TRANSFORMATION OF A KINETICALLY CONTROLLED NEMATIC PHASE OF A LINEAR POLYMER INTO ONE WHICH IS THERMODYNAMICALLY CONTROLLED VIA CYCLIZATION

The synthesis and characterization of cyclic main chain liquid-crystalline oligomers based on 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)butane (TPB) with 1,7-dibromoheptane (TPB-7(c)] are described. These oligomers were synthesized by the phase transfer catalysed polyetherification of TPB with 1,7-dibromoheptane under high dilution conditions and separated by column chromatography. Their cyclic structure was confirmed by 200 MHz H-1 NMR spectroscopy. The mesomorphic behaviour of TPB-7(c) was characterized by differential scanning calorimetry and polarized optical microscopy. The cyclic dimer is only crystalline, while the cyclic trimer, tetramer and pentamer exhibit an enantiotropic nematic mesophase. The high molecular weight linear homologue TPB-7(1) exhibits a nematic mesophase which has an isotropization transition temperature located in the very close proximity of its glass transition temperature. Therefore, this nematic phase is kinetically controlled. Due to the higher rigidity of cyclics versus linear structures the cyclic trimer, tetramer and pentamer exhibit higher isotropization transition temperatures than their linear homologue. Subsequently, the kinetically controlled nematic mesophase of the linear homologue is transformed into a thermodynamically controlled phase via cyclization.