Dendronized Poly(2-oxazoline) Displays within only Five Monomer Repeat Units Liquid Quasicrystal, A15 and σ Frank–Kasper Phases

Liquid quasicrystals (LQC) have been discovered in self-assembling benzyl ether, biphenylmethyl ether, phenylpropyl ether, biphenylpropyl ether and some of their hybrid dendrons and subsequently in block copolymers, surfactants and other assemblies. These quasiperiodic arrays, which lack long-range translational periodicity, are approximated by two Frank–Kasper periodic arrays, Pm3̅n cubic (Frank–Kasper A15) and P4₂/mnm tetragonal (Frank–Kasper σ), which have been discovered in complex soft matter in the same order and compounds. Poly­(2-oxazoline)­s dendronized with (3,4)nG1 minidendrons (where n denotes an alkyl chain, C_nH_2n+1) self-organize into the Pm3̅n cubic phase (n = 14 and 15) and, as reported recently, the P4₂/mnm tetragonal phase (n = 16). However, no LQC of a poly­(2-oxazoline) is yet known. Here we report the synthesis, structural and retrostructural analysis of a dendronized poly­(2-oxazoline) with n = 17 which self-organizes not only into the LQC but also in the above two Frank–Kasper approximants. All three phases are observed from the same polymer within a very narrow range of degree of polymerization that corresponds to only five monomer repeat units (5 ≤ DP ≤ 10). The formation of the Pm3̅n cubic, P4₂/mnm tetragonal and LQC phases from a single polymer chain within such a narrow range of DP raises the questions of how and why each of these phases is self-organized. This system may provide a model for theoretical investigations into the self-organization of soft matter into Frank–Kasper and related periodic and quasiperiodic arrays.