Among the polymers (b), preferable ones are those having repeating units of the formula: ##STR16## wherein R7 is a hydrogen atom, a straight or branched alkyl group having 1 to 4 carbon atoms or a group of the formula: R8 --OCO(CH2)z O--; R8 is a straight or branched alkyl group having 1 to 4 carbon atoms; z is zero or an integer of 1; R22 is an unsubstituted or substituted phenyl group; p, r and