At the Borderline between Glassy, Crystalline and Liquid Crystalline Macrocyclics1a

The synthesis and characterization of the macrocyclic oligopolyethers based on the conformationally flexible 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)butane (TPB) with 1,4-dibromobutane [TPB-(c)4(z), where z indicates the degree of oligomerization of the macrocyclic] and with 1,6-dibromohexane [TPB-(c)6(z)], respectively are described. These cyclic oligomers were synthesized by the phase transfer catalyzed polyetherification of TPB with 1,4-dibromobutane or 1,6-dibromohexane under high dilution conditions and the individual compounds were separated by column chromatography. Their cyclic structure was confirmed by 200 MHz 1H-NMR spectroscopy. The mesomorphic behavior of TPB-(c)4(z) and TPB-(c)6(z) was characterized by differential scanning calorimetry (DSC) and optical polarized microscopy. The macrocyclic derivative with the shortest spacer based on TPB and α,ω-dibromoalkanes which displays a liquid crystalline mesophase is the tetramer obtained with 1,4-dibromobutane [TPB-(c)4(4)]. However its nematic mesophase is only monotropic and its isotropization transition temperature is lower than that of its corresponding high molecular weight linear polyether TPB-(1)4. The macrocyclic polyether with the shortest spacer based on TPB which displays a nematic phase with an isotropization temperature which is higher than its high molecular weight linear homologue is the tetramer based on 1,6-dibromohexane [i.e., TBP-(c)6(4)]. TPB-(c)6(3) exhibits an unidentified monotropic X phase. The enthaply changes associated with the isotropization temperatures of the macrocyclics are lower than those of the corresponding linear polymers.