Comprehensive and Accurate Control of Thermosensitivity of Poly(2-alkyl-2-oxazoline)s via Well-Defined Gradient or Random Copolymerization

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Abstract

The lower critical solution temperature (LCST) of the poly(2-alkyl-2-oxazoline)s (POx) was precisely tuned over a broad range of temperatures via the well-defined gradient or random copolymerization between 2-n-propyl-2-oxazoline (nPrOx) and either 2-isopropyl-2-oxazoline (iPrOx) or 2-ethyl-2-oxazoline (EtOx). All the copolymerizations were cationically initiated by methyl p-tosylate at the optimum condition (42 °C in acetonitrile) for the living polymerization, resulting in an extremely narrow molecular weight distribution (Mw/Mn ≤ 1.05). It was determined from the composition analysis by 1H NMR and MALDI−TOF mass spectrometry that the respective monomer reactivity ratios were found to be 3.15 and 0.57 for nPrOx and iPrOx, respectively, sufficiently different to form the gradient copolymers P(nPrOx-grad-iPrOx), and 1.28 and 1.04 for nPrOx and EtOx, respectively, indicating the favorable formation of the random copolymers P(nPrOx-ran-EtOx). Both gradient and random copolymers followed a simple and practical rule during their LCST modulation, depending on the compositional variation between the hydrophilic/hydrophobic 2-alkyl-2-oxazoline monomers centering on the iPrOx. In particular, it was found that poly(2-n-propyl-2-oxazoline) (PnPrOx) independently exhibited a sharp LCST behavior near room temperature, even comparable to that of poly(2-isopropyl-2-oxazoline) (PiPrOx). Furthermore, the P(nPrOx-ran-EtOx) copolymers were not only synthesized much faster than the gradient copolymers with the iPrOx component but also showed a clear phase transition behavior over the wide temperature range from 23.8 to 75.1 °C, offering the most ideal choice, viz., a simple random distribution of the two monomers for tuning the LCST.

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