New thermoresponsive graft copolymers were synthesized by the cationic ring-opening polymerization of 2-methyl-2-oxazoline (MeOxa) or 2-ethyl-2-oxazoline (EtOxa), initiated by the random copolymers of chloromethylstyrene (CMS) and N-isopropylacrylamide (NIPAAm) using the “grafting from” method with a yield of 66−94%. The polymers were characterized by NMR, GPC, and DSC, and the conformational transition (lower critical solution temperature, LCST) of macroinitiators and graft copolymers was determined by the turbidity and DSC measurements. The transition temperature of the graft copolymers could be fine-tuned through the composition of the macroinitiator and the graft copolymer. An increasing quantity of the hydrophobic comonomer chloromethylstyrene in the macroinitiator lowered its LCST, while in the graft copolymer an increasing content of the hydrophilic segment of poly(2-methyl-2-oxazoline) or poly(2-ethyl-2-oxazoline) raised the transition temperature. For graft copolymers with a high content of long poly(2-alkyl-2-oxazoline) grafts, stabilized aggregates with a thermoresponsive core can be formed at the LCST instead of precipitation of the material.

New non‐ionic hydrogels were synthesized by radical homopolymerization of vinyl end‐functionalized poly(2‐methyl‐2‐oxazoline) bis(macromonomers), or by radical copolymerization of these bis(macromonomers) with N‐vinyl‐2‐pyrrolidone (NVP). The poly(2‐methyl‐2‐oxazoline) bis(macromonomers) were synthesized through “living” cationic ring‐opening polymerization of 2‐methyl‐2‐oxazoline (MeOXA), using, simultaneously, the known “initiating” and “end‐capping” method for synthesis of macromonomers. Chloromethyl styrene was used as initiator and N‐(4‐vinylbenzyl)‐piperazine was used as the terminating agent. Well defined poly(2‐methyl‐2‐oxazoline) bis(macromonomers) were obtained with Pn = 4, 11, and 17. The hydrogel structures were characterized by high‐resolution magic angle spinning NMR technique and their solvent absorption capacity was tested by swelling experiments in different solvents. The bis(macromonomers) were characterized by NMR spectroscopy and gel permeation chromatography.Schematic of polymerizationmagnified imageSchematic of polymerization

Amphiphilic graft copolymers with a thermosensitive PNiPAAm backbone and pH-sensitive hydrophilic poly(2-carboxyethyl-2-oxazoline) graft chains are synthesized. In aqueous solution, stable micelle-like aggregates are formed by increasing the temperature in the pH range 4.5-5.5. The micelles are crosslinked by electron-beam irradiation, yielding stable core-shell nanogels of about 100 nm diameter with reversible thermoand pH-dependent swelling behavior. The temperature sensitivity is provided by a conformational change in the PNiPAAm core, whereas the thickness of the poly(2-carboxyethyl-2-oxazoline) corona depends on pH. The reversible bisensitivity of core-crosslinked nanogels is verifi ed by DLS, while AFM measurements demonstrate the predicted core-shell structures of the aggregates. other sensitivities, that is, pH, light, magnetic fi eld, solvent quality, etc., and their effects on the reversible self-assembly in aqueous solutions or on hydrogel organization.Poly( N -isopropylacrylamide) (PNiPAAm) is the most frequently studied thermosensitive polymer and shows a phase transition in aqueous solutions at a physiological interesting temperature range. [ 2 ] The formation of PNiPAAm-based block copolymers with pH-responsive poly(acrylic acid) (PAA) [ 3 ] or poly( N -acryloylpyrrolidine) [ 1 , 4 ] offered new self-assembly possibilities. In dependence on the applied temperature and pH value, the PNiPAAmblock -PAA copolymers change their hydrophobic and hydrophilic balance and form different types of micelles. [ 3 ] Also random PNiPAAm-co -PAA and PAA-graft -PNiPAAm structures were realized by Chen and Hoffman, [ 5 ] and the temperature-induced phase transition over a wide range of pH values could be demonstrated. Topp et al. [ 6 ] reported on the synthesis of block copolymers of types A-B and A-B-A composed of poly(ethylene glycol) (PEG) and PNiPAAm as well as their micellization behavior. The design of these copolymers is based on the hydrophobic character of PNiPAAm above its lower critical solution temperature 4 5 6 7 8 9 1 0 30 35 40 45 50 55 60 R h (nm) pH 59 nm 44 nm 47 nm 34 nm Δ Δ Δ ΔT 25 °C 50 °C 12 % 100 % Degree of dissociation Δ Δ Δ ΔT

Graft copolymers with thermo‐sensitive PNIPAAm backbone and hydrophilic PEtOxa graft chains demonstrated typical amphiphilic behavior. For specific compositions stable micelle‐like aggregates were formed depending on the temperature. Applying long polyoxazoline side chains ($\overline {DP} $ > 120), stable reversible micelle‐like aggregates with hydrodynamic radii of 30–40 nm could be obtained between 33 and 55 °C. These graft copolymers have been successfully crosslinked by electron‐beam irradiation in the micellar state yielding core/shell type nanogels with thermo‐reversible swelling behavior. The temperature dependent volume change of the new thermo‐responsive nanogels due to the phase transition of the PNIPAAm core has been verified by DLS.magnified image

New polymethyloxazoline hydrogels are synthesized by the cationic ring‐opening copolymerization of 2‐methyl‐2‐oxazoline and 2,2′‐tetramethylenebis(2‐oxazoline), using random copolymers of chloromethylstyrene and methyl methacrylate, or of chloromethylstyrene and styrene as macroinitiators. The synthesis is carried out in benzonitrile at 110 °C in the presence of potassium iodide, which activates the chloromethyl group as initiating functionality. In general, the hydrogels are obtained very rapidly, with a yield of between 42 and 95 wt.‐%. Besides its initiating function, the macroinitiator also provides the possibility of introducing heterogeneity into the hydrogel structure by forming rigid hydrophobic domains. The hydrogel structures were characterized by high‐resolution magic angle spinning NMR spectroscopy, and their solvent absorption capacity was determined by swelling experiments in solvents of different polarity. The hydrogels showed a maximal swelling degree of 18 g of water and 40 g of methanol, respectively, per gram of the hydrogel. magnified image

New thermo‐sensitive functionalized graft copolymers characterized by a poly(N‐isopropylacrylamide) backbone and grafts containing 2‐ethyl‐2‐oxazoline and 2‐(2‐methoxycarbonylethyl)‐2‐oxazoline units were synthesized. The conformation transition temperatures of the graft copolymers could be modified by variation of the molar composition in the side chain, by different side chain structure (random distribution of both oxazolines vs. diblock structure) and by hydrolysis of the methylester to the acid form. Graft copolymers with long functional oxazoline side chains allowed the stabilization of aggregates above the phase transition temperature of the backbone until the LCST of the side chain. The temperature window allowing for the formation of stable aggregates was widened with acid functions in the corona.magnified image

Voit (2014) Synthesis and characterization of new pH-and thermoresponsive hydrogels based on N-isopropylacrylamide and 2-oxazolines, Designed Monomers and Polymers, 17:3, 208-216, DOI: 10.1080/15685551.2013 New pH-and thermo-responsive hydrogels (HG) were synthesized by free radical polymerization of N-isopropylacrylamide and a macromonomer, which was a hydrolyzed random copolymer of 2-carboxyethyl-and 2-methyl-2-oxazoline, using a bisacrylamide as crosslinker. The polymerization was carried out in a mixture of water and ethanol at room temperature and was initiated by ammonium peroxodisulfate. The HG showed conformational transitions with variation of temperature and/or pH-value and as a function of hydrogel composition. This property was shown macroscopically as hydrogel contraction or expansion. The HG structures were characterized by high-resolution magic angle spinning (HRMAS) NMR spectroscopy. The thermal properties, in particular the lower critical solution temperatures, were determined by temperature-dependent HRMAS NMR measurements and differential scanning calorimetry. The pH responsibility was determined by swelling experiments in water at different pH values.

This study reports on a facile and versatile approach for modification of macroscopic surface via grafting of multifunctional poly(2-oxazoline) molecules in brush-like conformation. For this purpose, carboxyl-terminated poly(2-isopropyl-2-oxazoline) molecules have been synthesized by ring-opening cationic polymerization and subsequently grafted on underlined substrates by exploiting the "grafting to" approach. A systematic variation in thickness of the grafted poly (2-isopropyl-2-oxazoline) brushes has been demonstrated. Polymer-modified surfaces have been characterized by means of a number of analytical tools including ellipsometry, X-ray photoelectron spectroscopy, ultraviolate spectroscopy, attenuated total reflection infrared spectroscopy and atomic force microscopy. Interestingly, poly(2-isopropyl-2-oxazoline) molecules have been found to retain their physical properties even after grafting on macroscopic surfaces. Finally, fabricated polymer brushes have been used as platform for stabilization of inorganic nanoparticles on macroscopic surfaces.