Highly ordered dielectric mirrors via the self-assembly of dendronized block copolymers

Article abstract of DOI:10.1021/ja4081502

Dendronized block copolymers were synthesized by ruthenium-mediated ring-opening methathesis polymerization of exo-norbornene functionalized dendrimer monomers, and their self-assembly to dielectric mirrors was investigated. The rigid-rod main-chain confo

Full text of DOI:10.1021/ja4081502

Article
pubs.acs.org/JACS
Highly Ordered Dielectric Mirrors via the Self-Assembly of
Dendronized Block Copolymers
Victoria A. Piunova, Garret M. Miyake, Christopher S. Daeffler, Raymond A. Weitekamp,
and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratories for Chemical Synthesis, Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
ABSTRACT: Dendronized block copolymers were synthesized by ruthenium-mediated ring-opening methathesis polymer-
ization of exo-norbornene functionalized dendrimer monomers, and their self-assembly to dielectric mirrors was investigated. The
rigid-rod main-chain conformation of these polymers drastically lowers the energetic barrier for reorganization, enabling their
rapid self-assembly to long-range, highly ordered nanostructures. The high fidelity of these dielectric mirrors is attributed to the
uniform polymer architecture achieved from the construction of discrete dendritic repeat units. These materials exhibit light-
reflecting properties due to the multilayer architecture, presenting an attractive bottom-up approach to efficient dielectric mirrors
with narrow band gaps. The wavelength of reflectance scales linearly with block-copolymer molecular weight, ranging from the
ultraviolet, through the visible, to the near-infrared. This allows for the modulation of photonic properties through synthetic
control of the polymer molecular weight. This work represents a significant advancement in closing the gap between the
precision obtained from top-down and bottom-up approaches.
INTRODUCTION
Additionally, BCPs are a class of soft materials that can be
shaped, molded, or applied to any geometric specification.
■
5
Materials that can selectively control light by efficient reflection,
directed propagation, or enhanced confinement have numerous
applications as optical elements and devices.
Chain entanglement is a fundamental characteristic of
polymers that greatly influences their bulk materials properties.
Chain entanglement also presents an energetic barrier for the
self-assembly of BCPs that slows the rate of equilibration to
ordered morphologies. Furthermore, it complicates the syn-
thesis of ultrahigh molecular weight (MW) polymers and
generally impedes them from assembling to large domain sizes
1
,2
Photonic
crystals (PCs) are a class of nanostructured materials with
tunable reflection due to their periodic dielectric function,
which creates a photonic band gap where there are no allowed
frequencies at which light can propagate through the
1
,2
material.
The simplest 1D PC architecture, termed a
6
necessary to interact with long wavelength visible light. Thus,
dielectric mirror, is constructed from alternating layers of a
low and a high refractive index material. The wavelength of
reflected light (λ) is proportional to the domain size (d) and
the refractive index (n) of the layers [mλ = 2(d n + d n )].
1
most BCP-based PCs do not have high enough MWs or cannot
overcome chain entanglement to an extent that allows them to
1
assemble to long-wavelength-reflecting PCs. Traditionally7,
1
1
2 2
BCP-based PCs reflect wavelengths of light as long as green,
Conventional methods to fabricate dielectric mirrors utilize
top-down approaches, such as layer-by-layer depositions and
8
9
and domain swelling with solvents or homopolymers can
enable interaction with longer wavelengths. Our strategy to
overcome this hurdle has been to exploit polymer architectures
with reduced chain entanglement to minimize the energetic
barrier for reorganization and thus facilitate self-assembly.
One polymer architecture that is known to have a reduced
degree of chain entanglement is the molecular brush
3
coextrusion. These approaches can produce highly precise
structures but require complex setups and high costs. Recently,
we have been focusing on the bottom-up approach of block-
copolymer (BCP) self-assembly as an inexpensive and simple
alternative route to dielectric mirrors with an ultimate goal of
4
making paintable PCs. Although lacking in precision, BCPs
remain attractive as dielectric mirrors because their materials
properties can be synthetically controlled to produce materials
with tailored chemical, mechanical, and optical properties.
Received: August 6, 2013
©
XXXX American Chemical Society
A
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Figure 1. Structures of wedge-type monomers and their ROMP to dendronized BCPs. The schematic represents the corresponding synthesis and
self-assembly to narrow bandwidth dielectric mirrors.
1
0
15
copolymer. This architecture is composed of a sterically
congested array of low MW, polymeric side chains that are
unified through a common main chain. The steric interactions
between the densely packed polymeric repeat units enforce the
polymeric main chain to adopt a highly elongated conformation
of linear BCPs to uniform morphologies, we hypothesized
that minimizing the elements of polydispersity in the polymers
could enable the fabrication of more uniform and efficient
dielectric mirrors. Despite the fact that the polymeric side
chains align parallel with the lamellae, their dispersity could
influence nonuniform packing, promoting lamellae curvature,
domain size variability, and bandwidth broadening. Addition-
ally, near the polymer chain ends the degree of steric
congestion is reduced and the polymeric side chains become
perpendicular to the lamellae. In this region the polydisperse
polymeric side chains could further increase the dispersity in
the lengths of the brush BCPs, leading to nonuniform assembly.
Intuitively, it would be assumed that main-chain dispersity
would have the greatest impact on the ability of the polymer to
assemble to highly ordered morphologies. However, we have
recently shown that main-chain dispersity is not a large
deterrent, as blending two different MW brush BCPs (with
identical side chains and different degrees of main-chain
polymerization) can afford well-ordered morphologies where
the domain sizes scale with the weight incorporation of the two
11
with drastically reduced entanglement. Molecular brush
copolymers can be constructed via grafting-from, grafting-to,
or grafting-through procedures. Our interest in this field has
been in the grafting-through approach, utilizing ruthenium-
mediated ring-opening metathesis polymerization (ROMP)
of exo-norbornene functionalized low-MW polymers to
produce ultrahigh-MW molecular brush BCPs. This strategy
ensures quantitative grafting density and promotes the rapid
self-assembly of highly congested brush BCPs to long
12
13
4
wavelength reflecting dielectric mirrors. The brush BCPs
assemble in a manner that the side chains align parallel with the
lamellae, and the resulting domain size is directly modulated by
the degree of polymerization through the polymer main chain,
while the MW of the side chain has a much lesser impact on the
domain size. Thus, the peak wavelength of reflection (λmax) of
these materials can be predictably and precisely tuned by
adjusting the degree of polymerization of the polymer main
chain. Despite this capability, the bandwidths of the PCs are
somewhat large and broaden with increasing λmax, which
inhibits their potential in applications where more precision is a
1
4
4c
components. Regardless, in the context of precision dielectric
mirrors, domain size dispersity must be eliminated.
Here we explore the possibility of utilizing discrete
monomers that can still produce BCPs with a highly elongated,
rigid polymer architecture to reduce the dispersity in the system
to increase the fidelity of the dielectric mirrors. It has been
demonstrated that dendronized polymers exhibit a minimal
degree of chain entanglement due to the steric repulsion
between pendant monodisperse wedge side groups. Thus, we
believed dendronized polymers to be an ideal platform to test
our hypothesis. Additionally, as the molar mass of a dendritic
monomer can be considerably less than a macromonomer, an
equivalent degree of polymerization within the main chain for
both types of polymers would produce dendronized polymers
with a significantly reduced MW in comparison to brush BCPs.
The reduced MW of the dendronized polymer would
presumably increase diffusion rates and enhance the rate of
self-assembly. Here we report the sequential ROMP of exo-
norbornene functionalized wedge-type monomers to dendron-
ized BCPs and their self-assembly to highly ordered stacked
4
requirement. The ability to produce photonic crystals with
controlled bandwidths is necessary for specific applications. For
example, IR reflecting windows target a broad bandwidth
1
6
(
several hundred nanometers) to reject all nonvisible light from
passing while filter or display applications call for a bandwidth
of a few nanometers. This report makes a significant
advancement in improving the fidelity of BCP dielectric
mirrors, which results in a narrowing of the bandwidth, further
enhancing their application potential.
We partially attribute the broad bandwidth in our previous
systems to the nonuniform domain sizes achieved through BCP
self-assembly. Within the molecular brush BCP architecture
there exist three polydisperse components: the two different
polymeric repeat units and the unifying polymeric main chain.
Although polydispersity is known to enhance the self-assembly
B
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lamellar nanostructures with light reflecting properties (Figure
1).
RESULTS AND DISCUSSION
■
In order to synthesize BCPs with pendant dendritic groups we
designed two norbornene wedge-type monomers function-
alized with decyl (AW) and benzyl ether groups (BnW)
(
Figure 1). Monomers AW and BnW are AB -type, Newkome
3
dendrimers where branches radiating from the central core
constitute one complete generation in the dendrimer core.
The AW and BnW monomers were synthesized in high
1
7,18
Figure 2. (Left) Plot of reflectance as a function of wavelength for thin
films of the dendronized BCPs before (solid line) and after (dashed
line) thermal annealing. Color scheme corresponds to the BCP MW,
1
9
yields. The ROMP of the wedge monomers was efficient over
where M = 480 kDa (purple), 570 kDa (blue), 1250 kDa (red), 1390
a broad range of monomer to catalyst ratios, ranging from
w
kDa (black). Samples were prepared by controlled evaporation from
DCM solutions (2 g/L) and annealed for 24 h at 100 °C under
vacuum. The plot on the right demonstrates a linear correlation
between λmax of the primary reflectance peak and the molecular weight
of the dendronized BCPs.
2
00:1 to 2000:1, producing polymers with MWs (weight
average MW = M ) ranging from M = 427 to 2932 kDa while
w
w
low polydispersity indices (PDIs) (PDI = 1.01−1.27) (Table 1)
Table 1. Summary of the ROMP of BnW and AW Mediated
a
by 1
(
65.5%), narrow bandwidth with λmax of 330 nm. The brightly
colored violet film (M = 570 kDa) also showed a narrow
reflectance profile with λmax = 400 nm. The red-reflecting film
w
monomer
M)
conversion
time
(min)
Mw
(kDa)
PDI
w
b
c
c
(
[M]:[1]
(%)
(M /M )
n
(
M = 1250 kDa) exhibited a broad profile with λ = 768 nm.
BnW
BnW
BnW
BnW
BnW
AW
200:1
97
86
99
99
99
96
5
30
40
40
40
5
427
1539
2077
2358
2932
390
1.01
1.15
1.27
1.16
1.19
1.03
w max
The film fabricated from the polymer with M = 1390 kDa
reflected light in the near IR (λmax = 1390 nm). Polymers with
higher MWs did not reflect light.
Most importantly, the films fabricated from the dendronized
BCPs possessed significantly narrower bandwidths than our
500:1
w
1000:1
1500:1
2000:1
200:1
a
previously reported isocyanate brush BCPs with comparable
Polymerizations were performed in 2.00 mL of THF at ambient
4
temperature, [Bn(A)W] = 0.18 mM. Determined by 1H NMR.
b
λmax
.
To directly compare the bandwidths of these
c
dendronized BCP dielectric mirrors with our previously
Determined by light scattering.
reported results, we calculated the gap−midgap ratio (GMR)
by dividing the measured full width at half the λ (fwhm =
max
were maintained. After demonstrating the efficient ROMP of
BnW and AW, we proceeded to synthesize well-defined BCPs
by addition of AW after the polymerization of BnW had been
completed. The dendronized BCPs were isolated in high yields,
with MWs ranging from 480 to 3340 kDa and low PDIs (PDI =
Δλ) by λmax (Table 3). The GMRs of the dielectric mirrors
Table 3. Comparison between Bandwidths of Dendronized
BCPs and Isocyanate Brush BCPs
1
.05−1.23) (Table 2). All BCPs had nearly equimolar
dendronized BCPs
brush BCP
1
incorporation of each monomer, as determined by H NMR.
To investigate the ability of the BCPs to assemble to
dielectric mirrors, we prepared thin films on glass slides by
controlled evaporation from dichloromethane (DCM) sol-
utions (concentration = 2 g/L). The fabricated films visually
appeared transparent (M = 480 kDa), brightly colored [M =
Mw
λmax
b
GMR
Mw
a
λmax
b
GMR
c
a
c
,
d
(
kDa)
(nm)
(%)
(kDa)
(nm)
(%)
4
5
80
70
330
393
768
9 (5)
13 (4)
18 (2)
1512
2918
4167
334
511
664
17
25
27
1
250
w
w
a
b
Determined by light scattering. Determined by spectrophotometer
5
70 kDa (violet) and 1250 kDa (red)], or slightly translucent
c
with integrating sphere. GMR = gap−midgap ratio, determined by
(Mw = 1390, 1940 kDa). To quantify the light-reflecting
d
fwhm/λ
. The number in parentheses represents the theoretical
max
properties of these materials, reflectance spectra were recorded
on a spectrophotometer with an integrating sphere diffuse
reflectance accessory (Table 2, Figure 2). The BCP with the
values of the gap−midgap ratio with zero dispersity.
lowest MW (M = 480 kDa) demonstrated a high-intensity
w
a
Table 2. Summary of the Copolymerization of BnW and AW Mediated by 1
b
b
c
d
d
[
BnW]:[AW]:[1]
Mw (kDa)
PDI (M /M )
BnW (mol %)
yield (%)
λmax (nm)
λmax (nm) annealed
w
n
2
3
5
7
1
1
2
00:200:1
480
570
1.05
1.05
1.10
1.24
1.32
1.30
1.23
49.7
52.0
49.2
51.3
52.6
51.2
50.7
98.2
94.0
98.0
97.0
95.5
97.2
96.4
330
393
757
888
440
468
50:350:1
00:500:1
1250
1390
1940
2900
3340
960
50:750:1
1223
000:1000:1
500:1500:1
000:2000:1
a
b
c
Polymerizations performed in 2.00 mL of THF at ambient temperature, [BnW] = [AW] = 0.18 mM. Determined by light scattering. Determined
1
d
by H NMR. Determined by spectrophotometer with integrating sphere.
C
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Article
produced from dendronized BCPs were much smaller (GMR =
data. Dispersity in the domain thicknesses and partial phase
decoherence were incorporated to account for disorder in the
4
b,c
9
−18%) than from the isocyanate-based BCPs
(GMR =
23
17−27%) for dielectric mirrors that reflected light across the
multilayer. A detailed description of the simulation procedure
4a
visible spectrum (λ = 334−768 nm). To compare the current
can be found in our previous report. The 1D transfer matrix
simulations coincided with the experimentally obtained
max
dielectric mirrors with theoretically perfect reflectors, we
computationally calculated the GMRs for dielectric mirrors
possessing identical refractive indices as the dendronized BCPs
but no layer thickness dispersity (vide infra). The perfect
dielectric mirrors with identical λmax as the dendronized BCPs
had GMRs ranging from 5 to 2%.
reflection spectra for all the dielectric mirrors (M = 480−
w
1250 kDa). Overall, modeling results support that a multilayer
stack is obtained through the rapid self-assembly of the
dendronized BCPs, giving rise to 1D photonic crystals.
To gain insight into the decreased bandwidths of these
materials, we investigated the morphology of the films
produced from DCM solutions by controlled evaporation.
We attribute the striking difference between the BCP systems
to the fact that the polydispersity in the side groups was
eliminated by replacing macromolecular repeat units with well-
defined discrete groups, yielding more unified “building blocks”
for the formation of highly ordered stacked lamellar
morphologies. Additionally, the reduced polymer MW for
dendronized BCPs, because of the lower MW of the repeat
unit, most likely enables faster diffusion and assembly. In both
systems, as the MW of the BCPs is increased, bandwidth
broadening is observed, suggesting increased disorder in the
bulk morphology. In order to overcome the kinetics for
reorganization, the samples were annealed at 100 °C for 24 h.
Cross sections of freeze-fractured films were stained with RuO
4
and examined by scanning electron microscopy (SEM). For the
films produced by dendronized BCPs with MW lower than
1390 kDa, stacked lamellar morphologies were observed, as
24
expected for the symmetric BCPs (Figure 4a−c). Remarkable
long-range order was observed throughout the entire bulk of
the film, ranging from ∼100−200 uniform layers of alternating
domains for the samples with M = 480 and 570 kDa (Figure
w
4a,b). These multilayer films show unprecedented order for
BCPs in this size regime. Unfortunately, an increase in MW
resulted in a rapid loss in ordering (Figure 4c,d), which was
attributed to the dominating influence of chain entanglement
that increases the energetic barrier for reorganization.
Comparison of the SEM cross sections before and after
annealing (Figure 5) revealed some improvement in long-range
Thermal annealing induced a noticeable red shift (Δλ
max
varying from 75 to 345 nm) along with increased bandwidth
broadening and decreased reflectance intensity (Figure 2,
dashed lines) for samples with MW ranging from 480 to 1390
kDa (vide infra).
It is established that for 1D photonic crystals, the wavelength
order for samples with M = 1390 and 1940 kDa samples, in
w
of reflected light is directly proportional to the domain spacing,
addition to lamellae thickening (1.3 times on average) for all
samples. A significant overall evolution in morphology was
6
which in turn is related to the MW of the polymer. A strictly
2
detected for the film prepared from the BCP with M = 1390
linear dependence (R = 0.996) between the λ of the primary
w
max
kDa (Figure 5i). Initially, a poorly defined morphology,
consisting of only a few domains resembling lamellae, was
observed at the polymer−air interface. After annealing, stacked
lamellae with domain sizes ranging between 380 and 400 nm
were observed.
reflectance peak and the molecular weight was observed for
dendronized BCPs (Figure 2), which is in accord with our
4
previously reported systems. These data are in contrast to
linear polymer analogs, where the domain size is theoretically
2
/3 20
proportional to MW
.
This unique behavior of brush or
The film produced from the highest MW polymer (M =
dendronized BCPs originates from the rigid architectures that
exhibit a reduced degree of chain entanglement. As a result, the
assembly to large domain sizes is achieved through the
incorporation of a fewer number of monomer repeat units
w
1
940 kDa) also demonstrated a drastic change in morphology.
The original sample produced by controlled evaporation from
DCM lacked long-range order and exhibited scattered
elongated “raspberry-like” (∼1−1.5 μm long and ∼0.25 μm
wide) features (Figure 5e). Upon annealing, an overall
reorganization to a somewhat periodic arrangement, resembling
a cylindrical morphology, was observed (Figure 5j). Similarly,
the raspberry-like features were noted for the 1390 kDa sample
at the polymer−substrate interface before thermal annealing.
These similarities suggest that before the system reaches the
thermodynamic minimum, corresponding to a lamellar
morphology, it undergoes a series of metastable states, which
become kinetically trapped with the high MW polymers. Even
extended annealing (120 h) was unable to annihilate the
21
than standard random-coil BCPs.
To further support the origin of the reflecting properties in
22
dendronized BCPs, we performed transfer matrix simulations
to generate reflectance spectra for the dielectric mirrors (Figure
). The simulation parameters were determined from
spectroscopic ellipsometry, SEM cross sections, and reflectance
3
morphological disorder for the polymer with M = 1940 kDa.
w
The lack of morphological order for the ultrahigh-MW
dendronized BCPs explains their lack of light reflecting
properties. SEM analysis revealed a minimal change in
morphology, which remotely resembled stacked lamellae. We
hypothesize that the lamellar morphology evolution occurs
through several stages, proceeding through a disordered
morphology into a lower energy, metastable morphology, en
Figure 3. Plot of reflectance as a function of wavelength for dielectric
mirrors fabricated from the dendronized BCPs with M = 480 kDa
w
25
route to the final thermodynamic minimum. This reorganiza-
(
purple), 570 kDa (blue), 1250 kDa (red), 1390 kDa (black). Dashed
tion becomes exceedingly slow with an increase in the BCP
MW. The combination of the reflectance and SEM analysis
suggest that the reduced MW of dendronized BCPs, in
lines represent corresponding simulated spectra. The shaded regions
represent photonic bandgaps simulated for perfectly alternating
multilayers with no layer thickness dispersity.
D
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Figure 4. SEM cross sections of dendronized BCPs films prepared by controlled evaporation from DCM with M = 480 (a), 570 (b), 1250 (c), and
w
1
390 (d) kDa. Scale bar = 1 μm.
Figure 5. SEM cross sections of dendronized BCPs films prepared by controlled evaporation from DCM with M = 480 kDa (a, f), 570 kDa (b, g),
w
1
250 kDa (c, h), 1390 kDa (d, i), and 1940 kDa (e, j). Scale bar = 1 μm.
comparison to brush BCPs, allows their more rapid self-
assembly to highly ordered dielectric mirrors. However, the less
rigid architecture of dendronized BCPs experiences a higher
degree of chain entanglement, which inhibits the ultrahigh-MW
polymers from rapidly reaching equilibrium, instead trapping
them in a metastable state. Taking into account the annealing-
induced morphological evolution observed by SEM, we attempt
to explain the changes in the reflectance profiles of dendronized
BCPs upon annealing. The shift in λmax is caused by the
increase in domain spacing induced by enthalpy-driven
thickening of lamellae. However, the thickening was not
uniform throughout the bulk of the material, resulting in the
domain dispersity and the broadening of the reflectance peak.
CONCLUSIONS
■
We have synthesized a series of well-defined dendronized BCPs
with MWs ranging from 480 to 3340 kDa. Rapid self-assembly
of these BCPs from a volatile solvent afforded one-dimensional
periodic nanostructured materials capable of reflecting light
across the visible spectrum from the UV, through the visible, to
the near-IR. The dielectric mirrors fabricated from dendronized
BCPs demonstrated extremely narrow bandwidths of high
intensity. The highly ordered nanostructures in this size regime
have not been previously obtained via the self-assembly of
BCPs. This work closes the gap between the precision obtained
from bottom-up and top-down approaches for the fabrication
of synthetic photonic crystals. With the ability to fabricate high-
fidelity multilayers under ambient conditions through the self-
E
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assembly of BCPs, we are working to develop paintable,
functional, and tunable optical elements.
95% EtOH to give a white solid. The white solid was suspended in
Et O (100 mL). Concentrated HCl (6 mL) was added to the ethereal
2
suspension, followed by the precipitation of potassium chloride. Water
(
50 mL) was added then separated from the organic phase. The
EXPERIMENTAL SECTION
■
organic phase was washed with water (2 × 50 mL) and brine (1 × 50
mL) and dried over sodium sulfate. The solution was filtered and the
solvent was removed by rotary evaporation to yield 3b as a white solid
Materials and Methods. (H IMes)(PPh )(Cl) RuCHPh was
2
3
2
received as a research gift from Materia Inc. and was converted to 1
26
1
(3.6 g, 5.3 mmol, 89%). H NMR (500 MHz, CDCl ): δ 7.32 (2H, s),
via literature procedure. N-(hydroxyethyl)-cis-5-norbornene-exo-2,3-
3
2
7
4
.04 (2H, t, J = 7 Hz), 4.02 (4H, t, J = 7 Hz), 1.82 (4H, quintet, J = 7
dicarboximide (2) was prepared according to literature procedure.
All other chemicals were purchased from Sigma-Aldrich. Solvents were
Hz), 1.75 (2H, quintet, J = 7 Hz), 1.48 (6H, quintet, J = 7 Hz), 1.39−
1
.22 (48H, bs), 0.88 (9H, t, J = 7 Hz). 13C{ H} NMR (126 MHz,
1
purified by passage through solvent purification columns and further
28
CDCl ): δ 171.02, 152.85, 143.16, 123.47, 108.56, 104.99, 73.55,
degassed with argon. All polymerizations were performed in a
nitrogen-filled glovebox in 20 mL scintillation vials. NMR spectra were
recorded on a Varian Inova 500 MHz spectrometer. Chemical shifts
were referenced to internal solvent resonances and are reported as
parts per million relative to tetramethylsilane. High-resolution mass
spectra were provided by the California Institute of Technology Mass
Spectrometry Facility. Polymer molecular weights were determined by
multiangle light-scattering (MALS) gel permeation chromatography
3
6
2
9.20, 31.95, 31.93, 30.34, 29.76, 29.74, 29.73, 29.70, 29.67, 29.64,
9.57, 29.40, 29.37, 29.28, 26.08, 26.05, 22.70, 14.12. HRMS (ES):
calcd 673.5771, found 673.5771.
Alkyl Wedge Monomer (AW). A 100 mL Schlenk flask equipped
with a stir bar was flame-dried under vacuum. The cooled flask was
backfilled with argon and charged with 3b (3.8 g, 5.6 mmol, 1 equiv),
alcohol 2 (1.3 g, 6.2 mmol, 1.1 equiv), 4-dimethylaminopyridine (66
mg, 0.56 mmol, 0.1 equiv), and CH Cl (25 mL). The solution was
(
GPC), with THF as the eluent, using a miniDAWN TREOS light-
2
2
cooled to 0 °C in an ice bath, with precipitation of some reagents.
Dicyclohexylcarbodiimide (1.3 g, 6.2 mmol, 1.1 equiv) was added to
the cooled solution, and the reaction was stirred at 0 °C for 30 min.
The reaction was warmed to room temperature and stirred for 18 h.
The resulting suspension was filtered and the solid was washed with
scattering detector, a Viscostar viscometer, and an OptilabRex
refractive index detector, all from Wyatt Technology. An Agilent
1
200 UV−vis detector was also present in the detector stack. Absolute
molecular weights were determined using dn/dc values calculated by
assuming 100% mass recovery of the polymer sample injected into the
GPC. Polymer thin films were prepared from the controlled
evaporation of polymer solutions (∼2 g/L) in dichloromethane onto
glass slides that had been previously washed with methanol and
hexane. After the solvent was allowed to evaporate, the samples were
dried under vacuum overnight. SEM images were taken on a ZEISS
CH Cl (25 mL). Solvent was removed from the filtrate by rotary
2 2
evaporation to yield very viscous oil. Ethanol (95%, 100 mL) was
added to the oil and stirred for 3 h. The resulting white solid was
filtered and residual solvent was removed in vacuo to yield the alkyl
1
wedge monomer AW (3.4 g, 3.9 mmol, 70%). H NMR (500 MHz,
1
550 VP field emission SEM. Reflection measurements were
CDCl ): δ 7.20 (2H, s), 6.27 (2H, t, J = 2 Hz), 4.40 (2H, t, J = 5 Hz),
3
performed on a Cary 5000 UV/vis/NIR spectrophotometer, equipped
with an integrating sphere diffuse reflectance accessory (Internal DRA
4.01 (6H, t, J = 1 Hz), 3.90 (2H, t, J = 5 Hz), 3.23 (2H, m), 2.69 (1H,
d, J = 2 Hz), 1.82 (4H, quintet, J = 7 Hz), 1.73 (2H, quintet, J = 7 Hz),
1
800). All measurements were referenced to a LabSphere Spectralon
1.55 (1H, s), 1.48 (6H, m), 1.43−1.21 (50H, br), 0.88 (9H, t, J = 7
Hz). 13C{ H} NMR (126 MHz, CDCl
1
): δ 177.69, 166.06, 152.81,
3
9
9% certified reflectance standard. The samples were illuminated
through a Spectralon-coated aperture with a diameter of 1 cm, with a
beam area of approximately 0.5 cm . The samples were scanned at a
rate of 600 nm/min, with a 1 nm data interval, from 1800 to 200 nm,
with a detector crossover (InGaAs to PMT) at 875 nm. The frequency
dependent refractive indices were generously measured and fit by Ron
Synowicki and Nina Hong at J.A. Woollam Co., Inc.
142.46, 137.77, 124.01, 107.98, 73.47, 69.12, 61.63, 47.85, 45.25,
42.69, 37.53, 31.95, 31.93, 30.34, 29.76, 29.74, 29.73, 29.71, 29.70,
29.67, 29.66, 29.58, 29.41, 29.40, 29.37, 29.33, 26.11, 22.70, 14.12.
HRMS (ES+): calcd 864.6717, found 864.6716.
2
Benzyl Wedge Monomer (BnW) Synthesis. Methyl 3,4,5-
Tribenzylbenzoate (4a). A 250 mL round-bottomed flask equipped
with a stir bar was charged with DMF (50 mL). The solution was
sparged for 30 min with argon. After sparging, the flask was
sequentially charged with methyl gallate (1.9 g, 10 mmol, 1 equiv),
benzyl bromide (4.8 mL, 40 mmol, 4 equiv), and potassium carbonate
(8.5 g, 60 mmol, 6 equiv). The flask was then equipped with a Vigreux
column and heated to 80 °C for 12 h. Upon cooling to room
temperature, the reaction mixture was diluted with water (100 mL)
and extracted with diethyl ether (2 × 100 mL). The combined organic
phases were washed with water (100 mL) and then 50% brine (100
mL) and dried over magnesium sulfate. The combined organic phases
were filtered through a plug of basic alumina. The solvent was
removed by rotary evaporation to yield an oil, which became white
Synthesis of Monomers. Methyl 3,4,5-Tris(decyloxy)benzoate
3a). A 250 mL round-bottomed flask equipped with a stir bar was
(
charged with DMF (50 mL). The solution was sparged for 30 min
with argon. After sparging, the flask was sequentially charged with
methyl gallate (1.9 g, 10 mmol, 1 equiv), bromodecane (10 mL, 40
mmol, 4 equiv), and potassium carbonate (8.5 g, 60 mmol, 6 equiv).
The flask was then equipped with a Vigreux column and heated to 80
°
C for 12 h. Upon cooling to room temperature, the reaction mixture
was diluted with water (100 mL) and extracted with diethyl ether (2 ×
00 mL). The combined organic phases were washed with water (100
1
mL) and then 50% brine (100 mL) and dried over magnesium sulfate.
The combined organic phases were filtered through a plug of basic
alumina. The solvent was removed by rotary evaporation to yield an
1
solid 4a (3.9 g, 98.5 mmol, 85%) in vacuo. H NMR (500 MHz,
1
oil, which became white solid 3a (6.2 g, 9.1 mmol, 91%) in vacuo. H
CDCl
3
): δ 7.48−7.44 (4H, m), 7.43−7.38 (8H, m), 7.37−7.33 (2H,
NMR (500 MHz, CDCl ): δ 7.25 (2H, s), 4.00 (2H, t, J = 6.5 Hz),
m), 7.30−7.26 (3H, m), 5.16 (4H, s), 5.14 (2H, s), 3.91 (3H, s).
3
13
1
3
1
(
.99 (4H, t, J = 6.5 Hz), 3.87 (3H, s), 1.80 (4H, quintet, J = 7 Hz),
C{ H} NMR (126 MHz, CDCl ): δ 166.62, 152.55, 142.41, 137.43,
3
.73 (2H, quintet, J = 7 Hz), 1.46 (6H, quintet, J = 7 Hz), 1.38−1.19
136.65, 128.53, 128.51, 128.17, 128.01, 127.93, 127.53, 125.21, 109.99,
109.07, 75.12, 71.23, 52.22. HRMS (FAB): calcd 454.17.80, found
454.1782.
3,4,5-Tribenzylbenzoic Acid (4b). A 250 mL round-bottomed flask
equipped with a stir bar was charged with 4a (3.9 g, 8.5 mmol, 1
equiv), potassium hydroxide (4.8 g, 85 mmol, 10 equiv), and 95%
EtOH (43 mL). The round-bottom flask was equipped with a water-
cooled condenser. The suspension was refluxed (∼80 °C) for 4 h.
Upon cooling, the reaction mixture thickened substantially. The solid
48H, bs), 0.87 (9H, t, 7 Hz). 13C{ H} NMR (126 MHz, CDCl ): δ
1
3
1
3
2
66.93, 152.81, 142.36, 124.64, 107.97, 104.99, 73.48, 69.16, 52.09,
1.95, 31.93, 30.33, 29.75, 29.74, 29.73, 29.70, 29.67, 29.64, 29.57,
9.40, 29.37, 29.31, 26.08, 26.06, 22.70, 22.68, 14.14, 14.12. HRMS
(
FAB+): calcd 689.6084, found 689.6095.
,4,5-Tris(decyloxy)benzoic Acid (3b). A 250 mL round-bottomed
3
flask equipped with a stir bar was charged with 3a (4.1 g, 6 mmol, 1
equiv), potassium hydroxide (3.4 g, 60 mmol, 10 equiv), and 95%
EtOH (30 mL). The round-bottom flask was equipped with a water-
cooled condenser. The suspension was refluxed (∼80 °C) for 4 h.
Upon cooling, the reaction mixture thickened substantially. The solid
̈
was filtered with a Buchner funnel and washed with cold (−20 °C)
95% EtOH to give a yellow solid. The solid was suspended in Et O
2
(100 mL). Concentrated HCl (6 mL) was added to the ethereal
suspension, followed by the precipitation of potassium chloride. The
̈
was filtered with a Buchner funnel and washed with cold (−20 °C)
F
dx.doi.org/10.1021/ja4081502 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
suspension was filtered and washed with water. The filtrate was
AUTHOR INFORMATION
■
dissolved in acetone and dried over MgSO . Filtration and solvent
4
1
Corresponding Author
rhg@caltech.edu
removal by rotary evaporation yielded 4b (1.85 g, 4.2 mmol, 49%). H
NMR (500 MHz, acetone-d ): δ 7.55 (4H, d, J = 7 Hz), 7.48−7.44
6
(
4H, m), 7.41 (4H, tt, J = 7 Hz, 1.5 Hz), 7.35 (2H, tt, J = 7 Hz, 1.5
Notes
13
1
Hz), 7.30−7.25 (3H, m), 5.23 (4H, s), 5.14 (2H, s). C{ H} NMR
(
The authors declare no competing financial interest.
126 MHz, acetone-d ): δ 166.23, 152.62, 142.17, 137.95, 137.20,
6
1
7
28.41, 128.28, 128.04, 127.86, 127.73, 127.64, 125.70, 108.87, 74.59,
0.76. HRMS (FAB): calcd 441.1702, found 441.1682.
Benzyl Wedge Monomer (BnW). A 100 mL Schlenk flask equipped
ACKNOWLEDGMENTS
■
We gratefully acknowledge Ron Synowicki and Nina Hong at
J.A. Woollam Co., Inc. for their measurement and fitting of the
refractive indices of these polymers. Reflection measurements
were collected at the Molecular Materials Research Center of
the Beckman Institute of the California Institute of Technology.
This work was supported by a Dow-Resnick Bridge Award.
This research was conducted with Government support under
and awarded by DoD, Air Force Office of Scientific Research,
National Defense Science and Engineering Graduate (NDSEG)
Fellowship (32 CFR 168a).
with a stir bar was flame-dried under vacuum. The cooled flask was
backfilled with argon and charged with 4b (1.7 g, 3.8 mmol, 1 equiv),
alcohol 2 (865 mg, 4.2 mmol, 1.1 equiv), 4-dimethylaminopyridine
(
46 mg, 0.38 mmol, 0.1 equiv), and CH Cl (20 mL). The solution
2 2
was cooled to 0 °C in an ice bath, with precipitation of some reagents.
Dicyclohexylcarbodiimide (862 mg, 4.2 mmol, 1.1 equiv) was added to
the cooled solution, and the reaction was stirred at 0 °C for 30 min.
The reaction was warmed to room temperature and stirred for 18 h.
The resulting suspension was filtered and the solid was washed with
CH Cl (25 mL). Solvent was removed from the filtrate by rotary
2
2
evaporation to yield an off-white solid. Ethanol (95%, 100 mL) was
added to the oil and the mixture stirred for 3 h. The resulting white
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(
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dx.doi.org/10.1021/ja4081502 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX