Positive resist composition, method of forming resist pattern, and polymeric compound

A positive resist composition comprising a base component (A′) which exhibits increased solubility in an alkali developing solution under action of acid and generates acid upon exposure,

• the base component (A′) comprising a polymeric compound (A1′) comprised of a structural unit (a5-1) represented by general formula (a5-1) shown below, a structural unit (a0-1) represented by general formula (a0-1) shown below and a structural unit (a0-2) that generates acid upon exposure,
• the structural unit (a0-2) containing a group represented by general formula (a0-2′) shown below:
• wherein each R independently 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 hydrocarbon group of at least 10 carbon atoms;
• R2 represents an arylene group which may have a substituent;
• each of R3 and R4 independently represents an organic group, wherein R3 and R4 may be mutually bonded to form a ring with the sulfur atom;
• R5 represents a substituent;
• a represents an integer of 0 to 4;
• each R7 independently represents a hydrogen atom or an alkyl group of 1 to 5 carbon atoms;
• Y represents a divalent linking group;
• and X− represents an anion moiety represented by any one of general formulas (1) to (5) shown below,
• X0-Q1CF2pSO3⊖ (1)
• X0-Q2-OCF2SO3⊖ (2)
• X10-Q3-Y10—N⊖—SO2—Y11 (3)
• A⊖ (4)
• Y12CH2qSO3⊖ (5)
• wherein each X0 independently represents a hydrocarbon group of 3 to 30 carbon atoms which may have a substituent;
• Q1 represents a divalent linking group containing a carbonyl group;
• each p independently represents an integer of 1 to 3;
• Q2 represents a single bond or an alkylene group;
• X10 represents a hydrocarbon group of 1 to 30 carbon atoms which may have a substituent;
• Q3 represents a single bond or a divalent linking group;
• Y10 represents —C(═O)— or —SO2—;
• Y11 represents an alkyl group of 1 to 10 carbon atoms which may have a substituent or a fluorinated alkyl group of 1 to 10 carbon atoms which may have a substituent;
• A− represents a carboanion;
• Y12 represents a cyclic alkyl group of 4 to 20 carbon atoms which may have an oxygen atom (═O) as a substituent;
• and q represents 0 or 1.