IMAGEABLE ELEMENT AND COMPOSITION COMPRISING THERMALLY REVERSIBLE POLYMERS

Abstract: The present invention also includes an imageable element, comprising a substrate and a thermally imageable composition comprising a thermally sensitive polymer which exhibits an increased solubility in an aqueous developer solution upon heating. The thermally sensitive polymer includes at least one covalently bonded unit and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each of the non-covalently bonded unit. The present invention also includes a method of producing the imaged element. The present invention still further includes a thermally imageable composition comprising a thermally sensitive polymer according to the present invention and a process for preparing the thermally sensitive polymer, which is a supramolecular polymer. The process includes contacting a hydrocarbly-substituted isocytosine and a diisocyanate to produce a mono-adduct and contacting the mono-adduct and a polyfunctional material to produce the supramolecular polymer.

Description: IMAGEABLE ELEMENT AND COMPOSITION COMPRISING THERMALLY REVERSIBLE POLYMERS BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to an imageable element comprising a substrate and a thermally imageable composition coated on a surface of the substrate. The thermally imageable composition comprises a thermally sensitive polymer, which comprises at least one covalently bonded unit and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each non-covalently bonded unit. More particularly, the present invention relates to a thermally sensitive supramolecular polymer prepared from a hydrocarbyl-substituted isocytosine, a diisocyanate and a poiyfunctional material such as phenolic resin, acrylic resin, polyester resin and polyurethane resin. 2. DESCRIPTION OF THE PRIOR ART WO 98/14504 discloses a supramolecular polymer containing monomeric units, which in pairs, form at least 4 H-bridges with one another. The article in J. Org. Chem., 53, 5787-9 (1988) discloses bifunctional compounds that can associate into polymers or oligomers by 2-center H-bond units. Supramolecular polymers based on 3-center H- bond units are disclosed in Macromolecules, 28, 782-83 (1995). None of the above references discloses the use of these polymers in thermal imaging. EP 969 966 discloses phenolic polymers together with a non- photosensitive solubility inhibitor which provides acceptor sites for H- bonding and EP 985 166 discloses the corresponding methods. The present invention is directed to a supramolecular polymer having non- covalent interactions, such as H-bonding, as an integral part of the polymer structure. The H-bonding in the present invention is not between a H- donor polymer and a solubility inhibitor having H-acceptor sites, but is an integral part of the polymer structure. WO 99/01795 discloses phenolic polymers, which are modified with groups that provide acceptor sites for H-bonding with other phenolic polymers. The present invention is directed to a supramolecular polymer having non-covalent interactions as an integral part of the polymer structure. An article by B. J. B. Folmer et al., Advanced Materials, Vol. 12 (No. 12), pages 874-878 (2000), discloses supramolecular polymer materials. The article does not disclose polymers derived from poiyfunctional materials, such as, poiyfunctional phenolic resin, acrylic resin, polyester resin or polyurethane resin, nor does it disclose the use thereof in thermal imaging. SUMMARY OF THE INVENTION The present invention includes an imageable element comprising a substrate and a thermally imageable composition according to the present invention coated on a surface of the substrate. The thermally imageable composition according to the present invention comprises: a thermally sensitive polymer which exhibits an increased solubility in an aqueous developer solution upon heating, said thermally sensitive polymer comprising: at least one covalently bonded unit; and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each non-covalently bonded unit. The present invention further includes a method of producing an imaged element. The method comprises the steps of providing an imageable element comprising a substrate and a thermally imageable composition according to the present invention coated on a surface of the substrate; exposing the imageable element to thermal radiation to produce imagewise exposed regions; and contacting the exposed imageable element and a developer within a period of time after the exposing step to remove the exposed regions and thereby produce the imaged element. The present invention still further includes a process for preparing a supramolecular polymer. The process comprises contacting a hydrocarbyl- substituted isocytosine and a diisocyanate to produce an isocytosine/isocyanate mono-adduct; and contacting the isocytosine/isocyanate mono-adduct and a poiyfunctional material selected from the group consisting of: poiyfunctional phenolic resin, acrylic resin, polyester resin, polyurethane resin, and a combination thereof, at a temperature and for a period of time sufficient to produce the supramolecular polymer. In addition to the thermally imageable composition, the present invention includes a monomer represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 ; R 3 is a phenolic residue derived from a poiyfunctional phenolic resin represented by the formula R 3 (OH) n ; and n is at least 1. The present invention also includes a supramolecular polymer derived from monomers represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 ; R 3 is a phenolic residue derived from a poiyfunctional phenolic resin represented by the formula R 3 (OH) n ; and n is at least 1. The present invention further includes an isocytosine/isocyanate mono-adduct in a monomeric or quadruple hydrogen bonded dimeric form. The monomeric form of the isocyanate mono-adduct can be represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl and Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 , the diisocyanate being selected from the group consisting of isophorone diisocyanate, methylene-bis-phenyl diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, tetramethylxylylene diisocyanate, dimers thereof, adducts thereof with diols, and mixtures thereof. The present invention still further includes an isocytosine/isocyanate bis-adduct in a monomeric or quadruple hydrogen bonded polymeric form. The monomeric form of the bis-adduct can be represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl and Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 . In the present invention, the image areas are reinforced with strong two or more centered H-bond links. Therefore, the image areas exhibit long press life and increased resistance to press chemicals when compared with the systems of the prior art, which include relatively weak single H-bond units. Thus, the thermally reversible polymers of the present invention can have relatively strong bonds and provide thermal solubilization systems that have enhanced differentiation between exposed and non-exposed areas. These polymers can undergo thermal solubilization and provide a durable, non-exposed area, corresponding to the etch resist in a PCB or to the image area in a printing plate. The imagewise exposure and development steps of the present invention do not require an intermediate pre-development heating step. In addition, superior press life is achieved without the need of a post- development bake because of the strength of the two or more centered H- bond links, such as, 4-centered H-bond units, in the image areas. The superior press life resulting from the strongly bonded image is unexpected for a no-preheat, digitally imaged composition. Furthermore, the inventive element of the present invention, particularly when it comprises a single layer coating composition, is simple to manufacture. Accordingly, the present invention provides thermally reversible polymers that can be used in simple and cost effective methods that are useful in thermal imaging of lithographic plates and circuit boards. DETAILED DESCRIPTION OF THE INVENTION The thermally imageable composition of the present invention is useful in digital imaging applications, including printing plates and printed circuit boards. Lithographic printing is based on the immiscibility of oil and water. Ink receptive areas are generated on the surface of a hydrophilic surface. When the surface is moistened with water and then ink is applied, the hydrophilic background areas retain the water and repel the ink. The ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is thereafter reproduced. Lithographic printing plate precursors, i.e., imageable elements, typically include a radiation-sensitive coating applied over the hydrophilic surface of a support material. If after exposure to radiation, the exposed regions of the coating become soluble and are removed in the developing process, revealing the underlying hydrophilic surface of the support, the plate is called a positive-working printing plate. Conversely, if exposed regions of the plate become insoluble in the developer and the unexposed regions are removed by the developing process, the plate is called a negative-working plate. In each instance, the regions of the radiation- sensitive layer that remain (i.e., the image areas) are ink-receptive and the regions of the hydrophilic surface revealed by the developing process accept water and repel ink. The present invention is useful in positive-working printing plates and includes a thermally imageable composition comprising a thermally sensitive polymer, which comprises at least one covalently bonded unit and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond in each unit. Preferably, the thermally reversible non-covalently bonded unit is a two-, three or four-centered H-bonded unit. In one embodiment, the thermally reversible non-covalently bonded unit includes a two-centered H- bonded unit, which comprises two 2-pyridone groups. In another embodiment, the thermally reversible non-covalently bonded unit includes a three-centered H-bonded unit, which comprises a cyclic imide group and a 2,6-diaminotriazine groups. In yet another embodiment, the thermally reversible non-covalently bonded unit includes a four-centered H-bonded unit, which comprises two isocytosine groups. The term "unit" in the context of the present invention refers to any chemical group, moiety or functionality. Examples of such units include covalently bonded units, such as those derived from novolak resins, and non-covalently bonded units, i.e., two or more centered H-bond links, such as those derived from 4-centered H-bonded units. Such 4-centered H-bonded units in a preferred embodiment have a structure resulting from association of the two sites capable of quadruple hydrogen bonding, namely two isocytosine units. The association of the two quadruple hydrogen bonding sites and the quadruple hydrogen bonded unit resulting therefrom, i.e., the 4-centered H-bonded unit, is schematically represented below for the "keto" tautomer, which is one of the two possible tautomeric forms of the isocytosine ring system. The schematic representation is for an isocytosine having the substituent -CH 2 CH 2 CH 2 -R: The 4-centered H-bonded unit can result from the association of two sites that are in the "enol" tautomeric form in which the hydrogen is on the oxygen atom instead of being on the nitrogen. The "keto" and "enol" tautomeric forms of an isocytosine derivative are described and proposed structures shown in the previously cited WO 98/14504, the contents of which are incorporated herein by reference in its entirety. Accordingly, whenever a composition of present invention is represented by the formula of the "keto" form, it should be understood that the actual compound will exist either in the "keto" or in the "enol" tautomeric forms or in a combination of the "keto" and "enol" tautomeric forms. As an example, the formation of such a 4-centered H-bonded unit according to the present invention resulting from a 4-centered H-bonding interaction of two 6-methylisocytosinyl groups, each attached to a group R, is schematically represented below: Formation of a 4-centered H-bonded "unit" The term "supramolecular polymer" in the context of the present invention refers to a polymer which derives it's polymeric properties through a combination of covalent bonds and specific secondary interactions, which includes hydrogen bonding, particularly two or more centered H-bond links. Such secondary interactions provide high bond strength and contribute substantially to the polymeric behavior. The non-covalently bonded unit acording to the present invention includes at least a two-centered H-bond within each unit. Preferably, the non-covalently bonded unit incudes a two, three or four centered H-bond within each unit. H-bonds that are higher than four centered can also be useful in the present invention. In addition to the two or more centered H-bonding, the non- covalently bonded unit in the thermally imageable composition of the present invention further includes one or more additional secondary interactions, such as, van der Waals associations, hydrophobic associations, ionic associations and a combination thereof. Furthermore, in addition to the two or more centered H-bonded units, the thermally sensitive polymer itself can have additional intermolecular or intramolecular interactions, including intermolecular or intramolecular H-bonding. To achieve developability, the thermally sensitive polymer in the thermally imageable composition must have at least one base-soluble functional group having a pKa of less than 14. Such base-soluble functional groups include, for example, carboxylic, sulfonic, imide, N-acyl sulfonamide and phenolic hydroxy groups. The term "hydrocarbyl" in the context of the present invention refers to a linear, branched or cyclic alkyl, alkenyl, aryl, aralkyl or alkaryl of 1 to 22 carbon atoms, substituted derivatives thereof, wherein the substituent group is selected from halogen, hydroxy, hydrocarbyloxy, carboxyl, ester, ketone, cyano, amino, amido and nitro groups. Hydrocarbyl groups in which the carbon chain is interrupted by oxygen, nitrogen or sulfur are also included in the term "hydrocarbyl". The term "hydrocarbylene" in the context of the present invention refers to a linear, branched or cyclic alkylene, vinylene, arylene, aralkylene or alkarylene of 1 to 22 carbon atoms, substituted derivatives thereof, wherein the substituent group is halogen, hydroxy, carboxyl, hydrocarbyloxy, ester, ketone, cyano, amino, amido and nitro groups. Hydrocarbylene groups in which the carbon chain is interrupted by oxygen, nitrogen or sulfur are also included in the term "hydrocarbylene". Bifunctional compounds, which can associate into an oligomer or polymer by two centered H-bond units are disclosed in J. Org. Chem., 53, 5787-9 (1988). Supramolecular polymers based on three centered H-bond units are disclosed in Macromolecules, 28, 782-83 (1995). The contents of these articles are incorporated herein by reference for all purposes. Representative of the compounds that can associate into an oligomer or polymer by two centered H-bond units are bridged bipyridone derivatives such as those disclosed in the previously incorporated J. Org. Chem., 53, 5787-9 (1988), provided that such systems have at least one base-soluble functional group, such as, carboxylic, sulfonic, imide, N-acyl sulfonamide or phenolic hydroxy groups. The non-covalently bonded unit in this embodiment includes a bridged bipyridone derivative capable of forming a two-centered H-bond. The thermally sensitive polymer in the thermally imageable composition according to the present invention includes a supramolecular polymer derived from monomers represented by the formula: wherein each R' and R" can independently be H, linear, branched or cyclic alkyl, aryl, aralkyl, alkaryl, substituted aryl, alkenyl, halogen, cyano, nitro, alkoxy, aryloxy, alkoxycarbonyl, amido, acyl, aminocarbonyl, carboxylic, sulfonic, imide, N-acyl sulfonamide or phenolic hydroxy, provided that at least one of the R' and R" groups is carboxylic, sulfonic, imide, N-acyl sulfonamide or phenolic hydroxy. A can be a bridging group, such as, alkylene, arylene, aralkylene, alkarylene, substituted arylene, -O-, -S-, NR'", -CH=CH- or -C≡C-. Representative of the compounds that can associate by three centered H-bond units are maleimide/styrene copolymers associated with 4-vinyl-2,6-diaminotriazine/ styrene copolymers, such as those disclosed in the previously incorporated Macromolecυles, 28, 782-83 (1995), provided that such copolymers have at least one base-soluble functional groups, such as, carboxylic, sulfonic, imide, N-acyl sulfonamide or phenolic hydroxy groups. The non-covalently bonded unit in this embodiment can be a group capable of forming one or more three centered H-bonds with another same or different such group to form a three centered H-bonded pair. In one embodiment, the thermally sensitive polymer in the thermally imageable composition according to the present invention includes a supramolecular polymer derived from maleimide/styrene copolymers associated with 4-vinyl-2,6-diaminotriazine/ styrene copolymers. Terpolymers thereof are also suitable for use as the thermally sensitive polymer in the thermally imageable composition according to the present invention. The covalently bonded unit according to the present invention is derived from a poiyfunctional material covalently bonded to sites that are also bonded to the non-covalently bonded units. At least one, preferably more than one, same or different poiyfunctional materials can be bonded to such sites. The supramolecular polymer can be any polymer or resin that is capable of producing properties required for thermal imaging. For example, to achieve developability, the poiyfunctional material has at least one base-soluble functional group having a pKa of less than 14. Such base-soluble functional groups include, for example, carboxylic, sulfonic, imide, N-acyl sulfonamide and phenolic hydroxy groups. The preferred poiyfunctional materials include poiyfunctional phenolic resins, acrylic resins, polyester resins, polyurethane resins, and combinations thereof. Preferably, the poiyfunctional phenolic resin is a phenol/cresol novolak, polyvinyl phenol polymer, vinyl phenol/hydrocarbyl acrylate copolymer or a mixture thereof. In a preferred embodiment, the thermally sensitive polymer in the thermally imageable composition according to the present invention is a supramolecular polymer derived from monomers represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 ; R 3 is a phenolic residue derived from a poiyfunctional phenolic resin represented by the formula R 3 (OH) n ; and n is at least 1. Preferably, the poiyfunctional phenolic resin is phenol/cresol novolak, polyvinyl phenol polymer, vinyl phenol/hydrocarbyl acrylate copolymer, and a mixture thereof. The diisocyanates that are suitable for use in the present invention include any diisocyanates, also referred to herein as "difunctional isocyanates." Preferably, the diisocyanate is selected from isophorone diisocyanate, methylene-bis-phenyl diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, tetramethylxylylene diisocyanate, dimers thereof, adducts thereof with diols, and mixtures thereof. Because the poiyfunctional phenolic resin has n functional groups, up to n hydroxy groups can react with an equal number of isocyanate functional groups, each attached to a site bearing a non-covalently bonded unit. Preferably, in the above-described supramolecular polymer n is 1 , 2 or 3. Thus, when n=2 in the above preferred embodiment, two hydroxy groups from a poiyfunctional phenolic resin react with an equal number of isocyanate functional groups, each attached to a site bearing a non- covalently bonded unit, to produce a linear polymer in which the chain growth is the result of association of sites that are capable of four-centered hydrogen bonding to produce a quadruple hydrogen bonding unit. When n=3, three hydroxy groups from a poiyfunctional phenolic resin react with an equal number of isocyanate functional groups, each attached to a site bearing a non-covalently bonded unit, to produce a crosslinked polymer in which crosslinking is achieved through association of sites that are capable of four-centered hydrogen bonding to produce three quadruple hydrogen bonding units. Thus, the thermally sensitive polymer can be crosslinked. Preferably, such crosslinks are thermally reversible. In another preferred embodiment, the thermally imageable composition of the present invention further comprises a photothermal converter material. The supramolecular polymer can be prepared by a process comprising the steps of contacting a hydrocarbyl-substituted isocytosine and a disocyanate to produce an isocytosine/isocyanate mono-adduct and contacting the isocytosine/isocyanate mono-adduct and a poiyfunctional material, such as, a poiyfunctional phenolic resin, acrylic resin, polyester resin, polyurethane resin, and a combination thereof, at a temperature and for a period of time sufficient to produce the supramolecular polymer. Preferably, the hydrocarbyl groups in the above process are independently alkyl groups of 1 to 22 carbon atoms and the diisocyanate is isophorone diisocyanate, methylene-bis-phenyl diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, tetramethylxylylene diisocyanate, dimers thereof, adducts thereof with diols, or mixtures thereof. The poiyfunctional phenolic resin can be phenol/cresol novolak, polyvinyl phenol polymer, vinyl phenol/hydrocarbyl acrylate copolymer, or a mixture thereof. The isocytosine/isocyanate mono-adduct can be in a monomeric or quadruple hydrogen bonded dimeric form having a quadruple hydrogen bonded unit. The monomeric form is represented by the formula: wherein each R 1 and R 2 can independently be hydrogen and hydrocarbyl; Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 , the diisocyanate being selected from isophorone diisocyanate, methylene-bis-phenyl diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, tetramethylxylylene diisocyanate, dimers thereof, adducts thereof with diols, and mixtures thereof. The dimeric form of the isocytosine/isocyanate mono-adduct has a structure resulting from association of the two sites capable of quadruple hydrogen bonding, namely two isocytosine units. The association of the two quadruple hydrogen bonding sites and the quadruple hydrogen bonded unit resulting therefrom, i.e., the 4-centered H-bonded unit, is schematically represented below for a related system in which the -Y-NCO group is replaced with the group -CH 2 CH 2 CH 2 -R: The isocytosine/isocyanate bis-adduct can be in a monomeric form or it can be in a quadruple hydrogen bonded oligomeric or polymeric form. The quadruple hydrogen bonded oligomeric or polymeric form results from association of two sites capable of quadruple hydrogen bonding, each from a different isocytosine/isocyanate bis-adduct, to produce an oligomer or polymer having repeating units of associated quadruple hydrogen bonding units, i.e., the 4-centered H-bonded units. The monomeric form of the isocytosine/isocyanate bis-adduct is represented by the formula: wherein each R 1 and R 2 can independently be hydrogen and hydrocarbyl and Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 . Reaction of 1 mole of an isocytosine derivative with 1 mole of diisocyanate, such as isophorone diisocyanate, produces the isocytosine/isocyanate 1 :1 adduct, which will spontaneously dimerize to form a dimeric mono-adduct having a thermally reversible 4-center H-bond. The resulting dimeric mono-adduct has free isocyanate groups on each end. This dimeric mono-adduct can then be used to form thermally reversible crosslinks with phenolic polymers. Any unreacted diisocyanate, such as isophorone diisocyanate, can also crosslink the phenolic polymers, however such crosslinking is not reversible. To avoid crosslinking by the unreacted diisocyanate, an excess of isocytosine, i.e., about 10-20 % molar excess, is preferably used. Excess isocytosine can further react with the mono-adduct, which can also dimerize spontaneously to give a diisocyanate bis-adduct having two or more thermally reversible 4-center H-bonds. Using "I" for the diisocyanate and "M" for isocytosine derivative, the reactions can be illustrated as follows: l + M ^ l-M l-M M- monomeric dimeric mono-adduct mono-adduct l-M M-l + M ^ l-M M-l-M ^ l-M M-l-M M-l - l-M M-l-M M-l-M M-l To maximize the formation of lower order adducts, isocytosine is added slowly to the diisocyanate to ensure that an excess diisocyanate is present at the early stages of the reaction. The present invention further comprises an imageable element which comprises a substrate and a thermally imageable composition according to the present invention coated on a surface of the substrate. Preferably, the surface of the substrate underlying the thermally imageable composition is hydrophilic. The present invention is suitable for use in single as well as multilayer thermally imageable elements that are useful in lithographic printing, including lithographic printing plates that can be thermally imaged by imagewise exposure with a laser or a thermal printing head. The multilayer thermally imageable element is useful as a precursor for a lithographic printing member. In a multilayer thermally imageable case, the element has a bottom layer comprising a polymeric material which can be removed by treatment with an aqueous alkaline solution, and a top layer comprising the thermally sensitive polymer which exhibits an increased solubility in an aqueous developer solution upon heating and comprising: at least one covalently bonded unit; and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each of the non- covalently bonded units. The bottom layer contains a polymeric material, such as, a terpolymer of methacrylic acid, acrylamide an d N-phenyl maleimide. The bottom polymeric material is soluble in aqueous alkaline developer, but the top layer is insoluble in aqueous alkaline developer. However, the top layer becomes soluble or penetrable to the aqueous alkaline developer following thermal exposure. In addition to the thermally imageable layer, the thermally imageable element can have additional layers, such as, an underlying layer. Possible functions of an underlying layer include: (1) to enhance developability of the imagewise unexposed areas; and (2) to act as a thermal insulating layer for the imagewise exposed areas. Such thermal insulating polymeric layer prevents otherwise rapid heat dissipation, for example, through the heat conducting aluminum substrate. This allows more efficient thermal imaging throughout of the thermally imageable layer, particularly in the lower sections. In accordance with these functions, the underlying layer should be soluble or at least dispersible in the developer and, preferably, have a relatively low thermal conductivity coefficient. The thermally imageable element can further have an overlying layer. Possible functions of an overlying layer include: (1) to prevent damage, such as scratching, of the surface layer during handling prior to imagewise exposure; and (2) to prevent damage to the surface of the imagewise exposed areas, for example, by over-exposure which could result in partial ablation. The overlying layer should be soluble, dispersible or at least permeable to the developer. The present invention still further includes a method of producing an imaged element. The method comprises the steps of providing an imageable element comprising a substrate and a thermally imageable composition according to the present invention coated on a surface of the substrate, exposing the imageable element to thermal radiation to produce imagewise exposed regions and contacting the exposed imageable element and a developer within a period of time after the exposing step to remove the exposed regions and thereby produce the imaged element of the invention. The developer composition is dependent on the nature of the polymeric substance, but is preferably an aqueous composition. Common components of aqueous developers include surfactants, chelating agents, such as, salts of ethylenediamine tetraacetic acid, organic solvents, such as, benzyl alcohol, and alkaline components, such as, inorganic metasilicates, organic metasilicates, hydroxides and bicarbonates. Preferably, the aqueous developer is an alkaline developer containing inorganic metasilicates or organic metasilicates when the polymeric substance is a phenolic resin. Typically, the step of exposing the imageable element to thermal radiation is carried out using an infrared laser. However, other methods such as visible or UV laser imaging may also be used, provided that a photoconverter, i.e., a photothermal converter, is present. Thus, for exposure with such visible or UV radiation sources, the imageable composition generally includes a photothermal converting material. The Applicants have unexpectedly discovered that despite the strong multi-centered H-bonding, it was possible to thermally reverse such bonding during IR laser exposure in which the pixel dwell time of the laser is less than 100 μs. Thermally reversing the bonding by exposing at a pixel dwell time of not more than 100 μs enables one to produce an imaged element economically and within a short period of time, thereby providing a significant cost advantage over methods that require exposures at a pixel dwell time of more than 100 μs. Accordingly, the thermal radiation in the method of the present invention preferably has a pixel dwell time of not more than 100 μs. Furthermore, despite the expected ease of reformation of strong multi-centered H-bonding, the Applicants have discovered surprisingly that, following exposure, it was possible to remove the exposed areas with an aqueous developer prior to reformation of the multi-centered H-bonded units. A characteristic feature of this process is transiency of thermal solubilization in that developability decreases with time following exposure. Based on the examples, imageable elements should preferably be developed within up to 1 hour, more preferably within up to 30 minutes, most preferably within up to 10 minutes following exposure. For example, in the case of infrared laser imaging, the period of time between the steps of exposing and treating with a developer is preferably less than about 10 minutes. The method of the present invention can be used in one or two layer systems. In the two layer systems, the bottom layer is soluble or dispersible in a developer, whereas the top layer is insoluble in the developer. Imagewise thermal exposure of such composite layer increases the rate of removal of the exposed areas of both layers. The rate of removal of the exposed areas can be increased by enhanced rate of dissolution or dispersibility of the bottom layer and by enhanced permeability of the top layer. The supramolecular polymers of the present invention can be used in the top layer. The thermally imageable composition of the present invention is useful in digital imaging applications, including printing plates and printed circuit boards. Synthetic route: The synthetic route describes in detail the synthesis of "RESIN A". Further syntheses were undertaken to produce "RESIN B" and "RESIN C" as described in table A. Synthesis of the quadruple hydrogen bonding entity (QHBE) Stage 1 Into a 250 ml flask were added 100 g of dried tetrahydrofuran (THF) and 13.78 g (0.11 mole) of dried 6-methylisocytosine. To this mixture was added 22.24 g (0.1 mole) of isophorone diisocyanate and the flask sealed from atmospheric moisture with a conical stopper. The mixture was left at ambient temperature for five days. The resulting mixture represented 0.1 mole of the quadruple hydrogen bonding entity, which would then be reacted with polymers containing phenolic -OH functionality (see Science, 278, 1601-1604 (1997) and in J. Polym. Sci., Part A: Polym. Chem., 37, 3657-3670 (1999). Stage 2 110.75 g of Bakelite resin LB6564 was dissolved into 400 g of dried tetrahydrofuran (THF) in a 1 litre flask and 40% of the stage 1 mixture (0.04moles of QHBE) added slowly with stirring. Slow stirring was continued for 1 - 2 days at ambient temperature. The mixture was tested for isocyanate functionality using FT-IR. The reaction was considered to be complete when the presence of an isocyanate functionality could no longer be detected by Infrared Spectroscopy (IR). Chemical Reactions: Preparation of QHB-functional phenolic resin nate novolak The QHB-functional phenolic resin and quadruple H-bonded unit resulting therefrom are represented schematically below. Phenolics having 2 QHB units will undergo chain extension, whereas phenolics having 3 or more QHB units are capable of undergoing crosslinking. Schematic representation of QHB structure QHB The QHB group can form a four centered H-bond as shown below: Formation of a 4-centered H-boπded "unit" Isolation of the QHB polymer The product was isolated by precipitation into water. The mixture was slowly poured as a thin stream into 5 litres of vigorously stirred distilled water using a Silverson/Rotamix mixer. Initially, the product precipitated as a sticky viscous mass, but tended to harden and crumble with further stirring as the tetrahydrofurane (THF) was extracted from the polymer. The aqueous phase was decanted and replaced with fresh water and left overnight. The mixture was again vigorously stirred to form a fine precipitate, which was washed with water while being filtered, using a Buchner funnel. The damp cake was crumbled into a drying tray and dried at 40°C until constant weight was obtained. Table A Resin LB6564 Lyncur M(S-1) Lyncur CBA Phenolic Resin (equiv wt) 116.43 108.14 262.15 Phenolic Resin (equivalents) 0.9512 0.476 0.238 Resin weight (g) 110.75 51.43 62.34 QHBE (moles) 0.04 0.02 0.01 QHBE weight (g) 13.91 6.954 3.477 QHBE Solvent T.H.F. T.H.F. T.H.F. PHENOL/QHBE equiv ratio 23.78 23.78 23.78 Solvent T.H.F. T.H.F. T.H.F. Solvent weight (g). 400 200 250 Yield (g). 119.7 32 46.4 Batch name RESIN A RESIN B RESIN C Definitions: 1. LB6564: a phenol/cresol novolak marketed by Bakelite, UK. 2. M(S-1): Lyncur M(S-1), a polyvinylphenol polymer having Mw = 1600 to 2400 and Mn = 1100 to 1500, as supplied by Siber Hegner, Beckenham, UK. 3. CBA: Lyncur CBA (PVPh-butylacrylate), a copolymer of polyvinylphenol having the structure: n/m = 8 2 - 6/4, Mw = 10 000 - 30 000 Mn = 5000 - 5500 4. RESIN A: the resin obtained when 110.75 g of LB6565 is reacted with 13.91 g of QHBE. 5. RESIN B: the resin obtained when 51.43 g of M(S-1) is reacted with 6.954 g of QHBE. 6. RESIN C: the resin obtained when 62.34 g of Lyncur CBA resin is reacted with 3.477 g of QHBE. 7. KF654B PINA: an IR dye as supplied by Riedel de Haan Ltd, Middlesex, UK, having the following structure: Θ Br 8. IR dye B: an IR dye having the structure: 9. Crystal violet: basic violet 3, C.I. 42555, Gentian violet, as supplied by Aldrich Chemical Company, Dorset, UK. 10. Oxonol Blue: a blue dye having the structure: 11. Paris Blue: a blue pigment supplied by Kremer Pigmente, Aichstetten, Germany. 12. Developer A: 14%wt sodium metasilicate pentahydrate in water. 13. Developer B: 0.2M NaOH in water. 14. Copper substrate A: double sided copper laminate of overall thickness 254 microns, having copper cladding 18 microns thick on each side of an insulating substrate, catalogue number N4105-2, 0.008, H/H THE, as supplied by New England Laminates (UK) Ltd of Skelmersdale, UK. The copper substrate is then brush grained using a mechanical grainer, stock number 4428, supplied by PCB machinery Ltd, Haslingden, Rossendale, UK, rinsed with distilled water for 10 seconds and allowed to air dry, prior to coating. 15. Etching solution A: Ferric chloride hexahydrate crystals, catalogue no. 551-227 as supplied by RS components, Corby, UK. 16. Stripper A: Catalogue no. 690-855 as supplied by RS components. Plate examples and press test result: Formulation A and B solution concentrations were selected to provide the specified dry film compositions with coating weights of 2 g/m 2 after thorough drying, at 100°C for 90 seconds in a Mathis oven. In each case the substrate used was an aluminum sheet that had been electrograined and anodised and post-anodically treated with an aqueous solution of an inorganic phosphate. The coating solutions were coated onto the substrate by use of a wire wound bar. For formulation A, RESIN A (see Table A) and IR dye B were used in the ratio of 98:2 (w:w). For formulation B, Paris Blue, and RESIN A (at a ratio of 1 :4, w:w) were ball milled together for 4 days such that the dispersed mill base had a solids content of 30 wt% in 1-methoxypropan-2- ol and a particle size of <10 microns as determined by grind gauge. Thus, the formulation B coating was prepared as a solution on 1-methoxypropan- 2-ol. The precursors were imaged using an internal test pattern, on a Creo Trendsetter at an imaging energy density of 250 mJ/cm 2 . They were then processed using a Kodak Polychrome Graphics Mercury Mark V processor containing developer A at 22.5°C at a process speed of 750 mm/min. Performance was assessed by comparing actual screen densities with expected densities using a Gretag D19C densitometer (available from Colour Data Systems Ltd, The Wirral, UK) Density Results: The printing forms were press tested on a Heidelberg Speed Master 52 printing press, using Duo laser 80 g paper, Gibbons Geneva A1 colour ink and a standard fount solution (comprising 87% water, 10.5% isopropyl alcohol and 2.5% Goldfount surfactants). Each printing form ran cleanly, with no inking in background areas. Density tests were carried out on the solid image areas (100% screen areas) at a range of runlengths. Formulations A and B maintained 100% screen density beyond 23,500 impressions and the plates ran cleanly on press. There was no evidence of coating failure throughout the printing test. Latent image stability: Coating formulation C was prepared as a solution in 1- methoxypropan-2-ol. The solution concentration was selected to provide the specified dry film composition of 2% IR dye B and 98% RESIN A with a coating weight of 2 g/m 2 after thorough drying at 100°C for 90 seconds. The precursor was then imaged using an internal test pattern with 100% coverage, on a Creo Trendsetter 3244 at an imaging energy density of 220 mJ/cm 2 . Next, separate samples of this precursor were cut up and the time between imaging and developing was increased (0, 1 , 2, 3 and 4 hours). Additional samples were stored at elevated temperature (40°C) for 0, 4 and 21 hours. Samples were then processed using a Kodak Polychrome Graphics Mercury Mark V processor containing developer A at 22.5°C at a process speed of 750 mm/min. In this way, the latent image stability of the formulation was evaluated using a Gretag densitometer. Latent Image Stability Results, room temperature storage: Latent Image Stability Results. 40°C storage: Developer resistance tests: The solution concentrations were selected to provide the specified dry film compositions with coating weights of 2 g/m 2 after thorough drying, at 100°C for 90 seconds in a Mathis oven. In each case the substrate used was an aluminum sheet that had been electrograined and anodised and post-anodically treated with an aqueous solution of an inorganic phosphate. The coating solutions were coated onto the substrate by use of a wire wound bar. Precursor samples were then developed for increasing amounts of time at different ages, with developer A at 25°C and a visual assessment was made of whether any coating remained, ie was the coating resisting the developer. Fresh Plates (non-imaged): "Coating visible" = coating is unattacked by developer "Some coating left" = coating is partially dissolved by developer "All coating is removed" = developer has dissolved coating away 1 Day old plates, stored at ambient (non-imaged): All coating was removed from Formulations G through L. Day old plates, stored at ambient (non-imaged): All coating was removed from Formulations G through L. Day old plates, stored at 40°C (non-imaged): All coating was removed from Formulations G through L. 6 Day old plates, stored at 40°C (non-imaged): Formulations G through I were not tested All coating was removed from Formulations J through L. As shown in Table A, the QHBE/phenol group equivalent ratio is about 1/24 in these formulations. Developer resistance as a function of QHBE level was also determined, using a series of derivatives of LB6564 of different QHB level shown in the table below and compared with unmodified LB6564. These results show that increasing levels of QHBE enhance developer resistance. Accordingly, those skilled in the art will be able to adjust the QHB/Resin ratio to provide an appropriate resistance for resins used in practicing the present invention, such as resins C and D. 2-Laver Plate Example Stage 1 : Synthesis of the Quadruple Hydrogen Bonding Entity (QHBE): Dry 6-methylisocytosine (13.78 g) (0.11 mole) and dry 2-butanone (MEK) (100 g) were charged into a 250-mL flask under N 2 atmosphere. With the temperature held constant at 30°C, isophorone diisocyanate (22.24 g) (0.1 mole) was added dropwise, after which the temperature was raised to 45°C and the reaction was continued for 8 hr. The % unreacted NCO was determined by titration to be 13.0 (11.7 theoretical), corresponding to 0.1 mole QHBE (26.5% solids). Stage 2: Preparation of QHB-Functional Phenolic Resin Novolac resin LB-6564 (110.8 g) and tin octoate (0.4 g) were dissolved in dry MEK in a 1-L flask, followed by the addition of QHBE (48.5 g, corresponding to 40 wt% of the stage 1 product) (0.04 mol) over a period of 1 hr with stirring. After heating under N 2 for 8 hr at 80°C, all of the NCO functionality was reacted, as determined by titration. The MEK was removed by rotary evaporation and replaced by 2- methoxyethanol (100 g). The resulting mixture was slowly poured as a thin stream into distilled water (5 L), which was vigorously stirred using a Silverson/Rotamix mixer. The resulting damp cake was dried at 40°C to provide the QHB-functional phenolic resin, UR 4309. Plate Preparation and Evaluation A formulation of copolymer ACR-1478 (described below) (4.25 g) and IR dye B (0.75 g), dissolved in a mixture of dioxalane (43 g), methanol (43 g) and methyl lactate (14 g) was spin coated onto a lithographic aluminum substrate to provide a dry coating wt of 1.4 g/m 2 . The plate substrate had been electrolytically grained, anodized and hydrophilized with polyvinyl phosphonic acid prior to coating. A solution of the QHB-phenolic resin UR 4309 (3 g) in a mixture of toluene (70 g) and 2-methoxypropanol (30 g) was spin coated onto the above coated substrate to provide a second layer, having a dry coating wt of 0.7 g/m 2 . The composite printing plate precursor was further dried for at 95°C for 3 min and imagewise exposed to a dose of 150 mJ/cm 2 , using a Creo trendsetter 3244 at a 10W power setting. The imaged plate precursor was processed in a PC32 processor (available from Kodak Polychrome Graphics), using Kodak Polychrome Graphics developer 956. One pass through the processor at a speed of 2 m/min resulted in complete removal of the imagewise exposed areas and provided the imaged plate having an excellent image and a clean background. The image quality remained good after a second pass through the processor, which demonstrated good processor latitude. The plate precursor was also shown to be white-light safe. Preparation of Copolymer ACR-1478 Dioxalane (75 g), 95% ethanol (75 g), methacrylic acid (14.7 g), N- phenylmaleimide (59.1 g) and methacrylamide (26 g), followed by dioxalane (75 g) and ethanol (75 g), were charged into a 4-neck, 4-L flask, equipped with a mechanical stirrer, condenser, temperature controller and nitrogen purge. The reaction mixture was heated to 60°C, using a heating mantle. Azo-bis-isobutyronitrile (Vazo-64, from DuPont) (0.135 g) was added and the reaction was maintained at 60°C for 22 hr. After cooling to room temperature, the reaction mixture was slowly poured onto a mixture of ethanol (1.22 kg) and distilled water (250 g) contained in a 2-gallon plastic container over a period of 45 min and then stirred for 30 min. The precipitated product was filtered, re-suspended in ethanol (920 g), filtered and dried at 40°C overnight. The ACR-1478 copolymer of methacrylic acid : N-phenylmaleimide : methacrylamide (20.9 : 41.6 : 37.4 mol%) exhibited an acid number of about 96. PCB Example: The coating formulation M was prepared as a solution in 1- methoxypropan-2-ol / DMF (75% : 25%, v:v). The substrate used was copper substrate A. The coating solution was coated onto one side of the copper substrate by means of a wire wound bar. The solution concentration was selected to provide the specified dry film composition with a coating weight of 4 g/m 2 after thorough drying at 130°C for 150 seconds. Coating Formulation M A sample of the printed circuit board precursor was imaged on the Trendsetter 3244, using the internal test pattern, plot 0 at 400 mJ/cm 2 . The exposed precursor was then processed by immersion in developer B for 30 seconds at 20°C. At this imaging and developing condition, the 50% dots laid down by the imagesetter were complete 50% dots on the substrate. The precursor was then etched (uncovered copper removed) by immersion in the etching solution A, diluted with water (50% w:w) which was constantly being stirred, for between 5 and 10 minutes at 40°C. The precursor was then rinsed with water. Finally, the precursor was cleaned by immersion in Stripper A diluted with water (20% w:w) (to remove the remaining coating) at 20°C for 60 seconds. The printed circuit board sample had a copper pattern remaining that was an accurate copy of the precursor above after the development stages. The present invention has been described with particular reference to the preferred embodiments. It should be understood that variations and modifications thereof can be devised by those skilled in the art without departing from the spirit and scope of the present invention. Accordingly, the present invention embraces all such alternatives, modifications and variations that fall within the scope of the appended claims.

Claims: WHAT IS CLAIMED IS: 1. An imageable element comprising: a substrate; and a thermally imageable composition coated on a surface of said substrate; wherein said thermally imageable composition comprises a thermally sensitive polymer which exhibits an increased solubility in an aqueous developer solution upon heating, said thermally sensitive polymer comprising: at least one covalently bonded unit; and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each said non-covalently bonded unit. 2. The imageable element of claim 1 , wherein said two or more centered H-bond is a four centered H-bond. 3. The imageable element of claim 1 , wherein said four- centered H-bond comprises two isocytosine groups. 4. The imageable element of claim 1 , wherein said covalently bonded unit is derived from a poiyfunctional material that is soluble or dispersible in an aqueous developer solution. 5. The imageable element of claim 1 , wherein said covalently bonded unit is derived from a poiyfunctional material selected from the group consisting of: poiyfunctional phenolic resin, acrylic resin, polyester resin, polyurethane resin, and a combination thereof. 6. The imageable element of claim 1 , wherein said poiyfunctional phenolic resin is selected from the group consisting of: phenol/cresol novolak, polyvinyl phenol polymer, vinyl phenol/hydrocarbyl acrylate copolymer, and a mixture thereof. 7. The imageable element of claim 1 , wherein said thermally sensitive polymer comprises a supramolecular polymer derived from monomers represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; wherein Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 ; wherein R 3 is a phenolic residue derived from a poiyfunctional phenolic resin represented by the formula R 3 (OH) n ; and wherein n is at least 1. 8. The imageable element of claim 7, wherein said poiyfunctional phenolic resin is selected from the group consisting of: phenol/cresol novolak, polyvinyl phenol polymer, vinyl phenol/hydrocarbyl acrylate copolymer, and a mixture thereof. 9. The imageable element of claim 7, wherein said diisocyanate is selected from the group consisting of: isophorone diisocyanate, methylene-bis-phenyl diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, tetramethylxylylene diisocyanate, dimers thereof, adducts thereof with diols, and mixtures thereof. 10. The imageable element of claim 7, wherein n is 1 , 2 or 3. 11. The imageable element of claim 1 , wherein said thermally reversible non-covalently bonded unit includes a two-centered H-bond, which comprises two 2-pyridone groups. 12. The imageable element of claim 11 , wherein said thermally sensitive polymer comprises a supramolecular polymer derived from monomers represented by the formula: wherein each R' and R" is independently selected from the group consisting of: H, linear, branched or cyclic alkyl, aryl, aralkyl, alkaryl, substituted aryl, alkenyl, halogen, cyano, nitro, alkoxy, aryloxy, alkoxycarbonyl, amido, acyl, aminocarbonyl, carboxylic, sulfonic, imide, N- acyl sulfonamide and phenolic hydroxy with the proviso that at least one of said R' and R" groups is selected from the group consisting of: carboxylic, sulfonic, imide, N-acyl sulfonamide and phenolic hydroxy; and wherein A is a bridging group selected from alkylene, arylene, aralkylene, alkarylene, substituted arylene, -O-, -S-, NR'", -CH=CH- and -C≡C-. 13. The imageable element of claim 1 , wherein said thermally reversible non-covalently bonded unit includes a three-centered H-bond, which comprises a cyclic imide group and a 2,6-diaminotriazine group. 14. The imageable element of claim 1 , wherein said thermally sensitive polymer comprises a maleimide/styrene copolymer associated with a 4-vinyl-2,6-diaminotriazine/ styrene copolymer, with the proviso that said copolymers comprise at least one base-soluble functional group selected from the group consisting of: carboxylic, sulfonic, imide, N-acyl sulfonamide and phenolic hydroxy. 15. The imageable element of claim 1 , further comprising a photothermal converter material. 16. The imageable element of claim 1 , further comprising between said substrate and said thermally imageable composition a bottom layer that is soluble or dispersible in a developer. 17. A thermally imageable composition comprising: a thermally sensitive polymer which exhibits an increased solubility in an aqueous developer solution upon heating, said thermally sensitive polymer comprising: at least one covalently bonded unit; and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each said non-covalently bonded unit. 18. The thermally imageable composition of claim 17, wherein said thermally sensitive polymer comprises a supramolecular polymer derived from monomers represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; wherein Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 ; wherein R 3 is a phenolic residue derived from a poiyfunctional phenolic resin represented by the formula R 3 (OH) n ; and wherein n is at least 1. 19. A method of producing an imaged element comprising the steps of: providing an imageable element comprising: a substrate; and a thermally imageable composition coated on a surface of said substrate; said thermally imageable composition comprising: a thermally sensitive polymer which exhibits an increased solubility in an aqueous developer solution upon heating, said thermally sensitive polymer comprising: at least one covalently bonded unit; and at least one thermally reversible non- covalently bonded unit, which includes a two or more centered H-bond within each said non-covalently bonded unit; exposing said imageable element to thermal radiation to produce imagewise exposed regions; and contacting said exposed imageable element and a developer within a period of time after said exposing step to remove said exposed regions and thereby produce said imaged element. 20. The method of claim 19, wherein said thermally imageable composition comprises a photothermal converting material. 21. The method of claim 19, wherein said step of exposing said imageable element to thermal radiation is carried out using an infrared laser. 22. The method of claim 19, wherein said period of time is up to 1 hour. 23. The method of claim 19, wherein said thermal radiation has a pixel dwell time of not more than 100 μs. 24. A process for preparing a supramolecular polymer, said process comprising: contacting a hydrocarbyl-substituted isocytosine and a diisocyanate to produce an isocytosine/isocyanate mono-adduct; and contacting said isocytosine/isocyanate mono-adduct and a poiyfunctional material selected from the group consisting of: poiyfunctional phenolic resin, acrylic resin, polyester resin, polyurethane resin, and a combination thereof, at a temperature and for a period of time sufficient to produce said supramolecular polymer. 25. The process of claim 24, wherein said hydrocarbyl is an alkyl of 1 to 22 carbon atoms. 26. The process of claim 24, wherein said diisocyanate is selected from the group consisting of: isophorone diisocyanate, methylene- bis-phenyl diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, tetramethylxylylene diisocyanate, dimers thereof, adducts thereof with diols, and mixtures thereof. 27. The process of claim 24, wherein said poiyfunctional phenolic resin is selected from the group consisting of: phenol/cresol novolak, polyvinyl phenol polymer, vinyl phenol/hydrocarbyl acrylate copolymer, and a mixture thereof. 28. A supramolecular polymer derived from monomers represented by the formula: wherein each R 1 and R 2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; wherein Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO) 2 ; wherein R 3 is a phenolic residue derived from a poiyfunctional phenolic resin represented by the formula R 3 (OH) n ; and wherein n is at least 1.

Inventors: HUANG JIANBING, MONK ALAN, PAPPAS S PETER, SARAIYA SHASHIKANT, HUANG, JIANBING, MONK, ALAN, PAPPAS, S., PETER, SARAIYA, SHASHIKANT
Applicants: KODAK POLYCHROME GRAPHICS L L, KODAK POLYCHROME GRAPHICS, L.L.C.
Application year: 2001
Main classification: C08G85
IPC codes: B41C1, B41M5, C08G18, C08G73, G03F7