wherein R3 is a hydrogen atom or a methyl group, R4 and R5 each is a hydrogen atom or a univalent alkyl, aryl or aralkyl group containing 1 to 20 carbon atoms and such R4 and R5 groups may be bonded together at the respective other ends, R6 is ???C(O)O??? (ester group), ???C(O)??? (keto group) or an o-, m- or p-phenylene group, R7 is a direct bond or a bivalent organic group containing 1 to 20 car