Nitroso-dimer-containing compositions and photoimaging process

A method for producing an image on a substrate by the process which comprises

• a. applying to the substrate a layer of a photopolymerizable coating composition which comprises
• 1. at least one, nongaseous, ethylenically unsaturated compound capable of forming a high polymer by free-radical initiated chain addition propagation,
• 2. 3-95% by weight, based on the total composition, of an organic polymeric binder having a molecular weight of at least 4000,
• 3. 0.1-5% by weight, based on the total composition, of a nitroso dimer which is a noninhibitor of free-radical polymerization but thermally dissociates to nitroso monomer which is an inhibitor of free-radical polymerization, and is characterized by
• a. a dissociation constant of 10.sup.-2 - 10.sup.-10 in solution at 25.degree. C, and
• b. a rate of dissociation having a half-life of at least 30 seconds in solution at 25.degree. C, and
• 4. 0.1 - 2% by weight, based on total composition, of an organic radiation-sensitive free-radical generating system, with the proviso that when the amount of free-radical generating system is 1-2%, the weight ratio of nitroso dimer to free-radical generating system is greater than 2 to 1,
• b. imagewise exposing the photopolymerizable layer through an image-bearing transparency at a temperature of 20.degree.-65.degree. C to radiation which activates the free-radical generating system to generate free-radicals, whereby the free-radicals are consumed by reaction with the nitroso monomer,
• c. essentially completely deactivating the nitroso dimer inhibitor system,
• d. reexposing at least the previously unexposed portion of the photopolymerizable layer to radiation which activates the free-radical generating system to generate free-radicals while continuing to maintain the nitroso dimer inhibitor system in the essentially completely deactivated state, whereby photopolymerization takes place, and
• e. developing the resulting image.