Non-charged, water soluble dendronized polymers

Article abstract of DOI:10.1039/c1nj20517j

The synthesis of core/shell amphiphilic dendronized polymers is described which provides access to peripherally OEG-decorated and, thus, non-charged, water soluble cylindrical objects of varying cross-sectional diameter (thickness). Shell and core consist

Full text of DOI:10.1039/c1nj20517j

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PAPER
Non-charged, water soluble dendronized polymerswz
Baozhong Zhang and A. Dieter Schluter*
¨
Received (in Montpellier, France) 13th June 2011, Accepted 4th August 2011
DOI: 10.1039/c1nj20517j
The synthesis of core/shell amphiphilic dendronized polymers is described which provides access
to peripherally OEG-decorated and, thus, non-charged, water soluble cylindrical objects of
varying cross-sectional diameter (thickness). Shell and core consist of a branching unit with three
methyl-terminated triethylene glycol arms and of 1, 2, 3, or 4 generations of known amide-based
dendrons, respectively, which are all held together by a PMMA-type main chain backbone. The
adsorption behavior of these novel polymers on mica is investigated with the AFM and
semi-quantitatively compared to the amide-based parent polymers.
however are rather special in higher order structure formation
1. Introduction
because subtle changes in conditions can have a substantial
impact on the outcome. To nevertheless broaden the structural
basis for such studies, we aimed at an easy access to non-charged,
yet water soluble dendronized polymers of varying thickness. We
have recently reported an oligoethylene glycol (OEG)-based
dendron 1 and its use in the synthesis of ‘‘all-OEG’’ first and
second generation dendronized polymers as well as an
‘‘all-OEG’’ third generation oligomer (OEG-PG3) by the macro-
monomer route (Fig. 1).9 All these polymers are fully water
soluble at room temperature and in a relatively wide pH range.
We figured that it may be sufficient to achieve full water solubility
by just decorating the existing series of dendronized polymers
with an outermost layer of the branched OEG part of 1 (Fig. 1).
This view is supported by the observation that the dehydration
and collapse of OEG-PG1 and OEG-PG2 are greatly influenced
by the periphery and much less so by the interior.10
Dendronized polymers consist of a linear backbone surrounded
by a dense, structurally regular dendritic branchwork anchored
to each repeat unit and theoretically have an ‘‘infinite’’ number
of ends.1 These ends allow for functionalization and mediate
responsivity to external stimuli in terms of contraction/expansion
of both the main backbone length and the cross-section
diameter.2 The cross-section diameter of dendronized polymers
(thickness) can be systematically tuned by varying the dendron
generation.3 Recently we have reported a virtually structure-
perfect fifth generation representative, PG5 (Fig. 1), the thickness
of which amounts to 10 nm in solution and to B8 nm when
adsorbed in the dry state on substrates.4 Its length reaches the
micrometre range. With such dimensions PG5 not only is the
largest synthetic linear structure with molecular precision to
date (M E 200 MDa) but also it mimics the sizes of biological
objects such as microfilaments and some cylindrical viruses.5
In biology, hierarchical structure formation is common for
reaching size and function. For example, ‘‘small’’ structure
proteins driven by weak intermolecular forces assemble linearly
into filaments which then bundle up into ever thicker, eventually
macroscopic fibers.6 Having provided access to cylindrical
molecular objects7 in a size range relevant for such biological
processes, it was of obvious interest to render them water
soluble. This would allow for studying, e.g., the self-assembly
behavior of synthetic and biological structures aiming at
unprecedented hybrid structures. We have earlier shown that
charged dendronized polymers are fully water soluble and can
give rise to a peculiar assembly behavior.8 Charged species
We here report on the decoration of our working horse first
through fourth generation dendronized polymers PG1-4 with
dendron 1 to give the hybrid polymers hPG2-5 (Fig. 2), the
outermost generation of which is based on OEG.11 The
polymers were investigated by atomic force microscopy
(AFM) to determine their heights when adsorbed on mica as
single entities. To do so the different generations of polymers
hPG2-5 were also compared with their non-OEG counter
parts PG2-5 and referenced to absolute heights of PG2-5 from
previous TEM studies.4 Finally it will be briefly discussed in
qualitative terms which impact the more polar outer shell of
hPG2-5 have in terms of surface-induced flattening.
2. Results and discussion
Department of Materials, Polymer Chemistry, ETH Zurich, Wolfgang
Pauli Str. 10, HCI J541, 8093 Zurich, Switzerland.
E-mail: ADS@mat.ethz.ch
2.1. Synthetic issues
w This article is part of the themed issue Dendrimers II, guest-edited
by Jean-Pierre Majoral.
The syntheses of hPG2-5 were done as exemplary shown for
hPG5 in Scheme 1. For a particular target generation (e.g.
hPG5) the starting dendronized polymer of the same generation
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c1nj20517j
c
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Fig. 1 Chemical structures of PG5, the all-OEG-based OEG-PG3 and the OEG-based first generation dendron shown as focal point active ester
1. PG5 represents the homologous working horse series of PG1-5, the branched repeat units of which all have identical (non-OEG based)
structures.
in order to ensure an as high as possible conversion. For
purification the original protocol was changed from chromato-
graphy to precipitation of the hybrid polymers into
diethylether. This way, yields of 80–95% could be achieved
throughout the generations.12 The OEG-decorated polymers
hPG2-5 were obtained as white, finely fibrous materials when
freeze-dried from dioxane (typical concentration: 50–100 mg Lꢀ1).
They were fully soluble in water (by visual inspection)
and in organic solvents of a wide range of dielectric constants,
such as chloroform, dichloromethane, methanol, DMF. This
however applied to freeze-dried material only. If the solvent of
any of these solutions was removed in vacuum, the resulting
solid material could be partially swollen but not fully
re-dissolved. Additionally, aqueous solutions of hPG2-5
Fig. 2 The hybrid structure hPG5 obtained by dendronization of a
changed with time. Already after a few hours they turned
turbid and later formation of a precipitate was observed, the
nature of which was not analyzed. Charged dendronized
polymers in aqueous medium are known to aggregate through
duplex formation.13 A theoretical model for this behavior is
available according to which duplex formation enables the
hydrophobic interior parts of the polymers to better protect
themselves against the aqueous surrounding.14 A similar
mechanism may apply to the non-charged hybrid dendronized
polymers discussed here as well. This solution behavior and
particularly the fact that once dried polymers do not
re-dissolve did not facilitate determination of structure
PG4 with OEG-dendron 1 at the maximum possible peripheral amine
functional ends. The PG4 used for this transformation had a GPC
number average degree of polymerization Pn E 1000.
minus one was used in its deprotected form (e.g. de-PG4). The
starting polymers were prepared using the procedure we reported
earlier and had Pn E 1000 and Pw E 1500 determined by gel
permeation chromatography (GPC).3 The deprotection was
performed in neat trifluoroacetic acid and the subsequent
dendronizations were done in full analogy to the homologous
series PG1-5.3,4 5 equiv. of active ester 1 per terminal amino
group and total reaction times of up to 30 days were employed
Scheme 1 Synthesis of the hybrid dendronized polymer, hPG5, from deprotected PG4 and the OEG-dendron active ester 1. Note that because of
the threefold branching multiplicity of compound 1, the molar mass of deprotected PG4 almost doubles by the dendronization (assuming 100%
conversion).
c
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New J. Chem., 2012, 36, 414–418 415

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perfection by the methods that proved successful so far. This is
why this aspect was not dealt with the same rigorosity and
comprehensiveness as in previous cases.3 Nevertheless some
semi-quantitative insight could be gained for hPG3, which was
used as example. Any eventually non-dendronized terminal
amines were spectroscopically amplified with 2,4-dinitrofluoro-
benzene, the Sanger reagent used for protein-labeling.15 This
turned them into UV tags, the absorption of which was
quantified against a standard. The tagging had to be done in
DMF solution. All attempts failed to replace this solvent after
the synthesis step without irreversible precipitation of the
polymers. This turned out to be somewhat problematic
because of the dimethylamine impurity always present in
DMF and the fact that the Sanger reagent reacted with it to give
an aniline derivative absorbing in the same range as the tagged
polymers. The absorption of the UV-labeled polymer was therefore
normalized to the solvent/Sanger reagent background.16
Nevertheless we failed to reach the same level of accuracy that
we had repeatedly encountered when applying this method in,
e.g., tetrachloroethane,3 particularly because of the much
higher concentration of the dimethylamine impurity compared
to non-dendronized amines in the polymer. Because of this
unfortunate complication, the reliability of the method was
first checked by investigating the reaction between excess
Sanger reagent and deprotected PG2. For this polymer all
amines should in principle react and a UV labeling degree of
100% was expected, the experiment, however, afforded 93%
conversion. While this indicates some uncertainty, it was
nevertheless considered acceptable for estimating the coverage
with an accuracy of approximately ꢁ10%. If now applied to
hPG3 a coverage of 94% was obtained, indicating that a few
percent of its terminal amines were not dendronized. Thus the
dendronization was near quantitative and it was assumed that
this would also apply to the other congeners of the series. High
coverage was also indicated by the NMR spectra of hPG2-5,
where the ratio between the signals for the OEG groups and
the aromatic protons were in good agreement.
2.2. AFM imaging of hPG2-5
Polymers hPG2-5 were imaged by tapping mode AFM on
mica as single components and in direct comparison with their
respective non-OEG decorated counterparts, PG2-5. Fig. 3
shows some representative images obtained after spin-coating
the samples from chloroform and aqueous solutions. Other
images can be found in the ESI.z For all generations, individual
polymer chains were observed when the samples were prepared
from chloroform. As already found for the parent series
PG1-5,4 the chains stretch out more and more with increasing
generations indicating an increasing persistence length. This is
a result of the increasing steric load along the main chain back-
bone and can qualitatively be seen by comparing Fig. 3a and b.
AFM heights (hAFM) were determined for all new polymers and
also PG1-5 by co-preparing hybrid and parent dendronized
Fig. 3 Height and phase contrast tapping mode AFM images of dendronized polymers on mica under ambient conditions. Samples (a)–(d) were
prepared by spin-coating from chloroform solutions and samples (e) and (f) by spin-coating from aqueous solution. (a) hPG5, (b) hPG3, (c) PG5
co-adsorbed with hPG5, (d) PG4 and hPG4, (e) hPG5 after air-drying for 1 d, (f) hPG3 after air-drying for 3 d. For images of hPG2 and hPG4, and
co-adsorbed PG2/hPG2 and PG3/hPG3 see the ESI.z
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Table 1 AFM heights of hPG2-5 [hAFM(hPG2-5)] and PG2-5 [hAFM(PG2-5)] from this study compared to known TEM heights of the parent
polymers PG2-5 [hTEM(PG2-5)] as reference, Dh = hTEM(PG2-5) ꢀ hAFM(PG2-5), and the corrected AFM heights of hPG2-5 [hAFM,corr.(hPG2-5)].
All samples prepared by spin-coating from chloroform on mica, all data in nm
n
hAFM(hPG2-5)
hAFM(PG2-5)
hTEM(PG2-5)
Dh
hAFM, corr.(hPG2-5)
2
3
4
5
0.1 ꢁ 0.1
0.8 ꢁ 0.1
2.8 ꢁ 0.3
2.9 ꢁ 0.2
1.2 ꢁ 0.2
1.7 ꢁ 0.3
3.8 ꢁ 0.2
4.6 ꢁ 0.3
2.3 ꢁ 0.4
3.4 ꢁ 0.5
4.9 ꢁ 0.3
7.3 ꢁ 0.2
1.1
1.7
1.1
2.7
1.2
2.5
3.9
5.6
polymers of the same generation (e.g., Fig. 3c and d). This way
AFM(hPG2-5) and hAFM(PG2-5) were obtained under
3. Conclusion
h
Water-insoluble Boc-protected amine-terminated dendronized
polymers can be converted into non-charged yet water-soluble
derivatives by a simple post-polymerization dendronization
with the triply branched OEG dendron active ester 1 to result
in novel hybrids. The coverage for this step could not be
rigorously quantified but seems to be near quantitative for all
generations investigated. The hybrids hPG2-5 have an
amphiphilic core/shell type, cylindrical structure, reminiscent
of bottlebrush polymers21 with linear block copolymers as side
chains,22 which upon adsorption onto the negatively charged
substrate mica flatten out somewhat more than their parent
counter parts. The degree to which this happens will be on the
order of 45% and is thus smaller than for bottlebrushes under
comparable conditions.23 The new hybrid structures also have
some similarity to cylindrical micelles self-assembled from
block copolymers.24 The fact that the hybrids precipitate from
aqueous solution after some time points towards an interesting
aggregation behavior worthy of exploration. The hybrids
hPG2-5 represent the first covalent cylindrical objects with
diameters in the several nanometre range that because of their
non-charged nature may be compatible with the bio world.
identical conditions (Table 1). hAFM values are known to
deviate from real heights17 and to easily vary by B1 nm from
preparation to preparation. Absolute heights of PG1-5 (hTEM
)
had previously been determined by TEM on mica substrate
and suggested themselves as an internal reference.4,18,19 The
difference (Dh) between hAFM(PG2-5) and hTEM(PG2-5) was
determined and used to correct hAFM(hPG2-5); this afforded
h
AFM,corr.(hPG2-5) (Table 1).
Finally, Fig. 3e and f show exemplary the different behavior
of the hybrid polymers when prepared from aqueous solution.
Presumably because of forces during the drying process,
single-layered islands are observed rather than single chains.
Interestingly, these islands could be resolved to the molecular
level for the highest generation sample hPG5. Ridges with an
average spacing of 8.2 ꢁ 0.7 nm can be clearly identified. As
has been reported for related cases, this spacing is considered a
measure for the width of chains on the substrate.20 hAFM for
this sample is 4.9 ꢁ 0.2 nm which is in full agreement with the
value measured from chloroform.
The results of Table 1 show that hAFM of all hybrid
polymers hPG2-5 is markedly lower than the parent polymers
of the respective generation. Assuming that this is a reflection
of structural differences between the two kinds of polymers
rather than of substantially different tip/object interactions, we
consider two effects to be particularly responsible. First, the
adsorption force between the polar OEG periphery of hPG2-5
and the partially negatively charged mica should be larger
than that between the less polar periphery of PG2-5 and the
same substrate. This results in a stronger distortion of the
cross-section geometry from circular, or flattening, than is
observed for PG2-5. Second, the hybrid rather than the parent
polymers should have an intrinsic tendency to phase segregate,
which supports the above trend. It is noteworthy that these
effects seem to be operative despite the fact that the molar
masses of hPG2-5 per repeat unit are larger by some 20% than
of PG2-5. The width value obtained for hPG5 from the AFM
image in Fig. 3e [wAFM(hPG5) = 8.2 ꢁ 0.7 nm] and the height
value hAFM,corr.(hPG5) = 5.6 nm can in principle be used to
roughly estimate the volume available for each repeat unit
(Vru). Assuming an all-stretched backbone conformation
(which is not necessarily correct) the length of a repeat unit
is l = 0.25 nm. With an assumed density of r = 1.2 g cmꢀ3
Vru = 11.5 nm3. Given the maximal molar mass per repeat
unit of Mru = 13 277 g molꢀ1 is expected Vru = 18 nm3. While
the experimental value is B35% lower, the order of magnitude
is the same and the deviation can be related to the
experimental uncertainties.
4. Experimental part
4.1. Syntheses and materials
All syntheses performed and the analytical data obtained for
the new compounds are described in the ESI.w The starting
polymers, deprotected de-PG1-4 were synthesized according to
the reported procedure, and their degree of polymerization DP
was determined by GPC to be B1000.3 Tetrahydrofuran
(THF) was refluxed over lithium aluminium hydride (LAH)
and dichloromethane (DCM) distilled from CaH2. Other
reagents and solvents were used as purchased. All reactions
were performed under a nitrogen protection. Macherey-Nagel
precoated TLC plates (silica gel 60 G/UV254, 0.25 mm) were
used for thin-layer chromatography (TLC) analysis. Silica gel
60 M (Macherey-Nagel, 0.04–0.063 mm, 230–400 mesh) was
used for column chromatography.
4.2. Measurements
1H and 13C NMR spectra were recorded on Bruker AV 300, 500
and 700 HMz spectrometers. HR-MS analyses were performed
by the MS service of the Laboratorium fur Organische Chemie,
¨
ETH Zurich, on IonSpec Ultra instruments. Elemental analyses
¨
were performed by the Mikrolabor of the Laboratorium fur
¨
Organische Chemie, ETH Zurich. AFM measurements were
¨
c
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New J. Chem., 2012, 36, 414–418 417

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performed on a Nanoscopes IIIa Multi Mode Scanning probe
microscope (Digital Instruments, San Diego, CA) operated in
the tapping mode with an ‘‘E’’ scanner (scan range 10 mm ꢂ
10 mm) under ambient conditions. Olympus silicon OMCL-
AC160TS cantilevers (Atomic Force F&E GmbH, Mannheim,
Germany) were used with a spring constant around 42 N mꢀ1
and a resonance frequency of 250 to 350 kHz. The samples were
prepared by spin-coating (2000 rpm) the polymer solution
(3–4 mg Lꢀ1 in chloroform or water) onto freshly cleaved
mica (from PLANO W. Plannet GmbH, Wetzlar, Germany).
11 For dendronized polymers with different branched repeat units in
the side chain, see: (a) J. L. Mynar, T. Choi, M. Yoshida, V. Kim,
´
C. J. Hawker and J. M. J. Frechet, Chem. Commun., 2005, 5169;
(b) W. Li and A. Zhang, Sci. China, Ser. B: Chem., 2010, 53, 2509;
(c) L. Shu, I. Gossl, J. P. Rabe and A. D. Schluter, Macromol.
¨
Chem. Phys., 2002, 203, 2540.
¨
12 If chromatography through silica gel is used instead, the yields
drop to 0–30%.
13 W. Zhuang, E. Kasemi, Y. Ding, M. Kroger, A. D. Schluter and
¨
J. P. Rabe, Adv. Mater., 2008, 20, 3204.
¨
¨
¨
14 (a) Y. Ding, H. C. Ottinger, A. D. Schluter and M. Kroger,
¨
¨
J. Chem. Phys., 2007, 127, 094904; (b) M. Kroger, O. Peleg,
¨
Y. Ding and Y. Rabin, Soft Matter, 2008, 4, 18.
15 F. Sanger and E. O. P. Thompson, Biochem. J., 1953, 53, 353.
16 There are procedures to reduce the dimethylamine content in
DMF. We found however the purified DMF to be inferior because
its lower level of dimethylamine did not stay constant in time,
which posed an additional complication. For the purification of
DMF, see: A. B. Thomas and E. G. Rochow, J. Am. Chem. Soc.,
1957, 79, 1843.
17 (a) S. J. T. Van Noort, K. O. Van der Werf, B. G. De Grooth,
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(b) W. Zhuang, C. Ecker, G. A. Metselaar, A. E. Rowan,
Acknowledgements
This work was financially supported by the Swiss National
Science Foundation (NRP 62 ‘‘Smart Materials’’) which is
gratefully acknowledged. We thank Prof. N. D. Spencer and
M. Textor, ETHZ, for the access to the AFM.
´
R. J. M. Nolte, P. Samorı and J. P. Rabe, Macromolecules, 2005,
38, 473.
18 B. Zhang, R. Wepf, M. Kroger, A. Halperin and A. D. Schluter,
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