Optically clear polymer compositions containing an interpenetrant

Disclosed are optically clear xerogel polymer compositions containing an interpenetrant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is directed to optically clear xerogel polymer compositions 
containing an interpenetrant. These compositions are characterized by the 
presence of hydroxyl functionalities which are blocked with a removable 
blocking group which, after removal of the blocking groups and hydration 
of the composition, will have a water content of at least 10 weight 
percent and preferably from about 35 to about 70 weight percent and a 
modulus of at least about 2 Mdyne/cm.sup.2. 
This invention is further directed to methods for the preparation of 
optically clear hydrogel polymer compositions containing hydroxyl 
functionality and an interpenetrant. 
2. State of the Art 
Hydrogel polymer compositions and the use of these compositions in 
ophthalmic devices, especially contact lenses, are well known in the art. 
Such hydrogel polymer compositions are typically manufactured as 
copolymeric systems, optionally cross-linked which are formed in the 
xerogel state where they are hard materials. This xerogel, in the presence 
of water or other water containing solvent, hydrates and undergoes a 
change so that it attains the hydrogel state. Upon hydration, the 
resulting polymer composition contains water and, accordingly, becomes 
softer and more pliable as compared to the polymer composition prior to 
hydration. The degree of softness and pliability is related to the amount 
of water incorporated into the polymer composition after hydration. 
Additionally, contact lenses derived from polymer compositions having 
large amounts of water provide greater wearer comfort and higher oxygen 
permeability. Accordingly, the art has generally been directed to the 
incorporation of large amounts of water into such polymer compositions. 
However, notwithstanding the desirability of incorporating large amounts of 
water into hydrogel polymer compositions, one problem typically 
encountered is that as the water contents increases, the structural 
rigidity of the polymer composition, as measured by its modulus, decreases 
and can reach a point where the structural rigidity is less than 
desirable. Accordingly, the hydrogel polymer composition is typically 
formulated to balance the need for a large water content and for a 
suitable modulus and the values achieved for both parameters is often 
compromised from ideal values. 
In regard to the above, it is known in the art that an interpenetrant 
incorporated into a polymer composition increases the structural rigidity 
of the composition thereby providing a means to obtain a desired level of 
water content while retaining suitable structural rigidity. 
However, a problem is encountered in the area of ophthalmic devices when a 
large amount of an interpenetrant, i.e., greater than about 1.5 weight 
percent (based on the dry weight of the polymer composition), is 
incorporated into a hydrogel polymer composition comprising hydroxyl 
groups. Specifically, it has been found that the use of such a large 
amount of interpenetrant in such hydrogel polymer compositions renders the 
resulting composition optically opaque. Without being limited to any 
theory, it is believed that the hydroxyl comprising polymer compositions 
have poor solubility for the interpenetrant and, accordingly, optical 
opacity for the resulting composition arises from phase separation of the 
interpenetrant from the polymer. In any event, optically opaque materials 
cannot be used in ophthalmic devices because optical clarity is an 
overriding criticality in these devices. 
In one embodiment, the art has circumvented this problem by including large 
quantities of an organic solvent (e.g., about 80-95 weight percent or 
more), such as dimethyl sulfoxide (DMSO), with an interpenetrant 
chemically modified to include a reactive vinyl group. See, for example, 
European Patent Application Publication No. 0 456 611. The organic solvent 
acts to solubilize the interpenetrant as well as the monomer/polymer 
composition and the reactive vinyl group acts to covalently incorporate 
the interpenetrant into the polymer backbone during polymerization. 
After polymerization, the resulting polymer is solvated (i.e., not a 
xerogel). The inclusion of large amounts of solvent in the polymers via 
such methods complicates the manufacturing process of ophthalmic devices 
from hydrogel materials because such materials are typically cast or 
formed in the xerogel state, and once solvated, become soft and pliable 
which makes machining the solvated materials difficult. Accordingly, the 
final shape and other physical characteristics of the polymeric article 
are preferably formed during the xerogel state, i.e., in the absence of 
significant amounts of any solvent. The inclusion of large amounts of 
solvent in the prior art methods for forming an optically clear polymer 
composition containing an interpenetrant will, however, preclude the 
formation of such a xerogel composition. 
In view of the above, the art has heretofore been seeking, without success, 
an optically clear xerogel polymer composition comprising hydroxyl groups 
on the polymer and having incorporated therein at least about 1.5 weight 
percent of an interpenetrant. 
SUMMARY OF THE INVENTION 
This invention is directed, in part, to optically clear xerogel polymer 
compositions comprising a polymer and at least about 1.5 weight percent of 
an interpenetrant (based on the weight of the xerogel) wherein the polymer 
comprises blocked hydroxyl functional groups wherein the blocking groups 
are removable. The xerogel polymer compositions are further characterized 
as forming, upon deblocking and hydration, a hydrogel polymer composition 
having a water content of at least 10 weight percent and preferably from 
about 35 to about 70 weight percent and a modulus of at least about 2 
Mdynes/cm.sup.2. 
Accordingly, in one of its composition aspects, this invention is directed 
to an optically clear xerogel polymer composition comprising: 
a polymer comprising blocked hydroxyl functional groups, and 
at least about 1.5 weight percent of an interpenetrant based on the total 
weight of the xerogel polymer composition 
wherein said composition has a sufficient optical clarity to permit the 
passage of at least 80% of visible light through a 0.1 millimeter (mm) 
thick sample of the composition. 
In a preferred embodiment, the polymer composition described above is 
cross-linked. In a further preferred embodiment, the polymer composition, 
after deblocking and hydration, has a water content of at least 10 weight 
percent and even more preferably from about 35 to about 70 weight percent 
water based on the total weight of the hydrated hydrogel polymer 
composition and a modulus of at least about 2 Mdynes/cm.sup.2. 
In a further preferred embodiment, the hydrated hydrogel polymer 
composition prepared from the xerogel polymer described above has a 
modulus of from 2 to 50 Mdyne/cm.sup.2, more preferably 5 to 30 
Mdyne/cm.sup.2 and still more preferably greater than about 12 
Mdyne/cm.sup.2, a percent elongation of greater than about 100% and more 
preferably greater than about 175% and an oxygen permeability of greater 
than about 10 Dk units and more preferably greater than about 18 Dk units. 
In a still further preferred embodiment, the hydrated hydrogel polymer 
composition has a water content of from about 45 to 70% and more 
preferably about 50%. 
This invention is also directed, in part, to the unexpected discovery that 
the preparation of such optically clear hydrogel polymer compositions can 
be obtained by placing a removable block group on the hydroxyl groups of 
the monomer component(s) prior to polymerization and incorporation of an 
interpenetrant therein. After polymerization and interpenetrant 
incorporation, the blocking groups are removed and the xerogel polymer 
composition hydrated to provide for an optically clear hydrogel polymer 
composition. 
Accordingly, in one of its method aspects, this invention is directed to a 
method for the preparation of an optically clear xerogel polymer 
composition comprising a polymer comprising hydroxyl functionalities which 
functionalities are blocked with a removable blocking group, and at least 
about 1.5 weight percent of an interpenetrant based on the total weight of 
the xerogel polymer composition which method comprises: 
(a) selecting a monomer composition wherein each component thereof 
comprises a reactive vinyl functionality and at least one of the 
components of the composition comprises at least one hydroxyl functional 
group; 
(b) blocking the hydroxyl functionalities on each of the hydroxyl 
containing monomer components selected in (a) above with a removable 
blocking group; 
(c) combining said monomer composition with at least 1.5 weight percent of 
an interpenetrant based on the total weight of the composition; and 
(d) polymerizing the composition produced in (c) above to provide for an 
optically clear xerogel polymer composition. 
In a preferred embodiment, the method described above further comprises: 
(e) removing the blocking groups from said hydroxyl groups; and 
(f) hydrating the composition produced in (e) above. 
In another preferred embodiment, an effective amount of a cross-linker is 
incorporated into the monomer composition prior to polymerization 
procedure (d).

DETAILED DESCRIPTION OF THE INVENTION 
As noted above, this invention is directed, in part, to optically clear 
xerogel polymer compositions containing an interpenetrant and methods for 
preparing such compositions. However, prior to discussing this invention 
in further detail, the following terms will first be defined: 
The term "hydrogel polymer composition" refers to the polymer compositions 
described herein which, after polymer formation, are hydratable when 
treated with water and, accordingly, can incorporate water into the 
polymeric matrix without itself dissolving in water. Typically, water 
incorporation is effected by soaking the polymer composition in a water 
solution for a sufficient period so as to incorporate at least 10 weight 
percent water and preferably from about 35 to about 70 weight percent 
water, and more preferably about 50 weight percent water, into the polymer 
composition based on the total weight of the polymer composition. 
The term "xerogen polymer composition" refers to the composition formed in 
the absence of large quantities of added solvent wherein any solvent in 
the polymer composition is typically less than about 5 weight percent of 
the total composition and more preferably less than about 2 weight percent 
of the total composition. 
The term "removable blocking group" refers to any group which when bound to 
one or more hydroxyl groups prevents reactions from occurring at these 
hydroxyl groups and which protecting groups can be selectively removed by 
conventional chemical and/or enzymatic procedures to reestablish the 
hydroxyl group. The particular removable blocking group employed is not 
critical and preferred removable hydroxyl blocking groups include 
conventional substituents such as benzyl, benzoyl, acetyl, chloroacetyl, 
trichloroacetyl, fluoroacetyl, trifluoroacetyl, t-butylbiphenylsilyl and 
any other group that can be introduced onto a hydroxyl functionality and 
later selectively removed by conventional methods in mild conditions 
compatible with the nature of the product. In a particularly preferred 
embodiment, the removable blocking group is a solvolyzable blocking group. 
In another preferred embodiment, the removable blocking group is selected 
such that upon hydration and removal of the removable blocking group, 
little or no dimensional change occurs in the polymer. More preferably, 
the extent of dimensional change, as measured by change in percent 
expansion, is controlled to less than about .+-.25 % and even more 
preferably to less than about .+-.10%. 
The term "solvolyzable" or "solvolyzable blocking groups" refers to groups 
capable of cleavage into a carboxyl containing compound and an alcohol, in 
the presence of a nucleophile, for example, a hydroxyl ion in water or a 
weak base such as ammonia or an organic amine or a C.sub.1 to a C.sub.4 
alcohol Solvolyzable blocking groups include acyl and haloacyl blocking 
groups of from 2 to 8 carbon atoms as well as a haloacyl blocking group of 
the formula X.sub.3 CC(O)O-- wherein each X is independently selected from 
the group consisting of fluoro and chloro. Preferably, the solvolyzable 
groups are capable of being solvolyzed under mild solvolysis conditions 
such as in aqueous solutions of a pH of from greater than 7 to less than 
about 12 and a temperature of less than about 60.degree. C. Such 
solvolyzable leaving groups are well known in the art and a list of such 
solvolyzable leaving groups is described in, for example, European Patent 
Application Publication No. 0 495 603 A1, U.S. Pat. No. 4,638,040 and U.S. 
Pat. No. 5,362,768, all of which are incorporated herein by reference in 
their entirety. 
The term "interpenetrant" refers to structurally rigid, high molecular 
weight materials which are soluble, at the levels employed, in at least 
one of the components utilized in preparing the polymer compositions 
described herein. Such interpenetrants are well known in the art and 
include, by way of example, siloxane, polyurethane, cellulose acetate 
butyrate, cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose, 
hydroxyethyl hydroxypropyl cellulose, mixtures of interpenetrants, as well 
as interpenetrants chemically modified to include a polymerizable group 
such as vinyl groups, epoxide groups, isocyanates, etc. (see, for example 
European Patent Application Publication No. 0 456 611) and the like. Such 
interpenetrants are either commercially available or can be prepared by 
art recognized techniques from commercially available starting materials. 
The particular interpenetrant employed is not critical. Preferably, the 
interpenetrant has a molecular weight of from about 1,000 to about 
50,000,000 and more preferably from about 5,000 to about 500,000. 
Interpenetrants are considered as structurally rigid if 1.5% of the 
interpenetrant increases the modulus of a polymer composition by at least 
1 Mdyne/cm.sup.2 as compared to the same polymer composition in the 
absence of the interpenetrant. 
The term "compatible ethylenically unsaturated monomers free of hydroxyl 
groups" refers to monomers which do not contain either hydroxyl groups or 
blocked hydroxyl groups; which can participate in polymer formation with a 
monomer containing hydroxyl groups blocked with a removable blocking 
group; and which, when so incorporated into the polymer composition 
provide for a polymer composition which, after solvolysis and hydration, 
is suitable for use in ophthalmic devices, i.e., the hydrogel polymer is 
clear so as to transmit visible light. Suitable compatible ethylenically 
unsaturated monomers free of hydroxyl groups include, by way of example, 
methyl acrylate, methyl methacrylate, trifluoromethyl methacrylate, 
trifluoromethyl acrylate, 2',2',2'-trifluoroethyl methacrylate, 
2',2',2'-trifluoroethyl acrylate, ethyl methacrylate, ethyl acrylate, 
styrene, and the like. Such materials are well known in the art and are 
either commercially available or can be prepared by methods known per se 
in the art. 
Preferably, the compatible ethylenically unsaturated monomer free of 
hydroxyl groups solubilizes, in whole or in part, the interpenetrant 
employed. A particularly preferred combination of a compatible 
ethylenically unsaturated monomer free of hydroxyl groups and an 
interpenetrant is methyl methacrylate and cellulose acetate butyrate. 
Another preferred compatible ethylenically unsaturated monomer free of 
hydroxyl groups is phenoxyethyl methacrylate which also solubilizes 
cellulose acetate butyrate, although less efficiently than methyl 
methacrylate. 
The term "cross-linking agent" refers to a monomer containing at least two 
reactive groups capable of forming covalent linkages with reactive groups 
found on at least one of the monomers used to prepare the polymer 
compositions described herein. Suitable reactive groups include, for 
example, vinyl groups which can participate in the polymerization 
reaction. The reactive groups are typically substituents on a suitable 
backbone such as a polyoxyalkylene backbone (including halogenated 
derivatives thereof), a polyalkylene backbone, a glycol backbone, a 
glycerol backbone, a polyester backbone, a polyamide backbone, polyurea 
backbone, a polycarbonate backbone, and the like. 
Cross-linking agents for use in the preferred compositions described herein 
are well known in the art and the particular cross-linking agent employed 
is not critical. Preferably, however, the reactive vinyl group is attached 
to the backbone of the cross-linking agent via an ester bond such as that 
found in acrylate and methacrylate derivatives such as urethane 
diacrylate, urethane dimethacrylate, ethylene glycol diacrylate, ethylene 
glycol dimethacrylate, polyoxyethylene diacrylate, polyoxyethylene 
dimethacrylate, and the like. However, other suitable vinyl compounds 
include, by way of example, di- and higher- vinyl carbonates, di- and 
higher-vinyl amides (e.g., CH.sub.2 .dbd.CH-C(O)NHCH.sub.2 CH.sub.2 
NHC(O)CH.dbd.CH.sub.2), and the like. 
Preferred cross-linking agents include, by way of example, ethylene glycol 
dimethacrylate, ethylene glycol diacrylate, diethylene glycol 
dimethacrylate, diethylene glycol diacrylate, triethylene glycol 
dimethacrylate, triethylene glycol diacrylate, tetradecaethylene glycol 
dimethacrylate, tetradecaethylene glycol diacrylate, allyl methacrylate, 
allyl acrylate, trimethylol-propane trimethacrylate, trimethylolpropane 
triacrylate, 1,3-butanediol dimethacrylate, 1,3-butanediol diacrylate, 
1,4-butanediol dimethacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol 
dimethacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol dimethacrylate, 
1,9-nonanediol diacrylate, 1,10-decanediol dimethacrylate, 1,10-decanediol 
diacrylate, neopentyl glycol dimethacrylate, neopentyl glycol diacrylate, 
2,2'bis[p-(.gamma.-methacryloxy-.beta.-hydroxypropoxy)phenyl]propane, 
pentaerythritol triacrylate, pentaerythritol trimethacrylate, 
pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, 
1,4-cyclohexanediol diacrylate, 1,4-cyclohexanediol dimethacrylate, 
ethoxylated bis-phenol-A-diacrylate, ethoxylated 
bis-phenol-A-dimethacrylate, bis-phenol-A-dimethacrylate, 
bis-phenol-A-diacrylate, N,N'-methylenebisacrylamide, as well as 
difunctional macromers having a molecular weight of from about 1,000 to 
about 1,000,000. Such materials are well known in the art and are either 
commercially available or can be prepared by methods known per se in the 
art. 
The cross-linking agent preferably has at least 2 and more preferably from 
2 to about 6 vinyl functionalities and preferably has a number average 
molecular weight of from about 100 to about 2,500 (except for the 
macromers described above). More preferably, the vinyl functionalities are 
acrylate or methacrylate groups attached to a polyoxyalkylene backbone 
(including halogenated derivatives thereof), a polyalkylene backbone, a 
glycol backbone, a glycerol backbone, a polyester backbone, or a 
polycarbonate backbone. 
Formulations 
The polymer compositions described herein are prepared by first preparing a 
suitable formulation containing the requisite components and then 
polymerizing the formulation. Specifically, the formulation comprises a 
monomer composition and an interpenetrant. 
The monomer composition comprises at least one monomer component comprising 
a reactive vinyl functionality and at least one hydroxyl functional group 
wherein the hydroxyl groups are blocked with a removable blocking group. 
Suitable hydroxyl monomer components (prior to blocking) include 
hydroxyethyl methacrylate (HEMA), hydroxyethyl acrylate, glycidyl 
methacrylate, glycidyl acrylate, hydroxypropyl methacrylate, hydroxypropyl 
acrylate, butanediol monomethacrylate, mixtures of such components, and 
the like. Suitable blocking groups include, by way of example only, 
benzyl, benzoyl, acetyl, chloroacetyl, trichloroacetyl, fluoroacetyl, 
trifluoroacetyl, t-butyl-biphenylsilyl groups, and the like. When the 
monomer component contains more than one hydroxyl group, e.g., glycidyl 
methacrylate, the removable blocking groups employed therewith may be the 
same or different groups but, for ease of synthesis, are preferably the 
same. 
The monomer composition can optionally contain one or more compatible 
ethylenically unsaturated monomers free of hydroxyl groups. When the 
monomer composition does contains such ethylenically unsaturated monomers 
free of hydroxyl groups, the composition preferably comprises sufficient 
hydroxyl containing monomers such that the resulting hydrogel polymer 
composition will absorb at least 10 weight percent and more preferably 
from 35 to about 70 weight percent water. In a particularly preferred 
embodiment, the monomer composition comprises at least about 20 weight 
percent of monomer component(s) comprising a reactive vinyl functionality 
having at least one hydroxyl functional group wherein the hydroxyl groups 
are blocked with a removable blocking group and more preferably from about 
50 to about 100 weight percent based on the total weight of the monomer 
composition and still more preferably from about 80 to 100 weight percent. 
The formulation also contains an interpenetrant which is employed in the 
amount of at least about 1.5 weight percent based on the total weight of 
the formulation (in the absence of any water) and preferably from about 5 
to about 60 weight percent and more preferably from about 5 to about 30 
weight percent. The use of higher concentrations of interpenetrant may 
decrease the water content of the resulting hydrated polymer composition. 
The specific amount of interpenetrant employed is selected so that the 
hydrogel polymer composition has a modulus of at least 2 Mdynes/cm.sup.2, 
preferably 2 to 50 Mdynes/cm.sup.2 and more preferably 2 to 30 
Mdynes/cm.sup.2. 
The compositions of this invention are preferably cross-linked and, 
accordingly, one of the components of a preferred formulation is a 
cross-linking agent. When employed, the cross-linking agent is employed in 
an amount sufficient to provide a cross-linked product but preferably is 
employed in an amount of from about 0.1 to about 30 weight percent, more 
preferably from about 0.1 to about 5 weight percent and still more 
preferably from about 0.2 to about 3 weight percent based on the total 
weight of the formulation. The use of higher amounts of cross-linker 
appears to correlate to polymer compositions having a higher modulus but 
lower water content and a lower percent elongation. 
The formulation can optionally contain one or more additional components 
such as initiators, colorants, etc. which are conventionally employed in 
the art. 
These formulations as well as the reagents employed to prepare these 
formulations are preferably stored and formulated in containers which 
prevent premature polymerization of one or more of the reagents. For 
example, the use of amber bottles for storing reagents inhibits 
photo-induced polymerization. 
Methodology 
The formulations described above are readily polymerized by conventional 
techniques such as thermal, UV, .gamma. irradiation, or electron beam 
induced polymerization to provide for the polymer composition. For 
example, thermal induced polymerization can be achieved by combining a 
suitable polymerization initiator into the mixture of monomers under an 
inert atmosphere (e.g., argon) and maintaining the mixture at an elevated 
temperature of from about 20.degree. C. to about 75.degree. C. for a 
period of time from about 1 to about 48 hours. 
Ultraviolet (UV) induced polymerization can be achieved by combining a 
suitable polymerization initiator into the mixture of monomers under an 
inert atmosphere (e.g., argon) and maintaining the mixture under a 
suitable UV source. Preferably, UV induced polymerization is conducted at 
ambient conditions for a period of from about 5 minutes to 24 hours. 
Suitable polymerization initiators are well known in the art including 
thermal initiators such as t-butyl peroxy pivalate (TBPP), t-butyl peroxy 
neodecanoate (TBPN), benzoyl peroxide, methyl ethyl ketone peroxide, 
diisopropyl peroxycarbonate and the like and UV initiators such as 
benzophenone, Darocur 1173 (available from Ciba Geigy, Ardsley, N.Y., 
USA), bis-azoisobutyronitrile and the like. 
The particular thermal or UV initiator employed is not critical and 
sufficient initiator is employed to catalyze the polymerization reaction. 
Preferably, the initiator is employed at up to about 1 weight percent 
based on the total weight of the composition. 
Polymerization achieved by either electron beams or .gamma. irradiation 
does not require the use of an initiator and the formulation to be 
polymerized is merely exposed to the electron beam or .gamma. irradiation 
using conventional methods. 
Polymerization is typically conducted in a manner so as to facilitate 
manufacture of the finished contact lens. For example, polymerization can 
be conducted in molds which correspond to the structure of the contact 
lens. Alternatively, polymerization can be conducted so as to form a 
polymer rod which can be machined (lathed) to provide contact lenses of 
suitable dimensions. In this latter embodiment, polymerization can be 
conducted in a silylated glass test tube and after polymerization, the 
test tube is broken to provide for the polymeric rod. The rod, in the form 
of the xerogel, can be machined, for example, lathed, cut, milled, and 
consequently, the rod can be made into contact lenses by cutting small 
cylinders or buttons from the rod and subsequent lathing. In still another 
alternative embodiment, polymerization can be conducted in a base curve 
mold to provide a button suitable for forming a contact lens. 
In any event, after polymerization, a post-curing procedure is optionally 
employed to complete the polymerization process which post-curing step 
typically increases the hardness of the polymer. The post-curing procedure 
can comprise heating the polymer to a temperature of from about 60.degree. 
C. to 120.degree. C. for a period of from about 2 to about 24 hours. 
Alternatively, the post-curing step can employ .gamma. irradiation of from 
about 0.1 to about 5 Mrad. Combinations of these two procedures can also 
be employed. 
The polymer compositions described above, preferably in the contact lens 
forms described, are then subjected to removal of the removable blocking 
group and hydrolysis. The conditions for removal of the removable blocking 
group are dependent, of course, on the blocking group employed and it is 
well within the skill of the art to select the appropriate conditions 
relative to the blocking group employed. Either during or after removal of 
the removable blocking group, the composition is then subjected to 
conventional hydration to provide for the hydrated form of the 
composition. 
In a particularly preferred embodiment, the removable blocking group is a 
solvolyzable blocking group and solvolysis of the blocking groups and 
hydration of the polymer composition occurs simultaneously. Solvolysis is 
preferably conducted by suspending the contact lens in an aqueous solution 
in the presence of a nucleophile, for example, a hydroxyl ion in water or 
a weak base such as ammonia or an organic amine or a C.sub.1 to a C.sub.4 
alcohol. Preferably, the solvolyzable groups are capable of being 
solvolyzed under mild solvolysis conditions such as in aqueous solutions 
of a pH of from greater than 7 to less than about 12 and a temperature of 
from about 10.degree. C. to about 60.degree. C. 
Under these conditions, hydration of the polymer material also occurs. 
However, if desired, a separate hydration step can be employed. Hydration 
is continued until the polymer composition is fully hydrated which, in the 
present case, means that the water content of the hydrogel is from about 
35 to about 70 weight percent. 
In still another embodiment, water can be included in the polymerization 
step resulting in direct inclusion of water into the polymer composition. 
Utility 
The xerogel polymer compositions described herein are suitable for use in 
medical and non-medical applications such as water absorbent materials 
useful in a variety of applications. After water incorporation, the 
polymer compositions described herein are particularly suitable for use in 
ophthalmic devices such as contact lenses providing requisite optical 
clarity, water content, high strength, no deterioration over time, 
relatively slow release of hydrated water upon exposure to air, and good 
optical properties including transparency. 
When formed into contact lenses, the lenses are typically dimensioned to be 
from about 0.02 to about 0.15 millimeters in thickness and preferably from 
about 0.05 to about 0.10 millimeters in thickness (all thicknesses 
measured in the xerogel state). 
The invention will now be illustrated by way of examples which are provided 
for the purpose of illustration only and are not intended to be limiting 
in the present invention. 
In the following examples, the following abbreviations represent the 
following: 
BPAGMA=bis-phenol-A 2-hydroxypropyl dimethacrylate 
CAB=cellulose acetate butyrate 
cm=centimeter 
EGDMA=ethylene glycol dimethacrylate 
EWC=equilibrium water content 
EX33 =Esperox 33.RTM.(t-butylperoxyneodecanoate) 
GMA=glycidyl methacrylate 
HCEGMA=di-trichloroacetate ester of glyceryl methacrylate 
LE=linear expansion 
Mdynes=megadynes 
min=minute 
mm=millimeter 
MMA=methyl methacrylate 
ppm=parts per million 
EXAMPLES 
In the examples set forth below, polymer compositional values are set forth 
for the Equilibrium Water Content ("EWC"), linear expansion and tensile 
properties (i.e., tensile strength, percent elongation and modulus). 
Unless otherwise indicated, these values were determined as follows: 
Measurement of Equilibrium Water Content 
Equilibrium Water Content (EWC) is determined by soaking the polymer 
samples in phosphate buffered saline solution for overnight. The samples 
are removed, lightly blotted dry with a tissue and subsequently weighed. 
The hydrated samples are then placed in a vacuum oven at 60.degree. C. 
overnight. The next flay, the sample is reweighed. Equilibrium Water 
Content is calculated using the following equation: 
##EQU1## 
where X=mass of hydrated polymer 
Y=mass of dehydrated polymer 
The EWC is sometimes referred to as the % water. 
Measurement of Linear Expansion 
Linear Expansion factor is determined by measuring the diameter of the 
xerogel polymer sample using the Nikon Profile Projector V-12 (available 
from Nippon Kogaku K.K., Tokyo, Japan). The sample is then soaked 
overnight in phosphate buffered saline solution. The diameter is 
subsequently remeasured in phosphate buffered saline. Linear Expansion is 
calculated using the following equation: 
##EQU2## 
where X=hydrated polymer diameter 
Y=Initial (dry) polymer diameter 
Measurement of Tensile Properties 
From a disc or a lens, a "dumb-bell" shaped specimen is cut. The sample is 
inspected under a microscope for nicks and cuts. If these are observed the 
sample is discarded. The thickness of the specimen is then measured. 
The sample is then placed between the clamps on an Instron tensile tester 
(available from Instron Corp., Canton, Massachusetts, USA) or an 
equivalent instrument. The initial grip separation used is 10 mm. The 
sample is placed under saline during measurement to prevent drying out. 
The experiment is then started with the cross-head speed set to 5 mm/min. 
The Instron records the force required to pull on the sample as a function 
of cross-head displacement. This information is then converted into a 
stress-strain plot. The experiment continues until the sample breaks. 
From the stress strain plot are calculated the following: 
Tensile strength: The maximum stress the sample can withstand before 
breaking. 
Elongation: The amount of extension (expressed as a percent of original 
length/grip separation) the sample undergoes before breaking. 
Modulus: Is the slope of the initial linear portion of the stress-strain 
curve. 
The experiment is usually repeated with 5 samples from the same batch of 
polymer and the average and standard deviation are reported. 
Comparative Example A and Example 1 below illustrate that blocking of the 
hydroxyl groups on the hydrophilic monomer is essential to preparing an 
optically clear xerogel polymer composition incorporating an 
interpenetrant. Examples 2-3 exemplify that dimensional change occurring 
during hydration can be controlled by selection of the polymer composition 
relative to the removable blocking group. Examples 4-21 illustrates 
further examples of polymer compositions of this invention. Example 22 
illustrates enhancements in the amount of surface wettability achieved for 
molded ophthalmic devices from polymer compositions made via the methods 
of this invention. 
COMATIVE EXAMPLE A and EXAMPLE 1 
Two xerogel polymer compositions were prepared by incorporating an 
interpenetrant into the polymer composition which polymer compositions in 
the hydrogel form contained hydroxyl functionality. Specifically, the 
first composition, Comparative Example A, was prepared such that the 
hydroxyl functionalities on the glycidyl methacrylate were not blocked 
with a removable blocking group during polymer formation. Contrarily, in 
Example 1, the hydroxyl groups were blocked with a removable blocking 
group (i.e., as the trichloroacetate ester). 
Specifically, the formulations for Comparative Example A and Example 1 are 
as set forth in Table I below: 
TABLE I 
______________________________________ 
Monomer A 
MMA/CAB.sup.1 
BPAGMA.sup.2 
EX33.sup.3 
______________________________________ 
Example 1 
HCEGMA.sup.4 
2.19 g 0.274 g 0.107 g 
(17.72 g) 
Comparative 
GMA 4.38 g 0.548 g 0.107 g 
Example A 
(12.58) 
______________________________________ 
.sup.1 MMA/CAB = 30% wt:wt cellulose acetate butyrate in methyl 
methacrylate 
.sup.2 BPAGMA = bisphenol-A 2hydroxypropyl dimethacrylate (1:2 wt:wt 
BPAGMA in DMSO) 
.sup.3 EX33 = Esperox 33 
.sup.4 HCEGMA = ditrichloroacetate ester of glyceryl methacrylate (stored 
at least at -5.degree. C. and preferably at -5.degree. C.) 
These formulations were prepared by combining monomer A with both the 
methyl methacrylate/cellulose acetate butyrate composition and the 
cross-linker (BPAGMA). The composition was then mixed for 1 hour and, 
afterwards, degassed for 6 minutes. At this point, the initiator (EX33) 
was added to the composition and the formulation was again degassed, this 
time for 30 seconds. Degassing was conducted in order to avoid 
contamination of the reaction vessel with oxygen which may have an adverse 
effect on the degree of polymerization. The resulting formulation was 
polymerized and cured in a programmable oven ramped at 10.degree. 
C./minute in the following manner to provide for a xerogel polymer 
composition: 
1) 40.degree. C./2 hours 
2) 55.degree. C./2 hours 
3) 70.degree. C./2 hours 
4) room temperature/4 hours 
Afterwards, the xerogel polymers of Comparative Example A and Example 1 
were subjected to hydrolysis using a 5% solution of ammonium hydroxide 
which, in the case of Example 1, resulted both in removal of the blocking 
groups (via solvolysis) and hydration of the polymer composition. The 
clarity/opaqueness of the resulting polymer compositions are set forth in 
Table II below: 
TABLE II 
______________________________________ 
POLYMER OPTICAL PROPERTY 
OPTICAL PROPERTY 
OF AS THE XEROGEL AS THE HYDROGEL 
______________________________________ 
Comparative 
optically opaque 
optically opaque 
Example A 
Example 1 
optically clear optically clear 
______________________________________ 
Other physical properties for the polymer of Example 1 were determined to 
be as follows: tensile strength=13.8.+-.5.8 Mdynes/cm.sup.2 ; percent 
elongation=211.+-.89; modulus=22.1.+-.12.3; and an EWC=52.3.+-.2.5. 
The results of this comparison establish that hydrogel polymer compositions 
containing an interpenetrant require the blocking of the hydroxyl groups 
on the monomers prior to polymerization in order to achieve optical 
clarity in either the xerogel or hydrogel composition. The physical 
properties of the polymer of Example 1 establish that this polymer 
possesses tensile strength, percent elongation, modulus and EWC values 
suitable for use in ophthalmic devices. 
EXAMPLES 2 and 3 
The following examples illustrate that selection of the removable blocking 
group can be made to control of dimensional change arising from hydrating 
the polymer composition. Specifically, the formulations for Examples 2 and 
3 are as set forth in Table III below: 
TABLE III 
______________________________________ 
HCEGMA.sup.5 MMA/CAB.sup.6 
BPAGMA.sup.7 
EX33.sup.8 
______________________________________ 
Example 
94.24 wt % 5.236 wt % 0.523 wt % 
0.4 wt % 
Example 
89.22 wt % 9.345 wt % 0.9345 wt % 
0.4 wt % 
3 
______________________________________ 
.sup.5 HCEGMA = ditrichloroacetate ester of glyceryl methacrylate (stored 
at least at -5.degree. C. and preferably at -5.degree. C.) 
.sup.6 NMA/CAB = 30% wt:wt cellulose acetate butyrate in methyl 
methacrylate 
.sup.7 BPAGMA = bisphenol-A 2hydroxypropyl dimethacrylate (1:2 wt:wt 
BPAGMA in DMSO) 
.sup.8 EX33 = Esperox 33 
These formulations were polymerized and cured in the manner described above 
for Comparative Example A and Example 1 to provide for optically clear 
xerogel polymer compositions. 
Afterwards, the xerogel polymers of Examples 2 and 3 were subjected to 
hydrolysis using a 5% solution of ammonium hydroxide which resulted both 
in removal of the blocking groups (via solvolysis) and hydration of the 
polymer composition. The resulting compositions were both optically clear 
and had the physical properties set forth in Table IV below: 
TABLE IV 
______________________________________ 
Linear 
Ex. Expan- 
No. Modulus.sup.A 
Tens..sup.B 
% Elong. 
EWC sion 
______________________________________ 
2 4.4 .+-. 1.1 
4.1 .+-. 1.6 
117 .+-. 28 
64.4 .+-. 0.9% 
22.2% 
3 12.5 .+-. 1.7 
14.8 .+-. 6.7 
140 .+-. 40 
51.2 .+-. 0.7% 
5.5% 
______________________________________ 
.sup.A = in Mdynes/cm.sup.2 
.sup.B = in Mdynes/cm.sup.2 
In both cases, the percent of linear expansion was maintained to less than 
25% evidencing a degree of control of expansion arising from hydration. 
Example 3, in particular, exemplifies a polymer composition having 
approximately 513% water which undergoes minimal expansion during 
hydration. 
Accordingly, by selecting the removable blocking groups relative to the 
amount of water to be absorbed, it is possible to provide for a polymer 
composition having little dimensional change during hydration. 
EXAMPLES 4-21 
The following examples are examples of optically clear polymer 
compositions, both as the xerogel and the hydrogel, within the scope of 
this invention. These polymer compositions were prepared in the manner 
described above and hydrated in a manner similar to that also described 
above. The formulations employed to prepare these polymer compositions are 
described in Table V below: 
TABLE V 
______________________________________ 
HCEGMA.sup.5 MMA/CAB.sup.6 
BPAGMA.sup.7 
EX33.sup.8 
______________________________________ 
Example 4 
94 8 0.5 0.4 wt % 
Example 5 
97 8 0.5 0.4 wt % 
Example 6 
94 8 1.0 0.4 wt % 
Example 7 
97 8 1.0 0.4 wt % 
Example 8 
94 12 0.5 0.4 wt % 
Example 9 
97 12 0.5 0.4 wt % 
Example 10 
94 12 1.0 0.4 wt % 
Example 11 
97 12 1.0 0.4 wt % 
Example 12 
96 11 0.4 0.4 wt % 
Example 13 
98 11 0.4 0.4 wt % 
Example 14 
96 13 0.4 0.4 wt % 
Example 15 
98 13 0.4 0.4 wt % 
Example 16 
96 11 0.4 0.4 wt % 
Example 17 
98 11 0.4 0.4 wt % 
Example 18 
96 13 0.4 0.4 wt % 
Example 19 
98 13 0.4 0.4 wt % 
Example 20 
97 12 0.4 0.4 wt % 
Example 21 
96 11 0.4 0.4 wt % 
______________________________________ 
.sup.5 HCEGMA = ditrichloroacetate ester of glyceryl methacrylate (stored 
at -5.degree. C.) 
.sup.6 MMA/CAB = 30% wt:wt cellulose acetate butyrate in methyl 
methacrylate 
.sup.7 BPAGMA = bisphenol-A 2hydroxypropyl dimethacrylate (1:2 wt:wt 
BPAGMA in DMSO) 
.sup.8 EX33 = Esperox 33 
In Examples 4-21, HCEGMA, MMA/CAB and BPAGMA amounts are all reported in 
parts by weight. 
EXAMPLE 22 
The purpose of this example is to illustrate the enhancement in surface 
wettability of molded ophthalmic devices (contact lenses) made via the 
methods of this invention as compared to conventional prior art methods. 
Specifically, when polymerization of the monomer mix is conducted in a 
polypropylene mold, the hydrophobic nature of the mold tends to orient the 
molecules during polymerization such that the resulting polymer surface 
contains a more hydrophobic nature than the interior of the polymer. 
This difference can be quantified by comparing the contact angle of the 
surface of the polymer to that of a surface formed by lathing the polymer 
such that the interior of the polymer is exposed. Typically, when the 
hydroxyl groups of the monomer are not blocked prior to polymerization, 
the resulting polymer composition will have a significant increase in the 
contact angle of the surface formed during polymerization as opposed to 
the contact angle of a surface formed after polymerization by lathing. The 
increase in contact angle corresponds to a reduction in surface 
wettability. 
In the present case, a polymer composition formed in the manner described 
above was tested for its contact angle for both the surface formed during 
polymerization as opposed to the contact angle of a surface formed after 
polymerization by lathing. In both cases, the contact angle was 
40.degree..+-.2.degree. evidencing that there was no reduction in surface 
wettability between the surface formed during polymerization and the 
interior of the polymer. Without being limited to any theory, it is 
believed that the blocking groups employed on the hydroxyl groups of this 
invention alter the hydrophilic nature of the monomer to a more 
hydrophobic nature thereby permitting orientation of these groups on the 
surface of the polymer. After polymer formation, the removal of these 
blocking groups exposes hydroxyl groups on the surface of the polymer. In 
any event, the enhanced surface wettability is a beneficial attribute of 
the polymers of this invention. 
By following the procedures set forth above, other optically clear polymer 
compositions containing an interpenetrant and hydroxyl groups can be 
prepared merely by substitution of an appropriate reagent for the reagent 
recited in the examples above. For example, a polymer composition 
employing EGDMA can be prepared as above merely by substituting the BPAGNA 
cross-linker with the EGDMA cross-linker. In such a case, 0.0355 grams of 
the EGDMA can replace 0.274 grams of the BPAGMA/DMSO cross-linker. Other 
substitutions can be readily done which substitutions are well within the 
skill of the art.