For the first time, we report 3D printing of RAFT-based formulations to fabricate functional objects in a layer-by-layer fashion.

The photopolymerization-based 3D printing process is typically conducted by using free radical polymerization, which leads to fabrication of immutable materials. An alternative 3D printing of polymeric materials by using trithiocarbonate (TTC) reversible addition–fragmentation chain transfer (RAFT) agents has always been a challenge for material and polymer scientists. Herein we report 3D printing of RAFT-based formulations that can be conducted fully open to air using a standard digital light processing (DLP) 3D printer and under mild conditions of visible light at blue (λmax = 483 nm, 4.16 mW/cm2) or green (λmax = 532 nm, 0.48 mW/cm2) wavelength. Our approach is based on activation of TTC RAFT agents using eosin Y (EY) as a photoinduced electron-transfer (PET) catalyst in the presence of a reducing agent (triethylamine (TEA)), which facilitated the oxygen tolerant 3D printing process via a reductive PET initiation mechanism. Reactivation of the TTCs present within the polymer networks enables postprinting monomer insertion into the outer layers of an already printed dormant object under a second RAFT process, which provides a pathway to design a more complex 3D printing. To our best knowledge, this is the first example of oxygen tolerant EY/TEA catalyzed PET-RAFT facilitated 3D printing of polymeric materials. We believe that our strategy is a significant step forward in the field of 3D printing.

Light-responsive polymeric networks have shown diverse applications as spatiotemporally tunable materials. Herein, we present a straightforward and facile strategy to fabricate photoexpandable/transformable-polymer networks (PET-PNs) that can undergo a growth mechanism via visible light-induced radical polymerization. Our PET-PN fabrication and its subsequent photogrowth process is based on using a trithiocarbonate chain transfer agent, dibenzyl trithiocarbonate (DBTTC), that can be activated by either photoredox catalysis or direct photolysis (photoiniferter) mechanisms. We first demonstrated the use of a photoredox catalyst (5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP) for initiating the reversible addition–fragmentation chain-transfer (RAFT) polymerization of a cross-linkable system consisting of difunctional monomers (i.e., tetra­(ethylene glycol) diacrylate (TEGDA)) alongside with a monofunctional monomer (i.e., oligo­(ethylene glycol) methyl ether acrylate) (OEGA)) under red LED light (λ max = 635 nm, 0.7 mW/cm2) to fabricate polymer networks. Photogrowth of these networks were achieved through a photoredox-catalyzed insertion of new monomers (such as OEGA) into the network strands when exposed to red LED light. We further investigated the photoiniferter properties of DBTTC for the formation and subsequent photogrowth of polymer networks via direct photolysis under blue LED light (λ max = 460 nm, 0.7 mW/cm2) without the presence of external initiators or catalysts. Visible light-induced monomer insertion and photogrowth of the parent networks were demonstrated by measuring the mass increase and the swelling capacity of the networks. Finally, we demonstrated the facile light-induced welding of networks, suggesting that our simple PET-PN system facilitates fabrication of reprocessable materials.

Controlled modification in the structure and properties of three-dimensional (3D) printed polymers, as in the broader context of cross-linked polymer networks, in response to an external stimulus has been of great importance to meet the demands of advanced applications and environmental sustainability concerns. In this study, a dynamic covalent di­(meth)­acrylate cross-linker containing a reversible addition–fragmentation chain transfer (RAFT) trithiocarbonate (TTC) functionality was synthesized and used for the formation of living photoexpandable/transformable polymer networks (PET-PNs). The network-bound TTC functionalities were activated in a postsynthesis stage via a visible light-controlled photoredox-catalyzed RAFT polymerization, enabling monomer addition into the existing scaffolds. This approach allowed controllable and successive postsynthesis photogrowth, photofunctionalization, and/or photowelding reactions. The expandable RAFT-capable TTC cross-linker (TTC-XL) was also exploited to manufacture living 3D materials via a layer-by-layer photopolymerization process facilitated by a modified digital light processing (DLP) 3D printer. The 3D printed materials were also capable of undergoing successive postprinting reactions (e.g. functionalization) via a photoredox-catalyzed RAFT process under a red light-emitting diode (LED) light irradiation. From the viewpoint of material sustainability and recyclability, this study is a great step forward and it will open up additional possibilities in the field of 3D printing for the fabrication of advanced functional materials.