Base material and method for producing structure including phase-separated structure

A base material for bringing a block copolymer-containing layer to phase separation, the base material comprising:

• a block copolymer (A) comprising a block (A1) and a block (A2),
• wherein the block (A1) has a constituent unit represented by the following Formula (a1),
• the block (A2) has a constituent unit represented by the following Formula (a2),
• the block copolymer (A) has no constituent unit derived from an alkyl (meth)acrylate,
• the block copolymer (A) has a molar ratio of the constituent unit of the block (A2) of more than 0 mol % and 40 mol % or less with respect to a total of the constituent unit of the block (A1) and the constituent unit of the block (A2),
• wherein Ra11 represents 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, Ra12 represents a substituent, and n is an integer of 0 or more and 5 or less, and
• wherein Ra21 represents 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, L1 represents a single bond or a divalent linking group, and Y1 represents a divalent linking group having 1 or more and 15 or less carbon atoms.