Polypropylene macroradicals

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. 

Upon photolysis of polypropylene hydroperoxide (PP—OOH) a major absorption at 1726 and 1718 cm has been observed in the IR spectrum, which is attributed to the carbonyl groups. Sometimes the macroradical having free radical site reacts with a neighboring newly born hydroperoxide causing the formation of a macroalkoxy radical [116]. 

The transfer of hydrogen to peroxyl radicals may proceed intra- or inter-molecularly. Intramolecular transfer reaction (isomerization) of peroxyl macroradicals of polypropylene occurs during the oxidation of the polymer in a solution of inactive solvent [75] while the intermolecular transfer is preferred during the oxidation in reactive solvent or in the crystalline state [76]. 

The majority of packaging plastic materials consists of polyolefins and vinyl polymers, namely polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). Obviously, these polymers have many other applications not only as packaging materials. Chemically they are all composed of saturated hydrocarbon chains of macro-molecular size their typical thermal decomposition pathway is free radical one initiated by the homolytic scission of a backbone carbon-carbon bond. In spite of the basic similarity of the initial cleavage, the decomposition of the hydrocarbon macroradicals is strongly influenced by fhe nafure of the side groups of the main chain. 

The final structure of a grafted product will depend on the kjk, kjk, and ratios. For PE macroradicals, high values of k are typical and grafting is followed by cross-linking of the chains. For polypropylene, chain degradation is typical because of high fes values. In the case of ethylene-propylene rubber, there is observed a competition between the two side reactions. 

If irradiation of polypropylene is carried out in the presence of acetylene, the efficiency of crosslinking increases substantially [72]. This may be explained by a reaction of alkyl macroradicals with acetylene to give radicals of the allyl type which are known to combine preferentially. 

Rate constants of macroradical decay in isotactic polypropylene chtinge in relation to the content of the crosslinks [74]. For the first stage of crosslink formation, the rate constant of radical decay decreases by about 1 order for higher conversions of ca osslinking the rate constant increases. The initial decrease of the rate constant seems to be associated with a reduced mobility of macromolecular segments, while the subsequent increase with a gradual reduction of polymer crystallinity. 

The behavior depicted in Fig. 7.2 is observed with many polymers upon exposure to sunlight, including with commercial polyalkenes such as polyethylene and polypropylene. In the latter cases, impurity chromophores act as initiators of the autoxidation process (see Scheme 7.4 in Section 7.1.3). Important elementary reactions determining the autoxidation process are described in the following. Free radicals Rx formed during the initiation phase abstract hydrogen atoms from macromolecules PH, thus forming macroradicals P [Eq. (7-11)]. 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

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. 

Only one of the amine groups of this stabiliser is oxidised to ARs during photolysis. The part of monoradicals formed recombines with alkyl macroradicals of polypropylene, and then the second amine group is converted into ARs. 

As with smaller radicals, the reactivity of a macroradical much depends on the electron density of the atom bearing the unpaired electron. If reactivity is low, the chance of a radical being trapped before abstraction can occur increases with decreasing reactivity. For radicals formed by degradation, the lifetimes follow the order P- < PO- < POz. Free isolated POz in polypropylene may have a half-lifetime as long as 500 s at ambient temperature. To account for this exceptional stability, and in analogy to the post-gel effect in free-radical polymerization, it may be... 

Baramboim and Rakityanskii [136-139] produced graft copolymers based on polyamides. The extrusion of a polypropylene-polycaprolactam mixture at 200-210°C produced radicals with a resultant change in polypropylene molecular weight and formation of block and graft copolymers. The intensity of degradation and, consequently, the extent of polypropylene reaction decreased with an increase in the amount of polyamide in the mixture as noted by a reduction of the mixture melt viscosity. Radical acceptors interfere with copolymerization by interacting with the macroradicals [139]. 

Another distinction of polymer autoxidation fixim the oxidation of hydrocarbons concerns reactions involving alkyl radicals. Under conditions of liquid-phase oxidation, usually all alkyl radicals are transformed into peroxyl radicals (the reaction of R- with O2 occurs very rapidly, see Chapter 4). In solid polymer the alkyl macroradical reacts wifli O2 much more slowly (see Chapter 6). This results for polypropylene (PP) oxidation, for example, in the fact that the following two reactions compete with commensurable rates ... 

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