The effect of molecular architecture on the thermotropic behavior of poly[11-(4′-cyanophenyl-4"-phenoxy)undecyl acrylate] and its relation to polydispersity

Contrary to theoretical predictions on rodlike molecules (mixture of axial ratios) and previous experimental speculations on side chain liquid crystalline polymers (mixture of molecular weights), the breadth of the isotropization transition of poly[11-(4'-cyanophenyl-4 "-phenoxy)undecyl acrylate] is not broadened by polydispersity in chain length alone. Instead, it is broadened by the limited miscibility of a mixture of branched structures. SCLCPs and their mesogenic side chains can therefore be treated as mixtures of random coils and monodisperse rods. This was demonstrated by comparing the thermotropic behavior of linear and three-arm star poly[11-(4'-cyanophenyl-4 "-phenoxy)undecyl acrylate]s prepared by atom-transfer radical polymerization (ATRP), and their binary blends and unmixed composites. The biphasic region of Linear polymers with pdi = 1.15-1.49, and of their binary mixtures, is extremely narrow. Although 3-arm star polymers with pdi = 1.11-2.20 also exhibit extremely narrow isotropization transitional binary mixtures of the star polymers with significant differences in branching density have limited miscibility and broad isotropization transitions. The broad isotropization transition of unfractionated poly[11-(4'-cyanophenyl-4 "-phenoxy)undecyl acrylate] prepared by standard radical polymerization indicates that it also contains a mixture of branched structures due to chain transfer to polymer at high monomer conversion. 11-(4'-Cyanophenyl-4 "-phenoxy)undecyl acrylate is the most highly functionalized monomer to be polymerized by ATRP, and the resulting 3-arm star polymers are the first star polymers synthesized by ATRP.