6.2 Hybridization of Thiol–Ene Chemistry Hydrogels for Biomedical Applications

Light-triggered thiol–ene polymerization is a powerful tool for synthesizing hydrogels that are aimed to be applied in situ or used as 3D scaffolds. Thiol–ene reactions are a class of click transformations that involve free-radical-mediated addition of electron-rich thiol groups to electron-poor carbon–carbon double bonds. When tuned with homopolymerization of the carbon–carbon double bonds, the resultant hydrogel properties can be finely adjusted. In this review, commonly used methods for modifying polymers with thiol groups or double bonds are discussed, and strategies to overcome flaws in thiol–ene hydrogels are provided. Emphasis is given to the application and outlook of thiol–ene cross-linked hydrogels.

Amphiphilic and Perfluorinated Poly(3-Hydroxyalkanoate) Nanocapsules for 19F Magnetic Resonance Imaging

Nanoparticles have recently emerged as valuable tools in biomedical imaging techniques. Here PEGylated and fluorinated nanocapsules based on poly(3-hydroxyalkanoate) containing a liquid core of perfluorooctyl bromide PFOB were formulated by an emulsion-evaporation process as potential 19F MRI imaging agents. Unsaturated poly(hydroxyalkanoate), PHAU, was produced by marine bacteria using coprah oil and undecenoic acid as substrates. PHA-g-(F; PEG) was prepared by two successive controlled thiol-ene reactions from PHAU with firstly three fluorinated thiols having from 3 up to 17 fluorine atoms and secondly with PEG-SH. The resulting PHA-g-(F; PEG)-based PFOB nanocapsules, with a diameter close to 250–300 nm, are shown to be visible in 19F MRI with an acquisition time of 15 min. The results showed that PFOB-nanocapsules based on PHA-g-(F; PEG) have the potential to be used as novel contrast agents for 19F MRI.

Lawsone Derivatives as Efficient Photopolymerizable Initiators for Free-Radical, Cationic Photopolymerizations, and Thiol—Ene Reactions

Two new photopolymerizable vinyl (2-(allyloxy) 1,4-naphthoquinone, HNQA) and epoxy (2-(oxiran-2yl methoxy) 1,4-naphthoquinone, HNQE) photoinitiators derived from lawsone were designed in this paper. These new photoinitiators can be used as one-component photoinitiating systems for the free-radical photopolymerization of acrylate bio-based monomer without the addition of any co-initiators. As highlighted by the electron paramagnetic resonance (EPR) spin-trapping results, the formation of carbon-centered radicals from an intermolecular H abstraction reaction was evidenced and can act as initiating species. Interestingly, the introduction of iodonium salt (Iod) used as a co-initiator has led to (1) the cationic photopolymerization of epoxy monomer with high final conversions and (2) an increase of the rates of free-radical polymerization of the acrylate bio-based monomer; we also demonstrated the concomitant thiol–ene reaction and cationic photopolymerizations of a limonene 1,2 epoxide/thiol blend mixture with the HNQA/Iod photoinitiating system.

Enzymatic C-N Bond Formation via Aerobic Nitroso Ene Reactions

<p><i>A novel biocatalytic protocol enables the direct and selective introduction of nitrogen functionalities via activation of allylic C-H bonds. Utilizing an oxidase/peroxidase couple for the formal dehydrogenation of N-hydroxycarbamates and hydroxamic acids with air as terminal oxidant, acylnitroso species are generated under particularly mild aqueous conditions. The reactive intermediates undergo C-N bond formation through an ene-type mechanism and provide high yields both in intramolecular and intermolecular enzymatic aminations. Investigations on alternative reaction pathways and labelling studies provide more insights into this unprecedented biocatalytic promiscuity of classical oxidoreductases as catalysts for nitroso ene-based transformations.</i></p>

Enzymatic C-N Bond Formation via Aerobic Nitroso Ene Reactions

<p><i>A novel biocatalytic protocol enables the direct and selective introduction of nitrogen functionalities via activation of allylic C-H bonds. Utilizing an oxidase/peroxidase couple for the formal dehydrogenation of N-hydroxycarbamates and hydroxamic acids with air as terminal oxidant, acylnitroso species are generated under particularly mild aqueous conditions. The reactive intermediates undergo C-N bond formation through an ene-type mechanism and provide high yields both in intramolecular and intermolecular enzymatic aminations. Investigations on alternative reaction pathways and labelling studies provide more insights into this unprecedented biocatalytic promiscuity of classical oxidoreductases as catalysts for nitroso ene-based transformations.</i></p>

Rheometer Evidences for the Co-Curing Effect of a Bismaleimide in Conjunction with the Accelerated Sulfur on Natural Rubber/Chloroprene Rubber Blends

The rheometer curing curves of neat natural rubber (NR) and neat chloroprene rubber (CR) with maleide F (MF) exhibit considerable crosslinking torque at 180 °C. This indicates that MF can crosslink both these rubbers via Alder-ene reactions. Based on this knowledge, MF has been introduced as a co-crosslinking agent for a 50/50 blend of NR and CR in conjunction with accelerated sulfur. The delta (Δ) torque obtained from the curing curves of a blend with the addition of 1 phr MF was around 62% higher than those without MF. As the content of MF increased to 3 phr, the Δ torque was further raised to 236%. Moreover, the mechanical properties, particularly the tensile strength of the blend with the addition of 1 phr MF in conjunction with the accelerated sulfur, was around 201% higher than the blend without MF. The overall tensile properties of the blends cured with MF were almost retained even after ageing the samples at 70 °C for 72 h. This significant improvement in the curing torque and the tensile properties of the blends indicates that MF can co-crosslink between NR and CR via the Diels–Alder reaction.