Johannes Elbert

Individually Addressable Thermo‐ and Redox‐Responsive Block Copolymers by Combining Anionic Polymerization and RAFT Protocols

A novel diblock copolymer consisting of poly(vinylferrocene) (PVFc) and poly(N,N-diethylacrylamide) (PDEA) is synthesized via a combination of anionic and RAFT polymerization. The use of a novel route to hydroxyl-end-functionalized metallopolymers in anionic polymerization and subsequent esterification with a RAFT agent leads to a PVFc macro-CTA (M¯n = 3800 g mol(-1) ; Đ = 1.17). RAFT polymerization with DEA affords block copolymers as evidenced by (1) H NMR spectroscopy as well as size exclusion chromatography (6400 ≤ M¯n≤ 33700 g mol(-1) ; 1.31 ≤ Đ 1.28). Self-assembly of the amphiphilic block copolymers in aqueous solution leads to micelles as shown via TEM. Importantly, the distinct thermo-responsive and redox-responsive character of the blocks is probed via dynamic light scattering and found to be individually and repeatedly addressable.

Asymmetric Faradaic systems for selective electrochemical separations

Ion-selective electrochemical systems are promising for liquid phase separations, particularly for water purification and environmental remediation, as well as in chemical production operations. Redox-materials offer an attractive platform for these separations based on their remarkable ion selectivity. Water splitting, a primary parasitic reaction in aqueous-phase processes, severely limits the performance of such electrochemical processes through significant lowering of current efficiencies and harmful changes in water chemistry. We demonstrate that an asymmetric Faradaic cell with redox-functionalization of both the cathode and the anode can suppress water reduction and enhance ion separation, especially targeting organic micropollutants with current efficiencies of up to 96% towards selective ion-binding. A number of organometallic redox-cathodes with electron-transfer properties matching those of a ferrocene-functionalized anode, and with potential cation selectivity, were used in the asymmetric cell, with cobalt polymers being particularly effective towards aromatic cation adsorption. We demonstrate the viability and superior performance of dual-functionalized asymmetric electrochemical cells beyond their use in energy storage systems; they can be considered as a next-generation technology for aqueous-phase separations, and we anticipate their broad applicability in other processes, including electrocatalysis and sensing.

A triblock terpolymer vs. blends of diblock copolymers for nanocapsules addressed by three independent stimuli

Stimuli-responsive micro- to nano-scale containers have gained increasing attention due to their unique potential to selectively release payloads under specific environmental conditions. We report here novel triple stimuli-responsive nanocapsules that selectively respond to changes in temperature, pH value, and redox potential. The nanocapsules were prepared from either a triple responsive triblock terpolymer or a blend of responsive diblock copolymers, both synthesized by sequential anionic polymerization. We then compared the release performance of nanocapsules under oxidative conditions and changes of temperature or pH value. Our results reveal the close correlation between the release properties of stimuli-responsive nanocontainers and the microstructure of the polymer shell. In fact, the microphase separation between the responsive diblock copolymers across the shell significantly hinders the triggered release of the payload from the nanocapsules. These results demonstrate that the fine morphology of triblock terpolymers can be exploited to achieve the triggered release of payloads from polymer nanocontainers upon application of three different external triggers.

One for all: cobalt-containing polymethacrylates for magnetic ceramics, block copolymerization, unexpected electrochemistry, and stimuli-responsiveness

Novel cobalt-containing homo- and diblock copolymers with poly(methyl methacrylate) (PMMA) are synthesized by atom transfer radical polymerization (ATRP) of a neutral cobalt-complex methacrylate. An efficient route for a single-step synthesis of the cobalt precursor based on easily-available starting materials followed by esterification with methacrylic acid is presented. The cobalt-methacrylate monomer is furthermore polymerized by thermal, free radical and statistical copolymerization with MMA and investigated with respect to (absolute) molar masses, polymer composition, and thermal properties. ATRP affords block copolymers as evidenced by 1H NMR spectroscopy, size exclusion chromatography (SEC) and differential scanning calorimetry (DSC). The cobalt-containing homopolymers are investigated and tailored with respect to their thermal conversion into magnetic cobalt oxides and elemental cobalt which is evidenced by X-ray diffraction (XRD), Raman spectroscopy, and superconducting quantum interference device (SQUID) magnetometry measurements. The (reversible) electrochemistry of the cobalt-containing polymethacrylates and block copolymers thereof are thoroughly addressed by cyclic voltammetry (CV) studies. Interestingly, the prepared metalloblock copolymers exhibit redox-responsiveness (both reduction and oxidation) and thus structure formation in the presence of a reduction or oxidation reagent are demonstrated by transmission electron microscopy (TEM).

Multi-stimuli responsive block copolymers as a smart release platform for a polypyridyl ruthenium complex

A variety of applications of amphiphilic block copolymers result from the control of their self-assembled structures. Herein, the synthesis and structure formation of block copolymers (BCPs) consisting of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as one segment and poly(methyl methacrylate) (PMMA) or a statistical copolymer (PDMAEMA-co-PMMA) as a second segment, is described. The BCPs provide molar masses between 8.9 kg mol−1 and 35.6 kg mol−1 with low polydispersity index values, Đ = 1.05–1.13. BCPs are synthesized via sequential anionic polymerization strategies while structure formation in water is investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The PDMAEMA-containing micelles in water are loaded with a Ru(II) polypyridyl complex, i.e. [Ru(bipy)2-dppz-7-hydroxymethyl][PF6]2 (bipy = 2,2′-bipyridine; dppz = dipyridophenazine), which was previously shown to act as a potential photosensitizer in photodynamic therapy (PDT). Successful loading of the BCP micelles is evidenced by TEM measurements after dialysis in water. Stimulus-responsive release of the Ru(II) complex from the BCP micelles is shown using ultrasound, change of pH or temperature as external triggers. The quantification and release profiles for the Ru(II) complex are obtained by atomic absorption spectrometry (AAS). As a result, PDMAEMA-b-PMMA is not capable of releasing the Ru(II) complex in a controlled manner after application of, for instance, ultrasound or temperature change as external triggers due to the shielding (stealth effect) of the BCP. On the contrary, micelles made of BCPs featuring PDMAEMA and PDMAEMA-co-PMMA segments reveal excellent Ru(II) complex release profiles due to the tailored molecular composition of the underlying block segments as evidenced by temperature-dependent DLS and AAS measurements. Thus, these smart PDMAEMA-containing BCPs pave the way to a variety of applications for selective triggered release of small molecules.

Surface‐Initiated Anionic Polymerization of [1]Silaferrocenophanes for the Preparation of Colloidal Preceramic Materials

A novel strategy for the preparation of poly(ferrocenylsilane) (PFS) immobilized on the surface of cross-linked polystyrene (PS) nanoparticles is reported. The ferrocene-containing core/shell architectures are shown to be excellent candidates as preceramic polymers yielding spherical ceramic materials consisting of iron silicide (Fe3 Si) and metallic iron after thermal treatment. For this purpose, dimethyl- and hydromethyl[1]silaferrocenophane monomers are polymerized by surface-initiated ring-opening polymerization upon taking advantage of residual vinylic moieties at the PS particle surface. A strategy for selective chain growth from the particle surface is developed without the formation of free PFS homopolymer in solution. The grafted particles are characterized using transmission electron microscopy (TEM) and dynamic light scattering (DLS). These particles are excellent precursors for ceramics as studied by thermogravimetric analysis (TGA). The composition of the ceramics is studied using X-ray diffraction (XRD) measurements, while the morphology is probed by scanning electron microscopy (SEM) revealing the original spherical shape of the precursor particles. Obtained ceramic materials- predominantly based on iron silicides-show ferromagnetic behavior as investigated by superconducting quantum interference device (SQUID) magnetization measurements at different temperatures.

CHAPTER 5:Recent Advances in Immobilized Ferrocene-Containing Polymers

The interesting properties of redox-responsive ferrocene-containing polymers attached or confined on (non-)functional flat substrates, nanoparticles, in pores, in gels or at the surface of electrodes have led to manifold interesting applications utilizing their stimuli-responsive nature. Synthetic aspects for different immobilization strategies will be covered briefly, as will addressing the polarity of such materials (electro)chemically (e.g. for switching surface properties, permeability and for controlled release), host–guest complexation of ferrocene with cyclodextrins (e.g. for creating self-healing materials); novel sensing applications, as ceramic precursors and optical applications. The concepts presented herein show that confined and immobilized metallopolymers are promising candidates for the preparation of a variety of next generation high-tech materials.

Molecular ring rotation in poly(vinylferrocene)

We investigate the ring rotation dynamics in poly(vinylferrocene) (PVFc) using incoherent neutron spectroscopy. PVFc contains ferrocene units laterally attached to a polymer backbone, allowing for one cyclopentadienyl ring of the organometallic sandwich structure of ferrocene to undergo rotational jump diffusion. The barrier of rotation is found to be broadly distributed, but the dynamics can be well described using a rotation rate distribution model which is well known from the description of methyl group rotation in glassy polymers. As necessary information for the analysis of quasielastic scattering data, we measure the static structure factor of the polymer using polarized neutron diffraction. Neutron time-of-flight and backscattering data are then combined and consistently modeled over the large temperature range from 80 K to 350 K yielding an Arrhenius behavior of the jump rate distribution. The mean value of potential barrier distribution is found to be 〈EA〉 = 9.61(2) kJ mol-1 with a root mean square width of σE = 3.12(1) kJ mol-1, being the result of superposition of constant intramolecular and heterogeneous intermolecular rotational barriers.

Vibrational spectra of ferrocene, ferrocene-containing polymers and their oxidized compounds

Ferrocene-containing polymers are an important member in the class of redox- responsive polymers, changing e.g. their solubility or conformation upon external stimuli. We present a study of their vibrational spectra using inelastic neutron scattering, focusing on the central building block of these polymers, the organometallic ferrocene complex. The vibrational modes of a bulk ferrocene sample are compared to those of poly(vinylferrocene), poly(ferrocenyldimethylsilane) and poly(ferrocenylmethylsilane). In the former polymer, the ferrocene complex is laterally attached to the polymer chain and the vibrational spectrum shows a slight shift and broadening of the fingerprint modes in the range of 100-800 cm−1 except for the ring-metal-ring stretching mode which was not detectable anymore. The latter two polymers, where the ferrocene complex is part of the polymer backbone, exhibit larger differences to the vibrational spectrum of bulk ferrocene. Moreover, several contributions to the spectra caused by methyl groups in these polymers could be identified. In order to study the influence of oxidation on ferrocene and redox-responsive polymers, we investigated the ionic compound ferrocenium triiodide and oxidized poly(vinylferrocene). It is observed that the weakening of the η5-complex bond by a missing electron leads to a significant shift of the fingerprint modes to lower frequencies.