Anionic polyelectrolyte-stabilized nanoparticles via RAFT aqueous dispersion polymerization

Article abstract of DOI:10.1021/la203991y

We report the synthesis of anionic sterically stabilized diblock copolymer nanoparticles via polymerization-induced self-assembly using a RAFT aqueous dispersion polymerization formulation. The anionic steric stabilizer is a macromolecular chain-transfer

Full text of DOI:10.1021/la203991y

ARTICLE
pubs.acs.org/Langmuir
Anionic Polyelectrolyte-Stabilized Nanoparticles via RAFT Aqueous
Dispersion Polymerization
M. Semsarilar, V. Ladmiral, A. Blanazs, and S. P. Armes*
Dainton Building, Department of Chemistry, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
S Supporting Information
b
ABSTRACT: We report the synthesis of anionic sterically
stabilized diblock copolymer nanoparticles via polymeriza-
tion-induced self-assembly using a RAFT aqueous dispersion
polymerization formulation. The anionic steric stabilizer is a
macromolecular chain-transfer agent (macro-CTA) based on
poly(potassium 3-sulfopropyl methacrylate) (PKSPMA), and
the hydrophobic core-forming block is based on poly(2-hydro-
xypropyl methacrylate) (PHPMA). The effect of varying syn-
thesis parameters such as the salt concentration, solids content,
relative block composition, and anionic charge density has been
studied. In the absence of salt, self-assembly is problematic
when using a PKSPMA stabilizer because of lateral repulsion between highly charged anionic chains. However, in the presence of
added salt this problem can be overcome by reducing the charge density within the coronal stabilizer layer by either (i) statistically
copolymerizing the KSPMA monomer with a nonionic comonomer (2-hydroxyethyl methacrylate, HEMA) or (ii) using a binary
mixture of a PKSPMA macro-CTA and a poly(glycerol monomethacrylate) (PGMA) macro-CTA. These diblock copolymer
nanoparticles were analyzed by 1H NMR spectroscopy, gel permeation chromatography (GPC), dynamic light scattering (DLS),
transmission electron microscopy (TEM), and aqueous electrophoresis. NMR studies suggest that the HPMA polymerization is
complete within 2 h at 70 °C, and DMF GPC analysis confirms that the resulting diblock copolymers have relatively low polydispersities
(Mw/Mn < 1.30). NMR also suggests a significant degree of hydration for the core-forming PHPMA chains. Depending on the
specific reaction conditions, a series of spherical nanoparticles with mean diameters ranging from 50 to 200 nm with tunable anionic
surface charge can be prepared. If a binary mixture of anionic and nonionic macro-CTAs is utilized, then it is also possible to access a
vesicular morphology.
’ INTRODUCTION
micelles, wormlike micelles, or vesicles.15ꢀ30 For example, Hawker’s
group synthesized lightly cross-linked poly(N,N0-dimethylacrylamide)-
poly(N-isopropyl acrylamide) diblock copolymers via reversible
additionꢀfragmentation transfer (RAFT) polymerization at
70 °C in aqueous solution.15 On cooling, these spherical copolymer
particles became nanogels due to the swelling of the poly-
(N-isopropyl acrylamide) chains below their LCST of around
32 °C.15 This concept has been recently extended to include
other thermosensitive core-forming monomers by An et al.16,17
Pan and co-workers have published a series of papers describing
the synthesis of block copolymer spheres, wormlike particles, and
vesicles via RAFT alcohol dispersion polymerization. However,
monomer conversions were always substantially incomplete
(typically only 30ꢀ70%), and the removal of large volumes of
unreacted monomer makes such formulations unsuitable for
industrial scale up.24ꢀ27 Charleux et al. have recently developed
rather more efficient aqueous emulsion polymerization formula-
tions based on the living radical polymerization of water-immiscible
Amphiphilic block copolymers can self-assemble into a wide
range of complex morphologies in dilute aqueous solution, in-
cluding micelles,1 vesicles,1ꢀ5 tubules,6ꢀ9 toroids,10 and large
complex micelles.11 The precise morphology obtained is dictated
by various parameters, including the hydrophilic/hydrophobic
balance, the copolymer molecular weight, the copolymer con-
centration, the choice of solvent, and the processing route.12 In
the latter case, various protocols have been developed to promote
self-assembly, including dialysis, pH switching, and thin film
rehydration. For example, Eisenberg and co-workers reported a
range of morphologies for asymmetric amphiphilic diblocks by
initial dissolution in a good solvent for both blocks (e.g., DMF),
followed by slow dialysis against water. More specifically, reduc-
ing the hydrophilic block (typically either poly(acrylic acid) or
poly(ethylene oxide)) while fixing the hydrophobic polystyrene
block led to morphological transitions from micelles to rods to
vesicles to large compound micelles.1,13 Increasing the hydro-
phobic block led to morphological transitions from spheres to
rodlike or vesicular structures.13,14
Received: October 12, 2011
More recently, there has been increasing interest in exploiting
polymerization-induced self-assembly for the production of spherical
Revised:
Published: November 24, 2011
November 23, 2011
r
2011 American Chemical Society
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commodity monomers such as styrene and n-butyl acrylate to
prepare diblock copolymer spheres, worms, or vesicles. In par-
ticular, it was shown that the hydrophilic/hydrophobic balance
dictates the final copolymer morphology and that self-assembly is
much more problematic if polyelectrolytic stabilizer chains are
utilized because their mutual repulsion hinders in situ self-
assembly.18,22,23 This difficulty could be circumvented by either
reducing the stabilizer charge density via statistical copolymer-
ization with nonionic comonomers and/or by the addition of
sufficient salt to screen the strong electrostatic repulsive forces.19
We recently reported a robust RAFT aqueous dispersion poly-
merization formulation for the production of well-defined nano-
objects.30 This protocol is based on the use of a poly(glycerol
monomethacrylate) (PGMA) stabilizer block and 2-hydroxypro-
pyl methacrylate (HPMA) as a core-forming monomer. HPMA
is one of only a few vinyl monomers that are water-miscible (up
to 13% at 25 °C) but produce a water-insoluble polymer. This is
an essential prerequisite for a successful aqueous dispersion
polymerization formulation. Our initial report focused on the
production of either low-polydispersity spherical nanoparticles
ranging from 26 to 105 nm diameter or relatively poly-
disperse vesicles of around 500 nm. GPC analyses of such block
copolymer nanoparticles revealed relatively high polydispersities,
which were due to a dimethacrylate impurity in the HPMA
monomer. More recently, we have shown that if this impurity is
removed polydispersities of less than 1.20 can be obtained, even
when targeting mean degrees of polymerization of up to 1000 for
the core-forming block.28 Moreover, very high HPMA conver-
were kindly donated by Cognis UK Ltd. (Hythe). HPMA comprises
approximately 75% 2-hydroxypropyl methacrylate and 25 mol % 2-
hydroxyisopropyl methacrylate.31
Synthesis of 4-Cyano-4-(2-phenylethane sulfanylthiocarbonyl)
Sulfanylpentanoic Acid (PETTC). 2-Phenylethanethiol (10.5 g,
76.0 mmol) was added over 10 min to a stirred suspension of sodium
hydride (60% in oil, 3.15 g, 79.0 mmol) in diethyl ether (150 mL) at a
temperature of between 5 and 10 °C. The vigorous evolution of hy-
drogen was observed, and the grayish suspension became a viscous white
slurry of sodium phenylethanethiolate over 30 min. The reaction mixture
was cooled to 0 °C, and carbon disulfide (6.00 g, 79.0 mmol) was grad-
ually added to provide a thick yellow precipitate of sodium 2-phenyl-
ethanetrithiocarbonate, which was collected by filtration after 30 min
and used in the next step without purification. A suspension of sodium
2-phenylethanetrithiocarbonate (11.6 g, 0.049 mol) in diethyl ether
(100 mL) was treated by the gradual addition of solid iodine (6.30 g,
0.025 mol). The reaction mixture was stirred at room temperature for
1 h, and the insoluble white sodium iodide was removed by filtration.
The yellow-brown filtrate was washed with an aqueous solution of sodium
thiosulfate to remove excess iodine and dried over sodium sulfate, and
the solvent was removed under reduced pressure to leave a residue of
bis-(2-phenylethane sulfanylthiocarbonyl) disulfide (∼100% yield). A
solution of 4,40-azobis(4-cyanopentanoic acid) (ACVA, 2.10 g, 7.50
mmol) and bis-(2-phenylethane sulfanylthiocarbonyl) disulfide (2.13 g,
5.00 mmol) in ethyl acetate (50 mL) was degassed by nitrogen bubbling
and held at reflux under a dry nitrogen atmosphere for 18 h. After the
volatiles were removed under vacuum, the crude product was washed
with water (five 100 mL portions). The organic phase was concentrated
and purified by silica column chromatography (7:3 petroleum ether/ethyl
acetate, gradually increasing to 4:6) to afford 4-cyano-4-(2-phenyletha-
nesulfanylthiocarbonyl)sulfanylpentanoic acid as a yellow oil (yield 78%).
1H NMR (400.13 MHz, CD2Cl2, 298 K): δ 1.89 (3H, ꢀCH3),
2.34ꢀ2.62 (m, 2H, ꢀCH2), 2.7 (t, 2H, ꢀCH2), 3.0 (t, 2H, ꢀCH2), 3.6
(t, 2H, ꢀCH2), 7.2ꢀ7.4 (m, 5H, aromatic).
1
sions (>99% as judged by H NMR) are routinely achieved
within 2 h at 70 °C, with an enhanced rate of polymer-
ization being observed just after the onset of nucleation. Careful
sampling of the reaction solution confirmed that spheres were
first converted to worms and then vesicles, with useful insights
into the precise nature of the worm-to-vesicle transition being
obtained.28 In a separate study, the first detailed phase diagram
for any polymerization-induced self-assembly formulation was
elucidated for a PMPC-PHPMA formulation (where PMPC is
poly(2-(methacryloyloxy)ethyl phosphorylcholine)). This is im-
portant because it enables the predictable and reproducible
synthesis of pure phases of spheres, worms, or vesicles.29 Finally,
the effect of cross-linking the core-forming PHPMA chains with
ethylene glycol dimethacrylate was studied, and it was shown that
a novel “lumpy rod” phase could be accessed, albeit over a relatively
narrow range of conditions.29
13C NMR (400.13 MHz, CD2Cl2, 298 K): δ 24.2 (CH3), 29.6
(CH2CH2COOH), 30.1(CH2Ph), 33.1 (CH2CH2COOH), 39.9 (SCH2-
CH2Ph), 45.7 (SCCH2), 118.6 (CN), 127.4, 128.8, 129.2, 144.3 (Ph),
177.4 (CdO), 222.2 (CdS).
Synthesis of Poly(potassium 3-sulfopropyl methacrylate)
[PKSPMA] Macro-CTA. In a typical experiment, a round-bottomed
flask was charged with KSPMA (5.00 g, 20.0 mmol), PETTC (0.197 g,
0.580 mmol, dissolved in 1.0 mL dioxane), ACVA (33.0 mg, 0.220 mmol),
and an aqueous 1:7 acetic acid/sodium acetate buffer (4.00 g, 100 mM,
pH 5.5). The sealed reaction vessel was purged with nitrogen and placed
in a preheated oil bath at 70 °C for 2 h. The resulting PKSPMA macro-
CTA (96% conversion; Mn = 9800 g molꢀ1, Mw = 11 500 g molꢀ1
,
In the present work, we have replaced the nonionic poly-
(glycerol monomethacrylate) stabilizer block with an anionic
polyelectrolytic block based on poly(potassium 3-sulfopropyl
methacrylate) (PKSPMA). A systematic study of the effect of
varying the diblock copolymer composition, the overall solids
content, and the effect of added salt is presented for this RAFT
aqueous dispersion polymerization formulation. In addition, we
have explored the effect of diluting the coronal charge density by
either (i) the statistical copolymerization of KSPMA with HPMA
or (ii) using binary mixtures of PKSPMA and PGMA macro-
molecular chain-transfer agents (macro-CTA).
Mw/Mn = 1.18) was purified by dialysis against 9:1 water/methanol and
isolated by freeze-drying overnight. The mean degree of polymerization
(DP) for this macro-CTA was calculated by 1H NMR spectroscopy by
comparing the integrated aromatic proton signals due to the PETTC
chain end at 7.2ꢀ7.4 ppm to those due to the methacrylic polymer
backbone at 0.4ꢀ2.5 ppm.
Synthesis of Poly(potassium 3-sulfopropyl methacrylate-stat-
2-hydroxyethyl methacrylate) P(KSPMA-stat-HEMA) Macro-
CTA. In a typical experiment, a round-bottomed flask was charged with
KSPMA (1.00 g, 4.10 mmol), 2-hydroxyethyl methacrylate (HEMA,
2.64 g, 20.3 mmol), PETTC (0.270 g, 0.800 mmol, dissolved in 1.0 mL
dioxane), ACVA (45.5 mg, 0.160 mmol) and 1:7 acetic acid/sodium
acetate buffer (3.0 g, 100 mM, pH 5.5). The sealed reaction vessel was
purged with nitrogen and placed in a preheated oil bath at 70 °C for 2 h.
The resulting polymer (97% conversion, Mn = 13 300 g molꢀ1, Mw =
15 600 g molꢀ1, Mw/Mn = 1.17) was purified by dialysis against 9:1
water/methanol and isolated by freeze-drying overnight. The mean DP
of the purified macro-CTA was calculated as described previously.
’ EXPERIMENTAL SECTION
Materials. All reagents were purchased from Sigma-Aldrich and
were used as received, unless otherwise noted. 4,40-Azobis-4-cyanopen-
tanoic acid (ACVA, >98%) was used as an initiator. 2-Hydroxyethyl
methacrylate (HEMA) and 2-hydroxypropyl methacrylate (HPMA, 97%)
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Scheme 1. RAFT Aqueous Dispersion Polymerization of 2-Hydroxypropyl Methacrylate to Produce Sterically-Stabilized Diblock
Copolymer Nanoparticles at 70 °C Using (a) Anionic Poly(potassium 3-sulfopropyl methacrylate) Macro-CTA and (b) Anionic
Poly(potassium 3-sulfopropyl methacrylate-stat-2-hydroxyethyl methacrylate) Macro-CTA
Block Copolymer Nanoparticle Synthesis via RAFT Aque-
ous Dispersion Polymerization. For a typical aqueous dispersion
polymerization synthesis conducted at 10 wt % solids, the protocol was
as follows. HPMA (1.00 g, 6.90 mmol), ACVA (0.780 mg, 0.003 mmol),
and P(KSPMA6-stat-HEMA29) macro-CTA (73.0 mg, 0.014 mmol)
were codissolved in deionized water (9.66 g). The reaction mixture was
sealed in a round-bottomed flask and purged with nitrogen for 20 min
before being placed in a preheated oil bath at 70 °C for 24 h. The higher
solids content (15, 20, and 25 wt %) formulations were treated similarly
by reducing the amount of solvent, as required.
dried under ambient conditions. DLS studies were conducted using a
Malvern Instruments Zetasizer Nanoseries instrument equipped with a
4 mW HeꢀNe laser operating at 633 nm, an avalanche photodiode
detector with high quantum efficiency, and an ALV/LSE-5003 multiple
τ digital correlator electronics system. Aqueous electrophoresis studies
were performed on aqueous 0.01 wt % copolymer solutions using the
same Malvern Instruments Zetasizer Nanoseries instrument. The solu-
tion pH was adjusted by the addition of 0.01 M HCl or 0.01 M KOH
using an autotitrator, and the background electrolyte was 10ꢀ3 M NaCl.
Copolymer Characterization. Block copolymer molecular weight
distributions were assessed by DMF gel permeation chromatography
(GPC) using two Polymer Laboratories PL gel 5 μm mixed C columns
and one PL polar gel 5 μm guard column arranged in series and maintained
at 60 °C, followed by a Varian 390 LC refractive index detector. The eluent
contained 10 mM LiBr, and the flow rate was 1.0 mL minꢀ1. Ten near
monodisperse poly(methyl methacrylate) standards (ranging from
625 g molꢀ1 to 618 000 g molꢀ1) were used for calibration (K = 2.094 ꢁ
10ꢀ3, α = 0.642).
’ RESULTS AND DISCUSSION
In this work, we examined the synthesis of sterically stabilized
diblock copolymer nanoparticles via the RAFT aqueous disper-
sion polymerization of HPMA using an anionic polyelectrolyte
stabilizer as a macro-CTA (Scheme 1). The RAFT synthesis of
the block copolymer nanoparticles was performed in two steps.
First, KSPMA was polymerized using 4-cyano-4-(2-phenyletha-
nesulfanylthiocarbonyl)sulfanyl pentanoic acid (PETTC) as the
CTA and 4,40-azobis(4-cyanopentanoic acid) as the free radical
initiator (CTA/initiator molar ratio = 5:1) in an aqueous buffer
solution at pH 5.5 so as to minimize the hydrolysis of the RAFT
chain ends32 and hence ensure good block efficiencies. A small
amount of 1,4-dioxane cosolvent was used to ensure complete
dissolution of the CTA. The solution polymerization was termi-
nated after 2 h, and the resulting PKSPMA macro-CTA was
purified by dialysis against a 9:1 v/v water/methanol mixture.
Polydispersities ranged between 1.18 and 1.20, as indicated by
aqueous GPC. The second step was performed under RAFT
aqueous dispersion polymerization conditions using this near
monodisperse macro-CTA, which was dissolved in distilled water
along with HPMA and 4,40-azobis(4-cyanopentanoic acid) initiator.
Molecular weight distributions for the PKSPMA macro-CTA pre-
cursors were determined using aqueous GPC. The standard GPC pro-
tocol involved using a Pharmacia Biotech Superose 6 column connected
to a Polymer Laboratories ERC-7517A refractive index detector. The
eluent was an aqueous solution of 0.20 M NaNO3 containing 50 mM
Trizma buffer at pH 7. Calibration was achieved using a series of near
monodisperse poly(ethylene oxide) standards ranging from 440 to
288 000 g molꢀ1
.
1H NMR spectra were acquired using either a 250 or 400 MHz Bruker
spectrometer in d6-DMSO or D2O. Sodium 3-(trimethylsilyl)-1-propa-
nesulfonate was used as an internal standard. TEM studies were con-
ducted using a Philips CM 100 instrument operating at 100 kV. To prepare
TEM samples, 5.0 μL of a dilute aqueous copolymer solution was placed
onto a carbon-coated copper grid, stained using uranyl formate, and then
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The macro-CTA/initiator molar ratio was fixed at 5.0, and
HPMA polymerization was conducted at 70 °C. Initially, the
reaction mixture was transparent, but it became increasingly turbid
as the polymerization proceeded, indicating in situ phase separa-
tion. This is because the HPMA monomer is water-soluble but
the growing PHPMA chains are hydrophobic, which leads to
polymerization-induced self-assembly after around 45 min and
ultimately to colloidally stable sterically stabilized dispersions.
A series of PKSPMA34-PHPMAx diblock copolymers were
synthesized in aqueous media with mean target degrees of poly-
merization (x) of 100, 200, or 300 simply by adjusting the
[HPMA]/[macro-CTA] molar ratio. This systematic approach
led to a series of spherical nanoparticles exhibiting a range of
mean diameters. However, the mean nanoparticle diameter did
not simply increase monotonically as longer PHPMA chains were
targeted, as described previously by Li and Armes for diblock
copolymer particles prepared via RAFT aqueous dispersion poly-
merization using a nonionic stabilizer.30 This qualitative differ-
ence is most likely due to the strong polyelectrolyte nature of the
PKSPMA block, which leads to lateral repulsive electrostatic
forces between neighboring stabilizer chains, thus impeding their
efficient self-assembly. This results in the formation of rather
large, ill-defined micellar aggregates comprising relatively hydrated
PHPMA cores. This interpretation is supported by inspecting a
1H NMR spectrum (Figure 1) recorded for diblock copolymer
PKSPMA34-PHPMA300 dissolved in D2O because characteristic
PHPMA signals are clearly visible at 3.8 ppm. Moreover, DLS
indicates a relatively large hydrodynamic diameter of about 200 nm
(and a relatively low scattered-light intensity), and TEM studies
reveal rather ill-defined particles (Figure S1a). In principle, the
covalent stabilization of the core-forming PHPMA block could
minimize this problem. Thus, 1 mol % ethylene glycol dimethacrylate
(EGDMA) cross-linker was added to a PKSPMA34-PHPMA300
formulation, but unfortunately no improvement in particle
morphology was observed (Figure S1b).
Alternatively, the addition of salt leads to charge screening,
which should facilitate the close packing of the PKSPMA stabilizer
chains within the corona. This approach has been recently explored
by Charleux et al. in the context of RAFT aqueous emulsion
polymerization.19 In the present study, a series of PKSPMA34-
PHPMA100ꢀ300 diblock copolymer nanoparticles were prepared
in the presence of 0.10, 0.20, or 0.30 M NaCl. Table S1 sum-
marizes the dynamic light scattering results obtained for this
series of dispersions. Relatively large particles are observed at
each salt concentration for the PKSPMA34-PHPMA100 diblock
copolymer (244, 293, and 152 nm diameter, respectively).
Increasing the DP of the core-forming chains reduces the mean
diameter to 40ꢀ60 nm, presumably because of their more hydro-
phobic character and more efficient charge screening. Compar-
ing TEM images obtained for diblock copolymer nanoparticles of
the same chemical composition suggests that the addition of salt
is an effective means of screening the anionic charge density and
hence promoting self-assembly, as expected (Figure 2). More-
over, if the target DP of the core-forming PHPMA chains is
increased up to 1000 or 5000 in the absence of any salt, then the
hydrodynamic diameter increases monotonically from 130 to
175 nm (Table S2).
An alternative approach to salt screening is to reduce the
charge density on the polyelectrolyte stabilizer chains. However,
because the PKSPMA chains are strong polyelectrolytes, their
anionic character cannot be varied by adjusting the solution pH.
Instead, KSPMA was statistically copolymerized with a neutral
comonomer, 2-hydroxyethyl methacrylate, to reduce the charge
density within the coronal layer and hence promote more efficient
chain packing. In practice, a series of P(KSPMA-stat-HEMA)
statistical copolymers were synthesized, whereby the overall
DP of the stabilizer chains was fixed at approximately 34 (for
comparison with the performance of the PKSPMA34 homopol-
ymer stabilizer) and the number of KSPMA units per chain
Figure 1. 1H NMR spectra obtained for (a) PHPMA50 homopolymer
in CD3OD, (b) PKSPMA34 homopolymer in D2O, and (c) PKSPMA34-
PHPMA300 in D2O.
Figure 2. Representative transmission electron micrographs obtained for PKSPMA34-PHPMA300 spherical nanoparticles synthesized by RAFT
aqueous dispersion polymerization at 70 °C in the presence of (a) no salt, (b) 0.10 M NaCl, (c) 0.20 M NaCl, and (d) 0.30 M NaCl (entries 3, 6, and 9 in
Table S1).
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Figure 3. Transmission electron micrographs obtained for (a) P(KSPMA11-stat-HEMA24)-PHPMA500 with zero NaCl, (b) P(KSPMA11- stat-
HEMA24)-PHPMA500 with 0.20 M NaCl, (c) P(KSPMA11- stat-HEMA24)-PHPMA500 with 0.30 M NaCl, (d) P(KSPMA6-stat-HEMA29)-PHPMA500
with zero NaCl, (e) P(KSPMA6-stat-HEMA29)-PHPMA500 with 0.20 M NaCl, and (f) P(KSPMA6-stat-HEMA29)-PHPMA500 with 0.30 M NaCl at
10 wt % solids.
1
Figure 5. Evolution of the number-average molecular weight Mn
(vs poly(methyl methacrylate) calibration standards) and polydispersity
(Mw/Mn) with monomer conversion as judged by DMF GPC when
using a P(KSPMA6-stat-HEMA29) macro-CTA for the RAFT aqueous
dispersion polymerization of HPMA at 70 °C, 10 wt % solids, and 0.10 M
NaCl. The targeted diblock composition was P(KSPMA6-stat-HEMA29)-
Figure 4. Conversion vs time data calculated from H NMR spectra
(D2O) for the RAFT aqueous dispersion polymerization of HPMA
using a P(KSPMA6-stat-HEMA29) macro-CTA at 70 °C, 10 wt % solids,
and 0.10 M NaCl. The targeted diblock composition was P(KSPMA6-
stat-HEMA29)-PHPMA500
.
was 6, 11, or 25.33 A series of PHPMA nanoparticles were synthesized
via RAFT aqueous dispersion polymerization at 70 °C at 10 wt %
solids using each of these copolymer macro-CTAs as the steric
stabilizer (Table S3). Judging by the TEM images (Figure S2)
and the DLS data (Table S3), it is clear that reducing the anionic
charge density leads to the formation of smaller spherical nano-
particles with little or no change in polydispersity. Furthermore,
conducting the HPMA polymerization in the presence of added
salt using these P(KSPMA-stat-HEMA) macro-CTAs produces
larger nanoparticles when targeting longer core-forming blocks
(Table S4 and Figure S3). Typically, the hydrodynamic particle
diameter increases from 40 to 80 nm as the target DP of the
PHPMA500
.
PHPMA chains increases from 100 to 500. Also, fused particles
and short worms are observed when a macro-CTA containing
the lowest proportion of KSPMA (six repeat units) is employed
in the presence of either 0.20 or 0.30 M NaCl (Figure 3e,f).
The effect of the copolymer concentration (solids content)
was also investigated to examine whether either worms or vesicles
could be obtained, as recently reported by Sugihara and co-
workers.29 When copolymer stabilizer macro-CTA contained 11
KSPMA residues, increasing the solids content in the presence of
salt still produced spherical particles (Figure S4) but higher-order
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Figure 8. Aqueous electrophoresis data obtained for PKSPMA34-
PHPMA500 (green 1), P(KSPMA25-stat-HEMA11)-PHPMA500 (blue 2),
P(KSPMA11-stat-HEMA24)-PHPMA500 (red b), and P(KSPMA6-
stat-HEMA29)-PHPMA500 (black 9). In each case, these aqueous disper-
sions were prepared at 10 wt % solids in the presence of 0.20 M NaCl.
Electrophoretic measurements were made on 0.01 wt % dispersions in the
presence of 10ꢀ3 M background NaCl.
Figure 6. Evolution of GPC traces (refractive index detector) with time
for the RAFT aqueous dispersion polymerization of HPMA using a
P(KSPMA6-stat-HEMA29) macro-CTA at 70 °C, 10 wt % solids, and
0.10 M NaCl. The targeted diblock composition was P(KSPMA6-stat-
HEMA29)-PHPMA500
.
Scheme 2. Schematic Preparation of Diblock Copolymer
Nanoparticles with Mixed Coronas via RAFT Aqueous Dis-
persion Polymerization of HPMA at 70 °C Using a Binary
Mixture of an Anionic PKSPMA Macro-CTA and a Nonionic
PGMA Macro-CTAa
a The latter stabilizer reduces the overall coronal charge density and
hence enables polymerization-induced self-assembly to occur in the
presence of added salt.
Figure 7. Evolution of the intensity-average particle diameter and
polydispersities (PDI) with time as judged by dynamic light scattering
for the RAFT aqueous dispersion polymerization of HPMA using a
P(KSPMA6-stat-HEMA29) macro-CTA at 70 °C, 10 wt % solids, and
0.10 M NaCl. The targeted diblock composition was P(KSPMA6-stat-
In all cases, the polymerization of HPMA was monitored by
1H NMR spectroscopy, and monomer conversions were calcu-
lated on the basis of the disappearance of the monomer vinyl
signals relative to a convenient nonvolatile internal standard
(sodium 3-(trimethylsilyl)propyl sulfonate). However, GPC
analysis was limited to those diblock copolymers synthesized
using the macro-CTA containing the lowest anionic charge
density (i.e., P(KSPMA6-stat-HEMA29)-PHPMAy) because
all other copolymers proved to be insufficiently soluble in
the DMF eluent. A kinetic study was performed for this least-
anionic formulation: it was found that approximately 100%
conversion was attained within around 2 h (Figure 4). An
enhanced rate of polymerization was observed after around
45 min in the semilog plot inset shown in Figure 4, which
corresponds to the onset of turbidity in the reaction solution
due to the nucleation of the growing PHPMA chains. The
number-average molecular weight, Mn, of the diblock copoly-
mer increased linearly with conversion, with polydispersities
remaining below 1.25 (Figure 5). This suggests good pseudo-
living character for the HPMA polymerization, as expected for
a RAFT formulation.
HEMA29)-PHPMA500
.
morphologies were observed as the solids content was increased
on reducing the KSPMA content to an average of 6 residues per
copolymer stabilizer chain in the presence of 0.20 M salt (Table
S5 and Figure S5). Thus, spherical particles of around 30 nm
were observed by TEM at 15 or 20 wt % for a target PHPMA DP
of 100 or 200, but short worms were formed at a DP of 300
(Figure S5e) and micrometer-sized aggregates of worms were
observed at a DP of 500 (Figure S4b). Block copolymer vesicles
were obtained at a solids content of 25% when targeting a
PHPMA DP of 300 (Figure S5f), but increasing the target DP
further to 1000 produced only colloidally unstable aggregates
(image not shown). The same conditions (i.e., using a PKSPMA6-
stat-HEMA29 macro-CTA at 25 wt %) employed in the presence
of 0.30 M NaCl resulted in spherical particles for PHPMA DPs of
less than 300 but only unstable aggregates when the PHPMA DP
exceeded 300 (image not shown).
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Figure 9. Electrophoretic mobilities obtained for 0.10% aqueous dispersions of the following diblock copolymer nanoparticles at pH 4.50: (a) PGMA60-
PHPMA500, (b) PKSPMA34-PHPMA500, and (c) (1 PKSPMA34 + 4 PGMA60)-PHPMA500. Originally (a) was prepared at 10 wt % solids in the absence
of any salt and (b) and (c) were prepared at 10 wt % solids in the presence of 0.20 M NaCl.
Moreover, the unimodal GPC traces shown in Figure 6 suggest
a high blocking efficiency for the P(KSPMA6-stat-HEMA29)
macro-CTA because there is no evidence of any unreacted
macro-CTA. The high molecular weight shoulder that becomes
more prominent at higher conversion is due to a small amount of
a dimethacrylate impurity (∼0.26 mol %) present in the HPMA
monomer.28 The evolution of hydrodynamic particle diameter
with time was monitored by DLS for the same RAFT aqueous
dispersion polymerization of HPMA at 70 °C using the P-
(KSPMA6-stat-HEMA29) macro-CTA (Figure 7). Initially, no
particles could be detected (up to 20% monomer conversion).
Then nucleation occurs as the PHPMA chains become suffi-
ciently hydrophobic to overcome the unfavorable electrostatics
and induce block copolymer self-assembly: at this point, the for-
mation of colloidal particles in the reaction solution is also confirmed
visually by the increase in turbidity. The mean intensity-average
diameter reported by DLS gradually increased from around 50 to
59 nm as the HPMA polymerization proceeded to full conversion
within 2 to 3 h at 70 °C. The final sterically stabilized nanopar-
ticles have a relatively low polydispersity of 0.05. A closer inspection
of Figures 4 and 7 confirms that the onset of nucleation at around
45 min corresponds to an enhanced rate of polymerization.
(Note the change in slope in the semilogarithmic kinetic plot.)
Similar observations have been reported by Blanazs and co-workers28
for a RAFT aqueous dispersion polymerization formulation based
on a nonionic macro-CTA. The most likely explanation is that,
once the block copolymer nanoparticles are formed, some of the
unreacted HPMA monomer dissolved in the continuous phase
becomes solubilized within the PHPMA micelle cores, leading to
a relatively high local monomer concentration and hence a faster
rate of polymerization.28
A series of P(KSPMAx-stat-HEMAy)-PHPMA500 diblock co-
polymer dispersions prepared using four different macro-CTAs
were characterized by aqueous electrophoresis (Figure 8). As ex-
pected, nanoparticles prepared using the PKSPMA homopoly-
mer macro-CTA (x = 34, y = 0) exhibited the most negative zeta
potentials (ꢀ55 to ꢀ60 mV), with little or no pH dependence
over the pH range studied. However, gradually diluting the anionic
charge density within the coronal chains using the nonionic HEMA
comonomer allows the zeta potential of these nanoparticles to be
systematically adjusted. Moreover, just six KSPMA comonomer
units per stabilizer chain produce a zeta potential of ꢀ30 to ꢀ35 mV,
again exhibiting almost no pH dependence.
A second strategy was also explored to modulate the coronal
charge density in these block copolymer nanoparticles (Scheme 2):
HPMA was polymerized using a binary mixture of two macro-CTAs.
From an entropic viewpoint, this approach should produce block
copolymer nanoparticles with mixed coronas. Hence the nonionic
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Figure 10. Transmission electron micrographs obtained for the RAFT aqueous dispersion polymerization of HPMA at 70 °C using xPKSPMA34
+
yPGMA60 binary mixtures (where the x/y molar ratio is 1:1, 3:7, 1:4, 1:9, and 1:19) under the following conditions: (aꢀe) zero salt, 10 wt %; (fꢀj)
0.20 M NaCl, 10 wt %; and (kꢀo) 0.20 M NaCl, 20 wt %.
PGMA60 stabilizer chains should dilute the lateral electrostatic repul-
sions between the highly anionic PKSPMA34 chains within the
coronal layer, facilitating polymerization-induced self-assembly.
The proportion of PKSPMA macro-CTA chains was system-
atically varied from 5 to 50 mol %, and the mean DP of the core-
forming PHPMA chains was fixed at 500 at 10 w/w % solids in
the presence of 0.20 M NaCl. This approach also provided good
control over the electrophoretic footprint exhibited by the block
copolymer nanoparticles (Figure S6). At present, we cannot explain
the remarkable nonlinear nature of the zeta potential curves
observed in this case, although it is worth emphasizing that these
data sets were replicated three times using two different Nano-
sizer instruments, with 10ꢀ3 M NaCl being used as a back-
ground electrolyte in each case.
To verify that true entropic mixing of the two types of stabilizer
chains had occurred with the coronal layer, we examined the raw
mobility data obtained at pH 4.25 prior to calculating the zeta
potentials. As expected, the nonionic PGMA60-PHPMA500 block
copolymer nanoparticles prepared as a reference exhibited essen-
tially zero mobility. In contrast, PKSPMA34-PHPMA500 nanoparticles
exhibited a highly negative mobility of almost ꢀ4μm cm/V s whereas
the mixed-corona (1 PKSPMA34 + 4 PGMA60)-PHPMA500
nanoparticles prepared using 20 mol % PKSPMA34 macro-CTA
had an intermediate negative mobility of around ꢀ2 μm cm/V s
(Figure 9). Moreover, this binary mixture strategy not only
afforded control over the electrophoretic footprint of the nano-
particles but also enabled higher-order morphologies to be
accessed (Figure 10). Thus, HPMA polymerizations performed
at a solids content of 10 wt % in the presence of 0.20 M NaCl
using 5 to 20 mol % PKSPMA macro-CTA led to the formation
of block copolymer vesicles (Figure 10hꢀj). TEM studies also
confirmed the presence of some short worms (formed by the partial
fusion of two or more spheres) when using a 30 mol % PKSPMA
stabilizer (Figure 10g), whereas only spheres were observed when
50 mol % PKSPMA stabilizer was employed (Figure 10f). Finally,
regardless of the precise binary mixture of anionic/nonionic
macro-CTAs employed, HPMA polymerizations conducted at a
higher solids content (20 wt %) always resulted in the formation
of block copolymer vesicles (Figure 10kꢀo).
’ CONCLUSIONS
A range of anionic polyelectrolyte-stabilized diblock copoly-
mer nanoparticles were prepared via RAFT aqueous dispersion
polymerization of 2-hydroxypropyl methacrylate at 70 °C using
a PKSPMA-based macro-CTA. The strongly anionic character of
the PKSPMA stabilizer hinders polymerization-induced self-assem-
bly due to the strong lateral repulsive forces between adjacent
chains, which leads to either no particle formation or the formation
of loose block copolymer aggregates comprising hydrated PHPMA
chains. This problem can be overcomeby the addition of salt, which
screens these repulsive forces and leads to the formation of
significantly denser, more well defined nanoparticles. Alternatively,
the anionic charge density can be diluted by statistically copoly-
merizing KSPMA with a nonionic comonomer such as 2-hydro-
xyethyl methacrylate. The latter approach allows the electro-
phoretic behavior of these anionic nanoparticles to be precisely
tuned. A third strategy for promoting in situ self-assembly involves
diluting the anionic PKSPMA macro-CTA with a nonionic poly-
(glycerol monomethacrylate) macro-CTA to modulate the elec-
trostatic repulsive forces. This approach enables higher-order mor-
phologies such as vesicles to be obtained, particularly for syntheses
conducted in the presence of salt in more concentrated solution.
’ ASSOCIATED CONTENT
S
Supporting Information. Additional TEM images, DLS
b
diameters (and polydispersities), and aqueous electrophoresis
data obtained for these anionic diblock copolymer nanoparticles.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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’ AUTHOR INFORMATION
(30) Li, Y.; Armes, S. P. Angew. Chem., Int. Ed. 2010, 49, 4042–4046.
(31) Save, M.; Weaver, J. V. M; Armes, S. P.; McKenna, P. Macro-
molecules 2002, 35, 1152–1159.
Corresponding Author
*E-mail: s.p.armes@sheffield.ac.uk.
(32) Thomas, D. B.; Convertine, A. J.; Hester, R. D.; Lowe, A. B.;
McCormick, C. L. Macromolecules 2004, 37, 1735–1741.
(33) Poly(2-hydroxyethyl methacrylate) is soluble in water up to a
mean degree of polymerization of 45, although it exhibits LCST-type
behavior at elevated temperature. See Weaver, J. V. M; Bannister, I.;
Robinson, K. L.; Bories-Azeau, X.; Armes, S. P.; Smallridge, M.;
McKenna, P. Macromolecules 2004, 37, 2395–2403. Statistical copolym-
erization with an ionic monomer such as KSPMA eliminates this inverse
temperature solubility behavior.
’ ACKNOWLEDGMENT
We thank EPSRC for postdoctoral support (EP/G007950/1
and EP/E012949/1).
’ REFERENCES
(1) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728–1731.
(2) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967–973.
(3) Christian, D. A.; Tian, A.; Ellenbroek, W. G.; Levental, I.;
Rajagopal, K.; Janmey, P. A.; Liu, A. J.; Baumgart, T.; Discher, D. E.
Nat. Mater. 2009, 8, 843–849.
(4) Massignani, M.; LoPresti, C.; Blanazs, A.; Madsen, J.; Armes,
S. P.; Lewis, A. L.; Battaglia, G. Small 2009, 5, 2424–2432.
(5) Howse, J. R.; Jones, R. A. L.; Battaglia, G.; Ducker, R. E.; Leggett,
G. J.; Ryan, A. J. Nat. Mater. 2009, 8, 507–511.
(6) Won, Y.-Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960–963.
(7) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P.
Science 2004, 306, 98–101.
(8) Cui, H.; Chen, Z.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Science
2007, 317, 647–650.
(9) Wang, X.; Guerin, G.; Wang, H.; Wang, Y.; Manners, I.; Winnik,
M. A. Science 2007, 317, 644–647.
(10) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L.
Science 2004, 306, 94–97.
(11) Jain, S.; Bates, F. S. Science 2003, 300, 460–464.
(12) Hayward, R. C.; Pochan, D. J. Macromolecules 2010,
43, 3577–3584.
(13) Kellum, M., G.; Smith, A. E.; Xu, X.; McCormick, C. L. ACS
Symp. Ser. 2009, 1024, 195–215.
(14) Jitchum, V.; Kakwere, H.; Ladmiral, V.; Perrier, S. ACS Symp.
Ser. 2009, 1024, 279–292.
(15) An, Z.; Shi, Q.; Tang, W.; Tsung, C.-K.; Hawker, C. J.; Stucky,
G. D. J. Am. Chem. Soc. 2007, 129, 14493–14499.
(16) Liu, G.; Qiu, Q.; Shen, W.; An, Z. Macromolecules 2011,
44, 5237–5245.
(17) Shen, W.; Chang, Y.; Liu, G.; Wang, H.; Cao, A.; An, Z.
Macromolecules 2011, 44, 2524–2530.
(18) Boissꢀe, S.;Rieger, J.;Belal, K.;Di-Cicco, A.;Beaunier, P.;Li, M.-H.;
Charleux, B. Chem. Commun. 2010, 46, 1950–1952.
(19) Boissꢀe, S.; Rieger, J.; Pembouong, G.; Beaunier, P.; Charleux,
B. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 3346–3354.
(20) Charleux, B.; D’Agosto, F.; Delaittre, G. Preparation of Hybrid
Latex Particles and CoreꢀShell Particles through the Use of Controlled
Radical Polymerization Techniques in Aqueous Media. In Hybrid Latex
Particles; van Herk, A. M., Landfester, K., Eds.; Springer: Berlin, 2011;
Vol. 233, pp 125ꢀ183.
(21) Qiu, J.; Charleux, B.; Matyjaszewski, K. Prog. Polym. Sci. 2001,
26, 2083–2134.
(22) Rieger, J.; Zhang, W.; Stoffelbach, F. O.; Charleux, B. Macro-
molecules 2010, 43, 6302–6310.
(23) Zhang, X.; Boissꢀe, S.; Zhang, W.; Beaunier, P.; D’Agosto, F.;
Rieger, J.; Charleux, B. Macromolecules 2011, 44, 4149–4158.
(24) Wan, W.-M.; Pan, C.-Y. Polym. Chem. 2010, 1, 1475–1484.
(25) Cai, W.; Wan, W.; Hong, C.; Huang, C.; Pan, C. Soft Matter
2010, 6, 5554–5561.
(26) He, W.-D.; Sun, X.-L.; Wan, W.-M.; Pan, C.-Y. Macromolecules
2011, 44, 3358–3365.
(27) Huang, C.-Q.; Pan, C.-Y. Polymer 2010, 51, 5115–5121.
(28) Blanazs, A.; Madsen, J.; Battaglia, G.; Ryan, A. J.; Armes, S. P.
J. Am. Chem. Soc. 2011, 133, 16581.
(29) Sugihara, S.; Blanazs, A.; Armes, S. P.; Ryan, A. J.; Lewis, A. L.
J. Am. Chem. Soc. 2011, 133, 15707.
922
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