Poly(p-phenylene)s with mesogenic side groups : A potential class of NII 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 methylenic groups in the alkyl spacer and their Ni(0)-catalyzed polymerization to produce poly(p-phenylene)s (PPP) containing mesogenic side groups are described. The resulting side chain liquid crystalline polymers (SCLCPs) have both a rigid rodlike mesogenic backbone and rigid rodlike mesogenic side groups. All polymers display only a broad enantiotropic nematic phase and no side-chain crystallization. The isotropization temperature of these SCLCPs exhibits a steeper increase with the degree of polymerization (DP) than that 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 classic odd-even dependence on the spacer length. These unusual trends may indicate that the director of the nematic phase of these SCLCPs is determined by the mesogenic backbone and not by the mesogenic side groups, and therefore, these polymers may provide the first examples of the N II phase.