol % recognition unit of polymer.
broad peaks; these probably resulted from a slow exchange rate
between the self-associated and free UG units.12a The addition
of 4b to a solution of 3a entirely converted the broad downfield
peaks to sharp resonances similar to these seen in the inter-
molecular UG‚DAN heterocomplex. Thus, the 1H NMR signals
of a mixture of 3a‚4b with a 1:1 mole ratio shifted significantly
downfield from their position in the individual components.15
It is notable that the resonances of the amido NHs are broad
and that their chemical shift depended on the mole ratio of DAN
and UG units, indicating a fast exchange rate between free and
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Figure 2. 1H NMR of 12 and polystyrene 4c containing DAN in
chloroform-d at room temperature. Vinylic hydrogens (marked as green)
in 12 were not detected in 4c.
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Miscible Supramolecular Polymer Blend
A R T I C L E S
Figure 4. (a) Photographs of PBMA‚4c and 3b‚4c films prepared from
slow drying of a chloroform solution. (b) Tapping mode AFM images of
PBMA‚PS and 3a‚4b thin films prepared from solution-casting on mica.
However, the thin film of a mixture of 3a and 4b both having
4 mol % recognition unit, prepared in the same way as that
above, was smooth, with no features evident on the nanometer
scale. These observations are consistent with the formation of
a miscible blend driven by the DAN‚UG recognition. Although
the structure of the polymer assembly is not known, it is likely
that a supramolecular network is formed.
Figure 3. 1H NMR spectra in chloroform-d at various [UG]/[DAN] ratios
(room temperature). 3a exhibited unidentified, weak self-association of UG,
which appeared in polymeric systems containing excess UG moiety (1).
Concentration of mixtures: 3.5 g/dL for 3a‚4b, 1.5 g/dL for 3b‚4b.
Differential scanning calorimetry (DSC) was performed on
mixtures of 3b and 4c, which were prepared by manual melt
mixing on glass for ∼5 min at 120 °C. To erase any thermal
history, we took the sample through a 10 min isothermal stage
before the first cooling step. A control study was carried out
using a mixture of 3b and PS prepared identically to the 3b‚4c
mixture. The DSC of the 3b‚PS mixture exhibited two endo-
thermic relaxation peaks at 43 and 104 °C, indicating phase
separation.18 The two glass-transition temperatures (Tg) for the
mixture corresponded to those observed for the individual
polymers. Consistent with the formation of homogeneous
blends,18 1:1 and 1:2 w/w mixtures of 3b‚4c exhibited one
endothermic peak that corresponded to the enthalpic relaxation
of a homogeneous, glassy amorphous material. The glass-
transition temperatures (Tg), 73 and 80 °C, respectively, are
located between those of 3b (43 °C) and 4c (104 °C) and
depend on the ratio of the polymers.18,19 It is noticeable that no
clear transition was observed for the 2:1 w/w mixture of 3b
and 4c.
complexed DAN on the NMR time scale. In addition, there is
no self-association of DAN (4b in Figure 3), consistent with
previous observations.12a As a result, the excess (uncomplexed)
DAN units appear to be involved in the intermolecular hetero-
complexation of UG units through fast exchange with com-
plexed units. In contrast, when UG is in excess, the exchange
between free and complexed UG was slow on the NMR time
scale, as evidenced by the lack of change in the peak positions
of the complexed UG and the appearance of a weakly self-
associated species (1 in Figure 3).
Morphology Study of a Supramolecular Polymer Blend.
Figure 4a shows films of PBMA‚4c and 3b‚4c prepared from
the slow drying of a chloroform solution (4 g/dL, 1:1 w/w).
The 3b‚4c film was homogeneous, colorless, and transparent.13a,16
In contrast, a mixture of PBMA and 4c gave a solid with thick,
white domains (millimeter dimensions) that likely contain mostly
4c. AFM images provide nanoscopic evidence for the homo-
geneous mixing without phase separation (Figure 4b).13,17 The
thin film prepared by casting a 1.0 g/dL solution of PBMA
and PS in choroform on mica exhibited islands with lateral
dimensions ranging from ca. 100 to 40 nm (Figure 4b).
SEC Study of the Aggregation of an Immiscible Polymer
Pair through Molecular Recognition. The aggregation of UG-
PBMA and DAN-PS in toluene solution was examined using
size-exclusion chromatography (SEC) with a UV detector set
to 345 nm, a wavelength specific to the DAN moiety (Figure
6). Tailing of supramolecular assemblies can be caused by at
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Park and Zimmerman
Figure 5. DSC thermograms (2nd heating) of three mixtures of 3b and 4c
(2:1, 1:1, and 1:2 w/w) and three controls (4c, 3b, and a mixture of 3b and
PS). Experimental conditions: melt mixing for the mixtures, 20 °C/min
(1st heating) f 10 min of isothermal stage at 130 °C f 20 °C/min (cooling)
f 20 °C/min (2nd heating). Baseline was not corrected.
Figure 7. Plots of specific viscosity (ηsp) of mixtures (1:1 w/w) of various
DAN-PS 4 and UG-PBMA 3 with two control polymer mixtures (PBMA‚
4c and 3b‚PS) vs concentration.
at a concentration of ∼2.3 g/dL. Larger assemblies could not
be observed because of the high viscosity at higher concentra-
tion.
Solution Viscosity Behavior and Aggregate Growth. The
blends were further characterized by measuring the solution
viscosity of mixtures of PS-DAN and UG-PBMA in chloroform,
using an Ubbelohde viscometer (Figure 7). Plots of specific
viscosities (ηsp) of two control polymer mixtures (PBMA‚4c
and 3b‚PS) vs concentration were linear over the concentration
range 1-7 g/dL, indicating that there were no significant
physical entanglements or noncovalent interactions between the
random polymer coils at least within the range of molecular
weights of the polymers used in this study.21 However,
consistent with the formation of suprastructures,3,4 1:1 (w/w)
mixtures of DAN-PS 4 and UG-PBMA 3 exhibited large
increases in ηsp with increasing concentration.
The increase in viscosity observed in Figure 7 depended on
the mole ratio of recognition unit within the polymer and the
molecular weight of the polymer. In general, the viscosity of a
polymer solution depends on the size of the macromolecule,
according to the Mark-Houwink equation. The aggregation of
macromolecular chains in dilute solution driven by hydrogen
bonding might lead to an unentangled reversible supramolecular
polymer network, because entanglement of low-molecular-
weight macromolecular chains would not be expected.23 Thus,
the viscosity increase may result from the altered dynamic
motion of the polymer coil, which is constrained and slowed
by reversible hydrogen bonding between neighboring polymer
coils. The extent of this effect would likely depend on the
lifetime or the binding constant of the hydrogen-bonding
interactions and the number of interactions between polymer
Figure 6. SEC of 4c and various weight ratios of 3b and 4c in toluene on
a single column (HR4E). Ten microliters of the solution (∼2.3 g/dL) were
injected. A UV detector was used to detect DAN at 345 nm.
least two phenomena.20,21 First, polar hydrogen-bond donor and
acceptor groups may cause absorption of the materials to the
cross-linked polystyrene packing in SEC columns. Secondary
dissociation of units in dynamic equilibrium may become
physically separated because of the flow and thus lose the chance
to re-form a complex. Indeed, significant tailing due to the
interaction with the solid phase was observed in 4c containing
DAN. However, with increasing amounts of 3b added, the
retention time and the tailing were progressively reduced, giving
a sharp and narrow peak, as the strong complexation of UG
and DAN minimized the interaction between DAN units and
the packing material. The result is both consistent with the
formation of a supramolecular polymer and multivalence
pairings of PS and PBMA chains. The highest degree of
aggregation was observed in a 1:1 wt % mixture of 3b and 4c
(20) (a) Zeng, F. W.; Zimmerman, S. C.; Kolotuchin, S. V.; Reichert, D. E. C.;
Ma, Y. G. Tetrahedron 2002, 58, 825-843. (b) Lohmeijer, B. G. G.;
Schubert, U. S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1413-
1427.
(22) Wu, Y.; Feng, S. J. Appl. Polym. Sci. 2001, 80, 975-978.
(23) (a) Rubinstein, M.; Semenov, A. N. Macromolecules 1998, 31, 1386-1397.
(b) Semenov, A. N.; Rubinstein, M. Macromolecules 1998, 31, 1373-
1385.
(21) Xu, J.; Fogleman, E. A.; Craig, S. L. Macromolecules 2004, 37, 1863-
1870.
9
11586 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006

Miscible Supramolecular Polymer Blend
A R T I C L E S
weight ratios of the polymers. These two polymers each have
4 mol % recognition units and similar molecular weights. As
seen in Figure 9a, when the amount of 3a is increased (more
UG units) the viscosities are higher and the slope of the log-
log plot is higher. This is fully consistent with the conclusion
above that excess UG units participate by weakly self-associat-
ing. Indeed, in examining PBMA, 3a, and 3b alone, we find
that the weak self-association of UG units produces slopes in
the viscosity plot of 1.09, 1.65, and 1.98, respectively (Figure
9b).
Aggregation driven by the weak self-association of UG units
occurs even in dilute concentrations. Thus, the hydrodynamic
radius (Rh) of 3b, measured by dynamic light scattering (DLS),
at the overlap concentration26 C* (∼0.8 g/dL) in chloroform
was 6.4 nm, which is almost 3-fold bigger than that in THF
(2.3 nm), where UG self-association is minimal. In addition,
with increasing concentration over C*, formation of very large
particles was observed (Figure 10a). In contrast, the Rh values
of PBMA as controls were 3.1 and 2.7 nm at the overlap
concentration (C* ≈ 0.7 g/dL) in THF and chloroform,
respectively. Furthermore, the Rh of PBMA in chloroform
decreased with increasing concentration, a typical behavior for
noninteracting polymer coils.26
In looking at the viscosity increases in Figure 9a, it was
interesting to note the presence of a universal transition viscosity,
characterized by a change in slope in the log(ηsp) vs log(c) plots.
This transition viscosity is dependent on concentration, occurring
at log(ηsp) ) 0.25-0.50 with the slope increasing from 1.52-
1.99 to more than 3.79 (Figure 9a). Occurring at relatively
constant ηsp, and therefore similarly rigid aggregates, the
transition may represent the concentration at which supra-
molecular aggregates start to overlap. This concentration can
be defined as Csupra* ) 3Mw,supra/NA4πRg,supra, where Mw,supra
and Rg,supra are the molecular weight and radius of gyration for
supramolecular aggregates, respectively, and NA is Avogadro’s
number.27 Therefore, the higher Csupra* values would correspond
to denser supramolecular structures at dilute concentration (and
vice versa). For example, the excess DAN units in the 1:2 weight
ratio of 3b‚4b are involved in the heterocomplexation of UG
units within the same aggregate, whereas excess UG units (2:1
w/w) weakly link the less-dense supramolecular aggregates,
creating interparticle connections.
Figure 8. Photographs of a control (PBMA‚4b) and a supramolecular
polymer (3a‚4b) solution in chloroform at the same concentration (15 g/dL).
chains. A number of factors can lead to a higher-viscosity
polymer solution. For example, higher MWs generally correlate
with higher viscosities. It was anticipated that a higher density
of recognition units would be a critical parameter in obtaining
high viscosity increases when comparing polymers of similar
molecular weights. Indeed, the viscosity increases observed for
chloroform solutions of 3b‚4b or 3b‚4c were much larger than
those of 3a‚4b or 3a‚4c, even though the molecular weight of
3b (16 kD) is smaller than that of 3a (28 kD). Polymer 4a,
with an average of 1.4 recognition units per chain, showed only
a modest increase in viscosity with concentration. Thus, it
appears that more than a single connection between polymer
chains is required.
The supramolecular polymer could be visualized macroscopi-
cally with a 2:1 (w/w) mixture of 3a and 4b in chloroform
forming a gel, whereas at the same concentration (15 g/dL), a
solution of PBMA and 4b lacking any specific interpolymer
interaction flowed freely (Figure 8). The molecular weight of
PBMA (29 kD) is similar to that of 3a (28 kD), so that the
difference in properties qualitatively indicates that intermolecular
recognitions of UG and DAN units on the polymers 3a and 4b,
respectively, are responsible for the significant increases in
viscosity.
More details of the aggregation driven by strong complexation
of UG and DAN are found in the double logarithmic plots of
ηsp and concentration (g/dL), as shown in Figure 9a. The slopes
for mixtures of 3b‚4b (1:1 w/w) and 3a‚4b (2:1 to 1:2 w/w)
over a certain concentration range from 3.98 to 5.68, values
that are higher than that (∼3.6) predicted4a,c,21 from the reptation
model of a telechelic type of living polymer (F ) 2).24 The
larger slopes are due to the higher mole percent of the
recognition units (F > 4) in the polymers used in this study.
The DLS data in panels b and c of Figure 10 show the growth
of aggregates as well as their distribution. Polymer 3b fixed at
two concentrations (0.56 and 1.13 g/dL) was titrated with 4b.
At the lower concentration of 3b (0.56 g/dL), the degree of
aggregation was lower and the distribution of the aggregates
was more homogeneous. This result suggests a dynamic process
whereby most of the aggregates have sufficient freedom to
exchange polymers and form uniform particles. On the other
1
The data are consistent with the H NMR results described
above, both sets of data indicating that the UG and DAN units
on the polymers fully participate in intermolecular interactions.
Interestingly, the slope for a 1:1 (w/w) mixture of 3b‚4b ([UG]/
[DAN] ) 2.5) is higher than that of 3a‚4b ([UG]/[DAN] )
1.0) despite the fact that the molecular weight of 3a (28 kD) is
(25) (a) Dondos, A.; Tsitsilianis, C. Polym. Int. 1992, 28, 151-156. (b) Pierre,
E.; Dondos, A. Eur. Polym. J. 1987, 23, 347-351.
(26) (a) Negadi, A.; Duval, M.; Benmouna, M. Polym. Bull. 1999, 43, 261-
267. (b) Vancso´, G. Makromol. Chem. 1989, 190, 1119-1131. (c) Venohr,
H.; Fraaije, V.; Strunk, H.; Borchard, W. Eur. Polym. J. 1998, 34, 723-
732.
1
higher than that of 3b (16 kD). As observed in the H NMR
experiments (Figure 3), the excess UG units on 3b are not
involved in the heterocomplexation of DAN units on 4b, but
rather increase viscosity through weak self-association.
To examine this issue in more detail, additional viscometric
measurements were carried out on blend of 3a‚4b with different
(27) (a) For noninteractive polymers, the overlap between polymer coils occurs
at an overlap concentration C* ) 3Mw/NA4πRg, where Mw and Rg are the
molecular weight and the radius of gyration for polymers. Therefore, to
clarify Mw and Rg for supramolecular polymers that exhibit concentration-
dependence, the subscript supra is added to the parameters, Csupra*, Mw,supra
,
and Rg,supra. (b) For more details, see: Pan, C.; Maurer, W.; Lodge, T. P.;
Stepanek, P.; von Meerwall, E. D.; Watanabe, H. Macromolecules 1995,
28, 1643-1653.
(24) Cates, M. E. Macromolecules 1987, 20, 2289-2296.
9
J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11587

A R T I C L E S
Park and Zimmerman
Figure 9. Double logarithmic plots of ηsp and concentration (g/dL): (a) Effect of recognition unit ratios, [UG]/[DAN], on the slopes; 1:1 w/w for 3b‚4b
(2.5) and 3a‚4b (1.0), 2:1 for 3a‚4b (2.0), 1:2 for 3a‚4b (0.67). The values in parentheses indicate recognition unit ratios, [UG]/[DAN], on polymers. (b)
Comparison of slopes in PBMA and UG-PBMA (3a and 3b).
Figure 10. Distribution of the hydrodynamic radius (Rh) of PBMA, 3b, and reversible supramolecular network polymers, measured by DLS in chloroform
at room temperature. (a) Concentration dependence of noninteractive (PBMA) and interactive polymer (3b). (b) Titration of 3b with 4b at a concentration
of 0.56 g/dL. (c) Titration of 3b with 4b at a concentration of 1.13 g/dL. Plots are normalized with an arbitrary scattered intensity.
Figure 11. Dependences of specific viscosities (ηsp) of supramolecular network polymers on external stimuli. (a) Temperature: (-9-) 2.8 g/dL of 3a and 4b
(2:1 w/w) in 1,2-dichlorobenzene, (0) recovered viscosity of a supramolecule after cooling from 80 °C, (-b-) a mixture of PBMA and 4b as control. (b) %
DMSO: 6.6 g/dL of 3a and 4b (2:1 w/w) in chloroform. (c) Mole percent DAN added to a solution of 3a and 4b (6.6 g/dL, 2:1 w/w) in chloroform.
hand, larger aggregates lead to viscous solutions where larger
heterogeneous particle distribution results from a slow dynamic
exchange (Figure 10c).
Reversibility of Supramolecular Polymer Formation. The
reversibility of supramolecular polymer formation was inves-
tigated because the ability to form or break the hydrogen bonds
by external stimuli is a key reason for the interest in these
materials. Panels a and b of Figure 11 show the dependence of
viscosities on temperature and solvent polarity, respectively. The
viscosity of a mixture of 3a and 4b (2.8 g/dL, 2:1 w/w) in 1,2-
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Miscible Supramolecular Polymer Blend
A R T I C L E S
and reagents were of reagent grade and were used as received without
further purification.
dichlorobenzene exhibited an exponential decrease in ηsp with
increasing temperature, in contrast to the results of the control
mixture (PBMA and 4b). In the latter case, the dynamic motion
of a polymer coil is faster at higher temperatures, leading to
the decreased viscosity, whereas for 3a‚4b, the rapid decrease
in ηsp can be ascribed to weakened hydrogen bonding, resulting
in network disassembly. Upon being cooled to 26 from 80 °C,
the aggregates spontaneously re-formed, returning to the
original, higher viscosity.
A similar result was observed upon the addition of DMSO
to a chloroform solution of a mixture of 3a and 4b (6.6 g/dL,
1:1 w/w), in which the viscosity decreased progressively. The
addition of ca. 20% v/v DMSO resulted in the loss of viscosity
obtained by hydrogen bonding, indicating complete disassembly
of the aggregates. In addition to heat and solvent polarity, it
was anticipated that the aggregates might be disassembled by
the addition of 2. Thus, free DAN would compete with DAN
linked to PS and displace it, resulting in breaking the inter-
molecular contacts between polymer coils. Indeed, a dramatic
decrease in viscosity was observed upon addition of DAN
(Figure 11c). As the concentration of free DAN reached the
point where it was 20-40% that of polymer-bound DAN, the
ηsp dropped to ca. 5-10% of its original value.
Melting points were measured on a Thomas-Hoover melting point
apparatus and are uncorrected. Brine refers to a saturated aqueous
solution of NaCl. All NMR spectra were acquired in the VOICE
laboratory, University of Illinois, Urbana-Champaign, with Varian Unity
500 MHz instrument. Chemical shifts are in parts per million and
coupling constants (J) are in Hertz. For 1H spectra, chemical shifts are
referenced to the residual protio solvent peak: 7.26 ppm for chloroform-
d, 2.50 ppm for DMSO-d6. For 13C spectra, chemical shifts are
referenced to the solvent peak at 77.5 ppm in chloroform-d and 39.5
ppm in DMSO-d6.
Molecular weights against polystyrene standards were estimated at
30 °C in THF at a flow rate of 1.0 mL/min using a Viscotek SEC
(TDA model 300) equipped with a Hitachi autosampler (L-7250), a
Hitachi pump (L-7100), and a Viscotek refractive index detector (300
RI). SEC experiment for study of the formation of aggregates was
carried out at room temperature in toluene at a flow rate of 1.0 mL/
min using SEC (single column, HR4E) equipped with a Hitachi pump
(L-6000) and a Hitachi UV detector (L-4000L). Viscosity was measured
in chloroform using an Ubbelohde viscometer (Cannon, 75L50) at 26
°C. Hydrodynamic radius (Rh) and distribution were collected at room
temperature using a protein solution DLS (DynaPro) and analyzed with
Dynamics V6 software. For the DLS and viscometry experiments,
solutions of polymers and solvents were filtered using a 0.2 µm PTFE
filter (Millipore). The filtrate was precipitated into methanol, collected,
and dried. Glass-transition temperatures (Tg) were determined at heating
and cooling rates of 20 °C/min under nitrogen using a Pyris Perkin-
Elmer DSC and reported as the midpoint of the second heating. Surface
topography was examined at a scan rate of 1.969 Hz using a Digital
Instrument atomic force microscope (multimode AMF) in tapping mode
with a TappingMode silicon probe (NanoDevices Metrology tip, type
TAP300). The collected images were flattened using Nanoscope III
software (Digital Instrument).
Conclusion
Several recently reported, multiply hydrogen-bonded com-
plexes have created an important toolkit for chemists interested
in creating supramolecular polymers. For the formation of
supramolecular blends from an immiscible polymer pair, where
strong repulsive forces exist, UG and DAN have been found to
be quite useful as a result of the high stability and fidelity of
their heterocomplex. Specifically, DAN and UG units were
displayed along the backbone of immiscible polymers, PS and
PBMA (DAN-PS and UG-PBMA), respectively. Both polymers
were readily prepared via radical copolymerization. It was found
that intermolecular recognition between two different polymer
coils occurred, connecting PS and PBMA coils at the molecular
level. The mixture of UG-PBMA and DAN-PS formed colorless
and transparent films with no evidence of phase separation on
either the nano- or macroscopic scale. A differential scanning
calorimetry study further demonstrated the formation of a
homogeneous blend with a single endothermic peak, corre-
sponding to the enthalpy relaxation of an amorphous material
with one major composition. SEC, viscosity, and DLS studies
also demonstrated the formation of a supramolecular polymer.
The properties of the resulting supramolecular blends are tunable
as well as thermoreversible. The key finding is that miscible
blends were formed even though a low mol % of the recognition
units was incorporated into the polymers. This result underscores
the importance of high-affinity and high-fidelity hetero-
complexation in the creating of new supramolecular polymer
architectures.
Succinic Acid 6-[2-(3-Butyl-ureido)-6-oxo-1,6-dihydro-purin-9-
yl]-2,2-dimethyl-tetrahydro-furo[3,4-d][1,3]dioxol-4-ylmethyl Ester
2-(2-Methyl-acryloyloxy)-ethyl Ester (UG-BMA, 7). A mixture of
780 mg (6.0 mmol) of 2-hydroxyethyl methacrylate, 2.1 g (4.0 mmol)
of carboxylated UG 5,12a 866 mg (4.4 mmol) of EDCI, and 66 mg (0.6
mmol) of DMAP in 30 mL of DMF was stirred overnight at room
temperature. The solvent was removed using a rotary evaporator
equipped with a high vacuum. The crude gel was dissolved in 50 mL
of chloroform, and 30 mL of water was poured into the solution. The
organic layer was collected, and the aqueous layer was extracted three
times with 30 mL of chloroform. The combined organic layers were
washed with 30 mL of brine, dried over MgSO4, filtered, and reduced.
The crude material was purified by column chromatography (SiO2, Rf
) 0.25, 9:1 chloroform:methanol) to give 1.78 g (71%) of 7 as a white
solid. Mp: 69.0-71.5 °C (chloroform/methanol). 1H NMR (500 MHz,
DMSO- d6): δ 11.95 (1H, s), 9.91 (1H, s), 8.00 (1H, s), 7.17 (1H, s),
6.03 (1H, d, J ) 1.8), 5.97 (1H, s), 5.63 (1H, s), 5.25 (1H, dd, J ) 2.1,
6.3), 5.02 (1H, dd, J ) 3.4, 5.8), 4.33 (1H, q, J ) 2.1, 6.3), 4.22 (5H,
m), 4.13 (1H, m), 3.14 (2H, m), 2.46-2.40 (4H, m), 1.81 (3H, s), 1.51
(3H, s), 1.46-1.40 (2H, m), 1.31-1.26 (5H, m), 0.88 (6H, t, J ) 6.8).
13C NMR (125 MHz, DMSO- d6): δ 172.4, 172.3, 167.0, 155.5, 149.1,
137.8, 136.2, 126.7, 120.1, 114.1, 89.8, 84.7, 84.6, 81.6, 64.7, 63.0,
62.6, 39.4, 32.3, 32.0, 29.0, 28.9, 27.6, 26.0, 25.8, 25.0, 20.1, 18.5,
and 14.2. m/z (ESI): 635.2650 [(M + H)+]; C28H39N6O11 requires M,
635.2677.
Experimental Section
General Experimental. All reactions were carried out under a dry
nitrogen atmosphere. Reaction temperatures reported are the temper-
atures of the heating medium. DMF was distilled from sodium hydride
under a vacuum prior to use. DMSO was distilled from calcium hydride
under a vacuum prior to use. Butyl methacrylate and styrene were
passed over aluminum oxide (activated, basic) and then distilled from
calcium hydride at reduced pressure prior to use. All other chemicals
Decanedioic Acid Mono-(4-vinyl-benzyl) Ester (St-CO2H, 11). A
mixture of 2.64 g (17.3 mmol) of 4-vinylbenzyl chloride, 7.0 g (34.6
mmol) of sebasic acid, 7.2 g (51.9 mmol) of K2CO3, and 53 mg (0.2
mmol) of 18-crown-6 in 150 mL of DMF was stirred overnight at 110
°C. Water (10 mL) was added to the mixture, and the solvent was
removed using a rotary evaporator equipped with high vacuum. Ethyl
acetate (200 mL) was poured into the remaining solid, and the resulting
9
J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11589

A R T I C L E S
Park and Zimmerman
slurry was stirred. The insoluble solid was filtered off. The filtrate was
washed three times with 40 mL of water. The organic layer was
collected, dried over MgSO4, filtered, and reduced. The crude material
was purified by column chromatography (SiO2, Rf ) 0.55, ethyl acetate)
to give 2.2 g (40%) of 11 as a white solid. Mp: 62.5-64.0 °C
(chloroform/methanol). 1H NMR (500 MHz, CDCl3): δ 10.44 (1H, br
s), 7.40 (2H, d, J ) 8.2), 7.31 (2H, d, J ) 8.2), 6.72 (H, dd, J ) 10.8,
17.6), 5.76 (H, d, J ) 17.6), 5.26 (H, d, J ) 10.8), 5.10 (2H, s), 2.36-
2.32 (4H, m), 1.66-1.59 (4H, m), 1.33-1.29 (4H, m). 13C NMR (125
MHz, CDCl3): δ 179.9, 173.9, 136.6, 128.7, 128.6, 126.6, 125.9, 114.5,
66.1, 65.9, 65.7, 34.5, 34.1, 29.1, 25.1 and 24.8. m/z (ESI): 319.1897
[(M + H)+]; C19H27O4 requires M, 319.1909.
9-(7-Heptanoylamino-[1,8]naphthyridin-2-ylcarbamoyl)-non-
anoic Acid 4-Vinyl-benzyl Ester (DAN-St, 12). A mixture of 1.57 g
(4.93 mmol) of St-CO2H 10, 1.34 g (4.93 mmol) of amino naphthyridine
13,12a 0.95 g (4.9 mmol) of EDCI, and 60 mg (0.5 mmol) of DMAP in
30 mL of DMF was stirred overnight at room temperature. The solvent
was removed using a rotary evaporator equipped with a high vacuum.
The crude gel was dissolved in 100 mL of chloroform, and 50 mL of
water was poured into the solution. The organic layer was collected,
and the aqueous layer was extracted three times with 30 mL of CHCl3.
The combined organic layers were washed with 30 mL of brine, dried
over MgSO4, filtered, and reduced. The crude material was purified
by column chromatography (SiO2, Rf ) 0.33, 95:5 chloroform:
methanol) to give 2.65 g (94%) of 12 as a white solid. Mp: 135-137
°C (chloroform/methanol). 1H NMR (500 MHz, CDCl3): δ 8.43 (2H,
d, J ) 8.8), 8.40 (2H, br s), 8.12 (2H, d, J ) 8.8), 7.39 (2H, d, J )
8.2), 7.30 (2H, d, J ) 8.2), 6.70 (H, dd, J ) 10.8, 17.6), 5.75 (H, d, J
) 17.6), 5.25 (H, d, J ) 10.8), 5.08 (2H, s), 2.44 (4H, q, J ) 7.0 and
13.9), 2.33 (2H, t, J ) 7.8), 1.78-1.69 (4H, m), 1.62 (2H, t, J ) 7.0),
1.39-1.26 (10H, m), 0.88 (3H, t, J ) 6.8). 13C NMR (125 MHz,
CDCl3): δ 173.9, 173.0, 172.9, 154.4, 153.8, 139.3, 137.7, 136.6, 135.8,
128.7, 126.6, 118.4, 114.5, 113.8, 66.0, 38.0, 37.9, 34.5, 31.7, 29.3,
29.2, 29.0, 25.4, 25.1, 22.7 and 14.2. m/z (ESI) 573.3421 [(M + H)+]
C34H45N4O4 requires M, 573.3441.
methanol. This process was done again. The collected gel was dried
under a vacuum at 40 °C. 3a: The general procedure for the radical
polymerization using 253.7 mg (0.4 mmol) of UG-BMA 7, 1.42 g (10
mmol) of BMA, and 32.8 mg (0.2 mmol) of AIBN in 42 mL of DMSO
provided 3a. 3b: The general procedure for the radical polymerization
using 634.3 mg (1.0 mmol) of UG-BMA 7, 1.42 g (10 mmol) of BMA,
and 32.8 mg (0.2 mmol) of AIBN in 42 mL of DMSO provided 3b.
1H NMR (500 MHz, DMSO- d6): δ 11.96 (s, NH), 9.92 (s, NH), 8.00
(s, CH), 7.16 (s, NH), 6.01 (s, CH), 5.25 (s, CH), 5.02 (s, CH), 4.30
(s, CH), 4.22-3.82 (br m, CH and OCH2), 3.14 (s, CH2), 2.00-0.88
(br m, CH2 and CH3).
DAN-Attached Polystyrene (DAN-PS, 4). The radical copolym-
erization under a nitrogen atmosphere in a 50 mL round-bottom flask
sealed with a rubber septum followed a general procedure as outlined
below. A solution of DAN-St 12, styrene, and AIBN in DMSO was
stirred for 48 h at 90 °C and then precipitated into methanol. The
resulting solid was collected, redissolved in chloroform, and reprecipi-
tated into methanol. This process was done again. The collected solid
was dried under a vacuum at 40 °C. 4a: The general procedure for the
radical polymerization using 100 mg (0.17 mmol) of DAN-St 12, 1.8
g (17.3 mmol) of styrene, and 12 mg (0.073 mmol) of AIBN in 20 mL
of DMSO provided 4a. 4b: The general procedure for the radical
polymerization using 790 mg (1.38 mmol) of DAN-St 12, 3.44 g (33.1
mmol) of styrene, and 23 mg (0.14 mmol) of AIBN in 50 mL of DMSO
provided 4b. 4c: The general procedure for the radical polymerization
using 367 mg (0.64 mmol) of DAN-St 12, 752 mg (7.23 mmol) of
styrene, and 5.2 mg (0.031 mmol) of AIBN in 10 mL of DMSO
1
provided 4c. H NMR (500 MHz, CDCl3): δ 8.52 (d, J ) 8.5, Ar-H
in the naphthyridine), 8.35 (s, NH), 8.16 (d, J ) 8.5, Ar-H in the
naphthyridine), 7.35-7.00 (br s, Ar-H in styrene unit), 6.80-6.45 (br
s, Ar-H in styrene unit), 5.10 (s, Ar-CH2O), 2.44 (br s, CH2), 2.33
(br s, CH2), 2.14-1.79 (br s, H in styrene unit overlapped with CH2),
1.74-1.36 (br s, H in styrene unit overlapped with CH2), 1.23-1.15
(m, CH2), 1.04-0.96 (m, CH3).
UG-Attached Poly(butyl)methacrylate (UG-PBMA, 3). The radi-
cal copolymerization under a nitrogen atmosphere in a 50 mL round-
bottom flask sealed with a rubber septum followed a general procedure
as outlined below. A solution of UG-BMA 7, BMA, and AIBN in
DMSO was stirred for 24 h at 60 °C. The reaction generated gel inside
the flask. The solvent was decanted, and the resulting gel was then
dissolved in chloroform followed by precipitation into methanol. The
unreacted UG-BMA 7 was soluble in methanol. The precipitated gel
was collected, redissolved in chloroform, and reprecipitated into
Acknowledgment. Funding of this work by the NSF (CHE-
0212772) and by the U.S. Department of Energy, Division of
Materials Science, under Award DEFG02-91ER45439 through
the Frederick Seitz Materials Research Laboratory at the
University of Illinois at Urbana-Champaign is gratefully
acknowledged.
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11590 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006