Active ester resin, method for producing the same, thermosetting resin composition, cured product of the thermosetting resin composition, semiconductor encapsulating material, pre-preg, circuit board, and build-up film

A method for producing an active ester resin, the method comprising reacting a polyfunctional phenolic compound (a1) with a monofunctional aromatic carboxylic acid or its chloride (a2) and an aromatic dicarboxylic acid or its chloride (a3) in such a manner that

• an amount of the monofunctional aromatic carboxylic acid or its chloride (a2) is 0.46 to 0.95 moles and
• an amount of the aromatic dicarboxylic acid or its chloride (a3) is 0.27 to 0.025 moles
• per mole of phenolic hydroxyl groups of the polyfunctional phenolic compound (a1), the polyfunctional phenolic compound (a1) being represented by structural formula (1) below:
• (wherein Ar represents a benzene ring, a naphthalene ring, a benzene ring nuclear-substituted by an alkyl group having 1 to 4 carbon atoms, or a naphthalene ring nuclear-substituted by an alkyl group having 1 to 4 carbon atoms;
• X represents a methylene group, a divalent cyclic aliphatic hydrocarbon group, a phenylene dimethylene group, or a biphenylene-dimethylene group;
• and n represents the number of repeating units and an average thereof is in a range of 0.5 to 10).

A curable resin composition comprising an active ester resin (A) and an epoxy resin (B) as essential components, the active ester resin (A) having a resin structure produced by reacting a polyfunctional phenolic compound (a1) with a monofunctional aromatic carboxylic acid or its chloride (a2) and an aromatic dicarboxylic acid or its chloride (a3) in such a manner that

• an amount of the monofunctional aromatic carboxylic acid or its chloride (a2) is 0.46 to 0.95 moles and
• an amount of the aromatic dicarboxylic acid or its chloride (a3) is 0.27 to 0.025 moles
• per mole of phenolic hydroxyl groups of the polyfunctional phenolic compound (a1), the polyfunctional phenolic compound (a1) being represented by structural formula (1) below:
• (wherein Ar represents a benzene ring, a naphthalene ring, a benzene ring nuclear-substituted by an alkyl group having 1 to 4 carbon atoms, or a naphthalene ring nuclear-substituted by an alkyl group having 1 to 4 carbon atoms;
• X represents a methylene group, a divalent cyclic aliphatic hydrocarbon group, a phenylene dimethylene group, or a biphenylene-dimethylene group;
• and n represents the number of repeating units and an average thereof is in a range of 0.5 to 10).