HK1000251B - Process for graft copolymerization on surfaces of preformed substrates to modify surface properties - Google Patents

Description

This invention is to a process for modifying the surfaces of preformed polymer substrates by the controlled graft polymerization thereon of selected ethylenically unsaturated monomers and to products made by said process.

Graft polymerization per se has long been known in the art with many graft copolymers such as ABS (acrylonitrile/butadiene/styrene) resins achieving considerable commercial success.

It has also been known in the art that various vinylic monomers can be graft polymerized onto polymer substrates which have been first treated with ionizing radiation in the presence of oxygen or with ozone to form peroxy groups on the surface of said substrate. US-A-3,008,920 and US-A-3,070,573 teach the grafting of selected monomers onto ozonated polymer substrates.

While such a process would in theory seem to be a panacean method to modify at will the surface characteristics of any polymer substrate, such is not the case as is seen in the teachings of US-A-4,311,573 and US-A-4,589,964.

The objective of such graft polymerization is to modify the surface of the polymer substrate without causing major changes in the physical characteristics of the substrate as a whole.

Problems have arisen when such a graft polymerization process is carried out. One serious complication involves graft polymerization of the vinylic monomer onto the substrate as desired, but with the simultaneous and undesired homopolymerization of the vinylic monomer. This problem can be minimized by carrying out the graft polymerization process in the presence of a metal redox system using a variable valence metal ion in the reduced state to convert any hydroxyl free radical present to hydroxyl ion and thus minimize the simultaneous homopolymerization problem. See US-A-3,008,920, US-A-4,311,573 and US-A-4,589,964.

US-A-4,311,573 and US-A-4,589,964 teach that another problem encountered in the surface grafting of a preformed polymeric substrate concerns depth and density control of the graft. If the bulk properties of the substrate are to be retained, then the graft depth should be no greater than necessary to modify the surface characteristics of the article. Grafts of excessive depth, grafts of insufficient density to achieve the desired property modification and the swelling and degradation of the substrate article during the process are serious problems plaguing this panacean process.

US-A-4,311,573 and US-A-4,589,964 teach a method aimed at inhibiting homopolymerization, at controlling graft depth and at accelerating graft polymerisation to increase graft density, namely by carrying out the graft polymerization in the presence of a variable metal ion (ferrous) and a complexing agent (squaric acid) to control mobility of said ions.

US-A-3,676,190 describes polymer articles, modified by grafting, which are previously ozonized in a bath of halocarbon liquid. Said polymers may be ozonized in the presence of swelling liquids or non-swelling liquids, the thickness of the ozonized skin may thereby be varied.

DE-B-1,250,635 describes a process for making graft polymers, characterized in that ozonation and/or graft copolymerization of preformed articles may be carried out in the presence of liquids.

One object of this invention is to provide a facile process for modifying the surface characteristics of a preformed polymeric substrate to impart desired properties thereto.

Another object of this invention is to prepare contact lenses, biomedical devices or other useful materials by the process of this invention.

The instant invention is to a process for modifying the surface characteristics of a preformed polymer substrate to impart hydrophilicity, hydrophobicity, optical properties, transmission properties, dyeability or tintability, opacity, diffraction differences, wettability, bonding characteristics, oxygen permeability, bactericidal properties, lubricity or multilayer membrane technology thereto by graft polymerization on said substrate, having peroxy and hydroperoxy groups on said polymer, of an ethylenically unsaturated monomer, which comprises contacting the polymer substrate with a solution which is or contains a chain transfer agent, preferably a primary or secondary C₁-C₄-alkanol, to saturate or swell said polymer, said solution being insoluble in the perhalogenated liquid medium used subsequently during the ozonation step, to limit subsequent hydroperoxidation and peroxidation to the surface of said polymer, ozonating the saturated or swollen polymer with ozone dissolved in a perhalogenated liquid medium insoluble in the solution being or containing the chain transfer agent, and then graft polymerizing an ethylenically unsaturated monomer onto essentially only the surface of the polymer substrate.

The substrate after exposure to ozone will have on its surface both peroxy (-O-O-) and hydroperoxy (-OOH) groups. Upon thermal or otherwise induced decomposition, the peroxy groups cleave into two active free radicals attached to the surface of the polymeric substrate offering sites on the surface to initiate graft polymerization with the ethylenically unsaturated monomer.

On the other hand upon thermal or otherwise induced decomposition, the hydroperoxy groups also cleave into two active free radicals. One is attached to the polymer surface and is capable of initiating graft polymerization thereon while the other is a free hydroxyl radical not attached to the surface. This latter free radical is available to initiate homopolymerization of the monomer unless such homopolymerization is inhibited or suppressed.

US-A-3,008,920 and US-A-4,589,964 teach that an effective homopolymerization-inhibiting agent is the cuprous, ferrous or other variable valence metal ion such as those of cobalt, manganese, molybdenum, tin, indium, cerium, chromium, thallium and vanadium. A preferred metal salt providing such metal ion is ferrous ammonium sulfate although other ferrous salts such as ferrous sulfate, ferrous chloride, ferrous iodide and ferrous bromide can be used as well.

These reduced valence (-ous) salts, e.g. ferrous ammonium sulfate, react with the hydroxyl free radical in a redox system to produce the hydroxyl radical and the oxidized (-ic) salt, e.g. ferric ammonium sulfate. With the concentration of hydroxyl free radical thus minimized or eliminated, there is no initiator for the homopolymerization which is now effectively suppressed.

Since in general the presence of homopolymer unattached to the surface of the substrate is undesirable leading to high extractables and unstable surface characteristics, a homopolymerization-inhibiting agent is usually present in the graft polymerization step of the instant process.

However, while the ferrous ion inhibits homopolymerization, there is a limitation in its use since such ions subsequently penetrate into the polymeric substrate allowing for the desired graft polymerization to occur at an undesired spot, namely in the interior of the substrate.

The effect of this graft polymerization at the wrong place is a distortion of the substrate with a concomitant loss in physical properties and dimensional stability and integrity. Such distortion is generally undesirable for obvious reasons and in the contact lens field is intolerable.

The preformed polymeric substrate which can be used in this process can be any fabricated polymeric product such as a film, fiber, pellicle, device or object including contact lenses whose surface characteristics are in need of modifying in some fashion to impart hydrophilicity, hydrophobicity, optical properties, transmission properties dyeability (tinting), opacity, diffraction differences, wettability, bonding characteristics, oxygen permeability, bactericidal properties, lubricity, and multilayer membrane technology.

The only requirement is that the polymer from which the fabricated product is made must itself have a hydrocarbon group somewhere in its structure making it amenable to peroxidation and hydroperoxidation when exposed to ozone to form peroxy and hydroperoxy groups on the preformed polymeric substrate surface.

Polymeric materials useful in this instant invention include inter alia polyolefins, polyesters, polyamides, cellulosics, polyurethanes, non-silicone hydrogels, hydrophilic polysiloxanes, hydrophobic polysiloxanes, polymers containing poly(alkylene oxide) units, polycarbonates, silicone rubber, natural and synthetic rubber, epoxy resins, polyvinyl chloride, polystyrene, poly(methyl methacrylate) and copolymers.

The peroxy and hydroperoxy groups are conveniently introduced onto the surface of the preformed polymeric substrate by subjecting said substrate to ozone (O₃). This can be done by appropriately suspending, placing or otherwise fixing the preformed substrate in a chamber or vessel so that the surfaces to be modified will be intimately contacted with ozone in a gaseous carrier such as ozonated air or ozonated oxygen or with ozone dissolved in a perhalogenated solvent for a period of time sufficient to result in the requisite uptake of ozone onto the polymer surface to form the desired peroxy and hydroperoxy groups. Generally this time required is less than one hour, usually about 30 minutes.

The reaction temperature is generally not critical, and the reaction can be conducted over a wide temperature range from between 0° and 100°C. For convenience ambient temperatures are preferred.

In order to facilitate the reaction between the polymer substrate and ozone to form the hydroperoxidized substrate, it is preferable for the reaction to be carried out in the presence of a small amount of moisture. Indeed, with hydrogel materials the polymeric substrate can be saturated with water before ozonization is carried out.

Ozone can be conveniently prepared in admixture with a carrier gas by passing an oxygen containing gas, such as air or pure oxygen, through a standard ozone generator. In the case of air, about 2 % ozone by weight is generally produced. In the case of pure oxygen, about 4 % ozone by weight is characteristically produced.

The ozone prepared by the ozone generator can also be dissolved in a perhalogenated solvent such as inter alia carbon tetrachloride, 1,1,2-trichloro-1,2,2-trifluoroethane, octafluorocyclobutane, perfluorohexane, perfluoroheptane, perfluoro-(1,3-dimethylcyclohexane), perfluorocyclohexane, 1,1,1-trichloro-2,2,2-trifluoroethane, 1,1,1,2-tetrachloro-2,2-difluoroethane and 1,1,2,2-tetrachloro-1,2-difluoroethane. Preferably carbon tetrachloride, perfluoro-(1,3-dimethylcyclohexane), 1,1,2-trichloro-1,2,2-trifluoroethane or perfluorohexane is the perhalogenated solvent of choice.

In the instant invention, the preformed polymer substrate is first saturated or swollen with a solution which is or contains a chain transfer agent before the subsequent ozonization step is carried out using ozone dissolved in a perhalogenated solvent. The solution containing the chain transfer agent is insoluble in the liquid medium containing the ozone. The perhalogenated solvents are those mentioned above particularly perfluoro-(1,3-dimethylcyclohexane).

The chain transfer agent is preferably a primary or secondary alkanol of 1 to 4 carbon atoms such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butyl alcohol or isobutyl alcohol. The solution containing the chain transfer agent can also contain water or a tertiary lower alkanol, such as tert-butyl alcohol, to assist in solubilizing the chain transfer agent.

While other materials such as mercaptans are also effective chain transfer agents, mercaptans are precluded from serious consideration for that purpose for aesthetic reasons (poor odor properties).

Isopropanol is a particularly preferred chain transfer agent in the instant process.

Two benefits accrue from the embodiment of the instant invention especially in the case of substrates which are hydrogels.

These are: First, the ozonization occurs on the polymer substrate in the swollen or extended state. The subsequent graft copolymerization can then occur with the substrate already in the normal physical state and size in which the end-use product, i.e. biomedical device, contact lens, will be used minimizing structural and dimensional changes which might otherwise occur during use.

Second, the primary or secondary alcohol acts as a chain transfer agent. The presence of such material inside the swollen polymer substrate helps prevent any subsequent graft polymerization from occurring in the interior of the polymer substrate later in the instant process by limiting the peroxidation and hydroperoxidation to the surface of the substrate.

Another aspect of the instant invention is the ozonation of polysiloxane polymer substrates, particularly contact lenses, in the presence of a perhalogenated hydrocarbon liquid, particularly 1,1,2-trichloro-1,2,2-trifluoroethane. Polysiloxane contact lenses have high surface tack making them stick together when ozonated in gaseous or in aqueous media causing irreparable damage to said lenses when their separation is attempted.

The polysiloxane swells in the perhalogenated hydrocarbon liquid and ozone is highly soluble in said liquid leading to a large (up to 13 times) increase in peroxy and hydroperoxy sites, compared to ozonation in water, on the suface of the polysiloxane lenses suitable for later graft polymerization.

Following the exposure of the preformed polymeric substrate to ozone in liquid medium, the ozonated substrate is allowed to air dry at ambient temperature to eliminate any residual ozone. While ozonation has occurred primarily at sites on the exposed surfaces, some peroxidized and hydroperoxidized groups may also be present in any adventitious internal interstices or recesses available to the ozone.

Since the ozonated substrate contains peroxy and hydroperoxy groups which are unstable when raised to elevated temperatures, the ozonated substrate can be kept for long periods of time (several months) at low temperature (0 to 20°C) in an atmosphere of nitrogen without loss of the peroxy and hydroperoxy groups.

In order to prevent undesired changes in overall polymer properties involving the basic integrity of the substrate itself, it is desirable to prevent or at least to minimize any subsequent grafting of the modifying monomer by graft polymerization anywhere on the preformed polymer substrate except on the surface of said substrate.

To prevent the penetration of the grafting monomer into the polymer substrate to any appreciable depth, the ozonated substrate after air drying to remove residual ozone may be treated by several routes before graft polymerization is attempted. In each case, the ozonated substrate is purged with nitrogen so that subsequent graft polymerization is not impeded.

Generally, it requires only a relatively small amount of material (by weight) to be actually grafted onto the surface of a polymer substrate to achieve the desired modification in the substrate surface properties.

The graft polymerization is generally carried out using an aqueous solution of an ethylenically unsaturated monomer or mixture of monomers capable of undergoing graft addition polymerization onto the surface of the substrate. In those cases where the monomer is not appreciably soluble in water, a cosolvent, preferably tert-butyl alcohol, is used to enhance the solubility of the monomer in the aqueous graft polymerization system.

If desired, the graft polymerization mixture may contain a catalytic amount of a conventional catalyst characteristically employed in polymerizing vinylic compounds, preferably a free radical catalyst. Of particular interest are the conventional peroxide and azo catalysts such as hydrogen peroxide, benzoyl peroxide, tert-butyl peroctoate or azobis(isobutyronitrile). In many cases, an added initiator is not needed due to the innate activity of the ozonated substrate with its peroxy and hydroperoxy groups.

Additionally where indicated, the graft polymerization can be carried out in the presence of actinic radiation with or without the presence of a photoinitiator.

The choice of the monomer or monomers depends on the nature of the substrate and on the particular surface modification desired. Thus the monomers may be hydrophilic, hydrophobic, crosslinking agents, dyesites, bactericidal or with any of a wide gamut of properties as required to achieve the modification desired.

Suitable hydrophilic monomers include generally water soluble conventional vinyl monomers such as: acrylates and methacrylates of the general structure where R₁ is hydrogen or methyl and R₂ is hydrogen or is an aliphatic hydrocarbon group of up to about 10 carbon atoms substituted by one or more water solubilizing groups such as carboxy, hydroxy, amino, lower alkylamino, lower dialkylamino, a polyethylene oxide group with from 2 to about 100 repeating units, or substituted by one or more sulfate, phosphate, sulfonate, phosphonate, carboxamido, sulfonamido or phosphonamido groups, or mixtures thereof; acrylamides and methacrylamides of the formula where R₁ and R₂ are as defined above; acrylamides and methacrylamides of the formula where R₃ is lower alkyl of 1 to 3 carbon atoms and R₁ is as defined above; maleates and fumarates of the formula         R₂OOCCH=CHCOOR₂ wherein R₂ is as defined above; vinyl ethers of the formula         H₂C=CH-O-R₂ where R₂ is as defined above; aliphatic vinyl compounds of the formula         R₁CH=CHR₂ where R₁ is as defined above and R₂ is as defined above with the proviso that R₂ is other than hydrogen; and vinyl substituted heterocycles, such as vinyl pyridines, piperidines and imidazoles and N-vinyl lactams, such as N-vinyl-2-pyrrolidone.

Included among the useful water soluble monomers, although not all of them are comprised by the above mentioned general formulae, are: 2-hydroxyethyl-; 2- and 3-hydroxypropyl-; 2,3-dihydroxypropyl-; polyethoxyethyl-; and polyethoxypropyl-acrylates, methacrylates, acrylamides and methacrylamides; acrylamide, methacrylamide, N-methylacrylamide, N-methylmethacrylamide, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide; N,N-dimethyl- and N,N-diethyl-aminoethyl acrylate and methacrylate and the corresponding acrylamides and methacrylamides; 2- and 4-vinylpyridine; 4- and 2-methyl-5-vinylpyridine; N-methyl-4-vinylpiperidine; 2-methyl-1-vinylimidazole; N,N-dimethylallylamine; dimethylaminoethyl vinyl ether; N-vinylpyrrolidone; acrylic and methacrylic acid; itaconic, crotonic, fumaric and maleic acids and the lower hydroxyalkyl mono and diesters thereof, such as the 2-hydroxyethyl fumarate and maleate, sodium acrylate and methacrylate; maleic anyhdride; 2-methacryloyloxyethylsulfonic acid and allylsulfonic acid.

Preferred water soluble monomers include 2-hydroxyethyl methacrylate, N,N-dimethylacrylamide, acrylic acid and methacrylic acid, and most preferably 2-hydroxyethyl methacrylate.

Suitable hydrophobic copolymerizable monomers include water insoluble conventional vinyl monomers such as: acrylates and methacrylates of the general formula where R₁ is as defined above and R₄ is a straight chain or branched aliphatic, cycloaliphatic or aromatic group having up to 20 carbon atoms which is unsubstituted or substituted by one or more alkoxy, alkanoyloxy or alkyl of up to 12 carbon atoms, or by halo, especially chloro or preferably fluoro, or C₃-C₅polyalkyleneoxy of 2 to about 100 units; acrylamides and methacrylamides of the general formula where R₁ and R₄ are as defined above; vinyl ethers of the formula         H₂C=CH-O-R₄ where R₄ is as defined above; vinyl esters of the formula         H₂C=CH-OCO-R₄ where R₄ is as defined above; maleates and fumarates of the formula         R₄OOC-HC=CH-COOR₄ where R₄ is as defined above; and vinylic substituted hydrocarbons of the formula         R₁CH=CHR₄ where R₁ and R₄ are as defined above.

Useful hydrophobic monomers include, although not all of them are comprised by the above mentioned general formulae, for example: methyl, ethyl, propyl, isopropyl, butyl, ethoxyethyl, methoxyethyl, ethoxypropyl, phenyl, benzyl, cyclohexyl, hexafluoroisopropyl or n-octyl-acrylates and -methacrylates as well as the corresponding acrylamides and methacrylamides; dimethyl fumarate, dimethyl maleate, diethyl fumarate, methyl vinyl ether, ethoxyethyl vinyl ether, vinyl acetate, vinyl propionate, vinyl benzoate, acrylonitrile, styrene, alpha-methylstyrene, 1-hexene, vinyl chloride, vinyl methyl ketone, vinyl stearate, 2-hexene and 2-ethylhexyl methacrylate.

Suitable crosslinking agents are diolefinic monomers such as: allyl acrylate and methacrylate; alkylene glycol and polyalkylene glycol diacrylates and dimethacrylates, such as ethylene glycol dimethacrylate and propylene glycol dimethacrylate; trimethylolpropane triacrylate; pentaerythritol tetraacrylate, divinylbenzene; divinyl ether; divinyl sulfone; bisphenol A diacrylate or methacrylate; methylene-bisacrylamide; diallyl phthalate; triallyl melamine; and hexamethylene diacrylate and dimethacrylate.

The following examples are presented for the purpose of illustration only and are not to be construed to limit the nature or scope of the instant invention in any manner whatsoever.

Example 1: Effect of Ozonation of Polysiloxane in a Halocarbon on Hydroperoxide Yield

To determine the relative ozonation rates of polysiloxane films in water as compared to perhalogenated hydrocarbons, separate samples of the same polysiloxane film are placed in water and in 1,1,2-trichloro-1,2,2-trifluoroethane (Freon® TF or 113) into which ozone, prepared in a standard ozone generator, is passed at room temperature for 30 minutes. The solubility of ozone in water is 4.5 ppm while in Freon® TF or 113 is 491 ppm.

The samples of polysiloxane films in the water system clump together very quickly. Analysis of hydroperoxide content (iodometric titration method) in said films shows 0.924 mg/g or a 0.09 % hydroperoxide content.

The samples of polysiloxane film ozonated in the Freon® TF or 113 system stay separate and analysis of hydroperoxide content on said films shows 12 mg/g or a 1.2 % hydroperoxide content.

Clearly ozonation of substrate materials in the Freon® system leads to higher hydroperoxide contents in the substrate materials after ozonation.

Example 2: A silicone macromer film is ozonated in water for five minutes at ambient temperature, allowed to air-dry for thirty minutes and is then placed in a beaker of deionized water with a nitrogen purge for 15 minutes. The film is then placed in a grafting solution which is made up of 100 g of deionized water, 1.0 g of N,N-dimethylacrylamide, 0.14 g of methylene-bisacrylamide and 0.3 g of ferrous ammonium sulfate hexahydrate. The film is kept in the grafting solution for 15 minutes under nitrogen before removal and evaluation. The film is found to be very lubricious, but is also opaque and distorted due to deep penetration of the grafting material into the substrate film.

Example 3: Effect of a Chain Transfer Agent During Ozonation for limiting the Graft to the Surface of the Substrate

A siloxane macromer film equilibrated in an aqueous 10 % isopropanol solution (90 g of water and 10 g of isopropyl alcohol) is dipped into a beaker of deionized water for five seconds, is then placed into a cylinder containing perfluoro-(1,3-dimethylcyclohexane) and ozonated for five minutes. After air drying for thirty minutes, the film is placed in a beaker of deionized water with a nitrogen purge for 15 minutes before being placed in a grafting solution which is made up of 100 g of deionized water, 1.0 g of N,N-dimethylacrylamide, 0.14 g of methylene-bisacrylamide and 0.3 g of ferrous ammonium sulfate hexahydrate.

Grafting is carried out under nitrogen for 15 minutes. The film is then removed and evaluated. The grafted film is clear, lubricious and undistorted since the grafting has been limited to the surface of the substrate due to the chain transfer characteristics of the aqueous isopropanol system.

This is in contrast to the grafted film prepared in example 2 which is opaque and distorted due to graft penetration.

Example 4: An experiment to show the effect of the chain transfer agent on subsequent grafting is carried out following the general procedure given in example 3 except that the amount of time the equilibrated silicone macromer film is dipped in water is reduced from five (5) seconds to three (3) seconds.

After the grafting step is completed, the film is not as lubricious as the film obtained in example 3.

It is clear that the amount of chain transfer agent (isopropanol in this case) present in the equilibrated substrate film is determinative of the amount of subsequent grafting which can take place. In this case there is more isopropanol present, and consequently less grafting takes place.

Example 5: The effect the amount of chain transfer agent present has on the amount of subsequent grafting is also demonstrated when the procedure of example 3 is exactly repeated except for the concentration of the aqueous 50 % isopropanol solution (50 g of water and 50 g of isopropyl alcohol) used to equilibrate the siloxane macromer film.

After the grafting step is completed, the film is not as lubricious as the film obtained in example 3 again showing that an increased amount of chain transfer agent (isopropanol in this case) present in the equilibrated substrate reduces the amount of subsequent grafting which can occur.

Example 6: A silicone macromer film is placed in an aqueous 10 % isopropanol solution (90 g of water and 10 g of isopropyl alcohol) and ozonated for five minutes at ambient temperature. After air-drying for 15 minutes, the film is placed in a beaker of water with a nitrogen purge for 15 minutes. It is then placed in a grafting solution of 100 g deionized water, 1.0 g N,N-dimethylacrylamide, 0.14 g of methylene-bisacrylamide and 0.3 g of ferrous ammonium sulfate hexahydrate for thirty minutes.

After this time the film is removed and found to be clear and ungrafted totally unlike the grafted film of example 2 which is very lubricious, opaque and distorted.

The only difference between the procedures of example 2 and of example 6 is the presence of an excess amount of chain transfer agent (isopropanol) in this example which prevents any grafting from occurring.

Example 7: A polysiloxane-polyurethane film is equilibrated in isopropyl alcohol and then ozonated for ten minutes in perfluoro-(1,3-dimethylcyclohexane) at ambient temperature.

A control film, not first equilibrated in isopropanol, is also ozonated for ten minutes in perfluoro-(1,3-dimethylcyclohexane) at ambient temperature.

Each ozonated film is then placed in a beaker of water. The film equilibrated with isopropanol remains clear while the control film, not equilibrated with isopropanol, becomes very opaque.

Polar hydroperoxide groups form in the ozonated control film allowing for the water uptake which leads to opacity.

In the ozonated film, first equilibrated with isopropanol, hydroperoxy groups are not present since the chain transfer agent (isopropanol) prevents their formation by transfer of a hydrogen atom to the free radical produced on the film during ozonization. Thus, the film is protected from hydroperoxide formation and film clarity is preserved.

Claims (9)

1. A process for modifying the surface characteristics of a preformed polymer substrate to impart hydrophilicity, hydrophobicity, optical properties, transmission properties, dyeability or tintability, opacity, diffraction differences, wettability, bonding characteristics, oxygen permeability, bactericidal properties, lubricity or multilayer membrane technology thereto by graft polymerization on said substrate, having peroxy and hydroperoxy groups on said polymer, of an ethylenically unsaturated monomer, which comprises contacting the polymer substrate with a solution which is or contains a chain transfer agent to saturate or swell said polymer, said solution being insoluble in the perhalogenated liquid medium used subsequently during the ozonation step, to limit subsequent hydroperoxidation and peroxidation to the surface of said polymer, ozonating the saturated or swollen polymer with ozone dissolved in a perhalogenated liquid medium insoluble in the solution being or containing the chain transfer agent, and then graft polymerizing an ethylenically unsaturated monomer onto essentially only the surface of the polymer substrate.
2. A process according to claim 1 wherein the chain transfer agent is a primary or secondary alkanol of 1 to 4 carbon atoms.
3. A process according to claim 2 wherein the chain transfer agent is dissolved in water or in an aqueous solution containing tert-butyl alcohol.
4. A process according to claim 2 wherein the chain transfer agent is isopropanol.
5. A process according to claim 1 wherein the perhalogenated liquid medium is carbon tetrachloride, 1,1,2-trichloro-1,2,2-trifluoroethane, perfluorohexane or perfluoro-(1,3-dimethylcyclohexane).
6. A process according to claim 5 wherein the perhalogenated liquid medium is perfluoro-(1,3-dimethylcyclohexane).
7. A process according to claim 1 wherein the graft polymerizing step is carried out in the presence of a variable metal ion to suppress homopolymerization during grafting of the ethylenically unsaturated monomer.
8. A process according to claim 7 wherein the metal ion is ferrous.
9. A process according to claim 1 wherein the polymer substrate is a contact lens.