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 comprising at least two 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:

wherein 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 at least one sensitizer of said mixture is an asymmetric sensitizer wherein R 1 ≠R 4 or R 2 ≠R 5 or R 3 ≠R 6 and wherein the total amount of asymmetric sensitizers in said mixture is at least 25 percent by weight relative to the total weight of said mixture.

2. The lithographic printing plate precursor according to claim 1 , wherein said mixture of sensitizers comprises at least three different sensitizers.

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

4. The lithographic printing plate precursor according to claim 3 , 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.

5. The lithographic printing plate precursor according to claim 4 , wherein for at least one sensitizer R 5 or R 2 represents a branched alkoxy group having 3 to 15 C-atoms.

6. The lithographic printing plate precursor according to claim 5 , wherein for at least one sensitizer R 5 or R 2 represents a branched alkoxy group having 3 to 15 C-atoms.

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

8. The lithographic printing plate precursor according to claim 7 , 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.

9. The lithographic printing plate precursor according to claim 8 , wherein for at least one sensitizer R 5 or R 2 represents a branched alkoxy group having 3 to 15 C-atoms.

10. The lithographic printing plate precursor according to claim 9 , wherein said branched alkoxy group comprises an asymmetric C-atom.

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

wherein

R 1 to R 6 have the same meaning as in Formula I;

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

wherein

m is 1 or 2;

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

R and R′ represent an optionally substituted alkyl group.

12. The lithographic printing plate precursor according to claim 1 , wherein the photopolymerizabie coating further comprises a hexaarylbisimidazole compound as photo-initiator.

13. The lithographic printing plate precursor according to claim 12 , wherein the photopolymerizable layer further comprises a mercaptobenzothiazole compound as co-initiator.

14. 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 emitting light having a wavelength of from 300 to 500 nm;

optionally pre-heating said exposed precursor; and

developing said precursor.

15. The method according to claim 14 , wherein said light has an energy density, measured on the surface of the plate, of 100 μJ/cm 2 or less.

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