Patent ID: 7442485

Claim:
A lithographic printing process which comprises the steps of: imagewise exposing to infrared light a presensitized lithographic plate which comprises a hydrophilic support and an image-forming layer containing an infrared absorbing agent, a radical polymerization initiator, an ethylenically unsaturated polymerizable compound and a binder polymer, said polymerization initiator being a salt of a sulfonium ion with an anion selected from the group consisting of (1) hydrogensulfate ion, (2) a sulfate ester ion, (3) a polymer having a carboxylate ion, (4) a polymer having a sulfonate ion, (5) a polymer having an anion of —S0 2 —N − —R 1 (wherein R 1 is carboxyl, formyl, an aliphatic group, an aromatic group, a heterocyclic group, —O—R, —S—R, —CO—R, —O—CO—R or —CO—O—R, and wherein R is an aliphatic group, an aromatic group or a heterocyclic group), (6) a carboxylate ion having an α-carbon atom substituted with an acyl group, carbamoyl, a substituted carbamoyl group or cyano, (7) a carboxylate ion having an α-carbon atom substituted with two or more aromatic groups, and (8) a carboxylate ion having an α-carbon atom to which a non-metallic atom other than carbon and hydrogen is attached, to cause a radical polymerization reaction of the polymerizable compound within the exposed area; removing the image-forming layer within the unexposed area while mounting the lithographic plate on a cylinder of a printing press; and then printing an image with the lithographic plate while mounting the lithographic plate on the cylinder of the printing press.