Main-chain Chiral Poly(2-oxazoline)s: Influence of Alkyl Side-chain on Secondary Structure Formation in Solution
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Bloksma, Meta M.; Rogers, Sarah; Schubert, Ulrich S.; Hoogenboom, Richard
- Abstract:
- 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.
- Year:
- 2011
- Type of Publication:
- Article
- Journal:
- Journal of Polymer Science: Part A: Polymer Chemistry
- Volume:
- 49
- Pages:
- 2790 - 2801