Synthesis of sterically-stabilized polystyrene latexes using well-defined thermoresponsive poly(N-isopropylacrylamide) macromonomers

Article abstract of DOI:10.1021/ma2016584

A series of well-defined thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) macromonomers was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization followed by aminolysis and nucleophilic Michael addition. 2-(Dodecylthiocarbonothioylthio)-2-methylpropanoic-2-phenoxyethyl ester (CTA) was used as the RAFT chain transfer agent to prepare six PNIPAM-CTA precursors with target degrees of polymerization of 20, 30, 40, 50, 60, and 75. These NIPAM polymerizations were conducted in 1,4-dioxane and proceeded with good control and low polydispersities (M_w/M_n < 1.10) up to more than 90% conversion. The PNIPAM trithiocarbonate end-groups were then converted to methacrylate end-groups by combining (i) aminolysis and (ii) nucleophilic Michael addition using the bifunctional reagent, 3-(acryloyloxy)-2-hydroxypropyl methacrylate (AHPMA), in a one-pot reaction. The resulting PNIPAM macromonomers were evaluated as reactive steric stabilizers for latex syntheses. Near-monodisperse submicrometer-sized latexes were obtained by alcoholic dispersion polymerization of styrene in methanol, as judged by scanning electron microscopy (SEM) and dynamic light scattering (DLS). In contrast, a latex synthesized in the presence of a PNIPAM-CTA had a bimodal size distribution, while thiol-capped PNIPAM chains produced ill-defined nonspherical particles and styrene polymerization conducted in the absence of any stabilizer led to macroscopic precipitation. These control experiments confirm that using the methacrylate-capped macromonomers is essential for successful latex syntheses. ¹H NMR analysis confirm the presence of PNIPAM chains in the latex particles and XPS measurements indicate that the stabilizer is located on the particle surface, as expected. The well-known thermo-responsive nature of the stabilizer was successfully transferred to these latexes, which exhibit reversible flocculation upon heating above the LCST of the PNIPAM chains.

Full text of DOI:10.1021/ma2016584

ARTICLE
pubs.acs.org/Macromolecules
Synthesis of Sterically-Stabilized Polystyrene Latexes Using
Well-Defined Thermoresponsive Poly(N-isopropylacrylamide)
Macromonomers
J. R. McKee,†,‡,§ V. Ladmiral,†,* J. Niskanen,‡ H. Tenhu,‡ and S. P. Armes†,*
†Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire, S3 7HF, U.K.
‡Department of Chemistry, University of Helsinki, PB 55, FIN-00014 HY Helsinki, Finland
S Supporting Information
b
ABSTRACT: A series of well-defined thermo-responsive poly-
(N-isopropylacrylamide) (PNIPAM) macromonomers was
prepared by reversible additionꢀfragmentation chain transfer
(RAFT) polymerization followed by aminolysis and nucleophi-
lic Michael addition. 2-(Dodecylthiocarbonothioylthio)-2-meth-
ylpropanoic-2-phenoxyethyl ester (CTA) was used as the
RAFT chain transfer agent to prepare six PNIPAM-CTA
precursors with target degrees of polymerization of 20, 30, 40,
50, 60, and 75. These NIPAM polymerizations were conducted
in 1,4-dioxane and proceeded with good control and low polydispersities (Mw/Mn < 1.10) up to more than 90% conversion. The
PNIPAM trithiocarbonate end-groups were then converted to methacrylate end-groups by combining (i) aminolysis and (ii)
nucleophilic Michael addition using the bifunctional reagent, 3-(acryloyloxy)-2-hydroxypropyl methacrylate (AHPMA), in a one-
pot reaction. The resulting PNIPAM macromonomers were evaluated as reactive steric stabilizers for latex syntheses. Near-
monodisperse submicrometer-sized latexes were obtained by alcoholic dispersion polymerization of styrene in methanol, as judged
by scanning electron microscopy (SEM) and dynamic light scattering (DLS). In contrast, a latex synthesized in the presence of a
PNIPAM-CTA had a bimodal size distribution, while thiol-capped PNIPAM chains produced ill-defined nonspherical particles and
styrene polymerization conducted in the absence of any stabilizer led to macroscopic precipitation. These control experiments
confirm that using the methacrylate-capped macromonomers is essential for successful latex syntheses. 1H NMR analysis confirm
the presence of PNIPAM chains in the latex particles and XPS measurements indicate that the stabilizer is located on the particle
surface, as expected. The well-known thermo-responsive nature of the stabilizer was successfully transferred to these latexes, which
exhibit reversible flocculation upon heating above the LCST of the PNIPAM chains.
’ INTRODUCTION
macromonomers via anionic polymerization using a 4-vinylbenzyl
alkoxide initiator. Similarly, the preparation of poly(methyl vinyl
ether) macromonomers by cationic polymerization using an
acrylate initiator was recently reported.12 Allyl-, vinyl acetate- and
vinyl ether-based initiators have also been used to prepare well-
defined macromonomers via atom transfer radical polymerization
(ATRP).13 Postpolymerization modification usually involves a
two-step protocol and is therefore generally more time-consum-
ing. Nevertheless, it is often much more versatile, since it allows
the incorporation of vinyl functionalities which would otherwise
participate in the chain-growth polymerization. This approach
has been successfully employed in conjunction with free-radical
polymerization, ATRP and anionic polymerization.3,14 The well-
known orthogonality of so-called “click” chemistry, in particular
the copper(I)-catalyzed 1,3-dipolar cycloaddition of azides to alkynes,
has been exploited to produce well-defined macromonomers.15
Macromonomers are polymer chains that contain one or more
polymerizable groups. This broad definition encompasses a large
number of possible architectures.1 In practice, the term “macro-
monomer” is often used to designate macromolecules containing
a single vinyl group located at one chain-end. Such macromono-
mers can be readily copolymerized using conventional free radical
polymerization, which is one of the most industrially relevant
polymerization methods. Macromonomers have been widely
used to prepare sterically stabilized latexes,2,3 graft copolymers,4
dendritic polymers,5 or so-called “bottle brush” polymers.6 They
can be prepared by two main methods: (i) polymerization from an
initiator containing a vinyl group that does not participate in the
polymerization process and (ii) postpolymerization modification
of the chain-end functionality. In addition, catalytic chain transfer
polymerization (CCTP) also provides a facile route to methacrylic
macromonomers,7,8 although this approach produces terminal
vinyl groups that do not exhibit the same copolymerizability as
more conventional methacrylates.9 Both Lascelles et al.10 and
Nagasaki et al.11 prepared a range of styrene-functionalized
Received: July 19, 2011
Revised:
August 16, 2011
Published: September 14, 2011
r
2011 American Chemical Society
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Macromolecules
Scheme 1. RAFT Synthesis of the Well-Defined Methacrylate-Terminated PNIPAM Macromonomers Used in This Work
ARTICLE
Homopolymer precursorsobtainedbyATRPcanalsobeconverted
into macromonomersbysubstitutionoftheterminalhalogenatom,16
although this method may suffer from reduced efficiency due to
premature loss of halide from the chain-ends toward the end of
the polymerization. Recently, Boyer et al. combined reversible
additionꢀfragmentation chain transfer (RAFT) polymerization17
with thiolꢀene chemistry to obtain model macromonomers by
reacting polymeric thiols with diacrylates or dimethacrylates.18
Poly(N-isopropylacrylamide) (PNIPAM) is a nonionic hy-
drophilic polymer that is soluble in water below its lower critical
solution temperature (LCST), but above this critical tempera-
ture the chains undergo a coil-to-globule transition and become
insoluble.19 The precise cloud point depends on the PNIPAM
molecular weight, its chain-end groups20,21 and its architecture,22
but it is generally accepted to be around 31ꢀ32 °C for mean
degrees of polymerization of 9 and above.23 Controlled radical
polymerization techniques have been used to prepare PNIPAM
homopolymers and copolymers. Nitroxide-mediated polymeri-
zation was used to graft PNIPAM chains onto a polystyrene star
polymer.24 The ATRP of acrylamides is generally more challen-
ging than that of methacrylates or acrylates due to a smaller ATRP
equilibrium constant,25 potential deactivation of the copper catalyst
and in situ displacement of the terminal halogen atom, but this
chemistry has also been utilized to prepare well-defined PNI-
PAM-based copolymers.20b,26 For example, we recently reported
a new class of thermo-responsive biochemically degradable
gelators based on PNIPAM-containing ABA triblock copolymers
[where A = PNIPAM and B = poly(2-(methacryloyloxy)ethyl
phosphorylcholine)] prepared via ATRP.27 However, according
to the literature the most widely used technique for the synthesis
of well-defined PNIPAM appears to be RAFT polymerization.28
The grafting of PNIPAM homopolymer (or copolymer) onto
solid surfaces has been used in a range of applications, such as
liquid chromatography,29 permeation-controlled filters,30 chemical
sensors,31 cell culture,32 controlled bioadhesion,33 protein adhesion,34
and functional composite surfaces.35 Thermo-responsive colloidal
particles are of particular interest for biomolecule separation29a,36
and cell culture scaffolds.37 The preparation of thermo-respon-
sive colloidal particles was pioneered by Pelton and Chibante,38
who reported the synthesis of PNIPAM-based microgel particles
via aqueous dispersion polymerization.39 Subsequently, Kawaguchi
and co-workers extended this work to include the aqueous
emulsion copolymerization of styrene with various N-substituted
acrylamides.40 Pichot et al. used surfactant-free emulsion poly-
merization to prepare poly(styrene-N-isopropylacrylamide-
2-aminoethyl methacrylate hydrochloride) latexes using a batch
process or a ‘shot-growth’ strategy to ensure incorporation of the
cationic comonomer at the surface of the latex particles.41 Below
32 °C, these latexes were shown to be electrosterically stabilized
by a combination of the thermo-responsive PNIPAM chains
and the cationic surface charge conferred by the 2-aminoethyl
methacrylate hydrochloride residues. In contrast, the latexes were
mainly stabilized by electrostatic repulsion above 32 °C, since the
PNIPAM chains collapse onto the latex particle surface above
their LCST.42 In addition, the covalent attachment of antibodies
to the surface of such microgels was also studied.43 Ballauff and
co-workers studied the thermo-responsive properties of poly-
styreneꢀPNIPAM coreꢀshell particles.44 The high Tg cores
were synthesized first via seeded emulsion polymerization, then
NIPAM was copolymerized with a N,N0-methylenebis(acrylamide)
to form a cross-linked shell. Tenhu and co-workers synthesized
similar coreꢀshell particles45 and used a combination of DLS, 1H
NMR and differential scanning microcalorimetry to examine
their thermo-responsive behavior in aqueous solution. The
critical swelling temperature for the cross-linked PNIPAM shell
was shifted to higher temperature and exhibited a broader phase
transition compared to PNIPAM microgels. Moreover, this
thermal transition is also affected by the shell thickness, although
variable temperature DLS studies suggested only a relatively
small change in the particle diameter.
In the present work, PNIPAM precursor chains were obtained
by RAFT and readily converted into well-defined macromono-
mers via a convenient one-pot protocol that combines aminolysis
and thia-Michael addition and utilizes a commercially available
methacrylate-acrylate reagent. This approach produces well-
defined PNIPAM macromonomers with somewhat higher atom
efficiency than those synthesized by Boyer et al.18 or the prepara-
tion of α, ω-hydroxy telechelic polymers described by Qiu and
Winnik.46 These methacrylate-capped macromonomers were
then used as steric stabilizers for the dispersion polymerization
of styrene in methanol to afford model thermo-responsive sterically
stabilized polystyrene latexes.
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ARTICLE
’ EXPERIMENTAL DETAILS
added to the PNIPAM65-CTA solution and degassing was conducted for
a further 10 min. Then, 150 μL aliquots were extracted at regular time
intervals and immediately added to vials containing 200 μL of a DMF
solution of acetic acid (1 mg mLꢀ1) and 750 μL DMF. Then, 100 μL of
each of these solutions were then diluted further to 1.0 mL with DMF and
their UV absorption spectra were recorded (λmax at 313 nm) at 20 °C.
Homopolymerization of NIPAM via RAFT. NIPAM polymer-
izations were performed at 70 °C. The [NIPAM]/[CTA] molar ratio
was adjusted so that an average DP of 20, 30, 40, 50, 60, or 75 was
achieved when the monomer conversion reached 90ꢀ92%. Each poly-
merization was quenched by rapidly cooling using an ice bath. The reac-
tion mixtures were dialyzed against methanol (cutoff: 1000 g molꢀ1).
After dialysis, methanol was removed under vacuum to yield a solid
yellow product. The PNIPAMn-CTA molecular weights were deter-
mined by 1H NMR and the polydispersity was obtained via DMF GPC
(using poly(methyl methacrylate) calibration standards). In a typical
run, freshly recrystallized NIPAM (4.0 g; 35.35 mmol), DDMPEAT
(317.00 mg; 0.65 mmol), and ACVA (18.35 mg; 0.065 mmol) were
dissolved in 1,4-dioxane (10 mL) in a glass vial fitted with a rubber seal
and the solution was degassed for 20 min using a dry nitrogen purge. The
reaction mixture was then placed in an oil bath set at 70 °C and ter-
minated after approximately 90 min.
Materials. The (2-(dodecylthiocarbonothioylthio)-2-methylpropa-
noic acid) precursor CTA was synthesized according to the method
described by Skey and O’Reilly.47 2-Bromo-2-methyl-propionic acid,
3-(acryloyloxy)-2-hydroxypropyl methacrylate (AHPMA), 4-(dimethyl-
amino) pyridine (DMAP), 4,40-azobis(4-cyanovaleric acid) (ACVA),
dimethylphenylphosphine, n-dodecanethiol, n-hexylamine, carbon di-
sulfide (CS2), K3PO4, NaH, N,N0-dicyclohexylcarbodiimide (DCC),
2-phenoxyethanol and triethylamine (TEA) were all purchased from
Aldrich and used as received. Anhydrous MgSO4 and Na2SO4 were pur-
chased from Fisher and used as received. 2,20-Azobis(isobutyronitrile)
(AIBN) was purchased from BDH laboratories and used as received.
N-Isopropylacrylamide (NIPAM) was purchased from Aldrich (97%)
and recrystallized from n-hexane and toluene (25:1). Styrene (Aldrich)
was passed through a column of basic alumina to remove its inhibitor
and stored at ꢀ20 °C prior to use. 1,4-Dioxane, dichloromethane
(DCM), diethyl ether, ethyl acetate, methanol and petroleum ether were
all purchased from Fisher as HPLC grade solvents and used as received.
Deionized water was used in all experiments. Silica gel 60 (0.0632ꢀ
0.2 mm) was obtained from Merck (Darmstadt, Germany). NMR solvents
(D2O, CD3OD, DMSO-d6 and CDCl3) were purchased from Goss
Scientific Instruments Ltd.
Synthesis of Thiol-Capped PNIPAM (PNIPAM50-SH). Thiol-
ended PNIPAM (PNIPAM50-SH) was synthesized by aminolysis in the
presence of dimethylphenylphosphine (DMPP) as a reducing agent.
A 3-fold excess of n-hexylamine and 0.1 equiv of DMPP was used with
respect to PNIPAM50-CTA. The reaction was conducted in THF and
the product was isolated as a white powder after purification by pre-
cipitation (twice) into excess diethyl ether.
Synthesis of 2-(dodecylthiocarbonothioylthio)-2-methyl-
propanoic-2-phenoxyethyl Ester (DDMPEAT). DDMPEAT was
synthesized by dissolving (2-(dodecylthiocarbonothioylthio)-2-methyl-
propanoic acid) (DDMAT) (3.0 g, 8.23 mmol) and 2-phenoxyethanol
(1.137 g, 8.23 mmol) into 20 mL of anhydrous DCM at 0 °C. N,N0-
dicyclohexylcarbodiimide (DCC) (2.04 g, 9.89 mmol) and 4-(dimethyl-
amino)pyridine (DMAP) (100.65 mg, 0.823 mmol) were dissolved in
15 mL of anhydrous DCM and added dropwise into the reaction mixture,
see Scheme 1. The reaction solution was stirred at 20 °C for 48 h under
nitrogen and a white precipitate of (N,N0-dicyclohexylurea) (DCU) was
formed. The crude product was further purified by first removing the
DCU by filtration. Then the supernatant was evaporated under vacuum
and the dark yellow crude product was dissolved in petroleum ether before
being purified by flash chromatography (20:1 petroleum ether/ethyl
acetate). The final yield of the purified DDMPEAT product (>99%
purity as judged by 1H NMR) was 75%.
Macromonomer Synthesis. The aminolysis and subsequent
Michael addition were conducted using a one-pot protocol. In a typical
reaction, PNIPAM50-CTA (2.00 g; 0.312 mmol) and 3-(acryloyloxy)-2-
hydroxypropyl methacrylate (AHPMA) (200.00 mg; 0.938 mmol) were
dissolved in DMF (49 mL). The yellow solution was degassed by
nitrogen bubbling for 20 min, after which 1.0 mL of a degassed solution
of n-hexylamine (126.00 mg; 1.25 mmol) and TEA (31.60 mg; 0.312 mmol)
in DMF was injected. Nitrogen bubbling was continued for a further
10 min and the reaction solution was then stirred at 20 °C for 15 h, lead-
ing to the disappearance of its yellow coloration. The resulting PNI-
PAM50-MA macromonomer was first dialyzed in methanol (cutoff: 1000
g molꢀ1) to remove any water-insoluble impurities, then dialyzed in
deionized water. The final purified macromonomer was isolated as a
white powder after freeze-drying from water overnight. Optimization of
the one-pot aminolysis Michael addition was performed on one-tenth of
the scale, i.e., using 200 mg PNIPAM50-CTA dissolved in 5.0 mL of DMF.
Alcoholic Dispersion Polymerization of Styrene. A typical
polystyrene latex was synthesized by dissolving PNIPAM50-MA macro-
monomer (50.0 mg) into methanol (4.50 g) along with styrene (500 mg)
and AIBN (5.0 mg). Oxygen was removed by purging with dry nitrogen
for 30 min. The polymerization was initiated by placing the vessel into an
oil bath at 70 °C. After 1 h, the reaction mixture became slightly turbid,
and 3 h later it became milky-white. The reaction was stirred at 70 °C for
24 h. The latexes were purified by five centrifugation/redispersion cycles,
twice in methanol and then three times in deionized water. Precisely the
same protocol was used for the control experiments using PNIPAM50-
SH and PNIPAM50-CTA. The resulting sterically stabilized latexes were
1H NMR (400.13 MHz, CDCl3, 298 K), δ (ppm): 0.9 (3H, ꢀCH3);
1.3 (18H, ꢀCH2ꢀ); 1.6 (2H, CH2ꢀCH2ꢀS); 1.7 (6H, ꢀ(CH3)2); 3.2
(CH2ꢀSꢀ); 4.2 (2H, ꢀCH2ꢀOPh); 4.5 (2H, COOꢀCH2ꢀ); 6.9 and
7.3 (5H, ꢀAr).
13C NMR (100 MHz, CDCl3, 298 K), δ (ppm): 14.2 (CH3ꢀCH2ꢀ);
22.8, 27.9, 29.0, 29.2, 29.4, 29.5, 29.6, 29.7, 32.0 (10C, dodecyl chain);
25.4 (C(CH3)2)); 37.0 (ꢀCH2ꢀS); 55.9 (C(CH3)2)); 64.2, 65.6 (ꢀOꢀ
CH2ꢀCH2ꢀO); 114.7, 121.1, 129.5 (CH, Ar); 158.6 ((ꢀCH)2ꢀCꢀO);
173.0 (CdO); 221.4 (CdS).
Kinetics of Thia-Michael Addition. n-Dodecylthiol (18.89 mg;
93 μmol) and TEA (3.15 mg; 31 μmol) were dissolved in DMF-d7
(0.90 mL) and this solution was placed in an NMR tube. Then AHPMA
(20.0 mg; 93 μmol) was injected into the NMR tube and this was taken
to be zero time. Spectra were recorded at 20 °C every 10 min for 2.5 h.
Kinetics of Aza-Michael Addition. n-Hexylamine (9.44 mg;
93 μmol) and TEA (3.15 mg; 31 μmol) were dissolved in DMF-d7
(0.90 mL) and this solution was placed in an NMR tube. Then AHPMA
(20.0 mg; 93 μmol) was injected into the NMR tube and this was taken
to be zero time. Spectra were recorded at 20 °C every 10 min for 2.5 h.
Kinetics of Aminolysis. PNIPAM65-CTA (100 mg; 12.8 μmol)
was dissolved in DMF (2.0 mL) along with AHPMA (8.23 mg; 38.4
μmol) and the mixture was degassed for 20 min. In a separate vial,
n-hexylamine (50.7 μL) and TEA (17.8 μL) were dissolved in DMF
(5.0 mL) and this solution was also degassed for 10 min. n-Hexylamine
(0.50 mL; 38.4 μmol) and TEA (12.8 μmol) of this stock solution were
1
analyzed by H NMR, dynamic light scattering, X-ray photoelectron
spectroscopy and scanning electron microscopy (see below).
’ POLYMER CHARACTERIZATION
Dynamic Light Scattering (DLS). The intensity-average hydro-
dynamic diameter of each latex was recorded using a Malvern Nanosizer
ZS instrument. Methanolic solutions of 0.1% w/v% latex were analyzed
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Figure 1. 1H NMR spectra recorded for (a) DDMPEAT (CDCl3), (b)
PNIPAM50-CTA homopolymer (CD3OD:D2O = 4:1 v/v), and (c)
PNIPAM50-MA macromonomer (D2O:CD3OD = 10:1 v/v). Note: the
polymers are fully soluble in CD3OD, D2O helps to mask the amide
protons of PNIPAM . PNIPAM50-CTA gives a slightly turbid solution in
pure D2O at the concentration required to perform accurate end-group
analysis due to its two hydrophobic chain-ends; thus, a mixture of D2O/
CD3OD was used.
Figure 2. Kinetic data obtained for the RAFT homopolymerization of
NIPAM using DDMPEAT CTA: (a) evolution of conversion and
monomer consumption as a function of time; (b) evolution of molecular
weight and polydispersity as a function of monomer conversion.
Conditions: 4.00 g NIPAM in 10.0 mL of 1,4-dioxane at 70 °C;
[NIPAM]0:[DDMPEAT]:[ACVA] = 100:1:0.1.
using disposable plastic cuvettes and data were averaged over three
consecutive runs.
photoelectron spectrometer equipped with a monochromatic Al KR
X-ray source (hν = 1486.6 eV) and operating at a base pressure of 10ꢀ8
to 10ꢀ10 mbar. Latex particles were dried onto silicon wafers and eva-
cuated to ultrahigh vacuum prior to XPS measurements.
Gel Permeation Chromatography (GPC). The molecular
weights and polydispersities of the various PNIPAM-MA macromono-
mers and their corresponding PNIPAM-CTA precursors were deter-
mined by DMF GPC at 60 °C. The GPC setup comprised two Polymer
Laboratories PL gel 5 μm Mixed-C columns maintained at 60 °C in
series with a Varian 390 LC refractive index detector. The flow rate was
1.0 mL minꢀ1, and the mobile phase contained 10 mmol LiBr. A total of
’ RESULTS AND DISCUSSION
10 near-monodisperse PMMA standards (Mp = 625 to 618 000 g molꢀ1
)
PNIPAM-MA Macromonomers. The CTA synthesis and the
synthetic route adopted for PNIPAM-MA macromonomers are
both outlined in Scheme 1. S-Dodecyl-S0-(α,α0-dimethyl-α00-acetic
acid) trithiocarbonate, (DDMAT) was synthesized according to
the method described by Skey and O’Reilly.47 A phenoxyethyl
moiety was introduced into DDMAT via esterification of the
R group. This substituent provides a very convenient NMR label
for the determination of number-average molecular weights via
end-group analysis, as well as the extent of Michael addition. The
ethyl proton signals due to the phenoxyethyl group at 3.8 and
4.0 ppm were shifted downfield to 4.2 and 4.8 ppm respectively
due to ester formation. Esterification was stopped after 48 h at
77% conversion. After column chromatography, the purity of
were used for calibration.
1
NMR Spectroscopy. All H NMR and 13C NMR spectra were
recorded in either CD3OD, CDCl3, DMSO-d6, DMF-d7, or D2O using
either a 250 MHz Bruker Avance 250 or a 400 MHz Bruker Avance 400
spectrometer.
Scanning Electron Microscopy (SEM). SEM studies were per-
formed using a FEI Sirion field emission scanning electron microscope at
a beam current of 244 μA and an operating voltage of 5 kV. Samples were
dried onto aluminum stubs and sputter-coated with a thin layer of gold
prior to examination to prevent sample charging.
UVꢀVisible Spectroscopy. All spectra were recorded with a
1.0 cm quartz cuvette using a Perkin-Elmer Lambda 25 spectrometer
operating at a scan speed of 480 nm minꢀ1
.
1
DDMPEAT was estimated to be more than 99% by H NMR
X-ray Photoelectron Spectroscopy (XPS). X-ray photoelec-
tron spectra (XPS) were acquired using a Kratos Axis Ultra DLD X-ray
(see Figure 1).
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The homopolymerization of NIPAM in 1,4-dioxane using
100:1:0.1 proportions for [NIPAM]:[DDMPEAT]:[ACVA] at
70 °C was well controlled. Polydispersities remained below 1.15
throughout the polymerization and the evolution of molecular
weight with conversion was linear, see Figure 2. The slight deviation
from the theoretical molecular weight line observed above 40% con-
version arises from the use of PMMA standards for GPC calibration.
After a short induction period of approximately 7 min the polym-
erization proceeded rapidly, with around 90% conversion being
attained in 90 min. The rate of polymerization then slowed consi-
derably, with no apparent adverse effect over the polydispersity.
A series of near-monodisperse PNIPAM homopolymer pre-
cursors (PNIPAM-CTA) with mean DPs ranging from 20 to 75
were synthesized in high yield and with good control. High end-
group fidelity was maintained while minimizing monomer waste
by using NIPAM/DDMPEAT molar ratios such that the desired
DPs were attained at high conversion. More specifically, poly-
merizations were quenched at 88ꢀ94% conversion to minimize
the possibility of termination by combination (see Table 2) and
experimental DPs close to those targeted were obtained. Mean
DPs were calculated using 1H NMR (see Figure 1) by comparing
the integrated aromatic protons due to the CTA end-group at
6.8ꢀ7.2 ppm to that of the pendent methine proton due to the
NIPAM residues at 3.8 ppm. This analysis suggested a CTA effi-
ciency of more than 90%. DMF GPC analysis confirmed excellent
control over the RAFT polymerization of NIPAM. The Mn values
obtained from GPC are in reasonably good agreement with the
NMR analyses (see Supporting Information) and the final poly-
dispersity of the PNIPAM chains remained below 1.10 (see Table 2).
The PNIPAM-CTA precursor is then converted into thiol-
capped PNIPAM chains and nucleophilic Michael addition of the
terminal thiol with the acrylate group on AHPMA affords the
desired PNIPAM-MA macromonomer (see Scheme 1). In principle,
these last two steps can be conducted separately or via a one-pot
protocol. Conversion of the Z-groups of RAFT-synthesized poly-
mers into thiols is well-documented and can be achieved by
aminolysis,18,48,49 hydrolysis50 or the action of metal hydrides.51
However, this transformation can be somewhat problematic. The
formation of disulfide bonds by combination of two thiols via
oxidative coupling can lead to bimodal molecular weight dis-
tributions and prevents subsequent reaction of the thiol. In
addition, a recent study by Harrisson reported a radical-catalyzed
side reaction between thiols and either trithiocarbonates or
dithioesters which yields not only disulfides but also non-thiol-
functionalized polymers.52 Finally, Xu et al.53 have reported that
thiol-terminated poly(methyl methacrylate) chains obtained by
aminolysis of the Z-group can irreversibly form thiolactones.
This latter reaction proceeds via intramolecular backbiting of the
thiol to the carbonyl carbon of the ester group on the penulti-
mate monomer unit of the polymer chain. It is therefore only
likely to occur with poly(meth)acrylates. This literature pre-
cedent suggests that it may be difficult to achieve quantitative
conversion of Z-groups into free thiols. Thus, the best method to
isolate thiol-terminated polymers is to conduct the aminolysis
in the presence of a reducing agent such as sodium dithionite54
or a phosphine.39,55 In the present study dimethylphenyl phos-
phine was used to obtain thiol-capped PNIPAM. Neverthe-
less, the GPC trace shown in Figure 3 suggests the presence of
a small amount of disulfide-containing species. When the thiol-
capped polymer is not the desired final product but merely a
reaction intermediate, it may be advantageous to conduct the
reactions using a one-pot protocol without isolating the
Figure 3. Kinetic plots for the aminolysis of PNIPAM65-CTA, and the
Michael additions of n-dodecylthiol and n-hexylamine to AHPMA in
DMF at 20 °C.Reactions conditions: Aminolysis: [PNIPAM65-CTA ]:-
[AHPMA]:[n-hexylamine]:[TEA] = 1:3:3:1, [PNIPAM65-CTA ]0
=
5.12 mmol Lꢀ1 in DMF. Thia-Michael addition: [n-dodecylthiol]:-
[AHPMA]:[TEA] = 3:3:1, [n-dodecylthiol]0 = 0.1 mol Lꢀ1 in DMF-
d7. Aza-Michael addition: [n-hexylamine]:[AHPMA]:[TEA] = 3:3:1,
[n-hexylamine]0 = 0.1 mol Lꢀ1 in DMF-d7.
Table 1. Summary of Data Obtained for One-Pot Aminolysis-
Michael Addition of a PNIPAM65-CTA Precursor (DP = 65,
Mn = 7800 g/mol, Mw/Mn = 1.07) at 20 °C for 15 ha
entry PNIPAM65
-
no.
CTA
AHPMA amine TEA aminolysis/% conversion/%
1
2
3
4
5
6
7
8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.5
3.0
2.5
1.0
2.0
3.0
4.0
7.5
2.0
3.0
1.0
1.0
1.0
1.0
1.0
7.5
0
100
62
90
55
3.0
98
75
3.0
100
100
100
97
>95
>95
>95
73
3.0
10.0
3.0
3.0
0
100
84
a The PNIPAM65-CTA, AHPMA, n-hexylamine, and TEA columns
indicate the respective molar equivalents of these reagents relative to
the PNIPAM65-CTA precursor, the conversion and aminolysis columns
refer to the yield of the thia-Michael addition product and the extent of
the aminolysis respectively.
intermediate thiol species. Indeed, the Michael addition proceeds
at such a high rate56 that the known thiol side reactions may be
suppressed. This approach has been investigated previously. Qiu
and Winnik46 reported the one-pot synthesis of thioethers
starting from telechelic PNIPAM precursors prepared by RAFT
polymerization. In this study, an efficient synthetic methodology
was established and the order of reactivity for electron-deficient
olefins in thia-Michael addition was confirmed as: acrylate .
methacrylate > acrylamide > methacrylamide. Boyer et al.18 used
a very similar approach to prepare a range of ω-functional polymers,
including macromonomers from dimethacrylates and diacrylates.
Herein we adapted this one-pot protocol for the synthesis of
macromonomers in high yields with a significant reduction in the
quantity of excess divinyl reagent required to produce well-defined
macromonomers. 3-(Acryloyloxy)-2-hydroxypropyl methacrylate
(AHPMA) is an asymmetric diene containing both an acrylate
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and a methacrylate moiety; this reagent was chosen to investigate
the selectivity of the thia-Michael addition. n-Hexylamine was
used rather than shorter, more volatile primary amines so as to
minimize loss of reactant during nitrogen purging; this reagent
is known to efficiently convert trithiocarbonates into thiols.18 DMF
was chosen as the reaction medium because thia-Michael addition
proceeds faster in polar solvents that are able to stabilize thiolate
anions. The catalytic role of triethylamine mentioned by Boyer
et al. was also investigated.
Table 1 summarizes the experimental data obtained for our
optimization of the macromonomer synthesis. All experiments
were carried out using a near-monodisperse (Mw/Mn = 1.07)
PNIPAM precursor of 7800 g molꢀ1 for a fixed reaction time of
15 h. Whether a large or small stoichiometric excess of AHPMA
was used (see entries 1 and 8), no addition of thiol to methacrylate
was observed. These results confirm those of both Winnik’s group46
and Prestwich et al.57 and suggest a much faster rate of addition
of thiols to acrylates than to methacrylates; according to Davis
and co-workers, the reaction with methacrylates usually requires
heating.58 Under ideal conditions (i.e., in the absence of any side
reactions), the theoretical molar ratios for aminolysis and the
subsequent thia-Michael addition to acrylate should be [PNIPAM65-
CTA]:[AHPMA]:[n-hexylamine] = 1:1:2. Complete aminolysis
of a polymer prepared using a trithiocarbonate-based CTA yields
a thiourea and two thiol species (a small molecule thiol and a
polymeric thiol). In the case of a one-pot synthesis, the possibility
of aza-Michael addition must be considered in addition to the
various side reactions discussed earlier. Indeed, the efficiency
of this one-pot synthesis is very sensitive to the amount of
n-hexylamine employed (see entries 1 to 5 in Table 1). When
the n-hexylamine/trithiocarbonate molar ratio is below 3.0, the
overall yield of the thia-Michael addition product is significantly
reduced. This is presumably because aza-Michael addition com-
petes with aminolysis of the trithiocarbonate end-groups. In order to
assess the extent of influence of the aza-Michael addition, the
kinetics of each of the three main reactions taking place during
the macromonomer synthesis were investigated. Figure 3 shows
the conversion vs time plots obtained for the aza- and thia-
Michael additions and also for the aminolysis of the trithiocar-
bonate-capped PNIPAM chains. Aminolysis and thia Michael
addition are both much faster than the addition of n-hexylamine
to AHPMA. For example, addition of n-dodecylthiol to AHPMA
reaches 90% conversion within 25 min and 80% of the trithio-
carbonate is converted into thiol within 90 min at 20 °C. In
contrast, addition of n-hexylamine to acrylate only proceeds to
about 30% conversion after 90 min. Moreover, the thiol formed
via aminolysis of the PNIPAM-CTA is likely to be even more
reactive than n-dodecylthiol, since Lowe et al.56 recently reported
that n-alkanethiols were much less reactive toward acrylates than
thioglycolates. Overall, our results suggest that the primary amine
reacts preferentially with the trithiocarbonate moieties, rather
than with the acrylate. Moreover, competition between the
intermediate thiol and the remaining excess amine for Michael
addition across the acrylic double bond strongly favors the thiol.
Furthermore, Haddleton et al. recently demonstrated that pri-
mary amines can also behave as a nucleophilic catalyst for thia-
Michael addition.59 In summary, aza-Michael addition should
only have minimal impact on the macromonomer synthesis
under the stated synthetic conditions.
Figure 4. DMF GPC curves recorded for PNIPAM50-MA macromo-
nomer and the corresponding PNIPAM50-CTA and PNIPAM50-SH
precursors. Note the weak shoulder observed at higher molecular weight
in the PNIPAM50-SH chromatogram, which is attributed to the forma-
tion of a disulfide species.
Table 1. This is due to the significantly higher reactivity of
secondary thiols compared to tertiary thiols.60 Triethylamine
appears to be a mild catalyst for the thia-Michael addition. When
two equivalents of n-hexylamine was used (see entries 3 and 7,
Table 1) very similar yields were obtained. When using three
equivalents of n-hexylamine, a modest increase in yield is
observed in the presence of TEA (see entries 4 and 8, Table 1).
These observations are consistent with a recent study by Lowe
et al., which suggests that TEA has very little effect on the kinetics
of thia-Michael addition.56 In our case, the optimum reaction
stoichiometry for the one-pot “aminolysis plus thia-Michael
addition” synthesis appears to be [PNIPAM-CTA]:[AHPMA]:-
[n-hexylamine]:[TEA] = 1:3:4:1 (entry 5, Table 1). These con-
ditions were used for all the macromonomers synthesized in this
study. Compared to the previous one-pot route reported by
Boyer et al.,18 who synthesized closely related (meth)acrylate-
functionalized PNIPAM-macromonomers using a ten-fold ex-
cess of either diacrylates or dimethacrylates, our method is rather
more atom-efficient. This is because using an acrylate-methacrylate
adduct (AHPMA) allows a significantly smaller excess of the
unsaturated substrate, since coupling of two PNIPAM chains is
strongly disfavored. Indeed, given the much higher reactivity of
acrylates compared to methacrylates toward thia-Michael addi-
tion, such unwanted side-products were not observed even when
using a relatively modest excess of AHPMA (see Figure 4).
This one-pot “aminolysis plus thia-Michael addition” route
was used to prepare a series of six well-defined PNIPAM-MA
macromonomers (see Table 2). The mean degrees of polymer-
1
ization and end-group fidelities were calculated by H NMR
spectroscopy (see Figure 1) in CD3OD/D2O mixtures and their
molecular weight distributions were characterized by DMF GPC
(see Figures 4 and 6). The degree of aminolysis was readily
assessed (albeit qualitatively) by the disappearance of the char-
acteristic yellow color originating from the trithiocarbonate end-
groups. Complete loss of the characteristic maximum absorption
due to trithiocarbonate end-groups at λ = 313 nm was observed
by UV spectroscopy, which indicated that very high levels of
Despite insufficient AHPMA to react with the secondary
macromolecular thiol and tertiary thiol formed by aminolysis
of the trithiocarbonate, a yield of 90% is obtained for entry 1 in
1
aminolysis were achieved (Figure 5). H NMR studies of the
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Table 2. Summary of Monomer Conversions, Molecular Weights, Polydispersities, and Extents of Chain-End Functionalization
for the Six PNIPAM-MA Macromonomers and Two Corresponding PNIPAM-Based Precursors Prepared in This Study
b
sample
target DP
DPa
conv. (%)a
initiator efficiency (%)
Mn (g/mol)a
Mw/Mn
extent of Michael addition (%)
PNIPAM50-CTA
PNIPAM50-SH
PNIPAM20-MA
PNIPAM30-MA
PNIPAM40-MA
PNIPAM50-MA
PNIPAM60-MA
PNIPAM75-MA
54
54
23
33
44
54
65
75
51
51
21
33
38
51
63
75
93
93
91
93
90
93
88
94
99
99
99
93
96
99
91
94
6200
6200
2900
4200
5000
6200
7600
9000
1.05
1.05
1.08
1.06
1.06
1.05
1.06
1.08
n.a.
n.a.
>95
>95
>95
>90
>90
>90
a Calculated by 1H NMR. b Measured by DMF GPC.
Figure 6. DMF GPC curves for the six PNIPAM-MA macromonomers
(see Table 2) labeled according to their respective mean degree of
polymerizations as determined by 1H NMR spectroscopy.
Figure 5. UV absorption spectra recorded in 1,4-dioxane for the
PNIPAM50-CTA precursor (0.33 g/L) and the corresponding PNI-
PAM50-MA macromonomer (1.0 g/L).
Styrene does not dissolve in water, hence our choice of methanol
to fulfill the criterion for a dispersion polymerization
purified macromonomers isolated after dialysis (see Figure 1)
confirm the appearance of characteristic methacrylic vinyl peaks
at 5.6 and 6.0 ppm, suggesting successful aminolysis and sub-
sequent Michael addition of AHPMA. The extent of Michael
addition was determined by comparing the integrated phenyl
end-group signals at 6.9 and 7.3 ppm to that of the terminal
methacrylate protons at 5.6 and 6.0 ppm (see Table 2).
PNIPAM-PS Latexes. Most previous studies dealt with the use
of PNIPAM as a steric stabilizer to prepare aqueous latexes at
close to ambient temperature, i.e., below its LCST. However,
methanol was chosen as the polymerization medium in this
study. Since PNIPAM does not exhibit LCST behavior in pure
methanol, the dispersion polymerization of styrene can be con-
ducted at 70 °C without compromising the latex stability. The
PNIPAM-MA macromonomers were investigated as potential
reactive steric stabilizers for the synthesis of model polystyrene
latexes prepared by dispersion polymerization in methanol at
70 °C (see Table 3). One reviewer suggested that more well-
defined particles might have been obtained by preparing latexes
in water at a temperature below the LCST of PNIPAM. This may
be true, but we are particularly interested in developing alcoholic
dispersion polymerization formulations in our research group.
Three control experiments were conducted to demonstrate
that the presence of vinyl-functionalized macromonomer is
essential for the formation of well-defined sterically stabilized
latex. When the dispersion polymerization of styrene was conducted
in the absence of any stabilizer (data not shown), only macro-
scopic precipitation occurred, as expected.3a In a second control
experiment (see entry 1 in Table 3), the styrene polymerization
was conducted using the PNIPAM50-CTA precursor as a stabi-
lizer. In this case, a colloidally stable latex was formed. However,
SEM studies confirmed that this latex had a broad particle size
distribution (see Figure 7a). In this case, AIBN initiator was
utilized in a four-fold excess compared to the PNIPAM macro-
CTA. Thus, it is reasonable to assume that the majority of the
styrene was polymerized via conventional free radical polymer-
ization under these conditions, rather than under RAFT control
(since RAFT syntheses are normally conducted at CTA/initiator
molar ratios of 5 to 10). Nevertheless, some background RAFT poly-
merization may partly account for the highly polydisperse particles
observed by SEM. Moreover, growth of a polystyrene block from
a polyacrylamide-based macro-CTA agent has been reported to
be relatively inefficient, leading to incomplete chain extension
and a bimodal molecular weight distribution.61 Such a poor block-
ing efficiency may also explain the broad latex size distribution
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Table 3. Summary of the Effect of Varying the DP and Concentration of the Macromonomer Stabilizer on the Mean Latex Particle
Diameter, Polydispersity, Styrene Conversion, Stabilizer Grafting Density, Stabilizer Content, and Stabilizer Surface Coverage
stabilizer
(wt %)
d (PDI)
(nm)a
monomer
conv. (%)c
stabilizer content
(wt %)c
stabilizer surface
coverage (%)d
no.
stabilizer
d (nm)b
Γ (chains/nm2)c
Γ (mg/m2)c
1
2
3
4
5
6
7
8
PNIPAM50-CTA
PNIPAM50-SH
PNIPAM40-MA
PNIPAM50-MA
PNIPAM50-MA
PNIPAM50-MA
PNIPAM60-MA
PNIPAM75-MA
10
10
10
10
20
30
10
10
70 (0.13)
410 (0.18)
340 (0.03)
680 (0.03)
510 (0.22)
320 (0.15)
780 (0.06)
830 (0.09)
bimodale
93
93
88
93
82
86
87
91
-
-
-
-
43
46
40
62
75
66
67
f
-
-
-
480
570
390
290
730
780
0.04
0.03
0.03
0.03
0.04
0.02
0.34
0.31
0.33
0.27
0.55
0.37
4.0
3.1
4.8
5.3
4.3
2.7
a As determined by DLS. b As determined by SEM. c As determined by 1H NMR. d As determined by XPS. e Mean diameter range: 500ꢀ1800 nm and
50ꢀ250 nm. f Mean diameter range: 250ꢀ1000 nm.
Figure 7. SEM images of sterically stabilized polystyrene latexes prepared by alcoholic dispersion polymerization in methanol at 70 °C with the
following stabilizers: (a) PNIPAM50-CTA precursor; (b) PNIPAM50-SH intermediate; (c) PNIPAM40-MA macromonomer; (d) PNIPAM60-MA
macromonomer. In each case, the stabilizer concentration was 10% based on styrene monomer.
observed in the present study. In a third control experiment,
PNIPAM50-SH was assessed as a potential steric stabilizer since
thiols are well-known to be very effective chain transfer agents
and thiol-terminated PEO has been successfully used to prepare
polystyrene latex using an alcoholic dispersion polymerization
formulation.62 Latex particles were indeed obtained using the
PNIPAM50-SH stabilizer, but SEM studies again revealed a broad
size distribution and also some evidence of interparticle fusion
(see Figure 7b). In contrast, the PNIPAM-MA macromonomers
proved to be much more effective steric stabilizers. In all cases,
the styrene polymerization proceeded to very high conversion
within 24 h at 70 °C without any observable formation of
coagulum. Macromonomers with mean DPs ranging from 40
to 75 gave colloidally stable latex particles, as judged by both
visual inspection and DLS. SEM studies confirmed a near-
monodisperse spherical morphology, with mean number-average
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Figure 8. 1H NMR spectrum recorded for a PNIPAM60-MA-stabilized polystyrene latex dissolved in CDCl3. Signal f at 4.1 ppm is assigned to the
methine proton of the NIPAM residues in the stabilizer chains. Note: the weak signal at 3.5 ppm in this inset is due to CH3OH.
the adsorbed amount of PNIPAM chains remained approxi-
mately constant at around 0.3 mg mꢀ2. These observations are
similar to previously reported findings for the alcoholic disper-
sion polymerization of styrene using hydrophilic macromono-
mer stabilizers. According to the literature, the higher stabilizer
concentration allows a greater latex surface area to be accom-
modated, which accounts for the observed reduction in particle
size.10,62ꢀ64 It is noteworthy that PNIPAM macromonomers of
DP = 20 or 30 (and, to a lesser extent, DP = 40) led to colloidally
stable latex particles, but with relatively broad size distributions.
For macromonomers of DP = 50 or higher, the particle size
distribution becomes much narrower (see Figure 7). Increasing
the DP of the macromonomer stabilizer at a fixed concentration
of 10 wt % (based on styrene) tended to produce an increase in
the mean latex diameter (compare entries 3, 4, 7 and 8, see
Table 3). Presumably, this is due to the reduced number of
stabilizer chains available for latex stabilization.
In order to further confirm that the PNIPAM chains are
Figure 9. X-ray photoelectron spectra recorded for a PNIPAM60-MA-
stabilized polystyrene latex (see entry 7 in Table 3). The original
PNIPAM60-MA macromonomer and a charge-stabilized polystyrene
latex were used as reference materials.
indeed located at the latex surface, we used X-ray photoelectron
spectroscopy (XPS).65 With a typical sampling depth of 2 to
10 nm, XPS is a powerful technique for the interrogation of
surfaces. This technique has been previously used extensively by
us,3b,66 and also by others,67 to investigate the surface of various
colloidal particles, including latexes. A typical X-ray photoelec-
tron survey spectrum obtained for polystyrene latex particles
prepared using a PNIPAM60-MA stabilizer is shown in Figure 9.
Characteristic signals for N 1s, O 1s, C 1s, and Si 2s/Si 2p confirm
the presence of these elements at the (near-)surface of the latex.
The oxygen and nitrogen signals confirm the presence of PNIPAM
stabilizer chains on the particle surface. The silicon signals suggest
some small degree of contamination, possibly by silicone-based
pump oil or from the underlying silicon wafer onto which the
latex sample was deposited. This contamination could also explain
why the N1s/O1s signal ratio is less than unity but stays constant
at around 0.5 for all latexes (and also the PNIPAM60-MA macro-
monomer). Comparing the relative intensities of the N1s signal due
to the macromonomer alone and the PNIPAM60-MA-stabilized
latex allows the surface coverage of the latex by the stabilizer chains
to be estimated. This stabilizer surface coverage increases when
higher macromonomer concentrations and molecular weights are
utilized in the latex syntheses, as expected (see Table 3).
diameters ranging from 290 to 780 nm depending on the
macromonomer chain length and concentration used in the
formulation (see Figure 7, parts c and d). Because of polydis-
persity effects, DLS reported intensity-average diameters of 320
to 830 nm for the same latexes (see Table 3). The stabilizer con-
tent for each latex was calculated using 1H NMR. Unfortunately
the PNIPAM-MA signal due to the methine protons at 4.05 ppm was
too weak for reliable quantitative analysis. Thus, the integrated
signal for the two aromatic styrene protons at 6.25 to 6.90 ppm was
compared to that corresponding to the two methylene protons in
the polystyrene and PNIPAM backbone at 0.8 to 2.4 ppm and
also that of the six methyl protons due to the pendent isopropyl
group on the NIPAM residues (see Figure 8). These stabilizer
contents were then used, together with the mean SEM diameters,
to estimate the adsorbed amount of stabilizer (Γ, in mg mꢀ2) for
each latex. These data are summarized in Table 3.
The mean latex diameter is reduced on increasing the macro-
monomer concentration, as expected (compare entries 4ꢀ6 in
Table 3). Higher latex stabilizer contents are obtained on increasing
the macromonomer concentration used in the formulation, while
Finally, the thermo-responsive behavior of a 1.0% aqueous
dispersion of a PNIPAM50-MA-stabilized polystyrene latex
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stabilizer chains were located at the latex surface, as expected. All
latexes were colloidally stable as aqueous dispersions at 20 °C,
with macroscopic aggregation being observed on heating above
35 °C. This transition proved to be reversible. Hence the well-known
LCST behavior of the PNIPAM chains dictates the colloidal
stability of these model thermo-responsive sterically stabilized
latexes.
’ ASSOCIATED CONTENT
S
Supporting Information. Approximate linear relationship
b
between the NMR-derived molecular weight and GPC molecular
weight for various PNIPAM precursors. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 10. Visual appearance of (a) 1.0 wt % aqueous solution of
PNIPAM50-MA macromonomer at 20 °C and (b) the same solution
heated up to 50 °C and a 1.0 wt % aqueous dispersion of PNIPAM50-
MA-stabilized polystyrene latex at (c) 20 and (d) 50 °C, respectively.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: (V.L.) v.ladmiral@sheffield.ac.uk; (S.P.A.) s.p.armes@
sheffield.ac.uk.
(see entry 7 in Table 3) and also a dilute aqueous solution of
the corresponding PNIPAM50-MA macromonomer was briefly
investigated over the temperature range from 20 to 50 °C (see
Figure 10). The macromonomer was water-soluble at 20 °C but
precipitated at 50 °C, since the latter temperature is well above its
characteristic LCST of 32 °C and hence leads to the well-known
coil-to-globule transition.17a Similarly, the latex particles are
colloidally stable at 20 °C, but become flocculated at 50 °C
due to collapse of the chemically grafted PNIPAM chains on the
latex surface. Visual inspection of a magnetically stirred aqueous
latex dispersion gradually heated up with the aid of an oil bath
confirmed that colloidal aggregation actually commences at
around 35 °C, which is close to the LCST for linear PNIPAM
chains. This thermally induced colloidal aggregation proved to be
fully reversible, since the sedimented aggregates redispersed
completely at 20 °C.
Current Address
§Department of Applied Physics, Aalto university, P.O.Box
15100, Fin-00076 Aalto, Espoo, Finland.
’ ACKNOWLEDGMENT
EPSRC is thanked for postdoctoral support for V.L. (EP/
E012949/1).
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