Pu Xiao

Photopolymerization upon LEDs: new photoinitiating systems and strategies

This paper is focused on: (i) the recent developments in LED technology, allowing the design of novel and efficient light sources for the free radical and cationic photopolymerization of various monomers, the synthesis of interpenetrating polymer networks (IPNs) or thiol–ene photopolymerization; (ii) the existing and emerging applications in these areas; and (iii) the development of novel photoinitiators and photoinitiating systems specifically adapted for LED excitation.

Naphthalimide based methacrylated photoinitiators in radical and cationic photopolymerization under visible light

The abilities of two naphthalimide derivatives with a methacryloyl group to initiate, when incorporated in multi-component systems, a ring-opening polymerization of epoxides and a radical polymerization of acrylates under different irradiation sources (e.g. very soft halogen lamp irradiation, laser diode at 457 nm, laser diode at 405 nm and blue LED bulb at 462 nm) have been investigated. One of them is particularly efficient for the cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure and leads to the formation of an interpenetrated polymer network IPN (30 s for getting tack free coatings). The migration stability of one of these naphthalimide derivatives is found to be excellent in the cured polyacrylates and IPNs. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping and laser flash photolysis techniques.

Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization

Summary In the present paper, the photoredox catalysis is presented as a unique approach in the field of photoinitiators of polymerization. The principal photocatalysts already reported as well as the typical oxidation and reduction agents used in both reductive or oxidative cycles are gathered. The chemical mechanisms associated with various systems are also given. As compared to classical iridium-based photocatalysts which are mainly active upon blue light irradiation, a new photocatalyst Ir(piq)2(tmd) (also known as bis(1-phenylisoquinolinato-N,C 2’)iridium(2,2,6,6-tetramethyl-3,5-heptanedionate) is also proposed as an example of green light photocatalyst (toward the long wavelength irradiation). The chemical mechanisms associated with Ir(piq)2(tmd) are investigated by ESR spin-trapping, laser flash photolysis, steady state photolysis, cyclic voltammetry and luminescence experiments.

Green light sensitive diketopyrrolopyrrole derivatives used in versatile photoinitiating systems for photopolymerizations

The monomeric (DPPDT) and polymeric (PDQT) diketopyrrolopyrrole–thiophene derivatives combined with an iodonium salt or an amine (and optionally an additive) are used as photoinitiating systems to initiate the cationic polymerization of epoxides or divinyl ethers, radical polymerization of acrylates and thiol–ene polymerization under different irradiation sources (i.e. very soft halogen lamp, laser diodes at 473 nm, 532 nm and 808 nm). The DPPDT based systems are very efficient upon the green light exposure. Remarkably, these systems are characterized by a higher efficiency than references based on camphorquinone, a well known photosensitizer for visible light. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis and electron spin resonance spin trapping techniques.

Chalcone derivatives as highly versatile photoinitiators for radical, cationic, thiol–ene and IPN polymerization reactions upon exposure to visible light

Chalcone derivatives are proposed as novel photoinitiators for polymerization upon exposure to visible light (even under soft irradiation conditions: blue LED bulb at 462 nm, halogen lamp, and sunlight). Remarkably, these structures are highly versatile allowing access to photoinitiating systems for (i) the free radical polymerization of acrylates, (ii) the cationic polymerization of epoxides and vinyl ethers, (iii) the synthesis of interpenetrated polymer networks (IPNs) and (iv) the thiol–ene processes. Excellent polymerization profiles are obtained. The initiation mechanisms are analyzed through ESR, steady state photolysis, cyclic voltammetry, laser flash photolysis and fluorescence experiments.

Red-light-induced cationic photopolymerization: perylene derivatives as efficient photoinitiators.

Nine different perylene derivatives are prepared and their ability to initiate, when combined with an iodonium salt (and optionally N-vinylcarbazole), a ring-opening cationic photopolymerization of epoxides under very soft halogen lamp irradiation is investigated. One of them is particularly efficient under a red laser diode exposure at 635 nm and belongs now to the very few systems available at this wavelength. The photochemical mechanisms are studied by steady-state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques.

A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs

Curcumin exhibits broad ground state light absorption and can act as a photoinitiator for the free radical photopolymerization of methacrylates under air upon exposure to different household LED bulbs. The effects of temperature and various additives on the photoinitiation efficiency of curcumin-based systems have been investigated. The curcumin-based system exhibits the highest photoinitiation efficiency at 25 °C. Additives also play an important role in the photoinitiation efficiency, and well-designed systems can even demonstrate higher efficiency than the commercial type I photoinitiator [phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (XBPO)] and type II photoinitiator [camphorquinone (CQ)]. Interestingly, the curcumin/diphenyliodonium hexafluorophosphate/triphenylphosphine combination is a capable multicolor photoinitiating system able to initiate free radical photopolymerization under air upon exposure to UV, blue, green, yellow, red, and warm white household LED bulbs. In addition, reversible addition–fragmentation chain transfer (RAFT) photopolymerization of N-isopropylacrylamide can also be achieved using a curcumin-based system under the irradiation of a blue LED bulb. The photochemical mechanisms associated with the generation of radicals from the investigated photoinitiating systems are investigated by different techniques (fluorescence, steady state photolysis, and electron spin resonance spin-trapping methods) and discussed in detail. More interestingly, the polymer sample produced through the photopolymerization process using the curcumin-based photoinitiating system demonstrates almost no toxicity to human fibroblast Hs-27 cells, endowing this photoinitiating system with great potential for the fabrication of biocompatible polymeric materials.

Cellular Uptake and Movement in 2D and 3D Multicellular Breast Cancer Models of Fructose-Based Cylindrical Micelles That Is Dependent on the Rod Length

While the shape effect of nanoparticles on cellular uptake has been frequently studied, no consistent conclusions are available currently. The controversy mainly focuses on the cellular uptake of elongated (i.e., filaments or rod-like micelles) as compared to spherical (i.e., micelles and vesicles) nanoparticles. So far, there is no clear trend that proposes the superiority of spherical or nonspherical nanoparticles with conflicting reports available in the literature. One of the reasons is that these few reports available deal with nanoparticles of different shapes, surface chemistries, stabilities, and aspects ratios. Here, we investigated the effect of the aspect ratio of cylindrical micelles on the cellular uptake by breast cancer cell lines MCF-7 and MDA-MB-231. Cylindrical micelles, also coined rod-like micelles, of various length were prepared using fructose-based block copolymers poly(1-O-methacryloyl-β-d-fructopyranose)-b-poly(methyl methacrylate). The critical water content, temperature, and stirring rate that trigger the morphological transition from spheres to rods of various aspect ratios were identified, allowing the generation of different kinetically trapping morphologies. High shear force as they are found with high stirring rates was observed to inhibit the formation of long rods. Rod-like micelles with length of 500-2000 nm were subsequently investigated toward their ability to translocate in breast cancer cells and penetrate into MCF-7 multicellular spheroid models. It was found that shorter rods were taken up at a higher rate than longer rods.

Amino and nitro substituted 2-amino-1H-benzo[de]isoquinoline-1,3(2H)-diones: as versatile photoinitiators of polymerization from violet-blue LED absorption to a panchromatic behavior

Four different 2-amino-1H-benzo[de]isoquinoline-1,3(2H)-dione derivatives (NDNs) bearing nitro- or amino- substituents have been prepared and incorporated into photoinitiating systems together with an iodonium salt, N-vinylcarbazole, an amine or 2,4,6-tris(trichloromethyl)-1,3,5-triazine. The ring-opening cationic polymerization of epoxides as well as the free radical polymerization of acrylates upon exposure to various visible LEDs (e.g. LEDs at 405 nm, 455 nm or 470 nm) could be performed. Reactions under green or red light (e.g. laser diodes at 532 or 635 nm) are also feasible. Compared to the well-known camphorquinone-based systems, some of the novel NDN-based combinations employed here exhibit a much higher polymerization efficiency. A panchromatic behavior can also be observed with a specifically designed compound bearing two nitro groups (e.g. radical polymerization or thiol–ene processes can be promoted upon blue, green or red light). This outstanding performance paves the way to polymerization under soft conditions. Their photochemical properties and structure/efficiency relationships have been investigated using steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin trapping techniques.

Copper complexes: the effect of ligands on their photoinitiation efficiencies in radical polymerization reactions under visible light

Twenty-three copper complexes (CuC) containing different ligands were synthesized. Most of them exhibited light absorption in the visible range (adapted to LEDs, laser diodes and halogen lamp exposure). The CuC/iodonium salt (Iod), CuC/Iod/N-vinylcarbazole, CuC/alkyl bromide/methyl diethanolamine photoinitiating systems (PISs) allow good monomer conversion profiles for the free radical polymerization of trimethylolpropane triacrylate. Some of them are better than bisacylphosphine oxide or camphorquinone based systems used as references. Real-Time Fourier Transform Infrared spectroscopy and steady state photolysis were used as analytical methods. The results revealed that the ligands of the copper complexes played an important role towards the photoinitiation ability of these PISs. The use of CuC as photocatalysts to initiate thiol–ene reaction is also discussed.