Common industrial methods for producing styrene typically include separation and purification processes such as distillation to remove unwanted impurities. Unfortunately, purification processes carried out at elevated temperatures result in an increased rate of undesired polymerization. Distillation is generally carried out under vacuum to minimize loss of monomer. The presence of oxygen, which is typically excluded from styrene distillation, will also promote polymerization of the monomer.
This polymerization results not only in loss of desired monomer end-product, but also in the uses of production efficiency caused by polymer formation and/or agglomeration of polymer on process equipment. Thermal polymerization, which typically occurs during distillation, of styrene monomer results in the formation of normal (i.e., linear) polymer. This resulting polystyrene polymer is characterized by its glassy and transparent appearance and its solubility in the styrene monomer and many organic solvents.
The compounds generally used commercially to inhibit polymerization of vinyl aromatic monomers are of the dinitrophenol family. For example, U.S. Pat. No. 4,105,506, Watson et al. teaches the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds. U.S. Pat. No. 4,466,905, Butler et al. teaches that a combination of 2,6-di-nitro-p-cresol and p-phenylenediamine will inhibit polymerization in a distillation column when oxygen is present. U.S. Pat. No. 4,774,374, Abruscato et al. teaches compositions for inhibiting the polymerization of vinyl aromatic compounds. The composition is an oxygenated product of the reaction of N-aryl-N'-alkyl-p-phenylenediamine with oxygen. U.S. Pat. Nos. 5,426,257 and 5,489,718, Arhancet, teach methods and compositions for inhibiting the polymerization of vinyl aromatic monomers comprising an oxime compound and a hydroxylamine compound and/or a phenylenediamine.
The use of hydroxylamine compounds for preventing polymerization of vinyl aromatic compounds is disclosed in U.S. Pat. No. 2,965,685, Campbell. Their use in combination with phenylenediamines is disclosed in U.S. Pat. Nos. 5,396,004 and 5,510,547, Arhancet et al.
U.S. Pat. Nos. 4,003,800 and 4,040,911, Bacha et al., teach methods for inhibiting the polymerization of styrene utilizing a quinone alkide compound without or with a hindered phenol compound, respectively. Preferably, the quinone alkide is a methide such as 2,6-di-t-butyl-4-methenyl quinone methide.
U.S. Pat. No. 5,616,774, Evans et al., discloses processes and compositions for inhibiting the polymerization of vinyl aromatic monomers using a 7-aryl quinone methide. U.S. Pat. No. 5,583,247, Nesvadba et al., teaches inhibiting the polymerization of ethylenically unsaturated monomers with a 7-substituted quinone methide.
None of these references has taught or suggested employing a hydroxylamine compound and a quinone methide derivative together to inhibit the polymerization of a vinyl aromatic monomer.