Synthesis and characterization of amine-functionalized polystyrene nanoparticles

Article abstract of DOI:10.1021/ma0507286

Functionalized polymer nanoparticles have been synthesized through an intramolecular cross-linking reaction with polystyrene as the backbone and an amino group as the functional moiety. The intramolecular cross-linking reaction was performed under ultradi

Full text of DOI:10.1021/ma0507286

5886
Macromolecules 2005, 38, 5886-5891
Synthesis and Characterization of Amine-Functionalized Polystyrene
Nanoparticles
Jing Jiang and S. Thayumanavan*
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
Received April 8, 2005; Revised Manuscript Received May 4, 2005
ABSTRACT: Functionalized polymer nanoparticles have been synthesized through an intramolecular
cross-linking reaction with polystyrene as the backbone and an amino group as the functional moiety.
The intramolecular cross-linking reaction was performed under ultradilute conditions, and the cross-
linked polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance
(NMR), dynamic light scattering (DLS), and atomic force microscopy (AFM). The availability of the amino
moiety in the nanoparticle for further functionalization was confirmed by its reaction with trimethylacetyl
chloride.
from commercial sources and purified through flash column
chromatography. The RAFT reagent, 2-(2-cyanopropyl)dithio-
benzoate, was synthesized according to the reported proce-
dure.20 All other chemicals were used directly without further
purification.
1. Introduction
Nanoparticles have emerged as one of the important
building blocks in the construction of a wide range of
materials that could find use in medical, mechanical,
and electronic applications.1,2 Especially, functionalized
nanoparticles have attracted considerable attention
because of their potential applications as building blocks
for a variety of nanotechnology applications, ranging
from vectors for drug and DNA delivery systems to the
nanoscale devices such as single-electron transistors and
molecular switches.3-10 However, most of the surface
functionalizations in these materials are obtained
through modification of metal and semiconductor nano-
particles by self-assembly, organic reaction, or polym-
erization to form functional monolayers or polymeric
“shells”.5,11-16 Beyond the use of polymer micelles
through self-assembly to form the nanoparticles, a new
strategy involving the collapse and intramolecular
coupling of single-polymer chains to give discrete nano-
particles has been developed.17,18 More recently, an
intramolecular radical cross-linking reaction was
achieved, where the reaction was performed under
ultradilute solution.19 Under these conditions, the co-
valent links are exclusively formed between segments
of the same polymer chain, leading to a unimolecular
particle with size ranging from 3 to 15 nm. While these
seminal reports demonstrate possibility of achieving
polymeric nanoparticles by intramolecular cross-linking,
the possibility of further functionalizing these nanopar-
ticles has not been reported. Such a capability would
significantly expand the scope of these polymeric nano-
particles in a variety of applications. With this objective
in mind, we report here a polystyrene-based nanopar-
ticle that displays amino moieties. The capability for
further functionalization, using these amino groups as
the handle, is demonstrated using a simple reaction
with a carboxylic acid chloride.
1H NMR spectra were recorded on a 400 MHz NMR
spectrometer using the residual proton resonance of the
solvent as the internal standard. Chemical shifts are reported
in parts per million (ppm). When peak multiplicities are given,
the following abbreviations are used: s, singlet; d, doublet; t,
triplet; q, quartet; quin, quintet; m, multiplet; br, broad. Flash
chromatography was performed with silica gel. Analytical thin-
layer chromatography was performed on silica plates with a
F-254 indicator, and the visualization was accomplished by a
UV lamp. The molecular weights were estimated by gel
permeation chromatography (GPC) with polystyrene as a
standard and with a refractive index detector, and the sample
was eluted with anhydrous THF. A digital correlator and a
goniometer were used for the DLS measurements. The light
source was a solid-state laser system, operating at 514 nm.
The temperature was kept constant at 25 °C. Possible dust
particles were eliminated using filters with 0.20 µm pore size
for THF solutions. All measurements were done at a correla-
tion time of 1.0 min. The particle sizes reported are the average
of at least five readings. Tapping-mode scanning atomic force
microscopy (AFM) images were obtained in both height- and
phase-contrast modes with a Digital Instruments Nanoscope
II scanning force microscope with etched silicon tips on
cantilevers, with spring constants ranging from 40.0 to 60.0
N/m. Thin films were prepared by the spin coating of 0.5 wt
% 1,4-dioxane solutions of the copolymers on polished silicon
wafers, with a spin casting speed of 1500 rpm.
Synthesis of 4-N-Boc-vinylaniline (1). 4-Vinylaniline (2.4
g, 20 mmol) was dissolved in 40 mL anhydrous THF, and then
di-tert-butyl dicarbonate (4.0 g, 24 mmol) in 40 mL THF was
added dropwise to the mixture. After refluxing overnight, the
mixture was allowed to cool to ambient temperature and then
partitioned between water and dichloromethane. The aqueous
layer was extracted with CH2Cl2 twice, and the organic layers
were combined and washed with water then dried over sodium
sulfate. After filtration, the solvent was removed under
reduced pressure; the residue was purified by flash chroma-
tography over silica gel eluted with hexane/ethyl acetate to
afford 1 (3.5 g, 80% yield). 1H NMR (400 MHz, CDCl3): δ 7.34
(s, 4H), 6.69 (q, 1H), 6.33 (s, 1H), 5.66 (d, 1H), 5.12 (d, 2H),
1.57 (s, 9H).
2. Experiment
THF and toluene were dried over sodium and freshly
distilled before use. Acetone was dried over potassium carbon-
ate. 4-Vinylbenzyl chloride and 4-vinylaniline were purchased
General Procedure for the Synthesis of Poly(4-N-Boc-
aminostyrene)-co-(4-chloromethylstyrene) (3). 4-N-Boc-
vinylaniline (1) (1.0 equiv), 4-vinylbenzyl chloride (1.0 equiv
for 3a, 0.5 equiv for 3b), 2-(2-cyanopropyl) dithiobenzoate
* Author to whom correspondence should be addressed.
E-mail: thai@chem.umass.edu.
10.1021/ma0507286 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/08/2005

Macromolecules, Vol. 38, No. 14, 2005
Amine-Functionalized Polystyrene Nanoparticles 5887
Scheme 1. Synthetic Route for the Polystyrene
Copolymer Backbone.
Scheme 2. Synthetic Route for the Crosslinked
Nanoparticles.
Figure 1. GPC of the un-cross-linked copolymers 5a, 5b and
the cross-linked copolymers 6a, 6b.
Table 1. GPC and DLS Results of the Precursors and the
Nanoparticles
precursor copolymer 5
cross-linked nanoparticle 6
x:ya
Mn
PDI Rh (nm)c
Mn
PDI
Rh (nm)
b
a 1:1 21 000 1.66
b 2:1 19 000 1.50
26.1
13.3
12 000
13 000
1.31
1.33
16.9
10.8
a The ratio of x:y was detected by NMR. b Number-average
molecular weight was measured by GPC using polystyrene as
standard. c Hydrodynamic radius was measured by DLS in THF.
and a catalytic amount of 18-crown-6 were dissolved in
anhydrous acetone and refluxed for 48 h under nitrogen. After
cooling to room temperature, the mixture was filtered, con-
centrated, and then precipitated from methanol. The target
copolymer 5 with a double bond in the pendant group was
obtained after filtration as a light-yellow powder.
5a (x:y ) 1:1): 0.42 g, yield 76%; Mn ) 21 000, PDI ) 1.66.
1H NMR (400 MHz, CDCl3): δ 7.61-7.25 (br, 4H), 7.24-6.21
(br, 12H), 5.72 (br, 1H), 5.22 (br, 1H), 4.92 (br, 4H), 2.14-
1.25 (br, 15H).
5b (x:y ) 2:1): 0.46 g, yield 83%; Mn ) 19 000, PDI ) 1.50.
1H NMR (400 MHz, CDCl3): δ 7.52-7.31 (br, 4H), 7.25-6.18
(br, 16H), 5.72 (br, 1H), 5.25 (br, 1H), 4.98 (br, 4H), 2.14-
0.85 (br, 27H).
General Procedure for Cross-linked Nanoparticles 6a
and 6b. A solution of 5 (200 mg) and AIBN (10 mg) in
anhydrous THF (500 mL) was bubbled with nitrogen for 30
min and stirred at 60 °C for 48 h. After cooling to room
temperature, the mixture was concentrated and then poured
into hexane. The precipitate was collected by filtration to afford
6 as a light-yellow solid.
6a (x:y ) 1:1): 190 mg, yield 95%; Mn ) 12 000, PDI ) 1.31.
1H NMR (400 MHz, CDCl3): δ 7.66-6.12 (br, 16H), 5.01 (br,
4H), 2.14-0.78 (br, 18H).
6b (x:y ) 2:1): 185 mg, yield 93%; Mn ) 13 000, PDI ) 1.33.
1H NMR (400 MHz, CDCl3): δ 7.71-6.12 (br, 20H), 4.01 (br,
4H), 2.24-0.85 (br, 30H).
(RAFT reagent) and AIBN (0.01 equiv) were dissolved in
anhydrous toluene. The mixture was subjected to freeze-
pump-thaw cycles 4 times, then stirred at 90 °C for 24 h. After
cooling to room temperature, the solution was poured into
hexane, and the precipitate was collected by filtration. After
drying under vacuum, polymer 3 was obtained as a white
powder.
3a (x:y ) 1:1): 1.2 g, yield 40%; Mn ) 15 000, PDI ) 1.27.
1H NMR (400 MHz, CDCl3): δ 7.51-6.21 (br, 8H), 4.52 (br,
2H), 2.18-1.17 (br, 15H).
3b (x:y ) 2:1): 1.6 g, yield 60%; Mn ) 14 000, PDI ) 1.22.
1H NMR (400 MHz, CDCl3): δ 7.51-6.21 (br, 12H), 4.52 (br,
2H), 2.16-1.17 (br, 27H).
Synthesis of 4-[(3-Hydroxyphenoxy)methyl]styrene
(4). Resorcinol (4.4 g, 40 mmol), 4-vinylbenzyl chloride (6.1 g,
40 mmol), and potassium carbonate (14 g, 0.1 mol) were
dissolved in 200 mL of acetone and refluxed for 18 h under
nitrogen. After cooling to room temperature, the mixture was
partitioned between water and dichloromethane. The aqueous
layer was extracted with dichloromethane twice, and the
organic layers were combined and washed with water then
dried over sodium sulfate. After filtration, the solvent was
removed under reduced pressure, and the residue was purified
by flash chromatography over silica eluted with hexane/ethyl
acetate to afford 4 (3.3 g, 35% yield). 1H NMR (400 MHz,
CDCl3): δ 7.42 (m, 4H), 7.15 (t, 1H), 6.69 (t, 1H), 6.55 (q, 2H),
5.71 (d, 1H), 5.26 (d, 2H), 5.07 (s, 2H), 4.66 (s, 1H).
General Procedure for the Precursor Copolymers 5a
and 5b. Copolymer 3 (1.0 equiv), 4-[(3-hydroxyphenoxy)-
methyl]styrene 4 (2.0 equiv), potassium carbonate (3.0 equiv),
General Procedure for the Synthesis of Functional-
ized Nanoparticles 7a and 7b. A solution of 6 (1 equiv) in
methanol was placed in a sonicator for 10 min to form a
suspension. In another flask, acetyl chloride (2 equiv) was
added dropwise to methanol and stirred for 30 min at 0 °C,
and then added dropwise to the suspension at 0 °C. After
addition, the mixture was stirred at room temperature until
it became a clear solution. The solution was concentrated and
then poured into ether. This process was repeated twice, and
the precipitate was collected by filtration to afford 7 as a light-
yellow solid.
7a (x:y ) 1:1): 51 mg, yield 90%. 1H NMR (400 MHz,
MeOD): δ 7.86-6.08 (br, 16H), 2.26-0.58 (br, 9H).
7b (x:y ) 2:1): 54 mg, yield 93%. 1H NMR (400 MHz,
MeOD): δ 7.98-6.02 (br, 20H), 2.24-0.52 (br, 12H).

5888 Jiang and Thayumanavan
Macromolecules, Vol. 38, No. 14, 2005
Figure 2. 1H NMR spectra of the copolymers before and after cross-linking: (a) 5b; (b) 6b.
Figure 3. AFM images of the (a) un-cross-linked copolymer 5a and (b) the cross-linked nanoparticles 6a. All the samples are
spin coated on silicon wafers at the speed of 1500 rpm with a 0.5 wt % 1,4-dioxane solution. Scan areas are 10 µm × 10 µm for
all cases.
General Procedure for Functional Nanoparticles 8a
and 8b. A solution of 7 (1 equiv) and Et3N (3 equiv) was
dissolved in dichloromethane, and then trimethyl-
acetyl chloride (1 equiv) was added dropwise at 0 °C. After
the addition, the mixture was stirred at room temperature
overnight, then precipitated in methanol. This precipitation
process was repeated two more times, and the precipitate was
collected by filtration to afford 8 as a light-yellow solid.
8a (x:y ) 1:1): yield 95%. 1H NMR (400 MHz, CDCl3): δ
7.23-6.08 (br, 16H), 5.24-4.67 (br, 4H), 1.88-0.82 (br, 18H).
8b (x:y ) 2:1): yield 93%. 1H NMR (400 MHz, CDCl3): δ
7.98-6.02 (br, 20H), 5.35-4.60 (br, 4H), 2.65-0.52 (br, 27H).
3. Results and Discussion
The target polymer’s structure involves a polymer-
containing 4-N-Boc-vinylaniline (1) and 4-chloromethyl-

Macromolecules, Vol. 38, No. 14, 2005
Amine-Functionalized Polystyrene Nanoparticles 5889
Scheme 3. Synthetic Route for the Functionalized
Nanoparticle.
was carried out under radical polymerization conditions
using AIBN (0.02 mg/mL) as the initiator in refluxing
THF. After allowing this reaction to proceed for 24 h,
the reaction mixture was concentrated, and the product
polymers 6a and 6b were obtained by precipitation in
methanol.
The elution profiles in GPC for these polymers are
shown in Figure 1. The reason for the high-molecular-
weight shoulder in polymers 5b and 6b is not clear at
this time. Repeating the polymerization reaction af-
forded polymer products that exhibit this shoulder
reproducibly. The molecular weights were estimated
from GPC using polystyrene standards. Subjecting
polymers 5a and 5b to the cross-linking reaction condi-
tions should not result in a decrease in the molecular
weight. Therefore, the decrease in Mn is attributed to a
change in architecture of the macromolecules, presum-
ably from a random coil to a nanoparticle.
If the hypothesized change in architecture were to
occur in our polymers, then the hydrodynamic radius
of the polymers obtained before and after cross-linking
should be significantly different. For this purpose, we
measured the Rh for polymers 5 and 6 using DLS, as
shown in Table 1. The Rh decreased from 26.1 to 16.9
nm from polymers 5a and 6a, whereas this value
decreased from 13.3 to 10.8 nm from 5b to 6b. The
percentage decrease in Mn (by GPC) and Rh (from DLS)
in polymers 5a to 6a and 5b to 6b is consistent with
the cross-linking density in these polymers. Similar
GPC and DLS behavior was also observed in the
literature, which has been attributed to an intramo-
lecular cross-linking and the ensuing change in the
architecture of the polymer.19
styrene (2). The protected amino group monomer would
provide the necessary functionality display in the final
polymeric nanoparticle, while the chloromethyl styrene
monomer affords the handle for incorporating the
functionality needed for intramolecular cross-linking.
Monomer 1 was obtained from the commercially avail-
able 4-vinylaniline and di-tert-butyl dicarbonate. Com-
pound 1 was copolymerized with 2 under reversible
addition-fragmentation chain transfer (RAFT) using
AIBN as the initiator, as shown in Scheme 1. Poly(4-
N-Boc-aminostyrene)-co-(4-chloromethylstyrene) (3a and
3b) were obtained using two different feed ratios of
monomers 1 and 2 with Mn and PDI of 15 000 and 1.27
for 3a, and 14 000 and 1.22 for 3b, respectively. An
estimate of the monomer ratio in polymers 3a and 3b
corresponds well with the feed ratio of 1:1 and 2:1,
The products of reactions outlined in Scheme 1 and
1
Scheme 2 were also analyzed by H NMR. The NMR
spectra of the un-cross-linked polymer 5b and cross-
linked polymer 6b are shown in Figure 2a and b,
respectively. In polymer 5b, the peaks at 5.25 and 5.72
ppm are assigned to be the protons in the double bond
of the pendant styrene. As expected, these peaks almost
disappeared upon cross-linking to form 6b (Figure 2b).
The residual peak at 5.72 ppm could be due to the un-
cross-linked double bond, the integration of which
suggests a cross-linking yield of nearly 90% for 6b. The
above-mentioned characterizations using NMR, GPC,
and DLS clearly show that intramolecularly cross-linked
polymer nanoparticles were obtained using this proce-
dure.
1
respectively, as determined by H NMR.
To functionalize the polymer with a cross-linkable
functionality, polymers 3a and 3b were treated with
compound 4 to afford the corresponding vinyl-function-
alized polymers 5a and 5b. Compound 4 was obtained
from the reaction of compound 2 with resorcinol (Scheme
2). Polymers 5a and 5b were subjected to the intramo-
lecular cross-linking protocol under high dilution condi-
tions (0.40 mg/mL) in THF. This cross-linking reaction
We also investigated the nature of un-cross-linked and
cross-linked polymers 5a and 6a using AFM. The AFM
images of the un-cross-linked copolymers 5a and cross-
Figure 4. 1H NMR spectra of the functionalized nanoparticle 7b (a) and recovered nanoparticle 8b (b).

5890 Jiang and Thayumanavan
Macromolecules, Vol. 38, No. 14, 2005
linked nanoparticles 6a are shown in Figure 3. Figure
3a and b represent the height (left) and phase (right)
images of the un-cross-linked copolymers 5a and cross-
linked nanoparticles 6a, respectively. From these im-
ages, one can clearly see that there are significant
differences between the un-cross-linked copolymer and
the cross-linked nanoparticles. The un-cross-linked
copolymer 5a exhibits “wormlike” structure, which could
be attributed to the aggregation of these polymers.
However, very different AFM images were observed
with the cross-linked nanoparticles (6a). The wormlike
structures in Figure 3a totally disappeared, and instead,
narrowly dispersed nanoparticles were obtained through
the cross-linking reaction. Note that the concentration
of the polymer and the speed of spin coating are
identical in both polymers. Similar behavior was also
obtained with copolymer 5b and cross-linked nanopar-
ticle 6b. These results also agree with those obtained
through DLS measurements, which further confirm that
an intramolecular cross-linking process is involved in
the formation of these nanoparticles.
Summary
A new class of functionalized polymeric nanoparticles
was successfully achieved. The nanoparticle is based on
a styrene-based copolymer backbone with amino moi-
eties as functional group displays. These particles were
achieved using an intramolecular collapse of the poly-
mer by carrying out a cross-linking reaction under high
dilution conditions. Analyses of the reactions using
NMR, GPC, DLS, and AFM confirm the intramolecular
cross-linking process. Our data also show that the size
of the nanoparticles can be tuned by controlling the
cross-linking density. Similarly, the number of amino
functionalities could also be tuned by varying the
monomer feed ratio. It is easy to imagine that such
control could also be achieved by controlling the molec-
ular weight. Finally, the functionalized intramolecular
cross-linked nanoparticles with amine groups were
reacted with trimethylacetyl chloride to demonstrate
that the amino moiety is available for further function-
alization. These nanoparticles have the potential to find
applications in a wide range of areas; an example of such
an application is drug delivery because of the easily
controllable size, cross-link density, and number of
functional groups.21 Investigations in this direction will
be part of the research efforts in our laboratories.
After the synthesis of the intramolecularly cross-
linked copolymers, 6 was reacted with acetyl chloride
in methanol to deprotect the Boc group to afford the
functionalized nanoparticle bearing amino functional-
ities 7, as shown in Scheme 3. The intramolecular cross-
linked nanoparticle 6 is soluble in common organic
solvents such as THF, DCM, and chloroform. However,
after the deprotection of the Boc group, the functional-
ized nanoparticle 7 was found to be insoluble in most
organic solvents except methanol. To confirm that the
deprotective reaction was achieved, we compared the
1H NMR of 7b with 6b (compare Figure 4a with Figure
2b). It is clear that the Boc group was removed because
the peak corresponding to tert-butyl moiety at around
Acknowledgment. We thank the U. S. Army Re-
search Office for partial support of this work. We also
thank the support from the NSF-MRSEC program of
the University of Massachusetts.
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