. " well as derivatives of these polymers: ##STR3## wherein R represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms such as a methyl group or ethyl group, or a halogen atom such as chlorine or bromine; M represents a hydrogen atom, an alkali metal atom such as sodium or potassium, an alkali earth metal atom such as calcium or magnesium, or an organic base such as trimethylamine or tri" . . . .