Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2000-02-07
2003-02-11
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C562S598000, C560S004000
Reexamination Certificate
active
06518452
ABSTRACT:
The present invention relates to a method of stabilizing esters of meth(acrylic) acid, which are present as a pure substance or as a constituent of mixtures which comprise no Brönsted acid (no protic acid) whose Brönsted acid strength is greater than the Brönsted acid strength of acrylic acid, against unwanted free-radical polymerization by adding a polymerization inhibitor comprising a p-phenylenediamine.
In this document (meth)acrylic acid is used as a shortened form of writing “acrylic acid and/or methacrylic acid”.
Because of their activated vinyl group, esters of (meth)acrylic acid have a pronounced tendency to undergo free-radical polymerization. This is of advantage insofar as it makes (meth)acrylates outstandingly suitable for targeted preparation of polymers by the method of initiated free-radical polymerization, such polymers being required, for example, to produce adhesives. At the same time, however, the pronounced tendency to undergo free-radical polymerization is of disadvantage insofar as both the storage and the chemical and/or physical processing (for example, distillation or rectification) of the (meth)acrylates or of mixtures comprising such esters may, especially under the action of heat and/or light, be accompanied by unwanted, spontaneous free-radical polymerization of the (meth)acrylic esters. As well as the fact that such uncontrolled free-radical polymerizations of the (meth)acrylates or mixtures comprising them constitute a considerable hazard potential (the free-radical polymerization is highly exothermic and, consequently, in many cases explosive) they also have diverse other consequences. For example, polymer formed unintentionally during the distillation of mixtures comprising (meth)acrylic esters may deposit on the surface of the evaporator (where the high temperatures increase the propensity to form polymer), thereby undesirably reducing heat transfer. Alternatively, the polymer formed may clog the internals of rectification columns, causing unwanted pressure losses. Both phenomena necessitate the interruption of the rectification process for laborious removal of the polymer formed.
It is therefore common practice to add to these esters of (meth)acrylic acid and mixtures comprising such esters, in the course of both storage and chemical and/or physical processing, compounds which act as inhibitors of free-radical polymerization of the (meth)acrylates and hence stabilize them against unwanted free-radical polymerization.
From WO 92/01665 it is known to use as such an inhibitor a mixture of p-phenylenediamines with defined N,N-substitution. These p-phenylenediamines are said to be favorable polymerization inhibitors especially in the presence of strong acids. U.S. Pat. No. 4,797,504 recommends the use of p-phenylenediamines in combination with hydroxylamines as polymerization inhibitors for (meth)acrylic esters.
GB-A 1,064,845 contains the very general teaching to add organic compounds having a nitroso group, ie. an atomic grouping —N═O, as polymerization inhibitors to monomers having a vinyl group.
U.S. Pat. No. 5,322,960 discloses the addition to esters of (meth)acrylic acid of a polymerization-inhibiting mixture which includes, inter alia, a nitroxyl radical (a compound having at least one >N—O. group).
The teaching of JP-A 5/320217 is, inter alia, to stabilize (meth)acrylic acid against unwanted free-radical polymerization by adding a mixture that includes a nitroxyl radical and at least one polymerization inhibitor from the group consisting of phenothiazine, p-phenylenediamine, diphenylamine, hydroquinone and hydroquinone monomethyl ether. Such an inhibitor mixture is said also to reduce the tendency of (meth)acrylic acid to undergo polymerization in the course of its esterifications catalyzed by strong acids. JP-A 5/320217 also teaches that polymerization inhibitors suitable for reducing the polymerization tendency of (meth)acrylic acid are not necessarily appropriate for inhibiting the polymerization of their esters as well, which is supposed to be attributable to the fact that the ester no longer possesses the carboxyl group. Accordingly, JP-A 5/320217 contains no indication whatsoever that the inhibitor systems it recommends for (meth)acrylic acid might also be suitable for inhibiting the polymerization tendency of (meth)acrylic esters.
It is an object of the present invention, therefore, to provide a method of stabilizing esters of (meth)acrylic acid, which are present as a pure substance or as a constituent of mixtures which comprise no Brönsted acid whose Brönsted acid strength is greater than the Brönsted acid strength of acrylic acid, against unwanted free-radical polymerization by adding a polymerization inhibitor comprising a p-phenylenediamine, said process being more effective in inhibiting polymerization than the prior art processes acknowledged above.
We have found that this object is achieved by a method of stabilizing esters of (meth)acrylic acid, which are present as a pure substance or as a constituent of mixtures which comprise no Brönsted acid whose Brönsted acid strength is greater than the Brönsted acid strength of acrylic acid, against unwanted free-radical polymerization by adding a polymerization inhibitor comprising a p-phenylenediamine, which comprises adding to the pure substance or to the mixture a polymerization inhibitor which comprises:
a) at least one nitroxyl radical and/or at least one organic compound having at least one nitroso group (inhibitors a) and
b) at least one p-phenylenediamine (inhibitors b).
A measure of Brönsted acid strength is the equilibrium constant (acid constant) of the reaction HA+H
2
O⇄A
⊖
+H
3
O
⊕
at 25° C. and 1 atm, where HA is the Brönsted acid and A
⊖
the associated conjugated base (cf. Christen, Grundlagen der allgemeinen und anorganischen Chemie, Verlag Sauerländer, Aarau, 1973, pp. 353/354).
Nitroxyl radicals (also known as N-oxyl radicals) suitable for the purposes of the invention are in particular those derived from a secondary amine that carries no hydrogens on the a carbons (in other words, the N-oxyl groups are derived from corresponding secondary amino groups). Particularly suitable among these are those N-oxyl radicals specified in EP-A 135280, prior application DE-A 19651307, U.S. Pat. Nos. 5,322,912, 5,412,047, 4,581,429, DE-A 1618141, CN-A 1052847, U.S. Pat. Nos. 4,670,131, 5,322,960, prior application DE-A 19602539, EP-A 765856 and JP-A 5/320217.
Examples of such suitable, stable N-oxyl radicals deriving from a secondary amine are those of the formula I
where
R
1
,R
2
,R
5
and R
6
=the same or different straight- or branched-chain, substituted or unsubstituted alkyls and
R
3
and R
4
=the same or different straight- or branched-chain, substituted or unsubstituted alkyls, or
R
3
CNCR
4
=a substituted or unsubstituted cyclic structure.
Compounds I that are suitable in accordance with the invention are in particular those specified in EP-A 135 280, prior application DE-A 19651307, U.S. Pat. Nos. 5,322,912, 5,412,047, 4,581,429, DE-A 16 18 141, CN-A 1052847, U.S. Pat. Nos. 4,670,131, 5,322,960 and prior application DE-A 19602539.
Examples of these are those stable N-oxyl radicals of the formula I in which R
1
, R
2
, R
5
and R
6
are (identical or different) C
1
-C
4
-alkyls, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or tert-butyl, linear or branched pentyls, phenyls or substituted derivatives thereof and R
3
and R
4
are (identical or different) C
1
-C
4
-alkyls, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or tert-butyl, linear or branched pentyls, substituted derivatives thereof or, together with CNC, the cyclic structure
where n is an integer from 1 to 10 (frequently from 1 to 6), including substituted forms of such cyclic structures. Typical examples are 2,2,6,6-tetramethyl-1-oxyl-piperidine, 2,2,5,5-tetramethyl-1-oxyl-pyrrolidine and 4-oxo-2,2,6,6-tetramethyl-1-oxyl-piperidine.
The N-oxyl radicals I can be prepared by oxidizing the corresponding secondary
Aichinger Heinrich
Herbst Holger
Nestler Gerhard
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