Method for producing ethylene-vinyl alcohol copolymer and ethylene-vinyl ester copolymer, method for producing ethylene-vinyl alcohol copolymer

Abstract

This invention provides an ethylene-vinyl alcohol copolymer produced using a RAFT agent, in which discoloration is suppressed. [Solution] An ethylene-vinyl alcohol copolymer having a disulfide bond content of 0.01 to 0.20 mol% relative to vinyl alcohol units and an ethylene structural unit content of 20 to 60 mol%.

Description

[Technical Field]

[0001] The present invention relates to a method for producing ethylene-vinyl alcohol copolymers and ethylene-vinyl ester copolymers, and a method for producing ethylene-vinyl alcohol copolymers. [Background technology]

[0002] Ethylene-vinyl alcohol copolymer (hereinafter sometimes referred to as "EVOH resin") has excellent transparency, gas barrier properties such as oxygen, fragrance retention, solvent resistance, oil resistance, and mechanical strength, and is molded into films, sheets, bottles, etc., and is widely used as a packaging material for various products such as food packaging materials, pharmaceutical packaging materials, industrial chemical packaging materials, and agricultural chemical packaging materials.

[0003] The aforementioned EVOH resin is obtained by saponifying an ethylene-vinyl ester copolymer, which is a copolymer of ethylene and vinyl ester monomers. Radical polymerization is widely used industrially as the polymerization method for this product. The reaction mechanism of the aforementioned radical polymerization includes an initiation reaction, a growth reaction, and a termination reaction, and a chain transfer reaction may occur as a side reaction. In radical polymerization, once an initiation radical is generated, it reacts with monomers one after another, and the reaction terminates when the growing radical is deactivated. Since the deactivation of the growing radical occurs through recombination termination reactions, disproportionation termination reactions, or side reactions such as chain transfer reactions, polymers with inconsistent molecular lengths are produced.

[0004] Therefore, as a method to control the molecular weight of the resulting polymer, polymers are synthesized by living radical polymerization. Living radical polymerization is a polymerization method that consists only of an initiation reaction and a growth reaction in the polymerization process, and does not involve reactions that inactivate growing radicals, such as termination reactions or chain transfer reactions, so polymers with uniform molecular lengths can be obtained. In addition, RAFT (Reversible Addition-Fragmentation Chain Transfer) polymerization, which is a living radical polymerization using a specific chain transfer agent, is also known as a living radical polymerization.

[0005] For example, Patent Document 1 describes a method for producing an EVOH resin having a thiol structure at its terminus by synthesizing a block polymer of ethylene-vinyl ester copolymer using a RAFT agent having a cyano group and then performing saponification. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2007-246639 [Overview of the project] [Problems that the invention aims to solve]

[0007] Although the RAFT agent having a cyano group described in Patent Document 1 is suitable for producing block polymers, when a random copolymer of ethylene and vinyl ester monomer is produced using this RAFT agent, the conversion rate and the molecular weight of the resulting polymer are low, resulting in poor productivity. Furthermore, EVOH resins obtained using RAFT agents are generally known to be prone to discoloration due to their thiol terminology, which necessitates a removal process and limits their applications.

[0008] Accordingly, an object of the present invention is to provide a method for producing an ethylene-vinyl ester copolymer having high conversion rate and high molecular weight and excellent productivity when ethylene and a vinyl ester monomer are randomly copolymerized using a RAFT agent under such background. Another object of the present invention is to provide an EVOH resin produced using a RAFT agent, wherein the EVOH resin has suppressed coloration. [Means for Solving the Problem]

[0009] However, in view of such circumstances, the present inventor has conducted intensive studies, and as a result, has found that by using a specific RAFT agent having no cyano group as the RAFT agent, the conversion rate and molecular weight become higher when ethylene and a vinyl ester monomer are randomly copolymerized, and thus completed the present invention. Furthermore, the present inventor has found that coloration can be suppressed by further reacting the thiol groups of thiol-terminated EVOH resin obtained using a specific RAFT agent to form disulfide bonds, and adjusting the amount of the disulfide bonds within a specific range, and thus completed the present invention.

[0010] That is, the present invention has the following aspects. [1] An EVOH resin, wherein the amount of disulfide bonds relative to vinyl alcohol units is 0.01 to 0.20 mol%, and the content of ethylene structural units is 20 to 60 mol%. [2] The EVOH resin according to [1], wherein the EVOH resin is a random copolymer. [3] A method for producing an ethylene-vinyl ester copolymer obtained by copolymerizing ethylene and a vinyl ester monomer, the method comprising performing copolymerization of the ethylene and the vinyl ester monomer in the presence of a radical polymerization initiator and a RAFT agent represented by the following general formulas (1) to (4). [Chemical Formula] [Chemical Formula] [ka] [ka] [4] A method for producing an EVOH resin obtained by saponifying an ethylene-vinyl ester copolymer, wherein the ethylene-vinyl ester copolymer is an ethylene-vinyl ester copolymer obtained by the method for producing an ethylene-vinyl ester copolymer described in [3]. [Effects of the Invention]

[0011] According to the manufacturing method of the present invention, the conversion rate and molecular weight are increased when ethylene and vinyl ester monomers are randomly copolymerized, resulting in excellent productivity. Furthermore, the EVOH resin of the present invention can suppress discoloration despite being manufactured using a RAFT agent. [Modes for carrying out the invention]

[0012] The present invention will be described below based on examples of embodiments for carrying out the present invention. However, the present invention is not limited to the embodiments described below.

[0013] In this specification, "x and / or y (where x, y are any configuration)" means at least one of x and y, and can mean x only, y only, or x and y. In this specification, when "X~Y" (where X and Y are any numbers) is used, unless otherwise specified, it means "greater than or equal to X and less than or equal to Y," and also includes the meanings of "preferably greater than X" or "preferably less than Y." In this specification, when we use the expressions "X or greater" (where X is any number) or "Y or less" (where Y is any number), we also mean "preferably greater than X" or "preferably less than Y." With respect to the numerical ranges described stepwise in this specification, the upper limit or lower limit of a numerical range in one step may be arbitrarily combined with the upper limit or lower limit of a numerical range in another step. Further, in the numerical ranges described in this specification, the upper limit or lower limit of the numerical range may be replaced with the values shown in the examples.

[0014] As used herein, the term "main component" refers to a component that greatly affects the properties of the target object. The content of this component is usually 50% by mass or more in the target object, preferably 55% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, and may be 100% by mass.

[0015] <EVOH resin> EVOH resin is generally a resin obtained by saponifying an ethylene-vinyl ester copolymer, which is a copolymer of ethylene and a vinyl ester monomer, and is a water-insoluble thermoplastic resin. The EVOH resin obtained in this manner mainly contains ethylene-derived structural units and vinyl alcohol structural units, and contains a small amount of vinyl ester structural units remaining without being saponified.

[0016] The EVOH resin according to one embodiment of the present invention (hereinafter referred to as "the present EVOH resin") has a disulfide bond content of 0.01 to 0.20 mol% relative to the vinyl alcohol units, and an ethylene structural unit content of 20 to 60 mol%.

[0017] The aforementioned "disulfide bond" is a structure derived from the RAFT agent used as a chain transfer agent in the production of this EVOH resin. Normally, when ethylene and vinyl ester monomers are copolymerized using a RAFT agent, an ethylene-vinyl ester copolymer having a thiocarbonylthio terminus is obtained. Subsequently, when the ethylene-vinyl ester copolymer having a thiocarbonylthio terminus is saponified, the thiocarbonylthio terminus is converted to a thiol group, resulting in an EVOH resin having a thiol group. Such EVOH resins having a thiol group are known to be prone to discoloration. On the other hand, in the present invention, it has been found that by further reacting the thiol group to form a disulfide bond and setting the amount of this disulfide bond within a specific range, discoloration can be suppressed to a degree comparable to that of an EVOH resin that does not use a RAFT agent.

[0018] The amount of disulfide bonds to the vinyl alcohol unit is 0.01 to 0.20 mol%, preferably 0.01 to 0.10 mol%, and more preferably 0.01 to 0.05 mol%. Keeping the amount of disulfide bonds within this range suppresses discoloration of the EVOH resin. Note that the amount of disulfide bonds is 1 This can be calculated by measuring H-NMR.

[0019] Furthermore, the copolymer form of this EVOH resin may be either a random copolymer or a block copolymer, but a random copolymer is preferred from the viewpoint of gas barrier properties.

[0020] The ethylene structural unit content of this EVOH resin is 20 to 60 mol%, preferably 22 to 50 mol%, and particularly preferably 25 to 35 mol%. When the ethylene structural unit content is within the above range, excellent gas barrier properties are achieved. The ethylene structural unit content can be controlled by the pressure of the ethylene during copolymerization of the vinyl ester monomer and ethylene. Furthermore, the ethylene structural unit content is... 1 This can be calculated by measuring H-NMR.

[0021] The 1,2-glycol bond content of this EVOH resin is typically 0.1 to 5 mol%, preferably 0.2 to 3 mol%, and more preferably 0.5 to 2.5 mol%. When the 1,2-glycol bond content, i.e., the heterogeneous bond content, is within the above range, the crystallinity of the EVOH resin tends to increase, resulting in superior gas barrier properties. 1 This can be calculated by measuring H-NMR.

[0022] The degree of saponification of this EVOH resin is typically 90-100 mol%, preferably 95-100 mol%, and particularly preferably 99-100 mol%. When the degree of saponification is above the lower limit, it tends to exhibit superior gas barrier properties, thermal stability, and moisture resistance. The degree of saponification can be controlled by the amount of saponification catalyst (usually an alkaline catalyst such as sodium hydroxide) used to saponify the ethylene-vinyl ester copolymer, as well as the temperature and time. Furthermore, the degree of saponification is... 1 This can be calculated by measuring H-NMR.

[0023] The weight-average molecular weight (Mw) of this EVOH resin is typically 3,000 to 200,000, preferably 5,000 to 100,000, and more preferably 10,000 to 50,000.

[0024] Furthermore, the number-average molecular weight (Mn) of this EVOH resin is typically 1,000 to 50,000, preferably 2,000 to 40,000, and more preferably 5,000 to 30,000.

[0025] Furthermore, the molecular weight distribution (PDI) [Mw / Mn] of this EVOH resin is typically 1.0 to 5.0, preferably 1.1 to 4.0, more preferably 1.2 to 3.0, and even more preferably 1.3 to 2.6. When the molecular weight distribution is within the above range, the crystallinity of the EVOH resin tends to be high, resulting in excellent gas barrier properties.

[0026] The weight-average molecular weight and number-average molecular weight are values ​​calculated on a standard polystyrene basis by gel permeation chromatography (GPC), and can be measured by the method described in the examples below.

[0027] The yellow index (YI) of this EVOH resin is usually 60 or less, preferably 50 or less, more preferably 30 or less, and even more preferably 10 or less. The aforementioned yellow index can be measured by the method described in the embodiments below.

[0028] Such EVOH resins can be obtained, for example, by producing an ethylene-vinyl ester copolymer using a RAFT agent and saponifying it, preferably by saponifying an ethylene-vinyl ester copolymer obtained according to the manufacturing method described below. The following describes a method for producing ethylene-vinyl ester copolymers.

[0029] A method for producing an ethylene-vinyl ester copolymer according to one embodiment of the present invention (hereinafter referred to as "this production method") involves copolymerizing ethylene and a vinyl ester monomer in the presence of a radical polymerization initiator and a RAFT agent represented by the following general formulas (1) to (4).

[0030] [ka]

[0031] [ka]

[0032] [ka]

[0033] [ka]

[0034] The aforementioned RAFT agent is a compound for controlling living radical polymerization. It acts as a chain transfer agent, reacting with the ends of growing polymer chains to stop polymer growth and simultaneously generating new polymerization initiation radicals.

[0035] As the vinyl ester monomer, vinyl acetate is typically used due to its market availability and efficient impurity removal during manufacturing. Other vinyl ester monomers besides vinyl acetate include, for example, aliphatic vinyl esters such as vinyl formate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, vinyl stearate, and vinyl versatate, and aromatic vinyl esters such as vinyl benzoate. However, aliphatic vinyl esters having 3 to 20 carbon atoms, preferably 4 to 10 carbon atoms, and particularly preferably 4 to 7 carbon atoms are usually used. These are usually used individually, but multiple types may be used simultaneously as needed.

[0036] Furthermore, the copolymerization of ethylene and vinyl ester monomer may also contain the following comonomers as copolymerization components, to the extent that they do not hinder the effects of the present invention (for example, 10 mol% or less of the copolymerization component). Examples of the comonomers include olefins such as propylene, 1-butene, and isobutene; hydroxyl group-containing α-olefins such as 3-buten-1-ol, 3-buten-1,2-diol, 4-penten-1-ol, and 5-hexen-1,2-diol, and their esterified and acylated derivatives; hydroxyalkylvinylidenes such as 2-methylenepropane-1,3-diol and 3-methylenepentane-1,5-diol; 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, and 1,3-dibuty Hydroxyalkylvinylidene diacetates such as lyloxy-2-methylenepropane; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, (anhydride) phthalic acid, (anhydride) maleic acid, (anhydride) itaconic acid, or their salts, or mono or dialkyl esters with 1 to 18 carbon atoms in the alkyl group; acrylamide, N-alkylacrylamide with 1 to 18 carbon atoms in the alkyl group, N,N-dimethylacrylamide, 2-acrylamidopropanesulfonic acid, or their salts, acrylamidopropyldimethylamine, or its salts, or its quaternary salts Acrylamides such as methacrylamide, N-alkylmethacrylamide with 1 to 18 C1 of the alkyl group, N,N-dimethylmethacrylamide, 2-methacrylamidepropanesulfonic acid or its salts, methacrylamidopropyldimethylamine or its salts or its quaternary salts, etc.; N-vinylamides such as N-vinylpyrrolidone, N-vinylformamide, N-vinylacetamide, etc.; vinyl cyanides such as acrylonitrile, methacrylnitrile, etc.; alkyl vinyl ethers with 1 to 18 C1 of the alkyl group, hydro Examples include vinyl ethers such as xyalkyl vinyl ethers and alkoxyalkyl vinyl ethers; vinyl halogenated compounds such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, and vinyl bromide; vinyl silanes such as trimethoxyvinylsilane; allyl halogenated compounds such as allyl acetate and allyl chloride; allyl alcohols such as allyl alcohol and dimethoxyallyl alcohol; and comonomers such as trimethyl-(3-acrylamido-3-dimethylpropyl)-ammonium chloride and acrylamide-2-methylpropanesulfonic acid.These can be used individually or in combination of two or more types.

[0037] The radical polymerization initiator is not particularly limited and known polymerization initiators can be used, for example, azo compounds such as 2,2'-azobis-(2,4-dimethylvaleronitrile), 2,2'-azobisisobutyronitrile (AIBN), 2,2'-azobis-(4-methoxy-2,4-dimethylvaleronitrile), and 2,2'-azobis-(2-methylpropionamidine)dihydrochloride, alkyl peresters such as t-butylperoxyneodecanoate and t-butylperoxypivalate, and bis-(4-t-butylcyclohexyl Examples include peroxy-dicarbonates such as peroxy-dicarbonate, dicyclohexyl peroxy-dicarbonate, bis(2-ethylhexyl) peroxy-dicarbonate, di-sec-butyl peroxy-dicarbonate, and di-isopropyl peroxy-dicarbonate; and peroxides such as acetyl peroxide, diacetyl peroxide, dilauroyl peroxide, didecanoyl peroxide, dioctanoyl peroxide, dipropyl peroxide, and dibenzoyl peroxide. These may be used individually or in combination of two or more.

[0038] The RAFT agent used in this manufacturing method is a compound represented by the following general formulas (1) to (4). Furthermore, it is preferable that the RAFT agent used in this manufacturing method does not have a cyano group from the viewpoint of reactivity, and it is more preferable that it is of the dithiocarbonate type. Among these, the compounds represented by general formulas (1) and (2) are preferred because they can further suppress the discoloration of the EVOH resin when the ethylene-vinyl ester copolymer is saponified to produce the EVOH resin.

[0039] [ka]

[0040] In the above general formula (1), R 1 , R 3Examples of alkyl or aryl groups consisting of carbon, hydrogen, and a heteroatom include aliphatic hydrocarbon groups having 1 to 20 carbon atoms, which may have substituents, and aryl groups having 5 to 20 carbon atoms, which may have substituents.

[0041] Examples of the aliphatic hydrocarbon groups having 1 to 20 carbon atoms include linear, branched, or cyclic aliphatic hydrocarbon groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and adamantyl groups.

[0042] Examples of the aryl group having 5 to 20 carbon atoms include the phenyl group, naphthyl group, fluorenyl group, anthryl group, phenantrenyl group, pyridine group, phthalimide group, and anisole group.

[0043] Examples of the substituents include aliphatic hydrocarbon groups having 1 to 20 carbon atoms, alkoxy groups such as methoxy groups, ethoxy groups, propoxy groups, butoxy groups, and tert-butoxy groups, and halogen (F, Cl, Br, I) atoms.

[0044] In the above general formula (1), R 1 The carbon group is preferably an aliphatic hydrocarbon group having 1 to 20 carbon atoms, which may have substituents; more preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms, which may have substituents; even more preferably an ethyl group, an isopropyl group, and particularly preferably an isopropyl group.

[0045] In the above general formula (1), R 3 The carbon group is preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms, more preferably a branched aliphatic hydrocarbon group having 1 to 10 carbon atoms, even more preferably a branched aliphatic hydrocarbon group having 1 to 6 carbon atoms, and particularly preferably an isobutyl group.

[0046] In the above general formula (1), R2 Hydrogen or an aliphatic hydrocarbon group having 1 to 6 carbon atoms is preferred. Examples of the aliphatic hydrocarbon group having 1 to 6 carbon atoms include linear, branched or cyclic aliphatic hydrocarbon groups such as methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, sec-butyl group, tert-butyl group, pentyl group, hexyl group, cyclopropyl group, cyclobutyl group, cyclopentyl group and cyclohexyl group. Among these, hydrogen and linear aliphatic hydrocarbon groups having 1 to 6 carbon atoms are preferred, and hydrogen and methyl groups are more preferred.

[0047]

Chemical Formula

[0048] In the general formula (2), R 4 , R 6 Examples of the alkyl group consisting of carbon, hydrogen and heteroatoms or the aryl group include an optionally substituted aliphatic hydrocarbon group having 1 to 20 carbon atoms and an optionally substituted aryl group having 5 to 20 carbon atoms.

[0049] As the optionally substituted aliphatic hydrocarbon group having 1 to 20 carbon atoms, those described for R 1 , R 3 in general formula (1) can be mentioned. As the optionally substituted aryl group having 5 to 20 carbon atoms, those described for R 1 , R 3 in general formula (1) can be mentioned.

[0050] In the general formula (2), R 4 is preferably an optionally substituted aliphatic hydrocarbon group having 1 to 10 carbon atoms or an optionally substituted aryl group having 6 to 10 carbon atoms, more preferably ethyl group, isopropyl group, trifluoromethyl group, adamantyl group, perfluoroalkyl group having 2 to 10 carbon atoms, or phenyl group.

[0051] In the general formula (2), R 6The group is preferably a linear aliphatic hydrocarbon group having 1 to 10 carbon atoms, more preferably a linear aliphatic hydrocarbon group having 1 to 6 carbon atoms, and particularly preferably a methyl group or an ethyl group.

[0052] In the above general formula (2), R 5 The aliphatic hydrocarbon group is preferably hydrogen or an aliphatic hydrocarbon group having 1 to 6 carbon atoms. The aliphatic hydrocarbon group having 1 to 6 carbon atoms is R of general formula (1). 2 The examples given are those explained above. Among these, hydrogen and linear aliphatic hydrocarbon groups having 1 to 6 carbon atoms are preferred, and hydrogen and methyl groups are more preferred.

[0053] [ka]

[0054] In the above general formula (2), R 7 , R 9 Examples of alkyl or aryl groups consisting of carbon, hydrogen, and a heteroatom include aliphatic hydrocarbon groups having 1 to 20 carbon atoms, which may have substituents, and aryl groups having 5 to 20 carbon atoms, which may have substituents.

[0055] The aliphatic hydrocarbon group having 1 to 20 carbon atoms that may have the substituents is R of general formula (1). 1 , R 3 The examples given are those explained above. The aryl group having 5 to 20 carbon atoms that may have the substituent is R of general formula (1). 1 , R 3 The examples given are those explained above.

[0056] In the above general formula (3), R 7 The carbon group is preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms, which may have substituents, and more preferably an ethyl group.

[0057] In the above general formula (3), R 9 It is preferably an aryl group having 5 to 20 carbon atoms, which may have substituents, and more preferably a phthalimide group.

[0058] In the above general formula (3), R 8 The aliphatic hydrocarbon group having 1 to 20 carbon atoms is preferably hydrogen or an aliphatic hydrocarbon group having 1 to 6 carbon atoms. The aliphatic hydrocarbon group having 1 to 20 carbon atoms is R of general formula (1). 1 The following are examples of what was explained earlier. Among them, hydrogen is preferred.

[0059] [ka]

[0060] In the above general formula (4), R 10 , R 12 Examples of alkyl or aryl groups consisting of carbon, hydrogen, and a heteroatom include aliphatic hydrocarbon groups having 1 to 20 carbon atoms, which may have substituents, and aryl groups having 5 to 20 carbon atoms, which may have substituents.

[0061] The aliphatic hydrocarbon group having 1 to 20 carbon atoms that may have the substituents is R of general formula (1). 1 , R 3 The examples given are those explained above. The aryl group having 5 to 20 carbon atoms that may have the substituent is R of general formula (1). 1 , R 3 The examples given are those explained above.

[0062] In the above general formula (4), R 10 The carbon atom is preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms, which may have substituents, or an aryl group having 5 to 20 carbon atoms, which may have substituents, and more preferably an ethyl group or a pyridine group.

[0063] In the above general formula (4), R 12 The group is preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms, more preferably a linear aliphatic hydrocarbon group having 1 to 6 carbon atoms, and particularly preferably a methyl group.

[0064] In the above general formula (4), R 11The aliphatic hydrocarbon group is preferably hydrogen or an aliphatic hydrocarbon group having 1 to 6 carbon atoms. The aliphatic hydrocarbon group having 1 to 6 carbon atoms is R of general formula (1). 2 The examples given are those explained above. Among these, hydrogen and a linear aliphatic hydrocarbon group having 1 to 6 carbon atoms are preferred, and more preferably, hydrogen and a methyl group.

[0065] Furthermore, the RAFT agent may be a dimer of a compound represented by general formulas (1) to (4).

[0066] Next, we will explain the specific method for producing ethylene-vinyl ester copolymers (this manufacturing method). In this manufacturing method, vinyl ester monomer, ethylene, a radical polymerization initiator, and a RAFT agent are added to the reactor to initiate the copolymerization reaction. These may be added all at once or in any order.

[0067] The copolymerization reaction between ethylene and vinyl ester monomer can be carried out by known polymerization methods, such as solution polymerization, suspension polymerization, emulsion polymerization, bulk polymerization, etc., and solution polymerization is preferred from the viewpoint of uniform diffusion. Furthermore, the polymerization method may be continuous or batch.

[0068] The aforementioned solution polymerization is typically carried out by adding vinyl ester monomer, a radical polymerization initiator, a RAFT agent, and a solvent to a reactor, stirring, and then injecting ethylene under pressure and heating to polymerize the ethylene with the vinyl ester monomer. The ethylene pressure is usually around 0.1 to 10 MPa.

[0069] Alcohols are preferred as the solvent, but in addition to alcohols, organic solvents that can dissolve ethylene, vinyl acetate, and ethylene-vinyl ester copolymers (for example, dimethyl sulfoxide, etc.) can be used. As the aforementioned alcohols, for example, aliphatic alcohols having 1 to 10 carbon atoms such as methanol, ethanol, propanol, n-butanol, and t-butanol can be used. These can be used individually or in combination of two or more. Among these, methanol is particularly preferred.

[0070] The amount of radical polymerization initiator added is typically 10 to 10,000 ppm, preferably 50 to 1,000 ppm, and more preferably 100 to 500 ppm, relative to the mass of the charged vinyl ester monomer.

[0071] Furthermore, the amount of RAFT agent added is typically 0.1 to 20 parts by mass, preferably 0.2 to 10 parts by mass, and more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the charged vinyl ester monomer. When the amount of RAFT agent added is within the above range, an ethylene-vinyl ester copolymer with a uniform molecular weight tends to be obtained. Moreover, the EVOH resin obtained by saponifying this ethylene-vinyl ester copolymer tends to have excellent gas barrier properties and can suppress discoloration.

[0072] The copolymerization reaction is preferably carried out under an inert gas atmosphere that is inert to the vinyl ester monomer. Examples of inert gases include nitrogen and argon.

[0073] The temperature in the copolymerization reaction is typically 20 to 90°C, preferably 40 to 80°C, and more preferably 50 to 70°C. When the temperature is within this range, there is a tendency to further reduce discoloration when the EVOH resin is obtained. Furthermore, the copolymerization reaction time is usually 2 to 15 hours, preferably 3 to 11 hours. In the case of a continuous polymerization method, it is preferable that the average residence time in the polymerization tank is approximately the same for all stages.

[0074] Afterward, once the predetermined polymerization rate is reached, a polymerization inhibitor is added to stop copolymerization. The polymerization rate is typically 10 to 90 mol%, preferably 30 to 80 mol%, relative to the vinyl ester monomer added. The resin content in the solution after polymerization is typically 5 to 85% by mass, preferably 20 to 70% by mass.

[0075] Examples of polymerization inhibitors that can be used include N,N-dialkylhydroxylamines, styrene derivatives, hydroquinone derivatives, quinone derivatives, piperidine derivatives, and conjugated polyenes. These can be used individually or in combination of two or more. Among these, conjugated polyenes are preferred, and sorbic acid is particularly preferred.

[0076] The amount of polymerization inhibitor added is usually 0.0001 to 3 parts by mass, preferably 0.0005 to 1 part by mass, and more preferably 0.001 to 0.5 parts by mass, per 100 parts by mass of the charged vinyl ester monomer.

[0077] After stopping the polymerization, the ethylene-vinyl ester copolymer can be recovered from the solution containing the ethylene-vinyl ester copolymer (ethylene-vinyl ester copolymer solution) by removing unreacted ethylene gas and unreacted vinyl ester monomers as needed.

[0078] The unreacted ethylene gas can be removed, for example, by evaporation. Furthermore, as a method for removing unreacted vinyl ester monomers, for example, a method can be employed in which the ethylene-vinyl ester copolymer solution is continuously supplied at a constant rate from the top of a column packed with Raschig rings, organic solvent vapor such as methanol is blown in from the bottom of the column, a mixed vapor of the organic solvent such as methanol and the unreacted vinyl ester monomer is discharged from the top of the column, and the ethylene-vinyl ester copolymer solution from which the unreacted vinyl ester monomers have been removed is taken out from the bottom of the column.

[0079] The ethylene-vinyl ester copolymer obtained by this manufacturing method preferably has the following composition.

[0080] The ethylene structural unit content in the ethylene-vinyl ester copolymer is typically 20-60 mol%, preferably 25-50 mol%, and more preferably 25-35 mol%, as measured according to ISO 14663.

[0081] The vinyl acetate conversion rate in the ethylene-vinyl ester copolymer is typically 1 to 90% by mass, preferably 5 to 80% by mass, and more preferably 8 to 70% by mass. The vinyl acetate conversion rate can be measured according to the vinyl acetate conversion rate measurement method described later.

[0082] The weight-average molecular weight (Mw) of the ethylene-vinyl ester copolymer is typically 3,000 to 200,000, preferably 4,000 to 150,000, and more preferably 5,000 to 110,000.

[0083] Furthermore, the number-average molecular weight (Mn) of the ethylene-vinyl ester copolymer is typically 1,000 to 50,000, preferably 1,500 to 40,000, and more preferably 3,000 to 35,000.

[0084] Furthermore, the molecular weight distribution (PDI) [Mw / Mn] of the ethylene-vinyl ester copolymer is typically 1.0 to 6.0, preferably 1.1 to 5.0, more preferably 1.2 to 4.0, and even more preferably 1.3 to 3.0. When the molecular weight distribution is within the above range, the EVOH resin obtained by saponifying this ethylene-vinyl ester copolymer tends to have excellent gas barrier properties.

[0085] The ethylene-vinyl ester copolymer produced as described above can be used for various purposes, and by further saponification, EVOH resin can be produced.

[0086] The saponification can be carried out by known methods, and typically, the saponification reaction is performed by adding an alkaline catalyst (saponification catalyst) to an ethylene-vinyl ester copolymer solution from which the unreacted vinyl ester monomer has been removed. Both continuous and batch saponification methods are possible.

[0087] Examples of the aforementioned alkaline catalyst include sodium hydroxide, potassium hydroxide, and alkali metal alkoxides.

[0088] The saponification conditions vary depending on the catalyst used, the ethylene content of the ethylene-vinyl ester copolymer, and the desired degree of saponification. However, in the present invention, it is preferable to carry out saponification in two separate steps. By carrying out saponification in two separate steps in this manner, the thiocarbonylthio terminus of the ethylene-vinyl ester copolymer is converted to a thiol group, and further, this thiol group becomes a disulfide bond, resulting in an EVOH resin having a disulfide bond.

[0089] For the first saponification (primary saponification) reaction, the temperature is usually 40-90°C, and the amount of saponification catalyst used is usually 0.001-0.6 equivalents (per vinyl ester group), which is preferable. The saponification time depends on the saponification conditions and the desired degree of saponification, but is usually selected from 0.1-2 hours.

[0090] In the primary saponification process, the solid content concentration of the ethylene-vinyl ester copolymer solution is typically 10-30% by mass, preferably 15-25% by mass.

[0091] Furthermore, the reaction conditions for the second saponification (secondary saponification) can be the same as those for the primary saponification.

[0092] After the saponification reaction is complete, the alkaline catalyst is neutralized as needed, followed by solid-liquid separation, washing, and drying to obtain EVOH resin.

[0093] The EVOH resin obtained in this way can be suitably used as a molding material for various molded products in various forms such as pellets, powders, liquids, and fibers, and is particularly suitable for gas barrier applications. [Examples]

[0094] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the invention. In the examples, "parts" and "%" refer to mass.

[0095] Prior to the examples, the following RAFT agents were prepared.

[0096] [ka]

[0097] [ka]

[0098] [ka]

[0099] [ka]

[0100] [ka]

[0101] [ka]

[0102] [ka]

[0103] [ka]

[0104] [ka]

[0105] [ka]

[0106] [ka]

[0107] [ka]

[0108] [ka]

[0109] [ka]

[0110] [ka]

[0111] <Example 1> In an induction-stirred autoclave under a nitrogen atmosphere, 460 parts vinyl acetate, 75.4 parts methanol, and 1.09 parts RAFT agent 1 were added to the shell side, and azobisisobutyronitrile (AIBN) was added to the inner tube as a methanol solution to a concentration of 178 ppm by mass (based on vinyl acetate). Next, AIBN was introduced into the cylinder of the induction-stirred autoclave from the inner tube, and the temperature and pressure were increased to 67°C and the ethylene pressure to 1 MPa to start copolymerization. After stirring for 1 hour, a methanol solution containing 1 part sorbic acid was added to stop the copolymerization. Subsequently, the copolymerized solution was recovered from the autoclave, residual vinyl acetate and methanol were removed by distillation using an evaporator, and then dried under reduced pressure to obtain the ethylene-vinyl ester copolymer of Example 1.

[0112] <Example 2> The ethylene-vinyl ester copolymer of Example 2 was obtained in the same manner as in Example 1, except that 1.04 parts of RAFT agent 2 were used.

[0113] <Example 3> The ethylene-vinyl ester copolymer of Example 3 was obtained in the same manner as in Example 1, except that 1.31 parts of RAFT agent 3 were used.

[0114] <Example 4> The ethylene-vinyl ester copolymer of Example 4 was obtained in the same manner as in Example 1, except that 1.11 parts of RAFT agent 4 were used.

[0115] <Example 5> The ethylene-vinyl ester copolymer of Example 5 was obtained in the same manner as in Example 1, except that 1.57 parts of RAFT agent 5 were used.

[0116] <Example 6> The ethylene-vinyl ester copolymer of Example 6 was obtained in the same manner as in Example 1, except that 1.36 parts of RAFT agent 6 were used.

[0117] <Example 7> The ethylene-vinyl ester copolymer of Example 7 was obtained in the same manner as in Example 1, except that 2.63 parts of RAFT agent 7 were used.

[0118] <Example 8> The ethylene-vinyl ester copolymer of Example 8 was obtained in the same manner as in Example 1, except that 2.07 parts of RAFT agent 8 were used.

[0119] <Example 9> The ethylene-vinyl ester copolymer of Example 9 was obtained in the same manner as in Example 1, except that 1.18 parts of RAFT agent 9 were used.

[0120] <Example 10> The ethylene-vinyl ester copolymer of Example 10 was obtained in the same manner as in Example 1, except that 1.40 parts of RAFT agent 10 were used.

[0121] <Example 11> The ethylene-vinyl ester copolymer of Example 11 was obtained in the same manner as in Example 1, except that 1.35 parts of RAFT agent 11 were used.

[0122] <Comparative Example 1> Comparative Example 1's ethylene-vinyl ester copolymer was obtained in the same manner as in Example 1, except that 1.44 parts of RAFT agent A were used.

[0123] <Comparative Example 2> Comparative Example 2, an ethylene-vinyl ester copolymer, was obtained in the same manner as in Example 1, except that 1.40 parts of RAFT agent B were used.

[0124] <Comparative Example 3> Comparative Example 3, an ethylene-vinyl ester copolymer, was obtained in the same manner as in Example 1, except that 1.10 parts of RAFT agent C were used.

[0125] <Comparative Example 4> Comparative Example 4, an ethylene-vinyl ester copolymer, was obtained in the same manner as in Example 1, except that 1.30 parts of RAFT agent D were used.

[0126] <Example 12> In an induction-stirred autoclave under a nitrogen atmosphere, 460 parts vinyl acetate, 37.7 parts methanol, and 0.54 parts RAFT agent 1 were added to the shell side, and methanol solution was added to the inner tube so that the AIBN concentration reached 178 ppm by mass (based on vinyl acetate). Next, AIBN was introduced into the cylinder of the induction-stirred autoclave from the inner tube, and the temperature and pressure were increased to 67°C and the ethylene pressure to 4 MPa to start copolymerization. After stirring for 6 hours, a methanol solution containing 1 part sorbic acid was added to stop the copolymerization. The copolymerized solution was then recovered from the autoclave and dried under reduced pressure to obtain the ethylene-vinyl ester copolymer of Example 12.

[0127] <Example 13> The ethylene-vinyl ester copolymer of Example 13 was obtained in the same manner as in Example 12, except that 0.55 parts of RAFT agent 4 were used.

[0128] <Example 14> The ethylene-vinyl ester copolymer of Example 14 was obtained in the same manner as in Example 12, except that 0.68 parts of RAFT agent 6 were used.

[0129] <Example 15> The ethylene-vinyl ester copolymer of Example 15 was obtained in the same manner as in Example 12, except that 1.31 parts of RAFT agent 7 were used.

[0130] <Example 16> The ethylene-vinyl ester copolymer of Example 16 was obtained in the same manner as in Example 12, except that 0.70 parts of RAFT agent 10 were used.

[0131] <Example 17> The ethylene-vinyl ester copolymer of Example 17 was obtained in the same manner as in Example 12, except that 0.67 parts of RAFT agent 11 were used.

[0132] <Comparative Example 5> In an induction-stirred autoclave under a nitrogen atmosphere, 460 parts vinyl acetate and 37.7 parts methanol were added to the shell, and methanol solution was added to the inner tube to achieve an AIBN concentration of 178 ppm by mass (based on vinyl acetate). Next, AIBN was introduced into the cylinder of the induction-stirred autoclave from the inner tube, and the temperature and pressure were increased to 67°C and the ethylene pressure to 4 MPa to start copolymerization. After stirring for 1 hour, a methanol solution containing 1 part sorbic acid was added to stop the copolymerization. The copolymerized solution was then recovered from the autoclave and dried under reduced pressure to obtain the ethylene-vinyl ester copolymer of Comparative Example 5.

[0133] <Comparative Example 6> In Comparative Example 5, the copolymer of Comparative Example 6 was obtained in the same manner as in Comparative Example 5, except that the copolymerization time was changed to 6 hours.

[0134] The following measurements were performed on the ethylene-vinyl ester copolymers obtained from Examples 1-17 and Comparative Examples 1-6. The results are shown in Tables 1 and 2 below.

[0135] [Ethylene structural unit content (ethylene content)] The obtained ethylene-vinyl ester copolymer was dissolved in deuterated chloroform and measured under the following conditions. 1 The ethylene content (mol%) was calculated by measuring 1H-NMR. (Measurement conditions) • Equipment: Ascend-400 (manufactured by Bruker) ·Measurement temperature: 25℃ • Total number of times: 16

[0136] [Vinyl acetate conversion rate] In each example and comparative example, a portion of the solution after copolymerization was sampled and weighed. After drying thoroughly until all volatile components were removed, the solution was weighed again. From these two weighings, the non-volatile content (%) was determined, and the yield of the copolymer was estimated. From this copolymer yield and the ethylene content (mol%), the amount of vinyl acetate converted was determined, and the vinyl acetate conversion rate (%) was calculated by dividing this by the amount of vinyl acetate used.

[0137] [Weight average molecular weight (Mw), number average molecular weight (Mn), molecular weight distribution (PDI)] Approximately 10 mg of the obtained ethylene-vinyl ester copolymer was collected in a glass vial, and tetrahydrofuran containing 2,6-di-t-butyl-4-methylphenol (dibutylhydroxytoluene) (BHT) (BHT concentration = 0.03%) was added as a stabilizer to adjust the ethylene-vinyl ester copolymer concentration to 0.1%. This solution was filtered through a 0.45 μm pretreatment filter (GL Sciences "Chromatodisk 13N") to prepare the sample. Next, using Shodex HK-404L (4.6 × 150 mm, 3.5 μm) and Shodex HK-401 (4.6 × 150 mm, 3 μm) columns manufactured by Showa Denko Corporation, GPC measurements were performed using a Tosoh Corporation HLC-8420GPC equipped with an RI detector under the following measurement conditions. (Measurement conditions) • Sample solution injection volume: 20 μL Column temperature: 40°C • Eluent: Special grade tetrahydrofuran (stabilizer: contains BHT) ·Flow rate: 0.6mL / min Furthermore, for the calculation of each molecular weight, commercially available monodisperse polystyrene was used as a standard sample. A calibration curve was created from the retention time and molecular weight data of the obtained polystyrene standard sample, and the weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular weight distribution (PDI) were calculated based on the calibration curve. For the polystyrene standards used, we used F-380, 128, 80, 40, 20, 10, 4, 2, 1, A-5000, 2500, 1000, and BHT manufactured by Tosoh Corporation, and the approximate equation for the calibration curve was a cubic equation.

[0138] [Table 1]

[0139] [Table 2]

[0140] The results in Tables 1 and 2 show that the ethylene-vinyl acetate copolymers of Examples 1 to 17, obtained by random copolymerization of ethylene and vinyl acetate using RAFT agents represented by general formulas (1) to (4), exhibited superior productivity compared to the ethylene-vinyl acetate copolymers of Comparative Examples 1 to 4, obtained using RAFT agents containing cyano groups, due to their higher molecular weight and vinyl acetate conversion rate. Furthermore, the ethylene-vinyl acetate copolymers of Examples 1 to 17 obtained using RAFT agents represented by general formulas (1) to (4) had small molecular weight distributions, indicating that polymerization was appropriately controlled by the RAFT agents represented by general formulas (1) to (4).

[0141] <Example 18> The ethylene-vinyl ester copolymer obtained in Example 12 was placed in a round-bottom flask, and a predetermined amount of methanol was added. The polymer was then dissolved in a 70°C water bath, and the methanol was removed by distillation using an evaporator. This process was repeated three times to remove residual vinyl acetate from the polymer. Next, a predetermined amount of methanol was added to the ethylene-vinyl ester copolymer in the round-bottom flask to prepare a 21% solution, which was then dissolved in a 70°C water bath. While stirring the aforementioned solution, a NaOH methanol solution (2% Na concentration) was added little by little. Then, a reflux condenser was attached to the round-bottom flask, and after raising the temperature to 70°C, primary saponification was carried out by heating under reflux for 20 minutes. After the primary saponification was complete, methanol was added to bring the solution concentration to 12.5%, and the added methanol was removed by distillation using an evaporator. Next, a predetermined amount of methanol was added to a round-bottom flask to prepare a 21% solution. This solution was dissolved in a 70°C water bath, and while stirring, a NaOH methanol solution (2% Na concentration) was gradually added. After that, a reflux condenser was attached to the round-bottom flask, and after raising the temperature to 70°C, secondary saponification was performed by heating under reflux for 20 minutes. After secondary saponification was complete, water was added to achieve a water-to-methanol mass ratio of 14 / 86, a reflux condenser was attached, and the solution was heated in an 80°C water bath to completely dissolve the precipitate. Afterward, the resin was concentrated in an evaporator to a concentration of 20-30%. After concentration, the contents were cooled in a tray filled with ice to solidify the resin. The solidified resin was cut with scissors, placed in a 1L beaker, and washed by adding 500mL of 2.5% acetic acid solution, stirring for 30 minutes, and then filtering. This washing process was repeated a total of three times. Next, the EVOH resin was obtained by washing it three times with 1000 mL of water and drying it at 100°C.

[0142] <Example 19> In Example 18, the EVOH resin of Example 19 was obtained in the same manner as in Example 18, except that the ethylene-vinyl ester copolymer obtained in Example 13 was used.

[0143] <Example 20> In Example 18, the EVOH resin of Example 20 was obtained in the same manner as in Example 18, except that the ethylene-vinyl ester copolymer obtained in Example 14 was used.

[0144] <Example 21> In Example 18, the EVOH resin of Example 21 was obtained in the same manner as in Example 18, except that the ethylene-vinyl ester copolymer obtained in Example 15 was used.

[0145] <Example 22> In Example 18, the EVOH resin of Example 22 was obtained in the same manner as in Example 18, except that the ethylene-vinyl ester copolymer obtained in Example 16 was used.

[0146] <Example 23> In Example 18, the EVOH resin of Example 23 was obtained in the same manner as in Example 18, except that the ethylene-vinyl ester copolymer obtained in Example 17 was used.

[0147] <Comparative Example 7> In Example 18, the EVOH resin of Comparative Example 7 was obtained in the same manner as in Example 18, except that the ethylene-vinyl ester copolymer obtained in Comparative Example 6 was used.

[0148] The following measurements were performed on the EVOH resins obtained from Examples 18-23 and Comparative Example 7. The results are shown in Table 3 below.

[0149] [Degree of saponification of EVOH, ethylene structural unit content (ethylene content), disulfide bond content, 1,2-glycol bond content] The obtained EVOH resin was dissolved in heavy DMSO and measured under the following conditions: 1 The degree of saponification, ethylene content (mol%), disulfide bond amount (mol%), and 1,2-glycol bond amount (mol%) were calculated by measuring 1H-NMR. (Measurement conditions) • Equipment: Ascend-400 (manufactured by Bruker) ·Measurement temperature: 50℃ • Total number of times: 16

[0150] [Weight average molecular weight (Mw), number average molecular weight (Mn), molecular weight distribution (PDI)] Approximately 10 mg of the obtained EVOH resin was collected in a glass vial, and hexafluoroisopropanol (HFIP) containing 5 mmol / L sodium trifluoroacetate was added to adjust the concentration of the EVOH resin to 0.1%. This solution was filtered through a 0.45 μm aperture filter (DISMIC 13HP045AN, manufactured by Advantec Toyo Co., Ltd.) to prepare the sample. Next, as the column, two HFIP-type columns, Tosoh TSKgel GMHHR-M, were connected in series to a Tosoh TSKgel guardcolumn SuperH-L manufactured by Tosoh Corporation. Using a Tosoh HLC-8420GPC equipped with an RI detector, GPC measurements were performed under the following measurement conditions. (Measurement conditions) • Sample solution injection volume: 10 μL Column temperature: 40°C • Eluent: Hexafluoroisopropanol (HFIP) containing 5 mmol / L sodium trifluoroacetate ·Flow rate: 0.2mL / min Molecular weight was calculated using commercially available monodisperse polymethyl methacrylate as a standard sample. A calibration curve was created from the retention time and molecular weight data of the obtained polymethyl methacrylate standard sample, and the weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular weight distribution (PDI) were calculated based on this calibration curve. Agilent Technologies' InfinityLab EasiVial PM was used as the polymethyl methacrylate standard, and a cubic equation was used as the approximation formula for the calibration curve.

[0151] [Yellow Index (YI)] The colorability of the obtained EVOH resin was evaluated using a spectrophotometer / colorimeter SE7700 manufactured by Nippon Denshoku Industries Co., Ltd. under the following measurement conditions. (Measurement conditions) ·Conditions: 4-item screen reflective LAV Reference values: X=90.99, Y=96.04, Z=103.05 The average value of N=4 was used.

[0152] [Table 3]

[0153] From the results in Table 3, the ethylene content of the EVOH resins of Examples 18-23, which were produced using RAFT agents represented by general formulas (1)-(4), was equivalent to that of the EVOH resin of Comparative Example 7, which did not use a RAFT agent. Furthermore, the EVOH resins of the examples, which had fewer disulfide bonds, had a lower yellow index and less discoloration. In particular, the EVOH resin of Example 19, which used RAFT agent 4, had a lower yellow index and less discoloration than the EVOH resin produced without using RAFT agents represented by general formulas (1)-(4). Furthermore, the results in Table 3 above show that the EVOH resins produced using the RAFT agents represented by general formulas (1) to (4) in the examples had a high degree of crystallinity due to the low amount of 1,2-glycol bonds. Furthermore, the weight-average molecular weight of the EVOH resins in Examples 18-23 was similar to that of the EVOH resin in Comparative Example 7, which did not use a RAFT agent, and the molecular weight distribution was smaller than that of the EVOH resin in Comparative Example 7. Based on these findings, it is presumed that the EVOH resins in Examples 18-23 possess high gas barrier properties.

[0154] <Example 24> In an induction-stirred autoclave under a nitrogen atmosphere, 460 parts vinyl acetate, 67.6 parts methanol, and 0.592 parts RAFT agent 1 were added to the shell side, and 2,2'-azobis(2,4-dimethylvaleronitrile) was added to the inner tube as a methanol solution to a concentration of 732 ppm by mass (based on vinyl acetate). Next, 2,2'-azobis(2,4-dimethylvaleronitrile) was introduced into the cylinder of the induction-stirred autoclave from the inner tube, and the temperature and pressure were increased to 50°C and the ethylene pressure to 3 MPa to start copolymerization. After stirring for 6 hours, a methanol solution containing 1 part sorbic acid was added to stop the copolymerization. Subsequently, the copolymerized solution was recovered from the autoclave, residual vinyl acetate and methanol were removed by distillation using an evaporator, and then the mixture was dried under reduced pressure to obtain the ethylene-vinyl ester copolymer of Example 24.

[0155] <Example 25> The ethylene-vinyl ester copolymer of Example 25 was obtained in the same manner as in Example 24, except that 0.603 parts of RAFT agent 2 were used.

[0156] <Example 26> The ethylene-vinyl ester copolymer of Example 26 was obtained in the same manner as in Example 24, except that 0.853 parts of RAFT agent 5 were used.

[0157] <Example 27> The ethylene-vinyl ester copolymer of Example 27 was obtained in the same manner as in Example 24, except that 0.738 parts of RAFT agent 6 were used.

[0158] <Example 28> The ethylene-vinyl ester copolymer of Example 28 was obtained in the same manner as in Example 24, except that 1.427 parts of RAFT agent 7 were used.

[0159] <Example 29> The ethylene-vinyl ester copolymer of Example 29 was obtained in the same manner as in Example 24, except that 1.124 parts of RAFT agent 8 were used.

[0160] <Example 30> The ethylene-vinyl ester copolymer of Example 30 was obtained in the same manner as in Example 24, except that 0.641 parts of RAFT agent 9 were used.

[0161] <Example 31> The ethylene-vinyl ester copolymer of Example 31 was obtained in the same manner as in Example 24, except that 0.768 parts of RAFT agent 10 were used.

[0162] <Example 32> The ethylene-vinyl ester copolymer of Example 32 was obtained in the same manner as in Example 24, except that 0.733 parts of RAFT agent 11 were used.

[0163] <Comparative Example 8> In an induction-stirred autoclave under a nitrogen atmosphere, 460 parts vinyl acetate and 67.6 parts methanol were added to the shell, and 2,2'-azobis(2,4-dimethylvaleronitrile) was added to the inner tube as a methanol solution to a concentration of 732 ppm by mass (based on vinyl acetate). Next, 2,2'-azobis(2,4-dimethylvaleronitrile) was introduced into the cylinder of the induction-stirred autoclave from the inner tube, and the temperature and pressure were increased to 50°C and the ethylene pressure to 3 MPa to start copolymerization. After stirring for 6 hours, a methanol solution containing 1 part sorbic acid was added to stop the copolymerization. The copolymerized solution was then recovered from the autoclave and dried under reduced pressure to obtain the ethylene-vinyl ester copolymer of Comparative Example 8.

[0164] For the ethylene-vinyl ester copolymers obtained in Examples 24-32 and Comparative Example 8, the ethylene structural unit content, vinyl acetate conversion rate, weight-average molecular weight, number-average molecular weight, and molecular weight distribution were measured according to the method described above. The results are shown in Table 4 below.

[0165] [Table 4]

[0166] The results in Table 4 show that the ethylene-vinyl acetate copolymers of Examples 24-32 obtained using RAFT agents represented by general formulas (1)-(4) had a small molecular weight distribution, and the polymerization was appropriately controlled even when the polymerization temperature was changed by the RAFT agents represented by general formulas (1)-(4).

[0167] <Example 33> The ethylene-vinyl ester copolymer obtained in Example 24 was placed in a round-bottom flask, and a predetermined amount of methanol was added. Then, the polymer was dissolved in a 70°C water bath, and the methanol was removed by distillation in an evaporator. This process was repeated three times to remove residual vinyl acetate from the polymer. Next, a predetermined amount of methanol was added to the ethylene-vinyl ester copolymer in the round-bottom flask to prepare a 21% solution, which was then dissolved in a 70°C water bath. While stirring the aforementioned solution, a NaOH methanol solution (2% Na concentration) was added little by little. Then, a reflux condenser was attached to the round-bottom flask, and after raising the temperature to 70°C, primary saponification was carried out by heating under reflux for 20 minutes. After the primary saponification was complete, methanol was added to bring the solution concentration to 12.5%, and the added methanol was removed by distillation using an evaporator. Next, a predetermined amount of methanol was added to a round-bottom flask to prepare a 21% solution. This solution was dissolved in a 70°C water bath, and while stirring, a NaOH methanol solution (2% Na concentration) was gradually added. After that, a reflux condenser was attached to the round-bottom flask, and after raising the temperature to 70°C, secondary saponification was performed by heating under reflux for 20 minutes. After secondary saponification was complete, water was added to achieve a water-to-methanol mass ratio of 14 / 86, a reflux condenser was attached, and the solution was heated in an 80°C water bath to completely dissolve the precipitate. Afterward, the resin was concentrated in an evaporator to a concentration of 20-30%. After concentration, the contents were cooled in a tray filled with ice to solidify the resin. The solidified resin was cut with scissors, placed in a 1L beaker, and washed by adding 500mL of 2.5% acetic acid solution, stirring for 30 minutes, and then filtering. This washing process was repeated a total of three times. Next, the EVOH resin was obtained by washing it three times with 1000 mL of water and drying it at 100°C.

[0168] <Example 34> In Example 33, the EVOH resin of Example 34 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 25 was used.

[0169] <Example 35> In Example 33, the EVOH resin of Example 35 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 26 was used.

[0170] <Example 36> In Example 33, the EVOH resin of Example 36 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 27 was used.

[0171] <Example 37> In Example 33, the EVOH resin of Example 37 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 28 was used.

[0172] <Example 38> In Example 33, the EVOH resin of Example 38 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 29 was used.

[0173] <Example 39> In Example 33, the EVOH resin of Example 39 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 30 was used.

[0174] <Example 40> In Example 33, the EVOH resin of Example 40 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 31 was used.

[0175] <Example 41> In Example 33, the EVOH resin of Example 41 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Example 32 was used.

[0176] <Comparative Example 9> In Example 33, the EVOH resin of Comparative Example 9 was obtained in the same manner as in Example 33, except that the ethylene-vinyl ester copolymer obtained in Comparative Example 8 was used.

[0177] The EVOH resins obtained from Examples 33-41 and Comparative Example 9 were measured for weight-average molecular weight, number-average molecular weight, molecular weight distribution, and yellow index according to the method described above. The results are shown in Table 5 below.

[0178] [Table 5]

[0179] From the results in Table 5, the ethylene content of the EVOH resins of Examples 33-41, produced using RAFT agents represented by general formulas (1)-(4), was equivalent to that of the EVOH resin of Comparative Example 9, which did not use a RAFT agent. Furthermore, the EVOH of Examples 33-41, which had a low disulfide bond content, had a low yellow index and low coloration. In particular, the EVOH resins of Examples 33 and 39, which used RAFT agent 1 and RAFT agent 9, had a lower yellow index and suppressed coloration than the EVOH resin produced without using RAFT agents represented by general formulas (1)-(4). Thus, it was found that the optimal RAFT agent for improving coloration differs depending on the polymerization temperature. Furthermore, as shown in the results in Table 5 above, the EVOH resins of Examples 33 to 41, which were produced using the RAFT agents represented by general formulas (1) to (4), had a similar amount of 1,2-glycol bonds as Comparative Example 9, which did not use a RAFT agent, indicating that they had a high degree of crystallinity. Furthermore, the weight-average molecular weight of the EVOH resins in Examples 33-41 was similar to that of the EVOH resin in Comparative Example 9, which did not use a RAFT agent, and the molecular weight distribution was smaller than that of the EVOH resin in Comparative Example 9. Based on these findings, it is presumed that the EVOH resins of Examples 33 to 41 possess high gas barrier properties. [Industrial applicability]

[0180] This EVOH resin exhibits suppressed discoloration and high gas barrier properties, making it particularly useful as a packaging material for various foods, as well as condiments such as mayonnaise and dressings, fermented foods such as miso, oily foods such as salad oil, beverages, cosmetics, and pharmaceuticals.

Claims

1. An ethylene-vinyl alcohol copolymer having a disulfide bond content of 0.01 to 0.20 mol% relative to vinyl alcohol units and an ethylene structural unit content of 20 to 60 mol%.

2. The ethylene-vinyl alcohol copolymer according to claim 1, wherein the ethylene-vinyl alcohol copolymer is a random copolymer.

3. A method for producing an ethylene-vinyl ester copolymer obtained by copolymerizing ethylene and a vinyl ester monomer, wherein the copolymerization of ethylene and the vinyl ester monomer is carried out in the presence of a radical polymerization initiator and a RAFT agent represented by the following general formulas (1) to (4). 【Chemistry 1】 【Chemistry 2】 【Transformation 3】 【Chemistry 4】

4. A method for producing an ethylene-vinyl alcohol copolymer obtained by saponifying an ethylene-vinyl ester copolymer, wherein the ethylene-vinyl ester copolymer is an ethylene-vinyl ester copolymer obtained by the method for producing an ethylene-vinyl ester copolymer described in claim 3.

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