Phase Transfer Pd(O) Catalyzed Polymerization Reactions. 2. Thermal Characterization of Liquid Crystalline 1,2-(4,4′-Dialkoxyaryl)acetylene Derivatives

Abstract

A homologous series of 1,2-(4,4′-di-n-alkoxyphenyl)acetylenes with n = 1,3-12 was prepared by a one pot phase transfer Pd(O)/Cu(I) catalyzed three step coupling of an aryl halide with 2-methyl-3-butyn- 2-ol, followed by deprotection of the carbinol group and finally a second coupling with either the same or another aryl halide. The lower members of this series with n = 1-3 are crystalline. In addition to a nematic phase, the compounds with from four to eight methylenic units in the alkoxy substituents present crystalline polymorphism due to the proximity of the crystallization temperatures and the low rates of crystallization. Therefore, under the nonequilibrium conditions at which the compounds were analyzed, these immiscible crystalline phases coexist. The higher homologs of n = 10-12 display an unusual enantiotropic smectic C phase directly below a nematic phase. These compounds also exhibit an additional monotropic smectic phase, possibly smectic B, below the smectic C phase. Methyl branches were introduced into either the aromatic ring(s) of the mesogen or the alkoxy substituent of several 1,2-(4,4′-di-n-alkoxyphenyl)acetylenes. This acts to depress liquid crystallinity more than crystallinity. However, the assymetrically substituted branched derivatives display a nematic phase in addition to the crystalline phase(s).