The copolymers of formula (III) in which Y is --OH are commercially available or may be prepared by copolymerizing monomers as shown below: ##STR16## The copolymers of formula (III) in which Y is Cl may be prepared by treating the corresponding copolymer having Y???--OH with a chlorinating agent such as thionyl chloride in an inert solvent such as an alkane (hexane or heptanes) or an aromatic solv