Block copolymer, its manufacturing method, and absorbent and absorbent article using the same

Abstract

This block copolymer (A) contains a vinyl alcohol-based polymer block (b), and an ionic polymer block (c) including a vinyl alcohol-based monomer and a monomer unit having an ionic group forming a salt. The ionic group is a carboxylate group, a sulfonate group, or an ammonium group. The number average molecular weight (Mnb) of the polymer block (b) is 15,000-220,000. The contained amount of the vinyl alcohol-based monomer unit with respect to all monomer units in the ionic polymer block (c) is 5-95 mol%. The number average molecular weight (MnA) of the block copolymer (A) is 20,000-440,000. A ratio (Mnb / MnA) of the number average molecular weight (Mnb) with respect to the number average molecular weight (MnA) is 0.1-0.9. Such block copolymer has high absorbability with respect to water, salt water, and the like, and has both gel shape retaining ability and water solubility.

Description

[Technical Field]

[0001] The present invention relates to a block copolymer having excellent absorbency for water, saltwater, etc., and a method for producing the same. The present invention also relates to an absorbent and an absorbent article using the block copolymer. [Background technology]

[0002] Superabsorbent polymers have been widely used in hygiene materials such as sanitary products and disposable diapers, soil water retention agents, etc. Known examples of such superabsorbent polymers include crosslinked polyacrylates, self-crosslinked polyacrylates, crosslinked starch-acrylate graft copolymers, hydrolyzates of crosslinked acrylamide copolymers, neutralized crosslinked isobutylene-maleic anhydride copolymers, and crosslinked carboxyalkyl cellulose salts.

[0003] However, these polymers are crosslinked and insoluble in water, so they cannot be flushed down the drain, resulting in the generation of large amounts of waste, which has been problematic.Furthermore, these polymers are not sufficiently biodegradable, so they may cause soil pollution, marine pollution, etc., and have also been problematic from an environmental perspective.

[0004] Patent Document 1 describes a highly water-absorbent resin using a crosslinked polymer of an ethylenically unsaturated monomer having acrylic acid and / or an acrylate salt as a main constituent unit. However, such a crosslinked polymer is insoluble in water and also has insufficient biodegradability. Patent Document 2 describes an absorbent obtained by crosslinking a biodegradable water-absorbent resin, such as a carboxyalkyl cellulose salt, with an amino acid or the like, and then further crosslinking the resin with a surface crosslinking agent. However, the absorbent is insoluble in water and sometimes has insufficient biodegradability.

[0005] On the other hand, polyvinyl alcohol is a crystalline, water-soluble polymer material, and its excellent water solubility and film properties (strength, oil resistance, film-forming ability, oxygen gas barrier properties, etc.) are utilized for a wide range of applications, including emulsifiers, suspending agents, surfactants, fiber processing agents, various binders, paper processing agents, adhesives, and films.

[0006] Non-Patent Documents 1 and 2 describe block copolymers containing a polyacrylic acid block and a polyvinyl alcohol block, and block copolymers containing a potassium polyacrylate block and a polyvinyl alcohol block. However, these block copolymers sometimes have insufficient water absorption properties or difficulty in retaining their shape after absorbing water. Furthermore, these documents do not describe the water absorption properties of the block copolymers. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 7-033818 [Patent Document 2] Japanese Patent Application Publication No. 8-196901 [Non-patent literature]

[0008] [Non-Patent Document 1] Highly Stretchable Free-Standing Poly(acrylic acid)-block-poly(vinyl alcohol) Films Obtained from Cobalt-Mediated Radical Polymerization, Macromolecules, 2017, vol.50, p6054-6063 [Non-patent document 2] Synthesis of Novel Well-Defined Poly(vinyl acetate)-b-poly(acrylonitrile) and Derivatized Water-Soluble Poly(vinyl alcohol)-b-poly(acrylic acid) Block Copolymers by Cobalt-Mediated Radical Polymerization,Macromolecules 2008, vol41, p2353-2360 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made to solve the above problems, and an object of the present invention is to provide a block copolymer that has high absorbency for water, salt water, etc., as well as gel shape retention and water solubility. [Means for solving the problem]

[0010] The above-mentioned problem is solved by a block copolymer (A) comprising a vinyl alcohol-based polymer block (b) and an ionic polymer block (c) containing a monomer unit having an ionic group forming a salt and a vinyl alcohol-based monomer unit, wherein the ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group, and the number average molecular weight (Mn b ) is 15,000 to 220,000, the content of vinyl alcohol-based monomer units in the ionic polymer block (c) relative to the total monomer units is 5 to 95 mol %, and the number average molecular weight (Mn A ) is 20,000 to 440,000, and the number average molecular weight (Mn A ) to the number average molecular weight (Mn b ) ratio (Mn b / Mn A ) is solved by providing a value between 0.1 and 0.9.

[0011] In this case, the ionic group is preferably a carboxylic acid group. It is also preferable that the counter ion of the ionic group is an ion of an element of Group 1, 2, 12, 13 or 17 of the periodic table. The content (J) of the monomer units having the ionic group forming the salt relative to the total monomer units in the block copolymer (A) is A It is also preferable that the content (K ) of the monomer units having an ionic group forming the salt relative to the total monomer units in the polymer block (c) is 2 to 90 mol %. c ) is also preferably 5 to 95 mol %.

[0012] It is also preferable that the degree of saponification of the block copolymer (A) is 80 to 99.99 mol %. A / Mn A ) is preferably 1.05 to 1.95. It is also preferable that the block copolymer (A) is biodegradable.

[0013] It is preferable that the amount of deionized water absorbed per 0.1 g of block copolymer (A) at 20° C. is 20 g or more. It is also preferable that the amount of 0.9 mass % aqueous sodium chloride solution absorbed per 1 g of block copolymer (A) at 20° C. is 20 g or more.

[0014] The soluble content when dissolved in water at 95°C is preferably 95% by mass or more, and more preferably 95% by mass or more when dissolved in water at 20°C.

[0015] An absorbent material containing the block copolymer (A) is a preferred embodiment of the present invention. The absorbent is preferably in the form of particles. An absorbent article containing the absorbent material is a more preferred embodiment. The absorbent article is preferably for hygiene, daily necessities, construction and civil engineering, industrial, agricultural, medical, or food use.

[0016] The above-mentioned problems can also be solved by providing a method for producing a block copolymer (A), the method comprising: a polymerization step of polymerizing a vinyl ester monomer by controlled radical polymerization in the presence of a radical polymerization initiator and a control agent and copolymerizing the vinyl ester monomer with an ionic monomer having an ionic group or a derivative thereof to obtain a vinyl ester block copolymer including a vinyl ester polymer block (b1) and an ionic polymer block (c1) containing a vinyl ester monomer unit and an ionic monomer unit; and a saponification step of saponifying the vinyl ester monomer unit in the vinyl ester block copolymer obtained in the polymerization step to form a vinyl alcohol monomer unit, as an essential step; and a salt formation step of forming a salt from the ionic monomer unit as an optional step. [Effects of the Invention]

[0017] The block copolymer (A) of the present invention has high absorbency for water, saltwater, etc., as well as gel shape retention and water solubility. Therefore, the block copolymer (A) is suitable for use in absorbents and absorbent articles for water, saltwater, etc. The production method of the present invention allows the production of the block copolymer (A). DETAILED DESCRIPTION OF THE INVENTION

[0018] The block copolymer (A) of the present invention comprises a vinyl alcohol-based polymer block (b) and an ionic polymer block (c) containing a monomer unit having an ionic group forming a salt and a vinyl alcohol-based monomer unit, wherein the ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group, and the number average molecular weight (Mn b ) is 15,000 to 220,000, the content of vinyl alcohol-based monomer units in the ionic polymer block (c) relative to the total monomer units is 5 to 95 mol %, and the number average molecular weight (Mn A ) is 20,000 to 440,000, and the number average molecular weight (Mn A ) to the number average molecular weight (Mn b ) ratio (Mn b / Mn A ) is between 0.1 and 0.9.

[0019] The block copolymer (A) of the present invention has high absorbency for water, salt water, etc., as well as gel shape retention and water solubility. The high absorbency is believed to be due to the ionic polymer block (c), and the gel shape retention and water solubility are believed to be due to the vinyl alcohol-based polymer block (b).

[0020] A preferred method for producing the block copolymer (A) of the present invention comprises, as essential steps, a polymerization step in which vinyl ester monomers are polymerized by controlled radical polymerization in the presence of a radical polymerization initiator and a polymerization inhibitor, and then a vinyl ester monomer and an ionic monomer having an ionic group or a derivative thereof are polymerized to obtain a vinyl ester block copolymer containing a vinyl ester polymer block (b1) and an ionic polymer block (c1) containing vinyl ester monomer units and ionic monomer units, and a saponification step in which the vinyl ester monomer units in the vinyl ester block copolymer obtained in the polymerization step are saponified to form vinyl alcohol monomer units. The production method is described in detail below.

[0021] First, the polymerization step will be described. In the polymerization step, polymerization of a vinyl ester monomer and polymerization of the vinyl ester monomer and an ionic monomer are carried out by controlled radical polymerization in the presence of a radical initiator and a control agent. A vinyl ester polymer block (b1) is synthesized by polymerizing a vinyl ester, and an ionic polymer block (c1) is synthesized by polymerizing a vinyl ester monomer and an ionic monomer. In the synthesis step of the ionic polymer block (c1), an ionic polymer block (c1), which is a copolymer containing ionic monomer units and vinyl ester monomer units, is synthesized by copolymerizing an ionic monomer and a vinyl ester monomer, thereby improving the gel shape retention of the resulting block copolymer (A).

[0022] Examples of vinyl ester monomers that can be used in the production method of the present invention include vinyl formate, vinyl acetate, vinyl trifluoroacetate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl versatate. From an economical viewpoint, vinyl acetate is preferably used.

[0023] The ionic monomer used in the production method of the present invention is a monomer having an ionic group or a derivative thereof. The ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group. These may be used alone or in combination of two or more. A carboxylic acid group is preferred as the ionic group. The monomer having the ionic group may or may not form a salt.

[0024] Examples of the monomer having an ionic group or a derivative thereof that can be used as the ionic monomer include a monomer having a carboxylic acid group, such as (meth)acrylic acid, maleic acid, itaconic acid, or fumaric acid, or a derivative thereof; a monomer having a sulfonic acid group, such as vinyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, or 2-(meth)acryloylpropanesulfonic acid, or a derivative thereof; and a monomer having an ammonium group, such as vinyloxyethyl trimethylammonium chloride, vinyloxybutyl trimethylammonium chloride, N-acrylamidomethyl trimethylammonium chloride, 3-(methacrylamido)propyl trimethylammonium chloride, N-acrylamidoethyl trimethylammonium chloride, allyl trimethylammonium chloride, or methallyl trimethylammonium chloride, or a derivative thereof. Among these, a monomer having a carboxylic acid group or a derivative thereof is preferred, and a monomer having a carboxylic acid group or an ester thereof is more preferred.

[0025] The monomer having a carboxylic acid group or its ester is preferably a (meth)acrylic acid ester. Examples of the (meth)acrylic acid ester include methyl (meth)acrylate, ethyl (meth)acrylate, N-propyl (meth)acrylate, i-propyl (meth)acrylate, N-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate, with methyl (meth)acrylate being more preferred. The (meth)acrylic acid ester may be either a methacrylic acid ester or an acrylic acid ester, with acrylic acid ester being more preferred.

[0026] The monomer having a sulfonic acid group is preferably a (meth)acrylamide monomer, more preferably 2-(meth)acrylamide-2-methylpropanesulfonic acid, and more preferably 2-acrylamide-2-methylpropanesulfonic acid.

[0027] The monomer having an ammonium group is preferably a (meth)acrylamide monomer, and more preferably 3-(methacrylamide)propyltrimethylammonium chloride.

[0028] Furthermore, the block copolymer (A) of the present invention may contain a monomer unit derived from an ethylenically unsaturated monomer (e) copolymerizable with a vinyl ester monomer and the ionic monomer, within a range that does not impair the effects of the present invention. Examples of the ethylenically unsaturated monomer (e) include olefins such as ethylene, propylene, 1-butene, and isobutene; acrylamides such as acrylamide, N-alkyl (C1 to C18) acrylamide, and N,N-dimethylacrylamide; methacrylamides such as methacrylamide, N-alkyl (C1 to C18) methacrylamide, and N,N-dimethylmethacrylamide; N-vinylamides such as N-vinylpyrrolidone, N-vinylformamide, and N-vinylacetamide; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl ethers such as alkyl (C1 to C18) vinyl ether, hydroxyalkyl vinyl ether, and alkoxyalkyl vinyl ether; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, and vinyl bromide; vinyl silanes such as trimethoxyvinylsilane; and allyl compounds such as allyl acetate, allyl chloride, allyl alcohol, and dimethylallyl alcohol. When the ethylenically unsaturated monomer (e) is used, it may be copolymerized during the polymerization of the vinyl ester polymer block (b1), or may be copolymerized during the polymerization of the ionic polymer block (c1), or may be polymerized separately from the polymerization of the vinyl ester polymer block (b1) and the ionic polymer block (c1).

[0029] The content of the ethylenically unsaturated monomer (e) in the vinyl alcohol polymer block (b) and the ionic polymer block (c) constituting the block copolymer (A) is preferably 10 mol % or less, more preferably 3 mol % or less, even more preferably 1 mol % or less, based on the total monomer units in each block, and it is particularly preferred that the ethylenically unsaturated monomer (e) is substantially absent. Furthermore, the content of the ethylenically unsaturated monomer (e) in the total monomer units of the block copolymer (A) is preferably 10 mol % or less, more preferably 3 mol % or less, even more preferably 1 mol % or less, and it is particularly preferred that the ethylenically unsaturated monomer (e) is substantially absent.

[0030] The controlled radical polymerization employed in the production method of the present invention is a polymerization reaction in which the terminal of a propagating radical (active species) is in equilibrium with a covalently bonded species (dormant species) bonded to a control agent, and the reaction proceeds. Examples of control agents used in the production method of the present invention include organic cobalt complexes, organic iodine compounds, thiocarbonyl compounds, organic tellurium compounds, organic compounds having a redox center, and organic compounds having a stable radical, with organic cobalt complexes being preferred.

[0031] The organic cobalt complex may, for example, contain a divalent cobalt atom and an organic ligand. Suitable organic cobalt complexes include cobalt(II) acetylacetonate [Co(acac)2] and cobalt(II) porphyrin complexes. Among these, cobalt(II) acetylacetonate is preferred from the viewpoint of production costs.

[0032] Examples of polymerization methods include known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Among these, bulk polymerization, which involves polymerization without a solvent, and solution polymerization, which involves polymerization in various organic solvents, are usually used. In order to obtain a polymer with a narrow molecular weight distribution, bulk polymerization, which does not use a solvent or a dispersion medium that may cause side reactions such as chain transfer, is preferred.

[0033] On the other hand, solution polymerization may be preferable in terms of adjusting the viscosity of the reaction solution and controlling the polymerization rate. Examples of organic solvents used as solvents during solution polymerization include esters such as methyl acetate and ethyl acetate; aromatic hydrocarbons such as benzene and toluene; and lower alcohols such as methanol and ethanol. Among these, esters and aromatic hydrocarbons are preferred to prevent chain transfer. When using a solvent, the amount of solvent used may be determined in accordance with the number-average molecular weight of the desired block copolymer (A) and taking into consideration the viscosity of the reaction solution. For example, the mass ratio (solvent / monomer) may be selected from the range of 0.01 to 10. The mass ratio (solvent / monomer) is preferably 0.1 or more and 5 or less.

[0034] The radical initiator used in the polymerization step may be appropriately selected from conventionally known azo initiators, peroxide initiators, redox initiators, etc. Examples of azo initiators include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), etc. Examples of peroxide initiators include percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanate, α-cumyl peroxyneodecanate, and t-butyl peroxyneodecanate; acetylcyclohexylsulfonyl peroxide; diisobutyryl peroxide; and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate. Furthermore, the above initiators can be combined with potassium persulfate, ammonium persulfate, hydrogen peroxide, etc. Redox initiators include those obtained by combining the above peroxides with reducing agents such as sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, L-ascorbic acid, and Rongalit. The amount of initiator used varies depending on the polymerization catalyst and cannot be determined in general, but is selected as appropriate depending on the polymerization rate.

[0035] In the controlled radical polymerization used in the present invention, dormant species are first formed by covalent bonding of the control agent to the radicals at the growing ends of short-chain polymers produced by the decomposition of a radical initiator. For a certain period after the start of the reaction, short-chain polymers are simply produced and converted to dormant species, and high polymerization does not substantially progress. This period is called the induction period. After the control agent is consumed, the reaction enters a growth period in which high polymerization progresses, and the molecular weights of most molecular chains in the reaction system increase uniformly in proportion to the polymerization time. This allows the production of vinyl ester-based block copolymers with a narrow molecular weight distribution. The time required for the monomer polymerization process, including the induction period and growth period, is typically 3 to 50 hours.

[0036] As described above, in the controlled radical polymerization of the present invention, theoretically, one polymer chain is produced from one molecule of added control agent. Therefore, the amount of control agent added to the reaction solution is determined taking into consideration the target number average molecular weight and polymerization rate. Usually, it is preferable to use 0.001 to 1 mole of control agent per 100 moles of monomer.

[0037] Unless the number of moles of radicals generated exceeds the number of moles of the control agent, the polymerization reaction proceeds solely through the thermal dissociation of the control agent from the dormant species, resulting in extremely slow polymerization rates depending on the reaction temperature. Therefore, considering that a radical initiator generates two radicals, the number of moles of radical initiator used must be more than half the number of moles of control agent. Generally, the amount of active radicals supplied by an initiator depends on the initiator efficiency, so some initiators are deactivated without being used to form dormant. Therefore, the number of moles of radical initiator used is preferably at least 1 times the number of moles of control agent, more preferably at least 1.5 times. On the other hand, if the number of moles of radicals generated exceeds the number of moles of control agent, the proportion of uncontrolled radical polymerization increases, resulting in a broadened molecular weight distribution. The number of moles of radical initiator used is preferably no more than 10 times the number of moles of control agent, more preferably no more than 6 times.

[0038] The method for mixing the radical initiator, the control agent, and the monomer is not particularly limited, as long as it is a method that can generate dormant species and control the polymerization degree of the polymer. Examples of such methods include a method of mixing the radical initiator and the control agent and then mixing the resulting mixture with the monomer, a method of mixing the radical initiator, the control agent, and the monomer all at once, and a method of mixing the control agent and the monomer and then mixing the resulting mixture with the radical initiator. The radical initiator, the control agent, and the monomer may also be mixed in portions. For example, a method may be used in which the radical initiator, the control agent, and a portion of the monomer are mixed to generate dormant species in which the control agent is covalently bonded to a short-chain polymer terminal, and then the dormant species is mixed with the remainder of the monomer to achieve a high polymerization degree. The dormant species may also be isolated as a macroinitiator and then mixed with the remainder of the monomer to achieve a high polymerization degree.

[0039] In the polymerization step, either the synthesis of the vinyl ester polymer block (b1) or the synthesis of the ionic polymer block (c1) may be carried out first. When the synthesis of the vinyl ester polymer block (b1) is carried out first, the polymerization of the vinyl ester monomer is initiated by mixing the vinyl ester monomer, optionally the ethylenically unsaturated monomer (e), a radical initiator, and a radical control agent in the manner described above. It is preferable not to use an ionic monomer when synthesizing the vinyl ester polymer block (b1).

[0040] After the number-average degree of polymerization of the vinyl ester polymer block (b1) reaches the target value, the vinyl ester monomer and the ionic monomer are polymerized to synthesize the ionic polymer block (c1). At this time, the remaining vinyl ester monomer may be removed and then the ionic monomer added to the reaction solution to initiate polymerization of the ionic monomer. However, from the viewpoint of further improving gel shape retention, it is preferable to add the ionic monomer to the reaction solution without removing the vinyl ester monomer to initiate copolymerization of the remaining vinyl ester monomer and the ionic monomer. The method for adding the ionic monomer is not particularly limited, and examples include a method of adding it all at once or a method of feeding it over time. The latter method is preferred from the viewpoint of further improving gel shape retention by uniformly introducing the ionic monomer units. If necessary, additional vinyl ester monomer or an ethylenically unsaturated monomer (e) may be added together with the ionic monomer. The number-average degree of polymerization of the polymer can be confirmed by gel permeation chromatography (GPC), specifically, the method described in the Examples below is employed.

[0041] When synthesizing the vinyl ester polymer block (b1) first, it is preferable to terminate the reaction before the disappearance of the ionic monomer in order to obtain a binary vinyl ester block copolymer having one vinyl ester polymer block (b1) and one polymer block (c1) containing a vinyl ester monomer unit and an ionic monomer unit. On the other hand, when obtaining a ternary or higher multi-component vinyl ester block copolymer, it is preferable to continue the polymerization to synthesize the vinyl ester polymer block (b1) even after the disappearance of the ionic monomer. This method allows the production of a ternary vinyl ester block copolymer consisting of block (b1)-block (c1)-block (b1). In the present invention, the portion obtained by polymerizing the vinyl ester when the molar ratio of the ionic monomer unit to the vinyl ester in the reaction solution (ionic monomer / vinyl ester) is 0.00001 or less is referred to as the vinyl ester polymer block (b1). The point at which the molar ratio (ionic monomer / vinyl ester) reaches 0.00001 is determined by the method described in the Examples. The reaction can be stopped when the number-average degree of polymerization of the vinyl ester block copolymer reaches a target value. Alternatively, to obtain a quaternary or higher multi-component vinyl ester block copolymer, the ionic monomer can be added again to the reaction solution of the ternary vinyl ester block copolymer to continue the polymerization.

[0042] In the polymerization step, when the synthesis of the ionic polymer block (c1) is carried out first, the polymerization is initiated by mixing the ionic monomer, the radical initiator, and the organic cobalt complex by the method described above. At this time, it is preferable to further add a vinyl ester monomer to further improve gel shape retention. Furthermore, an ethylenically unsaturated monomer (e) may be further added as necessary. After the polymer block (c1) containing the vinyl ester monomer unit and the ionic monomer unit is synthesized in this way, each block is formed sequentially.

[0043] The polymerization temperature when synthesizing the vinyl ester polymer block (b1) and the ionic polymer block (c1) is preferably, for example, 0°C to 80°C. If the polymerization temperature is below 0°C, the polymerization rate will be insufficient, and productivity will tend to decrease. From this perspective, the polymerization temperature is more preferably 10°C or higher, and even more preferably 20°C or higher. On the other hand, if the polymerization temperature exceeds 80°C, the molecular weight distribution of the resulting block copolymer (A) will tend to be broad. From this perspective, the polymerization temperature is more preferably 65°C or lower, and even more preferably 50°C or lower.

[0044] In the polymerization step, when the number-average degree of polymerization or polymerization rate of the vinyl ester-based block copolymer reaches the target value, it is preferable to perform a termination step in which a polymerization terminator is added to terminate the polymerization reaction. Examples of the polymerization terminator include 1,1-diphenylethylene; styrene compounds such as styrene, α-methylstyrene, and 4-tert-butylstyrene; hydroxyaromatic compounds such as p-methoxyphenol, hydroquinone, cresol, t-butylcatechol, and p-nitrosophenol; quinone compounds such as benzoquinone and naphthoquinone; conjugated carboxylic acids such as muconic acid and sorbic acid; thioethers such as phenothiazine, distearyl thiodipropionate, and dilauryl thiodipropionate; aromatic amines such as p-phenylenediamine and N-nitrosodiphenylamine; nitroxides such as 2,2,6,6-tetramethylpiperidine 1-oxyl and 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl; and transition metal salts such as copper acetate, copper dithiocarbamate, and manganese acetate. Among these, 1,1-diphenylethylene, sorbic acid and benzoquinone are preferred, and 1,1-diphenylethylene is more preferred.

[0045] The number of moles of the polymerization terminator added is preferably 1 to 100 moles per mole of the added inhibitor. If the number of moles of the polymerization terminator is too small, the radicals at the polymer terminals may not be sufficiently captured, and the color tone of the resulting block copolymer (A) may deteriorate. On the other hand, if the number of moles of the polymerization terminator is too large, the production cost may increase.

[0046] The temperature of the reaction solution in the termination step may be any temperature at which the polymerization terminator can react with the terminal radicals of the vinyl ester block copolymer, and is preferably 0 to 80° C. The time required for the termination step is usually 10 minutes to 5 hours.

[0047] When a cobalt complex is used as the inhibitor, it is preferable to carry out an extraction step prior to the saponification step, in which the obtained vinyl ester-based block copolymer solution is contacted with an aqueous solution containing a water-soluble ligand to extract and remove the cobalt complex from the vinyl ester-based block copolymer solution. In this way, by removing the cobalt complex contained in the vinyl ester-based block copolymer solution in advance and then carrying out the saponification step, a block copolymer (A) with a good hue can be obtained. Specifically, the aqueous solution and the vinyl ester-based block copolymer solution, which are not soluble in each other, are vigorously stirred to increase the interfacial area between them, then allowed to stand, and the mixture is separated into an oil layer and an aqueous layer, after which the aqueous layer is removed. This operation may be repeated multiple times.

[0048] The water-soluble ligand used in the extraction step is preferably an acid having a pKa of 0 to 12 at 25°C. When a strong acid with a pKa of less than 0 is used, it is difficult to efficiently extract the cobalt complex, so the pKa is preferably 2 or greater. When a weak acid with a pKa of more than 12 is used, it is also difficult to efficiently extract the cobalt complex, so the pKa is preferably 7 or less. When the acid is a polyvalent acid, it is necessary that the first dissociation constant (pKa1) is within the above range. As for the water-soluble ligand, the acid having a pKa of 0 to 12 is preferably a carboxylic acid or phosphoric acid (pKa1 is 2.1), more preferably a carboxylic acid. Of these, acetic acid (pKa is 4.76) is particularly preferred.

[0049] The pH of the aqueous solution containing the water-soluble ligand is preferably 0 to 5. The pH is more preferably 1 or higher, and even more preferably 1.5 or higher. The pH is more preferably 4 or lower, and even more preferably 3 or lower.

[0050] In the saponification step, vinyl alcohol monomer units are formed by saponifying the vinyl ester monomer units contained in the vinyl ester polymer block (b1) and the ionic polymer block (c1) in the vinyl ester block copolymer obtained in the polymerization step. The saponification step converts the vinyl ester polymer block (b1) into the vinyl alcohol polymer block (b).

[0051] In the saponification step, the vinyl ester block copolymer produced by the above-mentioned method is dissolved in alcohol and saponified, thereby converting the vinyl ester monomer units in the vinyl ester block copolymer to vinyl alcohol monomer units. Furthermore, when a vinyl ester block copolymer obtained using an acrylic acid ester as an ionic monomer is saponified, the acrylic acid ester monomer units in the copolymer may be converted to acrylic acid monomer units, or the converted acrylic acid monomer units may form salts. Furthermore, the acrylic acid ester monomer units or acrylic acid monomer units may form lactone rings with adjacent vinyl alcohol monomer units.

[0052] Examples of the alcohol used in the saponification reaction include lower alcohols such as methanol and ethanol, with methanol being particularly preferred. The alcohol may be a hydrous alcohol or a dehydrated alcohol. The alcohol used in the saponification reaction may contain a solvent such as acetone, an ester such as methyl acetate or ethyl acetate, or toluene. Examples of catalysts used in the saponification reaction include alkali metal hydroxides such as potassium hydroxide and sodium hydroxide; alkali catalysts such as sodium methylate; and acid catalysts such as inorganic acids. The temperature of the saponification reaction is preferably in the range of 20 to 70°C. If a gel-like product precipitates as the saponification reaction proceeds, the product is crushed at this point, washed, and then dried.

[0053] The production method of the present invention optionally includes a salt-forming step in which a salt is formed on the ionic monomer units in the block copolymer after the saponification step. The method for forming a salt on the ionic monomer units is not particularly limited, and any known method may be used depending on the type of ionic monomer unit. The saponification and salt-forming steps convert the ionic polymer block (c1) into an ionic polymer block (c) containing a monomer unit having an ionic group that forms a salt and a vinyl alcohol-based monomer unit. If the ionic polymer block (c1) after the saponification step contains a monomer unit having an ionic group that forms a salt, the block (c1) corresponds to the block (c), and therefore the block copolymer (A) of the present invention can be obtained without the salt-forming step.

[0054] When an acrylic acid ester is used as the ionic monomer, it is preferable to perform the salt formation process on the block copolymer after the saponification process. This converts the acrylic acid ester units, acrylic acid units, and lactone rings into acrylic acid monomer units that form a salt. Specific methods include mixing the block copolymer after the saponification process with an aqueous solution of a metal hydroxide and alcohol. Examples of the metal hydroxide include sodium hydroxide, potassium hydroxide, and calcium hydroxide. Examples of the alcohol include methanol and ethanol. The salt formation process is suitably performed, for example, at a temperature in the range of 20 to 100°C.

[0055] The monomer unit having an ionic group in the ionic polymer block (c) is formed by the ionic monomer described above. The ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group, and among these, a carboxylic acid group is preferred. The monomer unit having an ionic group is as described above, but among these, a monomer unit having a carboxylic acid group is preferred, and an acrylic acid monomer unit is more preferred.

[0056] The monomer unit having an ionic group in the ionic polymer block (c) forms a salt. The counter ion of the monomer unit is not particularly limited, but is preferably an ion of an element in Group 1, 2, 12, 13, or 17 of the periodic table, and among these, cations such as sodium ion, potassium ion, magnesium ion, calcium ion, zinc ion, and aluminum ion, and anions such as chloride ion are preferred. As the cation, sodium ion and calcium ion are more preferred, and sodium ion is even more preferred.

[0057] The block copolymer (A) of the present invention thus obtained contains a vinyl alcohol-based polymer block (b) and an ionic polymer block (c) containing a monomer unit having an ionic group forming a salt and a vinyl alcohol-based monomer unit. The block copolymer (A) may be a diblock copolymer consisting of one block (b) and one block (c), a triblock copolymer consisting of one block (b) and two blocks (c), or two blocks (b) and one block (c), or a multiblock copolymer consisting of a total of four or more blocks (b) and (c). Among these, the block copolymer (A) is preferably a diblock copolymer or a triblock copolymer. The bond between the block (b) and the block (c) is preferably linear. The block copolymer (A) may contain blocks other than the block (b) and the block (c), but the content thereof is preferably 10 mol % or less, more preferably 1 mol % or less.

[0058] The saponification degree of the block copolymer (A) of the present invention is preferably 80 to 99.99 mol%. In the present invention, the saponification degree refers to the ratio (mol%) of the number of moles of vinyl alcohol monomer units to the total number of moles of vinyl ester monomer units and vinyl alcohol monomer units in the block copolymer (A). If the saponification degree is less than 80 mol%, the crystallinity of the block copolymer may be too low, resulting in a decrease in gel shape retention and the desired absorption performance may not be achieved. The saponification degree is preferably 85 mol% or more, more preferably 90 mol% or more, and even more preferably 95 mol% or more. On the other hand, if the saponification degree exceeds 99.99 mol%, production of the block copolymer (A) tends to become more difficult. The saponification degree is preferably 99.95 mol% or less. The saponification degree of the vinyl alcohol-based block copolymer (A) 1 It can be determined by H-NMR measurement, specifically by the method described in the Examples.

[0059] The content (Z) of units derived from ionic monomers having an ionic group or a derivative thereof relative to the total monomer units in the block copolymer (A) of the present invention A When an acrylic acid ester is used as the ionic monomer, the units derived from the ionic monomer include acrylic acid monomer units that form a salt, acrylic acid monomer units that do not form a salt, acrylic acid ester monomer units, and lactone rings, and the content (Z A ) is the total amount of these units.

[0060] Content (Z A When the content (Z) is 2 mol % or more, the absorbency of the block copolymer (A) is further increased. A ) is more preferably 3 mol% or more, further preferably 4 mol% or more, and particularly preferably 5 mol% or more. A When the content (Z) is 90 mol % or less, the gel shape retention of the block copolymer (A) is further improved. A ) is more preferably 60 mol % or less, further preferably 40 mol % or less, particularly preferably 20 mol % or less.

[0061] The content (J) of the monomer units having an ionic group forming the salt relative to the total monomer units in the block copolymer (A) of the present invention is A The content (J) is preferably 2 to 90 mol %. A When the content (J) is 2 mol % or more, the absorbency of the block copolymer (A) is further increased. A ) is more preferably 3 mol% or more, further preferably 4 mol% or more, and particularly preferably 5 mol% or more. A When the content (J) is 90 mol % or less, the gel shape retention of the block copolymer (A) is further improved. A ) is more preferably 60 mol % or less, further preferably 40 mol % or less, particularly preferably 20 mol % or less.

[0062] The content of vinyl alcohol-based monomer units in the vinyl alcohol-based polymer block (b) relative to all monomer units is preferably 80 mol% or more, more preferably 90 mol% or more, even more preferably 95 mol% or more, and particularly preferably 99 mol% or more. In the present invention, the vinyl alcohol-based monomer units refer to vinyl ester monomer units and vinyl alcohol monomer units, and the content of vinyl alcohol-based monomer units refers to the total content of vinyl ester monomer units and vinyl alcohol monomer units.

[0063] The content of vinyl alcohol monomer units in the vinyl alcohol polymer block (b) relative to all monomer units is preferably 80 mol% or more, more preferably 90 mol% or more, even more preferably 95 mol% or more, and particularly preferably 99 mol% or more.

[0064] The content of the units derived from the ionic monomer relative to all the monomer units in the vinyl alcohol polymer block (b) is usually less than 0.1 mol %.

[0065] The content of the units derived from the ionic monomer relative to the total monomer units in the ionic polymer block (c) (R c The content (R ) is preferably 5 to 95 mol %. c When the content (R) is 5 mol % or more, the absorbency of the block copolymer (A) is further increased. c ) is more preferably 6 mol % or more, even more preferably 8 mol % or more, and particularly preferably 10 mol % or more. On the other hand, from the viewpoint of further improving the gel shape retention of the block copolymer (A), the content (R c ) is more preferably 80 mol % or less, further preferably 40 mol % or less, particularly preferably 25 mol % or less.

[0066] The content of the monomer units having an ionic group forming the salt relative to the total monomer units in the ionic polymer block (c) (K c The content (K) is preferably 5 to 95 mol %. c When the content (K) is 5 mol % or more, the absorbency of the block copolymer (A) is further increased. c ) is more preferably 6 mol % or more, even more preferably 8 mol % or more, and particularly preferably 10 mol % or more. On the other hand, from the viewpoint of further improving the gel shape retention of the block copolymer (A), the content (K c ) is more preferably 80 mol % or less, further preferably 40 mol % or less, particularly preferably 25 mol % or less.

[0067] The ionic polymer block (c) contains vinyl alcohol-based monomer units. This improves the gel shape retention and absorbency of the block copolymer (A). The content of vinyl alcohol-based monomer units (total content of vinyl alcohol units and vinyl ester units) relative to all monomer units in the ionic polymer block (c) is 5 to 95 mol%. If this content is less than 5 mol, the effects of improving the gel shape retention and absorbency of the block copolymer (A) are not obtained. The content is preferably 20 mol% or more, more preferably 60 mol% or more, and even more preferably 75 mol% or more. On the other hand, if the content of the vinyl alcohol-based monomer units exceeds 95 mol%, the absorbency of the block copolymer (A) decreases. The content is preferably 94 mol% or less, more preferably 92 mol% or less, and even more preferably 90 mol% or less.

[0068] The number average molecular weight (Mn A The molecular weight distribution is narrow and the number average molecular weight (Mn A ) can be obtained. A When Mn is 20,000 or more, the absorbability of the block copolymer (A) is improved. A is preferably 30,000 or more, more preferably 40,000 or more, even more preferably 50,000 or more, and particularly preferably 60,000 or more. A When Mn is 440,000 or less, the gel shape retention is improved. A The number average molecular weight (Mn) is preferably 300,000 or less, more preferably 250,000 or less, and even more preferably 200,000 or less. A ) and molecular weight distribution (Mw A / Mn A ) is a value obtained by measuring the block copolymer (A) by the GPC method using polymethyl methacrylate as a standard substance and a tetrahydrofuran (THF) column. The measurement method is as described in the Examples.

[0069] The number average molecular weight (Mn b When the block copolymer (A) contains a plurality of vinyl alcohol-based polymer blocks (b), the total number average molecular weight of each vinyl alcohol-based polymer block (b) is defined as the number average molecular weight (Mn b ) Mn b By setting the Mn to 15,000 or more, the gel shape retention is improved while maintaining the absorbency. b is preferably 20,000 or more, more preferably 30,000 or more, even more preferably 35,000 or more, and particularly preferably 40,000 or more. b If Mn exceeds 220,000, the absorbency decreases. b is preferably 180,000 or less, more preferably 150,000 or less, and even more preferably 120,000 or less. b The polymer was sampled from the reaction solution during polymerization and analyzed by GPC. 1 After H-NMR measurement, the number average molecular weight of the obtained polymer and the content of each monomer unit are used to calculate the molecular weight. Specifically, the method described in the Examples below is used.

[0070] The number average molecular weight (Mn A ) to the number average molecular weight (Mn b ) ratio (Mn b / Mn A ) is 0.1 to 0.9. b / Mn A When the ratio (Mn) is 0.1 or more, the gel shape retention and water solubility of the block copolymer (A) are improved. b / Mn A ) is preferably 0.2 or more, more preferably 0.3 or more. b / Mn A When the ratio (Mn) is 0.9 or less, the absorbency of the block copolymer (A) is improved. b / Mn A ) is preferably 0.8 or less, more preferably 0.7 or less.

[0071] The molecular weight distribution (Mw A / Mn A ) is preferably 1.05 to 1.95. By polymerizing by controlled radical polymerization, a block copolymer (A) having a narrow molecular weight distribution can be obtained. A / Mn A ) is preferably 1.80 or less, more preferably 1.65 or less, and even more preferably 1.55 or less. A / Mn A When the ratio of block (b) to block (c) is within the above range, block (b) and block (c) form a dense phase-separated structure, improving absorbency and shape retention.

[0072] In the polymerization step, a vinyl ester block copolymer containing a vinyl ester polymer block (b1) and an ionic polymer block (c1) can be obtained by polymerizing a monomer containing a vinyl ester monomer and a monomer containing an ionic monomer and a vinyl ester monomer. In a preferred embodiment, the saponification step and optional salt formation step are performed on the vinyl ester block copolymer to obtain a block copolymer (A) containing a vinyl alcohol polymer block (b) and an ionic polymer block (c) containing a monomer unit having an ionic group that forms a salt and a vinyl alcohol monomer unit.

[0073] When (meth)acrylic acid or a (meth)acrylic acid ester is used as the ionic monomer, the molar ratio of lactone rings to the total of acrylic acid monomer units and lactone rings (V A ) [lactone rings / total of acrylic acid monomer units and lactone rings] is preferably 0.75 or more.

[0074] By subjecting the block copolymer (A) to a heat treatment in an acidic aqueous solution, acrylic acid monomer units adjacent to vinyl alcohol monomer units and capable of forming lactone rings with the vinyl alcohol monomer units are converted into lactone rings. On the other hand, when the acrylic acid monomer units are continuous, the acrylic acid monomer units remain as lactone rings without forming lactone rings even after the treatment. Therefore, when the proportion of the portions where vinyl alcohol monomer units and acrylic acid monomer units are alternately arranged is high, i.e., when the proportion of the portions where acrylic acid ester monomer units and acrylic acid monomer units are continuous is low, the molar ratio (V A ) [lactone ring / total of acrylic acid monomer unit and lactone ring] becomes higher. A ) [lactone ring / total of acrylic acid monomer unit and lactone ring] is an index of the randomness of the ionic polymer block (c) containing vinyl alcohol-based monomer units and acrylic acid monomer units forming salts. A When the molar ratio (V) [lactone ring / total of acrylic acid monomer units and lactone rings] is 0.75 or more, the gel shape retention of the block copolymer (A) is further improved. A ) [lactone ring / total of acrylic acid monomer units and lactone ring] is more preferably 0.80 or more, even more preferably 0.85 or more, and particularly preferably 0.90 or more. The conditions for the heat treatment and drying of the block copolymer (A) are the same as those described in the Examples.

[0075] The crystalline melting temperature (Q) of the polymer obtained by resaponifying the block copolymer (A) to a degree of saponification of 99 mol% or more A ) [°C] is preferably 210°C or higher. A ) further improves gel shape retention. A More preferably, the temperature is 215°C or higher, and even more preferably, 220°C or higher. A can be measured by the method described in the Examples.

[0076] It is preferable that the amount of deionized water absorbed per 0.1 g of block copolymer (A) at 20°C is 20 g or more. There is no particular upper limit, but it is usually less than 100 g. It is also preferable that the amount of 0.9 mass % sodium chloride aqueous solution absorbed per 1 g of block copolymer (A) at 20°C is 20 g or more. There is no particular upper limit, but it is usually less than 100 g. The amount of deionized water or sodium chloride aqueous solution absorbed is measured by the method described in the Examples. Block copolymer (A) with such high absorbency for deionized water or saline solution is suitable for various applications.

[0077] When 1 g of block copolymer (A) is dissolved in 1000 ml of water at 95°C, the soluble content is preferably 95% by mass or more, and more preferably 95% by mass or more when dissolved in 1000 ml of water at 20°C. Such block copolymer (A) with high solubility in water is environmentally friendly. Furthermore, since the block copolymer (A) can be made to absorb water, salt water, etc. and then discharged into the sewer, waste can be reduced. Furthermore, it is believed that the performance of such block copolymer (A) can be utilized to develop new applications. The soluble content is calculated by the method described in the Examples.

[0078] The block copolymer (A) preferably has a high biodegradability. A high biodegradability is environmentally friendly. Furthermore, a high soluble content when dissolved in water and a high biodegradability as described above can further reduce waste from the block copolymer (A). The biodegradability is preferably 20% or more, more preferably 40% or more. The biodegradability can be determined by the method described in the Examples.

[0079] The absorbency and water solubility of the block copolymer (A) of the present invention can be appropriately changed by designing the block copolymer (A). For example, the absorbency of deionized water per 0.1 g is X DIW [g], and the soluble amount when 1 g of block copolymer (A) is dissolved in 1000 ml of water at 95°C is Y 95[mass %], and the soluble amount when 1 g of block copolymer (A) is dissolved in 1000 ml of water at 20°C is Y 20 When expressed as [mass%], X DIW is 20 or more, Y 95 is 95 or more, Y 20 Copolymer with a value of 95 or more; X DIW is 20 or more, Y 95 is 95 or more, Y 20 A copolymer having a value of 0 to 95; X DIW is 40 or more, Y 95 is 95 or more, Y 20 Copolymer with a value of 95 or more; X DIW is 40 or more, Y 95 is 95 or more, Y 20 It is possible to prepare a block copolymer (A) having a value of 0 to 95. In addition, the amount of soluble matter in water at temperatures other than the above-mentioned 95°C and 20°C may be adjusted depending on the application.

[0080] An absorbent material containing block copolymer (A) is a preferred embodiment of the present invention. The form of the absorbent material is not particularly limited, but examples include particles, sheets, tapes, gels, plasters, films, fibers, etc., of which particles are preferred. The content of block copolymer (A) in the absorbent material is not particularly limited, but is preferably 1% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more.

[0081] As components other than the block copolymer (A) in the absorbent, materials that prevent the block copolymer (A) from falling off and that do not hinder liquid permeation are appropriately used, such as paper (e.g., tissue paper), pulp, and various nonwoven fabrics (e.g., spunbond nonwoven fabric, meltblown nonwoven fabric, thermalbond nonwoven fabric, needle-punched nonwoven fabric, spunlace nonwoven fabric, and airlaid nonwoven fabric). These may be subjected to water-solubilizing treatment, water-decomposable treatment, hydrophilizing treatment, or pore-opening treatment as necessary. Polymers such as polyvinyl alcohol, polyacrylic acid, polyacrylamide, polyurethane, polylactic acid, starch, cellulose, polyoxyalkylene, polyethylene, polypropylene, and polyethylene terephthalate may be used alone or in combination.

[0082] An absorbent article having the absorbent material is a more preferred embodiment. The absorbent article is suitably used for hygiene, daily necessities, construction and civil engineering, industrial, agricultural, medical, or food applications. Specifically, the absorbent article includes hygienic absorbent articles such as disposable diapers, urine pads, sanitary napkins, breast pads, incontinence pads, sweat bands, and water-soluble sanitary materials; daily necessities such as gel air fresheners, disposable body warmers, cat litter, pet sheets, deodorants, and portable toilets; construction and civil engineering absorbent articles such as drip absorbents, sealing materials, concrete curing materials, lost circulation prevention materials, lost water prevention materials, sealing materials, and desert greening materials; anti-condensation sheets for containers, fire extinguishers, Suitable applications include water-stopping materials for communication cables, alkaline battery materials, water-swelling paints, water-wettable paints, artificial snow compounds, and oil-based moisture removers for other industrial absorbent articles; agricultural absorbent articles such as soil water-retaining agents, seedling raising sheets, seed coatings, fertilizer slow-release agents, and pesticide and fertilizer disintegration aids; medical absorbent articles such as moisturizers, wound dressings, blood solidification agents, medical underpads, fabric stimulants, and body fluid absorbents; and food absorbent articles such as ice packs and freshness-preserving agents. Absorbent articles with absorbents composed of water-soluble and water-decomposable components can be easily disposed of in water, either as the absorbent article itself or the absorbent portion detached from the absorbent article, after absorbing body fluids. Therefore, such absorbent articles can be used as sanitary materials that can be flushed down the drain without pretreatment such as crushing. The use of highly biodegradable block copolymers allows for decomposition by activated sludge, enabling disposal in sewage. In addition, it will be possible to control the release of absorbent materials that absorb and retain ingredients such as detergents, pesticides, and fertilizers, triggered by the amount of water. [Example]

[0083] The present invention will be explained in more detail below using examples.

[0084] [Materials used in the examples] Cobalt(II) acetylacetonate (Co(acac)2) Cobalt(II) tetramesitylporphyrin (Co(TMP)) [2,2'-Azobis(4-methoxy-2,4-dimethylvaleronitrile)] (V-70) Azobisisobutyronitrile (AIBN) Vinyl acetate (VAc) Methyl acrylate (MA) 2-Acrylamido-2-methylpropanesulfonic acid (AMPS) 3-(Methacrylamido)propyltrimethylammonium chloride (MAPTAC) 1,1-Diphenylethylene (1,1-DPEt)

[0085] [Number average molecular weight (Mn A ), number average molecular weight (Mn b ) and molecular weight distribution (Mw A / Mn A )] The number average molecular weight (Mn) and molecular weight distribution (Mw / Mn) of the polymer were measured using a gel permeation chromatography device manufactured by Shimadzu Corporation under the following measurement conditions. Column: Two tetrahydrofuran-based columns "KF-806M" manufactured by Showa Denko K.K. connected in series Standard sample: Polymethyl methacrylate Solvent and mobile phase: tetrahydrofuran (THF) Flow rate: 1.0mL / min Temperature: 40℃ Sample solution concentration: 0.2% by mass (filtered through a 0.45 μm filter) Injection volume: 100μL Detector: RI

[0086] [Content of ionic monomer units in vinyl ester block copolymer (U)] The content (U) (mol %) of acrylic acid ester monomer units (ionic monomer units) in the vinyl ester block copolymer was determined by the following method. 1H-NMR measurement was performed. The integral value (4.8 ppm) of the peak derived from the methine proton (-CHCH(OCOCH)-) of the vinyl acetate monomer unit was defined as T, and the integral value (3.6 ppm) of the peak derived from the side chain proton (-CHCH(COOCH)-) of the methyl acrylate monomer unit was defined as S. The content (U) (mol %) of the acrylate monomer unit in the vinyl ester block copolymer was calculated using the following formula: (U) (mol%) = (S / 3) / (S / 3 + T) × 100

[0087] When MAPTAC was used as the ionic monomer, the integrated value of the peak derived from N-methyl proton (-N(CH3)3) (peak detected in the range of 3.1 to 3.3 ppm) was defined as S', and the content (U) (mol %) of MAPTAC monomer units in the vinyl ester block copolymer was calculated using the following formula. (U) (mol%) = (S' / 9) / (S' / 9 + T) × 100

[0088] When AMPS was used instead of the acrylic acid ester monomer, the integrated value of the peak derived from the side chain methylene proton (-CH2SO3H) (peak detected in the range of 2.9 to 3.2 ppm) was defined as S", and the content (U) (mol%) of the AMPS monomer unit in the vinyl ester block copolymer was calculated using the following formula. (U) (mol%) = (S" / 2) / (S" / 2 + T) × 100

[0089] [Number average degree of polymerization DP of block copolymer (A)] A and the number average degree of polymerization DP of polymer block (b) b ] Number average degree of polymerization DP of polymer block (b) in copolymer (A) b was calculated as follows:

[0090] The polymer block (b) is formed by saponifying the vinyl ester polymer block (b1) in the vinyl ester block copolymer. b Since the number average degree of polymerization does not change substantially before and after saponification, the number average degree of polymerization DP of polymer block (b) was calculated by the GPC measurement of the polymer before saponification. b Similarly, the number average degree of polymerization determined from the results of GPC measurement of the polymer after polymerization termination and before saponification was used as the number average degree of polymerization DP of copolymer (A). A It was decided.

[0091] Here, the vinyl ester polymer block (b1) refers to a block (b1) obtained by polymerizing a vinyl ester in the absence of an ionic monomer such as an acrylic acid ester, and a block (b1) obtained by polymerizing a vinyl ester in the presence of an ionic monomer at the initial stage of polymerization or during polymerization to obtain a copolymer block (c1) containing vinyl ester monomer units and ionic monomer units, and then polymerizing a vinyl ester-based monomer in a state in which the ionic monomer is consumed before the vinyl ester, resulting in a molar ratio of the ionic monomer to the vinyl ester in the reaction solution (ionic monomer / vinyl ester) of 0.00001 or less.

[0092] The boundary between the "vinyl ester polymer block (b1)" and the "copolymer block (c1) containing vinyl ester-based monomer units and ionic monomer units" was determined as follows: Sampling was carried out appropriately during polymerization, and the number-average degree of polymerization (DP) and the content (U) (mol%) of ionic monomer units of the polymer at each sampling point were measured by GPC and 1 The point at which the molar ratio (ionic monomer / vinyl ester) reaches 0.00001 was measured by H-NMR and calculated using the Mayo-Lewis equation, which is the theoretical copolymerization formula, and the reactivity ratio (r VAc =0.01, r MA = 30). In this case, the content of the ionic monomer unit contained in the vinyl ester polymer block (b1) formed in a state where the molar ratio (ionic monomer / vinyl ester) is 0.0001 or less is less than 0.1 mol %.

[0093] The number average degree of polymerization (DP) of the sampled polymer was determined by GPC and 1 The molecular weight (Mn) of the polymer was calculated by the following formula using the number average molecular weight (Mn) of the polymer obtained by H-NMR, the content (U) (mol %) of acrylic acid monomer units, and the molecular weights of the acrylic acid ester monomer units and vinyl ester monomer units (MA: 86, VAc: 86). (DP)=Mn / {(U / 100)×86+[(100-U) / 100]×86}

[0094] Number average degree of polymerization DP of copolymer (A) A The GPC and 1 The number average degree of polymerization (DP) of the polymer block (b) was calculated by the above formula using the values ​​obtained by H-NMR. When a vinyl ester polymer block is obtained at the beginning of polymerization, a sample of the polymer was measured just before the addition of the ionic monomer. b When a vinyl ester polymer block is obtained after copolymerization of an acrylic ester and a vinyl ester, the number average degree of polymerization DP of the polymer block (b) was calculated from the difference between the number average degree of polymerization measured by measuring the polymer sampled at the boundary of the block and the vinyl ester block copolymer after termination of the polymerization. b asked for.

[0095] [Content of units derived from ionic monomers in block copolymer (A) (Z A ) (mol%)] When an acrylic acid ester was used as the ionic monomer, the obtained copolymer (A) was stirred in a hydrochloric acid solution of pH 2 at 100°C for 1 hour, and then dried at 120°C to convert all of the acrylic acid-based monomer units (acrylic acid units, acrylic acid ester units, acrylate units) in the copolymer into acrylic acid monomer units or lactone ring structures (lactone rings are formed by the reaction of acrylic acid monomer units or acrylic acid ester monomer units with adjacent vinyl alcohol monomer units). The copolymer was washed with methanol to remove salts, and then dried at 90°C for 2 days under reduced pressure. In this way, a polymer that had been heat-treated in an acidic aqueous solution and then dried was obtained. A nuclear magnetic resonance spectrometer "LAMBDA 500" manufactured by JEOL Ltd. was used to analyze the copolymer (polymer that had been heat-treated in an acidic aqueous solution and then dried) at 40°C and 95°C. 1 H-NMR measurement was carried out. DMSO-d6 was used as the solvent. The content of acrylic acid-based monomer units (Z) relative to the total monomer units of copolymer (A) was A ) (mol %) was calculated as follows:

[0096] The content of acrylic acid-based monomer units (Z) relative to the total monomer units of copolymer (A) was calculated using the following formula: A ) (mol %) was calculated. (Z A )(mol%)=(X+Y) / (W+2X+Y+(P / 3))×100 The meanings of the symbols in the formula are as follows: Y: Integrated value of the peak derived from the side chain protons of acrylic acid (-CH2CH(COOH)-) (broad peak detected in the range of 11.0 to 13.0 ppm) X: The methine proton of the main chain of acrylic acid in the lactone ring (-CH2C H Total integral value (double peak between 2.6 ppm and 3.0 ppm) of peaks derived from (R1)CH2CH(R2)-) (where R1-R2 form a bond with each other, and -R1-R2- represents a -CO-O- structure) W: Total integral value of the peaks derived from the methine protons of vinyl alcohol (-CH2CH(OH)-) (peaks between 3.6 ppm and 4.0 ppm) P: Integration value of the peak derived from the side chain protons of vinyl acetate (-CH2CH(OCOCH3)-) (1.9 ppm to 2.0 ppm)

[0097] When MAPTAC was used instead of the acrylate monomer, the N-methyl proton (-N(C H 3) The integral value of the peak derived from 3) (the peak detected in the range of 3.1 to 3.3 ppm) was designated as F2, and the content of MAPTAC monomer units in copolymer (A) (Z A ) (mol %) was calculated. When AMPS was used instead of the acrylic acid ester monomer, the side chain methylene proton (-C H The integral value of the peak derived from the copolymer (2SO3H) (peak detected in the range of 2.9 to 3.2 ppm) was designated as F3, and the content of AMPS monomer units in the copolymer (A) (Z A ) (mol %) was calculated. (Z A ) (mol%) = (F2 / 9) / ((F2 / 9) + W + (P / 3)) × 100 (Z A ) (mol%) = (F3 / 2) / ((F3 / 2) + W + (P / 3)) × 100

[0098] [molar ratio of lactone ring (V A ) [lactone ring / total of acrylic acid monomer units and lactone rings] Using the above X and Y, the molar ratio of lactone rings (V) to the total of acrylic acid monomer units and lactone rings in the polymer (polymer that has been heat-treated in an acidic aqueous solution and then dried) can be calculated using the following formula: A ) [lactone ring / total of acrylic acid monomer units and lactone ring] was calculated. (V A )=X / (X+Y)

[0099] [Content of units derived from ionic monomers in polymer block (c) (R c )] The content of the acrylic acid-based monomer units (R ) relative to the total monomer units in the polymer block (c) containing the vinyl alcohol-based monomer units and the acrylic acid-based monomer units in the copolymer (A) c ) (mol %) is the number average degree of polymerization DP of copolymer (A). A , the number average degree of polymerization DP of the polymer block (b) b was calculated using the following formula: (R c )(mol%)=(Z A )×DP A / (DP A -DP b )

[0100] [Content of units derived from vinyl alcohol-based monomers in polymer block (c) (H c )] The content of vinyl alcohol-based monomer units (H ) relative to the total monomer units in the polymer block (c) containing vinyl alcohol-based monomer units and acrylic acid-based monomer units in the copolymer (A) c ) (mol %) was calculated according to the following formula. (H c ) (mol%) = 100 - (R c )

[0101] [Saponification degree] Using the above P and W, the degree of saponification (mol %) of the copolymer (A) was calculated according to the following formula. Degree of saponification (mol%)=1-(P / 3) / (W+(P / 3))×100

[0102] [Content of monomer units having ionic groups forming salts in block copolymer (A) (J A )] The content of monomer units having an ionic group forming a salt relative to the total monomer units in the copolymer (A) (J A ) is, for example, the copolymer (A)1 The methine protons of W, X, Y, P, and acrylates (-CH2C H It was calculated using the integral value F1 of the peak derived from (COOM)- (peak detected in the range of 2.4 to 2.7 ppm) according to the following formula: where M represents the counter cation of the acrylate salt. (J A ) (mol%) = F1 / (F1+W+2X+Y+(P / 3)) × 100

[0103] When AMPS or AMPS is used instead of the acrylic acid ester monomer, the amount is calculated by the following formula using the above-mentioned W, P, F2, and F3. (J A ) (mol%) = (F2 / 9) / ((F2 / 9) + W + (P / 3)) × 100 (J A ) (mol%) = (F3 / 2) / ((F3 / 2) + W + (P / 3)) × 100

[0104] [Content of monomer units having ionic groups forming salts in the ionic polymer block (c) (K c )] The content of salt-forming ionic monomer units in the polymer block (c) (K c ) is calculated using the following formula: (K c )(mol%)=(J A )×DP A / (DP A -DP b )

[0105] [Crystal melting temperature (Q A )] To 100 parts by mass of copolymer (A), 1860 parts by mass of methanol and 50 parts by mass of sodium hydroxide were added and heated at 40°C for 2 hours to completely saponify the remaining vinyl ester groups (saponification degree ≥ 99.9 mol%). If saponification was insufficient, additional sodium hydroxide was added and the reaction was continued until the remaining vinyl ester groups were completely saponified. Next, phenolphthalein solution was added, and the mixture was washed with methanol until no alkaline reaction was observed in the washings (methanol), and the sodium hydroxide and sodium acetate were removed. The washed polymer was dried at 120°C until no methanol remained, yielding a polymer for measuring the crystalline melting temperature.

[0106] The crystalline melting temperature (Q ) of the polymer was measured under a nitrogen atmosphere using a differential scanning calorimeter (DSC25) manufactured by TA Instruments. A ) was measured. 3 mg of the polymer that had been dried under reduced pressure at 90°C for 2 days was sealed in an aluminum container and set in a differential scanning calorimeter. The container was heated from 40°C to 250°C at a rate of 10°C per minute, held for 1 minute, and cooled to -80°C at a rate of 10°C per minute, and held for 1 minute. Thereafter, the temperature of the maximum point of the endothermic peak observed between 150°C and 250°C when the temperature was raised to 250°C at a rate of 10°C per minute was measured (Q A ) (℃).

[0107] [Deionized water (DIW) absorption evaluation of block copolymer (A)] 0.1 g of copolymer (A) was placed in a 100 mL beaker, and 10, 20, 30, 40, 50, or 60 g of deionized water was gently added to the beaker at 20°C and allowed to stand for 10 minutes. Immediately after that, the contents of the beaker were transferred onto a mesh with a 74 μm opening. If no water droplets dripped from the mesh within 1 minute, the total amount of deionized water was considered to have been absorbed. The maximum amount of deionized water that could be absorbed [deionized water (g) / copolymer (A) (g)] was determined, and this value was evaluated as "absorbency."

[0108] [Saline absorption and retention evaluation of block copolymer (A)] 1 g of copolymer (A) was placed in a 100 mL beaker, and 10, 20, 30, 40, 50, or 60 g of 0.9% by weight sodium chloride solution was gently added to the beaker at 20°C and allowed to stand for 10 minutes. Immediately after this, the contents of the beaker were transferred onto a 74 μm mesh. Absorption was determined when no droplets dripped from the mesh within 1 minute. The maximum amount of aqueous solution that could be fully absorbed [aqueous solution (g) / copolymer (A) (g)] was determined, and this value was evaluated as "absorbency." Furthermore, after the 10-minute stand, the beaker was then placed on its side. The maximum amount of aqueous solution [aqueous solution (g) / copolymer (A) (g)] at which the contents did not leak out of the beaker was evaluated as "retention."

[0109] [Evaluation of the amount of block copolymer (A) dissolved in deionized water (DIW)] 1 g of copolymer (A) was added to 1000 mL of deionized water, stirred for 60 minutes in an atmosphere of 20°C or 95°C, and then filtered through a 200-mesh metal sieve. The residue on the sieve was heated and dried at 120°C for 4 hours, and its mass was taken as D (g). The soluble fraction E (solubility in water), which is an index of water solubility, was calculated using the following formula. E(mass%)=(1-D)×100

[0110] [Biodegradability evaluation of block copolymer (A)] According to JIS K 6951, 20 mg of copolymer (A) was added to 200 ml of a standard test culture solution prepared by dissolving 8.5 g of anhydrous potassium dihydrogen phosphate, 21.75 g of anhydrous dipotassium hydrogen phosphate, 33.4 g of disodium hydrogen phosphate, and 0.5 g of ammonium chloride in 1000 ml of distilled water. Standard activated sludge was then added to the solution to a concentration of 20 ppm. The culture solution was cultured at 22°C for 28 days with stirring. The amount of carbon dioxide generated during the culture period was measured periodically to determine the total amount of carbon dioxide generated, F (mg). The total amount of carbon dioxide generated from the culture solution without the addition of copolymer (A), G (mg), was also calculated in the same manner. Furthermore, the biodegradation rate (%) was calculated using the calculated amount of carbon dioxide generated upon complete decomposition of copolymer (A), H (mg), according to the following formula: Biodegradation rate (%)={(FG) / H}×100

[0111] [Example 1] <Polymerization process> [Synthesis of block b] A reactor equipped with a stirrer, reflux condenser, and initiator addition port was charged with 0.19 parts by mass of Co(acac)2 and 0.67 parts by mass of V-70 as an initiator, and the reactor was evacuated and then purged with inert gas by introducing nitrogen three times. 500 parts by mass of VAc purified by simple distillation was then added, and the reactor was immersed in a water bath and heated to an internal temperature of 30°C while stirring.

[0112] [Synthesis of block c] While stirring, sampling was carried out as needed to check the progress of polymerization from the solids concentration. When the conversion of VAc reached 19% by mass, 7.8 parts by mass of MA were added (corresponding to the lump-sum addition of additional monomer in Table 1). At a conversion of 19% by mass, the number average molecular weight (Mn) of the polymer was 129,000. Sampling was continued as needed to check the progress of polymerization from the solids concentration. When the total conversion of VAc and MA reached 26% by mass, it was determined that MA had been completely consumed [the molar ratio (acrylic acid ester / vinyl ester) was less than 0.00001]. 1 The number average molecular weight (Mn) was 182,600 after confirmation by H-NMR and addition of 0.66 parts by mass of 1,1-DPEt as a polymerization inhibitor.

[0113] The induction period of polymerization to obtain block b was 6 hours, and the growth period from the start of high polymerization to the addition of the polymerization inhibitor was 4 hours.

[0114] [Purification of block copolymers] After adding the polymerization inhibitor, the internal temperature was raised to 60°C and the mixture was heated and stirred for 1 hour. 500 parts by weight of a 25% by weight aqueous acetic acid solution (pH 2.0) was added, stirred for 5 minutes, and then allowed to stand for 30 minutes to separate into two layers. The aqueous layer was removed. The mixture was connected to a vacuum line, and unreacted monomers were removed under reduced pressure at 30°C. Methanol was then added to dissolve the vinyl ester-based block copolymer, and the resulting solution was added dropwise to deionized water to precipitate the vinyl ester-based block copolymer. The vinyl ester-based block copolymer was recovered by filtration and dried in a vacuum dryer at 40°C for 24 hours to obtain the vinyl ester-based block copolymer. Details of the above polymerization process are shown in Table 1.

[0115] <Saponification process> Next, 100 parts by mass of the obtained vinyl ester-based block copolymer and 1,833.6 parts by mass of dehydrated methanol were added to the same reactor as above and dissolved, and then the water bath was heated and stirred until the internal temperature reached 40°C. 66.4 parts by mass of a methanol solution of sodium hydroxide (concentration 14% by mass, 9.3 parts by mass of sodium hydroxide) was added thereto. The thus-prepared vinyl ester-based block copolymer solution with a concentration of 5% by mass was subjected to a saponification reaction at 65°C for 1 hour.

[0116] <Salt formation process> To the saponified product obtained after deliquoring, 46.5 parts by mass of sodium hydroxide, 2,000 parts by mass of dehydrated methanol, and 210 parts by mass of ion-exchanged water were added, and heating was continued for another hour at 65°C. After deliquoring, phenolphthalein solution was added to the washing liquid (methanol), and the mixture was washed with methanol until no alkaline reaction was observed, thereby removing the sodium hydroxide and sodium acetate. The solid obtained after centrifugal dehydration was dried in a vacuum dryer at 40°C for 24 hours, yielding the target copolymer (A) (a diblock copolymer of block b and block c).

[0117] <Physical properties of block copolymer (A)> The various physical properties of the copolymer (A) obtained were measured, and the performance was evaluated. A ) is 99,800, and the number average molecular weight (Mnb ) is 66,100, and the ratio (Mn b / Mn A ) is 0.66, and the molecular weight distribution (Mw A / Mn A ) was 1.30, and the degree of saponification was 99.9 mol %. The above results are summarized in Table 2.

[0118] [Example 2] Copolymer (A) (a diblock copolymer of block b and block c) was obtained in the same manner as in Example 1, except that the [Synthesis of block c] in the <Polymerization step> in Example 1 was changed as shown in Table 1. The evaluation results of the obtained copolymer (A) are shown in Table 2.

[0119] [Example 3] <Polymerization process> [Synthesis of block b] A reactor equipped with a stirrer, reflux condenser, initiator inlet, and feed pump was charged with 0.19 parts by mass of Co(acac)2 and 0.67 parts by mass of V-70 as an initiator. The reactor was evacuated and then purged with inert gas by introducing nitrogen three times. 500 parts by mass of VAc purified by simple distillation was then added, and the reactor was immersed in a water bath and heated to an internal temperature of 30°C while stirring.

[0120] [Synthesis of block c] While stirring, samples were taken as needed to check the progress of polymerization from the solids concentration. When the conversion of VAc reached 13% by mass, 1.3 parts by mass of MA was added (corresponding to the lump-sum addition of additional monomers in Table 1). The number average molecular weight (Mn) of the polymer at a conversion of 13% by mass was 86,000. Immediately thereafter, polymerization was carried out while feeding a mixture of 25 parts by mass of VAc and 6.2 parts by mass of MA over time. The progress of polymerization was checked from the solids concentration, and the sampled polymer was subjected to GPC measurement and 1 H-NMR measurement was carried out, and the feeding was completed when the total conversion rate of VAc and MA reached 18 mass % (corresponding to the feeding in Table 1).

[0121] After that, when the total conversion rate of VAc and MA reached 22%, it was confirmed that MA was completely consumed [the molar ratio (acrylic ester / vinyl ester) was less than 0.00001]. 1 The number average molecular weight (Mn) was confirmed by H-NMR. When the total conversion rate of VAc and MA reached 22% by mass, 0.66 parts by mass of 1,1-DPEt was added as a polymerization inhibitor.

[0122] The induction period of polymerization to obtain block b was 6 hours, and the growth period from the start of high polymerization to the addition of the inhibitor was 3 hours.

[0123] [Purification of block copolymers] Thereafter, purification was carried out in the same manner as in Example 1 to obtain a vinyl ester-based block copolymer. Table 1 shows the details of the above polymerization steps.

[0124] <Saponification step and salt formation step> Thereafter, the saponification step and salt formation step were carried out in the same manner as in Example 1 to obtain the target copolymer (A) (a diblock copolymer of block b and block c). The results of the measurements and evaluations of the obtained copolymer (A) are summarized in Table 2.

[0125] [Example 4] Copolymer (A) (a diblock copolymer of block b and block c) was obtained in the same manner as in Example 3, except that the [Synthesis of block c] in the <Polymerization step> in Example 3 was changed as shown in Table 1. The evaluation results of the obtained copolymer (A) are shown in Table 2.

[0126] [Example 5] The target copolymer (A) (a diblock copolymer of block b and block c) was obtained in the same manner as in Example 3, except that the [Synthesis of block c] in the <Polymerization step> in Example 3 was changed as shown in Table 1. The evaluation results of the obtained copolymer (A) are shown in Table 2.

[0127] [Example 6] The target copolymer was obtained in the same manner as in Example 3, except that in the [Synthesis of block b] and [Synthesis of block c] of the <Polymerization step> in Example 3, changes were made as shown in Table 1. The evaluation results of the obtained copolymer are shown in Table 2.

[0128] In Example 6, in the procedure corresponding to [Synthesis of Block c], polymerization was continued even after the comonomer MA was completely consumed, and then a polymerization terminator was added, so the final copolymer was a triblock copolymer consisting of block b, block c, and block b.

[0129] [Example 7] The target copolymer (A) (a diblock copolymer of block b and block c) was obtained in the same manner as in Example 1, except that in the saponification step in Example 1, 46.5 parts by mass of calcium hydroxide was used instead of 46.5 parts by mass of sodium hydroxide, as shown in Table 1. The evaluation results of the obtained copolymer are shown in Table 2.

[0130] Specifically, the following changes were made: "46.5 parts by mass of sodium hydroxide, 2000 parts by mass of dehydrated methanol, and 210 parts by mass of ion-exchanged water were added to the obtained saponified product," instead of "46.5 parts by mass of calcium hydroxide, 2000 parts by mass of dehydrated methanol, and 210 parts by mass of ion-exchanged water were added to the obtained saponified product."

[0131] The procedure for "adding 66.4 parts by mass of a methanol solution of sodium hydroxide (concentration 14% by mass, 9.3 parts by mass of sodium hydroxide)" was the same as in Example 1.

[0132] [Comparative Example 1] <Polymerization process> [Synthesis of random copolymers] A reactor equipped with a stirrer, reflux condenser, argon inlet, initiator addition port, and feed pump was charged with 640 parts by mass of VAc, 1.1 parts by mass of MA (corresponding to the initial monomer in Table 1), and 250 parts by mass of methanol, and the atmosphere inside the reactor was replaced with an inert gas for 30 minutes while bubbling with nitrogen. The water bath was heated to start heating the reactor, and when the internal temperature reached 60°C, 0.15 parts by mass of AIBN was added as an initiator to start polymerization.

[0133] Polymerization was carried out while gradually feeding a 40% by mass solution of MA in methanol, and appropriate sampling was conducted to check the progress of polymerization based on the solids concentration. When the total conversion of VAc and MA reached 35% by mass, 0.15 parts by mass of p-benzoquinone was added to terminate the polymerization. The total amount of MA fed over time at this point was equivalent to 11.4 parts by mass (corresponding to the additional monomer feed in Table 1).

[0134] The induction period of polymerization to obtain block c was 0 hours, and the growth period from the start of high polymerization to the addition of the inhibitor was 3 hours.

[0135] Thereafter, purification, saponification, and salt formation were carried out in the same manner as in Example 1 to obtain the target random copolymer. The evaluation results of the obtained copolymer are shown in Table 2.

[0136] Comparative Example 2 The <saponification step> in Example 3 was changed as follows. 46.5 parts by mass of acetic acid and 2,000 parts by mass of methanol were added to 100 parts by mass of the vinyl ester-based block copolymer obtained in the <polymerization step> of Example 3, and the mixture was heated at 65°C for 1 hour. After deliquoring, the mixture was washed with methanol and centrifuged to obtain a solid, which was then dried in a vacuum dryer at 40°C for 24 hours to obtain the target block copolymer. The <salt formation step> was not performed. The measurement and evaluation results of the obtained block copolymer are summarized in Table 2.

[0137] Comparative Example 3 Copolymer (A) (a diblock copolymer of block b and block c) was obtained in the same manner as in Example 1, except that the [Synthesis of block b] and [Synthesis of block c] in the <Polymerization step> in Example 1 were changed as shown in Table 1. The evaluation results of the obtained copolymer (A) are shown in Table 2.

[0138] Comparative Example 4 Table 2 shows the results of the evaluation carried out using a chemically crosslinked polyacrylic acid-based superabsorbent resin (manufactured by Sumitomo Seika Chemicals Co., Ltd., trade name: AQUAKEEP SA60S).

[0139] Comparative Example 5 <Polymerization process> [Synthesis of block c] A reactor equipped with a stirrer, reflux condenser, and initiator inlet was charged with 1.56 parts by mass of Co(TMP) and 0.91 parts by mass of AIBN as an initiator. The reactor was evacuated and then purged with inert gas by introducing nitrogen three times. 160.0 parts by mass of MA purified by simple distillation and 480 parts by mass of toluene (corresponding to the initial monomers in Table 1) were then added. The reactor was then immersed in a water bath and heated to an internal temperature of 60°C with stirring. Sampling was performed as needed to confirm the progress of polymerization based on the solids concentration. When the MA conversion reached 18%, the polymerization was terminated by cooling to 30°C. The number-average molecular weight (Mn) of the polymer at 18% conversion was 15,500. The reactor was then connected to a vacuum line, and the remaining MA and toluene were distilled off under reduced pressure at 30°C.

[0140] [Synthesis of block b] After adding 640 parts by mass of VAc (corresponding to the lump-sum addition of additional monomer in Table 1), the mixture was heated and stirred until the internal temperature reached 60°C. Sampling was performed as needed, and the progress of polymerization was confirmed based on the solids concentration. When the VAc conversion rate reached 22% by mass, 1.68 parts by mass of 1,1-DPEt was added as a polymerization inhibitor. The number average molecular weight (Mn) at this time was 91,200.

[0141] The induction period until block b polymerization was 5 hours, and the growth period until the addition of the inhibitor was 4 hours.

[0142] Thereafter, purification, saponification, and salt formation were carried out in the same manner as in Example 1 to obtain the target copolymer (A) (a diblock copolymer of block c and block b). The evaluation results of the obtained copolymer are shown in Table 2.

[0143] [Table 1]

[0144] [Table 2]

[0145] The block copolymers (A) of Examples 1 to 7 were excellent in DIW and saline absorbability, saline retention (gel shape retention), and water solubility (water solubility). Although the biodegradability was measured only for Example 6, it is believed that all of Examples 1 to 8 exhibited equally good biodegradability.

[0146] The copolymer of Comparative Example 1 is a random copolymer having neither block (b) nor block (c). The copolymer of Comparative Example 2 has block (b) but does not have ionic polymer block (c) containing a monomer unit having an ionic group that forms a salt. The copolymer of Comparative Example 3 is a copolymer having Mn A is less than 20,000 and Mn b The copolymer of Comparative Example 5 is a copolymer in which the ionic polymer block (c) does not contain a vinyl alcohol-based monomer unit. The copolymers of Comparative Examples 1 to 3 and 5 were insufficient in DIW and saline absorption and saline retention. The copolymer of Comparative Example 2 was also poor in water solubility. The polyacrylic acid-based superabsorbent resin of Comparative Example 4 did not have block (b) and was excellent in water absorption performance, but was poor in water solubility (water solubility) and biodegradability.

Claims

1. A block copolymer (A) comprising a vinyl alcohol-based polymer block (b) and an ionic polymer block (c) containing a monomer unit having an ionic group forming a salt and a vinyl alcohol-based monomer unit, the ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group; The number average molecular weight (Mn ｂ ) is 15,000 to 220,000; the content of vinyl alcohol-based monomer units in the vinyl alcohol-based polymer block (b) relative to all monomer units is 90 mol % or more; the content of vinyl alcohol-based monomer units in the ionic polymer block (c) is 5 to 95 mol % relative to all monomer units; the content (J A ) of the monomer units having an ionic group forming a salt relative to all the monomer units in the block copolymer (A) is 2 to 90 mol %, The number average molecular weight (Mn Ａ ) is 20,000 to 440,000, and Number average molecular weight (Mn Ａ ) to the number average molecular weight (Mn ｂ ) ratio (Mn ｂ / Mn Ａ ) is 0.1 to 0.

9.

2. The block copolymer (A) according to claim 1 , wherein the ionic group is a carboxylic acid group.

3. The block copolymer (A) according to claim 1 or 2, wherein a counter ion of the ionic group is an ion of an element of Group 1, 2, 12, 13 or 17 of the periodic table.

4. The content of the monomer units having an ionic group forming the salt relative to the total monomer units in the ionic polymer block (c) (K ｃ 4. The block copolymer (A) according to claim 1, wherein the amount of the copolymer (A) is 5 to 95 mol %.

5. The block copolymer (A) according to any one of claims 1 to 4, wherein the degree of saponification of the block copolymer (A) is 80 to 99.99 mol%.

6. The molecular weight distribution (Mw Ａ / Mn Ａ 6. The block copolymer (A) according to any one of claims 1 to 5, wherein the saturation coefficient (S) is 1.05 to 1.

95.

7. The block copolymer (A) according to any one of claims 1 to 6, which is biodegradable.

8. 8. The block copolymer (A) according to claim 1, which has an absorption amount of deionized water of 20 g or more per 0.1 g of the block copolymer (A) at 20°C.

9. The block copolymer (A) according to any one of claims 1 to 8, wherein the block copolymer (A) absorbs 20 g or more of a 0.9% by mass aqueous sodium chloride solution at 20°C per 1 g of the block copolymer (A).

10. The block copolymer (A) according to any one of claims 1 to 9, which has a soluble content of 95% by mass or more when dissolved in water at 95°C.

11. The block copolymer (A) according to claim 10, which has a soluble content of 95 mass% or more when dissolved in water at 20°C.

12. An absorbent material comprising a block copolymer (A), the block copolymer (A) comprises a vinyl alcohol-based polymer block (b) and an ionic polymer block (c) containing a monomer unit having an ionic group that forms a salt and a vinyl alcohol-based monomer unit; the ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group; the number average molecular weight (Mn b ) of the vinyl alcohol polymer block (b) is 15,000 to 220,000; the content of vinyl alcohol-based monomer units in the ionic polymer block (c) is 5 to 95 mol % relative to all monomer units; the content (J A ) of the monomer units having an ionic group forming a salt relative to all the monomer units in the block copolymer (A) is 2 to 90 mol %, The number average molecular weight (Mn A ) of the block copolymer (A) is 20,000 to 440,000, and An absorbent material having a ratio (Mn b / Mn A ) of number average molecular weight (Mn b ) to number average molecular weight (Mn A ) of 0.1 to 0.

9.

13. 13. The absorbent material of claim 12 which is a particle.

14. An absorbent article comprising the absorbent material according to claim 12 or 13.

15. The absorbent article according to claim 14, which is for hygiene, daily necessities, construction and civil engineering, industrial, agricultural, medical or food use.

16. A method for producing a block copolymer (A), comprising: the production method includes a polymerization step of polymerizing a vinyl ester monomer by controlled radical polymerization in the presence of a radical polymerization initiator and a control agent, and copolymerizing the vinyl ester monomer with an ionic monomer having an ionic group or a derivative thereof, to obtain a vinyl ester block copolymer including a vinyl ester polymer block (b1) and an ionic polymer block (c1) containing a vinyl ester monomer unit and an ionic monomer unit; and the method includes, as an essential step, a saponification step of forming vinyl alcohol monomer units by saponifying vinyl ester monomer units in the vinyl ester-based block copolymer obtained in the polymerization step, The method includes an optional salt formation step of forming a salt from the ionic monomer unit, the resulting block copolymer (A) comprises a vinyl alcohol-based polymer block (b) and an ionic polymer block (c) containing a monomer unit having an ionic group that forms a salt and a vinyl alcohol-based monomer unit, the ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group; the number average molecular weight (Mn b ) of the vinyl alcohol polymer block (b) is 15,000 to 220,000; the content of vinyl alcohol-based monomer units in the ionic polymer block (c) is 5 to 95 mol % relative to all monomer units; the content (J A ) of the monomer units having an ionic group forming a salt relative to all the monomer units in the block copolymer (A) is 2 to 90 mol %, The number average molecular weight (Mn A ) of the block copolymer (A) is 20,000 to 440,000, and A method for producing a block copolymer (A), wherein the ratio (Mn b / Mn A ) of the number average molecular weight (Mn b ) to the number average molecular weight (Mn A ) is 0.1 to 0.

9.

17. The method for producing the block copolymer (A) according to claim 16, wherein the control agent is an organic cobalt complex.

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