Synthesis and arm dissociation in molecular stars with a spoked wheel core and bottlebrush arms

Article abstract of DOI:10.1021/ja506780y

Unique star-like polymeric architectures composed of bottlebrush arms and a molecular spoked wheel (MSW) core were prepared by atom transfer radical polymerization (ATRP). A hexahydroxy-functionalized MSW (MSW_6-OH) was synthesized and converted into a six-fold ATRP initiator (MSW_6-Br). Linear chain arms were grafted from MSW_6-Br and subsequently functionalized with ATRP moieties to form six-arm macroinitiators. Grafting of side chains from the macroinitiators yielded four different star-shaped bottlebrushes with varying lengths of arms and side chains, i.e., (450-g-20)₆, (450-g-40)₆, (300-g-60)₆, and (300-g-150)₆. Gel permeation chromatography analysis and molecular imaging by atomic force microscopy confirmed the formation of well-defined macromolecules with narrow molecular weight distributions. Upon adsorption to an aqueous substrate, the bottlebrush arms underwent prompt dissociation from the MSW core, followed by scission of covalent bonds in the bottlebrush backbones. The preferential cleavage of the arms is attributed to strong steric repulsion between bottlebrushes at the MSW branching center. Star-shaped macroinitiators may undergo aggregation which can be prevented by sonication.

Full text of DOI:10.1021/ja506780y

Article
pubs.acs.org/JACS
Synthesis and Arm Dissociation in Molecular Stars with a Spoked
Wheel Core and Bottlebrush Arms
Joanna Burdynska,† Yuanchao Li,‡ Anant Vikas Aggarwal,§ Sigurd Hoger,*,§ Sergei S. Sheiko,*,‡
́
̈
and Krzysztof Matyjaszewski*,†
†Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh,
Pennsylvania 15213, United States
‡Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
§Kekule-
53121 Bonn, Germany
́
Institut fur Organische Chemie und Biochemie, Rheinische Friedrich-Wilhelms-Universitat Bonn, Gerhard-Domagk-Straße 1,
̈
̈
S
* Supporting Information
ABSTRACT: Unique star-like polymeric architectures com-
posed of bottlebrush arms and a molecular spoked wheel
(MSW) core were prepared by atom transfer radical polymer-
ization (ATRP). A hexahydroxy-functionalized MSW
(MSW6‑OH) was synthesized and converted into a six-fold
ATRP initiator (MSW6‑Br). Linear chain arms were grafted from
MSW6‑Br and subsequently functionalized with ATRP moieties to
form six-arm macroinitiators. Grafting of side chains from the
macroinitiators yielded four different star-shaped bottlebrushes
with varying lengths of arms and side chains, i.e., (450-g-20)6,
(450-g-40)6, (300-g-60)6, and (300-g-150)6. Gel permeation
chromatography analysis and molecular imaging by atomic force
microscopy confirmed the formation of well-defined macromolecules with narrow molecular weight distributions. Upon
adsorption to an aqueous substrate, the bottlebrush arms underwent prompt dissociation from the MSW core, followed by
scission of covalent bonds in the bottlebrush backbones. The preferential cleavage of the arms is attributed to strong steric
repulsion between bottlebrushes at the MSW branching center. Star-shaped macroinitiators may undergo aggregation which can
be prevented by sonication.
approaches: “grafting through”,13 “grafting onto”,14 and
INTRODUCTION
■
“grafting from”.4,8,15−17 CRP techniques, in particular atom
transfer radical (ATRP),3,18−20 reversible addition−fragmenta-
tion chain transfer (RAFT,)21,22 and nitroxide-mediated
polymerizations (NMP),23 have provided a convenient route
for the preparation of well-defined molecular brushes. Both the
grafts and backbones may have different branching topologies
and chemical compositions that can be altered separately, which
exceedingly expands the range and sophistication of molecular
architectures, and thus materials’ properties as compared to
those of linear polymers. Up to now, a variety of densely grafted
copolymers with different architectures were prepared including
brush-coil,24 diblock (both within the backbone and side
chains),21,25−30 gradient,31,32 and multi-arm star-like brushes.33
Such unique macromolecules have also been extensively
investigated in areas of supersoft elastomers,34,35 pho-
tonics,10,28,29,36 organic nanotubes,11,21 biomimetic materi-
als,37−39 networks and porous materials,27,40 and lithography/
nanofabrication.41,42
Continuous development in the areas of controlled radical
polymerization (CRP) and organic synthesis has given
researchers access to a variety of complex macromolecules
with a vast number of well-defined topologies.1−3 In particular,
molecular bottlebrushes, a unique type of graft copolymers with
side chains densely packed along a linear polymer chain, have
garnered a great deal of attention.4−11 The high grafting density
generates strong steric repulsion between the tethered side
chains, resulting in the corresponding increase in the
persistence length and disentanglement of bottlebrushes. A
distinct worm-like conformation of molecular bottlebrushes,
along with their exceptional length12 (up to several micro-
meters), affords the ability to image individual molecules using
atomic force microscopy (AFM). Therefore, molecular
bottlebrushes have been widely used as model systems for
experimental studies of single-molecule ordering, motion, and
reactivity at interfaces.8
In synthetic chemistry, dense grafting has been explored as a
versatile platform for the design of complex molecular and
supramolecular systems with bottlebrushes as shape-persistent
mesoblocks. Bottlebrushes can be prepared via three different
Received: July 8, 2014
© XXXX American Chemical Society
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Figure 1. (a) Synthetic approach used for the preparation of molecular spoked wheels (MSWs) and chemical structures of the rim (blue), spoke
(red), hub (green), and MSW derivatives. Reagents and conditions: i, Pd(PPh3)2Cl2, CuI, PPh3, THF, piperidine, μw (microwave irradiation), 120
°C, 12 min, 92%; ii, K2CO3, THF, methanol, rt, 2 h, 100%; iii, Pd2dba3, CuI, dppf, THF, piperidine, 80 °C, 17 h, 81%; iv, TBAF, THF, 0 °C to rt, 3 h,
74%; v, 2-bromo-2-methylpropionyl bromide, pyridine, CH2Cl2, 0 °C, 2 h. (b) NMR spectra of MSW6‑OH and MSW6‑Br in CD2Cl2. The spectra are
very similar, but the methyl groups on the 2-bromo-2-methylpropanoate unit MSW6‑Br give rise to a new sharp singlet signal at 1.92 ppm. (c)
MALDI-TOF of MSW6‑Br showing the high purity of the 6-fold initiator.
The ATRP initiator can also be designed to modify the
architecture of densely grafted copolymers. Previously, the
multifunctional ATRP initiators were used to prepare three-
and four-armed architectures, which combined the properties of
both polymeric stars and molecular bottlebrushes.33 AFM
imaging of individual molecules provided a visual confirmation
of star-shaped bottlebrushes, also allowing for the correlation of
the type of a catalytic system with the structural quality of
obtained polymers. Star-like bottlebrushes displayed a distinct
ordering of monolayers, showing transition under compression,
from a dendritic to a disk-like conformation.43
In this work, we have designed a novel type of molecular
spoked wheel (MSW) ATRP initiator, which allows for the
synthesis of molecular star bottlebrushes with a higher number
of arms than reported previously, and a planar topology that is
forced by the disk-like structure of the initiator. MSWs form a
new class of conjugated macrocycles with multiple aromatic
rings and alkyne groups covalently linked together44−48 to form
exceptionally rigid and highly fluorescent structures.49−53 These
unique molecules can be potentially utilized as optically active
materials,54 molecular wires,55 or environmentally confined
reaction containers.56 It was also shown previously that they are
useful tools for fundamental studies of the understanding of the
microscopic electronic structure of conjugated polymers.50 For
the purpose of this study, we have synthesized a six-fold,
hydroxy-functionalized MSW (MSW6‑OH) and converted it to a
well-defined ATRP initiator (MSW6‑Br). The ATRP function-
alized MSW allowed for the preparation of star-like
bottlebrushes with a distinct hexa-arm topology. Star-shaped
bottlebrushes with varying lengths of arms and side chains were
generated by grafting poly(n-butyl acrylate) from star-like
backbones under normal ATRP conditions. Polymers were
characterized by gel permeation chromatography (GPC) and
AFM, proving the formation of star-shaped polymers. AFM
analysis allowed for the direct visualization of star-shaped
bottlebrushes with distinctive topological differences resulting
from varied lengths of both grafts and arms.
The goal of this work was to synthesize new star-like
bottlebrush architectures from a radial, six-fold MSW initiator.
We expected to enhance mechanical tension on the MSW
through the incorporation of bottlebrush arms, thus causing a
distortion of the highly conjugated core. A buildup of steric
repulsion at the MSW core led to prompt dissociation of
bottlebrush arms, which was observed and studied by AFM.
However, no significant changes in electronic properties were
detected, as indicated by emission experiments (see SI, Figures
S6−S8).57
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Molecular
Spoked Wheel-Based Six-Fold ATRP-Initiator MSW6‑Br
.
The synthetic approach used for the preparation of six-fold
MSW6‑Br ATRP initiator was based on the previous reports on
analogous MSWs.50,58 In this paper, we introduced synthetic
adjustments that allowed for the synthesis of MSW6‑Br. A
complete description of synthetic procedures including all
organic precursors and sample characterization is given in the
Supporting Information (SI).
The synthesis of MSW6‑Br relied on the preparation of its
hexa-hydroxy precursor, MSW6‑OH. The incorporated hydroxyl
functionalities allow post-cyclization modification of the MSW
providing access to new, more complex MSWs which can be
potentially used in the generation of novel advanced materials.
Furthermore, due to the strong propensity of previously
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Scheme 1. Synthesis of Star-like Bottlebrushes with Hexa-arm ATRP Initiator MSW6‑Br via Double “Grafting-from” Approach
prepared MSWs analogues toward the aggregation, we also
aimed to eliminate such tendency in MSW6‑Br, thus ensuring
the polymerization exclusively from unimolecular initiator
species.58 This was accomplished through the replacement of
linear hexadecyl side chains of the spoke, with branched 2-
hexyldecyl groups (Figure 1a). As proved by dynamic light
scattering (DLS) measurements in chloroform and toluene
(Figure S5), the incorporation of the branching in MSWs
resulted in the successful suppression of their aggregation. It
was ascribed to the reduced ability of MSWs to interact via π−π
stacking as well as their improved solubility as compared to
structures with linear hexadecyl groups.
MSW6‑OH was obtained via a convex modular approach,
based on the design of individual modules: the rim (blue),
spoke (red) and hub (green). All the building blocks were
linked together prior the cyclization (Figure 1a). The most
crucial step was the design of the functional rim (Figure 1a,
blue; Figure S2, 8) with incorporated hydroxyl functionality.
The structure of the rim was based on m-terphenyl-scaffold
modified with 3-[(triisopropylsilyl)oxy]propyl linker
(TIPSO−), which allowed for the incorporation of a protected
hydroxyl group into the molecule (Figure 1a; Figure S2, 8).
Next, the rim was attached to the spoke (Figure 1a, red; Figure
S2, 16) to form a spoke-rim module (Figure 1a, Figure S2, 18),
which was then selectively deprotected and linked to the hub
(Figure 1a, green; Figure S2, 20). A subsequent deprotection of
the assembled product yielded a MSW precursor with six free
hydroxyl (Figure 1a, Figure S2, 22) groups, which was directly
cyclized under pseudo high-dilution conditions to form the
desired MSW6‑OH (Figure 1a). Then, MSW6‑OH was converted
into a well-defined ATRP initiator, MSW6‑Br through a six-fold
esterification reaction with α-bromoisobutyryl bromide and
pyridine (Figure 1a). The esterification of MWS6‑OH proceeded
with high yields (89%) and efficiency, as confirmed by the
appearance of the signal at 1.94 ppm corresponding to CH3−
protons of ATRP moieties in the 1H NMR spectra of MSW6‑Br
(Figure 1b). MALDI-TOF analysis of MWS6‑Br showed the
presence of two peaks corresponding to pure molecules and
molecules that lost one bromine atom upon ionization
(molecular mass 7148.6 and 7068.6 Da, respectively), thus
confirming a very high purity of the final ATRP hexa-initiator.
“grafting-from” approach under normal ATRP conditions.
First, (2-trimethylsiloxyethyl) methacrylate (HEMA-TMS)
was polymerized from the multifunctional ATRP initiator
(MSW6‑Br), followed by a subsequent esterification with ATRP-
active moieties. Finally, poly(n-butyl acrylate) side chains were
grafted from six-arm ATRP macroinitiators, resulting in star-
shaped molecular bottlebrushes (Scheme 1).33 P(HEMA-TMS)
was grafted from MSW6‑Br under normal ATRP conditions to
form arms of stars with degrees of polymerization (DPs) of 450
((450-TMS)6) and 300 ((300-TMS)6). Reactions were
performed with high molar ratios of [M]:[I] (9600 and 6000
respectively) while keeping polymerizations at limited mono-
mer conversions (∼30%), thereby suppressing the intermo-
lecular termination between growing arms, and subsequent
star−star coupling. To ensure high initiation efficiencies from
the multifunctional initiator, MSW6‑Br, the polymerizations of
HEMA-TMS were performed with CuICl/dNbpy complex as a
catalytic system.33,59 Earlier results revealed that ATRP
reactions under halogen exchange conditions produce well-
defined stars with narrow arm length distributions and
improved grafting efficiency, as compared to traditionally
used CuIBr-based catalyst.33 [P(HEMA-TMS)450
]
((450-
6
TMS)6) and P(HEMA-TMS)300]6 ((300-TMS)6) were char-
acterized by GPC confirming the formation of star-polymers
(Figure 2, black) with narrow molecular weight distributions
(MWDs), Mw/Mn = 1.22 and 1.12, respectively (Table 1,
samples 1 and 4). The number-average molecular weights (Mn)
for the macroinitiators (450-TMS)6 and (300-TMS)6 were
lower than the corresponding theoretical values (Mn,th). This
was attributed to a compact structure of star-shaped P(HEMA-
TMS) polymers, which resulted in hydrodynamic volumes
smaller than the dimensions of linear poly(methyl methacry-
late) (PMMA) standards.
The functionalization of (450-TMS)6 and (300-TMS)6 with
ATRP reactive groups was performed via one pot, two-step
process, following a previously reported procedure.4 First,
TMS-blocked hydroxyl groups were deprotected in the
presence of KF/TBAF, followed by the subsequent addition
of α-bromoisobutyryl bromide, resulting in the formation of
respective PBiBEM macroinitiators (450-Br)6 and (300-Br)6.
1H NMR spectroscopy of polymers confirmed the full
deprotection of OTMS together with the quantitative
incorporation of ATRP functionalities, as indicated by the
disappearance of TMS signals and the downfield shift of both
adjacent CH2 peaks of P(HEMA-TMS).60 GPC character-
Synthesis and Characterization of (PBiBEM300/450
)
Hexa-arm Macroinitiators. The synthetic pathway for the
synthesis of star-shaped brushes is shown in Scheme 1.
Molecular bottlebrush arms were prepared via double
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During the preparation of (450-Br)6, it was noticed that the
macroinitiator could undergo aggregation in THF (Figure S9
and S10). However, the process could be readily prevented
through sonication of the solution, thus allowing for a full
recovery of single (450-Br)6 species (Figure S9). We did not
observe any measurable backbone scission during sonication at
different durations. As shown in Figure S9, sonication causes
low MW shift until single star-species are regenerated; however,
no peaks at lower MWs were observed. The lack of molecular
degradation was corroborated by DLS, as no decrease of the
hydrodynamic volume was observed after dissociation of the
aggregates. The aggregation was ascribed to changes in the
solubility of MSW after the attachment of long polymeric
chains, which also might induce a phase separation between
hydrophobic core and polymer arms. A more detailed
discussion of the process is given in the Supporting
Information.
Synthesis and Characterization of (PBiBEM300/450-g-
PnBA20/40/60/150)6 Star-Shaped Bottlebrushes. Due to the
tendency of (450-Br)6 to form aggregates, the solution of the
polymer was sonicated prior to attempting the SCs grafting
step, thus allowing for the restoration of individual macro-
initiator species. Poly(n-butyl acrylate) (PnBA) side chains
were grafted from (450-Br)6 using the “grafting from”
approach. Two polymerizations from the (450-Br)6 were
performed under normal ATRP conditions utilizing CuIBr/
dNbpy catalysts with a ratio of [nBA]:[BiBEM] = [400]:[1].
The polymerizations were stopped at different monomer
conversions, generating star-like bottlebrush polymers with
DPs of PnBA grafts = 20 ((450-g-20)6) and 40 ((450-g-40)6).
GPC analysis of (450-g-20)6 and (450-g-40)6 showed the shift
of (450-Br)6 peak signal toward higher MWs (Figure 2a, blue
and green); however, some broadening of MWDs was also
observed (Table 1, samples 3a and 3b). The higher Mw/Mn
values of (450-g-20)6 and (450-g-40)6 were related to the
presence of small fractions of high and/or low molecular weight
shoulders in both GPC traces (Figure 2a, blue and green). The
formation of high-molecular-weight (HMW) GPC peaks was
likely due to small amounts of the residual macroinitiator
aggregates that persisted during the side chains grafting as well
as the possibility of the intermolecular coupling between
growing macromolecules. On the other hand, the origin of the
Figure 2. GPC traces of (a) [P(HEMA-TMS450)]6 ((450-TMS)6,
black), (PBiBEM450
) ((450-Br)6, red), (PBiBEM450-g-(PnBA20)6
6
((450-g-20)6, blue), and (PBiBEM450-g-(PnBA40)6 ((450-g-40)6,
green); (b) [P(HEMA-TMS300)]6 ((300-TMS)6, black), (PBi-
BEM300)6 ((300-Br)6, red), (PBiBEM300-g-(PnBA60)6 ((300-g- 60)6,
blue), and (PBiBEM300-g-(PnBA150)6 ((300-g-150)6, green).
ization of (450-Br)6 and (300-Br)6 gave respective Mw/Mn =
1.39 and 1.20 (Table 1, samples 2 and 5), showing no
significant shift of signals as compared to the corresponding
P(HEMA-TMS) materials (Figure 2, red). Broadening of the
MWD of (450-Br)6 (Table 1, sample 2) was related to the
presence of a high molecular weight signal in the GPC trace
(Figure 2a, red), which was ascribed to a small fraction of
polymeric aggregates present in the sample.
Table 1. Characterization of Backbones and Star-like Bottlebrush Polymers Prepared from MSW6‑Br
GPC
AFM
a
b
c
c
d
e
sample
polymer structure
name/DParm‑g‑SC
Mn,th, ×10−6
Mn, ×10−6
Mw/Mn
Mn, ×10−6
Mw/Mn
1
[P(HEMA-TMS450)]6
(450-TMS)6
(450-Br)6
0.55
0.76
7.67
14.6
0.37
0.51
14.3
35.1
0.27
1.22
N/A
N/A
3.7
N/A
N/A
1.27
1.30
N/A
N/A
1.26
1.28
f
f
2
(PBiBEM450
)
0.28
1.39
6
3a
3b
4
(PBiBEM450-g-PnBA20)6
(PBiBEM450-g-PnBA40)6
[P(HEMA-TMS300)]6
(450-g-20)6
(450-g-40)6
(300-TMS)6
(300-Br)6
1.24
1.06
0.16
0.22
1.11
0.19
1.48
1.61
1.12
1.20
1.20
10.2
5.3
N/A
N/A
6.4
5
(PBiBEM300)
6
6a
6b
(PBiBEM300-g-PnBA60)6
(PBiBEM300-g-PnBA150
(300-g-60)6
(300-g-150)6
)
10.3
6
a
1
Calculated from the monomer conversion obtained by H NMR spectroscopy. DParm = [ACH (polymer)/ACH (monomer+polymer)] × [DPtarget/6]; where
2
2
A
CH (monomer+polymer) and ACH (polymer) are the respective areas under the curve corresponding to −CH2− protons of the polymer only and the monomer
2
2
and the polymer total, and DPtarget is a targeted DP of HEMA-TMS vs MSW6‑Br; DPSC = [1 − (A[nBA]/A[nBA] )] × DPtarget; where A[nBA] and A[nBA] are
0
0
b
the integrations of vinyl protons of nBA vs anisole as an internal standard. Mn,th = 6 × DParm × (MWarm,monomer + DPSCs × MWnBA) + MWMSW
.
c
d
Determined by THF GPC using linear PMMA standards. Number-average molecular weight determined as a ratio of mass per unit area from LB
film transfer and number of molecules per unit area from molecular imaging by AFM. Molecular weight dispersity calculated by assuming a constant
molecular weight per unit arm length for each sample. Analyzed by DMF GPC using linear PMMA standards.
e
f
D
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Figure 3. AFM height micrographs of (a) (PBiBEM450-g-PnBA20)6 ((450-g-20)6), and (d) (PBiBEM300-g-PnBA60)6 ((300-g-60)6) spin-cast from
dilute chloroform solutions onto mica, and (b) (PBiBEM450-g-PnBA20)6 ((450-g-20)6), (c) (PBiBEM450-g-PnBA40)6 ((450-g-40)6), (e) (PBiBEM300
g-PnBA60)6 ((300-g-60)6), and (f) (PBiBEM300-g-PnBA150
-
)
((300-g-150)6) prepared by Langmuir−Blodgett deposition using mica as the
6
substrates. Blue circles indicate aggregated or/and cross-linked molecular stars, and red circles highlight structures with imperfections/missing arms/
cross-links.
low-molecular-weight (LMW) peak was not fully understood. It
has been tentatively ascribed to single arms, i.e., the linear
bottlebrush. It was suspected that the highly congested
structure of a star-shaped bottlebrush might be inducing a
spontaneous cleavage of arms from the core. The loss of an arm
would lead to relaxation of the highly strained architecture of
the hexa-armed bottlebrush through the reduction of the steric
hindrance between their bulky arms. GPC traces of (450-g-20)6
and (450-g-40)6 showed in both cases a presence of LMW
polymers, which might indicate that the scission of arms from
the highly congested core could occur in a solution for brush-
stars arms with relatively short side chains.
In order to prepare a smaller star-shaped macroinitiator,
shorter arms with DP = 300 ((300-TMS)6) were polymerized
from MSW6‑Br. The reduction of the arm length should
suppress the tendency of the polymer to form aggregates. The
“grafting from” (300-Br)6 was performed with a higher ratio of
[nBA]:[BiBEM] = [1400]:[1], enabling longer side chains at
lower monomer conversions and diminishing an intermolecular
termination. Also, dNbpy was replaced with a more active
ligand, PMDETA, to avoid a significant drop in the
polymerization rate caused by dilution of the catalytic system.
The reaction was stopped at 4.5% monomer conversion, giving
PnBA molecular bottlebrush with average DPSC = 60 of side
chains ((300-g-60)6). The GPC characterization showed a clear
shift of (300-Br)6 signal toward higher molecular weights
(Figure 2b, blue), while keeping a low value of Mw/Mn = 1.20
(Table 1, sample 5). Neither LMW nor HMW impurities were
observed for (300-g-60)6, which suggests that shorter arms
(300 vs 450) result in less steric hindrance and provide enough
space for the accommodation of brushes with longer side
chains. The same polymerization conditions were used to graft
longer side chains, reaching 11.1% monomer conversion and
thus obtaining DPSC = 150 of grafts ((300-g-150)6).
Interestingly, when compared to (300-g-60)6, the star-brush
with longer side chains displayed a much broader MWD, Mw/
Mn ≈ 10 (Table 1, sample 6b) and a significant tailing toward
LMWs in the GPC trace (Figure 2b, green). This indicated that
noticeable degradation of the material occurred during the
polymerization, suggesting that both grafts and the arm lengths
contribute to the steric crowding in star-bottlebrushes, and
most likely inducing scission of arms from the MSW core.
More accurate MWD data for star-shaped bottlebrushes were
obtained by using a combination of molecular imaging by AFM
and the Langmuir−Blodgett (LB) technique, which was shown
to be particularly suitable for large branched macromolecules.45
AFM Characterization of Star-Shaped Bottlebrushes.
AFM was employed to image both single molecules prepared
by spin-casting and individual stars within dense monolayer
prepared by the LB technique. The imaging process is greatly
favored by spreading of the side chains, which increases both
the persistence length of adsorbed bottlebrushes and the
distance between the neighboring molecules.5,33,61,62 Figure
3d,e depicts high-resolution images of single molecules that
demonstrate distinct “octopus”- and “starfish”-like (with six or
five arms) topologies for (450-g-20)6 and (300-g-60)6
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respectively. AFM micrographs of LB monolayers of respective
bottlebrushes (450-g-20)6, (450-g-40)6, (300-g-60)6, and
(300-g-150) are depicted in Figure 3b,c,e,f. Similar to the
single molecules, star-bottlebrushes exhibit a gradual change of
the conformation from a worm-like shape to a rigid rod-like
shape with increasing length of the grafts. This behavior is in
agreement with literature reports on extension of the polymer
backbone and the corresponding increase of the persistence
length of bottlebrush macromolecules upon synthetic elonga-
tion of the side chains.7,63
High-resolution imaging of individual molecules by AFM was
employed to characterize both MWD and length distribution of
individual arms. Table 1 outlines the MWD data obtained by
measuring mass per unit area from LB and number of
molecules per unit area by AFM. The AFM-LB combination
was shown to be particularly suitable for large branched
macromolecules.45 As expected, the AFM-LB numbers for Mn
are higher than the corresponding numbers obtained by GPC
that are usually underestimated for large and branched
macromolecules. Clear resolution of the individual arms
enabled accurate statistical analysis of the number arm length
⟨Ln⟩ as well as the arm length distribution ⟨Lw⟩/⟨Ln⟩, as
summarized in Table 2. The narrow length distributions (⟨Lw⟩/
could be related to the limited initiation efficiency from
MSW6‑Br initiator as reported previously for the fabrication of
three- and four-armed brush polymers.33 The second cause is
mechanical cleavage of the arms due to the strong tension
enhancement at the branching core of the bottlebrush stars.
As reported previously, bottlebrush structures are able to
generate significant tension in their backbone due to steric
repulsion between densely grafted side chains.64 Depending on
the side-chain length and grafting density, and the interaction
with the surrounding environment (solvent, substrate,
neighboring macromolecules), the backbone tension can be
amplified from the pico-Newton to nano-Newton range.
Mechanical forces of this magnitude are sufficient to break
covalent bonds.7,65 Additional amplification of bond tension
occurs in tethered bottlebrushes that also exhibit a gradient of
tension due to steric repulsion between the neighboring
bottlebrushes.66 As shown in Figure 4, for the star-brush
(300-g-150)6, after adsorption on water/2-propanol (99.5/0.5
wt/wt %) mica surface for 3 h, the number fraction of
molecules with one and two arms increased from ∼25% to
35%, accompanied by the corresponding decrease in the
number fractions of molecules with three, four, five, and six
arms, while the average length of arms hardly changed. This
indicates that bottlebrush stars undergo preferential cleavage of
the arms from the MSW core. After 24 h, there were almost no
molecules with multiple arms remained. In addition, the
average arm length decreased by ∼20 nm, suggesting scission of
covalent bonds in the backbone of individual bottlebrush arms,
which occurred at a slower rate than the arm cleavage process.
The preferential cleavage of the arms was ascribed to
stronger steric repulsion of neighboring arms at the MSW core
(Figure 4f). In addition, the ester bond connecting the core and
the arm could be more vulnerable than C−C bonds under
aqueous conditions,64 which might also favor the cleavage of
the arms. In contrast, neither the cleavage of the arms nor the
backbone scission within the arms was observed for the star-
brushes with shorter side chains within 24 h, i.e., (450-g-20)6,
(450-g-40)6 and (300-g-60)6 (Figure S11). This was due to the
short grafts that were not able to generate large enough tension
along the backbone to break a covalent bond.
Table 2. AFM Characterization of Star-Bottlebrushes
⟨Ln⟩
⟨Lw⟩/
b
a
c
sample
name
(nm)
⟨Ln⟩
DParm,NMR ⟨Lmon⟩ (nm)
3a
3b
6a
6b
(450-g-20)6
(450-g-40)6
(300-g-60)6
(300-g-150)6
105
3
3
2
2
1.09
1.07
1.09
1.12
450
450
300
300
0.23 0.01
0.25 0.01
0.23 0.01
0.32 0.01
112
69
95
a
Number-average contour length of the individual arms of star-like
b
bottlebrushes imaged by AFM. Length dispersity: the ratio of weight-
average (⟨Lw⟩) and number-average (⟨Ln⟩) arm lengths, determined as
c
2
⟨Lw⟩/⟨Ln⟩ = ⟨Ln ⟩/⟨Ln⟩2. ⟨Lmon⟩ = ⟨Ln⟩/DParm,NMR
.
⟨Ln⟩ ≈ 1.1) of arms in all star-like bottlebrushes proved the
uniformity of structures, thus confirming well-controlled
polymerization processes at each synthetic step (Table 2). In
every case, arms in star-shaped bottlebrushes were extended,
which was reflected in lengths of repeating units falling in the
range of 0.23−0.25 nm. The values were close to the
monomeric unit length in a fully extended zigzag chain (0.25
nm) and showed an excellent agreement of AFM results with
the DPs of arms obtained from the monomer conversion by 1H
NMR spectroscopy (Table 2). However, the monomeric length
of the arm in (300-g-150)6 was noticeably larger than 0.25 nm,
which could be due to overestimation of the arm length in
bottlebrushes with short arms and relatively long side chains.
AFM images also allowed clear visualization of several types
of structural imperfections in the obtained star-shaped
bottlebrushes (Figure 3). First, we observed single molecules
with more than six arms (blue circles), which could be ascribed
to physical aggregation of (450-Br)6 macroinitiators during the
side chain grafting. Second, a few irregular structures with
interconnected arms linking several star-like brushes were also
noticed (blue circles). This was attributed to the coupling
between growing arms of P(HEMA-TMS) stars ((450-TMS)6
and (300-TMS)6) during ATRP processes. Third, a significant
fraction of macromolecules with the number of arms lower than
six (red circles) was observed which might have two origins: (i)
synthetic and (ii) physical. The less than 6 number of arms
SUMMARY
■
In conclusion, a rational design of a rigid, well-defined
molecular spoked wheel hexa-ATRP initiator allowed for the
preparation of polymeric materials with novel properties,
allowing for focusing tension to specific bonds and potentially
selective bond activation/scission. Star-shaped bottlebrushes
were synthesized via the combination of advanced synthetic
methods and ATRP. Side chains were grafted from star-shaped
macroinitiators yielding the following polymers: (450-g-20)6,
(450-g-40)6, (300-g-60)6 and (300-g-150)6. AFM imaging of
such architectures confirmed the unusual “starfish”- and
“octopus”-like topologies of star-brush polymers, proving the
successful synthesis of these complex polymeric architectures.
During the synthesis, it was discovered that PBiBEM
macroinitiators (450-Br)6 demonstrate a strong tendency to
form physical aggregates through the hydrophobic interactions
of MSW cores. The association process was prevented by
sonication of polymer solutions prior to the analysis and brush
synthesis. Reported star-like brush polymers represent a new
class of polymers that integrate the properties of both
bottlebrushes and MSWs. The cleavage of the arms from the
MSW core occurred preferentially, which may shed light on the
F
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Figure 4. AFM height micrographs of LB films of (300-g-150)6 transferred from water/2-propanol (99.5/0.5 wt/wt %) surface onto mica substrates
after (a) 2 min, (b) 3 h, and (c) 1 day. (d) The number distribution of contour length of (300-g-150)6 after 2 min (red), 3 h (blue), and 1 day
(green) (more than 400 molecules counted). (e) The number fraction of objects with different numbers of arms. (f) A schematic representation of
the side chains tapering near the core in solution and the amplification of force on the substrate.
elsewhere.67 The samples for AFM measurement were prepared by
design of molecular tensile machines that can focus mechanical
either LB deposition or spin-casting from dilute solutions. LB films
tension on specific bonds.
were transferred onto freshly cleaved mica substrates at a constant
surface pressure of 0.5 mN/m and a controlled transfer ratio. Imaging
of individual molecules was performed in PeakForce QNM mode
EXPERIMENTAL SECTION
■
using a multimode AFM (Bruker) with a NanoScope V controller. We
̈
The following describes the synthesis and characterization of
polymers.
used silicon probes with a resonance frequency of 50−90 Hz and a
spring constant of ∼0.4 N/m. In-house developed computer software
was used to analyze the AFM images with respect to arm size and
MWD of star-like bottlebrushes. Typically, ensembles of 200−300
molecules were analyzed to ensure standard deviation of the mean
below 10%.
Materials. n-Butyl acrylate (nBA, 99%, Acros) and (2-
trimethylsiloxy)ethyl methacrylate (HEMA-TMS, Scientific Polymer
Products) were purified by passing the monomer through a column
filled with basic alumina to remove the inhibitor. All other reagents:
copper(I) bromide (CuIBr, 99.999%), copper(II) bromide (CuIIBr2,
99.999%), copper(I) chloride (CuICl, 99.995%), copper(II) chloride
(CuIICl2, 99.999%), 4,4′-dinonyl-2,2′-dipyridyl (dNbpy, 97%)
N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), po-
tassium fluoride (KF, 99%), tetrabutylammonium fluoride (TBAF, 1.0
M in THF), α-bromoisobutyryl bromide (98%), 2,5-di-tert-butylphe-
nol (DTBP, 99%), triethylamine (TEA, ≤99%), and solvents were
purchased from Aldrich and used as received without further
purification.
Synthesis of 1, [P(HEMA-TMS)450]6 (450-TMS)6. A 10 mL
Schlenk flask was charged with MSW6‑Br (0.0137 g, 0.0019 mmol),
HEMA-TMS (4.0 mL, 18.4 mmol), dNbpy (0.023 g, 0.056 mmol),
CuIICl2 (0.62 mg, 0.0046 mmol), and anisole (1.0 mL). The solution
was degassed by three freeze−pump−thaw cycles. During the final
cycle, the flask was filled with nitrogen, and CuICl (22.6 mg, 0.0230
mmol) was quickly added to the frozen reaction mixture. The flask was
sealed, evacuated and back-filled with nitrogen five times, and then
immersed in an oil bath at 60 °C. Polymerization was terminated after
Characterization. The conversion of nBA was determined from
1
1H NMR spectra recorded in CDCl as a solvent using Bruker 300
69 h reaching 26.1% conversion as determined by H NMR, which
̈
3
corresponded to DP ≈ 450 per arm. Apparent molecular weight
determined by THF GPC: Mn,GPC = 2.70 × 105, and Mw/Mn = 1.22.
The reaction mixture was diluted with chloroform, passed through
neutral alumina to remove the catalyst, then concentrated and used for
the next step without further purification.
Synthesis of 2, (PBiBEM450)6 (450-Br)6. A 50 mL round-bottom
flask was charged with 1 (1.711 g, 5.80 mmol), KF (0.513 g, 8.70
mmol), DTBP (0.120 g, 0.580 mmol), and then dry THF (15 mL)
was added under nitrogen. The reaction mixture was cooled down in
an ice bath, followed by the injection of TBAF (0.12 mL, 1.0 M in
THF, 0.120 mmol) and subsequent dropwise addition of α-
bromoisobutyryl bromide (2.67 g, 1.43 mL. 11.6 mmol) over the
MHz spectrometer. The particle size was measured using a Zetasizer
Nano from Malvern Instruments. The sonication was performed using
Ultrasonic cleaner model FS20 from Fisher Scientific with a sweep
frequency of 40 kHz. MWDs of the polymers were characterized by
GPC using Polymer Standards Services columns (guard, 105, 103, and
102 Å), with THF eluent at 35 °C, flow rate 1.00 mL/min, and
differential refractive index detector (Waters, 2410). The apparent
number-average molecular weights (Mn) and molecular weight
dispersities (Mw/Mn) were determined with a calibration based on
linear PMMA standards and diphenyl ether as an internal standard,
using WinGPC 6.0 software from Polymer Standards Services. In
addition, the Mn was measured by the AFM-LB method described
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Article
course of 10 min. Upon addition, the reaction mixture was allowed to
reach room temperature and was stirred for another 16 h. Afterward
solids were filtered off and the mixture was precipitated into
methanol/water (70/30), re-dissolved in chloroform (30 mL), and
passed through the column filled with basic alumina. The product 2
was re-precipitated three times in hexanes and dried overnight under
vacuum. Apparent molecular weight determined by THF GPC: Mn,GPC
= 2.80 × 105, and Mw/Mn = 1.39.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed synthesis and characterization of the initiator,
MSW6‑Br, and all its precursors; discussion and characterization
of the aggregation of (450-Br)6; and distribution of number of
arms in star-shaped objects. This material is available free of
charge via the Internet at http://pubs.acs.org.
Synthesis of 3a, (PBiBEM450-g-PnBA20)6 (450-g-20)6. A 25 mL
Schlenk flask equipped with a stir bar was charged with macroinitiator
2 (0.0870 g, 0.3086 mmol of BiBEM groups), nBA (17.6 mL, 123.4
mmol), dNbpy (0.126 g, 0.309 mmol), CuIIBr2 (1.7 mg, 0.0077
mmol), and anisole (2.5 mL). The solution was degassed by three
freeze−pump−thaw cycles. During the final cycle CuIBr (21.0 mg,
0.1466 mmol) was quickly added to the frozen reaction mixture under
nitrogen atmosphere. The flask was sealed, evacuated and back-filled
with nitrogen five times, and then immersed in an oil bath
thermostated at 60 °C. The polymerization was stopped after 20 h,
and the monomer conversion was determined by both gravimetry and
1H NMR, resulting in the brush polymer 3a with DP ≈ 20 of side
chains. The polymer was purified by three precipitations from cold
methanol, and dried under vacuum at room temperature, to a constant
mass. Apparent molecular weight was determined using THF GPC:
Mn,GPC = 1.24 × 106, and Mw/Mn = 1.48.
AUTHOR INFORMATION
■
Corresponding Authors
hoeger@uni-bonn.de
sergei@email.unc.edu
km3b@andrew.cmu.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support from the National Science Foundation (DMR-
0969301 and DMR-1122483), the DFG (SFB 624), and the
Fonds der Chemischen Industrie is acknowledged.
Synthesis of 3b, (PBiBEM450-g-PnBA40)6 (450-g-40)6. The
reaction was set up and analyzed in the same way as 3a except the
reaction temperature was 70 °C. The amounts of reagents used for the
polymerization: macroinitiator 2 (0.0554 g, 0.1964 mmol of BiBEM
groups), nBA (11.2 mL, 78.6 mmol), dNbpy (0.080 g, 0.196 mmol),
CuIIBr2 (1.1 mg, 0.0049 mmol), anisole (1.25 mL) and CuIBr (13.3
mg, 0.0933 mmol). The polymerization was stopped after 23 h 20 min,
giving the brush polymer, 3b, with DP = 40 of side chains. Apparent
molecular weight determined by THF GPC: Mn,GPC = 1.06 × 106, and
Mw/Mn = 1.61.
Synthesis of 4, [P(HEMA-TMS)300]6 (300-TMS)6. The reaction
was set up and analyzed in the same way as 1. The amounts of
reagents used for the polymerization: MSW6‑Br ATRP initiator (0.0164
g, 0.0023 mmol), HEMA-TMS (3.0 mL, 17.2 mmol), dNbpy (0.017 g,
0.041 mmol), CuIICl2 (0.46 mg, 0.0034 mmol), anisole (0.75 mL), and
CuICl (1.7 mg, 0.0172 mmol). The polymerization was stopped after
39 h 10 min reaching 30.1% conversion, giving the polymer, 4, with
DP ≈ 300 per arm. Apparent molecular weight determined by THF
GPC: Mn,GPC = 1.64 × 105, and Mw/Mn = 1.12.
Synthesis of 5, (PBiBEM300)6 (300-Br)6. The reaction was set up
and analyzed in the same way as 2. The amounts of reagents used for
the functionalization: the polymer 4 (0.50 g, 2.5 mmol), KF (0.161 g,
2.72 mmol), DTBP (0.051 g, 0.248 mmol), dry THF (15 mL), TBAF
(0.013 mL, 1.0 M in THF, 0. 013 mmol), and α-bromoisobutyryl
bromide (0.626 g, 0.37 mL, 2.72 mmol) over the course of 10 min.
The reaction was performed twice to ensure quantitative functionaliza-
tion of the macroinitiator 5. The polymer was purified by dialysis
against THF using 50 kDa MWCO membranes. The polymer was
stored as a solution in anisole to avoid aggregation. Apparent
molecular weight determined by THF GPC: Mn,GPC = 2.23 × 105, and
Mw/Mn = 1.20.
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