Ordered chiral structures in the crystals of main-chain chiral poly(2-oxazoline)s

The formation of a crystalline structure with the presence of chiral secondary structures was investigated for the enantiopure polymer, p-R-2-butyl-4-ethyl-2-oxazoline (p-R-BuEtOx), its enantiomer p-S-BuEtOx, and the racemic p-RS-BuEtOx by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and circular dichroism (CD) of a polymer film. The DSC results revealed that the glass transition temperatures (Tg) of the enantiopure polymers and the racemic polymer are approximately the same with a Tg of~50 _C. However, the enantiopure polymers showed an additional transition due to cold-crystallization (Tcc) followed by two melting peaks (Tm1 andTm2). Extended thermal investigations indicated that the double melting peak is caused by the melt-recrystallization mechanism. The XRD results confirmed that the enantiopure polymers are semicrystalline and p-RS-BuEtOx is amorphous and that no change in crystalline structure appears during the melting transitions. Furthermore, comparison of the DSC results with the CD measurements of the polymer film revealed that the crystals formed during the cold crystallization have an ordered chiral structure, which almost completely disappears during melting.