l groups or alkylene moieties preferablycontain up to 6 carbons, e.g. C.sub.1-6 or C.sub.1-3 ; cyclic groups preferably have ring sizes of 3 to 8 atoms; and fused multicyclic groups preferably contain 8 to 16 ring atoms.It will be appreciated that the provisos in the definition of Lp exclude compounds having two heteroatoms bonded directly together or separated by an alkyl group having only one ca