Abstract
Abstract A number of novel ester group containing aromatic and cycloaliphatic diisocyanates: 4,4′-diisocyana-tophenyl benzoate, 4,4′-diisocyanato-trans-cyclohexyl benzoate and several derivatives containing lateral substituents were synthesized by reaction of appropriate trimethylsiloxy phenyl or cyclohexyl isocyanates with isocyanatobenzoyl chlorides. They form nematic liquid crystalline phases. The isocyanate group greatly stabilizes the mesophase with regard to other three atom terminal groups. The influence of lateral methyl substituents in various positions on the thermal transitions was studied. They reduce the nematic range. 4-Isocyanatophenyl 4-methoxybenzoate and the corresponding diazetidinedione (dimer with a higher aspect ratio) were also synthesized. The pure dimer gives an isotropic melt due to a cycloreversion to the monomer. When present as impurities diazetidinediones decrease the melting point and increase the clearing temperature of the parent isocyanate.
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