| abstract |
Disclosed is a viscosity index improver that has a high viscosity index improving effect and is excellent in metal fatigue resistance and metal wear resistance. An alkyl (meth) acrylate (a) having an alkyl group having 1 to 36 carbon atoms, a hydroxyl group-containing monomer (b1), a carboxyl group-containing monomer (b2), a phosphate ester group-containing monomer. Monomer (b3), phosphono group-containing monomer (b4), amino group-containing monomer (b5), amide group-containing monomer (b6), thiol group-containing monomer (b7), and sulfonic acid group-containing One or more monomers (b) selected from the group consisting of the structural unit (b8) are used as essential structural monomers, and the average side chain length (sb) of the monomer (b) and the monomer (a ) Average side chain length (sa) ratio [(sb) / (sa)] is a viscosity index improver comprising a copolymer (A) having a ratio of 0.1 to 12.0. [Selection figure] None |