Synthesis and properties of a new photoinitiator of polymerization by visible irradiation based on 3,5-di-tert-butyl-o-benzoquinone

A new sterically hindered trialkyl-substituted o-benzoquinone, 4,6-di-tert-butyl-3-[(3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl]cyclohexane-3,5-diene-1,2-dione was synthesized. Its spectral, electronic, and structural characteristics were determined, which were found to be similar to those for analogous 3,6- and 3,5-di-tert-butyl-o-benzoquinones. The following sequence was observed for the reactivity in the photoreduction reaction in the presence of N,N-dimethylaniline (DMA): 3,6-di-tert-butyl-o-benzoquinone > 4,6-di-tert-butyl-3-[(3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl]cyclohexane-3,5-diene-1,2-dione > 3,5-di-tert-butyl-o-benzoquinone. The new quinone, like 3,6- and 3,5-di-tert-butyl-o-benzoquinones, in combination with DMA was found to initiate radical polymerization under the irradiation by LED sources in the range from 395 to 630 nm. It was found that the introduction of the pyrazole-containing fragment at position 6 of the quinonoid ring of 3,5-di-tert-butyl-o-benzoquinone led to a sharp increase in the activity of the new compound as a photoinitiator compared to 3,5-di-tert-butyl-o-benzoquinone and made the new quinone comparable with 3,6-di-tert-butyl-o-benzoquinone in the photoinitiating activity. At the same time, a photopolymerizable composition containing a photoinitiating system based on 4,6-di-tert-butyl-3-[(3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl]cyclohexane-3,5-diene-1,2-dione was found to be more stable than the photoinitiating system based on 3,6-di-tert-butyl-o-benzoquinone.