Oil-in-Oil-Emulsions: Tailor-Made Amphipolar Emulsifiers - ACS

Chapter 6

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Oil-in-Oil-Emulsions: Tailor-Made Amphipolar Emulsifiers M. S. Hoffmann, R. Haschick, M. Klapper,* and K. Müllen Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. *E-mail: [email protected]

In order to obtain stable biphasic solvent mixtures such as emulsions and miniemulsions, distinct emulsifiers are required for the liquid/liquid interface. Until recently, nanoscale polymer particles have been mainly gained from waterborne heterophase techniques in radical processes, herein nonaqueous emulsions are presented. For these biphasic systems consisting of two aprotic organic nonmiscible solvents, high molecular weight amphipolar block copolymers were developed which show selective solubility in both phases and give additional stability by sterical shielding. Different emulsifiers, ranging from low molecular weight surfactats up to long chain block copolymers, were designed for the stabilization of hydrocarbon/perfluorocarbon mixtures and the metallocene-catalyzed synthesis of polyolefins therein. Furthermore, PI-b-PMMA emulsifiers were applied for mixtures of DMF dispersed in n-hexane. By these emulsions high molecular weight polyester and polyurethane particles were recieved, while further developments led to more sophisticated morphologies like core-shell structures.

Introduction Emulsions and Emulsion Polymerization Emulsion polymerization was first introduced in the beginning of the 20th century, and it became a well-established and widely utilized method in polymer synthesis within the last decades (1). This and other heterophase techniques © 2011 American Chemical Society In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


such as micro- and miniemulsion polymerization require appropriate emulsifiers which yield stable biphasic solvent mixtures and getting them compatibilized (2). While viscosity and thermal problems due to exothermic polymerization reactions can be significantly reduced in comparison to bulk polymerization, this technique simultaneously allows access to both high molecular weights and high reaction rates (3). Furthermore, because of the distinct control over the product morphology, i.e. size and shape of the obtained material, emulsion polymerization as well as miniemulsion and microemulsion has become the most common technique for industrial synthesis of organic nanoparticles. These methods allow the control of the resulting particle diameters from a few nanometers up to the micrometer range in addition to giving good processability over the obtained latex dispersions. Organic nanoparticles have gained tremendous interest in recent years and have found manifold applications, especially as nanofunctional materials due to their well-defined morphology, surface and size, resulting in tailor-made unique chemical and physical properties. The fields of applications are numerous, such as coatings (4), paints and pigments (5), but also in drug-delivery (6), diagnostics (7) and catalysis (8, 9). As the polymerization media are usually based on an aqueous continuous phase, the emulsions are very attractive both from the economical point of view due to low costs and availability of the solvent and also from the ecological point of view due to its environmental friendliness. Unfortunately, one can only benefit from this advantage when applying radical chain growth mechanism and using compounds which are not sensitive to an aqueous environment (3). Thus, classical waterborne emulsion and miniemulsion polymerizations do not allow the use of acid chlorides or isocyanates in step growth polymerizations or the application of moisture-sensitive catalysts, e.g. metallocenes for polyolefin synthesis. Side reactions might occur, resulting in hydrolysis of acid chlorides, formation of urea or decomposition of an active metal center. Therefore, due to the shift of the stoichiometric ratio or overall decomposition, only low molecular weights can be achieved often along with broad molecular weight distributions. Hence, nonaqueous systems needed to be developed which are suitable to carry out such sensitive polymerization methods. The most critical issue was the stabilization of two nonmiscible aprotic organic solvents which required new, optimized polymeric surfactants. Nonaqueous Emulsions Several examples of two nonmiscible organic solvent combinations – so-called oil-in-oil emulsions – were described in the literature based on alcohols etc. (10), but they do not solve the problem of side reactions with acidic protons which were mentioned above. Therefore, emulsions consisting of two nonmiscible and aprotic organic solvents needed to be designed. First examples of solvent mixtures consisting of DMF / n-hexane or cyclohexane / acetonitrile, resp., were reported by Riess et al. already in 1970 (11). These mixtures were stabilized by PS-b-PI and PS-b-PMMA block copolymers, resp., yielding droplet diameters not smaller than 1 µm. Stabilization of this kind of solvent mixtures – possessing a lower interfacial tension than aqueous systems – can hardly be 92 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


achieved with low molecular weight surfactants, but it is reported in the literature that this can be accomplished by high molecular weight copolymers (20,000 up to 100,000 g mol−1) (12, 13). These emulsifiers can be adapted precisely to the employed solvents according to the Hansen parameters; specific affinity and solubility in one of the phases can be achieved by tuning the monomer polarity, the segment lenth and the block length ratio (14, 15). Especially block copolymers turned out to be capable stabilizers for these heterogeneous systems (2). In contrast to low molecular weight tensides, the polymeric surfactants provide kinetic stability as the long chains are much more restricted in their mobility. Furthermore, the entanglement leads to “frozen” assemblies, whereby formation of the micellar core is the main enthalpic driving force (16). Above the critical micelle concentration (cmc), the molecules start to associate with the insoluble block in the core and the soluble block dissolved in the surrounding solvent. Usually the value of cmc is reported to be significantly lower than those needed for low molecular weight amphiphiles (17, 18). Additionally, block copolymers provide a much better sterical shielding against coalescence than low molecular weight emulsifiers because of their increased spatial demanding properties (19). The biphasic mixtures of aprotic solvents reported so far provide good emulsion stability whilst we continued this concept in order to perform highly sensitive polymerizations in the confined geometry of the dispersed droplets. A decisive issue within our development of new types of nonaqueous emulsions was the synthesis of suitable emulsifiers. Additionally, in order to gain control over the product morphology, these biphasic systems need to result in heterogeneous solvent mixtures with narrow size-distributed droplets in a tunable size range to obtain well defined and uniform nanoparticles.

Results and Discussion Perfluorinated Carbon / Hydrocarbon Emulsions – Polyolefin Synthesis For the synthesis of polyolefins, by far the most produced commodity plastics, metallocene catalysts have recently attracted great interest. They allow for the synthesis of previously inaccessible polymers and feature excellent activities, high stereoselectivities as well as high molecular weights together with low polydispersities (20). A major drawback of these catalysts is their high moisture sensitivity. Furthermore, the product morphology (size and shape of the obtained polymer particles) cannot be controlled properly when applied under homogeneous conditions. Additionally, reactor fouling is commonly observed due to local overheating of the resulting product. These problems can be overcome by supporting the catalyst on inorganic or organic supports, however, this demands additional preparation of the carrier material, e.g. thermal treatment to remove hydroxyl groups etc. (21) Alternatively, aqueous emulsions were proven to work for polyolefin synthesis (22). Though, due to the water sensitivity of the well-estbalished catalysts, only specially designed metal complexes can be used, often yielding only low activities. In order to circumvent the aforementioned drawbacks, we developed nonaqueous emulsions and successfully applied them for this demanding purpose 93 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


(23, 24). An apolar aprotic solvent was needed as the dispersed phase in order to provide sufficient solubility for both olefin monomers and metallocene/cocatalyst systems. A severe decrease in polymerization activity would be caused by the complexation of a polar solvent to the active metal species. In contrast to the first demand, a solvent which was immiscible with the first one and also chemically as far as possible inert was prerequisite. Therefore, oil-in-oil emulsions based on a perfluorinated solvent as the continuous phase and a non-coordinating hydrocarbon such as toluene as the dispersed phase, hosting the metallocene catalyst, were utilized for the polymerization of gaseous ethylene and propylene. In order to stabilize these novel emulsions, appropriate emulsifiers needed to be designed. First attempts via a low molecular weight emulsifier with a lipophilic and a fluorophilic moiety (Figure 1) did not provide sufficiently stable emulsions, probably due to the aforementioned reasons. In numerous examples high molecular weight emulsifiers provided good droplet stability due to enlarged specifically soluble moieties even in solvent mixtures with low polarity differences (15). Therefore, statistical copolymers based on poly(4-hydroxy styrene) with Mw = 20,000 g mol−1 were synthesized by two subsequent Williamson etherifications (Figure 2) (23). These amphiphilic structures, especially with RF = CH2(CF2)10CF3, provided a satisfying emulsion stability and, therefore, allowed the polymerization of gaseous ethylene and propylene, resp., in toluene droplets dispersed in perfluoromethylcyclohexane. The resulting droplet diameters showed the expected dependency on the emulsifier amount and could be decreased down to 90 nm. The fluorous oil-in-oil emulsions were exploited subsequently for the synthesis of polyolefin particles by metallocene catalysis from gaseous ethene or propene (Figure 3; Activity: 400 kg PP (mol Zr hr bar)−1 at 1 bar and 60 °C with MBI catalyst ([dimethylsilanediylbis(3,3′-(2-methylbenz[e]indenyl))]-zirconium dichloride)). After formation of the micelles by stirring and ultrasonication treatment, the hydrocarbon and the cocatalyst (methylaluminoxane MAO) were added and the emulsion was saturated with the gaseous monomer. Subsequent addition of the metallocene-based precatalyst initiated the polymerization. As the metallocene/MAO catalyst system is only barely soluble in the continuous phase, it is assumed that the reaction takes place in the confined geometry of the dispersed hydrocarbon droplets (“nanoreactors”). The nonaqueous fluorous emulsions gave high molecular weight polyolefin particles (e.g. Mn,PE = 450,000 g mol−1, PDI = 3.1), and the diameters of the resulting spheres (Figure 4) were controlled by parameters like polymerization time as well as pressure. A decrease of the particle diameters was accomplished by decreasing them, which is typical for a waterborne emulsion. Thus, these new emulsions allow polymerization under pseudo-homogeneous conditions. In order to overcome the diffusion limitation and to alter the conditions towards miniemulsion conditions, polymerization of liquid propylene in fluorous emulsions without any additional solvent as the dispersed phase was developed (24). Due to these more demanding conditions, stabilization of the emulsions needed to be improved by modification of the emulsifiers. High molecular weight block copolymer emulsifiers were synthesized in a nitroxide mediated controlled 94 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 1. Low molecular weight lipophilic/fluorophilic emulsifier.

Figure 2. Statistical high molecular weight lipophilic/fluorophilic emulsifier.

Figure 3. Schematic description of olefin polymerization in perfluorinated nonaqueous emulsions with MBI catalyst.

Figure 4. SEM micrograph of PE particles after 30 min at 60 °C and 40 bar reaction with MBI/MAO catalyst. 95 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


polymerization of styrene (S) and pentafluorostyrene (FS) (Figure 5). Two high molecular weight blocks were utilized which have a specific affinity to the hydrocarbon and the perfluorinated solvent, respectively. Increased affinity of the fluorinated block to the surrounding medium was accomplished by an additional perfluorinated alkyl chain in p-position. These structures turned out to be the most suitable for the stabilization of the oil-in-oil emulsions. Light scattering measurements indicated the formation of stable droplets (24), and the classical dependence on the amount of emulsifier was observed, while there was no change in droplet size over a broad temperature range – even close to the boiling point of the continuous phase. With the amounts of up to 10 wt.-% emulsifier concerning the dispersed phase droplet sizes were decreased to 50 nm. Switching completely to miniemulsion conditions resulted in a significant increase of the metallocene catalyzed polymerization of liquified propene (3500 kg PP (mol Zr hr)–1 at 25 bar and 60 °C with MBI catalyst; see Figure 3). With this new method, monomer diffusion through the continuous phase was avoided whilst additional organic solvents were omitted. Due to the surrounding perfluorinated solvent, heat transfer was promoted from this highly exothermic process, and therefore reactor fouling, which is caused by molten polymer under homogeneous conditions, was supressed. Besides pure olefin polymerization, these stable heterogeneous systems allowed the synthesis of core-shell structures. For example particles were obtained based on a core consisting of isotactic polypropylene with the shell of poly(n-butyl acrylate). Thus, the surrounding shell with its relatively low Tg showed good film forming properties of the stiff core material and provides an alternative to similar core-shell structures gained from polystyrene and poly(n-butyl acrylate). Polar/Nonpolar Nonaqueous Emulsions – Step Growth Polymerization In order to conduct polymerizations of polar monomers to form e.g. polyurethanes or polyesters, the aforementioned solvent mixtures could not be applied. In contrast, emulsions consisting of a polar dispersed phase and a nonpolar continuous phase were formed. Different solvent combinations were suitable for this approach; for example N,N-dimethyl formamide (DMF) dispersed in n-hexane or acetonitrile dispersed in cyclohexane. These mixtures were already used by Riess et al. (11) in order to prepare emulsions stabilized by copolymers of poly(styrene) (PS) and poly(isoprene) (PI) or poly(methyl methacrylate) (PMMA). On the basis of this concept new block copolymers consisting of PI and PMMA synthesized by anionic polymerization (25, 26) were developed within our group (Figure 6). These turned out to be most suitable to disperse a polar organic solvent as DMF in a nonpolar phase like n-hexane. The PMMA part of the copolymer is selectively soluble inside the polar dispersed phase and the PI sequence in the nonpolar sourrounding medium. The different polarities and, therefore, also the solubilities of the segments ensured good stabilization behavior in the two phases. Additionally, the investigation of the appropriate block lengths and the block length ratios played a crucial role in the specific designing of the surfactants (Table I). 96 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 5. High molecular weight lipophilic/fluorophilic PS-b-PFS block copolymer.

Figure 6. PI-b-PMMA block copolymer for polar/apolar oil-in-oil emulsions. Table I. Block lengths and block ratios as well as phase stabilization behavior of different block copolymers (27)

a

Polymer

PI-blocka Mn / g mol−1

PMMAblockb Mn / g mol−1

Mn/Mw

Ratio PI/PMMA

Hydrodyn. diameterc / nm

1

7,500

27,500

1.4

29:71

phase separation

2

3,000

4,000

1.3

52:48

phase separation

3

15,500

15,500

1.1

59:41

58

4

5,500

2,500

1.2

76:24

37

5

15,500

7,000

1.2

76:24

42

6

23,000

7,000

1.3

83:17

32

b

1H

by GPC in THF vs. PI standards; by ratio calculation via NMR in CDCl3 at 250 MHz and 298 K; c by DLS of 3 g MeCN and 0.5 g PI-b-PMMA in 24 g cyclohexane at scattering angle θ = 90 °.

Table I demonstrates that only copolymers with a PI fraction of at least 50% and a molecular weight of 8,000 g mol−1 resulted in stable emulsions. With respect to the droplet size and stability, the optimal ratio of the block copolymer consisted of approximately 75 mol-% PI and 25 mol-% PMMA. Concerning the molecular weight dependency, it is known that the stabilization efficiency of low molecular weight surfactants like SDS is unsatisfactory for emulsions with low polarity differences as it was already shown above for the perfluorinated systems. SDS molecules not only arrange on the droplet surface but are also soluble in the continuous phase and form micelles there. This results in a depletion of the surfactant from the particles destabilizing the emulsion. In contrast to that, high 97 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


molecular weight emulsifiers possess much lower dynamics and this kinetical effect leads to a better stabilization of the droplets. While the data concerning the emulsifier composition (Table I) were measured using a constant block copolymer concentration, Table II shows the effect on the particle size by changing the concentration of emulsifier in a constant mixture of 15 mL n-hexane and 1.5 mL of DMF. The droplet size decreased with a decreasing amount of emulsifier while at a mass concentration of approximately 5 mg copolymer per gram hexane, phase separation occured and no droplet formation was detected. Additionally, it was observed that at a concentration of about 25 mg PI-b-PMMA / 1 g n-hexane a plateau value was reached and the droplet size did not decrease any more due to a equilibrium between umulsifier whish is bound in a micelle and free one. In order to investigate the time dependency of the emulsion stability, the hydrodynamic radii of the droplets resulting of 0.2 g block copolymer (PI700-b-PMMA160), 0.5 g DMF in 12 g n-hexane were measured (Figure 7). Over a period of 3 hours the hydrodynamic radii remained almost constant in a size range of approximately 50 nm and no aging effects were determined. Obviously, the mixture was stable enough to keep its droplet size without stirring and sonication for a long time. Figure 8 shows the development of the droplet radii during a slow increase of temperature to 45 °C. Cooling the heated emulsion to room temperature led to an increase of the droplet radius to approximately 50 nm, which was in the size range of the droplets at the beginning of the measurement. Thus, raising the temperature caused a reversible decrease in droplet size and vice versa. A potential reason for this behavior might be the slight solubility of DMF in n-hexane which increased with rising temperature. It was assumed that the shrinking effect of the droplets was caused by a partial enrichment of DMF molecules of the dispersed phase in the continuous phase. This assumption was supported by 1H NMR experiments. It was proven that 1 mL of n-hexane solved at 20 °C approximately 0.02 mL of DMF, whereas at 40 °C the dissolved amount increased to 0.05 mL and according to the literature both phases mix at 68 °C (28). The synthesis of particles in these solvent mixtures can usually be considered as an emulsion polymerization based on a diffusion controlled process. Hitherto, there are also approaches available using compounds which are insoluble in the continuous phase (29). Thus, there is no diffusion control but only “nanovessels” are present (mini-emulsion process). In our standard procedure for particle formation by emulsion polymerization, the emulsifier (PI-b-PMMA) was dispersed homogeneously inside the continuous phase and, finally, micelle formation occurred (Figure 9). The addition of the polar phase containing compound A (e.g. a diol for the formation of polyurethanes or polyesters) and several minutes of ultrasonication led to a stable biphasic system. By dropwise addition of compound B (e.g. a diisocyanate for polyurethanes, a diacid dichloride for polyesters or a catalyst for ROMP) and allowing it to diffuse into the droplets, initiation of the the reaction took place. Since component A is barely soluble in the continuous phase, the reaction occured mainly inside the droplets. This was presented particularly for different types of reactions such as polyadditions, polycondensations and catalytic reactions (15). 98 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Table II. Effect of the emulsifier concentration on the droplet size of mixtures of 15 mL n-hexane and 1.5 mL DMF


PI-b-PMMAb m / ga

ω10−2 / wt%

Droplet diam.c / nm

0.40

3.97

40 (±14)

0.29

2.88

41 (±19)

0.21

2.11

56 (±28)

0.15

1.49

79 (±34)

0.10

0.99

98 (±43)

0.05

0.50

phase separation

0.20

phase separation

0.02 a

b

amount of emulsifier; weight portion of emulsifier considering Hexane; a scattering angle θ = 90 °.

c

by DLS at

Figure 7. Time dependency of the droplet radii in a mixture of 12 g n-hexan, 0,5 g DMF and 0,2 g PI700-b-PMMA160. The use of this nonaqueous emulsions opened access to versatile polymerization procedures and, therefore, different materials for the preparation of nanoparticles (9, 15). One important example is the polyaddition reaction which, for example, was used to form polyurethane particles (30). In order to accomplish such a synthesis, a diol and a diisocyanate were chosen to be compound A and B. In most cases the polar phase was DMF dispersed in n-hexane. However, acetonitrile dispersed in cyclohexane or N-methylpyrrolidone in tetradecane were also used. It was possible to prepare polyurethane particles with molecular weights as high as Mn = 40,000 g mol−1 (PDI = 2.0) which equals – in regards to the Carothers equation – a conversion higher than 99%. It was assumed that two main factors were responsible for the high molecular weights and conversions: (i) the absence of water during the reaction, which diminished the amount of 99 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


side reactions of the isocyanates such as urea formation; (ii) Schotten-Baumann conditions, meaning that the 1:1 stoichiometry which is necessary for the polyaddition was achieved by the diffusion of the monomers through the interface of the continuous and the dispersed phase. As the diols were predominantly soluble in the dispersed droplets, the stoichiometry of the reaction was controlled by the diffusion of the diisocyanate component into the micelles. PU particles which were prepared by this method possessed a very narrow size distribution and a diameter of approximately 20 to 70 nm (Figure 10). The particles had a well defined spherical morphology and did not agglomerate (monomodal distribution according to DLS measurement). The slight aggregation visible in the SEM image was caused by the sample preparation during solvent evaporation. Besides the synthesis of particles consisting solely of non-porous PU, more complex morphologies such as core-shell or porous structures were achieved by these types of emulsions (31). For instance, the preparation of porous particles by the slow and controlled addition of a well defined amount of water was possible. Hereby, some of the isocyanate groups underwent side reactions with water, forming urea derivatives. Simultaneously, carbon dioxide was released forming pores inside the viscous polymeric material. These particles with diameters from several hundreds of nanometers up to several micrometers had pore sizes in the range from 30 to 500 nm. Also, experiments concerning the use of porous PU particles as a support for the metallocene polymerization were accomplished. Another possible morphology for these PU particles is the preparation of core-shell particles. It was possible to create a polymeric shell from different methacrylate monomers by free radical polymerization around the existing PU core. These particles were prepared with small diameters of about 100 to 300 nm and high molecular weights (Figure 11). Besides the preparation of core-shell particles via physical absorption of one polymer on the surface of another, the chemical attachment of surface functionalizing groups were investigated as well. Therefore, different model compounds were chosen in order to accomplish click type reactions in nonaqueous emulsions. Even though click reactions were performed in an aqueous environment, the possibility to use such a versatile tool in our system offers the possibility to attach manifold functional compounds to the polymer particles like dyes, polyethylene oxide chains or even DNA or peptides. Such a functionalization will result in water dispersible, fluorescent or biofunctional particles. In order to investigate this application, poly(propargyl methacrylate)-copoly(methyl methacrylate) (PgMA-co-PMMA) particles were prepared and converted with different azides (e.g. 1-amino-11-azido-3,6,9-trioxaundecane or 5-azidopentanoic acid). These azides were chosen due to their polarity which is necessary to ensure their diffusion into the dispersed phase. Depending on the amount of PgMA in the copolymer, the performed “click”-reaction gave a hardly soluble polymer which was most probably due to the attached side groups leading to strong H-bonds. However, a PgMA concentration of approximately 10 mol-% inside the PgMA-MMA-copolymer yielded a soluble polymer even after the polymer analogous “click”-reaction. Via 1H NMR (for soluble samples) and IR spectroscopy (for insoluble polymers) it was proven that the signal intensity 100 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 8. Temperature dependency of the droplet radii in a mixture of 12 g n-hexan, 0,5 g DMF and 0,2 g PI700-b-PMMA160.

Figure 9. Schematic description of the polymerization in nonaqueous emulsion. (see color insert)

Figure 10. left) SEM image of PU particles at 120 V drop casted on Si waver; right) DLS curve from diluted primary PU particle dispersion at 90 ° scattering angle. of the triple bonds was decreasing (1H NMR at 250 MHz and 298 K in THF-d8: 4.8 ppm (s, 2H); ATR-IR: 2100 cm–1) whereas a signal caused by the triazole ring appeared (1H NMR at 250 MHz and 298 K in THF-d8: 7.8 ppm (s, 1H)). 101 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 11. left) SEM image of porous PU particles at 750 V drop casted on Cu grid; right) TEM image of PU-core PMMA-shell particles at 200 kV.

Conclusion It is demonstrated that for new types of hetereogeneous mixtures such nonaqueous emulsions, the design of appropriate stabilizing agents is essential. Block-structured surfactants provide the best stability due to their long segments. It was crucial to optimize the distinct solubilities and sterical shielding while the polarities and block lengths were adjusted carefully to the applied biphasic solvent mixtures. The biphasic systems which are presented herein have been stabilized by PS-b-PFS and PI-b-PMMA block copolymers, respectively. Emulsions consisting of n-hexane / DMF and toluene / perfluoromethyl cyclohexane were obtained which allow for the polymerization of moisture sensitive monomers or catalysts within the confined geometry of the stable droplets. While mainly solvent mixtures have been previously applied, these nonaqueous systems allow for the application of highly sensitive metallocene catalysts for the olefin polymerization as only aprotic solvents are used. Also polycondensations and polyaddition reactions of water sensitive monomers are accessible, yielding nanoparticles of polyester and polyurethane nanoparticles. In all cases the decisive point was the design of high molecular weight surfactants capable to form stable biphasic systems.

Acknowledgments The authors acknowledge Thomas Wagner and Jürgen Thiel for the synthesis of the PI-b-PMMA copolymers, as well as Gunnar Glasser for the SEM measurements and Sandra Seywald for GPC analysis. Financial support of the Stiftung Stipendien-Fonds des Verbandes der Chemischen Industrie e.V. and providing of the metallocene catalyst by LyondellBasell is also gratefully acknowledged.

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Oil-in-Oil-Emulsions: Tailor-Made Amphipolar Emulsifiers - ACS

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