A photopolymerizable composition is disclosed, which comprises a photopolymerizable compound and a photopolymerization initiator comprising a cationic dye, an organic boron compound and a compound having an --SH group in the molecule thereof, and which can be used for making printing plates, as resist materials for making printed circuit boards, as color proof materials, etc. The photopolymerizable composition can be applied for an image-forming process utilizing microcapsules, which comprises the steps of encapsulating the photopolymerizable composition with a color image-forming material in the core of microcapsules, coating the microcapsules on a support to form a photosensitive sheet, imagewise exposing the photosensitive sheet, superposing the photosensitive sheet on an image-receiving sheet, pressing the whole surface of the assembly to rupture the microcapsules at the unexposed portions, and transferring the color image-forming material into the image-receiving sheet. The color image obtained by using the photopolymerizable composition exhibits high sensitivity.

FIELD OF THE INVENTION 
This invention relates to a novel photopolymerizable composition. More 
specifically, the invention relates to a high-sensitive and spectrally 
sensitized photopolymerizable composition which can be used for making 
lithographic printing plates and resin relief plates, as resists or 
photomasks for making printed boards, and making black and white or color 
transfer sheets or coloring sheets. 
BACKGROUND OF THE INVENTION 
A photopolymerizable composition fundamentally contains a 
photopolymerization initiator and an addition polymerizable compound 
having at least two ethylenically unsaturated bonds in the molecule 
(hereinafter, the polymerizable compound is referred to as "polyfunctional 
monomer"). When the composition is irradiated by light, the composition is 
hardened to change the tackiness thereof, and becomes insoluble in the 
solvent. By utilizing these properties, these photopolymerizable 
compositions have been widely used for photography, printing, surface 
processing of metals, inks, etc. The utilization methods and application 
examples of such a composition are described, for example, in J. Kosar, 
Light Sensitive Systems, pages 158-193, published by J. Wiley & Sons, New 
York (1965) and K.I. Jacobson and R.E. Jacobson, Imaging Systems, pages 
181-222, published by J. Wiley & Sons, New York (1976). 
Also, recently, as an image forming method utilizing a photopolymerizable 
composition, an image forming system utilizing light-sensitive 
microcapsules containing a photopolymerizable composition in the 
microcapsules has been proposed. For example, JP-A-57-124343, 
JP-A-57-179836 and JP-A-57-197538 (the term "JP-A" as used herein 
indicates an "unexamined published Japanese patent application" disclose a 
method of forming dye images by imagewise exposing the coloring sheet 
having a coated layer of microcapsules containing a dye and a 
photopolymerizable composition composed of a vinyl compound and a 
photopolymerization initiator, superposing the coloring sheet onto an 
image-receiving sheet, and applying pressure to the whole assembly. 
Also, an attempt of spectrally sensitizing the photopolymerizable 
composition up to the visible light wavelength region and forming digital 
images using a laser as a light source and an application of such a 
photopolymerizable composition to full color photographic light-sensitive 
materials has been investigated. 
For example, in Nihon Shasin Gakkai-shi (Journal of The Society of 
Photographic Science and Technology of Japan), Vol. 49, No. 3, page 
230(1986) and Kino Zairyo (Functional Materials), pages 48-60, (September, 
1983), describe a spectral sensitization method of photopolymerizable 
compositions for laser recording. Also, JP-A-59-189340 describes a method 
of spectrally sensitizing an organic peroxide initiator with an organic 
dye. Furthermore, EP-A 223587 discloses organic boron anion salts of 
organic cationic dyes. 
In particular, a method of using the organic boron compound anionic salt of 
the organic cationic dye as a photosensitive initiator, has a wide 
selection range for cationic dye compounds and is useful in designing a 
photopolymerization initiator composition sensitive to optional 
wavelengths. 
However, by such a method, the photopolymerization initiator composition 
obtained, having sensitivity to visible light through the light 
sensitivity thereof, is still unsatisfactory. Thus, in EP-A-223587, the 
sensitivity is increased by combining the aforesaid composition with an 
N,N-dialkylaniline derivative, in particular 
2,6-diisopropyl-N,N-dimethylaniline. However, the light sensitivity 
obtained by using the aforesaid combination is yet insufficient. 
For photopolymerizabion initiators having light sensitivity to mainly 
ultraviolet light, various methods for improving the sensitivity have 
hitherto been known. 
For example, a method is known of combining an aminophenyl ketone and an 
active methyl compound or an amino compound (as described in U.S. Pat. No. 
3,661,588), a combination of Michler's ketone and benzophenone (as 
described in U.S. Pat. No. 3,682,641), a combination of a 
2,4,5-triphenylimidazolyl dimer and an organic mercapto compound (as 
described in U.S. Pat. No. 3,479,185), a combination of an aromatic ketone 
or a polynuclear quinone and a mercaptoimidazoline or a 
mercaptobenzoxazole (as described in JP-A-53-702), and a combination of a 
p-dialkylaminostilbene derivative, a hexaarylbiimidazole, and a 
mercaptobenzoxazole or a mercaptobenzimidazole (as described in JP 
A-59-56403). 
However, a high sensitivity imparting technique for a novel combination of 
an organic cationic dye and an organic boron compound anionic salt 
photopolymerization initiator having light sensitivity to light of longer 
wavelength than visible light has not yet been known. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a novel photopolymerizable 
composition having high sensitivity and having light sensitivity to a 
light source of visible light or longer wavelength region. More 
particularly, the object of this invention is to provide a 
photopolymerizable composition having high sensitivity and having a light 
sensitivity of a visible light or longer wavelength region (for example, 
to a laser light source), and which is advantageously utilized for making 
lithographic printing plates and resin relief plates, and as a resist or a 
photomask for making printed boards. 
Another object of this invention is to provide microcapsules having light 
sensitivity to a light source of a visible light or longer wavelength 
region and capable of being advantageously used for the formation of black 
and white or color images. 
As a result of various investigations, the inventors have discovered that 
the aforesaid objects can be attained by a photopolymerizable composition 
comprising: 
(a) a polymerizable compound having at least one ethylenically unsaturated 
bond in the molecule, 
(b) an organic boron compound anionic salt of an organic cationic dye 
represented by formula (I) shown below, and 
(c) a compound having an -SH group in the molecule thereof, as represented 
by formula (II) shown below; 
##STR1## 
wherein D.sup.+ represents a cationic dye, and R.sup.1, R.sup.2, R.sup.3, 
and R.sup.4, which may be the same or different, each represents an alkyl, 
aryl, aralkyl, alkaryl, alkenyl, alkinyl, alicyclic, heterocyclic, or 
allyl group having carbon atoms less than 18 or a group selected from the 
derivatives of these groups; with the proviso that at least two of 
R.sup.1, R.sup.2, R.sup.3, and R.sup.4 may combine to form a ring 
structure; and 
##STR2## 
wherein X represents NR' (wherein R' represents a hydrogen atom, an alkyl 
group having from 1 to 12 carbon atoms, a phenyl group or a phenyl group 
substituted with a halogen atom or an alkyl group having from 1 to 4 
carbon atoms), an oxygen atom, a sulfur atom, a methylene group or a 
methine group; and A represents a residual group to form a 5- or 
6-membered heterocyclic ring together with the moiety --N.dbd.CX--X-- or a 
residual group being composed of 2 or 3 carbon atoms, which forms a part 
of an aromatic ring.