Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

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Introduction of Fluorine and Fluorine-Containing Functional Groups

Die wichtigsten konzeptionellen Fortschritt auf dem Gebiet der Fluorierungen der letzten zehn Jahre wurden durch die Organo- und Ubergangsmetallkatalyse ermoglicht. Die schwierigste Umwandlung ist weiterhin die Bildung der C-F-Stammbindung, was in erster Linie eine Folge der hohen Hydratationsenergie von Fluorid, starker Metall-Fluor-Bindungen und der hoch polarisierten Bindungen zu Fluor ist. Den meisten Fluorierungen fehlt es immer noch an Allgemeingultigkeit, Vorhersagbarkeit und Kosteneffizienz. Trotz der Einschrankungen sind fluorierte Verbindungen mit modernen Fluorierungsmethoden leichter zuganglich als je zuvor. Vor allem beginnen sich die modernen Methoden auf Forschungsgebiete auszuwirken, die keine grosen Materialmengen benotigen, z. B. die Wirkstoffentwicklung und die Positronenemissionstomographie. Diese Entwicklungen werden in diesem Aufsatz zusammengefasst und vor dem Hintergrund konventioneller Fluorierungsmethoden diskutiert.

Einführung von Fluor und fluorhaltigen funktionellen Gruppen

ilicone elastomers are promising materials for dielectric elastomer transducers (DETs) due to their superior properties such as high efficiency, reliability and fast response times. DETs consist of thin elastomer films sandwiched between compliant electrodes, and they constitute an interesting class of transducer due to their inherent lightweight and potentially large strains. For the field to progress towards industrial implementation, a leap in material development is required, specifically targeting longer lifetime and higher energy densities to provide more efficient transduction at lower driving voltages. In this review, the current state of silicone elastomers for DETs is summarised and critically discussed, including commercial elastomers, composites, polymer blends, grafted elastomers and complex network structures. For future developments in the field it is essential that all aspects of the elastomer are taken into account, namely dielectric losses, lifetime and the very often ignored polymer network integrity and stability.

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The Current State of Silicone-Based Dielectric Elastomer Transducers.

From landfilling to vitrimer chemistry in rubber life cycle

Photo-crosslinkable, self-healable and reprocessable rubbers

Fluorination of elastomer materials

At present, the most common used crosslinking process for carboxylated nitrile butadiene rubber (XNBR) latex is an accelerated sulfur curing system with zinc oxide. To avoid allergenic reactions related to residual accelerator levels in dipped XNBR latex articles such as medical gloves, a dual curing process has been developed combining thermal and photochemical crosslinking reactions. The two-step procedure involves the formation of covalent and ionic bonds to ensure good mechanical properties of the final products. The photochemical thiol-ene reaction is used to generate covalent crosslinks between the remaining CC double bonds of the butadiene units whereas the carboxylic moieties are conventionally cured with divalent metal oxides (ZnO) under elevated temperature (formation of ionic crosslinks). The photochemical curing step is carried out both in the latex phase using a falling film photoreactor (prevulcanization) as well as in the solid phase by UV irradiation of dried XNBR films (postvulcanization). The mechanical properties and crosslink densities of the cured XNBR films are determined and the influence of selected curing parameters is assessed. The results give evidence that a combined approach of thermal prevulcanization and photochemical postvulcanization makes the production of latex articles (e.g., gloves) with tailored properties and good skin compatibility feasible. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Dual crosslinking of carboxylated nitrile butadiene rubber latex employing the thiol-ene photoreaction

Photo-vulcanization using thiol-ene chemistry: Film formation, morphology and network characteristics of UV crosslinked rubber latices

The photochemical prevulcanization of natural rubber (NR) latex via the thiol-ene reaction is a new approach aiming at the replacement of noxious processing agents used in conventional sulfur vulcanization processes (e.g., accelerators) together with cost saving options. The crosslinking reaction involves the excitation of a selected photoinitiator with ultraviolet (UV) light which is followed by the formation of thioether links due to the thiol-ene addition reaction. The photochemical process is carried out in a falling film photoreactor which provides not only a continuous prevulcanization process but also exhibits a technology which is already commercially well established. The main advantage of the falling film process lies in the short prevulcanization time and the mild reaction temperature. Following the idea of the manufacture of low-allergenic surgical gloves made from NR latex without compromising on the glove quality by means of physical performance, crucial process parameters of the falling film process have been determined and characterized in this study. Surgical gloves were made from the photocured NR latex which was prevulcanized using selected process conditions. The physical properties including tensile strength, elongation, modulus, and crosslink density were examined together with the aging stability and the stability against high-energy radiation (sterilization with gamma rays). It was found that the UV light intensity, the number of illumination cycles, the choice of photoinitiator, and the thiol crosslinker play an important part in the glove quality. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Armin Holzner

Papers

Fluorination of elastomer materials

Dual crosslinking of carboxylated nitrile butadiene rubber latex employing the thiol-ene photoreaction

Photo-vulcanization using thiol-ene chemistry: Film formation, morphology and network characteristics of UV crosslinked rubber latices

Characteristics of the photochemical prevulcanization in a falling film photoreactor

Investigation of different influences on the fatigue behaviour of industrial rubbers