Methods for manufacturing polyvinyl alcohol-based polymers, dispersants and stabilizers and ethylene-based polymers

Abstract

This invention provides a polyvinyl alcohol-based polymer, etc. In this invention, polyvinyl alcohol polymers are made to fully satisfy the following requirement 1, as well as the following requirement 2 and / or requirement 3. Requirement 1: Possesses polymeric unsaturated bonds Requirement 2: The YI of the 4% by mass aqueous solution is below 13. Requirement 3: YI is 18 or below

Description

Methods for manufacturing polyvinyl alcohol-based polymers, dispersants and stabilizers and ethylene-based polymers This invention relates to a polyvinyl alcohol polymer, various uses of the polyvinyl alcohol polymer [e.g., a dispersion stabilizer [e.g., a dispersion stabilizer for suspension polymerization of ethylene monomers (especially vinyl chloride monomers)]], and a method for manufacturing ethylene polymers [especially vinyl chloride polymers (resins)] using the polyvinyl alcohol polymer (or dispersion stabilizer). Industrially, vinyl chloride-based resins are typically manufactured using batch suspension polymerization. This process involves dispersing vinyl chloride and other vinyl monomers in an aqueous medium in the presence of a dispersant and stabilizer, and then polymerizing using an oil-soluble polymerization initiator. Factors influencing the quality of vinyl chloride-based resins during the polymerization process include: polymerization rate, the ratio of aqueous medium to monomer, polymerization temperature, the type and amount of polymerization initiator, the type of polymerization tank, stirring speed, and the type and amount of dispersant and stabilizer. Among these, the effect of the dispersant and stabilizer is particularly significant. The function of a dispersant stabilizer used in the suspension polymerization of vinyl chloride-based resins is to disperse the monomers in an aqueous medium, forming stable droplets, to uniformly adjust the size of the repeatedly dispersed and combined droplets, and to control the agglomeration of the polymer particles. Therefore, the required properties of this dispersant stabilizer can be exemplified as follows: <1> Controlling the particle size of the obtained vinyl chloride-based resin particles within an appropriate range; <2> Increasing the plasticizer absorption of the obtained vinyl chloride-based resin particles, thereby improving molding and processability; <3> Setting the porosity of the obtained vinyl chloride-based resin particles within a certain range, making the removal of residual monomers easier; <4> Improving the color of the obtained vinyl chloride-based resin particles. That is, for the above-mentioned dispersant stabilizers, the following requirements are made, for example: to exert excellent dispersing power (protecting colloidal properties) and to control the particle size and particle shape of vinyl chloride resins to an appropriate state. As the aforementioned dispersing stabilizers, polyvinyl alcohol resins (hereinafter sometimes simply referred to as PVA, etc.), cellulose derivatives, etc., are usually used alone or in appropriate combinations. For example, Non-Patent Document 1 describes a method for using PVA with high emulsifying power, such as a viscosity-average degree of polymerization of 2000 and a saponification degree of 88 mol% or 80 mol%, as a dispersant stabilizer for the suspension polymerization of vinyl chloride, or using PVA with a viscosity-average degree of polymerization of 600-700 and a saponification degree of around 70 mol%. Furthermore, Patent Document 1 discloses a dispersion stabilizer for suspension polymerization, characterized by containing a polyvinyl alcohol polymer (B) with double bonds in its side chains, obtained by acetalizing a polyvinyl alcohol polymer (A) with a monoaldehyde having olefinic unsaturated double bonds. [Prior Art Documents] [Patent Documents] [Patent Document 1] International Publication No. 2015 / 182567 [Non-Patent Document] [Non-Patent Document 1] "POVAL", Polymer Publishing Association, published in 1981. [The problem the invention aims to solve] The object of this invention is to provide a polyvinyl alcohol (PVA) based polymer, etc. [Technical Means for Solving the Problem] As mentioned above, PVA is used as a dispersant and stabilizer for the suspension polymerization of vinyl chloride and the like, and technologies are being developed to further improve PVA used as such a dispersant and stabilizer. In particular, Patent Document 1 relates to a technology developed by the inventors, which aims to improve polymerization stability or dispersibility (protective colloid properties) by modifying PVA with monoaldehydes having olefinic unsaturated double bonds (acetalization). On the other hand, according to the inventors' research, it has been found that the PVA used can affect the color (hue) of the resulting resin (such as vinyl chloride resin), and may even cause the resulting resin to become colored. However, the reasons why different PVAs have what kind of effect on the color of the resin are unknown, and it is extremely difficult to explore which PVAs are useful for reducing or inhibiting coloration. Furthermore, this effect of coloring still exists even when modified with monoaldehydes having olefinic unsaturated double bonds (acetalization) as in Patent Document 1 (especially without improvement), making the exploration of PVAs that can improve polymer stability or dispersibility and achieve improvement or suppression of coloring extremely difficult. In this case, the inventors discovered that the coloring and hue (or degree) of PVA itself can affect the coloring of the resulting resin. Further research revealed that, based on PVA that fully meets specific requirements (coloring characteristics, etc.), the coloring of the resulting resin can be improved or suppressed. In particular, even for PVA with unsaturated bonds (unsaturated double bonds) as in Patent Document 1, this tendency does not change significantly. It can even improve polymerization stability or dispersing power and achieve improvement or suppression of coloring. Through repeated research, the inventors completed the present invention. That is, the present invention relates to the following inventions, etc. [1] A polyvinyl alcohol polymer (A) that satisfies the following requirements 1, and the following requirements 2 and / or requirements 3. Requirements 1: having polymerizable unsaturated bonds Requirements 2: the YI of a 4% by mass aqueous solution is 18 or less Requirements 3: the YI is 13 or less [2] The polyvinyl alcohol polymer (A) as described in [1], wherein the ratio of polymerizable unsaturated bonds is 3 μmol / g or more. [3] The polyvinyl alcohol polymer (A) as described in [1] or [2], which satisfies the following requirements: the YI of a 4% by mass aqueous solution is 15 or less and / or the YI is 12 or less. [4] The polyvinyl alcohol polymer (A) as described in any one of [1] to [3], which further satisfies the following requirement 4. Requirement 4: The transmittance of a 1% by mass aqueous solution at 430 nm is 90% or more [5] The polyvinyl alcohol polymer (A) described in any one of [1] to [4], wherein the ratio of polymeric unsaturated bonds is 5 to 500 μmol / g, the YI of a 4% by mass aqueous solution is 15 or less, the YI is 12 or less, and the transmittance of a 1% by mass aqueous solution at 430 nm is 95% or more. [6] The polyvinyl alcohol polymer (A) described in any one of [1] to [5] contains an acetal skeleton (a) having polymeric unsaturated bonds. [7] The polyvinyl alcohol polymer (A) described in any one of [1] to [6] contains an acetal skeleton (a) having polymeric unsaturated bonds, the acetal skeleton (a) comprising the skeleton represented by the following formula (a1). [Chemical 1] (where R' represents a group having polymerizable unsaturated bonds) [8] The polyvinyl alcohol polymer (A) described in any of [1] to [7] contains an acetal skeleton (a) having polymerizable unsaturated bonds, the acetal skeleton (a) comprising the skeleton represented by the above formula (a1), and the content of the acetal skeleton (a) is 0.05 to 5 mol% relative to each monomer unit. [9] The polyvinyl alcohol polymer (A) described in any of [1] to [9] further contains an ionic skeleton (b).

[10] The polyvinyl alcohol polymer (A) described in any of [1] to [9] further contains an ionic skeleton (b), and the content of the ionic skeleton (b) is 0.01 to 5 mol% relative to each monomer unit.

[11] The polyvinyl alcohol polymer (A) described in any of [1] to

[10] has a saponification degree of 50 to 90 mol.

[12] The polyvinyl alcohol polymer (A) described in [1] to

[11] has a viscosity (20°C) of 1 to 300 mPa·s (e.g., 1 to 100 mPa·s, 2 to 100 mPa·s) of a 4% by mass aqueous solution.

[13] A dispersion stabilizer containing the polyvinyl alcohol polymer (A) described in any one of [1] to

[12] .

[14] The agent described in

[13] is a dispersion stabilizer for polymerization.

[15] The agent described in

[13] or

[14] is a dispersion stabilizer for suspension polymerization.

[16] The agent described in any one of

[13] to

[15] is a dispersion stabilizer for suspension polymerization of vinyl monomers containing vinyl chloride.

[17] A method for manufacturing a vinyl polymer, wherein vinyl monomers are polymerized in the presence of the polyvinyl alcohol polymer (A) described in any one of [1] to

[16] or the agent.

[18] The manufacturing method described in

[17] , wherein the polymerization is suspension polymerization.

[19] The manufacturing method described in

[17] or

[18] involves suspension polymerization of an ethylene monomer containing vinyl chloride.

[20] Use of a polyvinyl alcohol polymer (A) described in any one of [1] to

[12] , wherein it is used as a dispersion stabilizer (dispersant).

[21] The use described in

[20] , wherein the dispersion stabilizer is a dispersion stabilizer for suspension polymerization.

[22] The use described in

[20] or

[21] , wherein the dispersion stabilizer is a dispersion stabilizer for suspension polymerization of an ethylene monomer containing vinyl chloride.

[23] The use of a polyvinyl alcohol polymer (A) described in any one of [1] to

[12] , wherein it is used for the polymerization of ethylene monomers.

[24] The use described in

[23] , wherein the polymerization is suspension polymerization.

[25] As described in

[23] or

[24] , wherein the polymerization is a suspension polymerization of an ethylene monomer comprising vinyl chloride. [Effects of the Invention] According to the present invention, a PVA (novel or specific PVA) or its use (dispersion stabilizer, etc.) may be provided. This type of PVA (dispersion stabilizer, etc.) has a specific hue (coloring properties). In particular, although this type of PVA (dispersion stabilizer, etc.) has unsaturated double bonds, it also has excellent hue. Furthermore, this type of PVA also possesses properties as a dispersing stabilizer. For example, it can exhibit excellent dispersing power (protective colloidal properties), achieve high polymerization stability, and obtain resins with excellent plasticizer absorption or color (such as vinyl chloride resins and other ethylene polymers). Therefore, it possesses the properties of such a dispersing stabilizer and can also simultaneously improve or inhibit the coloration of the resin. The following describes in detail the embodiments used to implement the present invention. However, the present invention is not limited to the embodiments described below. The polyvinyl alcohol-based polymers of the present invention (polyvinyl alcohol-based polymer (A), PVA-based polymer (A), PVA (A), PVA) particularly satisfy (sufficiently satisfy) the following requirement 1 and at least one of the following requirements 2 to 4. Requirement 1: Possesses polymerizable unsaturated bonds [e.g., polymerizable unsaturated bonds determined by bromine titration (detected or quantified by bromine titration)] (e.g., unsaturated double bonds). Requirement 2: The YI of a 4% by mass aqueous solution is 13 or less. Requirement 3: The YI is 18 or less. Requirement 4: The transmittance of a 1% by mass aqueous solution at 430 nm is 90% or more. Furthermore, this invention includes various uses of such polyvinyl alcohol-based polymers (polyvinyl alcohol-based polymer (A)), particularly dispersants (dispersion stabilizers, such as dispersion stabilizers for suspension polymerization) containing polyvinyl alcohol-based polymers (polyvinyl alcohol-based polymer (A)). In the aforementioned various uses [e.g., dispersants (dispersion stabilizers) such as dispersion stabilizers for suspension polymerization], one or more PVA-based polymers (A) may be used. Hereinafter, this invention will be described in detail. [Polyvinyl alcohol polymer (A)] The PVA polymer (A) satisfies one or more of the following requirements 1 and requirements 2 to 4: Requirement 1: Possesses polymerizable unsaturated bonds; Requirement 2: The YI of a 4% by mass aqueous solution is 13 or less; Requirement 3: The YI is 18 or less; Requirement 4: The transmittance of a 1% by mass aqueous solution at 430 nm is 90% or more. In requirement 1, polymerizable unsaturated bonds can be exemplified by, for example, double bonds (unsaturated double bonds), triple bonds (unsaturated triple bonds), etc. Polymerizable unsaturated bonds can usually be double bonds (especially carbon-carbon double bonds) [at least double bonds (especially carbon-carbon double bonds) can be included]. In the PVA-based polymer (A), the ratio of polymerizable unsaturated bonds can be selected from a range of 1 μmol / g or more (e.g., 2 μmol / g or more), preferably 3 μmol / g or more, and even more preferably 5 μmol / g or more (e.g., 8 μmol / g or more, 10 μmol / g or more, 12 μmol / g or more, 15 μmol / g or more, 20 μmol / g or more). In PVA-based polymers (A), there is no particular upper limit to the ratio of polymerizable unsaturated bonds, such as 3000 μmol / g or less, 2000 μmol / g or less, 1000 μmol / g or less, 800 μmol / g or less, 600 μmol / g or less, 500 μmol / g or less, 450 μmol / g or less, 400 μmol / g or less, etc. The ratio of polymeric unsaturated bonds can also be set as a range formed by appropriately combining the lower and upper limits of the above range (hereinafter, the description of the range is also set in the same way). In a representative example, the ratio of polymerizable unsaturated bonds in PVA-based polymers (A) can be 1–2000 μmol / g, preferably 3–1000 μmol / g, and even more preferably 5–500 μmol / g. In the PVA-based polymer (A) (the PVA-based polymer (A) that fully satisfies requirement 1), the ratio of polymerizable unsaturated bonds can fully satisfy the following skeleton ratio [for example, the ratio of the acetal skeleton (a) can be fully satisfied (for example, 0.001 mol% or more, 0.05 to 5 mol%, 0.1 to 3 mol%, 0.2 to 2 mol% etc. relative to each monomer unit). If the content of polymerizable unsaturated bonds (e.g., unsaturated double bonds) is within the range described above, the effects of the present invention can be easily and efficiently achieved. In particular, if the content of polymerizable unsaturated bonds (e.g., unsaturated double bonds) is not too low (e.g., 5 μmol / g or more), the following effects are easily achieved, and therefore preferred: when used in suspension polymerization, the polymerization stability is excellent, the adhesion of scale to the polymerization tank is suppressed, and the particle size of the resulting vinyl resin is not easily coarsened. On the other hand, if the content of polymerizable unsaturated bonds (e.g., unsaturated double bonds) is not too high (e.g., 500 μmol / g or less), the following effects are preferred: vinyl resins with high plasticizer absorption are easily obtained. The polymeric unsaturated bonds contained in PVA-based polymers (A) can be detected or quantified, for example, by bromine titration [to determine their content (content ratio)]. The method of bromine titration is not particularly limited; it utilizes the reaction of polymeric unsaturated bonds (unsaturated double bonds, etc.) contained in PVA-based polymers (A) with bromine, and calculates the content (μmol / g) of polymeric unsaturated bonds (unsaturated double bonds, etc.) contained in PVA-based polymers (A) from the mass (moles) of bromine reacting with these bonds. As described above, the PVA-based polymer (A) satisfies any one or more of the following requirements 2 to 4. Requirement 2: The YI of a 4% by mass aqueous solution is less than 18. Requirement 3: The YI is less than 13. Requirement 4: The transmittance of a 1% by mass aqueous solution at 430 nm is 90% or higher. In most cases, PVA-based polymers (A) fully satisfy requirements 2 and / or 3. In particular, it is preferable to satisfy any two or more of requirements 2 to 4, and even more preferably, to satisfy all of requirements 2 to 4. Requirement 2 specifies that the yellowness index (YI) of a 4% by mass aqueous solution of PVA-based polymer (A) is 18 or less, preferably 16 or less (e.g., 15, 13, or 12), and even more preferably 11 or less (e.g., 10, 9, or 8). If Requirement 2 (YI of the 4% by mass aqueous solution) is within the range described above, in addition to its excellent hue, it is preferable to use it as a dispersant and stabilizer to easily suppress or improve the coloring of the resulting vinyl resin. Furthermore, the YI of the 4% by mass aqueous solution of PVA-based polymer (A) can be measured, for example, at 20°C using a UV-Vis spectrophotometer (quartz cell with an optical path length of 10 mm), and calculations can be performed based on the obtained data. Requirement 3 specifies that the YI (yellowness, yellow index) of the PVA-based polymer (A) (in itself, in solid form) is 13 or less, preferably 12 or less, and even more preferably 11 or less (e.g., 10 or less), and can also be 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, etc. If Requirement 3 (the YI of itself, in solid form) is within the range described above, in addition to having excellent hue, it is preferable to use it as a dispersing stabilizer to easily suppress or improve the coloring of the resulting vinyl resin. Furthermore, the YI of the PVA-based polymer (A) can be obtained, for example, by measuring it with a colorimeter. In the measurement, the PVA-based polymer (A) is usually in powder form, and the measurement can be performed by appropriately pulverizing it or controlling the particle size according to its morphology. For example, the YI value can be obtained by measuring the test powder obtained by passing PVA [e.g., in powder form (such as that which has been pulverized)] through a metal wire mesh (sieve) with a mesh size of 0.5 mm (30 mesh) using a colorimeter. Requirement 4 specifies that the transmittance of a 1% by mass aqueous solution of PVA-based polymer (A) at 430 nm is 90% or higher (e.g., exceeding 90%), preferably 91% or higher (e.g., 92% or higher), and even more preferably 93% or higher (e.g., 94%, 95%, 96%, 97% or higher). If Requirement 4 (transmittance of a 1% by mass aqueous solution at 430 nm) is within the range described above, in addition to its excellent hue, it is preferable to use it as a dispersant and stabilizer to easily suppress or improve the coloring of the resulting vinyl resin. Furthermore, regarding the transmittance of a 1% by mass aqueous solution of PVA-based polymer (A) at 430 nm, for example, the transmittance of the 1% by mass aqueous solution at 430 nm can be measured at 20°C using a UV-Vis spectrophotometer (quartz cell with an optical path length of 20 mm, blank sample: pure water). The PVA-based polymer (A) is not particularly limited as long as it fully satisfies the above requirements. As a state having polymerizable unsaturated bonds (fully satisfying requirement 1), it may preferably contain an acetal backbone (a) having polymerizable unsaturated bonds (e.g., vinyl unsaturated double bonds). In this invention, even if it contains such polymerizable unsaturated bonds (ethylene unsaturated double bonds, etc.) [for example, an acetal skeleton (a) having polymerizable unsaturated bonds (ethylene unsaturated double bonds, etc.)], it is easy to achieve the following: excellent color, and when used for suspension polymerization, it is easy to obtain ethylene resins with excellent color. [Acetal skeleton (a)] In the acetal skeleton (a), there is no particular limitation on the number of polymerizable unsaturated bonds, as long as it is 1 or more (e.g., 1 to 5, 1 to 3, 1 to 2, 1, etc.). In the acetal skeleton (a), the acetal can be either a cyclic acetal or an acyclic (chain) acetal, preferably a cyclic acetal. A representative acetal skeleton with polymerizable unsaturated bonds contains the skeleton (structural unit) represented by the following formula (a1). Therefore, the acetal skeleton (a) may contain the skeleton represented by the following formula (a1). [Chemistry 2] (In the formula, R' represents a group with polymerizable unsaturated bonds) In formula (a1) above, R' is a group having a polymerizable unsaturated bond. R' can be the polymerizable unsaturated bond itself, or it can be a group containing a polymerizable unsaturated bond (e.g., a hydrocarbon group). Furthermore, in addition to the polymerizable unsaturated bond, the group having a polymerizable unsaturated bond can also have substituents. The substituents can be appropriately selected according to the type of group having a polymerizable unsaturated bond, and there is no particular limitation. Examples include: hydroxyl, halogen atom, acetyl, ester, alkoxy, nitro, substituted amino group, and groups different from the group that forms the base (e.g., aromatic groups such as aryl). Substituents can be substituted alone or in combination of two or more. Examples of groups having polymerizable unsaturated bonds [especially double bonds (ethylene double bonds)] include: groups having one polymerizable unsaturated bond {e.g., alkenyl [e.g., vinyl, allyl, propenyl (1-propenyl, 2-propenyl, etc.), butenyl, pentenyl, 6-methyl-5-hexenyl, decenyl, 2-(dimethylamino)vinyl, cyclohexenyl, 2-phenylvinyl, etc., having 2 or more carbon atoms (e.g., 2 to 30, preferably 2 to 14, and even more preferably 2 to 10 carbon atoms) hydrocarbon groups (which may have substituents)]}; and groups having two or more polymerizable unsaturated bonds {e.g., alkadienyl [e.g., 1,3- Alkadienyl groups (which may have substituents, such as alkyladienyl, 2,6-dimethyl-1,5-hexadienyl, cyclohexadienyl, propenylcyclohexenyl, etc., having 4 or more carbons (e.g., 4 to 30, preferably 4 to 14, and even more preferably 4 to 10)), alkyltrienyl groups (e.g., alkyltrienyl groups having 6 or more carbons (e.g., 6 to 30, preferably 6 to 24)), alkyltetraenyl groups (e.g., alkyltetraenyl groups having 8 or more carbons (e.g., 8 to 30, preferably 8 to 24)), alkylpentenyl groups (e.g., alkylpentenyl groups having 10 or more carbons (e.g., 10 to 30, preferably 10 to 24)), etc.) The acetal skeleton having polymerizable unsaturated bonds {e.g., the group represented by formula (a1) (or R'-< in formula (a1))} can be derived from the corresponding carbonyl compound (e.g., aldehydes, their acetals, ketones, etc.), especially aldehydes [e.g., R'CHO (R' is an aldehyde with a hydrocarbon group having polymerizable unsaturated bonds) etc.]. Furthermore, as mentioned above, carbonyl compounds may have substituents. Examples of such carbonyl compounds include: enaldehydes [e.g., acrolein, crotonaldehyde, methacrolein, 3-butenal, 3-methyl-2-butenal, 2-methyl-2-butenal, 2-pentenal, 3-pentenal, 4-pentenal, 2-hexenal, 3-hexenal, 4-hexenal, 5-hexenal, 2-ethylcrotonaldehyde, 2-methyl-2-pentenal, 3-(dimethylamino)propenal, 10-undecenal, myristaldehyde, palmaldehyde, olealdehyde, transolealdehyde, isolealdehyde, codaldehyde, sinenal, tetradecenal, linolenic acid, citronellol, cinnamaldehyde, etc., with 3 to 15 carbon atoms, preferably with 3 to 10 carbon atoms], alkyldienal [ Examples of unsaturated aldehydes (especially monoaldehydes) include 2,4-pentadienal, 2,4-hexadienal, 2,6-nonadienal, citral, perillaldehyde, etc., which have 5 to 15 carbon atoms, preferably 5 to 10 carbon atoms; alkanetrienal (e.g., linolenic acid aldehyde, tung oil aldehyde, etc., which have 7 to 30 carbon atoms, preferably 7 to 25 carbon atoms); alkanetetraenal (e.g., octadecanetetraenal, arachidonic acid, etc., which have 9 to 30 carbon atoms, preferably 9 to 25 carbon atoms); and alkanepental (e.g., eicosaponeal, etc., which have 11 to 30 carbon atoms, preferably 11 to 25 carbon atoms). Corresponding ketones and acetals are also included. Furthermore, when a carbonyl compound has isomers (such as cis-trans isomers), it includes any one of the isomers (such as both the cis and trans isomers). As mentioned above, acetals, which are condensations of aldehydes and alcohols, can also be used as carbonyl compounds. There are no particular limitations on acetals; for example, condensations with primary alcohols (such as methanol) can be cited. These carbonyl compounds can be used alone or in combination of two or more. Furthermore, from the perspective of water solubility, carbonyl compounds are preferably composed of monocarbonyl compounds (monoxalates, etc.). Even when using polycarbonyl compounds (such as dioxalates, etc.), in most cases, reducing their amount can ensure the level of water solubility. Furthermore, the acetal skeleton with polymerizable unsaturated bonds (e.g., the acetal skeleton represented by formula (a1) above) can be a skeleton that can be introduced via hydroxyl groups, or it can be an acetal skeleton introduced via, for example, two adjacent hydroxyl groups (e.g., the hydroxyl groups of a vinyl alcohol unit). For example, when using a carbonyl compound (aldehyde, ketone, etc.) with polymerizable unsaturated bonds, for example, by acetalizing the two adjacent OH groups in a PVA-based polymer with a carbonyl compound with polymerizable unsaturated bonds, a PVA-based polymer (A) containing an acetal skeleton (a) with polymerizable unsaturated bonds can be obtained. An acetal skeleton with polymerizable unsaturated bonds (such as the acetal skeleton represented by formula (a1) above) may or may not have ionic groups (ionic skeleton). PVA-based polymers (A) may contain an acetal skeleton with polymerizable unsaturated bonds alone or in combination of two or more acetal skeletons with polymerizable unsaturated bonds. In the PVA-based polymer (A), the content of the acetal backbone (a) [or polymerizable unsaturated bond, such as the backbone represented by formula (a1)] in the PVA-based polymer (A) can be selected from a range of 0.001 mol% or more (e.g., 0.005 mol% or more) relative to each monomer unit, for example, it can be 0.01 mol% or more, preferably 0.05 mol% or more, even more preferably 0.1 mol% or more, especially 0.2 mol% or more, and can be 10 mol% or less [e.g., 8 mol% or less (e.g., 5 mol% or less, 3 mol% or less), preferably 2 mol% or less, even more preferably 1 mol% or less]. Furthermore, as mentioned above, the ranges (upper and lower limits) can be appropriately combined to select a range (e.g., 0.01–3 mol%, 0.05–5 mol%). Specifically, the content of the acetal skeleton (a) (or polymeric unsaturated bond) in the PVA-based polymer (A) is, relative to each monomer unit, 0.05 to 5 mol%, preferably 0.1 to 3 mol%, and more preferably 0.2 to 2 mol%. Furthermore, the so-called 1 mole% content refers to the situation where, relative to the total number of monomer units (such as the total number of monomer units such as vinyl alcohol units, vinyl ester units, etc.), there is one acetal skeleton (a) (for example, the skeleton represented by formula (a1)). If the content is as described above, the performance as a dispersant (dispersion stabilizer) can be achieved efficiently (e.g., excellent polymerization stability, and efficient acquisition of vinyl chloride resins with appropriate average particle size or excellent plasticizer absorption). Furthermore, by setting the upper limit value to a level that is not too high, it is easy to improve the preparability or storage stability of the aqueous solution and its dispersibility in warm water. Furthermore, there are no particular limitations on the method for determining the content of the acetal skeleton (a); for example, NMR can be used. As a specific example, PVA-based polymers (A) can be dissolved in d6-DMSO solvent, and the content can be determined by... 1 H-NMR was used to analyze the signal from the polymerizable unsaturated bonds (such as vinyl double bonds) in the acetal skeleton (a) to determine the content of the acetal skeleton (a). The PVA-based polymer (A) may contain an ionic framework (b). If the PVA-based polymer (A) contains an ionic framework (b), PVA aqueous solutions can be easily prepared, and the stability of the PVA aqueous solution (and consequently its dispersibility in warm water) is improved. When the PVA aqueous solution is stored in a tank or the like, or when the PVA aqueous solution is added to a polymerizer (warm water at 40-70°C), the PVA will not precipitate in the polymerizer. Furthermore, by introducing the ionic framework (b), such effects can be achieved [e.g., excellent dispersibility (e.g., dispersibility relative to water or warm water)]. However, on the other hand, if only the ionic framework (b) is introduced, there is a risk of color deterioration. In this invention, even if the ionic framework (b) is introduced in this way, at least one of the requirements (2) to (4) can be fully satisfied, and excellent dispersibility can be achieved, as well as excellent color, etc., with high efficiency. [Ionic framework (b)] The ionic framework (b) has ionic groups. Examples of ionic groups include: anionic groups {e.g., acid groups [e.g., carboxyl groups, sulfonic acid groups (-SO)]} 3H), phosphate groups, etc., cationic groups [e.g., amino groups, ammonium (ammonium cations)], and their salts (the groups formed by these salts). As a salt, its properties depend on whether it is anionic or cationic. Examples include: metal salts (such as alkali or alkaline earth metals (such as lithium, sodium, potassium, magnesium, and calcium salts)) and halides (such as chlorides, bromides, and iodides). When the ionic group is a polybasic acid, the salt can be a single (same) salt or a combination of two or more salts. Among these plasma groups, acid groups (especially carboxyl groups and sulfonic acid groups) and their salts {acid group salts, such as carboxylates [e.g., -COOM (M is an alkali metal such as sodium (or its cation))], sulfonates [e.g., -SO]} 3M (M is an alkali metal such as sodium (or its cation))]}. The ionic skeleton (b) is not particularly limited in its state as long as it has an ionic group. Examples include: (1) an acetal skeleton (acetal group, acetal unit) with an ionic group, (2) a skeleton corresponding to (or from) a monomer with an ionic group, and (3) a skeleton corresponding to (or from) other compounds that can introduce ionic groups {e.g., chain transfer agents with ionic groups [e.g., alcohols, carbonyl compounds (aldehydes, ketones, especially aldehydes), thiols, etc.]}. Furthermore, an ionic framework (b) (one ionic framework (b)) only needs to have one or more ionic groups, or it can have two or more. The PVA-based polymer (A) may have the plasma backbone (b) alone or in combination of two or more of the plasma backbones (b). In the PVA-based polymer (A), the content (ratio, percentage) of the ionic backbone (b) (e.g., the backbones (1), (2) and / or (3)) relative to each monomer unit can be selected from a range of 0.001 mol% or more (e.g., 0.005 mol% or more), for example, it can be 0.01 mol% or more, preferably 0.03 mol% or more, and even more preferably 0.05 mol% or more, and can be 10 mol% or less [e.g., 8 mol% or less (e.g., 5 mol% or less, 3 mol% or less), preferably 2 mol% or less, and even more preferably 1 mol% or less]. Specifically, the content of the ionic framework (b) (e.g., the frameworks (1), (2) and / or (3) above) relative to each monomer unit may be 0.01 to 5 mol%, preferably 0.03 to 2 mol%, and even more preferably 0.05 to 1 mol%. Furthermore, the content of 1 mole% refers to the situation where, relative to the total number of monomer units (such as the total number of monomer units such as vinyl alcohol units, vinyl ester units, etc.), there is one ionic skeleton (b) (such as the skeletons (1), (2) and / or (3) mentioned above). If the content is as described above, it is easy to improve the preparability or storage stability of aqueous solutions of PVA-based polymers (A), as well as their dispersibility in warm water. Furthermore, by setting the upper limit value to a level that is not too high, the performance of PVA-based polymers (A) as dispersants and stabilizers can be achieved efficiently (e.g., excellent polymerization stability, and the efficient acquisition of vinyl chloride-based resins with appropriate average particle size or plasticizer absorption). Furthermore, the method for determining the content of ionic groups can be selected based on the type of skeleton containing ionic groups, and there are no particular limitations. For example, it can be determined using NMR, titration, UV absorbance, etc. As a specific example, the skeleton content represented by the following formula (b1-1) can be determined by dissolving the PVA-based polymer (A) in d6-DMSO solvent, using... 1 H-NMR is used to determine the content of carboxyl groups by analyzing signals from substituents (e.g., hydrogen) on the benzene ring. Alternatively, the PVA-based polymer (A) can be completely saponified, and the sample after Soxhlet extraction (e.g., removal of sodium acetate) can be dissolved in water. A small amount of sodium hydroxide (NaOH) is added, and conductivity titration is performed using dilute hydrochloric acid. The amount of carboxyl groups can then be determined from the titration volume of the hydrochloric acid. Furthermore, when the acetal skeleton with ionic groups has a structure that absorbs UV (ultraviolet light), the content of the acetal skeleton with ionic groups can be determined by measuring the UV absorbance of an aqueous solution containing the PVA-based polymer (A). Furthermore, in the PVA-based polymer (A), the content of the acetal backbone (a) (content per monomer unit) relative to the ionic backbone (b) (e.g., the aforementioned backbones (1), (2) and / or (3)) is 1 mol, which may be 50 mol or less (e.g., 30 mol or less, 20 mol or less), preferably 15 mol or less, and even more preferably 10 mol or less, and may be 0.05 mol or more (e.g., 0.1 mol or more, 0.5 mol or more), preferably 1 mol or more, even more preferably 2 mol or more, and especially 3 mol or more. If this ratio is used, it is easy to balance the excellent preparation and storage stability of the aqueous solution with its excellent performance as a dispersant and stabilizer. Furthermore, the introduction of the ionic backbone (b) can be carried out using known methods depending on its state. Examples of such introduction methods include: (1) acetalizing a PVA-based polymer (sometimes called PVA-based polymer (C)) with a carbonyl compound (aldehyde, acetal, ketone, etc., especially aldehyde) having an ionic group, thereby obtaining a PVA-based polymer (B-1) having an ionic group; (2) obtaining a polyethylene ester-based polymer having an ionic group by copolymerizing a monomer having an ionic group with vinyl ester, and then saponifying it, thereby obtaining a PVA-based polymer (B-3) having an ionic group; (3) saponifying a polyethylene ester-based polymer (having an ionic group) obtained by polymerizing vinyl ester in the presence of a chain transfer agent (alcohol, aldehyde, thiol, etc.) having an ionic group, thereby obtaining a PVA-based polymer (B-4) having an ionic group. The following is a detailed description of the various states of the ionic framework (b). ((1) Acetal skeleton with ionic group) As described above, the ionic group is present in the acetal skeleton (acetal group, acetal unit) (substitution on the acetal skeleton). Acetals can be either cyclic acetals or acyclic (chain) acetals, with cyclic acetals being preferred. A representative acetal skeleton with ionic groups contains the skeleton (structural unit) represented by the following formula (b1). Therefore, an acetal skeleton with ionic groups may contain the skeleton represented by the following formula (b1). [Chemistry 3] (In the formula, R represents a radical with ionic properties) In the above formula (b1), R is a base having an ionic radical. R can be the ionic radical itself, or it can be a linker group having an ionic radical (a base consisting of an ionic radical and a linker group that replaces the ionic radical). As a linking group (becoming a base group), examples include hydrocarbon groups. Examples of hydrocarbon groups include: aliphatic hydrocarbon groups [e.g., alkyl groups [e.g., chain alkyl groups (e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, etc. C]]. 1-30 Alkyl), cycloalkyl (e.g., cyclopentyl, cyclohexyl, etc. C 3-10 Cycloalkyl and other saturated aliphatic hydrocarbon groups], aromatic hydrocarbon groups [e.g. aryl (e.g. phenyl, tolyl, xylyl, naphthyl, etc. C] 6-20 Aryl), aralkyl (e.g., benzyl, phenethyl, etc. C 6-20 Aryl-C 1-4 Alkyl groups, etc. In addition to ionic groups, the linking group (hydrocarbon group) may also have substituents (substituents that are not ionic groups). There are no particular limitations on substituents; examples include: hydroxyl groups, halogen atoms, acetyl groups, ester groups, alkoxy groups, nitro groups, and groups different from the base group (e.g., aromatic groups such as aryl groups). Substituents can substitute alone on the linking group (hydrocarbon group) or in combination of two or more substituents on the linking group (hydrocarbon group). In a linker group (such as a hydrocarbon group) with an ionic group, the number of ionic groups only needs to be one or more, or two or more ionic groups can be substituted on the linker group. As a specific acetal skeleton with an ionic group (the skeleton represented by formula (b1)), examples include: the skeleton of R in the above formula (b1) being an ionic group (e.g., a carboxyl group and its salts), and the skeleton represented by the following formula (b1-1). [Chemistry 4] (where R is in the formula) 1~R 5 represents a hydrogen atom or a substituent. Wherein, R... 1~R (At least one of the 5 is an ionic group) In the above formula (b1-1), the examples shown above can be used as ionic groups and substituents. R 1~R At least one of the five groups is an ionic group, preferably any one of them can be an ionic group. For example, it could be R. 1~R One of the five is an ionic group (such as a carboxyl group, a sulfonic acid group, or a salt thereof), and the other four are hydrogen atoms. Furthermore, the acetal skeleton with an ionic group {e.g., the group represented by formula (b1) (or R-< in formula (b1))} can be derived from the corresponding carbonyl compound (e.g., aldehyde, its acetal, ketone, etc.), especially aldehydes (e.g., RCHO). Furthermore, the carbonyl compound may have substituents. Examples of such carbonyl compounds include: alkaloids having an ionic group (e.g., glyoxylic acid, methylacetic acid, methylpropionic acid, and their salts), aromatic carbon aldehydes having an ionic group (e.g., methylbenzoic acid (e.g., 4-methylbenzoic acid), methylbenzenesulfonic acid (e.g., 2-methylbenzenesulfonic acid, 4-methylbenzene-1,3-disulfonic acid), their salts), etc.), aldehydes (especially monoaldehydes), and corresponding ketones, acetals, etc. Furthermore, when the carbonyl compound contains isomers (e.g., cis-trans isomers), it includes any one of the isomers (e.g., both the cis and trans isomers). Furthermore, as long as an ionic group can be formed in the PVA-based polymer (A), the ionic group in the carbonyl compound can be derivatized (esterified, anhydrinated, etc.). For example, even if it is an ester (e.g., an alkyl ester) or an anhydride, it can also be used as long as it can form (e.g., by hydrolysis) the corresponding acid group (carboxyl group, sulfonic acid group) or its salt in the PVA-based polymer (A) (the same applies to ionic groups below). These carbonyl compounds can be used alone or in combination of two or more. Furthermore, from the perspective of water solubility, carbonyl compounds are preferably composed of monocarbonyl compounds (monoxalates, etc.). Even when using polycarbonyl compounds (such as dioxalates, etc.), in most cases, reducing their amount can ensure the level of water solubility. Furthermore, the acetal skeleton with ionic groups (e.g., the acetal skeleton represented by formula (b1) above) can be a skeleton that can be introduced via hydroxyl groups, or it can be an acetal skeleton introduced via, for example, two adjacent hydroxyl groups (e.g., the hydroxyl groups of a vinyl alcohol unit). For example, when using carbonyl compounds (aldehydes, ketones, etc.) with ionic groups, for example, by acetalizing the two adjacent OH groups in a PVA-based polymer with a carbonyl compound with ionic groups, a PVA-based polymer (A) containing an acetal skeleton with ionic groups can be obtained. An acetal skeleton with ionic groups (such as the acetal skeleton represented by formula (b1) above) may or may not have polymerizable unsaturated bonds. PVA-based polymers (A) may contain an acetal skeleton with ionic groups or may not contain an acetal skeleton with ionic groups. PVA-based polymers (A) may contain an acetal skeleton with ionic groups alone or in combination of two or more acetal skeletons with ionic groups. There are no particular limitations on the method of introducing (introducing) an acetal skeleton with ionic groups (e.g., an acetal skeleton with carboxyl groups, sulfonic acid groups or salts thereof) into a PVA-based polymer (A), and conventional methods can be used. In a representative method, the PVA-based polymer (C) can be acetalized using a carbonyl compound (aldehyde, acetal, ketone, etc.) with an ionic group, as described below. Furthermore, by acetalizing the PVA-based polymer (C) using a carbonyl compound with an ionic group in this manner, a PVA-based polymer (B-1) with an ionic group can be obtained. During acetalization, the carbonyl compound with polymerizable unsaturated bonds coexists, thereby obtaining a PVA-based polymer (A) that simultaneously possesses an acetal backbone (a) and an ionic group, which is therefore preferable. Examples of aldehydes with ionic groups include glyoxylic acid, 2-methylbenzoic acid, 4-methylbenzoic acid, sodium 2-methylbenzenesulfonate, sodium 4-methylbenzenesulfonate, disodium 4-methylbenzene-1,3-disulfonic acid, etc., with 4-methylbenzoic acid or sodium 2-methylbenzenesulfonate being more preferred. As mentioned above, acetals, which are condensations of aldehydes and alcohols, can also be used as carbonyl compounds. There are no particular limitations on acetals; for example, condensations with primary alcohols (such as methanol) can be cited. Carbonyl compounds can be used alone or in combination of two or more. ((2) Corresponding to the skeleton of a monomer with ionic groups) There are no particular limitations on the monomer with ionic groups, and it can be appropriately selected according to the type of ionic groups. Specific examples of monomers include: monomers with acid groups [e.g., monomers with carboxyl groups [e.g., monocarboxylic acids (e.g., aliphatic unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and butenoic acid), polycarboxylic acids (e.g., aliphatic unsaturated dicarboxylic acids such as iconic acid, maleic acid, and trans-butenoic acid), and their salts], monomers with sulfonic acid groups [e.g., alkenyl sulfonic acids (e.g., vinyl sulfonic acid and allyl sulfonic acid), alkenyl aromatic sulfonic acids (e.g., styrene sulfonic acid), acetylamine monomers with sulfonic acid groups (e.g., 2-acrylamide-2-methylpropanesulfonic acid), and their salts]], and monomers with other ionic groups [e.g., monomers with amino groups (e.g., (meth)acrylamide-propyl dimethylamine), and their salts], etc. Furthermore, as mentioned above, if ionic groups can be introduced into the PVA-based polymer (A) (e.g., through hydrolysis, ultimately forming ionic groups in the PVA-based polymer (A)), then these can also be derivatives [e.g., acid anhydrides (e.g., maleic anhydride), esters (e.g., alkyl esters)]. In other words, such derivatives can also be called derivatives capable of forming ionic groups. As a specific example, even when using acrylates, it is sufficient to introduce acrylic acid or its salts into the final PVA-based polymer (A). Therefore, acrylates can introduce a backbone corresponding to acrylic acid or its salts into the PVA-based polymer (A). These monomers can be used alone or in combination of two or more. As described above, for example, by saponifying a polyethylene ester polymer containing ionic groups obtained by copolymerizing such a monomer with ionic groups and vinyl ester, a PVA polymer containing ionic groups (B-3) can be obtained. ((3) Corresponding to the skeleton of other compounds capable of introducing ionic groups) In such a skeleton (3), as compounds capable of introducing ionic groups, examples as described above include: alcohols having ionic groups, carbonyl compounds having ionic groups (aldehydes, ketones, especially aldehydes), thiols having ionic groups, etc. These can generally function as chain transfer agents. Among these, thiols are preferred from the perspective of their higher chain transferability (and therefore easier introduction of ionic groups). Examples of thiols having ionic groups include: thiols having acid groups [e.g., thiols having carboxyl groups [e.g., thiols with thiol saturated fatty acids such as 3-mercaptopropionic acid, thiosuccinic acid, etc.], thiols having sulfonic acid groups [e.g., thioalkylsulfonic acids such as 3-mercapto-1-propanesulfonic acid], and their salts (e.g., sodium 3-mercapto-1-propanesulfonate)]. As described above, for example, by polymerizing ethylene esters in the presence of a chain transfer agent (alcohol, aldehyde, thiol, etc.) with ionic groups, ionic groups from the chain transfer agent can be introduced into the ends of the ethylene ester polymer. Subsequently, by saponifying the ethylene ester polymer, a PVA-based polymer (B-4) containing ionic groups at the ends can be obtained. Furthermore, PVA-based polymers (A) may have other acetal skeletons (acetal groups, acetal units) that do not fall under the category of acetal skeletons (a) or acetal skeletons with ionic groups. Examples of such other acetal skeletons include those in formula (a1) where R' is a group without ionic groups and polymerizable unsaturated bonds (e.g., aliphatic groups, aromatic groups, etc.). Examples of such groups include: aliphatic groups [e.g., alkyl groups (e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, etc. C] 1-30Alkyl), cycloalkyl (e.g., cyclopentyl, cyclohexyl, etc. C 3-20 cycloalkyl groups (e.g., aryl groups, such as phenyl, naphthyl, etc.) and aromatic groups (e.g., aryl groups, such as phenyl, naphthyl, etc.). 6-20 Aryl groups, etc. There are no particular limitations on the method of introducing other acetal skeletons; conventional methods can be used, such as acetalizing PVA-based polymers (C) by using an aldehyde corresponding to the other acetal skeleton. Furthermore, in this method, other acetal skeletons are usually formed from two adjacent vinyl alcohol units. Examples of such aldehydes include: alkane aldehydes [e.g., acetaldehyde, propionaldehyde, butyraldehyde, pentanal, hexanal, heptanal, octanal, nonanal, decanal, undecaldehyde, dodecaldehyde, 2-methylbutanal, 2-ethylbutanal, 2-methylpentanal, 2-ethylhexanal], cycloalkane carbon aldehydes [e.g., cyclopentane carboxyl aldehyde (cyclopentane carbon aldehyde), cyclohexane carboxyl aldehyde (cyclohexane carbon aldehyde) etc.] and other aliphatic aldehydes, or aromatic carbon aldehydes (e.g., benzaldehyde, naphthaldehyde etc.) and other aromatic aldehydes. The PVA-based polymer (A) has at least vinyl alcohol units and may have vinyl alcohol units and unhydrolyzed (saponified) units [e.g., vinyl ester units (or units derived from vinyl ester monomers, such as vinyl acetate units, etc.)]. Furthermore, PVA-based polymers (A) may also have other units (vinyl alcohol units, unhydrolyzed units, acetal skeletons (a), ionic skeletons (b), etc., other than those exemplified above) as needed. Examples of such units include units from other monomers exemplified in the section on PVA-based polymers (C) below. The degree of saponification of the PVA-based polymer (A) can be, for example, 20 mol% or more (e.g., 25 mol% or more), preferably 30 mol% or more (e.g., 35 mol% or more), and even more preferably 40 mol% or more (e.g., 45 mol% or more), especially 50 mol% or more (e.g., 55 mol% or more, 60 mol% or more). The upper limit of the degree of saponification of PVA-based polymer (A) may be, for example, 95 mol% or less (e.g., 93 mol% or less), preferably 90 mol% or less (e.g., 88 mol% or less), and even more preferably 85 mol% or less (e.g., 80 mol% or less). Specifically, the degree of saponification of the PVA-based polymer (A) can be, for example, 20 to 90 mol% (e.g., 50 to 90 mol%), preferably 55 to 85 mol%, and even more preferably 60 to 80 mol%. If the degree of saponification is not too low, the aqueous solution exhibits excellent preparation and storage stability, as well as good dispersibility in warm water, making it superior. If the degree of saponification is not too high, it is superior in terms of easily exerting its excellent properties as a dispersant (e.g., excellent polymerization stability, and the ability to efficiently obtain vinyl chloride resins with appropriate average particle size or high plasticizer absorption). Furthermore, the degree of saponification can be determined, for example, by the method for determining the degree of saponification of PVA as specified in JIS K 6726. When the PVA-based polymer (A) has vinyl ester units, the ratio of the ionic backbone (b) (in terms of monomer unit units) relative to 100 mol of vinyl ester units can be 10 mol or less, preferably 5 mol or less, and even more preferably 3 mol or less, and can be 0.01 mol or more (e.g. 0.05 mol or more, 0.1 mol or more), preferably 0.2 mol or more, and even more preferably 0.3 mol or more. If this ratio is used, it is easy to balance the excellent preparation and storage stability of the aqueous solution with its excellent performance as a dispersant and stabilizer. The viscosity (20°C) of a 4% by mass aqueous solution of PVA-based polymer (A) is not particularly limited. For example, it can be selected from the range of 1 mPa·s or more (e.g., 1.5 mPa·s or more), preferably 2 mPa·s or more (e.g., 2.2 mPa·s or more), more preferably 2.5 mPa·s or more (e.g., 2.7 mPa·s or more), and even more preferably 3 mPa·s or more (e.g., 3.2 mPa·s or more, 3.4 mPa·s or more, 3.6 mPa·s or more), etc. There is no particular limitation on the upper limit of the viscosity (20°C) of a 4% by mass aqueous solution of PVA-based polymer (A). For example, it can be selected from the range of 2000 mPa·s or less (e.g., 1500 mPa·s or less, 1000 mPa·s or less, 500 mPa·s or less). Representatively, it can be selected from the range of 300 mPa·s or less (e.g., 250 mPa·s or less, 200 mPa·s or less, 150 mPa·s or less, 120 mPa·s or less, 100 mPa·s or less, 80 mPa·s or less, 50 mPa·s or less). It can also be 30 mPa·s or less (e.g., 20 mPa·s or less), preferably 15 mPa·s or less (e.g., 10 mPa·s or less), and even more preferably 9 mPa·s or less (e.g., 8 mPa·s or less). Specifically, the viscosity (20°C) of a 4% by mass aqueous solution of PVA-based polymer (A) can be, for example, 1–500 mPa·s (e.g., 2–300 mPa·s, 1–100 mPa·s, 2–100 mPa·s, 2.5–30 mPa·s), or less than 20 mPa·s (e.g., 3–15 mPa·s, 3.2–10 mPa·s, 3.4–9 mPa·s, 3.6–8 mPa·s). There is no particular limitation on the (average) degree of polymerization of the PVA-based polymer (A), for example, it can be 100 or more (e.g., 120 or more), preferably 150 or more (e.g., 160 or more), and even more preferably 180 or more (e.g., 200 or more, 220 or more, 250 or more, 280 or more, 300 or more), etc. There is no particular limitation on the upper limit of the (average) degree of polymerization of the PVA-based polymer (A). For example, it can be selected from the range of less than 10,000 (e.g., less than 8,000, less than 5,000), less than 3,000 (e.g., less than 2,500), preferably less than 2,000 (e.g., less than 1,500), and even more preferably less than 1,000 (e.g., less than 800). Specifically, the (average) degree of polymerization of the PVA-based polymer (A) can be, for example, 120 to 3000 (e.g., 200 to 2000), preferably 250 to 1500, and even more preferably 300 to 1000. If the viscosity or degree of polymerization of a 4% by mass aqueous solution of PVA-based polymer (A) is not too low, it is advantageous in terms of polymerization stability, inhibition of scale adhesion, and inhibition of coarsening of the resulting ethylene-based resin. Furthermore, if the viscosity or degree of polymerization of a 4% by mass aqueous solution is not too high, it is advantageous in terms of the ease of preparation and storage stability of the aqueous solution, and excellent dispersibility in warm water. Furthermore, the viscosity of a 4% by mass aqueous solution (20°C) can be determined, for example, by the method specified in JIS K 6726. Also, the degree of polymerization can be determined, for example, by the method specified in JIS K 6726, or it can be a calculated (converted) value based on other analytical methods [for example, a calculated (converted) value based on the viscosity of a 4% by mass aqueous solution]. The cloud point of a 4% by mass aqueous solution of PVA-based polymer (A) is preferably above 20°C (e.g., above 20°C, above 22°C, above 23°C, above 24°C, above 25°C), more preferably above 27°C, and may also be above 30°C. There is no particular upper limit to the cloud point of a 4% by mass aqueous solution of PVA-based polymer (A), and it can be, for example, 75℃, 70℃, 65℃, 60℃, 55℃, 50℃, etc. For example, the cloud point of a 4% by mass aqueous solution of PVA-based polymer (A) can be between 25℃ and 50℃. If this is the type of turbidity point, then the aqueous solution has excellent preparability or storage stability. Furthermore, the cloud point of a 4% by mass aqueous solution can be adjusted by the degree of saponification, degree of polymerization, and content of ionic skeleton (ionic group) of the PVA-based polymer (A). [Aqueous Liquid] PVA-based polymer (A) can be used directly as a dispersing stabilizer (dispersant), or it can be used in the form of an aqueous liquid dissolved in water. The aqueous liquid of the present invention only needs to contain PVA-based polymer (A) and water. For example, the aqueous liquid is formed by dispersing or dissolving PVA-based polymer (A) in water as a dispersing phase. In aqueous solutions, the content of PVA-based polymers (A) is not particularly limited. For example, it can be 1% or more by mass (e.g., 2% or more by mass, 3% or more by mass) or less than 80% by mass (e.g., less than 70% by mass, less than 60% by mass, less than 50% by mass, less than 40% by mass, less than 30% by mass). The aqueous solution of this invention has good stability. From the perspective of improving stability, aqueous solutions can contain water-soluble organic solvents. Examples of water-soluble organic solvents include: alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, and isobutanol; esters such as methyl acetate and ethyl acetate; and glycol derivatives such as ethylene glycol, ethylene glycol monomethyl ether, and ethylene glycol monoethyl ether. Furthermore, two or more of these organic solvents can be mixed for use. In cases where water-soluble organic solvents are included, the ratio of water-soluble organic solvents to the total solvent may be, for example, 70% by mass or less (e.g., 60% by mass or less), preferably 50% by mass or less, and even more preferably 30% by mass or less. In particular, from the viewpoint of environmental considerations or improved workability, the content of organic solvents relative to the total solvent or aqueous liquid is preferably 5% by mass or less. [Manufacturing Method] In this invention, there is no particular limitation on the method of manufacturing PVA-based polymer (A). For example, PVA-based polymer (C) can be acetalized by using aldehydes or the like that have polymerizable unsaturated bonds, thereby obtaining PVA-based polymer (A-1). When the PVA-based polymer (A) contains ionic groups, a carbonyl compound with polymerizable unsaturated bonds (e.g., a monoaldehyde) and a carbonyl compound with ionic groups (e.g., an aldehyde) are used to simultaneously acetalize the PVA-based polymer (C) (which does not have ionic groups), thereby obtaining a PVA-based polymer (A-2) containing ionic groups. Furthermore, a PVA-based polymer (B-3 or B-4) with ionic groups is acetalized using a carbonyl compound with polymerizable unsaturated bonds (e.g., a monoaldehyde), thereby obtaining PVA-based polymers (A-3, A-4) containing ionic groups. Therefore, the steps for manufacturing PVA-based polymers (A-1, A-2, A-3, A-4) can be divided into, for example, the steps of manufacturing PVA-based polymer (C) or PVA-based polymers with ionic groups (B-3 or B-4), and the step of acetalizing any of these PVA-based polymers (acetalization step). There are no particular limitations on the method for manufacturing PVA-based polymers (C) or PVA-based polymers containing ionic groups (B-3 or B-4), and previously known methods may be used. The following details the PVA-based polymers (B-3 or B-4) with ionic groups and PVA-based polymer (C), as well as the acetalization step. [PVA-based polymers (B-3), (B-4) and (C)] There is no particular limitation on PVA-based polymer (C). For example, PVA-based polymers obtained by saponifying (reacting) vinyl ester polymers [saponification of vinyl ester polymers (polymers with vinyl ester monomers as polymerizing components)] can be used. Furthermore, PVA-based polymers (B-3) and (B-4) can be obtained, for example, by using other monomers containing monomers with ionic groups and chain transfer agents containing chain transfer agents with ionic groups in the manufacture of PVA-based polymer (C) as described below. This vinyl ester polymer can be obtained by polymerizing at least a vinyl ester monomer (polymerizing it as a polymerizing component). There is no particular limitation on the polymerization method; previously known methods can be used, such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. However, considering the control of the degree of polymerization or the saponification reaction after polymerization, solution polymerization using methanol as a solvent or suspension polymerization using water or a water / methanol mixture as a dispersion medium is preferred, but not limited to these methods. There are no particular limitations on the vinyl ester monomers that can be used in the above polymerization. Examples include vinyl acetate, vinyl formate, vinyl propionate, vinyl octanoate, vinyl kappaate, and other fatty acid vinyl esters. One or more of these vinyl ester monomers can be used. Among them, vinyl acetate is preferred from an industrial point of view. In the polymerization of vinyl ester monomers, as long as the effects of this invention are achieved, vinyl ester monomers can be copolymerized with other monomers. In other words, the polymerization composition of vinyl ester polymers can include vinyl ester monomers and other monomers. There are no particular limitations on the other monomers that can be used; examples include: α-olefins (e.g., ethylene, propylene, n-butene, isobutene, etc.), (meth)acrylic acid and its salts, (meth)acrylates [e.g., alkyl (meth)acrylates (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tributyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, octadecyl (meth)acrylate, etc., C64 (meth)acrylate] 1-20 Alkyl esters, etc.; (meth)acrylamide; (meth)acrylamide derivatives (e.g., N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-hydroxymethyl(meth)acrylamide, etc.); vinyl ethers (e.g., methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tributyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, etc.). 1-20 Alkyl vinyl ethers, nitrile compounds (e.g., acrylonitrile, methacrylonitrile), vinyl halide compounds (e.g., vinyl chloride, vinyl fluoride), vinylidene dihaloethylene compounds (e.g., vinylidene chloride, vinylidene fluoride), allyl compounds (e.g., allyl acetate, allyl chloride), vinyl silane compounds (e.g., vinyltrimethoxysilane), and fatty acid alkenyl esters (e.g., isopropyl acetate). One or more of these other monomers may be used. Here, by using other monomers containing monomers with ionic groups as other monomers, PVA-based polymers (B-3) can be obtained. Examples of ionic monomers include those listed above, such as monomers with acid groups [e.g., monomers with carboxyl groups [e.g., monocarboxylic acids (e.g., aliphatic unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and butenoic acid), polycarboxylic acids (e.g., aliphatic unsaturated dicarboxylic acids such as iconic acid, maleic acid, and fumaric acid), and their salts], monomers with sulfonic acid groups [e.g., alkenyl sulfonic acids (e.g., vinyl sulfonic acid and allyl sulfonic acid), alkenyl aromatic sulfonic acids (e.g., styrene sulfonic acid), acetylamine monomers with sulfonic acid groups (e.g., 2-acrylamide-2-methylpropanesulfonic acid), and their salts], and monomers with other ionic groups [e.g., monomers with amino groups (e.g., (meth)acrylamide-propyl dimethylamine), and their salts], etc. When using other monomers, the content of the other monomers can be appropriately selected according to the monomers used, for example, relative to the total amount of polymer components, such as 0.1 to 20% by mass. Furthermore, during the polymerization of vinyl ester monomers, chain transfer agents can be coexisting to adjust the degree of polymerization of the resulting vinyl ester polymer. There are no particular limitations on chain transfer agents; examples include aldehydes such as acetaldehyde, propionaldehyde, butyraldehyde, and benzaldehyde; ketones such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; thiols such as 2-hydroxyethanethiol, dodecyl mercaptopropionic acid, mercaptosuccinic acid, and sodium 3-mercapto-1-propanesulfonate; and organohalogens such as carbon tetrachloride, trichloroethylene, and perchloroethylene. According to the inventors' research, if aldehydes or organohalides, which are commonly used chain transfer agents, are used, there is a tendency for the color to easily deteriorate (colorization) (the reason is not yet determined, but it is presumed that structures that cause color deterioration (colorization) are easily introduced into the terminal portions, etc.). Therefore, it is ideal that even when using aldehydes or organohalides, the requirements 2 to 4 are sufficiently met. Here, by using a chain transfer agent containing a chain transfer agent with ionic groups as a chain transfer agent, a PVA-based polymer (B-4) can be obtained. Examples of chain transfer agents having ionic groups include the above-mentioned examples, such as alcohols having ionic groups, carbonyl compounds having ionic groups, thiols having ionic groups {e.g., thiols having acid groups [e.g., thiols having carboxyl groups [e.g., thiols with thiol groups (e.g., thiols with thiol groups such as 3-mercaptopropionic acid, thiosuccinic acid, etc.], thiols having sulfonic acid groups [e.g., thioalkylsulfonic acids (e.g., 3-mercapto-1-propanesulfonic acid)], and their salts (e.g., sodium 3-mercapto-1-propanesulfonate)]}, etc. The amount of chain transfer agent added is determined based on the chain transfer constant of the added chain transfer agent and the degree of polymerization of the target vinyl ester polymer. Ideally, it should be 0.1 to 10 by mass relative to the total amount of polymerizing components. By subjecting the ethylene ester polymer obtained in the above manner to a saponification reaction, PVA-based polymers (C) (and further (B-3), (B-4)) can be manufactured. There are no particular limitations on the method for saponification of vinyl ester polymers, and previously known methods can be followed. For example, alcoholysis or hydrolysis reactions using acidic catalysts such as alkali metal hydroxides (sodium hydroxide, potassium hydroxide, etc.), inorganic acids (hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, etc.), and organic acids (formic acid, acetic acid, oxalic acid, p-toluenesulfonic acid, etc.) can be used. Examples of solvents used in the saponification reaction include: alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; and aromatic hydrocarbons such as benzene and toluene. These can be used alone or in combination of two or more. When a gel-like product precipitates as the saponification reaction proceeds, polyvinyl alcohol polymer can be obtained by pulverizing and drying the gel. It is preferable to neutralize any remaining catalyst before drying. As a neutralizing agent, acidic substances such as acetic acid or phosphoric acid are used when using an alkaline catalyst, and alkaline substances such as sodium hydroxide or potassium hydroxide are used when using an acidic catalyst. The drying of polyvinyl alcohol polymers (C) (and subsequently (B-3), (B-4)) can be carried out in a moderately oxidizing atmosphere in air or in an inert atmosphere (such as nitrogen). Furthermore, the drying temperature can be at room temperature (natural drying) or under heating or at high temperatures. From the viewpoint of efficient drying, temperatures typically above 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, and 70°C are commonly used. There is no upper limit to the drying temperature; for example, it can be 250°C, 220°C, 200°C, 180°C, 160°C, 150°C, 140°C, 130°C, 120°C, 110°C, 100°C, 90°C, and 80°C. According to the inventor's research, drying in an oxygen-rich atmosphere such as air or at high temperatures tends to cause color deterioration (coloring) (although the reason is not determined, it is presumed that structures that cause color deterioration (coloring) are easily introduced into the main chain). Therefore, it is ideal that even when drying is carried out in an oxidizing atmosphere, under heating, or at high temperatures, the requirements 2 to 4 are fully satisfied. From this perspective, drying is preferably carried out in an inert atmosphere, and also preferably at a temperature that is not too high (e.g., below 120°C, below 120°C, below 115°C, below 110°C, below 100°C, below 90°C, 70-110°C, etc.). Furthermore, there is no particular limitation on the drying time; it can be selected according to factors such as the drying temperature, for example, it can be 1 to 12 hours. [Acetalization] In this invention, the method for acetalizing PVA-based polymers [(C), (B-3), (B-4)] using a carbonyl compound (such as an aldehyde) having a polymerizable unsaturated bond or a carbonyl compound having an ionic group is not particularly limited, and known acetalization methods can be used. By acetalizing PVA-based polymer (C) using a carbonyl compound having a polymerizable unsaturated bond, PVA-based polymer (A-1) can be obtained. By acetalizing PVA-based polymer (C) using both a carbonyl compound having a polymerizable unsaturated bond and a carbonyl compound having an ionic group, PVA-based polymer (A-2) can be obtained. Furthermore, by acetalizing PVA-based polymers (B-3, B-4) having an ionic group using a carbonyl compound having a polymerizable unsaturated bond, PVA-based polymers (A-3, A-4) can be obtained. In acetalization, there is no particular limitation on the amount of carbonyl compound used. For example, it can be 0.05 to 50 parts by mass relative to 100 parts by mass of PVA polymer, preferably 0.1 to 20 parts by mass, and even more preferably 0.2 to 10 parts by mass. Furthermore, the acetalization reaction is preferably carried out in the presence of an acidic catalyst. There are no particular limitations on the acidic catalyst, and examples include inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid; and organic acids such as formic acid, acetic acid, oxalic acid, and p-toluenesulfonic acid. There is no particular limitation on the amount of acid catalyst used, but it is, for example, 0.1 to 10 parts by weight relative to 100 parts by weight of PVA-based polymer. Specific acetalization methods include, for example, the following: (i) using an alkaline catalyst such as sodium hydroxide to saponify a vinyl ester polymer in a solvent such as methanol to obtain a solution of a PVA polymer, then adding an aldehyde or the like and an acidic catalyst to perform acetalization, followed by neutralization with an alkaline substance to obtain a solution of PVA polymer (A); (ii) using an acidic catalyst as a saponification catalyst to saponify a vinyl ester polymer in a solvent such as methanol to produce a PVA polymer, then adding an aldehyde or the like, directly using the acidic catalyst used in the saponification reaction to perform an acetalization reaction, followed by neutralization with an alkaline substance to obtain a solution of PVA polymer (A); (iii) In the presence of an acidic catalyst and aldehydes, a vinyl ester polymer undergoes simultaneous saponification and acetalization reactions in a solvent, followed by neutralization with an alkaline substance to obtain a solution of PVA polymer (A); (iv) an aldehyde is added to an aqueous solution of the PVA polymer, and the reaction is carried out in the presence of an acidic catalyst, followed by neutralization with an alkaline substance to obtain an aqueous solution of PVA polymer (A); (v) an aldehyde is directly added to a slurry or powdered PVA polymer, or a liquid prepared by dissolving or dispersing aldehydes in an organic solvent or water is added, and the reaction is carried out in the presence of an acidic catalyst, followed by neutralization with an alkaline substance to remove excess solvent, thereby obtaining PVA polymer (A). In methods (i) to (iii), the solvent can be dried to obtain a solid form, or the solvent can be replaced with water to prepare an aqueous solution. In method (iv), PVA polymer (A) can be obtained in the form of an aqueous solution, and therefore can be directly used for suspension polymerization of vinyl chloride, etc. (v) The reaction method in the slurry state can obtain PVA-based polymers (A) in solid form, which is easy to operate. Furthermore, in the methods (i) to (v), there are no particular limitations on the method of preparing the PVA-based polymer into an aqueous solution, or the methods of saponification, neutralization, dissolution, dispersion and drying, and conventional methods can be used. Furthermore, there are no particular restrictions on the alkaline substances used for neutralization; for example, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide can be cited. From the viewpoint of reaction rate, the pH of the reaction solution during the acetalization reaction is preferably below 3.0, and more preferably below 1.0. Furthermore, the pH of the neutralized reaction solution is preferably between 4.7 and 9.0, and more preferably between 7.0 and 8.5. The drying of polyvinyl alcohol polymer (A) can be carried out in the same way as that of polyvinyl alcohol polymer (C) (and further (B-3), (B-4)). The drying conditions [drying atmosphere, drying temperature, drying time (e.g., 1 to 12 hours, etc.)] and the preferred state (and the reasons thereof) are also as described above (e.g., drying is carried out in an inert atmosphere such as nitrogen at a temperature that is not too high (e.g., 70 to 110°C). [Applications, Manufacturing Methods of Ethylene-Based Polymers, etc.] PVA-based polymers (A) can be used for various applications (e.g., dispersants, membrane applications, etc.), and as described above, they are particularly preferred as dispersion stabilizers [or dispersants, such as dispersion stabilizers (dispersants) used in polymerization (e.g., suspension polymerization)]. Therefore, the following describes the use of the dispersion stabilizer of the present invention (or PVA-based polymer (A, hereinafter the same)) or the manufacturing method of ethylene-based polymers carried out by polymerization (especially suspension polymerization) of ethylene monomers using the dispersion stabilizer. The suspension polymerization in this invention refers to the following polymerization method: adding an insoluble ethylene monomer and an oil-soluble polymerization initiator to an aqueous medium and stirring, thereby forming tiny droplets containing the ethylene monomer, and carrying out polymerization in these droplets. Here, the aqueous medium that can be used is not particularly limited, and examples include: water, aqueous solutions containing various additives, and mixtures of water and organic solvents that are miscible with water. The PVA-based polymer (A) described above in this invention can be used as a dispersion stabilizer during suspension polymerization of ethylene monomers. There are no particular limitations on the ethylene monomer used; preferred examples include vinyl chloride, vinylidene chloride, styrene, acrylates, methacrylates, vinyl acetate, acrylonitrile, and other ethylene monomers commonly used in suspension polymerization, with vinyl chloride monomers being particularly preferred. Examples of vinyl chloride monomers include vinyl chloride monomer (vinyl chloride), and mixtures of vinyl chloride monomer and other monomers that can be copolymerized with it. Examples of other monomers that can be copolymerized with vinyl chloride monomer include: vinylidene chloride, vinyl acetate, ethylene, propylene, acrylic acid, acrylate, methacrylic acid, methacrylate, styrene, vinylalkoxysilane, maleic acid, hydroxyalkyl acrylate, allyl sulfonic acid, vinyl sulfonic acid, etc. Therefore, the dispersion stabilizer of the present invention is suitable for suspension polymerization of vinyl monomers containing vinyl chloride monomers (especially vinyl chloride), and is particularly preferably used for homopolymerization of vinyl chloride by suspension polymerization. It can also be used for binary or multi-component copolymerization of vinyl chloride with one or more known monomers capable of copolymerizing with vinyl chloride by suspension polymerization, and is particularly preferably used as a dispersion stabilizer in copolymerization of vinyl chloride and vinyl acetate by suspension polymerization. Vinyl chloride resins can be obtained by suspension polymerization of vinyl monomers containing vinyl chloride. In the manufacture of vinyl chloride resins, it is preferable that 50 to 100 moles (or 50 to 100% by mass) of vinyl chloride is used relative to the total amount of vinyl monomers used. Polymerization initiators used in the suspension polymerization of ethylene monomers are also known, such as: percarbonate compounds such as diisopropyl peroxide, di(2-ethylhexyl) peroxide, and di(ethoxyethyl) peroxide; perester compounds such as benzoyl peroxide, tert-butyl peroxyneodecanate, α-isopropylphenyl peroxyneodecanate, and tert-butyl peroxyneodecanate; peroxides such as acetocyclohexylsulfonylurea peroxide and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetic acid ester; peroxides such as 2,2'-azobisisobutyronitrile, azobis-2,4-dimethylpentanitrile, and azobis(4-methoxy-2,4-dimethylpentanitrile); benzoyl peroxide; lauryl peroxide; and so on. Furthermore, they can also be used in combination with these compounds such as potassium persulfate, ammonium persulfate, and hydrogen peroxide. The primary function of a dispersant stabilizer in the suspension polymerization of ethylene monomers is to stabilize droplets containing ethylene monomers and their polymers, preventing polymer particles generated from the droplets from fusing together and forming larger clumps. The dispersant stabilizer of this invention, due to its excellent dispersing properties, can form stable droplets with a smaller dosage, thus preventing the formation of clumps caused by fusing. Furthermore, "stabilizing droplets" means ensuring that small and generally uniformly sized droplets are stably dispersed in the dispersion medium of the suspension polymerization. In the suspension polymerization of ethylene monomers, there is no particular limitation on the amount of the dispersant stabilizer (or PVA-based polymer (A)) used in this invention. Generally, it is 5 parts by mass or less relative to 100 parts by mass of the ethylene monomer, preferably 0.005 to 1 part by mass, and even more preferably 0.01 to 0.2 parts by mass. Typically, the dispersant stabilizer of this invention is also used in the same manner as conventional dispersant stabilizers, pre-dissolved in the dispersion medium of the suspension polymerization using conventional methods before adding the ethylene monomer. As a dispersant and stabilizer in the suspension polymerization of ethylene monomers, the dispersant and stabilizer of this invention can be used alone or in combination with other dispersant and stabilizers. Examples of such other dispersant and stabilizers include known dispersant and stabilizers used in the suspension polymerization of ethylene monomers such as vinyl chloride in an aqueous medium, such as PVA or modified PVA polymers other than those of this invention with an average degree of polymerization of 100 to 4500 and a degree of saponification of 30 to 100 moles, water-soluble cellulose ethers such as methylcellulose, hydroxyethylcellulose, and hydroxypropylcellulose, water-soluble polymers such as gelatin, oil-soluble emulsions such as sorbitan monolaurate, sorbitan trioleate, glyceryl tristearate, and ethylene oxide-propylene oxide block polymers, water-soluble emulsifiers such as polyoxyethylene glyceryl oleate and sodium laurate. One or more of these other dispersants can be used simultaneously. In this invention, the dispersant stabilizer is preferably a combination of two or more PVA-based polymers with different degrees of polymerization and saponification. More preferably, one or more of these polymers are designated as the PVA-based polymer (A) used as the dispersant stabilizer of this invention. Even more preferably, the dispersant is a combination of a PVA-based polymer with a degree of polymerization of 1700 or higher and a PVA-based polymer with a degree of polymerization of 1000 or lower, with one or more of these polymers designated as the PVA-based polymer (A) of this invention. In suspension polymerization using the dispersant stabilizer of the present invention, various known dispersing aids may also be used concurrently. As such a dispersing aid, low-saponification-degree PVA, etc., with a saponification degree preferably of 30-60 mol%, more preferably 35-55 mol%, is preferred. Furthermore, PVA, etc., with an average degree of polymerization preferably of 160-900, more preferably 200-500, is preferred as the dispersing aid. In addition to dispersants, chain transfer agents, polymerization inhibitors, pH adjusters, scale inhibitors, crosslinking agents, and other additives known in the suspension polymerization of ethylene compounds can also be used. There are no restrictions on the polymerization temperature in suspension polymerization. It can be selected arbitrarily according to the type of ethylene monomer used, the degree of polymerization of the target polymer, and the polymerization yield. Generally, 40–70°C is preferred. There are also no particular restrictions on the polymerization time, as long as it is set appropriately according to the target polymerization yield. The ethylene-based polymers obtained by the manufacturing method of the present invention described above can be processed into various molded articles. In particular, vinyl chloride-based resins, for example, can be obtained with high efficiency, exhibiting an average particle size within an appropriate range and excellent plasticizer absorption, and in most cases, show good processability for various molded articles. [Example] The present invention will be further described in detail below with examples, but the present invention is not limited to these examples. Furthermore, in the following examples and comparative examples, "%" and "parts" refer to "mass %" and "parts by mass" unless otherwise specified. First, the evaluation methods for PVA polymer (A) and vinyl chloride polymer (vinyl chloride resin) in this embodiment are presented below. (Determination of Degree of Polymerization and Viscosity of 4% Aqueous Solution (20°C) (4% Viscosity)) The determination was performed according to the method specified in JIS K 6726. (Method for determining saponification degree) The determination shall be performed in accordance with the method specified in JIS K 6726. (Method for determining the content of polymeric unsaturated bonds in PVA) In an Erlenmeyer flask, to prepare an aqueous solution by dissolving 5 g of the sample (PVA) in 150 g of pure water, add 0.5 mol / L bromoacetic acid solution dropwise until the bromine color (yellow) disappears. Divide the mass of bromine required for titration (μmol) by the weight of PVA (g) to calculate the content of polymeric unsaturated bonds in PVA (μmol / g). (Method for determining the YI of PVA aqueous solution) Prepare a 4% aqueous solution and measure its UV-Vis spectrum (quartz cell with an optical path length of 10 mm) at 20°C using a UV-Vis spectrophotometer (manufactured by Japan Spectrophotometer Co., Ltd.; V-730). Calculate the YI of the 4% aqueous solution from the obtained data. (Method for determining the YI of PVA) The YI was determined using a colorimeter (manufactured by Nippon Denshoku Kogyo Co., Ltd.; NW-12). The test sample (test powder) was obtained by passing pulverized PVA (powder) through a metal wire mesh (sieve) with a mesh size of 0.5 mm (30 mesh). (Method for determining the transmittance of PVA aqueous solution at 430 nm) A 1% aqueous solution of PVA was prepared, and its transmittance at 430 nm was determined at 20°C using a UV-Vis spectrophotometer (manufactured by Japan Spectrophotometer Co., Ltd.; V-730) (quartz cell with an optical path length of 20 mm, blank sample: pure water). (Method for determining the cloud point of PVA aqueous solution) A 4% PVA aqueous solution at 20°C was placed in a quartz cell with an optical path length of 10 mm. Using a UV-Vis spectrophotometer (manufactured by Japan Spectrophotometer Co., Ltd.; V-730), the transmittance was continuously measured at 430 mm from 20°C with a heating rate of 2°C / min. The temperature at which the transmittance relative to the blank sample (pure water) becomes 50% was taken as the cloud point. (Evaluation Method for Storage Stability of PVA Aqueous Solution) Place a beaker containing 4% PVA aqueous solution into a constant temperature water bath at 30°C. Visually confirm the state of the aqueous solution after 24 hours and evaluate it according to the following criteria: 〇: The aqueous solution remains homogeneous. ×: The aqueous solution separates into two layers. (Evaluation of vinyl chloride polymers) Vinyl chloride polymers are evaluated as follows. <Average Particle Size> The average particle size is determined by measuring the particle size distribution using a Rotap vibrating screen (using a JIS screen). <Plasticizer Absorption> The obtained resin was added to a cylindrical container with a bottom lined with glass fiber. Excess dioctyl phthalate (DOP) was added, and the mixture was left to stand for 30 minutes to allow the DOP to permeate into the resin. The resin was then centrifuged at 3000 rpm to remove excess DOP. The weight of the resin was measured, and the DOP absorption per 100 parts of polymer was calculated. A higher DOP absorption indicates better plasticizer absorption and superior molding processability. <Evaluation of the Colorimetric Properties of Vinyl Chloride Polymers> A mixture of 100 parts by weight of a vinyl chloride polymer, 50 parts by weight of di-2-ethylhexyl phthalate, 0.8 parts by weight of a dioctyltin mercapto stabilizer, 0.1 parts by weight of a polyethylene lubricant, and 0.8 parts by weight of a calcium-zinc composite (one-pack) stabilizer was kneaded at 160°C for 5 minutes using a test roll to form a sheet with a thickness of 0.45 mm. Multiple sheets were stacked and pressed at 160°C for 5 minutes to produce test pieces measuring 40 mm × 40 mm × 15 mm (thickness). The YI of the test piece was measured using a colorimeter. [Example 1] (Synthesis of PVA-based polymer (C)) 55 parts methanol and 45 parts vinyl acetate monomer were pre-added to a reactor equipped with a stirrer, condenser, nitrogen inlet, and initiator inlet. Nitrogen gas was circulated into the system while the temperature was raised to 60°C. 5 parts of a 1% methanol solution of 2,2'-azobis(2,4-dimethylpentanonitrile) (ADVN) as an initiator were added to begin polymerization. During polymerization, the system was maintained at 60°C, and nitrogen gas was circulated into the system while 90 parts of vinyl acetate monomer were continuously added over a period of 4 hours after the start of polymerization. One part of a 1% methanol solution of ADVN was added at 1 hour and 2 hours after the start of polymerization. The system was cooled when the vinyl acetate yield reached 85%, ending the polymerization. Furthermore, the acetaldehyde concentration at the end of polymerization was 100 ppm. Methanol vapor was added to the obtained polymer while the remaining vinyl acetate monomer was distilled off to obtain a 50% methanol solution of polyvinyl acetate. Then, 14 parts of methyl acetate and 6 parts of a 3% methanol solution of sodium hydroxide were added to 100 parts of the obtained 50% methanol solution of polyvinyl acetate and mixed thoroughly. A saponification reaction was carried out at 40°C. The resulting gel was pulverized and impregnated in a mixed solvent (200 parts) of 100 parts methanol and 100 parts methyl acetate. Acetic acid was added until the pH reached 9 for neutralization. The sample obtained by solid-liquid separation was then dried at 80°C for 5 hours under a nitrogen atmosphere. Based on the analysis results, a PVA-based polymer (C) powder with a saponification degree of 76 mol% and a viscosity (20°C) of 5.5 mPa·s for a 4% by mass aqueous solution was obtained. (Synthesis of PVA-based polymer (A-1)) 100 parts of the obtained PVA-based polymer (C) powder were impregnated in a mixed solvent of 150 parts methanol and 300 parts methyl acetate. 0.8 parts acrolein were added, and the mixture was kept at 50°C for 1 hour. Then, 5 parts of a 50% methanol solution of p-toluenesulfonic acid were added, and the reaction was carried out at 50°C for 1 hour. Subsequently, neutralization was performed using 10 parts of a 5% methanol solution of sodium hydroxide. The pH value after neutralization was 7.5. After removing the solvent by centrifugation, the mixture was dried at 80°C for 5 hours under a nitrogen atmosphere to obtain the PVA-based polymer (A-1). The analytical values ​​of this PVA-based polymer (A-1) were: degree of saponification 77 mol%, viscosity of a 4% (w / w) aqueous solution (20°C) 5.8 mPa·s (average degree of polymerization approximately 600), and cloud point of a 4% aqueous solution 35°C. Furthermore, the degree of saponification and degree of polymerization were determined according to the method specified in JIS K 6726. The content of double bonds from acrolein was 80 μmol / g, the YI of the PVA powder was 2, the YI of the 4% by weight aqueous solution was 7, and the transmittance at 430 nm was 98%. Moreover, even when a 4% PVA aqueous solution was kept at 30°C for 24 hours, the aqueous solution remained homogeneous. (Suspension Polymerization of Vinyl Chloride) Using the PVA-based polymer (A-1) obtained above as a dispersant and stabilizer, suspension polymerization of vinyl chloride was carried out under the conditions shown below. 120 parts of deionized water and 1.5 parts of a 4% aqueous solution of the PVA-based polymer (A-1) obtained above were added to a pressure-resistant stainless steel polymerizer (0.06 parts of PVA-based polymer (A-1) relative to 100 parts of vinyl chloride monomer). The pressure inside the polymerizer was then reduced to 50 mmHg using a vacuum pump. After degassing, 100 parts of vinyl chloride monomer were added, followed by 0.06 parts of tributyl peroxyneodecanate as a polymerization initiator. The mixture was stirred, and the temperature was raised. Suspension polymerization was carried out while maintaining the internal temperature of the polymerizer at 57°C. The polymerization reaction was stopped when the conversion rate of vinyl chloride reached 88%. Then, unreacted monomers were recovered using a vacuum trap, and the polymer slurry was extracted from the polymerizer for dehydration and drying to obtain vinyl chloride polymer (vinyl chloride resin). [Examples 2-12] The PVA-based polymers shown in Table 1 (A-1) were synthesized in the same manner as in Example 1, except that the polymerization conditions, saponification conditions, type and amount of aldehyde used in the acetalization reaction were appropriately modified. The obtained PVA-based polymer (A-1) was used to carry out suspension polymerization of vinyl chloride in the same manner as in Example 1 to obtain vinyl chloride polymer. [Comparative Examples 1-4] The PVA-based polymer (A-1) shown in Table 1 was synthesized as follows. Using the obtained PVA-based polymer (A-1), vinyl chloride was subjected to suspension polymerization in the same manner as in Example 1 to obtain a vinyl chloride polymer. The evaluation results of the PVA-based polymer (A-1) and the resulting vinyl chloride polymer are summarized in Table 1. [Table 1] As shown in the table above, when the PVA-based polymer (A-1) obtained in Examples 1 to 12 is used for suspension polymerization of vinyl chloride, the polymerization stability is excellent, and vinyl chloride resin with an average particle size in an appropriate range, a large amount of plasticizer absorption, reduced sheet colorability, and excellent hue can be obtained. [Comparative Example 1] PVA-based polymer (C) was dried at 150°C for 5 hours in air. Otherwise, PVA-based polymer (A-1) was synthesized in the same manner as in Example 1. The color of the resulting PVA-based polymer (A-1) deteriorated. Suspension polymerization of vinyl chloride was attempted in the same manner as in Example 1, but the colorability of the resulting vinyl chloride sheet deteriorated. [Comparative Example 2] A PVA-based polymer (C) without double bonds, with a viscosity (20°C) of 5.5 mPa·s (average degree of polymerization of about 600) and a saponification degree of 77 moles of aqueous solution of 4% by mass, was used as the PVA-based polymer (A-1). Suspension polymerization of vinyl chloride was attempted in the same manner as in Example 1, but the vinyl chloride resin was block-formed and polymerization could not proceed normally. [Comparative Example 3] Vinyl acetate was polymerized in the presence of carbon tetrachloride to obtain polyvinyl acetate, which was then saponified according to Example 1 to obtain a PVA-based polymer with a viscosity (20°C) of 5.5 mPa·s (average degree of polymerization of about 600) of 4% by mass aqueous solution, a degree of saponification of 77 moles, a double bond content of 80 μmol / g, a YI of 35 for PVA powder, a YI of 40 for 4% by weight aqueous solution, and a transmittance of 80% at 430 nm for 1% by weight aqueous solution. Using this PVA-based polymer, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1, but the colorability of the resulting vinyl chloride resin sheet deteriorated. [Comparative Example 4] Vinyl acetate was polymerized in the presence of acetaldehyde to obtain polyvinyl acetate, which was then saponified according to Example 1 to obtain a PVA-based polymer with a saponification degree of 77 mol%, a viscosity (20°C) of 5.5 mPa·s for a 4% by weight aqueous solution (average degree of polymerization approximately 600). This polymer was dried at 130°C for 5 hours under a nitrogen atmosphere to obtain a PVA-based polymer with a double bond content of 80 μmol / g, a YI of 45 for the PVA powder, a YI of 55 for a 4% by weight aqueous solution, and a transmittance of 65% at 430 nm for a 1% by weight aqueous solution. Using this PVA-based polymer, suspension polymerization of vinyl chloride was performed in the same manner as in Example 1, but the colorability of the resulting vinyl chloride resin sheet deteriorated. [Examples 13-15] The saponification conditions, the type (2 types) of aldehyde used in the acetalization reaction, and the amount used were appropriately modified, except that the PVA-based polymers shown in Table 2 were synthesized in the same manner as in Example 1. Using the obtained PVA-based polymer (A-2), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain a vinyl chloride polymer. The evaluation results of the PVA-based polymer (A-2) and the obtained vinyl chloride polymer are summarized in Table 2. [Table 2] [Example 16] (Synthesis of PVA-based polymer (B-3)) 55 parts methanol and 45 parts vinyl acetate monomer were pre-added to a reactor equipped with a stirrer, condenser, nitrogen inlet, and initiator inlet. Nitrogen gas was circulated into the system while the temperature was raised to 60°C. Six parts of a 1% methanol solution of 2,2'-azobis(2,4-dimethylpentanonitrile) (ADVN) as an initiator were added to begin polymerization. During polymerization, the system was maintained at 60°C, and nitrogen gas was circulated into the system while 90 parts vinyl acetate monomer and 2 parts of a 20% methanol solution of itonic acid were continuously added over a period of 4 hours after the start of polymerization. One and two parts of a 1% methanol solution of ADVN were added at 1 hour and 2 hours after the start of polymerization, respectively. The system was cooled when the vinyl acetate yield reached 85%, ending the polymerization. Methanol vapor was added to the obtained polymer while the remaining vinyl acetate monomer was distilled off to obtain a 50% methanol solution of polyvinyl acetate. Then, 14 parts of methyl acetate and 6 parts of a 3% methanol solution of sodium hydroxide were added to 100 parts of the obtained 50% methanol solution of polyvinyl acetate and mixed thoroughly. A saponification reaction was carried out at 40°C. The resulting gel was pulverized and impregnated in a mixed solvent (200 parts) of 100 parts methanol and 100 parts methyl acetate. Acetic acid was added until the pH reached 9 for neutralization. The sample obtained by solid-liquid separation was then dried at 80°C for 5 hours under a nitrogen atmosphere. Analysis revealed a PVA-based polymer (B-3) powder with a saponification degree of 71 mol%, a 4% by mass aqueous solution viscosity (20°C) of 5.5 mPa·s (average degree of polymerization approximately 600), and an itconic acid content of 0.2 mol%. (Synthesis of PVA-based polymer (A-3)) The PVA-based polymer (A-3) shown in Table 3 was synthesized in the same manner as in Example 1. (Suspension polymerization of vinyl chloride) Using the obtained PVA-based polymer (A-3), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain a vinyl chloride polymer. [Examples 17-20] Except for appropriate changes to various conditions, the PVA-based polymers (A-3) shown in Table 3 were synthesized in the same manner as in Example 16. Using the obtained PVA-based polymer (A-3), vinyl chloride was subjected to suspension polymerization in the same manner as in Example 1 to obtain vinyl chloride polymers. The evaluation results of the PVA-based polymer (A-3) and the obtained vinyl chloride polymers are summarized in Table 3. Furthermore, in the table, "AMPS" refers to sodium 2-acrylamide-2-methylpropanesulfonate. [Table 3] [Example 21] (Synthesis of PVA-based polymer (B-4)) 20 parts methanol, 80 parts vinyl acetate monomer, and 0.02 parts 3-mercaptopropionic acid were pre-added to a reactor equipped with a stirrer, condenser, nitrogen inlet, and initiator inlet. Nitrogen gas was circulated into the system while the temperature was raised to 60°C. 1.5 parts of a 1% methanol solution of 2,2'-azobis(2,4-dimethylpentanonitrile) (ADVN) as an initiator were added to begin polymerization. During polymerization, the system was maintained at 60°C, and nitrogen gas was circulated into the system while 2 parts of a 10% methanol solution of 3-mercaptopropionic acid were continuously added over 4 hours after the start of polymerization. Then, 0.5 parts of a 1% methanol solution of ADVN were added at 1 hour and 2 hours after the start of polymerization. The system was cooled when the yield of vinyl acetate reached 80%, ending the polymerization. Methanol vapor was added to the obtained polymer while the remaining vinyl acetate monomer was distilled off to obtain a 50% methanol solution of polyvinyl acetate. Then, 14 parts of methyl acetate and 6 parts of a 3% methanol solution of sodium hydroxide were added to 100 parts of the obtained 50% methanol solution of polyvinyl acetate and mixed thoroughly. A saponification reaction was carried out at 40°C. The resulting gel was pulverized and impregnated in a mixed solvent (200 parts) of 100 parts methanol and 100 parts methyl acetate. Acetic acid was added until the pH reached 9 for neutralization. The sample obtained by solid-liquid separation was then dried at 80°C for 5 hours under a nitrogen atmosphere. Analysis revealed a PVA-based polymer (B-4) powder with a saponification degree of 71 mol%, a 4% by mass aqueous solution viscosity (20°C) of 5.5 mPa·s (average degree of polymerization approximately 600), and a 3-mercaptopropionic acid content of 0.2 mol%. (Synthesis of PVA-based polymer (A-4)) The PVA-based polymer (A-4) shown in Table 4 was synthesized in the same manner as in Example 1. (Suspension polymerization of vinyl chloride) Using the obtained PVA-based polymer (A-4), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain a vinyl chloride polymer. [Examples 22-24] Except for appropriate changes to various conditions, the PVA-based polymers shown in Table 4 (A-4) were synthesized in the same manner as in Example 21. Using the obtained PVA-based polymer (A-4), vinyl chloride was subjected to suspension polymerization in the same manner as in Example 1 to obtain a vinyl chloride polymer. The evaluation results of the PVA-based polymer (A-4) and the resulting vinyl chloride polymer are summarized in Table 4. Furthermore, in the table, "MPS" refers to sodium 3-mercapto-1-propanesulfonate. [Table 4] The PVA-based polymers (A-2, A-3, A-4) obtained in Examples 13-24 exhibit excellent polymerization stability when used in the suspension polymerization of vinyl chloride. This results in vinyl chloride resins with an appropriate average particle size, high plasticizer absorption, reduced sheet colorability, and excellent hue. [Industrial Applicability] This invention provides a specific polyvinyl alcohol-based polymer. This polymer is preferably used as a dispersing stabilizer (dispersant), etc.

Claims

1. A polyvinyl alcohol polymer (A) that satisfies the following requirements 1, 2 and / or 3: requirement 1: having polymerizable unsaturated bonds; requirement 2: the YI of a 4% by mass aqueous solution is 18 or less; requirement 3: the YI is 13 or less.

2. The polyvinyl alcohol polymer (A) of claim 1, wherein the ratio of polymerizable unsaturated bonds is 3 μmol / g or more.

3. The polyvinyl alcohol polymer (A) of claim 1 or 2 satisfies that the YI of a 4% by mass aqueous solution is less than 15 and / or less than 12.

4. The polyvinyl alcohol polymer (A) of claim 1 or 2 further satisfies requirement 4 below: requirement 4: the transmittance of a 1% by mass aqueous solution at 430 nm is 90% or more.

5. The polyvinyl alcohol polymer (A) of claim 1 or 2, wherein the ratio of polymerizable unsaturated bonds is 5 to 500 μmol / g, the YI of a 4% by mass aqueous solution is 15 or less, the YI of a 1% by mass aqueous solution is 12 or less, and the transmittance at 430 nm of a 1% by mass aqueous solution is 93% or more.

6. The polyvinyl alcohol polymer (A) of claim 1 or 2, which contains an acetal backbone (a) having polymerizable unsaturated bonds.

7. The polyvinyl alcohol polymer (A) of claim 1 or 2, which contains an acetal skeleton (a) having polymerizable unsaturated bonds, the acetal skeleton (a) comprising the skeleton represented by the following formula (a1), [Chemical 1] (where R' represents a group having polymerizable unsaturated bonds).

8. The polyvinyl alcohol polymer (A) of claim 1 or 2, comprising an acetal skeleton (a) having polymerizable unsaturated bonds, the acetal skeleton (a) comprising the skeleton represented by the above formula (a1), the content of the acetal skeleton (a) being 0.05 to 5 mol per monomer unit.

9. The polyvinyl alcohol polymer (A) of claim 1 or 2 further contains an ionic backbone (b).

10. The polyvinyl alcohol polymer (A) of claim 1 or 2 further contains an ionic backbone (b), the content of which is 0.01 to 5 moles per monomer unit.

11. The polyvinyl alcohol polymer (A) of claim 1 or 2 has a saponification degree of 50 to 90 moles.

12. The polyvinyl alcohol polymer (A) of claim 1 or 2 has a 4% by mass aqueous solution viscosity (20°C) of 2 to 100 mPa·s.

13. A dispersion stabilizer comprising a polyvinyl alcohol polymer (A) as claimed in claim 1 or 2.

14. The agent as described in claim 13 is a dispersion stabilizer for polymerization.

15. The agent as claimed in claim 13 is a dispersion stabilizer for suspension polymerization.

16. The agent as claimed in claim 13 is a dispersant stabilizer for suspension polymerization of ethylene monomers containing vinyl chloride.

17. A method for manufacturing an ethylene-based polymer, wherein ethylene monomers are polymerized in the presence of a polyvinyl alcohol-based polymer (A) as claimed in claim 1 or 2.

18. The manufacturing method as described in claim 17, wherein the polymerization is suspension polymerization.

19. The manufacturing method of claim 17, wherein an ethylene monomer containing vinyl chloride is subjected to suspension polymerization.