Abstract:
The present invention relates to a process comprising the steps of reacting a substituted epoxide, and preferably a silicone containing substituted epoxide with at least one lithium acrylic acid salt, at least one acrylic acid and optionally at least one inhibitor at a temperature above about 60° C. to form a substituted hydroxy acrylate.

Description:
RELATED PATENT APPLICATIONS 
     This patent application claims priority of a provisional application, U.S. Ser. No. 60/483,846, which was filed on Jun. 30, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to processes for the production of silicone monomers and particularly bis(trimethylsilyloxy)silylalkyl glycerol methacrylates. 
     BACKGROUND OF THE INVENTION 
     Various silicone containing monomers have found utility as starting materials in the production of medical devices, such as ophthalmic devices and particularly, soft contact lenses having improved permeability to oxygen. One class of suitable monomers includes tris and bis(trimethylsilyloxy)silylalkylglycerol methacrylates (“SiAGMA”). One process for making SiAGMA includes reacting the epoxide of the SiAGMA with methacrylic acid and either the sodium, potassium or lithium salt of methacrylic acid and an inhibitor, such as hydroquinone monomethyl ether. Reaction conditions include heating for about 15 hours, and yields SiAGMA having a purity of between about 75 to 95% and a number of byproducts, including dimethacrylated byproducts. When included in the monomer mixes used to make ophthalmic devices such as contact lenses, the dimethacrylated byproducts can act as crosslinkers, which even in small quantities can change the modulus of the resulting device. Accordingly, the concentration of these difunctional byproducts must either be tightly controlled or minimized. Removal of the difunctional byproducts is conventionally done by a cumbersome silica gel column chromatography step. 
     Thus, there remains in the art for an improved process for the production of SiAGMA type compounds, and particularly one which minimizes the formation of difunctional byproducts. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a process comprising the steps of reacting a substituted epoxide, and preferably a silicone containing substituted epoxide with at least one lithium acrylic acid salt, at least one acrylic acid and optionally at least one inhibitor at a temperature above about 60° C. to form a substituted hydroxy acrylate. Specifically, the present invention relates to a process comprising the step of reacting a first reaction mixture comprising substituted epoxide with at least one lithium salt, at least one acrylic acid at a temperature above about 60° C. for up to about one day to form a substituted hydroxy acrylate and said substituted epoxide 
    
    
     DESCRIPTION OF THE INVENTION 
     Suitable substituted epoxides include those of Formula I, below: 
                                
Wherein R 1  is any substituent which would not react with a nucleophilic compound.
 
Preferred epoxides include those shown in Formula II, below:
 
                                
Wherein R 2  is a C1 to C12 alkyl substituted with at least one Si containing moiety and preferably at least one silicone. Suitable Si containing compounds include compounds of the formula III:
 
                                
wherein A 1  and A 2  are the same or different and are selected from lower alkyl, alkyloxy and B;
 
B is lower alkyl, alkoxy or a group of the structure:
 
                                
wherein R 3 , R 4 , R 5 , R 8  and R 9  are the same or different and are selected from the group including lower alkyl, phenyl, benzyl, and tri-alkyl siloxy substituents and y is an integer from 1 to 25. As used herein the term “lower alkyl” refers to alkyl groups comprised of 1 to 4 carbon atoms.
 
Specific examples of suitable epoxides include those of formula IV:
 
                                
Wherein R 6  and R 7  are independently selected from alkyl groups having 1 to 4 carbon atoms, n is an integer between 1 and 12 and x is an integer between 0 and 3. Even more specifically, the epoxide may be the epoxide of (3-glycidoxypropyl)bis(trimethylsiloxy)methylsilane.
 
     Epoxides may be formed in a number of ways including, but not limited to, oxidation of alkenes with peroxyacids, formation by an intramolecular S N 2 reaction of halohydrins, addition of a nucleophilic oxidizing agent (such as a basic solution of hydrogen peroxide) to an α,β-unsaturated carbonyl compound, and the reaction of a sulfonium ylide with a carbonyl compound. Alternatively, epoxides substituted with a Si containing group may be prepared by the hydrosilylation of an already formed epoxide containing an olefinic functionality. Such methods are well known to those skilled in the art and this list of synthetic routes to epoxides and epoxides substituted with a Si containing group, in no way limits the scope of this invention to these preparations. 
     According to the process of the present invention the epoxide is reacted with at least one acrylic acid and at least one lithium salt of said acrylic acid. Suitable acrylic acids comprise between 1 and 4 carbon atoms. Preferably said acid is methacrylic acid. The reaction between the epoxide and the acrylic acid may be equimolar, however, it may be advantageous to add an excess of acrylic acid. Accordingly, the acrylic acid may be used in amounts between about 1 and about 3 moles of acrylic acid per mole of the epoxide. 
     The lithium salts of the present invention comprise lithium and at least one acrylic acid comprising between 1 and 4 carbon atoms. Preferably said lithium salt is the Li salt of methacrylic acid. The lithium salt is added in an amount sufficient to catalyze the reaction, and preferably in an amount up to about 0.5 equivalents, based upon the epoxide. 
     An inhibitor may also be included with the reactants. Any inhibitor which is capable of reducing the rate of polymerization may be used. Suitable inhibitors include sulfides, thiols, quinines, phenothiazine, sulfur, phenol and phenol derivatives, mixtures thereof and the like. Specific examples include, but are not limited to hydroquinone monomethyl ether, butylated hydroxytoluene, mixtures thereof and the like. The inhibitor may be added in an amount up to about 10,000 ppm, and preferably in an amount between about 1 and about 1,000 ppm. 
     The reaction is conducted at elevated temperatures, preferably greater than about 60° C. and more preferably between about 80° C. and about 110° C. Suitable reaction times include up to about a day, preferably between about 4 and about 20 hours, and more preferably between six hours and about 20 hours. It will be appreciated by those of skill in the art the temperature and reaction time are inversely proportional, and that higher reaction temperatures may allow for decreased reaction times and vice versa. However, in the process of the present invention it is desirable to run the reaction to or near completion (for example, greater than about 95% conversion of substituted epoxide, and preferably greater than about 98% conversion of substituted epoxide). 
     It has been found that by using lithium as the metal ion in the salt, less impurities are generated at a given set of reaction conditions. Thus, the present invention provides a process for the production of substituted hydroxy acrylates in higher purities than by conventional methods. The substituted hydroxy acrylates may be further purified using super critical fluid extraction. Suitable extraction fluids are non-reactive with the silicone containing compounds to be purified and have critical points below the range which would cause degradation of the silicone containing compound. Examples include carbon dioxide, ethane, ethylene, propane, propylene, chlorotrifluoromethane, mixtures thereof, and the like. Carbon dioxide is a preferred supercritical fluid because it has a low critical point, is generally non-reactive with the silicone-containing compounds, non-flammable and environmentally benign. 
     Suitable conditions include one or more separation zones, where the supercritical fluid in the first zone comprises a density between about 0.5 and 0.7 g/ml and a density of 0.1 g/ml to about 0.3 g/ml. The desired densities may be achieved by controlling the pressure and temperature within the reaction zone. Any number of additional separation zones may be included. 
     In order to illustrate the invention the following examples are included. These examples do not limit the invention. They are meant only to suggest a method of practicing the invention. Those knowledgeable in contact lenses as well as other specialties may find other methods of practicing the invention. However, those methods are deemed to be within the scope of this invention. 
     The following abbreviations are used in the examples below: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 SiMAA2 
                 bis(trimethylsilyloxy)methylsilylpropylglycerol methacrylate 
               
               
                   
                 (CA Index name is 2-propenoic acid, 2-methyl, 2-hydroxy-3- 
               
               
                   
                 [3-[1,3,3,3-tetramethyl-1- 
               
               
                   
                 [(trimethylsilyl)oxy]disiloxanyl]propoxy]propyl) 
               
               
                 MEHQ 
                 hydroquinone monomethyl ether 
               
               
                 Epoxide 
                 (3-glycidoxypropyl)bis(trimethylsiloxy)methylsilane 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 1 
     To a three-neck, 5000 mL round bottom reaction flask equipped with a magnetic stir bar, condenser with an attached drying tube, and a thermocouple, was added 92 g dry lithium methacrylate (1 mol, 0.17 equivalents) and 1023 grams methacrylic acid (11.91 mol, 2 equivalents). MEHQ (4.65 g, 0.037 mol, 0.006 equivalents) was added to the reaction flask. The reaction was stirred. With stirring, was added 2000 grams of Epoxide (obtained from Wright Corporation, 5.95 mol). The reaction mixture was heated to 90° C. 
     After about fifteen hours, the reaction mixture was removed from heat, allowed to cool to about 50° C. and transferred to a separatory funnel using ≈3200 mL hexanes (to give a 1:1 ratio of reaction mixture to hexanes) for transfer and to dilute the reaction mixture. The hexanes layer was washed successively with 4×≈3200 ML and 1×2000 mL 0.5 M aqueous NaOH, and 3×3200 mL 2.5 weight % aqueous NaCl. The organic layer was then dried over 250 gm Na 2 SO 4  and filtered. 
     To the filtrate was added 800 g of flash grade silica gel. The heterogeneous mixture was agitated for three hours at room temperature and filtered over a fritted glass funnel. The filtrate was then concentrated on the rotary evaporator, at 55° C., to give SiMAA2. The resulting SiMAA2 was analyzed by LC-MS for purity. Purity results are listed in Table 1, below. 
     EXAMPLE 2 
     To a three-neck, 5000 mL round bottom reaction flask equipped with a magnetic stir bar, condenser with an attached drying tube, and a thermocouple, was added 59 g dry potassium methacrylate (0.476 mol, 0.08 equivalents) and 1023 grams methacrylic acid (11.91 mol, 2 equivalents). MEHQ (4.65 g, 0.037 mol, 0.006 equivalents) was added to the reaction flask. The reaction was stirred. With stirring, was added 2000 grams of Epoxide (obtained from Silar, 5.95 mol). The reaction mixture was heated to 100° C. 
     After about fifteen hours, the reaction mixture was removed from heat, allowed to cool to room temperature and transferred to a separatory funnel using ≈2000 mL hexanes for transfer and to dilute the mixture. The hexanes layer was washed successively with 3×≈5000 mL 0.5 M aqueous NaOH, and 3×≈3500 mL 2.5 weight % aqueous NaCl. The organic layer was then dried over Na 2 SO 4  and filtered. The filtrate was then concentrated on the rotary evaporator, at 55° C., to give SiMAA2. The resulting SiMAA2 was analyzed by LC-MS for purity. Purity results are listed in Table 1, below. 
                                                   TABLE 1                       Example 1   Example 2                                        Total Purity (%)   85.9   84           Difunctional   4.92   8.97           impurities(%)           Ethyl Acetate(%)   &lt;0.02   0.43           Hexanes(%)   &lt;0.06   &lt;0.6           Epoxide(%)   0.59   &lt;0.06           Glycol(%)   0.49   0.76                        
Difunctional impurities include the following compounds
 
                                
Glycol is
 
     
       
                 
         
             
             
         
      
     
     EXAMPLES 3-8 
     To a three-neck, 125 mL round bottom reaction flask equipped with a magnetic stir bar, condenser with an attached drying tube, and a thermocouple, was added 6.88 gm methacrylic acid and methacrylic acid salt listed in Table 2, in the amounts listed in Table 2. The reaction was stirred. With stirring, was added 13.44 g Epoxide (obtained from Silar, 40 mmol). The reaction mixture was heated to 90° C. 
     After fourteen hours, the reaction mixture was removed from heat, allowed to cool to about 50° C. and transferred to a separatory funnel using ≈21.5 mL hexanes for transfer and to dilute the mixture. The hexanes layer was washed successively with 3×≈33.3 mL 0.5 N aqueous NaOH, and 3×≈33.3 mL 2.5 weight % aqueous NaCl. The organic layer was then dried over Na 2 SO 4  and filtered. The resulting SiMAA2 was analyzed by LC-MS for purity. Purity results are listed in Table 2, below. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Ex # 
                 Salt 
                 MAA salt (g) 
                 Toluene (mL) 
                 Purity 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 3 
                 Li 
                 0.575 
                 — 
                 87.8 
               
               
                 4 
                 K 
                 0.776 
                 — 
                 82.8 
               
               
                 5 
                 Na 
                 0.676 
                 — 
                 86.2 
               
               
                 6 
                 Li 
                 1.8 
                 10 
                 89.9 
               
               
                 7 
                 K 
                 2.425 
                 10 
                 38.8 
               
               
                 8 
                 Na 
                 2.11 
                 10 
                 86.1