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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 Polymerizationa 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:
[email protected] 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:
[email protected]
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 ina
: 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.
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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
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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.
vessels did not provide a more efficient stirring; hence no baffles were used in the polymerization reactions. Visual * of approximately 360 rpm and experiments revealed an Nvis 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 Nvis * . 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 Nvis * 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.
Results and Discussion
Emulsion Polymerization of Styrene
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 Nvis * , 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
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 styrene 20.0 vinyl acetate – SDS 1.17 SPS 0.02 SC 0.02 NDM – stirring speed a) [rpm] 306
78.93 19.73 – 0.71 0.25 0.02 0.36 307
78.69 – 20.09 1.18 0.02 0.02 – 305
78.88 – 19.73 0.71 0.31 0.02 0.35 306
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]
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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.
Fouling of the Reactor
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.-% ().
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.
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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
Exp1 Exp2 Exp3 Exp4 Exp5 Exp6
conventional automated synthesizer automated synthesizer conventional automated synthesizer automated synthesizer
CCTA
rpm
wt.-%
306 366 400 307 366 366
– – – 1.08 1.02 1.80
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
M w 106
CI 102 kmol dm
3
1
0.17 0.15 0.52 0.73 0.73 1.3
M n 106
M w/M n
1
g mol
g mol
2.39 2.26 1.76 0.38 1.7 0.72
1.69 1.14 0.55 0.09 0.76 0.15
1.5 1.9 3.2 4.18 2.25 4.6
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 dm3 reactor and the automated synthesizer (75, 27 and 13 mL reaction vessels). Experiment
Exp1 Exp2 Exp3 Exp4
conventional automated synthesizer automated synthesizer automated synthesizer
V
Dp,v
< 10%
25%
50%
75%
90%
ml
nm
nm
nm
nm
nm
nm
400 75 27 13
79 77 79 81
56 55 55 57
65 64 65 67
77 76 77 79
91 89 91 93
106 102 105 108
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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
Ni [rpm] 305 a) 366 b) Xfinal 0.97 0.96 M 0.25 0.25 CE [kmol dm3 0.05 0.05 w ] CI [kmol dm3 0.005 0.005 w ] 68 72 Dp,v [nm] M w 106 [g mol1] 1.4 1.5 M n 106 [g mol1] 0.4 0.7 a) b)
PVAc 1 þ CTA 306 a) 0.97 0.45 0.3 0.01 65 0.9 0.17
PVAc 2 þ CTA 366 b) 0.96 0.45 0.3 0.01 67 0.8 0.2
Stirring speed for turbine impeller. 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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