Themonovalent organic group preferably has 100 or less carbon atoms.Examples of a substituent which each of the above groups may have include: a halogen atom such as a fluorine atom, a chlorine atom, and an iodine atom; an alkyl group such as a methyl group, an ethyl group, and a propyl group; and an aryl groupsuch as a phenyl group.The average number of repeating structural units a each represent