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Hexaarylbiimidazole (HABI) and its derivatives with various hydrogen-atom donor coinitiators are widely used as efficient photoinitiators of free radical polymerization of acryl ate monomers [1,2]. Tri aryl i mi dazol yl radicals (lophyl radicals) which are formed by photochemical decomposition of HABI rapidly oxidize the coinitiators by hydrogen-atom abstraction or by electron transfer from the coinitiators, and generate initiating free radicals in polymerization. Sensitization mechanism of HABI, especially of bis[2-(o-chlorophenyl)-4,5diphenylimidazole] (o-Cl-HABI) has recently been studied using laser flash photolysis technique [3,4]. In the dichloromethane solution of o-C1-HABI in the presence of a visible light photosensitizing dye, 2,5-bis[(2,3,6,7-tetrahydro1H,5H-benzo[i,j]quinol izin-1-yl)methylene]cyclopentanone (JAW), it is concluded that lophyl radicals are formed through dissociation of the radical anion of oC1-HABI which is formed by electron transfer from the singlet excited state of JAW to o-C1-HABI [3]. In the case of the acetoni tri 1 e solution of o-Cl -HABI sensitized by the triplet state of 2-i sopropyl thi oxanthone and 4,4'-bi s(N,N'di methyl ami no)benzophenone (Michler's ketone), an encounter complex formation between o-CI-HABI and the triplet sensitizer is suggested [4]. o-C1-HABI can also be used as an efficient sensitizer in photocrosslinking reaction of solid polymer films such as bromoand chl oromethyl ated polystyrene (BCMS). Because sensitizers in solid polymer matrices behave differently from those in the solution [5], the study of sensitization mechanism in solid polymer matrices is important to develop new polymer materials. Futhermore, photocrossl i nki ng efficiency of BCMS was reported to be affected by an external

Photochemical behavior of hexaarylbiimidazole in solid polymer matrices.