Defoaming agent and lubricating oil composition

A defoaming agent obtainable by a process, the process comprising:

• (i) polymerizing (B) at least one defoaming agent monomer in (A) a polymerization solvent under coexistence of (C) a polymer soluble in the polymerization solvent,
• wherein the (B) at least one defoaming agent monomer comprises a first monomer, the first monomer giving a repeating unit represented by the following general formula (1) upon polymerization:
• wherein in the general formula (1), X1 is a repeating unit obtainable by polymerization of an (meth)acryloyl group;
• Y1 is a side chain comprising a linear or branched polysiloxane structure, the polysiloxane structure comprising a repeating unit represented by the following general formula (2) and having a polymerization degree of 5 to 300;
• and Z1 is a linking group linking the repeating unit X1 and the side chain Y1;
• wherein in the general formula (2), R1 and R2 are each independently a C1-18 organic group or combination of two or more C1-18 organic groups;
• the polymer (C) is at comprises at least one polymer other than the component (B);
• in the (i) polymerizing, the polymer (C) coexists with the monomer (B) in the solvent (A) even before the polymerizing is initiated;
• and
• the polymer (C) is at least one polymer selected from the group consisting of non-dispersant polyalkyl (meth)acrylate, dispersant polyalkyl (meth)acrylate, a comb-shaped polyalkyl (meth)acrylate, a star-shaped polyalkyl (meth)acrylate, an olefin copolymer, a hydrogenated product of styrene-diene copolymer, a hydrogenated product of polyisoprene, a polyisobutylene, a maleated polyisobutylene, an imidated product of maleated polyisobutylene, a hydrogenated polybutadiene, an oil-soluble polyester, and an ethylene-propylene-diene rubber.