JP7543645B2 - Method for producing highly polymerizable N-vinyl carboxylic acid amide monomer - Google Patents

Description

The present invention relates to a method for producing a highly polymerizable N-vinyl carboxylic acid amide monomer. More specifically, the present invention relates to a method for producing an N-vinyl carboxylic acid amide monomer with improved polymerizability with high productivity.

N-vinyl carboxylic acid amide polymers derived from N-vinyl carboxylic acid amides are water-soluble polymers that dissolve not only in water but also in polar solvents such as alcohol and dimethyl sulfoxide (DMSO). As they are non-ionic polymers, they are not affected by salts or pH, and have the properties of being highly weather-resistant, especially stable against heat. Taking advantage of these properties, they are used industrially as binders, dispersants, adhesives, thickeners, flocculants, etc.

Many methods have been proposed for producing N-vinyl carboxylic acid amides, such as a method in which an intermediate N-(1-alkoxyethyl) carboxylic acid amide is produced from a carboxylic acid amide, acetaldehyde, and an alcohol as raw materials, and then the intermediate is synthesized by thermal or catalytic decomposition.
In the synthesis reaction of this N-vinyl carboxylic acid amide, in addition to N-vinyl carboxylic acid amide, intermediates such as unreacted N-(1-alkoxyethyl) carboxylic acid amide and carboxylic acid amide are contained. These compounds have physical properties such as boiling point, vapor pressure, solubility, etc. close to those of N-vinyl carboxylic acid amide, and it has been difficult to purify N-vinyl carboxylic acid amide by distillation. Therefore, a method for purifying N-vinyl carboxylic acid amide has also been proposed.

Japanese Patent No. 2619204 discloses a purification method using pressure crystallization, which allows N-vinyl carboxylic acid amides showing high polymerizability to be obtained in relatively high purity. However, this method requires a large investment in equipment, and there is a problem that products cannot be provided at low cost unless they are produced industrially on a large scale. In addition, Japanese Patent No. 5126764 discloses a solvent crystallization method using alcohol and aliphatic hydrocarbon. However, this solvent combination requires a large amount of poor solvent and has low volumetric efficiency, resulting in poor productivity.
Furthermore, as a polymerization inhibitor of N-vinyl carboxylic acid amide, Japanese Patent No. 4099831 specifies N-1,3-butadienyl carboxylic acid amide, and WO 17/145569 specifies unsaturated aldehydes. However, there were cases where good polymerizability could not be obtained even after removing the above.

Patent No. 2619204 Patent No. 5126764 Patent No. 4099831 WO 17/145569

The object of the present invention is to provide a method for efficiently purifying N-vinyl carboxylic acid amide and producing a highly polymerizable N-vinyl carboxylic acid amide monomer. Furthermore, the object of the present invention is to produce a high molecular weight N-vinyl carboxylic acid amide polymer using the produced N-vinyl carboxylic acid amide monomer.

As a result of extensive research into the production method of highly polymerizable N-vinyl carboxylic acid amide monomers, we discovered that it is possible to produce highly polymerizable N-vinyl carboxylic acid amide monomers by solvent crystallization using toluene and an aliphatic hydrocarbon having 6 to 7 carbon atoms, and thus completed the present invention.

That is, the present invention is configured as follows.
[1] A method for producing a purified N-vinyl carboxylic acid amide monomer, comprising the steps of: dissolving crude N-vinyl carboxylic acid amide monomer crystals containing 85 to 99% by mass of an N-vinyl carboxylic acid amide monomer in a mixed solvent of toluene and an aliphatic hydrocarbon having 6 to 7 carbon atoms; carrying out solvent crystallization; and recovering a purified N-vinyl carboxylic acid amide monomer product by solid-liquid separation.
A method for producing highly polymerizable N-vinyl carboxylic acid amide monomers.
[2] The mass ratio of toluene/N-vinyl carboxylic acid amide monomer crystals in the solvent crystallization step is 0.01 or more and 0.5 or less;
The method for producing a highly polymerizable N-vinyl carboxylic acid amide monomer according to [1], characterized in that a mass ratio of an aliphatic hydrocarbon having 6 to 7 carbon atoms/N-vinyl carboxylic acid amide monomer crystals in the solvent crystallization step is 0.5 or more and 3.0 or less.
[3] The method for producing a highly polymerizable N-vinyl carboxylic acid amide monomer according to [1] or [2], characterized in that the mass of the toluene is 0.003 to 1.0 relative to the mass of the aliphatic hydrocarbon having 6 to 7 carbon atoms.
[4] The method for producing a highly polymerizable N-vinyl carboxylic acid amide monomer according to any one of [1] to [3], wherein the solvent crystallization is carried out by dissolving the N-vinyl carboxylic acid amide monomer crystals in a mixed solvent of toluene and an aliphatic hydrocarbon having 6 to 7 carbon atoms at 30° C. to 80° C., and then cooling to −20° C. to 35° C.
[5] The method for producing a highly polymerizable N-vinyl carboxylic acid amide monomer according to any one of [1] to [4], characterized in that the aliphatic hydrocarbon having 6 to 7 carbon atoms used in the solvent crystallization is at least one selected from normal hexane, cyclohexane, normal heptane, cycloheptane, and methylcyclohexane.
[6] The method for producing a highly polymerizable N-vinyl carboxylic acid amide monomer according to any one of [1] to [5], characterized in that the solid-liquid separation method in the solvent crystallization step is filtration separation.
[7] The method for producing a highly polymerizable N-vinyl carboxylic acid amide monomer according to any one of [1] to [5], wherein the N-vinyl carboxylic acid amide monomer is an N-vinyl acetamide monomer.

According to the present invention, it is possible to produce highly polymerizable N-vinyl carboxylic acid amide monomers with higher productivity than conventional methods.

A method for carrying out the present invention comprises the following steps.
A process of dissolving crude N-vinylcarboxylic acid amide monomer crystals containing 85 to 99% by mass of the N-vinylcarboxylic acid amide monomer in a mixed solvent of toluene and an aliphatic hydrocarbon having 6 to 7 carbon atoms, followed by solvent crystallization and solid-liquid separation to recover a purified N-vinylcarboxylic acid amide monomer.

In this specification, the term "crude N-vinyl carboxylic acid amide monomer" means that the N-vinyl carboxylic acid amide monomer contains impurities as well as other components, i.e., it is an "unpurified" N-vinyl carboxylic acid amide monomer.

Examples of the N-vinyl carboxylic acid amide monomer include N-vinyl formamide, N-vinyl acetamide, N-methyl-N-vinyl acetamide, N-vinyl benzamide, N-vinyl-N-methyl formamide, N-vinyl-N-ethyl formamide, N-vinyl-N-ethyl acetamide, and N-vinyl pyrrolidone. N-vinyl acetamide is preferred.

The crude N-vinyl carboxylic acid amide monomer may be obtained by any production method. For example, it may be obtained by synthesizing N-(1-alkoxyethyl) carboxylic acid amide from carboxylic acid amide, acetaldehyde and alcohol, and subjecting it to thermal or catalytic decomposition (see JP-A-50-76015). It may also be obtained by synthesizing ethylidene bis carboxylic acid amide from carboxylic acid amide and acetaldehyde, and subjecting it to thermal decomposition (see JP-A-61-106546).
The crude N-vinyl carboxylic acid amide monomer mainly contains, other than the N-vinyl carboxylic acid amide monomer, N-(1-alkoxyethyl) carboxylic acid amide, carboxylic acid amide, alcohols having 1 to 5 carbon atoms, etc., which are impurities derived from the production method of the N-vinyl carboxylic acid amide monomer, etc.

The crude N-vinyl carboxylic acid amide monomer contains the N-vinyl carboxylic acid amide monomer in an amount of 85 to 99% by mass, preferably 90 to 98% by mass, and more preferably 92 to 98% by mass. When the N-vinyl carboxylic acid amide monomer is contained in this range, the recovery rate of the N-vinyl carboxylic acid amide monomer can be increased, and the obtained N-vinyl carboxylic acid amide monomer has high purity and excellent polymerizability, which is desirable. When the amount of the N-vinyl carboxylic acid amide monomer contained in the crude N-vinyl carboxylic acid amide monomer is in the range of 50 to 85% by mass, the content of the N-vinyl carboxylic acid amide monomer may be appropriately adjusted by operations such as crystallization, distillation, and extraction. Insoluble components in the crude N-vinyl carboxylic acid amide monomer can also be removed in advance by such operations.

Normally, the upper limit of the content of N-vinyl carboxylic acid amide monomer in crude N-vinyl carboxylic acid amide monomer obtained by known production methods is within the above-mentioned range, but even if crude N-vinyl carboxylic acid amide monomer higher than this upper limit is treated by the production method of the present invention, it is possible to obtain a purified N-vinyl carboxylic acid amide monomer.

Solvent Crystallization Step In the solvent crystallization step, the recovered N-vinyl carboxylic acid amide monomer crystals are dissolved in toluene, which is a crystallization solvent, in the presence of an aliphatic hydrocarbon having 6 to 7 carbon atoms, which is a poor solvent, and then crystallized. The recovered N-vinyl carboxylic acid amide monomer crystals contain the N-vinyl carboxylic acid amide monomer in an amount of 88% by mass or more, preferably 90% by mass or more, and more preferably 92% by mass or more. The N-vinyl carboxylic acid amide monomer crystals may contain the crystallization mother liquor and the mother liquor used for rinsing, in addition to the N-vinyl carboxylic acid amide monomer.

The ratio of the mass of toluene used to the mass of the N-vinylcarboxylic acid amide monomer crystals used is preferably 0.01 to 0.5, more preferably 0.02 to 0.4, and even more preferably 0.03 to 0.3.
In addition, the aliphatic hydrocarbon having 6 to 7 carbon atoms used is preferably normal hexane, cyclohexane, methylcyclohexane, normal heptane, or cycloheptane, and more preferably cyclohexane or methylcyclohexane. Aliphatic hydrocarbons having 5 or less carbon atoms have a low boiling point and are difficult to handle, while aliphatic hydrocarbons having 8 or more carbon atoms have a high boiling point and may not be completely removed from the crystals. In addition, the mass of the aliphatic hydrocarbon having 6 to 7 carbon atoms is preferably a ratio of 0.5 to 3.0, more preferably 0.5 to 2.0, relative to the mass of the N-vinyl carboxylic acid amide monomer crystals used. This ratio range is desirable because it allows the recrystallization of the N-vinyl carboxylic acid amide monomer to be sufficiently precipitated, does not increase the amount of solvent used, and can increase the volumetric efficiency of the crystallizer.
The mass of toluene used is preferably 0.003 to 1.0, more preferably 0.005 to 0.8, and even more preferably 0.01 to 0.5, relative to the mass of the aliphatic hydrocarbon having 6 to 7 carbon atoms used.

The temperature for dissolving the N-vinylcarboxylic acid amide monomer crystals is preferably 30 to 80° C., more preferably 30 to 60° C., and further preferably 35 to 45° C. This temperature range is desirable because the N-vinylcarboxylic acid amide is sufficiently dissolved without being denatured.
Next, the dissolved N-vinyl carboxylic acid amide monomer is cooled to precipitate recrystallization. The cooling temperature is preferably −20° C. to 35° C., more preferably −15° C. to 30° C., and even more preferably 10° C. to 30° C. This temperature range is desirable because it reduces equipment costs and provides a sufficient crystallization yield.
The difference between the temperature at which the N-vinylcarboxylic acid amide monomer crystals are dissolved and the temperature at which the dissolved N-vinylcarboxylic acid amide monomer is cooled to precipitate recrystallization is preferably 1 to 100°C, more preferably 3 to 40°C, and even more preferably 5 to 30°C.
The recrystallized N-vinyl carboxylic acid amide monomer is recovered to produce a purified N-vinyl carboxylic acid amide monomer.

As a method for efficiently separating the recrystallized N-vinyl carboxylic acid amide monomer precipitated in the solvent crystallization step, separation by filtration is preferred. There are no limitations on the filtration method, but from the viewpoint of separation from the crystallization mother liquor and productivity, centrifugal filtration, pressure filtration, etc. are preferred. In addition, rinsing after filtration is also preferred in terms of improving separation from the mother liquor. As the solvent used for rinsing, toluene and aliphatic hydrocarbons having 6 to 7 carbon atoms, similar to the crystallization solvent, are preferred. The mass of toluene used for rinsing is preferably used in a ratio of 0.01 to 0.3, more preferably 0.01 to 0.2, and even more preferably 0.02 to 0.1, relative to the mass of the N-vinyl carboxylic acid amide monomer crystals recovered from step (A). In addition, the mass of the aliphatic hydrocarbons having 6 to 7 carbon atoms used for rinsing is preferably used in a ratio of 0.1 to 3.0, more preferably 0.1 to 2.0, and even more preferably 0.1 to 1.5, relative to the mass of the N-vinyl carboxylic acid amide monomer crystals recovered from step (A).

The highly polymerizable N-vinyl carboxylic acid amide monomer produced in this way has high purity and high polymerizability.

The mother liquor recovered in the solvent crystallization step contains raw materials for synthesizing an N-vinyl carboxylic acid amide monomer, such as alcohol, carboxylic acid amide, N-(1-alkoxyethyl) carboxylic acid amide, etc. These may be recycled by being sent to a process for producing a crude N-vinyl carboxylic acid amide monomer, for example, a process for synthesizing an N-(1-alkoxyethyl) carboxylic acid amide.
The highly polymerizable N-vinyl carboxylic acid amide monomer thus produced can be used after appropriate polymerization. For example, it can be used as a homopolymer obtained by polymerizing a monomer containing the high purity N-vinyl carboxylic acid amide monomer obtained by the above-mentioned production method, or as a copolymer obtained by polymerizing the N-vinyl carboxylic acid amide monomer with another monomer. These (co)polymers are water-soluble and can be used for various applications.

Examples of the other monomers include (meth)acrylic acid or its salts, methyl (meth)acrylate, ethyl (meth)acrylate, (iso)propyl (meth)acrylate, butyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, methoxyethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and polyoxyalkylene glycol mono(meth)acrylate (meth)acrylic acid-based monomers; (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-methylol(meth)acrylamide, and 2-(meth)acrylamide-2-methylpropanesulfonic acid or its salts. , N-isopropyl (meth)acrylamide and other (meth)acrylamide-based monomers; vinyl ester-based monomers such as vinyl acetate, vinyl butyrate, and vinyl valerate; styrene-based monomers such as styrene, α-methylstyrene, p-methylstyrene, p-methoxystyrene, and m-chlorostyrene; vinyl ether-based monomers such as methyl vinyl ether, butyl vinyl ether, and vinyl benzyl ether; dicarboxylic acid-based monomers such as maleic anhydride, maleic acid or its salt, fumaric acid or its salt, maleic acid dimethyl ester, and fumaric acid diethyl ester; allyl-based monomers such as allyl alcohol, allyl phenyl ether, and allyl acetate; and monomers such as (meth)acrylonitrile, vinyl chloride, ethylene, and propylene.

The other monomers may be used alone or in combination of two or more. The amount of the other monomers may be appropriately determined depending on the application of the copolymer, but it is desirable to use them in an amount of usually 60% by mass or less, preferably 40% by mass or less, of the total monomers.

A polymerization initiator may be used when polymerizing the highly polymerizable N-vinyl carboxylic acid amide monomer of the present invention. As the polymerization initiator, any initiator generally used in the radical polymerization of vinyl compounds can be used without any limitation. For example, a redox-based polymerization initiator, an azo compound-based polymerization initiator, and a peroxide-based polymerization initiator can be mentioned.
These may be used alone or in combination of two or more.
The above (co)polymers are suitably used in a wide range of fields by utilizing their functions such as thickening effect, dispersing effect, etc. Specific examples are given below, but are not limited to these.

(1) Industrial dispersants: For example, they are used as dispersants for various inorganic and organic powders. More specifically, they are used as dispersants for various polar solvents such as water for inorganic powders such as silica, alumina, titania, calcium carbonate, etc.; mineral powders such as talc, kaolin, etc.; various pigment powders such as carbon black, etc.; resin powders such as polyurethane, polyacrylic ester, polyethylene, etc.; and organic powders such as stearates.

(2) Thickeners and dispersants for paints, inks, etc. For example, they are used as additive dispersants, viscosity and leveling regulators, and wettability improvers for paints, inks, etc.

(3) Treatment and collection agents for water and oil. (4) Cosmetics. For example, in cosmetics such as shampoo, rinse, and lotion, they are used as emulsion stabilizers, lubricants, emulsion-type cosmetics (used as an emulsifier), film-type packs, and setting agents.

(5) Toiletry products For example, it is used as a thickening agent in liquid detergents (for clothing, kitchen, toilets and tiles), toothpaste, cleansers, fabric softeners, industrial cleaners, etc.

(6) Adhesives and their auxiliaries (7) Medical field For example, they are used for retaining and sustained-release of drugs in base materials for patches such as tablets (sustained-release drugs), enteric-coated drugs, poultices and plasters, external ointments, drug release controlled preparations, intragastric floating sustained-release preparations, mucosal administration preparations, compositions for epidermis (medical films), wound covering protective materials, dental materials, oral absorbents, interdental cleaning tools, etc., as well as lubricants for medical instruments such as urethral catheters and enema devices which are heated in a disinfecting autoclave for repeated use, and as viscosity adjusters for diagnostic drugs.

(8) Water absorbents, water retention agents, sealing agents, and cooling agents. (9) Others. For example, they are used as papermaking treatment agents, deodorizing agents, drying agents, fermentation aids, packing materials, and stripping agents for old walls, as well as thickening agents for toys, sweat-absorbing accessories, contact media for ultrasonic flaw detection, ultrasonic probes, and electrolyte supports for batteries and sensors.

The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.
The analytical method for the highly polymerizable N-vinylacetamide monomer obtained by the production method of the present invention is described below. However, the analytical method is not limited to the following, and any known method can be used.

Composition Analysis The purity of the N-vinylacetamide monomer obtained in the Examples and Comparative Examples is determined by GC analysis under the following conditions.
Equipment: High-performance general-purpose gas chromatograph "GC-2014" (Shimadzu Corporation)
Column: DB-WAX (φ0.25 mm × 30 m, manufactured by Agilent Technologies)
Carrier gas type: He
Carrier gas flow rate: 1 mL/min Split ratio: 40
Column temperature: Set the temperature rise program in the following order: 40°C (7 min) → temperature rise (25°C/min) → 130°C (15 min) → temperature rise (30°C/min) → 220°C (7 min) Injection temperature: 200°C
Detector: Flame ionization detector (FID)
Detector temperature: 230°C
The N-vinylacetamide monomer was confirmed and identified by ultraviolet-visible spectroscopic absorption spectrum by high performance liquid column chromatography (HPLC) under the following measurement conditions.
Column: Showa Denko K.K.: Shodex (registered trademark) SIL-5B
Eluent: isopropyl alcohol (IPA)/n-hexane = 1/9 (mass ratio)
Column temperature: 40°C
Flow rate: 1.0 mL/min Detector: UV-Vis spectrometer, 254 nm

Polymerization Test The polymerizability of the resulting N-vinylacetamide monomer purified product is evaluated by the following polymerization test.

[1] Prepare a 100 ml glass container equipped with a catalyst injection tube, a nitrogen gas inlet tube, a nitrogen gas exhaust tube, and a thermometer.
[2] 20 g of purified N-vinylacetamide monomer and 58 g of ion-exchanged water are weighed into the glass container [1].
[3] Heat to 30° C. in a water bath while bubbling nitrogen gas at 50 cm ³ /min. Continue passing nitrogen gas through until the end of polymerization.
[4] As a polymerization initiator, 1.6 g of 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (V-044 manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) is dissolved in 48.4 g of ion-exchanged water.
[5] 4.0 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a polymerization initiator is dissolved in 46.0 g of ion-exchanged water.
[6] After one hour of nitrogen gas aeration, 1.0 g of the polymerization initiator solution of [4] and then 1.0 g of the polymerization initiator solution of [5] are added via syringe.
[7] The glass container is removed from the water bath, the moisture on the surface of the glass is removed with paper, and then the container is transferred to an insulated container to continue polymerization.
[8] The polymerization temperature was monitored, and the standard temperature peak arrival time (the time from the injection of the polymerization initiator to the arrival of the temperature peak) was used as an index of polymerization. The polymerization was judged to be good or bad based on the standard temperature peak arrival time, with a time of less than 120 minutes being considered good.

[Synthesis Example 1]
224 g of acetaldehyde, 325 g of methanol, and 100 g of acetamide were reacted at 40° C. under a sulfuric acid catalyst to synthesize N-(1-methoxyethyl)acetamide with a pH of 1.2. A 48% by mass aqueous solution of sodium hydroxide was added to the reaction solution to adjust the pH to 8.3. Thereafter, low boiling point components were distilled off at a temperature of 60 to 70° C. and a pressure of 33 kPa (absolute pressure) using a simple distillation apparatus, and then water and methanol were distilled off at a temperature of 70° C. and a pressure of 0.3 kPa (absolute pressure) to obtain 151 g of N-(1-methoxyethyl)acetamide with a purity of 92% by mass.
Next, N-(1-methoxyethyl)acetamide was fed at a rate of 1.5 g/min to a reactor (a tubular reactor having an inner diameter of 20 mm and a length of 240 mm) maintained at 400° C. and 20 kPa (absolute pressure). A mixture of N-vinylacetamide and methanol produced by the thermal decomposition reaction was condensed in a cooling tube installed at the reactor outlet to obtain a crude N-vinylacetamide recovered product. The conversion rate of N-(1-methoxyethyl)acetamide was 90%.

Next, a 0.3% Pd-Al ₂ O ₃ catalyst was packed into the column (the amount packed was set to 1 ml of catalyst per 20 g of the crude N-vinylacetamide recovered product), and the crude N-vinylacetamide recovered product was circulated and circulated at a reaction temperature of 40° C., a hydrogen gas pressure of 0.03 MPa (gauge pressure), and a space velocity (SV value) in the catalyst-packed column of 100/hour, and N-1,3-butadienylacetamide produced as a by-product in the thermal decomposition reaction was reduced by hydrogenation.
The reaction was continued until the amount of N-1,3-butadienylacetamide was 30 ppm by mass or less. The crude N-vinylacetamide recovered product with reduced N-1,3-butadienylacetamide was distilled using a simple distillation apparatus under conditions of a vacuum degree of 0.3 kPa (absolute pressure) or less and a bottom temperature of 60° C. or less to remove methanol, thereby obtaining 120 g of crude N-vinylacetamide monomer. The N-vinylacetamide monomer purity in this crude N-vinylacetamide monomer was 75% by mass.

[Synthesis Example 2]
The crude N-vinylacetamide monomer obtained in Synthesis Example 1 was heated to 40°C in a crystallizer to melt it, cooled from 40°C to 10°C for 6 hours to cause crystallization, transferred to a centrifugal filter, subjected to solid-liquid separation using the centrifugal filter, and then rinsed with 4 g of toluene/77 g of methylcyclohexane to obtain 52 g of crystals of N-vinylacetamide monomer. The purity of N-vinylacetamide at this time was 95% by mass.

[Example 1]
10 g of toluene and 57 g of methylcyclohexane were added to 52 g of the N-vinylacetamide monomer crystals obtained in Synthesis Example 2, and the mixture was heated to 40° C. to dissolve, and then cooled to 30° C. at 5° C./hour to perform solvent crystallization, thereby precipitating recrystallized N-vinylacetamide monomer. Thereafter, the slurry containing the recrystallized product was transferred to a centrifugal filter, and solid-liquid separation was performed using the centrifugal filter, followed by rinsing with 1 g of toluene/25 g of methylcyclohexane, and the recrystallized product was recovered to obtain a purified product of N-vinylacetamide monomer. The obtained purified product weighed 41 g, and the purity of the N-vinylacetamide monomer was 99.7% by mass.
The resulting purified N-vinylacetamide monomer was subjected to a polymerization test, and the result was 100 minutes.

[Example 2]
Recrystallization of N-vinylacetamide monomer was precipitated in the same manner as in Example 1, except that in the solvent crystallization, the amounts of toluene and methylcyclohexane used were 31 g and 57 g, respectively, and the cooling end point temperature of the solvent crystallization was 20° C. Thereafter, the slurry containing the recrystallization was sent to a centrifugal separator for solid-liquid separation, and after rinsing, the recrystallization was recovered to obtain a purified product of N-vinylacetamide monomer. The obtained purified product weighed 47 g and had a purity of 99.8% by mass. The result of the polymerization test was 112 minutes.

[Example 3]
Recrystallization of N-vinylacetamide monomer was precipitated in the same manner as in Example 1, except that in the solvent crystallization, the amounts of toluene and methylcyclohexane used were 52 g and 57 g, respectively, and the cooling end point temperature of the solvent crystallization was 15° C. Then, the slurry containing the recrystallization was sent to a centrifugal separator for solid-liquid separation, and after rinsing, the recrystallization was recovered to obtain a purified product of N-vinylacetamide monomer. The obtained purified product weighed 47 g and had a purity of 99.8% by mass. The result of the polymerization test was 108 minutes.

[Example 4]
Recrystallization of N-vinylacetamide monomer was precipitated in the same manner as in Example 1, except that normal hexane was used instead of methylcyclohexane in the solvent crystallization and rinsing. Thereafter, the slurry containing the recrystallization was sent to a centrifugal filter, and solid-liquid separation was performed using the centrifugal filter. After rinsing, the recrystallization was recovered to obtain N-vinylacetamide monomer. The obtained purified product weighed 46 g and had a purity of 99.8% by mass. The result of the polymerization test was 104 minutes.

[Example 5]
Recrystallization of N-vinylacetamide monomer was precipitated in the same manner as in Example 1, except that cyclohexane was used instead of methylcyclohexane in the solvent crystallization and rinsing. Thereafter, the slurry containing the recrystallization was sent to a centrifugal filter, and solid-liquid separation was performed using the centrifugal filter. After rinsing, the recrystallization was recovered to obtain N-vinylacetamide monomer. The obtained purified product weighed 43 g and had a purity of 99.8% by mass. The result of the polymerization test was 98 minutes.

[Example 6]
N-vinylacetamide monomer having a purity of 99.7% by mass and a polymerization test result of 106 minutes, which was obtained by carrying out the same operations as in Synthesis Examples 1, 2 and Example 1, was stored at 20° C. for 1.5 years. After storage, the purity was 97.2% by mass, and the result of the polymerization test was 130 minutes. A purified product of N-vinylacetamide monomer was obtained from 52 g of this N-vinylacetamide monomer in the same manner as in Example 1. The obtained purified product weighed 40 g and had a purity of 99.7% by mass. The result of the polymerization test was 102 minutes.

[Comparative Example 1]
A polymerization test was carried out on 52 g (purity 95% by mass) of the crystals of the N-vinylacetamide monomer obtained in Synthesis Example 2. The polymerization time was 160 minutes, which was a poor result.

[Comparative Example 2]
To 100 g of crude N-vinylacetamide monomer obtained in the same manner as in Synthesis Example 1 after removing methanol, 20 g of toluene and 110 g of methylcyclohexane were added and the mixture was heated to 40° C. to dissolve, and then cooled to 5° C. over 7 hours to carry out solvent crystallization. However, even when the mixture was further cooled to 0° C. over 1 hour, no crystal precipitation was observed, and N-vinylacetamide monomer could not be obtained.

[Comparative Example 3]
Recrystallization of N-vinylacetamide monomer was precipitated in the same manner as in Example 1, except that in the solvent crystallization, 20 g of toluene was used, no methylcyclohexane was used, and the end point temperature of the solvent crystallization was set to 10°C. Although the amount of solvent was small and the fluidity of the slurry was considerably deteriorated, the recrystallized N-vinylacetamide monomer was transferred to a centrifugal filter, and solid-liquid separation was performed using the centrifugal filter, and then rinsing was performed with 1 g of toluene/25 g of methylcyclohexane to recover the recrystallized product. The obtained purified product weighed 41 g and had a purity of 99.7% by mass. The result of the polymerizability test was 116 minutes.

[Comparative Example 4]
In the solvent crystallization, 31 g of toluene and 208 g of methylcyclohexane were used, so that it became necessary to enlarge the crystallization apparatus, and the volume efficiency was greatly reduced to one-fourth. In addition, the end point temperature of the solvent crystallization was set to 10°C. Except for that, the recrystallization of N-vinylacetamide monomer was precipitated in the same manner as in Example 1. Since the amount of solvent used was large, the precipitate containing the recrystallized N-vinylacetamide monomer was divided and charged into a centrifugal filter, and solid-liquid separation was performed using the centrifugal filter, and then rinsing was performed with 1 g of toluene/25 g of methylcyclohexane to recover the recrystallization. The obtained purified product had a weight of 49 g and a purity of 99.7% by mass. The result of the polymerizability test was 180 minutes.
The crystallization conditions and the evaluation results are shown in Table 1.

The N-vinyl carboxylic acid amide monomer obtained by the present invention can be used to produce N-vinyl carboxylic acid amide polymers that are used as flocculants, thickeners, dispersants, adhesives, etc.

Claims (5)

The method includes a step of dissolving crude N-vinyl carboxylic acid amide monomer crystals containing 85 to 99% by mass of the N-vinyl carboxylic acid amide monomer in a mixed solvent of toluene and an aliphatic hydrocarbon having 6 to 7 carbon atoms, followed by solvent crystallization and solid-liquid separation to recover a purified N-vinyl carboxylic acid amide monomer product,
the mass of the toluene is 0.003 to 1.0 relative to the mass of the aliphatic hydrocarbon having 6 to 7 carbon atoms;
The N-vinyl carboxylic acid amide monomer is an N-vinyl acetamide monomer;
Method for producing N-vinyl carboxylic acid amide monomer. the mass ratio of toluene/N-vinyl carboxylic acid amide monomer crystals in the solvent crystallization step is 0.01 or more and 0.5 or less;
The method for producing an N-vinyl carboxylic acid amide monomer according to claim 1, characterized in that a mass ratio of an aliphatic hydrocarbon having 6 to 7 carbon atoms/N-vinyl carboxylic acid amide monomer crystals in the solvent crystallization step is 0.5 or more and 3.0 or less. The method for producing an N-vinyl carboxylic acid amide monomer according to claim 1 or 2, wherein the solvent crystallization is carried out by dissolving the N-vinyl carboxylic acid amide monomer crystals in a mixed solvent of toluene and an aliphatic hydrocarbon having 6 to 7 carbon atoms at 30°C to 80°C, and then cooling the mixture to -20°C to 35°C. The method for producing an N-vinyl carboxylic acid amide monomer according to any one of claims 1 to 3, characterized in that the aliphatic hydrocarbon having 6 to 7 carbon atoms used in the solvent crystallization is at least one selected from normal hexane, cyclohexane, normal heptane, cycloheptane, and methylcyclohexane. The method for producing an N-vinyl carboxylic acid amide monomer according to any one of claims 1 to 4, characterized in that the method for solid-liquid separation in the solvent crystallization step is filtration separation.