. "A method of introducing hydroxyl groups into the above-described copolymers comprises, for example, reacting the copolymers with molecular halogen, such as chlorine or bromine, in the presence of a catalyst, such as ferric chloride or boron fluoride, to halogenate the copolymers, and hydrolyzing the resulting copolymers by boiling them in water together with sodium hydroxide and the like." . . . .