tly hydrogen orC.sub.1-C.sub.18 hydrocarbyl; and L has the formula --O-A.sup.1-Y-A.sup.2-O-- wherein A.sup.1 and A.sup.2 are independently a monocyclic divalent a C.sub.6-C.sub.16 aromatic radical, and Y is a single bond, an oxygen, or a C.sub.1-C.sub.16 bridgingradical in which one or two atoms separate A.sup.1 from A.sup.2.25.