Ab initio emulsion polymerization by RAFT-controlled self-assembly

Article abstract of DOI:10.1021/ma048787r

A method is developed to enable emulsion polymerization to be performed under RAFT control to give living character without the problems that often affect such systems: formation of an oily layer, loss of colloidal stability, or loss of molecular weight control. Trithiocarbonate RAFT agents are used to form short stabilizing blocks from a water-soluble monomer, from which diblocks can be created by the subsequent polymerization of a Hydrophobic monomer. These diblocks are designed to self-assemble to form micelles. Polymerization is initially performed under conditions that avoid the presence of monomer droplets during the particle formation stage and until the hydrophobic ends of the diblocks have become sufficiently long to prevent them from desorbing from the newly formed particles. Polymerization is then continued at any desired feed rate and composition of monomer. The polymer forming in the reaction remains under RAFT control throughout the polymerization; molecular weight polydispersities are generally low. The number of RAFT-ended chains within a particle is much larger than the aggregation number at which the original micelles would have self-assembled, implying that in the early stages of the polymerization, there is aggregation of the micelles and/or migration of the diblocks. The latexes resulting from this approach are stabilized by anchored blocks of the hydrophilic monomer, e.g., acrylic acid, with no labile surfactant present. Sequential polymerization of two hydrophobic monomers gives completely novel core - shell particles where most chains extend from the core of the particles through the shell layer to the surface.

Full text of DOI:10.1021/ma048787r

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Macromolecules 2005, 38, 2191-2204
2191
Ab Initio Emulsion Polymerization by RAFT-Controlled Self-Assembly§
Christopher J. Ferguson,† Robert J. Hughes,†,) Duc Nguyen,† Binh T. T. Pham,†
Robert G. Gilbert,† Algirdas K. Serelis,‡ Christopher H. Such,‡ and
Brian S. Hawkett*,†
Key Centre for Polymer Colloids, Chemistry School F11, University of Sydney,
New South Wales 2006, Australia, and Dulux Australia, McNaughton Road, Clayton,
Victoria 3168, Australia
Received June 19, 2004; Revised Manuscript Received December 27, 2004
ABSTRACT: A method is developed to enable emulsion polymerization to be performed under RAFT
control to give living character without the problems that often affect such systems: formation of an oily
layer, loss of colloidal stability, or loss of molecular weight control. Trithiocarbonate RAFT agents are
used to form short stabilizing blocks from a water-soluble monomer, from which diblocks can be created
by the subsequent polymerization of a hydrophobic monomer. These diblocks are designed to self-assemble
to form micelles. Polymerization is initially performed under conditions that avoid the presence of monomer
droplets during the particle formation stage and until the hydrophobic ends of the diblocks have become
sufficiently long to prevent them from desorbing from the newly formed particles. Polymerization is then
continued at any desired feed rate and composition of monomer. The polymer forming in the reaction
remains under RAFT control throughout the polymerization; molecular weight polydispersities are
generally low. The number of RAFT-ended chains within a particle is much larger than the aggregation
number at which the original micelles would have self-assembled, implying that in the early stages of
the polymerization, there is aggregation of the micelles and/or migration of the diblocks. The latexes
resulting from this approach are stabilized by anchored blocks of the hydrophilic monomer, e.g., acrylic
acid, with no labile surfactant present. Sequential polymerization of two hydrophobic monomers gives
completely novel core-shell particles where most chains extend from the core of the particles through
the shell layer to the surface.
Introduction
problems: poor colloidal stability, poor molecular weight
control, or high polydispersity.3,9-14 There have been
various mechanisms postulated to explain these prob-
lems,9 for example, problems with RAFT agent trans-
port across the aqueous phase.15 This postulate has been
supported by evidence from Prescott et al.16 who showed
that these problems may be circumvented by ensuring
that all of the RAFT agent is located in seed particles
before polymerization is commenced. The procedure
used by these authors is limited to cases where seed
particles are used and thus does not allow ab initio
(unseeded) emulsion polymerization under RAFT con-
trol (much of the final polymer comes from the seed and
is thus not grown under controlled conditions). Another
approach that has been used with success is miniemul-
sion polymerization.17-20 This method removes the need
for the RAFT agent to migrate across the aqueous
phase, as the particles form from the droplets initially
present in the emulsion.
The successful application of RAFT (Reversible Ad-
dition-Fragmentation chain Transfer) techniques to both
bulk and solution polymerizations is well documented.1-4
A RAFT agent has the generic formula Z-C(dS)S-R,
where R is a leaving group that mediates the first
transfer event and thereafter remains the non-active
end group of the growing polymer chain and Z is an
activating group. The essential steps in RAFT polym-
erization are
Pm• + Z-C(dS)S-Pn h Z-C•(-SPn)S-Pm h
•
Z-C(dS)S-Pm + Pn (1)
•
Propagation occurs with the polymeric radicals Pn ,
whose “free” lifetime is very brief compared to the time
that they spend as nonpropagating SdC(Z)SPn entities.
The control that this method allows over molecular
weight, polydispersity, and chain architecture has al-
lowed many novel polymers to be produced.5-8
This paper expands on our earlier Communication21
describing an alternative method of applying RAFT
techniques to ab initio emulsion polymerization (as
distinct from miniemulsion).
While RAFT is well developed for use in homogeneous
polymerizations, the same cannot be said for dispersed
polymerization systems. Almost all published experi-
ments for RAFT polymerization in conventional emul-
sion systems have reported one or more of the following
Proposed Particle Formation Model
In a conventional ab initio emulsion polymerization,
particle formation can occur by three mechanisms:
micellar entry, homogeneous nucleation, and droplet
nucleation.22 Whichever nucleation route is occurring
in a conventional emulsion polymerization, in the pres-
ence of RAFT agents, this agent must still migrate into
the newly formed particles (from monomer droplets,
from monomer swollen micelles, or from the aqueous
phase) for the reaction to proceed under RAFT control.
§ Preliminary publication of some of this work appeared as a
Communication: Ferguson, C. J.; Hughes, R. J.; Pham, B. T. T.;
Hawkett, B. S.; Gilbert, R. G.; Serelis, A. K.; Such, C. H.
Macromolecules 2002, 35, 9243-9245.
* Author to whom correspondence should be addressed.
E-mail: b.hawkett@chem.usyd.edu.au.
† University of Sydney.
‡ Dulux Australia.
) Deceased March 17 2003.
10.1021/ma048787r CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/10/2005

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2192 Ferguson et al.
Macromolecules, Vol. 38, No. 6, 2005
Figure 1. Process used to create particles.
If the diffusion of RAFT agent is slow on the time scale
of particle formation and/or if there is significant RAFT
agent in the monomer droplets while polymerization is
occurring, then the previously reported problems will
occur for the reasons suggested by Prescott et al.9,16
The novel technique used in the present work ensures
that the RAFT agent will be located exclusively in the
particles as they form and, thus, that the entire polym-
erization will be under RAFT control. The fundamental
idea is that the polymerization process is such that (a)
all species formed from the RAFT agent are surface
active and therefore able to self-assemble into micellar
structures and (b) no other species capable of stabilizing
particles are present. Propagation will occur to a point
where chains are too hydrophobic to desorb from mi-
celles, and particles will thus be created. As the chains
in this newly formed particle grow and the size in-
creases, other diblocks may absorb onto the newly
created surface and in turn propagate to a point where
they are also not able to desorb from the surface. The
sequence of events then becomes similar to that which
occurs when particles are nucleated from micelles in a
conventional emulsion polymerization, with the obvious
distinction that the surfactant molecules in this case
are increasing in size and overall hydrophobicity. For
this mechanism to operate, the growing RAFT agent
must be able to self-assemble to form micelles in water,
preferably with the active RAFT end located in the
micelle interior to facilitate polymerization of hydro-
phobic monomer to yield particles. This model is more
general than that put forward in our preliminary
communication,21 where it was suggested that it was
necessary to have the formation of rigid micelles from
the diblocks.
that will self-assemble into micelles. This can be done
by the sequential polymerization of first a hydrophilic
and then a hydrophobic monomer in the presence of the
RAFT agent. Thus, a hydrophilic monomer such as
acrylic acid can be polymerized in the presence of a
RAFT agent to yield a macro-RAFT agent containing a
small number of hydrophilic groups (typically five in this
work). Further polymerization with a hydrophobic
monomer yields a diblock macro-RAFT agent that is
capable of forming micelles. Subsequent polymerization
will cause the diblock polymer to grow, and the micelles
then evolve into latex particles, which will continue to
grow under RAFT control. This process is illustrated in
Figure 1.
When diblock structures are formed using RAFT-
controlled polymerization, the original RAFT function-
ality is largely retained at the end of the chain. While
any RAFT agent will suffice for the process of self-
assembly by hydrophilic/hydrophobic growth, it is more
useful to start with a RAFT agent that is amphipathic,
specifically where one end is hydrophobic and the other
is hydrophilic. This presents the opportunity of forming
the amphipathic macro-RAFT agents by polymerizing
appropriate monomers, either in separate synthetic
steps or in situ. If they are surface active, these diblock
polymers are able to self-assemble to form micelles in
water. Creating the diblock polymer in situ provides a
method that ensures the polymer is able to form micellar
structures. The strategy of using an amphipathic RAFT
agent ensures that each end of a diblock polymer chain
will have character similar to that of the adjacent
monomer unit. Having a hydrophilic R group reduces
the likelihood of stabilizing chain ends associating
strongly with other R groups or the latex particle surface
to give rise to reduced stabilizer efficiency and high latex
viscosity or colloidal instability through bridging floc-
One way to carry out the process described above is
to use the RAFT agent to create amphipathic diblocks

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Macromolecules, Vol. 38, No. 6, 2005
Emulsion Polymerization by RAFT-Controlled Self-Assembly 2193
culation. A hydrophobic Z group will ensure that the
RAFT process will occur in the particle interior rather
than at the surface (although the latter might be
desirable in some instances).
Figure 2. Structures of RAFT agents, 2-{[(butylsulfanyl)-
carbonothioyl]sulfanyl}propanoic acid, (a), and 2-{[(dodecyl-
sulfanyl)carbonothioyl]sulfanyl}propanoic acid, (b).
By this approach being empolyed, the problem of
RAFT transport across the aqueous phase is avoided.
The RAFT agent remains soluble in the aqueous phase
until sufficient hydrophobic monomer has been added
to make it nonlabile, at which point the RAFT agent
will be located exclusively in the proto-particles. Gail-
lard et al.23 also used RAFT to synthesize AA/BA
diblocks of varying sizes and then cleaved the RAFT
end. They found that these diblocks behave as conven-
tional surfactants in emulsion polymerization. This
supports the basic ideas behind the protocols developed
in the present paper.
It is important to avoid the presence of monomer
droplets,9,16 at least up to the point where the RAFT
agents have propagated with sufficient hydrophobic
monomer to become locked into the particles since
otherwise the macro-RAFT agents can migrate to the
droplet surface and stabilize the monomer/droplet/water
interface. In a conventional emulsion polymerization,
labile surfactant can desorb from droplets as monomer
is transported to the growing particles, thereby allowing
the droplets to shrink. However, if one simply added
the RAFT agent to a conventional emulsion polymeri-
zation with monomer droplets present, growth of the
chain would then immobilize the macro-RAFT agent on
the droplet surface and reduce monomer loss from the
droplets, thereby giving rise to droplet nucleation.16
Having polymerization occurring in the droplets is
undesirable, as the relative concentrations of RAFT
agent and monomer will be different from what they
are in the particles, thereby giving rise to a broader
molecular weight distribution, as well as creating
particles which are of a different size to those created
from micelles and which are prone to coalescence.
This approach has many potential benefits. The most
obvious is controlling polymer molecular weight and
microstructure in a way not hitherto available in
emulsion polymerization. This is a result of the particles
all having RAFT agent present and attached to es-
sentially all the chains from the time they were created.
Another benefit is that the particles can be formed in
the absence of free surfactant, as colloidal stability will
be afforded by the hydrophilic chain ends located on the
particle surface (either a species which can be charged,
such as acrylic acid, or uncharged, such as acrylamide,
or any combination of ionizable and nonionizable mono-
mers), which cannot subsequently desorb from the
particle. The absence of free surfactant is desirable in
many technical applications. The method by which the
particles are created also provides the opportunity to
functionalize the particle surface (by beginning with a
functionalized RAFT agent or adding functional mono-
mer with the hydrophilic monomer) and control particle
morphology (by changing the feed composition after the
particles have formed).
The structures of the chosen RAFT agents, which are
trithiocarbonates and where the R group is propanoic
acid, are shown in Figure 2: 2-{[(butylsulfanyl)carbono-
thioyl]sulfanyl}propanoic acid (a) and 2-{[(dodecylsul-
fanyl)carbonothioyl]sulfanyl}propanoic acid (b). The car-
boxylic acid group was chosen to be similar to acrylic
acid, which will be polymerized as the hydrophilic block.
The R group yields a secondary radical on cleavage from
the RAFT agent, which is sufficiently stable to allow
both the cleavage process and monomer addition to
occur under the reaction conditions chosen. This trithio-
carbonate class of RAFT agents has a low susceptibility
to hydrolysis in the pH range used here (although
hydrolysis can occur under more alkaline conditions24).
This low susceptibility to hydrolysis, which we con-
firmed by UV-visible spectroscopic analysis of a water
solution of the RAFT agent, contrasts to the higher
susceptibilityofthecommonerdithioesterRAFTagents.25-29
Acrylic acid was chosen as a convenient monomer to
make the hydrophilic block, as it polymerizes well under
the control of the chosen RAFT agent, and short
anchored blocks are able to stabilize latex particles
when the acid groups are neutralized. Butyl acrylate
was chosen as a convenient second-stage monomer due
to its high propagation rate coefficient30,31 and favorable
reaction kinetics with the RAFT agent and acrylic acid.
This ensures that the polymerization will proceed at a
reasonable rate.
The process could be sub-optimal during the aqueous
polymerization step as a result of the monomer concen-
tration being very low (due to the low water solubility
of most monomers used in emulsion polymerization).
This leads to a reduced rate of propagation in relation
to termination, which will result in some terminated
aqueous diblock species. While radicals would propagate
mainly in micelles once they formed, aqueous-phase
termination might still occur to some extent in the very
early stages of the process, when there is some lability
of chains (as inferred in a later section), leading to some
transfer of radical activity between micelles. Any new
chains that are initiated in the aqueous phase without
a RAFT agent will still have a carboxylic group from
the initiator, but this will be less effective in stabilizing
the particles than the acrylic acid block, as well as
producing chains that are free to desorb from the
particles. Thus, it is important to optimize polymeriza-
tion during this period to ensure that as many chains
as possible have both the targeted acrylic acid block on
the end and RAFT functionality.
To optimize the yield of the desired product and have
an easily characterizable system, it was decided to
preform the acrylic acid-containing macro-RAFT agent
in concentrated aqueous solution. The initiator chosen
for this purpose was 4,4′-azobis(4-cyanopentanoic acid)
(V-501), as it is water-soluble and carries an acid
functionality similar to the R group from the RAFT
agent. An additional benefit is that this initiator does
not contain a peroxide group, a class of compound that
is thought to be responsible for oxidation of some RAFT
agents.1
Selection of Reaction Conditions
The basic procedure described could be carried out in
a multitude of ways. In this work, it was considered
important to have a system that was as simple as
possible and also able to be easily characterized. As
described above, it was desired to utilize a RAFT agent
with a hydrophobic Z group and a hydrophilic R group.

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2194 Ferguson et al.
Macromolecules, Vol. 38, No. 6, 2005
The second stage involves the formation of diblock in
water. Diblocks containing both hydrophobic and hy-
drophilic groups will form micelles as they become
surface active. The strategy for formation of the hydro-
phobic part of the diblock is not straightforward, on
account of the low water solubility of butyl acrylate. The
proposed mechanism calls for all the chains to grow to
a length where they are able to self-assemble as mi-
celles, which will then swell with monomer, which in
turn will give rise to an acceleration of the polymeri-
zation rate (the monomer concentration at the locus of
polymerization increases dramatically at this stage).
There are many strategies for optimization of this
second step. For example, the use of a more water-
soluble monomer (e.g., methyl acrylate) would speed up
this process. Counterbalancing this is the requirement
that diblocks are able to come to a pseudo-equilibrium
distribution (see eq 1) before further polymerization
begins to lock them into particles. In another strategy,
the number of aqueous propagation steps required could
be reduced by beginning the diblock formation in an
organic solvent. While this will have the desired effect
of reducing the time taken to form micelles, the down-
side is that some organic solvent will still be present in
the latex synthesis, which may be undesirable if not
removed.
The other consideration for achieving RAFT-con-
trolled emulsion polymerization by the present mech-
anism is to avoid the presence of monomer droplets,
especially at early times in the reaction. To this end,
the reactions were run under a controlled feed of
monomer, with a feed profile chosen using conversion-
time data from monomer-flooded experiments, i.e.,
experiments where excess monomer was always present.
Monomer concentration needs to be kept as high as
possible (while avoiding the presence of droplets) so as
to maximize propagation in relation to chain-stopping
events. Once all the particles have formed (and thus the
RAFT agents are all locked into the particles), the
presence of monomer droplets should not present a
problem, and the monomer-saturated conditions in the
particles will maintain the polymerization rate at a high
value.
Figure 3. Raw GPC traces showing the effects of eluent and
sample preparation: (a) latex sample dissolved directly and
run in THF eluent, (b) dried latex sample dissolved in THF
and run in THF/acetic acid, and (c) from procedure of dissolv-
ing latex directly and run in THF/acetic acid (the optimal
procedure).
acid) (V-501) (Wako) were used as received. Dioxane (Aldrich)
was distilled before use.
Electrospray Mass Spectrometer Analysis was carried
out using a Finnigan Mat LCQ MS detector with Finnigan
LCQ Data Processing and Instrument Control Software. Ten-
microgram samples were dissolved in 10 mL of 50:50 methanol/
H2O and fed into the electrospray ionization unit at 0.2 mL
min-1. The electrospray voltage was 5 kV, the sheathing gas
was nitrogen at 415 kPa, and the heated capillary was 200
°C.
GPC Analysis. Molecular weight distributions were deter-
mined using gel permeation chromatography (GPC). Analyses
were carried out using a Shimadzu system fitted with a series
of Waters columns (HR4, HR3, and HR2). Molecular weight
was determined from refractive index data analyzed with
Polymer Laboratories Cirrus software, with all molecular
weights being relative to polystyrene standards and converted
to poly(butyl acrylate) or poly(methyl acrylate) using “universal
calibration”32 and the following Mark-Houwink parameters:
33 styrene, K ) 11.4 × 10-5 dL g-1, a ) 0.716; BA, K ) 12.2 ×
10-5 dL g-1, a ) 0.70; MA, K ) 26.1 × 10-5 dL g-1, a ) 0.659.
The acid groups on the initiator and macro-RAFT
agent need to be at least partially neutralized in order
for the RAFT agent and initiator to dissolve in water.
This was accomplished by adding sufficient NaOH to
bring the total added molar concentration (which in-
cludes that added during the macro-RAFT agent syn-
thesis) to the same as that of the added acrylic acid.
This ensures that there is some buffering action from
the carboxylic acid species. This degree of neutralization
leads to a measured pH of around 6 for latex prepara-
tion. Maintaining an acidic pH was considered impor-
tant to minimize hydrolysis of the RAFT agent, as has
been observed by a number of authors.28,29 The ioniza-
tion of the acid groups ensures that the acid groups are
able to provide electrostatic stabilization to the particles
as they form.
Sample preparation for GPC analysis was an important
aspect of this work. The presence of acrylic acid at the chain
end was capable of producing artifacts in the chromatograms
if precautions were not taken. It appears that acrylic acid on
the chain ends can be responsible for two phenomena. (1) The
first of these is that an irregular peak structure was often
observed at longer elution times than that of the main polymer
peak when the samples were analyzed using tetrahydrofuran
(THF) as the eluent. Figure 3 shows the raw GPC traces with
DRI detection for the same polymer sample run under different
conditions. Trace (a) is typical of polymer samples that have
been run with 100% THF as the eluent. An improved proce-
dure was to mask carboxylic acid interactions34 by adding 5%
acetic acid to the eluent. (2) The other phenomenon is
aggregation of polymer chains due to the low solubility of the
acrylic acid chain ends in THF, which would also lead to a
larger apparent molecular weight. This is assumed to be the
reason it was very difficult to solubilize the polymer films in
order to prepare GPC samples. This problem was overcome
by adding a little water to the THF used to dissolve the dried
polymer, or alternatively dissolving the latex directly into the
eluent. If neither of these measures were taken, the GPC
showed a peak at approximately 20 times the molecular weight
of the main peak, as shown in trace (b) of Figure 3, which was
assigned as an inverse-micelle structure forming in the THF.
Experimental Section
Reagents. Milli RO water was used in the synthesis of
latexes and acrylic acid-containing RAFT agents. Acrylic acid
(Sumika) and styrene (Synthetic Resins) were purified by
distillation under reduced pressure. Butyl acrylate and methyl
acrylate (Aldrich) had inhibitor removed by passing them
through an inhibitor-removal column (Aldrich). Sodium hy-
droxide (NaOH) (Aldrich) and 4,4′-azobis(4-cyanopentanoic