Method for producing methacrylated benzophenones

The invention relates to a process for preparing methacrylated benzophenones and to the uses thereof.

Described in the state of the art is a methacrylic anhydride process for preparing the above-stated esters (JP2003261506, Mitsubishi Rayon). Triethylamine is used as catalyst. Since the amine forms a salt with the methacrylic acid produced during the reaction, it must be made equimolar with respect to the benzophenone. Accordingly, equimolar quantities of salt are obtained, which require disposal as waste. Consequently the process is not very economical.

Further methods of the prior art are the reaction of methacryloyl chloride with hydroxy-functional benzophenone, and the reaction of this raw material with glycidyl methacrylate. When using methacryloyl chloride it is necessary to take account of the corrosive and caustic properties. On contact with water, moreover, HCl is released.

DE 1720603 describes a process for preparing aqueous dispersions of readily crosslinkable polymers. In that process, acrylic and methacrylic esters are copolymerized with photoactive, olefinically unsaturated monomers, where appropriate with the accompanying use of photoactive, non-ionogenic emulsifiers.

EP 0346788 describes a process for preparing radiation-sensitive carbamoylbenzo- and -acetophenones having at least one methacrylate or acrylate end group. That process reacts isocyanatoalkyl (meth)acrylates with hydroxyacetophenones or hydroxybenzophenones, using a basic catalyst. It is necessary to operate in the absence of moisture. Moreover, only dried, non-nucleophilic solvents can be used.

The object was to provide an improved process for preparing (meth)acrylic esters of hydroxy-functional benzophenones.

The object has been achieved by a process for preparing benzophenone (meth)acrylates, characterized in that hydroxybenzophenones and (meth)acrylic anhydride are reacted in the presence of catalytic amounts of acid, then the catalyst is neutralized, and subsequently the crude monomer is purified.

The notation “(meth)acrylate” here denotes methacrylate, such as methyl methacrylate, ethyl methacrylate, etc., and acrylate, such as methyl acrylate, ethyl acrylate, etc., and also mixtures of both.

Surprisingly it has been found that, with the process of the invention, high conversions are achieved, and the amount of by-products is greatly reduced. It has been found that the process of the invention is burdened only by a low salt load, which comes about when the catalyst acid is neutralized in the course of work-up. The by-product methacrylic acid can be used as a comonomer in the subsequent polymerization of the benzophenone monomer, or can be recycled for the preparation of new methacrylic anhydride.

The reaction can take place in the presence of common alkylsulphonic or arylsulphonic acids, preferably with sulphuric acid.

Preference is given to reacting 4-hydroxybenzophenone and (meth)acrylic anhydride in the presence of catalytic amounts of concentrated sulphuric acid.

(Meth)acrylic anhydride is added in a slight excess relative to the hydroxybenzophenone. The reaction takes place at temperatures between 50 to 120° C., preferably at 80° C. to 100° C., over 4 to 8 hours, preferably over 5.5 to 6.5 hours.

The catalytically employed acid is neutralized with aqueous bases, preferably with alkali metal hydroxide solution or ammonia solution.

The subsequent work-up of the crude monomer takes place by addition of water. In this way, the impurities are dissolved and can be separated off without problems. The water-soluble impurities of the [(meth)acryloyloxy]benzophenone melt are removed preferably by the addition of water.

[(Meth)acryloyloxy]benzophenone is precipitated by the addition of excess water into the reaction mixture, and is isolated in solid form by filtration.

The benzophenone (meth)acrylates, prepared in high purity, can be stored and reacted further in solution with methyl methacrylate, n-butyl methacrylate, isobutyl methacrylate or styrene.

Benzophenone (meth)acrylates can be used for subsequent photocrosslinking of polymers by daylight or UV light, and also as polymeric photoinitiators.

The benzophenone (meth)acrylates can be used, furthermore, as comonomer for polymerization reactions.

The examples given below are given for better illustration of the present invention, but are not apt to restrict the invention to the features disclosed herein.

Apparatus: 4 l four-neck round-bottomed flask with mechanical stirrer, reflux condenser, Pt100 temperature sensor, air introduction tube, Anschutz attachment, dropping funnel, electrically heated oil bath

Batch:

3.5 mol of 4-hydroxybenzophenone, 99.7%: 695.9 g 3.85 mol of methacrylic anhydride (stabilized with 2000 ppm of 2,4-dimethyl-6-tert-butylphenol): 618.4 g 0.020 mol of concentred sulphuric acid: 1.99 g 1864 mg of hydroquinone monomethyl ether 932 mg of 2,4-dimethyl-6-tert-butylphenol

Neutralization of the catalyst acid with 1.8 g of sodium hydroxide in solution in 10 g of water

Esterification of the excess methacrylic anhydride with 22.4 g of methanol Theoretical yield: 930.0 g

Procedure:

The batch was weighed out completely and then heated to 90° C. with stirring and introduction of air. Reaction time at 90° C.: 6 h. It was then cooled to about 60° C. and the sodium hydroxide in solution in water, for neutralizing the sulphuric acid catalyst, and also the methanol, for esterifying the unreacted methacrylic anhydride, were added. This was followed by stirring at 60° C. for 1 h, after which the batch was poured as thin jet, with stirring (metallic paddle stirrer, stirring motor), into 3 l of water. The mixture was stirred for 0.5 h and the precipitate was then isolated by suction on a glass filter frit, washed with twice 2 l of water, and subsequently subjected to preliminary drying with air on the suction filter. The solid was subsequently dried in air.

Yield: 924.6 g (99.4% of theory)

Analyses: water content: 0.08%

hydroquinone monomethyl ether: 6 ppm 2,4-dimethyl-6-tert-butylphenol: 174 ppm

Gas chromatography:

0.047% methyl methacrylate 0.013% methacrylic acid 0.637% 4-hydroxybenzophenone 97.56% 4-(methacryloyloxy)benzophenone Pt—Co colour number as 20% strength solution in acetone: 150

Apparatus: 4 l four-neck round-bottomed flask with mechanical stirrer, reflux condenser, Pt100 temperature sensor, air introduction tube, Anschütz attachment, dropping funnel, electrically heated oil bath

Batch:

1.5 mol of 4-hydroxybenzophenone: 303 g 1.65 mol of methacrylic anhydride (stabilized with 2000 ppm of 2,4-dimethyl-6-tert-butylphenol): 262 g 0.0087 mol of concentred sulphuric acid: 0.84 g 798 mg of hydroquinone monomethyl ether 399 mg of 2,4-dimethyl-6-tert-butylphenol

Procedure:

The batch was weighed out completely and then heated to 90° C. with stirring and introduction of air. Reaction time at 90° C.: 6 h. It was then cooled to about 60° C. and the sodium hydroxide in solution in water, for neutralizing the sulphuric acid catalyst, and also the methanol, for esterifying the unreacted methacrylic anhydride, were added. The batch was subsequently stirred at 60° C. for 1 h and then admixed with 1566 g of methyl methacrylate, with stirring. The resulting solution was cooled to room temperature with stirring, and filtered. The solution of the 4-(methacryloyloxy)benzophenone in methyl methacrylate has the following composition as determined by gas chromatography:

56.016% methyl methacrylate 6.954% methacrylic acid 2.399% 4-hydroxybenzophenone 32.717% 4-(methacryloyloxy)benzophenone.

The water content is 0.27%; the stabilizer content is 113 ppm of 2,4-dimethyl-6-tert-butylphenol and 4 ppm of hydroquinone monomethyl ether. The Pt—Co colour number is 169.