Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2001-07-03
2003-02-11
Killos, Paul J. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S225000, C562S598000, C526S083000, C526S315000, C568S448000, C568S459000, C544S194000, C544S195000, C585S005000, C585S832000, C585S435000
Reexamination Certificate
active
06518453
ABSTRACT:
The present invention relates to a process for stabilizing chemical compounds which are present as pure substance or as a component of mixtures and have at least one ethylenically unsaturated group to undesired free radical polymerization, in which polymerization inhibitors which contain:
a) at least one nitroxyl radical (inhibitors a)) and
b) at least one chemical compound containing the element phosphorus in chemically bonded form (inhibitor b))
are added to the pure substance or to the mixture.
Chemical compounds having at least one ethylenically unsaturated group have a pronounced tendency to free radical polymerization owing to their ethylenically unsaturated groups. This is advantageous in that chemical compounds having at least one ethylenically unsaturated group are very suitable for the controlled preparation of polymers by free radical polymerization, as required, for example, for the production of adhesives. At the same time, however, the pronounced tendency to free radical polymerization is disadvantageous in that, both during storage and during the chemical and/or physical processing (e.g. distillation or rectification) of the chemical compounds having at least one ethylenically unsaturated group or mixtures containing them, undesired, spontaneous, free radical polymerization of said chemical compounds can occur, in particular under the action of heat and/or light. Apart from the fact that such uncontrolled free radical polymerizations of the chemical compounds having at least one ethylenically unsaturated group or mixtures containing them constitute a considerable potential danger (free radical polymerizations are highly exothermic and as a consequence frequently explosive), they moreover give rise to a variety of other secondary effects. For example, polymer formed in an undesirable manner during the distillation of mixtures containing chemical compounds having at least one ethylenically unsaturated group may be deposited on the surface of the evaporator (the tendency to polymer formation is higher there owing to the high temperatures) and thus reduce the heat transfer in an undesired manner. However, the polymer formed can also block the baffles in rectification columns, which results in undesired pressure losses. Both require stopping of the rectification process in order to remove the resulting polymer by an expensive procedure.
It is therefore general practice to add both to chemical compounds having at least one ethylenically unsaturated group and to mixtures containing such chemical compounds, both during storage and during the chemical and/or physical processing, compounds which act as inhibitors or retarders of free radical polymerization of the chemical compounds having at least one ethylenically unsaturated group and thus stabilize these chemical compounds to undesired free radical polymerization.
U.S. Pat. No. 5,322,960 discloses the addition of esters of (meth)acrylic acid as polymerization inhibitors to a mixture which contains, inter alia, a nitroxyl radical (a compound which has at least one >N—O.group). An example of a possible nitroxyl radical is, inter alia, 4,4′,4″-tris-(2,2,6,6-tetramethylpiperidinooxyl)phosphite, which may also be used as a mixture with other nitroxyl radicals.
EP-A 685 447 likewise recommends the presence of 4,4′,4′-tris-(2,2,6,6-tetramethylpiperidinooxyl)phosphite for inhibiting the free radical polymerization of (meth)acrylic acid (in this publication, (meth)acrylic is used as abbreviated notation for “acrylic and/or methacrylic”) and/or their esters. Similarly, CN-A 1 052 847 recommends the presence of N-oxyl phosphite esters as free radical polymerization inhibitors. The disadvantage of these polymerization inhibitors containing intramolecularly bonded phosphorus is that the ratio of N-oxyl groups to phosphorus is not variable.
DE-A 21 33 921 relates to a process for preventing the free radical polymerization of &agr;,&bgr;-monoethylenically unsaturated carboxylic acids, in which a combination of at least one hydroxyl-containing compound, e.g. cresole, and at least one compound containing the element phosphorus in chemically bonded form, e.g. phosphoric acid or derivatives thereof, is used as a polymerization inhibitor. Phosphoric acid contains the total amount of the phosphorus chemically bonded therein with the oxidation number +5. According to DE-A 21 33 921, the combination of a hydroxyl-containing phenolic compound and a compound containing the element phosphorus in chemically bonded form results in an increased inhibitory effect in a synergistic manner.
DE-A 29 13 218 discloses a process for the preparation of (meth)acrylic esters, in which a mixture of at least one organic ester of phosphorous acid and at least one monohydric or dihydric phenol is added to suppress free radical polymerization.
U.S. Pat. No. 4,187,382 relates to the esterification of organic diols with acrylic acid. It recommends pretreating the diol with triphenyl phosphite in order thus to reduce the tendency of the reaction mixture to undergo free radical polymerization. The use of a conventional phenolic polymerization inhibitor is recommended.
EP-A 810 196 recommends, as polymerization inhibitors for (meth)acrylic acid and its esters, the combination of a nitroxyl radical and a tertiary phosphine in which the phosphorus is covalently bonded to three carbon atoms and has the oxidation number +3. According to EP-A 810 196, the presence of the tertiary phosphine increases the inhibitory effect of nitroxyl radicals in a synergistic manner. The presence of the latter for stabilizing (meth)acrylic esters to undesired free radical polymerization is also recommended in DE-A 19734171.
It is an object of the present invention to provide a process for stabilizing chemical compounds which are present as pure substance or as the component of mixtures and have at least one ethylenically unsaturated group to undesired free radical polymerization, in which polymerization inhibitors which contain at least one nitroxyl radical and at least one further chemical compound which is capable of increasing the inhibitory effect of nitroxyl radicals in a synergistic manner are added to the pure substance or to the mixture.
We have found that this object is achieved by a process for stabilizing chemical compounds which are present as pure substance or as a component of mixtures and have at least one ethylenically unsaturated group to undesired free radical polymerization, in which polymerization inhibitors which contain:
a) at least one nitroxyl radical (inhibitors a)) and
b) at least one chemical compound containing the element phosphorus in chemically bonded form (inhibitor b))
are added to the pure substance or the mixture, wherein the at least one inhibitor b) contains on the one hand no N-oxyl group and on the other hand at least a part of the phosphorus chemically bonded with an oxidation number ON of −3≦ON≦+5 (frequently +1<ON≦+5, often +3≦ON≦+5), this phosphorus simultaneously being covalently bonded to at least one hetero atom different from carbon.
Initiators b) whose total amount of phosphorus present satisfies the abovementioned definition are preferred.
The oxidation number of an atom within a covalent compound of known structure is intended here to be understood as meaning a number having a positive or negative sign which indicates the charge which the atom would have if the bonding electron pairs of those of the covalent bonds in which it participates are assigned to the electronegative atom in each case within the covalent bond. In the case of electron pairs from covalent bonds between two identical atoms, each atom receives one electron.
In this publication, the electronegativity is a measure of the ability of an atom to attract electrons in a covalent bond as attributable to pauling.
The electronegativities relevant in this publication are those according to H. R. Christen, Grundlagen der allgemeinen und anorganischen Chemie, Verlag Sauerländer, Aarau, Diesterweg.Sa
Jaworek Thomas
Keller Harald
Müller-Engel Klaus Joachim
Schliephake Volker
Schröder Jürgen
BASF - Aktiengesellschaft
Killos Paul J.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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