Remarkable Versatility of Silane/Iodonium Salt as Redox Free Radical, Cationic, and Photopolymerization Initiators

Photoinitiated Cationic Ring-Opening Polymerization of Octamethylcyclotetrasiloxane

Photochemical techniques have recently been revitalized as they can readily be adapted to different polymerization modes to yield a wide range of complex macromolecular structures. However, the implementation of the photoinduced cationic methods in the polymerization of cyclic siloxane monomers has scarcely been investigated. Octamethylcyclotetrasiloxane (D4) is an important monomer for the synthesis of polydimethylsiloxane (PDMS) and its copolymers. In this study, the cationic ring-opening polymerization (ROP) of D4, initiated by diphenyl iodonium hexafluorophosphate (DPI), has been studied. Both direct and indirect initiating systems acting at broad wavelength using benzophenone and pyrene were investigated. In both systems, photochemically generated protonic acids and silylium cations are responsible for the polymerization. The kinetics of the polymerization are followed by viscosimetry and GPC analyses. The reported approach may overcome the problems associated with conventional methods and therefore represents industrial importance for the fabrication of polysiloxanes.

Thiophenium Salts as New Oxidant for Redox Polymerization under Mild- and Low-Toxicity Conditions

In mild conditions (under air, room temperature, no monomer purification and without any energy activation), redox free radical polymerization (RFRP) is considered as one of the most effective methods to polymerize (meth)acrylate monomers. In the past several years, there has been a growing interest in research on the development of new redox initiating systems (RISs), thanks mainly to the evolution of toxicity labeling and the stability issue of the current RIS based on peroxide and aromatic amine. In this study, a new, low-toxicity RIS based on thiophenium salt as the oxidant species is presented with various reductive species. The reactivity and the stability of the proposed RISs are investigated and the synthesis of new thiophenium salts reported.

A Sprayable and Visible Light Rapid-Cured Strippable Film for Surface Radioactive Decontamination

Strippable film is effective for removing radioactive contamination. However, it still has some limitations, such as the long curing time (about 30 min~24 h) and the requirement of organic solvents. To address these issues, we report a simple protocol to prepare strippable decontamination films using liquid polybutadiene (LPB) and tert-butyl acrylate (TBA) as the raw materials without solvent and using camphorquinone/ethyl 4-dimethylaminobenzoate (CQ/EDB) as a photoinitiator, where the film was formed under household LED panel light or daylight irradiation for about 540 s. After a thorough study of viscosity, real-time Fourier transform infrared (RT-FTIR spectra), gel and volatile organic compound (VOC) contents, mechanical properties and decontamination efficiency, the optimum composition and curing conditions were determined for the decontamination strippable film. VOC content is as low as 12.7 ± 0.7% and the resultant strippable film exhibits good mechanical performances with a tensile strength of up to 5.4 ± 0.4 MPa and elongation of up to 66.6 ± 13%. Most important, the decontamination efficiencies of this strippable film for ¹³³CsCl on glass, ceramic and metal surfaces reach up to 98.1%, 94.3% and 97.6%, respectively.

A case of chain propagation: α-aminoalkyl radicals as initiators for aryl radical chemistry

The generation of aryl radicals from the corresponding halides by redox chemistry is generally considered a difficult task due to their highly negative reduction potentials. Here we demonstrate that α-aminoalkyl radicals can be used as both initiators and chain-carriers for the radical coupling of aryl halides with pyrrole derivatives, a transformation often employed to evaluate new highly reducing photocatalysts. This mode of reactivity obviates for the use of strong reducing species and was also competent in the formation of sp² C-P bonds. Mechanistic studies have delineated some of the key features operating that trigger aryl radical generation and also propagate the chain process.

Review on UV-Induced Cationic Frontal Polymerization of Epoxy Monomers

Ultraviolet (UV)-induced cationic frontal polymerization has emerged as a novel technique that allows rapid curing of various epoxy monomers upon UV irradiation within a few seconds. In the presence of a diaryliodonium salt photoinitiator together with a thermal radical initiator, the cationic ring opening polymerization of an epoxide monomer is auto-accelerated in the form of a self-propagating front upon UV irradiation. This hot propagating front generates the required enthalpy to sustain curing reaction throughout the resin formulation without further need for UV irradiation. This unique reaction pathway makes the cationic frontal polymerization a promising route towards the efficient curing of epoxy-based thermosetting resins and related composite structures. This review represents a comprehensive overview of the mechanism and progress of UV-induced cationic frontal polymerization of epoxy monomers that have been reported so far in literature. At the same time, this review covers important aspects on the frontal polymerization of various epoxide monomers involving the chemistry of the initiators, the effect of appropriate sensitizers, diluents and fillers.

Chemiluminescence-Induced Free Radical-Promoted Cationic Polymerization

Chemiluminescence (CL) has recently been featured as a new external light source for various photoinduced reactions with attractive features such as eliminating continuous energy supply and advanced light source setups. In the present study, the free-radical-promoted cationic polymerization of cyclohexene oxide, n-butyl vinyl ether, and N-vinyl carbazole under CL irradiation is described. The method is based on the visible-light-induced generation of electron donor radicals from bis-(4-methoxybenzoyl)diethyl germane (BAG), bis(2,4,6-trimethylbenzoyl) phenyl phosphinate, and camphorquinone by CL illumination followed by electron transfer to diphenyl iodonium hexafluorophosphate (Ph₂ I⁺ PF₆ ^- ) to form corresponding cations capable of initiating cationic polymerization. The applicability of the process to network formation is also demonstrated by using a bifunctional monomer, tri(ethylene glycol) divinyl ether.

Photoinduced free radical promoted cationic polymerization 40 years after its discovery

Free radical promoted cationic photopolymerization has been described with its historical background, main principles and usage in polymer synthesis.

Silane/iodonium salt as redox/thermal/photoinitiating systems in radical and cationic polymerizations for laser write and composites

Methylphenylsilane (MPS) and octadecylsilane (ODS) are originally proposed here combined with iodonium salt as a highly versatile triple initiating system for redox, photo and thermal polymerizations (both radical and cationic).