Monodisperse, micron-sized reactive low molar mass polymer microspheres by two-stage living radical dispersion polymerization of styrene

Article abstract of DOI:10.1021/ma061321j

Living/controlled radical dispersion polymerization of styrene was carried out in ethanol and in ethanol-water mixtures in the presence of perfluorohexyl iodide (C₆F₁₃I, 2-5 wt % based on styrene) as a degenerative chain transfer (DCT) agent or 1-cyano-1-methylpropyl dithiobenzoate (CMPDB, 0.5 and 1.0 mol % based on styrene) as a reversible addition-fragmentation chain transfer (RAFT) agent. These reagents disrupt the nucleation stage of particle formation when present at the start of the reaction. If their addition is delayed until the nucleation stage is complete, the reaction acquires the characteristics of a living/controlled radical polymerization: M_n increases linearly with the monomer conversion, and the molar mass distribution is much narrower (minimal M_w/M _n: 1.2) than polystyrene prepared without C₆F ₁₃I or CMPDB. For reactions in ethanol, at low molar mass, the particles formed have a broad size distribution as a consequence of the solubility of the polymer at the reaction temperature, followed by precipitation upon cooling. When the reactions were run in a more polar medium (95 wt % aqueous ethanol), the polymer molecules precipitated as they were formed and were absorbed by the existing particles in the solution. The particles formed had an average diameter in the range 1-3 μm with a very narrow size distribution (CV < 1%). The polymer chains in these particles are reactive and chain-extendible.

Full text of DOI:10.1021/ma061321j

8318
Macromolecules 2006, 39, 8318-8325
Monodisperse, Micron-Sized Reactive Low Molar Mass Polymer
Microspheres by Two-Stage Living Radical Dispersion Polymerization
of Styrene
Jing-She Song and Mitchell A. Winnik*
Department of Chemistry, UniVersity of Toronto, 80 St. George Street,
Toronto, Ontario M5S 3H6, Canada
ReceiVed June 13, 2006; ReVised Manuscript ReceiVed September 11, 2006
ABSTRACT: Living/controlled radical dispersion polymerization of styrene was carried out in ethanol and in
ethanol-water mixtures in the presence of perfluorohexyl iodide (C6F13I, 2-5 wt % based on styrene) as a
degenerative chain transfer (DCT) agent or 1-cyano-1-methylpropyl dithiobenzoate (CMPDB, 0.5 and 1.0 mol %
based on styrene) as a reversible addition-fragmentation chain transfer (RAFT) agent. These reagents disrupt
the nucleation stage of particle formation when present at the start of the reaction. If their addition is delayed
until the nucleation stage is complete, the reaction acquires the characteristics of a living/controlled radical
polymerization: Mn increases linearly with the monomer conversion, and the molar mass distribution is much
narrower (minimal Mw/Mn: 1.2) than polystyrene prepared without C6F13I or CMPDB. For reactions in ethanol,
at low molar mass, the particles formed have a broad size distribution as a consequence of the solubility of the
polymer at the reaction temperature, followed by precipitation upon cooling. When the reactions were run in a
more polar medium (95 wt % aqueous ethanol), the polymer molecules precipitated as they were formed and
were absorbed by the existing particles in the solution. The particles formed had an average diameter in the range
1-3 µm with a very narrow size distribution (CV < 1%). The polymer chains in these particles are reactive and
chain-extendible.
Introduction
This goal remains elusive and has to be approached in several
steps. One can imagine using chain transfer agents to reduce
the molar mass of the seed particles. The chain transfer agent
(CTA) serves the dual role of terminating growing chains and
subsequently initiating new chains. The choice of a chain
transfer agent in a given reaction depends on a balance between
the transfer rate of the CTA and the propagation rate of the
monomer. For optimum chain transfer agents, these two rates
should be similar, and the chain transfer constant has a value
near unity.
Over the past 25 years there has been a strong interest in the
synthesis of polymer particles with micrometer dimensions and
a narrow size distribution. These types of particles are used as
spacers in liquid crystal displays and as carriers in biodiagnostic
applications. Several research groups have described the prepa-
ration of micron-sized polymer particles by a stepwise seeded
swelling polymerization procedure in which a swelling step by
monomer is an essential feature of forming large particles.1-3
The monodisperse seed particles they used are typically sub-
micron in diameter and are prepared by emulsion polymeriza-
tion. In addition, the many subtle features of this approach to
particle synthesis limit the scope and scale of these reactions.
Dispersion polymerization represents an alternative approach
to the synthesis of micrometer-sized particles.4,5 The reaction
can be carried out in one pot, and is easily scalable, but has
other disadvantages. While the particles obtained are often
characterized by a very narrow size distribution, there are
problems with batch-to-batch control over particle diameter.
Until recently, it has been also difficult to prepare functional
or cross-linked particles by this route.
In principle, it should be possible to combine these techniques,
to use dispersion polymerization to prepare monodisperse seed
particles with diameters larger than 1 µm that could be grown
to a much larger size via a suspension polymerization in water.
This would work best if the particles prepared as the seed
contained only low molar mass polymer, so that it could be
effectively swollen by the monomer added to the seed particles
in water. This approach might be even more attractive if the
polymer in the seed particles were synthesized by a living/
controlled polymerization reaction so that the chains in the seed
could be extended and incorporated into the final polymer as
the particle size increased.
While many groups have investigated the use of chain transfer
agents in solution polymerization and emulsion polymerization,6-8
there have been fewer studies of chain transfer in dispersion
polymerization. We know of only two publications that exam-
ined these reagents in any detail.9,10 Both report problems related
to the presence of a chain transfer agent in the reaction. For
example, in the dispersion polymerization of styrene in ethanol,4
micron-sized monodisperse particles are obtained when no CTA
is present. However when even small amounts of a chain transfer
agent such as carbon tetrabromide (CBr4)10 were added to the
reaction, many attractive features of the reaction disappeared.
The polymerization rate decreased, and the particle size distribu-
tion broadened considerably. In the presence of 0.36 wt %
butanethiol chain transfer agent, polydisperse particles were
obtained. The GPC chromatograms of these particles contained
several broad peaks, and the ratio Mw/Mn was around 100.9
We recently discovered that many of the problems associated
with dispersion polymerization could be overcome if one
delayed the addition of problematic reagents until the nucleation
stage was complete, and the particle number in the reaction
became constant. We called this methodology “two-stage”
dispersion polymerization.11 The fundamental hypothesis was
that the nucleation stage in dispersion polymerization reactions
is short-lived but very sensitive to perturbation, whereas the
particle growth stage is more robust. In this way, for dispersion
* Corresponding author. E-mail: mwinnik@chem.utoronto.ca.
10.1021/ma061321j CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/04/2006

Macromolecules, Vol. 39, No. 24, 2006
Low Molar Mass Polymer Microspheres 8319
copolymerization of styrene in ethanol, we were able to prepare
monodisperse PS particles containing a covalently bound
fluorescent dye,12 PS particles cross-linked with ethylene glycol
dimethacrylate (EGDMA) or divinylbenzene (DVB),13 and
carboxylic acid-functionalized PS particles.14 The same approach
works for CBr4 as a chain transfer agent to lower the molar
mass of polystyrene, and one obtains particles of the same size
and narrow particle size distribution with or without chain
transfer agent if one delays adding the CBr4 until the reaction
turns turbid.15
zation via dispersion polymerization has remained a challenge.
In this work, we present our results of a study of the controlled
radical dispersion polymerization of styrene in the presence of
chain transfer agents. One set of experiments employed per-
fluorohexyl iodide (C6F13I) as a degenerative chain transfer
(DCT) agent. The second set of experiments use 1-cyano-1-
methylpropyl dithiobenzoate (CMPDB) as a RAFT agent. Other
groups have obtained interesting results with the C6F13I system,
both for bulk polymerizations18 and for miniemulsion polym-
erization.23 CMPDB was used as the RAFT agent to mimic the
structure of the initiator 2, 2′-azobis(2-methylbutyronitrile)
(AMBN). Using the two-stage dispersion polymerization strat-
egy, we obtained monodisperse, micron-sized PS particles
consisting of chain-extendible low molar mass polymer.
These kinds of low molar mass PS particles have several
disadvantages as seed particles for further seeded polymeriza-
tion. One disadvantage is that PS chains in seed particles may
be incompatible (or immiscible) with the polymer produced by
polymerization of the second monomer used to swell the seeds.
Phase separation may occur in the resulting particles; non-
spherical particles may form, and coagulation may occur.
Another disadvantage is that these PS oligomers are not chain-
extendible or cross-linked when cross-linked particles are
needed. It is a difficult, time-consuming, and high-cost process
to remove these oligomers from the final particles.
Experimental Section
Reagents. All organic reagents were used without further
purification, including styrene (Aldrich), methanol, ethanol, poly-
(vinylpyrrolidone) (PVP, Mw ) 55 000 g/mol, Aldrich), Triton
X-305 (70% solution in water, Aldrich), 2,2′-azobis(2-methyl-
butyronitrile) (AMBN, Wako Pure Chemical Industries Ltd.),
perfluorohexyl iodide (C6F13I, 99%, Aldrich), and phenylmagnesium
bromide (1 N in THF, Aldrich).
It is well-known that living/controlled radical polymerization
is a useful technique for a preparation of chain-extendible
polymers. The different methods that lead to controlled radical
polymerization are based on either a reversible termination
(mainly nitroxide-mediated radical polymerization16 and atom
transfer radical polymerization17) or a reversible chain transfer
reaction.18,19 In the case of reversible transfer, the activation
process is based on a bimolecular reaction between an active
macromolecule and a dormant one, leading to the exchange of
the ω-end group. It can be a direct exchange as in the so-called
degenerative chain transfer (DCT) technique where an iodine
atom is exchanged.18 Another more recent approach is reversible
addition-fragmentation chain transfer (RAFT),19 in which case
chains are end-functionalized by a dithioester or trithiocarbonate
that is exchanged via an addition-fragmentation process. In
such systems, a conventional radical initiator is needed together
with the specific transfer agent, and a great advantage of the
reversible transfer technique is that the experimental conditions
can be very close to conventional ones, in terms of temperature,
monomer concentration, and reaction process.
Synthesis of RAFT Agent CMPDB. CMPDB was prepared
according to the method described in Patent WO9905099. A
solution of 0.03 mol of phenylmagnesium bromide in 30 mL of
anhydrous THF was added to a 150 mL flask. Carbon disulfide
(2.28 g, 0.03 mol) was added to the solution at room temperature
slowly, and the reaction was allowed to stir magnetically at room
temperature overnight. Then the mixture was poured into water
and acidified with dilute hydrochloric acid. The solution was
extracted with chloroform (20 mL × 3). After the solvent was
removed on a rotary evaporator, crude dithiobenzoic acid was
obtained, and it was used directly in the following step. The purity
of the crude dithiobenzoic acid is high (1H NMR, CDCl3, 300 MHz,
δ: 8.0-8.1 ppm (d, 2H, C6H4), δ 7.54-7.62 ppm (m, 1H, C6H4),
δ 7.35-7.43 ppm (m, 2H, C6H4), δ 6.3-6.5 ppm (s, 1H, SH)).
Crude dithiobenzoic acid (4.9 g, 0.032 mol) was added to ethyl
acetate (20 g) and treated with dimethyl sulfoxide (DMSO, 1.4 g,
0.018 mol) under nitrogen protection for 7 h at room temperature.
To this solution AMBN (4.6 g, 0.024 mol) was added, and the
mixture was heated at 80 °C for 16 h. After evaporating the solvent,
crude CMPDB was obtained. The pure CMPDB was obtained as a
dark red oil by chromatography on a silica gel column with hexane:
ether ) 9:1 as the eluent. (1H NMR, CDCl3, 300 MHz, δ: 7.85-
7.93 ppm (d, 2H, C6H4), δ 7.50-7.60 ppm (m, 1H, C6H4), δ 7.30-
7.40 ppm (m, 2H, C6H4), δ 2.0-2.4 ppm (m, 2H, -CH2-) δ 1.90-
1.93 ppm (s, 3H, -âCH3) δ 1.18-1.24 ppm (m, 3H, -CH3 in
-CH2CH3)). The yield is about 50% and the purity is about 96%.
One-Stage Dispersion Polymerization. The standard recipe for
the dispersion copolymerization of styrene with C6F13I in ethanol
is listed in Table 1. The following procedure was used: All the
ingredients were added to a 250 mL three-neck reaction flask
equipped with a condenser and a gas inlet. After a homogeneous
solution was formed at room temperature, the solution was
deoxygenated by bubbling nitrogen gas at room temperature for
30 min. Then the flask was placed in a 70 °C oil bath and stirred
mechanically at 100 rpm. The monomer conversion was determined
gravimetrically by removing aliquots during the polymerization.
Two-Stage Batch Dispersion Polymerization. The standard
recipe for the two-stage dispersion copolymerization of styrene with
C6F13I in ethanol is listed in Table 1. The following procedure was
In recent years, several research groups have explored the
possibility of using living/controlled polymerization techniques
in dispersion polymerization. Mu¨lhaupt and co-workers20 re-
ported using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-
mediated radical chemistry for the attempted dispersion polym-
erization of styrene in n-decane at 135 °C using a “Kraton”
stabilizer. SEM images revealed a very broad particle size
distribution (50 nm-10 µm). Armes and co-workers21 reported
living radical chemistry with TEMPO in both alcoholic and
aqueous alcoholic media using poly(N-vinylpyrrolidone) (PVP)
as a steric stabilizer at 112-130 °C. The addition of TEMPO
had a profound effect on the polymerization chemistry: only
moderate monomer conversions and only relatively low molar
mass polystyrene chains were obtained. All TEMPO-synthesized
latexes had spherical particle morphologies and very broad size
distributions. Choe’s group in Korea described experiments
involving dispersion photopolymerization of styrene plus poly-
(N-vinylpyrrolidone) stabilizer in ethanol in the presence of a
RAFT agent.22 They varied the amount of the RAFT agent, in
both the absence and presence of a conventional initiator, azobis-
(isobutyronitrile) (AIBN), at various temperatures. Both the
molar mass distribution and the particle size distribution were
very broad. Conducting living or controlled radical polymeri-

8320 Song and Winnik
Macromolecules, Vol. 39, No. 24, 2006
Table 1. Standard Recipe for the Dispersion Polymerization of
Styrene with Degenerative Transfer Agent in Ethanol (Amounts in
grams)
Chart 1
two-stage
materials
one-stage
1st stage
6.25
2nd stage
monomer
styrene
C6F13I
ethanol
PVP
6.25
0.125
18.75
1.0
0.35
0.25
6.25
0.25
18.75
DCT agent
medium
stabilizer
costabilizer
initiator
reaction in Chart 1, where ktr is the second-order rate constant
for transfer. Here R-X represents the DCT or RAFT agent,
∼∼Mi• refers to a propagating chain consisting of Mi monomers,
and ∼∼Mj-X is a dormant chain bound to the entity subject
to transfer. For normal chain transfer agents in uncontrolled free
radical polymerization and for DCT and RAFT agents at low
monomer conversion, the decrease in number-averaged chain
length is described by the Mayo equation
18.75
1.0
0.35
0.25
Triton X-305
AMBN
used: All of the stabilizer (PVP), the co-stabilizer (Triton X-305)
and initiator (AMBN), and half of the monomer and ethanol were
added to a 250 mL three-neck reaction flask equipped with a
condenser and a gas inlet. After a homogeneous solution formed
at room temperature, the solution was deoxygenated by bubbling
nitrogen gas at room temperature for 30 min. Then the flask was
placed in a 70 °C oil bath and stirred mechanically at 100 rpm.
The C6F13I or CMPDB was dissolved in the remaining styrene plus
ethanol at 70 °C under nitrogen. After the C6F13I or CMPDB had
dissolved and the polymerization reaction had run for 1 h, the hot
C6F13I or CMPDB solution was added into the reaction flask.
Aliquots were taken at different reaction times for GPC, SEM, and
conversion measurements.
[CTA]
[M]
1
xn
1
(xn)0
)
+ Ctr
(1)
where xn is the number-averaged degree of polymerization, and
the subscript 0 refers to the reaction in the absence of CTA.
[CTA] is the concentration of chain transfer agent, [M] is the
monomer concentration, and Ctr, the chain transfer constant, is
the ratio of ktr to the propagation rate constant kp.
Characterization of Particles. The particle size was examined
by both optical microscopy (Olympus, X41) and scanning electron
microscopy (SEM, Hitachi S-5200). To prepare samples for optical
microscopy, the final polymer particle suspension was diluted with
water, and a drop was placed on a clean glass microscope slide.
SEM samples were prepared with a drop of diluted suspension on
a mica film. The particle size and size distributions were examined
by SEM. A particle size histogram was constructed from measure-
ments of 200-300 individual particles in the electron micrographs.
The molar mass and molar mass distributions of the constituent
polymers were determined by gel permeation chromatography
(GPC) in tetrahydrofuran (THF) using polystyrene calibration
standards with molar mass ranging from 600 to 411 000. GPC
measurements were performed on a Waters liquid chromatograph
equipped with a Waters 480 R410 differential refractometer (RI)
detector. The polymer samples for GPC analysis were prepared as
follows: The particles were centrifuged at 2000 rpm for 10 min.
The precipitated particles were redispersed into fresh ethanol/water
(95/5, w/w) and centrifuged again. This process was repeated four
times. After the particles were dried, they were dissolved in THF,
and the solution was passed through a 0.45 µm filter before injecting
During a reversible chain transfer reaction mechanism, two
chain transfer constants should be considered. The first one (Ctr1)
describes the consumption of the chain transfer agent and
controls the increase in molar mass vs monomer conversion.
The second one (Ctr2) describes the degenerative exchange
reaction between an active chain and a dormant one and affects
the polydispersity index (Mw/Mn ) 1 + 1/Ctr2 at 100%
conversion).24 For C6F13I and styrene, a value of Ctr1 ) 1.4
and a value Ctr2 ) 3.8 were reported by Goto et al.25 For the
RAFT agent CMPDB and styrene, both Ctr1 and Ctr2 were larger
than 20.19
One of the main objectives in a living reversible transfer
reaction is a linear increase in Mn or xn with increasing
conversion. The living nature of chain propagation is enhanced
by early consumption of the transfer agent; thus, Ctr1 should
have a value much larger than unity. In addition, there should
be a large excess of transfer agent to initiator.
In a dispersion polymerization, the monomer, polymeric
stabilizer, initiator, and solvent constitute the initial reaction
mixture. The reaction begins as a homogeneous solution
polymerization, and grafting to the stabilizer chain competes
with homopolymerization. The homopolymer chains grow in
size until they become insoluble in the reaction medium. The
solubility of these polymers is a function of their molar mass,
composition, the reaction temperature, and the composition of
the reaction medium. A polymer with a molar mass larger than
a certain critical value (M > Mcrit) precipitates and aggregates
to form colloidally unstable precursor particles (nuclei). These
nuclei coalesce and adsorb stabilizers from the medium onto
their surface until they become colloidally stable. At this point,
particle nucleation ceases. Once the total number of particles
in the system is fixed, the nucleation stage of the reaction is
over. Once the nucleation stage is complete, polymerization
initiated in the continuous phase leads to growing radicals that
are eventually captured by existing particles. As these particles
grow in size, polymerization occurs predominantly within these
particles.
1
it into the GPC column. The H NMR spectra were recorded in
CDCl3 solutions by using a Mercury 300 MHz. Chemical shifts
are reported in ppm, downfield of internal TMS, and are referred
to as s (singlet), d (doublet), and m (multiplet).
Results and Discussion
Both RAFT and DCT are living radical polymerization
systems based on a reversible transfer mechanism. The reversible
reaction is between a dormant chain and an active radical, in
which an end group originating from the transfer agent is
exchanged between the two chains. With DCT, there is a direct
exchange involving, for example, an iodine atom. With RAFT,
an addition-fragmentation process is used to exchange a moiety
such as a dithioester between the two chains. With the reversible
transfer mechanism, a conventional initiator is used to initiate
the chains. The transfer agent is then consumed by the radicals
originating from the initiator decomposition. The total number
of chains in the system is the sum of the transfer agent and
primary radical molecules.
Before comparing DCT and RAFT reactions of styrene in
dispersion polymerization, we will review briefly important
concepts about chain transfer processes. The chain transfer step
of both DCT and RAFT reactions can be described by the
The addition of chain transfer agents decreases the molar mass
of the polymer formed in the reaction. As we will see below,
the lower molar mass has two consequences. First, it delays
the nucleation stage, leading to a loss of control over particle

Macromolecules, Vol. 39, No. 24, 2006
Low Molar Mass Polymer Microspheres 8321
Figure 2. SEM images of PS particles prepared in ethanol by the two-
stage method with 2 wt % C6F13I (based on total styrene) at different
reaction times: (A) 3 and (B) 24 h.
Figure 1. SEM images of PS particles prepared by conventional
dispersion polymerization in ethanol: (A) without any C6F13I, (B) with
2 wt % of C6F13I.
size distribution. This problem can be overcome by delaying
the addition of the chain transfer agent until after the nucleation
stage is complete. The second effect is that chains produced in
the reaction with M < Mcrit remain soluble in the medium and
only precipitate when the reaction is cooled. This also results
in a broad particle size distribution as new particles are formed
in this precipitation step. This problem can largely be overcome
for polystyrene by employing ethanol-water mixtures as the
reaction medium to reduce the value of Mcrit. As in our previous
report on CBr4 as a chain transfer agent,15 we find much better
control over particle size distribution for reactions run in 95/5
w/w ethanol/water than in ethanol itself.
C6F13I as a Degenerative Chain Transfer Agent. C6F13I is
a degenerative chain transfer agent for radical polymerization
of styrene, with a value of Ctr ) 1.4.23 The transfer process
takes place in two successive steps. The first is the conventional
transfer to the chain transfer agent C6F13I, producing new chains
with a perfluorohexyl group at one end and an iodine atom at
the other end (X ) I in Chart 1). The second step is the transfer
of the iodine atom from an end-functionalized chain to a
propagating macroradical, which is a thermodynamically neutral
exchange process. As shown in Chart 1, this second step does
not create new chains but contributes to the extension of the
existing chains. In this work, C6F13I was used in radical
dispersion polymerization of styrene in ethanol and in ethanol-
water mixtures.
Reactions in Ethanol. We compared two different dispersion
polymerization strategies. First we carried out traditional (one-
stage) dispersion polymerization reactions of styrene in which
all of the ingredients were present at the beginning of the
reaction.26 Figure 1A,B shows the SEM images of PS particles
prepared by this method. The recipe is given in Table 1. In the
absence of C6F13I, the resulting particles were monodisperse
and spherical (Figure 1A, O-1 in Table 2). In the presence of 2
wt % of C6F13I, the average diameter of PS particles increased
and the particle size distribution was broader (Figure 1B, O-2
in Table 2). These results are similar to those of Ahmad and
Tauer,10 who obtained polydisperse PS particles using only 0.5
wt % CBr4 as a chain transfer agent.
We found that when 2 wt % C6F13I was present at the
beginning of the reaction, the nucleation stage, as inferred from
the onset of turbidity, became much longer, increasing from
less than 10 min to more than 15 min. It should be noted that
under our preferred reaction conditions the initiator was added
at room temperature, and the solution needed more than 5 min
to reach 70 °C. As we mentioned above, a CT agent, such as
C6F13I, present at the beginning of a dispersion polymerization
reaction, leads to a significant fraction of chains with M < Mcrit
.
This delays the onset of nucleation and broadens the particle
size distribution.
In our two-stage dispersion polymerization strategy, we defer
the addition of the problematic reagents like CT agents until
the nucleation stage is complete. Yasuda et al.27 showed that
the nucleation stage for dispersion polymerization of styrene
in ethanol is complete at less than 1% of conversion. We often
take the onset of turbidity in a reaction as the signature of the
end of the nucleation stage but sometimes wait, as we do here,
for 1 h after the beginning of the reaction to add the CT agent.
This corresponds to ∼5% monomer conversion.
Figure 2 shows the results of a reaction in which 2 wt % of
C6F13I (based upon total styrene) mixed with styrene and ethanol
was added 1 h after the start of polymerization. Aliquots were
taken from the reaction flask at different polymerization times.
Surprisingly, at low monomer conversion, polydisperse particles
were formed (Figure 2A, 3 h, 13% conversion). As the reaction
continued, the particle size distribution narrowed considerably
(Figure 2B, T-1 in Table 2, 24 h, 90% conversion). Some
particles at 13% conversion appeared nonspherical in SEM
images, probably as a result of evaporation of unreacted
monomer during drying. The unreacted monomer is also likely
to soften the particles, leading to particle deformation when these
particles come in contact with each other. A similar result for
5 wt % C6F13I in ethanol (T-2 in Table 2) is presented in the
Supporting Information. The resulting PS particles at 13%
Table 2. Effect of C6F13I or CMPDB on the Molar Mass, Molar Mass Distribution, the Particle Size, and Size Distribution
no.
C6F13I wt %
Dav,aµm
CV,b %
Mexp
PDI
3.65
Mn,cal At 100% convf
method
medium
ethanol
ethanol
ethanol
O-1
O-2
T-1
0
2
2
2.0
2.5
3.3
<1
8
<3
21 200
one-stage
one-stage
two-stage
23 300
23 300
16 000
90%e
2.19
1.84
2.04
1.49
2.30
T-2
5
3.3
2.7
2.0
2.0
5.0
<2
10 670
9300
9290
two-stage
two-stage
two-stage
two-stage
ethanol
95%e
T-4c
T-5c
T-6c
5
11 240
95%e
ethanol/water
ethanol/water
ethanol/water
1.0d
0.5d
<3
4000
10 400
20 900
40%e
<2
12 900
50%e
a Dav: the average diameter of the particles. b CV ) 1/n∑in)1(|Di - Dav|/Dav). c Ethanol/water (95/5, wt/wt) was used as the reaction medium in the first
stage, and C6F13I mixed with styrene and ethanol/water (95/5, wt/wt) was added batch-wise 1 h after the start of polymerization. d CMPDB, mol % based
on styrene. e Monomer conversion of this sample. f Theoretical value of Mn, calculated as 104([M]0/[C6F13I]0).

8322 Song and Winnik
Macromolecules, Vol. 39, No. 24, 2006
mass of these PS chains exceeds Mcrit, they should be captured
by the existing seed particles. At high conversion, there was
nearly no free oligomer dissolved in ethanol at 70 °C. Therefore,
no new particles formed upon cooling, and monodisperse
particles were obtained. When larger amounts of C6F13I were
employed, more polymer chains with M < Mcrit were produced.
The size distribution of the resulting particles was also very
broad.
Reactions in Ethanol-Water Mixtures. To reduce the
solubility of PS to promote precipitation of PS oligomers onto
existing PS particles, we ran reactions in a more polar reaction
medium. A number of authors have shown that dispersion
polymerization of styrene can be run in water-ethanol mix-
tures.9,15,28 The overall influence of adding a small amount of
water to the traditional one-stage dispersion polymerization
reaction is to decrease particle size while maintaining the narrow
size distribution.9 This approach provides one strategy to
decrease the critical molar mass of the growing polymer.
Figure 3. Plot of ln([M]/[M]0) vs time for the two-stage dispersion
polymerization of styrene in ethanol in the presence of 2 wt %f C6F13I
(based on total styrene).
We investigated whether this change of reaction medium,
incorporated into the two-stage method, would maintain the
narrow particle size distribution and at the same time permit
the use of large quantities of the DCT agent. We carried out a
series of reactions in an ethanol/water (95/5 w/w) medium with
different amounts of C6F13I. When C6F13I (5 wt % based on
total styrene) mixed with styrene and ethanol/water (95/5) was
added 1 h after the start of the reaction, monodisperse PS
particles were obtained (Figure S2D in the Supporting Informa-
tion). It appears that as long as the system is kept above the
critical molar mass for the polymer, the two stage method is
effective at producing particles with a narrow size distribution.
Figure 4. Number-average molar mass and molar mass distribution
vs monomer conversion for the two-stage dispersion polymerization
of styrene in ethanol in the presence of 2 wt % of C6F13I. Linear Mn )
conversion × 104([M]0/[ C6F13I]0).
In the Supporting Information, we present a plot (Figure S3)
of ln([M]/[M]0) vs time for the second stage of a reaction in
ethanol-water (95/5) in the presence of 5 wt % C6F13I. This
result is very similar to that for the two-stage reaction in pure
ethanol in the presence of 2 wt % C6F13I and indicates that this
amount of C6F13I has no evident effect on the styrene polym-
erization rate for the two-stage method. GPC chromatograms
(Figure S4 in the Supporting Information) and the Mn vs
conversion curve (Figure S5) show that Mn increases linearly
with conversion. The Mw/Mn values are also much narrower
than that for PS prepared without C6F13I. The observed shift of
the GPC peak between 13% monomer conversion (Mn ) 7690,
Mw/Mn ) 2.10) and 77% (Mn ) 12 030, Mw/Mn ) 1.78) together
with the decrease of Mw/Mn is a good indication that a chain
extension process has occurred as expected when a degenerative
transfer reaction takes place. Moreover, the theoretical molar
mass was reached at the final conversion, indicating complete
consumption of C6F13I. This result also implies that most chains
have an iodine atom at the end and can be extended by the
degenerative transfer reaction when a second load of monomer
is added. We also note that the extrapolated value of Mn at low
conversion is much smaller than in the case of 2 wt % C6F13I.
In both reactions, the polymer chains produced by DCT agent
remain “living” and chain-extendible.
conversion exhibited a very broad size distribution (Figure S1A).
The resulting PS particles at 90% of conversion were relatively
uniform in size (Figure S1B), but a small fraction of smaller
and larger particles could be seen in some SEM images.
Information about the reaction kinetics is presented in Figure
3, where we show that the plot of ln([M]/[M]0) vs time is linear.
At the very beginning of the reaction, the polymerization rate
is slower due to the solution polymerization mechanism. GPC
traces show that the sample at 13% conversion had Mn ) 11 000,
slightly lower than the critical molar mass in ethanol at 70 °C
(12 000),27 and the sample at 90% conversion had Mn ) 16 000.
A similar result was also obtained for 5 wt % C6F13I in ethanol.
The indication of the controlled nature of the polymerization
in the presence of C6F13I is the linear increase of Mn with
monomer conversion up to about 50% conversion, as shown in
Figure 4. For this reaction, Mw/Mn values (ca. 1.8) were much
narrower than those obtained without C6F13I (Mw/Mn > 3.0)
but much broader than those obtained by ideal living polym-
erization systems due to the relatively low value of the chain
transfer constant.
Tests of the reaction without any C6F13I present indicated
that monodisperse seed particles were formed in-situ in less than
15 min. We infer that low molar mass polymer was formed in
the reaction in the presence of C6F13I and that much of this
polymer remained in solution at 70 °C. This polymer precipitated
when the reaction was cooled, forming new particles in an
uncontrolled fashion. Because these PS chains each have an
iodine atom at one end, these chains can be extended gradually
to higher molar mass at higher conversions. Once the molar
At very high conversions (above 70%), Mn leveled off and
Mw/Mn was broadened. For dispersion polymerization at high
conversion, most of the C6F13I was consumed, and most of the
remaining monomer and initiator were dissolved in the continu-
ous medium. Under these circumstances, there is not enough
monomer within the particles for the polymer chains in the
particles to grow. Uncontrolled radical polymerization in the
medium dominated the reaction. To avoid this problem, it is
necessary to stop the reaction below 70% conversion in order
to keep most polymer chains in the particles living.

Macromolecules, Vol. 39, No. 24, 2006
Low Molar Mass Polymer Microspheres 8323
Figure 5. SEM (A) and optical microscopy (B) images of PS particles
prepared by the two-stage controlled dispersion polymerization in
ethanol-water mixture (95/5, wt/wt) at a conversion of 6% (St:
CMPDB:AMBN ) 200:2:1, mole ratio): (A) precipitated particles, (B)
serum diluted with water.
Figure 6. GPC chromatograms of PS in precipitated particles and
serum at a low conversion (6%) for the two-stage method in ethanol-
water mixture (95/5, w/w) (St:CMPDB:AMBN ) 200:2:1, mole ratio).
Two-Stage RAFT Dispersion Polymerization. As in the
case of DCT, the challenge is to achieve reaction conditions
that yield living, extendible polymers while maintaining control
over the size and size distribution of the polymer particles
obtained. The chain transfer constant for RAFT is expected to
be much larger than that for C6F13I. The magnitude of Ctr is
estimated to be more than 20 for CMPDB as a RAFT agent for
radical polymerization of styrene.19a As a consequence, we
anticipated that we would be able to achieve narrower molar
mass polydispersities with RAFT dispersion polymerization than
with DCT, with a theoretical polydispersity index of PS using
CMPDB as a RAFT agent on the order of 1.05.
Dispersion polymerization reactions involving CMPDB as a
RAFT agent were run in ethanol-water (95/5). This medium
was used both for the nucleation stage in the absence of the
RAFT agent and for the second stage. CMPDB was added along
additional solvent and monomer 1 h after the start of the
reaction. Samples were taken at different conversions, diluted
with water, and examined by SEM and optical microscopy. We
found that at low conversions the recovered PS particles were
very polydisperse. To investigate what happened in this system,
a sample was taken at a low conversion (6%), centrifuged to
sediment the particles, and separated into a particle component
and a transparent serum. The precipitated particles were
redispersed into water to prevent any absorbed oligomer from
diffusing into the medium. The homogeneous serum was diluted
with water, and it immediately became milky due to the
precipitation of dissolved oligomer.
Figure 5A shows the SEM image of the precipitated particles,
indicating these particles are highly monodisperse. In this image,
one can see an example of two particles that fused into a dimer
particle as the sample dried on the grid. Particle fusion is likely
promoted by the low molar mass of the polymer. Figure 5B
shows an optical microscopy image of the diluted serum,
indicating the formation of polydisperse particles due to the
precipitation of the soluble low-molar-mass PS chains when
water was added.
Figure 6 shows the GPC curves of the polymer in the
precipitated particles and isolated from the serum at 6%
conversion. There are two well-resolved peaks for the polymer
obtained from the particles, a large and broad peak (Mn )
16 000, Mw/Mn ) 2.4) corresponding to the polymer formed in
the absence of the RAFT agent (in the first stage), and another
smaller, narrower peak (Mn ) 800, Mw/Mn ) 1.15) correspond-
ing to the oligomers formed in the presence of the RAFT agent.
There is only one main narrow peak (Mn ) 600, Mw/Mn ) 1.20)
for the serum corresponding to the oligomers formed in the
presence of the RAFT agent. Because the molar mass of these
PS chains is far below the critical molar mass in the medium at
the polymerization temperature, only a very small fraction of
the oligomers were absorbed by the seed particles present in
the reaction.
Figure 7. SEM images of PS particles prepared by the two-stage
method in the presence of RAFT agent in ethanol-water mixture (95/
5, w/w): (A) sample taken at the conversion of 40% (St:CMPDB:
AMBN ) 200:2:1, mole ratio); (B) sample taken at the conversion of
50% (St:CMPDB:AMBN ) 200:1:0.75, mole ratio).
Figure 8. GPC chromatograms of PS particles prepared by the two-
stage living dispersion polymerization in ethanol-water (95/5, w/w)
at different reaction times (St:CMPDB:AMBN ) 200:2:1, mole ratio)
(4.5 h: Mn ) 800, Mw/Mn ) 1.20; 8.5 h: Mn ) 1300, Mw/Mn ) 1.19;
21 h: Mn ) 2200, Mw/Mn ) 1.38; 28.5 h: Mn ) 2700, Mw/Mn ) 1.47;
46 h: Mn ) 4000, Mw/Mn ) 1.49).
Figure 7A shows an SEM image of the particles at 40%
conversion (Mn ) 4000, Mw/Mn ) 1.5), indicating these particles
are nearly monodisperse. A small fraction of larger particles
can be seen in other images of this sample. In a separate
experiment, we examined the particles formed at 50% conver-
sion (Figure 7B), when the molar mass of the PS chains (Mn )
12 900, Mw/Mn ) 2.3) was above the critical molar mass in the
medium at the polymerization temperature. Here the particles
are highly monodisperse. We infer that most or all of the
polymer molecules in the system exceed their solubility and
are captured by existing particles.
GPC curves for one reaction at different conversions are
presented in Figure 8. As the polymerization proceeds, the PS
chains grow longer. At low conversion, the peak due to polymer
formed in the first stage in the absence of RAFT agent is
prominent, but it becomes less significant as the reaction
proceeds. The solubility of the PS chains decreases with
increasing conversion for the PS formed in the second stage,

8324 Song and Winnik
Macromolecules, Vol. 39, No. 24, 2006
Conclusion
Dispersion polymerization of styrene in ethanol in the
presence of chain transfer agents leads to a substantial broaden-
ing of the particle size distribution and a delay in the onset of
particle nucleation. If the addition of a DCT agent such as C6F13I
or a RAFT agent such as CMPDB is delayed until the nucleation
stage is complete and the CT agent is added with a mixture of
additional monomer and solvent, one can obtain particles
containing lower molar mass polymer, but with the same particle
size and a similar narrow size distribution as in the case where
no chain transfer agent is present. Other complications arise if
the polymer molar mass is reduced below the critical value Mcrit
for precipitation from the medium at the reaction temperature.
These problems may be overcome by adding small to modest
amounts of water to the reaction medium to reduce the solubility
of the polymer in the reaction medium.
Figure 9. Volume of particles (D3) vs monomer conversion for the
two-stage method in ethanol-water mixture (95/5, w/w) (St:CMPDB:
AMBN ) 200:2:1, mole ratio). The solid line is the theoretical plot of
D3 vs monomer conversion, and 0% conversion refers to the point at
which the RAFT agent was added to the reaction.
The polymer chains formed in the presence of C6F13I or
CMPDB are chain-extendible. Mn increased linearly with the
increase of conversion. When the Mn of the polymer chains is
above Mcrit, they will be captured by the seed particles formed
during the nucleation stage in the absence of the DCT or RAFT
agent. Colloidally stable, monodisperse particles containing
extendible polymer chains were prepared for both living radical
polymerization systems.
Acknowledgment. We thank NSERC Canada and the
Province of Ontario through its ORDCF program for their
support of this project. J.-S. Song thanks the Province of Ontario
for an OGS scholarship.
Figure 10. Plot of ln([M]/[M]0) vs polymerization time for the two-
stage living dispersion polymerization in ethanol-water mixture (95/
5, w/w) (St:CMPDB:AMBN ) 200:2:1, mole ratio).
Supporting Information Available: Additional information
about reactions in the presence of 5 wt % C6F13I: SEM images of
particles prepared in ethanol and in ethanol-water, GPC plots as
a function of reaction time, kinetic data and plots of Mn and Mw/
Mn vs conversion. This material is available free of charge via the
Internet at http://pubs.acs.org.
References and Notes
(1) Ugelstad, J.; Mørk, P. C.; Kaggerud, K. H.; Ellingsen, T.; Berge, A.
AdV. Colloid Interface Sci. 1980, 13, 101.
(2) Okubo, M.; Shiozaki, M.; Tsujihiro, M.; Tsukuda, Y. Colloid Polym.
Sci. 1991, 269, 222.
(3) Tuncel, A.; Tuncel, M.; Ergun, B.; Alagoz, C.; Bahar, T. Colloids
Surf., A 2002, 197, 79.
(4) Lok, K. P.; Ober, C. K. Can. J. Chem. 1985, 63, 209.
(5) Hora´k, D.; SÄvec, F.; Fre´chet, J. M. J. J. Polym. Sci., Part A: Polym.
Chem. 1995, 33, 2961.
(6) Monteiro, M. J.; Barbeyrac, J. D. Macromolecules 2001, 34, 4416.
(7) Manders, B. G.; Morrison, B. R.; Klostermann, R. Macromol. Symp.
2000, 155, 53.
(8) Glockner, P.; Metz, N.; Rittter, H. Macromolecules 2000, 33, 4228.
(9) Paine, A. J.; Luymes, W.; McNulty, J. Macromolecules 1990, 23, 3104.
(10) Ahmad, H.; Tauer, K. Colloid Polym. Sci. 2003, 281, 686.
(11) (a) Song, J. S.; Tronc, F.; Winnik, M. A. J. Am. Chem. Soc. 2004,
126, 6562. (b) Song, J. S.; Winnik, M. A.; Metz, H. J.; Graciet, J. C.
WO05026225A1.
(12) Song, J. S.; Tronc, F.; Winnik, M. A. Polymer 2006, 47, 817.
(13) Song, J. S.; Winnik, M. A. Macromolecules 2005, 38, 8300.
(14) Song, J. S.; Chagal, L.; Winnik, M. A. Macromolecules 2006, 39,
5729.
(15) Song, J. S.; Winnik, M. A. Polymer 2006, 47, 4557.
(16) (a) Solomon, D. H.; Rizzardo, E.; Cacioli, P. U.S. Patent 4,581,429,
March 27, 1985. (b) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P.
M.; Hamer, G. K. Macromolecules 1993, 26, 2987. (c) Veregin, R. P.
N.; Georges, M. K.; Kazmaier, P. M.; Hamer, G. K. Macromolecules
1993, 26, 5316. (d) Kazmaier, P. M.; Moffat, K. A.; Georges, M. K.;
Veregin, R. P. N.; Hamer, G. K. Macromolecules 1995, 28, 1841. (e)
Puts, R. D.; Sogah, D. Y. Macromolecules 1996, 29, 3323.
(17) (a) Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614.
(b) Wang, J.-S.; Matyjaszewski, K. Macromolecules 1995, 28, 7572.
(c) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T.
Macromolecules 1995, 28, 1721. (d) Percec, V.; Barboiu, B.; Neumann,
A.; Ronda, J. C.; Zhao, M. Macromolecules 1996, 29, 3665.
Figure 11. Number-average molar mass and molar mass distribution
vs monomer conversion for the two-stage method in ethanol-water
mixture (95/5, w/w) (St:CMPDB:AMBN ) 200:2:1, mole ratio). Linear
Mn ) conversion × 104([M]0/[CMPDB]0).
and the fraction of the oligomer or polymer absorbed by the
existing particles becomes larger. In Figure 9 we present a plot
of D3 (D is the mean particle diameter) as a function of monomer
conversion. At late stages of the reaction, D3 increases linearly
with conversion. The fact that the particle volume at a low
conversion is lower than predicted also indicates some oligomers
are not absorbed by the existing seed particles.
As in the case of the DCT polymerization, the plot of ln-
([M]/[M]0) vs polymerization time curve is linear (Figure 10)
for the RAFT reaction. The data in Figure 11 show that Mn
increases linearly with conversion and extrapolate to Mn ≈ 0 at
zero time. The Mw/Mn values are also much narrower than for
the PS prepared without CMPDB. All these features are
characteristics of living radical polymerization and are consistent
with a large value of Ctr.

Macromolecules, Vol. 39, No. 24, 2006
Low Molar Mass Polymer Microspheres 8325
(18) (a) Wang, J.-S.; Gaynor, S. G.; Matyjaszewski, K. Polym. Prepr. (Am.
Chem. Soc., DiV. Polym. Chem.) 1995, 36, 465. (b) Matyjaszewski,
K.; Gaynor, S. G.; Wang, J.-S. Macromolecules 1995, 28, 2093. (c)
Gaynor, S. G.; Wang, J.-S.; Matyjaszewski, K. Macromolecules 1995,
28, 8051.
(19) (a) Le, T. P.; Moad, G.; Rizzardo, E.; Thang, S. H. PCT Int. Appl.
WO 9801478 A1980115; Chem. Abstr. 1998, 128, 115390. (b)
Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P.
T.; Mayadunna, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.;
Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559. (c) Chong,
Y. K.; Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S. H.
Macromolecules 1999, 32, 2071. (d) Moad, G.; Rizzardo, E.; Thang,
S. H. Aust. J. Chem. 2005, 58, 379. (e) Perrier, S.; Takolpuckdee, P.
J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5347. (f) Favier, A.;
Charreyre, M.-T. Macromol. Rapid Commun. 2006, 27, 653.
(20) Holderle, M.; Baumert, M.; Mu¨lhaupt, R. Macromolecules 1997, 30,
3420.
(22) Shim, S. E.; Jung, H.; Lee, H.; Biswas, J.; Choe, S. Polymer 2003,
44, 5563.
(23) Lansalot, M.; Farcet, C.; Charleux, B.; Vairon, J. P. Macromolecules
1999, 32, 7354.
(24) Mu¨ller, A. H. E.; Zhuang, R.; Yan, D.; Litvinenko, G. Macromolecules
1995, 28, 4326.
(25) Goto, A.; Ohno, K.; Fukuda, T. Macromolecules 1998, 31, 2809.
(26) We use a nonionic surfactant (Table 1) as a costabilizer in our reactions.
While some authors find that there is no need for a costabilizer in
dispersion polymerization of styrene in ethanol, we sometimes find a
narrower particle size distribution when TX-305 is included in the
reaction, and this represents our default recipe.
(27) Yasuda, M.; Seki, H.; Yokoyama, H.; Ogino, H.; Ishimi, K.; Ishikawa,
H. Macromolecules 2001, 34, 3261.
(28) Ober, C. K.; Lok, K. P. Macromolecules 1987, 20, 268.
(21) Gabaston, L. I.; Jackson, R. A.; Armes, S. P. Macromolecules 1998,
31, 2883.
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