Lithographic printing plate precursor

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 two different sensitizers according to Formula I,

• wherein
• R1 to R6 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 an optionally substituted divalent linking group selected from the group consisting of:
• n is an integer from 0 to 2;
• X represents S, O or NRx;
• * represents the linking positions of the linking group L to the phenyl groups of Formula I;
• R, R′ and Rx represent an optionally substituted alkyl group;
• wherein for at least one sensitizer of said mixture at least one of R1 to R6 contains an asymmetric C-atom and the total amount of sensitizers of which at least one of R1 to R6 contains an asymmetric C-atom is more than 60 percent by weight relative to the total weight of said mixture.

A process of preparing a mixture of sensitizers according to formula II,

• wherein
• R1 to R6 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;
• L1 is an optionally substituted divalent linking group selected from the list consisting of:
• wherein
• m is 1 or 2;
• * represents the linking positions of the linking group L1 to the phenyl groups of Formula II;
• and
• R and R′ represent an optionally substituted alkyl group
• by reacting at least two different aldehydes according to Formula III and IV,
• with a bis-phosphonate according to Formula V,
• wherein
• R1 to R6 have the same meaning as in Formula II;
• R″ represents an alkoxy group or a phenyl group;
• L1′ is a divalent linking group selected from the list consisting of:
• m is 1 or 2;
• * represents the linking positions of the linking group L1′;
• and
• R and R′ represent an optionally substituted alkyl group.