Photoacid-catalysed pigmentation of dyestuff precursors enhanced by acid amplifiers in polymer films

The pigmentation of BOC-protected pigment precursors catalysed by photogenerated acidic species in films of polystyrene (pSt) and poly(alpha-methylsytrene) according to the latent pigment technology was investigated to reveal that the process consists of three steps: the deprotection to give free indigo molecules, the subsequent diffusion of the molecules, and their aggregation to give the hydrogen-bonded pigment. The photoacid-catalysed regeneration of indigo protected with tert-butoxycarbonyl (BOC) residues is comprised of two processes: a fast process at the early stage, followed by a very slow one. The former likely corresponds to the reaction within reactive spheres in polymer films, whereas the subsequent slow process displays a suppressive effect of polymer matrices on the migration of the molecules from the reactive spheres so that the completion of the pigmentation requires prolonged heating. On the other hand, the addition of an acid amplifier, which decomposes autocatalytically to liberate new sulfonic acid molecules, results in the marked enhancement of the deprotection and consequently the pigmentation. Particle size analysis of solutions dissolving pigmented films showed that smaller particles of indigo of about 20 nm in diameter are formed under heating below the glass transition temperature (T-g) of pSt whereas larger ones of a few hundred nm become predominant when films are heated above the T-g, reflecting both the fast and slow processes.