Ketone peroxide initiators, decomposition

Oxidation of LDL can be divided into different stages i) initiation of lipid peroxidation ii) propagation of PUFA-mediated lipid peroxidation iii) decomposition of lipid hydroperoxides into reactive aldehydes and ketones, and iv) modification of apo B, leading to recognition of LDL by the macrophage scavenger receptor. 

Many peroxides affect pol mierization, but those used are available in quantity and the choice is based both on economics and performance. It has been shown that the activity of the organic peroxides in any polymerization is related to their decomposition rates at various temperatures. If elevated cure temperatures, 200- 250°F (93-121°C), are used, benzoyl peroxide is preferred. The amount required is about 1.0 per cent. It is preferred because a long catalyzed tank life results at room temperature. If lower temperatures in the 120 180 F (49-82°C) range are employed, hydroperoxides are more effective. Methyl ethyl ketone peroxide and cumene and ter- tiary butyl hydroperoxide all find application. Lauroyl peroxide, cyclohexanone peroxide, and <-butyl perbenzoate are used in limited amounts. Mixtures of two peroxides are often used, one to initiate the reaction and a second to promote the polymerization once it is started. 

Table 20-1. Half-Lives and Activation Energies of Decomposition of Some Free Radical Initiators AI BN, Azobisisobutyronitrile BPO, Dibenzoyl Peroxide MEKP, Methyl Ethyl Ketone Peroxide IPP, Diisopropyl Peroxide Dicarbonate Dicup, Dicumyl Peroxide CuHP, Cumyl Hydroperoxide...

Examples of some important initiators are ethyl methyl ketone peroxide, cyclohexanone peroxide, benzoyl peroxide and cumene peroxide. Accelerator is a reducing agent, such as cobalt octoate, which is added in very small quantity to catalyze decomposition of the initiator into free radicals. Aromatic amines such as dimethyl aniline or dimethyl /j-toluidine are added to promote that reaction. The two components are combined prior to application, ensuring even distribution of initiator in the system. The dosage of initiator, accelerator, promoter and inhibitor will determine the pot life, the longest period of time during which mixture is still usable and can be applied. 

Thermal decomposition of dihydroperoxides results in initial homolysis of an oxygen—oxygen bond foUowed by carbon—oxygen and carbon—carbon bond cleavages to yield mixtures of carbonyl compounds (ketones, aldehydes), esters, carboxyHc acids, hydrocarbons, and hydrogen peroxide. 

A general reaction mechanism for the grafting of MA onto EPM is given in Figure 13.3 [15,16]. Free-radical grafting of MA starts with the decomposition of the radical initiator, usually a peroxide [15,18]. The peroxide decomposes at elevated temperamres into the corresponding oxy radicals, which may further degrade to alkyl radicals and ketones. These oxy and alkyl radicals abstract... 

From the decomposition mechanism and the products formed it can be deduced that DCP primarily generates cumyloxy radicals, which further decompose into highly reactive methyl radicals and acetophenone, having a typical sweet smell. Similarly, tert-butyl cumyl peroxide (TBCP) forms large quantities of acetophenone, as this compound still half-resembles DCP. From the decomposition products of l-(2-6 rt-butylperoxyisopropyl)-3-isopropenyl benzene ( ), it can be deduced that the amount of aromatic alcohol and aromatic ketone are below the detection limit (<0.01 mol/mol decomposed peroxide) furthermore no traces of other decomposition products could be identified. This implies that most likely the initially formed aromatic decomposition products reacted with the substrate by the formation of adducts. In addition, unlike DCP, there is no possibility of TBIB (because of its chemical structure) forming acetophenone. As DTBT contains the same basic tert-butyl peroxide unit as TBIB, it may be anticipated that their primary decomposition products will be similar. This also explains why the decomposition products obtained from the multifunctional peroxides do not provide an unpleasant smell, unlike DCP [37, 38]. 

The mechanism of the oxidation may be interpreted by the following steps. (1) Oxygen attacks the methyl group at the aid of the longest open chain of the hydrocarbon to form water and an aldehyde, probably through the decomposition of initially formed peroxides. (2) The aldehyde is oxidized to a lower aldehyde, water, carbon monoxide, or carbon dioxide. (3) In the case of the branched isomers, this process continues until a branch in the molecule is reached, giving rise to a ketone instead of an aldehyde as the product. (4) Oxidation at low temperature slows down at this stage since ketones oxidize with more difficulty than aldehydes. 

Acrylic esters and unsaturated polyesters are commercially cured with peroxides or peresters. The choice of per compound is determined on the basis of price, the achievable polymerization rate, and the side products formed. The polymerization rate is determined by the decomposition rate of the initiator, when mixed with the material to be cured, as well as on the free radical yield. In addition, attention should be paid to the fact that many per compounds decompose slowly during storage, thus reducing the polymerization activity per unit initiator mass. For this reason, crystalline per compounds are more stable because of the lower diffusion than amorphous or dissolved per compounds. Side products of initiator compounds can have an unfavorable effect on the long-term thermoset properties dibenzoyl peroxide, for example, forms acids dicumyl peroxide forms ketones. Acids can hydrolyze the ester bonds of polyester chains, causing scission, and ketones can... 

Undissociated hydrogen peroxide also behaves as a nucleophile to some extent, being about 10 times as nucleophilic as water. For example, H2O2 readily adds to many carbonyl double bonds, giving hydroxyhydroper-oxides (peracetals and perketals). Compounds prepared thus are used as polymerisation initiators on account of their radical decomposition at moderate temperatures (—O—O— bond homolysis). However, they can be hazardous if formed accidentally in other synthesis reactions, for which ketones are therefore not recommended solvents. Neutral H2O2 can also... 

The oxidation of alkanes by r-butyl hydroperoxide (TBHP) has been catalysed by titanium alkoxides, producing the corresponding alcohols and ketones. A radical mechanism is proposed in which r-butoxyl radical formed from TBHP and titanium alkoxide initiates the reaction. The evolution of oxygen (from the decomposition of peroxide) and the abstraction of hydrogen from alkane to form alkyl radical occur competitively. A method for the determination of both the primary and secondary KIEs at a reactive centre based on starting-material reactivities allows the determination of the separate KIEs in reactions for which neither product analysis nor absolute rate measurements are applicable. It has been applied to the FeCls-catalysed oxidation of ethylbenzene with TBHP, which exhibits both a primary KIE and a substantial secondary KIE the findings are in accordance with previous mechanistic studies of this reaction. The oxidation of two l-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy-acids and TBHP catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin [Fe(ni)P] is a two-step process. In single turnover reactions, dye and Fe(in)P compete for the initially formed OFe(IV)P+ in a fast reaction and OFe(IV)P is produced the peroxy acid dye stoichiometry is 1 1. This is followed by a slow phase with 2 1 peroxy acid dye stoichiometry [equivalent to a... 

There was no induction period in the solution polymerization of vinyl chloride [52] initiated by the benzoyl peroxide-dimethylaniline system in various solvents such as tetrahydrofuran, ethylene dichloride, dioxane, cyclohexanone, methylethyl ketone, and so forth. The initial rate of polymerization and the conversion was directly and inversely proportional to the temperature, respectively. The polymerization was restricted to only 20% conversion, probably due to the complete consumption of benzoyl peroxide. Without the monomer, the extent of decomposition on benzoyl peroxide reaches a constant value regardless of the temperature and amount of di-methylaniline. It was seen that the greater the amount of dimethylaniline, the faster the initial rate of polymerization and the lower the conversion. The degree of polymerization of vinyl chloride obtained by the redox system benzoyl peroxide-dimethylaniline was generally lower than the polymer obtained by the benzoyl peroxide system alone. The activation energy of the polymerization by the redox system was lower than that of the benzoyl peroxide alone initiated polymerization and found to be 12.5 kcal mol The initial rate of polymerization could be expressed as... 

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