Preparation of (Meth)acrylated Alkyl Phosphates
Publication Date: 2004-Oct-14
The IP.com Prior Art Database
Reported herein is the utility of solid phase acidic resins as reaction catalysts and their application in simple high yielding processes of making (meth)acrylated alkyl phosphates that are otherwise made via laborious multi-step and wasteful procedures. The processes utilize Amberlyst-15, and the like, as the solid phase catalysts and require only a simple filtration of the recyclable catalyst at the end of the reaction in order to recover the desired product. Neither extra steps nor solvents are required. The acidic monomers produced by this method can be key ingredients in self-etching dental adhesive formulations. Discussion and Results ω-Methacryloyloxyalkyl phosphates (Scheme 1) serve as acidic etchants in self-etching dental adhesive formulations and, being polymerizable, have the capability to copolymerize with the other monomers in the formulation such that no hazards to the patient results after curing of the adhesive. Traditionally, this class of materials is prepared in two steps, the first of which is an esterification of a diol with methacrylic acid in the presence of a soluble acidic catalyst (see Scheme 2). The second step involves phosphorylating the methacrylated alcohol prepared in the first step to give the corresponding phosphate. A successful phosphorylation reaction, which results in a minimum of phosphoric acid by-product, requires a very dry alcohol from the first step. Several laborious work-up steps are required in the first step in order to ensure a complete removal of the soluble catalyst and that the intermediate alcohol is dry enough for the second step. U.S. Patent No. 4,539,382 (Omura et al.) discloses methods of making various methacrylated alkyl and aromatic alcohols by reacting methacrylic acid with diols in the presence of p-toluenesufonic acid (soluble acid catalyst) at 90ºC along with continuous removal of the water by-product by applying a vacuum. After the reaction is complete, the crude product is either taken neat or dissolved into some organic solvent and then washed with an aqueous sodium bicarbonate solution several times until alkaline, followed by several water washings (5 times on average). The resulting moist, catalyst-free product was then dried by anhydrous Na2SO4 followed by vacuum at 90ºC for several hours (Scheme 3). One complication of having residual water in the intermediate is the formation of undesired phosphoric acid in the final methacrylated phosphate product (Scheme 4). Although Omura et al indicated that water can be removed by the multi-step work-up shown in Scheme 3, we have found that it is extremely difficult to remove all the water under the described conditions. Also, applying a vacuum to the product at high temperature can result in undesirable polymerization. Also, when the methacrylated alcohol was made according to the Omura et al. methods, the amount of the phosphoric acid by-product in the phosphorylated product was much more than was observed with our solid phase catalyst method. For example, the Omura et al. procedure resulted in an average 13 mole% of phosphoric acid in the phosphorylation step compared to 0.9-1.1 mol% phosphoric acid when Amberlyst-15 resin was used as a solid phase catalyst. Additionally, the solid state catalyst is simple to use with much less labor required. Another advantage to the solid phase catalyst is the fact that these resins are recyclable. The reaction yield was improved by over 15% by changing from soluble catalyst to the solid phase catalyst. Experimental 6-(Hydroxyhexyl) methacrylate (MHP Precursor) 1,6-Hexane diol (154.25 g, 1.27 mol of 97% sample from Aldrich Chemical, Milwaukee, WI) was placed in a 500-ml 3-neck flask equipped with a mechanical stirrer, a thermocouple, and dry air stream blowing through the flask and into an oil bubbler. The solid diol was heated to 90ºC (all melted by 40-50ºC). With continuous stirring, methacrylic acid (125.5 g, 1.46 mol from Alfa Aesar) was added followed by 300 mg of BHT and 10 g of Amberlyst-15 resin (Alfa Aesar). The mixture was continuously stirred at 90ºC for 3 hours followed by a vacuum using tap water aspirator. The vacuum was applied for 5-10 minutes every 45-60 minutes with the air stream running through the flask all the time. After 20 hours of reaction time, NMR indicated that all the methacrylic acid reacted. The heat was turned off and the flask contents were cooled to room temperature. The desired product was isolated by vacuum filtration to give 230 g (90%) of a yellow oil. The Amberlyst resin was recovered, washed with methanol, and then dried for future use. The structure of the product was confirmed by 1H and 13C NMR and was found to contain 91.63 mol% desired monoester and 8.37 mol% diester. 6-(Methacryloyloxyhexyl) phosphate (MHP) Phosphorus (V) oxide (31 g, 0.109 mol P4O10, Alfa Aesar) was placed in a 500-ml 3-neck flask equipped with a mechanical stirrer and a dry nitrogen stream blowing through the flask and into an oil bubbler. To the flask was then added 50 ml of methylene chloride to make a slurry with the phosphorus (V) oxide. The flask was cooled in a dry ice-acetone bath for 5 minutes. With stirring, MHP precursor above (137.02 g, 0.655 mol of mono ester and 0.06 mol of diester) in 50 ml of methylene chloride was added slowly into the flask over 1 hour. The mixture was vigorously stirred in the dry ice bath for 1 hour and at room temperature for 3 hours. A reflux condenser was placed on one arm of the flask and the mixture was refluxed (39-40ºC) for one hour. The heat was turned off and, after cooling to room temperature, the solvent was removed in a rotary evaporator at 25ºC to give 155 g (92%) of a yellow oil. Phosphorylation was confirmed by 1H, 13C and 31P NMR which showed that the product was a mixture of 39.3 mol% pyro-MH-phosphate, 32 mol% Di-MH phosphate, 27.05 mol% MHP, and 0.93% phosphoric acid.