Undercoat agent and method of producing structure containing phase-separated structure

An undercoat agent which is used for phase separating a layer containing a block copolymer having a block of a structural unit derived from an (α-substituted) acrylate ester on a substrate, comprising:

• a resin component,
• wherein the resin component includes
• a structural unit represented by general formula (ba0-1), and/or
• a structural unit represented by general formula (ba0-2), and
• a structural unit (ba0-3) having a substrate interacting group:
• wherein in the formula (ba0-1), R represents a hydrogen atom, an alkyl group of 1 to 5 carbon atoms or a halogenated alkyl group of 1 to 5 carbon atoms, R1 represents a halogen atom, or a linear, branched, or cyclic organic group of 1 to 20 carbon atoms which may contain an oxygen atom, a halogen atom, or a silicon atom, or an organic group obtained by combining these, and n is an integer of 1 to 5;
• and wherein in the formula (ba0-2), R represents a hydrogen atom, an alkyl group of 1 to 5 carbon atoms or a halogenated alkyl group of 1 to 5 carbon atoms, and R2 represents a linear, branched, or cyclic organic group of 1 to 20 carbon atoms which may contain an oxygen atom, a fluorine atom, or a silicon atom, or an organic group obtained by combining these.