Cyclohexyl-substituted 4-hydroxy-pyrazole polymerization inhibitors

Abstract

The present invention relates to a mixture containing at least one cyclohexyl-substituted 4-hydroxy-pyrazole and at least one polymerizable compound.

Description

[0001] Cyclohexyl-substituted 4-hydroxypyrazole polymerization inhibitors

[0002] The present invention relates to a mixture containing at least one cyclohexyl-substituted 4-hydroxypyrazole and at least one polymerizable compound.

[0003] Chemical compounds that contain one or more ethylenically unsaturated groups have a pronounced tendency toward radical polymerization. Such compounds are therefore also referred to as polymerizable compounds. Due to their tendency toward radical polymerization, these compounds are used as monomers for the production of polymers. However, the pronounced tendency of these compounds toward radical polymerization is disadvantageous in that it can lead to undesirable, spontaneous radical polymerization during storage and transport, as well as during chemical and / or physical processing, such as distillation or rectification, particularly under the influence of energy, such as heat and / or light.Such uncontrolled polymerizations can lead to the gradual formation of polymer deposits, for example, on heated surfaces, necessitating their removal and thus often resulting in a reduction in operating times. Uncontrolled polymerizations can even be explosive.

[0004] It is therefore common practice to add compounds that inhibit or at least slow down unwanted, spontaneous radical polymerization to ethylenically unsaturated compounds that have a tendency toward radical polymerization, or to mixtures containing such compounds, both during storage and transport and during chemical and / or physical processing. Such substances are referred to as polymerization inhibitors.

[0005] Polymerization inhibitors can be used as individual chemical compounds or as mixtures of compounds. Depending on the application, specific requirements apply to the polymerization inhibitor. For a polymerization inhibitor to be suitable as a transport and / or storage stabilizer for ethylenically unsaturated compounds, it is important that the efficiency of the polymerization inhibitor, i.e., the extent of polymerization inhibition, can be regulated.Under the storage and / or transport conditions of the ethylenically unsaturated compounds, the polymerization inhibitor should sufficiently prevent or slow down the undesired, spontaneous radical polymerization, whereas the desired radical polymerization of the ethylenically unsaturated compounds should be possible under appropriate polymerization conditions without the need to first separate the polymerization inhibitor used during storage and / or transport. If the stabilizer used during storage and / or transport is not separated during the desired polymerization, it is important that it does not adversely affect the desired polymerization, for example, by acting unintentionally as a regulator.

[0006] Measured by global production volume, acrylic acid is certainly one of the most important ethylenically unsaturated compounds. Acrylic acid is typically stabilized against unwanted, spontaneous radical polymerization during storage and / or transport with 0.018 to 0.022 wt.% hydroquinone monomethyl ether (MEHQ), based on the amount of acrylic acid. For adequate stabilization of acrylic acid against unwanted, spontaneous radical polymerization using MEHQ, it is necessary that oxygen is dissolved in the acrylic acid in sufficient quantities. Sufficient amounts of oxygen are usually dissolved in the acrylic acid when acrylic acid is stored and / or transported under an atmosphere containing 5 to 21 vol.% oxygen.During the desired polymerization of acrylic acid, the oxygen content dissolved in the acrylic acid is reduced, thereby reducing the efficiency of MEHQ to inhibit polymerization to such an extent that acrylic acid can be polymerized in the presence of MEHQ.

[0007] In addition to its use as a polymerization inhibitor during storage and / or transport of acrylic acid, MEHQ is also used as a polymerization inhibitor during storage and / or transport of methacrylic acid, acrylic acid esters and / or methacrylic acid esters or mixtures containing one or more of the aforementioned compounds.

[0008] Due to its wide use, MEHQ is one of the most important storage and / or transport stabilizers for polymerizable compounds, especially for acrylic acid, methacrylic acid, acrylic acid esters and methacrylic acid esters.

[0009] Polymerizable compounds are stored in suitable, permanently installed containers, such as storage tanks or storage containers (see, for example, Acrylic Acid, A Summary of Safety and Handling, 4th Edition 2013, 7 Bulk Storage Facilities and Accessories). It is preferred that the polymerizable compounds remain in the respective containers for at least one minute during storage, more preferably at least 10 minutes, and most preferably at least 60 minutes. Although the storage period is theoretically unlimited under appropriate conditions, the storage period is generally reduced to a minimum for economic reasons. It is preferred that the polymerizable compounds remain in the respective containers for a maximum of 180 days, more preferably for a maximum of 90 days, and most preferably for a maximum of 30 days.Particularly preferred ranges result from any combination of the aforementioned lower and upper limits. Polymerizable compounds are usually transported in suitable, transportable containers such as tanks or containers by ship, railcar, and / or truck (see, for example, Acrylic Acid, A Summary of Safety and Handling, 4th Edition 2013, 9 Safe Transport of Acrylic Acid). Of course, the transportable containers can also be permanently installed on the corresponding means of transport, for example, ship tanks or rail tank cars. It is also possible for the containers to be stored at a specific location for a certain period of time before being transported to another location.Polymerizable compounds can also be transported in pipelines or hoses, for example, after purification of the polymerizable compounds to the storage tank, during loading from the storage tank into a transportable container, and / or during loading from one transportable container to another. It is preferred that the polymerizable compounds remain in the respective containers for at least 10 seconds during transport, preferably at least one minute, more preferably at least 10 minutes, and most preferably at least 60 minutes. Although the duration is theoretically unlimited under appropriate conditions, the duration is generally reduced to a minimum for economic and safety reasons.It is preferred that the polymerizable compounds remain in the respective containers for a maximum of 180 days during transport, particularly preferably for a maximum of 90 days, and most particularly preferably for a maximum of 30 days. Particularly preferred ranges result from any combination of the aforementioned lower and upper limits.

[0010] The object of the present invention was to provide mixtures containing at least one polymerizable compound, in particular acrylic acid, methacrylic acid, acrylic esters, and / or methacrylic esters, and at least one polymerization inhibitor. The polymerization inhibitors are intended to ensure sufficient stabilization of the polymerizable compounds against unwanted radical polymerization. However, in the case of desired radical polymerization, there should be no need to separate the polymerization inhibitors from the mixture prior to polymerization. The polymerization inhibitors should therefore not act as polymerization regulators and / or polymerization inhibitors in the desired radical polymerization.

[0011] The object is achieved by mixtures containing at least one compound of the general formula (I) where

[0012] R 1H or Ci- to Ce-alkyl, and

[0013] R 2 H or Ci- to Ce-alkyl, and at least one polymerizable compound.

[0014] Preferably, R 1 H or Ci- to C4-alkyl and R 2 Ci- to C4-alkyl.

[0015] Preferably, R 1 Ci- to Ca-alkyl and R 2 Ci-, C3- or C4-alkyl, in particular methyl, isopropyl or tert-butyl.

[0016] Particularly preferred is R 1 Methyl and R 2 is methyl, isopropyl or tert-butyl.

[0017] The alkyl groups can be straight, branched and / or cyclic, whereby branched and cyclic alkyl compounds would only be possible from Ca-alkyl onwards.

[0018] Suitable compounds of the general formula (I) are, for example, 3-cyclohexyl-1,5-dimethyl-1H-pyrazol-4-ol, 3-cyclohexyl-1-isopropyl-5-methyl-1H-pyrazol-4-ol or 1-(tert-butyl)-3-cyclohexyl-5-methyl-1H-pyrazol-4-ol.

[0019] In the mixture, the total amount of the compounds of the general formula (I) is preferably from 0.0001 to 0.1000 wt.%, preferably from 0.0002 to 0.0500 wt.%, particularly preferably from 0.003 to 0.0250 wt.%, very particularly preferably from 0.004 to 0.0100 wt.%, especially preferably from 0.005 to 0.0075 wt.%, in each case based on the total amount of polymerizable compounds.

[0020] In the mixture, the total amount of polymerizable compounds is preferably at least 5% by weight, preferably at least 50% by weight, particularly preferably at least 80% by weight, very particularly preferably at least 98% by weight, especially preferably at least 99% by weight, in each case based on the total weight of the mixture.

[0021] Polymerizable compounds are preferably mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acids, mono-, di- or triethylenically unsaturated C3 to Cs aldehydes, mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acid esters having 1 to 20 carbon atoms in the ester groups, mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acid amides, mono-, di- or triethylenically unsaturated C3 to Cs nitriles, mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acid anhydrides, vinyl esters of saturated C2 to C2o carboxylic acids, vinyl ethers of saturated C1 to C8 alcohols, vinyl aromatics, vinyl heteroaromatics, vinyl lactams having 3 to 10 carbon atoms in the ring, open-chain N-vinylamide compounds and N-vinylamine compounds, vinyl halides, aliphatic, optionally halogenated hydrocarbons with 2 to 8 carbon atoms and 1 or 2 ethylenic double bonds,Vinylidenes or any mixtures of two or more of the aforementioned compounds.

[0022] Particularly preferred are mono-, di- or triethylenically unsaturated C3- to C5-carboxylic acids, for example acrylic acid, methacrylic acid, dimethacrylic acid, ethacrylic acid, citraconic acid, methylenemalonic acid, crotonic acid, fumaric acid, mesaconic acid, itaconic acid and maleic acid, mono-, di- or triethylenically unsaturated C3- to C5-carboxylic acid esters having 1 to 20 carbon atoms in the ester groups, for example acrylic acid esters with C1- to C20-alkyl, methacrylic acid esters with C1- to C20-alkyl, dimethacrylic acid esters with C1- to C20-alkyl, ethacrylic acid esters with C1- to C20-alkyl, citraconic acid esters with C1- to C20-alkyl, methylenemalonic acid esters with C1- to C20-alkyl, crotonic acid esters with C1- to C20-alkyl, fumaric acid esters with C1- to C20-alkyl, me- saconic acid esters with Ci- to C2o-alkyl, itaconic acid esters with Ci- to C2o-alkyl and maleic acid esters with Ci- to C2o-alkyl, mono-, di- or triethylenically unsaturated C3- to Cs-carboxylic acid amides, for example acrylamide, methacrylamide,Dimethacrylamide, ethacrylamide, citraconicamide, methylenemalonicamide, crotonicamide, fumaramide, mesaconicamide, itaconicamide and maleamide, mono-, di- or triethylenically unsaturated C3 to Cs nitriles, for example acrylonitrile and methacrylonitrile, and mono-, di- or triethylenically unsaturated C3 to Cs carboxylic anhydrides, for example acrylic anhydride, methacrylic anhydride, itaconic anhydride and maleic anhydride.

[0023] Very particular preference is given to acrylic acid, methacrylic acid, acrylic acid esters with C1- to C8-alkyl, such as methyl acrylate, ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate, and methacrylic acid esters with C1- to C8-alkyl, such as methyl methacrylate. Other suitable polymerizable compounds are dipropylene glycol diacrylate, tripropylene glycol diacrylate, polyethylene glycol diacrylate, glycerol triacrylate, ethoxylated glycerol triacrylate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, butanediol monoacrylate, dicyclopentadienyl acrylate, 2-dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate, and 2-hydroxypropyl acrylate.

[0024] With acrylic acid as the polymerizable compound, the total amount of the compounds of the general formula (I) in the mixture is preferably from 0.0010 to 0.1000 wt.%, preferably from 0.0020 to 0.0500 wt.%, particularly preferably from 0.0025 to 0.0200 wt.%, very particularly preferably from 0.0030 to 0.0080 wt.%, in each case based on acrylic acid.

[0025] With methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and / or 2-ethylhexyl acrylate as polymerizable compound, the total amount of the compounds of the general formula (I) in the mixture is preferably from 0.0001 to 0.0100 wt.%, preferably from 0.0002 to 0.0075 wt.%, particularly preferably from 0.0003 to 0.0050 wt.%, very particularly preferably from 0.0005 to 0.0020 wt.%, in each case based on the total amount of methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and / or 2-ethylhexyl acrylate.

[0026] A further object of the present invention is the use of at least one compound of the general formula (I), as defined above, for inhibiting the polymerization of at least one polymerizable compound.

[0027] The present invention further provides a process for storing and / or transporting a mixture comprising at least one compound of general formula (I) as defined above and at least one polymerizable compound.

[0028] The mixture is usually stored and / or transported under an oxygen-containing atmosphere.

[0029] The mixture is preferably stored in a container under an oxygen-containing atmosphere with an oxygen content of 5 to 10 vol.%, and the mixture is regularly circulated within the container, for example, by completely pumping the tank contents around at least once a week. The relatively low oxygen content prevents flammable gas mixtures in the container. Circulation replaces consumed dissolved oxygen in the liquid polymerizable compound.

[0030] Mixtures according to the invention with acrylic acid as the polymerizable compound are particularly suitable for the production of water-swellable polyacrylic acids (superabsorbents) and water-soluble polyacrylic acids. The production of superabsorbents is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, Volume 35, pages 73 to 93.

[0031] Mixtures according to the invention with methyl acrylate, ethyl acrylate, n-butyl acrylate or 2-ethylhexyl acrylate as polymerizable compound are particularly suitable for the preparation of polymer dispersions.

[0032] Examples

[0033] 3-i 1-1,5-dimethyl-1H-pyrazole-4-ol

[0034] 1,5-Dimethyl-3-phenyl-1H-pyrazol-4-ol (2.1 g) was dissolved in 100 g of methanol and placed in a 300 ml Hastelloy autoclave. 0.5 g of catalyst (0.5% Ru on alumina) was added and the autoclave was closed. The reaction mixture was then hydrogenated at 120 °C and 200 bar of hydrogen for 48 h.

[0035] The autoclave was depressurized, and the catalyst was filtered from the reaction mixture. After removal of the solvent under vacuum, product A was obtained in 99% purity, determined by GC. Due to the small scale of the reaction and the multiple sampling during the reaction, no yield is given.

[0036] 1 H-NMR (400 MHz, CDCI3): ö in ppm = 1.16-1.39 (m, 3 H), (qd, J = 12.4, 3.1 Hz, 2H), 1.69 (m, 1 H), 1.73-1.91 (m, 4 H), 2.13 (s, 3 H), 2.61 (m, 1 H), 3.63 (s, 3 h), 4.79 (s, 1 h).

[0037] 3-Cyclohexyl-1-isopropyl-5-methyl-1H-pyrazole-4-ol (B):

[0038] To a solution of isopropylhydrazine hydrochloride (2.26 g, 20 mmol, 1.0 equiv) in n-butyl acetate (22 mL) was added sodium bicarbonate (2.52 g, 30 mmol, 1.5 g), followed by hexahydrobenzaldehyde (2.24 g, 20 mmol, 1.0 equiv). The mixture was heated at 80°C for 1.5 h, followed by the addition of methanol (20 mL). The reaction mixture was heated at 70°C for a further 2.5 h. The methanol was removed in vacuo, followed by the addition of hexane (20 mL). The formed precipitate was filtered and dried in vacuo. The resulting hydrazone (3.38 g, 64% purity by GC, 12.3 mmol, 62% yield) was used in the next step without further purification. The hydrazone (3.38 g, 64% purity by GC, 12.3 mmol, 1.0 equiv) was added to n-butyl acetate (28 mL), followed by the addition of acetic acid (2.55 g, 42.5 mmol, 3.5 equiv). Heating to 70°C yielded a clear solution.At this temperature, methylglyoxal (40% in water, 5.46 g, 30.3 mmol, 2.5 equiv) was added and the mixture heated to reflux. After 24 hours, the reaction was cooled to room temperature, and the pH was adjusted to 7 using 2 M aqueous sodium hydroxide solution. After extraction with water, the organic phase was dried (MgSO4) and the solvent removed in vacuo. The crude product was purified by column chromatography (silica, cyclohexane / ethyl acetate, gradient), affording product B (21% yield over two steps) as a colorless solid.

[0039] 1 H-NMR (400 MHz, CDCI3): ö in ppm = 1.11 - 1.35 (m, 9H), 1.47 (qd, J = 13.3, 12.6, 3.5 Hz, 2H), 1.64 (d, J = 12.1 Hz, 1 H), 1.73 (tt, J = 9.4, 3.4 Hz, 4H), 2.06 (s, 3H), 2.54 (dt, J = 11.8, 3.4 Hz, 1H), 4.24 (hept, J = 6.6 Hz, 1H), 7.18 (s, 1H).

[0040] 1-(tert-Butyl)-3-cyclohexyl-5-methyl-1H-pyrazole-4-ol (C):

[0041] Sodium bicarbonate (1.68 g, 20 mmol, 1.0 equiv) was added to a mixture of tert-butylhydrazine hydrochloride (2.49 g, 20 mmol, 1.0 equiv) in methanol (25 mL). The reaction mixture was stirred at room temperature for 1 h. Subsequently, hexahydrobenzaldehyde (2.24 g, 20 mmol, 1.0 equiv) was added, and the mixture was stirred at room temperature for a further 1 h. The solvent was removed in vacuo. The resulting hydrazone was dissolved in ethyl acetate (80 mL), followed by the addition of methylglyoxal (40% in water, 9.00 g, 50.0 mmol, 2.5 equiv) and acetic acid (1.92 g, 32.0 mmol, 1.6 equiv). The reaction mixture was heated at 70°C for 20 h. The pH was then adjusted to 7 using 2 M aqueous sodium hydroxide solution, followed by extraction with ethyl acetate (3 x 100 ml). The combined organic phases were dried (Na2SO4) and the solvent removed in vacuo.The crude product was purified by column chromatography (silica, cyclohexane / ethyl acetate, gradient) to give product C (39% yield) as a pale yellow solid.

[0042] 1 H-NMR (400 MHz, CDCI3): ö in ppm = 1.30 (m, 4 H) 1.55 (s, 9 H), 1.66 (m, 2 H), 1.79 (m, 2H),

[0043] 1.88 (m, 2 h), 2.29 (s, 3 h), 2.61 (m, 1 h), 3.53 (s, 1 h).

[0044] Experiments on polymerization inhibition

[0045] The monomer used was distilled twice to remove the polymerization inhibitor MEHQ. The resulting monomer was then mixed with the specified amount of the polymerization inhibitor (ppm = mg / kg).

[0046] 0.5 ml of each mixture was filled into a 1.8 ml ampoule and stored at 80 °C in a circulating air drying cabinet.

[0047] In each test series, three ampoules of each mixture were filled and tested, with the mean time to complete polymerization being recorded visually.

[0048] The relative efficacy is calculated from the quotient of the time until polymerization of the sample with the polymerization inhibitor under investigation and the time until polymerization of the sample with MEHQ. For MEHQ itself, the efficacy is thus 100%.

[0049] The results are summarized in Tables 1, 2 and 3:

[0050] Table 1: Mixtures with acrylic acid Table 2: Mixtures with ethyl acrylate

[0051] Table 3: Mixtures with n-butyl acrylate

[0052] MEHQ: hydroquinone monomethyl ether

[0053] A: 3-Cyclohexyl-1,5-dimethyl-1H-pyrazole-4-ol

[0054] B: 3-Cyclohexyl-1-isopropyl-5-methyl-1H-pyrazole-4-ol

[0055] C: 1-(tert-Butyl)-3-cyclohexyl-5-methyl-1H-pyrazole-4-ol

[0056] *) Comparison

[0057] The polymerization inhibitors according to the invention have improved efficacy compared to MEHQ.

[0058] Polymerization experiments

[0059] Under a nitrogen atmosphere, 450 g of water were placed in a reaction vessel. The reaction mixture was heated to 95°C with stirring. Once the temperature reached 95°C, three streams were added while stirring and maintaining the temperature.

[0060] Stream 1 : Dosage over 5 h, 500 g acrylic acid

[0061] Stream 2: Dosage over 4.75 h, 15 g sodium hypophosphite in 35 g deionized water Stream 3: Dosage over 5.25 h, 5 g sodium peroxodisulfate in 66.4 g deionized water

[0062] After the addition of the three streams, the reaction mixture was stirred for a further hour at 95°C. The reaction mixture was then cooled to room temperature and 80 g of water were added. The acrylic acid used in stream 1 was stabilized with 200 ppm by weight of 3-cyclohexyl-1,5-dimethyl-1H-pyrazol-4-ol or 200 ppm by weight of MEHQ.

[0063] The obtained polymers were analyzed by GPC (calibration with Na-PAA standard, eluent 0.01 mol / l phosphate buffer, pH=7.4 in distilled water, with 0.01 M NaNs).

[0064] Table 4: Polymerization of acrylic acid

[0065] MEHQ: hydroquinone monomethyl ether A: 3-cyclohexyl-1,5-dimethyl-1H-pyrazole-4-ol

[0066] *) Comparison

[0067] The polymerization inhibitor according to the invention resulted in a polyacrylic acid with comparable molecular weight.

Claims

Patent claims 1. Mixture containing at least one compound of general formula (I) where R 1 H or Ci- to Ce-alkyl, and R 2 H or Ci- to Ce-alkyl, and at least one polymerizable compound.

2. Mixture according to claim 1, wherein R 1 H, Ci- to C4-alkyl and R 2 Ci- to C4-alkyl.

3. Mixture according to claim 1, wherein R 1 Ci- to Ca-alkyl and R 2 is C1-, C3- or C4-alkyl.

4. Mixture according to claim 1, wherein R 1 Methyl and R 2 methyl, isopropyl or tert-butyl.

5. Mixture according to claim 1, wherein the compound of general formula (I) is 3-cyclohexyl-1,5-dimethyl-1H-pyrazol-4-ol, 3-cyclohexyl-1-isopropyl-5-methyl-1H-pyrazol-4-ol or 1-(tert-butyl)-3-cyclohexyl-5-methyl-1H-pyrazol-4-ol.

6. Mixture according to one of claims 1 to 5, wherein the total amount of the compounds of general formula (I) in the mixture is from 0.0001 to 0.1000 wt.%, based on the total amount of polymerizable compounds.

7. Mixture according to one of claims 1 to 6, wherein the total amount of polymerizable compounds is at least 5% by weight, based on the total weight of the mixture.

8. Mixture according to one of claims 1 to 7, wherein the polymerizable compound is acrylic acid and / or methacrylic acid.

9. Mixture according to claim 8, wherein the total amount of the compounds of general formula (I) in the mixture is from 0.0010 to 0.1000 wt.%, based on the total amount of acrylic acid and / or methacrylic acid.

10. Mixture according to one of claims 1 to 7, wherein the polymerizable compound is methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and / or 2-ethylhexyl acrylate.

11. Mixture according to claim 10, wherein the total amount of the compounds of general formula (I) in the mixture is from 0.0001 to 0.0100 wt.%, based on the total amount of methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and / or 2-ethylhexyl acrylate.

12. Use of at least one compound of general formula (I) as defined in any one of claims 1 to 5 for inhibiting the polymerization of at least one polymerizable compound.

13. A method for storing and / or transporting a mixture according to any one of claims 1 to 11.

14. The method according to claim 13, wherein the mixture is stored and / or transported under an oxygen-containing atmosphere.

15. The method according to claim 13, wherein the mixture is stored in a container under an oxygen-containing atmosphere with an oxygen content of 5 to 10 vol.% and the mixture is regularly circulated in the container.

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