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Characteristic features of heterophase polymerisation of styrene with simultaneous formation
of surfactants at the interface
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2001 Russ. Chem. Rev. 70 791
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Russian Chemical Reviews 70 (9) 791 ± 800 (2001)
# 2001 Russian Academy of Sciences and Turpion Ltd
DOI 10.1070/RC2001v070n09ABEH000669
Characteristic features of heterophase polymerisation of styrene with
simultaneous formation of surfactants at the interface
N I Prokopov, I A Gritskova
Contents
I. Introduction
791
792
II. Surface properties of the potassium salts of carboxylic acids with alkyl substituents of different lengths and
the composition of styrene emulsions prepared during their formation at the interface
III. Bulk properties of the salts of carboxylic acids sparingly soluble or virtually insoluble in water and the composition
of styrene emulsions formed in parallel with their synthesis at the interface
IV. Synthesis of polystyrene suspensions with a narrow particle size distribution under the conditions of formation
of surfactants at the interface
794
794
799
V. Conclusion
Abstract. Data on the heterophase polymerisation of styrene
under conditions of surfactant formation at the monomer ±
water interface are generalised. A new, in principle, approach is
proposed the essence of which is to obtain a monomer emulsion
simultaneously with the synthesis of an emulsifier at the mono-
mer ± water interface and with initiation of the polymerisation in
the interfacial layer. The preparation of surfactants at the inter-
face allows one to control efficiently the degree of dispersion and
the stability of the emulsions formed. By varying the nature of the
acid and the metal counter-ion used in the surfactant synthesis at
the interface, it is possible to change the interfacial tension, to
influence the microemulsification, disintegration of the monomer,
and the formation of structure of interfacial adsorption layers.
The mechanism of formation of polymer-monomeric particles as
well as their diameter and size distribution depend substantially
on the solubility of the resulting surfactants in water. The
bibliography includes 47 references.
spontaneous microemulsification of the monomer at phase inter-
face has been observed;5, 6 polystyrene microbeads with a narrow
size distribution have been prepared in the absence of surfac-
tants;7, 8 the governing role of the composition of monomer
emulsions in the mechanism of formation of polymer-monomeric
particles (PMP) and its influence on the particle size distribution
have been established.9 ± 14
Detailed study of emulsions of monomers with various
natures Ð styrene, methyl methacrylate, chloroprene, isoprene,
mixtures of polar and non-polar monomers (for example, styrene
and methacrylic acid) Ðhas shown 15 ± 19 that the emulsion com-
position and the degree of dispersion can be controllable. For
example, the addition of water-insoluble surfactants to the mono-
mer phase of the emulsion or to the other phase gives rise to
emulsions consisting mainly of monomer microdroplets. Mean-
while, emulsions prepared in the presence of sodium alkylsulfo-
nate insoluble in the monomer consist of emulsifier micelles and
monomer macrodroplets. Thus, the procedure used to prepare the
initial emulsion of the monomer (or monomers) can pre-determine
the particle diameter in the polymer suspension and the size
distribution of particles.
I. Introduction
For several decades, researchers' assumptions concerning the
compositions of monomer emulsions and the patterns of emulsion
polymerisation have been based on the Harkins ± Yurzhenko
views; these also underlie the only existing quantitative theory
(the Smith ± Ewart theory).1 ± 4 However, numerous deviations of
experimental data from theoretical predictions have stipulated the
search for new approaches to the description of this process. These
deviations have been detected even in a study of styrene polymer-
isation initiated by potassium persulfate carried out in the
presence of an ionic emulsifier, which is exactly the reaction that
served initially as the basis for the Smith ± Ewart theory. The
problem of description of emulsion polymerisation became espe-
cially topical after the appearance of results which could not be
predicted by existing theories. Thus the occurrence of a quasi-
It has been proposed to perform heterophase polymerisation
of monomers with simultaneous synthesis of a surfactant at the
phase interface in order to control the emulsion composition. This
polymerisation method differs essentially from the commonly
accepted one,20 ± 26 as becomes obvious already when the initial
monomer emulsion is prepared.
Traditionally, a monomer and an aqueous solution of an
emulsifier are stirred at a rotational velocity of 600 ± 800 rpm until
a uniform emulsion is produced. The size of monomer droplets in
this emulsion depends on the rotational velocity, interfacial
tension (s1,2) and temperature. The size of monomer droplets
and the nature of the surfactant influence the emulsion stability.
Various stability factors act in the interfacial adsorption layer
(IAL). The surfactant concentration in the IAL is determined by
the surfactant capability of being adsorbed at the surface of
droplets. Since the surfactant concentration is usually higher
than the critical micelle concentration (CMC), it is generally
accepted that the initial emulsion consists of surfactant micelles,
which solubilise the monomer, and of monomer macrodroplets.
The method used to prepare the initial emulsion for polymer-
isation under conditions of surfactant synthesis at the interface
differs in principle from the above-described one. In this case, a
long-chain carboxylic acid is dissolved preliminarily in the mono-
N I Prokopov, I A Gritskova M V Lomonosov Moscow State Academy of
Fine Chemical Technology, prosp. Vernadskogo 86, 117571 Moscow,
Russian Federation. Fax (7-095) 434 87 11. Tel. (7-095) 434 86 44.
E-mail: fun-latex@mtu-net.ru (N I Prokopov).
Tel. (7-095) 247 04 43 (I A Gritskova).
Received 26 March 2001
Uspekhi Khimii 70 (9) 890 ± 900 (2001); translated by Z P Bobkova

792
N I Prokopov, I A Gritskova
mer phase, while an alkali is dissolved in the aqueous phase. This
procedure gives a fine emulsion of the monomer, owing to the
significant decrease in the interfacial tension caused by the fact
that the emulsifier formed in the neutralisation reaction is
accumulated in the interfacial layer. In this case, the interfacial
tension decreases much more significantly than in the case when
the same emulsifier is adsorbed on the surface of droplets from the
aqueous phase in the conventional preparation of the emulsion
(<1 and *10 mN m71, respectively).
The degree of dispersion of the resulting emulsion must also
depend on the nature of the emulsifier synthesised at the interface.
For example, if the emulsifier is soluble both in the monomer and
in water, its mass transfer through the interface takes place in
conformity with the partition coefficient, the stability of the
interfacial boundary is degraded, and the monomer undergoes
disintegration and microemulsification. In this case, apart from
monomer droplets, the resulting emulsion contains monomer
microdroplets and emulsifier micelles if the emulsifier content in
the aqueous phase is greater than that required for stabilisation of
monomer droplets and microdroplets.
4
3
2
1
1.2
1
1.0
0.8
0.6
0.4
0.2
2
0
t /min
0
20
10
Figure 1. Content of potassium oleate in water (1) and oleic acid in
styrene (2) vs. duration of neutralisation.
s
1,2 /mN m71
The composition of the monomer emulsion determines the
mechanism of formation and the diameter distribution of the
PMP. When the PMP are formed from emulsifier micelles and
monomer microdroplets, the polymer suspension contains par-
ticles of different sizes (with diameters from 0.06 to 0.4 mm) and is
characterised by a broad size distribution.
Polymerisation of an emulsion consisting of monomer micro-
droplets affords a polymer suspension with an average particle
diameter of 0.08 to 0.7 mm, depending on the structure and the
surface-active properties of the emulsifier synthesised at the inter-
face.
40
30
20
10
0
1
2
The particle size distribution (PSD) depends on the stability of
the PMP during the synthesis. If some factors providing stabilisa-
tion of the interfacial adsorption layer arise at initial stages of
monomer conversion, the PSD is narrow.
0.02
0.06
0.10
Emulsifier concentration /mol litre71
This review describes new approaches to the control of the
composition of monomer emulsions. These methods allow the
preparation of polymer suspensions with particles of different
diameters and with a narrow PSD.
Figure 2. Interfacial tension at the interfaces `aqueous solution of
potassium laurate ± styrene' (1) and `aqueous solution of potassium
hydroxide ± styrene solution of the lauric acid' (2) vs. emulsifier concen-
tration.
II. Surface properties of the potassium salts of
carboxylic acids with alkyl substituents of different
lengths and the composition of styrene emulsions
prepared during their formation at the interface
In several studies,20 ± 26 salts of oleic acid, salts of saturated fatty
acids with the C11 ± C25 hydrocarbon chains, and the potassium
soap from disproportionated rosin have been used as the objects
of investigation.
The interfacial tension s1,2 at the styrene ± water interface
decreases from 42.2 to 30.8 mN m71 over the homologous series
from lauric to cerotic acid. For none of the acids does the
interfacial tension depend on the time the surface has existed,
i.e., adsorption of the acid is not the limiting stage of the surfactant
formation. However, the interfacial tension sharply decreases
with an increase in the alkali concentration to reach
*1 mN m71 for an alkali content in water of 1072 mass %.
Figure 1 presents the data on the neutralisation kinetics of
oleic acid with potassium hydroxide at the styrene ± water inter-
face at a temperature of 20 8C. The data were gained by gravim-
etry (curve 1) and refractometry (curve 2). It can be seen that
neutralisation is almost completed over a period of several
minutes. After 5 ± 10 min, the concentration of potassium oleate
in water becomes *4 mass %.
When potassium laurate is synthesised at the interface, the
interfacial tension is less than 1 mN m71, whereas this value for
the interface between an aqueous solution of potassium laurate,
on the one hand, and styrene, on the other hand, is
s
1,2 = 8 mN m71
The isotherms of interfacial tension were used to calculate the
.
limiting adsorption value (Gmax), the area occupied by one
surfactant molecule in a saturated adsorption layer (Smin) and
the surface activity of the surfactant (G) (Table 1). It can be seen
that for the formation of emulsifiers at the interface, the G value
markedly increases, Gmax also increases, and Smin decreases.
The procedure of formation of emulsions should have a
pronounced influence on the formation and properties of the
interfacial adsorption layers on the surface of monomer drop-
lets.27 ± 30 Indeed, when the monomer is emulsified by an aqueous
solution of an emulsifier, the interfacial layer is formed by the
molecules of a highly ionised carboxylic acid salt adsorbed from
the aqueous phase, and when the emulsifier is synthesised at the
interface, this layer is formed initially by the adsorbed molecules
of the carboxylic acid and after neutralisation, it is formed by salt
molecules.
The instability of the phase interface caused by the chemical
synthesis of the emulsifier taking place in the boundary layer and
the emulsifier mass transfer to the aqueous phase, which are
accompanied by the transformation of the chemical reaction
energy directly into the surface energy,31 ± 33 result in effective
fragmentation of monomer droplets and monomer microemulsi-
fication. This influences, first of all, the average diameter of the
droplets of the monomer emulsion. If the emulsion is prepared
The substantial difference between the interfacial tension at
the interface between styrene solutions of lauric acid of various
concentrations and aqueous solutions of alkali in the synthesis of
potassium laurate and the interfacial tension for the adsorption of
the same emulsifier from the aqueous phase is illustrated in Fig. 2.

Characteristic features of heterophase polymerisation of styrene with simultaneous formation of surfactants at the interface
793
Table 1. Surface properties of emulsifiers at the interface.
X-Ray diffraction and electron-microscopic investiga-
tions 22, 23 of styrene emulsions prepared with the synthesis of
potassium carboxylates at the interface showed that they contain
particles of various sizes, namely, monomer macrodroplets with a
size of not more than 10 mm, monomer microdroplets
0.03 ± 0.04 mm in diameter, and emulsifier micelles with a size of
0.003 ± 0.005 mm. The surfactant concentration calculated for the
aqueous phase equals 2 mass % ± 5 mass %, which is
50 ± 100 times as high as the CMC.
If the concentration of the emulsifier formed at the interface is
decreased to the level sufficient only for stabilisation of monomer
microdroplets, the surfactant content in the aqueous phase would
be lower than the CMC and the formation of emulsifier micelles
would become impossible. In this case, the emulsion would mainly
consist of monomer microdroplets. This situation has been
realised in the preparation of styrene emulsions under conditions
of synthesis of potassium carboxylates at the interface. It can be
seen from the data presented in Table 2 that the emulsifier
concentration corresponding to the minimum s1,2 values in the
aqueous phase is several times lower than that corresponding to
complete neutralisation of the acid. This means that some of the
surfactant is consumed for the formation of the adsorption layer
on the surface of monomer droplets and microdroplets.
Emulsifier
The way 106Gmax
of intro-
ducing the
emulsifier a
Smin
G
/mol m72
/nm2 /mN m2 mol71
Potassium laurate
Potassium myristate
Potassium palmitate
Potassium oleate
A
B
A
B
A
B
A
B
4.0
6.4
3.6
8.4
4.8
10.3
4.0
6.4
2.0
13.4
2.9
9.6
0.41
0.26
0.45
0.22
0.35
0.16
0.41
0.26
0.81
0.12
0.58
0.18
18
84
30
67
44
82
27
135
4
Potassium behenoate A
B
21
12
33
Potassium cerotate
A
B
a A Ð the emulsifier is introduced in the aqueous phase, B Ð the emulsifier
is formed at the interface.
under conditions of synthesis of potassium laurate and the
potassium soap from disproportionated rosin at the interface,
the average diameter of monomer droplets is *2 times smaller
than that for the case of monomer emulsification by an aqueous
solution of the surfactant (10 and 20 mm, respectively).
Table 2. Variation of the emulsifier concentration during neutralisation of
the acid under static conditions (25 8C, overall time of reaction 24 h).
Emulsifier
Total emulsifier
concentration
/mass %
Emulsifier concentration
in the aqueous phase
at s1,2 min /mass %
Study of monomer microemulsification under static condi-
tions has shown 20 that the intensity of this process depends on the
method for the preparation of emulsions. In a styrene emulsion
prepared with the formation of the emulsifier at the interface, the
volume of the microemulsion layer is several times as great as that
at the interface between styrene and an aqueous solution of the
emulsifier. This can be clearly seen when considering the prepara-
tion of styrene emulsions by these methods in the presence of rosin
soap (Fig. 3). A large volume of the microemulsion increases the
emulsion stability. For example, styrene emulsions prepared with
the formation of ionogenic surfactants at the interface are
5 ± 10 times as stable as the emulsions formed upon emulsification
of the monomer by an aqueous solution of the emulsifier.
Potassium laurate
(CMC 0.22%)
0.15
0.25
0.50
4.00
0.15
0.25
0.50
4.00
0.08
0.10
0.21
1.00
0.02
0.04
0.12
1.50
Potassium oleate
(CMC 0.04%)
Procedures making use of small-angle X-ray diffraction and
electron microscopy of liquid ± liquid systems have been devel-
oped to study the composition of styrene emulsions. Using these
procedures, the degrees of dispersion of liquid ± liquid systems can
be studied without hardening the monomer phase.34, 35
The critical micelle concentration of the emulsifier in the
aqueous phase is attained only in the case where the acid
concentration in the monomer phase exceeds 0.5 mass % with
respect to styrene.36 As was to be expected, the lithium salts of
acids have lower concentrations in the aqueous phase than the
potassium salts.
Since all the salts synthesised at the interface are soluble in
water, effective mass transfer of these salts to the aqueous phase
takes place as well as quasi-spontaneous emulsification of the
monomer and the formation of microemulsions as a quite visible
white belt.
15
2
10
Under dynamic conditions, the CMC of the emulsifier in the
aqueous phase is attained when the acid concentration in the
monomer phase is >1 mass %. This is twice that in the synthesis
of surfactants under static conditions; this is explained by the
increase in the interfacial area owing to stirring and to the
consumption of the emulsifier for the stabilisation of monomer
droplets and microdroplets.
5
5
1
Thus, if the concentration of the emulsifiers (potassium
carboxylates) formed at the interface is 0.5 mass % relative to
the monomer, the initial emulsion of styrene contains only
monomer microdroplets because the rest of the emulsifier is
spent for their stabilisation. This means that, upon initiation of
polymerisation in these emulsions, the polymer-monomeric par-
ticles would be formed from monomer droplets and the polymer
suspensions would have a narrow particle size distribution.
0
1.0
2.0
3.0
t /days
Figure 3. Variation of the height of a microemulsion layer under static
conditions at the interfaces `styrene ± aqueous solutions of the rosin soap'
(1) and `styrene solution of rosin ± an equimolar aqueous solution of
KOH' (2). Emulsifier concentration is 1.161072 mol litre71, 25 8C.

794
N I Prokopov, I A Gritskova
Table 4. Size of particles in an emulsion of styrene prepared with
simultaneous synthesis of surfactants of various natures at the interface
(styrene : water volume ratio of the phases 1 : 2, surfactant concentration
in the aqueous phase 0.084 mol litre71).
III. Bulk properties of the salts of carboxylic acids
sparingly soluble or virtually insoluble in water and
the composition of styrene emulsions formed in
parallel with their synthesis at the interface
Surfactant
Method used to determine
the particle size a
Particle
size b /nm
The solubility of lithium salts of carboxylic acids in water exceeds
only slightly the CMC. Thus the solubilities of lithium laurate,
stearate and oleate at 50 8C are 0.28 mass %, 0.10 mass % and
0.13 mass %, while the CMC values are 0.25 mass % ±
0.30 mass %, 0.07 mass % and 0.12 mass %, respectively.37 The
solubilities of the barium, calcium and zinc salts of the same acids
in water do not exceed 0.004 ± 0.008 g per 100 g of water. The Li,
Ba, Ca and Zn salts are also virtually insoluble in the organic
phase, in particular, in styrene.37
Analysis of the interfacial tension isotherms shows that the
surface tension for the formation of lithium, barium, calcium and
zinc myristates at the styrene ± water interface is of the order of
21 ± 31 mN m71, whilst potassium myristates decrease the inter-
facial tension almost to zero (Fig. 4).
Potassium stearate
SAXD
SAXD
EM
3.5
34
36
Lithium stearate
Lithium laurate
LCS
EM
120
115
210
200
500
490
LCS
EM
Barium stearate
Zinc stearate
LCS
LCS
a SAXD is small-angle X-ray diffraction, EM is electron microscopy, LCS
is laser correlation spectroscopy; b The first line in the column implies the
size of surfactant micelles, while the other numbers are the sizes of
monomer microdroplets.
s
1,2 /mN m71
50
40
30
20
10
0
It has been shown by electron microscopy and laser correla-
tion spectroscopy that, when poorly water-soluble or virtually
water-insoluble surfactants, namely, lithium and barium stea-
rates, are employed, the emulsions formed during their synthesis
at the interface consist of macro- and microdroplets of the
monomer, the size of the latter ranging from 120 to 500 nm, and
no emulsifier micelles are present.
The data on the degree of dispersion of the styrene emulsions
prepared simultaneously with the synthesis of various surfactants
at the interface are summarised in Table 4. These data indicate
that the composition of a styrene emulsion depends substantially
on the method of its preparation and that it is much more intricate
than it has been considered in the classical works by Smith ±
Ewart, Yurzenko, and their pupils.3, 4
4
3
2
5
1
0.02
0.10
0.06
Emulsifier concentration /mol litre71
The investigations have shown that the reaction of surfactant
formation at an interface affects substantially the surface phe-
nomena in the interfacial adsorption layers and can serve as a
powerful factor for controlling the dispersivity of emulsions, and,
hence, the mechanism of formation of particles.
Figure 4. Interfacial tension at the interfaces `styrene solution of myristic
acid ± aqueous solution of potassium (1), barium (2), calcium (3), zinc (4),
or lithium (5) hydroxide' vs. emulsifier concentration. The time of droplet
formation is 20 s, 20 8C.
IV. Synthesis of polystyrene suspensions with a
narrow particle size distribution under the
conditions of formation of surfactants at the
interface
The surfactants insoluble in both phases exhibit a much lower
(by a factor of 10 ± 20) surface activities than water-soluble
potassium myristate (Table 3). Lithium myristate occupies an
intermediate position; its surface activity at the styrene ± water
interface is 3 ± 6 times higher than those of the barium, calcium,
and zinc salts but is *3 times lower than the activity of the
potassium salt.
Emulsions prepared under the conditions of formation of
potassium myristate at the interface are 2 ± 3 times as stable as
emulsions prepared in parallel with the synthesis of lithium,
barium, calcium and zinc salts (all other conditions being the
same), which is due to the formation of microemulsions at the
surface of monomer droplets.
Research into the synthesis of polymer dispersions with a specified
diameter of particles and a narrow particle size distribution has
shown that these properties are mainly dictated by the granulo-
metric composition of the initial emulsion of the monomer, the
time of formation of the PMP, and the creation of conditions
which ensure stability of emulsions in the interfacial layer.38, 39
These factors depend on many parameters, most of all, on the
conditions chosen for the preparation of emulsions and for
polymerisation.
The granulometric composition of the initial emulsion of the
monomer and, hence, the mechanism of PMP formation can be
readily controlled by selecting appropriate surfactants and an
appropriate way of introducing them into the system (either as
separate solutions in water or in the monomer or as solutions both
in water and in the monomer the concentration of which is
determined by the surfactant solubility in these phases) and by
the synthesis of surfactants at the interface.14, 18, 26 Any of these
methods gives monomer emulsions characterised by different
degrees of dispersity due to different concentrations of the
surfactant at the interface. Correspondingly, the interfacial ten-
sions also differ significantly.
Table 3. Surface properties of emulsifiers at the styrene ± water interface.
Emulsifier
G /mN m2 mol71
s
1,2 min /mN m71
Potassium myristate
Lithium myristate
Barium myristate
Calcium myristate
Zinc myristate
67.0
22.5
7.3
< 0.5
20.6
27.1
28.7
30.2
5.4
3.5

Characteristic features of heterophase polymerisation of styrene with simultaneous formation of surfactants at the interface
795
The time of formation of the PMP is a very important factor
which largely determines the diameter and the size distribution of
particles; this factor depends considerably on the efficiency of
initiation of polymerisation and on the process temperature.
Finally, the stability of the PMP during polymerisation is
ensured by the existence of factors of PMP stability acting in the
interfacial adsorption layers (electrostatic or structure-mechan-
ical factor or both). These factors also influence the diameter of
particles and their size distribution.
The duration of the stage of PMP formation can vary from
several minutes to several hours. It depends appreciably on the
granulometric composition of the emulsion and the efficiency of
initiation. At this stage, particles can coalesce due to instability
giving rise to a coagulum. Such a phenomenon has been observed,
for example, in suspension polymerisation of styrene in the
presence of poly(vinyl alcohol), starch, or gelatin as stabilisers. It
is at the stage of PMP formation that the diameter of particles of a
polymer suspension can be controlled by affecting their coales-
cence in one or another way. This process is extremely complex. In
order to eliminate the coagulum formation, surfactants are added
to the system. They, in turn, can induce the formation of new PMP
and extend markedly the size distribution of particles.
When styrene polymerisation is carried out in the presence of
organosilicon surfactants, insoluble in water and incompatible
with the resulting polymer, this stage occurs in a different
way.40, 41 In this case, a strong interfacial layer consisting of the
surfactant displaced by the polymer and of the polymer itself is
formed on the PMP surface at relatively low degrees of monomer
conversion. The diameter of particles in a polymer suspension can
be controlled by varying the nature of the surfactant and the
temperature.
Performing styrene polymerisation with simultaneous syn-
thesis of a surfactant at the interface ensures, on the one hand, the
possibility of realisation of either emulsion or suspension poly-
merisation mechanism giving rise to polymer suspensions in which
the diameter of particles can differ by an order of magnitude and,
on the other hand, it ensures stability of these suspensions because
the surfactant concentration in the interface is higher than that in
the case where the surfactant is adsorbed onto the PMP surface
from the aqueous phase. Thus polymerisation can be carried out
at lower concentrations of the surfactant and gives polymer
suspensions more stable during storage or modification (e.g., by
proteins).
PMP become a sort of microreactors in which polymerisation
progresses involving the monomer and the radicals present
therein. In this case, the rate of polymerisation and the PMP
diameter should not depend on the concentration of the initiator,
and an increase in the surfactant concentration would entail an
increase in the polymer molecular mass due to the decrease
(caused by diffusion restrictions) in the concentration of radicals
which enter the PMP at an initial stage of the process.
Thus, by conducting polymerisation of monomers with simul-
taneous synthesis of surfactants at the interface, one can obtain
polymer suspensions which differ not only in the particle diameter
and size distribution but also in the morphology of particles. For
example, in the case of synthesis of water-soluble sodium or
potassium carboxylates, polymeric suspensions with a broad size
distribution of particles should be produced because the initial
emulsion contains monomer microdroplets and emulsifier
micelles from which the PMP are formed.
Preparation of polymer suspensions with a narrow particle
size distribution by polymerisation under conditions of the syn-
thesis of water-soluble surfactants at the interface proved possible
only when the concentrations of the monomer (volume ratio of the
phases) and the surfactant were such that the surfactant being
formed was consumed only for stabilisation of the monomer
microdroplets and of the PMP formed from them, i.e., when
virtually no surfactant micelles were present in the aqueous phase.
The foregoing is illustrated by experimental data. Figure 5
shows the kinetic curves for the emulsion polymerisation of
styrene under conditions of formation of potassium salts of
carboxylic acids at the interface. In these experiments, aliphatic
acids with alkyl substituents from C11 (lauric acid) to C25 (behenic
acid) were used; the conditions of polymerisation were identical. It
can be seen that polymerisation occurs without any induction
period, which could have corresponded to the stage of formation
of polymer-monomeric particles, and the high rate of the process
is retained almost to the complete monomer conversion. The
reaction system is stable, and the final polymer suspension
contains no coagulum. The stability of particles in the suspension
is largely ensured by the action of the electrostatic factor of
stabilisation, as indicated by the high x-potential of particles
(Table 5).
It follows from the data given in Table 5 that the polymer-
isation rate wp somewhat increases as the alkyl radical of the
The morphology of particles prepared under conditions of
surfactant synthesis at the interface should differ essentially from
that resulting from conventional synthesis.
6
4
100
80
60
40
20
0
5
3
2
When emulsions are prepared under ordinary conditions, the
interfacial adsorption layer is formed upon adsorption of the
emulsifier from the aqueous phase first on monomer droplets and
then on the PMP. When polymerisation is carried out with
synthesis of an emulsifier at the interface, the initial monomer
emulsion as such does not actually exist. In this case, simulta-
neously with vigorous disintegration (microemulsification) of
monomer droplets, initiation of polymerisation takes place; the
resulting microdroplets capture radicals being thus converted into
the PMP. In parallel, the interfacial adsorption layers are formed
from the acid molecules dissolved in the monomer, the salt
molecules resulting from neutralisation of the acid, and polymer
molecules (polymerisation occurs in the area of the PMP adsorp-
tion layers). The preliminary dissolution of the acid in the
monomer ensures the optimal orientation of the acid and salt
molecules in the interfacial layer, which gives rise to the electro-
static factor of stabilisation. Due to the high polymer concen-
tration in the interfacial layer, the viscosity of the IAL
substantially increases and the structure-mechanical factor of
stabilisation also starts to be effective.
1
40
80
120
t /min
Figure 5. Yield of polymer vs. time of styrene polymerisation in the
presence of emulsifiers.
Emulsifier: (1) potassium laurate; (2) potassium myristate; (3) potassium
palmitate; (4) potassium stearate; (5) potassium cerotate; (6) potassium
behenoate. Polymerisation temperature 50 8C, monomer : water = 1 : 2
(by volume), concentration of the initiator (azobisisobutyronitrile) in
styrene 561072 mol litre71, emulsifier concentration in the aqueous
The synthesis of water-insoluble surfactants at the interface
should yield particles with the `core ± shell' type structure. The
formation of these particles is responsible for the specific features
of polymerisation kinetics. Even at early stages of the process, the
phase 8.461073 mol litre71
.

796
N I Prokopov, I A Gritskova
a
Table 5. Properties of polymers and polymer suspensions prepared in the
polymerisation of styrene with simultaneous formation of the potassium
salts of fatty acids at the interface.
Percentage
6
Surfactant
wp
(% min71
1076 MM
d /nm
n
x-Poten-
tial /mV
5
)
4
3
2
Potassium laurate 0.68
2.3
3.3
102
93
1.19
1.22
752
747
Potassium
myristate
Potassium
palmitate
0.73
0.83
3.6
90
1.20
749
Potassium stearate 0.88
Potassium cerotate 0.92
4.0
4.2
4.2
80
76
70
1.20
1.07
1.07
750
746
745
1
63
158
398
25
Potassium
behenoate
0.94
b
Percentage
Note. Here and in other tables: MM is the viscosity-average molecular
mass, d is the average particle diameter, n is the polydispersity coefficient.
48
36
24
12
surfactant becomes longer; this is due to a slight change in the
number of PMP formed.
The molecular mass (MM), equal to (2.5 ± 4.2)6106, and the
molecular-mass distribution (MMD) of the polymers correspond
to known published data on the emulsion polymerisation of
styrene initiated by an oil-soluble initiator, namely, azobis(isobu-
tyronitrile) 39 (Fig. 6). For low degrees of monomer conversion
(1% and 5%), the MMD curves exhibit two peaks, one at lower
and one at higher molecular masses (MM & 100 000 and
MM > 1 000 000, respectively). When the degree of monomer
conversion increases to 90%, the lower-molecular-mass peak
substantially decreases or even disappears. The presence of the
lower-molecular-mass peak is due to the fact that polymerisation
occurs both in the bulk PMP and in the emulsifier micelles.13, 39
As noted above, an increase in the length of the alkyl radical is
accompanied by a decrease in the solubility of its salts in the
aqueous phase. Whereas potassium laurate, myristate, palmitate,
and stearate are soluble in water fairly readily (30 mass % ±
63 158
Particle diameter /nm
398
25
Figure 7. Particle size distribution for a polystyrene suspension prepared
with simultaneous formation of potassium laurate (a) and potassium
behenoate (b) at the interface.
70 mass %), potassium cerotate and behenoate are much less
soluble (3 mass % ± 5 mass %).24 This should influence the mech-
anism of PMP formation. In the former case, the particles are
formed by a mixed mechanism Ð both from micelles of the
surfactant which solubilises the monomer and from the monomer
microdroplets. In the latter case, the particles are mainly formed
from monomer microdroplets. Indeed, polymer suspensions
formed under conditions of synthesis of potassium laurate at the
interface have very broad particle size distributions, whereas
suspensions prepared with the synthesis of potassium behenoate
are virtually monodisperse (Fig. 7).
If the emulsifier formed at the interface is poorly soluble in
water (for example, lithium salts of carboxylic acids), the initial
emulsion of styrene, as shown above, contains only microdroplets
of the monomer, the emulsifier being spent entirely for their
stabilisation.
Polymerisation of styrene under conditions of the synthesis of
lithium carboxylates has some distinctive features (Fig. 8), which
are due, first of all, to the fact that these salts are less soluble than
the potassium salts. Thus the solubility of lithium myristate in
water does not exceed 0.28 mass % and the surface activity of this
compound is lower. Therefore, when styrene polymerisation is
conjugated with the synthesis of lithium salts of fatty acids, the
mean diameter of particles of polymer suspensions is 1.5 ± 3 times
greater than that attained with potassium salts, all other factors
being the same (Table 6). The particle size distribution in the
`lithium' suspensions is fairly narrow because all the emulsifier is
consumed for the formation of the interfacial adsorption layer of
droplets, microdroplets and the polymer-monomeric particles
formed from them.
105qw
16
a
14
6
4
2
0
105qw
b
18
16
14
6
4
2
0
105qw
c
4
2
0
1
3
5 1076 MM
The rate of polymerisation and the molecular mass of the
polymer prepared in the presence of lithium myristate are lower
than those with lithium laurate because the number of PMP
produced in this case is smaller.
Figure 6. Molecular-mass distribution (qw) of polystyrene prepared by
polymerisation of the monomer under conditions of formation of potas-
sium laurate at the interface for degrees of conversion of (%): (a) 1; (b) 5,
(c) 90.

Characteristic features of heterophase polymerisation of styrene with simultaneous formation of surfactants at the interface
797
1
100
80
60
40
20
0
80
60
40
20
0
2
3
1
2
3
t /min
20
60
100
t /h
6
10
2
Figure 8. Yield of polymer vs. time of styrene polymerisation in the
presence of emulsifiers. Emulsifier: (1) lithium oleate; (2) lithium laurate;
(3) lithium stearate. The volume phase ratio monomer : water = 1 : 10,
initiator concentration in water 0.1 mass %, emulsifier concentration in
water 0.5 mass %.
Figure 9. Yield of polymer vs. time of styrene polymerisation under
conditions of formation of emulsifiers at the interface. Emulsifier: (1)
barium myristate; (2) calcium myristate, (3) zinc myristate. Here and in
Figure 10: polymerisation temperature 70 8C, monomer : water volume
phase ratio 1 : 10, potassium persulfate concentration in relation to the
monomer 1 mass %, emulsifier concentration in the aqueous phase
0.4 mass %.
Table 6. Characteristics of the particles of polymer suspensions prepared
while performing styrene polymerisation simultaneusly with the synthesis
of ionogenic surfactants at the interface (temperature 70 8C, styrene :
water phase ratio 1 : 10, initiator concentration 0.1 mass %).
of particles over diameters. This implies that, from the very
moment of formation of the polymer-monomeric particles, a
strong interfacial layer appears on their surface and protects the
particles from coalescence.42, 43
Surfactant
c
d /nm
n 0 (%)
(see b)
x-Poten- 1076 MM
tial /mV
(mass %) a
The kinetic curves for styrene polymerisation under the
conditions of synthesis of barium, calcium and zinc myristate at
the interface are presented in Fig. 9. The curves are all S-shaped,
which is typical of heterophase polymerisation; the formation of
Lithium laurate 0.25
Lithium stearate 0.50
Potassium laurate 0.25
258
220
193
65
5
6
4
6
735
734
748
750
1.2
1.3
3.8
4.2
polymer-monomeric particles occurs over
a
period of
Potassium
stearate
0.50
120 ± 240 min, and the time it takes to achieve the complete
conversion of styrene is 10 ± 12 h.
a Emulsifier concentration in the aqueous phase; b size variation coeffi-
cient.
It is of interest to compare these data with the results obtained
for styrene polymerisation under conditions in which the mono-
mer emulsions were formed via emulsification of the monomer
with aqueous suspensions of insoluble salts of myristic acid. The
resulting polymer suspension had low stability, and the average
particle diameter or the size distribution could not be determined
due to the high content of coagulum (more than 30%). Thus,
polymerisation with the synthesis of surfactant at the interface has
certain advantages pointing to good prospects of this method.
As was to be expected, the change in the concentration of the
insoluble salt synthesised at the interface (Table 8) barely influ-
ences the rate of polymerisation, the particle diameter and the
particle size distribution, which remains narrow over the whole
range studied. The increase in the polymer molecular mass with an
increase in the concentrations of the insoluble salts synthesised at
the interface is, apparently, due to the decrease in the concen-
tration of radicals entering the PMP from the aqueous phase,
caused by the increase in the thickness and viscosity of the
Attempts to synthesise polymer suspensions consisting of
larger particles with a narrow size distribution failed: either the
reaction system was unstable, resulting in a substantial amount of
the coagulum, or the particle size distribution was too broad.
Polymerisation of styrene accompanied by the formation of
barium, calcium and zinc salts of carboxylic acids taken in the
same concentration occurs in a different way. This was demon-
strated in relation to myristates. In this case, the diameter of
particles scarcely depends on the metal nature and equals
*800 nm (Table 7). The average diameter of particles also
remained nearly constant as the degree of monomer conversion
changed, and the polymer suspensions had a narrow distribution
Table 7. Polymerisation of styrene under simultaneous synthesis of
myristates at the interface.
Table 8. Effect of the emulsifier concentration (barium myristate) on the
rate of styrene polymerisation, the size of particles in the polymer
suspensions and on the molecular mass of the resulting polymer.
Surfactant
wp 102
/mol litre71 s71
1075 MM d /nm
x-Poten-
tial /mV
(see a)
Emulsifier concentration wp 102
1075 MM
d
Barium
5.5
5.4
6.7
6.6
6.2
700
800
850
729.5
725.7
723.2
in the aqueous phase
(mass %)
/mol litre71 s71
/nm
myristate
Calcium
myristate
0.8
5.5
5.5
5.3
5.2
4.9
4.8
4.7
7.0
6.7
6.4
5.6
4.4
2.0
1.5
695
700
720
780
820
850
920
Zinc myristate 4.9
0.4
Note. Here and in Tables 8, 9 and 11: polymerisation temperature 70 8C,
volume ratio of the phases 1 : 10, potassium persulfate concentration in
relation to the monomer 1 mass %, emulsifier concentration in the
aqueous phase 0.4 mass %.
0.1
0.05
0.025
0.0125
0.00625
a Stability of all the particles to electrolytes was 0.2 mol litre71
.

798
N I Prokopov, I A Gritskova
interfacial adsorption layer, resulting in the appearance of diffu-
sion restrictions.
100
80
60
40
20
0
The stability of PMP during polymerisation is ensured by the
structure-mechanical and electrostatic factors of stabilisation
acing in the interfacial layer. The x-potential of the particles in a
polymer suspension is 727 mV, which is much lower than that for
styrene polymerisation under the same conditions but without an
emulsifier (770 mV).44 The low x-potential can be accounted for
by diffusion restrictions arising due to a definite orientation of the
functional groups of polymer chains at the interface.
3
4
1
2
5
Thus, styrene polymerisation accompanied by the synthesis of
salts of myristic acid insoluble in both phases makes it possible to
prepare polymer suspensions with a narrow particle size distribu-
tion (the particle diameter is 600 ± 900 nm). The conditions of
synthesis of these polymer suspensions are technologically simple
and the reaction system remains stable during polymerisation.
Due to the low x-potentials, the particles of a polystyrene
suspension stabilised by barium, calcium and zinc myristates
prove unstable during immobilisation of bioligands in an isotonic
solution. An increase in the stability of polymer suspensions
without substantial change in the particle diameter and without
deterioration of the narrow size distribution of particles can be
attained by using the synthesis of mixtures of potassium and
barium myristates at the interface during polymerisation.45 When
the content of the water-soluble potassium salt in the salt mixture
is 6.25 mass % to 12.5 mass %, polymerisation gives rise to stable
polymer suspensions with a mean particle diameter of 580 ±
680 nm and a narrow particle size distribution (Table 9). An
increase in the stability is explained by an increase in the x-
potential of the particles to 740 mV.
t /h
2
6
10
Figure 10. Yield of polymer vs. time of styrene polymerisation in the
presence of emulsifiers. Emulsifier: (1) potassium myristate; (2) lithium
myristate, (3) barium myristate, (4) calcium myristate; (5) zinc myristate.
both processes. As an example, we shall consider polymerisation of
styrene accompanied by the synthesis of myristic acid salts at the
interface.
The kinetic curves for styrene polymerisation with the syn-
thesis of the myristates under discussion are presented in Fig. 10.
They all have an S-like shape, typical of heterophase polymer-
isation, and differ from one another in the duration of the non-
steady-state stage of the formation of polymer-monomeric par-
ticles, the rate of polymerisation, the range of degrees of con-
version in which the rate of the process is constant, and in the time
required for the full monomer conversion. For example, polymer-
isation of styrene accompanied by the synthesis of potassium
myristate starts almost immediately, while in the case of synthesis
of barium, calcium, and zinc salts insoluble both in water and in
the monomer, the formation of polymer-monomeric particles
requires 120 ± 240 min. The time periods required to attain
complete conversion of styrene are also different: in the former
case, polymerisation occurs over a period of 2.5 ± 3 h and in the
latter case, this takes 10 ± 12 h.
Table 9. Dependence of the rate of styrene polymerisation, the molecular
mass of the polymers and the average diameter of particles in a polymer
suspension on the ratio of emulsifiers, barium and potassium myristates
(the total concentration of emulsifiers in the aqueous phase was
0.4 mass %).
Emulsifier percentages
102 wp
/mol litre71 s71
1075 MM
d /nm
(see a)
barium
myristate
potassium
myristate
For clarity, we shall compare the processes taking place in the
PMP formed in the styrene polymerisation performed simulta-
neously with the synthesis of soluble and insoluble salts of
carboxylic acids (Table 10). The concentration of free radicals
during bulk polymerisation is known 46 to be usually about
1077 ± 1078 mol litre71, or 1012 ± 1013 radical ml71, i.e., the vol-
ume per radical is, on average, 561071 ± 561072 mm3. The
microdroplets formed under conditions of synthesis of soluble
surfactants at the interface have a volume of 1073 ± 1074 mm3,
which is much less than the volume per radical for bulk polymer-
isation. Therefore, each particle does not contain more than one
radical; only when the termination rates are very low (for example,
when radicals are fixed in the polymeric matrix of the interfacial
layers), can a microdroplet contain several radicals. If the concen-
tration of microdroplets in the system is 1014 ml71, the rate of
initiation is close to that for bulk polymerisation
50
50
14.4
12.7
8.5
25
100
130
280
580
680
62.5
75
37.5
25
19
14
87.5
93.75
12.5
6.25
5.8
8.5
7.8
5.5
a For particles with sizes of 100 and 130 nm, the stability to electrolytes was
0.15 mol litre71; for other particles, it was 0.20 mol itre71
.
It of interest to compare the properties of the polymer suspen-
sions prepared under conditions of surfactant formation at the
interface with the properties of those prepared by conventional
polymerisation (i.e., with emulsification of the monomer with an
aqueous solution of an emulsifier) as well as the kinetic features of
Table 10. Characteristics of styrene bulk polymerisation (I) and polymerisation in microdroplets of various sizes (II, III).
Characteristics
I
II a
III b
The number of particles
7
7
1014 ± 1015
0.5
0.561014 ± 0.561015
1073 ± 1074
1012 ± 1013
10 ± 100
1012 ± 1013
5 ± 10
0.561013 ± 0.561014
1071 ± 1073
1012 ± 1013
5 ± 10
The number of radicals per particle
The total number of radicals in 1 ml of the system
Volume of the monomer per radical /mm3
Rate of initiation /radical ml71 s71
Average lifetime of radicals /s
1012 ± 1013
1071 ± 1072
1012 ± 1013
1
a The size of microdroplets is 0.05 ± 0.1 mm (water-soluble surfactants); b the size of microdroplets is 0.4 ± 0.8 mm (water-insoluble surfactants).

Characteristic features of heterophase polymerisation of styrene with simultaneous formation of surfactants at the interface
799
(1077 ± 1078 mol litre71 s71
,
or 1012 ± 1013 radical ml71 s71),
The synthesis of emulsifiers at the interface makes it possible
to control the interfacial tension, disintegration and microemulsi-
fication of the monomer, to form the structure of interfacial
adsorption layers of surfactants by an appropriate selection of
the carboxylic acid and the counter-ion.
and the initiation efficiency is equal to unity, the lifetime of
radicals in these particles should be fairly long (10 ± 100 s). With
this lifetime, the molecular mass of the polymer is determined by
the rate of chain transfer via the monomer. In the case of styrene,
MM = 16106 ± 26106.
A different situation arises when styrene polymerisation
proceeds in the monomer microdroplets formed in the synthesis
of water-insoluble surfactants at the interface. Their volume is
now 1071 ± 1073 mm3; therefore, they can accommodate several
radicals (5 to 10), the lifetime of which is several seconds, and the
polymer molecular mass is up to 105.
These differences are consistent with the data on the diameter
of the polymer-monomeric particles in which polymerisation
proceeds (Table 11). Indeed, in the styrene polymerisation accom-
panied by synthesis of potassium myristate at the interface, the
average diameter of the PMP is 63 nm, while for the synthesis of
barium myristate under the same conditions, the diameter is an
order of magnitude greater and equals 700 nm.
The mechanism of formation of the polymer-monomeric
particles and their diameter and size distribution are dictated by
the solubility in water of the surfactant formed at the interface.
When polymerisation is carried out under conditions of synthesis
of a soluble surfactant at the interface, the polymer-monomeric
particles are produced from the emulsifier micelles and monomer
microdroplets. This ensures the formation of polymer suspensions
with a broad particle size distribution. Owing to the low concen-
tration of the resulting surfactant, which is, however, sufficient for
providing the stability of polymer-monomeric particles, micelles
are excluded from the formation of the PMP, and polymer
suspensions with narrow size distributions of particles are formed.
When surfactants poorly soluble or insoluble in water are
synthesised at the interface, the PMP are mainly produced from
monomer microdroplets, and a narrow size distribution of par-
ticles in the resulting suspensions is achieved.
Table 11. Polymerisation of styrene in the presence of myristates.
Surfactant
102 wp
/mol litre s71
1075 MM
d /nm
(see a)
x-Poten-
tial /mV
References
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50
65
752
735
729
726
723
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Calcium myristate
Zinc myristate
11.3
5.5
5.4
4.9
15
270
6.7
6.6
6.2
700
800
850
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47. N I Prokopov, Doctoral Thesis in Chemical Sciences, Moscow
State Academy of Fine Chemical Technology, Moscow, 1999
a Ð Dokl. Chem. (Engl. Transl.)
b Ð Polym. Sci. (Engl. Transl.)
c Ð Colloid J. (Engl. Transl.)
d Ð Russ. J. Phys. Chem. (Engl. Transl.)