Improved synthesis of fullerynes by Fisher esterification for modular and efficient construction of fullerene polymers with high fullerene functionality

Article abstract of DOI:10.1016/j.polymer.2011.07.026

For the first time, the scope of Fisher esterification has been extended to fullerene derivatives to improve the synthesis of alkyne-functionalized fullerenes (fullerynes) using 1-chloronaphthalene as a solvent and a specially-designed, home-made reactor to promote high yield. The design allows for higher solubility of fullerene derivatives and a continuous azeotropic distillation for the removal of water to drive the reaction to completion with yields >90%. Both fullerynes (Fulleryne01 and Fulleryne02) were found to click to polymers, such as azide-functionalized poly(ε- caprolactone) (PCL-N₃), in high efficiency without the need for fractionation. As evidenced by ¹H NMR, ¹³C NMR, size exclusion chromatography, and matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, the fullerene polymers thus obtained possess well-defined structure, narrow polydispersity (~1.01 by MALDI-TOF mass spectrometry; ~1.03 by size exclusion chromatography), and high fullerene functionality (~100%). They can serve as model compounds for the investigations of polymer structures and dynamics.

Full text of DOI:10.1016/j.polymer.2011.07.026

Polymer 52 (2011) 4221e4226
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Improved synthesis of fullerynes by Fisher esterification for modular and efficient
construction of fullerene polymers with high fullerene functionality
,
Wen-Bin Zhang a, Jinlin He a b, Xuehui Dong a, Chien-Lung Wang a, Hui Li c, Fuai Teng c, Xiaopeng Li d,
,
,
a,
*
Chrys Wesdemiotis a d, Roderic P. Quirk a , Stephen Z.D. Cheng
*
a Institute and Department of Polymer Science, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, OH 44325-3909, USA
b College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
c School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
d Department of Chemistry, The University of Akron, Akron, OH 44325, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
For the first time, the scope of Fisher esterification has been extended to fullerene derivatives to improve
the synthesis of alkyne-functionalized fullerenes (fullerynes) using 1-chloronaphthalene as a solvent and
a specially-designed, home-made reactor to promote high yield. The design allows for higher solubility of
fullerene derivatives and a continuous azeotropic distillation for the removal of water to drive the
reaction to completion with yields >90%. Both fullerynes (Fulleryne01 and Fulleryne02) were found to
Received 13 June 2011
Received in revised form
18 July 2011
Accepted 20 July 2011
Available online 26 July 2011
“click” to polymers, such as azide-functionalized poly( 3 -caprolactone) (PCL-N3), in high efficiency
without the need for fractionation. As evidenced by 1H NMR, 13C NMR, size exclusion chromatography,
and matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, the
fullerene polymers thus obtained possess well-defined structure, narrow polydispersity (w1.01 by
MALDI-TOF mass spectrometry; w1.03 by size exclusion chromatography), and high fullerene func-
tionality (w100%). They can serve as model compounds for the investigations of polymer structures and
dynamics.
Keywords:
Click chemistry
Fisher esterification
Fullerene
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
fullerene polymers with high degree of functionality (w100%),
a variety of different compositions, and of complex architectures, in
Fullerenes are fascinating carbon nanostructures with wide-
ranging applications [1,2]. In order to improve pristine fullerene’s
compatibility with other functional materials, chemical modifica-
tions are often necessary, which has greatly expanded the scope of
fullerene materials and their applications [3]. Problems in doing
chemistry with fullerene usually involve, for example, the often
unavoidable multiple addition and drastic reaction conditions [4,5],
and result in a mixture of mono-addition product, multi-adducts,
and unfunctionalized C60. In small molecular fullerene deriva-
tives, chromatographic methods are effective in the separation and
purification of product. However, it can be a major concern for
polymers, biological systems, and other heterogeneous systems
where functional group reactivities are inherently low and purifi-
cation is often difficult, if possible. Although there have been many
reports on the synthesis of fullerene polymers of various architec-
tures with sufficient control, it is still challenging to prepare
a modular and efficient way [2,6]. Such polymer-tethered C60s are
typical model compounds for shape amphiphiles. Shape amphi-
philes usually refer to molecules possessing amphiphilic features
based on differences in the shape of the molecular segments and
are expected to show rich phase structure and self-assembling
behavior in bulk and solution [7e9]. In addition, since polymers
can often form supramolecular assemblies via self-assembly
processes, such as crystallization [10e12] and micellization
[13,14], it is also of great interest to study how the self-assembly of
polymers can assist the ordered arrangement of C60 units. Recently,
we have shown a “click” chemistry approach to fullerene polymers
by the model synthesis of well-defined C60-end-capped poly-
styrene with high fullerene functionality in high yield [15]. The Cu-
catalyzed Huisgen [3 þ 2] cyclcoaddition reaction [16e18] between
the alkyne-functionalized fullerene (thus named ‘fulleryne’) and
azide-functionalized materials proceeded efficiently at room
temperature without side reactions between azide and C60 core
(Scheme 1). This approach has also been reported by other
researchers in the synthesis of a variety of fullerene materials, such
* Corresponding authors. Tel.: þ1 330 972 6931; fax: þ1 330 972 8626.
E-mail addresses: rpquirk@uakron.edu (R.P. Quirk), scheng@uakron.edu
(S.Z.D. Cheng).
as porphyrin-C60 conjugate [19], fullerene sugar ball [20], C60
-
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.07.026

4222
W.-B. Zhang et al. / Polymer 52 (2011) 4221e4226
poly(N-isopropyl acryl amide) (PNIPAM) [21], C60-viral nano-
particle conjugates [22], and others [23e25]. No complicated
purification procedures, such as fractionation, were necessary in
these cases. Considering its modularity and efficiency, this
approach is quite advantageous in the construction of fullerene
materials for a variety of applications. In order for this approach to
be viable for practical applications, it is essential and critical to find
fullerynes that are readily available from C60 (synthetic facility),
very reactive (chemical reactivity), and potentially multifunctional
(material function). Fulleryne01 (Scheme 2), a C60 propiolate, has
been shown to possess “click” reactivity in reactions with azide-
functionalized polymer in our previous reports [15,26]. Its prac-
tical usage was hindered by its poor availability since the yield of
the final esterification step was only 17% when it was first reported.
In this article, we report an improved synthesis of Fulleryne01 with
up to 92% yield by Fisher esterification using 1-chloronaphthalene
as the solvent and a specially-designed, home-made reactor to
promote efficiency (Scheme 2). To the best of our knowledge, this is
the first example in doing Fisher esterification with fullerene
derivatives. This method was also applied with high yield to the
Scheme 2. Synthesis of Fulleryne01 and Fulleryne02 by Fisher esterification.
amount of product [31]. In the mixed anhydride route, the low
solubility of fullerene carbonochloridate in THF prevented its
effective reaction with sodium prop-2-ynoate [32]. The problems
are mostly associated with either the poor solubility of the fullerene
derivative in polar solvents or with side reactions, such as Michael
addition, that are promoted by the adjacent carbonyl group [32].
Fisher esterification is a classic reaction and has been widely
used to synthesize propiolates commonly in a high yield [33,34]. It
is known to be an equilibrium reaction between an alcohol and an
acid to give the desired ester and water under the catalysis of acidic
species (Scheme 2). Based on Le Chatelier’s Principle, with higher
concentrations of the starting materials, more products will form at
equilibrium. Moreover, if the side products, such as water, can be
continuously removed from the system, the equilibrium can be
pushed forward to the product side. Generally, Fisher esterification
works well with large scale preparations where a significant
amount of water is generated and removed. The DeaneStark trap is
often used in the laboratory to remove water by azeotropic distil-
lation with toluene or benzene [35]. The reaction usually involves
grams of sample and the aqueous and organic components forms
phase-separated layers in the trap, facilitating removal of water.
Alternatively, the Soxhlet extractor can also be used in combination
with activated molecular sieves for the effective removal of small
quantities of water [35].
synthesis of Fulleryne02 (Scheme 2), a derivative of phenyl-C61
-
butyric methyl ester (PCBM). With improved accessibility of full-
erynes, the “click” chemistry approach to fullerene materials can be
viable as a general, efficient method, and these fullerynes can serve
as versatile building blocks in the modular and efficient construc-
tion of advanced fullerene materials. Specifically as an example, in
the present work, C60-end-capped poly( -caprolactone)s (PCL-C60)
3
were synthesized with controlled molecular weight, narrow poly-
dispersity, and high fullerene functionality (w100%) by sequential
ring opening polymerization, nucleophilic substitution, and “click”
coupling with both Fulleryne01 and Fulleryne02 (Scheme 3). Being
a typical shape amphiphile with a semi-crystalline polymer tail,
PCL-C60s are expected to show interesting self-assembling behav-
iors based on the interplay between the crystallization of PCL and
aggregation of C60
.
2. Results and discussions
At first, the Fisher esterification did not seem to work well for
fullerene derivatives since they usually have low solubility in terms
of its molar concentration in toluene and benzene, which would
make the reaction very inefficient. For example, the solubility of C60
is only 2.1 ꢀ10ꢁ4 mol/L (1.7 mg/mL) in benzene and 4.0 ꢀ 10ꢁ4 mol/
L (2.8 mg/mL) in toluene [3]. Furthermore, considering the rela-
tively high molecular weight of fullerene derivatives (e.g., 792 Da
for fullerene alcohol 1), the water generated in this case is only at
the milligram scale if the reaction is carried out with a sample of
several hundreds of milligrams, which cannot be phase-separated
from toluene to be removed. For example, the water generated
from the reaction between 0.2 g of 1 and propiolic acid is 4.5 mg,
assuming 100% yield. If the solubility of 1 is similar to C60 in toluene
(2.8 mg/mL), the saturated toluene solution will have a volume of
71 mL, corresponding to a weight of 61.4 g (density of toluene is
0.86 g/mL). The concentration of water in toluene will be 0.0073 wt
2.1. Synthesis of fullerynes by Fisher esterification
It has been shown by both the mechanistic study and experi-
ment that the Huisgen cycloaddition reaction can proceed in high
efficiency under mild conditions when electron-deficient alkynes
are used [27e29]. Therefore, the terminal alkynes with an adjacent
carbonyl group, such as fullerene propiolates, are good candidates
for fullerynes with high reactivity. The preparation of propiolates
has been thoroughly studied in the literature, such as DCC/DMAP
mediated esterification of propiolic acid [30], the acyl chloride
method [31], and mixed anhydride route [32]. None of these
methods were effective in the preparation of fullerene propiolates
in our attempts. The addition of propynoyl chloride resulted in the
immediate formation of a resinous precipitate without detectable
Scheme 3. Synthesis of C60-end-capped poly( 3 -caprolactone)sa. a: (i) Sn(Oct)2, 3 -cap-
rolactone, toluene, 60 ꢂC, 55%; (ii) NaN3, DMF, 60 ꢂC, 94%; (iii) Fulleryne01 (or Full-
eryne02), CuBr, PMDETA, toluene, 85% (or 87% for Fulleryne02).
Scheme 1. A general functionalization methodology to fullerene materials via “Click”
chemistry.

W.-B. Zhang et al. / Polymer 52 (2011) 4221e4226
4223
%, far below the solubility limit of water in toluene (0.033 wt%) [36].
As a result, the reaction will be extremely sensitive to any residual
moisture in the system. Indeed, in a trial run of the reaction in
toluene using a DeaneStark apparatus overnight or using a Soxhlet
extractor with activated 4 Å molecular sieves, only trace amount of
product formation were observed (Table 1, Entry I). These issues
might account for the fact that there has not been any report on the
application of Fisher esterification to the synthesis of fullerene
derivatives. Nevertheless, by manipulating the reaction conditions,
it is still possible to improve the yield of Fisher esterification
because, after all, it is an equilibrium reaction.
The first approach was to increase the solubility of fullerene
derivatives in the system by using an uncommon solvent,
1-chloronaphthalene, as the reaction medium. Being a chlorinated
aromatic solvent with high boiling point (259 ꢂC/760 mmHg) and
inert under most reaction conditions, 1-chloronaphthalene is
known to be a relatively good solvent for fullerenes. Its solubility for
C60 is as high as 51 mg/mL (97 ꢀ 10ꢁ4 mol/L) [3]. Toluene was used
as a co-solvent to remove water azeotropically from the system. By
doing so, it has been shown that the reaction efficiency was greatly
improved. In the case of 1, the yield increased to 34% and 38% using
a Dean-Starks apparatus and a Soxhlet extractor, respectively
(Table 1, Entry II and III). For fullerene alcohol 2, which was
obtained by reduction of PCBM using DIBAL-H (in 93% yield, see
Supporting Information), the esterification was very efficient with
yields of 84% and 85%, respectively (Table 1, Entry II and III). Full-
eryne02 was then fully characterized by 1H NMR, 13C NMR, FT-IR,
and MALDI-TOF to confirm the chemical structure and purity (see
Supporting Information for characterization data and discussions).
An additional advantage of using 1-chloronaphthalene in Fischer
esterification is that, after the reaction is complete and toluene is
removed, the reaction mixture can be directly applied to the top of
a silica gel column for the recovery of 1-chloronaphthalene using
hexanes, the purification of the product using toluene, and the
recovery of unreacted alcohols by further eluting with toluene/
ethyl acetate (v/v ¼ 95/5), which increases the overall efficiency of
the process. The better yields obtained for 2 than 1 are probably
due to the higher solubility of 2 in the mixed solvent of 1-
chloronaphthalene/toluene. This process works sufficiently well
for the preparation of Fulleryne02, but more efforts are required to
optimize the preparation conditions of Fulleryne01.
Fig. 1. Schematic drawing of the reactor design for Fisher esterification of fullerene
derivatives.
flask (B), and toluene reservoir (C). All the reactants and solvents
were carefully purified and thoroughly dried before reaction. The
reactor was attached to a vacuum line and flame dried three times
before use. The reactants and 1-chloronaphthalene were placed in
flask A and the activated 4 Å molecular sieves were placed in flask B.
Being inert and efficient, molecular sieves were a perfect choice for
this purpose. Toluene was then transferred into flasks A and B by
vacuum distillation. After refilling the system with extra-dry
nitrogen, the flasks (A and B) were placed in two oil baths of
preset temperature (w120 ꢂC). Then a unidirectional circulating
system of toluene was formed from the reaction flask (A) to the
water-removal flask (B) to the toluene reservoir (C) and then back
to the reaction flask (A). The redistillation of toluene seemed
essential in the complete removal of water from the system, as
reflected by the results. The cycle went continuously to push the
reaction to completion. In this case, both Fulleryne01 and Full-
eryne02 were obtained in over 90% yields (Table 1, Entry IV), which
can be considered remarkable when compared to the trace amount
of product in the first trial. Both the syntheses of Fulleryne01 and
Fulleryne02 have been scaled up to several grams of product in our
laboratory using this method. We are trying to scale it up to even
larger quantities to make it commercially viable. With the ready
accessibility of fullerynes, the “click” approach to fullerene mate-
rials can be viewed as a modular and efficient one. As an example,
a series of C60-end-capped PCLs (PCL-C60) was synthesized using
both Fulleryne01 and Fulleryne02, which will be discussed in the
following section.
The next attempt was made to completely exclude the moisture
from the system and to drive the reaction equilibrium to the
product side. Similar to anionic polymerization where the most
stringent reaction conditions are required, it is possible to reduce
the water content down to parts per million (ppm) levels with
glass-blowing and high vacuum line techniques [37,38]. To this end,
a special reactor was designed based on our understanding of the
reaction mechanism and made by glass-blowing techniques (Fig. 1
and Fig. S1). Fig. 1 shows a schematic drawing of this reactor and
Fig. S1 (see Supporting Information) is a photograph of a real
reactor. It consists of three flasks: reaction flask (A), water-removal
2.2. Modular functionalization of PCL with fullerynes (synthesis of
PCL-C60
)
The synthesis of fullerene poly(
3
been reported by several groups in the past, using either a “graft-
ing-from” approach or a “grafting-to” approach. In the first
approach, varying numbers of hydroxyl groups were first installed
onto the fullerene core to initiate the ring opening polymerization
Table 1
Reaction conditions and yields for Fisher esterification of fullerene alcohol 1 and 2.
of -caprolactone [39e41]. In the latter approach, the PCL was first
3
Entry Solvent
Reactor
Fullerene Yield
alcohol
synthesized with reactive groups located at different positions
along the chain, typically an azide at the chain-end or along the
chain [5,42e44]. Their addition to C60s gave end-capped PCL or star
PCL with a C60 core, depending on the stoichiometry [5,44]. The
effects of conjugation on crystallization, degradation behavior, and
fiber preparation by electro-spinning were studied [5,41,42,44]. The
direct addition of azide-functionalized polymers to C60 is a simple
and straightforward method widely used in literature for the
preparation of fullerene polymers [6,45e47]. It is known to proceed
I
Toluene
DeaneStark apparatus
1
2
1
2
1
2
1
2
trace
trace
34%
84%
38%
85%
92%
93%
II
Toluene/1-Chloronaphthalene DeaneStark apparatus
Toluene/1-Chloronaphthalene Soxhlet apparatus
Toluene/1-Chloronaphthalene Home-made reactor
III
IV

4224
W.-B. Zhang et al. / Polymer 52 (2011) 4221e4226
the validation of theory predictions on the phase behavior of shape
amphiphiles [8,48]. Our approach offers a valuable alternative to
the literature results by the possibility to prepare such model
compounds by “click” chemistry.
The azide-functionalized PCLs were prepared from 3-bromo-1-
propanol by ring opening polymerization of -caprolactone and
3
subsequent nucleophilic substitution in a procedure similar to
literature reports (see Supporting Information) [5]. The ring
opening polymerization of -caprolactone catalyzed by Sn(Oct)2 is
3
known to proceed in a controlled fashion [49,50]. The presence of
halogen functionality, such as chloro or bromo, does not interfere
with the polymerization [5,43]. The polymer was obtained with
a molecular weight of 3.6 kg/mol and a polydispersity of 1.18 as
determined by size exclusion chromatography. The presence of
bromo end-functionality is demonstrated by the triplet in the 1H
NMR spectrum at
ring with sodium azide at 60 ꢂC in DMF overnight, the peak
completely shifts upfield to 3.39 ppm. The characteristic
d 3.48 ppm. After conversion to azide by stir-
d
absorption band of azide in FT-IR spectrum is also evident at
2098 cmꢁ1 (see Supporting Information, Fig. S5). As a modular
feature, the azide-functionalized PCL (PCL-N3) (1 equivalent) was
reacted with fullerynes (1.05 equivalents) under identical condi-
tions to give PCL-C60I and PCL-C60II, respectively for Fulleryne01
and Fulleryne02. The “click” procedure was as simple as mixing
the reactants with catalyst, degassing, and stirring at room
temperature overnight (see Supporting Information). After chro-
matographic removal of excess fullerynes and residual catalyst
followed by precipitation into methanol, PCL-C60s were obtained
as brown powders in high yields (85% and 86% for PCL-C60I and
PCL-C60II, respectively).
The success of the reaction was evidenced by the complete
disappearance of the characteristic absorption band of azide at
2098 cmꢁ1. The chemical structures were confirmed by 1H NMR
(Fig. 1), 13C NMR (Fig. S4), and MALDI-TOF mass spectrometry
(Fig. 3). The comparison of 1H NMR spectra of fullerynes and
corresponding fullerene polymers is shown in Fig. 2. For PCL-C60I,
the formation of a triazole ring was demonstrated by the sharp
peak appearing at
signal assignable to methylene protons adjacent to the azide shifts
from 3.39 ppm completely to 4.56 ppm. The rest of the protons
on Fulleryne01 can be clearly visible at 6.42, 3.29, and 2.90 ppm,
d
8.23 ppm in the 1H NMR spectrum, and the
Fig. 2. 1H NMR spectra of Fulleryne01 (a), PCL-C60I (b), Fulleryne02 (c), and PCL-C60II
(d). The * and # denote the residual solvent of toluene and water, respectively.
d
d
d
while some of them overlap with protons on the PCL backbone.
For PCL-C60II, the proton on the triazole ring can be seen as
with a less tendency toward multi-addition and crosslinking,
providing sufficient control in the synthesis of fullerene polymers
for most purposes. Nevertheless, well-defined structure, high C60
functionality (w100%), and high molecular uniformity are highly
desired for many physical studies, such as single crystal growth and
a sharp peak at
lene protons adjacent to the azide shifts from
completely to 4.47 ppm, which overlaps with the methylene
group of the ester group on Fulleryne02 (proton g). The other
d
8.04 ppm, and the signal assignable to methy-
d
3.39 ppm
d
Fig. 3. MALDI-TOF mass spectra of PCL-C60I (a), and PCL-C60II (b). The insets are the corresponding overview of the spectra.

W.-B. Zhang et al. / Polymer 52 (2011) 4221e4226
4225
shoulder in SEC trace could only be attributed to the formation of
aggregates.
3. Conclusions
In summary, we have successfully extended the scope of Fischer
esterification to fullerene derivatives for the efficient synthesis of
fullerynes including Fulleryne01 and Fulleryne02 in excellent
yields. A non-typical solvent, 1-chloronaphthlene, was used as the
reaction medium to increase the molar concentration of the reac-
tants. A special reactor was designed to allow the continuous
circulation of toluene to remove the water by azeotropic distillation
and moisture adsorption with molecular sieves. These ready avail-
able fullerynes can serve as versatile building blocks in the modular
and efficient construction of fullerene polymers, as demonstrated
by the synthesis of PCL-C60I and PCL-C60II. These fullerene polymers
possess well-defined structure and high degree of functionality of
C60 (w100%). They can serve as model compounds in the study of
the physics of fullerene polymers. Relevant work currently under
intense investigation in our group includes the phase behavior of
shape amphiphiles and the use of self-assembly of polymers, such
as single crystal growth and micellization, as templates to manip-
ulate the ordered structure formation of C60 in multiple dimensions
across different length scales.
Fig. 4. SEC overlay of PCL-Br (dashed blue), PCL-N3 (dashed red), PCL-C60I (solid
purple), and PCL-C60II (solid orange).
protons on Fulleryne02 essentially remain at the same chemical
shifts, at
spectra of both samples (Fig. S4), the sp2 carbons appear around
125e150 ppm, which are due to the presence of the C60 cage in
d
7.90, 7.48, 2.91, and 1.71 ppm, respectively. In 13C NMR
d
the polymer. In FT-IR spectra (Fig. S3), the characteristic sharp
absorption at 527 cmꢁ1 from the vibration of CeC bonds on C60
can also be seen. The most convincing evidence comes from
MALDI-TOF mass spectrometry. The spectra are shown in Fig. 3.
For both PCL-C60I and PCL-C60II, only a major single distribution
was observed in addition to several very minor distributions. The
m/z values of the major distribution match the calculated mono-
Acknowledgments
isotopic mass of the fullerene polymers very well. Since PCL-C60
I
and PCL-C60II have the same precursor PCL-N3, the peak of
maximum intensity for both samples are both found to be the 25-
This work was supported by NSF (DMR-0906898 to S. Z. D. C.;
DMR-0821313 and CHE-1012636 to C. W.). J. H. acknowledges the
China Scholarship Council for financial support. W.-B. Z. acknowl-
edges the Lubrizol Corp. for a fellowship.
mer. The calculated monoisotopic mass for the 25-mer of PCL-C60
I
is 3819.8 Da ([M25∙Na]þ) and the observed m/z equals 3820.3. The
experimental m/z for PCL-C60II is 3910.6, which fits the calculated
monoisotopic mass of 3909.9 ([M25∙Na]þ) for the 25-mer. The
minor distributions of very low intensity should be PCLs and/or
PCLs fragments with different chain end-groups that are possibly
generated during the ionization or post-source process. The clean
MALDI-TOF mass spectra without signals of homopolymers and
higher addition products are prime evidence of the products’ high
purity and high C60 functionality. The chemical structures of PCL-
C60 fullerene polymers are thus unambiguously proven. There is
only one C60 per chain connected with the polymer in a well-
defined linkage as proposed in the scheme. The polydispersity
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.polymer.2011.07.026.
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calculated from the MALDI-TOF spectra is 1.01 for both PCL-C60
and PCL-C60II, which is very narrow.
I
Fig. 4 shows the SEC overlay of PCL-Br, PCL-N3, PCL-C60I, and
PCL-C60II. The elution profile of both PCL-Br and PCL-N3 is a mon-
omodal, symmetric peak with relatively narrow molecular weight
distribution (PDI ¼ 1.18). Notably, although the attachment of a C60
to the PCL chain significantly increases the overall molecular
weight, the elution volumes of fullerene polymers remain essen-
tially the same as that of PCL-N3. The polydispersity determined by
SEC is 1.09 and 1.03 for PCL-C60I and PCL-C60II, respectively, which
is lower than that of the precursor PCL-N3 (PDI ¼ 1.18), but higher
than that determined by MALDI-TOF mass spectrometry. This is
because direct mass spectrometric analysis is known to under-
estimate the molecular weight and polydispersity of the mate-
rials [51]. In addition, there appears a high molecular weight
shoulder in the SEC trace of PCL-C60s. This is very likely due to the
aggregation of PCL-C60s in THF (a typical non-solvent for C60),
which is consistent with previous reports on fullerene polymers
[52]. To verify that it is due to aggregate formations rather than
multi-adducts, the samples were also analyzed by linear mode
MALDI-TOF mass spectrometry since it generally has higher
sensitivity than reflectron mode [53]. No high molecular weight
impurities could be observed in this case (Fig. S8). Therefore, the
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