A copolymer of 2-acrylamido-2methyl propane sulfonate and acrylamide. AMPS polymers are highly water-soluble anionic additives designed for high-salinity and high-temperature water-mud applications. (Alkyl-substituted acrylamide can be used instead of ordinary acrylamide, which lessens its vulnerability to hydrolysis at high temperature and high pH.) Polymers from 0.75 to 1.5 MM molecular weight are suggested for fluid-loss control in these difficult muds.Reference:Perricone AC, Enright DP and Lucas JM: "Vinyl Sulfonate Copolymers for High-Temperature Filtration Control of Water-Base Muds," SPE Drilling Engineering 1, no. 5 (October 1986): 358-364.