Secondary structure formation of main-chain chiral poly(2-oxazoline)s in solution

Abstract

The synthesis and microwave-assisted polymerization of new 2,4-disubstituted-2-oxazoline monomers, namely R-2-butyl-4-ethyl-2-oxazoline, RS-2-butyl-4-ethyl-2-oxazoline and S-2-butyl-4-ethyl-2-oxazoline, are reported. A kinetic investigation of the polymerization of the enantiopure and racemic monomers revealed comparable polymerization rates and in all cases a living mechanism. The difference in solubility between the racemic polymer and the enantiopure polymer gave a first indication that the enantiopure polymers might form ordered, most likely helical, structures in solution, while the racemic polymer forms a random coil. The dichroic Cotton effects obtained with circular dichroism (CD) confirmed the formation of a secondary structure of the enantiopure polymers in trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP), while the racemic polymer did not show a CD signal. In addition, CD revealed that the ordered structures in HFIP exhibit a unique temperature dependent change in the secondary structure presumably due to HFIP hydrogen bonding to the polymeric amide groups. However, small angle neutron scattering (SANS) data contradict the CD results. When the polymers are dissolved in trifluoroethanol-d₃, a good solvent, SANS indicated the presence of a random coil for both the enantiopure polymers and the racemic polymer. These observations indicate the formation of a flexible and dynamic structure of the chiral polymers so that only a certain fraction of the polymer adopts an ordered secondary structure in this good solvent. When the polymers are dissolved in a bad solvent like methanol-d₄, the SANS data reveal that the enantiopure polymers do form a more compact elongated structure compared to the racemic polymer, implying that the ordered structure of the enantiopure polymer is better preserved in a ‘bad’ solvent, i.e. the persistence length of the ordered segments increases. As such, these synthetic polymers form a dynamic secondary structure similar to polyproline type II helices.