Main‐Chain Chiral Poly(2‐oxazoline)s: Influence of Alkyl Side‐Chain on Secondary Structure Formation in the Solid State

Abstract

SummaryThe influence of the side‐chains of main‐chain chiral poly(2‐oxazoline) s on their thermal properties was investigated using differential scanning calorimetry (DSC) and the nature of the obtained melting endotherms was further investigated by thermal annealing of the polymers. Poly(R‐2‐ethyl‐4‐ethyl‐2‐oxazoline) (p‐R‐EtEtOx) was found to be amorphous, while polymers with longer side‐chains are semi‐crystalline. Previously we reported that the chirally ordered crystals of poly(R‐2‐butyl‐4‐ethyl‐2‐oxazoline) (p‐R‐BuEtOx) have a high melting temperature of more than 200 °C. in this work we demonstrate that elongation of the side‐chains from butyl to octyl results in a decrease in the crystallization rate and melting temperature suggesting that the chirally ordered crystals of p‐R‐BuEtOx are based on close packing of the main‐chain enhancing diploar interactions between the tertiary amide moieties. Crystallizaiton of chiral polymers with longer side‐chains results may then be driven by close packing of the side‐chains. This is supported by the observation that further elongation of the side‐chain length increases the crystallization rate. Moerover, an additional melting endotherm appears for these polymers at a lower temperature upon annealing ascribed to a dual crystal size population. Circular dichroism (CD) measurements of the semi‐crystalline main‐chain chiral polymer films revealed the presence of chirally ordered crystals while X‐ray diffraction (XRD) patterns revealed a closer packing of the chiral poly(2‐alkyl‐2‐oxazoline) s compared to the non‐chiral polymers, suggested to result form the chiral ordering in the crystals. Grazing incidence wide angle x‐ray scattering (GIWAXS) patterns indicated that the chiral crystals of p‐R‐BuEtOx do not form a helical structure, however, the substrate does influence the type of structure formed.