Precise control of lower critical solution temperature of thermosensitive poly(2-isopropyl-2-oxazoline) via gradient copolymerization with 2-ethyl-2-oxazoline as a hydrophilic comonomer

The lower critical solution temperature (LCST) of amphiphilic poly(2-isopropyl-2-oxazoline) (PiPrOx) was precisely tuned via the copolymerization with 2-ethyl-2-oxazoline (EtOx) as a hydrophilic comonomer. The copolymerization was cationically initiated by methyl p-tosylate at the optimum condition (42 degrees C in acetonitrile) for living polymerization, obtaining the copolymers with a narrow molecular weight distribution (M-w/M-n <= 1.02). The monomer reactivity ratios of 1.78 and 0.79 respectively were derived for EtOx and iPrOx from the cumulative and instantaneous compositions of the copolymers determined from the H-1 NMR and MALDI-TOF mass spectrometry. This set of the reactivity ratios are sufficiently different enough to form the gradient copolymers, in which each polymer chain has a trend of a gradually decreasing EtOx and an increasing iPrOx composition along the backbone from the alpha-terminal to omega-chain end. These gradient copolymers followed a rather simple rule in their thermosensitive behaviors to show a linear increase monomers, opening a new way to engineer the thermosensitivity of polymeric materials directing to particular applications.