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

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-<i>N</i>,<i>N</i>-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

Blue-to-Red Light Sensitive Push–Pull Structured Photoinitiators: Indanedione Derivatives for Radical and Cationic Photopolymerization Reactions

The actual photonitiators PI can only operate in a restricted part of the visible spectrum; as a consequence, several PIs are usually necessary to harvest all the emitted visible photons. In the present paper, new dyes based on a donor−π-acceptor structure (1,3-indanedione derivatives) are incorporated into visible light sensitive photoinitiating systems of polymerization. They exhibit an unusual and remarkable broad absorption lying from the blue to the red. When employed in the presence of an iodonium salt (Iod) and optionally <i>N</i>-vinylcarbazole (NVK), these dyes can efficiently initiate the radical photopolymerization of acrylates, the cationic photopolymerization of epoxide and vinylether monomers and the hybrid cure of acrylate/epoxide blends under exposure, e.g., at 405, 457, 473, 532, and 635 nm. They partly behave as organic photocatalysts. These particular light absorption properties and the initiation step mechanisms are investigated in detail

Organic Photocatalyst for Polymerization Reactions: 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene

A new organic photocatalyst (9,10-bis­[(triisopropylsilyl)­ethynyl]­anthracene, <b>An-Si</b>) is proposed here for the formation of free radicals under very soft irradiation conditions under air through a photoredox catalysis. It works according to an oxidative cycle that uses the combination of <b>An-Si</b>, a diphenyl iodonium salt along with a silane. This behavior is highlighted through an investigation of its excited state and redox properties. The different chemical intermediates are characterized by ESR experiments. In addition, the reversibility of the oxidation reaction of <b>An-Si</b> was investigated by cyclic voltammetry. This three-component system is able to promote the ring-opening photopolymerization of an epoxide as well as the free radical photopolymerization of an acrylate upon household LED bulb and Xe lamp exposure. Excellent polymerization profiles (mainly in ROP) are obtained. The specific properties of this catalyst are outlined

Tunable Organophotocatalysts for Polymerization Reactions Under Visible Lights.

New organic photocatalysts derived from pyrene, anthracene, naphthacene, and pentacene are presented here for the formation of free radicals through a photoredox catalysis. These OPCs can work according to <i>an oxidative cycle</i> in a three component system in combination with diphenyl iodonium salt and a silane or in <i>a reductive cycle</i> in combination with amine and alkyl halide. This OPC behavior is highlighted through an investigation of the associated excited state and redox properties. The free radicals generated are characterized by ESR or photolysis experiments. Upon household LED bulb or Xe lamp exposure, the oxidative three-component system is able to promote the ring-opening polymerization ROP of an epoxide whereas the reductive three-component system is very efficient to initiate the free radical photopolymerization FRP of an acrylate. This ability of OPCs to initiate different polymerization reactions (ROP and FRP) is clearly an outstanding property

New Push–Pull Dyes Derived from Michler’s Ketone For Polymerization Reactions Upon Visible Lights.

Among other photoinitiating systems, aromatic ketone based compounds have been largely exploited as photoinitiators (PIs). However, none of these compounds efficiently absorb above 420 nm. The search of novel architectures of PIs for getting an important red-shift of the absorption is crucial for the use of visible lights for polymer synthesis. Novel bifunctional dyes derived from the Michler’s ketone structure are proposed here as photoinitiators for the free radical polymerization of acrylates and the cationic polymerization of epoxides upon exposure to 457, 473, and 532 nm laser diodes and even to a green LED bulb at 514 nm. Excellent polymerization profiles are obtained. These original dyes exhibit a push–pull molecular character for a remarkable covering of the visible lights. The formation of the radicals and the ions in the two- and three-component photoinitiating systems is described and the initiation steps are discussed

Trifunctional Photoinitiators Based on a Triazine Skeleton for Visible Light Source and UV LED Induced Polymerizations

Trifunctional photoinitiators (tPIs) based on benzophenone, anthracene, and pyrene chromophores linked to a triazine moiety are proposed as new initiating systems. In combination with an iodonium salt and a silane, these structures are able to initiate the radical polymerization RP of acrylates and the cationic polymerization CP of epoxides and vinylethers under Xe–Hg lamp, LED and very soft irradiation (i.e., halogen lamp). Upon addition of an amine, these new photoinitiators were also able to start the radical polymerization of acrylates. Excellent RP and CP polymerization profiles are obtained i.e. better than those recorded using the reference compounds (benzophenone, anthracence and pyrene). In CP, some of these compounds combined with thianthrenium salts can also be used. The mechanisms involved in the different multicomponent initiating systems were analyzed by ESR, fluorescence, steady state photolysis, and laser flash photolysis experiments

New Cleavable Photoinitiator Architecture with Huge Molar Extinction Coefficients for Polymerization in the 340–450 nm Range.

A new Type I photoinitiator <b>Tr_DMPA</b> is described. It consists in three 2,2′-dimethoxy 2-phenyl acetophenone (DMPA) units grafted onto a truxene (<b>Tr</b>) scaffold. Compared to DMPA itself, the lowest electronic transition exhibits a ππ* character and the corresponding molar extinction coefficients ε are increased from about 400 M^–1 cm^–1 (at about 332 nm for DMPA) to 63 000 M^–1 cm^–1 (at 338 nm for <b>Tr_DMPA</b>); such huge values are exceptional in Type I photoinitiators at this wavelength. <b>Tr_DMPA</b> undergoes a fast cleavage and efficiently initiate an acrylate polymerization upon a Xe–Hg lamp, a halogen lamp or a laser diode exposure at 405 nm (upon very low light intensities: 2–12 mW/cm²) in the 300–450 nm range. The polymerization of epoxides or divinylethers is also feasible in the presence of an iodonium salt at 405 nm where reference photoinitiators cannot efficiently operate. The chemical mechanisms analyzed by ESR, fluorescence, steady state photolysis, and laser flash photolysis experiments are discussed

A Tris(triphenylphosphine)ruthenium(II) Complex as a UV Photoinitiator for Free-Radical Polymerization and <i>in Situ</i> Silver Nanoparticle Formation in Cationic Films

The characterization and the photochemical investigation of a Ru^II complex (Ru­(PPh₃)₃Cl₂) having phosphine ligands are reported. DFT calculations and ESR spin trapping experiments revealed for the first time that the photodecomposition of the complex is governed by a homolytic cleavage of the P–(C₆H₅) bond generating phenyl radicals Ph^• which are able to initiate the free radical polymerization of acrylate monomers. The addition of a H-donor HD plays a key role in the cationic photopolymerization of epoxides: (i) the reaction efficiency is enhanced using [Ru­(PPh₃)₃Cl₂]/HD and (ii) the <i>in situ</i> formation of Ag nanoparticles is observed in the presence of Ru­(PPh₃)₃Cl₂/HD/AgSbF₆ according to the following reactions: Ph^•/DH hydrogen abstraction yielding a D^• radical and oxidation of D^• by the silver salt

Oligomeric Photocatalysts in Photoredox Catalysis: Toward High Performance and Low Migration Polymerization Photoinitiating Systems.

In the present paper, four fluorescent materials currently used in organic light emitting diodes (OLEDs) are presented in an original way as high performance photocatalysts usable in polymerization photoinitiating systems. Their performance is excellent in free radical polymerization, cationic polymerization but also in the synthesis of interpenetrating polymer networks (IPNs). A coherent picture of the chemical mechanisms involved in these new photocatalytic systems is provided. Remarkably, an oligomeric and copolymerizable photocatalyst (PVD2) is proposed here for the first time, i.e., both the high molecular weight of PVD2 and the presence of reactive double bonds as end groups (which could be involved in a copolymerization reaction) ensure a very low migration of the catalyst from the synthesized polymer