Preparation of (meth)acrylates

Details Preparation of (meth)acrylates Preparation of (meth)acrylates

2000-05-10

2003-01-28

Rotman, Alan L. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C560S212000, C560S218000

Reexamination Certificate

active

06512138

ABSTRACT:

The present invention relates to a process for the preparation of (meth)acrylates, in which (meth)acrylic acid is esterified with alkanols in the presence of esterification catalysts. After the esterification, unconverted starting compounds and the (meth)acrylate to be formed are separated off by distillation, and an oxyester-containing bottom product is obtained. In addition to the oxyesters, oligomeric (meth)acrylic acid and oligomeric or polymeric (meth)acrylates are also formed. These compounds and their formation are first explained below.
Owing to their activated C═C double bond, alkyl esters of (meth)acrylic acid are important starting compounds for the preparation of polymers to be produced by free radical polymerization and used, for example, as adhesives.
Usually, the preparation of the esters is carried out by esterification of the. (meth)acrylic acid with alkanols at elevated temperatures in the liquid phase with or without a solvent and in the presence of, as a catalyst, acids other than (meth)acrylic acid, and subsequent isolation by distillation.
The disadvantage of this process is that, on the one hand, the formation of polymeric (meth)acrylates cannot be completely prevented in spite of the use of polymerization inhibitors and, on the other hand under the abovementioned esterification conditions, unconverted starting alkanols undergo an addition reaction at the double bond of already formed alkyl (meth)acrylate with formation of a compound of the formula I shown below and unconverted (meth)acrylic acid undergoes an addition reaction at said double bond with formation of a compound of the formula II, as secondary reaction (Michael addition).
Successive multiple addition is also possible. Furthermore, mixed types can occur. These adducts (alkoxyesters and acyloxyesters) are referred to as oxyesters for short:
 RO(—CH
2
—CHR′—CO
2
)
x
—R  (I)
CH
2
═CR′CO
2
(—CH
2
—CHR′—CO
2
)
y
—R  (II)
where x and y are each an integer from 1 to 5,
R is alkyl and
R′ is H or CH
3
.
The formation of oxyesters is described, for example, in DE-A 23 39 529. The formation of oxyesters takes place essentially independently of the specific esterification conditions. Oxyester formation in the preparation of acrylates is of very particular importance.
The term polymeric (meth)acrylates means the polymeric products formed by free radical polymerization of (meth)acrylates. In contrast to the Michael adducts, these polymers cannot be cleaved back into the starting monomers under conventional reaction conditions. They form when the (meth)acrylates are subjected to thermal stress, for example in the synthesis or working-up of the esterification mixture by distillation or in the purification by distillation.
The term (meth)acrylic acid refers to acrylic or methacrylic acid.
The term oligomeric (meth)acrylic acid means the Michael adducts of (meth)acrylic acid with itself and with the resulting secondary products. Such Michael adducts can be characterized by the formula (III)
CH
2
═CR′—CO
2
(—CH
2
—CHR′—CO
2
)
z
—H  (III)
where z is an integer from 1 to 5 and
R′ is H or CH
3
,
and should be distinguished here from (monomeric) (meth)acrylic acid and from (meth)acrylic acid polymers (which are obtainable by free radical polymerization of (meth)acrylic acid). What is important is that the Michael addition of (meth)acrylic acid with itself and with the resulting secondary products is reversible.
Oligomeric (meth)acrylic acid is obtained in the bottom product, for example in the distillative treatment of (for example crude) (meth)acrylic acid (the term “crude” indicates a small fraction of in particular aldehydic impurities which are still present).
Any esterification mixture is usually worked up by separating off unconverted starting compounds and the desired ester from the reaction mixture by distillation, the acid catalyst used for the esterification and unconverted (meth)acrylic acid being separated off previously, if required, by extraction by means of water and/or aqueous alkali (cf. for example Ullmann's Encyclopedia of Industrial Chemistry, Vol. A1, 5th Ed., VCH, page 167 et seq.). The bottom product remaining in such a working-up by distillation contains the oxyesters and the polymeric (meth)acrylates, which result in a considerably lower yield.
Attempts were therefore made to use a wide range of procedures to reduce the losses of desired products which are due to the occurrence of these byproducts, especially the Michael adducts.
DE-A-195 47 459 and DE-A-195 47 485 describe processes for the esterification of (meth)acrylic acid with an alkanol, in which, after the esterification reaction, the resulting (meth)acrylate and unconverted starting compounds are distilled off from the reaction mixture and an oxyester-containing bottom product forms. Either (meth)acrylic acid or oligomeric (meth)acrylic acid is added directly to the bottom product and the oxyesters contained in the bottom product are then cleaved by the action of elevated temperatures in the presence of acid catalysts which differ from (meth)acrylic acid or oligomeric (meth)acrylic acid, or the oxyesters are first separated from the bottom product by distillation and the distillate is treated as described above. The cleavage products obtained are recycled directly to the esterification. The cleavage residue remaining after the cleavage and removal of the cleavage products is not worked up further.
EP-A-0 765 860 describes a process for the esterification of (meth)acrylic acid with an alkanol, in which, after the reaction, resulting (meth)acrylates and unconverted starting compounds are separated from an oxyester-containing bottom product. The oxyesters contained in the bottom product separated off are isolated by distillation and the resulting distillate is cleaved at elevated temperatures in the presence of acids. No working up of the cleavage residue is described.
EP-A-0 767 163 describes a further process for the esterification of (meth)acrylic acid with an alkanol. After removal of resulting (meth)acrylates and unconverted starting compounds, the bottom product is separated off and is heated to 150 to 200° C. in the presence of acid, the pressure being set so that the cleavage products formed during cleavage of the oxyesters contained in the bottom product immediately evaporate off. No further treatment of the cleavage residue is described.
JP-A H8 183 756 describes the acid-catalyzed degradation of Michael adducts in the presence of water. However, the conversion is only about 60%.
EP-A-0 779 268 describes a process for the preparation of alkyl acrylates, in which the high-boiling residues (oxyesters) are hydrolytically cleaved, the alkanol formed is separated off by distillation and the hydrolysis residue is subjected to a cleavage reaction.
The process has the disadvantage that the recovery rate is unsatisfactory and that the alkyl acrylate component contained in the oxyester is hydrolytically cleaved into alkanol and acrylic acid and is not recovered as such.
It is an object of the present invention to provide a process for the esterification of (meth)acrylic acid with an alkanol, in which, compared with the above processes, the amount of byproducts is further reduced and more products which can be recycled to the esterification or removed as desired ester are obtained from the remaining residues.
We have found that this object is achieved, according to the invention, by a process for the esterification of (meth)acrylic acid with an alkanol in the presence of an esterification catalyst, in which unconverted starting compounds and the (meth)acrylate to be formed are separated off by distillation and an oxyester-containing bottom product is formed and is separated off, and
(a) in the bottom product separated off, the oxyesters present are cleaved, if required after the addition of (meth)acrylic acid and/or oligomeric (meth)acrylic acid, directly in the presence of acid catalysts differing from (meth)acrylic acid and/or oligomeric

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