The plastics material generally used in the fabrication of denture base is poly(methyl methacrylate) . Pol (methyl eth- aer late) 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 sometime. 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)aerylate ester into 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. Chloro-substituted 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, bromo-substituted polymers are preferred. A mixture of halo-substituents, such as both chloro-and bromo- substituents, 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 15% 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 (me h) erylate 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 alkenyl group of the formula: 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 par¬ ticularly 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 mono- or vie di-haloalkyl, preferably mono- or 2,3 di- halopropyl, especially mono- or 2,3 di-bromopropyl,(meth) er late. The polymer of this invention may be a homopolymer, such as poly(mono- or vie di-hal ^' oalkyl (meth)acrylate, preferably poly (mono- or 2,3 di-halopropyl(meth) er late) , especially poly(mono- or 2,3 di-bromopropyl(meth)aerylate) . Alternatively, it may be a random, block or graft, preferably random or block, copolymer with one or more further (meth)aer late esters, other vinyl monomers, or polymers, for example elastomers, polysaccharides or homo- or copoly J~(meth)aeryl te esters . The further (=eth)acr lat esters may comprise one or more further substituted or unsubstitute alkenyl (meth)aer late esters as hereinbefore defined or one or more substituted or unsubstituted alkyl, suitably C. to C, alkyl, especially methyl or ethyl, (meth)aer lates.

In accordance with another aspect of this invention there is provided a radio-opaque polymer as aforesaid which comprises a toughening dispersed elasto eric 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 radio-opacity. Suitable elastomers comprise polybutadienes such as natural rubber, butadiene/st rene rubbers such as styrene/ butadiene block copolymers which may comprise less than 40% by weight of polymerised styrene, butadiene/aer lonitrile rubber or butyl rubber.

The further (meth)aeryl te 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 15,000,000

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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 eopoly [(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 poly- merising 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 eopoly [(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:

R _{1 2} C ^« CH ₂ wherein:

R, and R~, which may be the same or different, each represent a (-M) group conjugated with the ethylenic bond. Preferably, R, or R- represents an aryl group; it is particularly preferred that R. and - both represent phenyl groups.

It is preferred that the temperature of the reaction mixture is maintained from -100°G to -40°C, preferably from -80°C to - 60°C. The process of this invention is preferably performed on a (meth) crylate 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 readi-ly be covalently bonded to the polymer by (hydro)halogenation, especially (hydro)bro ination, 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 resultin 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 eopoly meth)acrylate esterj, and if desired a grafting monomer, for example methyl (meth acrylate, into the reactant mixture with a free radical 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 radical poly¬ merising, preferably at an elevated temperature such as from 40 to 100 C, preferably 50 to 80 C, one or more (meth) crylate 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 eopoly ftmeth)acrylate esterj. A free radical initiator, for example a peroxide such as ben∑oyl 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 corres¬ ponding homo or poly- halo substituted alcohol; for example by reacting (meth)acryloyl chloride with 2,3-dibromopropanol. 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 coplymer, 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 sgent, for example a glycoldi-(meth) crylate 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 raio-opaque denture base whenever prepared by the process of this invention.

Furthermore, this invention provides a cement, for example for joining fractured or coa ted bone or endoprosthesis comprising a radio-opaque homo— or copolymer of this invention. Thus, beads - ^~ 3.de 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 meth¬ acrylate monomer containing about 2% of a tertiary aromatic ε=ine. 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

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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 tap 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-diphenyl- ethylene (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 -70 C 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 (62.8g) 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 -70 C. The reaction was terminated by the addition of methanol (5.0 ml) and the polymer

isolated by precipitating into petroleum ether (b.p. 60-80 C) . The polymer, poly(allyl ethacrylate) , was filtered off and dried under vacuum at room temperature, yield 62.2g (99%).

Poly(all lmethacr late) (20g) was dissolved in carbon tetra- chloride (600 ml) at room temperature. The solution was then filtered, cooled to 0°C, and a slight excess of bromine (26g) in carbon tetrachloride (100 ml) was added with vigorous shaking. An immediate orange precipitate was formed and the mixture was left at 0°C for 48 hours. The crude orange dibromide was filtered off, washed with carbon tetrachloride and dried at room temperature, yield 44.6g (98%). The product was dissolved in chloroform and reprecipitated into petrol ether (b.p. 60-80°C) , filtered and dried under vacuum at room temperature to give 1.4g of a white polymer, poly(2,3-dibromoρropylmethacrylate) . EXAMPLE 2 Preparation of beads from pre-formed radio-opaque polymer This Example describes the fabrication of the polymer of

Example 1- in a form familiar to dentaltechnicians. To facilitate curing after dough moulding benzoyl peroxide was incorporated inside the beads.

The polymer (lOg) together with benzoyl peroxide (O.lg) was dissolved in chloroform (50 ml) and added to a stirred solution of gelatine (6g) and Tepol(2g) in water (200 ml) at 40°C. The mixture was stirred under reduced pressure and the temperature maintained at 40 C 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 (40°C) and allowing them to settle out. Finally the polymer beads were filtered off and dried in vacuum at room temperature to give 80-90% yield. 10-20% of the polymer is lost as microfine beads that do not settle out during the washing sequence.

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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 (40g) was dissolved in chloroform (300 ml) and added to a stirred solution of gelatine (18g) and Tepol (6g) 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 80-90% yield. EXAMPLE 4

Preparation of radio-opaque graft- copolymer A solution of starch (8g) 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 60 C and a slow stream of nitrogen was then passed through the solution. Next, 2-azo-iso-but.yronitrile (1.25g) and lOg of the polymer of Example 1 were dissolved in methyl methacrylate (lOOg) and the solution added through the dropping funnel to the stirred starch solution. The temperature was raised in 5°C steps every hour and finally -held at S0°C 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 70-80% yield of graft copolymer.

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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 meth acrylate) marketed under the trade mark "TREVALON": B is a 20:80% by weight block of brominated poly(n-allyl meth acrylate) with methyl methacrylate; C is a 30:70% by weight block of brominated poly(n-allyl meth acrylate) with methyl methacrylate;

D is a 40:60% by weight block of brominated poly(n-allyl meth acrylate) with methyl methacrylate; E is a 50:50% by weight block of brominated poly(n-allyl meth¬ acrylate) with methyl methacrylate; F is pol (methyl methacrylate). EXAMPLE 5"

Preparation of a denture base from radio-opaque graft copolymer beads

The polymer beads (15g) were added to methyl methacrylate (7.5g), 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 could be easily packed into a mould and after curing (seven hour delay, three hours at 100 C) gave translucent products. Relevant material properties are tabulated in Table 1. TABLE 1 bromine content radio-opaque graft of radio-opaque copolyer (wt.% graft copolymer impact strength Young' s modulus cured acrylic) (wt.%) ft . lb/1 " notch dvn. cm

1

CONTROL 1 0 too brittle 7.87 x 10 ¹⁰ CONTROL 2 100 ^* 0 0.023 2.6 x 10 ^L0

1 58.3 34.5 0.022 2.34 x 10 ¹⁰

2 70.7 27.1 0.021 I im x 10 ¹⁰

3 80.2 20.1 0.019 2.39 x 10 ¹⁰

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 PTFΞ 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 (35g) was added and the red colour was discharged to give a pale yellow solution that thickened appreciably. Methyl methacrylate (15g) was added and the thick solution stirred for one hour at -70 C.

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 20g of allyl methacrylate. After the allyl methacrylate had been allowed to polymerise for one hour at -70°C, 30g of methyl methacrylate was added, and the solution was stirred for a further hour at -70 C.

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 (6.5g) and the block copolymer from Example 1, (15g) containing 47.1% bromine were dissolved in methyl methacrylate (45g) together with benzoyl peroxide (0.5g) and tert-dodecyl mercaptan (O.lg). The solution was flushed with nitrogen, stirred and heated to 60 C 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 100 C 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.

TABLE 2 impact strength flexural strength ft. lb/1" notch dyn. cm-

Conventional dental acrylic 0.022 • 1.82 x 10 ⁹

Material from Example 9 0.051 1.29 x 10 ⁹

Trevalon X-ray opaque 0.019 1.12 x 10 ⁹

Ray paque (X-ray opaque temporary crown and bridge material) 0.016 0.64 x I0 ⁹

EXAMPLE 10

Preparation of 2,3-dibromopτopyl methacrylate A solution of ethacryloyl chloride (53g) in dry ether

(100 ml) was added, ropwise to a vigorously stirred solution of pyridine (40g and 2,3-dibromopro anol (10.9g) in ether (100 ml), maintaining the reaction at a temperature from 0° to 5°C. The mixture was stirred overnight at a temperature from 0° to 5°C then the mixture was extracted successively with water, dilute hydrochloric acid and aqueous sodium bicarbonate solution. The

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ether solution was dried over sodium sulphate and solvent removed under reduced pressure at 0 C and the colourless monomer stored at -20°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 65°C 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 (20g) ,.methyl methacrylate (30g) and benzoyl peroxide (0.5g) was added to a vigorously stirred solution of 3% gelatin (400 ml) in water. The mixture was heated at 65 C for five hours and at 95 C 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 ζl.Og) in a mixture of 2,3- dibromopropyl methacrylate (15g) and methyl methacrylate (35g) containing benzoyl peroxide (0.5g) and tert-dodecylmercap an (0.2g) was stirred vigorously at 65 C. 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 80°C 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.