NEGATIVELY-WORKING LITHOGRAPHIC PRINTING PLATE PRECURSOR AND METHOD

1 . A negative-working, infrared radiation-sensitive lithographic printing plate precursor comprising:

a substrate having a hydrophilic surface, and

an infrared radiation-sensitive imageable layer that is disposed on the hydrophilic surface of the substrate, and comprises:

one or more free radically polymerizable compounds;

one or more infrared radiation absorbers;

an initiator composition that provides free radicals upon exposure of the infrared radiation-sensitive imageable layer to infrared radiation, the initiator composition comprising compound A represented by the following Structure (I) and, one or more compounds collectively as compound B that are represented by the following Structure (II) or Structure (III); and

a primary polymeric binder,

wherein:

R 1 , R 2 , R 3 , R 4 , R 5 and R 6 are independently substituted or unsubstituted alkyl groups or substituted or unsubstituted alkoxy groups each having 2 to 9 carbon atoms;

at least one of R 3 and R 4 is different from R 1 or R 2 ;

the difference between the total number of carbon atoms in R 1 and R 2 and the total number of carbon atoms in R 3 and R 4 is 0 to 4;

the difference between the total number of carbon atoms in R 1 and R 2 and the total number of carbon atoms in R 5 and R 6 is 0 to 4; and

X 1 , X 2 and X 3 are the same or different anions.

2 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein compound B comprises a compound that is represented by Structure (III), R 1 is the same as R 5 , and R 2 is the same as R 6 .

3 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 2 , wherein R 1 is the same as R 2 .

4 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein compound B comprises a compound that is represented by Structure (II), R 1 is the same as R 2 , and R 3 is the same as R 4 .

5 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 4 , wherein the difference in the number of carbon atoms between R 1 and R 3 is 0, 1, or 2.

6 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein R 1 , R 2 , R 3 , R 4 , R 5 and R 6 are independently substituted or unsubstituted alkyl groups.

7 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein each of R 1 , R 2 , R 3 , R 4 , R 5 and R 6 independently has 3 to 6 carbon atoms.

8 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein the molar ratio of compound A to compound B is from 10:90 to and including 90:10.

9 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein the sum of both compound A and compound B is at least 5 weight % and up to and including 18 weight %, based on the total dry weight of the infrared radiation-sensitive imageable layer.

10 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein at least one of X 1 , X 2 , and X 3 is a tetraarylborate anion comprising the same or different aryl groups.

11 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein each of X 1 , X 2 , and X 3 is tetraphenylborate.

12 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein at least one infrared radiation absorber is a cyanine dye comprising a tetraarylborate anion.

13 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein the primary polymeric binder is present in particulate form.

14 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein the primary polymeric binder comprises recurring units derived from a styrene and recurring units derived from acrylonitrile.

15 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein the primary polymeric binder comprises recurring units comprising polyalkylene oxide segments and recurring units comprising pendant cyano groups.

16 . The negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 , wherein:

compound B comprises a compound that is represented by Structure (III), R 1 is the same as R 5 , and R 2 is the same as R 6 ; or compound B comprises a compound represented by Structure (II), R 1 is the same as R 2 , R 3 is the same as R 4 , and the difference in the number of carbon atoms between R 1 and R 3 is 0, 1, or 2;

R 1 , R 2 , R 3 , R 4 , R 5 and R 6 are independently substituted or unsubstituted alkyl groups each having 3 to 6 carbon atoms;

the molar ratio of compound A to compound B is from 20:80 to and including 80:20;

the sum of the weight of both compound A and compound B is at least 7 weight % and up to and including 15 weight %, based on the total dry weight of the infrared radiation-sensitive imageable layer;

X 1 is a tetraphenylborate anion, and optionally each of X 2 and X 3 is a tetraarylborate anion;

at least one infrared radiation absorber is a cyanine dye comprising a tetraarylborate anion; and

the primary polymeric binder is present comprises recurring units derived from styrene and recurring units derived from acrylonitrile.

17 . A method for providing a lithographic printing plate, the method comprising in sequence:

imagewise exposing the negative-working, infrared radiation-sensitive lithographic printing plate precursor of claim 1 to infrared radiation to provide an exposed precursor comprising exposed regions and non-exposed regions in the infrared radiation-sensitive imageable layer,

mounting the exposed precursor onto a lithographic printing press, and

removing the infrared radiation-sensitive imageable layer in the non-exposed regions using a lithographic ink, fountain solution, or both a lithographic ink and a fountain solution to provide a lithographic printing plate.

18 . The method of claim 17 , wherein the negatively-working infrared radiation-sensitive lithographic printing plate precursor is defined by:

compound B comprises a compound that is represented by Structure (III), R 1 is the same as R 5 , and R 2 is the same as R 6 ; or compound B comprises a compound that is represented by Structure (II), R 1 is the same as R 2 , R 3 is the same as R 4 , and the difference in the number of carbon atoms between R 1 and R 3 is 0, 1, or 2;

R 1 , R 2 , R 3 , R 4 , R 5 and R 6 being independently substituted or unsubstituted alkyl groups each having 3 to 6 carbon atoms;

the molar ratio of compound A to compound B is from 20:80 to and including 80:20;

the sum of the weight of both compound A and compound B is at least 7 weight % and up to and including 15 weight %, based on the total dry weight of the infrared radiation-sensitive imageable layer;

X 1 is a tetraphenylborate anion, and optionally each of X 2 and X 3 is a tetraphenylborate anion;

at least one infrared radiation absorber is a cyanine dye comprising a tetraarylborate anion; and

the primary polymeric binder comprises recurring units derived from styrene and recurring units derived from acrylonitrile.