Visible Light-Induced Transformation of Polymer Networks

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.