GB1599281A

GB1599281A - Photopolymerisable composition

Info

Publication number: GB1599281A
Application number: GB723/77A
Authority: GB
Original assignee: IMI PRINTING PROD; Imperial Chemical Industries Ltd
Current assignee: IMI PRINTING PROD; Imperial Chemical Industries Ltd
Priority date: 1977-01-10
Filing date: 1977-01-10
Publication date: 1981-09-30
Also published as: DE2800754A1; BE862674A; IT7819094A0; LU78838A1; NL7800286A; JPS53104685A; DK10978A; FR2377052A1

Description

(54) A PHOTOPOLYMERISABLE COMPOSITION (71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company, and IMI PRINTING PRODUCTS LIMITED, Kynoch Works, Witton, Birmingham B6 7BA, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to photopolymerisable compositions and to methods of production and use thereof.

Photopolymerisable compositions are known comprising a soluble polymeric material together with an addition-polymerisable monomer. Such a composition may be employed for example to produce a relief printing plate by exposing a sheet of the composition to radiation, particularly light through a negative, so that further polymerisation occurs in those parts of the composition which hive been exposed to the radiation and subsequently removing the unexposed areas with a suitable solvent down to,the desired depth of relief.

Belgian Patent No. 820,404 describes a- photopolymerisable composition which can be developed (that is, unexposed areas removed) by treatment with water or an aqueous liquid, which composition comprises essentially: (a) a water-soluble or water-dispersible polymer or copolymer, or a polymer or copolymer mixture in which one or more of the components is water-soluble or water-dispersible; (b) an addition-polymerisable monomer which is preferably, though not essentially, water-insoluble but is soluble in or compatible with a solvent or dispersing medium for (a); and (c) a photoinitiator capable of initiating polymerisation of the monomer whereby the composition may be rendered water-insoluble by suitable radiation.

Belgian Patent No. 840,110 describes a photopolymerisable composition comprising (a) a water-soluble or water-dispersible polymer or copolymer, or a polymer or copolymer mixture in which one or more of the components is water soluble or water-dispersible, (b) an addition-polymerisable monomer which is soluble in or compatible with a solvent or dispersing medium for (a); and (c) at least one photo-sensitiser having the structure

where U is C=O, > CG, G2 or > CG3OG4, G, to G4, which may be the same or different are hydrogen or hydrocarbyl groups, n is 0 or 1 and the groups A, which may be the same or different are hydrocarbyl or substituted hydrocarbyl groups, the groups A being aromatic or substituted aromatic when n is 1 and U is > CG1Gz and when n is 0 both groups A are aromatic, polymeric component (a) being capable of reducing photosensitiser (c) when said photosensitiser is in an excited state to effect addition polymerisation of at least a proportion of component (b) of the composition.

Polymeric component (a) preferably comprises a reducing component having the structure

where M is an element of Group Vb of the Periodic Table of the Elements, at least one of the units D having a carbon atom which forms part of the polymer chain of the polymeric component (a) and at least one other of units D, being hydrogen, hydrocarbyl, substituted hydrocarbyl, or a group in which two units D together with the element M form a ring system, and where, when the element M is attached directly to an aromatic group, at least one of the other groups D has a

attached to M.

Preferably polymeric component (a) comprises a poly(N-vinyl pyrrolidone) and, optionally, a vinyl acetate component.

We have found that it is advantageous to include in the compositions described in the aforesaid patent specifications a proportion of an addition polymerisable vinyl-ended prepolymer having the structure: [CH2=C(R3)X NH(R7NHYR2YNH)mR1NHYlnB (I) (hereinafter referred to as "vinyl-ended prepolymer"), in which B is an organic group preferably a divalent organic group; Y is -CO-O-, -CONH- or -CO; X is a divalent organic group; R, and R2 are polyvalent organic groups, preferably divalent organic groups; R3 is hydrogen or a hydrocarbyl group, preferably an alkyl group containing one or two carbon atoms; n is an integer preferably 2, 3, 4, 5 or 6, and more preferably 2; m is 0 or an integer and need not necessarily be the same in each chain attached to B; and when n=l the prepolymer has the structure CH2=C(R3)X NH R1NHX(R3)C=CH2.

By divalent we mean that a group is bonded to two in-chain atoms of the prepolymer.

While Y may be the same or different throughout the prepolymer, preferably it is the same throughout the prepolymer and more preferably it is -CO-O-, i.e.

preparation of the vinyl-ended prepolymer comprises reacting a plurality of hydroxyl groups with a plurality of isocyanate groups, although we do not exclude the possibility that the preparation of the vinyl-ended prepolymer may comprise reacting carboxyl groups or amino groups with isocyanate groups.

Where n is more than 2, m is preferably 0. Where n is 3, B is, for example, glycerol, trimethyl propane or 1,2,4-butanetriol or an oxyalkylated e.g.

oxypropylated, derivative of these. Where n is 4, B is for example, pentaerythritol or an oxyalkylated, e.g. oxypropylated, derivative thereof. Where n is 6, B is, for example, sorbitol or mannitol or an oxyalkylated, e.g. oxypropylated, derivative of these. Choice of an appropriate polyhydroxy compound for any one particular application will present no problem to the skilled man and may readily be determined by simple experiment.

Preferably, however, for reasons of convenience and in particular for reasons of ease of preparation n is 2, i.e. where R, and R2 are divalent groups, the vinylended prepolymer is linear, and more preferably R2 and B are the same.

In order that a printing plate prepared by curing a composition according to the invention should possess good mechanical properties it is preferred that at least one of R1, R2 and B possess at least one in-chain cyclic group, preferably an aromatic group, and it is particularly preferred that the in-chain cyclic group is R2 and/or B.

Where a printing plate having particularly good mechanical properties is desired it is preferred that in the structure I, B, R1, R2, R3 X and n have the preferred designations previously ascribed to them, i.e. B and R2 are the same and are divalent and at least R1 or R2, preferably R2, possesses at least one in-chain cyclic group, and that m is less than 10 and that there are not more than 30 atoms in the chain between adjacent in-chain cyclic groups. Where a printing plate of higher strength is desired, preferably there are not more than 20 and more preferably not more than 12 atoms in the chain between adjacent in-chain cyclic groups, and where m is one and only R2 contains at least one in-chain cyclic group or where m is zero and only B contains at least one in-chain cyclic group there are not more than 35, preferably not more than 25, and more preferably not more than 15 atoms in the chain between the group

and the nearest in-chain cyclic group.

For reasons of ease of preparation of the vinyl-ended prepolymer the value of m preferably is not greater than 5 and more preferably not greater than 2, i.e. where R1, R2 and B are divalent, the molar ratio of isocyanate groups in the diisocyanate or mixture thereof to hydroxyl groups in the diol or mixture thereof from which the vinyl-ended prepolymer is produced is preferably 1.1:1 or greater, and more preferably 1.2:1 or greater, and more preferably still, m is not greater than one, i.e.

the molar ratio of isocyanate groups in the diisocyanate or mixture thereof to hydrocarbyl groups in the diol or mixture thereof from which the vinyl-ended prepolymer is produced is suitably 1.33:1 or greater.

Examples of diols containing cyclic groups which may be used in the preparation of the vinyl-ended prepolymer are cycloalkane diols, e.g. 1:3- and 1:4cyclohexane diol and a diol having the structure

in which case the group R2 and/or B in the prepolymer has the structure

respectively. Other suitable diols include, for example, cycloalkane dialkanols, e.g.

cyclohexane dimethanol or cyclohexane diethanol; polycycloalkane diols, polycycloalkane dialkanols, aryl dialkanols and oxyalkylated derivatives of aromatic compounds containing two phenolic groups.

Particularly preferred diols, on account of the desirable properties of the printing plates which may be produced from compositions comprising them are diols of the structure

that is, oxyalkylated derivatives of phenolic compounds, where R4 and R5 are hydrogen atoms or alkyl groups, e.g. methyl, and Ar is a divalent aromatic group in which each free valency is on an aromatic carbon atom, and in which a+b together preferably total not more than 8 and a is preferably not greater than b+3.

In this case the divalent group R2 has the structure

Ar may be mononuclear, e.g. as in phenylene, fused polynuclear as in naphthalene or anthracene, or preferably has the structure

in which Z is a divalent link, e.g. -0-, -SO2-, -CO- or -CH2- or substituted derivative of -CH2-, e.g.

Suitably, one of the groups R4 and R5 on the diol is hydrogen and the other methyl, or both R4 and R5 are hydrogen, that is, the diol may be prepared by reaction of propylene oxide or ethylene oxide with a phenolic compound having the structure H(#Ar-OH, preferably

Preferably a plus b is not greater than 4.

Examples of diols which do not have cyclic groups in the chain include, for example, ethylene glycol and propylene glycol in which case R2 has the structure H2-CH2- or

butylene glycol, diethylene glycol and derivatives thereof in which one or more of the carbon atoms are substituted by atoms or groups which are unreactive towards hydroxyl and isocyanategrous.

It will be appreciated by the skilled man that the diols or polyols hereinbefore described may be replaced by corresponding diamines or polyamines where applicable.

Diisocyanates, within which term we include mixtures having an average functionality of between 2 and 3, containing cyclic groups which may be used to prepare the vinyl-ended prepolymer include, for example, diisocyanates in which the chain between the free valencies is provided with at least one aromatic group or at least one cycloaliphatic group, or in which the chain between the free valencies includes in combination at least one aromatic and at least one cycloaliphatic group.

Cycloaliphatic diisocyanates which may be used include, for example, 3 isocyanatomethylene - 3,5,5 - trimethylhexylisocyanate and diisocyanates of the structure:

where -Z- is a divalent link which may be, for example, -CH2- or substituted derivative thereof, O,SO2, -CO-, and the isocyanate groups are linked meta or para to the group Z. A particular example is 4:4'-dicyclohexylmethane diisocyanate.

Aromatic diisocyanates which may be used include, for example, 2:4- or 2:6tolylene diisocyanates, or mixtures thereof, in which case the divalent group R, has the structure

or combination of said structures.

Another suitable aromatic diisocyanate is that having the structure

where p is a whole number chosen such that there are preferably not more than 30 atoms between cyclic groups in the vinyl-ended prepolymer derived therefrom. A suitable-diisocyanate having the latter structure is xylylene diisocyanate.

A particularly suitable aromatic diisocyanate is that having the structure:

where Z is a divalent link which may have the designations hereinbefore described and in which the isocyanate groups are linked meta or para to the Group Z. A preferred example is 4:4'-diisocyanatodiphenyl methane.

Diisocyanates which do not contain cyclic groups may be used in the production of the urethane prepolymer.

Suitably such diisocyanates include, for example, tetramethylene diisocyanate, pentamethylene diisocyanate and hexamethylene diisocyanate, in which case the divalent group R1 will have the structure CH2)4-, -(CH2)5 or CH2)6-.

The vinyl end-group of the vinyl-ended prepolymer may be obtained by reacting an ethylenically unsaturated compound or mixture thereof which has an iso cyan ate group or isocyanate-reactive group with, respectively, an isocyanatereactive group or isocyanate group. Preferably the ethylenically unsaturated compound or mixture thereof has an isocyanate-reactive group.

Where the ethylenically unsaturated compound or mixture thereof has an isocyanate-reactive group it may be, for example acrylic acid or a derivative thereof having the structure

where R3 is hydrogen or a hydrocarbyl group, preferably an alkyl group containing one, two or three carbon atoms, e.g. methyl. In this case the group -X- in the vinyl-ended prepolymer I has the structure

after the elimination of carbon dioxide from the initially formed product of reaction.

Preferably the ethylenically unsaturated compounds are hydroxyl containing compounds having, for example, the structure:

where R3 is hydrogen or a hydrocarbyl group, preferably an alkyl group containing one, two or three carbon atoms, e.g. methyl, and q is a whole number of at least 2, and is preferably 2, or derivatives of said compound in which one or more hydrogen atoms in the group --(CH,), are substituted by a hydrocarbyl group, for example, alkyl, e.g. methyl, although we do not exclude the possibility that the ethylenically unsaturated compound may be an amino containing compound or mixture thereof. Where the ethylenically unsaturated compound has the structure II, the group X in the vinyl-ended prepolymer I has the structure -COOCH2),-O-CO- Typical examples of ethylenically unsaturated compounds having the structure II, include hydroxyethyl or hydroxypropyl acrylate or methacrylate made by reaction of acrylic acid or methacrylic acid with ethylene oxide or propylene oxide, in which case, where the ratio of acid to oxide is 1:1 the group X in the vinyl-ended prepolymer I will have the structure

in which, respectively, both R, and R7 are hydrogen and one of R6 and R7 ishydrogen and the other is methyl. Where the ratio of acid to oxide is less than 1:1, the group X in the vinyl ended prepolymer will have the structure

where r is preferably less than 10.

Other suitable isocyanate-reactive ethylenically unsaturated compounds include allyl alcohol, in which case the group X in the vinyl-ended prepolymer has the structure -CH2-O-CO-, and acrylamide and methacrylamide, in which case the group X in the vinyl-ended prepolymer has the structure -CONH-C.

Where R1, R2 and B are divalent preferably an isocyanate-ended prepolymer is formed initially which is then reacted with an ethylenically unsaturated compound containing an isocyanate-reactive group, preferably a hydroxyl group, to produce the vinyl-ended prepolymer. In order that the isocyanate-ended prepolymer may have isocyanate end-groups it will be appreciated that a molar excess of the diisocyanate over the diol or mixture thereof must be used in the preparation of the isocyanate-ended prepolymer, the value of m in the prepolymer being dependent upon the molar proportion diisocyanate to diol used and decreasing as this ratio increases.

Where the valency of B is greater than 2 and m is 0, preferably the vinyl-ended prepolymer is formed by reacting an ethylenically unsaturated compound containing an isocyanate reactive group, preferably a hydroxyl group, with a diisocyanate to form a prepolymer having an isocyanate group at one end which is then reacted with a polyol.

The prepolymers having at least one isocyanate end-group i.e. the isocyanate ended prepolymers or the prepolymers having an isocyanate group at one end and an ethylenically unsaturated group at the other end, may be viscous and, especially where m is a large number, they may be solid, and it is desirable in these circumstances that preparation of the prepolymers containing at least one isocyanate end-group be effected in the presence of an inert diluent, which is preferably a solvent for at least one of the reactants and/or for the product.

Similarly, where the prepolymer containing at least one isocyanate end-group is very viscous or solid, reaction of the prepolymer containing at least one isocyanate end-group with the ethylenically unsaturated compound reactive therewith or with the polyol to form the vinyl-ended prepolymer may desirably be effected in the presence of an inert diluent, which is preferably a solvent for at least one of the reactants and/or for the product. The diluent should be substantially free of groups which are reactive with isocyanate groups, at least to such an extent that the diluent does not interfere with the formation of the prepolymer. The diluent may be, for example, the liquid ethylenically unsaturated comonomer which is copolymerisable with the vinyl-ended prepolymer.

Where the vinyl-ended prepolymer is prepared in an inert diluent, the vinylended prepolymer may be separated from the diluent, e.g. by evaporation of the diluent or by addition to the diluent of a non-solvent for the vinyl-ended prepolymer.

We, have found that the presence in the compositions described in the aforementioned Belgian patents of a proportion of a vinyl-ended prepolymer as hereinbefore described is advantageous in that photopolymerised products derived from compositions comprising the vinyl-ended prepolymer usually have better mechanical properties than those prepared without the use of the prepolymer.

The structure of the vinyl-ended prepolymer and the proportion of the vinylended prepolymer which may be employed according to this invention will be influenced by the hardness or flexibility required in the photopolymerised product.

For example, where a rigid product is to be obtained preferably both R, and R2 contain in-chain cyclic groups and the proportion of vinyl-ended prepolymer in the polymerisable composition may be up to 25% by weight of the composition, or even more, but if a flexible product, for example a printing plate, is required a lower concentration, up to 15%, e.g. 5% to 15%, conveniently about 10%, by weight, or a vinyl-ended prepolymer having a smaller proportion of in-chain cyclic groups, may be preferred. The optimum concentration of any particular vinyl-ended prepolymer may be determined by simple trial, since it may vary with the particular vinyl-ended polymer employed.

We prefer to employ a vinyl-ended prepolymer which is solid at ambient temperature since the resulting polymerised product even if all the vinyl-ended prepolymer is not incorporated in the polymer, will not contain a liquid unpolymerised prepolymer which may adversely affect is mechanical properties.

The vinyl-ended prepolymer should be compatible with the other components of the composition and with any solvent employed particularly so that phase separation does not occur.

Although the vinyl-ended prepolymer, or part of it, will usually be incorporated, or be present in the composition in the form of a solution in an appropriate solvent we do not exclude the possibility that it, or part of it, may be incorporated in the form of a solid usually particulate and very finely divided so that it may be easily dispersed.

The invention provides photopolymerisable compositions which may be developed (that is, unexposed areas removed) by treatment with water or an aqueous liquid, and the compositions may be employed for example in the preparation of coated sheet materials, carrying a layer of the composition. Such sheet materials may be employed for example as printing plates. Other uses for the compositions include photo-resist applications.

According to another aspect of the invention therefore we provide a composite product comprising a carrier having deposited thereon a photopolymerisable composition as hereinbefore described. Preferably the carrier will be a sheet and a layer of the composition will be deposited upon at least one surface thereof. A particular application of such a sheet product is in the production of relief plates which are capable, after suitable processing, of being used both for direct or offset letterpress printing, flexo-printing or as pattern plates as described fully in Belgian Patent No. 820,404, and accordingly another aspect of the invention provides a method of preparing relief plates by selectively exposing a composition according to the invention to appropriate radiation, for example through a negative, and developing the exposed composition (conveniently in the form of a sheet upon a carrier).

The invention is illustrated by the following examples:- Example 1 A printing pattern plate was prepared as follows:- A coating solution was prepared by mixing together 167 parts of a vinyl pyrrolidone:vinyl acetate:dimethylamino ethyl-methacrylate copolymer (weight ratio 50:50:2) in industrial methylated spirit (60% w/w solution), 20 parts ofa a vinyl- ended prepolymer, obtained as described in Example 3, in 20 parts of methyl ethyl ketone, 20 parts of ethylene glycol dimethacrylate 10 parts of diisobutyl phthalate and 1 part of fluorenone (photosensitiser).

This mixture was coated onto a steel plate and dried at 850C to give a solid coating 0.76 mm thick. The coating was exposed through a line and half tone negative for 70 seconds using a mercury vapour Uv source and processed by spraying with water at 250C for 3,5 minutes.

After drying at 1250C for 4 minutes the relief image was very hard and water resistant and could be used as a newspaper pattern plate.

A replica of the plate was made by pressing the plate in contact with a newspaper flong containing 15% moisture for 20 seconds at 100 psi gauge without the flong adhering to the plate.

Example 2 A printing plate was prepared as follows:- A coating solution was prepared as in Example 1, except that only 10 parts of the vinyl-ended prepolymer in 10 parts of methyl ethyl ketone were employed together with 20 parts of diisobutyl phthalate.

This composition was coated onto a steel plate and dried at 850C to give a solid layer 0.5 mm thick.

The plate was exposed through a line and half tone negative for 75 seconds using a mercury vapour source and then processed with a water spray at 250. After drying the plate could be flexed sufficiently to be fitted to a newspaper rotary letterpress printing cylinder, and gave multiple prints of good quality.

Example 3 704 parts of the condensate obtained by reacting 2,2 bis(p-hydroxy-phenyl) propane with propylene oxide in molar ratio 1:2, in 200 parts methylene chloride were reacted with 1000 parts of 4,4'-diisocyanatodiphenyl methane in 1200 parts methylene chloride containing dibutyl tin laurate (3 parts). After 2 hours 2hydroxyethyl methacrylate (600 parts), containing dibutyl tin laurate (3 parts), was added and the reaction continued for 66 hours. The solid vinyl-ended prepolymer was then isolated from the methylene chloride solution.

WHAT WE CLAIM IS: 1. A photopolymerisable composition comprising (a) a water-soluble or water-dispersible polymer or copolymer or a polymer or

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

appropriate solvent we do not exclude the possibility that it, or part of it, may be incorporated in the form of a solid usually particulate and very finely divided so that it may be easily dispersed.

The invention provides photopolymerisable compositions which may be developed (that is, unexposed areas removed) by treatment with water or an aqueous liquid, and the compositions may be employed for example in the preparation of coated sheet materials, carrying a layer of the composition. Such sheet materials may be employed for example as printing plates. Other uses for the compositions include photo-resist applications.

According to another aspect of the invention therefore we provide a composite product comprising a carrier having deposited thereon a photopolymerisable composition as hereinbefore described. Preferably the carrier will be a sheet and a layer of the composition will be deposited upon at least one surface thereof. A particular application of such a sheet product is in the production of relief plates which are capable, after suitable processing, of being used both for direct or offset letterpress printing, flexo-printing or as pattern plates as described fully in Belgian Patent No. 820,404, and accordingly another aspect of the invention provides a method of preparing relief plates by selectively exposing a composition according to the invention to appropriate radiation, for example through a negative, and developing the exposed composition (conveniently in the form of a sheet upon a carrier).

The invention is illustrated by the following examples:- Example 1 A printing pattern plate was prepared as follows:- A coating solution was prepared by mixing together 167 parts of a vinyl pyrrolidone:vinyl acetate:dimethylamino ethyl-methacrylate copolymer (weight ratio 50:50:2) in industrial methylated spirit (60% w/w solution), 20 parts ofa a vinyl- ended prepolymer, obtained as described in Example 3, in 20 parts of methyl ethyl ketone, 20 parts of ethylene glycol dimethacrylate 10 parts of diisobutyl phthalate and 1 part of fluorenone (photosensitiser).

This mixture was coated onto a steel plate and dried at 850C to give a solid coating 0.76 mm thick. The coating was exposed through a line and half tone negative for 70 seconds using a mercury vapour Uv source and processed by spraying with water at 250C for 3,5 minutes.

After drying at 1250C for 4 minutes the relief image was very hard and water resistant and could be used as a newspaper pattern plate.

A replica of the plate was made by pressing the plate in contact with a newspaper flong containing 15% moisture for 20 seconds at 100 psi gauge without the flong adhering to the plate.

Example
2 A printing plate was prepared as follows:- A coating solution was prepared as in Example 1, except that only 10 parts of the vinyl-ended prepolymer in 10 parts of methyl ethyl ketone were employed together with 20 parts of diisobutyl phthalate.

This composition was coated onto a steel plate and dried at 850C to give a solid layer 0.5 mm thick.

The plate was exposed through a line and half tone negative for 75 seconds using a mercury vapour source and then processed with a water spray at 250. After drying the plate could be flexed sufficiently to be fitted to a newspaper rotary letterpress printing cylinder, and gave multiple prints of good quality.

Example 3

704 parts of the condensate obtained by reacting 2,2 bis(p-hydroxy-phenyl) propane with propylene oxide in molar ratio 1:2, in 200 parts methylene chloride were reacted with 1000 parts of 4,4'-diisocyanatodiphenyl methane in 1200 parts methylene chloride containing dibutyl tin laurate (3 parts). After 2 hours 2hydroxyethyl methacrylate (600 parts), containing dibutyl tin laurate (3 parts), was added and the reaction continued for 66 hours. The solid vinyl-ended prepolymer was then isolated from the methylene chloride solution.

WHAT WE CLAIM IS: 1. A photopolymerisable composition comprising (a) a water-soluble or water-dispersible polymer or copolymer or a polymer or

copolymer mixture in which one or more of the components is water-soluble or water-dispersible; (b) an addition polymerisable monomer; (c) a photoinitiator capable of initiating polymerisation of the monomer whereby the composition may be rendered water-insoluble by suitable radiation; and (d) an addition polymerisable vinyl-ended prepolymer having the structure [CH2=C(R3)X NH (R1NHYR2YNH)rnR1NHYl B in which B is an organic group; Y is -CO-O-, -CONH- or CO; X is a divalent organic group; R1 and R2 are polyvalent organic groups; R3 is hydrogen or a hydrocarbyl group; n is an integer; m is 0 or an integer; and when n=l, the prepolymer has the structure CH2=C(R3)XNHRlNHX(R3)C=CH2 2. A composition according to claim 1, in which component (c) comprises a photosensitiser having the structure

in which A, U, G1 to G4 are as hereinbefore defined and n is 0 or 1, component (a) being capable of reducing photosensitiser (c) when said photosensitiser is in an excited state to effect addition polymerisation of at least a proportion of component (b) of the composition.
3. A composition according to claim 1 or 2 in which component (a) comprises units of an N-vinyl pyrrolidone.
4. A composition according to any one of the preceding claims in which component a) comprises units of vinyl acetate.
5. A composition according to any one of the preceding claims in which B is a divalent group.
6. A composition according to any one of the preceding claims in which the vinyl-ended prepolymer is obtained by reaction of a plurality of appropriately reactive groups with a plurality of isocyanate groups to give a prepolymer in which Y is -CO-.
7. A composition according to claim 6 in which the vinyl-ended prepolymer is prepared by reaction of a plurality of hydroxyl groups with a plurality of isocyanate groups.
8. A composition according to any one of the preceding claims comprising a vinyl-ended prepolymer in which at least one of R1, R2 and B possess at least one in-chain cyclic group.
9. A composition according to any one of the preceding claims in which the vinyl-ended prepolymer is present in an amount forming from 10% to 25% by weight of the composition.
10. A composition according to claim 9 in which the vinyl-ended prepolymer is present in an amount forming up to 15% by weight of the composition.
11. A composition according to any one of the preceding claims in which the vinyl-ended prepolymer is solid at ambient temperature.
12. A composite product comprising a composition according to any one of the preceding claims and a carrier.
13. A product according to claim 12 in the form of a sheet.
14. A method of preparing a relief printing plate which comprises the steps of exposing a plate comprising the composition of any one of claims 1 to 11 to polymerising radiation whereby at least a part of the composition is polymerised, and working the exposed plate with an aqueous liquid to remove unpolymerised composition.
15. A relief printing plate obtained by the method of claim 14.
16. A composition according to claim I and substantially as hereinbefore described.
17. A relief printing plate substantially as hereinbefore described.