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1 . HYDROPHILIC COPOLYMER COMPOSITIONS USEFUL AS CONTACT LENSES
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to hydrophilic plastic materials useful as soft contact lenses and, more particularly, relates to hydrophilic copolymers and to their preparation and use, which compositions are extremely resistant to clouding and/or discoloration in use.
2. The Prior Art
As is already known, compositions adapted for use as soft contact lenses have been developed from hydrophilic-type polymers which are softer and more easily accommodated by the eye then are the earlier hydrophobic-type polymers such as polymethyl methacrylate and the like. Hydrophilic polymers may be defmed typically as lightly cross-linked, essentially waterinsoluble copolymers derived from one or more monomers containing hydroxy groups for imparting to the polymers their affmity for water. These polymers may further be defined as coherent, 3-dimensional polymer structures or networks which have the ability to absorb or imbibe water, even in large quantities, e.g., up to 90 weight %, without dissolution. When containing water in any amount whatsoever, a hydrophilic polymer will expand correspondingly and, in its hydrated state, correctly may be designated as a hydrophilic polymer “gel,” or “hydrogel.” A specific class of polymer hydrogels which have gained particular commercial acceptance as soft contact lenses are those derived from acrylic esters. U.S. Pat. Nos. 2,976,576 and 3,220,960 issued to O. Wichterlc and D. Lim on Mar. 28, 1961, and on Nov. 30, 1965, respectively, are early patents which describe the use of methanol-insoluble hydrophilic acrylic ester polymer materials for the manufacture of soft contact lenses.
Acrylic ester hydrophilic polymers, for the most part, are derived by copolymerizing a mixture containing a major amount of a water-soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group, and a minor amount of a bifunctional diester of acrylic or methacrylic acid which cross-links the hydrophilic group-containing monomer as it polymerizes. The degree and type of cross-linking in the resulting polymer governs, to a large extent, its maximum water content, when fully hydrated.
Although accommodated much more comfortably by the wearer than the prior hard contact lenses, presently known soft contact lenses, as prepared from the aforedescribed polymer hydrogels, do have disadvantageous properties and have not been completely satisfactory. Hydrogel lenses favor the growth of pathological bacteria and fungi on their surfaces. If not regularly cleaned and sterilized or if they are stored in contaminated solutions, pathogens can be easily absorbed by the lens materials due to their flexible, hydrophilic polymer structure. Also, because of their aforesaid flexible, hydrophilic polymer structure, proteins and other normal substances in the eye environment can be easily diffused through the lenses with use. Accumulation of such substances in a soft contact lens causes its discoloration and clouding with repeated cleaning and sterilization techniques practiced by the wearer. Too, the lenses can lose sufficient amounts of water during use to deleteri
2 ously affect their dimensional stability and optical acuity.
It is an object of this invention, therefore, to provide a soft contact lens which is resistant to penetration by pathological organisms and chemicals damaging to the eye.
It is another object of this invention to provide a hydrophilic lens polymer which is sufficiently resistant to the diffusion of proteins and other migratory eye substances to prolong its life and optical effectiveness significantly.
It is yet another object of this invention to provide a soft contact lens which will retain a sufficient quantity of water during use to maintain its dimensional stability and optical acuity.
Theipresent invention is directed to novel hydrophilic copolymers adapted to the fabrication of desirable soft contact lenses, which copolymers contain a major portion of polymerized units of an hydroxyalkyl acrylate or methacrylate monomer with a minor portion of a nitro-substituted aryl acrylate or methacrylate monomer, and optionally with a minor amount of a further monomer. The further monomer may be present either as an impurity in the primary constituents of the copolymerization mixture or may be specifically added, e.g., to provide cross-linking sites for the developing polymeric chains from said primary monomer reactants. Without adding any significant amount of additional monomer in the copolymerization process, or without any modification of the surface of said copolymer products when shaped into lenses, the copolymer obtained herein is found to be extremely resistant to protein diffusion therethrough, thus exhibiting less clouding and discoloration in use by comparison to other acrylic ester-type hydrophilic polymer lenses in commerce at this time.
BRIEF DESCRIPTION OF THE DRAWING
Other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawing, FIG. 1. This drawing is a graph wherein the average disintegrations per minute (dpm) of lens samples fabricated from a copolymer product of this invention are plotted against the number of days these samples are stored in simulated tear solution containing 3H-lysozyme, compared to those exhibited by similarly tested lenses fabricated from an hydroxylethyl methacrylate (HEMA) homopolymer.»
DESCRIPTION OF THE PREFERRED ' EMBODIMENTS
The terms “polymer” and “copolymer” as used herein in the specification and claims in defming the hydrophilic, water-insoluble products of this invention refer to a macromolecular substance which has been produced by polymerizing two principal comonomers, although such product may incidentally contain polymerized units of one or more additional known monomers in minor amounts, for purposes such as cross-linking, increasing the wettability of soft contact lens products fabricated therefrom, or otherwise.
Likewise, as used herein in the specification and claims, the terms “hydrogel” and “polymer hydrogelf’ are each intended to refer to a shaped hydrophilic polymer, e.g., a soft contact lens, which contains imbibed
water in an amount ranging generally from less than 1% to 90% by weight of a shaped polymer. However, it is well recognized that to be completely comfortable to the eye, soft contact lenses for practical application normally contain at least about 25% water, preferably about 30% water, and still more preferably about 35% water, by weight. Accordingly, polymer hydrogels described herein as fabricated from the hydrophilic polymer products of this invention are those containing at least about 25% water by weight.
As previously stated, the hydrophilic copolymers of this invention are obtained by the simultaneous polymerization and cross-linking, in the presence of a free radical polymerization catalyst, of a mixture of:
(a) a hydroxy containing acrylate or alkacrylate;
(b) a nitro-substituted aryl acrylate or methacrylate;
(c) a cross-linking agent.
The hydroxy containing acrylates and alkacrylates and which are used in the copolymerization as component (a) may be represented by the structural fonnula:
*1‘ 1" CH2=C-‘COO-(-Cl-IZ—CI-I-—O'),7I~I
wherein R represents hydrogen or alkyl of from 1 to about 6 carbon atoms; R’ represents hydrogen or alkyl of from 1 to about 6 carbon atoms, preferably hydrogen, methyl or ethyl; and n is an integer of l to about 6. Such hydroxy containing acrylates and alkacrylates and may correctly be called glycol and polyglycol monoacrylates and monoalkacrylates. They are well-known in the art and may be obtained by the alkoxylation of an alphamethylene carboxylic acid, e.g., acrylic or methacrylic acid, with a vicinal alkylene oxide, such as ethylene oxide, propylene oxide or the like, until the desired amount of alkylene oxide has been reacted with the alpha-methylene carboxylic acid. Specific hydroxyalkyl acrylates and alkacrylates suitably employed as component (a) of the polymerization mixture include hydroxyethyl methacrylate (1-IEMA), hydroxypropyl acrylate (I-IPA), and hydroxypropyl methacrylate (HPMA). Normally, these monomers comprise at least about 93% by weight of the reaction mixture and of the copolymer product prepared. Percentages of the monomers preferred at present are from about 94% to 99.4%, by weight of the reaction mixture and product, with percentages ranging from 98% to 99.4% being the most preferred. The particular monomer presently preferred is hydroxyethyl methacrylate (I-IEMA).
The monomer used herein as component (b) of the copolymerization reaction mixture specifically is paranitrophenyl methacrylate. This compound has the formula:
1": r CH2-'=C"--'C—O—C15H4NO2
Units from this monomer generally may comprise only up to about 7% by weight of the reaction mixture and the prepared copolymer. Preferably, this monomer comprises from about 0.6% to 6% and still more preferably about 0.6% to 2% by weight of the reaction mixture and product.
As the optional component (c) of the copolymerization reaction mixture, i.e., the cross-linking agent, there
may be used an alkylene glycol diacrylate or dimethacrylate, represented by the formula:
1‘ ‘F CI-I;=C—COO—(CHg<)iOOC-'C=CI-I;
wherein R represents hydrogen or alkyl of 1 to 4 carbon atoms, and n is an integer of from l to about 20, preferably of from 1 to 4. As examples thereof may be mentioned ethylene glycol diacrylate, ethylene glycol dimethacrylate, propylene glycol diacrylate or dimethacrylate, and the like. Ethylene glycol dimethacrylate is the presently preferred difunctional cross-linking agent. When employed, this component normally is'incorporated in extremely minor concentrations, i.e., from about 0.10% to 0.99% by weight of the polymerization mixture.
Preparation of the copolymers herein may be effected by various techniques known in the art. The process may be carried out by bulk polymerization of the comonomer mixture in the presence of a free radical polymerization catalyst, such as any of the well-known inorganic or organic peroxy compounds. These catalysts may be employed in the range of about 0.05% to about 2% by weight of the monomer components. Typical catalysts include lauroyl peroxide, benzoyl peroxide, isopropyl percarbonate, azobisisobutyronitrile (AIBN) and known redox systems such as the ammonium persulfate-sodium metabisulfite combination and the like. Irradiation such as by ultraviolet light may also be employed to catalyze the copolymerization reaction. .
The copolymerization reaction generally is carried out at temperatures ranging between room temperature and 90° C., with temperatures of 50°—70° C. being preferred.
The copolymerization is advantageously carried out in bulk by preparing the comonomer mixture, adding the required quantity of free radical initiator thereto and then conducting the reaction in a sealed vessel at the selected temperature. According to presently preferred practice, the reaction mixture, after preparation, is cast into a depression on the upper surface of a glass plate. The glass plate is then covered with another having a similar depression on its lower surface. When positioned, the depression on the underside coincides with that of the lower plate to form a reservoir wherein copolymerization is effected upon heating the plate assembly at the desired reaction temperature. The reaction is conducted for a time period of 10-16 hours, depending upon the amount of reaction initiator employed, the relative proportions of the monomers and the nature of any solvent employed.
Alternatively, the reaction may be carried out in the presence of water-soluble solvents in which the monomer components are soluble. Suitable solvents include the lower aliphatic alcohols, dioxane, ethylene glycol, glycol esters or ethers, etc. When employed, the solvent will be present in the reaction medium in about equivalent volumetric proportions to the hydroxyalkyl methacrylate monomer. Upon completion of the reaction, the organic solvent may be removed by washing the reaction mixture with water, by distillation or by other known recovery procedures.
When employing an organic solvent, films may be formed by casting the copolymer solution onto a smooth surface, then drying and stripping the copolymer film therefrom. Also, polymeric products having a