A modified PVA that enables formation of a film superior in cold water solubility, mechanical strength, and chemical resistance is provided. The modified PVA comprises a monomer unit represented by formula (I), and a structural unit represented by formula (II):

##STR00001##

wherein: a content of the monomer unit (I) is from 0.05 mol % to 10 mol %, and a content of the structural unit (II) is from 0.001 mol % to 0.5 mol % with respect to the total monomer units in the modified PVA; a viscosity average degree of polymerization is from 300 to 3,000; and a degree of saponification is from 82 mol % to 99.5 mol %. In the formula (I): R.sup.1 represents a hydrogen atom, or a methyl group; and R.sup.2 represents —R.sup.3—SO.sub.3.sup.−X.sup.+, —R.sup.3—N.sup.+(R.sup.4).sub.3Cl.sup.−, or a hydrogen atom, R.sup.3 representing an alkanediyl group, X.sup.+ representing a hydrogen atom, a metal atom, or an ammonium group, and R.sup.4 representing an alkyl group.

The present invention provides: a water-soluble film that is superior in cold water solubility and in practical properties such as strength and stiffness and that can resist deterioration in various properties such as chemical resistance over time; and a modified polyvinyl alcohol used for the water-soluble film. The present invention relates to a modified polyvinyl alcohol containing 0.05 to 10 mol % of a monomer unit having two carboxyl groups. In .sup.1H-NMR spectroscopy of the modified polyvinyl alcohol, an integral (X) from 6.8 to 7.2 ppm and an integral (Y) from 5.3 to 5.5 ppm satisfy the following expression:

0.60≦X/(X+Y)<1.0.

The present invention provides: a water-soluble film that is superior in cold water solubility and in practical properties such as strength and stiffness and that can resist deterioration in various properties such as chemical resistance over time; and a modified polyvinyl alcohol used for the water-soluble film. The present invention relates to a modified polyvinyl alcohol containing 0.05 to 10 mol % of a monomer unit having two carboxyl groups. In .sup.1H-NMR spectroscopy of the modified polyvinyl alcohol, an integral (X) from 6.8 to 7.2 ppm and an integral (Y) from 5.3 to 5.5 ppm satisfy the following expression:

0.60≦X/(X+Y)<1.0.

A novel acrylic polymer is provided. The novel acrylic polymer is an acrylic polymer which has a lactone ring structure in the main chain and has a melt flow rate of 2 g/10 minutes or more as measured at 230° C. under a load of 3.8 kg according to JIS K 7210.

A novel acrylic polymer is provided. The novel acrylic polymer is an acrylic polymer which has a lactone ring structure in the main chain and has a melt flow rate of 2 g/10 minutes or more as measured at 230° C. under a load of 3.8 kg according to JIS K 7210.

A curable composition which enables the formation of a cured product having a low dielectric constant, low dielectric loss tangent, high heat resistance, good elongation and excellent tensile strength and which has excellent film forming properties; a cured product of the curable composition; and a method for forming an insulating film using the curable composition. This curable composition contains a modified maleimide compound, a radical generator and a thiol compound. The curable composition may contain a photo-radical generator as the radical generator. The curable composition may contain a protonic acid.

An ethylene-vinyl alcohol copolymer has an ethylene content of 20 to 60 mol % and includes a carboxylic acid group and a lactone ring group present at terminals thereof, wherein the molar concentration (Y/Z) of a lactone ring molar content (Y) to a sum (Z) of a carboxylic acid molar content (X) and the lactone ring molar content (Y) is not less than 58 mol %. The ethylene-vinyl alcohol copolymer is excellent in high-temperature thermal stability substantially without thermal decomposition, and is free from odor emanation and coloration even if being processed at a higher temperature.

An ethylene-vinyl alcohol copolymer has an ethylene content of 20 to 60 mol % and includes a carboxylic acid group and a lactone ring group present at terminals thereof, wherein the molar concentration (Y/Z) of a lactone ring molar content (Y) to a sum (Z) of a carboxylic acid molar content (X) and the lactone ring molar content (Y) is not less than 58 mol %. The ethylene-vinyl alcohol copolymer is excellent in high-temperature thermal stability substantially without thermal decomposition, and is free from odor emanation and coloration even if being processed at a higher temperature.

An ethylene-vinyl alcohol copolymer has an ethylene content of 20 to 60 mol % and includes a carboxylic acid group and a lactone ring group present at terminals thereof, wherein the molar concentration (Y/Z) of a lactone ring molar content (Y) to a sum (Z) of a carboxylic acid molar content (X) and the lactone ring molar content (Y) is not less than 58 mol %. The ethylene-vinyl alcohol copolymer is excellent in high-temperature thermal stability substantially without thermal decomposition, and is free from odor emanation and coloration even if being processed at a higher temperature.

Ethylene-vinyl alcohol copolymer pellets comprise an ethylene-vinyl alcohol copolymer and water, and have a water content of not higher than 0.5 wt. %, wherein the ratio (Y/Z) of a lactone ring content (Y) to the sum (Z) of a carboxylic acid content (X) and the lactone ring content (Y) in terminal structures of the ethylene-vinyl alcohol copolymer is not less than 60 mol %. The pellets are excellent in high-temperature thermal decomposition-suppressing effect and thermal stability, and are free from odor emanation and coloration even if being processed at a higher temperature.