C???O???C) therein) in the polyether represented by general formula (7) to form the organic group (???R2) represented by general formula (3). ???R3(???O???Xn???Zm???Y)p (3) R in the polyether represented by general formula (7) is a double bond-terminated p-valent hydrocarbyl group having 2 to 20 carbon atoms or is a group comprising said hydrocarbyl group having an ether linkage (C???O???C) therei