Ring opening metathesis polymerization on non-covalently functionalized single-walled carbon nanotubes

Article abstract of DOI:10.1039/b211194b

Norbornene polymerization has been initiated selectively on the surface of single-walled carbon nanotubes (SWNTs) via a specifically adsorbed pyrene-linked ROMP initiator, resulting in a homogeneous non-covalent poly(norbornene) coating.

Full text of DOI:10.1039/b211194b

Ring opening metathesis polymerization on non-covalently functionalized
single-walled carbon nanotubes†
Fernando J. Gómez, Robert J. Chen, Dunwei Wang, Robert M. Waymouth* and Hongjie Dai*
Department of Chemistry, Stanford University, Stanford, California, USA. E-mail: waymouth@stanford.edu,
hdai@stanford.edu; Fax: 650-725-0259
Received (in Purdue, IN, USA) 18th November 2002, Accepted 29th November 2002
First published as an Advance Article on the web 12th December 2002
Norbornene polymerization has been initiated selectively on
the surface of single-walled carbon nanotubes (SWNTs) via
a specifically adsorbed pyrene-linked ROMP initiator,
resulting in a homogeneous non-covalent poly(norbornene)
coating.
1
Scheme 1 Complete functionalization strategy. Path A: Adsorption of
organic precursors (e.g. 2 or 3) followed by cross-metathesis with a
ruthenium alkylidene. Path B: Adsorption of a pryene-substituted ruthe-
nium alkylidene (e.g. 4).
Since their discovery in 1991, carbon nanotubes (CNTs) have
been the focus of numerous investigations due to their unique
structural and electrical characteristics. Their outstanding
sensitivity to chemical doping,2 —as is manifested in dramatic
changes of resistance upon the adsorption of certain molecules
in the gas phase—coupled with their diminutive size, reveals a
vast potential for nanoelectronics and chemosensing applica-
tions. Key to bringing this potential to realization is a detailed
investigation into the nature of association phenomena between
,3
nanotube of approximately 40 nm diameter (polymer coating
thickness on the order of ~ 15 nm). A representative uncoated
nanotube of ~ 5 nm diameter is shown in the inset.21 Optimum
results in terms of selective functionalization were obtained
with strategies involving exposure of the nanotubes to solutions
of pyrene-substituted alkylidene 4 (Path B in Scheme 1):
homogeneous coatings of variable thickness were obtained in
almost all of the experiments which employed amide 4 and
polymerization times less than 2 h (Table 1, entries 4, 10 and
4,5
nanotubes and various chemical environments.
For this
purpose, and in order to enhance their compatibility with
various matrices and their solubility in conventional media, a
series of nanotube sidewall- and end-functionalization schemes
are currently under exhaustive investigation.6
–10
Furthermore,
1
1).
Initial experiments attempted to convert adsorbed organic
the controlled introduction of binding sites as a tool in the
tailoring of the sensing properties of CNTs while maintaining
their composition essentially intact poses new challenges to the
synthetic chemist. In this regard, polymer functionalization
constitutes an attractive alternative to covalent modification of
species (e.g. 2, 3) in-situ to a ruthenium initiator employing
cross-metathetical alkylidene exchange (Scheme 1, Path A).
Incubation of SWNT-containing TEM grids in solutions of
(isolated) norbornenylmethyl ester 2 in various solvents led to
inefficient functionalization, and no clear evidence of selective
PNBE coating was found after attempted alkylidene formation
CNTs,11–15
mainly due to the unique physicochemical proper-
ties of polymers, and the possibilities inherent to the introduc-
tion of specific functional groups in their primary structure.
We have engaged in the development of functionalization
strategies based on sidewall adsorption of a polymerization
initiator followed by polymer growth off the nanotube side-
walls. Such an approach not only ensures selective coating with
organic polymer but also provides a non-destructive modifica-
tion of the nanotube structure. In this communication, we report
on the preparation of polynorbornene-coated single-walled
CNTs (SWNTs) using ring opening metathesis polymerization
(
ROMP). The variety of functional groups and macromolecular
architectures amenable of ROMP have already made of it a
valuable tool in the functionalization of surfaces and parti-
cles.16–20
In a previous study, we demonstrated that polycyclic
aromatics such as pyrene could serve as anchors for selective,
non-covalent sidewall functionalization of SWNTs. With this
in mind, we explored two routes towards SWNTs function-
alized with ruthenium alkylidenes (Scheme 1) which make use
of cross-metathesis reactions both on and off the nanotube
walls. In a representative experiment (e.g. Scheme 1, Path B), a
gold TEM grid or silicon oxide substrate containing as-grown
Scheme 2 Synthesis of pyrene-funtionalized initiators and precursors.
3 2
Reagents: (a) 2-Norbornen-5-ylmethanol, Et N, DMAP (cat.), Et O; (b)
4
4
-vinylaniline, Et N, CH Cl ; (c) Cl (PCy RuNCHPh, CH
3
2
2
2
3
)
2
2
Cl
2
.
2 2
SWNTs‡ was immersed in CH Cl solutions of 4 (Scheme 2),
rinsed to remove non-specific adsorption, and exposed to
norbornene (NBE) solutions.
Fig. 1 shows a TEM image of SWNTs coated with
polynorbornene (PNBE). As implied by microscopy, this
particular sample (Table 1, entry 4) shows a PNBE-coated
Fig. 1 Transmission electron microscopy of a single walled carbon nanotube
after polymer functionalization (Table 1, entry 4). A representative as-
grown tube (diameter ~ 5 nm) can be observed in the inset (not to scale).
Note that SWNTs grown directly on our TEM grids are reproducible and
appear similar in yield and structure for different batches.21
†
Electronic supplementary information (ESI) available: full experimental
details for compounds 2–4, nanotube preparation and microscopy analysis.
See http://www.rsc.org/suppdata/cc/b2/b211194b/
1
90
CHEM. COMMUN., 2003, 190–191
This journal is © The Royal Society of Chemistry 2003

Table 1 Polynorbornene coating on SWNTsa
a cylindrical object at the molecular level. While the thickness
of the polynorbornene coatings on SWNTs increases with
increasing monomer concentrations as predicted (Table 1,
entries 4–7), it does not exhibit expected behavior with
increasing polymerization time (Table 1, entries 1–4). Reaction
times longer than 5–10 min, in fact, result in thinner coatings
until there is virtually none at 120 min. This may be due in part
to a decreasing ability of the pyrenyl group to continue to
effectively anchor the polymer chain onto the SWNT while
solubilization of the polymer becomes increasingly more
energetically favorable as it grows longer and longer. Further
efforts are directed at studying the effects that variables such as
polymer desorption can have on the polymer thickness. To
resolve this issue, we are currently exploring other polymeriza-
tion reactions on the sidewalls of SWNTs with polymers that are
less soluble in the reaction solvents.
Polym.
[Norbornene]/M time/min
Coating
thickness/nm
Entry
Pathway
1
2
3
4
5
6
7
8
B
B
B
B
B
B
B
A
A
B
B
0.5
0.5
0.5
0.5
0.25
0.10
0.05
0.01
0.5
120
60
20
5
5
5
None
< 5
< 5
10–20
< 5
< 5
None
None
None
5–10
5–10
5
b
25
25
1
b
9
c
1
1
a
0
0.01
0.01
c
1
5
2 2
Conditions: SWNTs on Au TEM grids. [4] ≈ 25 mM (CH Cl ), 2 h
b
incubation time, room-temperature polymerization, toluene solution. 2 (5
In conclusion, we have developed a flexible methodology for
the non-covalent functionalization of SWNTs with organic
polymer employing ring-opening metathesis polymerization.
The authors thank DARPA-MTO for funding (Grant
c
2
mM); 5 (10 mM). SiO flat substrates.
with Cl
tions of various NBE concentrations (Table 1, entries 8 and
).
Functionalization of as-grown SWNTs (using 10 mM NBE
solutions) deposited onto SiO substrates utilizing ferritin-
derived catalysts22 yielded samples suitable for AFM imaging
tapping mode). A representative nanotube of approximate
2 3 2
(PCy ) RuNCHPh (5) followed by exposure to solu-
#
N66001-02-1-8911), and Prof. Ken Wagener (U. of Florida)
for a gift of benzylidene 5. The contributions of Dr Wenhong
Fan and Mr. Yiming Li are highly appreciated.
9
2
(
Notes and references
diameter = 1.4 nm is shown in Fig. 2(a); Fig. 2(b) shows a
PNBE-coated SWNT (Table 1, entry 11) of approximate
diameter = 9.1 nm, corresponding to a polymer coating of ca.
‡
SWNTs on TEM microgrids21 and silicon oxide22 substrates were
prepared in a manner previously described. See ESI† for a detailed
experimental description.
7.7 nm, as determined by the respective cross-sectional height
profiles. As observed in the AFM image, the SiO surface was
2
1
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essentially free of PNBE, which provides further support for the
high selectivity of the functionalization step using polycyclic
aromatics.
A series of control experiments was incorporated into our
study as a probe for specific physisorption of the pyrenyl group
as the functionalization mechanism. For example, no polymer
was observed when the SWNTs were exposed to solutions of
ruthenium benzylidene 5 instead of the pyrenyl-functionalized
alkylidenes. Similarly, exposure of the SWNTs-containing
grids to solutions of polymer chains already grown using either
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Fig. 2 Atomic force microscopy of (a) a non-functionalized SWNT and (b)
a SWNT coated with polynorbornene (Table 1, entry 11).
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