wherein R3 is a hydrogen atom or a methyl group, R4 and R5 are the same or different and each is a hydrogen atom or an monovalent alkyl, aryl or aralkyl group containing up to 20 carbon atoms and R4 and R5 may be bound to each other at respective other termini, 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 divalent organic grou