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Issue 13, 2019
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A substituent para-to-ortho positioning effect drives the photoreactivity of a dibenzothiophene-based oxalate series used as LED-excitable free radical photoinitiators

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Abstract

Four dibenzothiophene (DBT)-based methyl oxalates, synthesized through a one-step Friedel–Crafts acylation reaction, were developed as high-performance photoinitiators (PIs) for the free radical polymerization of acrylate resins excitable using LED sources that emit in the near-UV and visible region (i.e. 365–425 nm). These type I PIs show good optical absorption properties (ε365 nm = 1 000–5300 M−1 cm−1). Their respective photophysical properties, as well as the sequential photocleavage mechanism, were investigated using a large set of theoretical and experimental methods. A relevant structure/reactivity relationship opening the way to enhancing the PI performance is highlighted. Indeed, a para-to-ortho positioning effect of the oxalate-based substituent regulates both the absorption and photocleavage abilities within this DBT series. Para-isomers consequently exhibit a photoinitiating reactivity, whose efficiency is more than one order of magnitude higher than that of their ortho homologues. Associated with a H-donor alkylamine co-reactant, these photoinitiating efficiencies can be amplified even more due to an additional reaction pathway that judiciously assigns a new role to the aryloyl radical by-products. These results correlate the PI photopolymerization performance with the photoinduced processes occurring in the excited states.

Graphical abstract: A substituent para-to-ortho positioning effect drives the photoreactivity of a dibenzothiophene-based oxalate series used as LED-excitable free radical photoinitiators

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Publication details

The article was received on 14 Jan 2019, accepted on 12 Feb 2019 and first published on 12 Feb 2019


Article type: Paper
DOI: 10.1039/C9PY00052F
Citation: Polym. Chem., 2019,10, 1599-1609

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    A substituent para-to-ortho positioning effect drives the photoreactivity of a dibenzothiophene-based oxalate series used as LED-excitable free radical photoinitiators

    H. Pan, S. Chen, M. Jin, J. Malval, D. Wan and F. Morlet-Savary, Polym. Chem., 2019, 10, 1599
    DOI: 10.1039/C9PY00052F

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