Interfacial polycondensation of 1,1’-diamino-4,4’-bipyridinium di-iodide, [H2N-(NC5H4)-N-+-C5H4N+-NH2]I-2, with aromatic di- or tri-acyl chlorides, affords a novel class of hydrophilic polymers. These materials are isoelectronic with polyesters but contain ylid linkages [N+-N--CO] in place of ester bonds [C-O-CO]. Non-crosslinked polymers of this type are insoluble in conventional organic solvents but dissolve readily in proton-donor solvents such as formic and trifluoroacetic acids. Single-crystal X-ray studies of the model oligomer C6H5CO-N--(N5H4)-N-+-C5H4N+-N--COC6H5 reveal an essentially rigid-rod structure and, in keeping with this, the poly-ylids derived from terephthaloyl chloride and naphthalene-2,6-diacyl chloride exhibit lyotropic liquid crystallinity. Interfacial syntheses of poly-ylids on ultrafiltration membranes afford supported membranes with nodular morphology and useful nanofiltration properties.