er of 1 or more, and m>k, and the polymer (b) has repeating units of the formula: ##STR50## 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; z is 0 or an integer of 1 or more; R21 is a hydrogen atom or a lower alkyl group; R22