HABIs as Photopolymerization Initiators

During the period up to 1963, hexaarylbiimidazoles were primarily studied for their physical, principally, photochromic properties. Work at the Organic Chemicals Department of the DuPont Company was primarily concerned with the color-forming interaction between HABls and triarylmethane leucodyes. The group working in this area had little familiarity with photopolymerization, but did periodically try to interest another company division, the Photo Products Department in examining various novel HABls as photoinitiators. 

and viscosity 9.0-13.5 poises) soln. in Me2CO 110.0, a 10% cellulose acetate (39.4% Ac 55% HOAc, viscosity 130-82 poises) soln. in Me2CO 32.5, pentaerythritol triacrylate 30.0, poly(ethylene oxide) (mol. wt. 400) (in 14 ml. MeOH) 2.0 g., and Me2CO to make 200.0 g. The standard soln. 

the dimer 2-(o-chlorophenyl)-4,5-diphenylimidazole 80, and CBr4,100-mg. were mixed, coated as a 0.15-mm. layer on a 0.03-mm. poly(ethylene terephthalate) support precoated with a layer of vinylidene chloride-Me acrylate-itaconic acid terpolymer, and dried for 30 min, at ambient temp, to give a 0.03-mm. film. The laminated product was contacted with an original, exposed to a photoflood lamp at a distance of 40.6 cm. for 30 sec., delaminated, developed, and passed with a receptor paper through 2 rollers heated at 125° to transfer the under exposed zones from the photopolymer layer to the receptor sheet thereby forming a well-contrasted and defined image on the paper. 

Even though the project was terminated because office copy arena became exclusively an opportunity for electrophotography, DuPont applied for a patent, which was the first of many that identified HABIs as photopolymerization initiators. Cescon, R.L. Cohen and Dessauer s patent, issued much later, as fiorther work was required to understand the extent of the chemistry involved therein. 

Much of the success of the RISTON and CROMALIN was derived from the availability of an exclusive, novel photoinifiator system based on hexaarylbiimidazole photoini-fiators. The exclusivity was derived from a determined effort earlier in the Organic Chemicals Department of DuPont to obtain composition of matter patents, which eliminated competitors from applying this technology to their products. These photoinitiators were patented in 1974 and provided DuPont with a unique photooxidants and photopolymerization initiators. 

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The two patent publications cited above were the beginnings of HABI-initiated photopolymerization. Since then, more than a thousand subsequent U.S. patents identify HABIs as photopolymerization initiators. 

Speed of the system then under intense study, we might actually design a system in which much higher speed could be realized. This did indeed work, and in the end, we had found that under certain conditions, HABI acted as photopolymerization initiators. 

In a parallel development, a group under Vaughn Chambers also observed that HABIs could indeed act as photopolymerization initiators. 

Even though there was an intense effort at DuPont s Photo Products Department to investigate the use of photopolymerization as a means of generating new products for the imaging industries, there seemed little interest there in critically evaluating the HABIs as initiators. We had submitted samples to our colleagues at the Parlin Photo Products Department, but their initial tests were unsuccessful, i.e., they could not effect polymerization of polyacrylates with HABIs. 

Hexaarylbiimidazoles (HABIs) were discovered in the early 1960s and aroused worldwide interest because of their unusual physical properties various stimuli such as heat, light and pressure converted these molecules to colored species, which were found to be relatively stable free radicals. Unlike Gomberg s triarylmethyl radicals, triarylimidazolyl radicals were oxygen insensitive. A research program directed at finding commercial applications for these materials at the DuPont Company resulted in the discovery that HABIs were useful as photooxidants and photopolymerization initiators. This in turn led to a number of very successful commercial products for the graphic arts, electronics, displays and communication industries. 

We found that all samples prepared were effective in initiating photopolymerization, and on returning from Europe, I submitted samples of several of these new HABIs to Photo Products Department for evaluation, but no response was received. Of course, it is difficult to develop much enthusiasm for incorporating yet another new material into existing coating recipes, as this would entail a long round of testing, examination, etc. 

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