ral formula (1) which are chemically bonded to the copolymer (a) to form abranched or crosslinked structure: ##STR3## wherein R represents a hydrogen atom or a lower alkyl group; X represents one or more groups selected from among --COOCH.sub.3, --COOC.sub.2 H.sub.5, COOC.sub.4 H.sub.9, --COOCH.sub.2 CH(C.sub.2H.sub.5)C.sub.4 H.sub.9, --CN and ##STR4## and n is an integer of 1 or above.5.