. . . . "alkenyl group or an alkenyloxy group having 1 to 20 carbon atoms, and R is a functional group having a bivalently bonding group expressed by a single bond, ???(CH2)r???, ???(CH2O)s???, ???(OCH2)t???, ???COO???, ???OCO???, ???S???, ???S???S???, ???(CH2)u???O???, ???O???(CH2)v??? or ???O??? (where r, s, t, u and v are integers, preferably integers from 1 to 12) and is bonded to the polymer main chai" .