MXPA99010985A - Method for aqueous emulsion polymerization initiated by free-radicals for producing an aqueous polymer dispersion - Google Patents

Abstract

The invention relates to a method for aqueous emulsion polymerization initiated by free-radicals. Monomers are polymerized in the presence of an N-oxyl radical by means of special radical polymerization initiators at temperatures above 100°C and at pressures above the steam pressure of the polymerization mixture.

Description

PREPARATION OF AN AQUEOUS DISPERSION OF POLYMERS BY POLYMERIZATION IN EMULSION, AQUEOUS INITIATED BY FREE RADICALS The present invention relates to an aqueous emulsion polymerization process initiated by free radicals for preparing an aqueous dispersion of polymers, wherein the compounds having at least one group with ethylenic unsaturation (monomers) are emulsified in an aqueous medium by means of of dispersants and polymerized by means of a free radical polymerization initiator in the presence of an N-oxyl radical (a compound having at least one group N which comes from a secondary amine, which does not carry hydrogens on the carbons to (in other words, the N-oxyl groups come from the corresponding secondary amino groups), to form an aqueous dispersion of polymers. In this document, the aforementioned N-oxyl radicals will be mentioned as stable N-oxyl radicals. Aqueous dispersions of polymers are fluid systems consisting of, like the dispersed phase, the polymer particles in stable dispersed distribution (Storage stability in general = 24 h, usually = 2-3 days, usually> one week) in an aqueous dispersion medium. The numerical average diameter of the polymer particles is generally from 0.01 to 1 μ.

Like polymer solutions when the solvent is evaporated, the aqueous dispersions of polymers have the property, when the aqueous dispersion medium is evaporated, of forming polymeric films, which is why aqueous polymer dispersions are widely used in the form of polymers. direct as binders, for example for paints or compositions for leather coating. However, in many cases the dispersed polymer is also separated by coagulation and used as a constituent in polymer blends to modify the chemical properties. For this purpose, the polymer separated from the aqueous polymer dispersion is extruded together with, for example, other thermoplastics, with or without customary additives such as colorants, pigments, lubricants, stabilizers or fillers. Aqueous dispersions of polymers are prepared mainly by aqueous emulsion polymerization initiated by free radicals of the compounds having at least one group with ethylenic unsaturation at less than 100 ° C. In this case, the monomers to be polymerized, which are mainly of low solubility in water, are emulsified in the aqueous medium without great effort, for example by customary stirring, with the addition of the dispersant and polymerized by the action of the initiators of free radical polymerization. The initiators of free radical polymerization are usually water-soluble peroxides, hydroperoxides and / or azo compounds which, at a certain temperature, generally = 100 ° C, dissociate in the reactive radicals which activate the polymerization. The term emulsion expresses the fact that the monomers and the water are present as a system of two liquids in more or less fine distribution and with little mutual solubility. The phrase aqueous emulsion expresses the fact that the aqueous phase forms the continuous phase. The preparation of an aqueous emulsion of monomers usually requires the addition of dispersants (eg Ullmanns Encyklopadie Chemie, Weinheim (1975), p.449), which prevents the direct combination of two monomeric droplets from colliding in the aqueous emulsion, which guarantees the stability of the resulting aqueous polymer dispersion. As a result of the low dispersion stress, the aqueous emulsion of the monomer used together with the aqueous free-radical emulsion polymerization commonly consists, mainly, of monomer droplets with a diameter of > 1 μ. Like all polymerizations initiated by free radicals of compounds that have at least one group with ethylenic unsaturation, the process of aqueous polymerization initiated by free radicals also has the disadvantage that the molecular weight of the polymer chains usually does not increase in the degree of conversion and that the resulting polymer chains are generally not of uniform molecular weight. In other words, and in terms of their molecular weight, the polymer obtainable is generally not monodisperse but normally has a PDI polydispersity index in this sense of >; 2 (PDI = MW / M ?? / where Mw = weighted average molecular weight and Mn = numerical average molecular weight), which is attributed in particular to the termination reactions as a consequence of the irreversible combination of the ends of the polymer chain that grows with free radicals. Another disadvantage of the aqueous emulsion polymerization initiated by free radicals is that a change made during the polymerization to the monomers to be polymerized generally does not give rise to segmented copolymers (block polymers) but usually, at best, to the core-shell polymer particles dispersed with a core composed of one type of monomer and a composite cover of the other type of monomer, the bond between the core and the shell being mainly non-chemical but rather physical. TRIP vol. 4, No. 6, June 1996, page 183 ff., US-A 5,322,912, WO 96/24620, US-A-4, 581, 429, US-A 5,412,047, EP-A 135 280 and the earlier application DE-A 19602539 describe that carrying out polymerizations initiated by free radicals at more than 100 ° C in the presence of an N-oxyl radical, stable (ie, one practically free of an initiating action) allows a certain degree of control of the polymerization initiated by free radicals. The mechanism on which the action is based is assumed to be that the stable N-oxyl radicals do not terminate irreversibly but simply temporarily the reactive radical ends in blocks of a growing polymer chain at elevated temperatures. The result of this is a reduction in the concentration in the steady state of the ends of the polymer chains by free radicals in growth, thereby reducing the possibility of irreversible termination of chain growth by combining two ends of polymer chain growing. This originates on average polymer chains that grow (linear in theory) in proportion to the conversion of the polymerization. The result of this is an average molecular weight that grows (ideally linear) in proportion to the conversion of the polymerization, with the resulting polymer having a polydispersity index of 1. According to US-A 5,322,912, column 10, line 65 et seq., the suitable reaction medium for a controlled, free radical initiated polymerization of this kind includes an emulsion. Other details related to the performance of this emulsion polymerization initiated by free radicals are not given in US Pat. No. 5,322,912. The same applies to DE-A 19602539. The only recommendation made in US-A 5,412,047, column 18, lines 54 et seq. for the case where the polymerization initiated by free radicals takes place in a multiphase system, as is the case with the aqueous emulsion polymerization initiated by free radicals, it is to use stable N-oxyl radicals which are particularly low in water solubility. The availability of an aqueous emulsion polymerization, initiated by free radicals, controlled, easy to perform to prepare an aqueous dispersion of polymers would be advantageous in that it would allow the molecular weight of the resulting polymer, present in the dispersed distribution, to be adjusted in a controlled way. This controlled establishment determines, for example, the cohesion and adhesion of the film resulting from the aqueous polymer dispersion. In general, there is an increase in the degree of cohesion as the molecular weight increases, while a decrease in molecular weight generally favors the surface tack of the film. In addition, the possibility of controlled adjustment opens direct access to the aqueous dispersions of the designed block copolymers, since the ends of the polymer chains of free radicals are not destroyed by the combination but only by the reversibility of the blocks. In other words, after the consumption of a first type of monomer, the polymerization can be continued with the addition of other types of monomers. To prepare an aqueous polymer dispersion by aqueous emulsion polymerization, initiated, controlled, Macromolecules 1997, _30 / pp. 324-326 recommends carrying out the polymerization so that a preformed aqueous polymer dispersion (seed latex) is charged to a polymerization vessel and that at this initial charge the monomers to be polymerized are added and also a hydrophobic compound which under the action of the heat dissociates into a stable N-oxyl radical and a free radical partner that initiates the polymerization. The reaction mixture is then left at room temperature to allow both monomers to be polymerized and the hydrophobic compound to diffuse into the seed polymer particles (swelling). After the swelling has taken place, the temperature rises (>; 100 ° C) to carry out the polymerization under superatmospheric pressure. The disadvantages of this process are that it requires the prior preparation of the comparatively complex hydrophobic compound, and the extremely slow swelling process. In addition, it absolutely requires the previous preparation of LU latex seed. An object of the present invention is to provide a more advantageous process of polymerization in aqueous emulsion initiated by free radicals, controlled to prepare aqueous polymer dispersions. We have found that this object is achieved by an aqueous emulsion polymerization process, initiated by free radicals to prepare an aqueous polymer dispersion, wherein the compounds having at least one ethylenically unsaturated group are emulsified in an aqueous medium by means of dispersants and polymerization by free radicals in the presence of a stable N-oxyl radical, consisting of: a) using a peroxide, a hydroperoxide and / or an azo compound as the initiator of free radical polymerization whose molal solubility at 25 ° C and 1 bar in water is greater than or equal to the corresponding molal solubility of the tert-butyl hydroperoxide in water and whose dissociation temperature in the polymerization medium is < 100 ° C, b) perform polymerization in aqueous emulsion, initiated by free radicals at more than 100 ° C, and c) perform polymerization in aqueous emulsion, initiated by free radicals at pressures that are higher than the vapor pressure of the mixture of polymerization present inside the vessel for polymerization. The stable N-oxyl radicals suitable according to the invention are all those specified in EP-A 135 280, former application DE-A 19651307, US-A 5,322,912, US-A 4,581,429, WO 96/24620, US-A 5,412,047, and earlier application DE-A 19602539. Examples of these suitable stable N-oxyl radicals which come from a secondary amine are those of the formula I: Rl R6 I R2- N C R5 ID »R3 OR R4 where R1, R2, R5 and R6 = are the same or different substituted or unsubstituted alkyls, straight or branched chain, and R3 and R4 the same or different substituted or unsubstituted alkyls, straight or branched chain, or 3 4 R CNCR = a substituted or unsubstituted cyclic structure. Particularly suitable compounds I are those specified in EP-A 135 280, the earlier application of DE-A 19651307, US-A 5,322,912, US-A 5,412,047, US-A 4,581,429, DE-A 16 18 141, CN-A 1052847 , US-A 4,670,131, US-A 5,322,960, the earlier application DE-A 19602539. Examples thereof are those stable N-oxyl radicals of the formula I in which R, R, R and R are (identical or different) ), methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, linear or branched pentyl, phenyl or substituted groups thereof and R and 4 R are (identical or different) methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, linear or branched pentyl, substituted groups thereof or together with CNC, the cyclic structure: where n is an integer from 1 to 10 (in many cases from 1 to 6), including the substituted forms of these cyclic structures. Common examples are 2, 2, 6, 6-tetramethyl-oxylpiperidine, 2, 2, 5, 5-tetramethyl-1-oxylpyrrolidine and 4-oxo-2,2,6,6-tetramethyl-1-oxylpiperidine. Suitable N-oxyl radicals can be prepared from the corresponding secondary amines by oxidation, for example with hydrogen peroxide. In general, these can be prepared as a pure substance. Suitable N-oxyl radicals which are particularly advantageous according to the invention are those whose molal solubility in the aqueous polymerization medium at 25 ° C and 1 bar is <10. 10 -6 mol / kg, preferably > 10- ^ mol / kg, more preferred > 10 ~ 4 mol / kg and, with special preference, > 10 ~ 3 mol / kg. In general, this solubility of the stable N-oxyl radicals to be used according to the invention is no greater than greater than 10 1 mol / kg. Stable N-oxyl radicals of improved solubility in an aqueous medium include, in particular, carboxylated, phosphonated, sulphonated and / or hydroxylated piperidine or pyrrolidine N-oxyls and di-N-oxyls of the following formulas II to IX: where I? I = from 2 to 10, R7, R8, R9 = independent of each other II H, N c- (CH2) q C00T M®-, NH2, O C (CH2) q COO® M® - COO © M®, - S03? M®, -P03? M®, OR P03 M2, OR S03? M ^, OH, O-f CR2 CH2 O) - H or O- { CH CH 2 -H, CH 3 provided that at least one of 7 8 9 the substituents R, R, and R present is different from hydrogen and M is a hydrogen ion or an alkali metal ion (especially K + or Na +). q = __ an integer from 1 to 10, R ', R', R 'and R f = independent of each other and independent of R, R, R and R are the same groups as R1, 1 or R = "C1 alkyl" -C4 -CH = CH2, -C = CH, -CN, OC ti NH2, -COO "M +, -COOCH3 OR -COOC2H, an organic radical with at least one primary, secondary (for example -NHR) or tertiary amino group , for example - NRlRt-, 2 ', or at least one ammonium group - N + R14RX5R16X ~, where X- = F ~, Cl, "Br ~, HS04, S022 ~, H2P04, HP022_ or P043" and R14, R , and R independent of each other are organic radicals (for example independent of each other and independent of R 1 the same groups as R1), R12 = independently of R 11 the same groups as R, 1"1'1" 1 'or -H , -OH, C, C alkyl, -C00"M +, -C = CH, - c NH2, CH3 and C2H5 C1-C4 alkyl substituted with hydroxy (for example hydroxy ethyl or hydroxypropyl), or R11, R together with the oxygen of a carboxyl group and OR 13 II -H, CH3 or • CH2 C-O0 ® Preferably R6 '= -CH3.

In many cases, the molal solubility of the N-oxyl radicals stable in water already corresponds to the desired solubility values according to the invention for the aqueous polymerization medium. If this is not the case, the preferred solubility value according to the invention for the aqueous polymerization medium can be adjusted in a manner known per se, especially when the stable N-oxyl radical has an acidic or basic group as a functional group, by varying the pH of the aqueous polymerization medium (for example by adding a base such as KOH or NAOH, or by adding an acid such as HCl, H2SO4 or H3P0). Common examples of the stable N-oxyl radicals which are suitable according to the invention are: 4-hydroxy-2, 2,6,6,6-tetramethyl-1-oxylpiperidine, 4-hydroxy-2,6-diphenyl-2, 6-dimethyl-l-oxylpiperidine, 4-carboxy-2,2,6,6-tetramethyl-l-oxylpiperidine, 4-carboxy-2,6-diphenyl-2,6-dimethyl-l-oxylpiperidiha, 3-carboxylic acid 2, 2,5, 5-tetramethyl-1-oxylpyrrolidine, 3-carboxy-2, 5-diphenyl-2,5-dimethyl-1-oxylpyrrolidine and the sodium or potassium salt of the 4-hydroxy-2 sulfuric monoester, 2,6,6-tetramethyl-l-oxylpiperidine. The preparation of 3-carboxy-2,2,5,5-tetramethyl-1-oxylpyrrolidine, for example, is given by Romanelli, M .; Ottaviani, M.F .; Martini, G .; Kevan, L., JPCH J: Phys. Chem., EN, 93, 1, 1989, pp. 317-322. The compounds (VI) and (VII) can be obtained according to US-A 4,665,185 (for example Example 7) and DE-A 19510184.

Other common, adequate examples are: Akiyoshi, Tetsuji; JA 8, Bull, EN, 65, 4, Beilstein Registration number Beilstein 6498805 (4-amino-2,2,6,6-tetramethyl-1-oxyl-piperidine); Registration number Beilstein 6800244 (C11H23 2O2); Registration number Beilstein 5730772 (N-methyl-4-amino-2,2,6,6-tetramethyl-l-oxyl-piperidine); Registration number Beilstein 5507538 (2,2,6,6-tetramethyl-4- (2-amino-ethylamino) -1-oxyl-piperidine); Beilstein Registration 4417950 (4 < bis (2-hydroxyethyl) > -amino- 2,2,6,6-tetramethyl-l-oxylpiperidine); twenty or Beilstein Registration number Beilstein 4139900 (4-amino-2,2,6-6-tetramethyl-4-carboxy-1-oxyl-piperidine); Beilstein registration number 4137088 (4-amino-4-cyano-2, 2,6,6,6-tetramethyl-1-oxyl-piperidine); or Beilstein Registration number Beilstein 1468515 (2,2,6,6-tetramethyl-4-hydr? Xi-4-acetyl-l-oxyl-piperidine); Registration number Beilstein 1423410 (2,2,4,6,6-pentamethyl-4-hydroxy-l-oxyl-piperidine); Registration number Beilstein 6205316 (4-carboxymethylene-10 2,2,6,6-tetramethyl-1-oxyl-piperidine); Registration number Beilstein 1395538 (4- <2-carboxybenzoyloxy> -2,6,6,6-tetramethyl-1-oxyl-piperidine); Registration number Beilstein 3546230 (4-carboxymethyl-2,2,6,6-tetramethyl-1-oxyl-piperidine); Registration number Beilstein 3949026 (4-carboxyl-2,2,6,6-tetramethyl-1-oxyl-piperidine); Registration number Beilstein 4611003 (ethylenediamine tetraacetic acid mono (l-? Xyl-2,2,6,6,20 tetramethylpiperidinyl) Beilstein o Beilstein Registration number Beilstein 5080576 (N- (2,2,6,6-tetramethyl-l-oxyl-4-piperidinyl) monoamide of succinic acid) / Registration number Beilstein 5051814 (4- (4-hydroxyubutanoylamino) -2,2,6,6-tetramethyl-1-oxyl-piperidine); 0 Registration number Beilstein 4677496 (2, 2, 6, 6-tetramethyl-4-oximino-1-oxyl-piperidine); Ro Beilstein O Beilstein registration number Registration number Beilstein 1423698 (4-ethyl-4-hydroxy-2,2,6,6-tetramethyl-1-oxyl-piperidine); Registration number Beilstein 5509793 (4-ethoxymethyl-4-hydroxy-2, 2,6,6-tetramethyl-l- O-oxy piperidine); N Beilstein registration number 3960373 (Ci0H19N2? 3); or Beilstein Registration number Beilstein 3985130 (2,2,6,6-tetramethyl-l-oxyl-4-piperidylidene) succinic acid.

Of course, it is also possible according to the invention to employ mixtures of the stable N-oxyl radicals. It is surprising that according to the invention it is possible to use stable N-oxyl radicals whose molal solubility in the aqueous polymerization medium, at 25 ° C and 1 bar is greater than the corresponding molal solubility in the monomers to be polymerized or in the mixture of monomers to be polymerized. The initiators of free radical polymerization, which are suitable according to the invention are, for example, azo compounds, such as 4,4'-azobiscyanovaleric acid, hydroperoxides such as tert-butyl hydroperoxide and / or peroxides such as hydrogen peroxide or peroxodisulfuric acid and the alkali metal salts thereof (especially the K + and Na + salt). Other suitable free radical polymerization initiators are provided in Ullmanns Encyclopaedia der Technischen Chemie, Verlag Chemie, Weinheim, 4th edition, volume 15, p. 187 ff. It is also possible to make use of combined systems composed of at least one organic reducing agent and at least one peroxide and / or hydroperoxide, for example tert-butyl hydroperoxide and the sodium hydroxymethansulfinic acid, or hydrogen peroxide and ascorbic acid. Also suitable are the combined systems which, in addition to the reducing agent and the peroxide, contain a small amount of a metal compound that is soluble in the polymerization medium and whose metallic component can exist in two or more valence states, examples being ascorbic acid / iron (II) sulfate / hydrogen peroxide.

Based on the molar amount of the monomers that are to be subjected to free radical polymerization, the amount of the free radical polymerization initiator employed in the novel process is generally from 10 to 2 mol%, usually 10". to 1 mol%, and is guided in a manner known per se by the desired molecular weight of the resulting polymer in the dispersed distribution.

The molar ratio of the stable N-oxyl radicals to the initiator of the free radical polymerization, in the case of the novel variant process a), is usually from 0.5 to 5, preferably from 0.8 to 4. By the addition of organic acids, such as canforsulf acid, ionic or p-toluene sulphonic acid (US-A 5,322,912), or by the addition of dimethylsulfoxide (US-A 5,412,047), or indoleacetic acid to the polymerization mixture it is generally possible to increase the rate of the polymerization in the novel process. The dispersants which are suitable according to the invention, in particular, the emulsifiers which are commonly employed in the context of aqueous emulsion polymerizations, initiated by free radicals. Examples of these are block copolymers of ethylene oxide and propylene oxide, ethoxylated mono-, di- and trialkylphenols (for example EO units: 3 to 50 and alkyl: C4-C9), ethoxylated fatty alcohols (for example EO units) : 3 to 50 and alkyl: C8-C36), and also alkali metal salts and ammonium salts of alkyl sulphates (for example C8-C3 alkyl), of sulfuric monoesters of ethoxylated alkanols (for example EO units: 4 to 30) and alkyl: C? 2-C3o) and of ethoxylated alkyl phenols (for example EO units: 3 to 50 and alkyl: C-C15), alkyl sulphonic acids (for example alkyl: C? 2-C3S) and alkyl aryl acids sulphonic (for example alkyl: C9-C3d) • Other suitable dispersants are compounds of Formula X where V and W are hydrogen or C4-C14 alkyl but are not hydrogen at the same ti, and G and H can be alkali metal ions and / or ammonium ions. Preferably, V and W are linear or branched alkyls of 6 to 18 carbons or hydrogen, and especially of 6, 12 and 16 carbons, and not simultaneously hydrogen. G and H are preferably sodium, potassium or ammonium ions, giving particular preference to sodium. Particularly advantageous compounds X are those in which G and H are sodium, V is a branched alkyl of 12 carbons and W is hydrogen or V. Frequent use is made of technical grade mixtures with a proportion of 50 to 90% by weight of the monoalkylated product, for example Do fax ® 2A1 (registered trademark of Dow Chemical Company). Compounds X are generally known, for example, from US-A 4 269 749, and can be obtained commercially. Based on the monomers to be polymerized, the amount of dispersant used according to the invention is, in general, from 0.1 to 10% by weight. At the onset of novel free radical initiated aqueous emulsion polymerization, the amount of emulsifier is generally chosen to be greater than the critical micelle concentration. Examples of the monomers having at least one ethylenically unsaturated group are defined as ethylene or propylene, vinylaromatic monomers such as styrene, 2-vinylnaphthalene and 9-vinylanthracene, substituted vinylaromatic monomers such as p-methylstyrene, α-methylstyrene, o-chlorostyrene, p -chlorostyrene, 2,4-dimethylstyrene and 4-vinylbiphenyl, esters of vinyl alcohol with Ci-Ciß monocarboxylic acids, such as vinyl acetate, vinylpropionate, vinyl n-butyrate, vinyl laurate and vinyl stearate, esters of α, β-monoethylenically unsaturated mono- and dicarboxylic acids, such as in particular acrylic acid, methacrylic acid, aleic acid, fumaric acid and itaconic acid, generally with O.-C20 alkanols, frequently C1-C12, usually C? -C8 , and with particular frequency of C1-C4, especially acrylate and methyl methacrylate, ethyl, n-butyl, isobutyl, tert-butyl and 2-ethylhexyl, dimethyl maleate or n-butyl maleate, the nitriles of the above-mentioned α, β-monoethylenically unsaturated carboxylic acids, such as acrylonitrile and methacrylonitrile, and conjugated dienes of C4-C8, such as 1, 3-butadiene and isoprene. These monomers generally constitute the main monomers which, based on the total amount of the monomers to be polymerized, normally constitute a proportion of more than 50% by weight. The monomers which when polymerized alone usually produce homopolymers of increased solubility in water are normally copolymerized only as modifying monomers in amounts, based on the total amount of the monomers to be polymerized, of less than 50% by weight, generally from 0 to 20% by weight and, in most cases / from 0 to 10% by weight. Examples of these monomers are the α, β-monoethylenically unsaturated C3 ^ C6 mono- and dicarboxylic acids, and their anhydrides and amides, the examples being acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, acrylamide and methacrylamide. and also the monoesters of these carboxylic acids with polyhydric alcohols, such as hydroxyethyl acrylate, hydroxypropyl acrylate and also vinyl sulfonic acid and N-vinylpyrrolidone. The novel process, preferably, can be applied to the monomers styrene, vinyltoluene, C?-C8 alkyl (meth) acrylates, especially n-butyl acrylate, 2-ethylhexyl acrylate or methyl methacrylate, and acrylonitrile and to the mixtures of compound monomers to the extent of at least 85% by weight of these monomers or mixtures of these monomers. According to the invention it is possible in a comparatively controlled manner to produce the random, alternating or segmented copolymers, especially the diblock and triblock copolymers, from the aforementioned monomers, present in the aqueous medium in dispersed distribution. Of particular importance is the novel preparation of poly (n-butyl acrylate) and block copolymers comprising poly (n-butyl acrylate) as at least one segment. By appropriately controlling the supply of the monomers to be polymerized it is also possible to prepare gradient polymers, ie polymers with a decreasing or increasing content of comonomer along the polymer chain. In the case of triblock copolymers block A-block B-block-C, blocks A and C can be composed of identical or different monomers. The glass transition temperature of the blocks can be chosen as desired. For example, the chemical composition of blocks A and C can be chosen so that their glass transition temperature will be; 0 ° C. At the same time, the chemical composition of block B can be chosen so that its vitreous transition temperature is < 0 ° C.

In this context, more than 70 wt.%, For example, of block B may be composed of C? -C8 (meth) acrylates in polymerized form. In this context, block B is often composed of n-butyl acrylate, 2-ethylhexyl acrylate or mixtures thereof in polymerized form. The comonomers which may be incorporated by polymerization may also, of course, be those which have more than one vinyl group. The result of this is cross-linked polymers. It is particularly advantageous according to the invention to prepare aqueous polymer dispersions whose dispersed polymer particles have a core / shell morphology. The advantage of this results, in particular, from an improved connection of the cover to the core. A core / shell morphology is generally obtained when, with the polymerization period, a change is made in the monomer and at the same time the new formation of the dispersed polymer particles is practically suppressed. Preferably, monomers with a crosslinking action are copolymerized in the core. The core may be composed, for example, of polystyrene or poly ethyl methacrylate or of a copolymer of styrene and acrylonitrile, and may have a glass transition temperature = 25 ° C. The first cover may consist, for example, of polybutadiene, poly-n-alkyl acrylate such as poly-n-butyl acrylate or copolymers with a glass transition temperature Tg < 0 ° C. This can be followed by one or more additional hard covers (for example composed of polystyrene, polymethyl methacrylate or poly-styrene-acrylonitrile copolymer with a Tg> 25 ° C. After their isolation, these core / shell polymer particles can be used to modify other plastics The molecular weight of the polymers obtainable according to the invention and present in dispersion in the aqueous medium can be stabilized in a simple way by reducing the temperature of the polymerization at the desired time point and thus freezing the blocking from the ends of the growing polymer chains by the stable N-oxyl radicals.In general, this is done below 100 ° C. This block can be inverted by increasing the temperature.An alternative way to establish molecular weight is to limit the amount of monomers that are going to be pplimerizados.To do is provided a new irreversible adjustment of molecular weight me by the addition of conventional molecular weight regulators, such as thioglycolic acid esters and 2-ethylhexanol or ter-dodecyl mercaptan. Upon addition, the ends of the polymer chains increase irreversibly and the polymer chains are released from the stable N-oxyl radicals which can be subsequently removed, for example, by suitable extraction. According to the invention, therefore, it is possible to obtain in a simple form aqueous dispersions of polymers whose weight average molecular weight Mw has specific values from > 100 to 250,000, or > 10,000 to 250.00. The polydispersity indices of the molecular weight can be < 2, frequently < 1.5. In the case of block copolymers, this applies to the individual segments as well. The polymerization temperature is for convenience, according to the invention > 100 ° C to 180 ° C, in particular from 120 to 150 ° C. It is essential for the invention that the polymerization be carried out at a pressure above the vapor pressure of the polymerization mixture at the appropriate polymerization temperature. This pressure can be > 1 bar at 1000 bar, advantageously from 2 to 20 bar and, with very special advantage, from 4 to 10 or from 5 to 7 bar. The desired pressure conditions can be established in a simple way by placing an initial pressure in the polymerization reactor, before the polymerization mixture is heated to the desired polymerization temperature, by means of inert gases such as methane., C02, CO, Ar, He, q N2. This initial pressure can usually be from 3 to 5 bar, for example. The closed polymerization reactor is then brought to the temperature of the polymerization. The novel, free radical initiated aqueous emulsion polymerization is usually performed in the absence of molecular oxygen. However, of course it is also possible to carry out the polymerization in the presence of molecular oxygen. In other words, the desired initial pressure can also be established by means of air, for example, or even by using gaseous monomers such as butadiene or ethylene, alone or in a mixture with the aforementioned gases. The initial pressure is usually set at temperatures <100 ° C, generally from 0 to 75 ° C or from 25 ° C to 75 ° C. Polymerization is often carried out in the presence of pH buffers such as sodium bicarbonate. The novel process can be carried out in a particularly simple manner by loading all the constituents of the polymerization mixture (including the aqueous phase) into the polymerization vessel with stirring, setting the desired initial pressure and then the desired polymerization temperature, while continuing the stirring inside the closed polymerization vessel, and then carrying out the polymerization to the desired degree of conversion while continuing the polymerization. In many cases the temperature is first established at a level from 50 ° C up to < 100 ° C, to_ activate the dissociation of the polymerization initiator by free radicals. The heating to the actual polymerization temperature is then carried out. Of course it is also possible to supply the monomers to be polymerized to the polymerization vessel in stages and / or under gradient. In the same manner, the stable N-oxyl radicals and the free radical polymerization initiator that is used can be added to the polymerization mixture before, during or after the end of a polymerization step. The solids content of the resulting aqueous polymer dispersion is generally from 20 to 50% by volume. However, if required it can be up to 75% in volume. The diameter of the dispersed polymer particles of the resulting aqueous polymer dispersion can be increased by chemical agglomeration and / or pressure agglomeration. To verify the diameters of the polymeric particles present in the resulting aqueous polymer dispersion it is possible according to the invention and of course to add seed latex. This can be done before or during the execution of the polymerization in aqueous emulsion initiated by free radicals, novel. Unlike the closest prior art process, however, in accordance with the invention it is not necessary to inflate these seed polymer particles. A seed process is used in particular when a wide diameter distribution of the resulting polymer particles is desired. The aqueous seed polymer dispersion used according to the invention is advantageously that which in the same way has been prepared according to the novel polymerization process. According to the invention it is possible to obtain aqueous polymer dispersions whose dispersed polymer particles consist of polymers with the following structure: wherein I = residue of the polymerization initiator by free radicals and ? / w ?? v? v ?? v? v / Vv, _ COp0? -,? - mere rami. , f = i • cad ^ o or l -, i. neal -,, If a free radical polymerization initiator is used which, with thermal dissociation, produces fragments that have more than one free radical functionality, the following structures are also possible: where m = 1 to 4. Similar structures are possible and stable, polyfunctional N-oxyl radicals are used, in other words, compounds having more than one N-oxyl radical group. Where ?????? / W is a block copolymer consisting of a hydrophobic block and a hydrophilic block, the aforementioned structures are suitable as dispersants (see, the above prior application DE-A 19648029). A disadvantage of the aqueous emulsion polymerization initiated by free radicals according to the invention is its, in some cases, comparatively low reaction rate. With this in mind it may be convenient to combine the novel process with a conventional free radical initiated aqueous emulsion polymerization. This can be done, for example, starting conventionally and then proceeding according to the invention, or vice versa.

In the latter case, for example, the initiator of the free radical polymerization is added in excess (based on the amount of the N-oxyl radical present) at the point of time considered appropriate. In both cases, the polydispersity of the resulting polymer increases Examples General observations The monomers and polymerization aids used in the following examples were used without purification, in other words, as purchased. The stable N-oxyl radicals that were used were 4-hydroxy-2,2,6,6-tetramethyl-1-oxylpiperidine from Hüls AG (4-hydroxy-TEMPO), 2,2,6,6-tetramethyl-l- Aldrich's oxylpiperidine (TEMPO) and the potassium salt of the sulfuric monoester of 4-2, 2, 6, 6-tetramethyl-l-oxylpiperidine, synthesized in our laboratory (TEMP0-0-S03K). The molar solubilities of the three stable N-oxyl radicals mentioned above are, at 25 ° C and 1 atm in water: TEMPO: 6. 5 - 10-5 mol / kg 4-hydroxy-TEMPO: 3. 25 - 10"3 mol / kg TEMP0-0-S03K: 2. 55 - 10" 3 mol / kg The molecular weights were determined by "CPG (gel permeation chromatography) with calibration by polystyrene standards. Potassium 2, 6,6-tetramethyl-l-oxyl-4-sulfate (TEMPO-O-SO3K) A solution of 15.9 (0.1 mol) of the sulfur trioxide / pyridine complex (1: 1) in 80 ml of acetonitrile was added dropwise at 80 ° C with stirring to a solution of 18 g (0.1 mol) of 4-hydroxy -TEMPO (from Hüls AG) in 70 ml of acetonitrile. The reaction mixture was subsequently stirred at the boiling temperature (reflux) for 5 hours and then at 25 ° C for a further 12 hours. Subsequently, the acetonitrile was removed by distillation under reduced pressure. Then a solution of 0.1 mol of KOH in 80 ml of water was added. The resulting aqueous solution was subjected to extraction 7 times in succession with 150 ml of lime acetate each time (to separate the pyridine formed and the initial compounds that did not react). The remaining aqueous phase was concentrated under reduced pressure and the residue was subsequently dried under high vacuum, yielding 20.2 g of TEMPO-0-S03K. (Additional synthesis options are provided, for example, in Melhorn, Rolf Joachim, Packer, Lester, CHCHAG, Can. J.C. EN, 60, 1982, pp. 1452 and in Sunamoto, Junzo; Akiyoshi, Kazunari, Kihara, Tetsuji; Endo, Masayuki, BCS JA8, Bull. Chem. Soc. Jpn., EN, 65.4, 1992, pp 1041-1046.

Example 1 A 5-liter pressure vessel with stirring was charged at 25 ° C with 1550 g of water, 4.7 g of sodium bicarbonate, 9.3 g of the potassium salt of C3o alkylsulfonic acid, 3.6 g of 4-hydroxycarboxylic acid. TEMPO, 3.8 g of potassium peroxodisulfate, 281 g of styrene and 94 g of acrylonitrile. Then an initial pressure of 4 bar was established in the polymerization vessel by means of molecular nitrogen and the polymerization mixture was heated to 95 ° C. This temperature was maintained for 30 minutes. Subsequently it was raised to 120 ° C and the polymerization was carried out at this temperature (and at a pressure of about 7 bar) for 12 hours). The mixture was subsequently cooled to 60 ° C, 200 g of N-butyl acrylate were added, the mixture was heated to 120 ° C and the polymerization was continued at 120 ° C and about 7 bar for another 12 hours. The resulting aqueous polymer dispersion had a solids content of 13% by weight and was free of clots. For this polymerization, the weight average molecular weight Mw of the resulting polymer was determined as a function of the total polymerization period at 120 ° C. The results are shown in Table 1.

Table 1 Polymer Total period of Mw polymerization [120 ° C, h] polystyrene- 4 9800 acrylonitrile polystyrene- 13600 acrylonitrile polystyrene- 12 28,000 acrylonitrile polystyrene- 15 31500 acrylonitrile-polybutylacrylate paraolistyrene- 18 35900 acrylonitrile polybutylacrylate polystyrene- 42,000 acrylonitrile polybutylacrylate The increase in molecular weight with the polymerization period is evidence of the controlled nature of the aqueous emulsion polymerization initiated by free radicals.

Comparative example Example 1 was repeated without setting an initial pressure above the vapor pressure of the polymerization mixture. After a short period of polymerization at 120 ° C (at about 4 bar) the aqueous polymer dispersion became unstable and coagulated.

Example 2 A 5 liter pressure vessel was charged, with stirring, at 25 ° C with 1550 g of water, 4.7 g of sodium bicarbonate, 9.3 g of potassium salt of C30 alkylsulfonic acid, 3.6 g of 4- hydroxy-TEMPO, 3.8 g of potassium peroxodisulfate, 375 g of styrene. After an initial pressure of 4 bar which was established in the polymerization vessel by means of molecular nitrogen, the polymerization mixture was heated to 95 ° C. This temperature was maintained for 30 minutes. Subsequently, it was raised to 120 ° C and the polymerization was carried out at this temperature (approximately 7 bar) for 44 hours. At a polymerization conversion of 70% by weight, the polymerization was terminated by cooling to room temperature. The resulting aqueous polymer dispersion had a solids content of 14.5% by weight and was free of coagulum. For this polymerization the weight average molecular weight Mw and the PDI of the resulting polymer was determined as a function of the total polymerization period at 120 ° C.

Results are shown in table 2 Table 2: Polymer Total period of Mw PDI polymerization [120 ° C, h] polystyrene 3 6800 1.45 polystyrene 19 18900 1.34 polystyrene 29 25300 1.42 polystyrene 44 33000 1.46 There was a remarkable increase in the average molecular weight with the polymerization period. The molecular weight distribution remains narrow even in high polymerization conversions.

Example 3 Example 2 was repeated but using 3.6 g of TEMPO instead of 3.6 g of 4-hydroxy-TEMPO. After polymerization for 44 hours at 120 ° C, the polymerization was terminated by cooling to room temperature. The conversion of the polymerization was 83.5% by weight. The solids content of the resulting aqueous polymer dispersion was 17.2% by weight. The "corresponding time dependent results are shown in Table 3J.

Table 3 Polymer Total period of Mw PDI polymerization [120 ° C, h] polystyrene 3 9800 1.31 polystyrene 19 10800 1.29 polystyrene 29 12600 1.33 polystyrene 44 1430 1.65 The increase in the weighted average molecular weight with the period of the polymerization is less pronounced than in Example 2. The polydispersity of the molecular weight in the same way increases more sharply with high conversions of the polymerization compared to Example 2.

Example 4 Example 2 was repeated, but using 5.3 g of TEMPO-0-SQ3K, instead of 3.6 g of 4-hydroxy-TEMPO. After polymerization for 7 hours at 120 ° C, the polymerization was terminated by cooling to room temperature. The conversion of the polymerization was 98.5% by weight. The solids content of the resulting aqueous polymer dispersion was 20.4% by weight. The corresponding time-dependent results are shown in Table 4.

Table 4 Polymer Total period of Mw PDI polymerization [120 ° C, h] polystyrene 0.5 247100 3.53 polystyrene 1.5 27"9000 2.96 polystyrene 2 292800 2.91 polystyrene 3 306700 2.76 polystyrene 6 323500 2.61 polystyrene 7 328000 2.72

Claims (3)

1. CLAIMS An aqueous emulsion polymerization process initiated by free radicals to prepare an aqueous polymer dispersion, wherein the compounds having at least one ethylenically unsaturated group are emulsified in an aqueous medium by means of dispersants and are polymerized by free radicals in the presence of a radical N-oxyl stable, which consists of: (a) use a peroxide, a hydroperoxide and / or an azo compound as the initiator of free radical polymerization whose molal solubility at 25 ° C and 1 bar in water is greater than or equal to the corresponding molal solubility of tert-butyl hydroperoxide in water whose dissociation temperature in the middle of the polymerization is < 100 ° C, b) carry out the polymerization in aqueous emulsion initiated by free radicals at more than 100 ° C, c) performing the aqueous emulsion polymerization initiated by free radicals at pressures that are above the vapor pressure of the polymerization mixture present within the vessel for polymerization.
2. 2. The process, as mentioned in claim 2, wherein the stable N-oxyl radical used is a compound of the formula I: Rl R6 R2 C N C R5 (I) R3 Rt where RX, R2, R5 and R6 = the same or different substituted or unsubstituted alkyls, straight or branched chain and R3 and R4 = the same or different substituted or unsubstituted alkyls, straight or branched chain, or R3CNR4 = a substituted or unsubstituted cyclic structure.
3. 3. The process as recited in claim 1, wherein the stable N-oxyl radical used is a compound of one of the following formulas II to IX: where m = from 2 to 10, R7, R8, R9 = independent of each other: II H, N C- (CH2) g COO ^ M © - NH2 O C (CH2) q- - COOe M® t COO © ® r - S03 ° M® r - '- PO3 ° M® 3 © Ó PO3 2 r 0 SO3® Vi®. OH, 0 - < -ca - -CH2 - - O) - H i CB3 provided that at least one of the substituents R7, R8, and R9 present is different from hydrogen M + = a hydrogen ion or an alkali metal ion q = an integer from 1 to 10, R1 ', R2', R5 'and R6 '= independent of each other, the same or different substituted or substituted alkyls, straight or branched chain, R1', R2 ', R5' and R6 '= independent of each other and independent of R1, R2, R5 and R6 the same groups such as R1, R10 = C3-C4 alkyl -CH = CH2, -G = CH, -CN, -COO "M +, -COOCH3 or -COOC2H5, Rn_ an organic radical with at least one primary or tertiary amino group or at least one ammonium group, R 1-2_ independently of R11 the same groups as R11 or - H, -OH, C1-C4 alkyl, -COO ~ M +, -C = CH, or O -C II HH2, - -c II- GH3, C HS or C? -C alkyl substituted with hydroxy, or R? ll, R = together with the oxygen of a carbonyl group and The process, as mentioned in claim 1, wherein the stable N-oxyl radial used is at least one representative of the group consisting of 4-hydroxy-2, 2,6,6-tetramethyl-1-oxylpiperidine, 4- hydroxy-2,6-diphenyl-2,6-dimethyl-l-oxylpiperidine, 4-carboxy-2, 2,6,6,6-tetramethyl-l-oxylpiperidine, 4-carboxy-2,6-diphenyl-2,6- dimethyl-l-oxylpiperidine, 3-carboxy-2, 2,5, 5-tetramethyl-l-oxylpyrrolidine, 3-carboxy-2, 5-diphenyl-2,5-dimethyl-l-oxylpyrrolidine and the sodium or potassium salt of the sulfuric monoester of 4-hydroxy-2, 2,6,6-tetramethyl-1-oxylpiperidine. The process, as mentioned in any of claims 1 to 4, wherein the stable N-oxyl radical used is at least one whose molal solubility in the aqueous polymerization medium at 25 ° C and 1 bar is > 10 ~ 6 mol / kg. The process, as mentioned in any of claims 1 to 4, wherein the stable N-oxyl radical used is at least one whose molal solubility in the aqueous polymerization medium at 25 ° C and 1 bar is > 10 ~ 3 mol / kg. The process, as mentioned in any of claims 1 to 4, wherein the stable N-oxyl radical used is at least one whose molal solubility in the aqueous polymerization medium at 25 ° C and 1 bar is greater than the molal solubility. corresponding to the monomers to be polymerized. The process, as mentioned in any of claims 1 to 7, wherein a free radical polymerization initiator additionally used is peroxodisulfuric acid and / or one of its alkali metal salts, The process, as mentioned in any of the claims 1 to 8, wherein the monomers to be polymerized are compounded to the extent of at least 85% by weight of the monomers from the group consisting of styrene, vinyltoluene, C? -C8 alkyl (meth) acrylates. and acrylonitrile. 10. The process, as mentioned in any of claims 1 to 9, wherein there is at least one change of the monomers to be polymerized during the course of the polymerization. 11. The process, as mentioned in any of claims 1 to 10, wherein the polymerization temperature is > 100 to 180 ° C. 12. The process, as mentioned in any of claims 1 to 11, wherein the polymerization pressure is from 2 to 20 bar. The process, as mentioned in any of claims 1 to 12, wherein the polymerization mixture is charged to the polymerization vessel at a temperature below 100 ° C, at this temperature a pressure is established above the vapor pressure of the polymerization mixture, and then the closed polymerization vessel is heated to the polymerization temperature which is higher than 100 ° C, and the polymerization is carried out. 14. An aqueous polymer dispersion obtainable by a process as mentioned in any of claims 1 to 13.