Amphiphilic Ferrocene-Containing PEG Block Copolymers as Micellar Nanocarriers and Smart Surfactants.

An important and usually the only function of most surfactants in heterophase systems is stabilizing one phase in another, for example, droplets or particles in water. Surfactants with additional chemical or physical handles are promising in controlling the colloidal properties by external stimuli. The redox stimulus is an attractive feature; however, to date only a few ionic redox-responsive surfactants have been reported. Herein, the first nonionic and noncytotoxic ferrocene-containing block copolymers are prepared, carrying a hydrophilic poly(ethylene glycol) (PEG) chain and multiple ferrocenes in the hydrophobic segment. These amphiphiles were studied as redox-sensitive surfactants that destabilize particles as obtained in miniemulsion polymerization. Because of the nonionic nature of such PEG-based copolymers, they can stabilize nanoparticles even after the addition of ions, whereas particles stabilized with ionic surfactants would be destabilized by the addition of salt. The redox-active surfactants were prepared by the anionic ring-opening polymerization of ferrocenyl glycidyl ether, with PEG monomethyl ether as the macroinitiator. The resultant block copolymers with molecular weights (Mn) between 3600 and 8600 g mol-1 and narrow molecular weight distributions (Mw/Mn = 1.04-1.10) were investigated via 1H nuclear magnetic resonance and diffusion ordered spectroscopy, size exclusion chromatography, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Furthermore, the block copolymers were used as building blocks for redox-responsive micelles and as redox-responsive surfactants in radical polymerization in miniemulsion to stabilize model polystyrene nanoparticles. Oxidation of iron to the ferrocenium species converted the amphiphilic block copolymers into double hydrophilic macromolecules, which led to the destabilization of the nanoparticles. This destabilization of nanoparticle dispersions may be useful for the formation of coatings and the recovery of surfactants.

Water-Soluble Metallocene-Containing Polymers.

Metallocenes are organometallic compounds with reversible redox profiles and tunable oxidation and reduction potentials, depending on the metal and substituents at the cyclopentadienyl rings. Metallocenes have been introduced in macromolecules to combine the redox-activity with polymer properties. There are many examples of such hydrophobic polymer materials, but much fewer water-soluble examples are found scattered across the polymer literature. However, in terms of drug delivery and other biological applications, water solubility is essential. For this very reason, all the synthetic routes to water-soluble metallocene containing polymers are collected and discussed here. The focus is on neutral ferrocene- and ruthenocene-containing and charged cobaltocenium-containing macromolecules (i.e., symmetrical sandwich complexes). The synthetic protocols, self-assembly behavior, and other benefits of the obtained materials are discussed.

Sequence-Controlled Polymers via Simultaneous Living Anionic Copolymerization of Competing Monomers.

Natural macromolecules, i.e., sequence-controlled polymers, build the basis for life. In synthetic macromolecular chemistry, reliable tools for the formation of sequence-controlled macromolecules are rare. A robust and efficient chain-growth approach based on the simultaneous living anionic polymerization of sulfonamide-activated aziridines for sequence control of up to five competing monomers resulting in gradient copolymers is presented. The simultaneous azaanionic copolymerization is monitored by real-time (1) H NMR spectroscopy for each monomer at any time during the reaction. The monomer sequence can be adjusted by the monomer reactivity, depending on the electron-withdrawing effect by the sulfonamide (nosyl-, brosyl-, tosyl-, mesyl-, busyl) groups. This method offers unique opportunities for sequence control by competing copolymerization: a step forward to well-engineered synthetic polymers with defined microstructures.

Triple-Stimuli-Responsive Ferrocene-Containing PEGs in Water and on the Surface.

Triple-stimuli-responsive PEG-based materials are prepared by living anionic ring-opening copolymerization of ethylene oxide and vinyl ferrocenyl glycidyl ether and subsequent thiol-ene postpolymerization modification with cysteamine. The hydrophilicity of these materials can be tuned by three stimuli: (i) temperature (depending on the comonomer ratio), (ii) oxidation state of iron centers in the ferrocene moieties, and (iii) pH-value (through amino groups), both in aqueous solution and at the interface after covalent attachment to a glass surface. In such materials, the cloud point temperatures are adjustable in solution by changing oxidation state and/or pH. On the surface, the contact angle increases with increasing pH and temperature and after oxidation, making these smart surfaces interesting for catalytic applications. Also, their redox response can be switched by temperature and pH, making this material useful for catalysis and electrochemistry applications. Exemplarily, the temperature-dependent catalysis of the chemiluminescence of luminol (a typical blood analysis tool in forensics) was investigated with these polymers.

Functional Group Distribution and Gradient Structure Resulting from the Living Anionic Copolymerization of Styrene and para-But-3-enyl Styrene.

The functional group distribution along the polymer backbone resulting from the living anionic copolymerization of styrene (S) and para-but-3-enyl styrene (pBuS) was investigated in cyclohexane at room temperature. A variety of copolymers with different comonomer contents x(S) = 0-0.84 have been synthesized with molecular weight dispersities Mw/Mn ≤1.12. All polymers have been characterized in detail by 1H NMR spectroscopy, size exclusion chromatography (SEC), and differential scanning calorimetry (DSC). A detailed understanding of the monomer sequence distribution during the copolymerization was achieved by real-time 1H NMR spectroscopy. This technique permits us to determine the changing monomer concentration of each monomer in stock throughout the reaction. Consequently, monomer incorporation and thus the probability of incorporation can be determined at any time of the copolymerization, and a precise determination of the functional group density along the polymer chain is possible. To demonstrate accessibility of the olefin side chains of the copolymer for transformations, quantitative thiol-ene addition of a cysteine derivative has been studied.

Enlarging the Toolbox: Epoxide Termination of Polyferrocenylsilane (PFS) as a Key Step for the Synthesis of Amphiphilic PFS-Polyether Block Copolymers.

Epoxide termination and functionalization of living poly(ferrocenyldimethylsilane) (PFDMS) is introduced by precapping the living PFDMS with a 4/2 molar mixture of 1,1-diphenylethylene and 1,1-dimethylsilacyclobutane acting as a "carbanion pump" system. Subsequent addition of allyl glycidyl ether (AGE) leads to quantitatively functionalized PFDMS-AGE polymers with molecular weights between 1500 and 15 400 g mol-1 and polydispersity indices ≤1.10, carrying one hydroxyl group and an additional allylic double bond. PFDMS-AGE was then applied as a macroinitiator for the living anionic ring-opening polymerization of ethylene oxide (EO) to generate amphiphilic and water-soluble poly(ferrocenyldimethylsilane-b-ethylene oxide) block copolymers with a low polydispersity index. All polymers have been characterized by 1H NMR spectroscopy, DOSY 1H NMR spectroscopy, size exclusion chromatography (SEC), and MALDI-ToF mass spectrometry. In addition, for the characterization of the morphology of the PFDMS-b-PEO block copolymers transmission electron microscopy (TEM) was performed in methanol, confirming the formation of cylindrical micelles with an organometallic core and polyether corona.