Polymorphism in an oligo(aryl ether ketone)

1,3-Bis(4-phenoxybenzoyl)benzene (I) shows a single melting peak at T-1=120 degrees C when rapidly crystallized from a methanol/benzene mixture (10:1 v/v) (sample A) and a double melting peak at T-1=120 degrees C and T-2=130 degrees C when crystallized slowly (overnight) (sample B). Slow recrystallization of 1 using ethanol instead of methanol induces the formation of large single crystals which only show a single melting peak at T-2=130 degrees C (sample C). Analysis of powder X-ray diffractograms obtained for A, B and C samples clearly point out the coexistence of two crystalline phases in sample B: phase 1 (observed for sample A) and phase 2 (sample C). From optical microscopy and thermal (DSC) observations, it can be concluded that, upon heating, phase I melts at T-1 and the molten material converts itself into phase 2 if some crystals of phase 2 are present. Single crystal structure determination of, both, phase 1 and 2 reveal conformational differences of the molecule in both phases which pack in an orthorhombic and a monoclinic unit cell, respectively.