) R.sub.0 is divalent when R.sub.0 and R.sub.0 are intramolecularly covalently bonded to each other and to the same dioxycarbon atom to form a heterocyclic ring or a heterocyclic ring substituted with an alkyl, alkoxy or alkenyl when R.sub.0 is an alkyleneoxy or alkenyleneoxy and R.sub.0 is an alkyleneoxy, alkenyleneoxy or alkylene; (