The invention relates to a microemulsion or microsuspension process for the production of a polymer dispersion. Moreover, it provides a polymer dispersion and polymer particles obtainable by the process.

The production of such polymer suspensions/dispersions (also called polymer latices) is of essential importance for the latex processing industry. Typical applications of polymer latices are e.g. the production of adhesives, binders for emulsion paints or the production of plastics and elastomers. Examples include poly(vinyl chloride) (PVC) and polystyrene (PS) and copolymers such as a styrene-acrylonitrile resin (SAN). Due to the constantly increasing demand for polymer latices, the proportion of rubber made from a synthetic polyisoprene latex, at around 15 Mt per year, is now larger than the amount obtained from natural latex deposits (approx. 12.5 Mt, as of 2016). The example clearly demonstrates that synthetic variants are becoming increasingly important as they are an independent latex source for natural latex. Even more important is the fact that the physicochemical properties of synthetic polymer dispersions can be specifically controlled by the monomer composition and the manufacturing process. This allows to gain control over the desired application properties, generally making polymer latex synthesis a superior technology regarding product design.

Various methods are known for producing synthetic polymer dispersions, for example solution polymerization, suspension polymerization and emulsion polymerization techniques (as well as the related microemulsion polymerization) to name the most important (E. Vivaldo-Lima, P. E. Wood, A. E. Hamielec and A. Penlidis, Ind. Eng. Chem. Res., 1997, 36, 939-965; A. M. van Herk, Ed., Chemistry and Technology of Emulsion Polymerisation, John Wiley & Sons, Oxford, 2nd edn., 2013; J. Ausa, J. Polym. Sci. Part A Polym. Chem., 2004, 42, 1025-1041 ). All of these methods have advantages and disadvantages. These relate in particular to the size control of the polymers produced (average polymer diameter, polydispersity), the conversion and the degree of contamination of the polymer by residues of monomers, auxiliaries or solvents. In solution polymerization processes, a control of the size of polymer particles is generally difficult to accomplish, especially without further addition of chain regulators. On the other hand, the polymerization in solution requires comparatively little effort for homogenization, and the solvent can be conveniently separated off in technical implementation.

In (micro-)suspension and (micro-)emulsion polymerization processes ( cf . J.S. Guo et al., Journal of Polymer Science Part A: Polymer Chemistry, 1989, 27, 691-710) the size distribution of polymers can be controlled significantly better. However, the addition of suspending agents and emulsifiers (surfactants) can have a negative influence on the properties of the obtained polymers. For example, surfactants have been shown to accelerate the release of small molecules (plasticizers) from a polymeric workpiece (W. Hou and J. Xu, Curr. Opin. Colloid Interface Sci., 2016, 25, 67-74; E. L. Teuten et al., Philosophical Transactions of the Royal Society B: Biological Sciences, 2009, 364, 2027-2045). Techniques for the separation of such substances are mostly inadequate and complex, since, for example, emulsifiers have a considerable foaming behavior in solution and adhere strongly to the polymer produced. In addition, a high level of energy is required for homogenization in two-phase systems such as suspensions and emulsions.

The surfactant-free emulsion polymerization, which is analogous to classic emulsion polymerization, uses hydrotropes instead of surfactants to control a polymerization in an emulsion. Macroscopic compartmentation of monomer- and water-rich phases is used (emulsion), which is kinetically stabilized by the hydrotrope to obtain an emulsion. However, similar to the conventional emulsion polymerization, this process requires a high level of energy and technical effort to homogenize and stabilize the two-phase system during the polymerization. Also, the size distribution is strongly dependent on the degree of dispersion of the emulsion and thus on the type of homogenization.

Raj et al. (W. R. Pa!ani Raj, M. Sasthav and H. Michael Cheung, Langmuir, 1991 , 7, 2586- 2591) describe the formation of surfactant free microemulsions of methyl methacrylate as a hydrophobic monomer together with acrylic acid as a hydrotropic monomer, and the polymerization thereof to provide porous solid materials and films. The authors found that these microemulsions exhibited similar characteristics as microemulsions formed using a surfactant (sodium dodecyl sulfate).

Yao et al. (Y. Yao et. al., Phys. Chem. Chem. Phys., 2019, 21, 10477-10487) describe a polymerization process wherein a surfactant free microemulsion (SFME) is formed using hydrotropic monomers and a hydrophobic substance. Following the polymerization of the hydrotropic monomers, the hydrophobic substance, which does not participate in the polymerization is enclosed in the resulting polymer.

Thus, a need remains for an approach in the production of polymer dispersion which combines the positive aspects of the various polymerization techniques available in the art.

To address this need, the present inventors have found that the characteristics of surfactant free microemulsions can be exploited in the production of polymer dispersions and polymer particles in line with the process defined in the annexed claims. Moreover, they have found that the process of the invention not only allows these polymer particles to be obtained in a convenient and efficient manner, but also with controlled particle characteristics and in a high purity.

Microemulsions which are formed without the use of traditional surfactants to stabilize the dispersed phase (known as surfactant free microemulsions, SFME) generally comprise water, a hydrotrope and a hydrophobic component (oil). Water and the hydrophobic component have a miscibility gap. Depending on the choice of the hydrotrope and the composition of the ternary mixture, mesoscopically structured solutions can be generated in which water-rich or oil-rich domains e.g. in the range from 1 to 100 nm are present (S. Sakai, S. Urano, H. Takatsuki, Waste Management, 2000, 20, 241-247; M. L. Klossek, D. Touraud, T. Zemb and W. Kunz, ChemPhysChem, 2012, 13, 4116-4119). In such SFME, different aggregate morphologies can be provided, similarly to surfactant-based microemulsions, i.e. oil-in-water, bicontinuous and water-in-oil (T. Lopian, S. Schbttl, S. Prevost, S. Pellet-Rostaing, D. Horinek, W. Kunz and T. Zemb, ACS Cent. Sci., 2016, 2, 467-475). The obtained microemulsions are thermodynamically stable, homogeneous and transparent (T. N. Zemb, M. Klossek, T. Lopian, J. Marcus, S. Schoettl, D. Horinek, S. F. Prevost, D. Touraud, O. Diat, S. Marcelja and W. Kunz, Proc. Natl. Acad. Sci., 2016, 113, 4260-4265). To stabilize the hydrophobic component in water, short-chain alcohols - to provide an example - were found to be suitable hydrotropes (T. Buchecker, S. Krickl, R. Winkler, I. Grillo, P. Bauduin, D. Touraud, A. Pfitzner and W. Kunz, Phys. Chem. Chem. Phys., 2017, 19, 1806-1816).

In the microemulsion polymerization of the present invention, the monomer takes on the role of the hydrophobic component. In this way, the advantages of solution polymerization and that of (micro)emulsion polymerization can be combined. A mesoscopically structured solution (microemulsion) is used which may be formed, in addition to the monomer, from easily separable solvents such as water and alcohols. Traditional surfactants, which are difficult to remove from the polymer dispersion and from the polymer particles contained therein, do not need to be present. Nevertheless, a complex homogenization of the solution is not necessary, since the SFME are homogeneous and thermodynamically stable. This leads to a better reproducibility with comparatively little technical and energy expenditure (such as caused by complex stirring systems).

At the same time, it is possible to achieve size control on the polymer growth by compartmentation into monomer-rich and water-rich domains. Thus, polymer latices can be provided in a controlled manner. Control mechanisms can be implemented in the process of the invention e.g. through the composition of the mixture water/hydrotrope/monomer, the use of mixtures of hydrotropes, the choice of initiator and its concentration, or by adding chain regulators. Furthermore, the resulting polymers can be easily isolated since volatile solvents can be used to provide the microemulsion. This allows a high-purity polymer to be obtained. Moreover, as components of the SFME, substances can be used which can be recycled (e.g. by distillation), which are inexpensive, readily biodegradable and have a toxicity that is comparatively low compared to surfactants.

The following items provide a summary of the aspects of the invention and of preferred embodiments thereof.

1. A process for the preparation of a polymer dispersion, comprising

(a) a step of providing an oil-in-water microemulsion, which oil-in-water microemulsion comprises

(i) dispersed domains comprising one or more types of radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water,

(ii) water,

(iii) a hydrotrope which comprises a compound selected from compounds of the formula R ¹ -(0-R ² ) _m -0-R ³ or of the formula R ⁴ -OH, or a mixture of two or more of these compounds, wherein

R ¹ is a hydrocarbyl group which is free of any moiety with more than 6 carbon atoms attached to each other, m is 1 or 2,

R ² is, independently for each occurrence if m is more than 1 , selected from ethanediyl (-CH2-CH2-) and isopropanediyl (-CH(CH3)-CH2-),

R ³ is selected from hydrogen, methyl, ethyl, propyl, and -CH2-COOH, and R ⁴ is a C3 to C6 alkyl group, and

(iv) a radical initiator, and wherein, if the total amount of all radically polymerizable monomers in the oil-in-water microemulsion is 100 mol%, a molar ratio of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water is 75 mol% to 100 mol%, and a molar ratio of other radically polymerizable monomers is 0 mol% to 25 mol%, and the oil-in-water microemulsion is substantially free of any surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with more than 6 carbon atoms, cationic surfactants and anionic surfactants, and,

(b) a step of polymerizing the monomers comprised by the oil-in-water microemulsion to provide the polymer dispersion.

2. The process in accordance with item 1 , wherein the dispersed domains have a hydrodynamic diameter, as determined by dynamic light scattering (DLS), of not more than 100 nm, more preferably of not more than 50 nm, still more preferably of not more than 15 nm.

3. The process in accordance with item 1 or 2, wherein the microemulsion comprises an amount of 1 to 30 wt%, more preferably 1 to 20 wt%, still more preferably 1 to 15 wt%, and most preferably 5 to 15 wt%, based on the total weight of the microemulsion, of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water.

4. The process in accordance with any of items 1 to 3, wherein the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water are monomers which have a solubility in water at 20 °C of 80 g/l or less.

5. The process in accordance with item 4, wherein the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water are monomers which have a solubility in water at 20 °C of 0.5 to 80 g/l, more preferably 0.5 to 30 g/l, and still more preferably from 1 to 20 g/l. 6. The process in accordance with any of items 1 to 5, wherein the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water comprise one or more types of monomers selected from methyl methacrylate, ethyl acrylate, methyl acrylate, styrene, acrylonitrile, vinyl chloride, vinyl acetate, butadiene, butene, isoprene, methyl styrene, and 2-ethylhexylacrylate.

7. The process in accordance with any of items 1 to 6, wherein, if the total amount of all radically polymerizable monomers in the oil-in-water microemulsion is 100 mol%, a molar ratio of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water is 80 mol% to 100 mol%, more preferably 90 mol% to 100 mol%, still more preferably 95 mol% to 100 mol%, and is most preferably 100 mol%, and a molar ratio of other radically polymerizable monomers is 0 mol% to 20 mol%, more preferably 0 mol% to 10 mol%, still more preferably 0 mol% to 5 mol%, and is most preferably 0 mol%..

8. The process in accordance with any of items 1 to 7, wherein the oil-in-water microemulsion comprises one or more other radically polymerizable monomers in addition to the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water.

9. The process in accordance with any of items 1 to 8, wherein the microemulsion is free of surfactants.

10. The process in accordance with any of items 1 to 9, wherein the microemulsion comprises the hydrotrope in an amount of 15 to 55 wt%, more preferably 20 to 50 wt%, still more preferably 25 to 50 wt%, and most preferably 30 to 50 wt%, based on the total weight of the microemulsion.

11. The process in accordance with any of items 1 to 10, wherein the hydrotrope is free of a compound comprising a C-C double bond.

12. The process in accordance with any of items to 11 , wherein R ¹ is a C2-C6 alkyl group, more preferably a C3-C6 alkyl group, and still more preferably a C3-C5 alkyl group.

13. The process in accordance with any of items 1 to 12, wherein the hydrotrope comprises or consists of a compound of the formula R ⁴ -OH, wherein R ⁴ is a C3 to C6 alkyl group, more preferably a C3 to C5 alkyl group, or a mixture of two or more of these compounds. 14. The process in accordance with any of items 1 to 13, wherein the logP value of the hydrotrope ranges from ranges from -1 to +2, more preferably from -0.16 to +0.85.

15. The process in accordance with any of items 1 to 14, wherein the microemulsion comprises the water in an amount of 30 to 80 wt%, more preferably 40 to 70 wt%, and still more preferably 45 to 65 wt%, based on the total weight of the microemulsion.

16. The process in accordance with any of items 1 to 15, wherein the radical initiator is selected from a water-soluble and an oil-soluble initiator.

17. The process in accordance with any of items 1 to 16, wherein the radical initiator has a half-life i2 of 1 h or less at a temperature of 95 °C.

18. The process in accordance with any of items 1 to 17, wherein the radical initiator is selected from an azo initiator, an inorganic peroxodisulfate and an organic peroxide.

19. The process in accordance with item 18, wherein the radical initiator is selected from an azo initiator and an organic peroxide comprised in the microemulsion in an amount of 0.01 mol% to 0.50 mol%, more preferably 0.03 mol% to 0.10 mol%, based on the molar amount of radically polymerizable monomers, or wherein the radical initiator is an inorganic peroxodisulfate comprised in the microemulsion in an amount of 1 to 5 mol%, based on the molar amount of radically polymerizable monomers.

20. The process in accordance with any one of items 1 to 19, wherein the radical initiator is selected from azobis(isobutyronitrile) (AZBN), di-(4-tert-butylcyclohexyl) peroxydicarbonate (BCC), tert-butylperoxyneodecanoate (PND) and di-(2-ethylhexyl) peroxydicarbonate (EPC- S).

21. The process in accordance with any of items 1 to 20, wherein the polymerization of the monomers is carried out in a temperature range of 0 to 150°C, more preferably in a range of 20 to 120°C, still more preferably 20 to less than 100 °C, and most preferably 40 to less than 100°C.

22. The process in accordance with any of items 1 to 21, which further comprises a step (c) of recovering at least a part of the hydrotrope after the polymerization from the obtained polymer dispersion, and recycling the recovered hydrotrope for the preparation of a further oil- in-water microemulsion.

23. The process in accordance with any of items 1 to 22, wherein the polymer dispersion is a dispersion of polymer nanoparticles and/or microparticles.

24. The process in accordance with item 23, wherein the polymer particles are non- crosslinked polymer particles.

25. A process for the preparation of polymer particles, comprising the steps of preparing a polymer dispersion in accordance with the process of any of items 1 to 24, and isolating the polymer particles from the polymer dispersion.

26. The process according to item 25, wherein the polymer particles are isolated from the dispersion via evaporation of the liquid phase of the dispersion.

27. A polymer dispersion, which is obtainable by the process in accordance with any of items 1 to 24, and which is substantially free of a surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with more than 6 carbon atoms, cationic surfactants and anionic surfactants.

28. Polymer particles, which are obtainable by the process in accordance with item 25 or 26, and which are substantially free of a surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with more than 6 carbon atoms, cationic surfactants and anionic surfactants.

29. The polymer dispersion of item 27 or the polymer particles of item 28 which are substantially free of surfactants.

30. Use of the polymer dispersion or the polymer particles in accordance with any of items 27 to 29 for the preparation of a paint.

31. Use of the polymer dispersion or the polymer particles in accordance with any of items 27 to 29 for the preparation of a medical device or a coating of a medical device. 32. Use of the polymer dispersion or the polymer particles in accordance with any of items 27 to 29 for the preparation of an interior part of a car.

33. Use of the polymer particles in accordance with item 28 or 29 in a polymer powder composition for 3D printing, preferably for laser sintering.

It will be understood that the foregoing summary forms a part of the general description of the invention, and that the information on features of the invention provided in the following also applies for the above items.

The process for the preparation of a polymer dispersion in accordance with the present invention comprises, as a step (a), a step of providing an oil-in-water microemulsion, which microemulsion comprises

(i) dispersed domains comprising one or more types of radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water,

(ii) water,

(iii) a hydrotrope, and

(iv) a radical initiator.

The term “microemulsion” is used herein in line with its established meaning in the field of microemulsion polymerization, with the difference that the oil-in-water microemulsion is substantially free of a surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with 6 or more carbon atoms, cationic surfactants and anionic surfactants, i.e. a surfactant as it is conventionally used in microemulsion polymerization processes. Preferably, the oil-in-water microemulsion does not contain, i.e. is free of, a surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with 6 or more carbon atoms, cationic surfactants and anionic surfactants.

A microemulsion comprises dispersed domains which are sufficiently small such that the microemulsion is transparent. Since, unlike in a conventional emulsion, a dispersed phase is not visible in a microemulsion, microemulsions are frequently referred to as single phase systems and/or the dispersed domains are referred to as “pseudo-phase”. Unlike emulsions, such microemulsions are thermodynamically stable and consequently do not demix over time or cannot be demixed by centrifugation. As will be understood by the skilled reader, in the oil- in-water microemulsion provided in the process of the present invention, the dispersed domains are oil-rich domains. The dispersed domains of a microemulsion, also in the context of the present invention, generally have a hydrodynamic diameter of not more than 100 nm. The hydrodynamic diameter can be conveniently determined by dynamic light scattering (DLS). It represents a mean diameter of the dispersed domains, and the values indicated herein are provided by a 2 ^nd order cumulant analysis of the DLS data.

Preferably, the hydrodynamic diameter of the dispersed domains in the microemulsion is not more than 50 nm, and more preferably not more than 15 nm. Moreover, the hydrodynamic diameter is preferably not less than 1 nm.

The dispersed domains of the microemulsion comprise one more types of radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water (typically water at 20°C).

Due to their insolubility or limited solubility in water, the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water may alternatively be referred to as radically polymerizable hydrophobic monomers.

The expression “insoluble in water” refers to a solubility of the monomer in water (at 20°C) of less than 0.1 g/l. The miscibility gap with water is expressed by a limited solubility of the concerned monomer in water (at 20 °C), typically a solubility of 80 g/l or less. Thus, in the context of the present invention, the monomer which is insoluble in water or which has a miscibility gap with water can also be identified as a monomer which has a solubility in water (at 20°C) of 80 g/l or less, preferably 30 g/l or less, and more preferably 20 g/l or less.

On the other hand, it is preferred that the monomer is a monomer which has a solubility in water (at 20°C) of 0.5 g/l or more, more preferably of 1 g/l or more.

Radically polymerizable monomers are typically monomers comprising a C-C double bond.

It is particularly preferred that the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water comprise or consist of one or more types of monomers selected from methyl methacrylate, ethyl acrylate, methyl acrylate, styrene, acrylonitrile, vinyl chloride, vinyl acetate, butadiene, butene, isoprene, methyl styrene, and 2- ethylhexylacrylate. More preferred are one or more types of monomers selected from methyl methacrylate, ethyl acrylate, methyl acrylate, vinyl chloride and vinyl acetate.

Preferably, the oil-in-water microemulsion comprises an amount of 1 to 30 wt%, more preferably 1 to 20 wt%, still more preferably 1 to 15 wt%, and most preferably 5 to 15 wt%, based on the total weight of the microemulsion, of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water.

It will be understood that a single type of radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water may be comprised in the dispersed domains of the oil-in-water microemulsion, in which case the process of the present invention yields a homopolymer. It is also possible to use two or more, e.g. two or three types of radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water may be comprised in the dispersed domains of the oil-in-water microemulsion.

In addition to the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water, the oil-in-water microemulsion may comprise other radically polymerizable monomers. As will be understood by the skilled reader, these other monomers have a higher solubility in water than the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water. Due to their higher solubility in water, these other monomers may alternatively be referred to as radically polymerizable hydrophilic monomers. Typically, such other monomers would be monomers with a solubility in water (at 20°C) of more than 80 g/l. The oil-in-water microemulsion may comprise one or more types of such other radically polymerizable monomers.

However, if the oil-in-water microemulsion comprises other radically polymerizable monomers in addition to the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water, their ratio in the total amount of radically polymerizable monomers contained in the microemulsion is limited. Thus, if the total amount of all radically polymerizable monomers in the oil-in-water microemulsion is 100 mol%, the molar ratio of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water is 75 mol% to 100 mol%, preferably 80 mol% to 100 mol%, more preferably 90 mol% to 100 mol%, still more preferably 95 mol% to 100 mol%, based on a total molar amount of radically polymerizable monomers contained in the oil-in-water microemulsion. Most preferably, the monomers contained in the microemulsion consist of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water as discussed above, including preferred embodiments thereof.

In line with the above, the other radically polymerizable monomers in addition to the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water may be present as an optional component of the oil-in-water microemulsion in a molar ratio of 0 to 25 mol%, preferably 0 to 20 mol%, more preferably 0 to 10 mol%, and still more preferably 0 to 5 mol%, based on the total molar amount of radically polymerizable monomers in the oil-in-water microemulsion as 100 mol%. Most preferably, such other radically polymerizable monomers in addition to the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water are not contained in the microemulsion. It will be understood that the molar ratios of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water and of the optional other radically polymerizable monomers indicated above add up to 100 mol%. Preferably, the monomers contained in the oil-in-water microemulsion consist of radically polymerizable monomers.

In the process in accordance with the present invention, it is not necessary that the polymer contains neutralized (deprotonated) acid groups as ionic groups which may facilitate the formation of a dispersion. Therefore, it is preferred that either the oil-in-water microemulsion provided in the process in accordance with the invention does not comprise radically polymerizable monomers which contain an acid group as a functional group, or that, if the oil- in-water microemulsion comprises radically polymerizable monomers which contain an acid group as a functional group, the process in accordance with the invention does not comprise a step before or during the polymerization of the monomers wherein a base is added to at least partially neutralize the acid group. As will be understood by the skilled reader, prominent examples of radically polymerizable monomers containing an acid functional group are monomers containing a C-C-double bond and a carboxylic acid group, such as acrylic acid or methacrylic acid. As noted above, such monomers can be present in the oil-in-water microemulsion as other monomers in addition to the monomers which are insoluble in water or which have a miscibility gap with water.

The polymer particles contained in the polymer dispersion provided by the process in accordance with the present invention are typically non-crosslinked polymer particles. Thus, a crosslinking agent, e.g. a compound comprising more than one radically polymerizable group, is typically not contained in the oil-in-water microemulsion. As noted above, the oil-in-water microemulsion provided in the context of the process in accordance with the present invention is substantially free of, preferably does not contain, any surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with more than 6 carbon atoms, cationic surfactants and anionic surfactants. Such surfactants are typically used in microemulsion polymerization processes of the prior art, but can be dispensed with in the process of the present invention. More preferably, the microemulsion is substantially free of, still more preferably free of, any surfactant selected from the group consisting of nonionic surfactants comprising a moiety with more than 6 carbon atoms attached to each other, cationic surfactants and anionic surfactants. As will be understood, a moiety with more than 6 carbon atoms attached to each other may be a linear chain of more than 6 carbon atoms, a branched chain of more than 6 carbon atoms, a ring or a ring system having more than 6 carbon atoms, or a substituted ring or ring system having more than 6 carbon atoms provided by the ring/ring/system and the substituent(s) in combination. Still more preferably, the microemulsion is substantially free, most preferably free, of surfactants.

As will be appreciated by the skilled reader, a composition which is “substantially free” of a certain component does not contain amounts of the component which would materially affect the essential characteristics of the composition.

Moreover, polymerization processes may use protective colloids which prevent an agglomeration of dispersed domains/dispersed phases or of polymer particles provided by the polymerization. Examples of such protective colloids are water soluble polymers such as polyvinyl acetate, polyvinyl alcohol, polyvinyl pyrrolidone, cellulose ethers, polyacrylates, starch, pectin, alginate or gelatin, pektins or gelatin, or Pickering additives. Preferably, the microemulsion provided in the context of the process of the present invention is free of any protective colloid selected from polyvinyl acetate, polyvinyl alcohol, polyvinyl pyrrolidone, cellulose ethers, polyacrylates, starch, pectin, alginate, gelatin, pektins or gelatin, and Pickering additives, and more preferably it is free of any protective colloid.

The hydrotrope which is contained in the oil-in-water microemulsion provided in step (a) of the process in accordance with the invention comprises a compound selected from compounds of the formula R ¹ -(0-R ² ) _m -0-R ³ or of the formula R ⁴ -OH, or a mixture of two or more of these compounds, wherein

R ¹ is a hydrocarbyl group which is free of any moiety with more than 6 carbon atoms attached to each other, m is 1 or 2,

R ² is, independently for each occurrence if m is more than 1 , selected from ethanediyl (-CH2-CH2-) and isopropanediyl (-CH(CH ₃ )-CH2-),

R ³ is selected from hydrogen, methyl, ethyl, propyl, and -CH2-COOH, and R ⁴ is a C3 to C6 alkyl group.

The hydrotrope is preferably contained in the oil-in-water microemulsion in an amount of 15 to 55 wt%, more preferably 20 to 50 wt%, still more preferably 25 to 50 wt%, and most preferably 30 to 50 wt%, based on the total weight of the microemulsion.

The hydrotrope is generally a liquid at the temperature at which the polymerization is carried out (and at atmospheric pressure of 1013.25 hPa). A liquid hydrotrope may also be referred to as a hydrotropic solvent. Moreover, it will be understood that the hydrotrope will generally not react in the polymerization of the radically polymerizable monomers. Thus, the hydrotrope is preferably free of a compound comprising a C-C double bond. The hydrotrope is able to interact with the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water to allow the formation of the dispersed domains and thus of the oil- in-water microemulsion.

The hydrotrope used in the process of the present invention comprises a compound selected from compounds of the following formula (iiia) or of the following formula (iiib), or a mixture of two or more of these compounds:

R ¹ -(0-R ² )m-0-R ³ (iiia)

R ⁴ -OH (iiib), wherein

R ¹ is a hydrocarbyl group which is free of any moiety with more than 6 carbon atoms attached to each other, and is more preferably a C2 to C6 hydrocarbyl group, m is 1 or 2, preferably 1 ,

R ² is, independently for each occurrence if m is more than 1 , selected from ethanediyl (- CH2-CH2-) and isopropanediyl (-CH(CH3)-CH2-),

R ³ is selected from hydrogen, methyl, ethyl, propyl, and -CH2-COOH, and is preferably hydrogen, and

R ⁴ is a C3 to C6 alkyl group, preferably a C3 to C5 alkyl group. Still more preferably, R ¹ in the above formula is a C2-C6 alkyl group, even more preferably a C3-C6 alkyl group, and most preferably a C3-C5 alkyl group,

In line with the above, the hydrotrope which is present as component (iii) in the oil-in-water microemulsion may comprise one or more compounds of formula (iiia) and/or one or more compounds of formula (iiib). Preferably, it comprises one or more compounds of formula (iiib). The hydrotrope preferably comprises the compound selected from compounds of the following formula (iiia) or of the following formula (iiib), or the mixture of two or more of these compounds, in an amount of more than 50 wt%, based on the total amount of the hydrotrope. More preferably, the hydrotrope consists of a compound selected from compounds of the formula (iiia) or of the formula (iiib), or of a mixture of two or more of these compounds. Still more preferably, the hydrotrope consists of one or more compounds of formula (iiib).

Thus, in line with a particularly preferred embodiment, the hydrotrope comprises or, still more preferably, consists of an alcohol which is selected from alcohols of the formula R ⁴ -OH, wherein R ⁴ is a C3 to C6 alkyl group, more preferably a C3 to C5 alkyl group, or a mixture of two or more of these alcohols. Examples of such alcohols are propanol, butanol or pentanol.

As will be understood from the above, the oil-in-water microemulsion may contain a single compound which acts as a hydrotrope, or a mixture of two or more, such as two or three, of such compounds. For example, mixtures of compounds acting as hydrotropes with different degrees of hydrophilicity may be used to exert an influence on the growth of the polymer particles.

Preferably, the logP value of the hydrotrope ranges from -1 to +2, more preferably from -0.16 to +0.85. As will be understood by the skilled reader, the logP value as referred to in this context represents the logarithm to the base of 10 of the partition coefficient Pow of the hydrotrope between water and 1-octanoS. The partition coefficient is defined (in line with the commission regulation (EU) No. 260/2014 of January 24, 2014), as the ratio of the equilibrium concentrations of a test substance in 1-octanol saturated with water (Co) and water saturated with 1-octanol (Cw). It is generally determined at 25 °C. A suitable method for the determination of the partition coefficient, and thus of the logP value, is also indicated in the commission regulation (EU) No. 260/2014 of January 24, 2014. However, in line with the REACH regulations (Registration, Evaluation, Authorisation and Restriction of Chemicals), logP values are generally available from safety datasheets provided for commercially available chemicals. If the hydrotrope comprises two or more compounds, each of them preferably shows the logP values discussed above.

The oil-in-water microemulsion provided in the context of the process in accordance with the invention preferably comprises the water in an amount of 30 to 80 wt%, more preferably 40 to 70 wt%, and still more preferably 45 to 65 wt%, based on the total weight of the microemulsion.

Since the hydrotrope can contribute to the volume of the water phase of the oil-in-water emulsion, the water does not need to provide the largest volume ratio of the components forming the emulsion. In line with the skilled person’s understanding, the oil-in-water microemulsion comprises a continuous phase which comprises water, but the hydrotrope can be used in weight ratios exceeding those of the water contained in the oil-in-water emulsion. However, the formation of an oil-in-water emulsion can be conveniently accomplished provided that the weight percentage of the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water is lower than the weight percentage of the water, based on the total weight of the oil-in-water emulsion.

As the radical initiator, initiators known in the art can be used, and the choice thereof is not particularly limited. For example, the radical initiator can be a water-soluble or an oil soluble initiator.

Preferably, a thermal initiator is used, and more preferably a thermal initiator with a half-life ivz of 1 h or less at a temperature of 95 °C.

As exemplary types of a radical initiator, an initiator selected from an azo initiator, an inorganic peroxodisulfate and an organic peroxide can be mentioned. Preferably, the radical initiator is selected from azobis(isobutyronitrile) (AZBN), di-(4-tert-butylcyclohexyl) peroxydicarbonate (BCC), tert-butylperoxyneodecanoate (PND) and di-(2-ethylhexyl) peroxydicarbonate (EPC- S).

As will be appreciated by the skilled reader, the amount of radical initiator can be adjusted to control the size of the polymer particles in the polymer dispersion provided by the process in accordance with the present invention. An increased amount of radical initiator leads to a decrease in particle size. If the radical initiator is selected from an azo initiator and an organic peroxide, it is preferably comprised in the microemulsion in an amount of 0.01 mol% to 0.50 mol%, more preferably 0.03 mol% to 0.10 mol%, based on the molar amount of all radically polymerizable monomers contained in the microemulsion. If the radical initiator is an inorganic peroxodisulfate, it is preferably comprised in the microemulsion in an amount of 1 to 5 mol%, based on the molar amount of all radically polymerizable monomers contained in the microemulsion.

In addition to the components discussed above, i.e. the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water, the optional further radically polymerizable monomers, the water, the hydrotrope and the radical initiator, the oil- in-water microemulsion can contain further components, if desired, provided that the microemulsion of the process in accordance with the invention does not contain any surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with 6 or more carbon atoms, cationic surfactants and anionic surfactants. As an example of further components, chain regulators can be mentioned which allow a control of the molecular weight of the polymer. However, such chain regulators are not required, and are typically absent.

Preferably, the oil-in-water microemulsion is free of any components which do not act as reactants in the polymerization reaction of step (b) besides the water and the hydrotrope. As should be understood, the radical initiator is considered as a reactant in the polymerization reaction in this regard.

More preferably, the oil-in-water microemulsion essentially consists of, still more preferably consists of, the radically polymerizable monomers which are insoluble in water or which have a miscibility gap with water, the optional other radically polymerizable monomers, the water, the hydrotrope and the radical initiator as the only components in order to minimize the possible influence of further components on the quality of the obtained polymer particles. As will be appreciated by the skilled reader, a composition which “essentially consists” of certain components does not contain amounts of further components which would materially affect the essential characteristics of the composition.

The oil-in-water microemulsion can be conveniently prepared by mixing its components as referred to above.

After the oil-in-water microemulsion has been prepared in step (a), the monomers comprised by the oil-in-water microemulsion are polymerized in step (b) to provide the polymer dispersion. As will be understood by the skilled reader from the definition of the components of the oil-in- water microemulsion, the step (b) is a step wherein the radically polymerizable monomers contained in the microemulsion are polymerized.

The polymerization conditions can be appropriately adjusted, and are similar to those applied in conventional microemulsion processes using a surfactant. Preferably, the polymerization of the monomers is carried out in a temperature range of 0 to 150°C, more preferably in a range of 20 to 120°C, still more preferably 20 to less than 100 °C, and most preferably 40 to less than 100°C. As will be understood, the polymerization temperature can be suitable selected within these ranges and preferred ranges to remain below the boiling point of all the components of the microemulsion. Alternatively, a closed reaction vessel can be used in order to avoid the loss of any of the components of the microemulsion via evaporation.

Likewise, the polymerization reaction can be carried out in a closed vessel to allow the polymerization of monomers which are in a gaseous state at the polymerization temperature.

After the polymerization has been allowed to proceed over an appropriate period of time, a polymer dispersion, i.e. a suspension of polymer particles, is obtained. The polymer dispersion can be processed by removing or by exchanging all or a part of the solvents contained therein or by other purification methods known in the art. Moreover, the polymer particles can be isolated from the polymer dispersion.

Thus, the invention further encompasses a process for the preparation of polymer particles, which process comprises the steps of preparing a polymer dispersion in accordance with the process for the preparation of a polymer dispersion discussed above, and isolating the polymer particles from the dispersion.

As noted above, the polymer particles can be conveniently isolated from a polymer dispersion obtained by the process in accordance with the invention via evaporation of the liquid phase of the dispersion. Due to the substantial absence, preferably the absence, of surfactants from the dispersion which are conventionally used in microemulsion polymerization processes, i.e. surfactants selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with more than 6 carbon atoms, cationic surfactants and anionic surfactants, the evaporation directly provides polymer particles of high purity.

Advantageously, the process for producing a polymer dispersion in accordance with the invention may further comprise a step (c) of recovering at least a part of the hydrotrope after the polymerization from the polymer dispersion, and recycling the recovered hydrotrope for the preparation of a further oil-in-water microemuision. For example, the hydrotrope can be recovered via distillation from the polymer dispersion.

As noted above, the invention also provides, as further aspects, a polymer dispersion which is obtainable by the process in accordance with the present invention for the preparation of a polymer dispersion, and polymer particles which are obtainable by preparing a polymer dispersion by the process in accordance with the present invention, and isolating the polymer particles from the dispersion.

As will be understood from the above, the polymer dispersion and the polymer particles are likewise substantially free, preferably free, of any surfactant selected from the group consisting of nonionic surfactants comprising a linear alkyl chain with more than 6 carbon atoms, cationic surfactants and anionic surfactants. More preferably, the polymer dispersion and the polymer particles are substantially free, still more preferably free, of any surfactant selected from the group consisting of nonionic surfactants comprising a moiety with more than 6 carbon atoms attached to each other, cationic surfactants and anionic surfactants. Still more preferably, they are substantially free, most preferably free, of surfactant.

It is preferred in line with the above that the polymer dispersion and the polymer particles are free of protective colloids, chain regulators, and/or other auxiliaries.

The polymer dispersion provided by the process in accordance with the invention typically comprises polymer particles in the form of nanoparticles, microparticles, or a combination of both.

The particle characteristics of the polymer particles, such as size and molecular weight, can be conveniently controlled according to need, e.g. by adjusting the monomer concentration or the type or concentration of the polymer initiator, by adjusting the time for which the polymerization is carried out, or via the composition of the mixture water/hydro trope /monomer, the use of mixtures of hydrotropes or by adding chain regulators. Thus, for example, polymer particles with a weight average (Mw) of 10,000 to 3,500,000 and Mw/Mn-ratios of 1 to 5, as determined by gel permeation chromatography (GPC), can be conveniently prepared.

As noted above, the polymer particles are typically non-crosslinked polymer particles. The polymer dispersion and the polymer particles provided by the present invention can be favorably used in various applications due to the absence of impurities which cannot be avoided in dispersions/particles prepared in a conventional manner. For example, the polymer dispersion or the polymer particles can be used for the preparation of paints or other coatings, or for the preparation of plastic articles and elastomers, e.g. for interior parts of cars. Other relevant applications include the use of the polymer dispersion or the polymer particles for the preparation of a medical device or a coating of a medical device. Furthermore, polymer powder compositions can be provided by the present invention which are suitable for 3D printing applications, e.g. as polymer powder compositions for laser sintering. Also in such applications, the inherent advantages of the process in accordance with the invention can be exploited, e.g. that polymer particles with a controlled particle size can be conveniently prepared, and/or that the particle surfaces are substantially free of, or free of, additives, in particular surfactants, which may interfere with a laser sintering process. As examples of polymers for 3D printing applications, reference can be made to ABS (acrylonitrile-butadiene- styrene) or ASA (acrylonitrile-styrene-acrylate) copolymers.

Thus, in a related aspect, the invention further encompasses a paint, an interior part of a car, a medical device or a polymer powder composition for 3D printing comprising the polymer particles provided in accordance with the invention.

Examples

Chemicals

Methyl methacrylate (MMA, 99%, stabilized), vinyl acetate (VAC, 99%, stabilized), n- propanol (NPA, 99.5%), n-butanol (NBA, 99.5%), sodium sulphite (98%) and potassium peroxodisulphate (PS, for analysis) were obtained from Merck (Darmstadt, Germany) tert- Butyl alcohol (TBA, 99%) was purchased from Carl Roth (Karlsruhe, Germany). Tetrahydrofuran (THF, analytical reagent grade) was obtained from Fisher Scientific (Schwerte, Germany). Poly(methyl methacrylate) standards (ReadyCal-Kit PSS-mmkitr1) were bought from PSS Polymer Standards Service (Mainz, Germany). Azobisisobutyronitrile (AZDN, 98%), b/s-(4-ferf-butylcyclohexyl)peroxydicarbonate (BCC, 95%), bis-( 2- Ethylhexyl)peroxydicarbonate (EPC-S, 95%), and tert- butyl peroxyneodecanoate (PND, 95%) were a gift from PERGAN (Bocholt, Germany). 1,2-Dichloroethane (DCE, 99.8%) and vinyl chloride (VC, 99.9%, stabilized) were a gift from VESTOLIT (Marl, Germany).

Prior to use, MMA and Vac were distilled at 40 °C and 60 mbars (MMA), respectively 240 mbars (Vac), using a rotary evaporator in order to remove the stabilizer. All other chemicals were used without further purification. Aqueous solutions were prepared using deionized water with a resistivity of 18 MDcm.

Methods and techniques

Ternary phase diagrams

Phase diagrams were recorded using a dynamic and static process, according to Clausse et at. (H. L. Rosano, M. Clausse (Eds.) Surfactant science series, Vol. 24, Dekker, New York, 1987). For this purpose, binary mixtures (each 3 g) were prepared in screwable tubes of borosilicate glass. The third component was added gradually until a visible change in the phase behaviour occurred. Measurements were carried out at 25 °C, 45 °C, and 65 °C, and phase transition was determined by the naked eye. The weight fractions were calculated from the mass of the individual components derived from precise weight measurements.

Density measurements

Density measurements were performed with a temperature-controlled DMA5000M density meter from Anton Paar (Graz, Austria). The samples were injected directly into an oscillating loop and evaluated by the provided software. Viscosity measurements

Dynamic viscosity measurements were done using a temperature-controlled AMVn Automated Microviscosimeter from Anton Paar (Graz, Austria). The falling ball viscosimeter is equipped with a glass tube (1.6 mm diameter) and a ball of stainless steel (diameter 1.5 mm, density 7.7 g cm ³ ). The samples were filled directly into the glass tubes and checked for air bubbles. The falling time was measured at an angle of 30° and 70° for ten times each at a certain temperature (25 °C, respectively 45 °C ± 0.01 °C). The viscosity was calculated as a mean value of all 20 measurements. For each setup, an instrument constant was calculated using millipore water.

Dynamic light scattering (DLS) measurements

Dynamic light scattering (DLS) experiments were done using a temperature-controlled CGS-3 goniometer system from ALV (Langen, Germany), connected to an ALV-7004/FAST Multiple Tau digital correlator and a vertically polarised 22 mW HeNe laser (l = 632.8 nm). Measurements were performed at a scattering angle of 90°. Samples were filtered into dust- free cylindrical measurement cells (10 mm outer diameter) using a 0.2 pm PTFE membrane filter.

Data points for surfactant-free microemulsion (SFME) systems (before polymerization) were recorded for 300 s at 25 °C, respectively, 45 °C. DLS spectra were evaluated mainly qualitatively due to their correlation function and their lag time as described by Buchecker, Krickl et al. (T Buchecker, S. Krickl, R. Winkler, I. Grillo, P. Bauduin, D Touraud, A Pfitzner, W. Kunz, Physical chemistry chemical physics : PCCP 2017, 19, 1806). Additionally, hydrodynamic radii were calculated by 2nd order cumulant analysis, using ALV-7004 Correlator Software and assuming the viscosity of pure alcohol-water mixtures (without monomers) and the refractive index of the pure monomer.

Data points for the initial polymer growth were recorded for 30 s every minute at 45 °C ± 1 °C. The evaluation of the hydrodynamic radii was done as described above.

Polymerizations

The initiators were mixed with the monomer before obtaining the SFME. The reaction mixtures (sample volume 6 mL) were filtered into rolled rim bottles and closed with crimp caps. Oxygen was replaced by flushing the reaction mixture with nitrogen for one minute. For polymerizations followed by DLS, a modified DLS measurement cell with screw-cap and septum (sample volume 3 mL) was used instead of the rolled rim bottles. To stop the polymerizations after 7 h, the reaction containers were cooled down to 0 °C, water was added, and the obtained polymers were dried by a Buchner flask under reduced pressure. After 1-3 hours, polymerizations were stopped by evaporating the reaction mixture in a rotary evaporator (40 °C, 30 mbars).

The sample containers were put into a temperature-controlled bath (25 °C, 45 °C, respectively, 65 °C ± 0.2 °C) for thermal initiation (BCC, PND, EPC-S). For UV-activated initiation (AZDN), a home-built gadget with six UV-LED CUNA66A1B (l=365 nm, 900 mW) mounted on a water-cooled plate (LED-TECH.de, Moers, Germany) was used. Each vial was irradiated by exactly one UV-LED for 4 minutes. For redox initiation, potassium persulfate and sodium sulfite were mixed separately in water. After combining both solutions, the reaction started after 10 to 60 seconds. For UV- and redox-initiated polymerizations, the sample containers were kept in a temperature-controlled bath at 25 °C ± 0.2 °C after the initiation.

Gel permeation chromatography (GPC)

Gel permeation chromatography (GPC) was performed using a Viscotek 270-03 setup equipped with a GPC1000 column from Malvern (Malvern, United Kingdom). Dry polymers were diluted in THF (approx. 1 mg mL ¹ ) and filtered into 1.5 mL GC vials using 0.2 pm PTFE filters. An autosampler connected to the GPC setup injected 150 pL with a 1 :45 split ratio. The flow rate was set to 1.0 mL min ¹ , and the detection was done using the refractive index detector. The evaluation of the molar masses and their distributions were done by provided software, using a PMMA ReadyCal kit PSS-mmrkitr1 from PSS Polymer Standards Service (Mainz, Germany) as a reference.

Handling of vinyl chloride

As vinyl chloride (VC) is gaseous at room temperature and standard pressure, investigations are quite complex without special equipment. Thus, a thick-walled glass reactor with three brazen pressure connections was used. These connections were used for VC-inlet, nitrogen-inlet, and outlet. All chemicals but VC itself were filled into the reactor at the beginning. Then, the reactor was closed, and the VC was added via the inlet. The VC mass was estimated by measuring the volume by a scale at the glass-reactor and using the literature density. Temperature control was obtained by immersing the glass reactor into a water bath. UV- irradiation was done by the gadget described in section “Polymerizations”, with the difference of using all six UV-LED lamps for a time of 17 minutes. Example 1 : Composition and investigation of potential SFME systems using the monomer methyl methacrylate (MMA)

The SFME systems were investigated by recording ternary phase diagrams and by performing DLS measurements at selected points. Obtained phase diagrams are printed in Figure 1.

For all mixtures, 5 wt% (percent by weight) of monomer were used in order to obtain oil-in-water SFME preferably. The content of alcohol as exemplary hydrotrope was successively varied from 50 wt% to 30 wt% by steps of 10 wt%. In addition, for each alcohol system, 10 wt% and 15 wt% MMA were tested in order to increase the space-time-yield. The exact compositions are shown in table 1.

Figure 1 shows the ternary phase diagrams for (a) water/n-propyl alcohol (NPA)/MMA at 25 °C and 65 °C and (b) water/t-butyl alcohol (TBA)/MMA at 25 °C and 65 °C. All compositions are given in parts per weight. The phase boundaries between one-phasic (1 F, top of the phase diagrams) and two-phasic (2 F, bottom of the phase diagrams) region are marked as a continuous black line (25 °C) and dashed line (65 °C). Black squares represent the compositions of the SFME in which the polymerization of MMA was carried out. The exact compositions are shown in table 1.

Table 1 : Compositions of SFME systems with methyl methacrylate (MMA) which were used for polymerizations ‘mixture of 17 wt% n-Butanol in n-Propanol The tendency towards the formation of SFME was determined with the help of dynamic light scattering (DLS) measurements. The obtained correlation functions of the systems with MMA are plotted in Fig. 2-5. For a first quantitative analysis, it is assumed that the formed oil- rich aggregates are larger and less fluctuating, the longer the decay time and the closer the Y- intercept is to the value 1 (T. Buchecker et al., loc. cit).

By using 2 ^nd order cumulant analysis, hydrodynamic radii were calculated. The obtained results are drawn in table 2. The viscosities were measured for the pure water-alcohol mixtures without the monomer. As a refractive index, pure MMA was assumed.

Figure 2 shows the correlation functions for the compositions B30, B40, and B50 (c.f. table 1 ) at (a) 25 °C and (b) 45 °C. Figure 3 shows the correlation functions for the compositions (a) B30, B10, and B15 at 45 °C and (b) P30, P10, and P15 at 45 °C (c.f. table 1 ). Figure 4 shows the correlation functions for the compositions P30, P40, and P50 (c.f. table 1) at (a)

25 °C and (b) 45 °C. Figure 5 shows the correlation functions for the composition N30, N40, and N50 at 25 °C (c.f. table 1).

In general, the most pronounced SFMEs can be found near the phase boundary, and their expression decreases with increasing alcohol concentration within a system. For the systems with NPA and TBA, the SFME is weakened by increasing the MMA content from 5 wt% to 15 wt%. According to the above-mentioned criteria, the tendency to form SFME (based on the choice of alcohol) decreases slightly from TBA to NPA. Besides, an increase in temperature according to expectations leads to weaker SFME (lower Y-intercepts), except for the system B30. On the other hand, the calculated hydrodynamic radii are enlarged by increasing the temperature to 45 °C (see table 2). This can be explained by a higher Brownian motion of the aggregates in a less viscous environment due to the higher temperature. When comparing the different alcohols, the calculated hydrodynamic radii behave in the same order as expected above (TBA>NPA).

Table 2 Calculated hydrodynamic radii for the samples at 25 °C and 45 °C. Samples marked with were not measured.

Example 2: Initial hours of polymerization using MMA and different initiators

Selection of the hydrotrope, initiation using AZDN (UV-activated)

In the following, the influences of the initiators, the hydrotrope, and the composition of the SFME system on the polymerization kinetics of MMA are shown on a model system (initiator AZDN). Figure 6 shows the course of polymerization of MMA with different SFME compositions (see fig. 1 ) and different AZDN concentrations. As can be seen from the development of the hydrodynamic radii, the choice of alcohol as a hydrotrope can have a direct influence on the polymerization kinetics (see above all fig. 6b).

The best growth control in this system was found at 25 °C in the system with n- propanol. However, the influence of the selected alcohol is dependent on the AZDN concentration and thus the seed concentration, which corresponds to the number of chain start reactions (influence more pronounced for higher AZDN concentrations).

Figure 6 shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) on the systems containing 5 wt% MMA, 30 wt% of different alcohols (n-propanol (P30) and tert-butyl alcohol (B30)) and the UV-sensitive initiator AZDN in the concentrations (a) 0.04 mol% built radicals and (b) 0.4 mol% built radicals (referring to the monomer). All reactions were initiated by UV-LED-light (l = 365 nm) and an exposure time of 4 mins and 25 °C.

The previous reactions were carried out with a constant composition with 30 wt% alcohol and 65 wt% water. However, the water/aicohol ratio also has an influence on the formation of SFME, as explained above. For this reason, fig. 7 shows the different polymerization kinetics with an increased alcohol concentration and a fixed initiator concentration (0.04 mol% AZDN, 25 °C) using the model system water/M MA/N PA. The hydrodynamic radii increase significantly, with the NPA concentration being increased from 30 wt% to 40 wt% and 50 wt%. As the alcohol to water ratio increases, size control during polymer growth decreases.

Figure 7 shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) of systems containing 5 wt% MMA, 30, 40 or 50 wt% NPA and the UV-sensitive initiator AZDN in the concentration 0.04 mol% built radicals (referring to the monomer). All reactions were initiated by UV-LED-light (l = 365 nm) and an exposure time of 4 mins and 25 °C.

Composition of the SFME, Polymerization with BCC

The same result can be obtained when using TBA and thermal activation with 0.08 mol% BCC as initiator (see fig. 8). To show the differences more clearly, a molar mass determination was carried out by gel permeation chromatography (GPC) after a polymerization time of 2 h. The results confirm the relationships shown so far and the hypothesis of the postulated mechanism of action of SFME on polymerization.

Figure 8 (a) shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) of systems containing 5 wt% MMA, 30, 40 or 50 wt% TBA and the temperature-sensitive initiator BCC in the concentration 0.08 mol% of built radicals (referring to the monomer). The reaction temperature was 45 °C. Figure 8 (b) shows the gel permeation measurement of the same polymers after a reaction time of 2 hours.

The retention times of GPC were evaluated using PMMA standards of a known molar mass distribution. The value M _n corresponds to the number average of the molar mass, M _w to the mass average of the molar mass and M _p to the molar mass at the distribution peak. The M _w /M _n -ratio corresponds to the polydispersity of the distribution.

The average molecular weight can be influenced by choice of the initiator concentration in the case of surfactant-free microemulsion and micro-suspension polymerization as well as with other, common types of polymerization. Contrary to general observations with other types of polymerization, the measured hydrodynamic radii of the polymers in the DLS kinetics measurement increase with increasing BCC concentration (see fig. 9). In contrast, the results from the determination of the molar masses by GPC suggest that the average molar mass of the polymers can be reduced by increasing the initiator concentration. This deviation between the results obtained by DLS and GPC is possibly due to the significantly faster kinetics and the larger number of initially growing polymerizing chains, which can no longer be shielded from other monomers simultaneously. Thus, it is believed that the DLS polymerization kinetics does only show the size of aggregates containing built polymer chains and their hydration shell. One aggregate may contain more than one growing polymer chain, especially at higher initiator concentrations. This behaviour is similar in the example of the water/MMA/NPA system with the UV-sensitive initiator AZDN at 25 °C (see fig. 9) and in the example of the water/MMA/TBA system with the temperature-sensitive initiator BCC at 45 °C (see fig 10).

Figure 9 (a) shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) of systems containing 5 wt% MMA, 30 wt% TBA and the temperature-sensitive initiator BCC in different concentrations between 0.02 mol% and 0.4 mol% of built radicals (referring to the monomer). The reaction temperature was 45 °C. Figure 9 (b) shows the gel permeation measurement of the same polymers after a reaction time of 2 hours.

It is assumed that the results from GPC are more reliable, since a few larger polymers can have a strong influence on the result in DLS measurements (scattering intensity proportional to GH ⁶ ! ).

Figure 10 (a) shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) of systems containing 5 wt% MMA, 30 wt% NPA and the UV-sensitive initiator AZDN in different concentrations between 0.04 mo!% and 0.4 mol% of built radicals (referring to the monomer). All reactions were initiated by UV-LED-light (l = 365 nm) and an exposure time of 4 mins and 25 °C. Figure 10 (b) shows the gel permeation measurement of the same polymers after a reaction time of 2 hours.

Mixtures of different alcohols, polymerization with persulfate

In the model system containing water/MMA/NPA and the initiator persulfate (activated by sulfite) at 25 °C the use of n-butanol/NPA mixtures instead of pure NPA was tested. This alcohol mixture can further strengthen the stabilizing effect of SFME on the polymerization kinetics (see fig. 11). This is due to the generation of an even more pronounced SFME and can be varied continuously through the mixing ratio NPA to NBA without changing the total alcohol content. In this way, it is possible to reduce the polymer size by a factor of 10 (compare results P50).

Figure 11 shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) of systems containing 5 wt% MMA, (a) 30, 40 and 50 wt% NPA and (b) a comparison between 30 wt% NPA and 30 wt% of a mixture containing 17 wt% n-Butanol in NPA. In both cases the redox-activated initiator persulfate in a concentration of 4 mol%, activated by 4 mol% sulfite (referring to the monomer). All reactions were done at 25 °C.

Consequently, it was possible to confirm once again that the polymerization behaviour of MMA to PMMA can be controlled both by the composition of the SFME and by the choice and concentration of the initiator. By using SFME it is possible to reduce the average size of the polymers and thus to get a size control over the polymer growth.

Example 3: Polymerizations with methyl methacrylate - longer polymerization times

Impact of the hydrotrope, SFME composition and initiator concentration

Further investigations were made after a polymerization time of 7 hours using the temperature-sensitive initiator BCC. Afterwards, polymer sizes were measured by GPC. The obtained molar masses (see fig. 12-14) differ by the use of different alcohols. Molar masses obtained by NPA systems are slightly larger compared to those obtained by TBA systems. This behaviour is in-line with the stability of the SFME systems (TBA > NPA, see fig. 1-5).

Figure 12 shows the impact of the alcohol concentration using a fixed MMA content (5 wt%) and 30, 40, and 50 wt% of the alcohols (a) NPA and (b) TBA. The molar masses were obtained as a mean value of three individual polymerizations by GPC using 0.4 mol% BCC, 45 °C and a polymerization time of 7 hrs. The standard deviation is plotted as error bar.

The impact of the alcohol concentration (and thus the SFME composition) is printed in figure 12. For both systems, the molar masses decrease with increasing alcohol content (P30 > P40 > P50, B systems consequently). At a first look, this seems to be a discrepancy compared to the results found after a polymerization time of 2 hours (see figure 8 (b)). But with the molar masses, also the yield is decreased by increasing the alcohol content. Yields of more than 80 % were only found at the systems near to the phase border. This leads to the assumption that the SFME systems have an impact on the initiator decomposition and the lifetime of the radicals, as well. Less stabilized systems (higher alcohol contents) seem to lead to faster kinetics and thus larger polymers at the first few hours (see figure 8 (b)), but the reaction stops before the conversion is completed. Thus, systems with lower alcohol content (and thus a better-structured SFME) allow slower kinetics and higher conversion rates. The impact of initiator concentrations was tested with the concentrations 4, 0.4, and 0.04 mol% BCC using the systems P30 and B30. The results are printed in figure 13. As expected, lower concentrations lead to higher polymer weights. This confirms the results obtained after a polymerization time of 2 hours (see figure 9 (b)). For a potential application, the choice of the initiator concentration might be the best option to fine-tune the former chosen SFME system.

Figure 13 shows the impact of the initiator concentration using a fixed MMA (5 wt%) and alcohol content (30 wt%) using the alcohols (a) NPA and (b) TBA. The molar masses were obtained as a mean value of three individual polymerizations by GPC using 0.04, 0.4, and 4 mol% BCC, 45 °C and a polymerization time of 7 hrs. The standard deviation is plotted as error bar.

Additionally, SFME systems with 10 and 15 wt% of monomer were investigated. The exact compositions are described in table 1 and figure 1 (X30, X10, and X15 for X=P, B). For both alcohols, polymerizations were possible with up to 15 wt%, as well. Results are printed in figure 14. By GPC, it was shown that the mean molar mass decreases by increasing MMA content. The effect is probably partly due to increased alcohol concentration and also due to worse structured SFME systems (c.f. fig. 12 and table 1 for the exact compositions). However, yields are still much better compared to the samples X40 and X50.

Figure 1 shows the impact of the MMA concentration using different mixtures along the phase border (compositions X30, X10, and X15 for X=P or B, see table 1 ) for the alcohols (a) NPA and (b) TBA. The molar masses were obtained as a mean value of three individual polymerizations by GPC using 0.04, 0.4, and 4 mol% BCC, 45 °C and a polymerization time of 7 hrs. The standard deviation is plotted as error bar.

Example 4: Extension of the concept to further monomers: vinyl acetate (VAC) and vinvi chloride (VC)

In order to show that the method of polymerization in SFME can generally be used for hydrophobic, vinylic monomers, an analogous polymerization of the monomer vinyl acetate (VAC) was examined.

Figure 15 shows the phase diagrams for the systems (a) water/n-propanol/vinyl acetate at 25 °C and (b)water/tert-butyl alcohol/vinyl acetate at 25 °C and 65 °C. Those phase diagrams are very similar to the systems with MMA shown above. Besides, measurements of the samples using DLS showed comparable, but slightly weaker correlation functions (see fig. 16, for comparison with MMA, see fig. 2-5).

Figure 16 shows the correlation functions of the SFME system (a) water/VAC/NPA at 25 °C and (b) water/VAC/TBA at 45 °C.

The polymerization works with all tested initiators and SFME systems containing VAC. Fig. 17 shows an example of the DLS polymerization kinetics of the systems water/VAC/NPA and water VAC/TBA with AZDN at 25 °C and water/VAC/TBA with BCC at 45 °C. This results in polymers of the order of Mw = 25-85 kDa.

Figure 17 shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) of systems containing (a) 5 wt% VAC, 30 wt% NPA respectively TBA and the UV- sensitive initiator AZDN in the concentration 0.4 mol% built radicals (referring to the monomer). Both reactions were initiated by UV-LED-light (l = 365 nm) and an exposure time of 4 mins and 25 °C. (b) Polymerization kinetics of the system containing 5 wt% VAC, 30 wt% TBA and the temperature-sensitive initiator BCC at a reaction temperature of 45 °C.

Also, the industrially very important monomer vinyl chloride (VC) was investigated for its polymerization in SFME. Phase diagrams and light scattering experiments were carried out with its industrial precursor 1 ,2-dichloroethane (DCE), as VC is gaseous under standard conditions. In the experiments, it was shown in particular that hydrophobic components that do not contain oxygen can form SFME as well. In the present systems containing water/DCE/NPA and water/DCE/TBA (see fig. 18) slightly higher mass concentrations of alcohol were required (35, 45 and 55 wt% instead of 30, 40 and 50 wt%) to form surfactant- free microemulsions containing 5 wt% DCE. The measured autocorrelation functions and thus, the formation of SFMEs are more pronounced than those of the other systems measured so far (see fig. 19).

Figure 18 shows phase diagrams for the systems (a) water/n-propanol/1 ,2-dichloroethane at 25 °C and (b) water/tert-butyl alcohol/1 ,2-dichloroethane at 25 °C.

Figure 19 shows correlation functions of the SFME system (a) water/DCE/NPA at 25 °C and (b) water/DCE/TBA at 25 °C. In experiments with VC in the ternary system water/NPA/VC, it was possible to show that all the prerequisites for the phase behaviour of the ternary mixture for a successful polymerization of VC are already fulfilled.

To show the possibility of PVC synthesis in an SFME, two systems containing water/NPA/VC with AZDN at 25 °C and water/TBA/VC with BCC at 45 °C were tested to check the method. In the first polymerization experiments with VC and an initiator (AZDN in the water/NPA system, BCC in the water/TBA system) it was already possible to show that polymerizations were taking place. In the case of an initiator concentration of about 0.8 mol% after a polymerization time of 2 hours, the molecular masses were in a region of Mw = 50 kDa (in the case of BCC) and Mw = 31 kDa (in the case of AZDN).

Example 5 (comparative example): Polymerizations in the molecular dispersed system

MMA/EtOH/Water

Like done for the hydrotropes TBA and NPA, ternary phase diagrams and DLS measurements were performed for the hydrotrope ethanol. With the hydrotrope ethanol we can observe a ternary phase diagram which is comparable to those derived with other hydrotropes (compare figures 1 and figure 20a). Like for all other hydrotropes, the mixtures E30-E50 were chosen to have the same composition. Samples E10 and E15 have a smaller hydrotrope content in comparison to the samples with 10 wt%, respectively 15 wt% monomer, as the ternary phase diagrams of TBA and NPA show a larger two-phase region. As the structures in SFME is known to decrease with bigger distance to the phase border, samples with higher EtOH contents were not considered for further investigation.

Figure 20 shows (a) the ternary phase diagram for the system water, EtOH, MMA at 25 °C and 45 °C and (b) the autocorrelation functions derived by DLS for the compositions E30,

E40, and E50 (see table 3) at 25 °C.

However, the autocorrelation functions derived by dynamic light scattering (compare figures 2-5 and figures 20b and 21 ) do not show any hints for a structuring, which is necessary for a surfactant-free microemulsion. Thus, this system does not form a SFME.

Figure 21 shows autocorrelation functions derived by DLS for (a) the compositions E30, E10, and E15, and (b) the compositions E30, E40, and E50 (see table 3) at 45 °C. Table 3: Compositions of SFME systems with ethanol and methyl methacrylate (MMA) which were used for polymerizations.

Polymerizations were performed for 7 hours with an initiator concentration of 0.4 mol% BCC (compare example 3). While a controlled microemulsion polymerization yielding small polymer particles could be observed forTBA (compare figure 12), the samples with EtOH reach a nearly 10 times higher molar mass, indicating that the polymerization rather proceeded as a solution polymerization than as a microemulsion polymerization. Polymerization samples with composition T30 derived Mn=206 kDa and Mw=632 kDa as mean value of three different samples, while E30 derived Mn=2066 kDa and Mw=4595 kDa as mean value of three different samples. This can be leaded back to the missing stabilization by the structuring of E30 in comparison to B30.

Experiments with the initiator persulfate and sulphite at 25 °C (compare Mixtures of different alcohols, polymerization with persulfate, example 2) were done with the hydrotrope ethanol, as well.

Figure 22 shows the measurement of the polymerization kinetics by dynamic light scattering (DLS) of systems containing 5 wt% MMA and 30, 40 and 50 wt% EtOH. The redox-activated initiator persulfate in a concentration of 4 mol%, activated by 4 mol% sulfite (referring to the monomer) was used. All reactions were done at 25 °C.

When comparing the results of EtOH and NPA/mixture of NPA and n-Butanol (see figures 11 and 22), a difference in the hydrodynamic radius of approxematly factor 5 can be observed for samples with same monomer and same hydrotrope concentration, but with EtOH instead of NPA as hydrotrope.

In both examples with different temperatures and different initiator systems, much larger polymer particles were observed in the system with EtOH which does not form a SFME. This demonstrates that the hydrotrope EtOH is not able to form distinct aggregates with the hydrophobic monomer. By that, missing stucturing leads to an uncontrolled polymer growth.