Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-08-26
2001-08-07
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C623S005110, C623S006120, C351S16000R
Reexamination Certificate
active
06271281
ABSTRACT:
FIELD OF THE INVENTION
The present invention broadly relates to ocular implants. Specifically, it relates to ocular implants made from homopolymers containing chemical crosslinkers for interlinking polymer chains. More specifically, the present invention is directed to ocular implants made from homopolymers containing chemical crosslinkers having the unique ability to produce stable elasticity in these homopolymers in conjunction with the production of other unique combinations of chemical and physical properties. The present invention is particularly well suited to the production of elastomeric, soft, optically clear, high refractive index, low tack homopolymers and to medical devices, including intraocular lenses, corneal implants, corneal overlays, and phakic retractive lenses, made from such homopolymers.
BACKGROUND
Generally speaking, “polymers” are commonly understood to be any of a wide variety of synthetically produced, nonmetalic or organic compounds which can be molded into various forms and hardened for commercial use. They are made from high molecular weight macromolecules produce by “polymerizin” or chemically linking individual chemical sub-units or “monomers.” There are essentially two types of polymers: homopolymers and copolymers. “Homopolymers” are made up of identical, repeating monomers chemically bonded together into polymer chains of various lengths. “Copolymers” are made from combinations of at least two different monomers which are polymerized to form chains of alternating different monomers or chains where the different monomers are randomly dispersed throughout.
There are both naturally occurring and synthetically produced polymers. Examples of natural polymers include, among others, proteins, polysaccharides, deoxyribose nucleic acid (DNA) and rubber, wherein the individual monomer sub-units are, respectively, amino acids, sugars, nucleic acids, and isoprene. Common synthetic polymers, which include plastics and silicones, are made from highly chemically reactive monomers including styrenes, acrylates, silanols and many others. Synthetic polymers have become one of the most important classes of molecules since their invention at the turn of the twentieth century. They have had a significant impact on every aspect of human life. However, significant efforts are continually underway to further our understanding of, and to advance the science of polymer chemistry. These efforts include the development of critically needed superior polymeric materials having presently unavailable combinations of physical and chemical properties.
The physical and chemical properties of both homopolymers and copolymers are dictated by the extent and the nature of polymer chain interactions within the polymers themselves. These interactions are, in turn, a function of the individual monomeric sub-units' sizes, weights, charges and chemical structures. The most important types of interactions between polymer chains are those chemical interactions which result in what is know in the art as “crosslinking.” Crosslinking can be defined as a chemical process which joins individual polymer chains together by forming chemical bridges between and among the polymer chains. These “crosslinks” lock the polymer chains together into immense single molecules wherein the individual polymer chains can no longer slip over or relative to one another.
There are essentially two mechanisms by which polymers can be crosslinked. The first crosslinking method utilizes an external energy source, such as high energy radiation or heat, to induce interactions between chemically reactive functional groups within the individual monomers of each polymer chain forming new chemical bonds between the polymer chains. Polymers crosslinked using such an external energy source must be composed of monomers that are susceptible to such chemical reactions. Typically, such monomers have pendent, exposed chemical functional groups (portions of the monomer that are chemically reactive and extend away from the polymer chain, also referred to as “residues”) which are capable of interacting with chemically compatible pendent groups on adjacent polymer chains. One example of this type of crosslinking involves the naturally occurring proteins found in animal skin. These proteins are complex polymers composed of numerous different monomers (amino acids) each containing highly reactive pendent chemical groups including sulfur, carboxylic acid and amine residues. As animals age, the cumulative effects of UV radiation (sun exposure) induce crosslinking between these protein molecules, changing the physical structure of these polymers and causing the skin to lose its natural elasticity and to become hard and wrinkled.
The second crosslinking mechanism utilizes the addition of exogenous crosslinking agents (an additional multifunctional molecule, not part of a polymer chain) in conjunction with the application of a chemical catalyst (or “accelerator”) which promotes the reaction between the crosslinking agents and the chemical functional groups within the polymer chains. Such chemical reactions among polymer chains using crosslinking agents are not limited to polymers with pendent chemical groups. Rather, this form of chemical crosslinking works equally well with smaller monomer sub-units (such as “isoprene” or natural rubber) in which the only reactive functional group is a double chemical bond that is sequestered within the linear portion of the molecule (the straight part of the polymer chain, not extending from the macromolecule). Therefore, the use of crosslinking agents, either alone or in conjunction with external energy sources such as heat and radiation, provides an extremely versatile crosslinking mechanism which can produce profound changes in the polymer's properties.
One example of the dramatic changes that such exogenous crosslinking agents can produce in a polymer is the “vulcanization” of rubber. Vulcanization is the process of chemically bridging or linking the polymer's chains of natural rubber (polyisoprene) using elemental sulfur as the exogenous crosslinking agent. Heat and compounds such as peroxides, metallic oxides, and chlorinated quinones are also used to catalyze the chemical reactions between the polyisoprene chains and the sulfur. Without vulcanization, naturally occurring raw rubber is an extremely tacky, amorphous mass that will not hold a shape and is easily solubilized or dissolved by organic compounds such as gasoline, oil, and acetone. After crosslinking the raw rubber hardens and becomes less tacky, more resistant to cold induced hardening or heat induced softening, and resistant to organic solvents. This crosslinked rubber can be formed into commercial articles and products while hot and fluid, and will retain the formed shape upon cooling. Without crosslinking, natural rubber would not possess these beneficial properties required for its wide range of industrial applications including tires, shoes, electric insulators and waterproof articles.
These crosslinking techniques are commonly employed with both natural and synthetic polymers in order to create polymer compounds having optimized properties for particular applications. However, crosslinking polymers is a technically difficult process that must be precisely controlled for good results. Crosslinking agents can be simple inorganic compounds such as the sulfur used for vulcanization discussed above, or can be more complex organic compounds such as the divinyl benzene used in a wide variety of more exotic plastics. The amount of crosslinker added, the rate at which the crosslinking reaction is allowed to occur, and the density of the crosslinkable chemical functional groups present on the polymer chains all contribute to the resulting polymer's physical and chemical properties.
Consequently, the polymer chemist is faced with a series of difficult and conflicting choices that often result in compromises necessary to achieve the appropriate final compounds for a given application or purpose. Further, it is essential for
Gulati Vijay
Liao Xiugao
Medennium, Inc.
Oppenheimer Wolff & Donnelly LLP
Yoon Tae H.
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