Synthesis of Metallopolymers and Direct Visualization of the Single Polymer Chain

Article abstract of DOI:10.1021/jacs.0c00110

During the past few decades, the study of the single polymer chain has attracted considerable attention with the goal of exploring the structure-property relationship of polymers. It still, however, remains challenging due to the variability and low atomic resolution of the amorphous single polymer chain. Here, we demonstrated a new strategy to visualize the single metallopolymer chain with a hexameric or trimeric supramolecule as a repeat unit, in which Ru(II) with strong coordination and Fe(II) with weak coordination were combined together in a stepwise manner. With the help of ultrahigh-vacuum, low-temperature scanning tunneling microscopy (UHV-LT-STM) and scanning tunneling spectroscopy (STS), we were able to directly visualize both Ru(II) and Fe(II), which act as staining reagents on the repeat units, thus providing detailed structural information for the single polymer chain. As such, the direct visualization of the single random polymer chain is realized to enhance the characterization of polymers at the single-molecule level.

Full text of DOI:10.1021/jacs.0c00110

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Article
Synthesis of Metallo-Polymers and Direct
Visualization of Single Polymer Chain
Zhikai Li, Yiming Li, Yiming Zhao, Heng Wang, Yuan Zhang, Bo Song,
Xiaohong Li, Shuai Lu, Xin-Qi Hao, Saw Wai Hla, Yingfeng Tu, and Xiaopeng Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00110 • Publication Date (Web): 09 Mar 2020
Downloaded from pubs.acs.org on March 10, 2020
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Synthesis of Metallo-Polymers and Direct Visualization of Single Polymer Chain
Zhikai Li,†,‡,┴ Yiming Li,‡,┴ Yiming Zhao,†,┴ Heng Wang,‡, ∥ Yuan Zhang,§,# Bo Song,‡ Xiaohong Li,†
Shuai Lu,‡,∇ Xin-Qi Hao,∇ Saw-Wai Hla,*,§ Yingfeng Tu,*,† Xiaopeng Li*,‡
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†Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, State and Local Joint Engineering
Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou, Jiangsu 215123, China
‡Department of Chemistry, University of South Florida, Tampa, Florida 33620, United States
∥ College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518055, China
§Nanoscience and Technology Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
#Department of Physics, Old Dominion University, Norfolk, Virginia 23529, United States
∇College of Chemistry, Zhengzhou University, Zhengzhou, Henan 450001, China
ABSTRACT: During the past few decades, the study of single polymer chain has attracted considerable attentions with the goal of
exploring the structure-property relationship of polymers. It still, however, remains challenging due to the variability and low atomic
resolution of amorphous single polymer chain. Here, we demonstrated a new strategy to visualize single metallo-polymer chain with
hexameric or trimeric supramolecule as repeat unit, in which Ru(II) with strong coordination and Fe(II) with weak coordination were
combined together in a step-wise manner. With the help of ultrahigh-vacuum, low-temperature scanning tunneling microscopy (UHV-
LT-STM) and scanning tunneling spectroscopy (STS), we were able to directly visualize both Ru(II) and Fe(II), which act as staining
reagents on the repeat units, and thus providing detailed structure information for the single polymer chain. As such, the direct
visualization of single random polymer chain is realized to enhance the characterization of polymers at single-molecule level.
Introduction
Over the past decades, in order to enhance the
chains;31-41 3) metalation of pendant groups through post-
polymerization modification.42-53 However, these approaches
have certain limitations. In the first approach, the
polymerization is typically centered on small and stable metal-
organic components, e.g., metallocenes, which are polymerized
through living radical polymerization, ring-opening metathesis
polymerization (ROMP), etc.54-56 In the second approach, the
understanding of the structure-property relationship of
polymers, characterization of single polymer chain has attracted
widespread attentions.1-2 Microscopy, such as scanning
tunneling microscopy (STM) or atomic force microscopy
(AFM), allows us to observe the molecular structures directly.
However, except for the large biomolecules such as DNA,3-7 the
imaging of structure as well as conformation is mainly limited
to crystalline, liquid crystalline, and the aggregation of polymer
chains.8-13 With densely grafted side-chains, the worm-like
polymer bottlebrushes can be observed directly,14-23 yet the
dense and long side chains severely affect the polymeric
backbone conformation.24-25 A formidable challenge still
remains to address the random coil conformations of
amorphous single polymer chain due to the variability and low
atomic resolution.
supramolecular polymerization is mainly achieved through
36-37, 39-41, 57
dynamic coordination interaction.32,
Therefore,
precise control of the polymerization is difficult for dynamic
supramolecular polymers with labile coordination bonds. In the
third approach, metalation of pendant groups after
polymerization is also limited to metal with weak coordination
in order to get low-defect polymers.42, 44-48 To date, very few
systems were able to harness both strong and weak coordination
metal ions together into metallo-polymers with discrete
supramolecules as repeat units.
Thanks to the recent flourish of 2D and 3D metallo-
supramolecular structures with precisely-controlled sizes and
shapes constructed by coordination-driven self-assembly,58-82
we introduced two discrete structures, hexameric and trimeric
metallo-supramolecules, as repeat units to construct metallo-
polymers due to the high symmetry as well as the appropriate
size for STM imaging (Scheme 1). Copper(I)-catalyzed azide-
alkyne cycloaddition (CuAAC) “click” reaction, which owns
the advantages of fast reaction rates, and high yields with mild
reaction conditions,83-86 was selected for the polymerization. In
this
As is known to all, metals or metal ions with high electron
density are able to afford relatively strong signals and enhanced
atomic resolution under microscopy like STM. Therefore, if one
can introduce these metal components into polymer chains as
“staining reagents”, the amorphous single polymer chain could
be easily observed and confirmed by STM depending on the
position of metal. Generally, metal components are introduced
into polymers through three approaches, viz., 1) polymerization
of monomers with metal ions in the main chains or pendant
groups;26-30 2) supramolecular polymerization through
coordination of metal ions with organic monomer in the main
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Scheme 1. Monomers (A, Hex-M; B, Tri-M and Alkyne-PEG), and Their Corresponding Metallo-Polymers with 2D
Hexameric (C) and Trimeric(D) Supramolecules as Repeat Units.
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design, we bridged 2,2ʹ:6ʹ,2ʺ-terpyridine (tpy) ligand and Ru(II)
with strong coordination interaction to create the open-form
metal-organic hexameric and trimeric monomers with free tpy.
strong connectivity of <tpy-Ru(II)-tpy>,87-94 by which six
ditopic tpy ligands bearing 120-degree angle were bridged
linearly with five Ru(II) ions. Similarly, two Ru(II) ions were
used to bridge three ditopic tpy ligands with 60-degree angle
together to afford Tri-M. Note that Hex-M and Tri-M are
functionalized by dual azide groups, which were designed for
the next polymerization by click reaction. The 1H NMR spectra
of Hex-M and Tri-M were shown in Figure 1A and 1E, with
proton labeling at the top of Scheme 1. The assignment of peaks
associated with the chemical structure was assisted by the two-
After polymerization by click reaction, through further
metalation of Fe(II) with weak coordination, both strong and
weak binding metal ions were harnessed into one polymer
system in a step-wise manner. Our initial assumption was that
the metalation of Fe(II) of metal-organic monomers would form
ring-closing hexagon or triangle as repeat units. And we could
measure the structure and conformation of the amorphous
single polymer chain based on the location of Ru(II) and Fe(II).
Unexpectedly, according to the results of ultrahigh-vacuum,
low-temperature scanning tunneling microscopy (UHV-LT-
STM) and scanning tunneling spectroscopy (STS), both
hexagon assembled by intramolecular coordination and linear
chains assembled by intermolecular assembly were observed.
In contrast, only intramolecular assembly into triangle existed
in trimeric repeat units.
1
dimensional (2D) H-1H gCOSY and NOESY NMR spectra
(Figures S41-S44, and S71-S74). After treating the precursors
(compounds 8 and 16) bearing chloride as terminal groups with
sodium azide, the triplet peak for methylene unit near chloride
at around 3.59 ppm totally disappeared with a new triplet peak
at 3.28 ppm appeared, demonstrating that the conversion was
completed. The integrals from corresponding peaks fit well
with the theoretical value, indicating the high purity of Hex-M
and Tri-M.
Results and Discussion
The Hex-M and Tri-M monomers were further
characterized by electrospray ionization-mass spectrometry
(ESI-MS). Hex-M (Figure 2A) was observed with a dominant
set of peaks with continuous charge states from 6+ to 10+, due
Scheme 1 depicts the two metal-organic monomers with
open-form structures, viz., hexameric monomer (Hex-M) and
trimeric monomer (Tri-M), which were prepared according to
the procedure as shown in Supporting Information (Schemes
S1-S5, Figures S1-S76). Briefly, Hex-M was synthesized
through a well-established step-wise strategy based on the
-
to successive loss of the counterions, PF6 . The experimental
isotope pattern of each charge state agrees well with its
theoretical simulation
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Figure 1. 1H NMR spectra of (A) Hex-M (600 MHz), (B) Hex-P (600 MHz), (C) Alkyne-PEG (400 MHz), (D) Tri-P (400 MHz) and (E)
Tri-M (400 MHz) in CD3CN. * represents the residue solvent CHCl3.
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result (Inset and Figure S46). The averaged molecular mass is
deconvoluted as 7019 Da, which matches the chemical
composition [C340H294N42O6Ru5P10F60]. Figure 2C shows the
ESI-MS of Tri-M with continuous charge states from 3+ to 4+.
The experimental isotope pattern of each charge state is also
consistent with the simulated one (Inset and Figure S76). The
averaged molecular mass is deconvoluted as 3375 Da, which
has a good agreement with the chemical composition
[C168H142N24O6Ru2P4F24]. Furthermore, traveling wave ion
mobility-mass spectrometry (TWIM-MS) spectra (Figure 2B
and 2D) show a series of narrowly distributed signals with
successive charge states for both Hex-M and Tri-M, indicating
high isomeric purity of the structures.
Information (Figures S80-S89). The triplet peak at 3.28 ppm
corresponding to methylene unit near azide group of the
monomers (Hex-M and Tri-M) disappeared, indicating
monomers were completely reacted. After the formation of
triazole by click reaction, this triplet peak was significantly
shifted downfield (~1.23 ppm) and the protons of alkynyl group
was dramatically shifted downfield (~4.99 ppm).
Due to the strong electrostatic interactions between charged
polymer backbones and counterions, the characterization data
by size exclusion chromatography, viscometry, and laser light
scattering, may come from the associations rather than the
single chains of the metallo-polymers, which can be regarded
as a kind of polyelectrolytes. Since all the solution-based
techniques have the similar problem, MALDI-TOF mass
spectrometry was further used to characterize the metallo-
polymers with open-form monomers in their solid state. In
contrast to losing multiple counterions for ESI-MS of
monomers, such metallo-polymers are mainly ionized by losing
Alkyne-terminated pentaethylene glycol (Alkyne-PEG,
Scheme S6, Figures S77-S79) was selected as another
component to polymerize with the monomers (Hex-M and Tri-
M) to construct the corresponding metallo-polymers (Hex-P
and Tri-P) by click reaction (Scheme S7). During the
polymerization process in DMF, a large amount of precipitation
was produced, suggesting the formation of metallo-polymers
(Hex-P and Tri-P). The products were fully characterized by
NMR and matrix-assisted desorption ionization-time of flight
(MALDI-TOF). Figure 1 shows the 1H NMR spectra of Hex-P
-
single counterion, that is PF6 in this study. Hex-P (Figure 3A)
shows a series of peaks with interval ca. 7340 Da, in a good
agreement with the molar mass of the repeat unit in Hex-P
(Hex-M with Alkyne-PEG, C356H320N42O12Ru5P10F60;
theoretical value: 7334 Da). High molecular weight signal up to
73400 (m/z) was detected with the degree of polymerization of
10, directly confirming the successful synthesis of
1
(Figure 1B) and Tri-P (Figure 1D), while 2D H-1H gCOSY
and NOESY NMR spectra are provided in Supporting
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corresponding metallo-polymer Hex-P. In terms of polymers
with large polydispersity index value (PDI),
polymer Tri-P. The average interval between adjacent peaks is
about 3684 Da, which agrees with the molar mass of the repeat
unit in Tri-P (Tri-M with Alkyne-PEG,
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C184H168N24O12Ru2P4F24; theoretical value: 3689 Da).
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Figure 3. MALDI-TOF MS of Hex-P (A) and Tri-P (B) with
corresponding numbers of repeat units in linear mode.
After polymerization, Fe(II) with weak coordination was
introduced for further metalation.35, 42, 98-103 In order to avoid the
undesired crosslink between different polymer chains, low
concentration (0.2 mg mL-1) of metallo-polymers (Hex-P and
Tri-P) was used in the metalation process. The metalation
results were supported by the Ultraviolet-visible (UV-vis)
absorption spectra as shown in Figure S90. In both metallo-
polymers, the expected peak at about 496 nm was ascribed to
the metal-to-ligand charge-transfer transition (MLCT) of <tpy-
Ru(II)-tpy> connectivity.31, 101, 104 After Fe(II) being introduced,
a new emerging peak at about 574 nm that was attributed to the
characteristic MLCT band of the <tpy-Fe(II)-tpy> was
observed, indicating successful metalation.35, 98-99, 101-104
Figure 2. ESI-MS of Hex-M (A), TWIM-MS plot (m/z vs drift
time) of Hex-M (B), ESI-MS of Tri-M (C), TWIM-MS plot of Tri-
M (D).
it is worth noting that MALDI-TOF generally displays a
nonsymmetrical distribution of peaks for polymers with higher
abundant signals at low m/z range due to the high ionization
efficiency of species with low molecular weight.95-97 This also
suggested that large oligomers with high polymerization degree
might exist in our system, however, were not ionized due to the
limitation of MALDI-TOF. With small monomer Tri-M, much
higher degree of polymerization was detected in Tri-P as shown
in Figure 3B. High molecular weight signal up to 70000 (m/z)
was obtained with the degree of polymerization of 19,
indicating the successful synthesis of corresponding metallo-
Furthermore, ultrahigh-vacuum, low-temperature scanning
tunneling microscopy (UHV-LT-STM) was then used in an
effort to characterize metallo-polymers and address the
structure and conformation of polymer chains. Samples were
dissolved in acetonitrile and deposited on an Ag (111) surface
by drop-casting. It should be noted that in order to clearly
observe a single metallo-polymer chain, low concentration
(0.05 mg mL-1) was used to prepare the STM sample. Due to
the octahedral
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Figure 4. (A) STM image of Hex-P on the supporting Ag(111) surface. (B) STM image of Hex-P with STS data acquisition locations on
<tpy-Ru(II)-tpy> unit shown in red. (C) STM image of Hex-P-C. (D) STS profiling of Hex-P-C with <tpy-Ru(II)-tpy> unit in red, and <tpy-
Fe(II)-tpy> unit in blue. (E) STM image of Hex-P-L. (F) STS profiling of Hex-P-L with <tpy-Ru(II)-tpy> unit in red, and <tpy-Fe(II)-tpy>
unit in blue. And the blue line in inset represents the polymer chain proposed by <tpy-metal(II)-tpy> units. dI/dV-V results of STS of (G)
Ru(II) (red) and (H) Fe(II) (blue) correspond to different electronic features in (D) and (F). Scale bar: 10 nm.
coordination geometry and higher electron density around the
metal ions, the <tpy-Ru(II)-tpy> or <tpy-Fe(II)-tpy> units as the
stained spots gave rise to a relatively strong signal in the form
of a bright lobe when compared to the organic portions.105
however caused severe background signals during UHV-LT-
STM imaging. After self-assembly with Fe(II), six bright lobes
including five <tpy-Ru(II)-tpy> units and one <tpy-Fe(II)-tpy>
unit are observed as a group (Figure 4C and Figure S93). As
expected, the repeat units of Hex-P underwent intramolecular
coordination into a ring-closed structure and six bright lobes
arranged in a cyclic hexagonal structure (Hex-P-C). The
distance between two adjacent bright lobes of the cyclic
hexagonal structure is around 2.1 nm, which is well consistent
with the calculated size by modeling (Figure S91). Also due to
the poor solubility, we were only able to observe one metallo-
polymer chain with three cyclic hexagonal repeat units (Figure
Figure 4A and Figure S92 show the STM images of Hex-
P, in which each of the five bright lobes located closely together
correspond to five connected <tpy-Ru(II)-tpy> units, indicating
the successful formation of metallo-polymers. Aggregation
occurred due to the poor solubility of Hex-P in acetonitrile,
making it difficult to observe longer polymer chains and only
large bright spots are observed for aggregates as shown in
Figure S92. We also used DMF and DMSO as solvents, which
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S93). Besides intramolecular coordination, intermolecular
coordination into a linear chain (Hex-P-L) was also observed
as shown in Figure 4E and Figure S93. It should originate from
the constraint of the repeat units in the metallo-polymer chain,
where Alkyne-PEG brings two monomers close enough for the
occurrence of intermolecular complexation.
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The positions of Fe(II) and Ru(II) ions in the repeat units
were identified by the means of dI/dV-V using scanning
tunneling spectroscopy (STS) that probed local density of states
(LDOS).106-107 Tunneling spectra were measured by positioning
the STM tip above each lobe of the repeat units on Ag (111)
surface at a fixed height with a bias range of ±2V. From dI/dV-
V spectroscopic data, the energy gap between the HOMO and
LUMO can be directly obtained.108 Thus, from the STS
measurements of each <tpy-metal(II)-tpy> unit, we were able
to differentiate the Fe(II) and Ru(II) centers based on their
different energy gaps. Among the six bright lobes of a group,
one bright lobe in each group in Figure 4C and 4E gives the
HOMO-LUMO gap of 2.4 eV (Figure 4H); while the
remaining five bright lobes provide the gap value of 2.8 eV
(Figure 4G). Thus, we can identify the site with a smaller gap,
viz. 2.4 eV, as Fe (II) and that of the larger gap, viz. 2.8 eV, as
the Ru (II) site. Then, STM imaging was able to directly read
the “sequence” of metal ion within the polymer chains (Figure
4D and 4F), in which red and blue dots represent <tpy-Ru(II)-
tpy> unit and <tpy-Fe(II)-tpy> unit, respectively. More
importantly, the polymer chain can be profiled depending on
the positions of metal ions as shown in Figure 4F, by which we
were able to deduce the conformation of the polymer chain.
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Figure 5. (A) STM image of Tri-P on the supporting Ag(111)
In the STM image of Tri-P, each of the two bright lobes
positioning closely together represents two connected <tpy-
Ru(II)-tpy> within a repeat unit (Figure 5A and Figure S94).
With the introduction of Fe(II), all of the repeat units carried out
intramolecular coordination to form a ring-closed structure,
where three bright lobes arranged into a triangle (Tri-P-C) as
evidenced by STM imaging (Figure 5C and Figure S95). And
the distance between two adjacent bright lobes of the cyclic
triangular structure is ca. 1.2 nm, which is well consistent with
the modeling size (Figure S91). It should be noted that with
enhanced solubility, up to 12 triangular repeat units were
observed in a single polymer chain, much higher than that in the
hexameric system. This result is consistent with the MALDI-
TOF MS data as shown in Figure 3. The positions of Fe(II) and
Ru(II) ions in the repeat unit are also identified by the means of
dI/dV-V obtained in STS (Figure 5E and 5F). We then labeled
<tpy-Ru(II)-tpy> unit and <tpy-Fe(II)-tpy> unit with red and
blue dots, respectively on the basis of STS results (Figure 5B
and 5D). The polymer chain is readily profiled using the blue
dots, because all <tpy-Fe(II)-tpy> units are located right in the
middle of the chain. Meanwhile, the conformation could be
estimated and proposed according to the polymer chain drawn
as shown in the inset of Figure 5D.It should be noted that
theoretically the molecular weight and polydispersity index for
the metallo-polymers can be calculated based on the observed
polymer single chains from STM images. However, to obtain
statistically average value, hundreds of high resolution STM
images without strong aggregation are needed which is not
practical currently.
surface. (B) STS profiling Tri-P with <tpy-Ru(II)-tpy> unit in red.
(C) STM image of Tri-P-C. (D) STS profiling of Tri-P-C with
<tpy-Ru(II)-tpy> unit in red, and <tpy-Fe(II)-tpy> unit in blue. And
the blue line in inset represents the polymer chain proposed by
<tpy-Fe(II)-tpy> units. dI/dV-V results of STS of (E) Ru(II) (red)
and (F) Fe(II) (blue) correspond to different electronic features in
(D). Scale bar: 5 nm.
Conclusions
In summary, a novel class of metallo-polymers were
synthesized by harnessing Ru(II) with strong interaction and
Fe(II) with weak interaction into one polymer system in a step-
wise manner. First, open-form metal-organic monomers with
Ru(II) were polymerized by click reaction to form the
corresponding metallo-polymers (Hex-P and Tri-P). After
further metalation by Fe(II), Hex-P and Tri-P displayed
different coordination or self-assembly behaviors. More
importantly, the structures and conformations of single polymer
chains can be directly visualized through STM and STS of the
<tpy-metal(II)-tpy> units. The metallo-polymers generated by
Hex-P include not only hexagons constructed by intramolecular
coordination, but also hexameric linear chains formed by
intermolecular coordination. In a sharp contrast, the further
coordination of Tri-P was carried out in a more controlled
manner with only intramolecular complexation observed to
form triangles as the repeat units. The different results are
attributed to the size and rigidity of the repeat units. Overall, our
endeavors will bring innovative aspects into polymer chemistry
field and shed light on the development of innovative
approaches to synthesize metallo-polymers and study the
structure and conformation of polymer chains at single-
molecule level. The characterization with the combination of
STM and STS could be expanded into other metallo-polymer
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Films for Poly[(R)-3-hydroxybutyric acid] and Its Copolymer. Polymer
2000, 41, 867-874.
systems to advance our understanding of the structure-property
relationship of polymers.
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Reiter, G.; Castelein, G.; Sommer, J.-U.; Röttele, A.; Thurn-
Albrecht, T. Direct Visualization of Random Crystallization and
Melting in Arrays of Nanometer-Size Polymer Crystals. Phys. Rev.
Lett. 2001, 87, 226101.
ASSOCIATED CONTENT
Supporting Information. Instrumentations, synthetic details,
molecular modeling, monomer and polymer characterization,
10.
Banno, M.; Wu, Z.-Q.; Nagai, K.; Sakurai, S.-i.; Okoshi, K.;
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6
7
8
1
including H NMR, 13C NMR, 2D COSY, 2D NOESY, ESI-MS,
Yashima, E. Two-Dimensional Bilayer Smectic Ordering of Rigid
Rod-Rod Helical Diblock Polyisocyanides. Macromolecules 2010, 43,
6553-6561.
UV-vis and additional STM images are included in supporting
information. This material is available free of charge via the
Internet at http://pubs.acs.org.
11.
Kumaki, J.; Kajitani, T.; Nagai, K.; Okoshi, K.; Yashima, E.
9
Visualization of Polymer Chain Conformations in Amorphous
Polyisocyanide Langmuir-Blodgett Films by Atomic Force
Microscopy. J. Am. Chem. Soc. 2010, 132, 5604-5606.
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AUTHOR INFORMATION
Corresponding Author
*shla@anl.gov
*tuyingfeng@suda.edu.cn
*xiaopengli1@usf.edu
12.
Banno, M.; Yamaguchi, T.; Nagai, K.; Kaiser, C.; Hecht, S.;
Yashima, E. Optically Active, Amphiphilic Poly(meta-phenylene
ethynylene)s: Synthesis, Hydrogen-Bonding Enforced Helix Stability,
and Direct AFM Observation of Their Helical Structures. J. Am. Chem.
Soc. 2012, 134, 8718-8728.
13.
Guan, W.; Wang, G.; Ding, J.; Li, B.; Wu, L. A
Approach of Modified Polyoxometalate
Author Contributions
┴These authors contributed equally.
Supramolecular
Polymerization and Visualization of a Single Polymer Chain. Chem.
Commun. 2019, 55, 10788-10791.
14.
Copolymer Brushes and Their Self-Assembly into Supramolecular
Tubes. Science 2008, 319, 1512-1515.
Schappacher, M.; Deffieux, A. Synthesis of Macrocyclic
Notes
The authors declare no competing financial interest.
15.
Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K.
Cylindrical Molecular Brushes: Synthesis, Characterization, and
Properties. Prog. Polym. Sci. 2008, 33, 759-785.
16.
Schmalz, H.; Ballauff, M.; Müller, A. H. E. Water-Soluble Organo-
Silica Hybrid Nanowires. Nat. Mater. 2008, 7, 718-722.
17.
Block Copolymers and Their Melt Self-Assembly into Large Domain
Nanostructures. Macromolecules 2009, 42, 2135-2141.
18.
Stimuli-Responsive Molecular Brushes. Prog. Polym. Sci. 2010, 35,
24-44.
19.
ACKNOWLEDGMENT
We acknowledge the support from National Institutes of Health
(R01GM128037) and National Science Foundation of China
(21774090). Use of the Center for Nanoscale Materials, an Office
of Science user facility, was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, under
Contract No. DE-AC02-06CH11357. We also acknowledge partial
support through University of South Florida Nexus Initiative (UNI)
Award, and the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD).
Yuan, J.; Xu, Y.; Walther, A.; Bolisetty, S.; Schumacher, M.;
Rzayev, J. Synthesis of Polystyrene-Polylactide Bottlebrush
Lee, H.-i.; Pietrasik, J.; Sheiko, S. S.; Matyjaszewski, K.
Nese, A.; Kwak, Y.; Nicolaÿ, R.; Barrett, M.; Sheiko, S. S.;
Matyjaszewski, K. Synthesis of Poly(vinyl acetate) Molecular Brushes
by a Combination of Atom Transfer Radical Polymerization (ATRP)
and Reversible Addition-Fragmentation Chain Transfer (RAFT)
Polymerization. Macromolecules 2010, 43, 4016-4019.
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