Laser flash photolysis of bismaleimides

Abstract

Bismaleimides containing between 2 and 12 methylene spacer units have been synthesized and characterized using laser flash photolysis spectroscopy. In the case of the monofunctional N-methylmaleimide, the 1,4-biradical was observed only at maleimide concentrations high enough to effectively quench the triplet state, indicating that the 1,4-biradical is formed from self-quenching of the triplet excited state. The transient spectral features and the transient lifetimes of bismaleimides containing 2 and 9 methylene spacer units closely resemble those of N-methylmaleimide and are attributed to triplet-state transients. N,N′-Alkylenebismaleimides with 3 and 6 methylene spacer groups exhibit a transient with a very short lifetime attributed to a singlet 1,4- biradical species formed from intramolecular interaction. The absence of triplet spectra in these latter bismaleimides indicate that the 1,4-biradical is formed directly from the excited singlet state. The large difference in the lifetime of the 1,4-biradical formed in monofunctional maleimide solutions and bismaleimide solutions is reflective of differences in the biradical spin-state multiplicity. The bismaleimide with a 12 methylene spacer group exhibits transient spectra corresponding to both a triplet state of the maleimide and a 1,4-biradical formed by exo interaction of two maleimide groups. As a demonstration of the ability of acrylate monomers to react with the short-lived singlet biradicals, it is shown that the photopolymerization of a multifunctional acrylate monomer is readily initiated by the 1,4-biradical formed from the bismaleimides with 3 and 6 methylene spacer groups, presumably by direct reaction of the biradical with the acrylate.