The copolymer of claim 1 wherein said anionic groups are acrylamide or methacrylamide groups of the general formula: ##STR7## where R' is H or an alkyl and R is an organic substituent with at least 1 carbon and Y is --COOM, --SO3 M or --OSO3 M in which M is a cation such as hydrogen, a metal or a nitrogen-containing base, such as an amine.