Polymer nanoparticles via intramolecular crosslinking of sulfonyl azide functionalized polymers

Article abstract of DOI:10.1016/j.polymer.2011.05.054

A new approach to prepare polymeric nanoparticles via intramolecular collapse of single chain of sulfonyl azide functionalized polymers is proposed. Upon heating, the sulfonyl azide functionalized linear copolymers lose nitrogen and form nitrene. This nit

Full text of DOI:10.1016/j.polymer.2011.05.054

Polymer 52 (2011) 3597e3602
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Polymer
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Polymer nanoparticles via intramolecular crosslinking of sulfonyl azide
functionalized polymers
*
Xiaoyu Jiang, Hongting Pu , Peng Wang
Institute of Functional Polymers, School of Materials Science & Engineering, Tongji University, Shanghai 201804, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new approach to prepare polymeric nanoparticles via intramolecular collapse of single chain of
sulfonyl azide functionalized polymers is proposed. Upon heating, the sulfonyl azide functionalized linear
copolymers lose nitrogen and form nitrene. This nitrene reacts with CeH bond of the backbone in dilute
solution and leads to the efficient intramolecular crosslinking and formation of nanoparticles where the
diameter of nanoparticles can be controlled by both the molecular weight and the content of sulfonyl
azide groups. A significant reduction in the hydrodynamic volume is observed on going from the starting
random coil of linear chains to the corresponding nanoparticles. The morphology and the dimension of
nanoparticles are characterized by using transmission electron microscope (TEM), atomic force
microscopy (AFM), as well as dynamic laser scattering (DLS).
Received 17 February 2011
Received in revised form
14 April 2011
Accepted 27 May 2011
Available online 6 June 2011
Keywords:
Nanoparticles
Sulfonyl azide
Intramolecular crosslinking
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
By intramolecular crosslinking of functionalized linear polymer
chains under dilute solution, a wide range of nanoparticles can be
Nanoparticles are nanometre-scale aggregates of atoms. Their
size gives them unusual optical, electronic, and magnetic properties
caused by quantum-mechanical and Coulomb-charging effects,
which have many potential applications [1e4]. The preparation of
polymeric nanoparticles with a controlled size and pre-determined
arrangement of functional groups has become an attractive
research topic in recent years [5e7]. This interest is driven by the
use of these tailor-made, functional nanoparticles in a variety of
applications in the fields of microelectronics [8e10], drug delivery
[11e13], polymer processing [14], etc. To address this need,
a diverse range of strategies have been developed to increase the
scope and availability of these systems including microemulsion
techniques [15e17], discrete polymer synthesis of spherical mole-
cules such as dendrimers [18,19], the self-assembly of block
copolymers into micelles followed by chemical crosslinking [20,21],
as well as supramolecular strategies to collapse nanoparticles [22].
Nanoparticles above 20 nm in size are easier to make but control of
their structure is more difficult. In direct contrast, smaller nano-
particles, such as dendrimers, allow access to the potentially more
interesting sub-20 nm regime and are structurally well defined,
though significantly more difficult to prepare. To achieve the goal of
developing a facile synthetic procedure for preparing well defined,
sub-20 nm nanoparticles, intramolecular chain collapse strategies
have been developed.
prepared with the chain collapse strategy relying heavily on the
reactivity and orthogonality of the crosslinking functionality. Up to
date, this technique has made use of free-radical crosslinking
of vinyl functionalization [23e25], the thermal crosslinking of
benzocyclobutene (BCB)[26,18]or BCB precursors using a continuous
addition technique [28], alternative o-qinodimethane precursors
[29], the room-temperature crosslinking of isocyanate linear copol-
ymers with diamine in dilute solution [30], click chemistry [31], and
photo-crosslinking of cinnamoyl groups [32], which has been shown
to form particles in the range of 5e20 nm. In present study, the linear
copolymers containing the crosslinkable sulfonyl azide groups are
synthesized. It is well-known that the sulfonyl azide groups can lose
nitrogen and form nitrene when raised to sufficiently high temper-
ature. The nitrene quickly reacts with CeH bond of the backbone
in diluted solution which leads to the efficient intramolecular
crosslinking and formation of nanoparticles of single chain.
2. Experimental
2.1. Materials
Styrene (St) and methyl methacrylate (MMA) were purchased
from Shanghai Chemical Reagent Co and distilled under reduced
pressure before use. Azobisisobutyronitrile (AIBN) was recrystal-
lized from methanol. The chain transfer agent, s-ethoxycarbonyl
phenylmethyl dithiobenzoate (ECPDB), was prepared using
* Corresponding author. Fax: þ86 21 69580143.
E-mail address: puhongting@tongji.edu.cn (H. Pu).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.05.054

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X. Jiang et al. / Polymer 52 (2011) 3597e3602
a literature procedure [33]. Tetrahydrofuran (THF) and chloroform
from Shanghai Chemical Reagent Co were dried by sodium, using
benzophenone as indicator. All other chemicals were purchased
from Shanghai Chemical Reagent Co and used without further
purification.
0.01 mmol) in DMF were added to the reaction mixture. After three
times freeze and thaw cycles, the tube was sealed under nitrogen
and heated for 5 h at 60 ꢀC. The resulting polymer solution was
diluted with 3 mL THF, then precipitated into a large excess of
ethanol for three times. Then the copolymer was dried in vacuum
and used without further purification. 1H NMR (CDCl3),
d
(ppm):
2.1.1. Synthesis of 4-styrenesulfonyl azide (SSAz) 3
7.76e6.85 (m, ArH), 3.58 (bt, CH3), 1.84 (bs, CH3), 1.82e1.25 (m, CH,
The method for the synthesis of SSAz is shown in Scheme 1. Na-4-
styrenesulfonate 1 (14.7 g, 72.6 mmol) was suspended in DMF
(75 mL) under nitrogen and cooled to 0 ꢀC. Afterwards thionyl
chloride (32 mL, 441 mmol) was added dropwise within 10 min. The
reaction mixture was stirred at 0 ꢀC for 30 min and subsequently at
room temperature for 1 h, during which a homogenous solutionwas
obtained. The solution was poured onto ice (380 g). The resulting
aqueous layer was extracted with diethyl ether (3 ꢁ 80 mL). After
removing the solvent in the collected organic layers, the resulted
styrene sulfonyl chloride 2 (2.27 g, 11.2 mmol) was dissolved in
acetone (35 mL) and an aliquot volume of water (35 mL) was added.
The turbid reaction mixture was cooled to 0 ꢀC and NaN3 (802 mg,
12.33 mmol) was added in small portions. After the reaction mixture
was stirred for 1.5 h at 0 ꢀC, the acetone was removed in vacuum
(30 ꢀC, 150 mbar) and the aqueous layer was extracted with diethyl
ether (3 ꢁ 20 mL). The combined organic layers were dried over
Na2SO4 and the solvent was evaporated at 30 ꢀC. The resulting oil 3
(2.19 g, 10.5 mmol, 94%) was dried in vacuum and used in the
CH2); FTIR (KBr, n
/cmꢂ1): 2127 (eN3), 1730 (eC]O), 1149,
1071ðAreSOꢂ2 Þ.
2.2. Collapse reaction of the copolymers in dilute solution
In a 100 mL three-necked flask equipped with an internal
thermometer, 25 mL of benzyl ether was heated at 190 ꢀC under
nitrogen. 0.1 g sulfonyl azide-functionalized linear polymer 5
(Mw ¼ 46 902, PDI ¼ 1.54) was dissolved in benzyl ether (12 mL)
and added dropwise via a peristaltic pump at ca. 12 mL/h with
vigorous stirring. After addition the reaction mixture was heated
for an additional 1.5 h, the solvent was distilled under reduced
pressure, and the remaining crude product was dissolved in THF
and precipitated into ethanol. Finally, the nanoparticle 8 was
obtained as a brown solid. The nanoparticle 9 was prepared from
polymer 7 with the same method.
2.3. Characterization methods
polymerization reaction without further purification. FTIR (KBr, n/
cmꢂ1): 3092(CeH), 2934(eCH2), 2127(eN3), 1631(eC]Ce), 1566,
1493, 1451(eC]C), 1161(eS]O), 758, 652(benzyl). 1H NMR (CDCl3,
1H nuclear magnetic resonance measurements were carried out
at room temperature using a Bruker AMX 300 spectrometer. CDCl3
was used as the solvent.
d/ppm): 7.91 (2H, C-Har), 7.61 (2H, C-Har), 6.78 (1H, eCH]CH2), 5.95
(1H, eCH]CHHcis), 5.52 (1H, eCH]CH Htrans).
FTIR analysis of the samples was carried out by a thermo Bruker
EQUINOXSS/HYPERION2000 spectrometer. KBr pellet method was
employed.
Gel permeation chromatography (GPC) was performed in
tetrahydrofuran on a Waters Alliance HPLC system. The molecular
weight of the polymers was calculated relative to linear polystyrene
standards.
Transmission electron microscopy (TEM) was performed with
a transmission electron microscope (TEM, Hitachi H-600), oper-
ating with an acceleration voltage of 200 kV. The TEM samples
were prepared by placing one drop of the diluted dispersion of the
nanoparticles on a 200 mesh carbon coated copper grid and left in
air to dry.
The height and distribution of the nanoparticles were also
determined by tapping-mode atomic force microscopy (AFM) (SPA-
300HV, Seiko Instruments Inc.) under ambient condition in air. The
standard silicon tips were used. The average particle size of the
nanoparticles was also determined on a commercial dynamic light
scattering (DLS) (Malvern Autosizer 4700). Differential scanning
calorimetry (DSC) of the polymers was performed on MDSC-Q100
(TA).
2.1.2. Synthesis of poly(styrene-co-(4-styrenesulfonyl azide)) 5
In a typical run of the polymerization process, a mixture of
styrene 4 (4.68 g, 45.0 mmol), 4-styrenesulfonyl azide 3 (1.05 g,
5.0 mmol), ECPDB (15.80 mg, 0.05 mmol), and AIBN (4.11 mg,
0.025 mmol) in DMF were added to the reaction mixture. After
three times freeze and thaw cycles, the tube was sealed under
nitrogen and heated for 5 h at 60 ꢀC. The resulting polymer solution
was diluted with 3 mL THF, then precipitated into a large excess of
ethanol for three times. Then the copolymer was dried in vacuum
and used without further purification. 1H NMR (CDCl3,
d/ppm):
7.26e6.57 (m, ArH), 2.56 (brs, CH), 1.83e1.26 (m, CH, CH2); FTIR
(KBr,
n
/cmꢂ1): 2127 (eN3), 1171, 1081ðAreSOꢂ2 Þ.
2.1.3. Synthesis of poly((methyl methacrylate)-co-(4-
styrenesulfonyl azide)) 7
In a typical run of the polymerization process, a mixture of
methyl methacrylate 6 (1.80 g, 18 mmol), 4-styrenesulfonyl azide 3
(0.418 g, 2 mmol), ECPDB (6.32 mg, 0.02 mmol), and AIBN (1.64 mg,
3. Results and discussion
Synthetically, the preparation of single-chain polymeric nano-
particles through thermally activated methods involves two steps.
The first step involves the incorporation of thermally activated
crosslinkable groups into linear polymer backbones. The second
strategy is collapsing via intramolecular crosslinking in ultradilute
solution. The advantages of BCB as the thermally activated cross-
linkable group are its stability to radical polymerization and high
reactivity under crosslinking condition [26,27]. However, the
disadvantage of BCB, which has a crosslinking temperature as high
as 250 ꢀC, precludes the presence of sensitive groups and reduces
the application of the nanoparticles. To overcome these challenges,
we investigate 4-styrenesulfonyl azide 3 with the ability to be
SOCI2
Scheme 1. Synthesis of 4-styrenesulfonyl azide.

X. Jiang et al. / Polymer 52 (2011) 3597e3602
3599
maximum peak of exothermic is at 194.52 ꢀC. Fig. 1 shows the
decomposition and binding mechanism of sulfonyl azide groups.
Upon heating, the sulfonyl azide groups lose nitrogen and form
nitrene. The further reaction for the nitrene has five possibilities,
(1) the polymer with nitrene abstracts the hydrogen in CH-bond of
another polymer chain and recombines with it; (2) the polymer
with nitrene inserts the CH-bond of another polymer chain; (3) the
polymer with nitrene recombines with itself or another polymer
chain with nitrene; (4) the polymer with nitrene inserts the groups
of sulfonyl azide of polymer chain to form the azo dimmers in
highly concentrated solution. (5) the polymer with nitrene
abstracts the hydrogen in CH-bond of its own chain and recombines
with it, then the intramolecular crosslinking structure is formed. In
dilute solution, the fifth reaction is quite easier than the other four
reactions. Thus, the nanoparticles based on the intramolecular
collapse of the single chain can be achieved via the continuous
addition strategy of the copolymer in dilution solution. In the
traditional strategy, the concentration of polymer solution needs to
be on the ultra-dilute reaction conditions (ca. 10ꢂ5e10ꢂ6 M) to
ensure the collapse and intramolecular coupling of single-polymer
chains to give nanoparticles. According to Hawkers’ work [26],
following a continuous addition strategy and this coupling event,
the traditional conditions of ultrahigh dilution need only be met for
the reactive intermediates and not for the polymers themselves,
which allows their concentration to increase to very high levels
(0.1e1.0 M) without intermolecular crosslinking reactions leading
to gelation or coupling of individual nanoparticles. We also adopt
the continuous addition strategy, and the concentration of polymer
solution is controlled in 0.1e1.0 M to avoid the intermolecular
crosslinking. A temperature of 190 ꢀC for the intramolecular chain
collapse process in benzyl ether can be used. In this approach,
a concentrated solution of the starting linear polymer 5, was
continuously added via peristaltic pump to the solvent. After the
addition of the polymer 5, the reaction continued for 1.5 h and the
solvent was removed, the nanoparticle 8 was isolated using normal
precipitation techniques (Scheme 3).
y
x
ECPDB
AIBN, DMF, 60oC
O
S
O
O
S
O
N3
N3
4
3
5
x
y
ECPDB
O
O
O
O
AIBN, DMF, 60oC
O
S
O
O
S
O
N3
N3
7
3
6
Scheme 2. Synthesis of poly(styrene-co-(4-styrenesulfonyl azide)) 5 and poly((methyl
methacrylate)-co-(4-styrenesulfonyl azide)) 7.
incorporated as a comonomer into the backbone of the linear
polymer and similar crosslinking characteristics at 180e200 ꢀC.
The synthetic strategy involves copolymerization of 4-styren
esulfonyl azide 3 with a variety of vinyl monomers, such as
styrene, methyl methacrylate, as shown in Scheme 2, to give the
desired polymers with sulfonyl azide functionality incorporated
along the backbone. To prepare the sulfonyl azide functionalized
copolymers with well-controlled molecular weight, polydispersity,
and contents of sulfonyl azide groups in the copolymers, we per-
formed a typical RAFT-mediated free radical polymerization, with
ECPDB as the chain transfer agent. The reaction temperature of
polymerization is at 60 ꢀC with AIBN as the initiator, because the
decomposition of the azide group only takes place at much higher
temperatures [34]. The SSAz-functionalized copolymer 5 can be
readily characterized by FTIR which shows the characteristic peak
of sulfonyl azide groups for the azide group at 2127 cmꢂ1 and the
Evidence of the conformational change from the random coil of
the linear polymer to the nanoparticle via intramolecular collapse
is obtained from an apparent molecular weight decrease of the
copolymer, as shown in Table 1. Comparison of GPC results for the
collapsed nanoparticles versus the starting linear polymer is an
efficient diagnostic technology for demonstrating the volume
change of a random coil of the polymer caused by an intramolecular
crosslinking of the single chain. A significant feature is the reduc-
tion in hydrodynamic volume of the random coil of the linear
polymer via the intramolecular collapse to give the final nano-
particles. As shown in Table 1 for styrene based copolymers con-
taining 5, 10, and 15 mol % of sulfonyl azide-functionalized repeat
S]O group at 1160 cmꢂ1
.
Since the seminal work of Breslow et al. in 1960s, it is well-
known that sulfonyl azide groups cleave off nitrogen and form
a nitrene when raised to sufficiently high temperature. The nitrene
has a high tendency to insert into almost CH-bonds and in some
cases even OH-bonds [35e37]. After successful incorporation of the
sulfonyl azide group into linear copolymers, differential scanning
calorimetry (DSC) results of styrene/SSAz (90/10) copolymer indi-
cate that the decomposition temperature of azide groups in the
linear copolymer is around 160e220 ꢀC and the temperature for the
SO2NH
SO2N
NO2S
P
+ N2
P
Dilute solution (5)
SO2N
(4)
P
heat or UV
N2
SO2N3
Sulf onyl nitrene inserted
into sulfonyl azide
SO2N3
SO2N
(1) P'
P
P
P
H
(2)
P
SO2N
(3)
H
P'
+
P'
P
SO2N
SO2N P'
P
H
H
SO2N
NO2S
SO2N
H
P
P
P'
P
H-abstraction and recombination
Formation of azo dimmer by
dimerization of sulfonyl nitrenes
CH-bond insertion
Fig. 1. The decomposition and binding mechanism of sulfonyl azide groups.

3600
X. Jiang et al. / Polymer 52 (2011) 3597e3602
b
a
1.5
1.0
0.5
0.0
20
25
30
35
Retention time (min)
Fig. 2. GPC traces for, (a) linear polymer 5 (Mw ¼ 46 902, PDI ¼ 1.54), (b) nanoparticle
8 (Mw ¼ 24 885, PDI ¼ 1.52).
a polystyrene derivative with 5% sulfonyl azide gives a nanoparticle,
whose hydrodynamic volume diminishes to 17% of the starting
linear polymer because of the intramolecular crosslinking. The
copolymer with 10% sulfonyl azide gives a nanoparticle, whose
hydrodynamic volume diminishes to 45% of the starting linear
polymer. However, the polystyrene derivative with 20% sulfonyl
azide gives a nanoparticle with the hydrodynamic volume dimin-
ishing to 56% of the starting linear polymer. The hydrodynamic
volume decreases with increasing density of crosslinking. The
hydrodynamic volume decreases with increasing level of sulfonyl
azide, thus the size of the nanoparticle also decreases with
increasing density of crosslinking. The data of dynamic light scat-
tering (DLS) also confirm the conclusion. The polystyrene deriva-
tive with 15% sulfonyl azide gives a nanoparticle with the average
diameter of 16.5 nm. Increasing the molecular weight and incor-
porating 20% sulfonyl azide gives a nanoparticle with the average
diameter of 15 nm (Fig. 3). A consequence of this is that the size of
the nanoparticle can be controlled by the structure and the func-
tionality of the starting linear polymer.
Scheme 3. Diagram of the preparation of nanoparticle
collapse of linear polymer 5.
8 via the intramolecular
units, a remarkable increase in molecular weight is observed when
the relative percentage of sulfonyl azide groups increases. After
crosslinking, the increased retention time corresponds to a reduc-
tion in hydrodynamic volume of the nanoparticle with increasing
intramolecular crosslink density. For example, the original linear
polymer
5
(styrene/SSAz
¼
85/15) has a molecular weight
Mw ¼ 46 902 (PDI ¼ 1.54). However, upon reaction the chain
decreases in size to give a nanoparticle with an apparent molecular
weight Mw ¼ 24 885 (PDI ¼ 1.52) (Fig. 2). For the same molecular
weight of the starting linear polymer 5, a systematic decrease in the
hydrodynamic volume of the nanoparticles is observed on
increasing the content of sulfonyl azide groups from 15% to 20%. It is
consistent with an increase in the level of intramolecular cross-
linking and a globular structure. It is also clear that no significant
intermolecular crosslinking is observed, and the GPC trace with
a slightly lower polydispersity than the linear precursor.
Furthermore, the confirmation of the structural change is
obtained from FTIR results. Comparison of FTIR spectra for the
starting linear polymer 5 and the nanoparticle 8, which show the
disappearance of the azide (2127 cmꢂ1) after reacting 1.5 h at
The data in Table 1 also demonstrate the inherent versatility of
this approach in controlling the size of the nanoparticle. The size
and density of crosslinking of the nanoparticle can be controlled by
the level of sulfonyl azide groups and the molecular weight of the
starting linear polymer. The change of the size of the nanoparticles
can be reflected via the change of the hydrodynamic volume of the
starting linear polymer and the nanoparticles. For example,
20
a
b
15
Table 1
10
5
Comparison of the equivalent molecular weights and PDI for the starting linear
polymers 5, 7, and the final nanoparticles 8, 9
Copolymers
SSAz%
Linear copolymers
Nanoparticles
Mw
PDI
Mw
PDI
SSAz-St
SSAz-St
SSAz-St
SSAz-St
SSAz-MMA
SSAz-MMA
5
10
15
20
5
10 340
21 880
46 902
47 105
9138
1.38
1.59
1.54
1.46
1.43
1.41
8536
12 239
24 885
20 604
7446
1.40
1.61
1.52
1.53
1.44
1.54
0
10
Diameter(nm)
10
25 457
15 599
Fig. 3. Size distribution for nanoparticle 8, (a) 20% SSAz, and (b) 15% SSAz.

X. Jiang et al. / Polymer 52 (2011) 3597e3602
3601
change, which indicates the similar structure and composition
between the linear polymer and the nanoparticle.
RAFT-mediated living free radical polymerization can also be
used for the synthesis of a variety of linear copolymers with other
vinyl monomers. As show in Table 1, starting linear polymers based
on methyl methacrylate (MMA) 6 can be employed as the precursor
polymer with no change in the efficiency of the intramolecular
collapse process. For example, the copolymerization of MMA/SSAz
(90/10) results in the well-defined random copolymer 7 with
a molecular weight Mw ¼ 25 457 (PDI ¼ 1.41). Addition of
a concentrated solution of 7 to benzyl ether at 190 ꢀC to give
nanoparticle 9 with an apparent molecular weight Mw ¼ 15 599
(PDI ¼ 1.54). From these data it can be concluded that a similar
volume reduction and crosslinking density are observed for the
styrenic and methacrylate based materials, illustrating the high
efficiency of the sulfonyl azide-based intramolecular crosslinking
reaction.
Thedimensionof thenanoparticles was measured byAFM(Fig. 5)
and TEM (Fig. 6). In Fig. 4 the nanoparticle 8 (15 mol% SSAz) gives
effective particle diameter of 14 ꢃ 0.2 nm, which is in basic agree-
ment with the size determined by TEM (15 ꢃ 0.5 nm). Dynamic light
scattering (DLS) was also performed in THF solution of the nano-
particles on the functionalized polymers and their respective
collapsed products. The results also support a change in effective
diameter from 25 nm as an extended coil to 16 nm in particle size for
the 15 KDa copolymers. Performing the light scattering measure-
ments in chloroform and toluene gives similar results.
Fig. 4. FTIR spectra of polymer 5 (a) and nanoparticle 8 (b).
190 ꢀC and the formation of the CeN bonds (Fig. 4). A new peak
appears at 1200 cmꢂ1 for stretching vibration of groups of CeN
bond. Furthermore, the peak intensity of polymer 5 at 1450 cmꢂ1 is
stronger than the nanoparticles 8 because of the appearance of
bending vibration of CeN bond. The peak at1160 cmꢂ1 is the groups
of eS]O. Meanwhile, the peaks of other groups have no significant
Fig. 5. Representative tapping-mode AFM image of nanoparticle 8, samples were prepared by drop deposition onto freshly cleaved mica and allowed to dry under ambient
condition.
Fig. 6. TEM micrograph of polymeric nanoparticle 8, average diameters are shown with the corresponding distribution and a representative image.

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X. Jiang et al. / Polymer 52 (2011) 3597e3602
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In summary, we have demonstrated the sulfonyl azide func-
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crosslinking of the polymer and form single-chain nanoparticles. By
using the continuous addition strategy, intermolecular crosslinking
can be effectively eliminated even at high concentration. The novel
crosslinking unit can react with a variety of vinyl monomers and
the temperature for the crosslinking reaction is lower than BCB. The
dimension of nanoparticles can be controlled in 5e20 nm by
varying the molecular weight and molar percentage of sulfonyl
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