Cross-linkable liquid crystal monomers containing hydrocarbon 1,3-diene tail systems

The unprecedented use of polymerizable hydrocarbon tail systems containing 1,3-diene groups for the design of thermotropic and lyotropic liquid crysta l (LC) monomers is described. Crosslinkable LC dienes are synthesized by at taching LC core units to modular omega -bromoalka-1,3-diene tails of variab le length. These modular diene tails are synthesized by the oxidation of lo ng chain omega -bromoalkanl-ols to the corresponding omega -bromoalkanals. Reaction of the omega -bromoalkanals with Matteson's reagent, followed by t reatment with triethanolamine and deoxysilylation under Peterson eliminatio n conditions, affords the desired omega -bromoalka-1,3-diene tails. The eff ect of the 1,3-diene group on the mesogenic behavior of certain thermotropi c and lyotropic LC systems was determined by examining 1,3-diene analogues of a thermotropic calamitic LC diacrylate and a taper-shaped lyotropic LC t riacrylate. Compared to their diacrylate analogues, the thermotropic LC bis (1,3-diene)s exhibit the same progression of nematic and smectic phases but with higher smectic C to nematic transition temperatures and higher cleari ng temperatures. Replacement of the three acrylate groups in the tapered-sh aped lyotropic LC monomer with 1,3-diene moieties had little effect on its tendency to form the inverted hexagonal phase at room temperature in the pr esence of water. The lyotropic LC diene phases also exhibit higher clearing temperatures than the corresponding LC triacrylate. The 1,3-diene group wa s found to be an efficient cross-linking unit for the photopolymerization o f lyotropic LC phases at ambient temperature because of its hydrophobicity, minimal phase perturbation, and the high degree of photopolymerization. Wi th thermotropic LC systems, however, Diels-Alder dimerization of adjacent d iene units was found to occur upon heating the thermotropic LC bis(diene) m onomers to ca. 90 degreesC or higher. Thus, as a photopolymerizable group i n LC monomer design, the practical utility of the 1,3-diene group appears t o be limited to temperature regimes below 90 degreesC.