The present invention provides an industrially advantageous method for producing vinyl alcohol polymers having a low degree of polymerization using a chain transfer agent having an aliphatic hydrocarbon group having 6 to 12 carbon atoms, the method being capable of producing a vinyl alcohol polymer whose aqueous solution has excellent viscosity stability. The present invention relates to a method for producing vinyl alcohol polymers, including: polymerizing a vinyl ester monomer (a) in the presence of a polymerization initiator (B) and a chain transfer agent (C) having an aliphatic hydrocarbon group having 6 to 12 carbon atoms to obtain a vinyl ester polymer (A); and saponifying the vinyl ester polymer (A) at a concentration of 40 mass % or more and 75 mass % or less in the presence of an acidic catalyst (D) to obtain a vinyl alcohol polymer, wherein the vinyl alcohol polymer has a viscosity-average degree of polymerization of 160 or more and 400 or less.

A process for preparing an ultra-high molecular weight polymer via emulsion polymerization at the room/indoor temperature includes using persulfate as initiator, water as medium, anionic surfactant as emulsifier, sodium bicarbonate as pH regulator, styrene, (meth) acrylate or vinyl acetate as monomer, which is subjected to a free radical polymerization at room temperature and the normal pressure. The ultra-high molecular weight polymers may have weight average molecular weights larger than 1,000,000 g/mol. The monomer conversion may be above 85% after reacting for 1˜6 hours. The process may be carried out at room temperature and normal pressure without the addition of other assist initiators. There temperature and pressure of the reaction may not be controlled, and the molecular weight and molecular weight distribution may be adjusted and controlled in a wider range.

A process for preparing an ultra-high molecular weight polymer via emulsion polymerization at the room/indoor temperature includes using persulfate as initiator, water as medium, anionic surfactant as emulsifier, sodium bicarbonate as pH regulator, styrene, (meth) acrylate or vinyl acetate as monomer, which is subjected to a free radical polymerization at room temperature and the normal pressure. The ultra-high molecular weight polymers may have weight average molecular weights larger than 1,000,000 g/mol. The monomer conversion may be above 85% after reacting for 1˜6 hours. The process may be carried out at room temperature and normal pressure without the addition of other assist initiators. There temperature and pressure of the reaction may not be controlled, and the molecular weight and molecular weight distribution may be adjusted and controlled in a wider range.

Provided is a method for producing a polyvinyl alcohol having improved mechanical strength without significantly impairing productivity. A method for producing a polyvinyl alcohol having an average degree of polymerization of 5.0×10.sup.3 or more and 1.5×10.sup.4 or less, the method including steps of: subjecting a vinyl ester-based monomer to suspension polymerization at a polymerization temperature of 10° C. or more and 50° C. or less with a polymerization rate of 10% or more and 70% or less to obtain a vinyl ester-based polymer; dissolving the vinyl ester-based polymer in an alcohol-based solvent; and carrying out a saponification reaction by adding a saponification catalyst to an alcohol-based solution of the dissolved vinyl ester-based polymer.

Provided is a method for producing a polyvinyl alcohol having improved mechanical strength without significantly impairing productivity. A method for producing a polyvinyl alcohol having an average degree of polymerization of 5.0×10.sup.3 or more and 1.5×10.sup.4 or less, the method including steps of: subjecting a vinyl ester-based monomer to suspension polymerization at a polymerization temperature of 10° C. or more and 50° C. or less with a polymerization rate of 10% or more and 70% or less to obtain a vinyl ester-based polymer; dissolving the vinyl ester-based polymer in an alcohol-based solvent; and carrying out a saponification reaction by adding a saponification catalyst to an alcohol-based solution of the dissolved vinyl ester-based polymer.

A method to purify a polymer is disclosed. The method is part of a manufacturing process wherein a virgin polymer is produced from a polymerization process employing at least a monomer or a comonomer as a feedstock. The monomer or the comonomer is selected from the group consisting of organic polar monomers, inorganic monomers, vinyl aromatic monomers, conjugated dienes, and mixtures thereof. In the method, the polymer in solid, liquid or molten state is brought into contact with a fluid solvent or an extraction fluid in a supercritical state or near supercritical state. The contact is at controlled temperature and pressure, allowing the fluid solvent to diffuse into the polymer and extract the impurities intended to remove. The method can be used to remove at least 10%, or at least 20%, or at least 50% of the target impurity from the polymer.

A method to purify a polymer is disclosed. The method is part of a manufacturing process wherein a virgin polymer is produced from a polymerization process employing at least a monomer or a comonomer as a feedstock. The monomer or the comonomer is selected from the group consisting of organic polar monomers, inorganic monomers, vinyl aromatic monomers, conjugated dienes, and mixtures thereof. In the method, the polymer in solid, liquid or molten state is brought into contact with a fluid solvent or an extraction fluid in a supercritical state or near supercritical state. The contact is at controlled temperature and pressure, allowing the fluid solvent to diffuse into the polymer and extract the impurities intended to remove. The method can be used to remove at least 10%, or at least 20%, or at least 50% of the target impurity from the polymer.

Provided are a polyvinyl alcohol-based resin having an excellent plugging effect and a plugging agent for underground treatment. A polyvinyl alcohol-based resin is disclosed, in which a degree of swelling (%) when the polyvinyl alcohol-based resin is immersed in water at 40° C. for 30 minutes and a solubility (%) when the polyvinyl alcohol-based resin is immersed in water at 65° C. for 5 hours satisfy the degree of swelling/the solubility≥3.0.

A water-resistant gas barrier film is provided with a gas barrier layer at least disposed on a substrate film. The gas barrier layer is made from coating liquid comprising (a) at least one of carboxy group-modified polyvinyl alcohol and partially saponified polyvinyl alcohol, (b) completely saponified polyvinyl alcohol, and (c) polyethyleneimine.

A water-resistant gas barrier film is provided with a gas barrier layer at least disposed on a substrate film. The gas barrier layer is made from coating liquid comprising (a) at least one of carboxy group-modified polyvinyl alcohol and partially saponified polyvinyl alcohol, (b) completely saponified polyvinyl alcohol, and (c) polyethyleneimine.