Mechanistic Investigation of the Silane, Germane, and Stannane Behavior When Incorporated in Type I and Type II Photoinitiators of Polymerization in Aerated Media

Electron Paramagnetic Resonance Spin Trapping (EPR–ST) Technique in Photopolymerization Processes

To face economic issues of the last ten years, free-radical photopolymerization (FRP) has known an impressive enlightenment. Multiple performing photoinitiating systems have been designed to perform photopolymerizations in the visible or near infrared (NIR) range. To fully understand the photochemical mechanisms involved upon light activation and characterize the nature of radicals implied in FRP, electron paramagnetic resonance coupled to the spin trapping (EPR–ST) method represents one of the most valuable techniques. In this context, the principle of EPR–ST and its uses in free-radical photopolymerization are entirely described.

Enhanced Mechanical and Thermal Properties of Stereolithography 3D Printed Structures by the Effects of Incorporated Controllably Annealed Anatase TiO2 Nanoparticles

Fabrication of low-cost, durable and efficient metal oxide nanocomposites were successfully synthesized and reinforced with photo-resin via 3-dimensional printing. Here, we put forward a novel approach to enhance the mechanical and thermal behaviors of stereolithography (SLA) 3D printed architecture by adding TiO₂ nanoparticles (TNPs) in different crystalline phases (anatase and rutile), which were obtained at different annealing temperatures from 400 °C to 1000°C. The heat-treated anatase TNPs were scrutinized by X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, diffusive reflectance spectroscopy (DRS), and transmission electron microscopy (TEM) analysis. Among all the samples, at 800 °C, annealed anatase TNPs exposed a highly crystalline anatase phase, having a low energy bandgap and a comparably high tensile strength (47.43 MPa) and high elastic modulus (2.261 GPa) for the 3D printed samples, showing improvement by 103% and 32%, respectively, compared with the printed pristine stereolithography resin (SLR) sample. Moreover, enhanced storage modulus and tan δ values were achieved via the better interfacial interactions between the incorporated nanofillers and the SLR matrix. In addition to this, enhanced thermal conductivity and thermal stability of the SLR matrix were also noted. The low energy bandgap and nanoscale size of the fillers helped to achieve good dispersion and allowed the UV light to penetrate at a maximum depth through the photo resin.

Organometallic vs organic photoredox catalysts for photocuring reactions in the visible region

Recent progresses achieved in terms of synthetic procedures allow now the access to polymers of well-defined composition, molecular weight and architecture. Thanks to these recent progresses in polymer engineering, the scope of applications of polymers is far wider than that of any other class of material, ranging from adhesives, coatings, packaging materials, inks, paints, optics, 3D printing, microelectronics or textiles. From a synthetic viewpoint, photoredox catalysis, originally developed for organic chemistry, has recently been applied to the polymer synthesis, constituting a major breakthrough in polymer chemistry. Thanks to the development of photoredox catalysts of polymerization, a drastic reduction of the amount of photoinitiators could be achieved, addressing the toxicity and the extractability issues; high performance initiating abilities are still obtained due to the catalytic approach which regenerates the catalyst. As it is a fast-growing field, this review will be mainly focused on an overview of the recent advances concerning the development of organic and organometallic photoredox catalysts for the photoreticulation of multifunctional monomers for a rapid and efficient access to 3D polymer networks.

Photo-polymerization properties of type-II photoinitiator systems based on 2-chlorohexaaryl biimidazole (o-Cl-HABI) and various N-phenylglycine (NPG) derivatives

A series of p-substituted NPG derivatives (Cl-NPG, OMe-NPG and NO₂-NPG) comprising different push-pull characteristics have been synthesized and characterized. The NPG derivatives have good thermal stability and red shifted absorption when compared with the original N-phenyl glycine (NPG) compound. These NPGs were selected in combination with 2-chlorohexaaryl biimidazole (o-Cl-HABI) for Type II free radical polymerization (FRP). Commercial NPG was also mixed with o-Cl-HABI for comparison. Their photo-polymerization properties were investigated by the gel fraction method in a nitrogen atmosphere. Electron transfer efficiencies for those Type II packages were studied by cyclic voltammetry (CV) and free energy change ΔG results.

A novel photoinitiating system producing germyl radicals for the polymerization of representative methacrylate resins: Camphorquinone/R3GeH/iodonium salt

OBJECTIVES

The aim of our study is to find an amine free photoinitiating system (PIS) for the polymerization of representative dental methacrylate resins. A photoinitiating system (PIS) based on camphorquinone (CQ)/triphenylgermanium hydride/diphenyl iodonium hexafluorophosphate is proposed and compared to the conventional CQ/amine couple. The polymerization monitoring of thin (∼20μm) and thick (1.4mm) samples of a bisphenol A-glycidyl methacrylate (Bis-GMA)/triethyleneglycol dimethacrylate (TEGDMA) blend (70%/30% w/w) and of a urethane dimethacrylate (UDMA) upon exposure to a commercial blue LED centered at 477nm under air or in laminate is described. Finally, the impact of the photoinitiating system composition on the final polymer color is evaluated in detail.

METHODS

FTIR and DSC experiments are used to record the photopolymerization profiles. ESR spectrometry and steady state photolysis are used to detect the produced radicals. Color measurements are carried out to determine the key parameters in the bleaching of the different dental formulations.

RESULTS

The efficiency of the newly proposed PISs for the photopolymerization of BisGMA/TEGDMA and UDMA for thin (20μm) or for thick (1.4mm) samples upon exposure to a dental blue LED under air is excellent. It is noticeably higher than that of the CQ/amine reference couple. Excellent bleaching properties are also observed under irradiation in presence of the new PISs. A good correlation is found between the sample bleaching and the amount of Ph3GeH in the formulation. The excited state processes could be established. The overall chemical mechanisms for the initiation step were also clarified.

Tris(trimethylsilyl)silane as a co-initiator for dental adhesive: Photo-polymerization kinetics and dynamic mechanical property

OBJECTIVES

The purpose of this study was to evaluate the polymerization behavior of a model dentin adhesive with tris(trimethylsilyl)silane (TTMSS) as a co-initiator, and to investigate the polymerization kinetics and mechanical properties of copolymers in dry and wet conditions.

METHODS

A co-monomer mixture based on HEMA/BisGMA (45/55, w/w) was used as a model dentin adhesive. The photoinitiator system included camphorquinone (CQ) as the photosensitizer and the co-initiator was ethyl-4-(dimethylamino) benzoate (EDMAB) or TTMSS. Iodonium salt, diphenyliodonium hexafluorophosphate (DPIHP) serving as a catalyst, was selectively added into the adhesive formulations. The control and the experimental formulations were characterized with regard to the degree of conversion (DC) and dynamic mechanical properties under dry and wet conditions.

RESULTS

In two-component photoinitiator system (CQ/TTMSS), with an increase of TTMSS concentration, the polymerization rate and DC of CC double bond increased, and showed a dependence on the irradiation time and curing light intensity. The copolymers that contained the three-component photoinitiator system (CQ/TTMSS/DPIHP) showed similar dynamic mechanical properties, under both dry and wet conditions, to the EDMAB-containing system.

SIGNIFICANCE

The DC of formulations using TTMSS as co-initiator showed a strong dependence on irradiation time. With the addition of TTMSS, the maximum polymerization rate can be adjusted and the network structure became more homogenous. The results indicated that the TTMSS could be used as a substitute for amine-type co-initiator in visible-light induced free radical polymerization of methacrylate-based dentin adhesives.

Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones

This review focuses on the advancements and progress in photoinitiated polymerization techniques mediated by thioxanthone (TX) and its derivatives.

Zirconocene Dichloride: An Efficient Cleavable Photoinitiator Allowing the in Situ Production of Zr-Based Nanoparticles Under Air

Cp₂ZrCl₂ is presented as both an effective photoinitiator and additive for radical photopolymerization reactions in aerated conditions. This compound is characterized by remarkable properties: (i) an efficiency higher than that of a reference Type I photoinitiator (2,2-dimethoxy-2-phenylacetophenone, DMPA), (ii) an excellent ability, when added to DMPA, to overcome the oxygen inhibition of the polymerization, and (iii) a never reported in situ photoinduced and oxygen-mediated formation of zirconium-based nanoparticles (diameter ranging from 50 to 70 nm). The photochemical properties of Cp₂ZrCl₂ are investigated by steady state photolysis and electron spin resonance (ESR) experiments. The high reactivity of this compound is ascribed to a bimolecular homolytic substitution S_H2 (clearly characterized by molecular orbital calculations) which converts the peroxyls into new polymerization-initiating radicals and oxygenated Zr-based nanoparticles.

Recent applications of the (TMS)3SiH radical-based reagent

This review article focuses on the recent applications of tris(trimethylsilyl)silane as a radical-based reagent in organic chemistry. Numerous examples of the successful use of (TMS)(3)SiH in radical reductions, hydrosilylation and consecutive radical reactions are given. The use of (TMS)(3)SiH allows reactions to be carried out under mild conditions with excellent yields of products and remarkable chemo-, regio-, and stereoselectivity. The strategic role of (TMS)(3)SiH in polymerization is underlined with emphasis on the photo-induced radical polymerization of olefins and photo-promoted cationic polymerization of epoxides.

New boryl radicals derived from N-heteroaryl boranes: generation and reactivity

Recently, boryl radicals have been the subject of revived interest. These structures were generated by hydrogen-abstraction reactions from the corresponding boranes (i.e., from amine or phosphine boranes). However, the classical issue remains their high B--H bond-dissociation energy (BDE), thereby preventing a very efficient hydrogen-abstraction process. In the present paper, new N-heteroaryl boranes that exhibiting low B--H BDE are presented; excellent hydrogen-transfer properties have been found. Both the generation and the reactivity of the associated boryl radicals have been investigated through their direct observation in laser flash photolysis. The boryl radical interactions with double bonds, oxygen, oxidizing agent, and alkyl halides have been studied. Some selected applications of N-heteroaryl boryl radicals as new polymerization-initiating structures are proposed to evidence their high intrinsic reactivity.

New thioxanthone and xanthone photoinitiators based on silyl radical chemistry

New cleavable photoinitiators are presented. As demonstrated here, combining the thioxanthone/xanthone (or other PI skeletons) and silyl chemistries should open up a new promising direction of research.

Recent advances in sunlight induced polymerization: role of new photoinitiating systems based on the silyl radical chemistry

Silyl radical chemistry opens new opportunities for photoinitiating systems for sunlight polymerization.

Photochemical Methods for the Preparation of Complex Linear and Cross-linked Macromolecular Structures

In this contribution, the current state of the art is summarized and an overview of photoinitiating systems for both radical and cationic polymerizations and their potential application in the preparation of complex linear and cross-linked macromolecular structures are described. Recent relevant studies have been devoted to developing novel free radical and cationic photoinitiators having spectroscopic sensitivity in the near-UV or visible range. Photoinitiated controlled radical polymerization methods leading to tailor-made polymers with predetermined structure and architecture are briefly presented. Several synthetic methodologies for the preparation of epoxy and (meth)acrylate based formulations containing clay or metal nanoparticles are also summarized. The nanoparticles are homogenously distributed in the network without macroscopic agglomeration. Applicability to both free radical and cationic systems is demonstrated.