Double benzylidene ketones as photoinitiators for visible light photopolymerization

Initiator-Free Thiol-Aldehyde Photo Polycondensation

Traditional photopolymerizations generally require an initiator for initiating the polymerization while few cases have created degradable chemical bonds. Moreover, the migration instability and cytotoxicity of photo initiators are posing issues to human health and the environment. In this work, we discovered an initiator-free photo polycondensation system (IFPPC) between polymercaptans and aldehyde monomers, producing elastic and high strength plastic materials with exchangeable and degradable dithioacetal groups. IFPPC proceeds through a radical mechanism and is applicable in producing linear polymers and crosslinked networks. Nano-composites can also be obtained via an organic solvent-free Pickering emulsion polymerization using cellulose nanofibers as the stabilizer. The dithioacetal groups afford excellent stability under hydrolytic conditions and repeated thermal processing of cross-linked polymers, while their cleavage in aqueous solution of salts (silver nitrate, zinc iodide, copper bromide, etc.) results in fast degradation for recovering the aldehyde monomer. This is a successful demonstration of producing recyclable and degradable plastics through photo irradiation while avoiding the problematic issues of photo initiators.

Fast Visible-Light 3D Printing of Conductive PEDOT:PSS Hydrogels

Functional inks for light-based 3D printing are actively being searched for being able to exploit all the potentialities of additive manufacturing. Herein, a fast visible-light photopolymerization process is showed of conductive PEDOT:PSS hydrogels. For this purpose, a new Type II photoinitiator system (PIS) based on riboflavin (Rf), triethanolamine (TEA), and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is investigated for the visible light photopolymerization of acrylic monomers. PEDOT:PSS has a dual role by accelerating the photoinitiation process and providing conductivity to the obtained hydrogels. Using this PIS, full monomer conversion is achieved in less than 2 min using visible light. First, the PIS mechanism is studied, proposing that electron transfer between the triplet excited state of the dye (3 Rf*) and the amine (TEA) is catalyzed by PEDOT:PSS. Second, a series of poly(2-hydroxyethyl acrylate)/PEDOT:PSS hydrogels with different compositions are obtained by photopolymerization. The presence of PEDOT:PSS negatively influences the swelling properties of hydrogels, but significantly increases its mechanical modulus and electrical properties. The new PIS is also tested for 3D printing in a commercially available Digital Light Processing (DLP) 3D printer (405 nm wavelength), obtaining high resolution and 500 µm hole size conductive scaffolds.

Recent Advances in Monocomponent Visible Light Photoinitiating Systems Based on Sulfonium Salts

During the last decades, multicomponent photoinitiating systems have been the focus of intense research efforts, especially for the design of visible light photoinitiating systems. Although highly reactive three-component and even four-component photoinitiating systems have been designed, the complexity to elaborate such mixtures has incited researchers to design monocomponent Type II photoinitiators. Using this approach, the photosensitizer and the radical/cation generator can be combined within a unique molecule, greatly simplifying the elaboration of the photocurable resins. In this field, sulfonium salts are remarkable photoinitiators but these structures lack absorption in the visible range. Over the years, various structural modifications have been carried out in order to redshift their absorptions in the visible region. In this work, an overview of the different sulfonium salts activable under visible light and reported to date is proposed.