GB2085012A - Radio-opaque material - Google Patents

Abstract

A homo- or copolymer of a (meth)acrylate ester into which atoms capable of absorbing x-radiation are incorporated by covalent bonding, the resulting polymer having a viscosity average molecular weight greater than 400,000.

Description

SPECIFICATION Radio-opaque material This invention relates to radio-opaque plastics material; more particularly, this invention relates to radio-opaque plastics material suited, inter alia, to use in the fabrication of denture base, to processes for preparing such materials, to methods of fabricating denture base therefrom, to dentures comprising such materials; and to cements for endoprosthesis comprising such materials.

The plastics material generally used in the fabrication of denture base is poly(methyl methacrylate). Poly(methyl methacrylate) is a highly satisfactory material in most respects for use as denture base: thus, it has adequate strength for most applications, high translucency (and hence aesthetic acceptability), and can readily be fabricated by the dental technician in simple low-cost moulds to give dimensionally accurate, well-fitting dentures. However, a disadvantage of poly(methyl methacrylate) is that it is radiolucent; and this has serious implications in matters of personal safety for denture wearers involved in a traumatic incident wherein they ingest all or part of their denture. Thus, the patient is often unaware that this has happened and when symptoms occur later (for example, sepsis, vomiting, coughing fits) X-ray diagnosis will reveal nothing.

The need for a radio-opaque plastics material suited to use in the fabrication of denture base has been known for some time. Methods to make denture base opaque have included the incorporation of metals in the form of wire, mesh, foil and powder, radio-opaque glasses, radio-opaque oxides and salts, and organoiodine compounds. However, none of these methods has led to an acceptable product because they either reduce the strength or alter the handling and working properties of the plastics material, or make it toxic, or less translucent.

This invention seeks to provide a radio-opaque plastics material in which the aforementioned disadvantages are reduced or overcome.

According, therefore, to one aspect of the present invention there is provided a homo- or copolymer of a (meth)acrylate ester in to which atoms, especially halogen atoms, capable of absorbing X-radiation are incorporated by covalent bonding, the resulting polymer having a viscosity average molecular weight greater than 400,000. By "capable of absorbing X-radiation" is meant herein that the resulting polymer is radio-opaque.

Halogen atoms capable of absorbing X-radiation are chlorine, bromine and iodine atoms. Chlorosubstituted polymers of the invention are found to be less effective than the corresponding bromo- ones while the iodo-substituted polymers tend to be too labile, eliminating iodine; accordingly, bromosubstituted polymers are preferred. A mixture of halo-substituents, such as both chloro- and bromosubstituents, may be present in the polymers of the invention.

In order to ensure that the polymers of this invention are capable of absorbing X-radiation, as herein defined, it is preferred that they comprise at least 5% by weight, preferably at least 10% by weight, of halogen. In practice, it is found 1 5% to 20% by weight of halogen gives very satisfactory results, though up to 30%, or even up to 55% by weight, of halogen may be used.

According to a preferred embodiment of this invention, there is provided a homo- or copolymer of a substituted or unsubstituted alkenyl (meth)acrylate ester, preferably an alkenyl methacrylate ester, into which atoms, preferably halogen atoms, especially chlorine or bromine atoms, capable of absorbing X-radiation are incorporated by covalent bonding. Preferably, the alkenyl group is an unsubstituted alkenyi group of the formula: (CH2)nCH=CHR wherein: R represents a hydrogen atom or a lower alkyl group, preferably a hydrogen atom; and n represents an integer from 1 to 3, preferably 1. A particularly preferred such group is an unsubstituted allyl group.

The homo- or copolymer of this invention preferably comprises the (hydro)halogenation, preferably the (hydro)bromination, product of the substituted or unsubstituted alkenyl ester; for example a monoor vic di-haloalkyl, preferably mono- or 2,3 dihalopropyl, especially mono- or 2,3 di-bromopropyl, (meth)acrylate.

The polymer of this invention may be a homopolymer, such as poiy(mono- or vic di-haloalkyl (meth)acrylate, preferably poly(mono- or 2,3 di-halopropyl(meth)acrylate), especially poly(mono- or 2,3 di-bromopropyl(meth)acrylate). Alternatively, it may be a random, block or graft, preferably random or block, copolymer with one or more further (meth)acrylate esters, other vinyl monomers, or polymers, for example elastomers, polysaccharides or homo- or copoly[(meth)acrylate esters. The further (meth)acrylate esters may comprise one or more further substituted or unsubstituted alkenyl (meth)acrylate esters as hereinbefore defined or one or more substituted or unsubstituted alkyl, suitably C, to C4 alkyl, especially methyl or ethyl, (meth)acrylates.

In accordance with another aspect of this invention there is provided a radio-opaque polymer as aforesaid which comprises a toughening dispersed elastomeric phase. Typically, the elastomer comprises a polymerised unsubstituted or halo-substituted diene; suitably the elastomer may be grafted to the radio-opaque moiety and, where the elastomer is halo-substituted, will enhance the radioopacity. Suitable elastomers comprise polybutadienes such as natural rubber, butadiene/styrene rubbers such as styrene/butadiene block copolymers which may comprise less than 40% by weight of polymerised styrene, butadiene/acrylonitrile rubber or butyl rubber.

The further (meth)acrylate esters, other vinyl monomers or polymers, for example elastomers, where present, may comprise from 5% to 80% by weight, preferably from 20% to 50% by weight, of the copolymer It is desirable that the polymer of this invention should have a viscosity average molecular weight from 400,000 to 1 5,000,000 preferably from 500,000 to 12,000,000. Below the lower limit the polymers tend to exhibit an undesirable lack of dimensional stability. Above the upper limit there are processing difficulties.

The polymers of this invention may be blended with one or more other compatible polymers, especially homo- or copoly[(meth)acrylate esters] such as poly(methyl methacrylate).

According to a further aspect of this invention, there is provided a process for the preparation of a homo- or copolymer as hereinbefore defined, which process comprises anionically polymerising one or more (meth)acrylate esters in the presence of, as catalyst, an alkali metal hydrocarbyl; covalently bonding to the polymer so formed atoms capable of absorbing X-radiation, and, if desired, polymerising the product with one or more further (meth)acrylate esters, other vinyl monomers or polymers and/or blending the radio-opaque polymer so formed with one or more other compatible polymers, especially homo- or copoly[(meth)acrylate ester]. It is preferred that the alkali metal is lithium; and that the hydrocarbyl moiety is a lower alkyl moiety.

According to a preferred embodiment of the process of this invention, there is incorporated into the reaction mixture a compound of the formula: R1 R2C=CH2 wherein: R, and2, which may be the same or different, each represent a (-M) group conjugated with the ethylenic bond. Preferably, R1 or R2 represents an aryl group; it is particularly preferred that R1 and R2 both represent phenyl groups.

It is preferred that the temperature of the reaction mixture is maintained from -1 000C to -400C, preferably from -800C to --600C.

The process of this invention is preferably performed on a (meth)acrylate ester which is a substituted or unsubstituted alkenyl ester, as hereinbefore defined. Where this is so and where it is desired that the atoms capable of absorbing X-radiation are halogen atoms these may readily be covalently bonded to the polymer by (hydro)halogenation, especially (hydro)bromination, of the alkenyl groups which are found to remain unchanged during the anionic polymerisation.

It is highly desirable that all reagents used in the process of this invention are anhydrous.

This invention further provides a process as hereinbefore defined which additionally comprises incorporating an anionically copolymerisable species into the reactant mixture. The anionically copolymerisable species may comprise a further (meth)acrylate ester or other vinyl monomer, for example styrene or (meth)acrylonitrile, which may be added ab initio, thereby resulting in a random copolymer, or after the initial (meth)acrylate ester is polymerised, thereby resulting in a block copolymer.Also, this invention provides a process as hereinbefore defined which additionally comprises incorporating a copolymerisable polymer, for example an elastomer, a polysaccharide or a homo- or copoly[(meth)acrylate ester, and if desired a grafting monomer, for example methyl (meth)acrylate, into the reactant mixture with a free radial initiator, for example a peroxide such as benzoyl peroxide and a chain transfer agent, for example an alcohol or mercaptan such as tertiary dodecyl mercaptan.

According to a still further aspect of this invention, there is provided a process for the preparation of a homo- or copolymer as hereinbefore defined, which process comprises radial polymerising, preferably at an elevated temperature such as from 400 to 1 000C, preferably SOC to 800 C, one or more (meth)acrylate esters into which atoms, especially halogen atoms, capable of absorbing X-radiation are incorporated by covalent bonding; and, if desired, polymerising the product with one or more further (meth)acrylate esters, other vinyl monomers, or polymers and/or blending the radio-opaque polymer so formed with one or more other compatible polymers, especially homo- or copoly[(meth)acrylate ester.

A free radical initiator, for example a peroxide such as benzoyl peroxide, is preferably present.

The atoms capable of absorbing X-radiation, for example halogen atoms, preferably bromine atoms, may be incorporated in the acid moiety or the alcohol moiety, or indeed both but preferably the latter, of the ester; preferably the alcohol moiety is mono- or poly-bromo substituted. Suitably the ester may be formed by reacting the corresponding acid halide or anhydride with a corresponding homo or poly-halo substituted alcohol; for example by reacting (meth)acryloyl chloride with 2,3dibromopropanol. Such materials have particularly good optical properties.

This invention also provides a radio-opaque homo- or copolymer, or polymer blend, whenever prepared by the process of this invention. The radio-opaque homo- or copolymer, or polymer blend of this invention may be in the form of beads, crumbs, sheets, rods, blocks or other forms of stock.

This invention further provides a dough moulding process for the preparation of a cured radio opaque polymeric mass, especially a denture base, which process comprises mixing a radio-opaque homo- or copolymer, or polymer blend, according to the invention with a liquid (meth)acrylic ester in the presence of a curing agent, for example a glycoldi-(meth)acrylate such as ethylene glycoldi (meth)acrylate in an amount of less than 10%, preferably less than 6%, by weight of the dough; and permitting the dough to cure, preferably at an elevated temperature and in a mould.

This invention also provides a radio-opaque denture base whenever prepared by the process of 'this invention.

Furthermore, this invention provides a cement, for example for joining fractured or coapted bone or endoprosthesis comprising a radio-opaque homo- or copolymer of this invention. Thus, beads made from the radio-opaque homo- or copolymer and containing 0.9% benzoyl peroxide, once sterilised either by gamma radiation or formaldehyde vapour, can be dough moulded with methyl methacrylate monomer containing about 2% of a tertiary aromatic amine. The dough described will set to a hard and robust mass in about five to seven minutes. Such a system may be used as a cement for endoprosthesis and has the advantage over radiolucent cements of being visualised on X-ray plates. Such a cement will have advantage over radio-opaque cements of the barium sulphate type both in strength, rheological properties and toxicology, there being no toxic barium ions to leach from the system.The following Examples illustrate the invention.

EXAMPLE 1 Preparation of poly(2,3-dibromopropylmethacrylate) Tetrahydrofuran (200 ml) was purified by heating under reflux for three hours over lithium aluminium hydride, the reflux condenser being protected from moisture with a calcium chloride drying tube. The mixture was cooled and then the tetrahydrofuran was distilled under reduced pressure onto a sodium mirror where it was kept sealed under vacuum.

An aliquot of allyl methacrylate, stored in the dark over calcium hydride, was distilled under reduced pressure onto a sodium mirror, then immediately from the mirror into a glass ampoule equipped with a teflon tape and protected from light. The ampoule was then attached to a three-necked 500 ml round bottomed flask, the whole apparatus being connected to a high vacuum line, evacuated and flamed.

The flask was immersed in liquid nitrogen and 1,1 -diphenylethylene (0.4 ml) was added from a syringe through a silicone rubber septum. The tetrahydrofuran was then distilled into the reaction flask from the sodium mirror. The apparatus was next immersed in an acetone bath cooled to -700C and the contents of the flask were stirred vigorously with a magnetic stirrer. Butyl lithium (5.0 ml of a 1.6 M solution in hexane) was added, and an orange colour developed immediately which darkened to deep red over a period of 10 minutes.The allyl methacrylate (62.8 g) was then added over a period of two minutes; after approximately five minutes the solution had begun to thicken and the reaction was allowed to proceed for an hour at --700C. The reaction was terminated by the addition of methanol (5.0 ml) and the polymer isolated by precipitating into petroleum ether (b.p. 60--800C). The polymer, poly(allylmethacrylate), was filtered off and dried under vacuum at room temperature, yield 62.2 g (99%).

Poly(allylmethacrylate) (20 g) was dissolved in carbon tetrachloride (600 ml) at room temperature.

The solution was then filtered, cooled to OOC, and a slight excess of bromine (26 g) in carbon tetrachloride (100 ml) was added with vigorous shaking. An immediate orange precipitate was formed and the mixture was left at OOC for 48 hours. The crude orange dibromide was filtered off, washed with carbon tetrachloride and dried at room temperature, yield 44.6 g (98%). The product was dissolved in chloroform and reprecipitated into petrol ether (b.p. 60-800C), filtered and dried under vacuum at room temperature to give 41.4 g of a white polymer, poly(2,3-dibromopropylmethacrylate).

EXAMPLE 2 Preparation of beads from preformed radio-opaque polymer This Example describes the fabrication of the polymer of Example 1 in a form familiar to dental technicians. To facilitate curing after dough moulding benzoyl peroxide was incorporated inside the beads.

The polymer (10 g) together with benzoyl peroxide (0.1 g) was dissolved in chloroform (50 ml) and added to a stirred solution of gelatine (6 g) and Tepol (2 g) in water (200 ml) at 400 C. The mixture was stirred under reduced pressure and the temperature maintained at 400C until all the chloroform had been evolved (approximately three hours). Water (600 ml) was then added and the mixture poured into a 1.1 measuring cylinder and the beads were allowed to settle out overnight. The polymer beads were washed repeatedly by suspending them in warm water (400 C) and allowing them to settle out. Finally the polymer beads were filtered off and dried in vacuum at room temperature to give 8090% yield.

1 0-20% of the polymer is lost as microfine beads that do not settle out during the washing sequence.

EXAMPLE 3 Preparation of beads from preformed radio-opaque polymer This Example describes an alternative procedure to Example 2; benzoyl peroxide was not added.

The polymer (40 g) was dissolved in chloroform (300 ml) and added to a stirred solution of gelatine (18 g) and Tepol (6 g) in water (600 ml) at 50 C. The mixture was stirred under a stream of nitrogen and the temperature was raised slowly to 70 C at such a rate (over five hours) as to prevent excessive foaming of the mixture. When the internal temperature had reached 70"C, water (400 ml) was added and the mixture was then poured into a 1.1 measuring cylinder and the polymer beads allowed to settle out overnight. The polymer beads were washed repeatedly by suspending them in hot water and allowing them to settle out. Finally the polymer beads were filtered off and dried in vacuum at room temperature to give 8090% yield.

EXAMPLE 4 Preparation of radio-opaque graft copolymer A solution of starch (8 g) in water (400 ml) was placed in a 1 litre resin kettle equipped with a stirrer, reflux condenser, nitrogen inlet and a dropping funnel. The apparatus was immersed in a waterbath at 600C and a slow stream of nitrogen was then passed through the solution. Next, 2-azoiso-butyronitrile (1.25 g) and 10 g of the polymer of Example 1 were dissolved in methyl methacrylate (100 g) and the solution added through the dropping funnel to the stirred starch solution. The temperature was raised in 50C steps every hour and finally held at 800C for two hours (total reaction time six hours). The suspension was then poured into a weak solution of a biological detergent ('Radiant') and the beads allowed to settle out over night.The polymer beads were washed repeatedly by suspending them in hot water and allowing them to settle out. Finally the polymer beads were filtered off and dried in vacuum at room temperature to give 7080% yield of graft copolymer.

By an essentially similar procedure, a variety of radio-opaque graft copolymers was prepared and formed into identical test specimens. The radiograph compares such specimens with one another and with conventional materials. In the radiograph: A is a specimen of a conventional radio-opaque poly(methyl methacrylate) marketed under the trade mark "TREVALON"; B is a 20:80% by weight block of brominated poly(n-allyl methacrylate) with methyl methacrylate; C is a 30:70% by weight block of brominated poly(n-allyl methacrylate) with methyl methacrylate; D is a 40:60% by weight block of brominated poly(n-allyl methacrylate) with methyl methacrylate; E is a 50:50% by weight block of brominated poly(n-allyl methacrylate) with methyl methacrylate; F is poly(methyl methacrylate).

EXAMPLE 5 Preparation of a denture base from radio-opaque graft copolymer beads The polymer beads (15 g) were added to methyl methacrylate (7.5 g), containing 5% ethylene glycoldi-methacrylate and 0.5% benzoyl peroxide, mixed thoroughly and left to stand. All four brominated samples gelled within four minutes to give a slightly translucent rubbery dough that couid be easily packed into a mould and after curing (seven hour delay, three hours at 1000C) gave translucent products. Relevant material properties are tabulated in Table 1.

TABLE 1 bromine content radio-opaque graft of radio-opaque copolymer (wt.% graft copolymer impact strength Young's modulus cured acrylic) (wt.%) ft. Lob/1" notch dyn. cam~2 CONTROL 1 Ol 0 too brittle 7.87 x 1010 CONTROL2 1002 0 0.023 . 2.6 x 10'0 1 58.3 34.5 0.022 2.34 x 1010 2 70.7 27.1 0.021 2.44 x 1010 3 80.2 20.1 0.019 2.39 x 1010 1. composed wholly of methyl methacrylate polymerised during cure 2. composed wholly of cured poly(methyl methacrylate) beads EXAMPLE 6 Preparation of block copolymer of allyl methacrylate and methyl methacrylate Tetrahydrofuran (200 ml) was distilled from a sodium mirror under reduced pressure into a reaction flask equipped with a PTFE stirrer bar and diphenyl ethylene (0.5 ml) was added followed by a solution of butyl lithium in hexane (5.0 ml of 1.6 M solution). Allyl methacrylate (35 g) was added and the red colour was discharged to give a pale yellow solution that thickened appreciably. Methyl methacrylate (1 5 g) was added and the thick solution stirred for one hour at -700C.

The block copolymer was isolated by precipitation to give a quantitative yield of a white polymer powder. This polymer was brominated in the manner described in Example 1 and isolated as a white powder.

EXAMPLE 7 Preparation of a block copolymer of allyl methacrylate with methyl methacrylate The procedure described in Example 6 was followed using 200 ml of tetrahydrofuran; 0.4 ml 1.1 diphenyl ethylene, 5.0 ml of butyl lithium (1.6) and 20 g of allyl methacrylate. After the allyl methacrylate had been allowed to polymerise for one hour at 700 C, 30 g of methyl methacrylate was added, and the solution was stirred for a further hour at -700C.

Isolation and subsequent bromination were performed in a similar manner to Example 6.

EXAMPLE 8 Polymerisation of methyl methacrylate in the presence of elastomer and brominated block copolymer Polybutadiene (6.5 g) and the block copolymer from Example 1, (15 g) containing 47.1% bromine were dissolved in methyl methacrylate (45 g) together with benzoyl peroxide (0.5 g) and tert-dodecyl mercaptan (0.1 g). The solution was flushed with nitrogen, stirred and heated to 600C until polymerisation was complete. The bulk polymer was dissolved in chloroform and converted to beads by the method described in Example 2. The final polymer had a rubber content of 10.9% and a bromine content of 11.8% by weight.

EXAMPLE 9 Dough fabrication of polymer beads 2.1 parts of the polymer bead material prepared in Example 8 were mixed with 1 part of monomeric methyl methacrylate containing 5% ethylene glycol dimethacrylate as a cross-linking agent and 0.5% benzoyl peroxide and the dough so formed could be packed into conventional plaster moulds, used for moulding denture base materials, heat cured at 1000C to give a white-translucent tough acrylic material.

This material exhibits typical craze whitening associated with rubber modified glassy polymer and relevant material properties are tabulated in Table 2 together with those of other acrylic denture base materials.

TABLE2 impact strength flexural strength ft. Lob/1" notch dyn. cm-2 Conventional dental acrylic 0.022 1.82 x 109 Material from Example 9 0.051 1.29 x 109 Trevalon X-ray opaque 0.019 1.12 x 109 Ray paque (X-ray opaque temporary crown 0.016 0.64 x 109 and bridge material) EXAMPLE 10 Preparation of 2,3-dibromopropyl methacrylate A solution of methacryloyl chloride (53 g) in dry ether (100 ml) was added dropwise to a vigorously stirred solution of pyridine (40 g) and 2,3-dibromopropanol (109 g) in ether (100 ml), maintaining the reaction at a temperature from 0 to 50C. The mixture was stirred overnight at a temperature from 0 to 50C then the mixture was extracted successively with water, dilute hydrochloric acid and aqueous sodium bicarbonate solution.The ether solution was dried over sodium sulphate and solvent removed under reduced pressure at 00C and the colourless monomer stored at 200 C.

EXAMPLE 11 Bulk polymerisation of 2,3-dibromopropyl methacrylate 2,3-dibromopropyl methacrylate containing 1% benzoyl peroxide was poured into a mould between glass plates and cured overnight at 650C to give a clear, colourless high density, X-ray opaque resin.

EXAMPLE 12 Copolymerisation of 2,3-dibromopropyl methacrylate with methyl methacrylate A mixture of 2,3-dibromopropyl methacrylate (20 g), methyl methacrylate (30 g) and benzoyl peroxide (0.5 g) was added to a vigorously stirred solution of 3% gelatin (400 ml) in water. The mixture was heated at 650C for five hours and at 950C for a further two hours. The fine polymer beads were washed with hot water and filtered off.

These polymer beads gel with methyl methacrylate and can be dough moulded to give an X-ray opaque resin as described previously.

EXAMPLE 13 Copolymerisation of 2,3-dibromopropyl methacrylate and methyl methacrylate in the presence of an elastomer A solution of polybutadiene (7.0 g) in a mixture of 2,3-dibropropyl methacrylate (1 5 g) and methyl methacrylate (35 g) containing benzoyl peroxide (0.5 g) and tert-dodecylmercaptan (0.2 g) was stirred vigorously at 650C. The clear solution became cloudy and immediately after phase inversion had occurred. An aqueous solution of 3% starch (400 ml) containing 1% Tepol was added and the mixture stirred at 800C for five hours. Finally the fine polymer beads were washed with hot water, filtered and dried.

Again these beads gelled with methyl methacrylate to give a tough X-ray opaque resin.

Claims (50)

1. A homo- or copolymer of a (meth)acrylate ester into which atoms capable of absorbing x-radiation are incorporated by covalent bonding, the resulting polymer having a viscosity average molecular weight greater than 400,000.

2. A homo- or copolymer according to Claim 1 wherein the atoms comprise halogen atoms capable of absorbing x-radiation.

3. A homo- or copolymer according to Claim 2 wherein the halogen atoms capable of absorbing x-radiation comprise bromine atoms.

4. A homo- or copolymer according to any preceding claim wherein the ester comprises a substituted or unsubstituted alkenyl ester.

5. A homo- or copolymer according to Claim 4 wherein the alkenyl group is an unsubstituted alkenyl group of the formula: (CH2)nCH=CHR wherein: R represents a hydrogen atom or a lower alkyl group; and n represents an integer from 1 to 3.

6. A homo- or copolymer according to Claim 5 wherein the alkenyl group is an unsubstituted allyl group.

7. A homo- or copolymer according to any of Claim 4 to 6 which is the (hydro) halogenation product of the substituted or unsubstituted alkenyl ester.

8. A homo- or copolymer according to Claim 7 wherein the ester comprises a mono- or vic- di-halo alkyl (meth)acrylate.

9. A homo- or copolymer according to Claim 8 wherein the ester comprises a mono- or 2,3-di-halo propyl (meth)acrylate.

10. A homo-or copolymer according to Claim 9 wherein the ester comprises a mono- or 2,3-dibromopropyl (meth)acrylate.

11. A homo- or copolymer according to any preceding claim wherein the ester comprises a methacrylate ester.

12. A homopolymer according to any preceding claim.

13. A random, block or graft copolymer according to any of Claims 1 to 11.

14. A copolymer according to Claim 13 wherein a comonomer or copolymer comprises a further (meth)acrylate ester, other vinyl monomer, elastomer, polysaccharide or homo- or copoly [(meth)acrylate ester].

15. A copolymer according to Claim 14 wherein a comonomer comprises a C1 to C4 alkyl (meth)acrylate.

16. A copolymer according to Claim 13 or 14 which comprises a toughening dispersed elastomeric phase.

1 7. A copolymer according to Claim 16 wherein the elastomer comprises a polymerised unsubstituted or halo-substituted diene.

1 8. A copolymer according to Claim 1 7 wherein the elastomer comprises polybutadiene, butadiene/styrene rubber, butadiene/acrylonitrile rubber or butyl rubber.

1 9. A copolymer according to Claim 1 8 wherein the elastomer comprises natural rubber.

20. A copolymer according to Claim 1 8 wherein the butadiene/styrene rubber is a styrene/butadiene block copolymer comprising less than 40% by weight of polymerised styrene.

21. A copolymer according to any of Claims 14 to 20 wherein the further (meth)acrylate esters, other vinyl monomers, or polymers comprise from 5% to 80% by weight of the copolymer.

22. A homo- or copolymer according to any preceding claim which has a viscosity average molecular weight from 400,000 to 15,000,000.

23. A homo- or copolymer according to any preceding claim blended with a poly [(meth)acrylate ester].

24. A process for the preparation of a homo- or copolymer according to any preceding claim, which process comprises anionically polymerising a (meth)acrylate ester in the presence of, a catalyst, an alkali metal hydrocarbyl, and covalently bonding to the polymer so formed atoms capable of absorbing x-radiation.

25. A process according to Claim 24 wherein the alkali metal is lithium.

26. A process according to Claim 24 or 25 wherein the hydrocarbyl moiety is a lower alkyl moiety.

27. A process according to Claim 25 or 26 wherein there is incorporated into the reaction mixture, as initiator, a compound of the formula: R,R2C=CH2 wherein: R and R2, which may be the same or different, each represent a (-M) group conjugated with the ethylenic bond.

28 A process according to Claim 27 wherein R1 or R2 represents an aryl group.

29. A process according to Claim 28 wherein R1 and R2 both represent phenyl groups.

30. A process according to any of Claims 24 to 29 wherein the ester is a substituted or unsubstituted alkenyl ester.

31. A process according to Claim 30, wherein the atoms capable of absorbing x-radiation are halogen atoms and are covalent bonded to the polymer by (hydro)halogenation of the alkenyl groups.

32. A process according to any of Claims 24 to 31 for the preparation of a copolymer which additionally comprises incorporating an anionically copolymerisable species into the reactant mixture.

33. A process according to Claim 32 wherein the anionically copolymerisable species comprises a further (meth)acrylate ester or other vinyl monomer which is added ab initio thereby resulting in a random copolymer.

34. A process according to Claim 32 or 33 wherein the anionically copolymerisable species comprises a further (meth)acrylate ester or other vinyl monomer which is added after the initial (meth)acrylate ester is polymerised thereby resulting in a block polymer.

35. A process according to any of Claims 24 to 34 for the preparation of a graft copolymer which additionally comprises incorporating a copolymerisable polymer and, if desired, a grafting monomer into the reactant mixture with a free radical initiator and a chain transfer agent.

36. A process according to Claim 35 wherein the copolymerisable polymer comprises an elastomer.

37. A process according to any of Claims 24 to 36 wherein the radio-opaque homo- or copolymer so formed is blended with a poly [(meth)acrylate ester].

38. A process for the preparation of a homo- or copolymer according to any of Claims 1 to 23, which process comprises radical polymerising a (meth)acrylate ester into which atoms capable of absorbing x-radiation are incorporated by covalent bonding.

39. A process according to Claim 38 wherein the atoms capable of absorbing x-radiation are incorporated into the alcohol moiety of the ester.

40. A process according to Claim 39 wherein the atoms capable of absorbing x-radiation are halogen atoms.

41. A process according to Claims 38 or 39 wherein the ester is prepared by reacting the corresponding acid halide or anhydride with a corresponding mono- or poly- halo substituted alcohol.

42. A process according to Claim wherein the polymerisation is effected at a temperature from 40 to 1000C.

43. A radio-opaque homo- or copolymer, or polymer blend, whenever prepared by the process of any of Claims 24 to 42.

44. A radio-opaque homo-or copolymer, or polymer blend, according to any of Claims 1 to 23 and 43 wherein the polymer is in the form of beads or crumbs.

45. A dough moulding process for the preparation of a cured radio-opaque polymeric mass, which process comprises mixing a radio-opaque homo- or copolymer, or polymer blend, according to any of Claims 1 to 23, 43 or 44 with a liquid (meth)acrylate ester in the presence of glycoldi(meth)acrylate; and permitting the mixture to cure.

46. A process according to Claim 45 wherein curing is effected, at elevated temperature, in a mould.

47. A cured radio-opaque polymeric mass prepared by the process of Claim 45 or 46.

48. A cured radio-opaque polymeric mass according to Claim 47 which is a denture base.

49. A radio-opaque homo- or copolymer, or polymer blend, according to any of Claims 1 to 23, 43 or 44 for use in animal or human endoprosthesis.

50. A radio-opaque homo- or copolymer, or polymer blend, according to Claim 49 used for cementing fractured or coapted bone.