Method for producing modacrylic resin

Abstract

To provide a method for producing a modacrylic resin that yields a modacrylic resin with a high polymerization ratio of acrylonitrile, wherein a halogen-containing monomer and a sulfonic acid group-containing monomer are employed as copolymer components, in a water-based medium without use of an emulsifier, while inhibiting scale formation.SOLUTION: The production method comprises the step of polymerizing, in a polymerization reactor substantially free of an emulsifier and in the presence of water and a water-soluble polymerization initiator, a monomer composition containing 65-84 mass% of acrylonitrile, 5-34 mass% of one or more halogen-containing monomers selected from the group consisting of vinyl halides and vinylidene halides, and 1-10 mass% of a sulfonic acid group-containing monomer, wherein the sulfonic acid group-containing monomer is at least one selected from the group consisting of 2-acrylamido-2-methyl-1-propanesulfonic acid, methallylsulfonic acid, vinylsulfonic acid, and metal salts thereof, and wherein the sulfonic acid group-containing monomer is supplied stepwise into the polymerization reactor.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a modacrylic resin having a high copolymerization ratio of acrylonitrile. [Background technology]

[0002] Modacrylic resins, which are copolymerized with acrylonitrile and halogenated monomers such as vinyl chloride, are widely used as raw materials for modacrylic fibers because they offer soft texture and flame retardancy when formed into fibers. Modacrylic fibers are typically produced by wet-spinning modacrylic resins. Therefore, to achieve a denser fiber structure during solidification of the spinning dope and to impart dyeability to the fibers, the modacrylic resins used as raw materials for modacrylic fibers are copolymerized with monomers having ionic functional groups, such as sulfonic acid group-containing monomers. Modacrylic resins can generally be produced by emulsion polymerization or suspension polymerization. For example, Patent Document 1 discusses the production of modacrylic resins, which are copolymerized with acrylonitrile, vinylidene chloride, and sulfonic acid group-containing monomers, by emulsion polymerization. Furthermore, Patent Documents 2 to 5 discuss the polymerization of a modacrylic resin obtained by copolymerizing acrylonitrile, vinylidene chloride, and a sulfonic acid group-containing monomer in an aqueous medium using an oxidizing agent such as a persulfate without using a surfactant (also called an emulsifier). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-130948 [Patent Document 2] Japanese Patent Application Publication No. 8-120006 [Patent Document 3] Japanese Patent Application Publication No. 62-209115 [Patent Document 4] Japanese Patent Application Laid-Open No. 63-105012 [Patent Document 5] Japanese Patent Application Publication No. 4-114018 Summary of the Invention [Problem to be solved by the invention]

[0004] On the other hand, in order to improve the heat resistance of modacrylic fibers (less likely to shrink at high temperatures, higher shrinkage initiation temperature), it is necessary to increase the copolymerization ratio of acrylonitrile in the modacrylic resin. However, in the case of emulsion polymerization as described in Patent Document 1, if the copolymerization ratio of acrylonitrile is 60% by mass or more, the stability of the latex is poor, and there is a problem that it solidifies during the polymerization. Furthermore, in the emulsifier-free continuous polymerization without using a surfactant described in Patent Documents 2 to 5, when the copolymerization ratio of acrylonitrile is 65% by mass or more, adhesion of aggregates containing the monomers used in the polymerization (hereinafter also referred to as scale) may occur inside the polymerization machine.

[0005] In order to solve the above-mentioned problems of the related art, the present invention provides a method for producing a modacrylic resin that can produce a modacrylic resin having a high polymerization ratio of acrylonitrile and that contains a halogen-containing monomer and a sulfonic acid group-containing monomer as copolymerization components in an aqueous medium without using an emulsifier, while suppressing the generation of scale. [Means for solving the problem]

[0006] One or more embodiments of the present invention relate to a method for producing a modacrylic resin, comprising: polymerizing a monomer composition in the presence of water and a water-soluble polymerization initiator in a polymerization reactor substantially free of an emulsifier; the monomer composition comprising 65 to 84 mass% of acrylonitrile, 5 to 34 mass% of one or more halogen-containing monomers selected from the group consisting of vinyl halides and vinylidene halides, and 1 to 10 mass% of a sulfonic acid group-containing monomer; the sulfonic acid group-containing monomer is at least one selected from the group consisting of 2-acrylamido-2-methyl-1-propanesulfonic acid, methallylsulfonic acid, vinylsulfonic acid, and metal salts thereof; the amount of water supplied is 150 to 240 mass parts, and the amount of the water-soluble polymerization initiator supplied is 0.20 to 0.44 mass parts, relative to 100 mass parts of the monomer composition; and the sulfonic acid group-containing monomer is supplied to the polymerization reactor in stages. [Effects of the Invention]

[0007] According to one or more embodiments of the production method of the present invention, a modacrylic resin containing a halogen-containing monomer and a sulfonic acid group-containing monomer as copolymerization components and having a high polymerization ratio of acrylonitrile can be obtained by polymerization in an aqueous medium without using an emulsifier, i.e., under conditions in which an emulsifier is substantially absent, while suppressing the generation of scale. DETAILED DESCRIPTION OF THE INVENTION

[0008] The inventors of the present invention have conducted extensive research to solve the above-mentioned problems. As a result, they have discovered that by using a specific sulfonic acid group-containing monomer, supplying a predetermined amount of water and a water-soluble polymerization initiator, and supplying the sulfonic acid group-containing monomer in a stepwise manner when polymerizing acrylonitrile, a halogen-containing monomer, and a sulfonic acid group-containing monomer using water as a medium in the substantial absence of an emulsifier, it is possible to obtain a modacrylic resin containing the halogen-containing monomer and the sulfonic acid group-containing monomer as copolymerization components and with a high polymerization ratio of acrylonitrile, while suppressing the generation of scale. In particular, in conventional continuous polymerization, it is difficult to copolymerize a halogen-containing vinyl such as vinyl chloride with acrylonitrile unless the mass ratio of the halogen-containing vinyl such as vinyl chloride to acrylonitrile is high. However, according to the production method of the present invention, even if the mass ratio of the halogen-containing vinyl such as vinyl chloride to acrylonitrile is low, a modacrylic resin containing acrylonitrile, a halogen-containing vinyl such as vinyl chloride, and a sulfonic acid group-containing monomer as copolymerization components can be obtained by continuous polymerization, and with a high polymerization ratio of acrylonitrile.

[0009] In this specification, when a numerical range is indicated with "to", the numerical range includes both end values (upper and lower limits). For example, a numerical range of "X to Y" is a range including both end values X and Y. Furthermore, when multiple numerical ranges are described in this specification, it is understood that it includes numerical ranges obtained by appropriately combining the upper and lower limits of different numerical ranges. Furthermore, when multiple upper and lower limits of a numerical range are described separately in this specification, it is understood that it includes numerical ranges obtained by appropriately combining the upper and lower limits.

[0010] A method for producing a modacrylic resin according to one or more embodiments of the present invention includes polymerizing a monomer composition in the presence of water and a water-soluble polymerization initiator in a polymerization reactor substantially free of an emulsifier. As used herein, the term "emulsifier" refers to a surfactant containing a hydrophilic group and a lipophilic group. The phrase "substantially free of emulsifier" as used herein means that no emulsifier is intentionally added to the polymerization reactor; if an emulsifier is present as a contaminant, the phrase "substantially free of emulsifier" applies.

[0011] The monomer composition contains 65 to 84 mass% of acrylonitrile, 5 to 34 mass% of one or more halogen-containing monomers selected from the group consisting of vinyl halides and vinylidene halides, and 1 to 10 mass% of a sulfonic acid group-containing monomer. This allows for the production of a modacrylic resin that contains a halogen-containing monomer and a sulfonic acid group-containing monomer as copolymerization components and has a high polymerization ratio of acrylonitrile, specifically, a modacrylic resin that contains 65 to 84 mass% of structural units derived from acrylonitrile, 5 to 34 mass% of structural units derived from the halogen-containing monomer, and 1 to 10 mass% of structural units derived from the sulfonic acid group-containing monomer.

[0012] When the modacrylic resin contains 65% by mass or more of structural units derived from acrylonitrile, the heat resistance of the modacrylic fiber made using the modacrylic resin is improved, and the fiber is less likely to shrink at high temperatures or has a higher shrinkage onset temperature. When the modacrylic resin contains 5% by mass or more of structural units derived from the halogen-containing monomer, the fiber can exhibit flame retardancy. When the modacrylic resin contains 1% by mass or more of structural units derived from a sulfonic acid group-containing monomer, the fiber can be made dyeable.

[0013] The monomer composition preferably contains 70 to 84% by mass of acrylonitrile, 5 to 29% by mass of the halogen-containing monomer, and 1 to 10% by mass of the sulfonic acid group-containing monomer. This allows for the production of a modacrylic resin that contains a halogen-containing monomer and a sulfonic acid group-containing monomer as copolymerization components and has a higher polymerization ratio of acrylonitrile, specifically, a modacrylic resin that contains 70 to 84% by mass of structural units derived from acrylonitrile, 5 to 29% by mass of structural units derived from the halogen-containing monomer, and 1 to 10% by mass of structural units derived from the sulfonic acid group-containing monomer. This further improves the heat resistance of modacrylic fibers produced using this modacrylic resin.

[0014] The method for supplying acrylonitrile to the polymerization reactor is not particularly limited, and may be, for example, a lump supply or a stepwise supply. However, from the viewpoint of increasing the conversion rate of the halogen-containing monomer, it is preferable to supply a part of the amount, for example, 5 to 30 mass% of the total amount, to the polymerization reactor in a lump at an early stage before the polymerization temperature is reached, and then supply the remaining amount, for example, 70 to 95 mass% of the total amount, stepwise after the polymerization temperature is reached. The stepwise supply may be an intermittent supply divided into multiple times, or a continuous supply for a certain period of time. The supply rate during continuous supply may be constant (uniform) or nonuniform, but from the viewpoint of polymerization stability, it is preferable that it be uniform.

[0015] The vinyl halide is not particularly limited, and examples thereof include vinyl chloride, vinyl bromide, and vinyl iodide. The vinylidene halide is not particularly limited, and examples thereof include vinylidene chloride, vinylidene bromide, and vinylidene iodide. These halogen-containing monomers may be used alone or in combination of two or more.

[0016] The method for supplying the halogen-containing monomer to the polymerization reactor is not particularly limited, and may be, for example, a lump-sum supply or a stepwise supply. However, from the viewpoint of increasing the conversion rate of the halogen-containing monomer, it is preferable to supply the entire amount to the polymerization reactor in a lump-sum supply at an early stage before the polymerization temperature is reached.

[0017] The sulfonic acid group-containing monomer is at least one selected from the group consisting of 2-acrylamido-2-methyl-1-propanesulfonic acid, methallylsulfonic acid, vinylsulfonic acid, and metal salts thereof, and is preferably at least one selected from the group consisting of 2-acrylamido-2-methyl-1-propanesulfonic acid, methallylsulfonic acid, and metal salts thereof. This allows for the production of a modacrylic resin containing a sulfonic acid group-containing monomer as a copolymerization component and with a high acrylonitrile polymerization ratio. The metal salt may be an alkali metal salt such as a sodium salt.

[0018] The sulfonic acid group-containing monomer is supplied to the polymerization reactor in stages. This allows for the production of a modacrylic resin containing a halogen-containing monomer and a sulfonic acid group-containing monomer as copolymerization components and with a high acrylonitrile polymerization ratio in an aqueous medium in the substantial absence of an emulsifier, while suppressing scale generation. The sulfonic acid group-containing monomer may be supplied to the polymerization reactor intermittently in multiple batches, or continuously for a certain period of time. From the viewpoint of increasing the conversion rate of the sulfonic acid group-containing monomer, it is preferable to continuously supply the entire amount of the sulfonic acid group-containing monomer to the polymerization reactor for a predetermined period of time after the temperature of the polymerization reactor reaches the polymerization temperature. If the polymerization period is defined as the period from the time the polymerization temperature is reached to the end of polymerization, it is preferable to continuously supply the entire amount of the sulfonic acid group-containing monomer to the polymerization reactor for a predetermined period of time in the first half of the polymerization period. The supply rate of the sulfonic acid group-containing monomer may be constant or non-uniform, but is preferably constant from the viewpoint of polymerization stability.

[0019] The amount of water supplied is 150 to 240 parts by mass, preferably 160 to 235 parts by mass, more preferably 170 to 230 parts by mass, even more preferably 180 to 225 parts by mass, even more preferably 190 to 220 parts by mass, and particularly preferably 200 to 220 parts by mass, per 100 parts by mass of the monomer composition. This allows for the production of a modacrylic resin containing a halogen-containing monomer and a sulfonic acid group-containing monomer as copolymerization components and with a high acrylonitrile polymerization ratio in an aqueous medium in the substantial absence of an emulsifier, while suppressing scale generation. The water is not particularly limited, and various types of water, such as ion-exchanged water, can be used as appropriate. The method for supplying water to the polymerization reactor is not particularly limited. For example, from the standpoint of heat removal, the majority of the water, for example, 40 to 100% of the total amount, may be supplied to the polymerization reactor all at once in the early stages before the polymerization temperature is reached.

[0020] The amount of the water-soluble polymerization initiator to be supplied is 0.20 to 0.44 parts by mass, and preferably 0.25 to 0.34 parts by mass, relative to 100 parts by mass of the monomer composition, thereby making it possible to obtain a modacrylic resin containing a halogen-containing monomer and a sulfonic acid group-containing monomer as copolymerization components and having a high polymerization ratio of acrylonitrile in an aqueous medium in the substantial absence of an emulsifier, while suppressing the generation of scale.

[0021] The water-soluble polymerization initiator may be any water-soluble polymerization initiator commonly used in polymerization, and is not particularly limited. Examples include water-soluble inorganic peroxides and water-soluble azo compounds, with water-soluble inorganic peroxides being preferred from the viewpoint of easy availability. Examples of water-soluble inorganic peroxides include persulfates and hydrogen peroxide, with persulfates being preferred from the viewpoint of ease of polymerization. Examples of persulfates include ammonium persulfate, sodium persulfate, and potassium persulfate.

[0022] The water-soluble polymerization initiator such as a persulfate (oxidizing agent) is preferably used in combination with a reducing agent from the viewpoint of enhancing polymerization efficiency. Examples of the reducing agent include sodium hydrogen sulfite, ammonium hydrogen sulfite, and sodium thiosulfate. The mass ratio of the oxidizing agent such as a persulfate to the reducing agent is not particularly limited, and may be, for example, 1 to 4 parts by mass of the reducing agent per 1 part by mass of the oxidizing agent such as a persulfate. Furthermore, from the viewpoint of enhancing polymerization efficiency, the oxidizing agent such as a persulfate and the reducing agent may be used in combination with a polymerization accelerator such as sulfuric acid, iron sulfate, or copper sulfate. 0.5 to 2 parts by mass of the polymerization accelerator may be used per 1 part by mass of the oxidizing agent such as a persulfate.

[0023] A portion of the water-soluble polymerization initiator such as the persulfate (oxidizing agent), for example, 1 to 25 mass% of the total amount, may be supplied all at once to the polymerization reactor at an early stage before the polymerization temperature is reached, and the remaining amount, for example, 75 to 99 mass% of the total amount, may be supplied continuously for a certain period after the polymerization temperature is reached. The supply rate of the remaining amount of the water-soluble polymerization initiator may be constant or may be non-uniform, but is preferably constant from the viewpoint of polymerization stability.

[0024] The method for supplying the reducing agent, the polymerization accelerator, and the like to the polymerization reactor is not particularly limited, and may be, for example, a lump supply or a stepwise supply. However, from the viewpoint of improving the yield, it is preferable to supply the entire amount to the polymerization reactor at once in the initial stage before the polymerization temperature is reached.

[0025] The polymerization reactor is not particularly limited as long as it is a reactor that can be used for polymerizing a modacrylic resin. The polymerization temperature is not particularly limited, but may be, for example, 40 to 70°C or 45 to 65°C from the viewpoint of heat removal from the polymerization reaction and resin quality.

[0026] The polymerization can produce a modacrylic resin in a slurry form. The modacrylic resin in a slurry form can be dehydrated and then dried to produce a powder modacrylic resin. The water content of the modacrylic resin after dehydration and before drying may be 120% or less. The drying method is not particularly limited and can be performed using, for example, a hot air dryer. The drying temperature may be 50 to 100°C or 55 to 85°C from the viewpoint of suppressing resin coloration. In this specification, the water content of the modacrylic resin after dehydration and before drying can be measured as described in the Examples.

[0027] The conversion rate of the sulfonic acid group-containing monomer is not particularly limited and may be, for example, 35% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more. In this specification, the conversion rate of the sulfonic acid group-containing monomer can be measured as described in the Examples.

[0028] The polymerization yield of the modacrylic resin is not particularly limited, and is preferably 50 to 100%, more preferably 60 to 100%, and even more preferably 70 to 100%, from the viewpoint of reducing the remaining proportion of auxiliary raw materials. In this specification, the yield of the modacrylic resin can be measured as described in the examples.

[0029] The modacrylic resin is not particularly limited, but for example, when used in wet spinning, from the viewpoint of increasing the resin concentration in the spinning dope, realizing a dense fiber structure, and achieving fiber strength, the weight average molecular weight is preferably 10,000 to 500,000, more preferably 20,000 to 400,000, and even more preferably 30,000 to 200,000.

[0030] The modacrylic resin is not particularly limited, but from the viewpoint of increasing fiber strength and easily obtaining modacrylic fibers with good combability, the molecular weight distribution (mass average molecular weight / number average molecular weight) is preferably 1.1 to 7.0, and more preferably 1.1 to 5.0. In this specification, the mass average molecular weight and number average molecular weight of the modacrylic resin can be measured as described in the examples.

[0031] The modacrylic resin is not particularly limited, but when dimethylformamide is used as a solvent, the specific viscosity at 30° C. is preferably 0.05 to 0.60, and more preferably 0.05 to 0.50. In this specification, the specific viscosity can be measured as described in the examples.

[0032] The modacrylic resin can be suitably used as a raw material for modacrylic fibers. The modacrylic resin contains structural units derived from sulfonic acid group-containing monomers, which allows it to form a dense structure when used in wet spinning and also allows it to impart dyeability to fibers. Because the modacrylic resin has a high copolymerization ratio of acrylonitrile, modacrylic fibers made from the modacrylic resin have good heat resistance, are less likely to shrink at high temperatures, or have a high shrinkage initiation temperature. [Example]

[0033] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0034] First, various measurement and evaluation methods will be described. (1) Polymerization yield of modacrylic resin The polymerization yield was calculated using the following formula based on the total mass (A) of the nonvolatile components used in the polymerization (parts by mass) when the total amount of the monomers used is taken as 100 parts by mass, the solids concentration (Sc%) of the slurry or latex after the polymerization reaction, the acrylonitrile content (AN%) calculated by elemental analysis of the modacrylic resin, and the content (S%) of ionic functional groups (sulfonic acid groups) determined by sulfur analysis. The nonvolatile components used in the polymerization refer to all components excluding vinyl chloride. Polymerization yield (%) = (Sc% × A) / (1 - Sc% × (1 - AN% - S%)) (2) Conversion rate of sulfonic acid group-containing monomer It is calculated using the following formula from the sulfonic acid group content (S%) determined by sulfur analysis and the polymerization yield (%) obtained in (1). In the formula below, the amount of sulfonic acid group-containing monomer charged is expressed in parts by mass when the total amount of the monomers used is taken as 100 parts by mass. Conversion rate (%) = S% × polymerization yield% × 100 / charge amount of sulfonic acid group-containing monomer (3) Moisture content of modacrylic resin The slurry after polymerization was dehydrated for 5 minutes using a centrifugal dehydrator at a centrifugal force of 640 × g to obtain a hydrous resin (W0), and the hydrous resin was dried in a hot air dryer at 60°C for 24 hours to obtain a dry resin (W1). The water content of the modacrylic resin was calculated based on the following formula. Moisture content (%)=[(W0-W1) / W1]×100 (4) Mass average molecular weight and number average molecular weight of modacrylic resin Measurements and calculations were performed by GPC using gel permeation chromatography ("HLC-8320GPC" manufactured by Tosoh Corporation). Polystyrene was used as the standard polymer, and dimethylformamide was used as the mobile phase (eluent). (5) Specific viscosity ηsp of modacrylic resin 1.0 g of modacrylic resin was dissolved in 500 mL of dimethylformamide, and the specific viscosity ηsp was measured at 30°C using an Ostwald viscometer. (6) Scale evaluation After the completion of polymerization, the slurry was discharged, and the polymerization reactor was filled with water, and the water was discharged to remove the remaining resin. Thereafter, the polymerization reactor was opened, and the reactor wall, stirring blades, and baffle plates were inspected visually and by touch, and scale evaluation was performed according to the following criteria. A: Almost no scale (aggregates containing the monomers used in polymerization) was found by visual inspection on the reactor wall, stirring blades, or baffle plates, and no roughness was felt by palpation. B: Almost no scale is visible on the reactor wall, stirring blades, and baffle plates of the polymerization reactor, but roughness is felt by touch. C: Polymerization can be continued, but scale formation can be visually confirmed throughout the polymerization reactor. D: A large amount of scale has formed on the entire polymerization reactor, making it impossible to continue polymerization.

[0035] Example 1 A polymerization reactor was charged with 17 parts by weight of vinyl chloride (VCM), 16 parts by weight of acrylonitrile (AN), 166 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium bisulfite, 0.003 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 64 parts by weight of acrylonitrile and 3.0 parts by weight of 2-acrylamido-2-methyl-1-propanesulfonate sodium (AMPSNa) dissolved in 27 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water were continuously fed at a constant rate from the start of polymerization to the end of polymerization. After completion of polymerization, unreacted monomer was recovered from the polymerization reactor, and the slurry was discharged. The resulting slurry was dehydrated and dried in a hot air dryer at 60°C for 24 hours to obtain a modacrylic resin. The resulting modacrylic resin consisted of 78.5% by mass of structural units derived from acrylonitrile, 19.6% by mass of structural units derived from vinyl chloride, and 1.9% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate. The mass-average molecular weight was approximately 239,000, the molecular weight distribution (mass-average molecular weight / number-average molecular weight) was 2.94, and the specific viscosity was 0.423. The polymerization yield was 98%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 62%.

[0036] Example 2 A polymerization reactor was charged with 27 parts by weight of vinyl chloride, 14 parts by weight of acrylonitrile, 166 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium bisulfite, 0.003 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 56 parts by weight of acrylonitrile and 3.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 27 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water were continuously fed at a constant rate from the start of polymerization to the end of polymerization. After polymerization was completed, unreacted monomer was recovered from the polymerization reactor, and the slurry was discharged. The resulting slurry was dehydrated and dried in a hot air dryer at 60°C for 24 hours to obtain a modacrylic resin. The resulting modacrylic resin consisted of 74.9% by mass of structural units derived from acrylonitrile, 23.1% by mass of structural units derived from vinyl chloride, and 2.0% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate, and had a mass-average molecular weight of approximately 201,000, a molecular weight distribution (mass-average molecular weight / number-average molecular weight) of 3.18, and a specific viscosity of 0.366. The polymerization yield was 85%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 57%.

[0037] Example 3 A modacrylic resin was obtained in the same manner as in Example 1, except that 3.0 parts by mass of sodium methallyl sulfonate (hereinafter also referred to as "SMS") was used instead of sodium 2-acrylamido-2-methyl-1-propanesulfonate. The resulting modacrylic resin consisted of 82.2% by mass of structural units derived from acrylonitrile, 16.2% by mass of structural units derived from vinyl chloride, and 1.6% by mass of structural units derived from sodium methallylsulfonate, and had a mass-average molecular weight of approximately 335,000, a molecular weight distribution (mass-average molecular weight / number-average molecular weight) of 3.79, and a specific viscosity of 0.613. The polymerization yield was 95%, and the conversion of sodium methallylsulfonate was 51%.

[0038] Example 4 A modacrylic resin was obtained in the same manner as in Example 2, except that 3.0 parts by mass of sodium methallyl sulfonate was used instead of sodium 2-acrylamido-2-methyl-1-propanesulfonate. The resulting modacrylic resin consisted of 78.1% by mass of structural units derived from acrylonitrile, 19.7% by mass of structural units derived from vinyl chloride, and 2.2% by mass of structural units derived from sodium methallylsulfonate, and had a mass-average molecular weight of approximately 288,000, a molecular weight distribution (mass-average molecular weight / number-average molecular weight) of 3.23, and a specific viscosity of 0.588. The polymerization yield was 74%, and the conversion of sodium methallylsulfonate was 40%.

[0039] Example 5 A modacrylic resin was obtained in the same manner as in Example 1, except that during the polymerization, 3.0 parts by mass of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 27 parts by mass of ion-exchanged water was continuously fed at a constant rate from immediately after the start of polymerization until 2.5 hours later. The resulting modacrylic resin consisted of 78.8% by mass of structural units derived from acrylonitrile, 18.2% by mass of structural units derived from vinyl chloride, and 3.0% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate. The mass-average molecular weight was approximately 213,000, the molecular weight distribution (mass-average molecular weight / number-average molecular weight) was 2.88, and the specific viscosity was 0.406. The polymerization yield was 98%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 98%.

[0040] Example 6 A polymerization reactor was charged with 18 parts by weight of vinyl chloride, 16 parts by weight of acrylonitrile, 175 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium bisulfite, 0.003 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature in the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 64 parts by weight of acrylonitrile was added from immediately after the start of polymerization until 5.5 hours later, 2.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 18 parts by weight of ion-exchanged water was added from immediately after the start of polymerization until 2.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was added continuously at a constant rate from the start of polymerization to the end of polymerization. After completion of polymerization, unreacted monomer was recovered from the polymerization reactor, and the slurry was discharged. The resulting slurry was dehydrated and dried in a hot air dryer at 60°C for 24 hours to obtain a modacrylic resin. The resulting modacrylic resin consisted of 80.1% by mass of structural units derived from acrylonitrile, 17.9% by mass of structural units derived from vinyl chloride, and 2.0% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate. The mass-average molecular weight was approximately 233,000, the molecular weight distribution (mass-average molecular weight / number-average molecular weight) was 2.79, and the specific viscosity was 0.406. The polymerization yield was 98%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 98%.

[0041] Example 7 A polymerization reactor was charged with 19 parts by weight of vinyl chloride, 16 parts by weight of acrylonitrile, 184 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium bisulfite, 0.003 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature in the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 64 parts by weight of acrylonitrile was added from immediately after the start of polymerization until 5.5 hours later, 1.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 9 parts by weight of ion-exchanged water was added from immediately after the start of polymerization until 2.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was added continuously at a constant rate from the start of polymerization to the end of polymerization. After completion of polymerization, unreacted monomer was recovered from the polymerization reactor, and the slurry was discharged. The resulting slurry was dehydrated and dried in a hot air dryer at 60°C for 24 hours to obtain a modacrylic resin. The resulting modacrylic resin consisted of 79.8% by mass of structural units derived from acrylonitrile, 19.2% by mass of structural units derived from vinyl chloride, and 1.0% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate. The mass-average molecular weight was approximately 236,000, the molecular weight distribution (mass-average molecular weight / number-average molecular weight) was 3.09, and the specific viscosity was 0.411. The polymerization yield was 98%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 98%.

[0042] Example 8 A modacrylic resin was obtained in the same manner as in Example 1, except that 3.0 parts by mass of methallylsulfonic acid (hereinafter also referred to as "AMPS-H") was used instead of sodium 2-acrylamido-2-methyl-1-propanesulfonate. The resulting modacrylic resin consisted of 80.2% by mass of structural units derived from acrylonitrile, 17.7% by mass of structural units derived from vinyl chloride, and 2.1% by mass of structural units derived from 2-acrylamido-2-methyl-1-propanesulfonic acid, and had a mass-average molecular weight of approximately 308,000, a molecular weight distribution (mass-average molecular weight / number-average molecular weight) of 3.24, and a specific viscosity of 0.556. The polymerization yield was 96%, and the conversion of 2-acrylamido-2-methyl-1-propanesulfonic acid was 67%.

[0043] Example 9 A polymerization reactor was charged with 27 parts by weight of vinyl chloride, 14 parts by weight of acrylonitrile, 166 parts by weight of ion-exchanged water, 0.03 parts by weight of ammonium persulfate, 0.79 parts by weight of sodium bisulfite, 0.004 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 56 parts by weight of acrylonitrile and 3.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 27 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 2.5 hours later, and 0.41 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water were continuously fed at a constant rate from the start of polymerization to the end of polymerization. After polymerization was completed, unreacted monomer was recovered from the polymerization reactor, and the slurry was discharged. The resulting slurry was dehydrated and dried in a hot air dryer at 60°C for 24 hours to obtain a modacrylic resin. The resulting modacrylic resin consisted of 69.3% by mass of structural units derived from acrylonitrile, 28.4% by mass of structural units derived from vinyl chloride, and 2.3% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate. Its mass-average molecular weight was approximately 111,000, its molecular weight distribution (mass-average molecular weight / number-average molecular weight) was 2.51, and its specific viscosity was 0.218. The polymerization yield was 98%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 78%. A small amount of scale formed in the polymerization reactor.

[0044] (Comparative Example 1) A polymerization reactor was charged with 20 parts by weight of vinyl chloride, 16 parts by weight of acrylonitrile, 193 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium hydrogen sulfite, 0.003 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 64 parts by weight of acrylonitrile was added from immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was continuously added at a constant rate from the start of polymerization to the end of polymerization. A large amount of scale was generated, and the polymerization could not be completed.

[0045] (Comparative Example 2) A modacrylic resin was obtained in the same manner as in Example 1, except that 3.0 parts by mass of sodium styrenesulfonate (hereinafter also referred to as "3S") was used instead of sodium 2-acrylamido-2-methyl-1-propanesulfonate. The obtained modacrylic resin consisted of 77.8% by mass of structural units derived from acrylonitrile, 19.0% by mass of structural units derived from vinyl chloride, and 3.2% by mass of structural units derived from sodium styrenesulfonate, and had a mass average molecular weight of approximately 276,000, a molecular weight distribution (mass average molecular weight / number average molecular weight) of 2.62, and a specific viscosity of 0.631. The polymerization yield was 78%, and the conversion of sodium styrenesulfonate was 88%, but a large amount of scale formed in the polymerization reactor.

[0046] (Comparative Example 3) A polymerization reactor was charged with 17 parts by weight of vinyl chloride, 16 parts by weight of acrylonitrile, 3.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate, 193 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium bisulfite, 0.003 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 64 parts by weight of acrylonitrile was added immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was continuously added at a constant rate from the start of polymerization to the end of polymerization. Unreacted monomer was recovered from the polymerization reactor, and the slurry was then discharged. The resulting slurry was dehydrated and dried in a hot air dryer at 60°C for 24 hours to obtain a modacrylic resin. The resulting modacrylic resin consisted of 79.0% by mass of structural units derived from acrylonitrile, 18.4% by mass of structural units derived from vinyl chloride, and 2.6% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate, and had a mass-average molecular weight of approximately 171,000, a molecular weight distribution (mass-average molecular weight / number-average molecular weight) of 2.32, and a specific viscosity of 0.326. The polymerization yield was 93%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 80%, but a large amount of scale formed in the polymerization reactor.

[0047] Comparative Example 4 A polymerization reactor was charged with 27 parts by weight of vinyl chloride, 14 parts by weight of acrylonitrile, 3.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate, 193 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium bisulfite, 0.003 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 56 parts by weight of acrylonitrile was added immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was continuously added at a constant rate from the start of polymerization to the end of polymerization. A large amount of scale was generated, and the polymerization could not be completed.

[0048] (Comparative Example 5) A modacrylic resin was obtained in the same manner as in Example 1, except that the amount of ion-exchanged water charged before the start of polymerization was changed to 306 parts by mass. The resulting modacrylic resin consisted of 80.6% by mass of structural units derived from acrylonitrile, 17.6% by mass of structural units derived from vinyl chloride, and 1.8% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate, and had a mass-average molecular weight of approximately 292,000, a molecular weight distribution (mass-average molecular weight / number-average molecular weight) of 3.17, and a specific viscosity of 0.481. The polymerization yield was 92%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 55%, but a large amount of scale formed in the polymerization reactor.

[0049] (Comparative Example 6) A polymerization reactor was charged with 37 parts by weight of vinyl chloride, 10 parts by weight of acrylonitrile, 166 parts by weight of ion-exchanged water, 0.04 parts by weight of ammonium persulfate, 1.06 parts by weight of sodium bisulfite, 0.005 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 50 parts by weight of acrylonitrile and 3.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 27 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 2.5 hours later, and 0.54 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water were continuously fed at a constant rate from the start of polymerization to the end of polymerization. After polymerization was completed, unreacted monomer was recovered from the polymerization reactor, and the slurry was discharged. The resulting slurry was dehydrated and dried in a hot air dryer at 60°C for 24 hours to obtain a modacrylic resin. The resulting modacrylic resin consisted of 60.2% by mass of structural units derived from acrylonitrile, 37.8% by mass of structural units derived from vinyl chloride, and 2.0% by mass of structural units derived from sodium 2-acrylamido-2-methyl-1-propanesulfonate, and had a mass-average molecular weight of approximately 89,000, a molecular weight distribution (mass-average molecular weight / number-average molecular weight) of 2.76, and a specific viscosity of 0.168. The polymerization yield was 95%, and the conversion of sodium 2-acrylamido-2-methyl-1-propanesulfonate was 63%, but a large amount of scale formed in the polymerization reactor.

[0050] (Comparative Example 7) A polymerization reactor was charged with 17 parts by weight of vinyl chloride, 16 parts by weight of acrylonitrile, 166 parts by weight of ion-exchanged water, 0.01 parts by weight of ammonium persulfate, 0.26 parts by weight of sodium bisulfite, 0.001 parts by weight of iron sulfate, and 0.35 parts by weight of sulfuric acid (64% by weight). The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 64 parts by weight of acrylonitrile and 3.0 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 27 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 2.5 hours later, and 0.14 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was continuously fed at a constant rate from the start of polymerization to the end of polymerization. A large amount of scale was generated, and the polymerization could not be completed.

[0051] (Comparative Example 8) A polymerization reactor was charged with 43 parts by weight of vinyl chloride, 4 parts by weight of acrylonitrile, 175 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium hydrogen sulfite, 0.003 parts by weight of iron sulfate, 0.35 parts by weight of sulfuric acid (64% by weight), and 0.87 parts by weight of sodium lauryl sulfate. The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 51 parts by weight of acrylonitrile and 2.0 parts by weight of sodium styrenesulfonate dissolved in 18 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water were continuously fed at a constant rate from the start of polymerization to the end of polymerization. The latex solidified during the polymerization, and the polymerization could not be completed.

[0052] Comparative Example 9 A polymerization reactor was charged with 51.8 parts by weight of vinyl chloride, 4 parts by weight of acrylonitrile, 173.2 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium bisulfite, 0.003 parts by weight of iron sulfate, 0.35 parts by weight of sulfuric acid (64% by weight), and 0.87 parts by weight of sodium lauryl sulfate. The temperature inside the polymerization reactor was then raised to 50°C to initiate polymerization, and polymerization was carried out at 50°C for 6 hours. During the polymerization, 42 parts by weight of acrylonitrile and 2.2 parts by weight of sodium 2-acrylamido-2-methyl-1-propanesulfonate dissolved in 19.8 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was continuously fed at a constant rate from the start of polymerization to the end of polymerization. The latex solidified during the polymerization, and the polymerization could not be completed.

[0053] (Comparative Example 10) A polymerization reactor was charged with 52.5 parts by weight of vinyl chloride, 4 parts by weight of acrylonitrile, 179.5 parts by weight of ion-exchanged water, 0.02 parts by weight of ammonium persulfate, 0.53 parts by weight of sodium hydrogen sulfite, 0.003 parts by weight of iron sulfate, 0.35 parts by weight of sulfuric acid (64% by weight), and 0.87 parts by weight of sodium lauryl sulfate, and the polymerization was carried out at a polymerization temperature of 50°C for 6 hours. During the polymerization, 42 parts by weight of acrylonitrile and 1.5 parts by weight of sodium methallylsulfonate dissolved in 13.5 parts by weight of ion-exchanged water were continuously fed from immediately after the start of polymerization until 5.5 hours later, and 0.27 parts by weight of ammonium persulfate dissolved in 17 parts by weight of ion-exchanged water was continuously fed at a constant rate from the start of polymerization to the end of polymerization. The latex solidified during the polymerization, and the polymerization could not be completed.

[0054] In the examples and comparative examples, the mass average molecular weight, number average molecular weight, polymerization yield, conversion rate of sulfonic acid group-containing monomer (hereinafter also referred to as SM), specific viscosity ηsp, and water content of the modacrylic resin were measured as described above, and the results are shown in the following Tables 1 and 2. In addition, the scale inside the polymerization reactor was evaluated as described above, and the results are shown in the following Tables 1 and 2.

[0055] [Table 1]

[0056] [Table 2]

[0057] As can be seen from Table 1 above, in the examples, without using an emulsifier, it was possible to obtain a modacrylic resin in an aqueous medium in which a halogen-containing monomer and a sulfonic acid group-containing monomer were copolymerized as copolymerization components and in which the polymerization ratio of acrylonitrile was increased, while suppressing the generation of scale.

[0058] On the other hand, in Comparative Example 1, in which no sulfonic acid group-containing monomer was used, Comparative Example 4, in which 2-acrylamido-2-methyl-1-propanesulfonate sodium was supplied all at once at the beginning, and Comparative Example 7, in which the amount of water-soluble polymerization initiator supplied was small, a large amount of scale was generated, and the polymerization reaction could not be completed. Scale was generated in Comparative Example 2, in which styrenesulfonate sodium was used as the sulfonic acid group-containing monomer, Comparative Example 3, in which the sulfonic acid group-containing monomer was supplied all at once at the beginning, and Comparative Example 5, in which a large amount of water was supplied. In Comparative Example 6, in which the amount of acrylonitrile supplied was low and the amount of water-soluble initiator supplied was large, the polymerization reaction proceeded, but scale was generated. In Comparative Examples 7 to 9, in which an emulsifier was used, the latex solidified during the polymerization, and it was not possible to polymerize a modacrylic resin with a high acrylonitrile polymerization ratio.

[0059] The present invention is not particularly limited, but it is desirable to include, for example, the following embodiments.

[0060] [1] A method for producing a modacrylic resin, comprising: polymerizing a monomer composition in the presence of water and a water-soluble polymerization initiator in a polymerization reactor substantially free of an emulsifier; the monomer composition contains 65 to 84 mass% of acrylonitrile, 5 to 34 mass% of one or more halogen-containing monomers selected from the group consisting of vinyl halides and vinylidene halides, and 1 to 10 mass% of a sulfonic acid group-containing monomer; the sulfonic acid group-containing monomer is at least one selected from the group consisting of 2-acrylamido-2-methyl-1-propanesulfonic acid, methallylsulfonic acid, vinylsulfonic acid, and metal salts thereof; the amount of the water supplied is 150 to 240 parts by mass, and the amount of the water-soluble polymerization initiator supplied is 0.20 to 0.44 parts by mass, relative to 100 parts by mass of the monomer composition; The method for producing a modacrylic resin comprises supplying the sulfonic acid group-containing monomer to the polymerization reactor in a stepwise manner. [2] The method for producing a modacrylic resin according to [1], wherein the halogen-containing monomer includes a vinyl halide. [3] The method for producing a modacrylic resin according to [1] or [2], wherein the water-soluble polymerization initiator is a persulfate. [4] The method for producing a modacrylic resin according to any one of [1] to [3], wherein the sulfonic acid group-containing monomer is continuously supplied to the polymerization reactor for a predetermined period after the temperature inside the polymerization reactor reaches the polymerization temperature. [5] The method for producing a modacrylic resin according to any one of claims [1] to [4], wherein the water-soluble polymerization initiator is used in combination with a reducing agent. [6] The method for producing a modacrylic resin according to any one of [1] to [5], wherein the amount of the water-soluble polymerization initiator supplied is 0.25 to 0.34 parts by mass per 100 parts by mass of the monomer composition.

Claims

1. A method for producing a modacrylic resin, comprising: polymerizing a monomer composition in the presence of water and a water-soluble polymerization initiator in a polymerization reactor substantially free of an emulsifier; the monomer composition comprises 65 to 84 mass% of acrylonitrile, 5 to 34 mass% of one or more halogen-containing monomers selected from the group consisting of vinyl halides and vinylidene halides, and 1 to 10 mass% of a sulfonic acid group-containing monomer; the sulfonic acid group-containing monomer is at least one selected from the group consisting of 2-acrylamido-2-methyl-1-propanesulfonic acid, methallylsulfonic acid, vinylsulfonic acid, and metal salts thereof; the amount of water supplied is 150 to 240 parts by mass, and the amount of water-soluble polymerization initiator supplied is 0.20 to 0.44 parts by mass, relative to 100 parts by mass of the monomer composition; The method for producing a modacrylic resin comprises supplying the sulfonic acid group-containing monomer to the polymerization reactor in a stepwise manner.

2. The method for producing a modacrylic resin according to claim 1 , wherein the halogen-containing monomer comprises a vinyl halide.

3. The method for producing a modacrylic resin according to claim 1 , wherein the water-soluble polymerization initiator is a persulfate.

4. 2. The method for producing a modacrylic resin according to claim 1, wherein the sulfonic acid group-containing monomer is continuously supplied to the polymerization reactor for a predetermined period of time after the temperature inside the polymerization reactor reaches the polymerization temperature.

5. The method for producing a modacrylic resin according to claim 1, wherein the water-soluble polymerization initiator is used in combination with a reducing agent.

6. 2. The method for producing a modacrylic resin according to claim 1, wherein the amount of the water-soluble polymerization initiator supplied is 0.25 to 0.34 parts by mass relative to 100 parts by mass of the monomer composition.

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