Polymers of carbonic acid, 4. Crystalline smectic polyurethanes derived from piperazine or 4,4′-bipiperidine and diphenols containing aliphatic spacers†

4,4′-Alkylenedioxydiphenols (3a–e), prepared from α,ω-dibromoalkanes and hydroquinone, were converted with phosgen to the corresponding dichloroformates 4a–e, which were reacted with piperazine, trans-2,5-dimethylpiperazine or 4,4′-bipiperidine under the conditions of interfacial condensation. The DSC measurements of polyurethanes derived from 2,5-dimethylpiperazine displayed one endotherm indicating a normal melting process, whereas the polyurethanes derived from piperazine or 4,4′-bipiperidine showed a reversible first order phase transition below the melting point. WAXS powder patterns indicate a change between two different smectic modifications. Thermomechanical analyses (penetration method) proved that also the high-temperature smectic modification is a solid state and not a liquid-crystalline one.