Patent Description:
One known example of a use of polyvinyl alcohol (hereinafter, also referred to as "PVA" for short) is a dispersion stabilizer for suspension polymerization of vinyl compounds, and a variety of PVAs are used to this end (see Patent Literatures <NUM> to <NUM>).

In order to improve polymerization stability during polymerization of vinyl chloride, some PVAs are used for polymerization reaction after being subjected to heat treatment (see Patent Literatures <NUM> to <NUM>) while others use a PVA satisfying expressions relating UV absorbance at a specific wavelength and molecular weight (see Patent Literature <NUM>). However, the polymerization stability of vinyl monomers resulting from the use of these PVAs as dispersion stabilizers in suspension polymerization of vinyl chloride is not always satisfactory. In fact, the polymerization stability is insufficient, and the product vinyl chloride polymer contains a large number of coarse particles, particularly when these dispersion stabilizers for suspension polymerization are used in small amounts.

Further, <CIT> describes a dispersing stabilizer for suspension homopolymerization or copolymerization of vinyl chloride, comprising a polyvinyl alcohol having a degree of hydrolysis of <NUM> to <NUM>% by mole, an absorbance of not less than <NUM> measured at a wavelength of <NUM> with respect to a <NUM>% by weight aqueous solution, a content of carboxyl group of <NUM> to <NUM>% by mole and a cloud point of not less than <NUM> measured with respect to the <NUM>% by weight aqueous solution;.

<CIT> describes a dispersion stabilizer for suspension polymerization of vinyl compounds comprising a vinyl alcohol polymer (A) which has a saponification degree of <NUM> mol% or more and less than <NUM> mol% and a viscosity-average polymerization degree (P) of more than <NUM> and less than <NUM>, and has a terminal alkyl group having <NUM> to <NUM> carbon atoms, and in which a content of monomer units having an oxyalkylene group is <NUM> mol% or less and the relationship between the viscosity-average polymerization degree (P) and a modification rate (S)(mol%) of the alkyl group satisfies Formula (<NUM>) <NUM> ≤ S × P / <NUM> < <NUM>; and.

<CIT> describes dispersing agent useful in a suspension polymerization process, the dispersing agent comprising:
a polyvinyl alcohol having:.

The present invention has been made to provide a solution to the foregoing problem, and it is an object of the present invention to provide a dispersion stabilizer for suspension polymerization of vinyl compounds that exhibits excellent polymerization stability even when used in reduced amounts, and provides excellent plasticizer absorptivity with little formation of coarse particles. Another object of the present invention is to provide a method for producing such a dispersion stabilizer.

After intensive studies, the present inventor found that the foregoing issue can be resolved with the use of a dispersion stabilizer for suspension polymerization comprising a vinyl alcohol polymer (A) that contains a specific amount of chlorine or bromine atoms, and in which the cloud point and absorbances at specific wavelengths fall in specific ranges in aqueous solutions. The present invention was completed on the basis of this finding.

Specifically, the present invention is set out in the appended set of claims.

When used for suspension polymerization of a vinyl compound, a dispersion stabilizer for suspension polymerization of the present invention exhibits excellent polymerization stability even when used in reduced amounts, and the product vinyl polymer excels in plasticizer absorptivity with little formation of coarse particles.

A dispersion stabilizer for suspension polymerization of the present invention comprises a specific vinyl alcohol polymer (A). The vinyl alcohol polymer (A) has a viscosity-average degree of polymerization of more than <NUM> and less than <NUM>,<NUM>, a degree of saponification of more than <NUM> mol% and less than <NUM> mol%, a degree of modification by a chlorine or bromine atom of <NUM>% by mass or more and less than <NUM>% by mass, a cloud point of more than <NUM> and less than <NUM> in a <NUM>% by mass aqueous solution, a UV absorption spectrum absorbance (x) at <NUM> of more than <NUM> and less than <NUM> in a <NUM>% by mass aqueous solution, and a UV absorption spectrum absorbance (y) at <NUM> of more than <NUM> and less than <NUM> in the same <NUM>% by mass aqueous solution. A dispersion stabilizer for suspension polymerization of the present invention may substantially consist of the vinyl alcohol polymer (A). As used herein, "substantially solely consisting of vinyl alcohol polymer (A)" means that the content of components other than the vinyl alcohol polymer (A) in the dispersion stabilizer for suspension polymerization is less than <NUM>% by mass, preferably less than <NUM>% by mass, even more preferably less than <NUM>% by mass. It is also to be noted that the numerical ranges recited in this specification may be combinations of upper and lower limits that are changed as appropriate from the upper and lower limits specified herein, provided that such changes are made within the specified ranges.

The vinyl alcohol polymer (A) used in the present invention (hereinafter, vinyl alcohol polymer will be referred to also as "PVA" for short) can be produced with a polymerization step of obtaining a vinyl ester polymer through polymerization of a vinyl ester monomer with addition of preferably carbon tetrachloride or carbon tetrabromide as a chain transfer agent, and a step of obtaining a vinyl alcohol polymer through saponification of the vinyl ester polymer obtained in the polymerization step. The polymerization may be carried out using a known method, for example, such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, or dispersion polymerization. Preferred from the viewpoint of industrial applications are solution polymerization, emulsion polymerization, and dispersion polymerization. The polymerization may be any of a batch, a semi-batch, and a continuous process.

Examples of the vinyl ester monomer include vinyl acetate, vinyl formate, vinyl propionate, vinyl caprylate, and vinyl versatate. Preferred from the viewpoint of industrial applications is vinyl acetate.

In the vinyl ester monomer polymerization step, it is preferable to use a chain transfer agent to adjust the degree of polymerization. From the viewpoints of compatibility with the vinyl compound used as the dispersion stabilizer for suspension polymerization of vinyl compounds, and the ability to introduce a modifying group having high radical reactivity, the chain transfer agent is preferably one selected from, for example, carbon tetrachloride, carbon tetrabromide, trichloromethane, tribromomethane, dichloromethane, and dibromomethane. Carbon tetrachloride and carbon tetrabromide are more preferred from the viewpoint of reactivity and ease of handling.

In the polymerization step, the polymerization conversion rate of the vinyl ester monomer is <NUM>% or more and less than <NUM>%. The reaction suffers from poor productivity when the polymerization conversion rate is less than <NUM>%. With a polymerization conversion rate of <NUM>% or more, the viscosity in a polymerization vessel becomes excessively high, which, in addition to making the production difficult, tends to decrease the degree of modification as a result of elimination of a modifying group due to extended exposure to radicals, though the extent of elimination depends on the type of the modifying group introduced. The polymerization conversion rate is preferably <NUM>% to <NUM>%, more preferably <NUM>% to <NUM>%, even more preferably <NUM>% to <NUM>%.

For the production of PVA(A), the chain transfer agent may be added at once in early stages of the polymerization step. It is, however, preferable to sequentially add carbon tetrachloride or carbon tetrabromide as the chain transfer agent because the chain transfer agent, when added at once, may cause gelation due to nonuniform reaction, or may cause decrease of the degree of modification by chlorine or bromine atoms. The chain transfer agent is added in an amount of preferably <NUM> parts by mass or more and <NUM> parts by mass or less, more preferably <NUM> parts by mass or more and less than <NUM> parts by mass, even more preferably <NUM> parts by mass or more and less than <NUM> parts by mass relative to <NUM> parts by mass of the vinyl ester monomer.

For the saponification reaction of vinyl ester polymer, a conventionally known alcoholysis or hydrolysis reaction is applicable that uses a basic catalyst such as sodium hydroxide, potassium hydroxide, or sodium methoxide, or an acid catalyst such as p-toluenesulfonic acid as a saponification catalyst. The saponification catalyst is used in an amount of <NUM> to <NUM>, more preferably <NUM> to <NUM>, even more preferably <NUM> to <NUM> in terms of a molar ratio with respect to the vinyl ester monomer unit of the vinyl ester polymer, though the amount used is not particularly limited. Examples of the solvent usable 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 may be used alone, or two or more thereof may be used in combination. For convenience, it is preferable to carry out the saponification reaction in the presence of basic catalyst sodium hydroxide, using methanol or a mixed solution of methanol and methyl acetate as solvent.

The water content of the saponification reaction solution (a feedstock vinyl ester polymer solution) used for the saponification reaction of vinyl ester polymer is not particularly limited, and is preferably less than <NUM>% by mass, more preferably <NUM>% by mass or less, even more preferably <NUM>% by mass or less. The concentration of the vinyl ester polymer in the saponification reaction solution is preferably <NUM>% to <NUM>% by mass, more preferably <NUM>% to <NUM>% by mass, even more preferably <NUM>% to <NUM>% by mass.

It is important that the degree of modification by a chlorine or bromine atom in PVA(A), that is, the chlorine or bromine atom content, be <NUM>% by mass or more and less than <NUM>% by mass. Preferably, the degree of modification by a chlorine or bromine atom is <NUM>% by mass or more and <NUM>% by mass or less because it makes the polymerization more stable, and improves the plasticizer absorptivity with less formation of coarse particles. With a degree of modification of less than <NUM>% by mass, PVA(A) fails to exhibit sufficient polymerization stability when used as a dispersion stabilizer for suspension polymerization of vinyl compounds. When the degree of modification is <NUM>% by mass or more, PVA(A) becomes overly compatible with a vinyl compound, and easily dissolves therein, with the result that PVA(A) loses its function as a dispersion stabilizer for vinyl compounds, and the polymerization stability becomes insufficient. A PVA(A) with a degree of modification of <NUM>% by mass or more also tends to undergo gelation during its synthesis, and is difficult to produce.

The degree of modification by a chlorine or bromine atom in PVA(A) can be measured using a known method. Specifically, it is convenient to use, for example, the ion chromatography technique described in JIS Z <NUM>-<NUM>:<NUM> (total chlorine content testing method), or the elemental analysis by ICP emission spectroscopy described in JIS K <NUM>:<NUM>. In order to determine the degree of modification by a chlorine or bromine atom in PVA(A), a PVA(A) sample was burned in an oxygen-filled flask, and the combustion gas was absorbed in water. The mass of chlorine or bromine atoms in PVA(A) was then determined by ion chromatography measurement to calculate the degree of modification.

It is important that PVA(A) have a viscosity-average degree of polymerization of more than <NUM> and less than <NUM>,<NUM>. Preferably, the viscosity-average degree of polymerization is more than <NUM> and less than <NUM>, even more preferably more than <NUM> and less than <NUM>. With a viscosity-average degree of polymerization of <NUM> or less, polymerization of vinyl compound becomes unstable, and the resulting vinyl polymer particles have a large average particle diameter with a large proportion of coarse particles. The plasticizer absorptivity of the resulting vinyl polymer particles decreases when the viscosity-average degree of polymerization is <NUM>,<NUM> or more. Here, the viscosity-average degree of polymerization is a measured value based on JIS K <NUM>:<NUM>. Specifically, in the case of a PVA with a degree of saponification of less than <NUM> mol%, the PVA was saponified until the degree of saponification reached at least <NUM> mol%, and the viscosity-average degree of polymerization (P) was determined from the limiting viscosity [η] (liter/g) measured in water at <NUM>, using the following formula.

It is important that the degree of saponification of PVA(A) be more than <NUM> mol% and less than <NUM> mol%. Preferably, the degree of saponification is more than <NUM> mol% and less than <NUM> mol%. A degree of saponification of <NUM> mol% or less impairs ease of handling, such as by causing PVA(A) to easily precipitate upon being dissolved in water, making it impossible to add PVA(A) in accurate quantities. With a degree of saponification of <NUM> mol% or less, the resulting vinyl polymer particles have a large average particle diameter with an increased proportion of coarse particles. When the degree of saponification is <NUM> mol% or more, the resulting vinyl polymer particles also have a large average particle diameter with an increased proportion of coarse particles, and the plasticizer absorptivity becomes insufficient. Here, the degree of saponification is a measured value based on JIS K <NUM>:<NUM>.

It is important that PVA(A) have a cloud point in a <NUM>% by mass aqueous solution of more than <NUM> and less than <NUM>. Preferably, the cloud point is more than <NUM> and less than <NUM>. A cloud point of <NUM> or less impairs ease of handling, such as by causing PVA(A) to easily precipitate in an aqueous solution, making it impossible to add PVA(A) in accurate quantities. When the cloud point is <NUM> or more, the resulting vinyl polymer particles have a large average particle diameter with an increased proportion of coarse particles. As used herein, "cloud point" means a temperature at which the transmittance of visible light of <NUM>-nm wavelength through a <NUM>% by mass aqueous solution of PVA(A) falls below <NUM>%. The cloud point can be measured with, for example, an ultraviolet-visible spectrophotometer (UV-<NUM>, UV-<NUM>, UV-<NUM> manufactured by Shimadzu Corporation).

It is important that PVA(A) have a UV absorption spectrum absorbance (x) at <NUM> of more than <NUM> and less than <NUM> in a <NUM>% by mass aqueous solution. Preferably, the UV absorption spectrum absorbance (x) is more than <NUM> and less than <NUM>. It is important that PVA(A) have a UV absorption spectrum absorbance (y) at <NUM> of more than <NUM> and less than <NUM> in the same <NUM>% by mass aqueous solution. Preferably, the UV absorption spectrum absorbance (y) is more than <NUM> and less than <NUM>. The UV absorption spectrum absorbance represents the amount or linkage of double bonds introduced to PVA(A), and with the absorbance falling in the foregoing ranges, adsorbability for vinyl compounds increases, and this contributes to polymerization stability when the PVA(A) is used as a dispersion stabilizer for suspension polymerization. When the absorbance is outside of the foregoing ranges, adsorbability for vinyl compounds may decrease, or PVA(A) may assume a dissolved state beyond adsorption, with the result that PVA(A) fails to exhibit its effect as a dispersion stabilizer for suspension polymerization. The absorbance (x) and absorbance (y) are measured using the device and conditions described in the Examples below.

The ratio (y/x) of absorbance (y) at <NUM> to absorbance (x) at <NUM> in a UV absorption spectrum of PVA(A) is preferably more than <NUM> and <NUM> or less, more preferably <NUM> or more and <NUM> or less, even more preferably <NUM> or more and <NUM> or less. Here, the UV absorption at <NUM>-nm wavelength derives from the structure [-CO-(CH=CH)<NUM>-] in PVA(A), and the UV absorption at <NUM>-nm wavelength derives from the structure [-CO-(CH=CH)<NUM>-] in PVA(A). This means that the amounts of double bonds with different linkages introduced into PVA(A) approach the same number as the absorbance ratio (y/x) becomes closer to <NUM>. The double-bond linkage in PVA(A) appears to contribute to polymerization stability, and to inhibit formation of coarse particles when PVA(A) is used as a dispersion stabilizer for suspension polymerization of vinyl compounds.

PVA(A) is preferred for use as a dispersion stabilizer for suspension polymerization of vinyl compounds. When used as a dispersion stabilizer for suspension polymerization of vinyl compounds, PVA(A) stabilizes the polymerization reaction, and the plasticizer absorptivity improves with reduced formation of coarse particles in the resulting vinyl polymers.

Another embodiment of the present invention is a vinyl polymer producing method in which a vinyl compound is subjected to suspension polymerization in the presence of a PVA(A)-containing dispersion stabilizer for suspension polymerization of the present invention. The method produces a particulate vinyl polymer.

There is no particular limitation on how to charge the PVA(A)-containing dispersion stabilizer for suspension polymerization of the present invention into a polymerization vessel. For example, the dispersion stabilizer may be charged into a polymerization vessel in the form of (i) an aqueous solution, or (ii) an as-processed powder form. Preferred for uniformity in a polymerization vessel is method (i).

A dispersion stabilizer for suspension polymerization of the present invention may contain various additives, in addition to PVA(A), provided that such additives are not detrimental to the gist of the present invention in the suspension polymerization of a vinyl compound. Examples of such additives include polymerization degree adjusters such as aldehydes, halogenated hydrocarbons, and mercaptans; polymerization inhibitors such as phenol compounds, sulfur compounds, and N-oxide compounds; pH adjusters; cross-linking agents; preservatives; mildewcides; antiblocking agents; antifoaming agents; and compatibilizing agents. The content of additives in the dispersion stabilizer for suspension polymerization is preferably less than <NUM>% by mass, more preferably less than <NUM>% by mass, even more preferably less than <NUM>% by mass relative to PVA(A).

Examples of the vinyl compounds include vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, and esters and salts thereof; maleic acid, fumaric acid, and esters and anhydrides thereof; styrene, acrylonitrile, vinylidene chloride, and vinyl ethers. Preferably, the dispersion stabilizer for suspension polymerization of the present invention is used for suspension polymerization of vinyl chloride alone, or for suspension polymerization of vinyl chloride and a monomer that is copolymerizable with vinyl chloride. Examples of the monomer that is copolymerizable with vinyl chloride include vinyl esters such as vinyl acetate and vinyl propionate; (meth)acrylic acid esters such as methyl (meth)acrylate and ethyl (meth)acrylate; α-olefins such as ethylene and propylene; unsaturated dicarboxylic acids such as maleic anhydride and itaconic acid; acrylonitrile, styrene, vinylidene chloride, and vinyl ethers.

For suspension polymerization of vinyl compounds, an oil-soluble or water-soluble polymerization initiator, which is conventionally used for polymerization of vinyl chloride, can be used. Examples of the oil-soluble polymerization initiator include: peroxycarbonate compounds such as diisopropyl peroxydicarbonate, bis(<NUM>-ethylhexyl)peroxydicarbonate, and bis(<NUM>-ethoxyethyl)peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanoate, t-butyl peroxypivalate, t-hexyl peroxypivalate, and α-cumyl peroxyneodecanoate; peroxides such as acetyl(cyclohexylsulfonyl)peroxide, <NUM>,<NUM>,<NUM>-trimethylpentyl-<NUM>-peroxyphenoxyacetate, bis(<NUM>,<NUM>,<NUM>-trimethylhexanoyl)peroxide, and lauroyl peroxide; and azo compounds such as <NUM>,<NUM>'-azobis(<NUM>,<NUM>-dimethylvaleronitrile), <NUM>,<NUM>'-azobis(isobutyronitrile), and <NUM>,<NUM>'-azobis(<NUM>-methoxy-<NUM>,<NUM>-dimethylvaleronitrile). Examples of the water-soluble polymerization initiator include potassium persulfate, ammonium persulfate, hydrogen peroxide, and cumene hydroperoxide. The oil-soluble or water-soluble polymerization initiators may be used alone, or two or more thereof may be used in combination.

In suspension polymerization of a vinyl compound, the polymerization temperature is not particularly limited, and may be about <NUM>, or higher than <NUM>, or may be about <NUM> to <NUM>. A polymerizer equipped with a reflux condenser may be used in order to increase the heat removal efficiency of the polymerization reaction system.

The dispersion stabilizer for suspension polymerization of the present invention shows high polymerization stability, particularly when used in reduced amounts. In suspension polymerization of a vinyl compound, the dispersion stabilizer for suspension polymerization of the present invention may be used in an amount (concentration) of <NUM>,<NUM> ppm or less, <NUM> ppm or less, <NUM> ppm or less, <NUM> ppm or less, or <NUM> ppm or less relative to the vinyl compound. Here, "ppm" means "ppm by mass".

The resulting vinyl polymer can be used for formation of various molded products by adding additives such as a plasticizer, as needed.

The following describes the present invention in greater detail by way of Examples. It should be noted that the present invention is in no way limited by the following Examples. In the following Examples and Comparative Examples, "part(s)" means "part(s) by mass", and "ppm" means "ppm by mass", unless otherwise specifically stated.

The viscosity-average degree of polymerization of PVA(A) was measured according to JIS K <NUM>:<NUM>. Specifically, when PVA(A) had a degree of saponification of less than <NUM> mol%, PVA(A) was saponified until the degree of saponification reached at least <NUM> mol%, and the resulting PVA was measured for viscosity-average degree of polymerization (P) from the limiting viscosity [η] (liter/g) measured in water at <NUM>, using the following formula.

The degree of saponification of PVA(A) was determined according to JIS K <NUM>:<NUM>.

PVA(A) was burned in a flask under an oxygen atmosphere. The combustion gas was then absorbed in water, and measured with an ion chromatograph ICS-<NUM> manufactured by Thermo Fisher Scientific Inc. The degree of modification (% by mass) was determined from the mass of the chlorine or bromine atoms contained in PVA(A) by comparing the peak area with a standard solution of a known concentration of chlorine or bromine.

A <NUM>% by mass aqueous solution of PVA(A) was prepared. The aqueous solution was charged into a cell having a <NUM>-cm light path, and visible light transmittance at <NUM> was measured at varying measurement temperatures using an ultraviolet-visible spectrophotometer (UV-<NUM>, manufactured by Shimadzu Corporation). The temperature at which the visible light transmittance fell below <NUM>% was determined as the cloud point.

A <NUM>% by mass aqueous solution of PVA(A) was prepared. The aqueous solution was charged into a cell having a <NUM>-cm light path, and was measured for absorbance at <NUM> and <NUM> using an ultraviolet-visible spectrophotometer (UV-<NUM>, manufactured by Shimadzu Corporation).

Vinyl acetate (<NUM>,<NUM> parts; hereinafter, vinyl acetate also will be referred to simply as "VAc") and methanol (<NUM> parts) were charged into a polymerization vessel. After replacement with nitrogen, the mixture was heated to <NUM>, and <NUM>,<NUM>'-azobis(isobutyronitrile) was added in an amount of <NUM>% by mass relative to VAc, along with <NUM> parts of methanol. This was immediately followed by addition of a methanol solution of carbon tetrachloride (a concentration of <NUM>% by mass; room temperature) to the polymerization vessel at a constant rate to initiate polymerization. A polymerization conversion rate reached <NUM>% upon addition of <NUM> parts of carbon tetrachloride, and the polymerization was ceased at this point. The remaining VAc was then removed from the system with methanol in a procedure performed under reduced pressure while adding methanol. This yielded a <NUM>% by mass methanol solution of vinyl ester polymer (hereinafter, also will be referred to simply as "PVAc"). The methanol solution of vinyl ester polymer was diluted to <NUM>% by mass with methanol solvent, and this was followed by a saponification reaction carried out for <NUM> hour at <NUM> with the saponification reaction solution containing <NUM>% by mass water, using sodium hydroxide as a saponification catalyst in a molar ratio of <NUM> with respect to PVAc. The reaction mixture was neutralized with methyl acetate, and dried. This produced a PVA(A1) that had a viscosity-average degree of polymerization of <NUM>, a degree of saponification of <NUM> mol%, a degree of modification by chlorine atoms of <NUM>% by mass, a UV absorption spectrum absorbance (x) at <NUM> of <NUM> in a <NUM>% by mass aqueous solution, a UV absorption spectrum absorbance (y) at <NUM> of <NUM> in the same <NUM>% by mass aqueous solution, a ratio (y/x) of <NUM>, and a cloud point of <NUM> in a <NUM>% by mass aqueous solution.

PVA(A2) to PVA(A14) were produced in the same manner as in Production Example <NUM>, except that the vinyl acetate and methanol contents, the type and content of chain transfer agent and the method of addition of chain transfer agent, the polymerization conversion rate, and saponification conditions such as the water content of saponification reaction solution were varied as shown in Tables <NUM> and <NUM>. Table <NUM> shows the production conditions and results. Table <NUM> shows the types of chain transfer agents used. It is to be noted that PVA(A13) was produced through polymerization that used a large quantity of chain transfer agent charged at once in an early stage of reaction, and the polymerization product had a gel form, preventing it from being used in subsequent steps.

PVA(A1) as a dispersion stabilizer for suspension polymerization was dissolved in deionized water, and <NUM> parts of the PVA(A1) aqueous solution was charged into an autoclave. Here, PVA(A1) was charged in an amount that would make the PVA(A1) concentration <NUM> ppm with respect to the amount of vinyl chloride charged. Thereafter, deionized water was added to make the total fraction of deionized water <NUM>,<NUM> parts. This was followed by addition of <NUM> parts of a <NUM>% by mass toluene solution of cumyl peroxyneodecanoate, and <NUM> parts of a <NUM>% by mass toluene solution of t-butyl peroxyneodecanoate into an autoclave, and nitrogen was introduced into the autoclave until the pressure reached <NUM> MPa. Nitrogen purging was conducted a total of five times to thoroughly replace inside of the autoclave with nitrogen and remove oxygen. After this procedure, <NUM> parts of vinyl chloride was added. The contents inside the autoclave were then heated to <NUM> to initiate polymerization of vinyl chloride, with stirring. The autoclave had an inner pressure of <NUM> MPa at the start of polymerization. After about <NUM> hours from the start of polymerization, the polymerization was ceased at the timing when the pressure inside the autoclave reached <NUM> MPa. The polymerization product was taken out after removing unreacted vinyl chloride. The product was then dried at <NUM> for <NUM> hours to obtain vinyl chloride polymer particles.

The vinyl chloride polymer particles were evaluated for (<NUM>) average particle diameter, (<NUM>) particle size distribution, and (<NUM>) plasticizer absorptivity, using the following methods. The evaluation results are presented in Table <NUM>.

The particle size distribution was measured according to the dry sieving method described in JIS Z <NUM>:<NUM>, using a sieve that complied with Tyler Standard Sieve Series. The measurement results were plotted to create a Rosin-Rammler distribution, and the average particle diameter (dp50) was calculated.

The content of vinyl chloride polymer particles (% by mass) that did not pass through a <NUM>-µm mesh sieve (equivalent to a JIS standard <NUM>-mesh sieve) was evaluated using the following criteria. Here, the content means the percentage of particles accumulating on the sieve. The sieve opening complies with the nominal opening W of JIS Z <NUM>-<NUM>-<NUM>.

The content of vinyl chloride polymer particles (% by mass) that passed through a <NUM>-µm mesh sieve but did not pass through a <NUM>-µm mesh sieve (equivalent to a JIS standard <NUM>-mesh sieve) was evaluated using the following criteria. Here, the content means the percentage of particles accumulating on the sieve. The sieve opening complies with the nominal opening W of JIS Z <NUM>-<NUM>-<NUM>.

With regard to the vinyl chloride polymer particles that did not pass through the <NUM>-µm mesh sieve and the vinyl chloride polymer particles that did not pass through the <NUM>-µm mesh sieve, smaller content values of these polymer particles mean higher polymerization stability with fewer coarse particles and a sharper particle size distribution.

The mass (A (g)) of a <NUM>-mL syringe filled with <NUM> of absorbent cotton was measured, and <NUM> of vinyl chloride polymer particles was added to the syringe and the combined mass (B (g)) was measured. After adding <NUM> of dioctyl phthalate (DOP), the syringe was allowed to stand for <NUM> minutes, and centrifuged at <NUM>,<NUM> rpm for <NUM> minutes. The resulting mass (C (g)) was then measured. The plasticizer absorptivity (%) was determined from the calculation formula below. Higher values of plasticizer absorptivity mean that the product is more easily processible, and that fewer defects, such as seeding, occur on the exterior of the product, particularly when the polymer is processed into a sheet.

Suspension polymerization of vinyl chloride was conducted in the same manner as in Example <NUM>, except that the PVAs(A) shown in Table <NUM> were used as dispersion stabilizers for suspension polymerization in the amounts given in the table relative to vinyl chloride. The dispersion stabilizers for suspension polymerization of the present invention do not produce polyvinyl chloride particles of coarse particle sizes, and provide desirable polymerization stability even when used in small amounts with respect to vinyl chloride.

Suspension polymerization of vinyl chloride was conducted in the same manner as in Examples <NUM> and <NUM>, except that PVA(A9) was used as PVA(A). PVA(A9) had an excessively low degree of polymerization, and the resulting vinyl chloride polymer particles had a large average particle diameter with a large proportion of coarse particles.

Suspension polymerization of vinyl chloride was conducted in the same manner as in Examples <NUM> and <NUM>, except that PVA(A10) was used as PVA(A). PVA(A10) had an excessively high degree of saponification, and the resulting vinyl chloride polymer particles had a large average particle diameter with a large proportion of coarse particles. The plasticizer absorptivity was also insufficient.

Suspension polymerization of vinyl chloride was conducted in the same manner as in Examples <NUM> and <NUM>, except that PVA(A11) was used as PVA(A). PVA(A11) had an excessively low degree of saponification, and the resulting vinyl chloride polymer particles had a large average particle diameter with a large proportion of coarse particles.

Suspension polymerization of vinyl chloride was conducted in the same manner as in Examples <NUM> and <NUM>, except that PVA(A12) was used as PVA(A). PVA(A12) had an excessively high degree of modification by chlorine atoms, and the compatibility with vinyl chloride was overly high. The resulting vinyl chloride polymer particles therefore had a large average particle diameter with a large proportion of coarse particles.

Suspension polymerization of vinyl chloride was conducted in the same manner as in Examples <NUM> and <NUM>, except that PVA(A14) was used as PVA(A). PVA(A14) had an excessively low degree of modification by chlorine atoms, and failed to produce a sufficient effect with the introduced modifying group. Accordingly, the polymerization stability was insufficient. The resulting vinyl chloride polymer particles therefore had a large average particle diameter with a large proportion of coarse particles.

As demonstrated in the Examples above, a vinyl polymer having excellent polymerization stability with a small average particle diameter and little formation of coarse particles can be obtained with the use of the dispersion stabilizer for suspension polymerization of vinyl compounds of the present invention that contains a PVA in which the degree of modification by a chlorine or bromine atom falls in a specific range, and in which ultraviolet absorbance values at specific wavelengths in an aqueous solution, and a cloud point in an aqueous solution fall in specific ranges, even when the dispersion stabilizer is used in small amounts. This makes the present invention highly useful in industry.

Claim 1:
A dispersion stabilizer for suspension polymerization of vinyl compounds that comprises a vinyl alcohol polymer (A) having a viscosity-average degree of polymerization of more than <NUM> and less than <NUM>,<NUM>, a degree of saponification of more than <NUM> mol% and less than <NUM> mol%, a degree of modification by a chlorine or bromine atom of <NUM>% by mass or more and less than <NUM>% by mass, a cloud point of more than <NUM> and less than <NUM> in a <NUM>% by mass aqueous solution, an ultraviolet absorption spectrum absorbance (x) at <NUM> of more than <NUM> and less than <NUM> in a <NUM>% by mass aqueous solution, and an ultraviolet absorption spectrum absorbance (y) at <NUM> of more than <NUM> and less than <NUM> in the same <NUM>% by mass aqueous solution,
wherein the viscosity-average degree of polymerization, the degree of saponification, the degree of modification, the cloud point, as well as the ultraviolet absorption spectrum absorbances (x) and (y) are measured as defined in the description.