New Donor-Acceptor Stenhouse Adducts as Visible and Near Infrared Light Polymerization Photoinitiators.

Polymerization photoinitiators that can be activated under low light intensity and in the visible range are being pursued by both the academic and industrial communities. To efficiently harvest light and initiate a polymerization process, dyes with high molar extinction coefficients in the visible range are ideal candidates. In this field, Donor-acceptor Stenhouse Adducts (DASA) which belong to a class of recently discovered organic photochromic molecules still lack practical applications. In this work, a series of DASA-based dyes are proposed as photoinitiators for the free radical polymerization of (meth)acrylates upon exposure to a near infrared light (laser diode at 785 nm).

Light-Induced Thermal Decomposition of Alkoxyamines upon Infrared CO2 Laser: Toward Spatially Controlled Polymerization of Methacrylates in Laser Write Experiments.

Systems combining photopolymerization and thermal polymerization have already been reported in the literature. Upon near-infrared (NIR) light exposure, this principle of polymerization is called photoinduced thermal polymerization or photothermal polymerization. Thanks to an NIR dye used as the light-to-heat convertor (called hereafter a heater), an alkoxyamine (e.g., BlocBuilder-MA) is dissociated upon NIR light irradiation, initiating the free-radical polymerization of methacrylates. In the present paper, a novel approach is presented for the first time to decompose the alkoxyamine through a direct heat generation upon mid-infrared irradiation by a CO2 laser at 10.6 µm. Compared with previous approaches, there is no additional heater used in this work, as the heat is directly generated by laser irradiation on the alkoxyamine/monomer system. The polymerization can be initiated for benchmark methacrylate monomers with spatial controllability, that is, only in the laser-irradiated area, opening the way for laser write or three-dimensional printing applications in the presence of fillers.

Fillers as Heaters for Photothermal Polymerization upon NIR Light.

Photo-induced thermal polymerization upon near-infrared (NIR) light irradiation has been reported in the literature. In this approach, a component able to convert the NIR light into heat must be used in combination with a thermal initiator to initiate the free-radical polymerization of (meth)acrylates. In recent studies, some absorbers have been presented as very efficient heat generators (called heaters). In the present work, different fillers are investigated as heaters and compared to organic NIR absorbers. An alkoxyamine (e.g., BlocBuilder-MA) is used as thermal initiator and is dissociated by the heat generated by the NIR photoexcitation of the fillers. In the present work, several fillers are examined: graphene oxide, graphene nanoplatelets, multi-walled carbon nanotubes, and silicon carbide. Due to the energy of the photon delivered, NIR light curing is challenging but offers several advantages compared to visible light. The most interesting feature is the deeper penetration of the light inside the photocurable resin, enabling the polymerization of thick samples. Parallel to this, incorporation of fillers in resins allows unique access to composites through photothermal polymerization of (meth)acrylates. Three different wavelengths of irradiation have been studied: 785, 940, and 1064 nm.

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.