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) (P i PrOx) 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 °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 i PrOx from the cumulative and instantaneous compositions of the copolymers determined from the 1 H 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 i PrOx composition along the backbone from the α-terminal to ω-chain end. These gradient copolymers followed a rather simple rule in their thermosensitive behaviors to show a linear increase in LCST with an increasing mol % of EtOx. Consequently, a series of P(EtOx- co - i PrOx) with finely tuned LCST in aqueous medium were obtained through the cationic copolymerization simply by varying the initial composition of both monomers, opening a new way to engineer the thermosensitivity of polymeric materials directing to particular applications.