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

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Abstract

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 °C in acetonitrile) for living polymerization, obtaining the copolymers with a narrow molecular weight distribution (Mw/Mn ≤ 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 1H 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 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-iPrOx) 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.

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