Unravelling the crystal and molecular structure of a 1,3-linked aromatic poly(ether-ketone)

Highly-oriented fibres of the aromatic polyetherketone [-O(1,4-Ar)CO(1,3-Ar)CO(1,4-Ar)-]n (Ar = phenylene) obtained by nucleophilic polycondensation of F(1,4-Ar)CO(1,3-Ar)CO(1,4-Ar)F with HO(1,4-Ar)CO(1,3-Ar)CO(1,4-Ar)OH afford, after annealing, very well-resolved X-ray fibre patterns. The new diffraction data are used to test two different published models for the crystal and molecular structure of this polymer, both based on a three-ring crystallographic repeat, and it is shown by diffraction-modelling that neither of these structures is compatible with the new fibre-data. In contrast, we report a novel structure, based on a six-ring crystallographic repeat, that gives a simulated fibre-diffraction pattern in very good agreement with the experimental pattern. Crystal data for the new structure are: orthorhombic, space group Pcam, a = 7.67, b = 6.13, c = 29.72 Å, two chains per cell, V = 1397 Å3 ρ = 1.43. Unusually, the two chains in the unit cell are related by an a-glide, meaning that the carbonyl groups in the two chains are aligned anti-parallel rather than having the more generally found parallel arrangement resulting from the presence of a b-glide. The new structure is shown to be fully compatible with previously published X-ray powder and electron diffraction data.