Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications

Over the past several decades, photopolymerization has become an active research field, and the ongoing efforts to develop new photoinitiating systems are supported by the different applications in which this polymerization technique is involved-including dentistry, 3D and 4D printing, adhesives, and laser writing. In the search for new structures, bis-chalcones that combine two chalcones' moieties within a unique structure were determined as being promising photosensitizers to initiate both the free-radical polymerization of acrylates and the cationic polymerization of epoxides. In this review, an overview of the different bis-chalcones reported to date is provided. Parallel to the mechanistic investigations aiming at elucidating the polymerization mechanisms, bis-chalcones-based photoinitiating systems were used for different applications, which are detailed in this review.

New Pure Organic and Peroxide-Free Redox Initiating Systems for Polymerization in Mild Conditions

Redox initiating systems (RISs) are highly worthwhile for polymerization in mild conditions (at room temperature—RT) without external thermal or light activation. With high performance redox initiating systems RIS, the free radical polymerization FRP can even be carried out under air and without inhibitors/stabilizers removal from the monomers/resins. However, efficient RISs are still based on peroxides or metal complexes. In this work, a pure organic and peroxide-free RIS is presented based on the interaction of a well-selected triarylamine derivative (T4epa) with iodonium salt used as reducing and oxidizing agents, respectively. The redox polymerization (Redox FRP) was followed through pyrometry and thermal imaging experiments. Remarkably, a full control of the work time as well as a high reactivity is observed for mild conditions.

Recent Advances on Copper Complexes as Visible Light Photoinitiators and (Photo) Redox Initiators of Polymerization

Metal complexes are used in numerous chemical and photochemical processes in organic chemistry. Metal complexes have not been excluded from the interest of polymerists to convert liquid resins into solid materials. If iridium complexes have demonstrated their remarkable photochemical reactivity in polymerization, their high costs and their attested toxicities have rapidly discarded these complexes for further developments. Conversely, copper complexes are a blooming field of research in (photo) polymerization due to their low cost, easy syntheses, long-living excited state lifetimes, and their remarkable chemical and photochemical stabilities. Copper complexes can also be synthesized in solution and by mechanochemistry, paving the way towards the synthesis of photoinitiators by Green synthetic approaches. In this review, an overview of the different copper complexes reported to date is presented. Copper complexes are versatile candidates for polymerization, as these complexes are now widely used not only in photopolymerization, but also in redox and photoassisted redox polymerization processes.

Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity

In recent years, polymerization processes activated by light have attracted a great deal of interest due to the wide range of applications in which this polymerization technique is involved. Parallel to the traditional industrial applications ranging from inks, adhesives, and coatings, the development of high-tech applications such as nanotechnology and 3D-printing have given a revival of interest to this polymerization technique known for decades. To initiate a photochemical polymerization, the key element is the molecule capable to interact with light, i.e., the photoinitiator and more generally the photoinitiating system, as a combination of several components is often required to create the reactive species responsible for the polymerization process. With the aim of reducing the photoinitiator content while optimizing the polymerization yield and/or the polymerization speed, photocatalytic systems have been developed, enabling the photosensitizer to be regenerated during the polymerization process. In this review, an overview of the photocatalytic systems developed for polymerizations carried out under a low light intensity and visible light is provided. Over the years, a wide range of organometallic photocatalysts has been proposed, addressing both the polymerization efficiency and/or the toxicity, as well as environmental issues.

Sulfonium salt based charge transfer complexes as dual thermal and photochemical polymerization initiators for composites and 3D printing

Charge transfer complexes (CTCs) exhibit excellent initiating abilities as safe photo, thermal, or photo/thermal dual initiators, which cannot be achieved by classical initiators.

Ferrocene-based (photo)redox polymerization under long wavelengths

Ferrocene-based photoredox catalysis is proposed here for the first time.

New Synthetic Route to an Highly Efficient Photoredox Catalyst by Mechanosynthesis

Photoredox catalysis based on the [Cu(neo)(DPEphos)]BF₄ copper complex allowed getting a significant improvement of the polymerization performances (e.g., thick samples, coatings...) compared to that obtained with other benchmarked photoinitiators in both cationic (CP) and free radical polymerizations (FRP). Nevertheless, as for other copper complexes classically used as photoinitiators in polymer science, the synthesis of these complexes is carried out in a solvent; this fact remains an obstacle to their widespread use because of the cost associated with the use of a solvent and the complex synthesis procedure. In the present study, on the contrary, an outstanding efficient mechanosynthesis of [Cu(neo)(DPEphos)]BF₄-purity ≥95% outranking the previous Cu(I) mechanosynthesis-allowed (i) to divide the synthesis time by 170-fold (as only 5 min is necessary to get the complex), (ii) to lower the environmental impact and cut the synthetic costs associated with solvent usage, and (iii) to access a new Cu(I) complex with a counteranion that is impossible to introduce under the traditional chemistry methods (e.g., I^-). Reactivities of the mechanosynthesized copper complexes in resins (FRP and CP) confirmed the very high purity of the obtained copper complex by ¹H nuclear magnetic resonance spectroscopy and high-resolution electrospray ionization mass spectrometry.

Charge-Transfer Complexes as New Inhibitors/Photoinitiators for On-Demand Amine/Peroxide Redox Polymerization

Redox free-radical polymerizations have widespread applications but still clearly suffer from poor time control of the reaction. Currently, the workability (delay of the gel time) in redox polymerization after mixing is possible thanks to two main types of inhibitors (radical scavengers): phenols and nitroxides. Out of this trend, we propose in this work an alternative strategy for time delaying of the redox polymerization, which is based on charge-transfer complexes (CTCs). Thanks to iodonium salt complexation, the amine (here 4-N,N-trimethylaniline) is proposed to be stored in a CTC equilibrium and is slowly released over a period of time (as a result of the consumption of free amines by peroxides). This alternative strategy allowed us to double the gel time (e.g., from 60 to 120 s) while maintaining a high polymerization efficiency (performance comparable to reference nitroxides). More interestingly, the CTCs involved in this retarding strategy are photoresponsive under visible LED@405 nm and can be used on demand as photoinitiators, allowing (i) spectacular increases in polymerization efficiencies (from 50 °C without light to 120 °C under mild irradiation conditions); (ii) drastic reduction of the oxygen-inhibited layer (already 45% C=C conversion at a 2 μm distance from the top surface) compared to the nonirradiated sample (thick inhibited layer of more than 45 μm); and (iii) external control of the redox polymerization gel time due to the possible light activation.

Stable copper acetylacetonate-based oxidizing agents in redox (NIR photoactivated) polymerization: an opportunity for the one pot grafting from approach and an example on a 3D printed object

Stable Cu(ii) ox. agent for redox (graft) polymerization.

Photopolymerization processes of thick films and in shadow areas: a review for the access to composites

The state of the art for the access to thick samples by photopolymerization processes as well as some perspectives are provided.

Mechanosynthesized copper(i) complex based initiating systems for redox polymerization: towards upgraded oxidizing and reducing agents

Significant improvements of the recently proposed Cu(i)/VitC + water/dibenzoyl peroxide (BPO) system for the redox free radical polymerization of methacrylates in air are presented here.