Process for producing a carboxyl group-containing crosslinked polymer or a salt thereof

Abstract

The present invention provides a method for producing a carboxyl group-containing crosslinked polymer or a salt thereof, by which it is possible to achieve a balance between the coatability and the coating film performance of a composition that contains this crosslinked polymer or a salt thereof.　This method for producing a carboxyl group-containing crosslinked polymer or a salt thereof includes a step for polymerizing monomer components including an ethylenically unsaturated carboxylic acid monomer by means of precipitation polymerization or dispersion polymerization in the presence of an exchange chain transfer mechanism type control agent. The exchange chain transfer mechanism type control agent is a polymer having a living radical polymerization active unit derived from an exchange chain transfer mechanism with one or more vinyl-based monomer polymer chains, and / or a polymer other than said polymer.

Description

Technical Field

[0001] The present invention relates to a method for producing a carboxyl group-containing crosslinked polymer or a salt thereof.

Background Art

[0002] Carboxyl group-containing polymers are used in various applications such as thickeners and viscosity modifiers for cosmetics, binders for non-aqueous electrolyte secondary battery electrodes, anti-settling agents for pigments, and dispersion stabilizers for metal powders. Among these applications, as a thickener for cosmetics, when the carboxyl group-containing polymer is linear, it has drawability and stickiness, but as the degree of crosslinking increases, the drawability decreases and a feeling of freshness is felt. Therefore, in cosmetics where drawability is not required and freshness is required, a carboxyl group-containing crosslinked polymer is often used because high thickening properties can be obtained with a small amount of use. Also, as a binder for lithium-ion secondary battery electrodes, a carboxyl group-containing crosslinked polymer is often used because it can impart good binding properties and cycle characteristics.

[0003] Here, as methods for producing carboxyl group-containing crosslinked polymers, various methods such as precipitation polymerization and dispersion polymerization are known.

[0004] The precipitation polymerization method is a polymerization method for obtaining a crosslinked polymer in a system where the polymerizable monomer is dissolved in a solvent but the resulting polymer is insoluble in the solvent and precipitates, and generally particles of several μm to several hundred μm are obtained. For example, in Patent Document 1, as a carboxyl group-containing crosslinked polymer, a slightly crosslinked acrylic acid-based polymer is produced by precipitation polymerization and its use as a binder for lithium-ion secondary battery electrodes is attempted.

[0005] In addition, the dispersion polymerization method is a method of obtaining a crosslinked polymer as primary particles by using a dispersion stabilizer or the like in precipitation polymerization. It is a polymerization method capable of controlling the particle diameter of the generated fine particles depending on the type and addition amount of the polymerizable monomer, solvent, and dispersion stabilizer, and is suitable for obtaining particles having a size of submicron to several micrometers. For example, in Patent Document 2, as a crosslinked polymer containing a carboxyl group, a slightly crosslinked acrylic polymer is produced by dispersion polymerization and its use as a binder for a lithium ion secondary battery electrode is attempted.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, according to the inventors' studies, when using a slightly crosslinked acrylic polymer produced by the precipitation polymerization method or dispersion polymerization method described in Patent Documents 1 and 2 as a binder for particles in a slurry (for example, a binder for an active material in a lithium ion secondary battery electrode slurry), while the crosslinking of the acrylic polymer can enhance the binding property between the particles in the slurry, the spread of the polymer in water increases and the viscosity greatly increases even with a small addition, so there is a limit to reducing the slurry viscosity, and it may be a problem that the coating property and the coating film performance (for example, the cycle characteristics of a lithium ion secondary battery) cannot be achieved simultaneously.

[0008] The present invention has been made in view of such circumstances, and an object thereof is to provide a method for producing the crosslinked polymer or a salt thereof that can achieve both the coating property and the coating film performance of a composition containing a crosslinked polymer containing a carboxyl group or a salt thereof.

Means for Solving the Problems

[0009] As a result of intensive studies to solve the above problems, the present inventors have found that by using a carboxyl group-containing crosslinked polymer or a salt thereof obtained by a production method comprising a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization in the presence of a metathesis chain transfer mechanism type controller, it is possible to exhibit excellent coating film performance while ensuring the coatability of a composition containing the crosslinked polymer or a salt thereof, and thus completed the present invention.

[0010] The present invention is as follows. 〔1〕A method for producing a carboxyl group-containing crosslinked polymer or a salt thereof, comprising a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization in the presence of a metathesis chain transfer mechanism type controller, wherein the metathesis chain transfer mechanism type controller is a polymer having a living radical polymerization active unit by a metathesis chain transfer mechanism with a polymerization chain of one or more vinyl monomers, and / or a metathesis chain transfer mechanism type controller other than the polymer. 〔2〕The production method according to 〔1〕, wherein the metathesis chain transfer mechanism type controller is a reversible addition-fragmentation chain transfer agent (RAFT agent). 〔3〕The production method according to 〔2〕, wherein the reversible addition-fragmentation chain transfer agent has a trithiocarbonate group in the molecule. 〔4〕The production method according to any one of 〔1〕 to 〔3〕, wherein the amount of the metathesis chain transfer mechanism type controller used is 0.0001 to 0.50 mol% based on the total amount of the monomer component containing the ethylenically unsaturated carboxylic acid monomer. 〔5〕The production method according to any one of 〔1〕 to 〔4〕, wherein the monomer component contains 50% by mass or more and 100% by mass or less of an ethylenically unsaturated carboxylic acid monomer based on the total amount thereof. 〔6〕The production method according to any one of 〔1〕 to 〔5〕, wherein the crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.1 part by mass or more and 2.0 parts by mass or less based on 100 parts by mass of the total amount of the non-crosslinkable monomers. [7] The production method according to any one of [1] to [6], wherein the crosslinked polymer or a salt thereof is neutralized to a neutralization degree of 80 to 100 mol% and has a particle diameter measured in an aqueous medium of 0.1 μm or more and 5.0 μm or less in terms of volume-based median diameter. [8] The production method according to any one of [1] to [7], wherein the crosslinked polymer or a salt thereof has a water swelling degree at pH 8 of 20 or more and 80 or less. [Advantages of the Invention]

[0011] According to the carboxyl group-containing crosslinked polymer or a salt thereof obtained by the production method of the present invention, it is possible to achieve both the coatability and the coating film performance of the composition containing the crosslinked polymer or a salt thereof. [Brief Description of the Drawings]

[0012]

Figure 1

[0013] The present invention is a production method comprising a step of producing a carboxyl group-containing crosslinked polymer (hereinafter also referred to as "the present crosslinked polymer") or a salt thereof by subjecting a monomer component containing an ethylenically unsaturated carboxylic acid monomer to precipitation polymerization or dispersion polymerization in the presence of a chain transfer type regulator.

[0014] Hereinafter, the present invention will be described in detail. In the present specification, "(meth)acryl" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Further, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group.

[0015] 1. Structural unit of the present crosslinked polymer [Structural Unit Derived from Ethylenically Unsaturated Carboxylic Acid Monomer] This crosslinked polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as “component (a)”), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of a monomer component containing an ethylenically unsaturated carboxylic acid monomer. Since this crosslinked polymer has such a structural unit and thus has a carboxyl group, the adhesiveness to the base material is improved and water swellability is imparted, so that the stability of a composition containing this crosslinked polymer or a salt thereof (hereinafter also referred to as “this composition”) can be enhanced. In particular, in the application of lithium-ion secondary batteries, the adhesiveness to the current collector as the base material is improved, and since it is excellent in the desolvation effect of lithium ions and ionic conductivity, an electrode with low resistance and excellent high-rate characteristics can be obtained.

[0016] Examples of the ethylenically unsaturated carboxylic acid monomer include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamidealkylcarboxylic acids such as (meth)acrylamidehexanoic acid and (meth)acrylamidedodecanoic acid; ethylenically unsaturated monomers having a carboxyl group such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, or (partial) alkali neutralized products thereof. One of these may be used alone, or two or more thereof may be used in combination. Among the above, a compound having an acryloyl group as a polymerizable functional group is preferable in that a polymer having a long primary chain length can be obtained due to a high polymerization rate and the adhesive strength of this crosslinked polymer becomes good, and acrylic acid is particularly preferable. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer having a high carboxyl group content can be obtained.

[0017] The content of component (a) in the present crosslinked polymer is not particularly limited, but for example, it can be contained in an amount of 10% by mass or more and 100% by mass or less based on all the structural units of the present crosslinked polymer. By containing component (a) within such a range, excellent adhesiveness to the base material can be easily ensured. The lower limit is, for example, 20% by mass or more, and also for example, 30% by mass or more, and also for example, 40% by mass or more. When the lower limit is 50% by mass or more, the dispersion stability of the present composition becomes good and a higher adhesive strength can be obtained, which is preferable. It may be 60% by mass or more, or 70% by mass or more, or 80% by mass or more. Further, the upper limit is, for example, 99.9% by mass or less, and also for example, 99.5% by mass or less, and also for example, 99% by mass or less, and also for example, 98% by mass or less, and also for example, 95% by mass or less, and also for example, 90% by mass or less, and also for example, 80% by mass or less. As the range, a range obtained by appropriately combining such lower and upper limits can be adopted. For example, it can be 10% by mass or more and 100% by mass or less, or 50% by mass or more and 100% by mass or less, or 50% by mass or more and 99.9% by mass or less, or 50% by mass or more and 99% by mass or less, or 50% by mass or more and 98% by mass or less, etc.

[0018] <Other structural units> In addition to component (a), the present crosslinked polymer can contain structural units derived from other ethylenically unsaturated monomers copolymerizable therewith (hereinafter also referred to as "component (b)"). Examples of component (b) include structural units derived from ethylenically unsaturated monomer compounds having an anionic group other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers. These structural units can be introduced by copolymerizing a monomer containing an ethylenically unsaturated monomer compound having an anionic group other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or a nonionic ethylenically unsaturated monomer.

[0019] (b) The proportion can be 0% by mass or more and 90% by mass or less with respect to all the structural units of the present crosslinked polymer. The proportion of component (b) may be 1% by mass or more and 60% by mass or less, may be 2% by mass or more and 50% by mass or less, may be 5% by mass or more and 40% by mass or less, or may be 10% by mass or more and 30% by mass or less. In particular, in the case of lithium-ion secondary battery applications, when component (b) is contained at 1% by mass or more with respect to all the structural units of the present crosslinked polymer, the affinity for the electrolyte is improved, and thus an effect of improving lithium-ion conductivity can also be expected.

[0020] As component (b), among those described above, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining a coating film with good flex resistance. Examples of the nonionic ethylenically unsaturated monomer include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, hydroxyl group-containing ethylenically unsaturated monomers, and the like.

[0021] Examples of the (meth)acrylamide derivative include N-alkyl(meth)acrylamide compounds such as isopropyl(meth)acrylamide and t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as N-n-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamide compounds such as dimethyl(meth)acrylamide and diethyl(meth)acrylamide. One of these may be used alone, or two or more thereof may be used in combination.

[0022] Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanoethyl (meth)acrylate and cyanomethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; and vinylidene cyanide. Among these, one of them may be used alone, or two or more of them may be used in combination. Among the above, acrylonitrile is preferred in terms of its high nitrile group content.

[0023] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have an aliphatic substituent such as (meth)acrylic acid cyclopentyl, (meth)acrylic acid cyclohexyl, (meth)acrylic acid methylcyclohexyl, (meth)acrylic acid t-butylcyclohexyl, (meth)acrylic acid cyclodecyl, and (meth)acrylic acid cyclododecyl; (meth)acrylic acid isobornyl, (meth)acrylic acid adamantyl, (meth)acrylic acid dicyclopentenyl, (meth)acrylic acid dicyclopentenyl oxyethyl, (meth)acrylic acid dicyclopentanyl; and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate. Among these, one of them may be used alone, or two or more of them may be used in combination.

[0024] Examples of the hydroxyl group-containing ethylenically unsaturated monomer include (meth)acrylic acid hydroxyethyl, (meth)acrylic acid hydroxypropyl, and (meth)acrylic acid hydroxybutyl. Among these, one of them may be used alone, or two or more of them may be used in combination.

[0025] The crosslinked polymer or a salt thereof preferably contains structural units derived from (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc., in terms of its excellent binding property to the particles in the slurry. In particular, in the case of lithium-ion secondary batteries, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as the component (b), it can exhibit a strong interaction with the electrode material and can exhibit good binding properties to the active material. As a result, a strong and well-integrated electrode binder layer can be obtained. Therefore, as the "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less", an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred. Furthermore, particularly in the case of lithium-ion secondary batteries, in terms of improving the cycle characteristics of the resulting secondary battery, it is preferable that the component (b) contains a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer, and it is preferable to contain 0.5% by mass or more and 70% by mass or less of the structural unit, more preferably 2.0% by mass or more and 50% by mass or less, and even more preferably 10.0% by mass or more and 50% by mass or less.

[0026] As other nonionic ethylenically unsaturated monomers, for example, (meth)acrylic acid esters may be used. Examples of the (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate; (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these may be used alone, or two or more thereof may be used in combination.

[0027] From the viewpoints of the binding property with particles in the slurry and the coating film performance, an aromatic (meth)acrylate compound can preferably be used. From the viewpoint of further improving the lithium ion conductivity and the high rate property, a compound having an ether bond, such as an alkoxyalkyl (meth)acrylate ester such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, is preferable, and 2-methoxyethyl (meth)acrylate is more preferable.

[0028] Among nonionic ethylenically unsaturated monomers, a compound having an acryloyl group is preferable in that a polymer having a long primary chain length can be obtained due to the high polymerization rate, and the adhesion of the present crosslinked polymer becomes good. Further, as the nonionic ethylenically unsaturated monomer, a compound having a glass transition temperature (Tg) of 0°C or lower of the homopolymer is preferable in that the obtained coating film has good flex resistance.

[0029] The present crosslinked polymer may be in the form of a salt in which part or all of the carboxyl groups contained in the polymer are neutralized. The type of the salt is not particularly limited, and examples include alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as calcium salts and barium salts; other metal salts such as magnesium salts and aluminum salts; ammonium salts and organic amine salts. In particular, for lithium ion secondary battery applications, among these, alkali metal salts and magnesium salts are preferable, and alkali metal salts are more preferable, because they are less likely to have an adverse effect on battery characteristics.

[0030] The present crosslinked polymer is a crosslinked polymer having a crosslinked structure. The crosslinking method in the present crosslinked polymer is not particularly limited, and for example, the following methods are exemplified. 1) Copolymerization of a crosslinkable monomer 2) Utilizing chain transfer to the polymer chain during radical polymerization 3) After synthesizing a polymer having a reactive functional group, adding a crosslinking agent as necessary for post-crosslinking Since the present crosslinked polymer has a crosslinked structure, the crosslinked polymer or a salt thereof can have excellent adhesive strength. Among the above, a method by copolymerization of a crosslinkable monomer is preferable because the operation is simple and it is easy to control the degree of crosslinking.

[0031] <Crosslinkable monomer> Examples of the crosslinkable monomer include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group.

[0032] The above polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as (meth)acryloyl group and alkenyl group in the molecule, and examples include polyfunctional (meth)acrylate compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl group and alkenyl group. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferable in terms of easily obtaining a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferable.

[0033] Examples of the polyfunctional (meth)acrylate compound include di(meth)acrylates of dihydric alcohols such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate; tri(meth)acrylates and tetra(meth)acrylates of polyvalent alcohols having a trivalent or higher valency such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of a trimethylolpropane ethylene oxide modified product, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate, i.e., poly(meth)acrylates; bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0034] Examples of the polyfunctional alkenyl compound include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl sucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene, etc.

[0035] Examples of the compound having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl (meth)acrylate, etc.

[0036] Specific examples of the monomer having the above self-crosslinkable crosslinkable functional group include hydrolyzable silyl group-containing vinyl monomers, N-methylol(meth)acrylamide, N-methoxyalkyl(meth)acrylate, etc. These compounds can be used alone or in combination of two or more.

[0037] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. For example, vinyl silanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate; silyl group-containing methacrylic esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; and silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate, etc. can be mentioned.

[0038] When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.05 parts by mass or more and 5.0 parts by mass or less, more preferably 0.1 parts by mass or more and 5.0 parts by mass or less, still more preferably 0.2 parts by mass or more and 4.0 parts by mass or less, and even more preferably 0.3 parts by mass or more and 3.0 parts by mass or less, based on 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). When the amount of the crosslinkable monomer used is 0.05 parts by mass or more, it is preferable in terms of better adhesive strength and stability of the composition. When it is 5.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be high. Similarly, the amount of the crosslinkable monomer used is preferably 0.02 to 1.7 mol%, more preferably 0.10 to 1.0 mol%, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer).

[0039] 2. Method for producing the present crosslinked polymer The crosslinked polymer is obtained by subjecting a monomer component containing the ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "the present monomer") to precipitation polymerization or dispersion polymerization in the presence of a chain transfer mechanism type controller. Here, precipitation polymerization is a method for producing a polymer by performing a polymerization reaction in a solvent that dissolves the monomer as a raw material but does not substantially dissolve the produced polymer. As the polymerization proceeds, the polymer particles grow larger by aggregation and growth, and a dispersion of polymer particles in which primary particles of several tens of nm to several hundreds of nm are secondarily aggregated into several μm to several tens of μm is obtained. A dispersion stabilizer can also be used to control the particle size of the polymer. In addition, the above secondary aggregation can also be suppressed by selecting a dispersion stabilizer, a polymerization solvent, etc. Generally, precipitation polymerization with suppressed secondary aggregation is also called dispersion polymerization.

[0040] Regarding the exchange chain transfer mechanism type controller Examples of the exchange chain transfer mechanism type controller according to the present invention include a controller in reversible addition-fragmentation chain transfer polymerization (RAFT method) (hereinafter also referred to as "RAFT agent"), a controller in iodine transfer polymerization, a controller in a polymerization method using an organic tellurium compound (TERP method), a controller in a polymerization method using an organic antimony compound (SBRP method), a controller in a polymerization method using an organic bismuth compound (BIRP method), and the like. As the exchange chain transfer mechanism type controller, a polymer having a living radical polymerization active unit by an exchange chain transfer mechanism with a polymerization chain of one or more vinyl monomers (hereinafter also simply referred to as "first polymer", which will be described in detail in the following paragraphs

[0052] to

[0078] ), and a controller other than the polymer can be used. The first polymer and the controller other than the polymer may be used alone or in combination. In the presence of the exchange chain transfer mechanism type controller, by subjecting the monomer to precipitation polymerization or dispersion polymerization, the primary chain length is shortened and the chain lengths are made uniform to form a uniform crosslinked structure, so that it is possible to increase the water swelling degree of the present crosslinked polymer. Along with this, it is presumed that good coatability due to reduction of the composition viscosity and excellent coating film performance can be achieved at the same time. Among these, the RAFT agent and the controller in the iodine transfer polymerization method are preferable, and the RAFT agent is more preferable, in that the crosslinked structure of the present crosslinked polymer can be made more uniform.

[0041] As the RAFT agent, a first polymer having a living radical polymerization active unit by the reversible addition-fragmentation chain transfer method (described in detail later), and / or a RAFT agent other than the first polymer (dithioester compound, xanthate compound, trithiocarbonate compound, dithiocarbamate compound, etc.) can be used. As the RAFT agent other than the above-mentioned first polymer, specifically, for example, 2-cyano-2-propyl benzodithioate, 2-phenyl-2-propyl benzodithioate, trithiocarbonate, 2-cyano-2-propyl dodecyl trithiocarbonate, 2-(dodecylthiocarbonothioylthio)propionic acid, 3-((1-carboxyethylthio)carbonothioylthio))propionic acid, methyl 2-(dodecylthiocarbonothioylthio)-2-methylpropanoate, 1,4-bis(n-dodecylsulfanylthiocarbonylsulfanylmethyl)benzene, dibenzyl trithiocarbonate, distyryl trithiocarbonate, dicumyl trithiocarbonate, cyanomethyl-N-methyl-N-phenyldithiocarbamate and the like can be mentioned. Among the RAFT agents, those having trithiocarbonate in the molecule are particularly preferable in that the crosslinked structure of the present crosslinked polymer can be made more uniform.

[0042] As the controller in the iodine transfer polymerization method, a first polymer having a living radical active unit by the iodine transfer polymerization method (described in detail later), and / or a controller other than the first polymer can be used. As the controller other than the above-mentioned first polymer, specifically, for example, methyl iodide, methylene iodide, iodoform, carbon tetraiodide, 1-phenylethyl iodide, perfluoroalkyl perfluoroalkyl iodide having 1 to 20 carbon atoms in the alkyl group such as benzyl iodide, ethyl 2-iodopropionate, methyl 2-iodoisobutyrate, ethyl 2-iodoisobutyrate, ethyl 2-iodo-2-phenylacetate, ethylene glycol bis(2-iodo-2-phenylacetate), ethylene glycol bis(2-iodoisobutyrate), 1,5-diiodo-2,4-dimethylbenzene, 2-iodopropionitrile and the like can be mentioned.

[0043] The exchange chain transfer mechanism type control agent may be monofunctional with one active site, or a polyfunctional agent with two or more active sites can also be used. The exchange chain transfer mechanism type control agent with two or more functional groups causes the polymerization chain to extend in two or more directions. From the perspective of manufacturing the present crosslinked polymer, it may be preferable to use a bifunctional or polyfunctional exchange chain transfer mechanism type control agent with three or more functional groups.

[0044] Regarding the usage amount of the exchange chain transfer mechanism type control agent, in terms of making the crosslinked structure of the crosslinked polymer more uniform, it is preferably 0.0001 to 0.50 mol% based on the total amount of the present monomer, more preferably 0.0001 to 0.40 mol%, still more preferably 0.0001 to 0.30 mol%, and even more preferably 0.0002 to 0.30 mol%.

[0045] As the polymerization initiator used together with the exchange chain transfer mechanism type control agent, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but it is not particularly limited. The usage conditions can be adjusted by known methods such as thermal initiation, redox initiation using a reducing agent in combination, and UV initiation so that an appropriate amount of radicals is generated. In order to obtain the present crosslinked polymer with a long primary chain length, it is preferable to set the conditions so that the amount of radicals generated is less within the range where the production time is acceptable. Among the above polymerization initiators, azo compounds are preferable in terms of being easy to handle safely and having few side reactions during radical polymerization. Specific examples of the above azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis[N-(2-propenyl)-2-methylpropionamide], 2,2'-azobis(N-butyl-2-methylpropionamide), and the like. The above radical polymerization initiator may be used alone or in combination of two or more.

[0046] The preferable amount of the polymerization initiator to be used is, for example, 0.001 to 2 parts by mass, for example, 0.005 to 1 part by mass, and for example, 0.01 to 0.1 part by mass when the total amount of the monomer components to be used is 100 parts by mass. If the amount of the polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, it is easy to obtain a polymer having a long primary chain length. The usage ratio of the polymerization initiator is not particularly limited, but from the viewpoint of making the crosslinked structure of the present crosslinked polymer uniform, the amount of the polymerization initiator used relative to 1 mol of the above-mentioned exchange chain transfer mechanism type controller is preferably 0.5 mol or less, and more preferably 0.2 mol or less. Further, from the viewpoint of stably carrying out the polymerization reaction, the lower limit of the amount of the polymerization initiator used relative to 1 mol of the exchange chain transfer mechanism type controller is 0.001 mol. Therefore, the amount of the polymerization initiator used relative to 1 mol of the exchange chain transfer mechanism type controller is preferably in the range of 0.001 mol or more and 0.5 mol or less, and more preferably in the range of 0.005 mol or more and 0.2 mol or less.

[0047] As the polymerization solvent, a solvent selected from water, various organic solvents, etc. can be used in consideration of the types of monomers to be used, etc. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent having a small chain transfer constant. Specific examples of the polymerization solvent include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These can be used alone or in combination of two or more. Alternatively, it may be used as a mixed solvent with these and water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20 °C of more than 10 g / 100 ml. Among the above, methyl ethyl ketone and acetonitrile are preferred in terms of small generation of coarse particles and adhesion to the reactor, good polymerization stability, difficulty in secondary aggregation of the precipitated crosslinked polymer (or easy dissociation in the aqueous medium even if secondary aggregation occurs), small chain transfer constant and obtaining a polymer with a large degree of polymerization (primary chain length), and easy operation during neutralization in the process described later.

[0048] Also, in order to make the neutralization reaction proceed stably and rapidly during neutralization in the process, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent. Examples of such highly polar solvents preferably include water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0% by mass, more preferably 0.1 to 10.0% by mass, still more preferably 0.1 to 5.0% by mass, and even more preferably 0.1 to 1.0% by mass based on the total mass of the medium. If the proportion of the highly polar solvent is 0.05% by mass or more, the effect on the above neutralization reaction can be recognized, and if it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. Further, in the polymerization of a highly hydrophilic ethylenically unsaturated carboxylic acid monomer such as acrylic acid, when a highly polar solvent is added, the polymerization rate is improved and it becomes easier to obtain a polymer with a long primary chain length. Among the highly polar solvents, water is particularly preferable because of its great effect of improving the polymerization rate.

[0049] The reaction temperature during the polymerization reaction in the presence of the exchange chain transfer mechanism type controller is preferably 30°C or higher and 120°C or lower, more preferably 40°C or higher and 110°C or lower, and still more preferably 50°C or higher and 100°C or lower. If the reaction temperature is 30°C or higher, the polymerization reaction can proceed smoothly. On the other hand, if the reaction temperature is 120°C or lower, side reactions can be suppressed and the restrictions on initiators and solvents that can be used are relaxed.

[0050] The dispersion of the crosslinked polymer obtained through the overlapping process can obtain the target crosslinked polymer in a powder state by performing a reduced-pressure and / or heat treatment, etc. in the drying process to distill off the solvent. At this time, before the above drying process, for the purpose of removing unreacted monomers (and their salts), following the polymerization process, it is preferable to include a solid-liquid separation process such as centrifugation and filtration, and a washing process using an organic solvent or a mixed solvent of an organic solvent / water. When the above washing process is provided, even when the crosslinked polymer is secondarily aggregated, it is easily disassembled during use, and further, since the remaining unreacted monomers are removed, good performance is also shown in terms of the coating film performance.

[0051] In the production method of the present invention, when an unneutralized or partially neutralized salt is used as the ethylenically unsaturated carboxylic acid monomer, an alkali compound may be added to the dispersion of the crosslinked polymer obtained by the polymerization process to neutralize the polymer (hereinafter, also referred to as "in-process neutralization"), and then the solvent may be removed in the drying process. Further, after obtaining the powder of the crosslinked polymer in the state of an unneutralized or partially neutralized salt, an alkali compound may be added when preparing the slurry composition to neutralize the polymer (hereinafter, also referred to as "post-neutralization"). Among the above, in-process neutralization is preferable because the secondary aggregates tend to be easily disassembled.

[0052] Here, as the exchange chain transfer mechanism type control agent, as described above, a polymer (first polymer) having a living radical polymerization active unit by an exchange chain transfer mechanism with a polymerization chain of one or more vinyl monomers (hereinafter, also simply referred to as "first monomer") (hereinafter, also simply referred to as "first polymerization chain") can be used.

[0053] In producing the present crosslinked polymer by polymerizing the present monomer in the presence of the first polymer, the first polymer can be used as a base point for the polymerization of the present monomer and as a dispersion stabilizer in the polymerization solvent of the crosslinked polymer. A present crosslinked polymer in which a polymer chain having a structural unit derived from the present monomer is bonded to the polymerization chain of the first polymer can be obtained as dispersed fine particles. By doing so, polymerization stability, that is, aggregation of the present crosslinked polymer during the polymerization process can be suppressed, generation of coarse aggregated particles can be suppressed, and a present crosslinked polymer having a small particle size and a narrow particle size distribution can be obtained.

[0054] In producing the present crosslinked polymer by polymerizing the present monomer in the presence of the first polymer, in order for the first polymer to function as a dispersion stabilizer, for example, the first polymer can be used in an amount of 0.3 parts by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the total mass of the present monomer. By using within such a range, while making the first polymer function as a dispersion stabilizer, a present crosslinked polymer mainly containing the present monomer can be produced. Also, when the amount of the first polymer is less than 0.3 parts by mass, it is difficult to obtain a sufficient dispersion stabilizing effect, and the particle size of the present crosslinked polymer tends to exceed 0.3 μm. Even when the amount exceeds 50 parts by mass, it is difficult to improve the functionality as a dispersion stabilizer, and the effect of reducing the particle size of the present crosslinked polymer also becomes small.

[0055] The first polymer can be used, for example, in an amount of 0.5 parts by mass or more, and for example, 1 part by mass or more with respect to 100 parts by mass of the total mass of the present monomer. Also, the first polymer can be used, for example, in an amount of 40 parts by mass or less, for example, 30 parts by mass or less, and for example, 20 parts by mass or less. The range of the amount of the first polymer used with respect to 100 parts by mass of the total mass of the present monomer can be set by appropriately combining the above upper and lower limits.

[0056] Method for producing the first polymer By polymerizing a monomer composition containing the first monomer in the presence of a known chain transfer exchange mechanism type controller, a first polymer having a first polymer chain having a structural unit derived from the first monomer and a living polymerization active unit by a chain transfer exchange mechanism can be obtained.

[0057] The polymerization conditions for producing the first polymer are well-known to those skilled in the art. Examples of the polymerization process include various processes such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Considering that it is the polymerization basis in the production of the present crosslinked polymer and functions as a dispersion stabilizer, for example, solution polymerization can be used. In addition, the types of exchange chain transfer mechanism regulators, the types and amounts of polymerization initiators used, polymerization solvents, reaction temperatures, and other polymerization conditions are appropriately selected according to the above paragraphs

[0040] to

[0043] and

[0045] to

[0049] . The amount of the exchange chain transfer mechanism regulator used is appropriately adjusted according to the number average molecular weight (Mn) of the target first polymer. As the exchange chain transfer mechanism regulator, RAFT agents and regulators in the iodine transfer polymerization method are preferable in that they can reduce the molecular weight distribution of the first polymer. Furthermore, the concentration when producing the first polymer is not particularly limited with respect to the total mass of the charged amounts such as the polymerization solvent and the first monomer. For example, it can be 10% by mass or more and 80% by mass or less, or for example, 15% by mass or more and 70% by mass or less, or for example, 20% by mass or more and 70% by mass or less.

[0058] Typically, when a monofunctional exchange chain transfer mechanism type regulator is used, a living polymerization active unit is provided at the end of the first polymer chain. When a bifunctional or higher functional exchange chain transfer mechanism type regulator is used, a living polymerization active unit branches in two or more directions and each has a first polymer chain. In any case, when there is another polymer chain, this other polymer chain is directly bonded to the living polymerization active unit, and the first polymer chain is bonded to the distal end of the other polymer chain so that the first polymer chain is provided on the more distal side with respect to the living polymerization active unit.

[0059] The first polymer can also include two or more first polymer chains. For example, after performing living radical polymerization or the like using one or more first monomers of a certain composition, living radical polymerization or the like is performed using one or more first monomers of another composition, whereby a first polymer having first polymer chains (blocks) having structural units derived from first monomers of different compositions can be obtained.

[0060] The number average molecular weight (Mn) of the first polymer is not particularly limited. For example, it is 3,000 or more, and for example, it is 5,000 or more, and for example, it is 7,000 or more, and for example, it is 8,000 or more, and for example, it is 10,000 or more. Also, the same Mn is 50,000 or less, and for example, it is 30,000 or less, and for example, it is 25,000 or less, and for example, it is 20,000 or less, and for example, it is 15,000 or less, and for example, it is 14,000 or less, and for example, it is 12,000 or less. As the range of Mn, the above-described lower limit and upper limit can be appropriately combined and set. For example, it is 5,000 or more and 25,000 or less, and for example, it is 10,000 or more and 25,000 or less, and for example, it is 10,000 or more and 15,000 or less, and for example, it is 10,000 or more and 14,000 or less.

[0061] The weight-average molecular weight (Mw) of the first polymer is not particularly limited, but for example, it is 5,000 or more, and for example, 7,000 or more, and for example, 9,000 or more, and for example, 10,000 or more, and for example, 13,000 or more, and for example, 15,000 or more. Also, the same Mw is 60,000 or less, and for example, 55,000 or less, and for example, 50,000 or less, and for example, 45,000 or less, and for example, 40,000 or less, and for example, 36,000 or less, and for example, 35,000 or less, and for example, 30,000 or less, and for example, 25,000 or less. As the range of Mw, the above-mentioned lower limit and upper limit can be appropriately combined and set. For example, it is 1,000 or more and 40,000 or less, and for example, 10,000 or more and 35,000 or less, and for example, 10,000 or more and 30,000 or less, and for example, 15,000 or more and 25,000 or less.

[0062] Incidentally, both Mw and Mn of the first polymer can be measured by gel permeation chromatography using polystyrene as a standard substance. As the details of the chromatography conditions, the conditions disclosed in the subsequent examples can be adopted.

[0063] The molecular weight distribution (Mw / Mn) of the first polymer is not particularly limited, but for example, it is 2.5 or less, and for example, 2.4 or less, and for example, 2.3 or less, and for example, 2.0 or less, and for example, 1.6 or less, and for example, 1.5 or less, and for example, 1.4 or less, and for example, 1.3 or less. Also, the molecular weight distribution is, for example, 1.1 or more, and for example, 1.2 or more, and for example, 1.3 or more, and for example, 1.4 or more, and for example, 1.5 or more. As the range of the molecular weight distribution, the above-mentioned lower limit and upper limit can be appropriately combined and set. For example, it can be 1.1 or more and 2.5 or less, and for example, 1.1 or more and 2.4 or less, and for example, 1.1 or more and 2.3 or less, and for example, 1.1 or more and 2.0 or less, etc.

[0064] The smaller the molecular weight distribution of the first polymer, the smaller the particle size of the resulting crosslinked polymer tends to be. It is preferably 2.4 or less, and in order to obtain a crosslinked polymer with a smaller particle size, it is preferably 1.7 or less, more preferably 1.6 or less, and even more preferably 1.4 or less.

[0065] <The first monomer> Examples of the first monomer include styrenes, (meth)acrylonitrile compounds, maleimide compounds, unsaturated acid anhydrides, and unsaturated carboxylic acid compounds. One or more of these can be used in combination.

[0066] Styrenes include styrene and its derivatives. Specific compounds include styrene, α-methylstyrene, β-methylstyrene, vinyltoluene, vinylxylene, vinylnaphthalene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, p-n-butylstyrene, p-isobutylstyrene, p-t-butylstyrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-chloromethylstyrene, p-chloromethylstyrene, o-chlorostyrene, p-chlorostyrene, o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, divinylbenzene, etc. One or more of these can be used. Among these, from the viewpoint of polymerizability, styrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-hydroxystyrene, m-hydroxystyrene, and p-hydroxystyrene are preferred.

[0067] Examples of the (meth)acrylonitrile compound include (meth)acrylonitrile, acrylonitrile, α-methylacrylonitrile, etc. For example, acrylonitrile is used.

[0068] As maleimide compounds, maleimide compounds include maleimide and N-substituted maleimide compounds. Specific examples of N-substituted maleimide compounds include N-alkyl-substituted maleimide compounds such as N-methylmaleimide, N-ethylmaleimide, N-n-propylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide, N-isobutylmaleimide, N-tert-butylmaleimide, N-pentylmaleimide, N-hexylmaleimide, N-heptylmaleimide, N-octylmaleimide, N-laurylmaleimide, N-stearylmaleimide, etc.; N-cycloalkyl-substituted maleimide compounds such as N-cyclopentylmaleimide, N-cyclohexylmaleimide, etc.; N-aryl-substituted maleimide compounds such as N-phenylmaleimide, N-(4-hydroxyphenyl)maleimide, N-(4-acetylphenyl)maleimide, N-(4-methoxyphenyl)maleimide, N-(4-ethoxyphenyl)maleimide, N-(4-chlorophenyl)maleimide, N-(4-bromophenyl)maleimide, N-benzylmaleimide, etc. One or more of these can be used. For example, N-phenylmaleimide is used.

[0069] In addition, examples of unsaturated acid anhydrides include maleic anhydride, itaconic anhydride, citraconic anhydride, etc., and one or more of these can be used.

[0070] Examples of unsaturated carboxylic acid compounds include unsaturated dicarboxylic acids such as (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, citraconic acid, cinnamic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, maleic anhydride, itaconic anhydride, and citraconic anhydride, and monoalkyl esters of unsaturated dicarboxylic acids, etc. One or more of these can be used.

[0071] Among these, as the first monomer, it is preferable to contain at least styrenes, for example. This is because styrenes are easy to undergo living polymerization and can impart appropriate hydrophobicity and affinity for organic solvents. Hydrophobicity or affinity for organic solvents can be imparted to the first polymer chain. By doing so, for example, when producing a crosslinked polymer by dispersion polymerization in a polar organic solvent, the first polymer tends to exist on the surface layer of the crosslinked polymer, and the dispersion stability of the crosslinked polymer is improved.

[0072] Styrenes are, for example, 20% by mass or more of the total mass of the first monomer. This is because when it is 20% by mass or more, living polymerization becomes easy and appropriate hydrophobicity and affinity for organic solvents can be imparted. Also, for example, it is 30% by mass or more, also for example, 35% by mass or more, also for example, 40% by mass or more, also for example, 50% by mass or more, also for example, 60% by mass or more, also for example, 65% by mass or more, also for example, 70% by mass or more, also for example, 75% by mass or more. Further, styrenes are 100% by mass or less of the total mass, also for example, 95% by mass or less, also for example, 90% by mass or less, also for example, 85% by mass or less, also for example, 80% by mass or less, also for example, 75% by mass or less. As the range with respect to the total mass of styrenes, the above-mentioned lower limit and upper limit can be set by appropriately combining them. For example, it is 20% by mass or more and 95% by mass or less, also for example, 30% by mass or more and 75% by mass or less, also for example, 35% by mass or more and 85% by mass or less.

[0073] (Meta)acrylonitrile compounds, maleimide compounds, acid anhydrides, and unsaturated carboxylic acid compounds can each be used alone, or it is preferable to use one or more of these four types in combination with styrenes. This is because all of these four types can maintain, regulate, or impart hydrophobicity or organic solvent affinity to the first polymer chain. Among them, one or more of (meta)acrylonitrile compounds such as acrylonitrile, maleimide compounds such as N-phenylmaleimide, and acid anhydrides. Among them, combinations such as styrene and acrylonitrile, styrene and N-phenylmaleimide are preferred. The unsaturated carboxylic acid compound is preferable in that it can easily change the polarity of the first polymer.

[0074] When used in combination with styrenes, the total amount of these one or more first monomers other than styrenes is, for example, 20% by mass or more of the total mass of the first monomers (the first monomer units of the first polymer chain) for polymerizing the first polymer chain. Also, for example, 25% by mass or more, also for example, 30% by mass or more, also for example, 35% by mass or more, also for example, 40% by mass or more, also for example, 50% by mass or more, also for example, 60% by mass or more. Further, the (meta)acrylonitrile compound is 80% by mass or less of the total mass, also for example, 75% by mass or less, also for example, 70% by mass or less, also for example, 65% by mass or less, also for example, 60% by mass or less, also for example, 55% by mass or less, also for example, 50% by mass or less. As the range with respect to the total mass of styrenes, the above-mentioned lower limit and upper limit can be appropriately combined and set. For example, it is 20% by mass or more and 65% by mass or less, also for example, 25% by mass or more and 50% by mass or less.

[0075] <The first polymer chain> The first polymer chain may be a polymer chain consisting only of the above-described first monomer, but if necessary, other vinyl monomers other than the above can be used as the first monomer. For example, known vinyl monomers such as (meth)acrylic acid and (meth)acrylic acid esters such as alkyl (meth)acrylates can be used. In addition, such other monomers are, for example, 10% by mass or less, for example, 5% by mass or less, for example, 3% by mass or less, for example, 1% by mass or less, and for example, 0.5% by mass or less of the total mass of the monomers constituting the first polymer chain.

[0076] Further, the first polymer may include a block (another polymer chain) different from the first polymer chain. Such another polymer chain may be added, for example, in a separate synthesis step after the formation of the first polymer chain. In this case, a radical polymerization initiator and another vinyl monomer are continuously or newly supplied to the first polymer having the first polymer chain, and another polymer chain (block) composed of units derived from monomers other than the first monomer having a composition different from that of the first polymer chain is obtained. By being provided so as to be directly connected to the living radical active unit described later and connected to the first polymer chain, a part of the monomer common to the present monomer used in the present crosslinked polymer can be provided in the first polymer in advance.

[0077] <Living radical polymerization active unit> Since the first polymer has a living radical polymerization active unit by an exchange chain transfer mechanism, various monomers can be selected for the precipitation polymerization or dispersion polymerization of the present monomer for the solubility of the first polymer in the polymerization solvent and the function as a dispersion stabilizer.

[0078] As the exchange chain transfer mechanism of the living radical polymerization active unit in the first polymer, there may be mentioned a reversible addition-fragmentation chain transfer polymerization method (RAFT method), an iodine transfer polymerization method, a polymerization method using an organic tellurium compound (TERP method), a polymerization method using an organic antimony compound (SBRP method), a polymerization method using an organic bismuth compound (BIRP method), and the like. Among these, the RAFT method and the iodine transfer polymerization method are preferable, and the RAFT method is more preferable, in terms of being able to reduce the particle size of the present crosslinked polymer.

[0079] 3. Characteristics of the present crosslinked polymer <Aqueous solution viscosity of the present crosslinked polymer> The present crosslinked polymer or a salt thereof preferably has a viscosity of 100 mPa·s or more in a 2 mass% aqueous solution. When the viscosity of the 2 mass% aqueous solution is 100 mPa·s or more, the storage stability of the composition containing the crosslinked polymer is high, and it becomes possible to exhibit excellent adhesive strength. The viscosity of the 2 mass% aqueous solution may be 1,000 mPa·s or more, may be 10,000 mPa·s or more, or may be 50,000 mPa·s or more. The aqueous solution viscosity is obtained by uniformly dissolving or dispersing a predetermined amount of the present crosslinked polymer or a salt thereof in water and then measuring the Brookfield viscosity (25°C) at 12 rpm according to the method described in the examples.

[0080] This crosslinked polymer becomes swollen in water by absorbing water. Generally, when the crosslinked polymer has an appropriate degree of crosslinking, the greater the amount of hydrophilic groups in the crosslinked polymer, the easier it is for the crosslinked polymer to absorb water and swell. Regarding the degree of crosslinking, the lower the degree of crosslinking, the easier it is for the crosslinked polymer to swell. However, even when the number of crosslinking points is the same, the greater the molecular weight (primary chain length), the more crosslinking points contribute to the formation of the three-dimensional network, so the crosslinked polymer becomes less likely to swell. Therefore, by adjusting the amount of hydrophilic groups, the number of crosslinking points, the primary chain length, etc. of the crosslinked polymer, the viscosity of the crosslinked polymer aqueous solution can be adjusted. At this time, the number of the above crosslinking points can be adjusted, for example, by the amount of crosslinkable monomer used, the chain transfer reaction to the polymer chain, the post-crosslinking reaction, etc. Also, the primary chain length of the polymer can be adjusted by setting conditions related to the amount of radical generation such as the initiator and polymerization temperature, and by selecting a polymerization solvent considering chain transfer, etc.

[0081] <Particle diameter of this crosslinked polymer> In this composition, it is preferable that the crosslinked polymer is well dispersed as water-swellable particles having an appropriate particle diameter without existing as large-particle-size lumps (secondary aggregates) because the crosslinked polymer can exhibit good adhesive strength.

[0082] When the crosslinked polymer in which the degree of neutralization based on the carboxyl group possessed by the crosslinked polymer is 80 to 100 mol% is dispersed in water, the particle diameter (water-swelled particle diameter) is preferably in the range of 0.1 μm or more and 5.0 μm or less in terms of volume-based median diameter. A more preferable range of the above particle diameter is 0.1 μm or more and 4.0 μm or less, a further preferable range is 0.1 μm or more and 3.0 μm or less, an even more preferable range is 0.2 μm or more and 3.0 μm or less, and a still more preferable range is 0.3 μm or more and 3.0 μm or less. If the particle diameter is in the range of 0.1 μm or more and 5.0 μm or less, it exists uniformly in the composition in a suitable size, so the stability of the composition is high and it is possible to exhibit excellent adhesive strength. If the particle diameter exceeds 5.0 μm, there is a risk that the adhesive strength will be insufficient as described above. Also, there is a risk that the coatability will be insufficient in that it is difficult to obtain a smooth coated surface. On the other hand, when the particle diameter is less than 0.1 μm, there are concerns from the viewpoint of stable manufacturability.

[0083] In the present crosslinked polymer, in the present composition, acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are preferably neutralized so that the degree of neutralization is 20 mol% or more and used in the form of a salt. The above degree of neutralization is more preferably 50 mol% or more, still more preferably 70 mol% or more, even more preferably 75 mol% or more, yet even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit value of the degree of neutralization is 100 mol%, and it may be 98 mol% or 95 mol%. The range of the degree of neutralization can be appropriately combined with the above lower limit value and upper limit value. For example, it may be 50 mol% or more and 100 mol% or less, 75 mol% or more and 100 mol% or less, or 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, it is preferable in that the water swelling property is good and the dispersion stabilizing effect is easily obtained. In the present specification, the above degree of neutralization can be calculated from the charged values of the monomer having an acid group such as a carboxyl group and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the carboxylate salt by subjecting the crosslinked polymer or its salt to IR measurement of the powder after drying treatment at 80°C for 3 hours under reduced pressure conditions.

[0084] <Water swelling degree of the present crosslinked polymer> In the present specification, the water swelling degree is based on the weight "(W A ) g" of the crosslinked polymer at the time of drying, and the amount of water "(W B ) g" absorbed when the crosslinked polymer is saturated and swollen with water, and is calculated based on the following formula. (Water swelling degree) = { (W A ) + (W B )} / (W A )

[0085] The crosslinked polymer or a salt thereof preferably has a water swelling degree at pH 8 of 20 or more and 80 or less. If the water swelling degree is within the above range, since the crosslinked polymer or a salt thereof swells moderately in an aqueous medium, when forming a coating film, it becomes possible to secure a sufficient adhesion area to the particles in the slurry and the substrate, and the binding property tends to be good. The above water swelling degree may be, for example, 21 or more, 23 or more, 25 or more, 27 or more, or 30 or more. When the water swelling degree is 20 or more, the crosslinked polymer or a salt thereof spreads on the surface of the particles or the substrate in the slurry, and a sufficient adhesion area can be secured, so that good binding property can be obtained. The upper limit value of the water swelling degree at pH 8 may be 75 or less, 70 or less, 65 or less, 60 or less, or 55 or less. When the water swelling degree exceeds 80, the viscosity of the present composition tends to increase, and as a result, the uniformity of the coating layer is insufficient, and sufficient binding force may not be obtained. In addition, there is a possibility that the coatability of the present composition may decrease. The range of the water swelling degree at pH 8 can be set by appropriately combining the above upper limit value and lower limit value. For example, it is 23 or more and 70 or less, and for another example, it is 25 or more and 65 or less, and for still another example, it is 25 or more and 55 or less. The water swelling degree at pH 8 can be obtained by measuring the swelling degree of the crosslinked polymer or a salt thereof in water at pH 8. As the water at pH 8, for example, ion-exchanged water can be used, and if necessary, the pH value may be adjusted using an appropriate acid or alkali, or a buffer solution or the like. The pH at the time of measurement is, for example, in the range of 8.0 ± 0.5, preferably in the range of 8.0 ± 0.3, more preferably in the range of 8.0 ± 0.2, and still more preferably in the range of 8.0 ± 0.1.

[0086] In addition, those skilled in the art can adjust the water swelling degree by controlling the composition and structure of the crosslinked polymer. For example, by introducing an acidic functional group or a highly hydrophilic structural unit into the crosslinked polymer, the water swelling degree can be increased. Also, by lowering the crosslinking degree of the crosslinked polymer, the water swelling degree usually becomes higher.

Examples

[0087] Hereinafter, the present invention will be specifically described based on examples. Note that the present invention is not limited by these examples. Hereinafter, "parts" and "%" mean parts by mass and mass %, respectively, unless otherwise specified.

[0088] ≪Evaluation of carboxyl group-containing crosslinked polymer (salt)≫ (Degree of water swelling at pH 8) The degree of water swelling at pH 8 was measured by the following method. The measuring apparatus is shown in FIG. 1. The measuring apparatus is composed of <1> to <3> in FIG. 1. <1> consists of a burette 1 with a side tube for air venting, a pinchcock 2, a silicone tube 3, and a polytetrafluoroethylene tube 4. <2> has a support cylinder 8 with many holes on the bottom surface above a funnel 5, and a filter paper 10 for the apparatus is further installed thereon. <3> A sample 6 (measurement sample) of the crosslinked polymer is sandwiched between two filter papers 7 for fixing the sample, and the filter papers for fixing the sample are fixed by an adhesive tape 9. Note that all the filter papers used are ADVANTEC No. 2 with an inner diameter of 55 mm. <1> and <2> are connected by a silicone tube 3. Also, the height of the funnel 5 and the support cylinder 8 with respect to the burette 1 is fixed, and the lower end of the polytetrafluoroethylene tube 4 installed inside the side tube of the burette and the bottom surface of the support cylinder 8 are set to be at the same height (dotted line in FIG. 1).

[0089] The measuring method will be described below. Remove the pinchcock 2 in <1>, pour deionized water from the upper part of the burette 1 through the silicone tube 3, and fill the deionized water 12 from the burette 1 to the filter paper 10 for the apparatus. Then, close the pinchcock 2 and remove the air from the polytetrafluoroethylene tube 4 connected to the side tube of the burette with a rubber stopper. In this way, the deionized water 12 is continuously supplied from the burette 1 to the filter paper 10 for the apparatus. Next, after removing the excess ion-exchanged water 12 that oozed out from the filter paper 10 for the apparatus, record the reading (a) of the scale of the burette 1. Weigh 0.1 - 0.2 g of the dry powder of the measurement sample and evenly place it in the center of the sample-fixing filter paper 7 as shown in <3>. Sandwich the sample with another filter paper, fasten the two filter papers with the adhesive tape 9, and fix the sample. Place the filter paper with the sample fixed on the filter paper 10 for the apparatus shown in <2>. Next, record the reading (b) of the scale of the burette 1 after 30 minutes have elapsed since the lid 11 was placed on the filter paper 10 for the apparatus. The total water absorption (c) of the measurement sample and the two sample-fixing filter papers 7 is obtained by (a - b). By the same operation, measure the water absorption (d) of only the two filter papers 7 without including the sample of the cross-linked polymer. Perform the above operations and calculate the water swelling degree from the following formula. Note that the solid content used in the calculation is the value measured by the method described later. Water swelling degree = {dry weight of the measurement sample (g) + (c - d)} / {dry weight of the measurement sample (g)} However, dry weight of the measurement sample (g) = weight of the measurement sample (g) × (solid content (%)÷100)

[0090] Here, the method for measuring the solid content is described below. Take about 0.5 g of the sample into a weighing bottle whose weight has been measured in advance [weight of the weighing bottle = B (g)], accurately weigh the weighing bottle with the sample [W0 (g)], then place the weighing bottle with the sample in a non-windy dryer and dry it at 155 °C for 45 minutes, and measure the weight of the weighing bottle at that time [W1 (g)], and obtain the solid content from the following formula. Solid content (%) = (W1 - B) / (W0 - B)×100

[0091] (Measurement of particle diameter (water-swollen particle diameter) in an aqueous medium) Weighed 0.25 g of the powder of the carboxyl group-containing crosslinked polymer salt and 49.75 g of ion-exchanged water into a 100 cc container, and set it on a rotation / revolution type stirrer (manufactured by Shinki Co., Ltd., Awatori Rentaro AR-250). Then, carried out stirring (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes), and further degassing (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) treatment to prepare a hydrogel in a state where the carboxyl group-containing crosslinked polymer salt was swollen in water. Next, the particle size distribution of the above hydrogel was measured with a laser diffraction / scattering type particle size distribution analyzer (manufactured by Microtrac Bell Co., Ltd., Microtrac MT-3300EXII) using ion-exchanged water as a dispersion medium. When an appropriate amount of the hydrogel that could obtain an appropriate scattered light intensity was introduced into the place where an excessive amount of the dispersion medium was circulating for the hydrogel, the particle size distribution shape measured after several minutes became stable. As soon as the stability was confirmed, the particle size distribution was measured to obtain the volume-based median diameter (D50) as a representative value of the particle diameter.

[0092] (Measurement of viscosity of 2 mass% aqueous solution) Weighed 2.0 parts of the powder of the carboxyl group-containing crosslinked polymer salt and 98 parts of ion-exchanged water into a container, and set it on a rotation / revolution type stirrer (manufactured by Shinki Co., Ltd., Awatori Rentaro AR-250). Then, stirring (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and further degassing (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) treatment were repeated until the unswollen powder part disappeared to prepare a hydrogel fine particle dispersion in a state where the carboxyl group-containing crosslinked polymer salt was swollen in water. After adjusting each obtained hydrogel fine particle dispersion to 25 °C ± 1 °C, the viscosity at a rotor speed of 12 rpm was measured using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd., TVB-10).

[0093] ≪Synthesis of the first polymer≫ (Polymer 1) Into a reactor equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube, 2.0 parts of a RAFT agent (dibenzyl trithiocarbonate: DBTTC), 0.410 parts of 2,2'-azobis(2-methylbutyronitrile) (manufactured by Nippon Fine Chemical Co., Ltd., trade name "ABN-E"), 75 parts of styrene (St), 25 parts of acrylonitrile (AN), and 67 parts of anisole were charged. After sufficient degassing by nitrogen bubbling, polymerization was initiated in a constant temperature bath at 80°C. After 4 hours, the reaction was stopped by cooling to room temperature. The above polymerization solution was reprecipitation purified from methanol / water = 90 / 10 (vol%) and vacuum dried to obtain Polymer 1. As a result of the test by gas chromatography, the reaction rate of the obtained Polymer 1 was 72%. The molecular weight of Polymer 1 was Mn 11,900, Mw 15,500, and Mw / Mn was 1.30. Note that styrene and acrylonitrile correspond to the first monomer.

[0094] (Method for Measuring the Molecular Weight of the First Polymer) The molecular weight of the first polymer was measured by gel permeation chromatography (GPC). That is, the number average molecular weight (Mn) and weight average molecular weight (Mw) in terms of polystyrene were obtained by THF-based GPC. In addition, the molecular weight distribution (Mw / Mn) was calculated from the obtained values. Note that GPC was performed under the following conditions.

[0095] Column: 4 columns of TSKgel SuperMultipore HZ-M manufactured by Tosoh Solvent: Tetrahydrofuran Temperature: 40°C Detector: RI Flow rate: 600 μL / min

[0096] ≪Production of Carboxyl Group-Containing Crosslinked Polymer Salt≫ (Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1) For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 100 parts of acrylic acid (hereinafter referred to as "AA"), 0.001 part of DBTTC, 0.90 part of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and 1.0 mol% of triethylamine corresponding to the above AA were charged into the reactor. After thoroughly purging the inside of the reactor with nitrogen, it was heated to raise the internal temperature to 55°C. After confirming that the internal temperature was stable at 55°C, 0.040 part of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. At this point, turbidity was observed in the reaction solution, so this point was taken as the polymerization start point. The monomer concentration was calculated to be 15.0%. When the cooling of the reaction solution was started 12 hours after the polymerization start point and the internal temperature dropped to 25°C, 52.4 parts of lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H2O") powder was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of a carboxyl group-containing polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

[0097] The obtained polymerization reaction solution was centrifuged to precipitate the polymer particles, and then the supernatant was removed. Thereafter, the precipitate was redispersed in acetonitrile having the same weight as the polymerization reaction solution, and then the washing operation of precipitating the polymer particles by centrifugation and removing the supernatant was repeated twice. The precipitate was collected and dried under reduced pressure at 80°C for 3 hours to remove volatile components, thereby obtaining a powder of a carboxyl group-containing polymer salt R-1. Since the carboxyl group-containing polymer salt R-1 has hygroscopicity, it was stored sealed in a container having a water vapor barrier property. The powder of the carboxyl group-containing polymer salt R-1 was measured by IR, and the neutralization degree was determined from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O of carboxylic acid Li. As a result, it was equal to the calculated value from the charging and was 90 mol%. The water swelling degree was 36.4, the particle diameter in an aqueous medium was 1.72 μm, and the viscosity of a 2 mass% aqueous solution was 9,110 mPa·s.

[0098] (Examples 2 to 15 and Comparative Examples 1 to 2: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-17) The same operations as in Production Example 1 were carried out except that the charged amounts of the monomer, crosslinkable monomer, and neutralizing agent were as described in Table 1, and a polymerization reaction solution containing carboxyl group-containing crosslinked polymer salts R-2 to R-17 was obtained. Next, the same operations as in Production Example 1 were carried out for each polymerization reaction solution to obtain powdery carboxyl group-containing crosslinked polymer salts R-2 to R-17. Each carboxyl group-containing crosslinked polymer salt was stored sealed in a container having a water vapor barrier property. The water swelling degree, particle diameter in an aqueous medium, and viscosity of a 2 mass% concentration aqueous solution of R-2 to R-17 are shown in Table 1. Regarding the particle diameter of R-3 (neutralization degree: 70 mol%) in an aqueous medium, the measurement was carried out after adjusting the neutralization degree to 90 mol% with LiOH·H2O.

[0099]

Table 1

[0100] Details of the compounds used in Table 1 are shown below. AA: Acrylic acid IBXA: Isobornyl acrylate BM1430: 2-(Dodecylthiocarbonothioylthio)propionic acid IBME: Methyl 2-iodoisobutyrate T-20: Trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20") P-30: Pentaerythritol triallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl P-30") TEA: Triethylamine AcN: Acetonitrile V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.) LiOH·H2O: Lithium hydroxide monohydrate Na2CO3: Sodium carbonate K2CO3: Potassium carbonate

[0101] ≪Evaluation of the composition containing a carboxyl group-containing crosslinked polymer salt≫ (Example 1: Evaluation of the Composition Containing the Crosslinked Polymer Salt R-1 with Carboxyl Group) <Preparation of the slurry composition> A material prepared by coating 10% carbon on the surface of SiOx (0.8 < x < 1.2) by CVD method (hereinafter referred to as "Si-based active material") was prepared, and a mixture of artificial graphite and Si-based active material was used as the active material. As the binder, a mixture of this crosslinked polymer salt R-1, styrene-butadiene rubber (SBR) latex, and carboxymethyl cellulose (CMC) was used. Using water as the diluting solvent, the solid content concentration of the slurry composition was adjusted to 50% by mass, and artificial graphite: Si-based active material: R-1: SBR: CMC = 90:10:1.0:1.0:1.0 (solid content) was mixed using T.K. Hibismix manufactured by Primix Corporation for 2 hours to prepare a slurry composition. The viscosity of the slurry composition was 3,670 mPa·s, which was a sufficiently low value. Using the obtained slurry composition, an electrode was fabricated, and its coatability and coating film performance were evaluated. The specific procedures and evaluation methods are shown below.

[0102] (Coatability of the Slurry Composition) The above slurry composition was coated on both sides of a copper foil (thickness: 20 μm) and dried to form a paste layer. Then, after rolling so that the thickness of the paste layer was 27 μm and the packing density was 1.3 g / cm 3 it was punched into a 3 cm square to obtain a negative electrode plate. The coatability of the slurry composition in the fabrication of the above negative electrode plate was evaluated based on the following criteria and was evaluated as "◎". <Evaluation Criteria> ◎: No appearance abnormalities such as streaks and bumps are observed on the surface. 〇: Slight appearance abnormalities such as streaks and bumps are observed on the surface. △: Some appearance abnormalities such as streaks and bumps are observed on the surface. ×: Prominent appearance abnormalities such as streaks and bumps are observed on the surface.

[0103] (Fabrication of a Lithium-Ion Secondary Battery) In an N-methylpyrrolidone (NMP) solvent, 100 parts of lithium iron phosphate (LFP) as a positive electrode active material, 0.2 parts of carbon nanotubes as a conductive agent, 2 parts of Ketjen black, and 0.6 parts of vapor-grown carbon fiber (VGCF) were mixed and added. Polyvinylidene fluoride (PVDF) was mixed as a binder for the electrode composition to prepare a positive electrode composition. A coating layer was formed by applying and drying the positive electrode composition on an aluminum current collector (thickness: 15 μm). Then, after rolling so that the thickness of the coating layer became 88 μm and the packing density became 3.1 g / cm 3 After rolling so that the thickness of the coating layer became 88 μm and the packing density became 3.1 g / cm

[0104] (Coating film performance) Regarding the performance of the coating film obtained from the above slurry composition, in this example, it was evaluated by measuring the cycle characteristics of the lithium-ion secondary battery. The lithium-ion secondary battery of the laminate type cell prepared by the above procedure was charged and discharged at a charge-discharge rate of 0.2C under the conditions of 2.7 to 3.4V by CC discharge, and the initial capacity C0 was measured. Further, charge and discharge were repeated in an environment at 25°C, and the capacity C50 after 50 cycles was measured. The cycle characteristic (ΔC) calculated by the following formula was 91.8%, and the cycle characteristic based on the following criteria was evaluated as "〇". Note that the higher the value of ΔC, the better the cycle characteristic. ΔC = C50 / C0 × 100 (%) <Evaluation criteria> ◎: Charge-discharge capacity retention rate is 95.0% or more 〇: Charge-discharge capacity retention rate is 90.0% or more and less than 95.0% △: Charge-discharge capacity retention rate is 85.0% or more and less than 90.0% ×: Charge-discharge capacity retention rate is less than 85.0%

[0105] (Examples 2 to 15 and Comparative Examples 1 and 2: Evaluation of Compositions Containing Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-17) A slurry composition was prepared by performing the same operations as in Example 1 except that the carboxyl group-containing crosslinked polymer salt was as described in Table 1, and the viscosity of the composition was measured. Further, the coatability of the composition and the cycle characteristics of the secondary battery obtained using the composition were evaluated. The results are shown in Table 1.

[0106] ≪Evaluation Results≫ As is clear from the results of Examples 1 to 15, the slurry compositions containing the crosslinked polymer salts obtained by the production method of the present invention all had good coatability and excellent coating film performance (in this example, the cycle characteristics of the secondary battery equipped with the electrode obtained using the composition). Among these, when comparing the cases where the water swelling degree and the particle size in the aqueous medium were equivalent (Examples 2, 12, 13), as the exchange chain transfer mechanism type control agent, when using the control agent in reversible addition-fragmentation chain transfer polymerization (Examples 2, 12), the coating film performance was superior to the case where an iodine transfer polymerization control agent was used (Example 13) (in this example, the charge-discharge capacity retention rate was high and the cycle characteristics were excellent). On the other hand, in the case of a slurry composition containing a crosslinked polymer produced without using an exchange chain transfer mechanism type control agent, either the coating film performance (cycle characteristics of the secondary battery) or the coatability was significantly inferior (Comparative Examples 1 and 2).

Industrial Applicability

[0107] The composition containing the carboxyl group-containing crosslinked polymer or its salt obtained by the production method of the present invention is excellent in both coatability and coating film performance, and thus is expected to be applied to various uses such as thickeners and viscosity modifiers for cosmetics, binders for non-aqueous electrolyte secondary battery electrodes, anti-settling agents for pigments, and dispersion stabilizers for metal powders.

Claims

1. A method for producing a carboxyl group-containing crosslinked polymer or a salt thereof, comprising: a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization in the presence of a chain transfer mechanism type controller; the chain transfer mechanism type controller is a polymer having a living radical polymerization active unit by a chain transfer mechanism with a polymerization chain of one or more vinyl monomers, and / or a chain transfer mechanism type controller other than the polymer; the crosslinked polymer or a salt thereof is neutralized to a neutralization degree of 80 to 100 mol%, and has a particle diameter measured in an aqueous medium of 0.1 μm or more and 5.0 μm or less in terms of volume-based median diameter.

2. The production method according to claim 1, wherein the chain transfer mechanism type controller is a reversible addition fragmentation chain transfer agent (RAFT agent).

3. The production method according to claim 2, wherein the reversible addition fragmentation chain transfer agent has a trithiocarbonate group in the molecule.

4. The production method according to any one of claims 1 to 3, wherein the amount of the chain transfer mechanism type controller used is 0.0001 to 0.50 mol% based on the total amount of the monomer component containing the ethylenically unsaturated carboxylic acid monomer.

5. The production method according to any one of claims 1 to 4, wherein the monomer component contains 50% by mass or more and 100% by mass or less of an ethylenically unsaturated carboxylic acid monomer based on the total amount thereof.

6. The crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.1 part by mass or more and 2.0 parts by mass or less based on 100 parts by mass of the total amount of the non-crosslinkable monomer. The production method according to any one of claims 1 to 5.

7. The production method according to any one of claims 1 to 6, wherein the crosslinked polymer or a salt thereof has a water swelling degree at pH 8 of 20 or more and 80 or less.

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