Resin composition for forming phase-separated structure, method for producing structure having phase-separated structure, and a block copolymer

A resin composition for forming a phase-separated structure, wherein the resin composition comprises a block copolymer (A),

• the block copolymer (A) has a block (A1), a block (A2), and a block (A3) in this order,
• the block (A1) comprises a polymer having a repeating structure of a constituent unit represented by formula (a1),
• the block (A2) comprises a polymer having a constituent unit represented by formula (a2-1),
• the block (A3) comprises a random copolymer having a structure in which a constituent unit represented by formula (a3-1) and a constituent unit represented by formula (a3-2) are randomly arranged,
• wherein, in the formula (a1), Ra1 is an alkyl group, and n1 is an integer of 0 or more and 5 or less,
• in the formula (a2-1), Ra21 is a substituent having a silicon atom, and n21 is an integer of 1 or more and 5 or less,
• in the formula (a3-1), Ra31 is an alkyl group which optionally has a silyl group, a fluorine atom, a carboxy group, an amino group, a hydroxy group, or a phosphoric acid group, and Ra32 is an alkylene group which optionally has a hydroxy group, and
• in the formulas (a1), (a2-1), (a3-1), and (a3-2), R1, R21, R31, and R32 are each independently a hydrogen atom, an alkyl group having 1 or more and 5 or less carbon atoms, or a halogenated alkyl group having 1 or more and 5 or less carbon atoms.