Lithographic printing plate precursor

1. A lithographic printing plate precursor comprising an image-recording layer, said image-recording layer being photo-polymerizable upon exposure with light having a wavelength of from 300 to 500 nm and comprising a mixture of sensitizers, said mixture comprises at least three different sensitizers according to Formula I,

wherein

R 1 to R 6 independently represent a hydrogen atom, an optionally substituted alkyl group, an optionally substituted alkoxy group, a cyano group, a halogen atom, a dialkylamino group, an alkylarylamino group or a diarylamino group;

L is a divalent linking group selected from the group consisting of:

n is an integer from 0 to 2;

X represents S, O or NR x ;

* represents the linking positions of the linking group L to the phenyl groups of Formula I;

R, R′ and R x represent an optionally substituted alkyl group;

wherein for at least two sensitizers of said mixture at least one of R 1 to R 6 contains an asymmetric C-atom and the total amount of two or more sensitizers of which at least one of R 1 to R 6 contains an asymmetric C-atom is more than 65 percent by weight relative to the total weight of said mixture.

2. The lithographic printing plate precursor according to claim 1 , wherein R 1 to R 6 are alkoxy groups.

3. The lithographic printing plate precursor according to claim 2 , wherein R 1 , R 3 , R 4 and R 6 are methoxy groups and R 5 and R 2 are alkoxy groups having 3 to 15 C-atoms.

4. A method of making a printing plate comprising the steps of:

providing a lithographic printing plate precursor as described in claim 3 ;

image-wise exposing said precursor with a laser having a wavelength from 300 to 500 nm;

optionally pre-heating said exposed precursor; and

developing said precursor.

5. The method according to claim 4 , wherein said light has an energy density, measured on the surface of the plate, of 100 μJ/cm 2 or less.

6. The method according to claim 4 , wherein the development is performed in a gum-solution.

7. The lithographic printing plate according to claim 1 , wherein said mixture of sensitizers comprises a mixture of sensitizers according to formula II,

wherein

R 1 to R 6 independently represent a hydrogen atom, an optionally substituted alkyl group, an optionally substituted alkoxy group, a cyano group, a halogen atom, a dialkylamino group, an alkylarylamino group or a diarylamino group;

L 1 is a divalent linking group selected from the list consisting of:

wherein

m is 1 or 2; and

represents the linking positions of the linking group L 1 to the phenyl groups of Formula II;

R and R′ represent an optionally substituted alkyl group.

8. The lithographic printing plate precursor according claim 1 , wherein the photopolymerizable coating further comprises a hexaarylbisimidazole compound as initiator.

9. The lithographic printing plate precursor according to claim 8 , wherein the photopolymerizable layer further comprises a mercaptobenzthiazole compound as co-initiator.

10. A method of making a printing plate comprising the steps of:

providing a lithographic printing plate precursor as described in claim 1 ;

image-wise exposing said precursor with a laser having a wavelength from 300 to 500 nm;

optionally pre-heating said exposed precursor; and

developing said precursor.

11. The method according to claim 10 , wherein said light has an energy density, measured on the surface of the plate, of 100 μJ/cm 2 or less.

12. The method according to claim 10 , wherein the development is performed in a gum-solution.

13. A method of making a printing plate comprising the steps of:

providing a lithographic printing plate precursor as described in claim 2 ;

image-wise exposing said precursor with a laser having a wavelength from 300 to 500 nm;

optionally pre-heating said exposed precursor; and

developing said precursor.

14. The method according to claim 13 , wherein the development is performed in a gum-solution.

15. The method according to claim 13 , wherein said light has an energy density, measured on the surface of the plate, of 100 μJ/cm 2 or less.