Revisiting 2-Isopropylthioxanthone: Direct and Reactivable Photopolymerization of Acrylates from Near-UV to Blue Light. Efficient 3D Printing and Surface Modification

Reversible-deactivation radical photopolymerization (photoRDRP) enables precise spatiotemporal control of polymer architectures. Because of embedded active chain ends, photoRDRP offers straightforward reactivation and chain extension, providing unmatched flexibility for fine-tuning structural, mechanical, and physicochemical properties for targeted applications. Nevertheless, its application to 3D printing remains limited by the cost and synthetic effort of the control agent, as well as by a limited reactivity under common near-UV and blue light sources, which requires printers working in the visible range and results in usually low printing rates. Herein, we show the unexpected ability of 2-isopropylthioxanthone (ITX) to promote both a direct acrylate photopolymerization, (with no need for any conventional co-initiator and a printing rate close to those of conventional photoinitiators) and a subsequent surface-initiated (SI) photopolymerization via reactivable surface-bound species, yielding postfunctionalization results that are equivalent to, or surpass, those of RAFT systems. Compared to benzyldithiocarbamate (BDC) and N-(tert-butyl)-N-(2-methyl-1-(4-benzoylphenyl)propyl)-O-(1-phenylethyl) hydroxylamine (MBA), two model photoinitiators for photoRDRP in the near-UV range, ITX enabled controlled SI photopolymerization of neat acrylate monomers over thicknesses of several hundred micrometers, with higher molar reactivity and a broader spectral response. This allowed for direct photopolymerization and photostructuring using wavelengths from 365 to 405 nm. This straightforward and scalable approach opens promising avenues for the on-demand functionalization and assembly of complex 3D-printed architectures and is expected to bridge the gap between academic precision and industrial accessibility.