Main-Chain Chiral Poly(2-oxazoline)s: Influence of Alkyl Side-chain on Secondary Structure Formation in Solution

The synthesis and microwave-assisted polymerization of a series of chiral 2-oxazolines with varying alkyl pendant groups, namely R-2-ethyl-4-ethyl-2-oxazoline (R-EtEtOx), R-2-butyl-4-ethyl-2-oxazoline (R-BuEtOx), R-2-octyl-4-ethyl-2-oxazoline, 2-nonyl-4-ethyl-2-oxazoline, and R-2-undecyl-4-ethyl-2-oxazoline (R-UndeEtOx), are reported. A kinetic investigation of the polymerization of R-EtEtOx revealed a living polymerization mechanism. The poly(2-oxazoline)s containing an ethyl, butyl, and octyl pendant group form similar chiral structures according to circular dichroism measurements. When the pendant group is further elongated, the chiral structure becomes more flexible in trifluoroethanol and the thermal response in hexafluoroisopropanol (HFIP) significantly changes. The short-range structure of poly-R-BuEtOx dissolved in HFIP is thermoresponsive in a complex way, due to HFIP hydrogen bonding to the polymeric amide groups, whereas the long-range structure determined from small angle neutron scattering is insensitive to temperature demonstrating that only the local secondary structure changes with temperature. In addition, the chiral structure of poly-R-UndeEtOx depends on the polarity of the solvent. The short-range structure becomes more flexible in polar solvents, most likely due to interactions with the amide groups disturbing the secondary structure. In contrast, the long-range structural transition from an ellipsoid in the apolar n-hexane to a rod structure in the polar n-butanol is ascribed to better solvation of the long aliphatic side chains. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2790-2801, 2011