One-pot radical polymerization of one inimer to form one-dimensional polymeric nanomaterials

Article abstract of DOI:10.1002/adma.201203230

A one-pot polymerization strategy is put forward for producing 1D polymeric nanomaterials directly from a single inimer. An inimer bearing an NMP initiating site is polymerized in the presence of a RAFT CTA to form a pearl-necklace structure constituted o

Full text of DOI:10.1002/adma.201203230

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One-Pot Radical Polymerization of One Inimer to Form
One-Dimensional Polymeric Nanomaterials
Pengju Ma, Hui Nie, Qingqing Zhou, Yuesheng Li,* and Hewen Liu*
Inspired by their unique properties and widespread applications
ranging from biological sciences to materials engineering, one-
dimensional (1D) organic nanomaterials kept attracting much
interest of researchers in developing novel synthetic strate-
gies which would have far-reaching impact in broad fields.[1]
1D polymer nanoparticles were usually obtained via either
tedious organic chemistry approaches to obtain covalently
linked moieties, such as dendronized polymers and fullerene-
based one-dimensional nanopolymers;[2,3] or supramolecular/
self-assembly approaches,[4] or physical confinement,[5] etc.
Molecular congregating under the influence of multifarious
intermolecular forces, for example, self-assembly of di/multi-
block copolymers or polymer brushes, formed colloidal 1D
nanoparticles.[4] Physical confinement, for example, electro-
spinning or crystallization inside cylindrical nanopores, could
likewise form nanofibers.[5] Though polymers played important
roles, as architectural units or templates, etc, in all the above-
mentioned strategies, few approaches were reported to form
covalent 1D nanomaterials in a one-pot way directly from mon-
omers as polymerization chemistry usually does.
Free radical polymerization is perhaps the most common
and facile reaction for making a wide variety of different
polymers directly from monomers. Extension of the power of
polymer chemistry has long been the target of research efforts,
and research in the field is ever increasing with new types of
materials and methodologies being continually discovered.[6,7]
It is well established that self-condensing polymerization of
an inimer yielded hyperbranched polymers.[6] Living radical
polymerizations, including atom transfer radical polymeriza-
tion (ATRP), nitroxide-mediated polymerization (NMP), and
reversible addition fragmentation transfer polymerization
(RAFT), and their concurrent/successive combinations have
become powerful tools for the design of macromolecular archi-
tectures, especially topological structures.[7] However, generally
only crosslinked structures or hyperbranched polymers could
be obtained in pursuing three-dimensional macromolecular
structures via one-pot polymerization chemistry.[8]
In this work, we intend to synthesize 1D nanomaterials
via one-pot concurrent living/controlled polymerizations of
inimers. It is proposed that the 1D nanomaterials could be
formed by the linkage of hyperbranched polymeric intermedi-
ates via trapping active end groups in the one-pot free radical
polymerization (Scheme 1). This approach could form 1D
pearl-necklace polymers directly whose molecular configuration
could be more stable than polymer brushes, especially in solu-
tions. Moreover, the 1D nano-structures could be constituted
with not only “homogeneous” building blocks but also “copoly-
meric” units.
We consider the concurrent living free radical polymeriza-
tions of an inimer bearing an ATRP/NMP initiating site in the
presence of a RAFT chain transfer agent (CTA). Both ATRP
and NMP can directly generate free radicals for propagation
if capping groups are activated; whereas RAFT seems to regu-
late the propagating way of radicals through balanced reactions
with existing radicals. Because of the balance as illustrated in
Scheme 1c, the contents of free radicals for either RAFT or
ATRP/NMP are the same. Both the kinetic length of chains
stemming from either RAFT or ATRP/NMP and the probability
of branching are nearly the same. Thus most probably, no het-
erogeneous propagation directions can be distinguished in the
formation of hyperbranched polymers. Though the intermo-
lecular linkage of hyperbranched polymers through the combi-
nation of active ATRP/NMP sites is suppressed to the lowest
possibility, however, RAFT CTA sites are apt to trap radicals
generated from active ATRP/NMP sites by nature. Especially
the so-called cross-termination can form stable bonds between
CTA and radicals.[9] Thus one hyperbranched polymer can joint
other hyperbranched polymers one by one to form a pearl-neck-
lace structure constituted of repeating hyperbranched polymer
“pearls” (Scheme 1d). The diameter of the 1D nanostructure is
determined by the size of the hyperbranched polymeric inter-
mediates. The ideal case is that only one RAFT CTA site in a
hyperbranched polymer can bind any one active ATRP/NMP
site in another hyperbranched polymer to form a linkage. This
is possible in a general RAFT system with a molar ratio of
monomer to CTA, for example, 100 or higher. An NMP inimer
is used in this work instead of ATRP inimers because of two
considerations, i.e. the ease in the view of the stoichiometry
and avoidance of metal ions’ interferences with RAFT CTA.
To verify the feasibility, we synthesized an inimer bearing an
NMP initiating site, 2,2,6,6-tetramethyl-1-(4-vinyl-benzyloxy)-
piperidine(PSt), via spin-trapping of the radicals generated in
a redox reaction between the chloride group of 4-vinylbenzyl
chloride and a transition metal catalyst. PSt was obtained as
colorless liquid. The yield of pure PSt was 64.2%. The struc-
ture of PSt was verified by 2D 1H-13C correlation NMR, and MS
(Figure S1-S3 in the Supporting information).
Prof. H. Liu, P. Ma, H. Nie, Q. Zhou
CAS Key Laboratory of Soft Matter Chemistry
Department of Polymer Science and Engineering
University of Science and Technology of China
Hefei, Anhui, China, 230026
E-mail: lhewen@ustc.edu.cn
Prof. Y. Li
State Key Laboratory of Polymer Physics and Chemistry
Changchun, China
E-mail: ysli@ciac.jl.cn
DOI: 10.1002/adma.201203230
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Scheme 1. Self-condensing polymerization of an inimer in the presence of RAFT CTA.
The free radical polymerization of PSt was carried out at
130 °C with addition of RAFT CTA ([PSt] = 0.244 M, molar
[PSt]/[CTA] = 110) to form concurrent RAFT and NMP dual
controlled polymerization systems. The [PSt]/[CTA] feed
ratio was chosen based on optimized results in the literature.
TEMPO-based NMP systems demanded high polymerization
temperature (125-145 °C) for the dissociation of the C–O bond
in the NMP initiating site. Two other kinds of polymerization
were carried out as well as control tests: TEMPO-mediated self-
condensing vinyl polymerization of PSt at 130 °C to form hyper-
branched polymers, and RAFT polymerization of PSt at 50 °C
with addition of azobisisobutyronitrile (AIBN) as the initiator to
form linear polymers. N-methyl-2-pyrrolidone was used as the
reaction media. When N-methyl-2-pyrrolidone was used as the
reaction media, the polymer yields in the concurrent NMP and
RAFT reached nearly 100% in 36 hours without gelling.
(Mw/Mn) during self-condensing NMP and concurrent NMP/
RAFT of PSt (Figure 1). GPC data manifested that the concur-
rent NMP/RAFT of PSt showed typical kinetic characteristics
of step condensation polymerization, differing from the kinetic
characteristics of the formation of hyperbranched polymers
in the self-condensing NMP. At the early stage of the concur-
rent NMP and RAFT, the polymers showed narrow molecular
weight distribution (Mw/Mn ∼ 1.1) which indicated a tightly
controlled manner in the dual controlled polymerization sys-
tems. The widening of molecular weight distribution in longer
polymerization time was not because of out-of-control on the
free radical polymerization. Concerning that the low Mw peak
always appeared in the wide molecular weight distribution, our
assumption that the polymers were formed by step-combina-
tion of hyperbranched pearls was reasonable.
Because of rigidity in structures, mainly signals arising
from TEMPO end groups and phenyl groups were observed in
either solid or solution 13C NMR of these polymers (Figure S4).
Gel permeation chromatography (GPC) was used to monitor
the weight average molecular weight (Mw) and its distribution
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Figure 1. Molecular weight and structural analysis via FTIR. (A) and (B) were the GPC chromatograms of polymers obtained inconcurrent NMP/RAFT
system and self-condensing NMP, respectively. Polymerization time in A (hours): a: 6; b: 12; c: 19.5; d: 30; e: 31; f: 34; and in B: g: 6; h: 12; i: 24; j:
32; k: 40. C and D was calibrated from A and B, respectively. (E) FTIR absorbance of the polymers obtained in the concurrent NMP/RAFT systems in
polymerization time of 6 (a), 10 (b), 15 (c), 20 (d), and 25 hours (e), and (F) The ratios of the absorbance at 1134 cm−1 to that at 1512 cm−1 versus
polymerization time.
In contrast, FTIR was an efficient tool to monitor the struc-
tural changes during the concurrent living polymerizations
of inimers. We studied the FTIR spectra of these polymers,
TEMPO and the monomer. A peak at 1134 cm−1 arose from the
skeletal vibration in TEMPO. Another peak at 1512 cm−1 was
due to a vibration mode of aromatic rings (Figure 1E). The two
peaks were sharp and strong, and were not affected by overlap
polymers were observed to decrease, and to reach a constant
value of ∼0.5. This indicated that the contents of aromatic struc-
tures increased with polymerization time, or vice versa, the
contents of TEMPO groups decreased. The trends elucidated
in the curve of Abs1134/Abs1512 versus time were in accord with
the analysis of the fractions of terminal groups in SCVP.[10] The
FTIR results revealed a highly branched nature of the polymers
obtained in the concurrent dual controlled polymerization. Fast
developed molecular weight but constant Abs1134/Abs1512 value
at the later stage of the polymerization supported the mecha-
nism of combination of hyperbranched intermediates.
with other peaks; thus the absorbance ratios of them (Abs1134
/
Abs1512) could be used to characterize the structural changes
during polymerization. The Abs1134/Abs1512 for the monomer
PSt was about 2.1, whereas was greater than 2.8 for a linear
polymer obtained by RAFT polymerization at relative low tem-
perature. Along with elongated polymerization time in the con-
current living polymerizations, the absorbance ratios in these
To verify the step-combination mechanism, the hyper-
branched “pearls” were isolated and purified, and then were
subjected to the polymerization conditions. The well-isolated
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polymer b with narrow molecular weight dis-
tribution (b in Figure 1) was chosen. After
being heated at 130 °C for 30 hours, polymer
parts with Mw above 106 were observed via
GPC in the solution of polymer b; whereas
Mw of the hyperbranched polymer h obtained
in Figure 1 was almost not changed in the
same conditions. This experiment verified
the recombination mechanism, as illustrated
in Scheme 1(d). Obviously, if different hyper-
branched polymer “pearls” obtained from dif-
ferent inimers were combined in the polymer-
ization, a pearl-necklace structure constituted
of “copolymeric” building blocks could be
produced. By using molecular weight control
techniques, Janus particles could be possibly
obtained as well.
The sulfur contents due to RAFT CTA
in the polymers were measured with X-ray
photoelectron spectroscopy (XPS), and were
found to be 0.13 0.1 atom% in the poly-
mers obtained from dually controlled systems
(Figure S7), which were close to the value
(0.14 0.1 atom%) found in the linear poly-
mers obtained via RAFT polymerization. The
additional results proved that RAFT CTA was
involved in the dually controlled polymeriza-
tion systems.
The obtained polymers were mixtures of
hyperbranched polymers and polymer nano-
rods, as evidenced by GPC and TEM observa-
tion. The polymer nanorods were well soluble
in tetrahydrofuran (THF), N-methyl-2-pyrro-
Figure 2. (A) TEM of sample d with average length of 137 15 nm and width of 30 5 nm.
(B) AFM of sample d. The sample was prepared by spin-coating THF solution of polymer d
(0.1 mg/mL) on mica. The particle in a cycle mark has a length of 125 nm and width of 25 nm.
(C) TEM of the products from the concurrent controlled polymerization in 30 hours where [PSt]
and [CTA] were twice as those for sample d. (D) Budding grafting structures found in the con-
current controlled polymerization. (E) Optical microscopy image of self-aggregated aqueous
suspension derived from sample d. Magnification 100X.
lidone and toluene, but not in dimethylformamide (DMF). Thus
the polymer nanorods were obtained by classification of a THF
solution of the raw polymers with precipitation of DMF. The
nanorods obtained from the sample d in Figure 1 were found to
have a length of about 137 nm and a diameter of about 30 nm in
average according to statistics on TEM image (Figure 2A). If the
concentrations of both [PSt] and [CTA] were doubled, Mw and
Mw/Mn obtained in the polymerization of 30 hours at 130 °C
were 16000 and 2.4, respectively, and we obtained nanorods
with lengths around 60 nm and diameters of around 15 nm,
indicative of regulatory lengths of these rods via the concurrent
controlled polymerization methodology (Figure 2C). SCVP of
an inimer was usually related to equilibrium between the acti-
vation and deactivation processes. High initiator (inimer) con-
centration favored a shift of the equilibrium toward the active
radicals, leading to a higher concentration of radicals,[11] a faster
polymerization rate but smaller molar mass.
Actually, hyperbranched blocks could randomly attach
towards each other to afford three kinds of topological arrange-
ments of hyperbranched blocks, including nanorods, botryoidal
shapes and budding grafting shapes. However, the botryoidal
shape could lose its identity from hyperbranched polymers. If
one branch of the botryoidal shape grew up to form “budding”,
a budding grafting shape could be obtained, as illustrated in
Figure 2D. The budding grafting shapes relied on the develop-
ment of huge molar mass. Overall, nanorods were the most
probably obtained structures after classification. AFM observa-
tions were performed on polymer d as well. The sample was
prepared by spin-coating THF solution of polymer d (0.1 mg/
mL) on mica, and the corresponding image was shown in
Figure 2B. The sample was scanned at different directions for
recognizing and avoiding possible AFM artifacts. The image
showed nanometer-sized rod-like particles with length of about
130 nm, and width between several to 30 nm. The average sizes
were coincided with TEM observation.
The structural order of the polymers produced in the con-
current NMP/RAFT system at different time was investigated
with X-ray powder diffraction (XRD). Wide-angle XRD patterns
of these polymers showed only a broad peak at around 20° cor-
responding to non-crystalline of the polymer molecular chains
without short range order. However, after being simply orientated
by rotary evaporation from THF solutions which was conducive
to orientation of nanorods, the polymers showed diffraction
peaks in small angles XRD (SA-XRD) at 2Q around 1.3° corre-
spondent to periodic spacing of about 6.6 nm (Figure 3A), indica-
tive of typical features of structures with long-range orders. There
was no long-range order presented in either the hyperbranched
polymer or the linear polymer of PSt. SA-XRD in combination
with wide-angle XRD results indicated that the long-range order
possibly stemmed from the nanorod-close-packing conforma-
tion. The structures seemed more regular than expected. The
polymer formed from a normal system in 30 hours showed the
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rotation through crossed polarizers, the
orientated sample changed from bright to
extinction cyclically. We measured the optical
density change of a certain place at each 5°
rotation of sample stage (Figure 3D). The var-
iation of optical density was cyclical and sym-
metrical, indicative of high orientation order.
The end TEMPO groups could initiate
further polymerization. We attached poly(N-
isopropylacrylamide), PNIPAM, to these
nanorods via NMP of N-isopropylacrylamide.
The obtained PNIPAM-modified nanorods
could be easily emulsified in water. However,
the aqueous suspension of PNIPAM-mod-
ified nanorods could also easily aggregate to
form macroscopic 1D structures following
the direction of the external magnetic field
(Figure 2E). The 1D aggregated structure
could be ascribed to these polymer nanorods.
However, the easy aggregation process was
likely related to the magnetic properties of
these nanorods. Because of the attachment
of TEMPO groups at the chain ends, those
nanorods showed super-paramagnetic proper-
ties, whereas TEMPO itself is a paramagnetic
material (Figure S8).
In conclusion, polymer nanorods were
formed by facile concurrent NMP/RAFT
polymerization of a styrene-based inimer.
Comprehensive analysis in combination with
TEM observation suggested that 1D nanos-
tructures were preferentially formed from
those systems with high monomer concentra-
Figure 3. Structural order in the polymer nanorods. (A) Small-angle X-ray powder diffraction tions and suitable proportion of [PSt]/[CTA].
patterns of polymer powders obtained in the concurrent NMP/RAFT system in different polym-
erization time: (m) 24, (d) 30, (e) 31, (f) 34 hours. Curve n arose from the polymer obtained
by recombination polymerization of a hyperbranched polymer in 30 hours. (B) Wider XRD
spectrum of curve d. (C) D-spacing correspondent to the maximum in (A). (F) Orthogonal
polarizing microscopy image of polymer d, and (E) its nature view; (D) Optical density varies
cyclically as the specimen is rotated against the polarizers.
The lengths of nanorods were regulatory via
adjustment of feed ratios or polymerization
time. The approach proposed in this work
was conducive to making novel nanomate-
rials. These rigid polymeric nanorods could
also reinforce composites. We tested rein-
forcement of epoxy resins by addition of pol-
ymer d of only 0.1 wt%, and found that the storage modulus in
a wide range from room temperature to 150 °C were increased
by 2-1.5 fold (Figure S9).
best regularity, evidenced by the appearance of multilevel diffrac-
tion peaks (d in Figure 3). However, we must point out that if
these nanorod samples were stored as dry powder, the periodic
spacing could continue to increase possibly because of combi-
nation among close-packed nanorods. The periodic spacing of
sample m increased from 6.4 nm to about 10.6 nm after about
4 months. These samples were stable if stored in solutions.
The SA-XRD of the sample obtained by recombination of
the isolated hyperbranched polymer b in Figure 1A in 30 hours
yielded a 2Q peak at about 1.6° correspondent to periodic
spacing of about 5.0 nm (n in Figure 3A), significantly smaller
than other rod-like samples obtained with continuous addition
reaction of monomers.
Furthermore, many different nitroxides other than TEMPO
have been designed for the initiators of NMP or NMP based
inimers, and have flourished NMP systems for the synthesis of
block copolymers and macromolecular architectures.[12] Basi-
cally the inimers based on these newly emerging nitroxides are
applicable for the approach of this work. Overall, the approach
and nanorods obtained in this approach are worthy of further
study, and will find widespread applicability.
One intriguing characteristic of the polymers m, d, e, f, and
e (correspondent to the samples in Figure 1A and Figure 3A)
obtained in the concurrent NMP/RAFT system were that they
showed liquid crystalline orientation in a polarizing microscope
(Figure 3F). When sample d was observed with 360-degree
Experimental Section
Synthesis of 2,2,6,6-tetramethyl-1-(4-vinyl-benzyloxy)-piperidine (PSt): A
flask was charged with 2,2,6,6-tetramethy1-1-piperidine-N-oxyl (TEMPO)
(5 g, 0.032 mol), 4-vinylbenzyl chloride (5 g, 0.033 mol), [2,2′]bipyridinyl
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(8 g, 0.051 mol), Cu(I)Cl (4.5 g, 0.046 mol) and THF (50 mL). The
reaction was continued for 72 h at 50 °C under pure N2. After solid
sediments were removed by filtration, the solution was passed through
a column of neutral alumina to remove the copper salts. Pure PSt was
obtained as colorless liquid by passing through a silica gel column
chromatography with petroleum ether as eluent (5.6 g). Yield: 64.2%.
13C NMR (100 MHz, CDCl3, δ): 17.0 (CH2), 20.3 (CH3), 33.2 (CH3), 40.0
(CH2), 59.0 (C(CH3)2), 77.5 (CH2-O), 112.5 (CH2 =). HRMS (EI) m/z
(%): 117.0893 (70) [C9H9+], 156.1376 (100) [C9H18NO+], [M]+ calcd for
C18H27NO, 273.2093; found, 273.2092.
Concurrent RAFT and NMP dual polymerization at 130 °C to afford
nanorod-like polymers: In a typical experiment, a Schlenk flask was flame-
dried under vacuum and charged with monomer PSt (0.1 g, 0.366 mmol)
and RAFT CTA, 2-benzyl-sulfanylthiocarbonylsulfanyl-2-methyl-propionic
acid, (0.92 mg, 3.2 × 10−6 mol) dissolved in N-methyl-2-pyrrolidone
(1.5 mL). The reaction mixture was degassed by three freeze-pump-thaw
cycles. The reaction was carried out at 130 °C. The raw polymers are
obtained by precipitation with methanol.
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Acknowledgements
Project supported by the National Natural Science Foundation of China
(21090354 and 21174137) and the Chinese ITER Specific Foundation
(2009GB104004).
Received: August 6, 2012
Published online:
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