A polymer composition according to claim 1, wherein the monomer unit having the functional group B is represented by the formula: ##STR42## wherein R6 is a hydrogen atom or a lower alkyl group; R7 is a hydrogen atom, a lower alkyl group, a lower alkoxy group, an acyloxy group, a saturated heterocyclic oxy group, or R8 O--CO--(CH2)z --O--; R8 is an alkyl group; and z is 0 or an integer of 1 or more