VISCOSITY INDEX IMPROVER, REFRIGERATOR OIL, AND WORKING FLUID COMPOSITION FOR REFRIGERATORS

The present invention relates to a viscosity index improver containing a copolymer (A) that contains, as essential constituent monomers, a fluorine atom-containing monomer (a) represented by the following formula (1) and a monomer (b) represented by the following formula (2),wherein the copolymer (A) has a solubility parameter of 8.1 to 10.0 (cal/cm3)1/2, and a mass ratio (b/a) of the monomer (b) to the monomer (a) in the monomers constituting the copolymer (A) is 0.01 to 42:wherein R1 represents a hydrogen atom or a methyl group; R2 is a C2-C4 alkylene group; p is an integer of 0 or 1; q is an integer from 0 to 20, each R2 may be the same as or different from each other when q is 2 or greater; and Y is a monovalent group in which one or more or all of hydrogen atoms in a C1-C40 hydrocarbon group are replaced by fluorine atoms, andwherein R3 represents a hydrogen atom or a methyl group; -X1- is a group represented by -O- or -NH-; R4 is a C2-C4 alkylene group; r is an integer of 0 or 1; s is an integer from 0 to 20, and each R4 may be the same as or different from each other when s is 2 or greater; and R5 is a C1-C40 hydrocarbon group.