Poly(p-phenylene)s with mesogenic side groups: A potential class of N-II side chain liquid crystalline polymers?

The synthesis and characterization of a series of n-[(4-cyano-4'-biphenylyl )oxy]alkyl-2,5 dichlorobenzoate monomers containing from n = 2 to 12 methyl enic groups in the alkyl spacer and their Ni(0)-catalyzed polymerization to produce poly(p-phenylene)s (PPP) containing mesogenic side groups are desc ribed. The resulting side chain liquid crystalline polymers (SCLCPs) have b oth a rigid rodlike mesogenic backbone and rigid rodlike mesogenic side gro ups. All polymers display only a broad enantiotropic nematic phase and no s ide-chain crystallization. The isotropization temperature of these SCLCPs; exhibits a steeper increase with the degree of polymerization CDP? than tha t observed for polymers with flexible or semiflexible backbones. Therefore, above plateau DPs their isotropization temperature is higher than that of polymers with flexible or semiflexible backbones, although it displays a cl assic odd-even dependence on the spacer length. These unusual trends may in dicate that the director of the nematic phase of these SCLCPs is determined by the mesogenic backbone and not by the mesogenic side groups, and theref ore, these polymers may provide the first examples of the N-II phase.