Interactions of carbon nanotubes with pyrene-functionalized linear-dendritic hybrid polymers

Article abstract of DOI:10.1002/pola.23855

A series of linear-dendritic hybrid polymers, containing pyrene units at the periphery of aliphatic polyester dendrons, were prepared for the purpose of dispersing shortened single-walled carbon nanotubes (SWNTs) in tetrahydro-furan (THF). The prepared hybrids contained 1, 2, 4, 8, or 16 (G0 through G4) pyrene units and a linear segment composed of polystyrene. It was found that a minimum of four pyrene units was necessary to form a strong enough interaction with SWNTs to enable steric stabilization in solution, when using a linear polymer segment of 11.5 kDa. Increasing either the number of pyrene units per polymer chain or the length of the polymer segment to 18.0 kDa did not improve nanotube solubility, whereas decreasing the polymer length resulted in significantly less effective nanotube dissolution. The G4 dendron alone, without the linear polystyrene segment, was also found to Impart solubility to the nanotubes in THF. Interactions between the series of linear-dendritic hybrids and full-length multiwalled carbon nanotubes were also investigated, and it was found that the polymers exhibited strong interactions with the multiwalled carbon nanotube surface, resulting in the formation of stable solutions.

Full text of DOI:10.1002/pola.23855

Interactions of Carbon Nanotubes with Pyrene-Functionalized
Linear-Dendritic Hybrid Polymers
GREG J. BAHUN, ALEX ADRONOV
Department of Chemistry and the Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada
Received 11 October 2009; accepted 22 November 2009
DOI: 10.1002/pola.23855
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A series of linear-dendritic hybrid polymers, con-
taining pyrene units at the periphery of aliphatic polyester
dendrons, were prepared for the purpose of dispersing short-
ened single-walled carbon nanotubes (SWNTs) in tetrahydro-
furan (THF). The prepared hybrids contained 1, 2, 4, 8, or 16
(G0 through G4) pyrene units and a linear segment com-
posed of polystyrene. It was found that a minimum of four
pyrene units was necessary to form a strong enough interac-
tion with SWNTs to enable steric stabilization in solution,
when using a linear polymer segment of 11.5 kDa. Increasing
either the number of pyrene units per polymer chain or the
length of the polymer segment to 18.0 kDa did not improve
nanotube solubility, whereas decreasing the polymer length
resulted in significantly less effective nanotube dissolution.
The G4 dendron alone, without the linear polystyrene seg-
ment, was also found to impart solubility to the nanotubes in
THF. Interactions between the series of linear-dendritic
hybrids and full-length multiwalled carbon nanotubes were
also investigated, and it was found that the polymers exhi-
bited strong interactions with the multiwalled carbon nano-
tube surface, resulting in the formation of stable solutions.
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2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 48: 1016–1028, 2010
KEYWORDS: block copolymers; dendrimers; nanocomposites;
supramolecular structures; synthesis
INTRODUCTION Carbon nanotubes (CNTs) have received a
great deal of attention in recent years as a result of their
extraordinary mechanical strength and electronic properties.1
These unique characteristics translate into potential applica-
tions within a variety of high-strength materials and elec-
tronic devices, including sensors, light-emitting diodes, and
photovoltaics.2 In many of these applications, the nanotubes
are incorporated within other pre-existing materials to pro-
duce functional composites. However, for optimal perform-
ance, a uniform distribution of the CNTs within the host ma-
terial is preferred and often essential.3 Given that CNTs form
bundles and are inherently insoluble in aqueous and com-
mon organic solvents, they must be exfoliated into individual
tubes if uniform mixing with other materials is to be
achieved. It is therefore necessary to chemically functionalize
CNTs with solubilizing groups, which enable exfoliation and
homogenous dispersion within solvents and various host
materials.
nitrenes,32–34 and carbenes,34,35 as well as dipolar cycloaddi-
tion chemistry,20,31,36–44 have been developed and enable
attachment of various structures to the nanotube walls. How-
ever, such covalent functionalization chemistry effectively
introduces defects along the nanotube walls, leading to a
decrease in the strength and conductivity of the tubes.45,46
An alternative approach that does not cause any change to
the chemical structure of the nanotubes involves supramo-
lecular functionalization, and has received increasing recent
attention.14,15,27,29,47–67
Both small molecules and macromolecules have been used
to disrupt nanotube bundles and provide individual
nanotubes in solution, strictly by supramolecular interac-
tions.68–70 The utilization of small molecules to noncova-
lently functionalize nanotubes has largely focused on deriva-
tives of polycyclic aromatic hydrocarbons (PAH’s), such as
naphthalene, phenanthrene, and pyrene.69–75 Pyrene can p-
stack to CNTs with a binding energy of up to 50 kJ/mol.76 In
addition to small molecules, it has also been shown that
introduction of PAH’s as side-chains on polymers enables
supramolecular grafting of the resulting conjugates to the
nanotube surface.77–85 Je´roˆme and coworkers have shown
that PMMA, functionalized with pyrene units as side-chains
randomly along the polymer backbone, could stabilize dis-
persions of multiwalled carbon nanotubes (MWNTs) in
Many methods have been developed to functionalize
CNTs.1,4–13 Much of the early work involved treatment with
harsh oxidizing agents to provide carboxylic acid functional-
ities at the nanotube ends and defect sites, enabling further
derivatization through esterification and amidation chemis-
try.1 Additionally, a number of other sidewall functionaliza-
tion methods, including reactions with radicals,1,6,14–31
Correspondence to: A. Adronov (E-mail: adronov@mcmaster.ca)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1016–1028 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
toluene, tetrahydrofuran (THF), and chloroform.77,78 Increas-
ing the concentration of pyrene on the polymers improved
the solution-phase stability of the resulting materials. Our
own work has investigated the role of polymer architecture
and composition with linear pyrene-functionalized polysty-
rene polymers.79 In our previous study, block copolymers,
featuring a pyrene block, were shown to be superior in dis-
persing single-walled carbon nanotubes (SWNTs) when
compared to polymers containing an equal amount of pyrene
randomly distributed throughout the backbone. It was also
shown that increasing the length of the pyrene-containing
block decreased the solubility of the polymer–nanotube
complexes.79
(SEC) using a Waters 2695 separations module equipped
with a Waters 2414 refractive index detector and three Poly-
mer Labs PLgel individual pore size columns, with 5 lm
bead size and pore sizes of 100, 103, and 105 Å, at a con-
ꢀ
stant temperature of 40 C. Polystyrene standards were used
for calibration, and THF was used as the mobile phase at a
flow rate of 1.0 mL/min. Raman spectroscopy was per-
formed on a Renishaw InVia Raman spectrometer equipped
with a 25 mW argon ion laser (514 nm), a 300 mW
Renishaw 785 nm laser, 1800 l/mm and 1200 l/mm gratings
for the two lasers, respectively, and a high-resolution map-
ping stage. The Raman system is also equipped with a Leica
microscope having 5ꢁ, 20ꢁ, and 50ꢁ objectives as well as a
USB camera for sample viewing. UV–Vis spectroscopy was
performed using a Varian Cary 50 spectrophotometer. Fluo-
rescence spectra were measured using a Jobin-Yvon SPEX
Fluorolog 3.22 equipped with a 450 W Xe lamp, double-exci-
tation and double-emission monochromators, and a digital
photon-counting photomultiplier. Slit widths were set to 0.9
nm bandpass on both excitation and emission. Correction for
variations in lamp intensity over time and k was achieved
using a reference silicon photodiode.
Despite these initial encouraging results, very little has been
reported about the effect of nonlinear polymer architectures
on the stability of supramolecular polymer–nanotube com-
plexes. Although branched macromolecules, such as den-
drimers and hyperbranched polymers, have been used to co-
valently functionalize CNTs, their effectiveness in
supramolecular functionalization has received comparatively
little attention.71 Fewer still are the studies that have relied
on p-stacking from the periphery of a dendron to increase
the interactions with nanotubes in nonaqueous solvents.86
We hypothesized that introduction of PAH’s, such as pyrene,
at the periphery of a dendrimer would not only provide a
compact, globular architecture that may preorganize the
peripheral pyrene groups around the nanotube, but would
also produce structures in which the number of binding
groups available in each macromolecule is precisely known.
Here, we report the preparation of pyrene-functionalized lin-
ear-dendritic polymer hybrids and their ability to form non-
covalent complexes with SWNTs that are soluble in organic
solvents.
Synthesis of TSe-G1-Py2 (11a) (General Procedure for
Coupling Pyrene Butyric Acid to Peripheral Alcohols)
TSe-G1-OH2 (4) (0.148 g, 0.468 mmol, 1 equiv) was added
to a flame-dried flask equipped with a magnetic stir bar
along with pyrene butyric acid (0.394 g, 1.3 mmol, 2.9
equiv), EDC (0.263 g, 1.4 mmol, 2.9 equiv), and DMAP
(0.026 g, 0.2 mmol, 0.8 equiv). Dry DCM was added (4 mL)
and the mixture was stirred overnight. The solvent was
evaporated in vacuo, and column chromatography on silica
gel was used to purify the product (eluent: 100% DCM, then
98:2 DCM:EtOAc). Yield: 0.32 g (80%).
1H NMR (600 MHz, CDCl3): d ¼ 1.14 (s, 3 H), 2.14 (quintet, J
¼ 7.4 Hz, 4 H), 2.32 (s, 3 H), 2.43 (t, J ¼ 7.2 Hz, 4 H), 3.33
(m, 6 H), 4.17 (m, 4 H), 4.40 (t, J ¼ 6 Hz, 2 H), 7.22 (d, J ¼
7.9 Hz, 2 H), 7.70 (d, J ¼ 8.1 Hz, 2 H), 7.79–8.25 (m, 18 H).
13C NMR (150 MHz, CDCl3): d ¼ 17.7, 21.6, 26.7, 32.8, 33.7,
46.4, 55.0, 58.3, 65.2, 123.3, 124.9, 125.0, 125.2, 126.0,
126.8, 127.4, 127.6, 128.1, 128.8, 130.1, 131.0, 131.5, 135.6,
136.2, 145.2, 172.4, and 173.0. MS Calc’d for C54H48O8S [M]þ
m/z ¼ 856.31, [M þ NH4]þ m/z ¼ 874.34. Found ES MS: [M
EXPERIMENTAL
Materials
SWNTs were purchased from Carbon Nanotechnologies
(Houston, TX) and shortened according to previously pub-
lished procedures.23 MWNTs (purchased from Nanocyl, Bel-
gium, Nanocyl-7000, 90% purity, Lot 257-1) were used as
received. Benzylidene anhydride (2) was synthesized accord-
ing to literature procedures.87 Compounds 3–10 were syn-
thesized according to previously published procedures.88 All
other reagents were obtained from Acros or Aldrich and
used without further purification. Column chromatography
was carried out with Silica Gel 60 (230–400 mesh, ASTM).
þ NH4]þ ¼ 874.4, high-resolution ES MS: [M þ NH4]þ
¼
874.3447.
Synthesis of HOOC-G1-Py2 (11b) (General Procedure
Removal of TSe Group)
Techniques
NMR spectra were measured on Bruker DRX 500 MHz and
The TSe-G1-Py2 (11a) (0.428 g, 0.5 mmol, 1 equiv) was
added to a round-bottom flask equipped with a stir bar. This
was dissolved in toluene (5 mL), before adding DBU to the
mixture (0.38 g, 2.5 mmol, 5 equiv). The reaction was moni-
tored by TLC (EtOAc:hexanes 2:1). Upon consumption of
starting material (Rf ¼ 0.75), ethyl acetate (10 mL) was
added and the solution was washed with 20% NaHSO4 (3 ꢁ
25 mL). The organic layer was concentrated down, and pre-
cipitated from an ethyl acetate solution into hexanes. The
solid was filtered and dried to give 0.30 g of product (88%).
1
Avance 600 MHz spectrometers. H spectra were recorded at
500 or 600 MHz and 13C NMR spectra were recorded at 125
or 150 MHz in CDCl3. The nondeuterated solvent signal was
used as the internal standard for both 1H and 13C spectra.
MALDI-TOF mass spectrometry was performed using
a
Micromass TofSpec 2E spectrometer in positive ion mode
using 2-(4-hydroxyphenyl-azo) benzoic acid (HABA) as the
matrix. Polymer molecular weight and polydispersity index
(PDI) were estimated by size exclusion chromatography
NANOTUBE-POLYMER HYBRID INTERACTIONS, BAHUN AND ADRONOV
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1H NMR (200 MHz, d6-DMSO): d ¼ 1.14 (s, 3 H), 1.96 (m, 4
H), 2.43 (t, J ¼ 7.1 Hz, 4 H), 3.25 (t, J ¼ 7.5 Hz, 4 H), 4.21
(m, 4 H), and 7.77–8.35 (m, 18 H). 13C NMR (125 MHz, d6-
DMSO): d ¼ 18.0, 27.1, 32.3, 33.6, 45.9, 65.6, 123.8, 124.6,
124.7, 125.2, 125.3, 126.5, 127.0, 127.7, 127.8, 128.6, 129.8,
130.8, 131.3, 136.5, 172.9, and 174.6. MS Calc’d for
17 equiv), EDC (2.224 g, 11.6 mmol, 19.2 equiv), and DMAP
(0.052 g, 0.17 mmol, 0.3 equiv) were added to a flame-dried
flask equipped with a magnetic stir bar. Dry DCM was added
(10 mL) and the mixture was stirred overnight. The solvent
was evaporated in vacuo, and the product was purified
by column chromatography on silica gel (5:0.6:0.4, DCM:
hexanes:EtOAc). Yield: 1.55 g (82%).
C
45H38O6 [M]þ m/z ¼ 674.78, [M þ NH4]þ m/z ¼ 692.30.
Found ES MS: [M þ NH4]þ ¼ 692.2, high-resolution ES MS:
1H NMR (500 MHz, CDCl3): d ¼ 1.15 (s, 3 H), 1.17 (s, 6 H),
1.19 (s, 12 H), 2.08 (quintet, J ¼ 7.5 Hz, 16 H), 2.18 (s, 3 H),
2.40 (t, J ¼ 7.2 Hz, 16 H), 3.24 (t, J ¼ 7.8 Hz, 18 H), 4.1–4.3
(m, 28 H), 4.34 (t, J ¼ 6.2 Hz, 2 H), 7.05 (d, J ¼ 8.7 Hz, 2 H),
7.60 (d, J ¼ 8.5 Hz, 2 H), 7.65–8.20 (m, 72 H). 13C NMR
(150 MHz, CDCl3): d ¼ 17.3, 17.6, 17.9, 21.5, 26.7, 32.7, 33.6,
46.6, 46.6, 46.8, 54.7, 58.3, 65.3, 65.4, 66.2, 123.3, 124.8,
124.9, 124.9, 125.0, 125.1, 125.9, 126.7, 127.3, 127.4, 127.5,
127.9, 128.7, 130.0, 130.9, 131.5, 135.6, 136.4, 145.0, 171.2,
171.6, 172.2, and 173.0. MS Calc’d for C204H180O32S [M]þ
m/z ¼ 3173.2, [M þ Na]þ m/z ¼ 3196.2. Found MALDI-TOF
MS: [M þ Na]þ m/z ¼ 3198.7.
[M þ NH4]þ ¼ 692.3000.
Synthesis of TSe-G2-Py4 (12a)
The general procedure for coupling pyrene butyric acid to
peripheral alcohols was followed. TSe-G2-OH4 (6) (0.148 g,
0.27 mmol, 1 equiv) pyrene butyric acid (0.394 g, 1.3 mmol,
5 equiv), EDC (0.263 g, 1.4 mmol, 5.1 equiv), and DMAP
(0.026 g, 0.2 mmol, 0.8 equiv) were added to a flame-dried
flask equipped with a magnetic stir bar. Dry DCM was added
(4 mL) and the mixture was stirred overnight. The solvent
was removed, and column chromatography on silica gel was
used to purify the product (100% DCM, then 98:2 DCM:E-
tOAc). Yield: 0.27 g (62%).
Synthesis of HOOC-G3-Py8 (13b)
1H NMR (500 MHz, CDCl3): d ¼ 1.04 (s, 3 H), 1.18 (s, 6H),
2.11 (q, J ¼ 7.5 Hz, 8 H), 2.23 (s, 3 H), 2.41 (t, J ¼ 7.2 Hz, 8
H), 3.25–3.31 (m, 10 H), 4.11 (broad s, 4 H), 4.21 (m, 8 H),
4.33 (t, J ¼ 5.8 Hz, 2 H), 7.14 (d, J ¼ 8.1 Hz, 2 H), 7.65 (d, J
¼ 8.2 Hz, 2 H), and 7.74–8.28 (m, 36 H). 13C NMR (150
MHz, CDCl3): d ¼ 14.3, 17.3, 17.9, 21.5, 26.7, 32.7, 33.6, 46.5,
54.8, 58.3, 60.5, 65.3, 65.5, 77.2, 123.3, 124.9, 124.9, 125.0,
125.1, 125.9, 126.8, 127.3, 127.5, 127.5, 128.0, 128.8, 130.0,
130.9, 131.5, 135.6, 172.1, and 173.0. MS Calc’d for
The general procedure for TSe removal was followed. TSe-
G3-Py8 (13a) (0.160 g, 0.1 mmol, 1 equiv) was combined
with DBU (0.030 g, 0.2 mmol, 2 equiv) in a round-bottom
flask equipped with a magnetic stir bar. Toluene was added
(1.5 mL) and the reaction was stirred at room temperature.
Upon consumption of starting material, CH2Cl2 (10 mL) was
added and the solution was washed with 20% NaHSO4 (3 ꢁ
25 mL). The organic layer was concentrated down, and pre-
cipitated from an ethyl acetate solution into hexanes. Yield:
0.124 g (82%).
C
104H92O16S [M]þ m/z ¼ 1629.90. Found MALDI-TOF MS: [M
þ Na]þ m/z ¼ 1629.
1H NMR (600 MHz, CDCl3): d ¼ 1.09 (s, 6 H), 1.14 (s, 12 H),
1.22 (s, 3 H), 2.05 (quintet, J ¼ 7.3 Hz, 16 H), 2.36 (broad m,
16 H), 3.21 (t, J ¼ 7.7 Hz, 16 H), 4.1–4.3 (m, 28 H), 7.6–8.2
(m, 72 H). 13C NMR (150 MHz, CDCl3): d ¼ 17.4, 17.6, 17.9,
26.7, 31.0, 32.5, 32.7, 33.6, 46.5, 46.8, 65.2, 65.4, 68.5, 123.3,
124.8, 124.9, 125.0, 125.0, 125.1, 125.9, 126.7, 127.3, 127.5,
127.5, 128.7, 130.0, 130.9, 131.4, 135.6, 172.2, 172.2, 172.4,
and 173.0. MS Calc’d for C195H170O30 [M]þ m/z ¼ 2991.2,
[M þ Na]þ m/z ¼ 3014.2. Found MALDI-TOF MS: [M þ Na]þ
m/z ¼ 3017.9.
Synthesis of HOOC-G2-Py4 (12b)
The general procedure for TSe removal was followed. TSe-
G2-Py4 (12a) (0.160 g, 0.1 mmol, 1 equiv) was combined
with DBU (0.030 g, 0.2 mmol, 2 equiv) in a round-bottom
flask equipped with a magnetic stir bar. Toluene was added
(1.5 mL) and the reaction was stirred at room temperature
for 1 h. Upon consumption of starting material (Rf ¼ 0.75),
ethyl acetate (10 mL) was added and the solution was
washed with 20% NaHSO4 (3 ꢁ 25 mL). The organic layer
was concentrated down, and precipitated from an ethyl ace-
tate solution into hexanes. Yield: 0.12 g (87%).
Synthesis of TSe-G4-Py16 (14a)
1H NMR (600 MHz, CDCl3): d ¼ 1.14 (s, 3 H), 1.18 (s, 6 H),
2.08 (quintet, J ¼ 7.7 Hz, 8 H), 2.39 (t, J ¼ 7.2 Hz, 8 H), 3.24
(t, J ¼ 7.7 Hz, 8 H), 4.18–4.26 (m, 12 H), 7.70–8.18 (m, 36
H). 13C NMR (150 MHz, CDCl3): d ¼ 17.5, 17.9, 26.7, 32.7,
33.6, 46.4, 46.5, 65.4, 65.7, 123.3, 124.9, 125.0, 125.1, 125.4,
125.9, 126.8, 127.3, 127.5, 127.8, 128.1, 128.7, 130.0, 130.9,
131.4, 135.5, 172.1, 173.1, and 175.5. MS Calc’d for
The general procedure for coupling pyrene butyric acid to
peripheral alcohols was followed. 1-Pyrenebutyric acid
(0.249 g, 0.86 mmol, 24 equiv), EDC, (0.172 g, 0.9 mmol, 25
equiv), DPTS (0.001 g, 3.6 ꢁ 10ꢂ6 mol, 0.1 equiv) and TSe-
G4-OH16 (10) (0.070 g, 3.6 ꢁ 10ꢂ5 mol) were added to 0.7
mL of dichloromethane in a round-bottom flask equipped
with a magnetic stir bar. The mixture was stirred for 3 days
after which MALDI indicated the reaction had not reached
complete conversion. Additional pyrene butyric acid (0.125
g, 0.4 mmol, 12 equiv) and EDC (0.090 g, 0.47 mmol, 13
equiv) were added and the reaction was stirred for further 2
days. At this time, the MALDI showed only the final product.
The product was purified using column chromatography
(4.0:0.5:0.5 DCM:hexanes:EtOAc as the eluent) and dried
C
95H82O14 [M]þ m/z ¼ 1447.66, [M þ Na]þ ¼ 1469.56.
Found MALDI-TOF MS: [M þ Na]þ m/z ¼ 1472.
Synthesis of TSe-G3-Py8 (13a)
The general procedure for coupling pyrene butyric acid to
peripheral alcohols was followed. TSe-G3-OH8 (8) (0.614 g,
0.6 mmol, 1 equiv), pyrene butyric acid (3.043 g, 10.6 mmol,
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ARTICLE
overnight in vacuo to yield 0.165 g (75%) of a white
powder.
Synthesis of PS-G1-Py2 (11c)
11.5 kDa Polystyrene (0.200 g, 0.017 mmol, 1 equiv) was
added to a round-bottom flask with KI (2 mg, 0.012 mmol,
0.7 equiv) and K2CO3 (2.7 mg, 0.02 mmol, 1.1 equiv). This
was dissolved in 0.5 mL of DMF, followed by addition of the
G1 acid (11b) (0.042 g, 0.06 mmol, 3.6 equiv), and the reac-
tion mixture was stirred at 40 ꢀC. The reaction was moni-
tored by TLC (3:1 DCM:hexanes) and after 2 days, the mix-
ture was diluted in DCM (5 mL), washed with 20% NaHSO4
(4 ꢁ 10 mL), and dried. The product was purified by column
chromatography (3:1 DCM:hexanes, 100% DCM) and precipi-
tated into hexanes to give 0.174 g of white solid (82%). SEC:
Mw ¼ 12,100 g/mol, Mw/Mn ¼ 1.07.
1H NMR (500 MHz, CDCl3): d ¼ 1.15 (broad m, 45H), 1.99
(quintet, J ¼ 6.8 Hz, 32 H), 2.31 (t, J ¼ 7.1 Hz, 35 H), 3.13 (t,
J ¼ 7.7 Hz, 34 H), 4.09–4.28 (broad m, 60 H), 4.31 (t, J ¼
5.4 Hz, 2 H), 6.88 (d, J ¼ 8 Hz, 2 H), 7.48 (d, J ¼ 8 Hz, 2 H),
and 7.55–8.08 (m, 144 H). 13C NMR (125 MHz, CDCl3): d ¼
17.2, 17.5, 17.6, 17.9, 26.6, 32.6, 33.6, 46.5, 46.7, 65.1, 65.2,
65.7, 66.6, 123.2, 124.8, 124.9, 125.0, 125.8, 126.7, 127.2,
127.4, 127.4, 127.8, 128.6, 129.9, 130.8, 131.4, 135.5, 171.5,
171.6, 171.7, 172.1, and 172.9. MS Calc’d for C404H356O64
S
[M]þ m/z ¼ 6262.4, [M þ K]þ m/z ¼ 6301.4. Found MALDI-
TOF MS: [M þ K]þ m/z ¼ 6318.0.
1H NMR (500 MHz, CDCl3): d ¼ 1.22–2.30 (broad m), 2.34–
2.44 (broad m, 4 H), 3.31 (broad, 4 H), 4.25–4.39 (broad m,
4 H), 5.00–5.13 (broad m, 2 H), 6.30–7.25 (broad m), 7.76–
8.26 (m, 18 H). 13C NMR (125 MHz, CDCl3): d ¼ 11.6, 14.2,
18.9, 20.9, 22.8, 25.5, 29.2, 31.7, 32.8, 34.7, 34.8, 40.6, 40.9,
44.1, 44.4, 65.6, 124.9–128.9, 145.3, 145.4, 145.5, 145.6,
145.8, and 146.0.
Synthesis of HOOC-G4-Py16 (14b)
The general procedure for TSe removal was followed. The
TSe-G4-Py16 (14a) (0.165 g, 2.6 ꢁ 10ꢂ5 mol, 1 equiv) was
dissolved in toluene (0.4 mL) and upon addition of DBU
(0.022 g, 0.1 mmol, 5 equiv) was stirred at room tempera-
ture. The mixture was stirred overnight, after which no start-
ing material could be observed by TLC. The mixture was
diluted with 5 mL of CH2Cl2, and then washed with 20%
NaHSO4 (3 ꢁ 10 mL) followed by brine (1 ꢁ 10 mL) before
being dried with Na2SO4. Filtering off the drying agent fol-
lowed by precipitation into hexanes yielded 0.145 g of the
acid (91%).
Synthesis of PS-G2-Py4 (12c)
11.5 kDa Polystyrene (0.200 g, 0.017 mmol, 1 equiv) was
added to a round-bottom flask with KI (2 mg, 0.012 mmol,
0.7 equiv) and K2CO3 (2.7 mg, 0.02 mmol, 1.1 equiv). This
was dissolved in 0.5 mL of DMF, followed by addition of the
G2 acid (12b) (0.145 g, 0.1 mmol, 5.8 equiv), and the reac-
tion mixture was stirred at 40 ꢀC. The reaction was moni-
tored by TLC (3:1 DCM:hexanes). After 3 days, the mixture
was diluted in DCM (5 mL), washed with 20% NaHSO4 (4 ꢁ
10 mL), and dried. The product was separated by column
chromatography (3:1 DCM:hexanes, 100% DCM), and pre-
cipitated into hexanes to yield 0.163 g of white powder.
1H NMR (500 MHz, CDCl3): d ¼ 1.12 (broad s, 42H), 1.20 (s,
3 H), 2.01 (quintet, J ¼ 7.5 Hz, 32 H), 2.32 (t, J ¼ 7.1 Hz, 32
H), 3.15 (t, J ¼ 7.8 Hz, 32 H), 4.10–4.28 (broad m, 60 H),
and 7.55–8.10 (m, 144 H). 13C NMR (125 MHz, CDCl3): d ¼
17.4, 17.5, 17.8, 26.5, 32.5, 33.5, 46.4, 46.7, 65.1, 65.3, 123.1,
124.7, 124.8, 124.9, 125.0, 125.7, 126.6, 127.1, 127.3, 127.4,
128.6, 129.9, 130.8, 131.3, 135.4, 171.4, 172.1, 172.2, and
172.9. MS Calc’d for C395H346O62 [M]þ m/z ¼ 6080.4, [M þ
K]þ m/z ¼ 6119.4. Found MALDI-TOF MS: [M þ K]þ m/z ¼
6139.1.
Yield: 0.163 g (72%). SEC: Mw ¼ 13,000 g/mol, Mw/Mn
¼
1.06.
1H NMR (600 MHz, CDCl3): d ¼ 1.35–2.35, 2.36–2.41 (t, J ¼
7.2 Hz, 8 H), 3.23–3.29 (t, J ¼ 7.6 Hz, 8 H), 4.14–4.25 (m, 12
H), 4.93–5.03 (broad m, 2 H), 6.28–7.25 (broad m), and 7.5–
8.3 (m, 36 H). 13C NMR (150 MHz, CDCl3): d ¼ 11.6, 14.3,
17.7, 17.9, 18.9, 20.9, 21.3, 22.4, 22.8, 22.8, 25.4, 26.7, 28.4,
28.6, 29.2, 29.5, 29.8, 31.7, 32.1, 32.7, 33.6, 33.9, 34.7, 34.8,
36.2, 40.4–40.8, 41.9, 42.5–42.9, 43.6, 44.0, 44.3, 45.0, 45.1,
45.6–46.3, 46.5, 46.6, 46.7, 65.3, 65.7, 123.3, 124.9, 124.9,
125.0, 125.1, 125.2, 125.6, 125.8, 125.9, 126.8, 127.3–128.4,
128.8, 130.1, 131.0, 131.5, 135.6, 145.0–146.2, 172.0, 172.2,
and 173.0.
Synthesis of PS-G0-Py (15)
11.5 kDa Polystyrene (0.200 g, 0.017 mmol, 1 equiv) was
added to a round-bottom flask with KI (2 mg, 0.012 mmol,
0.7 equiv) and K2CO3 (2.7 mg, 0.02 mmol, 1.1 equiv). This
was dissolved in 0.5 mL of DMF and stirred for 30 min.
Finally, pyrene butyric acid was added (18 mg, 0.06 mmol,
3.6 equiv) and the mixture was stirred at 90 ꢀC. After 2
days, the contents were extracted into DCM, and washed
with water (2 ꢁ 20) and 20% NaHSO4 (aq) (3 ꢁ 10). The
organic layer was dried and the product was purified by col-
umn chromatography (100% DCM) to yield 0.185 g (90%).
SEC: Mw ¼ 12,050 g/mol, Mw/Mn ¼ 1.09.
Synthesis of PS-G3-Py8 (13c)
11.5 kDa Polystyrene (0.300 g, 0.026 mmol, 1 equiv) was
added to a round-bottom flask with KI (2 mg, 0.012 mmol,
0.5 equiv) and K2CO3 (3 mg, 0.02 mmol 0.8 equiv). This was
dissolved in 1 mL of DMF, followed by addition of the G3
acid (13b) (0.289 g, 0.1 mmol, 3.7 equiv), and the reaction
mixture was stirred at room temperature. The reaction was
monitored by TLC (3:1 DCM:hexanes). After 5 days, the mix-
ture was diluted in DCM (5 mL), washed with 20% NaHSO4
(4 ꢁ 10 mL), and dried. The product was separated by col-
umn chromatography (3:1 DCM:hexanes, 100% DCM), and
1H NMR (600 MHz, CDCl3): d ¼ 1.25–2.29 (broad m), 2.50
(broad, 2H), 3.34–3.43 (broad m, 2 H), 5.00–5.13 (broad m,
2 H), 6.28–7.25 (broad m), and 7.80–8.30 (m, 9 H). 13C NMR
(150 MHz, CDCl3): d ¼ 11.6, 14.3, 14.5, 18.9, 19.6, 20.6, 20.9,
22.8, 25.4, 27.0, 27.8, 28.4, 29.2, 29.8, 31.7, 32.9, 34.0, 34.7,
34.8, 36.2, 40.5–40.8, 42.7, 43.6, 44.0, 44.3, 46.0–46.6, 66.6,
123.5–128.7, 130.1, 131.0, 131.6, 135.8, 145.2–146.2, and
173.4.
NANOTUBE-POLYMER HYBRID INTERACTIONS, BAHUN AND ADRONOV
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of G1 to G4 dendrons.
precipitated in hexanes to give 0.201 g of a white powder
(53%). SEC: Mw ¼ 13,350 g/mol, Mw/Mn ¼ 1.08.
accurate to five decimal places) with equimolar amounts (5
ꢁ 10ꢂ4 mmol) of each linear-dendritic polymer (G0 to G4)
in 5 mL of THF. The mixtures were ultrasonicated for 10
min and then filtered through a plug of glass wool (in a pip-
ette) until the filtrate was clear and homogeneous. The same
procedure was followed for full-length SWNTs and MWNTs,
using 2.0 mg of the nanotubes for each experiment. All spec-
trophotometric measurements were performed in triplicate,
and average values are reported.
1H NMR (600 MHz, CDCl3): d ¼ 1.05 (broad s, 6 H), 1.14
(broad s, 15 H), 1.30–2.3 (broad m), 2.36 (t, J ¼ 7.0 Hz, 16
H), 3.20 (t, J ¼ 7.5 Hz, 16 H), 4.16–4.26 (broad m, 28 H),
4.89–5.03 (m, 2 H), 6.3–7.5 (broad m), and 7.65–8.15 (m, 72
H). 13C NMR (150 MHz, CDCl3): d ¼ 14.3, 17.5, 17.9, 21.1,
22.3, 26.6, 28.4–28.6, 32.6, 33.6, 40.5, 40.8, 41.6–46.7, 53.5,
60.5, 65.2, 65.3, 66.1, 123.2–131.4, 135.5, 145.2–146.2,
171.2, 171.5, 172.1, and 172.9.
RESULTS AND DISCUSSION
Synthesis of PS-G4-Py16 (14c)
Preparation of aliphatic polyester dendrons that were peri-
pherally functionalized with pyrene units was accomplished
by a divergent synthetic strategy based on the 2,2-bis
(hydroxymethyl)-propanoic acid (bis-MPA) building block.87,89
The resulting poly(2,2-bis(hydroxymethyl)-propanoic acid)
(PMPA) dendrimer structures, initially reported by Ihre
et al.,90 can be easily and efficiently prepared by nucleophilic
attack on the benzylidene protected anhydride of bis-MPA.87
The easily removable core protecting group, 2-toluenesulfonyl
ethanol (TSe) (1) was used to react with anhydride 2 to pro-
duce the first generation protected dendron (3), as depicted
in Scheme 1.88 Deprotection using standard hydrogenolysis
conditions resulted in the first generation diol 4. Subsequent
iterative reaction with the anhydride 2, followed by hydroge-
nolysis, enabled divergent construction of dendrons up to the
fourth generation (Scheme 1). It should be noted that the TSe
protecting group was stable to all coupling and deprotection
reactions, and all products were purified by simple extraction,
followed by precipitation (i.e., no column chromatography was
required).88
11.5 kDa Polystyrene (0.300 g, 0.026 mmol, 1 equiv) was
added to a round-bottom flask with KI (2 mg, 0.012 mmol,
0.5 equiv) and K2CO3 (3 mg, 0.02 mmol 0.8 equiv). This was
dissolved in 0.5 mL of DMF, followed by addition of the G4
acid (14b) (0.590 g, 0.1 mmol, 3.7 equiv), and the reaction
mixture was stirred at room temperature. The reaction was
monitored by TLC (3:1 DCM:hexanes). After 7 days, the mix-
ture was diluted in DCM (5 mL), washed with 20% NaHSO4
(4 ꢁ, 10 mL), and dried. The product was separated by col-
umn chromatography (3:1 DCM:hexanes, 100% DCM), and
precipitated in hexanes to give 0.178 g of white solid (39%).
SEC: Mw ¼ 14,340 g/mol, Mw/Mn ¼ 1.07.
1H NMR (600 MHz, CDCl3): d ¼ 1.11 (broad s), 1.16 (s), 1.20
(broad s), 1.32–2.17 (broad m), 2.35 (broad triplet, J ¼ 7.0
Hz, 32 H), 3.16 (broad triplet, J ¼ 7.6 Hz, 32 H), 4.01–4.47
(broad m, 60 H), 4.95–5.05 (m, 2 H), 6.28–7.38 (broad m),
and 7.57–8.16 (m, 144 H). 13C NMR (150 MHz, CDCl3): d ¼
14.3, 17.4, 17.6, 17.9, 20.8–22.3, 26.6, 28.4, 28.6, 32.6, 33.6,
40.3–40.8, 43.6–46.8, 60.5, 65.1, 65.6, 123.1–131.4, 135.5,
145.1–146.2, 171.4, 171.5, 171.8, 172.1, and 172.9.
Introduction of pyrene groups at the dendron periphery was
accomplished by esterification with 4-(1-pyrenyl) butyric
acid (PyCOOH), using 1-[3-(dimethylamino)propyl]-3-ethyl-
carbodiimide (EDC) as the coupling agent (Scheme 2). For
Preparation of Nanotube–Polymer Complexes
Nanotube solutions were prepared by mixing exactly 2.0 mg
of shortened SWNTs (measured on an analytical balance
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ARTICLE
SCHEME 2 Synthesis of pyrene-functionalized dendrons.
generations 1 and 2 (G1, G2), the peripheral functionaliza-
tion was driven to completion with a small excess of
PyCOOH (1.2 equiv per OH), allowing the reaction to proceed
overnight. Not surprisingly, the G3 and G4 dendrons required
a significantly larger excess of PyCOOH (ꢃ2 equiv per OH)
and, in the case of G4, complete conversion also required a
longer reaction time, in excess of 5 days. These coupling
reactions could be easily monitored by MALDI-TOF mass
spectrometry. For the G4 species, the crude reaction after 48
h showed a distribution of products separated by 272 mass
units, attributed to structures having a varying number of
PyCOOH groups. After 5 days of stirring, this product distri-
bution coalesced into a single substance having a molecular
weight corresponding to the fully pyrene-functionalized G4
dendron. Figure 1 shows the MALDI-TOF MS data for the G2,
G3, and G4 dendrons.
conditions for deprotection of the TSe group are mild, high
yielding, and enable easy product purification by extraction
into ethyl acetate or dichloromethane followed by precipita-
tion into hexanes.88 The removal of the TSe group was con-
firmed by the disappearance of several diagnostic signals in
the 1H NMR spectrum (Fig. 2). The signals from the para-
substituted aromatic ring (7.05 and 7.60 ppm) were no lon-
ger visible, providing the clearest evidence for the successful,
high-yielding deprotection step. There was also a loss of the
signal at 4.31 ppm from the CH2 protons of the protecting
group, but overlap with signals from the dendron backbone
along with the butyl esters of the peripheral pyrene linkers
limited the diagnostic utility of this signal.
Several options are available when preparing linear-dendritic
hybrid polymers.91–95 In our case, we chose to prepare the
polymer and dendrons separately and then covalently link
the two. A sample of polystyrene (PS, Mw ¼ 11.5 kDa, PDI ¼
1.09) was first prepared by nitroxide-mediated radical poly-
merization (NMP) using the unimolecular nitroxide initiator
Removal of the TSe groups using DBU liberated the desired
pyrene-functionalized dendrons having a carboxylic acid
group at their core. This approach is advantageous as the
NANOTUBE-POLYMER HYBRID INTERACTIONS, BAHUN AND ADRONOV
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 1H NMR of TSe-G4-Py1613a (top) and COOH-G4-
Py1613b (bottom) showing the clean disappearance of signals
corresponding to the TSe protecting group (identified with
arrows). Insets show close-up views of the spectral region
between 6.8 and 8.2 ppm.
FIGURE 1 MALDI-TOF mass spectral data for (a) TSe-G2-Py4,
(b) TSe-G3-Py8, and (c) TSe-G4-Py16
. Calculated molecular
weights are 1630, 3173, and 6262 g/mol, respectively.
developed by Hawker and coworkers.96,97 The resulting poly-
styrene, having a chloromethyl group at one end, could be
coupled to the G1 to G4 dendrons having a carboxylic acid
group at their core under basic conditions (K2CO3, KI), as
shown in Scheme 3. Additionally, PyCOOH was coupled to
the polymer under the same conditions to produce a G0 con-
trol compound. Once again, as the dendron size increased,
increasing amounts of excess dendron were required to drive
the coupling reaction to completion. Purification of the lin-
ear-dendritic hybrids was accomplished by column chroma-
tography to remove the excess dendron and any unreacted
polystyrene from the mixture.
cantly different from that of the polymer, this data proves
that formation of the linear-dendritic hybrid was successful.
It should be noted that the small signal that appears at 22
min in this plot corresponds to a miniscule amount of resid-
ual unreacted dendron (much <1%), which proved impossi-
ble to remove by column chromatography. Table 1 provides
molecular weight data (based on GPC results) for all the lin-
ear-dendritic hybrid polymers prepared in this study.
Both the dendrons and the linear-dendritic hybrids were
studied by UV–Vis and fluorescence spectroscopy. The spec-
tral data for the linear-dendritic hybrid polymers are
depicted in Figure 4. Solutions of these polymers were pre-
pared by dissolving equimolar amounts of each polymer in
THF, and the data in Figure 4(A) shows the approximate
doubling of pyrene absorption expected with each increase
in dendron generation. The steady state fluorescence spectra
given in Figure 4(B) were normalized to monomer emission,
allowing clear observation of the increase in pyrene excimer
emission (centered around 480 nm) with increasing genera-
tion. This indicates that the pyrene groups at the dendrimer
periphery are relatively mobile, even at the higher genera-
tions, and should be capable of orienting themselves around
the sidewall of a CNT for optimal p-stacking interactions.
Characterization of the linear-dendritic hybrids was accom-
plished by 1H NMR and 13C NMR, and gel permeation chro-
matography (GPC). Proton NMR data clearly indicated the
presence of signals corresponding to the pyrene-functional-
ized dendrons, as well as the polystyrene chain. Using a pho-
todiode array detector, GPC provided conclusive evidence
that formation of the linear-dendritic hybrids had occurred.
The three-dimensional plot depicted in Figure 3 for the PS-
G4 hybrid shows that, at the retention time corresponding to
the polymer, pyrene absorption from the dendron is clearly
observable. Since the retention time of the dendron is signifi-
SCHEME 3 Preparation of linear-dendritic hybrid polymers.
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ARTICLE
FIGURE 3 Size exclusion chromatography data using a photo-
diode array UV–Vis detector for the characterization of the PS-
G4 linear-dendritic hybrid, showing the characteristic absorp-
tion spectrum of pyrene units at the retention time correspond-
ing to the polymer hybrid product.
With the linear-dendritic hybrids in hand, it was possible to
investigate their interactions with CNTs in solution. As illus-
trated in Figure 5, it was expected that the pyrene units of
the linear-dendritic hybrid polymers would p-stack to the
surface of the SWNTs, effectively anchoring them to the
nanotubes. This would result in polystyrene chains extending
from the nanotube surface into solution, serving as solubiliz-
ing groups for the polymer–nanotube complexes. To maintain
consistency with previous work, initial investigations were
conducted with shortened SWNTs (having lengths on the
order of 350 nm), as they were expected to have greater sol-
ubility than as-purchased, full-length SWNTs.79,23,36,98 Solu-
tions were prepared by separately mixing equimolar
amounts (5 ꢁ 10ꢂ4 mmol) of each pyrene-labeled linear-
dendritic hybrid polymer (G1 to G4, along with the control
G0 compound) with a constant mass (2 mg) of the shortened
SWNTs in 5 mL of THF. The resulting mixtures were ultraso-
nicated in a bath sonicator for 10 min, and the solutions
were subsequently filtered through a plug of glass wool (in a
FIGURE 4 (A) UV absorbance and (B) fluorescence spectra of
linear-dendritic hybrids in THF. Fluorescence spectra are nor-
malized to monomer emission at 376 nm.
Pasteur pipette) to remove any visible insoluble particulate.
The filtered solutions were then investigated to determine
nanotube concentration. It should be noted that allowing the
solutions to stand for a period of several days did not lead
TABLE 1 Molecular Weights and Polydispersities of
Pyrene-Containing Species, Before and After Coupling
to the 11.5 kDa Polystyrene
Hybrid
Starting Mol.
Wt. (g/mol)
Mol. Wt.a
Starting Material
(g/mol)
PDIa
Polystyrene
11,500a
288b
NA
1.09
1.10
1.07
1.06
1.08
1.06
Pyrene butyric acid
11b (G1 acid)
12b (G2 acid)
13b (G3 acid)
14b (G4 acid)
12,000
12,300
13,000
13,500
14,300
675b
1,630b
3,173b
6,262b
FIGURE 5 Schematic illustration of the interaction between a
linear-dendritic hybrid polymer (second generation dendron)
and a SWNT (molecular mechanics optimizations of the dendri-
tic segment in both the linear-dendritic hybrid and the poly-
mer–nanotube complex were carried out using the Universal
Force Field provided with the software package Avogadro).
a
Determined by GPC.
b
Calculated molecular weight of discrete pyrene-containing
species.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Linear-Dendritic Polymer Hybrids Mixed with
Nanotubes and Resulting Solubilities in THF
Number of
CNT Concentration
Polymer
Pyrenes per Polymer
(mg/L)
15
1
3 6 2
11c
12c
13c
14c
2
5 6 1
4
40 6 6
55 6 7
64 6 2
8
16
FIGURE 6 Photograph of SWNTs mixed with pyrene-labeled
polymer hybrids 15 (a), 11c (b), 12c (c), 13c (d), and 14c (e) in
THF.
results were found with polymers 13c and 14c (data not
shown).
to any observable precipitation of solubilized nanotubes.
Based on visual inspection, it was clear that nanotube mix-
tures with polymers coupled to higher generation dendrons
exhibited higher nanotube concentrations (see photograph of
the solutions, Fig. 6).
A more quantitative analysis of nanotube concentration in
these mixtures was conducted spectrophotometrically by
measuring the absorption of each solution at 500 nm, and
using the previously reported specific extinction coefficient
for the nanotubes at this wavelength (e ¼ 0.0103 L mgꢂ1
cmꢂ1), according to previously published procedures.23 The
measured nanotube concentration values, given in Table 2,
show that the concentration of nanotubes in solution exhib-
its a discontinuous increase when the number of pyrene
units per polymer chain increases from 2 to 4. Clearly, the
interaction strength between one or two pyrenes with the
surface of a CNT is not large enough to overcome the
entropic and enthalpic forces associated with polymer disso-
lution in THF, as well as the van der Waals forces responsi-
ble for bundling of the nanotubes. Therefore, the polymer
hybrids bearing one or two pyrene units do not impart any
appreciable solubility to the nanotubes. However, when py-
rene multiplicity at the dendron surface reaches a value of 4
or higher, the pyrene-nanotube interaction is large enough to
overcome the tendency for the polymer to dissociate into so-
lution and for the nanotubes to rebundle, thereby immobiliz-
ing the polymer chains on the nanotube surface. The anch-
ored polystyrene chains then impart the observed solubility
via steric stabilization.99 It is interesting to note that,
although the observed nanotube concentration continues to
increase beyond the second generation, the gain in solubility
does not increase proportionately to the increase in pyrene
number. This indicates that, for a PS chain having an Mw of
11.5 kDa, introduction of four pyrene units is enough to
anchor the chain to the nanotube surface. Increasing the py-
rene number beyond four results in diminishing returns,
possibly because all the pyrene units cannot simultaneously
bind to the nanotube surface, as they are crowded within a
small area. Some of the peripheral pyrene units may be
back-folded within the dendron structure, and do not come
in contact with the nanotube surface. A rudimentary molecu-
lar modeling/geometry optimization study using a Universal
Force Field (UFF) corroborates this hypothesis,100 as it indi-
cates that, starting with the third generation, there is no
enough space on the nanotube surface for all the pyrene
units on a given dendron to simultaneously bind (Fig. 8).
Certainly, at the fourth generation, the dendrimer is too large
to allow all the pyrene units to come in close proximity to
Raman spectroscopy, using 785 nm laser excitation, was
used to confirm that the observed solutions actually con-
tained SWNTs. Figure 7 compares the Raman spectra of pris-
tine shortened SWNTs, which were simply suspended in THF
by sonication, to shortened SWNTs that have been solubi-
lized by the linear-dendritic hybrid 12c (containing a G2
dendron). In both cases, a drop of the nanotube-containing
suspension/solution was deposited on a glass microscope
slide and allowed to dry prior to measurement. The spectra
in Figure
7 show that the characteristic Raman-active
stretching vibrations, including the graphitic G band at
ꢃ1590 cmꢂ1 and the radial breathing modes (RBM) in the
range of 150–300 cmꢂ1, are observable, indicating the pres-
ence of SWNTs within the samples. No significant differences
are observable between the nanotube samples before and af-
ter interaction with the linear-dendritic hybrids, indicating
that polymer adsorption does not lead to any structural
changes in the nanotube framework. Practically identical
FIGURE 7 Raman spectra (785 nm excitation) of drop-cast sam-
ples of pristine shortened SWNTs (A) and shortened SWNTs
that were functionalized with the linear-dendritic hybrids (B).
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ARTICLE
FIGURE 8 Molecular models showing optimized geometries of
the G2 (A), G3 (B), and G4 (C) dendrons interacting with the
nanotube surface (molecular mechanics optimization was
accomplished using the Universal Force Field provided with
the Avogadro software package). [Color figure can be viewed
in the online issue, which is available at www.interscience.
wiley.com.]
FIGURE 9 Photograph of SWNTs mixed with dendron acids
12b (a), 13b (b), and 14b (c) in THF.
(44 mg/L) were very similar to that of the 11.5 kDa hybrid.
This shows that there is no significant difference between
using a polymer chain of 11.5 kDa or 18 kDa when trying to
stabilize nanotubes in THF with these linear-dendritic
hybrids. This result also demonstrates that the G2 dendron
is still able to anchor the hybrid structure to the nanotubes
even though it is supporting a longer polymer chain.
the nanotube, and instead extends a number of its chain
ends into solution.
The effect of changing polymer molecular weight was also
investigated. Two shorter polymers were easily prepared
and coupled to the dendrons, while a longer polymer was
prepared by chain extending the original 11.5 kDa polysty-
rene. This longer polymer (Mw ¼ 18.0 kDa) was then also
coupled to the G2 dendron. We chose to investigate hybrids
with the Py4-[G2]-COOH dendron, as this is the smallest
structure that imparted appreciable solubility to the nano-
tubes. The final structures that were investigated included
the G2 hybrid polymers where the PS chains had Mw values
of 2.5, 4.6, and 18 kDa. Not surprisingly, in the two lower
molecular weight cases, the measured nanotube solubility, 7
6 1 and 29 6 5 mg/L, respectively, was lower than what
was found with the 11.5 kDa polymer. This indicates that,
although the interaction of the four pyrene units with the
nanotube surface in each of these polymer chains was strong
enough to anchor the polymer (based on results described
earlier), the shorter PS chains did not serve as strong
enough solubilizing groups to match the solubility observed
with the 11.5 kDa chains. In the case of the higher molecular
weight polystyrene, the nanotube concentrations achieved
To determine if the dendrons themselves were capable of
imparting any solubility, the dendron acids alone were mixed
with SWNTs in THF and treated as described earlier. These
experiments were performed with G2 to G4 dendrons, as it
was already determined that the G0 and G1 structures do
not bind the nanotubes with enough strength to overcome
the internanotube van der Waals forces (see earlier). From
these experiments, it was found that the G2 and G3 den-
drons also did not lead to any significant nanotube solubility
[Fig. 9(a,b)]. This was not surprising as these relatively small
dendrons are expected to bind to the nanotube surface, but
do not exhibit any structural feature that can interact with
solvent strongly enough to pull nanotubes into solution.
However, the G4 dendron produced a relatively dark solution
that, when measured by UV–Vis spectroscopy, exhibited a
nanotube concentration of 36 mg/L [Fig. 9(c)]. In this case,
the dendron seems to be large enough to simultaneously
allow some of the pyrene-functionalized branches to bind to
FIGURE 10 (A) Photograph of MWNTs mixed with 15 (a), 11c (b), 12c (c), 13c (d), and 14c (e) in THF. (B) Plot of MWNT absorp-
tion at 500 nm in THF as a function of generation number in the linear-dendritic hybrid polymers.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
the nanotube surface while others extend into solution,
imparting solubility to the dendron-nanotube complex. This
was consistent with what was found in the molecular model-
ing studies discussed earlier (see Fig. 8).
dendron generation, and therefore number of pyrene units
per polymer, gives higher nanotube solubility when mixed
with an equal number of polymer chains. A large increase in
shortened SWNT solubility was observed on increasing from
the first to the second dendron generation, indicating that
four pyrene units are necessary to anchor the polymer
chains to the nanotube surface and overcome the tendency
for nanotubes to form large insoluble bundles. The G4 den-
dron alone was found to provide stable nanotube solutions,
but the lower generation species were not effective. While
full-length SWNTs were not dissolved by any of the linear-
dendritic hybrids, full-length MWNTs were shown to interact
with all of the hybrids in THF, and gave an increasing solu-
bility with increasing dendron generation due to the
enhanced interaction between pyrene and a more mildly
curved nanotube surface. Here again, a large increase in
nanotube solubility was observed on increasing from the
second to the third dendron generation, indicating that four
pyrene units per polymer are needed to impart significant
solubility to MWNTs.
Considering that the interaction between the pyrene-func-
tionalized linear-dendritic hybrid polymers and shortened
SWNTs was strong enough to lead to nanotube solubility, we
wished to determine if the same polymers would also impart
solubility to full-length nanotubes that were not subjected to
any oxidation steps. Using identical procedures to those
described earlier (5 ꢁ 10ꢂ4 mmol of polymer, 2 mg of
unshortened SWNTs, and 5 mL of THF), it was found that
the SWNTs remained completely insoluble in THF after soni-
cation of the mixture for 10 min, or longer. This is consistent
with our previous results using pyrene-functionalized block
copolymers, which were also not capable of imparting solu-
bility to full-length SWNTs. The lack of apparent interaction
with full-length SWNTs indicates that the binding strength of
the polymer to the nanotube surface is not strong enough to
overcome the inherent tendency of the full-length nanotubes
to bundle together.
This work was supported by the Natural Science and Engineer-
ing Research Council of Canada (NSERC), the Premier’s
Research Excellence Award (PREA), the Canada Foundation for
Innovation (CFI), and the Ontario Innovation Trust (OIT).
With these results in mind, we turned our attention to
MWNTs, as several previous reports have described the
interaction of both pyrene-containing small molecules and
polymers with the surface of MWNTs.15,80–83,101–105 Theoreti-
cally, the greater diameter of MWNTs results in a decreased
surface curvature, which may improve the interaction
between the p-system of flat pyrene units and that of the
MWNTs. Using the same protocol as described earlier for
shortened SWNTs, it was found that the linear-dendritic
hybrids of low generation (G0 and G1) had very weak inter-
actions with the full-length MWNTs, whereas the G2 to G4
hybrids were able to stabilize them in solution to a signifi-
cantly greater extent (Fig. 10). Again, a discontinuous jump
in MWNT concentration was observed upon transition from
the first to second dendron generation, as clearly indicated
in the plot of absorbance values at 500 nm as a function of
the dendron generation number [Fig. 10(B)]. This indicates
that having four pyrene units per polymer was sufficient to
overcome the nanotube bundling interactions and to impart
steric stabilization to the MWNTs. Additional pyrene units
per polymer did not significantly change MWNT concentra-
tions. Interestingly, when the same MWNTs were mixed with
the G4 dendron alone (having no polymer attached) and sub-
jected to the aforementioned treatment, no nanotube solubil-
ity was observed. In this case, it is likely that the larger di-
ameter of the MWNTs, relative to SWNTs, allows more
pyrene units to bind to the nanotube surface, leaving fewer
end-groups to impart solubility. For full-length MWNTs, it
therefore appears as though the presence of the polymer
solubilizing chain is essential for achieving solubility.
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