To obtain an improved process for producing a (thickened) polymer wherein a usual nonpressurized reactor can be used by producing a carboxylic acid (co)polymer in a silicone oil.

The process comprises the step of polymerizing a carboxylic acid monomer in a silicone solvent in an inert atmosphere in the presence of an initiator. In the process, a water-insol. cross-linked thickened (co)polymer which gives a 0.5 wt.% water soln. having a viscosity of about 50,000-300,000 cP is obtd. by conducting the polymn. at a molar ratio of the carboxylic acid monomer to a cross-linking monomer of (1:0.03)-(1:0.10). A pref. initiator is a redox initiator. An esp. pref. carboxylic acid monomer is acrylic acid, and an esp. pref. cross-linking monomer is allkyl methacrylate. Examples of the silicone solvent are cyclomethylsiloxane, a linear polydimethylsiloxane, an arom. phenyl-contg. siloxane, a polymethylalkylsiloxane, and a fluorosilicone.