Cyclohexyl-substituted 4-hydroxypyrazole polymerization inhibitor

Abstract

The present invention relates to a mixture comprising at least one cyclohexyl-substituted 4-hydroxypyrazole and at least one polymerizable compound.

Description

Technology Field

[0001] The present invention relates to a mixture comprising at least one cyclohexyl-substituted 4-hydroxypyrazole and at least one polymerizable compound. Background Technology

[0002] Chemical compounds having one or more ethylenically unsaturated groups have a significant tendency toward free radical polymerization. Therefore, such compounds are hereinafter referred to as polymerizable compounds. The tendency of said compounds toward free radical polymerization implies that said compounds are used as monomers for the production of polymers. However, the significant tendency of said compounds toward free radical polymerization is also a disadvantage in that unwanted spontaneous free radical polymerization can occur during storage and transport, and also during chemical and / or physical processing (e.g., distillation or rectification), particularly under the action of energy (e.g., heat and / or light). Such uncontrolled polymerization can lead to the gradual formation of polymer precipitates, for example, on heated surfaces, which requires the removal of the polymer precipitates and thus often results in shortened operating times. Uncontrolled polymerization can even proceed explosively.

[0003] Therefore, it is common to add compounds that prevent or at least delay unwanted spontaneous free-radical polymerization during the storage and transport of ethylenically unsaturated compounds or mixtures containing said compounds that have a tendency toward free-radical polymerization, and also during chemical and / or physical processing. Such substances are referred to as polymerization inhibitors.

[0004] Polymerization inhibitors may be used in the form of individual chemical compounds or mixtures of compounds. Depending on their field of application, specific requirements are imposed on 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 degree of inhibition of polymerization) is controllable. Under the conditions of storage and / or transport of ethylenically unsaturated compounds, the polymerization inhibitor must adequately prevent or delay unwanted spontaneous free-radical polymerization, whereas the desired free-radical polymerization of the ethylenically unsaturated compound must be possible under appropriate polymerization conditions without the need to first detach the polymerization inhibitor used during storage and / or transport. It is important that if the stabilizer used during storage and / or transport is not detached for the desired polymerization, it does not negatively affect the desired polymerization by, for example, unintentionally acting as a chain transfer agent.

[0005] In terms of worldwide production volume, acrylic acid is undoubtedly one of the most important ethylenically unsaturated compounds. During storage and / or transport, acrylic acid is typically stabilized against unwanted spontaneous free-radical polymerization using hydroquinone monomethyl ether (MEHQ) in an amount of 0.018 wt% to 0.022 wt% based on the amount of acrylic acid. To properly stabilize acrylic acid against unwanted spontaneous free-radical polymerization using MEHQ, it is necessary for a sufficient amount of oxygen to be dissolved in the acrylic acid. When acrylic acid is stored and / or transported in an atmosphere containing 5 vol% to 21 vol% oxygen, a sufficient amount of oxygen is generally dissolved in the acrylic acid. In a given polymerization of acrylic acid, the content of dissolved oxygen in the acrylic acid decreases, so that the efficiency of MEHQ as a polymerization inhibitor is sufficiently reduced so that acrylic acid can undergo polymerization in the presence of MEHQ.

[0006] In addition to being used as a polymerization inhibitor for the storage and / or transport of acrylic acid, MEHQ is also used as a polymerization inhibitor for the storage and / or transport of methacrylic acid, acrylic acid esters and / or methacrylic acid esters, or mixtures containing one or more of these compounds.

[0007] The widespread use of MEHQ makes it one of the most important storage and / or transport stabilizers for polymerizable compounds, particularly acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters.

[0008] The polymerizable compound is stored in a suitable permanently installed container (e.g., a storage tank or other storage container) (see, for example, [Acrylic Acid, A Summary of Safety and Handling, 4th edition 2013, 7 Bulk Storage Facilities and Accessories]). The polymerizable compound is stored in each container for preferably at least 1 minute, particularly preferably at least 10 minutes, and very particularly preferably at least 60 minutes. Under appropriate conditions, the storage period is theoretically unlimited, but for economic reasons, the storage period is generally shortened to the minimum. The polymerizable compound is stored in each container for preferably not exceeding 180 days, particularly preferably not exceeding 90 days, and very particularly preferably not exceeding 30 days. The particularly preferred range is derived from a free combination of the aforementioned lower and upper limits.

[0009] Polymerizable compounds are generally placed in suitable transportable containers (e.g., tanks or other containers) and transported by ship, rail, and / or truck (see, e.g., Acrylic Acid, A Summary of Safety and Handling, 4th edition 2013, 9 Safe Transport of Acrylic Acid). Transportable containers may, of course, be permanently installed on the appropriate means of transport (e.g., tanker ships or railway tank cars). Of course, it is also possible for the containers to be stored at a specific location for a specific period of time before being transported to another location. Polymerizable compounds may also be transported via pipelines or hoses, for example, to storage tanks, after purification of the polymerizable compounds, during filling from storage tanks to transportable containers, and / or during filling from one transportable container to another. It is desirable for the polymerizable compounds to be transported in each container for at least 10 seconds, preferably at least 1 minute, particularly preferably at least 10 minutes, and very particularly preferably at least 60 minutes. Under appropriate conditions, the duration is theoretically unlimited, but the duration is generally shortened to the minimum for economic and safety reasons. The polymerizable compound is transported in each container preferably for 180 days or less, particularly preferably for 90 days or less, and very particularly preferably for 30 days or less. The particularly preferred range is derived from a free combination of the aforementioned lower and upper limits.

[0010] The object of the present invention was to provide a mixture comprising at least one polymerizable compound, in particular acrylic acid, methacrylic acid, acrylic acid ester and / or methacrylic acid ester, and at least one polymerization inhibitor. The polymerization inhibitor is intended to ensure sufficient stabilization of the polymerizable compound against unwanted free-radical polymerization. However, for the intended free-radical polymerization, it should not be necessary to separate the polymerization inhibitor from the mixture prior to polymerization. Therefore, the polymerization inhibitor should not act as a chain transfer agent and / or polymerization inhibitor in the intended free-radical polymerization.

[0011] The above objective is achieved by a mixture comprising at least one compound of the following formula (I) and at least one polymerizable compound:

[0012] (I)

[0013] In the above formula,

[0014] R 1 It is H or C1-C6-alkyl, and

[0015] R 2 is H or C1-C6-alkyl.

[0016] Preferably, R 1 is H or C1-C4-alkyl, and R 2 is a C1-C4-alkyl.

[0017] Preferably, R 1 is a C1-C3-alkyl and R 2 is a C1-, C3-, or C4-alkyl, particularly methyl, isopropyl, or tert-butyl.

[0018] More preferably, R 1 is methyl and R 2 is methyl, isopropyl, or tert-butyl.

[0019] The alkyl groups may each be straight-chain, branched, and / or cyclic, but branched and cyclic alkyl compounds will only be possible from C3-alkyl.

[0020] Suitable compounds of chemical formula (I) are, for example, 3-cyclohexyl-1,5-dimethyl-1H-pyrazole-4-ol, 3-cyclohexyl-1-isopropyl-5-methyl-1H-pyrazole-4-ol or 1-(tert-butyl)-3-cyclohexyl-5-methyl-1H-pyrazole-4-ol.

[0021] The total amount of the compound of formula (I) in the above mixture is, in each case, based on the total amount of polymerizable compound, preferably 0.0001 wt% to 0.1000 wt%, more preferably 0.000 wt% to 0.0500 wt%, particularly preferably 0.003 wt% to 0.0250 wt%, very particularly preferably 0.004 wt% to 0.0100 wt%, and particularly preferably 0.005 wt% to 0.0075 wt%.

[0022] The total amount of polymerizable compound in the above mixture is, based on the total weight of the mixture, preferably at least 5% by weight, more preferably at least 50% by weight, particularly preferably at least 80% by weight, very particularly preferably at least 98% by weight, and particularly preferably at least 99% by weight.

[0023] The above polymerizable compound is preferably a mono-, di-, or triethylenedimethyl unsaturated C3-C8-carboxylic acid, a mono-, di-, or triethylenedimethyl unsaturated C3-C8-aldehyde, a mono-, di-, or triethylenedimethyl unsaturated C3-C8-carboxyl ester having 1 to 20 carbon atoms in an ester group, a mono-, di-, or triethylenedimethyl unsaturated C3-C8-carboxamide, a mono-, di-, or triethylenedimethyl unsaturated C3-C8-nitrile, a mono-, di-, or triethylenedimethyl unsaturated C3-C8-carboxylic acid anhydride; saturated C2-C 20 -Vinyl ester of carboxylic acid; saturated C1-C 10- Vinyl ethers of alcohols; vinyl aromatics, vinyl heteroaromatics, vinyl lactams having 3 to 10 carbon atoms in a ring, open-chain N-vinylamide compounds and N-vinylamine compounds, vinyl halides; optionally halogenated aliphatic hydrocarbons having 2 to 8 carbon atoms and 1 or 2 ethylenetic double bonds; vinylidene; or any predetermined mixture of two or more of the aforementioned compounds.

[0024] Particularly preferred are mono-, di-, or triethylenedimethyl unsaturated C3-C8-carboxylic acids, e.g., 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 triethylenedimethyl unsaturated C3-C8-carboxylic acid esters having 1 to 20 carbon atoms in an ester group, e.g., C1-C 20 -Acrylic acid ester having an alkyl group, C1-C 20 -alkyl methacrylic acid ester, C1-C 20 - A dimethacrylic acid ester having an alkyl group, C1-C 20 Ethacrylic acid ester having an alkyl group, C1-C 20 -alkyl citraconic acid ester, C1-C 20 -alkyl methylenemalonic acid ester, C1-C 20 Crotonic acid ester having -alkyl groups, C1-C 20 -alkyl fumaric acid ester, C1-C 20 -alkyl mesaconic acid ester, C1-C 20 Itaconic acid ester having an alkyl group and C1-C 20Maleic acid esters having an alkyl group; mono-, di-, or triethylenedimethyl unsaturated C3-C8-carboxamides, e.g., acrylamide, methacrylamide, dimethacrylamide, ethacrylamide, citraconamide, methylenemalonamide, crotonamide, fumaramide, mesaconamide, itaconamide, and maleamide; mono-, di-, or triethylenedimethyl unsaturated C3-C8-nitriles, e.g., acrylonitrile and methacrylonitrile; and mono-, di-, or triethylenedimethyl unsaturated C3-C8-carboxylic acid anhydrides, e.g., acrylic acid anhydride, methacrylic acid anhydride, itaconic acid anhydride, and maleic acid anhydride.

[0025] Very particularly desirable are acrylic acid, methacrylic acid; acrylic acid esters having a C1-C8-alkyl group, such as methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate; and methacrylic acid esters having a C1-C8-alkyl group, such as methyl methacrylate.

[0026] Additional 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.

[0027] When acrylic acid is used as the polymerizable compound, the total amount of the compound of formula (I) in the mixture is, in each case, based on acrylic acid, preferably 0.0010 wt% to 0.1000 wt%, more preferably 0.0020 wt% to 0.0500 wt%, particularly preferably 0.0025 wt% to 0.0200 wt%, and very particularly preferably 0.0030 wt% to 0.0080 wt%.

[0028] When methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and / or 2-ethylhexyl acrylate is used as the polymerizable compound, the total amount of the compound of formula (I) in the mixture is, in each case, preferably 0.0001 wt% to 0.0100 wt%, more preferably 0.0002 wt% to 0.0075 wt%, particularly preferably 0.0003 wt% to 0.0050 wt%, and very particularly preferably 0.0005 wt% to 0.0020 wt% based on the total amount of methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and / or 2-ethylhexyl acrylate.

[0029] The present invention also provides the use of at least one compound of formula (I) as defined above for inhibiting the polymerization of at least one polymerizable compound.

[0030] The present invention also provides a method for storing and / or transporting a mixture comprising at least one compound of formula (I) as defined above and at least one polymerizable compound.

[0031] The above mixture is generally stored and / or transported in an oxygen-containing atmosphere.

[0032] It is preferable that the mixture be stored in a container in an oxygen-containing atmosphere having an oxygen content of 5 volume% to 10 volume%, and that the mixture in the container be recirculated periodically, for example, by pumping the contents of the entire tank at least once a week. A relatively low oxygen content prevents the flammable gas mixture in the container. The recirculation replaces the consumed dissolved oxygen in the liquid polymerizable compound.

[0033] The mixture of the present invention having acrylic acid as a polymerizable compound is particularly suitable for the production of water-swellable polyacrylic acid (superabsorber) and water-soluble polyacrylic acid. The production of superabsorbers is described, for example, in the literature [Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, volume 35, pages 73 to 93].

[0034] The mixture of the present invention having methyl acrylate, ethyl acrylate, n-butyl acrylate, or 2-ethylhexyl acrylate as a polymerizable compound is particularly suitable for the production of polymer dispersions. Specific details for implementing the invention

[0035] Examples

[0036] 3-cyclohexyl-1,5-dimethyl-1H-pyrazol-4-ol( A ):

[0037] 1,5-dimethyl-3-phenyl-1H-pyrazole-4-ol (2.1 g) was dissolved in 100 g of methanol and introduced into a 300 ml Hastelloy autoclave. 0.5 g of catalyst (0.5% Ru on alumina) was added and the autoclave was closed. Subsequently, the reaction mixture was hydrogenated at 120°C and 200 bar for 48 hours.

[0038] The autoclave was depressurized, and the catalyst was filtered out from the reaction mixture. After removing the solvent under reduced pressure, product A with 99% purity (determined by GC) was obtained. Due to the small scale of the reaction and the repeated sampling during the reaction, the yield is not reported.

[0039]

[0040] 1 H NMR (400 MHz, CDCl3): δ(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).

[0041] 3-cyclohexyl-1-isopropyl-5-methyl-1H-pyrazol-4-ol( B ):

[0042] Sodium bicarbonate (2.52 g, 30 mmol, 1.5 g) and subsequently hexahydrobenzaldehyde (2.24 g, 20 mmol, 1.0 equivalent) were added to a solution of isopropylhydrazine hydrochloride (2.26 g, 20 mmol, 1.0 equivalent) in n-butyl acetate (22 ml). After heating this mixture to 80°C for 1.5 hours, methanol (20 ml) was added. The reaction mixture was further heated to 70°C for 2.5 hours. After removing the methanol under reduced pressure, hexane (20 ml) was added. The formed precipitate was filtered and dried under reduced pressure. The resulting hydrazone (3.38 g, 64% purity by GC, 12.3 mmol, 62% yield) was used in the next step without further purification.

[0043] Hydrazone (3.38 g, 64% purity by GC, 12.3 mmol, 1.0 equivalent) was added to n-butyl acetate (28 ml), followed by the addition of acetic acid (2.55 g, 42.5 mmol, 3.5 equivalents). Upon heating to 70°C, a clear solution was obtained. At that temperature, methylglyoxal (40% in water, 5.46 g, 30.3 mmol, 2.5 equivalents) was added, and the mixture was heated under reflux. After 24 hours, the reaction was cooled to room temperature, and the pH was adjusted to 7 using a 2 M aqueous sodium hydroxide solution. After extraction with water, the organic phase was dried with (MgSO4), and the solvent was removed under reduced pressure. The crude product was purified by column chromatography (silicon dioxide, cyclohexane / ethyl acetate, gradient) to obtain product B as a colorless solid (21% yield over two steps).

[0044]

[0045] 1 H NMR (400 MHz, CDCl3): δ(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, 1H), 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).

[0046] 1-(tert-butyl)-3-cyclohexyl-5-methyl-1H-pyrazol-4-ol( C ):

[0047] in methanol (25 ml) tertSodium bicarbonate (1.68 g, 20 mmol, 1.0 equivalent) was added to a mixture of butylhydrazine hydrochloride (2.49 g, 20 mmol, 1.0 equivalent). The reaction mixture was stirred at room temperature for 1 hour. Subsequently, hexahydrobenzaldehyde (2.24 g, 20 mmol, 1.0 equivalent) was added, and the mixture was stirred at room temperature for an additional 1 hour. The solvent was removed under reduced pressure. The resulting hydrazone was dissolved in ethyl acetate (80 ml), and then methylglyoxal (40% in water, 9.00 g, 50.0 mmol, 2.5 equivalent) and acetic acid (1.92 g, 32.0 mmol, 1.6 equivalent) were added. The reaction mixture was heated to 70°C for 20 hours. Next, the pH was adjusted to 7 using a 2 M aqueous sodium hydroxide solution, and then extracted with ethyl acetate (3 x 100 ml). The combined organic phase was dried with (Na2SO4), and the solvent was removed under reduced pressure. The crude product was purified by column chromatography (silicon dioxide, cyclohexane / ethyl acetate, gradient) to obtain product C (39% yield) as a pale yellow solid.

[0048]

[0049] 1 H NMR (400 MHz, CDCl3): δ(ppm) = 1.30 (m, 4 H) 1.55 (s, 9 H), 1.66 (m, 2 H), 1.79 (m, 2H), 1.88 (m, 2 H), 2.29 (s, 3 H), 2.61 (m, 1 H), 3.53 (s, 1H).

[0050] Inhibition of polymerization experiments

[0051] The monomer used was distilled twice to remove the polymerization inhibitor MEHQ. To the obtained monomer, a specified amount of polymerization inhibitor (ppm = mg / kg) was added in each case.

[0052] 0.5 ml of each mixture was placed in a 1.8 ml ampoule and stored in a convection drying oven at 80°C.

[0053] In each test series, three ampoules of each mixture were filled and tested in each case, and the average time for complete polymerization was determined by visual inspection.

[0054] Relative efficacy is calculated as the ratio of the time to polymerization of the sample containing the polymerization inhibitor to be tested to the time to polymerization of the sample containing MEHQ. Therefore, the efficacy of MEHQ itself is 100%.

[0055] The results are summarized in Tables 1, 2, and 3 below:

[0056]

[0057]

[0058]

[0059] The polymerization inhibitor of the present invention exhibits improved efficacy compared to MEHQ.

[0060] Polymerization experiment

[0061] 450 g of water was initially added to a reaction vessel under a nitrogen atmosphere. The initially added reaction mixture was heated to 95°C while stirring. Once the temperature reached 95°C, three streams were metered in while maintaining the temperature and stirring:

[0062] Stream 1: 500 g of acrylic acid, metered in over 5 hours;

[0063] Stream 2: 15 g of sodium hypophosphite in 35 g of deionized water, metered introduction over 4.75 hours;

[0064] Stream 3: 5 g of sodium peroxodisulfate in 66.4 g of deionized water, metered and introduced over 5.25 hours.

[0065] After adding the three streams mentioned above, the reaction mixture was stirred at 95°C for an additional hour. Subsequently, the reaction mixture was cooled to room temperature, and 80 g of water was added.

[0066] The acrylic acid used in stream 1 was stabilized with 200 ppm by weight of 3-cyclohexyl-1,5-dimethyl-1H-pyrazole-4-ol or 200 ppm by weight of MEHQ.

[0067] The obtained polymer was analyzed by GPC (corrected with Na-PAA standard, eluent: 0.01 mol / l phosphate buffer (pH 7.4; in distilled water containing 0.01 M NaN3).

[0068]

[0069] The polymerization inhibitor of the present invention provided polyacrylic acid having a comparable molecular weight.

Claims

Claim 1 At least one compound of the following chemical formula (I); and a mixture comprising at least one polymerizable compound: (I) In the above formula, R 1 is H or C1-C6-alkyl, and R 2 is H or C1-C6-alkyl. Claim 2 In paragraph 1, R 1 is H or C1-C4-alkyl, and R 2 is a mixture that is a C1-C4-alkyl. Claim 3 In paragraph 1, R 1 is a C1-C3-alkyl, and R 2 A mixture that is a C1-, C3-, or C4-alkyl. Claim 4 In paragraph 1, R 1 is methyl, and R 2 is a mixture of methyl, isopropyl, or tert-butyl phosphorus. Claim 5 In claim 1, the compound of formula (I) is a mixture of 3-cyclohexyl-1,5-dimethyl-1H-pyrazole-4-ol, 3-cyclohexyl-1-isopropyl-5-methyl-1H-pyrazole-4-ol or 1-(tert-butyl)-3-cyclohexyl-5-methyl-1H-pyrazole-4-ol. Claim 6 A mixture according to any one of claims 1 to 5, wherein the total amount of the compound of formula (I) in the mixture is 0.0001 weight% to 0.1000 weight% based on the total amount of polymerizable compounds. Claim 7 A mixture according to any one of claims 1 to 6, wherein the total amount of the polymerizable compound is 5% by weight or more based on the total weight of the mixture. Claim 8 A mixture according to any one of claims 1 to 7, wherein the polymerizable compound is acrylic acid and / or methacrylic acid. Claim 9 A mixture according to claim 8, wherein the total amount of the compound of formula (I) in the mixture is 0.0010 weight% to 0.1000 weight% based on the total amount of acrylic acid and / or methacrylic acid. Claim 10 A mixture according to any 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. Claim 11 A mixture according to claim 10, wherein the total amount of the compound of formula (I) in the mixture is 0.0001% to 0.0100% by weight based on the total amount of methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and / or 2-ethylhexyl acrylate. Claim 12 Use of a compound of formula (I) defined in any one of claims 1 to 5 for inhibiting the polymerization of at least one polymerizable compound. Claim 13 A method for storing and / or transporting a mixture according to any one of claims 1 to 11. Claim 14 In paragraph 13, the method wherein the mixture is stored and / or transported in an oxygen-containing atmosphere. Claim 15 A method according to claim 13, wherein the mixture is stored in a container in an oxygen-containing atmosphere having an oxygen content of 5 volume% to 10 volume%, and the mixture in the container is regularly recirculated.

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