wherein n and R1 are as hereinbefore defined, M is a group containing two or more aromatic rings bridged by a rigid central linkage, R4 is a divalent hydrocarbon group having from one to about 12 carbon atoms and may be linear, branched, cyclic, aromatic or a combination thereof and may be substituted with one or more inert groups, such as, for example, a methoxy group, or may contain one or more