Active ester resin, thermosetting resin composition, cured product of same, semiconductor encapsulation material, prepreg, circuit board, and build-up film

An active ester resin having one resin structure selected from the group consisting of:

• a resin structure obtained by reacting a phosphorus-containing phenolic compound (A1x) having a structural segment (i) represented by structural formula (I) below:
• Ar represents a benzene ring or a naphthalene ring, Fc represents a hydrogen atom or a hydroxyl group, and Z represents a structural segment selected from the group consisting of partial structures represented by structural formula z1 or z3 below:
• with an aromatic dicarboxylic acid or an anhydride or dihalide of an aromatic dicarboxylic acid or a C2-6 saturated dicarboxylic acid or an anhydride or dihalide of a C2-6 saturated dicarboxylic acid (A2) so that hydroxyl groups in the structural segment (i) form ester bonds, the resin structure having two structural segments (i) bonded to each other via an ester residue of the (A2);
• a resin structure obtained by reacting a phosphorus-containing phenolic resin (A1y) having a novolac phenolic resin structure and having, as a substituent on an aromatic nucleus, a structural segment selected from the group consisting of partial structures represented by the structural formula z1 or z3 with an aromatic dicarboxylic acid or an anhydride or dihalide of an aromatic dicarboxylic acid or a C2-6 saturated dicarboxylic acid or an anhydride or dihalide of a C2-6 saturated dicarboxylic acid (A2) so that all or some of hydroxyl groups of the phosphorus-containing phenolic resin (A1y) form ester bonds, the resin structure having two hydroxyl groups bonded to each other via an ester residue of the (A2);
• and
• a resin structure obtained by reacting a phosphorus-containing phenolic resin (A1z) having a structural segment (ii) selected from the group consisting of partial structures represented by structural formula (II) below:
• wherein R6 represents a hydrogen atom or a C1-6 alkyl group, Z represents a structural segment selected from the group consisting of a hydrogen atom and partial structures represented by the structural formula z1 or z3;
• wherein at least one of the Z represents a structural segment selected from the partial structures represented by the structural formula z1 or z3 with an aromatic dicarboxylic acid or an anhydride or dihalide of an aromatic dicarboxylic acid or a C2-6 saturated dicarboxylic acid or an anhydride or dihalide of a C2-6 saturated dicarboxylic acid (A2) so that all or some of hydroxyl groups of the phosphorus-containing phenolic resin (A1z) form ester bonds, the resin structure having two hydroxyl groups bonded to each other via an ester residue of the (A2);
• wherein in the structural formula z1 or z3 above, R1, R2, R3, and R4 each independently represent a hydrogen atom, a C1-5 alkyl group, a chlorine atom, a bromine atom, a phenyl group, or an aralkyl group, R5 represents a hydrogen atom or a C1-5 alkyl group, R represents a C1-4 alkyl group, and n represents the number of substituents OR on an aromatic nucleus and is in the range of 1 to 3.