Synthesis and self assembling properties of rod-like, 2-ureido-4- pyrimidinone-based main chain supramolecular dendronized polymers

Article abstract of DOI:10.1021/ma101647j

A series of G1-G3 supramolecular dendronized polymers 6 bearing dimeric 2-ureido-4-pyrimidinone (UPy) units on the main chain and aliphatic hydrocarbon dendrons as side chain appendages was prepared. Because of the high crystallinity and poor solubility o

Full text of DOI:10.1021/ma101647j

Macromolecules 2010, 43, 8389–8399 8389
DOI: 10.1021/ma101647j
Synthesis and Self Assembling Properties of Rod-Like, 2-Ureido-4-
pyrimidinone-Based Main Chain Supramolecular Dendronized Polymers^
Chun-Ho Wong,† Wing-Shong Chan,† Chui-Man Lo,† Hak-Fun Chow,*,†,‡
To Ngai,† and Ka-Wai Wong§
†Department of Chemistry and The Center of Novel Functional Molecules, The Chinese University of
Hong Kong, Shatin NT, Hong Kong SAR, ‡Institute of Molecular Functional Materials, Areas of
Excellent Scheme, University Grants Committee, Hong Kong SAR, and §Department of Physics,
The Chinese University of Hong Kong, Shatin NT, Hong Kong SAR
Received July 22, 2010; Revised Manuscript Received September 1, 2010
ABSTRACT: A series of G1-G3 supramolecular dendronized polymers 6 bearing dimeric 2-ureido-4-
pyrimidinone (UPy) units on the main chain and aliphatic hydrocarbon dendrons as side chain appendages
was prepared. Because of the high crystallinity and poor solubility of such rigid rod polymers, only the G3
dendron could confer the resulting polymer 6 (n = 3) with enough solubility to enable its characterization and
property studies. It was found that the nature of the dendrons play an important role on the UPy binding
strength, solubility and self-assembly properties. The reversible nature of the polymerization process was
demonstrated in different solvent systems by viscosity studies. A double logarithmic plot of the specific
viscosity against concentration revealed a deflection point at 26 mM in CHCl3 at 26 and 40 ꢀC. Below this
critical concentration a straight line with a slope of 1.5 was obtained, while a slope of 4.0-4.2 was secured
above this concentration. The data suggested that the associative interaction between the di-UPy monomer 5
(n = 3) increased nonlinearly with increasing monomer concentration. While the above investigations
confirmed that these were main chain supramolecular dendronized polymers, UV-vis spectroscopic study
revealed a large bathochromic shift (32 nm) with increasing polymer concentration in CHCl3. This finding
was consistent with the formation of J-type aggregates via stair-case stacking between interchain UPy rings.
SEM morphological study also confirmed that the resulting polymers appeared as fibrous superbundles with
a very high aspect ratio. A model was purposed to rationalize how such bundles could be assembled from the
di-UPy dendritic macromonomer 5 via intrachain hydrogen bonding and interchain stacking interactions.
Introduction
Chart 1. Supramolecular dendronized polymers prepared via
(a) graft-to and (b) macromonomer polymerizations.
Dendronized polymers represent an interesting class of macro-
molecules with unique structural features such as rigidification of
the backbone with increasing size of the dendritic side chains.
They are generally prepared by graft-in, graft-from, and macro-
monomer polymerization approaches.1 Traditionally, preparation
of dendronized polymers mainly relies on covalent connection
between the dendritic monomers. However, examples of using
noncovalent methods, such as metal coordination, hydrogen bond-
ing, and/or host guest complexation to construct self-assembling
supramolecular dendronized polymers are rare, despite the fact
that these strategies have been successfully applied to the pre-
paration of many supramolecular nondendronized polymers.2
One particular interesting attribute of supramolecular polymers
lies on their controllable degree of polymerization (DP) through
changes in the solvents, temperature, and other external factors.
Hence, such polymers are actually self-healing yet degradable
systems and could prove extremely useful in materials applications.
The use of noncovalent methods to anchor dendrons to a
polymer backbone via the graft-to method has been exemplified.
For example, Stoddart employed host guest binding interaction
to graft cationic ammonium dendrons to a polystyrene or poly-
(acetylene) backbone containing crown ether side chain appen-
dages (Chart 1a).3 Similarly, Jiang disclosed that oligoether
dendrons bearing a carboxylic acid group at the focal point could
be grafted to a poly(4-vinylpyridine) chain through hydrogen
bonding interaction in chloroform.4 Main chain supramolecular
dendronized polymers based on macromonomer polymerization
had also been reported (Chart 1b). For example, Zimmerman dis-
closedthat dendronized Sn4þ-porphyrin macromonomers could
be linked together through metal-ligand iþnteraction via succinic
5
€
acid linkers. Wurthner also reported a Ag -promoted polymeri-
zation of dendronized bi(pyridine) macromonomers which led to
the synthesis of rigid rod dendronized metallopolymers.6 How-
ever, as metal-ligand interaction is relatively strong among the
many supramolecular interactions, in both cases, the reversibility
*Corresponding author. E-mail: hfchow@cuhk.edu.hk.
^ This paper is dedicated to Prof. Henry N. C. Wong on the occasion of his
60th birthday.
r 2010 American Chemical Society
Published on Web 09/28/2010
pubs.acs.org/Macromolecules

8390 Macromolecules, Vol. 43, No. 20, 2010
Wong et al.
Chart 2
of the self-assembly process, i.e., depolymerization, was therefore
not demonstrated.
bifunctional dendritic macromonomers should be relatively
straightforward. However, this construction process has several
inherent problems that are originated from the presence of the
dendritic appendage. First, the relatively larger size of the
dendron, especially the higher generation ones, can lower the
DP value of the polymer due to steric inhibition.6 Second, the
dendron itself can create a significant different microenvironment
polarity in comparison to that of an alkyl substituent normally
used in a model compound. This change of polarity can result in
an abrupt drop of binding constant and hence decreasing the DP
value. For example, Kaifer demonstrated that putting a G3
oligoamide-oligoester dendron at the urea end of a 2-ureido-4-
pyrimidinone (UPy) unit (4) lowered the dimerization constant
Instead of metal-ligand coordination, hydrogen bonding can
also be used as the driving force for self-assembly. The advantage
of using hydrogen bonding is that the self-assembly process is
readily reversible by change the solvent and temperature. How-
ever, despite many successes in using multiple hydrogen bonding
systems to prepare supramolecular nondendronized polymers,7
only a few examples were disclosed on applying this strategy to
prepare supramolecular dendronized polymers. For examples,
Zimmerman demonstrated that dendronized monomers bearing
different multiple hydrogen bonding motifs, such as bis(isophthalic
acid) (1),8 heterocyclic DDA AAD (2),9 and bis(ureidodeaza-
3
pterin) (3)10 could be used as linkages to construct supramolecular
dendronized polymers. However, due to preorganization of the
various hydrogen bonding moieties, the predominant species
formed were cyclic oligomers, and only a small amount of linear
oligomers/polymers was found. Percec also reported that amphi-
philic dendronized dipeptides could also self-assemble into helical
pores in highly nonpolar cyclohexane solutions.11 Hence, linear
supramolecular dendronized polymers were not deliberately pre-
pared as the target molecules in these studies.
from 107 to 3 M-1 12
. Meijer and Sijbesma also reported that an
appropriately placed oligo(ethylene oxide) chain could lead to a
102-103-fold drop of the strength of UPy binding.13 Third, the
dendron itself may induce a switch of the tautomeric equilibrium
of the heterocycle, which can again modify the binding constant
as well as the binding geometry. We recently demonstrated that
subtle changes of the steric congestion around the vicinity of the
UPy unit increased the % amount of the weaker binding DADA
4-pyrimidinol tautomer.14 Fourth, there is always the possibility
of forming cyclic oligomers when the dendritic monomer pos-
sesses a flexible backbone. In the case of using UPy as the binding
unit, Meijer again had reported many interesting findings in the
ring-chain equilibrium involving nondendritic di-UPy monomers.15
Generally, this cyclization problem is a minor issue if the monomer
concentration is well above the ring-chain transition critical
concentration and if the binding strength between the monomer
is very strong.16 Fifth, polymers with a rigid rod structure
generally possess poor solubility, and this can pose problems in
their synthesis, structural characterizations and property studies.
Last but not least, one should also ensure that the bifunctional
UPy dendritic monomer can be prepared in high structural
homogeneity, and the sample should be devoid of any mono-
functional UPy macromonomer, which can act as a chain stopper
and lower the polymerization efficiency. Herein we wish to report
our synthetic endeavor to tackle these problems, and to disclose
the first synthesis and self-assembling properties of rigid-rod
supramolecular dendronized polymers 6 based on the quadruple
hydrogen bonding between bifunctional UPy monomers 5 (see
Chart 2). Specifically, we noted that the size of the dendron had a
dramatic effect on the property of the resulting dendronized
polymers. Hence the lower generation dendronized polymers 5
At first glance, synthesis of rigid-rod, main-chain hydrogen
bond-mediated dendronized polymers from the self-assembly of

Article
Macromolecules, Vol. 43, No. 20, 2010 8391
(n = 1 and 2) are solids that are poorly soluble in most solvent
systems. On the other hand, the corresponding G3 analogue
possesses excellent solubility property in most organic solvents,
and allowed us to investigate its viscosity property even in highly
nonpolar solvents such as hexane. Based on UV spectroscopic
and SEM studies, it was found that the polymer chains further
self-assembled to form rod-like bundles due to stacking of the
UPy moieties in the form of J-type aggregates. This paper also
reveals that this self-assembling synthetic approach is a viable
method to generate new types of sterically congested dendronized
polymers with fascinating properties.
General Procedure for the Synthesis of Gn-Dibromides 10
(n = 1-3). To a mixture of Ph3P (6 mol equiv) and anhydrous
K2CO3 (1 mol equiv) in CH2Cl2 was added CBr4 (3 mol equiv) at
0 ꢀC. After stirring for 15 min at 25 ꢀC, a solution of
Gn-aldehyde 9 (1 mol equiv) in CH2Cl2 was added and the
resulting suspension was stirred at 25 ꢀC for 3 h. The reaction
mixture was concentrated under reduced pressure and hexane
was added. The suspension was filtered through a plug of silica
gel to obtain the crude product which was further purified by
flash chromatography (eluent: hexane) to give the target com-
pound.
G1-Dibromide 10 (n = 1). Starting from G1-aldehyde 9 (n =
1) (9.89 g, 0.054 mol), Ph3P (84.46 g, 0.32 mol), anhydrous
K2CO3 (7.42 g, 0.054 mol), and CBr4 (53.39 g, 0.16 mol) in
CH2Cl2 (300 mL), the target compound (11.80 g, 64%) was
obtained as a colorless liquid. Rf: 0.79 (hexane). δH: 0.88 (6 H, d,
J = 6.6, CH3), 0.89 (6 H, d, J = 6.7, CH3), 1.05-1.34 (6 H, m),
1.34-1.61 (4 H, m), 2.21-2.40 (1 H, m, CHC=C), 6.11 (1 H, d,
J = 9.8, CHdCBr2). δC: 22.7, 22.9, 28.3, 32.5, 36.4, 44.3, 87.þ9,
Experimental Section
Materials. Unless otherwise stated, all reagents were purchased
from commercial suppliers and used without further purification.
Tetrahydrofuran (THF) was freshly distilled prior to use from
sodium/benzophenone ketyl under N2. Dimethyl sulfoxide (DMSO)
and CH2Cl2 were freshly distilled from CaH2. N,N-Dimethyl-
formamide (DMF) was freshly distilled from CaSO4.
143.9. m/z (EI): 340 (Mþ). HRMS (EI): calcd for C13H24Br2
340.0219; found, 340.0216. SEC (RI): Rt 37.01.
,
Instrumentation. 1H NMR (300 MHz unless otherwise stated)
and 13C NMR (75.5 MHz) spectra were recorded on a Bruker
Avance DPX 300 nuclear magnetic resonance spectrometer. All
measurements were carried out at 25 ꢀC in CDCl3 unless other-
wise stated. Mass spectra analyses using electron spray ioniza-
tion (ESI) technique or electron impact (EI) technique were
obtained on a ThermoFinnigan MAT 95 XL double focusing
sector mass spectrometer, and MALDI-TOF analyses on a
MALDI Autoflex mass spectrometer. The reported molecular
mass (m/z) values, unless otherwise specified, were monoisoto-
pic mass. Elemental analyses were carried out at MEDAC Ltd.,
Brunel Science Center, Cooper’s Hill Lane, Egham, Surrey
TW20 0JZ, U.K. SEC measurements were performed on Waters
Styragel columns (HR1, HR2, HR3, and HR4 7.8 ꢀ 300 mm in
serial) at 40 ꢀC using THF as eluent (flow rate =1.0 mL/min) on
a Waters HPLC 515 pump equipped with a Waters 2489 tunable
UV absorbance detector or Viscotek LR40 laser refractometer.
The retention time (Rt) was reported in minute. Solution viscosity
was measured in CHCl3 solution (spectrophotometric grade,
stabilized with amylene) or hexane solution (distilled) at a specified
temperature in an Ubbelohde or a Zeitfuchs cross-arm viscom-
eter. Solutions were filtered over 5 mm filters before use. For
scanning electron microscopy measurements, the samples were
spin-dried with a CHEMAT Technology KW-4A spin-coater at
either low (300 rpm) or high spin rate (1500 rpm) at 25 ꢀC on a
silicon wafer, coated with Au particles using a Fisons Instruments
Polaron SC502 Stutter Coater with a spin rate of 6000 rpm and
subsequently studied by a Leo 1450 VP scanning electron
microscope at 20 kV and a 3.2 nA probe current.
Synthesis. All reactions were carried out under N2 unless
otherwise stated. The progress of the reactions was monitored
by thin layer chromatography (TLC) performed on Merck
precoated silica gel 60F254 plates, and compounds were visualized
with a spray of 5% (w/v) dodecamolybdophosphoric acid in
EtOH and subsequent heating. Flash chromatography was carried
out on columns of Merck Keiselgel 60 (230-400 mesh).
2-Amino-4-benzyloxy-6-methylpyrimidine (18). 2-Amino-4-
chloro-6-methylpyrimidine (17) (0.50 g, 3.48 mmol) was added
to a suspension of benzyl alcohol (0.54 mL, 5.22 mmol) and NaH
(0.21 g, 60% oil suspension, 5.22 mmol) in DMF (5 mL), and the
resulting mixture was heated to 50 ꢀC. After 24 h, the suspension
was filtered through a pad of Celite. After evaporation of solvent
under reduced pressure, the residue was purified by flash column
chromatography (eluent: hexane/EtOAc = 1/2) to give the target
compound (0.52 g, 72%) as a white solid. Mp: 110-112 ꢀC
(hexane/EtOAc). Rf: 0.39 (hexane/EtOAc = 1/2). δH: 2.26 (3 H,
s, CH3), 5.11 (2 H, s, NH2), 5.31 (2 H, s, CH2Ph), 6.01 (1 H, s,
pyrimidyl-H), 7.28-7.45(5 H, m, Ph). δC: 23.6, 67.4, 96.9, 128.0,
128.5, 136.7, 162.9, 168.3, 170.5. HRMS (MALDI-TOF): calcd
for C13H17N3O þ Hþ, 216.1131; found, 216.1130.
G2-Dibromide 10 (n = 2). Starting from G2-aldehyde 9
(n = 2) (4.95 g, 11.30 mmol), Ph3P (17.79 g, 67.82 mmol),
anhydrous K2CO3 (1.56 g, 11.30 mmol), and CBr4 (11.25 g,
33.91 mmol) in CH2Cl2 (30 mL), the target dibromide (5.81 g,
86%) was obtained as a colorless liquid. Rf: 0.84 (hexane). δH:
0.88 (24 H, d, J = 6.6, CH3), 1.03-1.58 (34 H, m), 2.30-2.46
(1 H, br s, CHCdC), 6.12 (1 H, d, J = 9.8, CHdCBr2). δC: 23.0,
24.4, 28.7, 31.4, 31.5, 33.9, 35.2, 36.1, 36.2, 37.9, 43.7, 88.0,
143.8. HRMS (MALDI-TOF): calcd for C31H60Br2 þ Agþ,
699.2094; found, 699.2069. Anal. Calcd for C31H60Br2:C, 62.83;
H, 10.20; Br, 26.97. Found: C, 62.93; H, 9.99; Br, 27.07. SEC
(RI): Rt 34.45.
G3-Dibromide 10 (n = 3). Starting from G3-aldehyde 9
(n = 3) (4.95 g, 11.30 mmol), Ph3P (3.63 g, 13.83 mmol), anhy-
drous K2CO3 (0.32 g, 2.31 mmol), and CBr4 (2.29 g, 6.92 mmol)
in CH2Cl2 (5 mL), the target product (2.35 g, 90%) was obtained
as a colorless liquid. Rf: 0.87 (hexane). δH: 0.90 (48 H, d, J = 6.6,
CH3), 1.02-1.43 (74 H, m), 1.51 (8 H, septet, J = 6.6, CHMe2),
2.30-2.50 (1 H, m, CHCdC), 6.12 (1 H, d, J = 9.9, CHdCBr2).
δC: 23.0, 23.87, 23.92, 24.5, 28.7, 31.5, 34.1, 34.3, 35.2, 36.2, 37.5,
38.0, 43.9, 87.9, 143.9. HRMS (MALDI-TOF): calcd for
C67H132Br2 þ Agþ, 1205.7730; found, 1205.7761. Anal. Calcd
for C67H132Br2: C, 73.32; H, 12.12; Br, 14.56. Found: C, 73.72;
H, 12.30; Br, 14.03. SEC (RI): Rt 32.37.
General Method for the Synthesis of Gn-Acetylenes 11 (n =
1-3). n-BuLi (1.6 M in hexane, 2.2 mol equiv) was added
dropwise to a solution of the Gn-dibromide 10 (1 mol equiv)
in THF at -78 ꢀC. After stirring for 30 min at -78 ꢀC and 30 min
at 25 ꢀC, the solution was poured into HCl (1 M) and extracted
with hexane. The combined extracts were washed with saturated
NaCl solution, dried (MgSO4) and filtered. After evaporation of
solvent under reduced pressure, the residue was purified by flash
column chromatography (eluent: hexane) to give the target
compound.
G1-Acetylene 11 (n = 1). Starting from G1-dibromide 10
(n = 1) (11.80 g, 0.035 mol) and n-BuLi (47.70 mL, 0.076 mol),
the target G1-acetylene (5.77 g, 92%) was obtained as a color-
less liquid. Rf: 0.62 (hexane). δH: 0.89 (12 H, d, J = 6.5, CH3),
1.15-1.64 (10 H, m), 2.04 (1 H, d, J = 2.4, CtCH), 2.16-2.32
(1 H, m, CHCtC). δC: 22.6, 22.9, 28.1, 32.2, 33.1, 36.7, 69.2,
88.1. m/z (EI): 179 (M - Hþ). HRMS (EI): calcd for C13H24-
Hþ, 179.1794; found, 179.1789. SEC (RI): Rt 37.20.
G2-Acetylene 11 (n = 2). Starting from G2-dibromide 10
(n = 2) (4.84 g, 8.17 mmol) and n-BuLi (11.23 mL, 17.97 mmol),
the target G2-acetylene (3.09 g, 87%) was obtained as a color-
less liquid. Rf: 0.71 (hexane). δH: 0.88 (24 H, d, J = 6.6, CH3),
1.02-1.31 (22 H, m), 1.31-1.58 (12 H, m), 2.03 (1 H, d, J = 2.3,
CtCH), 2.25-2.41 (1 H, m, CHCtC). δC: 22.9, 24.5, 28.6, 31.3,
31.5, 31.7, 33.6, 35.6, 36.0, 36.1, 37.9, 69.2, 88.3. HRMS

8392 Macromolecules, Vol. 43, No. 20, 2010
Wong et al.
(MALDI-TOF): calcd for C31H60 þ Agþ, 539.3740; found,
539.3719. Anal. Calcd for C31H60: C, 86.03; H, 13.97. Found: C,
85.83; H, 14.25. SEC (RI): Rt 34.52.
20 h, the suspension was filtered through a pad of Celite and the
filtrate was concentrated under reduced pressure to give the
target compound.
G3-Acetylene 11 (n = 3). Starting from G3-dibromide 10
(n = 3) (2.00 g, 1.82 mmol) and n-BuLi (2.51 mL, 4.01 mmol),
the target product (1.50 g, 88%) was obtained as a colorless
liquid. Rf: 0.80 (hexane). δH: 0.87 (48 H, d, J = 6.6, CH3),
1.03-1.55 (82 H, m), 2.03 (1 H, d, J = 2.3, CtCH), 2.23-2.40 (1
H, m, CHCtC). δC: 22.9, 23.75, 23.85, 24.6, 28.6, 31.4, 31.8,
33.8, 34.2, 34.3, 35.7, 36.1, 37.5, 37.9, 69.1, 88.4. HRMS
(MALDI-TOF): calcd for C67H132 þ Agþ, 1045.9387; found,
1045.9374. SEC (RI): Rt 32.32.
General Procedure for the Synthesis of Acetylenic G1-Diesters
14 (n = 1-3). A degassed solution of Gn-acetylene 11 (2.5-
3 mol equiv), ditriflate 13 (1 mol equiv), NEt3 (6 mol equiv),
PdCl2(PPh3)2 (0.15 mol equiv), and CuI (0.15-0.2 mol equiv) in
THF/DMF (1/1) was stirred at 25 ꢀC for 15 h. The reaction
mixture was taken up in a mixture of saturated NH4Cl solution,
hexane, and EtOAc. After filtration, the organic layer was
separated, washed with water and saturated NaCl solution,
dried (MgSO4), and filtered. After evaporation of solvent under
reduced pressure, the residue was purified as described in the
following text.
Acetylenic G1-Diester 14 (n = 1). Starting from G1-acetyl-
ene 11 (n = 1) (5.77 g, 32.00 mmol), ditriflate 13 (6.70 g, 12.93
mmol), NEt3 (10.80 mL, 77.49 mmol), PdCl2(PPh3)2 (1.27 g,
1.81 mmol), and CuI (0.50 g, 2.63 mmol). The crude product was
purified by precipitation from EtOH to give the target acetylenic
G1-diester (6.38 g, 85%) as a white solid. Mp: 80.5-82 ꢀC. Rf:
0.38 (hexane/EtOAc = 50/1). δH: 0.90 (12 H, d, J = 6.5, CH3),
0.91 (12 H, d, J = 6.5, CH3), 1.20-1.65 (26 H, m), 2.52 (2 H,
quint, J = 7.0, CHCtC), 4.38 (4 H, q, J = 7.1, OCH2), 7.93 (2
H, s, ArH). δC: 14.4, 22.6, 22.9, 28.2, 32.9, 33.4, 36.8, 61.6, 79.7,
101.7, 123.0, 134.7, 135.7, 165.7. m/z (ESI): 601 (M þ Naþ,
100%). HRMS (ESI): calcd for C38H58O4 þ Naþ, 601.4238;
found, 601.4247. Anal. Calcd for C38H58O4: C, 78.85; H, 10.10.
Found: C, 79.03; H, 10.34. SEC (UV): Rt 34.00.
Acetylenic G2-Diester 14 (n = 2). Starting from G2-acetyl-
ene 11 (n = 2) (3.09 g, 7.12 mmol), ditriflate 13 (1.23 g, 2.37
mmol), NEt3 (1.99 mL, 14.24 mmol), PdCl2(PPh3)2 (0.23 g, 0.33
mmol) and CuI (0.06 g, 0.33 mmol). The crude product was
purified by flash column chromatography (eluent: hexane/
EtOAc = 50/1) to get the target acetylenic G2-diester (2.31
g, 90%) as a yellow liquid. Rf: 0.66 (hexane/EtOAc = 50/1). δH:
0.849 (24 H, d, J = 6.6, CH3), 0.855 (24 H, d, J = 6.6, CH3),
1.03-1.34 (44 H, m), 1.34-1.65 (30 H, m), 2.52-2.69 (2 H, m,
CHCtC), 4.37 (4 H, q, J = 7.1, OCH2), 7.93 (2 H, s, ArH). δC:
14.5, 22.9, 24.7, 28.6, 31.3, 31.4, 33.0, 33.7, 35.6, 36.0, 36.1, 37.9,
61.5, 79.8, 101.6, 123.0, 134.7, 135.7, 165.6. m/z (ESI): 1106
(M þ Naþ, 100%). HRMS (ESI): calcd for C74H130O4 þ Naþ,
1105.9861; found, 1105.9866. Anal. Calcd for C74H130O4: C,
82.01; H, 12.09. Found: C, 81.61; H, 12.35. SEC (UV): Rt 31.99.
Acetylenic G3-Diester 14 (n = 3). Starting from G3-acetyl-
ene 11 (n = 3) (1.50 g, 1.60 mmol), ditriflate 13 (0.28 g, 0.53
mmol), NEt3 (0.45 mL, 3.20 mmol), PdCl2(PPh3)2 (0.056 g,
0.080 mmol), and CuI (0.015 g, 0.080 mmol). The crude product
was purified by flash column chromatography (eluent: hexane/
CH2Cl2 = 20/1) to get the target acetylenic G3-diester (0.56 g,
50%) as a yellow liquid. Rf: 0.16 (hexane). δH: 0.87 (96 H, d, J =
6.6, CH3), 1.01-1.64 (170 H, m), 2.50-2.70 (2 H, m, CHCtC),
4.38 (4 H, q, J = 7.1, OCH2), 7.95 (2 H, s, ArH). δC 14.5, 22.9,
23.7, 23.8, 24.8, 28.6, 31.4, 33.1, 33.9, 34.16, 34.24, 34.2, 35.7,
36.0, 37.4, 37.9, 61.5, 79.7, 101.7, 123.1, 134.5, 135.9, 165.5. HRMS
(MALDI-TOF): calcd for C146H274O4 þ Naþ, 2116.1164;
found, 2116.1180. Anal. Calcd C146H274O4: C, 83.75; H, 13.19.
Found: C, 83.82; H, 13.28. SEC (UV): Rt 30.31.
Saturated G1-Diester 15 (n = 1). Starting from acetylenic
G1-diester 14 (n = 1) (6.16 g, 10.64 mmol) and palladium black
(0.38 g), the target saturated G1-diester (6.25 g, 100%) was
obtained as a colorless liquid. Rf:: 0.50 (hexane/EtOAc = 50/1).
δH: 0.88 (24 H, d, J = 6.6, CH3), 1.02-1.22 (8 H, m), 1.22-1.35
(10 H, m), 1.40 (6 H, t, J = 7.1, CH2CH3), 1.44-1.57 (8 H, m),
2.80-2.92 (4 H, m, ArCH2), 4.38 (4 H, q, J = 7.1, OCH2), 7.64
(2 H, s, ArH). δC: 14.5, 22.9, 28.5, 31.2, 31.4, 36.0, 36.1, 38.3,
61.2, 132.6, 133.0, 141.7, 167.7. m/z (ESI): 609 (M þ Naþ,
100%). HRMS (ESI): calcd for C38H66O4 þ Naþ, 609.4864;
found, 609.4860. Anal. Calcd for C38H66O4: C, 77.76; H, 11.33.
Found: C, 77.94; H, 11.44. SEC (UV): Rt 33.89.
Saturated G2-Diester 15 (n = 2). Starting from acetylenic
G2-diester 14 (n = 2) (2.31 g, 2.13 mmol) and palladium black
(0.20 g), the target saturated G2-diester (2.30 g, 99%) was
obtained as a colorless liquid. Rf: 0.67 (hexane/EtOAc = 50/1).
δH: 0.87 (48 H, d, J = 6.6, CH3), 1.03-1.34 (60 H, m), 1.34-1.58
(20 H, m), 2.77-2.97 (4 H, m, ArCH2), 4.37 (4 H, q, J = 7.1,
OCH2), 7.64 (2 H, s, ArH). δC: 14.5, 22.9, 23.9, 28.6, 31.4, 31.6,
34.1, 34.4, 36.1, 36.5, 38.0, 38.1, 61.2, 132.6, 133.0, 141.7, 167.7.
HRMS (MALDI-TOF): calcd for C74H138O4 þ Naþ, 1114.0487;
found, 1114.0419. Anal. Calcd for C74H138O4: C, 81.40; H,
12.74. Found: C, 81.27; H, 12.55. SEC (UV): Rt 31.92.
Saturated G3-Diester 15 (n = 3). Starting from acetylenic
G3-diester 14 (n = 3) (0.37 g, 0.18 mmol) and palladium black
(0.01 g), the target saturated G3-diester (0.34 g, 92%) was
obtained as a colorless oil. Rf: 0.67(hexane/EtOAc = 50/1). δH:
0.87 (96 H, d, J = 6.6, CH3), 1.01-1.35 (146 H, m), 1.35-1.60
(30 H, m), 2.75-2.99 (4 H, m, ArCH2), 4.37 (4 H, q, J = 7.1,
OCH2), 7.64 (2 H, s, ArH). δC: 14.6, 23.0, 23.9, 24.0, 28.6, 31.5,
31.7, 34.3, 34.6, 36.2, 36.5, 37.6, 38.0, 38.4, 61.1, 132.8, 133.0,
141.9, 167.5. HRMS (MALDI-TOF): calcd for C146H282O4 þ
Naþ, 2124.1789; found, 2124.1760. SEC (UV): Rt 30.25.
General Procedure for the Synthesis of Gn-Diacids 16 (n = 1
and 2). A mixture of saturated Gn-diester 15 (n = 1 and 2)
(1 mol equiv) and aqueous KOH (1 M in H2O, 10-20 mol equiv)
in THF/CH3OH (1/1) was heated under reflux for 18 h. The
mixture was cooled to 25 ꢀC and acidified with diluted HCl (1 M)
to pH = 2. The aqueous phase was extracted with EtOAc and
the combined organic layers were washed with water and
saturated NaCl solution, dried (MgSO4), and filtered. The filtrate
was concentrated under reduced pressure to give the target
compound.
G1-Diacid 16 (n = 1). Starting from saturated G1-diester 15
(n = 1) (6.25 g, 10.64 mmol) and aqueous KOH (110 mL, 0.11
mol), the target G1-diacid (5.49 g, 97%) was obtained as a white
solid. mp: 216-218 ꢀC (CH3OH). Rf: 0.25 (hexane/EtOAc = 10/
1). δH (DMSO-d6): 0.84 (24 H, d, J = 6.6, CH3), 0.98-1.33 (18
H, m), 1.33-1.56 (8 H, m), 2.75-2.93 (4 H, m, ArCH2), 7.60 (2
H, s, ArH), 13.08 (2 H, s, COOH). δC (DMSO-d6): 22.5, 22.6,
27.8, 30.3, 30.5, 35.3, 35.5, 37.3, 132.1, 133.2, 140.8, 168.4.
HRMS (MALDI-TOF): calcd for C34H58O4 þ Naþ, 553.4227;
found, 553.4265. Anal. Calcd for C34H58O4: C, 76.93; H, 11.01.
Found: C, 77.07; H, 11.34. SEC (UV): Rt 34.10.
G2-Diacid 16 (n = 2). Starting from saturated G2-diester 15
(n = 2) (2.30 g, 2.10 mmol) and aqueous KOH (42 mL, 42 mmol),
the target G2-diacid (1.96 g, 99%) was obtained as a colorless
liquid. Rf: 0.49 (hexane/EtOAc = 30/1). δH: 0.85 (48 H, d, J =
6.6, CH3), 1.01-1.70 (74 H, m), 2.84-3.16 (4 H, m, ArCH2),
7.90 (2 H, s, ArH), 10.99-13.25 (2 H, br s, COOH). δC: 22.9,
23.9, 28.6, 31.4, 31.8, 34.1, 34.3, 36.1, 36.4, 38.0, 38.1, 132.1,
134.1, 143.5, 173.3. m/z (ESI): 1058 (M þ Naþ, 100%). HRMS
(ESI): calcd for C70H130O4 þ Naþ, 1057.9861; found, 1057.9866.
SEC (UV): Rt 31.99.
General Procedure for the Synthesis of Saturated Gn-Diesters
15 (n = 1-3). A mixture of the acetylenic Gn-diester 14 and
palladium black (3-8% weight of Gn-diester) in absolute
EtOH/THF (1/1) was stirred under H2 (1 atm) at 25 ꢀC. After
G3-Diacid 16 (n = 3). A mixture of potassium tert-butoxide
(0.28 g, 2.49 mmol) and water (0.01 mL, 0.62 mmol) in THF
(5 mL) was stirred at 0 ꢀC for 5 min. A solution of the saturated

Article
Macromolecules, Vol. 43, No. 20, 2010 8393
G3-diester 15 (n = 3) (0.33 g, 0.16 mmol) in THF (5 mL) was
added and the resulting suspension was stirred 25 ꢀC for 48 h.
The mixture was acidified with diluted HCl (1 M) to pH = 2.
The aqueous phase was extracted with CH2Cl2 (3 ꢀ 20 mL) and
the combined organic layers were washed with water and satura-
ted NaCl solution, dried (MgSO4) and filtered. After evapora-
tion of solvent under reduced pressure, the residue was purified
by flash column chromatography (eluent: hexane/EtOAc =
50/1) to afford the target G3-diacid (0.31 g, 98%) as a colorless
liquid. Rf: 0.76 (hexane/EtOAc = 20/1). δH: 0.87 (96 H, d, J =
6.6, CH3), 1.00-1.73 (170 H, m), 2.90-3.10 (4 H, m, ArCH2),
7.91 (2 H, s, ArH), 9.12-12.36 (2 H, br s, COOH). δC: 23.0, 23.8,
24.0, 28.6, 31.4, 31.9, 34.2, 34.3, 34.6, 36.1, 36.6, 37.5, 37.9, 38.4,
131.8, 134.1, 143.5, 171.8. HRMS (MALDI-TOF): calcd for
C142H274O4 þ Naþ, 2068.1163; found, 2068.1261. Anal. Calcd
for C142H274O4: C, 83.37; H, 13.50. Found: C, 83.40; H, 13.96.
SEC (UV): Rt 30.30.
General Procedure for the Synthesis of Gn-Bifunctional Pro-
tected Monomers (n = 1 and 2). A solution of Gn-diacid 16
(1 molequiv), NEt3 (2.5 molequiv), andDPPA(2.5-3 mol equiv)
in toluene was stirred at 25 ꢀC for 2 h. The amine 18 (2.3 mol
equiv) was then added, and the resulting mixture was heated
under reflux for 20 h. The reaction mixture was concentrated
and purified by precipitation from EtOAc to give the target
compound.
2.52-2.80 (4 H, m, ArCH2), 5.35 (4 H, s, CH2Ph), 6.26 (2 H, s,
pyrimidyl-H), 7.21 (2 H, s, ArNHCONH), 7.29-7.47 (10 H, m,
Ph), 7.69 (2 H, s, ArH), 11.18 (2 H, s, ArNH). δC: 22.9, 23.7,
23.9, 24.0, 28.6, 29.3, 31.4, 34.1, 34.3, 34.5, 34.6, 36.0, 37.5, 37.9,
38.2, 68.4, 101.2, 125.1, 128.3, 128.5, 128.7, 132.4, 133.5, 136.0,
152.5, 157.3, 166.9, 170.4. HRMS (MALDI-TOF): calcd for
C
166H298N8O4 þ Na, 2492.3286; found, 2492.3263. Anal. Calcd
for C166H298N8O4: C, 80.71; H, 12.16; N, 4.54. Found: C, 81.04;
H, 12.31; N, 4.73. SEC (UV): Rt 30.22.
G1 Supramolecular Dendronized Polymer 6 (n = 1). A mixture
of the G1-bifunctional protected monomer 19 (n = 1) (0.20 g,
0.21 mmol) and palladium black (0.06 g) in CH3OH (12 mL) and
CHCl3 (120 mL) was stirred under H2 (1 atm) at 25 ꢀC. After
24 h, the suspension was filtered through a pad of Celite and the
filtrate was concentrated under reduced pressure and purified by
precipitation from CH3OH to give the target G1-dendronized
polymer (0.12 g, 74%) as a white powder. δH (400 MHz, DMSO-
d6, signal of benzylic Hs were obscured by residue solvent
signals): 0.79 (24 H, d, J = 6.4, CH3), 0.98-1.13 (8 H, m), 1.13-
1.30 (10 H, m), 1.30-1.55 (8 H, m), 2.17 (6 H, s, pyrimidyl-
CH3), 5.84 (2 H, s, pyrimidyl-H), 7.48 (2 H, s, ArH), 9.48-10.29
(4 H, br s, NHCONH), 11.00-11.72 (2 H, br s, NH in UPy).
HRMS (MALDI-TOF): calcd for C44H80N8O4 þ Naþ, 797.5412;
found, 797.5420. Anal. Calcd for C44H80N8O4: C, 68.18; H,
9.10; N, 14.45. Found: C, 68.02; H, 9.63; N, 14.38.
G2 Supramolecular Dendronized Polymer 6 (n = 2). A mixture
of the G2-bifunctional protected monomer 19 (n = 2) (0.10 g,
0.07 mmol) and Pd(OH)2-C (0.04 g) in CH3OH (6 mL) and
CHCl3 (60 mL) was stirred under H2 (1 atm) at 25 ꢀC. After 24 h,
the suspension was filtered through a pad of Celite and the
filtrate was concentrated under reduced pressure and purified by
precipitation from CH3OH to give the target G2-dendronized
polymer (0.12 g, 80%) as a white powder. Once precipitated, the
compound could not be redissolved in any solvents to allow
structural characterization. HRMS (MALDI-TOF): calcd for
C80H142N8O4 þ Hþ, 1280.1227; found, 1280.1243.
G1-Bifunctional Protected Monomer 19 (n = 1). Starting
from G1-diacid 16 (n = 1) (2.57 g, 4.84 mmol), NEt3 (1.69 mL,
12.10 mmol), DPPA (4.00 g, 14.52 mmol), and compound 18
(2.40 g, 11.15 mmol), the G1-bifunctional protected monomer
(2.81 g, 61%) was obtained as a white solid. Mp: 206-207 ꢀC
(toluene). Rf: 0.18 (hexane/EtOAc = 5/1). δH: 0.77 (12 H, d, J =
6.6, CH3), 0.78 (12 H, d, J = 6.6, CH3), 0.93-1.45 (22 H, m),
1.47-1.66 (4 H, m), 2.38 (6 H, s, pyrimidyl-CH3), 2.55-2.76
(4 H, m, ArCH2), 5.35 (4 H, s, CH2Ph), 6.27 (2 H, s, pyrimidyl-H),
7.21 (2 H, s, ArNHCONH), 7.29-7.46 (10 H, m, Ph), 7.65 (2 H,
s, ArH), 11.18 (2 H, s, ArNH). δC: 22.7, 23.8, 28.4, 29.4, 31.2,
34.3, 35.9, 38.3, 68.4, 101.1, 125.3, 128.3, 128.4, 128.7, 132.5,
133.6, 136.0, 152.6, 157.3, 166.9, 170.4. m/z (ESI): 978 (M þ
Naþ, 100%). HRMS (ESI): calcd for C58H82N8O4 þ Naþ,
977.6362; found, 977.6367. Anal. Calcd for C58H82N8O4: C,
72.92; H, 8.65; N, 11.72. Found: C, 73.41; H, 8.88; N, 11.93. SEC
(UV): Rt 32.97.
G2-Bifunctional Protected Monomer 19 (n = 2). Starting
from G2-diacid 16 (n = 2) (1.73 g, 1.67 mmol), NEt3 (0.58 mL,
4.18 mmol), DPPA (1.15 g, 4.18 mmol), and compound 18 (1.15
g, 4.18 mmol), the G2-bifunctional protected monomer (1.68 g,
69%) was obtained as a white solid. Mp: 145-146 ꢀC (THF/
EtOH). Rf: 0.25 (hexane/EtOAc = 10/1). δH: 0.85 (48 H, d, J =
6.5, CH3), 0.99-1.28 (60 H, m), 1.28-1.37 (2 H, m), 1.45 (8 H,
septet, J = 6.5, CHMe2), 1.53-1.66 (4 H, m), 2.39 (6 H, s,
pyrimidyl-CH3), 2.59-2.75 (4 H, m, ArCH2), 5.35(4H, s, CH2Ph),
6.26 (2 H, s, pyrimidyl-H), 7.18 (2 H, s, ArNHCONH), 7.30-
7.46 (10 H, m, Ph), 7.68 (2 H, s, ArH), 11.15 (2 H, s, ArNH). δC:
22.86, 22.88, 23.8, 28.5, 29.3, 31.28, 31.33, 34.0, 34.3, 34.5, 35.98,
36.01, 37.9, 68.4, 101.1, 125.1, 128.3, 128.4, 128.7, 132.5, 133.4,
136.0, 152.5, 157.3, 166.9, 170.4. HRMS (MALDI-TOF): calcd
for C94H154N8O4 þ Hþ, 1461.2198; found, 1461.2208. SEC (UV):
Rt 31.62.
G3 Supramolecular Dendronized Polymer 6 (n = 3). A mixture
of the G3-bifunctional protected monomer 19 (n = 3) (0.12 g,
0.05 mmol) and Pd(OH)2-C (0.04 g) in CH3OH (6 mL) and
CHCl3 (60 mL) was stirred under H2 (1 atm) at 25 ꢀC. After 24 h,
the suspension was filtered through a pad of Celite and the
filtrate was concentrated under reduced pressure and purified by
precipitation from EtOAc to give the target G3-dendronized
polymer (0.09 g, 81%) as a white syrup. δH (400 MHz) 4[1H]-
pyrimidinone tautomer (76%): 0.86 (96 H, d, J = 6.3, CH3),
0.98-1.73 (170 H, m), 2.20 (6 H, s, pyrimidyl-CH3), 2.50-3.10
(4 H, s, ArCH2), 5.80 (2 H, s, pyrimidyl-H), 7.20 (2 H, s, ArH),
11.80 (2 H, br s, NHCONH), 13.28 (1 H, br s, NH in UPy);
4-pyrimidinol tautomer (24%): 0.86 (96 H, d, J = 6.3, CH3),
0.98-1.73 (170 H, m), 2.39 (6 H, s, pyrimidyl-CH3), 2.50-3.10
(4 H, s, ArCH2), 6.23 (2 H, s, pyrimidyl-H), 7.71 (2 H, s, ArH),
11.36 (2 H, br s, NHCONH), 13.28 (4 H, br s, NH in UPy). δC
(100 MHz, some aromatic signals were too weak to be observed):
22.9, 23.0, 23.8, 24.0, 28.6, 31.3, 31.4, 34.1, 34.2, 34.26, 34.31,
148.1, 155.0, 166.8, 172.6. HRMS (MALDI-TOF): calcd for
C
152H286N8O4 þ Naþ, 2312.2347; found, 2312.2375. SEC (UV):
Rt 30.00.
G3-Bifunctional Protected Monomer 19 (n = 3). A solution
of G3-diacid 16 (n = 3) (0.61 g, 0.30 mmol) and NEt3 (0.42 mL,
3.00 mmol) in toluene (5 mL) was stirred at 60 ꢀC for 2 h. The
reaction mixture was cooled to 25 ꢀC and DPPA (0.83 g, 3.00
mmol) was added. After the reaction was stirred at 25 ꢀC for 2 h,
compound 18 (0.19 g, 0.90 mmol) was then added, and the
reaction mixture was heated under reflux for 48 h. The reaction
mixture was concentrated and purified by flash column chromato-
graphy (eluent: hexane/EtOAc = 30/1) to get the target G3-
bifunctional protected monomer (0.36 g, 49%) as a colorless
liquid. Rf: 0.46 (hexane/EtOAc = 10/1). δH: 0.86 (96 H, d, J =
6.6, CH3), 0.99-1.69 (170 H, m), 2.39 (6 H, s, pyrimidyl-CH3),
Results and Discussion
1. Design of Di-UPy Macromonomers 5. The UPy moiety
was chosen as the binding unit due to its extremely strong
self-dimerization constant (Kdim = 6 ꢀ 107 M-1) in CHCl3 at
25 ꢀC. This will ensure that the supramolecular dendronized
polymer formed will be of high DP values. In principle, any
nonpolar dendron could be used as the dendritic component
so long as it does not create a highly polar environment or it
does not engage in backbiting to the UPy unit. Previously we
ꢀ
showed that Frechet-type oligo(benzyl ether) dendrons
could preserve the native binding strength of the UPy unit,

8394 Macromolecules, Vol. 43, No. 20, 2010
Scheme 1. Synthetic Route To Target Bifunctional Dendritic Monomers 5 (n = 1-3)
Wong et al.
and they also did not perturb the preferential formation of
the DDAA tautomer,17 and therefore were ideal candidates.
However, due to synthetic hurdles encountered during the
introduction of the two UPy units onto the aromatic core, we
needed to anchor the UPy moieties initially as their O-benzyl
protected derivatives (vide infra). Hence, in the final depro-
tection step of the O-benzyl groups, the oligo(benzyl ether)
dendrons could not withstand the hydrogenolysis condi-
tions. In the end, our previously reported aliphatic hydro-
carbon dendrons18 were chosen for this job. They are known
to possess excellent solubilzation properties in nonpolar
organic solvents than their linear counterparts, and can also
preserve the nonpolar microenvironment around the UPy
unit, although they were found to induce more (4-6%) of
the weaker binding DADA tautomers.14 To ensure that the
two UPy units were arranged in a linear fashion, they were
anchored to a dendronized aromatic ring in a para relation-
ship. However, we did notice in a recent report that cyclic
species were formed even when the UPy groups were pre-
organized in a fixed angle of 180ꢀ.19
First, compound 7 was only soluble in highly polar solvents
while the G1 diacid 16 (n = 1)—precursor of the G1
diisocyanate 8 (n = 1)—had a relatively poor solubility in
these polar solvents, therefore it was difficult to find a
suitable solvent to facilitate the reaction. Second, the amino
group in isocytosine 7 had a lower nucleophilicity, and hence
higher reaction temperature and longer reaction time were
required to force the reaction to proceed. Under such harsh
conditions the starting materials began to decompose and no
desired product could be isolated from the reaction.
To solve the problem stated above, it was decided to use an
O-benzyl-protected amino compound 18 to react with diiso-
cyanates 8 (Scheme 2). There are several advantages of using
this O-benzyl-protected compound. First, it has better solubility
in many common organic solvents. Second, the nucleophilicity
of compound 18 is higher than that of 6-methylisocytosine
(7). Third, after the reaction, any monocoupled product, if
present, can be easily separated from the doubly coupled
target compound 19. Fourth, the benzyl groups in com-
pounds 19 can be cleanly removed by hydrogenolysis in high
yield to give the target monomers 5. Hence, this modified
synthetic route was used for the synthesis of our G1-G3
dendronized supramolecular polymers 6. It should be men-
tioned here that the use of O-benzyl protected group in the
synthesis of UPy compounds had already been demonstrated
by Meijer.23
The amino compound 18 was prepared in 72% yield by
heating a mixture of benzyl alcohol, NaH, and commercially
available 2-amino-4-chloro-6-methylpyrimidine (17) in DMF
using a literature procedure.24 Reaction of 2 equiv of 18 with
the G1-G3 diisocyanates 8 proceeded smoothly to give the
doubly protected monomers 19 in 49-69% yield.
The two O-benzyl groups had to be removed from the
bifunctional protected monomers 19 in order to release the
target dendronized di-UPy monomers 5. However, two
critical issues had to be addressed in the final synthetic step.
First, hydrogenolysis of the two O-benzyl groups must go to
100% completion. If only one of them is removed, the
resulting mono-UPy compound will serve as a polymer chain
stopper that will inevitably lower the DP value. Second, once
the hydrogenolysis has proceeded to a certain extent, there
will be the presence of the mono-UPy and di-UPy compunds,
together with the supramolecular oligomers formed from
these two species. Under this circumstance, it is necessary to
maintain the solubility of the many species in the reaction
2. Synthesis. We initially employed the coupling reaction20
between 6-methylisocytosine (7) and a dendronized diiso-
cyanate 8 to arrive at the target dendronized polymers 6
(Scheme 1). Hence, the known dendritic aldehydes 918 (G1-
G3) were converted to the corresponding acetylenes 11 in
59-79% yield via the vinylic dibromides 10 using a Corey-
Fuchs reaction sequence.21 The terminal acetylenes (2.5-3.0
equiv) 11 were then coupled to a ditriflate 13, prepared from
diethyl 2,5-dihydroxyterephthalate (12) employing a litera-
ture procedure,22 in the presence of (Ph3P)2PdCl2, CuI, and
NEt3 to afford the unsaturated diesters 14 in 50-90% yield.
The triple bonds were then saturated in the presence of
Pd-black to yield the dendronized diesters 15, followed by
alkaline hydrolysis (KOH or KO-t-Bu) to produce the
diacids 16 in 90-98% overall yield. The diacids 16 were then
reacted with diphenyl phosphorazidate (DPPA) and NEt3 to
give the corresponding diacylzides, and immediately sub-
jected to Curtius rearrangement by heating at 120 ꢀC to
generate the diisocyanates 8 in situ.
The G1 diisocyanate analogue 8 (n = 1) was then allowed
to react with 2 equiv of 6-methylisocytosine 7 in an attempt
to prepare the G1 di-UPy derivative 5 (n = 1), which should
then self-assemble to produce the supramolecular G1 den-
dronized polymer 6 (n = 1) in nonpolar solvents. However,
problems were encountered during the coupling reaction.

Article
Macromolecules, Vol. 43, No. 20, 2010 8395
Scheme 2. Synthetic Route To Target Bifunctional Dendritic Monomers 5 (n = 1-3) via an O-Benzyl Protection Strategy
medium. If precipitation occurs during hydrogenolysis, the
mono-UPy unit, still carrying one O-benzyl moiety, located
at the oligomer chain ends will have difficulty to undergo
further hydrogenolysis due to phase separation. This will
lead to incomplete removal of the O-benzyl groups and hence
a poor DP value. This precipitation problem is particularly
acute for rigid rod supramolecular polymers such as those
described here. In principle, the latter problem could be
alleviated by adding hydrogen bond donor or acceptor
solvents such as acetic acid or alcoholic solvents in the
reaction medium, which can reduce the extent of oligomer
formation and also keeps the highly polar di-UPy monomer
in solution.
Hydrogenolysis of the G1 and G2 protected monomers 19
was first carried out using 10% palladium on charcoal or
palladium black in THF as the solvent. Removal of the first
O-benzyl group was very fast and resulted in the immediate
formation of dimers 20. However, dismantling of the second
one proceeded very slowly, and formation of insoluble
oligomeric species was noted. At this point, the reaction
failed to proceed further and the hydrogenolysis was just
partially completed. This was particularly problematic with
the G1 and G2 series, as they lacked larger hydrocarbon
dendrons to enhance their solubility. We also tried adding
EtOH or acetic acid to the reaction mixture in order to break
down the UPy quadruple hydrogen bonding interactions but
the precipitation problem still persisted. Furthermore, the
precipitated G1 and G2 oligomers could not be redissolved in
most organic solvents for structural characterization. It was
also difficult to monitor the extent of the reaction by thin
layer chromatography as significant tailing on the plates was
observed.
could be redissolved in polar solvents such as DMF and
DMSO, the G2 analogue (yield =80%) was devoid of any
solubility in any solvents. The G3 analogue (yield = 81%),
on the other hand, was highly soluble in nonpolar solvents.
3. Structural Characterizations. The structural identities of
the compounds were confirmed by a combination of 1H and
13C NMR spectroscopy, mass spectrometry (MS) and size
1
exclusion chromatography (SEC). The H NMR spectral
signals of the various bifunctional compounds 14-16 and 19
are well separated into distinctive regions. The first is the set
of signals originated from the hydrocarbon dendrons at δ
0.8-1.6 and δ 2.4-3.0. The second is due to the set of signals
of the nondendritic substituents attaching to the central
aromatic core, and the third is a singlet located at ∼ δ
7.6-7.9 due to the aromatic hydrogens of the central core;
its exact position is dependent on the nature of the nonden-
dritic substituents. After coupling of the pyrimidine rings to
the central core, the bifunctional O-benzyl protected mono-
mers 19 (n = 1-3) are readily diagnosed by the signal (δ
6.25) correspond to the pyrimidine nucleus and that (δ 5.3) of
the benzyl protons. Of particular interest is that the two urea
N-H protons resonate at very different positions (δ 11.2 and
7.2). The downfield signal is the one that can form an
intramolecular hydrogen-bond to the pyrimidyl nitrogen.
Incidentally, all the chemical shift values of these signals are
independent of the dendrimer generation.
The molecular mass of all compounds were also deter-
mined by mass spectrometry. For compounds with higher
molecular weight values, the use of MALDI-TOF was needed
in order to obtain the molecular ion peak. The obtained
experimental values agreed well with the theoretical values.
The purity of the synthesized compounds was also checked
by SEC (Styragel HR1, HR2, HR3, and HR4 columns in
serial, THF, 40 ꢀC) using polystryenes as the standards.
Because of the significant difference in the nature of our
compounds and polystyrenes, the calculated Mw values were
slightly different from the theoretical ones. In all cases, a
sharp peak with a polydispersity index (PDI) of less than 1.04
was found, confirming the compounds were of high homo-
geneity.
It was then decided to monitor the reaction by 1H NMR
spectroscopy. Hydrogenolysis of the G1 and G2 compounds
19 was carried out in diluted solutions (∼1 mM) using
catalytic Pd(OH)2 on charcoal in a 10/1 mixture of CDCl3/
CD3OD. Under the reaction conditions, no precipitate was
formed during the reaction and complete removal of the
O-benzyl groups was realized. Hence, compounds 19 could be
cleanly converted to the supramolecular dendronized poly-
mers 6 by conducting the hydrogenolysis under the above-
mentioned conditions. After that the catalyst was removed
by filtration and the products 5 were isolated by precipitation
with CH3OH (n = 1 and 2) or EtOAc (n = 3). While the
precipitated G1 dendronized polymer 5 (n = 1, yield =74%)
4. Supramolecular Polymerizations of 5. The G1 dendro-
nized di-UPy monomer 5 (n = 1) is insoluble in nonpolar
solvents and has very poor solubility even in polar solvents
such as DMSO and DMF. Therefore, NMR spectroscopy
could only be conducted in DMSO-d6 in diluted solution

8396 Macromolecules, Vol. 43, No. 20, 2010
Wong et al.
Figure 2. Partial 1H NMR (400 MHz) spectrum of 8.7 mM solutions of
supramolecular dendronized polymer 6 (n = 3) in (a) 10%, (b) 25% and
(c) 35% DMSO-d6 in CDCl3. O, b, and 0 indicate signal peaks of the
DDAA, DADA, and 4[3H]-pyrimidinone species, respectively.
the binding strength between the UPys should be slightly
weakened. Nonetheless, the positions of the NH signals in
both the DDAA and DADA forms did not change in the
concentration range between 58 mM and 10 μM (see Sup-
porting Information for details). Even at the lowest concen-
tration (10 μM), the polymeric signals were still observed and
no monomeric signals were found. Assuming the polymeric
species formation was over 90% at the lowest concentration,
the binding strength between the UPy units in the DDAA
form was estimated to be 107 M-1 in CDCl3 at 25 ꢀC.
The self-assembling properties of the G3 suparmolecular
dendronized polymer 6 (n = 3) was further investigated by
1H NMR spectroscopy in 10%, 25%, and 35% DMSO-d6 in
CDCl3 (8.7 mM) (Figure 2). It was found that signals (δ
8.8-11.3) corresponding to the monomeric 4[3H]-pyrimidi-
none appeared in all three solvent systems. In 10% DMSO-
d6, the monomer as well as the polymer signals (δ 11.5-13.2)
from both the DDAA and DADA forms were still observa-
ble. The intensity ratio of polymer to monomer signals was
about 1/1, which indicated that polymer formation was
greatly depressed. Incidentally the relative amount of DADA,
as compared to DDAA tautomeric form, also decreased.
This result was consistent with the literature finding, in
which polar solvent systems disfavored the formation of
DADA tautomer.20 When the percentage of DMSO-d6 was
increased to 25%, the DADA signals disappeared and the
intensities of the monomeric 4[3H]-pyrimidinone signals became
higher than those of DDAA. From the signal integrations,
only 10% of the UPy groups were in polymeric form. In 35%
DMSO, the polymer signals completely vanished and only
signals corresponding to the monomeric 4[3H]-pyrimidinone
were observed. The 13C NMR spectrum of polymer 6 (n = 3)
was also recorded in CDCl3. Because of its polymeric nature,
the peaks were very broad. While the signals due to the
aliphatic dendrons could be clearly identified, those of the
aromatic and heteroaromatic carbons gave very weak signals.
The G3 supramolecular polymer 6 (n = 3) was analyzed
by MALDI-TOF mass spectrometry. The molecular ion
peak (M þ Naþ: 2312.2375) matched well to the theoretical
value (2312.2347). However, no peak due to oligomeric
species could be observed. The polymer sample was also
subjected to SEC analysis in THF at 40 ꢀC. In this case, a
sharp peak was found with a PDI value of 1.01. Incidentally,
the retention time (Rt = 30.00 min) was very close to that
(Rt = 30.22 min) of the di-O-benzyl derivative 20 (n = 3),
suggesting the polymer dissociated into individual monomer
unit 5 (n = 3) under the column conditions.
Figure 1. 1H NMR spectrum (400 MHz, CDCl3, 8.7 mM) of G3
supramolecular dendronized polymer 6 (n = 3). O and b indicate
signal peaks of DDAA and DADA species, respectively.
(0.65 mM). As expected, the UPy unit existed in the mono-
meric 4[3H]-pyrimidinone form, in which the intramolecular
hydrogen-bonded NH resonated at the most downfield
region (δ 11.4) and the remaining two urea NH signals
appeared at δ 9.5-10.3. The precipitated G1 di-UPy mono-
mer 5 (n = 1) could also be characterized by MALDI-TOF
mass spectroscopy. The experimental molecular weight
(M þ Naþ: 797.5420) matched with the theoretical value
(797.5412). However, no peaks of oligomeric aggregates
were observed. As the compound was only soluble in very
polar solvents, in which it existed in monomeric state, and
hence its supramolecular properties were not investigated.
The G2 dendronized di-UPy monomer 5 (n = 2) was
insoluble in any solvents once precipitated from CH3OH,
characterization could only be done during the hydrogeno-
1
lysis. As mentioned earlier, its H NMR spectrum showed
the complete disappearance of the O-benzyl proton signal
(see Supporting Information for details). An aliquot from the
reaction mixture was taken out and subjected to MALDI-TOF
analysis and the experimental molecular ion peak (M þ Hþ:
1280.1243) matched with the theoretical value (1280.1227). On
the other hand, the precipitate solid sample failed to produce
any mass spectrum. Again, due to its insolubility, no further
studies could be done.
In sharp contrast, the G3 di-UPy monomer 5 (n = 3) is
soluble in most nonpolar solvents such as CHCl3, hexane,
and toluene, and could be characterized by 1H NMR spec-
troscopy in CDCl3 (Figure 1). Some notable features were
observed. First, the downfield signals at δ 11-13.5 corre-
sponding to the three hydrogen-bonded N-Hs were observed.
This confirmed that the monomeric units 5 (n = 3) were
hydrogen-bonded together to form the polymer species 6.
Second, all peak signals were broadened, suggesting they
were originated from a polymer structure. Third, two sets of
proton signals were observed, which corresponded to the
4[1H]-pyrimidinone DDAA and 4-pyrimidinol DADA tau-
tomeric structures. The dominant species could be assigned
to the 4[1H]-pyrimidinone in which the pyrimidyl proton
resonated at δ 5.8. The peak with a lower intensity at δ 6.2
was due to the pyrimidyl proton of 4-pyrimidinol.20 From
their relative integrations, the amount of DDAA and DADA
tautomeric species in CHCl3 was 3/1. The percentage of
DADA tautomer was very large as compared to the analo-
gous mono-UPy compounds bearing the same aliphatic
hydrocarbon dendron,14 in which the ratio was about 20/1.
A higher percentage of DADA tautomer also indicated that
Attempts were also made to obtain the molecular weight
(MW) and size distribution of the G3 supramolecular den-
dronized polymer 6 by dynamic laser light scattering (DLLS)
experiments. At low polymer concentrations (1-5 mg/mL),

Article
Macromolecules, Vol. 43, No. 20, 2010 8397
Figure 3. Double-logarithmic plots ofspecific viscosity vs concentrationofsupramolecular dendronizedpolymer6 (n = 3) inCHCl3 solutionsat (left)
26 ꢀC and (right) 40 ꢀC.
Figure 4. Specific viscosity of supramolecular dendronized polymer 6
Figure 5. UV absorption spectra of G3 supramolecular dendronized
(n = 3) in CHCl3 (b) and hexane (O) at 26 ꢀC.
polymer 6 at different concentrations in CHCl3 at 25 ꢀC.
a peak with an Rh of 20 nm was found, which corresponded
to a polymer chain consisted of 10-15 monomer units based
on the molecular dimension of the rigid backbone (see
Supporting Information for details). However, at a sample
concentration of 10 mg/mL, an additional aggregation peak
at 200 nm began to emerge. Because of this aggregation
effect, we were unable to extract the actual MW value of the
individual supramolecular polymer chain 6 at higher con-
centrations by DLLS.
5. Viscosity Properties of Supramolecular G3 Dendronized
Polymer 6. The solution viscosity of the G3 supramolecular
dendronized polymer 6 was measure0d at 26 and 40 ꢀC in
CHCl3 in a concentration range of 1-60 mM. It was found
that the specific viscosity (ηsp) increased nonlinearly with
concentration. The corresponding double-logarithmic plots
of ηsp against concentration showed a change of slope at
26 mM (6 g/dL) (Figure 3). Below this concentration the
slope of the line was around 1.5, while above this the slope
became 4.0-4.2. For covalent, noninteracting polymer
chains, the theoretical slope is 1.0. Hence, a slope of 1.5 at
the diluted concentration range suggested that the polymeric
species were interacting even in diluted solutions and the
degree of polymerization (DP) increased gradually. At the
higher concentration range, the relatively high slope of 4.0
indicated that very strong concentration dependent associa-
tion of individual monomer units, which was a characteristic
feature of supramolecular polymers. These values are also
consistent with other UPy-based supramolecular polymers
reported by Meijer.15,23a In addition, the ηsp value at 26 ꢀC
was twice as high as that at 40 ꢀC of the same concentration.
This fact was also reflected by the slightly higher slope (4.16)
at 26 ꢀC as compared to that (3.98) at 40 ꢀC, showing the
destabilizing entropiceffecton monomer association athigher
temperature.
Because of the presence of two G3 aliphatic hydrocarbon
dendrons, the supramolecular dendronized polymer 6 (n =
3) is also soluble in hexane. This unique property enabled us
to investigate its viscosity property and the association of the
UPy units in hexane. It was found that ηsp was about 20-30
times higher in hexane than in CHCl3 at the same concentra-
tion (Figure 4). For example, a solution of the polymer at
27 mM possessed a ηsp value of 400 in hexane, but a much
smaller value of 15 in CHCl3. Unfortunately, the polymer
slowly precipitated from hexane once the concentration was
above 30 mM. Hence, we were unable to conduct viscosity
measurements at higher concentrations to determine the slopes
of the double-logarithmic plot of ηsp against concentration
in hexane. However, based on the much higher viscosity values
in hexane, the polymers formed in hexane should have a
much higher DP value than that in CHCl3. The change of
viscosity properties of the G3 polymer 6 with temperature
and different solvent systems further demonstrated the
dynamic nature of the association process of such supramo-
lecular dendronized polymers.

8398 Macromolecules, Vol. 43, No. 20, 2010
Wong et al.
6. UV-Vis Spectroscopy. In addition to investigating the
hydrogen bond mediated one-dimension self-assembly of the
UPy units, it is also of interest to understand whether the
dimeric UPy rings, belonging to the same or different poly-
mer chains, can interact with each other. Sijbesma and
Meijer had demonstrated that dimeric UPys could stack on
top of each other to form columnar architecture.15c Hence,
concentration dependent UV-vis spectrophotometric stud-
ies were also carried out on the G3 dendronzied polymer 6.
UV-vis spectra of the polymer solutions of different
concentrations (0.01-5 mM) were recorded in CHCl3 at 25 ꢀC
(Figure 5). A reduction in the molar extinction coefficient
along with a significant bathochromic shift of the absorption
maximum from 281 to 313 nm were seen with increasing
concentration of supramolecular polymer 6. This observa-
tion was consistent with the formation of J-type aggregates
with increasing concentration of UPy units.25 Hence, it
appeared that the dimeric UPy units aligned themselves in
a stair-case fashion with a small slippage angle (Figure 6).
This stacking arrangement could also avoid interchain den-
dron repulsion. Furthermore, it should be noted that the
plane of the dimeric UPy rings might not align in a coplanar
manner to the central aromatic cores. As a result, the two
large dendrons are tilted away from the plane of the UPy
rings so as to avoid steric blocking of the UPy units from
dimerization. This proposed self-assembly structure is in
€
many ways reminiscent to that proposed by Wurthner on
hydrogen bond-mediated main chain supramolecular poly-
mers based on a perylene bisimide skeleton.26
7. Scanning Electron Microscopy. As the two UPy units are
aligned in an angle of 180ꢀ on a rigid aromatic spacer, the
resulting supramolecular polymer should possess a linear
structure. Hence, scanning electron microscopy (SEM) was
used to study the morphology of the polymer in the solid
state. Samples of the G3 supramolecular polymer 6 were
prepared from THF solutions containing different substrate
concentrations (0.1, 0.5, and 1.0 mg/mL) and then spin-
coated on a silicon wafer either at low (300 rpm) or high
(1500 rpm) angular speed. After that, the samples were
coated with gold particles before examinations (Figure 7).
It was found that fibrous structures with high aspect ratio
were predominantly found in the samples irrespective of
spinning rate and sample concentration. Linear arrangement
of the two UPy units as well as the large G3 hydrocarbon
dendrons forced the supramolecular polymer backbone to
become extended such that interdendron repulsion could be
minimized. Bent strings were also found but they appeared to
arise from interchain winding or overlaying of two separate
fibers (Figure 7b). The polymer fibers were of different
lengths but most of them were between 10-100 μm. Assum-
ing each dendronized monomer was fully stretched, the head
Figure 6. Schematic representation of self-assembly of macromonomer
5 via hydrogen bonding and formation of slipped J-typeaggregates. The
aromatic cores, UPy units and the dendrons are in purple, dark green,
and blue, respectively. For clarity, the dendrons are removed in some of
the structures.
Figure 7. SEM images of G3 supramolecular dendronized polymers 6 prepared under various spin coating conditions on silicon wafers: (a) 0.1 mg/mL
at 1500 rpm, (b) 0.5 mg/mL at 300 rpm, (c) 1.0 mg/mL at 300 rpm, and (d) 1.0 mg/mL at 1500 rpm.

Article
Macromolecules, Vol. 43, No. 20, 2010 8399
to tail distance between the two UPy units is about 2 nm.
Hence, a single polymer chain of 10 μm in length will have
approximately 5000 monomer units. Moreover, each fiber
has a relatively uniform width of 2-3 μm, which indicated
that the observed linear structure actually consisted of many
polymer chains bundling up together in a manner similar to
Schenning, A. P. H.; Sijbesma, R. P.; Meijer, E. W. Chem. Rev. 2009,
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units and hydrophobic interaction between the nonpolar
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€
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Conclusion
In this paper, we reported the synthesis and characterization of
main chain supramolecular dendronized polymers using a hydro-
gen bond mediated self-assembly approach. Because of the high
crystallinity and poor solubility of such rigid rod polymers, their
preparations could not have been accomplished without first
securing a clean and high yielding synthetic route to the bifunc-
tional UPy dendritic macromonomers 5. It was noticed that the
nature of the dendrons had a substantial influence on the UPy
binding strength, solubility and self-assembly properties. In this
study, we found that aliphatic hydrocarbon dendrons were well
suited for these purposes. Nonetheless, only the G3 dendrons
could confer the resulting supramolecular polymer 6 (n = 3) with
good solubility to facilitate its characterization and property
studies. 1H NMR and viscosity studies confirmed the reversible,
supramolecular nature of the resulting polymer 6. In particular,
viscometry data suggested the associative interaction between the
monomers increased nonlinearly with increasing monomer con-
centration. While the above findings confirmed that the indivi-
dual UPy units self-assembled to form main chain supramole-
cular dendronized polymer chains, UV-vis spectroscopic study
revealed that individual polymer chains stacked on top of each
other via the UPy rings in the form of J-aggregates. SEM morpho-
logical study also showed that the resulting polymer had a fibrous
structure with a very high aspect ratio. A model was purposed to
rationalize how such fibers could be assembled from the dendritic
macromonomers. Hence, the self-assembling synthetic approach
can offer an alternative route to create dendronized polymers
with new and fascinating properties. Having confirmed that the
hydrogen bond self-assembled approach could furnish dendro-
nized polymers with high DP values, our next goal is to examine
whether rigidification of the dendronized polymer backbone can
be realized from using di-UPy dendritic macromonomers having
a structural flexible backbone.
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Supporting Information Available: Figures showing 1H and
13C NMR spectra of all compounds, and DLLS data of G3
supramolecular dendronized polymer6. This material is available
free of charge via the Internet at http://pubs.acs.org.
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