Potentials and Limitations of Automated Parallel Emulsion ...

320 Macromol. Rapid Commun. 2003, 24, 320–324
Communication: The application of automated parallel
synthesizer robots for the investigation of polymerization
processes is of major interest at present. In this contribution
we describe the application of the emulsion polymerization
of styrene and vinyl acetate. The preparations of emulsions
and latexes were investigated in detail and compared to
‘‘conventional’’ stirred tank reactors. In particular the influence
of the vortex mixing as well as the limitations
regarding solid content and reactor fouling are addressed.
Potentials and Limitations of Automated Parallel
Emulsion Polymerization a
Dirk-Jan Voorn, 1 Martin W. M. Fijten, 2 Jan Meuldijk, 1 Ulrich S. Schubert,* 2 Alex M. van Herk* 1
1 Laboratory of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
E-mail: a.m.v.herk@tue.nl
2 Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute
(DPI), P.O. Box 513, 5600 MB Eindhoven, The Netherlands
E-mail: u.s.schubert@tue.nl
Keywords: automated synthesis; emulsion polymerization; high-throughput experimentation; parallel chemistry; polystyrene;
poly(vinyl acetate)
Introduction
The development of combinatorial and high-throughput
methods has created a new tool for accelerated processes
within synthetic organic and pharmaceutical research in the
last decade. [1–5] In the beginning, combinatorial libraries
of short peptides have been build in vast quantities by
Houghton et al. [6,7] Recently, parallel synthesis has also
become a more common place within catalyst and material
research. [8] In addition, fast screening techniques have
been applied in the fields of catalysis [9–12] and polymer
research. [13] It is well-known that many parameters in
polymerization processes, such as monomer conversion,
catalyst, initiator and temperature have a significant in-
a
: Supporting information (including the Experimental Part) for
this article is available on the journal’s homepage under
www.mrc-journal.de or from the author.
fluence on both the polymerization process and the
products. The rapid parallel synthesis approach widens
the number of parameters that can be varied [14] and enables
the combination of screening and robotic synthesis to
develop novel materials and process conditions.
Several different kinds of polymerization reactions have
been successfully conducted recently by using automated
parallel synthesis: condensation polymerization, [15] suspension
polymerization, [16,17] ring-opening polymerization,
[18,19] conventional free-radical polymerization, [20]
and controlled radical polymerization (NMRP, [21]
ATRP [22–25] and RAFT [26] ). To the best of our knowledge,
application of automated parallel methods in emulsion
polymerization has not been reported so far. Emulsion
polymerization is a free-radical polymerization process,
which involves the emulsification of monomers in a
continuous aqueous phase and stabilization of the initial
droplets and final latex particles by a surfactant. Surfactants
Macromol. Rapid Commun. 2003, 24, No. 4 ß WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003 1022-1336/2003/0403–320$17.50þ.50/0

Potentials and Limitations of Automated Parallel Emulsion Polymerization 321
have a large influence on the latex product properties, e.g.
particle size distribution, molecular weight and rheological
properties. Traditionally, latex products can be prepared in
different types of reactors. [27] Stirred tank reactors are
generally preferred, in particular for semi-batch operations.
[28] Optimization of emulsion polymerization conditions
are often very time-consuming (e.g., type of surfactant
and concentration). In order to investigate potential
applications of combinatorial and high-throughput chemistry
in emulsion polymerization, automated emulsion
polymerizations in five parallel reactors utilizing welldefined
systems of styrene and vinyl acetate were chosen.
For the first model experiments described here the potential
of the robot system has not been fully explored and
characterization was restricted to classical methods, i.e. not
optimized for HTE purposes yet.
Results and Discussion
Parallel emulsion polymerizations of Sty and VAc were
performed in a Chemspeed ASW2000, comprising up to 80
reactors of 13 mL and allowing fully automated sampling.
For the present model study only five parallel reactions
were chosen in order to evaluate general applicability.
Therefore, also sampling, work-up and characterization
procedures have not been optimized, but classical methods
and techniques were used. Due to the many different variables
in emulsion polymerizations, automated fast screening
can provide a less time-consuming approach. First of
all, however, it must be clearly demonstrated whether the
automated synthesizer provides reproducible results that
are comparable with those of conventional experiments.
Stirring is a prerequisite to keep the monomer emulsified, at
least in stage 1 and 2 of an emulsion polymerization. The
quality of emulsification is directly related to the final latex
properties in terms of particle concentration, particle size
distribution and, to a lesser extent, molecular mass distribution.
Traditionally, emulsification is obtained by mechanical
agitation of two or more liquids present in the system.
In contrast to that, the automated synthesizer utilizes vortex
stirring (0 to 1400 rpm). In order to successfully apply the
robot, the two agitation methods have to produce more or
less identical emulsions and latex products. Emulsification
can be determined by means of visual observation of the
lowest impeller speed required for sufficient emulsification
N vis * , defined by Skelland and Seksaria. [29] Kemmere
et al. [30] empirically determined this stirring speed for Sty
and VAc batch emulsion polymerizations with conventional
pitched blade impellers. To the best of our knowledge,
however, nothing is published on emulsification by
means of vortex stirring. Therefore, emulsification using
the automated synthesizer has been visually inspected
utilizing CDX2 for coloring the organic phase. The results
revealed the existence of both a lower and an upper critical
vortexing speed. Application of baffles in the reaction
vessels did not provide a more efficient stirring; hence no
baffles were used in the polymerization reactions. Visual
experiments revealed an N vis * of approximately 360 rpm and
an upper limit of 400 rpm for the emulsification of Sty and
VAc in the 75 mL reactors. An increase in monomer weight
fraction for both VAc and Sty from 0.25 to 0.50 only slightly
influenced N vis * . In addition, different sizes of reaction
vessels were investigated. Decreasing the vessel volume
required higher vortexing speeds for complete emulsification.
For the 27 mL vessels, an N vis * of 550 rpm has been
found and 650 rpm for the 13 mL vessels. The emulsification
experiments also indicated differences of the lateral
position of the vessels. Apparently, emulsification is
significantly more efficient when the vessels are placed in
the center of the reaction block. However, further investigations
are required in order to explain this phenomenon.
Emulsion Polymerization of Styrene
In the next set of experiments, batch polymerizations
have been performed in a conventional stirred tank reactor
(400 mL) and 75 mL reaction vessels of the automated
synthesizer. The recipes used in this study are collected in
Table 1. Comparing the reaction courses revealed almost
identical conversion/time histories for both conventional
and automated systems (Figure 1). However, it should be
noted that an induction period of about 15 min was found
for the automated reactions. b At present we do not have an
explanation for the fact. In Figure 1, corrected reaction
times for the conventional experiment were used.
Corresponding particle sizes as a function of reaction
time are depicted in Figure 2. The particle size distributions
were measured with TEM and dynamic light scattering
(DLS). All the latex products revealed corresponding
particle sizes and distributions, indicating that in both
approaches comparable latex particles were produced
Table 1. Emulsion polymerization recipes for styrene and vinyl
acetate in weight percentages (SDS: sodium dodecyl sulfate, SPS:
sodium sulfate, SC: sodium carbonate, NDM: dodecyl
mercaptane).
Ingredient Recipe 1 Recipe 2 Recipe 3 Recipe 4
water 78.79 78.93 78.69 78.88
styrene 20.0 19.73 – –
vinyl acetate – – 20.09 19.73
SDS 1.17 0.71 1.18 0.71
SPS 0.02 0.25 0.02 0.31
SC 0.02 0.02 0.02 0.02
NDM – 0.36 – 0.35
stirring speed a) [rpm] 306 307 305 306
a) Stirring speed for impeller-mixed polymerization.
b Similar results were obtained from automated parallel ATRP
experiments in solution using the same robot system. [24,25]

322 D.-J. Voorn, M. W. M. Fijten, J. Meuldijk, U. S. Schubert, A. M. van Herk
Figure 1. Conversion/time history for ab-initio emulsion polymerization
with different styrene to water ratios in the conventional
and the automated synthesizer (AS) reactors. Conventional
(), AS366 25 wt.-% (}), AS366 30 wt.-% (*), AS366 35 wt.-%
(~), AS366 50 wt.-% (*).
(Figure 3). For both reactor types, modest changes in
emulsifier or initiator concentration resulted in particle
sizes within the experimental error.
The molecular weight of the final products for the
emulsion polymerization of 25 wt.-% Sty are provided in
Table 2. Experiments conducted without CTA resulted in
rather high molecular weights. The molecular weights
obtained for all polymers are identical within the experimental
error. Increasing the initiator concentration and
adding CTA (NDM) revealed that, apparently, the CTA is
less effective for the small reactors in comparison to the
conventional reactor. Moreover, a longer tail in the lowmolecular-weight
region was observed for the polymers
produced in the automated synthesizer, resulting in an
increase in polydispersity from approximately 2 to around
5. Lack of heat transfer or insufficient mixing resulting in
concentration or temperature gradients may cause the
resulting broad molecular weight distributions. [31]
Sequential emulsion polymerization reactions on a
27 mL and 13 mL scale were also conducted and workedup
simultaneously. A solid content of 25 wt.-% Sty resulted
in the formation of polySty latexes. Particle size
distributions obtained from conventional and parallel
polymerization are reported in Table 3, demonstrating the
reproducibility of the sizes with different vessel volumes.
Molecular weights of the latexes were slightly influenced
by decreasing the reactor size. The data obtained so far,
however, shows clearly the comparability of emulsion
polymerization carried out in 75 mL reactors and using the
conventional set-up.
Emulsion Polymerization of Vinyl Acetate
As a second monomer VAc was utilized (for polymerization
recipes, cf. Table 1). This monomer is more polar and hence
more water-soluble than any of the other common monomers
whose polymers are insoluble in water. The main
feature of VAc emulsion polymerizations is clearly demonstrated
in Figure 4, i.e., the reaction pathway for both
conventional and 75 mL reactors is the same. Particle sizes
for both reactions are identical. Increasing the monomer
concentration did not result in a drastic change in particle
size and molecular weight. It should also be noted that no
coagulum formation could be observed for polyVAc
synthesis. In Table 4 the characteristics of the emulsion
polymers are collected. The VAc experiments conducted
with the automated synthesizer showed an increase in the
low-molecular-weight fraction when CTA was added.
Figure 2. Ab-initio emulsion polymerization with different
styrene to water ratios: particle diameter as a function of time,
conventional (*), AS366 25 wt.-% (~), AS366 30 wt.-% ().
Fouling of the Reactor
Reactor fouling often occurs in emulsion polymerization, in
particular with high solid contents at higher conversion.
The gas/liquid interface can be a source of coagulum
formation. [32] With the conventional reactor set-up no
significant coagulum formation in the latex, on the Teflon
stirrer or on the reactor wall could be observed. Polymer
build-up within the 75 mL vessels was present at the top of
the reaction vessel. Decreasing the scale of the polymerization
(i.e. reactor size) and vortex mixing increased the
specific gas/liquid surface area, and, therefore, a larger
build-up of polymer was observed. Addition of surfactant
to the recipe in order to increase the stability of the
latex resulted in less polymer build-up. However, further
investigation is required for a more detailed understanding.

Potentials and Limitations of Automated Parallel Emulsion Polymerization 323
Figure 3. Transmission electron microscopy images of polystyrene emulsion polymers:
(a) conventional 25 wt.-%, (b) automated synthesizer (366 rpm) 25 wt.-%.
Table 2.
Molecular weight of the obtained polymer products.
Experiment N C CTA C I 10 2 M w 10 6 M n 10 6 M w /M n
rpm wt.-% kmol dm 3 g mol
Exp1 conventional 306 – 0.17 2.39 1.69 1.5
Exp2 automated synthesizer 366 – 0.15 2.26 1.14 1.9
Exp3 automated synthesizer 400 – 0.52 1.76 0.55 3.2
Exp4 conventional 307 1.08 0.73 0.38 0.09 4.18
Exp5 automated synthesizer 366 1.02 0.73 1.7 0.76 2.25
Exp6 automated synthesizer 366 1.80 1.3 0.72 0.15 4.6
1
g mol
1
Conclusions
Combinatorial materials research represents a promising
tool for product and process development. It was shown for
the first time that an automated parallel synthesizer can be
applied to emulsion polymerization utilizing industrially
relevant polymer recipes. The preparation of Sty and VAc
emulsions and latexes were investigated in detail concerning
the comparability of the results to conventional stirred
tank reactors. Visual emulsification experiments for the
automated synthesizer utilizing vortex mixing revealed that
the critical stirring speed has a lower and an upper limit.
Vortexing speed must be higher for decreased reaction
vessels sizes. Batch emulsion polymerizations of Sty and
VAc have successfully been carried out in a model study and
clearly illustrated that the automated approach represents a
very promising tool for emulsion polymerizations opening
new venues for fast and efficient research in this direction.
However, a thorough investigation of the polymerization
conditions for each recipe is required and the limitations for
each case have to be investigated; e.g., in the case of Sty
polymerization, only solid contents up to 30 wt.-% could be
handled in the automated synthesizer so far. For the present
preliminary study only five polymerizations were performed
in parallel, which can easily be expanded to 16 or 80
parallel reactors (75 mL or 13 mL reactors, respectively).
The determination of molecular weight and polydispersity
index can be accelerated by utilizing online GPC (see, e.g.
ref. [18] ), shorter columns or rapid GPC systems. [33] However,
at the moment, the investigation of average particle
size and particle size distribution including sample preparation
presents the real bottleneck in the HTE emulsion
Table 3. Particle-size distribution measured with TEM of polystyrene emulsion polymerizations (25 wt.-%) in the ‘‘conventional’’
0.4 dm 3 reactor and the automated synthesizer (75, 27 and 13 mL reaction vessels).
Experiment V D p,v < 10% 25% 50% 75% 90%
ml nm nm nm nm nm nm
Exp1 conventional 400 79 56 65 77 91 106
Exp2 automated synthesizer 75 77 55 64 76 89 102
Exp3 automated synthesizer 27 79 55 65 77 91 105
Exp4 automated synthesizer 13 81 57 67 79 93 108

324 D.-J. Voorn, M. W. M. Fijten, J. Meuldijk, U. S. Schubert, A. M. van Herk
Figure 4. Conversion/time history of vinyl acetate emulsion
polymerization for the conventional and automated synthesizer
polymerization (AS): conventional 25 wt.-% (~), AS366
25 wt.-% (*).
Table 4. Characteristics of poly(vinyl acetate) latexes produced
in conventional reactor and automated synthesizer.
Entry PVAc 1 PVAc 2 PVAc 1
þ CTA
PVAc 2
þ CTA
N i [rpm] 305 a) 366 b) 306 a) 366 b)
X final 0.97 0.96 0.97 0.96
M 0.25 0.25 0.45 0.45
CE [kmol dm 3 w ] 0.05 0.05 0.3 0.3
CI [kmol dm 3 w ] 0.005 0.005 0.01 0.01
D p,v [nm] 68 72 65 67
M w 10 6 [g mol
] 1.4 1.5 0.9 0.8
M n 10 6 [g mol
] 0.4 0.7 0.17 0.2
a) Stirring speed for turbine impeller.
b) Stirring speed of automated synthesizer.
approach. Different new routes, including parallelized
sample preparation, automated AFM [34] and particle size
analysis are currently explored.
Acknowledgement: The authors would like to thank the
Foundation Emulsion Polymerization (SEP), NWO and DPI for
financial support as well as Chemspeed Ltd. for the excellent
collaboration.
Received: January 22, 2003
Revised: February 19, 2003
Accepted: February 21, 2003
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