Heat sensitive positive-working lithographic printing plate precursor

1. A heat-sensitive positive-working lithographic printing plate precursor comprising

(1) a support having a hydrophilic surface or which is provided with a hydrophilic layer, and

(2) a heat-sensitive coating comprising

an underlayer on said support and thereon an upperlayer,

an IR absorbing agent in at least one of said underlayer and upperlayer,

a phenolic resin in said upperlayer, and

a first polymer in said underlayer,

wherein said first polymer is an alkaline soluble polymer comprising a first monomeric unit having a structure according to formula I or formula II and a second monomeric unit having a structure according to formula III,

wherein

* denotes the binding sites of the monomeric unit in the polymer backbone,

each R 1 independently represents hydrogen or an alkyl group,

Z represents oxygen or —NR 2 —,

a and b independently are 0 or 1,

R 2 represents hydrogen or an optionally substituted alkyl, alkenyl or alkynyl group,

Ar 1 and Ar 2 are an aromatic group with the proviso that at least one of Ar 1 and Ar 2 is an optionally substituted heteroaromatic group,

R 3 represents hydrogen or an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted aralkyl group or an optionally substituted alkaryl group, or an aryl group which is not substituted by a sulphonamide group or a heteroaryl group which is not substituted by a sulphonamide group, and

Ar 3 is an aromatic group which is not substituted by a sulphonamide group.

2. The precursor according to claim 1 , wherein Ar 3 is an optionally substituted aryl group, with the proviso that it is not substituted by a sulphonamide group.

3. The precursor according to claim 1 , wherein Ar 3 is an optionally substituted phenyl group, with the proviso that it is not substituted by a sulphonamide group.

4. The precursor according to claim 3 , wherein said substituting group of said phenyl group does not comprise an acidic group selected from a hydroxyl group, a carboxylic acid group or a sulphonic acid group.

5. The precursor according to claim 1 , wherein R 3 is hydrogen.

6. The precursor according to any of claims 1 - 5 , wherein said first polymer further comprises at least one monomeric unit selected from the group consisting of an optionally N-substituted maleimide monomeric unit, an ester of a (meth)acrylic acid, polyoxyethylene (meth)acrylic acid ester, 2-hydroxy ethyl (meth)acrylate, an optionally substituted styrene, a styrene sulphonic acid, an o-vinyl benzoic acid, a p-vinyl benzoic acid, a m-vinyl benzoic acid, an optionally substituted vinyl pyridine, N-vinyl caprolactam, N-vinyl pyrrolidone, (meth)acrylic acid, itaconic acid, maleic acid, glycidyl (meth)acrylate, optionally hydrolysed vinyl acetate and vinyl phosphonic acid.

7. The precursor according to claim 1 , wherein said first polymer comprises said first monomeric unit in an amount of at least 10 mol % and said second monomeric unit in an amount of at least 5 mol %.

8. The precursor according to claim 1 , wherein said first polymer comprises said first monomeric unit in an amount ranging between 20 and 90 mol % and said second monomeric unit in an amount ranging between 10 and 80 mol %.

9. A method for making a lithographic printing plate comprising the steps of:

(1) providing a heat-sensitive lithographic printing plate precursor according to claim 1 ;

(2) image-wise exposing said precursor with IR-radiation or heat; and

(3) developing said image-wise exposed precursor.