Multiwalled carbon nanotubes grafted with hyperbranched polymer shell via SCVP

Article abstract of DOI:10.1021/ma047736r

A self-condensing vinyl polymerization (SCVP) strategy via atom transfer radical polymerization (ATRP) was employed to grow hyperbranched macromolecules on the surfaces of multiwalled carbon nanotubes (MWNTs). The MWNT surface fixed with ATRP initiators [MWNT-COOCH₂CH₂OCOC-(CH ₃)₂Br, MWNT-Br] was prepared by reaction of MWNT-COCl with excess of 2-hydroxyethyl 2-bromoisobutyrate. Then, using the MWNT-Br as polyfunctional initiator (B*_f), hyperbranched macromolecules were covalently grafted on MWNTs by self-condensing vinyl polymerization of 2-((bromobutyryl)oxy)ethyl acrylate (BBEA, AB* inimer) via the ATRP process. The resultant products were characterized by PT-IR, NMR, and TEM. TGA measurements showed that the weight ratio of the as-grown hyperbranched polymers on the MWNT surfaces reached 80%. The as-prepared MWNTs grafted with hyperbranched polymers exhibit relatively good dispersibility in organic solvents such as THF and chloroform.

Full text of DOI:10.1021/ma047736r

2606
Macromolecules 2005, 38, 2606-2611
Multiwalled Carbon Nanotubes Grafted with Hyperbranched Polymer
Shell via SCVP
Chun-Yan Hong,*,† Ye-Zi You,*,‡ Decheng Wu,§ Ye Liu,§ and Cai-Yuan Pan†
Department of Polymer Science and Engineering, University of Science and Technology of China,
Hefei, 230026, Anhui, P. R. China; Department of Polymer Science and Engineering, Hefei University
of Technology, Hefei, 230009, Anhui, P. R. China; and Institute of Material Research and Engineering,
3 Research Link, Singapore 117602, Singapore
Received November 3, 2004; Revised Manuscript Received January 17, 2005
ABSTRACT: A self-condensing vinyl polymerization (SCVP) strategy via atom transfer radical polym-
erization (ATRP) was employed to grow hyperbranched macromolecules on the surfaces of multiwalled
carbon nanotubes (MWNTs). The MWNT surface fixed with ATRP initiators [MWNT-COOCH2CH2OCOC-
(CH3)2Br, MWNT-Br] was prepared by reaction of MWNT-COCl with excess of 2-hydroxyethyl 2′-
bromoisobutyrate. Then, using the MWNT-Br as polyfunctional initiator (B*f), hyperbranched macro-
molecules were covalently grafted on MWNTs by self-condensing vinyl polymerization of 2-((bromobutyryl)-
oxy)ethyl acrylate (BBEA, AB* inimer) via the ATRP process. The resultant products were characterized
by FT-IR, NMR, and TEM. TGA measurements showed that the weight ratio of the as-grown
hyperbranched polymers on the MWNT surfaces reached 80%. The as-prepared MWNTs grafted with
hyperbranched polymers exhibit relatively good dispersibility in organic solvents such as THF and
chloroform.
Scheme 1
Introduction
The potential utility of carbon nanotubes in a variety
of technologically important applications, such as mo-
lecular wires and electronics, sensors, and probes, high-
strength fibers, field emission, and hydrogen storage,
is now well established.1,2 However, despite significant
research progress,3 successful applications of such com-
posite systems require well-dispersed nanotubes having
good adhesion with the host matrix and good solubility
in organic solvents, which, unfortunately, is not easily
realized. Processing is very difficult due to the poor
solubility of single-walled carbon nanotubes (SWNTs)
or multiwalled carbon nanotubes (MWNTs). Exfoliation
of nanotube bundles is a major challenge in fabricating
molecular electronics and producing homogeneously
dispersed conducting layers within electroluminescent
devices. Thus, many research groups have focused on
functionalizing carbon nanotubes with various organic
and organometallic structures and linear polymers to
increase their solubility.4
Hyperbranched polymers and dendrimers have dis-
tinctive, special chemical and physical properties, and
they have played a vital role in interface and surface
sciences, which can be used advantageously as func-
tional surfaces and interfacial materials.5 In addition,
dendrimers and hyperbranched polymers have good
solubility, lower melt viscosity, and extremely high
density of functional groups at the surface compared
with the linear analogues. Dendrimers and hyper-
branched polymers should therefore increase the solu-
bility of MWNTs and enable further functionalization
of MWNTs. Most molecular wires have poor solubility
and processing ability; however, when these insoluble
molecular wires are wrapped with highly branched
polymers, the branched polymers not only protect and
stabilize the central conjugated backbone of molecular
wires by preventing intermolecular actions but also
improve the solubility and processability without alter-
ing their electronic characteristics.6 Moreover, some
research groups reported that dendrimers or hyper-
branched polymers have been successfully immobilized
onto the surface of Au and silicon wafer;7 the hyper-
branched polymers can be covalently modified with a
broad range of functional groups, such as fluorophores,
electroactive groups, perfluorinated moieties, dyes, and
other linear polymers;7a and also the surface-confined
hyperbranched polymers are suitable for a number of
technical applications, including corrosion inhibition,
chemical sensing, cellular engineering, and micrometer-
scale patterning.4 Therefore, hyperbranched polymers
and dendrimers may be ideal materials to coat and
functionalize carbon nanotubes.
The recent discovery of SCVP made it easy to use
vinyl monomers for the one-step synthesis of hyper-
branched structures.6 This reaction is based on an
initiator-monomer (“inimer”) of the general structure
AB*, where the double bond is designated A and B* is
a functional group which can be transformed to an
active center (such as a cationic, radical, or anionic
active center) and initiates the polymerization of the
double bonds. The activated B* adds across the double
bond A to form the dimer. Scheme 1 shows the idealized
case of one addition of monomer followed by deactivation
to form the new group A*. The dimer, 2, can now be
reactivated at either B* or A*. Addition of monomer at
A* results in 3a, while addition at B* yields 3b. Further
† University of Science and Technology of China.
‡ Hefei University of Technology.
§ Institute of Material Research and Engineering.
* Corresponding authors. E-mail hongcy@ustc.edu.cn (C.-Y.H.)
or 1yzyou@ustc.edu (Y.-Z.Y.).
10.1021/ma047736r CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/24/2005

Macromolecules, Vol. 38, No. 7, 2005
Multiwalled Carbon Nanotubes 2607
Scheme 2. Synthetic Route of
MWNT-g-COOCH2CH2OCOC(CH3)2Br (MWNT-Br)
addition of monomer, or n-mer, to either 3a or 3b, can
result in a polymer that is highly branched. Matyjas-
zewski first reported that atom transfer radical polym-
erization (ATRP) can be applied to the syntheses of
hyperbranched polymers by SCVP. In the SCVP system
via ATRP, AB* should contain a double bond and a
halogen atom.8
Although many research groups have used linear
polymer chains to modify the surface of carbon nano-
tubes by ATRP, anionic, cationic, and ring-opening
polymerizations5 and Yan reported a SCVP via ring-
opening polymerization method to graft hyperbranched
polymer on MWNTs,9 there are still few reports on
highly branched polymers functionalizing the surface
of carbon nanotubes. Here, we report an easy method
to coat the surface of MWNT with a hyperbranched
polymer shell by self-condensing vinyl polymerization
(SCVP) via atom transfer radical polymerization (ATRP).
MWNT-COOH (1.4 g) was suspended in SOCl2 (30 mL). This
suspension was stirred at 70 °C for 16 h. The solid was then
separated by filtration and washed with anhydrous CH2Cl2.
Subsequently, it was dried under vacuum at 40 °C for 1 h,
obtaining MWNT-COCl (1.18 g, yield 77%). 2-Hydroxyethyl
2′-bromoisobutyrate (9.6 g) in toluene (15 mL) was added to
the one-neck flask that contained MWNT-COCl (1.0 g), and
the reaction was stirred at 100 °C for about 24 h under a pure
N2 atmosphere. After the reaction was finished, the solvent
was completely removed under vacuum, and the products were
washed several times with ethanol (500 mL) and filtered. The
Experimental Section
Materials. MWNT was provided by the Tsinghua-Nafine
Nano-Powder Commercialization Engineering Centre. Bromo-
isobutyric acid (98%), glycol (99%), bromobutyric acid (99%),
and CuBr (98%) were purchased from Aldrich. 1,1,4,7,7-
Pentamethyldiethylenetriamine (PMDETA, 99%) was pur-
chased from Acros. All chemical reagents were used directly
without further purification.
Synthesis of 2-Hydroxylethyl 2′-Bromoisobutyrate
(HEBrIB). HEBrIB was prepared as follows: Ethylene glycol
(42.0 g, 0.67 mol), 2-bromoisobutyric acid (11.2 g, 0.067 mol),
p-toluenesulfonic acid (0.2 g, 9.0 × 10-4 mol), and 50 mL of
benzene were heated at 96 °C while stirring for 36 h, and the
water generated by esterification reaction was removed by an
oil-water separator. Then the reaction mixture was poured
into 500 mL of distilled water, and the lower, light-yellow
organic phase was separated. The organic portion was dis-
solved in methylene chloride and dried over anhydrous mag-
nesium sulfate overnight. After filtration from the magnesium
salt, removal of solvent, the 2-hydroxylethyl 2′-bromoiso-
butyrate, was obtained as a colorless liquid (10 g, yield 70%).
1H NMR of HOCH2CH2OCOC(CH3)2Br (δ, ppm): 1.0 (s,
6H, 2-CH3), 3.7 (t, 2H, OCH2CH2OH), 4.30 (t, 2H, -COOCH2-
CH2-). The synthesis of 2-hydroxylethyl 2′-bromobutyrate
(HEBrB) was similar to that of HEBrIB. 1H NMR of HOCH2-
CH2OCOCH(C2H5)Br (δ, ppm): 1.05 (t, 3H, -CH2CH3), 2.0 (m,
2H, -CH2CH3), 3.7 (t, 2H, OCH2CH2OH), 4.12 (t,1H, CH(Br)-
CH2CH3), 4.30 (t, 2H, -COOCH2CH2-).
Synthesis of 2-((Bromobutyryl)oxy)ethyl Acrylate
(BBEA) (AB* Inimer). A solution of acrylyl chloride (5.0 g,
0.055 mol) in 10 mL of dichloromethane was added dropwise
to an ice-cooled solution of HEBrB (10.6 g, 0.05 mol) and
triethylamine (5.6 g, 0.55 mol) in dichloromethane (80 mL)
over 1 h under nitrogen. The reaction continued for 24 h under
room temperature. After removing the precipitate produced
during evaporation of dichloromethane by filtration, a yellow
oil was obtained, which was washed with distilled water three
times and dried over anhydrous magnesium sulfate for 24 h.
The solvents were evaporated under reduced pressure to obtain
a crude product. Pure BBEA (9.0 g, yield 66.7%) was obtained
by passing a solution of BBEA in dichloromethane through
silica gel column. 1H NMR (δ, ppm): 1.05 (t, 3H, -CH2-CH3),
2.0 (m, 2H, -CH2-CH3), 4.11 (t, 1H, -CH(Br)-), 4.30 (s, 4H,
-OCH2CH2O-), 5.88 (d, 1H, CH2dCH-), 6.18 (dd, 1H, CH2d
CH), 6.48 (d, 1H, CH2dCH-).
Figure 1. FT-IR spectra of multiwalled carbon nanotube
(MWNT) surface grafted with COOCH2CH2OCOC(CH3)2Br
group (MWNT-Br, A) and multiwalled carbon nanotube grafted
with hyperbranched polymers 1 (HP-MWNT, B).
Attachment of Initiators to the Surface of MWNTs. In
a typical experiment, crude MWNTs (2.0 g) were added to 60%
HNO3 aqueous solution (30.0 mL). The mixture was treated
with ultrasonic bath (40 kHz) for 10 min and stirred for 18 h
at reflux. Then, the mixture was vacuum-filtered through 0.22
µm Millipore polycarbonate membrane and washed with
distilled water until the pH of the filtrate was 7. The filtered
solid was dried under vacuum for 24 h at 50 °C, obtaining
MWNT-COOH (1.50 g).
Figure 2. TGA curves of pristine multiwalled carbon nano-
tubes (MWNTs, A), MWNT-COOCH2CH2OCOC(CH3)2Br
(MWNT-Br, B), MWNT grafted with hyperbranched polymers
1 (C, polymerization time was 15 h), MWNT grafted with
hyperbranched polymers 2 (D polymerization time was 24 h),
and hyperbranched polymer (E) under N2.

2608 Hong et al.
Macromolecules, Vol. 38, No. 7, 2005
Scheme 3. Detailed Procedure of Hyperbranched Polymers Growing from the Surface of MWNT
initiator-attached MWNTs were dried at 50 °C for 10 h under
vacuum. FT-IR (KBr): 2920 (C-H stretching), 1730 (CdO
stretching), 1460 (C-H bending). TGA: 10% total weight loss
at 250-420 °C. Anal. Found: C, 92.9; H, 0.47; Br, 3.8, which
corresponds to about 0.63 initiator groups per 100 carbons.
SCVP of AB* Using MWNT-Br as Polyfunctional Ini-
tiator. In a typical polymerization, MWNT-Br (100.0 mg, 0.042
mmol initiators) was placed in a clean two-neck flask, and a
solution of CuBr (7.2 mg, 5.0 × 10-5 mol) and ligand (1,1,4,7,7-
pentamethyldiethylenetriamine, 17.5 mg, 1.0 × 10-4 mol)
([CuBr]:[L] ) 1:2) in toluene was added into the flask with a
syringe under N2. The flask was placed in an oil bath
maintained at 100 °C, then BBEA (1.0 mL, 3.7 mmol) was
added into the flask slowly over 3 h, and the reaction was
stirred for another 24 h. After 24 h, the heterogeneous
polymerization solution was diluted with THF (50 mL) and
kept stirring in a round-bottom flask for 1 h to dissolve the
soluble polymer. The resultant products in THF were filtered
by using 0.22 µm Millipore polycarbonate membrane and
washed thoroughly with THF; this washing was repeated until
the filtrate contained no polymer. The polymer-grafted MWNTs
were dried at 40 °C for 24 h under vacuum.

Macromolecules, Vol. 38, No. 7, 2005
Multiwalled Carbon Nanotubes 2609
SCVP of AB* in the Presence of Pristine MWNT. For
comparison, SCVP of BBEA (1.0 mL, 3.7 mmol) catalyzed by
CuBr (7.2 mg, 5.0 × 10-5 mol) and 1,1,4,7,7-pentamethyldi-
ethylenetriamine (PMDETA, 17.5 mg, 1.0 × 10-4 mol) was
carried out at 100 °C for 36 h in the presence of pristine
MWNTs (80 mg). After finishing the polymerization, the
product was washed with THF thoroughly. The resultant
product was dried in an oven at 40 °C for 24 h under vacuum.
SCVP of AB* and tert-Butyl Acrylate Using MWNT-
Br as Polyfunctional Initiator. MWNT-Br (100.0 mg, 0.042
mmol initiators) was placed in a clean two-neck flask, and a
solution of CuBr (7.2 mg, 5.0 × 10-5 mol) and ligand (1,1,4,7,7-
pentamethyldiethylenetriamine, 17.5 mg, 1.0 × 10-4 mol)
([CuBr]:[L] ) 1:2) in toluene was added into the two-neck flask
with a syringe under N2. The flask was placed in an oil bath
maintained at 100 °C, then a mixture of BBEA (1.0 g, 3.7
mmol) and tert-butyl acrylate (1.0 g, 7.8 mmol) was added into
the flask slowly over 3 h, and the reaction was stirred for
another 24 h. After 24 h, the heterogeneous polymerization
solution was diluted with THF (100 mL) and kept stirring in
a round-bottom flask for 1 h to dissolve the soluble polymer.
The resultant products were filtered by using 0.22 µm Milli-
pore polycarbonate membrane and washed thoroughly with
THF; this washing was repeated until the filtrate contained
no polymer. The polymer-grafted MWNTs were dried at 40 °C
for 24 h under vacuum.
Results and Discussion
In SCVP via ATRP, when polyfunctional initiator
(B*f) was added in the SCVP system, B*f can participate
in the initiating polymerization of AB* to form Bf-
containing hyperbranched polymers.7 To attach a hy-
perbranched polymer shell to a multiwalled carbon
nanotube (MWNT), ATRP surface initiator, a bromo-
isobutyryl fragment immobilized on MWNT was pre-
pared as shown in Scheme 2. First, the carboxyl MWNT
(MWNT-COOH) was prepared by oxidation of the crude
MWNT with 60% HNO3. Then MWNT-COCl, obtained
from treating the MWNT-COOH with thionyl chloride,
was reacted with excess 2-hydroxyethyl 2′-bromoiso-
butyrate in the presence of triethylamine to produce
bromoisobutyrate functionalized MWNT (MWNT-Br).
MWNT-Br was washed thoroughly with ethanol until
the filtrate showed no 2-hydroxyethyl 2′-bromoiso-
butyrate. MWNT-Br was confirmed from the charac-
teristic C-H and carbonyl stretching vibrations cen-
tered at 2920 and 1730 cm-1, respectively, in the FT-
IR spectrum (Figure 1). The quantity of the bromoester
attached to the surface of MWNT was determined from
its thermogravimetric analysis (TGA), which showed
that the product was stable to 250 °C, upon which the
product degraded, but it becomes stable above 420 °C,
and the total weight loss was about 10%, corresponding
to the decomposition of the bromoester on the surface
of MWNT-Br (Figure 2B). The molar percent of the
bromoester (B*) is about 0.6% with respect to carbon,
which is consistent with the result obtained from
elemental analysis.
MWNT-Br was used as a polyfunctional initiator (B*f)
in the SCVP via ATRP to produce hyperbranched
macromolecule functionalized MWNTs, as illustrated in
Scheme 3.6,10 During the SCVP of BBEA (AB*) via the
ATRP process, because both the AB* inimer and the
MWNT-Br have initiating groups capable of initiating
the polymerization of vinyl groups, the chain growth can
be started from both B* in the MWNT-Br and B* group
in the inimer. Both the activated B* can add to the
double bond, A, to form the ungrafted or grafted dimer
with a new propagating center, A*. Further addition of
AB* inimer or dimer to A* and B* centers results in
Figure 3. 1H NMR spectrum of multiwalled carbon nanotubes
(MWNTs) grafted with hyperbranched polymers 1 (polymer-
ization time was 15 h).
Figure 4. Solubility in organic solvent: MWNT-g-COOCH2-
CH2OCOC(CH3)2Br in THF (A), MWNT grafted with hyper-
branched polymers 1 in THF (B) and CHCl3 (C).
hyperbranched macromolecules.6 From the structures
of B* of MWNT-Br and B* in AB* inimer, it is clear
that B* of MWNT-Br has a higher activity of adding to
the double bond than that of AB*;7,11 thus, most of the
B* immobilized on the surface of multiwalled carbon
nanotubes has participated in initiating self-condensing
vinyl polymerization via slow addition of AB* inimer.
Of course, some ungrafted hyperbranched polymers
were still produced during the polymerization. After the
polymerization, the reaction mixture was diluted with
THF, and the nanotubes were washed thoroughly with
THF and then filtered to remove soluble ungrafted
hyperbranched polymers. Washing was done until no
polymer was found in the filtrate. The weight of the gray
or black MWNT grafted with hyperbranched polymers
(HP-MWNT) was much greater than MWNT-Br initia-
tor added, indicating the presence of surface-grown
hyperbranched macromolecules.

2610 Hong et al.
Macromolecules, Vol. 38, No. 7, 2005
Figure 5. TEM images of MWNT-g-COOCH2CH2OCOC(CH3)2Br (A), MWNT grafted with hyperbranched polymers (B), and
dispersion of MWNT-g-COOCH2CH2OCOC(CH3)2Br in hyperbarnched polymer solution in THF (C).
For comparison, SCVP of AB* was carried out in the
presence of pristine MWNT. After the products was
washed with THF thoroughly, it was found that there
was no increased weight of MWNT and almost no
hyperbranched macromolecules absorbed on MWNT
from FT-IR result, which indicates that the ungrafted
hyperbranched polymers can be removed from MWNT
by washing with THF.
MWNT will decompose around 500 °C in the presence
of air and around 800 °C under nitrogen.5 The weight
of hyperbranched polymers grafted on MWNT can be
obtained from the TGA method. The TGA analysis of
HP-MWNT in the presence of nitrogen showed a major
decomposition in the temperature range at 280-420 °C,
corresponding to surface grown hyperbranched poly-
mers (Figure 2). Hyperbranched polymer was grown
from the surface of MWNTs. To further confirm that
the surface of MWNTs was grafted with the hyper-
branched polymer, the sample was characterized by FT-
1
IR and H NMR. Comparison of the FT-IR spectra of
HP-MWNT and MWNT-Br shows that the characteristic
C-H and carbonyl stretching vibrations centered at
2920 and 1730 cm-1 were strengthened very much
(Figure 1B), which indicates that the hyperbranched
polymers were successfully grafted from the surface of
MWNTs. Hyperbranched polymers grown from the
surface of MWNT are clearly seen in the 1H NMR
spectrum (Figure 3). The observed proton signals are
entirely due to hyperbranched polymers grown from
MWNT.
Figure 6. 1H NMR spectrum of multiwalled carbon nanotube
grafted with hyperbranched polymers containing tert-butyl
acrylate units.
individually; however, it is clear that (MWNT-Br)s are
bounded together. The TEM images of dispersion of
MWNT-Br in hyperbranched polymers solution in THF
were obtained and shown in Figure 5C; the images are
similar to those of MWNT-Br, and carbon naotubes are
also entangled together.
Dispersion of MWNT-Br into THF was very difficult
even after the carbon nanotubes were sonicated; sedi-
mentation of MWNT-Br from THF appeared a few
minutes after sonication. However, dispersion of HP-
MWNT in THF and chloroform was very easy. Figure 4
shows the solubility of MWNT-Br in THF and HP-
MWNT in THF and chloroform. It is clear that MWNT-
Br is insoluble in THF, and there was much sedimen-
tation of MWNT-Br at the bottom of the vial. However,
the HP-MWNT is soluble in THF and chloroform,
forming a homogeneous solution; there was no sedimen-
tation observed even after 10 days.
These MWNTs coated with hyperbranched shell have
many terminal bromo-functional groups; these bromo-
functional groups can be used as active sites for further
functionalizing. In addition, this SCVP method can be
extended to self-condensing atom transfer radical co-
polymerization to create MWNTs coated with a different
functional shell. This self-condensing atom transfer
radical copolymerization of the inimer (BBEA, AB*) and
tert-butyl acrylate can produce multiwalled carbon
nanotubes coated with hyperbranched polymer shell
1
containing tert-butyl acrylate units. Its H NMR spec-
trum is shown in Figure 6; it is clear that the charac-
teristic peaks for poly(tert-butyl acrylate) can be ob-
served besides the characteristic peaks for hyperbranched
polymers. The hydrolysis of the ester functionality of
branched poly(tert-butyl acrylate) creating acid-contain-
ing hyperbranched polymer shell coating MWNTs and
the corresponding properties are in progress.
For comparison, we attempted to disperse the mixture
MWNT-Br (20%) and hyperbranched polymers (80%)
into THF. This hyperbranched polymer is a poor dis-
persant for multiwalled carbon nanotubes in THF; some
MWNT-Br precipitated from solvent in 1 day. All of the
evidence supports solubilization of the MWNT by co-
valently bounded hyperbranched polymers and not
adsorbed hyperbranched polymers. The images of the
resultant MWNT obtained after 15 h polymerization
and MWNT-Br were detected with transmission elec-
tron microscopy (TEM), as shown in Figure 5. In
comparison with its precursor, we can find from TEM
images that the HP-MWNT had a thin hyperbranched
polymer layer (2-4 nm), and HP-MWNT was separated
Conclusion
The self-condensing atom transfer radical polymeri-
zation method is successfully applied to functionalize
MWNT, resulting in MWNT with a hyperbranched
polymer shell. MWNT with a hyperbranched polymer
shell has good solubility and is easy to disperse indi-

Macromolecules, Vol. 38, No. 7, 2005
Multiwalled Carbon Nanotubes 2611
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