Similarities and differences between the mesomorphic behaviour of oligomeric macrocyclics and of linear high relative molecular mass polyethers based on 1-(4′-hydroxybiphenyl-4-yl)-2-(4-hydroxyphenyl)butane and flexible spacers

Abstract

This paper describes the synthesis and mesomorphic behaviour of macrocyclics of 1-(4′-hydroxybiphenyl-4-yl)-2-(4-hydroxyphenyl)butane (TPB) with α,ω-dibromoalkanes containing 13 [TPB(c)13(z), where z defines the size of the macrocyclic, i. e., z= 1, monomer; z= 2, dimer, etc. ], 14 [TPB-(c)14(z)] and respectively 18 [TPB-(c)18(z)] methylenic groups. It also discusses the influence of spacer length, x, and ring size, z, on their NMR spectra and on the phase behaviour of TPB-(c)x(z) macrocyclics with x= 4–14 and 18, and with z= 1–5, in comparison with that of their linear highrelative- molecular- mass polyethers TPB-(I)x. Macrocyclic monomers are liquid, crystalline, or glassy. Only the macrocyclic dimers with x= 10 and 12–18 exhibit a monotropic nematic phase. The macrocyclic trimer with x= 6 displays a monotropic smectic phase. T hose with x larger than 6 display an enantiotropic nematic phase, while those with x= 8, 13 and 14 exhibit, in addition to the enantiotropic nematic phase, also an enantiotropic smectic phase. Only the macrocyclic tetramer with x= 4 displays a monotropic nematic phase. All others exhibit enantiotropic nematic phases. All cyclic pentamers exhibit an enantiotropic nematic phase. The isotropization transition temperatures of cyclic trimers show an inverse odd–even dependence on x(i. e., the higher temperature is displayed by odd spacers) while those of cyclic tetramers and pentamers show a conventional odd-even dependence. Owing to the higher rigidity of the macrocyclics than that of their linear low and high relative molecular mass homologues most macrocyclics exhibit higher isotropization temperatures than do even their corresponding high relative molecular mass linear derivatives. However, the enthalpy and entropy changes associated with the isotropization transitions of macrocyclics are lower than those of their linear high relative molecular mass homologues. These results demonstrate that macrocyclic compounds containing mesogenic groups represent a novel class of liquid crystals which displays a higher ability to form nematic and smectic phases than do the corresponding low and high relative molecular mass linear homologues.