AU688325B2 - Telomer composition - Google Patents

Description

WO 95/00577 PCT/AU94/00285 1 TELOMER COMPOSITION The present invention relates to the manufacture of plastic optical articles such as video discs and ophthalmic lenses. The most widely used plastic ophthalmic lens material is polymerised diethylene glycol bis (allyl carbonate).

This polymer has proved a satisfactory material for the manufacture of ophthalmic lenses because of a combination of features, including excellent transmission, resistance to discolouration, high strength and high impact resistance. The material has a reasonable abrasion resistance and can be coated to improve that resistance.

Lenses formed from polymers including divinyl benzene are generally brittle and have a high yellowness index.

In Australian Patent Appln. 81216/87, the entire disclosure of which is incorporated herein by reference, applicant describes a cross-linkable casting composition including at least polyoxyalkylene glycol diacrylate or dimethacrylate and at least one poly functional unsaturated cross-linking agent. Whilst the lenses produced from the prior art compositions provide advances in the art, difficulties may be encountered in certain applications. For example, difficulties may be encountered in some patients in fitting lenses made from such compositions. Moreover, such lenses have been found to be cosmetically unattractive.

Japanese Patent Appln. 63-309509 to Showa Denko KK discloses the formation of a prepolymer of divinyl benzene and a polythiol compound which may be used in the formation of optical materials with high refractive indexes.

Japanese Patent Appln. 90152702 to Mitsubishi Gas discloses a pentaery''ritol triacrylate or methacrylate polymer with a minor amount, e.g. by weight, of divinyl benzene in combination with pentaerythritol tetrabis (beta thiopropionate) or pentaerythritol tetra bis (thioglycolate). This polymer may be used to form optical lenses of high refractive index.

However, there remains a need in the prior art for optical articles of very high refractive indexes, very low densities and excellent colour, that still retain excellent mechanical properties.

Accordingly it is an object of the present invention to overcome, or at I I -2least alleviate, one or more of the difficulties related to the prior art.

Accordingly, the present invention provides a telomer formed from: approximately 40 to 90% by weight of the telomer, of a divinyl benzene or derivative thereof; and approximately 1 to 60% by weight of the telomer of a tetrasubstituted disoloxane or one of its higher oligomers, wherein the tetrasubstituted disiloxane is of the formula: H-Si-O-Si-H

I

R2 r4 10 wherein R 1

R

2

R

3 and R 4 which may be the same or different, are selected from alkyl groups of 1 to 10 carbon atoms.

A tetramethyl disiloxane (TMDS) is preferred Me Me H-Si-O-Si-H

II

Me Me The siloxane compound may be present in amounts of from approximately 15 10 to 60% by weight, preferably approximately 20 to 50% by weight, based on the total weight of the telomer composition.

The telomer composition according to the present invention may be utilised in the preparation of an optical article such as a contact lens. The optical article may be characterised by being thinner and/or lighter than known prior art articles whilst retaining good abrasion, impact and heat resistance. The optical article may exhibit a high refractive index, excellent colour, good flexibility, low shrinkage, high gas permeability, and very low density. The polymeric article may also exhibit a low Yellowness Index.

By the term "high refractive index" as used herein, we mean a polymer having a refractive index of at least approximately 1.55, preferably approximately 1.55 to 1.57.

By "low density", as used herein, we mean a density in the range of approximately 1.15 to 1.25 g cm' 3 -3- By "very low density" as used herein we mean a density in the range of to 1.15g cm' 3 The present invention also provides a process for the preparation of a telomer, which process includes providing: approximately 40 to 90% by weight of the telomer, of a divinyl benzene or derivative thereof; approximately 1 to 60% by weight of the telomer, of a tetrasubstituted disiloxane or one of its higher oligomers, wherein the tetrasubstituted disiloxane is 10 of the formula: :R R3 H-Si-O-Si-H R2 R4

I

wherein R 1

R

2

R

3 and R 4 which may be the same or different, are selected from alkyl groups of 1 to 10 carbon atoms; and 15 an hydrosilation catalyst; and mixing the monomers at elected temperature in the presence of the hydrosilation catalyst for a period sufficient to allow reaction therebetween.

The partially substituted siloxane compound may be tetramethyl disiloxane (TMDS). The hydrosilation catalyst may include a metal catalyst. The metal catalyst may be a platinum or rhodium catalyst. A platinum catalyst is preferred.

The platinum catalyst may be selected from H 2 PtCI 6 Pt(COD)CI2, Karstedt's catalyst and Lameroux catalyst. The platinum catalyst H 2 PtCl6, is preferred.

The reaction mixture may be heated to a temperature of approximately 500C to 900C, preferably 65°C to 750C. The reaction may continue for approximately 12 to 24 hours.

The reaction mixture may include a stoichiometric excess of aromatic olefin.

The telomer may include a mixture of isomers and oligomers. The telomer may be further purified to remove such oligomers.

The present invention also provides a cross-linkable prepolymer casting aq composition including a telomer formed from an effective amount of

MEN

-4an aromatic diolefin monomer and a partially substituted di- or polysiloxane compound; and a polymerisable comonomer selected from methacrylates, acrylates, vinyls, vinyl ethers, allyls, aromatic olefins, ethers and polythiols; wherein the partially substituted di- or polysiloxane compound is a tetrasubstituted disiloxane compound or its higher oligomers, the tetrasubstituted disiloxane compound being selected from the following: Ri

R

3 H-Si-O-Si-H

R

2

R

4 wherein Ri, R 2

R

3 and R 4 which may be the same or different, are selected from alkyl groups of 1 to 10 carbon atoms.

The polymerisable comonomer may be selected to improve the properties and/or processability of the prepolymer casting composition. The polymerisable comonomer may be selected from any suitable type, eg methacrylates, acrylates, vinyls, vinyl ethers, allyls, aromatic olefins, ethers, polythiols and mixtures thereof.

15 The polymerisable comonomer may be present in amounts of from approximately 1 to 60% by weight based on the total weight of the casting composition.

The polymerisable comonomer may preferably be selected from one or more of aromatic olefins, polyoxyalkylene glycol diacrylates or dimethacrylates, polymerisable bisphenol monomers capable of forming a homopolymer having a high refractive index of more than 1.55, urethane monomers having 2 to 6

M

WO 95/00577 PCT/AU94/00285 terminal acrylic or methacrylic groups, fluorene diacrylates or dimethacrylates, and thiodiacrylate or dimethacrylate monomers.

Where an aromatic olefin is included, the aromatic olefins may be selected from styrene, divinyl benzene (DVB), and 3,9-divinyl-2,4,8,10tetraoxaspiro[5.5]undecane (DTU). The aromatic olefins may be present in amounts of from approximately 1 to 60% by weight, preferably approximately 5 to by weight, based on the total weight of the casting composition.

Where a thiodiacrylate or dimethacrylate is included, the thiodiacrylate or dimethacrylates may be selected from bis(4-methacryloylthioethyl)sulfide 1i (BMTES) and bis(4-methacryloylthiophenyl)sulfide (BMTS). The thiodiacrylate or dimethacrylate may be present in amounts of from approximately 1 to 60% by weight, preferably approximately 5 to 40% by weight, based on the total weight of the casting composition.

Where a fluorene diacrylate or methacrylate is included, the fluorene diacrylate or dimethacrylate monomer may be selected from a bisphenol fluorene dihydroxy acrylate (BFHA) or a bisphenol fluorene dimethacrylate (BFMA) or mixtures thereof.

The fluorene diacrylate or dimethacrylate monomer may be present in amounts of from approximately 1 to 35% by weight, preferably approximately 5 to 20% by weight.

The polyoxy alkylene glycol diacrylate or dimethacrylate compound according to the present invention, when present, may include ethylene oxide or propylene oxide repeating units in its backbone. A polyethylene glycol dimethacrylate is preferred. Suitable materials are those sold under the trade names NKESTER 4G, 9G, 14G or 23G by Shin Nakamura.

The polyoxy alkylene glycol diacrylate or dimethacrylate component may be present in amounts of from approximately 1% by weight to 60% by weight, preferably approximately 5% to 40% by weight, based on the total weight of the casting composition.

The high index bisphenol monomer component in the cross-linkable casting composition when present may be selected from: dimethacrylate and diacrylate esters of bisphenol A; dimethacrylate and diacrylate esters of I I WO 95/00577 PCT/AU94/00285 6 4,4'bishydroxy-ethoxy-bisphenol A and the like.

A preferred high index bisphenol compound is a bisphenol A ethoxylated dimethacrylate. A bisphenol A ethoxylated dimethacrylate sold under the trade designation ATM20 by Anchomer or Diacryl 101 by AKZO have been found to be suitable. Halogenated high index bisphenol compounds which may be used include those sold under the trade designation and NK Ester 534M by Shin Nakamura.

The high index bisphenol monomer may be present in amounts of from approximately 1 to 50% by weight, preferably 5 to 40% by weight based on the io total weight of the casting composition.

The polythiol, when present, may be selected from Pentaerythritol Tetrakis (3-mercapto-propionate) [PTMP], Trimethylolpropane Tris (3-mercaptopropionate) [TTMP], 4-mercaptomethyl-3,6-dithia-1,8-octanedithiol [MDO], Pentaerythritol Tetrakis (3-mercaptoacetate) [PTMA], Trimethylolpropane Tris (3mercaptoacetate) [TTMA], 4-t-butyl-1,2-benzenedithiol, 2-mercaptoethylsulfide, 4,4'-thiodibenzenethiol and benzenedithiol, The polythiol may be present in amounts of from approximately 1 to by weight, preferably 20 to 50% by weight.

In a further aspect of the present invention the cross-linkable coating composition may further include at least one poly-functional unsaturated crosslinking monomer.

The poly functional unsaturated cross-linking monomer according to the present invention may be a tri- or tetra- functional vinyl, an acrylic or methacrylic monomer. The cross-linking monomer may be a short chain monomer for example trimethylol propane trimethacrylate, pentaerythritol triacrylate or tetracrylate, or the like.

A material sold under the trade name SR-454, which is an ethoxylated trimethylol propane triacrylate, has been found to be suitable.

The poly functional unsaturated cross-linking monomer may be present in amounts of from approximately 1 to 30% by weight, preferably approximately 5 to by weight based on the total weight of the casting composition.

The cross-linkable casting composition according to the present invention WO 95/00577 PCT/AU94/00285 7 may further include a cross-linking initiator. The cross-linking initiator may be a radical heat and/or ultraviolet cationic or radical initiator. A radical heat initiator is preferred. The compositions may be cured by a combination of UV radiation and/or heat. The combination of UV radiation and heat may reduce the possibility of incomplete curing for example due to the phenomenon known as "radical trapping".

The composition, with the addition of approximately 0.1% to 2.0% by weight of initiator may be exposed to UV radiation for between 0.5 and seconds.

Any commercially viable UV curing system may be used. We have used a Fusion System with microwave driven lamps.

Typically 2 4 passes under the UV lamps plus a period of heat treatment at 100°C completes the cure.

Alternatively or in addition to UV irradiation, the cross-linkable casting composition may be to heat curing. The length of heat cure may be between approximately 1 hour up to approximately 48 hours. Monomer filled moulds are placed in an oven. The oven is heated in accordance with a prescribed heating regime. Preferably curing time is between approximately 6 to 24 hours. Heating profiles depend upon the type of monomers to be cured and the initiators used to cure them.

The amount of initiator may vary with the monomers selected. It has been possible to operate with a relatively low level of initiator of between approximately 0.05 and preferably 0.4% to 1.0% by weight.

The following initiators have been found to be suitable.

AIBN (Azo radical heat initiator) Azodiisobutyronitrile TX-29 (Dialkyl Peroxide radical heat initiator) 1,1-di-(-butyl peroxy-3,3,5-trimethyl cyclohexane) -TBPEH (Alkyl Perester radical heat initiator) t-butyl per-2-ethylhexanoate (Diacyl Peroxide radical heat initiator) Benzoyl Peroxide WO 95/00577 PCT/AU94/00285 8 (Peroxy Dicarbonate radical heat initiator) Ethyl Hexyl Percarbonate (Ketone Peroxide radical heat initiator) Methyl ethyl ketone peroxide Cyracure UV1-6974 (cationic photoinitiator) Triaryl sulfonium hexafluoroantimonate SLucirin TPO (radical photoinitiator) 2,4,6-Trimethylbenzoyldiphenylphosphine oxide Vicure 55 (radical photoinitiator) methyl phenylglyoxylate benzoic acid Irgacure 184 or 651 (photoinitiators) 1 -hydroxy-cyclohexylphenylketone or 2-2-dimethoxy-l,2-diphenylethan-1one.

Initiator may be a single component or combination of initiator components.

Other additives may be present which are conventionally used in casting compositions such as inhibitors, dyes, UV stabilisers and materials capable of modifying refractive index. Mould release agents can be added but they are in general not required with the compositions used in the method of the present invention. Such additives may include: UV Absorbers including Ciba Geigy Tinuvin P 2(2'-hydroxy-5'methyl phenyl) benzotriazole Cyanamid Cyasorb UV 531 -2-hydroxy-4-n-octoxybenzo-phenone Cyanamid Cyasorb UV5411-2(2-hydroxy-5-t-octylphenyl)-benzotriazole Cyanamid UV 2098 2 hydroxy-4-(2-acryloyloxyethoxy) benzophenone National S C Permasorb MA 2 hydroxy-4-(2 hydroxy-3-methacryloxy) propoxy benzophenone Cyanamid UV24 2,2'-dihydroxy-4-methoxybenzophenone BASF UVINUL 400 2,4 dihydroxy-benzophenone BASF UVINUL D-49 2,2'-dihydroxy-4,4' dimethoxy-benzophenone BASF UVINUL D-50 4,4' tetrahydroxy benzophenone BASF UVINUL D-35-ethyl-2-cyano-3,3-diphenyl acrylate

I

WO 95/00577 PCT/AU94/00285 9 BASF UNINYL N-539-2-ethexyl-2-cyano-3,3-diphenyl acrylate Ciba Geigy Tinuvin 213 Hindered amine light stabilisers (HALS), including Ciba Geigy Tinuvin 765/292 bis (1,2,2,6,6-pentamethyl-4-piperidyl) sebacate SCiba Geigy 770 bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate Antioxidants including Ciba Geigy Irganox 245 triethylene glycol-bis-3-(3-tertbutyl-4-hydroxy-5methyl phenyl)propionate Irganox 1010 -2,2-bis[[3-[3,4-bis(1, 1-dimethylethyl)-4-hydroxyphenyl]-1 oxopropoxy]methyl]-1,3-propanediyl 3,5-bis(1,1-dimethyl ethyl)-4-hydroxy benzene propanoate Irganox 1076 octadecyl 3-(3',5'-di-tert-butyl(-4'- hydroxyphenyl) propionate Anticolourinq agents including Triphenyl phosphine 9, 10 dihydro-9-oxa-10-phosphaphenanthrene-1-oxide Cure modifiers including Dodecyl mercaptan Butyl mercaptan Thiophenol Other monomeric additives may be included to improve processing and/or material properties, these include: methacrylic acid, maleic anhydride, acrylic acid adhesion promoters/modifiers such as Sartomer 9008, Sartomer 9013, Sartomer 9015 etc.

dye-enhancing, pH-adjusting monomers like Alcolac SIPOMER 2MIM a charge-reducing cationic monomer to render the material more antistatic, example Sipomer Q5-80 or Q9-75 hydrophobic comonomers: Shin Nakamura NPG, P9-G etc. to reduce the water adsorption of the material viscosity modifiers In a further aspect of the present invention there is provided a polymeric WO 95/00577 PCT/AU94/00285 article formed from a cross- linkable prepolymer casting composition as described above. The polymeric article may be an optical article. The optical article may provide characteristics equal to or greater than those achievable with articles made from diethylene glycol bis(allyl carbonate) but with a considerably reduced cure time and substantially increased throughput. The optical article may be further characterised by having reduced weight and/or thickness relative to the prior art, low Yellowness Index, high gas permeability and very low density, whilst retaining good abrasion, heat and impact resistance.

The overall refractive index may be in the high refractive index range of 1o approximately 1.56 or higher, preferably 1.56 to 1.57.

The optical articles prepared by the method of this invention include camera lenses, ophthalmic lenses including contact lenses and video discs.

The casting composition may be formed into a suitable article by mixing in a convenient vessel the components making up the material, and then adding the curing catalyst and/or photo-initiator. The mixed material is then degassed or filtered. As the curing time is substantially reduced the casting process may be undertaken on a continuous or semi-continuous basis.

The present invention will now be more fully described with reference to the accompanying examples. It should be understood, however, that the description following is illustrative only and should not be taken in any way as a restriction on the generality of the invention described above.

EXAMPLE 1 Synthesis of divinylbenzene siloxane telomers Divinylbenzene 960 (DVB) which is a mixture of 96% divinylbenzene (both meta and para isomers) and 4% ethyl-vinyl benzene was used as a starting compound. The material as received was a clear pale yellow liquid.

The dropwise addition of TMDS to a 20% excess of neat DVB containing a catalytic amount of H 2 PtCI 6 at about 65-70 0 C with further stirring at this temperature range for 18 hours gave a very pale brownlcrimson liquid. The resultant colour was no more intense than the colour of the initial DVB. The colour was removed by passing the liquid through a column of neutral alumina.

The resulting clear colourless liquid was obtained in about 90-95% yield (see WO 95/00577 PCTIAU94/00285 11 Scheme 1, Figure The major losses were due to the passing of the material through alumina. If required further material could be isolated by washing the column with a suitable solvent and then removing that solvent.

Analysis of the resulting DVB siloxane telomer (designated DST) by 1H NMR (Figure 2) indicated the presence of a mixture of isomers and oligomers.

The 1H NMR shows that attachment of the Si atom has occurred at either the aor p carbon of the DVB. Though the diagram only shows the para DVB isomers, howe, ar the meta isomers will also be present.

Reproducibility of the DST reaction The initial synthesis of DVB was attempted on a 10 g scale, this scale has been increased to 50 g, 100 g, 250 g and 500 g (twice). On each of these occasions the NMR's and GPC's of the resulting products indicated that approximately the same reaction mixture was obtained, EXAMPLE 2 Product produced in Example 1 was first cast neat as flat sheets using 3 different initiators. Results are as follows: TABLE 1 Initiator Concentration Refractive Abbe Specific Index Gravity TX 29 1.0 1.562 32 1.040 TBPO 1.0 1.567 31 1.042 AIBN 0.5 1.565 33 1.041 TX29 1.1 di-tert butyl peroxy 3,3,5 trimethyl cyclohexane CH3 CH O-O-C(CH3 3 O-0-C(CH 3 3 CH 3 I WO 95/00577 PCT/AU94/00285 12 TBPO t-butyl peroxy octoate

(CH

3 3

-C-O-O-C-CH-(CH

2 3

-CH

3 II I O C 2

H

Hard transparent samples with a Barcol hardness of about 25 were obtained.

SG of the monomer 0.959.

Therefore, shrinkage is about 7.8% after polymerisation.

The low density appears to be a useful feature.

EXAMPLES 3- 11 Example 2 was repeated under similar conditions with monomer mixes as designated in Table 2 below. The results achieved are also given in Table 2.

Satisfactory lenses were achieved in Examples 3 to 5, 8 and 9 inclusive.

Examples 6 and 7 are comparative examples only.

TABLE 2 Example Monomer DST:Monomer Clarity Hardness (Barcol) 3 4G 50:50 4 24 4 ATM20 50:50 4 38 PTMP 70:30 4 9 6 DAIP 10:90 x 0 7 CR39 10:90 x 0 8 A-HPN 50:50 4 52 9 A-9300 50:50 4 33 WO 95/00577 CrA9/08 PCT/AU94/00285

CH

3

CH

3

CH

2

=C-C-(OCH

2

CH

2 4 0-C-C=CH2 11 11 PTMP C(CH 2 0 C CH 2

CH

2 SH)4 CR39

A-HPN

WO 95/00577 PCT/AU94/00285 14 0 o °o A-9300 -O-CH2CH=CH2 C-0-CH2CH=CH 0

DAIP

Finally, it is to be understood that various other modifications and/or alterations may be made without departing from the spirit of the present invention as outlined herein.

Claims (25)

1. A telomer formed from approximately 40 to 90% by weight of the telomer, of a divinyl benzene or derivative thereof; and approximately 1 to 60% by weight of the telomer of a tetrasubstituted disiloxane or one of its higher oligomers, wherein the tetrasubstituted disiloxane is of the formula: RI R 3 H-Si-O-Si-H I I R2 R4 wherein R 1 R 2 R 3 and R 4 which may be the same or different, are selected from Salkyl groups of 1 to 10 carbon atoms.

2. A telomer according to claim 1 wherein the tetrasubstituted disiloxane is a compound of formula: Me Me I- H-Si-O-Si-H 15 Me Me

3. A telomer according to claim 2 wherein the aromatic olefin monomer is present in an amount of from 50 to 80% by weight of the telomer.

4. A telomer according to claim 3 wherein the siloxane is present in an amount of approximately 20 to 50% by weight of the telomer. An optical article formed from a telomer according to any one of the preceding claims.

6. An optical article according to claim 5 wherein the optical article has a refractive index of at least 1.55.

7. An optical article according to claim 6 wherein the refractive index is in the range of 1.55 to 1.57.

8. An article according to any one of claims 5 to 7 wherein the optical article has a density in the range of 1.0 to 1.15 g cm 3 -16-

9. An article according to any one of claims 5 to 8 wherein the article is selected from the group consisting of an opthalmic lens, a contact lens or a camera or video lens. A process for the preparation of a telomer which process includes providing: approximately 40 to 90% by weight of the telomer, of a divinyl benzene or derivative thereof; approximately 1 to 60% by weight of the telomer, of a tetrasubstituted disiloxane or one of its higher oligomers, wherein the tetrasubstituted disiloxane is of the formula: R, R 3 I H-Si-O-Si-H R2 R I wherein R 1 R 2 R 3 and R 4 which may be the same or different, are selected from alkyl groups of 1 to 10 carbon atoms; and an hydrosilation catalyst; and mixing the monomers at elevated temperature in the presence of the hydrosilation catalyst for a period sufficient to allow reaction therebetween.

11. A process according to claim 10 wherein the tetrasubstituted disiloxane is a compound of formula: Me Me H-Si-O-Si--H Me Me

12. A process according to claims 10 or 11 wherein the divinyl benzene or derivative is present in a stoichiometric excess.

13. A process according to claim 12 wherein the hydrosilation catalyst is a platinum or rhodium catalyst.

14. A process according to claim 13 wherein the catalyst is selected from the group consisting of H 2 PtCle, Pt(COD)C 2 Karstedt's catalyst and Lameroux catalyst. -17- A process according to any one of claims 10 to 14 wherein the reaction mixture is heated to a temperature of approximately 50 0 C to 900C, and the reaction is carried out for a period of approximately 12 to 24 hours.

16. A process according to any or- of claims 10 to 14 wherein the telomer includes a mixture of isomers and oligomers and the telomer is purified to remove the oligomers.

17. A cross-linkable prepolymer casting composition including a telomer formed from an effective amount of an aromatic diolefin monomer and a partially substituted di- or polysiloxane compound; and a polymerisable comonomer selected from methacrylates, acrylates, vinyls, vinyl ethers, allyls, aromatic olefins, ethers and polythiols; wherein the partially substituted di- or polysiloxane compound is a Stetrasubstituted disiloxane compound or its higher oligomers, the tetrasubstituted 15 disiloxane compound being selected from the following: R R3 H-Si-O-Si-H R2 R4 wherein R 1 R 2 R 3 and R 4 which may be the same or different, are selected from alkyl groups of 1 to 10 carbon atoms.

18. A casting composition according to claim 17 wherein the aromatic monomer is divinyl benzene or a derivative thereof.

19. A casting composition according to claim 17 or 18 wherein the tetrasubstituted disiloxane is a compound of formula: Me Me H-Si-O-Si-H Me Me A casting composition according to any one of claims 17 to 19 wherein the copolymerisable monomer is selected from one or more of aromatic olefins, polyoxyalkylene glycol diacrylates or dimethacrylates, polymerisable bisphenol monomers capable of forming a homopolymer having a refractive index of more than 1.55, urethane monomers having 2 to 6 terminal acrylic or methacrylic -18- groups, fluorene diacrylates or dimethacrylates, and thiodiacrylate or dimethacrylate monomers.

21. A casting composition according to claim 20 wherein the copolymerisable monomer includes an aromatic olefin selected from styrene, divinyl benzene (DVB), and 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5] undecane (DTU) and is present in an amount of from approximately 5 to 40% by weight, based on the total weight of the casting composition.

22. A casting composition according to claim 20, wherein the copolymerisable monomer includes a thiodiacryiate or dimethacrylates selected from bis(4- methacryloylthioethyl)sulfide (BMTES) and bis(4-methacryloylthio-phenyl)sulfide (BMTS) and is present in an amount of from approximately 5 to 40% by weight, based on the total weight of the casting composition.

23. A casting composition according to claim 20 wherein the copolymerisable monomer includes a fluorene diacrylate or dimethacrylate monomer selected from 15 a bisphenol fluorene dihydroxy acrylate or bisphenol fluorene dimethacrylate or Smixtures thereof and is present in an amount of from approximately 1 to 35% by weight, based on the total weight of the casting composition.

24. A casting composition according to claim 20 wherein the copolymerisable monomer includes a polyoxy alkylene glycol diacrylate or dimethacrylate 20 compound having ethylene oxide or propylene oxide repeating units and is present in an amount of from approximately 5% by weight to 40% by weight, based on the total weight of the casting composition. A casting composition according to claim 20 wherein the copolymerisable monomer includes a bisphenol monomer selected from dimethacrylate and diacrylate esters of bisphenol A; dimethacrylate and diacrylate'esters of 4,4'bishydroxy-ethoxy-bisphenol A, bisphenol A ethoxylated dimethacrylate and the like and is present in amounts of from approximately 5 to 40% by weight, based on the total weight of the casting composition.

26. A casting composition according to claim 20 wherein the copolymerisable monomer includes a polythiol selected from Pentaerythritol Tetrakis (3-mercapto- propionate) [PTMPI, Trimethylolpropane Tris (3-mercaptopropionate) [TTMP], 4- mercaptomethyl-3, 6-dithia-1,8-octanedithiol [MDO], Pentaerythritol Tetrakis (3- mercaptoacetate) [PTMA], Trimethylolpropane Tris (3-mercaptoacetate) [TTMA], g -3 a u) -19- 4-t-butyl-1,2- benzenedithiol, 2-mercaptoethylsulfide, 4,4'-thiodibenzenethiol and benzenedithiol and is present in amounts of from approximately 20 to 50% by weight.

27. A casting composition according to any one of claims 17 to 26 further including at least one poly-functional unsaturated cross-linking monomer selected from tri- or tetra- functional vinyl, an acrylic or methacrylic monomer such as trimethylol propane trimethacrylate, trimethylol propane triacrylate, pentaerythritol triacrylate or tetraarylate or the like.

28. A casting composition according to any one of claims 17 to 27 further including a cross-linking radical heat and/or ultraviolet cationic or radical initiator.

29. A polymeric article formed from the casting composition according to any one of claims 17 to 28.

30. A telomer substantially as hereinbefore described with reference to any one of the examples. 15 31. A polymer article substantially as hereinbefore described with reference to any one of the examples. S