Copolymer, positive resist composition, and method of forming resist pattern

A copolymer comprising:

• a monomer unit (I) represented by formula (I), shown below:
• where, in formula (I), R1 is a halogen atom or a halogen atom-substituted alkyl group, R2 is a fluorine atom-containing organic group, and R3 and R4 are each a hydrogen atom, a halogen atom, an unsubstituted alkyl group, or a halogen atom-substituted alkyl group and may be the same as or different from each other;
• a monomer unit (II) represented by formula (II), shown below:
• where, in formula (II), R5 is a halogen atom or a halogen atom-substituted alkyl group, R6, R7, and R8 are each a hydrogen atom or an optionally substituted aliphatic group that does not include a fluorine atom and may be the same as or different from one another, R6, R7, and R8 may form a ring structure with a carbon atom that is bonded thereto, and R9 and R10 are each a hydrogen atom or an unsubstituted alkyl group and may be the same as or different from each other;
• and
• a monomer unit (III) represented by formula (III), shown below:
• where, in formula (III), R11 and R12 are each a hydrogen atom or an unsubstituted alkyl group and may be the same as or different from each other, R13 is an optionally substituted alkyl group, R14 is a halogen atom or an organic group, and p is an integer of not less than 0 and not more than 5.