Covalent surface functionalization of multiwalled carbon nanotubes through bergman cyclization of enediyne-containing dendrimers

Article abstract of DOI:10.1002/pola.24834

A method for covalent functionalization of multiwalled carbon nanotubes (MWCNTs) was developed using the free radicals generated through Bergman cyclization of enediyne-containing compounds. Four enediyne-bearing Frechet type dendrimers were synthesized in good quantities and characterized. Then, the enediyne-containing molecules were reacted with MWCNTs in N-methyl-2- pyrrolidinone at 206 °C under nitrogen. The structure and morphology of the resulting products were characterized by thermogravimetric analysis, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy. The dendrimer- functionalized MWCNTs showed good solubility/dispersibility in common organic solvents and polymer solutions. They were used in the formation of polymer composites through electrospinning with polycaprolactone. The results confirmed the surface functionalization of MWCNTs.

Full text of DOI:10.1002/pola.24834

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ARTICLE
Covalent Surface Functionalization of Multiwalled Carbon Nanotubes
Through Bergman Cyclization of Enediyne-Containing Dendrimers
Jianguo Ma,1,2 Sheng Deng,1 Xin Cheng,1 Wei Wei,1 Aiguo Hu1
1Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China
University of Science and Technology, Shanghai 200237, China
2Faculty of Chemistry Biology and Materials Science, East China Institute of Technology, Jiangxi 344000, China
Correspondence to: A. Hu (E-mail: hagmhsn@ecust.edu.cn)
Received 4 May 2011; accepted 13 June 2011; published online 29 June 2011
DOI: 10.1002/pola.24834
ABSTRACT: A method for covalent functionalization of multi-
walled carbon nanotubes (MWCNTs) was developed using the
free radicals generated through Bergman cyclization of enediyne-
containing compounds. Four enediyne-bearing Frechet type den-
drimers were synthesized in good quantities and characterized.
Then, the enediyne-containing molecules were reacted with
MWCNTs in N-methyl-2-pyrrolidinone at 206 ꢀC under nitrogen.
The structure and morphology of the resulting products were
characterized by thermogravimetric analysis, ultraviolet–visible
spectroscopy, Fourier transform infrared spectroscopy, Raman
spectroscopy, and transmission electron microscopy. The den-
drimer-functionalized MWCNTs showed good solubility/dispersi-
bility in common organic solvents and polymer solutions. They
were used in the formation of polymer composites through elec-
trospinning with polycaprolactone. The results confirmed the sur-
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face functionalization of MWCNTs. 2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 49: 3951–3959, 2011
KEYWORDS: Bergman cyclization; carbon nanotubes; dendrimers;
electrospinning; modification; nanocomposites
INTRODUCTION Carbon nanotubes (CNTs) have attracted
tremendous interest since Iijima’s1 land mark paper in 1991,
due to their unique chemical, electrical, and mechanical
properties.2–5 However, the great tendency of pristine CNTs
to establish strong van der Waals and p–p interactions cause
them to form tight bundles, which makes them insoluble and
indispersible in common solvents and seriously restrict their
applications. Consequently, many sidewall functionalization
methods have emerged in recent years and greatly improved
CNTs’ solubility and dispersibility.6–19 Among these methods,
covalent functionalization in particular has received signifi-
cant attentions, for it may supply CNTs with desired func-
tionality and without deteriorating their own unique proper-
ties. These methods relied on directly attacking sp2 carbons
of CNTs with fluorine,20,21 aryl radicals,22,23 diazonium,24,25
hydrogen,26,27 nitrenes,8,28,29 carbenes,30 free radicals,31–35
and 1,3-dipoles,11,36 etc. Extensive covalent functionalization
of CNT sidewalls may greatly improve their solubility, but
the conjugated p-system of the tubes could be disrupted and
their optical and electronic properties could be affected.
Therefore, functionalization of nanotubes with dendrimers
represents a particularly promising strategy to introduce suf-
ficient amount of functional groups onto the CNT surfaces
for succeeding processing purpose with a limited number of
sp2 carbon atoms are attacked.37 Dendritic functionalities
can be introduced through two strategies. One, growing den-
drimers from the surface of CNTs (grafting-from).37,38 Two,
attaching dendrimers onto nanotubes via their conical func-
tional groups (grafting onto).8,39,40 Herein, we wish to report
our work on surface functionalization of multiwalled CNTs
(MWCNTs) through attacking sidewalls of MWCNTs with dir-
adicals generated from Bergman cyclization and further ex-
ploration in generating MWCNT–polycaprolatone (PCL) com-
posites through electrospinning.
Enediyne compounds undergo Bergman cyclization41 to gener-
ate diradical intermediates when they are heated or shined
with ultraviolet light. Many naturally occurring enediyne com-
pounds show strong cytotoxicity arising from in situ triggered
Bergman cyclization. Recent years have witnessed explosive
research interests of Bergman cyclization on synthesis of biosi-
milar enediyne compounds for potential applications in antitu-
mor medicines.42–46 Bergman cyclization has also been used to
generate ill-defined polynaphthalene in mesoporous silica
channels,47 initiate free radical polymerizations,48,49 prepare
carbon enriched materials for thin-film lithography,50 and gen-
erate precursors for glassy carbon.51 Together with the facial
preparation and structural variability of enediyne compounds,
Bergman cyclization is prone to show wide application not
only in pharmaceutical research but also in polymer and mate-
rial science. However, the research in the later part is limited
to a few cases. Smith and coworkers52 showed that diradicals
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FIGURE 1 TGA curves (10 ꢀC
minꢂ1
in N2) of (A) pristine
,
MWCNTs (a), MWCNT–11a (b),
MWCNT–5a (c), and oligomers of
compound 5a (d); (B) pristine
MWCNTs (a), MWCNT–11b (b),
MWCNT–5b (c), and oligomers
of compound 11b (d).
from bis-ortho-diynylarene precursors react with carbon nano-
onion and greatly improve the solubility of these carbonous
materials. Recently, we studied the preparation of brush poly-
mers through Bergman cyclization of enediyne-containing mac-
romonomers, preparation of mesoporous carbon, and function-
alization of CNTs with polyesters through ‘‘grafting-from’’
strategy.53–55 Diradicals generated in situ were found to be
highly reactive in construction of those nanostructures. In this
work, we synthesized Frechet type dendrimers with peripheral
benzyl or long alkyl chains and conical enediyne functionalities
and conducted thermal Bergman cyclization in the presence of
MWCNTs. The diradicals generated in situ were shown to react
with the sidewalls of MWCNTs by radical addition. The solubil-
ity of functionalized MWCNTs in common organic solvents and
PCL matrix were greatly improved.
tions.56,57 Hawker and coworkers used this technique to syn-
thesize polymeric nanoparticles starting from benzocyclobu-
tene-containing linear polymer through intramolecular chain
collapse. A degassed solution of dendritic enediyne was thus
added slowly to a boiling suspension of MWCNTs in N-
methyl-2-pyrrolidinone (NMP) through a peristalsis pump. In
comparison with ‘‘mixed-all-together’’ method, the reaction
took place at much faster speed, while no cross-linked prod-
uct was generated.
Characterization with TGA
Thermogravimetric analyses (TGA) was usually used to dif-
ferentiate the surface bounded groups and carbonous mate-
rials, due to the different thermal stabilities of these two
parts. TGA curves of functionalized MWCNTs and oligomers
of 5a and 11b under nitrogen atmosphere are shown in Fig-
ure 1 (note: TGA curves of oligomers of 5b and 11a are not
shown in Figure 1 for clarity reason). Comparing TGA curves
of pristine and functionalized MWCNTs, weight losses of
25.3, 16.7, 12.8, and 10.4% are observed for MWCNTs func-
tionalized with 5a, 11a, 5b, and 11b, respectively, which are
attributed to the presence of compounds 5 and 11 on the
sidewalls of MWCNTs. The loading of dendrimer units per
carbon atoms on MWCNTs were thus calculated to be 0.63,
0.20, 0.48, 0.19%, respectively. Furthermore, the degrees of
functionalization of MWCNT-11 are lower than that of
MWCNT-5, these differences are due to the larger steric hin-
drance of compounds 11. Nevertheless, all the functionalized
MWCNTs are well dispersible in common organic solvents.
The peripheral groups are the key to improve the dispersi-
bility of functionalized MWCNTs even at low grafting
densities.
RESULTS AND DISCUSSION
Synthesis of Compound 5 and 11
In our research, compound 1 was chosen as the conical
group for dendrimers, due to its structural feature and facial
preparation. Hydroxy group can be easily modified with a va-
riety of reactions to introduce functional groups onto this
molecule. Trimethylsilyl (TMS) acetylene is a synthetic equiv-
alent of acetylene, and the steric effect of TMS group inhibits
unwanted Bergman cyclization or oxidative coupling of ter-
minal alkyne groups during preparation and storage. TMS
groups could be easily removed under mild condition. We
found that after removal of TMS groups, Bergman cyclization
of these enediyne compounds occurred at much lower tem-
perature.53 The dendritic parts were synthesized following
the procedure developed by Frechet et al. All the synthesis
took place smoothly to give desired products in high yields.
In addition, long alkyl chains were introduced to the periph-
ery of dendrimers (5a and 11a) to further improve the solu-
bility of final functionalized MWCNTs in common organic sol-
vents and polymer solutions.
Characterization with FTIR Spectroscopy
The Fourier transform infrared (FTIR) spectra of MWCNT-5a
[curve b in Fig. 2(A)] and MWCNT-11a [curve c in Fig. 2(A)]
showed CAH stretching vibrations at ꢁ2925 cmꢂ1 that did
not appear in the spectrum of pristine MWCNTs. These two
bands are corresponding to the methylene groups of the
compound 5a and 11a, respectively. The difference between
b, c, and a confirmed the occurrence of Bergman cyclization
on the sidewalls of MWCNTs. The presence of the CAH scis-
sor vibration of methylene groups at ꢁ1444 cmꢂ1 and the
CAO stretching vibrations of ester groups at ꢁ1259 cmꢂ1 in
Functionalization of MWCNTs Through ‘‘Slow Addition’’
Method
Slow addition strategy has been explored in many thermal
triggered reactions to effectively minimize the concentration
of reactive intermediates generated under high temperature
and limit the possibility of intermolecular crosslinking reac-
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FIGURE 2 FTIR spectra of (A,
left) pristine MWCNTs (a),
MWCNT–5a (b), MWCNT–11a (c),
and oligomers of compound 5a
(d); (B, right) pristine MWCNTs
(a), MWCNT–5b (b), MWCNT–11b
(c), and oligomers of compound
5b (d).
the spectra of b and c also provided the supports the cova-
lent grafting of MWCNTs. Similarly, the FTIR spectra in Fig-
ure 2(B) indicated CAH stretching vibrations and scissor
reaching to each other by extracting hydrogen atoms from
solvent molecules.
vibration of methylene group at 2921 and 1435 cmꢂ1
,
Characterization with Raman Spectroscopy
The Raman spectra in Figure 4(A,B) revealed that pristine
MWCNTs and functionalized MWCNTs all showed two char-
acteristic bands at ꢁ1581 and ꢁ1351 cmꢂ1, which are
denoted as G bands and D bands, respectively. G bands are
related to the in-plane E2g vibration mode of benzenoid
framework of the CNTs, whereas D bands are attributed to
sp3 hybridized carbon atoms in the walls.58 The intensity ra-
tio of these two bands ID/IG is typically used as an index of
graphitization of carbonous materials. In most work on side
wall functionalization of MWCNTs, the ratio ID/IG increased
due to the introduction of more defects onto MWCNTs. In
Figure 4(A,B), it is clearly shown that the ID/IG ratios were
all increased when comparing functionalized MWCNTs with
pristine MWCNTs. The ID/IG of pristine MWCNTs is 0.43,
whereas ID/IG of MWCNT-5a, MWCNT-5b, MWCNT-11a, and
MWCNT-11b are 0.81, 0.73, 0.76, and 0.69, respectively.
These phenomena further confirm the surface functionaliza-
tion of MWCNTs.
respectively. The results of FTIR characterization confirmed
the occurrence of reactions between the pristine MWCNTs
and enediyne-containing dendrimers.
Characterization with UV–Vis Spectroscopy
The Ultraviolet–visible (UV–Vis) absorption spectra of grafted
MWCNTs (Fig. 3) showed broad absorption signals between
380 and 500 nm, which are corresponding to the p–p* tran-
sition of oligonaphthalene moieties formed through Bergman
cyclization. According to generally accepted mechanism of
Bergman cyclization,41 thermal triggered reaction generates
benzyl 1,4-diradical intermediates, which are highly reactive
toward carbonous materials. After the covalent functionaliza-
tion of one side of the diradical intermediates to MWCNTs,
the other side could further couple with other diradical
intermediates to form oligomeric chains on the surface of
MWCNTs. If the concentration of diradical intermediates is
too high, indispersible three-dimensional polymeric networks
could form through coupling of terminal radicals of oligo-
meric chains growing from different nanotubes. For this rea-
son, slow addition method is highly important in functionali-
zation of MWCNTs through Bergman cyclization. Through
this method, the growing chains could be terminated before
Characterization with TEM
Functionalized MWCNTs were characterized with transmission
electron microscope (TEM). The samples were prepared by
drop-coated onto a copper grid and viewed through TEM.
FIGURE 3 UV–Vis spectra of (A)
pristine MWCNTs (a), MWCNT–
5a (b), and MWCNT–11a (c), (B)
pristine MWCNTs (a), MWCNT–
5b (b), and MWCNT–11b (c). All
spectra were obtained in CHCl3.
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FIGURE 4 Raman spectra of (A,
left) pristine MWCNTs (a),
MWCNT–5a (b), and MWCNT–5b
(c); (B, right) pristine MWCNTs
(a),
MWCNT–11a
(b),
and
MWCNT–11b (c).
TEM images in Figure 5 indicated the presence of small CNT
bundles and a substantial amount of amorphous organic layer
on the surface of MWCNTs. The inhomogeneous layers on the
sidewalls of tubes were attributed to random heterogeneous
addition reactions of diradicals, which were produced by Berg-
man cyclization of enediynes with sp2 carbons of MWCNTs.
was stirred for a while to obtain an ink-like solution, which
was stable even after long time standing. After electrospin-
ning, composite fibers were obtained, TEM images show that
all MWCNTs are well distributed in PCL fibers. All the axes
of MWCNTs are parallel to the direction PCL fibers (Fig. 6).
The alignment of CNTs in electrospun nanofibers were stud-
ied by Cheng and coworkers.60 It was proved to be the main
factor for enhancing the strength of polymer fibers by adding
CNTs. The uniformity of functionalized MWCNTs in polymer
nanofibers is another evidence of the effectiveness of our
functionalization method.
Electrospun Fibers of Functionalized MWCNTs in PCL
Electrospinning is recognized as an efficient technique for
the fabrication of polymer nanofibers. In an electric field of
several kV cmꢂ1, the elongation of polymer stream ejected
from electrode produces fibers with ultra low diameters.
MWCNTs and modified MWCNTs have been electrospun with
various polymers, like polyacryloylnitrile, polyethyleneoxide,
polyureathane, etc.59–62 The thermal stability and Young’s
modules of polymer fibers were improved after electrospun
with SWCNTs.61 Considering the good solubility of these den-
drimers modified MWCNTs, we explored fabrication of
MWCNT polymer composites through electrospinning with a
biocompatible and biodegradable polymer, PCL. A mixture of
functionalized MWCNTs, PCL, and Tetrahydrofuran (THF)
EXPERIMENTAL
Materials
THF and triethylamine were dried with sodium or calcium
hydride under an atmosphere of nitrogen before use. PCL
(Mn ꢁ 1 ꢃ 105) was obtained from Shenzhen BrightChina,
Industrial. MWCNTs were provided by the Chengdu Organic
Chemicals, Chinese Academy of Science. They were prepared
through chemical vapor deposition. All other chemicals were
FIGURE 5 TEM images of MWCNT–5a (left) and MWCNT–11a (right).
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FIGURE 6 TEM images of MWCNT–5a (left) and MWCNT–11b (right) in electrospun PCL fibers.
analytical grade and used as received without further purifi-
cation. All reactions were performed using Schlenk techni-
ques under an atmosphere of nitrogen unless otherwise
1H NMR (CDCl3, d, ppm): 7.16 (PhAH, 2H), 6.63 (PhAH, 1H),
3.96(AOACH2A, 4H), 3.90(ACOAOACH3, 3H), 1.75–1.79
(AOACH2ACH2ACH2A, 4H), 1.42–1.44 (AOACH2ACH2A,
4H), 1.26 (ACH2ACH2ACH2A, 48H), 0.88 (ACH2ACH3, 6H).
noted.
4-(2-(2-(trimethylsilyl)ethynyl)phenyl)but-3-yn-1-ol
(1) was synthesized according to literature procedure.63 Ene-
diyne-containing dendrimers 5b and 11b were synthesized
according to our recently published article.64
3,5-Bis(hexadecyloxy)benzoic Acid (3a)
To a solution of compound 2a (6.17 g, 10 mmol) in THF (40
mL) and methanol (8 mL), a saturated aqueous solution of
KOH (2.24 g, 40 mmol) was added. The reaction mixture
was refluxed overnight. After cooled to room temperature,
1M HCl was added and the pH of solution was adjusted to
3–5, the mixture was concentrated in vacuo, and, the residue
was extracted with dichloromethane, and washed with brine.
The organic layer was dried over anhydrous MgSO4, filtered,
the solvent was removed under reduce pressure and a white
solid was obtained without further purification (5.90 g,
98%).
Characterization
Nuclear magnetic resonance (NMR) spectra were recorded in
chloroform-d (CDCl3) on an Ultra Shield 400 spectrometer
(Bruker BioSpin AG, Magnet System 400 MHz/54 mm). Mass
spectra (MS) were generated using a Micromass LCT electro-
spray mass spectrometer or by chemical ionization on Micro-
mass GCT mass spectrometer. FTIR spectra were recorded
from KBr pallets on a Nicolet Magna 5700 FTIR spectrome-
ter. UV–Vis analyses were performed on a UV-2550PC dou-
ble-beam spectrometer from CHCl3 solutions at room tem-
perature. The size and morphology of the products were
observed using a JEOL-200CX TEM for low-resolution images
(LRTEM, 120 kV). TGA of the samples were conducted on a
TG/WRT-2P. Raman spectra were recorded using a Renishaw
in ViaþReflex Raman spectrometer.
1H NMR (CDCl3, d, ppm): 11.01 (PhACOOH, 1H), 7.18
(PhAH, 2H), 6.65 (PhAH, 1H), 3.94(AOACH2A, 4H), 1.78–
Methyl 3,5-bis(hexadecyloxy)benzoate (2a)
This compound was synthesized according to literature pro-
cedure with minor modification.65 To a solution of methyl
3,5-dihydroxybenzoate (5.04 g, 30 mmol) in dry acetonitrile,
1-bromohexadecane (22.90 g, 75 mmol) and anhydrous
K2CO3 (20.73 g, 150 mmol) wereꢀ added. The reaction mix-
ture was heated overnight at 100 C in a sealed vessel. After
cooled to room temperature, all volatile components were
removed in vacuo. The crude product was extracted with
dichloromethane and washed with brine, dried over anhy-
drous MgSO4, and concentrated. The residue was purified by
column chromatography using petroleum ether/dichlorome-
thane (1/1) and petroleum ether/dichloromethane (1/4; Rf
¼ 0.5) as elutes. Final product was obtained as a white pow-
der (18.12 g, 98 %).
SCHEME 1 Synthesis of enediyne-containing G-1 (5) and G-2
(11) dendrimers.
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1.74 (AOACH2ACH2A, 4H), 1.41–1.43 (AOACH2ACH2A,
4H), 1.24 (ACH2ACH2ACH2A, 48H), 0.86 (ACH2ACH3, 6H).
give product as white solid (8.65 g, 98%), and the product
was used for next step without further purification.
1H NMR (CDCl3, d, ppm): 6.50 (PhAH, 2H), 6.38 (PhAH, 1H),
4.62 (PhACH2AOH, 2H), 3.95 (PhAOACH2A, 4H),
2.17(PhACH2AOH, 1H), 1.76 (AOACH2ACH2A, 4H), 1.43
(ACH2ACH2ACH3, 4H), 1.26 (ACH2ACH2ACH2A, 48H), 0.88
(ACH2ACH3, 6H).
4-(2-(2-(Trimethylsilyl)ethynyl)phenyl)but-3-ynyl
3,5-bis(hexadecyloxy)benzoate (4a)
1 (1.10 g, 4.57 mmol), 3,5-bis(hexadecyloxy)benzoic acid
(3a; 4.13 g, 6.85 mmol), and 4-dimethylaminopyridine (0.11
g, 0.91 mmol) were dissolved in dry dichloromethane (40
mL), and a solution of N,N0-dicyclohexylcarbodiimide (1.32 g,
6.40 mmol) in dry dichloromethane (10 mL) was added
drop wise. This mixture was stirred for 24 h under nitrogen
at room temperature and then filtered and concentrated. The
residue was purified on a silica-gel column (ethyl acetate/pe-
troleum ether ¼ 1:20, Rf ¼ 0.8) to give pure product as a
yellow oil (3.60 g, 95%).
1-(Bromomethyl)-3,5-bis(hexadecyloxy)benzene (7a)
To a solution of the compound 6a (8.0 g, 13.6 mmol) and
CBr4 (9.02 g, 27.2 mmol) in dry THF was added PPh3 (7.13
g, 27.2 mmol) slowly, the mixture was stirred at room tem-
perature for 25 min under nitrogen and then washed with
saturated NaHCO3 and brine, respectively. The organic phase
was dried over anhydrous MgSO4, filtered, and concentrated
by rotary evaporation, and the crude product was purified
by column chromatography using petroleum ether, dichloro-
methane/petroleum ether (1/4), and dichloromethane/petro-
leum ether (4/1) as elute (Rf ¼ 0.7), respectively. The final
product was obtained as a pale yellow solid (6.92 g, 78%).
1H NMR (CDCl3, d, ppm): 7.38–7.46 (PhAH, 2H), 7.21–7.23
(PhAH, 2H), 7.19 (PhAH, 2H), 6.63 (PhAH, 1H), 4.52
(ACCACH2ACH2A, 2H), 3.95 (PhAOACH2A, 4H), 2.95
(ACCACH2A, 2H), 1.75 (PhAOACH2ACH2A, 4H), 1.43
(PhAOACH2ACH2A, 4H), 1.26 (ACH2ACH2ACH2A, 48H),
0.88 (ACH2ACH3, 6H), 0.26 (s, SiACH3, 9H). 13C NMR
(CDCl3, d, ppm): 166.1, 160.1, 132.0, 131.7, 128.1, 126.2,
107.7, 106.6, 103.5, 98.2, 89.5, 80.8, 68.3, 62.8, 31.9, 29.7,
29.4, 26.0, 22.7, 20.2, 14.1, 0.0. MS (ESIþ): m/z calcd. for
1H NMR (CDCl3, d, ppm): 6.50 (PhAH, 2H), 6.38 (PhAH, 1H),
4.40 (PhACH2AOH, 2H), 3.94 (PhAOACH2A, 4H), 1.76
(AOACH2ACH2A, 4H), 1.42 (ACH2ACH2ACH3, 4H), 1.26
(ACH2ACH2ACH2A, 48H), 0.88 (ACH2ACH3, 6H).
C
54H86O4Si: 827.6374 [(MþH)þ]; found: 827.6411[(MþH)þ].
Methyl 3,5-bis(3,5-bis(hexadecyloxy)benzyloxy)benzoate
(8a)
4-(2-Ethynylphenyl)but-3-ynyl 3,5-bis(hexadecyloxy)
benzoate (5a)
To a solution of compound 7a (6.52 g, 10 mmol), 3,5-dihy-
droxybenzoate (0.76 g, 4.54 mmol), potassium iodide (0.075
g, 0.45 mmol), and 18-crown-6 ether (0.24 g, 0.9 mmol) in
dry acetone, potassium carbonate (12.58 g, 91 mmol) was
added. The mixture was refluxed for 48 h under nitrogen
and then the solvent was removed in vacuo and the residue
was extracted with dichloromethane and washed with brine.
The organic phase was dried over anhydrous MgSO4, filtered,
and concentrated under reduced pressure. The residue was
purified by column chromatography using petroleum ether/
dichloromethane (1/4) and petroleum ether/dichlorome-
thane (4/1) as elute (Rf ¼ 0.8), respectively. Final product
was obtained as a white powder (4.76 g, 80%).
To a solution of compound 4a (3.30 g, 4 mmol) in THF (30
mL) was added tetrabutylammonium fluoride (TBAF; 5.20 g,
5 equiv. to TMS groups). After stirring for 30 min at room
temperature under nitrogen, the solution was passed
through a short silica gel column and eluted with ethyl ace-
tate, the crude product was purified by column chromatogra-
phy using a mixture solvent (ethyl acetate/petroleum ether
¼ 1/20, Rf ¼ 0.5) to get the product as a yellow oil (2.01 g,
66%). Note: This reaction was run in dark environment.
1H NMR (CDCl3, d, ppm): 7.40–7.48 (PhAH, 2H), 7.24
(PhAH, 2H), 7.20 (PhAH, 2H), 6.63 (PhAH, 1H), 4.52
(ACCACH2ACH2A, 2H), 3.95 (PhAOACH2A, 4H), 3.26
(ACCAH,
1H),
2.96
(ACCACH2A,
2H),
1.76
1H NMR (CDCl3, d, ppm): 7.28 (PhAH, 2H), 6.79 (PhAH, 1H),
6.55 (PhAH, 4H), 6.40 (PhAH, 2H), 4.98 (PhAOACH2APhA,
4H), 3.93 (PhAOACH2ACH2A, 8H), 3.90 (ACOAOACH3, 3H),
1.78 (AOACH2ACH2A, 8H), 1.43 (ACH2ACH3, 8H), 1.26
(ACH2ACH2ACH2A, 96H), 0.88 (ACH2ACH3, 12H).
(PhAOACH2ACH2A, 4H), 1.43 (PhAOACH2ACH2A, 4H),
1.26 (ACH2ACH2ACH2A, 48H), 0.88 (ACH2ACH3, 6H). 13C
NMR (CDCl3, d, ppm): 166.3, 160.1, 132.5, 131.7, 128.4,
126.4, 107.8, 106.5, 89.9, 82.1, 80.9, 80.5, 68.3, 62.9, 31.9,
29.7, 29.4, 26.0, 22.7, 20.3, 14.1. MS (ESIþ): m/z calcd. for
C
51H78O4: 793.5537 [(MþK)þ]; found: 793.5552 [(MþK)þ].
3,5-Bis(3,5-bis(hexadecyloxy)benzyloxy)benzoic acid (9a)
To a solution of compound 8a (3.93 g, 3 mmol) in THF (10
mL) and methanol (2 mL), a saturated solution of KOH (0.23
g, 8 mmol) wasꢀ added. The reaction mixture was refluxed
overnight at 55 C. The resulting mixture was purified simi-
lar to compound 5 and a white solid was obtained without
further purification (3.76 g, 97%).
(3,5-Bis(hexadecyloxy)phenyl)methanol (6a)
2a (9.26 g, 15.0 mmol) was dissolved in dry THF and added
drop wise to a slurry of LiAlH4 (0.86 g, 22.5 mmol) in dry
ꢀ
THF at 0 C, the mixture was stirred overnight at room tem-
perature under nitrogen, and then the reaction mixture was
quenched with a small amount of water and filtered. The fil-
trate was concentrated under reduced pressure, the crude
product was extracted with dichloromethane and washed
with brine, dried over anhydrous MgSO4, concentrated to
1H NMR (CDCl3, d, ppm): 11.04(ACOOH, 1H), 7.32 (PhAH,
2H), 6.82 (PhAH, 1H), 6.55 (PhAH, 4H), 6.41 (PhAH, 2H),
4.99 (PhAOACH2APhA, 4H), 3.75 (PhAOACH2ACH2A, 8H),
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1.78 (AOACH2ACH2A, 8H), 1.44 (ACH2ACH3, 8H), 1.25
(ACH2ACH2ACH2A, 96H), 0.88 (ACH2ACH3, 12H).
tube, 5a (100 mg) was dissolved in NMP (8 mL) and added
slowly by a peristalsis pump to the suspension of MWCNTs
under nitrogen, and the reaction mixture was further heated
for 4 days. After cooled to room temperature, the mixture was
filtered through a 0.2-lm polytetrafluoroethylene membrane
and washed with large excess of diethyl ether and THF.
4-(2-(2-(Trimethylsilyl)ethynyl)phenyl)but-3-ynyl
3,5-bis(3,5-bis(hexadecyloxy) benzyloxy) benzoate (10a)
This compound was synthesized similar to compound 4a
starting from 3,5-bis(3,5-bis(hexadecyloxy)benzyloxy)benzoic
acid (9a) and 1. Pure product was obtained as a light yellow
powder, yield 90%.
ꢀ
MWCNT-5a was obtained after dried at 90 C for 24 h under
vacuum as a black powder (28.5 mg).
Yield 90%. 1H NMR (CDCl3, d, ppm): 7.38–7.44 (PhAH, 2H),
7.32 (PhAH, 2H), 7.19–7.21 (PhAH, 2H), 6.80 (PhAH, 1H),
6.54 (PhAH, 4H), 6.41 (PhAH, 2H), 4.96 (PhAOACH2APhA,
4H), 4.52 (ACCACH2ACH2A, 2H), 3.93 (PhAOACH2ACH2A,
8H), 2.94 (ACCACH2A, 2H), 1.76 (AOACH2ACH2A, 8H), 1.43
(PhAOACH2ACH2A, 8H), 1.26 (PhAOACH2ACH2ACH2A,
96H), 0.88 (ACH2ACH3, 12H), 0.26 (s, SiACH3, 9H). 13C NMR
(CDCl3, d, ppm): 165.9, 160.5, 159.8, 138.5, 132.1, 131.8,
128.1, 126.2, 108.5, 107.3, 105.8, 103.5, 100.9, 98.3, 89.4,
80.9, 70.3, 68.1, 62.9, 31.9, 29.7, 29.4, 26.1, 22.7, 20.2, 14.1,
0.0. MS (ESIþ): m/z calcd. for C100H162O8Si: 1520.2118
[(MþH)þ]; found: 1520.2121 [(MþH)þ].
Formation of MWCNT-Polymer Composites
Electrospinning was carried out using a syringe and a stainless
steel needle with flat tip at an applied voltage of 22 kV. A
grounded aluminum plate located at a distance of 25 cm from
the tip of the needle was used as a fiber collecting device. The
fibers were electrospun with a mixture solution of functional-
ized MWCNTs/PCL/THF at weight ratio of 0.01:0.1:1.
Oligomers of Enediyne-Containing Compounds
Free oligomers were prepared by heating a solution of ene-
diyne-containing compounds in NMP to reflux under nitro-
gen for 4 days. The products were obtained after removal of
solvent as dark red oil.
4-(2-Ethynylphenyl)but-3-ynyl 3,5-bis(3,5-bis(hexadecyloxy)
benzyloxy)benzoate (11a)
CONCLUSIONS
This compound was synthesized similar to compound 5a
starting from 10a. Pure product was obtained as a yellow
oil, yield 93%.
We have developed a new method for modification of CNTs.
The diradicals produced by Bergman cyclization can be
grafted onto the surface of the nanotubes and formed cross-
linked oligomers. After functionalization, MWCNTs showed
much better solubility in organic solvent such as THF,
CH2Cl2, and NMP. All the functionalized MWCNTs were char-
acterized with TGA, TEM, FTIR, UV–Vis, and Raman spectros-
copy. Results confirmed the occurrence of side wall modifica-
tion of MWCNTs. The surface grafted MWCNTs were
electrospun with PCL, uniformly distributed MWCNTs in
polymer fibers were found. Further expansion of reaction
scope of Bergman cyclization in functionalization of carbon
nanomaterials and exploration of MWCNT composites in bio-
applications are underway in our laboratory.
1H NMR (CDCl3, d, ppm): 7.41–7.46 (PhAH, 2H), 7.32
(PhAH, 2H), 7.21 (PhAH, 2H), 6.80 (PhAH, 1H), 6.54
(PhAH, 4H), 6.40 (PhAH, 2H), 4.97 (PhAOACH2APhA, 4H),
4.53 (ACCACH2ACH2A, 2H), 3.93 (PhAOACH2ACH2A, 8H),
3.24 (ACCAH, 1H), 2.94 (ACCACH2A, 2H), 1.78
(AOACH2ACH2A, 8H), 1.43 (PhAOACH2ACH2A, 8H), 1.26
(PhAOACH2ACH2ACH2A, 96H), 0.88 (ACH2ACH3, 12H).
13C NMR (CDCl3, d, ppm): 166.0, 160.5, 159.7, 138.5, 132.5,
131.9, 128.4, 126.3, 108.5, 107.2, 105.7, 100.9, 89.8, 82.1,
80.9, 80.6, 70.3, 68.1, 62.9, 31.9, 29.7, 29.4, 26.1, 22.7, 20.2,
14.1. MS (ESIþ): m/z calcd. for C97H154O8: 1470.1439
[(MþNa)þ]; found: 1470.1459 [(MþNa)þ].
This work was supported by National Natural Science Founda-
tion of China (20874026 and 20704013), Shanghai Shuguang
Project (07SG33), New Century Excellent Talents in University,
and Shanghai Leading Academic Discipline Project (B502). The
author A.H. thanks Prof. Xiaoyong Deng and Prof. Zheng Jiao of
Shanghai University for their kindly help in TEM.
Purification of MWCNTs
MWCNTs were purified according to literature procedure.66
As-received MWCNTs (150 mg) were sonicated for 30 min
and washed with aqueous hydrofluoride for three times, and
then refluxed in 6 M nitric acid for 24 h. After cooled to
room temperature, the reaction mixture was first rinsed with
deionized water and then filtered through a 0.2-lm PTFE
membrane. The residue was annealed at 250 ꢀC for 2 h
under air followed by heating to 450 ꢀC for 2 h under
nitrogen.
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