The present invention relates to a mixture which contains one or more vinyl-containing compounds as a component (A) and, as a further component, a stabilizer (B) which contains one or more readily volatile nitroxyl compounds of secondary amines as component (b1), one or more sparingly volatile nitroxyl compounds of secondary amines as component (b2), if required as one or more aromatic nitro compounds as component (b3) and, if required, one or more iron compounds as component (b4).
The present invention furthermore relates to stabilizers (B), which contain the components (b1) and (b2), (b1) and (b2) and (b3), (b1) and (b2) and (b4), and (b1), (b2), (b3) and (b4), and a process for inhibiting the premature polymerization of vinyl-containing compounds during their purification or distillation by adding a stabilizer (B) or by adding the components of stabilizer (B) as individual substances or in at least two groups of the components.
It is known that many unsaturated compounds tend to undergo polymerization, as a rule free radical polymerization, while the temperature is increased. For example, vinylaromatic compounds, such as styrene or xcex1-methylstyrene, must be stabilized with suitable compounds in order to prevent premature polymerization during the distillative purification of the crude products obtained on a large industrial scale. Usually, these stabilizers or polymerization inhibitors are added, before or during the purification step, to the crude products to be distilled. In spite of this measure, considerable proportions of polymers are still obtained. In specific cases, particularly in the event of operating faults, complete polymerization of the monomers or monomer mixtures present may take place during the purification or distillation. This results in high costs owing to the very expensive purification and the stoppage of production.
USSR patents 1,027,150, 1,558,888 and 1,139,722 describe the stabilization of styrene by using nitroxyl or bisnitroxyl compounds.
In the prior German patent application 19 510 184.7, 4-acylaminopiperidine-N-oxyl derivatives are used for stabilizing monomers capable of free radical polymerization.
The prior German patent application DE 19 609 312.0 describes compositions which contain vinyl-containing monomers and at least one N-oxyl compound of a secondary amine, the latter having no hydrogen atoms on the N-bonded carbon atoms.
Mixtures of vinylaromatic compounds with sterically hindered nitroxyl compounds, which are activated by traces of oxygen, are described in WO 96/16921.
Japanese publication Hei 1-165 534 discloses I-piperidyloxy derivatives as polymerization inhibitors for styrene. U.S. Pat. No. 3,733,326 describes the inhibition of the polymerization of vinylmonomers by using free radical precursor compounds.
Mixtures of nitroxyl and nitro compounds are described in U.S. Pat. No. 5,254,760 and prior German patent application 19 622 498.5 for stabilizing vinylaromatic compounds or vinyl-containing monomers during the purification or distillation.
Mixtures which, in addition to vinyl-containing compounds, also contain nitroxyl compounds and iron compounds and, if required, nitro compounds and costabilizers are described in prior German patent application 19 651 307.3.
The effectiveness of the stabilizers described in these publications and the stability of the additive-containing monomer mixtures are good, the higher proportion of nitro compounds described in the prior German patent application 19 622 498.5 also achieving a better retardation/effect and the presence of iron compounds in the stabilizers described in the prior German application 19 651 307.3 having a clearly favorable effect on their effectiveness. If the supply of monomers and added stabilizers to the column is stopped, these stabilizers therefore achieve a better retarding effect until the onset of extensive polymerization reactions.
In the case of all stabilizers, attention has been focused to date mainly on the stabilization of the bottom part of the column, but stabilization of monomers present in the vapor phase has been neglected. Conventional procedures in this connection are spraying in the stabilizer, also used in the bottom, in a carrier solution, for example the monomer mixture to be distilled, and/or the wetting of the column walls with, for example, the same carrier solutions to which stabilizer has been added. An even distribution of the stabilizer in the vapor phase and hence the effective stabilization thereof during the distillation/purification is not adequately ensured by such measures.
However, since in these large-scale industrial processes even small portions of polymers accumulate to give large amounts of undesirable byproducts, there is a continuous need for even more effective polymerization inhibitors. Reduction in the proportion of nitro compounds is also desirable with a view to improving the handling by the operator and reducing possible environmental pollution.