HK1012721B - Improved itaconate copolymeric compositions for contact lenses - Google Patents

Description

This invention relates to improved itaconate copolymers useful as contact lens materials, and to contact lenses formed therefrom.

Early hard contact lenses were produced from polymethyl methacrylate (PMMA) or cellulose acetate butyrate (CAB). Later, rigid, gas permeable (RGP) contact lenses formed of silicone-containing copolymers were introduced, offering various advantages over PMMA and CAB lenses, particularly increased oxygen permeability. The original silicone-containing RGP lenses were based on copolymers of a silicone-containing monomer and methyl methacrylate, and such lenses remain on the market today.

A newer class of copolymers for silicone-containing RGP lenses is itaconate ester copolymers. U.S. Patent Nos. 4,152,508 (Ellis et al.), 4,330,383 (Ellis et al.) and 4,826,889 (Ellis et al.) disclose copolymers for contact lenses prepared from: a monofunctional siloxanyl ester monomer; an itaconate ester; an ester of a monohydric or polyhydric alkanol or phenol and a (meth)acrylic acid; a crosslinking agent; and preferably a hydrophilic monomer.

Known copolymers for RGP lenses also include copolymers of fluorinated itaconate esters, such as the copolymers disclosed in U.s. Patent Nos. 4,686,267 (Ellis et al.) and 4,996,275 (Ellis et al.), which are prepared from a fluorinated itaconate ester and an ethylenically unsaturated organosiloxane.

Other examples of itaconate ester copolymers for RGP lenses are disclosed in the following U.S. Patents: 4,602,074 (Mizutani et al.); 4,508,884 (Wittmann et al.) ; 4,743,667 (Mizutani et al.); 4,826,936 (Ellis); and 4,861,850 (Novicky).

As disclosed in U.S. Patent No. 4,152,508, the itaconate ester provides rigidity, hardness and some degree of wettability to the resultant copolymer. However, the inclusion of an itaconate ester tends to make the resultant copolymers more brittle.

Certain multifunctional organosiloxanes have been described as useful for adding higher impact strength and reducing brittleness of itaconate RGP copolymers. U.S. Patent No. 4,826,936 describes a class of multifunctional organosiloxanes having the formula: wherein n is 0 to 10, the total "a" values is at least 2, each Y' is an unsaturated polymerizable group, and the remaining variables have the meanings given in the patent. A preferred monomer for itaconate copolymers is 1,3-bis(methacryloxypropyl)-1,1,3,3 tetra(trimethylsiloxy) disiloxane (designated SM-6). Additionally, the aforementioned US Patents Nos 4,686,267 and 4,996,275, disclose that the fluorinated itaconate copolymers may include a multifunctional organosiloxane of U.S. 4,826,936. U.S. Patent No. 4,743,667 also discloses multifunctional organosiloxane monomers for RGP contact lens materials described as having high impact strength and reduced brittleness of itaconate ester copolymers. Preferred monomers include 1,5-bis(methacryloxypropyl)-1,1,3,3,5,5-hexamethyl trisiloxane (designated BiMAPPS-1) and 1,3-bis(methacryloxyethoxypropyl)-1,1,3,3-tetramethyl disiloxane (designated BiMAPPS-2). The approaches suggested in each of these patents involve employing a relatively rigid multifunctional organosiloxane wherein the number of siloxane units bridging the polymerizable functional groups is preferably 2 to 4.

The present invention relates to an improved itaconate ester copolymer useful as a contact lens material, particularly as a rigid gas permeable (RGP) lens material. The copolymer is the polymerization product of a mixture comprising:

• (a) from 5 to 60 weight percent of an itaconate;
• (b) from 2 to 60 weight percent of an ethylenically unsaturated, monofunctional organosiloxane monomer;
• (c) from 1 to 25 weight percent of a hydrophilic monomer; and
• (d) an amount of a siloxane compound represented by formula (I) : wherein: each A is independently an activated unsaturated radical;each R is independently selected from a C1-C12 monovalent hydrocarbon radical, a C1-C12 monovalent hydrocarbon radical containing ether linkages, a halogen-substituted C1-C12 monovalent hydrocarbon radical, and a halogen-substituted C1-C12 monovalent hydrocarbon radical containing ether linkages;each R' is independently a C1-C22 divalent hydrocarbon radical; andn averages from 15 to 50;and wherein the copolymer is clear and said amount of said siloxane compound of formula (I) is effective to provide the copolymer with a toughness of at least 1.2 MPa.mm/mm, as determined by the ASTM D 790M-86 standard on a 0.5 mm disk sample.

The copolymers represent improved itaconate-containing rigid gas permeable (RGP) materials having good toughness and reduced brittleness.

We have found, that the siloxane compounds of formula (I) i.e. component (d) of the polymerisation mixture, are especially effectice in improving the toughness of itaconate copolymers, in contrast to the above-described previous approaches wherein itaconate copolymers employed "shorter" and more rigid multifunctional siloxane compounds.

The itaconate esters used herein are known in the art and include compounds of formula (II): wherein X and Y, which may be the same or different, are : hydrogens C₁-C₁₈ alkyl or fluoro-substituted alkyl groups; C₅-C₁₈ cycloalkyl or fluoro-substituted cycloalkyl groups; C₂-C₆ alkenyl groups or fluoro-substituted alkenyl groups; phenyl groups or fluoro-substituted phenyl groups; benzyl or fluoro-substituted benzyl groups; phenethyl or fluoro-substituted phenethyl groups; or C₂-C₁₈ ether or fluoro-substituted ether groups; provided that at least one of X and Y is other than hydrogen.

Representative itaconates include methyl itaconate, dimethyl itaconate, phenyl itaconate, methyl phenyl itaconate, bis (1,1,1,3,3,3-hexafluoro-2-propyl) itaconate, bis (2,2,2-trifluoroethyl) itaconate, bis (1H,1H-perfluorooctyl) itaconate, bis (1H,1H,1H-perfluoroundecyl) itaconate, bis (perfluoro-t-butyl) itaconate, bis (pentafluorophenyl) itaconate, bis (2H,2H-perfluorobenzyl) itaconate, and bis (pentafluorophenylmethyl) itaconate.

The itaconate is employed at 5 to 60% by weight of the monomeric mixture from which the copolymer is prepared in order to provide copolymers having sufficient rigidity and hardness. According to preferred embodiments, the itaconate is present at 20 to 55 weight percent, with 30 to 50 weight percent being more preferred, in the monomeric mixture.

As recognized in the art, when an itaconate is used in place of, or in combination with, methyl methacrylate, the resultant copolymer has increased rigidity and hardness.. However, the inclusion of the itaconate ester also tends to make the resultant copolymer more brittle. RGP contact lens materials are frequently provided in the form of rods, buttons, or lens blanks, which are subsequently machined into contact lenses having desired lens surfaces. When the copolymeric material is brittle, difficulties can be encountered in machining such materials, such as chipping or flaking or even breakage of the material.

We have found that the inclusion in the copolymers of the compounds of formula (I) can effectively toughen the resultant copolymer, thereby overcoming the disadvantages attributed to the itaconate monomer. The copolymers represent improved itaconate-containing RGP materials having optical clarity, reduced brittleness, and improved toughness. Thus, the copolymers have a toughness of at least 1.2 MPa·mm/mm (as determined by ASTM D 790M-86 standards on 0.5 mm disk samples), and preferably a toughness of at least 1.5 MPa·mm/mm.

The compounds of formula (I) can be produced by general methods known in the art, such as the methods disclosed in U.S. Patent Nos. 4,153,641, the disclosure of which is incorporated herein by reference.

While various multifunctional organosiloxanes have been described as useful in contact lens formulations, including siloxane compounds of the general formula (wherein A, R' and R generally correspond to the definitions given for formula (I), and n' has various ranges), we have found that for itaconate copolymeric contact lens materials, a relatively narrow class of the formula (I) compounds provide consistently the desired effects.

In formula (I), n averages at least 15. Accordingly, the difunctional siloxane compound of formula (I) is relatively flexible. While not wishing to be bound by any particular theory, it appears that the terminal activated unsaturated groups on such "longer" and relatively flexible difunctional compounds can react with different itaconate portions of the resultant copolymer, thereby providing relatively "flexible" crosslinks between these itaconate portions. In turn, the copolymers have increased toughness and are less brittle.

In contrast, difunctional siloxane compounds corresponding to formula (I) which contain fewer siloxane units and are more rigid do not provide itaconate copolymers having the desired improvements attributed to the present invention.

On the other hand, when difunctional siloxane compounds which contain a relatively large number of siloxane units are employed in conjunction with itaconate ester monomers, phase separation of the individual components in the monomeric mixture appears to occur. This can result in a copolymer which is not transparent and/or has insufficient toughness.

Accordingly, n in formula (I) averages no more than 50.

In formula (I), A is an activated unsaturated radical, i.e. an unsaturated group that includes a substituent for facilitating free radical polymerization, preferably a vinyl-containing substituent. Representative A radicals include (meth)acryloxy, (meth)acrylamido and styryl. As used herein, the term "(meth)" denotes optional methyl substitution. Thus, a term such as "(meth)acrylate" denotes "acrylate or methacrylate". The methacryloxy radical is more preferred.

Each R' is independently a C₁-C₂₂ divalent hydrocarbon radical. Representative R' radicals include alkylene radicals, and preferred radicals include methylene, propylene and butylene.

Each R is independently selected from the group consisting of a C₁-C₁₂ monovalent hydrocarbon radical, a C₁-C₁₂ monovalent hydrocarbon radical containing ether linkages, a halogen-substituted C₁-C₁₂ monovalent hydrocarbon radical, and a halogen-substituted C₁-C₁₂ monovalent hydrocarbon radical containing ether linkages. Representative R radicals include alkyl, cycloalkyl, aryl, aralkyl, alkaryl radicals, alkoxyalkyl radicals, and halogen-substituted derivatives thereof. Preferred radicals include C₁-C₄ alkyl, with methyl being especially preferred.

The most preferred siloxane compound of formula (I) has the formula:    wherein n averages about 25.

The siloxane compound is employed in an amount effective to reduce the brittleness of the resultant copolymer. Generally, the siloxane compound is present at from about 3 to about 25 weight percent of the initial monomeric mixture, preferably at from 5 to 20 weight percent, with from 9 to 15 weight percent being especially preferred. One skilled in the art can readily determine optimal amounts for specific formulations.

The ethylenically unsaturated monofunctional organosiloxane monomer (b) is useful for increasing oxygen permeability of the copolymer. Preferred organosiloxanes are monofunctional organosiloxanes containing a (meth)acrylate radical, such as tris(trimethylsiloxy)methacryloxy propylsilane (TRIS), pentamethyldisiloxanylmethylmethacrylate, phenyltetramethyldisiloxanylethylacrylate, heptamethylcyclotetrasiloxanepropylmethacrylate, heptamethylcyclotetrasiloxanemethylmethacrylate and methyldi(trimethylsiloxy)methacryloxymethyl silane. Other organosiloxane monomers known in the art are described in U.S. Patent No. 4,686,267, the disclosure of which is incorporated herein by reference. The organosiloxane monomer is included in the monomeric mixture at from 2 to 60 weight percent, preferably at from 5 to 50 weight percent.

The hydrophilic monomer (c) is useful for increasing hydrophilicity and improving wettability of the resultant copolymer. Conventional hydrophilic monomers include: hydrophilic (meth)acrylates, such as 2-hydroxyethyl methacrylate; hydrophilic (meth)acrylamides, such as methacrylamide and N,N-dimethylacrylamide; (meth)acrylic carboxylic acids, such as methacrylic acid; and vinyl lactams, such as N-vinylpyrrolidone. Preferably, the hydrophilic monomer is methacrylic acid, N-vinylpyrrolidone or a mixture thereof. The hydrophilic monomer is included in the monomeric mixture at from 1 to 25 weight percent, preferably at from 5 to 15 weight percent.

Other materials known for contact lens formulations may optionally be employed in the monomeric mixture from which the itaconate copolymer is prepared.

A (meth)acrylate monomer (e) may be included which further modifies hardness of the copolymer. Such monomers are preferably an ester of a C₁-C₂₀ monohydric or polyhydric alkanol or phenol and (meth)acrylic acid. Representative monomers include: alkyl (meth)acrylates, such as methyl methacrylate, ethyl methacrylate, and neopentyl methacrylate; cycloalkyl-containing (meth)acrylates, such as cyclohexyl methacrylate; and phenyl methacrylate. This (meth)acrylate may be included in the monomeric mixture at from 0 to 50 weight percent, preferably at from 5 to 35 weight percent.

A conventional non-silicone containing crosslinking agent (f) may be employed. Crosslinking agents include polyfunctional derivatives of (meth)acrylic acid, (meth)acrylamide and other multivinyl substituted compounds. Representative crosslinking agents include: ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, neopentyl glycol dimethacrylate, trimethylolpropane trimethacrylate, hexamethylene bisacrylamide and divinyl benzene. The crosslinking agent may be included in the monomeric mixture at from 0 to 20 weight percent, preferably at from 1 to 10 weight percent.

Accordingly, preferred copolymers are prepared from a monomeric mixture comprising:

• (a) an itaconate ester at from 20 to 55 weight percent;
• (b) an ethylenically unsaturated, monofunctional organosiloxane monomer at from 5 to 50 weight percent;
• (c) a siloxane compound of formula (I) at from 5 to 20 weight percent, and more preferably from 9 to 15 weight percent;
• (d) a hydrophilic monomer at from 5 to 15 weight percent;
• (e) a (meth)acrylate monomer at from 0 to 50 weight percent, preferably from 5 to 35 weight percent; and
• (f) a crosslinking agent at from 0 to 20 weight percent, preferably from 1 to 10 weight percent.

Other optional components include conventional free radical initiators, which are generally employed at from 0.01 to 2% by weight, and coloring agents.

The monomeric mixtures can be polymerized by methods known in the art, preferably in the presence of heat or ultraviolet radiation, and if desired, the copolymers can be treated with gamma radiation to reduce any unreacted monomers. Preferably, the mixtures are molded in a shape which is subsequently machined into the form of a contact lens, such as rod stock, a lens button, or a lens blank containing one finished surface. Alternately, the mixtures can be molded directly in the form of a contact lens.

The following examples further illustrate preferred embodiments of the invention.

Various copolymers were polymerized from the monomeric mixtures listed in the following tables, wherein the amounts of the individual components are given in parts by weight. The mixtures were placed in cylindrical tubes, and the tubes were deoxygenated and sealed. The mixtures were polymerized by heating in a water bath (40°C for 3 days), followed by heating in an oven (65°C for 2 days). Various copolymers were subjected to post-polymerization treatment to reduce unreacted monomers, including exposure to gamma radiation in an inert atmosphere.

Toughness and modulus were determined according to ASTM-D 790M-86 standards on 0.5-mm disk samples cut from the polymerized rods. Standard deviation is listed parenthetically in the tables. Permeability was determined on 0.5-mm disk samples by the gas-to-gas method.

The results are summarized in the following tables, which include the following abbreviations:

AIBN

2,2-azobisisobutyronitrile (initiator)

AIVN

2,2-azobisisovaleronitrile (initiator)

BHI

bis(1,1,1,3,3,3-hexafluoro-2-propyl) itaconate

MAA

methacrylic acid

MMA

methyl methacrylate

NPGDMA

neopentyl glycol dimethacrylate

NPMA

neopentyl methacrylate

NVP

N-vinyl pyrrolidone

TRIS

tris(trimethylsiloxy)silylpropyl methacrylate

M2D15

Formula (I) wherein each R is methyl, each R' is butylene, each A is methacrylate, and n averages about 15

M2D25

Formula (I) wherein each R is methyl, each R' is butylene, each A is methacrylate, and n averages about 25

M2D50

Formula (I) wherein each R is methyl, each R' is butylene, each A is methacrylate, and n averages about 50

M2D100

Formula (I) wherein each R is methyl, each R' is butylene, each A is methacrylate, and n averages about 100

M2DF100

Formula (I) wherein each R is trifluoromethyl, each R' is butylene, each A is methacrylate, and n averages about 100

TABLE I

	EX 1	EX 2*	EX 3*
BHI	43.4	45.2	45.0
	13.0	7.2	3.5
MAA	3.5	5.7	5.0
MMA	16.2	10.3	10.0
NPGDMA	7.6	0	0
NVP	7.6	8.3	8.0
TRIS	8.7	25.2	45.0
AIBN	0.20	0.17	0.17
AIVN	0.059	0.050	0.050
Toughness- MPa·mm/mm	2.48 (0.57)	0.98 (0.07)	0.53 (0.03)
Modulus- MPa	1613 (46)	1252 (25)	1424 (45)
Clarity	C	C	C

TABLE I

* Denotes a comparative example

TABLE II

	EX 4	EX 5	EX 6	EX 7	EX 8
BHI	40.0	40.0	40.0	39.9	43.4
	12.0	11.0	10.0	12.0	13.0
MAA	5.0	5.0	5.0	3.2	3.5
MMA	0	0	0	0	16.2
NPGDMA	5.0	5.0	5.0	7.0	7.6
NPMA	25.0	27.0	29.0	22.9	0
NVP	5.0	5.0	5.0	7.0	7.6
TRIS	8.0	7.0	6.0	8.0	8.7
AIBN	0.17	0.17	0.17	0.17	0.20
AIVN	0.054	0.054	0.054	0.054	0.059
Toughness- MPa·mm/mm	2.13 (0.23)	1.83 (0.62)	1.56 (0.39)	1.63 (0.50)	2.80 (0.76)
Modulus- MPa	973 (3)	1057 (37)	1016 (24)	1422 (29)	1450 (44)
	72	66	59	80	40
Clarity	C	C	C	C	C

TABLE III

	EX 9*	EX 10*	EX 11*	EX 12	EX 13*	EX 14*
BHI	39.9	45.0	45.0	43.4	39.9	39.9
	12.0	3.5	14.0	13.0	0	0
	0	0	0	0	12.0	0
	0	0	0	0	0	12.0
MAA	3.2	5.0	5. 0	3.5	3.2	3.2
MMA	0	10.0	10.0	16.2	0	0
NPGDMA	7.0	0	0	7.6	7.0	7.0
NPMA	22.9	0	0	0	22.9	22.9
NVP	7.0	8.0	8.0	7.6	7.0	7.0
TRIS	8.0	25.0	25.0	8.7	8.0	8.0
AIBN	0.17	0.17	0.17	0.20	0.17	0.17
AIVN	0.054	0.050	0.050	0.059	0.054	0.054
Toughness- MPa·mm/mm	1.82 (0.45)	0.40 (0.02)	0.42 (0.05)	3.00 (0.55)	0.80 (0.15)	1.08 (0.08)
Modulus- MPa	1362 (36)	1415 (23)	712 (77)	977 (153)	703 (19)	1290 (31)
Clarity	H	C	C	C	H	C

TABLE III

* Denotes a comparative example

The data summarized in Table I demonstrate that compounds of formula (I) wherein n averages 15 approach the lower end of the range of this class of siloxane compounds which are effective to provide itaconate copolymers having sufficient toughness. Copolymers having satisfactory toughness were obtained with M₂D₁₅, but a relatively high amount of M₂D₁₅ was required to achieve the advantageous effects. Compare Example 1, with a toughness of 2.48 MPa·mm/mm, with Examples 2 and 3, where the toughness was well below the required minimum of 1.2 MPa·mm/mm.

The data summarized in Table III demonstrate that compounds of formula (I) wherein n averages 50 approach the upper end of the range of the siloxane compounds which are effective to provide itaconate copolymers having good toughness and sufficient clarity for contact lens materials. The copolymer of Example 12 was clear and exhibited very good toughness (3.00 MPa·mm/mm). Example 9 exhibited good toughness, but appeared hazy, and Examples 10 and 11 were clear but exhibited low toughness. In contrast, the copolymer of Example 13, which employed M₂D₁₀₀, had both low toughness and a hazy appearance.

Tables IV and V further illustrate the effect of the number of siloxane units in the formula (I) compounds when the compounds are employed in itaconate copolymers. Each of the formulations in Table IV, and each of the formations in Table V, employ different formula (I) compounds. Generally, copolymers having increased toughness were achieved by employing formula (I) compounds having higher "n" values. However, phase separation occurred with compounds having higher "n" values, resulting in lower toughness and/or haziness. Even though halogen substitution on the siloxane compounds (M₂DF₁₀₀) appeared to lessen the effects of phase separation, such compounds of formula (I), wherein n averages 100, still exhibited insufficient toughness. TABLE IV

	EX 1	EX 8	EX 12
BHI	43.4	43.4	43.4
	13.0	0	0
	0	13.0	0
	0	0	13.0
MAA	3.5	3.5	3.5
MMA	16.2	16.2	16.2
NPGDMA	7.6	7.6	7.6
NVP	7.6	7.6	7.6
TRIS	8.7	8.7	8.7
AIBN	0.20	0.20	0.20
AIVN	0.059	0.059	0.059
Toughness- MPa·mm/mm	2.48 (0.57)	2.80 (0.76)	3.00 (0.55)
Modulus- MPa	1613 (46)	1450 (44)	977 (153)
	42	40	44
Clarity	C	C	C

TABLE V

	EX 7	EX 9*	EX 13*	EX 14*
BHI	39.9	39.9	39.9	39.9
	12.0	0	0	0
	0	12.0	0	0
	0	0	12.0	0
	0	0	0	12.0
MAA	3.2	3.2	3.2	3.2
NPGDMA	7.0	7.0	7.0	7.0
NPMA	22.9	22.9	22.9	22.9
NVP	7.0	7.0	7.0	7.0
TRIS	8.0	8.0	8.0	8.0
AIBN	0.17	0.17	0.17	0.17
AIVN	0.054	0.054	0.054	0.054
Toughness- MPa·mm/mm	1.63 (0.50)	1.82 (0.45)	0.80 (0.15)	1.08 (0.08)
Modulus- MPa	1422 (29)	1362 (36)	703 (19)	1290 (31)
	80	89	122	60
Clarity	C	H	H	C

TABLE V

* Denotes a comparative example

Tables VI and VII illustrate additional copolymers which employ the more preferred formula (I) compound, M₂D₂₅. Generally, copolymers having higher toughness can be obtained by increasing the amount of this formula (I) compound. Additionally, these examples demonstrate that in order to provide copolymers having a toughness of at least 1.2 MPa·mm/mm, the minimum amount of the formula (I) compound will be about 3% by weight, depending on the specific formulation (in Examples 21-23 the required toughness was seaved with 2.8% and 2.9% of M₂D₂₅, whereas in Examples 24 and 25 a similar amount of M₂D₂₅ proved to be insufficient): TABLE VI

	EX 15	EX 16	EX 17	EX 18	EX 19	EX 20
BHI	45.0	34.1	46.6	46.6	41.4	39.9
	14.0	9.7	9.5	9.5	6.7	6.2
MAA	5.0	2.4	4.8	4.8	3.6	4.8
MMA	10.0	0	0	0	0	0
NPGDMA	0	11.7	11.4	6.6	8.7	11.4
NVP	8.0	6.3	3.8	8.5	6.4	8.6
TRIS	25.0	35.5	23.8	23.8	33.0	29.0
AIBN	0.17	0.19	0.20	0.18	0.18	0.21
AIVN	0.050	0.024	0	0.024	0.016	0
Toughness- MPa·mm/mm	3.77 (0.04)	3.28 (0.27)	2.34 (0.36)	3.84 (0.29)	2.29 (0.67)	2.05 (0.31)
Modulus- MPa	1005 (20)	1030 (10)	962 (81)	1141 (13)	1125 (18)	1330 (37)

TABLE VII

	EX 21	EX 22	EX 23	EX 24*	EX 25*	EX 26*
BHI	47.7	47.1	34.2	46.6	40.9	45.0
	2.9	2.9	2.8	2.8	2.8	3.5
MAA	2.4	3.6	4.8	4.8	4.8	5.0
MMA	O	0	0	0	0	10.00
NPGDMA	11.7	11.5	11.4	4.8	4.8	0
NVP	8.8	8.7	8.6	8.6	8.6	8.0
TRIS	26.3	26.0	38.0	32.3	38.0	25.0
AIBN	0.21	0.16	0.18	0.21	0.17	0.17
AIVN	0	0.048	0	0	0.048	0.050
Toughness- MPa·mm/mm	1.48 (0.10)	1.19 (0.11)	1.39 (0.23)	0.93 (0.08)	0.97 (0.05)	0.56 (0.03)
Modulus- MPa	1387 (15)	1422 (31)	1289 (15)	1269 (29)	1190 (28)	1390 (33)

TABLE VII

* Denotes a comparative example

Claims (18)

1. A copolymer useful as a contact lens material which is the polymerization product of a mixture comprising:

   (a) from 5 to 60 weight percent of an itaconate;

   (b) from 2 to 60 weight percent of an ethylenically unsaturated, monofunctional organosiloxane monomer;

   (c) from 1 to 25 weight percent of a hydrophilic monomer; and

   (d) an amount of a siloxane compound represented by formula (I) : wherein:

   each A is independently an activated unsaturated radical;

   each R is independently selected from a C₁-C₁₂ monovalent hydrocarbon radical, a C₁-C₁₂ monovalent hydrocarbon radical containing ether linkages, a halogen-substituted C₁-C₁₂ monovalent hydrocarbon radical, and a halogen-substituted C₁-C₁₂ monovalent hydrocarbon radical containing ether linkages;

   each R' is independently a C₁-C₂₂ divalent hydrocarbon radical; and

   n averages from 15 to 50;

   and wherein the copolymer is clear and said amount of said siloxane compound of formula (I) is effective to provide the copolymer with a toughness of at least 1.2 MPa.mm/mm, as determined by the ASTM D 790M-86 standard on a 0.5 mm disk sample.
2. A copolymer of Claim 1, wherein, in formula I,

   each A is independently a vinyl-containing radical;

   each R is independently a C₁-C₄ alkyl radical; and

   each R' is independently an alkylene radical.
3. A copolymer of Claim 2, wherein siloxane compound (d) has the formula: wherein n averages about 25.
4. A copolymer of any preceding claim, wherein itaconate (a) is a compound of the formula: wherein X and Y are independently selected from hydrogen; C₁-C₁₈ alkyl or fluoroalkyl groups; C₅-C₁₈ cycloalkyl or fluoro-substituted cycloalkyl groups; C₂-C₆ alkenyl or fluoro-substituted alkenyl groups; phenyl or fluoro-substituted phenyl groups; benzoyl or fluoro-substituted benzoyl groups; phenethyl or fluoro-substituted phenethyl groups; and C₂-C₁₈ ether or fluoro-substituted ether groups; with the proviso that at least one of X and Y is other than hydrogen.
5. The copolymer of Claim 4, wherein itaconate (a) is bis(1,1,1,3,3,3-hexafluoro-2-propyl) itaconate.
6. A copolymer of any preceding claim, wherein monofunctional organosiloxane monomer (b) is a (meth)acrylate radical-containing compound.
7. A copolymer of Claim 6, wherein the organosiloxane monomer (b) is tris(trimethylsiloxy)silylpropyl methacrylate.
8. A copolymer of any preceding claim, wherein the hydrophilic monomer (c) is selected from hydrophilic (meth)acrylates, hydrophilic (meth)acrylic carboxylic acids, vinyl lactams, and mixtures thereof.
9. A copolymer of Claim 8, wherein the hydrophilic monomer (c) is selected from methacrylic acid, N-vinylpyrrolidone, and mixtures thereof.
10. A copolymer of any preceding claim, wherein said mixture also comprises:

    (e) from 0 to 50 weight percent of a hardness modifying (meth)acrylate monomer; and

    (f) from 0 to 20 weight percent of a non-silicone containing crosslinking agent.
11. A copolymer of Claim 10, wherein the (meth)acrylate monomer (e) is at least one alkyl (meth)acrylate.
12. A copolymer of Claim 11, wherein the alkyl (meth)acrylate is selected from methyl methacrylate, neopentyl methacrylate, and mixtures thereof.
13. A copolymer of any one of Claims 10-12, wherein the crosslinking agent (f) is neopentyl glycol dimethacrylate.
14. A copolymer of any one of Claims 10-13, wherein said mixture comprises:

    (a) the itaconate at from 20 to 55 weight percent;

    (b) the ethylenically unsaturated, monofunctional organosiloxane monomer at from 5 to 50 weight percent;

    (c) the hydrophilic monomer at from 5 to 15 weight percent;

    (d) the siloxane compound of formula (I) at from 5 to 20 weight percent;

    (e) the (meth)acrylate monomer at from 5 to 35 weight percent; and

    (f) the non-silicone containing crosslinking agent at from 1 to 10 weight percent.
15. A copolymer of Claim 14, wherein said mixture comprises:

    (a) the itaconate at from 30 to 50 weight percent; and

    (d) the siloxane compound of formula (I) at from 9 to 15 weight percent;
16. A copolymer of any preceding claim, wherein said mixture consists essentially of: bis(1,1,1,3,3,3-hexafluoro-2-propyl) itaconate; the compound wherein n averages about 25; tris (trimethylsiloxy)silylpropyl methacrylate; methacrylic acid; N-vinylpyrrolidone; at least one monomer selected from methyl methacrylate and neopentyl methacrylate; and at least one free radical initiator.
17. A copolymer of any one of Claims 1-15, wherein said mixture consists essentially of: bis(1,1,1,3,3,3-hexafluoro-2-propyl) itaconate; the compound
18. A rigid, gas permeable contact lens formed of a copolymer according to any preceding claim.