Preferred examples of the polymers represented by the formula [2] are those of the formula [2] wherein R6 and R21 are independently a hydrogen atom, R7 is a hydrogen atom, a lower alkyl group, a lower alkoxyl group, an acyloxy group, a saturated heterocyclic oxy group or a group of the formula R8 O--CO--(CH2)z --O-- (wherein R8 represents a lower alkyl group and z is 0 or 1), one of R22 and R23 is