Self-sorting in polymers

Article abstract of DOI:10.1021/ma050755w

Random and block copolymers containing two different classes of hydrogen-bonding side-chains have been prepared by ring-opening metathesis polymerization. The resulting copolymers can be viewed as "universal polymer backbones" based solely on two competitive hydrogen-bonding pairs. The hydrogen-bonding side chains containing thymine and cyanuric acid-based recognition motifs are shown to self-assemble with their complementary diamido pyridine and isophthalic wedge moieties, respectively, even in the presence of competitive recognition sites, i.e., selective functionalization of the copolymers can be accomplished via a one-step orthogonal self-assembly approach displaying self-sorting in a competitive environment. These results clearly demonstrate the concept of self-sorting in synthetic polymers and suggest the design of complex polymeric materials containing competitive noncovalent interactions.

Full text of DOI:10.1021/ma050755w

Macromolecules 2005, 38, 7225-7230
7225
Self-Sorting in Polymers
Caroline Burd and Marcus Weck*
Department of Chemistry and Biochemistry, Georgia Institute of Technology,
Atlanta, Georgia 30332-0400
Received April 11, 2005; Revised Manuscript Received June 10, 2005
ABSTRACT: Random and block copolymers containing two different classes of hydrogen-bonding side-
chains have been prepared by ring-opening metathesis polymerization. The resulting copolymers can be
viewed as “universal polymer backbones” based solely on two competitive hydrogen-bonding pairs. The
hydrogen-bonding side chains containing thymine and cyanuric acid-based recognition motifs are shown
to self-assemble with their complementary diamido pyridine and isophthalic wedge moieties, respectively,
even in the presence of competitive recognition sites, i.e., selective functionalization of the copolymers
can be accomplished via a one-step orthogonal self-assembly approach displaying self-sorting in a
competitive environment. These results clearly demonstrate the concept of self-sorting in synthetic
polymers and suggest the design of complex polymeric materials containing competitive noncovalent
interactions.
Introduction
pairs play a key role in the storage and decoding of
genetic material. Recent research efforts have focused
on the manipulation of the Watson-Crick base pairs of
DNA5 and employing them as a synthetic tool, using
complementary DNA oligomers as a template to bring
reactants together, thereby discovering new reactions.6
Other investigations have utilized hydrogen-bonding
moieties as enzyme mimics to induce catalytic activity.7
In these examples, the inherent self-sorting of hydrogen-
bonding receptors is key. The polymeric system pre-
sented herein follows this trend by relying on hydrogen-
bonded dimers and a hydrogen-bonded enzyme mimic
as recognition units.
The copolymers introduced herein can be viewed as
a “universal polymer backbone”2c based solely on hy-
drogen-bonded recognition motifs that are able to self-
assemble with their corresponding receptor molecule in
the presence of competitive recognition units. The
copolymers are based on two monomers that encompass
three design elements: (1) norbornene as the polymer-
izable unit that propagates in a living fashion via ring-
opening metathesis polymerization (ROMP),10 (2) a
spacer molecule to increase solubility and to decouple
the polymer from the hydrogen-bonding recognition
units, and (3) the recognition units themselves that are
either thymine derivatives that can undergo three
hydrogen bonds to self-assemble with diaminopyridines3
or cyanuric acids that are able to self-assemble with
isophthalic wedge type receptor via six hydrogen bonds
(Chart 1).4,9 These recognition units were chosen be-
cause of their structural similarity and thus their
possibility to display competitive interactions during the
self-assembly process.
Self-sorting is the ability of objects such as molecules
to find and self-assemble selectively with their corre-
sponding recognition units. From complex systems, such
as DNA replication and transcription, to simple phe-
nomena, such as oil-water phase separation, self-
sorting is evident throughout our daily lives. In syn-
thetic systems, self-sorting based on hydrogen-bonding
has been explored as a general phenomenon in low
molecular weight organic molecules.1 For example, Isaac
and co-workers have shown that in the presence of
complex mixtures self-sorting of small-molecule recep-
tors via hydrogen-bonding still occurs regardless of
potential competitive interactions.1a However, to date,
the principle of self-sorting in polymeric and complex
un-natural systems that can be viewed as simple
synthetic analogue to biopolymers such as DNA has not
been explored. While in polymer science many reports
in the literature outline the use of pendant side-chain
polymers based on multiple recognition units,2 these
examples however are in highly controlled systems
without the presence of a competitive recognition unit.
In this manuscript, we investigate the concept of self-
sorting in polymers by employing norbornene copoly-
mers composed of two different hydrogen-bonding side-
chains. These hydrogen-bonding side-chains contain
thymine and cyanuric acid-based recognition units that
are able to self-assemble with diamido pyridine3 and
isophthalic wedge moieties,4 respectively. We demon-
strate that the principle of self-sorting can be translated
to synthetic side-chain-functionalized polymers, i.e.,
copolymer side-chains self-assemble with their corre-
sponding recognition unit in the presence of competitive
recognition sites.
Experimental Section
The concept of self-sorting in polymers is essential for
replication and translation of genetic material. The
Watson-Crick-type base pairs adenine-thymine, ad-
enine-uracil, and guanine-cytosine are the three domi-
nant sets of hydrogen-bonded dimers that nature offers.
The specific interactions within these complementary
Materials. Unless otherwise noted, all commercially avail-
able reagents and solvents were used without further purifica-
tion. Dueterated solvents were distilled over calcium hydride
and stored in the dark. Anhydrous dichloromethane and THF
were dried via passage through copper oxide and alumina
columns under argon. Column chromatography was carried
out on silica gel 60, 230 ( 400 mesh (Whatman). Gel-
permeation chromatographies (GPC) were carried out on
polymer solutions in THF at 30 °C (column combination: 2x
* To whom correspondence should be addressed. E-mail:
marcus.weck@chemistry.gatech.edu.
10.1021/ma050755w CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/27/2005

7226 Burd and Weck
Macromolecules, Vol. 38, No. 17, 2005
Chart 1. Fully Hydrogen-Bonded Copolymer and the Corresponding Recognition Units
Scheme 1. Synthesis of Monomers 1 and 2a
American Polymer Standards 10 µm particle size, linear mixed
bed packing, flow rate 1 mL min-1) with a Waters 1525 binary
pump coupled to a Waters 2414 refractive index detector.
Calibrations are based on poly(styrene) standards. NMR
spectra were recorded on a Varian Mercury 300 spectrometer
(1H, 300 MHz; 13C, 75 MHz). Chemical shifts are reported in
ppm on the δ scale relative to the solvent signal. Electrospray
ionization (ESI) mass spectra were obtained on a Micromass
Quattro LC spectrometer and fast atom bombardment (FAB)
mass spectra on a VG Instruments 70SE spectrometer.
Elemental analyses were performed by Atlantic Microlabs,
Norcross, GA. Monomer 24c and recognition units 53e and 64c,f
were prepared as previously reported.
exo-Bicyclo[2.2.1]hept-5-ene-2-carboxylic acid-11-(3-
methyl-4,6-trioxo-[1,5]diazinan-1-yl)-undecyl Ester (1).
To a stirred solution of 4 (4.0 g, 10.8 mmol)4c and thymine (108
mmol) in DMSO (100 mL), K2CO3 (3.0 g, 22 mmol) was added.
The reaction mixture was allowed to stir at room temperature
for 60 h, poured into a solution of NaHSO3(aq) (500 mL),
extracted with a 1:1 mixture of diethyl ether/dichloromethane,
and dried over magnesium sulfate. The solvent was removed
by rotary evaporation, and the residue was purified by passage
over silica (1:1 mixture EtOAc/hexanes). After drying on high
vacuum, 1 was obtained as a white solid in 53% yield. 1H NMR
(300 MHz CDCl3): δ ) 9.33 (br s, 1H, NH), 6.96 (br s, 1H),
6.10 (m, 2H, CHdCH), 4.07 (t, 2H, J ) 6.7 Hz, CH2O), 3.88 (t,
2H, J ) 7.5 Hz, CH2N), 3.03 (m,1H), 2.91 (m, 1H), 2.22 (m,
1H), 1.91 (m, 1H), 1.90 (s, 3H), 1.69-1.49 (m, 5H), 1.41-1.24
(m, 16H). 13C NMR (75 MHz CDCl3): δ ) 176.3, 164.5, 151.1,
140.6, 138.2, 135.9, 111.2, 64.9, 48.9, 46.9, 46.7, 43.5, 41.9,
30.64, 29.8, 29.5, 29.4, 29.0, 26.8, 26.3, 12.7. MS (EI): m/z (%)
) 416.26601 (M+, 416.26751 calcd.). Anal. Calcd for C24H36-
N2O4: C, 69.10; H, 8.64; N, 6.70. Found: C, 68.75; H, 8.83; N,
6.64.
a Key: (a) 180 °C, reflux 8 h, 70%; (b) 1. NaOH, H2O, 2. KI,
I2, H2O 44%; (c) DCC/DMAP, Br (CH2)11OH, 50 °C, 82%; (d)
thymine, K2CO3, DMSO, room temperature, 53%; and (e)
cyanuric acid, K2CO3, DMSO, room temperature, 36%.
1
the cyanuric acid moieties were monitored by H NMR. The
NMR data was evaluated using the computer program
ChemEquili.11
Results and Discussion.
Monomer Synthesis. The monomers were synthe-
sized as outlined in Scheme 1. Isomerically pure exo-
norbornene carboxylic acid was synthesized from the
endo:exo mixture by iodolactonaization using estab-
lished literature procedures.8 The exo-norbornene car-
boxylic acid was then functionalized with 11-bromoun-
decanol using DCC/DMAP to yield the corresponding
norbornene bromide 4.9 Monomers 1 and 2 were formed
in one step by reacting 4 with an excess of either
thymine or cyanuric acid, respectively. Dibutylamido
pyridine 5 was synthesized in one step from the acyla-
tion of diamino pyridine (recrystallized from chloroform)
using butyl chloride. The octyl-ether isopthalic wedge
receptor 6 was synthesized in close analogy to literature
procedures4c,f after functionalizing the phenolic hydroxyl
of dimethyl 5-hydroxy isopthalate with 1-bromo-octane
via a Williamson ether synthesis.
Polymer Synthesis and Living Characterization.
All monomers were subjected to ROMP using Grubbs’
first-generation catalyst.10 The polymerizations were
carried out in 0.2 M solutions of deuterated tetrahydro-
furan at room temperature with monomer-to-catalyst
ratios of 20:1-120:1. All polymerizations were moni-
tored via 1H NMR. A 50:1 monomer-to-catalyst ratio of
2 polymerizes within less than five minutes, while the
ROMP of 1 (50:1 monomer-to-catalyst ratio) is complete
within an hour. Figure 1 shows the kinetic data for the
ROMP of 1 (the detailed polymerization behavior of 2
has been reported before4c). All polymers were purified
and isolated by repeated precipitations into hexanes.
The polymers were characterized by GPC, and the
Polymerizations. To a 0.2 M stirred solution of monomer
in THF, 0.006 mmol of ruthenium catalyst was added. The
mixture was stirred at room temperature, and the reaction
progress was monitored by 1H NMR until completion. After
complete conversion, two drops of ethyl vinyl ether were added
to terminate the reaction. The polymers were purified by
precipitation into hexanes.
1
Polymer 1. H NMR (300 MHz, 15:85 dioxane-d6:CDCl3):
δ ) 9.8 (br s, 1H, NH), 6.90 (br s, 1H), 5.4-5.1 (m, 2H), 3.90
(br m, 2H), 3.60 (t, 2H, J ) 5.5 Hz, CH2N), 2.7-2.3 (br m, 5
H), 2.2-0.5 (br m, 23 H).
Copolymer 1:2. 1H NMR (300 MHz, 15:85 dioxane-d6:
CDCl3): δ ) 10.0 (br s, 2H, NH), 9.8 (br s, 1H, NH), 6.90 (br
s, 2H), 5.4-5.1 (m, 4H), 3.90 (br m, 4H), 3.60 (t, 4H, J ) 6.4
Hz, CH2N), 2.7-2.3 (br m, 10 H), 2.2-0.5 (br m, 46 H).
Self-Assembly. To a 0.04 M solution (15:85 dioxane:
chloroform) of polymer, 2.8 equiv of recognition unit (either 5,
6, or both) dissolved in 0.5 mL of 15:85 dioxane:chloroform
were added dropwise.
Titration Studies. Association constants were measured
by NMR titration of a 0.2 M solution (15:85 dioxane:chloro-
form) of the receptor molecules with a 0.04 M solution (based
on recognition units) of the corresponding polymer, where the
molarity is based on the number of recognition units. The
chemical shifts of the imide protons on either the thymine or

Macromolecules, Vol. 38, No. 17, 2005
Self-Sorting in Polymers 7227
bonded recognition units on polymers containing two
hydrogen-bonded moieties are able to find their comple-
mentary recognition units in the presence of each other,
we carried out self-assembly studies in 15:85 dioxane:
chloroform solutions. First we investigated the self-
assembly of small molecules onto the monomers and the
homopolymers of 1 and 2 by carrying out NMR titration
experiments of the monomers and the individual ho-
mopolymers thereby establishing the hydrogen-bonding
properties and measuring the association constants of
5 onto 1 and 6 onto 2 without the presence of competi-
tive moieties. All association constants were determined
by titration of the corresponding recognition unit (5 or
6) into a chloroform/dioxane solution of the polymer (1
and 2) and following the shift of the polymeric imide
signals by 1H NMR (Figures 3 and 4 and Table 2). The
association constants of the self-assembly steps of 5 onto
1 and 6 onto 2 were measured to be 1.0 × 102 and 3.0
× 102 M-1, respectively. These association constants are
very similar to the association constants between the
monomers and their corresponding units suggesting
that the bond strengths between the receptor units
along the polymer and their small molecule recognition
units are comparable to the bond strength of their
monomeric analogues. It is important to note that these
constants were measured in the presence of a competi-
tive solvent (mixture of dioxane:chloroform), thereby
reducing the association constant to reported values in
the literature that were obtained in pure halogenated
solvents.3,4 However, the use of this mixed solvent
system was necessary to tailor the solubility of both
homopolymers and copolymers throughout all self-
assembly experiments thereby allowing us to compare
all data.
To prove if the concept of self-sorting in synthetic
polymers is possible, we carried out titration experi-
ments on both block and random copolymers containing
50:50 mixtures of 1 and 2 using both recognition units
in a one pot procedure (Figures 5 and 6). Figure 6 shows
the amide region (10.5-13.5 ppm) of the 1H NMR
spectra of the homopolymers self-assembled with 2.8
equiv of their corresponding units in a mixture of 15:85
dioxane-d8:CDCl3 followed by both the block and ran-
dom copolymers assembled with 2.8 equiv of both
complementary units. The figure clearly shows that the
signals observed for the homopolymers (spectra A and
B) are a superposition with those observed for the
copolymers (spectra C and D). This superposition is
evident that self-sorting of these mixed solutions does
occur, i.e., each individual recognition unit finds its
receptor molecule in the copolymer mixtures with the
same fidelity as in the homopolymer solutions. We also
determined the binding constants of both side-chain
recognition units on the random and block copolymers.
The association constants were measured to be 1.0 ×
102 and 3.2 × 102 M-1 for the random copolymer and
6.0 × 101 and 2.5 × 102 M-1 for the block copolymer.
These numbers are strikingly similar to their noncom-
petitive homopolymer analogs which were measured to
be 1.0 × 102 and 3.0 × 102 M-1. It is important to note
that the association constants for the random and block
copolymers are the same within the error range of the
measurements. Combining these results on the associa-
tion constants measurements with the titration and
NMR data clearly demonstrates that, by simply select-
ing competitive recognition units that have well-defined
self-assembly motifs in equimolar concentrations, it is
Figure 1. Kinetic data for the ROMP of 1.
Table 1. Polymer Characterization Data for Polymers 1
and 2
[M]:[I]
Mn (10-3
)
Mw (10-3
)
PDI
poly-1
20
40
12.0
20.0
26.8
33.2
46.7
12.1
21.2
30.9
44.7
66.1
15.0
29.5
42.2
55.2
63.1
14.6
29.1
42.3
66.5
108
1.23
1.46
1.57
1.66
1.35
1.20
1.26
1.37
1.49
1.64
60
80
120
20
poly-2
40
60
80
120
obtained molecular weights and polydispersities are
displayed in Table 1.
For both monomers we explored the living nature of
the polymerization by investigating whether the stoi-
chiometry was decisive in the resultant polymers. A
linear relationship between Mn and the monomer-to-
catalyst ratios was found in both cases (Figure 2A). To
further characterize the living nature of the monomers,
we carried out homoblock copolymerizations in two
steps. A 20:1 [M]:[I] ratio of the desired monomer (1 or
2) was polymerized to completion and allowed to sit for
1 h. Subsequently, 100 equiv of additional monomer
were added. As shown in Figure 2B, a dramatic increase
in the molecular weight of 1 was observed for the
polymer after the addition of additional monomer. These
results in conjunction with the stoichiometric experi-
ments clearly prove the living nature of 1. While this
dramatic increase during the block copolymerization
experiment did not occur for polymers based on 2,
polymerization of 2 still displays a linear relationship
between Mn and monomer-to-catalyst ratios. This data
suggests but does not prove unequivocally the living
nature of the ROMP of 2.
After establishing the living nature of the ROMP of
1 and 2, we synthesized AB block copolymers starting
with monomer 1 followed by 2. A 50:1 [M]:[I] ratio of 1
was polymerized to completion. Subsequently, 50 equiv
of 2 were added and allowed polymerize to completion.
Random copolymers were synthesized by polymerizing
an equimolar solution of 1 and 2 (50 equiv each) to
completion. Both copolymers were characterized by GPC
1
and H NMR. Polymers were found to have molecular
weights between 1 and 7.0 × 104 vs poly(styrene)
standards, with polydispersities ranging from 1.2 to 1.6
(Table 1).
Self-Assembly. The pendant functional groups of our
polymers have been shown throughout the literature to
exhibit well-defined self-assembly behavior with their
corresponding recognition units, diamino pyridine for
1 and isophthalic wedge for 2.3,4 To address the question
whether self-sorting occurs on polymers, i.e., if hydrogen-

7228 Burd and Weck
Macromolecules, Vol. 38, No. 17, 2005
Figure 2. (A) Characterization of the living character of the ROMP of monomers 1 and 2: (9) stoichiometric polymerization of
1, (2) stoichiometric polymerization of 2. (B) GPC traces of polymers prepared using monomer 1. (blue) Polymer after complete
conversion ([M]:[I] ) 20:1, Mw ) 1.5 × 104, Mn ) 1.2 × 104, polydispersity index (PDI) ) 1.23). (red) Same polymer after standing
for 0.5 h followed by polymerization of 100 equiv ([M2]:[M1] ) 100:1, [M]:[I] ) 20:1, Mw ) 1.1 × 105, Mn ) 7.2 × 104, PDI ) 1.78)
of additional monomer.
Figure 3. Chemical shifts of the imide protons of poly-1 ([)
as a function of the equivalents of 5.
Figure 5. Self-sorting on polymers: NMR titration curve of
the addition of equimolar solutions of both recognition units
at once: (red circle) shift of the imide protons of the cyanuric
acid moieties of a cyanuric acid based homopolymers, (green
circle) shift of the imide protons of the cyanuric acids of a 50:
50 1:2-random copolymer, (blue triangle) shift of the imide
protons of the cyanuric acids of a 50:50 1:2-block copolymer,
(blue square) shift of the imide protons of the thymines of a
50:50 1:2-block copolymer, (green triangle) shift of the imide
protons of the thymines of a 50:50 1:2-random copolymer, and
(red rhombus) shift of the imide protons of the thymine
moieties of thymine homopolymers.
Figure 4. Chemical shifts of the imide protons of poly-2 ([)
as a function of the equivalents of 6.
Table 2. Association Constants Determined by 1H NMR
Data11
Ka [M-1
]
mers, the question arises whether individual recognition
units along the copolymers can be addressed by their
complementary receptors in the presence of a second
competitive recognition unit. To address this question,
we investigated the copolymers by measuring the as-
sociation constants using only one recognition unit at a
time, i.e., single self-assembly experiments in the pres-
ence of a competitive receptor (Figure 7). The binding
constants were determined by a stepwise titration of the
single recognition unit into a copolymer solution. The
dynamic equilibrium of the hydrogen bonds is respon-
sible for competition when a single-side chain’s pairing
nature is left unsatisfied. Because of the competitive
nature of the monomers (when there is only one
complementary receptor available for hydrogen-bond-
Association Constants of 1:5
monomer 1:5
1.0 × 102 ((4)
1.0 × 102 ((5)
8.0 × 101 ((8)
1.0 × 102 ((10)
poly-1:5
50:50 block copoly. 1:2:5
50:50 random copoly. 1:2:5
Association Constants of 2:6
monomer 2:6
poly-2:6
3.0 × 102 ((52)
3.0 × 102 ((60)
2.5 × 102 ((108)
3.2 × 102 ((134)
50:50 block copoly. 1:2:6
50:50 random copoly. 1:2:6
possible to translate the concept of self-sorting into
synthetic polymer chemistry.
With the previous result clearly demonstrating the
principle of self-sorting on random and block copoly-

Macromolecules, Vol. 38, No. 17, 2005
Self-Sorting in Polymers 7229
Figure 8. Chemical shifts of a 50:50 1:2 random copolymer
stepwise titration (first addition of 5 followed by the addition
of 6): ([) thymine imide proton shift associated with 5, (])
thymine imide proton shift associated with 6 after full as-
sembly of 5 (equivalents are for a combination of 5 and 6), (0)
cyanuric acid imide proton shifts associated with the addition
of 5, followed by the addition of 6 (9).
1
Figure 6. Self-sorting on polymers. Amide region of the H
NMR of the polymers after the addition of 2.8 equiv of
recognition units (A) homopolymer (50 repeating units) of 1
after the addition of 2.8 equiv of 5, (B) homopolymer (50
repeating units) of 2 after the addition of 2.8 equiv of 6, (C)
random copolymer of 1 and 2 after the addition of 2.8 equiv of
both 5 and 6, (D) block copolymer of 1 and 2 after the addition
of 2.8 equiv of both 5 and 6, (E) homopolymer (50 repeating
units) of 2 after the addition of 2.8 equiv of 5, and (F)
homopolymer (50 repeating units) of 1 after the addition of
2.8 equiv of 6.
Figure 9. Chemical shifts of a 50:50 1:2 random copolymer
stepwise titration, (first addition of 6 followed by the addition
of 5): ([) thymine imide proton shift associated with 6, (])
thymine imide proton shift associated with 5 after full as-
sembly of 6 (equivalents are for a combination of 5 and 6), (0)
cyanuric acid imide proton shifts associated with the addition
of 6, followed by the addition of 5 (9).
Figure 7. Chemical shifts of a 50:50 1:2 random copolymer
stepwise titration with only one recognition unit at a time:
(2) shift of the imide protons of the cyanuric acid moieties with
6; ([) shift of the imide protons of the thymine moieties with
5 on the copolymer.
when carrying out the second self-assembly titration,
these weak nonspecific hydrogen-bonding interactions
between the first receptor unit and its noncomplemen-
tary copolymer side-chain give way to a specific hydrogen-
bonding interaction between the second receptor and its
targeted recognition unit as evident by the strong and
selective titration curve. Furthermore, NO shifts of the
imide protons of the first targeted self-assembly pair are
observed, i.e., in Figures 8 and 9, the imide signals
corresponding to 1 and 2, respectively, do not change
or increase further; however the signals associated with
2 or 1 show a subsequent increase. The final placements
of each of the imide signals of both 1 and 2 in Figures
8 and 9 are an exact superposition of the full titration
of both of their corresponding homopolymers. Therefore,
while the binding constants in a competitive environ-
ment (only one recognition unit) are decreased, when
the competition is removed through the addition of a
second recognition unit, self-sorting is again observed.
ing), the association constants of the copolymers are
slightly decreased. The stronger interaction of 2:6 shows
less of a decrease from a Ka of 3.0 × 102 M-1 for the
homopolymer to 2.5 × 102 M-1 for the copolymer
systems, as opposed to that of the weaker interaction
of 1:5 from 1.0 × 102 M-1 to 6.0 × 101 M-1. This result
demonstrates that (a) competitive interactions take
place along the copolymer backbones and (b) by employ-
ing recognition units with stronger association constants
(such as 2:6) any interference from competitive moieties
can be minimized.
Finally, we investigated if the hydrogen-bonding and
self-sorting processes are truly dynamic processes. If the
notion of a truly dynamic process holds, a stepwise self-
assembly experiment onto the copolymers with both
receptor molecules should yield fully self-sorted copoly-
mers. Therefore, we carried out self-assembly experi-
ments on the random and block 50:50 copolymers in a
stepwise fashion using first one receptor (12 equiv)
followed by the second one and vice versa. Figures 8 and
9 show the results of these experiments. In both cases,
the titration curves of the first self-assembly step clearly
showed the expected shifts of the imide protons of the
targeted side-chain recognition unit along the copoly-
mers. However, the nontargeted recognition unit also
displayed some degree of hydrogen-bonding, which is
evident by the slow shifting of the imide protons of the
nontargeted side-chain recognition unit. Nevertheless,
Conclusion
We have clearly demonstrated that self-sorting can
be translated to synthetic side-chain-functionalized
polymers and occurs even in the presence of highly
competitive recognition units. This was shown by de-
signing, synthesizing (via ROMP), and self-assembling
copolymers containing both cyanuric and thymine acid-
based moieties with their corresponding recognition
units. The resulting copolymers can be viewed as
“universal polymer backbones” based solely on two

7230 Burd and Weck
Macromolecules, Vol. 38, No. 17, 2005
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competitive hydrogen-bonding recognition motifs. Selec-
tive functionalization of the resultant copolymers was
accomplished via a one-step orthogonal self-assembly
displaying self-sorting in a competitive environment.
This methodology, that can be viewed on a very primi-
tive level as a synthetic DNA analogue, has the potential
to create a variety of complex, rapidly functionalizable
materials for use in areas such as material science, drug
delivery, and biomimetic chemistry.
Acknowledgment. Financial support has been pro-
vided by the National Science Foundation (CHE-
0239385) and the Office of Naval Research (MURI,
Award No. N00014-03-1-0793). M.W. gratefully acknowl-
edges a 3M Untenured Faculty Award, a DuPont Young
Professor Award, an Alfred P. Sloan Fellowship, and a
Blanchard Assistant Professorship.
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