Materials Sciences and Applications, 2010, 1, 292-300
doi:10.4236/msa.2010.15043 Published Online November 2010 (http://www.SciRP.org/journal/msa)
Cationic Microemulsion Polymerization of Alkyl
Acrylates
Issa Katime1, Jesús Arellano2, Eduardo Mendizábal3, Jorge Flores3
1Grupo de Nuevos Materiales y Espectroscopia Supramolecular, Facultad de Ciencia y Tecnología, Bilbao, España; 2Departamento
de Ingeniería Química, Universidad de Guadalajara, Guadalajara, Jalisco, México; 3Departamento de Química, Universidad de
Guadalajara, Guadalajara, Jalisco, México.
Received September 13th, 2010; revised October 26th, 2010; accepted October 30th, 2010.
ABSTRACT
Here we present the polymerization of n-butyl acrylate (BA), ethyl acrylate (EA) and methyl acrylate (MA) in
tri-component microemulsions, using a cationic surfactant such as dodecyl trimetyl ammonium bromide in water, as a
function of temperature, initiator type and, monomer and initiator concentration. The final latexes are transparent and
blue color, with particle size ranging between 20 and 60 nm determined by quasielastic light scattering (QLD) and SEC
molar masses of the order of 106 g/mol. Reaction times are short and reaction rates are high with final conversions be-
tween 70 and 98% depending on the monomer and the reaction conditions.
Keywords: Microemulsion Polymerization, Particle Size, Nanoparticles, Quasielastic Light Scattering, Surfactant
1. Introduction
Microemulsion polymerization was born as an alternative
process for the production of polymeric latexes with
unique particle size, molar masses and structure. Syn-
thesis of stable latexes with particle size ranging from 10
a 35 nm has been possible through this process where a
fast polymerization affords high molar mass polymers
[1,2]. Stoffer and Bone [3,4] first reported microemul-
sion polymerization in 1980. Most microemulsion po-
lymerization reported in the 80’s, were carried out in four
and five component systems [2]. The first three compo-
nent microemulsion polymerization (water, surfactant
and monomer) was reported by Pérez-Luna et al. in 1990
[5,6]. Since then microemulsion polymerization of dif-
ferent monomers in three component systems have been
reported [7-18], where the influence of different parame-
ters on the polymerization kinetics and on the obtained
latexes have been studied [12,13,19-27]. Some of the
parameters are: 1) Initial monomer concentration in the
system, polymerization rate and conversion increase with
initial monomer concentration due to the higher number
and size of the drops being formed in the microemulsion.
Gan et al. [12] and Pérez-Luna et al. [6] report similar
behavior of the polymerization reaction rates for styrene
in three and four components, 2) Temperature, reaction
rates are higher when temperature is raised due to a quick
increase in the initiator decomposition rate. Final conver-
sion increases as the mobility of the macromolecules
increases with temperature. Guo et al. [19] and
Rodríguez-Guadarrama [10,11] report activation energy
values for microemulsion polymerization of methyl
methacrylate and, 3) The initiator and the surfactant, the
rate of polymerization and the rate of conversion increase
with initiator concentration. To understand the influence
of the type of initiator and surfactant the structure of
them has to be considered. Guo et al. [19] studied the
polymerization of styrene in SDS and pentanol and re-
ported higher reaction and conversion rates; with an ini-
tiator soluble in the aqueous phase KPS, than those ob-
tained in the oil phase AMBN. Gan et al. [13] study sty-
rene polymerizations with SDS or CTAB using different
initiators. They observed a higher reaction rate with SDS
than with CTAB. Similar results were observed for other
systems attributed to the “electrostatic charge effect” and
to a chain transfer reaction between the bromide ion of
the surfactant and the KPS free radical [23], 4) electro-
lyte addition, this effect depends on factors such as
structure and concentration of the electrolyte; addition of
the electrolyte greatly alters one phase region of these
systems decreasing the solubility between water and sur-
factants. Full et al. [23], studied the addition of KBr on
KPS initiated styrene microemulsion polymerization in

Cationic Microemulsion Polymerization of Alkyl Acrylates 293
microemulsion with dodecyl trimetyl ammonium bro-
mide, finding that reaction rate decreases, as well as par-
ticle size and molar masses as salt concentration in-
creases. Similar results were obtained when NaBr was
used as electrolyte in styrene microemulsion polymeriza-
tion [22,23] and, 5) alcohol addition, alcohols drastically
modify the one phase region of water-oil-surfactant, as
shown by Gan et al. [12] and Puig et al. [20]. Gan et al.
[13], reported higher molar masses for more amphifilic
surfactants. Puig et al. [20], report that reaction rate,
conversion degree and molar mass decrease with alcohol
content. They also found influence of the cosurfactant.
In this work we present the polymerization of n-butyl
acrylate (BA), ethyl acrylate (EA) and methyl acrylate
(MA) in tricomponent microemulsions, using a cationic
surfactant such as dodecyl trimetyl ammonium bromide
in water as a function of temperature, monomer and ini-
tiator concentration and initiator type. Particle size and
molar mass of the final latexes are of the order of 20 and
60 nm and 106 g/mol, respectively. High reaction rates
and conversions above 70% were found in all cases. Fi-
nal latexes were transparent and slightly blue.
2. Experimental
Materials. Aldrich 99% pure dodecyl trimethyl ammo-
nium bromide (DTAB) was used as surfactant. WAKO
2,2’-azobis(amidinopropane) V-50, Fluka potassium
persulfate KPS, and Merck 2,2-Azobisisobutyronitrile
(AIBN), all more than 99% pure, were used as initiators.
Acrylates such as ethyl (EA), methyl (MA) and n-butyl
(n-BA) from Scientific Polymer Products 99% were used,
after passing them through a DE-HIBIT 100 Scientific
Polymer Products column to remove the inhibitor. Merck
99% hydroquinone was used to inhibit the reaction. Wa-
ter was distilled twice. Chromatographic grade Merck
tetrahydrofurane (THF) was used as mobile phase in
molar mass SEC determinations.
Phase Diagram Determination. A weighed sample of
the monomer aqueous solution (50 wt %) was placed in a
25-mL vial and thermostated at 25ºC. Then, the oil phase
was slowly added, drop by drop, under vigorous stirring
until the turbid emulsion turned into an optically trans-
parent microemulsion. The final composition of the mi-
croemulsion was determined via weighing. The conduc-
tivity of the microemulsion showed that had a globular
structure, which was formed by micelles swollen with the
aqueous phase. The systems studied in this article were
stable for a period of at least several hours.
Synthesis and Kinetic Studies. Kinetics were followed
by dilatometry, polymerizations were carried out in a
small two mouth reactor (approximately 30 mL), with a
capillary tube connected to one of them (40 cm high and
0.1 cm diameter) and a small septum to the other one to
inject the initiator. An adequately prepared microemul-
sion is added to the reactor, placed in a constant tem-
perature bath with continuous stirring, under argon
stream for 45 minutes and them left until it reaches the
desired temperature. As the system increases its volume
due to thermal expansion, thermal equilibrium is thought
to be reached when the meniscus reaches a constant
height in the capillary. At this moment the solution of the
initiator is injected and timing of the reaction is started
by observing the decrease in the capillary height assum-
ing that volume change is proportional to conversion.
The dilatometer is calibrated calculating the conversion
degree by gravimetry of the final latexes in each reaction.
Particle Size. Particle size was determined using a qua-
sielastic light scattering (QLS) AMTEC apparatus equip-
ed with a He-Ne laser, 632.8 wavelength and 60 mW
power. The instrument has a BI-9000AT correlator. Cor-
relation data are analyzed by the cumulant method which
supplies an average r = q2, where q is the dispersion
vector and D the diffusion coefficient. Diffusion coeffi-
cient measurements are presented in terms of apparent
radius according to Stokes law and assuming that the vis-
cosity of the solvent is that of pure water. Molar masses
were calculated with a SEC instrument equipped with a
Knauer HPLC64 injection pump, a Rheodyne 7125 man-
ual injector, a Knauer differential refractive index detector,
two Polymer Laboratories PLGEL mixed-C columns, a
Polymer Laboratories PL-LALS interphase, and a PC-486
microcomputer with PL Caliber SEC Software. Merck
chromatographic grade tetrahydrofurane (THF) was used
as mobile phase.
3. Results and Discussion
Polymerization of n-butyl acrylate (n-BA) and ethyl
acrylate (EA) in three component microemulsions pre-
pared with dodecyl trimethyl ammonium bromide and
water are faster than those of methyl acrylate (MA) in the
same type of micro emulsions. Reaction times are similar
to those observed for these monomers in four component
microemulsions [28-30].
Reactions were carried out using a constant ratio of
dodecyl trimethyl ammonium bromide/water 15/85, and
varying different parameters in order to find out how
each one influenced the polymerization kinetics espe-
cially the initiation step. Polymer densities used to cal-
culate conversions are 1.22 g/mL, for poly(methyl acry-
late), 1.12 g/mL, for poly(ethyl acrylate) and 1.09 g/mL,
for de n-butyl acrylate [31].
Temperature effect. To study the temperature effect on
these polymerizations, in cationic microemulsions pre-
pared in DTAB and water, a 4% monomer composition
and 14.4% DTAB and 81.6% water were chosen. The

Cationic Microemulsion Polymerization of Alkyl Acrylates 294
reaction was initiated with 0.5% V-50 with respect to the
monomer and the different temperature were 50, 55, 60
and 65°C.
Monomer to polymer conversion plots as a function of
time, and polymerization rate as a function of conversion
for the three monomers show the same general trend at
the chosen temperatures. Figures 1 and 2 show the re-
sults obtained for the polymerization of n-butyl acrylate.
As can be seen, the polymerizations are fast and reach
degrees of conversion higher than 90% for n-butyl acry-
late, between 85 and 88% for ethyl acrylate at the chosen
temperatures, and higher than 70% for methyl acrylate
during 100 minutes at 60 and 65°C).
Initial reaction rate gets smaller as reaction tempera-
ture is decreased, however, at the end, similar conversion
degrees are reached in the different reactions with respect
020406080100 120 140 160 180 200
0.0
0.2
0.4
0.6
0.8
1.0
% Conversion
Time (min)
50 篊
55 篊
60 篊
65 篊
Figure 1. Conversion as a function of time for a polymeriza-
tion initiated with V-50 0.5% (with respect to the monomer)
at different temperatures in micro emulsions containing 4%
of n-butyl acrylate, 14.4% DTAB and 81.6% water.
0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
50 篊
55 篊
60 篊
65 篊
Rp
% Conversion
Figure 2. Reaction rate variation as a function of degree of
conversion for polymerizations initiated with 0.5% V-50
(with respect to the monomer) at different temperatures in
micro emulsions containing 4% n-buyl acrylate, 14.4%
DTAB and 81.6% water.
to each monomer. The latexes look transparent at the
beginning but become cloudy as the reaction proceeds. It
can be observed that the reaction rate only show two in-
tervals typical of micro emulsion polymerization, more
clearly in the case of n-butyl and ethyl acrylates than in
methyl acrylate, due to the smaller water solubility of the
first two. Solubilities of methyl, ethyl and n-butyl acry-
lates are 4.76, 1.48 and 0.2%, respectively [32]. Polym-
erization rates increase with reaction temperature. This
behaviour is due to the increase of free radical flow as
the initiator decomposition rate and the propagation con-
stant increase significantly with temperature [12,19].
Maximum polymerization rate depends on temperature
similar to Arrhenius prediction. Activation energy ob-
tained from the kinetic data is 132.6 kJ/mol for n-butyl
acrylate and 128.2 and 139.6 kJ/mol for ethyl and methyl
acrylate, respectively. These figures are higher than those
reported for different alkyl acrylates polymerized in a
similar manner [10,14].
In Figure 3, monomer to polymer conversion as a
function of reaction time for methyl, ethyl, and n-butyl
acrylates at 50 and 60°C are compared. N-butyl acrylate
polymerizations are faster than those of ethyl acrylate
and these ones faster than methyl acrylate at both tem-
peratures; they get faster as the substituent size increases.
An increase in the polymerization rate is observed as the
monomer water solubility decreases. It is also remarkable
that the conversion graphs change from a typical micro
emulsion polymerization to a solution polymerization
behaviour as the monomer water solubility increases.
In Figure 4, polymerization rate variations as a func-
tion of conversion for the different monomers at 50 and
60°C are shown. Two typical micro emulsion polymeri-
zation intervals are observed for n-butyl acrylate more
0 20406080100120140160
0.0
0.2
0.4
0.6
0.8
1.0
MA (50篊)
MA (60篊)
EA (50篊)
EA (60篊)
BA (50篊)
BA (60篊)
% Conversion
Time (min.)
Figure 3. Conversion as a function of time for a polymeriza-
tion initiated with V-50 0.5% (with respect to the monomer)
at 50 and 60°C in micro emulsions containing 4% of mono-
mer, 14.4% DTAB and 81.6% water for methyl, ethyl and
n-butyl acrylates.
%
%
%
%
%
%
%
%
MA (50%)
MA (60%)
EA (50%)
EA (60%)
BA (50%)
BA (60%)

Cationic Microemulsion Polymerization of Alkyl Acrylates 295
0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Rp
% Conversion
50 篊
55 篊
60 篊
65 篊
Figure 4. Reaction rates as a function of conversion for a
polymerization initiated with 0.5% V-50 (with respect to the
monomer) at 50 and 60°C in micro emulsions containing
4% of monomer, 14.4% DTAB and 81.6% water for methyl,
ethyl and n-butyl acrylates.
clearly than in the case of ethyl and methyl acrylates.
This behaviour is less clear for the last one due to its
higher water solubility. It is apparent that the reaction
rate decreases as the monomer water solubility increases.
Bearing in mind that initiation probability is directly
proportional to micelle concentration, as they contain the
monomer inside them (micellar nucleation), it is clear
that the more water soluble is the monomer and, as a
consequence the initiation step is more favorable, the
higher is the polymerization rate.
Table 1 shows that for the three monomers, particle
size and maximum polymerization rate increase with
temperature. Molar masses are of the order of 106 g/mol.
With respect to particle size, as the temperature increases,
so does the monomer diffusion rate which implies a lar-
ger amount of monomer per particle hence a larger size.
The maximum polymerization rate increases as the po-
Table 1. Particle size, Dp, Maximum polymerization rate,
Vpmax, mass average molar mass, Mw, of final latexes as a
function of temperature for polymerizations initiated with
0.5% V-50 (with respect to the monomer) in micro emul-
sions containing 4% n-butyl acrylate, 14.4% DTAB and
81.6% water.
Temperature (C) D
p (nm) Vpmax Mw × 106 g/mol
n-butyl acrylate
50 23 0.068 -
60 26 0.296 1.12
Ethyl acrylate
50 30.5 0.018 -
60 32 0.060 0.948
Methyl acrylate
50 42 0.003 -
60 53 0.011 1.05
lymerization rate increases with a higher radical flow
because of a higher initiator decomposition rate with
temperature.
Monomer concentration effect. In order to study the
effect of monomer concentration in the polymerization of
n-butyl, ethyl and methyl acrylates, in DTAB cationic
microemulsions 1, 2, 3, and 4% monomer and a constant
surfactant/water ratio were used. A fixed 15/85
DTAB/water ratio was polymerized. Reactions were car-
ried out at 60°C, initiated with 0.02 % V-50 with respect
to the total weight (initiator concentration was calculated
with respect to the initial monomer concentration where
this was 4%. This value was kept constant for the rest of
the reactions).
Monomer to polymer conversion with time plots, po-
lymerization rates with respect to conversion, for the
three monomers show the same trend. Figures 5 and 6
020406080100 120 140 160
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
% Conversion
Time (min. )
1%
2%
3%
4%
Figure 5. Monomer conversion with time for the polymeri-
zation of different methyl acrylate concentrations initiated
with 0.02% V-50 in 15/85 DTAB/water microemulsions at
60°C.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.000
0.002
0.004
0.006
0.008
0.010
0.012
Rp
% Conversion
1%
2%
3%
4%
Figure 6. Reaction rates with conversion for the polymeri-
zation of different methyl acrylate concentrations initiated
with 0.02% V-50 in 15/85 DTAB/water micro emulsions at
60°C.
%
%
%
%

Cationic Microemulsion Polymerization of Alkyl Acrylates 296
show monomer to polymer conversion plots with time
and polymerization rates with respect to conversion for
methyl acrylate at the same initial monomer different
concentrations. Conversion degrees for methyl acrylate
oscillate between 70 and 75% for the different monomer
concentrations; for ethyl acrylate around 85% except for
1% monomer concentration where it reaches 70%. On
the other hand final conversions for n-butyl acrylate are
all higher than 94%. In the conversion plots for the three
acrylates a monomer concentration effect is not observed
(see Figure 5 for methyl acrylate), except for 1% mono-
mer concentration. In general, latexes for the three
monomers are transparent at the beginning of the reac-
tion, except for a slightly blue color for higher initial
monomer concentrations. At the end of the reaction they
look cloudy with cloudiness increasing with monomer
concentration. Two intervals typical of microemulsion
polymerization are observed for the polymerization rate
in al cases (Figure 6 for butyl acrylate).
Figure 7 shows monomer to polymer conversion plots
as a function of reaction time for methyl, ethyl and
n-butyl acrylates at 2 and 4 weight percent with respect
to total weight. Acrylate polymerization rates follow this
order: n-butyl > ethyl > methyl. Polymerizations are
faster and an increase in polymerization rate with
monomer water solubility is observed.
Figure 8 shows polymerization rates with respect to
conversion for the above monomers at 2 and 4 monomer
weight percent with respect to total weight. Two intervals
typical of microemulsion polymerization are observed
although this behaviour is less clear as the monomer be-
comes less water soluble. It is clear how reaction rate
decreases as monomer water solubility increases.
Dependence of polymerization rate on monomer con-
centration has been reported for most microemulsion
0 20406080100120
0.0
0.2
0.4
0.6
0.8
1.0
%Conve3rsi髇
Time (min.)
MA(2 %)
MA(4 %)
EA(2 %)
EA(4 %)
BA(2 %)
BA(4 %)
Figure 7. Conversion with time for the polymerization of
methyl, ethyl and n-butyl acrylates initiated with 0.02%
V-50 in 15/85 DTAB/water micro emulsions with 2 and 4%
monomer concentration at 60°C.
0.0 0.2 0.4 0.6 0.8 1.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
MA (2 %)
MA (4 %)
EM (2 %)
EM (4 %)
BM (2 %)
BM (4 %)
Rp
% Conversion
Figure 8. Reaction rates with conversion for the polymeri-
zation of different methyl acrylate concentrations initiated
with 0.02% V-50 in 15/85 DTAB/water micro emulsions
with 2 and 4% monomer concentration at 60°C.
polymerizations as opposed to what is observed in emul-
sion [6,12,13]. In emulsion polymerizations the majority
of the monomer is dispersed as drops which do not sig-
nificantly participate in the initiation step as this takes
place in the micelles. It is well known that in microemul-
sion polymerization most of the monomer is located in
the swollen micelles and the rest is dissolved in water.
Therefore, a competition for radical capture takes place
between the monomer dissolved in water (homogeneous
nucleation) and the one in the micelles (micellar nuclea-
tion). This big increase in active sites, where the reaction
can be initiated, produces an increase in the initiation
step rate, the one controlling the reaction rate. In our case
an important effect of monomer concentration has not
been observed for the three acrylates at the concentra-
tions we used (smaller than those used by other authors)
which suggests a proportional variation in the interfacial
ratio: swollen micelles/water phase, at low monomer
concentration; this favors homogeneous nucleation and
polymerization rates do not show big change, this is our
case, which has also been reported for n-hexyl metha-
crylate [33]. Concluding, it is micellar nucleation which
has the highest impact on reaction rate increase and when
homogeneous nucleation is favored reaction rate tends to
decrease. When comparing different acrylates polymeri-
zation rates a decrease in final conversions and polym-
erization rates, when monomer water solubility increases,
is observed. In other words, by increasing the relative
amount of monomer in the aqueous phase, which favors
homogeneous nucleation, polymerization rate is de-
creased. In general, when micellar nucleation in a mi-
croemulsion polymerization is favored, reaction rate in-
creases, and when homogeneous nucleation is favors,
polymerization rate decreases.

Cationic Microemulsion Polymerization of Alkyl Acrylates 297
Table 2 shows how particle size increases with initial
monomer concentration and maximum polymerization
rate does not show a clear trend, for the three monomers
under study. Molar masses are of the order of 106 g/mol.
With respect to particle size, while for n-butyl acrylate
no change is observed, a size change is apparent for the
other two monomers as the initial monomer concentra-
tion is increased, which can be interpreted as a beginning
in particle coagulation due to the nature of the mono-
mers and to the similarity in the kinetics of their polym-
erizations. Maximum polymerization rate is very similar,
no change in reaction order is observed for the three
cases.
Effect of initiator (V-50) concentration. To study the
effect of initiator (V-50) concentration on n-butyl, ethyl
and methyl acrylates microemulsion cationic polymeri-
zations in DTAB/water, the following composition was
used: 4% monomer, 14.4% DTAB and 81.6% water.
Reactions were carried out at 60°C, V-50 concentrations
were 0.25, 0.50, 0.75 and 1.00%. Initiator concentration
is given as weight % with respect to initial monomer
concentration. V-50 is a water soluble initiator which
decomposes to give two cationic free radicals. Monomer
to polymer conversion vs. time plots and those for po-
lymerization rates Vs conversion for the three monomers,
show similar trends at the different initiator concentra-
tions employed. Figures 9 and 10 show monomer to
polymer conversion vs. time plots and polymerization
rates vs. conversion for ethyl acrylate at different V-50
concentrations.
This polymerization reach conversions between 86 and
93% for ethyl acrylate depending on initiator concentra-
tion, while for n-butyl acrylate, conversions higher than
95% are reached in all cases. On the other hand, for
methyl acrylate conversions are between 70 and 75%. In
general latexes look similar to those already described;
transparent at the beginning of the reaction they become
Table 2. Particle size, Dp, maximum polymerization rate,
Vpmax, mass average molar mass, Mw, for final latexes as a
function of initial monomer concentration, Cm, for 0.02%
V-50 initiated polymerizations in microemulsions with a
15/85 DTAB/water ratio.
Cm D
p (nm) Vpmax Mwx106
g/mol
n-butyl acrylate
2 24.7 0.230 -
4 26 0.296 1.12
Ethyl acrylate
2 20 0.075 -
4 32 0.060 0.948
Methyl acrylate
2 45 0.022 -
4 53 0.011 1.05
0 1020304050
0.0
0.2
0.4
0.6
0.8
1.0
% Conversion
Time (min.)
0.25%
0.50%
0.75%
1.00%
Figure 9. Polymer conversion (%) vs. time for polymeriza-
tions initiated with different concentrations of V-50 (with
respect to monomer) in microemulsions containing 4%
ethyl acrylate, 14.4% DTAB and 81.6% water, carried out
at 60°C.
0.0 0.2 0.4 0.6 0.8 1.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.25 %
0.50 %
0.75 %
1.00 %
Rp
%Conversion
Figure 10. Reaction rates vs. conversion for polymerizations
initiated with different concentrations of V-50 (with respect
to monomer) in microemulsions containing 4% ethyl acry-
late, 14.4% DTAB and 81.6% water, carried out at 60°C.
cloudy with reaction progress. Reaction rate decreases as
initiator concentration decreases. Two typical microemul-
sion polymerization intervals for reaction rate are only
observed in the case of less water soluble monomers. Po-
lymerization rate increases as initiator concentration in-
creases. This behavior is due to the increase in free radical
flow, as it was mention before, which highly increases the
probability of initiation in this reacting system.
In Figure 11, monomer to polymer conversion vs.
time is compared for methyl, ethyl and n-butyl acrylates
at V-50 concentrations of 0.5 and 1.0% (with respect to
monomer). N-butyl acrylate polymerizations are the
fastest followed by those of ethyl acrylate and methyl
acrylate (the slowest) Polymerizations are faster and as a
consequence an increase in polymerization rate, as

Cationic Microemulsion Polymerization of Alkyl Acrylates 298
0 20406080100
0.0
0.2
0.4
0.6
0.8
1.0
MA(0.50 %)
MA(1.00 %)
EA(0.50 %)
EA(1.00 %)
BA(0.50 %)
BA (1.00 %)
% Conversion
Time (min)
Figure 11. Conversion vs. time for polymerizations initiated
with different concentrations of V-50 (with respect to mono-
mer) in microemulsions containing 4% monomer, 14.4%
DTAB and 81.6% water, carried out at 60°C. The mono-
mers were methyl, ethyl and n-butyl acrylates.
monomer water solubility decreases, is observed. Mono-
mer to polymer conversion plots are less pronounced as
the monomer becomes more water soluble, until a solu-
tion polymerization behavior is reached. In Figure 12,
polymerization rate vs. conversion for the three alkyl
acrylates are presented. The two microemulsion polym-
erization typical intervals are observed in these plots
(these behavior is more noticeable as monomer water
solubility decreases). Once again, a reaction rate decrease
as monomer water solubility increases is observed.
A rapid polymerization rate increase is observed as
initiator concentration increases, mainly due to free radi-
cal flow increase [11,14,19,33], which increases the col
lision probability between monomer swollen micelles
and the radicals (micellar nucleation). In the same way,
0.00.20.40.60.81.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
MA(0.50 %)
MA(1.00 %)
EA(0.50 %)
EA(1.00 %)
BA(0.50 %)
BA (1.00 %)
Rp
% Conversion
Figure 12. Reaction rates vs. conversion for polymerizations
initiated with different concentrations of V-50 (with respect
to monomer) in microemulsions containing 4% monomer,
14.4% DTAB and 81.6% water, carried out at 60°C. The
monomers were methyl, ethyl and n-butyl acrylates.
the probability of oligomer formation with water soluble
monomer, which is then stabilized by surfactant or pene-
trate in monomer swollen particles to form new particles
(homogeneous nucleation), increases. In Table 3, particle
size for the three monomers stays constant and maximum
polymerization rate increases with initiator concentration,
due to an increase in free radical flow.
Initiator type influence. In order to study the influence
of the type of initiator, cationic microemulsion polym-
erizations of methyl, ethyl and n-butyl acrylates in water
and dodecyl trimethyl ammonium bromide were carried
out. Besides V-50, water soluble potassium persulfate
which decomposes in anionic free radicals (as opposed to
V-50 which decomposes in cationic free radicals) and
2,2-azobisisobutyronitrile (AIBN), oil soluble initiator,
were used under the same conditions: 4% monomer,
14.4% DTAB and 81.6% water, 0.5% initiator (with re-
spect to monomer) and 60°C.
Behavior upon variation of initiator type is very simi-
lar for ethyl and n-butyl acrylates. Monomer to polymer
conversion vs. time plots show that polymerizations ini-
tiated with V-50 are faster than those initiated with AIBN,
and the latter faster than those with KPS. In Figure 13,
monomer to polymer conversion plots for n-butyl acry-
late with the three initiators are compared (trends are
similar for the three monomers). Final conversions for
the three initiators are similar.
In Figure 14, polymerization rates vs. conversion for n
butyl acrylate with the three initiators are compared (the
other monomers show similar trends). The two intervals
typical of emulsion polymerization are observed for all
initiators. Reaction rates are higher for V-50 than for
AIBN and for KPS, as the decomposition rates of these
initiators follow the same order Decomposition constant
for V-50 3.2 × 10-5 s-1 at 60°C [34], AIBN’s is 1.25 × 10-5
s-1 at 60°C [35] and KPS’s is 3.1 × 10-6 s-1 at 60°C [35],
which greatly influences reaction rates. Although, in the
Table 3. Particle size Dp, maximum polymerization rate,
Vpmax, mass average molar mass, Mw, for final latexes as a
function of V-50 concentration, Ci, (with respect to mono-
mer) in microemulsions containing 4% monomer, 14.4%
DTAB and 81.6% water, carried out at 60°C.
Ci D
p (nm) Vpmax M wx106 g/mol
n-butyl acrylate
0.5 26 0.296 1.12
1.0 28 0.570 -
Acrilato de etilo
0.5 32 0.060 0.948
1.0 35 0.137 -
Acrilato de metilo
0.5 53 0.011 1.05
1.0 59 0.023 -

Cationic Microemulsion Polymerization of Alkyl Acrylates 299
050100 150 200 250 300 350 400
0.0
0.2
0.4
0.6
0.8
1.0
KPS
AIBN
V-50
% Conversion
Time (min.)
Figure 13. Conversion vs. time for polymerizations initiated
with 0.5% KPS, AIBN and V-50 al (with respect to mono-
mer) in microemulsions containing 4% n-butyl acrylate,
14.4% DTAB and 81.6% water, carried out at 60°C.
0.0 0.2 0.4 0.6 0.8 1.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
KPS
AIBN
V-50
Rp
% Conversion
Figure 14. Reaction rate vs. conversion for polymerizations
initiated with 0.5% KPS, AIBN and V-50 (with respect to
monomer) in microemulsions containing 4% n-butyl acry-
late, 14.4% DTAB and 81.6% water, carried out at 60°C.
case of water soluble initiators, other factors such as
electrostatic interactions between positively charged mi-
croemulsion drops and free radicals generated in initiator
decomposition should be taken into account.
KPS decomposes in anionic free radicals which inter-
act strongly with cationic microemulsion drops then the
free radicals are attracted to the drop surface where they
are “trapped”, reducing free radical concentration in the
aqueous phase hence decreasing the initiator efficiency.
This effect is not observed with V-50. This behavior has
been detected in styrene [6] and methyl methacrylate [11]
microemulsion polymerizations. It is well known that
free radicals liberated by KPS, suffer reaction transfer
with bromide counter ions present in the surfactant
[23,36], which is not the case with V-50. For oil soluble
AIBN whose radicals are inside the microemulsions drop,
the possibility of self termination is much higher than for
water soluble initiators, a major factor which makes ini-
tiation more difficult and decreases AIBN initiated over-
all polymerization rates.
4. Conclusions
Monomer solubility has a very strong effect on conver-
sion and reaction rate, as it plays a very important role in
the initiation step of microemulsion polymerizations.
Polymerization rate increases as monomer water solubil-
ity decreases, independent of other factors such as tem-
perature, monomer initial concentration, and initiator
type and concentration. As monomer water solubility
becomes higher, monomer to polymer conversion graphs
change from the typical monomer to polymer conversion
curves for microemulsion polymerizations to a typical
solution polymerization curve. Final conversion and re-
action rates do not show an initial monomer concentra-
tion effect, which only becomes slightly apparent at con-
centrations smaller than 1%. Final conversion and reac-
tion rates as a function of temperature, type and initiator
concentration change as observed in other systems. Final
particle sizes are between 20 and 60 nm, and final molar
masses are of the order of 106 g/mol.
5. Acknowledgements
Financial support for this work from Ministerio de Cien-
cia y Tecnología is gratefully acknowledged. Dr. Jesús
Arellano acknowledges the award of a fellowship from
the ICI (Instituto de Cooperación Iberoamericana).
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