Polypropylene peroxidation chain

Peroxide initiator is used to abstract hydrogen atoms attached to tertiary carbon atoms along the polypropylene molecular chain. This initial reaction step generates a number of free-radical reactive sites. 

MMA onto cellulose was carried out by Hecker de Carvalho and Alfred using ammonium and potassium persulfates as radical initiators [30]. Radical initiators such as H2O2, BPO dicumylperoxide, TBHP, etc. have also been used successfully for grafting vinyl monomers onto hydrocarbon backbones, such as polypropylene and polyethylene. The general mechanism seems to be that when the polymer is exposed to vinyl monomers in the presence of peroxide under conditions that permit decomposition of the peroxide to free radicals, the monomer becomes attached to the backbone of the polymer and pendant chains of vinyl monomers are grown on the active sites. The basic mechanism involves abstraction of a hydrogen from the polymer to form a free radical to which monomer adds ... 

Stearamide is one of many electron donors which donate an electron to the cationic moiety in excited MAH or in propagating -MAH chains. This results in the inhibition of the homopolymerization of MAH and decreases the crosslinking of polyethylene and the degradation of polypropylene which accompany the peroxide-catalyzed reaction of MAH with these polyolefins (8,9). ... 

The rhodium-catalyzed borylation of alkanes is applied to regiospecific functionalization of polyolefines.165,165a The reaction of polypropylenes (atactic, isotactic, and syndiotactic) with B2pin2 in the presence of Cp Rh( 74-C6Me6) catalyst at 200 °G affords the borylated polymers, which are treated with basic hydrogen peroxide in a mixture of THF and H20 to oxidize the boronate esters to the corresponding alcohols (Scheme 20). The hydroxylated polymers contain 0.2-1.5% hydroxymethyl side-chains. 

Polymer stabilization is another area in which the peroxide-decomposing and chain-breaking antioxidant properties of diorganotellurides has found utUity. Alone or in combination with phenol and phosphate antioxidants, electron-rich dialkylamino-substirnted diaryltellurides and alkylaryltellurides provided greatly enhanced polymer stability for a thermoplastic elastomer and for polypropylene. The effects were unique to the tellurides, with selenides not providing similar protective effects. ... 

Electron beam irradiation is one of the methods of cross-linking in fhis process. The other methods use peroxide, multifunctional azide, or an organofunctional silane. Polyethylene resins respond to electron beam irradiation well since the rate of cross-linking exceeds significanfly fhe chain scission. Polypropylene (PP) is prone to P-cleavage, which makes if difficult to cross-link by a free radical process. For fhaf reason, PP resins... 

The relationship of structure to activity is discussed on the basis of measurements performed in the presence of 0.05 mole of antioxidant per kg. of polypropylene. The results obtained at lower concentrations (0.01 and 0.025 mole/kg.) showed some differences in details. On the other hand, almost identical relationships were found (45) in stabilizing polypropylene at higher antioxidant concentrations (0.1 mole/kg.). Analysis of those concentration relationships supports our assumption that the activity of pyrocatechol derivatives is influenced above all by reactions between the peroxidic bodies and antioxidants in the oxidation chain-breaking mechanism. 

The phenolic initially gives up its labile hydrogen, which in turn reacts with the various radicals produced in chain reactions then the phenoxy radical becomes stabilized owing to its ability to form resonance structures. The resonance-stabilized forms of the phenoxy radical will not attack tertiary carbon—hydrogen bonds in the polypropylene chain but will react with other radicals such as a peroxide, resulting in the elimination of a second free radical. 

Inactive polypropylene powder, when treated with N,N-dimethyl-aminoethyl methacrylate and benzoyl peroxide in hexane at 30°C. did not possess detectable amounts of —COOR absorbance. The absence of —COOR absorbance in the resultant polymer gives further evidence that N,N-dimethylaminoethyl methacrylate either did not polymerize under the conditions used or formed only a very low molecular weight polymer which was washed out by the finishing procedure used. It also established that an active polymer chain is necessary and that poly (N,N-di-methylaminoethyl methacrylate) does not exist as a mixture with polypropylene but is tied into the polypropylene chain. 

Another crosslinking system uses peroxide and sulfur or a sulfur compound where the yield of insoluble gel is very senative to the ratio of peroxide/sulfur [127]. The optimum for the formation of insoluble gd is readied for the ratio of dicumyl peroxide/sulfur dore to 1 1. The role of sulfur in polyjaopylene crosslinking is similar to that of a multifunctional monomor. Sulfur suppresses side reactions of polypropylene macroradicals, and reduces the number of polymer main chain scissions. Addition of a multifunctional monomer as a third conponent of the crosslinking system does not increase the gel content, rather it reduces it. This effect can be explained when considering the scavenging of sulfur from the reaction system by its uneffective reactions with multifunctional monomers. 

Polypropylene is more susceptible to melt degradation than polyethylene, because of the presence of more reactive tertiary hydrogen atoms (attached to the carbon atom that is bonded to three other C atoms). At a temperature of 270 °C in injection moulding, tertiary alkyl free radicals R are generated thermally. If oxygen is present, a rapid reaction (R + 02- ROO ) produces a peroxide radical, which reacts further to form hydroperoxides (ROO -h RH ROOH-h R ). When the dissolved oxygen is used up, there is a greater chance of the chain scission reaction... 

Melt degradation is used commercially to narrow the molecular weight distribution, from a starting value of Mw/Mn = 5, to make it more suitable for fibre spinning and blow moulding. Random chain scission at tertiary C—H bonds eventually produces a most probable molecular weight distribution with M /Mn = 2. In practice, the distribution, produced by deliberately degrading the polypropylene with added peroxides, is broader, but... 

As was found by microphotometration, the concentration gradient of the grafted PAA inside the membrane and its profile depend very much on the amount of the monomer absorbed. It was also evident that, because of the method used for preparation, polymerization of the acid was initiated mainly by PO radicals formed on polypropylene chains by thermal and redox decomposition of POOH hydroperoxides or POOP peroxides. The homopolymer of polyacrylic acid which was also formed inside the film was later removed by long time elution with methanol. 

As has been shown, PP oxidation occurs predominantly intramolecularly, the kinetic chain moves along the macromolecule. Macroradical RO2, formed by the oxidation of polypropylene, reacts with a hydrogen atom from the tertiary C atom located in the P-position relative to the peroxide radical of their molecules. As a result, intramolecular transfer of a macromolecule oxidized PP formed "blocks" of several adjacent OH-groups. 

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