A copolymer includes (i) a plurality of ethylenic moieties A having a structure according to the following formula: ##STR00001##
wherein R.sup.2 and R.sup.3 independently represent hydrogen, a halogen, or an optionally substituted linear, branched, or cyclic alk(en)yl group, or an optionally substituted aromatic or heteroaromatic, and (ii) a plurality of acetal moieties B having a structure according to the following formula: ##STR00002##
wherein L represents a divalent linking group; x=0 or 1; and R.sup.1 represents an optionally substituted aromatic or heteroaromatic group including at least one hydroxyl group. Use of these polymers in the coating of lithographic printing plates provides a good abrasion resistance while the balance between the ink acceptance, arising from the ethylenic moieties, and the solubility in an alkaline developer, arising form the acetal moieties, can be controlled efficiently.

A lithographic printing plate precursor is disclosed which includes a support and a coating comprising a photopolymerisable layer including a polymerisable compound, an initiator and a pH sensitive colorant precursor and a protective overcoat layer provided above the photopolymerisable layer including a heat sensitive color-forming IR dye, which upon exposure to heat and/or light forms a print-out image which remains stable or increases during storage of the precursor in light or dark environment.

An object of the present invention is to provide an on-press development-type planographic printing plate precursor which suppresses color-forming defect after a lapse of time and has excellent visibility. Another object of the present invention is to provide a manufacturing method of a planographic printing plate and a printing method. The planographic printing plate precursor of the present invention is an on-press development-type planographic printing plate precursor including a support and an image-recording layer, in which the support has an aluminum alloy plate and an anodic oxide film disposed on the aluminum alloy plate, a density of intermetallic compounds having a major axis of 2.0 m or more on a surface of the support on a side opposite to the image-recording layer is 1,000 pieces/mm.sup.2 or less, the image-recording layer contains an infrared absorber and an acid color-forming agent, and a color-forming factor of the acid color-forming agent, which is determined by a predetermined measuring method, is 20,000 or more.

Provided are an on-press development type lithographic printing plate precursor including: a support; and an image-recording layer on the support, in which the image-recording layer contains an electron-accepting polymerization initiator, an electron-donating polymerization initiator, an infrared absorber, and a salt compound A, and an HOMO value of an anion constituting a salt in the salt compound A is 6.2 eV or more and 4.4 eV or less, and a method of preparing a lithographic printing plate or a lithographic printing method using the on-press development type lithographic printing plate precursor.

A lithographic printing plate precursor has a support and an image-recording layer on the support, in which the image-recording layer contains an infrared absorber, a polymerization initiator, and a polymer A. The weight-average molecular weight of the polymer A is more than 15,000 and 150,000 or less, and the ethylenically unsaturated bond valence of the polymer A is 3.0 mmol/g or more.

A positive-working lithographic printing plate precursor including a support and an image-recording layer formed on the support and consisting of a positive-working photosensitive resin composition, in which the positive-working photosensitive resin composition contains a compound having at least one electron-withdrawing group and at least one phenolic hydroxyl group in one molecule, an alkali-soluble resin, and an infrared absorber, and the electron-withdrawing group excludes a sulfonyl group; and a method of preparing a lithographic printing plate using the positive-working lithographic printing plate precursor.