WO2012143695A2

WO2012143695A2 - Polymers for contact lenses

Info

Publication number: WO2012143695A2
Application number: PCT/GB2012/000391
Authority: WO
Inventors: Abdul Rashid; Neil Graham; Roderick BOWERS
Original assignee: Ocutec Limited
Priority date: 2011-04-21
Filing date: 2012-04-19
Publication date: 2012-10-26
Also published as: GB201106774D0; WO2012143695A3

Abstract

The present invention relates to a polyurethane polymer compositions comprising polyethylene glycol) monoalkyl ether. The compositions may be either crosslinked (suitable for reaction cast moulding) or thermoplastic (suitable for injection and/or compression moulding). Either of the above varieties may contain carbinol terminated polydimethyl siloxane components. There is also provided a process for preparing such a polyurethane polymer composition. The invention also relates to a process for preparing a polyurethane xerogel in the form of a molded article, said process comprising the steps of. i. preparing a mixture of a. at least one poly(ethylene glycol) and/or at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having a functionality of greater than 2, b. at least one di-or poly-isocyanate, c. optionally at least one OH-terminated chain extender, d. optionally an additional compound comprising at least one hydroxyl group and at least one primary or secondary amine group, and e. at least one poly(ethylene glycol) monoalkyl ether; ii. dispensing the reaction mixture formed in step i) into a mold; iii. allowing the reaction mixture to react and cure; iv. removing the molded article from the mold; and v. hydrating the molded article. Optionally the process may comprise the step of hydrating the molded article to form a hydrated article.

Description

Polymers for Contact Leases

The present invention relates to poly(ethylene glycol), PEG based polyurethane polymers that have applications in the field of contact lens technology. In particular, the present invention relates to PEG based polyurethane polymers comprising poly (ethylene glycol) monoalkyl ether. The invention also relates to a process for preparing such materials, which can desirably and preferably be prepared in the complete absence of solvents.

Background to the Invention

Soft contact lenses offer a viable alternative to spectacles for the correction of visual defects such as myopia, hypermetropia and astigmatism. Early hydrogel lenses are derived almost exclusively from hydrophilic monomers such as 2-hydroxyethyl methacrylate, (HEMA). Although these lenses provided some comfort, they did not provide sufficient oxygen permeability to prevent problems associated with corneal hypoxia. Attempts to address this problem included copolymerising HEMA with hydrophilic monomers such as methacrylic acid and N-vinyl pyrrolidone. Although these polymers increased the level of oxygen permeability, the incorporation of these comonomers also leads to problems such as protein and lipid deposition, corneal desiccation, staining and lens dehydration.

More recently, a new generation of polymers has been developed to further increase the permeation of oxygen. These materials are based on the copolymerisation of silicone methacrylates with hydrophilic comonomers. The lenses produced from these materials were originally designed for extended wear, though daily wear products also exist now. Although successful in further increasing the oxygen permeability, these new materials still suffer from limitations such as lipid binding and dryness, all of which decrease lens on eye comfort.

There is therefore still a need for new contact lens polymers, which offer sufficient oxygen permeability for normal corneal metabolism during daily wear and for extended wear, and which provide a high level of comfort throughout the day. In particular, lenses having a high associated hydrophilicity are sought. In addition, contact lenses having a surface with good tear film stability promotes comfort, even for extended periods of wear and decreases the risk of infection.

One class of polymers which holds considerable promise for novel contact lens materials are PEG based polyurethanes.

Urethane chemistries have also been widely investigated in the field of biomedical devices. For example, US 3,786,034 discloses hard, hydrophilic polyurethane materials formed from reacting a specific polyol with a polyfunctional isocyanate. US 3,821,186 teaches similar such materials. Likewise, US 4,136,250 teaches a polymer formed by reacting a high molecular weight polydimethyl siloxane diol with 2 mole equivalents of isophorone di-isocyanate and then reacting with excess hydroxyl-containing monomers. Further urethane copolymers are disclosed in US 4,454,309 and US 4,359,553. US 6,930,196 discloses polyurethane hydrogel contact lenses prepared from prepolymers made by reacting (a) at least one multifunctional compound; (b) at least one di- isocyanate; and (c) at least one diol. The prepolymers so formed are then reacted with excess water to form a hydrogel polymer suitable for use as a contact lens. US 4,644,033 discloses a polyurethane hydrogel formed from the reaction of a polyoxyethylene and a polyfunctional isocyanate in a non-aqueous solvent. The materials can be molded into contact lenses.

US 5,932,200 discloses polyurethane formed from reacting a diol component and an organic di-isocyanate with critical selection of the amount of water in the reaction mixture and the diol component. The polyurethane is in the form of a gel that has applications in bum/wound care dressings and as surgical implants.

US 4,885,966 and US 5,175,229 disclose hydrophilic polymeric soft contact lenses prepared from prepolymers that are isocyanate-capped oxyethylene-based diols or polyols having a molecular weight of about 7000 to 30,000, wherein essentially all of the OH groups are capped with isocyanate. The prepolymers are hydrated to form polyurea- polyurethane polymers that are characterised by having a non-ionic surface which is resistant to non-specific protein adsorption.

It is well known anecdotally that poly(ethylene glycol) based polyurethane copolymers are associated with poor storage stability. Known polyurethane polymers based on poly(ethylene glycol), generally do not consistently maintain their properties after 6 months storage. In particular, known PEG based polyurethane polymers have a tendency not to retain their shape upon storage. In addition, stress cracks may appear on known PEG based polyurethane polymers, in particular following hydration.

The storage stability of polymers forming medical devices is clearly paramount. The properties of materials used in the manufacture of medical devices must be maintained upon extended periods of storage. The properties of such materials must be predictable and consistent. Materials having low and/or inconsistent storage stability are not suitable for use in the manufacture of medical devices such as contact lenses, regardless of any other promising properties.

Contact lenses with a high modulus are generally associated with fairly poor wearer comfort and increased risk of infection. However, reducing the modulus of a polyurethane polymer is generally associated with adversely affecting the structural stability of an article made from it.

In addition, current contact lenses are often associated with a low biocompatibility, such lenses on eye trigger foreign body response where biomolecules such as protein, lipids, immunoglobulins and complement proteins bind at the surface of the lens. This in particular reduces the tear film stability which is not desirable. This causes the eye to feel dry, and for the contact lenses to feel uncomfortable after prolonged periods of wear. The more hydrophobic the surface of a contact lens, the greater the likelihood of the tear component being adsorbed and hence higher the chances of forming the dry spots. This also increases discomfort and the risk of infection. In addition, the higher the contact angle and the higher the frictional properties associated with the surface of a contact lens, the less comfortable the lens and the greater the risk of associated eye infection. The present invention seeks to provide new polyurethane-based materials that are suitable for use in the contact lens industry. Ideally, polyurethane-based materials of the invention exhibit exemplary physical properties, in particular, in terms of surface hydrophilicity, associated contact angle, frictional properties and modulus. In addition, the polymers of the present invention generally is expected to have exceptionally high biocompatibility, in particular high tear film stability and very low limbal hyperaemia. The polymeric material of the present invention generally retains its shape well upon storage, having good structural stability, for instance stress cracks do not form on the articles made from polymers of the present invention upon storage.

Statement of Invention

According to a first aspect of the present invention there is provided a polyurethane polymer composition comprising polyfethylene glycol) monoalkyl ether having the structure shown below:

Alkyl-fOC¾CH₂]_n-X

Where X represents COOH, OH, NH₂ or NHCH₃. Typically X represents OH or NH₂, preferably X represents OH.

Alkyl represents an optionally substituted straight or branched chain alkyl group having a carbon backbone of 1 to 10 carbon atoms. Typically the alkyl group is a straight chain alkyl group having a carbon backbone of 1 to 5 carbon atoms. Preferably the alkyl group is a methyl, ethyl, propyl or butyl group.

N is 1 to 50, more preferably 5 to 20 even more preferably 10 to 20.

The use of poly(ethylene glycol) monoalkyl ether compounds is applicable in the manufacture of cross-linked polyurethane polymer compositions as well as thermoplastic polyurethane compositions.

Preferably the polyethylene glycol) monoalkyl ether is a polyethylene glycol) monomethyl ether compound (PEG ME) having the structure shown below: CH₃-[OCH₂CH₂]„-X

According to a second aspect of the present invention there is provided a polyurethane polymer composition prepared by reacting a mixture comprising:

a) at least one poly(ethylene glycol) and/or at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2,

b) at least one di- or poly-isocyanate;

c) optionally at least one OH-terminated chain extender;

d) optionally an additional compound comprising at least one hydroxyl group and at least one primary or secondary amine group; and

e) at least one poly(ethylene glycol) monoalkyl ether.

Whilst the applicant does not wish to be bound by theory, it is believed that the incorporation of poly(ethylene glycol) monoalkyl ether (in particular PEG ME) in the reactant mixture acts to alter the properties of the surface of the resultant polymeric composition. The terminal hydroxyl group of the poly(ethylene glycol) monoalkyl ether reacts with the NCO group of the di- or poly-isocyanate, to form a urethane linkage. The terminal CH3O group of the poly(ethylene glycol) monoalkyl ether does not take part in the polymerization reaction. As a result, only one end of the poly(ethylene glycol) monoalkyl ether chain is bonded into the resultant polymeric composition. The poly(ethylene glycol) monoalkyl ether chain branches out from the backbone of the polymeric composition, typically as a pendant chain. The provision of such pendant chains has been found to alter the surface properties of the polymeric composition.

Polymeric material comprising poly(ethylene glycol) dialkyl ether was disclosed in a PCT application filed claiming priority from GB 0919411.9. Poly(ethylene glycol) dialkyl ether has two terminal CH₃O groups which do not react with NCO groups of di- or poly- isocyanates. As such the poly(ethylene glycol) dialkyl ether does not take part in the polymerization reaction. In contrast, the poly(ethylene glycol) monoalkyl ether used in the preparation of the materials of the present invention acts as a reactant due to the presence of a terminal reactable group (OH group). Thus pendant chains are available that are capable of interacting with water and possibly forming stable hydrates. These structures over a contact lens surface would more or less behave as a dynamic water layer sweeping across the whole lens surface. This will eliminate or significantly reduce foreign body response. It would lower the contact angle of the lens surface that will result much better tear film stability and hence the lens will be more comfortable to wear. The comfort will sustain on prolonged daily wear as a result of the pendent chains ("water layer") that are chemically linked to the back bone of the polymer matrix as opposed to the surfactant solutions used by contact lens industry, to enhance comfort, in the lens storage liquid. The effect of these surfactants deteriorates over the period of time and eventually the surfactants are washed away resulting in lack of comfort to the wearer.

Where the polymeric composition is formed using reaction cast molding (RCM) techniques, the poly(ethy!ene glycol) monoaikyi ether (in particular PEG ME) is added to the reactant mixture prior to polymerization. This results in a polymeric composition having a higher surface hydrophilicity and lower associated frictional properties than an equivalent polymeric composition formed without the addition of poly(ethyIene glycol) monoaikyi ether. In addition, the biocompatibility of the polymeric composition of the present invention is anticipated to be high, in particular, the tear film stability associated with the polymeric composition of the present invention is anticipated to be high. Where the polymeric composition is a thermoplastic polymer, the poly(ethylene glycol) monoaikyi ether is added after polymerization has occurred. In such applications, as the poly(ethylene glycol) monoaikyi ether is associated with a relatively low molecular weight and a relatively short chain length, it is expected to be more mobile within the polymer matrix and reduce the modulus of the resultant polymeric composition. Articles made from such materials, for example articles such as contact lenses, are anticipated to have improved hydrophilicity and biocompatibility at their surface. In addition, the associated contact angle and friction of the resultant surface of the article is expected to reduce as a result of reorientation/expression of the PEG monoaikyi ether at the surface of the article particularly in the hydrated state because of the terminal hydrophilic groups e.g OH group. These effects are also expected to be longer lasting because PEG monoaikyi ether molecules would be entangled and remain held within the polymer matrix, the extent of which would depend on their chain length. The incorporation of polyethylene glycol) monoalkyl ether in the reactant mixture increases the hydrophilicity of the resultant composition. In addition, the hydrophilicity of medical devices formed from the polymeric material of the present invention would result in greater uniformity across its surface. Medical devices formed from materials that do not contain PEG monoalkyl ether are associated with high irritation and low comfort levels. In particular, contact lenses formed from such materials have low tear film stability, and low associated tear film break up times as stable tear films do not form easily over such surfaces possibly due deposition of tear film components. Prolonged use of such contact lenses results in dry eyes and high irritation levels.

In addition, the incorporation of poly(ethylene glycol) monoalkyl ether in the reactant mixture reduces the contact angle of the surface of the polymeric material. This is believed to be a further effect of the provision of pendant chains extending from the polymer backbone of the cross-linked polymeric material due to the incorporation of poly(ethylene glycol) monoalkyl ether in the reactant mixture. Aqueous films can form more easily over the surface of a material having a reduced contact angle, and once formed, aqueous films will be maintained for longer on such materials. As such, the polymeric material of the present invention is associated with anticipated high biocompatibility when used in or on the human or animal body. Such use is also expected to afford high comfort levels and low risk of infection. In particular it is anticipated that the material of the present invention is likely to afford a contact lens with high tear film stability and hence a better comfort will be experienced by the wearer. According to one embodiment, the materials of the present invention have an associated contact angle of 25° or less, typically 20° or less, suitably around 15°.

The incorporation of poly(ethylene glycol) monoalkyl ether in the reactant mixture also expected to reduces the frictional properties of the surface of the polymeric material, or an article formed therefrom. According to a third aspect of the present invention there is provided a process for preparing a polyurethane hydrogel, said process comprising:

i) preparing a mixture of a) at least one polyethylene glycol) and/or at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2, b) at least one di- or poly-isocyanate,

c) optionally at least one OH-terminated chain extender,

e) at least one poly(ethylene glycol) monoalkyl ether;

ii) allowing the mixture formed in step i) to react appropriately to form a cross- linked polyurethane xerogei; and

iii) hydrating the xerogei using an aqueous medium to form a hydrogel.

A fourth aspect of the invention relates to a polymer obtainable by the above described process.

A fifth aspect of the present invention relates to a process for preparing a contact lens comprising the steps of:

i) preparing a mixture of

a) at least one poly(ethylene glycol) and/or at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2, b) at least one di- or poly-isocyanate,

c) optionally at least one OH-terminated chain extender,

e) at least one poly(ethylene glycol) monoalkyl ether;

ii) dispensing the reaction mixture formed in step i) into a contact lens mold; iii) allowing the reaction mixture to react and cure (generally with the assistance with energy, in particular thermal energy or radiation);

iv) removing the contact lens from the mold; and

v) hydrating the contact lens. A sixth aspect of the present invention relates to an article of manufacture comprising a polymer as described above.

A seventh aspect of the present invention relates to the use of a polymer as described above in the preparation of a contact lens.

Detailed Description

Definitions

Poly(ethylene glycol) may also be referred to as polyoxyethylene, or hydroxyl terminated polyoxyethylene.

The "functionality" of a compound is used to refer to the number of functional groups present in the compound that are capable of reacting in the system. The "functionality" of the poly-isocyanate compound refers to the number of NCO groups present in the poly- isocyanate compound.

'Tolyfunctiona ' is generally used to refer to a molecule, or a mixture of molecules having more than 2 functional groups that are capable of reacting in the system. "Difunctional" is generally used to refer to a molecule or a mixture of molecules having 2 functional groups that are capable of reacting in the system.

"D " is a measure of the oxygen permeability of a material provided in Barrer units where 1 Barrer = 10 ^"u cm².ml.mmHg.

The term "hydrogel" is used herein to refer to a polymer comprising 10 wt% or more water. A hydrogel in an aqueous medium will absorb water and retain its original dry shape but it will be enlarged. It will not dissolve in water to form a fluid solution unless it is significantly degraded.

The term "xerogel" is used herein to refer to a polymeric material which may form a hydrogel upon contact with sufficient water. Generally a xerogel is dry and comprises less than 5 wt% water. The term "substantially anhydrous" is used herein to refer to conditions in which the amount of water is sufficiently low so as to produce a polyurethane backbone that is substantially free from urea groups. Preferably the amount of water in the reactant mixture is less than about 0.3 wt. %, more preferably less than about 0.1 wt. %, even more preferably less than about 0.05 wt. %.

"Polyol" is referred to herein as a compound having more than 2 available hydroxyl groups. Polyols generally have a molecular weight less than or equal to 1000.

"Macropolyol" is generally used to refer to a compound having more than 2 available hydroxyl groups linked to polyoxyethylene and/or polyoxypropylene homo or copolymer, or a mixture of such copolymers, and generally has a molecular weight greater than 1000. "Diol" is referred to herein as a compound having 2 available hydroxyl groups.

The term "carbinol" is used to refer to a hydroxyl functional group attached to a carbon atom. The carbon atom may be attached to a carbon atom (in particular a carbon atom forming part of a hydrocarbon group), or a non-carbon atom including Si, N and O.

The term "small alkyl group" refers to an alkyl group having a carbon backbone of 1 to 6 carbon atoms, typically 1 to 4 carbon atoms.

The term "reaction cast molding" (RCM) is used to refer to molding techniques which involve the steps of mixing the reactants together, dispensing the reactant mixture into a mold and allowing the reactant mixture to react and cure (generally with the assistance with energy, in particular thermal energy or radiation).

An RCM polymer composition is a composition which is cross-linked and can be used to form articles, in particular contact lenses, which can be steam sterilised.

The term "injection molding" (IM) is used to refer to molding techniques. An IM polymer composition is a thermoplastic composition which is generally linear. Such polymers can be used to manufacture articles, in particular contact lenses through injection molding techniques. The term "contact angle" is used to refer to the angle a fluid makes with the surface of the material, for instance the angle the sides of a droplet of water make when formed on the surface of the material. Contact angle can also be measured by measuring the angle the sides of an air bubble make when formed on the surface of the material underwater. "Block copolymer" is used to refer to a polymer containing sequences of each of two (or more) monomeric species in which one or more of the monomers polymerise to units of the resulting polymer chains which comprise sequences of a number of identical monomer units. The individual sequences can be short or long. "Graft copolymer" is a polymer comprising a main chain and one or more side chains, generally said side chains being structurally distinct from the main chain.

The term "chain extender" is generally used to refer to a low molecular weight difunctional monomer (typically but not exclusively of the order of lOOOgmol-1).

Polymer Composition

As noted above, the present invention provides a polyurethane polymer composition comprising poly(ethylene glycol) monoalkyl ether. Typically the alkyl group of the poly(ethylene glycol) monoalkyl ether is a small alkyl group comprising no more than 1 to 6 carbon atoms, suitably 1 to 4 carbon atoms. According to one embodiment, the poly(ethylene glycol) monoalkyl ether is poly(ethylene glycol) monomethyl ether (PEG ME) or poly(ethylene glycol) monobutyl ether (PEG BE).

The polyurethane composition is generally prepared by reacting a mixture comprising: a) at least one poly(ethylene glycol) and/or at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2, b) at least one di- or poly-isocyanate; c) optionally at least one OH-terminated chain extender;

e) at least one poly(ethylene glycol) monoalkyl ether.

The polymer compositions include varieties that are cross-linked, linear thermoplastic and optionally may also contain units formed as a result of reacting carbinol terminated PD S which may constitute as polymer backbone andVor pendant chains. According to one embodiment, the polyurethane composition is cross-linked and is prepared by reacting a mixture comprising:

a) at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2, wherein the polyol, and/or macropolyol is generally of formula I,

wherein at least three of Xi, X₂, X₃, X4 and X₅ are each independently an OH- terminated group (typically an OH-terminated polyoxyalkylene chain, preferably OH-terminated polyoxyethylene or polyoxypropylene chains), and the remainder of X], X2, X3, X4 and X5 are each independently H or absent, and Z is a central linking unit;

b) at least one di- or poly-isocyanate;

c) optionally at least one poly(ethylene glycol);

d) optionally at least ohe OH-terminated chain extender;

e) optionally an additional compound comprising at least one hydroxyl group and at least one primary or secondary amine group; and

f) at least one poly(ethylene glycol) monoalkyl ether. Where at least one poly(ethylene glycol) is included in the reaction mixture, the ratio of isocyanate to hydroxyl groups in the reaction mixture NCO/OH is generally within the range 0.75 to 1.75 and is typically close to unity.

According to one embodiment, the reaction mixture used to form the cross-linked polyurethane compostion comprises at least one polyol and/or at least one macropolyol of formula I, at least one di- or poly-isocyanate and optionally at least one OH-tenninated chain extender. In such embodiments the reaction mixture does not generally comprise a polyoxyalkylene diol such as PPG or polyoxyethylene diol.

According to one embodiment, the reaction mixture used to form the cross-linked polyurethane composition comprises at least one polyoxyalkylene diol (typically PPG or a copolymer of ethylene oxide and propylene oxide). In such embodiments, the reaction mixture generally also comprises at least one poly(ethylene glycol).

According to a further embodiment, the polyurethane composition is thermoplastic and is prepared by reacting a mixture comprising:

(a) at least one poly(ethylene glycol);

(b) at least one di-isocyanate;

(c) at least one OH-terminated chain extender; and

(d) at least one poly(ethylene glycol) monoalkyl ether.

In general for such embodiments, the functionality of all reactants is two or less. Typically the ration of isocayante to hydroxyl groups in the reaction mixture ( CO:OH) is 0.75 to 1.75, preferably around 1.

Advantageously, where the polyurethane composition is thermoplastic, components (a), (b) and (c) are reacted together, and (d) is added to the product of this reaction.

The incorporation of poly(ethylene glycol) monoalkyl ether into the reaction mixture, or to the product of the reaction of components (a), (b) and (c), increases the hydrophilicity and lowers the contact angle of the surface of articles formed from the resultant composition. Body fluids (such as blood, urine, tears and sweat) are tolerated more easily by surfaces having a low contact angle. Aqueous films can form more easily, and once formed are maintained more easily on such surfaces. The resultant polyurethane composition is thus particularly suited to applications requiring compatibility in or on the human or animal body as a higher hydrophilicity anaVor a low contact angle would increase comfort of a contact lens and reduce the risk of infection, in particular to the eye.

The polymeric composition of the present invention is generally used to form a molded article, and the molding of the polymeric composition introduces stresses. The stresses introduced are particularly marked where the article is molded through injection molding processes. In particular, stress cracking often appears upon hydration of known PEG based polyurethane polymers. The compounding of poly(ethylene glycol) monoalkyl ether into thermoplastic polymers, i.e. following polymerisation but prior to the molding process appears to greatly reduce or eliminate the stresses introduced through the molding cycle. The stresses are dissipated. This greatly increases the structural integrity of molded articles formed from the polymeric material of the present invention. In particular, the shape of the molded articles generally doesn't change upon storage, and stress cracks are not formed upon hydration of the molded article. As noted above, the polyurethane composition of the present invention may be cross- linked, and may be molded using reaction cast molding ( CM). As only one end of the poly(ethyIene glycol) monoalkyl ether compound is capped with an alkyl chain, this compound takes part in the polymerization reaction resulting in the formation of a graft copolymer in addition to the block copolymer formed when poly(ethylene glycol) reacts with other reactants. As such, the poly(ethylene glycol) monoalkyl ether compound may act as a reactant, and take part in the polymerization reaction. For cross-linked RCM compositions it is advantageous for the reactant mixture to be dispensed into the mold at or around ambient temperature, and the incorporation of a poly(ethylene glycol) monoalkyl ether compound may reduce the melting point of the reactant mixture. Advantageously in one embodiment, the reactant mixture may be dispensed into the mold from around 20 to around 40 °C. Generally, the incorporation of a poly(ethylene glycol) monoalkyl ether compound increases the water content of the resultant polyurethane composition upon hydration. It may also act as a placticiser which may reduce the glass transition temperature of the material. This may increase the oxygen permeability of the poiyurethane composition.

According to a further aspect of the present invention there is provided a contact lens formed from the polymer composition. The properties of such a contact lens are extremely promising. The surface characteristics of the lenses are generally significantly improved than surface of the lenses produced from polyurethane polymers without a poly(ethylene glycol) monoalkyl ether compound. The lenses of the present invention are generally associated with a contact angle of 30° or less, typically 20° or less, suitably around 15°. This is far lower than -currently marketed contact lenses which typically have an associated contact angle of around 70°. The biocompatibility is increased, in particular the tear film break up time is increased and the tear film stability of the lenses of the present invention is significantly higher than lenses produced from polyurethane polymers formed without a poly(ethylene glycol) monoalkyl ether compound. In addition, the frictional properties of the lenses of the present invention is significantly lower than those formed without a poly(ethylene glycol) monoalkyl ether compound. The tensile properties of the lenses are good, with the modulus typically 0.4 to 0.6 MPa, generally around 0.5 MPa. Such a relatively low modulus is an attribute commonly associated with improved comfort and decreased risk of infection. Contact lenses formed from the polymer of the present invention generally have a water content of 50 to 70 wt %, typically around 60 wt %, and this is considered desirable in terms of the industry standard. The oxygen permeability of contact lenses formed from the polymer of the present invention is generally higher than most standard methacrylate hydrogel contact lenses (other than silicon-containing methacrylates). The D value of the polymer of the present invention is typically 20 to 45 Baner, generally 25 to 45 Baner, suitably around 40 to 45 Barrer.

Poly(ethylene glycol) monoalkyl ether Compound

The polyethylene glycol) monoalkyl ether compound has the structure shown below: Alkyl-[OCH₂C¾]_n-X

Where X represents COOH, OH, NH₂ or NHC¾. Typically X represents OH or NH₂, preferably X represents OH.

Alkyl represents an optionally substituted straight or branched chain alkyl group having a carbon backbone of 1 to 10 carbon atoms. Typically the alkyl group is a straight chain alkyl group having a carbon backbone of I to 5 carbon atoms. Preferably the alkyl group is a methyl, ethyl, propyl or butyl group.

N is 1 to 50 more preferably 5 to 20 even more preferably 10 to 20.

Suitably the poly(ethylene glycol) monoalkyl ether is a poly(ethylene glycol) monobutyl ether (PEG BE) compound or a poly(ethylene glycol) monomethyl ether compound (PEG ME). Preferably the poly(ethylene glycol) monoalkyl ether is a PEG ME compound having the structure shown below:

CH₃-[ CH₂CH₂]n-X Due to the polyethylene glycol) backbone of the poly(ethylene glycol) monoalkyl ether, this compound is very compatible with the poly(ethylene glycol) or polyol compounds used to form the polyurethane composition of the present invention. Due to the presence of one terminal hydroxyl group, the poly(efhylene glycol) monoalkyl ether compound participates in the polymerization reaction to form a graft copolymer rather than a block copolymer. The terminal reactive group e.g. hydroxyl or amine group reacts with the NCO groups of the di- or poly-isocyanate to form urethane or urea groups respectively during the polymerization reaction. The monoalkyl ether participates in the polymerization reaction but only as a side group. It does not take part in a main chain growth reaction which can only occur with difunctional molecules. The terminal alkyl group of the poly(ethylene glycol) monoalkyl ether compound does not react in the polymerization reaction and this end of the poly(ethylene glycol) monoalkyl ether does not participate in the polymerization reaction. The chain length of the graft or the pendant chain can be varied by varying the molecular weight of the polyethylene glycol) mono alkyl ether.

When the PEG monoaikyl ether is added to the composition intended to form cross-linked structures it acts as either a chain tennination agent or as a grafting agent. When the PEG monoaikyl ether compound is added prior to polymerisation into the reactant mixture, the PEG monoaikyl ether compound may act as a chain termination unit (chain terrninator) so it provides many relatively long terminal chains. These long PEG chains are hydrophilic and their formation increases the physical properties including biocompatibility and hydrophilicity of the surface of articles formed from the resultant polymer composition. These chains are likely to express at the surface of an article formed from the polymer compositions rendering it more biofriendly.

The poly( ethylene glycol) monoaikyl ether acts primarily as a surface enhancer additive of the resultant polymeric composition. In particular; the incorporation of a poly(ethylene glycol) monoaikyl ether compound both as a coreactant (in RCM) or as an additive (in thermoplastic polymer material) reduces the contact angle and increases the hydrophilicity of the surface of the article made from the resultant polymeric composition. In addition, or because of these features, the compatibility of the surface of the polymeric composition (eg a contact lens) with body fluids is increased. In particular, the stability of aqueous films such as tear films formed at the surface of, for example a contact lens is increased and the break up time at the lens surface is increased. This would significantly enhance the on eye comfort. However, the incorporation of this compound also affects other properties of the polyurethane composition. The poly(ethylene glycol) monoaikyl ether may act as a humectant, lubricant, process aid, viscosity reducer, compatibility enhancer, modulus modifier, placticizer and/or polymer matrix structure modifier. Advantageously, the poly(ethylene glycol) monoaikyl ether may also increase the oxygen permeability of the polyurethane composition.

The concentration of the polyethylene glycol) monoaikyl ether in a given composition can be adjusted to obtain the required properties of the material (in particular the required surface properties of the material), resulting in a medical device which would increase biocompatibility. In particular such a composition would provide a contact lens which is more comfortable to wear. Preferably, the poly(ethylene glycol) monoalkyl ether is present in an amount of about 0.1 to about 10 wt %, more preferably from about 0.1 to about 6 wt %, more preferably still, about 0.1 to about 2 wt % of the ceactants.

Polyethylene glycol) monoalkyl ether compounds of various molecular weights are suitable for use in the present invention. Typically the molecular weight of the polyethylene glycol) monoalkyl ether is 100 to 5000 including e.g. 250, 500, 1000, 2000. Suitably the molecular weight is 100 to 1000, more suitably 200 to 400, preferably around 250. Advantageously, the incorporation of po!y(ethylene glycol) monoalkyl ether compounds into the polymer compositions of the invention results in a polyurethane composition having an improved hydrophilicity. Typically the hydrophilicity of the surface of a polyurethane composition is increased. According to one embodiment, the incorporation of poly(ethylene glycol) monoalkyl ether compounds into the polymer compositions of the invention results in a polyurethane composition having a reduced contact angle. Typically the contact angle of the surface of a polyurethane composition is reduced by at least 20% upon incorporation of a poly(ethylene glycol) monoalkyl ether compound compared to an equivalent composition absent a poly(ethylene glycol) monoalkyl ether compound, suitably the contact angle of the surface is reduced by at least 30%, more suitably by at least 50% or more.

Suitably the contact angle of the surface of the polymer composition of the present invention is 40° or less, typically 30° or less, generally 20° or less, preferably 15° or less. Advantageously the contact angle o the surface of the contact lens made from the polymer composition of the present invention is around 15° in one embodiment. ^; According to one embodiment, the incorporation of poly(ethylene glycol) monoalkyl ether compounds into the polymer compositions of the invention results in a polyurethane composition having an increased biocompatibility, including an increased stability to aqueous films, in particular films of tears, blood, sweat or urine. This generally results in an increased tear film stability on eye and the break up time for a tear film which forms on articles (e.g. contact lenses) formed from the polymer composition of the present invention is greater than articles formed from polyurethane compositions without a poly(ethylene glycol) monoalkyl ether compound. The on eye tear film duration varies from individual to individual. The tear film break up time is generally 6 to 12 seconds with most polyurethane compositions. However, this could be increased by up to one minute for some compositions of the present invention. Typically the on eye tear film break up time associated with the surface of a polyurethane composition is increased by at least 50% upon incorporation of a poly(ethylene glycol) monoalkyl ether compound compared to an equivalent composition without a poly(ethylene glycol) monoalkyl ether compound, suitably the aqueous film break up time associated with the surface is increased by at least 100%, more suitably by at least 200% or more.

The incorporation of poly(ethylene glycol) monoalkyl ether compounds into the polymer compositions of the invention results in a polyurethane composition having reduced frictional properties. In particular, the frictional properties associated with the surface of a polyurethane composition is decreased by at least 10% upon incorporation of a poly(ethylene glycol) monoalkyl ether compound compared to an equivalent composition without a poly(ethylene glycol) monoalkyl ether compound, suitably the frictional properties associated with the surface is decreased by at least 15%, more suitably by at least 20%.

The incorporation of a poly(ethylene glycol) monoalkyl ether compound in the thermoplastic polymer reduces the modulus thereof, whilst optimising the storage stability of an article formed from the thermoplastic polymeric compound. The polyethylene glycol) monoalkyl ether compound may be incorporated into the cross- linked composition prior to or after polymerisation. The poly(ethylene glycol) monoalkyl ether compound may be added to the thermoplastic polymer post polymerisation. In particular, the modulus associated with a polyurethane composition is decreased by at feast 10% upon incorporation of a poly(ethylene glycol) monoalkyl ether compound compared to an equivalent composition without a poly(ethylene glycol) monoalkyl ether compound, suitably the modulus is decreased by at least 15%, more suitably by at least 20%, typically more than 20%. Advantageously, the polyurethane composition of the present invention has a modulus associated with good comfort levels upon prolonged contact with the human body, as well as good storage stability. Preferably, the modulus of lenses prepared from the polymer compositions of the invention is from about 0.1 to about 0.8 MPa, more preferably, about 0.3 to about 0.5 MPa.

Preferably the polyurethane composition of the present invention has a relatively high oxygen permeability compared to the methacrylate based hydrogel compositions used to form contact lenses. Generally the polyurethane composition of the present invention has an associated DK value of 20 to 40 Barrer, suitably 30 to 35 Barrer, typically around 30 Barrer.

Generally the reaction mixture is liquid at ambient temperature and may be dispensed at ambient temperature (20 to 30 °C) or slightly higher (up to 40 °C). However for some embodiments of the present invention, the material is dispensed at an elevated temperature, for example 70 to 90 °C.

The potential water content of the polymeric compositions of the present invention following hydration is also in the suitable range required for a medical device such as contact lenses, and is moderately increased compared to medical devices formed from equivalent polymeric compositions without a polyethylene glycol) monoalkyl ether. The water content of a polyurethane composition of the present invention is typically 10 to about 90 weight % following hydration, more preferably, from about 20 to about 80 weight %, more preferably, from about 25 to about 75 weight %, even more preferably, from about 30 to about 70 weight %, more preferably still, ftom about 40 to about 70 weight %. Generally, following hydration, the polyurethane composition of the present invention comprises from about 50 to about 70 weight % water. The equilibrium water content of a polyurethane composition is an important material attribute and plays a key role in determining the bulk, mechanical and physical properties of the material. Water provides the medium to transmit oxygen. Where the polyurethane composition is in the form of a contact lens, the water content and the modulus govern important on-eye properties of the lens.

Polyethylene glycol)

The present invention may involve the use of at least one poly(ethylene glycol) (PEG). Poly(ethylene glycol)s of varying molecular weights are commercially available and can be used to afford the polymeric materials of the present invention. Blends of two or more different molecular weight polyCethylene glycols can also be used.

Preferably, the poly(ethylene glycol) compound has a molecular weight of from about 500 to about 100,000, more preferably from about 1000 to about 50,000, even more preferably from about 3000 to about 10,000, more preferably still from about 5000 to about 8000.

In one highly preferred embodiment, the polyethylene glycol) is PEG 6000. In another highly preferred embodiment, the PEG is selected from PEG 6088, PEG 3350 and PEG 1000.

According to one embodiment, more than one PEG compound may be used. Typically, the composition may comprise one low molecular weight PEG compound and one high molecular weight PEG compound. The low molecular weight PEG compound may have a molecular weight of less than 1500, generally less than 1000, suitably 800 to 900. The high molecular weight PEG compound may have a molecular weight of 2000 to 20000, suitably from about 5000 to about 8000. Preferably, the poly(ethylene glycol) is used in an amount of from about 20 to about 80 wt % of the reactants, more preferably from about 30 to about 70 wt %, more preferabl from about 35 to about 60 wt %, more preferably still, from about 40 to about 60 wt % of the reactants. Generally where the reaction mixture comprises at least one polyethylene glycol) compound, a di-isocyanate is used as the coreactant. Where the reaction mixture comprises at least one poly(ethylene glycol) compound, the ration of isocyanate to hydroxyl groups in the reaction mixture (NCO/OH) is generally between about 0.75 to about 1.75, typically close to unity.

Polyol / Macropolyol

The reactant mixture used to form the composition of the present invention comprises at least one polyol or macropolyol having a functionality of more than two, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2.

According to some embodiments of the present invention, the polyol/macropolyol may comprise tertiary hydrogen atoms (in particular, where the polyol is glycerol). Generally, where the polyols of the present invention comprise tertiary hydrogen atoms, they have only one tertiary hydrogen atom per molecule.

The hydroxyl groups of the polyol macropolyol and the PEG compound react with the NCO groups of the di- or poly-isocyanate to form a polymer matrix. The proportion/concentration of the polyol macropolyol used in the reactant mixture affects the resultant material properties of the polyurethane composition formed. In particular, in cases where there is enough isocyanate to react with all of the hydroxyl groups, the greater the concentration of any given polyol/macropolyol in the reactant mixture, the greater the degree of cross-linking, leading to an increased modulus of the polyurethane composition formed. The amount of polyol/macropolyol used depends on the extent of cross-link density required, and the resultant tensile properties required. Increasing the concentration of polyol/macropolyol increases the modulus and reduces the water content of the resultant composition. Generally the reaction mixture of the present invention comprises 10 wt% or less polyol or macropolyol, typically 7 wt % or less, suitably 5 wt % or less. Generally the ratio of isocyanate functional groups to hydroxyl functional groups (NCO/OH) is within the range from about 0.75 to about 1.75, typically 0.9 to 1.1, suitably about 1. The polyol or macropolyol, di- or poly-isocyanate, polyoxyalkylene diol, PEG (and optionally the additional compound) react to eventually form a polymer matrix.

According to one embodiment, the polyol has the structure of formula I:

1

wherein at least three of Xi, X₂, X₃, X4 and X_s each independently comprise a terminal hydroxyl group with the remainder of X_1; X₂, X₃, X4 and X₅ being independently H or absent, and Z is a central linking unit.

The polyol is a moiety comprising initiating hydroxyl groups attached to a central linking group, which is generally essentially hydrocarbon. These hydroxyl groups can each be used to polymerise a polyalkyleneoxide chain terminated by a hydroxyl group. The finally produced central linking moiety Z is generally devoid of active hydrogen atoms, that is hydrogen atoms which can initiate polymerisation. Z may contain groups that are inert to reaction with NCO groups or the polyoxyalkylene chains which are being polymerised or copolymerised. Generally the Z group has a molecular weight of 1000 or less.

Generally, at least three of X_t, X₂, X3, X₄ an X₅ are each independently an OH- terminated polyoxyalkylene chain, preferably OH-terminated polyoxyethylene or polyoxypropylene chains.

The polyol of formula I is preferably a macropolyol. As used herein, the term "macropolyol" refers to a macromer bearing multiple OH groups. As used herein, the term "macromer" (also referred to as "macromonomer") refers to a polymer or oligomer that has a functional group capable of participating in further polymerisation. Generally 3, 4 or 5 of X_l5 X₂, X₃, 4 and X5 comprise a terminal hydroxyl group.

According to one embodiment one or more of X_ls X₂, X₃, X4 and X₅ independently represents a hydroxyl group or an OH terminated, optionally substituted, polyoxyalkylene chain. Typically 3, 4 or 5 of Xi, X₂, X3, X4 and X₅ independently represent a hydroxyl group or an OH terminated, optionally substituted, polyoxyalkylene chain.

Typically the polyoxyalkylene chain comprises up to 6 carbon atoms, generally up to 3 carbon atoms. The polyoxyalkylene chain may be substituted with one or more ether groups. According to one embodiment the polyoxyalkylene chain is not substituted.

One or more of X_t, X2, X3, 4 and X₅ may independently represent an OH-terminated polyoxyalkylene chain wherein the polyoxyalkylene chain preferably does not comprise any tertiary hydrogen atoms. In particular 3, 4 or 5 of Xj, X₂, X₃, X4 and X₅ may independently represent an OH-terminated polyoxyalkylene chain. Preferably, the polyoxyalkylene chains are polymers and/or co polymers of ethylene oxide and/or propylene oxide in which the terminal hydroxyl groups may be primary or secondary hydroxyls, or a mixture thereof., More preferably, the polyoxyalkylene chains are OH- terminated polyoxyalkylene chains selected from polyoxyethylene and polyoxypropylene units or a mixture thereof. However copolymers of ethylene oxide and propylene oxide can also be used. Generally such copolymers comprise terminal hydroxyl groups.

Where the polyol of formula I is derived from the polymerization of ethylene propylene oxides, the polyol will have the same number of terminal hydroxyl groups the number of hydroxyl groups present in the compound from which it is derived. Generally where one or more of X X₂, X₃, ₄ and X₅ represents an OH-terminated polyoxyalkylene chain, in particular where 3, 4 or 5. of

X₂, X3, ₄ and X₅ represents an OH-terminated polyoxyalkylene copolymer chain, the polyol is maintained as a liquid at ambient temperature or temperatures slightly above (20 to 40 °C). Copolymers of ethylene oxide with propylene oxide can be made which maintain fluidity at room temperature. Such a polyol provides several advantages. In particular it is easier to handle and dispense at ambient temperature.

The polyol of formula I can be derived from various multi hydroxyl compounds e.g. a polyol comprising three polyoxyalkylene chains can be derived from the polymerization or co-polymerisation of ethylene or propylene oxides above or from a starter molecule of trimethylol propane, similarly a polyol comprising four polyoxyalkylene chains can be derived from pentaerythritol, and a polyol comprising five polyoxyalkylene chains can be derived from pentanepentols and/or sugar molecules bearing at least five hydroxyl groups. These can normally be purchased from commercial suppliers.

According to one embodiment the Z group is trivalent, suitably the polyol is glycerol, trimethylpropane (TMP) or hexanetriol (HT), in particular 1, 2, 6-hexanetriol. According to a further embodiment, the Z group is tetravalent, suitably the polyol is pentaerythritol.

Pentaerythritol

According to a further embodiment, when the Z group is pentavalent, suitably the polyol is pentanepentol (in particular 1, 2, 3, 4, 5-Pentanepentol), or is derived from pentanepentol (see formula II below).

1 ,2,3,4,5-Pentanepentol

I I

Where Xi, X₂, X_3> X4 and X₅ independently represent H, or an optionally substituted OH terminated polyoxyalkylene chain.

In one preferred embodiment, the polyol is a macropolyol of formula la,

la wherein each of Xi, X₂ and X₃ is independently a hydroxyl group or an optionally substituted OH terminated polyoxyalkylene chain; and

X4 is H or as defined for Xi , X₂ and X3.

In one preferred embodiment, the polyol is of formula la, each of X_ls X₂ and X₃ is independently an OH-terminated, unsubstituted polyoxyalkylene chain and X4 is H. Alternatively the polyol is of formula la, each of Xj, X2 and X₃ is independently an OH- terminated, polyoxyalkylene chain and X is H, where the polyoxyalkylene. chain does not comprise any tertiary hydrogen atoms.

In another preferred embodiment, the polyol is of formula la, each of Xi, X₂ and X₃ and X4 is independently an OH-terminated unsubstituted polyoxyalkylene chain, or alternatively an OH-terminated polyoxyalkylene chain where the polyoxyalkylene chain does not comprise any tertiary hydrogen atoms.

In another preferred embodiment, the macropolyol is of formula lb,

lb wherein each p is from about 3 to about 25 and R and R" represent H. More preferably, p is about 25.

According to one embodiment the polyol is selected from the group consisting of trimethylolpropane (TMP), 1,2,6-hexanetriol (HT) and pentaerythritol (PER).

1,2,6-hexanetriol (HT) is a clear, colourless viscous liquid at room temperature. HT acts as a 3D crosslinking agent. Increasing the concentration of HT in the reactant mixture, increases the modulus and reduces the water content of the resultant composition. Generally where the polyol is HT, the amount of polyol is 1 wt % or less, typically 0.2 to 0.7 wt%, generally 0.3 to 0.6 wt %.

Pentaerythritol is a white solid at room temperature. PER contains four hydroxyl groups and is used as covalent crosslinker to provide mechanical and thermal stability to the three dimensional polymer matrix of articles formed from the resultant composition (e.g. a contact lens). Increasing the concentration of PER in the reactant mixture, increases the modulus and reduces the water content of the resultant composition. Generally where the polyol is PER, the amount of polyol is 1 wt % or less, typically 0.05 to 0.2 wt%, generally 0.05 to 0.1 wt %. Trimethylolpropaae (TMP) with its three primary alcohol groups is used as a three dimensional crosslinking agent. It is a white solid at room temperature and easily melts to give a clear liquid at temperatures above its melting range of 58 - 60°C. Increasing the amount of TMP present in the reactant mixture., increases the modulus and reduces the water content of the resultant composition. Generally where the polyol is TMP, the amount of polyol is 5 wt % or less, typically 0.5 to 3 wt%.

Preferably, the polyol or macropolyol is a fluid at ambient temperatures. Preferably, the macropolyol has a molecular weight of from about 500 to about 20,000, more preferably from about 500 to about 15,000.

In one highly preferred embodiment, the macropolyol is an polyoxyethylene/polyoxypropylene copolymerisate, typically having four hydroxy groups. According to one embodiment, the macropolyol has the structure of formula lb above.

Such polyoxyethylene/polyoxypropylene copolymerisates are available from Clariant under reference P41, in particular P41/200, P41/300, P41/3000 and P41/12000 may be used.

In one highly preferred embodiment, the macropolyol is P41/300. Various grades of P41/300 are commercially available and can be used to afford the material of the present invention.

P41/300 has a molecular weight of -5000, P41/3000 has a molecular weight o -15,000, whereas P41/12000 has a molecular weight of -20,000.

Advantageously, the use of macropolyols of the invention (particularly P41/300, P41/3000 or P41/12000 and related compounds) gives rise to a liquid reaction mixture in which all the reaction components and additives are maintained in the fluid state for subsequent dispensing into molds at the ambient temperature, thereby allowing the reaction and curing to take place. The curing step may take place with or without additional heating.

Preferably, the polyol is used in an amount from about 10 to about 95 wt % of the reactants, more preferably from about 30 to about 70 wt % of the reactants.

The macropolyol used in the compositions of the invention is preferably a tetxafunctional hydroxyl terminated macromolecule (e.g. of formula la, lb or Ic). Preferably, the terminal OH groups are secondary hydroxyls (e.g. derived from polyoxypropylene unit, such as compounds of formula lc wherein m is greater than zero) that react with isocyanate groups (e.g. Desmodur W). The reactivity ratio of the secondary hydroxyl groups is generally lower than primary hydroxyls with isocyanate.-

Generally where the reaction mixture comprises at least one polyol or macropolyol, a poly- or di-isocyanate may be used. Advantageously a di-isocyanate is used.

According to one embodiment, the polyol may comprise silicon, in particular Z may comprise silicon. Typically the polyol is a polydialkyl siloxane diol, generally comprising at least one terminal carbinol group, suitably all of the hydroxyl functional groups are in the form of terminal carbinol groups.

According to one embodiment, the polyol may have the structure of Formula VII:

R R R

HO(CH₂]j ¼ - W— Si - 0-f~f→-)fc†— YN- {CH.₂}jOH)₂

R R R Formula VII where R represents a small alkyl group, typically methyl, x is an integer from 1 to 324, Y is an alkyl group (generally having a carbon backbone of 1 to 25 carbon atoms, typically 1 to 6 carbon atoms), J is an integer from 1 to 25 (generally 1 to 5, typically 2).

According to one embodiment, more than one polyol and/or more than one macropolyol may be used in the reactant mixture. As noted above, t £ polyol, and/or macropolyol is generally only included for the preparation of cross-linked polyurethane compositions. For the preparation of thermoplastic polyurethane compositions, the functionality of all reactants is generally two or less.

Di-isocyanate

The polymer composition of the invention is prepared using at least one di-isocyanate. Preferably, the di-isocyanate is an aliphatic di-isocyanate. The di-isocyanate performs a number of different functions. Firstly, it acts as a coupling agent for the poly(ethylene glycol) or polyol component to produce the soft segment. Secondly, it acts as a coupling agent to produce urethane-rich hard segments. Thirdly, it acts as a coupling agent for the soft and hard segments to build up the molecular weight of the resulting polymer.

The diisocyanate is preferably an aliphatic dusocyanate. Aliphatic diisocynates which are fluid at ambient temperatures are particularly preferred,

Preferably, the di-isocyanate is of the formula OCN-R₍-NCO, wherein Ri is a linear or branched C₃-Cig-alkylene, an unsubstituted or Ci-C₄-alkyl-substituted or CrC₄-alkoxy- substituted Q-Qo-arylene, a C7-Cig-aralkylene, a C6-Cio-arylene-Ci-C2-alkylene-C₆-Ci<>- arylene, a C₃-Cg-cycloalkylene, a C3-Cg-cycloalkylene-C|-C6-alkylene, a C₃-Cg- cycloaIkylene-Ci-C6-alkylene-C3-C8-cycloalkylene or a Ci-C6-alkylene-C3-Cg-cyclo- alkylene-C i -CValkylene.

Examples of preferred diisocyanates and suitable diisocyanates include methylene dicyclohexyl diisocyanate, isophorone diisocyanate, toluene-2,4-diisocyanate, toluene- 2,6-diisocyanate, mixtures of toIuene-2,4 and 2,6-diisocyanates, ethylene diisocyanate, ethylidene diisocyanate, propylene- 1,2-diisocyanate, cyclohexyIene-l,2-diisocyanate, cyclohexylene-l,4-diisocyanate, m-phenylene diisocyanate, 4,4"-biphenylene diisocyanate, 3,3'-dichloro4,4.'-biphenylene diisocyanate, 1,6-hexamethylene diisocyanate, 1,4-tetramethylene diisocyanate, 1,10-decamethylene diisocyanate, cun ene- 2,4-diisocyanate, 1,5-napthalene diisocyanate, 1,4-cyclohexylene diisocyanate, 2,5- fluorenediisocyanate, polymeric 4,4'-diphenylmethane diisocyanate or methylene dicyclohexyl diisocyanate.

The diisocyanate is preferably an aliphatic diisocyanate. Aliphatic diisocynates which are fluid at ambient temperatures are particularly preferred.

In one highly preferred embodiment, the di-isocyanate is Desmodur W (methylene bis (4,4'-cyclohexyl isocyanate), MW - around 262.5). Preferably, the di-isocyanate is used in an amount of from about 0.05 wt % to about 55 wt %. Where a poly(ethylene glycol) is used, the di-isocyanate is preferably used in an amount from about 10 wt % to about 40 wt %, more preferably, from about 20 wt % to about 40 wt % of the reactants. Where a polyol according to Formula I is used, the diisocyanate is preferably used in an amount from about 9 wt% to about 50 wt%, more preferably from about 20 wt% to about 35 wt%.

The amount of di-isocyanate in any given composition can be adjusted to modify the properties/attributes of the resultant polyurethane composition. Poly-isocyanate

The polymer composition of the invention may be prepared using at least one poly- isocyanate i.e., having more than two functional reactive groups. Preferably, the poly- isocyanate is an organic poly-isocyanate. The poly-isocyanate performs a number of different functions. Firstly, it acts as a coupling agent for the macropolyol component to produce the soft segment. Secondly, it acts as a coupling agent to produce urethane-rich hard segments. Thirdly, it acts as a coupling agent for the soft and hard segments to build up the molecular weight of the resulting polymer. It may also act as a crosslinker.

Suitable poly-isocyanates for use in the compositions of the present invention include, trifunctional trimer (isocyanurate) of isophorone diisocyanate, trifunctional trimer (isocyanurate) of hexamethylene diisocyanate and polymeric 4,4'-diphenylmefhane diisocyanate.

More preferably, the poly-isocyanate is aliphatic. Preferably, the poly-isocyanate is liquid at ambient temperature.

Preferably, the poly-isocyanate is used in an amount of from about 9 wt% to about 50 wt%, more preferably from about 20 wt% to about 35 wt%. The amount of poly- isocyanate in any given composition can be adjusted to modify the properties/attributes of the resultant po lyurethane composition.

The stoichiometery (NCO/OH ratio) of the reacting mixture plays an important part in determining the extent of cross-linking. For example, the cross-link density, and hence the molecular weight/modulus of the material, is expected to be relatively higher for a 1:1 NCO:OH stoichiometry, and such a material is also expected to afford relatively lower water content on hydration than the polymer afforded by a composition that has a non stoichiometric ratio (e.g. more NCO groups than OH groups). The skilled person would thus appreciate that the NCO:OH stoichiometry can be adjusted so as to obtain a material with the desired modulus and to some degree water content.

In one particularly preferred embodiment, the reactants are employed in such proportions as to provide an overall NCO/OH ratio in the range of from NCO/OH 2.0:1.1 - 2.0:2.8, more preferably, 1.7:2.0. Such NCO/OH ratios are particularly preferred where the polymer composition is prepared using a polyol of formula I. In particular such NCO/OH ratios are preferred where the polyurethane composition is prepared using CM techniques.

According to a further preferred embodiment, the reactants are employed in such proportions as to provide an overall NCO/OH ratio of 0.75 to 1.75, typically of less than 1.2, preferably from 0.8 to about 1.1, more preferably from about 0.85 to about 0.99, more preferably from about 0.95 to about 0.98. Such NCO/OH ratios are particularly preferred where the polymer composition is prepared using a polyethylene glycol) compound. In particular such NCO/OH ratios are preferred where the polyurethane composition is to be used for injection moulding.

It will be appreciated that the incorporation of the poly(ethylene glycol) monoalkyl ether compound increases the amount of hydroxyl groups in the reactant mixture due to the presence of the terminal hydroxyl group in the poly(ethylene glycol) monoalkyl ether compound therefore, the quantity of isocyanate used is to be adjusted accordingly.

OH-terminated Chain Extender

The polyurethane composition of the present invention may comprise one or more chain extenders.

Preferably, the chain extender is a di-functional chain extender comprising two hydroxyl groups which may be either primary or secondary hydroxyls.

Preferably, the number average molecular weight of the difunctional chain extender is less than or equal to 1000.

In one preferred embodiment, the chain extender is selected from poly(ethylene glycol)s and/or polypropylene glycols or glycols comprising copolymers of ethylene oxide and propylene oxide.

The chain extender may be a diol, in particular of formula II:

wherein n is an integer from 1 to 25, preferably 1 to 10, more preferably 1 to 4. According to a further preferred embodiment, the n is an integer from 2 to 25, preferably

2 to 10. Preferably, where the reactant mixture comprises at least one po!y(ethylene glycol), one or more diols of formula II are added.

Preferred chain extenders for use in the present invention include triethylene glycol, 1,4- butanediol, tetraethylene glycol, ethylene glycol, hexanediol, propylene glycol, 2-ethyl- 1,3-hexanediol, 1 ,5-pentanediol, 1,3-propanediol, 1,3 -butanediol, 2,3 -butanediol, 1,2- dimethyl- 1 ,2-cyclopentanediol, 1 _;2-cyelohexanediol, 1 ,2-dimethyl- 1 ,2-cyclohexanediol, and polymers of ethylene oxide and copolymers of ethylene oxide with propylene oxide having a number average molecular weight of less than or equal to 1000 may also be employed.

In one embodiment, the chain extender is diethylene glycol (DEG), i.e. n is 2.

In one highly preferred embodiment, the chain extender is triethylene glycol (TEG), i,e, n is 3. Advantageously, compositions derived from TEG produce thermoformable polymers that show high light transmissibility in the fully water swollen state.

In one highly preferred embodiment, the chain extender is tetraethylene glycol (TTEG), i.e. n is 4.

Preferably, the chain extender is used an amount of from 2 to about 45 wt % of the reactants, typically 5 to about 45 wt % of the reactants, more preferably from about 10 to about 25 wt % of the reactants. Where the chain extender is EG, preferably it is used in an amount of from about 2 to about 0 wt % of the reactants, more preferably from about 2 to about 6 wt % of the reactants.

Where the chain extender is DEG, preferably it is used in an amount of from about 5 to about 20 wt % of the reactants, more preferably, from about 10 to about 16 wt % of the reactants. Where the chain extender is TEG, preferably it is used in an amount of from about 8 to about 45 wt % of the reactants, more preferably from about 14 to about 30 wt % of the reactants, even more preferably from about 10 to about 25 wt % of the reactants, typically from about 15 to about 25 wt % of the reactants.

Where the chain extender is TTEG, preferably it is used in an amount of from about 20 to about 30 wt % of the reactants. The mixtures of chain extenders can also be used.

The proportion of the chain extender within a given composition can also affect the material properties. The chain extender reacts with NCO groups of the di- isocyanate (e.g.Desmodur W) to form the "hard" blocks within the resultant polymer matrix that affords the strength (tensile properties) to the material. The skilled person would thus appreciate that the proportion of the given chain extender can be adjusted in order to fine tune the tensile properties of the resulting material.

Preferably, the chain extender is of formula II where the reactant mixture comprises one or more PEG compounds. Generally the chain extender of formula II is used in an amount of about 2 to about 60 mole equivalents to the amount of PEG, preferably, from about 5 to about 30 mole equivalents, even more preferably, from about 14 to about 22 mole equivalents relative to the PEG in a given composition.

Additional Compound

According to one embodiment, the reactant composition may include an additional compound comprising one or more hydroxyl groups and one or more primary or secondary amine groups. The additional compound may react with isocyanate groups to form urea, urethane and/or imine groups. The addition of such an additional compound in the reactant mixture provides the resultant composition with increased modulus and associated mechanical stability. In addition, the use of the additional compound reduces the water content of the resultant composition and thus provides secondary controls for water content and the physical dimensions of an article formed from the composition of the present invention. The properties of the composition of the present invention may be controlled by using the appropriate amount of the additional compound. Generally, the reactant mixture comprises from about 0.5 wt % or less of the additional compound, typically from about 0.01 to about 0.4 wt %, suitably from about 0.05% to about 0.2 wt %. The additional compound is generally a difunctional amine, in which the functionality is provided by the presence of one hydroxyl group and one primary or secondary amine group. According to one embodiment, the additional compound is ethanolamine (EA) (in particular 2-ethanolamine). 2-Ethanolamine is a pungent, clear liquid at room temperature. EA reacts with isocyanate to form urea groups. In order to rrunimise the formation of urea groups, where the additional compound is or comprises an ethanolamine (EA), the amount of additional compound is generally limited to no more than 0.1 wt %. Typically the amount of EA present is 0.01 to 0.05 wt %.

Polydialkyl Siloxane Diol

According to one preferred embodiment, the composition of the present invention is prepared from at least one polydialkyl siloxane diol. Generally the polydialkyl siloxane diol comprises one or two terminal carbinol groups, typically two tenrinal carbinol groups. The reaction of the invention involves reacting the OH groups of the polydialkyl siloxane diol and polyol component with isocyanate groups to form a polyurethane. Polydialkyl siloxanes are substantially hydrophobic, whereas the polyol component is substantially hydrophilic. In order to overcome any potential compatibility problems, the hydride terminated polydialkyl siloxane is first reacted with a monoalIylpoly(ethylene glycol) in a hydrosilylation reaction to form a polydialkyl siloxane diol (also referred to hereinafter as the "silicone macromer") as follows:

Formula ill Formula IV

Formula V

Polydlaikyt siloxane dlol ("silicone macromer") where R is alkyl, p is an integer from 1 to 110 and x-is an integer from 1 to 324.

Other allyl glycols may also be used in the above reaction instead of the compound of formula IV. For example, alternative reactants include the following:

where q is an integer from 1 to 40, r is an integer from 1 to 10 and s is an integer from 1 to 25.

Preferably, the hydrosilylation is carried out in the presence of a catalyst. More preferably, the catalyst is a palladium catalyst. Alternatively, the catalyst may be platinum(0)-l,3-divinyl-l,l,3,3-tetramethyldisiloxane complex solution in xylene, Pt -2%.

The hydrosilylation reaction changes the hydrophobic nature of the polydialkyl siloxane to a relatively hydrophilic reactive monomer with OH functional groups. This improves the compatibility with the other co-reactants in the reaction mixture. W

In one particularly preferred embodiment, the polydialkyl siloxane is prepared by reacting hydride terminated polydimethyl siloxane (PDMS) with molecule comprising at least one hydroxyl group, one all l group and a polyoxyethylene unit which induces compatability. 5 In particular, the molecule may be a monoalkylpoly(ethylene glycol), such as poly(ethylene glycol) monoallyl ether.

According to one embodiment, the polydialkyl siloxane diol comprises two terminal carbinol groups.

10

In particular, the polydialkyl siloxane diol may comprise a hydrocarbyl group between the siloxane group and each carbinol group. The hydrocarbyl group may be substituted or unsubstituted, typically with one or more small alkyl groups. Alternatively or additionally, the hydrocarbyl group may comprise one or more ether, or ester groups. 15 Typically the hydrocarbyl group is unsubstituted. Alternatively, the hydrocarbyl group comprises an ether group. According to a further embodiment, the hydrocarbyl group comprises an ester group.

The hydrocarbyl group typically has a carbon backbone of 5 to 150 carbon atoms. 20 According to one embodiment, the hydrocarbyl group is unsubstituted and has a carbon backbone of 1 to 10 carbon atoms, generally 1 to 5 carbon atoms, typically 3 to 5 carbon atoms.

Alternatively, the hydrocarbyl group comprises an ether group and has a carbon backbone 25 of 5 to 50 carbon atoms, typically 5 to 40 carbon atoms.

According to a further embodiment, the hydrocarbyl group comprises an ester group and has a carbon backbone of 90 to 150 carbon atoms, typically 100 to 150 carbon atoms.

30 Generally the polydialkyl siloxane diol has a molecular weight of 500 to ίΟΟΟΟ, typically 1000 to 7000. Typically the alkyl groups of the polydialkyl siloxane diol are small alkyl groups. According to one embodiment, the polydialkyl siloxane diol is a polydimethyl siloxane diol. The term "Silicone Macromer" or "Carbinol terminated polydialkyl siloxane" is generally used to refer to a dihydroxy teraiinated block copolymer oxyethylene - dimethylsiloxane - oxyethylene (eg.,formula V described in this document) or oxypropylene - dimethylsiloxane - oxypropylene or caprolactone - dimethylsiloxane - caprolactone of different molecular weights containing different weight % of non-siloxane units. Some such compounds are also available commercially e.g., Gelest Inc. supplies compounds like DMS-C15 having a molecular weight of around 1000, and a non-siloxane content of around 20 wt %, DBE-C25 having a molecular weight of around 3500-4500, and a non- siloxane content of around 60 wt%}, DBP-C22 having a molecular weight of around 2500-3200, and a non-siloxane content of around 45-55 wt %, DBL-31 having a molecular weight of around 5700-6900, and a non-siloxane content of around 50 wt%.

The polydialkyl siloxane diol typically has the structure of Formula V:

Alternatively the polydialkyl siloxane diol has the structure of Formula VI:

HO

where R represents a small alkyl group, typically methyl, Y represents an alkyl group (generally having a carbon backbone of 1 to 25 carbon atoms, typically 1 to 6 carbon atoms), p is an integer from 1 to 110, x is an integer from 1 to 324 and A in an integer from 1 to 25, typically 1 to 10, generally 3 to 7, suitably 5. According to one embodiment the polydialkyl siloxane diol has the structure of Formula V and has an associated molecular weight of 600 to 10000.

According to further embodiment, the polydialkyl siloxane diol has the structure of Formula VI and has a molecular weight of 5500 to 7000.

According to one embodiment, the polydialkyl siloxane diol is an oxyethylene - dimethylsiloxane - oxyethylene block polymer. Alternatively the polydialkyl siloxane diol is a oxypropylene - dimethylsiloxane - oxypropylene block copolymer. According to a further embodiment the polydialkyl siloxane diol is a caprolactone - dimethylsiloxane - caprolactone block copolymer.

The polydialkyl siloxane diol may comprise a mixture of more than one of the compounds described above. In particular, the polydialkyl siloxane diol may include more than one compound of Formula V and/or Formula VI having different molecular weights.

In one preferred embodiment, the polydialkyl siloxane diol is hydroxyethoxy-propyl terminated PDMS.

In one highly preferred embodiment, the polydialkyl siloxane diol is a polydimethyl siloxane diol, i.e. R is methyl in formula III.

Preferably, the starting polydialkyl siloxane dihydride terminated has a molecular weight of from about 200 to about 12,000, even more preferably, from about 500 to about 2000.

Preferably, the all lpoly glycol has a molecular weight of from about 200 to about 2000, even more preferably, from about 500 to about 1200. In one particularly preferred embodiment, the silicone macromer is 2780 which is manufactured from allyl polyglycol 1100 and PDMS hydride terminated ( W=580). In another particularly preferred embodiment, the silicone macromer is 1580 which is manufactured from ally polyglycol 500 and PDMS hydride terminated (MW=580).

Similarly Carbinol (hydroxyl) terminated polydimethyl siloxanes such as copolymers of general architecture (oxyethylene)-(dimethylsiloxane)-(oxyethylene), (oxypropylene)- (dimethylsiloxane)-(oxypropylene) and (caprolactone)-(dimethylsiloxane)-(caprolactone) of different molecular weights and containing different non-siloxane content can be used. Any of these for simplicity may be referred to herein as the silicone macromer. Catalysts may be used to speed up the polyurethane formulation and any of those catalysts normally used by those skilled in the art may be employed. For example, suitable catalysts include dibutyltin dilaurate, stannous octoate, tertiary amines such as triethylarriine and the like. In one highly preferred embodiment, the catalyst is dibutyl tin dilaurate (DBTDL).

Preferably, the catalyst is used in an amount of from about 0.02 wt % to about 1.0 wt % of the reactants, more preferably, from about 0.05 wt % to about 0.5 wt %, even more preferably, from about 0.05 wt % to about 0.2 wt %, of the reactants. The second step of the reaction involves reacting the OH groups of the polydialkyl siloxane diol, PEG and diol components and the poly(ethylene glycol) monoalkyl ether compound with NCO groups of the di- or poly- isocyanate compounds to form a polyurethane. The reaction of the invention proceeds with the di- or poly- isocyanate compounds reacting randomly with the PEG, diol, poly(ethylene glycol) monoalkyl ether compound and silicone macromer to form a polymer matrix. Advantageously, the resulting polymer matrix allows high flux of oxygen, resulting in a high DK lens.

Additional Components

In one preferred embodiment, the composition further comprises one or more antioxidants. Suitable antioxidants include hindered phenols, BHA (butylated hydroxyl anisole), BHT (butylated hydroxytoluene) and ascorbic acid. Preferably, the antioxidant is BHA. Preferably, the antioxidant is used in an amount of about 0.01 to about 10 wt % of the reactants, more preferably from about 0.1 to about 5 wt %, even more preferably from about 0.2 to about 1 wt % of the reactants in any given composition. According to one embodiment, the antioxidant is present at 1 to 3 wt%.

In one preferred embodiment, the composition of the invention further comprises one or more tinting agents. By way of example, suitable tinting agents commonly used in the contact lens industry include the following: benzene sulfonic acid, 4-(4,5-dihydro-4-((2- methoxy-5-methyl-4-((2-(sulfooxy)ethyl)sulfonyl)phenyl)azo-3 -methyl-5-oxo- 1 H- pyrazol-l-yl); [2-naphthalene-sulfonic acid, 7-(acetylamino)-4-hydroxyl-3-((4-((sulfo- oxyethyl)sulfonyl)phenyl)azo)-]; [5 (4,6-dichloro-l,3,5-triazm-2-yl)amino-4-hydroxy-3- ((l-sulfo-2-naphthalenyl)azo-2,7-naphthaIene-disulfonic acid, trisodium salt]; [copper, 29H, 31 H-phthalocyaninato(2-)-N₂9,N₃o,N5 , ,N₃₂)-,sulfo((4((2-sulfooxy)ethyl)sulfonyl)- phenyl)amino) sulfonyl derivative]; and [2,7-naphthalenesulfonic acid, 4-amino-5- hydroxy-3,6-bis((4-((2-(sulfooxy)ethyl)sulfonyl) phenyl)azo)-tetrasodium salt] .

Particularly preferred tinting agents for use in the present invention are phthalocyanine pigments such as phthalocyanine blue and phthalocyanine green, chromic-alumina- cobaltous oxide, chromium oxides, and various iron oxides for red, yellow, brown and black colours, and others well known in the art. The use of organic pigments, particularly phthalocyanine pigments, more particularly copper phthalocyanine pigments, and even more particularly copper phthalocyanine blue pigment (e.g., Colour Index Pigment Blue 15, Constitution No. 7 160) is preferred. Opaquing agents such as titanium dioxide may also be incorporated. For certain applications, a mixture of colours may be employed for better simulation of natural iris appearance.

In one preferred embodiment, the tinting agent is a handling tint such as Reactive Blue 4.

Preferably, the weight percentage of the tinting agent is from about 0.0001 % to about 0.08 %, more preferably, 0.0001 % to about 0.05 %. In one preferred embodiment, the tinting agent is present in an amount of from about 0.005 to 0.08 wt %. In one preferred embodiment, the weight percentage of the tint is from about 0.0001 % to about 0.04 %, more preferably, from about 0.0001 % to about 0.03 wt % of the reactants. ^: 01

In one preferred embodiment, the composition of the invention further comprises one or more UV blockers or UV absorbers. A UV absorber may be, for example, a strong UV absorber that exhibits relatively high absorption values in the UV-A range of about 320- 380 nanometers, but is relatively transparent above about 380 nm. Preferably, the UV Blocker is a commercially available UV Blocker such as AEHB (acryloxyethoxy hydroxybenzophenone; CigHieOs).

Generally speaking, a UV absorber, if present, is provided in an amount from about 0.5 wt % to about 1.5 wt % of the reactants. Particularly preferred are compositions which include from about 0.6 wt % to about 1.0 wt % UV absorber, more preferably, about 1.0 wt % of the reactants.

Catalysts may be used to speed up the polyurethane formulation and any of those catalysts normally used by those skilled in the art may be employed. For example, suitable catalysts include those based on iron, tin, zinc, bismuth or zirconium, or catalysts comprising a tertiary amine or a tertiary polyamine containing compound. Exemplary catalysts include dibutyltin dilaurate, stannous octoate, tertiary amines such as triethylamine and the like. In one highly preferred embodiment, the catalyst is dibutyl tin dilaurate (DBTDL).

Preferably, the catalyst is used in an amount of from about 0.02 wt % to about 1.0 wt % of the reactants, more preferably, from about 0.05 wt % to about 0.5 wt %, even more preferably, from about 0.05 wt % to about 0.2 wt %, of the reactants.

Process

Another aspect of the invention relates to a process for preparing a cross-linked polyurethane hydrogel, said process comprising:

i) preparing a mixture of

a) at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2,

b) at least one di- or poly-isocyanate, c) optionally at least one poly(ethylene glycol), d) optionally at least one OH-terminated chain extender,

f ) at least one poIy(ethylene glycol) monoalkyl ether; ii) allowing the mixture formed in step i) to react appropriately to form a cross-linked polyurethane xerogel;

iii) hydrating the xerogel using an aqueous medium to form a hydrogei.

Generally, at least one poly(ethylene glycol) and at least one polyol of formula I are incorporated into the reactant mixture. Typically the ratio of NCO/OH in the reactant mixture is 0.5 to 2, suitably 0.75 to 1.75, more suitably around 1. Typically the polyol and/or macropolyol are of formula I.

Advantageously, the process of the invention involves curing the reactants in step (i) directly to form a polyurethane xerogel without the need for the addition of water as a reactant. The process of the present invention generally proceeds under substantially anhydrous conditions (less than or equal to 0.05 wt % moisture). This results in a polyurethane backbone that is substantially free from urea groups, in contrast to methods known in the art. The absence of water (as far as practicable) prevents any significant formation of urea groups which can increase the modulus of the material to a degree that is undesirable for medical devices such as contact lenses.

In one preferred embodiment of the invention the reactants in step (i) are mixed and dehydrated under vacuum. Preferably, the reactants are dehydrated under vacuum at a temperature of about 95°C for at least 90 minutes. Preferably, the reactants in step (i) are degassed under vacuum using a rotary evaporator. Advantageously, the process comprises a processing step. Said processing step typically involves injection or compression molding the material into the shape of a lens. Other suitable processing techniques include cast molding, spin cast molding and lathing. According to a further embodiment, there is provided a process for preparing a thermoplastic polyurethane hydrogel, said process comprising:

i) preparing a mixture comprising:

(a) at least one poly(ethylene glycol),

(b) at least one di-isocyanate,

(c) at least one OH-terminated chain extender,

ii) allowing the mixture to polymerise;

iii) adding at least one poly(ethylene glycol) monoalkyl ether to the polymerized mixture;

iv) hydrating the resultant mixture with an aqueous medium to form a hydrogel.

As noted above, in general the functionality of all components in the mixture to be polymerised is two or less. As such, a polyisocyanate would not be used to form a thermoplastic polyurethane composition. Typically the ratio of NCO/OH in the reactant mixture is 0.5 to 2, suitably 0.75 to 1.75, more suitably around 1.

A further aspect relates to a polymer obtainable by the process of the invention. Yet another aspect relates to the use of a polymer according to the invention in the preparation of a contact lens.

A fifth aspect of the present invention relates to a process for preparing a contact lens formed from a cross-linked polyurethane composition comprising the steps of:

i) preparing a mixture comprising

a) at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having a functionality of greater than 2, b) at least one di- or poly-isocyanate,

c) optionally at least one polyethylene glycol),

d) optionally at least one OH-terminated chain extender,

e) optionally an additional compound comprising at least one hydroxyl group least one primary or secondary amine group; and f) at least one poly(ethylene glycol) monoalkyl ether;

ii) dispensing the reaction mixture formed in step i) into a contact lens mold;

iii) allowing the reaction mixture to react and cure;

iv) removing the contact lens from the mold; and

v) hydrating the contact lens.

Typically the ratio of NCO/OH in the reactant mixture is 0.5 to 2, suitably 0.75 to 1.75, more suitably around 1. In one preferred embodiment, the reactants in step (i) are dispensed into a female lens mold and the male part of the lens mold is then placed over the liquid contained in the female part and subsequently closed by a machine or other method.

According to a further aspect of the present invention, there is provided a process for preparing a contact lens formed from a thermoplastic polyurethane composition comprising the steps of:

i) preparing a mixture comprising:

(a) at least one poly(ethylene glycol),

(b) at least one di-isocyanate,

(c) at least one OH-terrninated chain extender,

ii) allowing the mixture to polymerise;

iii) adding at least one poly(ethylene glycol) monoalkyl ether to the polymerised mixture;

iv) injection molding the polymerised mixture into a contact lens; and

v) hydrating the contact lens.

For all of the embodiments described above, the reaction may take place at a temperature of from about 70 °C to about 120 °C, more preferably, from about SO °C to about 110 °C. In one highly preferred embodiment, the reaction takes place at a temperature of from about 90 °C to about 100 °C.

Preferably, the mixture is reacted for about 0.5 to about 24 hours, more preferably, for about 3 to about 8 hours. Even more preferably, the mixture is reacted for at least about 5 hours, more preferably, at 8 hours. The disappearance of the NCO absorption band at around 2260 cm^"1 in the FTIR spectrum of the resulting product signifies that the reaction is complete. Preferably, the molds are allowed to cure for about 0.5 to about 24 hours, more preferably, for about 3 to about 8 hours. Even more preferably, the molds are allowed to cure for at least about 5 hours. Optionally the curing .can also be done in the oven under a dry nitrogen flow. Preferably, the molds are removed from the oven and allowed to cool to ambient temperature.

The lens molds may then be physically separated (at ambient temperature) and the part containing the lens is immersed in excess of saline for 5-150 minutes, more preferably for 60-90 minutes, more preferably still for 30-60 minutes, to demold the lens. Optionally the saline may contain a surfactant and/or a tinting agent.

Yet another aspect relates to the use of a polyurethane xerogel or polyurethane hydrogel according to the invention in the preparation of a contact lens.

Process for preparing a molded article

Another aspect of the invention relates to a process for preparing a cross-linked polyurethane xerogel in the form of a molded article, said process comprising the steps of:

i) preparing a mixture of

a) at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having a functionality of greater than 2 b) at least one di- or poly-isocyanate,

c) optionally at least one polyethylene glycol),

d) optionally at least one OH-terminated chain extender,

f) at least one poly(ethylene glycol) monoalkyl ether;

ii) dispensing the reaction mixture formed in step i) into a mold;

iii) allowing the reaction mixture to react and cure (generally with the assistance with energy, in particular thermal energy or radiation);

iv) removing the molded article from the mold; and

v) hydrating the molded article.

Typically the ratio of NCO/OH in the reactant mixture is 0.5 to 2, suitably 0.75 to 1.75, more suitably around 1.

Another aspect of the invention relates to a process for preparing a thermoplastic polyurethane xerogel in the form of a molded article, said process comprising the steps of:

i) preparing a mixture comprising

(a) at least one poly(ethylene glycol),

(b) at least one di-isocyanate,

(c) at least one OH-terminated chain extender,

ii) allowing the mixture to polymerise;

iv) injection molding the polymerised mixture into the form of a molded article; and v) hydrating the molded article.

In one preferred embodiment, the polymerised mixture may be granulated or peiletized, and optionally dried under vacuum or dry air, prior to injection molding. The injection molding preferably takes place using conventional injection molding apparatus (such as a BOY 50M), that will be familiar to one of ordinary skill in the art. In general the molded article is in the form of a contact lens.

Article of manufacture

Another aspect of the invention relates to an article of manufacture comprising a polymer as described above.

Generally the article of manufacture has good structural integrity upon storage. Typically, the article of manufacture maintains its shape upon storage for up to at least 12 months, generally at least 24 months, advantageously 5 years or more.

Preferably, the article of manufacture is in the form of a contact lens.

A contact lens should not preferably produce irritation on the eye, even after prolonged daily wear. The lower the contact angle associated with the surface of a contact lens, the lower the chance of irritation. The lenses of the present invention are generally associated with a contact angle of 30° or less, typically 20° or less, suitably around 15°. This is far lower than currently marketed contact lenses which typically have an associated contact angle of around 70°. The tear film stability of the lenses of the present invention is significantly higher than lenses produced from polyurethane polymers formed without a poly(ethylene glycol) monoalkyl ether compound. In addition, the hydrophilicity associated with the surface of the contact lenses of the present invention is generally very good, and all of these factors results in a contact lens which has high levels of comfort and, low levels of irritation even after prolonged periods of wear. A contact lens must be permeable to oxygen in order for the lens to facilitate normal corneal metabolism. Preferably, contact lenses prepared using the polymer composition of the invention exhibit a DK value of a least 10 Barrers more preferably, at least 20, even more preferably, at least 30 Barrers. More preferably still, the lenses have a DK of at least 40 Barrers.

In one preferred embodiment, the lenses have a DK of about 15 to about 45 Barrers more preferably, from about 25 to about 45 Barrers. A contact lens must be able to transmit light in the visible region in order to function effectively in correcting visual defects. Preferably, contact lenses prepared using the polymer composition of the invention exhibit a light transmissibility of at least 80 %, more preferably at least 90 %, even more preferably, at least 95 % or 97 %. Preferably, the light transmissibility is from about 90 to about 100 %, more preferably from about 95 to about 100 %, more preferably still, 100 %.

Preferably, contact lenses prepared using the polymer composition of the invention exhibit a modulus of from about 0.1 to about 0.8 MPa, more preferably from about 0.25 to about 0.6 MPa.

The modulus of a contact lens pays a key role in controlling the mechanical properties of a soft contact lens. In addition, the on-eye performance is directly affected by the modulus. A value of greater than 1.25 MPa is likely to cause corneal staining whilst a modulus below 0.1 MPa is likely to lead to a lens with poor handling properties.

The present invention is further described with reference to the following non-limiting examples. EXAMPLES

Materials & Treatment

Example 1

Polyethylene glycol) 6000 (two different batches), 3500 and 8000 (ex Clariant) were separately dehydrated under vacuum at 95°C for 4 hours and their number average rriolecular weights (Mn) were determined by the end group analysis. These analyses respectively afforded Mn = 5527 & 5931 (of a different batch of PEG 6000), 3880 and 8095 these values were used in calculating the stoichiometry and the material referred herein after as PEG 5527, PEG 5931. PEG 3880, PEG 8095 as indicated in the tables of examples. Subsequently the dehydrated materials were used in the manufacture of polymer/lenses.

PoIy(propylene) glycol of molecular weight 430 (ex Atlas polymers), referred herein after as PPG 430, was also dehydrated under vacuum at 95°C using a rotary evaporator to reduce the moisture content to <0.030% before use. Dicyclohexylmethane-4,4'- diisocyanate, Desmodur W, was also sourced from Atlas Polymers UK and used without further purification. Ethanol arrdne, EA, ferric chloride (anhydrous), hexanetriol (HT), trimethylol propane (TMP), Poly (ethylene) glycol monomethyl ether (PEG ME) and butylated hydroxyanisole (BHA), were sourced from Sigma Aldrich and used as supplied.

Using an analytical balance accurately weigh, into a 100 ml round bottom quickfit flask, the appropriate quantities (as detailed in Tables 1 and 2) of PPG 430 (7.3398g), HT (0.9136g), ferric chloride (0.0167g), BHA (0.0182g) and PEG ME 750 (0.4119g). The flask is stoppered and placed in a microwave for few seconds to dissolve the contents. EA (0.0204g) and the dehydrated PEG 5527 (20.02 l ) were then added to the flask. The contents were mixed using a magnetic stirring rod and-Desmodur W (8.3500g) was added to the flask through a syringe. The stirring was continued for one more minute and the flask was attached to a rotary evaporator to degas. This material was then dispensed into the female part of the preheated polypropylene lens moulds, the male part of the moulds were carefully touch dropped and subsequently closed by using a machine or other method. These moulds were then cured for 22 hours in an oven at 95°C to complete the reaction. The lens moulds containing the product were allowed to cool to ambient temperature, chilled in a freezer, separated, the products (contact lenses) were demolded. These lenses were subsequently immersed individually into saline contained in glass vials. After ~2 hours of hydration these were steam sterilised (122 ° C for 20 minutes) using a standard autoclave and method.

Optionally the sahne may contain a tinting agent such as Reactive Blue 4.

Example 2

The required quantities of the reactants and additives described in Table 3 below were accurately weighed into a round bottom Quickfit flask using a 4-place balance and stoppered. The contents after mixing were dehydrated under vacuum at 95°C for at least 90 minutes using a Buchi rotary evaporator. Table 3 (Formulation V7A)

PEG 800 = 9.2165g

PEGme 750 = 0.4ll9g

TMP = 0.1241g

Desmodur w = 3.676g

BHA = 0.1328g

DBTDL = 0.0212g

Cured for 22 hours at 95 deg C

The flask was lifted out of the oil bath and allowed to cool down to ambient temperature. Once cooled, the required amount of the catalyst (DBTDL) was added through a needled syringe (by the difference of weight of the syringe containing DBTDL before and after the addition to the flask containing the dehydrated components). The flask was quickly stoppered and shaken. Finally the appropriate quantity of Desm dur W was added into the flask (by the weight difference of a syringe containing the required amount of the Desmodur W) in a fume hood. Optionally the order of mixing the catalyst and Desmodur W can also be reversed for convenience of better mixing and subsequent dispensing into the molds. The contents of the flask were mixed vigorously (ensuring the flask remains stoppered) and quickly degassed under vacuum for ~\ minute using a rotary evaporator (without using oil bath) to eliminate/reduce the bubbles. The mixture was then dispensed into female polypropylene lens molds (25-55 microhtre per lens as appropriate for a given mold variety). The male part of the lens mold was then placed carefully over the liquid contained in the female part and subsequently closed using a machine appropriately preset for the mold type used. This operation was repeated until all the lens molds were closed. These molds were then placed in a tray, put in an oven set at 95°C and allowed to cure for 5 hours. Optionally, the curing can be done under dry nitrogen. The molds were then removed from the oven, allowed to cool to ambient temperature and chilled in a freezer (set at -80°C) for at least 20 minutes. The lenses were demolded from the molds by separating the male and female parts and subsequently immersing in saline contained in glass vials. Using an analytical balance the required amount of BHA and the dehydrated molten PEG are accurately weighed into a 100 ml polypropylene cup and sealed with the screwable lid. The cup is placed in an oven at 95°C for few minutes until the BHA dissolves. Using a syringe and a fine needle, the required quantity of the catalyst (DBTDL) is added into the cup and stirred to mix the contents. The stirring can be achieved by the aid of a heated glass rod left in the cup. Finally the required amount of the Desmodur is added to the cup through a syringe. The contents are mixed thoroughly and sealed with the lid. The cup is then placed in an oven at 95°C for 8 hours to complete the reaction. The disappearance of the NCO absorption band at around 2260 cm^"1 in the FTTR spectrum of the resultant product confirmed complete reaction.

Water content

Water content is calculated after measurement of dry weight and weight of a fully hydrated lens by using the following equation:

Water Content (%) = (W hydrated lens - dry _lens) / W hydiaied lens 00

Five hydrated lenses, with excess surface water removed, are separately weighed on an analytical balance and average value is taken as Wh_yd_r_ted lens- The lenses are then dried in an oven at 75 °C for 2 hours and weighed again separately. The average value is taken as dr lens-

% Transmittance

% Transmittance was determined under the guidance of ISO 8599 by using a double beam UV spectrophotometer (Jasco V530). A lens is placed into a cuvette containing standard saline solution. The cuvette is placed in the sample compartment. A matching cuvette containing saline is placed in the reference compartment of the UV spectrophotometer and a spectrum as percent transmittance was recorded between 200- 780nm. The test was repeated a further four times and the mean value (% transmittance) at 550nm was recorded.

The contact lenses thus produced can be steam sterilised by a conventional method or may be sterilised by a other techniques that terminally sterilise the objects. DK Measurement

DK Measurement (i.e., oxygen permeability) was carried out by the polarographic technique as briefly described below: Ten lenses were placed into the Gallenkamp incubator set at 35 +/-0.5 ⁰ C for 24 hours. The centre thickness (CT) of each of the ten lenses were measured by Render ET-3 Electronic Thickness Gauge and these lenses were stacked as follows: A single lens stack, two lens stack, three lens stack, and four lens stack. The CT of each stack was measured three times and a mean value for each was calculated and fed into a spread sheet specifically developed for the method. Also recorded was the atmospheric pressure into the spread sheet. The stack of lenses were replaced into the incubator set at 35 +/-0.5 ° C and humidity > 98%.

Each stack was separately placed on to the electrode (Rehder Permeometer with 8.7 mm electrode) ensuring that there are no bubbles entrapped between the lenses and the electrode. When the current reached its lowest point the reading was recorded in the relevant section of the spread sheet. This test was repeated for all the stacks.

The dark current reading (background) of the measurement system, when no oxygen is able to pass through to the electrode, was recorded and subtracted from all test material current values. Data was analysed taking into consideration the partial pressure of oxygen and the surface area of the polarographic sensor used and finally corrected for the edge effect. A graph of D t versus thickness (cm) was then plotted and the inverse of the gradient of the best fit taken to represent the oxygen permeability (DK) of the lens material.

Modulus Data

Modulus data was measured for contact lenses prepared in accordance with the invention by tensile testing using the Instron 5842 Tensile testing system with Merlin Software.

Correlation to Standards/Regulation: ISO 9001 :2008 (Quality Standards: Par 7.6; ISO 13485:2003 Medical Device Directive: Par 7.6; FDA Part 820 QS Regulation Subpart G: Control of inspection, monitoring and test equipment 820.72. Process (Material Preparation)

Thickness readings for each lens were obtained using the ET-3 Thickness gauge. The lenses were placed flat on the cutting mat and two long pieces were cut from around the centre of the flat lens using a razor blade. These cut pieces were put into saline solution in a sample dish. The sample was loaded on to clamps using tweezers carefully going for the top clamp first and then the bottom. The gap in between the clamps was set at 10 mm using a calibrated vernier caliper. Once set, the "Reset GL" button was pressed to set the Gauge Length". Once the sample was loaded, the balance load was set to O.0O0N and the test was started using the console controls.

Tables 1 2 and 3 show examples of the presently claimed compositions. Example 3

The compositions detailed in Table 4 are currently being investigated. The compositions are in the process of being prepared. Such compositions would be prepared by accurately weighing the components of the compositions into a round bottom Quickfit flask using a 4-place balance and then stoppering the flask. The contents after mixing are dehydrated under vacuum at 95°C for at least 90 minutes. The properties of the compositions are in the process of being measured in accordance with the methods detailed above.

Example 4

The required quantities of the reactants and additives described in Table 7 below were accurately weighed into a round bottom Quickfit flask using a 4-place balance and stoppered. The contents after mixing were dehydrated under vacuum at 95°C for at least

90 minutes using a Buchi rotary evaporator.

COMPOSITION V7A_^5R

Wt/g

PEG 800 9.2085

PEGme 720 0.2873

TMP 0.1235

Desmodur W 3.6180

BHA 0.1325

DBTDL 0.0224 Cured for 22 hours at 95 deg C COMPOSITION V5 -13

Wt/g

PEG 5931 = 50.0000

TMP = 0.1131

PPG 430 =25.3751

ETHANOL AMINE =0.0437

DESMODUR W =18.7981

BHA =0.9433

FeCb =0.0943

Cured for 22 hours at 95 deg C

COMPOSITION V6A-75

Wt/g

PEG 800 = 9.7965

TMP = 0.1148

PEGme 250 =0.2719

DESMODU W =3.7310

BHA =0.1353

DBTDL =0.0149

Cured for 22 hours at 95 deg C

The flask was lifted out of the oil bath and allowed to cool down to ambient temperature. Once cooled, the required amount of the catalyst (DBTDL) was added through a needled syringe (by the difference of weight of the syringe containing DBTDL before and after the addition to the flask containing the dehydrated components). The flask was quickly stoppered and shaken. Finally the appropriate quantity of Desmodur W was added into the flask (by the weight difference of a syringe containing the required amount of the Desmodur W) in a fume hood. Optionally me order of mixing the catalyst and Desmodur W can also be reversed for convenience of better mixing and subsequent dispensing into the molds. The contents of the flask were mixed vigorously (ensuring the flask remains stoppered) and quickly degassed under vacuum for ~1 minute using a rotary evaporator (without using oil bath) to eliminate reduce the bubbles. The mixture was then dispensed into female polypropylene lens molds (25-55 microlitre per lens as appropriate for a given mold variety). The male part of the lens mold was then placed carefully over the liquid contained in the female part and subsequently closed using a machine appropriately preset for the mold type used. This operation was repeated until all the lens molds were closed. These molds were then placed in a tray, put in an oven set at 95°C and allowed to cure for 5 hours. Optionally, the curing can be done under dry nitrogen. The molds were then removed from the oven, allowed to cool to ambient temperature and chilled in a freezer (set at -80°C) for at least 20 minutes. The lenses were demolded from the molds by separating the male and female parts and subsequently immersing in saline contained in glass vials.

Using an analytical balance the required amount of BHA and the dehydrated molten PEG are accurately weighed into a 100 ml polypropylene cup and sealed with the screwable lid. The cup is placed in an oven at 95°C for few minutes until the BHA dissolves. Using a syringe and a fine needle, the required quantity of the catalyst (DBTDL) is added into the cup and stirred to mix the contents. The stirring can be achieved by the aid of a heated glass rod left in the cup. Finally the required amount of the Desmodur W is added to the cup through a syringe. The contents are mixed thoroughly and sealed with the lid. The cup is then placed in an oven at 95°C for 8 hours to complete the reaction. The disappearance of the NCO absorption band at around 2260 cm^'1 in the FTI spectrum of the resultant product confirmed complete reaction. The contact angles of the resultant compositions were measured. The contact angle associated with compositions comprising PEGme were much lower than the contact angle of the composition tested which did not comprise PEGme.

Various modifications and variations of the described aspects of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes of carrying out the invention which are obvious to those skilled in the relevant fields are intended to be within the scope of the following claims.

Claims

A polyurethane polymer composition comprising a polyethylene glycol) monoalkyl ether compound having the structure of Formula A:

Alkyl-[OCH₂CH₂]„-X

Formula A

where alkyl represents an optionally substituted straight or branched chain alkyl group having a carbon backbone of 1 to 5 carbon atoms;

X represents COOH, OH, N¾ or NHy where y represents an alkyl group having up to five carbon atoms (typically NHCH3; and

n is 1 to 50.

The polymer composition of claim 1 wherein the poly(ethylene glycol) monoalkyl ether compound has the structure of Formula B:

CH₃-[OCH₂CH₂]_n-X

Formula B

The polymer composition as claimed in either one of claims 1 and 2 prepared by reacting a mixture comprising:

a) at least one poly(ethylene glycol) and/or at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macTopolyols having an averaged functionality of greater than 2,

b) at least one di- or poly-isocyanate,

c) optionally an OH-terminated chain extender,

e) at least one polyethylene glycol) monoalkyl ether.

4. The polymer composition as claimed in any preceding claim wherein the poly(ethylene glycol) monoalkyl ether compound is poly(ethylene glycol) monomethyl ether (PEG ME), polyethylene glycol) monoethyl ether, poly(ethylene glycol) monopropyl ether, poly(ethylene glycol) monobutyl ether, mono amino poly(ethylene glycol) monomethyl ether, mono amino poly(ethylene glycol) monoethyl ether, mono amino poly(ethylene glycol) monopropyl ether or mono amino poly(ethylene glycol) monobutyl ether.

The polymer as claimed in any preceding claim wherein the poly(ethylene glycol) monoalkyl ether is present in an amount of from about 0.1 to about 2.5 wt % of the reactants.

The polymer as claimed in any preceding claim wherein the poly(ethylene glycol) monoalkyl ether has a number average molecular weight of around 200 to 800.

The polymer composition of any one of claims 3 to 6 wherein the mixture comprises at least one poly(ethylene glycol) and at least one polyol of formula I.

The polymer as claimed in any one of claims 3 to 7 wherein the mixture comprises at least one poly(ethylene glycol) having a molecular weight of 5000 to 8000, preferably 6000.

9. The polymer composition of any one of claims 3 to 8 wherein the polyol and or macropolyol is of formula I

formula I

wherein at least three of XI, X2, X3, X4 and X5 are each independently an OH- terminated group (typically an OH-terminated polyoxyalkylene chain, preferably OH-terminated polyoxyethylene or polyoxypropylene chains), and the remainder of XI, X2, X3, X4 and X5 are each independently H or absent, and Z is a central linking unit.

10. The polymer composition as claimed in claim 9 wherein Z represents a hydrocarbyl group, optionally comprising one or more ether, ester and tertiary amine groups.

11. The polymer as claimed in either one of claims 9 and 10 wherein Z has a number average molecular weight of 000 or less.

12. The polymer as claimed in any one of claims 3 to 11 wherein the polyol is glycerol, trimethylpropane (TMP) or hexanetriol (HT).

13. The polymer composition of claim 3 to 6 wherein the mixture does not comprise a polyol of formula I.

14. The polymer as claimed in any one of claims 3 to 13 wherein the mixture comprises at least one di-isocyanate selected from the group consisting of methylene dicyclohexyl diisocyanate, isophorone diisocyanate, toluene-2,4- diisocyanate, tohiene-2,6-diisocyanate, mixtures of toluene-2,4 and 2,6- diisocyanates, ethylene diisocyanate, ethylidene diisocyanate, propylene- 1,2- diisocyanate, cyclohexylene-l ,2-diisocyanate, cyclohexylene-l,4-diisocyanate, m- phenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dichloro4,4'- biphenylene diisocyanate, 1,6-hexamethylene diisocyanate, 1,4-tetramethylene diisocyanate, 1,10-decamethylene diisocyanate, cumene-2,4-diisocyanate, 1,5- napthalene diisocyanate, 1,4-cyclohexylene diisocyanate, 2,5-fluorenediisocyanate and polymeric 4,4'-diphenylmethane diisocyanate.

15. The polymer as claimed i any one of claims 3 to 13 wherein the mixture comprises at least one poly-isocyanate selected from the group consisting of trifunctional trimer (isocyanurate). of isophorone diisocyanate and trifunctional^■' trimer (isocyanurate) of hexamethylene diisocyanate and polymeric 4,4'- diphenylmethane diisocyanate.

16. The polymer as claimed in any one of claims 3 to 14 wherein the OH-terminated chain extender is a diol, having the structure of formula II:

HO-[CH₂CH₂0]„-H

Formula II

wherein n is an integer from 3 to 25.

17. The polymer as claimed in any one of claims 3 to 15 wherein the OH-terminated chain extender is selected from the group consisting of " 1,4-butanediol, tetra(ethylene) glycol, hexanediol, propylene- 1,2-glycol, 2-ethyI-l,3-hexanediol, 1,5-pentanedioL 1,3-propanediol, 1 ,3-butanediol, 2,3-butanediol, 1,2-dimethyl- 1,2-cyclopentanediol, 1,2-cyclohexanediol, l,2-dimethyl-l,2-cyclohexanediol, and polymers of ethylene oxide and copolymers of ethylene oxide with propylene oxide having a nvunbei average of less than or equal to 1000.

18. The polymer as claimed in any preceding claim wherein the ratio of isocyanate functional groups to hydroxyl functional groups (NCO/OH) in the reaction mixture is 0.75 to 1.75, preferably about 1.

19. The polymer according to any one of claims 3 to 18 wherein the mixture comprises one or more polydialkyl siloxane dicarbinols.

20. The polymer according to any one of claims 3 to 19 wherein the mixture comprises one or more antioxidants, preferably one or more of the group consisting of hindered phenols and BHA (butylated hydroxyl anisole).

21. The polymer as claimed in claim 20 wherein the mixture comprises 0.1 to 3 wt% antioxidant

22. The polymer as claimed in any one of claims 3 to 21 wherein the mixture further comprises a catalyst.

23. The polymer as claimed in claim 22 wherein the catalyst is based on iron, tin, zinc, bismuth or zirconium, or the catalyst is a tertiary amine or a tertiary polyamine containing compound.

24. The polymer as claimed in claim 23 wherein the catalyst is selected from dibutyltin dilaurate, FeC¾, stannous octoate and tertiary amines, preferably wherein the catalyst is dibutyl tin dilaurate (DBTDL).

25. The polymer as claimed in any one of claims 3 to 24 wherein the mixture comprises one or more tinting agents, preferably Reactive Blue 4.

26. The polymer as claimed in any one of claims 3 to 25 wherein the mixture comprises one or more UV blockers, preferably AEHB (acryloxyethoxy hydroxybenzophenone).

27. A polyurethane hydrogel comprising the polymer of any preceding claim in hydrated form.

28. A process for preparing a cross-linked polyurethane hydrogel, said process comprising:

i. preparing a mixture of

a. at least one polyol or macropolyol having a functionality greater than 2, or a mixture of such polyols or macropolyols having an averaged functionality of greater than 2,

b. at least one di- or poly-isocyanate,

c. optionally at least one polyethylene glycol),

d. optionally at least one OH-terminated chain extender, e. optionally an additional compound comprising at least one hydroxyl group and at least one primary or secondary amine group; and f. at least one poly(ethylene glycol) monoalkyl ether;

ii. allowing the mixture formed in step i. to react appropriately to form a cross-linked polyurethane xerogel; and

iii. hydrating the xerogel using an aqueous medium to form a hydrogel.

29. A process as claimed in claim 28 wherein the polyol or macropolyol is of formula I.

30. A process for preparing a thermoplastic polyurethane hydrogel, said process comprising:

i) preparing a mixture comprising:

(a) at least one poly(ethylene glycol),

(b) at least one di-isocyanate,

(c) at least one OH-terminated chain extender, ii) allowing the mixture to polymerise;

iv) hydrating the resultant mixture with an aqueous medium to form a hydrogel.

31. A process as claimed in claim 30 wherein the functionality of all components in the mixture to be polymerised is two or less.

32. A polyurethane hydrogel obtainable by the process of any one of claims 28 to 31.

33. A process for preparing a cross-linked polyurethane xerogel in the form of a molded article comprising the steps of:

i) preparing a mixture of

b) at least one di- or poly-isocyanate, c) optionally at least one poly(ethylene glycol),

d) optionally at least one OH-terminated chain extender, e) optionally an additional compound comprising at least one hydroxyl group and at least one primary or secondary amine group, and

f) at least one poly(ethyIene glycol) monoalkyl ether; ii) dispensing the reaction mixture formed in step i) into a mold; iii) allowing the reaction mixture to react and cure;

iv) removing the molded article from the mold; and

v) hydrating the molded article.

34. A process as claimed in claim 33 wherein the ratio of isocvanate functional to hydroxyl functional groups (NCO:OH) in the reaction mixture is 0.75 to 1.75, preferably around 1.

35. A process as claimed in either one of claims 33 and 34 wherein the polyol and/or macropolyol is of formula I.

36. The process of any one of claims 33 to 35, wherein the process is a reactive cast molding process, said mixture comprising at least one poly(ethylene glycol) and at least one polyol and/or macropolyol of formula I.

37. A process for preparing a thermoplastic polyurethane xerogel in the form of a molded article, said process comprising the steps of:

i) preparing a mixture comprising

(a) at least one poly(ethylene glycol),

(b) at least one di-isocyanate,

iv) injection molding the polymerised mixture into the form of a molded article; and

v) hydrating the molded article.

38. A process as claimed in claim 37, the functionality of all components in the mixture to be polymerised is two or less.

39. A process according to any one of claims 34 to 38 wherein the molded article is a contact lens.

40. Use of a polymer according to any one of claims 1 to 26, or a polyurethane hydrogel according to claim 27 or claim 32 in the preparation of a contact lens.