Styrene autoxidation reaction

Tables I and II include data for the co-oxidations of styrene and butadiene in chlorobenzene and ferf-butylbenzene solutions, as well as with no added solvent. These solvents were chosen because the rate of oxidation of cyclohexene varies significantly in them at the the same rate of initiation (6). There is a variation in the over-all rate of oxidation under these solvent conditions, but there appears to be no significant difference in the measured ra and rb (Table II). If the solvent does affect the propagation reaction in autoxidation reactions, it affects the competing steps to the same degree.

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Howard and Ingold (7) proposed participation of a first-order termination reaction in the autoxidation of styrene. If such contributions from first-order terminations are real and widespread in autoxidation, more knowledge about them becomes essential. 

In a variation on this theme cobaltphthalocyaninetetrasulfonate (CoPcTs) was bound via the anionic sulfonate groups to styrene-divinylbenzene copolymer latexes containing quaternary ammonium ions.46 The resulting colloidal catalyst was used to effect the autoxidation of 2,6-di-tert-butylphenol in aqueous solution, to the corresponding diphenoquinone (reaction 21). The rate of oxidation was ten times faster than with homogeneous CoPcTs in water. 

Further evidence against initiation by direct oxygen activation in the oxidation of olefins is provided by the following two observations.185 First, no reaction was observed between olefins (e.g., cyclohexene, 1-octene, and styrene) and metal-dioxygen complexes, such as I, II, and V, when they were heated in an inert atmosphere (nitrogen). Second, no catalysis was observed with these metal complexes in the autoxidation of olefins, such as styrene, that cannot form hydroperoxides. 

TBHP vide supra). The autoxidation of EB is performed at 120-160 °C and 1- bar. MBA and acetophenone (ACP) are formed as by-products via the facile termination of the secondary 1-methylbenzylperoxy radicals. In order to minimize by-product formation by further oxidation of MBA and ACP, the autoxidation is carried out to only low conversions (< 12 %). This solution (ca. 10 %) of EBHP in EB is used in the epoxidation step, i.e., EB is the solvent for the latter step. A high propene/EBHP molar ratio is used and reaction conditions are similar to those of the TBHP process vide supra). The PO selectivity is reported to be 90 % at 92 % EBHP conversion [30] but in practice it may be higher. For comparison the heterogeneous Ti /SiOa catalyst in fixed-bed operation reportedly gives 93-94 % PO selectivity at 96 % EBHP conversion [11]. The products are separated by distillation and MBA is dehydrated to styrene in the vapor phase over a Ti02 catalyst. 

The reactions of phosphites with peroxy radicals continue to attract attention because of the use of phosphites as anti-oxidants. The autoxidation of a variety of hydrocarbons, e.g. tetralin, cumene, styrene, and cyclohexane, is inhibited by zinc dialkyldithiophosphates (60). In order to assess the reactivity... 

For example, the cobalt(II) complex for phthalocyanine tetrasodium sulfonate (PcTs) catalyzes the autoxidation of thiols, such as 2-mercaptoethanol (Eq. 1) [4] and 2,6-di(t-butyl)phenol (Eq. 2) [5]. In the first example the substrate and product were water-soluble whereas the second reaction involved an aqueous suspension. In both cases the activity of the Co(PcTs) was enhanced by binding it to an insoluble polymer, e.g., polyvinylamine [4] or a styrene - divinylbenzene copolymer substituted with quaternary ammonium ions [5]. This enhancement of activity was attributed to inhibition of aggregation of the Co(PcTs) which is known to occur in water, by the polymer network. Hence, in the polymeric form more of the Co(PcTs) will exist in an active monomeric form. In Eq. (2) the polymer-bound Co(PcTs) gave the diphenoquinone (1) with 100% selectivity whereas with soluble Co(PcTs) small amounts of the benzoquinone (2) were also formed. Both reactions involve one-electron oxidations by Co(III) followed by dimerization of the intermediate radical (RS or ArO ). 

In comparing the effectiveness of AT-alkylanilines and M-arylanilines as inhibitors for the autoxidation of styrene, Brownlie and Ingold consider a number of cases of increasing complexity [22,23]. Considering first the reaction sequence... 

By-products formed during their preparation (e.g., ethylbenzene and divin-ylbenzenes in styrene acetaldehyde in vinyl acetate) added stabilizers (inhibitors) autoxidation and decomposition products of the monomers (e.g., peroxides in dienes, benzaldehyde in styrene, hydrogen cyanide in acrylonitrile) impurities that derive from the method of storage of the monomer (e.g., traces of metal or alkali from the vessels, tap grease etc.) dimers, trimers, and polymers that are generally soluble in the monomer, but sometimes precipitate, for example, polyac-rylOTiitrile from acrylonitrile. Likewise, in polycondensation reactions it is important to remove reactive impurities because they can cause considerable interference during the polyreaction. 

Fragmentation (Sgi) of the vicinal peroxides yields 4-H(P)NE and hydroper-oxy epoxides in a reaction that is similar to the mechanism of styrene/oxygen co-polymerization and subsequent degradation to benzaldehyde and formaldehyde [57,65,66], An equivalent cross-chain mechanism is likely to contribute to the formation of 4-H(P)NE during autoxidation of cardiolipin [67]. 

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