Thermal switchable composition and imaging member containing cationic IR dye and methods of imaging and printing

[0002] This invention relates in general to thermal imaging compositions, and to lithographic imaging members (particularly lithographic printing plates) prepared therefrom. The invention also relates to a method of imaging such imaging members, and to a method of printing using them.

[0003] The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.

[0004] Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.

[0005] Thermally sensitive printing plates are becoming more common. Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haley et al). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.

[0006] It has been recognized that a lithographic printing plate could be created by ablating an IR absorbing layer. For example, Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). Such plates were exposed to focused near IR radiation with a Nd⁺⁺YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. Similar plates are described in Research Disclosure19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. Other publications describing ablatable printing plates include U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,339,737 (Lewis et al), U.S. Pat. No. 5,353,705 (Lewis et al), U.S. Pat. No. Reissue 35,512 (Nowak et al), and U.S. Pat. No. 5,378,580 (Leenders).

[0007] While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable for resulting printing quality. Such plates generally require at least two coated layers on a support.

[0008] Thermally switchable polymers have been described for use as imaging materials in printing plates. By “switchable” is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic or, conversely from hydrophilic to relatively more hydrophobic, upon exposure to heat. U.S. Pat. No. 4,034,183 (Uhlig) describes the use of high powered lasers to convert hydrophilic surface layers to hydrophobic surfaces. A similar process is described for converting polyamic acids into polyimides in U.S. Pat. No. 4,081,572 (Pacansky). The use of high-powered lasers is undesirable in the industry because of their high electrical power requirements and because of their need for cooling and frequent maintenance.

[0009] U.S. Pat. No. 4,634,659 (Esumi et al) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. While this concept was one of the early applications of converting surface characteristics in printing plates, it has the disadvantages of requiring long UV light exposure times (up to 60 minutes), and the plate's use is in a positive-working mode only.

[0010] U.S. Pat. No. 4,405,705 (Etoh et al) and U.S. Pat. No. 4,548,893 (Lee et al) describe amine-containing polymers for photosensitive materials used in non-thermal processes. Thermal processes using polyamic acids and vinyl polymers with pendant quaternary ammonium groups are described in U.S. Pat. No. 4,693,958 (Schwartz et al). U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having cationic quaternary ammonium groups that are heat-sensitive. However, the materials described in this art require wet processing after imaging.

[0011] WO 92/09934 (Vogel et al) describes photosensitive compositions containing a photoacid generator and a polymer with acid labile tetrahydropyranyl or activated ester groups. However, imaging of these compositions converts the imaged areas from hydrophobic to hydrophilic in nature.

[0012] In addition, EP-A 0 652 483 (Ellis et al) describes lithographic printing plates imageable using IR lasers, and which do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups. Imaging such compositions converts the imaged areas from hydrophobic to relatively more hydrophilic in nature, and thus requires imaging the background of the plate, which is generally a larger area. This can be a problem when imaging to the edge of the printing plate is desired.

[0013] Copending U.S. Ser. No. 09/162,905, filed on Sep. 29, 1998, U.S. Ser. No. 09/163,020 filed on Sep. 29, 1998, U.S. Ser. No. 09/309,999 filed on May 11, 1999, U.S. Ser. No. 09/310,038 filed on May 11, 1999 and U.S. Ser. No. 09/156,833 filed on Sep. 18, 1998, are directed to processless direct write printing plates that include an imaging layer containing heat sensitive polymers. The polymer coatings are sensitized to infrared radiation by the incorporation of an infrared absorbing material such as an organic dye or a fine dispersion of carbon black. Upon exposure to a high intensity infrared laser, light absorbed by the organic dye or carbon black is converted to heat, thereby promoting a physical change in the polymer (usually a change in hydrophilicity or hydrophobicity). The resulting printing plates can be used on conventional printing presses to provide, for example, negative images. Such printing plates have utility in the evolving “computer-to-plate” printing market.

[0014] Some of the heat-sensitive polymers described in the copending applications, particularly the polymers containing organoonium or other charged groups, have a tendency to undergo physical interactions or chemical reactions with the organic dye or carbon black, thus compromising the effectiveness of both polymers and heat-absorbing materials. In particular, while carbon black is an infrared radiation absorbing material of preference because of its low cost and absorption of light throughout the infrared region of the electromagnetic spectrum, its use also creates problems. For example, it cannot be readily dispersed out of water or the alcoholic solvents of choice. Special carbon black products that are designed to be water-dispersible (that is, have special surface functionalities), however, often agglomerate in the presence of polymers (including organoonium polymers) containing ionic groups due to chemical interactions.

[0015] Organic dye salts, by nature, are often partially soluble in water or alcoholic coating solvents and are thus preferred as IR dye sensitizers. However, many such salts have been found to be unacceptable because of insufficient solubility, because they react with the charged polymer to form hydrophobic products that can result in scummed or toned images, or because they offer insufficient thermal sensitization in imaging members having aluminum supports.

[0016] Thus, the graphic arts industry is seeking an alternative means for providing processless, direct-write lithographic imaging members that can be imaged without ablation, or the other problems noted above in relation to known processless direct write printing plates. It would also be desirable to have heat-sensitive imaging members that include IR dye sensitizers that are highly effective to convert light exposure into heat and that can be coated out of water or other environmentally suitable solvents.

[0017] The problems noted above are overcome with a composition useful for thermal imaging comprising:

[0018] a) a hydrophilic heat-sensitive ionomer,

[0019] b) water or a water-miscible organic solvent, and

[0020] c) an aromatic infrared radiation sensitive dye that is soluble in water or the water-miscible organic solvent, and has at least two quaternary ammonium groups.

[0021] This invention also provides an imaging member comprising a support and having disposed thereon a hydrophilic heat-sensitive layer that is prepared from the composition described above.

[0022] Still further, this invention includes a method of imaging comprising the steps of:

[0023] A) providing the imaging member described above, and

[0024] B) imagewise exposing the imaging member to provide exposed and unexposed areas in the imaging layer of the imaging member, whereby the exposed areas are rendered more hydrophobic than the unexposed areas by heat provided by the imagewise exposure.

[0025] Still again, a method of printing comprises the steps of carrying out steps A and B noted above, and additionally:

[0026] C) contacting the imaging member with a fountain solution and a lithographic printing ink, and imagewise transferring that printing ink from the imaging member to a receiving material.

[0027] As used herein, the term “ionomer” refers to a charged polymer having at least 20 mol % of the recurring units negatively or positively charged. These ionomers are generally referred to as “charged polymers” in the following disclosure.

[0028] The imaging members of this invention have a number of advantages, and avoid the problems of previous printing plates. Specifically, the problems and concerns associated with ablation imaging (that is, imagewise removal of a surface layer) are avoided because the hydrophilicity of the imaging layer is changed imagewise by “switching” (preferably, irreversibly) exposed areas of its printing surface to be less hydrophilic (that is, become more hydrophobic when heated). Thus, the imaging layer stays intact during and after imaging (that is, no ablation occurs). These advantages are achieved by using a hydrophilic heat-sensitive polymer having recurring ionic groups within the polymer backbone or chemically attached thereto. Such polymers and groups are described in more detail below. The polymers used in the imaging layer are readily prepared using procedures described herein, and the imaging members of this invention are simple to make and use without the need for post-imaging wet processing. The resulting printing members formed from the imaging members of this invention are generally negative-working in nature. In some cases, the polymers are crosslinked upon exposure and provide increased durability to the imaging members. In other and preferred cases, the polymers are crosslinked upon application to a support and curing.

[0029] Positively charged polymers, such as organoonium polymers that are preferred in the practice of this invention are typically coated out of water and methanol, solvents that readily dissolve these water-soluble polymeric salts.

[0030] The organic aromatic infrared radiation-sensitive dyes (“IR dyes” herein) used in this invention are desired sensitizers for thermal imaging members because they can be selected to have maximum absorption at the operating wavelength of a laser platesetter (generally 700 nm or more). Moreover, they can be coated in a dissolved (that is molecularly dispersed) state, providing for maximized utilization of energy as well as maximized image resolution capability. Water and alcoholic solvents used for dissolving the positively charged polymers also readily dissolve the organic IR dyes of because of the multiple quaternary ammonium groups on the dye molecule. Thus, homogeneous compositional coatings are possible on any type of imaging member support, including aluminum supports. Furthermore, we have not observed adverse effects that normally accompany an interaction of the polymers and the IR dyes described herein.

[0031] The imaging members of this invention comprise a support and one or more layers thereon that include a dried heat-sensitive composition. The support can be any self-supporting material including polymeric films, glass, ceramics, cellulosic materials (including papers), metals or stiff papers, or a lamination of any of these materials. The thickness of the support can be varied. In most applications, the thickness should be sufficient to sustain the wear from printing and thin enough to wrap around a printing form. A preferred embodiment uses a polyester support prepared from, for example, polyethylene terephthalate or polyethylene naphthalate, and having a thickness of from about 100 to about 310 μm. Another preferred embodiment uses aluminum sheets having a thickness of from about 100 to about 600 μm. The support should resist dimensional change under conditions of use.

[0032] The support may also be a cylindrical support that includes printing cylinders on press as well as printing sleeves that are fitted over printing cylinders. The use of such supports to provide cylindrical imaging members is described in U.S. Pat. No. 5,713,287 (Gelbart). The heat-sensitive polymer composition can be coated or sprayed directly onto the cylindrical surface that is an integral part of the printing press.

[0033] The support may be coated with one or more “subbing” layers to improve adhesion of the final assemblage. Examples of subbing layer materials include, but are not limited to, gelatin and other naturally occurring and synthetic hydrophilic colloids and vinyl polymers (such as vinylidene chloride copolymers) that are known for such purposes in the photographic industry, vinylphosphonic acid polymers, sol gel materials such as those prepared from alkoxysilanes (including glycidoxypropyltriethoxysilane and aminopropyltriethoxysilane), epoxy functional polymers, and various ceramics.

[0034] The backside of the support may be coated with antistatic agents and/or slipping layers or matte layers to improve handling and “feel” of the imaging member.

[0035] The imaging members, however, preferably have only one layer on the support, that is a heat-sensitive surface layer that is required for imaging. This hydrophilic layer is prepared from a composition of this invention and includes one or more heat-sensitive charged polymers and an aromatic IR dye as a photothermal conversion material (both described below). Because of the particular polymer(s) used in the imaging layer, the exposed (imaged) areas of the layer are rendered more hydrophobic in nature. The unexposed areas remain hydrophilic in nature.

[0036] In the heat-sensitive imaging layer of the imaging member, only the one or more charged polymers and one or more aromatic IR dyes are essential for imaging. The charged polymers generally are comprised of recurring units, of which at least 20 mol % include ionic groups. Preferably, at least 30 mol % of the recurring groups include ionic groups. Thus each of these polymers has a net charge provided by these ionic groups. Preferably, the ionic groups are cationic groups.

[0037] The charged polymers (ionomers) useful in the practice of this invention can be in any of three broad classes of materials:

[0038] I) crosslinked or uncrosslinked vinyl polymers comprising recurring units comprising positively-charged, pendant N-alkylated aromatic heterocyclic groups,

[0039] II) crosslinked or uncrosslinked polymers comprising recurring organoonium groups, and

[0040] III) polymers comprising a pendant thiosulfate (Bunte salt) group.

[0041] Each class of polymers is described in turn. The imaging layer can include mixtures of polymers from each class, or a mixture of one or more polymers of two or more classes. The Class II polymers are preferred.

[0042] Class I Polymers

[0043] The Class I polymers generally have a molecular weight of at least 1000 and can be any of a wide variety of hydrophilic vinyl homopolymers and copolymers having the requisite positively-charged groups. They are prepared from ethylenically unsaturated polymerizable monomers using any conventional polymerization technique. Preferably, the polymers are copolymers prepared from two or more ethylenically unsaturated polymerizable monomers, at least one of which contains the desired pendant positively-charged group, and another monomer that is capable of providing other properties, such as crosslinking sites and possibly adhesion to the support. Procedures and reactants needed to prepare these polymers are well known. With the additional teaching provided herein, the known polymer reactants and conditions can be modified by a skilled artisan to attach a suitable cationic group.

[0044] The presence of a cationic group apparently provides or facilitates the “switching” of the imaging layer from hydrophilic to hydrophobic in the areas that have been exposed to heat in some manner, when the cationic group reacts with its counterion. The net result is the loss of charge. Such reactions are more easily accomplished when the anion is more nucleophilic and/or more basic. For example, an acetate anion is typically more reactive than a chloride anion. By varying the chemical nature of the anion, the reactivity of the heat-sensitive polymer can be modified to provide optimal image resolution for a given set of conditions (for example, laser hardware and power, and printing press needs) balanced with sufficient ambient shelf life. Useful anions include the halides, carboxylates, sulfates, borates and sulfonates. Representative anions include, but are not limited to, chloride, bromide, fluoride, acetate, tetrafluoroborate, formate, sulfate, p-toluenesulfonate and others readily apparent to one skilled in the art. The halides and carboxylates are preferred.

[0045] The aromatic cationic group is present in sufficient recurring units of the polymer so that the heat-activated reaction described above can provide desired hydrophobicity of the imaged printing layer. The groups can be attached along a principal backbone of the polymer, or to one or more branches of a polymeric network, or both. The aromatic groups generally comprise 5 to 10 carbon, nitrogen, sulfur or oxygen atoms in the ring (at least one being a positively-charged nitrogen atom), to which is attached a branched or unbranched, substituted or unsubstituted alkyl group. Thus, the recurring units containing the aromatic heterocyclic group can be represented by the Structure I:

[0046] In this structure, R₁is a branched or unbranched, substituted or unsubstituted alkyl group having from 1 to 12 carbon atoms (such as methyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, methoxymethyl, benzyl, neopentyl and dodecyl). Preferably, R₁is a substituted or unsubstituted, branched or unbranched alkyl group having from 1 to 6 carbon atoms, and most preferably, it is substituted or unsubstituted methyl group.

[0047] R₂can be a substituted or unsubstituted alkyl group (as defined above, and additionally a cyanoalkyl group, a hydroxyalkyl group or alkoxyalkyl group), substituted or unsubstituted alkoxy having 1 to 6 carbon atoms (such as methoxy, ethoxy, isopropoxy, oxymethylmethoxy, n-propoxy and butoxy), a substituted or unsubstituted aryl group having 6 to 14 carbon atoms in the ring (such as phenyl, naphthyl, anthryl, p-methoxyphenyl, xylyl, and alkoxycarbonylphenyl), halo (such as chloro and bromo), a substituted or unsubstituted cycloalkyl group having 5 to 8 carbon atoms in the ring (such as cyclopentyl, cyclohexyl and ⁴-methylcyclohexyl), or a substituted or unsubstituted heterocyclic group having 5 to 8 atoms in the ring including at least one nitrogen, sulfur or oxygen atom in the ring (such as pyridyl, pyridinyl, tetrahydrofuranyl and tetrahydropyranyl). Preferably, R₂is substituted or unsubstituted methyl or ethyl group.

[0048] Z″ represents the carbon and any additional nitrogen, oxygen, or sulfur atoms necessary to complete the 5- to 10-membered aromatic N-heterocyclic ring that is attached to the polymeric backbone. Thus, the ring can include two or more nitrogen atoms in the ring (for example, N-alkylated diazinium or imidazolium groups), or N-alkylated nitrogen-containing fused ring systems including, but not limited to, pyridinium, quinolinium, isoquinolinium acridinium, phenanthradinium and others readily apparent to one skilled in the art.

[0049] W⁻ is a suitable anion as described above. Most preferably it is acetate or chloride.

[0050] Also in Structure I, n is defined as 0 to 6, and is preferably 0 or 1. Most preferably, n is 0.

[0051] The aromatic heterocyclic ring can be attached to the polymeric backbone at any position on the ring. Preferably, there are 5 or 6 atoms in the ring, one or two of which are nitrogen. Thus, the N-alkylated nitrogen containing aromatic group is preferably imidazolium or pyridinium and most preferably it is imidazolium.

[0052] The recurring units containing the cationic aromatic heterocycle can be provided by reacting a precursor polymer containing unalkylated nitrogen containing heterocyclic units with an appropriate alkylating agent (such as alkyl sulfonate esters, alkyl halides and other materials readily apparent to one skilled in the art) using known procedures and conditions.

[0053] Preferred Class I polymers can be represented by the following Structure II:

[0054] wherein X represents recurring units to which the N-alkylated nitrogen containing aromatic heterocyclic groups (represented by HET⁺) are attached, Y represents recurring units derived from ethylenically unsaturated polymerizable monomers that may provide active sites for crosslinking using any of various crosslinking mechanisms (described below), and Z represents recurring units derived from any additional ethylenically unsaturated polymerizable monomers. The various repeating units are present in suitable amounts, as represented by x being from about 20 to 100 mol %, y being from about 0 to about 20 mol %, and z being from 0 to 80 mol %. Preferably, x is from about 30 to about 98 mol %, y is from about 2 to about 10 mol % and z is from 0 to about 68 mol %.

[0055] Crosslinking of the polymers can be provided in a number of ways. There are numerous monomers and methods for crosslinking that are familiar to one skilled in the art. Some representative crosslinking strategies include, but are not necessarily limited to:

[0056] a) reacting an amine or carboxylic acid or other Lewis basic units with diepoxide crosslinkers,

[0057] b) reacting an epoxide units within the polymer with difunctional amines, carboxylic acids, or other difunctional Lewis basic unit,

[0058] c) irradiative or radical-initiated crosslinking of double bond-containing units such as acrylates, methacrylates, cinnamates, or vinyl groups,

[0059] d) reacting a multivalent metal salts with ligating groups within the polymer (the reaction of zinc salts with carboxylic acid-containing polymers is an example),

[0060] e) using crosslinkable monomers that react via the Knoevenagel condensation reaction, such as (2-acetoacetoxy)ethyl acrylate and methacrylate,

[0061] f) reacting an amine, thiol, or carboxylic acid groups with a divinyl compound (such as bis (vinylsulfonyl) methane) via a Michael addition reaction,

[0062] g) reacting a carboxylic acid units with crosslinkers having multiple aziridine units,

[0063] h) reacting a crosslinkers having multiple isocyanate units with amines, thiols, or alcohols within the polymer,

[0064] i) mechanisms involving the formation of interchain sol-gel linkages [such as the use of the 3-(trimethoxysilyl)propylmethacrylate monomer],

[0065] j) oxidative crosslinking using an added radical initiator (such as a peroxide or hydroperoxide),

[0066] k) autooxidative crosslinking, such as employed by alkyd resins,

[0067] l) sulfur vulcanization, and

[0068] m) processes involving ionizing radiation.

[0069] Monomers having crosslinkable groups or active crosslinkable sites (or groups that can serve as attachment points for crosslinking additives, such as epoxides) can be copolymerized with the other monomers noted above. Such monomers include, but are not limited to, 3-(trimethoxysilyl)propyl acrylate or methacrylate, cinnamoyl acrylate or methacrylate, N-methoxymethyl methacrylamide, N-aminopropylacrylamide hydrochloride, acrylic or methacrylic acid and hydroxyethyl methacrylate.

[0070] Additional monomers that provide the repeating units represented by “Z” in the Structure II above include any useful hydrophilic or oleophilic ethylenically unsaturated polymerizable monomer that may provide desired physical or printing properties to the hydrophilic imaging layer. Such monomers include, but are not limited to, acrylates, methacrylates, isoprene, acrylonitrile, styrene and styrene derivatives, acrylamides, methacrylamides, acrylic or methacrylic acid and vinyl halides.

[0071] Representative Class I polymers are identified hereinbelow as Polymers 1 and 3-6. Mixtures of these polymers can also be used. Polymer 2 below is a precursor to a useful Class I polymer.

[0072] Class II Polymers

[0073] The Class II polymers also generally have a molecular weight of at least 1000. They can be any of a wide variety of vinyl or non-vinyl homopolymers and copolymers.

[0074] Non-vinyl polymers of Class II include, but are not limited to, polyesters, polyamides, polyamide-esters, polyarylene oxides and derivatives thereof, polyurethanes, polyxylylenes and derivatives thereof, silicon-based sol gels (solsesquioxanes), polyamidoamines, polyimides, polysulfones, polysiloxanes, polyethers, poly(ether ketones), poly(phenylene sulfide) ionomers, polysulfides and polybenzimidazoles. Preferably, such non-vinyl polymers are silicon based sol gels, polyarylene oxides, poly(phenylene sulfide) ionomers or polyxylylenes, and most preferably, they are poly(phenylene sulfide) ionomers. Procedures and reactants needed to prepare all of these types of polymers are well known. With the additional teaching provided herein, the known polymer reactants and conditions can be modified by a skilled artisan to incorporate or attach a suitable cationic organoonium moiety.

[0075] Silicon-based sol gels useful in this invention can be prepared as a crosslinked polymeric matrix containing a silicon colloid derived from di-, tri- or tetraalkoxy silanes. These colloids are formed by methods described in U.S. Pat. Nos. 2,244,325, 2,574,902 and 2,597,872. Stable dispersions of such colloids can be conveniently purchased from companies such as the DuPont Company. A preferred sol-gel uses N-trimethoxysilylpropyl-N,N,N-trimethylammonium acetate both as the crosslinking agent and as the polymer layer forming material.

[0076] The presence of an organoonium moiety that is chemically incorporated into the polymer in some fashion apparently provides or facilitates the “switching” of the imaging layer from hydrophilic to oleophilic in the exposed areas upon exposure to energy that provides or generates heat, when the cationic moiety reacts with its counterion. The net result is the loss of charge. Such reactions are more easily accomplished when the anion of the organoonium moiety is more nucleophilic and/or more basic, as described above for the Class I polymers.

[0077] The organoonium moiety within the polymer can be chosen from a trisubstituted sulfur moiety (organosulfonium), a tetrasubstituted nitrogen moiety (organoammonium), or a tetrasubstituted phosphorous moiety (organophosphonium). The tetrasubstituted nitrogen (organoammonium) moieties are preferred. This moiety can be chemically attached to (that is, pendant) the polymer backbone, or incorporated within the backbone in some fashion, along with the suitable counterion. In either embodiment, the organoonium moiety is present in sufficient repeating units of the polymer (at least 20 mol %) so that the heat-activated reaction described above can occur to provide desired hydrophobicity of the imaging layer. When chemically attached as a pendant group, the organoonium moiety can be attached along a principal backbone of the polymer, or to one or more branches of a polymeric network, or both. When chemically incorporated within the polymer backbone, the moiety can be present in either cyclic or acyclic form, and can also form a branching point in a polymer network. Preferably, the organoonium moiety is provided as a pendant group along the polymeric backbone. Pendant organoonium moieties can be chemically attached to the polymer backbone after polymer formation, or functional groups on the polymer can be converted to organoonium moieties using known chemistry. For example, pendant quaternary ammonium groups can be provided on a polymeric backbone by the displacement of a “leaving group” functionality (such as a halogen) by a tertiary amine nucleophile. Alternatively, the organoonium group can be present on a monomer that is then polymerized or derived by the alkylation of a neutral heteroatom unit (trivalent nitrogen or phosphorous group or divalent sulfur group) already incorporated within the polymer.

[0078] The organoonium moiety is substituted to provide a positive charge. Each substituent must have at least one carbon atom that is directly attached to the sulfur, nitrogen or phosphorus atom of the organoonium moiety. Useful substituents include, but are not limited to, substituted or unsubstituted alkyl groups having 1 to 12 carbon atoms and preferably from 1 to 7 carbon atoms (such as methyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, methoxyethyl, isopropoxymethyl, substituted or unsubstituted aryl groups (phenyl, naphthyl, p-methylphenyl, m-methoxyphenyl, p-chlorophenyl, p-methylthiophenyl, p-N,N-dimethylaminophenyl, xylyl, methoxycarbonylphenyl and cyanophenyl), and substituted or unsubstituted cycloalkyl groups having 5 to 8 carbon atoms in the carbocyclic ring (such as cyclopentyl, cyclohexyl, 4-methylcyclohexyl and 3-methylcyclohexyl). Other useful substituents would be readily apparent to one skilled in the art, and any combination of the expressly described substituents is also contemplated.

[0079] The organoonium moieties include any suitable anion as described above for the Class I polymers. The halides and carboxylates are preferred.

[0080] Representative Class II non-vinyl polymers are identified herein below as Polymers 7-8 and 10. Mixtures of these polymers can also be used. Polymer 9 is a precursor to Polymer 10.

[0081] In addition, vinyl Class II polymers can be used in the practice of this invention. Like the non-vinyl polymers, such heat-sensitive polymers are composed of recurring units having one or more types of organoonium group. For example, such a polymer can have recurring units with both organoammonium groups and organosulfonium groups. It is also not necessary that all of the organoonium groups have the same alkyl substituents. For example, a polymer can have recurring units having more than one type of organoammonium group. Useful anions in these polymers are the same as those described above for the non-vinyl polymers. In addition, the halides and carboxylates are preferred.

[0082] The organoonium group is present in sufficient recurring units of the polymer so that the heat-activated reaction described above can occur to provide desired hydrophobicity of the imaged printing layer. The group can be attached along a principal backbone of the polymer, or to one or more branches of a polymeric network, or both. Pendant groups can be chemically attached to the polymer backbone after polymer formation using known chemistry. For example, pendant organoammonium, organophosphonium or organosulfonium groups can be provided on a polymeric backbone by the nucleophilic displacement of a pendant leaving group (such as a halide or sulfonate ester) on the polymeric chain by a trivalent amine, divalent sulfur or trivalent phosphorous nucleophile. Pendant onium groups can also be provided by alkylation of corresponding pendant neutral heteroatom groups (nitrogen, sulfur or phosphorous) using any commonly used alkylating agent such as alkyl sulfonate esters or alkyl halides. Alternatively a monomer precursor containing the desired organoammonium, organophosphonium or organosulfonium group may be polymerized to yield the desired polymer.

[0083] The organoammonium, organophosphonium or organosulfonium group in the vinyl polymer provides the desired positive charge. Generally, preferred pendant organoonium groups can be illustrated by the following Structures III, IV and V:

[0084] wherein R is a substituted or unsubstituted alkylene group having 1 to 12 carbon atoms that can also include one or more oxy, thio, carbonyl, amido or alkoxycarbonyl groups with the chain (such as methylene, ethylene, isopropylene, methylenephenylene, methyleneoxymethylene, n-butylene and hexylene), a substituted or unsubstituted arylene group having 6 to 10 carbon atoms in the ring (such as phenylene, naphthylene, xylylene and 3-methoxyphenylene), or a substituted or unsubstituted cycloalkylene group having 5 to 10 carbon atoms in the ring (such as 1,4-cyclohexylene, and 3-methyl-1,4-cyclohexylene). In addition, R can be a combination of two or more of the defined substituted or unsubstituted alkylene, arylene and cycloalkylene groups. Preferably, R is a substituted or unsubstituted ethyleneoxycarbonyl or phenylenemethylene group. Other useful substituents not listed herein could include combinations of any of those groups listed above as would be readily apparent to one skilled in the art.

[0085] R₃, R₄and R₅are independently substituted or unsubstituted alkyl group having 1 to 12 carbon atoms (such as methyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, hydroxymethyl, methoxymethyl, benzyl, methylenecarboalkoxy and a cyanoalkyl), a substituted or unsubstituted aryl group having 6 to 10 carbon atoms in the carbocyclic ring (such as phenyl, naphthyl, xylyl, p-methoxyphenyl, p-methylphenyl, m-methoxyphenyl, p-chlorophenyl, p-methylthiophenyl, p-N,N-dimethylaminophenyl, methoxycarbonylphenyl and cyanophenyl), or a substituted or unsubstituted cycloalkyl group having 5 to 10 carbon atoms in the carbocyclic ring (such as 1,3- or 1,4-cyclohexyl). Alternatively, any two of R₃, R₄and R₅can be combined to form a substituted or unsubstituted heterocyclic ring with the charged phosphorus, sulfur or nitrogen atom, the ring having 4 to 8 carbon, nitrogen, phosphorus, sulfur or oxygen atoms in the ring. Such heterocyclic rings include, but are not limited to, substituted or unsubstituted morpholinium, piperidinium and pyrrolidinium groups for Structure V. Other useful substituents for these various groups would be readily apparent to one skilled in the art, and any combinations of the expressly described substituents are also contemplated.

[0086] Preferably, R₃, R₄and R₅are independently substituted or unsubstituted methyl or ethyl groups.

[0087] W⁻ is any suitable anion as described above for the Class I polymers. Acetate and chloride are preferred anions.

[0088] Polymers containing quaternary ammonium groups as described herein are most preferred vinyl Class II polymers.

[0089] In preferred embodiments, the vinyl Class II polymers useful in the practice of this invention can be represented by the following Structure VI:

[0090] wherein X′ represents recurring units to which the organoonium groups (“ORG”) are attached, Y′ represents recurring units derived from ethylenically unsaturated polymerizable monomers that may provide active sites for crosslinking using any of various crosslinking mechanisms (described below), and Z′ represents recurring units derived from any additional ethylenically unsaturated polymerizable monomers. The various recurring units are present in suitable amounts, as represented by x′ being from about 20 to about 99 mol %, y′ being from about 1 to about 20 mol %, and z′ being from 0 to about 79 mol %. Preferably, x′ is from about 30 to about 98 mol %, y′ is from about 2 to about 10 mol % and z′ is from 0 to about 68 mol %.

[0091] Crosslinking of the vinyl polymer can be achieved in the same way as described above for the Class I polymers.

[0092] Additional monomers that provide the additional recurring units represented by Z′ in Structure VI include any useful hydrophilic or oleophilic ethylenically unsaturated polymerizable monomer that may provide desired physical or printing properties to the imaging layer. Such monomers include, but are not limited to, acrylates, methacrylates, acrylonitrile, isoprene, styrene and styrene derivatives, acrylamides, methacrylamides, acrylic or methacrylic acid and vinyl halides.

[0093] Representative vinyl polymers of Class II include Polymers 11-18 as identified herein below, and Polymer 14 is most preferred. A mixture of any two or more of these polymers can also by used.

[0094] Class III Polymers

[0095] Each of the Class III polymers has a molecular weight of at least 1000, and preferably of at least 5000. For example, the polymers can be vinyl homopolymers or copolymers prepared from one or more ethylenically unsaturated polymerizable monomers that are reacted together using known polymerization techniques and reactants. Alternatively, they can be addition homopolymers or copolymers (such as polyethers) prepared from one or more heterocyclic monomers that are reacted together using known polymerization techniques and reactants. Additionally, they can be condensation type polymers (such as polyesters, polyimides, polyamides or polyurethanes) prepared using known polymerization techniques and reactants. Whatever the type of polymers, at least 20 mol % (preferably 30 mol %) of the total recurring units in the polymer comprise the necessary heat-activatable thiosulfate groups.

[0096] The Class III polymers useful in the practice of this invention can be represented by the Structure VII wherein the thiosulfate group (or Bunte salt) is a pendant group:

[0097] wherein A represents a polymeric backbone, R₆is a divalent linking group, and Y is hydrogen or a cation.

[0098] Useful polymeric backbones include, but are not limited to, vinyl polymers, polyethers, polyimides, polyamides, polyurethanes and polyesters. Preferably, the polymeric backbone is a vinyl polymer or polyether.

[0099] Useful R₆linking groups include —(COO)_n(Z₁)_m— wherein n is 0 or 1, m is 0 or 1, and Z, is a substituted or unsubstituted alkylene group having 1 to 6 carbon atoms (such as methylene, ethylene, n-propylene, isopropylene, butylenes, 2-hydroxypropylene and 2-hydroxy-4-azahexylene) that can have one or more oxygen, nitrogen or sulfur atoms in the chain, a substituted or unsubstituted arylene group having 6 to 14 carbon atoms in the aromatic ring (such as phenylene, naphthalene, anthracylene and xylylene), or a substituted or unsubstituted arylenealkylene (or alkylenearylene) group having 7 to 20 carbon atoms in the chain (such as p-methylenephenylene, phenylenemethylenephenylene, biphenylene and phenyleneisopropylenephenylene). In addition, R₆can be an alkylene group, an arylene group, in an arylenealkylene group as defined above for Z₁.

[0100] Preferably, R₆is an alkylene group of 1 to 3 carbon atoms, an arylene group of 6 carbon atoms in the aromatic ring, an arylenealkylene group of 7 or 8 carbon atoms in the chain, or —COO(Z₁)_m— wherein Z₁is methylene, ethylene or phenylene. Most preferably, R₆is phenylene, methylene or —COO—.

[0101] Y₁is hydrogen, ammonium ion, or a metal ion (such as sodium, potassium, magnesium, calcium, cesium, barium, zinc or lithium ion). Preferably, Y₁is hydrogen, sodium ion or potassium ion.

[0102] As the thiosulfate group is generally pendant to the backbone, preferably it is part of an ethylenically unsaturated polymerizable monomer that can be polymerized using conventional techniques to form vinyl homopolymers of the thiosulfate-containing recurring units, or vinyl copolymers when copolymerized with one or more additional ethylenically unsaturated polymerizable monomers. The thiosulfate-containing recurring units generally comprise at least 20 mol % of all recurring units in the polymer, preferably they comprise from about 30 to 100 mol % of all recurring units. A polymer can include more than one type of repeating unit containing a thiosulfate group as described herein.

[0103] Polymers having the above-described thiosulfate group are believed to crosslink and to switch from hydrophilic thiosulfate to hydrophobic disulfide (upon loss of sulfate) with heating.

[0104] Thiosulfate-containing molecules (or Bunte salts) can be prepared from the reaction between an alkyl halide and thiosulfate salt as taught by Bunte, Chem. Ber.7, 646, 1884. Polymers containing thiosulfate groups can either be prepared from functional monomers or from preformed polymers. Polymers can also be prepared from preformed polymers in a similar manner as described in U.S. Pat. No. 3,706,706 (Vandenberg). Thiosulfate-containing molecules can also be prepared by reaction of an alkyl epoxide with a thiosulfate salt, or between an alkyl epoxide and a molecule containing a thiosulfate moiety (such as 2-aminoethanethiosulfuric acid), and the reaction can be performed either on a monomer or polymer as illustrated by Thames, Surf. Coating,3 (Waterborne Coat.), Chapter 3, pp. 125-153, Wilson et al (Eds.).

[0105] Representative synthetic methods for making ethylenically unsaturated polymerizable monomers and Class III polymers (Polymers 19-28) are illustrated below. Vinyl polymers can be prepared by copolymerizing monomers containing the thiosulfate functional groups with one or more other ethylenically unsaturated polymerizable monomers to modify polymer chemical or functional properties, to optimize imaging member performance, or to introduce additional crosslinking capability.

[0106] Useful additional ethylenically unsaturated polymerizable monomers include, but are not limited to, acrylates (including methacrylates) such as ethyl acrylate, n-butyl acrylate, methyl methacrylate and t-butyl methacrylate, acrylamides (including methacrylamides), an acrylonitrile (including methacrylonitrile), vinyl ethers, styrenes, vinyl acetate, dienes (such as ethylene, propylene, 1,3-butadiene and isobutylene), vinyl pyridine and vinylpyrrolidone. Acrylamides, acrylates and styrenes are preferred.

[0107] The imaging layer of the imaging member can include one or more Class I, II or III polymers with or without minor amounts (less than 20 weight %, based on total dry weight of the layer) of additional binder or polymeric materials that will not adversely affect its imaging properties.

[0108] In the composition used to provide the heat-sensitive layer, the amount of charged polymer is generally present in an amount of at least 1% solids, and preferably at least 2% solids. A practical upper limit of the amount of charged polymer in the composition is about 10% solids.

[0109] The amount of charged polymer(s) used in the imaging layer is generally at least 0.1 g/m², and preferably from about 0.1 to about 10 g/m²(dry weight). This generally provides an average dry thickness of from about 0.1 to about 10 μm.

[0110] The imaging layer can also include one or more conventional surfactants for coatability or other properties, dyes or colorants to allow visualization of the written image, or any other addenda commonly used in the lithographic art, as long as the concentrations are low enough so they are inert with respect to imaging or printing properties.

[0111] It is essential that the heat-sensitive imaging layer includes one or more photothermal conversion materials to absorb appropriate radiation from an appropriate energy source (such as a laser), which radiation is converted into heat. Thus, such materials convert photons into heat. Preferably, the radiation absorbed is in the infrared and near-infrared regions of the electromagnetic spectrum. The photothermal conversion materials useful in this invention are IR dyes that comprise multiple quaternary ammonium groups and one or more aromatic carbocyclic or aromatic or nonaromatic heterocyclic groups within the molecules. There are at least two quaternary ammonium groups anywhere in the molecule. Preferably, at least two of the quaternary ammonium groups are attached to one or more of the aromatic carbocyclic or aromatic or nonaromatic heterocyclic groups.

[0112] It is also essential that the IR dye be soluble in water or any of the water-miscible organic solvents that are described below as useful for preparing coating compositions. Preferably, the IR dyes are soluble in either water or methanol, or a mixture of water and methanol. Solubility in water or the water-miscible organic solvents means that the IR dye can be dissolved at a concentration of at least 0.5 g/l at room temperature.

[0113] The IR dyes are sensitive to radiation in the near-infrared and infrared regions of the electromagnetic spectrum. Thus, they are generally sensitive to radiation at or above 700 nm (preferably from about 800 to about 900 nm, and more preferably from about 800 to about 850 nm).

[0114] The IR dyes useful in this invention can be generally cyanine dyes having two nitrogen atoms conjugated to a polymethine chain that is terminated with 2 cyclic groups. One or more aromatic carbocyclic or aromatic or nonaromatic heterocyclic groups are also conjugated with the polymethine chain, that is either as part of the polymethine chain, or at either or both ends of the polymethine chain. Various aromatic carbocyclic and aromatic or nonaromatic heterocyclic groups are defined in more detail below as well as possible polymethine chains.

[0115] Particularly useful IR dyes useful in the practice of this invention include, but are not limited to, the compounds represented by Structure DYE shown as follows:

[0116] wherein “A” and “B” are independently substituted or unsubstituted cyclic groups that are either completely aromatic in nature, or that include an aromatic moiety fused to a non-aromatic heterocyclic or carbocyclic ring.

[0117] Useful aromatic carbocyclic groups generally include 6 to 10 carbon atoms in the ring including but not limited to, phenyl, naphthyl and tolyl groups (that can be substituted for example with halo, alkyl, alkoxy, aryl, sulfo, carboxy, acetyl or hydroxy groups). Useful aromatic or nonaromatic heterocyclic groups generally include 6 to 10 of any chemically possible combination of carbon, nitrogen, oxygen, sulfur and selenium atoms. Examples of such heterocyclic groups include, but are not limited to, substituted or unsubstituted pyridyl, pyrimidyl, quinolinyl, phenathridyl, indolyl, benzindolyl and naphthindolyl groups (that can be substituted for example with halo, quaternary ammonium, hydroxy, hydroxyalkyl, alkyl or aryl groups).

[0118] Preferably, the useful aromatic carbocyclic groups are substituted or unsubstituted phenyl or naphthyl groups, and the useful nonaromatic heterocyclic groups are substituted or unsubstituted indolyl, benzindolyl or naphthindolyl groups. More preferably A and B are independently substituted or unsubstituted indolyl or benzindolyl groups.

[0119] In Structure DYE shown above, “L” is a substituted or unsubstituted chromophoric chain conjugated to both A and B to provide sensitivity to near infrared or infrared radiation as described above (that is at least 700 nm). In one embodiment, L includes a nitrogen atom at one or both ends when A or B (or both) are carbocyclic groups. In another embodiment, A and B are N-heterocyclic groups and L is connected to nitrogen atoms in those groups. Additionally, L comprises a chain of at least 3 carbon atoms having alternating single and double bonds to provide conjugation with A and B groups (with or without nitrogen atoms). Preferably, L comprises at least 5 carbon atoms, and more preferably, L comprises from 7 to 9 carbon atoms. Any hydrogen atom in the conjugated chain can be replaced with any desirable substituent, or any two adjacent carbon atoms can be part of a cyclic moiety, as long as the conjugation and IR sensitivity of the molecule are not adversely affected.

[0120] R₇, R₈, R₉and R₁₀are the same or different substituents that include, but are not limited to, quaternary ammonium groups, substituted or unsubstituted alkyl groups (having 1 to 10 carbon atoms, branched or linear), substituted or unsubstituted alkoxy groups (having 1 to 10 carbon atoms), halo groups, substituted or unsubstituted aryl groups (having 6 to 10 carbon atoms in the ring) and any other substituents that would be readily apparent to a skilled worker in the art. Preferably at least two of these groups are quaternary ammonium groups, and at least two of these groups are attached to a nitrogen atom in a heterocyclic group (A or B).

[0121] By the term “quaternary ammonium” is meant to include the —N(ALKYL)₃⁺ moiety as well as groups that include that moiety connected indirectly to A or B through a substituted or unsubstituted organic group including, but not limited to, substituted or unsubstituted arylene groups having from 6 to 10 carbon atoms in the aromatic ring substituted or unsubstituted alkylene groups having 1 to 14 carbon atoms, substituted or unsubstituted alkenylene having 2 to 14 carbon atoms, substituted or unsubstituted alkynylene having 2 to 14 carbon atoms, or substituted or unsubstituted arylenealkylene groups or alkylenearylene groups having 7 to 20 carbon atoms in the chain. One skilled in the art would readily understand the nature and composition of such groups that link the quaternary ammonium moiety to the A or B aromatic group. Such linking groups can also be substituted with additional substituents other than quaternary ammonium groups that would be readily apparent to one skilled in the art. In addition, Structure DYE can also have additional quaternary ammonium groups beyond those represented by R₇-R₁₀. Such additional groups can be located anywhere in the molecule as long as the compound retains the desired IR sensitivity.

[0122] ALKYL can be the same or different substituted or unsubstituted alkyl groups having 1 to 7 carbon atoms (such as substituted or unsubstituted methyl, ethyl, isopropyl, hexyl and benzyl groups). Methyl and ethyl are preferred and methyl is most preferred.

[0123] In preferred embodiments, the quaternary ammonium groups in the IR dye can be represented by the pendant —R—⁺NR₃, R₄R₅group shown above in Structure V wherein R, R₃, R₄and R₅are as defined above. Those groups can be the same or different in the molecule.

[0124] In Structure DYE, M is a suitable anion of appropriate charge to balance the positively charged portion of the aromatic IR dye. Useful anions include, but are not limited to, halides, triflates and others readily apparent to one skilled in the art. Thus, “w” and “z” are integers that provide the desired charge to balance “x⁺” that represents the overall charge of the dye cation.

[0125] Examples of such useful aromatic IR dyes include, but are not limited to, the following compounds:

[0126] IR Dye 2 is like IR Dye 1 except that the anion is iodide, and IR Dye 3 is like IR Dye 1 except that the anion is triflate.

[0127] The heat-sensitive compositions and imaging layers can include additional photothermal conversion materials, although the presence of such materials is not preferred. Such optional materials can be other IR dyes, carbon black, polymer-grafted carbon, pigments, evaporated pigments, semiconductor materials, alloys, metals, metal oxides, metal sulfides or combinations thereof, or a dichroic stack of materials that absorb radiation by virtue of their refractive index and thickness. Borides, carbides, nitrides, carbonitrides, bronze-structured oxides and oxides structurally related to the bronze family but lacking the WO_2.9component, are also useful. Useful absorbing dyes for near infrared diode laser beams are described, for example, in U.S. Pat. No. 4,973,572 (DeBoer). Particular dyes of interest are “broad band” dyes, that is those that absorb over a wide band of the spectrum.

[0128] Alternatively, the same or different photothermal conversion material (including an aromatic IR dye described herein) can be included in a separate layer that is in thermal contact with the heat-sensitive imaging layer. Thus, during imaging, the action of the additional photothermal conversion material can be transferred to the heat-sensitive imaging layer.

[0129] The heat-sensitive composition of this invention can be applied to a support using any suitable equipment and procedure, such as spin coating, knife coating, gravure coating, dip coating or extrusion hopper coating. In addition, the composition can be sprayed onto a support, including a cylindrical support, using any suitable spraying means for example as described in U.S. Pat. No. 5,713,287 (noted above).

[0130] The heat-sensitive compositions of this invention are generally formulated in and coated from water or water-miscible organic solvents including, but not limited to, water-miscible alcohols (for example, methanol, ethanol, isopropanol, 1-methoxy-2-propanol and n-propanol), methyl ethyl ketone, tetrahydrofuran, acetonitrile and acetone. Water, methanol, ethanol and 1-methoxy-2-propanol are preferred. Mixtures (such as a mixture of water and methanol) of these solvents can also be used if desired. By “water-miscible” is meant that the organic solvent is miscible in water at all proportions at room temperature.

[0131] The imaging members of this invention can be of any useful form including, but not limited to, printing plates, printing cylinders, printing sleeves and printing tapes (including flexible printing webs), all of any suitable size or dimensions. Preferably, the imaging members are printing plates or on-press cylinders.

[0132] During use, the imaging member of this invention is exposed to a suitable source of energy that generates or provides heat, such as a focused laser beam or a thermoresistive head, in the foreground areas where ink is desired in the printed image, typically from digital information supplied to the imaging device. A laser used to expose the imaging member of this invention is preferably a diode laser, because of the reliability and low maintenance of diode laser systems, but other lasers such as gas or solid state lasers may also be used. The combination of power, intensity and exposure time for laser imaging would be readily apparent to one skilled in the art. Specifications for lasers that emit in the near-IR region, and suitable imaging configurations and devices are described in U.S. Pat. No. 5,339,737 (Lewis et al), incorporated herein by reference. The imaging member is typically sensitized so as to maximize responsiveness at the emitting wavelength of the laser.

[0133] The imaging apparatus can operate on its own, functioning solely as a platemaker, or it can be incorporated directly into a lithographic printing press. In the latter case, printing may commence immediately after imaging, thereby reducing press set-up time considerably. The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the imaging member mounted to the interior or exterior cylindrical surface of the drum.

[0134] In the drum configuration, the requisite relative motion between an imaging device (such as laser beam) and the imaging member can be achieved by rotating the drum (and the imaging member mounted thereon) about its axis, and moving the imaging device parallel to the rotation axis, thereby scanning the imaging member circumferentially so the image “grows” in the axial direction. Alternatively, the beam can be moved parallel to the drum axis and, after each pass across the imaging member, increment angularly so that the image “grows” circumferentially. In both cases, after a complete scan by the laser beam, an image corresponding to the original document or picture can be applied to the surface of the imaging member.

[0135] In the flatbed configuration, a laser beam is drawn across either axis of the imaging member, and is indexed along the other axis after each pass. Obviously, the requisite relative motion can be produced by moving the imaging member rather than the laser beam.

[0136] While laser imaging is preferred in the practice of this invention, imaging can be provided by any other means that provides or generates thermal energy in an imagewise fashion. For example, imaging can be accomplished using a thermoresistive head (thermal printing head) in what is known as “thermal printing”, described for example in U.S. Pat. No. 5,488,025 (Martin et al). Such thermal printing heads are commercially available (for example, as Fujisu Thermal Head FTP-040 MCS001 and TDK Thermal Head F415 HH7-1089).

[0137] Imaging of heat-sensitive compositions on printing press cylinders can be accomplished using any suitable means, for example, as taught in U.S. Pat. No. 5,713,287 (noted above), that is incorporated herein by reference.

[0138] After imaging, the imaging member can be used for printing without conventional wet processing. Applied ink can be imagewise transferred to a suitable receiving material (such as cloth, paper, metal, glass or plastic) to provide one or more desired impressions. If desired, an intermediate blanket roller can be used to transfer the ink from the imaging member to the receiving material. The imaging members can be cleaned between impressions, if desired, using conventional cleaning means.

[0139] The following examples illustrate the practice of the invention, and are not meant to limit it in any way. The synthetic methods are presented to show how some of the preferred heat-sensitive polymers and aromatic IR dyes can be prepared.

[0140] Polymers 1, 3-6 are illustrative of Class I polymers (Polymer 2 is a precursor to Polymer 3), Polymers 7-8 and 10 are illustrative of Class II non-vinyl polymers (Polymer 9 is a precursor to Polymer 10), Polymers 11-18 are illustrative of Class II vinyl polymers, and Polymers 19-28 are illustrative of Class III polymers.

[0141] Synthetic Methods

[0142] Preparation of Polymer 1: Poly(1-vinyl-3-methylimidazolium chloride-co-N-(3-aminopropyl)methacrylamide hydrochloride)

[0143] A] Preparation of 1-Vinyl-3-methylimidazolium methanesulfonate monomer Freshly distilled 1-vinylimidazole (20.00 g, 0.21 mol) was combined with methyl methanesulfonate (18.9 ml, 0.22 mol) and 3-t-butyl-4-hydroxy-5-methylphenyl sulfide (about 1 mg) in diethyl ether (100 ml) in a round bottomed flask equipped with a reflux condenser and a nitrogen inlet and stirred at room temperature for 48 hours. The resulting precipitate was filtered off, thoroughly washed with diethyl ether, and dried overnight under vacuum at room temperature to afford 37.2 g of product as a white, crystalline powder (86.7% yield).

[0144] B] Copolymerization/ion Exchange

[0145] 1-Vinyl-3-methylimidazolium methanesulfonate (5.00 g, 2.45×10 mol), N-(3-aminopropyl)methacrylamide hydrochloride (0.23 g, 1.29×10-³mol) and 2,2′-azobisisobutyronitrile (AIBN) (0.052 g, 3.17×1 mol) were dissolved in methanol (60 ml) in a 250 ml round bottomed flask equipped with a rubber septum. The solution was bubble degassed with nitrogen for ten minutes and heated at 60° C. in a water bath for 14 hours. The viscous solution was precipitated into 3.5 liters of tetrahydrofuran and dried under vacuum overnight at 50° C. to give 4.13 g of product (79.0% yield). The polymer was then dissolved in 100 ml methanol and converted to the chloride by passage through a flash column containing 400 cm³DOWEX® 1X8-100 ion exchange resin.

[0146] Preparation of Polymer 2: Poly(methyl methacrylate-co-4-vinylpyridine)(9:1 Molar Ratio)

[0147] Methyl methacrylate (30 ml), 4-vinylpyridine (4 ml), AIBN (0.32 g, 1.95×10⁻³mol), and N,N-dimethylformamide (40 ml, DMF) were combined in a 250 ml round bottomed flask and fitted with a rubber septum. The solution was purged with nitrogen for 30 minutes and heated for 15 hours at 60° C. Methylene chloride and DMF (150 ml of each) were added to dissolve the viscous product and the product solution was precipitated twice into isopropyl ether. The precipitated polymer was filtered and dried overnight under vacuum at 60° C.

[0148] Preparation of Polymer 3: Poly(methyl methacrylate-co-N-methyl-4-vinylpyridinium formate) (9:1 Molar Ratio)

[0149] Polymer 2 (10 g) was dissolved in methylene chloride (50 ml) and reacted with methyl p-toluenesulfonate (1 ml) at reflux for 15 hours. NMR analysis of the reaction showed that only partial N-alkylation had occurred. The partially reacted product was precipitated into hexane, then dissolved in neat methyl methanesulfonate (25 ml) and heated at 70° C. for 20 hours. The product was precipitated once into diethyl ether and once into isopropyl ether from methanol and dried under vacuum overnight 60° C. A flash chromatography column was loaded with 300 cm³of DOWEX® 550 hydroxide ion exchange resin in water eluent. This resin was converted to the formate by running a liter of 10% formic acid through the column. The column and resin were thoroughly washed with methanol, and the product polymer (2.5 g) was dissolved in methanol and passed through the column. Complete conversion to the formate counterion was confirmed by ion chromatography.

[0150] Preparation of Polymer 4: Poly(methyl methacrylate-co-N-butyl-4-vinylpyridinium formate) (9:1 Molar Ratio)

[0151] Polymer 2 (5 g) was heated at 60° C. for 15 hours in 1-bromobutane (200 ml). The precipitate that formed was dissolved in methanol,precipitated into diethyl ether, and dried for 15 hours under vacuum at 60° C. The polymer was converted from the bromide to the formate using the method described in the preparation of Polymer 3.

[0152] Preparation of Polymer 5: Poly(methyl methacrylate-co-2-vinylpyridine) (9:1 Molar Ratio)

[0153] Methyl methacrylate (18 ml), 2-vinylpyridine (2 ml), AIBN (0.16 g,), and DMF (30 ml) were combined in a 250 ml round bottomed flask and fitted with a rubber septum. The solution was purged with nitrogen for 30 minutes and heated for 15 hours at 60° C. Methylene chloride (50 ml) was added to dissolve the viscous product and the product solution was precipitated twice into isopropyl ether. The precipitated polymer was filtered and dried overnight under vacuum at 60° C.

[0154] Preparation of Polymer 6: Poly(methyl methacrylate-co-N-methyl-2-vinylpyridinium formate) (9:1 Molar Ratio)

[0155] Polymer 5 (10 g) was dissolved in 1,2-dichloroethane (100 ml) and reacted with methyl p-toluenesulfonate (15 ml) at 70° C. for 15 hours. The product was precipitated twice into diethyl ether and dried under vacuum overnight at 60° C. A sample (2.5 g) of this polymer was converted from the p-toluenesulfonate to the formate using the procedure described above for Polymer 3.

[0156] Preparation of Polymer 7: Poly(p-xylidenetetrahydro-thiophenium chloride)

[0157] Xylylene-bis-tetrahydrothiophenium chloride (5.42 g, 0.015 mol) was dissolved in 75 ml of deionized water and filtered through a fritted glass funnel to remove a small amount of insolubles. The solution was placed in a three-neck round-bottomed flask on an ice bath and was sparged with nitrogen for fifteen minutes. A solution of sodium hydroxide (0.68 g, 0.017 mol) was added dropwise over fifteen minutes via addition funnel. When about 95% of the hydroxide solution was added, the reaction solution became very viscous and the addition was stopped. The reaction was brought to pH 4 with 10% HCl and purified by dialysis for 48 hours.

[0158] Preparation of Polymer 8: Poly[phenylene sulfide-co-methyl(4-thiophenyl)sulfonium chloride]

[0159] Poly(phenylene sulfide) (15.0 g, 0.14 mol-repeating units), methanesulfonic acid (75 ml), and methyl triflate (50.0 g, 0.3 mol) were combined in a 500 ml round bottomed flask equipped with a heating mantle, reflux condenser, and nitrogen inlet. The reaction mixture was heated to 90° C. at which point a homogeneous, brown solution resulted, and was allowed to stir at room temperature overnight. The reaction mixture was poured into 500 cm³of ice and brought to neutrality with sodium bicarbonate. The resultant liquid/solid mixture was diluted to a final volume of 2 liters with water and dialyzed for 48 hours at which point most of the solids had dissolved. The remaining solids were removed by filtration and the remaining liquids were slowly concentrated to a final volume of 700 ml under a stream of nitrogen. The polymer was ion exchanged from the triflate to the chloride by passing it through a column of DOWEX® 1x8-100 resin. Analysis by ¹H NMR showed that methylation of about 45% of the sulfur groups had occurred.

[0160] Preparation of Polymer 9: Brominated poly(2,6-dimethyl-1,4-phenylene oxide)

[0161] Poly(2,6-dimethyl-1,4-phenylene oxide) (40 g, 0.33 mol repeating units) was placed dissolved in carbon tetrachloride (2400 ml) in a 5 liter round bottomed 3-neck flask with a reflux condenser and a mechanical stirrer. The solution was heated to reflux and a 150 Watt flood lamp was applied. N-bromosuccinimide (88.10 g, 0.50 g) was added portionwise over 3.5 hours, and the reaction was allowed to stir at reflux for an additional hour. The reaction was cooled to room temperature to yield an orange solution over a brown solid. The liquid was decanted and the solids were stirred with 100 ml methylene chloride to leave a white powder (succinimide) behind. The liquid phases were combined, concentrated to 500 ml via rotary evaporation, and precipitated into methanol to yield a yellow powder. The crude product was precipitated twice more into methanol and dried overnight under vacuum at 60° C. Elemental and ¹H NMR analyses showed a net 70% bromination of benzyl side chains.

[0162] Preparation of Polymer 10: Dimethyl sulfonium bromide derivative of poly(2.6-dimethyl-1,4-phenylene oxide)

[0163] Brominated poly(2,6-dimethyl-1,4-phenylene oxide) described above (2.00 g, 0.012 mol benzyl bromide units) was dissolved in methylene chloride (20 ml) in a 3-neck round bottomed flask outfitted with a condenser, nitrogen inlet, and septum. Water (10 ml) was added along with dimethyl sulfide (injected via syringe) and the two-phase mixture was stirred at room temperature for one hour and then at reflux at which point the reaction turned into a thick dispersion. This was poured into 500 ml of tetrahydrofuran and agitated vigorously in a chemical blender. The product, which gelled after approximately an hour in the solid state, was recovered by filtration and quickly redissolved in 100 ml methanol and stored as a methanolic solution.

[0164] Preparation of Polymer 11: Poly[methyl methacrylate-co-2-trimethylammoniumethyl methacrylic chloride-co-N-(3-aminopropyl)methacrylamide hydrochloride](7:2:1 Molar Ratio)

[0165] Methyl methacrylate (24.6 ml, 0.23 mol), 2-trimethylammoniumethyl methacrylic chloride (17.0 g, 0.08 mol), n-(3-aminopropyl)methacrylamide hydrochloride (10.0 g, 0.56 mol), azobisisobutyronitrile (0.15 g, 9.10×10⁻⁴mol, AIBN), water (20 ml) and dimethylformamide (150 ml) were combined in a round bottom flask fitted with a rubber septum. The solution was bubble degassed with nitrogen for 15 minutes and placed in a heated water bath at 60° C. overnight. The viscous product solution was diluted with methanol (125 ml) and precipitated three times from methanol into isopropyl ether. The product was dried under vacuum at 60° C. for 24 hours and stored in a dessicator.

[0166] Preparation of Polymer 12: Poly[methyl methacrylate-co-2-trimethylammoniumethyl methacrylic acetate-co-N-(3-aminopropyl)methacrylamide](7:2:1 Molar Ratio)

[0167] Polymer 11 (3.0 g) was dissolved in 100 ml of methanol and neutralized by passing through a column containing 300 cm³of tertiary amine functionalized crosslinked polystyrene resin (Scientific Polymer Products #726, 300 cm²) with methanol eluent. That polymer was then converted to the acetate using a column of 300 cm³DOWEX® 1x8-100 ion exchange resin (that is, converted from the chloride to the acetate by washing with 500 ml glacial acetic acid) and methanol eluent.

[0168] Preparation of Polymer 13: Poly[methyl methacrylate-co-2-trimethylammoniumethyl methacrylic fluoride-co-N-(3-aminopropyl)methacrylamide hydrochloride] (7:2:1 Molar Ratio)

[0169] Polymer 11 (3.0 g) was dissolved in 100 ml of methanol and neutralized by passing through a column containing 300 cm³tertiary amine functionalized crosslinked polystyrene resin (Scientific Polymer Products #726, 300 cm²) with methanol eluent. The polymer was then converted to the fluoride using a column of 300 cm³DOWEX® 1x8-100 ion exchange resin (that is, converted from the chloride to the fluoride by washing with 500 g of potassium fluoride) and methanol eluent.

[0170] Preparation of Polymer 14: Poly[vinylbenzyl trimethylammonium chloride-co-N-(3-aminopropyl)methacrylamide hydrochloride](19:1 Molar Ratio)

[0171] Vinylbenzyl trimethylammonium chloride (19 g, 0.0897 mol, 60:40 mixture of p,m isomers), N-(3-aminopropyl)methacrylamide hydrochloride (1 g, 0.00562 mol), 2,2′-azobis(2-methylpropionamidine)dihydrochloride (0.1 g), and deionized water (80 ml) were combined in a round bottom flask fitted with a rubber septum. The reaction mixture was bubble degassed with nitrogen for 15 minutes and placed in a water bath at 60° C. for four hours. The resulting viscous product solution was precipitated into acetone, dried under vacuum at 60° C. for 24 hours, and stored in a dessicator.

[0172] Preparation of Polymer 15: Poly([vinylbenzyltrimethyl-phosphonium acetate-co-N-(3-aminopropyl)methacrylamide hydrochloride] (19:1 Molar Ratio)

[0173] A] Vinylbenzyl bromide (60:40 mixture of p,m isomers) Vinylbenzyl chloride (50.60 g, 0.33 mol, 60:40 mixture of p,m isomers), sodium bromide (6.86 g, 6.67×10⁻²mol), N-methylpyrrolidone (300 ml, passed through a short column of basic alumina), ethyl bromide (260 g), and 3-t-butyl-4-hydroxy-5-methyl phenyl sulfide (1.00 g, 2.79×10⁻³mol) were combined in a 1 liter round bottomed flask fitted with a reflux condenser and a nitrogen inlet and the mixture was heated at reflux for 72 hours at which point the reaction was found to have proceeded to >95% conversion by gas chromatography. The reaction mixture was poured into 1 liter of water and extracted twice with 300 ml of diethyl ether. The combined ether layers were extracted twice with 1 liter of water, dried over MgSO₄, and the solvents were stripped by rotary evaporation to yield yellowish oil. The crude product was purified by vacuum distillation to afford 47.5 g of product (53.1% yield).

[0174] B] Vinylbenzyl trimethylphosphonium bromide

[0175] Trimethylphosphine (50.0 ml of a 1.0 Molar solution in tetrahydrofuran, 5.00×10⁻²mol) was added via addition funnel over about 2 minutes into a thoroughly nitrogen degassed dispersion of vinylbenzyl bromide (9.85 g, 5.00×10⁻²mol) in diethyl ether (100 ml). A solid precipitate began to form almost immediately. The reaction was allowed to stir for 4 hours at room temperature, then was placed in a freezer overnight. The solid product was isolated by filtration, washed three times with 100 ml of diethyl ether, and dried under vacuum for 2 hours. Pure product (11.22 g) was recovered as a white powder (82.20% yield).

[0176] C] Poly[vinylbenzyltrimethylphosphonium bromide-co-N-(3-aminopropyl)methacrylamide] (19:1 Molar Ratio)

[0177] Vinylbenzyltrimethylphosphonium bromide (5.00 g, 1.83×10⁻²mol), N-(3-aminopropyl)methacrylamide hydrochloride (0.17 g, 9.57×10⁻⁴mol), azobisisobutyronitrile (0.01 g, 6.09×10⁻⁵mol), water (5.0 ml), and dimethylformamide (25 ml) were combined in a 100 ml round bottomed flask sealed with a rubber septum, bubble degassed for 10 minutes with nitrogen, and placed in a warm water bath (55° C.) overnight. The viscous solution was precipitated into tetrahydrofuran and dried under vacuum overnight at 60° C. The liquids were filtered off, concentrated on a rotary evaporator to a volume of about 200 ml, precipitated again into tetrahydrofuran, and dried under vacuum overnight at 60° C. About 4.20 g was recovered. (81.9% yield).

[0178] D] Poly[vinylbenzyltrimethylphosphonium acetate-co-N-(3-aminopropyl)methacrylamide hydrochloride] (19:1 Molar Ratio)

[0179] DOWEX® 550 a hydroxide anion exchange resin (about 300 cm³) was poured into a flash column with 3:1 methanol/water eluent. About 1 liter of glacial acetic acid was passed through the column to convert it to the acetate, followed by about 3 liters of 3:1 methanol/water. 3.0 g of the product from step C in 200 ml of 3:1 methanol/water was passed through the acetate resin column and the solvents were stripped on a rotary evaporator. The resulting viscous oil was thoroughly dried under vacuum to afford 2.02 g of a glassy, yellowish material (Polymer 15, 67.9% yield). Ion chromatography showed complete conversion to the acetate.

[0180] Preparation of Polymer 16: Poly[dimethyl-2-(methacryloyloxy)ethylsulfonium chloride-co-N-(3-aminopropyl)methacrylamide hydrochloride] (19:1 Molar Ratio)

[0181] A] Dimethyl-2-(methacryloyloxy)ethylsulfonium methylsulfate

[0182] 2-(Methylthio)ethylmethacrylate (30.00 g, 0.19 mol), dimethyl sulfate (22.70 g, 0.18 mol), and benzene (150 ml) were combined in a 250 ml round bottomed flask outfitted with a reflux condenser and a nitrogen inlet. The reaction solution was heated at reflux for 1.5 hours and allowed to stir at room temperature for 20 hours at which point the reaction had proceeded to about 95% yield by ¹H NMR. The solvent was removed by rotary evaporation to afford brownish oil that was stored as a 20 weight % solution in dimethylformamide and used without further purification.

[0183] B] Poly[dimethyl-2-(methacryloyloxy)ethylsulfonium methylsulfate-co-N-(3-aminopropyl)methacrylamide hydrochloride] (19:1 Molar Ratio)

[0184] Dimethyl-2-(methacryloyloxy)ethylsulfonium methylsulfate (93.00 g of 20 wt. % solution in dimethylformamide, 6.40×10⁻²mol), N-(3-aminopropyl) methacrylamide hydrochloride (0.60 g, 3.36×10⁻³mol), and azobisisobutyronitrile (0.08 g, 4.87×10 mol) were dissolved in methanol (100 ml) in a 250 ml round bottomed flask fitted with a septum. The solution was bubble degassed with nitrogen for 10 minutes and heated for 20 hours in a warm water bath at 55° C. The reaction was precipitated into ethyl acetate, redissolved in methanol, precipitated a second time into ethyl acetate, and dried under vacuum overnight. A white powder (15.0 g) was recovered (78.12% yield).

[0185] C] Poly[dimethyl-2-(methacryloyloxy)ethylsulfonium chloride-co-N-(3-aminopropyl)methacrylamide hydrochloride] (19:1 Molar Ratio)

[0186] The precursor polymer (2.13 g) from step B was dissolved in 100 ml of 4:1 methanol/water and passed through a flash column containing 300 cm³of DOWEX® 1x8-100 anion exchange resin using 4:1 methanol/water eluent. The recovered solvents were concentrated to about 30 ml and precipitated into 300 ml of methyl ethyl ketone. The damp, white powder collected was redissolved in 15 ml of water and stored in a refrigerator as a solution of Polymer 16 (10.60% solids).

[0187] Preparation of Polymer 17: Poly[vinylbenzyldimethylsulfonium methylsulfate]

[0188] A] Methyl(vinylbenzyl)sulfide

[0189] Sodium methanethiolate (24.67 g, 0.35 mol) was combined with methanol (250 ml) in a 1 liter round bottomed flask outfitted with an addition funnel and a nitrogen inlet. Vinylbenzyl chloride (41.0 ml, 60:40 mixture of p- and o-isomers, 0.29 mol) in tetrahydrofuran (100 ml) was added via addition funnel over 30 minutes. The reaction mixture grew slightly warm and a milky suspension resulted. This was allowed to stir at room temperature for 20 hours at which point only a small amount of vinylbenzyl chloride was still evident by thin layer chromatography (2:1 hexanes/CH₂Cl₂eluent). Another portion of sodium methanethiolate was added (5.25 g, 7.49×10⁻²mol) and after ten minutes, the reaction had proceeded to completion by thin layer chromatography. Diethyl ether (400 ml) was added and the resulting mixture was extracted twice with 600 ml of water and once with 600 ml of brine. The resulting organic extracts were dried over magnesium sulfate, a small amount (about 1 mg) of 3-t-butyl-4-hydroxy-5-methyl phenyl sulfide was added, and the solvents were stripped by rotary evaporation to afford a yellowish oil. Purification by vacuum distillation through a long Vigreux column yielded 43.35 g (91%) of the pure product as a clear liquid.

[0190] B] Dimethyl(vinylbenzyl)sulfonium methylsulfate

[0191] Methyl (vinylbenzyl)sulfide (13.59 g, 8.25×10⁻²mol), benzene (45 ml), and dimethyl sulfate (8.9 ml, 9.4×10⁻²mol) were combined in a 100 ml round bottomed flask equipped with a nitrogen inlet. The mixture was allowed to stir at room temperature for 44 hours, at which point two layers were present. Water (20 ml) was added and the top (benzene) layer was removed by pipette. The aqueous layer was extracted three times with 30 ml of diethyl ether and a vigorous stream of nitrogen was bubbled through the solution to remove residual volatile compounds. The product was used without further purification as a 35% (w/w) solution.

[0192] C] Poly[dimethyl(vinylbenzyl)sulfonium methylsulfate]

[0193] All of the dimethyl(vinylbenzyl)sulfonium methylsulfate solution from the previous step (approximately 5.7×10⁻²mol) was combined with water (44 ml) and sodium persulfate (0.16 g, 6.72×10⁻⁴mol) in a 200 ml round bottomed flask fitted with a rubber septum. The reaction solution was bubble degassed with nitrogen for ten minutes and heated for 24 hours in a water bath at 50° C. As the solution did not appear viscous, additional sodium persulfate (0.16 g, 6.72×10⁻⁴mol) was added and the reaction was allowed to proceed for 18 more hours at 50° C. The solution was then precipitated into acetone and immediately redissolved in water to give 100 ml of a solution of Polymer 17 (11.9% solids).

[0194] Preparation of Polymer 18: Poly[vinylbenzyldimethylsulfonium chloride]

[0195] The aqueous product solution of Polymer 17 (16 ml, 4.0 g solids) was precipitated into a solution of benzyltrimethylammonium chloride (56.0 g) in isopropanol (600 ml). The solvents were decanted and the solids were washed by stirring for 10 minutes in 600 ml of isopropanol and quickly dissolved in water to give 35 ml of a solution of Polymer 18 (11.1% solids). Analysis by ion chromatography showed >90% conversion to the chloride.

[0196] Synthesis of poly(chloromethyl-ethylene oxide-co-sodium thiosulfate methyl-ethylene oxide) from polymer: Polymers 19-21

[0197] Poly(epichlorohydrin) (Aldrich Chemical Company, M_n=700,000) (10 g) was dissolved in 250 ml of anhydrous dimethylsulfoxide (DMSO) and anhydrous sodium thiosulfate (17.0 g) was added. The mixture was heated at 65° C. for 24 hours. After cooling to room temperature, the hazy reaction mixture was dialyzed against water. A small amount of the resulting polymer (Polymer 19) solution was freeze dried for elemental analysis and the rest of the polymer solution was subject to imaging testing. Elemental analysis indicated the reaction conversion to sodium thiosulfate was 16 mol %.

[0198] In another reaction of the same scale, the reaction mixture was heated at 85° C. for 40 hours. Elemental analysis of the resulting polymer (Polymer 20) indicated the conversion to sodium thiosulfate was 26 mol %. When the reaction was carried out at 65° C. for 18 hours, the conversion to sodium thiosulfate was 13 mol % (Polymer 21).

[0199] Synthesis of Polymers 22 and 23

[0200] Synthesis of poly(vinyl benzyl thiosulfate sodium salt-co-methyl methacrylate) from polymer: Polymer 22

[0201] Vinyl benzyl chloride (10 g, 0.066 mol), methyl methacrylate (15.35 g, 0.153 mol) and AIBN (0.72g, 4 mmol) were dissolved 120 ml of toluene. The solution was purged with dry nitrogen and then heated at 65° C. overnight. After cooling to room temperature, the solution was dropwise added to 1200 ml of isopropanol. The resulting white powdery polymer was collected by filtration and dried under vacuum at 60° C. overnight. ¹H NMR analysis indicate that the copolymer contained 44 mol % of vinyl benzyl chloride.

[0202] This polymer (16 g) was dissolved in 110 m of N,N′-dimethylformamide. To this solution was added sodium thiosulfate (12 g) and water (20 ml). Some polymer precipitated out. The cloudy reaction mixture was heated at 90° C. for 24 hours. After cooling to room temperature, the hazy reaction mixture was dialyzed against water. A small amount of the resulting polymer solution was freeze dried for elemental analysis and the rest of the polymer solution was subject to imaging testing. Elemental analysis indicated that all the vinyl benzyl chloride was converted to sodium thiosulfate salt.

[0203] Poly(vinyl benzyl thiosulfate sodium salt-co-styrene) (Polymer 23) can be similarly prepared.

[0204] Synthesis of IR Dyes

[0205] Synthesis of IR Dyes 1 and 3

[0206] 1,1,2-Trimethyl-1H-benz[e]indole (174 g, Fisher Scientific), butyronitrile (500 ml) and 3-bromopropyl-N,N,N-trimethylammonium bromide (270 g, prepared by adding trimethylamine to a solution of 1,3-dibromopropane at 0° C.) were heated to reflux in a round-bottom flask fitted with a condenser and mechanical stirrer. The reaction mixture was heated under a nitrogen atmosphere for 24 hours, then cooled and concentrated to an oil by rotary evaporation. A solution of CH₃CN (500 ml) and tetrabutylammonium bromide (270 g, Aldrich Chemical) and acetone (2500 ml) were added to the oil, and stirred at room temperature for 6 hours. The resulting intermediate, 1-(N,N,N-trimethylammoniumpropyl)-2,3,3 -trimethylbenz(e)indolenium bromide, was collected by filtration and washed with acetone. The material was quickly transferred to a vacuum oven and dried at 40° C. for 16 hours. Yield: 250 g, 75%.

[0207] A 5 liter flask fitted with a mechanical stirrer and heating mantle was charged with acetic anhydride (340 ml). The intermediate bromide of the prior paragraph (100 g) and N-[(3-(anilinomethylene-2-chloro-1-cyclohexan-1-yl)methylene]aniline monohydrochloride (40 g) were added along with triethylamine (30 g) to the flask and heated to 1 00° C. for 15 minutes. The reaction mixture was then cooled to room temperature. The contents of the reaction mixture were then added to a large flask containing diethyl ether (4 liters). After stirring, the diethyl ether was decanted and replaced with acetone (500 ml) and water (150 ml) containing sodium iodide (50 g) and heated to reflux (copper colored crystals appeared). The reaction mixture was again cooled to room temperature and IR Dye 1 was collected by filtration, washed with acetone (until washings were no longer colored by the solid), and then dried under vacuum at 40° C. Weight: 69 g.

[0208] A sample of IR Dye 1 (69 g) was slurried in CH₃CN (1000 ml). Propylene oxide (230 ml) was added and then trifluoromethanesulfonic acid (38 g) was added dropwise at room temperature over a 1-hour period. A solution appeared and after 4 hours of additional stirring, diethyl ether (2 liters) was added. The diethyl ether was then decanted and replaced with ethanol (1 liter) and heated to 60° C. (with stirring). A resulting crystalline solid (IR Dye 3) was collected by filtration, washed with ethanol and dried under vacuum for 16 hours. Weight: 66 g, λ_max=818 nm in methanol, ε_max=252,400 (methanol). The purity was determined to be about 99% by high pressure liquid chromatography.

[0209] Synthesis of IR Dye 2

[0210] In a round bottom flask (500 ml) was added a sample (8 g) of IR Dye 1 in CH₃CN (300 ml) and AMBERLITE IRA-400(Cl) ion exchange resin (Aldrich Chemical Co.). The resulting slurry was stirred for 16 hours. The desired product was extracted from the ion exchange resin by dissolving in boiling CH₃CN (200 ml). It was then filtered hot and then chilled at 5° C. for 5 hours at which time IR Dye 2 precipitated. The solid was washed with cold CH₃CN and dried under vacuum. Weight: 3 g, λ_max=819 nm in methanol, ε_max=251,000 (methanol). The purity was determined to be about 99% by HPLC.

[0211] Printing Plate Containing IR Dye A

[0212] Polymer 14 (0.508 g) and IR Dye A (0.051 g) identified below were dissolved in a 3:1 mixture (w/w, 8.74 g) of methanol and water. After mixing and just before coating, a solution of bis(vinylsulfonyl)methane (BVSM) crosslinking agent (0.705 g, 1.8% by weight in water) was added. The resulting solution was coated using a conventional wire wound rod (K Control Coater, Model K202, RK Print-Coat Instruments Ltd.) to a wet thickness of 25.4 μm on both a gelatin-subbed polyethylene terephthalate and mechanically grained and anodized aluminum supports. The coatings were dried in an oven for four minutes at 70-80° C. The resulting printing plates comprised a heat-sensitive imaging layer containing crosslinked Polymer 14 (1.08 g/m²) and IR Dye A (108 mg/m²) on either a polyester or aluminum support. The light green coatings on the polyester support exhibited a reddish reflex indicating the presence of crystallites in the coating. Thus, the coatings were not homogeneous.

[0213] The printing plates were exposed on a platesetter having an array of laser diodes operating at a wavelength of 830 nm each focused to a spot diameter of 23 μm. Each channel provided a maximum of 450 mWatts (mW) of power incident upon the recording surface. The plates were mounted on a drum whose rotation speed was varied to provide for a series of images set at various exposures as listed in TABLE I below. The laser beams were modulated to produce halftone dot images.

[0214] The exposed printing plates were mounted on a commercial A.B. Dick 9870 duplicator press and prints were made using VanSon Diamond Black lithographic printing ink and Universal Pink fountain solution containing PAR alcohol substitute (Varn Products Company). In the case of the plates having a polyester support, the exposed areas of the printing plates readily accepted ink and printed over 500 impressions of good quality at all exposure conditions, even though the optimum exposure was clearly above 360 mJ/cm2. In the case of the plate having an aluminum support, no substantial image was obtained at any of the exposure conditions.

[0215] Printing Plate Containing IR Dye B

[0216] Polymer 14 (0.508 g) and IR Dye B (0.051 g) identified above were dissolved in a 3:1 mixture (w/w, 8.74 g) of methanol and water. After mixing and just before coating, a solution of BVSM (0.705 g, 1.8% by weight in water) was added. The resulting solution was coated using a conventional wire wound rod (K Control Coater, Model K202, RK Print-Coat Instruments Ltd.) to a wet thickness of 25.4 μm on a gelatin-subbed polyethylene terephthalate support. The coatings were dried in an oven for four minutes at 70-80° C. The printing plates comprised a heat-sensitive imaging layer containing crosslinked Polymer 14 (1.08 g/m²) and IR Dye B (108 mg/m²) on a polyester support. The imaging layer was clear and blue-green in color (apparently free of crystallites).

[0217] The resulting printing plate was exposed as described in Comparative Example 1. A negative image came up early in the press run but scumming was quickly observed and the plate provided only a very poor image through 1000 impressions. It is believed that the scumming was caused by the interaction of the ionomer and IR Dye B.

[0218] Printing Plate Containing IR Dye 1

[0219] Polymer 14 (0.508 g) and IR Dye 1 (0.051 g) were dissolved in a 3:1 mixture (w/w, 8.74 g) of water and methanol. After mixing and just before coating, a solution of BVSM (0.705 g, 1.8% by weight in water) was added. The resulting solution was coated using a conventional wire wound rod (K Control Coater, Model K202, RK Print-Coat Instruments Ltd.) to a wet thickness of 25.4 μm on both gelatin-subbed polyethylene terephthalate and mechanically grained and anodized aluminum supports. The coatings were dried in an oven for four minutes at 70-80° C. The printing plates comprised heat-sensitive imaging layers containing crosslinked Polymer 14 (1.08 g/m²) and IR Dye 1 (108 mg/m²) on either polyester or aluminum supports. The resulting plates were clear and light green in color (apparently free of crystallites).

[0220] The printing plates were exposed and used in printing as described in Comparative Example 1. Unlike in Comparative Example 1, the exposed areas of both types of plates readily accepted ink and printed over 500 impressions of good quality at all exposure conditions (the optimum exposure was above 360 mJ/cm²). Neither type of plate exhibited scumming in the background of the prints.

[0221] Printing Plate Containing IR Dye 3

[0222] Polymer 14 (0.508 g) and IR Dye 3 (0.051 g) were dissolved in a 3:1 mixture (w/w, 8.74 g) of water and methanol. After mixing and just before coating, a solution of BVSM (0.705 g, 1.8% by weight in water) was added. The resulting solution was coated using a conventional wire wound rod (K Control Coater, Model K202, RK Print-Coat Instruments Ltd.) to a wet thickness of 25.4 μm on a gelatin-subbed polyethylene terephthalate and mechanically grained and anodized aluminum supports. The coatings were dried in an oven for four minutes at 70-80° C. The printing plates comprised heat-sensitive imaging layers containing crosslinked Polymer 14 (1.08 g/m²) and Dye 3 (108 mg/m²) on polyester or aluminum supports.

[0223] The printing plates were exposed and used in printing as described in Comparative Example 1. However unlike the plates in Comparative Example 1, the exposed areas of both types of plates readily accepted ink and printed over 500 impressions of good quality. Neither type of plate exhibited scumming in the background of the prints. The optimum exposure for the plate having a polyester support was above 360 mJ/cm².

[0224] The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.