t C.sub.(1-14) hydrocarbon radicals and substituted monovalent C.sub.(1-14) hydrocarbon radicals, R2 is selected from the class consisting of the same or different C.sub.(1-14) divalent hydrocarbon radicals, or substituted C.sub.(1-14) divalent hydrocarbon radicals, and R4 is a divalent aryl radical selected from the class consisting of ##STR13## X1 is a member selected from the class consisting o