nitrogen, oxygen or sulfur, there being up to 15 carbon atoms in each Z, two of which form a bridge between the carbon atoms connected by the adjacent Z groups, R is an aromatic group of up to 18 carbon atoms and up to two aromatic rings, inclusive; R' is an aliphatic group of up to 10 carbon atoms, inclusive; n or r each independently is zero or 1; X is a direct valence bond or X is alkylene of