Molecular containers based on amphiphilic PS-b-PMVE dendrigraft copolymers: Topology, organization, and aqueous solution properties

Article abstract of DOI:10.1021/ja0440203

The synthesis, characteristics, and properties of amphipatic, water-soluble dendrigrafts, with a polystyrene core and polystyrene-b-poly(methyl vinyl ether) (PS-b-PMVE) diblock as external branches, are described. The dendrigrafts are observed by AFM and TEM as egglike or long cylindrical objects which can self-organize intramolecularly in segregated subdomains forming flowerlike or strings of flowerlike objects. In organic solvents the dendrigrafts behave as fully soluble isolated macromolecules and show in water a low critical solubility temperature (LCST) at t > 30°C. The ability of the amphiphilic PS-b-PMVE dendrigrafts to complex and transport in water organic (pyrene) and metallo-organic (manganese tetraphenyl porphyrin) molecules is investigated. The possibility to stabilize the high oxidation state of metallo-porphyrin complexes through their encapsulation into the dendrigraft is shown.

Full text of DOI:10.1021/ja0440203

Published on Web 02/08/2005
Molecular Containers Based on Amphiphilic PS-b-PMVE
Dendrigraft Copolymers: Topology, Organization, and
Aqueous Solution Properties
Michel Schappacher,† Jean Luc Putaux,‡ Christelle Lefebvre,† and Alain Deffieux*,†
Contribution from the Laboratoire de Chimie des Polyme`res Organiques, UMR 5629
ENSCPB-CNRS, UniVersite´ Bordeaux 1,16 aVenue Pey Berland, 33607 Pessac Cedex, France,
Centre de Recherches sur les Macromole´cules Ve´ge´tales, ICMG-CNRS, Joseph Fourier
UniVersity of Grenoble, BP 53, F-38041 Grenoble Cedex 9, France
Received October 1, 2004; E-mail: deffieux@enscpb.fr
Abstract: The synthesis, characteristics, and properties of amphipatic, water-soluble dendrigrafts, with a
polystyrene core and polystyrene-b-poly(methyl vinyl ether) (PS-b-PMVE) diblock as external branches,
are described. The dendrigrafts are observed by AFM and TEM as egglike or long cylindrical objects which
can self-organize intramolecularly in segregated subdomains forming flowerlike or strings of flowerlike
objects. In organic solvents the dendrigrafts behave as fully soluble isolated macromolecules and show in
water a low critical solubility temperature (LCST) at t > 30 °C. The ability of the amphiphilic PS-b-PMVE
dendrigrafts to complex and transport in water organic (pyrene) and metallo-organic (manganese tetraphenyl
porphyrin) molecules is investigated. The possibility to stabilize the high oxidation state of metallo-porphyrin
complexes through their encapsulation into the dendrigraft is shown.
Introduction
original morphologies such as the recently observed grapelike
organization.5 Such specific spatial arrangement in nanometer-
In recent years, important research activity has been devoted
to the synthesis and study of branched and hyper-
branched macromolecules with starlike, comblike, and dendritic
architectures.1-3 Dendrigrafts which belong to this last family
are highly branched molecules characterized by several branch-
ing levels. We have recently reported a strategy of synthesis of
such architectures based on the successive use of anionic and
cationic living polymerizations for the preparation of the
elementary macromolecular building bricks.4 These successive
construction steps can be exploited to build macromolecules
constituted of macromolecular domains with distinct chemical
composition. This strategy was recently applied to the prepara-
tion of dendrigrafts possessing external branches constituted of
styrene and protected hydrophilic poly(vinyl ether) diblocks.
After a deprotection step, water-soluble polystyrene-b-poly-
(hydroxyethylvinyl) ether (PS-b-PVEOH) dendrigrafts were
obtained.5 Since polystyrene and poly(vinyl ether) blocks are
incompatible, they tend to self-organize intramolecularly, within
the macromolecule, to form segregated subdomains yielding
size domains can be explained by the capacity of macromo-
lecular blocks constituting a branch (polystyrene and poly(vinyl
ether)) to get in contact and associate with the blocks of the
same nature in neighboring branches, within the dendrigraft
macromolecule. Their capacity to self-assemble and the sub-
domain size will depend on their relative proximity, block
length, and limited degree of freedom, since each block
copolymer branch is linked at one end to the dendrigraft
backbone.
Continuing this work on the synthesis and properties study
of amphipatic dendrigrafts, we have investigated the preparation
of polystyrene dendrigrafts with polystyrene-b-poly(methyl vinyl
ether) (Ps-b-PMVE) diblock as external branches, since the
PMVE presents very interesting features. First it is the unique
alkyl vinyl ether monomer for which living cationic polymer-
ization directly yields a hydrophilic and water-soluble polymer,
without the necessary use of any protection/deprotection steps.3
Moreover PMVE chains exhibit in water a low critical solubility
temperature (LCST) and shows dissolution/precipitation char-
acteristics, at about 37 °C, that could be of great interest when
associated to the already reported complexing ability of am-
phiphilic dendrigrafts toward organic and metal organic guest
molecules. These characteristics could be valuably used, for
instance, to complex or transport selectively compounds dis-
persed in water media and extract them by temperature raising.
† Universite´ Bordeaux 1.
‡ Joseph Fourier University of Grenoble.
(1) (a) Branched Polymers I and II. AdVances in Polymer Science; Roovers,
J., Vol. Ed.; Springer-Verlag: Berlin, 1999; Vols. 142 and 143. (b)
Dendrimers and other Dendritic Polymers, Frechet, J. M. J.; Tomalia, D.
A.; Eds.; Wiley: Chichester, 2001.
(2) Star and hyperbranched Polymers. Plastics Engineering Series; Mishra,
M. K., Kobayashi, S., Eds.: Marcel Decker, Inc.: New York, 1999; Vol.
53.
(3) Teertstra S. J.; Gauthier M. Prog. Polym. Sci. 2004, 29, 277.
(4) Muchtar, Z.; Schappacher, M.; Deffieux, A. Macromolecules 2001, 34 (22),
7595-7600.
(5) Schappacher, M.; Deffieux, A.; Putaux, J.-L.; Viville, P.; Lazzaroni, R.
Macromolecules 2003, 36 (15), 5776-5783.
The present paper reports on the synthesis of PS dendrigrafts
with PS-b-PMVE external branches, their molecular character-
istics, and solubility parameters. Their internal organization in
9
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J. AM. CHEM. SOC. 2005, 127, 2990-2998
10.1021/ja0440203 CCC: $30.25 © 2005 American Chemical Society

Amphiphilic PS-b-PMVE Dendrigraft Copolymers
A R T I C L E S
Scheme 1
Table 1. Composition, Hydrodynamic Radius Rh in nm, and Polydispersity Index in Different Solvents of PS Dendrigafts with PS or
PS-b-PMVE as External Branches
composition
structure of external
branches (ref)
(PCEVE-g(-PS-b-PCEVE-
Rh
THFd
Rh
MeOHd
Rh
H2Od
g-(PS-b-PMVE)))
PS (1-PS)a
200-50-50-50
43 (1.15)e
75 (1.20)
94 (1.19)
120 (1.34)
PS-b-PMVE (1-PMVE)
200-50-50-50-360c
1000-80-170-70
1000-80-170-70-165c
63 (1.26)
97 (1.32)
60 (1.16)
92 (1.33)
PS (2-PS)b
PS-b-PMVE (2-PMVE)
b
a Mw ) 3.8 × 107 g/mol as determined by SLS in THF5; experimental number of external branches: 7600.
Mw was too high to be measured by SLS
(non linearity of Debye plots). Estimation by calculation assuming quantitative PS grafting and CEVE polymerization reactions yields a theoretical value of
about 1 × 109 g/mol and a number of external branches of 170 000 (1000 × 170) per dendrigraft. c DPn of the different building blocks. d At T ) 25 °C.
e Values in parentheses are polydisperity indexes.
(Scheme 1). The absence of ungrafted PS was checked by SEC. The
dimensions of these hyperbranched polymers are collected in Table 1.
selective and nonselective organic solvents was also studied
using atomic force microscopy and cryo-electron transmission
microscopy. Finally, their complexation properties for extraction/
transport purpose were investigated.
Synthesis of Amphipatic PS Dendrigrafts with an Hydrophilic
PMVE Shell (1-PMVE and 2-PMVE). The dendrigrafts were prepared
as previously described6 by initiation of living cationic polymerization
of methyl vinyl ether from the diethylacetal ends of external PS branches
of the hyper-branched PS cores. A typical sequential polymerization
procedure was as follows: 5 g of dendrigraft (1-PS) (corresponding to
1 mmol of diethyl acetal ends) and 200 µL (1.4 mmol) of TMSI were
added successively to 150 mL of dry toluene under vacuum at -35
°C. After 45 min, 23.2 g (0.4 mol) of MVE were added, and the
polymerization was started by addition of 13 mg of ZnCl2 (0.1 mmol)
dissolved in 1.5 mL of diethyl ether. Every 2 h a small sample of the
reaction mixture was taken for characterization purpose, and the MVE
conversion was followed by SEC analysis. After complete MVE
conversion (6 h), a lutidine/methanol solution (10% v/v) was added to
deactivate the system. The reaction mixture was washed with 10%
aqueous sodium thiosulfate solution and water and dried over MgSO4
and then under vacuum (yield 18.5 g). Linear PMVE which forms as
side product was removed by selective fractionation. Complete removal
was confirmed by SEC. The average PMVE block length was calculated
Experimental Section
Materials. Toluene (99,5%, J.T. Baker, Deventer, The Netherlands)
was purified by distillation over calcium hydride and stored over
polystyryllithium seeds. Diethyl ether (99%, J.T. Baker, Deventer, The
Netherlands) was purified by distillation over calcium hydride and
stored over potassium/benzophenone. Trimethylsilyl iodide (TMSI,
Aldrich 97%) and ZnCl2 (Aldrich 99,99%) were used without further
purification. Gaseous methyl vinyl ether (MVE, Fluka Chemika 99%)
was distilled over calcium hydride twice before use. 5,10,15,20-
Tetraphenyl-21H,23H-porphine manganese(III) chloride (MnClTPP)
(99%, Sigma-Aldrich France) and pyrene (98%, Sigma-Aldrich France)
were used as received. Sodium hypochlorite (NaOCl) (Sigma-Aldrich,
solution 10-13% available chlorine) was also used as such.
Synthesis of the PS Dendrigraft Precursors. The dendrigraft
precursors, (1-PS) 200/50/50/50-A (DPnPCEVE1/DPnPS1/DPnPCEVE2
/
DPnA-PS2, A ) R-diethylacetal) and (2-PS) 1000/80/170/70-A, used in
1
by H NMR from the proportion between MVE and styrene units in
this work were elaborated according to a previously reported procedure6
the copolymer, assuming quantitative initiation from all the PS acetal
ends.
(6) (a) Deffieux, A.; Schappacher, M. Macromolecules 1999, 32, 1797-1802.
(b) Deffieux, A.; Schappacher, M. Macromolecules 2000, 33, 7371-7377.
(c) Schappacher, M.; Billaud, C.; Paulo, C.; Deffieux, A. Makromol. Chem.
Phys. 1999, 200, 2377-2386.
Preparation of PS-PMVE Dendrigraft Aqueous Solutions. For
studies in water, 10 g of dendrigraft 1-PMVE were dissolved in 50
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A R T I C L E S
Schappacher et al.
mL of ethanol and purified and transferred into water by dialysis
(Spectra/Por7, molecular weight cutoff ca. 1000) for 3 days against
pure water at 20 °C.
Dynamic Light Scattering (DLS). Light scattering measurements
were performed using an ALV laser goniometer, which consists of a
22 mW HeNe linear polarized laser with 632.8 nm wavelength and an
ALV-5000/EPP multiple τ digital correlator with 125 ns initial sampling
time. Solutions were put in 10 mm diameter glass cells and kept at a
constant temperature of 25 °C. Scattering measurements were done
from 30° to 130 or 150° with a step of 20° or from 30° to 120° with
a step of 30°: only data at 90 °C are presented. The data acquisition
was done with ALV correlator control software, and the counting time
varied for each sample from 300 up to 600 s.
Other Characterization Techniques and Measurements. 1H NMR
spectra were recorded in CDCl3 on a Bruker Avance 400 MHz FT
apparatus. Size exclusion chromatography (SEC) analysis in THF
(distilled from CaH2) was performed at 25 °C at a flow rate of 0.7
mL/min on a Varian apparatus equipped with refractive index/laser light
scattering (Wyatt technology) dual detection and fitted with four TSK
columns (300 × 7.7 mm2, 250 Å, 1500 Å, 104 Å, 105 Å). UV-vis
spectroscopy measurements were performed at 20 °C on a Varian Cary
measurements 3E apparatus in UV quartz cells of 5 mm. Fluorescence
spectra were recorded on a Safas Monaco FLX spectrofluorometer at
310 nm excitation wavelength at room temperature.
Preparation and Study of Pyrene/PS-PMVE Dendrigraft Com-
plexes. Pyrene (4.7 mg, 2.3 × 10-6 mol) was first dissolved in 0.5 mL
of dichloromethane in a small flask, and the solvent was evaporated
until dryness. A solution of PS-PMVE dendrigraft (1-PMVE) (20 mL
1.3 mg/mL, 1.7 × 10-10 mol) in pure water (milli-Q) was then added
under vigorous stirring at 20 °C, and the UV-visible absorption spectra
of the water solution was recorded at increasing time.
Preparation and UV-visible Study of Metalloporphyrin/PS-
PMVE Dendrigraft Complexes. The metalloporphyrin (MnIIIClTPP)
(1.7 mg as fine powder, 2.4 × 10-6 mol) was poured into the pure
water (milli-Q) solution of PS-PMVE dendrigraft (1-PMVE) (30 mL
1.3 mg/mL, 2.5 × 10-10 mol) at 20 °C, pH ) 7 under vigorous stirring.
The UV-visible absorption spectra were recorded at increasing time
until the intensity of the absorption spectra remained constant (12 h)
indicating complete encapsulation.
Atomic Force Microscopy (AFM). Samples for AFM analysis were
prepared by solvent casting at ambient conditions by spin coating on
substrates starting from solutions in dichloromethane for the polystyrene
core dendrigraft or methanol and water for the PMVE derivated
dendrigrafts. Practically, 20 µL of a dilute solution (0.01 wt %) were
spin cast on a 1 × 1 cm2 freshly cleaved mica or on a highly oriented
pyrolytic graphite (HOPG) substrate. Samples were analyzed after
complete evaporation of the solvent at room temperature. All AFM
images were recorded in air with a Dimension microscope (Digital
Instruments, Santa Barbara, CA), operated in tapping mode. The probes
were commercially available silicon tips with a spring constant of 40
N/m, a resonance frequency lying in the 270-320 kHz range, and a
radius of curvature in the 10-15 nm range. In this work, both the
topography and the phase signal images were recorded with the highest
sampling resolution available, i.e., 512 × 512 data points.
Transmission Electron Microscopy (TEM). Droplets of 0.01 mg/
mL dendrigraft solutions were deposited onto carbon-coated microscopy
grids. To limit the reconcentration resulting from the fast evaporation
of organic solvents, the liquid in excess was immediately blotted with
filter paper. A droplet of aqueous uranyl acetate (2 wt %) was deposited
onto the specimen before drying. The excess of stain was blotted, and
the remaining liquid film was allowed to dry. As uranyl acetate is known
not to dissolve into organic solvents, our concern was that salt
recrystallization artifacts would appear when the aqueous solution was
in contact with the residual film of organic solvent. Surprisingly, as
shown in the Results section, negative staining worked well using this
method.
Cryo-TEM has been successfully used to observe various polymer
nanoparticles dispersed in water,7,8 but there exists very few examples
of systems in organic solvents that were studied with this technique.9-11
The main reason is that most organic solvents are known to dissolve
in liquid ethane which is the cryogen commonly used for fast-freezing.
Thus, organic solvents have to be quench-frozen in liquid nitrogen.
However, due to a low freezing efficiency, the resulting film is often
crystalline, and that creates detrimental diffraction effects during TEM
observation.
Oxidation of Dendrigraft Complexed MnIIIClTPP with Sodium
Hypochlorite: Generation of Oxo-MnIVTPP. In a quartz cuvette (5
mm optical path) containing an aqueous solution of MnIIIClTPP/
dendrigraft (1-PMVE) (0.17 mg of MnIIIClTPP in 3.9 mg of 1-PMVE
in 3 mL water, pH 7, 20 °C) was introduced in small portions a water
solution of sodium hypochlorite (1 µL aliquots of a 1% solution), and
the UV-visible spectrum of the solution was monitored with time.
The resulting visible spectrum (λmax(nm) ) 424, 473, 520, 625) is
characteristic of red oxo-MnIVTPP complex. Stability of the MnIV/
1-PMVE complex versus time was then followed by the intensity
decrease of the characteristic absorption bands with time.
In the present study, two organic solvents were used: methanol and
dichloromethane. Thin films of 1 mg/mL dendrigraft suspension in
methanol were formed on NetMesh (Pelco) “lacey” carbon membranes
and immediately plunged into liquid ethane (-171 °C) using a Leica
EM CPC fast-freezing device.8 As checked by cryo-TEM, methanol
was successfully vitrified using liquid ethane. Dendrigraft suspensions
in dichloromethane had to be quenched in liquid nitrogen, using a
homemade guillotine-type plunging system and a glass Dewar contain-
ing the cryogen. The specimens were then mounted onto a Gatan 626
cryo-holder cooled with liquid nitrogen and transferred into the
microscope. All samples were observed at -180 °C using a Philips
CM200 “Cryo” microscope operated at 80 kV. One major problem
was that vitreous solvents appeared to be highly radiation sensitive.
Bubbles rapidly developed under electron irradiation. The only way to
record images at magnifications of 15 000-30 000× was to use the
Low Dose System (FEI/Philips): the region of interest was chosen at
low magnification and low illumination while focusing was performed
at a higher magnification on an area nearby. Micrographs were recorded
on Kodak SO163 films.
Results and Discussion
The synthetic pathway used for the synthesis of dendrigrafts
with a PS core and PS-b-PMVE external branches is directly
derived from the previously reported synthesis and purification
of polystyrene-b-poly(vinyl ether) dendrigrafts. PMVE blocks
were introduced by initiation of MVE cationic polymerization
from a PS dendrigraft bearing an acetal function terminus at
each of its external PS branches (Scheme 1). Initiation from
this multifunctional precursor requires the preliminary trans-
formation of the acetal end group into an R-iodoether by addition
of trimethylsilyl iodide (TMSI). Propagation is then triggered
by adding zinc dichloride as catalyst and the monomer. In
agreement with previous data5 the efficiency of the initiation
reaction was estimated by proton NMR higher than 90%. The
very high increase of molar mass at each dendrigraft building
step allowed an easy fractionation of the dendrigrafts from the
linear PMVE homopolymers by selective precipitation.
(7) Chalaye, S.; Bourgeat-Lami, E.; Putaux, J. L.; Lang, J. Macromol. Symp.
2001, 169, 89-96.
(8) Durrieu, V.; Gandini, A.; Belgacem, M. N.; Blayo, A.; Eisele´, G.; Putaux,
J. L. J. Appl. Polym. Sci. 2004, 94-2, 700-710.
(9) Oostergetel, G. T.; Esselink, F. J.; Hadziioannou, G. Langmuir 1995, 11,
3721-3724.
(10) Boettcher, C.; Schade, B.; Fuhrhop, J. H. Langmuir 2001, 17-3, 873-877.
(11) Butter, K.; Bomans, P. H.; Frederik, P. M.; Vroege, G. J.; Philipse, A. P.
Nature Materials 2003, 2-2, 88-91.
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A R T I C L E S
Figure 1. Hydrodynamic radius and polydispersity of PS and PS-b-PMVE dendrigrafts in different solvents as determined by DLS.
The architectural characteristics of the initial PS dendrigrafts
1-PS and 2-PS and final PS-b-PMVE amphiphilic dendrigrafts
1-PMVE and 2-PMVE and their dimensional parameters in
different solvents are collected in Table 1.
Good solubility of PS and PS-b-PMVE dendrigrafts is
observed in THF and dichloromethane which are solvents of
both PS and PMVE blocks. DLS measurements performed at
various angles (30° to 120°) indicates the presence of a single
population for dendrigrafts 1-PMVE and 2-PMVE. As indicated
by their hydrodynamic radius and narrow size distribution in
solution, the hyperbranched macromolecules (Figure 1) behave
as isolated molecules without any detectable formation of
intermolecular aggregates. The important increase of the
hydrodynamic radius in THF after MVE polymerization,
comparing 1-PS with 1-PMVE and 2-PS with 2-PMVE, Table
1, confirms the important growth of the macromolecule due to
the formation of the PMVE shell. The PS-PMVE core-shell-
like macromolecules are also soluble in methanol and water (at
T < 25 °C), which are selective solvents of the PMVE shell.
As it may be noticed in Table 1, a strong reduction of the
hydrodynamic radius of the PS-PMVE dendrigrafts is observed
in these solvents as compared to THF. This can be explained
by the internal contraction of the PS core in methanol and water
nonsolvents. However, the influence of changes in solvent
quality for the PMVE material may also contribute to this
phenomenum. Although the PS-b-PMVE dendrigrafts exhibit
a much more compact PS core, thanks to the PMVE shell they
remain soluble and behave as monodisperse unimolecular
objects, as indicated by DLS data Figure 1. This can be related
to the highly protecting effect of the PMVE shell which limits
dendrigraft-dendrigraft associations through interaction between
their PS core.
Figure 2. Variation of the hydrodynamic radius of PS-b-PMVE dendrigraft
(1-PMVE) in water as a function of temperature (60 min between two
measurements). Inset: aqueous solution of (MnIIITPP)OH in dendrigraft (1-
PMVE): (a) at 20 °C; (b) after raising the temperature at 35 °C for 60 min.
Dendrigraft (1-PMVE) concentration: ∼1 × 10-8 M; (MnIII TPP)Cl ≈
1.10-4
M
solutions. The average hydrodynamic radius of dendrigrafts
1-PMVE as a function of the temperature is shown in Figure
2. A small decrease of the hydrodynamic radius at temperatures
ranging from 25 to 30 °C is first observed. This can be related
to a volume contraction of the PMVE shell associated to its
solubility decrease when temperature increases. This phenom-
enon is followed at 30 °C by a rapid increase of the apparent
hydrodynamic radius and a size distribution broadening of the
objects, suggesting aggregation between dendrigraft molecules
prior their precipitation. This is in agreement with the LCST
properties brought by the PMVE shell to the dendrigraft.12,13
However, contrarily to the reversibility of LCST processes
reported for linear homo- and block PMVE copolymers, the
solubilization/precipitation process is not reversible with the
dendrigrafts. Considering that water is not a particularly good
solvent for that polymer, this is likely due to strong intra- and
In methanol, the PS-PMVE dendrigrafts exhibit very similar
characteristics and behavior (almost same size and polydisper-
sity, no aggregation) in the whole temperature range examined,
whereas a strong influence of temperature is observed in water
(12) Verdonck, B.; Goethals, E. J.; Prez, F. E. D. Macromol. Chem. Phys. 2003,
204 (17), 2090-2098.
(13) Folder, C.; Patrickios, C. S.; Armes, S. P.; Billingham, N. C. Macromol-
ecules 1996, 29, 8160-8169.
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Schappacher et al.
Figure 3. TMAFM images of PS dendrigrafts deposited from dichloromethane solutions: (a) 1-PS (phase: 1 × 1 µm); (b) 2-PS (phase: 1 × 1 µm). Scale
bars: 100 nm.
Figure 4. TMAFM phase images of PS-b-PMVE dendrigraft (1-PMVE) deposited from (a) methanol (750 × 750 nm2) and (b) water (1000 × 1000 nm2).
The ratio of “tapping” amplitude to “free” cantilever oscillation amplitude was equal to 0.7 for image a and 0.95 for image b.
intermolecular associations yielding intermolecular entangle-
ments between PMVE blocks. A similar temperature dependence
was observed also with 2-PMVE, although temperatures of the
different phenomena were slightly lower due to the difference
in the dendrigraft chemical composition, molar masses, and
possibly their shape.
and 2-PS, the presence of smaller objects attributed to dendri-
graft fragments can also be noticed. AFM images of PS-b-
PMVE dendrigrafts (1-PMVE) were obtained from deposits of
their methanol and water solutions, after drying. Images obtained
from THF deposits show spherical objects similar to those
presented in Figure 3a but with a larger diameter of 100 nm, in
agreement with the formation of the PMVE blocks. Those
obtained from methanol deposits, a nonsolvent of the PS core,
exhibit a quite different structure with a specific intramolecular
organization (Figure 4a) corresponding to hard and soft sub-
domains of different chemical composition, as previously
reported for other amphiphilic dendrigrafts.5 The image obtained
from aqueous deposits does not show any specific internal
organization (Figure 4b). This may be explained by the presence
of remaining water inside the dendrigrafts which does not allow
us to distinguish hard and soft phases.
The shape and internal organization of the dendrigrafts were
further examined in the solid using AFM and TEM. AFM
images obtained from dichloromethane solution deposits of PS
dendrigrafts 1-PS and 2-PS, corresponding to the hydrophobic
core of the PS-PMVE amphiphilic dendrigrafts, are shown in
Figure 3. They appear as monolayer aggregates of uniform
objects which sizes (75 × 50 nm2 and 300 × 100 nm2,
respectively) in good agreement with the dimensions of
the different building blocks. Their specific shape, egglike for
1-PS and cylindrical for 2-PS, is determined by the ratio
between their length and cross section. The length is mainly
governed by the DPn of the central poly(chloroethyl vinyl
ether) (PCEVE) backbone (DPnPCEVE1 200 and 1000, respec-
tively), whereas the other constitutive blocks PS1, PCEVE2, PS2,
contribute to the cross section of the objects. In the case of 1-PS
Figure 5 shows TEM micrographs of dendrigraft preparations
after negative staining. Dendrigrafts 1-PMVE appear as ovoid
particles with a width of 80-100 nm and a length of 100-120
nm (Figure 5a). Their multilobular aspect is similar to that of
previously studied dendrigrafts dispersed in water.5 We have
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A R T I C L E S
Figure 5. TEM images of dendrigrafts from samples 1-PMVE and 2-PMVE suspended in methanol (a,d and b,e, respectively) as well as sample 2-PMVE
suspended in dichloromethane (c,f). In images a-c, the dry specimens were negatively stained with uranyl acetate, whereas, in images d-f, the dendrigrafts
are seen embedded in thin films of vitrified methanol (b,e) and dichloromethane (f). Scale bars: 100 nm.
Figure 6. Evolution of the UV-visible spectrum of a pyrene (1.15 × 10-4 M)/dendrigraft (1-PMVE) (8.5 × 10-9 M) mixture in water at increasing contact
time: (a) 1 min; (b-j) 15, 30, 105, 180, 300, 1320 min. Inset: fluorescence spectrum emission (λ
) 310 nm) of pyrene/dendrigraft in water.
excitation
shown that this peculiar morphology was not the result of a
drying artifact but was indeed related to the reorganization of
hydrophobic PS branches into chemically distinct subdomains.
These domains are formed by the intramolecular aggregation
between nonmiscible hydrophilic PMVE (soft phase) on one
hand and hydrophobic PS blocks (hard phase) on the other hand,
the volume fraction of PCEVE anchoring sites being negligible.
Since PS and PMVE are miscible in the bulk, the segregation
process likely takes place in the methanol solution used for the
sample preparation.
the main central PCEVE backbone breaks into smaller units,
although these units could not clearly be seen as individual
objects. Dendrigrafts 2-PMVE suspended in dichloromethane,
in which all types of branches are soluble, were also observed
after negative staining. As seen in Figure 5c, the flowerlike
morphology has disappeared in agreement with the fact that
the macromolecules are fully swollen in dichloromethane and
that no chemical segregation is detected. The dendrigrafts also
appear as strings of 100 nm spheroid subunits which confirm
the fragmentation of the central PCEVE backbone observed in
methanol. Surprisingly, these 100 nm units were not detected
either by SEC or DLS. It is thus difficult to know if the
fragmentation took place during the synthesis, the storage in
solution, or the sample preparation for imaging where the
Negatively stained dendrigrafts (2-PMVE) built on a longer
PCEVE backbone and dispersed in methanol appear as strings
of flowerlike particles with a 100 nm diameter (Figure 5b),
similar to individual dendrigrafts (1-PMVE). This suggests that
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A R T I C L E S
Schappacher et al.
Figure 7. Kinetics of decomposition of MnIIIClTPP in dichloromethane in the presence of NaOCl monitored by UV-visible absorption spectroscopy at 20
°C (10 min between two scans). [MnIIIClTPP] ) 1.1 × 10-5
Figure 8. UV-visible absorption spectrum in water of MnIII(OH)TPP (formed from MnClTPP) encapsulated into PS-b-PMVE dendrigrafts before and after
addition of several aliquots of NaOCl at 20 °C. [MnClTPP] ) 1.1 × 10-5
M
molecules may be submitted to important mechanical constraints
associated to their overcrowded structures and the phase
segregation process.
The capacity of amphipatic PS-PMVE dendrigrafts to
solubilize and transport into water small organic molecules and
organometallic complexes (pyrene and manganese(III) tetraphe-
nyl porphyrin, MnIII(X)TPP, were chosen as models) has been
investigated in aqueous media. Solid pyrene (1.15 × 10-4 M)
was contacted with an aqueous solution of 1-PMVE (8.5 ×
10-9 M), and the turbid mixture was maintained under stirring
at 20 °C until it turned to an homogeneous colorless solution.
The evolution of the absorption spectrum of the pyrene/
dendrigraft mixture at increasing contact time (Figure 6)
confirms the transportation of pyrene into water through its
complexation by 1-PMVE. The characteristics of the pyrene
absorption bands in the water/dendrigraft system (λmax(nm) )
263, 275, 297, 309, 323, 339) are very close to those observed
in conventional organic solvents (λmax(nm) ) 262, 274, 295,
307, 321, 337 in dichloromethane). According to the UV-
visible spectroscopy, at completion about 13 500 molecules of
pyrene are contained in one molecule of dendrigraft.
Samples 1-PMVE and 2-PMVE were also observed using
cryo-TEM. Figure 5d and e show images of dendrigrafts
embedded in vitreous methanol (fast-freezing in liquid ethane).
The contrast in this organic solvent is clearly lower than that
of particles previously observed in vitreous ice.5 Moreover,
while defocus values of 1-3 µm are usually chosen for cryo-
TEM imaging, we used higher defoci (4-6 µm) in order to
enhance the phase contrast and reveal the particles. In the
resulting images, only the denser parts, which likely correspond
to subdomains of insoluble PS branches, are detected, whereas
the PMVE corona is too diffuse to be seen. When sample
2-PMVE is observed in vitreous dichloromethane (frozen in
liquid nitrogen), only a weak amplitude contrast is detected on
the particles (Figure 5f). This confirms that the branches are
soluble and that the dendrigrafts are fully swollen in this solvent.
However, the cryo-TEM images of sample 2-PMVE suspended
in methanol or dichloromethane confirm that the peculiar
morphology observed on negatively stained specimens was not
a drying artifact. In both cases, the dendrigrafts appear as strings
of 100 nm units, suggesting that the fragmentation occurred in
an earlier preparation step.
Under UV light irradiation (λ ) 365 nm), the pyrene/
dendrigaft aqueous solution exhibits a fluorescent blue emission
very similar to that of pyrene in organic solvent. The fluores-
cence spectrum (inset in Figure 6) shows an intensity ratio IIII/
II of about 0.85, a value much higher than generally reported
for pyrene in water (IIII/II ) 0.59) and polar media and in the
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Amphiphilic PS-b-PMVE Dendrigraft Copolymers
A R T I C L E S
Figure 9. Kinetics of decomposition in water at 20 °C of MnIVoxoTPP encapsulated into PS-b-PMVE dendrigrafts as monitored by UV-visible absorption
spectroscopy (20 min between two scans).
range of the fluorescence spectrum of pyrene in aromatic
solvents (IIII/II ) 0.82 to 1.0).15 These observations suggest that
the pyrene molecules captured in the dendrigraft are located
within the polystyrene core and behave as individual molecules
such as in an organic solution, despite the surrounding water
medium and the high number of pyrene molecules per dendri-
graft.
chromium are known for their unusual high reactivity in oxygen
activation and transfer11 and their ability to oxidize organic
substrates. However, during these reactions, the metallopor-
phyrins act also frequently as a substrate for the oxidant and
mutually destroy in an intermolecular process. This is illustrated
in Figure 7 for MnIIICl TPP: addition of a small amount of
NaOCl as oxidizing agent results in a very rapid disappearance
of MnIIICl TPP without any observable formation of the oxidized
MnIVOTPP derivatives. To enhance the metalloporphyrin stabil-
ity toward oxidant, it is necessary to introduce bulky substituents
on their phenyl and pyrrole groups. We found that dendrigraft
encapsulated MnIIIOH TPP can be oxidized to yield relatively
stable MnIV TPP species. As shown in Figure 8, upon addition
of the oxidizing agent the main absorption band of encapsulated
MnIII(OH)TPP (λmax ) 468 nm) disappears to the benefit of a
new soret band (λ ) 426 nm) corresponding to the formation
of MnIVoxoTPP species16,21,22 which remains stable during
several hours. The half-life time of dendrigraft complexed
(MnIVTPP)oxo can be estimated from the half decay time of
its soret band to 5 h at 20 °C (Figure 9). Since the main
decomposition mechanism of the porphyrin ring in the oxidation
process is believed to proceed via a bimolecular reaction, the
strong stabilization effect observed with dendrigraft complexed
(MnIVTPP)oxo suggests that the metalloporphyrins are isolated
from each other inside the PS-b-PMVE dendrigraft, despite more
than 10 000 molecules can be contained in one dendrigraft
molecule. This could indicate that the metalloporphyrins are
individually trapped in small and nonconnected cavities (of
about 10 nm3) delimited by the polystyrene chains. The location
of (MnIVTPP)oxo molecules within the dendrigraft PS core is
further supported by the independence of the oxo complex
stability toward the pH of the solution from 7 to 14, contrarily
to tetra(4-sulfonatophenyl) porphinato manganese(IV) X)/water
systems15 which show a stability increase toward NaOCl at
increasing pH.
Complexation and transport of large metallo-organic mol-
ecules into water was investigated in the same way using
manganese tetraphenylporphyrin chloride, (MnIIITPP)Cl, as
starting molecule. As shown by the homogeneous green-colored
solution, the metalloporphyrin (∼1 × 10-4 M), added to the
water solution of the PS-PMVE dendrigraft (∼1 × 10-8 M)
as a fine powder, completely dissolves in less than 12 h at 20
°C (see inset a, Figure 2). The observed absorption bands are
characteristic of (MnIIITPP)OH (λmax ) 468 nm) which forms
by hydrolysis of the chloride derivative when in contact with
water. Interestingly, due to the LCST properties of the PMVE
dendrigraft shell upon raising the temperature of the solution
at about 35 °C, the metalloporphyrin/dendrigraft complex
precipitates yielding a colorless solution (inset Figure 2) free
of metalloporphyrin. This supports the fact that all the metal-
loporphyrin molecules are strongly trapped inside the dendri-
grafts and can be easily and efficiently removed from water
through this simple temperature-controlled precipitation process.
As indicated before, resolubilization of the dendrigrafts at lower
temperature was not possible.
Oxidized synthetic metalloporphyrin complexes have been
extensively studied as simple active site models for monooxy-
genase15 and peroxidase16,17 as well as catalysts for the
epoxidation reactions of organic substrates.19,20 Indeed, structur-
ally characterized oxometalloporphyrins of manganese, iron, and
(14) Schappacher, M.; Deffieux, A. Polymer 2004, 45, 4633-4639.
(15) Collman, J. P.; Gagne, R. R.; Gray, H. B.; Hare, J. J. Am. Chem. Soc.
1974, 96, 6522.
(16) Groves, J. T.; Stern, M. K. J. Am. Chem. Soc. 1988, 110, 8628-8638.
(17) Liu, M.; Su, Y. O. J. Electroanal. Chem. 1997, 426, 197-203.
(18) Benaglia, M.; Danell, T.; Fabris, F.; Sperandio, D.; Pozzi, G. Org. Lett.
2002, 24, 4229-4232.
(21) Groves, J. T.; Watanabe, Y.; McMurry, T. J. J. Am. Chem. Soc. 1983,
105, 4489-4490.
(22) Schappacher, M.; Weiss, R. Inorg. Chem. 1987, 26, 1190-1192.
(23) Turk, H.; Berber, H. Int. J. Chem. 2000, 32, 271-278.
(24) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039-
2044.
(19) Magno, S. G. D.; Dussault, P. H.; Schultz, J. A. J. Am. Chem. Soc. 1996,
118, 5312-5313.
(20) Davoros, E. M.; Diaper, R.; Dervissi, A.; Tornaritis, M. J.; Coutsocelos,
G. J. Porphyrins Phthalocyanines 1998, 2, 53-60.
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A R T I C L E S
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The use of dendrigraft-stabilized oxidized manganese por-
phyrin complexes as potential catalysts for oxygen activation
and transfer as well as for the oxidation of organic substrates
will be further investigated. The preparation of stable oxidized
forms of other metalloorganic complexes through dendrigraft
encapsulation is also in progress.
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2998 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005