In this current contribution, we provide a detailed investigation into the photochemistry and the free radical photoinitiating reactivity of LED light-sensitive photoinitiators (PIs). This series was designed on the basis of a judicious association of a carbazole-coumarin fused subunit and an O-acyl-α-oxooxime branch, which integrates an N−O photocleavable bond. Within this molecular framework, several substitution changes affecting specifically two distinctive sites of the oxime group have been proposed to rationalize some relevant structure−reactivity relationships. We show that the photobleaching rates of the oxime esters (OXEs) are clearly influenced by an ethyl-to-isopropyl substitution effect on the oxime methine carbon whereas the photoinitiating efficiency is mainly driven by a O-benzyl-to-O-acetyl substitution change. Of particular interest, we show that the photoinitiating efficiencies of these OXEs largely depart from their respective absorption spectra in such manner that their photopolymerization performance can be amplified by more than 2 orders of magnitude between 365 and 425 nm LED irradiation. This effect clearly outperforms the photoinitiating efficiency of the commercially available Irgacure OXE-02 oxime ester used as a reference. In the proposed mechanism that accounts for this original wavelength-dependent photopolymerization property, we highlighted the role of an imine-based transient species whose reactivity toward the acrylate monomer can be phototriggered promoting thereby an alternative competing reaction sequence.

Three‐dimensional (3D) printing and especially VAT photopolymerization leads to cross‐linked materials with high thermal, chemical, and mechanical stability. Nevertheless, these properties are incompatible with requirements of degradability and re/upcyclability. We show here that thionolactone and in particular dibenzo[c,e]‐oxepane‐5‐thione (DOT) can be used as an additive (2 wt %) to acrylate‐based resins to introduce weak bonds into the network via a radical ring‐opening polymerization process. The low amount of additive makes it possible to modify the printability of the resin only slightly, keep its resolution intact, and maintain the mechanical properties of the 3D object. The resin with additive was used in UV microfabrication and two‐photon stereolithography setups and commercial 3D printers. The fabricated objects were shown to degrade in basic solvent as well in a homemade compost. The rate of degradation is nonetheless dependent on the size of the object. This feature was used to prepare 3D objects with support structures that could be easily solubilized.