wherein R13 is a hydrogen atom or a methyl group, R14 and R15 each is a hydrogen atom or a monovalent alkyl group having 1 to 20 carbon atoms, aryl or aralkyl group and R14 and R15 may be bound to each other at respective other termini, R16 is ???C(O)O???(ester group), ???C(O)??? (keto group) or an o-, m- or p-phenylene group, R17 is a direct bond or a divalent organic group having 1 to 20 carbon