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 requirements 1 and 2. Requirement 1: Possesses polymeric unsaturated bonds Requirement 2: The temperature at which the weight loss rate determined by thermogravimetric analysis exceeds 0.5% / min within a temperature range above 150°C is above 255°C.

Description

Polyvinyl alcohol polymer, dispersion stabilizer, and method for producing ethylene polymer The present invention relates to a polyvinyl alcohol-based polymer, various uses of the polyvinyl alcohol-based polymer [such as a dispersion stabilizer [such as a dispersion stabilizer for suspension polymerization of vinyl monomers (especially vinyl chloride monomers)]], and a method for producing a vinyl polymer [especially a vinyl chloride polymer (resin)] using the polyvinyl alcohol-based polymer (or dispersion stabilizer). The industrial production of vinyl chloride resins is typically carried out through batch suspension polymerization, in which vinyl chloride and other vinyl monomers are dispersed in an aqueous medium in the presence of a dispersion stabilizer and then polymerized using an oil-soluble polymerization initiator. Factors in the polymerization process that influence the quality of vinyl chloride resins 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 dispersion stabilizer, with the dispersion stabilizer having a particularly significant impact. The function of a dispersion stabilizer used in suspension polymerization to produce vinyl chloride resin is to disperse the monomer in an aqueous medium to form stable droplets, uniformly adjust the size of the repeatedly dispersed and coalesced droplets, and control the agglomeration of the polymerized particles. Therefore, the following properties are required of such a dispersion stabilizer: <1> Controlling the particle size of the resulting vinyl chloride resin particles within an appropriate range; <2> Increasing the plasticizer absorption of the resulting vinyl chloride resin particles to improve moldability; <3> Maintaining the porosity of the resulting vinyl chloride resin particles within a certain range to facilitate the removal of residual monomers; and <4> Improving the thermal stability of the resulting vinyl chloride resin particles. That is, the above-mentioned dispersion stabilizer is required to exhibit excellent dispersing power (protective colloid property) and to control the particle size and particle shape of the vinyl chloride resin to an appropriate state, for example. As the above-mentioned dispersion stabilizer, polyvinyl alcohol-based resins (hereinafter sometimes referred to as PVA, PVA-based resins, PVA-based polymers, etc.), cellulose derivatives, etc. are generally used alone or in appropriate combination. For example, Non-Patent Document 1 describes a method of using PVA with a viscosity-average degree of polymerization of 2000 and a saponification degree of 88 mol% or 80 mol% as a dispersion stabilizer for suspension polymerization of vinyl chloride, or a method of using PVA with a viscosity-average degree of polymerization of 600 to 700 and a saponification degree of around 70 mol%. Patent Document 1 proposes a dispersion stabilizer for suspension polymerization, characterized by containing a polyvinyl alcohol polymer (B) having double bonds in the side chains, obtained by acetalizing a polyvinyl alcohol polymer (A) with a monoaldehyde having an olefinic unsaturated double bond. [Prior Art Document] [Patent Document] [Patent Document 1] International Publication No. 2015 / 182567 [Non-Patent Document] [Non-Patent Document 1] "POVAL", Polymer Publishing Association, published in 1981 [Problems to be solved by the invention] The present invention aims to provide a polyvinyl alcohol polymer (PVA). [Technical means for solving the problem] As described above, PVA is used as a dispersion stabilizer for suspension polymerization of vinyl chloride, etc., and technology is being developed to further improve PVA used as such a dispersion stabilizer. In particular, Patent Document 1 relates to a technology developed by the present inventors, which aims to improve polymerization stability or dispersibility (protective colloid properties) by modifying PVA with a monoaldehyde having an olefinic unsaturated double bond (acetalization). On the other hand, the inventors' research has discovered that the type of PVA used can affect the thermal stability of the resulting resin (such as vinyl chloride resin), even reducing it. However, the factors that determine how each PVA affects the thermal stability of the resin are unknown, making the search for a PVA that effectively improves thermal stability extremely difficult. Furthermore, this effect on thermal stability still exists (especially does not improve) even when the PVA is modified (acetalized) with a monoaldehyde having an olefinic unsaturated double bond as in Patent Document 1, and the search for PVA that can improve polymerization stability or dispersibility and achieve improved thermal stability (impart sufficient thermal stability) remains extremely difficult. Under such circumstances, the present inventors discovered that the thermal properties of PVA itself can affect the thermal stability of the resulting resin. As a result of further research, they found that PVA with specific thermal properties (thermal characteristics) can improve the thermal stability of the resulting resin (impart sufficient thermal stability). In particular, even with PVA having unsaturated bonds (unsaturated double bonds) as in Patent Document 1, this tendency does not change significantly, and it can even improve polymerization stability or dispersibility and achieve improved thermal stability (impart sufficient thermal stability). Continuing their research, they completed the present invention. That is, the present invention relates to the following inventions, etc. [1] A polyvinyl alcohol polymer (A) that fully satisfies the following requirements 1 and 2. Requirement 1: Possessing a polymerizable unsaturated bond Requirement 2: The temperature at which the rate of weight loss exceeds 0.5% / min as determined by thermogravimetric measurement in a temperature range of 150°C or higher is 255°C or higher [2] The polyvinyl alcohol polymer (A) as described in [1], wherein the ratio of polymerizable unsaturated bonds is 3 μmol / g or higher. [3] The polyvinyl alcohol polymer (A) as described in [1] or [2], wherein the temperature at which the rate of weight loss exceeds 0.5% / min as determined by thermogravimetric measurement in a temperature range of 150°C or higher is 258°C or higher. [4] The polyvinyl alcohol polymer (A) as described in any one of [1] to [3], wherein the ratio of polymerizable unsaturated bonds is 5 to 500 μmol / g, and the temperature at which the weight loss rate determined by thermogravimetric measurement exceeds 0.5% / min in a temperature range of 150°C or above is 260°C or above. [5] The polyvinyl alcohol polymer (A) as described in any one of [1] to [4], which contains an acetal skeleton (a) having polymerizable unsaturated bonds. [6] The polyvinyl alcohol polymer (A) as described in any one of [1] to [5], which contains an acetal skeleton (a) having polymerizable unsaturated bonds, and the acetal skeleton (a) includes a skeleton represented by the following formula (a1). [Chemical 1] (wherein R' represents a group having a polymerizable unsaturated bond) [7] The polyvinyl alcohol polymer (A) as described in any one of [1] to [6], which contains an acetal skeleton (a) having a polymerizable unsaturated bond, the acetal skeleton (a) includes the skeleton represented by the above formula (a1), and the content of the acetal skeleton (a) is 0.05 to 5 mol% per monomer unit. [8] The polyvinyl alcohol polymer (A) as described in any one of [1] to [7], which further contains an ionic skeleton (b). [9] The polyvinyl alcohol polymer (A) as described in any one of [1] to [8], which further contains an ionic skeleton (b), and the content of the ionic skeleton (b) is 0.01 to 5 mol% per monomer unit.

[10] The polyvinyl alcohol polymer (A) as described in any one of [1] to [9], which has a saponification degree of 50 to 90 mol%.

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

[10] , wherein the viscosity of its 4% by mass aqueous solution (20°C) is 1 to 300 mPa·s (e.g., 1 to 100 mPa·s, 2 to 100 mPa·s).

[12] A dispersion stabilizer comprising the polyvinyl alcohol polymer (A) as described in any one of [1] to

[11] .

[13] The agent as described in

[12] , which is a dispersion stabilizer for polymerization.

[14] The agent as described in

[12] or

[13] , which is a dispersion stabilizer for suspension polymerization.

[15] The agent as described in any one of

[12] to

[14] , which is a dispersion stabilizer for suspension polymerization of vinyl monomers containing vinyl chloride.

[16] A method for producing an ethylene polymer, comprising polymerizing an ethylene monomer in the presence of the polyvinyl alcohol polymer (A) as described in any one of [1] to

[15] or the agent.

[17] The production method as described in

[16] , wherein the polymerization is suspension polymerization.

[18] The production method as described in

[16] or

[17] , wherein the suspension polymerization is performed on vinyl monomers including vinyl chloride.

[19] A use of the polyvinyl alcohol polymer (A) as described in any one of [1] to

[11] , which is used as a dispersion stabilizer (dispersant).

[20] The use as described in

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

[21] The use as described in

[19] or

[20] , wherein the dispersion stabilizer is a dispersion stabilizer for suspension polymerization of vinyl monomers including vinyl chloride.

[22] A use of the polyvinyl alcohol polymer (A) as described in any one of [1] to

[11] , which is used for the polymerization of vinyl monomers.

[23] The use as described in

[22] , wherein the polymerization is suspension polymerization.

[24] The use as described in

[22] or

[23] , wherein the polymerization is a suspension polymerization of vinyl monomers including vinyl chloride. [Effects of the Invention] According to the present invention, there is provided a PVA (novel or specific PVA) or a use thereof (dispersion stabilizer, etc.). This type of PVA (dispersion stabilizer, etc.) has specific thermal properties. In particular, although this type of PVA (dispersion stabilizer, etc.) has unsaturated double bonds, it also has excellent thermal stability. Furthermore, this type of PVA also possesses the properties of a dispersion stabilizer. For example, it can exert excellent dispersing power (protective colloid properties), achieve high polymerization stability, and produce resins with excellent plasticizer absorption or thermal stability (for example, vinyl polymers such as vinyl chloride resins). Therefore, while it possesses the properties of a dispersion stabilizer, it can also simultaneously improve the thermal stability of the resin (imparting sufficient thermal stability). Hereinafter, the embodiments for implementing the present invention will be described in detail. However, the present invention is not limited to the embodiments described below. The polyvinyl alcohol polymer of the present invention (polyvinyl alcohol polymer (A), PVA polymer (A), PVA (A), PVA) particularly satisfies (sufficiently satisfies) the following requirements 1 and 2. Requirement 1: Possessing a polymerizable unsaturated bond [e.g., a polymerizable unsaturated bond determined (detected or quantified by bromine titration)] (e.g., an unsaturated double bond). Requirement 2: The temperature at which the rate of weight loss (determined by thermogravimetric measurement) exceeds 0.5% / min in a thermogravimetric measurement conducted in a temperature range of 150°C or higher is 255°C or higher. The present invention also encompasses various uses of such polyvinyl alcohol-based polymers (polyvinyl alcohol-based polymers (A)), particularly dispersants (dispersion stabilizers, such as dispersion stabilizers for suspension polymerization) containing such polyvinyl alcohol-based polymers (polyvinyl alcohol-based polymers (A)). In these various uses [e.g., dispersants (dispersion stabilizers) such as dispersion stabilizers for suspension polymerization], one or more PVA-based polymers (A) may be used. The present invention is described in detail below. [Polyvinyl alcohol polymer (A)] PVA polymer (A) generally satisfies the following requirements 1 and 2. Requirement 1: Contains a polymerizable unsaturated bond Requirement 2: The temperature at which the weight loss rate (determined by thermogravimetric measurement) exceeds 0.5% / min in a thermogravimetric measurement performed in a temperature range of 150°C or higher is 255°C or higher In requirement 1, the polymerizable unsaturated bond may be, for example, a double bond (unsaturated double bond) or a triple bond (unsaturated triple bond). The polymerizable unsaturated bond may generally be a double bond (particularly a carbon-carbon double bond) [and may include at least a double bond (particularly a carbon-carbon double bond)]. In the PVA-based polymer (A), the ratio of polymerizable unsaturated bonds can be selected from a range of, for example, 1 μmol / g or more (e.g., 2 μmol / g or more), preferably 3 μmol / g or more, and 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 the PVA-based polymer (A), the upper limit of the ratio of polymerizable unsaturated bonds is not particularly limited, and may be, for example, 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, or 400 μmol / g or less. The ratio of the polymerizable unsaturated bonds may be set to a range obtained by appropriately combining the lower limit value and the upper limit value of the above range (hereinafter, the same applies to the description of the range). Typically, the ratio of the polymerizable unsaturated bonds in the PVA polymer (A) is about 1 to 2000 μmol / g, preferably 3 to 1000 μmol / g, and more preferably 5 to 500 μmol / g. In the PVA-based polymer (A) (PVA-based polymer (A) that fully satisfies requirement 1), the ratio of polymerizable unsaturated bonds can fully satisfy the following skeleton ratio [for example, it can fully satisfy the ratio of acetal skeleton (a) (for example, relative to each monomer unit, 0.001 mol% or more, 0.05 to 5 mol%, 0.1 to 3 mol%, 0.2 to 2 mol%, etc.)]. If the content of polymerizable unsaturated bonds (e.g., unsaturated double bonds) is within the above range, 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 greater), the following effects are easily achieved, making it preferable: when used in suspension polymerization, polymerization stability is excellent, fouling on the polymerization tank is suppressed, and the resulting vinyl resin particles are less likely to coarsen. 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 preferable: vinyl resins with high plasticizer absorption are easily obtained. The polymerizable unsaturated bonds contained in the PVA polymer (A) (the polymerizable unsaturated bonds possessed by the PVA polymer (A)) can be detected or quantified [their content (content ratio) can be determined] by, for example, bromine titration. The bromine titration method is not particularly limited, but utilizes the reaction between the polymerizable unsaturated bonds (unsaturated double bonds, etc.) contained in the PVA polymer (A) and bromine. The polymerizable unsaturated bonds (unsaturated double bonds, etc.) contained in the PVA polymer (A) (μmol / g) is calculated from the amount (mole) of bromine that reacts with the polymerizable unsaturated bonds (unsaturated double bonds, etc.) contained in the PVA polymer (A). In requirement 2, the temperature (T) at which the weight loss rate exceeds 0.5% / min is an indicator of the thermal stability of the PVA-based polymer (A). A higher value indicates better thermal stability. T can be 255°C or higher, preferably 258°C or higher (e.g., 260°C or higher), more preferably 262°C or higher (e.g., 264°C or higher, 265°C or higher), and can also be 267°C or higher (e.g., 268°C or higher, 269°C or higher, 270°C or higher). The upper limit of T is not particularly limited and can be, for example, 450°C, 400°C, 350°C, 320°C, 300°C, 290°C, etc. Furthermore, if the weight loss rate does not exceed 0.5% within the measurement temperature range of thermogravimetric analysis, T may be set to a value "exceeding" the upper limit of the measurement temperature range (e.g., if the upper limit of the measurement temperature range is 400°C, T exceeds 400°C). With such T, the effects of the present invention can be easily and efficiently achieved (for example, when used in suspension polymerization, an ethylene resin having excellent thermal stability can be easily and efficiently obtained). Furthermore, this requirement 2 (temperature T) is not particularly limited. In addition to adjusting (selecting) the monomer composition of the PVA-based polymer (A), for example, by adjusting (selecting) the introduction method of the polymerizable unsaturated bond, the drying treatment conditions, etc. as described below, this requirement 2 (temperature T) can be easily and efficiently satisfied. Requirement 2 (weight loss rate) can be determined by thermogravimetric (TG) measurement. The conditions for thermogravimetric measurement (measurement conditions) are not limited, but for example, the heating rate can be a predetermined rate (e.g., 10°C / min). Other conditions (conditions other than the heating rate) are also not limited and can be arbitrarily selected. For example, a nitrogen atmosphere, a sample weight of 5-6 mg, and a measurement temperature range of 50°C or higher (e.g., 50-300°C, 50-400°C, 50-500°C) can be used. Other conditions can be set arbitrarily. In requirement 2, the PVA polymer (A) may be used directly in the thermogravimetric measurement (as a sample for the thermogravimetric measurement) or may be used after a purification treatment (such as extraction) as needed. For example, the PVA polymer (A) may be used after Soxhlet extraction using methanol as a solvent (it may be used after extraction). The PVA polymer (A) may contain impurities such as sodium acetate, which may affect the value of thermogravimetric measurement. However, extraction treatment (extraction treatment using methanol) can remove such impurities, making it easier to perform efficient thermogravimetric measurement (thermogravimetric measurement of the PVA polymer (A) itself) (easily eliminating the influence of impurities and measuring the original thermal stability of the PVA polymer (A)). Furthermore, even if impurities such as sodium acetate are contained, when used as a dispersant (dispersion stabilizer), they do not substantially function. Furthermore, when the particle size of the measurement sample (PVA-based polymer (A)) is relatively large, the particle size can be adjusted by, for example, crushing the sample with a crusher and then used in the thermogravimetric measurement. The PVA-based polymer (A) is not particularly limited as long as it satisfies the above-mentioned requirements 1 and 2. As an embodiment having a polymerizable unsaturated bond (sufficiently satisfying requirement 1), it may preferably contain an acetal skeleton (a) having a polymerizable unsaturated bond (e.g., an ethylenically unsaturated double bond). In the present invention, despite containing such polymerizable unsaturated bonds (ethylenically unsaturated double bonds, etc.) [for example, an acetal skeleton (a) having polymerizable unsaturated bonds (ethylenically unsaturated double bonds, etc.)], the following effects can be easily achieved: it is easy to prepare one with excellent thermal stability, and it is easy to obtain an ethylene-based resin with excellent thermal stability when used for suspension polymerization. [Acetal Skeleton (a)] In the acetal skeleton (a), the number of polymerizable unsaturated bonds is not particularly limited, and may be 1 or more (eg, 1 to 5, 1 to 3, 1 to 2, 1, etc.). In the acetal skeleton (a), the acetal may be either a cyclic acetal or a non-cyclic (chain) acetal, and is preferably a cyclic acetal. A typical acetal skeleton having a polymerizable unsaturated bond includes a skeleton (structural unit) represented by the following formula (a1). Therefore, the acetal skeleton (a) may include a skeleton represented by the following formula (a1). [Chemistry 2] (wherein R' represents a group having a polymerizable unsaturated bond) In the above formula (a1), R' is a group having a polymerizable unsaturated bond. R' may be a polymerizable unsaturated bond group itself, or may be a group containing a polymerizable unsaturated bond (e.g., a hydrocarbon group). Furthermore, the group having a polymerizable unsaturated bond may also have a substituent in addition to the polymerizable unsaturated bond. The substituent may be appropriately selected according to the type of the group having a polymerizable unsaturated bond, and is not particularly limited. Examples thereof include: a hydroxyl group, a halogen atom, an acyl group, an ester group, an alkoxy group, a nitro group, a substituted amino group, and a group different from the base group (e.g., an aromatic group such as an aryl group). The substituent may be substituted alone or in combination with two or more. Examples of the group having a polymerizable unsaturated bond [particularly a double bond (ethylenic double bond)] include a group having one polymerizable unsaturated bond {e.g., an alkenyl group [e.g., a hydrocarbon group (which may have a substituent) having 2 or more carbon atoms (e.g., 2 to 30, preferably 2 to 14, and more preferably about 2 to 10 carbon atoms, such as vinyl, allyl, propenyl (1-propenyl, 2-propenyl, etc.), butenyl, pentenyl, 6-methyl-5-hexenyl, decenyl, 2-(dimethylamino)vinyl, cyclohexenyl, 2-phenylvinyl, etc.]}, a group having two or more polymerizable unsaturated bonds {e.g., an alkadienyl group [e.g., 1,3-

[0014] hydrocarbon groups (which may have a substituent, such as an alkadienyl group having 4 or more carbon atoms (e.g., 4 to 30, preferably 4 to 14, and further preferably about 4 to 10) such as pentadienyl, 2,6-dimethyl-1,5-hexadienyl, cyclohexadienyl, and propenylcyclohexenyl)], alktrienyl groups [e.g., alktrienyl groups having 6 or more carbon atoms (e.g., 6 to 30, preferably about 6 to 24)], alkatetraenyl groups [e.g., alkatetraenyl groups having 8 or more carbon atoms (e.g., 8 to 30, preferably about 8 to 24)], and alkopentaenyl groups [e.g., alkopentaenyl groups having 10 or more carbon atoms (e.g., 10 to 30, preferably about 10 to 24)]], etc. The acetal skeleton having a polymerizable unsaturated bond {e.g., the group represented by formula (a1) (or R'-< in formula (a1))} can be derived from a corresponding carbonyl compound (e.g., an aldehyde, its acetal, a ketone, etc.), particularly an aldehyde [e.g., R'CHO (R' is an aldehyde having a hydrocarbon group having a polymerizable unsaturated bond)]. Furthermore, as described above, the carbonyl compound may have a substituent. Examples of such carbonyl compounds include alkenals [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)acrolein, 10-undecenal, myristoleic acid, palmitoleic acid, oleic acid, elaidic acid, isoleic acid, gadaldehyde, sinaphenic acid, tetracosene aldehyde, linolenic acid, citronellal, cinnamaldehyde, etc., alkenals having 3 to 15 carbon atoms, preferably alkenals having 3 to 10 carbon atoms], alkanedienals [ For example, unsaturated aldehydes (especially monoaldehydes) such as alkadienal having 5 to 15 carbon atoms, such as 2,4-pentadienal, 2,4-hexadienal, 2,6-nonadienal, citral, and perillaldehyde, preferably alkadienal having 5 to 10 carbon atoms], alktrienal [such as linolenic acid aldehyde and tungal, preferably alktrienal having 7 to 30 carbon atoms, such as alkanetrienal, preferably alkanetrienal having 7 to 25 carbon atoms], alkatetraenal [such as octadecatetraenal and arachidonic acid, preferably alkanetetraenal having 9 to 30 carbon atoms, such as alkanetetraenal, preferably alkanetetraenal having 9 to 25 carbon atoms], alkopentaenal [such as eicosapentaenal, preferably alkanepentaenal having 11 to 30 carbon atoms, such as alkanepentaenal], and corresponding ketones and acetals. Furthermore, when the carbonyl compound has isomers (eg, cis-trans isomers), any isomer (eg, both cis and trans isomers) is included. As described above, acetals, which are condensates of aldehydes and alcohols, can also be used as carbonyl compounds. Acetals are not particularly limited, and examples thereof include condensates of aldehydes and primary alcohols (such as methanol). These carbonyl compounds may be used alone or in combination of two or more. Furthermore, from the perspective of water solubility, the carbonyl compound is preferably composed of a monocarbonyl compound (monoaldehyde, etc.). Even when a polycarbonyl compound (for example, a polyaldehyde such as a dialdehyde) is used, in most cases it is used at a level that can ensure water solubility, for example, by reducing the amount of the polycarbonyl compound. Furthermore, the acetal skeleton having a polymerizable unsaturated bond (e.g., the acetal skeleton represented by the above formula (a1)) may be a skeleton that can be introduced via a hydroxyl group, or may be an acetal skeleton derived from (introduced via) two adjacent hydroxyl groups (e.g., the hydroxyl groups of a vinyl alcohol unit). For example, when a carbonyl compound having a polymerizable unsaturated bond (e.g., an aldehyde, a ketone, etc.) is used, two adjacent OH groups in the PVA polymer may be acetalized with the carbonyl compound having a polymerizable unsaturated bond, thereby obtaining a PVA polymer (A) containing an acetal skeleton (a) having a polymerizable unsaturated bond. The acetal skeleton having a polymerizable unsaturated bond (for example, the acetal skeleton represented by the above formula (a1)) may or may not have an ionic group (ionic skeleton). The PVA-based polymer (A) may contain an acetal skeleton having a polymerizable unsaturated bond alone or in combination of two or more acetal skeletons having a polymerizable unsaturated bond. In the PVA-based polymer (A), the content of the acetal skeleton (a) [or a polymerizable unsaturated bond, such as a skeleton represented by formula (a1)] in the PVA-based polymer (A) can be selected from a range of about 0.001 mol% or more (e.g., 0.005 mol% or more) per monomer unit, for example, it can be 0.01 mol% or more, preferably 0.05 mol% or more, further preferably 0.1 mol% or more, especially 0.2 mol% or more, etc., 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, further preferably 1 mol% or less]. Furthermore, as described above, these ranges (upper limit and lower limit) may be appropriately combined to select a range (for example, 0.01 to 3 mol%, 0.05 to 5 mol%, etc.). Specifically, the content of the acetal skeleton (a) (or polymerizable unsaturated bond) in the PVA-based polymer (A) can be 0.05 to 5 mol%, preferably 0.1 to 3 mol%, and further preferably 0.2 to 2 mol% per monomer unit. The content of 1 mol% refers to the situation where there is one acetal skeleton (a) (e.g., the skeleton represented by formula (a1)) per 100 monomer units (e.g., the total of vinyl alcohol units, vinyl ester units, etc.). When the content is as described above, the performance as a dispersant (dispersion stabilizer) can be efficiently achieved (for example, excellent polymerization stability, efficient acquisition of a vinyl chloride resin having an appropriate average particle size or excellent plasticizer absorption, etc.). Furthermore, by setting the upper limit value not too high, it is easy to improve the preparation properties of the aqueous solution, the storage stability, and the dispersibility in warm water. Furthermore, the method for measuring the content of the acetal skeleton (a) is not particularly limited, and it can be measured by, for example, NMR. As a specific example, the PVA polymer (A) can be dissolved in a d6-DMSO solvent, and the content of the acetal skeleton (a) can be measured by, for example, NMR. 1 This was measured by H-NMR, and the signal derived from the polymerizable unsaturated bond (ethylenic double bond, etc.) possessed by the acetal skeleton (a) was analyzed to determine the content of the acetal skeleton (a). The PVA polymer (A) may contain an ionic skeleton (b). If the PVA polymer (A) contains an ionic skeleton (b), the PVA aqueous solution can be easily prepared, and the stability of the PVA aqueous solution (and thus the 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 to 70°C), the PVA can be prevented from precipitating in the polymerizer. Furthermore, by introducing the ionic skeleton (b), such effects [such as excellent dispersibility (for example, dispersibility with respect to water or warm water)] can be achieved, but on the other hand, if only introduced, there is a risk of reduced thermal stability. In the present invention, even if the ionic skeleton (b) is introduced in this way, requirement (2) can be fully satisfied, and even excellent dispersibility can be achieved, and improvement of the thermal stability of the resin (imparting sufficient thermal stability) can be achieved efficiently. [Ionic skeleton (b)] The ionic skeleton (b) has an ionic group. Examples of the ionic group include an anionic group {e.g., an acid group [e.g., a carboxyl group, a sulfonic acid group (-SO 3H), phosphate group, etc.], cationic group [such as amine group, ammonium (ammonium cation)], salts thereof (such groups forming salts), etc. Examples of salts include metal salts [e.g., alkali or alkaline earth metal salts (e.g., lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts), halides (e.g., chlorides, bromides, iodides), etc.], depending on the anionic or cationic properties. When the ionic group is a polyacid, the salt may be a single salt (of the same type) or a combination of two or more salts. Among the ionic groups, preferred are acid groups (especially carboxyl groups and sulfonic acid groups) and their salts {salts of acid groups, 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 form of the ionic skeleton (b) is not particularly limited as long as it has an ionic group. Examples thereof include: (1) an acetal skeleton (acetal group, acetal unit) having an ionic group, (2) a skeleton corresponding to (or derived from) a monomer having an ionic group, (3) a skeleton corresponding to (or derived from) other compounds capable of introducing ionic groups {such as chain transfer agents having ionic groups [such as alcohols, carbonyl compounds (aldehydes, ketones, etc., especially aldehydes, etc.), thiols, etc.], etc.}, etc. Furthermore, the ionic skeleton (b) (one ionic skeleton (b)) only needs to have one or more ionic groups, and may have two or more ionic groups. The PVA-based polymer (A) may have the ionic skeleton (b) alone or in combination of two or more ionic skeletons (b). In the PVA-based polymer (A), the content (ratio, content ratio) of the ionic skeleton (b) (e.g., the above-mentioned skeletons (1), (2) and / or (3)) per monomer unit can be selected from a range of about 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 further preferably 0.05 mol% or more, etc., 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 further preferably 1 mol% or less]. Specifically, the content of the ionic skeleton (b) (e.g., the above-mentioned skeletons (1), (2) and / or (3)) per monomer unit can be 0.01 to 5 mol%, preferably 0.03 to 2 mol%, and further preferably 0.05 to 1 mol%. Furthermore, the so-called 1 mol% content refers to the situation where there is one ionic skeleton (b) (for example, the above-mentioned skeletons (1), (2) and / or (3)) for every 100 monomer units (for example, the total of monomer units such as vinyl alcohol units and vinyl ester units). When the content is as described above, the preparation efficiency and storage stability of the aqueous solution of the PVA-based polymer (A) and the dispersibility in warm water can be easily improved. Furthermore, by setting the upper limit value not too high, the performance of the PVA-based polymer (A) as a dispersion stabilizer can be efficiently achieved (for example, excellent polymerization stability, and the ability to efficiently obtain a vinyl chloride-based resin having an appropriate average particle size or excellent plasticizer absorption, etc.). Furthermore, the method for determining the content of the ionic group can be selected according to the type of the skeleton having the ionic group, and is not particularly limited. For example, it can be determined by NMR, titration, UV absorbance, etc. As a specific example, the content of the skeleton represented by the following formula (b1-1) can be determined by dissolving the PVA polymer (A) in d6-DMSO solvent and using 1 H-NMR analysis is performed to determine the amount of carboxyl groups by analyzing the signals from substituents (e.g., hydrogen) on the benzene ring. Alternatively, the PVA polymer (A) can be completely saponified, and a sample obtained after Soxhlet extraction (e.g., after removing the sodium acetate) can be dissolved in water. After adding a small amount of sodium hydroxide (NaOH), conductivity titration can be performed with dilute hydrochloric acid. The amount of carboxyl groups can be determined from the amount of hydrochloric acid titrated. Furthermore, if the acetal skeleton containing an ionic group has a UV (ultraviolet) absorbing structure, the content of the acetal skeleton containing an ionic group can be determined by measuring the UV absorbance of an aqueous solution containing the PVA polymer (A). Furthermore, in the PVA-based polymer (A), the content of the acetal skeleton (a) (content per monomer unit) relative to 1 mol of the ionic skeleton (b) (for example, the above-mentioned skeletons (1), (2) and / or (3)) may be 50 mols or less (for example, 30 mols or less, 20 mols or less), preferably 15 mols or less, and further preferably 10 mols or less, and may be 0.05 mols or more (for example, 0.1 mols or more, 0.5 mols or more), preferably 1 mol or more, further preferably 2 mols or more, and especially 3 mols or more. With such a ratio, it is easy to achieve both excellent preparation properties and storage stability of the aqueous solution and excellent performance as a dispersion stabilizer. The method for introducing the ionic backbone (b) can utilize a known method depending on the embodiment. Examples of such introduction methods include the following: (1) Acetalizing a PVA polymer (sometimes referred to as a PVA polymer (C)) with a carbonyl compound having an ionic group (aldehyde, acetal thereof, ketone, etc., particularly aldehyde) to obtain a PVA polymer (B-1) having an ionic group; (2) Saponifying a polyvinyl ester polymer having an ionic group obtained by copolymerizing a monomer having an ionic group with a vinyl ester to obtain a polyvinyl ester polymer having an ionic group; (3) Saponifying a polyvinyl ester polymer (having an ionic group) obtained by polymerizing a vinyl ester in the presence of a chain transfer agent having an ionic group (alcohol, aldehyde, thiol, etc.) to obtain a PVA polymer (B-4) having an ionic group. Hereinafter, various aspects of the ionic skeleton (b) will be described in detail. ((1) Acetal Skeleton Having an Ionic Group) As described above, the ionic group is possessed by the acetal skeleton (acetal group, acetal unit) (substituting on the acetal skeleton). The acetal may be either a cyclic acetal or a non-cyclic (chain) acetal, and is preferably a cyclic acetal. A typical acetal skeleton having an ionic group includes a skeleton (structural unit) represented by the following formula (b1). Therefore, the acetal skeleton having an ionic group may include a skeleton represented by the following formula (b1). [Chemistry 3] (wherein R represents a group having an ionic group) In the above formula (b1), R is a group having an ionic group. R may be an ionic group itself or a linking group having an ionic group (a group consisting of an ionic group and a linking group substituted with the ionic group). Examples of the linking group (the base) include hydrocarbon groups. Examples of the hydrocarbon group include aliphatic hydrocarbon groups [such as alkyl groups [such as chain alkyl groups (such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, etc.)] 1-30 Alkyl), cycloalkyl (such as cyclopentyl, cyclohexyl, etc. C 3-10 cycloalkyl) and other saturated aliphatic hydrocarbon groups], aromatic hydrocarbon groups [such as aryl (such as phenyl, tolyl, xylyl, naphthyl, etc. C 6-20 Aryl), aralkyl (such as benzyl, phenethyl, etc. C 6-20 Aryl-C 1-4 Alkyl) etc.] etc. In addition to the ionic group, the linking group (hydrocarbon group) may also have a substituent (a substituent other than the ionic group). The substituent is not particularly limited, and examples thereof include a hydroxyl group, a halogen atom, an acyl group, an ester group, an alkoxy group, a nitro group, and a group different from the base group (e.g., an aromatic group such as an aryl group). A substituent may be present on the linking group (hydrocarbon group) alone or in combination of two or more. In the linking group (hydrocarbon group, etc.) having an ionic group, the number of the ionic group may be one or more, and two or more ionic groups may substitute on the linking group. Specific examples of the acetal skeleton having an ionic group (skeleton represented by formula (b1)) include a skeleton in which R in the above formula (b1) is an ionic group (such as a carboxyl group and its salts), and a skeleton represented by the following formula (b1-1). [Chemistry 4] (Where R 1~R 5 represents a hydrogen atom or a substituent. 1~R 5) at least one of which is an ionic group In the above formula (b1-1), the ionic group and the substituent may be those exemplified above. 1~R At least one of 5 is an ionic group, and preferably any one of them is an ionic group. 1~R One of the five is an ionic group (such as a carboxyl group, a sulfonic acid group, or a salt thereof), and four are hydrogen atoms. Furthermore, the acetal skeleton having an ionic group {e.g., the group represented by formula (b1) (or R< in formula (b1))} can be derived from a corresponding carbonyl compound (e.g., an aldehyde, its acetal, a ketone, etc.), particularly an aldehyde (e.g., RCHO). Furthermore, the carbonyl compound may have a substituent. Examples of such carbonyl compounds include alkanals having an ionic group (e.g., alkanals having an acid group or salts thereof, such as glyoxylic acid, formylacetic acid, formylpropionic acid, and salts thereof), aromatic hydrocarbon carbonaldehydes having an ionic group (e.g., aromatic hydrocarbon carbonaldehydes having an acid group or salts thereof, such as formylbenzoic acid (e.g., 4-formylbenzoic acid), formylbenzenesulfonic acid (e.g., 2-formylbenzenesulfonic acid, 4-formylbenzene-1,3-disulfonic acid), and salts thereof), and aldehydes (especially monoaldehydes), corresponding ketones, and acetals. Furthermore, when the carbonyl compound has isomers (e.g., cis-trans isomers), any isomer (e.g., both cis and trans isomers) is encompassed. Furthermore, the ionic groups in the carbonyl compound may be derivatized (esterified, anhydrified, etc.) as long as they can form ionic groups in the PVA polymer (A). For example, even esters (e.g., alkyl esters) or acid anhydrides may be used as long as they can form (e.g., by hydrolysis) the corresponding acid groups (carboxyl groups, sulfonic acid groups) or their salts in the PVA polymer (A) (the same applies to ionic groups hereinafter). These carbonyl compounds may be used alone or in combination of two or more. Furthermore, from the perspective of water solubility, the carbonyl compound is preferably composed of a monocarbonyl compound (monoaldehyde, etc.). Even when a polycarbonyl compound (for example, a polyaldehyde such as a dialdehyde) is used, in most cases, the amount thereof is reduced to a level that can ensure water solubility. Furthermore, the acetal skeleton having an ionic group (e.g., the acetal skeleton represented by formula (b1) above) may be a skeleton that can be introduced via a hydroxyl group, or may be an acetal skeleton derived from (introduced via) two adjacent hydroxyl groups (e.g., the hydroxyl groups of a vinyl alcohol unit). For example, when using a carbonyl compound having an ionic group (e.g., an aldehyde, a ketone, etc.), two adjacent OH groups in the PVA polymer may be acetalized with the carbonyl compound having an ionic group, thereby obtaining a PVA polymer (A) having an acetal skeleton having an ionic group. The acetal skeleton having an ionic group (for example, the acetal skeleton represented by the above formula (b1)) may or may not have a polymerizable unsaturated bond. The PVA-based polymer (A) may or may not contain an acetal skeleton having an ionic group. The PVA-based polymer (A) may contain an acetal skeleton having an ionic group alone or in combination of two or more acetal skeletons having an ionic group. The method for incorporating (introducing) an acetal skeleton having an ionic group (eg, an acetal skeleton having a carboxyl group, a sulfonic acid group, or a salt thereof) into the PVA polymer (A) is not particularly limited, and conventional techniques can be used. In a representative method, the PVA polymer (C) can be acetalized with a carbonyl compound having an ionic group (aldehyde, its acetal, ketone, etc.) as described below. Furthermore, by acetalizing the PVA polymer (C) with a carbonyl compound having an ionic group in this manner, a PVA polymer (B-1) having an ionic group can be obtained. During acetalization, the carbonyl compound having a polymerizable unsaturated bond coexists, thereby obtaining a PVA polymer (A) having both an acetal skeleton (a) and an ionic group, which is preferred. Examples of the aldehyde having an ionic group include glyoxylic acid, 2-formylbenzoic acid, 4-formylbenzoic acid, sodium 2-formylbenzenesulfonate, sodium 4-formylbenzenesulfonate, and disodium 4-formylbenzene-1,3-disulfonate. 4-Formylbenzoic acid and sodium 2-formylbenzenesulfonate are preferred. As described above, acetals, which are condensates of aldehydes and alcohols, can also be used as carbonyl compounds. Acetals are not particularly limited, and examples thereof include condensates of aldehydes and primary alcohols (such as methanol). The carbonyl compound can be used alone or in combination of two or more. ((2) Skeleton corresponding to the monomer having an ionic group) The monomer having an ionic group is not particularly limited and can be appropriately selected depending on the type of the ionic group. Specific examples of the monomer include monomers having an acid group [e.g., monomers having a carboxyl group [e.g., monocarboxylic acids (e.g., aliphatic unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and crotonic acid), polycarboxylic acids (e.g., aliphatic unsaturated dicarboxylic acids such as itaconic acid, maleic acid, and fumaric acid), and salts thereof], monomers having a sulfonic acid group [e.g., alkenylsulfonic acids (e.g., vinylsulfonic acid, allylsulfonic acid), alkenyl aromatic hydrocarbon sulfonic acids (e.g., styrenesulfonic acid), amide monomers having a sulfonic acid group (e.g., 2-acrylamido-2-methylpropanesulfonic acid), and salts thereof]], and monomers having other ionic groups [e.g., monomers having an amino group (e.g., (meth)acrylamidopropyldimethylamine), and salts thereof]. Furthermore, as described above, if an ionic group can be introduced into the PVA polymer (A) (for example, by hydrolysis, ultimately forming an ionic group in the PVA polymer (A)), such a derivative may also be a derivative [e.g., an anhydride (e.g., maleic anhydride), an ester (e.g., an alkyl ester)]. In other words, such a derivative may also be referred to as a derivative capable of forming an ionic group. As a specific example, even when using an acrylic acid ester, it is sufficient to introduce acrylic acid or a salt thereof into the final PVA polymer (A). Therefore, the acrylic acid ester can introduce a backbone corresponding to acrylic acid or a salt thereof into the PVA polymer (A). These monomers can be used alone or in combination of two or more. As described above, for example, by saponifying a polyvinyl ester polymer containing an ionic group obtained by copolymerizing such a monomer having an ionic group with vinyl ester, a PVA polymer (B-3) containing an ionic group can be obtained. ((3) Skeletons corresponding to other compounds capable of introducing ionic groups) In this skeleton (3), as described above, examples of compounds capable of introducing ionicity include alcohols having ionic groups, carbonyl compounds having ionic groups (aldehydes, ketones, etc., especially aldehydes), and thiols having ionic groups. These compounds generally function as chain transfer agents. Among these, mercaptans are preferred from the viewpoint of high chain transfer properties (thus, easy introduction of ionic groups). Examples of the thiol having an ionic group include thiols having an acid group [e.g., thiols having a carboxyl group [e.g., mercapto saturated fatty acids (e.g., mercaptoalkanoic acids such as 3-mercaptopropionic acid and mercaptosuccinic acid)], thiols having a sulfonic acid group [e.g., mercaptoalkanesulfonic acids (e.g., 3-mercapto-1-propanesulfonic acid)], and salts thereof (e.g., sodium 3-mercapto-1-propanesulfonate)], etc.], and the like. As described above, for example, by polymerizing vinyl esters in the presence of such chain transfer agents (alcohols, aldehydes, thiols, etc.) having ionic groups, the ionic groups from the chain transfer agent can be introduced into the terminals of the vinyl ester polymer. Subsequently, by saponifying the vinyl ester polymer, a PVA polymer (B-4) containing ionic groups at the terminals can be obtained. Furthermore, the PVA polymer (A) may have other acetal skeletons (acetal groups, acetal units) that do not fall within the scope of the acetal skeleton (a) or the acetal skeleton having an ionic group. Examples of such other acetal skeletons include skeletons in which R' in the above formula (a1) is a group (such as an aliphatic group, an aromatic group, etc.) that does not have an ionic group and a polymerizable unsaturated bond. Examples of such groups include: aliphatic groups [such as alkyl groups (such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, etc.); 1-30Alkyl), cycloalkyl (such as cyclopentyl, cyclohexyl, etc. C 3-20 Cycloalkyl etc.) etc.], aromatic group [such as aryl (such as phenyl, naphthyl etc. C 6-20 aryl) etc.] etc. The method for introducing such an alternative acetal skeleton is not particularly limited and conventional methods may be used, for example, a method of acetalizing the PVA polymer (C) with an aldehyde corresponding to the alternative acetal skeleton. Furthermore, in this method, the alternative acetal skeleton is generally formed from two adjacent vinyl alcohol units. Examples of such aldehydes include aliphatic aldehydes such as alkanals [e.g., acetaldehyde, propionaldehyde, butyraldehyde, valeraldehyde, hexanal, heptanal, octanal, nonanal, decanal, undecanal, dodecanal, 2-methylbutanal, 2-ethylbutanal, 2-methylvaleraldehyde, 2-ethylhexanal], cycloalkane carboaldehydes [e.g., cyclopentanecarboxaldehyde (cyclopentanecarboaldehyde), cyclohexanecarboaldehyde (cyclohexanecarboaldehyde)], and aromatic aldehydes such as aromatic carboaldehydes (e.g., benzaldehyde, naphthaldehyde). The PVA-based polymer (A) has at least a vinyl alcohol unit, and may have a vinyl alcohol unit and an unhydrolyzed (saponified) unit [such as a vinyl ester unit (or a unit derived from a vinyl ester-based monomer, such as a vinyl acetate unit)]. Furthermore, the PVA polymer (A) may optionally contain other units (units other than those exemplified above, such as vinyl alcohol units, unhydrolyzed units, acetal skeletons (a), and ionic skeletons (b)). Examples of such units include units derived from other monomers exemplified in the section of the PVA polymer (C) below. The saponification degree 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), further preferably 40 mol% or more (e.g., 45 mol% or more), and especially 50 mol% or more (e.g., 55 mol% or more, 60 mol% or more). The upper limit of the saponification degree of the 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 further preferably 85 mol % or less (e.g., 80 mol % or less). Specifically, the saponification degree of the PVA-based polymer (A) may be, for example, 20 to 90 mol % (eg, 50 to 90 mol %), preferably 55 to 85 mol %, and further preferably 60 to 80 mol %. A saponification degree that is not too low is preferred because it provides excellent preparation of aqueous solutions, storage stability, and warm water dispersibility. A saponification degree that is not too high is preferred because it facilitates the development of excellent dispersant properties (e.g., excellent polymerization stability, ease of efficiently obtaining vinyl chloride resins with an appropriate average particle size or high plasticizer absorption). The saponification degree can be determined, for example, by the method for measuring the saponification degree of PVA specified in JIS K 6726. When the PVA-based polymer (A) has vinyl ester units, the ratio of the ionic backbone (b) (the ratio calculated as monomer units) relative to 100 mols of vinyl ester units can be 10 mols or less, preferably 5 mols or less, and more preferably 3 mols or less, and can be 0.01 mol or more (for example, 0.05 mol or more, 0.1 mol or more), preferably 0.2 mol or more, and more preferably 0.3 mol or more. With such a ratio, it is easy to achieve both excellent preparation properties and storage stability of the aqueous solution and excellent performance as a dispersion stabilizer. The viscosity of a 4% by mass aqueous solution (20°C) of the PVA-based polymer (A) is not particularly limited, and can be selected from a range of about 1 mPa·s or more (e.g., 1.5 mPa·s or more), and can be 2 mPa·s or more (e.g., 2.2 mPa·s or more), preferably 2.5 mPa·s or more (e.g., 2.7 mPa·s or more), and further 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. The upper limit of the viscosity of a 4% by mass aqueous solution (20° C.) of the PVA-based polymer (A) is not particularly limited. For example, it can be selected from a range of approximately 2000 mPa·s or less (e.g., 1500 mPa·s or less, 1000 mPa·s or less, 500 mPa·s or less). Typically, it can be selected from a range of approximately 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 may 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 the PVA-based polymer (A) may be, for example, about 1 to 500 mPa·s (e.g., 2 to 300 mPa·s, 1 to 100 mPa·s, 2 to 100 mPa·s, 2.5 to 30 mPa·s), or may be about 20 mPa·s or less (e.g., 3 to 15 mPa·s, 3.2 to 10 mPa·s, 3.4 to 9 mPa·s, 3.6 to 8 mPa·s). The (average) degree of polymerization of the PVA-based polymer (A) is not particularly limited, and may be, for example, 100 or more (e.g., 120 or more), preferably 150 or more (e.g., 160 or more), and further preferably 180 or more (e.g., 200 or more, 220 or more, 250 or more, 280 or more, 300 or more), etc. The upper limit of the (average) degree of polymerization of the PVA-based polymer (A) is not particularly limited. For example, it can be selected from a range of about 10,000 or less (for example, 8,000 or less, 5,000 or less), and can be 3,000 or less (for example, 2,500 or less), preferably 2,000 or less (for example, 1,500 or less), and further preferably 1,000 or less (for example, 800 or less). Specifically, the (average) degree of polymerization of the PVA-based polymer (A) may be, for example, 120 to 3000 (eg, 200 to 2000), preferably 250 to 1500, and more preferably about 300 to 1000. If the viscosity of a 4% by mass aqueous solution or the degree of polymerization of the PVA polymer (A) is not too low, it is advantageous in terms of polymerization stability, suppression of scale adhesion, and suppression of coarsening of the resulting vinyl resin. Furthermore, if the viscosity of a 4% by mass aqueous solution or the degree of polymerization is not too high, it is advantageous in terms of ease of preparation and storage stability of the aqueous solution, and excellent warm water dispersibility. The viscosity of a 4 mass % aqueous solution (20° C.) can be determined, for example, by the method specified in JIS K 6726. The degree of polymerization can also be determined, for example, by the method specified in JIS K 6726, or may be a calculated (converted) value based on other analytical techniques [e.g., a calculated value (converted value) based on the viscosity of a 4 mass % aqueous solution]. The cloud point of a 4 mass % aqueous solution of the PVA-based polymer (A) is preferably 20° C. or higher (e.g., higher than 20° C., higher than 22° C., higher than 23° C., higher than 24° C., higher than 25° C.), more preferably higher than 27° C., and may be higher than 30° C. The upper limit of the cloud point of a 4% by mass aqueous solution of the PVA polymer (A) is not particularly limited, and may be, for example, 75°C, 70°C, 65°C, 60°C, 55°C, 50°C, etc. Typically, the cloud point of a 4% by mass aqueous solution of the PVA polymer (A) may be, for example, 25 to 50°C. When the cloud point is such, the aqueous solution is excellent in preparation efficiency and storage stability. The cloud point of a 4% by mass aqueous solution can be adjusted by adjusting the saponification degree, polymerization degree, and ionic skeleton (ionic group) content of the PVA-based polymer (A). [Aqueous Liquid] The PVA polymer (A) can be used directly as a dispersion stabilizer (dispersant), or it can be used as an aqueous liquid dissolved in water. The aqueous liquid of the present invention only needs to contain the PVA polymer (A) and water. For example, the aqueous liquid is one in which the PVA polymer (A) is dispersed or dissolved in water as a dispersant. The content of the PVA-based polymer (A) in the aqueous liquid is not particularly limited, and may be, for example, 1% by mass or more (e.g., 2% by mass or more, 3% by mass or more), or 80% by mass or less (e.g., 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less). The aqueous liquid of the present invention has good stability. From the perspective of improving shelf stability, the aqueous liquid may contain a water-soluble organic solvent. 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 may be mixed. When a water-soluble organic solvent is included, the ratio of the water-soluble organic solvent to the total solvent can be, for example, 70 mass % or less (e.g., 60 mass % or less), preferably 50 mass % or less, and further preferably 30 mass % or less. In particular, from the perspective of environmental considerations or improved workability, the content of the organic solvent is preferably 5 mass % or less relative to the total solvent or the aqueous liquid. [Production Method] In the present invention, the method for producing the PVA polymer (A) is not particularly limited. For example, the PVA polymer (A-1) can be obtained by acetalizing the PVA polymer (C) with an aldehyde having a polymerizable unsaturated bond. When the PVA polymer (A) contains an ionic group, a carbonyl compound having a polymerizable unsaturated bond (e.g., a monoaldehyde) and a carbonyl compound having an ionic group (e.g., an aldehyde) are simultaneously subjected to an acetalization reaction on the (non-ionic) PVA polymer (C), thereby obtaining a PVA polymer (A-2) containing an ionic group. Alternatively, a PVA polymer (B-3 or B-4) containing an ionic group is subjected to an acetalization reaction using a carbonyl compound having a polymerizable unsaturated bond (e.g., a monoaldehyde), thereby obtaining PVA polymers (A-3, A-4) containing an ionic group. Therefore, the steps for producing the PVA polymers (A-1, A-2, A-3, A-4) are, for example, divided into a step of producing the PVA polymer (C) or the PVA polymer (B-3 or B-4) containing an ionic group, and a step of acetalizing one of these PVA polymers (acetalization step). The method for producing the PVA polymer (C) or the ionic group-containing PVA polymer (B-3 or B-4) is not particularly limited, and conventionally known methods can be used. The ionic group-containing PVA polymer (B-3 or B-4), the PVA polymer (C), and the acetalization step are described in detail below. [PVA polymers (B-3), (B-4) and (C)] The PVA polymer (C) is not particularly limited, and for example, a PVA polymer obtained by saponifying (reacting) a vinyl ester polymer [a saponified product of a vinyl ester polymer (a polymer having a vinyl ester monomer as a polymerization component)] can be used. Furthermore, PVA polymers (B-3) and (B-4) can be obtained, for example, by using another monomer containing a monomer having an ionic group, and a chain transfer agent containing a chain transfer agent having an ionic group, respectively, in the production of PVA polymer (C) as described below. The vinyl ester polymer can be obtained by polymerizing at least a vinyl ester monomer (polymerizing as a polymerization component). The polymerization method is not particularly limited and can be based on conventionally known methods, such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Considering the control of the degree of polymerization or the saponification reaction after polymerization, solution polymerization using methanol as the solvent or suspension polymerization using water or water / methanol as the dispersion medium is preferred, but the present invention is not limited to these methods. The vinyl ester monomers that can be used in the polymerization are not particularly limited. Examples include fatty acid vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl octanoate, and vinyl chorate. One or more of these vinyl ester monomers can be used. Among these, vinyl acetate is preferred from an industrial perspective. When polymerizing vinyl ester monomers, as long as the effects of the present invention are exerted, vinyl ester monomers and other monomers can be copolymerized. In other words, the polymerization components of the vinyl ester polymer may include vinyl ester monomers and other monomers. There are no particular limitations on other monomers that can be used, and examples include: α-olefins (such as ethylene, propylene, n-butene, isobutylene, etc.), (meth)acrylic acid and its salts, (meth)acrylates [such as (meth)acrylate alkyl esters (such as (meth)acrylate methyl, (meth)acrylate ethyl, (meth)acrylate n-propyl, (meth)acrylate isopropyl, (meth)acrylate n-butyl, (meth)acrylate isobutyl, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, octadecyl (meth)acrylate, etc.), (meth)acrylate C 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, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, etc.) 1-20 alkyl vinyl ethers, etc.), nitriles (e.g., acrylonitrile, methacrylonitrile, etc.), vinyl halides (e.g., vinyl chloride, vinyl fluoride, etc.), vinylidene halides (e.g., vinylidene chloride, vinylidene fluoride, etc.), allyl compounds (e.g., allyl acetate, allyl chloride, etc.), vinylsilane compounds (e.g., vinyltrimethoxysilane, etc.), fatty acid alkenyl esters (e.g., isopropenyl acetate, etc.), etc. These other monomers may be used alone or in combination. Here, by using other monomers including a monomer having an ionic group as other monomers, a PVA-based polymer (B-3) can be obtained. Examples of the ionic monomer include those exemplified above, monomers having an acid group [e.g., monomers having a carboxyl group [e.g., monocarboxylic acids (e.g., aliphatic unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and crotonic acid), polycarboxylic acids (e.g., aliphatic unsaturated dicarboxylic acids such as itaconic acid, maleic acid, and fumaric acid), and salts thereof], monomers having a sulfonic acid group [e.g., alkenylsulfonic acids (e.g., vinylsulfonic acid, allylsulfonic acid), alkenyl aromatic hydrocarbon sulfonic acids (e.g., styrenesulfonic acid), amide monomers having a sulfonic acid group (e.g., 2-acrylamido-2-methylpropanesulfonic acid), and salts thereof], and monomers having other ionic groups [e.g., monomers having an amino group (e.g., (meth)acrylamidopropyldimethylamine), and salts thereof]. When other monomers are used, the content of the other monomers can be appropriately selected according to the monomers used, and can be, for example, 0.1 to 20% by mass relative to the total amount of the polymerization components. Furthermore, during the polymerization of vinyl ester monomers, a chain transfer agent may be co-present for purposes such as adjusting the degree of polymerization of the resulting vinyl ester polymer. Chain transfer agents are not particularly limited, and examples include: aldehydes such as acetaldehyde, propionaldehyde, butyraldehyde, and benzaldehyde; ketones such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; mercaptans such as 2-hydroxyethanethiol, dodecyl mercaptan, 3-mercaptopropionic acid, mercaptosuccinic acid, and sodium 3-mercapto-1-propanesulfonate; and organic halogens such as carbon tetrachloride, trichloroethylene, and perchloroethylene. The present inventors have found that the use of conventional chain transfer agents such as aldehydes or organic halides tends to decrease the temperature (T) at which the weight loss rate exceeds 0.5% / min (the reason for this is not yet determined, but it is presumed that thermally unstable structures are easily introduced into the terminal portion, etc.). Therefore, even when using aldehydes or organic halides, it is desirable to limit the use to a level that fully satisfies requirement 2. Here, by using a chain transfer agent including a chain transfer agent having an ionic group as the chain transfer agent, a PVA-based polymer (B-4) can be obtained. Examples of the chain transfer agent having an ionic group include those exemplified above, for example, alcohols having an ionic group, carbonyl compounds having an ionic group, and thiols having an ionic group {for example, thiols having an acid group [for example, thiols having a carboxyl group [for example, mercapto saturated fatty acids (for example, mercaptoalkanoic acids such as 3-mercaptopropionic acid and mercaptosuccinic acid)], thiols having a sulfonic acid group [for example, mercaptoalkanesulfonic acids (for example, 3-mercapto-1-propanesulfonic acid)], and salts thereof (for example, sodium 3-mercapto-1-propanesulfonate)], etc.} and the like. The amount of the chain transfer agent added is determined by the chain transfer constant of the added chain transfer agent and the polymerization degree of the target vinyl ester polymer, and is generally preferably 0.1 to 10% by mass relative to the total amount of the polymer components. By subjecting the vinyl ester polymer obtained as described above to a saponification reaction, the PVA polymer (C) (and further (B-3) and (B-4)) can be produced. The saponification reaction method for vinyl ester polymers is not particularly limited and can be carried out according to conventional methods. For example, alcoholysis or hydrolysis reactions using acidic catalysts such as alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; 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 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 solvents can be used alone or in combination. If a gel-like product precipitates as the saponification reaction proceeds, the gel can be crushed and dried to obtain a polyvinyl alcohol polymer. It is preferable to neutralize the remaining catalyst before drying. When using an alkaline catalyst, an acidic substance such as acetic acid or phosphoric acid is used as the neutralizing agent; when using an acidic catalyst, an alkaline substance such as sodium hydroxide or potassium hydroxide is used. The polyvinyl alcohol polymer (C) (and further (B-3) and (B-4)) can be dried in an oxidizing atmosphere such as air, or in an inert atmosphere (e.g., nitrogen). The drying temperature may be room temperature (natural drying), heated, or at a high temperature. From the perspective of efficient drying, the drying temperature is typically 30°C or higher, 35°C or higher, 40°C or higher, 45°C or higher, 50°C or higher, 55°C or higher, 60°C or higher, or 70°C or higher. The upper limit of the drying temperature is not particularly limited, and examples thereof include 250°C, 220°C, 200°C, 180°C, 160°C, 150°C, 140°C, 130°C, 120°C, 110°C, 100°C, 90°C, or 80°C. However, according to the inventors' research, drying in an oxygen-rich atmosphere such as air or at high temperatures tends to lower the temperature (T) at which the weight loss rate exceeds 0.5% / min (the reason is not yet certain, but it is presumed that this easily introduces thermally unstable structures into the main chain, etc.). Therefore, even when drying in an oxidizing atmosphere, under heating, or at high temperatures, it is desirable to limit the drying to a level that fully satisfies requirement 2. From this viewpoint, drying is preferably performed under an inert atmosphere, and is also preferably performed at a temperature not too high (e.g., below 120°C, less than 120°C, below 115°C, below 110°C, below 100°C, below 90°C, or 70-110°C). The drying time is not particularly limited and can be selected according to the drying temperature and the like, and can be, for example, about 1 to 12 hours. [Acetalization] In the present invention, the method for acetalizing the PVA polymers [(C), (B-3), (B-4)] using a carbonyl compound (aldehyde, etc.) having a polymerizable unsaturated bond or a carbonyl compound having an ionic group is not particularly limited, and a known acetalization method can be used. By acetalizing the PVA polymer (C) using a carbonyl compound having a polymerizable unsaturated bond, a PVA polymer (A-1) can be obtained. By acetalizing the PVA polymer (C) using a carbonyl compound having a polymerizable unsaturated bond and a carbonyl compound having an ionic group, a PVA polymer (A-2) can be obtained. Furthermore, by acetalizing the PVA polymers (B-3, B-4) having an ionic group using a carbonyl compound having a polymerizable unsaturated bond, PVA polymers (A-3, A-4) can be obtained. In the acetalization, the amount of the carbonyl compound used is not particularly limited, and may be, for example, 0.05 to 50 parts by mass, preferably 0.1 to 20 parts by mass, and more preferably 0.2 to 10 parts by mass, relative to 100 parts by mass of the PVA polymer. The acetalization reaction is preferably carried out in the presence of an acidic catalyst. The acidic catalyst is not particularly limited, and examples thereof 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. The amount of the acidic catalyst used is not particularly limited, and is, for example, 0.1 to 10 parts by mass relative to 100 parts by mass of the PVA-based polymer. Specific acetalization methods include, for example, the following methods: (i) saponifying a vinyl ester polymer in a solvent such as methanol using an alkaline catalyst such as sodium hydroxide to obtain a PVA polymer solution, then adding an aldehyde and an acidic catalyst to carry out acetalization, and then neutralizing with an alkaline substance to obtain a PVA polymer (A) solution; (ii) saponifying a vinyl ester polymer in a solvent such as methanol in the presence of an acidic catalyst as a saponification catalyst to obtain a PVA polymer, then adding an aldehyde and the like, and carrying out an acetalization reaction directly using the acidic catalyst used in the saponification reaction, and then neutralizing with an alkaline substance to obtain a PVA polymer (A) solution; (iii) In the presence of an acidic catalyst and an aldehyde, a vinyl ester polymer is subjected to simultaneous saponification and acetalization reactions in a solvent, followed by neutralization with an alkaline substance to obtain a solution of a PVA polymer (A); (iv) an aldehyde is added to an aqueous solution of a PVA polymer, the reaction is carried out in the presence of an acidic catalyst, and the reaction is then neutralized with an alkaline substance to obtain an aqueous solution of a PVA polymer (A); (v) an aldehyde is added directly to a slurry or powder of a PVA polymer, or a solution prepared by dissolving or dispersing the aldehyde in an organic solvent or water is added to the slurry or powder of the PVA polymer, the reaction is carried out in the presence of an acidic catalyst, and the reaction is then neutralized with an alkaline substance, and the excess solvent is removed to obtain a PVA polymer (A). In methods (i) to (iii), the solvent can be subsequently dried to obtain a solid, or the solvent can be replaced with water to obtain an aqueous solution. In method (iv), the PVA polymer (A) can be obtained as an aqueous solution, and thus can be directly used in suspension polymerization of vinyl chloride, etc. The method (v) of reacting in a slurry state can obtain the PVA polymer (A) in a solid form, making it easy to handle. Furthermore, in methods (i) to (v), the method for preparing the PVA polymer into an aqueous solution, saponification, neutralization, dissolution, dispersion, and drying are not particularly limited, and conventional methods may be used. The alkaline substance used for neutralization is not particularly limited, and examples thereof include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. From the viewpoint of reaction rate, the pH of the reaction solution during the acetalization reaction is preferably 3.0 or less, more preferably 1.0 or less. The pH of the reaction solution after neutralization is preferably 4.7 to 9.0, more preferably 7.0 to 8.5. The polyvinyl alcohol polymer (A) can be dried in the same manner as the polyvinyl alcohol polymer (C) (and further (B-3) and (B-4)). The drying conditions [drying atmosphere, drying temperature, drying time (e.g., 1 to 12 hours, etc.)] and preferred aspects (and reasons therefor) are also as described above (e.g., drying in an inert atmosphere such as nitrogen at a moderate temperature (e.g., 70 to 110°C, etc.)). [Applications, Method for Producing Vinyl Polymers, etc.] The PVA polymer (A) can be used in various applications (e.g., as a dispersant, for film applications, etc.). As described above, it is particularly preferably used as a dispersion stabilizer [or dispersant, for example, a dispersion stabilizer (dispersant) for polymerization (e.g., suspension polymerization)]. Therefore, the following describes the use of the dispersion stabilizer of the present invention (or PVA polymer (A), hereinafter the same) and the method for producing a vinyl polymer by polymerization (particularly suspension polymerization) of vinyl monomers using the dispersion stabilizer. Suspension polymerization, as used herein, refers to a polymerization method in which an insoluble vinyl monomer and an oil-soluble polymerization initiator are added to an aqueous medium and stirred to form minute droplets containing the vinyl monomer, within which polymerization proceeds. The aqueous medium is not particularly limited and includes, for example, water, aqueous solutions containing various additives, and mixed solvents of water and a water-miscible organic solvent. The PVA polymer (A) described above in the present invention can be used as a dispersion stabilizer during suspension polymerization of vinyl monomers. The vinyl monomers are not particularly limited, but are preferably vinyl chloride, vinylidene chloride, styrene, acrylates, methacrylates, vinyl acetate, acrylonitrile, and other vinyl monomers commonly used in suspension polymerization. Vinyl chloride monomers are particularly preferred. Examples of the vinyl chloride monomer include vinyl chloride monomer (vinyl chloride), and mixtures of vinyl chloride monomer and other monomers copolymerizable therewith. Examples of other monomers copolymerizable with the vinyl chloride monomer include vinylidene chloride, vinyl acetate, ethylene, propylene, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, styrene, vinylalkoxysilane, maleic acid, hydroxyalkyl acrylate, allylsulfonic acid, and vinylsulfonic acid. Therefore, the dispersion stabilizer of the present invention is suitable for the suspension polymerization of vinyl monomers including vinyl chloride monomers (especially vinyl chloride), and can be preferably used in the homopolymerization of vinyl chloride carried out by suspension polymerization. It can also be used in the binary or multi-component copolymerization of vinyl chloride with one or more known monomers copolymerizable with vinyl chloride carried out by suspension polymerization. Among them, it can be particularly preferably used as a dispersion stabilizer in the copolymerization of vinyl chloride and vinyl acetate carried out by suspension polymerization. Vinyl chloride resins can be obtained by suspension polymerization of vinyl monomers including vinyl chloride. In the production of vinyl chloride resins, preferably 50 to 100 mol% (or 50 to 100 mass%) of vinyl chloride is present relative to the total amount of vinyl monomers used. The polymerization initiator used in the suspension polymerization of vinyl monomers may be any known one. Examples of the initiator include percarbonate compounds such as diisopropyl peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, and di(ethoxyethyl) peroxydicarbonate; perester compounds such as benzoyl peroxide, t-butyl peroxyneodecanoate, α-isopropylphenyl peroxyneodecanoate, and t-butyl peroxydecanoate; peroxides such as acetylcyclohexylsulfonyl peroxide and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; azo compounds such as 2,2'-azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, and azobis(4-methoxy-2,4-dimethylvaleronitrile); benzoyl peroxide and lauryl peroxide. These compounds may also be used in combination with potassium persulfate, ammonium persulfate, hydrogen peroxide, and the like. The primary function of a dispersion stabilizer in the suspension polymerization of vinyl monomers is to stabilize the droplets containing the vinyl monomers and their polymers, preventing the polymer particles generated from the droplets from fusing and forming larger agglomerates. Due to its excellent dispersing properties, the dispersion stabilizer of the present invention can be used in relatively small amounts to form stable droplets, thus preventing the formation of agglomerates caused by fusion. Furthermore, stabilizing the droplets means ensuring that small, roughly uniform droplets are stably dispersed in the dispersion medium of the suspension polymerization. In the suspension polymerization of vinyl monomers, the amount of the dispersion stabilizer of the present invention (or PVA polymer (A)) used is not particularly limited, but is generally 5 parts by mass or less, preferably 0.005 to 1 part by mass, and more preferably 0.01 to 0.2 parts by mass, relative to 100 parts by mass of the vinyl monomer. Generally, the dispersion stabilizer of the present invention is used in the same manner as conventional dispersion stabilizers, dissolved in the dispersion medium of the suspension polymerization prior to adding the vinyl monomer. As a dispersion stabilizer in the suspension polymerization of vinyl monomers, the dispersion stabilizer of the present invention may be used alone or in combination with other dispersion stabilizers. Examples of such other dispersion stabilizers include known dispersion stabilizers used in suspension polymerization of vinyl monomers such as vinyl chloride in aqueous media, for example, PVA or modified PVA polymers other than those of the present invention having an average degree of polymerization of 100-4500 and a saponification degree of 30-100 mol%, 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. These other dispersants may be used alone or in combination of two or more. In the present invention, it is preferred to use a combination of two or more PVA polymers having different degrees of polymerization and saponification as the dispersion stabilizer, and it is preferred that one or more of these serve as the PVA polymer (A) serving as the dispersion stabilizer of the present invention. It is more preferred to use a combination of a PVA polymer having a degree of polymerization of 1700 or higher and having high dispersion stability, and a PVA polymer having a degree of polymerization of 1000 or lower, and to use one or more of these as the PVA polymer (A) of the present invention. During suspension polymerization using the dispersion stabilizer of the present invention, various known dispersing aids may be used in combination. Examples of such dispersing aids include low-saponification PVA, preferably with a saponification degree of 30-60 mol%, more preferably 35-55 mol%. Furthermore, examples of such dispersing aids include PVA, preferably with an average degree of polymerization of 160-900, more preferably 200-500. In addition to the dispersing aid, various additives known in the suspension polymerization of vinyl compounds, such as a chain transfer agent, a polymerization inhibitor, a pH adjuster, an antifouling agent, and a crosslinking agent, may be used in combination. The polymerization temperature in the suspension polymerization is not limited and can be arbitrarily selected according to the type of vinyl monomer used, the degree of polymerization of the target polymer, the polymerization yield, etc., and is generally preferably 40 to 70° C. The polymerization time is also not particularly limited and can be appropriately set according to the target polymerization yield, etc. The ethylene polymers obtained by the above-described production method of the present invention can be processed into various molded articles. In particular, vinyl chloride resins, for example, can be efficiently obtained with an average particle size within an appropriate range and excellent plasticizer absorption, and in many cases, they have good processability for various molded articles. [Examples] The present invention is further described in detail below with reference to Examples, but the present invention is not limited by these Examples. Furthermore, in the following Examples and Comparative Examples, "%" and "parts" refer to "mass %" and "mass parts" unless otherwise specified. First, the evaluation method of PVA and the evaluation method of vinyl chloride polymer (vinyl chloride resin) in this embodiment are shown below. (Determination of polymerization degree and viscosity of 4% by mass aqueous solution (20°C) (4% viscosity)) The measurement was carried out in accordance with the method specified in JIS K 6726. (Determination of saponification degree) The saponification degree was measured according to the method specified in JIS K 6726. (Determination of the Polymerizable Unsaturated Bond Content in PVA) In a conical flask, dissolve 5 g of a PVA polymer in 150 g of pure water. Add a 0.5 mol / L bromine solution in acetic acid dropwise to the solution and titrate until the yellow bromine color disappears. Divide the amount of bromine required for titration (μmol) by the weight of the PVA (g) to calculate the polymerizable unsaturated bond content (μmol / g) in the PVA. (Thermogravimetric Measurement Method) Measurements were performed using a NEXTA STA200RV manufactured by Hitachi High-Tech Science Co., Ltd. under the following conditions: ・Nitrogen flow rate: 100 mL / min ・Sample weight: 5-6 mg ・Measurement temperature range: 50-500°C ・Heating rate: 10°C / min. The polymerization reduction rate (% / min) was calculated using the thermogravimetric measurement results, and the temperature (T) at which this rate reached 0.5% / min within the temperature range of 150°C or higher was determined. Furthermore, considering the possibility that weight loss at temperatures below 150°C is primarily due to the reduction of volatile components (water, methanol, or residual volatile components such as methyl acetate) that may be present in the measurement sample, a temperature of 150°C or higher was set. A sample was Soxhlet-extracted PVA polymer (A) using methanol as the solvent. The resulting sample was dried under reduced pressure at 120°C for 1 hour, pulverized using a grinder, and passed through a 60-mesh sieve. (Method for Determining the Turbidity Point of a 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-visible spectrophotometer (V-730, manufactured by JASCO Corporation), the transmittance was continuously measured from 20°C to 430 mm at a heating rate of 2°C / min. The temperature at which the transmittance reached 50% of that of a blank sample (pure water) was defined as the turbidity point. (Evaluation Method for Storage Stability of PVA Aqueous Solution) Place a beaker containing a 4% PVA aqueous solution in a 30°C constant-temperature water bath. Visually inspect the aqueous solution for 24 hours and evaluate according to the following criteria. ○: The aqueous solution remains homogeneous. ×: The aqueous solution separates into two layers. (Evaluation of vinyl chloride polymer) The vinyl chloride polymer was evaluated in the following manner. <Average Particle Size> The average particle size was determined by measuring the particle size distribution using a Rotap vibrating sieve (using a JIS sieve). <Plasticizer Absorption> The resulting resin was placed in a cylindrical container lined with glass fiber. An excess of dioctyl phthalate (DOP) was added and allowed to stand for 30 minutes to allow the DOP to penetrate the resin. The container was then centrifuged at 3000 rpm to remove excess DOP. The resin weight was then measured and the DOP absorption per 100 parts of polymer was calculated. A higher DOP absorption indicates better plasticizer absorption and superior moldability. <Evaluation of Thermal Stability of Vinyl Chloride Polymer> 100 parts by mass of vinyl chloride polymer, 0.5 parts by mass of a dioctyltin mercapto-based stabilizer, 0.8 parts by mass of a fatty acid ester lubricant, and 0.5 parts by mass of calcium stearate were melt-kneaded using a plastometer (Laboplastomill, manufactured by Toyo Seiki Co., Ltd.) at a stirrer temperature of 180°C and a rotor speed of 40 rpm. The time from the start of kneading until the resin blackened and the rotor torque began to increase was measured. A longer time indicates better thermal stability. [Example 1] (Synthesis of PVA-based polymer (C)) 55 parts of methanol and 45 parts of vinyl acetate monomer were pre-charged into a reactor equipped with a stirrer, a condenser, a nitrogen inlet, and an initiator inlet. While nitrogen was flowing through the system, the temperature was raised to 60°C. Five parts of a 1% methanol solution of 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) as an initiator were added to initiate polymerization. During the polymerization, the system was maintained at 60°C, nitrogen was flowing through the system, and 90 parts of vinyl acetate monomer were continuously added over a period of four hours after the start of polymerization. One part of a 1% methanol solution of ADVN was added one hour and two hours after the start of polymerization. When the reaction yield of vinyl acetate reached 85%, the system was cooled, and polymerization was terminated. The acetaldehyde concentration at the end of polymerization was 100 ppm. Methanol vapor was added to the resulting polymer while the remaining vinyl acetate monomer was distilled off, yielding a 50% methanol solution of polyvinyl acetate. Subsequently, 14 parts of methyl acetate and 6 parts of a 3% methanol solution of sodium hydroxide were added to 100 parts of the 50% methanol solution of polyvinyl acetate obtained above, mixed thoroughly, and saponified at 40°C. The resulting gel was pulverized and immersed in a 200 parts mixed solvent 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 polymer (C) powder with a saponification degree of 76 mol% and a 4% aqueous solution viscosity (20°C) of 5.5 mPa·s. (Synthesis of PVA Polymer (A-1)) 100 parts of the powder of the PVA polymer (C) obtained above was immersed in a mixed solvent of 150 parts of methanol and 300 parts of methyl acetate. 0.8 parts of acrolein was added and the mixture was maintained at 50°C for 1 hour. Then, 5 parts of a 50% methanol solution of p-toluenesulfonic acid was added and the mixture was reacted at 50°C for 1 hour. Subsequently, the mixture was neutralized with 10 parts of a 5% methanol solution of sodium hydroxide. The pH after neutralization was 7.5. Subsequently, the solvent was removed by centrifugation and the mixture was dried at 80°C under a nitrogen atmosphere for 5 hours to obtain PVA polymer (A-1). The analytical values ​​of the PVA polymer (A-1) showed a saponification degree of 77 mol%, a 4% by mass aqueous solution viscosity (20°C) of 5.8 mPa·s (average degree of polymerization of approximately 600), and a cloud point of a 4% aqueous solution of 35°C. The saponification degree and polymerization degree were measured according to the methods specified in JIS K 6726. The content of double bonds derived from acrolein was 80 μmol / g. Thermogravimetric analysis showed that the temperature (T) at which the weight loss rate exceeded 0.5% / min was 272°C. Furthermore, even when a 4% PVA aqueous solution was maintained at 30°C for 24 hours, the aqueous solution remained homogeneous. (Suspension Polymerization of Vinyl Chloride) Using the PVA polymer (A-1) obtained above as a dispersion stabilizer, suspension polymerization of vinyl chloride was carried out under the following conditions. To a pressure-resistant stainless steel polymerization machine, 120 parts of deionized water and 1.5 parts of a 4% aqueous solution of the PVA polymer (A-1) obtained above (0.06 parts of PVA polymer (A-1) per 100 parts of vinyl chloride monomer) were added. The interior of the polymerization machine was then depressurized to 50 mmHg using a vacuum pump. After degassing, 100 parts of vinyl chloride monomer was added. Furthermore, 0.06 parts of t-butyl peroxyneodecanoate as a polymerization initiator was added, followed by stirring and temperature increase. While maintaining the internal temperature of the polymerization machine at 57°C, suspension polymerization was carried out, and the polymerization reaction was terminated when the vinyl chloride conversion reached 88%. Unreacted monomers were then recovered using a vacuum trap, and the polymer slurry was withdrawn from the polymerization machine, dehydrated, and dried to obtain a vinyl chloride polymer (vinyl chloride resin). [Examples 2-12] PVA polymers (A-1) shown in Table 1 were synthesized in the same manner as in Example 1, except that the polymerization conditions, saponification conditions, and the type and amount of aldehyde used in the acetalization reaction were appropriately changed. Using the resulting PVA polymers (A-1), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain vinyl chloride polymers. [Comparative Examples 1 to 4] The PVA polymer (A-1) shown in Table 1 was synthesized as follows. Using the obtained PVA polymer (A-1), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain a vinyl chloride polymer. Table 1 summarizes the evaluation results of the PVA polymer (A-1) and the obtained vinyl chloride polymer. [Table 1] As shown in the above table, when the PVA polymer (A-1) obtained in Examples 1 to 12 is used for suspension polymerization of vinyl chloride, the polymerization stability is excellent, and a vinyl chloride resin having an average particle size within an appropriate range, a large plasticizer absorption amount, and excellent thermal stability can be obtained. [Comparative Example 1] PVA polymer (A-1) was synthesized in the same manner as in Example 1, except that PVA polymer (C) was dried at 150°C for 5 hours in an air atmosphere. The resulting PVA polymer (A-1) had low thermal stability. Suspension polymerization of vinyl chloride was attempted in the same manner as in Example 1, but the resulting vinyl chloride had low thermal stability. [Comparative Example 2] Using a PVA polymer (C) containing no double bonds, having a 4% by mass aqueous solution viscosity (20°C) of 5.5 mPa·s (average degree of polymerization of approximately 600) and a saponification degree of 77 mol%, as the PVA polymer (A-1), suspension polymerization of vinyl chloride was attempted in the same manner as in Example 1. However, the vinyl chloride resin was blocked, and polymerization could not proceed normally. [Comparative Example 3] Polyvinyl acetate was obtained by polymerization of vinyl acetate in the presence of carbon tetrachloride. This polyvinyl acetate was then saponified according to Example 1 to obtain a PVA polymer having a 4% by mass aqueous solution viscosity (20°C) of 5.5 mPa·s (average degree of polymerization of approximately 600), a saponification degree of 77 mol%, a double bond content of 80 μmol / g, and a temperature (T) of 220°C. This PVA polymer was used to carry out suspension polymerization of vinyl chloride in the same manner as in Example 1. However, the resulting vinyl chloride resin had lower thermal stability. [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 polymer with a saponification degree of 77 mol%, a 4% aqueous solution viscosity (20°C) of 5.5 mPa·s (average degree of polymerization of approximately 600). This polymer was dried at 130°C under a nitrogen atmosphere for 5 hours to obtain a PVA polymer with a double bond content of 80 μmol / g and a temperature (T) of 252°C. Using this PVA polymer, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1, but the resulting vinyl chloride resin had lower thermal stability. [Examples 13-15] PVA polymers (A-2) shown in Table 2 were synthesized in the same manner as in Example 1, except that the saponification conditions, the type (two types) and the amount of aldehyde used in the acetalization reaction were appropriately changed. Using the resulting PVA polymers (A-2), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain vinyl chloride polymers. The evaluation results of the PVA polymers (A-2) and the resulting vinyl chloride polymers are summarized in Table 2. [Table 2] [Example 16] (Synthesis of PVA-based polymer (B-3)) 55 parts of methanol and 45 parts of vinyl acetate monomer were pre-charged into a reactor equipped with a stirrer, a condenser, a nitrogen inlet, and an initiator inlet. While nitrogen was flowing through the system, the temperature was raised to 60°C. Six parts of a 1% methanol solution of 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) as an initiator were added to initiate polymerization. During the polymerization, the system was maintained at 60°C, and while nitrogen was flowing through the system, 90 parts of vinyl acetate monomer and two parts of a 20% methanol solution of itaconic acid were continuously added over a period of four hours after the start of polymerization. One hour and two hours after the start of polymerization, 1.2 parts of a 1% methanol solution of ADVN were added. When the reaction yield of vinyl acetate reached 85%, the system was cooled, and the polymerization was terminated. While adding methanol vapor to the resulting polymer, the remaining vinyl acetate monomer was distilled off to obtain a 50% methanol solution of polyvinyl acetate. Subsequently, 14 parts of methyl acetate and 6 parts of a 3% methanol solution of sodium hydroxide were added to 100 parts of the 50% methanol solution of polyvinyl acetate obtained above, mixed thoroughly, and saponified at 40°C. The resulting gel was pulverized and immersed in a 200 parts mixed solvent 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 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 of approximately 600), and an itaconic acid content of 0.2 mol%. (Synthesis of PVA-Based Polymer (A-3)) In the same manner as in Example 1, the PVA-based polymer (A-3) shown in Table 3 was synthesized. (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] The PVA polymers (A-3) shown in Table 3 were synthesized in the same manner as in Example 16, except that various conditions were appropriately modified. Using the resulting PVA polymers (A-3), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain vinyl chloride polymers. The evaluation results of the PVA polymers (A-3) and the resulting vinyl chloride polymers are summarized in Table 3. In the table, "AMPS" stands for sodium 2-acrylamido-2-methylpropanesulfonate. [Table 3] [Example 21] (Synthesis of PVA-based polymer (B-4)) 20 parts of methanol, 80 parts of vinyl acetate monomer, and 0.02 parts of 3-mercaptopropionic acid were pre-charged into a reactor equipped with a stirrer, a condenser, a nitrogen inlet, and an initiator inlet. While nitrogen was flowing through the system, the temperature was raised to 60°C, and 1.5 parts of a 1% methanol solution of 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) as an initiator was added to initiate polymerization. During the polymerization, the system was maintained at 60°C, nitrogen was flowing through the system, and 2 parts of a 10% methanol solution of 3-mercaptopropionic acid were continuously added over a period of 4 hours after the start of polymerization. Furthermore, 0.5 parts of a 1% methanol solution of ADVN were added 1 hour and 2 hours after the start of polymerization. When the reaction yield of vinyl acetate reached 80%, the system was cooled, and the polymerization was terminated. Methanol vapor was added to the resulting polymer while the remaining vinyl acetate monomer was distilled off, yielding a 50% methanol solution of polyvinyl acetate. Subsequently, 14 parts of methyl acetate and 6 parts of a 3% methanol solution of sodium hydroxide were added to 100 parts of the 50% methanol solution of polyvinyl acetate obtained above, mixed thoroughly, and saponified at 40°C. The resulting gel was pulverized and immersed in a 200 parts mixed solvent 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 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 of approximately 600), and a 3-mercaptopropionic acid content of 0.2 mol%. (Synthesis of PVA-Based Polymer (A-4)) In the same manner as in Example 1, the PVA-based polymer (A-4) shown in Table 4 was synthesized. (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 to 24] PVA polymers (A-4) shown in Table 4 were synthesized in the same manner as in Example 21 except that various conditions were appropriately changed. Using the resulting PVA polymers (A-4), suspension polymerization of vinyl chloride was carried out in the same manner as in Example 1 to obtain vinyl chloride polymers. The evaluation results of the PVA polymer (A-4) and the obtained vinyl chloride polymer are summarized in Table 4. In the table, "MPS" refers to sodium 3-mercapto-1-propanesulfonate. [Table 4] When the PVA polymers (A-2, A-3, and A-4) obtained in Examples 13 to 24 were used in suspension polymerization of vinyl chloride, they exhibited excellent polymerization stability, yielding vinyl chloride resins with an appropriate average particle size, a high plasticizer absorption capacity, and excellent thermal stability. [Industrial Applicability] The present invention provides a specific polyvinyl alcohol-based polymer that can be preferably used as a dispersion stabilizer (dispersant).

Claims

1. A polyvinyl alcohol polymer (A) that satisfies the following requirements 1 and 2: requirement 1: having polymerizable unsaturated bonds; requirement 2: the temperature at which the rate of weight loss determined by thermogravimetric analysis in a temperature range of 150°C or above exceeds 0.5% / min is 255°C or above.

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

3. For the polyvinyl alcohol polymer (A) of claim 1 or 2, the temperature at which the rate of weight loss determined by thermogravimetric analysis in the temperature range above 150°C exceeds 0.5% / min is 258°C or above.

4. The polyvinyl alcohol polymer (A) of claim 1 or 2, wherein the ratio of polymerizable unsaturated bonds is 5 to 500 μmol / g, and the temperature at which the rate of weight loss determined by thermogravimetric analysis in the temperature range above 150°C exceeds 0.5% / min is above 260°C.

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

6. 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).

7. 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.

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

9. 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.

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

11. 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.

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

13. The agent as requested in item 12 is a dispersion stabilizer for polymerization.

14. The agent as requested in item 12 is a dispersion stabilizer for suspension polymerization.

15. The agent as claimed in claim 12 is a dispersion stabilizer for suspension polymerization of ethylene monomers containing vinyl chloride.

16. A method for manufacturing an ethylene-based polymer, comprising polymerizing ethylene monomers in the presence of a polyvinyl alcohol-based polymer (A) as claimed in claim 1 or 2.

17. The manufacturing method as described in claim 16, wherein the polymerization is suspension polymerization.

18. The manufacturing method of claim 16, wherein an ethylene monomer containing vinyl chloride is subjected to suspension polymerization.