Process for preparing fluorine-containing polymer and photoresist composition

A process for preparing a fluorine-containing polymer having a structural unit (M1) derived from a monomer (m1) which provides an aliphatic ring structure in the polymer trunk chain and may have fluorine atom, in which said monomer (m1) being capable of providing an aliphatic ring structure in the polymer trunk chain is subjected to radical polymerization by using an organic peroxide having a structural unit represented by the formula (1 -1):

• wherein R is selected from monovalent hydrocarbon groups having 3 or more carbon atoms, in which hydrogen atom may be substituted with fluorine atom or monovalent hydrocarbon groups having ether bond, in which the total number of carbon atoms and oxygen atoms is 3 or more and hydrogen atom may be substituted with fluorine atom, and when in R, carbon atoms or carbon atoms and oxygen atoms in the case of having ether bond are counted from the carbon atom C1, at least one of the fourth atoms is a carbon atom to which at least one hydrogen atom is bonded;
• X1 and X2 are the same or different and each is hydrogen atom, halogen atom or a hydrocarbon group having 1 to 10 carbon atoms, in which a part or the whole of hydrogen atoms may be substituted with fluorine atoms, or the formula (1-2):
• wherein R′ is selected from divalent hydrocarbon groups having 4 or more carbon atoms, in which hydrogen atom may be substituted with fluorine atom or divalent hydrocarbon groups having ether bond, in which the total number of carbon atoms and oxygen atoms is 4 or more and hydrogen atom may be substituted with fluorine atom, and when in R′, carbon atoms or carbon atoms and oxygen atoms in the case of having ether bond are counted from the carbon atom C1, at least one of the fourth atoms is a carbon atom to which at least one hydrogen atom is bonded;
• X1 is hydrogen atom, halogen atom or a hydrocarbon group having 1 to 10 carbon atoms, in which a part or the whole of hydrogen atoms may be substituted with fluorine atoms;
• n is 0 or 1.