Secondary structure formation of main-chain chiral poly(2-oxazoline)s in solution
-
Bloksma, Meta M.; Rogers, Sarah; Schubert, Ulrich S.; Hoogenboom, Richard
- 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.
- Year:
- 2010
- Type of Publication:
- Article
- Journal:
- Soft Matter
- Volume:
- 6
- Pages:
- 994 - 1003