Nanoscale Control of the Surface Functionality of Polymeric 2D Materials.

Typically, 2D nanosheets have a homogeneous surface, making them a major challenge to structure. This study proposes a novel concept of 2D organic nanosheets with a heterogeneously functionalized surface. This work achieves this by consecutively crystallizing two precisely synthesized polymers with different functional groups in the polymer backbone in a two-step process. First, the core platelet is formed and then the second polymer is crystallized around it. As a result, the central area of the platelets has a different surface functionality than the periphery. This concept offers two advantages: the resulting polymeric 2D platelets are stable in dispersion, which simplifies further processing and makes both crystal surfaces accessible for subsequent functionalization. Additionally, a wide variety of polymers can be used, making the process and the choice of surface functionalization very flexible.

RNA-inspired intramolecular transesterification accelerates the hydrolysis of polyethylene-like polyphosphoesters.

To synthesize new (bio)degradable alternatives to commodity polymers, adapting natural motives can be a promising approach. We present the synthesis and characterization of degradable polyethylene (PE)-like polyphosphoesters, which exhibit increased degradation rates due to an intra-molecular transesterification similar to RNA. An α,ω-diene monomer was synthesized in three steps starting from readily available compounds. By acyclic diene metathesis (ADMET) polymerization, PE-like polymers with molecular weights up to 38 400 g mol-1 were obtained. Post-polymerization functionalization gave fully saturated and semicrystalline polymers with a precise spacing of 20 CH2 groups between each phosphate group carrying an ethoxy hydroxyl side chain. This side chain was capable of intramolecular transesterification with the main-chain similar to RNA-hydrolysis, mimicking the 2'-OH group of ribose. Thermal properties were characterized by differential scanning calorimetry (DSC (T m ca. 85 °C)) and the crystal structure was investigated by wide-angle X-ray scattering (WAXS). Polymer films immersed in aqueous solutions at different pH values proved an accelerated degradation compared to structurally similar polyphosphoesters without pendant ethoxy hydroxyl groups. Polymer degradation proceeded also in artificial seawater (pH = 8), while the polymer was stable at physiological pH of 7.4. The degradation mechanism followed the intra-molecular "RNA-inspired" transesterification which was detected by NMR spectroscopy as well as by monitoring the hydrolysis of a polymer blend of a polyphosphoester without pendant OH-group and the RNA-inspired polymer, proving selective hydrolysis of the latter. This mechanism has been further supported by the DFT calculations. The "RNA-inspired" degradation of polymers could play an important part in accelerating the hydrolysis of polymers and plastics in natural environments, e.g. seawater.

Terpyridine-Induced Folding of Anisotropic Polyphosphoester Platelets.

The folding of macromolecules is of great importance in nature. Also in synthetic polymer chemistry, single-chain nanoparticles, i.e. folding synthetic macromolecules, are a current research topic to mimic protein folding and to generate well-defined structures. Here, we present the "folding" of anisotropic polymer platelets to further mimic natural folding processes on the (sub)micrometer scale. We report on the synthesis of terpyridine-functionalized long-chain polyphosphoesters by acyclic diene metathesis polymerization that can crystallize in dilute solution into anisotropic polymer crystal platelets. As the terpyridine units are expelled to the platelet surface, terpyridine-metal interactions could be induced by adding nickel(II) bis(acetylacetonate) (Ni(acac)2) to the platelet dispersion in ethyl acetate. These polymer crystals were "folded" to homogeneous nanoparticles with a wrinkled structure, which were visualized by transmission electron microscopy (TEM). The size and size distribution of the obtained assemblies could be altered by varying the concentration of Ni(acac)2. In contrast, no wrinkled structures but rather intrachain cross-linking was observed, when Ni(acac)2 was added to the homogeneous polymer solution before crystallization. We believe that this concept of "folding" anisotropic polymer platelets will further enhance the control of morphologies on (sub)micrometer particles and might be useful for catalysis or separation.

Vitamin C Loaded Polyethylene: Synthesis and Properties of Precise Polyethylene with Vitamin C Defects via Acyclic Diene Metathesis Polycondensation.

A polyethylene-like polymer with an in-chain vitamin C group was synthesized by olefin metathesis polymerization. Here, we describe both the synthesis and a comprehensive physical characterization. Because of the olefin metathesis synthesis, the vitamin C groups are equidistantly arranged in the polyethylene (PE) main chain. Their separation was adjusted to 20 CH2 units. After hydrogenation, a semicrystalline polymer is obtained that is soluble in polar solvents. Because of its size and steric effect, the vitamin C acts as a chain defect, which is expelled from the crystal lattice, yielding a lamellar crystal with a homogeneous thickness corresponding to the interdefect distance. The physical properties were examined by various methods including differential scanning calorimetry, X-ray scattering, and transmission electron microscopy. We show that vitamin C retains its radical scavenger properties despite being incorporated into a polyethylene chain. Furthermore, we demonstrate that it is degrading in alkaline conditions. To complete its suitability as a biocompatible material, cytotoxicity and cell uptake experiments were performed. We show that the polymer is nontoxic and that it is taken up in nanoparticular form via endocytosis processes into the cytoplasm of cells.

One-Step Ring Opening Metathesis Block-Like Copolymers and their Compositional Analysis by a Novel Retardation Technique.

Using a one-step synthetic route for block copolymers avoids the repeated addition of monomers to the polymerization mixture, which can easily lead to contamination and, therefore, to the unwanted termination of chain growth. For this purpose, monomers (M1-M5) with different steric hindrances and different propagation rates are explored. Copolymerization of M1 (propagating rapidly) with M2 (propagating slowly), M1 with M3 (propagating extremely slowly) and M4 (propagating rapidly) with M5 (propagating slowly) yielded diblock-like copolymers using Grubbs' first (G1) or third generation catalyst (G3). The monomer consumption was followed by 1 H NMR spectroscopy, which revealed vastly different reactivity ratios for M1 and M2. In the case of M1 and M3, we observed the highest difference in reactivity ratios (r1 =324 and r2 =0.003) ever reported for a copolymerization method. A triblock-like copolymer was also synthesized using G3 by first allowing the consumption of the mixture of M1 and M2 and then adding M1 again. In addition, in order to measure the fast reaction rates of the G3 catalyst with M1, we report a novel retardation technique based on an unusual reversible G3 Fischer-carbene to G3 benzylidene/alkylidene transformation.

One-Step Preparation of Fuel-Containing Anisotropic Nanocapsules with Stimuli-Regulated Propulsion.

One of the dreams of nanotechnology is to create tiny objects, nanobots, that are able to perform difficult tasks in dimensions and locations that are not directly accessible. One basic function of these nanobots is motility. Movements created by self-propelled micro- and nanovehicles are usually dependent on the production of propellants from catalytic reactions of fuels present in the environment. Developing self-powered nanovehicles with internally stored fuels that display motion regulated by external stimuli represents an intriguing and challenging alternative. Herein, a one-step preparation of fuel-containing nanovehicles that feature a motion that can be regulated by external stimuli is reported. Nanovehicles are prepared via a sol-gel process confined at the oil/water interface of miniemulsions. The nanovehicles display shapes ranging from mushroom-like to truncated cones and a core-shell structure so that the silica shell acts as a hull for the nanovehicles while the core is used to store the fuel. Azo-based initiators are loaded in the nanovehicles, which are activated to release nitrogen gas upon increase of temperature or exposure to UV light. Enhanced diffusion of nanovehicles is achieved upon decomposition of the fuel.

Long-Chain Polyorthoesters as Degradable Polyethylene Mimics.

The persistence of commodity polymers makes the research for degradable alternatives with similar properties necessary. Degradable polyethylene mimics containing orthoester groups were synthesized by olefin metathesis polymerization for the first time. Ring-opening metathesis copolymerization (ROMP) of 1,5-cyclooctadiene with four different cyclic orthoester monomers gave linear copolymers with molecular weights up to 38000 g mol-1. Hydrogenation of such copolymers produced semicrystalline polyethylene-like materials, which were only soluble in hot organic solvents. The crystallinity and melting points of the materials were controlled by the orthoester content of the copolymers. The polymers crystallized similar to polyethylene, but the relatively bulky orthoester groups were expelled from the crystal lattice. The lamellar thickness of the crystals was dependent on the amount of the orthoester groups. In addition, the orthoester substituents influenced the hydrolysis rate of the polymers in solution. Additionally, we were able to prove that non-hydrogenated copolymers with a high orthoester content were biodegraded by microorganisms from activated sludge from a local sewage plant. In general, all copolymers hydrolyzed under ambient conditions over a period of several months. This study represents the first report of hydrolysis-labile and potentially biodegradable PE mimics based on orthoester linkages. These materials may find use in applications that require the relatively rapid release of cargo, e.g., in biomedicine or nanomaterials.