LIQUID-CRYSTALLINE POLYETHERS BASED ON CONFORMATIONAL ISOMERISM .28. MACROCYCLES WITH CLEARING TEMPERATURES HIGHER THAN THEIR LINEAR HIGH-MOLECULAR-WEIGHT HOMOLOGS

The synthesis and characterization of macrocycles based on the conformationally flexible 1-(4-hydroxybiphenyl-4'-yl)2-(4-hydroxyphenyl)butane (TPB) with 1,11-dibromoundecane [TPB-(c)11(z)], and TPB with 1,12-dibromododecane [TPB-(c)12(z)] (where c=cyclic and z defines the degree of oligomerization of the macrocycle, i.e. z=2 is a dimer; z=3 is a trimer etc.) are described. These cyclic oligomers were synthesized by the phase-transfer catalyzed polyetherification of TPB with 1,11-dibromoundecane or 1,12-dibromododecane under high dilution conditions and were separated by column chromatography. Their cyclic structure was confirmed by 200 MHz H-1 NMR spectroscopy. Most of the chemical shifts of the protons of the TPB unit are highly dependent on ring size owing to the conformational isomerism of the TPB mesogenic unit. The mesomorphic behaviour of TPB-(c)11(z) and TPB-(c)12(z) was characterized by differential scanning calorimetry (DSC) and optical polarized microscopy. TPB-(c(11(z) with z=3,4 or 5 and TPB-(c)12(z) with z-3 or 4 display equal or higher isotropization temperatures of their nematic mesophases than those of their linear high-molecular-weight homologues. Therefore, the mesophases of these macrocycles are stable over a broader range of temperature than those of their linear homologues. This was explained by the higher rigidity of the cyclic structure than that of the linear polymers, the absence of chain ends in the cyclic structure, and a suitable correlation between the TPB and the flexible spacer lengths.