Ferrocenyl dendronized polymers

Article abstract of DOI:10.1039/b819604d

Styrenic dendrons functionalized with ferrocenyl and chloromethylsilyl termini were synthesized and polymerized by the AIBN-initiated radical polymerization procedure yielding a ferrocenyl dendronized polymer and a chloromethylsilyl dendronized polymer, r

Full text of DOI:10.1039/b819604d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Ferrocenyl dendronized polymerswz
Elodie Boisselier,a Anita Chan Kam Shun,a Jaime Ruiz,a Eric Cloutet,b
Colette Belina and Didier Astruc*a
Received (in Montpellier, France) 4th November 2008, Accepted 5th December 2008
First published as an Advance Article on the web 9th January 2009
DOI: 10.1039/b819604d
Styrenic dendrons functionalized with ferrocenyl and chloromethylsilyl termini were synthesized
and polymerized by the AIBN-initiated radical polymerization procedure yielding a ferrocenyl
dendronized polymer and a chloromethylsilyl dendronized polymer, respectively. The latter
polymer was functionalized with ferrocenyl groups using ‘‘click’’ chemistry. The ferrocenyl
dendronized polymers are soluble in common organic solvents such as tetrahydrofuran,
dichloromethane, and chloroform. They show a reversible ferrocenyl wave in cyclic voltammetry
(CV) and form derivatized Pt electrodes. Their sizes and shapes were examined by size exclusion
chromatography (SEC), atomic force microscopy (AFM), dynamic light scattering (DLS) and
1
DOSY H NMR spectroscopy.
Introduction
Dendronized polymers1 are large nano-objects that can be
synthesized either by the dendronization of a functional polymer
(Scheme 1, route A) or the macromonomer route involving
polymerization of a pre-formed dendron (Scheme 1, route B).1–5
Their size, shape and functionalities bring about specific potential
applications, for instance in materials science3,4 and nano-
medicine.5 It has been shown that dendronized polymers are
Scheme 1 Two routes to dendronized polymers.
either spherical or cylindrical depending on the size of the
dendronic side chains.4 Given their large size, the visualization
of the shape of dendronized polymers can be carried out by
microscopy techniques such as atomic force microscopy (AFM).
Metal-containing polymers are of great interest for their
materials properties including catalysis, molecular electronics,
sensors and semi-conductors.6 Although ferrocene-containing
polymers are well known for such applications,6 only one
example of dendronized polyferrocene is known: Manners and
co-workers recently reported substitution of the chloro group
in poly(ferrocenylchloromethylsilane),7 with a Percec-type
dendron4 containing a benzylate focal point (i.e. Scheme 1,
route A), as well as several physico-chemical studies including
AFM observation of spherical cocoons for the single chains
of the dendronized polymer and elongated single-chain
structures.3,8
In this paper, we report an alternative method to synthesize a
ferrocene-containing dendronized polymer, i.e. the polymerization
of new triferrocenylsilyl and tris-(chloromethylsilyl) dendrons
containing a styrenyl group at the focal point (i.e. Scheme 1, route
B). In recent work, we reported the attachment of triferrocenylsilyl
dendrons containing other focal groups to the termini of
dendrimers including gold nanoparticle (AuNP)-centered dendri-
mers as sensors for inorganic anions including ATP.10 Thus,
modification of this family of dendrons by the introduction of a
styrenyl group now brings about the key entry to ferrocenyl
dendronized polymers.
Experimental
1. General data
Continuing our interest in redox-robust systems in nano-
objects,9 we wished to investigate the synthesis of ferrocenyl
dendronized polymers including their redox properties and
functions.
All operations were performed under a nitrogen atmosphere
using standard Schlenk techniques. Prior to use, THF was dried
over sodium benzophenone ketyl and freshly distilled under a
nitrogen atmosphere. CH2Cl2 was dried over calcium hydride
and freshly distilled under a nitrogen atmosphere. All other
reagents were obtained from commercial sources and used with-
out further purification. All glassware was previously dried in an
oven and cooled under a nitrogen flow. Ferrocenyldimethylsilane
was synthesized according to ref. 11.
a
´
Institut des Sciences Moleculaires, UMR CNRS No. 5255,
Universite Bordeaux 1, 33405 Talence Cedex, France.
´
E-mail: d.astruc@ism.u-bordeaux1.fr
`
b
Laboratoire de Chimie des Polymeres Organiques, UMR CNRS No.
5629, Universite´ Bordeaux 1, 33607 Pessac Cedex, France
w Electronic supplementary information (ESI) available: 1H NMR,
13C NMR and IR spectra, size exclusion chromatograms (SEC) and
cyclic voltammograms (CV) of the ferrocenyl dendronized polymers.
See DOI: 10.1039/b819604d
2. Physical measurements
1
The H NMR spectra were recorded at 25 1C on a Bruker AC
250 (250 MHz), a Bruker Advance 300 (300 MHz), and a Bruker
z This article is dedicated to our distinguished colleague Dr
Jean-Pierre Sauvage, on the occasion of his 65th birthday.
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DPX 400 (400 MHz) spectrometer. 13C NMR spectra were
recorded in the pulse FT mode on a Bruker AC 300 spectrometer
at 75.50 MHz. 29Si NMR spectra were recorded on a Bruker AC
300 spectrometer at 59.6 MHz. All chemical shifts are reported in
parts per million (d, ppm) with reference to SiMe4 (TMS). Mass
AIBN (101 mL) was then added, the solution was degassed
under vacuum, and the reaction mixture was stirred for 15 h in
the closed Schlenk tube under a nitrogen atmosphere at 100 1C
(pressure was kept with caution using the specially equipped
Schlenk tube). The solvent was removed under vacuum, and
the orange solid residue was partly dissolved in dichloro-
methane. The orange solid residue (37 mg) was insoluble in
all solvents. The soluble fraction was concentrated to 2 mL
and precipitated using 20 mL methanol, yielding an orange
waxy product that was then reprecipitated twice from dichloro-
methane solutions with methanol, leaving a waxy orange
product (44 mg, 30% yield).
1H NMR (CDCl3, 300 MHz), dppm (broad signals): 0.5
(Si-CH3); 0.6 (Si-CH2); 1.13 (CH2); 1.56 (Cq-CH2); 3.99 (2H,
Cp Si-C-CH); 4.05 (5H, Cp); 4.26 (2H, Cp, Si-C-CH); 5.3
(CH2-O); 6.83 (H vinyl); 7.13 (H arom.).
13C NMR (CDCl3, 75.47 MHz), dppm: ꢁ1.35 (Si-CH3);
18.01 (CH2); 18.51 (CH2); 42.54 (CH2); 43.63 (Cq);
68.88–73.73 (Cp); 77.61 (CH2-O); 127.87 and 128.18
(CH arom.); 156.94 (Cq arom.).
spectra were obtained at the CESAMO, Universite Bordeaux 1,
´
on a PerSeptive Biosystems Voyager Elite (Framingham, MA)
time-of-flight mass spectrometer. This instrument was equipped
with a nitrogen laser (337 nm), a delayed extraction, and a
reflector. It was operated at an accelerating potential of
20 kV in both linear and reflection modes. The mass spectra
shown represent an average over 256 consecutive laser shots
(3 Hz repetition rate). Peptides were used to calibrate the mass
scale using the two points calibration software 3.07.1 from
PerSeptive Biosystems. Elemental analyses were performed at
the Center of Microanalysis of the CNRS, Lyon Villeurbanne,
France. All electrochemical measurements were recorded in
degassed CH2Cl2 at 20 1C. Supporting electrolyte: [n-Bu4N][PF6]
0.1 M; working and counter electrodes: Pt, quasi-reference
electrode: Ag; internal reference: [Fe(Z5-C5Me5)2]+/0 15
;
scan
rate: 0.200 V sꢁ1
.
29Si NMR (CDCl3, 59.62 MHz), dppm: ꢁ2.658 (s, Si).
Size exclusion chromatography (SEC): M = 160 kDa;
PDI = 1.9.
3. Synthesis and polymerization
Synthesis of the monomeric triferrocenyl dendron 3. The
phenol triferrocenyl dendron 3 (0.562 g; 5.4 ꢂ 10ꢁ4 mol),
p-iodomethylstyrene (0.152 g, 6.23 ꢂ 10ꢁ4 mol), and potas-
sium carbonate (0.436 g; 3.15 ꢂ 10ꢁ4 mol) were successively
introduced into a Schlenk tube. DMF (30 mL) was then added
into the Schlenk tube in the dry lab. This reaction mixture was
stirred for one day at ambient temperature. DMF was then
evaporated under reduced pressure, and the crude residue was
dissolved in dichloromethane, potassium carbonate was then
filtered, and the concentrated solution was chromatographed
over a silica gel column using dichloromethane as the eluant.
Removal of dichloromethane under reduced pressure yielded
0.420 g of 3 as an orange–red solid (67%).
1H NMR (CDCl3, 300 MHz), dppm: 0.6 (s, 6H, Si-CH3); 0.6
(t, 2H, Si-CH2); 1.12 (m, CH2); 1.58 (t, 2H, Cq-CH2); 4.11 (s, 2H,
Cp, Si-C-CH); 4.15 (s, 5H, Cp); 4.36 (s, 2H, Si-C-CH-CH); 5.04
(s, 2H, CH2O); 5.28 (m, 1H, vinyl); 5.76 (d, 1H, vinyl); 6.92
(q, 1H, vinyl); 7.16 (d, 2H, arom.); 7.19 (d, 2H, arom.); 7.19
(d, 2H, arom.); 7.42 (s, 4H, arom.).
13C NMR (CDCl3, 75.47 MHz), dppm: 0.00 (Si-CH3); 19.49
(CH2); 20.06 (CH2); 44.13 (CH2); 45.26 (Cq); 70.79–75.63
(Cp); 78.59 (CH2-O); 116 (CH2 vinyl); 128.39 and 129.73
(CH, arom.); 133.51 (Cq arom.); 138.94 (Cq arom.); 153.27
(Cq arom.).
29Si NMR (CDCl3, 59.62 MHz), dppm: ꢁ2.67 (s, Si).
Anal. calc. for (C61H76Fe3OSi3)n: C 68.02, H 7.11; found:
C 67.61, H 7.21.
Synthesis of the tris(chloromethylsilyl) dendron 7. The triallyl
phenol dendron 2 (100 mg, 0.18 mmol) and iodomethylstyrene
(66 mg, 0.271 mmol) were introduced into a Schlenk flask, and
dry DMF (20 mL), then K2CO3 (125 mg, 0.904 mmol) were
added to the solution. The mixture was stirred for 3 days at
ambient temperature. At the end of the reaction, DMF was
removed, and the product was extracted with CH2Cl2, washed
with water, and purified by chromatography (CH2Cl2 100%),
which gave 86 mg of 7 as a yellow oil (70% yield).
1H NMR spectrum of the dendron 7 (CDCl3, 250 MHz):
7.43 (CH arom. core), 7.17 and 6.79 (CH arom. dendron), 6.73
(CHQCH2), 5.74 and 5.24 (CHQCH2), 5.06 (CH2O), 2.74
(CH2Cl), 1.63 (CH2CH2CH2Si), 1.09 (CH2CH2CH2Si), 0.59
(CH2CH2CH2Si), 0.07 (Si(CH3)2).
13C NMR (CDCl3, 62 MHz): 153.35 (Cq-O-CH2), 139.96
(Cq arom. and CHQCH2), 127.86 (CH arom. dendron), 115.13
(CH arom. styrene), 70.26 (O-CH2, arom.), 43.42 (Cq arom.
and CH2CH2CH2Si), 30.85 (CH2Cl), 17.95 (CH2CH2CH2Si),
14.79 (CH2CH2CH2Si), ꢁ4.10 (Si(CH3)2).
Polymerization of the tris(chloromethylsilyl) monomer 7. The
monomeric dendron 7 (140 mg, 0.205 mmol) was dried in a
Schlenk tube. A solution of azobisisobutyronitrile (AIBN) was
prepared with 36 mg of AIBN in 10 mL of distilled THF. 100 mL
of this solution (0.36 mg, 2.05 ꢂ 10ꢁ6 mol, 1% molar) were added
to the monomer. The mixture was stirred for 15 h at 100 1C under
a nitrogen atmosphere. At the end of the reaction, the product
was extracted with CH2Cl2 and purified by precipitation
with methanol. The precipitate was filtered and recovered with
dichloromethane, and reprecipitated three times using methanol,
yielding 8 as a yellow oil (70 mg, 50% yield).
MALDI TOF mass spectrum (m/z): calc.: 1077.05; found:
1044.34 (M ꢁ CH3)+; 1076.38 (M)+.
Polymerization of the dendron 3. Radical polymerization was
carried out using a 0.013 molar THF solution of AIBN11 that
was prepared by introducing 20 mg of AIBN in 10 mL of THF.
The dendron 3 (0.146 g; 1.36 ꢂ 10ꢁ4 mol) was introduced into
a Schlenk flask containing a side entry. The THF solution of
1H NMR of 8 (CDCl3, 250 MHz): 7.17 and 6.89 (CH arom.),
4.87 (CH2O), 2.71 (CH2Cl), 1.93 (CH2I), 1.62 (CH2CH2CH2Si),
1.09 (CH2CH2CH2Si), 0.58 (CH2CH2CH2Si), 0.05 (Si(CH3)2).
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13C NMR of 8 (CDCl3, 62 MHz): 157.00 (Cq-O-CH2), 139.93
(Cq arom.), 134.61 (CHCH2), 127.77 (CH arom.), 70.26
(O-H2 arom.), 46.85 (CqCH2), 42.28 (CH2CH2CH2Si), 30.79
(CH2Cl), 17.92 (CH2CH2CH2Si), 13.93 (CH2CH2CH2Si), ꢁ4.40
(Si(CH3)2). SEC: M = 33.6 kDa g molꢁ1. Polydispersity:
PDI = 1.58.
with distillated water, dried in air, and polished using cerium oxide
powder (5 MU). The dendronized polymer 5 was electro-
deposited onto the platinum-disk electrodes (A = 0.0078 cm2) from
degassed CH2Cl2 solutions (9.3 ꢂ 10ꢁ6 M) and [n-Bu4N][PF6]
(0.1 M) by continuous scanning (0.20 V sꢁ1) up to 50 cycles
between 0.0 and 0.9 V vs. [Fe(Z5-C5Me5)2]+/0. The coated
electrode was washed with CH2Cl2 in order to remove the
solution from material and dried in air. This modified
electrode was characterized by cyclic voltammetry (CV) using
freshly distilled CH2Cl2 as solvent containing only the
supporting electrolyte. It showed a single symmetrical CV
wave, and the linear relationship of the peak current
with potential sweep rate was verified. The surface coverage
G (mol cmꢁ2) by the ferrocenyl dendronized polymer 5
was determined from integrated charge of the CV wave.
G = Q/nFA, where Q is the charge, n is the number of
electrons transferred, F is the Faraday constant, and A is the
area. Thus, the surface coverage for the electrode modified
with 5 was 8.3 ꢂ 10ꢁ9 mol cmꢁ2 (ferrocenyl sites), corres-
ponding to 1.34 ꢂ 10ꢁ10 mol cmꢁ2 of 5. The dendronized
polymer 10 was electrodeposited in the same way as 5, and
the surface coverage G for the electrode modified with 10 was
7. 4 ꢂ 10ꢁ10 mol cmꢁ2 (ferrocenyl sites).
Functionalization of the polymer 8 and ‘‘click’’ reaction. The
polymer 8 (0.06 g, 87.8 mmol) was dissolved in DMF (8 mL),
and an excess of NaN3 (0.068 mg, 1.05 mmol) was added. The
reaction mixture was stirred at 60 1C for 12 h. DMF was
removed, the crude product was dissolved in 10 mL of dichloro-
methane, and the salts were filtered. Dichloromethane was
removed under vacuum, and the dendronized polymer 9 was
obtained as a yellow oil in 85% yield.
1H NMR (CDCl3, 250 MHz): 7.16 and 6.89 (8H arom.),
6.50 (1H, CH arom.), 5.42 (2H, CH2 CH arom.), 4.86
(2H, CH2O), 2.7 (2H, CH2N3), 1.62 (2H, CH2CH2CH2Si),
1.11 (2H, CH2CH2CH2Si), 0.55 (2H, CH2CH2CH2Si), 0.03
(6H, Si(CH3)2).
13C NMR (CDCl3, 62 MHz): 157.04 (Cq-O-CH2), 139.84
(Cq arom.), 134.61 (CHCH2), 127.68 (CH arom.), 70.31 (O-CH2
arom.), 43.47 (CqCH2), 42.27 (CH2CH2CH2Si), 41.40 (CH2N3),
17.93 (CH2CH2CH2Si), 15.26 (CH2CH2CH2Si), ꢁ3.67 (Si(CH3)2).
Infrared nC–N3: 2094 cmꢁ1. SEC: M = 34600 g molꢁ1
.
4. Dynamic light scattering measurements (DLS)
The dendronized polymer 9 (0.028 g, 45.2 mmol, 1 eq.) and
ethynylferrocene (0.019 g, 90.5 mmol, 2 eq. per branch) were
dissolved in THF. CuSO4 was added at 0 1C (4 eq. per branch,
1 M water solution), followed by dropwise addition of a freshly
prepared solution of sodium ascorbate (8 eq. per branch, 1 M
water solution) in order to obtain a ratio of solvent equal to 1 : 1
(THF–water). The solution was stirred for 12 h at 25 1C under
nitrogen. After removing THF under vacuum, CH2Cl2 and an
aqueous ammonia solution were added. The mixture was stirred
for 10 min in order to remove all the CuI trapped inside the
dendrimer as Cu(NH3)6+. The organic phase was washed twice
with water, dried with sodium sulfate and the solvent was removed
under vacuum. The product was precipitated with CH2Cl2–
pentane, and then with CH2Cl2–ether in order to remove excess
ethynylferrocene (yield = 70%).
The DLS measurements were made using a Malvern Zetasizer
3000 HSA instrument at an angle of 901, in dichloromethane
solution at 25 1C. Measurements were carried out at different
concentrations until the hydrodynamic diameter was found
to be constant at three different concentrations (for high
concentration, higher hydrodynamic diameter values were
found, due to aggregation).
5. Atomic force microscopy (AFM) experiments
The AFM samples were prepared by spin coating of a suitable
solution (1 mg mLꢁ1) (adjusted on a trial and error basis) of
dendronized polymer in CH2Cl2. Before spin coating, the
mica surface was cleaved with Scotch tape. The freshly cleaved
highly oriented mica surface was covered with the solution and
spinned at 1000 rpm with subsequent 10–15 second extra spinning
at 3000 rpm for complete drying in air. The AFM apparatus
is a Thermomicroscope CP Research capable of obtaining
measurements in multiple modes, and the sample imaging is
achieved in air immediately after spin coating. The tapping mode
was used giving the weakest interaction with the surface and
therefore the less chance of alteration. The cantilever–tip systems
used were nanosensors (PPPNCL, spring constant = 40 N mꢁ1),
with typical tip (silicon tip) radius of curvature of 6 nm. For
each sample, the images of topography, amplitude and phase
were obtained using the software Image Processing and Data
Analysis 20.0. A sample was considered as good when a
monolayer of ferrocenyl dendronized polymer was measured by
AFM. In order to estimate the size of the polymers, it is necessary
to consider the radius of curvature of the tip only if the measured
nano-object is smaller than the tip. In the present case, the tip
(6 nm) is much smaller than the measured object (22 nm), thus
one does not need to eliminate the tip-shape induced impact by
deconvolution.
1H NMR (CDCl3, 250 MHz): 7.42 (CH triazole), 7.14 and
6.89 (CH arom.), 4.69 (CH, CH2O), 4.69, 4.26, 4.04 (9H, Cp),
3.81 (2H, SiCH2N), 1.58 (2H, CH2CH2CH2Si), 1.11
(2H, CH2CH2CH2Si), 0.58 (2H, CH2CH2CH2Si), 0.06
(6H, Si(CH3)2).
13C NMR (CDCl3, 62 MHz): 157.0 (CqO), 146.1 (Cq of
triazole), 139.9 (Cq arom.), 134.6 (CH arom.), 126.7 (CH2CH
arom.), 120.0 (CH of triazole), 114.5 (CH arom.), 73.4 (Cq of
Cp), 70.9 (CH2O), 69.9, 68.9 and 66.9 (CH of Cp), 41.2
(CH2CH2CH2Si), 40.8 (CH2N), 15.2 (CH2CH2CH2), 14.6
(CH2CH2CH2Si), ꢁ3.5 (SiMe2).
Infrared nC–N3: disappearance of the band at 2094 cmꢁ1
.
DOSY 1H NMR: D = 6.65 (ꢃ0.6) ꢂ 10ꢁ11 m2 sꢁ1; rh = 8.6
(ꢃ0.8) nm; (D: diffusion coefficient; rh: hydrodynamic radius).
Dynamic light scattering: rh = 9.25 (ꢃ0.9) nm.
Derivatization of the Pt electrodes with the ferrocenyl
dendronized polymers 5 and 10. A platinum electrode (Sodimel,
Pt 30) was dipped into 10% aqueous HNO3 for 3 h, then rinsed
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6. Size exclusion chromatography
bulk becomes large around the focal point, which inhibits
further coupling. Therefore, the structure of this dendron was
designed to be located far enough from the bulky termini, and
the number of ferrocenyl termini was kept low at the first
generation although higher generations are known.10 The
radical polymerization, carried out using the standard
procedure,12 yielded an insoluble red powder (25% of the
total mass that could be a high-molecular-weight polymer)
and a polymer soluble in dichloromethane that was reprecipi-
tated three times from dichloromethane solutions using
methanol. This soluble polymer was analyzed using 1H and
13C NMR (see Experimental section) including DOSY NMR,
dynamic light scattering (DLS), size exclusion chromato-
graphy (SEC) and AFM. The DOSY experiment was not
successful, because the polymer was too large, and this
technique does not work with very large nano-objects. DLS
measurements reproducibly provided an apparent diameter
size of 28 ꢃ 2 nm. A large size was expected from the failure of
the DOSY NMR experiment, but this latter value appears to
be very large, presumably because of the solvent sphere
around the polymer and the non-spherical shape of the poly-
mer. Even larger values were observed for the other ferrocenyl
dendronized polymers by Manners et al.3 SEC provided a
molecular weight of 160 kDa with a polydispersity index (PDI)
of 1.9 (Fig. 1).
Size exclusion chromatography (SEC) or gel permeation
chromatography (GPC) was performed using THF as eluant
at 40 1C and a flow rate of 1 mL minꢁ1 through 4 columns
(TSK G5000HXL (9 mm), G4000HXL (6 mm), G3000HXL
(6 mm), and G2000HXL (5 mm)) and connected to Varian
refractometer and UV-visible spectrophotometer calibrated
against linear polystyrene standards.
Results and discussion
Synthesis of the triferrocenyl dendron 4
The new triferrocenyl dendron 4 containing a styrenyl group was
synthesized by Williamson reaction of the triferrocenyl dendron
310b with p-iodomethylstyrene, as shown in Scheme 2. The
precursor triferrocenyl dendron 3 was synthesized according to
a previously reported procedure by hydrosilylation of the
triallylphenol dendron
2
using ferrocenyldimethylsilane11
(Scheme 2). The substantial advantage of the catalyzed hydro-
silylation reaction in the present chemistry is that it is compatible
with the phenol group and thus does not require a tedious
protection–deprotection procedure. Likewise, the ferrocenyl
dendronic branch termini do not prevent the Williamson
reaction to attach the styrenyl group, and this reaction proceeds
smoothly in 67% yield.
Synthesis and polymerization of a tris(chloromethylsilyl)
dendron and ‘‘click’’ ferrocenylation
Polymerization of the dendron 4 to the dendronized polymer 5
An alternative synthesis of a ferrocenyl dendronized polymer
involves the synthesis of a tris(chloromethylsilyl) dendron that
might be functionalized with ferrocenyl termini subsequent to
polymerization. This strategy is a variant of the former one
also starting with the polymerization of a dendron, but the
latter method adds an additional series of two reactions after
polymerization (Scheme 3).
The best working procedure in our hands turned out to be the
AIBN-initiated radical polymerization of the styrenyl deriva-
tive bearing the dendronic group as a para substituent.12
A
possible problem of the dendron polymerization is the bulk
around the reactive focal point, as with any convergent
molecular construction.13 When the generation increases, the
Scheme 2 Synthesis and AIBN-induced radical polymerization of the triferrocenyl styrenyl dendron 4. The triallyl phenol dendron 2 and the
triferrocenyl phenol dendron 3 were synthesized according to ref. 10a,e and 10b, respectively.
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from a dichloromethane solution using methanol, and this same
type of purification was carried out for the ‘‘clicked’’ poly-
ferrocenyl dendronized polymer. Using this method, a ferrocenyl
dendronized polymer of dispersity PDI = 1.2 was obtained,
which is satisfactory given the type of polymerization reaction
that was used (Scheme 4). This good PDI value was obtained,
because the polymer was purified by reprecipitation three times.
The virtually quantitative conversion 8 - 9 - 10 (Scheme 4) is
1
verified by H NMR, because the CH2Cl (and CH2N3) signal
completely disappears upon ‘‘click’’ reaction of 9.
The DOSY 1H NMR experiments provided a mean diameter
of 17.2 nm, whereas the dynamic light scattering (DLS)
experiments yielded a diameter value of 18.5 nm, which is a
reasonably good agreement consistent with the relatively low
PDI value (Table 1).
Fig. 1 Size exclusion chromatogram (SEC) of 5 (left) using polystyrene
as the reference and 1,3,5-trichlorobenzene as a marker (shown on
the right). The SEC was carried out using a refractometer. Dn is
the variation of the refraction index as a function of time (seconds).
Cyclic voltammetry shows the reversible behavior of
the ferrocenyl oxidation wave in both 5 and 10 as with
ferrocenyl-terminated dendrimers.14 It is also a good means
of determining the average number of ferrocenyl units in a
polymer using the Bard–Anson equation.15 This equation only
applies in the absence of adsorption, however, because ad-
sorption increases the peak intensity value, and the number of
redox units would be provided in excess. In dichloromethane,
the ferrocene oxidation wave of 10 at 0.69 V vs. decamethyl-
ferrocene–decamethylferrocenium16 on a Pt anode indeed
appears reversible without adsorption as indicated by a
Ep,oxꢁEp,red value of 50 mV and a ratio between the anodic
and cathodic peak current that is about unity (Fig. 2).
The number of ferrocenyl units calculated for 10 with this
equation yields an average value of 153 ferrocenyl units
(51 triferrocenyl dendrons, corresponding to 67 kDa). This
method could not be applied to the other ferrocenyl dendro-
nized polymer 5, because adsorption was too strong, even in
dichloromethane. This is due to the larger size of ferro-
cenyl dendronized polymer 5 compared to 10. Scanning
around the ferrocenyl potential value easily led to the
formation of fully derivatized Pt electrodes17 for both 5 and
10 (Fig. 3 and S1, ESIw).
Scheme 3 Modification of route B (Scheme 1) with the functionalization
of a dendronized polymer applied to the ferrocenylation reaction by
‘‘click’’ chemistry (Scheme 4).
These reactions are the substitution of the chloro group by the
azido group followed by click reaction with ethynylferrocene.
The initial polychloro polymer 8 was purified by reprecipitation
Scheme 4 Synthesis of a ferrocenyl dendronized polymer by functionalization of a poly(chloromethylsilyl) dendronized polymer.
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Table 1 Compared physico-chemical data for the ferrocenyl dendronized polymers 5 and 10
1
DOSY H NMR
AFM
SEC
PDI
DLS
Diameter/nm
Diffusion coefficient/m2 sꢁ1
Diameter/nm
Height/nm
Width/nm
Fc-dendronized polymer 10
Fc-dendronized polymer 5
1.2
1.9
6.65 (ꢃ0.6) ꢂ 10ꢁ11
17.2 (ꢃ1.7)
18.5 (ꢃ1.8)
28 (ꢃ2.8)
2.4 (ꢃ0.2)
5 (ꢃ0.5)
22.5 (ꢃ2.2)
100 (ꢃ10)
Not applicable
Fig. 2 Cyclic voltammetry of the ferrocenyl dendronized polymer 10
using decamethylferrocene as the internal reference (right wave).
E1/2 = 0.47 V. Solvent: CH2Cl2; temperature: 20 1C; supporting
electrolyte: [n-Bu4N][PF6] 0.1 M; working and counter electrodes:
Fig. 4 AFM image of 5 and 10 on mica surface. (a) Topographic
2D AFM image of 5. (b) Topographic 3D AFM image of 5.
(c) Amplitude mode 2D AFM image of 10. (d) Topographic mode
3D AFM image of 10.
Pt; reference electrode: Ag; scan rate: 0.400 V sꢁ1
.
AFM of the ferrocenyl dendronized polymers
Both ferrocenyl dendronized polymers 5 and 10 were examined
by AFM on mica surfaces.18 The largest dendronized polymer 5
gave pictures containing spots that were typically 5 nm high and
100 nm wide whereas pictures of 10 gave spots that were 22 nm
wide and 2.5 nm high (Fig. 4). The spots of both dendronized
polymers look globular as for classic polymers, i.e. like if
polymers did not aggregate among one another. It does not
appear that the dendronized polymer 10 agglomerates in its
condensed state on the mica surface, because the width of
1
22 nm is close to the height measured by DOSY H NMR and
DLS (in deuterated chloroform and dichloromethane solution,
respectively). On the other hand, the size of the spots obtained
for 10 corresponds to clusters of aggregated dendronized
polymers. One could observe by AFM that elongated shapes
were not found, as expected with dendronized polymers in which
the dendrons are not bulky. Elongated dendrimers are observed
by AFM when dendrons of several generations are constructed,
whereas no dendritic construction was elaborated in the present
study. At some places in 10, however, it can be seen that several
polymeric units seem to stick together to form a chain fragment.
Fig. 3 Cyclic voltammograms of the ferrocenyl dendronized polymer
5 using decamethylferrocene as a reference (a and b): (a) in dichloro-
methane solution; (b) of an electrode modified with 5; (c) of an
electrode modified with 5 at various scan rates.
Concluding remarks
Two closely related routes to ferrocenyl dendronized polymers
were conducted via AIBN-initiated radical polymerization of
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functionalized styrenyl dendrons in this work: a direct poly-
merization of a triferrocenyl dendron and a ferrocenylation of
a dendronized polymer using ‘‘click’’ chemistry.
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radical reactions terminating the chains, thus shortening the
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for the ferrocenylized dendronic polymer (PDI = 1.2), which
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DLS was an excellent technique to evaluate the size of these
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1
10), confirmed by DOSY H NMR in the case of the smaller
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AFM also shows the flattening of these materials on a mica
surface in the condensed state18 (height: 5 nm for 5 and 2.5 nm
for 10). The diameter found is not significantly different for 10
(22.5 nm) from that determined by DLS (18.5 nm), suggesting
that the dendronic polymeric units of 10 do not aggregate on
mica, whereas they do with the larger dendronized polymer 5
(100 nm width spots).
Both ferrocenyl dendronized polymers show a reversible
ferrocene oxidation wave (which indicates that the ferrocenyl
units are located at the periphery of the dendronized polymers)19
and form stable derivatized Pt electrodes. The Bard–Anson
equation can be used for 10 to determine the number of
ferrocenyl groups (153) given the absence of adsorption in
dichloromethane, but not for 5 due to adsorption.
Acknowledgements
Financial assistance from the Institut Universitaire de France
(IUF), the Agence Nationale de la Recherche (ANR) (project
no. ANR-06-NANO-026-02), the Universite Bordeaux 1 and
´
the Centre National de la Recherche Scientifique (CNRS) is
gratefully acknowledged.
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