Evaluation of N-Aromatic Maleimides as Free Radical Photoinitiators:  A Photophysical and Photopolymerization Characterization

Abstract

Photopolymerizable compositions were prepared using acrylate monomers in combination with various N-aromatic maleimides. N-aromatic maleimides were segregated into two groups: those that could adopt a planar conformation and those that could not adopt a planar conformation. The maleimides were characterized using single-crystal X-ray diffraction spectroscopy, laser flash photolysis spectroscopy, UV−vis absorption spectroscopy, and photodifferential scanning calorimetry. Planar N-aromatic maleimides were found to have a low relative excited-state triplet yield, showing significant shift of the primary maleimide UV absorption band with changes in solvent polarity, and did not initiate free radial polymerization upon direct UV excitation. Twisted N-aromatic maleimides have a higher relative triplet yield, show negligible shift of the primary maleimide UV absorption band, with solvent polarity, and initiate free radical polymerization upon direct excitation. Addition of benzophenone was found to dramatically increase the initiation efficiency of both planar and twisted N-aromatic maleimides to levels approaching that of conventional cleavage photoinitiators.

ESR and photopolymerization study of polymerizable benzophenone photoinitiators containing functional maleimide groups

“…The other three polymerizable ones all initiate HDDA more efficiently than BP and HBP, with the R pmax and H max almost two times of BP. The result is quite consistent with Miller et al [28] in the physical mixture of BPs and MIs on the photopolyerization of HDDA. This may be ascribed to the efficient electron/proton-transfer process described by Miller, in which MIs can quench the BPs triplet and MDEA.…”

“…According to the ESR results and the reports described by Miller et al [28] planar Naryl MIs do not generate radicals and initiate photopolymerization. The photopolymerization initiated by these four polymerizable BPs should be ascribed to the structure of BP.…”

“…In each of these cases, the MIs participate in producing radical species and will also be consumed by the copolymerization with the photo-curing resin [28], i.e., they do not remain in the final film as an extractable or photoreactive contaminant [22]. Meanwhile, it is well known that maleimide and its derivatives can polymerize to give thermally stable polymers, and the copolymerization with various vinyl monomers has been verified to improve the thermal properties of the polymers, especially for N-aryl MIs [29].…”

“…However, the photoefficiency of these MIs is usually very low. According to the report by Miller and his co-workers [28], N-aliphatic, alkyl, or twisted N-aryl MIs all initiate polymerization in the presence of transferable hydrogens contrary to the planar Naryl MIs.…”

A Highly Efficient Polymerizable Photoinitiator Comprising Benzophenone, Thio Moieties, and N‐Arylmaleimide

“…Moreover, maleimides, as well as the coinitiator DMAEMA, will also quench the excited triplet BP moieties, which will decrease the reaction probability of DMAEMA with BP moieties. [30] However, the planar N-aryl maleimides cannot initiate vinyl polymerization. [14,30,31] Therefore, the effective radicals for polymerization, derived from the reaction between BP moieties and DMAEMA, will be decreased with increasing maleimides introduced to BP structure, leading to much lower photoefficiency of MMTBP than that of MTPBP.…”