Infrared-sensitive photosensitive composition

#The present invention relates to an infrared-sensitive photosensitive composition, in particular, relates to an infrared-sensitive photosensitive composition for use in the image-forming layer of a positive lithographic printing plate precursor for so-called direct plate-making capable of directly plate-making from digital signals such as computer data.

#As the system of directly plate-making from digital signals such as computer data, systems by (1) electrophotography, (2) photopolymerization by combination of exposure by Ar laser and post-heating, (3) lamination of a silver salt photosensitive material on a photosensitive resin, (4) a silver master type, and (5) breakdown of a silicone rubber layer by discharge breakdown and laser beams are known.

#However, a method of using electrophotography of (1) is complicated in processes such as electrification, exposure and development, and requires intricate and large-scaled equipment. A method (2) requires highly sensitive printing materials over and above a post-heating process, and handling in daylight is difficult. Since methods (3) and (4) use silver salts, processing is complicated and uneconomical. A method (5) is a method of comparatively reaching completion, but a problem of the elimination of silicone residues on a plate remains. On the other hand, the development of laser in recent years is remarkable, in particular, small-sized solid state lasers and semiconductor lasers having emission regions in near infrared to infrared of high output are easily available. Laser is very useful as the exposure light source of direct plate-making from digital signals such as computer data.

#2,2-Bis(hydroxymethyl)propionic acid (14.6 g) (0.109 mol), 13.3 g (0.0686 mol) of tetraethylene glycol, and 2.05 g (0.0228 mol) of 1,4-butanediol were put in around bottom three-neck flask equipped with a cooling pipe and a stirrer, and dissolved in 118 g of N,N-dimethylacetamide. 4,4′-Diphenylmethane diisocyanate (30.8 g) (0.123 mol), 13.8 g (0.0819 mol) of hexamethylene diisocyanate, and 0.1 g of di-n-butyl tin dilaurate as a catalyst were added to the above solution, and the reaction mixture was heated at 90° C. for 7 hours with stirring. N,N-Dimethylacetamide (100 ml), 50 ml of methanol and 50 ml of acetic acid were added to the reaction solution and stirred, and the reaction mixture was poured into 4 liters of water with stirring, thereby a white polymer was precipitated. The polymer was filtered out, washed with water, and then dried under reduced pressure, thereby 60 g of a polymer (Polyurethane Resin (a)) was obtained.

#The molecular weight of Polyurethane Resin (a) measured by gel permeation chromatography (GPC) was 70,000 on weight average (in terms of polystyrene). The carboxyl group content measured by titration was 1.43 meq/g.

#Polyurethane Resins (b) to (f) were synthesized in the same manner as in Synthesis Example 1 by using the diisocyanate compounds shown in Table A and the diol compounds shown in Table B. The oxygen content measured by titration and the molecular weight measured by GPC of each polyurethane resin are shown together in Table B.