Phototropic (also known as photochromic and photothermochromic) compounds are generally known as compounds which isomerize or tautomerize reversibly to another form when exposed to actinic light of a suitable wave length. When this wavelength of irradiation is removed and either heat or light of a different wavelength of irradiation is applied, the reaction is reversed to give the original phototropic compound.
It has now been discovered that a specific class of phototropic compounds, when irradiated to an aci- or acidic form, are effective as photoinitiators for cationic polymerization of various monomers resulting in polymers thereof and such photo-induced reaction is eminently suitable for use in the graphic arts or as light-cured coatings.
Vinyl ethers and epoxy resins have been photopolymerized employing various catalysts and promoters therefor. For example, U.S. Pat. No. 3,708,296 issued Jan. 2, 1973 to S. I. Schlesinger and commonly assigned herewith, employs photosensitive catalyst precursors which are aryl diazonium salts of complex halogenides which release a Lewis acid upon irradiation to initiate polymerization of epoxide monomers and prepolymers.
Another procedure, U.S. Pat. No. 3,347,676 employs metal salts such as complex silver borates in combination with organic halogens.
None of the prior procedures are without their particular difficulties however. For example, epoxides and related compositions containing photosensitive catalyst precursors have a tendency to gel on standing, often necessitating the need for gelation inhibitors to prevent premature reaction, while silver compounds are becoming increasingly expensive, and are not practical alternatives for large scale commercial production. Accordingly, it is desirable to identify and discover new monomer catalyzing agents useful in the photopolymerization process which are not subject to and overcome the deficiencies now existing in the art.