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

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 ( n PrOx) and either 2-isopropyl-2-oxazoline ( i PrOx) 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 ( M w / M n ≤ 1.05). It was determined from the composition analysis by 1 H NMR and MALDI−TOF mass spectrometry that the respective monomer reactivity ratios were found to be 3.15 and 0.57 for n PrOx and i PrOx, respectively, sufficiently different to form the gradient copolymers P( n PrOx- grad - i PrOx), and 1.28 and 1.04 for n PrOx and EtOx, respectively, indicating the favorable formation of the random copolymers P( n PrOx- 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 i PrOx. In particular, it was found that poly(2- n -propyl-2-oxazoline) (P n PrOx) independently exhibited a sharp LCST behavior near room temperature, even comparable to that of poly(2-isopropyl-2-oxazoline) (P i PrOx). Furthermore, the P( n PrOx- ran -EtOx) copolymers were not only synthesized much faster than the gradient copolymers with the i PrOx 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.