Dumbbell-shaped dendrimers - Synthesis and self-assembly

Article abstract of DOI:10.1139/V08-027

We report on the synthesis and detailed characterization of dendrimers that evolve symmetrically from a linear core of 2-butyne-1,4-diol with 3,5-dihydroxybenzyl alcohol based dendron arms. The divergent layer-by-layer build-up of the dumbbell-shaped dendrimers is based on simple acid-base hydrolytic chemistry of bis(dimethylamino)dimethylsilane with OH-terminated molecules. The self-assembly of these dendrimers in THF and water is significantly influenced by their generation number, the backbone structure, and the solvent.

Full text of DOI:10.1139/V08-027

540
Dumbbell-shaped dendrimers — Synthesis and
self-assembly1
Tatiana Vassilieff and Ashok Kakkar
Abstract: We report on the synthesis and detailed characterization of dendrimers that evolve symmetrically from a lin-
ear core of 2-butyne-1,4-diol with 3,5-dihydroxybenzyl alcohol based dendron arms. The divergent layer-by-layer build-
up of the dumbbell-shaped dendrimers is based on simple acid–base hydrolytic chemistry of
bis(dimethylamino)dimethylsilane with OH-terminated molecules. The self-assembly of these dendrimers in THF and
water is significantly influenced by their generation number, the backbone structure, and the solvent.
Key words: dendrimers, divergent synthesis, macromolecules, self-assembly
Résumé : On a effectué la synthèse et une caractérisation totale de dendrimères qui radient à partir d’un noyau linéaire
de but-2-yne-1,4-diol avec des ramifications à base d’alcool 3,5-dihydroxybenzylique. La synthèse divergente, faite
couche par couche, des dendrimères en forme d’haltère court est basée sur une chimie d’hydrolyse acide–base simple
de bis(diméthylamino)diméthylsilanes dans lesquels les molécules se terminent par des groupes OH. L’autoassemblage
de ces dendrimères dans un mélange de THF et d’eau est fortement influencé par le nombre de générations, la struc-
ture du squelette, et le solvant.
Mots-clés : dendrimères, synthèse divergente, macromolécules, autoassemblage.
[Traduit par la Rédaction] Vassilieff and Kakkar 547
Introduction
shaped dendrimers. The synthetic elaboration and detailed in-
vestigation of the properties of dumbbell-shaped dendrimers
have been much less explored than those of globular
dendrimers (6). We report herein a route to the synthesis of
dumbbell-shaped dendrimers that are constructed from the
Hyperbranched and monodisperse macromolecules that are
commonly referred to as dendrimers are becoming increas-
ingly important in designing smart and efficient nanomaterials
for a variety of applications in medicine, catalysis, and elec-
tronics (1). A significant effort in the design and syntheses of
these globular architectures has been invested, which has led
to inside-out (divergent) or outside-in (convergent) methodol-
ogies (2). Changing the central core leads to many structural
variations and novel types of morphologies in the resulting
dendrimers (3). We have been investigating the role of the
core, backbone, and linking units in the evolving morphology
of the dendrimers, and have established a simple and efficient
divergent route to the syntheses of such dendritic
macromolecules (4), based on the acid–base hydrolytic chem-
istry of amino-silanes and -stannanes with organic acids (5).
We were intrigued by the possibility of developing
hyperbranched structures using symmetric growth on either
side of a bifunctional linear core, giving rise to dumbbell-
bifunctional
core
2-butyne-1,4-diol
using
3,5-
dihydroxybenzyl alcohol (DHBA) as wedges. The synthetic
elaboration is based on the controlled addition of
bis(dimethylamino)dimethylsilane (Me2Si(NMe2)2) to the
bifunctional core, followed by the addition of DHBA. The
process is then repeated with Me2Si(NMe2)2) and DHBA in
sequence, with amine as the only by-product, and the purifi-
cation of dendrimers can be easily achieved by simple extrac-
tion into deuterium oxide. Self-organization of dendrimers by
means of suitable peripheral groups continues to be an area of
much significance and provides opportunities for designing
nanomaterials for molecular encapsulation and delivery (4, 7,
8). The dumbbell-shaped dendrimers reported here have suit-
ably placed OH groups for interdendrimer interactions, and
we have examined their self-assembly through such interac-
tions in THF and water, using UV–vis spectroscopy and
transmission electron microscopy. In THF, generations 1–3
require a higher critical aggregation concentration (cac) to
form aggregates than does generation 4. Because of the open
structure in the lower generations, the linear core inhibits self-
assembly, whereas in generation 4, the more globular archi-
tecture with more prominent hydrogen bonding sites facili-
tates self-assembly at lower concentrations.
Received 9 November 2007. Accepted 25 January 2008.
Published on NRC Research Press Web site at
canjchem.nrc.ca on 18 April 2008.
Dedicated to Professor Robert H. Marchessault for his
contributions to Science in general and to Macromolecular
Chemistry in particular.
T. Vassilieff and A. Kakkar.2 Department of Chemistry,
McGill University, 801 Sherbrooke St. West, Montreal,
Quebec H3A 2K6, Canada.
Results and discussion
1This article is part of a Special Issue dedicated to Professor
R. Marchessault.
Dendrimers were synthesized using a quantitative acid–
base hydrolytic reaction of aminosilanes with molecules con-
2Corresponding author (e-mail: ashok.kakkar@mcgill.ca).
Can. J. Chem. 86: 540–547 (2008)
doi:10.1139/V08-027
© 2008 NRC Canada

Vassilieff and Kakkar
541
Scheme 1.
taining terminal OH groups (5). The choice of the linear core
around which dendrons are built was based on rigidity, solu-
bility, and commercial availability, and 2- butyne-1,4-diol
was considered to be an ideal candidate for developing the
desired dendrimers. The dendron backbone was chosen to be
3,5-dihydroxybenzyl alcohol because it has been successfully
employed in our laboratory in the past to synthesize globular
dendritic architectures (4). For synthesizing the first genera-
tion dendrimer, 1 molar equivalent of 2-butyne-1,4 diol was
added dropwise over a period of 2 h to an ice-bath-cooled so-
lution of 2 equivalents of bis(dimethylamino)dimethylsilane
in THF. The solution was stirred at ice-bath temperature for
an additional six hours, and then left to warm to room tem-
perature. The solution was transferred to an addition funnel,
and added slowly to a solution of 2 equivalents of DHBA in
THF, and stirred overnight. After removal of the solvent in
vacuo, the product was extracted into deuterium oxide (D2O).
The solution was filtered and D2O was then removed under
vacuum, to afford a light pink powder 1. Continuation of this
methodology by carrying out reactions under controlled con-
ditions led to the synthesis of dendrimers 2–4 in very good
yields. Dendrimers were characterized using a combination of
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Can. J. Chem. Vol. 86, 2008
Fig. 1. (A) UV–vis absorption spectra of dendrimer 1 at concentrations of 1 to 10 mg/mL in THF and (B) plot of absorbance vs con-
centration of dendrimer 1 in THF at 340 nm.
1H and 13C{1H} NMR, matrix-assisted laser desorption/ion-
ization time of flight (MALDI-TOF), and FT-IR
spectroscopies, and elemental analyses. These data confirmed
the identity of the dendrimers as shown in Scheme 1.
provide essential information related to the morphology of
the resulting aggregates (9). We first studied the aggregation
behaviour of dendrimers 1–4 in THF at concentrations of 4
and 8 mg/mL (concentrations below and above cac). At
4 mg/mL, a general globular morphology for the aggregates
(Fig. 2) was observed for dendrimers 1–4. The particle size
distribution in these aggregates showed that, whereas the
size distribution for dendrimer 1 aggregates was found to
range between 30 and 135 nm (Fig. 2A), dumbbell
dendrimers 2–4 formed mostly small aggregates of 5 to
25 nm in size, below cac (Figs. 2B–2D). The largest aggre-
gates for those from generation 4, for example, were 55 nm
in size. This type of behavior has also been observed earlier
(10), and can be explained by considering that, generally,
dendrimers of lower generations have a more open structure.
This facilitates their interaction with other dendrimers.
Self-assembly of dendrimers via intermolecular interac-
tions between peripheral groups is a topic of current interest
(7, 8), and we have also investigated the aggregation behav-
iour of globular DHBA based dendrimers in solution and at
interfaces (4). We were intrigued by the self-assembly be-
haviour of dendrimers in which the linear core as well as the
evolving shape of the dendrimers could influence their asso-
ciation. In addition, dendrimers 1–4 contain peripheral
hydroxyl groups that increase in number exponentially from
4 to 32, and are expected to contribute to enhanced inter-
dendrimer interactions as the generation number increases.
The aggregation behaviour of dendrimers was first investi-
gated using UV–vis spectroscopy at concentrations of 1 to
10 mg/mL in THF. The dendrimers generally depicted three
peaks around 240, 260, and 290 nm at 1 mg/mL (Fig. 1A),
which become broad as the concentrations were increased and
the solutions become turbid. The cac for each generation was
calculated by plotting the intensity of scattered light at a given
wavelength against concentration (Fig. 1B), and it was found
to be 4.9 mg/mL for dendrimers 1–3. The latter is much
higher than the globular DHBA based dendrimers of genera-
tions 1-3, which was calculated to be 3.7 mg/mL (4g). This
suggests that the more open structure of dendrimers, con-
structed around the linear core, allows more freedom for these
molecules. In addition, the central core in these open struc-
tures might participate in packing via π–π interactions, and
these competing interactions might create an association–dis-
sociation equilibrium which will delay aggregation.
The dendrimer 4 showed a cac of 4.1 mg/mL, which was
quite comparable to the DHBA based dendrimers, suggest-
ing clearly that the structure was becoming more globular at
generation 4. The aggregation of dendrimers through partici-
pation of terminal OH groups in hydrogen bonding was also
confirmed by FT-IR that showed a broad peak at approxi-
mately 3300 cm–1 for the terminal OH groups. The latter
clearly suggested that terminal OH groups in these
dendrimers are strongly hydrogen-bonded at concentrations
above cac; below this concentration, a peak at around
3500 cm–1 is commonly observed, which is indicative of the
absence of hydrogen bonds.
When the concentration was increased to 8 mg/mL in
THF, it was observed that the globular shape of dumbbell
dendrimers 1–4 was generally retained. However, there were
quite significant differences in particle size distribution of
the aggregates from those at 4 mg/mL. For example,
dendrimer 1 formed a larger number of smaller aggregates,
in the range of 5 to 15 nm in size (Fig. 3). This can be ex-
plained by considering that at much higher concentrations,
large aggregates can collapse, yielding small-size clusters.
We also studied the aggregation of these dendrimers in a
more polar solvent at a concentration of 4 mg/mL and found
that aggregation behaviour changed significantly with the
change in the solvent from THF to water. The dumbbell-shaped
dendrimers possess a hydrophobic interior with polar OH
groups at the periphery. In lower generations, there may be hy-
drophobic–hydrophilic repulsion between dendrimers that en-
hances their coagulation in water. Water will also compete with
interdendritic hydrogen bonding, and it is expected that much
smaller aggregates will emerge. The transmission electron mi-
croscopy (TEM) study confirmed these expectations, and the
effects were quite evident especially in generations 1 and 2 of
dendrimers 1 and 2, which seem to avoid any contact with wa-
ter, and yield random assemblies of various confirmations
(Fig. 4). In addition, the size of these aggregate assemblies was
much smaller than in THF at 4 mg/mol.
Self-assembly of dendrimers 3 and 4 in water was found
to be quite different from that of first and second generation
dendrimers. It suggested that small aggregates of 5 to 25 nm
are first formed, which then assemble into larger clusters of
55 to 75 nm size (Fig. 4D). This is in agreement with the ob-
Transmission electron microscopy is a useful technique
for examining the self-assembly of macromolecules, and can
© 2008 NRC Canada

Vassilieff and Kakkar
543
Fig. 2. Transmission electron micrographs and particle-size distributions of dendrimers (A) 1, (B) 2, (C) 3, and (D) 4 in THF at 4 mg/mL
servation that generation 3 and 4 dendrimers will start to
adopt a more globular structure, and there will be much less
contact of the hydrophobic interior with hydrophilic exterior
(water), thereby reducing repulsion.
shaped dendrimers of generations 1–4. These dendrimers
evolve from a very open structure in lower generations to a
more globular architecture in higher generations. Molecular
self-assembly of these dendrimers is influenced by their back-
bone structure, as indicated by much higher critical aggrega-
tion concentration for generations 1–3 in THF, compared to
that for globular dendrimer counterparts, and for generation 4.
The influence of the solvent in their self-assembly was exam-
ined using THF and water; in lower generations, the inability
Conclusions
Symmetric growth of DHBA based dendrons on a linear
core has been successfully carried out to construct dumbbell-
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Can. J. Chem. Vol. 86, 2008
Fig. 3. Transmission electron micrographs and particle-size distributions of dendrimers (A) 1, (B) 2, (C) 3, and (D) 4 in THF at 8 mg/mL.
of the more polar solvent to interact with dendrimer back-
bones leads to aggregates that are formed by minimizing the
Experimental
repulsive forces. In higher generations, the major interaction
of the solvent with the dendrimer is at the periphery which
has a higher concentration of polar OH groups, and it does
not interfere significantly with their ability to self-associate.
The influence of the solvent in controlling morphology and
sizes of dendrimer aggregates is of interest, as it allows one to
tailor their properties for molecular encapsulation and release.
Materials and Measurements
2-butyne-1,4-diol (Sigma-Aldrich), 3,5-dihydroxybenzyl al-
cohol (Sigma-Aldrich), and bis(dimethylamino)dimethylsilane
(Gelest) were used as received. All manipulations were per-
formed under a nitrogen atmosphere using either standard
Schlenk line techniques or an Innovative Technology (Braun)
Labmaster MB-150-M dry box. All solvents were stored un-
© 2008 NRC Canada

Vassilieff and Kakkar
545
Fig. 4. Transmission electron micrographs and particle-size distributions of dendrimers (A) 1, (B) 2, (C) 3, and (D) 4 in H2O at 4 mg/mL.
der nitrogen and used after distillation over sodium–benzo-
phenone. NMR spectra were measured on a 270 MHz JEOL
spectrometer at ambient temperature. The samples were pre-
pared inside the dry box using deuterated solvents (Cam-
bridge Isotope laboratories, Inc.), and the chemical shifts in
ppm are reported relative to tetramethylsilane as an internal
ter using LiBr–dithranol as the matrix. Elemental analyses
were performed by Laboratoire d’Analyse Elementaire de
l’Université de Montréal. Infrared spectra were measured on a
Bruker IFS-48 Fourier transform infrared spectrometer using
a standard resolution of 4 cm–1 for transmission. UV–vis spec-
tra were recorded in water on a Hewlett Packard 8453 with a
resolution of 2 nm. TEM measurements were carried out on a
JEOL 2000FX microscope operating at an acceleration volt-
age of 80 kV.
1
standard for H and 13C spectra. Mass spectra were obtained
on a Hewlett Packard 5973 mass spectrometer and MALDI-
TOF spectra on a Kratos Kompact Maldi 3.v.4.0.0 spectrome-
© 2008 NRC Canada

546
Can. J. Chem. Vol. 86, 2008
(1h)
to
a
solution
of
4
equiv.
of
First generation dendrimer (1)
bis(dimethylamino)dimethylsilane (0.170 mL, 0.929 mmol)
in 5 mL of THF, and stirred overnight at room temperature.
The solution was added dropwise (1h) to a solution of 4
equiv. of DHBA (130 mg, 0.929 mmol) in 10 mL of THF,
and after stirring overnight, it was added dropwise (1h) to a
solution of 8 equiv. of bis(dimethylamino)dimethylsilane
(0.341 mL, 1.858 mmol) in 5 mL of THF, and stirred for
14 h. Finally it was added to a solution of 8 equiv. of DHBA
(260 mg, 1.858 mmol) in 15 mL of THF, and stirred over-
night. THF was removed under vacuum to afford a gel. D2O
(4–5 mL) was added to the gel and stirred for 5–10 minutes.
The solution was filtered and the D2O was then removed un-
A solution of 2-butyne-1,4-diol (0.200 g, 2.32 mmol) dis-
solved in 5 mL of dry THF was added dropwise to a solu-
tion of
2 equiv. of bis(dimethylamino)dimethylsilane
(0.852 mL, 4.65 mmol) in 5 mL of dry THF cooled to 0°C.
Stirring at ice bath temperature was maintained for an addi-
tional 6 h, and the resulting solution was subsequently
warmed to room temperature. The solution was then added
dropwise to a solution of 2 equiv. of DHBA (0.651 g,
4.65 mmol) in 10 ml of dry THF. The resulting solution was
stirred overnight, and the THF was then removed under vac-
uum to afford a sticky gel. D2O (2–3 mL) was added to the
gel and was stirred for 5–10 minutes. The solution was fil-
tered and the D2O was then removed under vacuum to afford
1
der vacuum to afford a light pink powder (402 mg, 61%). H
1
a light pink powder (778 mg, 70%). H NMR (270 MHz,
NMR (270 MHz, D2O-d2) δ: 0.01, 0.05 (m, 84H, OSiMe2O),
4.10 (s, 4H, OCH2C), 4.34 (s, 28H, C6H3-CH2O), 6.18 (s,
14H, C6H para), 6.30 (m, 28H, C6H2 ortho). 13C NMR
(68 MHz, D2O-d2, ppm) δ: –1.7, –0.77 (SiCH3), 49.6
(OCH2C), 63.6 (C6H3-CH2), 83.5 (OCH2C), 101.8, 106.3,
143.7, 157.0 (C6H3). FT-IR (KBr, cm–1): 3378 (νOH), 2892
(νC-H arom.), 1154 (δSi-CH3), 1042, 954 (νSi-O). MALDI-TOF
m/z: 2837.9 (expected 2833.9, including m/z Li+). Anal. calcd.
for C130H174O44Si14: C 55.10, H 6.19; found: C 55.12, H 6.38.
D2O-d2, ppm) δ: 0.05 (s, 12H, OSiMe2O), 4.11 (s, 4H,
OCH2C), 4.36 (s, 4H, C6H3-CH2O), 6.18 (s, 2H, C6H2 para),
6.30 (t, 4H, C6H2 ortho).13C NMR (68 MHz, D2O-d2) δ: –
1.7, –0.7 (SiCH3), 49.7 (OCH2C), 63.6 (C6H3-CH2), 83.6
(OCH2C), 101.9, 106.4, 143.8, 157.1 (C6H3). FT-IR
(KBr, cm–1): 3243 (νOH), 2896 (νC-H arom.), 1242 (δSi-CH3),
1041 (νSi-O). EI-MS m/z: 479.1 (expected: 478.6). Anal. calcd.
for C22H30O8Si2: C 55.21, H 6.32; found: C 55.36, H 5.96.
Second generation dendrimer (2)
Fourth generation dendrimer (4)
A solution of 1 equiv. of 2-butyne-1,4-diol (50 mg,
0.581 mmol) in 10 mL of THF was added to a solution of 2
equiv. of bis(dimethylamino)dimethylsilane (0.213 mL,
1.161 mmol) in 5 mL of THF, and stirred for 6 h at ice bath
temperature. The resulting solution was added dropwise over
a period of 1h to 2 equiv. of DHBA (163 mg, 1.161 mmol)
dissolved in 10 mL of THF. After stirring overnight, the
mixture was then added dropwise (1h) to a solution of
4 equiv. of bis(dimethylamino)dimethylsilane (0.426 mL,
2.323 mmol) in 5 mL of THF. The resulting solution was
added dropwise (1h) to 4 equiv. of DHBA (325 mg,
2.323 mmol) dissolved in 15 mL of THF, and stirred over-
night. The removal of the solvent under vacuum afforded a
sticky gel. D2O (4–5 mL) was added to the gel and stirred
for 5–10 minutes. The solution was filtered and the D2O was
then removed under vacuum to afford a light pink powder
A solution of 1 equiv. of 2-butyne-1,4-diol (20 mg,
0.232 mmol) in 10 mL of THF, was added to a solution
2 equiv. of bis(dimethylamino)dimethylsilane (0.085 mL,
0.465 mmol) in 5 ml THF, and stirred for 6 h at ice bath
temperature. The mixture was added dropwise (1h) to a so-
lution of 2 equiv. of DHBA (65.1 mg, 0.465 mmol) in
10 mL of THF, and stirred overnight at room temperature. It
was then added dropwise (1h) to a solution of 4 equiv. of
bis(dimethylamino)dimethylsilane (0.170 mL, 0.929 mmol)
in 5 mL of THF, and stirred overnight at room temperature.
The latter was added dropwise (1h) to a solution of 4 equiv.
of DHBA (130 mg, 0.929 mmol) in 10 mL of THF, and after
stirring overnight, it was added dropwise (1h) to a solution
of 8 equiv. of bis(dimethylamino)dimethylsilane (0.341 mL,
1.858 mmol) in 5 mL of THF, and stirred for 14 h. Then it
was added to a solution of 8 equiv. of DHBA (260 mg,
1.858 mmol) in 15 mL of THF, and stirred overnight. The
resulting solution was added dropwise to 16 equiv. of
bis(dimethylamino)dimethylsilane (0.681 mL, 3.717 mmol)
in 10 mL of THF. Stirring was continued overnight at room
temperature. The resulting solution was then added dropwise
(1h) to a solution of 16 equiv. of DHBA (521 mg,
3.717 mmol) in 10 mL of THF. The reaction mixture was
stirred for 36 h at room temperature. Solvent removal under
vacuum afforded a sticky gel. D2O (4–5 mL) was added to
the gel and was stirred for 5–10 minutes. The solution was
filtered and D2O was then removed under vacuum to afford
1
(455 mg, 62%). H NMR (270 MHz, D2O-d2) δ: 0.05 (m,
36H, OSiMe2O), 4.15 (s, 4H, OCH2C), 4.36 (s, 12H, C6H3-
CH2O), 6.18 (s, 6H, C6H para), 6.30 (m, 12H, C6H2 ortho).
13C NMR (68 MHz, D2O-d2, ppm) δ: –1.7, –0.1 (SiCH3), 50.3
(OCH2C), 64.3 (C6H3-CH2), 83.6 (OCH2C), 102.6, 107.0,
144.5, 157.8 (C6H3). FT-IR (KBr, cm–1): 3378 (νOH), 2962
(νC-H arom.), 1262 (δSi-CH3), 1084, 890 (νSi-O). MALDI-TOF
m/z: 1261.3 (expected 1263.7, including m/z Li+). Anal. calcd.
for C58H78O20Si6: C 55.11, H 6.22; found: C 55.95, H 6.26.
Third generation dendrimer (3)
A similar procedure as described above for the second
generation was used to prepare the third generation. A solu-
tion of 1 equiv. of 2-butyne-1,4-diol (20 mg, 0.232 mmol) in
10 mL of THF was added to a solution 2 equiv. of
bis(dimethylamino)dimethylsilane (0.085 mL, 0.465 mmol)
in 5 ml THF, and stirred for 6 h at ice bath temperature. The
mixture was added dropwise (1h) to a solution of 2 equiv. of
DHBA (65.1 mg, 0.465 mmol) in 10 mL of THF, and stirred
overnight at room temperature. It was then added dropwise
1
a pink powder (833 mg, 60%). H NMR (270MHz, D2O-d2)
δ: –0.06, 0.00, 0.22 (m, 180H, OSiMe2O), 4.10 (s, 4H,
OCH2C), 4.31 (s, 60H, C6H3-CH2O), 6.15 (s, 30H, C6H
para), 6.25 (m, 60H, C6H2 ortho). 13C NMR (68 MHz, D2O-
d2, ppm) δ: –1.54, –1.033, 0.40 (SiCH3), 49.5 (OCH2C), 63.9
(C6H3-CH2), 83.7 (OCH2C), 100.5, 105.9, 143.1, 158.8
(C6H3). FT-IR (KBr, cm–1): 3379 (νOH), 2889 (νC-H arom.),
1263 (δSi-CH3), 1042, 970 (νSi-O). MALDI-TOF m/z: 5978.8
© 2008 NRC Canada

Vassilieff and Kakkar
547
(expected: 5974.3, including m/z Li+). Anal. calcd. for
A.K. Kakkar. Coord. Chem. Rev. 233–234, 223 (2002); (j)
M.B. Peori and A.K. Kakkar. Organometallics, 21, 3860
(2002); (k) M.G.L. Petrucci, V. Guillemette, M. Dasgupta, and
C
274H366O92Si30: C 55.14, H 6.18; found: C 55.26, H 6.31.
A.K. Kakkar. J. Am. Chem. Soc. 121, 1968 (1999).
5. (a) K. Jones and M.F. Lappert. J. Organomet. Chem. 3, 295
Acknowledgments
(1965); (b) K. Jones and M.F. Lappert. J. Chem. Soc. 1944
(1965); (c) A.K. Sawyer. Organotin Compounds, Vol 2. New
York, NY. 1971; (d) A.G. Davies. Organotin Chemistry, 2nd
Ed. Wiley-VCH, Weinheim. 2004.
We would like to thank the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) and the Cen-
ter for Self-Assembled Chemical Structures (FQRNT,
Quebec, Canada) for financial assistance.
6. (a) Z. Ge, D. Chen, J. Zhang, J. Rao, J. Yin, D. Wang, X. Wan,
W. Shi, and S. Liu. J. Polym. Sci. Part A: Polymer Chem. 45,
1432 (2007); (b) B. Huang, A.R. Hirst, D.K. Smith,
V. Castelletto, and I.W. Hamley. J. Am. Chem. Soc. 127, 7130
(2005); (b) X.-D. Tang, Q.-Z. Zhang, A.-X. Li, X.-H. Fan, X.-
F. Chen, and Q.-F. Zhou. Chin. J. Chem. 23, 11 (2005); (c)
M. Lee, Y.-S. Jeong, B.-K. Cho, N.-K. Oh, and W.-C. Zin.
Chem. Eur. J. 8, 876 (2002); (d) G. R. Newkome, X. Lin,
C. Yaxiong, and G.H. Escamilla. J. Org. Chem. 58, 3123 (1993).
7. (a) A.W. Bosman, H.M. Janssen and E.W. Meijer. Chem. Rev.
99, 1665 (1999); (b) M.J. Laufersweiler, J.M. Rhode, J.-
L. Chaumette, D. Sarazin, and J.R. Parquette. J. Org. Chem.
66, 6440 (2001); (c) V. Percec, C.-H. Ahn, T.K. Bera, G.
Ungar, and D.J.P. Yeardley. Chem. Eur. J. 5, 1070 (1999);
(d) R.-M. Sebastian, G. Margo, A.-M. Caminade, and J.-P.
Majoral, Tetrahedron, 56, 6269 (2000); (e) M. Brettreich, S.
Burghardt, C. Bottcher, T.S. Bayerl, and A. Hirsch. Angew.
Chem. Int. Ed. 39, 1845 (2000).
8. (a) K. Tanaka, S. Dai, and T. Kajiyama. Langmuir, 19, 1196
(2003); (b) V. V. Tsukruk, H. Shulha, and X. Zhai. Appl. Phys.
Lett. 82, 907 (2003); (c) M. Sano, J. Okamura, A. Ikeda, and
S. Shinkai. Langmuir, 17, 1807 (2001); (d) A. P. H. J. Schenning,
C. Elissen-Roman, J.-W. Weener, M. W. P. L. Baars, S. J. van
der Gaast, and E. W. Meijer. J. Am. Chem. Soc. 120, 8199
(1998); (e) M. J. Laufersweiler, J. M. Rohde, J. L. Chaumette,
D. Sarazin, and J. R. Parquette, J. Org. Chem. 66, 6440
(2001); (f) W. Zhang and E. E. Simanek. Tetrahedron Lett. 42,
5355 (2001).
References
1. (a) P. Antoni, D. Nystrom, C. J. Hawker, A. Hulta, and M.
Malkoch. Chem. Comm. 2249 (2007); (b) J.K. Vohsa and B.
D. Fahlman. New J. Chem. 31, 1041 (2007); (c) M. Gingras, J-
M. Raimundo and Y. M. Chabre. Angew. Chem. Int. Ed. 46,
1010 (2007); (d) B. Helms and E. W. Meijer. Science, 313,
929 (2006); (e) G. R. Newkome, C. N. Moorefield, and F.
Vögtle. Dendrimers and Dendrons: Concepts, Syntheses and
Applications. Wiley-VCH, Weinheim. 2002; (f) S. M. Grayson,
J. M. J. Fréchet. Chem. Rev. 101, 3819 (2001); (g) D.-L. Jiang
and T. Aida. Nature, 388, 454 (1997); (h) Y. Sayed-Sweet, D.
M. Hedstrand, R. Spinder, and D. A. Tomalia. J. Mater. Chem.
7, 1199 (1997).
2. (a) Dendrimer and Other Dendritic Polymers. Edited by J. M.
J. Fréchet and D. A. Tomalia. John Wiley & Sons, West Sus-
sex, UK. 2001; (b) D. K. Smith. Chem. Commun. (Cambridge,
UK), 34 (2006); (c) D. Astruc and F. Chardac. Chem. Rev.
101, 2991 (2001); (d) Y. Ishida, M. Jikei, and M. Kakimoto.
Macromolecules, 33, 3202 (2000).
3. I. Washio, Y. Shibasaki, U. Mitsuru. Macromolecules, 38,
2237 (2005).
4. (a) A. A. Czerniewski, X. J. Liu, O. Rolland, R. Hourani, M.
Four, and A.K. Kakkar. J. Mater. Chem. 17, 2737 (2007);
(b) R. Hourani, A.K. Kakkar, and M.A. Whitehead. J.
Mol.Struct: THEOCHEM, 807, 101 (2007); (c) T. Vassilieff, F.
Quist, and A.K. Kakkar. J. Colloid Interface Sci. 297, 455
(2006); (d) R. Hourani, A.K. Kakkar, and M.A. Whitehead. J.
Mater. Chem. 15, 2106 (2005); (e) O. Bourrier, J. Butlin,
R. Hourani, and A.K. Kakkar. Inorg. Chim. Acta, 357, 3836
(2004); (f) O. Bourrier and A.K. Kakkar. Macromolec. Symp.
209, 97 (2004); (g) O. Bourrier and A.K. Kakkar. J. Mater.
Chem. 13, 1306 (2003); (h) A.K. Kakkar. Macromolec. Symp.
196, 145 (2003); (i) M. Dasgupta, M.B. Peori, and
9. J. C. H. Spence. Mater. Sci. Eng. R. 26, 1 (1999).
10. (a) K. Esumi, A. Suzuki, A. Yamahira, and K. Torigoe.
Langmuir, 16, 2604 (2000); (b) K. Esumi and K. Torigoe.
Prog. Colloid Polym. Sci. 80, 117 (2001); (c) C.Y. Bao,
M. Jin, R. Lu, T.R. Zhang, and Y.Y. Zhao. Mater. Chem. Phys.
81, 160 (2003); (d) F. Tarazona-Vasquez and P.B. Balbuena. J.
Phys. Chem. B, 108, 1599 (2004); D.-M. Xu, K.-D. Zhang,
and C.-L. Zhu. J. Appl. Polym. Sci. 104, 422 (2007).
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