Self-organizable vesicular columns assembled from polymers dendronized with semifluorinated janus dendrimers act as reverse thermal actuators

Article abstract of DOI:10.1021/ja2118267

The synthesis and structural analysis of polymers dendronized with self-assembling Janus dendrimers containing one fluorinated and one hydrogenated dendrons are reported. Janus dendrimers were attached to the polymer backbone both from the hydrogenated and from the fluorinated parts of the Janus dendrimer. Structural analysis of these dendronized polymers and of their precursors by a combination of differential scanning calorimetry, X-ray diffraction experiments on powder and oriented fibers, and electron density maps have demonstrated that in both cases the dendronized polymer consists of a vesicular columnar structure containing fluorinated alkyl groups on its periphery. This vesicular columnar structure is generated by a mechanism that involves the intramolecular assembly of the Janus dendrimers into tapered dendrons followed by the intramolecular self-assembly of the resulting dendronized polymer in a vesicular column. By contrast with conventional polymers dendronized with self-assembling tapered dendrons this new class of dendronized polymers acts as thermal actuators that decrease the length of the supramolecular column when the temperature is increased and therefore, are called reverse thermal actuators. A mechanism for this reversed process was proposed.

Full text of DOI:10.1021/ja2118267

Article
pubs.acs.org/JACS
Self-Organizable Vesicular Columns Assembled from Polymers
Dendronized with Semifluorinated Janus Dendrimers Act As Reverse
Thermal Actuators
Virgil Percec,* Mohammad R. Imam, Mihai Peterca, and Pawaret Leowanawat
Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323,
United States
S
* Supporting Information
ABSTRACT: The synthesis and structural analysis of
polymers dendronized with self-assembling Janus dendrimers
containing one fluorinated and one hydrogenated dendrons
are reported. Janus dendrimers were attached to the poly-
mer backbone both from the hydrogenated and from the
fluorinated parts of the Janus dendrimer. Structural analysis of
these dendronized polymers and of their precursors by a
combination of differential scanning calorimetry, X-ray diffrac-
tion experiments on powder and oriented fibers, and electron
density maps have demonstrated that in both cases the
dendronized polymer consists of a vesicular columnar structure containing fluorinated alkyl groups on its periphery. This
vesicular columnar structure is generated by a mechanism that involves the intramolecular assembly of the Janus dendrimers into
tapered dendrons followed by the intramolecular self-assembly of the resulting dendronized polymer in a vesicular column. By
contrast with conventional polymers dendronized with self-assembling tapered dendrons this new class of dendronized polymers
acts as thermal actuators that decrease the length of the supramolecular column when the temperature is increased and therefore,
are called reverse thermal actuators. A mechanism for this reversed process was proposed.
continue to be investigated for the polymerization of a diversity
of monomers containing dendrons. These methodologies lead
to dendronized polymers with conjugated and nonconjugated
backbones.5,11 Historical developments of this field and recent
reviews are available.1 Most of the research in this area involves
dendrons that are attached to the polymer backbone from their
apex.1−12 However, new architectural complexity was achieved
when dendrons were attached to the polymer backbone from
alternative positions than their apex or when other dendritic
building blocks were used in the design of dendronized poly-
mers.13 Here we report the synthesis and structural analysis of
dendronized polymers obtained by the polymerization of self-
assembling semifluorinated14a Janus dendrimers.14 This new
class of dendronized polymers produces self-organizable vesic-
ular columns by an intramolecular self-assembly of the Janus
dendrimer repeat units into supramolecular tapered dendrons
followed by an intramolecular self-assembly of the resulting
dendronized polymer. By contrast with self-organizable
dendronized polymers containing conventional self-assembling
tapered dendrons as side groups,5f this new class of dendroni-
zed polymers acts as thermal actuators that decrease their length
upon the increase of temperature. Therefore, these thermal
actuators are called reverse thermal actuators.
INTRODUCTION
■
Traditional dendronized polymers are synthetic or natural
linear backbones containing a dendritic group, which is most
frequently a dendron, attached to each of their repeat units.1
They are synthesized via covalent divergent,2 convergent fol-
lowed by covalent3 or supramolecular4 attachment to the
backbone as well as by dendritic macromonomer,5 strategies.
Recently, cyclic dendronized polymers1h,6 have been reported,
and it is expected that even more complex topologies will
emerge. The dendritic group attached to the backbone can be
self-assembling7 or not.8 Self-assembling dendritic groups
attached to polymer backbones generate dendronized polymers
that self-organize in periodic9 and quasiperiodic9d arrays. The
analysis of these arrays by complementary methods, including
small and wide-angle X-ray diffraction on powder and oriented
fiber combined with electron density maps, electron diffraction
combined with transmission electron microscopy (TEM), and
atomic force and scanning force microscopy experiments,
endow precise information on the 2- or 3-D structure of the
dendronized polymer.9 In addition, monomers derived from
self-assembling dendrons provide access to dendronized
polymers with extremely narrow molecular weight distribution
when polymerized by conventional methods in dilute solu-
tion9,10 and to extremely high molecular weight dendronized
polymers when polymerized in self-assembled state.9a,10a−c
Various living polymerization methods have been5a,e,f,6,9f,11 and
Received: December 19, 2011
Published: February 6, 2012
© 2012 American Chemical Society
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Figure 1. Topologies generated from linear covalent and supramolecular polymers dendronized with self-assembling dendrons, twin dendritic
molecules, and Janus dendrimers.
(Figure 1b,c). A major difference between twin dendritic
molecules and Janus dendrimers is that Janus dendrimers can
be attached to the polymer backbone via the two different
dendrons forming the Janus dendrimer (compare panel g with
panels h and i in Figure 1).
Synthesis of Self-Assembling Semifluorinated Janus-
Dendrimers Containing Methacryloyl Groups. Scheme 1
outlines the synthesis of hydrogenated, 9, and fluorinated, 10,
first generation dendrons containing methacryloyl groups. Both
9 and 10 were prepared by the reaction of methyl gallate, 1,
with triethyl orthoformate in the presence of Amberlyst 15 in
benzene at reflux to generate compound 2 containing two
protected phenol groups in 90% yield.
Alkylation of 2 with HO(CH2)11Br in DMF by using K2CO3
as base at 65 °C produced 3 in 80% yield after 4 h of reaction
time. 3 was deprotected with HCl to yield 94% of 4. Alkylation
of 4 with 1-bromododecane in DMF at 70 °C for 4 h in the
presence of K2CO3 produced 5 in 95% yield. Saponification of
5 with KOH in aqueous ethanol at reflux followed by
acidification with HCl yielded 7 in 94% yield. 6 was obtained
by the alkylation of 4 with 12-bromo-1,1,1,2,2,3,3,4,4,5,5,6,6,
7,7,8,8-heptadecafluorodecane14a,17 in 90% yield. Esterification
of 7 and 8 with methacryloyl chloride in CH2Cl2 in the
presence of Et3N followed by reflux with pyridine/H2O for 2 h
cleaved the carboxylic acid anhydride intermediate to produce 9
in 71% and 10 in 86% yields.
RESULTS AND DISCUSSION
■
Topologies Generated from Polymers Dendronized
with Self-Assembling Dendrons and Dendrimers. The
topologies investigated so far with linear polymers dendronized
with self-assembling dendritic building blocks, including the
one reported in this publication, are summarized in Figure 1.
Early work started with self-assembling dendrons and with
dendrons that do not self-assemble, being attached to the
polymer backbone either via conventional chains,5,9a,10b or
by living5f,g,11 polymerizations of dendronized monomers. The
divergent growth of a dendron from each repeat unit of a
polymer,2 the convergent attachment directly to the polymer
backbone3 and the attachment to the backbone via a flexible
spacer2c,15 are also part of the pioneering strategies (Figure
1a,b). Attachment of the dendrons to the backbone via supra-
molecular donor−acceptor,4a ionic15 and other interactions
followed (Figure 1c). The construction of supramolecular back-
bones via H-bonding,9e,16 donor−acceptor,4a ionic, and other
interactions15 provided additional methodologies (Figure 1d).
The topologies outlined in Figure 1a−d were accomplished
both with tapered and conical self-assembling dendrons.7,15
Subsequently, tapered self-assembling dendrons were attached
directly or via a spacer from their periphery rather than from
their apex (Figure 1e,f). Self-assembling twin dendritic mole-
cules,13a,b which are the simplest class of dendrimers, were also
used to dendronize polymers (Figure 1g).
Self-assembling twin dendritic molecules13a,b,16a are the
closest architectural motives resembling Janus dendrimers14
Scheme 2 outlines the final step in the synthesis of the Janus
dendrimers containing the methacryloyl group.
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Scheme 1. Synthesis of Hydrogenated and Fluorinated Dendrons Containing Methacryloyl Groups
Reagents and conditions: (i) (EtO)3CH, Amberlyst 15, benzene, reflux, 18 h; (ii) HO(CH2)11Br, K2CO3, DMF, 65 °C, 4 h; (iii) HCI,MeOH,
reflux, 12 h; (iv) R-Br, K2CO3, DMF, 65 °C, 1 h; (v) aq. KOH, EtOH, reflux, 2 h; (vi) HCI, 15 min; (vii) Et3N, CH2Cl2, 6 h; (viii) pyridine-H2O,
reflux, 2 h.
Scheme 2. Synthesis of Semifluorinated Janus Dendrimers Containing Methacryloyl Groups and of Their Corresponding
Polymers
Reagents and conditions: (i) DCC, DPTS, α,α,α-trifluorotolune, 50 °C, 4 h; (ii) AIBN, α,α,α-trifluorotolune, 80 °C, 48 h.
1113a was reacted with 10 in the presence of DCC/DPTS at
50 °C in α,α,α-trifluorotoluene for 4 h to yield 12 in 83% yield.
Amidation of 1314a with 9 under the same reaction conditions
produced 14 in 70% yield. The corresponding dendronized
polymers were obtained by radical polymerization of
12 and 14 with AIBN in α,α,α-trifluorotoluene at 80 °C
for 48 h to produce the isolated yields and the number
average molecular masses relative to polystyrene standards
(Mn) reported in Scheme 2. The absolute molecular
weights of dendronized polymers are 3−6 times larger
than those determined by gel permeation chromato-
graphy with polystyrene standards.10b For structural
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Scheme 3. Structures and Short Names of the Hydrogenated (a) and Fluorinated Twin Dendritic Molecules (b), of the
Semifluorinated Janus Dendrimers (c and d), and of the Polymers Dendronized with Janus Dendrimers (e and f)
analysis experiments, lower molecular mass and soluble
polymers that exhibit faster dynamics and can be
characterized in solution and in solid state are desirable.
Therefore, no experiments to synthesize higher molar mass
polymers were required at this time.
self-assembly and self-organization of the Janus dendrimers, of
polymers dendronized with Janus dendrimers and of their
mixtures was performed by a combination of differential scann-
ing calorimetry, temperature dependent optical polarized micro-
scopy, wide and small-angle X-ray diffraction experiments
on powder and oriented fibers, and experimental density.
Table 1 summarizes the X-ray diffraction data of the
compounds investigated. The assignment of phases
presented in Table 1 is based on small-angle powder
diffraction data and on the analysis of the wide-angle X-ray
diffraction patterns collected as a function of temperature
from oriented fibers.
The XRD data presented in Figures 3−5 and in Table 1
demonstrate that the polymers dendronized with Janus
dendrimers followed the pathways of self-assembly and self-
organization of their Janus dendrimers that are their precursors.
The relative intensity of the higher order X-ray powder diffrac-
tion peaks of the hexagonal (Φh) and centered rectangular
(Φr‑c) 2D columnar phases shown in Figure 3 are weak for all
of the structures. Both Janus dendrimers and polymers den-
dronized with Janus dendrimers have similar powder diffraction
intensity profiles and lattice dimensions (Figure 3 and Table 1).
Furthermore, the XRD patterns of the oriented fibers collected
in the low temperature orthorhombic crystalline phases (Φok) as
well as in the high temperature Φh and Φr‑c phases exhibit
similar 4.9−5.3 Å stacking features in the wide angle region
(Figure 4a,b).
Self-Assembly of Twin Dendritic Molecules, of
Polymers Dendronized with Twin Dendritic Molecules
and of Janus Dendrimers. Scheme 3 outlines the structures
of all compounds studied. They follow the same color code as
in their schematic representation from Figure 2. The left side of
Figure 2 outlines the self-assembly of hydrogenated (blue) twin
dendritic molecules and of polymers dendronized with twin
dendritic molecules. Twin dendritic molecules self-assemble in
a supramolecular column that self-organizes in a 2D columnar
hexagonal phase.13a,b,16a Polymers dendronized with twin
dendritic molecules self-assemble in an architecture consisting
of a polymer chain coated with a three-cylindrical bundle
supramolecular dendrimer.13a This polymer self-organizes in a
thermotropic nematic liquid crystal phase.13a Co-assembly of
the polymer coated with a three-cylindrical bundle supramolec-
ular dendrimer with twin dendritic molecules produces a novel
2D hexagonal column superlattice.13a Fluorinated (yellow) twin
dendritic molecules also self-assemble in a supramolecular
column forming a hexagonal columnar 2D lattice.14a Semifluo-
rinated Janus dendrimers self-assemble in vesicular columns
with diameter two times lager than that of the corresponding
columns generated from fluorinated or hydrogenated twin
dendritic molecules.14a These vesicular columns self-
organize in 2D hexagonal columnar lattices. During the
self-assembly process the Janus dendrimer changes its
conformation from Janus dendrimer to tapered dendron
(right side of Figure 2).14a
Interestingly, all Janus dendrimers and polymers dendronized
with Janus dendrimers exhibit at low temperature a Φok phase
self-organized from supramolecular columns that are strongly
correlated. Their column-to-column correlations are demon-
strated by the wide-angle off-meridional features on the 2 ×
(4.9−5.2) Å L = 1 layer lines, marked in Figure 4a. These
features together with the intense and sharp meridional maxima
observed on the 4.9−5.2 Å L = 2 layer lines (Figures 4a and 5)
Structural and Retrostructural Analysis of Janus
Dendrimers, of Polymers Dendronized with Janus
Dendrimers, and of their Mixtures. The analysis of the
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Figure 2. Self-assembly of hydrogenated twin dendritic molecules, of polymers dendronized with hydrogenated twin dendritic molecules, and their
coassembly (all in blue). Self-assembly of fluorinated twin dendritic molecules (in yellow) and of semifluorinated Janus dendrimers (half in blue and
half in yellow).
suggest that every second layer within the supramolecular
columns are in registry and that there are long-range helical
correlations. One possible helical packing of the Janus dendri-
mers within the supramolecular columns consistent with the
observed wide-angle diffraction features of the oriented fibers in
the Φok phases shown in Figures 4a and SF1 is a criss-cross one
demonstrated previously from single crystal X-ray experiments
performed on twin dendritic molecules.16a Therefore, the helix
parameter φ, that corresponds to the angle of rotation around
the column axis of adjacent column strata, is 360°/(2 μ) where
μ is the number of Janus dendrimers forming the supra-
molecular layer of the column (Table 2). The helix parameter
φ varies between ∼30° for (3,4,5)12G1-(3,4,5Poly)4F8G1-BzA
to ∼36° for (3,4,5Poly)12G1-(3,4,5)4F8G1-BzA. The helix
parameter corresponding to the translation of the layer along
the column axis varies in the narrow range of 4.9−5.2 Å, as
shown in Figures 4a and 5a,b (t values listed in Table 2). The
criss-cross type of helical packing of the supramolecular col-
umns self-organized in the Φok phases was most probably
facilitated by the formation of the intermolecular H-bonding
interactions. This mechanism is indirectly supported by the
observation that the structure lacking the potential of forming
H-bonding due to the methylation of its −CONH− group,
(3,4,5)12G1-(3,4,5)4F8G1-BzA-CH3, which self-organizes in a
simple orthorhombic columnar phase, Φsk‑o, exhibits wide-angle
off-meridional features only on the 6.3 Å L = 1 (Figure 6a).
The absence of secondary layers of wide-angle off-meridional
features in the XRD fiber pattern, shown in Figure 6a, sug-
gests that the supramolecular columns self-assembled from
(3,4,5)12G1-(3,4,5)4F8G1-BzA-CH3 are not forming the
helical criss-cross packing observed in the Janus dendrimers
and polymers dendronized with Janus dendrimers that are capa-
ble of forming H-bonding along the long axis of their column.
Figure 5 presents the meridional plots of the wide-angle
region of the XRD fiber patterns shown in Figure 4. In the high
temperature range of the Φh and Φr‑c phases the degree of
order within the supramolecular columns spans through about
5−7 column strata, as estimated from the width of their wide-
angle meridional maxima (Figure 5c,d). Interestingly, in the low
k
temperature range of the Φo phase the dendronized polymer
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA exhibits a degree of long-
range order of ξ ≈ 35 layers comparable with that of the Janus
dendrimer (ξ ≈ 48 layers, Figure 5b). This is expected con-
sidering that the Janus dendrimer is attached to the polymer
backbone through an alkyl group spacer containing 11 methyl-
enic units. On the other hand, in the case of the dendronized
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Table 1. XRD Data Collected from the Janus Dendrimers, Polymers Dendronized with Janus Dendrimers, and their Mixtures
d10, d11, d20, d21, d30 (Å)
d20, d40, d02, d20, d51, d42, d60 (Å)
a
compound
(3,4,5)12G1-(3,4,5Poly)4F8G1-BzA
T (°C) phase
a, b, c (Å)
56.3
d20, d02, d12, d03, d40 (Å)
130
25
Φh
48.9, -, 24.3, 18.3, 16.2
Φok
97.5, 51.8, 10.4
103.3, 49.5
49.0, 24.4, -, 22.9, -, 17.8, 16.3
51.9, 25.9, 24.8, -, 19.1, -, 17.2
49.9, 24.9, -, -, -, 17.2, 16.7
51.9, 25.9, 24.9, -, 19.1, -, 17.2
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA
100
25
Φr‑c
Φok
99.8, 47.6, 9.8
103.3, 49.7
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(80/20 molar ratio)
120
Φr‑c
25
Φok
98.8, 47.3, 9.8
101.0, 49.6
49.6, 24.8, -, -, -, 17.1, 16.5
50.7, 25.3, 24.8
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(70/30 molar ratio)
140
Φr‑c
25
Φok
Φh
97.6, 46.3, 9.8
58.8
49.0, 24.4, -, -, -, 16.8, 16.3
50.9, -, 25.5, 19.3, 17.0
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(50/50 molar ratio)
140
30
Φok
Φh
97.6, 46.8, 9.8
58.9
49.0, 24.4, -, -, -, 16.9, 16.3
51.0, -, 25.5, 19.3, 17.0
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(30/70 molar ratio)
140
25
Φok
Φr‑c
Φh
95.0, 46.6, 9.8
96.2, 47.8
56.9
47.5, 23.8, 23.3, -, -, 16.7, 15.8
48.1, 24.1, 24.0, 21.4, -, 16.9, 16.0
49.4, -, 24.6, 18.6, 16.4
98
(3,4,5)12G1-(3,4,5Poly)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(50/50 molar ratio)
140
30
60
Φok
95.8, 46.8, 9.8
63.0, 52.2, 6.3
47.9, 23.9, -, 21.1, -, 16.8, 15.9
31.5, 26.1, 24.1, 17.5, 15.8
(3,4,5)12G1-(3,4,5)4F8G1-BzA-CH3
Φsk‑o
a
Phase notations: Φh, columnar hexagonal 2D phase; Φok, columnar orthorhombic crystalline; Φr‑c, centered rectangular columnar 2D phase; and
Φsk‑o, columnar simple orthorhombic crystalline phase.
Figure 3. Small-angle powder X-ray diffraction data collected at the indicated temperatures from twin dendritic molecules, Janus dendrimers,
polymers dendronized with Janus dendrimers, and of their 1/1 molar mixtures. All structures are detailed in Scheme 3.
polymer of (3,4,5)12G1-(3,4,5Poly)4F8G1-BzA, the degree of
order is about 4 times lower (ξ ≈ 9 layers, Figure 5b).
Therefore, for this dendronized polymer the alkyl spacer
connecting the polymer to the Janus dendrimer is embedded in
the fluorinated region, and, therefore, is subjected to additional
steric constrains that limit the degree of order and the corre-
lation length within the supramolecular column.
Reconstruction of the Electron Density Distributions.
The similarity of the X-ray powder diffraction intensity profiles
of the Janus dendrimers and of the polymers dendronized with
Janus dendrimers (Figure 3) and of their wide angle XRD fiber
patterns (Figure 4b), all collected in the high temperature 2D
phases, demonstrated that the internal organization of their
supramolecular columns is similar. Based on this observation,
the relative electron density maps shown in Figure 7, recon-
structed from the powder XRD data shown in Figure 3, suggest
that the fluorinated chains are forming the periphery of the
supramolecular columns of the high temperature columnar
hexagonal phases regardless of the place of the Janus dendrimer
from which it is attached to the polymer backbone.
The relative electron density histograms constructed from
the corresponding distributions shown in Figure 7 are pre-
sented in Figure 8. The histograms exhibit two maxima: one in
the low electron density region that corresponds mainly to
the aliphatic region “continuum” and a second one in the high
electron density region that corresponds to the aromatic and
fluorinated region of the supramolecular columns. These maxi-
ma are in agreement with the expected aliphatic−aromatic−
fluorinated microphase segregation within the supramolecular
assemblies. The relative variation of the histogram profiles,
which is proportional to the number of electrons within the
hydrogenated and fluorinated chains (Figure 8) support the
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Figure 4. Wide-angle X-ray patterns collected from oriented fibers of Janus dendrimers and polymers dendronized with Janus dendrimers at the
indicated temperatures. The name of the compound, the fiber axis, their phases and diffraction features are indicated.
Figure 5. Meridional plots of the wide-angle X-ray fiber patterns shown in Figure 4. The short name of the compounds, the temperature, their phase,
their meridional maxima features, and their average correlation length ξ are indicated. All structures are detailed in Scheme 3.
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Table 2. Structural and Retrostructural Analysis of the Janus Dendrimers, of Polymers Dendronized with Janus Dendrimers,
and of Their Mixtures
a
a
a
a
a
compound
(3,4,5)12G1-(3,4,5Poly)4F8G1-BzA
T (°C)
phase
a, b (Å)
56.3
γ (deg)
t (Å)
A (Å2)
Mwt
μ
130
25
Φh
60.0
62.0
64.4
64.5
64.3
5.3
5.2
5.3
4.9
5.3
5490.1
5050.5
5113.4
4750.5
5134.0
1984
1984
2290
2290
2276
6.9
6.3
5.6
4.8
5.7
Φok
97.5, 51.8
103.3, 49.5
99.8, 47.6
103.3, 49.7
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA
100
25
Φr‑c
Φok
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(80/20 molar ratio)
120
Φr‑c
25
Φok
Φr‑c
98.8, 47.3
64.4
63.8
5.0
5.3
4673.2
5009.6
2276
2269
4.9
5.5
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(70/30 molar ratio)
140
101.0, 49.6
25
Φok
Φh
97.6, 46.3
58.8
64.6
60.0
5.0
5.3
4518.9
5988.5
2269
2255
4.7
6.7
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(50/50 molar ratio)
140
30
Φok
Φh
97.6, 46.8
58.9
64.3
60.0
5.0
5.3
4567.7
6008.8
2255
2241
4.8
6.7
(3,4,5Poly)12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(30/70 molar ratio)
140
25
Φok
Φr‑c
Φh
95.0, 46.6
96.2, 47.8
56.9
63.9
63.6
60.0
5.0
5.0
5.3
4427.0
4598.4
5607.7
2241
2241
2137
4.7
4.8
6.6
98
(3,4,5)12G1-(3,4,5Poly)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA
(50/50 molar ratio)
140
30
Φok
95.8, 46.8
63.9
5.2
4483.4
2137
5.2
a
Phase and symbol notations: Φh, columnar hexagonal 2D phase; Φok, columnar orthorhombic crystalline; and Φr‑c, centered rectangular columnar
2D phase; γ = arctan(a/b), a unit cell parameter corresponding to the angle between a and a + b directions; t, the thickness of the column stratum
obtained from the X-ray pattern of the oriented fiber; A, area of the unit cell where A = a × b (for Φok and Φr‑c) and A = a × a√3 for Φh; μ, is the
number of Janus dendrimers forming a column stratum of thickness t. The calculations of all parameters were performed as reported in previous
publications.7
Figure 6. Structure of the (3,4,5)12G1-(3,4,5)4F8G1-BzA-CH3 Janus dendrimer (a). The wide-angle X-ray pattern collected from the oriented fiber
of this Janus dendrimer at the indicated temperature (b) and the corresponding small angle powder XRD data (c).
proposed phases of the powder diffraction amplitudes pre-
sented in Figure 7. Therefore, the relative electron density
distributions shown in Figure 7 suggest that upon self-assembly
all Janus dendrimers and polymers dendronized with Janus den-
drimers form supramolecular columns with a fluorinated peri-
phery, regardless if the polymeric backbone is attached to the
hydrogenated or fluorinated region of the Janus dendrimer.
This result is also supported by the observation that the
fluorinated chains have a decreased conformational freedom in
comparison with the hydrogenated ones. Therefore, the more
flexible alkyl spacer connecting the polymer backbone to the
Janus dendrimer was expected to form the core region of the
supramolecular columns since such confinement requires an
increased statistical distribution of its gauche conformers.
Reversed Thermal Nanoactuators Generated from
Polymers Dendronized with Janus Dendrimers. First
order transitions between columnar phases of conventional
dendronized polymers (Figure 1a,b) accompanied by changes
of the diameter of the supramolecular columns have been
demonstrated to generate nanomechanical functions.5f Ori-
ented fibers produced from these dendronized polymers extend
their length with up to 42% when the temperature is increased
from 20 to 110 °C.5f This change is associated with a novel
helix−helix transition. For example, libraries of dendronized
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Figure 7. Reconstructed relative electron density maps of the Janus dendrimers, polymers dendronized with Janus dendrimers and of their 1/1 molar
mixtures self-assembled in the columnar hexagonal phases from Table 2. The phases of the powder diffraction amplitudes used in the reconstruction:
(10)-, (20)+, (21)-, and (30)-.
Figure 8. Histograms of the electron density distributions shown in Figure 7 and their correspondence with the number of electrons within the
hydrogenated and fluorinated regions, respectively, calculated from the chemical structures of the Janus dendrimers and of the polymers dendronized
with Janus dendrimers.
helical polyphenylacetylenes have been used to fabricate molec-
ular actuators capable of displacing 250 times their weight.5f
The mechanism of the nanomechanical function was attributed
to a decrease of the diameter and an increase of the length of
the supramolecular columns upon the increase of temperature.
This change is mediated by the unwinding of the helical
conformation or the polymer backbone, that is confined to its
cylindrical dendritic coat, as the temperature increases. The
decrease of the diameter of the dendronized polymer
corresponds to a reduction of the number of dendrons forming
the supramolecular column stratum from μ = 5 at low
temperature, to μ = 4 at high temperature.5f By contrast, for the
polymers dendronized with Janus dendrimers the number of
Janus dendrimers that form the column stratum increases at
higher temperatures (Table 2). Therefore, the expectation was
that all oriented fibers prepared from Janus dendrimers and
polymers dendronized with Janus dendrimers should exhibit a
thermally induced nanomechanical function that is the
“reversed” of that generated from polymers dendronized with
self-assembling tapered dendrons attached to the backbone
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from their apex (Figure 1a,b). In other words, the polymers
dendronized with Janus dendrimers must be capable of dis-
placing large weights upon the decrease of temperature and
therefore, will provide additional molecular motives needed to
fabricate complex nanomachines.
Figure 9 demonstrates that indeed oriented samples prepared
from polymers dendronized with Janus dendrimers exhibit an
heating and that the additional column-to-column correlations
of the ordered phases might play an important role in facili-
tating this change. Such column-to-column correlations were
not present in the previously reported dendronized polymers
containing self-assembling dendrons attached from their apex
on each repeat unit of their backbone.5f These conventional
dendronized polymers exhibit the opposite function of chain
expansion upon heating.5f In contrast with the polymers den-
dronized with Janus dendrimers, the Janus dendrimers and their
mixtures with the polymer dendronized with Janus dendrimers
exhibit a slight expansion along the axis of the supramolecular
column upon the increase of temperature (Figure 10a,b).
Figure 9. Unusual contraction along the fiber axis upon the increase of
temperature of the polymers dendronized with Janus dendrimers (a
and b). In both dendronized polymers the contraction is associated
with the transition from a crystalline columnar orthorhombic phase to
a 2D columnar phase that is accompanied by an unusual increase of
the column diameter.
Figure 10. Slight expansion along the fiber axis prepared by extrusion
from Janus dendrimer (a) and from its mixture with the polymer
dendronized with Janus dendrimers (b).
unusual thermal contraction along the fiber axis upon the
increase of temperature, as expected from the decrease of μ
(Table 2). Within the error of the calculation, the contraction
along the fiber axis, which coincides with the axis of the supra-
molecular columns, observed for polymers dendronized with
Janus dendrimers correlates with the increase of the diameter of
the supramolecular columns calculated from the XRD data. For
example, the macroscopic scale contraction of the (3,4,5)12G1-
(3,4,5Poly)4F8G1-BzA is 0.52 mm/0.56 mm ≈ 0.928 0.015
(Figure 9a).
The microscopic scale data, extracted from the XRD results
reported in Tables 1 and 2, demonstrated that the contraction
is expected to be proportional with the inverse of the unit cell
area: (5490.1 Å2)−1/(5050.5 Å2)−1 ≈ 0.919. This calculation
was based on the assumption that the density of the compound
does not change upon the increase of temperature. Similarly,
the thermal contraction calculated from the macroscopic and
microscopic scale data of (3,4,5Poly)12G1-(3,4,5)4F8G1-BzA
are 1.08 mm/1.14 mm ≈ 0.947 0.015 and (5113.4 Å2)−1/
(4750.5 Å2)−1 ≈ 0.929, respectively. Interestingly, the thermal
contraction upon the increase of temperature was observed at
the transition from the ordered rectangular phase to either the
hexagonal or to the rectangular 2D liquid crystalline phases.
These results suggest that the change of the lattice symmetry,
from “triangular” for columnar hexagonal and “rectangular” for
the columnar orthorhombic phases, is most probably not corre-
lated with the unusual thermal induced contraction upon
In these structures, the macroscopic scale expansion is in
contradiction with the microscopic scale changes established
from the analysis of the XRD data. For example, the (3,4,5Poly)-
12G1-(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA 30/
70 molar mixture was expected to exhibit a contraction of
(6008.8 Å2)−1/(4427.0 Å2)−1 ≈ 0.736 (Tables 1 and 2). There
are at least two possibilities that can account for this difference.
First, it is possible that the density of the compounds might
decrease upon the increase of temperature. Second, it is possi-
ble that the degree of the column-to-column interdigitation
changes as a function of temperature.
The results presented in Figures 9 and 10 suggest that the
unusual thermal induced contraction upon heating and expan-
sion upon cooling of the oriented fibers observed only in the
case of polymers dendronized with Janus dendrimers was
induced by the additional dendrimer−dendrimer connectivity
that interconnects the polymer backbones from different
columns. One possible mechanism that explains the increase
of the column diameter upon the increase of temperature is the
gradual reduction of column-to-column interdigitation. At low
temperatures, the fluorinated chains are stiff and therefore, their
rigidity favors a larger degree of column-to-column correlation
via the interdigitation of mostly all-trans fluorinated alkyl
chains. Wide angle fiber XRD data collected in the low
temperature Φok phases shown in Figure 4a demonstrated the
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Figure 11. Thermally induced changes of the supramolecular columns self-assembled from polymers dendronized with Janus dendrimers (a) and the
self-repairing of the polymers dendronized with Janus dendrimers by mixing with Janus dendrimers (b). Notations: a1 = lattice dimension for
assemblies generated from twin dendritic molecules (see Figure 2); a = lattice dimension for assemblies generated from Janus dendrimers (see
Figure 2).
presence of long-range column-to-column correlations and a
reduced conformational freedom of the fluorinated chains
(Figure 5a,b). In the Φh and Φr‑c phases, the presence of diffuse
and broad wide-angle features observed in the fiber XRD patt-
erns suggest that there are no significant long-range column-to-
column correlations (Figure 6b). This indicates that the degree
of column-to-column interdigitation is significantly reduced due
to the increased conformational freedom of the fluorinated
chains (Figure 5c,d). Therefore, the transition from strongly
coupled and interdigitated columns forming the columnar
orthorhombic crystalline phase, to uncoupled and less
interdigitated columns forming the high temperature 2D liquid
crystalline phase can explain the unexpected increase of the
column diameter. For example, the cross-section of the supra-
molecular columns self-assembled from the (3,4,5Poly)12G1-
(3,4,5)4F8G1-BzA + (3,4,5)12G1-(3,4,5)4F8G1-BzA 30/70
molar mixture increases with more than 36% from 4427.0 Å2/2
to 6008.8 Å2/2 upon the increase of temperature (Table 2). This
variation is attributed to the suggested mechanism involving a
certain degree of column-to-column interdigitation that, in addi-
tion, does not require any other thermal induced change, including
the change in the density of the compound.
mixing with Janus dendrimer with molar ratios larger than
50:50 (Table 1) is illustrated in Figure 11b.
CONCLUSIONS
■
The synthesis and structural analysis of polymers dendronized
with Janus dendrimers containing one fluorinated and one
hydrogenated dendron are reported. The Janus dendrimers
were attached to the polymer backbone both from the hydro-
genated and from the fluorinated parts of the Janus dendrimer.
Structural analysis of the polymers dendronized with Janus
dendrimers, of their model Janus dendrimers,14a and of the
corresponding hydrogenated and fluorinated twin dendritic
molecules14a demonstrated the assembly of these dendronized
polymers into vesicular columns. The surface of these vesicular
columns contains fluorinated alkyl groups regardless of whether
the Janus dendrimer is attached to the backbone from its
hydrogenated or fluorinated parts. The diameter of these vesic-
ular columns is two times larger than that of the corres-
ponding supramolecular columns assembled from the hydro-
genated or fluorinated twin dendritic molecules. These vesicular
columnar structures are generated by a mechanism that
involves an intramolecular self-assembly of the Janus dendritic
side groups into tapered dendrons followed by the intra-
molecular self-assembly of the resulting dendronized polymer
in a vesicular column. By contrast with conventional polymers
dendronized with self-assembling tapered dendrons this new
class of dendronized polymers acts as actuators that decrease
the length of the supramolecular column when the temperature
Figure 11a summarizes the thermal induced changes of the
vesicular supramolecular columns generating the contraction
along the column axis upon the increase of temperature and
also illustrates the criss-cross helical packing in the Φok phase.
The self-repairing process observed in the high temperature
phase of the polymer dendronized with Janus dendrimers by
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Journal of the American Chemical Society
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450−451. (b) Percec, V.; Heck, J.; Lee, M.; Ungar, G.; Alvarez-
Castillo, A. J. Mater. Chem. 1992, 2, 1033−1039. (c) Freudenberger,
is increased and, therefore, are called reverse thermal actuators.
A mechanism for this reverse process was advanced. This new
class of reverse nanoactuators are complementary to the
previously reported direct nanoactuators5f and to many other
examples of molecular architectures capable of undergoing
directional motion18 that are expected to provide access to
complexity currently unavailable in synthetic nanomachines.
R.; Claussen, W.; Schluter, A. D.; Wallmeier, H. Polymer 1994, 35,
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures with complete spectral and structural
analysis and complete reference 14b. This material is available
free of charge via the Internet at http://pubs.acs.org.
1368−1375. (i) Hawker, C. J.; Frec
́
het, J. M. J. Polymer 1992, 33,
1507−1511. (j) Zhang, A. F.; Zhang, B.; Wachtersbach, E.; Schmidt,
̈
M.; Schluter, A. D. Chem.Eur. J. 2003, 9, 6083−6092.
̈
(6) (a) Boydston, A. J.; Holcombe, T. W.; Unruh, D. A.; Frec
́
het, J.
AUTHOR INFORMATION
Corresponding Author
percec@sas.upenn.edu
■
M. J.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 5388−5389.
(b) Laurent, B. A.; Grayson, S. M. J. Am. Chem. Soc. 2011, 133,
13421−13429.
Notes
(7) Rosen, B. M.; Wilson, D. A.; Wilson, C. J.; Peterca, M.; Won, B.
C.; Huang, C.; Lipski, L. R.; Zeng, X.; Ungar, G.; Heiney, P. A.; Percec,
V. J. Am. Chem. Soc. 2009, 131, 17500−17521.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(8) (a) Hawker, C. J.; Frec
7638−7647. (b) Grayson, S. M.; Frec
́
het, J. M. J. J. Am. Chem. Soc. 1990, 112,
het, J. M. J. Chem. Rev. 2001,
■
́
Financial support by the National Science Foundation (Grants
DMR-1066116 and DMR-1120901) and by P. Roy Vagelos
Chair at the University of Pennsylvania is gratefully acknowl-
edged.
101, 3819−3867. (c) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.;
Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17,
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Baker, G. R.; Gupta, V. K. J. Org. Chem. 1985, 50, 2003−2004. (f) de
Brabander-van den Berg, E. M. M.; Meijer, E. W. Angew. Chem., Int. Ed.
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