Steffen Wiedmann

Manuel Luitz

Benjamin Kerscher

Jean-François Lutz

Rolf Mülhaupt
email: rolfmuelhaupt@web.de

Programmable thermoresponsive micelle-inspired polymer ionic liquids as molecular shuttles for anionic payloads

To enable the programmable thermoresponsive transport of anions between two phases, multifunctional hyperbranched polymer ionic liquids (hyperPILs) exhibiting micelleinspired architectures are tailored as molecular shuttles. These polyelectrolytes consist of a hyperbranched poly(3-ethyl-3-hydroxymethyloxetane) (PEHO) core, an inner polyionic imidazolium (Im + ) and an outer thermoresponsive polyoxazoline (POx) shell, which exhibits lower critical solution temperature (LCST) behavior in aqueous medium. Key step of the hyperPIL synthesis is the efficient chain termination of the cationic ring-opening 2-oxazoline polymerization by addition of polyfunctional imidazole-terminated PEHO. The resulting covalent attachment of the LCST-polyoxazoline shell renders hyperPIL polyelectrolytes thermoresponsive. As a function of the polyoxazoline chain length and the oxazoline monomer type, the hyperPIL cloud points (TCP) vary over a wide temperature range. The inner imidazolium shell enables the immobilization and transport of various anionically charged organic and inorganic payloads via anion exchange. Due to the thermal switching of the hydrophilicity/hydrophobicity balance, hyperPILs function as programmable molecular shuttles for anionic payloads transported back and forth between the phases of ethyl acetate and water. Thus, switchable hyperPILs qualify for manifold potential applications like catalytic processes with facile recycling of homogeneous catalysts via phase transfer.

INTRODUCTION

Micelles and micellar nanostructures play an outstanding role in biological and industrial processes. [START_REF]Micelles: Structural biochemistry, formation and functions and usage[END_REF][START_REF] Dreiss | [END_REF][START_REF] Feng | Smart wormlike micelles: Design, characteristics and applications[END_REF][START_REF] Sorella | Recent advances in catalysis in micellar media[END_REF][5] Therefore, the exploration of micelle-inspired polymer structures represents a fascinating field in polymer chemistry, since they can be used as functional nanocompartments and transporters. Typically, such micellar polymer systems are based on self-assembly of amphiphilic block copolymers. However, many of the resulting supramolecular assemblies are highly sensitive to shear and changes of pH or temperature, which frequently account for disintegration of the micelles accompanied by loss of their function. This risk is avoided by creating robust nanostructured core/shell polymers resembling unimolecular micelles. [START_REF] Moorefield | Unimolecular micelles: supramolecular use of dendritic constructs to create versatile molecular containers[END_REF] Polymers containing either repeat units or functional groups similar to ionic liquids (ILs)often referred to as polymer ionic liquids (PILs) -have established themselves in the group of polyelectrolytes as an intriguing subclass of materials exhibiting exceptional characteristics. [START_REF] Kohno | Thermoresponsive polyelectrolytes derived from ionic liquids[END_REF] Recently, owing to their unique combination of properties and related expected applications, the number of studies on thermoresponsive PILs, in particular ones with lower critical solution temperature (LCST) behavior in water, has increased rapidly. [START_REF] Kohno | Thermoresponsive polyelectrolytes derived from ionic liquids[END_REF] One strategy towards thermoresponsive PILs is homopolymerization of already thermoresponsive IL monomers. This strategy, pioneered by the groups of Ohno and Yuan, [START_REF] Kohno | Key Factors to Prepare Polyelectrolytes Showing Temperature-Sensitive Lower Critical Solution Temperature-type Phase Transitions in Water[END_REF][START_REF] Men | Poly(tetrabutylphosphonium 4styrenesulfonate): A poly(ionic liquid) stabilizer for graphene being multi-responsive[END_REF] was exploited for the synthesis of a variety of LCST PILs with polycationic or polyanionic backbone. [START_REF] Kohno | Ionic liquid-derived charged polymers to show highly thermoresponsive LCST-type transition with water at desired temperatures[END_REF][START_REF] Kohno | Temperature-Driven and Reversible Assembly of Homopolyelectrolytes Derived from Suitably Designed Ionic Liquids in Water[END_REF][START_REF] Men | Cationic Poly(ionic liquid) with Tunable Lower Critical Solution Temperature-Type Phase Transition[END_REF][START_REF] Men | Thermoresponsive polymerized gemini dicationic ionic liquid[END_REF] The second approach is the design of random or block copolymers of an IL monomer and a non-ionic monomer with at least one of them showing thermoresponsive behavior in polymerized form. Most prominent examples are copolymers of different imidazolium-type IL monomers and Nisopropylacrylamide (NIPAM). [START_REF] Mori | RAFT Polymerization of N -Vinylimidazolium Salts and Synthesis of Thermoresponsive Ionic Liquid Block Copolymers[END_REF][START_REF] Tauer | Core-shell particle interconversion with di-stimuliresponsive diblock copolymers[END_REF][START_REF] Soll | Double Stimuli-Responsive Copolymer Stabilizers for Multiwalled Carbon Nanotubes[END_REF][START_REF] Mori | Temperature-responsive self-assembly of star block copolymers with poly(ionic liquid) segments[END_REF][START_REF] Karjalainen | Diblock copolymers consisting of a polymerized ionic liquid and poly(N-isopropylacrylamide). Effects of PNIPAM block length and counter ion on self-assembling and thermal properties[END_REF][START_REF] Men | Double-stimuli-responsive spherical polymer brushes with a poly(ionic liquid) core and a thermoresponsive shell[END_REF] Similar to thermoresponsive PILs, stimulus-responsive hyperbranched macromolecules are attracting great attention. [START_REF] Wang | Synthesis and applications of stimuli-responsive hyperbranched polymers[END_REF] In a recent advance and as a rare example of thermoresponsive PILs with highly branched architecture, Tsukruk et al. tailored core/shell polymers composed of hyperbranched carboxylate-functionalized polyester cores and ammonium-terminated poly(N-isopropylacrylamide) (PNIPAM) chains bonded to the core molecules as thermoresponsive corona by ionic interactions. [START_REF] Korolovych | Thermally Responsive Hyperbranched Poly(ionic liquid)s: Assembly and Phase Transformations[END_REF] In the past, hyperbranched polymer ionic liquids (hyperPILs) with onion-like compartmentalized topology have been developed in our group, which combine the features of PIL materials with the benefits of unimolecular micellar architecture. [START_REF] Schüler | Hyperbranched polymeric ionic liquids with onion-like topology as transporters and compartmentalized systems[END_REF] They are composed of a hyperbranched poly(3-ethyl-3-hydroxymethyloxetane) (PEHO) core, an inner ionic shell of covalently attached imidazolium cations with tosylate counter anions, and a non-polar outer shell of alkyl chains. These micelle-inspired polyelectrolytes function as highly effective molecular transporters in two-phase systems and enable phase transfer of molecules and nanomaterials from water into immiscible non-polar organic solvents. [START_REF] Kerscher | Chapter 7. Polymeric Ionic Liquids with Micelle-like Topologies and Functions[END_REF] However, they only allow for the unidirectional and irreversible transport and do not enable shuttling of payloads back and forth between two phases. The programmable thermal switching of the transport between two phases is highly desirable for many applications including catalyst recycling. Hence, aiming at making this phase transfer reversible and switchable we have developed stimulus-responsive micellar core/shell hyperPILs as molecular shuttles. This is achieved by equipping the hyperPILs with an outer shell of thermoresponsive polyoxazoline segments exhibiting LCST behavior in aqueous media. Whereas this outer LCST-polyoxazoline shell enables thermal switching of the hydrophilicity/hydrophobicity balance, the cationic imidazolium coupling groups as inner shell account for the immobilization of anionic payloads via anion exchange.

During recent years, non-ionic thermoresponsive polymer systems have been tailored as molecular shuttles for the reversible transport of payloads back and forth between two liquid phases in a controlled manner. Most of them exploit self-assembly and the formation of block copolymer micelles. For example, since 2006, the Lodge group has pioneered such systems based on micelle-forming poly((1,2-butadien)-b-ethylene oxide), poly(N-isopropylacrylamide-bethylene oxide), and poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) block copolymers, which can reversibly transport dye molecules as payloads back and forth between water and a water-immiscible IL. This shuttling process is controlled by temperature changes. [START_REF] He | The micellar shuttle: Thermoreversible, intact transfer of block copolymer micelles between an ionic liquid and water[END_REF][START_REF] Bai | A Thermoreversible Micellization-Transfer-Demicellization Shuttle between Water and an Ionic Liquid[END_REF][START_REF] Bai | Block copolymer micelle shuttles with tunable transfer temperatures between ionic liquids and aqueous solutions[END_REF][START_REF] Bai | Thermodynamics and mechanism of the block copolymer micelle shuttle between water and an ionic liquid[END_REF][START_REF] Bai | Pluronic micelle shuttle between water and an ionic liquid[END_REF] Schubert's group demonstrated that also polyoxazoline block copolymers function as thermoreversibly switchable transporters in the two-phase systems used by Lodge. [START_REF] Guerrero-Sanchez | Controlled thermoreversible transfer of poly(oxazoline) micelles between an ionic liquid and water[END_REF][START_REF] Guerrero-Sanchez | Micellar dye shuttle between water and an ionic liquid[END_REF] Apart from block copolymers, appropriately designed polymer brush-grafted "hairy particles" were shown to reversibly travel back and forth between two phases not only in water/IL systems, [START_REF] Horton | Spontaneous phase transfer of thermosensitive hairy particles between water and an ionic liquid[END_REF][START_REF] Horton | Temperature-and pH-triggered reversible transfer of doubly responsive hairy particles between water and a hydrophobic ionic liquid[END_REF] but also in two-phase systems consisting of water and an organic solvent. [START_REF] Bao | Stimuli-Triggered Phase Transfer of Polymer-Inorganic Hybrid Hairy Particles Between Two Immiscible Liquid Phases[END_REF][START_REF] Li | Temperature-Induced Transport of Thermosensitive Hairy Hybrid Nanoparticles between Aqueous and Organic Phases[END_REF] For example, in octane/water, thermoresponsive hairy particles, like poly(methoxytri(ethylene glycol) methacrylate) grafted on a cross-linked poly(acrylic acid) core, were used as carriers for palladium nanoparticles, which catalyzed hydrogenation reactions and were easily recycled. [START_REF] Li | Thermosensitive water-dispersible hairy particle-supported pd nanoparticles for catalysis of hydrogenation in an aqueous/organic biphasic system[END_REF] However, to the best of our knowledge, the use of polyionic micelle-inspired macromolecules as thermoresponsive shuttles in two-phase systems has not been reported to date.

The novel hyperPILs presented herein are readily obtained by termination of the cationic ringopening 2-oxazoline polymerization with polyfunctional imidazole-terminated hyperbranched PEHO to covalently attach LCST-polyoxazolines to the PEHO core via cationic imidazolium groups as IL moieties. By the facile variations of the size and composition of the polyoxazoline segments, the cloud point (TCP) of the core/shell polyelectrolytes in aqueous solution is readily adjusted. Special attention is paid to temperature-induced transport in two-phase water/organic solvent systems using various payloads to unveil the transport mechanism and to demonstrate the concept of switchable hyperPILs as thermoresponsive molecular shuttles operating in two-phase systems.

RESULTS AND DISCUSSION

The synthetic route for the preparation of the thermoresponsive hyperPILs with micelleinspired core/shell architectures is shown in Scheme 1. The hyperPIL unimolecular micelles are readily obtained via a "grafting-to" process in which the cationic ring-opening polymerization of 2-oxazolines is terminated with polyfunctional imidazole-terminated hyperbranched poly(3ethyl-3-hydroxymethyloxetane) (PEHO-Im), which is prepared in a three step synthesis following previously reported procedures. [START_REF] Schüler | Hyperbranched polymeric ionic liquids with onion-like topology as transporters and compartmentalized systems[END_REF][START_REF] Kerscher | Nanostrukturierte polyionische Flüssigkeiten auf Basis hyperverzweigter Polyoxetane als Transporter, Dispergiermittel und Hybridmaterialien[END_REF] This chain termination reaction affords coupling of PEHO-Im and poly(2-oxazoline)s via the nucleophilic attack of the imidazole groups at the terminal oxazolinium rings to produce the inner shell consisting of imidazolium cations with tosylate counter anions. This strategy of using imidazole-functionalized polymers as macroterminating agents for living polyoxazoline chains is very versatile and has been presented as an efficient method for the synthesis of ionic polyisobutylene-b-poly(2-ethyl-2-oxazoline) diblock copolymers. [START_REF] Kerscher | Thermoresponsive Polymer Ionic Liquids and Nanostructured Hydrogels Based upon Amphiphilic Polyisobutylene-b-poly(2-ethyl-2-oxazoline) Diblock Copolymers[END_REF] Synthesis of the hyperPILs. The synthesis of PEHO-OH was performed by cationic ringopening polymerization of 3-ethyl-3-hydroxymethyloxetane (EHO) using BF3•OEt2 as initiator, following a published procedure. [START_REF] Bednarek | Branched polyether with multiple primary hydroxyl groups: polymerization of 3-ethyl-3hydroxymethyloxetane[END_REF] The obtained hyperbranched PEHO-OH was identified by 1 H and [START_REF] Men | Thermoresponsive polymerized gemini dicationic ionic liquid[END_REF] C NMR spectroscopy (for Figures of NMR spectra, SEC, TGA, and DSC curves of PEHO-OH and its derivatives see SI). Direct investigation of the molecular weight distribution using size exclusion chromatography (SEC) failed due to its poor solubility in the solvents commonly used for this purpose. The degree of branching of the PEHO-OH is 46 % as determined by [START_REF] Men | Thermoresponsive polymerized gemini dicationic ionic liquid[END_REF] C inverse-gated NMR spectroscopy of the trifluoroacetylated derivative (PEHO-TFA).

In order to enable the substitution of the hydroxy groups by imidazole, the PEHO-OH was subsequently treated with tosyl chloride to form the tosylate derivative PEHO-OTs, as previously described by our group. [START_REF] Schüler | Hyperbranched polymeric ionic liquids with onion-like topology as transporters and compartmentalized systems[END_REF] According to 1 H NMR spectroscopy, a high degree of functionalization (DF) of 90 % was achieved. SEC analysis revealed a number-average molecular weight (Mn) of 3800 g mol -1 and, as a result of chain transfer occurring as side reaction during EHO polymerization, [START_REF] Bednarek | Multihydroxyl Branched Polyethers. 2. Mechanistic Aspects of Cationic Polymerization of 3-Ethyl-3-(hydroxymethyl)oxetane[END_REF] a dispersity (Ð) of 2.0. However, since calibration of SEC with linear standards usually underestimates the molecular weight as well as the disperisty of hyperbranched polymers, these numbers have to be considered with appropriate care. From Mn of PEHO-OTs, the degree of polymerization (DP) of the PEHO core was calculated to be 15, which corresponds to a molar mass of the original PEHO-OH of 1700 g mol -1 . Finally, the tosyl groups were substituted by imidazole, [START_REF] Kerscher | Nanostrukturierte polyionische Flüssigkeiten auf Basis hyperverzweigter Polyoxetane als Transporter, Dispergiermittel und Hybridmaterialien[END_REF] thereby obtaining PEHO-Im with a high DF of 82 %. SEC measurement failed due to interactions of the imidazole groups with the stationary phase.

However, based on DP of PEHO-OTs and taking into account DF, Mn of PEHO-Im was calculated to be 2500 g mol -1 .

The yields of the polymerization and the polymer-analogous reactions were very high and amounted to at least 90 % each, which resulted in a total yield of remarkable 78 % over all the three steps. As expected, thermal analysis showed that all PEHO derivatives were amorphous substances with strongly end group-dependent glass transition (Tg) and onset decomposition temperatures (TO). Generally, the Tg values of hyperbranched macromolecules increase with higher end group polarity, i.e. stronger end group interaction, which is reflected in the higher values for PEHO-OH (40 °C) and PEHO-Im (52 °C) compared to PEHO-OTs (23 °C). It is worth to mention the extraordinary thermal stability of the PEHO-Im, which does not show significant mass loss up to temperatures of almost 400 °C, even under air. The substantially lower decomposition temperature of PEHO-OTs is explained, in analogy to the structurally related neopentyltosylate, [START_REF] Sudo | Ring-opening polymerization of 1,3-benzoxazines by p-toluenesulfonates as thermally latent initiators[END_REF] by rearrangement of the end groups followed by elimination of p-toluene sulfonic acid. The most important characteristics of PEHO-OH and its derivatives are summarized in Table 1. a End group fraction of the name-giving functional groups, according to 1 H NMR spectroscopy; b determined by SEC (Ð = 2.0; calculated from Mn: DP = 15); c calculated from DP and average molar mass of the respective linear repeat unit (for further explanations see SI); d determined by DSC; e determined by TGA.

To synthesize the thermoresponsive hyperPILs, poly(2-alkyl-2-oxazoline)s were grafted onto PEHO-Im core molecules in a "grafting-to" process. The oxazoline polymerization was performed by means of a cationic ring-opening process with methyl tosylate (MeOTs) as initiator. After complete consumption of the monomer, PEHO-Im was added to the solution as terminating agent of the cationic oxazoline polymerization in which most chains contained cationic oxazolinium rings as end groups. Dimethylformamide (DMF) was chosen as solvent, which, due to its polar aprotic nature, enables both the cationic ring-opening oxazoline polymerization and the dissolution of PEHO-Im. The monomers used were 2-ethyl-2-oxazoline (EtOx), 2-n-propyl-2-oxazoline (nPrOx), and a 1:1 molar mixture of the two monomers (EtOx/nPrOx) producing random copolymers. In addition, the DP of the polyoxazoline segments was varied by the molar ratio of monomer to initiator (10:1 or 20:1). A ratio of 1.1:1 was chosen as the molar ratio of initiator to imidazole groups of the PEHO-Im to achieve complete functionalization of the imidazole groups. Six hyperPILs with different polyoxazoline shells were synthesized, which are abbreviated as PEHO-IL-POxm (m = theoretical DP of the polyoxazoline shell according to monomer:initiator ratio).

The thermoresponsive hyperPILs were isolated by precipitation of the reaction mixture in Et2O, which in the case of the two PEHO-IL-PnPrOx samples and PEHO-IL-P(EtOx/nPrOx)10 afforded core/shell polymers with acceptable purity (max. 4 wt.% low-molecular-weight components, which correspond to the fraction below 5 kg mol -1 for m = 10 and below 10 kg mol -1 for m = 20 in the SEC curves; see Table S2 + S3 and Figure S14). For the other samples, no satisfactory purity was achieved in this process due to poor solubility of the respective free polyoxazolines in Et2O. To separate the oxazoline homopolymer byproducts, the two PEHO-IL-PEtOx samples and PEHO-IL-P(EtOx/nPrOx)20 were therefore subjected to a further purification step. For this purpose, highly concentrated aqueous solutions (c = 100 g L -1 ) of the polymers were heated a few degrees above their TCP, which led to a macroscopic temperature-induced phase separation (TIPS). At temperatures only slightly above TCP of the respective hyperPIL, the free polyoxazoline chains were still well soluble in water and, thus, remained in the water-rich upper phase, while the purified hyperPILs were isolated from the polymer-rich bottom phase. In this way, the amount of non-attached polyoxazoline chains was significantly reduced, as indicated by the decrease of the low-molecular-weight fraction to maximally 7 wt.%. However, the TIPS purification process was accompanied by a massive loss of product, resulting in moderate total yields of the respective PEHO-IL-POx samples of only 24 -56 %, whereas the PEHO-IL-PnPrOx samples and PEHO-IL-P(EtOx/nPrOx)10 were obtained in much higher yields ranging between 74 and 85 % (see Table 2).

Table 2: Properties of the hyperPILs.

Sample code

Polyoxazoline chain length

Mn [g mol -1 ] a To determine Mn and Ð of the hyperPILs from the SEC curves, the corresponding highmolecular signal was used without taking into account any low-molecular-weight impurities. The Mn values of the hyperPILs calculated from the 1 H NMR spectra varied between 18 000 and 21 000 g mol -1 for the PEHO-IL-POx10 samples and between 33 000 and 40 000 g mol -1 for the PEHO-IL-POx20 samples. Of course, as SEC results of the PEHO-OTs intermediate are included in these calculations, these molar masses are afflicted with some uncertainty. However, even though SEC analysis of the hyperPILs is similarly problematic due to the hyperbranched core/shell architecture, the Mn values measured by SEC matched very well those ones determined by [START_REF]Micelles: Structural biochemistry, formation and functions and usage[END_REF] H NMR spectroscopy and, depending on the polyoxazoline shell, fell in the range of 19 000 g mol -1 (PEHO-IL-PEtOx10) to 37 000 g mol -1 (PEHO-IL-P(EtOx/nPrOx)20). Thus, these numbers at least illustrate the magnitude of the molar masses of the hyperPILs. The dispersities varied from 1.6 to 2.0, however, due to the calibration with linear PMMA standards, SEC tends to underestimate these values. Table 2 summarizes the polyoxazoline chain lengths and the Mn and Ð values of the hyperPILs.

Ð Yield [M]0/[I]0 1 H NMR 1 H NMR SEC a SEC a [%] PEHO-IL-PEtOx10 10
The thermal analysis of the hyperPILs by DSC revealed Tg values varying between 24 and 45 °C. As expected due to the longer alkyl side-chains in nPrOx, [START_REF] Rettler | Thermal, Mechanical, and Surface Properties of Poly(2-N-alkyl-2-oxazoline)s[END_REF]42 the PEHO-IL-PnPrOx samples had slightly lower Tg values as compared to the PEHO-IL-PEtOx or PEHO-IL-P(EtOx/nPrOx) samples (see Table 3). The TO values determined by TGA under air ranged between 340 and 360 °C, whereas under nitrogen atmosphere slightly higher TO values between 350 and 370 °C were detected. In general, the differences in the decomposition temperatures of the hyperPILs are very small and a rational relation to their structure is not evident. Under air, no mass loss was detectable up to at least 200 °C, under nitrogen even up to at least 300 °C. Hence, the novel thermoresponsive hyperPILs exhibited the same high thermal stability as their alkylmodified predecessors. [START_REF] Schüler | Hyperbranched polymeric ionic liquids with onion-like topology as transporters and compartmentalized systems[END_REF] Table 3: Thermal properties of the hyperPILs.

Sample code

Tg Upon heating above a concentration-dependent and material-specific temperature, clouding of the solution was observed. By subsequent cooling, the solution cleared again. This behavior, which is typical for LCST-polyoxazolines, can be explained by phase separation into a polymerrich and a water-rich phase at higher temperatures. Below TCP the polyoxazoline chains are well dissolved due to the hydrogen bond formation between water and the polymer. Above TCP intraand intermolecular hydrogen bonds between polyoxazoline chains are favored. 43 This results in aggregation of the hyperPILs and produces polymer droplets of sufficient size to scatter light, resulting in the observed turbidity (see Scheme 2). The TCP of all hyperPILs at the investigated concentrations are graphically displayed in Figure 3 (the TCP values are listed in Table S3). The hyperPILs equipped with a PEtOx shell (PEHO-IL-PEtOx10 and PEHO-IL-PEtOx20) showed thermoresponsive behavior only at relatively high concentrations of 75 g L -1 and 25 g L -1 , respectively, whereas for the other hyperPILs, phase transitions were detected already at much lower concentrations of 5 g L -1 . Hence, the turbidimetric characterization of these PILs therefore focused on low concentrations (5 -25 g L -1 ), while the PEHO-IL-PEtOx samples were examined up to a concentration of 100 g L -1 . Especially the TCP values of PEHO-IL-PEtOx solutions were found to be strongly dependent on the size of the shell. The same trend was observed for the other hyperPILs, but the differences were less pronounced and significant changes only occurred at the lowest concentration of 5 g L -1 . The effect of longer alkyl side-chain substituents on TCP is in accord with linear polyoxazolines and can be explained by the resulting decrease of hydrophilicity. In fact, the observed influences of concentration and molar mass on TCP were very similar to those observed for polyoxazoline homo-and copolymers. 44 As a result of the turbidimetric investigations, it can be concluded that by appropriately selecting the ratio of the oxazoline monomers and the DP of the polyoxazoline chains, the thermoresponsive behavior can be readily adjusted to tailor hyperPILs with programmable TCP.

TEM. The representative TEM images of PEHO-IL-P(EtOx/nPrOx)10 and PEHO-IL-(PEtOx/nPrOx)20 displayed in Figure 4 show single circular particles with an average diameters with standard deviation of 20.7 ± 4.3 nm and 22.4 ± 4.8 nm respectively. Surprisingly, the length of the polyoxazoline segments has no substantial influence on the particle size. Therefore, the particles seem to represent aggregates of a small number of core-shell macromolecules, A single hyperPIL molecule has an average diameter not larger than 6 or 9 nm respectively, depending on the length of the polyoxazoline segments, as estimated by distance measurements on simulated 3D models. Samples prepared from more diluted solutions showed empty grids instead of the expected single macromolecules, probably because the small amount of deposited material was removed by the staining procedure with uranyl acetate solution. However, the size distribution of the particles is rather homogeneous, suggesting an energetic minimum at the observed diameter of about 20 nm. Images obtained from the other hyperPILs show particles with diameters around 20 nm aswell, yet the contrast is too low for a solid evaluation of the average size (see Figure S19). In general, the results obtained from TEM can only give an idea about the micellar morphology of the dissolved hyperPILs as upon drying previously individually disolved macromolecules might fuse together to the observed larger structures. Cryo-TEM meassurements, which would give a more accurate picture of the conditions in solutions, unfortunately showed no material. HyperPILs as thermoresponsive transporters. The thermal switching of the polarity and the hydrophilicity/hydrophobicity balance of the polyoxazoline shell accounted for a temperaturedependent phase change of the hyperPILs in two-phase systems consisting of water and an immiscible organic solvent. At temperatures below TCP, the hyperPILs were soluble in the aqueous phase, provided that water is the better solvent as compared to the organic phase. Above TCP, the hyperPILs became less polar and were rendered insoluble in water, prompting them to migrate into the organic phase. Thus, the hyperPILs can travel back and forth between aqueous and organic medium, depending on the temperature. This holds great prospects for the controlled and reversible shuttling of payloads between the two phases.

In order to investigate the temperature-induced phase change and to evaluate potential applications of the hyperPILs as programmable shuttles, the water-soluble anionic dye Congo red was used as model substance to be transported. The PEHO-IL-PEtOx samples were excluded from these transport experiments, since they showed thermoresponsive behavior only at relatively high concentrations and because their TCP values were above the boiling points of many common organic solvents. Depending on the selection of the organic solvent, different observations were made. In case of relatively polar solvents such as chloroform, an immediate phase transfer of hyperPIL and dye into the organic phase occurred already at room temperature.

When an aqueous Congo red/hyperPIL solution was added to chloroform, discoloration of the aqueous phase and coloration of the organic phase was observed. This state was maintained regardless of temperature. Thus, chloroform was always a better solvent for the hyperPILs than water, which prevented shuttling in this two-phase system. On the other hand, highly non-polar solvents such as n-hexane did not dissolve the hyperPILs, irrespective of the temperature, which impeded any phase transfer of the dye in such systems.

In the ethyl acetate/water system, however, the hyperPILs showed a temperature-dependent phase change and enabled the desired switchable shuttling of Congo red. The best results regarding reversibility and completeness of the phase transfer were achieved with PEHO-IL-PnPrOx10. Figure 5 illustrates this hyperPIL-mediated Congo red transport. At temperatures below the TCP of the hyperPIL, the dye was dissolved in the aqueous phase. Upon heating to 60 °C, the hyperPIL transported the dye into the organic medium. Cooling to 4 °C induced shuttling via the reversed transport of both hyperPIL and dye into the aqueous phase. The employed PEHO-IL-PnPrOx10 carrier contains about 0.6 mmol g -1 of cations and shuttled 0.8 g of Congo red anions per 1 g of hyperPIL. Although maximal transport capacity was not studied, this illustrates the substantial shuttling capability of the hyperPILs. This shuttling process was repeated several times without encountering noticeable differences. For elucidation of the transport mechanism, identical experiments were performed with a water-soluble cationic dye (methylene blue) and a water-insoluble non-ionic dye (Sudan IV) (see Figure S18 for structures of the investigated dyes). In both cases, a temperature-induced hyperPIL-mediated shuttling could not be observed. The polar methylene blue always remained in the aqueous phase, whereas the non-polar Sudan IV always stayed in the ethyl acetate phase.

In view of these results, it is apparent that shuttling only worked with anionic dye payloads as they were immobilized on hyperPILs via anion exchange for the tosylate counter ions of imidazolium cations in the inner ionic hyperPIL shell.

Going beyond the shuttling of anionic organic dyes, the switchable hyperPILs also turned out to be effective with respect to shuttling anionic palladium complexes (see Figure 6). The watersoluble palladium complex used was formed from the dipotassium salt of the triphenylphosphine ligand bis(p-sulfonatophenyl)phenylphosphine and disodium tetrachloropalladate (see Figure S20 for proposed structure of the complex). Similar to the transport of Congo red, PEHO-IL-PnPrOx10 was used as thermoresponsive shuttle. At 0 °C, the Pd complex was dissolved in the aqueous phase. Upon heating to 70 °C, the hyperPIL migrated into the ethyl acetate phase together with the palladium complex payload. By cooling to 0 °C, the complex was transported back from the ethyl acetate phase to the aqueous phase. Just like the Congo red transport, this process was repeated several times without impairing shuttling. Hence, PEHO-IL-PnPrOx10 serves as shuttle for both organic and inorganic anions.

Figure 6: PEHO-IL-PnPrOx10 as thermoresponsive shuttle for an anionic water-soluble Pdphosphine complex in the ethyl acetate/water two-phase system.

CONCLUSION

Polyfunctional polyelectrolyte carriers and molecular shuttles for anionic organic and inorganic payloads are readily tailored via the chain termination of the cationic 2-oxazoline ringopening polymerization with imidazole-terminated hyperbranched poly(3-ethyl-3hydroxymethyloxetane) (PEHO-Im). In this efficient "grafting-to" process, the cationic oxazolinium end groups of the poly(2-alkyl-2-oxazoline)s alkylate the imidazole groups of PEHO-Im to produce cationic alkyl imidazolium coupling groups as IL moieties incorporated within the hyperbranched polyelectrolyte backbone. The resulting hyperbranched polymer ionic liquids (hyperPILs) exhibit nanostructures resembling micelles but are much more robust with respect to their thermal and chemical stabilities as well as their low shear and pH sensitivity. The covalent attachment of LCST-polyoxazolines derived from 2-ethyl-and 2-n-propyl-2-oxazoline renders hyperPILs thermoresponsive and enables the programmable thermal switching of the hyperPIL polarity and its hydrophilicity/hydrophobicity balance. The monomer type, the polyoxazoline chain length and copolymer compositions govern the cloud point (TCP) of hyperPILs in water, which varies from 21 to 93 °C. As verified by the addition of an anionic organic dye (Congo red), both hyperPILs and dye molecules are shuttled back and forth between water and ethyl acetate phases at temperatures below and above the corresponding TCP. In contrast, substitution of LCST-polyoxazolines by long-chain alkyl groups exclusively permits irreversible temperature-independent dye transport in one direction from water to the organic phase. Since shuttling in water/ethyl acetate is achieved only by using anionic dyes, it appears highly likely that anionic dyes are immobilized on hyperPILs via anion exchange for tosylate counter ions of the imidazolium moieties. Successful shuttling in water/ethyl acetate in the presence of hyperPILs is also demonstrated for inorganic anions such as palladium complexes containing bis(p-sulfonatophenyl)phenylphosphine as a ligand. In principle, this thermal switching process using programmable thermoresponsive hyperPILs as polyfunctional carriers for anionic payloads holds great prospects with respect to the facile recovery of homogeneous catalysts. Upon completing the catalytic reaction in an organic medium at a temperature above TCP, the temperature is lowered to recover the catalyst by extracting it with water. It is then reused by the subsequent shuttling back into an organic phase at elevated temperature.

EXPERIMENTAL SECTION

Materials. General remarks. All syntheses were performed in dry conditions and under a nitrogen atmosphere using standard Schlenk techniques. In the calculation of the end group equivalents of the PEHO derivatives, the degrees of functionalization determined from the respective 1 H NMR spectra were considered.

Synthesis of PEHO-OH.

To a solution of BF3•OEt2 (5.5 mL, 6.2 g, 44 mmol, 1.0 equiv.) in dry CH2Cl2 (100 mL) 3-ethyl-3-hydroxymethyloxetane (50 mL, 51 g, 0.44 mol, 10 equiv.) was added dropwise under cooling in a water bath over a period of 2 h. Then, the reaction mixture was stirred at room temperature for 48 h. Afterwards, the reaction was stopped by addition of ethanol (10 mL) and the mixture was concentrated under reduced pressure until it became highly viscous but was still pourable. Subsequently, the mixture was precipitated in cyclohexane. The precipitate was dissolved in ethanol (75 mL) under gentle heating and the resulting polymer solution was precipitated in water (300 mL). The solid part was first dried at 60 °C in an oven and then at 80 °C under high vacuum. The product was obtained as a colorless solid (48.4 g; 95 %).

Synthesis of PEHO-

Synthesis of PEHO-Im.

A mixture of PEHO-OTs (10.6 g, 37.4 mmol OTs groups, 1.0 equiv.)

and imidazole (55.3 g, 826 mmol, 22 equiv.) was heated to 120 °C, causing the imidazole to melt. The mixture was stirred at 120 °C for 24 h. After cooling, the solidified mixture was dissolved in CH2Cl2 (200 mL) and the resulting solution was washed with aqueous NaOH (2M, 2 × 200 mL), water (200 mL), and saturated aqueous NaCl (200 mL). The organic phase was dried over MgSO4 and concentrated under reduced pressure. The residue was dried at 100 °C under high vacuum to yield PEHO-Im as a slightly brown solid (6.38 g; 90 %).

Synthesis of the hyperPILs. The syntheses of the hyperPILs were performed according to the following general procedure (weights, volumes and durations of polymerizations are listed in Table S1). To a solution of MeOTs (1.0 equiv.) in DMF the oxazoline monomer (10 or 20 equiv.) was added and the reaction solution was stirred in a preheated oil bath at 90 °C until the monomer was fully consumed. A solution of PEHO-Im (0.9 equiv. imidazole groups) in DMF was then added and stirred at 90 °C for another 20 h. After cooling, the solution was precipitated in Et2O (3 times the volume of the reaction mixture). The precipitate was dissolved in CH2Cl2 (3 mL per gram of precipitate) and again precipitated in Et2O (10 times the volume of the CH2Cl2 solution). The precipitation was performed one more time. The obtained precipitate was filtered off and dried under high vacuum. In the case of PEHO-IL-PEtOx10, PEHO-IL-PEtOx20, and PEHO-IL-P(EtOx/nPrOx)20, after precipitation in Et2O the hyperPILs, according to SEC analysis, still contained a considerable amount of low-molecular-weight free polyoxazolines. Therefore, a temperature-induced phase separation (TIPS) procedure was performed for further purification. For this purpose, the polymer to be purified was dissolved in water (c = 100 g L -1 ) and heated in a closed vessel slightly above TCP for 24 h (see Table S2 andS3). As a result two phases formed. The upper phase contained the main part of the free polyoxazolines, the lower phase the hyperPIL in purified form. The upper phase was pipetted off and the solvent was removed from both phases by freeze-drying. Yields and purities of the hyperPILs, which were obtained as white or yellow solids, are listed in Table S4.

HyperPIL-mediated temperature-switchable phase transfer of Congo red. Solutions of

Congo red in water (c = 0.3 g L -1 ) and of PEHO-IL-PnPrOx10 in EtOAc (c = 0.35 g L -1 ) were prepared. 2 mL of each of the solutions were mixed, resulting in a two-phase system consisting of a colorless upper EtOAc phase and a red lower aqueous phase. In a closed vessel, the twophase system was then kept at 60 °C for 23 h in an oven. As a result, the upper EtOAc phase was colored red, whereas the lower aqueous phase was almost completely colorless. The two-phase system was then kept in a refrigerator at 4 °C for 18 h, whereupon the pattern was reversed. The EtOAc phase was almost colorless and the aqueous phase was colored red. This process was repeated several times with the same observation.

HyperPIL-mediated temperature-switchable phase transfer of a Pd complex. A solution of bis(p-sulfonatophenyl)phenylphosphine dihydrate dipotassium salt (13 mg, 0.036 mmol, 3.0 equiv.) and disodium tetrachloropalladate (3.5 mg, 0.012 mmol, 1.0 equiv.) in degassed water (3 mL) was prepared under nitrogen atmosphere. This solution was let to stand at room temperature for 8 d to ensure complete complex formation. In addition, a solution of PEHO-IL-PnPrOx10 in degassed water (c = 1.5 g L -1 ) was prepared. A part of the Pd complex solution (0.25 mL) and a part of the polymer solution (2.25 mL) were mixed and degassed EtOAc (2.5 mL) was added. At this stage, the EtOAc phase was clear and colorless, while the aqueous phase was orange. The mixture was heated at 70 °C in an oil bath for 100 min under vigorous stirring. After letting the mixture rest for a short time for phase separation, a completely decolorized aqueous phase and an orange colored EtOAc phase were obtained. Then, the mixture was cooled to 0 °C in an ice bath, stirred vigorously for 30 min and let to rest for 35 min without stirring for phase separation. By this means, the state before heating was restored. The aqueous phase was colored orange and the EtOAc phase was colorless. This heating-cooling cycle was repeated several times with the same observation.

DSC. DSC curves were recorded on a DSC Seiko 6200 with heating and cooling rates of 10 K min -1 . The glass transition temperatures (Tg) were determined from the second heating curve using the inflection point of the transition.

Determination of TCP. The transmission measurements were performed on photometers with

Peltier element from Analytik Jena (S 300 VIS; for TCP < 60 °C) or ThermoFisher Scientific (Evolution 201; for TCP > 60 °C) at a wavelength of 500 nm and a heating/cooling rate of 1 K min -1 . The reference was demineralized water at 20 °C. The inflection point of the transmission-temperature curve was used to determine TCP. 45 TEM. TEM images were captured with a FEI Talos L120C transmission electron microscope.

The samples were prepared at room temperature by dropcasting of clear 1 wt.% aqueous hyperPIL solutions on a 200 mesh carbon-coated copper grid (Science Services, Munich, Germany) and were stained with aqueous uranyl acetate solution.

PEHO-OH and its derivatives 1.General remarks

For the sake of convenience, the structural formulas in the following NMR spectra only show the linear repeat unit of PEHO-OH and its derivatives despite the hyperbranched architecture.

To calculate the average molecular mass of a repeat unit of PEHO-OH and its derivatives, the molecular masses of the linear repeat units were used in good approximation. In case of different end groups being present, weighting was based on the corresponding degree of functionalization (DF). This is demonstrated exemplary in the following equation for a PEHO derivative PEHO-B, which was obtained from PEHO-A by polymer analogous reaction and contains not just B groups, but also still a certain amount of non-reacted A groups.

𝑀 𝑟𝑒𝑝 (𝑃𝐸𝐻𝑂-𝐵) = [1 -𝐷𝐹 (𝑃𝐸𝐻𝑂-𝐵)] * 𝑀 (𝑃𝐸𝐻𝑂-𝐴) + 𝐷𝐹 (𝑃𝐸𝐻𝑂-𝐵) * 𝑀 (𝑃𝐸𝐻𝑂-𝐵)
To calculate the equivalents of the functional end group B, the molar mass M (B) was used, which indicates the mass of PEHO-B that contains one mole of B groups: Since SEC measurements of PEHO-OH were not possible due to its limited solubility in the solvents commonly used for this purpose, Mn was determined by multiplying the degree of polymerization (DP) determined for PEHO-OTs (see Section 1. This results in the following proportions of the structural units: d = 23 %; l = 53 %; t = 24 %.

𝑀 (𝐵) = 𝑀 𝑟𝑒𝑝 (𝑃𝐸𝐻𝑂-𝐵) 𝐷𝐹 (𝑃𝐸𝐻𝑂-𝐵)
The degree of branching (DB) was calculated from the proportions of the structural units according to the equation of Frey et al. [START_REF] Dreiss | [END_REF] :

𝐷𝐵 = 2𝑑 2𝑑 + 𝑙 = 0.46

PEHO-OTs

Figure S3: 1 H (top) and [START_REF] Men | Thermoresponsive polymerized gemini dicationic ionic liquid[END_REF] C NMR spectrum (bottom) of PEHO-OTs. a Phase separation was induced by heating to the temperature indicated in brackets; b purity is defined as the percentage by weight of the fraction with molar masses larger than 5 kg mol -1 (for PEHO-IL-POx10 samples) or 10 kg mol -1 (for PEHO-IL-POx20 samples), as determined by SEC analysis; c for the purification by TIPS only a part of the crude product obtained after Et2O precipitation was used (1.0 g for each sample).

SEC curves

Procedure for calculating Mn from the 1 H NMR spectra

In the 1 H NMR spectra of the hyperPILs, the signals of the imidazole groups either completely disappeared or are at least of such low intensity that a meaningful quantitative analysis by integration is not possible. Therefore, in good approximation, a quantitative alkylation of the imidazole groups with polyoxazoline chains can be assumed for estimation of the molecular weights. Accordingly, the integral of a proton of the imidazolium rings and the polyoxazoline chains corresponds to 0.82 times (DF (PEHO-Im) = 0.82) the integral of a proton of a repeat unit of the PEHO core. To normalize the integrals, the signal of the CH2 group (H 11 ) of the polyoxazoline repeat unit directly bonded to the imidazolium cation was selected and its integral was set to 1.64.

The DP of the attached polyoxazoline chains can now be determined from the signal of the

The 1 H

 1 NMR analysis of the hyperPILs (Figure 1 exemplarily shows the 1 H NMR spectrum of PEHO-IL-PnPrOx10, for 1 H NMR spectra of the other hyperPILs see SI) revealed almost quantitative functionalization of the imidazole groups, as reflected by disappearance of the imidazole signals. Only the NMR spectrum of PEHO-IL-P(EtOx/nPrOx)20 showed clearly recognizable signals of unreacted imidazole groups, but reliable quantification was prevented by overlapping of the signals. By comparing the signal intensities of the polyoxazoline repeat unit directly bound to the imidazolium cation with the remaining polyoxazoline backbone, the DP of the attached polyoxazoline chains was determined. In accordance with the SEC analysis, the remaining traces of free polyoxazoline chains were neglected in this calculation. The chain lengths calculated in this way are always slightly larger than the monomer:initiator ratio, but the deviation was usually less than 15 %. Only the sample PEHO-IL-P(EtOx/nPrOx)20 showed a higher deviation of 33 %.

Figure 1 : 1 H

 11 Figure 1: 1 H NMR spectrum of PEHO-IL-PnPrOx10 (top) and magnification of the spectrum

a

  Determined by DSC; b determined by TGA. Thermoresponsive behavior of the hyperPILs. Special attention was payed to the investigation of the thermoresponsive properties of the micellar polyelectrolytes in aqueous solution. Due to the covalent attachment of the LCST-polyoxazoline shell, all six hyperPIL polyelectrolytes exhibited LCST behavior when exceeding sufficiently high concentrations.

Scheme 2 :

 2 Scheme 2: Clouding/clearing of an aqueous PEHO-IL-P(EtOx/nPrOx)20 solution by

Figure 2 :

 2 Figure 2: Transmission-temperature curves of aqueous solutions of PEHO-IL-PnPrOx10.

Figure 3 :

 3 Figure 3: TCP of aqueous hyperPIL solutions of different concentrations.

Figure 4 :

 4 Figure 4: TEM images of PEHO-IL-P(EtOx/nPrOx)10 (left) and PEHO-IL-P(EtOx/nPrOx)20

Figure 5 :

 5 Figure 5: Temperature-controlled shuttling of Congo red in ethyl acetate/water using PEHO-IL-

  TFA. A solution of PEHO-OH (474 mg, 4.08 mmol OH groups, 1.0 equiv.) in trifluoroacetic anhydride (10 mL, 15 g, 71 mmol, 17 equiv.) was stirred under reflux for 4 h. Excess trifluoroacetic anhydride and formed trifluoroacetic acid were distilled off and the remaining highly viscous liquid was dried at 70 °C under high vacuum. A colorless liquid (872 mg; 100 %) was obtained.Synthesis of PEHO-OTs.To a solution of PEHO-OH (25.0 g, 215 mmol OH groups, 1.0 equiv.) in pyridine (125 mL), a solution of p-tosyl chloride (82.0 g, 430 mmol, 2.0 equiv.) in pyridine (63 mL) was added at 0 °C over a period of 30 min. The reaction mixture was then stirred at room temperature for 7 d. Toluene (63 mL) was added to the reaction solution and the resulting precipitates were filtered off. Then, saturated aqueous K2CO3 (63 mL) was added portion wise to the filtrate. The aqueous phase was separated and the organic phase was concentrated under reduced pressure. The residue was taken up in CHCl3 (200 mL) and washed with water (2 × 200 mL), aqueous HCl (10 %, 2 × 200 mL), water (2 × 200 mL), and saturated aqueous NaCl (200 mL). The organic phase was dried over Na2SO4 and concentrated under reduced pressure. The residue was dried at 80 °C under high vacuum. PEHO-OTs was obtained as a slightly yellow solid (52.8 g; 91 %).

( 42 )

 42 Hoogenboom, R.; Fijten, M. W. M.; Thijs, H. M. L.; Lankvelt, B. M. van; Schubert, U. S. Microwave-assisted synthesis and properties of a series of poly(2-alkyl-2-oxazoline)s. Des. Monomers Polym. 2005, 8, 659-671, DOI: 10.1163/156855505774597704. (43) Rosa, V. R. de la; Woisel, P.; Hoogenboom, R. Supramolecular control over thermoresponsive polymers. Mater. Today 2016, 19, 44-55, DOI: 10.1016/j.mattod.2015.06.013. (44) Hoogenboom, R.; Thijs, H. M. L.; Jochems, M. J. H. C.; van Lankvelt, B. M.; Fijten, M. W. M.; Schubert, U. S. Tuning the LCST of poly(2-oxazoline)s by varying composition and molecular weight: Alternatives to poly(N-isopropylacrylamide)? Chem. Commun. 2008, 5758-5760, DOI: 10.1039/b813140f. (45) Osváth, Z.; Iván, B. The Dependence of the Cloud Point, Clearing Point, and Hysteresis of Poly(N -isopropylacrylamide) on Experimental Conditions: The Need for Standardization of Thermoresponsive Transition Determinations. Macromol. Chem. Phys. 2017, 218, 1600470, DOI: 10.1002/macp.201600470.

2 . 3 )Figure S2: 1 H

 231 Figure S2: 1 H (top) and 13 C IG NMR spectrum (bottom, with magnification of signal 3) of PEHO-TFA. Assignment of the different structural units to the respective signals of C 3 is according to Bednarek et al. 1

  The proportion of the respective structural units (d = dendritic, l = linear, t = terminal) was calculated from the integrals of the associated signals of 3d (43.0 -43.7 ppm), 3l (42.2 -43.0 ppm), 3t (41.5 -42.2 ppm), and the total integral of all signals associated to C 3 (41.5 -43.7 ppm) in the[START_REF] Men | Thermoresponsive polymerized gemini dicationic ionic liquid[END_REF] C inverse-gated (IG) NMR spectrum. Signal assignment is according to Bednarek et al.[START_REF]Micelles: Structural biochemistry, formation and functions and usage[END_REF]

Figure

  Figure S4: SEC curve of PEHO-OTs.

+ C 5 ) 3 .

 53 Figure S6: DSC thermograms (2 nd heating curve) of PEHO-OH and its derivatives.

Figure S7 :

 S7 Figure S7: TGA curves of PEHO-OH and its derivatives measured under air (left) and nitrogen atmosphere (right).

Figure S8: 1 H

 1 Figure S8: 1 H NMR spectrum of PEHO-IL-PEtOx10 with magnification below.

Figure S9: 1 H

 1 Figure S9: 1 H NMR spectrum of PEHO-IL-PEtOx20 with magnification below.

Figure S12: 1 H

 1 Figure S12: 1 H NMR spectrum of PEHO-IL-PnPrOx10 with magnification below.

Figure S13: 1 H

 1 Figure S13: 1 H NMR spectrum of PEHO-IL-PnPrOx20 with magnification below.

  0

Figure S14 :

 S14 Figure S14: SEC curves of the hyperPILs after the respective purification steps (TIPS = temperature-induced phase separation, for details see experimental section).

  -P(EtOx/nPrOx) 10 M / g mol -1 PEHO-IL-P(EtOx/nPrOx) 20 M / g mol -1 PEHO-IL-PnPrOx 10 M / g mol -1 PEHO-IL-PnPrOx 20

  polyoxazoline backbone or from the signals of the CH2-und CH3-units of the side chains. The expected contributions of any overlapping signals are subtracted from the integral value of the respective signal. The remaining integral value is then divided by the product of DF (PEHO-Im) and the number of protons per repeat unit that contribute to the signal, which gives DP of the polyoxazoline chains. In case of using the polyoxazoline backbone signal, the obtained value must be increased by 1, since the signals of the repeat unit directly bonded to the imidazolium cation do not overlap with those of the remaining repeat units. If several signals are suitable for determining DP, the arithmetic mean of the calculated DP values based on different signals is given.To calculate the Mn of the hyperPILs, DP of the polyoxazoline chains is first multiplied with the molar mass of the respective monomer and the molar mass of the initiator (MeOTs) is added to obtain the Mn of a single polyoxazoline chain. This value is multiplied with the number of imidazole units per PEHO-Im molecule, which is calculated by multiplying DF (PEHO-Im) (0.82) with DP of the PEHO core(15). Finally, to the Mn of the polyoxazoline shell obtained in this way, the Mn of PEHO-Im (2500 g mol -1 ) is added to obtain Mn of the hyperPILs. This calculation is shown below by the example of PEHO-IL-PnPrOx10. DP1, DP2 etc. denote the degrees of polymerization determined from different signals. From these values, the arithmetic mean is formed.

𝐷𝑃

Table 1 :

 1 Properties of PEHO-OH and its derivatives.

	Sample code	DF a	Mn	Yield	Tg d	TO e (air)	TO e (N2)
		[%]	[g mol -1 ]	[%]	[°C]	[°C]	[°C]
	PEHO-OH	100	1700 c	90	40	340	401
	PEHO-OTs	90	3800 b	91	23	235	299
	PEHO-Im	82	2500 c	95	52	402	404

  3-Ethyl-3-hydroxymethyloxetane (EHO) was kindly provided by Perstorp Polyols.

	BF3•OEt2 and 2-ethyl-2-oxazoline (99 %) were purchased from Alfa Aesar. Trifluoroacetic
	anhydride (99 %), tosyl chloride (98 %), methyl tosylate (98 %), and Congo red were obtained
	from Merck. Imidazole (99 %), Na2PdCl4 (98 %), and bis(p-sulfonatophenyl)phenylphosphine
	dihydrate dipotassium salt (97 %) were purchased from Sigma-Aldrich. 2-n-Propyl-2-oxazoline
	was synthesized according to a literature procedure. 42 Solvents were of analytical grade and
	purchased from different suppliers. DMF, 2-ethyl-2-oxazoline and 2-n-propyl-2-oxazoline were
	distilled and stored over molecular sieves (4 Å) prior to use. All other chemicals were used as
	received.

Table S4 :

 S4 Yields and purities of the hyperPILs after precipitation in Et2O and, if applied, subsequent TIPS.

	Sample code	Purification procedure a	Purity b [wt.%]	Yield [g | %]
	PEHO-IL-PEtOx10	precipitation in Et2O + TIPS (95 °C)	95	0.62 c | 56
	PEHO-IL-PEtOx20	precipitation in Et2O + TIPS (80 °C)	96	0.27 c | 24
	PEHO-IL-P(EtOx/nPrOx)10	precipitation in Et2O	97	1.33 | 85
	PEHO-IL-P(EtOx/nPrOx)20	precipitation in Et2O + TIPS (40 °C)	93	0.37 c | 36
	PEHO-IL-PnPrOx10	precipitation in Et2O	99	1.98 | 74
	PEHO-IL-PnPrOx20	precipitation in Et2O	96	2.45 | 80

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from the German Research Foundation (DFG) in the framework of the IRTG Soft Matter Science (GRK 1642). The authors thank Tobias Michael Trötschler for helpful discussions, Ivonne Knauer, Kristin Anne Lehmann and Carolin Daniela Guth for assistance in laboratory work, Andreas Warmbold for TGA & DSC measurements, Marina Hagios for SEC measurements, Dr. Victor Pacheco Torres as well as Dr. Manfred Keller for NMR measurements, and Prof. Philipp Kurz for the possibility to use spectroscopic equipment for turbidity measurements.

Scheme 1: Synthesis and molecular architectures of thermoresponsive hyperPILs.

ANALYSIS AND CHARACTERIZATION

NMR. NMR spectra were generally recorded in deuterated DMSO at 110 °C on a Bruker Avance III HD 500 MHz spectrometer (for 1 H) or at 27 °C on a Bruker Avance II 300 MHz spectrometer (for [START_REF] Men | Thermoresponsive polymerized gemini dicationic ionic liquid[END_REF] C). As an exception, PEHO-TFA was measured in deuterated chloroform at 27 °C on a Bruker Avance II 300 MHz spectrometer ( 1 H and [START_REF] Men | Thermoresponsive polymerized gemini dicationic ionic liquid[END_REF] C IG). The chemical shifts were referenced by the solvent signal.

SEC. SEC measurement of PEHO-OTs was performed on an Agilent 1260 Infinity System

with RI detector and chloroform as eluent at a flow rate of 1 mL min -1 and a temperature of 22 °C. The injection volume was 100 µL. SDV columns (5 µm) by PSS were used: 1 x precolumn (8 * 50 mm), 1 x 1000 Å, 1 x 10 000 Å, 1 x 100 000 Å and 1 x 1 000 000 Å (each 8 * 300 mm). For calibration, narrow PS standards (from PSS) in the mass range 162 -1 800 000 g mol -1 were used. Toluene was used as internal standard. SEC measurements of the hyperPILs were performed on an Agilent 1260 Infinity System with RI detector and DMAc/LiBr(0.5 wt.%) as eluent at a flow rate of 1 mL min -1 and a temperature of 50 °C. The injection volume was 50 µL. GRAM columns (10 µm) by PSS were used: 1 x precolumn (8 * 80 mm), 1 x 30 Å und 2 x 1000 Å (each 8 * 300 mm). For calibration, narrow PMMA standards (from PSS) in the mass range 410 -988 000 g mol -1 were used. To determine the average molecular weights and dispersities of the hyperPILs, the high-molecular weight peak was evaluated, in case of the presence of minor low-molecular-weight impurities (the lower integration limit was chosen from the minimum between the two peaks).

TGA. TGA curves were recorded on a STA 409 or a STA 449F5 from Netzsch with a heating rate of 10 K min -1 . To characterize the mass loss curves, the onset temperatures (TO) were determined.

Supporting Information

Programmable Thermoresponsive Micelle-Inspired Polymer Ionic Liquids as Molecular Shuttles for Anionic Payloads Steffen Wiedmann †, ‡, § , Manuel Luitz †, ‡, ⊥ , Benjamin Kerscher †, ‡ , Jean-François Lutz

Listings of low-molecular-weight fractions, yields and purities

Table S2: Percentage of low-molecular-weight impurities in PEHO-IL-POx10 samples after precipitation in Et2O and, if applied, subsequent TIPS. Table S3: Percentage of low-molecular-weight impurities in PEHO-IL-POx20 samples after precipitation in Et2O and, if applied, subsequent TIPS. a Determined from the inflection point of the transmission-temperature curve during heating ( = 500 nm; heating rate = 1 K min -1 ); -= no clouding detected, nd = not determined.

Sample code

Sample code

TEM images

(1) Bednarek, M.; Biedron, T.; Helinski, J.; Kaluzynski, K.; Kubisa, P.; Penczek, S. Branched polyether with multiple primary hydroxyl groups: polymerization of 3-ethyl-3hydroxymethyloxetane. Macromol. Rapid Commun. 1999, 20, 369-372.

(2) Hölter, D.; Burgath, F.; Frey, H. Degree of branching in hyperbranched polymers. Acta Polym. 1997, 48, 30-35.


M Alepidis

A Bouchard

M Bawedin

I Ionica

C Delacour

Keywords: equilibrium, body potential, silicon on insulator substrates, SOI, metal-silicon contacts

This article focuses on one of the floating body effects in siliconon-insulator (SOI) substrates, the out-of-equilibrium body potential. Unlike the already existing studies which are exploring either SOI fully-fabricated devices or SOI wafers measured with pressure probes, here the out-ofequilibrium body potential is explored in SOI substrates with deposited metal contacts. Chromium and gold/titanium contacts of various geometries were deposited on silicon films of SOI and the floating body potential was examined in each case. The effect of annealing on the device behavior was also investigated. The performance of the deposited metal contacts is evaluated through current and potential monitoring under quasi-static conditions. Depending on the workfunction difference between metal and silicon film, the current measurements show that the contact behavior is mainly ohmic-like or Schottky-like. While body potential manifests itself with Schottky-like contacts, it does not seem to exhibit a significant response with ohmic contacts. The influence of the contact architecture on the body potential signature is illustrated, enabling the selection of the proper contact layout for an enhanced body potential response.

I. INTRODUCTION

Although fully depleted silicon on insulator (SOI) field-effect transistors (FET) have improved performance compared to partially depleted devices, they are still not completely immune to floating body effects [1][2][3][4][5]. Even though the floating body effects are usually not desirable for the transistor operation [5], they can be exploited for practical applications [3]. For example, one of the floating body effects is the out-of-equilibrium body potential (V B ); it was extensively described for FD-SOI devices and used for DRAM applications [6]. The body potential variation in the device stems from the lack or excess of carriers in the Si film. A lack of carriers occurs while the gate voltage drives the Si film from depletion into inversion/accumulation and carriers are not instantaneously provided by the contacts. Under these conditions, the body potential simply follows the gate voltage and perishes after the injection of the carriers needed for the return to equilibrium. This effect was also measured in SOI wafers with the pseudo-MOSFET method [7], in which metallic pressure-probes are used as source/drain contacts and the silicon bulk serves as a back gate [2]. In this configuration, the body potential proved to be suitable for biochemical detection purposes since it exhibits a shift when charged molecules are placed on the top surface of the silicon film [7]. Consequently, the potential of applications is huge, since V B monitoring can extend the current measurements [8,9] that are typically used for the biochemical detection with FETs. However in the present status, V B was measured on SOI with a specific probe station with controlled pressure applied on the probes, which is known to provide ohmic contacts to the Si film [2]. A pragmatic use for sensing purposes requires fabrication of realistic integrated devices with metal contacts pads on top of the Si films which are not dependant on a particular measurement system. Metal contacts on SOI, inspired by the pseudo-MOSFET, have already been successfully fabricated previously [10][11][12][13][START_REF] Arsalan | IEEE SOI-3D-Subthreshold Microelectronics Technology Unified Conference (S3S)[END_REF], with widely used metals and alloys. These studies, focus on current measurements for device characterization and sometimes consider the Schottky barriers between the metal contacts and the low doped Si film. However, they do not discuss the body potential response and the key question is whether the out-of-equilibrium body potential manifests itself in such devices configuration.

The aim of this paper is to give insights on the physical phenomena and the technological choices provoking an out-ofequilibrium body potential response in an SOI with metal deposited contacts, when the backgate is scanned from accumulation to inversion and vice-versa. We explore the use of chromium (Cr) and gold/titanium (Au/Ti) pads on both drain current and body potential. The effects of thermal annealing and geometry of pads are also investigated.

II. EXPERIMENTAL PROCEDURE

The devices under test are fabricated on SOI with 70nm Si film and 145nm buried oxide (BOX) thicknesses. The Si film is p-type with a doping concentration of N a ≈10 15 cm -3 . Silicon mesas of 5x5cm 2 were isolated by lithography and reactive ion etching. The top native oxide layer present on the Si was removed in a 5% HF solution for 5 seconds, prior to the metal deposition. The chromium patterns were fabricated by sputtering (300nm Cr layer thickness) on top of the wafer maintained at 120 o C, followed by lithography and etching. The Au/Ti contacts were fabricated with a lift-off process after evaporating 5nm Ti and 30nm Au layers at room temperature, in vacuum. The metal pads are either circular with a radius of 100μm or square of 100μm side and they are positioned at various distances between them (d=0.3mm, 0.5mm and 1mm), as illustrated in the schematic of Fig. 1. The electrical characterization was realised in a standard semiconductor probe station. An Agilent B1500 semiconductor analyser and four source/measure units were used for each one of the device terminals (Fig. 1a). The drain current I D , gate current I G and body potential V B were measured during sweeping the gate voltage V G from accumulation to inversion (forward scan) and vice-versa (reverse scan), with a step of 50mV. The hold time (before the first Out-of-equilibrium body potential measurement on silicon-on-insulator with deposited metal contacts Miltiadis Alepidis, Aude Bouchard, Cecile Delacour, Maryline Bawedin, Irina Ionica  measurement) and delay time (before each measurement point) were set to 30s and 100ms respectively, insuring an equilibrium starting point and reduced fluctuations in the monitored currents. The source was grounded, the drain voltage V D was set to 0.1V and zero current condition was imposed to body contact for all the measurements. For I D versus V D , the voltage step was set to 20mV .The V G values selected correspond to subthreshold, low inversion and strong inversion regimes for the device under test.

III. RESULTS AND DISCUSSIONS

A. Effect of the metal on the device response Fig. 2 illustrates the I D -V G characteristics curves for Cr and Au/Ti circular contacts placed in the centre of the device at a distance d=1mm and a typical pseudo-MOSFET curve obtained with pressure probes of the same wafer. In strong inversion (V G >0), both metals have an I D in the same order of magnitude as the pseudo-MOSFET measurement, with higher ON-current and a steeper subthreshold slope for Au/Ti contacts. In depletion regime, for the Cr contacts I D drops in the order of 100 pA (similarly to the pseudo-MOSFET curve), while for Au/Ti it has a minimum value ~1nA. In accumulation (V G <0), I D increases back with Cr contacts while it stays constant around 20nA for Au/Ti, contrary to pseudo-MOSFET which has a hole current in the µA range. It is evident that for the fabricated devices, the Cr contacts produce an I D -V G shape similar to the pressure controlled probes on Si films with lower current values. Note however that a better electrical contact for electrons is obtained with Au/Ti pads. Fig. 3 presents the I D -V D curves of the same devices as in Fig. 2 for various V G voltages. The V G values were selected in order the Si film to be in the subthreshold or inversion/accumulation regimes for each device. In all cases, I D has a linear behaviour for low V D values. Au/Ti contacts have a clear ohmic response for V G >0 and they show better conductivity compared to Cr. For V G <0 with Cr contacts, I D quickly deviates from the linear behaviour when V D increases while Au/Ti contacts show very small current values with a lesser dependence on V G . This is consistent with Fig. 2, since for accumulation we observe a low value plateau current for Au/Ti contacts.

Theoretically, the type of the contact is defined by the workfunction difference between the metal and the semiconductor [START_REF]Semiconductor Material and Device Characterization[END_REF]. For the p-type silicon used, the workfunction is calculated at 4.91eV. Consequently, we expect a rectifying behaviour for the Cr (ϕ Cr =4.5eV [START_REF] Michaelson | [END_REF]) contacts and a non-rectifying for Au/Ti (ϕ Au ≥5.1eV [START_REF] Michaelson | [END_REF]). Note that the Ti layer is deposited for adhesion between Si and Au and for 5nm layer thickness, Ti should diffuse into the grain boundaries of Au [17], as a result it should not affect the electrical response [18]. However, the experimental I D -V D curves are not entirely ohmic or Schottky on the full range of the V G , probably because the actual carrier concentration in the Si film (including under the contacts) is modified by V G . The aim here is not to provide a full understanding of this rather complex problem. It is important to underline that for low V D , most of the curves show a fairly linear behaviour. We can therefore conclude that carriers should be provided in all the cases and we can proceed to the V B measurements. Fig. 4 depicts the V B response of the devices under test. V B with the Cr contacts is constant in accumulation and inversion regimes with the value of 50mV (corresponding to V D /2). In the depletion regime it follows V G and drops quickly to the equilibrium value. The rise of V B is associated to the out-of-equilibrium lack of electrons at the beginning of the depletion/low inversion regime. The drop appears when the carriers are efficiently injected and the device returns to equilibrium; this is also evident through the I G peak illustrated in the inset of the Fig. 4. The Au/Ti contacts present small variations of V B for negative V G and reaches 50mV for positive V G . The typical response of out-of-equilibrium V B for the pseudo-MOSFET is similar to the one observed with Cr, but lower in value. The shift and the range of the V B response between Cr and pressure probes is agreement with Fig. 2 since it is associated with the depletion regime of each device. Additionally, SOI MOSFETs give a comparable V B signature as reported in the literature [6,7,19]. Besides a stronger V B signature, the Cr contacts provided less I D current than Au/Ti leading to the conclusion that a higher resistivity contact might be more appropriate for a strong V B response. In order to check this hypothesis, we proceed to annealing of the devices, which is expected to improve the contacts.

B. Effect of annealing

The wafers were annealed for one hour in an oven at 300 o C under high vacuum. The wafers with Au/Ti metal pads showed slightly increased contact conductivity after annealing (higher I D ) but no increase in V B response and therefore we exclude them from the following studies. For a device with Cr contacts, a typical I D -V G curve before and after annealing is presented in Fig. 5. We notice higher ON-currents for accumulation and strong inversion, and an increase of the current values in the depletion region. Additionally, the off-region of the device is narrowed after annealing, similar to results previously obtained in the literature for SOI with pressure probes [20]. The resistivity of the contacts after annealing is reduced, especially for holes, and the I D -V D curves (presented as an inset of Fig. 5) have a linear response for both types of carriers after annealing (unlike in Fig. 3d). The annealing process clearly improves the contact conductivity for the chromium contacts, but what is its effect on the V B response? Fig. 6 illustrates V B -V G and I G -V G (inset) curves for the same device before and after annealing. The V B response is obviously diminished for both scanning directions. The deformation of the gate current (inset of Fig. 6) shape signifies that the electrical quality of the contact affects the minority carrier injection in the silicon film. Before annealing, the minority carriers cannot be provided instantaneously. After annealing, we notice a significant decrease in the displacement current which implies that the respective amount of carriers is injected almost instantly in the silicon film, therefore, the device does not enter an out-of-equilibrium regime and V B remains constant. The effects of the materials and the process in the above analysis reveal that an excellent contact quality, with low resistance, does not guarantee a favorable V B response. Indeed, a small contact resistance allows rapid carrier injection, leading to the suppression of the outof-equilibrium phenomenon. On the contrary, out-of-equilibrium V B necessitates existence of barriers preventing the contacts to provide the carriers instantly. Annealing of the Cr contacts transforms towards a purely ohmic behaviour, eliminating the Schottky barrier and hence the V B variations vanish.

For the next study of the effect of the distance between the contacts and the shape of the pads for V B engineering, we'll focus on devices with chromium contacts, without annealing.

C. Distance between metal contacts

The channel length and width of FD-MOSFETs affect significantly their drive current. Considering the unconventional 3D geometry in pseudo-MOSFET and the necessary modifications in the models to accurately describe its current [2], we investigate the effect on V B . While we can hardly correlate a parameter to the width, the channel length can be easily associated to the distance between the metal contacts. Initially, we monitored V B in a three terminal configuration as in Fig. 1a. We measured three devices with distances between the contacts of 0.3mm, 0.5 mm and 1mm. The curves are illustrated in Fig. 7. The body potential variation is pronounced in all devices but the values are lower for d=0.3mm. For d=0.5mm and d=1mm the difference between the curves seems minor since the signature is identical in shape and only slightly shifted. To eliminate any variability issues between the devices, V B was also monitored between various contacts on the same device having four circular metal pads and a distance of 1mm between them (Fig 8). The first contact (P1), which acts like the source, is grounded and V B was monitored on one of the other three contacts (P2, P3, P4). Under these experimental conditions, the drain contact is floating, hence no V D is applied. Fig. 8 illustrates the response of the body potential and the respective I G (inset). Body potential response is affected by the distance from the source, unlike the gate current which is perfectly superposed for all the experiments. Although, the value of the body potential stays constant for each position during the linear increase, V B returns to the equilibrium value in a sharper way if the distance between the contacts increases. I G peak value always occurs when V B returns to the equilibrium value, clarifying that the BOX capacitance charge concerns the full area of the Si mesas and is the same for a given mesa regardless of the distance. Larger distance between the contacts results also in higher V B values. When the Si film is depleted, channel time constant value is large [21]. Consequently, the extra time needed for the carriers to be transferred at distant contacts, drives V B to higher values.

Contact layout can have an important role on the tuning of the device and more specifically on V B . Another factor that affects the electric field distribution in the silicon mesa is the contact shape which we examine next.

D. Shape of the contact

Square and circular metal contacts are compared in terms of body potential response in Fig. 9. In the graph, the comparison is realized for three distances (d) between the contacts. For d=1mm, the response is almost not affected by the contact shape. However, the impact of the shape becomes strong on the V B -V G curves when the distance between the contacts is in the same order of magnitude as their own size. This effect is probably related to the current density distribution in the thin Si film, which is associated to the electric field distribution. Simulations in pseudo-MOSFET with point contacts have shown a strong current density around the contacts in the Si film [22]. In the case of 100µm size pads, spaced by 300µm, the electric field lines are affected by their shape and they might interact. The V B response, which is related to the way that the carriers are injected in the film, will be more sensitive to the shape of the contacts if they are closed to one another. For practical applications, it is more convenient to use contacts with a stronger separation in order to reduce this effect and have a V B less dependent on the contact in-plane geometry.

IV. CONCLUSIONS

This work focuses on the out-of-equilibrium body potential obtained on SOI wafers with metal-deposited contacts and gives an insight on the respective physics. The structures were inspired by the biochemical detection capability of the body potential in SOI wafers measured with pressure-controlled probes (pseudo-MOSFET configuration). The devices developed here are more adapted for applications since they replace the probes of a particular probestation with metal contacts directly deposited on the Si film. We selected Cr and Au/Ti for the contacts since they are theoretically expected to form Schottky or ohmic for the Si doping used. While Cr contacts show higher contact resistance and hence lower ON-current, they clearly give strong and pronounced V B response, similar to the pseudo-MOSFET configuration. On the contrary, Au/Ti devices have higher current, but almost no body potential response. Annealing of the samples reduces the contact resistivity, but also reduces significantly the V B signal. This illustrates the importance of having some barriers between the contacts and the Si film that prevent the carriers to be instantly provided during the V G scan. The architecture of the devices was also considered in the V B response through the investigation of the distance between the metal contacts and the contact shape. The results show that when the contacts are placed too close to each other, V B weakens. The distance between the contacts affects also the path that the body potential follows to return to the equilibrium value. The comparison with I G suggests that it stems from the diffusion of the carriers in the silicon film. Finally, the shape of the metal contact seems to be important when it is comparable in size with the distance between the contacts.

The overall outcome of this work indicates that for applications based on V B response (e.g charge sensing), the use of Schottky contacts would be preferable to the use of ohmic contacts.

Fig. 1 :

 1 Fig. 1: Device schematics. a) Cross-section view with the three metal contacts that correspond to source, drain and body. b) Top view of the device.

Fig. 2 :

 2 Fig. 2: ID-VG characteristic curves on semi-log scale Cr, Au/Ti and controlled pressure probes on top of 70nm thick Si on 145nm thick BOX. These curves are representative of all the devices tested.

Fig. 3 :

 3 Fig. 3: ID-VD curves for Au/Ti contacts with a) VG>0 (Si film in inversion) and b) VG<0 (Si film in accumulation) and for Cr contacts with c) VG>0 and d) VG<0. Same devices as in Fig.2

Fig. 4 :

 4 Fig. 4: VB-VG and IG-VG (inset) curves for devices with deposited contacts of Cr, Au/Ti and with controlled pressure probes. The right y-axis corresponds to the body potential of Au/Ti contacts, given in a smaller scale than Cr and pseudo-MOSFET (left y-axis).

Fig. 5 :

 5 Fig. 5: ID-VG curves in semi-log scale for a device with Cr contacts before and after annealing.

Fig. 6 :

 6 Fig. 6: VB-VG and IG-VG (inset curves for a device with Cr pads before and after annealing. The right y-axis corresponds to the body potential after annealing in a smaller scale.

Fig. 7 :

 7 Fig. 7: VB-VG curves for devices with Cr pads and various distances between the contacts.

Fig. 8 :

 8 Fig. 8: VB-VG curves for various contacts configurations. IG-VG curves (bottom inset) and schematic top view of the device (top inset).

Fig. 9 :

 9 Fig. 9: Comparison of VB-VG curves for square (of 100µm side) and circular (of 100µm diameter) metal pads, with various distances between the contacts.

ACKNOWLEDGMENTS

The authors thank SOITEC for providing the wafers in this study and especially Dr. Frédéric Allibert for the inspiring conversations.