JP2022120627A5 - - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a polyvinyl ester polymer, and more particularly to a method for producing a polyvinyl ester polymer by polymerizing a vinyl ester monomer in the presence of a radical polymerization initiator.

BACKGROUND ART Polyvinyl ester polymers have conventionally been widely used as raw materials for polyvinyl alcohol (hereinafter sometimes referred to as “PVA”). Polyvinyl alcohol is a crystalline water-soluble polymer material obtained by saponifying polyvinyl ester. , emulsifiers, suspending agents, surfactants, fiber processing agents, various binders, paper processing agents, adhesives, films, etc. PVA is a crystalline, water-soluble polymer material obtained by saponifying polyvinyl ester polymers. It is used in emulsifiers, suspension agents, surfactants, fiber processing agents, various binders, paper processing agents, adhesives, films, and the like.

Polyvinyl ester polymers are obtained by polymerizing vinyl ester monomers, and radical polymerization, which can be applied to the polymerization of various vinyl compounds and is easy to handle reaction systems, is an industrially applicable polymerization method. is also widely used. The reaction mechanism of radical polymerization includes initiation reaction, propagation reaction and termination reaction, and chain transfer reaction may occur as a side reaction. In radical polymerization, when an initiating radical is generated, it reacts with monomers in turn and the reaction is terminated by deactivation of the propagating radical. Inactivation of propagating radicals occurs due to recombination termination reaction, disproportionation termination reaction, chain transfer reaction as a side reaction, and the like, so that a polymer having nonuniform molecular lengths is produced.

Therefore, as a method for controlling the molecular weight of the resulting polymer, synthesis of the polymer by living radical polymerization is performed. Living radical polymerization is a polymerization method that consists only of an initiation reaction and a propagating reaction in the polymerization process. A coalescence is obtained.

Various techniques for synthesizing a polyvinyl ester-based (co)polymer using a living radical polymerization method have been studied. As a method for producing such a copolymer, a method of copolymerizing a vinyl ester monomer and a vinyl ether monomer in the presence of a chain transfer agent having a specific structure, a radical polymerization initiator, and a Lewis acid has been proposed. It is

JP 2013-237748 A

However, in the production method using the conventional living radical polymerization method, the chain transfer agent used reacts with the terminal of the polyvinyl ester polymer, resulting in the formation of a terminal structure derived from the chain transfer agent that causes coloration. However, there is a problem that the polyvinyl alcohol-based resin obtained by saponifying the obtained polyvinyl ester-based polymer is colored.

Accordingly, an object of the present invention is to provide a novel method for producing a polyvinyl ester-based polymer capable of obtaining a polyvinyl alcohol-based resin with suppressed coloring.

As a result of extensive studies, the present inventors have found that the terminal structure of the polyvinyl ester polymer is involved in the coloring of the polyvinyl alcohol resin. When the system monomer is polymerized, by treating it with a peroxide polymerization initiator after polymerization, it is possible to highly suppress the coloring of the polyvinyl alcohol resin obtained by saponifying the polyvinyl ester polymer obtained. The discovery led to the completion of the present invention.

That is, the present invention is characterized by the following (1) to (5).
(1) A step of polymerizing a vinyl ester monomer in the presence of a compound represented by the following general formula (A), and a step of treating the vinyl ester polymer with a peroxide polymerization initiator in this order. A method for producing a polyvinyl ester polymer characterized by the above.

(In formula (A) above, R is an alkyl group having 1 to 5 carbon atoms.)

(2) The polyvinyl ester polymer according to (1), wherein the amount of the peroxide polymerization initiator added is 0.1 to 500 with respect to the vinyl ester polymer 1. manufacturing method.
(3) The method for producing a polyvinyl ester polymer according to (1) or (2) above, wherein the peroxide polymerization initiator is an alkyl perester.
(4) The method for producing a polyvinyl ester polymer according to any one of (1) to (3) above, wherein the vinyl ester monomer is vinyl acetate.

According to the production method of the present invention, it is possible to produce a polyvinyl ester-based polymer from which a polyvinyl alcohol-based resin with suppressed coloration can be obtained.

Although the present invention will be described in detail below, these show examples of preferred embodiments, and the present invention is not limited to these contents.
In the present specification, (meth)allyl means allyl or methallyl, (meth)acryl means acrylic or methacrylic, and (meth)acrylate means acrylate or methacrylate.

The method for producing a polyvinyl ester-based polymer of the present invention comprises the steps of polymerizing a vinyl ester-based monomer in the presence of a compound represented by the following general formula (A), and peroxidizing the resulting vinyl ester-based polymer. It has a step of treating with a system polymerization initiator.

(In formula (A) above, R is an alkyl group having 1 to 5 carbon atoms.)

Living radical polymerization (specifically, RAFT (Reversible Addition-Fragmentation Chain Transfer) polymerization) using a specific chain transfer agent used in the present invention When a vinyl ester monomer is polymerized, reaction with the chain transfer agent A terminal structure derived from the chain transfer agent is generated at the end of the vinyl ester polymer, which is presumed to cause coloration. It is presumed that coloring of the polyvinyl alcohol-based resin obtained by saponifying the polyvinyl ester-based polymer of the present application can be suppressed, probably because the structure can be cleaved.

The method for producing a polyvinyl ester polymer of the present invention comprises the steps of: polymerizing a vinyl ester monomer in the presence of a compound represented by the general formula (A); and a step of treating with

First, the step of polymerizing a vinyl ester monomer in the presence of the compound represented by formula (A) will be described.

Vinyl ester monomers used in the production method of the present invention include, for example, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl myristate, and palmitin. Vinyl acid, vinyl stearate, vinyl cyclohexanecarboxylate, vinyl pivalate, vinyl octylate, vinyl monochloroacetate, vinyl adipate, vinyl methacrylate, vinyl crotonate, vinyl sorbate, vinyl benzoate, vinyl cinnamate, trifluoroacetic acid vinyl and the like. These may be used individually by 1 type, and may be used in combination of 2 or more type. Among these, vinyl acetate is preferably used from the viewpoint of price and availability.

The polyvinyl ester polymer produced in the present invention may be copolymerized with other monomers copolymerizable with the vinyl ester monomer as long as the effects of the present invention are not impaired. Other monomers include olefins such as ethylene, propylene, isobutylene, α-octene, α-dodecene, α-octadecene; acrylic acid, methacrylic acid, crotonic acid, maleic acid, maleic anhydride, and itaconic acid. Unsaturated acids or salts thereof, mono- or dialkyl esters thereof, etc.; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefins such as ethylenesulfonic acid, allylsulfonic acid and methallylsulfonic acid Sulfonic acid or its salts; Alkyl vinyl ethers; N-acrylamidomethyltrimethylammonium chloride; Allyltrimethylammonium chloride; Dimethyl allyl vinyl ketone; Polyoxyalkylene (meth) allyl ethers such as propylene (meth) allyl ether; Polyoxyalkylene (meth) acrylates such as polyoxyethylene (meth) acrylate and polyoxypropylene (meth) acrylate; Polyoxyethylene (meth) acrylamide, Polyoxyalkylene (meth)acrylamide such as polyoxypropylene (meth)acrylamide; Polyoxyethylene [1-(meth)acrylamide-1,1-dimethylpropyl] ester; Polyoxyethylene vinyl ether, polyoxypropylene vinyl ether such as polyoxypropylene vinyl ether Alkylene vinyl ether; Polyoxyalkylene allylamine such as polyoxyethylene allylamine and polyoxypropylene allylamine; Polyoxyalkylene vinylamine such as polyoxyethylene vinylamine and polyoxypropylene vinylamine; 3-buten-1-ol, 4-pentene- Examples include hydroxy group-containing α-olefins such as 1-ol and 5-hexene-1-ol, and derivatives such as acylated products thereof.

In addition, 3,4-dihydroxy-1-butene, 3,4-diacyloxy-1-butene, 3-acyloxy-4-hydroxy-1-butene, 4-acyloxy-3-hydroxy-1-butene, 3,4- diacyloxy-2-methyl-1-butene, 4,5-dihydroxy-1-pentene, 4,5-diacyloxy-1-pentene, 4,5-dihydroxy-3-methyl-1-pentene, 4,5-diacyloxy- 3-methyl-1-pentene, 5,6-dihydroxy-1-hexene, 5,6-diacyloxy-1-hexene, glycerin monoallyl ether, 2,3-diacetoxy-1-allyloxypropane, 2-acetoxy-1 - allyloxy-3-hydroxypropane, 3-acetoxy-1-allyloxy-2-hydroxypropane, glycerin monovinyl ether, glycerin monoisopropenyl ether, vinylethylene carbonate, 2,2-dimethyl-4-vinyl-1,3-dioxolane compounds having a diol such as

The compound represented by the above general formula (A) is a compound for controlling radical polymerization, and reacts with the end of a growing polymer chain to stop polymer growth and at the same time generate a new polymerization initiation radical. , is a chain transfer agent.
The polymerization mechanism of living radical polymerization will be explained by the following Scheme I, taking as an example production of polyvinyl acetate using vinyl acetate as a vinyl ester monomer. The radical Y· of polyvinyl acetate is added to the dithiocarbonyl compound 1 (dormant species) to which the polymer in the course of polymerization is added as a radical, and the intermediate radical 2 is generated. Further, another polyvinyl acetate radical Y′· is eliminated from the intermediate radical 2 to generate a new dormant species (dithiocarbonyl compound 1′) and another polyvinyl acetate radical Y′·. Polymerization proceeds by repeating addition and elimination in this way. In Scheme I below, X is OR (R is an alkyl group having 1 to 5 carbon atoms).

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Scheme I

In general formula (A), R is an alkyl group having 1 to 5 carbon atoms. Examples of alkyl groups having 1 to 5 carbon atoms include methyl group, ethyl group, propyl group, isopropyl group, n-butyl group, isobutyl group, sec-butyl group, tert-butyl group and pentyl group.

From the viewpoint of ease of production, the compound represented by the general formula (A) is preferably a compound in which R is an alkyl group having 1 to 4 carbon atoms, and a compound in which R is an alkyl group having 2 to 4 carbon atoms. more preferred. The compounds represented by formula (A) may be used singly or in combination of two or more.

Specific examples of the compound represented by the general formula (A) include S-1-isobutoxyethyl O-isopropyl xanthate (a compound in which R is an isopropyl group), S-1-isobutoxyethyl O-ethyl xanthate (R is an ethyl group, compound) and the like.

The step of polymerizing the vinyl ester monomer in the presence of the compound represented by formula (A) in the present invention is carried out in the presence of a polymerization initiator. Examples of polymerization initiators include 2,2′-azobis-(2,4,4-trimethylvaleronitrile), 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis-( 2-methylpropionamidine) dihydrochloride and other azo compounds, t-butyl peroxyneodecanoate, t-butyl perpivalate and other alkyl peresters, bis-(4-t-butylcyclohexyl)peroxy-di- peroxy-di-carbonates such as carbonate, di-cyclohexylperoxy-di-carbonate, bis(2-ethylhexyl) di-sec-butylperoxy-di-carbonate, di-isopropylperoxy-di-carbonate, acetyl peroxy known radical polymerization catalysts such as peroxides, diacetyl peroxide, di-lauroyl peroxide, di-decanoyl peroxide, di-octanoyl peroxide, di-propyl peroxide, di-benzoyl peroxide, etc. mentioned. Also, 2,2'-azobis-(2,4-dimethylvaleronitrile), 2,2'-azobis-(4-methoxy-2,4-dimethylvaleronitrile) and other low-temperature active radical polymerization catalysts for radical initiation agents can be mentioned. One of these polymerization catalysts may be used alone, or two or more thereof may be used simultaneously.

The step of polymerizing the vinyl ester monomer in the presence of the compound represented by formula (A) is carried out in the presence of an alcohol solvent. Examples of the alcohol solvent used in this step include methanol, ethanol, propanol, isopropyl alcohol, ethyl acetate and the like. These may be used individually by 1 type, and may be used in combination of 2 or more type.

Other components can be added to the polymerization reaction solution as long as they do not impair the effects of the present invention. Other additives include, for example, colorants, dyes, antifoaming agents, preservatives, antifungal agents, antioxidants, ultraviolet absorbers, surfactants, etc., and saturated fats such as stearamide. unsaturated fatty acid amides such as group amides and oleic acid amides, bis fatty acid amides such as ethylenebisstearic acid amides, known lubricants such as low-molecular-weight polyolefins, release agents, polyhydric alcohols such as ethylene glycol, glycerin, and hexanediol, In particular, known plasticizers such as aliphatic polyhydric alcohols, acids such as acetic acid and phosphoric acid and metal salts thereof such as alkali metals and alkaline earth metals, metal compounds such as metal oxides and hydroxides, boron Known thermal stabilizers such as boron compounds such as acids or metal salts thereof may be included.

Next, the step of treating the obtained vinyl ester polymer with a peroxide polymerization initiator will be described.

In the step of treating a vinyl ester polymer with a peroxide polymerization initiator, examples of the peroxide polymerization initiator used include alkyl peresters such as t-butyl peroxyneodecanoate and t-butyl perpivalate. bis-(4-t-butylcyclohexyl)peroxy-di-carbonate, di-cyclohexylperoxy-di-carbonate, bis(2-ethylhexyl)di-sec-butylperoxy-di-carbonate, di-isopropyl peroxy Peroxy-di-carbonates such as oxy-di-carbonate, acetyl peroxide, diacetyl peroxide, di-lauroyl peroxide, di-decanoyl peroxide, di-octanoyl peroxide, di-propyl peroxide, di- Examples include radical polymerization catalysts such as peroxides such as benzoyl peroxide.

Examples of alcohol solvents used in the step of treating the vinyl ester polymer with the peroxide polymerization initiator include methanol, ethanol, propanol, isopropyl alcohol, and ethyl acetate. These may be used individually by 1 type, and may be used in combination of 2 or more type.

Next, a specific method for producing the polyvinyl ester polymer of the present invention will be described.
In the step of polymerizing a vinyl ester monomer in the presence of a compound represented by general formula (A) in the production method of the present invention, a reactor is charged with a vinyl ester monomer, an alcoholic solvent, general formula (A ) and a polymerization initiator are added to initiate the polymerization reaction. Each raw material may be added at once, or may be added in any order. In addition, each raw material may be added all at once, or may be added in an arbitrary number of times at an arbitrary timing during the polymerization.

In the polymerization step, the reaction is preferably carried out in an inert gas atmosphere that is inert to the vinyl ester monomer. Examples of inert gases include nitrogen and argon.

The amount of the polymerization initiator added is preferably 0.00001 to 10 mol % with respect to the amount of the vinyl ester monomer added. Too much polymerization initiator may make it difficult to control the polymerization, and too little may cause problems in productivity. The amount of the polymerization initiator added is more preferably 0.00003 to 8 mol %, still more preferably 0.00005 to 6 mol %, relative to the amount of the vinyl ester monomer added.

The amount of the compound represented by formula (A) to be added is preferably 0.0001 to 10 mol % with respect to the amount of the vinyl ester monomer added. If the amount of the compound represented by the formula (A) is too large, the number of free polymerization terminal radicals becomes too small, and polymerization may not proceed. It is difficult to obtain a polymer. The amount of the compound represented by formula (A) to be added is more preferably 0.0003 to 8 mol%, still more preferably 0.0005 to 6 mol%.

The amount of the alcohol-based solvent added is preferably in the range of more than 0 and not more than 10 with respect to 1 unit of the vinyl ester-based monomer in mass ratio. If the amount of the alcohol solvent is too large, the resulting polymer may have a low molecular weight. It is more preferable to add the alcohol-based solvent in a range of more than 0 to 8 or less, more preferably more than 0 to 6 or less, relative to 1 of the vinyl ester-based monomer.

The temperature of the reaction solution in the polymerization step may be appropriately set in consideration of productivity, and is preferably carried out in the range of 0 to 150°C, more preferably 5 to 140°C, and even more preferably 10 to 130°C. If the temperature of the reaction solution is too high, it may become difficult to control the molecular weight or control the polymerization.

The reaction time may be appropriately set so as to obtain a polymer having the desired molecular weight, but in consideration of productivity, it is preferably carried out in the range of 0.1 to 48 hours, more preferably 0.3 to 36 hours. Preferably, 0.5 to 24 hours is more preferable. If the reaction time is too long, a problem may arise in productivity, and if it is too short, the molecular weight may not be uniform, and a vinyl ester polymer having a uniform molecular weight may not be obtained.

In the present invention, the termination step is performed by adding a polymerization terminator when the target polymerization rate is reached.

Examples of the polymerization terminator include quinone compounds such as hydroquinone, hydroquinone monomethyl ether and parabenzoquinone, and nitrobenzene compounds such as o-dinitrobenzene, m-dinitrobenzene and p-dinitrobenzene. These may be used individually by 1 type, and may be used in combination of 2 or more type. Among these, hydroquinone monomethyl ether is preferably used from the viewpoint of coloring and toxicity.

The amount of the polymerization terminator to be added is not particularly limited as long as it is an amount capable of terminating the polymerization, but it is preferably 0.0001 to 10000 ppm with respect to the amount of the vinyl ester monomer added. If the amount of the polymerization terminator is too small, the radicals at the ends of the polymer cannot be sufficiently captured, and the polymerization cannot be terminated sufficiently, which may result in a wide molecular weight distribution. The polyvinyl alcohol-based resin obtained by saponifying may be easily colored. The amount of polymerization terminator added is more preferably 0.0003 to 9000 ppm, more preferably 0.0005 to 8000 ppm, relative to the amount of vinyl ester monomer added.

In the termination step, the temperature of the reaction solution may be a temperature at which the polymerization terminator can scavenge radicals at the polymer terminal, preferably in the range of 0 to 100°C, more preferably 3 to 60°C, and further preferably 5 to 50°C. preferable. If the temperature of the reaction solution is too high, polymerization will proceed in addition to the termination reaction, which may result in a wide molecular weight distribution. It becomes difficult to mix uniformly.

The reaction time in the stopping step is preferably in the range of 0.01 to 48 hours, more preferably 0.03 to 36 hours, and even more preferably 0.05 to 24 hours, in consideration of productivity.

After the terminating step, the solution containing the vinyl ester polymer is heated, vacuum dried, etc. to remove the solvent and unreacted monomers, thereby obtaining the vinyl ester polymer.

In the subsequent step of treating the resulting vinyl ester polymer with a peroxide polymerization initiator, the vinyl ester polymer, peroxide polymerization initiator, and alcohol solvent are added to the reactor to initiate the reaction. Let Each raw material may be added at once, or may be added in any order.

In the step of treating the vinyl ester polymer with the peroxide polymerization initiator, the reaction is preferably carried out in an inert gas atmosphere which is inert to the vinyl ester polymer. Examples of inert gases include nitrogen and argon .

The addition amount of the peroxide polymerization initiator is preferably 0.1 to 500, more preferably 0.5 to 300, even more preferably 1 to 200, relative to 1 vinyl ester polymer.

The amount of the alcohol solvent to be added is preferably in the range of more than 0 and 10 or less with respect to 1 part of the vinyl ester polymer in terms of mass ratio. The alcohol solvent is more preferably added to the vinyl ester polymer 1 in the range of more than 0 to 8 or less, more preferably more than 0 to 6 or less.

The temperature of the reaction solution in the step of treating the vinyl ester polymer with the peroxide polymerization initiator may be appropriately set in consideration of productivity, preferably in the range of 0 to 150 ° C., 5 to 140 °C is more preferred, and 10 to 130°C is even more preferred.

The reaction time may be appropriately set to a time sufficient to cut the terminal structure derived from the chain transfer agent of the vinyl ester polymer, but it should be carried out in the range of 0.1 to 48 hours in consideration of productivity. is preferred, 0.3 to 36 hours is more preferred, and 0.5 to 24 hours is even more preferred. If the reaction time is too long, problems may arise in productivity, and if it is too short, the terminal structure derived from the chain transfer agent may not be sufficiently cleaved.

After completion of the reaction, the product is separated and purified by a known method, and the solvent and the like are distilled off by heating and vacuum drying to obtain the polyvinyl ester polymer of the present invention.

The polyvinyl ester-based polymer obtained by the production method of the present invention can be saponified to obtain a polyvinyl alcohol-based resin (hereinafter also referred to as a PVA-based resin).
For the saponification of the obtained polyvinyl ester-based polymer, a conventionally known saponification method can be employed. That is, it can be carried out using an alkali catalyst or an acid catalyst in a state in which the polyvinyl ester polymer is dissolved in an alcohol or water/alcohol solvent.
As the alkali catalyst, for example, hydroxides and alcoholates of alkali metals such as potassium hydroxide, sodium hydroxide, sodium methylate, sodium ethylate, potassium methylate and lithium methylate can be used.
Generally, the transesterification reaction using an alkali catalyst in an anhydrous alcoholic solvent is preferably used in terms of the reaction rate and the ability to reduce impurities such as fatty acid salts.

The reaction temperature for the saponification reaction is usually 20 to 60°C. If the reaction temperature is too low, the reaction rate tends to decrease and the reaction efficiency tends to decrease. In addition, when saponification is performed under high pressure using a tower-type continuous saponification tower with high pressure resistance, saponification can be performed at a higher temperature, for example, 80 to 150 ° C., and a small amount of saponification catalyst is used. It is also possible to obtain a high degree of saponification in a short period of time.

The polyvinyl ester-based polymer obtained in the present invention may be subjected to the saponification step as it is after the termination step without removing the solvent and unreacted monomers to obtain the PVA-based resin.

Since the polyvinyl ester polymer obtained by the production method of the present invention has the terminal structure derived from the chain transfer agent, which is presumed to cause coloring, has been cut, the PVA resin obtained using the polyvinyl ester polymer is suppressed, and the coloration of molded articles using the PVA-based resin is also suppressed.

EXAMPLES The present invention will be described in more detail below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. In the following examples and comparative examples, "parts" and "%" are based on mass unless otherwise specified.

(Synthesis Example 1: Production of S-1-isobutyloxyethyl O-isopropyl xanthate (iPrBEX))
100 mL (0.1 mol in Et ₂ O) of 1.0 M HCl·Et ₂ O (hydrogen chloride·diethyl ether) was placed in a 200 mL eggplant flask cooled to 0°C. 13 mL (0.1 mol) of isobutyl vinyl ether (IBVE) was added dropwise to a flask containing HCl.Et ₂ O while stirring, and the mixture was kept stirring at 0° C. for about 1 hour to obtain an IBVE-HCl adduct. Waste HCl was released as a gas and purified. 11.6 g (0.067 mol) of isopropyl potassium xanthate was placed in an eggplant flask cooled to 0° C., and 76 mL (0.076 mol) of the IBVE-HCl adduct solution synthesized above was added. From there, the mixture was stirred with a stirrer at 0° C. to room temperature for 24 hours. Then, it was filtered using a glass funnel, and the filtrate was transferred to a separating funnel and washed with 0.2M NaHCO ₃ three times and with pure water twice. This was transferred to a dry Erlenmeyer flask and pre-dried over anhydrous sodium sulfate overnight. Anhydrous sodium sulfate was removed and decanted into a dried eggplant flask. After evaporating the solvent, it was purified by _two vacuum distillations (b.p. 78oC/0.6kPa) over CaH2 under dry nitrogen. S-1-isobutyloxyethyl O-isopropyl xanthate represented by the following formula (1) was obtained as a yellow liquid with a synthesis yield of 51%.

(Polymerization Example 1: Production of vinyl ester polymer)
Predetermined amounts of vinyl acetate (100 g, 1.16 mol), ethyl acetate (20 g, 2.27×10 ⁻¹ mol), iPrBEX prepared in Synthesis Example 1 (129 mg, 5.5×10 ⁻⁴ mol, ethyl acetate 1 g 0.047 mol % dissolved in vinyl acetate) was added to the separable flask. After nitrogen was bubbled through this for 20 minutes with stirring, 2,2'-azobisisobutyronitrile (9 mg, 5.48×10 ^-5 mol, dissolved in 1 g of ethyl acetate) was added. After that, boiling point polymerization was carried out for 7 hours.

The polymerization rate of the polymer present in the resulting polymerization reaction solution was 48.4%. 30 ppm of a polymerization terminator (m-dinitrobenzene) was added to the polymerization reaction solution to terminate the polymerization reaction. By vacuum-drying the polymerization reaction solution after termination of the polymerization, the solvent and unreacted monomers were distilled off to obtain a polymer.

(Example 1)
A 10% by mass solution of the polymer obtained in Polymerization Example 1 was prepared with ethyl acetate (20 g, 2.27×10 ⁻¹ mol), and 10 g of the solution was added to an eggplant flask. Furthermore, 2.22 g of t-butyl perpivalate was added as a peroxide initiator and mixed to obtain a reaction solution. After nitrogen was blown into this for 10 minutes, a reflux tube was attached to the eggplant flask, and the reaction was carried out at an external temperature of 81° C. for 4.5 hours. After completion of the reaction, the product was ice-cooled, purified by reprecipitation with hexane, and dried in vacuum at 50°C for 8 hours to obtain polyvinyl acetate.

(Measurement of reduction rate of terminal structure derived from chain transfer agent presumed to be involved in coloring in terminal structure of vinyl ester polymer)
The chain transfer agent-derived terminal structure reduction rate of the obtained polyvinyl acetate was measured by the following method. The reduction rate of the terminal structure derived from the chain transfer agent was obtained from the integrated value of the peak at 5.64 ppm in ¹ H-NMR measurement. Table 1 shows the results.

(Coloring evaluation)
The obtained polyvinyl acetate was saponified by a conventional method to obtain polyvinyl alcohol, and the coloring was visually evaluated as follows.
0: white 1: very light yellow 2: light yellow 3: yellow 4: slightly dark yellow 5: dark yellow The results are shown in Table 1.

(Comparative example 1)
The polymer was treated in the same manner as in Example 1, except that 4 g of 2,2'-azobisisobutyronitrile was used instead of t-butyl perpivalate.

The chain transfer agent-derived terminal structure reduction rate of polyvinyl acetate obtained in the same manner as in Example 1 was measured by ¹ H-NMR. Further, the obtained polyvinyl acetate was saponified to polyvinyl alcohol, and coloration was evaluated by visual observation. Table 1 shows the results.

(Comparative example 2)
The polymer was treated in the same manner as in Example 1, except that 0.8 g of 2,2'-azobisisobutyronitrile was used instead of t-butyl perpivalate.

The chain transfer agent-derived terminal structure reduction rate of polyvinyl acetate obtained in the same manner as in Example 1 was measured by ¹ H-NMR. Further, the obtained polyvinyl acetate was saponified to polyvinyl alcohol, and coloration was evaluated by visual observation. Table 1 shows the results.

(Comparative Example 3)
The polymer was treated in the same manner as in Example 1, except that 1.6 g of 2,2'-azobis-(2,4-dimethylvaleronitrile) was used instead of t-butyl perpivalate.

The chain transfer agent-derived terminal structure reduction rate of polyvinyl acetate obtained in the same manner as in Example 1 was measured by ¹ H-NMR. Further, the obtained polyvinyl acetate was saponified to polyvinyl alcohol, and coloration was evaluated by visual observation. Results are shown in Table 1.

Table 1 collectively shows the above examples and comparative examples.

From Table 1, Example 1 using a peroxide polymerization initiator as a polymerization initiator used in the step of treating a vinyl ester polymer with a peroxide polymerization initiator is a comparative example using an azo polymerization initiator. The terminal structure reduction rate derived from the chain transfer agent of the obtained polyvinyl acetate is very high compared to 1 to 3, and the coloring of polyvinyl alcohol obtained by saponifying the obtained polyvinyl acetate is remarkably suppressed. I found out.

Claims (4)

characterized by including, in this order, a step of polymerizing a vinyl ester monomer in the presence of a compound represented by the following general formula (A), and a step of treating the vinyl ester polymer with a peroxide polymerization initiator. A method for producing a polyvinyl ester polymer.

(In formula (A) above, R is an alkyl group having 1 to 5 carbon atoms.) 2. The method for producing a polyvinyl ester polymer according to claim 1, wherein the addition amount of the peroxide polymerization initiator is 0.1 to 500 with respect to 1 part of the vinyl ester polymer. 3. The method for producing a polyvinyl ester polymer according to claim 1, wherein the peroxide polymerization initiator is an alkyl perester. 4. The method for producing a polyvinyl ester polymer according to any one of claims 1 to 3, wherein the vinyl ester monomer is vinyl acetate.