| abstract |
The present invention provides a viscosity index improver having an excellent viscosity index-improving effect and a lubricating oil composition having excellent low-temperature viscosity characteristics in which a shear-induced reduction in viscosity during long-term use is small and which generates no shear-induced precipitate. The present invention provides a viscosity index improver containing a copolymer (A) whose essential constituent monomer is a monomer (a) having a number average molecular weight of 800 to 4,000 represented by the following formula (1), the copolymer (A) having a solubility parameter in the range of 9.00 to 9.40:wherein R<sup>1</sup>is a hydrogen atom or a methyl group; -X<sup>1</sup>- is a group represented by -O-, -O(AO)<sub>m</sub>-, or -NH-, AO is a C2-C4 alkyleneoxy group, m is an integer of 1 to 10, each AO may be the same or different when m is 2 or more, and the (AO)<sub>m</sub>moieties may be randomly bonded or block-bonded; R<sup>2</sup>is a residue after removal of one hydrogen atom from a hydrocarbon polymer whose essential constituent monomer is butadiene in which the butadiene is present in a proportion of 50% by weight or more based on the weight of R<sup>2</sup>, or after removal of one hydrogen atom from a polymer formed by partial hydrogenation of the hydrocarbon polymer; and p represents a number of 0 or 1. |